Search Results

A method to simulate digital human running using an optimization-based approach is presented. The digital human is considered as a mechanical system that includes link lengths, mass moments of inertia, joint torques, and external forces. The problem is formulated as an optimization problem to determine the joint angle profiles. The kinematics analysis of the model is carried out using the Denavit-Hartenberg method. The B-spline approximation is used for discretization of the joint angle profiles, and the recursive formulation is used for the dynamic equilibrium analysis. The equations of motion thus obtained are treated as equality constraints in the optimization process. With this formulation, a method for the integration of constrained equations of motion is not required. This is a unique feature of the present formulation and has advantages for the numerical solution process.

Our previous formulation for optimization-based dynamic motion simulation of a serial-link human upper body (from waist to right hand) is extended to predict the motion of a tree-structured human model that includes the torso, right arm, and left arm, with various applied external loads. The dynamics of tree-structured systems is formulated and implemented. The equations of motion for the tree structures must be derived carefully when dealing with the connection link. The optimum solution results show realistic dual-arm human motions and the required joint actuator torques. In the second part of this paper, a new method is introduced in which the constraint forces and moments at the joints are calculated along with the motion and muscle-induced actuator torques. A set of fictitious joints are modeled in addition to the real joints.

Reliability-based design optimization (RBDO), which includes design optimization in design space and inverse reliability analysis in standard normal space, has been recently developed under the assumption that all input variables are independent because it is difficult to construct a joint probability distribution function (PDF) of input variables with limited data such as the marginal PDF and covariance matrix. However, since in real applications, it is common that some of the input variables are correlated, the RBDO results might contain a significant error if the correlation between input variables for RBDO is not considered. In this paper, Rosenblatt and Nataf transformations, which are the most representative transformation methods and have been widely used in the reliability analysis, have been studied and compared in terms of applicability to RBDO with correlated input variables.

The objective of this paper is to present our method of predicting and simulating visually realistic and dynamically consistent human stair-climbing motion. The digital human is modeled as a 55-degrees of freedom branched mechanical system with associated human anthropometry-based link lengths, mass moments of inertia, and centers of gravity. The joint angle profiles are determined using a B-spline-based parametric optimization technique subject to different physics-based, task-based, and environment-based constraints. The formulation offers the ability to study effects of the magnitude and location of external forces on the resulting joint angle profiles and joint torque profiles. Several virtual experiments are conducted using this optimization-based approach and results are presented.

Active steering systems can help the driver to master critical driving situations. This paper presents a fuzzy logic control strategy on active steering vehicle based on a multi-body vehicle dynamic model. The multi-body vehicle dynamic model using ADAMS can accurately predict the dynamic performance of the vehicle. A new hybrid steering scheme including both active front steering (applying an additional front steering angle besides the driver input) and rear steering is presented to control both yaw velocity and sideslip angle. A set of fuzzy logic rules is designed for the active steering controller, and the fuzzy controller can adjust both sideslip angle and yaw velocity through the co-simulation between ADAMS and the Matlab fuzzy control unit with the optimized membership function. To ensure the design of high-quality fuzzy control rules, a rule optimization strategy is introduced.

The objective of this research was to determine fatigue behavior of SAE 1045 steel subjected to very high tensile mean stress for unnotched, mildly notched, and sharply notched test specimens, and to determine if common S-Nf and ε-Nf mean stress fatigue life models are applicable. High tensile mean stress fatigue tests for R ratios of 0.8 and 0.9 were conducted using unnotched and notched, Kt=1.65 and Kt=3.65, axial loaded SAE 1045 steel specimens with hardness levels of Rc=10, 37, and 50. The monotonic notch strength ratio, NSR, for 5 of 6 test conditions was greater than 1, which allowed many notched cyclic test values of Smax or Sm to exceed the unnotched ultimate tensile strength. Much notched specimen fatigue resistance at these high R ratios was superior to that of unnotched specimens. However, cyclic creep/ratcheting, particularly for Rc=10 and 37, was a predominant cause of failure.

The objective of this research was to determine the feasibility of applying strain based fatigue life calculation models, which are commonly used for metals, to smooth SRIM polymer matrix composite axial specimens subjected to variable amplitude loading. A thorough investigation of the monotonic and strain controlled constant amplitude low cycle fatigue behavior of this material was conducted, including the effects of mean strains/stresses on the fatigue life of smooth specimens. Using these results, mean stress life calculations were made on the constant amplitude tests, as well as on smooth specimens subjected to strain controlled variable amplitude loading, using the Morrow and SWT mean stress models. These results were compared to experimental data, and it was found that the correlation between experimental and calculated lives was very poor, for both the constant amplitude and variable amplitude tests.

