Recent research works have been focusing on the development of tensegrity-based, planar, lightweight manipulators having variable stiffness capabilities, which is essential for payload manipulations while safely interacting with the environment (see link 1 and link 2). This research concentrates on the development of elementary, spatial, cable-operated variable-stiffness joints, such as the quaternion joint (refer Fig. 1), which can be further developed into fully-fledged spatial manipulators.

INVESTIGATIONS ON THE QUATERNION JOINT

1. DEVELOPMENT OF VIRTUAL PROTOTYPES AND VIRTUAL KINEMATIC MODELS

• Fabrication of un-actuated prototypes for motion studies.

• Development of virtual prototypes using kinematic modelling. Useful for mapping independent task-space variables to the configuration space. Also maps task-space variables to actuation cable lengths.

2. STATIC MODELLING OF THE MECHANISM

• Estimation of the wrench-feasible workspace (WFW) of the mechanism. For a given set of mechanism parameters (dimensions, masses, spring constants), WFW is the range of motion of the mechanism, where the wrench contributed by the input cable forces fully encloses the wrench contributed by gravity and springs. 

• Derived conditions to maintain static equilibrium.

3. STIFFNESS ANALYSIS

• Derivation of 2x2 stiffness matrix.

• Analysed dependence of stiffness matrix on input parameters.

• Explored maximum and minimum stiffness limits of the mechanism, and their dependence on cable actuation forces. 

4. STABILITY ANALYSIS

• Identified configurations at which mechanism is stable.