24-10-2008 20:52

Daniel_LaFleur

Re: pic: Concept 8wd Drivetrain

Couple of questions:

1> HDPE is rather slick, are you planning on covering it with roughtop?
2> Are the center 4 wheels lowered? If so, by how much/. If not, how are you overcoming friction?
3> Whats the diameter of the wheels and whats it's expected speeds?

24-10-2008 23:25

AndyB

Re: pic: Concept 8wd Drivetrain

I'll let sdcantrell56 answer officially for himself,

but I'm pretty sure there is a tread on the wheels already (If you look closely). I think he meant that the hub itself was made out of HDPE. The two sets of center wheels also appear to be slightly lowered by my eye, but I could be fooling myself. The wheel diameter looks to be 4".

25-10-2008 01:35

R.C.

Re: pic: Concept 8wd Drivetrain

Quick Question, why not uhmw for the wheels. Just wondering, I've heard of teams having success with them.

-rc

25-10-2008 02:02

sanddrag

Re: pic: Concept 8wd Drivetrain

Nice work on the approach and departure angles. I really like that. Every year we've ever had something to climb, it pains me to see teams with huge wheels that still can't do it, because the wheels are not positioned well in relation to the frame.

Robots don't always need large wheels or high ground clearance to climb things. This design is a fine example.

25-10-2008 02:18

AndyB

Re: pic: Concept 8wd Drivetrain

UHMW and HDPE are two very similar materials. They do have differences though. HDPE stands for High Density Polyethylene which is long strands of ethylene monomers. UHMW-PE (Ultra High Molecular Weight Polyethylene) is similar, but has even longer monomers, generally making it stronger with better wear properties. HDPE will hold it's shape a little bit better and some argue that it can be machined easier. UHMW tends to be a bit more expensive than HDPE depending on the scenerio.

Honestly, when it comes to something like a wheel, using one or the other shouldn't make a big difference. It probably comes down to what you can get your hands on easier.

25-10-2008 03:20

R.C.

Re: pic: Concept 8wd Drivetrain

Thanks for clearing that up, I wasn't sure what the difference was. Does anyone know how long those wheels last and nice job on the drawing and I totally agree with Sanddrag.

25-10-2008 12:04

sdcantrell56

Re: pic: Concept 8wd Drivetrain

Ok I will try to answer everyone. First of all I was planning on UHMW for the wheels. Something about having 2 hours of sleep made my brain function...well not function at all actually. Also the center wheels are lowered .175 inches for easy turning. The wheels are 4.5" diameter with roughtop tread. One of the nice things about using uhmw is we can buy enough material for 20 wheels for right around $70 dollars. From what I have heard UHMW makes a very durable wheel. It is stronger in impact situations and easier to attach tread (sheet metal screws). I will post up a picture of the wheel assembly with sprockets later but the entire thing with rim, tread, 2 sprockets, and 3 bearings weighs .5lbs.

Right now the speed is set right at 5fps and 14fps although that can easily be changed by switching the drive sprockets. Needless to say this drivetrain will not be getting pushed around.

Sandrag thanks for the compliments. I'm thinking there will be some type of obstacle next season and I hate big wheels so this is the result.

25-10-2008 17:22

DarkFlame145

Re: pic: Concept 8wd Drivetrain

besides traction, what does a 8WD offer that a 4 or 6WD doesn't offer? To me it just takes up extra weight, but that's just me.

26-10-2008 01:41

AndyB

Re: pic: Concept 8wd Drivetrain

If UHMW is buoyant (which I have absolutely no clue), it would aid in keeping it afloat in next year's water game...

Other than that, it's pretty much traction. Also, there is less "rock" to the chassis as 2 sets of wheels are lowered instead of just 1. This could make the turning a little more stable, but I don't really think it's too big of an issue.

I would recomend going with 6wd, with a couple idler wheels that don't contact the ground to avoid rubbing on the bottom of your frame on a ramp (that is, if there is a ramp.) 8wd is really just a design choice. There is an added coolness factor as well which I think should be factored into any design decision.

