Chih Hsing Liu

Dynamic Modeling of Damping Effects in Highly Damped Compliant Fingers for Applications Involving Contacts

In many industries, it is often required to transfer objects using compliant fingers capable of accommodating a limited range of object shapes/sizes without causing damage to the products being handled. This paper presents a coupled computational and experimental method in time domain to characterize the damping coefficient of a continuum structure, particularly, its applications for analyzing the damping effect of a highly damped compliant finger on contact-induced forces and stresses. With the aid of Rayleigh damping and explicit dynamic finite element analysis (FEA), this method relaxes several limitations of commonly used damping identification methods (such as log-decrement and half-power methods) that are valid for systems with an oscillatory response and generally estimate the damping ratio for a lumped parameter model. This damping identification method implemented using off-the-shelf commercial FEA packages has been validated by comparing results against published data; both oscillatory and nonoscillatory responses are considered. Along with a detailed discussion on practical issues commonly encountered in explicit dynamic FEA for damping identification, the effects of damping coefficients on contact between a rotating compliant finger and an elliptical object has been numerically investigated and experimentally validated. The findings offer a better understanding for improving grasper designs for applications where joint-less compliant fingers are advantageous. [DOI: 10.1115/1.4005270]

Explicit dynamic finite element analysis of an automated grasping process using highly damped compliant fingers

This paper has been motivated by the need to reduce the number of live animal tests in the development of an automated live-bird transfer system (LBTS) for the poultry meat-processing industry. Simulation-based models have been developed, which carefully address key engineering issues prior to live animal tests so that physical experiments can be focused on understanding reflex issues such as fear and escape behavior. To gain insights into the effects of operational timing on the LBTS handling performance, the multibody dynamics is modeled numerically based on the method of explicit dynamic finite element analysis (FEA) using off-the-shelf FEA packages. The findings also offer information on contact forces and their locations acting on the objects body and legs by the compliant fingers and grippers respectively for optimizing designs and avoiding damage to the object. Specifically, this paper discusses computational issues such as time-step considerations and highly damped behavior of compliant fingers when modeling using dynamic FEA methods. The FEA model has been validated by comparing simulated handling of an ellipsoidal object by a pair of robotic hands with multiple compliant fingers against published experimental data. It is expected that the FEA-based method presented here can be extended to a spectrum of applications where flexible multibody dynamics involving large deformable contacts and highly damped behaviors plays an important role.

Interior Head Impact Analysis of Automotive Instrument Panel for Unrestrained Front Seat Passengers

This study presents the numerical and experimental interior head impact analysis of automotive instrument panel according to the United Nations Economic Commission for Europe Regulation 21 (ECE R21). To minimize the possible injury risk for unrestrained front seat passengers due to the interior head impact with the instrument panel, the panel design needs to meet the ECE R21 standard which defines a pendulum-type head form as the impactor. The measured acceleration response of the head form should not exceed 80g continuously for more than 3ms. Motivated by the need to develop a simulation-based technique to evaluate the design of the instrument panel, a numerical model based on the explicit dynamic finite element analysis (FEA) by using the commercial FEA solver, LS-DYNA, is developed. To minimize the experimental cost, a gravity-based impactor with a smaller impact speed is develop as the test apparatus for verification purpose. The simulated results agree well with the experimental data; the average accuracy for the maximum value of impact acceleration at the head form is 95.4%. After the verification, the standard test conditions (with higher impact speed) are performed to evaluate the design. The outcome of this study can provide an efficient and cost-effective method to predict and improve the design of the instrument panel for interior head impact protection.

Explicit finite element analysis of a flexible multibody dynamic system with highly damped compliant fingers

Many industries require transferring objects from conveyors to a processing line at production rate. In food processing, grasping mechanisms with highly damped compliant fingers must be capable of accommodating a limited range of object shapes/sizes without causing damages on the products being handled. Most existing models, however, are inadequate to predict the dynamics of a compliant mechanism with large deformation, contact nonlinearity, and complex 3D geometries. This paper investigates the explicit finite-element (FE) method for industrial automation applications, where both geometric and operational parameters must be evaluated. Specifically, this paper discusses the effects of several key factors (that include material properties and element types as well as the numbers of nodes) on a FE computation. Along with an experiment /computation method (that relaxes limitations of a log-decrement method generally valid for systems with an oscillatory response), the procedure to account for the damping effect in simulating the dynamics of a compliant grasping system is numerically illustrated with experimental validation against published data.

