Detectors for Security

PIRDetectors for Security
Day or night, PIR devices detect motion by sensing any thermal contrast between a moving object or person and a stationary background. As a result, they抮e being incorporated into a growing number of security devices.
Andreas Hartmann, ASIM Technologies, Inc.

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 SensorAt 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 t齟 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 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梬avelengths 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 祄. The radiation of the human body is strongest between 8 and 14 祄. 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抯 target application. Sensing 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抯 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. emergency button Wireless Alarm System
The OpticsThe 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
PIR detector