Transcript
The Solar Stroller By Jeffrey Calhoun Jamie Padilla Michael Replogle Design Review for ECE 445, Senior Design, Spring 2016 TA: Katherine O’Kane 28 March 2016 Project No. 18
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Content 1. Introduction…………………………………………………………………………………....………....4 1.1 Statement of Purpose………………………………………………………………………...…........4 1.2 Objectives…………………………………………………………………………………...……......4 1.2.1 Goals and Benefits………………………………………………………………….…..........4 1.2.2 Functions and Features…………………………………………………...………….……....4 2. Design…………………………………………………………………………………...….....……........5 2.1 Block Diagram……………………………………………………………………………...…….….5 2.2 Block Descriptions……………………………………………………………………...……….…...5 2.2.1 Charging Block………………………………………………………………..……….…….5 2.2.1.1 Solar Panel..…………………………………………………………..………..….5 2.2.1.2 PMOS 1 OVLO Switch…………………………………………………...….….5 2.2.1.3 Over Voltage Lock Out (OVLO)…………………………………………….....…6 2.2.1.4 LiPo Battery…………………………………………………………..………...…6 2.2.1.5 Under Voltage Lock Out (UVLO)……………………………………..……….…6 2.2.1.6 Charge Notification…………………………………………………..…………....6 2.2.2 Power Delivery Block………………………………………………………………………..7 2.2.2.1 Direct Solar Supply DCDC Regulator……………………………………….......7 2.2.2.2 PMOS 2 Regulator Switch………………………………………………...…….7 2.2.2.3 Battery Supply DCDC Buck Converter…...………………………………...…...7 2.2.2.4 PMOS 3 Converter Switch…………………………………………………....…7 2.2.3 Control Block………………………………………………………………………………..7 2.2.3.1 Microcontroller (MCU)…………………………………………………………..7 2.2.4 Load……………………………………………………………………….……………..…..8 2.2.4.1 Light Switch…………………………………………………………………..…..8 2.2.4.2 LED Light Strip……………………………………………….…………………..9 2.2.4.3 LED Headlight..…………….……………………………………………………..9 2.2.4.4 USB Port…………………………………………………………………………..9 2.3 Detailed Schematics………………………………………………………………………………….9 2.3.1 BatterytoLoad DCDC Buck Converter…………………………………………………....9 2.3.2 Charge Notification……….……………………………………………………...………...10 2.3.3 Direct Solar Supply DCDC Regulator…………………………………………..………...10 2.3.4 OverVoltage Lock Out…………………………………….……………………………….11 3. Simulations……………………………………...……………………………………………….……...12 3.1 DCDC Buck Converter……………………………………………………………………….…....12 3.2 Solar Panel……………………………………………………………………………………….…15 4. Calculations………………………………………………………………………………………..…....17 4.1 DCDC Buck Converter……………………………………………………………………….…....17 5. Requirements and Verification…………………………………………………………………….…....18 6. Tolerance Analysis……………………………………………………………………………………...22 2
7. Cost and Schedule……………………………………………………………………………………....26 7.1 Cost Analysis……………………………………………………………………………………….26 7.1.1 Labor……………………………………………………………………………………..26 7.1.2 Parts……………………………………………………………………………………....26 7.1.3 Total Project Cost………………………………………..…………………………….….29 7.2 Schedule……………………………………………………………………...……………..….…29 8. Ethical Analysis…………………………………………………………………………………....…31 9. Safety Statement…………………………………………………………………………………...…32 9.1 Project Safety………………………………………………………………………………….…32 9.2 User Safety…………………………………………………………………………………….…33 10. References…………………………………………………………………………………………..33
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1. Introduction 1.1 Statement of Purpose Strollers have been a popular method of baby transport used across different cultures and time periods for children of all ages. Strolling makes life a little easier for the parent in terms of less work of carrying the baby, as well as everything that comes along: the diaper bag, snacks, bottles, toys, and their personal items. Our project aims to make a parent’s life more convenient while onthego with their child. The Solar Stroller allows parents to remain active and productive with their child while keeping their electronic devices charged by harnessing the energy from the sunlight or artificial light through photovoltaic (PV) modules on the stroller’s sunroof canopy. Parents can now be worryfree about being away from home with an uncharged phone, tablet, or camera. With this portable source of power, the Solar Stroller can also activate its external LED lighting for visual purposes to others, as well as provide path visibility for the parent in a nighttime setting to ensure the safety of all. Although there are a few strollers on the market with similar powering capabilities, the Solar Stroller differentiates itself by being an allinone solution in providing a power bank for onboard charging and illumination via a solar panel.
1.2 Objectives 1.2.1 Goals and Benefits ● ● ● ● ● ● ●
Portable USB to charge small electronic devices while onthego ○ Convenience, Emergencies, Directions Increase the parent’s use of stoller during nighttime with stroller lighting Battery powers LED headlight and LED strips on stroller for safer nighttime travel Small user notification to show available battery charge Solar panel to harness both sun and artificial light for maximum energy storage Onboard battery can be charged while stationary via sun or artificial light Waterproof outer circuitry for safe use in rain or snow
1.2.2 Functions and Features ● ● ● ● ● ● ● ● ●
Thin solar panel on sunroof canopy of stroller Rechargeable 7.4V / 8000mAh LiPo battery for energy storage MOSFETs to feed power via solar panel regulator or battery buck converter Overcharge and Undercharge protection schemes for safe use of LiPo battery Charge notification to let user know available stored energy Microcontroller to control PMOS control signals, adjust duty ratios, and activate loads USB 2.0, Type A port located in the bottom storage compartment LED light strips and LED headlight that can be activated for nighttime use Simple pushbutton light switch to illuminate lighting features
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2. Design 2.1 Block Diagram
Figure 1. System Block Diagram
2.2 Block Descriptions 2.2.1 Charging Block 2.2.1.1 Solar Panel The 20W Semi Flexible Sun Power ( JGN20WSPF) module on the sunroof canopy of the stroller harnesses both natural and artificial light to charge the battery and power various loads. The module is rated at 20 W and is responsible for all of the power generation on the Solar Stroller. It will be placed horizontally, a tilt of zero degrees (due to geometry of stroller canopy being flat), to ensure every direction of travel provides adequate production. This solar panel is rated at 12 V, however it will output around 17.521.5 V. The power generated will be fed into the charge controller and on to the battery. 2.2.1.2 PMOS 1 OVLO Switching The PMOS’ seen throughout the circuit will control the flow of power. These FETs are needed because if the solar panel and the battery are providing power and there is a voltage mismatch, current will be forced back into the converters which will cause damage. To avoid this, PMOS 1 & 2 will be operated asynchronously with PMOS 3. In essence, the solar panel will either be charging the battery and powering the load, or isolated entirely. When the solar panel is isolated entirely, the battery will be used to power the load. Each of these MOSFETs Vishay PMOS rated at 12Vdc and 6A (DC) and will receive a high or low control signal from the MCU to control the ON/OFF state.
