GB2164824A - Access control system - Google Patents

Abstract

An access control system of the kind in which an identification card bearing tuned circuits has a predetermined absorption effect upon a sensing or interrogation coil carrying a swept frequency. The sensing coil (1), or drive coil (2) associated with the sensing coil (1), is a null-balanced coil having balanced portions (A, B), which become unbalanced by the presence of the identification card. This enables quicker interrogation and simplified analysing circuitry. <IMAGE>

Description

SPECIFICATION Access control system This invention relates to access control systems.

An existing type of access control system requires that the person desiring access should offer an identifying card to a sensor placed at the point of entry. The sensor is a coil carrying an alternating current, the frequency of which is varied repetitively so as to embrace a spectrum which might, typically, extend from 2MHz to 27MHz, giving a total bandwidth of 25MHz. Each identifying card carries one or more tuned circuits, their respective resonant frequencies being chosen so as to represent a discreet identifying code.

The act of presenting the card, so as to couple with the sensing coil, will result in the several tuned circuits each absorbing energy at the frequency to which it is tuned. The Q of any given tuned circuit will determine the bandwidth over which energy will be absorbed, in practical examples being between 0.5MHz and 1MHz. Hence, with the examples given, the total interrogation bandwidth might be divided into 25 discrete identifying regions, each 1MHz wide. A card tuned circuit, made to resonate within a given identifying region, may thus signal one digit of the identifying code. A system of cards, each bearing, typically, four tuned circuits, may therefore cover a total of 12650 possible alternative codes.

In a typical existing system, the interrogating signal in the sensing coil is swept across the frequency spectrum in a repetitive sawtooth manner. With a card presented, as each tuned frequency on the card is traversed, the sensing coil impedance will be modified by the coupling to the card circuit, giving rise to a change in the signal voltage developed across the sensing coil. This effect is extremely small relative to the interrogating signal amplitude, making it necessary to integrate a number of successive samples in order to extract the card signals from the noise and interference. The consequently extended response time of the system can be a significant operational disadvantage.

it is an object of the present invention to overcome these difficulties.

According to the present invention there is provided an access control system of the kind which is enabled by an identification card bearing tuned circuits having a predetermined absorption effect upon a sensing coil carrying a swept frequency, characterised in that the sensing coil, or a drive coil associated with the sensing coil is a null balanced coil which is unbalanced by the presence of the identification card.

At a practical card separation, these pulses rise well above the residual background, and hence might readily be detected during a single frequency sweep, without the need for the integration of successive samples. Experimental results show that a response time of less than 10 milliseconds can comfortably be achieved.

Oscillograph photographs, comparing the sensor outputs of a typical existing system-the SCHLAGE Model 708 Card Reader-and the null-balanced sensor here described, are shown in Figure 4, later to be discussed in greater detail.

In order that the invention can be more clearly understood, reference will now be made to the accompanying drawings, in which: Fig. 1 illustrates the principles of the invention in a simple embodiment thereof; Fig. 2 shows another embodiment of the invention which is symmetrical, Fig. 2A showing the reverse side; Fig. 3 shows a further embodiment more compact than the embodiment of Figs. 1 and 2; Fig. 4 shows oscillograph photographs of the sensor output of Fig. 3 compared with the output of an existing prior art system.

Fig. 5 is a circuit block diagram of a frequency recognition decoding arrangement, and Fig. 6 is a circuit block diagram of a time delay decoding arrangement.

Referring to Figure 1, the basic principle is illustrated. A drive coil 1, which carries the swept-frequency interrogating waveform, is divided into two symmetrical halves A and B, their relative polarities being disposed so that, if, at any instant, the field generated by coil A is rising out of the Figure, the field of coil B is descending into it. The detection coil 2 is positioned so as to couple equally with both halves A and B of the drive coil, the balanced condition resulting in zero output from the detection coil. At practical operating frequencies between 2MHz and 30MHz, single-turn coils of about 150mm mean diameter have been found to be appropriate, and this is reflected in the drawings Figure. It might, nevertheless, be appropriate to use larger numbers-of turns in systems operating at lower frequencies.

Critical balance may be achieved by adjusting the relative areas of the two halves of the drive coil 1 by moving central conductor X sideways. A simpler method is to place a small eddy-current loop 3 (for example, a copper disc or plate) to couple with either field A or field B, depending upon the direction of residual unbalance. In practice, the eddy-current loop 3 is placed so as to overlap both fields and is then adjusted in one direction or the other until an output minimum is achieved.

The balance of the system is disturbed when a tuned-circuit card is presented either to the field of coil A or field of coil B. The effect is a frequencyselective equivalent of inserting the eddy-current loop; as the interrogating signal sweeps through the series reso nant frequency of a card tuned circuit, the circuit will couple a low impedance into that side of the sensor, producing an identifying unbalance signal in the detection coil 2.

