GB2493399A - Inductive position sensor with multiple resonant circuits of differing resonant frequencies - Google Patents

Abstract

Sensor 1 for sensing the relative position of relatively moveable members (e.g. portions 5,7 of a robot arm 3) wherein: multiple magnetic field generators 13 (e.g. resonant circuits) are fixed relative to the first member and spaced apart over a measurement path or region; receive aerial 17a,b is fixed relative to the second member and defines a detection path or region which overlaps part of the measurement region in dependence on the relative position of the two members (Fig.11); a signal induced in the receive aerial by one of the magnetic field generators is processed to determine the position of that magnetic field generator relative to the detection region; the magnetic field generation properties (e.g. resonant frequencies) of some of the magnetic field generators differ and their arrangement is such that the induced signal indicates the part of the measurement region overlapped by the detection region.

Description

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POSITION SENSING APPARATUS AND METHOD

This invention relates to a position sensing apparatus, and a corresponding method, for generating information indicative of the relative position between two members. The invention is particularly concerned with an inductive position sensor.

Various types of inductive position sensor are known for determining the relative position between two members. In these inductive position sensors, typically transmit and receive aerials are fixed relative to one member and an intermediate coupling element is fixed relative to the other member. As the two members move relative to each other along a measurement path, .the electromagnetic coupling between the transmit aerial and the receive aerial, via the intermediate coupling element, varies. Therefore, the signal induced in the receive aerial in response to an excitation signal being applied to the transmit aerial is indicative of the relative position of the first and second members. Such inductive sensors are described in, for example, WO 03/038379 and WO 2009/153580.

The electromagnetic coupling between the transmit aerial and the receive aerial typically varies in accordance with a sinusoidal function, with the period of the sinusoidal function being determined by the layout of one or both of the transmit aerial and the receive aerial. In some embodiments, the transmit aerial and the receive aerial are configured to track movement over multiple periods of the sinusoidal function. In such embodiments, the signal induced in the receive aerial does not provide an unambiguous position measurement unless some further mechanism is employed. One way of increasing the range of movement over which an unambiguous position measurement can be made is to utilise two sensors, with each sensor having an electromagnetic coupling which varies over the range of movement in accordance with a respective different spatial frequency, In this way, a Vernier-type arrangement is formed in which the position can be determined using the signals induced in both the receive aerials.

The present invention addresses the problem of increasing the unambiguous range of position measurement in a new way. Aspects of the invention are set out in the accompanying claims.

In brief, in an embodiment of the invention, a receive aerial is fixed relative to one member and a plurality of magnetic field generators are fixed relative to the other member at respective different positions. The dimensions of the receive aerial are such that the receive aerial cannot overlap all of the magnetic field generators. The magnetic field generators have different magnetic field generation properties, which enables the identity of an overlapped magnetic field generator to be determined from the signal induced in the receive aerial by the magnetic field generator. This allows calculation of a coarse, unambiguous measurement of the relative position of the two members. Preferably, the position of the magnetic field generator relative to the receive aerial can also be determined from the signal induced in the receive aerial. In this way, a fine measurement of the relative position of the two members can be calculated.

In an embodiment, the magnetic field generators are resonant circuits having respective resonant frequencies. Preferably, a transmit aerial is fixed on the same member as the receive aerial, such that a signal can be induced in a resonant circuit by applying an excitation signal at its resonant frequency to the transmit aerial. In this way, no direct electrical connection to resonant circuits is necessary, which is useful in many applications.

