Presence detection at the workstation using passive infrared (PIR) sensors

Figure 3.227: The detection range of a passive infrared sensor for presence and motion detection.

  • In a wide detection range, movement can be detected when it occurs tangentially (across the sensor’s "viewing direction") in a pattern comparable to walking movements.

  • In a more narrow detection range, radial movements resembling the pattern of walking movements towards the sensor are also detected.

  • In the presence detection range, small movements (e.g. writing tasks) are also registered.

Generally, PIR sensors are used to detect presence at indoor workstations. As passive sensors, they are characterised by simple construction, low maintenance requirements, relatively small dimensions, an advantageous price-performance ratio and low energy requirements. Beyond that, PIR sensors are available with highly varying detection ranges.

The functional principle of this sensor type can be described as follows:

  • It is based on detecting the body heat emitted by living beings into their environment in the form of infrared radiation.

  • The sensor is passive insofar as it does not need to emit a signal of its own but merely receives and evaluates signals from its environment.

  • The signal is evaluated through segmentation by the receiver, whereby each segment registers the incoming IR radiation of a solid angle range of the environment.

  • When movement occurs, the IR radiation in one or more segments changes, which the light management system interprets as presence.

  • There must be "visual contact" between the sensor and the moving person for the movement to be detected.

  • With increasing distance from the sensor, larger movements are required for detection to work. This is why a distinction is frequently made between the close range of presence detection (small movements suffice) and the extended range of motion detection (movement) (see figure).

  • The IR sensors power supply is usually managed through the corresponding control device’s sensor cable.

Similar to optical radiation, infrared radiation can be simply bundled or scattered through lenses and thus focused or expanded. It is therefore possible to generate entirely different detection characteristics which can be used according to the respective application's room geometry. For example, there are narrow-range sensors with great reach for corridors, sensors for elevated installation heights in racking warehouses with elliptical detection areas, wide-range sensors for offices and educational facilities as well as other detection characteristics. In specific cases, detection parameters such as range and sensitivity can be set at the sensor.

It depends on the respective electronic control device whether it merely evaluates an individual sensor’s detection range or whether it facilitates an extension of the detection range. Also a segmentation of the room into several areas by combining several sensors can be possible. The communication between sensor and control device is usually system-specific. A standardised interface for presence sensor systems is available in the context of the DALI-II standard (see section ).

Typical applications are e.g.

  • offices,

  • educational facilities,

  • sports halls,

  • rooms in the healthcare sector,

  • storage areas,

  • corridors,

  • and other indoor areas where persons generally move around in clothing that is not intended for outdoor winter activities (see below, subsequent section).

When positioning IR presence sensors it is important to observe that the sensors provide sufficient presence capture. Furthermore, sensor function should not be impaired by sources of interference in the detection range.

  • Stationary heat sources, for instance, can cause the sensors to switch on the light if they heat up or cool off rapidly, meaning they cause a change in the IR signal received by the sensor.

  • In adjacent but independently switched areas, IR radiation generated by indirect (fluorescent-lamp) lighting components in adjacent areas can trigger automatic switch-on.

Interference of this nature occurs rather rarely and can usually be fixed by blocking off the source of interference or changing the sensor’s position.