United States Patent - 6,188,318, Katz, et al. February 13, 2001  

Abstract: A dual-sensing intrusion detection device for detecting an intruder comprising a PIR sensor and a microwave sensor. The device comprises PIR processing means and microwave processing means, means for summing the processed PIR signal with the processed microwave signal to generate a summed signal, and means for comparing the summed signal to a sum threshold value to determine if an alarm condition exists. The sum threshold value is selected for optimal discrimination between a human intruder and an animal presence. An additional feature of the intrusion detection device is the generation of a PIR difference signal. This feature allows the device to compensate for the limited bandwidth of the PIR sensor and to be more sensitive to a human intruder. The processing of the sensor signals includes an integration technique that sums the amplitude values of the signal and causes the sum to decay at a slow rate. In addition, the slow decay rate causes the signal to be spread out in time, thereby allowing the signals from both detectors to be above the threshold at the same time.

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