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Design Innovation Papers

Automation of the Harvard Apparatus Pulsatile Blood Pump

[+] Author and Article Information
Charles E. Taylor

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

Zachary W. Dziczkowski

Department of Computer Science,
Virginia Commonwealth University,
401 West Main Street RM4225,
Richmond, VA 23220
e-mail: dziczkzw@gmail.com

Gerald E. Miller

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

Manuscript received January 6, 2012; final manuscript received August 7, 2012; published online October 15, 2012. Assoc. Editor: Paul A. Iaizzo.

J. Med. Devices 6(4), 045002 (Oct 15, 2012) (10 pages) doi:10.1115/1.4007637 History: Received January 06, 2012; Revised August 07, 2012

Producing accurate physiological circulatory conditions in vitro is integral to the evaluation of cardiac assist technologies. The ability to simulate cardiac function, normal or pathological, is dependent on the capabilities of the pump deployed for this purpose. Presented is a reference standard for this in vitro analysis, with automation features targeted for robust bench-top testing. Cardiac performance is typically described in terms of stroke volume, heart rate, and percent systole. Respectively, these three settings prescribe the volume of fluid ejected, the rate of pumping, and the percentage of the pumping cycle spent in ejection. A pump that provides settings for each of these parameters and precise repeatability allows for accurate construction of simulation conditions. These capabilities are present in the commercially available Harvard Apparatus 1423 pulsatile blood pump. Modifications have been made to this particular model that allow for the automation of its function and real-time performance determination. Discussed in this publication is the design and performance of a modified 1423 pump that employs universal serial bus (USB) communication in the control of its stroke volume, heart rate, and percent systole. The percent systole is denoted as the phase ratio on the hardware. Utilization of an embedded microcontroller (MCU) allows for not only the digital communication via a computer terminal, but process control of the subsystems maintaining each parameter. Care was taken to preserve the mechanical design employed by Harvard Apparatus; the modifications were not invasive to the mechanical driveline of the pump. Electromechanical design characterization was performed in Simulink® using the following Simscape™ block sets: Simscape™ Foundation Library, SimElectronics®, and SimMechanics™. This provided an accurate model of the systems during the design process, which assisted in the deployment of the process controllers with minimal prototype construction. Communication with the MCU is achieved with American Standard Code for Information Interchange (ASCII) commands delivered through a LabVIEW VI interface. Continuous readbacks on fill/ejection rate, pump rate (HR), percent systole (PS), and stroke volume (SV) are possible with these modifications. The deployed upgrade allows for complete automation of the Harvard Apparatus 1423 pulsatile blood pump, with the capability to run sequences of conditions without the need for manual intervention.

FIGURES IN THIS ARTICLE
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© 2012 by ASME
Topics: Engines , Design , Pumps , Blood
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References

Figures

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Fig. 1

Isometric view of the Modified Harvard Apparatus Model 1423 pulsatile blood pump. This perspective on the device is able to show all the modifications made to the assembly: fastener connecting stroke volume actuator to lead screw (1), manual control knobs for heart rate and percent systole (2), switch for selecting manual knob or communication-based settings control (3), BNC output connector for logical indication of pumping phase (4), USB connection for communication with the device (5), and bootloader connection for updating the firmware for the device (below USB connection). All the electronics are mounted on the black side panel of this modified pump; the only components not included on this panel are in the stroke volume actuation assembly and motor shaft encoder.

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Fig. 2

A model of the drivetrain, designed in SolidWorks, and later imported into Simulink™ via the SimMechanics™ Link tool. The arrows indicate the movement of the respective linkages. The elements of the drivetrain are the (1) cam connected to the driveshaft of the motor, (2) swing arm, (3) third linkage acting as a class 1 lever, (4) fulcrum of lever, and (5) translating linkage that attached to the hydraulic piston.

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Fig. 3

Schematic of the run-time process control code developed for the Harvard Model 1423 design updates. The 200 Hz loop processes the cam timing events, which serve to change the rpm settings, determination of heart rate, and calculation of percent systole. This timing loop is also responsible for the PI controller with feed-forward (FF) section that dictates the pump motor speed. The capture interrupt processes the optical encoder data and determines the speed. A slower 10-Hz timing loop processes the positioning of the stroke volume position and resulting actuation. The communication section is placed in an interrupt section to immediately process incoming (Rx) data. The low and high designations pertain to the priority of that interrupt; the ranking within a low/high designation is available in the device manual. The pins utilized by particular functions are designated next to each trace line. The blocks external to the run-time code section illustrate the systems this MCU is interfaced to, with dashed lines representing assemblies.

