MINIATURIZATION OF LVAD ELECTRONICS PACKAGE

Transcription

1 Proceedings of the Multi-Disciplinary Senior Design Conference Page 1 Project Number: P11021 MINIATURIZATION OF LVAD ELECTRONICS PACKAGE Andrew Hoag Computer Engineer Juan Jackson Electrical Engineer Smiha Sayal Industrial Engineer Zachery Shivers Electrical Engineer Nicole Varble Mechanical Engineer Jason Walzer Mechanical Engineer ABSTRACT BACKGROUND The goal of this project was to miniaturize the electronics package for RIT s custom levitating Left Ventricular Assist Device (LVAD). Since this system is still in development, its end-users are the physicians and engineers involved in its development. As a lifecritical system, the LVAD was required to be highly reliable and safe while being robust. Proper system functionality was achieved by using the H-bridges, analog signal conditioning and a microcontroller. Portability was achieved by placing them in a 180x82x103 mm waterproof ABS plastic enclosure. NOMENCLATURE LVAD Left Ventricular Assist Device ADC Analog to Digital Converter AMB Active Magnetic Bearing ABS Acrylonitrile Butadiene Styrene CAD Computer Aided Design DAC Digital to Analog Converter h Convection coefficient (W/m 2 K) HESA Hall Effect Sensor Array k Thermal conductivity coefficient (W/mK) L Layer thickness LED Light Emitting Diode P power (W) PID Proportional-Integral-Derivative PWM Pulse Width Modulation RIT Rochester Institute of Technology T inf ambient temperature ( o C) T s Surface temperature ( o C) USB Universal Serial Bus Figure 1 - Previous electronics package The current left-ventricle assist device (LVAD) system is centered on a custom, levitating impeller blood pump for use in human patients with cardiac disease to boost blood flow. A unique feature of the blood pump is its ability to magnetically levitate the impeller within the pump's housing, drastically reducing friction losses and increasing pump lifetime as compared to mechanical bearing pumps. Even though the system in its current state is fully functional, the external package that houses the electronics is too large. Therefore, the purpose of this project was to miniaturize the system such that it can be scaled to more portable configurations, while sustaining the functionality of the pump. As a lifecritical system, it must be highly reliable and safe. Previously, teams have successfully decreased the volume of the device, but failed to create an overall working system that matches the original. This was caused by inappropriate microcontroller selection. Therefore, the team paid special attention to creating a system with a manageable electronics control system. Copyright 2011 Rochester Institute of Technology

2 Proceedings of the Multi-Disciplinary Senior Design Conference Page 2 DESIGN System Architecture The pump consists of the magnetic levitation coils (collectively known as the active magnetic bearing system), a three-phase motor to spin the impeller, and several hall-effect sensors to sense the x-y position of the front and back of the impeller. To keep the pump levitating, the force applied by the magnetic coils is constantly readjusted to compensate for positional error of the impeller axis. The control system consists of a microcontroller with an ADC for the hall-effect sensors and motor control circuitry for the AMB coils and the 3-phase motor. This microcontroller must be powerful enough to handle the control algorithm which keeps the pump levitating and rotating. The entire external assembly needs to be robust to water splashes and drops from regular operating height. Figure 2 - Overall system architecture Enclosure Design The size of the enclosure and the electronics determine the overall volume of the system, and these sizes are interdependent. In order to accommodate any design changes in the future due to resizing or restructuring of electronics, a CAD model was created using the software Autodesk Inventor. By carefully entering design constraints into the CAD design, the enclosure could be resized to accommodate the size of the PCB. The user interface components ultimately determined the minimum size of the box instead of the PCB. Figure 3 - Custom waterproof enclosure with user interface facing outward Enclosure material was selected based on the customer constraints and manufacturability. The following options were considered: rapid prototyping of ABS plastic, stereo lithography, injection molding and machining of metals or polymers. The team decided to use ABS plastic for this project as it was cost effective, easy to manufacture, feasible to manufacture within the timeline and would enabled the team to make quick revisions even with complex shapes. Despite its favorable manufacturability, ABS plastic has several undesirable characteristics. Concern for durability from falling was addressed through drop tests. This drop test was performed by releasing the enclosure from 1.5m above the ground. 3D printed ABS, unlike molded or machined ABS, is porous. Initial tests showed that a coating was required to waterproof the enclosure. The prototype was sprayed on with a rubberized coating and was tested again for water leakage. The results from these tests are discussed in further sections. Without heat sinks, the sealed enclosure will not be able to dissipate the heat generated by the electronics. In order to verify that the internal temperature of the enclosure will not exceed a safe operation temperature for the electronics, a heat transfer analysis was conducted. Several assumptions were made in performing the analysis. First, free convection was assumed to occur on the outside of the box, occurring on all sides with the exception of the side facing the body and the top which contains most of the user interface components. This is expressed using Equation (1) below: q" = h air (T in -T ) (1) [1] Project P11021