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PMOS 1, the OVLO FET, will be used to isolate the solar panel from the battery. It will open if the solar panels are no longer triggering the voltage regulator (implying the sun has set and V(regulator) < 5V). The reasoning for this is that the only load during the day will be a mobile phone and as such the solar panel will be able to supply more than enough power to charge the phone. This will also ensure that battery charge is not being wasted while the solar panels are still able to produce power. 2.2.1.3 OverVoltage Lock Out (OVLO) The LT3652 by Linear Technology will be used as the OVLO to prevent backfeed current into the solar panel, isolate the battery when fullycharged, and isolate the battery when charging at a rate below “C/10”. The maximum charge current for the LT3652 is 2A and our LiPo batteries can be charged at a rate below “1C”, which corresponds to 8A maximum. In order to minimize our battery charging time, 3 LT3652 chips will be placed in parallel to increase the maximum charge rate to 6A without surpassing the maximum charge rate of the battery.. The OVLO receives anywhere from 17.5V21.5V in from the solar panel, and adjusts it to the nominal 7.4V of the LiPo battery. The OVLO allows for the LiPo batteries to be charged optimally without overcharging. The OVLO is a critical subset to the charging system because it protects the battery life, as well as the user and their child from the harm of overcharged LiPo batteries. 2.2.1.4 LiPo Battery The Solar Stroller uses a 7.4V, 8000mAh LiPo battery. This battery is large enough to power a 5W cell phone from dead to fullcharge and LED lights for at least 2 hours on a fullcharge. The battery stores the energy in the charging block and has the ability to be discharged from the various loads. The battery will be charged using the “1C” rule and discharged using the “20C” rule. The LiPo battery receives power from the OVLO if the battery is not fully charged, nor charging at a rate below “C/10”. The LiPo battery then discharges through the UVLO scheme before the loads. 2.2.1.5 UnderVoltage Lock Out (UVLO) As LiPo batteries discharge, their voltage beings to drop. In our case, our battery pack consists of two batteries in series. Neither of the two cells can drop below 3.2 V in order to preserve battery life and have the batteries safely operate. The overdischarge circuit prevents the batteries from supplying power to our loads when the battery pack drops below 6.1V, which is adjusted for overhead of other circuit elements. During this condition, the battery is isolated from the load and instead provides a large shunt resistance with microamps. 2.2.1.6 Charge Notification One helpful feature in the charging block is the user display that shows the current available battery charge. With this, the user can determine whether or not to use the stroller at night before recharging it during the day or with artificial light. This allows the user to know approximately how much capacity the battery will be able to power the stroller’s LED lighting features and USB port for charging
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their electronic devices. The user can estimate the amount of available power left for the stroller based off of 10% LED increments that correspond to the battery capacity remaining. 2.2.2 Power Delivery Block 2.2.2.1 Direct Solar Supply DCDC Regulator The DCDC Regulator is used to directly feed power from the solar panel to the loads during periods of the daytime when energy harvesting, instead of charging and discharging the LiPo battery simultaneously. The DCDC Regulator used for this project is the Texas Instruments LMZ23608 SIMPLE SWITCHER. This regulator will step down the DCDC voltage from the solar panel input, 17.521.5V, to the 5V load and can drive up to 8A of load. 2.2.2.2 PMOS 2 Regulator Switch PMOS 2, the Regulator FET, will be used to isolate the solar panel from the load whenever the battery is discharging. Having two DC voltage sources in parallel can be very tricky, therefore isolating them provides the functionality needed without unnecessary circuitry to balance the voltages. 2.2.2.3 Battery Supply DCDC Buck Converter The next step in the power path from the battery to the load is a DCDC converter to allow our loads to be powered off the battery. In order to make the 7.4V battery applicable for the 5V outputs for the LEDs and USB Port, a buck converter is used to stepdown the voltage while transporting energy from the charging block to the load block. This converter is essential to be able to utilize the harvested energy by the LiPo battery while keeping the power to the loads are an optimum level. 2.2.2.4 PMOS 3 Converter Switch PMOS 3, the Converter FET, will be used to isolate the battery from the load whenever the solar panels are producing power. Again, during the day the only load will be a mobile device, so as long as the solar panels are outputting enough power to trigger the voltage regulator, there is no need to engage the battery. Whenever the solar panels turn off due to lack of light, PMOS 3 will close and engage the battery to feed the load. 2.2.3 Control Block 2.2.3.1 Microcontroller (MCU) An Arduino Pro Mini will be used in this design to control the three various FETs as well as the duty ratio of the Buck converter. The Pro Mini model was chosen for its very low power consumption of 115uA per hour given that the battery pack is oversized this miniscule power consumption is negligible, but will be accounted for. For the Buck converter, the Arduino will monitor the output voltage with a resistor divider setup linked to an analog pin to check if the voltage is within the desired range (4.75V 5.25V). The resistor divider setup is used to ensure the voltage fed into the analog pin is within the correct range. If the voltage is below 4.75V the controller will modify the PWM signal coming off of one 7
of the digital pins to incrementally increase the duty ratio by 0.1 until the output voltage is back within the desired range. Conversely, if the output voltage is above 5.25V, the controller will lower the duty ratio incrementally. In addition to controlling the gate drive for the Buck converter, the Arduino will also be used to control three PMOS switches throughout the circuit. In order to do this, the Arduino will output digital signals from three different digital pins on the board. These signals will open or close the PMOS via HIGH (5V) and LOW (0V) signals to control where power is being delivered in the circuit. In order to properly control the FETs, the Arduino must be able to detect if there is a load. Load detection will be implemented via a shunt resistor near the load. In essence, this resistor will be used to determine the voltage difference across it. If a load is attached, current will be flowing, and we will see a noticeable voltage drop across the resistor. This will tell the MCU that a load is actually attached. A flowchart illustrating the functionality of the MCU is shown in Figure 2 below.
