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Unread 24-11-2003, 08:45
Joe Johnson's Avatar Unsung FIRST Hero
Joe Johnson Joe Johnson is offline
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Trantorques and Biased motors...

As to keeping the center distance, M. Krass is right, all you have to do is keep the gear set such that the gears add up to the same number of teeth.

I often refer to it as "flipping a tooth" from on gear to another. For example, I may use 13:47 and 14:46 on the same center distance.

The problem with the above scenario is it is not much of a ratio shift in this case and more importantly, they are gears that are almost certainly not available from PIC Design, W.M. Berg, McMaster, Stock Drive Products, or Small Parts Inc. (In addition it is a bad example because unless the module is huge -- meaning the D.P. is small -- a 13 Tooth gear is not likely to fit on a Trantorque).

While I am thinking about it, I always add a bit (5-10 thousandths of an inch) to the theoretical center distance. Backlash will almost never hurt you. Gear tooth interference from out of round gears is a killer.

Back to the changeable gear ratio gearbox, it is not as simple as making sure that Trantorques fit in the center of the gear and that off the shelf gears are available. You have to really layout the gearbox to make sure all the gears actually package in your gearbox. Sometimes I have to move boltholes between the plates so that they miss the gears in extreme cases. It is easy to put the holes in place while I am making the plates. The key is to plan them from the start not to have to add them after the fact. You also sometimes have to reduce shaft diameter or move a motor or whatever in order to make sure all your gear combinations can be accommodated.

Now for this whole, motor bias issue. Essentially this is when a motor performs differently in one direction vs. the other. When you have a robot drive system that has the right and left sides mirror images of each other, the motors drive in opposite directions when the robot goes forward or backward. This leads to a robot that wants to turn (especially at the start).

Yes I agree that motor bias is real, particularly on drill motors. Where I disagree with folks is where the fix should be. Many people feel very strongly that it should be fixed by either putting another gear stage in one of the motors to flip it's direction or doing something else to make sure the motors turn in the same direction.

My take on this is that if this is easy, be my guest, but if it is driving ANY complexity at all, it is not worth the bother. Given a competent driver and drive time, it is very easy for a driver to learn how to compensate for this problem. Alternatively, in 2 lines of code, you can have the computer compensate somewhat for this effect. Many folks even go so far as to use the Yaw Rate Sensor to allow the computer to fix this problem.

Last year, for whatever reason, we had one side that was a bit less efficient than the other side; this made the machine want to go in an arc. WE LEFT IT ALONE. Our driver was comfortable making the corrections himself and we were comfortable working on other, more pressing problems.

I am sure that I will get a flood of zealots on the other side of this issue calling me a heretic, but... ...so be it. I have to call 'em how I see 'em.

Joe J.

Last edited by Joe Johnson : 24-11-2003 at 08:48.
 


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