3 Proceedings of the Multi-Disciplinary Senior Design Conference Page 3 where q is the heat flux, h air is the convection coefficient of air, T in is the temperature inside the enclosure and is the ambient temperature. Also, the board was considered to be placed in the middle of the enclosure having no contact with the ABS plastic. Thus, conduction was assumed to occur from the board through both the air and then the ABS plastic. The thermal conductivity of ABS plastic is 0.17 W/mK [2]. It was expressed using Equation (2) below: q"= k(t in -T )/t (2) [1] Where k is the conduction coefficient and t is the thickness. Additionally, the electronic components were said to dissipate a maximum of 5 Watts. The analysis was assumed to be steady state and the ambient temperature was assumed to be body temperature (37 C). From the convection and conduction expressions the total thermal resistance was calculated using Equation (3) below: R = (1/(h air A)+t 1 /(Ak ABS )+t 2 /(Ak air )) (3) Using the above equations, the temperature inside the box was calculated to be 79 C, which is under the critical operating temperature of 85 C for the electronics components in the enclosure. Since these temperatures are very close, in-depth analysis for heat dissipation should have been conducted, however due to time constraints the team deemed it unnecessary to perform future research. Microcontroller The microcontroller sub-system is the core of the entire control system. Since this was one of the most crucial components, special attention was paid when considering feasible options. The factors considered for controller selection were as follows: Controller should be able to interface with 5V sensors and logic It should be able to handle floating point PID calculations It should have at least eight channels of 12-bit ADC Must be able to generate multiple PWM outputs for AMBs and motor control Two options were considered for the microcontroller, DSPIC and MSP430. The team chose the Texas Instruments MSP430F5438A 16-bit microcontroller as it met the project needs, is easy to use and due to team's prior experience in working with this controller. The following table lists the relevant specification of the microcontroller: Table 1-Specifications for Microcontroller Microcontroller Project Requirement Feature 12-bit SAR A/D converter 256 KB Flash Memory / 16 KB RAM 87 general purpose I/O 4x SPI/I2C Peripherals JTAG Sufficient resolution for PID inputs More than sufficient program and RAM space considering the simplicity of the algorithm and user interface. I/O for all general digital inputs from H-Bridges, user interface (buttons, LEDs, LCD) Interface to AMB H-bridges, D/A (compatibility) Reprogrammable and debuggable 2x 16-bit Timers Total of 8 timer channels The software was written in C using Texas Instrument s Code Composer Studio IDE and compiler. A TI MSP-FET430UIF USB to JTAG debug tool allowed for programming and debugging the microcontroller from a PC using the Code Composer tools. The MSP430 and built-in peripherals are largely interrupt driven. This includes the timers that are used to generate the PWM signals. The PWM signal for the PHX-35 motor controller is a standard R/C format, with a 50Hz frequency, and duty cycle varying between 1 and 2 ms. The AMB PWM signals provided to the H-bridges have a 20 khz frequency that normally operates between 30% and 70% duty cycle. [4] Analog to digital sampling for all HESA signals occurs 5000 times per second. Pump impeller levitation is controlled with a closed feedback loop, PID. The HESA signals are the feedback from changes made to the AMBs. Device status is displayed via the 32x100 LCD display and debug information is sent over UART serial at baud. The UART was connected to a FTDI FT232RL, which provided a virtual serial port over USB. Copyright 2011 Rochester Institute of Technology

