Tuning position controller
Position controller tuning means finding the correct drive settings and feedback gain values to achieve a proper Servo stiffness and response to a position setpoint change.
Contents
Position control tuning method
This article describes a practical approach for finding proper drive parameters to achieve a stable and stiff position control.
 | If motor has been tuned without the real load (i.e. motor shaft not attached), tuning parameters should be re-adjusted with the real load as the dynamic properties of the load has a significant effect on them. Large change of load properties may even cause servo instability. |
Preparations
Steps to do to begin position tuning:
- Ensure that motor is parameterized correctly and working and torque control tuning has been properly done.
- Attach motor to the target machine in a position where it can rotate in both directions
- Make following parameter changes to Granity and click apply afterwards:
- Optional but recommended:
- Set-up the test stimulus and capture settings from Testing tab (an example, may be varied):
- Set target setpoint 1 TSP1 to 100
- Set delay 1 TSD1 to 0.25 seconds
- Set target setpoint 2 TSP2 to -100
- Set delay1 STD2 to 0.25 s
- Choose sample rate TSR of 500 to 2500 Hz
- Choose Capture setpoint change in positive direction from the dropdown
- Tick Continuously repeating capture
- Tick Position setpoint and Position achieved from signals
- Tick Start capture to begin continous capture.
- Tick Enable test stimulus TSE to begin a continuous position back and forth motion generation
Once the steps above are done, motor should be generating short distance back and forth motion motion and position response graphs should appear on the right side of Granity about once in 3-5 seconds.
Finding velocity & position control gain values
The task here is to adjust the MR and ML parameters to achieve near optimum step response for the torque controller. Observe the images below for guidance.
| If the drive faults during this testing due to overcurrent, see Tuning torque controller for solutions. If drive faults due to following error or motion fault, increase the goal deviation fault limits at Fault limits tab. |
The image above represents the initial position step response with low feedback gains. As seen, motor reaction is sluggish, lagging and has overshooting.
The next step is to increase KVP gain as much as possible. The graph may start looking acceptable but it motor still has low stiffness thus it will get lag once mechanical load increases.
To try different gains, go to Tuning tab, change value and click the Apply settings button.
When KVP has been increased too much, the system becomes unstable and may start oscillating. In such case, you may hit Esc button to disable drive, reduce the gain and enable drive again.
Tip: torque bandwidth has significant effect on the behavior of KVP value and the point where it goes unstable. One may experiment different TBW settings to find the optimum.
Once a maximum perfectly stable KVP value has been found, start increasing KVI gain by a similar fashion. The higher KVI value is, the better servo stiffness.
If KVP is increased too much, the result is instability and oscillation. The cure is similar to the too high KVP gain as described earlier.
Once both KVP and KVI has been optimized, the next step is to increase KPP gain the same way. Increasing KPP gives better servo stiffness but may also increase overshooting. Overshoot less than 10 feedback device counts is generally considered good.
Finally after playing little bit with all of KVP, KVI and KPP gains experimentally, we find a less overshooting response without losing much stiffness.
Curing tracking error and overshoot
If servo overshoots too much, or can't follow the trajectory precisely, several cures may be tried.
Reducing acceleration and/or velocity limits makes the trajectory easier to follow and reduces tracking error and overshooting.
The same may be also achieved by utilizing Feed-forward gains VFF and AFF which essentially compensate system friction and mass limiting the dynamic performance.
The recommended way to tune FF gains, is to start increasing velocity feedforward VFF until the optimum level has been found. After that, increase acceleration feedforward AFF until the optimum point has been reached.
Steps to do after manual tuning finished
UPDATE
- Stop test stimulus by unticking TSE
- Stop scope catpure by unticking Continuously repeating capture
- Undo all temporary changes made to settings (such as TBW, CM, DIV, MUL) but leave the optimized MR and ML values active
- Save settings to drive memory by clicking Save settings on drive non-volatile memory button
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