| Compared to the traditional rigid joints or constant stiffness joint, the variable stiffness joint has a wider application prospect with adjustable stiffness. The structure is more complex, a single joint system requires two motors, a position motor to regulate joint position, another rigid motor to adjust joint stiffness. With higher requirements, the study of structure and control algorithm of variable stiffness joints is a very challenging task.The traditional stiffness regulation principle are mostly against spring, elastic material and lever principle. While against spring is energy-intensive; elastic material is strict on the material properties, and the stiffness range is limited; lever principle is adopted frequently, but the alterations to the elastic end arm length or the load end arm length realizes a limited range of stiffness regulation. While changing lever fulcrum position can realize a wide range, from zero to rigid. The structure using lever principle is more complex, and exists many problems such as large volume, high quality, gear meshing clearance, sliding friction in the roller slot, slow response of ball screw and so on. The control researches are not in-depth, and trajectory tracking experiment is not satisfied.This paper presents a stiffness regulation principle based on the characteristic of torsional spring. The structure is simple, energy-saving, and wide stiffness range, from zero to rigid. Wire rope transmission adopted can bear large bearing force, and avoids the gear meshing transmission clearance, and high quality, large volume and slow response happened at the ball screw. Based on the idea of modularization, the mechanical design embeds the stiffness regulation module and included angle measurement module into the joint, making the structure more compact.The existing nonlinear elastic part causes controller design difficult and easy affected by external disturbance, this paper puts forward a PD controller based on the virtual torque, and introduces a disturbance compensator for external disturbances and other uncertain factors to improve the trajectory tracking and dynamic response performances. As showed by experiment results, PD controller is verified to achieve good tracking effect and disturbance compensator introduced makes an improvement.In addition, this paper verifies the feasibility of stiffness regulation principle based on torsional spring through the stiffness measurement experiment, and presents the variable stiffness joint has a good practical significance. |
