Design and analysis of cable-driven manipulators with variable stiffness

• Cable-driven manipulator with significant stiffness regulation.
• Stiffness of the cable-driven manipulator is a function of the cable tensions.
• Variable stiffness is achieved by attaching novel variable stiffness devices.
• Design and analysis of the variable stiffness device is presented.
• A stiffness-oriented tension resolution algorithm is proposed.

A manipulator with variable stiffness allows the manipulator to adjust its stiffness to suit for different task requirements. In this paper, a cable-driven manipulator with the ability to significantly regulate its stiffness through tension resolution is introduced. Variable stiffness for the cable-driven manipulators is achieved by attaching a novel variable stiffness device along each driving cable, in which the stiffness of the device is a function of the cable tension. As the cable-driven manipulator has actuation redundancy, the tension distribution can be manipulated even at a stationary pose. Such property allows the cable-driven manipulator to adjust the stiffness of each variable stiffness device, thereby regulating the stiffness of the manipulator. The design and analysis of the variable stiffness device are presented. The variable stiffness device uses commercial torsion springs, and has a compact and light-weight design. For a particular pose, a stiffness-oriented tension resolution algorithm is proposed to optimize the tension distribution to achieve an isotropic stiffness matrix with high stiffness magnitude. Experimental and simulation results verified that cable-driven manipulators with such variable stiffness devices are able to achieve significant stiffness regulation.