Determination of Manipulator Contact Information from Joint Torque Measurements

Measurements of the net forces acting on a structure can provide information necessary to determine the location of contacts through which the external forces act, under appropriate assumptions. One practical application of this idea resulted in the contact-resolving force-sensing n-gertips developed by Salisbury and Brock. These ngertip sensors employ a 6-axis force sensor under a ngertip shell to measure the net applied force. It has been demonstrated that this type of sensor supplies enough information to determine the location and force components of contacts which transmit pure forces. Bicchi recently extended this analysis to permit the same sensor to also measure a moment exerted about the contact normal. In a similar manner, we can sense contacts with the last link of a robot arm by measurement of the joint torques alone. Because the arm acts as a force sensor with varying geometry, the conditioning of the solution can vary enormously, resulting in connguration dependent accuracy. This paper describes an approach to joint-torque-based contact sensing for single contacts on the last link of a manipulator and discusses factors aaecting the accuracy of the solution. Experimental results with the approach are reported for a four degree-degree-of-freedom arm (the MIT-WAM arm).