bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Investigation of the Distribution of Residual Strains in the Artery Wall
Authors: Greenwald S. E. , Moore J. E. , Rachev A. , Kane T. P. C. , Meister J.-J.
Published: February 2008

Arterial wall stresses are thought to be a major determinant of vascular remodeling both during normal growth and throughout the development of occlusive vascular disease. A completely physiologic mechanical model of the arterial wall should account not only for its residual strains but also for its structural nonhomogeneity. It is known that each layer of the artery wall possesses different mechanical properties, but the distribution of residual strain among the different mechanical components, and thus the true zero stress state, remain unknown. In this study, two different sets of experiments were carried out in order to determine the distribution of residual strains in artery walls, and thus the true zero stress state. In the first, collagen and elastin were selectively eliminated by chemical methods and smooth muscle cells were destroyed by freezing. Dissolving elastin provoked a decrease in the opening angle, while dissolving collagen and destroying smooth muscle cells had no effect. In the second, different wall layers of bovine carotid arteries were removed from the exterior or luminal surfaces by lathing or drilling frozen specimens, and then allowing the frozen material to thaw before measuring residual strain. Lathing material away from the outer surface caused the opening angle of the remaining inner layers to increase. Drilling material from the inside caused the opening angle of the remaining outer layers to decrease. Mechanical nonhomogeneity, including the distribution of residual strains, should thus be considered as an important factor in determining the distribution of stress in the artery wall and the configuration of the true zero stress state.
Oxygen Mass Transfer Calculations in Large Arteries
Authors: Moore J. A. , Ethier C. R.
Published: February 2008

The purpose of this study was to model the transport of oxygen in large arteries, including the physiologically important effects of oxygen transport by hemoglobin, coupling of transport between oxygen in the blood and in wall tissue, and metabolic consumption of oxygen by the wall. Numerical calculations were carried out in an 89 percent area reduction axisymmetric stenosis model for several wall thicknesses. The effects of different boundary conditions, different schemes for linearizing the oxyhemoglobin saturation curve, and different Schmidt numbers were all examined by comparing results against a reference solution obtained from solving the full nonlinear governing equations with physiologic values of Schmidt number. Our results showed that for parameters typical of oxygen mass transfer in the large arteries, oxygen transport was primarily determined by wall-side effects, specifically oxygen consumption by wall tissue and wall-side mass transfer resistance. Hemodynamic factors played a secondary role, producing maximum local variations in intimal oxygen tension on the order of only 5–6 mmHg. For purposes of modeling blood-side oxygen transport only, accurate results were obtained through use of a computationally efficient linearized form of the convection-diffusion equation, so long as blood-side oxygen tensions remained in the physiologic range for large arteries. Neglect of oxygen binding by hemoglobin led to large errors, while arbitrary reduction of the Schmidt number led to more modest errors. We conclude that further studies of oxygen transport in large arteries must couple blood-side oxygen mass transport to transport in the wall, and accurately model local oxygen consumption within the wall.
Monte Carlo Model for Determination of the Role of Heat Generation in Laser-Irradiated Tissue
Authors: Welch A. J. , Gardner C. M.
Published: February 2008

A Monte Carlo model is described for modeling photo propagation in a scattering medium. The fraction of locally absorbed photons is proportional to the local rate of heat generation in laser-irradiated tissue and the associated distribution of light (fluence rate) is obtained by dividing the rate of heat generation by the local absorption coefficient. Examples of computed distributions of the rate of heat generation are presented for situations where light scattering in tissue is important. The method is applied to analyze treatment of Port Wine Stain and the selection of laser wave-lengths for cyclophotocoagulation.
A Mixture Theory for Charged-Hydrated Soft Tissues Containing Multi-electrolytes: Passive Transport and Swelling Behaviors
Authors: Gu W. Y. , Lai W. M. , Mow V. C.
Published: February 2008

A new mixture theory was developed to model the mechano-electrochemical behaviors of charged-hydrated soft tissues containing multi-electrolytes. The mixture is composed of n + 2 constituents (1 charged solid phase, 1 noncharged solvent phase, and n ion species). Results from this theory show that three types of force are involved in the transport of ions and solvent through such materials: (1) a mechanochemical force (including hydraulic and osmotic pressures); (2) an electrochemical force; and (3) an electrical force. Our results also show that three types of material coefficients are required to characterize the transport rates of these ions and solvent: (1) a hydraulic permeability; (2) mechano-electrochemical coupling coefficients; and (3) an ionic conductance matrix. Specifically, we derived the fundamental governing relationships between these forces and material coefficients to describe such mechano-electrochemical transduction effects as streaming potential, streaming current, diffusion (membrane) potential, electro-osmosis, and anomalous (negative) osmosis. As an example, we showed that the well-known formula for the resting cell membrane potential (Hodgkin and Huxley, 1952a, b) could be derived using our new n + 2 mixture model (a generalized triphasic theory). In general, the n + 2 mixture theory is consistent with and subsumes all previous theories pertaining to specific aspects of charged-hydrated tissues. In addition, our results provided the stress, strain, and fluid velocity fields within a tissue of finite thickness during a one-dimensional steady diffusion process. Numerical results were provided for the exchange of Na+ and Ca++ through the tissue. These numerical results support our hypothesis that tissue fixed charge density (cF) plays a significant role in modulating kinetics of ions and solvent transport through charged-hydrated soft tissues.
Identification and Determination of Material Properties for Porohyperelastic Analysis of Large Arteries
Authors: Simon B. R. , Kaufmann M. V. , McAfee M. A. , Baldwin A. L. , Wilson L. M.
Published: February 2008

