Project lab 5 - Distributor car design & build

Date: 06/06/2013-07/06/2013
Duration of activity: 10:00-24:00 (06/06), 09:00 --> 13:00 (07/06) --> 16 hours
Group members participating: Ole Rasmussen and Kasper Føns

1. Goal

The goal of this activity is to design and build the distributor car of the distributor module.

2. Plan

• Design & build the transporter car by experimenting with car speeds (e.g gearing vs. precision) and find out which one is the most precise while still being fast.

3. Design & build the transporter car

The transporter car is the unit that will be transporting the tray with the Lego pieces around. To ensure it's as fast as possible we'll conduct a few experiments with different gearing ratios. In these experiments we will measure the precision of the car while keeping track of its speed.

3.1 Experiment setup

The setup for the experiments is quite simple. What we want to test is the precision and speed of the car.  We construct the simplest car possible built around an NXT. We attach the tray to this car, and construct a track with toothed bars that the car can drive on, and place it on some rubbers to avoid sliding. This is illustrated in Figure 1.

To measure precision we put a piece of paper underneath the car where it starts. We attach a beam to the side of the car, and put a mark on the paper representing the cars starting position. The precision of the car is measured by letting the car drive forward and backward a few times and measuring how far aware from the starting mark the beam is when it's done. This setup is shown in Figure 2.

There's an issue of what speed and acceleration values to use for the RegulatedMotor. What we do is to use the maximum speed and default acceleration. If the car runs fine with these settings we'll not touch them. However, if the car turns out to be unstable or too fast we'll adjust the values to see if we can make it better. The actual values we use isn't important since we will perform extensive tests for the best acceleration and speed values in a later lab. The only reason we want to adjust them in these experiments is to see if its possible to obtain a somewhat precise and fast car with a specific gear ratio. 

Figure 1: The setup for all experiments consists of the car with tray attached and a track for it to drive on.

Figure 2: The precision of the car is measured by how far away the car ends from the starting point after having driven forward and backward a few times.

3.2 Experiment 1 - Gear ratio of 5

We start with down-gearing the car to 1/5th of the original speed with a Z8 gearwheel driving a Z40 gearwheel. This is equivalent to a gear ratio of 40/8 = 5. The gear construction can be seen on Figure 3. A run of the car can be seen in the video in Figure 4. Figure 5 shows the result (i.e. deviation from the starting point). 

While the car seems to be 100% precise with this gearing (Figure 5) it's also really really slow (Figure 4). Therefore, we'll decrease the gear ratio for the next experiment.

Figure 3: Experiment 1 uses a car with a gear ratio of  5.

Figure 4: With a gear ratio of 1/5 the car is really slow

Figure 5: Experiment 1 result. The deviation from the starting position is zero, meaning the car is 100% precise.

3.3 Experiment 2 - Gear ratio of 1

In this experiment we change the car so that the we have a Z24 gearwheel driving another Z24 gearwheel. This results in a gear ratio of 1. The gear construction can be seen on Figure 6. A run of the car can be seen in the video in Figure 7.

Figure 6: Experiment 2 uses a car with a gear ratio of 1

Figure 7: With a gear ratio of 1 and acceleration of 6000 the car wobbles a bit when it changes direction.

As can be seen from video in Figure 7 the car wobbles a lot when it switches direction. This is fixed in experiment 2a.

3.3.1 Experiment 2a - Wobble fix

To fix the wobbling we decreased the acceleration from the default value of 6000 to 2800. Figure 8 shows the result. With the new acceleration value the car is much more stable. Figure 9 shows the precision measurement. Compared to experiment 1 this car is much faster (5 times actually) but it's still just as precise. In the next experiment we will decrease the gear ratio again to see if we can make it even faster.

Figure 8: Setting the acceleration to 2800 fixed the wobbling.

Figure 9: Experiment 2a result. The deviation from the starting position is zero, meaning the car is 100% precise

3.4 Experiment 3: gear ratio of 1/5

In this experiment the car is driving the wheels with a Z40 gearwheel connected to a Z8 gearwheel. The result is a gear ratio of 8/40 = 1/5. The gear construction can be seen on Figure 10. A run of the car can be seen in the video in Figure 11.

Figure 10: Experiment 3 uses a car with a gear ratio of 1/5.

Figure 11: With a gear ratio of 1/5 the default acceleration is too high.

As can be seen in the video in Figure 11 this test didn't go so well. It turns out that with a gearing ratio of 1/5 the car is so fast that it drives off the track. We fix this in experiment 3a.

