Understanding Passive Infra Red (PIR) Movement Sensors
All Remote Monitored or Event Driven CCTV systems require a reliable means of detecting a person or persons entering into the protected area.
There are many different types of sensors from video motion analytics to microwave Doppler detectors but by far the most reliable and cost effective way is to use a PIR detector.
These detectors are very common in internal burglar alarm systems where they operate in controlled environments. Design considerations are different for external PIR's because they have to operate reliably in an uncontrolled environment. These detectors are subject to rain, high humidity and large temperature changes from sub zero to tropical climates. They have to cope with strong winds which cause trees and foliage to move about in the field of view. Visible light from car headlights will flash across their lenses on dark nights. Pets and wild animals will roam around in the field of view.
Modern powerful micro controllers are now very affordable for the PIR detector market and allow the designer to implement a high degree of digital filtering to counter these changing conditions.
This filtering can be adjusted by the user and is normally described as Pulse Counting. Original analogue detectors simply counted the pulses from the comparator outputs but modern detectors apply different algorithms to filter the digitised infrared signal. It is common to still call this filtering, Pulse Counting but it is not the same.
Despite the degree of sophistication and filtering in these detectors it is still prudent to consider carefully the positioning of the detector. Correct positioning will reduce the need to set a high level of filtering and therefore the detector will be more sensitive to intruders. Detectors should always be mounted facing into the site to avoid detecting outside the protected area. This is in fact a requirement in the BS8418 British Standard for Remote Monitored Systems. Detectors should be mounted at the correct height as specified by the manufacturer as this will affect the detection pattern. Many external detectors are designed to be mounted at no more than 2.5 metres to comply with health and safety requirements. Some detectors are designed to be mounted at greater heights but this will then require the use of a cherry picker.
How the Passive Infrared Detectors Work
Infrared radiation enters through the front of the sensor, known as the sensor face. At the core of a PIR sensor is a lithium tantalate Pyrolectric Sensor exhibiting both piezoelectric and pyroelectric properties.
The sensor is often manufactured as part of an integrated circuit and may consist of two or four elements of equal areas of the pyroelectric material known as Dual or Quad. Pairs of the sensor elements may be wired as opposite inputs to a differential amplifier. In such a configuration, the PIR measurements cancel each other so that the average temperature of the field of view is removed from the electrical signal; an increase of IR energy across the entire sensor is self-cancelling and will not trigger the device. This allows the device to resist false indications of change in the event of being exposed to flashes of light or field-wide illumination. At the same time, this differential arrangement minimizes common-mode interference; this allows the device to resist triggering due to nearby electric fields.
The multi faceted fresnel lens is the most common method of focusing the emmited infrared body heat radiation onto the pyrolectric sensor.
Alternatively, some PIR's are manufactured with internal plastic, segmented parabolic mirrors to focus the infrared energy. Where mirrors are used, the plastic window cover has no Fresnel lenses molded into it. This filtering window may be used to limit the wavelengths to 7-14 micrometers which is closest to the infrared radiation emitted by humans. The PIR can be thought of as a kind of infrared camera which remembers the amount of infrared energy focused on its surface. A person entering the monitored area is detected when the infrared energy emitted from the intruder's body is focused by the Fresnel lens or mirror segment and overlaps a section on the chip which had previously been looking at some much cooler part of the protected area. That portion of the chip is now much warmer than when the intruder wasn't there. As the intruder moves, so does the hot spot on the surface of the chip. This moving hot spot causes the pyrolectric sensor to produce tiny electrical signals that are amplified and anylised by the processor to determine if the signal is a genuine alarm condition or just an environmental change.
PIR detectors have traditionally been hard wired and mounted underneath the camera on the same mast. PIR positioning is limited due to the cost of routing the cables and the resultant civil works. A multi core cable supplies the 12 volts to the detector and takes away the alarm and tamper contacts to the alarm input on the CCTV system. One cable is required for each PIR detector.
Some wired detectors use a connective protocol such as RS485. In this way many detectors can be connected to one multi core cable which is able to carry significantly more information to the CCTV system. The limitations still apply due to the logistics of running cables.
Wireless PIR detectors
With wireless detectors the cable running problem does not exist and the detector may be placed where it will perform best without compromise. It is vital that the radio communication is good and there are product on the market that have radio communication distances of 1000 metres and beyond. Although this distance may seem to be far in excess of what is required it should be noted that the signal only travels in straight lines and will have to bounce (Multipath) of adjacent buildings and objects. Each time this happens the signal will be reduced and therefore it is better to start with a system that has the best range. Some PIR detectors have Repeaters that may be deployed to extend the range and overcome difficult sites where the signal is poor.
Wireless PIR detectors should be Supervised (Pole) which means that they automatically communicated periodically to the receiver which monitors their health and performance. In this way if a detector fails or is disabled it will indicate on the system just as a hard wired detector would.
Wireless PIR detectors are powered by non re-chargeable batteries and must have a very low quiescent operating current to preserve the battery voltage. Some detectors will have a charge pump power supply which maintains the detectors optimum performance even though the batteries are diminishing. It is important that the PIR detector reports when its batteries are running low and this is usually sent as a message attached to the normal poling communications.
Article supplied by Graham Creek of Luminite Electronics Ltd. Click here to view information on Luminate PIR detectors. Please contact us with any questions you have on this subject.