GB1600430A - Ultrasonic type motion detector - Google Patents

Description

(54) ULTRASONIC TYPE MOTION DETECTOR (71) We, MATSUSHITA ELECTRIC WORKS LTD., a corporation organised under the laws of Japan of 1048 Oaza Kadoma, Kadoma-shi, Osaka, Japan, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to ultrasonic type motion detectors and, more particularly, to improvements in ultrasonic type motion detectors utilizing Doppler effect.

Ultrasonic type watching devices of the kind referred have been well known, and the one which is generally appreciated to be on the highest technical level today is disclosed in the United States Patent No.

3,665,443 granted to Galvin in May, 1972.

The watching device disclosed in this patent is devised to generate no mis-warning due to any turbulence of air caused by movements of curtain, variations in ambient temperature, influences of air-conditioner and the like, but is defective in that the detection is not accurate enough against such very large air turbulence as is caused by a bell or any air turbulences different from the one caused by the movements of curtain or the flow of air, that many complicated circuits are required, and that required cost is high. The present invention has been suggested to remove such defects.

A primary object of the described embodiment of the present invention is to provide an ultrasonic type motion detector which is high in the detecting sensitivity to human movements and is low in the probability of mis-operations by other physical phenomena than the human movements.

Another object is to provide an ultrasonic type motion detector which is simple in the structure, low in the cost and easy to manufacture.

The present invention therefore provides an ultrasonic type motion detector, comprising, a signal generator circuit which is driveable by an oscillator so as to generate first and second output signals of the same frequency but different phase, an ultrasonic transmitter driven by the first output signal of the signal generator circuit, an ultrasonic receiver positioned to receive ultrasonic waves emitted by the transmitter which have been Doppler shifted in frequency by reflection from a moving object, and moving object detecting means comprising, first and second quadrature detecting circuits connected to the ultrasonic receiver and to the signal generator circuit, each of which detecting circuits passes the received signal therethrough when the respective one of the first and second output signals of the signal generator circuit is beyond a preset level so as to give respective first and second beat signals, first and second phase difference detecting circuits each having an input connected to the output of one of the quadrature detecting circuits and means for generating a pulse signal in phase with a beat signal applied to the input, the pulse signal having a positive pulse at each rising edge of the beat signal, and an output for the pulse signal, the two phase difference detecting circuits being interconnected so that when one pulse signal is in phase advance of the other pulse signal the positive pulses of the said other pulse signal appear at its output, inverting means connected to invert one of the outputs of the phase difference detecting circuits, an in-phase component removing circuit connected to the inverted output and the non-inverted output so as to subtractively combine the pulse signals and cancel any components which are in-phase, a converting circuit connected to receive the combined pulse signal and convert it to an analogue voltage dependent on the frequency and polarity of the pulses in the signal, and two voltage comparator circuits connected to the converting circuit, each having a different threshold voltage defining upper and lower limits respectively for the analogue voltage and generating an alarm if the analogue voltage moves outside these limits.

