Light Guide:
Occupant Sensors

Occupant 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 occupant sensor 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 ability to detect minor motion, and increasing adjustment capabilities.

System Components While the entire occupant sensing and switching system is commonly referred to as an occupant sensor, the sensor itself is a system that is made up of several components, including a motion detector, electronic control unit, relay and power supply. The motion detector uses either ultrasonic sound waves or infrared radiation technologies for sensing motion. The electronic control unit collects the information supplied from the sensor and determines the occupancy status of the space. In some cases, the control unit can be calibrated to adjust the sensitivity of the sensors to motion. The controller also incorporates a programmable timing device that will turn off the lights after the room is unoccupied for a specific period of time. Output from the control unit energizes or de energizes the electromagnets of a relay. The relay opens or closes the lamps' circuit. Relay contacts must be properly sized to handle the line voltage and current. A power supply is also an element of the system and is needed to transform the 120 or 277 AC line voltage for powering the control unit's circuit and for sending output to the relay. The relationship between the power supply, relay, controller and motion detector are shown in Figure 10-1.

In most occupant sensor systems, the motion detector and controller are housed in one package; the power supply and relay comprise another integral unit, sometimes called a control unit or switchpack. In wallbox-type sensors, components are all integrated into one compact package, designed to fit into an existing switch box. The solid state switches often used in these packages are rated for relatively small loads.

Occupant sensors can also be connected to low-voltage relay and building automation systems. If the sensor is connected to a low-voltage relay system, the low-voltage operating power is derived from the relay panel's low-voltage supply.

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 the 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 many 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 10-2, coverage gaps occur between the cones of vision of alternate segments of the lens. These gaps widen with distance. At 40 feet from the sensor, for instance, coverage gaps of up to 8 feet 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 10 feet, arm and upper torso movement up to 20 feet, and full body movement up to about 40 feet. The sensitivity range of PIR sensors can vary substantially, however, depending on product quality and electronic circuiting design.

Ultrasonic Sensors Ultrasonic occupant sensors activate a quartz crystal that emits ultrasonic waves throughout 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 10-3 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 25 feet, arm and body torso detected out to 30 feet and full body motion can be detected out to over 40 feet. (The sensitivity range of different products will vary significantly.)

Detector Comparisons Passive infrared and ultrasonic occupant 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 line of site. In some circumstances, however, the movement of occupants behind partitions may be detectable by ultrasonic sensors, due to reflectance of the emitted sound waves around the partitions.

Other Sensor Technologies In the lighting controls industry, passive infrared and ultrasonic sensors currently dominate the occupant sensor market. Sensors that use microwave technology are also available; however, at the present time, these are primarily limited to the security and alarm industries. Other sensors may be based on sensing motion indirectly. One type responds to audible noise; others are tripped by either a foot pad, lever, or light beam. Indirect motion sensors can only be used in special applications, and so far, they have been of limited utility.

Hybrid occupant sensors, now available from at least one manufacturer, employ both infrared and ultrasonic capabilities in the same unit, offering improved operation with a minimum of false triggering.

Life Span It is difficult to adequately assess the life span of occupant sensor systems. Only one manufacturer has been making occupant sensors for 15 years, so any manufacturer's claim of life spans in excess of 15 years have yet to be proven empirically. Life cycle testing procedures seem to suggest that a reasonable life span estimate for most occupant sensors would range between 12 to 15 years. Control units, on the other hand, are estimated to have a life expectancy of between 6 and 10 years. Generally, control unit failures are caused by a deterioration of the transformer or relay within them. Deterioration may be exacerbated by high humidity environments and/or temperature extremes.

Adapted from the Advanced Lighting Guidelines: 1993 (Second Edition), originally published by the California Energy Commission.

 

Adapted from the Advanced Lighting Guidelines: 1993 (Second Edition), originally published by the California Energy Commission.

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