Drive Train design roadblocks

[Adam Shapiro]

As Henry pointed out to me just a few minutes ago (which I'm very surprised never occurred to me), it seems the second current reading is for the total draw from both gearboxes. A simple, well put answer, it would seem.

Now I am faced with figuring out what factor is causing my calculations to produce such high currents at (theoretical) max speeds much lower than those of other teams. For instance, with the gearing I'm now working with I have a current draw of 97A (194A total) at a max speed of 10.5 ft/s. Any suggestions?

[Ken Patton]

Adam-

You are right to be doing some detailed calculations like the JVN calcs as a way to figure out the potential performance of your drive system. Even if you don't end up using the design, the calculation exercise is still educational. So don't give up on that approach - just keep refining the input assumptions as you get more info or data.

My first inclination is to suggest that your assumed efficiencies (you quoted 60% overall, I think) are too low. This definitely will drive your top speed to a lower value and your currents (at a given speed) to a higher value. The efficiencies posted earlier in this thread are pretty accurate, and if you do a good design job (rolling elements, get rid of sliding contact wherever possible, minimize misalignment under load) you can hit the high end of those values.

One other source of efficiency loss is the tire-to-carpet interface. This is maybe a 93-95% type of number. Soft sticky tires are lower (90-93% maybe).

I think a good system with a simple spur gear drivetrain (no planetaries, no belts, just gears and chains and wheelchair wheels) should be able to be in the mid-80's efficiency-wise.

Advanced topic:
For people who are doing more than a closed-form calculation (i.e., they are using calculus to derive a simple equation that they can numerically integrate - the type of stuff you would do in 1st year of engineering school) I would suggest adding a loss term that is a constant with respect to speed. For a *very* efficient gearbox I use a 0.2 Nm spin loss applied to the output of the gearbox, and a 12 N drag loss applied to the robot mass.

Hope this helps,
Ken

[Matt Adams]

Originally Posted by Adam Shapiro

I figured no more than 60A per side as the main breaker can only handle 120.

Ah... this is a great time to throw some information out there. This is a Matt Adams story about engineering, tears and tragedy. I hope you have Kleenex.

461 was running 4 motors all designed around keeping them around the 40A mark and we had no problems... we were drawing around 150A pretty regularly without problems... until we ran over a dozen back to back matches in the finals at nationals. Despite the fact of this untimeliness, they held up very well above listed capacity for said reasons. (Hint: Keep circuit breaker cool.)

Overall, don't worry about the 120A breaker. Check out the spec sheet. It has no problem going 30% beyond that for 20 seconds or so. You typically won't be tripping that in a match, or even back to back matches if you make an effort to keep them cool.

The 40A breakers, however, I think are much less forgiving. I would be much more concerned about keeping these guys in check. My advice is to design your motors and loads evenly around the 40A breakers.

Good luck!

Matt

[JVN]

Originally Posted by Adam Shapiro

...but now I seem to be stuck on one factor on the JVN calc sheets..

Adam,
The things already posted in this thread should be sufficient to answer your questions. It's all good stuff!

I appologize for the lateness of this reply, but I've been on vacation for the past week+ (a little calm, before the storm).

I intended to get around to revising the spreadsheet, and polishing some of the less intuitive sections before kickoff this year. Unfortunately, it probably won't happen in the near future. Ya never know, we'll see how much free time I have.

Hope you solved your design problems.

Good luck,
John