Turning a pyroelectric infrared motion sensor into a high-accuracy presence detector by using a narrow semi-transparent chopper
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abstract
Pyroelectric infrared (PIR) sensors are the most popular presence detectors. However, their applications are limited to motion detection only, as the pyroelectric sensing element is only sensitive to radiated heat power fluctuation. This letter reports a chopped PIR (C-PIR) presence sensor, capable of detecting both the stationary and the moving occupants at high accuracy by introducing a narrow semi-transparent optical chopper to shutter the incident radiation periodically. More importantly, even though the narrow chopper can efficiently shutter the unidirectional infrared (IR) power radiated by stationary occupants, it has near-zero influence on the omnidirectional power radiated from the ambient environment. Therefore, the voltage difference generated by the C-PIR sensor between occupied and unoccupied scenarios can be directly used for identifying the presence of stationary occupants with high-accuracy. For moving occupant detection, the optical chopper can barely affect the variation of the radiated power and thus lead to the same high-accuracy detection as the on-board PIR. An approximate theoretical model is created to illustrate the physics and dynamics of IR radiation shuttered by the optical chopper. Parametric studies, experimental characterization, and empirical analysis were performed to obtain the optimal setting of the chopper in terms of its thickness and chopping frequency. Experimental results reveal that the C-PIR sensor can detect stationary occupants for up to 4.0m and moving occupants for up to 8.0m, while maintaining an accuracy of 100%. The field of view of the C-PIR was found at 110 horizontally and 90 vertically, consistent with that of the on-board PIR sensor.
