Capacitive proximity sensors use 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.
Proximity sensors, capacitive 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. Common body styles for capacitive proximity sensors are barrel, limit switch, rectangular, slot style, and ring.
Electrical connections for proximity sensors, capacitive can be fixed cable, connector(s), and terminals. A fixed cable is an integral part of sensor and often includes "bare" stripped leads. A sensor with connectors has an integral connector for attaching into an existing system. A sensor with terminals has the ability to screw or clamp down. Important specifications for capacitive proximity sensors include operating distance, repeatability, and switching frequency. Rated operating distance is the distance at which switching takes place. Repeatability is the distance within which the sensor repeatably switches. It is a measure of precision. The switching frequency is the frequency at which the switch may be turned on and off.
Load configurations are also important parameters to consider. Proximity sensors may switch an AC load or a DC load. DC load configurations can be NPN or PNP. NPN is a transistor output that switches the common or negative voltage to the load; load connected between sensor output and positive voltage supply. PNP is a transistor output that switches the positive voltage to the load; load connected between sensor output and voltage supply common or negative. Wire configurations are 2-wire, 3-wire NPN, 3-wire PNP, 4-wire NPN, and 4-wire PNP. Switch types can be normally open (NO) or normally closed (NC).
Other important parameters to consider when specifying capacitive proximity sensors include housing materials, dimensions, whether or not the sensor is shielded and intrinsically safe, and environmental operating condition parameters.