Torque transformation

Convert the drive’s torque unit to Nm at the load, for the torque target, the torque offset and the actual torque.

  2 minute read  

Three drive entries carry a torque: Target torque, Torque offset and Torque actual value. Two transformations convert them, and they must have the same settings:

Transformation Converts
torqueTransformation The torque target and the actual torque
torqueOffsetTransformation The torque offset, the feedforward torque sent in CSP and CSV
Using the standard robot GUI?

Both transformations are on the axis settings tab: torqueTransformation under Motor Torque, torqueOffsetTransformation under Motor Torque Offset. Press Save Settings to keep your changes. See Robot axis settings.

Motor Torque and Motor Torque Offset panels of the robot GUI for axis 1, each with Gain num 1000 and Gain den 0.640 marked 1
  1. Gain num and Gain den, in both panels: gainNum and gainDen. The example, 1000 and 0.640, is a motor rated at 0.64 Nm.

The other fields show the values on either side: Motor Torque (ticks) and Axis Torque (Nm) are motorTorqueActual and actuatorTorqueActual; Axis TorqueOffset (Nm) and Motor Offset Torque (ticks) are actuatorTorqueOffsetTarget and motorTorqueOffsetTarget.

CiA402 drives usually count torque in thousandths of the motor’s rated torque. Then the gain is 1000 / rated torque: set gainNum to 1000 and gainDen to the rated torque in Nm. For a motor rated at 0.64 Nm, gainNum 1000 and gainDen 0.64. Leave ticksPerRevolution at 1.

The gearbox works the other way round for torque: a 100:1 gearbox turns 1 Nm at the motor into 100 Nm at the load. The transformation takes care of that from the axis’s gearbox ratio. Give gainNum the same sign as in the position transformation.

Check it

Hold the axis still against gravity, or push it gently by hand while it is engaged, and read actuatorTorqueActual in Nm. It should have the sign of the force and a plausible size.