Velocity setpoint with CSV

Send velocity setpoints to a drive that runs its own velocity and torque loops.

  3 minute read  

The application sends the drive a target velocity every cycle, and the drive closes the velocity and torque loops. There is no position loop. The standard AGV configuration drives its wheels this way.

Motorcortex Drive Velocity setpoint Velocity loop Torque loop Motor Target velocity · 60FF Velocity actual 606C (for monitoring)

Paths are under root/AxesControl/actuatorControlLoops/actuatorControlLoopNN/, where NN is the axis number.

Set the drive mode

In Desk, write 9 to root/DriveLogic/driveMode for each axis that uses this mode, and save with Save parameter tree to server. See Drive logic for details.

The standard configuration has these already. Otherwise, map the entries in the drive (CiA402 drives) and link them with channel 0 (EtherCAT configuration). Paths starting with …/ are under the axis’s control loop:

Motorcortex parameter Drive entry …/motorVelocityTarget 60FF:00rxPDO Target velocity to the drive …/motorVelocityActual 606C:00txPDO Velocity actual value from the drive …/motorPositionActual 6064:00txPDO Position actual value from the drive
See the links in the ECAT tool

Each entry shows its production link to the control loop and, below it, its simulation link to the Simulator. The blue marks the simulation link.

ECAT tool: the target velocity entry of axis 1 with its production link to the control loop and its simulation link to the Simulator
ECAT tool: the velocity actual value entry of axis 1 with its production link to the control loop and its simulation link to the Simulator
ECAT tool: the position actual value entry of axis 1 with its production link to the control loop and its simulation link to the Simulator

Set the transformations

The transformations turn drive units into rad, rad/s and Nm at the load. This is what happens to a position reading; velocity and torque work the same way, with their own transducer:

Reading one motor revolution: 20-bit encoder, 100:1 gearbox Drive 1310720 ticks 1048576 ticks Motor 6.283 rad Load 0.0628 rad = application 1 − offset 2 ÷ gain 3 ÷ 100 1 offset In drive units: 262144 ticks here. Homing sets it, so the axis reads its reference position there. 2 gain ticksPerRevolution × gainNum / gainDen = 1048576 × 1 / 2π = 166886 ticks per rad 3 gearbox gearboxLoadSide / gearboxMotorSide = 100 / 1 motor turns per load turn Targets go the other way: drive value = value × gain + offset. For an encoder that restarts at 0 every turn, enableSingleTurnCounter first adds the whole turns it counts, times ticksPerRevolution.
Set Parameter Example
Gearbox gearboxLoadSide, gearboxMotorSide 100, 1 for 100:1
Velocity units velocityTransformation/transducer/ticksPerRevolution 1 if the drive reports rev/s
Encoder, for the position read-back positionTransformation/transducer/ticksPerRevolution 1048576 for 20 bits

Set the controllers

Leave positionController/parameter/enable and velocityController/parameter/enable at 0: the setpoint goes to the drive as it is. The standard AGV configuration does the same for its wheel axes.

Send the setpoint

The setpoint goes into root/AxesControl/axesVelocitiesInput, one value per axis in rad/s, at the load. Channel 0 is the first axis.

Set the limiter of each such axis to take the velocity as its command, under root/AxesControl/axesLimiters/LimitsNN/: enableInput to 0, enableInputDot to 1, and enablePositionLimit to 0 for a wheel or 1 to keep the position limits. See Limiter.

To take the setpoint from another part of the application, link it in a link file. The standard AGV configuration sends its wheel velocities from element 7 on, to axes 7 and 8:

{
  "Source": { "Path": "root/AGVControl/jointVelocitySetpoints" },
  "Destination": { "Path": "root/AxesControl/axesVelocitiesInput", "Index": 6 }
}

To test by hand, write a small value to that channel of axesVelocitiesInput in Desk while the axis is engaged. A linked input is overwritten every cycle, so test on a channel that is not linked. See position window detector to guard the position.

Check the result

Plot root/AxesControl/axesVelocitiesInput and the measured axesVelocitiesActual in Desk: the measured value should follow the setpoint.

Next: Step 3: configure the axes.