Servo drives , also known as servo controllers or servo motor controllers, are control boards used to operate AC servo motors. Their function is similar to that of a frequency converter for general AC motors, belonging to the servo control system and primarily used in high-precision mobile positioning systems. They typically control AC servo motors using methods such as position, speed, and torque to achieve highly precise transmission positioning, and are currently high-end products in transmission technology.
Currently popular servo controllers all use large digital signal processors (DSPs) as the control core, which can support very complex control algorithms and achieve intelligent, digital, and smart systems. Power electronic devices widely use optocoupler circuits with intelligent power control modules ( IPMs ) as the key design. The IPM integrates optocoupler circuits and has fault test and protection circuits for overvoltage, overcurrent, overtemperature, and undervoltage. A soft-start circuit is also added to the main control loop to reduce the impact on the controller during the startup process.
The output power driver module first rectifies the input power or voltage using a three-phase full-bridge rectifier circuit to obtain the corresponding AC power. The rectified power or voltage is then used to drive a three-phase permanent magnet synchronous AC servo motor via a three-phase sinusoidal PWM voltage-type inverter. The entire process of the power drive unit can be simply described as an AC-DC-AC process. The main topology of the rectifier unit ( AC-DC ) is a three-phase full-bridge uncontrolled rectifier circuit.
With the large-scale application of servo systems, the use, debugging, and maintenance of servo drives have become important technical issues. More and more industrial control technology operators are conducting in-depth research on servo drives.
Servo drives are an important component of modern motion control and are widely used in automated equipment such as industrial robots and CNC machining centers. Servo drives for controlling AC permanent magnet synchronous motors, in particular, have become a research hotspot both domestically and internationally. Current AC servo drive designs commonly employ a three-loop control algorithm based on vector control, encompassing current, speed, and position. The rationality of the speed closed-loop design within this algorithm plays a crucial role in the overall performance of the servo control system, especially in terms of speed control performance.
Generally, servo drives have three control modes: position control, torque control, and speed control.
1. Position control: Position control mode generally determines the rotation speed by the frequency of externally input pulses and the rotation angle by the number of pulses. Some servos can also directly assign values to speed and displacement through communication. Because position mode can have very strict control over both speed and position, it is generally used in positioning devices.
2. Torque Control: Torque control is achieved by setting the output torque of the motor shaft through external analog input or direct address assignment. The torque can be changed by changing the analog input in real time, or by changing the corresponding address value through communication.
The main applications are in winding and unwinding devices that have strict requirements on the material, such as winding devices or optical fiber drawing equipment. The torque setting must be changed at any time according to the change of the winding radius to ensure that the stress on the material does not change with the change of the winding radius.
3. Speed Mode: Rotation speed can be controlled via analog input or pulse frequency. With an external PID control system connected to a higher-level controller, speed mode can also be used for positioning, but the motor position signal or the position signal from the direct load must be fed back to the higher-level controller for calculation. Position mode also supports direct load external loop position signal detection. In this mode, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the detection device at the direct final load end. This reduces errors in the intermediate transmission process and increases the overall positioning accuracy of the system.
If there are no requirements for the speed or position of the motor, and all that is needed is a constant torque output, then torque mode is the appropriate choice.
If you have certain accuracy requirements for position and speed, but are not very concerned about real-time torque, torque mode is not very convenient, and speed or position mode is better.
If the host controller has good closed-loop control, speed control will be more effective. If the requirements are not very high or there are basically no real-time requirements, position control can be used.
The main control methods of servo drivers for motors
The main control methods of servo drives for motors are: position control, speed control, and torque control.
Position control refers to the driver's control over the motor's speed, angle, and torque. The host computer sends pulse trains to the driver to control the speed and angle. The input pulse frequency controls the motor's speed, and the input pulse number controls the motor's rotation angle.
Speed control: This refers to the driver controlling only the speed and torque of the motor. The rotation angle of the motor is controlled by the CNC taking the A , B , and Z encoder signals fed back by the driver. The CNC sends analog (voltage) signals to the driver, ranging from +10V to -10V . Positive voltage controls the motor to rotate forward, and negative voltage controls the motor to rotate in reverse. The magnitude of the voltage value determines the number of rotations of the motor.
Torque control: This refers to the servo driver controlling only the motor's torque. The motor's output torque does not change with the load; it only responds to the input torque command. The host computer sends an analog (voltage) signal to the driver, ranging from +10V to -10V . Positive voltage controls the motor to rotate forward, and negative voltage controls it to rotate in reverse. The magnitude of the voltage value determines the motor's output torque. The motor's speed and angle are controlled by the host computer.
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