Servo control systems have three control modes: speed control, torque control, and position control.
Basic knowledge
I. Servo System Components (from top to bottom)
Controller: PLC, frequency converter, motion control card, and other control devices, also known as host computer; Servo drive: connects the host computer and the servo motor, functioning similarly to a frequency converter for a regular AC motor. Servo motor: the actuator that receives control signals from the drive; Mechanical device: converts the circular motion of the servo motor (or the linear motion of the linear motor) into the required motion; Various sensors and relays: detect various signals in the industrial control environment and send them to the host computer or drive as criteria for judging certain actions.
II. Servo Control Method
There are three control methods: speed control, torque control, and position control. Speed control and torque control are both controlled using analog signals, while position control is controlled by sending pulses.
▶If you don't have requirements for the speed or position of the motor, and only need to output a constant torque, then of course you should use torque mode.
▶If you have certain requirements for position and speed accuracy, but are not very concerned about real-time torque, it is better to use speed or position mode.
▶If the host controller has good closed-loop control function, speed control will be more effective.
▶If the requirements are not very high, or there is basically no real-time requirement, the position control method does not place high demands on the upper-level controller.
In terms of servo drive response speed, torque mode has the least computational load and the fastest response to control signals; position mode has the greatest computational load and the slowest response to control signals.
When high dynamic performance is required during motion, real-time adjustments to the motor are necessary. If the controller itself has a slow processing speed (such as a PLC or a low-end motion controller), position control is used. If the controller has a faster processing speed, speed control can be used, moving the position loop from the driver to the controller, reducing the driver's workload and improving efficiency. If a better host controller is available, torque control can also be used, moving the speed loop away from the driver as well; this is generally only possible with high-end dedicated controllers. A relatively intuitive way to compare the quality of driver control is by its response bandwidth.
When torque or speed control is applied, a square wave signal is given to the motor through a pulse generator, causing the motor to continuously rotate forward and reverse, and the frequency is continuously increased. The oscilloscope displays a frequency sweep signal. When the vertex of the envelope reaches 70.7% of the highest value, it indicates that the motor has lost synchronization. At this point, the frequency level indicates the quality of control. Generally, the current loop can achieve more than 1000Hz, while the speed loop can only achieve tens of hertz.
1. Torque control
Torque control is achieved by setting the output torque of the motor shaft through external analog input or direct address assignment. Specifically:
For example, 10V corresponds to 5Nm. When the external analog signal is set to 5V, the motor shaft output is 2.5Nm. If the motor shaft load is below 2.5Nm, the motor rotates forward; if the external load is equal to 2.5Nm, the motor does not rotate; and if the load is greater than 2.5Nm, the motor rotates in reverse (usually occurring under gravity load). The torque setting can be changed in real-time by altering the analog signal setting, or it can be achieved by changing the corresponding address value via communication.
The main applications are in winding and unwinding devices where there are strict requirements on the stress on the material, such as wire 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.
2. Position control
Position control mode typically determines the rotational speed by the frequency of externally input pulses and the rotational angle by the number of pulses. Some servo systems can also directly assign speed and displacement values via communication. Because position control mode allows for very strict control over both speed and position, it is generally used in positioning devices. Applications include CNC machine tools, printing machinery, and more.
3. Speed control
Rotation speed can be controlled by analog input or pulse frequency. In the outer loop PID control with a host control device, speed mode can also be used for positioning, but the position signal of the motor or the position signal of the direct load must be fed back to the host for calculation.
The position mode also supports direct load outer loop detection of position signals. In this case, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the detection device at the final load end. The advantage of this is that it can reduce the error in the intermediate transmission process and increase the positioning accuracy of the entire system.
Servo 3-loop: Servo motors are generally controlled by three loops, which are three closed-loop negative feedback PID control systems.
The innermost PID loop is the current loop, which operates entirely within the servo driver. It uses Hall effect sensors to detect the output current of each phase of the motor and provides negative feedback to the current setting for PID adjustment, thereby ensuring that the output current is as close as possible to the set current. The current loop controls the motor torque, so the driver's computation is minimized and the dynamic response is fastest in torque mode.
The second loop is the speed loop, which uses the signal from the motor encoder for negative feedback PID control. Its PID output is directly set by the current loop. Therefore, speed loop control includes both speed loop and current loop. In other words, the current loop must be used in any mode. The current loop is the foundation of control. While controlling speed and position, the system is also controlling the current (torque) to achieve corresponding control of speed and position.
The third loop is the position loop, which is the outermost loop. It can be built between the driver and the motor encoder, or between the external controller and the motor encoder or the final load, depending on the actual situation. Since the output of the position control loop is the setting of the speed loop, the system performs calculations for all three loops in position control mode. At this time, the system has the largest computational load and the slowest dynamic response speed.