An embodiment of the present invention will now be described by way of example and which reference to the accompanying drawings, in which: Figs. 1A and 1B show when combined a block diagram of an embodiment of the detecting device according to the present invention; Figs. 2A and 2B show when combined a circuit of a practical embodiment of the detecting device according to the present invention:: Figs. 3A and 3B are respectively an explanatory view showing an ultrasonic wave transmitting and receiving state and a frequency spectrum diagram of received wave signals in the case when no moving object is present; Figs. 4A and 4B are respectively an explanatory view showing an ultrasonic wave transmitting and receiving state and a frequency spectrum diagram of received wave signals in the case when a moving object is present; Figs.SA and 5B are respectively a wave form diagram showing a combined state of ultrasonic waves received through different paths and their frequency spectrum diagram; Fig. 6 is an actual frequency spectrum diagram in the case when a moving object is present; Figs. 7A and 7B are respectively an explanatory view showing an ultrasonic wave transmitting and receiving state and a wave form diagram of a received wave signal in the case when sounds of a bell or the like are generated; Figs. 8A and 8B are respectively frequency spectrum diagrams of random spectra by bell sounds or the like at respectively different times; Figs. 9A through 9F are wave form diagrams of transmitted and received wave signals and quadrature detecting outputs in the present invention;; Figs. I OA and lOB show signal wave forms at respective parts of the device of Fig. 1; Fig. 11 shows signal wave forms at certain parts of the device of Fig. 1 in the case when an approaching object is present; Fig. 12 shows signal wave forms also at certain parts in the case when a moving object approaching and retreating from the detector is present; Fig. 13 shows signal wave forms at certain parts of the detector of the present invention in the case when wave components are present on both side wave bands substantially equally in the received wave signals; Fig. 14 shows wave forms for explaining control operation of limit detecting voltages of upper and lower limit discriminating circuits in the present invention; Fig. 15 shows a conventional quadrature detecting circuit in a block diagram;; Fig. 16 is a block diagram of a conventional ultrasonic type motion detector; and Figs. 17A and 17B are explanatory wave form diagrams in a case of excessively large signals and detected quadrature outputs substantially of the same phase.

Referring to an embodiment of the present invention shown in the block diagram of Figs. IA and 1 B, 1 is an oscillating circuit of high frequency, for example, 100 KHz. The output of this oscillating circuit 1 is connected to a 1/2 frequency divider 2. The output of this 1/2 frequency divider 2 is connected to a 900 phase difference generating and frequency dividing circuit 3 comprising flip-flops It4~1 and It4~2 (see Fig 2A). The signal from the 1/2 frequency divider 2 is further 1/2 frequency-divided by this frequency dividing circuit 3 so as to be a signal of about 25 KHz.

The output from the frequency dividing circuit 3 is amplified by an amplifying circuit 4 and transmitted outward as an ultrasonic wave by a transmitting element 5.

Any reflected back ultrasonic wave is received by a receiving element 6 and is amplified by an amplifying circuit 7. The output from the amplifying circuit 7 is connected to respective first and second quadrature detecting circuits 8 and 9. In the respective detecting circuits 8 and 9, outputs corresponding to the received wave signals will be obtained, only when there are Q output signals of the flip-flops It4~1 and Ic4~2, connected respectively through diodes Dl and D2 (see Fig. 2A).

The outputs of the respective first and second quadrature detecting circuits 8 and 9 are connected to first and second phase difference detecting circuits 14 and 15 respectively, through low-pass filters 10 and 11 for converting them to average level signals and further through amplifying and shaping circuits 12 and 13 which shape the wave forms for the inputs of the phase difference detecting circuits 14 and 15. In said phase difference detecting circuits 14.

and 15, a differential signal will be obtained at either of their outputs when the other input is at a "high" level depending on the outputs of the amplifying and shaping circuits 12 and 13.

The output from the first phase difference detecting circuit 14 is connected to an in-phase component removing circuit 19 and the output from the second phase difference detecting circuit 15 is also connected to this circuit 19 through an inverting circuit 16. The in-phase component removing circuit 19 couples the first and second phase difference detecting circuits 14 and 15 to a later described converting circuit 20 through diodes Dz1 and Dz2 (see Fig. 2B), and the output of the converting circuit 20 varies in response to the states of the outputs of the first and second phase difference detecting circuits 14 and 15.

The output from the in-phase component removing circuit 19 is connected to the converting circuit 20, which is a circuit for integrating an input pulse signal to give an analogue signal. Further, the output from the first phase difference detecting circuit 14 is connected to a first detection integrating circuit 17. The output from this circuit 17 is connected to a lower limit discriminating circuit 22. In the integrating circuit 17, the output of the first phase difference detecting circuit 14 is inverted and applied to a voltage setting capacitor C1 connected to a comparator Ic1 1 of the lower limit discriminating circuit 22 (see Fig.

