Week 7: Team Update

Heading into week 8, the team has several tasks that we will need to complete to stay on track. After speaking with Professor Brei, the team gained some pointers on how to start modeling our system. She suggested that we first go online and find companies that make motor/generator systems. This will give us a good sense of what's available and what companies make them. We will need some reliable suppliers with good spec sheets, also making sure that the components these companies make are small enough for our application.

In the meantime, we have begun to draw the system out in detail and come up with a gear-train design that will actually work. It was recommended that we find a motor first and then figure out what kind of transmission we need based on the gear ratio. The input power must be estimated to then find a motor that can accommodate it within a factor of safety. Once we've chosen a motor, we can use motor calculations and data from the spec sheets to figure out the optimal motor torque and speed and the gear ratio. This process will involve a lot of tweaking though, and we will probably have to it more than once.

It was implied that eventually we would model the entire system and solve it as a whole to find optimum values. How we plan on doing that is somewhat ambiguous, but the lectures helped the team at least gain insight as to how optimization can be used as a helpful design tool when dealing with problems that are unclear where to start from (like our gear-train/motor problem).

The modeling process is going to involve a lot of repetitive and confusing math, but where to start is outlined above. We have divided into sub-teams so that two group members can just focus on modeling, and the others can worry about the administrative/gate review 3 tasks. We are looking at a very busy week next week.

Week 7 - What's Next

Gate review 3 is due in a few weeks, and we still have a lot of work to do before we get there. Finishing our mathematical model is key to being able to move on to the next step of creating a detailed CAD model. We were given some more specific instructions about modeling in office hours with Professor Brei, but are not yet satisfied with the one we have created. It will take several iterations and adjustments before we can feel that we have adequately considered all aspects of the design. 

In addition to designing for engineering, it is important to consider product aesthetics and ergonomics in the next stage as well. We will need to consider how the user interacts with the product, how it will fit in their hand and how they intend to carry it. It will be interesting to see what Sou says about these topics in her lecture next week. 

Week 7: Challenges

Challenges that our team is going to face in the coming week will be creating a model of our dynamic pulley-string-generator system and then solving it to find the optimum values that would maximize efficiency. Setting up an optimizer in Excel will also require a strong set of analytical skills, since nobody on the team has much experience with optimization tools. With Gate Review 3 over our shoulders, staying on track time-wise is going to be difficult due to the solid effort the team needs to put into the design in the mean-time. Updating the Gantt chart will help gauge how we are managing our time, but that is another task in itself. Selecting a drivetrain that can transmit the power from the pulley in a compact/efficient package is a challenge because we cannot drive the DC motor with a worm gear on the shaft as originally thought, and requires more volume.

Week 7: Interesting / Exciting Findings

This past week, after learning about optimization, our team worked a lot on the different constraints of our design. Working with our alpha prototype, we discussed what our limitations were and where we could be more lenient. Right away, we had many constraints that were non-negotiable, such as the size of the motor housing needing to be less than or equal to the width of the phone case. Furthermore, we needed to constrain the total size of the case so that it would fit a majority of phones. Since we decided to prototype for an iPhone, that was a constraint we couldn't be lenient with because it the case could not exceed the size of a phone. However, the most interesting findings that we had are the aspects of the design that we could be lenient with, such as the length of the string/cord, and more of the aesthetic aspects of the design. Overall, this step in design was something our team had not previously thought about and therefore discovered a whole new collection of design details we could change.

Week 6: Team Project Update

This week the team used down-selection matrices to make a final decision on the product we are going to design. After conducting feasibility calculations for the work output and time/effort required, the team figures it can design a pull-string device that will be able to charge a smartphone in a reasonable amount of time. One pull should provide around 8 Joules, taking into account inefficiencies in the drivetrain and motor. The total charge time is estimated to be around 5 hours, but the team is striving to lower this time by keeping the total system efficiency higher than 20%.

Essentially, the device will be a case that attaches to the back of an iPhone and features a pull-string handle that the user tugs on. The force of the user pulling outputs a torque to the pulley, which will transmit that torque through an optimized gear-train and to a DC electric motor (acting as a generator). The generator converts the mechanical energy to electrical, and from here the energy will be stored in a large capacitor or small buffer battery. This energy will then be output at 5V to a charge port on the phone and hence will charge the phone battery.

We delivered a solid presentation Thursday for Gate Review 2 that really emphasized the feasibility of our product. The alpha prototype was demonstrated in class that was made from cardboard and a small-tape measure to show the retracting motion. All in all, it was quite a busy yet productive week and the team is looking forward to begin designing the physical product!

Week 6: Exciting / Interesting Findings

By making the alpha prototype this past week, one of the most interesting findings that our team discovered was the placement of every mechanic within our product, and just how much where each component is placed can affect the overall efficiency and design of the phone case. Previously, we had talked about getting it as compact as possible—even the possibility of making it an attachment rather than an entire phone case. However, after various design choices, we ultimately decided that making an entire case actually wouldn't be as bad as we thought. Most of all, we really learned this week how difficult it is to mesh aesthetic design with mechanical design. Furthermore, we discovered that if we assume a 20% motor efficiency, which is not very good, we can still achieve a certain level of feasibility. We calculated that although it takes about 5 hours of pulling to get a full charge on a smartphone battery, about 30 seconds of pulling will get a minute of talk time. Overall, although we hope to increase the efficiency, the results we currently have are not terrible.

Week 6 - Challenges

Making the device as efficient as possible will be our main challenge as we move forward. If the device isn't efficient enough the design may become less comfortable and the product could fail. In order to ensure that this doesn't happen, the areas where losses may occur have to be carefully analyzed. Measurements must be taken to ensure that the electronics are configured and working properly. The device will also have to be tested over and over again. Faculty will also be consulted to help make sure the device is as efficient as possible.

Additionally, we will have the challenge of figuring out what materials will be included in the final design and how they will be set up. The right type of string to pull the gears must be found. Also we will need to figure out the correct type of gears to use. We must also find the right electronics to use.