Final Design

Main Assembly Overview

Figure 1. CAD of Final Design

Description of the Design:

Main components of final design - The exploded view is shown in Figure 2 below

• Shell

• A LCD Display

• LED's

• Force sensitive resistors

• A Microcontroller and a circuit board

• A Load cell

• A Probe

Figure 2. Exploded View of the Design

Individual Components Analysis

• Shell

 The shell is the casing that protects all internal components, and is 3D printed in two parts the assembled with four M4 screws. This design was motivated by a flight game controller, and provides ergonomic grip for users. The material of shell is made of P430 ABS, which stands for Acrylonitrile Butadiene Styrene, similar to Acrylic but stronger than Styrene, and it is most commonly used for 3D printing. The size of shell is 20 cm (Height), 10 cm (Width), and 7 cm (Depth).

Figure 3. 3-D printed shell

• LCD Display

The main role of  LCD Display is output communication that provides the measured pressure with unit mmHG by using five buttons for easy interface. The display is embedded into the shell for simple design.

• LED's

 The LED's helps the user with easily finding correct location where the pressure being measured on patient. There are four LED's, placed above the LCD display with horizontally collinear to two LED's and vertically collinear to the rest. Each LED is connected to a force sensitive resistor, and is bicolored that provides two signals; Red and green. If the force acting on the force sensitive resistor exceeds the limited capacity, then the LED emits red. The green lights will be on only if the user correctly places them on the contact area of the patient. Otherwise it won't give the user any light.

• Force Sensitive Resistors

The force sensitive resistors mainly demonstrates the applied force on the contact area that is flatted for measurement. If all four force sensitive resistors are located correctly on the flattened area, then the measurements will be taken.

Figure 4. Force Sensitive Resistors around load cell

• Microcontroller

The microcontroller, Arduino Uno, is the main functional component that controls the output of the load cell and force sensitive resistor. The Arduino also amplifies their values if external force is within the threshold range.

Figure 5. Arduino of CAD design

• Load Cell

The load cell, one of the most important component, works like with a cantilever beam. Once the external force is by the bulging area, it causes deformation. The more force that is applied, the more deformation occurs, and the greater voltage output.

• Probe

The probe transfers the force applied on flattened area to the load cell.

Figure 6: Signal Flow Chart

Performance Results

Figure 7. The test result with force sensitive resistors

The prototype with force sensitive resistors were tested in the desired range of 0-100 mmHg. The resulting linear best-fitted line was , where x is the digital count and y is the pressure in cmH2O. The coefficient of determination R2 was determined to be 0.977. The amount of sample size drastically increased from 13 to 43, and the R2 did not decrease significantly, indicating this data is repeatable over a large range of pressure ranges. Though the measurements in high pressure values did not conform to the linear fit as well as the pressure values in the middle, the most needed measurements are in the range from 5 mmHg to 30 mmHg