Jingzhou Yang

Experimental study of micropolar and couple stress elasticity in compact bone in bending

Abstract Generalized continuum theories such as couple stress theory and micropolar theory have degrees of freedom in addition to those of classical elasticity. Such theories are thought to be applicable to materials with a fibrous or granular structure. In this study we observe size effects in quasistatic bending of compact bone. The effects are consistent with micropolar theory. From them we evaluate two nonclassical elastic constants.

Multi-objective Optimization for Upper Body Posture Prediction

*† ‡ § ** The demand for realistic autonomous virtual humans is increasing, with potential application to prototype design and analysis for a reduction in design cycle time and cost. In addition, virtual humans that function independently, without input from a user or a database of animations, provide a convenient tool for biomechanical studies. However, development of such avatars is limited. In this paper, we capitalize on the advantages of optimization-based posture prediction for virtual humans. We extend this approach by incorporating multi-objective optimization (MOO) in two capacities. First, the objective sum and lexicographic approaches for MOO are used to develop new human performance measures that govern how an avatar moves. Each measure is based on a different concept with different potential applications. Secondly, the objective sum, the min-max, and the global criterion methods are used as different means to combine these performance measures. It is found that although using MOO to combine the performance measures generally provides reasonable results especially with a target point located behind the avatar, there is no significant difference between the results obtained with different MOO methods.

Towards a new generation of virtual humans

This paper presents work from an ongoing project towards developing a new generation of virtual human models that are highly realistic in appearance, movement, and feedback. Santos™, an anatomically correct human model with more than 100 degrees of freedom, is an avatar that exhibits extensive modelling and simulation capabilities, resides in a virtual environment, and conducts human-factors analysis. The paper presents an optimisation-based approach to posture and motion prediction that allows the avatar to operate with autonomy rather than depending on stored animations and data or being restricted by inverse kinematics. It also presents approaches to determining reach envelopes and workspace zone differentiation, and discusses methods for evaluating the physiological status of the virtual human as it completes tasks. Muscle modelling including muscle wrapping, muscle force and stress determination is also discussed. Finally, the process of building a 25-DOF hand model is described. The result is an exciting step towards a virtual human that is more extensive and complete than any other.

Transient Study of Couple Stress Effects in Compact Bone: Torsion

Couple stress theory, which admits an internal moment per unit area as well as the usual force per unit area, is a generalization of classical elasticity. Experimentally we have demonstrated the existence of couple stress by measuring the effect of size on apparent stiffness of compact bone in quasi-static torsion. From these measurements, we obtain the characteristic length for bone in couple stress theory.

Towards understanding the workspace of human limbs

Significant attention in recent years has been given towards obtaining a better understanding of human joint ranges, measurement, and functionality, especially in conjunction with commands issued by the central nervous system. Studies of those commands often include computer algorithms to describe path trajectories. These are typically in ‘open-form’ with specific descriptions of motions, but not ‘closed form’ mathematical solutions of the full range of possibilities. This paper proposes a rigorous ‘closed form’ kinematic formulation to model human limbs, understand their workspace (also called the reach envelope), and delineate barriers therein where a path becomes difficult or impossible owing to physical constraints. The novel ability to visualize barriers in the workspace emphasizes the power of these closed form equations. Moreover, this formulation takes into account joint limits in terms of ranges of motion and identifies barriers therein where a person is required to attain a different posture. Examples include the workspaces of a typical forearm and a typical finger. The wrists range of motion is used to illustrate the visualization of the progress in the functionality of a wrist undergoing rehabilitation.

A new digital human environment and assessment of vehicle interior design

Vehicle interior design directly relates to driver performance measures such as comfort, efficiency, risk of injury, and vehicle safety. A digital human is a convenient tool for satisfying the need to reduce the design cycle in order to save time and money. This paper presents a digital human environment, Santos(TM), developed at The University of Iowa, and its assessment as applied to the interior design of a Caterpillar vehicle. The digital human environment involves male models and accommodates a large percentage of the operator population (from the 5th percentile to the 95th percentile). It has a user-friendly interface and includes various tools such as posture prediction, reachability check, zone differentiation, and biomechanics assessment for the upper body and hand. The key difference from a traditional digital human environment is that Santoss environment is optimization-based. This can answer design questions regarding whether the operator can reach relevant controls, what the comfort level is if one can reach the control, and what strength is required of the operator to pull a shift, etc. The illustrative example of a Caterpillar cab is demonstrated using this digital human environment.

