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Clifford
goes to Treasure Island
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Design Journal
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Clifford
goes to Treasure Island
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January 19
The robotics design studio elves must’ve been working hard last night, because we walked in this morning to find 3 50 lb. bags of sand, a magnet sensor and a set of treads for the car sitting by our station.
More Work on the Car
We’re going to rebuild the car so it’s smaller (using the cricket instead of the handy board) and uses the treads. For now, we plan to keep the same gearing – we’ll see how it does on sand with a 27:1 gear ratio, and we’ll adjust it as needed.
Problem: with the bigger wheels, the gearing was high enough off the ground that it didn’t hit. But now, the treads are small, so the gearing hits if we leave the wheels in the same configuration as before. We solved this by idling vertically with 2 16 gears. So the gearing is now: 8 to 24 (rubber band), 8 to 24, 8 to 24, 8 to 24, 16 to 16. It’s still a 27:1 ratio.
We were a little confused about how to connect the treads, but we figured it out (putting a 16 on the inside). Then we decided to move all the gears down so that the wheels could be at the back rather than in the middle (our gearing train had started at the back and went forward – placing the wheels an inch or two from the back, which was practically the middle of our now smaller/shorter vehicle).
Vehicle with the treads: our car now has 4 “wheels” – each connected to a tread. Each wheel is connected via it’s own separate axel (since we still need it to turn, which means one side of the vehicle must be free to turn w/o the other side, or with the other side turning backwards). The car still has rear-wheel drive, and is 14 FLUs long.
Finding the Treasure
Sensing the treasure: We are using a magnet sensor to find the “treasure” (which will be magnetized). We tested the sensor with different strength magnets and different depths of sand. We found that if we stuck 2 of the large magnets together, the sensor would work through about an inch of sand. If we stuck together about 8 magnets (2 of the larger and 6 of the medium size), the sensor would work in up to 3 inches of sand. We probably don’t want the sand to get that deep, and we also have to take into account that the magnets will be in a box of some sort and will be decorate to look like jewels.
Testing the Car
The car was able to run and turn fine on the carpet. It could go straight ok in the sand, but we could hear a lot of friction coming from the gear train and a lot of sand was getting into the gearing. Another issue was that the body of the car was just barely above the bottom of the treads, which meant that it was dragging in the sand. When we tried to turn the car, it was successful for a little while, but then too much sand entered the gearing and the car just started digging itself into a hole. Lyn took a look at our car, and suggested that we use the motors that had internal gearing since there was so much friction in our gear train. He also noticed that our rubber bands were at a diagonal, and were slipping a lot when we ran the car.
We are going to try putting in the motors with internal gearing, which means we don’t need to gear down as much (we plan to try 9:1 ratio, which means we have two 8 to 24 gear downs).
We spent all day playing around with the gearing. We got rid of the rubber band, so that the motor had a size 8 gear that turned a crown gear. Then we had one 8 to 24 gear down, and two 16s meshing vertically. The gear train isn’t stable at this point, it’s very prone to breaking. The car still wasn’t turning well in the sand, so we tried the big wheels again. They worked fine for going straight, but we’re still having problems turning. Part of this is because the gear train breaks so easily, but the other part might be b/c of the treads. We plan to work on the gear train more tomorrow. We will try moving things around so that the crown gears aren’t hitting the edges of the full bushings and so that they aren’t sliding out of place so much.
Trip to the Craft Store
There was a trip to the craft store. Since we already have our sand, the only thing we really need for now is a big box or container of some kind. Karen found little cardboard treasure boxes. We’re not sure what we’re going to do about the large container yet, maybe on the next trip we can find some kind of under bed storage thing or we can steal the turtle pool from the other group.