A “porohyperelastic” (PHE) material model is described and the theoretical frame-work presented that allows identification of the necessary material properties functions for soft arterial tissues. A generalized Fung form is proposed for the PHE constitutive law in which the two fundamental Lagrangian material properties are the effective strain energy density function, We, and the hydraulic permeability, k˜ij. The PHE model is based on isotropic forms using We=Ue(φ)=1/2C0(eφ−1) and the radial component of permeability, k˜RR=k˜RR(φ), with φ=C′1(I¯1−3)+C′2(I¯2−3)+K′(J−1)2. The methods for determination of these material properties are illustrated using experimental data from in situ rabbit aortas. Three experiments are described to determine parameters in Ue and k˜RR for the intima and media of the aortas, i.e., (1) undrained tests to determine C0, C′1, and C′2 (2) drained tests to determine K′; and (3) steady-state pressurization tests of intact and de-endothelialized vessels to determine intimal and medial permeability (adventitia removed in these models). Data-reduction procedures are presented that allow determination of k˜RR for the intima and media and Ue for the media using experimental data. The effectiveness and accuracy of these procedures are studied using input “data” from finite element models generated with the ABAQUS program. The isotropic theory and data-reduction methods give good approximations for the PHE properties of in situ aortas. These methods can be extended to include arterial tissue remodeling and anisotropic behavior when appropriate experimental data are available.
Finite Element Formulation of Biphasic Poroviscoelastic Model for Articular Cartilage
Authors: Suh Jun-Kyo , Bai Shi
Published: February 2008

The purpose of the present study was to develop a computationally efficient finite element model that could be useful for parametric analysis of the biphasic poroviscoelastic (BPVE) behavior of articular cartilage under various loading conditions. The articular cartilage was modeled as the BPVE mixture of a porous, linear viscoelastic, and incompressible solid and an inviscid and incompressible fluid. A finite element (FE) formulation of the BPVE model was developed using two different algorithms, the continuous and discrete spectrum relaxation functions for the viscoelasticity of the solid matrix. These algorithms were applied to the creep and stress relaxation responses to the confined compression of articular cartilage, and a comparison of their performances was made. It was found that the discrete spectrum algorithm significantly saved CPU time and memory, as compared to the continuous spectrum algorithm. The consistency analysis for the present FE formulation was performed in comparison with the IMSL, a commercially available numerical software package. It was found that the present FE formulation yielded consistent results in predicting model behavior, whereas the IMSL subroutine produced inconsistent results in the velocity field, and thereby in the strain calculation.
A Finite Element Based Method to Determine the Properties of Planar Soft Tissue
Authors: Flynn D. M. , Peura G. D. , Grigg P. , Hoffman A. H.
Published: February 2008

A finite element based method to determine the incremental elastic material properties of planar membranes was developed and evaluated. The method is applicable to tissues that exhibit inhomogeneity, geometric and material nonlinearity, and anisotropy. Markers are placed on the tissue to form a four-node quadrilateral element. The specimen is loaded to an initial reference state, then three incremental loading sets are applied and the nodal displacements recorded. One of these loadings must include shear. These data are used to solve an over-determined system of equations for the tangent stiffness matrix. The method was first verified using analytical data. Next, data obtained from a latex rubber sheet were used to evaluate experimental procedures. Finally, experiments conducted on preconditioned rat skin revealed non-linear orthotropic behavior. The vector norm comparing the applied and calculated nodal force vectors was used to evaluate the accuracy of the solutions.
Postsurgical Changes of the Opening Angle of Canine Autogenous Vein Graft
Authors: Han Hai-Chao , Zhao Li , Huang Min , Hou Li-Sheng , Huang Yiao-Tian , Kuang Zhen-Bang
Published: February 2008

