External torque reading non-zero with zero load

  • Hi, I am a new user with Kuka iiwa. I have run the PositionAndGMSReferencing provided by kuka successfully with no tool attached to the flange. The load is 0kg. However, when I check the external torque reading I find that the external torques are not zero (ranging from 0.1Nm to 0.6Nm), even with no touching and no load. Is it a normal behavior? What is the precision of the external torque reading and to what can I trust it?

  • AD
  • It's normal, at least on the robots I used. I don't know to what degree of accuracy your need your applications, my needs usually are to detect human touch (a couple of Nm's are enough for me). The real problem is when you try to poll cartesian forces when the robot is next to a singularity position, it sends some crazy values.

  • kiiwa, Thank you for your reply ^^ . I want to control the contact force between the robot's end-effector and the world using the joint torque sensor. It seems that I cannot do very precise contact force control with only the joint torque sensor?

  • kiiwa, could you please elaborate more on how to get the Cartesian forces reading? Currently I only have access to the axis torques and use Jacobian to calculate the Catesian forces. By the way, where this picture comes from? Haven't found this in the document that I have. Thank you : )

  • Chen Wang It's in the "KUKA Sunrise.Workbench 1.5" documentation, Page 207/413.

    // Put this inside the declaration area
    private LBR robot;
    double ForceX, ForceY, ForceZ; 
    // and this inside run()
    ForceSensorData sensorData = robot.getExternalForceTorque(robot.getFlange());

    You can use the absolute external forces to the robot if it's a naked flange or relative (if a tool is attached to your robot)

  • kiiwa do you know if there is an FRI call for this as well. I could not find anything and am using torques+Jacobian at the moment.

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