GB2218591A - Apparatus for sensing a person - Google Patents

Abstract

Pulsed Doppler radar apparatus for sensing the presence of a person is described where the radiated power is so low that interference requirements are met, whereby the carrier frequency employed may be outside the allocated channel for the apparatus. Correlation detection techniques (10) are used to recover the resultant low level echo signals. Specially modulated electromagnetic wave pulses are so phased and frequency differenced (alternately antiphased), that the equipment has a greater sensitivity to the target than it has for close up objects. The invention applies innovative modifications to Doppler radar techniques to make them suitable for this specialised task. Also discussed is a method whereby its immunity to large interfering signals can be increased. <IMAGE>

Description

APPARATUS FOR SENSING A PERSON This invention relates to apparatus for sensing a person. Currently, person sensors use passive and active infra-red, sonar Doppler or microwave Doppler. Only microwave Doppler is recommended for outdoor use since other systems have a high propensity for false alarms when used outdoors. Sonar systems use the air as the transmission vehicle which becomes very 'noisy' in windy conditions and passive infra-red systems see the small thermal change in air pockets during high winds.

Also, the nature of the clothing of the target person can greatly affect the echo strength returned on both infra-red and sonar systems. Although the effective range of the soar system can be defined by signal processing, since the ec return times are measured in milliseconds.

no such containment can be applied to the passive infra-red method. A passing exhaust pipe at 50 metres can be just as effective as a person at 5 metres.

Microwrive Doppler at 1OGHz is usually considered to be the best solution since it does not suffer from many of the disadvantages mentioned above. Electromanetic waves are readily reflected from the large water mass in any person regardless of what he is wear ng. However, all continuous wave (CW) microwave systems suffer from the disadvantage that the returned microwave power diminishes as the fourth power of distance to target. This means that a butterfly at 1 foot might return the same strength signal as a man at 10 feet. Also at 1OGH2, the licensed frequency band for such systems the wavelength is only 3cm. This means that very small objects can return sigrsificant energies.Thus small entities such as rotating metal bladed fans and unstable discharge paths in fluorescent strip lights can cause false alarms.

Another disadvantage of the microwave Doppler system is that the noise sideband energy in the oscillator becomes converted into audio by means uf delay paths c-reatinA a phase discriminator from the receiving mixer. This greatly limits the sensitivity at lo person velocities unless the oscillator is spectrally pure. This is normally only possible if the oscillator is synthesized which would be impractically expensive.

The present invention seeks to use electromasnetic wave sensing of perSC,r, with the advantages already mentioned, but using a system configuration that avoids the disadvantages.

According to the invention, there is provided apparatus for sensing a person comprising means arranged to use such low power electromagnetic wave levels that the emissions conform to all relevant National Radio Frequency Interference (RFI) requirements. The present invention brings together ideas used in radar and the superheterodyne receiver to produce a system whose sensitivity is more nearly constant with distance to target. In order tu transmit radar like pulses without incurring spectrum authorisation problems, the power level has to be well below the relevant National RFI levels requiring narrow ,and correlation techniques to recover the low level returned signals. Although superficially the problem seem identical to that addressed by Doppler radar. the problein is quite different.The range distance is measured in fee rather than tens of miles, the peak transmitted power is in microWatts rather that. Megawatts, the pulse widths and range times are measured in nanoseconds rather than milliseconds and the Doppler frequencies are sub-Hert: rather than at hundreds of Hertz . Also because the radiated power is below all National RFI specifications the operating frequencies can be chosen to suit the range and target and not limited to an approved band.

Advantageuusly, means are arranged to radiate pulses of r. f. energy alternately different in phase on successive pulses and means for performing signal correlation of a returned signal against a local oscillator whose frequency is identical to that of the radiated signal. Because the transmit power is so low, the local oscillator leakage becomes a transmit signal in itself and can become the dominant signal at close range if some means of local oscillator cancellation is not included.

Preferrably, means are arranged wherein the returned signal, after correlation with a local oscillator operating at the same frequency as the transmitted signal, is a.c coupled, amplified and time gated by a sampling gate. The transmitted pulse contains so few cycles of carrier signal that conventional i.f. matched filtering techniques are inappropriate.

Advantageously the sampling gate is also arranged to route the alternate phases of the transmitted signals to appropriate inputs of a differencin amplifier. This is to route the recovered signals from the correlator, corresponding to the alternate in-phase and anti-phase transmitted signals, to the appropriate inputs of a differencing amplifier thereby substantially cancelling residual local oscillator leak.

Advantageously the transmit oscillator is caused to transmit not only alternate r.f. pulses of different phase but also alternate pairs of r.f. pulses at slightly different carrier frequencies in such a phase relationship that their energies cancel at close range but add after the round trip to the target . This method can be used Separately or in conjunction with the above to increase the sensitivity at a distance and to reduce it close to the antenna.