4 Proceedings of the Multi-Disciplinary Senior Design Conference Page 4 Power Supplies An overview of the worst-case voltage and current requirements is detailed in Table 2. Note that these estimates are very conservative; they represent worstcase startup currents for the AMB levitation and rotation. Operating power will be significantly less. Table 2 - Estimated worst-case power supply requirements Supply Subsystem Current (ma) Power (mw) +3.3V Microcontroller, on-board LEDs +5V Interfaces (HESA power, PWMs, UI LEDs, etc) +12V AMB H-bridge logic VBAT AMB H-bridge power Total Total power (only one capacitor and one resistor) and its common use in other A/D circuitry [ref]. The maximum signal swing of the hall-effect sensor is 2.5V to 4.0V. Since the microcontroller's A/D is only able to accept signals from 0 to 3.3V, this signal had to be stepped down in voltage while maximizing its resolution. To do this, a voltage divider, bias manipulation, and a 3x gain stage were implemented. It was desirable to keep power losses through conversion to a minimum to reduce the heat dissipated in the enclosure. To achieve this, the 3.3V and 5V power supplies were designed with step-down converters. These step-down converters, commonly called buck converters, have conversion efficiencies of more than 90%. The 12V power supply was designed with a linear power regulator, which has comparatively low efficiencies; however, this was an acceptable design trade-off since the amount of current required for this voltage was very low, resulting in negligible heat dissipation. A wall power supply was chosen instead of selecting batteries due to time constraints and the difficulty of finding off-the-shelf batteries which could supply the necessary power. Compared to the previous prototype system, up to 8A continuous draw was anticipated and it was exceedingly difficult to find batteries that were able to supply this level of current for more than 6 hours while maintaining portability. Analog Signal Conditioning The AMB system requires feedback to keep the impeller levitating in the same position, approximately centered within the pump. Hall Effect sensors are used to sense the position of the impeller for the front and rear parts of the axis. An anti-aliasing filter ideally removes all frequencies above the Nyquist frequency of the incoming signal in order to eliminate aliasing effects introduced by the analog to digital converter. An RC filter was chosen instead of a higher order filter because of its simplicity Figure 4 - HESA signal conditioning circuit AMB System An AMB system was used to control rotor motion via the use of a standard H-bridge scheme. Current through the AMB coils controlled the force of the electromagnet. An H-bridge configuration allows both positive and negative current. High current consumption was a concern with the AMB subsystem; therefore, it was important that power efficiency was maximized in an effort to decrease thermal output. Miniaturizing AMBS components is also of interest. The Texas Instruments DRV8412 was selected as an integrated H-bridge solution. The DRV8412 provides two H-bridges per chip with 3A continuous and 6A peak currents. It features high power efficiency, PWM frequencies up to 500 khz, and integrated self protection circuits. This IC contains the power MOSFETs, biasing, and monitoring circuitry required. Printed Circuit Board The board was manufactured by Sierra Circuits and returned to us within five business days. It was populated by hand in the CIMS lab at RIT using surface mount reflow equipment, vastly decreasing the amount of time needed to assemble. A 4-layer PCB Project P11021

