Optical Encoder Speed

Formula

Description

An optical (or magnetic) encoder generates a fixed number of pulses per revolution (PPR). By counting pulses over a known time period, the rotational speed can be calculated. The count divided by PPR gives the number of revolutions in the measurement period, and multiplying by 60/t converts to RPM. This frequency-measurement method works best at moderate to high speeds. At very low speeds, the period-measurement method (measuring time between individual pulses) provides better resolution.

Variables

• n — Rotational speed (RPM)
• count — Number of encoder pulses counted in the measurement period
• PPR — Encoder pulses per revolution
• t — Measurement time window (s)

Practical Notes

Common encoder resolutions: 100-1000 PPR for general industrial, 2000-10000 PPR for precision servo. Quadrature encoders (A/B channels) multiply the effective resolution by 4× when counting all edges. At low speeds, quantization error is significant: for a 1000 PPR encoder at 1 RPM measured over 100 ms, only about 1.67 pulses are counted. Use longer measurement windows or the period-measurement method for low-speed applications.