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They Turn Off the Lights

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Figure 1. Infrared occupancy sensors contain at least one pyroelectric detector located behind an infrared transmitting lens with etchings. This lens design results in horizontal and vertical "fan-shaped" detection zones, as illustrated in this figure. The sensor responds to motion across these zones and must detect motion across one or more zones within a specified period to keep the lights on. For wall-mounted sensors, the field of view is defined by a horizontal angle ranging up to 180 degrees and a vertical angle ranging up to 90 degrees. For ceiling-mounted sensors, the field of view is often defined by a solid angle and can be imagined as a cone extending down and outward from the sensor. Manufacturers report the field of view for ceiling-mounted sensors in one of two ways: Some cut the cone horizontally and report the resulting circle as the field of view, i.e. 360 degrees. Others cut the cone vertically and report the angle at the apex of the resulting triangle as the field of view, i.e. 0 to 180 degrees. The latter method gives a more useful description of the field of view because the former method always results in 360 degrees, regardless of the size of the cone.

Ultrasonic occupancy sensors (see figure 2 below) operate by responding to the change in reflected sound waves in a space caused by a moving object. These products use transmitter and receiver technology to send and receive high frequency sound waves (ultrasonic) above the range of normal human hearing (between 25kHz and 40kHz). These waves are reflected back to the receiver in the sensor by room surfaces and objects. Motion in the space causes a change in the return frequency to the sensor which is interpreted as motion. Ultrasonic sensors do not require a direct line-of-sight to detect motion, unlike passive infrared sensors, and they will detect smaller motions.

Figure 2. A simplified wave pattern in an empty room illustrates the motion detection concepts for ultrasonic sensors. High-frequency sound waves are transmitted from the sensor, reflected from room objects and surfaces and eventually returned to the receiver located in the sensor. When an occupant enters or moves within the room, the reflected sound-wave pattern is altered. The receiver detects the change and turns the lights on. Because ultrasonic sensors transmit energy which essentially fills the volume of the space, they do not require a direct line-of-sight to the occupant.

Dual Technology occupancy sensors, also referred to as hybrid sensors, combine two technologies into one product. Passive infrared and ultrasonic are the two technologies commonly used in dual-technology sensors, although one US manufacturer has just introduced a dual-technology sensor that combines passive infrared and microphonic technology. Both products keep lights on if either technology detects motion, and turn lights off only if neither technology detects motion. By requiring only one technology to detect motion to keep luminaires on, these products reduce the likelihood of luminaires turning off while the space is occupied. To minimize the risk of the sensor turning on when no one is in the room, the passive infrared/ultrasonic sensor turns lights on only when both technologies detect motion.

OPERATIONAL FEATURES
Occupancy sensors have five primary operational features: sensitivity, field of view, coverage area, coverage pattern, and time delay. In addition, some products have daylight sensing features.

Sensitivity refers to the responsiveness of the sensor to motion in the space. Most products have an adjustable sensitivity setting. Adjusting the sensitivity will affect both the coverage area of the sensor and its responsiveness to motion. Increasing the sensitivity will increase the coverage area, but a very high sensitivity adjustment may result in the sensor not turning lights off when the space is unoccupied, particularly for ultrasonic products.

Field of view refers to the angular boundary within which the occupancy sensor will detect motion. The field of view is typically reported for only infrared sensors, and is a characteristic of the optical pattern etched in the lens. It determines both the sensor coverage area and the coverage pattern.

For some infrared products, the field of view may be adjusted by applying stickers or special tape to the sensor, usually provided by the manufacturer.

Coverage area defines the physical limits of the sensor's ability to detect motion. Most occupancy sensor manufacturers publish their coverage areas for the maximum sensitivity setting at a specific mounting height, although this may not be clearly stated in the product literature. The actual coverage area will depend on the mounting location and orientation of the sensor in the room, room geometry, furniture layout, and sensor sensitivity setting.

Coverage pattern. The coverage pattern, like the field of view, is typically reported only for infrared sensors, and is determined by the optical pattern etched in the lens. It is described by a geometric shape such as square, circle, rectangle, or ellipse. Coverage patterns can be vertically or horizontally oriented, depending on the mounting configuration of the sensor. It is important to compare a product's coverage pattern to the room geometry when specifying an occupancy sensor. An occupancy sensor with a long and narrow, rectilinear coverage pattern will perform better in a corridor than will a sensor with a square pattern, for instance.

Time delay. Most of the products offered in the United States allow the user to select a time interval for the luminaires to remain on after the space is vacated. Some European products have an adaptive time delay circuit that increases the time delay if there is not much motion in the room, but decreases it if motion is frequently detected in the room.

Time delay also prevents the luminaires from switching off during intervals when people are actually in the room but move too little or too slowly to be detected by the sensor. Energy may be wasted by choosing too long a time delay, whereas a short time delay may cause frequent switching in a space where people repeatedly enter and exit, such as a copy room. This frequent switching could reduce lamp life.

Daylight sensing. Spaces with abundant daylight do not always require electric lighting when they are occupied. Therefore, some occupancy sensors include a photocell for monitoring daylight in a space. The photocell monitors the amount of daylight in the space and compares it to a predetermined illuminance set by the end user. The luminaires are turned on only if the daylight illuminance is below the predetermined illuminance and the room is occupied.