A practical implementation of the symmetrical arrangement described above is illustrated in Figure 2, and is constructed from doublesided printed circuit board. Here, the two halves of the drive coil 10 are connected in parallel, a convenient configuration for construction as a double-sided printed circuit board. Since the simple arrangement of Figure 1 might suffer balancing problems due to capacitive coupling between the input and output coils 1 and 2, additional, earthed, open-circuited conductors 11A, 11B are positioned so as to embrace the output coil 12. For conditions of extreme external interference, it would be advantageous to supplement these screening conductors with provision for totally enclosing the output coil in a conventional gapped screen.In a printed-circuit version, this condition can be partially achieved by providing, on the back of the board, a similar earthed, open-circuited conductor 14 shown in Figure 2A, of sufficient width to embrace both the screening conductors and the output conductor. The screening might be carried further towards completion by adding a similar wide conductor (not shown) on the front face of the board, suitably insulated from the output coil. Although not essential, conventional printed-circuit techniques might be adopted to connect the several screening conductors together so as to encircle the output conductor completely. A balancing slug 13 is shown for establishing null-balance.

In the symmetrical arrangement of Figures 1 and 2, presentation of a card to coil A will produce an output in antiphase to that produced when the card is presented to coil B.

This distinction can be established by reference to the phase of the drive waveform. In certain applications, this feature could provide an additional coding facility, since the card user might signal alternatives-such as 'in' or 'out'-by the appropriate choice of sensor area.

However, for a compact sensor design, it is appropriate and natural to the user, that the sensing region should lie in the centre of the sensor housing. To this end, the coil configuration can be rearranged as shown in Figure 3, which is largely self-explanatory. Balancing coil position B of drive coil 20 has been redistributed to occupy the triangular corner areas of a rectangular plane surrounded by the output coil 22 screened by earthed open circuit conductors 21A, 21B. The octagonal area embraced by the inner conductors of the B coils and their interconnections becomes the functional field A. The instantaneous magnetic flux, emerging from the Figure in field A, reenters the paper shared equally between the four B fields. Hence, under balanced conditions, no resultant flux interacts with the output coil 22 and no signal output ensues.As before, critical balance may be achieved by adjusting an eddy-current slug 23 between the A field and any one of the B fields.

Presentation of a card to the centre of field A will, as before, unbalance the system so as to provide the appropriate coded output.

In Figure 3, the two ends of the output coil are shown unconnected. Satisfactory results have been achieved with one end of the coil connected to ground and the other to a single-ended amplifier. However, further improvement of the null balance condition might be achieved by adopting a balanced amplifier and grounding the centre point of the output coil.

The same approach might be applied, with advantage, to the drive coil and its input circuit.

it should be noted that Figure 3 is a fullsized reproduction of an actual experimental circuit board.

Figures 4 (a) and 4 (b) show a comparison of the sensing coil outputs of the SCHLAGE Model 708 Card Reader and the balanced sensor here described, using the same commercially available identification card in each. In the right-hand set of curves, showing the output from the balanced sensor, the X axis shows a drive-frequency sweep extending linearly from 2MHz on the left to 20MHz on the right. The oscilloscope Y amplifier is driven directly from the single-turn output coil.

The left-hand curves show the voltage developed across the SCHLAGE 708 sensor coil.

It will be appreciated that this shows the drive (interrogating) waveform, which becomes modified by the presence of a tuned-circuit card, and from which must be extracted the coded card signal. Although these signals are, relatively, so small as to be imperceptible on Figure 4, except when the card is in actual contact with the sensor, the card reader is able-by a process of successive sampling and integration to decode them at card separations up to about 150 millimetres. In this instance, the X axis sweep is generated by the card reader equipment, but corresponds roughly with the frequency sweep of the righthand curves.

In order to recognise the presence, or otherwise, of tuned circuits in the several identifying channels of the frequency spectrum, it is necessary for an interrogating signal to be present in each channel, for at least part of the time during which the card is offered. A first, crude, method would be to have a separate RF oscillator tuned to the centre of each channel. However, a major circuit simplification would be achieved by making a single oscillator step progressively from one channel to the next. This presents no problem, provided that the cycle is completed at least once during the time that the card is presented, but the use of a number of discreet frequencies could encounter the difficulty that any drift in the resonant frequency of the card tuned circuits could result in inadequate coupling with the spot interrogation frequency.

The preferred method, therefore, is to sweep the spectrum progressively, so that each card tuned circuit is interrogated regardless of its precise resonant frequency, the requirement being only that the tuned circuit frequency should not have drifted outside the bandwidth of its intended identifying channel.

It is desirable that the spectrum should be swept at constant velocity, both to ensure identical interrogation of all the channels and to simplify the synchronisation of the detection and decoding circuits. This requirement can be satisfied by a sawtooth scan-a mode adopted in existing commercial equipment.

