GB2428342A

GB2428342A - Sensing the proximity of machinery relative to a person

Info

Publication number: GB2428342A
Application number: GB0514291A
Authority: GB
Inventors: Geoffrey Pegman
Original assignee: ROBOTICS Ltd CONST
Current assignee: ROBOTICS Ltd CONST
Priority date: 2005-07-12
Filing date: 2005-07-12
Publication date: 2007-01-24
Anticipated expiration: 2025-07-12
Also published as: GB0514291D0; GB2428342B

Abstract

A proximity monitoring apparatus is disclosed comprising a transmitter 2 mounted on a heavy vehicle 1 and a detector unit 6 mounted upon a hard hat 5 worn by a person 4. The transmitter 2 radiates a given electromagnetic signal 7A and a higher frequency other electromagnetic signal 7B from the antenna 3. The wavelength of the given signal is selected such that the near-field of the radiated signal has a range which fully encompasses the full range of normal separation distances R which will occur as between the transmitter 2 and the person 4 in normal circumstances. The wavelength of the other (higher frequency) signal is selected such that the near-field of the radiated signal has a range which falls short of the full range of separation distances R. The detector unit 6 is responsive to both the given and other received signals to determine the separation of between the transmitter 2 and the detector unit 6, and is arranged to indicate to the person 4 wearing the detector unit 6 when the measured distance is less than a predetermined value corresponding to a danger/alarm separation.

Description

Proximity Sensing The present invention relates to apparatus and methods

for sensing the proximity of an object relative to another object. In particular, though not exclusively, the present invention relates to the sensing of the proximity of machinery relative to a person.

In many application areas, but particularly within the construction industry, people can come to harm through large movable machinery crushing them or striking them.

Traditional audio warning devices are often ineffective in alerting people of the proximity of machinery in noisy environments, particularly where the wearing of ear defenders may be a requirement. Other approaches to sensing moving objects involve continuous tracking of the positions of persons and/or machinery, but these suffer either from the expense of the tracking equipment or from the low reliability of the system due to the difficulty of determining the distance with the required accuracy to ensure a correct warning without issuing too many false alarms.

The present invention aims to provide a cost-effective solution to the problem of providing warning to people in a reliable manner and without significant false alarms.

At its most general, the present invention proposes proximity sensing using a near field component of a radiated electromagnetic field to determine the separation between the articles whose relative proximity is to be sensed. This ensures that the advantages of the structure of the near-field spatial component of the electromagnetic may be taken advantage of in normal use, as discussed with reference to figure 1.

The invention may provide is a proximity detection system that utilises the propagation features associated with the near-field of an electromagnetic field to accurately detect the proximity of an object carrying the transmitting unit. The near field is the field close to field source or the transmitter (i.e. less than one radian wavelength distance) and is contrasted with longer scale distances in which the normal, or far field, effects are seen. For the purposes of the current invention it is relevant to note that the primary near field effect that is of importance is the spatial field gradient - the rate of field attenuation with respect to distance between the transmitter and the receiver.

The equation below shows the magnetic field of a radiated electromagnetic signal received at a receiver (e.g. a receiving coil) at a distance from the field source (e.g. a transmitting coil) when the two coils are coplanar: H = jwi0mfl 1 +__1__exp(-jflR) Equation (1) 4ri /3R (/3R)2 (fiR)3) where p0 is the permeability of free space, /3 = 2. 'r / A is the inverse of the radian wavelength of the electromagnetic signal of wavelength 2, w is the angular frequency of the signal, i is the impedance of free space, j = /1i, and R is the separation between the receiver and the transmitter. The quantity /1 is also known in the art as the electrical length per metre of wavelength and may also be given by /1 = w/c where c is the speed of light in a vacuum.

This equation, which is found from a solution of Maxwell's Equations, can be divided into three different components: (a) a "radiation term" proportional to R1, which represents the flow of energy away from the transmitter and is the dominant component at large separations A; (b) an "induction component" proportional to R2 which represents the energy stored in the electromagnetic field of the signal during one quarter cycle and then returned to the transmitter the next quarter cycle, the contribution from this component exceeds that from the radiation component at relatively small separations; (c) an "electrostatic field component" proportional to R3 and results from an accumulation of charge within parts of the antenna of the transmitter, this component dominates the field of the signal at relatively small separations R. Figure 1 illustrates the variation of the magnitudes of these three components as a function the separation R between transmitter and receiver in multiples of the radian wavelength (i. e. the quantity fiR = 2rR / 2) and clearly illustrates that all three terms become equal in magnitude when 1[3R = 1.0 meaning that R = 2/2it. That is to say, when the separation between transmitter and receiver is equal to one radian wavelength of the transmitted electromagnetic signal, all three components have equal magnitude. When the separation exceeds a radian wavelength of the signal (R> 2/27r), then the radiation component dominates and this is known as the "far- field" of the signal, however, when the separation is less than a radian wavelength (R < 2 / 2.7r) the electrostatic field component dominates, this is known as the "near-

field" of the signal.

The present invention is born of the realisation that the near field of the signal has a relatively high spatial field gradient and so enables much more sensitive separation measurements when using the detected magnitude of the received signal in determining the separation since variations in detected magnitude (e.g. through noise) will equate to relatively small variations (e.g. errors) in the value of the separation determined from the detected signal magnitude. Thus, by appropriately selecting the wavelength of the radiated signal, one may generate a signal with a near-field which always envelops the range of separations A which would occur in normal use of the transmitter and receiver, such as on a building, construction, mining or quarrying site which may typically be of the order of a few metres or less to a few tens of metres at the most.

The invention relates to, for example, safety devices and concerns the sensing of objects and particularly vehicles that may cause harm to a person by impacting or crushing them. The invention is concerned with e.g. , active electro-magnetic sensing and based on an exploitation of a property of the near-field of a transmitter. The invention is particularly applicable to outdoor applications involving large vehicles or movable machinery, but may have application within factories or other indoor environments. The invention may employ a tactile stimulus to warn a user of vehicle/machinery proximity rather than (or in addition to) audio warning, to enable it to be used in noisy environments. The invention may provide a wearable device which can easily be carried from place to place to provide effective protection in noisy environments.

Accordingly, in a first of its aspects, the present invention may provide a detector for a proximity monitoring apparatus, the detector including a receiver means arranged to wirelessly receive electromagnetic signals radiated by a wireless transmitter means of the proximity monitoring apparatus, the detector being arranged to determine the separation between the receiver means and the transmitter means according to the near-field of a given electromagnetic signal from the transmitter means in conjunction with the far-field of another electromagnetic signal from the transmitter means. In this way, the aforementioned increased accuracy of measurement stemming from the use of the higher spatial field gradients of the near- field component of the received given signals may be taken advantage of. The given electromagnetic field most preferably includes a signal selected/scaled such that the near-field spatial component of the given signal encompasses most or all of the range normal separations of the objects whose separation is to be monitored in normal use. The detector is preferably arranged to generate an indicator (e.g. warning) signal indicating when the separation is determined thereby to be less than a predetermined value, in which the predetermined value is less than the radian wavelength of given signal, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 2rc.

Most preferably, the detector is arranged to determine the separation according to a measure of the far-field of this other electromagnetic signal determined relative to a measure of the near-field of the given electromagnetic signal.

For example, the measured or detected far-field of the other electromagnetic signal received by the receiver means may be scaled or normalised using the measured or detected near-field of the given electromagnetic signal at the transmitter means. The result would be a measure of the size of the far-field of the other electromagnetic signal expressed in terms of (e.g. as a fraction, ratio or proportion) of the size of the near-field of the given electromagnetic signal. Since the farfield and near-field components have sizes which scale differently as a function of the separation between transmitter and receiver means (i.e. separation A), then this relative measure of the far-field of the other electromagnetic signal will itself be dependent upon (e.g. be a function of) the separation in question. Consequently, the detector means may be preferably arranged to calculate the separation using the separationdependent relative measure of the far-field of the other electromagnetic signal as described above. The use of such a relative measure has the advantage of exploiting the sensitivity inherent in the high spatial gradient of the near-field of the given electromagnetic signal used in forming the relative measure.

For example, the detector may be arranged to determine the separation according to the ratio of the amplitudes or magnitudes of: said other electromagnetic signal representing said far-field, and; said given electromagnetic signal representing said near-field. Given that a nearfield predominantly scales as the inverse cube of the separation (R3) while a far-field predominantly scales as the inverse of separation (R1), the ratio will predominantly scale as the inverse of the square of the separation (R2).

The detector is preferably arranged to determine said separation using the square- root of said ratio.

The detector may be arranged to determine said separation using said nearfield and said far-field of said given and said other electromagnetic signals, respectively, substantially simultaneously received by the receiver means.

Alternatively, the given and other signals may be received in succession, but not together or not wholly together.

The detector preferably includes a receiver means tuned to be preferentially responsive to said given electromagnetic signals from the transmitter which have a radian wavelength greater than the separation between the transmitter and the detector in normal use of the proximity monitoring apparatus. For example, the detector may be a wireless receiver including an antenna and wireless receiver circuitry such as would be readily apparent to the skilled person. The antenna/receiver circuitry may be tuned to resonate at a frequency corresponding to the radian wavelength of the given electromagnetic signal. The frequency of the given signal may be between about 20 MHz and about 100 KHz so as to produce a signal having a near-field component extending to a distance from the transmitter of between about a few metres to about 500 hundred metres. Most preferably, the frequency is about 125KHz thereby ensuring that the near-field of the signal fully and adequately encompasses the range of normal separations (e.g. from several tens of metres to less than a metre) between persons and machinery/vehicles in a typical industrial scene.

