How does an occupancy sensor work?

Answer 1

The general answer is that occupancy sensors work by detecting human presence or activity. The specific answer is that occupancy sensors use one or more of three different technologies to detect or "sense" human presence and/or activity. The most common technology is passive infrared (PIR). A PIR sensor works by sending out invisible infrared beams that read or respond to temperature differences. If there is no one in the range of the sensor the sensor does not respond. When a warmer object, such as a human body, crosses two or more beams the sensor responds to the difference in temperature between the person and the background. This is the tecnology used in basic security sensors. In the case of a basic security sensor, there will be typically 4 - 10 beams which means each beam is fairly wide. This is why you can sometimes walk directly towards a security sensor and nothing happens. It is because you are moving within one beam and not crossing two beams. Only when you cross at least two beams will the sensor activate. In modern PIR occupancy sensors there can be over 1400 individual beams which makes them much more responsive to small movements such as sitting at a desk and typing on a keyboard. PIR works very well in most commercial environments and can detect at a long range, up to 40 meters, in warehouses and underground car parks. PIR has several limitations, however; it requires line of sight and will not read through any material including glass. In addition, PIR does not work well in extremely warm envorinments as it cannot distinguish between the temperature of the human body and the background. The second common technology is microphonics. As the name implies, occupancy sensors that incorporate microphonics work by picking up noise that is typical of human occupancy or activity. Advanced microphonics sensors incorporate Automatic Gain Control (AGC) and can "learn" background noises such as the steady hum of a central air conditioner or the sound of a circulating fan. They are also sensitive enough to pick up sounds typical of human activity such as typing on a keyboard, talking on the phone, a phone ringing, tapping on a desk, a chair squeaking, walking on a wood floor and numerous other sounds that are typically generated by human presence or activity. Microphonics type sensors do not require line of sight and can hear around corners. This type of sensor works very well in large open plan office and commercial areas as they can cover a large area without having to "see" all of it. The limitation of microphonics sensors is that they do not work in very noisy envorinments such as a factory or where a recurring noise not necessarily related to human activity such as a TV that stays on in the background or an individual, window type air conditioner that comes on and off whether there is a person in the room or not. The third common tecnnology is high frequency (HF) ultrasound or ultrasonics sensors. HF works exactly the same as a bat's "radar" or a submarine's "sonar." The sensor sends out a high frequency sound wave that bounces around the space and returns to the sensor. Any movement that disturbs the wave activates the sensor. HF sensors can be very sensitive and can read through many materials including the most commonly used interior construction materials and glass. From a human occupancy standpoint, HF's big limitation is that it responds to any motion, not just human presence. For example, an open window and rustling curtains or a moving window shade or, in an office environment a piece of paper coming out of a printer, can activate an HF sensor even if no one is there. To enhance the strengths and minimize the limitations of each of the three technologies, sensor manufacturers sometimes combine technoligies in "dual technology" sensors that typically include a combinaton of PIR and either microphonics or HF. The most imortant issue for a successful occupancy sensor installation is to understand the strengths and limitations of each type of sensor and to choose the appropriate sensor for th application. Occupancy sensors can save significant amounts of energy and take the human factor out of remembering to turn off the lights, the air conditioner and any other equipment that only needs to be on when there is human presence.