The opening angles of 30 canine autogenous vein grafts were measured to determine the postsurgical change of residual strain in the vein graft. Canine femoral veins were grafted to femoral arteries in the end-to-end anastomosis fashion. When harvested, the vein grafts were cut into short segments and the segments were cut open radially. The opened-up configurations were taken as the zero-stress states of the vessels. Opening angle, defined as the angle between the two lines from the middle point to the tips of the inner wall, was used to describe the zero-stress states. Results show that the opening angles (mean ± SD] are 63.0 ± 30.6 deg for normal femoral veins, and −0.4 ± 4.6, 6.1 ± 19.4, 25.4 ± 20.1, and 47.8 ± 11.4 deg for vein grafts at 1 day, 1 week, 4 and 12 weeks postsurgery, respectively. The postsurgical changes in opening angle reveal nonuniform transmural tissue remodeling in the vascular wall. The relations between the changes in opening angle and the changes in the morphology of the vein grafts are discussed. Intimal hyperplasia is correlated to the opening angle and is suggested to be the main factor for the postsurgical increase in opening angle. The longitudinal strain in the vein graft is found to decrease postsurgically.
Alteration of Mean Wall Shear Stress Near an Oscillating Stagnation Point
Authors: Hazel A. L. , Pedley T. J.
Published: February 2008

The site opposite an end-to-side anastomosis, resulting from femoral bypass surgery, and the carotid sinus are two regions well known to be prone to fibrous intimal hyperplasia or atherogenesis, respectively. The blood flow at these two sites features a stagnation point, which oscillates in strength and position. Mathematical models are used to determine some of the features of such a flow; in particular, the mean wall shear stress is calculated. The positional oscillations cause a significant change in the distribution and magnitude of the mean wall shear stress from that of the well-studied case of a stagnation point that oscillates only in strength. It is therefore proposed that the recorded effect of time dependence in the flow upon atherogenesis could still be a result of the distribution of the mean and not the time-varying components of the wall shear stress.
Bolus Contaminant Dispersion in Oscillating Flow in Curved Tubes
Authors: Eckmann D. M.
Published: February 2008

The investigation of longitudinal dispersion of tracer substances in unsteady flows has biomechanical application in the study of heat and mass transport within the bronchial airways during normal, abnormal, and artificial pulmonary ventilation. To model the effects of airway curvature on intrapulmonary gas transport, we have measured local gas dispersion in axially uniform helical tubes of slight pitch during volume-cycled oscillatory flow. Following a small argon bolus injection into the flow field, the time-averaged effective diffusion coefficient 〈Deff/Dmol〉 for axial transport of the contaminant was evaluated from the time-dependent local argon concentration measured with a mass spectrometer. The value of 〈Deff/Dmol〉 is extracted from the curve of concentration versus time by two techniques yielding identical results. Experiments were conducted in two helical coiled tubes (δ = 0.031, λ = 0.022 or δ = 0.085, λ = 0.060) over a range of 2 < α < 15, 3 < A < 15, where δ is the ratio of tube radius to radius of curvature, λ is the ratio of pitch height to radius of curvature, α is the Womersley parameter or dimensionless frequency, and A is the stroke amplitude or dimensionless tidal volume. Experimental results show that, when compared to transport in straight tubes, the effective diffusivity markedly increases in the presence of axial curvature. Results also compare favorably to mathematical predictions of bolus dispersion in a curved tube over the ranges of frequency and tidal volume studied.
Pulsatile Flow Computational Simulations of Mitral Regurgitation
Authors: Hopmeyer J. , Wilkerson P. W. , Thorvig K. M. , Levine R. A. , Yoganathan A. P.
Published: February 2008

The noninvasive quantification of mitral regurgitation remains an important clinical goal. Recently, the flow convergence method was developed to estimate the regurgitant flow rate. This study used three-dimensional pulsatile flow computational simulations to evaluate the accuracy of the flow convergence method in the presence of complicating factors such as ventricular confinement, noncircular orifice shape, and the presence of aortic outflow. Results showed that in the absence of aortic outflow and ventricular confinement, there was a plateau zone where the calculated flow rate by the hemispheric formula approximated the true flow rate, independent of the orifice shape. In the presence of aortic outflow and in chambers of physiologic dimensions, there was no longer a clear zone where the hemispheric formula was valid. The hemi-elliptic modiflcation of the flow convergence method worked in all cases, independent of the degree of ventricular confinement or the presence of aortic outflow. Therefore, application of the hemi-elliptic formula should be considered in future clinical studies.
Thermal Expansion Measurements of Frozen Biological Tissues at Cryogenic Temperatures
Authors: Rabin Y. , Taylor M. J. , Wolmark N.
Published: February 2008