2B). The lower side detecting limit voltage of the comparator It1~4 is thus made controllable so as to become lower by means of the voltage on the output side of the upper side wave band. In the lower limit discriminating circuit 22, the output of the converting circuit 20 is compared with the lower side detecting limit voltage controlled by the first detection integrating circuit 17 to provide an output.

The output from the second phase difference detecting circuit 15 is connected to the second detection integrating circuit 18 and the output from this circuit 18 is connected to the upper limit discriminating circuit 21. In the second detection integrating circuit 18, under the same principle as of the operation in the above described first detection integrating circuit 17, the upper side detecting limit voltage of the comparator It1~3 is made controllable so as to be higher with the voltage on the output side of the upper side wave band. In the upper limit discriminating circuit 21, the output of the converting circuit 20 is compared with the upper side detecting limit voltage controlled by the detection integrating circuit 18 to provide an output.

The output of the upper limit discriminating circuit 21 and lower limit discriminating circuit 22 are connected to an output circuit 23.

Referring briefly to a practical circuit arrangement based on the above described block diagram as shown in Figs. 2A and 2B, the same reference numerals are given respectively to the parts which performs the same operation as the respective blocks shown in Fig. 1, and details of this embodiment shall be omitted here.

The operating principle of the present invention shall now be explained in the following.

When there is no change within a watching zone in which ultrasonic waves are radiated from the transmitting element 5 as shown in Fig. 3A, the frequency fo of received ultrasonic waves will coincide with the frequency f of the radiated ultrasonic waves and a signal of a single frequency f=fO having no side wave band as shown in Fig.

3B will be received by the receiving element 6. When such a moving object M as a human object is present within the watching zone as shown in Fig. 4A, representing a Doppler frequency by Af, a sound velocity of the ultrasonic waves By c, a velocity of the moving object M by v and a radiated ultrasonic wave frequency byf, the relation of 2vf Af c will hold. The value will be either positive or negative depending on the moving direction of the moving object M and will be included as a side wave band in the received wave signal. As shown in Fig. 4B, when the moving object M approaches, the upper side wave band component will be produced but, when the moving object moves away, the lower side wave band component will be produced.Thus, a received wave signal will be obtained as a single side wave band (SSB), as it is called in communication systems. However, as the ultrasonic waves radiated in the space are received generally by the receiving element 6 as a combination of sound waves reflected by various parts within the radiating zone, in the case when an object having a movement not required to be detected is present within the watching zone or the air moves the ultrasonic wave signal from each of the above referred parts will vary in phase and amplitude and the combined signal will vary as shown in Fig. 5A, having double side wave band waves (DSB waves) distributed on both upper and lower sides as shown in Fig. 5B.Therefore, when a moving object M is present within the watching zone, the received wave signal will have such resultant spectrum as shown in Fig. 6 obtained by combining the waves of Figs.

3B, 4B and 5B.

Thus, in order to perform the detection of the moving object M, it will be evident from the above explanation that the detecting operation should preferably be made when the distribution of the two upper and lower side wave bands is non-uniform.

Therefore, in order to extract the respective upper and lower side wave bands, there are provided the first and second quadrature detecting circuits 8 and 9, low-pass filters 10 and 11, amplifying and shaping circuits 12 and 13 and first and second phase difference detecting circuits 14 and 15 in the block diagram shown in Fig. 1. However, while the first and second quadrature detecting circuits 8 and 9 make it possible to extract a signal in the case of an input of a single side wave band, the amplifying and shaping circuits 12 and 13 will be in-phase in the case of DSB waves in which both side wave bands are present at the input. This in-phase component is unnecessary for detecting the moving object M.Further, such sounds which are as strong in generated sound waves as the sounds of the bell B will disturb the ambient air which is acting as a medium of the ultrasonic waves, as shown in Fig. 7A, and will produce strong DSB signals in the received signals, as shown in Fig 7B, and it is necessary to cut out such DSB signals. In such a case, an opposite phase separation of the received wave signals can be performed since the phase difference detecting circuits 14 and 15 do not produce any output when one of the amplifying and shaping circuits 12 and 13 is of "high" level output, but the removal of the in-phase component will not be able to be made.The removal of this inphase component is made possible by causing in-phase signals entering both ends of the in-phase component removing circuit 19 to cancel each other so that the resultant input signals to the converting circuit 20 will be invariable. This is because, as will be described later, even if both outputs of the phase difference detecting circuits 14 and 15 are generated simultaneously, the noninverted output and inverted output will be troltage-divided by the resistances so that their signals will be cancelled with each other and the reference voltage of the converting circuit 20 will keep 1/2 Vcc.

