Re: Shifting Gears
 

Wow. This thread has been great for me. The sensors and coding I can handle with our students, but I'm very worried about our drive system for our team.

If I have followed this thread correctly, experience suggests that wise goals for a drive are...

a) Top speed of 10-12 ft/s and
b) A pushing force of 150 lbs.

These goals are a good target for most of the prior years of FIRST competition relative to the max amps and motors in the FIRST kit. Did I read correctly?

Now, given the above, possible schemes for achieving these goals (again with the restrictions of amps and motors) is to...

A) Use a pair motor configuration (one on each side),
B) Use A) with a shifter,
C) Use mutliple pair motor configuration (more than one motor on each side), or
D) Use C) with a shifter.

After reading the thread, my comments are that A) does not lead to competitive capabilities but what is the easiest for a Rookie team to achieve. Some form of B) and C) are the suggested routes for a competitive robot but require significant but equal investiment in terms of design, fabrication, blood, sweat, and tears. Finally, D) offers no competitive advantage over B) or C) and only compounds the blood, sweat, and tears.

Using my active listening (reading) skills, I've wanted to echo this back to the thread to see if I've digested the thread properly and also to refocus the thread/debate to more of a tutorial/guidance/sage suggestions for Rookie teams.

Re: Shifting Gears
 

I am very glad to have the summary. Thanks.

Except, I disagree with the declarative statement that multiple motors combined with shifters provides NO competitive advantage.

They clearly can provide benefits -- there are 2 questions you need to ask:

#1 Is there any benefit in your particular situation, given your robot and game strategy, ?
#2 Are the extra pain and suffering required for your team to implement the system worth the possible benefit?

I think that the answer is very likely that for most teams and most game strategies, the extra benefit is not worth the cost, but that is not always the case.

Ask the TechnoKats how important a shifting multimotor drive was to them the year they had to auto-balance with two goals on the on the ramp (2001)?

Ask Las Guerrillas how important it was to them the year they grabbed & lifted the goals (2002)?

It can be EXTREMELY important.

But perhaps not for your team depending on how you choose to play next year's game.

Joe J.

Re: Shifting Gears
 

Since I'm the one who made the strong and somewhat intentionally controversal comments regarding multiple motors, shifters, "competitive" definition, etc... I suppose I should comment.

To meet the two "competitive" objectives:
A. Top speed of 10-12 ft/s and
B. A pushing force of 150 lbs.

This can be accomplished with any of the following:
1. 4 or more motors coupled together
2. 2 motors coupled with a multi-gearset transmission
3. 4 or motors coupled with a multi-gearset transmission

What is factual is that you can accomplish A and B with either 1 or 2.
Dr. Johnson has no qualms with this.

His concern from my understanding is that "competitive" is not universal. He gave two perfect examples of sitautions where the needs of certain teams exceeded that of A and B, and 1 and 2 would not suffice.

i.e.
If your requirements exceed that of A and B, or if you would like to be "extra competitive", option 3 can (and will) provide that.

My thesis is that for MOST teams, MOST of the time, in MOST games of the past, you could be "competitive" using MAINSTREAM strategies without using method 3 above. Because you could do it without 3, anything more than 1 or 2, I feel, is wasteful (over engineered).

Matt

Re: Shifting Gears
 

I admit that the qualifications of A and B went past me upon first reading. Given those qualifications, then I agree, there is no reason to have BOTH multiple motors and shifters.

Joe J.

Re: Shifting Gears
 

Disclaimer: Beware this post... actual math may be involved!

Allright... I'm going to have to humbly disagree, and challenge this statement.

When I read this... it essentially shattered my little "shifting is necessary" paradigm, and it scared me enough that I actually decided to check the numbers. (School is out... I actually have time to run some numbers!) So... let's walk through the calculations I did, and see the results. Does the above statement hold up?

Sorry Joe, Sorry Matt... my numbers say you are both wrong.

