"Motors and Drive train edition" of Fresh From the Forum

[Ken Leung]

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IV. Drive train
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Drive train is the most important component on you robot, especially with this years game where speed and pushing force is really important. After all, what good would a robot do if it can’t move around and do what it’s designed for.

A few of us were trying to understand how a drive train work, and we came to a few conclusion:

A drive train will go as fast as its drive motors spinning at free speed after reduced by the gear ratio, and slow down by internal friction. (Just the robot running by itself on the field)
And the drive train will push as strong as the drive motors pushing with stall torque after multiply by the gear ratio, OR with a torque just before the motor start tripping the circuit breaker, OR pushing as strong as possible before wheels start slipping.
Check out this thread for more detailed discussion
http://www.chiefdelphi.com/forums/sh...&threadid=1114


About Drive train advice…
Check out this thread:
http://www.chiefdelphi.com/forums/sh...&threadid=1179


calculation for speed...

It’s not easy to figure out how fast your robot goes just by calculation, with all the internal friction in drive train, and different combination of motors. You best bet is still put the drive train together, run it with enough weight of your robot, and time the robot. You will be surprised how easy it is to put on different gears and try out different speed/torque combination.
http://www.chiefdelphi.com/forums/sh...&threadid=1907

Once you tested the robot for speed, it is quite easy to figure out how much overall internal friction is in your system.
“You can see by the figures for free speed and the geared down speed (multiplied back up to the motor speed input to the gears) how much "friction" the gear box represents. Then, with the a Speed-Torque curve for the motor in question in front of you, enter on the torque axis, go up to the line, the at the line, move horizontally to the left, and read the speed. ” -Lloyd Burns
http://www.chiefdelphi.com/forums/sh...&threadid=2025


Gear shifting

With the different demand on the drive train this year (speed to get to goals FAST, and torque to push/pull goal(s) with other robot(s)), a lot of people are considering gear shifting as the solution. By shifting to between two gear ratio, your drive train can run either with high speed/low torque, or high torque/low speed. A lot of teams take advantage of the gear shifter on drill transmission, but there are a lot of other ways as well.
“Last year we used drill motors for propulsion. We used servos to shift gears. The problem is you had to be standing still to shift.” - Wayne Doenges
“See if someone has a picture of the HOT BOT from last year they had a trick shifting mechanism using little air cylinders. I don't know if they used it but it looked really cool” -Matt Reiland
“Our team is going to try to shift on the fly by having 2 sets of drive wheels that are always spinning (one set for high gear, one set for low gear) and putting the high gear wheels down with a pneumatic.” - thedillybar
“A transmissiom would be cool - anyome thought of using something smple like a bicycle chain dereiler” -Dr. Bot
http://www.chiefdelphi.com/forums/sh...&threadid=1845
67’s shifter, and 131’s wheel switcher
http://www.chiefdelphi.com/forums/sh...=9845#post9845
Team 45 Technokat’s servo gear shifter, one of the best design for using servo to switch gears. However, I do recommend you spending time developing your own design… It is a really good learning experience that way (taking apart drill transmission to understand the gear shifter... and figuring out a good mechanical design to apply the servo).
http://www.chiefdelphi.com/forums/sh...&threadid=1800


Multiple motors on drive train

Another solution to needing speed and torque on the drive train is adding more motors. More mechanical power means additional speed AND torque, although it is hard to get two type of motors working together (not impossible). The idea is that you have to match up the motors such that they are going at the same speed, and have similar reaction under load. You should figure out exactly what you want your robot do, before deciding if you want to gear shift, or use multiple motors…
“The bottom line is that the speeds are required to be in some ratio that is fixed by the gearing. Assuming the same voltage to the motors, the motors then simply act like the SUM of the two motors. The speeds are equal (or geared to a ratio) the torque out is like the sum of each motor at that speed.” -Joe J.
“Last year we connected the drill and Fisher-Price motors end to end, ran the output of both through the drill gearbox, and drove our track drive from the output of the gearbox… We had them running at the same speed and it worked pretty well.” -Kit Gerhart

One concern people have is that more motors might draw too much current. However, turns out the 60 amp circuit breaker is more tolerant that it says. Both from people’s experience, and from team update 2:
“According to a post on another thread, at a load of 100 A, the breaker takes nearly two minutes to trip and takes 8 seconds at 200 A.” -Kit Gerhart
http://www.chiefdelphi.com/forums/sh...&threadid=1623
http://www.chiefdelphi.com/forums/sh...&threadid=1175
“Also, you should design your drive system so that it is difficult/impossible to actually stall your drive motors” -Joe J.
http://64.119.166.94/forums/showthre...&threadid=1571


About using chiaphua’s as drive motor…
“If you look at the specs, you will see a top speed of 5500 RPM. I'd say that would take more than a 3:1 or 4:1 reduction to acheive a drivable speed.” - Jeff Waegelin
http://64.119.166.94/forums/showthre...&threadid=1728


About pairing up Drills and chiaphua’s…
Want to use both drills and chiaphuas in the drive train? Take a look at this thread…
http://www.chiefdelphi.com/forums/sh...&threadid=1544
And this:
“I suppose that you might get away with gearing up the Chiaphua’s and stuffing both into the Drill Trans. One problem is going to be inertia. You may be able to get away with the static loads, but the rotary inertia of the two motor (and gearing) may cause a very large dynamic loading condition.” -Joe J.
“Last year's game gave us a false sense of security when it comes to linking motors together.” -Paul Copioli
http://www.chiefdelphi.com/forums/sh...&threadid=1693


Care to share how much motors you are using on the drive train? Cast a vote in this poll…
What motors for your drive train? Poll
http://www.chiefdelphi.com/forums/sh...&threadid=1644