Encoder resolution
A wheel encoder converts travel into counts. One revolution covers π·D of surface, and produces PPR pulses per channel. Instruments normally decode both channels on both edges, giving a quadrature multiplier of four, so the resolution is PPR × 4 / (π·D) counts per millimetre.
A 50.8 mm wheel with a 500 pulse encoder in quadrature gives about 12.5 counts per millimetre, or 0.08 mm per count – comfortably finer than any scan resolution in normal use. The rule is that the encoder step must be several times finer than the scan resolution, otherwise the position of each stored line jitters by a whole count.
Calculate the value, then calibrate it. Wheel diameter changes with wear and with the load pressing it onto the surface, and on a painted or scaled surface the wheel slips. Every procedure worth following requires the encoder to be verified over a known distance – typically 500 mm – before and after a scan, and the calculated value is only the starting point for that check.
Worked example
| Encoder wheel diameter | 50.8 mm |
| Pulses per revolution | 500 |
| Decoding multiplier | 4 |
| Calibration travel | 500 mm |
| Scan axis resolution | 1 mm |
| Wheel circumference | 159.593 mm |
| Counts per revolution | 2000 |
| Counts per millimetre | 12.532 |
| Travel per count | 0.0798 mm |
| Counts expected over the calibration travel | 6265.9 |
A 50.8 mm wheel rolls π × 50.8 = 159.593 mm per turn. 500 pulses per revolution decoded in quadrature gives 2000 counts per turn, so 2000/159.593 = 12.532 counts per millimetre, or 0.0798 mm per count. Over a 500 mm verification the instrument should report 6265.9 counts.
Use at your own risk — verify before you act
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