Figure 2. Flowchart for MCU 2.2.4 Load 2.2.4.1 Light Switch The Light Switch is a simple, standard pushbutton switch and transmits power to the LED Light Strip and LED Headlight to power on when the switch contacts are closed. The switch is implemented with a pushbutton in the load block for the user’s convenience. The light switch communicates via control signals to and from the MCU. At most, the switch needs to deliver 5V, 1A so we chose a switch that was rated beyond our needs.
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2.2.4.2 LED Light Strip A 5V LED Light Strip acts as an optional load to the battery for nighttime use. The light strip provides external lighting around the stroller for its visibility to others. The energy for the light strip is fed through the buck converter in the control block from the LiPo battery in the charging block. The light switch controls whether the light strip is off or on, corresponding with the switch being open and closed. 2.2.4.3 LED Headlight The 1 Watt LED Headlight acts as an optional load to the battery for nighttime use. The headlight provides path lighting in front of the stroller for the parent’s use. The energy for the LED Headlight is fed through the buck converter in the control block from the LiPo battery in the charging block. The light switch controls whether the LED Headlight is off or on, corresponding with the switch being open and closed. 2.2.4.4 USB Port The single USB 2.0, type A port acts as an optional load to the battery for the user to portably charge their 5V, 1A small electronic devices. The USB Port is fed through the buck converter in the power delivery block from the LiPo battery in the charging block. The MCU controls the buck converter’s duty ratio to appropriate serve the equivalent load attached.
2.3 Detailed Schematics 2.3.1 BatterytoLoad DCDC Buck Converter The LTSpice Circuit Schematic of the Buck Converter is shown in Figure 3 below.
Figure 3. Battery DC/DC Buck Converter to 5V Output with UVLO
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2.3.2 Charge Notification The circuit for the charge notification with LM3914 is shown in Figure 4 below.
Figure 4. LED DOT Configuration using the LM3914 Using the LM3914, we can specify a reference voltage that will adjust the LEDs to light according to what level the battery is at. For this configuration, the resistive values were chosen to provide the individual lights to come on for different voltage levels of the battery. For example, a low voltage of 6.1V will light LED1 and none of the rest. On the other end, a battery voltage of 8.4V will light only LED10 and nothing else. So for the whole battery voltage range of 6V to 8.4V, there will be a corresponding LED that we can label to signify when the battery reaches a certain voltage level. 2.3.4 Direct Solar Supply DCDC Regulator In order to drive the load while the solar panel is harvesting light, we have chosen the LMZ23608 regulator chip to buck down the voltage to the 5V load output. This chip is rated for up to 32V and 8A, which satisfies all of our solar panel specifications. The typical circuit application is shown below with a table of values to size the resistors. As shown in Figure 5, Vout should be 5V, which means the Vin range can be between 736V, where the 17.521.5V solar panel input satisfies.
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Figure 5. DCDC Regulator Schematic [7] 2.3.5 OverVoltage Lock Out To protect the LiPo battery from overcharging, we are using the LT3652 as an OVLO. This chip regulates the voltage going into the battery by adjusting the voltage and current, and isolating the battery when it is fullycharged. This chip is rated for a wide input voltage range between 4.95V to 32V, which satisfies the input range from the solar panel that is between 17.521.5V. The maximum charge current through the chip is 2A, which is rather small in comparison to the 8A maximum charging rate of the battery. In order to make sure the amount of time to charge our battery is reasonable, we will be using 3 chips in parallel operation which is possible according to Linear Technology [1]. Figure 6 displays three LT3652 connected in parallel operation for a 3S,4.2V battery.
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Figure 6. OVLO LT3652 in Parallel Operation [1]
3. Simulations 3.1 DCDC Buck Converter For simulation purposes, the battery is assumed to be producing a constant voltage of either 8.4V (full charge), 7.4V (nominal), or 6.4V (undervoltage cutoff). For the various loads, a resistor can be added to simulate the load that the converter will operate for. As seen in Figure 3, both loads are active requiring a total power output of 20W. In order to supply a constant 5V to these loads, the equivalent resistance to test for is given by Equation (1): 2
RLoad = VP =
(5V )2 (20W)
= 1.25Ω
(1)
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This allows for us to not only measure the voltage across the load, but also the current. Specifying the input voltage to be 6.4V, the buck converter is simulated by measuring the output voltage and current to verify that it is within the given specifications. During this simulation, the voltage from the battery (V_bat) will decrease linearly from 6.4V to 6.0V in 20ms, simulating a condition for lowvoltage. The voltage of the battery will remain at this level for 100ms, and then gradually increase to 6.4V over the span of 200ms to simulate a condition of the battery becoming readily available for use of powering the load again. When ran for a total simulation duration of 300ms, the following converter output voltage, V(v_out), and current, I(R8), were plotted in comparison to the battery voltage, V(v_bat).
Figure 7. UVLO test for converter under lockout conditions This simulation shows that our UVLO circuit will work for the specified condition when our battery dips below a certain threshold. If we analyze the LT1495 comparator reference voltages, then we can see the exact moment when the battery voltage will undergo a lockout condition and when that restriction will be lifted.
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Figure 8. Battery Voltage vs. Time UVLO Analysis This also shows us the compensation of hysteresis in determining a safe state for the battery to resume powering the load. This shows us that a minimum change in voltage of 22mV is needed to prevent the battery from being locked in a continuous state of isolating and connecting itself back to the load. Examining the steadystate ripple on these components, the zoomedin images are shown in Figure 9.
Figure 9. Average Output Voltage & Ripple with Average Output Current & Ripple 14
This shows that the output voltage is roughly within 5 ± .25V, and the current ripple is within 30% of the total output current. Using this circuit in testing the various load combinations (no load, USB only, LEDs only, USB and LEDs) along with the full range of battery voltages (6.2V, 7.4V, 8.4V), the resulting power is determined through simulation and compared to the worst case power output. A summary of the simulation results for this modified circuit with an inductance of 100μH and an output capacitance of 120μF is shown in Table 1. Table 1. Simulation Results for Various Load Combinations and Battery Voltages Battery Input Voltage
USB LEDs Power On On Output (Worst Case)
Duty Cycle
Load Resulting Resulting Resulting Resistance Current Voltage Power
6.2 V
X
5 W
.806
5 Ω
0.974 A
4.87 V
4.74 W
6.2 V
X
15 W
.806
1.6 Ω
2.84 A
4.75 V
13.49 W
6.2 V
X
X
20 W
.806
1.25 Ω
3.75 A
4.69 V
17.59 W
7.4 V
X
5 W
.75
5 Ω
1.07 A
5.39 V
5.76 W
7.4 V
X
15 W
.75
1.6 Ω
3.14 A
5.25 V
16.485 W
7.4 V
X
X
20 W
.75
1.25 Ω
4.15 A
5.18 V
21.497 W
8.4 V
X
5 W
.61
5 Ω
1.07 A
5.35 V
5.72 W
8.4 V
X
15 W
.61
1.6 Ω
3.12 A
5.22 V
16.29 W
8.4 V
X
X
20 W
.61
1.25 Ω
4.12 A
5.15 V
21.218 W
3.2 Solar Panel The solar panel chosen for this project is the Sun Power ( JGN20WSPF) module. In order to simulate our solar panel, we have chosen to use System Advisor Model (SAM), a renewable energy modeling software created by the National Renewable Energy Laboratory (NREL). In order to model the power loss due to no maximum power point tracking (MPPT), we introduced a 20% efficiency loss on the simulated converter [2]. As stated previously, this solar panel will sit on top of the stroller canopy at a zero degree tilt angle. This ensures that regardless of the direction of travel, the panel receives the same angle of incidence in terms of light. The solar panel data was inputted directly from the data sheet and is shown in Figure 10.
