Minibot climb rate

[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.