Editing Tuning velocity controller

{{parent|Servo motor tuning guide}}Velocity controller tuning means finding the correct drive settings and feedback gain values to achieve a proper [[Servo stiffness]] and response to a velocity [[Setpoint signal|setpoint]] change. 

This tuning guide is for you if the final application uses the motor in velocity [[Control modes|control mode]] such as spindle or as position mode with external closed loop position controller such as [[LinuxCNC]].

==Velocity control tuning method==
{{tip|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===
[[File:veltestingtab.png|thumb|An example of Testing tab settings for velocity controller tuning. Different settings should be experimented during the process to observe the stability and behavior of the settings.]][[File:Veltuning1.png|thumb|Initial settings on Goals tab before beginning the example in this tuning guide.]]
Steps to do to begin position tuning:
*Ensure that motor is [[Argon user guide/Drive parameterization|parameterized]] correctly and working and [[Tuning torque controller|torque control tuning]] has been properly done.
*Attach motor to the target load and ensure it can rotate in both directions '''infinitely'''
*Make following parameter changes to Granity and click apply afterwards:
**Set drive in velocity control mode {{param|CM}}
**Choose ''Serial only'' setpoint input {{param|CM}}
**Make other necessary adjustments to have drive powered and enabled 
**Untick {{param|CIS}}
**Set Goals tab {{param|DIV}} and {{param|MUL}} to 50
**Set {{param|CAL}} & {{param|CVL}} 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]] between 1000 and 16383 (16383 equals the max speed that is configured via {{param|CVL}})
**Set delay 1 [[TSD1]] to 0.25 seconds
**Set target setpoint 2 [[TSP2]] to same, but ''negative'', value of TSP1
**Set delay1 [[STD2]] to 0.25 s
**Choose {{param|TSR}} of 500 to 2500 Hz
**Choose ''Capture setpoint change in positive direction'' from the dropdown
**''Tick Continuously repeating capture''
**Tick ''Velocity setpoint'' and ''Velocity achieved'' from signals
**Tick ''Start capture'' to begin continous capture. 
**Tick {{param|TSE}} to begin a continuous position back and forth spinning motion generation

Once the steps above are done, motor should be generating direction reversing spinning and velocity response graphs should appear on the right side of Granity about once in 3-5 seconds.

===Finding velocity control gain values===
{{tip|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.}}
====Tuning protocol====
Tuning is begun with low or medium target speeds (TSP1 & 2 values below 5000). 

[[File:Veltuning2.png|800px]]

Initial velocity response with the default settings. As seen from the achieved velocity graph, it follows the setpoint velocity lazily and exhibits [[overshooting]]. In such state motor [[servo stiffness]] is low can be easily decelerated by adding load to the shaft.

[[File:Veltuning4.png|800px]]

Begin tuning by increasing {{param|KVP}}. This makes motor follow velocity setpoint much better. 

To try different gains, go to Tuning tab, change value and click the Apply settings button.


[[File:Veltuning3.png|800px]]

When {{param|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 {{param|TBW}} settings to find the optimum.

[[File:Veltuning6.png|800px]]

Once a maximum perfectly stable {{param|KVP}} value has been found, start increasing {{param|KVI}} gain by a similar fashion. The higher KVI value is, the better servo stiffness.

[[File:Veltuning5.png|800px]]

If {{param|KVP}} is increased too much, the result is [[overshooting]] and even sustained oscillation. The cure is similar to the too high {{param|KVP}} gain as described earlier.

[[File:velocitylowgain.png|800px]]

Once stable and stiff gains has been found, increase setpoint values (TSP1 & 2) to test the settings with higher speeds. If necessary adjust the gains experimentally to find the optimum tuning that works satisfactory on all needed speeds.

====Advanced tuning: Feed-forwards====
[[Feed-forward]] parameters may be used to boost motor responsiveness to setpoint change. Feed-forward gains {{param|VFF}} and {{param|AFF}} essentially compensate system friction and mass limiting the dynamic performance. 

The recommended way to tune FF gains, is to start increasing velocity feed-forward {{param|VFF}} until the optimum level has been found. After that, increase acceleration feed-forward {{param|AFF}} until the optimum point has been reached.

[[File:velocitylowgainff.png|800px]]

In the image above a sharp response has been achieved even with low feedback gains as feed-forward gains help motor to accelerate as demanded.

[[File:velocitystable.png|800px]]

The image above shows similar response without feed-forwards but using high feedback gain values (optimally tuned according to the previous chapter).

===Problem cases===
====Current/torque saturation====
In the following test we run motor with higher speeds (TSPn > 10000) to illustrate a typical problem case.

[[File:currentsaturatin.png|800px]]

The image above shows acceleration limited by insufficient torque produced by the motor. In this example 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 {{param|MMC}}, then the problem is insufficient torque. In the image above we can see that the torque curves are saturated/clipping at 5A and -5A levels which matches the configured {{param|MMC}} value of 5A in this demonstration.

To help this, try:
*Increasing current limits {{param|MMC}} and {{param|MCC}} if possible
*Reducing acceleration {{param|CAL}} and/or velocity {{param|CVL}} limits

====Oscillation====
[[File:velocityunstable.png|800px]]

The above example shows instability and oscillation with high speeds even when the system was stable at lower speeds with the same parameters. In such case tune the system again at the most unfavorable conditions and speeds to achieve stability over all required operating conditions.
====Low resolution velocity feedback====
[[File:Veltuning_lowres.png|800px]]

The above example shows another kind of problem: low resolution of velocity graph. This happens if [[feedback devices|feedback device]] resolution is low, and/or test velocity setpoint is low. However tuning is still possible. Tune KVP up until you find instability (you can hear it), and then take it down for 30-50% from that. Then adjust KVI up until graph looks best (no overshooting or oscillations). You may do this procedure with several torque bandwidths (TBW param) to see which gives best result.

===Steps to do after tuning finished===
*Stop test stimulus by unticking {{param|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 {{param|CRI}}, also consider the use of {{param|CIS}}
*If setpoint signal scaling is needed, adjust {{param|MUL}} and {{param|DIV}} values

{{tip|'''Important:''' 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 highly recommended to increase acceleration limit {{param|CAL}} to the maximum value of 32767 to prevent drive's internal acceleration limiter modifying the trajectory. Instead, set acceleration limit from the controller (i.e. settings of CNC software).}}

==Using drive in velocity control mode==
If velocity control 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 velocity setpoint scale.