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Figure 10. Solar Panel Characteristics for the Sun Power ( JGN20WSPF) Module The main focus of simulating the solar panel is to determine how long it will take the battery to charge up completely. In order to determine the charge time we extracted the hourly production data from SAM at various locations. Data for June 20th, the summer solstice in Champaign, and June 21st is shown in Table 2 below. Table 2. Predicted Power Generation by Location and Date
Power Generated by System (W) June 20th, 2016
June 21st, 2016
Location
10A M
11AM
12PM
10AM
11AM
12PM Total (Wh)
Champaign, IL
12.0
5.02
12.6
10.9
9.79
7.48
57.8
San Diego, CA
14.5
15.2
15.1
13.8
15.1
15.0
88.7
Denver, CO
14.1
13.4
13.4
12.2
14.5
8.04
75.6
Tampa, FL
12.1
13.6
14.0
12.7
14.1
14.4
80.9
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Referring to Table 2, we can see that in all four locations tested, the solar panel would be able to charge the battery completely in roughly 46 hours given that the maximum capacity of the battery is 59.2Wh. It is important to note that this is assuming that all of the power is delivered to the battery and no loads are running during this charging time.
4. Calculations 4.1 DCDC Buck Converter Our buck converter must accomplish the following list of specifications: ● ● ●
Step down the voltage from the 7.4V nominal LiPo battery to 5V ± 0.25V Supply enough current for the LEDs (2.2 A) and phonecharging loads (1 A) Have less than 30% output current ripple
This buck converter must appropriately serve our loads: ● ●
LED Light Strip & LED Headlight: (5V)(2A)+(5V)(0.2A) = 11W USB Port (Phone Charger): (5V)(1A) = 5W
Thus, for the case when both loads are to be powered by the battery, there is a total power output of 16W. In this case, an output current of 3.2 A is required to maintain an output voltage at 5V. The following relationship between the input and output voltage of a buck converter in regards to the duty cycle ( D ) to find the time during a period when our switch is on is described in Equation (2):
D=
V out V in
5V = 7.4V x Ƞ ≈ 0.75
(2)
This is assuming we have an efficiency of 90%, which is not far from expected for buck converters using real components. Using this duty cycle, a switching frequency can arbitrarily be chosen, 100 kHz, for sizing purposes. The minimum value of the inductor is determined by performing a voltsecond balance and using a ripple chosen ripple of 30% of the total output current, which is about to 1 A. The minimum inductor value is found using Equations (3)(4): di
V L = V in − V out = L dtL = L Lmin =
(V in−V out)(D) f sw∙ΔiL,pk−pk
ΔiL,pk−pk DT
(3)
−5V )(0.75) = (7.4V (100kHz)(1.0A) = 15μH
(4)
Using the voltage ripple from our inductor calculations, the minimum output capacitance can be calculated by knowing the output voltage ripple:
V c,out = V out dV c,out dt
ic,out = C out
(5)
(6)
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Using these Equations (5)(6), a conservation of charge calculation is performed between the inductor and capacitor currents to receive the minimum output capacitance value: Δi
C out,min = 8∙ΔVL,pk−pk ∙f sw = out
(1.0A) (8)(0.5V )(100kHz)
= 3.75μF
(7)
These calculated values for the buck converter are for the requirements that we specified, but for our application, they will need to be increased in value for varying voltage conditions. Table 3 summarizes the design parameters for the Buck Converter when scaled to larger, realistic values. These components in Table 3 were used to build a circuit schematic using the program LTSpice. Table 3. Designed Buck Converter Values Device Name
Designed Value
Input Capacitor C1
33 μF
Output Capacitor C2
120 μF
Inductor L1
100 μH
5. Requirements and Verification The complete Solar Stroller should meet the following requirements as summarized by module below. In order to determine if the requirements have been met, the verification checklist will be analyzed. Table 4. Requirements and Verification Table Requirement
Verification
Points
Solar Panel 1) Provide less than 5V output under nighttime conditions (PMOS switching threshold programmed to MCU) 2) Provide 532V output to OVLO & voltage regulator under direct sunlight (minimum & maximum requirements for OVLO & regulator) 3) Spatially fit on the canopy of the stroller (13.5’’ x 18.5’’)
1) Connect solar panel to a DMM. Measure voltage at night. Confirm voltage is less than 5V 2) Connect leads of solar panel to a DMM. Measure voltage under direct sunlight. Confirm voltage is within 532V 3) Measure dimensions of solar panel using a measuring tape. Confirm dimensions are 13.5’’ x 18.5’’. Also confirm there is a 1 inch buffer area on the stroller canopy around the outside of the panel
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Arduino Pro Mini
1) Power Arduino Pro Mini with 5V
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1) Digital 0 corresponds to ≤ 0.5V 2) Digital 1 corresponds to ≥ 3.75V 3) PWM Generation via Duty Cycle declaration 4) Control PMOS switches via HIGH (5V ± .2V) and LOW (0V ± .2V) signals to gate
DC power supply. Set digital pin output to LOW and measure output voltage with multimeter. Confirm voltage is less than 0.5 2) Power Arduino Pro Mini with 5V DC power supply. Set digital pin output to HIGH and measure output voltage with multimeter. Confirm voltage is greater than 3.75 3) Upload sample test code from Arduino website. Set duty ratio to 0.5 and probe the output of the digital pin on the oscilloscope. Confirm duty cycle is being generated at 50% 4) Connect 100 ohm resistor to PMOS source, connect 100 ohm resistor to PMOS drain, connect DC power supply to resistor on the source side. Drive the DC power supply at 5V. Send HIGH signal to gate, ensure no current is delivered to the drain resistor via a DMM. Send LOW signal to gate, ensure current is delivered to the load @ 4.75 to 5.25V.
