Occupancy or motion sensors are devices that sense the presence or absence of people within their monitoring range. Unlike scheduling controls, occupancy sensors do not operate on a time schedule, they merely detect whether a space is occupied or not. They may be used in restrooms, utility rooms, conference rooms, coffee rooms, locker rooms, and many other spaces. Typically, the sensor and a control unit can be enclosed in one unit, such as a wall box, but for larger facilities, the sensor is tied to a relay panel. The control unit or the relay is programmed to turn lights on when the presence of people is sensed by the motion detector, and may be programmed to turn the lights off when the space is unoccupied for a predetermined time period. The sensitivity of the sensor may also be adjustable.
Figure 6.2 The emergence of LEDs.
There are several types of motion sensors available, including passive infrared (PIR), active ultrasound, and hybrid technologies, such as combinations of PIR and active ultrasound, or PIR and audible sound. These sensors are typically used in locations such as hallways, lobbies, private offices, conference rooms, restrooms, and storage areas.
Ultrasonic sensors emit high frequency sound waves (ultrasonic) and sense the frequency of the reflected waves as they return to the device. Movement in the area where the waves are emitted changes the frequency of the reflected waves, causing the sensor to turn the lights on. Ultrasonic motion detectors provide continuous coverage of an area and are best suited for use in open areas, such as offices, classrooms, and large conference rooms. Mechanical devices that produce vibrations or changes in airflow, such as HVAC systems, can trigger ultrasonic occupancy sensors and cause lights to turn on.
PIR sensors detect radiation, that is, the heat energy that is released by bodies. They are labeled passive because they only accept infrared radiation and do not emit anything. PIRs operate in a line of sight and have to see an area, so they cannot be obstructed by open area partitions or tall furniture. PIRs use a lens to focus heat energy so that it may be detected. However, the lens views the covered area through multiple beams or cones and may create coverage gaps. Any objects that prevent the sensors from seeing portions of its designated area will cause the sensor to assume the area is unoccupied and turn the lights off, even when it isn't.
Careful placement of occupancy sensors is required to prevent false (nonintruder caused) alarms. Occupancy sensors should not be mounted in the direction of a window. Although the wavelength of infrared radiation to which the chips are sensitive does not penetrate glass very well, a strong infrared source such as from a vehicle headlight or sunlight reflecting from a vehicle window can overload the chip with enough infrared energy to fool the electronics and cause a false alarm. A person moving on the other side
Figure 6.3 Using lighting for safety and egress.
of the glass however would not be seen by the device. Devices should not be placed in such a position that an HVAC vent would blow hot or cold air onto the surface of device. Although air has very low emissivity (emits very small amounts of infrared energy), the air blowing on the plastic window cover could change the plastic's temperature enough to fool the electronics.
Other technologies and approaches to motion detection include sensing audible noise. Hybrid sensors (PIR and ultrasound, PIR and audible) offer the most effective occupancy detection and have maximum sensitivity without triggering false detections.
The placement of occupancy sensors is key to their working properly. Sensors can be mounted on walls or ceilings, and using multiple sensors can sometimes provide more accurate detection, especially in large or irregularly shaped areas. Occupancy sensors must be able to detect motion in their assigned space while ignoring mechanical vibrations and other false signals. Any malfunctioning of an occupancy sensor could be dangerous, especially if the area were a stairwell or other location where illumination is important for safety. Occupancy controls are best used in applications where occupancy does not follow a set schedule and is not predictable.