On the other hand, the spectrum of the sounds of the bell will be distributed not only to the audible frequency wave band but also to the ultrasonic wave band, and this distributed state will not be always constant so that, at one moment, it will be such spectrum-distribution as shown in Fig. 8A but, at another moment, it will be as in Fig.

8B. Thus, a very random spectrum distribution varying momentarily is provided. The spectrum for the transmitted wave signals is a side wave band distributed at random above and below. If this random distribution is well balanced above and below, there will be no problem but, if this spectrum distribution is unbalanced on the average, there is performed a detecting operation in the detector circuit.An obervation has been made that, when the movement in one direction of the human object continues for a fixed time, the received wave signal due to the movement of the human object will be a signal of the upper or lower side wave band continuing for a fixed time but the ultrasonic wave component included in the sounds of the bell or the like will produce side wave bands at random above and below, and any occurrence of a mis-operation by the bell sounds or the like has been successfully prevented by having an operation performed so that, in the above described case, each detecting limit level is adjusted in response to the level of the side wave band on the other side.

Operations of the respective parts in the detector of the present invention shall be referred to in the followings.

In Figs. 1 and 2, a square wave signal of, for example, about 100 KHz is generated by the oscillating circuit 1 and the generated signal is frequency-divided to be of 26300 Hz by the frequency-dividing circuits 2 and 3 and amplified by the amplifying circuit 4.

The transmitting element 5 is vibrated by the amplified signal to transmit ultrasonic waves into the air and the ultrasonic waves reflected back are received by the receiving element 6 and converted to an electric signal which will be amplified by the amplifying circuit 7. In this case, only a frequency band tuned with an LC resonating circuit connected to the collector of a transistor Tr2 will be selected and amplified. An automatic gain controlling circuit 7' is provided in the amplifying circuit 7.

Now, Figs. 3A through 9F show respective wave forms of outputs of flipflops of the 90" phase difference generating and frequency dividing circuit 3 and quadrature outputs of transmitted and received wave signals. Fig. 9A shows a transmitted wave signal, and Fig. 9D shows a received wave signal. The operation of the quadrature detecting circuits shall be explained with reference to the wave forms of Figs. 9A to 9F. In the first quadrature detecting circuit 8, the diode is switched by the "Q" output f' of It4~2 of the frequency dividing circuit 3 as shown in Fig. 9B, and, in the second quadrature detecting circuit 9, the diode is switched by the "Q" output f of It4~1 of the frequency dividing circuit 3 as in Fig. 9C and such outputs respectively as in Figs. 9E and 9F are obtained. That is, only while the "Q" outputs of IC4 r and It4~2 are on the "high" level will the received wave signal be taken out respectively at the outputs of the quadrature detecting circuits 8 and 9. Therefore, for the outputs of the quadrature detecting circuits 8 and 9, there are obtained the beat signals shown in Figs.

9E and 9F.

Fig. 10 shows wave forms of the respective parts in the detector of the present invention in the case when the object to be detected moves away. When the outputs of the quadrature detecting circuits 8 and 9 are put through the low-pass filters 10 and 11, the outputs Vp and V, will be as shown in (f) and (g) in Fig. 10. When these outputs are connected respectively to the non-inverting amplifiers It1~1 and It1~2 of the amplifying and shaping circuits 12 and 13, such outputs as in (h) and (i) in Fig.

