PIR Types
A Pyroelectric Infrared (PIR) sensor is an electronic component that measures infrared light radiating from objects in its field of view. Motion is detected when an infrared source with one temperature passes in front of another temperature source such as a wall. These devices do not recognise the movement of objects, such as doors opening nor do they operate through glass.
PIR’s come in different forms; Single Element, Dual Element and Quad Element. As you would expect the quad element has superior detection capabilities to that of either a single or dual element. When ceiling mounted behind a 360 degree Fresnel lens it is essential that a quad element is used to ensure continuous 360 degree visibility and reliability. For example a dual element device will be more responsive on two sides than the other two.
Reliability can also be an issue in the design of the electronic circuit implemented to interface the PIR to control circuitry. The PIR will produce just a few micro volts and will require an electronic amplifier to increase this to a working voltage in the region of 3-4 volts. The design of these amplifiers are very critical and if perfection is not achieved either the full performance of the PIR is not realized or the device will be subject to reliability issues. The ultimate in reliability and performance is when the amplifier is integral within the same metal housing as the PIR, as used in the Steinel Control Pro Quattro Series.
Unreliability normally shows up with ‘false triggering’ when a detector will respond to an electrical event such as a nearby light turning on or off. This can result in a building having the odd area of light on when empty. Unfortunately this is a common site in city centres.
Fresnel Lenses
All PIR’s have to be mounted behind a Fresnel Lens to become Presence/Movement sensor. The Fresnel lens reduces the amount of material and subsequently the size required compared to a conventional spherical lens by breaking the lens into a set of annular sections known as Fresnel Lenses. The lens will always have a focal point where the PIR must be positioned. If this is not right then the performance will be impaired. A more ‘bulbous’ lens will outperform a flatter lens in this application. The physical size of the lens does not mean better performance.
Single Head –v– Quad Head
In the commercial building market there are both single head sensors (one PIR per device) and quad head sensors (four PIR per device). The quad head devices cover a larger area than the single head devices . The advantage of quad head devices is that less devices are needed to cover the same area. The disadvantages are that when installed in an empty area that subsequently gets furnished obstructions will appear causing ‘dead spots’.
Quad head devices should only be used in areas where tall furniture or screens are unlikely to be installed.
Techniques
• Presence Detection — Both automatic on and automatic off. No user interaction
• Absence Detection — On and Off manually (switch) and automatic off to prevent lights being left on.
• Daylight Harvesting — When there is sufficient natural light the light switches off. Also known asDaylight Linking
• Constant Daylight — When the light is automatically adjusted to maintain a constant light level at the working plane.
• Scene Setting — Setting different levels of light in conjunction with Constant Daylight
• Inhibit — The ability to turn the Presence Detector off and prevent the lights turning on allowing a ‘Last man out switch’.
• Force — The technique used to ‘lock on or off’ individual lights to allow a security mode.
Example Applications
Offices
• Open Plan offices and circulation areas — Presence Detection
• Cellular Offices — Absence Detection
• Presentation Rooms — Absence Detection with Scene Setting
Schools
• Classrooms — Absence Detection (with Scene setting)
• Corridors — Presence Detection
• Toilets — Presence Detection
Retail
• Shops — Presence Detection with Force
• Back of shop — Presence or Absence Detection
Daylight Harvesting or Constant Daylight can be used in combination with the above applications to achieve even greater energy savings by utilising natural light entering the environment through windows.
Note:
It is generally recommended that the timer setting on a sensor are set to the light manufacturers recommended minimum time. This can be anything up to 20 minutes. If this figure is set to low then the benefit gained will be offset by the shortened lamp life.
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