Selection guide
Incremental vs Absolute Encoder Disc: Differences and How to Choose
Choose an absolute disc when the machine must know its position immediately after power loss (robot joints, multi-axis servo, valve actuators). Choose an incremental disc for speed or relative position measurement where cost matters.
Incremental encoder discs
An incremental disc carries equally spaced lines around the circumference — say 2500 — plus a separate index (Z) track. The readhead outputs two square waves, A and B, 90° apart. The controller reads direction from which channel leads and counts every edge (×4): 2500 lines × 4 = 10,000 counts per revolution.
An incremental disc tells you how far the shaft has turned, not where it is. After power-up the system must home on the Z pulse or an external switch.
Sine/cosine discs are incremental too: analog sin/cos outputs are interpolated to far higher resolution than square waves.
Absolute encoder discs
Every angular position has a unique code, so one reading at power-up gives the position — no homing. Two common layouts:
- Multi-track Gray code: n bits need n concentric tracks. Only one bit changes between adjacent positions, so readings don’t jump at transitions. More bits mean more tracks and a larger disc.
- Single-track pseudo-random code: a pseudo-random sequence on one track is read several bits at a time by a detector array, with an incremental track added for interpolation. The disc can be smaller — well suited to high bit counts.
Multi-turn counting uses a gear train driving extra discs (mechanical) or a battery / energy-harvesting counter (electronic).
Comparison
| Criterion | Incremental | Absolute |
|---|---|---|
| Output | A/B pulses or sin/cos | Unique code per position |
| At power-up | Must be homed | Position known immediately |
| Tracks | Incremental + index | Several Gray code tracks, or pseudo-random + incremental |
| Resolution expressed as | Lines × multiplication/interpolation | Bits (2ⁿ) |
| After pulses lost to noise | Error accumulates until next homing | Recovers at next reading |
| Disc pattern | Simple | Complex — demands high lithographic accuracy and track alignment |
| Cost | Lower | Higher |
Selection
- Spindle speed on CNC machines, conveyors, general motor feedback → incremental
- Robot joints, multi-axis servo, machines that can’t be homed after power loss → absolute
- Very high resolution on a small disc → sin/cos incremental track + pseudo-random absolute track
What to specify
Incremental: line count, Z index yes/no, square-wave or sin/cos. Absolute: bit count, code type, number of tracks and reading radius of each, readhead or decoder chip model. Full list: RFQ checklist.