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#1
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I need more power Cap'n!
I disagree with the statement that there is no reason to have multiple motors if you have a shifter.
They cover similar problems, but not EXACTLY the same problems. If you need or want more power (e.g. for faster accelerations) multiple motors gives you that. Notice that I use the term "power" in the engineering sense. I do NOT mean "more low end torque" as many do when they use the term on these fori. POWER = WORK PER SECOND <-- SPEED X TORQUE in our case. Switching gear ratios can help you get more power up to a point by choosing to load the motor such that it is providing its peak power (i.e. at 1/2 its stall torque) but if you need or want more power than that, more motors are your only real solution. Joe J. |
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#2
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Re: I need more power Cap'n!
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Everything you said is completely correct, as I would expect out of any Purdue alum... ![]() I guess that my two cents come in that I think that you can have "enough acceleration" and your max speed can be "fast enough" and your max torque can certainly be calculated to be sufficient with one pair of motors and a well designed shifting gear box. And by "enough", I mean reasonably competitive. Obviously a robot with 4 motors will be faster and accelerate more quickly than one with only a single pair. My opinion comes in saying that the estimated 10-14 feet per second you can get out of a high speed ratio gear set gives sufficiently competitive speed and acceleration for most applications. This is only an opinion. I belive that often for many teams, the weight required and additional engineering time could be better used elsewhere. Certainly that is debatable, and your point is definitely well taken. Matt |
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#3
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"Enough" is relative...
I agree that depending on the game, your choosen strategy and the skill of the operator, it is quite possible to make competitive robots with only one motor per side and a shifter. I'll even go farther than that. It is possible to make competitive robots without a shifter and only one motor per side (again, depending on the game, your strategy and the skill of your drivers).
My point was that power is a main reason for adding motors, even with a shifter. How much power is "enough" is up to each team to figure out. One more comment on this more power issue. When I say "more acceleration" I do not just mean quicker changes in velocity. While it is possible that this will be an important design consideration, I think that it is quite unlikely that the time savings alone are sufficient to justify needing more power in your drive system. Think about it. Is it likely that the half second or so you saved because you got up to full speed in 1 second rather than 2 seconds is going to win you a match? I doubt it. Having more power for acceleration is more than just having a high DV/DT. It also includes moving at a constant speed up a ramp for example. Another reason that acceleration is important is that until your robot gets moving, your motors are effectively stalled. The breakers cannot tell the difference between stalled motor current and current from accelerating your robot. So... ...by having more acceleration your motors are operating at this "stalled" condition for a shorter time period. Translation: your breakers will not trip as often from hard accels and decels. Cooler Breakers give you more margin for operation. You will not have to shift out of high gear for a minor skirmish with an opponent because your breakers will have that reserve of current for just that much longer than they would have. Margin of error for the operators. Now THAT wins matches! By the way, I am not a huge fan of multiple motors per side, but I just think that the decision should be made with a fair accessment of the merits of each case rather than a hard a fast rule that shifting is the answer, or multiple motors is the answer, or that multiple motors + shifting is the answer, etc. Just some more things to ponder as you weigh things in the balance. Joe J. Last edited by Joe Johnson : 10-12-2003 at 13:44. |
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