Design guide
Does Temperature Affect Encoder Disc Accuracy? Substrate Thermal Expansion
When a disc heats uniformly every line keeps its angle, so the angle reading barely changes. Watch three things instead: expansion mismatch between disc and hub (can create eccentricity), change of reading radius and pitch (large for PC), and gap change on reflective readheads.
1. Expansion coefficients of common substrates
| Material | Approx. ×10⁻⁶/K |
|---|---|
| Fused silica | 0.5 |
| Borosilicate glass | 3.3 |
| Soda-lime glass | 9 |
| Stainless steel | 16–17 |
| Aluminum alloy | 23 |
| Polycarbonate (PC) | 65–70 |
Typical values from common references; use your material supplier’s data for design.
2. Worked example: +60 K at R20 mm, 2500 lines
Pitch at R20 mm with 2500 lines ≈ 50.27 µm. Heating from 25 °C to 85 °C:
| Substrate | Change in reading radius | Change in pitch |
|---|---|---|
| Borosilicate glass | ≈ 4.0 µm | ≈ 0.01 µm |
| Soda-lime glass | ≈ 10.8 µm | ≈ 0.03 µm |
| Stainless steel (16) | ≈ 19.2 µm | ≈ 0.05 µm |
| PC (70) | ≈ 84 µm | ≈ 0.21 µm |
Glass and metal barely move. A PC disc’s reading radius shifts by nearly 0.1 mm, so check radial readhead tolerance and pitch matching at operating temperature.
3. Disc-to-hub mismatch
Glass bonded to an aluminum hub differs by about 14×10⁻⁶/K. With a 4 mm bore radius and +60 K, the radial difference is about 3.4 µm. A uniform adhesive layer absorbs it; an uneven one can let the disc shift after temperature cycling, leaving a few micrometers of eccentricity — 3 µm at R20 mm ≈ 31″. See eccentricity error.
4. Reflective readhead gap
Reflective readheads are gap-sensitive. Axial expansion of shaft, bearing housing and enclosure changes the gap and the signal amplitude — calculate it over the operating range and keep it within the readhead tolerance.
5. Recommendations
- Wide temperature range, high accuracy → glass disc with a matched-expansion hub or a uniform flexible adhesive layer
- Vibration, shock, thin design → reflective metal disc; check gap vs temperature
- Cost-sensitive, narrow temperature range → PC disc; check radius and pitch change
Tell us the operating temperature range, hub material and readhead model and we will recommend a substrate.