Bike Buddy Group 15 Sponsored By: Ari Nacius Progress Energy Nowook Park Ethan Pemble Nick Quinlan
Introduction Speed: 18 MPH Dir:28.53˚N,-81.20 ˚W T:85 ˚F P=4.2W • Bike Buddy uses a portable AC generator to harness power from pedaling. • It attaches to the bicycle and displays riding information. • Speed (mph) and direction • Lat./Long. coordinates • Ambient temperature • Power generated by pedaling (Watts) • It also supplies power to USB devices. [picture]
Goals & Motivation • Current portable bicycle generators are primarily used to power headlights. • Our goal is to expand on possible applications of this alternative energy source by providing additional features to the bike rider. • Provide accurate data to the user while efficiently powering all systems with the AC generator.
Power Flow Diagram AC generator Pedal the bike Power System DC converter Battery switcher Battery charger 6v regulator 5v regulator 3.3v regulator Display System USB µC GPS LCD
Building the Generator • Initially we wanted to design our own custom generator. • Instead of spending time on designing a generator we decided to concentrate on the capabilities of the LCD and sensing functions. • Because it was vital to provide constant power for the rest of the project to work, we thought it best to purchase one instead. Example of a home-made electric generator
Choosing a Generator • Pros: • Less energy loss to friction • Sleek design • Cons: • More expensive • Custom Wheel Needed • Low Power Output • Pros: • Higher Power Output • Cheap • Cons: • Energy Loss in wet or muddy conditions • Produces buzzing noise
Power Supply AC/DC conversion • No need for a step-up or step-down transformer. • Full Bridge rectifier using 4 schottky diodes for low voltage drops. • A 50V 2200 uF electrolytic capacitor is used to minimize the ripple before regulation. • A voltage regulator (LM317) regulates the voltage to a constant 10V. Formulas to find the average DC Voltage from the generator , When Vrms = 30V, Vdc = (30V x 1.414)/3.14 = 13.5V
Specifications & Requirements Lithium-Ion BatteryPack (2 Cell) • Capacity: 1400mAh • Voltage: 7.4 V (8.4 V pk) • Dimensions: 51mm x 38.1mm x 19mm • Weight: 2.5 oz • Maximum Charge Current: 1C or 1.4A • Maximum Current Draw: 0.87 A
Li-Ion Battery Charger Charger Comparison Table
Li-Ion Battery Charger Typical Charge Profile • MCP73842 manufactured by Microchip to charge an 8.4V Li-Ion battery. • Programmable Charge Current. • Programmable Safety Charge Timers. • Preconditioning of Deeply Depleted Cells. • Automatic End-of-Charge Control. • Continuous Cell Temperature monitoring • Automatic power-down when input power is removed to prevent battery discharge. Ctimer = 0.033uF Tprecon= (Ctimer/0.1uF) X 1 hr = 19.8 mns Tfast-charge = (Ctimer/0.1uF) X 1.5 hrs = 29.7 mns Tterm= (Ctimer/0.1uF) X 3 hrs = 59.4 mns
Li-Ion Battery Charger (cont’d) For a maximum charge current of 2A, RSENSE is calculated using the formula in the datasheet. RSENSE = 120mV / 2A = 60m • The charger turns off when the battery reaches a temperature limits of 10 F and 80 F. • Those temperature limits are set using two resistors Rt1 and Rt2 Rt1 = (2 x 10 x 100)/(100 - 10) = 22 Ohms Rt2 = (2 x 10 x 100)/(100 - 3 x 10) = 28.57 Ohms Practical values: Rt1 = 22.22 Ohms Rt2 = 29.00 Ohms *maufacturer-recommended design configurationation.