Thermal expansion data are essential for analyses of cryodestruction associated with thermal stresses during cryopreservation protocols as well as during cryosurgery. The present study tests a commonly used hypothesis that the thermal expansion of frozen tissues is similar to that of pure water ice crystals. This study further provides insight into the potential effect of the presence of cryoprotectants on thermal expansion. A new apparatus for thermal strain measurements of frozen biological tissues within a cryogenic temperature range is presented. Results are presented for fresh tissue samples taken from beef muscle, chicken muscle, rabbit muscle, rabbit bone, and pig liver. Pilot studies of the effect of cryoprotectants on thermal expansion are further presented for rabbit muscle immersed in dimethyl sulphoxide (2 mols/l) and glycerol (2 mols/l), and for pig liver perfused with dimethyl sulphoxide (2 mols/l). Thermal expansion of frozen soft biological tissues was found to be similar to that of water ice crystals in the absence of cryoprotectant. Thermal expansion of the rabbit bone was found to be about one half of that of frozen soft tissues. A significant reduction in the thermal expansion at higher temperatures was observed in the presence of cryoprotectants. A rapid change of thermal strain near −100°C was also observed, which is likely to be associated with the glass transition process of the cryoprotectant solutions.
A Device for Measuring Relative Angular Displacement
Authors: McCabe R. P. , Kohles S. S. , Chelikani S. V. , Vanderby R.
Published: February 2008

A simple, inexpensive, and accurate way to measure relative segmental rotations resulting from torsional loadings locally is described. To measure these rotations, we fabricated a planar spatial linkage (open-loop kinematic chain) requiring only one rotational displacement transducer. This paper describes this device, defines its kinematics, and examines its accuracy.
Robust Optimization of Total Joint Replacements Incorporating Environmental Variables
Authors: Chang P. B. , Williams B. J. , Santner T. J. , Notz W. I. , Bartel D. L.
Published: February 2008

Direct search techniques for the optimal design of biomechanical devices are computationally intensive, requiring many iterations before converging to a global solution. This, along with the incorporation of environmental variables such as multiple loading conditions and bone properties, makes direct search techniques infeasible. In this study, we introduced new methods that are based on the statistical design and analysis of computer experiments to account efficiently for environmental variables. Using data collected at a relatively small set of training sites, the method employs a computationally inexpensive predictor of the structural response that is statistically motivated. By using this predictor in place of the simulator (e.g., finite element model), a sufficient number of iterations can be performed to facilitate the optimization of the complex system. The applicability of these methods was demonstrated through the design of a femoral component for total hip arthroplasty incorporating variations in joint force orientation and cancellous bone properties. Beams on elastic foundation (BOEF) finite element models were developed to simulate the structural response. These simple models were chosen for their short computation time. This allowed us to represent the actual structural response surface by an exhaustive enumeration of the design and environmental variable space, and provided a means by which to validate the statistical predictor. We were able to predict the structural response and the optimal design accurately using only 16 runs of the computer code. The general trends predicted by the BOEF models were in agreement with previous three-dimensional finite element computer simulations, and experimental and clinical results, which demonstrated that the important features of intramedullary fixation systems were captured. These results indicate that the statistically based optimization methods are appropriate for optimization studies using computationally demanding models.
An Analytical Approach to Determine the in Situ Forces in the Glenohumeral Ligaments
Authors: Debski R. E. , Wong E. K. , Woo S. L-Y. , Fu F. H. , Warner J. J. P.
Published: February 2008

The purpose of this study was to use an analytical approach to determine the forces in the glenohumeral ligaments during joint motion. Predictions from the analytical approach were validated by comparing them to experimental data. Using a geometric model, the lengths of the four glenohumeral ligaments were determined during anterior-posterior loading simulations and forward flexion-extension. The corresponding force in each structure was subsequently calculated based on length data via load-elongation curves obtained experimentally. During the anterior loading simulation at 0 deg of abduction, the superior glenohumeral ligament carried up to 71 N at the maximally translated position. At 90 deg of abduction, the anterior band of the inferior glenohumeral ligament had the highest force of 45 N during anterior loading. These results correlated well with those found in previous experimental studies. We believe that this validated analytical approach can be used to predict the forces in the glenohumeral ligaments during more complex joint motion as well as assist surgeons during shoulder repair procedures.
Biomechanical Studies of the Rabbit Patellar Tendon After Removal of Its One-Fourth or a Half
Authors: Yamamoto N. , Hayashi K. , Hayashi F. , Yasuda K. , Kaneda K.
Published: February 2008

Effects of the overstressing induced by the harvest of grafts from the patellar tendon on the mechanical properties and morphometry of remaining tendon were studied using a rabbit model. The width of the patellar tendon was reduced by one-fourth or one-half equally removing the medial and lateral portions; by this surgery, the cross-sectional area was decreased by 25 or 50 percent from the original area. After all the rabbits were allowed unrestricted activities in cages for 3 to 12 weeks, their patellar tendons were harvested for mechanical and histological studies. The one-fourth removal induced no significant changes in the mechanical properties, but significantly increased the cross-sectional area. In the case of one-half removal, tensile strength and tangent modulus did not change in some tendons, although the cross-sectional area increased significantly. In the other central half tendons, mechanical strength decreased markedly, while the cross-sectional area increased; hypercellular areas and breakage of collagen bundles were observed in these tendons. These results indicate that the patellar tendon has an ability of functionally adapting to overstressing by changing the cross-sectional area, while keeping the mechanical properties unchanged, if the extent of overstressing is less than 30 percent.
A Fibril-Network-Reinforced Biphasic Model of Cartilage in Unconfined Compression
Authors: Soulhat J. , Buschmann M. D. , Shirazi-Adl A.
Published: February 2008

