Article

# A Method to 3D Print a Programmable Continuum Actuator with Single Material Using Internal Constraint

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## Abstract

Soft continuum robots require differential control of channel pressure across several modules to trace 3D trajectories at the tip. For current designs of such actuators, sheathing is required to prevent radial expansion when the chambers are pressurized. With the recent development of soft materials additive manufacturing, 3D printing has become a promising fabrication method for soft continuum robots. However, most current designs for continuum actuators are based on molding, which are not designed for 3D printing. This paper proposes an internal constraint-based soft continuum actuator for single material 3D printing, with tunable design parameters to render pre-defined motions. The internal constraint method maximizes the superiority of the rapid prototyping solution in terms of customizing the soft continuum actuators with high fabrication speed and design freedom. The internal constraints come in the form of internal beam elements that not only limit the undesired radial expansion (up to ∼14% of conventional design) but also allows the actuator to be pressurized at a higher driving pressure (up to ∼160%) and higher maximum bending angle (up to ∼320%) compared to conventional no-beam design. By tuning the design parameter q (determined by the number of constraint beams n per radial cross-section and the number of such sections k along the axial direction), we can render the actuator to the desired movement under specific driving pressure p. We show numerical simulation and hardware experiment results for a soft actuator to achieve specified bending and twisting motions following this design approach.

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... These valuable characteristics lead to the development of various pneumatic actuators with different structures, action types, pressure ranges, and fabrication methods [2]. However, most soft pneumatic actuators still require conventional electronic controllers and hard valves to regulate the internal fluid and control the actuation, sacrificing some benefits of being soft [3,4,5]. The existence of these tethered external rigid pressure systems severely limits the mobility and application fields of soft robots. ...
... However, these valves work with only pneumatic actuators with large working stroke, as it requires sufficient deformation to kink the elastomeric tubes and block the flow. Soft material 3D printing technique [11,4] provided another possible approach to create bi-stable structures with enhanced model repeatability, increased design flexibility, and ease in fabrication. Compared with silicone casting, 3D printing of thin bi-stable membrane brings reliable consistency in each batch of fabrication, which guarantees the repeatability of the snapthrough behaviour. ...
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Although research studies in pneumatic soft robots develop rapidly, most pneumatic actuators are still controlled by rigid valves and conventional electronics. The existence of these rigid, electronic components sacrifices the compliance and adaptability of soft robots.} Current electronics-free valve designs based on soft materials are facing challenges in behaviour consistency, design flexibility, and fabrication complexity. Taking advantages of soft material 3D printing, this paper presents a new design of a bi-stable pneumatic valve, which utilises two soft, pneumatically-driven, and symmetrically-oriented conical shells with structural bistability to stabilise and regulate the airflow. The critical pressure required to operate the valve can be adjusted by changing the design features of the soft bi-stable structure. Multi-material printing simplifies the valve fabrication, enhances the flexibility in design feature optimisations, and improves the system repeatability. In this work, both a theoretical model and physical experiments are introduced to examine the relationships between the critical operating pressure and the key design features. Results with valve characteristic tuning via material stiffness changing show better effectiveness compared to the change of geometry design features (demonstrated largest tunable critical pressure range from 15.3 to 65.2 kPa and fastest response time $\leq$ 1.8 s.
... This paper was recommended for publication by Editor Kyu-Jin Cho upon evaluation of the Associate Editor and Reviewers' comments. This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) Grant EP/V000748/1; S. Wang was supported by the CSC-PAG Oxford Scholarship for his DPhil study.) 1 soft pneumatic actuators still require conventional electronic controllers and hard valves to regulate the internal fluid and control the actuation, sacrificing some benefits of being soft [3,4,5]. The existence of these tethered external rigid pressure systems severely limits the mobility and application fields of soft robots. ...
... However, these valves work with only pneumatic actuators with large working stroke, as it requires sufficient deformation to kink the elastomeric tubes and block the flow. Soft material 3D printing technique [11,4] provided another possible approach to create bi-stable structures with enhanced model repeatability, increased design flexibility, and ease in fabrication. Compared with silicone casting, 3D printing of thin bi-stable membrane brings reliable consistency in each batch of fabrication, which guarantees the repeatability of the snap-through behaviour. ...
Article
Although research studies in pneumatic soft robots develop rapidly, most pneumatic actuators are still controlled by rigid valves and conventional electronics. The existence of these rigid, electronic components sacrifices the compliance and adaptability of soft robots. Current electronics-free valve designs based on soft materials are facing challenges in behaviour consistency, design flexibility, and fabrication complexity. Taking advantages of soft material 3D printing, this paper presents a new design of a bi-stable pneumatic valve, which utilises two soft, pneumatically-driven, and symmetrically-oriented conical shells with structural bistability to stabilise and regulate the airflow. The critical pressure required to operate the valve can be adjusted by changing the design features of the soft bi-stable structure. Multi-material printing simplifies the valve fabrication, enhances the flexibility in design feature optimisations, and improves the system repeatability. In this work, both a theoretical model and physical experiments are introduced to examine the relationships between the critical operating pressure and the key design features. Results with valve characteristic tuning via material stiffness changing show better effectiveness compared to the change of geometry design features (demonstrated largest tunable critical pressure range from 15.3 to 65.2 kPa and fastest response time ≤ 1.8s).
Chapter
A multi-material 3D printed soft actuator is presented that uses symmetrical, parallel chambers to achieve bi-directional variable stiffness. Many recent soft robotic solutions involve multi-stage fabrication, provide variable stiffness in only one direction or lack a means of reliably controlling the actuator stiffness. The use of multi-material 3D printing means complex monolithic designs can be produced without the need for further fabrication steps. We demonstrate that this allows for a high degree of repeatability between actuators and the ability to introduce different control behaviours into a single body. By independently varying the pressure in two parallel chambers, two control modes are proposed: complementary and antagonistic. We show that the actuator is able to tune its force output. The differential control significantly increases force output with controllable stiffness enabled within a safe, low-pressure range ($$\le 20$$ kPa). Experimental characterisations in angular range, repeatability between printed models, hysteresis, absolute maximum force, and beam stiffness are presented. The proposed design demonstrated a maximum bending angle of 102.6$$^\circ$$, maximum output force 2.17N, and maximum beam stiffness 0.96mN m$$^2$$.
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Multigait soft robot, Proceedings of the national academy of sciences
• R F Shepherd
• F Ilievski
• W Choi
• S A Morin
• A A Stokes
• A D Mazzeo
• X Chen
• M Wang
• G M Whitesides
R. F. Shepherd, F. Ilievski, W. Choi, S. A. Morin, A. A. Stokes, A. D. Mazzeo, X. Chen, M. Wang, G. M. Whitesides, Multigait soft robot, Proceedings of the national academy of sciences 108 (2011) 20400-20403.