Team Update #18

[martin417]

Update from Peachtree:

We deployed our minibot in 2 practice matches, and had no issues triggering the tower. For reference, the minibot weighs about 2.8 lb.s and runs the pole in about 1.6-1.8 sec.s We are using a 3-way household light switch to turn off the juice and short the motor leads for the return trip. I haven't measured the force required to flip that switch, but it is more than 4N.

[boomergeek]

Originally Posted by sanddrag

Thank you Ether! This is the first post I've seen all season to do a proper (yet quite simple) calculation of how fast a minibot of given weight can get up the pole. I've been meaning to do this with my students all season long, but was too busy with the arm and other things. I think now is the time to revisit it.

Trial and error is one thing. Knowing what you're shooting for, because you did some calculation first, is quite a different (and much better) thing.

I believe Ether's number are based on actual lab trials.
I.e., the specification provided by Tetrix was no where near sufficient to extrapolate to a 10.8 ft/sec estimate. It was only a team member that went in a lab with dynamometer capability and did lab trials with and without the gearbox attached that allowed for the calculation.
Thank Richard for the creating the data AND sharing the data that allows physics teachers to let this rise above tinkering status.
http://www.chiefdelphi.com/forums/sh...+dynamomet er

[Ether]

Originally Posted by boomergeek

I believe Ether's number are based on actual lab trials.
I.e., the specification provided by Tetrix was no where near sufficient to extrapolate to a 10.8 ft/sec estimate. It was only a team member that went in a lab with dynamometer capability and did lab trials with and without the gearbox attached that allowed for the calculation.
Thank Richard for the creating the data AND sharing the data that allows physics teachers to let this rise above tinkering status.
http://www.chiefdelphi.com/forums/sh...+dynamomet er

He (along with many others) already thanked Richard in post#5 of the thread you linked. But thanks once again, Richard :-)

Be aware that the calculation does not include any consideration of the time it takes the bot to accelerate to the indicated top speed. So if you use the top speed to calculate how long it will take to get to the top of the pole, the answer will be optimistic.

I'm going to take a look at a calculation that includes acceleration. I'll have to eyeball data from Richard's power vs torque curve (I didn't see raw data posted anywhere - is it available?). From other Tetrix curves I have seen, the motors are a lot less linear than other motors we've worked with, so I'll least-squares-fit a polynomial. Since the torque depends nonlinearly on speed, and the speed is the integral of acceleration which in turns depends on torque it's an interesting differential equation.

[boomergeek]

Originally Posted by Ether

He (along with many others) already thanked Richard in post#5 of the thread you linked. But thanks once again, Richard :-)

He was giving you credit for seemingly being the first to provide the physics formulas I think Richard had provided that in post #7 of that now famous thread.

I guess there is always a healthy back and forth between thinking of the academic physics (thought experiments) and going into the lab to make fundamental measurements AND sharing them.

Those that have sophisticated tools like a dynamometer and share the results with everyone are making the math/physics very valuable.
Speed is a relatively easy thing for most teams to measure: actual motor performance is not and the vendor published curves are not always sufficient to the challenge.

FIRST also inadvertently made understanding of physics more important in the trigger mechanism. It's all good. There are physics lessons everywhere.

[RMiller]

Going through the thread and looking for last week's events:
San Diego had issues (Cory (254), Jon (987))
Waterford worked okay (Kara (1189) (assume this is where she was), Zach (2337))
Wisconsin worked fine (Al (111))
Kansas City had issues (Jeff (1986))
Pittsburgh seemed to work (Ken (527))
Florida worked mostly (Alex (744), Jared (341)), Jared mentioned some issues, but did not have specifics
Lake Superior worked (me (next to the field all four days))
WPI had an issue for team 358 but few other issues (Chris (2791))
NYC and Israel have no reports in this thread.

For this week:
Detroit has had success (Chris (51))
Peachtree is working (Martin (1771))

[JesseK]

Here's a fun image, generated here. Red line is Ether's theoretical diameter, and the motor load is indeed at max power. Yet the iterations show that a slightly different diameter can squeeze out a bit more due to acceleration being a slightly larger-than-anticipated factor.

Assumes:
Battery voltage of 14.4V
6755 RPM Free Speed
0.095 Nm Stall Torque
(Extrapolated the stall/free currents based upon a realistic efficiency curve)
2 motors

100% wheel-to-pole efficiency
direct-drive minibot straight off the motor
2.3 lb weight
Minibot goes exactly 7.5 feet straight up

[Ether]

Originally Posted by boomergeek

He was giving you credit for seemingly being the first to provide the physics formulas I think Richard had provided that in post #7 of that now famous thread.

