GB2237470A - Intruder alarm system - Google Patents

Abstract

An intruder alarm system using FMCW radar. An FMCW signal (16) is generated (21) and mixed (25) with reflected signals (17) to produce an output signal (15) having a frequency dependent on range. The output signal (15) is applied to a threshold circuit (34) which triggers (T) the alarm (31, 32) on detecting the relatively sudden change in signal amplitude caused by a disturbance. The threshold circuit produces a dynamic threshold comprising upper and lower thresholds either side of a threshold determined by the running average of the output signal. Only sudden changes relative to background are thus detected. The output signal (15) is filtered (28, 29) so that the system is responsive only to disturbances occuring within predetermined range limits. The filter (28) characteristic is chosen so as to tend to suppress the effect of range on the amplitude of the reflected signal (17), thus providing a substantially constant sensitivity to disturbances over the operative range. <IMAGE>

Description

INTRUDER ALARM SYSTEM This invention relates to an intruder alarm system which uses FMCW (frequency-modulated continuous wave) radar to detect a disturbance.

Intruder alarms are known which use microwave c.w.

(continuous wave) radar to detect the presence of an intruder.

Detection is achieved by making use of the doppler effect, whereby a moving "target", the intruder, causes a change in the frequency of the reflected carrier wave, the change being dependent on the target speed. In such systems, reflections from stationary objects and from targets moving at speeds outside those of interest can be removed by passing the doppler frequency output through a bandpass filter having appropriate low and high frequency cut-offs.

However, the simple c.w. radar is not capable of determining the range of an object because the amplitude of the reflected signal which is a function of range, also varies with the size of the object. The capability of measuring the range of an object is useful An an intruder alarm as it allows the possibility of discounting targets which are found to lie outside certain range bands so that selected areas can be monitored.

The inability of a simple c.w. radar to measure range can be overcome by frequency-modulating the c.w. signal in some know periodic manner and comparing the frequency of the return signal with that of the transmitted signal. For example, if the transmitted signal is modulated with a sawtooth waveform so that its frequency increases linearly with time from a given starting frequency in eyery cycle, then, by heterodyning the return signal with a portion of the transmitted signal, a beat frequency is produced which is directly related to the time taken for the transmitted signal to travel from the transmitter to the target and back from the target to the receiver, so providing a measure of target range.

In the FMCW (frequency-modulated c.w.) radar described above, the doppler effect is not used and the carrier frequency and modulation rate are chosen so that any doppler frequency change resulting from target motion is sufficiently small compared to the beat frequency that its effect on the range measurement can be disregarded. Since target motion is no longer the criterion for detecting an intruder, it is now necessary to look for a relatively sudden change in the continual "background" return signal from objects within range to trigger the alarm. It is thus necessary to set a threshold on the receiver output so that a return signal which exceeds that threshold will trigger the alarm.Determining where to set the threshold will depend not only on the required sensitivity to a disturbance, but also on the magnitude of the background return signal, i.e. the reflections from stationary objects in the monitored zone. Thus, it can be seen that the choice of the threshold has to be determined according to, among other factors, the physical environment in which the alarm is situated. Consequently, there is a need to individually and manually set the trigger threshold of such an alarm system according to the operating circumstances. Further, if a change is made to the local environment, for example a large piece of furniture is moved, it is oft-en necessary to reset the trigger threshold to take account of the change in the background return which results.

In a conventional radar system the detection sensitivity for a given size of target is dependent on the magnitude of the return signal, which is a function of its range. In fact, from the basic radar equations it is known that the response to a target of fixed size decreases as the fourth power of the distance of the target from the radar. In an intruder -alarm application this is an undesirable form of response as it produces an ill-defined edge to the area in which a target can be reliably detected.

It is an object of the present invention to provide an intruder alarm system which alleviates the aforementioned problems.

According to the invention an intruder alarm system comprises a radar transmitter adapted to produce an FMCW signal, a receiver responsive to reflected signals, and a threshold circuit for detecting a relatively sudden change in the magnitude of the receiver output, the threshold circuit comprising averaging means for producing a time-averaged value of said magnitude, means for deriving from the time-averaged value a threshold level which is offset from the time-averaged value by an amount which is a predetermined percentage of the time-averaged value, and comparator means adapted to detect an excursion of the receiver output magnitude through the threshold level.

The means for deriving a threshold level may be adapted to derive two threshold levels which are respectively above and below the time-averaged value to form a window straddling the time-averaged value, the comparator means being adapted to detect an excursion of the receiver output magnitude outside the window.

In a preferred embodiment of the invention the receiver includes filter means adapted to select exclusively those reflected signal5 originating within a predetermined range band, so that the system is responsive only to disturbances occurring within the range band. Preferably, the filter means has a response characteristic tending to cause suppression of the effect of range on the magnitude of the receiver output.

