P16214: Bicycle Power Meter
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Preliminary Detailed Design

Table of Contents

This page shows our MSD team's progress as we began to finalize our design decisions. The key goal of the Preliminary Detailed Design phase is to take our highest risk items and focus on narrowing down the design decisions for these items. The high risk items will be mitigated by conducting more analysis and testing on these particular risk items. This page shows the results of the further testing and analysis.

The original documents for this phase can all be found in the Preliminary Detailed Design Documents sub-folder.

Team Vision for Preliminary Detailed Design Phase

Our vision for this design phase, the preliminary detailed design phase, is to focus on the high risk items and mitigate any issues. We are on track to completing the bicycle power meter by ImagineRIT. The purpose of this phase is to have most of our design concepts completed and finalized.

Prototyping, Engineering Analysis, Simulation

Strain Gauge Circuitry

After conducting some more analysis and calculations with sampling signals from the strains gauges our MSD team came to the realization that the strain gauges being used would not be able to produce a large enough signal that could be read at the microcontroller and used to distinguish variation in the forces applied. This led the team to look at other possible designs that could be used to amplify the signal to a value large enough that our microcontroller would be able to notice variations in the strain gauges. After performing some extra research and speaking with a Subject Matter Expert, our team decided that we would amplify the signal coming from the strain gauges using a 'wheat-stone bridge'. The circuit for the wheat stone bridge is shown below.

For our purposes, Ra and Rb are fixed to the nominal value of the strain gauges, R1 and R2. Nodes 1 and 2 will be connected to an instrumentation amplifier in order to achieve the desired gain/signal strength.

Ra = 350 ohm;

Rb = 350 ohm;

R1: Strain gauge; nominal value = 350 ohm;, maximum change = 0.35 ohm;

R2: Strain gauge; nominal value = 350 ohm;, maximum change = 0.35 ohm;

If we choose V+ (supply voltage) to be = 1V

R1 and R2 will change by an equal amount in opposite directions; i.e. if there is maximum change, R1 = 350.35 ohm; and R2 = 349.65 ohm;. Thus:

public/Photo Gallery/Preliminary Detailed Design/Wheat Stone Bridge Equation_1.JPG

We want our amplified signal to be 1.1V, which is the reference voltage which will yield the highest accuracy for data acquisition. Thus:

public/Photo Gallery/Preliminary Detailed Design/Wheat Stone Bridge Equation_2.JPG

Due to these calculations and analysis we need to choose an R value for our amplifier in order to achieve approximately 33dB gain.

Original Documents

The original documents for the strain gauge wheat-stone bridge analysis and calculations can be found in the Preliminary Detailed Design Documents sub-folder.

Crankset Component Placement

Further analysis was also taken to verify that all of our selected components would in fact be able to fit onto the crankset. The following pictures show the prototyping that was performed using cardboard cutouts of the actual dimensions of our selected components that will be used.

1.) public/Photo Gallery/Crankset Component Placement_1.jpg 2.) public/Photo Gallery/Crankset Component Placement_5.jpg 3.) public/Photo Gallery/Crankset Component Placement_6.jpg

4.) public/Photo Gallery/Crankset Component Placement_2.jpg 5.) public/Photo Gallery/Crankset Component Placement_7.jpg 6.) public/Photo Gallery/Crankset Component Placement_3.jpg

7.) public/Photo Gallery/Crankset Component Placement_4.jpg 8.) public/Photo Gallery/Crankset Component Placement_8.jpg

The pictures show the location where each component will be placed. Image 3.) shows that the accelerometer will be placed inside of the spindle of the crankset. The reason for doing this is to allow the accelerometer to be placed directly at the center of the axis of rotation for more accurate results. Another reason for placing the accelerometer inside of the spindle is to protect it from getting accidentally getting kicked or damaged.

Power Consumption Analysis

The System Components that will be used in the power meter that will consume power are listed below:

The following shows the calculations and analysis that were performed for each system component to see how much of the total power available they will use.

Battery:

Microcontroller:

  1. Maximum possible instantaneous power consumption
    • PMAX = (VMAX)(IMAX) = (5V)(500mA) = 2500mW = 2.5W
  2. Maximum possible total accumulated power consumption
    • Pa MAX = (PMAX)(Event Length) = (2.5W)(10hr) = 25Whrs
  3. Expected average instantaneous power consumption
    • PAVG = (Vin)(IAVG) = (5V)(18mA) = 0.09W
  4. Expected average total accumulated power consumption
    • Pa AVG = (Vin)(IAVG)(Event Length) = (5V)(18mA)(10hrs) = 0.90Whrs

The following chart shows all of the compiled results of the power consumption of the system components:

public/Photo Gallery/Preliminary Detailed Design/Power Consumption Chart_11-18-15.JPG

The graph above shows the total battery power (green), the maximum power consumption our microcontroller is able to draw from a power supply (blue), and the total power consumption we expect to see (red).

The above analysis shows that the total battery we choose to use will provide enough energy to supply our system for the 10 hours of the Imagine RIT Event.