26-10-2008 10:48

gorrilla

Re: pic: Concept 8wd Drivetrain

well i know that 1251 had an 8 wheels and they did great but they were pretty easy to move, maybe not the wheels, and maybe the weight of their robot, or the distribution of their weight towards the front but i think most robots have this problem unless the have a good weight distribution over the right places

having lots of traction dosent matter if you have all the weight in the wrong places

26-10-2008 11:13

AndyB

Re: pic: Concept 8wd Drivetrain

Keep in mind that increasing the number of wheels will no gain your more traction. You'll get the same traction out of a 6wd that you will out of an 8wd. At least theoretically.

You may gain some traction if using wedgetop tread due to it's 'stickiness'. You may also gain traction from using roughtop tread which will actually cling to the carpet. These situations are both relative to a similar setup with less wheels.

The tread you are using looks like standard rubber, unless you are using something else and just didn't model it, which is completely understandable. If you are using this tread, you will likely gain little to no advantage in traction over a 6wd.

26-10-2008 11:23

gorrilla

Re: pic: Concept 8wd Drivetrain

well i also dont see why teams are usig such small wheels all the time there cant be any benefit except maybe weight

26-10-2008 11:44

vivek16

Re: pic: Concept 8wd Drivetrain

Exactly, why use larger wheels when you can save 2-3 pounds and have the same functionality? Not to mention money.

-Vivek

26-10-2008 11:47

gorrilla

Re: pic: Concept 8wd Drivetrain

well do you think its really worth it i mean 1557 was 119.1 pounds and that was mostly in our frame and scissor lift

and with smaller wheels wouldent you have less traction because of less surface area

26-10-2008 12:22

s_forbes

Re: pic: Concept 8wd Drivetrain

Smaller wheels also let you set your whole robot down lower to the ground (low center of gravity, yay!) and they allow you to make your wheelbase about 2" longer than one with 6" wheels (less tippy, yay!). There's lots of good reasons to use them.

We put 4" wheels on our robot last year since it didn't have to do any climbing, and I didn't notice any difference in performance between it and our previous year's robot (with 6" wheels). Both were set up with 6 wheel drive and wedgetop tread.

I've also been kicking around a wooden eight wheel drive base in my head for a while, but it looks quite a bit different... I like the eight wheel drive concept, it has several things going for it that make it preferable over six wheel drive (IMO). Having four center wheels instead of only two slows down tread wear, and as someone else stated doesn't let your robot "rock" while it's driving around. It also gives you a bigger contact patch on the ground to avoid being spun. By bigger contact patch I don't mean more traction, but traction where you need it. Six wheel drive bots spin pretty easily when pushed on a corner since all their weight is on their two center wheels. But an eight wheel drivebase has four center wheels that carry the weight, and they're all distributed further out from the center.

And the obvious reason for using an eight wheel drivebase is that it allows you to climb over all those ramps on the field without high-centering.

26-10-2008 12:29

AndyB

Re: pic: Concept 8wd Drivetrain

No, because the overall robot weight is the same. If the surface area goes down, then the concentration of weight on the area goes up. Traction does not change.

Small wheels provide less cost, less size, and you need less reduction before the wheel. The only thing they lack is provided ground clearance. But that can be worked around.

26-10-2008 12:56

gorrilla

Re: pic: Concept 8wd Drivetrain

but were would one get four or 5inch wheels? without custom machining

26-10-2008 13:03

Andrew Schreiber

Re: pic: Concept 8wd Drivetrain

http://www.robotmarketplace.com/prod...on_wheels.html comes to mind quickly, Banebots also has some.

26-10-2008 13:15

AndyB

Re: pic: Concept 8wd Drivetrain

IFI has 4", Andymark has 4", 5", and will have 4" plaction soon (as far as I know). Banebots has 3", 4", and 5". Colson makes smaller wheels as well I believe.

26-10-2008 16:39

jimbot

Re: pic: Concept 8wd Drivetrain

Call me ignorant but wouldn't an 8wd chasis with 4 wheels lowered simply be a 4wd chasis that has a small wheel base? If so what exactly would the advantage of an 8wd have over a 4wd chasis?

26-10-2008 16:54

Daniel_LaFleur

Re: pic: Concept 8wd Drivetrain

Ground clearence and wheel size have little to do with each other. Relationship of the frame to the axles has more to do with ground clearence and CoG than wheel size. We've used 6" pneumatic wheels for the last few years, but have had less than 1" ground clearence since our axles have been positioned above our frame.

