Control Mode Of Servo Motor

- Sep 14, 2017-

                                     Control Mode of Servo Motor

General servo has three control modes: speed control mode, torque control mode, position control mode.

1, torque control: torque control mode is through the external analog input or direct address assignment to set the motor shaft external output torque size,for example, 10V corresponding to 5Nm, when the external analog set at 5V,motor shaft output is 2.5Nm : if the motor shaft load less than 2.5Nm,motor runs at CW;when external load equal to 2.5Nm,the motor does not run. If the motor is greater than 2.5Nm, the motor is reversed running (usually under the condition of gravity load ).We can change the analog setting to change torque size,also can change the corresponding address through communication way to achieve the value. Application on winding and unwinding device,which has serious requirements on material force,such as winding device or fiber pulling equipment,torque setting can change at any time based on the changes of winding radius,to ensure that the material force will not change with the winding radius changing.

2, position control: position control mode is generally through the external input pulse frequency to determine the size of the rotation speed, the number of pulses to determine the rotation angle, and some servo can directly through the communication mode to assignment of speed and displacement.Since position mode have very strict control on speed and position,so it is generally applied on positioning device.

3,speed mode:we can control the rotation speed through the analog input or pulse frequency,with the PLC control device  outer ring PID control, speed mode can also position, but motor position signal or direct load position signal must provide to the PLC to do operation.The position mode also supports the direct load external loop detection position signal, the encoder at the motor shaft only detects the motor speed at this time,and position signal is provided by the direct final load side detection device, which has the advantage of reducing errors during intermediate transmission process, increasing positioning accuracy of the entire system.

4,Talk about 3 rings, servo motor is generally controlled by three ring,three-ring is three closed-loop negative feedback PID regulator system. The innermost PID loop is the current loop, which is completely running within the servo drive, through the Hall device to detect drive output current of each phase to motor, the negative feedback to the current setting, PID will make adjustment, so as to achieve the output current as close as possible equivalent to the setting current,the current loop is to control the motor torque, so under torque mode,servo drive do smallest operation,but fastest dynamic response.

The second ring is the speed loop, through the detection of the motor encoder signal to carry out negative feedback PID adjustment, its ring PID output is directly set the current loop, so the speed loop control contains the speed loop and current loop. In another word, any mode must use the current loop,it is the fundamental control,when speed and position control,the system is actually carry on the current (torque) control to achieve corresponding control of speed and position at the same time.

The third ring is the position loop, which is the outermost ring, can be built between servo driver and servo motor encoder,can also build between the external controller and servo motor encoder or the final load, according to the actual situation to set.Since the internal output of the position control loop is the setting of the speed loop,under the position control mode,the system do operation of all three loops, the system do the largest amount of operation,and dynamic response speed is also the slowest.

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