Proceedings of the Multi-Disciplinary Senior Design Conference Page 7

Project Number: 11022

Transcutaneous Signal Transmission to the Left Ventricular Assist device

Chrystal M. Andreozzi/Lead Engineer / Vince A. Antonicelli/Electrical Engineer
Craig T. LaMendola /Mechanical Engineer / Yevgeniy Popovskiy/Computer Engineer

Proceedings of the Multi-Disciplinary Senior Design Conference Page 7

Abstract

The objective of the Transcutaneous Signal Transmission project was to develop a system that could transmit signals and power across the skin as necessary for the Left Ventricular Assist Device (LVAD). The designed system should reduce the number of wires that have to pass through the skin. A previous team of engineers had been developing a similar product but failed to produce a functioning device. Learning from the failures of the previous team, a working device was designed to successfully reduce the size of the transcutaneous cable, without compromising the operation of the LVAD. The original cable going to the LVAD consisted of 23 wires and had a diameter of 8 mm. Under this team’s design the cable was reduced to 10 wires, including 2 power wires, 4 signal wires and 4 repetitive signal wires. This reduced the cable to 2.97 mm in diameter. This paper will outline the design, including the results of preliminary testing.

Nomenclature

ADC: Analog to digital converter

DAC: Digital to analog converter

DIP: Dual In-line Package

FPGA: Field Programmable Gate Array

LADS: Signal from the main controller to Linear Amplifiers

LAOG: Linear Amplifier Output Return going to Magnetic Barings

LAP: Power to Linear Amplifiers

LVAD: Left Ventricular Assist Device

MCC: Speed Control Signal from the main controller to the Motor Controller

MCO: 3-phase Output of Motor Controller going to Motor

MCP: Power to Motor Controller

MUX: Multiplexer

PAAS: Analog Signals from the main controller that contains information from PWM Amplifiers as an alternative to PADS.

PADS: Signal from the main controller to PWM Amplifiers

PAOP: PWM Generator Output and Return going to Magnetic Barings

PAP: Power to PWM Generator

PAT: PWM Amplifier Trigger Signal, which serves as the enable for the Amplifiers

SOIC: Small Outline Integrated Circuit

PWB: Printed Wire Board

PWBA: Printed Wire Board Assemble

PWM: Pulse Width Modulation

RIT: Rochester Institute of Technology

SA: Signal from Position Sensors to the main controller

SP: Power to Sensors

TSSOP: Thin Shrink Small Outline Package

background

Our customer, Dr. Day, and his team have developed a Left Ventricular Assist Device (LVAD). The LVAD is a mechanical circulatory device designed to assist a patient with a failing heart. Typically, a patient will receive one for temporary use after a heart attack or major heat surgery. The LVAD is also design to assist a patient who is waiting for a heart transplant. The LVAD uses a large transcutanious cable to transmit the power and control signals needed between the internal pump, and the external control system. The LVAD system developed at RIT has a large cable, 8mm in diameter, that has 23 wires that are a combination of power and signal wires. The large diameter and lack flexibility in the cable not only causes a discomfort to the patient, but is also a health risk, because a thick cable exposes the inside tissue, and may serve as an entry point to potentially fatal infections. The flexibility and size of the cable piercing the skin are critical component in decreasing the chance of inflections. It is with this motivation that our customer has come to us seeking a solution.

A previous team, Senior design team 10022, worked on this project achieved the objective of reducing the number of wires in the cable, but in their case the overall system’s functionality was compromised. The main objective of this senior design group is to make sure the system continues to work, while reducing the number of wires going from the exterior system to the LVAD. The reliability of the system is crucial for project success.

This project also builds on the work of previous senior design project 10021, who had attempted to miniaturize the LVAD, replacing the large computer and peripheral circuitry with smaller alternatives. Project 10021 was also unsuccessful, and another team is tasked with achieving those objectives, with their project designated Project 11021. In the interest of maximally useful product, a secondary objective was to make our product compatible with intercepting the transcutaneous wire of that project.

Figure 1: Original Blood Pump Layout

Figure 2: Implemented Blood Pump Layout

methodology

A methodical approach was taken when improving the design of the signal and power transmission of the Left Ventricular Assist Device. The strategy was to improve the flexibility of the cable by eliminating wires. Considering the available technology, an analysis was completed to determine the best technical solution for improving the performance and reliability of the LVAD.

The first step was to gain an understanding on how the original system and the 10022 project design functions. We also had to analyze were Project 10022 and Project 10021 failed (and succeeded). Inspired by Project 10021, specific electrical components were identified that could be placed inside the body to eliminate larger wires passing through the skin. With these considerations a system was proposed that was able to function with the original controller and the Electronics Miniaturization Team, Project 11021. Figure x shows the un-augmented working layout of the system. Figure x shows our layout, which has the motor controller and PWM generator (used in place of the linear amplifier) placed inside the body. In the diagram, the main controller box can be either the present working LVAD controller, or the product of Project 11021.

Team 10021 and 10022 Failures

The analysis of the previous team’s failures was an important part of our design process. It is important to understand their failures in order avoid making the same mistakes. A major contributor to the previous teams failure was incorrect calculations of the bandwidth and clock speed, the lack of understand of requirement to program a chip, the lack of understanding how electrical component will come together, and the lack of debugging time. To minimize the risk of repeating the same errors made previously all calculation were double check all with outside personnel. The use of a bread board was implemented to insure less error and correct assembly. Lastly, 3 weeks of debugging time to our project plan.

