Apparatus for and method of doppler motion detection with standing wave drift compensation
This invention was made under a contract with the Defense Nuclear Agency. 
    
    
     RELATED APPLICATION 
     Ser. No. 675,033 filed 11-26-85 &#34;APPARATUS FOR AND METHOD OF DOPPLER MOTION DETECTION WITH STANDING WAVE DRIFT COMPENSATION.&#34; 
     BACKGROUND OF THE INVENTION 
     This invention relates to intrusion detection systems and more particularly to an acoustic doppler motion detection system used to detect moving objects in a confined space. 
     Acoustic doppler motion detection systems are used in enclosed volumes such as rooms and the like and operate by transmitting or radiating a signal and monitoring the return signal for changes caused by motion of an object within the room. Many of the such rooms are reverberant and as such, under steady state conditions, produce a fixed standing wave pattern with nulls and peaks throughout the room. Because this standing wave pattern exists in the room, the level or magnitude of the signal at the receiving tansducer depends on the location of the latter relative to the pattern. If the location of contents of the protected space, such as crates in a warehouse of furniture in an office, remains unchanged indefinitely, the standing wave pattern is essentially stationary (unless there is a change in temperature) and the level of the received signal is substantially constant. However, if the contents are moved within the space or if the air temperature changes, the standing wave pattern changes so that the receiving transducer may be located at a null resulting in poor signal strength and degradation of system performance. 
     One technique that has been proposed to overcome this problem is the use of a distributed microphone such as an elongated electret cable or tape which is sensitive throughout its length. The rationale for this approach was that differences in signal strength due to the standing wave pattern would average out over the length of the microphone. In practice, however, this was not the case and the technique proved to be unsuccessful. 
     This invention is directed toward a solution of this problem. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     A general object of the invention is the provision of an acoustic doppler detection system in which standing wave drift is effectively compensated. 
     A further object is the provision of such a system in which compensation for standing wave drift is achieved electronically. 
     Another object is the provision of a novel method of automatically compensating standing wave drift in an acoustic doppler motion detection system. 
     These and other objects of the invention are achieved with apparatus for and a method of changing the operating frequency of the acoustic wave generator when the strength of the received carrier wave is below a predetermined threshold. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic and block diagram of an acoustic doppler detection apparatus embodying the invention. 
     FIG. 2 is a block diagram of an algorithm for operating the computer shown in FIG. 1. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, FIG. 1 illustrates a preferred embodiment of the invention in a confined space represented as a room R and comprises doppler detection apparatus 10 having a receiving transducer 11 and a radiating transducer 12 spaced apart in room R as shown. Receiving transducer 11 preferably is a microphone and radiating transducer 12 may be a loud speaker or tweeter which radiates an acoustic wave at, for example, 2 kHz into the entire room. This carrier wave is reflected from the room walls and objects within the room. Because the room is reverberant an acoustic standing wave pattern with peaks and nulls is set up throughout the room. Receiving transducer 11 and radiating transducer 12 are mounted on fixed supports 14 and 15, respectively. 
     The output of receiving transducer 11 is electrically connected through a bandpass filter 17 to a receiving circuit 18 consisting of a preamplifier 19, mixer 20, demodulator 21 and alarm processor 22. Doppler detector 18 compares the output of demodulator 21 with a threshold voltage and produces an alarm signal output when there is movement of an object in room R. The output of alarm processor 22 energizes an alarm device such as a bell or visual display. Receiving circuit 18 is essentially the same as that described in our copending application Ser. No. 675,033 (which is incorporated herein by reference) and does not per se constitute this invention. 
     Radiating transducer 12 is electrically connected to signal generator 24 through a power amplifier 25. The output from generator 24 is also connected to mixer 20 in the receiving circuit as shown for processing of the received signal as is well known in the art. 
     In accordance with this invention, the output of filter 17 is also connected to an analog-to-digital converter 27 which converts the received signal to digital form for transmission to a digital microcomputer 28, the output of which is reconverted to analog form by a digital-to-analog converter 29 and passed to signal generator 24. Computer 28 is programmed by software 30 to cause the received signal to be periodically sampled and to produce an analog control signal which incrementally adjusts the operating frequency of signal generator 24 when the amplitude of the received signal at transducer 11 falls below a predetermined threshold level. Software 30 is based on an algorithm shown in FIG. 2. Thus the standing wave pattern of acoustic waves propagated into the room R by the radiating transducer 12 is altered to compensate for any drift detected by this level sensing circuit. 
     In operation, transducer 12 radiates a tone at a predetermined frequency, such as 2 kHz, into room R and transducer 11 receives the signal which includes waves reflected from objects in the room. If there is no movement within the room, the output from demodulator 21 is less than the threshold level of alarm processor 22 which then does not produce an output to the alarm device. If an object, such as a person, in room R does move, such movement amplitude modulates the carrier wave which is demodulated by demodulator 21 to produce an output to alarm processor 22 above the threshold thereof and to cause activation of the alarm device.