Position encoders rely on the quality of the scale as much as on the sensor electronics.
In optical encoders, the scale carries the periodic grating that the sensor reads to determine position. When the grating degrades in linearity, accuracy degrades proportionally.
Glass and metal tape are the two available scale materials, and the choice between them has direct, quantifiable effects on system accuracy, particularly in thermally variable environments.
Glass vs. Metal Tape: The Three Differentiating Parameters
1. Coefficient of Thermal Expansion (CTE)
Thermal expansion is the dominant accuracy-limiting mechanism for optical encoder scales in typical industrial and robotics environments. As scale material heats up, the grating pitch expands, introducing a systematic position error that is linear with temperature deviation and scale length.
| Scale Material | Typical CTE |
|---|---|
| Soda lime glass | ~8 ppm/°C |
| Borosilicate glass | ~3.3 ppm/°C |
| Steel (typical) | ~11–17 ppm/°C |
| Aluminum | ~23 ppm/°C |
At 8 ppm/°C (soda lime glass, one of the most common optical encoder scale materials), a 100 mm scale subjected to a 30°C temperature rise expands by 24 µm.
A comparable steel tape scale would expand by 33–51 µm under the same conditions, a factor of 1.4–2.1 more thermal error.
In compact robot joints, where multiple heat sources (servo drive, brake, motor) are enclosed in a small volume, sustained temperature rises of 30–50°C above ambient are operationally realistic.
The CTE difference between glass and metal tape directly affects achievable position accuracy over an operating shift.
2. Grating Uniformity (Scale Linearity)
The accuracy of an optical encoder is bounded by the uniformity of the grating pitch on the scale. Non-uniform pitch produces periodic errors, systematic position errors that repeat once per revolution (or per unit length for linear scales).
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Glass production processes (specifically chrome deposition through a photolithographic mask) allow tighter grating pitch tolerances than laser-etching on metal foil.
Chromium lines deposited on glass have sharper, more consistent edges than laser-etched lines, which exhibit finite kerf width variation.
The result is that glass scales can support higher accuracy classes: ±2 arc-seconds for rotary applications (as specified for soda lime glass scales at diameters up to 120 mm) versus lower accuracy in equivalent metal tape configurations.
3. Rigidity and Scale Integrity
Glass scales are more rigid than metal foil. Greater rigidity supports:
- Higher grating density (smaller pitch, higher resolution) without distortion during handling and installation.
- Resistance to bending that would alter grating geometry after installation.
- Dimensional stability under mounting preloads that could cause foil distortion.
The mechanical disadvantage of glass is its brittleness.
Glass scales are not suitable for applications where the scale may be subject to impact or bending loads, conditions where metal tape’s flexibility is an actual advantage.
When Glass Scales Are Technically Required
Glass scales are the correct specification when any of the following conditions apply:
- Accuracy class ±2 arc-seconds or better is required for a rotary application.
- Thermal stability across a wide operating range is necessary, e.g., multi-hour production runs with motor heating.
- High resolution through interpolation: interferential optical systems achieve resolution down to 1.2 nm on a 20 µm pitch glass grating. Equivalent performance on metal tape requires a contamination-tolerant optical system that partially compensates for the lower uniformity.
- Surgical robotics or medical imaging: applications where sustained accuracy during extended procedures is critical and environmental conditions are controlled.
Linear Application: Accuracy at ±2 µm
For linear encoding, glass scale systems achieve positional accuracies of ±2 µm across full travel. This performance class is unachievable with metal tape for equivalent scale lengths.
Rotary Application: Accuracy at ±0.005°
For rotary encoding with glass scales, absolute accuracy of ±0.005° is achievable. This corresponds to ±18 arc-seconds — at the high-accuracy end of what interferential optical encoders can deliver.
When Metal Tape Scales Are Acceptable
Metal tape scales are the appropriate specification when:
- The application is cost-driven and high-accuracy class is not required.
- The scale must be long (several meters) where glass would be mechanically impractical.
- The installation environment requires contamination-tolerant optics (where the optical system already compensates for scale imperfections).
- Impact risk exists during installation or in service.
Contamination-tolerant optical architectures (using low-coherence LED sources and wide-area averaging detector arrays) are specifically designed to extract reliable position signals from metal tape scales in semi-industrial environments. These systems filter out signal disturbances from scale scratches, contamination, and typical flatness variations, making metal tape viable at accuracy classes below ±5–10 µm.
Installation Constraints Specific to Glass Scales
Glass scales require care during mounting. Key considerations:
- No bending loads: Glass fractures under bending. Mounting surfaces must be flat and support the full scale back surface.
- Thermal compensation: When the glass scale is bonded to a metal mounting hub, differential thermal expansion between glass (8 ppm/°C) and metal (11–17 ppm/°C) can introduce stress. Manufacturer mounting procedures specify compliant adhesives or mounting fixtures that allow differential expansion without stress buildup.
- Eccentricity for rotary discs: Scale discs mounted to hubs must be concentric with the axis of rotation. Error formula: Angular error = arctan(eccentricity / radius). For a 50 mm diameter disc, 25 µm eccentricity introduces approximately 3.4 arc-minutes of angular error.
For related insights, feel free to explore our breakdown of
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