When the application scenarios require precise control of position, speed and torque (or a combination of the two), servo motor systems are generally used. According to the controlled parameters, the servo motor system can operate in torque mode, speed mode or position mode. Each mode requires a control loop, which is monitored and controlled by the servo drive and controller and sends commands to the motor to achieve the required performance.
Servo motor control-torque mode
In the torque mode, the current loop controls the motor, and the torque is proportional to the current. Therefore, the controller of the servo system will obtain the actual motor current from the drive and use this to determine the actual motor torque.
Then, it compares the actual torque value with the required torque, and adjusts the current of the motor in real time to achieve the required torque.
The amount of torque produced by the motor depends on the amount of current it receives. Torque determines the acceleration of the motor, which affects speed and position. Therefore, the current control loop of the servo system is necessary.
The current control loop is usually adjusted by a PI (Proportional Integral) controller, and the current loop parameters are usually set by the manufacturer.
The application range that requires torque mode control is wide, from winding (which must maintain a constant tension on the material web during winding) to injection molding (which must apply a constant clamping force on the mold).
Servo motor control-speed mode
When the application scenario requires the motor to maintain the set speed under changing loads, this will use the speed mode.
In speed mode, the motor speed is controlled by the voltage sent to the motor. But to change the speed (acceleration or deceleration) of the motor needs to increase or decrease the motor torque, so the current control loop is also needed in the speed mode.
When multiple control loops are used, the current control is the innermost loop, and the speed control loop is added "around" the current loop.
When using a position control loop, add it around the speed loop to form the outermost loop.
According to the loop adjustment from inside to outside, first adjust the current loop, then adjust the speed control loop, and then adjust the position control loop.
Many advanced servo controllers can "dynamically" switch between control modes, for example, they will not switch from speed mode to torque mode when the system is running, and will not be unstable or interrupted.
The speed control loop mentioned above obtains speed information from the encoder to determine the error between the actual and commanded speeds, and uses the error to determine which current (torque) the motor needs to correct the speed error.
The speed control loop is usually a PI controller, and the servo system running in the speed mode sometimes contains parameters that can smoothly accelerate or decelerate to minimize the influence of errors.
Examples of applications that use speed mode include conveyor tracking, distribution, and machining processes (such as grinding or polishing), during which the motor load changes, but the speed needs to be maintained throughout the process.
When the application scenario requires three control cycles, the servo system can also operate in position mode, allowing the motor to move the load to a precise position relative to the starting position or based on the absolute position.
In order to realize the position mode in servo control, three control loops are usually required: torque, speed and position. This is because the speed of the motor must be monitored to determine its position, and the torque must be monitored to determine how much current the motor needs to reach the commanded position without undershoot or overshoot. The position control loop uses PI or PID (proportional integral derivative) controller.