Does temperature change affect encoder disc accuracy? How to evaluate substrate thermal expansion
Uniform thermal expansion does not change the angular positions of the graduations, so its effect on the angle measurement itself is very small; what really matters is the expansion mismatch between the disc and the hub, the change in grating pitch at the reading radius, and the gap of reflective readheads. This article gives expansion coefficients for common substrates and worked examples.
2026-10-05 · Langfang Jingzheng Optoelectronic Technology Co., Ltd.
Bottom line: when an encoder disc is heated uniformly, every line stays at its original angle, so temperature has very little effect on the angle reading itself. Three things need attention: the expansion mismatch between the disc and the hub and shaft, the change in grating pitch at the reading radius, and the mounting gap of reflective readheads.
1. Expansion coefficients of common substrates
| Material | Coefficient of linear expansion (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 |
The table shows typical values from common references; refer to the material supplier's data for actual values.
2. Worked example: how much do disc dimensions change with a 60 K temperature rise?
Take a reading radius of 20 mm and 2500 lines as an example: the grating pitch is about 50.27 μm. With the temperature rising from 25 ℃ to 85 ℃ (a 60 K rise):
| Substrate | Change in reading radius | Change in grating pitch |
|---|---|---|
| Borosilicate glass | approx. 4.0 μm | approx. 0.01 μm |
| Soda-lime glass | approx. 10.8 μm | approx. 0.03 μm |
| Stainless steel (16) | approx. 19.2 μm | approx. 0.05 μm |
| PC (70) | approx. 84 μm | approx. 0.21 μm |
Glass and metal discs change very little. The reading radius of a PC disc changes by close to 0.1 mm, so the readhead's radial mounting tolerance and pitch matching must be checked at the operating temperature.
3. Expansion mismatch between disc and hub
When a glass encoder disc is bonded to an aluminum hub, their expansion coefficients differ by about 14×10⁻⁶/K. With a bore radius of 4 mm and a 60 K temperature rise, the radial dimensional difference is about 3.4 μm. With a uniform adhesive layer, this difference is absorbed by the adhesive; with an uneven layer, the disc may shift as a whole after temperature cycling, resulting in an eccentricity of a few micrometers. At a reading radius of 20 mm, 3 μm of eccentricity corresponds to an angular error of about 31″. For how to calculate the effect of eccentricity, see How much angular error does 5 μm of disc eccentricity cause.
4. Reflective readhead gap
Reflective readheads are relatively sensitive to the gap to the disc. Axial expansion of the shaft, bearing housing and enclosure when heated changes the gap and affects the signal amplitude. At the design stage, calculate the gap change over the operating temperature range and keep it within the readhead's allowable tolerance.
5. Selection recommendations
- Wide temperature range, high accuracy: glass encoder disc, with a hub made of a material with a similar expansion coefficient, or a flexible, uniform adhesive layer
- Vibration and shock, thin design: reflective metal encoder disc; check the change in readhead gap with temperature
- Cost-sensitive, narrow temperature range: PC encoder disc; check the changes in reading radius and grating pitch
In your RFQ, state the operating temperature range, hub material and readhead model, and our engineers will recommend a substrate accordingly.
This article was compiled by the engineering department of Langfang Jingzheng Optoelectronic Technology Co., Ltd. Jingzheng Optoelectronics manufactures glass encoder discs, reflective metal encoder discs, PC encoder discs and custom encoder discs. Website: www.encoderdisk.cn; phone: 0316-8933331.