Bradley E. Paden

An adaptive nonlinear predictive controller

Abstract The design and implementation of a new adaptive nonlinear predictive controller is presented using a general nonlinear model and variable transformations. The resulting controller is similar in form to standard linear model predictive controllers and can be tuned analogously. Alternative, the controller can be tuned using a single parameter. The design is computationally efficient. The controller is updated on-line without recalculating the controller gain matrix, which involves a matrix inversion. The new controller is compared to a PI controller and to an adaptive linear predictive controller through simulations of a continuous stirred-tank reactor. The effects of modeling errors on the new controller are also shown with simulations

On trajectory generation for flexible robots

A trajectory based on a Gaussian velocity profile is presented as an alternative to the double square pulse acceleration profile. The new trajectory leads to the fast positioning of the tip of a flexible robot with a minimal excitation of high-frequency modes. The torques necessary to move the robot according to this trajectory show a very smooth behavior. The absence of high-frequency content, present when double square pulse accelerations are considered, eliminates the Occurrence of undesired residual vibrations produced by modeling uncemnties at high frequencies. The excellent results obtained suggest the use of this new trajectory for fast and precise positioning of flexible robots.

Computational fluid dynamics analysis of blade tip clearances on hemodynamic performance and blood damage in a centrifugal ventricular assist device.

An important challenge facing the design of turbodynamic ventricular assist devices (VADs) intended for long-term support is the optimization of the flow path geometry to maximize hydraulic performance while minimizing shear-stress-induced hemolysis and thrombosis. For unshrouded centrifugal, mixed-flow and axial-flow blood pumps, the complex flow patterns within the blade tip clearance between the lengthwise upper surface of the rotating impeller blades and the stationary pump housing have a dramatic effect on both the hydrodynamic performance and the blood damage production. Detailed computational fluid dynamics (CFD) analyses were performed in this study to investigate such flow behavior in blade tip clearance region for a centrifugal blood pump representing a scaled-up version of a prototype pediatric VAD. Nominal flow conditions were analyzed at a flow rate of 2.5 L/min and rotor speed of 3000 rpm with three blade tip clearances of 50, 100, and 200 microm. CFD simulations predicted a decrease in the averaged tip leakage flow rate and an increase in pump head and axial thrust with decreasing blade tip clearances from 200 to 50 microm. The predicted hemolysis, however, exhibited a unimodal relationship, having a minimum at 100 microm compared to 50 microm and 200 microm. Experimental data corroborate these predictions. Detailed flow patterns observed in this study revealed interesting fluid dynamic features associated with the blade tip clearances, such as the generation and dissipation of tip leakage vortex and its interaction with the primary flow in the blade-blade passages. Quantitative calculations suggested the existence of an optimal blade tip clearance by which hydraulic efficiency can be maximized and hemolysis minimized.

An Adaptive Nonlinear Predictive Controller

The design and implementation of a new adaptive nonlinear predictive controller is presented using a general nonlinear model and variable transformations. The resulting controller is similar in form to standard linear model predictive controllers and can be tuned analogously. This design allows the controller to be updated on-line without recalculating the controller gain matrix, which involves a matrix inversion. The new controller is compared to a PI controller and to an adaptive linear predictive controller through simulations.

Position sensed and self-sensing magnetic bearing configurations and associated robustness limitations

We obtain and compare robustness limits for a magnetic bearing system controlled using two configurations; position and current measurement and current measurement only (sometimes referred to as the self-sensing configuration.) We show how these limits vary for varying system parameter values.

Elimination of adverse leakage flow in a miniature pediatric centrifugal blood pump by computational fluid dynamics-based design optimization.

Fetal bypass presents several perfusion challenges, including the need for high arterial flow rates using flexible arterial and small venous cannulae. We hypothesized that vacuum-assisted venous drainage (VAVD) would improve drainage and allow perfusion at higher flow rates which are thought to prevent placental dysfunction induced by fetal bypass. We conducted bypass for 60 minutes in 14 fetal lambs (90-105 days gestation; ∼1-1.5 kg) using a roller pump and various angled venous cannulae (8–12 Fr). VAVD at –20 mm Hg or –40 mm Hg was compared with gravity drainage. Average flow using gravity drainage was 139 ml/kg/min; after VAVD, we achieved average flows of 285 ml/kg/min (range, 109–481 ml/kg/min). VAVD at –40 mm Hg caused right atrial trauma in four fetuses; no injury was seen at –20 mm Hg. Venous air entrainment during repair of the injuries did not result in any apparent air embolism. Spontaneous pulmonary hemorrhage occurred in two fetuses at the highest flows (≥ 400 ml/kg/min). In all but one case, termination of bypass was followed by placental dysfunction within 120 minutes. VAVD can be safely applied during fetal bypass provided pressures are kept ≤ –20 mm Hg. However, the achieved higher flow rates do not prevent postbypass placental dysfunction and may indeed be detrimental to the fetus.

