Rather stupid questions ahead! Consider my rookie year and the only year I was a student was 2009, so I never got the normal drivetrain lessons I assume anyone else has.
All other things (rate of acceleration, climbing over large bumps, swerve drives, etc) aside, say I’m building a one speed drivetrain designed to drive at a decent speed and to push for a few seconds here and there, like I had to with 2791. I’ve picked up this from Chief:
The two ways to maximize pushing force are to maximize robot weight and to use wheels with a high CoF.
When pushing, you want your wheels to be traction limited so that your wheels slip as your motors draw 40A current; this way you do not trip the PDB’s circuit breakers.
With those two things in mind, I ran some numbers to figure out how to best gear the Toughboxes we were using. Of course, being a genius I ran the numbers for an 8 CIM drivetrain, not a 4 CIM drivetrian, so the robot was torque limited. Not living that one down anytime soon.
Running the numbers again, I had a lot more difficulty getting a drivetrain that used roughtop while remaining traction limited at 40 amps current draw. Something like 4 feet per second would end up being your maximum speed if you followed assumption 2 to the letter. So I have some questions…
Is assumption 2 wrong? Do teams get away with pushing using a drivetrain and pulling 60 amps or so for the 1-2 seconds you’re pushing? Do they rely on needing less than “maximum” pushing force since once your opponent starts moving the force goes way down due to the change to kinetic friction?
Do people use roughtop tread if they don’t plan on direct pushing matches? I mean, it’s used everywhere, and while I’m willing to bet most drivetrains from all the top teams shift between a traction limited and torque limited gear, I bet at least some don’t plan on doing a lot of pushing. Is it used on normal drivetrains for high resistance to pushing, even if they don’t have a method of becoming traction limited?
The main thing stopping me from saying “yes” to both of those are reading posts by people like Paul Copioli and whatnot that advocate being traction limited. The particular one that stumps me is way back from 2005:
(I use this one for reference since my team was using a very similar thought process when selecting transmissions this year)
I read this post and then got really confused. Even lowering robot weight to 130 (pre-bumper 2005 robots) got me numbers way different than in this post for drivetrains that run at that speed under load. So, maybe my math is completely off and you can get 6 fps out of a drivetrain that’s traction limited, and more with an FP in there, or the motor specs were lowered since then, or something.
So basically, can some of you smarter people give me some tips for gearing drivetrains in the future?
In general, limiting yourself to 40A before you lose traction is overkill. You almost never need to push against a “wall” for more than a few seconds at a time, in which case you can use a much higher figure (easily 2x as much) before worrying about tripping the breaker.
In the drive designs that I have done, I generally first look at things without worrying about the thermal breakers. Assume that you can draw the specified motor stall torque and current for brief durations of time without any issues, and design around that instead. Then, verify that your calculated currents with normal loads and at maximum tractive force aren’t going to be major problems. (For a space shuttle, I would model everything. For a FIRST robot I make assumptions and then verify that I wasn’t too far off).
If I recall correctly, a 4CIM drivetrain with roughtop and a maxed-out robot reaches a point where gearbox torque and maximum tractive force are approximately equal somewhere around 8-9 feet per second (depending on what efficiencies you assume). We verified this experimentally when testing our drive this season (when pushing against a wall, you could force the wheels to stall by pushing down onto the robot - thereby increasing normal force and therefore maximum tractive force; and you could force the wheels to slip by applying a slight amount of “lift” to the chassis).
JVN’s design calculator white paper can help you do all of the above.
I feel like rerunning these numbers again, so here I am. It’s been about 4 years since I ran them last, and I got numbers very similar to Paul last time.
Lets assume that the torque is linear in current. It’s a pretty good assumption, if you check the physics. We can then use the following data off the datasheet to get the torque at 40 Amps.
343.4 Oz-In stall at 133 Amps.
0 Oz-In free load at 2.7 Amps.
This gives us 98.3 Oz-In at 40 Amps. Which is 6.14 inch lbs.
Lets assume a coefficient of friction of about 1.2. That’s not too far off for Roughtop. Assuming a 150 lb robot, our wheels will slip at 180 lbs of pushing force. Lets choose an arbitrary wheel size of 4". So, the torque at slip on the sum total of all our wheels is 360 inch-lbs.
To achieve that torque at 40 amps with 4 CIMS, we need to have a gear down of 360 / (6.14 * 4) = 14.6 to 1. Now, let’s use the free speed of the motor to calculate our top speed.
Off by 10% from the 6.5 ft/sec number, which is pretty much loose change given that I probably don’t know my weight or coefficient of friction that well.
While it may seem overkill, we definitely we’re spinning our wheels pushing for nearly 10 seconds strait this year.
We had a 4 CIM base with 7" diameter wheels and use the AndyMark Super Shifter with the optional gear package.
Low Gear: 30.1:1
High Gear: 7.5:1
I just posted the latest version of our analysis file as it is significantly better then the previous ones. We used it to determine all of our gear ratios: http://www.chiefdelphi.com/media/papers/2292
Another design consideration is your operating voltage. Your motor power is proportional to the square of your operating voltage. We’ve started to monitor our voltage during the matches and found it can easily drop below 11V under load (in some cases 7V! [bad battery] :yikes:). Hence - much to my dislike as a mechanical guy - we’ve started to design with a nominal voltage of 11V.
With a single speed gearbox, speed and accelleration are trade offs.
As you increase your pushing force, you can speed up faster, stop faster, and turn faster. But your maximum speed drops.
In this year’s game we sacrificed top speed for accelleration and pushing force, due to the fact that it was pretty apparent that there would almost never be a time when a robot would get up to full speed due to the congestion on the floor.
We got the 14:1 upgrade kit for our Toughboxes, and… wow… that had to be our best performing drivetrain EVER. (2CIMs per side, through 14:1 toughbox, direct to 6"x1.5" IFI traction wheels with roughtop).
I think I’d go with the slower, quicker robot again, even in an open field competition. Robots tend to spend more time maneuvering than driving in a long straight line.
It will change from game to game, of course… but what good is a theoretical top speed that you never reach?
We chose to make our robot so small this year we ended up with both.
At 106lbs with 4 cims and 2 toughboxes chained to 6 plaction and 4 2008 kit wheels it maxed out around 13fps and accelerated to full speed in roughly 15 feet. As a bonus we could push pretty well too
It sure was a shock to all of us how our drivetrain performed this year, I will say it’s the best one we have ever made.