Various exemplary embodiments of the present invention will now be described in more detail with reference to the accompanying figures, in which: Figure 1 shows a schematic perspective view of a position sensor according to the present invention mounted on a robot arm to measure the relative rotary displacement of two components of the robot arm; Figure 2 shows another view of the position sensor of Figure 1, showing in more detail components of the position sensor; Figure 3 shows conductive tracks formed on an antenna member fomung part of the position sensor illustrated in Figure 1; Figure 4A shows the layout of a transmit aerial formed on the antenna member; Figure 413 shows the layout of a receive aerial formed on the antenna member; Figure 4C shows the layout of a feedback aerial formed on the antenna member; Figure 5 shows the conductive tracks of a sensor element which forms part of the position sensor illustrated in Figure 1; Figure 6 is a block diagram schematically showing the main components of the position sensor illustrated in Figure 1; Figure 7 is a block diagram showing the components of excitation signal generating and detection signal processing circuitry illustrated in Figure 6; Figure 8 is a flow chart schematically showing processing operations performed by the position sensor illustrated in Figure 1; Figure 9 shows a graph illustrating threshold values for the output of the excitation signal generating and pre-processing circuitry versus position relative to the antenna member; Figure 10 shows a graph illustrating the conversion of the output of the excitation signal generating and detection signal pre-processing circuitry into position information; and Figure 11 shows a schematic perspective view of an alternative position sensor for measuring linear movement.

FIRST EMBODIMENT

A first embodiment of the invention will now be described with reference to Figures 1 to 10. As shown in Figures 1 and 2, a position sensor 1 is mounted onto a robot arm 3 to measure rotary displacement between a first elongate arm portion 5 and a second elongate arm portion 7 which are generally aligned along a longitudinal axis of the robot arm 3. An annular collar 9 is fixed to the end of the first arm portion S that is adjacent the second arm portion 7. A first circumferential track I Ia and a second circumferential track 1 lb are formed around the circumference of the annular collar 9, the first circumferential track I Ia being axially spaced from the second circumferential track 1 lb. A first series of sensor elements 13 are positioned along the first circumferential track 11 a at regular intervals so that each sensor element 3 of the first series is circumferentially separated from the neighbouring sensor elements 13 of the first series by a distance d, which in this embodiment is 80mm. A second series of sensor elements 13 are positioned along the second circumferential track 1 lb at regular intervals so that each sensor element 13 of the second series is circuinferentially separated from the neighbouring sensor elements 13 of the second series by the distance d. The first series of sensor elements 13 is circumfercntially offset from the second series of sensor elements 13 by half the distance d, in this embodiment 40mm. Each sensor element 1 3 of the first series has a resonant circuit with a characteristic resonant frequency and each sensor element 13 of the second series has a resonant circuit with a characteristic resonant frequency. In this embodiment, all the resonant circuits have different characteristic resonant frequencies.

A sensor mount 15 is fixed to the second arm portion 7 at a point adjacent the first arm portion 5, and includes a portion which extends over the annular collar 9 which is fixed to the first arm portion 5. A first aerial member 17a is fixed to a surface of the sensor mount 15 facing the annular collar 9 in a position overlapping the first circumferential track I la, and a second aerial member 17b is fixed to the surface of the sensor mount facing the annular collar

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9 in a position overlapping the second circumferential track 1 lb. A printed circuit board (PCB) 19 is also fixed to the sensor mount 15 on a surface away from the annular collar 9, and circuitry on the PCB 19 is connected to the first and second aerial members 17a,l7b by electrical connections.

In this embodiment, the first aerial member I 7a comprises a first transmit aerial and a first receive aerial and the second aerial member 17b comprises a second transmit aerial and a second receive aerial. The dimensions of the first and second transmit aerials and the first and second receive aerials are such that for any relative rotary position at least one sensor element 13 is overlapped. In use, excitation signals are applied to one or both of the first and second transmit aerials to induce a resonant signal in any overlapped sensor element 13. The induced resonant signal in a sensor element 13 in turn induces a signal in the corresponding receive aerial. By determining the frequency of the excitation signal at which resonance occurs for an overlapped sensor element 13, a coarse position of the first arm portion 5 relative to the second arm portion 7 is determined. Further, in this embodiment the signal induced in the receive aerial corresponding to the overlapped sensor element 13 is processed to determine the position of the overlapped sensor element 13 relative to the corresponding aerial member 17, and by combining this relative position with the rough absolute position, a fine absolute position of the first arm portion 5 relative to the second arm portion 7 is determined.

