3 Servo Motor Control Modes Explained

1. Servo motor pulse control mode

Servo motors are widely used in various applications due to their precision and reliability. One of the most common methods of controlling servo motors, especially in small stand-alone equipment, is through pulse control. This method is straightforward and easy to understand, making it a popular choice for motor positioning.

Basic Control Concept

The pulse control mode operates on two fundamental principles:

1. Total Pulse Count: This determines the motor’s displacement. The number of pulses sent to the servo motor corresponds directly to the distance the motor will move. For example, if a servo motor requires 2000 pulses to complete one full rotation, sending 1000 pulses will result in a half rotation.
2. Pulse Frequency: This determines the motor’s speed. The frequency at which pulses are sent to the motor dictates how fast the motor will move. A higher pulse frequency results in a faster motor speed, while a lower frequency results in slower movement.

Implementation of Pulse Control

To implement pulse control for a servo motor, follow these steps:

1. Select Pulse Control Mode: Ensure that the servo motor and its controller are set to operate in pulse control mode. This can usually be done through the motor’s configuration settings or by selecting the appropriate mode in the controller’s software.
2. Determine Pulse Requirements: Refer to the servo motor’s manual to understand the relationship between pulses and motor movement. The manual will provide a table or formula indicating how many pulses are required for specific movements.
3. Generate Pulses: Use a pulse generator or a microcontroller to generate the required pulses. The pulse generator should be capable of producing pulses at the desired frequency and count.
4. Send Pulses to Motor: Connect the pulse generator to the servo motor’s input. The motor will receive the pulses and move accordingly. Ensure that the connections are secure and that the pulse generator is configured correctly.

Example Table from Servo Motor Manual

Here is an example of what a typical table from a servo motor manual might look like:

Command pulse form	Signal name	Positive direction command	Negative direction command
90 bit phase difference   2-phase pulse A + B phase	PULS SIGN	B is 90 degrees faster than phase a	B is 90 degrees slower than phase a
Positive pulse train + negative pulse train	PULS SIGN
Pulse + symbol	PULS SIGN

Advantages of Pulse Control

• Simplicity: Pulse control is easy to implement and understand, making it suitable for various applications.
• Precision: By controlling the number and frequency of pulses, precise positioning and speed control can be achieved.
• Flexibility: Pulse control can be used with various types of servo motors and controllers, providing flexibility in system design.

Pulse-Controlled Motor Driver Implementation Methods

In the realm of motor control, particularly for high-speed applications, pulse control methods are crucial for determining the rotation direction and speed of the motor. Below, we explore three distinct methods of pulse control, each with its unique characteristics, advantages, and limitations.

Method 1: Differential Control

Implementation:

• The driver receives two high-speed pulses, labeled as 𝑎a and 𝑏b.
• The rotation direction of the motor is determined by the phase difference between these two pulses.
  • If pulse 𝑏b leads pulse 𝑎a by 90 degrees, the motor rotates in the positive direction.
  • If pulse 𝑏b lags pulse 𝑎a by 90 degrees, the motor rotates in the reverse direction.

Characteristics:

• Alternating Pulses: The two-phase pulses alternate, leading to the term “differential control.”
• Anti-Interference: This method exhibits higher anti-interference capabilities, making it suitable for environments with strong electromagnetic interference.

Limitations:

• Resource Intensive: Requires two high-speed pulse ports for a single motor shaft, which can be a limitation in systems with limited high-speed pulse port availability.

Method 2: Sequential Pulse Control

Implementation:

• The driver still receives two high-speed pulses, but they do not exist simultaneously.
• When one pulse is active, the other must be inactive.
• One pulse controls the positive direction, while the other controls the negative direction.

Characteristics:

• Sequential Pulses: Ensures that only one pulse is output at any given time, preventing simultaneous pulse conflicts.

Limitations:

• Resource Intensive: Similar to differential control, this method also requires two high-speed pulse ports for a single motor shaft.

Method 3: Single Pulse with Directional IO Signal

Implementation:

• The driver receives a single high-speed pulse signal.
• The direction of the motor’s rotation is controlled by an additional directional IO signal.

Characteristics:

• Simpler Control: This method simplifies the control logic by reducing the number of pulse signals required.
• Resource Efficient: Occupies fewer high-speed pulse ports, making it ideal for smaller systems with limited resources.

Limitations:

• Lower Anti-Interference: While simpler, this method may not offer the same level of anti-interference capability as differential control.

Summary

Each pulse control method offers distinct advantages and is suitable for different application scenarios:

• Differential Control: Best for high-interference environments due to its superior anti-interference capabilities but requires more resources.
• Sequential Pulse Control: Offers a balance between control complexity and resource usage but still requires two high-speed pulse ports.
• Single Pulse with Directional IO Signal: Ideal for small systems with limited resources, offering simplicity and efficiency at the cost of potentially lower anti-interference capabilities.

When selecting a control method, consider the specific requirements of your application, including the level of interference, resource availability, and control complexity.

2. Servo Motor Analog Control Mode

In applications where precise speed control of a servo motor is required, analog control can be an effective method. This mode utilizes an analog signal to regulate the motor’s speed, offering a straightforward and flexible approach.

Analog Quantity Selection

The analog control signal can be either a voltage or a current. Each method has its own advantages and considerations:

Voltage Mode

In voltage mode, the control signal is a specific voltage applied to the servo motor’s control input. This method is relatively simple to implement and can even be achieved using a potentiometer for manual adjustments. Here are the key points:

• Implementation: Apply a specific voltage to the control signal end.
• Ease of Use: Simple setup, often using a potentiometer.
• Suitability: Ideal for straightforward applications with minimal environmental interference.