Nevertheless, possibly undesirable flyback transients could be eliminated by using a triangular scan waveform, scanning first up, and then down, the frequency spectrum.

The arrangement described above, in which the balanced coil carries the drive current, is inherently less likely to generate external interference than a single, unbalanced coil, since the field A is neutralised by fields B. However, the associated output coil is not so balanced in relation to incoming interference, and will exhibit the normal characteristics of a screened loop aerial. This will normally present no problem, since the amount of pickup proves, in practice, to be negligible compared with the unbalance signal due to the presentation of a card. An alternative to this arrangement is to reverse the roles of the drive and output coils. In this case, the balanced output coil may be expected to be less sensitive to incoming interference, but the drive coil will lose the advantage of self-neutralisation.Although this latter point would not appear to present any great problem, since the single, unneutralised sensing coil of the card reader we have used radiates in an identical manner, experiments suggest that the balanced-drivecoil arrangement is to be preferred.

Referring now to the remaining Figures, there are shown circuit block diagrams for decoding the code of an access control sensor card using sensing drive coil arrangements as described above.

Let it be assumed that the interrogating sweep extends from 2 to 20MHz, and that this sweep is divided, for coding purposes, into 17 regions, each 1MHz wide. The access card will be provided with several tuned circuits say, four for example-each circuit being tuned to the frequency of one of the 17 interrogating regions. The combination so selected will form the identifying code for that particular card. As the interrogating frequency sweeps through any given one of the 17 regions, a sensing-coil output will result only if the presented card carries a tuned circuit tuned to that particular region. Hence, on completion of a full sweep of all 17 regions, a four-circuit card will have generated four RF pulses, each pulse occuring within the period during which the interrogating frequency is passing through the region to which a given tuned circuit relates.

Let it also be assumed, for simplicity of description, that each interrogating sweep occupies a total period of 17 milliseconds. Consequently, in each full sweep, the interrogating signal will occupy each frequency region in turn for a period of one millisecond. If, for example, the card is tuned to the region lying between 2 and 3MHz, the resulting output RF pulse will contain some 2000 cycles of RF at a frequency somewhere between 2 and 3MHz, depending upon the exact frequency and bandwidth of the tuned circuit. It is also significant, for the purpose of the preferred decoding circuit here to be described, that the output pulse will occur during the first millisecond after commencement of the sweep.

Taking another example, suppose one of the card circuits to be tuned within the 10 to 11MHz region. During the 1 millisecond period in which the interrogating signal sweeps through this region the RF pulse from the sensing coil will comprise some 10 thousand cycles at a frequency somewhere between 10 and 11MHz, again depending upon the precise frequency and bandwidth of the card tuned circuit. It will also be apparent that the output pulse due to this tuned circuit will occur during the 8th millisecond after the commencement of the sweep.

Thus, during each interrogating sweep, a four-tuned-circuit card will generate four characteristic RF output pulses, each of rather less than one millisecond duration. It follows that the repetitive sweep, such as would be used in a practical system, will give rise to four characteristic trains of output pulses, which would persist for so long as the card remained in proximity to the sensing coil.

The output pulses possess two, independent, identifying characteristics. The first of these is the frequency of the RF component of the pulse. This remains unaffected by the time at which the pulse might occur after the start of the interrogating sweep. Alternatively, the pulse may be identified by its time delay after the commencement of the sweep. In this case, the frequency of its RF component is of no significance.

This permits the following two alternative decoding techniques, decoding by frequency recognition or decoding by time delay recognition.

1). Frequency Recognition Assuming a four-tuned-circuit 35 (Fig. 5), the interrogating drive circuit gives a repetitive sweep produced by a frequency sweep waveform generator 30 and a swept radio frequency oscillator 31 driving drive coil 32 which in turn drives source coil 33. Amplifier 34 amplifies the sensing coil output which will comprise four interleaved pulse trains, each train having an RF component at a frequency corresponding with that of its associated card tuned circuit. Using, still, the example of 17 coding regions, this output is fed to a parallel array of 17 bandpass filters, each 1MHz wide and each tuned to embrace one of the 17 regions as shown in Fig. 5. Each filter is provided with its own threshold detector, demodulator, integrator and storage circuit, so as to be capable of contributing one digit to a static four out-of-eighteen recognition code.Hence, any given card, having its circuits tuned to a specific four of the 17 possible regions, will stimulate outputs only from the four related filter circuits, resulting in a static code identifying that particular card. This code may be transferred into a subsequent processor for appropriate action.

It will be apparent that the above method does not depend upon the use of linear frequency-sweep, so that, although a linear sweep is to be preferred as the means of ensuring that all the regions are equally interrogated, other sweep waveforms, such as sinusoidal or random excursions are also permissible. A further variation is to replace the swept-frequency generator 30 with a separate generator for each of the interrogating regions.

