Capacitive Proximity Detectors
Although proximity detectors are generally equated with metal detectors, there are other types that sense the presence of objects that do not contain metal of any kind. One type of these detectors operates on a change of capacitance. When an object is brought into the proximity of one of these detectors, a change of capacitance causes the detector to activate. Several different types of capacitive proximity detectors are shown in Figure 16 – 7.
Since capacitive proximity detectors do not depend on metal to operate, they will sense virtually any material such as wood, glass, concrete, plastic, and sheet rock. They can even be used to sense liquid levels through a sight glass. One disadvantage of capacitive proximity detectors is that they have a very limited range. Most cannot sense objects over approximately one inch or 25 millimeters away. Many capacitive prox- imity detectors are being used to replace mechanical limit switches since they do not have to make contact with an object to sense its position. Most can be operated with a wide range of voltages such as 2 to 250 volts AC, or 20 to 320 volts DC.