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Design Innovation Papers

Mock Circulatory Loop Compliance Chamber Employing a Novel Real-Time Control Process

[+] Author and Article Information
Charles E. Taylor

e-mail: taylorce@mymail.vcu.edu

Gerald E. Miller

e-mail: gemiller@vcu.edu
Department of Biomedical Engineering,
Virginia Commonwealth University,
401 West Main Street RM1229,
Richmond, VA 23220

Manuscript received January 6, 2012; final manuscript received September 21, 2012; published online November 21, 2012. Assoc. Editor: Paul A. Iaizzo.

J. Med. Devices 6(4), 045003 (Nov 21, 2012) (8 pages) doi:10.1115/1.4007943 History: Received January 06, 2012; Revised September 21, 2012

The use of compliance chambers in mock circulatory loop construction is the predominant means of simulating arterial compliance. Utilizing mock circulatory loops as bench test methods for cardiac assist technologies necessitates that they must be capable of reproducing the circulatory conditions that would exist physiologically. Of particular interest is the ability to determine instantaneous compliance of the system, and the ability to change the compliance in real-time. This capability enables continuous battery testing of conditions without stopping the flow to change the compliance chamber settings, and the simulation of dynamic changes in arterial compliance. The method tested involves the use of a compliance chamber utilizing a circular natural latex rubber membrane separating the fluid and air portions of the device. Change in system compliance is affected by the airspace pressure, which creates more reaction force at the membrane to the fluid pressure. A pressure sensor in the fluid portion of the chamber and a displacement sensor monitoring membrane center deflection allow for real-time inputs to the control algorithm. A predefined numerical model correlates the displacement sensor data to the volume displacement of the membrane. The control algorithm involves a tuned π loop maintaining the volume distention of the membrane via regulation of the air space pressure. The proportional integral (PI) controller tuning was achieved by creating a computational model of the compliance chamber using Simulink™ Simscape® toolboxes. These toolboxes were used to construct a model of the hydraulic, mechanical, and pneumatic elements in the physical design. Parameter Estimation™ tools and Design Optimization™ methods were employed to determine unknown physical parameters in the system, and tune the process controller used to maintain the compliance setting. It was found that the resulting control architecture was capable of maintaining compliance along a pressure-volume curve and allowed for changes to the compliance set point curve without stopping the pulsatile flow.

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References

Figures

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Fig. 1

One of the membrane cartridges developed for the compliance chamber. The latex sheet is adhered to acrylic rings and cut to size. These cartridges allow for ease of replacement in the membranes, tracking of membrane use to ensure they are replaced after appropriate durations of use and prevention of prestretch during installation into the compliance chamber.

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Fig. 2

The compliance chamber design illustrating the various components utilized: (1) pneumatic lines, respectively, connected to the pressure regulation block and the emergency pressure relief valve: (2) laser displacement sensor monitoring the membrane center deflection, (3) pneumatic pressure sensor, (4) air bleed line, (5) hydraulic pressure sensor, (6) hydraulic chamber, (7) membrane cartridge between two black O-rings, (8) air chamber, (9) clamps used to create a hermetic seal between the compartments, (10) and the mounting rail for the assembly. The arrow indicates the direction of flow through the chamber. Not shown are the pneumatic pressure regulation block and the amplifier boards for the pressure sensors.

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Fig. 3

Simulink™ model of the compliance chamber indicating the three different domains the model is able to simulate. The hydraulic section is comprised of a flow rate source with pipe sections leading to and one exiting from the hydraulic chamber. The resistor creates the backpressure to the flow, with the 655 value indicating the stepper motor position needed for a 0.022 in2 equivalent hydraulic orifice. The membrane section illustrates the interface of the mechanical element of the membrane, the pneumatic contribution from the air pressure above the membrane, and the hydraulic pressure acting on the membrane. The motion sensor in this section provides the analogous reading of membrane deflection that the laser displacement sensor is providing in the physical system. The pneumatic section simulates the pressure regulation system that drives the air space pressure changes. Controller calculations are conducted in blocks located in the upper right of the model, which are interfaced with sensors connected to the appropriate domains of the system.

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Fig. 4

A diagram of the run-time loops programmed for the compliance control, with functional connections to external components, is shown. A fast 500 Hz timed loop with a low priority interrupt is used to capture the sensor data. The displacement sensor is used to determine the current volume of the membrane. The fluid pressure is used in the computation of the volume set point. The air pressure is monitored to ensure operation within the safety limits. After the volume error is determined, the PI controller actuates the pneumatic system to maintain the compliance desired. The communication with the computer is a high priority interrupt that processes the set point changes and returns measurements to the data logging system.

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Fig. 5

Experimental performance of the compliance chamber process controller. The pulsatile flow settings were 50 bpm, 30% systole and 80 ml stroke volume. The peripheral resistance was simulated to be equivalent to a 0.022 in2 hydraulic orifice. The first plot includes the fluid pressures of the experimental and computational models. The middle plot contains the flowmeter data, which is interpreted as left ventricular output. The bottom plot indicates the curve slope used to control the compliance and the determined slope from regression analysis. The calculation of the compliance through linear regression of the pressure-volume curves is plotted against the setpoints to illustrate the performance of this controller. Large deviations at the transitions to new curve settings are seen, but are expected in this state of set point change.

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Fig. 6

Pressure versus volume (PV) curves for the data presented in Fig. 5. The dashed lines indicate the set point curves used to control the compliance. The black curves are the plots of the experimental PV. The light gray line segments represent the regression performed on the systolic and diastolic sections of the experimental PV curves. The numbers at the top of each set point curve indicate the slope of that setting. The black traces between the curves result from the controller’s migrating to a new curve. Congruency of the set point, experimental and regression plots illustrate this control method’s ability to accurately execute control of the system compliance using a curve-based set point method.

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Fig. 7

Screenshot of the LABVIEW control panel for the compliance chamber during an experiment. The compliance graph presents the current volume-pressure value present in the chamber (white) with the set point curve overlaid (gray). The dial gauge indicates the valve opening in percentage; negative indicates venting and positive corresponds to positive pressure opening. A switch to activate or deactivate the controller is included for safety in the case of instability, or for investigations without compliance control. The led indicates that the controller is appropriately enabled or disabled. The reset graph button clears the compliance graph at the top, which is useful when a new compliance curve is actuated. The inputs in the lower right are for the compliance curve of interest; slope and intercept pertain to the volume-pressure curve that is to be followed by the controller. The ability to set and read the integrator value is useful in expediting controller response and monitoring controller performance, respectively. The update settings button communicates to the MCU the controller state and the curve values to be used.

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