The Powers of Induction Log
Week of 1/22
This week we decided to change the direction of our project. Instead of creating optical character recognition software, we decided to instead build an wireless charger for mobile smart devices. We are planning on creating a stand for an iPad that allows the user to charge while watching videos or surfing the web.
Week of 1/29
We met on Tuesday, brushed up on some electrical engineering basics, and learned about the underlying principles behind inductive power transfer. We learned that iPads and iPhones do not contain receiver coils, but instead require an additional adapter to be able to receive power from an inductive charger. Many smart phones created by another companies such as Samsung do contain receiver coils. We decided to build a charging pad that will work with compatible smart phones. In addition, we decided to design and 3D print an iPhone case that contains an adapter so that audience members at the demo can try out the charger even if they have an iPhone.
Week of 2/5
Blake met with Andrew on Monday and researched Wireless Power transfer and the Qi standard for wireless charging. In addition, he watched a few tutorials on MultiSim, the circuit simulation software available in the ESE Teaching Lab. Elizabeth watched tutorials on AutoCAD and researched circuit design. In light of the feedback from our proposal last week, we are considering changing the priorities of our project and what our demo will look like. There may not be enough information for us to reliably design a charging station that will be compliant with the Qi adapter used by iPhones. We also need to specify our goals in more depth. What are the elements of our design that we really care about? It's likely that our current proposal lacks focus and sufficient detail. Instead of charging stranger's phones (which comes with the additional liability of damage), we are considering having our device charge something besides phones.
Week of 2/12
We met separately from Andrew to discuss the focus of our project and came to a better understanding of what we're trying to do. Our modified goal is to output charge to a wireless port instead of the more finicky Qi adapter. Elizabeth made a circuit diagram, and Blake worked on the design and refined the budget based on design changes. We both learned the functions of the various parts required for our design. We met with Andrew on Monday and explained our new focus then rewrote our Wiki page with our new information in mind. Our project seems more set in stone and realistic now that we have the details decided, and we are ready to order our parts as soon as it's approved.
Week of 2/20
On Monday, Blake continued trying to get a simple simulation to work on MultiSim. For some reason, the current and voltage coming out of the drain of the MOSFET was much lower than the supply side voltage. He varied the frequency of the pulsing gate source (which is standing in for the Arduino) to no avail. He fiddled with it and is now considering using an amplifier to increase the magnitude of the switching voltage on the drain side of the MOSFET. That way the magnitude of the voltage can be manipulated while the temporal frequency of the switching is preserved. Additionally, there are no bridge rectifier (although it was easy to construct a diode bridge with 4 diodes) or 7805 Voltage regulators easily available on MultiSim so it is difficult to test the validity of anything he's doing. In addition, he tried varying the coupling coefficient between the inductors to see if that made any impact. Perhaps he simply grounded something he shouldn't have. We communicated with Humberto (through email and a meeting) to discuss specifics of the circuit. He recommended we read a few chapters in The Art of Electronics, which we did. We met on Sunday to pull together some details for the evaluation on Monday. Elizabeth also modified the Gantt chart.
Week of 2/27
We all met with Humberto on Monday and discussed our project from a long-term perspective. Humberto pointed out that we still had to order our USB port, and we might need more coils and other backup parts in case something went wrong. So we compiled a list of parts we had, and ordered any missing parts, as well as some extras. Humberto also helped us consider what the motivation for our project is (since a wireless charger isn't a new concept) and how we would present this concept in a demo, so we began brainstorming about that. One idea we had was that the concept of our project (a portable wireless charger) could be used in a wireless-charging laptop bag. We also began considering the packaging of the project, but most of that will come later (though Elizabeth is starting to think about it from a SolidWorks standpoint). Elizabeth also designed a simplistic version of the Arduino switching code, and we messed around with an oscilloscope to start testing things.
Week of 3/6
Blake tried determining the inductance of the two coils with several different methods, each yielding nonsensical results. The methods employed include the following: measuring the time constant of a RL series circuit for which the value of R is known, changing the frequency for an RL series circuit until the voltage across the resistor is equal to the voltage across the inductor, and changing the frequency of an RLC series circuit until the entire voltage is across the resistor (finding the resonant frequency where the capacitor's complex impedance cancels with the inductors complex impedance. Any one of these methods should allow one to calculate the inductance with ease yet the disagreement in his solutions was disconcerting. He will try again with more meticulous care in the near future. In the meantime, he moved on to testing the transistor by making a small switching circuit with the Arduino. At a frequency of about 1 kHz (a total delay time of 1 ms), he was able to detect the square waveform across the load on the oscilloscope. Once an approximate inductance value is known for the transmitter coil he will try to (safely) connect the switching circuit to the inductor in parallel with a 1N4007 diode. This was a heavy exam week. Elizabeth helped with all this and researched switching algorithms and physical design ideas.
