Re: Minibot climb rate
 

Hey Bill

Perhaps there is wheel slip in the beginning of the climb, thus prolonging the acceleration in the beginning of the climb. 0.83 lbs is a lot of friction for the minibot. My guess is the the friction is lower and the minibot can accelerate quicker, but slippage losses in the beginning of the climb slow down the time. It overcomes the slip when motor toque is lower and the rpm higher, thus higher up on the pole. From here is starts accelerating with its full potential. Overall it appears to accelerate the entire length. It would be like pushing (or distorting) the velocity curve to the right.

On our revised minibot, it appears to slip a bit in the beginning but accelerates quickly after it. We run a 2.5 pound minibot with a 3/8 od wheel. It climbs in about 1.5 seconds. We are defiantly running higher magnets forces then yours. By increasing the magnet force, you can prevent wheel slip and prevent the robot from flying off.

I will see if your numbers compare well to my calculator later tonight.

Re: Minibot climb rate
 

I think you may be right:

http://www.chiefdelphi.com/forums/sh...76#post1045576

Re: Minibot climb rate
 

We finished our last regional and did not implement our secret weapon:
(although we did tell the Boston Regional judges about it)
Rubber on rubber (1.2 CoF sliding friction) for the first 3 inches of vertical climb before switching to rubber to steel (0.3-0.6 CoF sliding friction).
We projected that would save us 0.2 to 0.3 seconds and still keep normal force low.

The rubber on rubber is minibot contact to a hostbot ramp that runs vertically parallel to the tower pole and then switches onto the pole before the minibot crosses the 18" line.

Re: Minibot climb rate
 

OK, more info and a video.

Kevin,
Shifting the battery up to the top and placing some surgical tubing at the back of the battery did the trick. Now our pivot point is the farthest point from the pole. This video is taken with a fairly depleted battery.

http://www.youtube.com/watch?v=P89k0hlHd1E

Either,

More accurate measurements:

Weight is 10052g, or, 2.32 lb.
Wheel diameter is effectively .45" when compressed against the pole.

So, based on our earlier discussions, it is really close to ideal.

Re: Minibot climb rate
 

2.215 lbs right now but we are a little slow at the beginning of our climb at about 1 second, we are going to tinker with our magnets to try and decrease the amount of time. We shall see how well this works in the next few days.

Re: Minibot climb rate
 

billbo911 - Team 1351's minibot had the exact same problem with falling off the pole. I'll try to implement changes similar to what you did; hopefully we can avoid catastrophe at Calgames. :) We already came close to destroying our extremely fragile Minibot at the Silicon Valley regional. It fell off of the pole twice during matches, where nobody could catch it - the .090 aluminum that forms its chassis barely survived.

Mark (or anyone else) - Other than increasing magnet strength, how can one reduce the wheel slip? I know there are obvious options like changing the tread material, but I'm wondering if any other techniques could also work.

Re: Minibot climb rate
 

Take a close look at the top of the battery. That "bump" is just a couple pieces of surgical tubing under some electrical tape. The battery is also raised up so that it is level with the robot frame. By doing this, the point that hits the sensor plate first is the bumper on the battery. The tubing compresses helping to dissipate some energy and also forces the minibot into the poll instead of away from it.

Re: Minibot climb rate
 

This sounds like the best solution:


I like this idea a lot. It is unfortunate that you did not have time to implement it. I hope your team will be able to try it out at an off season competition.

Bill our calculations show that your minibot should climb in about 1.3 seconds. I think its a little off, I am going to tinker with it a bit. Its set up to have drive-train efficiency different at low rpm than high rpm. Just to be clear, this is a separate efficiency from motor efficiency.

Re: Minibot climb rate
 

You can also modify the transmission or just take it off all together. Do the calculations with no transmission and a 3/8 shaft with surgical tubing as a wheel. I got under 1.5 seconds at max efficiency.

Re: Minibot climb rate
 

At the beginning of the build season, someone had suggested magnets and we quickly dismissed the idea, thinking with the potential weights involved, magnets would not be practical, except perhaps to hold a latching mechanism closed. As the build season progressed we experimented with many differnt concepts and prototypes. I favored a balanced approach as you describe. I found that anytime we would get close, one tweak would throw the whole thing off. When someone put together a prototype with a rare earth magnet, it became very simple. No muss no fuss, just get it to the pole and climb. That was the direction we went and it worked very well.

Re: Minibot climb rate
 

Yeah my team was in the same boat as you. I honestly thought magnets would not work. Now my students are amused that mentors can be wrong too.:)

Bill,
I fixed the calculations on my end, I am getting a climb rate of about 1.10 seconds. So I think slipping is defiantly slowing you down.

I have also been comparing my minibot to the the polycarbonate one here:
http://www.chiefdelphi.com/forums/sh...&postcount=191

Ours is the same mass but defiantly slower. I think another variable to consider is the accuracy of shaft placement. Ours is defiantly poor, the shafts are not perfectly perpendicular to the pole, so we are defiantly loosing efficiency. I don't know if this is common for others, but when we attach our un-powered minibot to the pole, it won't roll down unless we move it a bit. An accurate minibot should just slide down when not powered. Ours still slides down after the climb because it bounces off the top plate.

Re: Minibot climb rate
 

What?? Mentors wrong!! Seriously, never happens.:D

I've been thinking about the slipping. Though I know we have some really grippy "wheels", it is possible. It could also be that the shafts are spinning in the "wheels". We will try a couple experiments today to see if we can resolve it.

The reality is, we are going from a dead stop to max torque instantly, and fighting gravity while we are at it. The best option to overcome that is a curved ramp. I seriously doubt we have time to redesign out deployment system now. So.... stopping the slip is our best bet.

Re: Minibot climb rate
 

Given:

0.5 inch diameter wheel

0.09 Nm stall torque

0.4 coefficient of friction between pole and wheel (estimated, YMMV)


Find: required normal force to prevent slipping


Solution:


1) calculate the motive force at the wheel/pole interface produced by stall torque:

(0.09 Nm)/[(0.5/2 inches)*(0.0254 meters/inch)] = 14.17 Newtons = 3.19 lbs


2) divide the motive force by the coefficient of friction:

(3.19 lbs)/(0.4) = 8 pounds normal force required for no wheel slip


Edit:

To be clear, the above calculation is for the normal force on ONE wheel (shaft) powered by ONE motor.

If the wheel (shaft) is powered by two motors, then the number would double.

Re: Minibot climb rate
 

Different excercise from Ether's:

Using the second derivative of Ether's equation x(t), the acceleration versus time is:
a(t) = -D*e^(-B*t)
To find out when there's no more acceleration, set a(t) = 0 -- since ln(0) is negative infinity, we basically see that the minibot is always technically accelerating. HOWEVER we can see when it gets "close" to its max. speed by substituting in a very small number for a(t). In my case I chose 0.01 m/s^2 since that's small enough to become "unnoticable" IMO.

Sovling the equation for t, to see when the minibot is no longer accelerating gives us
t = ln(0.01/-D)*(1/-B)

Then we can plug that 't' back into the original equation to figure out the height at which the minibot is no longer accelerating.

Presuming:

• 0.40" Wheel Diameter
• 2.35lb Weight
• 0.8225lbs of friction (anecdotally determined)
• 2 motors, drive driven to the wheels
• No slip

The time of acceleration = 0.86 seconds
The distance of acceleration = 5.57 feet

Re: Minibot climb rate
 

Our frame which we are mounting our motors is actually holding up even better than we had expected even though it is only .040" thick. But we tinkered with the magnets and are now running even faster and have now begun to launch off the pole as we approach <1 second....