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 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 capabilities, speed control will be more effective.
If the requirements are not very high, or there are basically no real-time requirements, position control does not place high demands 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.
Generally speaking, when it comes to judging the quality of a driver control, each manufacturer claims theirs is the best. However, there's now a more intuitive way to compare them called response bandwidth. During torque or speed control, a square wave signal is sent to the motor via a pulse generator, causing it to continuously rotate forward and reverse, constantly increasing the frequency. The oscilloscope displays a frequency sweep signal. When the peak of the envelope reaches 70.7% of the maximum value, it indicates that synchronization has been lost. The frequency at this point reveals which product is superior. Typical current loops can achieve over 1000Hz, while speed loops can only reach tens of hertz.
To put it in a more professional way:
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, 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 typically determines the rotation speed by the frequency of externally input pulses and the rotation angle by the number of pulses. Some servo systems can also directly assign speed and displacement values via communication. Because position control allows for very strict control over both speed and position, it is generally used in positioning devices.
Application areas include CNC machine tools, printing machinery, etc.
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.
4. Let's talk about the three-loop system. Servo systems generally use three-loop control, which refers to 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 achieving an output current as close as possible to the set current. The current loop controls the motor torque, so in torque mode, the driver's computation is minimized, resulting in the fastest dynamic response.
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.
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