Homeland Security

PIR Detectors for Security

April 1, 2003 By: Andreas Hartmann, ASIM Technologies, Inc.


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PIR devices can detect a person moving into or through a detection zone with high reliability. The slightest positive or negative thermal radiation change in contrast to a background, focused by the appropriate optics, triggers the sensor element. There is no interference between neighboring units due to the passive nature of the detection principle. The use of differential or dual-channel sensor technology and advanced digital signal processing reduce false alarms caused by turbulence. Similarly, precision optics accurately define the field of view, allowing consistent and long-range coverage.

The Sensor

At the heart of every PIR detector is the pyroelectric sensor. Typical sensors use pyroelectric materials, such as tri-glycine sulphate (TGS) or lithium tantalite, which undergo a change in polarization if the temperature of the material is altered (in the case of PIR detectors by thermal radiation). If electrodes are placed on opposite faces of a thin slice of the material to form a capacitor, the change in polarization can be observed as an induced voltage across the slice if the external impedance is comparatively high. The sensor will only produce an electrical output signal when the temperature of the incident radiation changes. A differential sensor uses

Figure 1.

Curie point Tc: Materials lose their magnetic properties when raised past a certain temperature called the Curie point.

two elements of opposite polarity so that equal radiation changes on both elements are compensated to zero output. Both TGS and lithium tantalite exhibit a large spontaneous electrical polarization below Tc, known as the Curie point (see Figure 1). Lithium tantalite is more frequently used, thanks to its large pyroelectric coefficient and excellent chemical stability.

Spectral Sensitivity. The spectral response of pyroelectric sensors is limited to the far IR spectrum—wavelengths longer than the near IR of LEDs and shorter than microwaves. The wavelength of the incident radiation is not a determining factor because pyroelectric sensors have a flat response over a very wide spectral range. The spectral range is limited by the window material used by the manufacturer of the sensor component. Most off-the-shelf pyroelectric sensors limit the spectral response to all or parts of a range between 0.15 and 20 µm. The radiation of the human body is strongest between 8 and 14 µm. The goal of every filter is to make the sensor immune to unwanted radiation and pass a desired band, both of which are defined by the sensor's target application. Sensing

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Figure 2. Pyroelectric sensors have a band-pass type of response. The graph shows the current and voltage step-response of a sensor. Pass band centers around 1 Hz.

elements usually have built-in impedance converters or pre-amplifiers to lower the number of external components and minimize the costs of the circuitry.

Response Characteristics. Pyroelectric sensors have a bandpass type of response (see Figure 2), with the response rising at 20 dB per decade at low frequencies until reaching the peak output at 1 Hz. At higher frequencies the response falls at 20 dB per decade. Each pyroelectric sensor is designed to operate over a particular range of frequencies. Manufacturing techniques can move the average peak output at 1 Hz slightly, shifting the sensor's target response rate (the peak within the passband) so as not to coincide with the noise frequency. Typical outdoor detectors, for example, respond to objects moving between 0.2 and 5 m/s. Note that the electrical response characteristics are unrelated to the spectral sensitivity of the sensor.

The Optics

The optics come in various forms and shapes, and they focus the incoming radiation from the area that the detector covers. The longer the distance a detector covers, the higher the quality of the optics necessary.

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Figure 3. A Fresnel lens is a plano convex lens that has been collapsed on itself to form a flat lens. It retains the optical characteristics, yet it is flexible enough to serve as detector window.

Fresnel Lens. A Fresnel lens is a plano convex lens that has been collapsed on itself to form a flat lens while retaining its optical characteristics (see Figure 3). The resulting lens is thinner and has lower absorption losses, and it is the most extensively used lens for a wide range of detectors. The white plastic window often associated with motion detectors is, in fact, the flat side of a Fresnel lens. Among its advantages, this type of lens offers low cost and eliminates the need for a separate window for most applications. Detectors using Fresnel lenses are normally short- to medium-range devices, detecting objects up to about 30 m/100 ft. away.

