Eddy Current Proximity Sensors

Eddy current proximity sensors and switches detect the proximity or presence of a target by sensing the magnetic fields generated by a reference coil. They also measure variations in the field due to the presence of nearby conductive objects. Field generation and detection information is provided in the kHz to MHz range.  They can be used as proximity sensors to detect presence of a target, or can be configured to measure the position or displacement of a target.

Proximity sensors, eddy current 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 eddy current proximity sensors are barrel, limit switch, rectangular, slot style, and ring.

Target materials for eddy current proximity sensors can be magnetic, non-magnetic, ferrous, and non-ferrous. Magnetic target materials are magnetized, usually with a permanent magnet component.  Non-magnetic detection targets do not require magnetization.  Ferrous targets for position detection include iron or iron-based materials such as steel, stainless steel, etc.  Non-ferrous target materials are metallic but are not iron or steel-based, such as aluminum, brass, and copper.  Electrical connections for proximity sensors, eddy current 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. 

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 or NAMUR.  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).  A NAMUR-style proximity switch is a 2-wire DC sensor that operates at an intrinsically safe level of 8.2 V and current of 8 mA with on-off switch points operating between 1.2 to 2.1 mA.  NAMUR switches require a remote amplifier for operation and are used in applications where low signal levels are required for safety.

Other important parameters to consider include switched output types, position or distance output type, housing materials, dimensions, and environmental operating parameters.

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