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Three-loop control principle of servo motor and factors affecting system control
With the continuous advancement of industrial automation, servo control technology, power electronics, and microelectronics have rapidly developed, leading to the maturation of servo motion and control technologies. As a result, Motor motion control platforms have become widely adopted as a high-performance testing method. People’s expectations for servo performance continue to rise.
First and Third Loop Control Principles
1. The first loop is the current loop. This loop operates entirely within the servo driver. A Hall sensor detects the current output of each phase of the driver to the motor, and uses negative feedback to adjust the current setting through PID control, ensuring the output current remains as stable as possible. The current loop controls motor torque, making it most effective in torque mode, where the driver's operation is minimal and its dynamic response is fastest.
2. The second loop is the speed loop. This loop adjusts the signal from the servo motor encoder via negative feedback PID control. The output of the PID in this loop is directly set by the current loop, meaning the speed loop includes both the speed loop and the current loop. In other words, any mode of operation must utilize a current loop. The current loop acts as the follower, while simultaneously performing speed and position control by implementing current (torque) control to achieve the desired speed and position control.
3. The third loop is the position loop, which is the outermost loop. It can be constructed either between the driver and the servo motor encoder or between an external controller and the motor encoder or final load, depending on the specific situation. Since the internal output of the position control loop sets the speed loop, the system performs all three loops in position control mode. At this point, the system requires the most computation and has the slowest dynamic response.
[Figure 1]
Factors Affecting Control
1. The speed loop primarily performs PI (proportional and integral) adjustments. The proportionality is the gain, so we need to adjust the speed gain and the speed integral time constant to achieve the desired effect.
2. The position loop mainly involves P (proportional) adjustments. For this, we just need to set the proportional gain of the position loop. When adjusting the position loop, especially when there are specific requirements for position mode, it’s best to adjust the speed loop first. The parameter adjustments for the position loop and speed loop don’t have fixed values; they depend on the external load’s mechanical transmission connection mode, the movement pattern of the load, the load inertia, and the speed, acceleration requirements, as well as the rotor and output inertia of the motor itself. These are determined by numerous conditions. A simple adjustment method is to start by gradually increasing the gain parameter within the general range of experience based on the external load. The integral time constant should be adjusted from large to small, aiming for a steady-state value without vibration or overshoot, to reach the optimal value.
[MES-100 Control Method]
1. The MES-100 motion control platform consists of a motor and loading system, a motor driver debugging system, a data acquisition and power system. From the motor to the drive, it builds a complete hardware and software experimental environment, offering a fully open software and hardware interface. This provides a rich and scalable teaching experience, allowing for motor identification, stall testing, motor efficiency tests, motor parameter measurements, motor TN curve tests, motor motion control, encoder vector torque, and non-inductive vector speed analysis among other tests. The test results are illustrated in Figure 3.
[Image 3]
2. The speed and torque control of the servo motor are managed by analog quantities, while position control is handled by pulses. If there are no specific requirements for the motor's speed and position, the constant torque mode can be used. The set torque can be changed by altering the analog setting or modifying the corresponding address value through communication. If there are specific requirements for the position and speed, the speed or position mode can be employed. The position control mode typically determines the rotational speed based on the externally input pulse frequency and confirms the rotational angle by the number of pulses. It offers strict control over speed and position, making it widely used in various industries.
In conclusion, the MES-100 platform is a versatile tool for servo motor testing and control, offering flexibility and precision across multiple modes of operation. Its ability to handle a variety of scenarios makes it an essential component in modern automation systems.