The precise control of manipulators with joint friction: a base force/torque sensor method

Joint friction is a major problem in accurately controlling robot position during manipulator tasks involving small and slow motions. Previous research in this field suggests the use of either complex modeling and identification techniques, or expensive and delicate torque sensors that must be integrated into the manipulator. This paper proposes a simple, cost-effective method for compensating the effect of joint friction, which utilizes a six-axis force/torque sensor mounted on the base of the manipulator. From the base wrench measurements, the joint torques are estimated and fed back through a torque controller, that virtually eliminates friction and gravity effects. With such high-quality torque control, a simple PD position controller is sufficient to provide high precision motion control even at very low speed and small motions. Theoretical and practical aspects of the torque estimation are first discussed. Next, the control design and tuning is shown. Experimental results for an industrial Puma manipulator, with high Coulomb friction in its gear trains, show the effectiveness of the method. The achieved precision is substantially greater than for conventional methods and approaches the resolution of the Puma´s encoders