The 3 major factors here are:
1. 10 fps max speed
2. 150 lb linear bot pushing force
3. 4 Motors, no shifter.

First off... Set up some assumptions:
I'm gonna say, let's be nice. I'll assume 100% efficiency for the whole gearbox, (cuz I'm a nice guy so I'll break the laws of physics for you gentleman ;)). Notice, I'm also using the SLOWEST "competitive max speed" you mentioned (10fps). I'll also design to achieve this at the motor's free speed (again, cuz I'm a nice guy... and we'll assume no speed loss anywhere in the gearbox).

What does this mean?

Basically... I'm giving Joe and Matt the benefit of the doubt. I'm setting things up so they are COMPLETELY in Matt/Joe's favor. I WANT to prove myself wrong.



So, let's begin...
First, we'll design our combiner.

I'm going to use the Drill and the Chip, the 2 most powerful motors in our kit (again, just to be mean to myself, and nice to Matt/Joe). I design to match free speeds at 3688 RPM. (A 15:80 reduction off the Drill, and a 12:18 reduction off the Chip... gives us about a 99% speed match). I'm happy.
Based on my combiner theory this means the new "super motor" has a stall torque of 7.97N*m, a stall current of 234 Amps, and a free speed of 3688RPM. All of this is done at 12V for both motors.



Now, let's find our "10fps" ratio.

I'm going to choose a 6" dia wheel, simply because it's common. If we want our robot to go 10 feet per second, using a 6" wheel the wheel needs to be spinning at about 382 RPM. Now, we know our "super motor" free speed is 3688 RPM, so what reduction do we need between the motor and the wheel?
(382/3688 = .104).
So, we'll use this as our gearbox reduction.



Now... let's see what kinda max torque our new box is capable of.

Our "super motor" stall torque is 7.97N*m, which means (1 side of) our robot has a stall torque of (7.97 / .104 = 76.63 N*m), again with a 6"dia wheel, this is about 113 lbs of linear pushing force. Using 2 gearboxes this yields 226 lbs of pushing force (at stall). So yes... your gearbox has fulfilled the design requirement of 150lbs. However... the super motor is at stall, and drawing 234 Amps... which is bad.
I'm not happy with it, are you? Let's see how bad it is.



So, let's see what kinda current our robot draws if it's pushing 150 lbs.

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. To calculate the torque load on our "super motor" we simply say: 50.84 *.104 = 5.287 N*m. Now... how will our super motor perform under a 5.287N*m load? Well... by distributing the load, and calculating, I show that the Drill Motor is drawing 85.09 Amps, and the Chiaphua is drawing 71.76 Amps. These values are unacceptable for our competition. You'd burn up breakers if you tried to push for any extended amount of time (which is often necessary).

Simple solution to this?
Add a shifter to your robot.


Okay Joe/Matt... am I missing something?
I come from the "shifting side of the tracks" and am a huge fan of using a shifter to solve all my drivetrain problems. I'm also super-conservative when it comes to motor loading, and I always worry I'm going to draw too much current.

Based on my calculations, your above statement is wrong.
If you want to achieve Matt's "competitive speed" and his "competitive pushing force" in the same bot, using only 4 motors... you NEED a shifter. Otherwise... you're going to pop the breaker and sit there dead the whole match.

Prove me wrong... ;)
John (who sometimes has issues with arithmatic and algebra)


PS - I want to emphasize this was all in good fun, and in the interest of learning. Can anyone show me if I went wrong somewhere?

Man.. this was fun! A real live enginerding debate on CD that involves... *gasp* NUMBERS!

Re: Shifting Gears
 

No, you wouldn't be dead the whole match. It would be more like a sputtering car that is in the process of running out of gas. The robot would go full speed for about 30 seconds, you would get an advantage in the match, and then it would start cutting out on you. It would go for about 4-5 seconds and then stop. This would repeat until the end of the match. ... unless you had a shifter. Then you could just shift into low and continue on with the match.

