R-3 Repository :: Browsing by Author "Ghorbel, Fathi H."

Browsing by Author "Ghorbel, Fathi H."

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A model of the human cardiopulmonary system
(2001) Lu, Kun; Ghorbel, Fathi H.
This study developed a mathematical model of human cardiopulmonary system which consists of component models such as ventricular mechanics, hemodynamics of the systemic and pulmonic circulations, baroreflex control of arterial pressure, airway/lung mechanics and gas transport. Instantaneous elastance functions were used to describe the mechanics of the heart chambers. The resistive, compliant and inertial properties of the circulatory system were characterized by a lumped equivalent hydraulic circuit. Transfer functions were employed to represent the input-output relations of baroreflex pathways. On the pulmonary side, the airways were characterized using a lumped pneumatic model containing a mid-airway collapsible segment. Description of lung mechanics included the resistive, compliant properties of the lung tissue, which exhibited hysteresis. Gas transport was characterized by a distributed compartmental system containing ten contiguous segments. With suitable parameter adjustment, the nominal case simulation yielded realistic predictions of pressure, volume and flow waveforms that agreed well with published data. In addition, it predicted the temporal behavior of variables that are not routinely collected in cardiac catheterization or pulmonary laboratories, and which are difficult to measure. The model also demonstrated stability under large amplitude perturbations of the physiological variables, such as Valsalva maneuver. This model maybe employed usefully to show the detailed nature of normal human cardiopulmonary interactions and baroreflex control (e.g. ventricular interaction, Valsalva maneuver). It also provides methodologies for the development of more specific models of abnormal behavior, and as such, may serve as an aid in clinical diagnosis.
A singular perturbation approach to modeling closed kinematic chains
(2000) Gonzalez Garcia, Jorge Alberto; Ghorbel, Fathi H.
The purpose of this work is to develop a singular perturbation-based approach to modeling closed kinematic chains. First, a kinematical analysis is developed to show that closed kinematic chains cannot be modeled in general using only independent generalized coordinates. Second, the Lagrangian formulation is used to develop the DAE system for closed kinematic chains. Next, differential algebraic equations (DAEs) are described, followed by discussion of standard techniques for their solution and the limitations of the standard techniques with respect to model-based control design. Then, a singular perturbation approach to solving the DAE that arise from closed kinematic chains is developed. Using this model makes it possible to solve an ODE which is an approximation of the DAE. Finally, the technique is illustrated using the Rice Planar Delta Robot.
A three-pulse algorithm for minimum-fuel rotational maneuvers
(2004) Lowry, Nathan; Ghorbel, Fathi H.
Spacecraft equipped with a Reaction Control System (RCS) for attitude control typically utilize a "bang-off-bang" control algorithm for rotational maneuvers. This type of algorithm, which commands two distinct control periods to initiate and terminate a maneuver, can be fuel-suboptimal for maneuvers in which neither the initial nor the final state is at rest. This work introduces a rotational control algorithm for inertially axisymmetric spacecraft that uses three distinct control periods in order to minimize propellant consumption for large-angle maneuvers with non-trivial initial and final angular rates.
AFM-Based Mechanical Nanomanipulation
(2011) Landolsi, Fakhreddine; Ghorbel, Fathi H.
Advances in several research areas increase the need for more sophisticated fabrication techniques and better performing materials. Tackling this problem from a bottom-up perspective is currently an active field of research. The bottom-up fabrication procedure offers sub-nanometer accurate manipulation. At this time, candidates to achieve nanomanipulation include chemical (self-assembly), biotechnology methods (DNA-based), or using controllable physical forces (e.g. electrokinetic forces, mechanical forces). In this thesis, new methods and techniques for mechanical nanomanipulation using probe force interaction are developed. The considered probes are commonly used in Atomic Force Microscopes (AFMs) for high resolution imaging. AFM-based mechanical nanomanipulation will enable arranging nanoscale entities such as nanotubes and molecules in a precise and controlled manner to assemble and produce novel devices and systems at the nanoscale. The novelty of this research stems from the development of new modeling of the physics and mechanics of the tip interaction with nanoscale entities, coupled with the development of new smart cantilevers with multiple degrees of freedom. The gained knowledge from the conducted simulations and analysis is expected to enable true precision and repeatability of nanomanipulation tasks which is not feasible with existing methods and technologies.
