HexA Challenge Design Report
Liger: Craig Baron, Drew Somers, and Rebecca White
ELE 100, Introduction to Engineering
HexA Challenge
Project done between October and December 2004
Report due 12/10/04
Project Summary
The HexA Challenge involved the design, construction, and testing, and demonstration of
a small car driven by a DC motor and powered by two AAA batteries that would carry weight up
a ramp. The design process was informed by several class lab reports, independent research,
group discussion, and practical experience in the course of building and testing the vehicle.
Our team succeeded in designing and executing a strategy for our car; we did not succeed
in the demonstration (the “race”) itself. Our goal was to travel up the ramp slowly and carry a lot
of weight. True to our plan, the car went slowly—it took over thirty minutes to travel up the
ramp, and carried 964 grams. This was disappointing because it was too slow; the “race” allowed
us a maximum of five minutes. We also realized that we would need to carry more weight in
order to make this strategy worthwhile.
Problem Definition
Our team was challenged to design, build, test, document, and demonstrate an
autonomous machine/car that would carry the most mass up a ramp in the least amount of time.
We were given a small DC motor and 2 AAA batteries, so there were a number of variables we
had to consider when designing and building our car.
Process
In class, we did three formal labs relating to the planning of our car. In the first lab we
found the energy in a single AAA battery, so that we would know how much energy our car
would have available. The second lab focused on our motor and gear ratios, illustrating how
much faster each gear turns compared to the previous one. The third lab connected batteries with
the motor. It showed us what voltage to expect our motor to run at, and guidelines for the torque,
power, and efficiency of our motor.
As a team, we began by meeting together and brainstorming. The easiest place to start
was figuring out a basic strategy. We decided that we would go slowly, so that all of our time
and the battery’s energy would be spent transporting payload. We knew also that the vehicle
should be light, so that minimal energy was spent transporting the car itself, and that the car
should be rigid, so that no energy would be wasted in flexing. We then made a list of options and
ideas for the way the car would work. We considered:
- belts versus gears
- gearing and size of the wheels
- shape of the car
- 4 wheel drive
- Other ways of bringing weight up the ramp, such as pulling it up from the top with a
string
Choosing between belts and gears, we decided that we would wait and see which was more
practical when we built the car. Because we were aiming to go slowly, we decided for a higher
gear ratio, which would drive the wheels slowly but have more torque. The shape of the car
wasn’t terribly important; we chose a trapezoid. We knew that we would want to put weight over
all the wheels, so that there would be good traction for the drive wheels (rear) and the front
wheels would stay straight. Four wheel drive was discarded as an idea because it is inefficient.
We also discarded the idea of pulling up weights with a winch, because we weren’t sure how to
transfer power from the wheels to the winch once the vehicle reached the top of the ramp. It was
difficult to calculate how much weight to carry, because we had not yet done the motor and
battery lab.
Construction
Most of the materials in our car were from class and on hand in the skunk works. The
body of the car is cut out of Plexiglas. This was the beginning of our adjustments: once we
realized how heavy the Plexiglas was, and how big our original sketches were, we nearly halved
the size of the car.
At first we did not know what to use for wheels, but using wheels made from plastic and
rubber from a toy car solved that problem. We also used the chassis of one of these cars as the
underbody of our car, holding the axles in place.
The biggest problem we faced was figuring out how to mount the motor so the gears
would mesh properly. We tried flipping the chassis over so that the axle would be on top and
closer to the drive train instead of underneath the car. However, that would have meant that we
needed larger wheels than our plan called for. In the end, we epoxied the motor mount to the
back of the car and used a rubber band as a belt to link the gear train and rear axle.
The belt also presented logistical problems: the belt drives were not aligned properly.
Because the motor mount was epoxied to the body, we could not move the motor, so we added
another gear. this brought us closer to our original plan (we had revised it to aim for mid-range
times and weights) and meant that we would get more torque and be able to carry more weight.
the elastic band still rode off of the drive after a few seconds, so we used the only narrower band
we had on hand (it would ride up to the edge and roll over on itself, but did not climb up and off
the drive).
Materials
- Plexiglas frame.
- Two axles, for the front and back.
4 cm
6.32 cm
- Four wheels from a toy car.
- Three gears, two belt drives and a belt (rubber
band).
- Chassis from a toy car and four lego pieces with
holes in them, for the axles.
4 cm
- Eight nuts, six in front, two in back. They keep the
wheels in the same spot and keep them from
rubbing against the chassis.
- One screw and nut to hold the fiber glass chassis,
the toy car chassis and DC motor tightly together.
8 cm
- One DC motor
- One battery case(for two batteries)
- Two AAA batteries.
