Minibot climb rate

[PVCMike]

I too am curious to know what the sub 3 second mini-bots are using for gearing. Thank you to the few people who shared videos! No proof like video!

[wireties]

Originally Posted by rbaker8734

I think that you are forgetting about torque!!!

And gravity!

[inkspell4]

Can anyone give hints on haw to make the climb rate as fast as possible any ideas will bee much appriciated

[liam.larkin]

Some quick math and some analysis of the torque/stall curves for the motors have shown us that the most optimal gearing rate is .5/1 please check my math. Additionally using this gearing rate we found that a robot if carefully constructed can weight about 5-6 pounds. This gets you to the top in about 4.0-4.75 seconds. This has been pretty much confirmed by our present prototype minibot design.

These individuals claiming 2-3 seconds I have questioned unless the robot is like 3 pounds or they are direct driving but with that said you direct drive the motors get really close to stalling after only a few pounds. I would love to see a video of a team beating 4.0 seconds

[cmass]

your 5-6 pounds is way wrong.

Start with only what you need and then make that lighter. I'm certain that 4# is possible and teams will do better than that.

With standard out of the kit parts and not modified we got to 3 seconds and < 5#.

You are missing something.

[cmass]

Originally Posted by klmx30302

With 4" wheels the motors will backdrive easily, the way we get ours down is by putting some long bolts on the bottomside of the robot. next get a pole and attach a ziptie in a large loop at the top, use it to gently pull on the bottom of the minibot and it should backdrive all the way down.
Here is a video of our minibot prototype:
http://www.youtube.com/watch?v=8IKLSgfXqcM

No ladders, no poles on the field, look at the Q&A forum. Keep working!

[Gary Dillard]

Originally Posted by liam.larkin

Some quick math and some analysis of the torque/stall curves for the motors have shown us that the most optimal gearing rate is .5/1 please check my math. Additionally using this gearing rate we found that a robot if carefully constructed can weight about 5-6 pounds. This gets you to the top in about 4.0-4.75 seconds. This has been pretty much confirmed by our present prototype minibot design.

These individuals claiming 2-3 seconds I have questioned unless the robot is like 3 pounds or they are direct driving but with that said you direct drive the motors get really close to stalling after only a few pounds. I would love to see a video of a team beating 4.0 seconds

OK let me check your math, 2 ways

Suppose your minibot weighs 5 pounds, you use 2 motors with a .5:1 gear ratio, and you use 4 inch diameter wheels (using nominal numbers and ignoring start up and inefficiencies for now). The robot exerts a torque on the wheel of 10 inch pounds (weight times radius), and the gear ratio makes the torque on the motors 20 inch pounds, or 10 inch pounds on each motor. Looking at the motor curve, they run at 77.8 rpm at 10 inch pounds load (sorry, I always work in English units). 77.8 rpm on the motor divided by the gear ratio and divided by 60 gives you 2.593 revs per second at the wheel, times pi times the diameter gives you 33 inches per second up the pole. If you start at the top of the line (30 inches off of the floor) you have to travel 92.25 inches (122 minus 30 plus 1/4), which equals 2.83 seconds.

Now for optimum performance (tweaking the gearbox and/or the wheel diameter), you want to run the motors at peak power which is 9.36 Watts times 2 or 18.72 watts, which converts to 165.67 inch pounds per second. Moving 5 pounds times 92.25 inches and dividing by the power gives you 2.784 seconds feasible.

So you can see that for a robot less than 5 pounds, even including some inefficiencies 3 seconds is absolutely feasible.

[billbo911]

Do the math. Save the world!

[IKE]

The "little bits" here and there can kill you on the mini-bot. Our initial prototypes were 0.5 lbs heavier than we estimated (5 lbs). We had "geared" to be on peak power for those runs, but the 10% heavy also meant we needed about 10% more torque which took us down about 10% in power. Thus the 10% more weight would have added about 22% (1.1/0.9=1.21), but weight there is more... This 10% more mass also added additional friction which meant more torque, and again lower speeds therefore (1.15/0.85=1.35). Thus a mini-bot that we thought would climb in a little over 3 seconds ended up taking over 4 seconds (3.25 * 1.35 = 4.4 seconds). With certain mini-bot designs, a little weight can kill the performance for that design. This was initially very frustrating, but after review it is much more encouraging.