Original Documents

The original documentation for the power consumption analysis can be found within the Preliminary Detailed Design Documents sub-folder.

Drawings, Schematics, Flow Charts, Simulations

The following figure shows the schematic that was built for the wheat-stone bridge amplifier circuit:

Amplifier Circuit Schematic

Amplifier Circuit Schematic

This circuit was simulated using Orcad CIS and produced the following results:

Amplifier Circuit Simulation Results

Amplifier Circuit Simulation Results

Bill of Material (BOM)

The updated bill of material is shown below.

Bill of Material

Bill of Material

Original Documents

The original and up to date document for the bill of material can be found within the Project Management sub-folder.

Test Plans

The following table shows the updated test plan for our subsystems. The updated test plan now has an owner for each test being performed as well as a time for when the test should be completed by.

Updated Test Plan

Updated Test Plan

Some of the selected tests were completed during this phase while other tests have been started and are in progress. The following items show a more in depth description of each test being performed with results for the tests that have been completed.

Battery

The battery selected for our power meter is rated at a capacity of 950mAh. This rating will be tested by using a simple circuit to draw the more than the maximum amount of current our microcontroller can draw (500mA) at any one time. An ammeter and a voltage meter will be used to measure the current and voltage of the battery and these values will be logged every 5 minutes until the test is complete. The data points will then be plotted to show the lifetime characteristics of the battery.

Strain Gauges

The strain gauge used for this test is an omega 90 degree rosette. The strain gauge will be mounted to the crank arm to measure bending strain. The crankset will be clamped, and force will be applied to the pedal via hanging calibrated weights. The objective of this test is to be able to predict the force that is being applied to the pedal from the strain gauge signal. Since the crank arm length is constant, the force can be found by dividing the torque by the moment arm. The amplifying circuit will be used to create a more readable signal from the strain gauge. The amplified signal will be imported to labview using a NI Daq device for data analysis. The test procedure is outlined below:

  1. Solder leads to strain gauge
  2. Mount strain gauge to top of crank arm
  3. Clamp crankset
  4. Connect strain gauge to amp
  5. Connect amp to NI Daq device
  6. Apply calibrated weights to pedal
  7. Import data, and create force vs. strain signal curve

Microcontroller

The microcontroller to be used for this project is the DFRobot Bluno Nano Arduino BLE Microcontroller. A Bluno Nano will be mounted on the crank arm of the RIT Cycling Bike Blender for the final product and a development board version, the DFRobot Bluno Arduino BLE Microcontroller, will be used for testing.

Section 4.1: BLE

The first tests to be run for the microcontroller subsystem will involve the BLE communications. Items to be tested include the functionality of the BLE communication, the BLE range between the Bluno (development board) and the mobile phone, the BLE range between the Bluno Nano and the mobile phone, and the BLE range between the Bluno Nano and the Bluno.

BLE Test Diagram

BLE Test Diagram

Section 4.1.1: BLE Functionality Test:

The functionality of the BLE communications will be tested for each of the cases listed above by following the procedure below:

  1. Connect the microcontroller to the phone using Evothings Workbench and its provided simple sketch.
  2. Verify that a connection is made.

The test results will be measured using a binary scale. If the microcontroller is able to connect to the phone (or other microcontroller), the result of the test is pass; if the microcontroller is unable to connect to the phone (or other microcontroller), the result of the test is fail. It is expected that each item will pass this test.

The test is still on going and the results of this test have not been completed yet.

Section 4.1.2: BLE Range Test:

The range of BLE communications will be tested for each of the three cases/items listed above. This will be done by the following procedure:

  1. Connect the microcontroller to the phone using Evothings Workbench and the provided Arduino sketch
  2. Once the BLE is connected, begin to increase the distance between the devices
  3. Continue to increase the distance until the connection is lost
  4. Find the maximum distance by decreasing the distance between the devices until the connection is re-established
  5. Measure the distance between devices
  6. Repeat steps 2 - 5 three times and take the smallest recorded distance as the maximum BLE range

It is expected that the BLE range will be greater than or equal to 20 meters.

This test is also on-going and the results of this test have not yet been completed.

Crankset Layout

This test focuses on the Component layout as a mock up. Cardboard cutouts of each components were made and placed on the crankset. The test is used to confirm that each component will fit on the crankset.

This image shows the proposed locations for the microcontroller, amp circuit and accelerometer

This image shows the proposed locations for the microcontroller, amp circuit and accelerometer

 This image shows the proposed location for the battery and drive side amp circuit

This image shows the proposed location for the battery and drive side amp circuit

The results from the Crankset Layout test are shown below:

Results of the Crankset Layout Test

Results of the Crankset Layout Test

All of the components fit on the crankset. The proposed layout will be used to make the cad drawings for the full assembly.

