Quote:
Originally Posted by EricH
I don't follow this line of reasoning. 488 is on top of 3743. If there is any reason 488 is not on top of 3743, then you need to get another picture before you post again.
If any load path of 3743 is passing through 488, it is going upwards. Therefore, 3743 is supporting 488. You cannot logically make the conclusion that because 3743 and 488 are in contact, 3743 is not fully supported by the bridge, without presupposing that 1) 488 is not fully supported by the bridge and 2) any contact between two robots on a bridge causes one of them to be supported by the other.
I shouldn't have to remind you that two robots being in contact does not necessarily mean that one is supporting the other. If that's the case, I would figure that many of the balances this weekend need to be negated due to robot bumpers touching. Therefore, one of your two presuppositions is out the window based on precedent and simple engineering. This does not negate the other, but it does make it very difficult to make the argument that 3743 is not fully supported by the bridge.
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That's kind of what I mean by a fine point: it's not obvious, because we ordinarily don't pay much attention to small deflections and small forces. They just don't matter for ordinary strength and fit calculations, so we just ignore them.
But FIRST has not specified a scale, and at small (e.g. microscopic) scales, every real-world surface is uneven, is deflecting and exerts small amounts of weight. Additionally, things stick together at these scales, so small amounts of weight can be supported in both tension and compression. That's why, in the real world, contact implies support with unfathomably large certainty. I grant that's not perceptible—so let's put that aside for the moment. Maybe FIRST intended the support to be visible—although that's far from what they said.
At much larger scales, you can perceive many things which you wouldn't ordinarily care about. Here I'm thinking of things like deflections of bumpers. And before you object, consider the precedent: if any robot part, no matter how insubstantial, touches the key, you're considered in. As a practical matter, the referees limit themselves to what they can see, even if it's not a major piece of the robot. So why not here too?
Is there
not one point along the length of bumper-to-bumper contact where 3743's bumper fabric visibly weighs down on a fold of 488's fabric? A pair of interlocking weaves in the fabric, or the edge of a bumper number being pushed back? Or more concretely, what about that red plastic panel on 3743? Isn't it pressing on the (full-width) bumper of 488? What about the rocking action of 3743's drivetrain? Or indeed given any component in contact with 488 at all, a slight tilt is all it takes for the weight of a component to contribute (slightly) to the normal force.
I'm contending that even if you draw the line at what's
visible, the likelihood of some weight being transferred through 488 is very high (even if the magnitude of that weight is very small). And if you draw the line at the microscopic, it's essentially a certainty. I'm also contending that there's no reason in the rules or "technical common sense" to arbitrarily pick a threshold larger than the visible (e.g. specifying a quantity of weight that triggers the support rule).
Another way of looking at the ruling comes down to playing the odds. If you want to take the sure bet, you have to treat contact as implying support, unless otherwise demonstrated in a particular circumstance. If you want referees to have arguments about how much support is enough to trigger the rule, and whether that support is present, you can go with a different standard.
As for negating balances, FIRST ruled that when a robot is on top of another robot (in whole or part), and both robots are otherwise supported by the bridge, they're fully supported by the bridge. So I wouldn't be too concerned—this would not affect the outcome most of the time.
In any event, if 488 is not touching any robot but 3743, the question of support goes away—however I premised my call on the assumption that 2952 is in contact (and that such contact is obviously weight-bearing).
Quote:
Originally Posted by EricH
To do the test another way, and the classic FRC way: If 488 was to be removed from the bridge, would 3743 change position? (Considering a 6WD drop center's rock on shifting CG not to be a change of position here.) In the context of this picture, 3743 would not change position unless it rocked to the other side of the robot.
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The real test is: If 488 was to be removed from the bridge, and substituted with an equivalent force distribution
on the bridge and on 2952, would
any part of 3743 change position? (You can't neglect the rocking, because it implies that 3743 was being supported.)
That's a familiar test, and decently easy to implement in practice, but it's not strictly what the rules call for today.
So to put it all in perspective, if we say that contact implies support, the only thing that changes is the situation where a robot is in contact with a robot that is not itself fully supported (and where robot-robot support is not already obvious).
The other takeaway point is that we don't have to perceive the amount of weight being transferred—only the support which inevitably leads to a quantum of weight being transferred.