Last year we were unable to perform much analysis on our motor selection or pay close attention to driving strategy. By performing intensive motor testing this year and developing a MATLAB model of our vehicle dynamics, we hope to fully understand the efficiency spectrum of our motors and derive an optimized driving strategy.
When developing a MATLAB model for the car we started with deriving an equation of motion for our vehicle. The forces considered in our model include, vehicle weight, aerodynamic drag, rolling friction, inertial forces, as well as input motor torque. A simple force balance allowed us to derive the equation of motion shown below.
After developing an equation of motion for our car, we set out to determine experimental values for the parameters involved in our equation. These parameters include vehicle weight, coefficient of rolling friction, aerodynamic drag coefficient, as well as frontal area.We were easily able to weigh our 2013 vehicle to determine weight. When determining an experimental value for the coefficient of rolling friction however, we set up an experimental procedure in which we rolled the car down a ramp and measured how far it was able to free roll. By combining our experimental procedure with a simple energy balance we were able to calculate experimental values for our vehicle coefficient of friction. We performed the same tests with both our outboard and in-hub motor in order to be able to compare energy losses associated with each motor type. When determining experimental values for our frontal area and coefficient of drag we looked to existing competitor data. Since we plan to optimize both frontal area and drag coefficient for our 2014 car we looked at the PAC-Car (http://www.paccar.ethz.ch/) to settle on benchmark values for our initial model.
After determining our critical vehicle parameters we plugged them into our equation of motion and created a time marching algorithm. This allowed us to keep track of the position, velocity, acceleration, as well as energy usage of our vehicle while testing out several different motor torque inputs or driving strategies.
In order to actually develop efficiency values for our vehicle however, we needed to find a way to model our motor efficiency at a variety of power inputs. We are currently working on creating experimental motor efficiency values for our motors through the use of the test rig shown below.
By using the test rig to measure input current and voltage, as well as the output torque and speed we will be able to fully characterize our motor efficiency over a wide spectrum of torque and speed values. We will input these efficiencies into our MATLAB model and iterate through driving strategies to find the one that gives us the highest predicted overall vehicle efficiency. Currently we plan on testing our outboard motor as well as and in-hub motor, which was purchased from E-BikeKit. Both motors are brushless DC motors. The E-BikeKit motor provides optimal space saving because it does not require a chain (this makes it great for converting road bicycles to electric bicycles, which is what it is designed to do) but we hope to determine if it provides us with a better overall vehicle efficiency than our outboard motor. More information about E-BikeKit and the variety of electric bicycle motors that they design and manufacture can be found at www.ebikekit.com. We will keep you posted with the results of our testing!
The Safety Team ensures competition safety requirements are met by overall vehicle design, including body/frame strength, electrical circuit components, and steering & braking.
We started out by reading the Shell Competition Official Rules and Regulation booklet for a general idea of all the rules and marking which rules had changed from the previous year. Those were especially emphasized because they required large-scale design changes from the year before. For example, this year, all cars were required to have a solid floor that prevented the driver from touching the ground as well as a self-build motor controller. From these changes alone, we had to redesign the body of the car and delegate a team to build the controller from scratch.
Aside from these key changes, there were numerous other rules, ranging from the size of stickers on the car to complex engine isolation system safety. To ensure that no rules were overlooked, the Safety Team went through the rules again in much greater detail to compile and sort them into which of the design teams they best pertained to: Steering/Braking, Frame, Body, Electronics/Communication, Driver, and Other/Miscellaneous. Each list was then distributed to its corresponding team. After that, the Safety Team becomes a “check” for each of the design teams in the case of any rule-related question/issue. After the building is almost complete, the Safety Team will revisit each team to verify that all regulations were met.
To view the compiled list of safety rules:
This day marks the start of the CNC milling process of our car’s frame. Our frame was designed and tested using solidworks. The finite element analysis work predicts that the frame will only deflect 6 millimeters when subjected to a total distributed load of 800lbs (4 times max expected load)
The frame design was opened in Mastercam to create the G-code needed for the CNC mill. We used two end mills: a 1/2″ for cutting out the general triangular shape and a 3/16″ to sharpen the corners of our triangular shapes. Since or bar is 56″ long, we used two separate passes for the top side of the bar (this process was repeated for the bottom side of the frame rail).
The estimated completion time for each bar was 8 hours: 2 hours for each segment, including the tool change. The segments are top-front, top-back, bottom-front, and bottom-back. The total completion time for the whole frame is predicted to be about 22 hours for the two 56″ parallel bars as well as the 13″ front bar.
We still have a long way to go!
— Nils Albertsen
10/29/13 – Prototype Build
On this date, the steering and drive team began construction of a 1:1 model of this year’s frame. This was an important step forward for the design process. Once complete, this prototype will help us to decide on how to arrange the steering and brakes on the front end of the car. We ended the day with about a 60% complete model.
11/9/13 – Prototype Build (continuation)
On this date, the Steering and Drive Team finished construction of the 1:1 model of this year’s frame. To reiterate what was said in my last post, this a critical step forward for our team. This allows us to better visualize possible complications that arise when designing the front end of the car; in particular, this allows us to consider the clearance between the wheels and the body of the driver. This specific consideration is essentially the last obstacle that must be overcome in designing the front of the car; our next step is to determine the smallest wheelbase that will still clear the driver’s body (even with the wheels turned). The following step will be to finalize the dimensions of the front end of the frame (very dependent on the wheelbase).
The Duke Eco-Marathon team plans to attack the problem by engineering a super efficient and environmentally friendly automobile.