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Group 27-SenseWalk . Mark Applegate (EE) Sara Belichki (CpE) Matthew Czarniak (CpE) Nicholas Heintze (EE). Motivation & Project Goals. Improve the independence and quality of life for the visually impaired. Optimize a tool commonly used by the visually impaired. How to we do this?.
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Group 27-SenseWalk Mark Applegate (EE) Sara Belichki (CpE) Matthew Czarniak (CpE) Nicholas Heintze (EE)
Motivation & Project Goals • Improve the independence and quality of lifefor the visually impaired. • Optimize a tool commonly used by the visually impaired.
How to we do this? • Sonar:detect upcoming obstacles and vibrate a motor attached at the user’s wrist. • GPS: the SenseWalk will hold the user’s saved routes to guide them when needed. • Bluetooth:the user will be alerted of upcoming instructions as they go about a route.
Sense Walk System Design MCU GPS Sonar SD Card Digital Compass Bluetooth Power Supply
Microcontroller MCU GPS Sonar SD Card Digital Compass Bluetooth Power Supply
Microcontroller Requirements • The SenseWalk requires a microcontroller that contains • High flash memory-between 256 KB-512 KB • High frequency • ADC peripheral • Supports UART connection • Low power
Microcontroller Options MSP430F6659 ATmega2560
Sonar Design MCU GPS Sonar SD Card Digital Compass Bluetooth Power Supply
Sonar Specs and Requirements • Distance From 1 meter to 3 meters • Sonar operate at 40kHz • Not affected by noise • Microcontroller calculate distance • Vibrate a motor at different magnitudes proportional to distance
Ultrasonic Drive Circuit Options • 555 Timer • Square wave is highly distorted by load. • Phase shift Oscillator • Square wave is distorted by load. • 40kHz Crystal- CMOS Inverter Oscillator • Minimum distortion
Ultrasonic Transmitter Circuit Hex Inverter
Ultrasonic Receiver Circuit Quad Op Amp
Bluetooth MCU Sonar Bluetooth GPS Digital Compass SD Card Power Supply
Bluetooth Requirements • Only needs to be able to cover a short distance (Class 1 BT modules have a range of ~100 m, Class 2 ~10 m). • Needs to have a good data rate to ensure no delay. • Use UART to connected to embedded system. • Low power consumption. • Must have appropriate Bluetooth protocol to send out audio messages to headset.
Bluetooth Comparison PAN1315 RN-42
Navigation Design MCU GPS Sonar SD Card Digital Compass Bluetooth Power Supply
SD Card & Breakout Board • San Disk 4 GB • Petite FatFS • SD Library
Loading Waypoints on SD Card • Use route planner to create CSV / GPX • Upload CSV file with waypoints to SD
Software Class Diagram SenseWalk SDCard Compass Bluetooth Sonar GPS Init() getDirection() getCurrentCoordinates() getCurrentWaypoint() sendBluetooth() calculateSonar() determineRelativePos() determineCorrection() Sonar SDCard Bluetooth Compass GPS determineDistance() sendMotorCommand() ConnectHS() Transmit() OpenFile() CloseFile() ParseWaypoints() CurrentWaypoint() calibrateCompass() currentDirection() establishConnection() currentCoordinates()
Power Supply MCU GPS Sonar SD Card Digital Compass Bluetooth Power Supply
AC-DC Efficiency Requirements • Energy Star Requirements have become industry standard “AC-DC External Power Supply- An external AC-DC power supply is an EPS designed to convert line voltage ac input into lower voltage dc output.” [1] Output Power: 14.4W Minimum Efficiency Requirement: 73% [1] http://www.energystar.gov/ia/partners/prod_development/revisions/downloads/FinalSpecV2.pdf
Flyback Switching Converter • High Efficiency 80+ % capable. • Least costly of all isolated switching power supplies. • Full isolation capable. • Ideal for 0-200Watts. • Limited to 200Watts due to high peak switch currents. Magnetic & Capacitive Component Issues Controller & FET selection EMI design issues
Continuous Conduction Mode VS Discontinuous Conduction Mode CCM DCM DCM • Lower energy in transformer= smaller transformer • Less windings = lower I2R Losses • Higher ripple current = output capacitor ESR requirements • Higher transistor and diode peak current requirements CCM Lower ripple current Lower transistor and diode peak current requirements Right Half Plane Zero requires complicated control loop Higher energy in transformer = larger transformer
Transformer Design DCM • Primary Inductance: 1.078mH • Primary Peak Current: 0.75A • Max Input RMS Current: 0.32A • Max Output RMS Current: 2.08A • Turn Ratio: 8.94 • Primary & Secondary Wire Size • Core Material Selection • Gap Length, Total Winding Area
MOSFET Selection • Power MOSFET required - 600V and >2 Amps. • Experiment with MOSFETs to find best fit for efficiency. • Switching losses - Low parasitic capacitances and fast rise/fall times will combat switching losses. • Conduction losses - Low Rds(on) will combat conduction losses.
Li-ion Switching Battery Charger • Three states: Preconditioning, Constant Current, Constant Voltage [2] Courtesy of Texas Instruments
Li-ion Battery • Capacity: 2600 mAh • Working Voltage: 7.2V • Peak Voltage: 8.4V • Cut off voltage: 5.5 V • Max Charging Current: 2 Amp • Length: 2.8" • Width: 1.45" • Height: 0.8" • Weight: 3.5 Oz
Linear Vs Switching Regulators Linear • Low Cost • Low Design Complexity • Low Noise • Small size • Poor Efficiency <40% • High Heat Dissipation Switching • High Efficiency 80+% • Extended Battery Life • Higher Cost • Higher Design Complexity