Cartilage mechanical function relies on a composite structure of a collagen fibrillar network entrapping a proteoglycan matrix. Previous biphasic or poroelastic models of this tissue, which have approximated its composite structure using a homogeneous solid phase, have experienced difficulties in describing measured material responses. Progress to date in resolving these difficulties has demonstrated that a constitutive law that is successful for one test geometry (confined compression) is not necessarily successful for another (unconfined compression). In this study, we hypothesize that an alternative fibril-reinforced composite biphasic representation of cartilage can predict measured material responses and explore this hypothesis by developing and solving analytically a fibril-reinforced biphasic model for the case of uniaxial unconfined compression with frictionless compressing platens. The fibrils were considered to provide stiffness in tension only. The lateral stiffening provided by the fibril network dramatically increased the frequency dependence of disk rigidity in dynamic sinusoidal compression and the magnitude of the stress relaxation transient, in qualitative agreement with previously published data. Fitting newly obtained experimental stress relaxation data to the composite model allowed extraction of mechanical parameters from these tests, such as the rigidity of the fibril network, in addition to the elastic constants and the hydraulic permeability of the remaining matrix. Model calculations further highlight a potentially important difference between homogeneous and fibril-reinforced composite models. In the latter type of model, the stresses carried by different constituents can be dissimilar, even in sign (compression versus tension) even though strains can be identical. Such behavior, resulting only from a structurally physiological description, could have consequences in the efforts to understand the mechanical signals that determine cellular and extracellular biological responses to mechanical loads in cartilage.
Quantification of Structural Loading During Off-Road Cycling
Authors: De Lorenzo D. S. , Hull M. L.
Published: February 2008

To provide data for fatigue life prediction and testing of structural components in off-road bicycles, the objective of the research described herein was to quantify the loads input to an off-road bicycle as a result of surface-induced loads. A fully instrumented test bicycle was equipped with dynamometers at the pedals, handlebars, and hubs to measure all in-plane structural loads acting through points of contact between the bicycle and both the rider and the ground. A portable data acquisition system carried by the standing rider allowed, for the first time, this loading information to be collected during extended off-road testing. In all, seven experienced riders rode a downhill trail test section with the test bicycle in both front-suspension and full-suspension configurations. The load histories were used quantitatively to describe the load components through the computation of means, standard deviations, amplitude probability density functions, and power spectral density functions. For the standing position, the coefficients of variation for the load components normal to the ground were greater than 1.2 for handlebar forces and 0.3 and 0.5–0.6 for the pedal and hub forces, respectively. Thus, the relative contribution of the dynamic loading was much greater than the static loading at the handlebars but less so at the pedals and hubs. As indicated by the rainflow count, high amplitude loading was developed approaching 3 and 5 times the weight of the test subjects at the front and rear wheels, respectively. The power spectral densities showed that energy was concentrated in the band 0–50 Hz. Through stress computations and knowledge of material properties, the data can be used analytically to predict the fatigue life of important structural components such as those for steering. The data can also be used to develop a fatigue testing protocol for verifying analytical predictions of fatigue life.
Dynamics and Control of Instrumented Harmonic Drives
Authors: Kazerooni H.
Published: February 2008

Since torque in harmonic drives is transmitted by a pure couple, harmonic drives do not generate radial forces and therefore can be instrumented with torque sensors without interference from radial forces. The installation of torque sensors on the stationary component of harmonic drives (the Flexipline cup in this research work) produce backdrivability needed for robotic and telerobotic compliant maneuvers [3, 4, 6]. Backdrivability of a harmonic drive, when used as torque increaser, means that the output shaft can be rotated via finite amount of torque. A high ratio harmonic drive is non-backdrivable because its output shaft cannot be turned by applying a torque on it. This article first develops the dynamic behavior of a harmonic drive, in particular the non-backdrivability, in terms of a sensitivity transfer function. The instrumentation of the harmonic drive with torque sensor is then described. This leads to a description of the control architecture which allows modulation of the sensitivity transfer function within the limits established by the closed-loop stability. A set of experiments on an active hand controller, powered by a DC motor coupled to an instrumented harmonic drive, is given to exhibit this method’s limitations.
Simulation of Flexible-Link Manipulators With Inertial and Geometric Nonlinearities
Authors: Damaren Chris , Sharf Inna
Published: February 2008