Advantageously the apparatus uses r. f. sources which are oscillators that are stopped and started b synchronised drive sources to reduce the low frequency noise between the two sources and to minimise oscillator radiation leakage. The high speed, high isolation modulation requirements for the generation of the alternately phased pulsed carrier and local oscillator are difficult ' to achieve inexpensively by conventional modulators. Also, the phase jitter requirements for the low Doppler frequencies involved are very demanding. This part of the invention offers economic solutions to both of these problems allowing such radar technique to be applied to the Person Sensor problem.

Advantaeously the timing of the in-phase and antiphase r. f. bursts is so arranged that the notches ih the received spectrum of the signal can be positioned to filter out unwanted narrowband interfering signals by slightly adjusting the centre frequency of the oscillators using suitable control circuitry. Because the sisal levels are so low, the system is susceptible to local high power transmissions such as one may get from a nearby mobile radio transmitter. This part of the invention addresses that aspect.

Some ways in which the invention may be performed are now described by way of example with reference to the accompanying drawing in which: Figure 1 schematically illustratus apparatus in accordance with. the invention Figure 2 schematically illustrates another apparatus in accordance with the invention and Figure 3 is an explanatory diagram relating to the operation of the apparatus as shown Figure 1 and Figure 2.

A Transmit Oscillator 3, for example at about 1GHz. is turned on for a period in the order of lOnsec, by the drive pulse from a delay pulse generator 1. The repetition rate of this drive pulse may suitably be a few hundred of kiloHertz. The delay with respect to the clock pulse input transition, varies by half the period of the transmitted frequency on alternate clock edges. This gives alternate r. f. pulse bursts with the carrier in opposite phases. The divide-by-two circuit 2 switches this state. The short r. f.

burst is coupled via attenuator 4 to an antenna 5 tuned to the Transmit Oscillator frequency. A low peak power pulse at (say) -20dBm, is radiated from this antenna and any energEf radiated back from targets such as the man 6 in the diagram will send a delayed signal back to the antenna.

This returned signal will be fed from the antenna via the previous path back t an r.f. amplifier 7 where it will be amplified and passed on to a mixer 10. The amplifier 7 is included to improve the noise figure and reduce local oscillator leakage. The antenna can be of any type and can be made directional if the system is required to sense perks only in some given direction.

A Local Oscillator 9 is turned on by pulse generator 8 at about the same time as the Transmit Oscillator, but is turned off much later, (say) after gOnsec. This ensures that the mixer is still being driven when the reflected energy returns. The returned signal will be correlated with the Local Oscillator in the mixer giving a narrow pulse (lOnsec for this example) at its output, the amplitude dependirla: on the size of the returned signal and its relative phase with respect to the Local Oscillator. This narrow pulse is passed via short time constant coupling capacitor 11 and amplified by video amplifier 12.

By having the two Oscillators, 3 and 9, started in this synchronised manner we virtually eliminate low frequency phase jitter between them. By turning the Oscillators, 3 s and 9, off when not required, instead of using switched modulators, it is muc-h easier to get the low mean levels of radiation required by the system.

The a.c. coupled output from amplifier 12 is fed to a pair of sampling gates 15 and 16 along wit their memory capacitors 1 7 and 18. These are driven by waveforms 'e' and 'f' in such a manner as to connect in-phase outputs to the positive input of the differencing amplifier 19 and out-of-phase outputs into the negative input of 19. Thus any Local Oscillator leak (which itself becomes equivalent to a returned CW signal for close targets) is cancelled out since it is identical for each transmitted pulse.Any returned signals oriinating from the Transmit Oscillator will be of opposite phase on alternate pulses and will thus add in the differencing amplifier.

Finally this output is a.c. coupled via a large time constant 20 to any suitable detector circuit and threshhold detector as indicated by 21. This part of the Schematic is purely illustrative and may well be implemented by means of a d.c. coupled A-D convertor with appropriate mean level adjustment provided fro time to time by additional control circuits. This arrangement gives a person sensor whose sensitivity to the presence of persons is at a maximum at some chosen distance from the antenna.It also has virtually zero sensitivity for obJects beyond the chosen range Clearly te use of another such identical channel phased in quadrature could be used as a reference path to detect whether the person was advancing or receding from the antanna as is common practice in existing Doppler systems.

By way of a further example, Figure 2 shows a more complicated arrangement that gives another level of close up rejection by using alternate transmissions of in- and out-of-phase signals at slightly different frequencies.

They are transmitted in antiphase and thus cause near cancellation at the antenna but become in phase after the chosen round trip delay to the target. The identification numbers used on Figure 2 are the same as on Figure 1 where the items are ths same Because of the increased complexity, the drive circuits have bee shown as a single block 22 and the reader is referred to the waveforms at the bottom of the figure in order to follow the appropriate timings.

