Shell Eco-Marathon 2020

Acknowledgements

Project Advisor: William Sekulic

Technical Advisors: 

• • Dr. Derrek Rodriguez
  • Dr. Greg Vanderbeek
  • Dr. Stephen Geer
  • Jake Schrader

Final thank you to all of those that donated to this project. Your donations helped us achieve so much more than we thought we could

Elevator Pitch

Please watch the video for a short overview of the Colorado School of Mines 2019/2020 Shell Eco-Marathon team.

Design Approach

Chassis: Many design strategies were put into place to redesign the entire steering system . First, the team brainstormed several rough designs. After creating this rough idea, the team analyzed the forces that this system would undergo with a free body diagram (FBD). Once this was completed, the team split up and 3D modeled components on SolidWorks. The FBD was then used to create constraints for a Finite Element Analysis model which was later confirmed by a faculty member at Mines. Using the results from the FEA model, the decision matrix helped us arrive at the safest, easiest to manufacture design. The team then moved forward to fabrication. During fabrication, the design was changed to be manufactured more easily while still executing our design intent.

Figure 3: Upright Assembly	Figure 4: Steering Shaft Assembly

Body: The body was designed to be aerodynamic as well as safe for the driver during competition. As a result, our design included a one piece shell that enclosed all elements of the car, all wheels and tires included. As for dimensions, our car had to fit within a rectangular prism of 1000mm x 1300mm x 3500mm (height x width x length), and track width had to be at least 500mm and a wheelbase minimum of at least 1000mm. The max weight allowed was 140kg. Enclosing all wheels of the vehicle was a priority as this was not done in previous years, yet has a major aerodynamic benefit. The driver also couldn’t be exposed to the wheels or propulsion unit within the vehicle. Only front wheel steering was allowed, and there were to be no sharp corners anywhere on the body. The efficiency of our designed shell was verified using CFD analysis in Solidworks and proved to minimize drag at the intended speeds of our vehicle (refer to Figures 4 and 5). For the driver to have the ability to exit the vehicle in under ten seconds as required by the Shell competition, a one piece hatch system was designed. For driver visibility, the team designed a 180 degree view windshield for driver safety and view as the rules require driver visibility directly ahead and to 90 degrees on each side. We also included an approximately 600 mm crumple zone though the rules only require it to be 100 mm. This was to further our goals of driver safety should an unfortunate crash occur. 

Figure 4: CFD Velocity Cut Plot @ 15 MPH Vehicle Speed	Figure 5: CFD Velocity Cut Plot @ 25 MPH Vehicle Speed

Propulsion: We started with an understanding of what we needed to make the system to work while working within the restraint of the rules and budget, and considered choices that prioritized energy efficiency. First of all, reading over the rules we realized the importance of building a safety electric vehicle, therefore in order to accomplish that all the electric circuits must be protected, the battery must be placed outside the Driver’s compartment and securely mounted. Secondly, the battery must be lithium-based and could not exceed 48 V nominal and 60 V maximum and had to have a battery management system to control cell balancing, overvoltage protection and most importantly to protect the battery against risk of fire.  Lastly, the rules require a custom-built motor controller. After consulting the rules, we understood exactly which system components we needed to design ourselves and how they should perform. The following image models the energy flow through the propulsion system from the battery to the motor. 

We began with a theoretical analysis to model our design choices. This included the characteristics of our motor including power and speed, the drivetrain and its components, the size of the battery to power the system, and the control scheme to run our motor. For each system characteristic, we considered multiple options and estimated the complexity and cost of implementation along with the efficiency of that choice. The decision required finding the right balance between cost, complexity, and efficiency, and this balance was achieved by using design matrices to weigh the different categories and find the best overall option. Our initial testing confirmed that the components would function. When time was available, we made adjustments to make the system more efficient and easier for future teams to use.

Figure 6: Energy Flow Through the Propulsion System

Design Solution

Chassis: After completing the design process, the chassis subteam fabricated the full steering system, milled new uprights, welded new mounting points to the chassis, cut new tie rods, used a water jet to fabricate a new steering rack, and 3D printed a new steering wheel. All of these can be seen in Figure 7, mostly assembled.

Figure 7: Steering System

One of the seat belt mounting points was impacting the tailbone of the previous driver, and thus was cut and rewelded beneath the floor to prevent driver impact with the bolt head. Finally, we needed to integrate the other subsystems onto the car. To integrate the propulsion subsystem, we fabricated a motor mount and helped build parts for power transmission to the rear wheel. To allow the chassis to fit with a new streamlined design, the front roll bar was cut, lowered, and re-welded to the chassis.

