Coupling Effect of Nonlinear Stiffness of Tape Spring Hinges and Flexible Deformation of Panels during Orbit Maneuvers

Many solar panels for spacecrafts are deployed by Tape Spring Hinges (TSHs) which have changeable stiffness. The stiffness of TSH is small when panels are folded, and it becomes large quickly in its deployed status. Since the solar panel is a thin sheet, flexible deformation is easily generated by orbit maneuvers. The coupling effect between the nonlinear TSHs and the flexible panels generates obvious vibration which affects the operational stability of the satellite. To investigate this coupling effect, non-deformable, linear deformable and nonlinear deformable panels were modelled by rigid body, modal order reduction method (MORM) and finite element method (FEM), respectively. The driving torque of TSH was described as a function of the rotation angle and angular velocity. The nonlinear properties of the TSH were reflected by one angle-stiffness spline multiplied by one stiffness coefficient. Dynamic responses of a satellite in deployment and orbit steering were analyzed by numerical simulations. Analysis results indicate the local deformation of panels keeps the stiffness of the TSH within a large range which accelerates the orbit maneuvers. However, much vibration is generated by the coupling effect if the luck-up status is broken up. The coupling effect affects the sequence of deployment, overshoot phenomenon and acceleration magnitude of the panels. Although the MORM is more efficient than FEM in computation, we propose FEM is better suited in the design of TSH and in studying the precise control of spacecraft with flexible solar panels and TSHs.

Application of Absolute Nodal Coordinate Formulation in Calculation of Space Elevator System

The space elevator system is a space tether system used to solve low-cost space transportation. Its high efficiency, large load, reusability and other characteristics have broad application prospects in the aerospace field. Most of the existing mechanical models are based on “chain-bar” and a lumped mass tether model, which cannot effectively reflect the flexible behaviour of the rope of space elevator system. To establish an accurate mechanical model, the gradient deficient beam elements of the absolute nodal coordinate formulation (ANCF) are used to build the mechanical model of the space elevator system. The universal gravitation and centrifugal force in the model are derived. The calculation results of the ANCF model are compared with the results of the finite element method (FEM) and lumped mass (LM) models. The results show that the calculation results of the ANCF method are not very different from the results of the FEM and LM models in the case of axial loading. In the case of lateral loading, the calculation results of the ANCF method are basically the same as the results of the FEM and LM models, but can better reflect the local flexible deformation of the space elevator rope, and have a better calculation stability than FEM. Under the same calculation accuracy, the ANCF method can use fewer elements, and the speed of convergence is faster than the FEM and LM models.

Soft Sensor Model for Estimating the POI Displacement Based on a Dynamic Neural Network

To satisfy the increasingly demanding requirements in throughput and accuracy, more lightweight structures and a higher control bandwidth are highly desirable in next-generation motion stages. However, these requirements lead to a more flexible deformation, causing the estimation accuracy of the point of interest (POI) displacement to be guaranteed under the rigid-body assumption. In this study, a soft sensor model is constructed using a dynamic neural network (DNN) to estimate the POI displacement. This model can reflect the dynamic characteristics of the POI and realize accurate estimations. Moreover, a method combining stepwise and weight methods is proposed to analyze the influence of different DNNs, and a performance measure is presented to evaluate the soft sensor model. In the simulation, the DNN with the hidden feedbacks is proved to be the most suitable soft sensor model. The relative error and correlation coefficient obtained were less than 2% and 0.9998, respectively, during training and 5% and 0.9989, respectively, during testing. Compared with the data-driven model using the least-squares method, the proposed method exhibits a higher precision, and the relative error is within the setting range using the proposed performance measure.

Design and analysis of a lightweight lower extremity exoskeleton with novel compliant ankle joints

BACKGROUND: The exoskeleton for lower limb rehabilitation is an uprising field of robot technology. However, since it is difficult to achieve all the optimal design values at the same time, each lower extremity exoskeleton has its own focus. OBJECTIVE: This study aims to develop a modular lightweight lower extremity exoskeleton (MOLLEE) with novel compliant ankle joints, and evaluate the movement performance through kinematics analysis. METHODS: The overall structure of the exoskeleton was proposed and the adjustable frames, active joint modules, and compliant ankle joints were designed. The forward and inverse kinematics models were established based on the geometric method. The theoretical models were validated by numerical simulations in ADAMS, and the kinematic performance was demonstrated through walking experiments. RESULTS: The proposed lower extremity offers six degrees of freedom (DoF). The exoskeleton frame was designed adjustable to fit wearers with a height between 1.55 m and 1.80 m, and waist width from 37 cm to 45 cm. The joint modules can provide maximum torque at 107 Nm for adequate knee and hip joint motion forces. The compliant ankle can bear large flexible deformation, and the relationship between its angular deformation and the contact force can be fitted with a quadratic polynomial function. The kinematics models were established and verified through numerical simulations, and the walking experiments in different action states have shown the expected kinematic characteristics of the designed exoskeleton. CONCLUSIONS: The proposed MOLLEE exoskeleton is adjustable, modular, and compliant. The designed adjustable frame and compliant ankle can ensure comfort and safety for different wearers. In addition, the kinematics characteristics of the exoskeleton can meet the needs of daily rehabilitation activities.

