Thanks for the kind words. We are a third-year team, with lots of students and an active fund-raising program. This robot absorbed about 2/3 of our aluminum (I think we have four kits worth). Zippy is one of five VRC robots we are building or rebuilding. Using the Vex speed chart, the speed is about 3 feet per second. It’s pretty fast, but it feels even faster with the omni wheels and light weight. Zippy changes speed and direction really quickly.
Another secret to the acceleration is the use of 2.75" wheels. A lot of the energy in the motors is spent spinning the wheels up to speed over and over. The standard 4" wheels weigh about three ounces, and the 2.75" green wheels weigh less than two. Since we have lighter wheels with the mass closer to the point of rotation, the rotational inertia of the four 2.75" wheels on Zippy is a lot less than if it had 4" wheels. I’ll try to upload some video to You Tube and post the link. It’s pretty cool – and a great way for the students to learn the old engineering adage “simplify and add lightness.”
I had a question on Zippy in the gallery (which most probably don’t read), and I thought the answer might be interesting to more people, so I’m posting it here, too. It was about why we chose the 84-tooth “pizza” gears coupled with 36-tooth gears in the drive train.
The drive train was a compromise between the desire for high speed, wanting to use the small lightweight wheels, and trying to keep the number of components low. Because of the choice to use a fairly small base (only about 12 inches square) the first choice of direct-driving 5" wheels was out. The four inch wheels were a bad compromise for this design – too slow when direct-driven, they take up a lot of chassis room, and nearly as heavy as the five inch wheels. Once you decide to use the 2.75 inch wheels you then have to find a way spin them quickly.
The small wheels have a diameter of 2.75 inches, which is a circumference of about 8.6 inches. The Vex motors spin (at full battery power) a maximum of 100 RPM, so if you direct drive 2.75" wheels you have a maximum speed of 860 inches per minute, or 860/60 = 14.3 inches per second, or about 1.2 feet per second. At this rate it would take 10 seconds to cross the field. That’s way too slow. We know from experience that anything much above 3 feet per second is hard to accomplish with four drive motors (we rejected using six to save weight and room on the 'bot), especially if we wanted to climb the ramps, so we targeted that as our desired speed – 3 feet per second. To achieve that we needed about 250 RPM, a ratio of 2.5:1, out of the drive system. Playing with the available gears, we could get 3:1 with 36:12, but we’ve had quite a few 12-tooth gears strip in high-torque applications over the last three years, so we ended up with a “pizza gear” to a 36, which is a ratio of 84:36, or 7:3, which is lower than our target of 3:1, but we feel it’s an acceptable compromise. We’ve hand-timed Zippy crossing the field in about three to four seconds, which is plenty good enough.
Now that the high-strength gears are available, though, we might just switch to 36-tooth gears driving 12s. Just a leetle more speed!
Thanks for the kinds words. I’ll pass them along. We’ll see you at Gladstone.
The students of VRC 575 took the chassis and redid the drive train and changed the belt system. They’ve been calling it “Son of Zippy” but I think they’ll come up with a “real” name. The students who were originally 420 decided to stick to FTC instead. Sometimes, I can’t even keep up – and I’m the team’s lead mentor.
Just out of curiosity, did your teams have more luck than we did programming the ultrasonic on zippy? We tried using an ultrasonic on ours and it didn’t turn out too well since a good amount of Elevation has sloped surfaces such as the sides of goals that don’t return accurate/consistent results. Do you guys use it to turn until a certain value or just to go forward until a certain value is reached? We had pretty good luck getting the latter to work but the former didn’t work as well as we thought, even when we backed up and had it point at the side of a goal during autonomous.