Reference
Encoder Disc Glossary: 30 Key Terms Explained
Short answer
Encoder disc, encoder disk, code disc, code wheel and codewheel all mean the same part: the patterned disc that an optical encoder reads to measure angle. This glossary defines the 30 terms used most in disc drawings, RFQs and inspection.
| Term | Also called | Definition |
|---|---|---|
| Encoder disc | encoder disk, code disc, code wheel, codewheel, optical scale | Disc carrying a pattern of light/dark or high/low-reflectance areas; the angle-measuring core of an optical encoder |
| Optical encoder | — | Sensor converting rotation into electrical signals using a light source, an encoder disc and a photodetector |
| Incremental | — | Outputs pulses or sin/cos signals as the shaft turns; must be homed after power-up |
| Absolute | — | Each angular position has a unique code; position is known at power-up |
| Transmissive | — | Light passes through the disc; source and receiver on opposite sides |
| Reflective | — | Light reflects off the disc; source and receiver on the same side |
| Track | — | Annular band carrying one pattern type: incremental, index or absolute |
| Index | reference mark, Z | Mark occurring once per revolution, used for homing |
| Line count | lines per revolution, PPR, CPR | Number of lines around the incremental track, e.g. 2500 |
| Grating pitch | — | Distance between adjacent lines at the reading radius = 2πr ÷ line count |
| Duty cycle | — | Fraction of one pitch that is transparent (or high-reflectance); typically 1:1 |
| Reading radius | — | Distance from the readhead light-spot center to the disc center |
| Edge multiplication | ×4, quadrature | Counting rising and falling edges of A and B to get 4× the line count |
| Interpolation | subdivision | Dividing one sin/cos period into many parts to raise resolution |
| Sin/cos signals | analog quadrature | Two analog signals 90° apart, used for interpolation |
| Gray code | — | Code where only one bit changes between adjacent positions; common on multi-track absolute discs |
| Pseudo-random code | PRBS | Single-track absolute code; reading a few consecutive bits gives the position |
| Resolution | — | Smallest distinguishable angular step |
| Accuracy | — | Maximum deviation of reading from true angle; set by graduation error, eccentricity and interpolation error |
| Graduation error | — | Difference between actual and theoretical line positions |
| Eccentricity | — | Offset between pattern center and rotation center; causes once-per-revolution angular error |
| Concentricity | pattern-to-bore concentricity | Deviation between pattern center and bore center; a common inspection item |
| Optical density | OD | Light-blocking ability of opaque areas; OD 3 = 0.1 % transmittance |
| Reflectance | — | Fraction of light reflected by high- and low-reflectance areas |
| Contrast | — | Difference between light- and dark-area signals; sets amplitude and signal-to-noise ratio |
| Photolithography | lithography | Transferring a mask pattern to the substrate with photoresist, exposure and development |
| Chrome coating | Cr | Chrome layer on glass forming the opaque areas; graduations are etched into it |
| AOI | automated optical inspection | Machine-vision inspection of graduation integrity and surface defects |
| Readhead | read head | Component holding the light source and receiver that reads the pattern |
| Lissajous figure | XY plot | sin vs cos on an oscilloscope in XY mode; used to judge signal quality |
Related: incremental vs absolute · line count and resolution