Quote:
Originally Posted by Gdeaver
I'm a little surprised that all the posts mentioned FPS. A measure of velocity and no body had an comments on acceleration. What good is a robot that has a top speed of XX fps if it takes the entire length of the playing field to reach that speed. A robot with a low top speed and very quick acceleration can be very useful in some games.
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Quote:
Originally Posted by AdamHeard
I think in most cases, unless your going with some really high speeds, you will have very fast acceleration. Our 'bot this year had a top speed of 9.5 fps, and hit 9 fps in .2 seconds according to JVN's calc. I know in person it was slightly slower, but still no more than .5s to near top speed. Also, the in 05 and 06 we ran the old KOP trans with two small CIMs and had similar similar accelerations.
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We actually tune our bot's high gear to time over a distance as that is how most races are measured... i.e. 50 yard dash. Within JVN's drivetrain calculator
(
http://www.chiefdelphi.com/media/papers/1469) there is a modeler that allows you to figure how long it will take your bot to reach a reference distance. By iterating with you gear ratios you can determine a minimum time to a key distance in the game.
Example:
We had a 6x6 skid steer chassis with 8" wheels. Power was delivered via 2 small CIM's per side through AM gen 2 transmissions.
This leaves us with only the final stage to 'futz' with in tuning the system. After looking at the game rules we decided that the key distance for the game was 13 ft. The packaging constraints of our system left us with a doman of sprockets that fit. Through the miracles of spreadsheets we iterate through each of the possible combinations and get a time to reach our reference distance. Then we just pick the sprockets that coorespond to the lowest time and call that our optimized high gear ratio. I have included a brief spreadsheet (
http://www.chiefdelphi.com/media/papers/2046) that hopefully clarifies some of the grey spots in my description. In the spreadsheet I have also included a plot that shows how quickly the time increases as you move away from the optimum.
The low gear calcs are explained well in Ken and Paul's FRC Conference Presentation so I won't reiterate them here. They also address drivetrain performance with current limits. i.e. breakers Great stuff.
http://www.chiefdelphi.com/media/papers/1682
Disclaimers:
- Yes, I realize that this optimized high gear ratio may need to be shifted to accomodate the low gear needs. This is an engineering decision based upon what your team deems important for the game.
- Yes, the iteration is annoying, but it takes less that 10 min to accomplish and the return on investment seems worth it. I have tried both the Goal Seek and Solver functions to automate this, but the table lookup that is used seems to cause them difficulty.