1. Principles and practices

This week, I started brainstorming my project idea. Because of my interest in both medicine and engineering, I choose a CPR training vest. This vest would use force sensors to collect data on compression rate and possibly compression depth.

Research

I was inspired by a presentation by Jenni Buckley who explained how to make the physical components of the vest using springs and metal plates. I hope to focus more on digitalizing the compression data.

After listening to Jenni’s presentation, I researched the amount of force applied during CPR. According to the CPR Certified Blog, around 60 lbs of force is applied if the compressions are 2 inches deep.

Sketch

I began by drawing a sketch on graph paper based on this design.

Here is my napkin and digital sketch.

Materials

In order to keep my project around $100-$200, I researched the foundational technologies needed for my project.

• Force Sensitive Resistor ($11.25)

• Load Sensor ($10.95)

In regards to the CPR dummy that will encase my device, I hope to borrow one of my school’s! After this course, I may look into developing my own dummy.

Origionally, I was considering testing on humans, but after discussion with my Fab Academy instructor (Mr. Taylor) and guru (Dr. Fagan), I found the ethical implications of human trials would be too complicated for my preliminary prototypes.

Our discussion also lead me to take interest into the innerworkings of a CPR dummy. I found that you can buy a compression assembly replacement! I will most likely begin assembling on this.

Code

This is sample code for a force sensitive resistor. I found the code while reading a the hookup guide. It relates force (g) and resistance (ohms).

Force Sensitive Resistor Hookup Guide

/******************************************************************************
Force_Sensitive_Resistor_Example.ino
Example sketch for SparkFun's force sensitive resistors
  (https://www.sparkfun.com/products/9375)
Jim Lindblom @ SparkFun Electronics
April 28, 2016

Create a voltage divider circuit combining an FSR with a 3.3k resistor.
- The resistor should connect from A0 to GND.
- The FSR should connect from A0 to 3.3V
As the resistance of the FSR decreases (meaning an increase in pressure), the
voltage at A0 should increase.

Development environment specifics:
Arduino 1.6.7
******************************************************************************/
const int FSR_PIN = A0; // Pin connected to FSR/resistor divider

// Measure the voltage at 5V and resistance of your 3.3k resistor, and enter
// their value's below:
const float VCC = 4.98; // Measured voltage of Ardunio 5V line
const float R_DIV = 3230.0; // Measured resistance of 3.3k resistor

void setup() 
{
  Serial.begin(9600);
  pinMode(FSR_PIN, INPUT);
}

void loop() 
{
  int fsrADC = analogRead(FSR_PIN);
  // If the FSR has no pressure, the resistance will be
  // near infinite. So the voltage should be near 0.
  if (fsrADC != 0) // If the analog reading is non-zero
  {
    // Use ADC reading to calculate voltage:
    float fsrV = fsrADC * VCC / 1023.0;
    // Use voltage and static resistor value to 
    // calculate FSR resistance:
    float fsrR = R_DIV * (VCC / fsrV - 1.0);
    Serial.println("Resistance: " + String(fsrR) + " ohms");
    // Guesstimate force based on slopes in figure 3 of
    // FSR datasheet:
    float force;
    float fsrG = 1.0 / fsrR; // Calculate conductance
    // Break parabolic curve down into two linear slopes:
    if (fsrR <= 600) 
      force = (fsrG - 0.00075) / 0.00000032639;
    else
      force =  fsrG / 0.000000642857;
    Serial.println("Force: " + String(force) + " g");
    Serial.println();

    delay(500);
  }
  else
  {
    // No pressure detected
  }
}

Downloads

In preparation for the course, I also downloaded the following programs:

• Arduino
• AutoDesk Eagle
• AutoDesk Fusion360
• Inkscape
• GitHub Desktop
• Brackets (Because of Gitlab, I beleive my use of this application will be minimal)

Markdown

I also started learning how to write in Markdown. Here is a key that I made using this cheatsheet.