Magnetic proximity sensors are noncontact proximity devices utilize inductance, Hall effect principles, variable reluctance or magneto resistive technology. Magnetic proximity sensors are characterized by the possibility of large switching distances, available from sensors with small dimensions. They detect magnetic objects (usually permanent magnets), which are used to trigger the switching process. As the magnetic fields are able to pass through many non-magnetic materials, the switching process can also be triggered without the need for direct exposure to the target object. By using magnetic conductors (e.g., iron), the magnetic field can be transmitted over greater distances so that, for example, the signal can be carried away from high temperature areas. 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. Proximity sensing is the technique of detecting the presence or absence of an object using a critical distance. A position sensor determines an object's coordinates (linear or angular) with respect to a reference, displacement means moving from one position to another for a specified distance (or angle). In effect, a proximity sensor is a threshold version of a position sensor.
The body style of proximity sensors, magnetic 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 object passes through center of ring. Electrical connections for proximity sensors, magnetic 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 magnetic proximity sensors 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. Proximity sensors, magnetic 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 magnetic proximity sensors include power requirements, housing materials, dimensions, special features, and environmental operating conditions.