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

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. To resolve this fatal shortcoming, we have developed the variable stiffness pad (VSP) and its pressure supply system, which can be integrated to typical rigid robots. It allows one to modify the contact stiffness appropriately for the task and environment at hand. By the controlled pressured of 0–100 kPa, the prototype VSP attached to a rigid gripper can vary its stiffness linearly in the range of 23.5–42.0 kN/m during contact with time constant of 130 ms. This unique feature is used to modify contact stiffness properly. Moreover, contact stress is reduced since the effective contact area is enlarged thanks to the adaptation of the VSP local contact surface to the object. A concept of hybrid robots, where several VSPs are installed at the interaction surface of rigid robot, is proposed. Unlike traditional robots, both manipulation impedance and contact impedance 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.