Magnetoresistance minimizes encoder size…

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Micro-motors demand micro-encoders. Hall effect sensors are available, however there are disadvantages: low pulse count, no index pulse, no complementary signals. They also require a relatively strong magnetic field, limiting model size and tolerance levels.

Compact encoder requires little power and is insensitive to radial and axial play, mechanical tolerances, and fluctuating temperatures.

A new incremental encoder series, based on the anisotropic magnetoresistive (AMR) effect, matches encoder size to motors as small as 10 mm diameter while adding no more than 6 mm to overall length. Developed by maxon motor, the MR Digital Encoder also incorporates a special ASIC and sensor architecture to achieve high resolution where as many as six binary or decimal incremental figures can be selected.

Arranged as Wheatstone's Bridges, active sensor surfaces are shifted against each other by a quarter of a pole pitch.

The MR Encoder's principle benefit over Hall sensors is a much smaller detectable magnetic field. That is because magnetoresistive effects are expressed as a change in the electrical resistance of a material when a magnetic field is created. The amount of resistance depends not on the magnetic field's amplitude, but on the angle between the magnetization direction and the current direction. Maximum resistance occurs when current direction and magnetization direction are parallel; minimum resistance occurs when the angle between current and magnetization direction is 90 degrees.

Using a smaller detectable magnetic field, in turn, allows a significant reduction in encoder size. Instead of conventional Hall sensor construction, the MR Encoder employs a specially magnetized cogwheel pressed onto a motor shaft extension. Cogwheel diameter and pole breadth, adjusted to match the sensor's dimensions, produce the encoder's pulse count.

The AMR sensor chip works as a "strong field" sensor; sensor magnetization follows the cogwheel's stronger magnetic field. Since sensor signals are dependent only on the resulting angle between the direction of magnetic field and current, the amount of magnetization is not critical. The sensor chip, therefore, measures a mere 0.5 x 1.8 mm 2. The strong field principle also produces a signal that is widely independent of mechanical tolerances.

To create a fixed 90-degree phase relationship between channels A and B, the AMR sensor chip comprises two sets of four ferromagnetic metal strips. A Wheatstone's Bridge arrangement shifts one against the other by a quarter of the cogwheel's pole pitch. Each magnetic pole, consequently, gives a complete and practically harmonic-free sinusoidal signal that is suitable for multiplying signals using interpolation. The index signal is produced digitally, prompted by the signal of an additional AMR sensor in the magnetic disc's index pole.

The MR encoder's ASIC strengthens and interpolates the sensor signals, producing TTL-compatible encoder and complementary signals. In addition, the possible multiplication factors for interpolating the sinusoidal signals (decimal: 5, 10, 20; binary: 4, 8, 16) are adjustable. Up to six possible encoder pulse counts are produced for each cogwheel. The end result, claims maxon motor, is a micro-encoder good for all high-quality servo drivers.