Servo motors generally have three control methods: speed control, torque control, and position control.
Speed control and torque control are both achieved using analog signals. Position control is achieved by sending pulses. The specific control method used depends on the customer's requirements and the desired motion function.
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, torque mode is inconvenient; speed or position mode is preferable. If the host controller has good closed-loop control capabilities, speed control will be more effective. If the requirements are not very high, or if there are virtually no real-time requirements, position control places less emphasis on the host 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's processing speed is slow (e.g., a PLC or a low-end motion controller), position control is used. If the controller's processing speed is fast, speed control can be used, moving the position loop from the driver to the controller, reducing the driver's workload and improving efficiency (as seen in most mid-to-high-end motion controllers). 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, and in this case, servo motors are completely unnecessary.
In other words:
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 torque setting 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 directly obtained from the input.
