Fire detection with a combined ultrasonic-microwave Doppler sensor

The combination of ultrasonic and microwave sensors offers versatile sensor applications and new solutions in terms of reliability of decision and robustness against distortions. For motion detection in an intruder alarm system, the redundant sensor information from the Doppler multisensor is used to distinguish a moving person from distortions. The superior performance is based on the fact that a moving person causes coherent frequencies of the ultrasound and the microwave Doppler signal, whereas noise and distortions influence both waves differently. In this paper, a new complementary approach is described to recognise a fire as the source of a corrupted coherence between the ultrasonic and the microwave Doppler signal. As a result, a comprehensive protection system for both motion tracking and fire detection is presented which is very sensitive, fast in response, and robust against false alarms. Results from measurements with 40 kHz ultrasound and 24 GHz microwaves show the potential of the new approach.