2 comparators to monitor and compare the battery voltage levels with a reference voltage of 3.5V. 6 p-channel mosfets used to switch the batteries when the source battery reaches 3.5V. A zener diode is used to keep the reference voltage at a constant 3.5V. The 100uF capacitor is to ensure that the output doesn’t change during the switch. Battery Switcher
Battery Switcher Switcher Profile • One battery powers the unit while the other is being charged. • Switch happens when the battery powering the unit reaches 3.5V. • 3.5V is the minimum input voltage range of switching regulators that power the subsystems. • The power source switch does not affect the operation of the unit.
Switching Regulators Vref=1.5 V, 10kΩ≤R1≤ 500kΩ R2=R1*(Vout/Vref-1) For Vout=6V: R1=10kΩ, R2=30kΩ
Switching Regulators • The MAX608 low-voltage step-up controller operates • from a 1.8V to 16.5V input voltage range. • Pulse-frequency-modulation (PFM) control provides high efficiency at heavy loads, while using only 85μA (typical) when operating with no load. • In addition, a logic-controlled shutdown mode reduces supply current to 2μA typical. The output voltage is factory-set at 5V or can be adjusted from 3V to 16.5V with an external voltage divider. • The MAX608 operates in “bootstrapped” mode only (with the chip supply, OUT, connected to the DC-DC output). The two bootstrap capacitors and are employed on both sides of inductor to provide gate voltage to high side input switch through high side driver in any mode of operation. This allows the regulator to work in all three modes of operation without different external components or configurations depending on the mode.
The ACS756 current sensor needs a single +3 to+5V supply. Ultra-low power loss: 130uOhm internal resistance. 13kVRMS isolation voltage between terminals 4/5 and pins 1/2/3. Output voltage proportional to AC and DC current. 20mV/A output sensitivity. Nearly Zero magnetic hysteresis Current Sensor
The ADC pins have a voltage range of 0V to 5V. But since the internal reference voltage is 2.56V, our input voltage must not reach that level. We use a voltage divider to prevent the attempted maximum voltage from the generator from reaching 2.56V on the ADC pins. Voltage interval = 2.56V / 1023 = 0.0025V At every 2.5mV increment, a binary data is recorded and stored in a data register. Since we want the recorded voltage to be accurate to 1/10 of a volt, we select resistor values that will increment the stored binary data at every 1/10 of a volt. 0.0025 = (1/10)(R2/(R2+R1)) 1/40 = R2/(R1+R2) R1 = 39K, R2 = 1K Analog-to-Digital Converter
The ADC is used to measure the power generated by the generator by monitoring the voltage and current. The current sensor output is connected to a similar voltage divider as the one on the right for the battery. Since we don’t want to drain the batteries, we use a to isolate the batteries from the voltage divider. Analog-to-Digital Converter (cont’d)
Display System Goals • Small and power efficient • Peripheral sensors to provide information to the rider: • Speed and direction • Power generated • Global position • Ambient temperature • Time of day Lat: 28.60265 12:37 PM Lon: -81.23185 67 °F NE 6.73 MPH Generating 3.7 Watts µC Power sensor GPS
Liquid Crystal Display • Serial Graphic LCD from sparkfun • Provides simple 1-wire serial interface with built-in commands and character display. • 128x64 pixel space • Software-scalable backlighting for indoor/outdoor use • Operates at 6v, average current draw ~125 mA (with full backlighting)
GPS Receiver: LS20031 • The LS20031 GPS unit has an embedded antenna and simple TTL serial interface. • Built-in battery stores satellite positions for rapid startup. • 3.3v @ 41 mA
Development Board: STK-300 • RS-232 port for USART communication. • Simple USB programmer for quick prototyping. • Provides 8 buttons and LEDs for testing. • External 8 MHz crystal provided for source clock. • Includes C compiler (WinAVR) and AVR Studio 4 development environment.
