OVERVIEW | DESIGN PROCESS | PROTOTYPE | INTERMEDIATE DESIGN | FABRICATION | FINAL PROTOTYPE | FINAL TESTING | CONCLUSION | ABOUT THE TEAM
Team Pedal Power is developing a device that will generator and store power to charge cell phones, GPS devices, cameras and other portable electronic devices, simply from riding your bike. By harvesting a small portion of energy from the rotation of the wheel and storing it, we can give your electronic device that boost it needs after your daily bike commute, a ride around town, or while you are on an epic cross-country bike tour.
In modern day life, we have many portable electronics that require periodic to frequent charging in order to remain functional. Such items include cell phones, tablets, laptops, flashlights, GPS devices, and cameras, just to name a few. At times we may find ourselves in situations where these products cannot be readily recharged, and therefore become useless. The inability to charge can mean the loss of communication for personal or emergency needs, the inability to navigate to one’s destination, and the loss of the ability to make a record of events. Finding a solution to this problem will be the main focus of this project, with a specific customer set in mind.
One of the primary groups of individuals that face the problem of having their electronic products running out of energy, and yet have a readily available power source at their disposal, are cyclists. The problem for the cyclist is how to convert a portion the power that they produce while riding the bicycle into electrical power that can be harnessed to charge their device. The purpose of the product is to convert the kinetic energy of the moving bicycle into electrical energy for charging electronics. However, it is important to further define the potential customer beyond the general “cyclist”. Two primary customer bases have been taken into consideration, although their needs are somewhat different: the sports enthusiast and the commuter. Individuals who cycle for exercise, excitement, and the thrill of sports differ from those who ride for transportation needs. Sports enthusiasts will want more focus on product durability and performance, while the transportation base will require a product focused on endurance or longevity. One may want the ability for the product to easily disengage to increase the cyclist’s efficiency, while the other may have security concerns relating to theft. While both uses are cyclists riding similar equipment, their needs are different enough to warrant individually weighted design criteria.
The Design Concept that we are pursuing is of a DC generator that will be in a spring loaded housing mounted to the rear wheel triangle. The generator wheel will contact the rear wheel rim and be held in place by the force of the spring. This housing will also allow the generator to be disengaged from the wheel to reduce drag on the cyclist when it is not needed. The charging/discharging circuitry and the battery will be housed inside a weather resistant enclosure that will slide into a standard bicycle water bottle cage. It will be able to be freely disconnected from the generator allowing it to be taken with the cyclist as backup power away from the bicycle.
The Proof of Concept is to ensure the ability of our chosen generator to create enough voltage and amperage to initialize the charging circuit and to examine the performance of each. The generator characteristics were chosen based upon the assumed RPM of the generator wheel in contact with the wheel of a bicycle traveling at an average speed of 12 mph. The charging circuit chosen was an off-the-shelf Lithium Polymer charging circuit capable of accepting voltages ranging from 5 to 12 Volts with current ranging from 100mA to 2A. This charging circuit was chosen due to its wide range of acceptable inputs.
The Testing Platform was a standard road bike attached to a bicycle roller stand. A cyclo-computer was used to measure the speed that the bicycle would be traveling on the road based upon the rotation of the wheel. The generator was temporarily attached to the bicycles rear triangle and the generator wheel was put into contact with the bicycle wheel. Using a breadboard we could connect the generator to the charging circuit and easily measure the voltage across the terminals as well as the current in the circuit. The bicycle was then ridden to simulate traveling and pedaling. Readings of voltage and current were taken in 2 mph intervals beginning at 8 mph and increasing to 18 mph.
The Results obtained from the proof of concept testing have been enlightening, and in one instance, unanticipated. It was shown that our setup will indeed charge the battery given the parameters chosen. However, it was found that the charging circuit created a substantial amount of electrical resistance, reducing the anticipated current substantially, until a threshold of approximately 100mA was reached. At this point the circuit would begin charging the battery and resistance in the circuit would drop significantly. Once the charging was initialized and the resistance dropped, the current would then jump to approximately 400-500mA. The approximate speed of the bicycle required to meet this threshold is 16 mph, after which the bicycle speed could be lowered to around 12 mph and still maintain charging. We feel that this speed required to initiate charging is prohibitive to the products usefulness and many avenues are being examined in order to overcome this.
The Battery Assembly will contain the charging circuit as well as the 2500 mAh battery. The case will be made of aluminum in two parts, the bottom (shown) and a fit lid that will be sealed in place. All charging and electrical terminals will have a gasket as well as a water resistant dust plug to ensure water resistant durability.
The Generator Assembly includes a generator housing that is weather proofed similar to the battery and circuit housing. The generator housing as attached to a spring loaded hinge that provides enough force to keep the generator wheel in contact with the bicycle wheel while also providing a means to disengage the generator from the wheel when charging is either not needed or not wanted which will decrease the drag on the cyclist. A latching mechanism can be seen which provides manual latching of the generator in the disengaged position. On the bottom of the assembly are two worm drive fasteners which allow the entire assembly to be mounted to the widest range of bicycle rear triangle tube sizes as possible. This ensures that the generator can be mounted to almost any bicycle.
