Occupancy Sensors Not All the Same

Occupancy sensors can help save energy by automatically turning off lights when areas are not occupied. However, not all occupancy sensors are the same. Also, improper settings and placements of occupancy sensors can make building occupants view the system as more of an annoyance and than an improvement. Therefore, installing a lighting system for a building requires careful planning and understanding.

According to the U.S. Department of Energy, lighting accounts for 40 percent of the electricity used in commercial buildings. Occupancy sensor technology can lead to savings when lights turn on when someone enters a room and turn off when no one is in the room. Incorrect placement and settings can make the system less than optimal in terms of energy efficiency and can even be dangerous because of the poor or unsafe lighting conditions that they may cause.

Standard occupancy sensors use passive infrared (PIR) or ultrasonic technology to detect the presence or absence of occupants in an area. Other, less common “dual” sensors use a combination of PIR and and ultrasonic technology to overcome some of the weaknesses of each.

PIR Motion sensors employ infrared sensors detect infrared radiation “heat” emitted or reflected from an object in several directions or through multiple slots. The PIR motion sensor detects an occupant’s presence when it senses the difference between the heat emitted by moving people and background heat.

PIR sensors tend to be less prone to false positives, they cannot be triggered by airflow, and because they are passive, they consume minuscule amounts of power. However, PIR sensors require an unobstructed view. So applications around cubicles or bathroom stalls may not work as effectively.

A good *explanation of Occupancy Sensor technology was written by Leslie Hodges at the University of Michigan.

According to Hodges, PIR sensors cannot easily discern between humans and small animals. They are susceptible to “dead spots,” which are areas where motion cannot be detected within the field of view.

A PIR sensor is less sensitive the further away one gets from it. Also, PIR sensors cannot determine the distance of the user from the sensing device. For example, having one PIR sensor at the end of a long hallway is probably not enough coverage.

Another technology commonly used for occupancy sensors is ultrasonic technology. Ultrasonic-based occupancy sensors also have certain advantages and disadvantages. Unlike PIR sensors, ultrasonic sensors do not have gaps in the coverage zone. According to Hodges, ultrasonic sensors are “sensitive to slight motions at nearly twice the distance” of PIR sensors.

Ultrasonic sensors can detect motion despite a partial obstruction of the view, and they can calculate the distance to a moving object with the Doppler effect. The high sensitivity of ultrasonic sensors can lead to false triggers. The motion of air, fans, of an HVAC system, can trigger a conventional ultrasonic sensor the sensitivity is not set properly.

The most sophisticated and if used properly the most accurate occupancy sensors use both a PIR sensor and an ultrasonic sensor together. A micro controller acts as the brains of a “dual” sensor.

A particular dual sensor may have a proprietary algorithm that allows the sensor technologies to work together to create useful results. Additional functionality can be added with wireless communication technology. While properly installed dual sensors tend to be more accurate in detecting occupants, dual sensor technology adds significant cost to a project.

The PIR sensors and ultrasonic sensors have to be compatible and they both have to be used with the correct settings for a particular application. In addition to the cost of purchase, the cost of designing and installing and configuring a dual sensor system is often greater than that of installing either a PIR sensor system or an ultrasonic-based system and is usually more complex a task.

Another application for sensor technology is that it can provide a stream of data that if combined with ongoing data about power usage, scheduling, temperature, and other information, can create very useful information through data analysis with the use of a software backend.

It is data analysis applications such as these where the IoT possibilities can shine.

*Hodges, Leslie. “Ultrasonic and Passive Infrared Sensor Integration for Dual Technology User Detection Sensors,” Michigan State University, 2009.

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