Back to some more iterations.

[JesseK]

Originally Posted by Gary Dillard

So you can see that for a robot less than 5 pounds, even including some inefficiencies 3 seconds is absolutely feasible.

Check your math. The short story is -- equations only tell us where to start with MINIBOT prototyping, yet only real prototypes will tell us if the theory is sound.

The problem with such simple Power->Torque conversion in this case is that it does not account for the increased time the MINIBOT will take to get to its max speed under a higher torque load. Kinematic equations are non-linear with that respect when combined with the inverse relationship of an electric motor's speed-versus-torque relationship, thus analyzing a distance-versus-time chart between the two options may surprise you.

The problem is, distance-versus-time isn't quite as straight forward as it seems. In my analysis, I have to piecewise the graphs into acceleration sections and max-speed sections based upon the calculated time it take to accelerate to max speed under load.

So while I don't claim to have my calculations be 100% precise to real-world conditions, they do show that MINIBOTS with a 0.5:1 ratio with 4" wheels and a 5-lbs of weight will spend over 75% of their climb time in the acceleration phase, resulting in a 6+ second climb. 0.5:1 ratios with 3" wheels have 4-4.5 second climbs (50% of which is acceleration). Direct-drive 4" wheels have about the same times due to greater efficiency motor-to-pole coupled with less mass due to no gearing (all else equal), even though its max speed is technically slower than 0.5:1x3" wheels on a flat field.

...or they are direct driving but with that said you direct drive the motors get really close to stalling after only a few pounds

Actually, putting a 0.5:1 ratio on the MINIBOT puts it's torque load twice as close (ish) to stall as direct drive does.

[Gary Dillard]

Originally Posted by JesseK

... will spend over 75% of their climb time in the acceleration phase,

That doesn't pass the sanity check. If it takes 3-4 seconds to get to 33 inches per second, that's an acceleration of about 10 inches per second^2, or .03 g's. It doesn't start with a load of 0 g's, it starts with 1g plus acceleration, so that's about 3% reduction in available power using your numbers.

[Kevin Sevcik]

Originally Posted by JesseK

... 5-lbs of weight will spend over 75% of their climb time in the acceleration phase, resulting in a 6+ second climb....

I'll ditto Gary here. Check your math on this one. I was thinking along similar lines two weeks ago and whipped up a spreadsheet to do all the numerical integration for me. The acceleration phase was stupidly short, in the tenths of a second. But we don't need a complicated spreadsheet to tell us that. If we gear for peak power, that's 1/2 stall torque. So full stall torque is 2 times the weight of the bot, is 1 g instantaneous acceleration at the beginning. When you get up to half your final speed, you're still doing 0.5g. At 3/4 final speed, it's still 1/4g, and so on.

In fact, if you toss out friction, this is a very nice linear system to model. The fact that your torque/force decreases linearly with your speed makes it just another damping term, so it actually works out to a simple mass-damper system with an external force. So if I wasn't so lazy, I could tell you the exact time constant based solely off the robot's mass and motor characteristics.

[Steve_Alaniz]

...... And then... there was this

http://www.youtube.com/watch?v=sO4uNj44oZE

I'm SOOOOOOoooo envious!

Steve

[JB987]

Don't believe everything you see...all may not be as it seems. See the other related thread highlighting 1625's minibot climb. Do you clearly see the 1.5 lb. battery on the skyrocketing minibot? Do you vaguely see it?

[ttldomination]

Originally Posted by JB987

Don't believe everything you see...all may not be as it seems. See the other related thread highlighting 1625's minibot climb. Do you clearly see the 1.5 lb. battery on the skyrocketing minibot? Do you vaguely see it?

Right towards the end of its climb, you can see the battery fly up past the robot and fall down.

Or some other relatively heavy objected connected via wires.

- Sunny