Occupancy Sensors 101

April 1, 2007
Five steps to an effective installation. State energy codes in the United States, which must be at least as stringent as the ASHRAE/IES 90.1-1999 standard, require automatic shutoff of lighting in commercial buildings greater than 5,000 square feet in size, with few exceptions. Scheduling devices, such as intelligent control panels or occupancy sensors, can accomplish this task. According to the U.S. Environmental Protection Agency, energy

State energy codes in the United States, which must be at least as stringent as the ASHRAE/IES 90.1-1999 standard, require automatic shutoff of lighting in commercial buildings greater than 5,000 square feet in size, with few exceptions. Scheduling devices, such as intelligent control panels or occupancy sensors, can accomplish this task.

According to the U.S. Environmental Protection Agency, energy savings from using such devices can range from 40% to 46% in classrooms, 13% to 50% in private offices, 30% to 90% in restrooms, 22% to 65% in conference rooms, 30% to 80% in corridors, and 45% to 80% in storage areas. Besides providing a means of minimizing energy consumption, additional uses of occupancy sensors include security (by indicating that an area is occupied), and minimizing light pollution (by reducing the usage of lighting operating at night), whether it be outdoor lighting or indoor lighting emitting through windows or skylights.

Occupancy sensors are ideally suited for applications that require a higher granularity of control than can be economically achieved using scheduling (e.g., a floor of an office building with perimeter offices that must be controlled individually). Sensors are also considered most suitable when the space is intermittently occupied, meaning it is unoccupied for two or more hours per day, and where the lights are typically left on when the space is unoccupied. Appropriate applications include offices, classrooms, copy rooms, restrooms, storage areas, conference rooms, warehouses, break rooms, corridors, filing areas, and other spaces.

In contrast, scheduling typically is most suitable for larger projects, such as entire buildings and multiple spaces tied together on the same schedule. Scheduling is also often appropriate for public spaces where occupancy is typically predictable and based on a schedule — or where the lights must remain on even when the space is unoccupied, such as lobbies.

To provide proper automatic lighting shutoff for a building, a combination of scheduling devices and occupancy sensors may be desirable. Occupancy sensors can provide local control for private spaces, while scheduling panels can provide global control for the building depending on its operating schedule and public spaces where the lights must remain on even when the space is unoccupied.

Although they have matured since their introduction, and various manufacturers offer a robust offering of products, occupancy sensors remain application-sensitive devices. The goal is to gain the full advantages of occupancy sensor operation while avoiding possible negative outcomes, most notably nuisance switching (lights are switched under false conditions, such as turning off while somebody is still in the room but has been motionless for several minutes). As a result, thoroughly understanding the characteristics and requirements of the application and subsequently making appropriate design decisions — sensing technology, coverage patterns, sensor location, special features, and commissioning — are critical to ensuring a trouble-free application.

Let's look at the five key steps involved in determining the best sensor technology for a given application.

Step 1: Choose the right sensor technology

Occupancy sensors use different technologies to detect the presence or absence of people in a space, including passive infrared (PIR), ultrasonic, and dual-technology — each of which has its own advantages and disadvantages. Regardless of which sensor type is selected, it should activate the lights as soon as the person enters the room, but should not monitor the area outside the door to avoid nuisance switching. Additionally, the door swing should not obstruct the view of the sensor.

PIR sensors sense the difference in heat emitted by humans in motion from that of the background space. These sensors detect motion within a field of view that requires a line of sight; they cannot “see” through obstacles and have limited sensitivity to minor (hand) movement at distances typically greater than 15 feet. The sensor is most sensitive to movement laterally across the sensor's field of view, which can be adjusted.

PIR sensors are most suitable for smaller, enclosed spaces (wall switch sensors), spaces where the sensor has a view of the activity (ceiling- and wall-mounted sensors), and outdoor areas and warehouse aisles. Incompatible application characteristics include low motion levels by occupants, obstacles blocking the sensor's view, mounting on sources of vibration, or mounting within 6 feet to 8 feet of HVAC air diffusers.

Ultrasonic sensors use the Doppler principle to detect occupancy through emitting an ultrasonic high-frequency signal throughout a space, sense the frequency of the reflected signal, and interpret change in frequency as motion in the space. These sensors do not require a direct line of sight and instead can “see” around corners and objects, although they may need a direct line of sight if fabric partition walls are prevalent. In addition, ceiling-mounted sensor effective range declines proportionally to partition height. They are more effective for low motion activity, with high sensitivity to minor (hand) movement, typically up to 25 feet. The sensor is most sensitive to movement to and from the sensor. Ultrasonic sensors typically have a larger coverage area than PIR sensors. The sensor's view cannot be adjusted.

Ultrasonic sensors are most suitable for open spaces, spaces with obstacles, restrooms, and spaces with hard surfaces. Incompatible application characteristics include high ceilings (greater than 14 feet), high levels of vibration or air flow (which can cause nuisance switching), and open spaces that require selective coverage (such as control of individual warehouse aisles).

