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.
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:
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.
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:
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.
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.
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:
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.
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.
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.
Question: How much strain will be
expected due to the poisson effect?
- Poisson's Ratio Aluminum: 0.32
- 0.32 = X/0.00029
- Strain due to Poisson Effect: X = 0.0000928m/m
Question: How much strain will be
placed on the top of the crank arm?
- 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
M=.1725 * 624 = 107.64 NM
I= (bh^3)/12 = (.020 * .040^3)/12 = 1.07*10^7
strain= (107.64 * .020) / (69*10^9 * 1.07*10^7) = .00029m
- Length: 172.5mm
- Width: 38mm
Available Mounting Space:
- Drive Side ~ 86mm x 38mm
- Non-Drive Side ~ 170mm x 38mm
- Microcontroller Dimensions: 53mm x 19mm
- Accelerometer: 21mm x 18mm
- Battery: 25mm diameter
- Strain Gauges: 7.5mm x 10.8mm
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.
Single Axis of 3-Axis Accelerometer:
Sensing Range = 0-1 G (G = 9.8 m/s2)
Angle Change = 90°
Accelerometer Spec = 230 mV/G
Accelerometer 0G Voltage = 750 mV
Bluno Nano Microcontroller:
Analog to Digital Converter (ADC) = 10 bit (210 = 1024)
Internal Reference Voltage = 1.1V
Location of Accelerometer (Worst Case Scenario):
- Based on placing the accelerometer in a location where it will experience more than just the acceleration due to gravity
- This location would be anywhere on the crank arm other than the axis of rotation
- Analog Inputs
- Sensors require a total of 6 analog inputs
- Microcontroller has 7 available analog inputs
- Input Voltage Required
- Microcontroller supply voltage must be low to keep battery size low
- Bluno Nano has a supply voltage of 5.5V max
- Must be powerful enough to sample sensor data and package for wireless transmission through BLE
- Bluno Nano uses an Atmel 20MHz Atmega328 processor with an 8-channel 10-bit ADC
- Very little memory needed; only storing algorithm and small packages of data for short periods of time
- Atmega328 has 32K flash, 1K EEPROM, 2K RAM
- Ability for iOS App to process data from the microcontroller
- iPhone capable of receiving BLE
- Available iPhones to use during ImagineRIT: 3
Question: How much strain will be expected due to the poisson effect?
- Will be based on the accelerometer and strain gauge and how much power they consume
- Efficiency of algorithm will also determine how long the battery will last
- Will be narrowed down once algorithm is finalized
- Worst Case: Buy multiple batteries and stack them if the Bicycle Power Meter requires more power
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 Test PlanOur 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:
Original DocumentsThe 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.
|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|
The original and live documents for the Bill of Material can be found in the Bill of Materials Spreadsheet.
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:
The original and live documents for the risk management can be found in the Subsystem Level Design Documents
Design Review MaterialsThe 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:
- Break out how much power each component draws so that
we can be more sure of how much battery capacity we need
for our battery selection
- Calculate the worst-case power draw for all subsystems
- Compare this value to what we plan to use for battery
- Put the backup plan, being the backup microcontroller and display out, into our budget. (In case the Smartphone app falls through)
- Build a schematic for the strain gauge and
- From the strain gauge to the microcontroller – we may need some added circuitry here
- From the accelerometer to microcontroller
- Show this schematic to Prof. Slack or another subject matter expert to make sure that they don’t see any issues with our design and they think that it will work
- Finalize the housing/protection for accelerometer,
microcontroller, and other components
- There is the risk of kids kicking the device getting onto and off of the bicycle
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:
- Look into housing/protection for components and draw up a preliminary CAD design
- Purchase our microcontroller development board and start to tinker with it to familiarize ourselves with it for some preliminary prototyping
- Update the risk assessment
These action items were incorporated into our MSD team's plans for the next phase being the preliminary detailed design phase.
Plans for next phaseAfter 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:
- Gather Bike Crankset Parts
- Continue to Research App Development iOS
- Draw Preliminary CAD Design for the Bicycle Power Meter
- Finalize Subsystem Prices
- Purchase Subsystem Parts
- Finalize Detailed Design