Magnetostrictive Position Sensors


Image Credits: Balluff, MTS Sensors, Novotechnik, U.S.

 

Magnetostrictive position sensors are non-contact linear position sensors that use the momentary interaction of two magnetic fields to produce a strain pulse that moves along a waveguide. One field is from a magnet that moves along the outside of the waveguide. The other field is from the waveguide itself. Magnetostrictive position sensors that provide absolute measurements produce a unique signal for each point along the axis of travel. The advantage to this type of sensor is that it is non-contact and there is no wear or friction. It is also not affected by vibrations so there is no limit on the number of operating cycles. The disadvantage is the dead band on both sides of the sensor which cannot be reduced to zero.

 

How Magnetostrictive Position Sensors Work

Magnetrostriction is a property of ferromagnetic materials to expand or contract when placed in a magnetic field. The sensor senses the position of the permanent (position) magnet to determine the distance between the permanent magnet and the sensor head.
There are five main components of the magnetostrictive sensor: Waveguide, position magnet, electronics, strain pulse detection system, and damping module.

Magnetostrictive Position Sensor. Image Credit: MTS Magnetostrictive Sensor Principle

 

Typically, the waveguide wire is enclosed within a protective cover and attached to the device that is being measured. Applying a current pulse generates a sonic wave that travels along the waveguide to a small piece of magnetostrictive material that passes through a coil and is magnetized by a small, permanent magnet. The stress induced by the sonic wave causes a wave of changed permeability in the magnetostrictive material, resulting in a change in its magnetic flux and the production of voltage output from the coil. Electronic circuitry detects the voltage pulse and conditions it into the desired output.

Image Credit: Sensorland.com

This video illustrates.


Video Credit: Multiprox

 

At the sensor rod tip, at the end opposite the head, there is an unusable area called the dead zone. The system must be designed so that the front face of the position magnet will come no closer to the tip than the specified dead zone distance.

 

Magnetostrictive Position Sensor Characteristics


Encoder Reading

Depending on the sensor's characteristics, measurements can be either absolute or incremental. Magnetostrictive position sensors are generally absolute.

  • Absolute- absolute reading means the position is accurately known at power-on, without the need for setting a zero position. If there is a power outage, position can be determined immediately when the power returns.
  • Incremental- incremental measurements do not produce unique position signals. They are zeroed after power-down and rely upon an external switch or sensor for initial reference.

Magnetostrictive Position Sensor Output

Absolute Code

Typically, the position is read after power cycling. There are several types of outputs. 

  • Gray outputs require the use of a unit-distance coder in which only one bit changes with the transition from one measuring step to the next.
  • Binary outputs are represented as a series of ones and zeroes.

  • Binary coded decimal (BCD) outputs are represented by decimal numbers (integers) in which each digit is signified by four bits.

Incremental Signal

Incremental signals are digital or analog outputs.

  • Digital- square digital output signal lines produce one or two square waves that can be resolved via quadrature to four counts per line.
  • Analog- sine analog outputs use two channels that are 90° out of phase to read and combine sine and cosine signals. The theoretically infinite resolution is limited only by the resolution of the external digitizing device.
  • Single channel magnetostrictive position sensors provide one count per physical line.
  • Pulse and direction devices combine a unidirectional channel with either single-channel counting or multiple-channel counting with quadrature.

  • Reference or index signals produce a once-per-revolution mark or home position.

Electrical/Digital Output

There are many electrical and digital outputs for magnetostrictive sensors. Some examples include:

  • Serial synchronous interface (SSI) is a general standard for absolute measurement devices. Serial outputs such as RS232, RS422 and RS485 are commonly available.
  • Parallel outputs for magnetostrictive sensors include the general-purpose interface bus (GPIB), a standard which is also known as Hewlett-Packard interface bus (HPIB) and IEEE 488.
  • Analog current outputs include 4 - 20 mA and 0 - 2 mA.

  • Analog voltage outputs include 0 - 10V. Sensors that can transmit position data along optical fibers are also available.

Performance Specification

Magnetostrictive position sensors vary in terms of performance specifications, dimensions, and features. Performance specifications include:

 

  • Measurement range is the full range of travel that can be measured. A single sensor model may have selectable or configurable ranges.
  • Resolution is the smallest unit of distinction that optical triangulation position sensors can detect.
  • Operating temperature and environment describe the conditions the sensor will be placed in during operation. The environment may affect the material and accuracy of the measurement.
  • Dimensions of the waveguide rod are available. The cross-section height and cross-section width should be considered to ensure the device will fit into the system.
  • Sealed sensors prevent the ingress of contaminants such as dust, dirt, or swarf.

  • Probe/plunger sensors include a tip or plunger that can be placed directly against the surface of the measured device. Typically, these probe-style sensors are biased with a spring or light air pressure.

Applications

These sensors are ideal for applications which require precision measurement, "non-wearing" parts, and shock and vibration resistance. Magnetostrictive sensors benefit food and beverage, liquid level, medical, metalworking, paper converting, plastics, wood and testing equipment applications.

 

Chart Credit: MTS

 

References

How Magnetostrictive Sensor Works
How They Work: Magnetostrictive Linear Position Sensors
Magnetostrictive Linear-Position Sensor
Sizing and Applying Magnetostrictive Linear Position Sensors

Another helpful video with information can be found here 

 


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