10 will be obtained. These outputs are connected respectively to the phase difference detecting circuits 14 and 15.

the diodes Dx and Dy were not present in said phase difference detecting circuit, the input signals would be differentiated by the capacitors Cx and Cy and signals of such wave forms as in (j) and (k) of Fig. 10 would be obtained. In practice, however, the diodes Dx and Dy are present so that, as in (1) and (m) in Fig. 10, pulses will be generated only in the voltage Vu. This voltage is impressed on the in-phase component removing circuit 19 and converting circuit 20 but, as the voltage Vt is in-phase with the voltage Vv and the voltage Vw is inverted relative to the voltage Vu, the voltages Vv and Vw will be as in (n) and (o) of Fig. 10.The voltage Vo initially corresponds to a voltage 1/2 Vcc given by the resistances R1 and R2, but the diode Dz is of a polarity such that it will be able to give no influence on the voltage Vo when the voltage Vw is on the "high" level but, when Vw is on the "low" level, it will act to reduce the voltage Vo. The voltage Vo will be smoothed by the capacitor Cc so as to be of such wave form as in (p) in Fig. 10. Now, in the upper and lower limit discriminating circuits 21 and 22, the reference voltages Vz1 and Vz2 of the comparators It1~3 and It1~4 are assumed to be Vz2 < l/2Vcc < Vz1.

When Vo is smaller than Vz2, It1~4 will be turned on, Ic1 3 will remain as it is, the transistor Tr3 will be ON, and the relay Ry will be operated. The wave forms of It1~4, lc13, Tr3, Ry and LED are shown respectively in (r), (s), (t), (u) and (v) in Fig.

10.

While the above described case is one in which the received wave signal contains the lower side wave band component, that is, the moving object is moving in the direction of leaving the detector, wave forms in a case where the upper side wave band component is present in the received wave signal, that is. the moving object is approaching, will be respectively as shown in (a) to (g) in Fig. 11, and It1~3 will be ON. The explanation of the operation of this shall be omitted.

Next, in the case when the upper and lower side wave band components are alternately present in the received wave signal, that is, when the moving object is moving forward and rearward, as shown in (c) of Fig. 12, the voltage Vo will swing above and below within the range of Vz1 and Vz2 substantially on the basis of 1/2 Vcc and, therefore, It1~3 and It1~4 will not be ON.

Further, in the case when both side wave band components are present in the received wave signal substantially uniformly and only their levels fluctuate, that is, when such AM-like component as an air turbulence is present, Vv and Vw will be cancelled with each other and the voltage Vo will be 1/2 Vcc as shown in (e) in Fig. 13.

Therefore, if the outputs are separately provided by It1~3 and It1~4 as required, it is possible to discriminate whether the moving object moves away or approaches.

Now, in the case when such Doppler frequency wave component as of bell sounds is irregularly generated as shown in (a) and (b) in Fig. 14, the voltages Vt, Vu, Vv, Vt" Ww, Vu" and Vo will be as shown in (a) to (g) in Fig. 14 and the outputs Vt and Vu of the phase difference detecting circuits 14 and 15 will be random and irregular as shown in (a) and (b) in Fig. 14 and, therefore, will not be cancelled with each other even by the in-phase component removing circuit 19. As shown in (g) in Fig.

14, the voltage Vo would exceed the detecting limit voltages Vz1 and Vz2 of the upper and lower limit discriminating circuit 21 and 22 and the comparators It1~3 and Ic,, would inverse at random and the relay Ry would be repeatedly made ON and OFF.

Therefore, one of the outputs Vt and Vu of the phase difference detecting circuits 14 and 15 is inversed to be Vt" but, the other is not inversed and is made Vu", the first detection integrating circuit 17, in which the lower side detecting limit voltage Vz2 of the lower limit discriminating comparator is controlled to be lower by the voltage Vt" from the output side of the upper side wave band, is connected to the comparator Ic1a, and the second detection integrating corcuit 18, in which the upper side detecting limit voltage Vz1 of the upper limit discriminating comparator is controlled to be higher by the voltage Vu" from the output side of the lower side wave band, is connected to the comparator It1~3 so as to vary the upper and lower detecting limit voltages Vz1 and Vz2 and to make the voltages Vz'1 and Vz'2 as in (h) in Fig. 14 to eliminate the mis-operation.

