What is an Encoder?
An encoder is a device used in the operation of motors to monitor parameters such as current, speed, and the relative angular position of the shaft in real-time. This monitoring helps in determining the state of the motor and the equipment it drives, allowing for real-time control of both the motor and the equipment to achieve functionalities like servo control and speed adjustment.
Here, encoders serve as front-end measurement components, significantly simplifying the measurement system while offering precision, reliability, and robustness.
Encoder Description:
- An encoder is a rotary sensor that converts the physical quantities of position and displacement into a series of digital pulse signals. These signals are then collected and processed by control systems to issue commands that adjust or change the operational state of the equipment. Encoders can also measure linear motion when combined with mechanisms like rack and pinion or lead screws.
Classification of Encoders
Basic Classification:
- Code Disk and Code Scale: Encoders converting linear displacement into electrical signals are called code scales, whereas those converting angular displacement are known as code disks.
- Incremental Encoders: They provide information on position, angle, and number of turns, with resolution defined by pulses per revolution.
- Absolute Encoders: These provide position, angle, and turn count with each angle increment assigned a unique code.
- Hybrid Absolute Encoders: They output two sets of information - one for detecting magnetic pole positions with absolute value features, and another identical to incremental encoder outputs.
Common Motor Encoders:
- Incremental Encoder: Uses optoelectronic conversion to output three square wave pulse phases (A, B, Z). Phases A and B are 90 degrees out of phase, which helps determine the direction of rotation; Z phase provides one pulse per revolution for reference point location. Advantages include simple structure, long mechanical life, strong anti-interference capabilities, high reliability, and suitability for long-distance signal transmission. The downside is the inability to output absolute position information.
- Absolute Encoders: Output digital signals directly. The circular code disk has concentric tracks with alternating transparent and opaque sectors. The number of sectors doubles with each track, representing binary digits. Light sources and photodetectors translate this into binary signals. This encoder type does not require a counter; it can read a fixed digital code corresponding to any position. The resolution increases with more tracks; current products can have up to 16-bit resolution.
Encoder Working Principle
- An encoder features a photoelectric code disk with radial lines creating alternating light and dark patterns. Optoelectronic devices read these to generate four sine waves (A, B, C, D), with a 90-degree phase difference. Signals C and D are inverted and combined with A and B to stabilize the signal. A Z pulse is emitted once per revolution for zero reference.
Encoder Structure
- With A and B phases 90 degrees apart, the direction of rotation can be determined by which phase leads. The Z pulse provides a zero reference. Encoder disks can be made from glass, metal, or plastic, each with different precision, thermal stability, and durability characteristics.
Resolution:
- Defined by how many light/dark lines pass in one 360-degree rotation, typically ranging from 5 to 10,000 lines per revolution.
Position Measurement and Feedback Control
- Encoders are crucial in applications like elevators, machine tools, material processing, motor feedback systems, and various measurement and control devices. They convert light signals into TTL (or HTL) electrical signals, which are analyzed to reflect motor rotation angle and position accurately, enabling precise control when combined with inverters in closed-loop systems.
Summary
In summary, encoders are divided into incremental and absolute types, both converting other signals (like light) into electrical signals for control and analysis. Common applications like elevators and machine tools rely on precise motor adjustment through encoder feedback, paired with inverters for exact control.