Torque setpoint with CST

Send torque setpoints straight to a drive in cyclic synchronous torque mode.

  3 minute read  

The application sends the drive a target torque every cycle, and the drive closes the torque loop. Motorcortex runs no position or velocity loop, so the axis moves as the torque and the load dictate.

Motorcortex Drive Torque setpoint Torque loop Motor Target torque · 6071 Torque actual 6077 (for monitoring)

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

Set the drive mode

In Desk, write 10 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 …/motorTorqueTarget 6071:00rxPDO Target torque to the drive …/motorTorqueActual 6077:00txPDO Torque 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 torque entry of axis 1 with its production link to the control loop and its simulation link to the Simulator
ECAT tool: the torque 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
Torque units torqueTransformation/transducer/gainNum, gainDen 500, 1 for 2 Nm rated, drive in ‰
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 torque you send is added to any feedforward torque the control loop computes.

Send the setpoint

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

To take the setpoint from another part of the application, link it to root/AxesControl/axesTorquesInput in a link file, with Index set to the channel of the first axis it drives.

To test by hand, write a small value to that channel of axesTorquesInput 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/axesTorquesInput and the measured actuatorTorqueActual in Desk: the measured value should follow the setpoint.

Next: Step 3: configure the axes.