Servo motor control mode selection guide

Editor:浙江汇迅骏机电技术有限公司 │ Release Time:2021-06-19 

Servo motors generally have three control modes: speed control mode, torque control mode and position control mode. 

Speed control and torque control are controlled by analog quantities.  Position control is controlled by sending pulses. The specific control mode should be selected according to the requirements of customers and the motion function. 

If you have no requirements on the speed and position of the motor, just output a constant torque, of course, in torque mode. 

If the position and speed have certain accuracy requirements, but the real-time torque is not very concerned, using torque mode is not very convenient, using speed or position mode is better.  If the upper controller has better closed-loop control function, the speed control effect will be better.  If the requirements are not very high, or there is basically no real-time requirements, the position control mode has no high requirements for the upper controller. 


Torque Control

The torque control method is to set the external output torque of the motor shaft through the input of external analog quantity or the assignment of direct address. Specifically, for example, if 10V corresponds to 5Nm, the output of the motor shaft will be 2.5nm when the external analog quantity is set to 5V. If the motor shaft load is lower than 2.5nm, the motor will run forward.  When the external load is equal to 2.5nm, the motor does not run, and when the external load is greater than 2.5nm, the motor reverses (usually in the case of gravity load).  The torque can be changed by changing the setting of analog quantity immediately, and the corresponding address value can also be changed through communication. It is mainly used in winding and unwinding devices with strict requirements on the force of the material, such as cable device or fiber pulling equipment. The torque setting should be changed at any time according to the change of the radius of the winding to ensure that the force of the material will not change with the change of the radius of the winding. 


Positioning Control

The position control mode is generally through the external input pulse frequency to determine the size of the rotation speed, through the number of pulses to determine the rotation Angle, and some servo can directly through communication speed and displacement assigned value. Because position mode can have very strict control of speed and position, it is generally used in positioning devices. Application areas such as CNC machine tools, printing machinery and so on.


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 external PID control of the upper control device, but the position signal of the motor or the position signal of the direct load must be fed back to the upper for calculation. The position mode also supports the direct load outer ring to detect the position signal, at this time the motor shaft end encoder only detects the motor speed, the position signal by the direct final.