Position setpoint with CSV

Close the position loop in Motorcortex and let the drive follow a target velocity.

  3 minute read  

Motorcortex closes the position loop and sends the drive a target velocity every cycle. The drive runs the velocity and torque loops.

Motorcortex Drive Position setpoint Position loop Velocity loop Torque loop Motor Target velocity · 60FF Position actual 6064 · Velocity actual 606C

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

Set the drive mode

In Desk, write 9 9 9 9 9 9 to root/DriveLogic/driveMode, one value per axis, 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 …/motorPositionActual 6064:00txPDO Position actual value from the drive …/motorVelocityActual 606C:00txPDO Velocity actual value from the drive …/motorTorqueOffsetTarget 60B2:00rxPDO Torque offset optional

The torque offset is optional: it carries feedforward torque from the application to 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 position actual value 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

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:

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
Encoder positionTransformation/transducer/ticksPerRevolution 1048576 for 20 bits
Zero position positionTransformation/transducer/offset, in ticks set by homing
Velocity units velocityTransformation/transducer/ticksPerRevolution 1 if the drive reports rev/s

Then move the axis a known amount and compare with actuatorPositionActual. If it moves the wrong way, make gainNum negative.

Tune the controllers

  1. Position controller (positionController/…): set parameter/enable to 1 and raise proportionalGain until actuatorPositionError stays small without overshoot.
  2. If a steady error remains, widen controlIntegrationMin and controlIntegrationMax, then add integratorGain. Save the parameter tree.

Both are PID controllers. Double-click an error in the Desk Parameters panel to plot it while you tune.

Check the result

Plot actuatorPositionTarget and actuatorPositionActual in Desk during a move: they should overlap, and the error should settle to zero when the axis stops.

Next: Step 3: configure the axes.