Final Design

The final design is a digital system that was designed to allow a user to monitor their CPR performance by providing valuable feedback in the forms of compression depth, compression pressure, and compression rate on an external monitor.

Final & Testing Setup

Internal Sensor Hub and Display Module

Design Systems Overview

The two subsystem setup for our design is meant to serve the purpose of data collection and data analysis that adheres to our sponsor's requirements.

The internal assembly serves the purpose of collecting values from the chest compression an individual applies to the mannikin.
The external assembly serves the purpose of receiving those values and interpreting them into usable, visual feedback for the compression depth, compression pressure, and compression frequency.

Internal Subsystem

External Subsystem

Each of the components included in the final design can be found below along with the justifications for why they were selected.

Accelerometer

Compression Depth and Frequency

Accelerometers are small axis-based devices that can be affixed to items and indicate orientation. These sensors are typically integrated into other devices.
When the microscopic crystals within the sensor are stressed by acceleration, they produce a voltage. The accelerometer translates the changes in voltage to velocity or impact and orientation.
The accelerometer is used to detect acceleration. Its application in this project detects the change in acceleration due to the compressions and transforms that acceleration data into compression depth.

Diagram of process to measure the displacement data

Plots of data acquired during CPR tests conducted on the mannikin

Troubleshooting

Raw accelerometer data produces a lot of noise due to its high sensitivity. Filtering out the noise was necessary to acquire the accurate displacement data.

For each compression, the initial condition of displacement data was reset, thus acquiring a better result on displacement reading.

Testing Results

Data acquired through direct integration doesn’t provide accurate data due to noise produced from the sensor and microcontroller. To remove the interference, a simple moving average filter was implemented to obtain smoother acceleration data which in turn provides more accurate displacement data.

To measure the compression rate, numbers of compression were measured every 3 seconds and displayed as the compression per minute data. Counter function was implemented, where every time displacement data reached a threshold depth, the counter value would increase by 1 and provide the user number of successful compression.

Force Sensing Resistors

Pressure Readings

A force-sensing resistor is used to measure force, pressure, or mechanical stress that is being applied.
FSRs have high resistance, when not actuated, but when a force is applied to the sensors, the conductive film makes contact with the print, lowering the resistance of the sensor which provides a high voltage output.
FSRs were utilized to measure and provide data to the user regarding the force applied during each compression.

Plot of saturated voltage readings from FSR402

Mannikin placed in Instron Load Tester to measure force from desired compression

Initial Testing

Initial Choice: Interlink Electronics FSR402
Initial Test: FSR was placed inside a section of the mannikin and compressed repeatedly for approximately 15 seconds.

The FSR402 yielded a favorable performance due to its clear sinusoidal pattern that reflected the motions of compressions and decompressions. However, it was noted during the compression test that the maximum voltage was quickly reached, suggesting that the signal was saturating before the full range of compression was completed.

Force Calibration and Design Iteration

Validity Test: An Instron Load tester was used to compress the mannikin to the maximum compression depth which provided a set of force readings measured at various depths.
Conclusion: The amount of force required to achieve the compression range was in the hundreds of Newtons. A new sensor would need to be implemented into the final design.

FSR402 sensor had a range of 0 to 10 Newtons. In comparison, the new sensor chosen, FlexiForce A301 sensor by Tekscan was proven to have the necessary force range for our design application. Further calibration testing was performed on the A301 sensors to verify that the pressure measured during compressions was accurate.

Microcontroller

Teensy 4.0

A microcontroller unit (MCU) transforms the range of voltage signals acquired by the sensors into meaningful values to be acquired by the user.
The microcontroller unit must also be able to control and maintain the human periphery interface with which the user receives real-time feedback.

Testing and Analysis

From the tests conducted, it was determined that the Teensy outperforms the Arduino in every metric as pertinent to the project.

Clock speed considerations were put to rest due to the high computational speed of the Teensy.
As the screen size and consequently the number of pixels on the display increased for different display sizes, the computational load increased fairly linearly.

The only drawback of the Teensy is the high current requirement however by using a regulator for the power system we can provide sufficient power for desired performance.

Plot of Screen Size (X-Axis) vs PPS

Liquid-Crystal Display (LCD)

Human Peripheral Interface

A thin-film-transistor liquid-crystal display (TFT LCD) was chosen as the design solution as it provided the ideal screen size range with high fidelity. High fidelity is important as minuscule changes must be easily picked up during CPR.
The wiring for the display was divided into two main categories; electrical wires for directing power to the display pixels and the backlit LCD lights (Two different 5V inputs), and the main communication bus. The communication buses we tried to utilize were the Serial Peripheral Interface (SPI) and the 8 Bit parallel Interface.

Printed Circuit Boards

The purpose of designing a printed circuit board for the final solution was to make the subsystems more robust. The printed circuit board is a wired layered board that serves as the foundation for connections between electrical components. These wire traces are typically made from copper and replace the need for malleable jumper wires.

In this design, there are two printed circuit boards, one that performs as an internal sensor hub for the accelerometer and force sensing resistors and the other external printed circuit board that houses the microcontroller, display screen, and battery unit.

Internal: Sensor Hub

The internal printed circuit board has allotted space for:

two accelerometers
four force sensing resistors with voltage divider circuits
3 resistors in parallel within the voltage divider circuits
output connectors for all the sensors
the power supply connector

Any additional slots were added as a safety precaution during the testing phases. The force sensors and accelerometer were placed along the perimeter of the board and the output connectors to feed out the data into the microcontroller were centered around the end of the board along with the power supply space.

External: User Relay

The external printed circuit board houses:

the microcontroller
display
a voltage divider circuit for the battery
the power insert from the battery

The external board was created after the internal board which influenced the design choices for this board. In order to make the connections more robust, the screw terminal block that was used for the internal board was replaced by a JST XH connector which requires wire clamping to create a stronger hold for the components.

Prototyping Concern

During final assembly, it was revealed that the microcontroller was consuming a lot more power than anticipated. Thus, changes were applied to the external PCB setup. Instead of using 2 resistors from the voltage divider circuit, a voltage regulator was used along with a capacitor. The new changes in the PCB solved the issue and ran the program without any issues.

Case Assembly

Internal & External Device Case

Materials Considered:

Poly Carbonate
Acrylonitrile Butadiene Styrene

Materials used:

Polyoxymethylene (Delrin)
Acetal Plastic
Metal Sheet (as cover for the lid)

Materials Recommend:

High-density Polyethylene (HDPE)

Battery

A 9V battery was chosen to supply power to the entire system. It was deemed easier to step down the voltage to the necessary amount than increase the 3V provided by the alternatives.

Using 3V for the display results in an extremely dimly lit display and thus 5V is required for visual acuity. This would also result in the highest watt hours resulting in minimal battery changes needed.

Below is the spreadsheet outlining the electrical components used in the final design and their power drawings.

Battery Budget

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