Image Credit: Grainger Industrial Supply | Carlo Gavazzi | TURCK Inc.

 

Inductive proximity sensors are non-contact proximity devices that set up a radio frequency field with an oscillator and a coil. The presence of an object can alters this field and the sensor is able to detect this alteration.

 

Sensor Composition

An inductive proximity sensor is comprised of 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 or magnetic object (commonly called the target) 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.

 

Basic Functionality of Inductive Proximity Sensors

Video Credit: Balluff

 

 

 

Sensor Specifications

Performance Criteria

 

 

 

 

Depending on the sensor there can be minimum target size requirements.

 

  • 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 option.
  • 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. 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.

 

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.

 

 

Body Type

Inductive proximity sensors are typically housed in either a barrel or rectangular housing, although limit switch, slotted or "U" channel switches and ring or "doughnut" configurations are also available.

Threaded barrel and rectangular housing. Image Credit: OMEGA Engineering, Inc. | Baumer Ltd.

 

  • 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 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 objects pass through the center of the ring. 

 

Electrical Connections

Electrical connections for inductive 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.

 

Features

There are a number of features that may be important to consider when selecting inductive 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. 

 

Resources

 

Fargo Controls, Inc. - Inductive Sensor Operating Principles

 

Steven Engineering - Theory of Operation...Inductive Proximity Sensor


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