Modular Continuum Manipulator: Analysis and Characterization of Its Basic Module: History Edit

We present the basic module of a modular continuum arm (soft compliant manipulator for broad applications (SIMBA)). SIMBA is a robotic arm with a hybrid structure, namely a combination of rigid and soft components, which makes the arm highly versatile, dexterous, and robust. These key features are due to the design of its basic module, which is characterized by a three-dimensional workspace with a constant radius around its rotation axis, large and highly repeatable bending, complete rotation, and passive stiffness. We present an extensive analysis and characterization of the basic module of the SIMBA arm in terms of design, fabrication, kinematic model, stiffness, and bending behavior. All the theoretical models presented were validated with empirical results. Our findings show a positional typical error of less than ≈6% in module diameter (highly repeatable) with a passive stiffness of 0.8 N/mm (≈1 kg load). Our aim is to demonstrate that this kind of robotic element can be exploited as an elementary module of a more complex structure, which can be used in any application requiring high directional stiffness but without the need for an active stiffness mechanism, as is the case in daily activities (e.g., door opening, water pouring, obstacle avoidance, and manipulation tasks).

  • continuum manipulator
  • soft robot
  • modular arm
  • compliant structure
  • large deformation
  • constant curvature
  • tendon-driven actuation
  • kinematic modeling
  • planar spring
  • beam theory
In the current work, we have presented kinematic and cantilever beam models to describe the positions and stiffness of the basic module for a modular continuum arm, respectively. The kinematic model demonstrates its accuracy and repeatability (Figure 11) with only an error at a maximum of ≈6% in the module diameter. In both models we used the constant curvature assumption and adopted the principle of the Euler–Bernoulli beam theory for large deformation. We thus demonstrated that the cantilever beam model has a good prediction regarding experimental data of up to ≈100° of bending. The nonlinearity obtained by the experimental results for bending greater than 100° can be attained by reciprocal mapping of kinematic and stiffness domains, thus also facilitating the implementation of more precise control strategies.
The proposed module is very versatile and can easily act not only as arm component but can be integrated with all applications where directional stiffness, compliancy, rotational, and bending capabilities are required. For instance, alternative applications of the module could be as a leg for an alligator-inspired robot where stiffness is required in the plane perpendicular to the motion, and the inherent compliance of the module could be exploited for unstructured terrains [34,35]. Similarly, the module could be adopted for snake-inspired robots, where directional stiffness and axis rotation can be exploited for different snake locomotion modalities (sidewinding, corkscrewing, and strafing) 
The high compliancy and robustness demonstrated by the module, and even by an entire arm based on the presented design, also opens several other opportunities for applications in unstructured environments, for pipe inspections, or manipulation in hazardous environments.