Design Considerations for a Hyper-Redundant Pulleyless Rolling Joint With Elastic Fixtures

Compliant joints composed of slit tubes or springs are commonly utilized for steerable endoscopes and catheters. Because this type of joint is actuated by means of its elasticity, it is difficult to bend it with a small radius of curvature. In order to overcome this problem, a hyper-redundant Pulleyless Rolling joint with Elastic Fixtures (PREF joint) is proposed. To enable miniaturization, the PREF joint does not have a pulley, and the rolling contact motion aids the actuation cables to maintain the tension. The elastic fixtures guarantee a stable rolling motion at the center of the rolling surfaces. In this paper, the fundamental characteristics for the unit and the hyper-redundant PREF joints are presented. Moreover, design considerations are proposed for the PREF joint including the rolling tooth shape, structure of the elastic fixtures, and cable actuation method. The validity of the design is verified using finite-element analysis and a 10 mm diameter prototype composed of 12 units of PREF joints for a 1 degree-of-freedom motion. As a result, the 55 mm long joint can be bent up to ±180° with low actuation tension. The presented guidelines will assist in the development of various steerable instruments for medical applications.