Occupancy sensors are the most common lighting control devices used in facilities today. Previous Controls Corner articles have offered some guidance on how to best use sensors. However, there remains a need for a deeper understanding of how these sensors work, and how best to employ them for a code-compliant and trouble-free installation. This article is intended to cover the basics of occupancy sensing as concisely as possible.
Occupancy Sensor Technologies
There are four primary occupancy sensing technologies on the market. Each has their own operating characteristics, as well as strengths and weaknesses. For many applications, optimal operation will come from sensors that use more than one technology.
- Passive Infrared (PIR): Passive infrared sensors pick up body heat in motion within a space. This is a direct line of sight technology, meaning that if you can’t see the sensor, it can’t see you. Due to the way the sensor picks up movement, these sensors are more effective for large motions that are lateral to the sensor; motion directly towards the sensor is not detected as effectively. These sensors operate with switching zones that radiate from the sensor. The sensor is triggered when two of these zones are crossed. The farther you are from the sensor, the wider the zones, and the more motion it will take to trigger the sensor. Lastly, due to this being a passive technology, the energy draw for these sensors is very low. As we move towards wireless sensors, most battery powered sensors will use PIR. Active technologies, such as ultrasonic, draw more current and will drain the battery too quickly to be practical.
- Ultrasonic: Ultrasonic sensors operate by continuously putting out high frequency sound waves into the space. These sound waves are reflected by the walls and other objects in the space, and then heard by the sensor again. If there is a change in how the sound waves bounce back, the sensors are triggered. Due to how sound waves propagate, ultrasonic sensors can “see” around corners, making these sensors suitable for locations where there are obstructions, such as cubical walls or bathroom stalls. Ultrasonic sensors are better at picking up smaller motion than PIR sensors are. Because sound waves are continuously broadcast, this is an active technology. These sensors draw considerably more energy than PIR sensors, and will not be suitable for battery operation. They also require more power packs to cover large spaces than PIR sensors would.
- Microphonic: These sensors pick up sound in the space, and have specific sound filters and other digital logic to discern between ambient noise and human made sounds. This is a passive technology, and will have a smaller energy draw than ultrasonic. However, this technology is not as reliable as ultrasonic due to the wide variety of noise that can be present in a space. It is used in sensors that also have PIR sensors, and is set so microphonics alone do not trigger the sensor to turn on. With these limitations, microphonics can be considered an enhancement to PIR sensing technology. Microphonic technology is not equivalent to ultrasonic which is a far more accurate technology. Sensors with microphonic sensors will be less expensive than sensors that have ultrasonic technology.
- Dual Technology: Dual technology sensors incorporate both PIR and ultrasonic, offering the best capabilities of both technologies. These sensors are suitable for the widest range of applications, and have a number of different settings to tailor their operation to the specific application. These settings typically include sensitivity adjustments that are independent for each technology, trigger settings, retrigger settings, and time delay. Trigger is a setting to determine what will turn the lights on. It can be set to have either of the technologies alone, both together, or one or the other. Retrigger is similar, but this is the sensor configuration that will keep the lights on once they are on. Adjustability of sensors is important to ensure that sensors operate optimally in each space where they are installed.
Sensor Suitability for Common Applications
- Private Offices: Dual Technology is important in these spaces as it is crucial to be able to pick up both small and large motions, as well as to be able to see around objects.
- Bathrooms: Dual Technology offers the best ability to see around stall walls, as well as pick occupants up as soon as the door is opened.
- Classrooms: Dual Technology is the most useful to get the maximum coverage and reduce the number of sensors needed. However, sensor placement is critical. Many times, a single sensor is placed in the center of the room to try and save cost. However, this can create blind spots in the corners of the room which is where teachers are most likely to have their desks. When there are no students in the classroom, teachers may experience false off problems.
- Janitor Closets: PIR only is fine for these spaces as there typically will not be occupants in the space for long, or corners to see around.
- Corridors: Directional Ultrasonic only are good for these spaces. These sensors broadcast the sound waves a great distance from only two sides of the sensor, allowing very wide spacing between sensors while minimizing the number of sensors needed. The directional coverage pattern will also help to reduce lights turning on when there is motion in adjacent spaces.
- Conference Rooms: Ceiling Mounted Dual Technology installed in the center of the room is a good choice to be sure all occupants can be seen around the table to avoid false offs.
- Electrical Rooms: None! Automatic lighting controls are not allowed in electrical rooms per the National Electrical Code.
- Open Office: Ceiling Mounted Dual Technology will offer the widest coverage pattern with the ability to best detect motion in spite of cubical walls.
- Warehouses: PIR only is suitable for these spaces, as all motion tends to be large motion, and the technology allows for sensors that are well suited for high ceiling mounting. Sensors should be arranged down every aisle of the warehouse.
- Data Centers: PIR only, as a rule. The high level of airflow in data centers can cause ultrasonic sensors to remain tripped and hold the lights on. In general, occupancy sensing does not work well in data centers. Other control methods, such as time based, are highly recommended.
Adjustments for Optimal Operation
Occupancy sensors can operate ineffectively in only two ways. Either the lights don’t turn on when they should, or the lights won’t turn off when no occupants are present. Following are some adjustments that can be made to resolve each.
- When the lights don’t turn on, or when you have to walk too far into the space to have them turn on: The first step is to turn up the sensitivities to make sure the sensor is seeing the entrance points to the space. The second step, for dual technology sensors, is to change the initial trigger mode to turn the lights on when either of the technologies detects motion, instead of requiring both technologies to detect motion.
- When the lights don’t turn off: Essentially, the opposite of the adjustments above should be made. When lights don’t turn off, or turn on when no one is present, it is typically the ultrasonic portion of a dual tech sensor that is the culprit. First, ensure that the sensor is not installed too close to an air diffuser; moving air can trip ultrasonic sensors. Following that, turn down the ultrasonic sensitivity and set trigger modes to require both technologies to sense before the lights turn on (or to keep them on).
The use of occupancy sensors is the most prevalent control strategy to save energy and meets code requirements. In future editions of the energy code, occupancy sensors will be required in even more areas. In order to maximize energy savings while not sacrificing appropriate operation, careful sensor technology selection and operational adjustments are important. Following the above guidelines will help in ensuring a trouble-free installation.
As always, don’t hesitate to contact the controls team at MH for further information or to review specific applications. And remember, whenever you have a technical controls question, on any project or application, simply email firstname.lastname@example.org. This email address is monitored by our entire controls team. During our normal business hours, we are committed to a four-hour response time for all emails sent to this email address!