In Vitro Evaluation of Multiobjective Hemodynamic Control of a Heart-Assist Pump

Ventricular assist devices now clinically used for treatment of end-stage heart failure require responsive and reliable hemodynamic control to accommodate the continually changing demands of the body. This is an essential ingredient to maintaining a high quality of life. To satisfy this need, a control algorithm involving a trade-off between optimal perfusion and avoidance of ventricular collapse has been developed. An optimal control strategy has been implemented in vitro that combines two competing indices: representing venous return and prevalence of suction. The former is derived from the first derivative of diastolic flow with speed, and the latter derived from the harmonic spectra of the flow signal. The responsiveness of the controller to change in preload and afterload were evaluated in a mock circulatory simulator using a HeartQuest centrifugal blood pump (CF4b, MedQuest Products, Salt Lake City, UT). To avoid the need for flow sensors, a state estimator was used, based on the back-EMF of the actuator. The multiobjective algorithm has demonstrated more robust performance as compared with controllers relying on individual indices.

Towards the development of a pediatric ventricular assist device.

The very limited options available to treat ventricular failure in children with congenital and acquired heart diseases have motivated the development of a pediatric ventricular assist device at the University of Pittsburgh (UoP) and University of Pittsburgh Medical Center (UPMC). Our effort involves a consortium consisting of UoP, Childrens Hospital of Pittsburgh (CHP), Carnegie Mellon University, World Heart Corporation, and LaunchPoint Technologies, Inc. The overall aim of our program is to develop a highly reliable, biocompatible ventricular assist device (VAD) for chronic support (6 months) of the unique and high-risk population of children between 3 and 15 kg (patients from birth to 2 years of age). The innovative pediatric ventricular assist device we are developing is based on a miniature mixed flow turbodynamic pump featuring magnetic levitation, to assure minimal blood trauma and risk of thrombosis. This review article discusses the limitations of current pediatric cardiac assist treatment options and the work to date by our consortium toward the development of a pediatric VAD.

Magnetic levitation design for the PediaFlow ventricular assist device

Over the past few decades, we have seen a tremendous progress in the development of implantable ventricular assist devices (VAD). However, these devices are mainly developed for adult patients. For the patients under age 5 who have chronic heart failures, physicians must resort to extracorporeal circulatory support devices which often result in infection, thromboembolism, or excessive blood transfusions. In this paper, we describe a design process for a pediatric ventricular assist device (PVAD). The central part of the device is a magnetically levitated rotating pump which creates a pressure rise (~100 mmHg) at a required flow rate (~0.5 L/min) suitable for infants and small children. We have considered several different pump topologies, of which an axial mixed-flow pump configuration was chosen for further development. The pump impeller is supported by two radial permanent-magnet passive bearings. The rotor-dynamics analysis of the pump shows that the critical speeds of the pump are affected by the radial and yaw stiffnesses of the PM bearings. Hence, analytical expressions for the stiffnesses are derived and verified through FEA. In contrast to the radial suspension, the axial motion of the impeller is actively controlled using a voice-coil actuator. A toroidally-wound motor drives the pump with high efficiency and little additional negative radial stiffness. The design process relies heavily on optimization at the component-level and system-level. The preliminary results of the design optimization are presented in this paper

In-vitro evaluation of multi-objective hemodynamic control of heart-assist pump

Ventricular assist devices (VADs) now clinically used for treatment of end-stage heart failure require responsive and reliable control to accommodate the continually changing demands of the body. This is an essential ingredient to maintaining a high quality of life. To satisfy this need, a control algorithm involving a tradeoff between optimal perfusion and avoidance of suction is developed. An optimal control strategy has been implemented in-vitro that combines two competing indices: representing venous return and prevalence of suction. The former is derived from the first derivative of diastolic flow with speed, and the latter derived from the harmonic spectra of the flow signal. The responsiveness of the controller to change in preload and afterload were evaluated in a mock circulatory simulator using a maglev centrifugal blood pump (CF4b, MedQuest Products). To avoid the need for flow sensors, an estimator is utilized, based on the back-EMF of the actuator. The multi-objective controller has demonstrated more robust performance as compared to controllers relying on individual indices.