5 Proceedings of the Multi-Disciplinary Senior Design Conference Page 5 was selected for this design with the two internal layers dedicated to power routing and ground plane. A ground plane acts to remedy many noise and current loop issues, and is recommended whenever possible for PCB designs. Top and bottom layers are used for signal routing. Figure 6- Water Ingress Test Figure 5 - Top layer of PCB design The board has at least four distinct, functional sections: analog conditioning, AMB, power supplies, and microcontroller. The main constraint which had to be satisfied for layout is the placement of all connectors close to their locations on the enclosure. LED and button connectors are placed on the edges, large power connectors are placed near the AMBs and power supplies for easy routing of high power traces, and the microcontroller is placed as close as possible to the analog condition section to minimize trace length. TESTING & RESULTS The first test consisted of dropping the enclosure from 1.5 m above the ground in order to assess the material robustness. The results of the drop test indicated that the material could withstand a fall from the operating height, as no cracks or fissures were observed on the enclosure. For the purpose of the test a 50x50x70 mm ABS plastic enclosure was used. While a more appropriately sized enclosure weighing approximately the same as the actual enclosure could be used for the drop tests, the team deemed it unnecessary to risk the enclosure failure. The water ingress test was performed by placing the assembled enclosure under a stream of water with a flow rate of two gallons per minute for one minute as shown in Figure 6. During this test no water entered the enclosure. An additional test was performed were the enclosure was fully submerged in water. During this test some water did enter the enclosure. It was determined that the enclosure can be subjected to flowing water but could not withstand the pressure of full submersion. Only analytical calculations were done to verify that heat dissipation would not be an issue. Because of the poor thermal conductivity of both air and ABS plastic, it is recommended that the final enclosure be made of a different material. However, because of the ease in manufacturing, quick lead time, and the ability to create complex shapes with ease, it was found that this would be the most time and cost effective option. The generated PWM signals provided to the motor controller and AMBs were verified using an oscilloscope. Signal frequencies were correct and duty cycle ranges were within operating range. The system clocks, analog to digital functionality and signal conditioning CONCLUSIONS When worn on the hip, many of the team members found that the enclosure was comfortable for a short period of time. In order to prove that the device is comfortable for extended use, further testing, analyzing ergonomics of the enclosure should be performed. A decrease in height may also make the enclosure more comfortable to wear. Although the volume of the external enclosure did not meet the customer specification, the sacrifices to size was made to implement the LCD screen, accommodate the PCB board and assure that it would fit comfortably into the enclosure without interfering with the connectors. The printed circuit board functions properly and passed all functional testing after the first assembly with no cut traces or reworking: the analog section provides the proper linear transformations, the FTDI serial chip is recognized and communicates with a PC, the power supplies are all capable of providing their voltages within 0.01 volts, and the microcontroller is programmable via JTAG. The connectors on the board, however, were not the most effective choice as the extraction force is too large; this caused many unanticipated failures in the wires attached to the connectors. Copyright 2011 Rochester Institute of Technology

6 Proceedings of the Multi-Disciplinary Senior Design Conference Page 6 REFERENCES [1] Fowler, Kim., Electronic Instrument Design, Oxford University Press, USA, Ch. 8. [2] Incropera, Frank P., DeWitt, David P. Theodore L. Bergman Adrienne S. Lavine. Fundamentals of Heat and Mass Transfer, Sixth Edition [3]"Plastic Reference Data." Temperature Solutions Electronic Development Labs, Inc. HOME PAGE. < data.htm>. [4] Gomez, Arnold D. Control of a magnetically levitated ventricular assist device. Rochester Institute of Technology [5] Khare, Aditi. Estimation and control of the pump pressure rise and flow from intrinsic parameters for a magnetically-levitated axial blood pump. Rochester Institute of Technology ACKNOWLEDGMENTS Our team would like to thank: Dr. Steven Day, Dr. Shanbao Cheng (customers) Edward Hanzlik (tireless guide) Team P11022 for their collaboration for compatibility John Bonzo in the Brinkman Lab (for 3D printing our enclosure) Everyone in the Biomedical Laboratory (for their advice and assistance with construction of cabling and connectors Project P11021

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