Understanding Servo Motor and Photoelectric Encoder Application Circuits

Encoders are critical sensors in automation, CNC machining, robotics, and industrial control systems. They translate motion—whether rotational or linear—into electrical signals for precise control and feedback. This article explains the fundamentals of servo motor encoders and photoelectric encoders, explores real-world application circuits, and highlights common challenges with recommended improvements.

01. Servo Motor Encoders: Principles and Applications

A servo motor encoder is an electromechanical device that provides feedback about the motor’s position, speed, and rotation direction. It ensures that the control system knows exactly where the motor shaft is at any given time.

Servo encoders are typically mounted on the motor shaft and come in two main types:

• Incremental Encoders – Generate pulse signals proportional to shaft rotation. Each pulse corresponds to a fixed angular increment. Direction is determined by comparing phase-shifted signals (A/B channels).

• Absolute Encoders – Provide a unique digital code for each shaft position, ensuring precise angle detection even after power loss.

Key Functions of Servo Motor Encoders:

• Enable closed-loop control for accurate positioning

• Provide real-time speed feedback

• Detect shaft rotation direction

• Improve efficiency and reduce overshoot in servo systems

Application Example:
In CNC machine tools, servo encoders measure spindle and axis rotation. This data ensures cutting accuracy, synchronizes multi-axis movements, and provides stability under high-speed operations.

02. Photoelectric Encoder Application Circuits

A photoelectric encoder converts angular or linear displacement into electrical signals using a light source, a code disk with alternating transparent and opaque sections, and a photodetector. These encoders are widely used due to their high precision, reliability, and durability.

2.1 EPC-755A Photoelectric Encoder in a Car Driving Simulator

The EPC-755A encoder is selected for its excellent angle measurement, anti-interference capability, and stable pulse output.

• Resolution: 360 pulses per revolution

• Circuit Type: Open-collector output

• Application: Steering wheel angle detection in driving simulators

Working Principle:

• When rotating clockwise, Channel A leads Channel B by 90°. The circuit directs pulses to the count-up input of a bidirectional counter (74LS193).

• When rotating counterclockwise, Channel A lags Channel B by 90°, and pulses are directed to the count-down input.

Counting Range Example:

• With ±2.5 turns of steering rotation, the encoder outputs up to 900 pulses.

• A counting circuit of three 74LS193 chips extends the range, enabling precise tracking of steering angle for simulation control.

2.2 Photoelectric Encoders in Gravity Measuring Instruments

Incremental encoders are frequently used in gravimeters because of their simple construction and high resolution.

• Code Disk: Usually contains three tracks (A, B, and reference Z).

• Output: Two quadrature signals (A & B) with 90° phase difference, plus a reference pulse.

• Function: Direction is determined by whether A leads or lags B.

Problem of Erroneous Pulses:
Mechanical jitter or manual alignment can cause unwanted transitions, generating false counts.

Improved Solution – Quadrature Subdivision Circuit:

• A four-frequency subdivision method uses D-type flip-flops and a clock generator.

• Converts 1000 pulses/rev into 4000 pulses/rev, improving resolution to 0.09°.

• Cancels out false pulses from vibration, ensuring accuracy. 

Basic Waveforms and Circuitry of Incremental Photoelectric Encoders：

03. Issues and Improvement Measures in Encoder Applications

3.1 Common Issues

• Mechanical vibrations causing misalignment or unstable signals

• Environmental stress such as high temperature or humidity damaging components

• Electromagnetic interference (EMI) distorting output waveforms

3.2 Practical Improvement Measures

• Mounting Method: Use independent brackets instead of attaching encoders directly to motor housings to reduce vibration (down to 1.2 mm/s).

• Signal Transmission: Replace standard shielded cables with twisted-pair shielded cables, reducing EMI and ensuring clean signal transmission.

• PLC Software Integration: Monitor encoder signals in real-time to prevent errors during processes like continuous casting. Logic can be applied to filter false signals and synchronize mechanical movements.

Before the dummy bar delivery process is initiated, photoelectric signal 1 is "1".

After the dummy bar delivery process is started, in Phase A, the roller table is activated, and the dummy bar is fed upwards. When the dummy bar blocks the infrared light emitted by the photoelectric device, the photoelectric signal becomes "0". When the infrared light passes through the two small circular holes in the middle of the dummy bar, the photoelectric device emits signals 2 and 3, both of which are "1".

In Phase B of the dummy bar delivery process, the photoelectric signal is "0", the roller table stops, and the upward feeding of the dummy bar is paused. The 10th sector of the fan-shaped segment is pressed down, and the withdrawal straightener and "Synchronization 1" are activated, allowing the dummy bar to continue feeding upwards.

In Phase C of the dummy bar delivery process, the dummy bar continues to feed upwards and no longer blocks the infrared light. Photoelectric signal 4 becomes "1", activating "Synchronization 2" and halting "Synchronization 1". The dummy bar continues to feed upwards. At this point, the working process of the photoelectric device is completed.

The output signals of the PLC program are then input into the PLC's input module, replacing the original input signals of the photoelectric signals. The program flowchart is illustrated in Figure 6.

Conclusion

Servo motor encoders and photoelectric encoders are essential in modern automation, CNC machining, and precision instruments. While they deliver high accuracy and stability, challenges such as vibration, EMI, and environmental conditions must be addressed with proper mounting, cabling, and signal processing methods.

By combining servo motor feedback loops, photoelectric encoder circuits, and smart PLC integration, engineers can build reliable, high-performance motion control systems across industries.