However, voltage signals are susceptible to noise and interference, especially in complex environments. This can lead to unstable motor control, making it less reliable in such scenarios.

Current Mode

Current mode control requires a corresponding current output module to generate the control signal. Despite the additional hardware requirement, current signals offer significant advantages in terms of stability and reliability:

• Implementation: Requires a current output module.
• Anti-Interference: Strong resistance to environmental noise and interference.
• Suitability: Optimal for complex environments where stable control is critical.

The robust nature of current signals makes them preferable in industrial settings or other applications where environmental conditions may affect signal integrity.

3. Servo Motor Communication Control Mode

Servo motors are critical components in various industrial applications, providing precise control over motion. The communication control mode for servo motors is essential for achieving efficient and accurate operation, especially in complex and large-scale systems. Here, we explore the common communication protocols used for servo motor control and their benefits.

Common Communication Protocols

1. CAN (Controller Area Network)
   • Overview: CAN is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer.
   • Advantages: High reliability, real-time capabilities, and error detection mechanisms.
   • Applications: Widely used in automotive and industrial automation systems.
2. EtherCAT (Ethernet for Control Automation Technology)
   • Overview: EtherCAT is an Ethernet-based fieldbus system designed for real-time control applications.
   • Advantages: High-speed communication, low latency, and synchronization capabilities.
   • Applications: Ideal for high-performance applications such as robotics, CNC machines, and motion control systems.
3. MODBUS
   • Overview: MODBUS is a serial communication protocol originally published by Modicon for use with its programmable logic controllers (PLCs).
   • Advantages: Simplicity, ease of implementation, and wide adoption.
   • Applications: Commonly used in industrial environments for connecting electronic devices.
4. PROFIBUS (Process Field Bus)
   • Overview: PROFIBUS is a standard for fieldbus communication in automation technology.
   • Advantages: High data transmission speed, reliability, and extensive diagnostic capabilities.
   • Applications: Used in factory automation and process automation.

Benefits of Communication Control in Servo Motors

Using communication protocols to control servo motors offers several advantages, particularly in complex and large-scale system applications:

• Scalability: The system’s size and the number of motor shafts can be easily adjusted without significant changes to the infrastructure.
• Reduced Wiring Complexity: Communication control reduces the need for extensive control wiring, simplifying installation and maintenance.
• Flexibility: The built system is highly flexible, allowing for easy integration and reconfiguration of components.
• Enhanced Diagnostics: Communication protocols often include diagnostic features that help in monitoring and troubleshooting the system.
• Real-Time Control: Protocols like EtherCAT provide real-time control capabilities, essential for applications requiring precise timing and synchronization.

4. Expansion on Servo Motor Control Modes

1. Servo Motor Torque Control

Torque control mode allows for the precise setting of the motor shaft’s external output torque through the input of an external analog signal or direct address assignment. This mode is particularly useful in applications where maintaining consistent material stress is crucial.Example:

• If a 10V input corresponds to 5 Nm of torque, then a 5V input will result in 2.5 Nm of torque output.
• When the motor shaft load is less than 2.5 Nm, the motor rotates forward.
• When the external load equals 2.5 Nm, the motor remains stationary.
• When the load exceeds 2.5 Nm, the motor reverses direction (common in gravity-loaded systems).

Applications:

• Winding and unwinding devices, such as those used in textile manufacturing or optical fiber pulling equipment, where material stress must remain constant.
• Real-time adjustments to the torque setting can be made by altering the analog input or changing the value through communication protocols, ensuring consistent material stress despite changes in winding radius.

2. Servo Motor Position Control

In position control mode, the motor’s rotation speed is typically determined by the frequency of external input pulses, while the rotation angle is controlled by the number of pulses.Features:

• Some servo systems allow direct assignment of speed and displacement values through communication.
• This mode offers precise control over speed and position, making it ideal for applications requiring high accuracy.

Applications:

• Positioning devices
• CNC machine tools
• Printing machinery

3. Servo Motor Speed Mode

Speed mode allows for control of the motor’s rotation speed via analog input or pulse frequency.Features:

• With outer loop PID control from an upper control device, speed mode can also be used for positioning.
• The position signal from the motor or the direct load must be fed back to the upper computer for processing.
• Supports direct load outer ring position detection, where the motor shaft encoder only measures speed, and a separate device at the load end provides the position signal.

Advantages:

• Reduces errors in intermediate transmission
• Enhances overall system positioning accuracy

4. Understanding the Three Loops

Servo systems typically operate using three closed-loop negative feedback PID regulation systems: the current loop, the speed loop, and the position loop.Current Loop:

• The innermost loop, handled entirely within the servo driver.
• Detects and adjusts the output current of each motor phase using a hall device.
• Controls motor torque with minimal computational load and fast dynamic response.

Speed Loop:

• The second loop, utilizing feedback from the motor encoder.
• The PID output of the speed loop sets the current loop, meaning speed control inherently includes current control.
• Essential for any control mode, as the current loop forms the control foundation.

Position Loop:

• The outermost loop, which can be configured between the driver and motor encoder or between an external controller and the motor encoder/final load.
• The position control loop’s internal output sets the speed loop, necessitating the operation of all three loops in position control mode.
• This mode involves the most computational effort and has the slowest dynamic response.

Conclusion

Understanding the different control modes and the three-loop system of servo motors is essential for optimizing their performance in various industrial applications. Each mode offers unique advantages and is suited to specific tasks, ensuring precise control over torque, position, and speed.