Preferably, the given electromagnetic signal is generated such that any component of the received signal at the detector, in normal use, other than the component thereof which scales as the inverse of the cube of the distance from the transmitter (the "electrostatic field component"), is negligible in magnitude relative to the electrostatic field term. Thus, the signal is preferably generated such that, in normal use, the "radiation component" and the "induction component" of the given signal, which scale as the inverse and the inverse square, respectively, of the distance from the transmitter, are negligible relative to the "electrostatic field component". This ensures that the component of the given signal having the steepest spatial gradient is employed in determining separations. The components in question may be "magnetic field components" and/or "electrical field components".

Additionally, measurement of the "radiation component" of the other signal (which dominates in the far-field) is predominantly used in conjunction with measurement of the near-field component to determine the separation being monitored. For example, the contemporaneous measurement of the "electrostatic field component" (proportional to R3) of the given electromagnetic signal and "radiation field component" (proportional to R1) of the other electromagnetic signal (of higher frequency) may be combined to form a ratio of the two measurements. The magnitude of the ratio will be substantially proportional to the square of the separation being monitored (proportional to R2). The detector may be arranged to determine the separation using the square root of the ratio so determined. This is particularly useful is accounting for variations in detected signal magnitudes arising from variations in the orientation/alignment between the detector and the transmitters from which it receives signals. A lack of coplanar alignment as between the detector and a transmitter will result in a common reduction in the detected magnitude of the near-field component of the given two signal and the far-field component of the other of the two signals received contemporaneously from the same transmitter, and thus the ratio of contemporaneously detected near-field and far-field components will cancel the common reduction and will be substantially insensitive to such misalignments.

The detector being arranged to determine said separation using a farfield spatial component of the radiated other electromagnetic signal in conjunction with the near- field spatial component of the given electromagnetic signal, it is particularly suited to avoiding errors in the determination of the separation between transmitter and detector which might otherwise arise as a result of a detected reduction in received signal magnitude being interpreted as an increase in separation, where in fact it is wholly or partly the result of a change in transmitter/detector orientation or alignment.

This type of misinterpretation could have serious consequences where small separations are being monitored as between a person and a moving piece of heavy machinery.

Preferably, the near-field of the given electromagnetic signal is the parts thereof extending from the transmitter means to a distance not exceeding the radian wavelength thereof, and the far-field of the other electromagnetic signal is the parts thereof at any distance exceeding the radian wavelength thereof, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 27t.

The frequency of the other electromagnetic signal may be 400 MHz or higher.

The detector may be arranged to wirelessly detect the given electromagnetic signal and said other electromagnetic signal in which the radian wavelength of the other electromagnetic signal is less than the radian wavelength of the given electromagnetic signal. In this way, the detector is preferably arranged to use the near-field of the given electromagnetic signal in conjunction with the far-field of the other electromagnetic signal to determine said separation.

The detector (or the receiver means thereof) is preferably tuned to be preferentially responsive to the other electromagnetic signals from the transmitter which have a radian wavelength less than said separation. This is most preferably in addition to the detector (or receiver means) being preferentially responsive to the given electromagnetic signals. The receiver means may include one receiver antenna dedicated to detection of the given signals, and another receiver antenna dedicated to the other signals. These antennae may have dedicated receiver circuits or a common receiver circuit such as would be readily apparent to the skilled person.

Alternatively, a single de-tuned antenna may be employed as receiver means sufficiently de-tuned to be preferentially responsive to both the given signals and the other signals.

The receiver means preferably tuned to be preferentially responsive to said given electromagnetic signals from the transmitter means which have a radian wavelength greater than the separation between the transmitter means and the receiver means in normal use of the proximity monitoring apparatus.

Additionally or alternatively, the detector may be arranged to determine its relative orientation with respect to the transmitter from which it receives signals by employing an orthogonally disposed set of receiving antennae and by incorporating means for calculating the relative orientation using the relative magnitudes of concurrently received given (or other) signals thereat. The method of calculation could be any suitable method such as would be apparent to the skilled person and could exploit the fact that the relative magnitudes would be dependant upon the orientation of the transmitter relative to the axis about/along which the receiver antennae are separately aligned. The detector may include two (or three) antenna coils for receiving electromagnetic signals from said transmitter wherein each antenna coil is wound about a respective coil axis substantially orthogonal to the coil axis of the other antenna coil.

Preferably, the detector includes indicator means arranged to generate an indicator signal when the detector has determined that said separation is no greater than a predetermined distance within the near-field spatial component of the given electromagnetic signal thereby indicating the presence of the transmitter at or within a range not exceeding the predetermined distance. The indicator signal may be visual (e.g. a light), or audible (e.g. a buzzer or siren) or may be tactile (e.g. shaking or physical movement of the detector, or an actuator/rod included with the indicator means for prodding a wearer).

The detector may be arranged for use in a proximity monitoring apparatus comprising a plurality of said wireless transmitters. In such a case the detector is preferably arranged to periodically determine said separation with respect to any of said plurality of transmitters according to separate signals (e.g. in the form of separate signal pulses) received thereby from the transmitters in use, and to generate said indicator signal only after the detector has determined from two consecutive determinations (e.g. pulsed signals) using signals received from a given transmitter on separate occasions that said separation with respect to that transmitter is no greater than a predetermined distance. In this way, one may avoid detection errors arising from destructive interference between simultaneously received signals from different transmitters. Such destructive interference would result in at least a reduction, possibly complete, of both of the concurrently received separate signals potentially leading to an erroneous over-estimate of the separation between the detector and either/both transmitters. Once more, this overestimate could have serious consequences if the true separation were dangerously small. By effectively "double- checking" an initial detection of a separation within the predetermined range, the detector means is able to remove most or all false detections since the chances of two such false detections occurring immediately consecutively as a result of destructive signal interference is very small.

The detector is preferably arranged to determine said separation according to the magnetic field component of said received electromagnetic signals (given signals and other signals). Alternatively, or additionally, the electrical field component of the radiated signals may be used to determine said separation. Magnetic field intensity and/or electric field intensity detector means (e.g. a Hall Effect detector means) such as would be readily apparent to the skilled person may be employed.

It is most preferable that the detector may be worn by a person during activities involving vehicles or heavy moving machinery (lorries, quarrying/mining/construction equipment), and that transmitters to which the detector is responsive may be located upon such vehicles/machines. Preferably, the detector includes attachment means for attaching the detector to an item of apparel of a user in normal use. In another aspect, the invention may provide a hat including a detector according to any preceding claim.

The detector may include an antenna receiver means (or several of them) having a winding containing a ferrite core. The antenna may be connected to an amplifier means for receiving and amplifying signals received by the antenna receiver and for outputting amplified signals to a comparator means for determining a measure of said separation according to amplified signals received thereby from the amplifier means and for comparing the measure to a predetermined signal threshold value.

Preferably, the comparator means is operable to activate the indicator means when the predetermined threshold is exceeded, or when it is exceeded on two (or more) successive occasions.

It will be understood that the present invention may provide a transmitter means for generating said given electromagnetic signal, or said given and said other electromagnetic signals being scaled as discussed above to permit the detector to implement the invention described above.

Accordingly, in a second of its aspects, the present invention may provide a transmitter for a proximity monitoring apparatus arranged to wirelessly radiate a given electromagnetic signal and another electromagnetic signal each detectable by a wireless receiver of the proximity monttoring apparatus, the transmitter being arranged to generate the given electromagnetic signal to have a near-field extending a distance exceeding the separation between the transmitter and the detector in normal use and to generate the other electromagnetic signal to have a near-field extending a distance less than the separation between the transmitter and the detector in normal use, such that the detector may reside in the near-field of the given electromagnetic signal of the transmitter and simultaneously in the far-field of the other electromagnetic signal of the transmitter.

The transmitter is preferably tuned to preferentiafly radiate said given electromagnetic signals which have a radian wavelength greater than said separation in normal use of the proximity monitoring apparatus, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 27t.

The transmitter may be arranged to wirelessly radiate the other oscillating electromagnetic signal with a radian wavelength less than the radian wavelength of the given electromagnetic signal such that the nearfield spatial component of the other electromagnetic signal extends a distance less than the separation between the transmitter and the detector in normal use. The transmitter is preferably tuned to preferentially radiate the other electromagnetic signals having a radian wavelength less than said separation.

The transmitter may be arranged for use in a proximity monitoring apparatus comprising a plurality of such transmitters. In such a case, the transmitter is preferably arranged to radiate electromagnetic signals as signal pulses each transmitted only during a time period randomly temporally located within each respective one of a succession of separate predetermined time intervals. In this way, a succession of separate signal pulses may be radiated by the transmitter in a quasi- regular manner in the sense that a given pulse can will be generated within each successive time interval, while the exact time of generation is random. For example, the transmission time interval/window may be lOOms in duration, while a single pulse of ims duration is generated once only at a random instant within that lOOms transmission window. This means that when a plurality of such transmitters are used, the chances of simultaneous pulse overlap as between two transmitters is greatly reduced even if the two transmitters should accidentally (or deliberately) have synchronous transmission windows.