In this embodiment, the aerials formed on an aerial member, the excitation signal applied to the transmit aerial and the processing of the signals induced in the receive aerial is substantially as described in WO 2009/153580, the whole contents of which is hereby incorporated by reference. The components of the position sensor will now be described in more detail.

Aerial Layouts In this embodiment, each aerial member 17 is generally rectangular and has a receive aerial 21, a transmit aerial 23 and a feedback aerial 25 formed thereon. In particular, each aerial member 17 is a multi-layer PCB with conductive tracks being deposited on both surfaces (referred to hereafter as the top surface and the bottom surface for ease of reference), with connection between the conductive tracks on the top and bottom surfaces occurring through via holes. Each aerial member 17 is lengthwise aligned with the corresponding measurement track 11.

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In Figure 3 and the following Figures, conductive tracks deposited on the top surface are shown by solid lines whereas conductive tracks deposited on the bottom surface are shown by dashed lines. For ease of explanation, Figures 4A to 4C respectively show in isolation the conductive tracks forming the transmit aerial 23, the receive aerial 21 and the feedback aerial 25. As shown in Figures 4A to 4C, the transmit aerial 23 is almost exclusively formed by conductive tracks deposited on the bottom surface of the aerial member 17, whereas the receive aerial 21 and the feedback aerial 25 are predominantly formed on the top surface of the aerial member 17.

The transmit aerial 23 has a first set of three current loops around the periphery of the aerial member 17, with a second set of three current loops in the same sense as the first set positioned adjacent a first widthwise edge of the aerial member 17 and extending over about one fifth of the length of the aerial member 17.

The feedback aerial 25 has a first set of three current loops which extend from a second widthwise edge of the aerial member 17, opposite the first widthwise edge, over about four fifths of the length of the aerial member 17, and a second set of six current loops in the opposite sense to the first set of three current loops which extend over the remaining fifth ol the length of the aerial member 17 adjacent the first widthwise edge so that they are substantially aligned with the second set of three current loops of the transmit aerial 23. In this way, the transmit aerial 23 and the feedback aerial 25 arc balanced (i.e. an alternating electromagnetic field generated by the transmit aerial 23 directly induces substantially no signal in the feedback aerial 25) in a manner which is well known to those skilled in the art.

The receive aerial 2 1 extends over about four fifths of the length of the aerial member 17 from the second widthwise edge. The pattern of the conductive track of the receive aerial 21 is such that any perpendicular electromagnetic field component close to the position x equals zero passes through six current loops in one sense, and then moving in the measurement direction without changing sense the number of current loops reduces from five, then four, then three, then two and then one until finally at the central point of the length of the receive aerial 21 any perpendicular electromagnetic field component passes through no current loops. Continuing along the length of the aerial member 17, the sense of the current loops changes and the number of current loops increase from one, to two, to three, to four, to five and finally to six.

The particular arrangement of the receive aerial 21 results in the signal induced in the receive aerial 21 varying in dependence on the position of a sensor element 13 in a linear

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manner along a portion of the length of the aerial member 17, hereafter referred to as the measurement direction, from a minimum value to a maximum value. In this embodiment, the linear range extends over a distance of about 45mm along the measurement direction, in addition, the anti-symmetric arrangement of the current loops of the receive aerial 21 leads to the receive aerial 21 being substantially balanced with respect to the transmit aerial 23.

For completeness, Figure 5 shows that the inductor 29 of the resonant circuit 27 of a sensor element I 3 is formed by conductive track on a PCB arranged in a concentric set of current loops. The ends of the conductive track are joined by the capacitor 31 (not shown in Figure 5).

Circuitry Components Figure 6 schematically shows the main components of the position sensor illustrated in Figure 1. As shown, a processor 41 is connected to a clock 43, two programmable oscillators 45a,45b, an analog-to-digital converter (ADC) 47, a frequency look-up (able (LUT) 49 and an output port 51. Each programmable oscillator 45a,45b supplies an oscillating signal to corresponding excitation signal generating and detection signal pre-processing circuitry 53a,53b at a frequency controlled by the processor 41, and each excitation signal generating and detection signal pre-processing circuitry 53 outputs analog signals to the analog-to-digital converter 47.