Several important issues relevant to modeling of flexible-link robotic manipulators are addressed in this paper. First, we examine the question of which inertial nonlinearities should be included in the equations of motion for purposes of simulation. A complete model incorporating all inertial terms that couple rigid-body and elastic motions is presented along with a rational scheme for classifying them. Second, the issue of geometric nonlinearities is discussed. These are terms whose origin is the geometrically nonlinear theory of elasticity, as well as the terms arising from the interbody coupling due to the elastic deformation at the link tip. Accordingly, a general way of incorporating the well-known geometric stiffening effect is presented along with several schemes for treating the elastic kinematics at the joint interconnections. In addition, the question of basis function selection for spatial discretization of the elastic displacements is also addressed. The finite element method and an eigenfunction expansion techniques are presented and compared. All issues are examined numerically in the context of a simple beam example and the Space Shuttle Remote Manipulator System. Unlike a single-link system, the results for the latter show that all terms are required for accurate simulation of faster maneuvers. Hence, the conclusions of the paper are contrary to some of the previous findings on the validity of various models for dynamics simulation of flexible-body systems.
Synthesis of a Stable Discrete-Time Repetitive Controller for MIMO Systems
Authors: Sadegh Nader
Published: February 2008

This paper presents some new results on synthesis and stability analysis of a class of repetitive controllers for discrete-time multi-input multi-output (MIMO) linear time-invariant (LTI) systems. In particular, the necessary and sufficient conditions for exponential stability of the resulting closed-loop systems are derived. A simulation case study featuring a flexible mechanical system is also presented.
A Fourier Series Neural Network and Its Application to System Identification
Authors: Zhu C. , Paul F. W.
Published: February 2008

A distinctive neural network architecture, called the Fourier Series Neural Network (FSNN), is developed with particular consideration for applications in the area of system identification and control. This paper focuses on the theory of the FSNN and its application to system identification. This neural network is based on the topological structure of the multiple Fourier series, and is shown to be free of local minima. The global stability of the FSNN learning dynamics is guaranteed using the Delta learning rule. This paper demonstrates that the trained FSNN model approximates the Fourier series representation of an identified system with the network state weights approximating the coefficients of the Fourier series. This feature enables the FSNN to estimate the frequency spectrum of an unknown system, making the FSNN a powerful tool for controller design or on-line adaptive tuning based on system frequency response. The capabilities of the FSNN are demonstrated for linear and nonlinear systems by applying the FSNN to estimate the amplitude and phase spectrums of a second order linear transfer function and to model nonlinear inverse robot kinematics. These evaluations indicate that the FSNN modeling technique is applicable to both linear and nonlinear systems with multi-inputs and multi-outputs.
Direct Optimal Control of Nonlinear Systems Via Hamilton’s Law of Varying Action
Authors: Adigu¨zel E. , O¨z H.
Published: February 2008

Based on direct application of Hamilton’s Law of Varying Action in conjunction with an assumed-time-modes approach for both the generalized coordinates and input functions, a direct optimal control methodology is developed for the control of nonlinear, time varying, spatially discrete mechanical systems. Expansion coefficients of admissible time modes for the dependent variables of the dynamic system and those for the inputs constitute the states and controls, respectively. This representation permits explicit a priori integration in time of the energy expressions in Hamilton’s law and leads to the algebraic equations of motion for the system which replace the conventional differential state equations; therefore the customary extremum principles of calculus of variations involving differential form constraints are also bypassed. Similarly, the standard integral form of the quadratic regulator performance measure employed in the formulation of the optimality problem is transformed into an algebraic performance measure via assumed-time-modes expansion of the generalized coordinates and the control inputs. The proposed methodology results in an algebraic optimality problem from which a closed-form explicit solution for the nonlinear feedback control law is obtained directly. Simulations of two nonlinear nonconservative systems are, included.
Extraction of Transient Signals From Nonperiodic Dynamic Responses
Authors: Zheng Wanping , Kujath Marek R.
Published: February 2008

In practice, vibration signals generated by machines that are considered both linear and excited periodically are often slightly nonperiodic. A small variability in the periodic response may result from small variations of speed in the driving motors, in small variation in the system parameters, or nonlinearities causing mild chaotic motion. This paper deals with the problem of extracting transient components from almost periodic responses. One of the difficulties involved in the problem results from the presence of a broad range of frequencies in a transient response. The main difficulty, however, comes from the fact that the steady-state response is not purely periodic and the transient is totally submerged in it. Two major approaches of extracting the transient component have been investigated: algebraic subtraction and adaptive notch filtering. Both methods have been tested numerically before they were applied to the signals obtained from experiments. The results show that it is possible to extract transients from a dynamic response even if the period of the response varies.
Stability of Magnetic Bearing-Rotor Systems Having a Central Support Backup Bearing
Authors: Zhong P. , Townsend M. A.
Published: February 2008