Analysis and control of kinematic error in harmonic gear drive mechanisms
(1997) Were, Muhammed; Ghorbel, Fathi H.
Harmonic gear drives have found increasing application in high performance and precision servomechanisms due to their attractive physical and dynamic properties. However all harmonic gear drives exhibit undesireable nonlinear kinematic error and the ability to control and compensate this error is limited to obtaining an appropriately accurate and controllable representative mathematical model. A comprehensive literature search revealed a wide effort towards characterization with little or no effort to modeling and control. This work adopts existing models of kinematic error characterized in the literature and presents modeling techiques such that for any model of kinematic error used, the PD-control law will effectively regulate the control variable to the desired position. Additionally, asymptotic stability result reveals that in case of PD control, sensors should be placed to measure at the load end.
Characterization of a Submersible In-line Pump as a Thrust Generator for Swimming Robots
(2016-04-21) Karalomlu, Ali; Ghorbel, Fathi H.
Throughout history, machines have replaced human labor in many industries. Our machines have made it possible to transcend human limitations. In the present day, underwater robots have stretched the boundaries of what was previously thought impossible in terms of exploration, security and scientific discovery. These robots have become prevalent in a number of sectors and fields including the military, marine biology, oceanography and documentary production. As their applications become increasingly diverse, establishing precise control over the movement of these robots become more and more crucial. DC motors as force/torque generators for robots are well understood. We know exactly how much voltage must be applied to generate precise movement to achieve specifi c tasks. On the other hand, this is not the case for thrust generators for swimming robots due to the complexity of the dynamics of the combination of motor action with fluid/solid interactions. An in-line pump consists of a DC motor pumping fluid via a centrifugal pump through a tight space to generate thrust. Understanding of the dynamics of these interactions are vital if we are to establish accurate control over underwater robots. Through the mathematical modeling of ducted thrusters, computational analysis of fluid dynamics of ideal centrifugal pumps and a pendulum arm experimental setup, this research seeks to improve our understanding of the submersible in-line pumps as thrust generators for swimming robots.
Control of serial and parallel robots: Analysis and implementation
(1999) Gunawardana, Ruvinda Vipul; Ghorbel, Fathi H.
The research presented in this thesis is categorized into two areas. In the first part we address the issue of uniform boundedness of the elements of the equations of motion of serial robots, an important issue for the control of robots in this class. The second part is dedicated to the dynamic modeling and model based control of parallel robots. The field of serial robot control experienced tremendous growth over the past few decades resulting in a rigorous body of control results. An important assumption that is frequently made in establishing stability properties of these control laws is that the terms associated with the equations of motion of serial robots such as the inertia matrix, the Coriolis/centrifugal terms, and the Hessian of potential energy are uniformly bounded. This assumption however, is not valid for all serial robots. Since the stability conclusions of many control laws become local for robots that violate this assumption, it's important to be able to determine whether the terms in question are indeed uniformly bounded for a given robot. In the first part of this research we examine this issue and characterize the class of serial robots for which each of these terms are uniformly bounded. We also derive explicit uniform bounds for these terms which become important in control synthesis since the uniform bounds appear in the expressions of many control laws. The second part of this research is dedicated to parallel robots. Unlike in the case of serial robots, in parallel robots the independent generalized coordinates corresponding to the actuated joints do not uniquely determine the configuration of the robot. Therefore, an important issue that must be resolved in order to derive the dynamics of parallel robots is the existence of a transformation from the independent coordinates to a set of dependent coordinates that completely determine the robot configuration. The existence of such a transformation will enable the extension of most results in serial robots to parallel robots. In this research we characterize a region with specified boundaries where such a transformation exists and derive a numerical scheme for implementing the transformation in real time. Another contribution of this research is the design and construction of the Rice Planar Delta Robot which will serve as a test bed for results on parallel robots. This robot was used to experimentally verify the above result in a trajectory tracking experiment and a fast pick and place experiment.
