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Intrusion Sensors: PIRS or Dual Technology

ONE key difference between PIRs relates to processing. This can be either standard or microprocessor-controlled, with the latter type claimed by manufacturers to be able to “recognise” the waveform signal output from a pyro in the presence of IRE.

It’s true that well-designed sensors can monitor this waveform, which is a varying nonlinear quantity, recognising the waveform’s amplitude, cycle, frequency and wavelength and comparing what they see to what has been programmed into their memory. And microprocessor controlled sensors are able to achieve this comparison faster than analogue sensors. But bear in mind that the digital component will not make a badly designed PIR better than a well-engineered unit, though it can enhance a good sensor.

Another issue to consider is lens type. There are six primary lens patterns – and these can be applied generally to both fresnel lens types and mirror optics: Long range, dual zone, narrow coverage lenses; Long range, multizone, narrow coverage lenses; Multizone curtain lenses; Broad multizone, wide angle or volumetric and ceiling mount. The two most common lens types are fresnel and mirror optics, with mirror optics offering superior performance. Good fresnel lenses are capable, they’re just not as good as mirror optics.

When buying a PIR with an integral camera don’t compromise on sensor quality. Get the best sensor you can and don’t just buy a bad passive with the expectation that the video component will make a significant difference to the overall sensing ability. Most camera/PIR combos are of dubious quality.

Strengths and weaknesses of PIRs

Some of the advantages of PIRS include low false alarm rates when installed in compatible environments, low power drains, no energy emissions, no moving parts and simple electronics. PIRs have a long detection range, are cheaper than most other technologies, will not interfere with each other when installed in groups and can be adjusted in terms of lens pattern and sensitivity while in the field.

There are a number of disadvantages with the technology, too. For a start increases in ambient temperature will affect a PIR’s range and sensitivity. And it’s possible for intruders to wear a suit or carry a screen that reflects body heat to beat PIR sensing. There’s not the density or uniformity of coverage with PIRs that you get with other technologies like microwave, either. There are dead zones and it’s possible for furniture or pallets of stock to block some sensors’ views – it depends on design (some do have active detection of such blockages).

In addition, PIRs won’t alarm if the sensing element fails – something that beam sensors will do and a PIR sensor’s detection range is limited to 15m x 15m x 50m in wide angle and 50m in narrow beam, or a circular 15m pattern. Other disadvantages include the fact random IRE signals will cause false alarms and there’s a maintenance requirement. Lenses and mirrors need to be kept clean and insects and other pests kept at bay. In dry, dusty environments particles will be electrostatically drawn to the device where they’ll stick and cloud its vision.

Features you want

Features to look for include sensors with dual edge or quad sensing zones and an array with a lot of look-down zones, as well as more than one sensing range and the greatest possible number of discrete zones. Look for surge and low voltage protection, as well as RFI suppression, pyro electric sensing elements, adjustments letting you alter the range or mask zones and LED-supported latching. Walk-test is a valuable feature and a plug-in test meter is an advantage.

Other features to look for include fast-change sensor head, anti-masking (a low-power active infrared transceiver), tantallum capacitor-based RFI and EMI protection, a high signal-to-noise ratio, trouble log capability, auto self-test capability and temperature range of -10C to +50C.

Better PIRs should have silent alarm relay, multi-facted reflectors, low voltage signal, enhanced processing or design characteristics that allow sensitivity to be linked to rate and rise of threshold, duration of zone disturbance and intruder presence in both elements; a tamper contact, site-adjustable sensitivity and first-to-alarm memory.

They’ll also have temperature compensation. This features adjusts sensitivity in line with increases in environmental temperature in the target area taking into account that some locations may approach or exceed the temperature of the human body. When this occurs, an uncompensated sensor may have a detection range of 1-2 metres.

Dual technology sensors

When it comes to more challenging environments most installation teams go for dual technology sensors. They’re tough enough to handle sunlit windows and strong air currents and they’ll pick up small movements. Dual technology sensors combine a pair of technologies that are sensitive to 2 different types of disturbance – microwave and passive infrared – PIR. The thinking behind this is to ensure that each sensor supports the weaknesses of the other to eliminate false alarms.

It works thanks to the intrinsic nature of each of these two sensing technologies. PIR elements sense the level of IRE changing between zones over a set time at a set speed. There aren’t many environmental disturbances that mirror this sort of activity – but heat sources, especially warm air currents, spell trouble for PIRs.

Bear in mind with PIRs that the reflected IRE signals their pyro elements receive are minute. The low signal level means that amplification and filtration processes must be well engineered in order to achieve quality signals and this necessary processing slows response times as well as limiting coverage.

Microwave sensors

Microwaves work differently. They cover an area with a signal and then pick up variations between the signal sent and the signal reflected back. It’s called the Doppler shift. Microwave sensor technologies offer strong detection performance – they’ll pretty much detect anything that moves.

In terms of dual technology what’s vital is that PIRs are sensitive to movement across their zones, while microwave devices activate if they pick up a Doppler shift that moves either towards the sensor or away from it. Another issue with dual technology gear is each sensor can be tuned up more than would be possible if the sensors had to stand-alone.

Expect to pay from $30-100 for a good dual technology sensor bearing in mind there will be variations in price depending on things like lens design (mirror optic/fresnel), range, signal processing, active IRE and range gating. External units can be more expensive but remember that in all cases if a quality sensor saves you one trip across town the purchase price has been well and truly redeemed.

Fact file

When buying PIRs look for:

* Zone pattern and range
* Latching LED
* Window type
* Fresnel or mirror lens
* Recess options
* Walk test capability
* Mounting or tilting brackets
* Factory test EMI/RFI resistance
* Operating voltage, current draw
* Minimum operating temperature
* Modular electronics
* Anti-tamper, anti-mask
* Preferred mounting height
* Optimum operating temperature
* Mounting height.

Advantages of dual technology PIR/microwave sensors:

* Lower false alarm rate if environment changes
* Sensitive to all kinds of motion
* Can be used in hot, windy, changeful sites
* Normally includes features that are only
found in the best single tech sensors
* Sensitive movement in 2 directions.
* Still have detection capability if one sensor fails.
Disadvantages of dual technology sensors
* More expensive that PIRs
* Reduces overall probability of detection
compared to PIRs in their perfect environment
* Significantly greater complexity means
quality control is even more important.


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