- One toggle switch, to make a quick and efficient way to shut motor off after finishing its
runs.
- Glue for anything that needed to be held down (DC motor, battery pack, toy car chassis,
second belt gear attached to axle, and the four Lego pieces with holes in them).
Overall this vehicle had a mass of 184 grams.
Load, 964 g lead
DC Motor
Belt
Drive
s
Plexiglas
frame.
elt Drives
Batteries in
Battery Case
Toggle
Switch
Testing
We first put 100 grams on the car and started it up the ramp. We then increased the
weight to 964 grams and realized that the car traveled the same distance at about the same rate of
speed as previously observed. In hopes of gaining more traction, we changed the belt to a
thicker, more durable one. This would have helped, but belt drive system we had set up did not
line up properly and the belt kept slipping off the track. Since a thicker belt would not work, we
decided to use a shorter, thinner belt. This belt worked, but it still slipped a lot, and the car
moved very slowly. When loaded with 964 grams, our car took about 29 seconds to travel the
first foot up the ramp.
Predicted Performance
Based on the short trial we ran, our car would take a minimum of 16 minutes to get up the
ramp. In reality, we knew it would take longer because the batteries would drain. Our estimate, at
the time of our race, was the full 5 minutes available.
Gear Ratio
diameter
gear ratio to
previous
total gear ratio
(x:1)
motor
gear 1
0.4775
1.91
gear2
inner
0.4775
gear2
1.91
gear3
inner
0.4775
gear3
axle
wheel
1.91
1.6
2.2
4
4
4
0.84
1.4
4
16
64
53.6
74
Our car had a gear ratio up to the belt of 64:1. The belt drive on the motor had a diameter
of 1.91 cm, and on the axle had a diameter of approximately 1.56 cm. This gear ratio was .82, the
only reducing ratio in the drive train. The wheels were 2.2 cm, bringing the total gear ratio up to
about 74:1. We did not calculate this before the race, and so did not use it to make any
predictions.
Actual Performance
In reality, the car carried 964 grams up the ramp in over 31 minutes. This time isn’t
accurate at all, because we knew it wouldn’t make it to the top in 5 minutes and so tried to adjust
things during the run. We took the weight off, which didn’t make very much difference, and
gave it small pushes, which the motor resisted. The battery didn’t show signs of running down
until about two thirds of the way through the run. Near the end when the car was totally stopped,
the battery seemed to regain enough charge we switched the power off and on again to run the
car a few more inches. The belt slipped throughout the entire run. It seemed to “catch” on certain
parts of the drive gear, which was uneven and not completely circular.
Discussion of Results and Conclusions
The performance of our car was disappointing. Watching our car inch up the ramp for
half an hour, though, made us think about what we could have done better. Andrew from the
(team) Dance Hall Crashers mentioned that putting the belt earlier in the drive chain reduces the
belt’s likeliness to slip and dependence on tension; it moves faster and is transfers less torque.
The gear ratio should be reduced, so that the car will make it up the ramp in less than 5 minutes.
We would probably also look more closely at the labs, because the motor and battery lab seemed
to show that the motor was more efficient and more powerful at lower gear ratios.
Recommendations
The HexA challenge is a good project for the ELE 100 class. However, there are a few
things that could be modified to make the project better. One thing is the instructor(s) should let
the students know when they are actually starting the project. A timeline that showed the pacing
for the whole project would have been helpful When we did the battery and motor labs we did
not entirely realize that those labs were the start of the HexA project. Information could also be
tied together in a way that is less scattered and confusing to the students. The labs taught us quite
a bit, but it was confusing to have the mini-lectures after the labs, and to have so little structure
in the labs themselves.
Appendix
This design report is based on several documents, including:
- Lab guidelines and our lab reports
- Our team’s design PowerPoint presentation
- Notes from class
- Independent research by team members (general reading regarding the design of the car,
and research for our research papers)
- Our observations of race runs—ours and others’
- Input from other class members at the time of our race run
Team member contributions:
· Craig
- Organizer
- Began assignments that Rebecca and Drew were reluctant to start
- Made sure meetings happened
· Rebecca
- Made lists to get things done
- Finalized lab reports
· Drew
- Knew how to do things and what needed to get done in the Skunkworks
- Contributed original ideas for materials of the car
- Gathered final car parts
Team member contributions to this design report:
· Everyone
- We outlined the report in a group meeting, including basic ideas and
preliminary results
· Craig
- Procedure, diagram, labeled pictures, construction
· Drew
- Problem definition, process, construction, testing, recommendations
· Rebecca
- Predicted and actual performance, results and conclusions, appendix, project
summary. Overall revisions.