Encoder Overview
An encoder is an electromechanical device mounted on a motor shaft that tracks and feeds back the motor's speed and position by outputting digital pulses. Its core working principle is as follows: by counting the pulses generated by the encoder, the system can calculate the displacement of the motor's current position relative to the last known position, thereby verifying whether the motor has accurately reached the target position.
The basic structure of an encoder consists of a light source, a disk (code disk) with slots engraved along its edge, and a light receiver. When the code disk rotates together with the motor shaft, the slots chop the continuous static light source into a series of flashes. The light receiver detects these changes between light and dark and converts them into digital square-wave pulse signals, which are then output to the main controller. If the encoder's resolution matches the motor's step resolution, the encoder will generate one corresponding pulse for each step of motor movement.
I. Incremental Encoders
1. Working Principle and Output Signals
An incremental encoder operates by generating a series of pulses during motion. Its code disk has uniformly distributed slots. When the shaft rotates, a fixed photoelectric pickup detects the changes in transmitted light and outputs a continuous pulse sequence. A standard incremental encoder typically provides two square-wave signals with a 90° phase difference (Channel A and Channel B), known as "quadrature signals." The phase relationship between these two signals is used to accurately determine the direction of rotation.
2. Relativity of Position Information and the Power-Off Loss Issue
An incremental encoder outputs relative displacement information rather than absolute position. After the system is powered on, the encoder starts counting and outputting pulses, and an external counter or controller accumulates these pulses to calculate the current position. However, once power is lost, pulse output stops, and if the externally stored count value does not have backup power, it will be lost. After power is restored, the encoder cannot automatically know the current shaft position, and the count value will start again from zero.
3. Necessity of Homing (Returning to Reference)
Due to the above characteristics, systems using incremental encoders must perform a "homing" operation every time they start up or restart after a power failure. This operation usually drives the motor to move until a predefined physical reference point is triggered, such as a limit switch, a magnetic switch, or the Z-phase index pulse on the encoder disk. Once this point is found, the system resets or sets the position counter to a known value, which then serves as the absolute reference for all subsequent motion.
4. Advantages, Disadvantages, and Applications
Advantages: Relatively simple structure, low cost, and high reliability.
Disadvantages: Position information is lost after power-off and depends on a homing operation; anti-interference capability is relatively weak, and noise pulses may be mistakenly counted into the position.
Solution: For applications that require position retention after power-off, a backup battery can be used to supply power to the counter or storage unit.
II. Absolute Encoders
1. Core Principle: Unique Absolute Position Encoding
The fundamental characteristic of an absolute encoder is that each mechanical position on its code disk is assigned a unique digital code. This is usually achieved by manufacturing multiple concentric code tracks on the disk (each track representing one binary bit) and using multiple independent photoelectric sensors. Therefore, even when stationary or powered off, the output signal directly corresponds to the absolute angular position of the shaft.
2. Position Retention After Power-Off and Instant Availability on Power-Up
Since the position information is uniquely determined by the physical pattern of the code disk, an absolute encoder does not lose position after power-off. When the system is powered on again, the controller can immediately read the current absolute position code without performing any homing operation, achieving "power-on and ready-to-use," which greatly improves startup efficiency and safety.
3. Single-Turn and Multi-Turn Types
Single-turn absolute encoders provide a unique position value within one 360° rotation and are suitable for applications where the travel range is less than one revolution.
Multi-turn absolute encoders not only provide a unique value within one revolution, but also record the number of revolutions through an internal gearbox or electronic counting mechanism. They can provide a global absolute position over multiple turns (for example, up to 4096 turns) and are suitable for long-travel positioning applications.
4. Signals and Advantages
Output code: Gray code is commonly used, in which only one bit changes between adjacent positions, effectively preventing reading errors.
Anti-interference capability: The position is determined by the instant reading of the code disk pattern, so occasional electrical noise pulses are not accumulated, resulting in strong noise immunity.
High safety and flexibility: The position can be verified immediately at power-on, avoiding risks caused by starting from an unknown position; any point can be used as a programmable reference, making system design more flexible.
III. Mechanical Absolute Encoders (Magnetic Type)
This is a new type of absolute position sensing solution based on magnetic sensing principles, combining power-off memory with high environmental tolerance.
1. Single-Turn Position Detection Principle
Its core consists of a special composite magnet mounted at the center of the motor shaft (with bipolar magnetization at the center and multipole magnetization at the periphery) and corresponding magnetoresistive sensors. The sensor reads the direction of the central magnetic field to obtain a coarse absolute angle (for example, resolved to 180°), and at the same time detects the phase changes of the high-density peripheral magnetic field to obtain high-resolution angular subdivision. By combining the two, the precise single-turn absolute position can be calculated.
2. Multi-Turn Position Detection Principle
To achieve multi-turn absolute position detection, the system introduces a precision gear train. The main gear is mounted on the motor shaft and is followed by a series of reduction gears with specific tooth ratios. Each gear is equipped with its own magnet and sensor.
Working principle: When the motor shaft rotates, each gear rotates at a different speed. The magnets on these gears generate a unique combination of phase differences related to their positions. The system detects the magnetic flux phase of each gear and, by decoding this set of phase differences, can uniquely determine the absolute mechanical position of the motor shaft over a range of up to several thousand turns.
Design features: The gear tooth counts are specially designed so that the phase difference combination only repeats after reaching the maximum detectable number of turns (for example, 1800 turns), thereby ensuring the uniqueness of the position code. The gears are used only for detection and carry no power load. They are made of self-lubricating resin materials, ensuring long service life.
3. Core Advantages and Application Scenarios
No battery, permanent memory: The position information is determined by the physical positions of the mechanical gears and the magnet patterns, and is never lost even after complete power loss.
High environmental tolerance: With no precision optical components and a fully enclosed magnetic sensing design, it offers far better resistance to dust, oil contamination, condensation, vibration, and certain temperature shocks than optical encoders.
Balance between cost and reliability: Although the resolution may not match that of top-tier optical encoders, its robust structure, high reliability, and maintenance-free battery-free design make it an ideal choice for industrial applications that demand durability, safety, and the elimination of battery maintenance.
IV. Summary and Selection Reference
| Feature | Incremental Encoder | Optical Absolute Encoder | Mechanical (Magnetic) Absolute Encoder |
|---|---|---|---|
| Position information | Relative displacement | Absolute position available at power-on | Permanent absolute position (no battery) |
| After power-off | Position lost, homing required | Position retained (depends on battery or non-volatile memory) | Position permanently retained, no power required |
| Noise immunity | Average (noise pulses may be miscounted) | Good (position read instantly, noise not accumulated) | Good |
| Environmental tolerance | Good | Average (sensitive to dust and condensation) | Excellent (resistant to oil, vibration, temperature changes) |
| Cost | Low | Medium to high | Medium |
| Typical applications | Cost-sensitive systems where homing is acceptable, open-loop or simple closed-loop control | High-precision CNC, robotics, clean environments requiring power-on readiness | Outdoor equipment, heavy machinery, logistics equipment, and harsh industrial environments or applications concerned about battery maintenance |
Conclusion
For open-loop stepper motors or standard servo systems, incremental encoders remain the mainstream choice due to their high cost-effectiveness. In applications that require "power-on and ready-to-use," high safety, or complex positioning functions, absolute encoders are indispensable. Among absolute solutions, mechanical absolute encoders provide engineers with a powerful alternative that can simplify system design and improve long-term reliability, thanks to their battery-free permanent memory and excellent industrial durability.