I agree with the above analysis that John made, and I agree that shifters are extremely beneficial. We on team 45 have shifted gears every year since 1999 and we have learned that the #1 advantage to shifting is saving your breakers... and the side benefits are the immediate response of power or speed.

A circuit breaker on a FIRST robot can handle amps above it's rated limit, but just not for too long. If a robot is in high gear for 10-15 seconds, under harsh driving conditions, the breakers will hold up. However, after those 10-15 seconds, the robot driver better start thinking about shifting to low, because the breakers are getting low and starting to give.

The trick with these little circuit breakers is that they don't trip immediately at 40.1 amps. You can overload them for a while, but not too long.

In many matches over the past 5 years, we would run in high gear for the first 40-60 seconds (depending what we were doing) and then shift to low for the latter part of the match. This relief to the breakers would enable us to run in high during the last 5-10 seconds, if needed.

At least, that is from my experience.

Also, on a different note, I would contend that 150 lbs of pushing force may not be enough for a top-tier robot. There were robots which could push with more force than that during the last 2 years. Last year, many robots had unbelievable pushing power on the grating. I would up that number to 200-250 lbs. to be conservative.

Andy B.

Re: Shifting Gears
 

As I made that final blunt conclussion in my last post, I thought to myself, "You know what Matt "Number Cruncher" Adams... you never really did crunch numbers on using 4 motors. "

Furthermore, I think I concluded in my powerpoint presentation that using both motors will get you about 85% of 461's max speed last year with no worries of tripping breakers... which was theoretically 7 ft/sec.

Just now I cranked some numbers, and found the "best you can do" with only 4 motors and no shifter is about 6 feet per second without tripping breakers.

Of course, I also briefly ran over John's numbers and as best as I can tell...

John is correct.



Furthermore, I'd like to say in some respects, this is actually pretty obvious; 4 motors won't have the same advantages as shifting, and hence why John probably said, "Wait a second..."

If you look at both the drill and the chips, they're reasonably close, in terms of output torque at 40 AMPs at the same RPM. Since you have an argueable limit near 120 AMPs on your main breaker, you can only add an additional 40 amps in addition to the 80 A max you have from two drills (due to thier individual 40 breakers). Essentially, that's just like making your drivetrain 50% better. That's not enough to be truly competitive, though it's admittedly better than just a single pair with a single gear ratio. With a shifter, you can have torque ouput (or max speed) increase by factors of say 2.5 or 3. That's a big difference.

Thanks John!

Matt

P.S. To redeam myself, I'll crank out some numbers again with a shifter sometime soon to show how this is a different for those who are wondering.

Re: Shifting Gears
 

I also want to re-emphasize something that Joe mentioned: the definition of "competitive advantage" really depends upon the game.

I agree that in your typical game, the numbers you present above (especially the 150 lb pushing force number) would be about your pushing limit. However, in the 2002 game, we determined that our robot would need to push with a force of about 720 - 960 lbs before our treads would start to slip. For this game, having 150 lb. of pushing force would not be an advantage at all.

The point is: assuming a peak pushing force number before the game is announced can get you in trouble... unless you have a shifter. Then you can just pop in a new ultra-low ratio. However, your strategy may be such that a shifter is irrelevant.

Re: Shifting Gears
 

The what-if discussion has been very important for me (as a rookie mentor). I appreciate the hashing out of the various "theories". It may seem irrelevant to discuss strategies without knowing the game, but the experience that each of you have with regard to prior years and the debate relative to those years is very enlighting.

With regard to shifters, I'm assuming that teams do not design to shift on the fly. This thread has mainly discussed the design arguments and complexity of multi-motor/shifters.

My question would be what are the operational "gotchas" and "goodies" about the various theories. How well do the student operators handle shifting during competitions? How much awareness (training) does shifiting require? How does shifting affect autonomous mode?

