Design and analysis of parallel manipulators with 3 translational degrees of freedom used as an anti-balance perturbator, exactly constrained and overconstrained

The design of parallel manipulators with 3 translational degrees of freedom to deliver short-duration reactive inertial forces on the base is discussed. The intended application is a device that can apply perturbing forces on human limbs. The device, called an anti-balance perturbator, has to be mounted around the limb in a non-obtrusive way, to be lightweight with most mass attached to the moving platform, to have a large workspace with respect to the available space and to have a large bandwidth. Three designs are compared: an exactly constrained manipulator with three RUU legs, an overconstrained overactuated manipulator with four RUU legs and a manipulator with three overconstrained RRPaR legs. The designs contrast to common ones, because most mass is placed on the movable platform and because the base and the platform are almost in the same plane. A kinematic analysis addresses singularities and the sensitivity of the platform motion for clearance in the joints. Moreover, the compliance at the platform due to leg flexibility is determined. For these analyses, aggregate properties of the legs are used, which simplifies the analysis. Since the results show that the overconstrained manipulators are much less sensitive to clearance and much stiffer than the exactly constrained 3RUU manipulator, the design specifications can be more easily met. This makes the overconstrained designs preferable.