Occupancy sensors are switching devices that respond to the presence and absence of people in the sensor’s field of view. The system consists of a motion detector, an electronic control unit, and a controllable switch (relay). The motion detector senses motion and sends the appropriate signal to the control unit. The control unit then processes the input signal to either close or open the relay that controls power to the lights. The basic technology behind the occupancy sensor is derived from security systems developed for residential and commercial applications to detect intruders. However, the motion sensor has been refined so that it responds not only to the presence of occupants, but also to the absence of occupants in the space. Other enhancements of the technology have centered on reducing costs, increasing control intelligence, improving the ability to detect minor movements, and increasing adjustment capabilities.
Similar to photosensor, a “manual-on and auto-off” strategy is recommended for better energy efficiency. This strategy is also known as absence detection, requires the lights to be switched on manually by the building occupant via the manual switch. However, when the system detects that there is no movement in the room, it will automatically switch the lights off. This prevents the lights from being automatically switched on when a person only needs to retrieve a small item (pen, car key, hand phone, etc.) quickly from the room.
1 PASSIVE INFRARED (PIR)
Passive infrared (PIR) sensors react to the infrared heat energy emitted by people. PIR sensors are passive devices in that they only detect radiation; they do not emit it. They are designed to be maximally sensitive to objects that emit heat energy at a wavelength of around 10 microns (the peak wavelength of heat energy emitted by humans). PIR sensors are strictly line-of-sight devices. They cannot “see” around corners and a person will not be detected if there is an obstruction, such as a partition, between the person and the detector.
PIR sensors employ a pyroelectric transducer to detect infrared radiation. The device converts the IR energy into a voltage signal. A multi-faceted lens surrounds the transducer and focuses heat energy onto the detector. The lens views the area with a multitude of narrow and discrete beams or cones. As such, it does not view the area in a continuous fashion. As an occupant moves a hand, arm, or torso from one cone of vision to another, a positive signal is generated and sent to the controller. The detection pattern of PIR sensors is fan shaped – formed by the cones of vision seen by each segment of the faceted lens.
As shown in Figure 4.8, coverage gaps occur between the cones of vision of alternate segments of the lens. These gaps widen with distance. At 10m from the sensor, for instance, coverage gaps of up to 2m wide may be present. Since the sensor is most sensitive to motion that moves from one sensing cone to another, its sensitivity decreases with distance as the gaps between sensing cones widen. Most PIR sensors are sensitive to hand movement up to a distance of about 3.5m, arm and upper torso movement up to 6m, and full body movement up to about 13m. However, the sensitivity range of PIR sensors can vary substantially, depending on the product quality and electronic circuitry design.
0 5’ 10’ 15’ 15’ 7’ 0 7’ 15’ 20’ Wall-mounted sensor Infrared sensor range for detecting limb motion Ultrasonic sensor range for detecting full-body motion
Ultrasonic sensor range for detecting full-body motion Infrared sensor range for detecting full-body motion
82 | Building Energy Efficiency Technical Guideline For Active Design
7 Advanced Lighting Guidelines: 1993 (Second Edition), California Energy Commission.
2 ULTRASONIC SENSORS
Ultrasonic occupancy sensors activate a quartz crystal that emits ultrasonic waves throughout the space. The unit then senses the frequency of the reflected waves. If there is motion, the reflected wave’s frequency will shift slightly (Doppler effect). Ultrasonic sensors operate at frequencies that are above human sensitivity (20 kHz). Typical operating frequencies are 25, 30, and 40 kHz. Figure 4.8 shows the detection pattern of an ultrasonic sensor. The ultrasonic sound waves cover the entire area in a continuous fashion – there are no blind spots or gaps in the coverage pattern. For this reason, ultrasonic sensors are somewhat more sensitive to movement. For example, hand motion can be detected at a distance of about 8m, arm and torso motion detected at 10m and full body motion can be detected at over 13m. However, the sensitivity range of different products will vary significantly.
Comparisons between passive infrared (PIR) and ultrasonic occupancy sensors offer very similar characteristics in terms of overall performance. Ultrasonic sensors are more expensive, as a rule, but provide greater coverage than PIR detectors. However, increased sensitivity means that ultrasonic sensors are more susceptible to false triggering due to any movement in the space. For example, unless carefully calibrated, ultrasonic sensors will react to non-occupant movement such as breezes from open windows or HVAC systems. In most cases the sensitivity of the ultrasonic system, like the PIR system, is based on line-of-sight. In some circumstances, however, the movement of occupants behind partitions may be detectable by ultrasonic sensors, due to the reflectance of the emitted sound waves around the partitions.
3 OTHER SENSOR TECHNOLOGIES
In the lighting controls industry, passive infrared and ultrasonic sensors currently dominate the occupant sensor market. Sensors that use microwave and tomographic technologies are also available; however, at the present time, these are primarily limited to the security and alarm industries.
Hybrid occupant sensors, now available from many manufacturers, employ both infrared and ultrasonic capabilities in the same unit, offering improved operation with a minimal false triggering.
4 LIFE SPAN
It is difficult to adequately assess the life span of occupancy sensor systems. Life cycle testing procedures seem to suggest that a reasonable life span estimate for most occupancy sensors would range between 10 to 15 years.7
5 SUGGESTIONS
The price of motion sensors varies depending on the sensitivity requirements. For example, motion sensors for an office space requires detection of fine movement, whereas for a corridor space, the motion sensor is only required to detect large movements.
Motion sensors are particularly effective in spaces where the lights are typically left on when the space is unoccupied, such as infrequently used corridors or toilets.
It may be necessary in certain situations not to link all lights to the motion sensor, allowing a couple of lights to remain switched on at all times, so that the space will not be pitch black when the lights are switched off accidentally. In addition, it may also be unpleasant to walk into a pitch black room and have to wait a couple of seconds for the motion sensor to respond.
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In 2009, Pacific Gas and Electric Company in the United States published a research paper on a Low Ambient/ Task Lighting Pilot Project that showed a significant increase in user satisfaction and reduction in energy consumption.8 A summary of their study is provided in
Table 4.8 on the next page.
8 Heschong Mahone Group, Large Office (“Ziggurat” Building) Site Report, High Efficiency Office:
Low Ambient/Task Lighting Pilot Project, Pacific Gas and Electric Company, 2009.
4 DAYLIGHT SENSOR
Daylight sensors can detect a specific user-defined daylight level in a space and automatically switch lighting on, off or dim them. It uses photocell technology to measure the amount of available light.
When using a daylight sensor, it is very important to position the daylight sensor such that it is not confused by electrical lights. Designers should also be careful with motion sensors incorporated with both motion and daylight sensors as the ‘viewing direction’ for motion and daylight may be different.
For the Malaysian climatic zone with plenty of daylight from the hours of 8am to 6pm, the energy savings provided by dimming is rather small and is not required to be used. A simple on/off daylight sensor will provide very similar energy savings to a dimmable one. When daylight is harvested in an office space, it is recommended to practice the strategy of “auto-off and manual-on”. This means that the electrical lights are automatically switched off whenever the measured daylight is adequate, but to switch on the light when the daylight drops below the desired lux levels, the building occupants will have to go to the switch to flick it on manually. However, in public or common area spaces, it may be more convenient to program it to automatically switch the lights on as well when it drops below the desired lux value.