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Technology overview

Pneumatic cylinders contain a piston with a ring magnet that produces straight-line motion via compressed air. They are used to push and pull, transfer and clamp, etc. in applications such as workpiece positioning, sorting and robotics. Cylinder sensors detect the ring magnet and indicate the position of the cylinder stroke.

Ifm offers cylinder sensors using the Giant Magneto-Resistance (GMR) principle and the Anisotropic Magneto-Resistance (AMR) principle. We will discuss both principles and then compare.

GMR technology

GMR cells consist of three layers of material – two layers of cobalt (magnetizable) and one layer of copper (non-magnetizable). Without a magnetic field applied, the atoms of these layers do not allow electrons to pass through because of the high resistance between the layers. However, when a magnetic field is applied, the atoms align and the electrons can now pass through. The sensing element uses this physical property to generate a signal that can be used to indicate the position of a magnetic target.

GMR technology is very reliable; however, it has the risk of multiple switch points when the cylinder magnet is very strong. The travel distance (the maximum distance at which the sensor switches due to the magnet) is wide and the hysteresis is relatively large.

AMR technology

The AMR cell consists of a very thin layer of perm alloy, for example 20 nm of a nickel-iron alloy. When influenced by a magnetic field, the electrical resistance changes. This is very similar to GMR technology. AMR technology generates a more narrow travel distance and smaller hysteresis, so it is well suited for short-stroke and high-precision applications.

Comparison of GMR and AMR sensors to mechanical Reed switches