Shifting Gears

[Joe Johnson]

Originally Posted by George1083

What if you had treads driven by a 3" pully instead of 6" wheels? I'm pretty sure that would satisfy the 10 fps and 150 lbs requirements without burning up.

The size of the wheel is immaterial to the calculations.

If John wanted to do ALGEBRA rather than ARITHMATIC (he did not do MATH as he claimed ;-) he could have made the exact same arguments with the size of the wheel being any arbitrary size.

I will answer his comments in another message, but rest assured that the size of the wheel is not part of the answer.

Joe J.

[Joe Johnson]

John's analysis is right as far as it goes in that his arithmatic is correct.

What he shows is that a 2 motor solution is going to be MARGINAL at meeting the 10fps top speed and the 150 lbs pushing force criteria.

His main objection is that of current draw exceeding the rated current of the breakers at the 150 lbs rating. Yes this is true. But this is where you have to balance your particular plan for the game and your drive skill.

While you cannot stall for long periods of time, it the breakers can go a quite a bit longer than many of you folks seem to realize.

I don't have the Snap Action data sheet handy, but from the Bussman website, you can clearly see that a 40 Amp fuse can carry 80 Amps for 20 seconds before tripping.

20 seconds of pushing is a LOT of time if your driver is trained to know where the edge of the envelope is.

If you are Wildstang or Chief Delphi or one of the other teams that love Swerve, you can win not by brute force but by pulling at angles.

So... ...I feel I am just justified in the 10fps & 150 lbs with 2 motors per side. It is a close thing but not out of the bounds (especially if you recall that I discussed acceleration as essential a stall condition for the motors in an earlier part of this thread -- this is why you will likely have less than the calculated times to trip at the end of the match even if you did not get in any pushing fights during the earlier part of ther match).

FINALLY, I want to emphasize that I am NOT saying that I agree that 10fps and 150lbs is acceptable. But if is were, it is (just) possible with a shifter OR with 2 motors.

That's my story and I am sticking to it...

Joe J.

[JVN]

Originally Posted by Joe Johnson

What he shows is that a 2 motor solution is going to be MARGINAL at meeting the 10fps top speed and the 150 lbs pushing force criteria.

20 seconds of pushing is a LOT of time if your driver is trained to know where the edge of the envelope is.

FINALLY, I want to emphasize that I am NOT saying that I agree that 10fps and 150lbs is acceptable. But if is were, it is (just) possible with a shifter OR with 2 motors.

Joe, you are 100% correct.
I myself never double checked the breaker specs. While I DID know they were capable of exceeding their rating, I had no idea for how long. You are right, 10fps and 150lb are possible to achieve from the same gearbox, using nothing but 4-motors and a dream.

However... I should now revise my previous statement.
It may be possible, but it is not practical. (or at least... I'll NEVER do it).

Now this is more of a "personal preference" type debate... but when I design drivetrains/gearboxes: I keep my current draws down as far as possible. (Slower is safer!)

I almost always design for continual usage, because... who knows. In 2002, we saw matches where teams were forced to go into extended pushing matches for close to the full 2 minutes... since we're dealing with a hypothetical game... I guess you are correct. However, my read on Matt's statement is for 150 lbs of pushing continuously...

When I am designing, marginal is NEVER good enough.
I like my drivers as far away from the "edge of the envelope" as possible, at all times.

Why limit yourself to 20 seconds? Sometimes that extra second of pushing can mean the difference between victory and defeat, and a popped breaker (even one that resets soon after) can often mean you watch the rest of the competition from the stands.

Whenever one designs to live on the edge, he might eventually find himself plummeting over the side. This is why, I never push my motors too hard. (Those FP's sure have a distinctive smell when they're overworked and underpaid!)

In this competition, reliability is key.

So, you've gotta ask yourself...
Why not just downshift and call it a day?


John, who wonders if CD9 will have a shifter under the hood.

[JVN]

Originally Posted by Joe Johnson

If John wanted to do ALGEBRA rather than ARITHMATIC
(he did not do MATH as he claimed ;-)

Been there, done that... made a spreadsheet that does it for me.

Stay tuned folks... depending on how fast this break goes by, there may be a new whitepaper or two coming out.

In this particular instance, I thought a plug and chug solution could prove my point effectively. Remember folks: it's not the size of your formula, it's how you use it.

[Joe Johnson]

Originally Posted by JVN

Been there, done that... made a spreadsheet that does it for me.

Stay tuned folks... depending on how fast this break goes by, there may be a new whitepaper or two coming out.