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Fig. 4

The computer-aided design (CAD) model of the stroke volume actuator at an isometric view showing the top of the assembly. The gears are presented as cylinders with radial size equivalent to the pitch diameter. The interface of the actuator to the stroke volume lead screw (1). The back pane in this drawing (2) has been truncated to show only the section used for mounting the stroke volume actuator. The full back pane supports the drivetrain shown in Fig. 2. The elements of actuator motor assembly are a mounting spacer (3), gear box (4), and DC motor (5).

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Fig. 5

The Simulink™ model of the Harvard Apparatus model 1423 pulsatile blood pump. The subsystems that comprise the larger model are presented with masks to illustrate the physical assembly they represent. Operating parameters are set on the left side of the model: stroke volume (SV), heart rate (HR), percent systole (PS), and the power switch ON/OFF logic. The rpm determination block uses Eqs. (1) and (2) to produce the required rpm settings for each phase of pumping. The “SV to PivotPosition” lookup table in the upper left is an application of Eq. (3), providing the setting position from the SV value. The cam switch block actuates the rpm switch to enact the correct rpm setting. The PI controller operates on the error of the setpoint delivered from the preceding rpm switch and the driveshaft speed readback from the rotary encoder. The controller reset (lower left of PI block) is enacted by a change in the cam switch state, allowing the feed-forward control from the rpm switch block to dominate the speed change in the motor. The model has no outputs, as the hydraulic system this is connected to would need to be modeled. The pump head block represents the translation of the piston and the friction pertaining to that motion.

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Fig. 6

The Simulink™ model of the shunt wound motor, using Simscape™ elements, represents the electromechanical system of the motor accurately. An important factor in constructing a shunt wound motor model is the mutual inductance between the armature and the field coils, which is represented in the upper left. The mechanical elements presented are consistent with a DC motor model attached to a gearbox.

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Fig. 7

Simulink™ SimMechanics™ equivalent of the SolidWorks model presented in Fig. 2. The light gray elements in this model were created and connected automatically from the SimMechanics™ Link. These blocks represent the rigid bodies and constraints equivalent to the SolidWorks parts and assembly; the rigid bodies can be identified as those containing the center of gravity symbol. The black elements were added to interface the SimMechanics™ blocks with the model and provide an initial condition for the geometry. The “rot” connection on the left of the figure is interface of the motor driveshaft to the linkage, and the “trans” connector on the right of the figure is where the hydraulic piston is connected. The joint actuator for the pivot point controlling the stroke volume (SV) necessitates the inputs of control parameter.

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Fig. 8

The mock circulatory loop used to evaluate the performance of the upgraded design. Beginning at the modified Harvard Pump (1), the exiting passes through a flow meter (2) into the compliance chamber (3). The chamber has an elastic membrane that distends under pressure, providing a dampening effect on pressure waves. The resistor produces the backpressure through the occlusion of flow (4). A venous reservoir acts as a return tank for the loop and a supply for the pump (5). A centrifugal pump has been included to study the effects of preload on the Harvard Pump (6).

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Fig. 9

Control panel developed in LabVIEW to control the pump parameters. The slider bars are used to select the desired settings, and the “Update Settings” button actuates the communication to the microcontroller to update the parameters. The gray box at the bottom shows the performance values obtained from the microcontroller's process monitoring.

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Fig. 10

Pump output with a staircase change in heart rate, while maintaining 50% systole, and a stroke volume setting of 50 mL. The lower trace indicates the readbacks from each of the parameters, which were used as inputs to the computational model. The spurious values in percent systole and BPM at some of the transitions are due to the change to the new speed values midpumping cycle.

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Fig. 11

The 10% staircase change in percent systole from 30% to 70% and back with constant heart rate and stroke volume. The spurious readbacks at the transitions are due to the speed changes midstroke. These events are exacerbated in the computational model due to the simulated system response to these perceived rapid changes.

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Fig. 12

The functionality of the stroke volume change is illustrated by the continuous change from 20 mL to 95 mL, with heart rate and percent systole remaining unchanged

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