Design of a harmonic drive test apparatus for data acquisition and control
(1997) Hejny, Scott Wayne; Ghorbel, Fathi H.
Harmonic drive gear reducers were developed during the mid-1950's and are used in many industrial and military applications. The harmonic drive is a compact, "in-line" gear reducer capable of producing reduction ratios of up to 320:1. The devices are known for their efficiency and precision, but they also possess undesirable qualities characterized by nonlinear behavior. These undesirable aspects include the presence of both static and dynamic friction, flexibility (compliance), and kinematic, or positional, error. In the thesis, the mechanical design of a test platform for the study of the system nonlinearities was developed and fabricated. Experimental testing through computer controlled data acquisition validated the apparatus as a system for the examination and control of kinematic error. A new model for kinematic error was developed for the test system. Observations were made concerning the way in which flexibility effects the magnitude of the kinematic error. PD control schemes were implemented in both motor state and local state feedback control efforts. The load state feedback effort successfully compensated for the effects of kinematic error during regulation of the load position. The experimental results were compared to a model and discussed in detail.
Dynamic hysteresis modeling and applications
(2004) Dutta, Sushant M.; Ghorbel, Fathi H.
Hysteresis is a phenomenon which is widely observed in a variety of physical systems. It introduces a nonlinear and multivalued behavior in systems, making their modeling and control problematic. This thesis underlines the significance of dynamic hysteresis modeling from the perspectives of analysis and control. Toward that end, a widely accepted definition of hysteresis is adopted and some important properties of hysteresis are presented. Five general hysteresis models are discussed, along with some damping and friction models. Their properties are compared and contrasted. The Duhem model is shown to be a versatile dynamic hysteresis model, and it is adapted to two distinct physical systems. First, the evolution of dynamic hysteresis modeling of harmonic drive is studied, and a new dynamic model, based on Duhem model, is developed. It is more accurate than previous models and is used to prove, via the method of describing functions, that PID regulation control of harmonic drive can cause a limit cycle due to hysteresis. Second, a dynamic hysteresis model, based on Duhem model, is proposed for a shape memory alloy actuator, which yields a complete dynamic model of the actuator, linking its temperature, strain and electrical resistance together. Therefore, this thesis provides a foundation for dynamic hysteresis modeling in engineering systems and brings out the salient features of dynamic hysteresis modeling from the perspectives of analysis and control.
Dynamical analysis and control of longitudinal electromagnetic levitation
(2001) Artar, Remzi; Ghorbel, Fathi H.
Electromagnetic levitation is a unique technique both for measurements of the thermophysical properties of metals and for producing homogeneous melt in material processing. To maintain stability of the levitated sample is one of the most notable difficulties observed in this technique. The longitudinal levitator developed by Bayazitoglu and Shampine overcomes most of the drawbacks that are inherent in currently used levitator and can support more massive samples than those that can be supported by existing devices. On the other hand, the undesired sample oscillations have been experienced unless the sample is released very near its equilibrium position as levitation is begun. To approach this problem, a dynamical model of the process is needed. This study addresses dynamical analysis and control of the longitudinal electromagnetic levitation. The levitation force is analytically derived considering the effect of the sample's motion. Based on the dynamical analysis, a nonlinear dynamical model of the process is derived. Using the numerical solution of the dynamical model, the influences of the system parameters on the dynamical behavior of the sample are illustrated. To achieve stable levitation the derived dynamical model has been linearized and implementing a linear control technique, the sample's undesired oscillations have been successfully eliminated at very beginning of the levitation.
Foot Drop Gait: Analysis and Assistive Torque Design
(2019-04-19) Zhang, Sichao; Ghorbel, Fathi H.; Fregly, B.J.
