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Unread 02-04-2012, 13:46
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Nathan Streeter Nathan Streeter is offline
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Re: Moment of inertia and turning performance

Quote:
Originally Posted by Ether View Post
Segways turn without any* resistance.

A 6WD with omnis at all four corners is like a Segway with training wheels.


*well, for all practical purposes
I'm assuming that you are implying that you do often want a drive train that turns with negligible resistance, and that they can be controllable?

In the "real world" (i.e. with cars, airplanes, segways, etc.) you're definitely right - you basically always want no mechanical resistance in the drive mechanism. As a result, you have a drive mechanism that is capable of being quite responsive. However, if that mechanism is controlled carelessly, it could result in a vehicle that responds easily to accidental maneuvers. These potential side-effects are mitigated by control mechanisms with resistance or "feedback" (such as the yoke or stick on a plane), control motions that aren't inherently twitchy (two hands on a reasonably sized steering wheel), software damping of otherwise twitchy motions (a Wii uses some of this, I conjecture), and/or by training the manipulator to use the responsive mechanism advantageously (fighter pilot).

These are all better ways of getting good controlability out of an otherwise difficult-to-control system; however, I made several unspoken assumptions when I thought what I wrote. I figured teams would be working with control mechanisms that aren't perfect (the current joysticks at least have a little more resistance than the old ones...), that they'd be using control motions that aren't always easy to make steady (unsupported arms on two different joysticks), that most teams wouldn't be using software to damp out twitchiness (perhaps some teams do), and that teams wouldn't be requiring a level of responsiveness that could only be acquired with a resistance-free mechanism.

From our own team's experience, our drivers had a hard time getting accustomed to our very low-resistance drive trains... but we ended up with robots that could drive fairly well by compensating. We didn't want all the responsiveness we had, so we used software to map the distance from the neutral axes exponentially, so that small motions would be less significant. I consider this raising the tolerances, rather than "damping out twitchiness..." which I presume would be feasible but significantly more difficult, and perhaps not even preferable. We also used software so that when the driver hit a button they could limit the robot to half (or quarter) power for fine motions, such as placing a tube on a peg last year or lining up for a shot in 2010. Then we got the drivers to practice a lot.

So, in the FRC robot world, my experience has been that a hyper-sensitive robot is not preferable and that a good way to limit the sensitivity is to leave a little resistance in the system (i.e. put grippy tires at the corners rather than omnis); however, some teams may think otherwise. Perhaps your team disagrees... I could imagine some teams preferring hyper-sensitive robots that are driven well solely by the skill and practice of their drivers...

I apologize for my non-absolute statement that could easily be misinterpreted.
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Student: 2006-2010 (#1519)
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