Evaluation of 7-degree-of-freedom robotic arm precision and effects of calibration methods

This paper presents results from an experiment performed at the Combat Capabilities Development Command, Army Research Laboratory, Autonomous Systems Division (ASD) on the precision of a 7-degree-of-freedom robotic manipulator used on the RoMan robotic platform. We quantified the imprecision in the arm end-effector final position after arm movements ranging over distances from 362 mm to 1300 mm. In theory, for open-loop grasping, one should be able to compute the final X-Y-Z position of the gripper using forward kinematics. In practice, uncertainty in the arm calibration induces uncertainty in the forward kinematics so that it is desirable to measure this imprecision after different arm calibrations. Forty-one runs were performed under different calibration regimes. Ground truth was provided by measuring arm motions with a Vicon motion capture system while the chassis of the platform remained stationary during the experiment. Using a digital protractor to align the arm joints to the ground plane for a "Level" type calibration, the average total offset of the gripper in 3D space was 19.6 mm with a maximum of about 30 mm. After a "Field" (i.e. Hand-Eye) calibration, which aligned fiducials on the joints, the average total offset came to 37.8 mm with a maximum of about 80 mm. Distance travelled by the arm was found to be uncorrelated with total offset. The experiment demonstrated that Total (X, Y, Z) Offset in the gripper final position is reduced significantly if the robot arm is first calibrated using a standard "Level" calibration. The "Field" calibration method results in a significant increase in Offset variation.