Foot drop is a gait abnormality where patients slap the ground with the front foot at initial contact instead of the heel. Most of the time, this problem is caused by neurological disorder. The muscle tibialis anterior, which is responsible for dorsiflexion and picking up the foot when contacting the ground, will be most affected. Various assistive devices have been developed to solve the foot drop problem. Most of their design is based on the concept of preventing toe dragging and picking up the foot at initial contact, which are the two main features of the problem. This thesis proposes a simulation-based method to analyze the foot drop problem and explore assistive device torque designs to address it. A neuro-musculo-skeletal dynamic model for human locomotion that consists of 8 body parts, 20 muscles and 7 pairs of neural oscillators, is used to generate a normal gait pattern. This model was used as the basis for analyzing the foot drop gait and studying assistive device torque designs. Several metrics that characterize foot drop, such as joint motion profiles of lower limb and walking sequence, have been developed. The foot drop gait model that was produced in this thesis reproduced the muscular and neural aspects and was successfully checked against the developed foot drop metrics. With this model, different assistive device torque scenarios were studied and analyzed.
Human whole-body gas exchange and cerebral autoregulation studied using a cardiopulmonary model
(2004) Lu, Kun; Ghorbel, Fathi H.; Clark, John W., Jr.
The goal of this work is to study human whole-body gas exchange and cerebral autoregulation using a mathematical model. Previously, a human cardiopulmonary (CP) model [45, 47] was developed, which included heart, closed-loop blood circulation, gas exchange at lungs and baroreflex control of arterial pressure. In the current study, two major extensions to the model are made. First, a description of gas exchange in the peripheral tissues is added and is coupled with the lung gas exchanger via the circulatory loop with variable transport delays. A peripheral chemosensitive loop is also added to mimic the influence of blood gas composition on the heart and vasculature. The CP model is then used to predict the integrated cardiovascular and blood-tissue gas transport responses to pronounced changes in lung gas composition, and thus simulates changes encountered in apnea with and without passive oxygenation. The second extension of the CP model includes a more detailed description of cerebral circulation, cerebrospinal fluid (CSF) dynamics, brain gas exchange and cerebral blood flow (CBF) autoregulation. Two CBF regulatory mechanisms are described: autoregulation and CO2 reactivity. Central chemoreceptor control of ventilation is also added. This new model is subsequently used to study cerebral hemodynamic and brain gas exchange responses to test protocols commonly used in the assessment of CBF autoregulation (e.g., carotid artery compression and the thigh cuff test). The model closely mimics the experimental findings and provides biophysically based insights into the dynamics and interactions of the associated physiological systems. In summary, this work represents a bold effort in large-scale modeling of physiological systems. The presented model accurately describes the physiological systems and can explain how the cardiovascular, pulmonary and autonomic nervous systems interact in response to a variety of cardiopulmonary challenges, such as apnea, carotid artery compression and the thigh cuff test. With further refinement, the model may help investigators to better understand the complex biophysics of cardiopulmonary diseases such as sleep-related disorders of breathing (obstructive and central sleep apnea) and complications associated with head-injuries.
Integral manifold based control design of an electromechanical system
(1997) Sun, Xueqing; Ghorbel, Fathi H.
The control problem of an electromechanical system becomes more complex when the flexibility and friction are taken into account in the system model. To approach this problem, an electromechanical system is studied in this research as a system model which is composed of a motor driving a load through a flexible belt. The work of this thesis proposes a new integral manifold control design to achieve desired system performance based on the integral manifold control techniques. The compensation of friction is performed with a new friction compensation component in the integral manifold control algorithm. A detailed derivation of the integral manifold control law is given for regulation systems with a method of calculating control gains in the final control algorithm. A further study on the regulation system is conducted by selecting a set of control parameters based on the system characteristic equation, while a switching integral manifold phenomena is illustrated for tracking systems by exploiting system performance in state space. Simulation and experimental results are presented that show a consistent improvement in the system performance with the integral manifold control design strategy.