The transmitter preferably includes attachment means for attaching the transmitter to a vehicle or an item of machinery.

In a third of its aspects, the present invention may provide a proximity monitoring apparatus including a transmitter means arranged to wirelessly radiate electromagnetic signals and a detector including a receiver means arranged to wirelessly receive electromagnetic signals radiated by a transmitter means, the detector being arranged to determine the separation between the receiver means and the transmitter means according to the near-field of a given electromagnetic signal from the transmitter means in conjunction with the far-field of another electromagnetic signal from the transmitter means.

The detector of the proximity monitoring apparatus may be arranged to determine said separation according to a measure of the far-field of said other electromagnetic signal determined relative to a measure of the nearfield of said given electromagnetic signal.

The detector of the proximity monitoring apparatus may be arranged to determine said separation according to the ratio of the amplitudes of: said other electromagnetic signal representing said far-field, and; said given electromagnetic signal representing

said near-field.

The detector of the proximity monitoring apparatus is preferably arranged to determine said separation using the square-root of said ratio.

Preferably, the detector is arranged to determine said separation using said near- field and said far-field of said given and said other electromagnetic signals, respectively, substantially simultaneously received from the transmitter means.

In connection with the proximity monitoring apparatus most preferably the near-field of the given electromagnetic signal is the parts thereof extending from the transmitter means to a distance not exceeding the radian wavelength thereof, and the far-field of the other electromagnetic signal is the parts thereof at any distance exceeding the radian wavelength thereof, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 2it.

Preferably, the detector is arranged to wirelessly detect said given electromagnetic signal and said other electromagnetic signal in which the radian wavelength of the other electromagnetic signal is less than the radian wavelength of the given electromagnetic signal, wherein the detector is arranged to use the near-field of the given electromagnetic signal in conjunction with the far-field of the other electromagnetic signal to determine said separation.

The receiver means is preferably tuned to be preferentially responsive to said given electromagnetic signals from the transmitter means which have a radian wavelength greater than the separation between the transmitter means and the receiver means in normal use of the proximity monitoring apparatus..

The receiver means of the proximity monitoring apparatus preferably is tuned to be preferentially responsive to said other electromagnetic signals from the transmitter means which have a radian wavelength less than the separation between the transmitter means and the receiver means in normal use of the proximity monitoring apparatus.

The transmitter of the proximity monitoring apparatus is preferably tuned to preferentially radiate said given electromagnetic signals which have a radian wavelength exceeding the separation between the transmitter and receiver in normal use, and the detector is tuned to be preferentially responsive to said preferentially radiated given electromagnetic signals from the transmitter.

The proximity monitoring apparatus may be arranged such that the transmitter is arranged to wirelessly radiate said other electromagnetic signal with a radian wavelength less than the radian wavelength of thegiven electromagnetic signal such that the near-field of the other electromagnetic signal extends a distance less than the separation between the detector and the transmitter in normal use.

The detector of the apparatus is preferably arranged to generate an indicator (e.g. warning) signal indicating the when the separation is determined thereby to be less than a predetermined value, in which the predetermined value is less than the radian wavelength of given signal, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 2it.

The detector of the apparatus is preferably tuned to be preferentially responsive to saTd further electromagnetic signals from the transmitter which have a radian wavelength less than said separation. Similarly, the transmitter of the apparatus is preferably tuned to preferentially radiate said further electromagnetic signals having a radian wavelength less than said separation.

The detector of the apparatus may be arranged to determine said separation according to the ratio of the amplitudes of the received near- field and far-field components of the given electromagnetic signal and the further electromagnetic signal respectively.

The detector of the apparatus may include two antenna coils for receiving electromagnetic signals from said transmitter wherein each antenna coil is wound about a respective coil axis substantially orthogonal to the coil axis of the other antenna coil.

Preferably, the detector of the apparatus includes indicator means arranged to generate an indicator signal when the detector has determined that said separation is no greater than a predetermined distance within the near-field spatial component of the given electromagnetic signal thereby indicating the presence of the transmitter at or within a range not exceeding the predetermined distance.

The proximity monitoring apparatus may comprise a plurality of said transmitters each arranged in use to radiate said electromagnetic signals as signal pulses each transmitted only during a time period randomly temporally located within each a respective one of a succession of separate predetermined time intervals.

When the proximity monitoring apparatus comprises a plurality of said wireless transmitters, the detector may be arranged to periodically determine said separation with respect to any of said plurality of transmitters according to separate signal pulses received thereby from the transmitters in use, and to generate said indicator signal only after the detector has determined from two consecutive pulsed signals received from a given transmitter that said separation with respect to that transmitter is no greater than a predetermined distance.

In the proximity monitoring apparatus the detector is arranged preferably to determine said separation according to the magnetic field component of said received electromagnetic signals.

The detector of the proximity monitoring apparatus preferably includes attachment means for attaching the detector to an item of apparel of a user in normal use. For example, the proximity monitoring apparatus may include a hat to which said detector is attached. The proximity monitoring apparatus may include attachment means for attaching the detector to a vehicle or an item of machinery.

It is to be understood that the invention as described above implements a method of proximity detection, and that the present invention encompasses a corresponding such method or methods.

Accordingly, in a fourth of its aspects, the present invention may provide a method for determining the separation between an electromagnetic signal transmitter means and an electromagnetic signal receiver means including: wirelessly radiating a given electromagnetic signal from the transmitter means, the given field having a near-field extending a distance exceeding the separation between the transmitter and the receiver means in normal use; wirelessly radiating another electromagnetic signal from the transmitter means, the other field having a near-field extending a distance less than the separation between the transmitter and the receiver means in normal use; wirelessly detecting the radiated given electromagnetic signal and the other electromagnetic signal at the detector; determining said separation using the detected near-field of the given electromagnetic signal in conjunction with the detected far-field of the other electromagnetic signal.

The method may include preferentially radiating from the transmitter means said given electromagnetic signals with a radian wavelength exceeding the separation between the transmitter means and receiver means in normal use, and preferentially detecting said preferentially radiated given electromagnetic signals from the transmitter means, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 2m.

The method may include wirelessly radiating from the transmitter means the other oscillating electromagnetic signal with a radian wavelength less than the radian wavelength of the given electromagnetic signal such that the near-field spatial component of the further electromagnetic signal extends a distance less than the separation between the receiver means and the transmitter means in normal use.

The method may include preferentially detecting to said other electromagnetic signals from the transmitter which have a radian wavelength less than said separation Preferably, the method includes preferentially radiating from the transmitter said other electromagnetic signals having a radian wavelength less than said separation.

The method may include determining said separation according to the ratio of the amplitudes of the detected near-field and far-field components of the given electromagnetic signal and the other electromagnetic signal respectively.

Preferably, the method includes generating an indicator signal when it is determined that said separation is no greater than a predetermined distance within the near-field spatial component of the given electromagnetic signal thereby indicating the presence of the transmitter means at or within a range not exceeding the predetermined distance.

The method may include wirelessly radiating from a plurality of transmitters said electromagnetic signals as signal pulses each separately transmitted only during a time period randomly temporally located within a respective one of a succession of separate predetermined time intervals. The method may further comprise periodically determining with said detector said separation with respect to any of said plurality of transmitters according to separate signal pulses received thereby from the transmitters in use, and generating said indicator signal only after it has been determined from two consecutive pulsed signals received from a given transmitter that said separation with respect to that transmitter is no greater than a predetermined distance.

The method preferably includes determining said separation according to the magnetic filed component of said received electromagnetic signals.

In a further aspect, the present invention may provide a detector for a proximity monitoring apparatus, the detector including a receiver means arranged to wirelessly receive a given electromagnetic signal wirelessly transmitted by a transmitter means of the proximity monitoring apparatus, the detector being arranged to determine the separation between the receiver means and the transmitter means according to a mathematical root of a measure of the amplitude of the near-field of a received given electromagnetic signal from the transmitter means.

The term "mathematical root" may refer to a factor of a quantity that when multiplied by itself a specified number of times produces that quantity again. For example the th root of a quantity A (typically denoted Ji) produces the quantity A when multiplied by itself n times. Thus a square root of a quantity A (typically denoted /) produces the quantity A when multiplied by itself twice, while the cube root of the same quantity (typically denoted produces the quantity A when multiplied by itself three times.

The detector may be arranged to determine the cube root of said measure and to determine the separation according to the inverse of the cube root of the measure.

The near field amplitude (HNear) of the given electromagnetic field at a distance H from the field source varies predominantly as the inverse of the cube of the distance, i.e., IIa, B/R3 where B is a constant, such that R = The detector may include a receiver means tuned to be preferentially responsive to the given electromagnetic signals from the transmitter which have a radian wavelength greater than the separation between the transmitter and the detector in normal use of the proximity monitoring apparatus, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 27c.

The detector may include indicator means arranged to generate an indicator signal when the detector has determined that the separation is no greater than a predetermined distance within the near-field spatial component of the given electromagnetic signal thereby indicating the presence of the transmitter at or within a range not exceeding the predetermined distance.

