Giant Magneto Impedance Encoder

Operating Principle


General considerations

Advantages

The most notable advantages of this principle are:
  • no position hysteresis / backlash
  • scanning area over the entire circumference of a rotary measurement system
  • high signal to noise ratio
  • high accuracy
  • large mounting tolerances
The Giant Magneto Impedance phenomenon is known since 1930 and its main application was measuring extremely weak magnetic fields.
With its proprietary GMI technology (patent granted), FLUX GmbH Austria is the first encoder manufacturer which implemented this physical principle in position measurement systems.
The Giant Magneto Impedance (GMI) is a completely different physical phenomenon than the magnetoresistance and Hall effect which are usually applied in position measuring systems.
Magnetoresistance and Hall effect phenomenons can be explained by the Lorentz force that changes the flow of charge carriers (direct current) in a conductor, when it is placed in an external magnetic field. Consequently by changing the flow path in a conductor of direct current, the resistance of the path changes. The sensors based on these principles have some disadvantages like: position hysteresis (also known as backlash in a drive system), punctual scanning, low signal to noise ratio and relative low accuracy thus limiting their use mostly to “low end” applications. These disadvantages are specific to all types of magnetoresistance sensors: anisotropic (AMR), giant (GMR), tunnel (TMR). The Giant Magneto Impedance is based on the skin-effect. An external magnetic field changes the permeability and consequently the skin depth of a ferromagnetic material with GMI properties which is passed by an alternating current.

Giant Magneto Impedance principle

A GMI foil is a thin conductive ferromagnetic metallic foil. The impedance “Z” of a GMI foil changes when applying an external magnetic field like in Fig. 1.

Fig. 1. GMI foil in no respectively in external magnetic field
no magnetic field: | H0 | = 0 A/m
within magnetic field: | He | > 0 A/m
For a given frequency of the measuring alternating current (I ac ) the foil impedance (Z) in lower magnetic fields (| H0 | ~ 0 A/m) is bigger than the impedance of the foil when placed in a higher magnetic field (| He | > 0 A/m):
Z(| H0 |) > Z(| He | )
the depth of the flowing domain depends on the resistivity of the material influenced by the presence of the magnetic field. When such a GMI foil is placed above a magnetic scale coded with alternating polarity domains, areas with lower (darker) and higher (lighter) impedance arise like in Figure 2.

Fig.2. Areas with lower and higher impedance in a GMI foil
The position of the area with:
– low Z(|He|) – lighter color
– high Z(|H0|) – darker color
impedance in the GMI Foil changes with respect to the relative position of the magnetic scale in the direction of measurement “x”.
A position depending electrical signal can be generated by acquiring the impedance of the areas Z(|H0|) and Z(|He|) by the means of using a coil system like in Figure 3.

Fig.3. Coil System for impedance measurement
The emitter coil (E) is connected to a high frequency alternating current source in range of 1 up to 10MHz. The time variable magnetic field generated by the emitter coil induces eddy currents in the GMI foil. The eddy current amplitude and position will be determined by the impedance of the area below the coil system:
– area with low impedance (lighter color): eddy currents with high amplitude
– area with high impedance (darker color): eddy currents with low amplitude.
In Figure 4 the differential receiver coils (R+), (R-) generate an output signal proportional to the difference of the eddy current intensities of each impedance area. This electrical signal is conditioned and processed by the evaluation electronics and outputted as position information.

Fig.4. Coil system and induced eddy currents

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