Inductive proximity sensors are noncontact proximity devices that set up a radio frequency field with an oscillator and a coil. The presence of an object alters this field and the sensor is able to detect this alteration. An inductive proximity sensor comprises an LC oscillating circuit, a signal evaluator, and a switching amplifier. The coil of this oscillating circuit generates a high-frequency electromagnetic alternating field. This field is emitted at the sensing face of the sensor. If a metallic object (switching trigger) nears the sensing face, eddy currents are generated. The resultant losses draw energy from the oscillating circuit and reduce the oscillations. The signal evaluator behind the LC oscillating circuit converts this information into a clear signal. The measurement of proximity, position and displacement of objects is essential in many different applications: valve position, level detection, process control, machine control, security, etc.
The body style of inductive proximity sensors can be barrel, limit switch, rectangular, slot, or ring. A barrel body style is cylindrical in shape, typically threaded. A limit switch body style is similar in appearance to a contact limit switch. The sensor is separated from the switching mechanism and provides a limit of travel detection signal. A rectangular or block body style is a one piece rectangular or block shaped sensor. A slot style body is designed to detect the presence of a vane or tab as it passes through a sensing slot, or "U" channel. A ring shaped body style is a "doughnut" shaped sensor, where the object passes through center of ring. Electrical connections for proximity sensors, inductive 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 proximity sensors, inductive include operating distance, repeatability, field adjustable, and minimum target distance. 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. Field adjustable sensors can be adjustable while in use. Depending on the sensor's technology, there can be minimum target size requirements.
Load configurations are also important parameters to consider. Inductive 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). Switch specifications include whether or not the switch is normally open or normally closed. Switch repeatability and maximum switching frequency are important parameters to consider. The switch may be a NAMUR type switch, a specialized switch for switching a resistive load. Requires an external amplifier.
Other important parameters to consider when specifying inductive proximity sensors include power requirements, housing materials, dimensions, special features, and environmental operating conditions.