As shown in Figure 7, the oscillating signal output by a programmable oscillator 45 is input to transmit aerial drive circuitry 61 of the corresponding excitation signal generating and detection signal pre-processing circuitry 53. The oscillating signal output by the programmable oscillator 45 is also input to a 900 phase shifter 62 which applies a 900 phase shift to the oscillating signal to generate a phase shifted signal which is applied as a timing signal to first and second synchronous demodulators 63a,63b.

The transmit aerial drive circuitry 61 generates an excitation signal at the frequency of the oscillating signal from the programmable oscillator 45, and the excitation signal is applied to the transmit aerial 23 in order to generate a magnetic field which oscillates at frequency of the oscillating signal of the programmable oscillator 45. As will be described in more detail hereafter, the amplitude of the excitation signal is controlled by a feedback loop.

In this embodiment, the processor 41 controls the frequency of the oscillating signal output by the programmable oscillator 45 such that resonance is induced in the resonant circuit 27 of a sensor element 13 overlapped by the transmit aerial 23. As those skilled in the

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art will appreciate, the resonant signal induced in the resonant circuit 27 is shifted in phase by a quarter of a cycle from that of the oscillating magnetic field.

The resonant signal induced in the resonant circuit 27 itself generates an oscillating magnetic field, arid this osciflating magnetic field induces signals in the receive aerial 21 and the feedback aerial 25. In this embodiment, the amplitude of the oscillating signal induced in the feedback aerial 25 by the resonant signal in the resonant circuit 27 does not vary with the position of the overlapped sensor element 13 along the measurement direction, while the amplitude of the oscillating signal induced in the receive aerial 21 by thc rcsonant signal in the resonant circuit 27 varies linearly in accordance with the position of the overlapped sensor element 13 over a distance of 45mm along the measurement direction.

Although the signal induced in the feedback aerial 21 does not vary in accordance with the position of the sensor element 13 along the measurement direction, it may vary for a number of different factors including: -variations in the distance between the sensor element 13 and the aerial member 17 in a direction perpendicular to the measurement direction; -variations in the frequency output by the programmable oscillator 45; -variation in the amplitude of the current supplied to the transmit aerial 23 as a result of variation in the impedance of the drive circuitry 61 and the impedance of the conductive track forming the transmit aerial 23 with environmental factors such as temperature; and -variations in the resonant properties of the resonant circuit 27 with changing environmental factors, e.g. temperature, humidity.

in a similar fashion, these factors will cause a variation in the amplitude of the signal induced in the receive aerial 21, and uncorrected those variations would lead to inaccuracy in the measurement reading. Accordingly, in this embodiment a feedback control ioop is used to vary the amplitude of the excitation signal to cause the amplitude of the signal induced in the feedback aerial 25 to be set at a reference level.

As shown in Figure 7, the signal induced in the feedback aerial 25 is input to the first synchronous demodulator 63a which performs synchronous detection using the phase-shifted signal in order to take into account the phase shift introduced by the resonant circuit 27. The output of the first synchronous demodulator 63a passes through a low pass filter 65a to generate a DC signal corresponding to the amplitude of the signal induced in the feedback aerial 25. The output of the low pass filter 65a is input to a servo-system 67 along with a

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reference voltage level generated by a reference voltage level generator 69. The output of the servo-system 67 is input to the drive circuitry 61 to control the amplitude of the excitation signal applied to the transmit aerial 23.

ifl operation, the servo-system 67 controls the amplitude of the excitation signal so that the signal output by the low pass fitter 65a matches the reference voltage level, thereby forming a feedback control loop. As a result of this feedback control, the amplitude of the signal induced in the receive aerial 21 is predominantly dependent on the position of the sensor element 13 along the measurement direction.