The objective of this research was to obtain and compare fracture toughness, stress corrosion cracking and constant and variable amplitude fatigue behavior of AZ91E-T6 cast magnesium alloy in both an air and 3.5% NaC1 corrosive environment. An additional objective was to determine if commonly used models that describe fatigue behavior and fatigue life are applicable to this material and test environments. Fatigue tests included constant amplitude strain-controlled low cycle fatigue with strain ratio, R, equal to 0,−1 and −2, Region II constant amplitude fatigue crack growth with load ratio, R, equal to 0.05 and 0.5 and variable amplitude fatigue tests using keyhole notched specimens. In all fatigue tests, the corrosion environment was significantly detrimental relative to the air environment. The material was also susceptible to stress corrosion cracking. Low cycle fatigue models and the Paris equation properly represented the fatigue data in both environments.

A general methodology and associated computational algorithm for predicting realistic postures of digital humans (mannequins) in a virtual environment is presented. The basic plot for this effort is a task-based approach, where we believe that humans assume different postures for different tasks. The underlying problem is characterized by the calculation (or prediction) of the joint displacements of the human body in such a way to accomplish a specified task. In this work, we have not limited the number of degrees of freedom associated with the model. Each task has been defined by a number of human performance measures that are mathematically represented by cost functions that evaluate to a real number. Cost functions are then optimized, i.e., minimized or maximized subject to a number of constraints. The problem is formulated as a multi-objective optimization algorithm where one or more cost functions are considered as objective functions that drive the model to a solution.

The effect of restrictive clothing on functional reach and on balance and gait during obstacle crossing of five normal subjects is presented in this work using motion capture and stability analyses. The study has shown that restrictive clothing has considerably reduced participants' functional reach. It also forced the participants to change their motion strategy when they cross-higher obstacles. When crossing higher obstacles, the participants averted their stance foot, abducted their arms, flexed their torso, used longer stance time, and increased their hip angle in the medial-lateral (Rolling) and vertical (Yawing) directions. The stability analysis of a virtual human skeletal model with 18 links and 25 degrees of freedom has shown that participants' stability has become critical when they wear restrictive clothing and when they cross higher obstacles.

New methods for fast, adaptive motion prediction of a virtual human are proposed and tested. An optimal locomotion for gait-driven motions like pushing, climbing and pick-up/delivery are sought through gradient-based optimization and inverse-dynamics. Such gait-driven motion can be produced by adapting the normal gait motion to the case when a characteristic force is applied, which is called an applied force. The applied force is a resistance force for pushing case and an object weight for delivery case. The concept of the zero moment point is modified to assess the dynamic equilibrium of the motion in presence of the applied force. For fast calculation, analytical forms of the cost/constraint gradients are provided. Stepping patterns are specified a priori to ensure the continuity of the cost/constraint function gradients. Also, by varying knots for the B-spline curve approximation, the gait stage durations are optimized.

This paper presents an optimization-based algorithm for simulating the dynamic motion of a digital human. We also formulate the metabolic energy expenditure during the motion, which is calculated within our algorithm. This algorithm is implemented and applied to Santos™, an avatar developed at The University of Iowa. Santos™ is a part of a virtual environment for conducting digital human analysis consisting of posture prediction, motion prediction, and physiology studies. This paper demonstrates our dynamic motion algorithm within the Santos™ virtual environment. Mathematical evaluations of human performance are essential to any effort to compare various ergonomic designs. In fact, the human factors design process can be formulated as an optimization problem that maximizes human performance. In particular, an optimal design must be found while taking into consideration the effects of different motions and hand loads corresponding to a number of tasks.

Simulating human motion is a complex problem due to redundancy of the human musculoskeletal system. The concept of task-based motion prediction using single- or multi-objective optimization techniques provides a viable approach for predicting intermediate motions of digital humans. It is shown that task-based motion prediction is in fact a numerical optimal control problem. Alternative formulations for simulation of human motion are possible and can be solved by modern nonlinear optimization methods. Three techniques based on state variable elimination, direct collocation and differential inclusion are presented and compared. The basic idea of the formulations is to treat different combinations of the state variables, such as the joint profiles and torques or their parametric representations as independent variables in the optimization process.