Transmission Styles
 

Actually, many top tier teams do design to shift on the fly. One of the primary reasons teams use transmissions that do not shift on the fly is is due to the additional complexity in machining that's typically required for these sorts of designs.

However, if I had more time, I'd press the arguement that though most definitely a nice feature, shifting on the fly isn't a game breaking feature.

However let it be known that there's a difference between:

1. Shifting on the fly.
2. Shifting after being stopped momentarily.
3. Stoping, shifting, and slowly moving forward for components to engage.

I would strongly suggest that transmissions of type 1 or 2 be used...

If I didn't have finals this week.. there'd be much more to this post. I'm sure others will follow through with their thoughts as they have done so well before.

Let me also note that most of the time shifting is actually done through software, so "learning how to shift" for drivers isn't exactly the same difficulty of learning to drive stick on a car. When to implement it in a match, however, is a different matter.

Matt

Re: Shifting Gears
 

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.

Re: Shifting Gears
 

Not at all!
If we used the above designed gearbox with a 3" pully... it would be going half the speed, with double the output torque.

We're not talking about coeff of friction at all... sure the treads might end up being a little more "grippy" but... that doesn't matter in this debate. You can have all the traction in the world, but unless your gearbox can output the torque to back it up... you're just wasting your time. Just like you can have all the output torque in the world, but without traction, you're doing the same.

Given a set amount of power output, there is only a certain speed you can go while comfortably (without drawing too much current) pushing with a given force. The above proof I did, was to show that the drill and Chip alone, geared at 10fps cannot produce a 150lb linear force "comfortably".

Sure, gearing the system down to 5 fps would probably do it... but that's besides the point of this debate.

You can't have your cake and eat it to.
You can't have "tons of speed" and "tons of pushing force" at the same time, unless you have more power than our current motors allow (unless... you SHIFT!)



Again, the "competitive" level of speed and pushing force varies each year depending on the game. Some seasons 10 fps is enough, some seasons it isn't (like in 2002 if you wanted to beat 60/71 to the goals). Some seasons 150 lbs of linear pushing force is enough, some seasons it isn't (like in 2002 if you wanted to fight back against 308 when they had the goals lifted).

At the beggining of every drive-design process, we all have to set ourselves some specifications we want to fulfill, decide what the drive must accomplish to play the game to the level we want to play it. We basically need to do some guesswork about what our robots will need to deal with during the upcoming season. Any team in 2002 that thought "wow... our robot can push with 100lbs of force... we're unstoppable!" was... mistaken.

Also, if we decide "we are going to lift the goals so we can push harder". We then need to determine exactly how much torque the drive needs to be able to output, in order to back that up. Otherwise... as stated above, we're wasting our time. Imagine if 60 hadn't downshifted after lifting the goals in 2002... a pushing robot would have gotten in their way, and their motors would have stalled. No good! They needed their low-gear torque.

For those who are having trouble understanding some of the basic principles invovled in this debate, I recommend this presentation:
http://www.teamfordfirst.org/_docume...0(12-7-02).ppt

It's the best one I've seen in terms of throughly explaining drivetrain theory. (Paul's the man. :)) If you have any other questions, search around this forum, ask your friendly-neighborhood-FIRST-mentor, or drop me an email/IM.

John
(Who last season was on here learning for himself about drive physics, and never thought he'd be the one doing the ranting/teaching.)

Re: Shifting Gears
 

Actually, the above clips brings up a question....perhaps maybe should be a new thread....but what the pros/cons and arguments for tract vs wheeled drives (or other configurations)?

Re: Shifting Gears
 

Ahh... you're right... I'm an idiot.
::Kicks himself for forgetting elementary physics::

Re: Shifting Gears
 

I would like to STRONGLY echo what IrisLab says ... Although this is my third year as a mentor, it is the first year I am heavily involved in drivetrain design. While my career experience has been limited to static structures, these discussions, and especially this thread, have greatly increased my awareness of the important details to consider.

THANKS GUYS !!