Preliminary Detailed Design
Table of Contents
Team VisionThroughout the Subsystems-Level Design Phase, Team BASE has worked to accomplish:
- All sensors functioning, their accuracy and locations determined;
- An in-ear PPG PCB layout developed and ordered; and
- Functional and visionary CAD models
A functional tree was developed in order to visualize all the drawings that will be required for a successful design.
PCB Layout and Schematics
To measure SpO2 in the ear, two custom PCB boards were designed:
- In-ear sensor: 7x11mm
- Main processing board: 11x11.5mm
Sensor Feasibility Testing
Principles of measuring SpO2:
1. Hemoglobin (Hb) and oxygenated hemoglobin (HbO2) differ in their absorption of red and infrared light (see Figure)
2. The volume of arterial blood in tissue (and therefore, the light absorption) changes during the pulse.
3. The ratio of red and infrared absorption correlates to SpO2.
PPG Sensor Hardware
PPG Sensor = 2 LEDs + photodetector
The photodetector measures absorbance of IR and Red LEDs. We need both AC and DC components to measure SpO2.
An R-value is calculated from the following ratio: R = (ACred/DCred)/(ACir/DCir)
The R-value is linearly correlated to SpO2 (see Figure)
Benchtop Feasibility Testing
- 3-minute long measurements sitting still
- Gold standard sensor and our sensor both on the index fingers
- 5 people, 3 tests each (n = 15)
- Average Percent Difference = 2.30%
- Ambient Light
- Skin pigmentation
- Pressure on sensor
Sensor Placement Testing
- Location 1: Eliminated due to hair
- Location 2: Good perfusion and waveforms due to carotid artery
- Location 3: Testing put on hold until in-ear PPG sensor is ready
Location (2) Results
- Average Percent Difference = 5.80%
- Dicrotic notch not visible in waveforms
Temperature & Humidity Sensor
Benchtop Feasibility Testing
The Environmental sensor was tested on a benchtop as well as mounted on the head. The head attachment device was comprised of a breadboard with the sensor and microcontroller attached. A bandana was utilized to secure the breadboard to the head.
- Procedure A: stable conditions on benchtop
- Procedure B: fluctuating conditions on benchtop
- Procedure C: stable conditions on head
- Procedure D: fluctuating conditions on head
- Measurement length: > 20 minutes
- Gold Standard: General Tools LCR318
- Average Percent Difference (Temperature / RH):
- A: 0.82 / -3.22
- B: 1.08 / 2.87
- C: 10.56 / 51.8
- D: 8.84 / 91.13
- Small samples were taken with the sensor and lab equipment. Since atmospheric pressure changes slowly and over large areas, these samples were taken over a number of days.
- % Error: 1.04
- Let sensor run sitting on a benchtop, not moving to verify accuracy of sensor.
- Move sensor in just one of three directions (x,y, and z) to ensure proper detection. Repeat this for all directions.
- Move sensor in all directions at once to ensure proper tracking.
- Test sensor on someone walking.
- Have correct acceleration data, but still need to interpret it into something useful.
Bill of Material (BOM)
The budget for our project is $3,500. So far we have spent $416.85 (See Figure for breakdown of purchases). Prior to design review 4, a more comprehensive list with future purchases will be developed.
The following schedule was developed to illustrate the tasks that need to be completed by the end of the semester (MSD I):
Risk AssessmentBased off the chosen design, the following risks were identified. Actions to minimize each risk were developed and will be implemented.
Design Review Materials
Plans for next phaseBy Design Review 4, Team BASE hopes to accomplish the following tasks:
- Custom in-ear PPG sensor built and working (RP: Andrew, Htet, Tegan)
- Feasibility and placement testing completed for all sensors (RP: Terry, Tegan, Andrew, Alex)
- PCB layout completed and ordered (RP: Htet)
- 3D Printed Functional Prototype (RP: Andrew)
- Visionary model of project (RP: Byung)