Michael Baloh

Implantable centrifugal blood pump with hybrid magnetic bearings

Test methods and results of in vitro assessment of a centrifugal pump with a magnetically suspended impeller are provided. In vitro blood tests have been completed with a resulting normalized milligram index of hemolysis (NmIH) of 12.4 +/- 4.1, indicating that hemolysis is not a problem. Hydraulic characterization of the system with water has shown that a nominal pumping condition of 6 L/min at 100 mmHg was met at 2,200 rpm. Maximum clinically usable cardiac output is predicted be 10 L/min. The magnetic bearing supported impeller did not contact the housing and was shown to be stable under a variety of pumping conditions. The driving motor efficiency is 75% at the nominal condition. Finally, a description of the clinical version of the pump under development is provided.

Feedback control applications in artificial hearts

Artificial hearts are a commercially interesting technology which relies heavily on feedback control, particularly with centrifugal pump implementations using magnetic bearings. Several examples of feedback control in these pumps were explored in this paper, including parameter estimation for position sensing, observer based motor commutation, position control of the rigid impeller subjected to highly uncertain fluid dynamic forces, and pump discharge rate regulation (physiological control) to achieve uncertain objectives (due to insufficient experience) using difficult measurements on an extremely complex plant. Such problems are typical of an expanding range of biomedical applications for automatic control.

Magnetic Suspension Controls for a New Continuous Flow Ventricular Assist Device

A new continuous flow ventricular assist device (CFVAD III) using a full magnetic suspension has been constructed. The magnetic suspension centers the centrifugal impeller within the clearance passages in the pump, thus avoiding any contact. This noncontact operation gives very high expected mechanical reliability, large clearances, low hemolysis, low thrombosis, and relatively small size compared with current pulsatile devices. A unique configuration of a system of magnetic actuators on the inlet side and exit sides of the impeller gives full five axis control and suspension of the impeller. The bearing system is divided into segments that allow for three displacement axes and two angular control axes. For the first suspension tests, a decentralized set of proportional, derivative, and integral (PID) controllers acting along the modal coordinates are used to suspend the impeller. The controller design takes into account the blood forces acting on the magnetically suspended impeller, the unbalance forces on the impeller and gravitational loads during various body motions. In the final design, the bearing control axes will be coupled together through fluidic forces so the electronic feedback controller is a centralized multiple input, multiple output controller. The control system design must be robust against these types of externally imposed loads to keep the impeller centered and avoid blood damage. This article discusses the dynamic model, controller, and controller implementation for the magnetic suspension controller of CFVAD III. ASAIO Journal 1997; 43:M598-M603.

A Magnetic Bearing System for A Continuous Ventricular Assist Device

This article describes a prototype continuous flow ventricular assist device (CFVAD3) supported in magnetic bearings. The VAD is a small centrifugal four bladed pump. The pumps geometry is explained. The CFVAD3 is the first compact VAD completely supported in magnetic bearings. The magnetic bearings are composed of an inlet side actuator divided into eight pole sets, and an outlet side actuator, also divided into eight pole sets. The pump operating performance was tested and found to be within the design flow rate of up to 9 L/min, and head up to 170 mm Hg for human circulatory support. Magnetic bearing operation out of center positions under various operating orientations were measured and found to be < 1/6 of the bearing clearance, well within specifications. The expected magnetic bearing power loss has been calculated at approximately 6.5 watts.

Adaptive estimation of magnetic bearing parameters

As magnetic bearing applications become more complicated, the need for accurate models of the controlled bearing systems becomes more important. Ordinarily, accurate models are obtained by first levitating and then performing system identification. The method for measuring the actuator properties often involves lengthy tests that are performed manually. The paper presents initial research into using adaptive estimation to identify unknown parameters and disturbances for a simple one-dimensional magnetic bearing system. Analysis of the plant and construction of an estimator are presented with simulation results that show the estimator performance. Lastly, a more general model with hysteresis is parametrized and estimated.

Modeling and control of a magnetic bearing actuated beam

Presents a nonlinear coordinate transformation for a magnetic bearing system. The construction of this coordinate transformation follows a strict mathematical procedure. Once in the new coordinate system, a unique feedback linearization law exists which has several practical properties: the linearizing feedback is guaranteed regular, the resulting linearized system is guaranteed controllable, and beam angular position regulation is achievable. As a direct consequence of feedback linearization the system decouples into two systems: one governing the beam angle and the other a dynamic bias.

Nonlinear control of a benchmark beam balance experiment using variable hyperbolic bias

Magnetic bearing control schemes frequently use a bias current together with a superimposed control current to improve actuator gain and linearity. In this paper a variable bias scheme is developed to reduce electrical power loss while maintaining the control performance similar to conventional bias control. Two different bias schemes are proposed: a linear bias scheme and a hyperbolic bias scheme. A bias-dependent feedback linearization approach is applied to both systems to make the input/output relation of the system linear, and an observer-based stabilizing controller is designed to stabilize the nominal closed loop system. The resulting closed loop system dynamics are independent of bias except for amplifier saturation. The bias is minimized while avoiding amplifier saturation and singularity in feedback linearization with a separate bias control loop.