The values of the capacitors C1 and C2 in the first and second detection integrating circuits 17 and 18 are so selected that, in the case when the signal is present only in the upper side wave band or lower side wave band, the capacitor will be saturated and the voltages Vz'1 and Vz'2 will not vary more than is fixed to prevent any unfavorable influences.

As described above, not only when the Doppler component is present only in the one side side wave band but also when the Doppler component is present in an irregular form in each of both side wave bands as in the case of bell sounds, no misoperation will be caused.

Further, as shown in Fig. 2, in the first and second quadrature detecting circuits 8 and 9, the "Q" outputs of the cascade connected D type flip-flop circuits It4~1 and It4~2 and the inputs of the respective quadrature detecting circuits are connected respectively with each other through the diodes D, and D2 so as to obtain quadrature detecting circuits high in reliability without using such a conventional complicated and costly circuit as is shown in Fig. 15. Further, the D type flip-flop circuit is inexpensive as it also forms a frequency dividing circuit.

In the phase difference detecting circuits 14 and 15, as shown in Fig. 2, the outputs of the wave form shaping circuits 12 and 13 are connected respectively to the differentiating circuit 14' comprising the capacitor Cx and resistance Rx and the differentiating circuit 15' comprising the capacitor Cy and resistance Ry and are connected to the differentiating circuits of the others through the diodes Dx and Dy, therefore, differentiated detection signals will be present respectively at the output end of the second phase difference detecting circuit 15 when the input signal to the first phase difference detecting circuit 14 is advanced in the phase and at the output end of the first phase difference detecting circuit 14 when the input signal to the second phase difference detecting circuit 15 is advanced in the phase and phase difference detecting circuits are simplified.

In the in-phase component removing circuit 19 and converting circuit 20, as shown in Fig. 2, the resistances r0 are connected in series to the diodes Dz1 and Dz2 arranged to be of the same polarity between the two output terminals, a first integrating circuit consisting of the capacitor Ca and a second integrating circuit consisting of the capacitor Cb and resistance R3 are connected to the intermediate point of both resistances rO, whereby the removal of the in-phase component is made possible, an integrated analogue output corresponding to the input states of the pulse signals is obtained, and the input states of the two pulse signals are discriminated only by judging whether the analogue output is higher than, the same as or lower than the reference voltage or varies to be above or below it, so that the formations can be made simple. Further, as the limit detecting voltage of the upper and lower limit discriminating circuit is made to be controlled by the detection integrating circuit, even in the case when the Doppler frequency wave component is generated irregularly as in Fig. 8A, mis-operation will be able to be prevented from occurring.

In such a conventional detecting device as is shown in Fig. 16, the phase difference detecting circuit is formed so that, by sampling whether one of the quadrature detecting outputs is plus or minus with respect to the other quadrature detecting output as the standard, said one of the outputs will be converted. With such a wave signal including the Doppler frequency wave component as in Figs. 8A and 8B of, for example, bell sounds, the in-phase component cannot be removed and the compensation when the upper and lower side wave band components are generated at random cannot be made.

Further, there has been a defect that, in the case when both the quadrature detecting outputs are close to be in-phase, if the quadrature detecting output to be sampled is close to a sine curve as shown in Fig. 17A there will be no problem but, if there is an excess signal due to the bell sounds or the like as shown in Fig. 17B, the quadrature detecting output will be saturated to be close to a square wave and there will be the same signal output as in the case that there is a moving object.

Further, there has been a defect that, in the conventional phase difference detecting circuit, as one of the quadrature detecting output is given from one terminal by sampling the output with respect to the other quadrature detecting output as the standard, the voltage will be made zero in the case when there is no target object in the output voltage of this phase difference detecting circuit and, with this zero voltage as the standard, two current sources will be required to obtain a plus or minus voltage.

