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Unread 21-11-2003, 17:08
Joe Johnson's Avatar Unsung FIRST Hero
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Think Power...

There is a ton of good thinking in this message thread...

...but... there is another ton of sloppy thoughts in this message thread.



Too much to deal with on a case by case basis, so I will leave it as an exercise for the student to sort it all out.

but I do have a thought I two I would like to share.

#1 No one discussed the fact that the drill transmissions themselves have DOUBLE ACTING one way clutches in them.

The motor can drive the output, but the output cannot drive the input. Using such devices on two inputs to a planetary gearset (or its equivalent -- any of a family of devices with equations of the form w1 + (R2*w2) + (R3*w3) =0 where R2 & R3 are allowed to be negative), you can avoid the some of the problems that Paul has had with his Thunder Chicken Tranny -- you don't need to use a worm gear drive for example.

#2 People who design robots really should think in terms of power. The speed and torque of a motor are more or less just an accident of birth. Given the right amount of power, you can always use a ratio to trade one for the other.

Thinking in terms of power you can see the advantages of using 2 motors to drive a differential even though the input torques to the two motors have to balance -- the output speed doubles at any given torque -- this is another way of saying that the POWER doubles -- if you pick your ratios right, you can do twice as much work per second. This is often a good thing.

#3 To my mind, the main reason for multiple motor drives is to increase power. If you want to a very different speed/torque curve (more speed now, more torque later) I think you will almost always be better off with a shifting transmissionrather than motors that engage and disengage, especially if you want to use the drill transmission as your shifter.

Relatively robust systems can be made tolerably easliy if you are willing to live without shift on the fly. Using multiple motors makes using the standard drill tranmission a bit more tricky but not as difficult as rolling your own shifter (imho). CVT's and other more complicated systems quickly reach the point of diminishing returns.

I have more to say, but that will have to do for now.

Joe J.
 


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