Image Credit: ABB Cylindrical sensor. Image Credit: Honeywell
Eddy current proximity sensors and switches detect the proximity or presence of a target by sensing fluctuations in a magnetic field generated by a reference coil. The variations in this field are 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 the presence of a target, or can be configured to measure the position or displacement of a target.
Eddy Currents and Lenz's Law
Video Credit: National High Magnetic Field Laboratory
Eddy current proximity sensors can detect the position of conductive objects with high resolution. A coil at the face of the sensor is used to generate an alternating magnetic field. As a conductive object approaches the magnetic field, eddy currents are induced in the target object. In opposition to the magnetic field at the face of the sensor, the induced eddy currents generate an opposing magnetic field that draws its potential from the sensor. This destructive interference is used to calculate the position of the target. Since the signal response to the position of the target is highly linear, true eddy current sensors are typically used for the measurement of linear position, while simpler inductive sensors are typically used for proximity switches.
Eddy current model. Image Credit: Lion Precision
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.
- Rated operating distance is the critical distance at which switching takes place. It is important to choose a sensor that will operate in the required sensing range. This could be dictated by process requirements and mounting options.
- Repeatability is the distance within which the sensor repeatably switches. It is a measure of precision. Depending on the application, precision could be an important design criterion when selecting a sensor.
Electrical Switch Performance
Depending on the power options available, power requirements may be a key specification when selecting a sensor.
The device can be powered by either an AC or DC power source.
Load configurations are important parameters to consider. Eddy Current 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.
2-Wire DC circuitry typically includes a diode bridge in series with the sensor, allowing functionality either as NPN or PNP in a DC circuit.
Acceptable values of leakage current must be evaluated in this type of circuit
Switch types can be normally open (NO) or normally closed (NC).
Image Credit: Fargo Controls, Inc.
If the switch is a NAMUR type switch or a specialized switch for switching a resistive load. It will require an external amplifier.
Eddy current proximity sensors are typically housed in either a barrel or rectangular housing.
Threaded & non-threaded barrel
Image Credit: Panasonic Electric Works
A barrel body style is cylindrical in shape. The barrel of the sensor is typically threaded so that with the help of two lock-nuts the sensor can easily be adjusted closer or further from the target as necessary.
A rectangular or block body style is a one piece rectangular or block shaped sensor.
Electrical connections for eddy current proximity sensors can be fixed cable, connector(s), and terminals.
A fixed cable is an integral part of the 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.
There are a number of features that may be important to consider when selecting eddy current proximity sensors.
- Field adjustable sensors are required when adjustments need to be made while the proximity sensor is in use. Potentiometers are commonly used to control the magnetic field strength or detection range.
Self-teaching proximity sensors can be auto-calibrated to trigger the switch at a specific target location.
Materials of construction and enclosure ratings must be considered when the sensor will be located outdoors or exposed to extreme temperatures, or under the influence of wet, humid, dusty, dirty or corrosive process conditions.
|Enclosure ratings are specified by "National Electrical Manufacturers Association (NEMA)" or "Ingress Protection (IP)" ratings.|
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