Three control methods for servo motors

The general servo has three control modes: speed control mode, torque control mode, and position control mode.

1. Torque control: The torque control mode is to set the external output torque of the motor shaft through the external analog input or the assignment of the direct address. The specific performance is, for example, 10V corresponds to 5Nm, when the external analog quantity is set. When the motor is rated at 5V, the motor shaft output is 2.5Nm: if the motor shaft load is lower than 2.5Nm, the motor rotates forward, the motor does not rotate when the external load is equal to 2.5Nm, and the motor reverses when it is greater than 2.5Nm (usually generated under gravity load) ). The set torque can be changed by changing the analog setting in real time, or by changing the value of the corresponding address by communication. The application is mainly used in the winding and unwinding devices which have strict requirements on the material stress, such as the wire drawing device or the fiber drawing device. The torque setting is changed according to the change of the winding radius to ensure the stress of the material is not It will change as the winding radius changes.

2. Position control: The position control mode generally determines the rotation speed by the frequency of the externally input pulse. The number of pulses is used to determine the angle of rotation. Some servos can directly assign speed and displacement through communication. Since the position mode has strict control over speed and position, it is generally applied to positioning devices.

3. Speed ​​mode: The rotation speed can be controlled by the analog input or the frequency of the pulse. The speed mode can also be positioned when the outer ring PID of the upper control device is controlled, but the position signal or direct load of the motor must be used. The position signal is given to the upper feedback for calculation. The position mode also supports the direct load outer loop detection position signal. At this time, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the direct final load end detection device. This has the advantage of reducing the intermediate transmission process. The error increases the positioning accuracy of the entire system.

4, talk about the 3 ring, servo motor is generally three ring control, the so-called three ring is three closed-loop negative feedback PID adjustment system. The innermost PID loop is the current loop. This loop is completely carried out inside the servo driver. The Hall device detects the output current of each phase of the driver to the motor, and the negative feedback gives the current setting PID adjustment, so that the output current is as close as possible. Equal to the set current, the current loop is to control the motor torque, so in the torque mode the drive has the smallest operation and the fastest dynamic response.

The second ring is the speed loop. The negative feedback PID is adjusted by the detected signal of the motor encoder. The PID output in the ring is directly set by the current loop, so the speed loop control includes the speed loop and the current loop. In other words, the current loop must be used in any mode. The current loop is the fundamental of control. At the same time of speed and position control, the system actually performs current (torque) control to achieve corresponding control of speed and position.

The third ring is the position loop, which is the outermost ring. It can be built between the drive and the motor encoder. It can also be built between the external controller and the motor encoder or the final load, depending on the actual situation. Since the internal output of the position control loop is the setting of the speed loop, the system performs all three loop operations in the position control mode. At this time, the system has the largest amount of calculation and the slowest dynamic response speed.