Software Overview Initialize Serial Devices Power Switch ON Retrieve Sensor Data Retrieve Power Sensor Data (ATD) Retrieve GPS Data (USART1) Retrieve Temperature Data (TWI) Format numbers for display Update Display (USART0) Timer overflow? Stand-by
ATmega128 Timer • Timer1: 16-bit timer • System clock rate: 7.3728 MHz • Prescaler: divide-by-1024 • Tic: 7.2 kHz • Overflow: 9.1 ms • Desired period: 300ms or 2730 overflows The sensor update loop is driven by a timer, and executed every 300ms. The screen will update roughly 3 times per second.
USART on the ATmega128L USART is dependent on the internal system clock and is highly sensitive. To reduce data error rates, an external system clock rated at 7.3728 MHz is chosen. Both devices (the LCD and the GPS receiver) are configured to transmit at 9600 bps with 8 data bits, 1 stop bit, no parity bit. C1 = C2 = 15 nF
LCD Commands Wrapper functions Native Commands int lcd_clearScreen() int lcd_setBacklight(int) int lcd_setPixel(int,int) int lcd_setX(int) int lcd_setY(int) int lcd_drawLine(int,int,int,int) int lcd_drawCircle(int,int,int) int lcd_drawBox(int,int,int,int) int lcd_erase(int,int,int,int) Drawing
Parsing GPS Information • The only NMEA record used in the design is the Recommended Minimum Specific GNSS Data (RMC), which provides UTC time, date, latitude, longitude, speed over ground, and course over ground. $GPRMC,053740.000,A,2503.6319,N,12136.0099,E,2.69,79.65,100106,,,A*53
DS1625 • Data is read from / written via a 2-wire serial interface(open drain I / O lines) • Temperature measurements require no external components • Measures Temperatures from -55°C to +125°C (-67°F to +257°F) • Converts temperature to digital word in 500 ms • Temperature is read as a 9-bit value (two byte transfer)
DS1625 = -25°C
Two Wire Interface (TWI) • A popular serial peripheral interface bus • TWI stands for Two Wire Interface and for most parts this bus is identical to I²C. • The name TWI was introduced by Atmel and other companies to avoid conflicts with trademark issues related to I²C. • -More flexible than SPI (Serial Peripheral Interface ) • -Master and slave modes supported • -7-bit slave address • -Bidirectional, open-drain bus (device pulls down, resistors pull up) • -Two wires, SCL, (clock) and SDA (data) Typical TWI bus configuration
Two Wire Interface • A TWI transmission consists of • Startcondition • An address packet consisting of • -Read/Write indication and • -Slave acknowledge, (SLA+RW) • One or more data packets • Stopcondition • A Start condition initiates a transmission by a master. • Between Start and Stop conditions, the bus is busy and no other masters should try to initiate a transfer. • A Start condition is signaled by a falling edge of SDA while SCL is high.
Two Wire Interface Address packet • -Address packet is 9 bits long • -MSB first • -Address “000 0000” is reserved for broadcast mode • -7 address bits (driven by master) • -1 read/write control bit (driven by master) • -1 acknowledge bit (driven by addressed slave)
Two Wire Interface • Data packet • -All data packets are 9 bits long • -MSB first • -One data byte plus an acknowledge • -During a transfer, Master generates SCL, Receiver acknowledges • -Acknowledge (ACK): Slave pulls down SDA in the 9th SCL cycle • -Not Acknowledge (NACK): Slave does not pull down SDA in 9th cycle
WHY USB? • USB, became really popular nowadays to connect computer peripherals. • Not only for Data Source, but Power Source • A USB controller require to power one unit load, which is around 100mA. • such as fan, light, charging the batteries of mp3 players and cell phones.
Milestones Feb. 26 Assemble prototypes of hardware systems Mar. 27-28 Build and test power supply April 24 reBuildand retest power supply Mar. 13-14 Battery circuit built and tested Feb Mar Apr Feb. 27-28 Successfully implement USART devices Mar. 20-21 Finish programming Apr. 3-4 Assemble unit and attach to bicycle for final testing Feb. 19 Complete part acquisition Mar. 10 Complete basic software control flow