Dual-technology sensors employ both PIR and ultrasonic technologies, activating the lights only when both technologies detect the presence of people, which virtually eliminates the possibility of false-on, and requiring either one of the two technologies to hold the lights on, significantly reducing the possibility of false-off. Appropriate applications include classrooms, conference rooms, and other spaces where a higher degree of detection may be desirable.

Step 2: Select coverage pattern

A leading cause of application problems is improper coordination between required coverage area and required sensitivity. The designer must determine range and coverage area for the sensor, based on the desired level of sensitivity.

Manufacturers publish range and coverage area for sensors in their product literature, which may be different for minor (hand) motion and major (full-body) motion. Various coverage sizes and shapes are available for each technology. In a small space, one sensor may easily provide sufficient coverage. In a large space, it is recommended to partition the lighting load into zones, with each zone controlled by one sensor. The sensors are networked together by low-voltage wiring. When creating zones, sensor coverage should overlap by 20%.

Step 3: Layout

Proper sensor location ensures the smallest opportunity of nuisance switching, that the lights will turn on as soon as a person enters the space, and that the sensor maintains an unobstructed line of sight to task areas at all times. In addition to location, the sensor must also be oriented. Ultrasonic sensors, for example, should be oriented toward the area of greatest activity in the space. In addition, the designer must determine whether or not the sensor will be installed at the wall switch, wall/corner, ceiling, or task.

Ceiling-mounted sensors are appropriate for large areas that feature obstacles such as partitions, in addition to narrow spaces such as corridors and warehouse aisles. Units can be networked for control of areas that are larger than what can be controlled by a single sensor. These sensors typically present a two to three times higher installed cost than wall switch sensors, but can be very economical if controlling large zones.

High wall- and corner-mounted sensors are similarly appropriate for coverage of large areas that feature obstacles.

Wall switch (wall-box) sensors, relatively inexpensive and easy to install, are appropriate for smaller, enclosed spaces, such as private offices with clear line of sight between sensor and task area.

Workstation sensors are appropriate for individual cubicles and workstations. The sensor is connected to a power strip for simultaneous control of lighting and plug-in loads such as computer monitors, task lights, radios, and space heaters.

Step 4: Specify the sensor

Determine the specific feature set for the sensor and the power pack, and whether the sensor must be integrated with other control devices. The sensor may offer a number of special features, including:

  • Line-voltage sensors, which do not require a power pack and are suitable for applications where there is no plenum, or junction boxes are hard to access (but may only have the ability to switch about one-third the load of a ceiling sensor that uses a power pack).

  • Self-calibration — so-called “install and forget” sensors, designed with intelligence so that they are self-calibrating and self-adaptive.

  • Manual-on operation — human intervention is required to turn the lights on, but they are switched off automatically.

  • Bi-level switching (using two relays in the power pack).

  • Daylight switching (works with photo sensor).

  • Combination dimmer/occupancy sensor.

  • Isolated relay (separate low-voltage switch for interfacing with other loads such as HVAC).

  • Connectivity to a digital network.

  • Compact sensor integrated with light fixtures for simplified installation and lower installed cost, as well as a “blended-in” appearance.

Step 5: Installation and commissioning

To ensure occupant satisfaction with a sensor installation, the manufacturer and installer may collaborate on startup and field commissioning. This is because occupancy sensor settings should be calibrated to the specific application needs.

Commissioning begins during the design process, in ensuring that the right sensor technology was selected, and that it was placed correctly on the plans. For example, if an ultrasonic sensor is placed near a source of high airflow, it can experience false “ONs.” Similarly, if a PIR sensor is placed in such a way that it has a blind spot in part of the room, it will not detect occupancy in that part of the room and produce false “OFFs”.

The first step for the installer in performing field commissioning is to ensure that the wiring connecting the sensor or power pack to the power and loads is correct. Occupancy sensors must be installed according to manufacturer instructions and wiring diagrams. The next step is to verify proper placement and, if applicable, orientation of the sensors, so that they match the specifications and construction drawings.

Otherwise, there are two or three possible primary adjustments that may need to be tuned in the field. This should be coordinated with furniture placement, as occasionally furniture or equipment may be moved or relocated, which can affect sensor placement and/or orientation.

  1. The time delay provides control of the time in which the sensor will turn off the load after the room is unoccupied. While a lower time delay offers more energy savings, it might cause occupant dissatisfaction if the lights prematurely turn off. A longer time delay offers more assurances that the light will not turn off if there is little activity in the room. Generally, a 15-minute delay is recommended.

  2. The sensitivity setting allows the installer a way of controlling the range and sensitivity to movement of the sensor.

  3. The light level setting, available with models that offer a daylight-switching feature, allows the installer to hold off turning on the electric lights if the daylight level in the room is adequate.

The project may use self-calibrating and self-adaptive sensors, which automatically adjust their delay and sensitivity settings over time.

After commissioning is completed, users should be told about the intent and functionality of the controls. This is critical because if users do not understand the controls, they will complain and attempt to override or bypass them. Documentation and instructions should be given to the owner's maintenance personnel so that they can maintain and re-tune the system as needed.

DiLouie is the communications director for the Lighting Controls Association and principal of Zing Communications in Calgary.

About the Author

Craig DiLouie

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