Variable Stiffness Pad and Its Application to Hybrid Rigid-Soft Robots

Abstract

Robot manipulation can easily become unstable when interacting with stiff environment. This is due to high contact stiffness, which is usually cannot be controlled. Contact stiffness is an important factor for the robot to perform the tasks successfully. It helps determining the contact force, which in turn affects the robot response and stability of the interaction. Therefore, the variable stiffness pad (VSP) and its pressure supply system are developed. It allows one to modify the contact stiffness appropriately for the task and environment at hand. Moreover, since the VSP locally conforms to the surface of the object, effective contact area is enlarged so the induced contact stress is reduced. The prototype VSP for integrating with a rigid gripper can vary its stiffness linearly with time constant of 130 ms in the range of about 23.5-42.0 kN/mduring contact according to the controlled pressured of 0-100 kPa. This unique feature is used to modify contact stiffness properly. A concept of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hybrid robots</i> , where the VSPs are installed at the interaction surface of rigid robot, is proposed. Unlike traditional robots, both <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">manipulation impedance</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">contact impedance</i> of the hybrid robot may be adjusted in order to interact with unstructured environment properly. Analysis and experiments show that the hybrid rigid-soft gripper can manipulate objects with different shapes and stiffness safely.