Resolution vs Accuracy: They're Not the Same Thing
Resolution and accuracy are two distinct concepts that are often confused. An encoder can have very high resolution but poor accuracy, and vice versa. Understanding the difference is essential for correct selection.
Resolution
Resolution is the smallest angular increment the encoder can distinguish. It's expressed as:
- Pulses per revolution (ppr) for incremental encoders: 1024 ppr → increment = 360°/1024 = 0.352°
- Bits for absolute encoders: 13 bits → 2^13 = 8,192 positions → increment = 0.044°
Resolution is a digital parameter: it indicates how many "marks" the ruler has, not how precise those marks are.
Accuracy
Accuracy is the maximum deviation between the position read by the encoder and the actual shaft position. It's expressed in degrees, arc minutes, or arc seconds.
A 10,000 ppr encoder with ±0.05° accuracy has very closely spaced marks, but each one could be displaced up to 0.05° from the ideal position.
Practical Analogy
Imagine a ruler:
- Resolution = distance between marks (millimeters vs centimeters)
- Accuracy = how correctly each mark is positioned
A 0.1 mm ruler (high resolution) with poorly printed marks (low accuracy) is worse than a 1 mm ruler (low resolution) with perfectly aligned marks (high accuracy).
Repeatability
Repeatability is the ability to return to the same position multiple times with the same result. It's different from accuracy: an encoder can be very repeatable (always returns to the same point) but not accurate (that point is offset from the real position).
How to Choose
- Speed control: resolution matters (more pulses = smoother)
- Precise positioning: accuracy matters (error < mechanical tolerance)
- Precision angle measurement: both need to be high
- Safety applications (SIL): accuracy is the critical parameter
- Incremental encoders: accuracy ±0.05° — ±0.02°
- Single-turn absolute encoders: accuracy ±0.02° — ±0.01°
- Multi-turn absolute encoders: cumulative accuracy depends on the gearbox
