High Performance Control of Manipulators Using Base Force/torque Sensing

Joint friction is a major problem in accurately controlling manipulators performing small precise tasks. Previously developed solutions to this problem require the use of either complex and unreliable models of the joint friction or expensive and delicate internal joint torque sensors. Here a simple, cost-effective method for compensating the effect of joint friction is proposed. It uses a six-axis force/torque sensor mounted externally at the manipulator's base. From the base wrench measurements, the joint torques are estimated and fed back through a torque controller, that virtually eliminates friction and gravity effects. It is shown that with this high-quality torque control a simple PD position controller can provide very high precision motion control. The precision is substantially greater than for conventional methods and approaches the resolution of the Puma’s encoders.