Accelerometer

The reason for having the accelerometer is to allow for the cadence (revolutions per minute) to be measured. The accelerometer will be placed in the center of the spindle of the crank set for the Power Meter. The reasoning for having the accelerometer in the spindle is to protect it from any damage that may occur when riders are getting on and off of the bicycle or from riders whose feet slip off of the pedals and accidentally kick the accelerometer. Another reason for placing the accelerometer in the spindle is to allow for the accelerometer to be at the center of the axis of rotation so that the only acceleration that it will feel is that which is due to gravity. This will allow for the readings from the accelerometer to be most accurate. To ensure that the accelerometer is giving accurate results it must first be tested to show that it is producing the expected results.

The following are the steps to test the accelerometer accuracy:

  1. Place the accelerometer, and microcontroller onto a breadboard
  2. Connect the output from each axis of the accelerometer to an input pin on the microcontroller
  3. After powering the devices, choose one axis (x, y, or z) to isolate.
  4. Rotate the breadboard with the devices on it to known angles (0 deg, 90 deg, and -90 deg) and record the output voltage of the isolated axis.
    • Also record the number of bits used for each angle (using Arduino code for the Bluno
    • Make sure that each angle matches up with a particular orientation of the axis with respect to gravity (-1g, 0g, 1g)
  5. Repeat Step 4. for the other 2 axes

After performing this testing this will then allow the microcontroller to be coded to connect a particular angle with each different voltage that is produced. This test will be verified using the calculations that were done previously to determine the level of degree sensitivity of the accelerometer.

The results of this test are as follows:

After conducting the accelerometer testing it was seen that the x-axis and the y-axis produced values for the voltage that were close to the expected values as taken from the data sheet for the accelerometer. However, the z-axis did not produce the expected results taken from the data sheet. The recorded voltages for each axis are shown in the table below:

Results of the Accelerometer Test

Results of the Accelerometer Test

Due to the values of the z-axis not being the correct values, calculations were performed to verify that these incorrect values would be good enough to perform the task at hand. After the calculations it was determined that the z-axis could still detect the angle up to a 0.5 degree accuracy which is more than sufficient for the bicycle power meter application.

Strain Gauge Amplifier Circuit

Due to the very small range of change in signal value for the strain gauges, extra circuitry will be needed in order to amplify the strain gauge outputs before being measured by the microcontroller. A wheat-stone bridge will be used in order to create a circuit which compares the varying strain gauge signal with a non-varying signal. The difference in voltage between these two signals will be extremely small, in the range of 0V - .0005V, so the signals will be passed to an instrumentation amplifier in order to achieve a range of approximately 0V - 1.1V. This will allow the microcontroller to measure varying strain with acceptable accuracy. The amplifier will need to yield a gain of approximately 2000V/V, or 33dB.

The instrumentation amplifier to be used is the Texas Instruments INA2126. In order to test the amplifier, the circuit below will be constructed using a breadboard, the INA2126, and several resistors:

Amplifier Circuit Schematic

Amplifier Circuit Schematic

R5 will be chosen to be approximately 40.1 ohm, which will set the instrumentation amplifier to 2000V/V gain. R1 and R2 will represent the strain gauges, and will be chosen to be 349.65 ohm and 350.35 ohm in order to represent a maximum strain. R3 and R4 will be fixed to the nominal value of the strain gauges, 350 ohm. A sinusoidal voltage will be supplied to the wheat-stone bridge, then the input and output of the amplifier will be measured in order to verify the gain. The node between R1 and R2 (green probe) is the input measured and the output at the top op-amp (purple probe) is the output. It is expected that a gain of 2000V/V will be measured.

The test results of the amplifier circuit are as follows:

Amplifier Circuit Simulation Results

Amplifier Circuit Simulation Results

The output peak voltage in the simulation was 0.9974V and the input peak voltage was 499.815uV, corresponding to a gain of 1995.54V/V. This is very close to the expected value of 2000V/V.

Original Documents =

The original documents for the test plans can be found in the Preliminary Detailed Design Documents. Click the following link for the Updated Subsystem Test Plan. Also click the following link to see the Original Test Plan Document

Design and Flowcharts

Our MSD team came up with some design ideas for our bicycle power meter. The following image shows the updated systems architecture.

Updated Systems Architecture

Updated Systems Architecture

Our team also came up with a preliminary design concept for the crankarm housing that will protect our components and devices from being damaged. The following image shows the preliminary concept design.

Crankarm Housing Concept

Crankarm Housing Concept

Risk Assessment

After making some final design decisions our MSD team updated our risk assessment to show the most up to date risks that are team is facing going into the Detailed Design phase. The high risk items in this updated risk assessment will be key items and tasks to overcome in the Detailed Design phase. The most recent risk assessment is shown below.
Risk Management

Risk Management

Original Documents

The original and live document for the risk assessment can be found in the Preliminary Detailed Design Documents sub-folder.

Design Review Materials

The powerpoint presentation for the Preliminary Detailed Design Review can be found with the following link Preliminary Detailed Design Review

After the Preliminary Detailed Design review our MSD team was left with some take-aways and advice to look into during the Detailed Design phase. The take-aways from the review are as follows:

Plans for next phase

The plans for next phase are as follows:


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