Measuring end-point stiffness by means of a modular mechatronic system

Human arm muscular stiffness measurement is often a complex procedure which is of great interest for many disciplines from biomechanics to medicine and robotics. Modulation of impedance represents the principal mechanism underlying control of movements and interaction with external environment. Past literature proposed several methods to estimate multijoint hand stiffness while postural maintaining and dynamic tasks, mainly performed by means of planar robotic manipulanda. Despite these approaches are still considered robust and accurate, the computational burden of the robotic controller and hardware limitations make them not easy to implement. In the present paper a novel mechanism conceived for measuring multijoint planar stiffness by in single trial and in a reduced execution time is described and tested in different configurations. The device consisted in a mechanical rotary mechanism which applies cyclic radial perturbation to human arm of a known displacement and the force is acquired by means of a 6-axes commercial load cell. The outcomes suggest that the system is not only reliable in standalone mode but allows obtaining a reliable bi-dimensional estimation of arm stiffness even plugged in a planar manipulandum, dramatically reducing the amount of time for measurement and allowing to decouple the two controllers of the planar manipulator on which is mounted and the device itself.