Do not use one accuracy number for different errors
The word accuracy is incomplete without a measured quantity and reference. Critical dimension is a feature-size question; placement is a coordinate question; pitch is a relationship between repeated features. State which one matters and over what length or area.
| Requirement | What is measured | What it does not establish |
|---|---|---|
| CD or line width | Distance between defined edges | Position of the line centre |
| Pitch | Distance between corresponding repeated features | Absolute position from the mounting datum |
| Pattern placement | Feature coordinate relative to a datum | Local linewidth or installed instrument accuracy |
| Layer registration | Relationship between specified marks or layers | Overall glass dimensions |
Connect rotary geometry to angular error
For a small tangential displacement δs at reading radius r, the angular equivalent is approximately δθ = δs/r in radians. The same linear displacement therefore has a different angular meaning at a different radius. Define the actual reading radius rather than reporting only the disc outside diameter.
Check local pitch and accumulated error separately
A scale can have small neighbouring-pitch variation while gradually departing from the intended coordinate over a long travel. Conversely, a good overall length does not prove uniform local pitch. Specify whether the requirement concerns individual intervals, cumulative position from a reference or both.
Temperature, support and the definition of the reference end must accompany a long-scale measurement. For a grid, state whether coordinates are evaluated after removing a best-fit translation or rotation, or directly against physical datums. Those evaluations answer different questions.
Resolve direction before approving the code or pattern
A coded disc needs track order, bit significance, increasing-angle direction and the viewing side used for the code table. The same visible arrangement can be interpreted differently by a decoder when the track order or view is reversed. Confirm these conventions in the DWG and the code definition together.
A one-bit transition in ideal Gray code reduces ambiguity at adjacent code boundaries, but does not remove requirements for readhead alignment, illumination or boundary handling. The glass pattern and the decoder must be reviewed as a matched system.
State how an edge is detected
Measured line width can change with illumination, focus, threshold and the optical system used to locate an edge. Where a small tolerance is important, agree the edge definition and measurement setup. Comparing numbers from different threshold rules can create an apparent disagreement even on the same feature.
Keep component and assembled-system requirements distinct
Pattern geometry, eccentric mounting, readhead position, signal processing and temperature can all contribute to a system result. Put the component requirements on the DWG and describe the separate instrument validation. A disc or scale cannot establish the final system accuracy by itself.