Proximity sensors may be of the contact or non-contact type. Contact proximity sensors are the least expensive. Proximity sensors can have one of many technology types. These include capacitive, eddy current, inductive, photoelectric, ultrasonic, and Hall effect. Capacitive proximity sensors utilize the face or surface of the sensor as one plate of a capacitor, and the surface of a conductive or dielectric target object as the other. The capacitance varies inversely with the distance between capacitor plates in this arrangement, and a certain value can be set to trigger target detection. In an eddy current proximity sensor electrical currents are generated in a conductive material by an induced magnetic field. Interruptions in the flow of the electric currents (eddy currents), which are caused by imperfections or changes in a material's conductive properties, will cause changes in the induced magnetic field. These changes, when detected, indicate the presence of change in the test object. Magnetic inductive devices are identical in configuration to the variable reluctance type and generate the same type of signal. However, inductive pickoff coils have no internal permanent magnet and rely on external magnetic field fluctuations, such as a rotating permanent magnet, in order to generate signal pulse. Photoelectric devices are used to detect various materials at long range, using a beam of light. They detect either the presence or absence of light and use this information to read the data from the output transistor. An ultrasonic proximity sensor emits an ultrasonic pulse, which is reflected by surface and returned to sensor. Speed can be determined by measuring frequency difference (Doppler Effect). The basic "Hall Effect" sensing element is a semiconductor device which, when electrical current is sent through it, will generate an electrical voltage proportional to the magnitude of a magnetic field flowing perpendicular to the surface of the semiconductor.
The most important parameter to consider when specifying proximity sensors is the operating distance. This is the rated operating distance is the distance at which switching takes place. Common body styles for proximity sensors are barrel, limit switch, rectangular, slot style, and ring. Important dimensions to consider when specifying proximity sensors include barrel diameter, length, width, and height.
Proximity sensors can be a sensor element or chip, a sensor or transducer, an instrument or meter, a gauge or indicator, a recorder or totalizer, and a controller. A sensor element or chip denotes a "raw" device such as a strain gage, or one with no integral signal conditioning or packaging. A sensor or transducer is a more complex device with packaging and/or signal conditioning that is powered and provides an output such a dc voltage, a 4-20mA current loop, etc. An instrument or meter is a self-contained unit that provides an output such as a display locally at or near the device. Typically also includes signal processing and/or conditioning. A gauge or indicator is a device that has a (usually analog) display and no electronic output such as a tension gage. A recorder or totalizer is an instrument that records, totalizes, or tracks force measurement over time. Includes simple datalogging capability or advanced features such as mathematical functions, graphing, etc.