Abstract
Continuum robots exhibit promising adaptability and dexterity for soft manipulation due to their intrinsic compliance. However, this compliance may lead to challenges in modeling as well as positioning and loading. In this paper, a virtual work-based static model is established to describe the deformation and mechanics of continuum robots with a generic rod-driven structure, taking the geometric constraint of the drive rods into account. Following this, this paper presents a novel variable stiffness mechanism powered by a set of embedded Shape Memory Alloy (SMA) springs, which can make the drive rods become ‘locked’ on the body structure with different configurations. The resulting effects of variable stiffness are then presented in the static model by introducing tensions of the SMA and friction on the rods. Compared with conventional models, there is no need to predefine the actuation forces of the drive rods; instead, actuation displacements are used in this new mechanism system with stiffness being regulated. As a result, the phenomenon that the continuum robot can exhibit an S-shaped curve when subject to single-directional forces is observed and analyzed. Simulations and experiments demonstrated that the presented mechanism has stiffness variation of over 287% and further demonstrated that the mechanism and its model are achievable with good accuracy, such that the ratio of positioning error is less than 2.23% at the robot end-effector to the robot length.
Highlights
Using active control algorithms at actuation level to change the stiffness characteristics of continuum manipulators originates from the impedance control or hybrid motion/force control in traditional rigid robots but the strain energy was taken into account (Mahvash and Dupont, 2011; Bajo and Simaan, 2016)
This is because, since the actuation displacement of all the drive rods are limited to zero, this geometric constraint converts into a motion constraint, which exerts a non-negligible influence on the mechanical properties of the manipulator
5.5 Discussion on the variable stiffness performance The main structure of our continuum manipulator is composed of the NiTi alloy with its intrinsic compliance and light weight
Summary
2005; Dong et al, 2017; Kato et al, 2015; Rone and Ben-Tzvi, 2014b; Simaan et al, 2009; Zhang et al, 2016; Catalano et al, 2014), pneumatically-driven type (Walker et al, 2005; Kim et al, 2014a; Kang et al, 2013; Hawkes et al, 2017) and concentrictube type (Dupont et al, 2010; Webster et al, 2009; Rucker et al, 2010) Their flexible structures make it difficult for all types of soft continuum robots to withstand large forces and keep motion precision at the same time (Dai and Ding, 2005). The thin layers can keep the internal passage out of obstruction but a vacuum pump was still required as an extra power source
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