P16214: Bicycle Power Meter
/public/

Subsystem Design

Table of Contents

This page shows the progress that was made by our MSD team as we worked towards finalizing our subsystems designs. Our systems design was broken into the individual components and systems that help to make up the operational bicycle power meter. The subsystems for the bicycle power meter are the crankarm/crankset, the strain gauges, the microcontroller with bluetooth capabilities, the accelerometer, the battery to be used to power the devices, and the smartphone app.

All of the documents from this phase of the design can be found in the Subsystem Level Design Documents.

Team Vision for Subsystem-Level Design Phase

During the Subsystem-Level Design Phase our MSD team planned to break the finalized system design in its individual subsystems. Our team also wanted to have strong subsystem designs that would prepare us for the detailed design. During this phase our team took these subsystems and developed design concepts for each of them. The design concepts for the subsystems also included some of the devices and products that will be used to develop our bicycle power meter.

Subsystem Design Selection

Our MSD team sat down to discuss which subsystems designs would work best with our overall system for the bicycle power meter. The selection process for the subsystem design was similar to the process that was used for the system design. A Pugh analysis table was created for each subsystem and the team discussed each table to figure out the best options and designs for the subsystems.

Strain Gauges

A closer look was taken at the strain gauges subsystem to see which type of strain gauge would be the best fit for our power meter. The strain gauges will be used to sense and measure the various flex and tension of the crank arm while a person is riding the bicycle. The information gathered from the strain gauges will then be used assist in calculating the riders' power that is produced. A few different design concepts were created to see which strain gauges would be most practical and accurate for this system. The generated concepts were then compared using the Pugh analysis and our MSD team discussed the results of the Pugh. The following figure shows the Pugh analysis table for the stain gauges subsystem:

Pugh Analysis for Strain Gauge Subsystem

Pugh Analysis for Strain Gauge Subsystem

After conducting the Pugh analysis and discussion within the team along with some preliminary calculations we decided on using the Omega Tee 90 degree Rossete 350 ohm strain gauge.

Microcontroller

The next subsystem that our MSD team designed was for the microcontroller. The microcontroller will be used to sample the output readings from the strain gauges and to process this information into packages that would then be sent using bluetooth to the smartphone app. Along with the information coming from the strain gauges the microcontroller will also be receiving information from the accelerometer which will also be packed and sent to the smartphone app. The team decided that the decision process for the microcontroller would be to select one that had bluetooth communication capabilities. The following table shows the outcome of our Pugh analysis for possible microcontroller design concepts:

Pugh Analysis for Strain Gauge Subsystem

Pugh Analysis for Strain Gauge Subsystem

After conducting the Pugh analysis followed by some discussions within the team we decided that the best design for our system was the Bluno Nano Arduino BLE Bluetooth Microcontroller.

Accelerometer

Pugh Analysis for Strain Gauge Subsystem

Pugh Analysis for Strain Gauge Subsystem

The next subsystem that our MSD team designed was for the accelerometer. The purpose of the accelerometer is to be able to measure the cadence of the crank arm. The cadence is the measure of the revolutions per minute of the crankset. The information from the accelerometer will then be sent to the microcontroller through a wired connection. Similar to the information from the strain gauges, the information acquired from the accelerometer will be sent via bluetooth to the smaprtphone app. The figure depicted on the right shows the Pugh analysis table that was created by our MSD team. After our team discussed the results of the Pugh analysis we decided that the Freescale FXLN8361Q Accelerometer Breakout Board would be the best option for our accelerometer subsystem.

Battery

Next our MSD team designed concepts for how the devices would be powered. For our subsystem our team decided that we needed a battery that would be small enough to fit on either the crank arm or the crankset and would also provide enough power to power the devices used for the power meter. This led our team to decide on using a coin cell battery. The following shows the Pugh analysis that our team performed on various types of coin cell batteries:

Pugh Analysis for Strain Gauge Subsystem

Pugh Analysis for Strain Gauge Subsystem

After completing the Pugh analysis and our team discussion we decided that the Renata CR2477N coin cell battery would be the best design for our subsystem.

Smartphone App

Pugh Analysis for Strain Gauge Subsystem

Pugh Analysis for Strain Gauge Subsystem

Finally our MSD team discussed which operating system to use for the smartphone app. The app will be used to receive the information from the microcontroller via bluetooth. The smartphone app will be designed to receive this information and to process it into the riders power they are producing as well as to process the information into the calories burned by the rider while they are riding the bicycle. Our MSD team discussed whether the app should be created using the Android operating system or the IOS operating system. Our team also discussed the feasibility and risks associated with using each system. The figure depicted to the right shows our Pugh analysis that was created for the app operating system. After performing the analysis and team discussion we decided that the IOS operating system would be the best option for our system as it will be easier to complete within the timeline required for this project.

Original Documents

The original and live documents for all of the subsystems Pugh analysis can be found within the Subsystem Level Design Documents folder.

Feasibility: Prototyping, Analysis, Simulation

After all of the subsystem design concepts were selected our MSD team performed a feasibility analysis on each system to see if it would be possible to place these subsystem designs within our overall high level system. The feasibility analysis of each subsystem was performed by executing calculations, carrying out analysis, and creating simple diagrams to help visualize our subsystems.