MOUNTING LOCATIONS
The typical mounting locations for sensors are on a wall at the height of a typical wall switch, on the ceiling in the center of a room, or in the corner of a room mounted high on the wall or on the ceiling.

Wall-mounted sensors often are used to replace standard manual switches and are wired directly to the luminaire. Many of these sensors are available with a variety of manual control options that include automatic-on/automatic-off, manual-on/manual-off/automatic-off, two-level-on/automatic-off, and manual-on/automatic-off dimmer controls. Wall-mounted sensors are easier to install and are less expensive than ceiling-mounted products, but they are more susceptible to user tampering than are ceiling-mounted products.

Automatic-on/automatic-off sensors are commonly used. The sensor automatically turns the lights on when motion is detected and turns them off when motion is not detected. These products are appropriate in spaces where complete automatic control with no manual override is desired.

Manual-on/manual-off/automatic-off sensors must be switched on manually to turn on the luminaires. The sensor automatically turns off the luminaires when motion is no longer detected if luminaires are not manually turned off by the occupant. These products are appropriate in spaces where an occupant may choose to leave the luminaires off when daylight is adequate or when the occupant desires to maintain control of the lighting.

Two-level-on/automatic-off. A two-level sensor provides control similar to a two-level switching arrangement, which is designed to separately switch lamps within a luminaire or separately switch individual luminaires. The user has the option to manually select either a "half-on#&34; or "full-on" setting on the sensor.

Manual-on/automatic-off dimmers. These products operate in a manner similar to the manual-on/automatic-off type, except they incorporate a slide dimmer.

Ceiling-mounted sensors. Ceiling-mounted sensors are available in surface and recessed mounting options; however, most products are designed to be surface mounted onto the ceiling. Ceiling sensors are used in areas with coverage area requirements that exceed the capacity of wall-mounted versions in areas where the connected lighting loads exceed the capacity of wall-mounted versions, or where it is necessary to control multiple lighting circuits.

Ceiling-mounted sensors are usually wired to a separate control module, containing a transformer and a relay, which is remotely mounted in the ceiling. Multiple sensors and lighting circuits can be controlled by one control module.

Some ceiling-mounted sensors are designed with one-way, two-way, or three-way detection abilities. These products are usually designed to be mounted in a corner or on the side of a space, and are often used in corridors. Two- and three-way sensors contain more than one detector within the sensor housing: Two-way sensors are designed to "see" in two directions and are usually mounted in the center of a room or a corridor. Three-way sensors are designed to "see" in three directions and are usually mounted at a corridor intersection.

Corner-mounted sensors usually use infrared technology and are typically mounted in the corner of a room high up on the wall or on the ceiling. A corner mounting location is sometimes used in private offices at the corner closest to the door to aim the sensors' field of view into the room and avoid detecting motion beyond the office boundary. Other applications include large, open areas where sensors are used in two or more corners of a space for overlapping coverage.

FALSE TRIGGERING
False triggering occurs when the occupancy sensor incorrectly turns the lights on or off. The sensor may be too sensitive to external stimuli or not sensitive enough to occupancy. Examples of a "false positive" triggering would be lights turning on in response to a curtain moved by a breeze or air circulated by the HVAC system. A "false negative" event occurs when the sensor turns the lights off while the room is occupied. False triggering events can be minimized by following the manufacturer's recommendations for locating the sensor in the space, setting sensitivity levels, making time delay adjustments, and selecting a product with a coverage area appropriate for the tasks that occur in the space.

False negative events are the primary reason why some occupants are not satisfied with occupancy sensors. If a sensor continuously turns the lights off while a room is occupied, the occupant may become annoyed and tape over or disable the sensor. Occupants are not usually concerned if lights turn on while a room is unoccupied, but a building manager or owner would be. Thus, when retrofitting manual switches with wall-switch replacement occupancy sensors, many building owners and specifiers are now using products with manual on-and-off control in addition to the automatic shut-off feature. By using these products, occupants can have manual control over the lighting, and if they do not turn off the lights, they are automatically turned off by the occupancy sensor.

COMMISSIONING
Ultrasonic sensors are more difficult to commission than are infrared ones. The sensitivity of ultrasonic sensors must be tailored to the application. Some manufacturers recommend adjusting the sensitivity with the HVAC system on and off to be certain that air flow and vibrations won't cause the sensor to false trigger. Many infrared sensors do not have adjustable sensitivity, but sometimes the lens must be masked to restrict the sensor's field of view. For both technologies the required time delay needs to be set. After setting the time delay, the actual time delay should be checked for at least three repetitions to be certain that the required value is obtained because the potentiometers used in many products are not always accurate. For example, setting the time delay to 5 minutes may result in an actual time delay of 2 or 8 minutes. Dip switches are sometimes used in newer products and provide very accurate time delay settings.

Dorene Maniccia

The author works at the Lighting Research Center,
Rensselaer Polytechnic Institute,
Troy, N.Y., 12180-3590, USA
Fax: +1 518 276 4835
E-mail: manicd@rpi.edu

Nutek: Large-Scale Procurement Of Occupancy Sensors

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