Basics
Encoder Disc vs Code Wheel vs Codewheel: What They Are and How They Work
Encoder disc, encoder disk, code disc, code wheel and codewheel all mean the same part: the patterned disc that an optical rotary encoder reads to measure angle and speed. It is classified by optical path (transmissive or reflective), by code (incremental or absolute) and by substrate (glass, metal or plastic).
If you search for an encoder disc, you will also see encoder disk, code disc, code wheel and codewheel. They are the same component. This guide explains what the part does, how it works and how the main types differ — so you can talk to suppliers in the same language.
What an encoder disc does
An optical rotary encoder has four basic parts: a light source (usually an LED), the encoder disc, a photodetector, and signal-processing electronics. The disc is mounted on the rotating shaft. Its surface carries a precise radial pattern of light and dark areas. As the shaft turns, the pattern modulates the light reaching the detector, and the electronics convert that modulation into position, speed and direction signals.
Because the pattern is the measuring scale, the disc largely determines what the encoder can achieve:
- Line count sets the base resolution (see line count and resolution).
- Line edge quality and pitch uniformity affect signal quality and graduation error.
- Optical contrast — optical density on transmissive discs, reflectance contrast on reflective discs — sets signal amplitude.
- Pattern-to-bore concentricity affects accuracy: at a 20 mm reading radius, 5 µm of eccentricity causes about 52″ of angular error (see eccentricity error).
Names you will see
| Term | Typical usage |
|---|---|
| Encoder disc / encoder disk | General term, common with encoder and servo motor makers |
| Code disc / code disk | Common in technical literature and drawings |
| Code wheel / codewheel | Common in catalogs and component listings |
| Optical scale / grating disc | Used when emphasizing the measuring pattern |
Type 1 — by optical path
| Transmissive | Reflective | |
|---|---|---|
| How it reads | Light passes through clear areas, is blocked by opaque areas | Light reflects from high-reflectance areas, is absorbed by low-reflectance areas |
| Source and detector | Opposite sides of the disc | Same side of the disc |
| Typical substrate | Glass (chrome on glass), PC | Metal (stainless steel) |
| Typical benefit | High resolution, mature design | Thinner encoder, robust disc |
Type 2 — by code
- Incremental discs carry equally spaced lines plus an index (Z) mark. The readhead outputs A/B square waves 90° apart, or analog sine/cosine signals for interpolation. The system must home after power-up.
- Absolute discs give every angular position a unique code — multi-track Gray code, or a single pseudo-random track plus an incremental track — so the position is known at power-up.
More detail: incremental vs absolute encoder discs.
Type 3 — by substrate
| Substrate | Strengths | Watch-outs |
|---|---|---|
| Glass (chrome on glass) | Flat, low thermal expansion, very clean photolithographic lines | Sensitive to drops and impact |
| Metal (e.g. stainless steel) | Strong, does not shatter, suits thin reflective encoders | Higher thermal expansion than glass |
| Plastic (polycarbonate) | Light, tough, lowest cost in volume | High thermal expansion (about 65–70×10⁻⁶/K) |
How ENC7 can help
ENC7 makes all three substrates in its own factory — glass, reflective metal and PC encoder discs — plus custom code wheels built to your drawing. Not sure which one you need? Send us the application and readhead model and an engineer will recommend a disc type.