What is the difference between incremental and absolute encoder discs? How to choose
An incremental encoder disc outputs pulses and must be homed after power-up; an absolute encoder disc has a unique code for every angle, so the position is known at power-up. This article compares the two in terms of tracks, resolution, signals and cost, and gives selection advice.
2026-10-04 · Langfang Jingzheng Optoelectronic Technology Co., Ltd.
Bottom line: for equipment that must know its position immediately after a power loss and restart, such as robot joints, multi-axis servo systems and valve actuators, choose an absolute encoder disc; for equipment that only needs to measure speed or relative displacement and is cost-sensitive, choose an incremental encoder disc.
1. Incremental encoder discs
An incremental encoder disc carries equally spaced lines around the revolution, for example 2500 lines, plus a separate zero (Z) index track. The readhead outputs two square waves, A and B, with a 90° phase difference; the controller determines the direction of rotation from which of A and B leads, then counts with ×4 multiplication: 2500 lines × 4 = 10000 counts per revolution.
An incremental disc only tells you “how far it has turned”, not “where it is now”. After power-up it must first be homed by finding the Z signal or an external home switch; only then is the position meaningful.
Sine/cosine discs are also incremental. The readhead outputs analog sin/cos signals which, after interpolation (for example ×256 or ×1024), give a much higher resolution per revolution than square waves.
2. Absolute encoder discs
On an absolute encoder disc, each angular position corresponds to a unique code, so a single reading at power-up gives the current position without homing. There are two common approaches:
- Multi-track Gray code: n-bit resolution requires n concentric tracks. In Gray code only one bit changes between adjacent positions, so the reading does not jump significantly at code transitions on the tracks. The more bits, the more tracks and the larger the disc.
- Single-track pseudo-random code: a pseudo-random sequence is arranged on a single track; the readhead uses an array of photosensitive elements to read several consecutive bits at once and determine the absolute position, and an incremental track is added for interpolation. The disc can be made smaller, which suits high bit counts.
The resolution of an absolute disc is expressed in bits: 2ⁿ positions per revolution. 17 bits give 131072 positions of about 9.9″ each; 23 bits give about 0.15″. When the number of revolutions must also be recorded, a gear train drives several discs (mechanical multi-turn), or a battery or energy-harvesting circuit counts the revolutions (electronic multi-turn).
3. Comparison
| Criterion | Incremental | Absolute |
|---|---|---|
| Output | A/B pulses, or sin/cos | A unique code for each position |
| At power-up | Must be homed | Position known immediately |
| Tracks | One incremental track + index track | Several Gray code tracks, or one pseudo-random track + incremental track |
| Resolution expressed as | Line count × multiplication or interpolation factor | Bits (2ⁿ) |
| After pulses are lost to interference | Error keeps accumulating until the next homing | Recovers automatically at the next reading |
| Disc pattern | Simple | Complex; demands high photolithographic accuracy and track alignment |
| Cost | Low | High |
4. How to choose
- Spindle speed measurement on CNC machine tools, conveyor lines, general motor feedback: incremental
- Robot joints, multi-axis servo systems, equipment that cannot be homed after a power loss: absolute
- Very high resolution with a small disc: a sine/cosine incremental track combined with a pseudo-random absolute track
5. What to specify in your RFQ
For incremental discs, state the line count, whether a Z index is required, and square-wave or sine/cosine output; for absolute discs, state the bit count, code type (Gray code or pseudo-random code), the number of tracks and the reading radius of each, and the readhead or decoder chip model. For a detailed checklist, see Requesting a quote for a custom encoder disc: 20 parameters to send the manufacturer.
This article was compiled by the engineering department of Langfang Jingzheng Optoelectronic Technology Co., Ltd. Jingzheng Optoelectronics manufactures glass encoder discs, reflective metal encoder discs, PC encoder discs and custom encoder discs. Website: www.encoderdisk.cn; phone: 0316-8933331.