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What do accuracy, resolution and repeatability mean? Why is the accuracy of the encoder worse than resolution?

Accuracy
There is some jitter, caused by many different parts: ferrite particles in the ring, magnetization errors, sensor noise, etc. Additional errors make this noisy straight line to bend into a sine wave(s) with different frequencies. 1 sine per turn is eccentricity of the ring on a shaft. This is dominant and can reach up to ±0.1 deg. It can be cancelled out by self-calibration feature that will be included in the new generation of AksIM encoders. Less prominent higher frequency errors come, for example, from the bending of the ring when tightening the fasteners.

Resolution
It can go beyond accuracy. Good repeatability and zero hysteresis make high resolution usable, even though accuracy is lower than resolution.

Repeatability
Encoder position is repeatable and stable to the single count of resolution. This is what the readhead is capable of. Eventually it is even possible to map out the noise of the ring, though it would take a lot of points to accomplish that.

Generally speaking, the ring is responsible for accuracy and the readhead for resolution and repeatability.

*Note: SDE (sub divisional error) is inaccuracy caused by readhead, and it's not present on AksIM encoders.

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