The detector may be arranged for use in a proximity monitoring apparatus comprising a plurality of said wireless transmitters, the detector being arranged to periodically determine the separation with respect to any of the plurality of transmitters according to separate signal pulses received thereby from the transmitters in use, and to generate said indicator signal only after the detector has determined from two consecutive pulsed signals received from a given transmitter that the separation with respect to that transmitter is no greater than a predetermined distance.

The detector may be arranged to determine said separation according to the magnetic field component of said received electromagnetic signals.

The detector may include attachment means for attaching the detector to an item of apparel (e.g. a hat) of a user or building in normal use.

In another of its aspects, the present invention may provide a transmitter for a proximity monitoring apparatus arranged to wirelessly radiate a given oscillating electromagnetic signal detectable by a wireless receiver of the proximity monitoring apparatus, the transmitter being arranged to generate the given electromagnetic signal having a near- field spatial component extending a distance exceeding the separation between the transmitter and the detector in normal use such that the detector may determine said separation using the near-field spatial component of the given electromagnetic signal of the transmitter, wherein the transmitter is adapted for use in a proximity monitoring apparatus comprising a plurality of said transmitters, the transmitter being arranged in use to radiate electromagnetic signals as signal pulses each transmitted only during a time period randomly temporally located within each a respective one of a succession of separate predetermined time intervals.

Preferably the transmitter is tuned to preferentially radiate said given electromagnetic signals which have a radian wavelength greater than said separation in normal use of the proximity monitoring apparatus, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 2m.

The transmitter may be arranged to wirelessly radiate another oscillating electromagnetic signal with a radian wavelength less than the radian wavelength of the given electromagnetic signal such that the near-field spatial component of the other electromagnetic signal extends a distance less than the separation between the transmitter and the detector in normal use whereby the detector may determine said separation using the far-field spatial component of the other electromagnetic signal, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 27t.

The transmitter may be tuned to preferentially radiate the other electromagnetic signals having a radian wavelength less than said separation.

Preferably, the transmitter includes attachment means for attaching the detector to a vehicle or an item of machinery.

The detector of the invention in any of its aspects may be arranged to determine the square root of said measure and to determine said separation according to the inverse of said square root.

In the detector, preferably the receiver means is arranged to wirelessly receive another electromagnetic signal wirelessly transmitted by the transmitter means, and the detector is arranged to determine the square root of a measure of the amplitude of the far-field of a received other electromagnetic signal and to determine said separation according to the square root of the measure of the amplitude of the far- field of the other signal divided by said square root of said measure of the amplitude of the near-field of said given signal. The far field amplitude (HFar) of the given electromagnetic field at a distance A from the field source varies predominantly as the inverse of the distance, i.e. , Hfar = CIR where C is a constant, such that R = .j(B / C) x (Hr / Hear).

The present invention may provide a detector as described above according to any aforesaid aspect.

The present invention may provide a proximity monitoring apparatus including a detector according to any aforesaid aspect, and including said transmitter described above.

In yet a further of its aspects, the present invention may provide a method for determining the separation between an electromagnetic signal transmitter means and an electromagnetic signal receiver means including: wirelessly radiating a given electromagnetic signal from the transmitter means, the given signal having a near field extending a distance exceeding the separation between the transmitter means and the receiver means in normal use; wirelessly detecting the radiated given electromagnetic signal at the receiver means; determining a measure of the amplitude of the near-field of the detected given electromagnetic signal; determining the separation between the receiver means and the transmitter means according to a mathematical root of said measure.

The method may include determining the cube root of said measure and determining said separation according to the inverse of said cube root.

The method may include determining the square root of said measure and determining said separation according to the inverse of said square root.

The method may include wirelessly radiating another electromagnetic signal from the transmitter means, the other signal having a near-field extending a distance less then the separation between the transmitter means and the receiver means in normal use; wirelessly detecting the radiated other electromagnetic signal at the receiver means; determining a measure of the amplitude of the far-field of the detected other electromagnetic signal; determining said separation according to the square root of the measure of the amplitude of the far-field of the other signal divided by said square root of said measure of the amplitude of the near-field of said given signal.

Examples of the invention shall now be described with reference to the accompanying drawings in which: Figure 1 illustrates the spatial variation of components of the magnetic field of an electromagnetic field radiated from one antenna to another coplanar antenna; Figure 2 schematically illustrates a proximity monitoring apparatus in use; Figure 3 illustrates a hard hat including a detector for a proximity monitoring apparatus; Figure 4 illustrates the effects of a noise spike on errors in detected separation between and transmitter and detector of a proximity monitoring apparatus; Figure 5 illustrates the relative error in detected separation between a transmitter and a receiver arising from varying degrees of angular mis-alignment (non-coplanarity) between the two when only the far-field component of the transmitted signal is measurable; Figure 6 illustrates the relative error in detected separation between a transmitter and a receiver arising from varying degrees of angular mis-alignment (non-coplanarity) between the two when the near-field component of the transmitted signal is measured; Figure 7 schematically illustrates a transmitter circuit for a proximity monitoring apparatus; Figure 8 schematically illustrates a detector for a proximity monitoring apparatus; Figure 9 schematically illustrates a proximity monitoring apparatus in use

employing the near field of a transmitter thereof.

In the figures, like items are assigned like reference signs.

Figure 2 schematically illustrates a proximity monitoring apparatus employed by a person (4) on a construction site in normal use in the presence of a heavy vehicle (1).

The apparatus includes a signal transmitter device (2) mounted upon the heavy vehicle for wireless radiating a given electromagnetic signal (7A) and a higher frequency other electromagnetic signal (7B) from an antennae (3) thereof, the wavelength of the given signal being selected such that the near-field of the radiated given signal has a range (?/2t) which fully encompasses the full range of normal separation distances A which will occur as between the transmitter (2) and the person (4) in normal circumstances. In the present example, the range of separations in question is typically from about 20 metres to about half a metre or less as might be expected between an individual on the building site and the heavy vehicles used on site. The wavelength of the other (higher frequency) signal is selected such that the near-field of the radiated other signal has a range (X0/2ir) which falls far short of the full range of separation distances R. The proximity monitoring apparatus also includes a detector unit (6) mounted upon a hard hat (5) worn by the person in question, the hard hat being of the type to be worn by construction/industrial personnel. Figure 3 illustrates a suitable position of the detector unit (6) upon a hard hat, being at the front region of the hard hat when worn.

The detector unit includes an antenna signal receiver coil (60) comprising a conductive winding (e.g. wire) wound around a ferrite core and operably connected to an antennae receiver circuit unit (65) which is responsive to the electromagnetic signals (both the "given" and the "other") received by the receiver coil (60) (or coils, one for the given signals, one for the others) to determine a measure of the distance from the receiver coil to the transmitter of the sigansi so received, and to indicate to the person (4) wearing the detector unit when the measured distance is less than a predetermined value corresponding to a danger/alarm separation. The predetermined value is selected in conjunction with the signal wavelength (2) at which the transmitter is arranged to radiate its "given" signal, such that the predetermined value is less than the given signal radian wavelength so ensuring that the alarm condition is only determined in the near-field of the given signal. Similarly, the "other" signal wavelength (?) is chosen to produce a short near-field which is not entered by the receiver in normal use.

The antennae receiver coil (60) (or coils) and an antennae receiver circuit unit (65) are tuned to be preferentially responsive to the selected given and other signal wavelength values which the transmitter (2) is tuned to preferentially radiate. For example, the given signal wavelength may be a value corresponding to a signal frequency of about 125 KHz ( Ai2ir = 382 metres) which ensures that a predetermined danger/alarm separation of between about 20 metres and about half a metre or less is well within the near-field of the transmitter signal such that the operation of the proximity monitoring apparatus may benefit from the advantages that flow from this wavelength choice as discussed below. The signal wavelength of the other signal (7B) may correspond to a signal frequency of about 400 MHz or more (X/2it=0. 1 metres).

As is illustrated in figure 1, as discussed above, when the detector (6) is within the near-field of the given signal from the transmitter (2), the field strength of the given signal (7A) detected by the detector increases as the cube of the separation (A) as the receiver approaches the transmitter or vice versa. This contrasts with the situation where wavelength of the transmitted signal (7) is such that the detector is only within the far-field of the transmitter such that the detected field strength increases linearly as the detector approaches the transmitter and vice versa as is the case with the other signal (7B).

Several advantages follow from this. First, if the detection range is within the near- field of the given electromagnetic signal (7A) then a determination of the separation A using the magnitude of the detected given signal (e.g. in conjunction with the other signal) is more robust to extraneous noise that may arise from interfering emissions, multi-path reflections, or signal attenuation. In other words a given level of noise will result in a smaller range error than a similar noise. This effect is shown in figure 4.

Figure 4 shows the effect of a 25% noise spike (i.e. increases the detected signal by 25%) on a near-field part of the given signal (7A) and a 25% noise spike on a far-field part of the other signal (7B) at a similar field strength. It can clearly be seen, due to the much higher spatial field gradient of the near-field part of the given signal that the error (Al) in the calculated distance introduced by the noise spike within the near- field of the given signal is much less than the error (A2) in the calculated distance introduced by the noise spike within the far-field of the other signal. In turn this means that sensing a specific separation/distance can be achieved more accurately within the near-field of the given signal. Therefore as the range over which the near- field of the given signal (where the 1/A3 term dominates) extends is a function of radian wavelength of the given signal generated by the transmitter (see Equation 1), then the wavelength is chosen so that the separation/distance between transmitter and detector at which the detector is operable to produce a proximity alarm/indicator signal, lies well within the near-field of the given signal generated by the transmitter in normal use.