The unperturbed (fully nonlinear) stability of a magnetic bearing-rotor system having a single central backup bearing near the middle of the shaft is analyzed under conditions of magnetic bearing(s) failure for zero gravity using the Lyapunov second (direct) method; these results are shown to apply with gravity, based on observed similiaries of the nonlinear Lagrangian equations. These are completely general stability criteria for practical values of system parameters and conditions of system operation and failure. It turns out that the center-backup bearing configuration has some considerable advantages over conventional designs: in addition to known results for stability and instability, the system can be stable when one or both magnetic bearings fail (has negative stiffness). This “temporary stability” depends upon inherent gyroscopic forces and may be lost when dissipative forces are introduced. The results are applicable to other gyroscopic systems.
Dynamic Analysis of a Brushless D.C. Motor Using a Modified Harmonic Balance Method
Authors: Lee Ming-ran , Padmanabhan Chandramouli , Singh Rajendra
Published: February 2008

Analysis of brushless D.C. motor (BDCM) torque pulsations is an essential step in the diagnosis and control of vibration and noise generated by many electro-mechanical devices. The broad band spectral content of the torque pulsations, as predicted by a mathematical model which accounts for various complex effects, can often be obtained only by numerical integration which is time consuming while permitting little understanding of the dynamic interactions. Prior analytical approaches, such as the Fourier series technique or the d-q axis theory, are limited by the simplifying assumptions needed to compute the torque spectrum. This paper develops a new semi-analytical formulation for the analysis of nonlinear, time-varying BDCM’s which involve both spatial and temporal domains. A modified multi-term harmonic balance method, based on a transformation of the dual-domain problem to a spatial domain formulation, is developed here specifically to compute the magnitude of several harmonics of the pulsating torque. The interacting effects of key parameters, like dynamic eccentricity, magnetic saturation and open stator slots, on the time-varying inductances and rotor flux density distribution are included explicitly in the formulation. The predicted spectra compare very well with those obtained by direct time domain numerical integration. Yet, the proposed method is computationally efficient especially when the model dimension is reduced. It also provides better insight into the high frequency dynamics of the sample case.
Trajectory Preshaping for High-Speed Articulated Systems
Authors: Shiller Zvi , Chang Hai
Published: February 2008

This paper presents a method for reducing the tracking errors of articulated systems, moving along specified paths at high speeds. It consists of preshaping the reference trajectory to account for the dynamics of the feedback controller. The trajectory is assumed to be feasible, satisfying the known manipulator dynamics and the actuator constraints. The correction term, added to the nominal trajectory, is computed by filtering the nominal control inputs through the inverse of the feedback controller. A learning procedure is also presented to account for unknown parameters of the feedback controller and for unmodeled dynamics. It consists of an iterative parameter optimization that minimizes the tracking error along the path at each joint. The initial guesses for this optimization are selected to ensure that the tracking error is not worse than the error with no preshaping. The method is demonstrated experimentally for the UCLA Direct Drive Arm and for the AdeptOne industrial robot, achieving mean tracking errors as low as 0.11 mm at top speeds of 1.8 m/s.
The Discrete Event Control of Robotic Assembly Tasks
Authors: McCarragher B. J. , Asada H.
Published: February 2008

A new approach to process modeling, task synthesis, and motion control for robotic assembly is presented. Assembly is modeled as a discrete event dynamic system using Petri nets, incorporating both discrete and continuous aspects of the process. The discrete event modelling facilitates a new, task-level approach to the control of robotic assembly. To accomplish a desired trajectory a discrete event controller is developed. The controller issues velocity commands that direct the system toward the next desired contact state, while maintaining currently desired contacts and avoiding unwanted transitions. Experimental results are given for a dual peg-in-the-hole example. The experimental results not only demonstrate highly successful insertion along the optimal trajectory, but also demonstrate the ability to detect, recognize and recover from errors and unwanted situations.
Singularly Perturbed Bond Graph Models for Simulation of Multibody Systems
Authors: Zeid A. A. , Overholt J. L.
Published: February 2008

This paper develops a bond graph-based formalism for modeling multibody systems in a singularly perturbed formulation. As opposed to classical multibody modeling methods, the singularly perturbed formulation is explicit, which makes it suitable for modular simulation. Kinematic joints that couple rigid bodies are described by a set of differential equations with an order of magnitude smaller time scale than that of the system. Singularly perturbed models of joints can be used to investigate nonlinear properties of joints, such as clearance and friction. The main restriction of this approach is that the simulation may need to be computed using 64 bits precision because of the two-time scale nature of the solution. The formalism is based on developing bond graph models of an elementary set of graphical velocity-based constraint functions. This set can be used to construct bond graphs of any type of mechanical joint. Here, this set is used to develop bond graphs of several joints used in multibody systems and spatial mechanisms. Complex models of multibody systems may now be built by graphically concatenating bond graphs of rigid bodies and bond graphs of joints. The dynamic equations of the system are automatically generated from the resulting bond graph model. The dynamic equation derived from the bond graph are in explicit state space form, ready for numerical integration, and exclude the computationally intensive terms that arise from acceleration analysis.
A New Feedforward Neural Network Structural Learning Algorithm—Augmentation by Training With Residuals
Authors: Li C. James , Kim Taehee
Published: February 2008