OVLO 1) Limit voltage/current to battery based on each cell’s present charge 2) Ensure combined cells do not exceed 8.4V 3) Control the flow of charging current to the “1C” rule 4) Provide MPPT to maintain optimal module performance
1) Test fullycharged battery. Ensure OVLO limits voltage/current to prevent overcharge with multimeter 2) At full charge ensure each cell is at no more than ~4.2V 3) Measure when OVLO cuts off the PV input. Confirm the cell voltage is less than 4.2V 4) Measure charging current to battery with a multimeter. Verify current does not exceed “1C” charge rule (cannot be charged with more than 1 times the capacity)
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UVLO 1) Ensure undervoltage occurs and lockout happens below 6.1V 2) Ensure lockout condition is lifted when voltage level reaches above 6.2V
1) Attach voltage supply (powered off) to the “V_bat” pin of the buck converter. Attach a 1 Ohm load to the “V_out” pin of the buck converter and setup a volt meter to measure the voltage drop across the
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load 2) Set the voltage supply to 6.4V and power on the device to allow 6.4V across the input of the converter. Verify that there is voltage (46V) across the load 3) Lower the voltage supply in a 0.1V increment and verify that there is still voltage across the load 4) Repeat until 6.0V is applied to the input or until there is no voltage (0V +/ 1mV) across the load 5) Increase the Voltage in 0.1V increments until 6.4V is supplied or until voltage is again observed across the load 7.4 V LiPo Battery 1) Provide 7.4V ± 1V at full charge 2) Provide appropriate current to load without breaking the “20C” discharge rule.
1) At full charge, measure battery voltage with multimeter. Confirm it is ~8.4V 2) Test battery under max load. Use multimeter to confirm that battery does not break the “20C” discharge rule (cannot draw more than 20 times the capacity)
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7.4 V DC to 5.0 V DC Buck Converter 1) Modify voltage from battery to meet the load requirements of 5V ± .25V 2) Adjust duty ratio to maintain sufficient power to loads 3) Provide multiple outputs to run the various loads (5V 1A iPhone, 5V 1.6W LED Lamp, 5V 120mA/2.5” LED Strips).
1) Measure output voltage with multimeter under 5Ω, 1.6Ω, and 1.25Ω to ensure specification is met 2) Vary the input voltage with a DC power supply to confirm the duty ratio autoadjusts to maintain a steady output 3) Measure voltage outputs with various equivalent resistive loads attached using a multimeter. Confirm loads are receiving appropriate power 4) Probe output with oscilloscope. Confirm ripple is within specification
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Charge Notification 1) LED verification at lower voltage bound 2) Visual representation in LEDs for range of battery voltage (5.88.4V) 3) Cannot have two LEDs light at the same time with one voltage level
1) Setup a voltage supply to provide voltage to the “V_BATTERY” pin of the charge notification circuit. Start with a voltage of 5.8V and increment the voltage in steps of 0.1V and verify that LED1 lights up at 6.0V +/ 0.1V
5
20
through test range
2) Repeatedly increment the voltage supply until 8.5V verifying that the LEDs will light up one at a time in succession up to LED10 3) Lower the voltage supply to 10mV increment and decrease the voltage through the range of 8.5V down to 5.9V while verifying that no 2 LEDs are lit at the same time
PMOS 1) Block all power flow if |Vgs| < 0.7V 2) Transmit 3.2 ± .2A when |Vgs| > 1.5V 3) Stay below 60 degree C while transmitting less than 3.4A
1) Attach a 1.6 ohm power resistor (load) to the drain of the PMOS. Attach a 5V DC power supply to the source of the PMOS. Drive the gate voltage to 4.3V with another DC power supply and measure current through the resistor with a DMM. Confirm there is 0A flowing through the resistor. 2) Using the same setup as in step 1, drive the gate voltage to less than 3.5V. Confirm there is 3.0 3.4A flowing through the resistor using a DMM. 3) Using the same setup as in step 2, measure the PMOS temperature using a digital temperature gun
5
Light Switch 1) Deliver power to LED lighting features 2) Open/close contact in response to physical push from user
1) Inspect switch alone. Press to confirm metal contact closes. Press again to confirm metal contact returns to “open” position 2) Connect 50ohm resistor in series with switch. Using a multimeter measure resistance with switch open and closed. While closed multimeter should read value of resistor
1.25
LED Light Strip 1) 5V ≤ Vin < 6V 2) Max 5V @ 120mA/2.5” ± 10mA strip segment
1) Drive LEDs with DC power supply. Verify brightness peaks at 5V± 0.25V 2) While driving with DC power supply, measure current with multimeter. Confirm it is 120mA/2.5” ± 10mA
2.5
21
LED Headlight 1) Prated ≤ 1.6W 2) 4.5V < Vin < 5.5V
1) Drive LED with DC power supply. Verify brightness peaks at 5V± 0.25V 2) Measure voltage and current with multimeter to confirm 1.6W± 0.25W power consumption
2.5
USB Port 2.0, Type A 1) Vin = 5V ± 0.25V 2) Iin = 1A ± 0.1A
1) Attach a 5ohm resistor to output of USB port (iPhone equivalent resistance @ 5V, 1A) 2) Measure current and voltage via multimeter to confirm specifications are met
1.25
Temperature 1) Battery must operate safely between 32100 degrees F (safe temperature for prolonged exposure for small children) [6]
1) Confirm with local authority what the “safe” temperature range is for children outdoors
2.5
6. Tolerance Analysis In meeting our requirement of powering our LEDs for at least 2 hours and completely charging an iPhone, we need to have a battery that has the capacity to provide that. At max load, we have a total lighting consumption of 11W (2.2A @ 5V) and a requirement to charge a phone battery (approximately 2915mAh for a standard iPhone 6 battery). This would mean that our battery has to be able to supply:
P ower [W ] × T ime [hr] + Current Capacity (mAh) × V oltage [V ] = Energy [W h] (11W )(2 hours) + (2915mAh)(5V ) = 36.5W h Using this, batteries come in different capacities and multiple arrangements. To get a higher power rating, single lipo batteries are often connected in series in order to allow for larger electronics to be powered. In our case, the amount of energy needed by our system warrants the use of larger battery pack meaning that a twoseries LiPo orientation (7.4V) will be able to supply our load requirement. To ensure that other inefficiencies in the discharging part of our project are accounted for, we want to be able to oversize our battery. In this case, any battery with a capacity over 5000mAh should be sufficient. Oversizing the battery then allowed us to choose a 7.4V 8000mAh:
(7.4V )(8000mAh) = 59.2W h The energy from this battery should be more than enough to meet the power requirements for our loads. One of the most essential components of The Solar Stroller that affects the performance of this project is the power output of the solar panel. The solar panel power output controls the ability to charge 22
the battery and thus, provide a source of power to the USB port and various LED lights when the sun is not available. In order to ensure the user has the discussed 36.5Wh of power available every day the user must extract a certain percentage of the total available power each day. Referring to the SAM hourly data for June 20th the total available production for our system in Champaign, IL is 70.88Wh. Assuming a 90% efficiency on the Buck converter this leaves us with 63.79Wh delivered to the load throughout the day. With this in mind, if the user wants to extract 36.5Wh during the day in order to use the stroller at night the user would need to extract at least:
100 × (Demand [W h] ÷ (0.9 × Available [W h])) = P ercentage of Available Energy Needed [%] 100 × (36.575W h ÷ 63.79W h) = 57.34% of the available energy If the stroller is used every night, this could pose a problem for days later in the year, or if the stroller is rarely exposed to the sun. However, the ability to externally charge the onboard battery would alleviate this concern. Another key component of this project is the charge controller. It will be required to prevent overcharging, keep the battery at least 20% charged, and ensure each cell of the battery is balanced appropriately. Given that the battery being used is rated at 7.4V and 8000mAh, each cell will have a voltage of 4.2V when fully charged and will contain 4000mAh. With this in mind, each cell should never drop below 3.2V to prevent internal damage to the battery. In contrast, the charge controller must also ensure that the cumulative battery voltage does not exceed 8.4V. If it does, this will result in overcharging which causes internal damage to the battery. In addition, the charge controller must be able to follow the “1C” rule to ensure safe charging. This means that the battery will be charged with 1 times the capacity (8000mAh) until each cell reaches 4.2V ± 0.5%. Once each cell is fully charged the controller will work to keep a constant voltage of 4.2V on the battery until the charge current drops below 0.2C. The last feature of this charge controller is recharging the battery safely if the cell voltage should drop below ~2.8V. If this occurs, the controller must provide a 0.1C charge current until the cell is charged back to its safe zone of ~2.8V, it will then resume charging at 1C. There are different methods of implementing an undervoltage lockout circuit for battery protection. The benefit of using a circuit like this is to ensure that our LiPo battery does not become overdischarged where that can cause damage to the battery and possibly cause a fire if thermal runaway occurs. To combat this, the Linear Technology Corp. offers a solution [3] to isolate the battery from the circuit through by reading battery voltage, and switching off the load when it drops past a certain threshold. The main accomplishment of this circuit it to disconnect the battery without having it “floating” as some would say. This should not be done in practice. Instead, when the UVLO circuit detects a low voltage, it causes a disconnect from the load and shunts the output voltage of the batteries through a large resistance causing very little current (μA) draw from the battery. It is important to note that even a small discharge of the battery over time could lead to damage for the battery. After performing a power consumption analysis on the UVLO, it can be determined that while the battery is in lockout:
23
Figure 11. Battery Output Current vs. Time for Lockout Condition The simulation in LTSpice shows that around 3.783μA are drawn from the battery during a low voltage lockout. Noting that our battery will still have to power a lowpower MCU, we can estimate that our current draw in a lockout situation would be: 3.783μA + 115μ (from Arduino) = 118.683μA
(10)
If we have an expected charge useage capacity of 70% for our 8000mAh battery, then we would have depleted it to 30% of its capacity 2400mAh. If this were the case, then leaving the stroller alone in the dark would take a considerable amount of time to fully deplete the battery: 2400mAh / 118.683μA = 20204.9 hours or 2.3 years
(11)
Discharging at this slow rate means that the user will have a large amount of time (months) that can be taken in between uses without running the risk of completely depleting the battery. The UVLO also requires a tolerance of in its requirement. In order to have a very low power consumption when allowing the battery to power the load, the gate resistance proved to be the biggest factor in dissipating power. Most MOSFETs have a tradeoff between gate charge and onresistance so being able to choose a lowpower MOSFET comes with a price of having a charge delay associated with switching. We need to determine what delay the UVLO has and whether this is tolerable for our battery. We know that the lockout voltage occurs at the crossings of the voltage references to the comparator (refer to Figure 7). Once a low voltage is detected, the time it takes for the circuit to effectively lockout the battery can be measured: 24
Figure 12. Undervoltage detection and effective lockout timing This shows us that there is about a 100ms delay in recognizing a low voltage on the battery and being able to completely cutoff the load. Now, an analysis of the battery will need to be done to ensure this is tolerable. For the 7.4V / 8000mAh battery, there is a 25C discharge rate. This means that the battery should not exceed (25)(8A), or 200A discharge current. Since our maximum load is expected to be 4A for the scenario when all loads are on, this would mean that this battery is discharging at a rate of 0.5C. Since it will only be discharging this current for roughly 100ms until a cutoff is effectively performed, then it will only be allowed to discharge at max: (100ms) (4000mA) = 0.011mAh
(12)
This roughly 0.00013% of the battery 8000mAh capacity. This shows that this small delay in UVLO performance is tolerable to have the battery lockout without the delay causing the battery to deplete anywhere below a safe capacity level.