Secondly, the attenuation of the signal due to misalignment of the transmitter and receiver in the near field has a much smaller effect than in the far field. When the transmitter and detector employ e.g. antenna coils for signal transmission and detection respectively, then for maximum field strength at any distance, the transmitter and receiver coils should preferably be aligned such that the axes about which the windings of each antenna are wound are substantially parallel. If the detector antenna is moved out of alignment with the transmitter antenna then the received field strength will be lower than would be the case if alignment was achieved for the same separation distance. A similar effect occurs through equivalent miss-alignment of transmitter and detector antennas of other than the wound-coil type as will be readily appreciated by the skilled person.

This signal reduction is equivalent to moving an aligned detector antenna further away from the transmitter antenna. However, because of the more rapid field strength decrease with distance in the near-field, the distance error for any given misalignment angle is correspondingly smaller than in the far-field allowing for more accurate determination of separation.

Figure 5 illustrates the relative measured range (being the measured range value divided by the true range value) as determined from detector measurements of the far-field component of a transmitted signal as a function of varying degrees of inclination between the symmetry axis (e.g. antennae coil winding axis) of the antennae of the transmitter (2) and the detector (6). For the purposes of clarity, a zero inclination angle corresponds to parallel symmetry axis while a 900 inclination corresponds to perpendicular symmetry axis.

Due to the directionality (non-uniformity of signal transmission/reception strength or sensitivity in directions off the axis of symmetry of an antennae) misalignment between the transmitter and detector antennae can result in a reduced received signal strength at the detector antennae which may be erroneously interpreted as an increase in separation between the transmitter antennae and the detector antennae.

Figure 5 illustrates the extent of such an error when using only the farfield of the transmitted signal (7, 7A or 7B), and Figure 6 illustrates the extent of such an error when the near-field of the transmitted signal (7A) is used. In the former case, a 45 inclination results in a 41% overestimate in range, while in the latter case the same inclination angle results only in a 12% range over-estimation.

Figure 7 schematically illustrates a signal transmitter unit (2) in more detail. The transmitter unit includes a drive unit (19) containing a signal timer unit (not shown) for controlling signal transmission timings of an HF (radio frequency) oscillator unit (not shown), such as a quartz crystal controlled oscillator means, for generating an HF "given" signal of 125 KHz frequency, and an HF "other" signal of about 400 MHz, for output from the drive unit to drive the transmitter antennae (3). It is to be understood that timer and oscillator units such as would be readily apparent to the skilled person may be employed in this regard.

The drive unit is arranged to generate antennae drive signals as a succession of RF substantially square drive signal pulses at a pulse rate of about 10 Hz with each pulse being approximately 1 ms in duration.

The transmitter unit includes a first antenna circuit for generating the "given" signals comprising an antenna inductor coil (24A) of inductance LA wound around the axis of a ferrite rod (26A) and having an output end terminating at Ground and an input end connected to a drive signal output of the drive unit via a variable resistor (22A) and a variable capacitor unit (23A) of net capacitance CA. The capacitor unit may comprise a plurality of inter-connected capacitors only one or some of which are variable in nature. The antenna inductor coil (24A) and the capacitor unit (23A) are mutually arranged to form a series resonant tuned circuit which resonates at the selected signal frequency (f) of the drive signals emanating from the drive unit, such that: 2L4C In this case f=125KHz. The capacitance CA of the capacitor unit may be varied to accurately tune the antennae circuit to a given selected frequency of the drive signal received thereby from the drive unit (whether at 125 KHz, or some other frequency).

Variable resistor is operable/variable to adjust the output level of the transmitted signal, but may be replaced with a fixed-value resistor resulting in a fixed output level or signal strength (i.e. at a given distance).

The transmitter unit also includes a second antenna circuit comprising an antennae inductor coil (24B) of inductants LB wound around the axis of a ferrite rod (26B) and having an output end terminating at Ground and an input end connected to a drive signal output of the drive unit via a variable resistor (22B) and a variable capacitor (23B) of net capacitants CB. The second antenna circuit is substantially arranged in the same manner as the first antenna circuit to form a series resonance tuned circuit which resonates at the selected signal frequency (f) of the drive signals from the drive unit such that: 2 \JL,? C, In this case the resonant frequency is associated with the "other" electromagnetic signal such that f=400 KHz, this frequency being variable by varying the capacitants Cb to tune the antenna for the other electromagnetic signal as required.

Each of the first and second antennae of the transmitter is be arranged inuse to radiate the electromagnetic signals as signal pulses each transmitted only during a time period randomly temporally located within each a respective one of a succession of separate predetermined time intervals. The suitable choice of a time interval (e.g. 0.1 sec) ensures that signals will be transmitted from the transmitter on a suitably regular basis without undesirably (randomly located) long pauses between transmission pulses, while the randomisation of the instant of transmission within that time interval ensures that two or more separate transmitters cannot become synchronised in their transmissions. Such synchronisation could result in loss of signal reception or signal strength due to destructive interference between concurrently received separate transmitter signals at a given detector and is highly undesirable.

The drive unit (19) of the/each transmitter is, in this case, arranged to generate the aforementioned drive signals only during a time period randomly temporally located within each a respective one of a succession of separate predetermined time intervals which, of course, results in the aforementioned quasi-random transmission of electromagnetic signal pulses from the transmitter.

Since the transmitter (2) is arranged to wirelessly radiate another oscillating electromagnetic signal with a radian wavelength less than the radian wavelength of the given electromagnetic signal such that the nearfield spatial component of the other electromagnetic signal extends a distance less than the separation between the transmitter and the detector in normal use, the detector may then determine the separation using the far-field spatial component of the further electromagnetic signal as discussed below. The transmitter (2) is tuned to preferentially radiate said further electromagnetic signals having a radian wavelength less than said separation.

Accordingly, the drive unit (19) of the transmitter is then arranged to generate antenna drive signals of not only 125 KHz frequency, but is also arranged to generate drive signals of another frequency sufficiently high in value to ensure that the radian wavelength of the high frequency signal is much less than the normal separations typically encountered by the user. A signal frequency of about 400 MHz or more would be a suitable value for the high frequency signal, as discussed above.

The transmitter may include two antennae, one for transmitting the given electromagnetic signal and one for transmitting the other (higher frequency) signal.

Each may be served by a common drive circuitry or by separate drive circuitry.

Alternatively, one de-tuned (low 0) antenna may serve to transmit both signal types.

This provides even more immunity to external influences by arranging for transmission at two wavelengths. These wavelengths/frequencies are chosen according to the predetermined distance at which the detector will generate an alarm signal, indicating a dangerously low separation. The wavelength choices are such that the predetermined distance is well within the near-field of one given signal while being well within the far- field of the other (high frequency) signal.

Figure 8 schematically illustrates the detector (6) in more detail. The detector includes a receiver antenna (27), such as one or more coil antenna each comprising a conductive element wound around the axis of a ferrite core, operably connected to a signal amplifier unit (28) arranged to amplify electrical signals generated by the receiver antenna in response to receipt thereby of the electromagnetic signals radiated by the transmitter (2), and to output the amplified result to an evaluation unit (32). The evaluation unit (32) comprises a measurement unit (29) arranged to receive the amplified signals output from the amplifier unit (28), to determine a measure of the instantaneous magnitude (e.g. amplitude) of the "given" signal received by a receiver antenna, and the magnitude of the "other" higher frequency signal received by a receiver antenna, and to determine from the measure of the received signal magnitudes a measure of the separation (A) between the detector and the transmitter from which the received signal emanated, and to output the result. The evaluation unit also includes a comparator unit (30) having a first signal input port (+) arranged to receive the output of the measurement unit, and a second input port (-) arranged to simultaneously receive a predetermined separation/distance threshold value stored in a threshold store means (31) . The comparator unit is arranged to subtract the value of a signal received at its first signal input port (+) from the value of a signal simultaneously received at its second input port (-) and to output the result as the output of the evaluation unit (32). Thus, should the output of the evaluation unit be positive, this indicates that the measured separation exceeds the predetermined separation threshold value, while if the output is negative the measured distance is less than the threshold value.

An indicator unit (33) is operably connected to the evaluation unit so as to receive as an input signal the output signal generated by the evaluation unit, and to generate an indicator signal/alarm depending upon the nature of the signal input to it from the evaluation unit. The indicator unit is arranged to determine if the input (i.e. the output of the evaluation unit) is positive or negative, and in response to a positive signal the indictor unit does not generate an indicator alarm, while in response to a negative signal the indicator may generate an indicator signal/alarm warning of the dangerous proximity of the transmitter to the detector. The indicator unit may comprise an illumination means (e.g. LED, not shown) for generating a visible light signal, or an audible buzzer means, or a movement means for causing the detector to vibrate to indicate the alarm condition.