As shown in Figure 7, the signal induced in the receive aerial 21 is input to the second synchronous demodulator 63b which performs synchronous detection using the phase-shifted signal in order to take into account the phase shift introduced by the resonant circuit 27. The output of the second synchronous demodulator 63b is then input to a low-pass filter 65b, whose output is a DC signal having an amplitude which is dependent on the position of the sensor element 1 along the ineasurenient direction. Tins DC signal is then input to an offset/gain adjust circuit 71 which scales the DC signal so that the variation in magnitude of the scaled DC signal matches the dynamic range of the ADC 47. The sealed DC signal output from the offset/gain adjust circuit 71 is then input to the ADC 47, which outputs a corresponding digital value to the processor 41, which in turn converts the digital value to a position value using data stored in the frequency look-up table 49 as will be described in more detail hereafter.

As shown in Figure 7, the output of the servo-system 67 is also input to the ADC 47.

This signal is indicative of whether or not a sensor element 13 overlapped by the transmit aerial 23 is exhibiting resonance, and is used during calibration of the sensor.

Returning to Figure 6, the frequency look-up table 49 stores data associated with each of the sensor elements 13. Tn particular, the frequency look-up table 49 effectively stores a list of sensor elements 13 in order around the circumference of the robot arm, so that each sensor element 13 is positioned in the list between the neighbouring sensor elements 13 in the other circumferential track 11. For each sensor element 13, the frequency LUT 49 stores the corresponding resonant frequency and conversion data for converting a digital value input to the process into a position value. This conversion data include a slope value and an offset value, with a position value being generated by multiplying the received digital value by the slope value and then adding the offset value.

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Processor Operations The operation of the processor 41 will now be described with reference to the flow chart of Figure 8. Following power-up at Si, the hardware and software components are initialised at S3. The processor 41 then causes, at 55, the programmable oscillators 45 to scan their respective oscillation frequencies through an operational range to identify an overlapped sensor element 13 while the first arm portion 5 is stationary relative to the second arm portion 7.

Optionally, the processor 41 then initiates a calibration procedure during which the first arm portion 5 is moved by set increments relative to the second arm portion 7 and the oscillator frequencies are scanned at each incremental position to identify any sensor element 13 overlapped by the aerial member 17. An offset value is then calculated for convening the position of the overlapped sensor element 13 relative to the corresponding receive aerial into an absolute position value with relation to a reference relative position between the first arm portion 5 and the second arm portion 7. The position and the resonant frequency for an overlapped sensor element 13 is stored in the* frequency LUT 49, before moving the first portion 5 again by the set increment and the scanning the oscillator frequencies. This iterative incrementing and scanning proceeds until the position and resonant frequency of all the sensor elements 13 is stored in the frequency LUT 49.

The processor 41 then initiates monitoring of the relative position of the first arm portion 5 and the second arm portion 7, which may change due to movement of either arm portion. At 511, the processor 41 determines the digital value output by the ADC 47. At S 13, the processor 41 determines whether the digital value output by the ADC 47 is greater than a threshold HPSLh or less than a threshold LPS. As shown in Figure 9, digital values between LFSth and HPSth correspond to the overlapped sensor element 13 being adjacent a central portion of the corresponding aerial member 17. If the digital value corresponds to a position within the central portion of the corresponding aerial member 17, then the processor 41 applies, at S15, slope and offset calibration correction to the digital value, using data stored in the frequency LUT 49 in association with the resonant frequency of the overlapped sensor element 13, to generate absolute position information.

If the digital value is outside of the central portion of the corresponding aerial member 17, then the processor 41 prepares, at 517, for a possible switch to the next nearest sensor element 13 by determining which side of the central region overlaps the sensor element 13, looking up the resonant frequency of the next sensor element 1 3 to that side of the central region in the other circumferential track II in the frequency LLJT 49, and setting the programmable oscillator 45 for the other circumferential track 11 to that resonant frequency, if this has not been done already.

