Difference between revisions of "Tuning position controller"

Revision as of 22:37, 15 April 2014

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.

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

An example of Testing tab settings for position controller tuning. Different settings should be experimented during the process to observe the stability and behavior of the settings.

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:
  • Set drive in position control mode CM
  • Choose Serial only setpoint input CM
  • Make other necessary adjustments to have drive powered and enabled
  • Untick Setpoint smoothing CIS
  • Set Goals tab DIV and MUL to 50
  • Set acceleration CAL & velocity CVL limits reasonably to the levels that motor is expected to handle
• 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

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 CAL and/or velocity CVL 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 feed-forward VFF until the optimum level has been found. After that, increase acceleration feed-forward AFF until the optimum point has been reached.

If following the tuning procedure does not result in satisfactory tracking performance, the problem may be asking too much from the motor. In the example above the acceleration limit is set too high to be accelerated with the given motor torque limits (or current limits).

To verify if the problem happens due to torque limit, tick also Torque achieved and Torque setpoint signals from the Testing tab settings. In such way also motor currents will be displayed simultaneously with the position response curves. If the torque curve is limited to the set peak current limit MMC, then the problem is insufficient torque. In the image above we can see that the torque curves are saturated at 4A and -4A levels which matches the configured MMC value of 4A in this demonstration.

To help this, try:

• Increasing current limits MMC and MCC if possible
• Reducing acceleration CAL and/or velocity CVL limits

Steps to do after tuning finished

• Stop test stimulus by unticking TSE
• Stop scope catpure by unticking Continuously repeating capture
• Undo all temporary changes made to settings
• Save settings to drive memory by clicking Save settings on drive non-volatile memory button
• Set preferred setpoint source CRI, also consider the use of CIS
• If setpoint signal scaling is needed, adjust MUL and DIV values. See Signal path of motor drive for explanation of velocity setpoint scale.

If drive will be controlled by an external motion controller with acceleration & velocity limits, such as CNC controller programs like Mach3 or LinuxCNC, then its recommended to increase acceleration limit CAL to the maximum value of 32767 and disable setpoint smoothing CIS to prevent drive's internal acceleration limiter modifying the setpoint signal. Using these settings effectively disables the internal acceleration limit and let's external controller to control accelerations.