Trajectory Planning of Robot-assisted Abrasive Cloth Wheel Polishing Blade based on Flexible Contact

Industrial robot-assisted abrasive cloth wheel (ACW) accurately polish blades is considered to be a challenging task, and it is necessary to realize the digitalization of the process. Due to the flexible contact characteristics of abrasive cloth wheel and the change of blade surface curvature, the amount of microscopic material removal and the topographical errors of the blade surface are not uniform. So the surface roughness value is larger. In this paper, considering the flexible deformation of the blade and the abrasive cloth wheel during polishing contact, the polishing contact model of the blade and the abrasive cloth wheel is established and simulated. Firstly, based on the Preston equation and Hertz contact theory, the material removal profile in the contact area of the blade and the abrasive cloth wheel is analyzed. Secondly, the step length is optimized by considering the deformation of the abrasive cloth wheel in thedirection, and the line spacing is optimized by considering the material removal uniformity in thedirection. The NURBS curve is used to extract the blade polishing area curve and generate the polishing trajectory data points. Then, two methods of rigid trajectory planning and flexible adaptive trajectory planning of abrasive cloth wheel are simulated and analyzed by offline programming software. Finally, experiments were carried out on a four-station wheel changing polishing platform. Simulation and experiments results show that the proposed flexible adaptive trajectory planning method can make the surface roughness of the convex and the concave, the surface roughness of the leading and trailing edge, the total polishing efficiency increased by about 9.4%.

Research on loads applied on powered wheels and struts of civil aircraft at touchdown

Pre-rotating aircraft wheels is a valid method to reduce the drag loads applied on such wheels by the ground at touchdown. The drag loads have the probability to cause landing strut binding, which has negative effects on the landing gear and requires further analysis. To analyse the probability of the binding of the landing struts, a novel friction triangle model of the landing strut is established in a two-dimensional configuration. This paper builds a 6-degree-of-freedom aircraft dynamic model, consisting of an aircraft fuselage, landing struts, nose and main wheels. On the basis of such model, a set of dynamic analysis is performed to output the forces applied on the landing struts and wheels during spin-up and spring back processes. The maximum values of such forces are studied considering different pre-rotation speeds of the wheels. Through the new models applied in this paper, three conclusions are obtained. The first is that the main landing struts have the highest probability of strut binding at the moment of the first touchdown. The second is that the wheel pre-rotation decreases the probability of landing strut binding when the aircraft initially touches the ground. The third is that the multi-body dynamic model considering the flexible deformation of landing struts outputs drag loads with more and higher-frequency vibrations than the rigid strut model.

Simulation Study of Composite Materials Directional Pipe Based on Visual Prototype

When the flexible deformation of composite materials directional pipe which influences the movement of rocket projectile is considered, the visual prototype is a good approach that calculates the course of rocket projectile launch. In this thesis, the mechanical model of bearing load of composite materials directional pipe is established, and the theory and arithmetic of calculation are expatiated. After the simulation model is established and simulation calculation is done, not only the needed data and results are gained, but also the movement principles of rocket projectile in directional pipe and the dynamic mechanical capabilities of FRP directional pipe are held.

A Lab-Scale Underwater Glider With Flexible Camber Trailing Edge Wings

Wings are the main source of lift for the underwater gliders (UGs), and play a decisive role in the motion performance of UGs. A lab-scale UG with flexible camber trailing edge wings was proposed and developed to investigate influences of the trailing edge of the wings on the motion performance of UGs. The flexible deformation of the trailing edge was realized by the steer-by-wire actuator. Test results showed that the trailing edge of the wing can realize the maximum upward/downward sloping angles of +16°/-16°. Combining computational fluid dynamics simulations and tank experiments, the glide efficiency and stable margins of the lab-scale UG with variable camber trailing edge wings were obtained. Results showed that the angles of attack corresponding to the minimum lift-to-drag ratios were all negative in cases of downward sloping, and those were all positive in cases of upward sloping. Moreover, the suitable camber of the trailing edge, i.e., downward-sloping trailing edge on descending glides and upward-sloping trailing edge on ascending glides, can not only greatly improve glide efficiency, but also benefit the flight stability. The lift-to-drag ratios of the lab-scale UG with appropriate variable camber trailing edge wings (i.e., on descending glide with downward-sloping trailing edge, and on ascending glide with upward-sloping trialing edge) can increase by at least 10% compared with those with symmetric airfoil.

Approximate inertial manifold-based order reduction of rigid-flexible coupling manipulator

An order reduction method for the flexible deformation response analysis of rigid flexible manipulators is proposed based on the approximate inertial manifold theory. This method allows a lower dimensional simplified model to be constructed from a subspace smaller than the entire state space. In this paper, truncated three-order modes are used to construct a first-order system of AIM. Compared with the traditional Galerkin method, the results show that the proposed method can reduce the degree of freedom of the system and improve the computational efficiency without obviously losing the precision of the solution, which is convenient for the subsequent vibration analysis and controller design of the system.