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Localization for Autonomous Underwater Vehicles inside Harsh and GPS-Denied Environments
(2023-12-01) Ben Moallem, Issam; Ghorbel, Fathi H.
The localization of Autonomous Underwater Vehicles (AUVs) deployed for integrity inspection of liquid storage facilities, to prevent failure of the process, is a critical and challenging task. This is primarily due to the harsh and GPS-denied work environment, as well as to the high degree of accuracy required by such confined-space activities to ensure accurate motion control and navigation, and generate rigorous inspection data associated with their true physical locations. Conventionally, an AUV performing general surveying operations is equipped with an Inertial Navigation System (INS) and/or a Doppler Velocity Log (DVL) for real-time state estimation and positioning, with respect to some inertial reference frame, while navigating its local environment. Due to inherently accumulating measurement errors over time, an INS/DVL device usually relies on the GPS for periodic recalibrations, which requires surfacing of the submersible robot. In deep waters, this strategy is energy resource and time inefficient, hence costly. Furthermore, for covered and underwater environments such as storage tanks, GPS signals are not even accessible at the liquid surface. Moreover, neither the INS/DVL-GPS system nor the traditional baseline acoustic positioning systems, based on trilateration techniques, provide a satisfactory solution accuracy as demanded by precision tasks such as pinpointing defects in steel storage and underwater structures. To overcome the shortcomings of the conventional underwater localization techniques, and achieve high-fidelity mapping between inspection data and real physical locations, we propose in this thesis a novel, accurate, and robust method to solve the robot localization problem inside confined, harsh, and GPS-denied environments. This method uses affordable sensors and fast algorithms to develop new techniques that provide accurate positions of the mobile agent. Given the geometry of the asset under investigation, a map representation for the robot's workspace is constructed based on range measurements over its boundaries. Then, the robot's position and orientation are accurately estimated relative to some defined reference landmarks (features) extracted from the map. In the event that the robot fails to recognize any landmark, a point-set registration technique is employed. In this case, the robot recursively matches map observations while in motion, which yields a relative position with respect to the most recently determined landmark-based position. The devised localization method will unleash fully autonomous robotic operations in confined, harsh, and GPS-denied environments. It will also facilitate Risk-Based Inspection (RBI) by employing predictive capabilities to optimize maintenance planning. This method can be applied in the oil and gas industry for inspecting liquid storage assets such as Aboveground Storage Tanks (ASTs) and Floating Production Storage and Offloading units (FPSOs).
Magnetic flux leakage sensing: The forward and inverse problems
(2008) Dutta, Sushant M.; Ghorbel, Fathi H.
Nondestructive evaluation (NDE) is the inspection of samples for corrosion and physical defects without altering them in any way. NDE has a critical role in the robotic inspection of energy pipelines in order to prevent catastrophic failures. Magnetic flux leakage (MFL) sensing is by far the most effective technique for robotic inspection of ferromagnetic pipes and tubular specimens. Defect detection using MFL sensing is a mature area of work, but defect characterization using MFL sensing is an open research problem. Several issues involved in this process are not well understood---for example, the interplay of the components of the 3-dimensional MFL field for 3-dimensional defects, the spatial properties of the MFL field components, the effect of sensor lift-off on MFL signals, and the relationships between defect properties and MFL signal properties. This dissertation addresses these issues using a systematic approach. First the MFL sensing problem is decomposed into the forward and inverse problems. Subsequently, a tractable forward model is presented which is capable of predicting the 3-dimensional MFL field of a known 3-dimensional surface-breaking defect. Important properties of the MFL field and their correlation with sensing parameters and defect parameters are established using the model and simulation. A linear inversion technique is presented which exploits the structure and properties of the forward model to characterize defects based on measured MFL signals. This dissertation also proposes a general framework to solve the inverse problem independent of the NDE modality in use. This framework uses the principles of data fusion and neural networks, and is illustrated using both MFL signals as well as another NDE technique, namely ultrasonic testing. Finally, this dissertation addresses the problem of pipe wall thickness measurement as a special case of the inverse problem and develops a novel technique for pipe wall thickness measurement using MFL signals.
