Investigations of variable stiffness principles for compliant robotics

Abstract

Compared with conventional robots that require some sort of safety measures in operation, compliant robots capable of interacting with the environment in a gentle way and perform self-adjustment according to the surroundings are of greater need nowadays. To achieve compliance, robots with variable stiffness are highly desirable. Principles of stiffness control for conventional actuators and rigid robots, such as spring-based variable stiffness and impedance mechanisms, have been thoroughly studied. In this thesis, the focus is on the novel principles of variable stiffness, in particular, on geometry-based and jamming-based methods, and their innovative applications in robotics. The utilization of 3D printing technology in this research brings new insight into robotic research.

At first, a non-assembly design of a compliant pneumatic stepper motor based on the ratchet and pawl mechanism is proposed for robotic drives. The proposed stepper motor can generate rotary stepping motion in response to a train of pulse air pressure. In addition, the pawl is designed as a compliant mechanism instead of conventional rigid component. As a result, the whole mechanism can be fabricated by 3D printing without any post-assembly or installation of other components.

However, the ratchet and pawl mechanism based stepper motor has speed limitation and velocity discontinuity problems. An updated design of non-assembly pneumatic stepper motor allows velocity control, and the switch between continuous motion and stepping motion. The key element to realize the dual motion pattern is an innovative design of a roller valve inspired by the cam-follower mechanism. The roller valve can also modulate the stiffness of robots by air pressure control. The proposed concept of non-assembly robotic design and fabrication based on 3D printing and the proposed stepper motor designs can bring design innovation to various robots, such as household robots, industrial robots and so on.

The geometry-based principle is applicable for compliant robots with rigid parts. For better compliance, jamming technique is applied to a proposed soft robotic spine design. The soft robotic spine is proposed based on the integration of particle jamming and ball joint. The combined mechanism effectively overcomes the unpredictable behavior of jamming due to particle rearrangement and deformation which severely affects its repeatability and stability. The experimental studies have shown stiffness enhancement of 13 times. The compliant spine mechanism can be coupled with an actuation method such as wires/tendons, or soft actuators for robotic applications.

The research also finds that the particle jamming technology can bring unexpected effects into biomimetic robotic hand design. The proposed robotic finger design based on the integration of particle jamming and fiber-reinforced soft bending actuator provides an instructive model for robotic gripper applications. The prototype robotic hand demonstrates great performance of reliable grasping. Based on similar jamming interface, a bioinspired robotic palm is designed for the hand. The property of controllable stiffness of the palm allows a full contact of the palm with the object being grasped, thus providing a stable support for object grasping. The proposed palm design demonstrates a close emulation of human palm functions.