Overview
An encoder is a device that encodes or converts signals or data, such as bitstreams, into signal formats suitable for communication, transmission, and storage. An encoder converts rotary or linear displacement into electrical signals. Rotary displacement is detected by a code disk, while linear displacement is detected by a code strip.
Main Components
An encoder is mainly composed of a code disk (circular grating and indicator grating), a housing, a light source, and photosensitive elements.
1. Circular grating: The circular grating consists of alternating light and dark radial stripes formed by coating a transparent substrate or etching on metal. The distance between two adjacent stripes is called one grating period. The total number of grating periods around the circumference equals the encoder pulse count, i.e., the resolution.
2. Indicator grating: The indicator grating is a fixed plate whose window stripe pattern matches the circular grating pattern.
3. Housing: The housing carries and supports the circular grating, indicator grating, and other components.
4. Light source: The light source is typically an infrared LED.
5. Photosensitive elements: These are high-speed photosensors, typically silicon photodiodes and phototransistors.
Encoder Types by Operating Principle
Encoders are classified into incremental and absolute types based on their operating principle.
Incremental Encoder
1. Function: An incremental encoder converts displacement into periodic electrical signals, which are then converted into counting pulses. The number of pulses represents the magnitude of displacement.
Advantages and Disadvantages
Advantages:
- High accuracy (accuracy can be further increased using frequency multiplication circuits).
- Simple construction and lower cost. Suitable for angle measurement and speed measurement. Non-contact measurement offers high reliability and long service life.
Disadvantages:
- Requires homing at startup to find a zero reference.
- Cumulative errors can occur from interference during pulse transmission.
- A counter is required, and achievable speed is limited by system constraints.
Usage Considerations
- Plan for zero finding or homing on startup.
- Mitigate and account for cumulative errors due to interference.
- Balance the tradeoff between maximum rotational speed and resolution.
- Handle possible counter overflow.
Absolute Encoder
1. Function: An absolute pulse code disk is an absolute angular position detection device. Its position output is a digital signal in a specific format that represents the absolute position after displacement. To determine a displacement, the digital codes for the start and end absolute positions must be compared and processed.
2. Structure: The main parts are a rotating code disk, a light source, and photosensitive elements. Optical tracks on the disk have transparent and opaque regions distributed according to a defined pattern.
3. Principle: Light from the source passes through the optical system and the transparent regions of the code disk; after narrow slits it reaches photosensitive elements, which output "1" when light is received and "0" when blocked by opaque regions. The combined outputs from multiple tracks represent the angular position of the disk.
Advantages and Disadvantages
Advantages:
- High accuracy and non-contact operation, yielding long service life.
- No homing required at startup.
- No cumulative error from counting pulses.
- No external counter required; supports higher permissible rotational speeds.
Disadvantages:
- More complex structure and larger physical size.
- Higher cost compared with incremental encoders.
Usage Considerations
- Ensure a reliable power supply and consider voltage drop over long cable runs.
- Design for handling bit errors and signal integrity issues.
- Consider handling at zero-crossing or wrap-around points.