An intruder alarm system in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a schematic block diagram of an alarm system in accordance with the invention; Figure 2 is a block diagram of a threshold circuit for use in the alarm system of Figure 1; and Figures 3(a) and 3(b) are diagrams illustrating the operation of an FMCW radar.

Referring to the drawings, Figure 1 is a schematic block diagram of an FMCW radar intruder alarm. An FMCW signal is generated in conventional manner by FM modulator 21, microwave oscillator 22 of frequency fc and modulation generator 23. The periodic output waveform of frequency fm produced by the generator 23 may be of any convenient shape, a linear sawtooth waveform being shown by way of example. The FM modulator 21 produces an FMCW signal 16 having a frequency deviation df, which is transmitted by an antenna 19 arranged so that there is no significant 'back spill' of the transmitted signal to the antenna 20 which feeds the receiver section of the alarm. The reflected signal 17 received by the antenna 20 is amplified by an RF amplifier 24 and is fed to one input of a mixer 25.The other input of the mixer 25 is supplied with an attenuated version of the transmitted signal 16 extracted by a coupler 18. The mixer output 15 comprises a signal having a beat frequency related to the range of the 'target' object. The mixer output 15 is amplified further by a pre-amplifier 26 and then passed through an AGC amplifier 27 which has a gain which is adjusted according to the average signal level, which is dependent on the range of the target.

Range filters 28 and 29 (shown dotted) may optionally be provided to select signal returns from a predetermined range band and to modify their amplitude, as will be described shortly. The output of the range filter 29 or of the AGC amplifier 27 is supplied to a detector 33, the output I of which is fed to a threshold circuit 34. The threshold circuit 34 is adapted to detect a sudden change in the 'background' return level of signal 17 which occurs when an intruder approaches.

The AGC amplifier 27 strives to maintain the level of the signals applied to the filters 28 and 29 and to the detector 33 within their preferred operating values. The threshold circuit 34 provides an output signal 1M representing a time-averaged value of the signal level I. This output signal IM provides the control input for the AGC amplifier 27. A trigger output T of the threshold circuit 34 controls an alarm driver 30, which may be arranged to drive an audible warning device 31 and/or a visual indicator 32. Thus, the threshold circuit 34 monitors those components of the background return signal 17 picked up by the receive antenna 20 and passed by the filters 28,29, and produces a trigger signal T when there is a sudden change in signal level, such as may be caused by the presence of an intruder.

Figure 2 is a block diagram of the threshold circuit 34 of Figure 1 in accordance with the invention. As explained above, the circuit 34 is adapted to detect a relatively sudden change in the magnitude of an input signal I from the output of the detector 33 (Figure 1), such a change resulting in the generation of an output trigger signal T. The circuit 34 comprises an averaging circuit 1, which is supplied with the input signal I and acts upon it to produce a continuously calculated mean value 1M of the input signal I over a predetermined time interval. In its simplest form the averaging circuit 1 may comprise a shunt capacitor having an appropriate leakage resistance. The mean value IM of the input signal I is fed to two threshold generators 2, 3.The threshold generator 2 calculates an upper threshold level IU for the input signal I, which is a fixed percentage greater than the mean value IM. Similarly, the threshold generator 3 calculates a lower threshold level 1L for the input signal I, which is a fixed percentage smaller than the mean value IM. The respective percentage differences between the upper and lower threshold levels and the mean value 1M may be the same or different according to the change in the magnitude of the input signal that it is required to detect. The calculated upper and lower threshold levels IU and 1L are respectively supplied to the inputs 7 and 8 of comparators 4 and 5. The other inputs 9 and 10 of the comparators 4 and 5 are both supplied with the input signal I.The comparator 4 is adapted to produce an output 11 when the magnitude of the input signal I at input 9 exceeds the calculated upper threshold level IU at input 7. Similarly, the comparator 5 is adapted to produce an output 12 when the magnitude of the input signal I at input 10 falls below the calculated lower threshold level IL at input 8. The two threshold levels thus define a "window" which straddles the mean value 1M of the input signal I. When the magnitude of the input signal I changes to such an extent and sufficiently rapidly that it moves outside the window, i.e. it crosses one of the threshold levels, an output signal 11 or 12 is generated.The outputs of the comparators 4, 5 are conveniently combined by means of logic circuitry, for example the OR-gate 6 shown, so that when either of the output signals 11 or 12 is present, a single trigger signal T is generated to indicate that a sudden change in the magnitude of the input signal I has been detected. What constitutes a sudden change in the magnitude of the input signal is determined by the aforementioned predetermined time interval over which the input signal I is averaged by the averaging circuit 1. In general, the time interval will be chosen so that it is substantially longer than the duration of a disturbance that it is required to detect, so that the input signal magnitude crosses one of the threshold levels before the excursion has been 'absorbed' by the averaging circuit.

