This invention relates to improvements in "presence" detection systems of the so-called "passive infrared" variety which sense the presence of an animate object (e.g. an intruder or pedestrian) in a region under surveillance by sensing the infrared radiation (body heat) given off by such object. Moreover, it relates to improvements in pyroelectric detectors of the type used in such systems.
Whether for the purpose of sounding an alarm in response to unauthorized entry, automatically opening a door in a store or airport as a pedestrian approaches, controlling a "walk" sign at a street corner, etc., it is known to detect the presence or proximity of a human being (or other animal) in a region under surveillance by detecting the minute changes in temperature of such region occasioned by his sudden presence in the region. The "passive infrared" detection systems used to detect such changes typically comprise a pyroelectric detector adapted to provide an output signal proportional to the rate of change of infrared (IR) radiation incident thereon, an optical system for focusing IR radiation emanating from the region under surveillance onto the detector, and some sort of signal processing circuitry for distinguishing the changes in detector output produced by the object of interest from those changes produced by spurious background sources.
Pyroelectric detectors of the type used in passive IR detection systems commonly comprise one or more matched pairs of detector elements. Each element of the pair comprises two spaced parallel electrodes having a pyroelectric member, such as a polyvinylidene fluoride film, sandwiched therebetween. When subjected to a change in temperature, the pyroelectric member causes a signal to appear between its associated electrodes. A matched pair of detector elements, connected in opposition so as to produce identical signals of opposite polarity in response to a given change in temperature, affords the advantage of common mode rejection of spurious signals. The pyroelectric elements of each detector pair are usually arranged very close together so that both elements of a given detector pair "see" the same target at substantially the same time. Whereas only one pair of detector elements is used in passive infrared detection systems to provide narrow, so-called "corridor" protection, multiple pairs of detector elements arranged, for example, in a linear array are sometimes used in such systems to provide broad coverage. See, for example, the detectors disclosed in the commonly assigned U.S. Pat. No. 4,225,786, issued to D. E. Perlman. Of course, even a single detector can provide broad coverage when combined with a multifaceted optical system, such as disclosed, for example, in U.S. Pat. No. 3,703,718, issued to H. L. Berman.
In the never-ending struggle to rid false-alarms from passive infrared detection systems, some have taken a "redundancy" approach in which an alarm signal is produced only in the event two independent detection systems sense the same event or target substantially simultaneously. For example, in U.S. Pat. No. 4,614,938, issued to I. Weitman, there is disclosed a dual channel pyroelectric intrusion detection system comprising two pair (i.e. a total of four) of pyroelectric detector elements. The detector elements are interlaced so that the fields of view of one detector pair is, as nearly as possible, the same as the fields of view of the other detector pair. The respective outputs of the two detector pairs are processed independently and, only in the event both detector pairs provide an alarm output substantially simultaneously, is an alarm produced. While the redundancy inherent in this type of system affords considerable immunity from false alarms, it does so at the expense of requiring an additional detection system.
Another technique for minimizing false alarming in passive IR detection systems is the pulse-counting technique disclosed, for example, in U.S. Pat. No. 4,612,442, issued to Y. Toshimichi, and in U.S. Pat. No. 4,764,755 issued to D. F. Pedtke and G. E. Behlke. Here, the output of a single pair of detector elements is threshold detected to sense excursions above and below preset threshold levels. Such excursions occur as a moving target moves through the field of view of each detector element. For each excursion of the detector element output above threshold, a pulse is produced. These pulses are counted by a counting circuit, and an alarm signal is generated only in the event the number of pulses counted exceeds a preset number within a certain time interval. The higher the required pulse count and/or the shorter the time interval, the more immune the system is to false alarms. For high immunity from false alarms, a pulse count of three is required. To reach this count, either the target must pass through the field of view of one of the two detector elements twice, or the target temperature must be so different from ambient as to produce a "ringing" pulse which, like the normal pulses produced by the respective detector elements, has an amplitude exceeding the preset threshold.