Lego Racer

Our next project was to design a Lego vehicle with a single motor, powered by a PicoCricket, that can carry a 1.0 kg weight as fast as possible on a 4 meter course.  For our motors we had to use the old Lego rectangular motors that do not have internal gearing, so that we could learn how to make our own gear train.  For this project, my classmate Angel and I worked together! (Here's her blog: http://angelengr.blogspot.com/)

1. Motors and Gears
2. Our Lego Racer and Engineering Analysis
3. Reflection

Motors and Gears

Motors

We first learned about the different types of motors.  The one we had to use for our project was the 9V Lego Motor 2328c01 (which are the "old grey motors").  We compared this to the "new grey motors".  The old motor has a very high speed and low torque because there are no internal gears, and thus it is easier to stall the old one.  On the other hand the new motor is a little slower and has a bit more torque because it has internal gears, so it's harder to stall.

Torque is an important concept when working with motors and gears.   As you can see on the graph, torque and speed are inversely related.

 At one end, you get high torque, but a very slow speed.  And on the other end, you have high speed, but little torque.  So on either end, the vehicle will barely move. Thus, our goal for the project was to find the middle ground.

Also, consider the equation for power: Power = Torque * angular velocity.  We can see on the graph that this middle ground produces the maximum power.

Gears

We then learned about gears and gear trains, which is putting multiple gears together so they move and work together.  Adding gear trains increases the torque of the motor.  There are four types of Lego gears we use in this project:  40, 24, 16, and 8-tooth gears.  To start with a simple example, if we have an 8-tooth driving a 24-tooth gear, the 8-tooth will have to turn 3 times for the 24-tooth to turn 1 time.  These ratios can give us the gear ratio.  In our example, the gear ratio is 3:1 because of 3 turns:1 turn.

This brings up an important point:  Going from a small gear to a big gear lowers the speed and going from a big gear to a small gear increases the speed by a factor equal to the gear ratio.  Not only does the gear ratio affect speed, but it also affects torque.  Going from a small gear to a big gear increases the torque and from a big gear to a small gear decreases the torque.

We can also make longer gear trains, which is our goal for the project.  By starting with big gears and going to small gears, we can make the axle turn very fast.  But we have to think about torque at the same time.  Going from big to small gears will decrease the torque, and then our car wouldn't move!  This is why we need to find the right tradeoff of torque and speed.

Our Lego Car and Engineering Analysis

Angel and I started by experimenting with some different gear trains and measuring the speeds.  We made the body of the car with a simple gear train and the our iterations involved changing the gears to try to make it go as fast as possible.  Each time we changed the gear ratio, we timed the car to see how fast it went.  We went through 4 iterations. 

Gear Ratio:  2.5*3*5 = 37.5

Gear Ratio: 1.67*3*5 = 25

Gear Ratio: 1.67*3*3 = 15

 These gear trains gave us the following times:

      37.5:1     13.6 sec
         25:1     10.2 sec
         15:1     10.8 sec

We saw that as we got lower the time was going to go back up, so we wanted to make a gear train that was in between 25:1 and 15:1.



We took the 15:1 gear train and we changed the first gear to a 16-tooth.  This made the gear ratio 22.5:1.  When we timed the car it went significantly faster than our first tries, at 9.02 seconds.  We decided to keep this gear train for our final car.

Gear Ratio:  2.5*3*3 = 22.5    Time = 9.0 sec

Here is our final gear train on our finished Lego Racer.
We used two 24-tooth, one 40-tooth, one 16-tooth,
and two 8-tooth gears, for a total of six gears

A final change we made to our car was decreasing the amount of Legos we used as much as possible because wanted it to be light.  Angel and I tested it several more times and it finished the course in 9-10 seconds.  When we did a trial run with other students on Tuesday, it took 12 seconds which was longer than usual.  We think it was from testing it and the batteries were low, so we decided to change the batteries for our final race.

The final race took place on Friday.  We raced in pairs and Professor Banzaert recorded everyone's gear ratios and times.  Here is a video of our car:

Afterwards, the class looked at the results of all of our cars. The times are put in order from fastest to slowest.

          Gear Ratio  Time (sec)
              15:1            8.42
           22.5:1            8.92
           15.6:1            9.63
           20.8:1            9.68
           24.7:1          10.57
           20.8:1          10.93

So why isn't there a pattern to these times and gear ratios? We discussed several reasons in class why they is not, such as the size of the wheel, friction from gears and wheels, the number of wheels of the car, the distances of the wheels, and the weight of the car and the wheels.  Overall, it just the fact that we're doing this in real life, not as a computer simulation, and just like in all engineering, other factors come into play.

Reflection

We really enjoyed constructing our Lego racer for this project and are proud of our final car, which was the second fastest in the class!  There were a few interesting things I noticed when looking at the other team's times.  One was that the fastest car had a gear ratio of 15:1 and when we tested our car with a 15:1 ratio it was 2.5 seconds slower!  This really shows how the design of the car matters, like the wheels, the weight, and other aspects.  I think some improvements to our car could be using a larger third wheel instead of a small one.  We talked in class about how using larger wheels increases the angular velocity and this would probably make our car finish faster.  Another improvement would be trying to use the same gear train, but make the rest of the car as light as possible, by using less Legos, or lighter wheels. 

Our next project is very different than what we've been doing so far this semester.  We got out the Arduinos and will be doing some cool projects with them!