The processor 41 then determines, at S19, if the digital value is greater than a threshold value HEBSLh (which is larger than the threshold value HPSIh) or less than a threshold value LEBSth (which is less than the threshold value LPSth). As shown in Figure 9, digital values greater than the threshold value HEBSth correspond to one extreme of the linear measurement range, in this embodiment a 2.5mm region for which a neighbouring sensor element. 13 is overlapped by the other aerial member 17. Similarly, digital values less than the threshold value LEBSIB correspond to the other extreme of the linear measurement range, in this embodiment a 2.5mm region for which the other neighbouring sensor element 13 is overlapped by the other aerial member 17. These two extreme regions will hereafter be called blend regions. If the digital value does not correspond to a position in a blend region, then the processor 41 proceeds, at 515, with applying the slope and offset calibration correction to the digital value as described above. If, however, the processor 41 determines that the digital value does correspond to a blend region, then the processor 41 switches, at 521, to the next nearest sensor element 13 by applying the appropriate oscillating signal to the other excitation signal generating and detection signal pre-processing circuitry 53 and determining a digital value output by the ADC 47 corresponding to the signal output by the other excitation signal generating and detection signal pre-processing circuitry 53. The processor 41 then applies, at 523, respective slope and offset calibration to both of the digital values and then blends, at S25, the resultant position data to create absolute position information in the form of a blended position reading.

The blending process is schematically illustrated in Figure 10. The top graph in Figure 10 shows a variation of the digital value output by the ADC 47 with relative position.

A first linear response 81a corresponds to a signal induced in the receive aerial 21 by a first sensor element, a second linear response Sib corresponds to a signal induced in the receive aerial 21 by a second sensor element which neighbours the first sensor element, and a third linear response 81c corresponds to a signal induced in the receive aerial 21 by a third sensor element which neighbours the second sensor element on the other side to the first sensor element. In a first blend region 83a, signals are produced by both the first and second sensor elements. Similarly, in a second blend region 83b, signals are produced by both the second and third sensor elements. The bottom graph in Figure 10 shows the position readings calculated from the digital values illustrated in the top graph. In the blend regions 83, in this embodiment, the blending involves a weighted addition of the resultant position data, with the weighting values corresponding to the extent to which the digital values exceed HEBSth or are less than LFBSEh.

After the absolute position information is generated, whether from one position reading or a blend of two position readings, the processor 41 outputs, at S25, the absolute position information from the output port. The process then continues, at S27, until power down by determining a new position for the current sensor element 13 and then repeating the processing steps described above.

MODIFICATIONS AND FURTHER EMBODIMENTS

In the first embodiment, the invention has been described in the context of a rotary position encoder for measuring the relative rotary position of two portions of a robot arm which are rotatable relative to each other relative to a robot arm. Other applications for a rotary position encoder according to the claimed invention include machine tools such as lathes etc. Those skilled in the art will appreciate that the present invention could equally well be used for a linear encoder. In this regard, Figure 11 shows a linear arrangement having two adjacent linear tracks and two adjacent planar aerial members, each aerial member arranged for relative movement along a rectilinear measurement path with respect to a respective one of the two linear tracks. In addition, the position sensor could be adapted to measure linear position along a curved measurement path by varying the layout of the transmit aerial, the receive aerial and the feedback aerial in a manner which would be apparent to a person skilled in the art.

Although sensor elements are arranged in two tracks in the first embodiment, this is not essential. The sensor elements may be arranged in a single track, in which case only a single aerial member is required. In such an arrangement, the aerial member must overlap at least one sensor element at any one time. When more than one sensor element is overlapped, the transmit aerial may sequentially alternate between the resonant frequencies of the overlapped sensor elements. Alternatively, simultaneous excitation of the overlapped sensor elements may be performed if each of the overlapped sensor elements is arranged to generate a different magnetic field distribution and the receive aerial has plural windings each arranged to detect a particular one of the generated magnetic field distributions while being balanced with respect to the other generated magnetic field distributions.

The use of sensor elements which include aerials designed to generate magnetic fields having different field distributions, with the receive aerial being designed to distinguish between the different field distributions, can also be used to differentiate between sensor elements such that the need for a resonant circuit in the sensor clement is removed. All that is necessary is that the magnetic field generated by a sensor element has a property which can be detected to distinguish that sensor element from different sensor elements. It will be appreciated that this property need not be different for every sensor element. For example, in the first embodiment sensor elements which are sufficiently spaced from each other may have the same resonant frequency as the processor can determine which sensor element is producing the magnetic field from a history of previously detected sensor elements.