Mirror Optics. Long-range and high-precision devices often use mirrors similar to those found in telescopes that bundle and focus the incident radiation onto the sensor elements. These devices commonly feature a more precise detection zone and allow for detection of objects of up to 150 m/500 ft. away. Detectors using such mirrors need a separate window to protect the device against wind, dust, and other elements. The choice of the window material is important for detectors used in harsh environments to minimize maintenance. In addition, very few materials transmit the desired 8–14 µm radiation, adding to the cost of high-quality detectors.

Additional Detector Features

Common features of detectors include tamper switches and signal processing. Tamper switches are fitted into the housings and have a contact that opens when the detector housing is opened. If no separate tamper line is available, you can connect the contact in series with the detector's normally closed relay contact used to signal the alarm. Some units have logic that can distinguish between recurring patterns of radiation and nonrecurring patterns to minimize the number of false alarms while maintaining high sensitivity.

Special Features of Advanced PIR Detectors

Adaptive Threshold Discrimination. To optimally adjust the sensitivity to the level of background noise, the device constantly averages the background noise and adjusts the threshold levels for the alarm. This reduces the probability of nuisance alarms caused by wind, moving vegetation, or objects that produce a thermal contrast, even though the latter usually produce a thermal contrast weaker than that emitted by a human body.

Advanced Signal Shape Analysis. Precision detectors include logic analysis and signal processing, such as rate of rise and time windows between channels. Detectors equipped with multiple sensors that cover multiple areas independently often feature enhanced signal processing to further reduce the incidence of false alarms and increase sensitivity and reliability.

Internal Heating. Detectors that use mirrors instead of Fresnel lenses need heating elements for the optics to prevent steam buildup, especially if the devices are deployed in areas with high humidity or other rough environmental conditions.

Temperature Compensation. Contrast conditions can vary significantly in the course of the day and year. Temperature compensation maintains the detection and nuisance alarm properties under all possible conditions in an outdoor environment.

PIR Detectors for High Security

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Figure 4. Because PIR sensors are very sensitive, capable of detecting tiny thermal changes in contrast to a stationary background, they're frequently used in security applications to detect motion. For perimeter security, they're often paired with cameras and video motion detectors (VMDs); the PIR detects something moving and the camera or VMD then zeroes in on the area of the alarm.

In high-security applications, PIR detectors (see Figure 4) are often used to activate video motion detectors or focus closed-circuit TV cameras on areas where the motion detectors have triggered alarms. PIR detectors are not ideal as stand-alone components in such applications because manufacturers often do not guarantee the absence of false alarms. However, the detectors, unlike video-only surveillance systems, are not affected by the light level and work equally well day and night. They are also less affected by fog, as the longer IR waves travel about 50% further than the visible light, making PIR detectors good additions to outdoor perimeter control in harsh environments and poor weather conditions.

The presence of cameras lets you see if something is happening in the area where an alarm has occurred. For perimeter control, pan-tilt cameras (PTCs) are controlled by PIR detectors to minimize the number of comparatively expensive cameras used and/or to minimize the movement of cameras for no reason, lessening wear and tear on the mechanism that moves the camera.

Manufacturers are combining technologies and features to appeal to the large home security/consumer market. In addition to detectors that combine video and PIR in a single housing, there are devices that offer wireless transmission over distances up to 1000 ft. There are also high-end detectors that can be accessed via common bus protocols, such as RS-485. These allow easy integration into surveillance systems when used with PTCs and provide more detailed information about where and how far from a detector an alarm has occurred.

 
PIR Detector Pros and Cons

PROS

CONS

• The system is single ended, unlike light beam/laser barriers, which require aligned transmitters and receivers.

• The system is passive: Intruders can't detect the presence and location of the detectors (unlike radar systems).

• The detectors consume less power than IR- or radar-based units.

• They are unaffected by light, with the detectors working equally well day or night.

• Precision optics enable detectors to cover narrow areas accurately.

• The detectors complement cameras, allowing fewer cameras to cover the area.

• Small size and unobtrusive design help the detectors blend in with their surroundings.


• Can be fooled by someone moving especially fast or slow.

• Doesn't work well when the ambient temperature is 7°C below body temperature (30°C), as the detector reacts to the contrast between moving objects and a stationary background.


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