SI property class 12.9 high strength steel bolts were used to investigate the fatigue behavior of bolt threads rolled before/after heat treatment using two different thread profiles and five different preload values. Bolts were 3/8 UNRC-16 (coarse) and 3/8 UNRF-24 (fine) and preloads were taken as 1, 50, 75, 90, and 100% of roll before heat treatment proof stress. Maximum near surface residual compressive stresses, obtained via x-ray diffraction, ranged from -500 to -1000 MPa. Axial loads were applied through the nut and all fatigue failures occurred at the first thread of the nut/bolt interface. SEM evaluation indicated all fatigue crack growth regions contained multiple fatigue facets, while final fracture regions were ductile dimples.

In the Army mechanical fatigue subject to external and inertia transient loads in the service life of mechanical systems often leads to a structural failure due to accumulated damage. Structural durability analysis that predicts the fatigue life of mechanical components subject to dynamic stresses and strains is a compute intensive multidisciplinary simulation process, since it requires the integration of several computer-aided engineering tools and considerable data communication and computation. Uncertainties in geometric dimensions due to manufacturing tolerances cause the indeterministic nature of the fatigue life of a mechanical component.

A continuation of the SAEFDE round-robin fatigue test program was conducted to determine the influence of a finer microstructure on monotonic tension, strain-controlled low cycle fatigue, fatigue crack growth, and fracture toughness of A356-T6 cast aluminum alloy. The finer microstructure castings, referred to as material W, were obtained using a water-chilled sand casting procedure. Material W exhibited more desirable ductile behavior than the previous SAEFDE materials X, Y, and Z. Material W exhibited superior smooth specimen low cycle fatigue resistance at both short and long lives, when compared to materials X, Y, and Z. This was due in part to the higher ductility and lower porosity of material W over materials X, Y, and Z. Material W exhibited similar fatigue crack growth behavior, and slightly higher values of fracture toughness at the same thickness when compared to materials X, Y, and Z.

Axial strain-controlled low cycle fatigue behavior was obtained from smooth specimens machined from spokes of A356-T6 cast aluminum alloy wheels. Two different foundries cast the wheels. Three wheels were used from one production run at one foundry and two wheels were used from two different production runs at the other foundry. Specimens from the three wheels of the same production run had essentially the same monotonic tensile properties and low cycle fatigue resistance. Specimens from the two wheels of the different production runs had different monotonic tensile properties and different low cycle fatigue resistance. All these A356-T6 wheel specimens cyclic strain harden with hysteresis loops typically offset to the compression side by five percent or less. The usual log-log linear model for low cycle fatigue adequately described the low cycle fatigue behavior.

Fatigue crack growth behavior was obtained for the SAEFDE Committee's round-robin A356-T6 cast aluminum alloy program with crack growth rates between 10−11 and 10−6 m/cycle for R-ratios equal to 0.1 and 0.5. Three different mold temperatures resulted in secondary dendrite arm spacings (DAS) that varied from approximately 80 to 90 µm, resulting in only coarse microstructure. Threshold levels, ΔKth, and the Paris exponent, m, were approximately twice the values usually found for wrought aluminum alloys. The influence of R-ratio was quite pronounced and crack closure, as measured with a crack mouth COD gage, did not eliminate all threshold and near-threshold R-ratio differences. Roughness-induced crack closure appeared to be more important than plasticity-induced closure.

The results of the SAEFDE Committee's round robin low cycle fatigue test program with A356-T6 cast aluminum alloy indicated that the conventional low cycle fatigue model was not a satisfactory representation of the data. This occurred because the elastic strain amplitude-life curve was not log-log linear and this yielded a non-conservative fatigue life representation at both extremes of long and short lives. This paper involves a reanalysis of the A356-T6 composite all-laboratory data using two additional empirical models. These models are: 1. linear log-log total strain amplitude-life 2. bilinear log-log elastic strain amplitude-life Both proposed empirical models improve the representation of the data compared to the conventional low cycle fatigue model. The bi-linear log-log elastic equation, however, when added to the plastic equation, yields a discontinuous curve with non-conservatism in the region of the discontinuity.

We propose an algorithm of predicting dynamic biped motions of Santos™ human model. An alternative and efficient formulation of the Zero-Moment Point (ZMP) for dynamic balance and the approximated ground reaction forces/moments are derived from the resultant reaction loads, which includes the gravity, the externally applied loads, and the inertia. The optimization problem is formulated to address the redundancy of the human task, where the general biped and the task-specific constraints are imposed depending on the task requirements. The proposed method is fully predictive and generates physically feasible human-like motions from scratch without any input reference from motion capture or animation. The resulting generated motions demonstrate how a human reacts effectively to different external load conditions in performing a given task by showing realistic features of cause and effect.