The figures quoted here are for example only, and other frequencies and scan rates might also be adopted. in particular, the possible number of code alternatives might desirably be increased by adding more tuned circuits to the cards and/or by increasing the number of interrogating regions. This highlights one basic disadvantage of the frequency-recognition method, in that a separate filter circuit, with its associated detection and storage circuits, must be provided for each of the interrogating regions, leading to relatively expensive circuit complexity.

2). Time-Delay Recognition The preferred alternative decoding method relies upon recognition of the timing of the successive RF pulses in relation to the starting time of each frequency sweep, the actual frequency of the RF component of the feed the sensing coil output into a single demodulator and threshold detector, so as to produce a sequence of unidirectional pulses, time-distributed in a manner appropriate to the desired recognition code. Again using the example values previously adopted, the successive in~ terrogation regions may be designated by the numbers 1 to 17, starting from the moment of sweep commencement. The sweep will be assumed to progress, as before, at the rate of one region per millisecond.Upon presentation of a card having circuits tuned to, say, regions 7, 9, 12 and 15, the resulting output pulses, which will each be rather less than 1 millisecond in width, will appear, respectively, at 7, 9, 12 and 15 milliseconds after the start of each sweep. At a 1 millisecond clock rate, this sequence is stepped into a 17-bit serialto-parallel converter, from which it is transferred as a parallel code upon completion of each sweep. In this particular example the recognition code would be 00000010100100100. Thereafter, the code might be validated by comparison with permanently-stored information, or processed in any way desired.

A typical circuit block diagram is shown in Figure 6. A sinusoidal RF oscillator 40, 41, driving the sensing coil assembly 42, 43, is swept between the limits 2 and 20MHz by a linearly-increasing sawtooth waveform having a repetition rate such as to complete each sweep in a period of 17 milliseconds. A master clock generator 46 provides clock pulses at 1 millisecond intervals. These are applied, directly, to transfer the sensing coil output pulses-after demodulation 47 and shaping 48 -into the serial-to-parallel converter 49.

They are also divided by 17 to provide a transfer pulse for the completed parallel code, and a resetting, or synchronising, pulse for the sawtooth generator upon completion of each sweep.

An alternative method for producing the frequency sweep is to dispense with the sawtooth generator and to control the RF oscillator frequency with a voltage ramp generated by a digital-to-analogue converter driven by the master clock pulse generator. However, if the clock pulses remained at the previous 1 millisecond repetition rate, the D/A converter would be limited to an output staircase of 17 discrete levels. This could certainly be arranged to step the interrogating RF frequency into each of the 17 regions in turn, but the advantage of sweeping across the region, in search of the actual card-circuit tuning peak, would be lost. Consequently, this approach preferentially requires a much higher master clock rate and a D/A converter capable of many more increments of output amplitude.

The clock rate might, for example, be increased by a factor of ten. A 170-level D/A converter would then be used. From the start of a sweep cycle, the clock would step the D/A converter upward from zero to 170, resulting in 170 successive increments of the RF oscillator frequency during the course of the sweep. Thus, each of the 17 interrogation regions would experience 10 successive increments of the interrogating frequency, and the advantage of scanning across each region would be substantially retained. The higher clock rate would necessarily be divided by ten to provide the 1 millisecond pulse interval for stepping the serial-to-parallel converter, and by a further 17 to provide a resetting pulse at the end of each sweep cycle.

As before, the values quoted are exemplary only, and other values might be chosen to favour a particular system design.

The time-delay decoding method proposed above, by eliminating the major array of individual filter circuits required by the frequencyrecognition method, achieves the desired re sult with a significant economy of circuitry. In association with the null-balanced sensing coil the method offers levels of performance and manufacturing economy superior to those existing systems.

Claims (8)

1. An access control system of the kind which is enabled by an identification card bearing tuned circuits having a predetermined absorption effect upon a sensing coil carrying a swept frequency, characterised in that the sensing coil, or a drive coil associated with the sensing coil, is a null-balanced coil which is unbalanced by the presence of the identification card.

2. A system as claimed in Claim 1, characterised in that the balanced coil has a centrally located conductor adjusted sideways to establish the null-balance.

3. A system as claimed in Claim 1, characterised by comprising an eddy current loop for establishing the null-balance.

4. A system as claimed in any preceding Claim, characterised in that open loop screening conductors encircle the sensing coil.

5. A system as claimed in any preceding Claim, characterised in that the coils are formed by a printed circuit on a substrate.

6. A system as claim in Claim 5, characterised in that the reverse side of the substrate carries an earthed open loop screening conductor.

7. A system as claimed in any preceding claim, characterised in that the balancing portion of the balanced coil is distributed so that the sensing region lies generally central with respect to the coils.

8. A system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.