It can be seen that the circuit 34 provides a convenient and cheap way of achieving a constant sensitivity to change in the magnitude of the return signal level, independent of the actual magnitude of the signal, so that, whatever the signal level, the system is triggered by a preset percentage change in that signal level. The use of two threshold levels respectively above and below the mean level of the receiver output (i.e. the output of detector 33) takes into account the fact that, by virtue of his position with respect to surrounding stationary objects, an intruder may produce an additional return signal component which either interferes constructively with the background return to produce an overall increase in the receiver output level, or Interferes destructively with the background return to produce an overall decrease in the receiver output level.By catering for both these circumstances the probability of detecting the intruder is effectively doubled compared with a system in which a mere single threshold is used. Nonetheless it will be appreciated that, in its most basic form, the threshold circuit may be modified to detect only an excursion of the input signal in one direction, while still producing a useful result. In this case only one threshold generator and one comparator are needed and the OR-gate 6 may be dispensed with.

It can be seen that the threshold circuit 34 monitors the background return signal 17 picked up by the receive antenna 20 and produces a trigger signal T when there is a sudden change in signal level, such as may be caused by the presence of an intruder, which causes the magnitude of the input signal I to change by a percentage greater than a predetermined amount. The window constituted by the upper and lower threshold levels has a size which is at all times a fixed proportion of the mean input level IM. Thus, the circuit 34 provides the alarm with a constant sensitivity to signal change independent of the actual background return level. In this way the alarm adapts to its surroundings by learning the quiescent background level for its particular environment and adjusting the trigger threshold levels accordingly.

In a preferred embodiment of the invention, the range filters 28, 29 are used to define a sensitive range band of the system, so that disturbances occurring outside the specified range band do not trigger the alarm. The function of the filters 28,29 will now be explained with reference to Figures 3(a) and 3(b).

Assume first that the generator 23 (Figure 1) produces a triangular waveform so that the output of the FM modulator 21 comprises the linearly frequency-modulated signal 13 shown in Figure 3(a), the signal 13 having a deviation iSf about the frequency fcs i.e. that of the microwave oscillator 22. Now suppose a target object at a range corresponding to the time C produces a reflected version 14 of the transmitted signal 13, delayed by a delay td. The reflected signal 14 is picked up by the receive antenna 20 and mixed by the mixer 25 with a portion of the transmitted signal 13 to produce an output 15 having the difference or 'beat' frequency fB shown in Figure 3(b).

As can be seen from Figure 3(b), the beat frequency f8 remains essentially constant over the period T of the sawtooth waveform (assuming the target does not change its range substantially during this interval), except for the two 'glitches' at A and B which occur at the turn-around points A', B' of the signals 13,14. In the case of a linearly-modulated FMCW signal, as in this example, the beat frequency output 15 of the mixer 25 is directly dependent on the delay td of the reflected signal 14 relative to the transmitted signal 13, which delay is, of course, itself linearly related to the range of the target. Thus, if it is desired to monitor exclusively targets at particular ranges, this can be achieved by appropriate filtering of the mixer output 15.Referring again to Figure 1, it can be seen that in this example the filter 29 has a low-pass response characteristic which rejects signals of frequency higher than a selected cut-off frequency, and so defines a maximum sensitive range of the system. In one embodiment of the invention (not shown), the low-pass filter 29 may be replaced by a bandpass filter, the lower and upper cut-off frequencies of the response characteristic defining the sensitive range band of the system. The lower cut-off frequency defines the minimum range at which an object will be detected and the upper cut-off frequency defines the maximum range at which an object will be detected. The bandpass filter may be realised by a low-pass filter and a high-pass filter connected in cascade and these filters may be active filters employing operational amplifiers.The range resolution, that is the smallest difference in range that can be detected, is dependent on the magnitude of the deviation A f. Thus, a sharper range cut-off at the maximum sensitive range can be achieved by increasing the deviation f. If the filter 29 has a fixed low-pass characteristic, i.e. its cut-off frequency cannot be changed, the sensitive range of the system, i.e.

the location of the range limit, can be adjusted by altering the output frequency fm of the modulating waveform generator 23. This provides a convenient way of varying the sensitive range of the system.