25
7. Cost and Schedule 7.1 Cost Analysis 7.1.1 Labor Total cost of labor is determined by the hourly rate multiplied by the number of hours the project will take to complete, and multiplied by a cost factor, 2.5, to emulate the cost to the engineering company. The total cost of labor is summarized in Table 5 below. Table 5. Labor Costs Name
Hours Invested Hourly Rate
Cost Factor
Cost
Jeffrey Calhoun
220
$33.00
2.5
$18,150.00
Jamie Padilla
220
$33.00
2.5
$18,150.00
Mike Replogle
220
$33.00
2.5
$18,150.00
Total
$54,450.00
7.1.2 Parts For the creation of the Solar Stroller, the parts listed in Table 6 should be used. Table 6 also summarizes the estimated total cost for the parts. Table 6. Parts Costs Vendor/Item
Part Number
Quantity
Unit Cost
Total Cost
Ebay: Sunpower 20W SemiFlexible Panel
JGN20WSPF
1
$47.99
$47.99
Amazon: Vant Battery 7.4V 8000mAh 2S Cell 40C80C LiPo Battery Pack
VAN2S4690
1
$49.99
$49.99
Amazon: 360deal Waterproof Superbright 100cm White SMD Led Strip Light Lamp with USB Cable Port 5v
B00PJWDM5Y
1
$9.72
$9.72
Amazon: DC 3V5V 12LED Super Bright White Piranha LED Night Light Lamp Board
a14121800ux079 9
1
$6.41
$6.41
DigiKey: CNC Tech. USB 2.0 Receptacle
1751015ND
1
$0.89
$0.89
Craigslist: Stroller
1
$10.00
$10.00 26
Arduino: Arduino Pro Mini
1
$3.59
$3.59
Digikey: DCDC Regulator
LMZ23608TZ/N OPB
1
$4.75
$4.75
Digikey: CW Industries GPB Push Button Switch
SW644ND
1
$3.50
$3.50
Digikey: LT1495 IC OpAmp
LT1495CN8#PB FND
1
$5.15
$5.15
Digikey: Voltage Reference Shunt
LT1634CCZ1.25 #PBFND
1
$4.22
$4.22
Amazon: LiPo Battery Safety Bag
B00T01LLP8
1
$8.75
$8.75
Digikey: Linear Technology Battery Charger
LT3652
3
$7.16
$21.48
Mouser: PChannel MOSFET
SI7137DP T1 GE3
1
$2.29
$2.29
Mouser: NChannel MOSFET
IRFZ44NPbF
1
$1.57
$1.57
Mouser: PChannel MOSFET
SI2333DDST1
3
$0.42
$1.26
GE3 Digikey: Diode MBR735
MBR735E3/45G IND
1
$0.73
$0.73
Texas Instruments: TPS28225 Gate Driver
TPS28225
1
$1.87
$1.87
Digikey: 200MΩ Resistor
RNX200MCCT ND
1
$3.27
$3.27
UIUC ECE Machine Shop: PCB
1
$0
$0
Coilcraft: 100mH Inductor
PCV010405L
1
$0
$0
UIUC ECE Shop: 22 μF Capacitor
3
$0
$0
UIUC ECE Shop: 560 kΩ Resistor
3
$0
$0
UIUC ECE Shop: 470 kΩ Resistor
3
$0
$0
UIUC ECE Shop: 10 μF Capacitor
6
$0
$0
UIUC ECE Shop: 33 μF Capacitor
1
$0
$0
UIUC ECE Shop: 120 μF Capacitor
1
$0
$0
27
UIUC ECE Shop: 1.2 Ω Resistor
1
$0
$0
UIUC ECE Shop: 1 MΩ Resistor
1
$0
$0
UIUC ECE Shop: 10 MΩ Resistor
1
$0
$0
UIUC ECE Shop: 3.6 MΩ Resistor
1
$0
$0
UIUC ECE Shop: 2 MΩ Resistor
1
$0
$0
UIUC ECE Shop: 150 kΩ Resistor
1
$0
$0
UIUC ECE Shop: LEDs
12
$0
$0
UIUC ECE Shop: 10 kΩ Resistor
5
$0
$0
UIUC ECE Shop: 13 kΩ Resistor
1
$0
$0
UIUC ECE Shop: 12 kΩ Resistor
1
$0
$0
UIUC ECE Shop: 120 kΩ Resistor
1
$0
$0
UIUC ECE Shop: 1 μF Capacitor
1
$0
$0
UIUC ECE Shop: 330 μF Capacitor
2
$0
$0
UIUC ECE Shop: 5.62 kΩ Resistor
1
$0
$0
UIUC ECE Shop: 1.1 kΩ Resistor
1
$0
$0
UIUC ECE Shop: 0.47 μF Capacitor
1
$0
$0
UIUC ECE Shop: 4.7 nF Capacitor
1
$0
$0
UIUC ECE Shop: 1N4148 Diode
4
$0
$0
UIUC ECE Shop: 4.7 μF Capacitor
3
$0
$0
UIUC ECE Shop: 1N4732A Zener Diode
3
$0
$0
UIUC ECE Shop: 100 kΩ Resistor
4
$0
$0
UIUC ECE Shop: 270 kΩ Resistor
1
$0
$0
UIUC ECE Shop: 649 kΩ Resistor
1
$0
$0
UIUC ECE Shop: 28.7 kΩ Resistor
3
$0
$0
UIUC ECE Shop: 10V Zener Diode
1
$0
$0
UIUC ECE Shop: 390 μF Capacitor
1
$0
$0 28
UIUC ECE Shop: 0.05 Ω Resistor
3
$0
$0
Coilcraft: 10μH Inductor
0603AF103XJE U
1
$0
$0
UIUC ECE Shop: 1.4 MΩ Resistor
1
$0
$0
UIUC ECE Shop: MBR2340
1
$0
$0
Total
$187.43
7.1.3 Total Project Cost The total project cost is determined based on the sum of costs for labor and parts, which is summarized in Table 7 below. Table 7. Estimated Total Project Cost Section
Total
Labor
$54,450.00
Parts
$187.43
Grand Total
$54,637.43
7.2 Schedule Each member of the group is responsible for a specific task each week for the design, creation, and testing of the Solar Stroller, as well as signing up for presentation dates and submitting materials. In Table 8 below, ‘ALL’ corresponds to the team’s cumulative contribution. The individual responsibilities are split evenly amongst group members and are designated by their initials as following: JC for Jeffrey Calhoun, JP for Jamie Padilla, and MR for Mike Replogle. Table 8. Schedule and Division of Responsibilities Week 7Feb
Task Prepare and Finalize Project Proposal: ● Power Consumption Analysis ● Solar Panels ● Battery, Analog Controls
14Feb Prepare Mock Design Review:
Responsibility ALL JP MR JC ALL 29
● ● ● ●
Begin Solar Panel Specifications Begin Buck Converter Specifications Begin Charge Controller Safety and Ethical Analysis
MR JC JP JP
21Feb Sign Up Team for Design Review Prepare Design Review: ● Buck Converter Specifications ● Charge Controller ● Solar Panel Specifications
JP ALL JC JP MR
28Feb Analyze Project Proposal Feedback Finalize Design Review: ● Confirm Part Specifications ● Modify Tolerance Analysis for Solar Panel ● Confirm Requirements & Verification Submit Design Review 6Mar
JP ALL JC + MR MR JP JP
Order Parts: Begin Prototype Building: ● Solar Panel and PMOS ● Buck Converter ● OVLO & Loads
ALL MR JC JP
13Mar Continue Prototype Building: ● PCB Layout ● Buck Converter ● MCU Programming
JP JC MR
20Mar Construct Prototype: ● Order PCB ● MCU Programming & Connections
JP MR + JC
27Mar Submit Requirements & Verification (2) Run Initial System Testing: ● Control Block ● Power Delivery Block ● Charging & Load Block
JC MR JC JP
3Apr
MR MR JC JP
Submit Requirements & Verification (Final) Testing and Debugging System: ● Control Block ● Power Delivery Block ● Charging & Load Block
10Apr Optimization: ● PV Module & Tolerance Analysis Prepare and Finalize Mock Demo:
ALL 30
● ● ●
Control Block Power Delivery Block Charging & Load Block
17Apr Sign Up for Project Demonstration Sign Up for Mock Presentation Sign Up for Final Presentation Prepare Project Demonstration: ● Control Block ● Power Delivery Block ● Charging & Load Block 24Apr Finalize Project Demonstration Prepare Project Presentation Prepare Final Paper 1May
Finalize Project Presentation Finalize and Submit Final Paper Submit Laboratory Notebooks Laboratory Checkout Awards and Pizza Party
