The basic functions of a servo drive are motor driving and signal feedback. Most modern servo drives have an independent control system, typically using a digital signal processor, high-performance microcontroller, or FPGA as the main control chip. The control system outputs a digital signal with a relatively small current, which cannot directly drive the motor.
Servo drivers also need to convert digital signals into analog signals and amplify them to drive the motor. Internally, a servo driver integrates a main control system circuit, a drive circuit based on power devices, a current acquisition circuit, a Hall sensor acquisition circuit, and protection circuits for overvoltage, overcurrent, and temperature detection.
Servo driver working principle and control method
Servo drives all use digital signal processors (DSPs) as the control core, enabling the implementation of relatively complex control algorithms and achieving digitalization, networking, and intelligence. Power devices generally adopt drive circuits designed around intelligent power modules (IPMs). The IPM integrates the drive circuit and also has fault detection and protection circuits for overvoltage, overcurrent, overheating, and undervoltage. A soft-start circuit is also added to the main circuit to reduce the impact on the driver during startup.
First, the power drive unit rectifies the input three-phase power or mains power through a three-phase full-bridge rectifier circuit to obtain the corresponding DC power. The rectified three-phase power or mains power is then frequency-converted by a three-phase sinusoidal PWM voltage-source inverter to drive the AC servo motor. The entire process of the power drive unit can be simply described as an AC-DC-AC process, with the main topology of the rectifier unit (AC-DC) being a three-phase full-bridge uncontrolled rectifier circuit.
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.
