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Wednesday, September 21, 2011

PROXIMITY SENSORs


PROXIMITY SENSOR
A proximity sensor detects an object when the object approaches within the detection boundary of the sensor. Proximity sensors are used in various facets of manufacturing for detecting the approach of metal objects. 
Various types of proximity sensors are used for detecting the presence or absence of an object. The design of a proximity sensor can be based on a number of principles of operation, some examples include: variable reluctance, eddy current loss, saturated core, and Hall Effect. Depending on the principle of operation, each type of sensor will have different performance levels for sensing different types of objects. 

Common types of non-contact proximity sensors include inductive proximity sensors, capacitive proximity sensors, ultrasonic proximity sensors, and photoelectric sensors. Hall-effect sensorsdetect a change in a polarity of a magnetic field. 
Variable reluctance sensors typically include a U-type core and coils wound around the core legs. Inductive proximity sensors have a lossy resonant circuit (oscillator) at the input side whose loss resistance can be changed by the proximity of an electrically conductive medium. 
An electrical capacitance proximity sensor converts a variation in electrostatic capacitance between a detecting electrode and a ground electrode caused by approaching the nearby object into a variation in an oscillation frequency, transforms or linearizes the oscillation frequency into a direct current voltage, and compares the direct current voltage with a predetermined threshold value to detect the nearby object. 

Ultrasonic sensing systems provide a much more efficient and effective method of longer range detection. These sensors require the use of a transducer to produce ultrasonic signals. 
Eddy-current proximity sensors are well known and operate on the principle that the impedance of an ac-excited electrical coil is subject to change as the coil is brought in close proximity to a metallic object. 

Proximity sensors often are employed in manufacturing industries in which the sensors are exposed to harsh environmental conditions. Inductive proximity sensors are used in automation engineering to define operating states in automating plants, production systems and process engineering plants.
Magnetic proximity detectors are commonly used on ski lifts and tramways for detecting a derope condition of the steel cable used as a haul line or haul rope. 
Proximity sensors are widely used in the automotive industry to automate the control of power accessories. For instance, proximity sensors are often used in power window controllers to detect the presence of obstructions in the window frame when the window pane is being directed to the closed position.

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