The Encoder Disk: How Precision is Born
The encoder disk (code disk) is the component that determines the resolution and accuracy of an optical encoder. Disk quality is directly proportional to measurement quality.
Materials
Glass: tempered glass disks with chrome pattern deposited by photolithography. Highest precision (error < 5 arcsec), but fragile. Used in precision encoders Ø58-80 mm.
Mylar/Polyester: plastic film with printed pattern. More economical and robust than glass, but less precise. Used in standard industrial encoders.
Metal: steel disks perforated by EDM or laser. Extreme robustness, suitable for high-vibration environments. Limited resolution.
Incremental Pattern
The incremental disk has equidistant lines arranged radially. The number of lines corresponds to the base resolution (ppr). An additional inner track with a single window generates the index pulse (Z) — one reference per revolution.
Window geometry determines the signal duty cycle (on/off ratio). The ideal duty cycle is 50% (equal high and low pulse duration), achieved with windows exactly matching the pitch width.
Absolute Pattern
The absolute disk has concentric tracks, each corresponding to a resolution bit. A 13-bit disk has 13 tracks with increasing binary pattern from outside (least significant bit) to inside (most significant bit).
Gray code is preferred over natural binary because in a transition between adjacent positions, ONLY ONE BIT changes, eliminating read errors during movement.
Manufacturing Process
- Glass substrate cut to disk and polished
- Chrome deposition by sputtering
- Photoresist application
- UV exposure through precision mask
- Development and chemical etching of chrome
- Optical microscope inspection for quality verification
Maintenance
The disk is the most delicate encoder component. Never clean with aggressive solvents. In case of contamination, use filtered compressed air or isopropyl alcohol with optical cloth.
