Compliant continuum robots (CCRs) have slender and elastic bodies. Compared with a traditional serial robot, they have more degrees of freedom and can deform their flexible bodies to go through a constrained environment. In this paper, we classify CCRs according to basic transmission units. The merits, materials and potential drawbacks of each type of CCR are described. Drive systems depend on the basic transmission units significantly, and their advantages and disadvantages are reviewed and summarized. Variable stiffness and intrinsic sensing are desired characteristics of CCRs, and the methods of obtaining the two characteristics are discussed. Finally, we discuss the friction, buckling, singularity and twisting problems of CCRs, and emphasise the ways to reduce their effects, followed by several proposing perspectives, such as the collaborative CCRs.

The design of XY compliant parallel manipulators (CPMs) remains challenging considering the tradeoff between mirror-symmetry for better constrained undesired rotations and a small footprint, although a significant number of XY CPMs have been reported in extensive applications. This paper presents a new XY CPM using mirror-symmetry without increasing its footprint, mainly aiming to reduce the undesired parasitic rotations of input and output motion stages. The concept of mirror-symmetry is deployed to tackle the parasitic rotations, with the help of using a multilayer compact XY CPM design method. A nonlinear and analytical model of the proposed XY CPM is derived using free body diagrams and the beam constraint model (BCM) to accurately analyze its performance characteristics over a large range of motion. The designed XY CPM is then verified by the nonlinear finite element analysis (FEA) method. Finally, the proposed multilayer design is fabricated using two pieces of aluminum plate and it is mounted in a measurement system to experimentally validate several performance characteristics. The analytical, FEA, and/or experimental results show that the proposed design can sufficiently constrain undesired motions including parasitic rotation of input and output stages, cross-axis coupling error, and actuator isolation index. Compared with exiting designs, the proposed design also shows its merits in large motion range and out-of-plane stiffness.

This article presents a nonlinear model of an inversion-based generalized cross-spring pivot (IG-CSP) using the beam constraint model (BCM), which can be employed for the geometric error analysis and the characteristic analysis of an inversion-based symmetric cross-spring pivot (IS-CSP). The load-dependent effects are classified into two ways, including the structure load-dependent effects and beam load-dependent effects, where the loading positions, geometric parameters of elastic flexures, and axial forces are the main contributing factors. The closed-form load–rotation relationships of an IS-CSP and a noninversion-based symmetric cross-spring pivot (NIS-CSP) are derived with consideration of the three contributing factors for analyzing the load-dependent effects. The load-dependent effects of IS-CSP and NIS-CSP are compared when the loading position is fixed. The rotational stiffness of the IS-CSP or NIS-CSP can be designed to increase, decrease, or remain constant with axial forces, by regulating the balance between the loading positions and the geometric parameters. The closed-form solution of the center shift of an IS-CSP is derived. The effects of axial forces on the IS-CSP center shift are analyzed and compared with those of a NIS-CSP. Finally, based on the nonlinear analysis results of IS-CSP and NIS-CSP, two new compound symmetric cross-spring pivots are presented and analyzed via analytical and finite element analysis models.

Cable-driven compliant continuum robots (CCRs) can reach target areas in constrained spaces due to their elastic bodies being controlled remotely. They have been widely employed to inspect, maintain, or repair industrial machines such as gas turbine engines and oil pipes. However, their performances are usually limited by the low tip stiffness and the stiffness usually decreases with the increase of cable pulling forces. This paper aims to address the above problems by presenting a tip stiffness improved CCR formed by compliant anti-buckling universal joints (ACCR). The normalized nonlinear spatial models of the one-segment CCR and three-segment CCR are proposed and comprehensively verified using commercial nonlinear finite element analysis software. Given cable forces, prescribed cable displacements, tip loads, and gravity, the motion of any points on the CCRs can be analytically obtained. The performance characteristics of the one-segment CCRs and three-segment CCRs are extensively studied under different loading conditions, including the maximum CCR deformation, tip-location accuracy, shape dexterity, and tip stiffness. The results show that the tip stiffness of the ACCR is always much higher than that of the counterpart CCR under the same loading conditions. For example, the one-segment and three-segment ACCRs both have a high in-plane tip stiffness under in-plane actuation, which can increase by 49.0% and 31.3%, respectively; and they both have a high transverse tip stiffness under spatial actuation, which can increase by 48.9% and 31.2%, respectively. It is also confirmed that the ACCRs can increase tip stiffness by increasing cable actuation. Several preliminary planar experimental tests are carried out to validate the fabrication feasibility of the prototype, the accuracy of the analytical model, and/or the above-mentioned stiffness improvement.

This paper applies the kirigami technique to a non-rigid foldable tubular origami to make a rigid foldable tubular design, i.e., a radially closable kirigami (RC-kiri). The laminar emergent torsional (LET) compliant joint is applied to surrogate the crease, which makes the design applicable in practical engineering applications. By incorporating a non-flat folding design, the folding angles of each crease are minimized, leading to a reduction in the strain exerted on engineering materials. The kinetostatic theoretical model is constructed using the principle of virtual work, and its results are compared with those obtained from a simulation model in finite element analysis (FEA). A 3D printed physical model is tested to obtain the relationship between forces and displacements. FEA and experimental results match with theoretical findings. This study builds a bridge between origami and kirigami and expands the application of LET joints to the fabrication of tubular kirigami.