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3 Types of Servo Motor Control Modes

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3 Types of Servo Motor Control Modes

2023-12-27 09:46:19

Servo motor speed control and torque control are both controlled using analog signals, while position control is achieved through pulse control. The specific control mode to be used depends on the customer's requirements and the desired motion functionality.

Next, let me introduce the three control modes of servo motors:

  • If you don't have any specific requirements for motor speed and position, and only need a constant torque output, then torque mode is the appropriate choice.

  • If you have certain accuracy requirements for position and speed, and real-time torque is not a major concern, then speed or position mode is preferable.

  • If the upper-level controller has good closed-loop control capabilities, using speed control mode will yield better results. If the requirements are not very high, or there is minimal real-time requirement, using position control mode does not place high demands on the upper-level controller.

In terms of servo driver response speed: torque mode has the least computational load, resulting in the fastest response to control signals; position mode has the highest computational load, resulting in the slowest response to control signals.

When there is a high requirement for dynamic performance during motion, it is necessary to adjust the motor in real-time.

  • If the controller itself has a slow processing speed (such as PLC or low-end motion controllers), position mode should be used.

  • If the controller has a relatively fast processing speed, speed mode can be used to move the position loop from the driver to the controller, reducing the workload on the driver and improving efficiency.

  • If there is a better upper-level controller, torque mode can also be used, moving the speed loop away from the driver. However, this is generally only possible with high-end specialized controllers.

Generally, the quality of driver control can be compared using a more intuitive method called response bandwidth.

During torque or speed control, a square wave signal is applied to the motor using a pulse generator, causing the motor to continuously rotate in both directions and gradually increasing the frequency. The oscilloscope displays a sweeping frequency signal, and when the envelope reaches 70.7% of the maximum value, it indicates that the motor has lost synchronization. The frequency at this point can indicate the quality of control. Typically, the current loop can achieve frequencies above 1000Hz, while the speed loop can only achieve frequencies in the tens of Hz.

NO.1 Torque Control

Torque control mode sets the output torque of the motor shaft to the outside world by using an external analog signal input or direct addressing. For example, if 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 lower than 2.5Nm, the motor rotates in the forward direction; when the external load is equal to 2.5Nm, the motor does not rotate; when the load exceeds 2.5Nm, the motor rotates in the reverse direction (usually occurs in the presence of gravity load). The set torque can be changed instantly by changing the analog signal setting or by changing the corresponding address value through communication.

This mode is mainly used in winding and unwinding devices that have strict requirements on material forces, such as winding devices or fiber tensioning equipment. The torque setting needs to be changed at any time according to the variation of the winding radius to ensure that the material force does not change with the change of the winding radius.

NO.2 Position Control

Position control mode generally determines the rotation speed by the frequency of external input pulses and determines the rotation angle by the number of pulses. Some servos can directly assign values to speed and displacement through communication. Since position mode can have strict control over both speed and position, it is generally used in positioning devices.

Applications include CNC machine tools, printing machinery, and more.

NO.3 Speed Mode

The rotational speed can be controlled by analog input or pulse frequency. In speed mode with outer loop PID control by upper control device, it can also be used for positioning. However, the position signal of the motor or the position signal of the direct load must be fed back to the upper level for calculation. In position mode, it also supports direct load outer loop detection of position signal. In this case, the encoder at the motor shaft only detects motor speed, and the position signal is provided by the direct detection device at the final load end. The advantage of this is that it can reduce errors in the intermediate transmission process and increase the positioning accuracy of the entire system.

Talking about the 3 loops

Servo motors are generally controlled by three loops, which are three closed-loop feedback PID control systems. The innermost PID loop is the current loop, which is completely carried out inside the servo drive. It detects the output current of each phase of the motor through a Hall device, and provides negative feedback to adjust the set current through PID control, so as to make the output current as close as possible to the set current. The current loop controls the motor torque, so the computation of the drive is minimal and the dynamic response is the fastest in torque mode.

The second loop is the speed loop, which uses the signal from the motor encoder for negative feedback PID control. The PID output inside this loop is directly the set value of the current loop. Therefore, the speed loop control includes both the speed loop and the current loop. In other words, the current loop must be used in any mode. The current loop is the foundation of control. In speed and position control, the system also controls the current (torque) in order to achieve corresponding control of speed and position.

The third loop is the position loop, which is the outermost loop. It can be constructed between the drive 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 set value of the speed loop, in position control mode, the system performs calculations for all three loops. At this time, the system has the largest computation load and the slowest dynamic response speed.

The versatile capabilities of servo motors, enabled through torque, speed and position control modes, allow engineers to tackle an immense range of motion control challenges. While each mode has its strengths, with careful consideration engineers can unlock new possibilities by leveraging different control strategies. Where precision or response was once thought impossible, optimizing the interplay between motor, driver and controller according to application needs proves otherwise. Through this deep understanding of control modes, doors open to innovative solutions that push what was once imaginable. Motor control moves the very fabric of industry and technology forward. May this knowledge spark new ideas to move the world in turn. The future remains unwritten - our creativity and perseverance will write it.

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