3.4.1 Experiment 3a - Fixing acceleration

To fix the off-track driving, we decreased the acceleration from the default 6000 to 450. Figure 12 shows the result.

Figure 12: With a gear ratio of 1/5 and acceleration 450 the car drives much better. However,  it still isn't completely smooth when changing directions.

Figure 12 shows a much better run with the car. At least it stays within the track now while still being faster than the car in experiment 2a. Figure 13 shows the precision result; it's not very good. The car consistently doesn't return to the same initial position where it started. If we look closer at the video in Figure 12 we can see why. There's a slight oddity with the way the car drives. Instead of stopping and switching direction instantly it seems to use a bit of time to adjust to its final position and then it changes direction. This is actually because the low gear ratio puts a huge amount of force on the surrounding Lego, especially the axles through which the force is transferred. What can't be seen in the video is that even though the seems to stop for a while before switching directions, this isn't programmed behavior. To illustrate the problem we reset the acceleration and decreased the speed to 1/6 of the maximum. Figure 14 shows what is actually happening.

Figure 13: The precision of the car in experiment 3 is not very good.

Figure 14: Decreasing the speed to 1/6th of the original emphasizes the  problem with this car; the Lego can't handle so much acceleration, so it's being transferred in chunks.

Notice that in Figure 14 the car seems to move in bursts. The funny thing is that the program used to control the car is the exact same as for all the other experiments. The only thing that has changed is the gearing and the speed. Thus, what's actually happening here is a result of the force-transfer from the motors to the surrounding Lego: the motor transfers so much force to the wheels in such a short time that it physically bends the Lego axles. When all this force reaches the wheels the car is pushed quickly forward, but it also releases the force from the Lego. This is what causes the chopping movement. Figure 15 shows a slow-motion video of this behavior. Looking closely it can be seen that when the car is standing still the motor is actually still rotating, but the force just doesn't get transferred to the wheels fast enough to move the car instantly.

Figure 15: Slow-motion run of the car. Shows the force-transfer problem more clearly.

The cause of the problem is, as mentioned before, that the Lego axles can't handle the amount of force that is transferred through them. To alleviate this problem we'll try to construct a car specifically designed to minimize force transfer through Lego axles.

3.5 Experiment 4: gear ratio of 1/5, special design

In this experiment we construct a car designed specifically to avoid transferring force through long Lego axles. The design of this car is shown in Figure 16. It uses two motors to avoid the need for a long axle to reach out to each side of the car.

Figure 16: The new car uses 2 motors to eliminate the need for a long axle going out to each side of the wheels.

To minimize the force transfer through single axles we design a custom solution for attaching the gear wheels to the motor. This is shown in Figure 17. We have attached the gear wheels directly to the motors by the means of several 2-length axles. This ensures that all the force from the motor isn't transferred through a single axle like it usually is. Instead the force is distributed through each of the axles. Also, since the axles are so small they can store much less energy than longer axles.

Figure 17: Our custom design for transferring force between the motor and the gearwheel. This ensures almost no energy can be stored in the axles due to their short length.

A run of the new car can be seen in Figure 18. It's obvious that this car doesn't have the force transfer problem from experiment 3. Actually, now it almost transfers too much force too quickly to the wheels; it's almost so aggressive that it runs itself off the track!

Figure 18: The specialized car is a wild beast.

Just for fun we added a set of "guiders" to the car that graps underneath the track and keeps the car steady. The result can be seen in Figure 19. This helps a lot. What we have created here is a car that is super fast but nowhere near any kind of precision. This last experiment was mostly just for fun, to see how fast we could make the car without thinking about precision. 

Figure 19:  The specialized car with guiders. It's like taming a wild tiger with fishnets; it could (and will) break out any minute now...

4. Conclusion

In experiment 1 we constructed a car with a gear ratio of 5. This car turned out to be very precise but too slow. In experiment 2 we made the car faster with a gear ratio of 1, but it was a bit wobbly so we changed the acceleration and ended up with a car must faster but just as precise as in experiment 1. To see if we could make the car even faster we decreased the gear ratio further down to 1/5 in experiment 3. This introduced the problem with force transfer and energy storage in the Lego axles; the car was faster but not nearly as precise as the one in experiment 2. In experiment 4 we fixed the force transfer problem by constructing a car specifically built to minimize force transfer through Lego axles. It turned out to be extremely fast but also extremely imprecise. We had to add guiders that graps around the track just to keep the car steady.

The final result is that the car from experiment 2a was the most precise while still being relatively fast. Thus that is the car we will be using in the final distributor construction.

5. Next lab

In the next lab we will design and build the remaining parts of the distributor.