The Transmit Oscillator 3 is caused to couple alternately in-phase and out-of-phase r. f. bursts via attenuator 4 to antenna 5 and the returned signals coupled via short time constant 11, amplifier 12, sampling gates 15, 16 and differenced in amplifier 19 as before. However, now, both Oscillators include a frequency tuning control input such that their frequency can be altered by a small amount. This frequency offset would be +/-8 MHz for a 15 foot target distance. Waveform 'd' controls the tuning of the oscillators so that the first pair of transmitted pulses are at high frequency i1,008 MHz) whilst the second pair are transmitted at low frequency (99 MHz).By arransins the second set of sampling gate drive waveforms " and 'h' to select gates 23 and 24 during this second cycle, the output from differencing amplifier 27 will be near identical to the output from amplifier 19. Although the two transmitted pulses of different frequency are in-phase as they leave the antenna, the returned signals from some suitably ranged target (say) 15 feet, will be anti-phase due to the time delay (30nsec for a 15 foot target range) introduced to both signals. This means that these returned signals will be differenced in amplifier 2S to give a maximum response at the target range.

The combined effects of these two levels of suppression gives the system its desirable distance vrsas sensitivity characteristics. As with the single frequency system, the addition of an identical quadrature phase channel at either or both frequencies would allow the direction of movement of the target to be deduced.

The system works at such low mean signal power levels that paralysis due to a strong local signals, such as may be produced by a nearby mobile radio transmitter, is a possibility. It would be easy to detect this condition by simply having more sampling gates and switching the receiver part so that it received at times between the transmitted pulses. This could then allow an indication to be given that the sensor was overloaded and false alarms suppressed. It would however limit its usefulness since it could not sense persons in this state.

Figure 3 indicates how this situation can be greatly improved. Figure 3(a) shows the type of pulse trains so far considered; equally spaced alternate in- and out-of-phase r.f. bursts. Figure 2(b) shows the envelope of the spectrum of such a waveform. Since the positioning of these bursts is largely irrelevant to the operation of the system, they can be re-positioned in time as shown in Figure 3(c) to be spaced just in excess of the round time delay of the returned signal. Sjch a transmission will have a spectrum envelope as shown in Figure 3(d).Since the sampling gate timing will have the same temporal relationship as the transmitted pulses, the input sensitivity to any received signals will have a frequency response of very similar shape. By altering both Oscillator frequencies with a suitable measure-and-tune control circuit, the nearest notch t the interfering signal can be shifted t align it it, thus dramatically reducing its effect on the system. Since many measurements can be made between each pair of transmit pulses, this adjustment can be completed in a fraction of a millisecond after the detection of the interferins signal.By isolating the normal output by open circuiting sampling gates 15, 16 during this adJustment, the slow overall response time of the person sensor parts would be unaffected even if the interfering signal was being turned on an off and/or changing frequency.

The above solution applies only to the single frequency system. The two frequency system can be similarly improved if te two pulse are separated by exactly twice the round trip delay when the two separate spectra will have nulls at identical frequencies but offset by one null spacing. This just happens to be the optimum frequency spacing to cause the two frequency burst to sum at the target range. Thus if both oscillators are shifted simultaneously to align one of the notches onto the interfering signal the interference will be greatly reduced as in the single frequency case.

In both cases the interfering signal must be of limited bandwidth. With the values given in the example in Figure 3. rejection ratios of 30dB can be obtained for bandwidth of ',00hHv increasing to 50ds for OkHv bdwidts.

Claims (7)

1. Apparatus for sensing a person comprising means arranged to use such low power electromagnetic wave levels that the emissions conform to all international RFI requirements.

2. Apparatus as claimed in Claim 1 including means arranged to radiate pulses of r.f. energy alternately different in phase and means for performing signal correlation of a returned signal against a local oscillator whose frequency is identical to that of the radiated signal.

3. Apparatus as claimed in Claim 2 wherein the returned signal, after correlation with a local oscillator operating at the same frequency as the transmitted signal, is a.c.

coupled, amplified and time gated by a sampling gate.

4. Apparatus as claimed in Claim 3 wherein the sampling sate is arranged t route the alternate phases of the transmitted signals t appropriate inputs of a differencing amplifier thereby substantially cancelling residual local oscillator leak.

5. Apparatus as claimed in any preceding claim wherein a transmit oscillator is caused to transmit not only alternate r.f. pulses of different phase but also alternate pairs of r. f. pulses at slightly different carrier frequencies in such a phase relationship that their energies cancel at close range but add after the round trip t the target.

6. Apparatus as claimed in any preceding claim and including r. f. sources which are oscillators that are stopped and started by synchronised drive sources to reduce low frequency noise between the two sources and to minimize oscillator radiation leakages

7. Apparatus as claimed in any of Claims 2 to 6 preceding wherein the timing of in-phase and anti-phase r. f. bursts are so arranged that the notches in the received spectrum of the signal can be positioned by slightly adjusting the centre frequency of the oscillators to filter out unwanted narrowband interfering signals using suitable control circuitry.

8 Apparatus for sensing a person substantially as illustrated in and described with reference to the accompanying drawings.