Magneto-Resistive Sensor

Other Unique Engineering Ideas
The magnetoresistive effect is the change of the resistivity of a material due to a magnetic field; it has been discovered by Thomson in 1856 [1]. The improvement of the technology of thin ferromagnetic films (with a thickness of 10 – 50 nm) and the utilization of the Anisotropic Magnetoresistive (AMR) effect led to an increasing technical interest in this effect.

1. Description

2. Why

3. How

4. Future Trends

5. Related Links

Description 

The magnetoresistive effect is the change of the resistivity of a material due to a magnetic field; it has been discovered by Thomson in 1856 [1]. The improvement of the technology of thin ferromagnetic films (with a thickness of 10 – 50 nm) and the utilization of the Anisotropic Magnetoresistive (AMR) effect led to an increasing technical interest in this effect.Furthermore, a Giant Magnetoresistive (GMR) effect [2] has been discovered, which is based on the weak coupling between separate thin ferromagnetic films. The maximum resistivity change is up to 80% with the GMR effect, albeit at very high magnetic fields only. AMR sensors feature a high sensitivity at weak magnetic fields and a small consumption of energy.Their maximum change of resistance is of the order of 3 – 4 %. These sensors can be produced in large quantities and very cost-efficiently if the processes required for their production can be handled reproducibly.

Why

Magnetoresistive sensors are the perfect choice in a large number of diverse fields because of their many inherent advantages and sensing benefits: 

  • contactless operation, providing a long operating life and
  • wear-free measurement and detection
  • high reliability due to their rugged construction
  • high sensitivity
  • high operating temperature
  • wide operating frequency range (0 Hz to 1 MHz)
  • angle and rotational speed measurement
  • earth field detection
  • position measurement and detection
  • displacement measurement
  • current measurement

How 

Magnetoresistive sensor elements are magnetically controllable resistors. The effect whereby the electric resistance of a thin, anisotropic ferro-magnetic layer changes through a magnetic field is utilized in these elements. The determining factor for the specific resistance is the angle formed by the internal direction of magnetisation (M) and the direction of the current flow (I).

  • Resistance is largest if the current flow (I) and the direction of magnetisation run parallel.
  • The resistance in the base material is smallest at an angle of 90° between the current flow (I) and the direction of magnetisation (M).
  • Highly conductive material is also applied below an angle of 45°.
  • The current passing the sensor element takes the shortest distance between these two ranges 

This means that it flows at a preferred direction of 45°against the longitudinal axis of the sensor element. Without an external field, the resistance of the element is then in the medium range.An external magnetic field with a field strength (H) influences the internal direction of magnetisation, which causes the resistance to change as factor of the influence.

  • The actual sensor element is often designed with 4 magnetic field sensitive resistors interconnected to form a measuring bridge.
  • The measuring bridge is energised and supplies a bridge voltage.
  • A magnetic field which influences the bridge branches in different degrees leads to a voltage difference between the bridge branch which is then amplified and evaluated.
  • The sensor detects the movement of ferromagnetic structures (e.g. in gearwheels) caused by changes in the magnetic flow.
  • The sensor element is biased with a permanent magnet.
  • A tooth or a gap moving past the sensor influences the magnetic field at different degrees.

This causes changes in the magnetic field dependent resistance values in a magnetoresistive sensor. The changes in the magnetic field can therefore be converted into an electric variable and can also be conditioned accordingly. The output signal from the sensor is a square-wave voltage which reflects the changes in the magnetic field.Changes in the magnetic field cause the bridge voltage to be deflected. This voltage is amplified and supplied to a Schmitt trigger after conditioning. If the effective signal reaches an adequate level, the output  stage is set accordingly. 

Future Trends

Magnetoresistive sensors are designed specially for the rotational speed and zero speed detection in machines and equipment. They can be deployed for similar applications as inductive oscillatory sensors, with the difference that magnetoresistive sensors are also capable of detecting high frequencies and finer structures. Unlike Hall sensors, magnetoresistive sensors are not limited by a lower limiting frequency, provided that the sensors are directionally installed.Magnetoresistive sensors are used today, for example, in reading heads of magnetic data storage systems such as hard disks. They could find much wider spread applications, though, e.g., for measurements of the Earth’s magnetic field, as a gradiometre, a  compass, a position sensor, or for measurements of biomagnetic fields. The sensors require thin ferromagnetic films (e.g., permalloy)  

Keywords

Magnetoresistance, Thin-film, Sensors, Magnetic fields

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