Strain Gauge

  1. Question: How much strain will be expected due to the poisson effect?
     Drawing for Crank Arm Strain Calculations

    Drawing for Crank Arm Strain Calculations

    • Poisson's Ratio Aluminum: 0.32
    • 0.32 = X/0.00029
    • Strain due to Poisson Effect: X = 0.0000928m/m
  2. Question: How much strain will be placed on the top of the crank arm?
    • Assumptions:
      • Force of 624N applied at end of crank arm
      • Crank arm length (172.5mm)
      • Crank arm thickness (20mm)
      • Assume cantilever beam
      • Elastic modulus of aluminum 69Gpa
    • Solution:

M=.1725 * 624 = 107.64 N­M

I= (bh^3)/12 = (.020 * .040^3)/12 = 1.07*10^­7

strain= MC/EI

strain= (107.64 * .020) / (69*10^9 * 1.07*10^­7) = .00029m

Component Placement

 Drawing for Crank Arm Strain Calculations

Drawing for Crank Arm Strain Calculations

Crankset Dimensions:

Component Dimensions

It has been determined that all of the electronic components will be placed on the non-drive side crank arm. This crank arm will have approximately 170mm x 38mm of mounting space available, which is enough to ensure that all of the components will fit within the profile of the crank arm. The strain gauges will be mounted on the top of each crank arm, which has approximately 170mm x 11mm of available mounting space. The strain gauges are 7.5mm x 10.8mm in size, so the top of the crank arm has sufficient space.

Accelerometer

Representation of G-forces

Representation of G-forces

Single Axis of 3-Axis Accelerometer:

Sensing Range = 0-1 G (G = 9.8 m/s2)

Angle Change = 90°

Accelerometer Spec = 230 mV/G

public/Photo Gallery/Subsystem Design/Microcontoller_mV-per-degree.JPG

Accelerometer 0G Voltage = 750 mV

Bluno Nano Microcontroller:

Analog to Digital Converter (ADC) = 10 bit (210 = 1024)

Internal Reference Voltage = 1.1V

public/Photo Gallery/Subsystem Design/Accelerometer Calculations.JPG

Location of Accelerometer (Worst Case Scenario):

public/Photo Gallery/Subsystem Design/Accelerometer on Crank arm Diagram.JPG

Microcontroller

Battery

SmartPhone App

Question: How much strain will be expected due to the poisson effect?

Original Documents

The original and live documents for all of the feasibility analytics can be found in the Subsystem Feasibility sub-folder of the Detailed Design Documents folder.

Drawings, Schematics, Flow Charts, etc.

To help our MSD team realize the flow of power and information we designed a flow chart. The flow chart is shown below:
Subsystem Flow Chart

Subsystem Flow Chart

Subsystem Test Plan

Our MSD created a test plan which contains a list of all of the tests we will conduct on our device to ensure that it meets all of our customer and engineering requirements. The test plan is shown below:
Subsystem Test Plan

Subsystem Test Plan

Original Documents

The original and live document for the subsystem test plan can be found within the Test Plan Spreadsheet.

Bill of Materials (BOM)

After creating our subsystems designs our MSD team then created a preliminary Bill of Materials for the products that will be used in our entire system for the bicycle power meter. The table below shows the preliminary Bill of Materials that our team created.
Component Product Cost
Microcontroller Development Board Bluno Development Board $35.00
Strain Gauge Omega Tee 90 degree Rossete 350 ohm strain gauge $50.00 (ea)
Microcontroller Bluno Nano Arduino BLE Bluetooth Microcontroller $33.50 (ea)
Accelerometer Freescale FXLN8361Q Accelerometer Breakout Board $10.00 (ea)
Battery Renata CR2477N Coin Cell Battery $4.99 (ea)
Smartphone App iOS Developers Fee $100.00

Original Documents

The original and live documents for the Bill of Material can be found in the Bill of Materials Spreadsheet.

Risk Assessment

After designing all of the subsystems our MSD team revised our risk management table and made updates to it as necessary. The risk management table for our project is shown below:

Risk Management

Risk Management

Original Documents

The original and live documents for the risk management can be found in the Subsystem Level Design Documents

Design Review Materials

The following is a link to the powerpoint presentation for the Subsystems Design Gate Review

After presenting our subsystems design review to both our customer and our guide our MSD team took away some advice and notes from what our customer and guide had to say. The following is a list of points that both our guide and customer thought we should make sure to discuss as a team and make sure we accomplish in the next phase:

After obtaining these words of advice from our guide and customer, our MSD team then set up some action items to take care of in the next phase of the design. The action items are as follows:

These action items were incorporated into our MSD team's plans for the next phase being the preliminary detailed design phase.

Plans for next phase

After completing the subsystem design phase our MSD team is looking forward to the Preliminary Detailed Design phase. The following is a list of plans and actions that our MSD team will complete in the next design phase:

Home | Planning & Execution | Imagine RIT

Problem Definition | Systems Design | Subsystem Design | Preliminary Detailed Design | Detailed Design

Build & Test Prep | Subsystem Build & Test | Integrated System Build & Test | Integrated System Build & Test with Customer Demo | Customer Handoff & Final Project Documentation