The position sensor could also be used as a speed detector by taking a series of measurements of the position of the first member relative to the second member at known I ¶ timings.

In the described embodiments, the transmit aerial 23 is designed so that an alternating electromagnetic field generated by the transmit aerial 23 does not directly induce a signal in the receive aerial 21. In an alternative embodiment a proportion of the reference voltage level determined by the amplitude of the transmission drive signal is subtracted. In this way, any signal breakthrough proportional to the transmission drive signal may be compensated.

In the first embodiment, a feedback loop controls the amplitude of a drive signal applied to the transmit aerial 23 at a substantially fixed frequency. Alternatively, a voltage-controlled oscillator could be used with the feedback loop controlling the frequency of the oscillating signal generated by the voltage-controlled oscillator. In this way, the strength of the signal induced in the resonant circuit would be varied as the frequency of the excitation signal moved relative to the resonant frequency (i.e. increasing as the excitation frequency approaches the resonant frequency and decreasing as the excitation frequency moves away from the resonant frequency), and therefore the strength of the signal induced in the feedback aerial would vary.

In the exemplary embodiments, a passive resonant circuit on the sensor element 13 acts as an intermediate coupling element between the transmit aerial and both the receive aerial and the feedback aerial. Alternatively an active resonant circuit, including a power source for amplification, could be used.

The intermediate coupling element could include a non-linear element, such as a diode, so that a magnetic field oscillating at an excitation frequency induces a current in the intermediate coupling element having frequency components away from the excitation frequency. This would result in a signal being induced in the receive aerial at these new S frequency components, which could be detected and have improved noise immunity with respect to the excitation signal generating circuitry.

The intermediate coupling element effectively acts as a magnetic field generator which is powered through inductive coupling with the transmit aerial. In an alternative embodiment, a magnetic field generator on the sensor element could be directly powered, with the excitation signal applied being controlled in accordance with the signal induced in the feedback aerial. Such an arrangement has the advantage of removing the need of a transmit aerial, but also has the significant disadvantage of requiring electrical connections to the sensor element.

In the first embodiment, the present invention has been applied to a position sensor for sensing a relative position along a one-dimensional measurement path. It will be appreciated that the present invention could also be applied to a two-dimensional sensor by arranging a two-dimensional array of sensor elements on one member and providing an aerial member on a second member which can be moved over the array of sensor elements. In an embodiment, the aerial member includes two orthogonal transmit aerials which can be alternately excited.

A rough position measurement can be determined by identifying the sensor element or sensor elements currently overlapped, while a fine position measurement can be determined from the signals induced in the receive aerial as a result of applying excitation signals to each of the transmit aerials.

The first embodiment utilises an inductive position sensing arrangement substantially as described in WO 2009/153580. Alternative inductive position sensing arrangements could be used, for example those described in WO 03/038379 and WO 95/31696.

Claims (1)

