A Review of the Incremental Encoder

Introduction
Operating Principles of the Incremental Encoder
Signal Structure and Measurement Principles
Types of Technologies Used in Encoders
Electrical Output Types
Resolution and Measurement Accuracy
Error Sources and Compensation Methods
Application of the Incremental Encoder in CNC Systems
Industrial Installation and Maintenance Notes
Conclusion
References

1. Introduction

The incremental encoder is one of the most widely used position and speed sensors in industrial automation systems. This type of encoder is used in CNC machines, servo drives, and precision motion control systems as a feedback device for measuring the angular speed and shaft position of a motor.

2. Operating Principles of the Incremental Encoder

In the structure of an incremental encoder, a disc with a specific pattern of opaque and transparent lines is mounted on the motor shaft. A light source (LED) and a photodetector are responsible for generating electrical pulses as the lines pass in front of them. The main outputs consist of the A, B, and Z signals.

3. Signal Structure and Measurement Principles

The A and B signals are generated as square waves, with a 90-degree phase difference between them, and are used to determine the direction of rotation. The Z (or Index) signal produces one pulse per revolution, which is used as the zero reference.

4. Types of Technologies Used in Encoders

Incremental encoders are divided into three main groups based on their construction technology: optical, magnetic, and inductive. Each has specific characteristics and is selected depending on the operating environment.

5. Electrical Output Types

The encoder’s output must be compatible with the control circuit. The most important output types are TTL, HTL, Open Collector, Push-Pull, and Differential RS422; in noisy industrial environments such as CNC, RS422 is typically used.

6. Resolution and Measurement Accuracy

An encoder’s resolution is expressed in Pulses Per Revolution (PPR). The higher the PPR, the higher the measurement accuracy. In CNC applications, encoders with 1,000 to 10,000 PPR are typically used.

7. Error Sources and Compensation Methods

The most important possible errors in an incremental encoder include:

Jitter: random timing fluctuation in the spacing between pulse edges. In simple terms, the timing of the A or B edges is not perfectly uniform; it occurs slightly earlier or later than the ideal value.

Phase Error: in an incremental encoder, the A and B signals are normally generated with a 90-degree phase difference. If this phase difference is not accurate (for example, 85° or 95° instead of 90°), errors occur in determining the direction of rotation or the pulse count.

Signal Drop: the complete or temporary loss of the A, B, or Z signals — for example, due to a disconnection, excessive noise, or a voltage drop.

Mechanical Misalignment: if the encoder shaft is not precisely aligned with the motor shaft, vibration or play can cause non-linear variations in the pulses.

To reduce these errors, digital filters, shielded cables, and differential drivers are used.

8. Application of the Incremental Encoder in CNC Systems

In CNC machines, an incremental encoder is installed on the motion axes (X, Y, Z) and on the spindle to control the precise position of the tool. This data is used for closed-loop control to reduce motion error.

9. Industrial Installation and Maintenance Notes

For installing the encoder, a precise, play-free coupling should be used. Communication cables must be shielded and kept away from sources of noise. Keeping the sensor clean is very important in optical models.

10. Conclusion

As one of the key components of motion control systems, the incremental encoder plays an important role in the accuracy and stability of CNC machines. Choosing the right type of encoder and installing it correctly increases the system’s service life and efficiency.

11. References

Siemens — “Incremental Encoders for Motion Control”, Technical Manual, 2023.
Omron Industrial Automation — Encoder Selection Guide, 2022.
Heidenhain — “Fundamentals of Position Encoders”, 2021.
Rockwell Automation — “Feedback Devices in CNC Systems”, 2023.
Internal sources: technical documentation of Iranian industrial automation companies (2024).

Author: Nima Rad