A New Continuum Robot with Crossed Elastic Strips: Extensible Sections with Only One Actuator per Section

We propose a new kind of continuum robot based on crossed elastic strips. The actuator-specified location of the crossover point controls the lengths of the sections, enabling a wider range of configurations than would be possible with traditional fixedsection-length robots. Push-pull actuation of the crossed strips controls the curvature of the sections. We provide a model that describes the resulting configurations in terms of tangent circular arcs of varying lengths. Experiments with a prototype yield tip positions that agree with model predictions with an average error of 4.6% of the robot’s length. INTRODUCTION Continuum robots have unique properties among robotic manipulators. Their passive compliance and ability to take on a variety of shapes make them particularly well-suited to tasks in which they must operate in a delicate environment or maneuver around obstacles [1–3]. A variety of designs for continuum manipulators have been proposed, including pneumatically and hydraulically actuated manipulators [4–8], tendon-driven arms [9–13], designs with flexible push-pull actuation rods [14], and manipulators whose motion arises from the elastic interaction of flexible pre-curved concentric tubes [15, 16]. A typical design scheme among continuum robots involves multiple similar, individually actuated continuum sections, assembled in series to produce a multi-section arm. The ability to vary the length of the individual sections ∗Corresponding author. within a continuum robot is desirable, since it expands the range of shapes the robot can achieve [17]. However, the mechanical design and implementation of robots with this capability can be challenging for nearly all continuum robot designs. Perhaps the lone exception is pneumatic arms, since many pneumatic manipulators are inherently extensible [3]. However, pneumatic designs are not ideal for all tasks, and do not scale down to small sizes as well as some other designs [18]. When smaller diameters (on the order of several millimeters, for example) are required, tendonand rod-actuated designs are often selected. These designs typically have fixed backbone lengths for each section. However, there has been some research toward making them extensible. In [19], Blessing and Walker present a prototype of a tendon-driven robot whose section lengths can be adjusted and set prior to actuation, but not varied in real-time during robot operation. In this paper we present a design where the relative lengths of two sections can be varied during robot operation by the robot’s actuators. We also provide a model for this robot based on the constant curvature assumption. Constant curvature kinematic models which accommodate variable section lengths have been developed for general extensible multisection arms in [20–22]. Indeed, for some classes of continuum robots, extending fixed-section-length models to allow for varying section lengths is as simple as treating section length parameters as inputs rather than constants [19]. The notion of variable section lengths has also been used to model unintended length changes, which may occur in response to tendonapplied loads on the backbone, as in [12]. The new variable-section-length design we propose here is 1 Copyright c © 2015 by ASME inspired by the work of Yamada et al. in [23]. They propose a mechanism of bending for a small-scale continuum manipulator which can enable a variety of unusual robot shapes. The concept involves two flexible strips which are attached together at one end, crossed over one another one or more times, and inserted into a flexible outer sheath. Translating one of the strips into or out of the sheath causes the strips to push on the walls of the sheath and bend it into a curved shape. The key innovation in our design in contrast to that of Yamada et al. is to provide direct actuator control of the location of the crossover point. This provides improved control over the shape of the device, and enables a greater variety of achievable shapes. Coupling the location of the change in curvature to an input also has the benefit of enabling the use of the simple piecewise constant curvature kinematic model we present in this paper. Through experiments with a 4.5 mm diameter prototype, we demonstrate that the model can predict manipulator tip position to within an average of 4.6% of the robot’s length across a wide range of crossover point locations.