Week of 3/20
On Monday, Blake used the Arduino switching circuit (switching at 1kHz) to apply a pulsing voltage across the coil which was set in parallel with a diode. Instead of using the Li Ion battery as the DC power source, he simply used another Arduino pin set to HIGH (+5 V). The signal across the coil was convergent but highly irregular (as expected for the nonlinear diode element). He then tried moving another identical coil near the switching coil and measured the voltage across it. The signal's shape was preserved and at close distances the magnitude was almost equivalent. He then applied this varying voltage to a bridge rectifier circuit. The output DC voltage was about 3.2 V. Based on this result, Blake realized that either a higher applied voltage on the transmitter will need to be applied or by exploiting different inductance values on different coils or by both. If the 7.4 V Li ion battery does not supply enough power, a 12 V Li ion battery may be required. He's beginning to think about how to write an Arduino script for controlling the switching frequency. Blake tried to measure the coils' inductances again. He will figure it out this week. He doesn't know why this has been challenging. Over the last few weeks he has gotten much more familiar with oscilloscopes and function generators. Meanwhile, Elizabeth and Andrew discussed and began working on the physical design. Elizabeth plans to 3D print a universal phone case that will house the receiver part of the circuit, as well as a pocket of sorts that will fit inside a backpack and house the transmitter part of the circuit, as well as the battery. The two parts will fit together with how the pieces are designed, and there may also be magnets involved to keep the pieces in place.
Week of 3/27
On Monday, Elizabeth had a prototype for the phone case and backpack pocket, but the 3D printer was unavailable, so she plans to come back to the lab another day this week and print the design. In the meantime, she's looking at how all the electronic pieces will fit in her design. Andrew helped her install better software (she had been fumbling around with AutoCAD and 3D Builder, but Autodesk Inventor works better in terms of actually printing designs). She ordered two short charging cables, a micro usb and lightning cable, so that the device will be able to connect to most phones. She also has to figure out how the two halves of the universal phone case are going to be elastic (perhaps rubber bands will work, but she's unsure), and how to make the phone case relatively nice-looking and user-friendly. This is a really busy week for Elizabeth (tech week for her show), but she's doing her best. This week Blake measured all of the inductances of the coils with an RL circuit. He varied the frequency until the voltage across the resistor and the voltage across the inductor were equal in magnitude but phase shifted by 45 degrees. Under this condition, the inductance is simply the resistance divided by the frequency. The measured values were 17.6 uH, 22.7 uH, 21.2 uH, and 17 uH. He chose two use the inductor with the lowest inductance on the transmitter side and that with the highest inductance on the receiver side. This decision slightly boosts the voltage on the receiver end. He then decided to try powering the circuit with the 7.4 V Lithium ion battery and switching at ~10.8 kHz. He measured both the voltage across the transmitting coil and the voltage across the capacitor in parallel with the bridge rectifier on the receiver side. This trial was a humble success. The voltage on the receiver end was about 8 VDC, which is about what should be expected for the coils chosen. However, when he passed this output through the 7805 Voltage regulator the output was reduced to roughly 3 V instead of the expected 5 V. He found a 3.7 V Lithium Ion battery that would be compatible with the charger that we already have that could be wired in series with the 7.4 V Li Ion battery to produce a total of 11.1 applied volts. The battery only cost $10 and he went ahead and ordered it. This could provide the extra power needed to output to the USB. He additionally tried connecting a 1.5 ohm resistor between the source gate of the MOSFET and ground. He measured the analog voltage of this input with the Arduino and created a fail safe (if the voltage across this resistor exceeded 3 V, he would stop switching the circuit, leaving the switch between the battery and the coil/diode circuit open. This condition was never satisfied as the system was consistent throughout the trials. Adding this resistor did reduce the power transferred to the receiver end. NEXT STEPS: 1) When the new 3.7 V battery arrives try placing it in series with the 7.4 V battery. 2) Connect the output of the 7805 to the USB port. 3) Experiment with different switching frequencies. 4) Consider adding a dual polarity capacitor in parallel with the transmitting coil.
Week of 4/3
After the second evaluation with Humberto, we made an updated circuit diagram with a better configuration of resistors and diodes and transferred that design to our actual circuit. We also wired up the USB port. We're not outputting quite enough charge and will later experiment with adding an extra battery.
Week of 4/10
Blake finalized the circuit, adding an extra battery to make the charger output a constant 4.5 VDC. Elizabeth found an efficient layout for the circuit components and transferred the components of each side of the charger to prototype boards. Based on the size of the finished prototype boards, she finished the design for the 3D printed transmitter housing. Due to some confusion with 3D printer reservations, it won't be printed until next week.
Week of 4/17
Elizabeth began writing the tutorial article on using a prototype board, and Blake began working on the poster. The circuit boards were a bit sloppy, since Elizabeth was new to soldering at the time, so Elizabeth resoldered everything. Elizabeth put together the transmitter with the 3D printed housing, the magnets, and the circuit components.
Week of 4/24
We printed the poster, prepared everything for the demo, and demoed our project. Over the next week or so we finalized our wiki page.