Minibot climb rate

[Mark Sheridan]

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.

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/showpost.php?p=1045662&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.

[billbo911]

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.



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.

[Ether]

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.

[JesseK]

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

[crazyStone]

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....

[waialua359]

Our team is pretty excited about our new and improved minibot.
It is currently performing between 1.1 - 1.2 seconds from dead start (no ramp).
Our old one was good enough in weeks 2 and 4, but knew that it had to improve to stay with the mix of teams that are hovering above, at, or right below the 1 second mark.

[jblay]

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.

It seemed to me in the initial video like the plate on the top of your tower was slanted which may have caused your minibot to fly off. Its a problem we had before connecticut and then I realized that was the reason and we never had the issue again.

I don't know if you tested with a full battery or not but the battery being full would change the speed of the minibot I imagine and cause it to go slower and could be part of the fix for the minibot flying off the pole which isn't an actual fix.

[wireties]

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.

I surrender and am ordering magnets from K&J to try before the off-season events!

[Brandon Holley]

I don't know if you tested with a full battery or not but the battery being full would change the speed of the minibot I imagine and cause it to go slower and could be part of the fix for the minibot flying off the pole which isn't an actual fix.

I'm a little confused...you are saying that a full battery will cause the minibot to go slower??

-Brando

[Ether]

Our team is pretty excited about our new and improved minibot.
It is currently performing between 1.1 - 1.2 seconds from dead start (no ramp).


Could you tell us more about it?

- weight?

- wheel (shaft) diameter?

- wheel (shaft) tread material?

- magnets or something else?

- have you measured the friction?

[Alpha Beta]

I'm a little confused...you are saying that a full battery will cause the minibot to go slower??

-Brando

Well, of course. A fully charged battery has all of those extra electrons to lug around. Oh wait...that's not right. They're still there just switching between the anode and cathode. I guess I'm confused too. :rolleyes:

[Team 288]

Could you tell us more about it?

- weight?

- wheel (shaft) diameter?

- wheel (shaft) tread material?

- magnets or something else?

- have you measured the friction?




I can probably answer for this person, by inserting 931's minibot, instead of theirs.

931 has a minibot that climbs justbelow the 1 second mark.

Weight : 2.2 lbs

Wheel shaft diameter : .45 inches

Wheel shaft material : Electrical tape(standard)

27 lb pull magnet

No. =)

We'll have 6 of these at worlds and are willing to let other teams borrow (teams in Archimedes get priority)

Here's a video with a 7 foot pole. We got .7 seconds for total climb time.

http://www.youtube.com/watch?v=cUSCBbhu4JM

[jblay]

I'm a little confused...you are saying that a full battery will cause the minibot to go slower??

-Brando

Didnt see that I wrote that backwards the first time. I meant the battery not being full would slow the minibot down cause before the video he says that the battery was not full.