The measurement unit may be arranged to determine the measure of the separation by evaluating the square-root of the ratio of the measured magnitude of the received other signal and the received given signal. This stems from the fact that the near- field magnitude of the given signal predominantly varies as the inverse of the cube of the separation while the far-field magnitude of the other signal predominantly varies as the inverse of the separation. The ratio of the latter to the former thus varies predominantly as the square of the separation. The measurement unit may calculate the separation according to the square-root of the ratio.

This approach is to employ dual transmission and dual reception of two concurrent electromagnetic signals, a first ("given") signal being of relatively long wavelength and having a near-field enveloping the detector, and a second ("other") signal having a relatively short wavelength and having a far-field in which the detector is immersed.

The transmitter/detector pair operate at a frequency such that the distances/separations to be measured are all within the near-field of the first signal, whereas the other second signal has a frequency such the distances/separations to be measured are all are in the far-field of the second signal.

The detector is then arranged to determine the separation by calculating the ratio of the two signals received at the detector, and calculating the separation using the square-root of the ratio. From equation 1, it can be seen that the resulting ratio is approximately: Raiio=KR2 Where K is a constant found by calibration. Any signal magnitude reduction effects arising from antenna orientation will be the same in both the near-field and far-field signals and so will cancel out when the ratio of the two is formed.

The detector is preferably provided with two antenna windings (27) each separately tuned to a respective of the first and second signal frequencies. These two antennae may be served by the same amplifier unit (28), evaluator unit (32) and indicator unit (33).

In some embodiments the detector (6) may be arranged to operate in conjunction with a plurality of transmitters (2). In such a case, the transmitters in question are preferably arranged to emit pulsed signals, as discussed above, in which each transmitter transmits a signal pulse for only a short time randomly temporally located during a longer repeating regular transmission time frame/window. Since it is desirable for all transmitters to be the same, to increase versatility and ease of use, they preferably each have the same repetition rate for the transmission time window (i.e. a transmission window of common duration), and so there is still a likelihood of two or more transmitters becoming synchronised accidentally so that they repeatedly transmit at the same time were it not for the randomization of the signal pulse transmission instant within the transmission window.

In this way, even if two transmitters transmit at the same time during one cycle, they are unlikely to transmit at the same time during the next time frame. A simplified analysis of the benefits of using the random scheme is as follows. If a system has a transmission window which contains n, non-overlapping intervals within it, during which a transmitter can transmit a signal pulse, then, for the non-random scheme, the probability of a single pulse being corrupted by another transmitter is 1/n. The probability of m adjacent pulses being corrupted is also 1/n, since, once they are synchronised, they will continue to transmit at the same instants. When the randomised scheme is used, the probability of one occurrence of a simultaneous transmission is still 1/n, but the probability of m adjacent occurrences is (1/n)tm.

If the detector is made to require several pulses above the threshold before activation occurs, then using the randomised scheme means that the system will be much more likely to correctly determine range than the nonrandomised scheme. In practice, the detector may compare the incoming pulse amplitudes against a threshold. When several pulses exceeding the threshold have been received in close proximity, the warning device is activated. If a single, excessively high amplitude pulse is received because two nearby transmitters have transmitted simultaneously and combined in an additive manner, then this will not trigger the output. Similarly, a single low- amplitude pulse due to destructive interference will not interrupt the output.

Thus, the evaluator means (32) is this case is arranged to periodically determine the separation with respect to any of said plurality of transmitters according to separate signal pulses received thereby from the transmitters in use, and to generate an indicator signal only after the evaluator has determined from two consecutive pulsed signals received from a given transmitter that the separation with respect to that transmitter is no greater than a predetermined distance.

When the detector is attached to an item of apparel of a person, such as a hard hat (5) as illustrated in figure 3, the orientation of the receiver antenna (60) in normal use relative to a transmitter antenna axis is of importance. Since, in use, the hat-wearer may incline his or her head, it is necessary to insure that any errors introduced by this are not significant. The relative magnetic field strength (actual received strength as a fraction of maximum possible with fully aligned antennae) received by the receiver antenna (60) at an angle (0) relative to a vertically oriented the transmitting antenna (3) is given by the cosine of the angle of inclination between transmitter antenna axis and receiver antenna axis. Since transmitter antennae may typically be placed on vehicles and machinery it is a simple matter to mount them there such that the transmitter antenna axis is, and substantially remains, effectively vertical.

For an inclination angle range of +45 to -45 , the signal received will vary by 17% from the nominal which occurs at approximately 310. However, the range measured is proportional to the cube root of the field strength according to preferred embodiments of the invention, and so the 17 /o variation in field strength causes only a 5% error in measured range and so its effect is negligible. However, to ensure that the error does not exceed this amount, it is preferable to ensure that the wearer's head inclination does not cause the antenna inclination angle to exceed 45 otherwise the signal strength reduction would be very high. In fact, the measured field strength would decrease to zero at an inclination of 900. Since the wearer is much more likely to tilt his or her head forward rather than backwards, the receiver antenna is preferably tilted backwards (relative the forward direction of the wearer in normal use) by approximately 20 from the vertical when the wearer's head position is neutral. A suitable position for the antenna, which allows this angle to be conveniently achieved, is shown in figure 3 in which the receiver antenna is arranged to lie at 20 to the vertical. This means that a 5% ranging error is experienced for a head tilt range of 25 backwards to 65 forwards.

A further embodiment of the present invention shall now be described in which proximity monitoring may be undertaken using only the far field of a radiated given

electromagnetic field of the type discussed above.

Figure 9 schematically illustrates a proximity monitoring apparatus employed by a person (4) on a construction site in normal use in the presence of a heavy vehicle (1).

The apparatus includes a signal transmitter device (2) mounted upon the heavy vehicle for wireless radiating an electromagnetic signal (7) from an antennae (3) thereof, the wavelength of the signal being selected such that the near-field of the radiated signal has a range (X/2it) which fully encompasses the full range of normal separation distances R which will occur as between the transmitter (2) and the person (4) in normal circumstances. In the present example, the range of separations in question is typically from about 20 metres to about half a metre or less as might be expected between an individual on the building site and the heavy vehicles used on site.

The proximity monitoring apparatus also includes a detector unit (6) mounted upon a hard hat (5) worn by the person in question, the hard hat being of the type to be worn by construction/industrial personnel. Figure 3 illustrates a suitable position of the detector unit (6) upon a hard hat, being at the front region of the hard hat when worn.

The detector unit includes an antenna signal receiver coil (60) comprising a conductive winding (e.g. wire) wound around a ferrite core and operably connected to an antennae receiver circuit unit (65) which is responsive to the electromagnetic signals received by the receiver coil (60) to determine a measure of the distance from the receiver coil to the transmitter of the signal so received, and to indicate to the person (4) wearing the detector unit when the measured distance is less than a predetermined value corresponding to a danger/alarm separation. The predetermined value is selected in conjunction with the signal wavelength at which the transmitter is arranged to radiate its signal, such that the predetermined value is less than the signal radian wavelength so ensuring that the alarm condition is only

determined in the near-field of the signal.

The antennae receiver coil (60) and an antennae receiver circuit unit (65) are tuned to be preferentially responsive to the selected signal wavelength which the transmitter (2) is tuned to preferentially radiate. For example, the signal wavelength may be a value corresponding to a signal frequency of about 125 KHz (XI2it = 382 metres) which ensures that a predetermined danger/alarm separation of between about 20 metres and about half a metre or less is well within the near-field of the transmitter signal such that the operation of the proximity monitoring apparatus may benefit from the advantages that flow from this wavelength choice.

As is illustrated in figure 9, as discussed above, when the detector (6) is within the near-field of the transmitter (2), the field strength detected by the detector increases as the cube of the separation (R) as the receiver approaches the transmitter or vice versa. This contrasts with the situation where wavelength of the transmitted signal (7) is such that the detector is only within the far-field of the transmitter such that the detected field strength increases linearly as the detector approaches the transmitter and vice versa.

The transmitter of the apparatus of this embodiment may be substantially the same as the transmitted illustrated in Figure 7 in which only the antenna (24A) and associated circuitry are used for the production of a the given electromagnetic field within the near field of which the detector resides in use. Alternatively, the other antenna (24B) and associated circuitry for generation of the other electromagnetic

field may be dispensed with.

Similarly, the detector as illustrated schematically in Figure 8 may alternatively be arranged to determine the separation between transmitter and detector using only the near field of the given signal as follows. The detector includes a receiver antenna (27), such as a coil antenna comprising a conductive element wound around the axis of a ferrite core, operably connected to a signal amplifier unit (28) arranged to amplify electrical signals generated by the receiver antenna in response to receipt thereby of the electromagnetic signals radiated by the transmitter (2), and to output the amplified result to an evaluation unit (32). The evaluation unit (32) comprises a measurement unit (29) arranged to receive the amplified signals output from the amplifier unit (28), to determine a measure of the instantaneous magnitude (e.g. amplitude) of the signal received by the receiver antenna, and to determine from the measure of the received signal magnitude a measure of the separation (R) between the detector and the transmitter from which the received signal emanated, and to output the result. The evaluation unit also includes a comparator unit (30) having a first signal input port (+) arranged to receive the output of the measurement unit, and a second input port (-) arranged to simultaneously receive a predetermined separation/distance threshold value stored in a threshold store means (31). The comparator unit is arranged to subtract the value of a signal received at its first signal input port ( ) from the value of a signal simultaneously received at its second input port (-) and to output the result as the output of the evaluation unit (32). Thus, should the output of the evaluation unit be positive, this indicates that the measured separation exceeds the predetermined separation threshold value, while if the output is negative the measured distance is less than the threshold value.