1. <claim-text>CLAIMS1. A position sensor for sensing the relative position of a first and second members which are movable relative to each other along a measurement path, the position sensor comprising: a series of magnetic field generators fixed relative to the first member, the magnetic field generators being spaced apart relative to each other along the measurement path, a receive aerial fixed relative to the second member, the receive aerial defining a detection path which overlaps part of the measurement path, said overlapped part being dependent on the relative position of the first and second members, * a signal processor operable to process a signal induced in the receive aerial by at least one magnetic field generator to determine the position of one of the magnetic field generators relative to the detection path, wherein at least some of the magnetic field generators have tespective different magnetic field generation properties and the series of magnetic field generators are arranged such that said induced signal is indicative of the part of the measurement path overlapped by the detection path, and wherein the signal processor is operable to determine the position of the first member relative to the second member along the measurement path by determining the part of the measurement path overlapped by the detection path and the position of said one of the magnetic field generators relative to the detection path.</claim-text> <claim-text>2. A position sensor according to claim 1, wherein the series of magnetic field generators comprises a first row of magnetic field generators and a second row of magnetic field generators, the first row being transversely offset from the second row relative to the measurement path, and wherein the receive aerial comprises a first receive winding aligned with the first row and a second receive winding aligned with the second row.</claim-text> <claim-text>3. A position sensor according to claim 2, wherein the magnetic field generators of the first row and the magnetic field generators of the second row are staggered relative to each other along the measurement path.</claim-text> <claim-text>4. A position sensor according to claim 2 or 3, wherein the signal processor is operable to process signals induced in both the first receive winding and the second receive winding to determine the position of the first member relative to the second member.</claim-text> <claim-text>5. A position sensor according to any preceding claim, wherein in response to relative movement of the first and second members, the signal processor is arranged to switch between processing a signal induced in the detection aerial by one of the magnetic field generators and a signal induced in the detection aerial by a different one of the magnetic field generators to detennine the relative position of the first and second members, wherein said switching comprises a cross-over region in which said relative position is determined using a combination of the signal induced by said one of the magnetic field generators and the signal induced by said different one of the magnetic field generators.</claim-text> <claim-text>6. A position sensor according to any proceeding claim, further comprising a look-up table storing a position offset value for each of the magnetic field generators, wherein the signal processor is operable to calculate the relative position of the first and second members using the position of said one of the magnetic field generators relative to the detection path and the position offset value corresponding to said one of the magnetic field generators.</claim-text> <claim-text>7. A position sensor according to any preceding claim, wherein the series of magnetic field generators comprises a series of resonators, at least some of the resonators having respective different resonant frequencies.</claim-text> <claim-text>8. A position sensor according to claim 7, wherein the second member comprises a transmit aerial extending over said detection path, and the position sensor further comprises an excitation signal generator coupled to said transmit aerial, the excitation signal generator being operable to generate an excitation signal at the resonant frequency of a resonator overlapped by said detection path.</claim-text> <claim-text>9. A position sensor according to any preceding claim, wherein the measurement path is linear path.</claim-text> <claim-text>10. A position sensor according to any of claims I to 8, wherein the measurement path is a rotary path.</claim-text> <claim-text>11. A position sensor for sensing the relative position of a first and second members which are movable relative to each other in two dimensions, the position sensor comprising: an array of magnetic field generators fixed relative to the first member, the magnetic field generators being spaced apart relative to each other over a two-dimensional measurment region, a receive aerial fixed relative to the second. member, the receive aerial defining a detection region which overlaps part of the measurement region, said overlapped region being dependent on the relative position of the first and second members, a signal processor operable to process a signal induced in the receive aerial by at least one magnetic field generator to determine the position of one of the magnetic field generators relative to the detection path, wherein at least some of the magnetic field generators have respective different magnetic field generation properties and the series of magnetic field generators are arranged such that said induced signal is indicative of the part of the measurement region overlapped by the detection region.</claim-text> <claim-text>12. A position sensor according to claim 11, wherein the signal processor is operable to determine the position of the first member relative to the second member along the measurement path by determining the part of the measurement region overlapped by the detection path and the position of said one of the magnetic field generators relative to the detection region.</claim-text> <claim-text>13. A method of sensing the relative position of a first and second members which are movable relative to each other along a measurement path, wherein a series of magnetic field generators is fixed relative to the first member, the magnetic field generators being spaced apart relative to each other along the measurement path, and a receive aerial is fixed relative to the second member, the receive aerial defining a detection path which overlaps part of the measurement path, said overlapped part being dependent on the relative position of the first and second members, the method comprising:Sprocessing a signal induced in the receive aerial by at least one magnetic field generator to determine the position of one of the magnetic field generators relative to the detection path, wherein at least some of the magnetic field generators have respective different magnetic field generation properties and the series of magnetic field generators are arranged such that said induced signal is indicative of the part of the measurement path overlapped by the detection path, and wherein the method comprises determining the position of the first member relative to the second member along the measurement. path by determining the part of the measurement path overlapped by the detection path and the position of said one of the magnetic field generators relative to the detection path.</claim-text>