Several principles are available that can be used to build motion sensors.
- Passive Infra Red (PIR),
- Sound based,
- High frequency (HF / Radar) and
- Combinations of these principles.
Each has its own benefits and limitations. For instance, HF sensors can be very sensitive - however, this then comes at the disadvantage that the detection is not limited to a given space (a room) any longer. Essentially, they can see through walls. While useful in some instances, this behaviour is not desired in the majority of applications - e.g. persons walking along a corridor would trigger the sensors in individual offices.
For the main task of occupancy sensors, that is to reliably detect whether persons are present in a room, passive infra red (PIR) quickly appears to be the most suitable technology for a couple of reasons:
- PIR triggers based on temperature difference and motion - a fitting combination since a person's surfaces usually show some temperature difference to its surroundings, and
- the detection is bound to a confined space. PIR sensors can not see through walls (and not through glass walls or windows, either).
Unfortunately, the sensitivity of conventional PIR motion sensors is distinctly limited by electrical noise and other interferences and typical sensor designs are large und quite ugly. These disadvantages can be overcome by a new approach to PIR sensor design.