An Efficient Hybrid Method for Multibody Dynamics Simulation Based on Absolute Nodal Coordinate Formulation

This paper presents an efficient hybrid method for dynamic analysis of a flexible multibody system. This hybrid method is the combination of a penalty and augmented Lagrangian formulation with the mass-orthogonal projections method based on the absolute nodal coordinate formulation (ANCF). The characteristic of the ANCF that the mass matrix is constant and both Coriolis and centrifugal terms vanish in the equations of motion make the proposed method computationally efficient. Within the proposed method, no additional unknowns, such as the Lagrange multipliers in the Newmark method, are introduced, and the number of equations does not depend on the number of constraint conditions. Furthermore, conventional integration stabilization methods, such as Baumgartes method. are unnecessary. Therefore, the proposed method is particularly suitable for systems with redundant constraints, singular configurations, or topology changes. Comparing results from different methods in terms of efficiency and accuracy has shown that the proposed hybrid method is efficient and has good convergence characteristics for both stiff and flexible multibody systems.

Prediction and analysis of human motion dynamics performing various tasks

Several digital human softwares have shown the capabilities of simulating simple reach motions. However, predicting the dynamic effects on human motion due to different task loads is still immature. This paper presents an optimisation-based algorithm for simulating the dynamic motion of a digital human. The hypothesis is that human performance measures such as the total energy consumption governs human motion; thus the process of human motion simulation can be formulated as an optimisation problem that minimises human performance measures given at different constraints and hand loads, corresponding to a number of tasks. General equations of motion using Lagrangian dynamics method are derived for the digital human, and human metabolic energy is formulated in terms of joint space. Joint actuator torques and metabolic energy expenditure during motion are formulated and calculated within the algorithm, and it is applied to Santos™, a kinematically realistic digital human, developed at the University of Iowa. Results show that different external loads and tasks lead to different human motions and actuator torque distributions.

Optimization-based trajectory planning of the human upper body

This paper presents studies of the coordination of human upper body voluntary movement. A minimum-jerk 3D model is used to obtain the desired path in Cartesian space, which is widely used in the prediction of human reach movement. Instead of inverse kinematics, a direct optimization approach is used to predict each joints profile (a spline curve). This optimization problem has four cost function terms: (1) Joint displacement function that evaluates displacement of each joint away from its neutral position; (2) Inconsistency function, which is the joint rate change (first derivative) and predicted overall trend from the initial target point to the final target point; (3) The non-smoothness function of the trajectory, which is the second derivative of the joint trajectory; (4) The non-continuity function, which consists of the amplitudes of joint angle rates at the initial and final target points, in order to emphasize smooth starting and ending conditions. This direct optimization technique can be used for potentially any number of degrees of freedom (DOF) system and it reduces the cost associated with certain inverse kinematics approaches for resolving joint profiles. This paper presents a high redundant upper-body modeling with 15 DOFs. Illustrative examples are presented and an interface is set up to visualize the results.

Reach Envelope of a 9-Degree-of Freedom Model of the Upper Extremity

Abstract : This paper presents a rigorous mathematical formulation for modeling the upper extremity that is capable of considering a relatively large number of degrees of freedom, thus yielding a realistic model and associated envelope. Kinematic models are used to determine the reach envelope in closed-form and to better understand human motion. Joint ranges of motion are taken into account by transforming unilateral inequality constraints into equalities that are included in the formulation. Methods from geometry are implemented to analyze the motion and delineate barriers within the workspace. It is observed that these barriers are indeed surfaces where the limb has one or more joints at their limits, but also where the hands motion has encountered a kinematic singular configuration. Such a configuration is mathematically defined and is physically associated with two links being parallel at an instant in time or where two joints have their axes parallel (e.g., a fully extended arm yields a singular configuration). Barriers to motion can now be characterized in terms of different human performance measures, thus leading to a better understanding of the path trajectories assumed by humans as they execute tasks.