Every senior engineering student needs to complete a year long, capstone project. The projects involve extensive documentation detailing the background research, initial design proposal, testing plan, updates and a final report in addition to designing, building, and testing the project.
I had talked with the department last year about submitting a proposal for a senior project and was encouraged to. I wrote a proposal to start a team to compete in NASA's Human Exploration Rover Challenge (HERC for short). In this competition, teams design a collapsible vehicle capable of traversing a terrain similar to what we expect to find on other planets. The project was accepted by the department and I was put on a team with three fellow seniors.
Rover from last year's competition
NASA has a set of requirements for the rover:
- Must fit within a 5ft x 5ft x 5ft cube during it's collapsed state.
- Must be able to go from collapsed to operation-ready in under 1.5 minutes.
- Must use non-pneumatic tires designed and fabricated by the students
In addition to this, there are a obstacles and tasks that must be completed on the course. Obstacles include:
- Meteor impact crater
- 20-degree, 4ft tall inclined slope
While tasks include:
- Soil and water sample collection
- Solar panel deployment
- Flag deployment
In the beginning, we divided the rover into four sections - drivetrain, suspension, frame, and wheels - so we could each focus on one section. Since I had the most design experience, through Baja, and the suspension would likely be the most challenging to model. All modeling was done in Creo Parametric 4.0.
Initially, I wanted to design something that would require minimal fabrication from us since I knew that we would be on a time crunch. The main sponsor for Baja, Walker Mowers, has laser cut a lot of parts for Baja before. Taking inspiration from some off-road race trucks, a suspension arm made out of laser cut sections were initially designed. They were simple to design, minimal fabrication other than welding), and would be lightweight.
A stress analysis (done in Creo Simulate 4.0) showed that the highest areas of stress were the gaps between the sheets. The stress shown in the analysis was higher than the materials tensile strength, indicating a material failure under load. However, the load used was greater than we expected to see in competition and those sections in between the sheets could be filled with weld to better distribute the stress.
I then realized that it would be difficult to mount the shock since the other suspension arm would sit directly above the lower arm. This would require an awkward cut out in the upper arm, reducing the simplicity and increasing the weight of the arm, or the shock would have to mount off to the side of the arm, adding an unnecessary torsional load to the arm.
An A-Arm style suspension was designed to get around these issues. A-arms get their name because they look like the letter A. They're extensively used in motorsport because of their simplicity and high strength shape.
The width of the arms was kept as small as possible to reduce the amount of material used while still providing clearance for the axles and shock. The upper and lower shocks are identical except for the shock mounts on the lower arms. The shocks share a mount point with the upper A-Arm to, again, increase simplicity. In the images, the orange stripes are welds. I kept experiencing a mysterious path-error that I hadn't run into before and, even with working with my departments administration, couldn't figure out how to work around it. Because of this, I was able to model the part completely but was not able to run simulations in Creo Simulate. To ensure our parts were strong enough, we used wall thickness that our SAE Baja team was using since they will experience higher loads and stresses than our rover.
I was concerned about getting a consistent fit and weld with the suspension arm and wheel carriers, so I made some simple jigs to hold the parts together while they were tacked.
The suspension jig worked wonderfully and we were able to make all of the A-Arms efficiently and with minimal variations. The wheel carrier jig has not been used yet but will be used in the next few days when the center section is finished being turned down.
I personally welded the majority of the A-Arms and am very proud of my TIG welding.They aren't the most incredible welds, but they are solid and have good penetration. Some of the tubes had large gaps (1/4-1/2 inch) to be filled to attach to the center tube. It took some patience but I was able to TIG them with very solid welds.
One of the requirements set by NASA was that our wheels had to be non-pneumatic and we had to make all of it ourselves. After some research on past competitions and seeing what resources we had available, the wheel design that was chosen featured a plywood core, an aluminum rim and rubber tread made out of a drainage mat. A stencil was made from the CAD model and the plywood was cut using a jig saw. The aluminum was bent using a blowtorch and a jig we made. Once it was bent, we attached it to the wood core with gorilla glue and wood screws. The tread was attached to the rim with gorilla glue and cable ties. Once the first wheel was made, one of my teammates and I ran a tube through the center of the core and had it support our weight as we rolled it around the shop - it had no issues supporting us or rolling around. After this, we wanted to see how it would withstand some impacts. So, we through it down the stairs in the shop. It made its way down the stairs easily and experienced no damage. No stairs were harmed in the testing of this product.
Lastly, the frame design that was decided on used a rectangular box section tube, a hinge in the center, and a lot of holes to reduce the weight. Simulations in Creo Simulate were used to verify that the frame would be strong enough to support our weight with all of the holes. No issues were spotted in the simulation so we went ahead with the holes.
We wanted to use aluminum to further reduce the weight of the rover but we do not have the tools needed to weld aluminum in our shop so we went with thin walled steel and tried to reduce the weight in other ways. In the following pictures, you can see that the suspension tabs and center hinge has been welded on and the cut outs for the axle bearing have been done on one side of the frame.
The following rendering is of our final design. Not modeled is the drivetrain system, chairs, or tool storage. Due to limited access to Creo Parametric when not on campus, one of my teammates designed the drivetrain in Autodesk Fusion 360 with measurements taken off of our rover. The tools and tools storage will be made by students in our University's Intro to Engineering courses. We have acquired some folding chairs for the seating system on the rover. They will be mounted in the next few days.
We are currently on track to have the rover put together on time for the deadlines set by competition.
Once the rover is complete, we will begin testing. We have identified some areas around the university that have terrain similar to what will be encountered in competition. If any issues are identified, they will have to be resolved before we head to competition in the middle of April. We are also talking with the administration of the Engineering Department to identify how the rover can be reused next year in more senior projects and they sounded very interested in continuing the project.
In addition to competing in April, we also have some pieces of documentation due in the next month, mainly the final report and presentation.
This project has been very challenging but equally rewarding. Working with and leading my team has taught me a lot about leadership and the engineering problem solving process. My skills as a fabricator have also been pushed and I am very confident in all my skills in the shop, though I am most proud of my TIG welds.
I will write another update when we get back from competition and begin the last leg of documentation for our project. Stay tuned!