Design and Analysis of Automotive Bumper Covers in Transient Loading Conditions

This paper presents the analysis methods for design of automotive bumper covers. The bumper covers are plastic structures attached to the front and rear ends of an automobile and are expected to absorb energy in a minor collision. One requirement in design of the bumper covers is to minimize the bumper deflection within a limited range under specific loadings at specific locations based on the design guideline. To investigate the stiffness performance under various loading conditions, a numerical model based on the explicit dynamic finite element analysis (FEA) using the commercial FEA solver, LS-DYNA, is developed to analyze the design. The experimental tests are also carried out to verify the numerical model. The thickness of the bumper cover is a design variable which usually varies from 3 to 4 mm depending on locations. To improve the stiffness of the bumper, an optimal design for the bumper under a pre-defined loading condition is identified by using the topology optimization approach, which is an optimal design method to obtain the optimal layout of an initial design domain under specific boundary conditions. The outcome of this study provides an efficient and cost-effective method to predict and improve the design of automotive bumper covers.

Topology Synthesis and Optimal Design of an Adaptive Compliant Gripper to Maximize Output Displacement

This paper presents the optimal design process of an innovative adaptive compliant gripper (ACG) for fast handling of objects with size and shape variations. An efficient soft-add topology optimization algorithm is developed to synthesize the optimal topology of the ACG. Unlike traditional hard-kill and soft-kill methods, the elements are equivalent to be numerically added into the analysis domain through the proposed soft-add scheme. A size optimization method incorporating Augmented Lagrange Multiplier (ALM) method, Simplex method, and nonlinear finite element analysis with the objective to maximize geometric advantage (which is defined as the ratio of output displacement to input displacement) of the analyzed compliant mechanism is carried out to optimize the design. The dynamic performance and contact behavior of the ACG is analyzed by using explicit dynamic finite element analysis. Three designs are prototyped using silicon rubber material. Experimental tests are performed, and the results agree well with the simulation models. The outcomes of this study provide numerical methods for design and analysis of compliant mechanisms with better computational efficiency, as well as to develop an innovative adaptive compliant gripper for fast grasping of unknown objects.

Topology and size optimization of an adaptive compliant gripper to maximize the geometric advantage

This study presents a systematic optimal design procedure to develop an adaptive compliant gripper (ACG) for grasping objects with various sizes and shapes. A soft-add topology optimization algorithm is developed to synthesize the optimal layout of the ACG. One special characteristic of the proposed soft-add scheme is that the elements are equivalent to be numerically added into the analysis domain. A size optimization procedure by using a mixed method combing Augmented Lagrange Multiplier (ALM) method and Simplex method is also proposed to maximize the geometric advantage (GA, which is defined as output displacement divided by input displacement) of the ACG. After the optimal design is obtained, the explicit dynamic Unite element analysis and experimental tests are carried out to analyze the design. The results show the developed ACG is with good adaptability and highest GA. The average GA can improve 58% comparing to the previous design.

A multi-legged biomimetic stair climbing robot with human foot trajectory

This paper aims to develop a multi-legged stair climbing robot with the capability to steadily climb stairs with consistent human foot trajectory. The leg design is based on the eight-bar Jansen mechanism but a new set of leg configuration is numerically identified based on the optimal design method with the aim to mimic the human foot trajectory when climbing stairs. The kinematic analysis of the leg mechanism based on loop closure equation has been derived in order to identify the leg trajectories for various designs. The targeted foot trajectory is experimentally measured. The weighted sum and simplex methods are used to solve the multi-objective function in the optimal design process. The proposed design is an eight-leg robot; its dynamic performance and trajectories of the multibody motion when climbing stairs have been numerically verified by using the commercial CAE package, RecurDyn. The prototype of the biomimetic robot has been developed to proof the concept design. The experimental results show the multi-legged robot can step up and down stairs with steady human foot trajectory.

An evolutionary topology optimization method for design of compliant mechanisms with two-dimensional loading

This study aims to develop a general topology optimization method to synthesize compliant mechanisms with any combination of two-dimensional loading based on the concept of bi-directional evolutionary structural optimization (BESO). The objective function in this study is to maximize the output displacement. The optimal design of a compliant force-displacement inverter mechanism under both one-dimensional and two-dimensional loading conditions (including symmetric and non-symmetric loading cases) are discussed as the illustrative examples. The dynamic model solved by explicit dynamic finite element analysis (FEA) is used to analyze the dynamic performance of the inverter mechanism. The results show the geometric advantage of the optimal design ranges from 2.3 to 3.2 for compression mode, and 1.3 to 2.6 for extension mode in the given displacement-input ranges.

Dynamic analysis of the demolding process for PDMS microstructures with high aspect ratio

This study aims to investigate several peel demolding schemes through numerical simulations for demolding of the PDMS film with high aspect ratio microstructures. Motivated by the need to improve the yield rate of our first generation, rotating arm based peel demolding system, numerical models based on the explicit dynamic finite element method are developed in order to identify a minimum stress design of the process which can minimize the maximum stress of microstructures during demolding. A scale-up approach is proposed to reduce computational time for the micro-scale problems. The experimental tests are also carried out to verify the findings from numerical simulations. From this study, the roller based demolding system is identified as the optimal scheme in our analysis cases and is developed as our next generation automatic demolding system (the maximum pillar stress can reduce 20% comparing to the rotating arm scheme).