In this particular instance, I thought a plug and chug solution could prove my point effectively. Remember folks: it's not the size of your formula, it's how you use it.

All in fun John, All in fun. I had no doubt that you actually did the algebra and you are right that your example was better illustrated with a specific wheel diameter rather than the generalized solution. I was just having a bit of fun with you since you put your ***DANGER MATH ALERT*** disclaimer at the front of your posting.

Keep up the good work.

Joe J.

[Matt Adams]

Originally Posted by JVN

150/2 = 75 lbs per gearbox. Using a 6" wheel we can see that we need 50.84 N*m of output torque from each box to achieve 150lbs of linear pushing force.

Ah ha!

John is using the ole.... "I'll state a forumla and then use different units on the output" trick.

You need 75 lbs on each gearbox. You have a 6 inch wheel, with a 3" moment arm, which means you need 225 in-lbs, or 25.42 N-m. Maybe John used the diameter instead.

This halves the rest of the current requirements down to around 42.5 amps on the drill and 35 amps on the atwood... with a little tweaking they could be both modified to get under the 40 amp limit.

Admittedly, the numbers show that this will be pushing 160 amps altogether... which is a little too high for my blood.

and so continues the thread that never ends...

Matt

[JVN]

Originally Posted by Matt Adams

Ah ha!

John is using the ole.... "I'll state a forumla and then use different units on the output" trick.

You need 75 lbs on each gearbox. You have a 6 inch wheel, with a 3" moment arm, which means you need 225 in-lbs, or 25.42 N-m. Maybe John used the diameter instead.

Whoops...
Matt IM'd me earlier tonight, and YES... it's true. I was off by a factor of 2, early on in the calculation -- I used the diameter of the wheel for one of my calculations (not all of them) by accident.

Although... even with the correct calculations, the drivetrain is still under more load than I (personally) would find acceptable during a pushing match. However, it is now much closer to being arguably "good enough".

So Joe, Matt... I admit defeat.
As usual... a factor of 2 is the death of me!
(This is why I triple check ACTUAL design calculations I'm doing).

Eating his words,
John

[Joe Johnson]

Don't feel so bad. I didn't even do the calculations...

...when I am actually designing a robot, I do the calculations (though I sometimes still get it wrong -- ask me why our robot had a 6 tooth sprocket last year ;-), but for this thread I just relied on my experience with 2 motors per side in 1999, 2001, & 2002. I was confident that it could be done and just signed on the bottom line -- skipping the fine print.

I should have suspected something was wrong when John's calcs showed the system to be so marginal (which was not my experience), but I was blinded by the circuit breaker concern and I just missed it.

Thanks to Matt for checking the calcutions (by the way, everyone should take the time right now to give positive reps to both John and Matt for the work they have put into this entire thread -- yes do it right now -- it only take a few seconds).

Joe J.

[Matt Adams]

I'd just like to say that I really appreciate all of the contributions that so many people have made to this thread. I know that it was difficult for me to find comprehensive data on a lot of the subjects, but it seemed where I was missing out, there was always someone to fill in.

This post covered a lot of different topics, from transmission theory, to combining motors, to defining "competitive" robot capabilities, and some nice insight on how those 120 Amp breakers work in REALITY...

Big props to everyone involved. I'm really impressed and excited to have some good insight brought to this fundamental topic for everyone to see.

Matt

[Matt Adams]

Originally Posted by Joe Johnson

I agree with 80-90% of what Matt has to say, but the above quoted comment is one that I feel I need to say something about.
<SNIP>
This is not to say that there are not multiple reasons NOT to use N motors per side -- there are many, but tripping the 120 Amp breaker is not one of them (imho).
<SNIP>

Sit right back folks, it's story time from Matt Adams.

Sit, honest. This might get long.

I'm rehashing one of my very first posts (my very first?) and this reply made by Dr. Joe.

First off, I'd like to say that I absolutely respect Dr. Joe and many of the brilliant things that he's shared with the first community through Chief Delphi and other venues. He's a brilliant guy. So brilliant, in fact, that when he said the above statement back sevearl months ago, I really appreciated the engineering experience ahead of the the theoretical calculations I do in school. I used that advice while doing calculations for our machine, and we didn't have a single problem with our 120 Amp breaker over the course of the entire season, minus a single match. That single match is why I'm posting now.

Let me immediately remark that I am in no way trying to put any blame on Dr. Joe for the success or lacktherof of 461 this season. Ultimately any sort of machine failures are the responsibility of the design engineers and the people who give designs the stamp of approval (or signature in our case). That person was me.