Modeling and control of closed kinematic chains: A singular perturbation approach
(2005) Wang, Zhiyong; Ghorbel, Fathi H.
Closed kinematic chains (CKCs) are constrained multibody systems that contain closed kinematic loops. Nowadays, CKCs are used in a variety of applications ranging from flight simulators to medical instruments, and are becoming increasingly popular in the machine-tool industry and haptic interfaces due to their better performance in terms of accuracy, rigidity and payload capacity as compared to open-chain mechanisms. This document intends to present a novel methodology for the modeling and control of general CKCs. The dynamics of CKCs are characterized by index-3 differential algebraic equations (DAEs). Dynamic models in the form of DAEs pose difficulties in model-based control because most existing control design techniques are devised for explicit state space models. The control methodology presented in this document is based on a singular perturbation formulation (SPF), which has attractive properties including the minimum dimension of its slow dynamics and the large validity domain that contains the entire singularity-free workspace of the CKCs. The key issue of the model approximation error is addressed under different stability conditions. Explicit error bounds are derived and sufficient conditions for the exponential convergence of the approximation errors are established. For the control of CKCs, our approach transfers the control of the original DAE system to the control of an artificially created singularly perturbed system. Compared to control methods which directly solve the nonlinear algebraic constraint equations, the proposed method uses an ODE solver to obtain the dependent coordinates, hence eliminating the need for Newton type iterations and is amenable to real-time implementation. The closed loop system, when controlled by typical open kinematic chain schemes, achieves asymptotic trajectory tracking. The efficacy of the approach is illustrated by simulating the dynamics of a CKC mechanism, the Rice Planar Delta Robot, and then by validating the simulation results with experimental data. Thus, this work establishes a framework in which the control of CKCs can be systematically addressed.
Modeling and control of nonlinear transmission attributes in harmonic drive systems
(2001) Gandhi, Prasanna Subhash; Ghorbel, Fathi H.
Harmonic drives are special flexible gear systems widely used in space robots, the semiconductor industry, precision measuring devices, and military applications because of their advantages including near-zero backlash, high gear reduction, compact design, and light weight. On the other hand, they possess nonlinear transmission attributes including kinematic error, friction, flexibility, and hysteresis that are responsible for transmission performance degradation. Therefore, in-depth understanding, accurate modeling and control of transmission attributes are critical to the use of harmonic drives. There has been little research to date concerning these transmission attributes. This thesis characterizes harmonic drive transmission attributes and develops nonlinear control algorithms to enhance harmonic drive performance. The complete characterization of kinematic error in this research provides a new perspective to the understanding of the error. This thesis proposes an accurate hysteresis model in a differential equation form along with a constructive parameter identification scheme and extensive experimental validation. Furthermore, we extend a recently developed, accurate, dynamic friction model in a differential equation form to represent harmonic drive position dependent friction. Finally, several model-based nonlinear control algorithms are developed to improve harmonic drive performance. The most complex development compensates for kinematic error in presence of all other transmission attributes (flexibility, hysteresis and friction). Asymptotic stability with these algorithms is established using Lyapunov stability theory. The superior performance exhibited by these algorithms as compared to the traditional schemes is demonstrated using extensive simulation and experimental results. Thus, this thesis provides a solid foundation for performance improvement with harmonic drives as well as with other systems sharing one or more of the transmission attributes.
Nanomanipulation modeling and simulation
(2007) Mualim, Yanto; Ghorbel, Fathi H.
