Design and analysis of compliant universal joints and compliant continuum robots

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Date
2023
Authors
Li, Shiyao
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University College Cork
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Abstract
This dissertation presents the design and analysis of compliant universal joints and of cable-driven compliant continuum robots using the compliant universal joints. Rigid-body universal joints have been employed as transmission mechanisms in many applications such as vehicles and medical devices. However, they have problems associated with friction, lubrication, and assembly. Compliant mechanisms, transmitting motion and force by the deformation of elastic members, have many merits such as ease of fabrication, friction-free, no backlash, minimal assembly, and high precision. Inspired by these merits, compliant universal joints have been designed to obtain a lightweight and compact configuration without these problems mentioned earlier. Therefore, this dissertation starts from designing a compliant bidirectional-axial anti-buckling universal joint with four short tensile sheets and a wire beam. By an inversion-based arrangement of sheets, compressive forces exerted on the motion stage lead to tensile axial forces on each sheet to achieve stiffness improvement. The motions are analyzed in a linear method and the axial anti-buckling ability is verified by using nonlinear finite element analysis (FEA). To increase maximum deformation, four long tensile crossing sheets (i.e., two inversion-based cross-spring pivots) can be used to form the compliant anti-buckling universal joint. The planar nonlinear analysis of a generic inversion-based cross-spring pivot is necessarily derived at first in order to analyze the compliant universal joint. The load-dependent effects of the symmetric cross-spring pivot are particularly studied in this dissertation. The results show that the effective stiffness of a resulting mechanism depends on the equilibrium between the beam load-dependent effect and structure load-dependent effect. Two new compound cross-spring pivots are designed for large deformation with a minimized center shift and almost no load-dependent effects within a certain loading range, respectively. The nonlinear spatial analysis in terms of load-displacement relations of compliant universal joints is rarely investigated in the compliant mechanisms’ community, which is needed to show the physical insight into the constraint behavior of the universal joint. This dissertation addresses this gap and presents two types of nonlinear spatial models for compliant mechanisms consisting of elastic sheets. The performance characteristics of the anti-buckling universal joints are comprehensively studied, including center shifts, primary rotations, and load-dependent stiffness. The main characteristics of these compliant universal joints are the high customizable rotational compliance in the DoF direction and the high stiffness in the DoC (degrees of constraint) directions. Two compound-compliant anti-buckling universal joints are also designed, which are similar to the two compound cross-spring pivots. The anti-buckling robustness of these compliant universal joints is hopefully to benefit the stiffness improvement of a compliant continuum robot. Cable-driven compliant continuum robots (CCRs) can reach target areas in constrained spaces due to their elastic bodies being actuated remotely. They have been widely employed to inspect, maintain, or repair industrial machines such as gas turbine engines and oil pipes. However, their performance is usually limited by the low tip stiffness and small controllable stiffness range. The main reason is that their stiffness adversely decreases with the increase of cable pulling forces. This dissertation solves these problems by designing a tip-stiffness improved CCR formed by a series of compliant anti-buckling universal joints with two inversion-based cross-spring pivots. The merits of the load-dependent effects in the presentence of cable forces in a compliant anti-buckling universal joint are utilized to achieve the stiffness improvement of the CCR. The forward kineto-static models of the CCR using anti-buckling universal joints are studied by using the above nonlinear spatial method. Given any external loads (including cable forces, prescribed cable displacements, tip loads, and gravity), spatial displacements and rotations of any points on the CCR can be solved. The performance characteristics of the CCR under different loading conditions are well analyzed, including primary motions, maximum deformations, in-plane tip stiffness, out-of-plane tip stiffness, and spatial transverse tip stiffness. The results in all case studies show that the tip stiffness of the novel CCR is always higher than that of the counterpart traditional CCR under the same loading conditions. For example, the novel one-segment and three-segment CCRs both have a high in-plane tip stiffness under in-plane actuations, and the tip stiffness can increase by 49.0% and 31.3%, respectively. The novel one-segment and three-segment CCRs have a high transverse tip stiffness under spatial actuations, which can increase by 48.9% and 31.2%, respectively. All the theoretical results of both anti-buckling universal joints and the stiffness-improved CCRs are shown to be accurate using FEA simulations. The preliminary experimental tests are carried out to investigate the manufacturability of the anti-buckling universal joints and tip stiffness improved CCRs and to verify the theoretical models.
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Keywords
Compliant mechanisms , Compliant continuum robots , Anti-buckling universal joints , Rotational sequence , Nonlinear spatial analysis
Citation
Li, S. 2023. Design and analysis of compliant universal joints and compliant continuum robots. PhD Thesis, University College Cork.
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