A fully automatic feedforward neural network structural and weight learning algorithm is described. The Augmentation by Training with Residuals, ATR, requires neither guess of initial weight values nor the number of neurons in the hidden layer from users. The algorithm takes an incremental approach in which a hidden neuron is trained to model the mapping between the input and output of current exemplars, and is augmented to the existing network. The exemplars are then made orthogonal to the newly identified hidden neuron and used for the training of next hidden neuron. The improvement continues until a desired accuracy is reached. This new structural and weight learning algorithm is applied to the identification of a two-degree-of-freedom planar robot, a Van der Pol oscillator and a Mackay-Glass equation. The algorithm is shown to be effective in modeling all three systems and is far superior to a linear modeling scheme in the case of the robot.
Filtering for Linear Stochastic Systems With Small Measurement Noise
Authors: Aganovic Z. , Gajic Z. , Shen X.
Published: February 2008

In this paper we present a method which produces complete decomposition of the optimal global Kalman filter for linear stochastic systems with small measurement noise into exact pure-slow and pure-fast reduced-order optimal filters both driven by the system measurements. The method is based on the exact decomposition of the global small measurement noise algebraic Riccati equation into exact pure-slow and pure-fast algebraic Riccati equations. An example is included in order to demonstrate the proposed method.
The Effects of External Compression on Venous Blood Flow and Tissue Deformation in the Lower Leg
Authors: Dai Guohao , Gertler J. P. , Kamm R. D.
Published: February 2008

External pneumatic compression of the lower legs is effective as prophylaxis against deep vein thrombosis. In a typical application, inflatable cuffs are wrapped around the patient’s legs and periodically inflated to prevent stasis, accelerate venous blood flow, and enhance fibrinolysis. The purpose of this study was to examine the stress distribution within the tissues, and the corresponding venous blood flow and intravascular shear stress with different external compression modalities. A two-dimensional finite element analysis (FEA) was used to determine venous collapse as a function of internal (venous) pressure and the magnitude and spatial distribution of external (surface) pressure. Using the one-dimensional equations governing flow in a collapsible tube and the relations for venous collapse from the FEA, blood flow resulting from external compression was simulated. Tests were conducted to compare circumferentially symmetric (C) and asymmetric (A) compression and to examine distributions of pressure along the limb. Results show that A compression produces greater vessel collapse and generates larger blood flow velocities and shear stresses than C compression. The differences between axially uniform and graded-sequential compression are less marked than previously found, with uniform compression providing slightly greater peak flow velocities and shear stresses. The major advantage of graded-sequential compression is found at midcalf. Strains at the lumenal border are approximately 20 percent at an external pressure of 50 mmHg (6650 Pa) with all compression modalities.
Spatial and Temporal Variation of Secondary Flow During Oscillatory Flow in Model Human Central Airways
Authors: Tanaka G. , Ogata T. , Oka K. , Tanishita K.
Published: February 2008

Axial and secondary velocity profiles were measured in a model human central airway to clarify the oscillatory flow structure during high-frequency oscillation. We used a rigid model of human airways consisting of asymmetrical bifurcations up to third generation. Velocities in each branch of the bifurcations were measured by two-color laser-Doppler velocimeter. The secondary velocity magnitudes and the deflection of axial velocity were dependent not only on the branching angle and curvature ratio of each bifurcation, but also strongly depended on the shape of the path generated by the cascade of branches. Secondary flow velocities were higher in the left bronchus than in the right bronchus. This spatial variation of secondary flow was well correlated with differing gas transport rates between the left and right main bronchus.
Remodeling of a Collagenous Tissue at Fixed Lengths
Authors: Humphrey J. D.
Published: February 2008

Mature tissues can often adapt to changes in their chemical, mechanical, or thermal environment. For example, in response to sustained increases or decreases in mechanical loads, some tissues grow and remodel so as to restore the stress or strain to its homeostatic state. Whereas most previous work addresses gross descriptors of tissue growth, this paper introduces a possible cell-mediated mechanism by which remodeling may occur in a soft connective tissue—that the kinetics of collagen deposition and degradation is similar regardless of the configuration of the body at which it occurs. The proposed theoretical framework applies to three-dimensional settings, but it is illustrated by focusing on the remodeling of a uniaxial collagenous tissue that is maintained at a fixed length for an extended period. It is shown that qualitative features expected of such remodeling (e.g., an increased compliance and increased stress-free length when remodeling occurs at an extended length) are easily realized. Growth and remodeling are complex phenomena, however, and are likely accomplished via multiple complementary mechanisms. There is a need, therefore, to identify other candidate mechanisms and, of course, to collect experimental data suitable for testing and refining the possible theories.