Servo motor speed and torque control are both controlled using analog signals, while position control is achieved through pulse generation. The specific control method used depends on the customer's requirements and the desired motion function.
Next, we will introduce three control methods for servo motors.
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 a certain level of accuracy is required for position and speed, but real-time torque is not a major concern, it is better to use speed or position mode.
If the host controller has good closed-loop control capabilities, 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, the motor needs to be adjusted in real time. If the controller itself has a slow processing speed (such as a PLC or a low-end motion controller), position control should be used.
If the controller has a high processing speed, speed control can be used to move 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, removing the speed loop 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 through response bandwidth. During torque or speed control, a square wave signal is given to the motor via a pulse generator, causing it to continuously rotate forward and reverse, constantly increasing the frequency. The oscilloscope displays a swept frequency signal. When the peak of the envelope reaches 70.7% of the maximum value, it indicates loss of synchronization. The frequency at this point indicates the quality of control. Generally, the current loop can achieve speeds above 1000Hz, while the speed loop can only reach 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. For example, if 10V corresponds to 5Nm, then when the external analog input is set to 5V, the motor shaft output will be 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 (typically occurring under gravity loads). The set torque can be changed in real-time by altering the analog input setting, or by changing the corresponding address value via communication. This method is primarily used in winding and unwinding devices where strict requirements on material stress are required, such as wire winding devices or fiber optic drawing equipment. The torque setting must be adjusted continuously according to changes in the winding radius to ensure that the material stress does not change with 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 Mode
Rotational speed can be controlled via analog input or pulse frequency. In speed mode, positioning can also be achieved with outer-loop PID control from a higher-level control unit, but the motor position signal or the position signal from the direct load must be fed back to the higher-level unit for calculation. Position mode also supports direct load outer-loop position signal detection. 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 direct final load end. This reduces errors in the intermediate transmission process and increases the overall positioning accuracy of the system.
4. Discuss the three rings.
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 the 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.