The measurement unit is arranged to determine the measure of the separation by evaluating the cube root if the measured magnitude or amplitude of the received given signal. This stems from the fact that the near-field magnitude varies as the inverse of the cube of the separation, thus, the separation varies as the inverse of the cube root of the magnitude of the signal. The measurement unit may calculate the separation accordingly.

Variations and modifications of the above embodiments, such as would be readily apparent to the skilled person, may be made without departing from the scope of the present invention.

Claims

CLAIMS: 1. A detector for a proximity monitoring apparatus, the detector

including a receiver means arranged to wirelessly receive electromagnetic signals radiated by a wireless transmitter means of the proximity monitoring apparatus, the detector being arranged to determine the separation between the receiver means and the transmitter means according to the near-field of a given electromagnetic signal from the transmitter means in conjunction with the far-field of another electromagnetic signal from the transmitter means.
2. A detector according to any preceding claim arranged to determine said separation according to a measure of the far-field of said other electromagnetic signal determined relative to a measure of the near-field of said given electromagnetic signal.
3. A detector according to any preceding claim arranged to determine said separation according to the ratio of the amplitudes of: said other electromagnetic signal representing said far-field, and; said given electromagnetic signal representing said near-field.
4. A detector according to claim 3 arranged to determine said separation using the square-root of said ratio.
5. A detector according to any preceding claim arranged to determine said separation using said near-field and said far-field of said given and said other electromagnetic signals, respectively, substantially simultaneously received from the transmitter means.
6. A detector according to any preceding claim in which the near-field of the given electromagnetic signal is the parts thereof extending from the transmitter means to a distance not exceeding the radian wavelength thereof, and the far-field of the other electromagnetic signal is the parts thereof at any distance exceeding the radian wavelength thereof, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 27t.
7 A detector according to any preceding claim arranged to wirelessly detect said given electromagnetic signal and said other electromagnetic signal in which the radian wavelength of the other electromagnetic signal is less than the radian wavelength of the given electromagnetic signal, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 2it.
8. A detector according to any preceding claim in which the receiver means is tuned to be preferentially responsive to said given electromagnetic signals from the transmitter means which have a radian wavelength greater than the separation between the transmitter means and the receiver means in normal use of the proximity monitoring apparatus, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 27r.
9. A detector according to any preceding claim in which the receiver means is tuned to be preferentially responsive to said other electromagnetic signals from the transmitter means which have a radian wavelength less than the separation between the transmitter means and the receiver means in normal use of the proximity monitoring apparatus, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 2it.
10. A detector according to any preceding claim including two antenna coils for receiving electromagnetic signals from said transmitter wherein each antenna coil is wound about a respective coil axis substantially orthogonal to the coil axis of the other antenna coil.
11. A detector according to any preceding claim including indicator means arranged to generate an indicator signal when the detector has determined that said separation is no greater than a predetermined distance within the near-field of the given electromagnetic signal thereby indicating the presence of the transmitter means at or within a range not exceeding the predetermined distance.
12. A detector according to claim 11 for use in a proximity monitoring apparatus comprising a plurality of said wireless transmitter means, the detector being arranged to periodically determine said separation with respect to any of said plurality of transmitter means according to separate signal pulses received thereby from the transmitters in use, and to generate said indicator signal only after the detector has determined from two or more consecutive pulsed signals received from a given transmitter means that said separation with respect to the given transmitter means is no greater than a predetermined distance.
13. A detector according to any preceding claim arranged to determine said separation according to the magnetic filed component of said received electromagnetic signals.
14. A detector according to any preceding including attachment means for attaching the detector to an item of apparel of a user in normal use.
15. A hat or other apparel including a detector according to any preceding claim.
16. A transmitter for a proximity monitoring apparatus arranged to wirelessly radiate a given electromagnetic signal and another electromagnetic signal each detectable by a wireless receiver of the proximity monitoring apparatus, the transmitter being arranged to generate the given electromagnetic signal to have a near-field extending a distance exceeding the separation between the transmitter and the detector in normal use and to generate the other electromagnetic signal to have a near-field extending a distance less than the separation between the transmitter and the detector in normal use, such that the detector may reside in the near-field of the given electromagnetic signal of the transmitter and simultaneously in the far-field of the other electromagnetic signal of the transmitter.
17. A transmitter according to claim 16 in which the transmitter is tuned to preferentially radiate said given electromagnetic signals which have a radian wavelength greater than said separation in normal use of the proximity monitoring apparatus, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 2m.
18. A transmitter according to any of preceding claims 16 to 18 arranged to wirelessly radiate said other electromagnetic signal with a radian wavelength less than the radian wavelength of the given electromagnetic signal such that the near-field spatial component of the other electromagnetic signal extends a distance less than the separation between the transmitter and the detector in normal use, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 27c.
19. A transmitter according to any of preceding claims 16 to 18 for use in a proximity monitoring apparatus comprising a plurality of said transmitters, the transmitter being arranged in use to radiate said electromagnetic signals as signal pulses each separately transmitted only during a time period randomly temporally located within a respective one of a succession of separate predetermined time intervals.
20. A transmitter according to any of claims 16 to 19 including attachment means for attaching the detector to a vehicle or an item of machinery.
21. A proximity monitoring apparatus including a transmitter means arranged to wirelessly radiate electromagnetic signals and a detector including a receiver means arranged to wirelessly receive electromagnetic signals radiated by a transmitter means, the detector being arranged to determine the separation between the receiver means and the transmitter means according to the near- field of a given electromagnetic signal from the transmitter means in conjunction with the far-field of another electromagnetic signal from the transmitter means.
22. A proximity monitoring apparatus according to claim 22 arranged to determine said separation according to a measure of the far-field of said other electromagnetic signal determined relative to a measure of the nearfield of said given electromagnetic signal
23. A proximity monitoring apparatus according to claim 21 or 22 arranged to determine said separation according to the ratio of the amplitudes of: said other electromagnetic signal representing said far-field, and; said given electromagnetic signal representing said near-field.
24. A proximity monitoring apparatus according to claim 23 arranged to determine said separation using the square-root of said ratio.
25. A proximity monitoring apparatus according to any of claims 21 to 24 arranged to determine said separation using said near-field and said farfield of said given and said other electromagnetic signals, respectively, substantially simultaneously received from the transmitter means.
26. A proximity monitoring apparatus according to any of claims 21 to 25 in which the near-field of the given electromagnetic signal is the parts thereof extending from the transmitter means to a distance not exceeding the radian wavelength thereof, and the far-field of the other electromagnetic signal is the parts thereof at any distance exceeding the radian wavelength thereof, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 2m.
27. A proximity monitoring apparatus according to any of claims 21 to 26 arranged to wirelessly detect said given electromagnetic signal and said other electromagnetic signal in which the radian wavelength of the other electromagnetic signal is less than the radian wavelength of the given electromagnetic signal, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 2m.
28. A proximity monitoring apparatus according to any of claims 21 to 27 in which the receiver means is tuned to be preferentially responsive to said given electromagnetic signals from the transmitter means which have a radian wavelength greater than the separation between the transmitter means and the receiver means in normal use of the proximity monitoring apparatus, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 2t.
29. A proximity monitoring apparatus according to any of claims 21 to 28 in which the receiver means is tuned to be preferentially responsive to said other electromagnetic signals from the transmitter means which have a radian wavelength less than the separation between the transmitter means and the receiver means in normal use of the proximity monitoring apparatus, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 27t.
30. A proximity monitoring apparatus according to any of claims 21 to 29 in which the transmitter is tuned to preferentially radiate said given electromagnetic signals which have a radian wavelength exceeding the separation between the transmitter means and receiver means in normal use, and the receiver means is tuned to be preferentially responsive to said preferentially radiated given electromagnetic signals from the transmitter means, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 2it.
31. A proximity monitoring apparatus according to any of claims 21 to 30 in which the transmitter is tuned to preferentially radiate said other electromagnetic signal with a radian wavelength less than the radian wavelength of the given electromagnetic signal such that the near-field of the other electromagnetic signal extends a distance less than the separation between the receiver means and the transmitter means in normal use, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 2it.
32. A proximity monitoring apparatus according to any of claims 21 to 30 in which the detector is arranged to determine said separation according to the ratio of the amplitudes: of the received near-field and far-field of the given electromagnetic signal and the other electromagnetic signal respectively.
33. A proximity monitoring apparatus according to any of claims 21 to 31 in which the detector includes two antenna coils for receiving electromagnetic signals from said transmitter wherein each antenna coil is wound about a respective coil axis substantially orthogonal to the coil axis of the other antenna coil.
34. A proximity monitoring apparatus according to any of claims 21 to 32 in which the detector includes indicator means arranged to generate an indicator signal when the detector has determined that said separation is no greater than a predetermined distance within the near-field of the given electromagnetic signal thereby indicating the presence of the transmitter means at or within a range not exceeding the predetermined distance.
35. A proximity monitoring apparatus according to any of preceding claims 21 to 33 comprising a plurality of said transmitter means each arranged in use to radiate said electromagnetic signals as signal pulses each separately transmitted only during a time period randomly temporally located within a respective one of a succession of separate predetermined time intervals.
