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Motion detector
Monday, 16 October 2006

A motion detector is an electronic device that detects the physical movement in a given area and transforms motion into an electric signal. The motion detector may be electrically connected to devices such as security, lighting, audio alarms, and the like. Motion sensors are used in a wide variety of applications and as a result many different types of motion sensors are available. Motion detectors are mainly used in for security systems. For example, motion detectors are typically positioned near exterior doorways or windows of a building for monitoring the area around the building. Upon detecting motion, they generate an electrical signal that is transmitted to a preselected audible alarm or lighting device which is then activated. A number of different technologies are used for motion detection. Motion detectors typically employ ultrasound, passive infrared (PIR), or radar detection techniques. Among the most used and most effective type of intrusion detection devices are the passive IR detectors which in recent years have replaced other types such as microwave or ultrasonic detectors.

Infrared (IR) detectors are widely known in the arts of intrusion detection and in fire/smoke detection. These detectors have basically two forms: active and passive. Active IR detectors include a radiation source and an IR sensor which is sensitive to interruptions in the radiation sensed from the source. These detectors are used as intrusion detectors by providing a path of radiation from the source to the sensor in a place where the path is likely to be interrupted by an intruder. A passive infrared motion detection system detects heat energy radiated or emitted by an object, such as a body of a person, moving across a field of view of a heat sensor, such as a pyroelectric detector, of the motion detection system. Passive infrared motion detectors generally use an optical collection system and multiple sensing elements of alternating polarity to create a detection pattern in the volume of interest. Passive IR detectors generally employ a group of radiation sensors coupled through amplifiers to a logic circuit. The radiation sensors detect changes in ambient infrared radiation. The detection system has an electrical circuit operatively coupled to the heat sensor for producing a detection signal in response to the heat sensor detecting a change of temperature as, for example, caused by the body heat of a person entering the detection pattern. Passive infrared motion detectors and occupancy sensors employ an array of Fresnel lenses covering an entrance aperture. This lens array is illuminated by thermal infrared radiation from the object of interest.

PIR motion detectors are perhaps the most frequently used home security device. Passive IR motion detectors are usually designed to provide an indication to an alarm panel in response to detecting IR that is indicative of motion of the object. The alarm panel is responsive to receipt of the breach indication to cause an alarm condition to occur. Infrared motion detector devices are often used in automatic light switches and security systems to turn on a light or to activate some other form of alarm or warning indicator when a person or motor vehicle enters a monitored area. PIR motion detectors are commonly used in conjunction with outdoor light fixtures and indoor light switches to turn on a light in response to a person moving in the field of view monitored by the motion detector. The motion detector typically causes the light to remain on for a pre-set duration after motion has been detected. A typical motion detector responds to the movement of humans by generating a small electrical charge that is further conditioned to switch on electrical current to a lamp within the electrical light assembly that is to be activated by the motion detector. Activation of electrical lamp based on the detection of human motion is commonly used as a security arrangement. In a large number of installations, this arrangement is also used to conserve electricity. Electrical power remains available to the lighting fixture at all times, but it is the motion and ambient light sensors and associated logic circuits which determine if and when to turn the lights on. Passive IR sensors are also used as fire detectors. When used as fire detectors, the passive IR sensors are coupled to a circuit which monitors the rate of increase in ambient temperature. If the rate of increase in ambient temperature is above a certain preset value, a fire warning is activated.

Radar-based motion detectors may emit a continuous-wave (CW) microwave signal and compare the emitted and echo frequencies to produce a beat frequency that is proportional to range. Such microwave sensors for detecting the presence of moving objects, and the direction of movement, are found in a variety of applications such as door openers, security systems, and toilet flushers. Such detectors typically comprise a high frequency RF circuit comprising an oscillator for generating a high frequency oscillating signal, an antenna or antennas for transmitting and receiving the oscillating signal, and a mixer for modulating the oscillator signal with the received signal. In order to determine the direction of motion of an object, the oscillator signal is mixed with the receive signal to generate two output intermediate frequencies "IF", the combination of which determines the direction of movement of an object toward or away from the sensor. Doppler shift motion detectors are active motion detectors in which a wave transmitter transmits waves into a monitoring area, and then a wave receiver receives the reflected waves and produces a reception signal. By detecting the Doppler shift in the reflected signal, the detector circuitry detects whether a moving object is present in the area. Typical applications include intruder alarms and automatic door openers, for which a unit of conventional construction comprises a gunn diode oscillator mounted in a cavity resonator such as a simple waveguide tube. However, Doppler radar-based motion detectors are disadvantageous because of limited materials penetration, microphonics, frequency crowding, and poor short-range operation.

Ultrasonic motion detectors project and receive ultrasonic sound energy in a region of interest. Object motion within the region of interest and in the range of the ultrasonic motion sensor is detected and an alarm signal representative thereof is produced. The effective range of ultrasonic motion detectors differs from design range whenever the actual ambient atmospheric sound propogation conditions vary from the design or nominal atmospheric conditions. Ultrasonic motion detectors are commonly used for automatic door openers and security alarms. They are inexpensive and can operate with narrow beamwidths. However, installation options are limited because ultrasonic beams are easily blocked by thin materials, including paper. False triggering is easly caused by reflections from blowing curtains, pets, and flying insects. Some motion detectors operate on the principle of relative movement between a conductor and a magnetic field resulting in a current being induced in the conductor. Such induction-type sensors generally include a magnetic circuit with a permanent magnet, the magnetic circuit comprising a stator, a rotor and an electrical coil devised around the stator.