MR JC JP JP MR JC ALL MR JC JP ALL JC JP + MR JC JP ALL MR ALL
8. Ethical Analysis Our team vows to uphold the IEEE Code of Ethics [5] in the design, creation, and implementation of the Solar Stroller. The Solar Stroller provides external lighting and USB device charging for small electronics to increase the safety, health, and welfare of our users and the public. This ideology follows the first code of the IEEE Code of Ethics: “To accept responsibility in making decisions consistent with the safety, health, and welfare of the public, and to disclose promptly factors that might endanger the public or environment.” The Solar Stroller has been designed with the user in mind. The ideas behind the stroller began with a set amount of requirements and then built and prototyped to meet those needs. Such requirements include the amount of time in sunlight needed to charge the battery to full charge, and the amount of time expected for available use at full load with a certain percentage of battery. This ideology follows the second, third, and fourth code of the IEEE Code of Ethics: “ To avoid real or perceived conflicts of interest whenever possible, and to disclose them to affected parties when they do exist; To be honest and realistic in stating claims or estimates based on available data; To reject bribery in all its forms.“ Our team is aware and has addressed all safety concerns regarding the Solar Stroller and its technological components. We vow that our creation will be constructed with care for the safety of the user and our technical competence will meet the standards. This ideology follows the fifth and sixth code of the IEEE Code of Ethics: 31
“To improve the understanding of technology; its appropriate application, and potential consequences; To maintain and improve our technical competence and to undertake technological tasks for other only if qualified by training or experience, or after full disclosure of pertinent limitations.” Before finalizing our ideas and prototyping our design, the Solar Stroller has gone through many engineers, both professional and aspiring, for criticism in the hopes of improvement. All criticism has been analyzed and in return either resolved by changes or justified through proof of studies. The Solar Stroller is an item for anyone to use. This ideology follows the seventh and eighth code of the IEEE Code of Ethics: “ To seek, accept, and offer honest criticism of technical work, to acknowledge and correct errors, and to credit properly the contributions of others; To treat fairly all persons and to not engage in acts of discrimination based on race, religion, gender, disability, age, national origin, sexual orientation, gender identity, or gender expression.” The Solar Stroller will be designed to safely transport the user’s child and personal items. The appropriate safety precautions will be taken in order to ensure that neither the user nor the child becomes injured, and the user’s property is not damaged. This ideology follows the ninth code of the IEEE Code of Ethics: “To avoid injuring others, their property, reputation, or employment by false or malicious action.” Our team vows to function as a group, and provide support and resources for teammates in need of technical help in completing their tasks. The Solar Stroller design team will hold each other accountable with decisions in reference to the IEEE Code of Ethics. This ideology follows the tenth code of the IEEE Code of Ethics: “To assist colleagues and coworkers in their professional development and to support them in following this code of ethics.”
9. Safety Statement 9.1 Project Safety In assembling our Solar Stroller design, there is various circuitry that our team will take special precautions while handling. The most critical safety issue is correctly charging the LiPo battery on the stroller. Undercharging and overcharging the LiPo battery can result in a fire or explosion, which can be dangerous to our team and others, as well as damaging to our project and other design lab assets. It is essential for our overcharge protection and charge controller to work correctly for the successful and safe charging of our LiPo battery. The LiPo batteries will also not be exposed to flammable products or liquids during the assembly, nor will they be operated in conditions above 70°C (160 °F) as directed by the battery manufacturer. In order to provide safety to our loads, specifically our personal electronic devices for testing purposes, our Buck Converter must be designed to accurately step down from the 7.4V LiPo battery to a safe 5V for the USB port and LED lighting.
32
9.2 User Safety To ensure the safety of the user and their child, the circuitry components of the Solar Stroller should never be tampered with. The Solar Stroller contains LiPo batteries that can result in fire, explosions, and toxic smoke inhalation when mishandled. Due to the LiPo batteries sensitivity to temperature, do not operate in conditions above 70°C (160 °F). It is recommended that the user does not store flammable products or liquids near the charging battery.
10. References [1] Battery Charger’s Unique Input Regulation Loop Simplifies Solar Panel Maxiumum Power Point Tracking [Online], Available: http://cds.linear.com/docs/en/ltjournal/LTJournalV20N402dfLT3652Jay_Celani.pdf [2] Masters, Gilbert M. “Photovoltaic Materials and Electrical Characteristics, Photovoltaic Systems” in Renewable and Efficient Electric Power Systems, 2nd ed . Hoboken, NJ: John Wiley & Sons, 2004, ch. 6 & 7. [3] 4.5uA LiIon Battery Protection Circuit [Online], Available: http://cds.linear.com/docs/en/ltjournal/LT1389_0699_Mag.pdf [4] Test a Relay [Online], Available: http://www.wikihow.com/TestaRelay [5] IEEE Code of Ethics [Online], Available: http://www.ieee.org/about/corporate/governance/p78.html [6] Child Temperature Safety [Online], Available: https://www.ok.gov/health2/documents/weatherwatchforchildren2.pdf [7] LMZ23608 [Online], Available: http://www.ti.com/product/lmz23608#diagrams
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