Aluminum was chosen as the material of choice for both the battery/circuitry housing as well as the generator housing due to it being light weight and its high thermal conductivity. Each was machined from a solid piece of aluminum in the university’s machine shop by Ryan Stemmerik and Joshua McGuire with the guidance of Tony the machine shop genius. The picture below shows the battery case nearly completed. All that remains from this point is creating the penetrations for the electrical connections, USB and 2.1mm barrel connector. The lid will be sealed on with a silicone gasket seal to ensure water resistance, while the electrical connections will have plugs for the same purpose.
The Generator Housing shown below only requires a cap on the end of the housing, drilled to allow electrical connection to the battery pack. The electrical hole will be sealed with silicone to ensure water resistance. Point of the housing where the generator shaft penetrates is sealed with silicone grease for the same purpose of water resistance.
The Wheel and Shaft of the generator were made out of polyurethane and steel rod. A collar and setscrews were used to connect the shaft to the generator. Many materials were looked at for the wheel, many previous tests used material that did not last or did not make good frictional contact with the bike wheel. Ultimately a polyurethane similar to that used in skateboard wheels was used and proved to be a long lasting choice.
The Remainder of the fabrication was for the bracket which mounts the generator housing to the bicycle down tube and the water bottle mounting bracket. Both were fabricated in the UNR machine shop and can be seen in the final prototype photos. The battery case is mounted to the water bottle bracket using a quick release mechanism so that it can easily be take on and off the bicycle.
The Final Prototype was assembled and fitted to a fairly standard road bike. The water bottle cage mounting proved to be ideal and well placed. The generator assembly mounted nicely to the rear drops of the bicycle, and was shown to be mountable to a wide range of bike styles.
Because the water bottle mounting holes are standardized between all bicycle manufacturers and the clamping mechanism of the generator assembly will accept such wide array of bicycle tubing diameters, we feel very confident that this assembly will fit the lions share of the bicycles on the market.
Testing was done to ensure that the prototype met the original design criteria of water resistance and impact resistance. To meet the criteria of water resistance the battery pack was completely sealed using silicone sealant on the lid and epoxy sealant was used around all ports. In order to keep everything in place during impact. Silicone adhesive was used to adhere the circuit board to the case. The generator was sealed in much the same way, however, at the shaft exit from the case, silicone grease was used to seal the shaft and make it water resistant. Water testing was done by placing the components under a shower nozzle flowing at 2.5 GPM for 10 minutes then ensuring that they still functioned. The impact test was done by dropping the components from a height of 2 meters a total of three times and ensuring that they worked afterwards. In each instance the components passed testing.
A Capacitor can be seen in the picture above. In the Proof of Concept testing we noticed that the charging circuit would not begin to charge until it saw a 100mA current in its closed state, however once it saw this current, it would open and begin to charge at which point the resistance would drop and the current would rise to a fairly steady charging rate of 0.4-0.5A. Unfortunately this required us to ride the bicycle up to about 17 MPH to initialize charging. We could then drop the bike speed down to about 11 MPH and continue charging. We felt that this was not good enough performance; hence the small disk capacitor you see across the power in port in the upper right of the picture. The capacitor provides and initial spike of current to the charging circuit causing it to open. This can now be accomplished at a relatively modest 12 MPH.
The graph above shows the voltage and current to the charging circuit as the circuit opens to begin charging. Each spike is caused by the capacitor. We can see at the first spike, the current rises from approximately 0A to 0.5A which indicate the circuit beginning charging.
Ultimately the complete prototype design, which includes the generator bracket assembly and electronics pack, serves to meet the customer’s need, quantitative design objectives, and engineering specifications. These requirements are summarized with the primary functions, which include: device installation across a wide range of bicycles, having the capability to convert kinetic energy into electrical energy, storing the generated energy, and supplying a way to output stored energy to a variety of devices. Competitor research, engineering calculations, POC testing, and a structured revision process ensured that the finalized design was complete and operational.
We feel that we have successfully gone through the complete design process, iterations of design and need identification, and come out at the end of class with a successful product that is proven to work. Much has been gained through the process of this class and a better appreciation of the complexities of design, as well as the need for testing and prototyping was appreciated. This class and this process have been beneficial to us as students and as graduating engineers, and we feel that this experience is something that we can take into our future careers as an asset.
ABOUT THE TEAM
Andrew Childress is a senior mechanical engineering student, expected to graduate in spring 2014. Andrew enjoys mountain biking, skiing, weightlifting, and the great outdoors. He is interested in perusing a career in the aerospace/defense or internal combustion engine industries.
Aaron Stone is a senior in mechanical engineering expecting to graduate in spring 2014. Aaron is currently interning at Sustainable Energy Solutions where he performs thermodynamic and economic analysis as well as design of small to large scale energy projects. He most enjoys being outdoors skiing, biking, hiking and backpacking.
Ryan Stemmerik is a senior mechanical engineering student in the Honors program at UNR. He enjoys cooking, playing music, and playing games. He had interned at the Tracy Power Plant an hopes to pursue a career in robotics and prosthetics upon graduation.
Josh McGuire is a senior mechanical engineering student. Joshua likes to spend his free time playing video games, skiing, and swimming. He has interned at a power plant, and would like to pursue a career in energy after he graduates.
Luis Garcia is a senior mechanical engineering student. He likes to spend his time working out and spending time with his family. Luis currently interns in a project management position.