Magnetoresistive sensor operational principles.！

As the magnetoresistive elements pass through the magnetic field of the code wheel, the material resistance changes by approximately ±1.6 percent, depending on the polarity of the field. The sensor is constructed with an aspect ratio that is much wider than it is high. A change in resistance is observed when flux passes across the width, but when the magnetoresistive element is over a magnetic field that is normal to the conductor, no change in resistance is observed. When connected as shown in Fig. 10.11, if a 5-V potential is applied to the element, the output at phase A or phase B will vary by 0.040 V peak to peak. Because both magnetic poles affect the sensor in the same way but with opposite polarity, when a code wheel of radius r is rotated through an angle tan-1 rk, the output will complete one electrical cycle.
Methods of Fabrication. Magnetic encoders consist of a sensor element and a code wheel. In many cases, these are provided to the end user as separate components, and the application must provide suitable mounting structures.
The code wheel is often constructed as a cylinder, with magnetic poles recorded on the outer circumference. This allows the sensor to be placed along the outside surface of the drum. This is a major advantage for this type of sensor in that it provides for substantial motor end-play allowance. This is a benefit to motor manufacturers, and will sometimes allow lower-cost motors to be used. The code wheel is generally made by injection molding of an isotropic magnetic material which has been mixed with a plastic carrier, such as nylon or polycarbonate. It can be made in arbitrarily large or small sizes, depending only on the mechanical structure required to support it. For this reason it is well suited for large through-shaft requirements, such as in spindle motors.
The molded code wheel is magnetized in the same way as a recording tape. The wheel position is carefully controlled while a series of magnetic pole pairs are recorded onto the magnetic media. This provides a very flexible manufacturing approach. Any line count can be created by simply recording a different “song.” The minimum width that can be recorded is proportional to the coercivity of the magnetic material, so small pole pitches require low-coercivity magnets. Isotropic ferrite is most often used for high resolution.

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