Read an absolute sector
Each valid multi-track code word identifies a defined angular sector.
Gray Code Encoder Discs
Gray code for absolute position
A Gray-code disc arranges concentric optical tracks so adjacent valid position words differ by one bit. The reader samples those tracks and converts the Gray word into the position representation used by the controller.
Adjacent Gray-code words ideally change by one bit. This code sequence can reduce ambiguity at a sector boundary when compatible optical channels are sampled during a transition.
- Code sequence
- Gray code
- Nominal positions
- 2^n
- Line-width tolerance
- ±0.2 µm
- Mechanical cutting
- ±0.02 mm
What the optical pattern does
The code disc supplies a passive optical reference. The readhead, electronics, mechanical mounting and control algorithm determine how that reference becomes a usable position signal.
Manage boundary transitions
The one-bit adjacency of a confirmed Gray sequence limits simultaneous bit changes at a normal boundary.
Preserve the approved sequence
Manufactured tracks can be compared cell by cell with the released code map.
Glass, pattern and code
Common manufacturing capability and product-specific code geometry are shown together. Final dimensions and acceptance values follow the approved drawing.
| Property | Specification |
|---|---|
| PropertySubstrate | SpecificationB270 / K9 / soda-lime glass; fused silica by request |
| PropertyPattern medium | SpecificationChromium-on-glass or a specified clear/opaque pattern |
| PropertyPhotolithographic line-width tolerance | Specification±0.2 µm |
| PropertyMechanical cutting accuracy | Specification±0.02 mm |
| PropertyThickness | Specification≥0.4 mm |
| PropertyTotal thickness variation (TTV) | Specification≤2 µm |
| PropertySurface figure | Specificationλ/4 |
| PropertyOptical mode | SpecificationTransmissive or reflective, matched to the readhead wavelength |
| PropertyMaximum disc diameter | Specification1500 mm |
| PropertyPattern-to-bore concentricity | Specification≤0.02 mm |
| PropertyCode map | SpecificationApproved Gray-code sequence with bit count and every valid sector |
| PropertyTrack order | SpecificationMSB/LSB positions, track radii, widths and optical polarity |
| PropertyMechanical reference | SpecificationCode zero and reading direction related to the bore or mounting feature |
| PropertyTransition control | SpecificationAngular transition positions and registration between adjacent tracks |
Component tolerances do not state the accuracy of the complete encoder. Readhead geometry, eccentricity, bearing motion, interpolation and installation also contribute to system error.
Match Gray code to the decoder
Choose it for a decoder that expects a specified Gray sequence, track order and reading direction. It reduces simultaneous bit changes at neighbouring code boundaries.
Supply the actual readhead geometry and decoding convention with the drawing. Matching only the outside diameter is not enough to make two code discs interchangeable.
- Single-turn absolute optical encoders
- Precision rotary positioning with Gray-code decoding
- Existing instruments built around an approved Gray-code map
Match the disc to the reader
Substrate, chromium pattern, clear and opaque regions, outer geometry and mounting references are reviewed together. Transmissive and reflective reading require different optical definitions.
Substrate and outline
Select the glass, diameter, bore, thickness, edge finish and mounting datum for the mechanical assembly.
Optical contrast
Define clear-area transmission or the reflective contrast pair at the actual readhead wavelength and geometry.
Pattern registration
Relate tracks, transitions and zero marks to the bore or another stable mechanical datum.
Acceptance and drawing inputs
Confirm these items after the code type and core geometry are selected. They define how the finished disc is manufactured, checked and documented.
Inspection and acceptance
Verify the code sequence against the approved map, inspect angular transition positions, track-to-track registration and pattern-to-bore concentricity. For critical systems, define how code-word errors and missing or bridged features are reported.
What to provide in the drawing or inquiry
- Outer diameter, bore, thickness, edge format and mechanical datums
- Complete track drawing or code map, polarity and viewing direction
- Track radii, critical line widths, pitch and registration requirements
- Optical mode, operating wavelength, readhead gap and required contrast
- Zero datum, rotation direction and relationship to mounting features
- Inspection items, report format, quantity and drawing revision
Send the complete code map, mechanical drawing and readhead requirements for manufacturing and inspection review.
Discuss your encoder discSelection notes
Gray-code absolute discs arrange a Gray sequence across optical tracks to identify absolute angular position.
Adjacent standard Gray words change one bit. The artwork must also define bit order, increasing-angle direction and the transition across zero.
Information for your inquiry
Bit count, complete sequence, inner-to-outer bit order, rotation direction, zero and readhead window.
Is bit count enough to define the disc?
The same bit count can use different track order, reading direction and zero position. The complete map establishes how the read word corresponds to angle.