With that all said...
461 had 4 motors (no transmission) for their drive train this past year, as well as a few supplimental motors to control other functions, namely a fisher price for a small ball roller and a van door for a big ball grabber.

The reason I post now is to share some more real-life experience using this breaker, in hopes that similar mistakes are not made in the future.

Curie Field, 2004. Last match of the division finals. 461 had played at least 8 consecutive matches without swapping out with an alliance member for strategic reasons... both joysticks were pushed forward, and the robot does too... for about 5 seconds after autonomous before tripping the 120 Amp breaker... costing the alliance the match and perhaps winning the division or more.

The 120 Amp breakers are thermal devices... over the course of several matches, they heat up, and become more likely to trip. While several teams know to use compressed air to cool motors during the final rounds, most teams don't think about the main breaker. If you're pulling 160 amps or so through it for several consecutive matches, this is an important fact to remember.

The FIRST finals are truly a test for machines, and all of the design parameters need to be built around this. From personal experience, there's no feeling worse than losing a match because of a human error or an obvious potential failure that wasn't planned for. I hope that everyone who reads this post ensures they think carefully about power management, thermal conditions of their machine, and designing machines around their shining moments in the finals.

Good luck everybody!

Matt

[Matt Adams]

Originally Posted by Joe Johnson

I agree with 80-90% of what Matt has to say, but the above quoted comment is one that I feel I need to say something about.
<SNIP>
This is not to say that there are not multiple reasons NOT to use N motors per side -- there are many, but tripping the 120 Amp breaker is not one of them (imho).
<SNIP>

Sit right back folks, it's story time from Matt Adams.

Sit, honest. This might get long.

I'm rehashing one of my very first posts (my very first?) and this reply made by Dr. Joe.

First off, I'd like to say that I absolutely respect Dr. Joe and many of the brilliant things that he's shared with the first community through Chief Delphi and other venues. He's a brilliant guy. When he said the above statement back several months ago, I really appreciated the engineering experience instead of the theoretical calculations I do in school. I used that advice while doing calculations for our machine, and we didn't have a single problem with our 120 Amp breaker over the course of the entire season, minus a single match. That single match is why I'm posting now.

Let me immediately remark that I am in no way trying to put any blame on Dr. Joe for the success or lacktherof of 461 this season. Ultimately any sort of machine failures are the responsibility of the design engineers and the people who give designs the stamp of approval (or signature in our case). That person was me.

With that all said...
461 had 4 motors (no transmission) for their drive train this past year, as well as a few supplimental motors to control other functions, namely a fisher price for a small ball roller and a van door for a big ball grabber.

The reason I post now is to share some more real-life experience using this breaker, in hopes that similar mistakes are not made in the future.

Curie Field, 2004. Last match of the division finals. 461 had played at least 8 consecutive matches without swapping out with an alliance member for strategic reasons... both joysticks were pushed forward, and the robot does too... for about 5 seconds after autonomous before tripping the 120 Amp breaker... costing the alliance the match and perhaps winning the division or more.

The 120 Amp breakers are thermal devices... over the course of several matches, they heat up, and become more likely to trip. While several teams know to use compressed air to cool motors during the final rounds, most teams don't think about the main breaker. If you're pulling 160 amps or so through it for several consecutive matches, this is an important fact to remember.

The FIRST finals are truly a test for machines, and all of the design parameters need to be built around this. From personal experience, there's no feeling worse than losing a match because of a human error or an obvious potential failure that wasn't planned for. I hope that everyone who reads this post ensures they think carefully about power management, thermal conditions of their machine, and designing machines around their shining moments in the finals.

Good luck everybody!

Matt

[Joe Johnson]

Originally Posted by Matt Adams

...Let me immediately remark that I am in no way trying to put any blame on Dr. Joe for the success or lacktherof of 461 this season...

Let me just start by saying that all my remarks more or less come with a "your mileage may vary" disclaimer. I don't take blame or credit so easily.

Now to the 120Amp breaker issue. I am surprised that even with this condition you tripped the breaker, but I suppose that with all the improbablities piled up on top of eachother, it is possible. Well... ...to say "I suppose" is wrong. I should say, now I realize that given a long enough time of stalling 4 motors plus others you can trip the 120A breaker.

But evenso, I pretty much stand by my thought that it is almost impossible to trip it in a 2 minute match.

If anything, Matt's story is a cautionary tale to spray the 120A breaker with "cool in a can" while you are spraying your motors, especially if you are stalling many of the drive motors for significant portions of the match (search for my messages on "Cool in a Can" if you want to know my feelings about the importance of this product -- especially during elimination rounds where you play many matches in a row).

Joe J.