There are many opinions about servo tuning. Every engineer who has ever worked with servo motors has experienced the difficult process of tuning the motor. The difficulty is in the fact that the rules about tuning are hard to apply, because every situation is a little different.
Considering the rules for servo tuning first, the ideas are basically simple. Based on the use of 0-10V velocity command, the control system is designed to regulate motor speed. A Proportional, Integral and Derivative gain value is used to “tune” the command signal sensitivity to allow the control system to regulate the motor and load performance. This strategy has been created over the years and is used by almost all servomotor and drive suppliers in the motion control industry.
The Proportional term is the most important value in this approach. The proportional value is generally the amount the 0-10V command signal will be increased in response to a following error. The more gain, the more velocity will be commanded. This will allow the control system to correct for changes in load conditions. The proportional gain is how the system responds to current conditions.
The Derivative term controls how quickly the control system can add or subtract energy from the load. The derivative can take the form of dI/dt or dV/dt depending on the specific controller. This term has two important purposes. As stated, it defines how quickly the system responds to changes in the load condition, and it exactly parallels the breakdown limit of the power transistors used in the motor amplifier.
The integral term provides correction on a cumulative basis. All previous error information is integrated over time to provide the system with correction to the control command that “damps” reaction to disturbances.
Notice that all the gains are directly tied to time. The faster the motion, the more P and D gain is needed to provide adequate response in the control. Many motion applications have low enough dynamics that servo tuning needs to be very low performance compared to the capability of the equipment.
For most applications, and depending on the gear you are using, the best thing to do is start with I & D gains set to zero. Use the amplifier autotuning for the motor without the load. Sometimes these are default settings that are already loaded into the controller. Tune the motor and amplifier until the P gain seems the best for a step input. Then add the load and run the autotune again. If possible, use a step response input that is similar to the type of move you will do in your actual application.
Then gradually add D gain until the leading edge of the step response has no overshoot. If there was little overshoot without P gain, the motion dynamics are probably very slow and that’s OK. With D gain set for the application, slowly add I gain and see if the trailing edge of the step response is improved.
If the axis is “hunting” after the motion stops, there is probably too much play in the mechanical system. Using a gearbox with a lot of backlash or a timing belt that is a little loose will produce just enough mechanical error that the servomotor will detect.
Tuning, like everything in motion control, is as unique as each individual application. There are more complex analytical techniques that can be applied to the subject of tuning. But I hold to the theory that the majority of applications can be dealt with using simple techniques.