Simultaneous Stiffness Measurement Device for a Human Forearm

The paper proposes a stiffness measurement device composed of a measurement part that includes four indenters and an arm anchorage part that includes four linear servomotors. The device is able to make simultaneous stiffnesses measurements of the human forearm. An indenter using parallelogram mechanisms has two degrees-of-freedom and is designed to measure the stiffness at various locations. The kinematics of the parallelogram mechanism is computed to control the position of the indenters. Additionally, the admittance compensator for force control is designed so that the indenter can press the skin surface of the human forearm. According to the force level measured at loadcell installed on the indenter, the control is switched between pure position control and admittance control. Further, this device includes a two-axis potentiometer at the indenter endpoint. This potentiometer acts like a passive two degrees-of-freedom universal joint, allowing the indenter to press perpendicular to the measurement skin area. For this purpose, the mapping between the voltage output and the potentiometer angle was obtained by fitting for each axis. The proposed measurement device was tested for accuracy and repeatability. Ultimately, the measurement device was able to measure the stiffnesses of four regions of the human limb simultaneously.