36. A proximity monitoring apparatus according to claim 34 comprising a plurality of said wireless transmitter means, the detector being arranged to periodically determine said separation with respect to any one of said plurality of transmitter means according to separate signal pulses received thereby from the transmitter means in use, and to generate said indicator signal only after the detector has determined from two or more consecutive pulsed signals received from a given transmitter means that said separation with respect to the given transmitter means is no greater than a predetermined distance.
37. A proximity monitoring apparatus according to any of claims 21 to 35 in which the detector is arranged to determine said separation according to the magnetic filed component of said received electromagnetic signals.
38. A proximity monitoring apparatus according to any of claims 21 to 36 41 in which the detector includes attachment means for attaching the detector to an item of apparel of a user in normal use.
39. A proximity monitoring apparatus according to any of claims 21 to 37 including a hat or other item of apparel to which said detector is attached.
40. A proximity monitoring apparatus according to any of claims 21 to 38 including attachment means for attaching the detector to a vehicle or an item of machinery.
41. A method for determining the separation between an electromagnetic signal transmitter means and an electromagnetic signal receiver means including: wirelessly radiating a given electromagnetic signal from the transmitter ffieans, the given field having a near-field extending a distance exceeding the separation between the transmitter and the receiver means in normal use; wirelessly radiating another electromagnetic signal from the transmitter means, the other field having a near-field extending a distance less than the separation between the transmitter and the receiver means in normal use; wirelessly detecting the radiated given electromagnetic signal and the other electromagnetic signal at the receiver means; determining said separation using the detected near-field of the given electromagnetic signal in conjunction with the detected far-field of the other electromagnetic signal.
42. A method according to claim 40 including preferentially radiating from the transmitter means said given electromagnetic signals with a radian wavelength exceeding the separation between the transmitter means and receiver means in normal use, and preferentially detecting said preferentially radiated given electromagnetic signals from the transmitter means, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 2it.
43. A method according to any of claims 40 to 41 including wirelessly radiating from the transmitter means said other oscillating electromagnetic signal with a radian wavelength less than the radian wavelength of the given electromagnetic signal such that the near-field of the other electromagnetic signal extends a distance less than the separation between the receiver means and the transmitter means in normal use, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 2it.
44. A method according to any of claims 4o to 42 including determining said separation according to the ratio of the amplitudes of the detected near-field and far-field of the given electromagnetic signal and the other electromagnetic signal respectively.
45. A method according to any of claims 40 to 43 including generating an indicator signal when it is determined that said separation is no greater than a predetermined distance within the near-field of the given electromagnetic signal thereby indicating the presence of the transmitter means at or within a range not exceeding the predetermined distance.
46. A method according to any of preceding claims 4o to 44 including wirelessly radiating from a plurality of said transmitters said electromagnetic signals as signal pulses each separately transmitted only during a time period randomly temporally located within a respective one of a succession of separate predetermined time intervals.
47. A method according to claim 45 comprising periodically determining said separation with respect to any of said plurality of transmitter means according to separate signal pulses received thereby from the transmitter meansin use, and generating said indicator signal only after it has been determined from two or more consecutive pulsed signals received from a given transmitter that said separation with respect to the given transmitter means is no greater than a predetermined distance.
48. A method according to any of claims 32 to 40 including determining said separation according to the magnetic filed component of said received electromagnetic signals.
49. A detector for a proximity monitoring apparatus, the detector including a receiver means arranged to wirelessly receive a given electromagnetic signal wirelessly transmitted by a transmitter means of the proximity monitoring apparatus, the detector being arranged to determine the separation between the receiver means and the transmitter means according to a mathematical root of a measure of the amplitude of the near-field of a received given electromagnetic signal from the transmitter means.
50. A detector according to claim 49 wherein the detector is arranged to determine the cube root of said measure and to determine said separation according to the inverse of said cube root.
51. A detector according to any of claims 49 and 50 including a receiver means tuned to be preferentially responsive to said given electromagnetic signals from the transmitter which have a radian wavelength greater than the separation between the transmitter and the detector in normal use of the proximity monitoring apparatus, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 2it.
52. A detector according to any of claims 49 to 51 including indicator means arranged to generate an indicator signal when the detector has determined that said separation is no greater than a predetermined distance within the near-field spatial component of the given electromagnetic signal thereby indicating the presence of the transmitter at or within a range not exceeding the predetermined distance.
53. A detector according to claim 52 for use in a proximity monitoring apparatus comprising a plurality of said wireless transmitters, the detector being arranged to periodically determine said separation with respect to any of said plurality of transmitters according to separate signal pulses received thereby from the transmitters in use, and to generate said iqdicator signal only after the detector has determined from two consecutive pulsed signals received from a given transmitter that said separation with respect to that transmitter is no greater than a predetermined distance.
54. A detector according to any of claims 49 to 53 arranged to determine said separation according to the magnetic field component of said received electromagnetic signals.
55. A detector according to any of claims 49 to 54 including attachment means for attaching the detector to an item of apparel of a user or building in normal use.
56 A hat including a detector according to any of preceding claims 49 to 54.
57. A transmitter for a proximity monitoring apparatus arranged to wirelessly radiate a given oscillating electromagnetic signal detectable by a wireless receiver of the proximity monitoring apparatus, the transmitter being arranged to generate the given electromagnetic signal having a near-field spatial component extending a distance exceeding the separation between the transmitter and the detector in normal use such that the detector may determine said separation using the near-field spatial component of the given electromagnetic signal of the transmitter, wherein the transmitter is adapted for use in a proximity monitoring apparatus comprising a plurality of said transmitters, the transmitter being arranged in use to radiate electromagnetic signals as signal pulses each transmitted only during a time period randomly temporally located within each a respective one of a succession of separate predetermined time intervals.
58. A transmitter according to claim 57 in which the transmitter is tuned to preferentially radiate said given electromagnetic signals which have a radian wavelength greater than said separation in normal use of the proximity monitoring apparatus, wherein a radian wavelength of an electromagnetic signal is equal to its wavelength divided by 27t.
59. A transmitter according to any of preceding claims 57 to 58 arranged to wirelessly radiate another oscillating electromagnetic signal with a radian wavelength less than the radian wavelength of the given electromagnetic signal such that the near-field spatial component of the other electromagnetic signal extends a distance less than the separation between the transmitter and the detector in normal use whereby the detector may determine said separation using the far-field spatial component of the other electromagnetic signal, wherein an electrical length of an electromagnetic signal is equal to its wavelength divided by 27r.
60. A transmitter according to claim 59 tuned to preferentially radiate said other electromagnetic signals having a radian wavelength less than said separation.
61. A transmitter according to any of claims 57 to 60 including attachment means for attaching the detector to a vehicle or an item of machinery.
62. A detector according to claim 49 wherein the detector is arranged to determine the square root of said measure and to determine said separation according to the inverse of said square root.
63. A detector according to claim 62 wherein the receiver means is arranged to wirelessly receive another electromagnetic signal wirelessly transmitted by the transmitter means, and the detector is arranged to determine the square root of a measure of the amplitude of the far-field of a received other electromagnetic signal and to determine said separation according to the square root of the measure of the amplitude of the far-field of the other signal divided by said square root of said measure of the amplitude of the near-field of said given signal.
64. A detector according to claim 63 and any of claims ito 15.
A proximity monitoring apparatus including a detector according to any of claims 49 to 53, and including said transmitter according to any of claims 59 to 64.
66. A proximity monitoring apparatus according to any of claims 49 to 53 and any of claims 16 to 40.
67. A method for determining the separation between an electromagnetic signal transmitter means and an electromagnetic signal receiver means including: wirelessly radiating a given electromagnetic signal from the transmitter means, the given signal having a near field extending a distance exceeding the separation between the transmitter means and the receiver means in normal use; wirelessly detecting the radiated given electromagnetic signal at the receiver means; determining a measure of the amplitude of the near-field of the detected given electromagnetic signal; determining the separation between the receiver means and the transmitter means according to a mathematical root of said measure.
68. A method according to claim 67 including determining the cube root of said measure and determining said separation according to the inverse of said cube root.
69. A method according to claim 67 including determining the square root of said measure and determining said separation according to the inverse of said square root.
70. A method according to claim 69 including wirelessly radiating another electromagnetic signal from the transmitter means, the other signal having a near-field extending a distance less then the separation between the transmitter means and the receiver means in normal use; wirelessly detecting the radiated other electromagnetic signal at the receiver means; determining a measure of the amplitude of the far-field of the detected other electromagnetic signal; determining said separation according to the square root of the measure of the amplitude of the far-field of the other signal divided by said square root of said measure of the amplitude of the near-field of said given signal.
71. A method according to claim 69 and any of claims 41 to 48.
72. A detector substantially as described in any embodiment hereinbefore with reference to the accompanying drawings.
73. A transmitter substantially as described in any embodiment hereinbefore with reference to the accompanying drawings.
74. A proximity monitoring apparatus substantially as described in any embodiment hereinbefore with reference to the accompanying drawings.
75. A method substantially as described in any embodiment hereinbefore with reference to the accompanying drawings.