Body: With aerodynamic efficiency and driver safety in mind, the body team created a full-sized plug with the intention of utilizing this to construct a one-piece body as shown in Figures 8 through 12. The team laser-cut a skeleton out of 47 unique MDF panels to form an outline of the shape of the plug. This was then filled in using foam, and sanded down to match the outline of the skeleton panels. (Refer to Figure 11 below.) A layer of fiberglass was laid on top of the plug to create a rigid exterior and preserve the geometry of the plug. Cavities in the plug were filled in using bondo and smoothed down through additional sanding. This process of adding bondo and sanding it down was repeated multiple times to create a smooth exterior. The Next Steps section of this synopsis will outline further actions that can be taken to complete this process. Unfortunately, due to the halting of our build process and the cancellation of our competition, a full-sized plug was created, but the build was not completed.

Figure 8: Spine after MDF cutting	Figure 9: Body plug taping process
Figure 10: Body plug before sanding	Figure 11: Body plug after sanding
Figure 12: Bondo covered fiberglass plug waiting sanding

Propulsion: The custom software included a motor control algorithm using pulse width modulation and the driver controls mounted in the steering wheel. The control method used is Trapezoidal pulse width modulation, which is a simple and effective approach for controlling a brushless DC motor using input from the built-in Hall effect sensors. In the steering wheel, the drive controls included a deadman switch, a horn, and an accelerator button for the driver to control when power would be supplied to drive the motor. The motor was controlled by a homemade control system implemented as a printed circuit board (PCB). A schematic of the PCB design is shown below in Figure 13. With the motor and single speed drivetrain, the car would be capable of moving at 32 mph, and can be seen below in Figure 14. The system was fabricated on campus to integrate all components. The energy source built was a homemade lithium battery using 36 18650 battery cells to meet the capacity specifications outlined in the Shell competition rules. We also included protective elements such as fuses and a battery management system to protect it electrically, and electrically isolated casing to protect it physically. Figure 15 shows the battery and some of its protective elements.

Figure 13: Hardware schematic (PCB) for the motor control system	Figure 14: Drivetrain and Motor Attached to Chassis
Figure 15: Battery system with casing and BMS

Next Steps

Chassis: Due to the cancellations caused by COVID-19, work on this subsystem has halted, leaving the driver comfort integration incomplete. The seat was installed and the seat belt mounting points were moved. Unfortunately the bladder for drinking water was not installed, and we never did a full test fit of the driver with the interior of the car completely assembled. The entire steering system was almost complete, only requiring small calibrations and the attachment of the steering wheel to be race ready. The steering wheel needed only the buttons and LCD screen installed and wired before installation into the car. There were plans to attach the body with z-clips, but without the body completed this task was not started. On top of this, the steering system still needs to be aligned to allow for 0 degrees of camber and toe, and tested to see what amount of caster is ideal for drivability. To wrap up this assembly, the steering wheel flange needs to be attached to the shaft via a weld or a pin. The steering wheel can then be bolted onto the flange.

Body: Based on the current status of the plug, there are a few remaining tasks to be completed for a finalized body shell. The process of adding bondo and sanding it down needs to be further iterated for a smooth finish. From there, a female mold needs to be created and used to fabricate a final body shell. Plexiglass has been purchased and needs to be thermoformed into the proper shape for the body shell. Finally, the body shell needs to be attached to the chassis using clips. 

For the next year’s team, they should be able to make some major improvements to the body as they see fit. The team could afford new and sustainable material such as hemp biofiber, or another bio based material to pursue the sustainability award for the competition. Next year’s team could continue to build off of the current plug, and they should have the time to make physical molds. By utilizing the partially finished plug, a much nicer and cleaner body shell can be produced. In addition, the team could improve or design their own cockpit hatch for the vehicle as well as decide on how they want to best fasten this design to the shell of the vehicle.

Propulsion: All individual system components were completed, however, the final testing of the systems and integration of all of the components was not finalized. Testing needs to be completed on the trapezoidal control scheme software to ensure the expected pulse width modulated output. Additionally, testing needs to be completed on the new revision of the PCB to ensure functionality. The integration of components that needs to be done includes soldering of new revision of PCB, connecting buttons to interact with the PCB and horn, and final wiring between battery, fuses, PCB, and motor. As for recommendations to the next team, the team suggests using a derailleur to account for consistent chain tension in the next iteration, ordering PCBs from American manufacturers, and to begin integrating and testing electrical components at the end of the first semester.

Meet the Team