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Unread 03-07-2008, 12:54
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JVN JVN is offline
@JohnVNeun
AKA: John Vielkind-Neun
FRC #0148 (Robowranglers)
Team Role: Engineer
 
Join Date: May 2001
Rookie Year: 2000
Location: Greenville, Tx
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Re: 6WD Chassis Stiffness vs. Maneuverability

Quote:
Originally Posted by IKE View Post
Thanks. I really appreciate the great feedback so far.

Here are some observations of our chassis developement. There is a bit of a monkey wrench in the "CG" height thing.

During the build season, we took it for a test drive when it was still a bare chassis and balasted with sand bags up to competition weight. We used the sand bags to change for-aft weight distribution, but the "CG" height could not have been more than 7" off the floor. The same dynamic issues that we see in the fully prepped machine were there with the bare chassis.
The omnis were added to improve cornering. When we had omnis on all 4 corners the chassis predictably had a lot of whip. If the "CG" was not balanced for aft, it was extremely loose (unstable). When I ran the turning delta power calculations it was under 50 Watts (delta side to side) for a relatively high speed tight turn. This can be achieved by just the slightest release of inboard power with a 2 chip per side powertrain. When we had 2 KOP wheels on the back the chassis required a lot of reverse inside torque to turn even at high speeds (approximately 300 watts calculated for the same turn that was 50 watts above). This was correlated with motor temps as example 1 the inner motors stayed cool, while the motors were significantly hotter in example 2. For competition we went with 1 omni (inside rear) and 1 KOP outside rear, high traction wheels in the center and omnis up front. The calcualted turning power (again for similar turns) was about 150 watts. This set up has been good for our machine for this year, but would lack in 2007 games.
The fact that you are calculating turning power requirements is a cool thing. We never go into that level of detail. Turning is one of those things that we do experimentally (for better or for worse) and based on prior experience. (Guess and Check is probably a bad way to do it, but it seems to be working out so far. Negative reinforcement of the worst kind.)

What motors were you running in your drive this year?

-John
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In the interest of full disclosure: I work for VEX Robotics a subsidiary of Innovation First International (IFI) Crown Supplier & Proud Supporter of FIRST