This paper presents a novel six-degrees-of-freedom spatial hybrid manipulator and the analysis of its forward kinematics. The proposed manipulator has three limbs of identical architecture connecting a hexagonal fixed platform to a moving platform in the form of an equilateral triangle. The limbs are spaced symmetrically w.r.t. the fixed platform. Each limb consists of a planar five-bar mechanism at its base, the tip of which connects to the moving platform via a passive RS link. The forward kinematic problem of the manipulator is reduced to that of a circle intersecting an oct-circular curve of degree 16 in its own plane. As expected, this leads to a maximum of 16 isolated (complex) solutions, the reals among which lead to poses with a certain mirror-symmetry. Consequently, a polynomial of only degree 8 has to be solved eventually. Coefficients of this polynomial have been found as closed-form symbolic expressions in the architecture parameters and input variables. The theoretical results are demonstrated and validated numerically using an example.

Variable stiffness actuators (VSA) are finding wide applications in robotics to enhance safety during interactions with stiff environments. Researchers have proposed various design architectures like antagonistic actuation, which requires both the motors to be powered simultaneously for varying the stiffness or equilibrium position. In this paper, the design of a novel joint module, named as variable stiffness joint module (VSJM), is proposed, which consists of a lead-screw arrangement for varying the stiffness range and a cam based mechanism to change the stiffness within the set range quickly. The cam profile has been synthesized to maximize the stiffness variation as well as to maintain the cam and cam follower in static equilibrium when the output link is in the equilibrium position. This was achieved by properly positioning and orienting the friction cones at the contact points. By mechanically compensating the moment due to unbalanced forces at the contact points, the continuous usage of stiffness motor has been eliminated, leading to reduced power consumption. Details of the proposed mechanism are presented along with the mathematical model for cam profile synthesis and static analysis. A simplified prototype of the proposed design has been fabricated to perform the experiments. A hammering-a-nail experiment has been conducted to show the capability of the mechanism, and the results are presented.

A variable stiffness actuator (VSA) is an actuator in which the stiffness of the output shaft can be controlled either actively or manually. Most of the VSAs that exist in the literature use two motors that act independently or antagonistically to vary the stiffness and the joint position. The issue with antagonistic actuation is that the motors have to be continuously powered to maintain a stiffness level. Hence to avoid continuous powering of the motors, a variable stiffness joint module (VSJM) has been proposed which uses independent motors with a cam mechanism to change and maintain the stiffness. The VSJM-I is the first among the designs which incorporates a lead screw mechanism to manually change the mean stiffness apart from the cam mechanism to vary the stiffness electronically. This improves the stiffness range and hence the actuator can be used for a wider range of tasks.