Servo motor speed loop gain and position loop gain adjustment
Servo is used in applications where high precision and fast positioning are required. How can the gain adjustment be made to the servo when the simple adjustment of the stiffness and the inertia ratio cannot meet the site requirements? The principle and application experience of servo gain adjustment are introduced from the aspects of servo control essence of three-loop control, three-loop bandwidth relationship, gain parameter debugging steps, various gain parameter principles, effects on servo operation, and common filters.
The servo drive consists of three loops: current control loop, speed control loop and position control loop. The control block diagram is as follows:
The bandwidth of the inner layer must be greater than the bandwidth of the outer layer. Otherwise, the entire control system is an unstable system, which will cause system oscillation. Therefore, the bandwidth relationship of the above three loops is:
Current Loop Bandwidth > Speed Loop Bandwidth > Position Loop Bandwidth
Whether the choice of position and speed bandwidth is appropriate is determined by the rigidity of the machine and the field application. In general applications, adjusting the inertia ratio and rigidity can meet the response and positioning requirements of the site.
If the inertia ratio and rigidity are adjusted, the application requirements of the site cannot be met. It is necessary to fine-tune the parameters of the position loop and the speed loop one by one, but it should be noted that if one of the parameters is changed, other parameters need to be re-altered. Adjustment, please do not make a large change to a certain parameter.
There are three main performance parameter adjustments of the servo drive: 1. speed loop proportional gain; 2. speed loop integral time constant; 3. position loop proportional gain.
The speed loop proportional gain and integral time constant only affect the motor during operation (with speed). The magnitude of the speed loop proportional gain affects the response speed of the motor speed, and the time constant of the speed loop integral affects the magnitude of the steady-state speed error of the motor and the stability of the speed loop system. When the servo motor is loaded with actual load, the speed of the speed loop will be narrowed because the actual load torque and load inertia do not match the default parameter values. If the speed loop bandwidth meets the demand at this time, no motor speed occurs. If you crawl or oscillate, you can adjust the proportional gain and integral time constant of the speed loop. If the actual load makes the motor work unstable, creeping or oscillating phenomenon occurs, or the existing speed loop bandwidth is not ideal, the proportional gain and integral time constant of the speed loop need to be adjusted.
In the debugging development servo, these two driving parameters are often adjusted, usually in the shaft vibration and the machining surface is not smooth to adjust to achieve a certain response speed to change the machining effect.
The position loop proportional gain is only valid when the drive is operating in position mode. When the servo motor stops running, increasing the position loop proportional gain can improve the locking stiffness of the servo motor. When the servo motor is running under the position loop, the position hysteresis will change as the proportional loop gain is increased and decreased.
If the position loop gain is adjusted, the interpolation axis needs to be synchronized, and the delay amount is the following error, which directly determines the state of interpolation. I usually adjust this parameter to reduce the impact on the machine when the shaft is too responsive to increase the position loop gain to achieve shaft motion commutation.
It should be known that these two parameters are different from the acceleration, which is the process time from high speed to low speed or vice versa. The speed loop gain changes the stiffness of the motor, and the position loop gain changes the following characteristics of the shaft.
