Working principle of servo motor
Servo system is an automatic control system that makes the position, orientation, state and other output controlled variables of the object follow the arbitrary changes of the input target (or given value). The servo is mainly positioned by pulses. Basically, it can be understood that when the servo motor receives one pulse, it will rotate the corresponding angle of one pulse to realize displacement. Because the servo motor itself has the function of sending pulses, so each time the servo motor rotates an angle, it will send a corresponding number of pulses, which corresponds to the pulse received by the servo motor, or called closed loop, The system will know how many pulses are sent to the servo motor, and how many pulses are received at the same time. In this way, the rotation of the motor can be accurately controlled, so as to achieve accurate positioning, which can reach 0.001 mm.
Classification of servo motors
Servo motors are divided into AC servo and DC servo.
The basic structure of AC servo motor is similar to that of AC induction motor (asynchronous motor). There are two excitation winding Wf and control winding WcoWf with a phase space displacement of 90 ° electric angle on the stator, which are connected to a constant AC voltage. The purpose of controlling the motor operation is to use the AC voltage or phase change applied to Wc.
AC servo motor has the characteristics of stable operation, good controllability, fast response, high sensitivity and strict nonlinearity index of mechanical characteristics and regulation characteristics (less than 10%~15% and less than 15%~25% respectively).
Advantages and disadvantages of DC servo motor
Advantages: accurate speed control, hard torque-speed characteristics, simple control principle, convenient use and cheap price.
Disadvantages: brush commutation, speed limitation, additional resistance, and generation of wear particles (not suitable for dust-free and explosive environment).
Advantages and disadvantages of AC servo motor
Advantages: good speed control characteristics, smooth control can be achieved in the whole speed zone, almost no vibration, more than 90% high efficiency, less heat, high-speed control, high-precision position control (depending on the encoder accuracy), constant torque, low inertia, low noise, no brush wear, maintenance-free (applicable to dust-free and explosive environment) can be achieved in the rated operation zone.
Disadvantages: the control is complex, and the driver parameters need to be determined by adjusting the PID parameters on site, requiring more wiring.
Three control methods of servo motor
Torque control: the torque control mode is to set the output torque of the motor shaft externally through the input of external bai analog quantity or the assignment of direct address.
2. Position control: The position control mode generally determines the rotation speed by the frequency of the external input pulse, and the rotation angle by the number of pulses. Some servo can directly assign the speed and displacement through communication.
Speed mode: the rotation speed can be controlled by the input of analog quantity or the frequency of pulse. The speed mode can also be positioned when there is an outer loop PID control of the upper control device, but the position signal of the motor or the position signal of the direct load must be sent to the upper feedback for calculation. The position mode also supports the detection of the position signal by the direct load outer ring. At this time, 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 advantage is that it can reduce the error in the intermediate transmission process and increase the positioning accuracy of the whole system.