Not to start an argument here Dick, but the calculation I provided is somewhat different from the one Richard posted in #7. It's more similar to the one jreuter posted in #25. For the record, I hadn't read either of those posts until just now when I went back to re-read the thread after seeing your post. So Kudos to Richard and jreuter. I invite anyone interested to read that entire thread.

Does anyone know if raw (numerical) dyno data has been posted anywhere, which includes motor current and speed?

[Katie_UPS]

Originally Posted by RMiller

Wisconsin worked fine (Al (111))

While I'd normally take Al's word like one might of God, I'm not sure if this is true. There was an elims match where 234 deployed a minibot which didn't trigger, but definitely made it to the top. I was in the pits (save for 1675 matches), so this might've been the only time (freak incident or something like that).

[Dad1279]

Originally Posted by RMiller

.....
NYC and Israel have no reports in this thread.
.....

I reported only our experience at NY. There was one instance where our minibot did not trigger the top, but it was counted. All of our other deployments wee successful.

[Mike Soukup]

Originally Posted by Katie_UPS

While I'd normally take Al's word like one might of God, I'm not sure if this is true. There was an elims match where 234 deployed a minibot which didn't trigger, but definitely made it to the top. I was in the pits (save for 1675 matches), so this might've been the only time (freak incident or something like that).

In the match you're referring to, 234's minibot was moving up the pole so slowly that I doubt it hit the top with enough force to trigger the sensor. From what I saw, it pretty much died as it hit the top, if it even reached the top. All minibots that I saw climb the pole with decent speed, triggered the sensor.

[XaulZan11]

Originally Posted by Mike Soukup

234's minibot was moving up the pole so slowly that I doubt it hit the top with enough force to trigger the sensor. From what I saw, it pretty much died as it hit the top, if it even reached the top.

Yeah, the head ref had to take out a ladder and a piece of paper to determine if there was space inbetween the minibot and the top of the tower. The decision would have determined the match (and who advanced to the semifinals) if it wasn't for a red card on the other alliance.

[jspatz1]

Originally Posted by JesseK

Here's a fun image, generated here. Red line is Ether's theoretical diameter, and the motor load is indeed at max power. Yet the iterations show that a slightly different diameter can squeeze out a bit more due to acceleration being a slightly larger-than-anticipated factor.

Assumes:
Battery voltage of 14.4V
6755 RPM Free Speed
0.095 Nm Stall Torque
(Extrapolated the stall/free currents based upon a realistic efficiency curve)
2 motors

100% wheel-to-pole efficiency
direct-drive minibot straight off the motor
2.3 lb weight
Minibot goes exactly 7.5 feet straight up

This graph matches our experimental results. Our climb time was minimum at .40" dia.

[Lil' Lavery]

Just witnessed a false positive on the Bayou webcast. Minibot was about halfway up the tower when the lights triggered.

[Al Skierkiewicz]

Katie,
I discussed this in another thread I think. I was in the scoring area about ten feet from the minibot. It was spinning it's wheels as it climbed and when it reached the top, the wheels continued to spin but not enough force was applied to the plate. The Head Ref performed the paper test and initially determined that it had reached the top. Then he remembered a ruling about moving the plate. After checking the rules, I watched him and the rest of the refs do this, they determined that the plate had to move. Although I was not close enough to hear the refs conversation, I was sure that there was a a ref at the base of the tower when the minibot went up. A second check proved that the plate didn't move when the minibot was removed from the tower. His change in ruling was based on this second and critical check.

[Katie_UPS]

Originally Posted by Al Skierkiewicz

Katie,
I discussed this in another thread I think. I was in the scoring area about ten feet from the minibot. It was spinning it's wheels as it climbed and when it reached the top, the wheels continued to spin but not enough force was applied to the plate. The Head Ref performed the paper test and initially determined that it had reached the top. Then he remembered a ruling about moving the plate. After checking the rules, I watched him and the rest of the refs do this, they determined that the plate had to move. Although I was not close enough to hear the refs conversation, I was sure that there was a a ref at the base of the tower when the minibot went up. A second check proved that the plate didn't move when the minibot was removed from the tower. His change in ruling was based on this second and critical check.

I wasn't sure. I was on the sidelines rooting for their alliance. Thanks for clearing that one up