In the present device, the outputs of the phase difference detecting circuits are divided between the case when the target object approaches and the case when it moves away, the respective output signals being zero or plus signals, these two sets of outputs are distinguished by the in-phase component removing circuit and the target object is discriminated, with the 1/2 Vcc voltage as the standard when no object is present in the converting circuit, as plus to 1/2 Vcc in the case when the object approaches and as minus from 1/2 Vcc in the case when it moves away, so that it will be sufficient to use only one current source and the circuit formation can be simplified.

Claims (6)

WHAT WE CLAIM IS:

1. An ultrasonic type motion detector comprising, a signal generator circuit which is driveable by an oscillator so as to generate first and second output signals of the same frequency but different phase, an ultrasonic transmitted driven by the first output signal of the signal generator circuit, an ultrasonic receiver positioned to receive ultrasonic waves emitted by the transmitter which have been Doppler shifted in frequency by reflecting from a moving object, and moving object detecting means comprising, first and second quadrature detecting circuits connected to the ultrasonic receiver and to the signal generator circuit, each of which detecting circuits passes the received signal therethrough when the respective one of the first and second output signals of the signal generator circuit is beyond a preset level so as to give respective first and second beat signals, first and second phase difference detecting circuits each having an input connected to the output of one of the quadrature detecting circuits and means for generating a pulse signal in phase with a beat signal applied to the input, the pulse signal having a positive pulse at each rising edge of the beat signal, and an output for the pulse signal, the two phase difference detecting circuits being interconnected so that when one pulse signal is in phase advance of the other pulse signal the positive pulses of the said other pulse signal appear at its output, inverting means connected to invert one of the outputs of the phase difference detecting circuits, and in-phase component removing circuit connected to the inverted output and the non-inverted output so as to subtractively combine the pulse signals and cancel any components which are in-phase, a converting circuit connected to receive the combined pulse signal and convert it to an analogue voltage dependent on the frequency and polarity of the pulses in the signal, and two voltage comparator circuits connected to the converting circuit, each having a different threshold voltage defining upper and lower limits respectively for the analogue voltage and generating an alarm if the analogue voltage moves outside these limits.

2. An ultrasonic type motion detector according to claim 1, wherein each output of the phase difference detecting circuits is connected to a respective integrating circuit which converts the pulse signal at the said output to a voltage dependent on the frequency and polarity of the pulses in the signal and applies the voltage produced to a respective one of the voltage comparator circuits so as to thereby alter its threshold voltage by a corresponding amount, the lower limit being reduced by positive pulses at the output of the phase difference detecting circuits which is not connected to the said inverting means and the upper limit being increased by positive pulses at the other output thereof.

3. An ultrasonic type motion detector according to claim 1 or 2, wherein the first and second output signals of the signal generator circuit have a 90" phase difference between them and the quadrature detecting circuits each have a diode which is switched by a respective one of the said output signals.

4. An ultrasonic type motion detector according to any preceding claim, wherein first and second shaping circuits are respectively connected between the quadrature detecting circuits and the phase difference detecting circuits to convert the output of each quadrature detecting circuit to a square wave, each phase difference detecting circuit having at its input a capacitor connected to one end of a resistor which is earthed at the other end so as to differentiate the square wave received from the corresponding shaping circuit and having its output connected through a diode to the input of the other phase difference detecting circuit so as to inhibit the output when the square wave at the said input of the other phase difference detecting circuit is at a low level.

5. An ultrasonic type motion detector according to any preceding claim, wherein the in-phase component removing circuit comprises, a first diode, first and second resistors, and a second diode connected in series between the inverted and noninverted outputs of the phase difference detecting circuits, the diodes being connected in opposite senses with respect to the resistors, and the converting circuit includes a capacitor and resistor connected in parallel to the junction of the first and second resistors of the in-phase component removing circuit.

6. An ultrasonic type motion detector according to claim 1, substantially as herein described with reference to and as illustrated by the accompanying drawings.