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.
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.
Fresnel Lens
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.
Hard Wired
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. |
