A new overconstrained Gough-Stewart platform-based manipulator operating under force control

In vitro tests are essential to better understand the human joint behavior and to design more accurate prostheses and orthoses. In the literature, several test rigs have been presented that can be used to characterize the human joints for specific applications but poorly fit into the study when more general conditions are required. In 2014 the Group of Robotics, Automation and articular Biomechanics (GRAB) of the University of Bologna built an in vitro test rig that overcame these limitations. Even though the machine is capable to work with high accuracy, some improvements can be done in order to reduce the test time and simplify certain operations. Therefore, recently GRAB started the design of a new in vitro test rig, which can be regarded as an evolution of the previous one. The test rig comprises a number of subsystems one of which is the loading system that represents the core of the machine. In particular, this work presents a new manipulator whose features make it suitable as loading system for the new test rig. The new manipulator is an overconstrained Gough-Stewart platform-based robot. This property is useful since it makes it possible to realize a six degrees-of-freedom manipulator with a smaller number of joints and a higher stiffness with respect to a standard Gough-Stewart manipulator of the same size. Moreover, the manipulator does not require spherical joints and thrust journal bearings, thus allowing a larger workspace and simplifying the physical realization of the manipulator. The geometrical characteristics of the manipulator are shown, and the kinematic analysis is presented. A solution for the position analysis is proposed and the Jacobin matrix is derived. Furthermore, a singularity and workspace numerical analyses are presented. The geometrical interpretation of the singularities is provided.