It should be noted that an unwanted target at a range corresponding to the time D, where the times C and D are separated by the period T of waveform 13, or an integral multiple thereof, will produce a reflected signal generating the same beat frequency fB as the reflected signal 14 from the wanted target at the range corresponding to the time C. Thus the mixer output 15 does not represent an unambiguous range measurement. However, the amplitude of the reflection from the unwanted target will be considerably smaller than that of the reflection from the wanted target to the extent that it may not even be detectable.However, as a further safeguard, the period T of the waveform 13 may be made of such duration that it is equivalent to a range far in excess of the maximum range of interest, and the cut-off frequency of the low-pass filter 29 chosen so that the delay td corresponding to the maximum sensitive range is relatively short compared to the period T. This has the further advantage of reducing the duration of the glitches which occur in the beat frequency signal at the turn-around points in the triangular waveform.

The amplitude of the reflected signal 17 from a target of a given size is a function of its range. Thus, even within a specified range band determined by the filter 29 the amplitude of the reflected signal which it is required to detect will depend on the location of the particular target within the range band. It is known from the basic radar equations that the reflected signal level from a target of a fixed size decreases as the fourth power of the distance R of the target from the radar. Thus, even a relatively small increase in the range of a target will produce a substantial fall in the amplitude of the return signal. This is an undesirable characteristic of radar operation in an intruder detection application since it produces an ill-defined edge to the area in which a given target can be reliably detected and further requires a detector which operates over a wide dynamic range.However, the effect of range on the amplitude of the return signal can be substantially overcome by including a further filter 28 in the signal path between the mixer 25 and the detector 33. The filter 28 has a high-pass response characteristic and is designed to have a roll-off of approximately 12dB/octave, which substantially compensates for the fall-off in amplitude with range, (i.e. with frequency) over the pass-band of the filter 29, whether this is the low-pass filter as shown in Figure 1 or the bandpass filter described above. The effect of the filter 28, therefore, is that the signal fed to the detector 33 is substantially constant in amplitude independent of the location of the target within the specified range band. In this way, the system sensitivity to a target is substantially independent of its location.The response characteristic of the low-pass filter 29 may be shaped to produce a sharp cut-off in which the system sensitivity falls at a rate faster than l/R4. Similarly, when the filter 29 is replaced by a bandpass filter, its response characteristic may be shaped to produce a sharp cut-offs at the band edges, so that the system sensitivity falls rapidly at the maximum range and rises rapidly at the minimum range. Thus, a sensitive range band for the system can be established having well-defined edges.

In the case of a non-linear modulation, where the beat frequency output of the mixer is not linearly proportional to range, the effect of range on the amplitude of the return signal can still be overcome in the same manner by including within the overall filter transfer function a 'linearising' function compensating for the non-linearity of the modulating waveform.

It will be appreciated that the two filters 28,29 shown in Figure 1 are described by way of example only. All the required filter functions may be incorporated in a single filter design.

This filter may be realised as an analogue filter, or, with the inclusion of suitable A/D and D/A converters, as a digital filter.

Claims (7)

1. An intruder alarm system comprising a radar transmitter adapted to produce an FMCW signal, a receiver responsive to reflected signals, and a threshold circuit for detecting a relatively sudden change in the magnitude of the receiver output, said threshold circuit comprising averaging means for producing a time-averaged value of said magnitude, means for deriving from said time-averaged value a threshold level which is offset from said time-averaged value by an amount which is a predetermined percentage of said time-averaged value, and comparator means adapted to detect an excursion of the receiver output magnitude through said threshold level.

2. An intruder alarm system according to Claim 1, wherein said means for deriving a threshold level is adapted to derive two threshold levels which are respectively above and below said time-averaged value to form a window straddling the time-averaged value, said comparator means being adapted to detect an excursion of the receiver output magnitude outside said window.

3. An intruder alarm system according to Claim 1 or Claim 2, wherein said receiver includes filter means adapted to select exclusively those of said reflected signals originating within a predetermined range band, so that the system is responsive only to disturbances occurring within said range band.

4. An intruder alarm system according to Claim 3, wherein said filter means has a response characteristic tending to cause suppression of the effect of range on the magnitude of the receiver output.

5. An intruder alarm system according to Claim 4, wherein said filter means comprises a first filter and a second filter connected in cascade, the first filter having a high-pass response characteristic, the roll-off portion of which provides said suppression of the effect of range and the second filter having a low-pass response characteristic, the cut-off frequency of which defines a limit of said range band.

6. An intruder alarm system according to Claim 4, wherein said filter means comprises a first filter and a second filter connected in cascade, the first filter having a high-pass response characteristic, the roll-off portion of which provides said suppression of the effect of range and the second filter having a bandpass response characteristic, the cut-off frequencies of which define the limits of said range band.

7. An intruder alarm system substantially as hereinbefore described with reference to the accompanying drawings.