RESUMEN
Wheels have been commonly used for locomotion in mobile robots and transportation systems because of their simple structure and energy efficiency. However, the performance of wheels in overcoming obstacles is limited compared with their advantages in driving on normal flat ground. Here, we present a variable-stiffness wheel inspired by the surface tension of a liquid droplet. In a liquid droplet, as the cohesive force of the outermost liquid molecules increases, the net force pulling the liquid molecules inward also increases. This leads to high surface tension, resulting in the liquid droplet reverting to a circular shape from its distorted shape induced by gravitational forces. Similarly, the shape and stiffness of a wheel were controlled by changing the traction force at the outermost smart chain block. As the tension of the wire spokes connected to each chain block increased, the wheel characteristics reflected those of a general circular-rigid wheel, which has an advantage in high-speed locomotion on normal flat ground. Conversely, the modulus of the wheel decreased as the tension of the wire spoke decreased, and the wheel was easily deformed according to the shape of obstacles. This makes the wheel suitable for overcoming obstacles without requiring complex control or sensing systems. On the basis of this mechanism, a wheel was applied to a two-wheeled wheelchair system weighing 120 kilograms, and the state transition between a circular high-modulus state and a deformable low-modulus state was realized in real time when the wheelchair was driven in an outdoor environment.
RESUMEN
This paper presents a human-mimetic manipulator capable of shock absorption by using dislocation at the soft joint. A one degree-of-freedom (DOF) soft robot joint was developed based on the human elbow-joint structure, especially mimicking the humeroulnar joint in the elbow. Each component of the soft joint is combined by an elastic ligament, which is made up of elastic rubber and is attached to the pivot joint pin positioned at a predetermined place according to a specially designed pin guide. As an external impact is applied to the joint, the elastic ligament is elongated as the pivot joint pin is moved from the predetermined place. This state is defined as a dislocation, which is similar to the dislocation of a human joint when impacted. Dislocation in the proposed soft robot joint occurred when the external impact was larger than the predetermined threshold force. This threshold force can be predicted by the modulus of the elastic ligament and the shape of the pin guide, and the theoretical model was developed in this study. To evaluate the function of dislocation, dynamic and quasi-static impact tests were performed at the 1-DOF soft robot joint. Moreover, the human-mimetic manipulator is proposed based on the 2-DOF soft robot joint. This manipulator can realize four motions of a human arm using the pneumatic artificial muscles: flexion, extension, pronation and supination. Each artificial muscle and bone structure were similar to the human arm structure, and thus the configuration of each bone structure of the manipulator for each motion is similar to the configurations of a human skeletal structure. This manipulator was also capable of dislocation to absorb the external impact. The developed human-mimetic manipulator with a soft joint is expected to be applied to a naturally safe humanoid robot that works with humans in the same space.