A novel approach to better model nanomanipulation of a nanosphere lying on a stage via a pushing scheme is presented. Besides its amenability to nonlinear analysis and simulation, the proposed model is also effective in reproducing experimental behavior commonly observed during AFM-type nanomanipulation. The proposed nanomanipulation model consists of integrated subsystems that consistently define the dynamics of the nanomanipulator tip and nanosphere, friction between the nanosphere and the stage, and the contact deformation between the nanomanipulator tip and the nanosphere. The main feature of the proposed nanomanipulation model is the Lund-Grenoble (LuGre) dynamic friction model that reliably represents the stick-slip behavior of atomic friction experienced by the nanosphere. The LuGre friction model introduces a new friction state and has desirable mathematical properties making it a well-posed dynamical model that characterizes friction with fidelity. The proposed nanomanipulation model facilitates further improvement and extension of each subsystem to accommodate other physical phenomena that characterize the physics and mechanics of nanomanipulation. Finally, the proposed model is simulated and compared to existing modes in the literature to demonstrate its versatility and effectiveness.
Optimal open-loop CMG maneuvers
(2002) McCants, Edward; Ghorbel, Fathi H.; Spanos, Pol D.
In this thesis, a general method was developed to solve the momentum-optimal attitude command trajectory for a given reorientation. The solution method relied on converting a standard two-point boundary-value problem to an unconstrained optimization problem using Lagrange multipliers. This approach was applied to a realistic robotic assembly operation of the International Space Station. Results for the optimization method in this thesis were compared with alternative attitude command strategies. Unlike much of the previous research, the methods developed in this thesis account for spacecraft with arbitrary and/or changing inertia matrices, control torques which do not necessarily coincide with the principal axes of the spacecraft, the full nonlinear rotational dynamics, known external torques that are functions of attitude and changes in angular momentum and mass properties due to repositioning of spacecraft components. Using the methods developed in this thesis, the peak momentum use during a maneuver was minimized. Additionally, optimal maneuvers were able to remove accumulated momentum from the CMG system.
Respiratory function: A systems approach
(2000) Athanasiades, Athanasios; Ghorbel, Fathi H.; Clark, John W., Jr.
Metabolic production of CO2 varies according to task (exercise, speaking, etc.). The human body maintains homeostasis (i.e., a constant CO2 content in blood), by varying alveolar ventilation, V˙A. The regulatory mechanism involves the breathing apparatus (lungs, airways, diaphragm), affecting V˙A, and the medullary pontine respiratory center (MPRC) in the brain stem. The MPRC is responsible for generating the basic rhythm of breathing and corrects for disturbances. Neurons inside the MPRC are organized into a central pattern generator (CPG), the functionality of which is not clearly understood. Although intracellular recordings have established the stimulus-induced response of isolated respiratory neurons in specific locales of the brain stem, the emerging properties of the CPG have not been directly correlated with intrinsic neuronal behavior. We develop computer-based models that emulate the regulatory operation of respiration. We adopt a systems view of the process, whereby the breathing apparatus (mechanics model) represents the plant, actuated by the respiratory muscles and controlled by the CPG. The mechanics model has two degrees of freedom (for lung and airway motion) and exhibits hysteresis and other nonlinearities. A frequency domain analysis of a linearized model is used to estimate parameters. The mechanics model is validated against data (lung volume and intrapleural pressure) collected from volunteer human subjects in a pulmonary function lab. The CPG is modeled as a neuronal network. We develop and validate a Hodgkin-Huxley type neuronal model that can mimic, with biophysical realism, the response of isolated neurons in the ventral Nucleus Tractus Solitarius and Nucleus Ambiguus of experimental animals. Characteristic nonlinearities of neuronal behavior, such as spike frequency adaptation, delayed excitation and postinhibitory rebound are captured successfully. The proposed network generates a stable, realistic breathing rhythm and responds successfully to disturbances. Moreover, individual firing patterns of experimentally identified respiratory neurons (early-I, ramp-I, late-I, post-I, E2 and pre-I) mimic data closely. Computer simulations show that adaptation in the firing rate of specific neurons dictates the duration of respiratory phases (inspiration, post-inspiration and expiration) and provides a mechanism for phase switching.

Browsing by Author "Ghorbel, Fathi H."

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