Image Credits: Balluff Sensors, ifm efector, Measurement Specialties

 

Non-contact magnetoresistive linear position sensors and switches measure a magnetic object's distance and movement, or whether it has approached within a set distance.

How Magnetoresistive Linear Position Sensors and Switches Work

Magnetoresistance is a material property which reflects its change of resistance when exposed to a magnetic field. These sensors measure the angle direction of a field from the magnet (as opposed to the strength of the magnetic field). They provide an absolute reading from the moment they

are turned on; they do not need to be zeroed to an external reference.


Types of Magnetoresistive Linear Position Sensors and Switches 

Anisotropic magnetoresistance (AMR) depends on the angle between the electric current and the magnetization direction. The resistance is maximum when the magnetization direction is parallel to the current direction and minimum when the magnetization is perpendicular to the current.

 

A giant magnetoresistance (GMR) device consists of two or more layers of ferromagnetic metal separated by an ultra-thin non-magnetic metal spacer. When the magnetic layers are aligned in the same direction, electrical resistance decreases; if the magnetizations are opposing, electrical resistance increases. The 10-15% change in resistance found in GMR far exceeds that of AMR devices.

Tunneling magnetoresistance (TMR) is similar to GMR except that it uses an ultra-thin insulating layer to separate two magnetic layers. Electrons pass from one layer to the other through the insulator by quantum mechanical tunneling. The ease of tunneling (the electrical resistance) is modulated by the angle between the magnetization vectors in the two layers.


Specifications and Features


Device packaging may be a simple transducer or a fully self-contained sensor system.


Body styles include barrel, limit switch, rectangular/block, slot, or ring.


Sensor operating distance varies (e.g., 1.97 to 300 inches) or may be field-adjustable.

Repeatability (e.g., no more than 4 mils) is how quickly the device senses.


Devices can tolerate certain temperature ranges (e.g., -4 to 176 F).


Resistance value range is an indication of the full range of output values expected for the full range of input values.

Resistance tolerance is the "resolution" of the measured resistance value.


Digital interface options are serial, parallel, or other.


Analog, digital, and/or video display options exist.

Controls options include analog, digital, or computer.


Sensors may include a fixed cable, integral connector(s), terminals (screw or clamp down), and/or plastic or metal housing.

Sensors can be shielded (protected against EMI and RFI), weld field immune, short circuit protected, and/or intrinsically safe (cannot cause ignition of atmospheric mixtures).

 

Applications

These sensors boast maintenance-free, wearless operation; large permissible air gaps between sensor and target; precise linear measurement with high resolution; high bandwidth for monitoring dynamic processes; reliable and safe use in difficult operating conditions.

 

References

Finding the Right Sensor for Linear Displacement
How Magnetoresistive Sensor Works
Linear Position Sensing Using Magnetoresistive Sensors
Magnetoresistive Sensors
Magnetoresistive Sensors for Nondestructive Evaluation


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