Image Courtesy: Phoenix America, Inc.   Image Courtesy: Fargo Controls, Inc.       Image Courtesy: EMX Industries, Inc

 

Image Credit: Phoenix America, Inc. / Fargo Controls, Inc. / EMX Industries, Inc.

 

Proximity sensing is the technique of detecting the presence or absence of an object using a critical distance. Magnetic proximity sensors are non-contact proximity devices that are used to detect magnetic objects (e.g. permanent magnets). They sense the presence of a magnetic object, commonly referred to as the target. The target, characterized by its magnetic field, triggers the switching process when it enters the detection range of the sensor. The switch may be a reed switch or the device could operate due to inductive, variable reluctance, magneto resistive or hall effect operating principles.   

 

Sensors Specifications  

 

Performance Criteria

 

 

 

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

 

 

Electrical Switch Performance

Depending on the power options available, power requirements may be 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. Magnetic 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.

 

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: Structured Home Wiring.com

 

 

Body Type

Magnetic 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 : AutomationDriect.com / RS Components

  • 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 magnetic 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 

  • 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.
  • Depending on the sensor's technology, there can be minimum target size requirements.
  • 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.


Operating Principles

There are several operating principles used including reed switches, inductive, variable reluctance, magneto resistive or hall effect sensors.

  • Variable reluctance proximity sensors are comprised of a permanent magnet and a pick up coil.
  • Magneto-resistive proximity sensors measure the magneto-resistive effect or the impact of the resistivity of a ferromagnetic material in the presence of a magnetic field.
  • Reed Switches - Reed switches are magnetically actuated switches. They are typically manufactured with two ferromagnetic reeds (contact blades), which are sealed in a glass capsule. In the presence of a magnet, the blades (contacts) close.

 How a Reed Switch Works

Video Credit: MEDER electronic  

  • Inductive Proximity Sensors - 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.  

 Basic Functionality of Inductive Proximity Sensors

Video Credit: Balluff 

  • Hall Effect Proximity Sensors  - Hall effect sensors  function via the measurement of an electrical potential that is developed across an axis transverse to an applied current flow. The electrical potential measured is induced or amplified in the presence of a magnetic field. 

References

 

FARGO Controls, Inc.  - Operating Principles for Magnetic Sensors

 

KSR Kuebler - Magnetic Switches


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