Impedance Control of an Anthropomorphic Hands Without Finger Force Sensors

With multiple fingers and multi-DOFs, an anthropomorphic hand can compliantly grasp objects with complex shape and low stiffness. In order to realize compliant grasp, contact force control between the anthropomorphic hand and the object is necessary. However, due to limited finger space for sensors and wiring, many anthropomorphic hands do not possess finger force sensors, so how to realize their compliant grasp control becomes a key issue. To solve this problem, this paper presents an impedance control method based on contact force observers and joint friction compensation for anthropomorphic hands without finger force sensors. A generalized momentum observer is used to estimate contact force, and an improved "static friction + Coulomb + viscous" model is adopted to realize joint friction compensation. The proposed impedance control method is verified both by simulations in Simulink and grasp experiments of an anthropomorphic hand. All the results show the method can not only estimate contact forces accurately and compensate joint friction, but also compliantly grasp objects with low stiffness and complex shape. Note to Practitioners-This paper is motivated by the demand for grasp control in anthropomorphic hands without finger force sensors. Force feedback is important in grasp control of anthropomorphic hands, but installing force sensors is highly challenging due to limited finger space for sensors and wiring, and the attendant consequences of increasing weight, volume and expenses. In order to realize such sensor-less grasp control, we present an impedance control method by calculating contact forces with a generalized momentum observer and estimating finger joint friction by an improved speed-friction model. Simulation studies and prototype experiments show the proposed method can stably grasp objects with varying stiffness and complex shapes while realizing expected dynamic characteristics. The proposed method can be used for other anthropomorphic hands without finger force sensors to improve their grasping capability. In future research, we will test the proposed method on more anthropomorphic hands without finger force sensors.