Electrical Documentation – P13022
LVAD Breakaway Port
Jason Inman and Michael Edson
Electrical pictures can be found on EDGE in the photo gallery folder.
The final PCB files are located in the Final_System folder.
Our final programs for the microcontrollers are located in the Final_System folder.
For other info, refer to
Desired Operation of Electrical Components
Internal Microcontroller:
The internal microcontroller’s primary purpose is to send a periodic signal to the motor controller in the event of a disconnect in order to run the pump. This is performed using the Timer A module available on the microcontroller. In this setup, Timer A is run in continuous mode and counts all the way to 0xFFFF. The register TACCR0 is used to determine the width of the actual pulse and the output is 3.3 V during the duration of 0 to TACCR0. From TACCR0 to 0xFFFF the pulse is 0. With this method, the correct duty cycle can be achieved needed by the motor controller.
This microcontroller is also in charge of monitoring the internal battery during disconnect. This is done by monitoring the voltage of the battery using the analog to digital converter on the microcontroller. This information is then put in an ASCII string and sent wirelessly to the external microcontroller.
The final function is to monitor the temperature of the circuit during the charge cycle, as this produces the most heat. This is performed using voltage division between a 10 kΩ resistor and a thermistor rated for 10 kΩ getting power from the 5 V regulator such that the output is nominally 2.5 V. This voltage fluctuates based on the resistance of the thermistor which changes with temperature. This voltage is also read using an analog to digital converter and compared to an equation to determine the temperature.
External Microcontroller:
The primary purpose of the external microcontroller is to receive the battery voltage data using the UART and the receiver and use the voltage to determine an approximate lifetime remaining. The lifetime (in minutes) is then displayed on the two 7-segment displays for the user. When there are 30 minutes remaining, one beep from the alarm will occur. When there are 15 minutes, two beeps will be given. When the lifetime reaches zero, a solid beep is heard until reconnected. There is also a 15 minute safety factor built in, such that when the display says that there is zero minutes remaining there is actually 15 minutes of runtime left. This microcontroller also has access to a relay, which is used to toggle the alarm on and off as needed.
Voltage Regulators:
Four voltage regulator circuits are being used for this project. Two of these are the 3.3 V supplies for the microcontrollers on the internal and external boards. There is also a 5 V supply for the receiver circuit on the external board. These three regulators are all very similar Buck converters, and use the same external component (capacitors, inductors, and diodes) to ensure low ripple voltage and quick recovery times. The internal transmitter receives it’s 5 V power from the motor controller in order to save layout space. The other regulator is a 15 V boost circuit that converts the external battery power supply to 15 V for use in charging the battery. The external components were chosen through several calculations guided by the datasheet for the part (LM2577SX). Testing has shown that these regulators perform well under load conditions shown in the schematics.
Battery Selection:
Batteries have been chosen for this project to minimize size and weight while also achieving the 30 minute runtime required by the specifications. Cost could also have an impact but was not important for our design. 4.32 Whr batteries were chosen for the prototype design and have an approximate volume of 100 cm3. If the total voltage is 12.8 V roughly and the current draw is 0.5 A, the power usage is 6.4 W, which gives a runtime of roughly 162 minutes. Once real runtime data is collected, smaller size batteries may be considered for the final design.
Finished Electrical Work
Internal Microcontroller:
The internal controller at its present state has the ability to power the motor controller while running on the internal batteries. It sends out the required pulses to the motor controller to initiate and sustain the pump. Also, the vibration function was proven to work in testing.
External Microcontroller:
Currently, the external controller was shown to be able to display numbers from a table on the LED displays. An approximate battery lifetime data chart was placed in the code and the lifetime in minutes can be displayed to the user. Also, the relays trigger when the countdown reaches 0, which should alert the user that they are running out of time.
Connection with P13021:
An additional port was added to the internal case so that wireless energy transfer team could actually plug in there device to our circuit. Testing with their equipment was not performed, but the internal components successfully powered the pump using this connection.
PCB Design:
The final PCB design is smaller compared to the prototype with the idea of creating a small implant. Enough parts were ordered to make two of each circuit board as a back up. The PCB design files are located on EDGE in the Final_System folder.
Unfinished Electrical Work
Internal Microcontroller:
The programming for temperature sensing and battery life determination was not completed due to time restraints and lack of programming knowledge. The circuitry is all present for the programming to work though. The temperature is read as a voltage division between a resistor and a thermistor. This feeds into an ADC that can be used in the program to stop charging or turn off the device. The same is done for the battery voltage.
The transmission of the battery life data was not completed either. With more time, we would have looked into using the UART on both of the microcontrollers to send and receive data. The transmitter and receiver are in place on the boards and are located at the UART pins of the controllers. In theory, all that is required is the programming for the device to operate as desired.
External Microcontroller:
More testing could be done with the alarm feature using the microcontroller and the programming for receiving the data was not completed. Once again, the UART should be usable for the programming.
Batteries:
We have smaller batteries for the internal device that were not implemented in the final design due to an unexpected shortage of charging regulator boards. Instead, older batteries wrapped in electric tape were used as a quick fix. With more time, another charging board could be ordered and the smaller AA size batteries could be used as well as the cradle. Time testing would then need to be performed on these batteries to ensure they still meet the specs. It may also be possible to decrease the size of the implant by finding smaller batteries.
TI Motor Controller:
It is possible to reduce a large amount of internal circuitry by using the TI motor controller chip. This was attempted at the start of the quarter, but due to lack of programming knowledge was passed up for the microcontroller. If an EE or CE with good programming knowledge were to take part in the next iteration of the project, it is a strong recommendation that they attempt using this device. With it, the implant could become half the size it currently is.