Optical and magnetic encoders typically employ a readhead on the circumference of a scale. This approach can be sensitive to eccentricity error – the offset between the axis of rotation of the scale and the physical center of the scale (see Fig. 4). The offset can be caused by several factors including scale mounting concentricity, rotating shaft concentricity and bearing alignment. The problem with the offset is that the scale moves eliptically relative to the readhead. The readhead sees a longer or shorter arc depending on its position on the circumference.
Encoder manufacturers have a high degree of proficiency on mounting scales with minimal eccentricity. For the highest levels of performance, however, eccentricity error can be effectively eliminated by mounting a second readhead opposite the original readhead. As one readhead sees a longer arc the opposite readhead sees a shorter arc. The two readings can be combined for an average reading effectively free of eccentricity error. This does of course increase system cost but is frequently used when the highest levels of precision are required.
Figure 4 – Non-concentric optical disk and read head