Subsystem Build & Test
Team Vision for Subsystem Level Build & Test Phase
The goal of this phase was to create and test each subsystem in the project. The subsystems include the generator, voltage regulator, waterproof housing, and the attached turbine blades.
All of these subsystems were built and tested. While not all of them are working yet, we have made strides towards honing in on the most feasible solution through iterative testing and additional research. Many different aspects of the device are being rapidly tested in order to provide with other parts of the team with enough time for any potential redesign work.
Turbine & Housing
Multiple iterations of housing have been made to tweak minor details to address issues related to fit & finish, efficiency, and waterproofing. The newest design involves a new holder that decreases the losses from the bearing, and clips on to the bearing from both sides to secure it tightly no matter how bad the water conditions might be. Below are images describing the aforementioned change:
To address waterproofing, the tolerances have been decreased, and work on acetone vapor bath is ongoing.
The turbine print shown above is the new blade design to account for the new motor being purchased. The previous motor did not achieve the required output at the RPM specified, and the losses due to high torque would have been hard to achieve with the limited surface area the 3D printers at RIT could print at. The issue was addressed by altering the pitch angle by a factor of 4, to increase the RPM by the same factor. It is understood that the torque required by the water acting on the blades will have to increase, but since the new motor has a much lower required torque, this should be easier to achieve that the previous option. Adding a gearbox in the limited space was thought of, but we soon realized it wouldn't fit. Below are CAD diagrams of the old and new turbine blades. Old ones have been visually inspected and spin. We plan on identifying the speed they spin at after waterproofing is complete. We have not yet finished printing the new ones, and therefore they haven't been tested.
The first blade that was printed broke due to extremely thin features. The issue was addressed by making the thinnest sections that same thickness as the previous blades. The increased thickness in the rest of the sections should not decrease the modularity of these blades. Below is a picture of the failure points of the newly printed blade:
New Blade Version 1 Failure. Version 2 is currently being printed (Feb 26th, 2018) and should prevent this from happening again.
Mechanical Test Results & Updates
Stationary Waterproof Testing
Stationary waterproof testing was the first step to test our entire system. The stationary part means that water was not moving relative to the device, therefore the blades were not spinning. This involved the system being under water for 30 minutes. The system ultimately passed this test pushing us to the dynamic part of testing.
Dynamic Waterproof Testing & Driveshaft Optimization
Below is a frame from our video recording the dynamic testing (pardon the clarity, picture placed in only for visualization purpose):
The device failed this test. And it did so through the lip seal. We were expecting it to fail from exactly there. To fix this issue, the housing has slightly been altered, and post processing (acetone) methods on the housing will also be altered. Instead of a vapor bath, the housing will now be submerged in acetone to waterproof the outside AND the inside. Before, it was only being waterproofed on the outside. Secondly, the lip seal will also be a press fit now decreasing the chance of water passing through the gaps. Thirdly and most importantly, the drive shaft diameter will be increased. Below are all the different sizes (in inches) we plan on testing. We have to keep in mind that the higher the diameter, the higher to frictional losses.
Below is the result of one of the completed shafts (the one that failed waterproof testing due to the diameter being too small).
Electrical Test Results & Updates
The geared motor which we first ordered was tested and successfully generated the required power in front of our guide Chris. The problem with that motor was that it required too much torque and was very unstable when spinning during the drill machine test. At this point, we had no time to order a new one online and we found what we could from the local motor store and we as a whole team are not sure if it will work the way we want it to work. The problem with motors is that it is hard to find low torque and low rpm ones and this is where geared motors come into play. The only mechanical downside of geared motors is additional torque and stabilization required to keep the motor spinning steadily at the needed rpm. This is a thing both electrical and mechanical engineers have been struggling with. The struggle to find the perfect balance between torque, rpm, and power generated.
Below is a screenshot of the data obtained and plotted for a stepper motor.
The voltage regulation was a success and the voltage regulator works just as expected. The motor was and still is our biggest concern. There are too many things to be taken into consideration. We know one thing for sure that 6V-16V input to the voltage regulator will give 5V output to the power bank.
Finding the internal resistance of the power bank was quite a challenge. The vendor gave us a reading of 2.8 ohms and typically power banks are at most 10 ohms. It all depends on different internal circuitry as well as designs from different manufacturers.
We will be continuing to reach out to the vendor for a rated internal resistance as well as attempt to further verify the 2.8 ohm measurement on our own using the equipment in the electrical engineering labs.
The Updated Test Plan Data Sheets can be found here: Updated Test Plan Data Sheets
Bill of Materials
The BOM has not changed significantly since the last review. There are some miscellaneous parts (Aluminum stock, for example) that we have bought in order to continue testing various subsystems.
However, it may change drastically with upcoming purchases. We have planned well enough to where contracting out blade 3D prints to get the proper sizing will be expensive, but doable.
On the electrical side, we may need to order replacement USB connections as well as an additional geared motor if the one from Dan's Crafts and Things does not work up to our expectations.
The current BOM also includes pricing for the PVC piping for the mount. We have yet to purchase this pipe, but are very confident that the mount assembly should be a simple process. The largest possible complication is machining a drilling piece for the initial setup process.
Because of the general durability and modularity of the current design, the team had decided that arguably the best packaging for this device would be nearly none at all. The cable is the largest part of the device and can be wound around the housing and stowed or carried. The blades fit right on top of each other, so it is likely we will include an elastic band of sorts to keep them together. If we have time, we will visit the possibility of developing additional protective casing for the fins.
The current Bill of Materials can be found in the following link: Updated Bill of Materials
Risk Assessment and Problem Tracking
The Risk Assessment Document can be found here: Risk Assessment
The team has been tracking several larger, more complicated problems and the progress we are making to solve them.
The Problem Tracking Document can be found here: Problem Tracking
Updated Project Schedule & Milestones
Subsystem Testing Review Materials
The Review Outline can be found here: Subsystem Build & Test Review Outline
The Project Summary Sheet can be found here: Project Summary Sheet
Plans for next phase
The goal for the next design review is to have a working prototype of the device that we can simulate in action using a box fan and a mount. This will likely be the same way in which we will be presenting at Imagine RIT.