PRINCIPAL DETECTION ROLES
These roles can be classified broadly as external, perimeter or internal although there is often an overlap of techniques and duties and certain detectors will incorporate more than a single detection type to provide a dual technology approach.
External
These are often used to warn of an approach and to give short commands for CCTV/video starts and to switch lighting. Beam interruption devices such as, active infra red or microwave fall into this category and are the most widely used. Active infra reds comprise of a transmitter and receiver in a direct line and operate by using a signal break technique caused by an intruder interrupting the infra red signal. Microwave sensors function differently. They use their receiver to compare the received microwave energy in terms of wave form, amplitude or magnitude before causing an alarm signal.
Perimeter
For openings, this involves detectors such as magnetic reed switches or tilt contacts. As a perimeter device for the protection of openings such as doors and windows, the reed switch remains popular because it comes in a huge range of guises to cover low security doors through to heavy roller shutters. Tilt contacts are more specialised in their duty as a tip over device, being dependant on the switch's orientation relative to gravity.
For solid structures subject to attacking, forcing or breaking we use sensors such as inertia/vibration/breakglass or continuous wiring/foil on glass with programming for full alarm with the system set or configured to give 24 hour protection. Inertia/vibration/breakglass detectors have, in fact, largely taken over the role of continuous wiring/foil on glass and are used to respond to impact, shock or the sound frequency of breaking glass. These use either a piezo electric element that generates electricity by molecular vibration or they have an electromechanical mechanism with an integral or remote analyser to measure shock magnitude. Versions of this are used as the switching element in safe limpets. We also have the definitive glassbreak detector that uses an acoustic technique by reacting to the frequency of breaking glass or patterns of it.
Internal
Sensors for this duty give space or volumetric protection and in this we include PIR, ultrasonic, microwave, dual technology and capacitive (close proximity or touch) plus acoustic detectors.
The installer was by this time in a position of being able to choose from a vast array of low power consumption PIRs at a competitive price with a lens and coverage pattern to suit almost any duty.
For internal space protection the once popular ultrasonic device, using the doppler effect or apparent shift in frequency which occurs when energy waves are emitted by a moving object, is now rarely used and is in the main replaced by the passive infra red PIR.
The PIR commands the greatest attention as a detector and forms the nucleus of most intruder alarm systems so we will look at PIRs of today together with dual technology detectors in some depth a little later. The microwave detectors for internal duty tend to use the doppler effect by means of radio waves (that are considerably shorter than that of the ultrasonic) in preference to beam interruption technology. They are found in mid risk security applications and the protection of large open spaces, but they are losing favour to new generation PIRs.
Capacitive detectors respond to a change in capacitance to give volumetric or small area protection but because of their restricted target range they have limited appeal. Acoustic detectors are rather specialised being high security devices used only in rooms with hard surfaces such as bank vaults. They adopt a microphone, filter and integrator as their detection element to recognise intrusion sound and are generally supported by PIRs in the same area.
PIR DETECTORS
Passive infra reds are big business in electronic security. They are well established, used in high volume and can be easily understood in their use and mode of operation.
PIRs are 'passive' in the sense that they do not emit energy but are receivers of far-IR rays. The optical window of a detector captures IR and focuses it onto pyroelectric sensor elements that absorb the IR and transfer it into heat. When the amount of IR energy they receive alters, the elements themselves change temperature with the heating and cooling of the sensor and are then analysed by the detector in order to determine its signal response. The practice is to incorporate dual opposed sensors because these combine two elements each with opposing charges within one detector. An infrared source must then appear in an active zone on one section and then appear on the other within a given time scale. By using a time window the alarm relay will only activate to a number of given trigger strength signals.
This technology overcame the false alarm problem experienced with early generation PIRs that had single pyros which responded to any change in IR levels. Further developments brought about the introduction of quads with two dual output pyros fed to a signal processing unit. These will respond only to two signals from the quad system exceeding a predetermined threshold.
At the same time that the passive was gaining in popularity, attention was being paid to other general factors that have always been the cause of false alarms. These actors include radio frequency interference, electromagnetic interference, white light and high temperatures.
PIR's also became equipped with additional anti-false alarm features including dual slope temperature compensation, air draft protection, power supply transient filtering and sealed optical chambers to resist the ingress of dust and insects.
1 can personally confirm that the modern day PIR from the reputable manufacturer is inherently reliable and that the dual technology detector is virtually false alarm free.
The actual signal processing techniques that are now used by manufacturers to counter false alarms vary enormously and allow, in most cases, for selectable pulse counts and levels of sensitivity to be set by the installer on site.
The early days had seen the introduction of PIR's that gave either volumetric, curtain, corridor or pet alley coverage patterns. Progression then introduced options such as ceiling mount brackets and lens libraries, creep zones, tilt down lenses and anti masking protecting against spray paint attacks or deliberate lens blocking.
We also found 360 deg ceiling mount units and easily adjusted lenses for range and coverage together with blanking components to blind fingers of the pattern looking at false alarm sources. More sophisticated options such as built in microphones for event verification also appeared on the scene. The pet alley unit lens then lost some of its market to the intelligent pet immune device that can distinguish between humans and animals.
The installer was by this time in a position of being able to choose from a vast array of low power consumption PIRs at a competitive price with a lens and coverage pattern to suit almost any duty. Warranty periods became attractive and specialised units could be purchased if there was a need for latching or trouble signalling.
However, despite the excellent performance and reliability level of the PIR, the dual technology detector still claims to offer even greater resistance to false alarm triggering if selected and sited correctly.
A DUAL APPROACH
Dual technology, or combined detectors, incorporate two different technologies within a single head to negate environmental problems and confirm intrusion. The combined techniques respond to different forms of stimuli so an alarm condition can only occur in response to stimulus to which both detection techniques will react simultaneously. Combined breakglass/PIR's have been available for some time but the greatest inroads, are being made by the dual technology PIR/microwave for space protection. This has happened to such an extent that the term dual technology now infers a PIR/microwave sensor.
These are truly capable of nuisance free operation in unstable environments including areas that have problems with humidity. Such devices are proving excellent value for money and provide commercial grade performance in small, attractive packages. They have adopted particular band wavelengths using smaller microwave sources enabling these to be used alongside the PIR element in modem slim housings. They have opened a route into the domestic market because of their new attractive aesthetics and there is a huge range of options being made available to satisfy almost any duty including even greater pet immunity.
IN CONCLUSION
Competing manufacturers will always continue to promote their concepts, signal processing and options in different ways, but 1 can personally confirm that the modern day PIR from the reputable manufacturer is inherently reliable and that the dual technology detector is virtually false alarm free.
Source
Security Installer
