Development and Evaluation of a Quasi-Passive Stiffness Display

In the force feedback of physical human-robot interaction, stiffness is rendered to represent the hardness of the manipulated object, aiding users in executing accurate maneuvers. Active feedback based on electric motors usually cannot simultaneously achieve both high force and high backdrivability, leading to limitations in stiffness rendering. Therefore, this letter explores a quasi-passive stiffness display based on the variable stiffness mechanism (VSM). It can provide controllable stiffness feedback theoretically ranging from zero to infinity, while the feedback force mainly comes from the VSM's reaction to the user's press force. First, a VSM that decouples the output stiffness from the press displacement is proposed. Then, a one-degree-of-freedom stiffness display prototype based on the VSM is developed and evaluated through quantitative experiments. The experimental results demonstrate that the quasi-passive stiffness display can meet the requirements of diverse tasks within a wide stiffness/force range.