Nonlinear system identification framework of folding fins with freeplay using backbone curves

Due to wear and manufacturing tolerance,the freeplay is unavoidable in the hinges of folding fins,which exerts significant effects on the aerodynamic characteristics.This paper proposes a backbone-curve-based framework for the dynamical identification of folding fins containing the freeplay nonlinearity.With no need to measure the input force signal and the response signals of nonlinear related Degrees of Freedom(DOFs),the proposed method is more direct and elegant than most existing nonlinear identification approaches,and it contains three steps:Firstly,the underlying linear model of the folding fin structure is obtained through the modal test on its linear sub-parts,and then,the harmonic approximation solves the analytical expressions of the backbone curves of measurable DOFs.Secondly,response data measured from the sine-sweep test are used to extract the fitting points of backbone curves for these DOFs.Finally,the curve fitting approach is applied to identify the freeplay parameters.A series of numerical experiments verify the effectiveness of the proposed method.A real-life folding fin structure is also employed to illustrate how the method can be applied.These examples demonstrate that the identification framework can give an accurate dynamic model of the folding fin structure.

Motion-Planning Algorithm for a Hyper-Redundant Manipulator in Narrow Spaces

In this study,a hyper-redundant manipulator was designed for detection and searching in narrow spaces for aerospace and earthquake rescue applications.A forward kinematics equation for the hyper-redundant manipulator was derived using the homogeneous coordinate transformation method.Based on the modal function backbone curve method and the known path,an improved modal method for the backbone curves was proposed.First,the configuration of the backbone curve for the hyper-redundant manipulator was divided into two parts:a mode function curve segment of the mode function and a known path segment.By changing the discrete points along the known path,the backbone curve for the manipulator when it reached a specified path point was dynamically obtained,and then the joint positions of the manipulator were fitted to the main curve by dichotomy.Combined with engineering examples,simulation experiments were performed using the new algorithm to extract mathematical models for external narrow space environments.The experimental results showed that when using the new algorithm,the hyper-redundant manipulator could complete the tasks of passing through curved pipes and moving into narrow workspaces.The effectiveness of the algorithm was also proven by these experiments.

Inverse Displacement Analysis of a Hyper-redundant Elephant＇s Trunk Robot

This paper deals with inverse displacement analysis of a Hyper-redundant Elephant＇s Trunk Robot （HRETR）. The HRETR is connected in series with n modules of 3UPS-PRU parallel mechanism where the underline P denotes an active prismatic joint. Based on the idea of differential geometry, backbone curve of the robot is formulated by using a parametric fimction consisting of sub-fimctions and control parameters. A general algorithm for generating a backbone curve and fitting the modules to the backbone curve is proposed. In this way, the inverse displacement analysis of the robot can be carried out by solving the inverse displacement problem of each parallel mechanism module and taking into account the length limits of the links. A HRETR with 6 modules is taken as an example to demonstrate the applicability of the algorithm.