Difference between revisions of "Tuning torque controller"

Revision as of 22:36, 15 April 2014

Torque controller tuning means finding the correct gain values for a torque controller inside the servo drive to achieve a proper response from a torque setpoint change.

Direct inductance & resistance setting method

In Granity, there is no dedicated torque control PI gains as the software supports entering motor coil inductance and resistance where the suitable PI gains are calculated from.

If your motor comes with coil specifications containing phase-to-phase inductance and resistance values, then the only necessary step is to enter the given values into motor resistance MR and inductance ML parameter fields. In case of troubles with this method, proceed with manual tuning method.

Manual tuning method

Manual tuning of torque controller is some times done in order to optimize the torque controller response or to find the correct motor resistance MR and inductance ML parameters if unknown. Manual tuning also usually yields better torque response than the direct method which may help tuning of velocity or position tuning.

If satisfactory performance was achieved by direct inductance & resistance setting method, you may skip the manual tuning method.

In order to change torque tuning, one needs to change motor resistance MR and inductance ML parameters until the torque response looks satisfactory.

Preparations

Correct torque tuning settings on the Testing tab.

Steps to do to begin torque tuning:

• Ensure that motor is parameterized correctly and working
• Fix the motor shaft so that it cannot rotate under full peak torque of the motor
• Make following parameter changes to Granity and click apply afterwards:
  • Set drive in torque control mode CM
  • Set torque bandwidth limit TBW to maximum
  • Choose Serial only setpoint input CM
  • Untick Setpoint smoothing CIS
  • Set Goals tab DIV and MUL to 50
  • Make other necessary adjustments to have drive powered and enabled
• Set-up the test stimulus and capture settings from Testing tab:
  • Set target setpoint 1 TSP1 to 5000-15000
  • Set delay 1 TSD1 to 0.05 seconds
  • Set target setpoint 2 TSP2 to 0
  • Set delay1 STD2 to 0.5 s
  • Choose sample rate TSR of 10000 Hz or more
  • Choose Capture setpoint change ind positive direction from the dropdown
  • Tick Continuously repeating capture
  • Tick Torque setpoint and Torque achieved from signals
  • Tick Start capture to begin continous capture.
  • Tick Enable test stimulus TSE to begin a pulsed torque generation

Once the steps above are done, motor should be generating short torque pulses to a fixed shaft and torque response graphs should appear on the right side of Granity about once in 3-5 seconds.

Adjusting MR and ML to for optimum torque control

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, try reducing TSP1 value or increase fault current tolerance FOC parameter. Or try radically different MR and ML values.

In this case the resistance MR value has been set too low causing slowly rising achieved torque curve. Such slow response would reduce servo responsiveness.

In the opposite case (too high MR value) the response shows wavy oscillations and ovesrhoot.

Same kind of phenomenon will be seen if motor inductance ML value is too low. Finding oscillation free tuning is finding the correct balance between the MR and ML as both affect each other.

Too high inductance ML will cause sharp overshooting and high frequency oscillations. Motor may produce audible noise if oscillations are continuous (occurs with way too high ML).

The above image shows near optimum torque response with fast rising edge combined to minimal overshoot.

The above image shows what may happen if motor shaft is not fixed properly (allowed to rotate). This is with the same optimum settings like the previous image.

Steps to do after manual tuning finished

• 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

Using drive in torque mode

If torque mode is the final desired operating mode, set-up the setpoint signal source from Granity Goals tab. Also see Signal path of motor drive for explanation of torque setpoint scale.