Electronics Explanation

The task is to reduce the number of wires going through the skin. The Original Blood Pump Layout (Figure 1) shows the current layout of wires and components, with its 18 signals going through the skin, some of which are transmitted on multiple wires. To reduce this number, our team moved the motor controller inside the body, and replacing the linear amplifier outside the body with a set of PWM Amplifiers inside the body. These components are inside our internal enclosure. The Implemented Blood Pump Layout (Figure 2) shows this layout.

Power Transmission:

Supplied to us is a 15 Volt supply. This is used directly to power the motor controller and PWM amplifiers. It is also converted, inside the internal enclosures into a 5 Volt supply. This is used to power the circuits, and as the power to the sensors. Also supplied is a 5 Volt supply originally designed to power the sensors. This supply is used to power the digital logic to the outside circuit.

Signal Transmission:

There are three sets of signals that are transmitted: The sensor outputs, the digital control signals, and the magnetic barring control. With each of these there are three general stages: pre-processing, transition, and post-processing. The pre- and post-processing vary for each stage, but the transition is necessarily funneled through the same logical unit.

Transmission is achieved by a pair of analog multiplexers/demultiplexers (MUXs), each connected to the signal wire going through the skin which is used to select between different signals to send through based on the clock. This is time multiplexing. The analog MUX allows signals to be sent both ways and the timing mechanism makes sure that the two multiplexers are always on matching signals.

The LVAD contains a set of sensors that monitor the position of the magnetically elevated axel. These generate a signal that must be transmit, which are refer to as SA. These analog signals need to be converted to digital before transmission. This is achieved by a 12 bit, 8 channel input, and serial output analog to digital converter (ADC). This ADC will be made to output a serial signal for a portion of its operating time, sending 16 bits (of which 12 are useful) at 5MHz, during the time that it is set to transmit thought the MUX. The digital signal needs to be converted back to analog. This is achieved by a 12 bit, 8 channel output, and serial input digital to analog converter (DAC). This DAC will continuously output an analog signal on each of the eight output channels. It will update each channel as the correct signal comes in on the serial port.

There are two digital signals that need to be sent from the external controller to the motor inside the enclosure. There is a PWM signal, MCC, and a amplifier trigger, PAT. Both are treated as a digital signal in the projects designed electronics. No preprocessing is done to these two signals. These signals are stored in a pair of registers so that their value is maintained when they are not being transmitted. MCC is then designed to be fed to the customer provided motor controller, which converts it to a 3 phase output powering the LVAD motor. PAT similarly fed to the customer provided switch.

There are 4 signals that control the X and Y position of two magnetic bearings inside the pump. In the design, it was choose to take an analog signal, PAAS, and transform it to the needed signal PADS. The 4 analog signals, PAAS, are converted to digital using the same converter that is used for the sensor signal. As with those signals, the ADC will be made to output a serial signal for the portion of its operating time when it is connected to the MUX. After transmission, the four signals are converted back to analog, with a serial DAC in a manner identical to the Sensor signals. Then these analog voltages are converted to a PWM signal using a specially purposed microchip. This value is provided to the PWM amplifier, which processes them to output the correct magnetic barring high power output PWM signal.

Timing:

In order to keep all parts working together, proper timing circuitry is vital. A single clock was generated in the external enclosure, and send on a separate wire to the inside. On both sides, a 12 bit counter is used to subdivide the clock into fractional clocks, which are used as clock signals or control logic inputs as needed depending on the component. The fastest clock needed is 5MHz needed by the ADC, DAC, and associated control logic, so this is used for the master clock.

Initialization:

Another design component is the logic that ensures that the device starts in the proper state. There are three goals in this: to ensure that the internal and external counters are in sync, to bootstrap the analog to digital and digital to analog converters, and to ensure that all signals give acceptable values when they are first needed.

To ensure that both the internal and external counters are in sync, an active low clear signal is transmitted on the 0 channel of the analog multiplexer. A power-on-reset chip in the external enclosure generates this signal. This signal is maintained for the time it takes the internal analog MUX to cycle through all channels. Externally, this signal is used to as the clear/reset signal for all components including the counter. This has the effect of locking the MUX into transmitting the clear signal. On the internal end, the 0 channel is connected to a pull-up resister that maintains a high signal when the signal is disconnected. This channel is used as the internal clear/reset for all components. If the internal circuit is not started at the same time as the external circuit, then the internal circuit will cycle through all the channels on the MUX, until it reaches the 0 channel, at which point its counter will be reset. Both counters will then maintain the 0 state until the clear signal goes to high, at which point they will start counting in sync.

The ADCs needs to be bootstrapped on start up. This is achieved with Flip Flops that maintain a startup signal after the initial clear. The correct control register values are input in parallel to a shift register, one for each converter, which then converts it into the serial control stream the converter expects. The same shift register is also used to update which channel the ADC needs to transmit and the DAC is receiving.

Some effort was made to ensure that all signals give acceptable values when they are first needed applied. A major need was that the PWM amplifiers require valid data when they start up. To achieve this, a separate signal was used to communicate when that data is available. This signal has the designation PAT, and is transmitted as a digital signal much like MCC. The signal coming out of the designed board was fed to a control switch provided by the customer. Another requirement was that the sensor signals, SA, properly give an error state on startup. This was achieved by presetting the associated DAC to 0 values. Similarly the MCC signal was preset to 0 at the output flip-flop.