optical encoder disc working principle

An optical encoder disc (also known as code wheel) is the core optical‑mechanical component inside incremental or absolute optical rotary encoders. It converts mechanical rotational movement into readable electrical digital signals. By cooperating with light source, photodetector and signal processing circuit, the encoder disc enables accurate measurement of rotation angle, rotational speed, rotation direction and position feedback for motion‑control equipment.

Core Composition of Optical Encoder Assembly

A complete optical reading unit mainly includes four key parts:

1. Light source: Usually infrared LED, emits stable parallel light beam toward the encoder disc.
2. Optical encoder disc: Rotating disc with precisely patterned transparent slots and opaque blocking areas. It rotates synchronously with the target rotating shaft. Metal etched code discs and glass code discs are two mainstream substrate options.
3. Receiver / Photodetector: Photoelectric sensor placed on the opposite side of light source, receives light passing through disc slots and converts light intensity changes into electrical pulse signals.
4. Signal processing circuit: Amplifies, shapes and organizes raw pulses into square‑wave A/B/Z phase output for controller reading.

Step‑by‑Step Working Principle

1. The encoder disc is rigidly mounted to the motor or rotating shaft, spinning together with the shaft.
2. Infrared light from LED shines continuously onto the surface of rotating encoder disc.
3. When light hits transparent slots on the disc: light passes through and reaches photodetector → high‑level electrical signal is generated.
4. When light hits opaque solid area of the disc: light is blocked → low‑level electrical signal is generated.
5. Continuous rotation makes light alternately pass and be blocked. This produces periodic pulse‑wave signals.
6. Circuit module processes these pulses into standard A‑phase, B‑phase and Z‑phase index signals.
   • A phase & B phase: Two channels with 90‑degree phase difference. Controller counts pulses for speed calculation; judges rotation direction by comparing A/B sequence.
   • Z‑phase (index pulse): One single reference slot per full rotation. Provides zero‑position reference for position homing.

Difference Between Incremental and Absolute Encoder Disc

Incremental optical encoder disc

Carries repetitive equally‑spaced slot patterns. It outputs relative pulse changes during rotation. Cannot report absolute angle after power‑on reset; requires zero‑point homing via Z signal. Widely used for general‑purpose speed and motion feedback.

Absolute optical encoder disc

Has multiple concentric code tracks with unique coded slot patterns. Each angular position corresponds to one exclusive binary code. After power‑on, it directly outputs real‑time absolute angle position without homing. Suitable for high‑reliability position‑critical applications.

Metal‑Etched vs Glass Encoder Disc

Glass encoder disc

‑ High dimensional stability, extremely fine slot resolution. ‑ Brittle, anti‑vibration performance limited, higher cost. ‑ Mainly applied in high‑precision laboratory and optical instruments.

Photochemical etched metal encoder disc

‑ Thin stainless steel / nickel alloy substrate, good mechanical strength, shock‑resistant. ‑ Etched slot edges are burr‑free, stable dimensional accuracy. ‑ More resistant to vibration and temperature shock, cost‑effective for industrial mass‑production. ‑ Common for servo motors, stepper motors, automation equipment.

Key Performance Parameters of Encoder Disc

‑ Line count: Number of slots per full circle; determines angular resolution. Higher line count = finer angle measurement. ‑ Slot width & gap consistency: Poor slot uniformity will cause signal distortion, jitter and counting error. ‑ Substrate flatness: Warping will deflect light path and degrade signal quality. ‑ Outer diameter & inner mounting hole: Mechanical dimension matching for shaft assembly.

Common Industrial Applications

‑ Servo & stepper motor position feedback ‑ Robotics joint motion control ‑ CNC machine tools ‑ Medical diagnostic equipment ‑ Optical measuring instruments ‑ Automated production machinery

If you require custom optical encoder discs, submit your line count, dimension and drawing requirements for DFM review and quotation.