The Longer wheelbase of a 4" wheel is offset (in the case of this thread) by the number of wheels. The biggest advantage I've seen with smaller wheels is that they are lighter.

26-10-2008 16:59

techtiger1

Re: pic: Concept 8wd Drivetrain

As a member of a team that has designed, tested, and built and used a 8wd system for a year in competiton here are my thoughts. Look into the game first and see if you want 8 wheel drive these types of systems get heavy and can compromise other parts of the robot that need structure to be effective. 1251 exhibit A our claw this year which continually bent from trying to keep it light. Also make sure you have enough torque in the system to turn the robot. Raiseing the two wheels is pointless your better off using 6 wd in that case. Ours works by having the front wheels offset and 3 motors which allows us enough torque to be able to keep 6 wheels on the floor at all times and still turn. Finally the wheel size of 1.5in tread width is fine. With the direction FIRST is going I don't think the 8 wd drive is necessary that is my final thought for you. Look at it this way too if you don't use this system it was good cad practice.

My two cents,
Drew

26-10-2008 18:47

sdcantrell56

Re: pic: Concept 8wd Drivetrain

I don't know where to start with all the questions so I'll try my best.
First of all the reason I chose to do an 8 wheel drive design is due to a couple different factors. I will try to outline my thinking. I wanted to use a small diameter drive wheel to reduce the overall cg of the robot and to minimize material cost. I also thought that there is a decent chance of an obstacle being in this coming years game, hence the use of 8wd which equals the ability to climb a much steeper obstacle without bottoming out given the ground clearance. Also note the cuts in the front and rear of the frame to increase approach and departure angles also to help with climbing. 8wd is also inherently more stable as the rock is less dramatic. The 8 wheels in theory does not increase traction however in the world of FIRST it absolutely does. Not so much in the front-back direction but in the sideways direction. It will be much harder to push this robot sideways and to spin it without us wanting it spun. Finally I am trying to minimize the amount of machining we need for transmissions and such. With this design we can use an almost stock andymark 2 speed. The only thing changed is one gear is turned down and then a custom 7075 output shaft machined that is longer. With 8wd I can have 2 separate chains running one to each center wheel which creates a backup if one chain fails.

I am planning on Roughtop tread for the wheels so traction will be very high.

Additionally the issue of extra weight is very moot with this design. The entire drivetrain will be under 40lbs. that includes motors, transmissions, wheels and all the other little things, as well as all the mounting points for our superstructure. This means we have 80lbs to play with for electronics and the superstructure.

As for there not being much point to 8wd if we raise the outer wheels, look at 1270 from 2007. They raised the outer wheels and were one of if not the strongest pushing robot. They were also quite successful, making it all the way to einstein.

26-10-2008 19:59

gorrilla

Re: pic: Concept 8wd Drivetrain

if your going to run eight wheels wouldent it be easier(to implement not make unbreakable)and lighter to run actual treads? then you would have lots of traction


anyways it just seems unneccesary because if everyone knew we were going to be climbing lots of steep things then i know that i would consider treads before eight wheels simply becuase its more complex(Lots of chains and things to break) and harder to make everything straight and workig funcionally im not saying it would be harder to drive or climb stuff with just considering building and implementing things

26-10-2008 22:43

sdcantrell56

Re: pic: Concept 8wd Drivetrain

An 8 wheeled robot is actually much easier to make and design. All you have to do is run an extra axle and chain versus a 6 wheel design. For treads you really need at least 4 wheels/idlers per side so there is no reduction in complexity. The only thing easier about treads is the lack of chains. Although if you do a quick search you will see many many negatives about tank treads, namely there propensity to snap under side-loads and there high cost. Each tread will cost around $270 dollars.

The drive cogs for treads are heavy as well and the tread itself is heavy particularly compared to 8 plastic wheels and #25 chain. Unless there is a game that involves sand or other loose material, I as well as most people in FIRST will be hard pressed to consider treads as a viable option given the cost, weight, and reliability aspect.

Also how is it any harder to align 8 wheels versus 6 or versus 8-10wheels/idlers in the case of treads. If there is a problem with alignment then it will render any drivetrain useless, not just an 8 wheel drive robot

27-10-2008 01:36

David Guzman

Re: pic: Concept 8wd Drivetrain

First of all very nice CAD work. I also like the way you did the base in the front and back for climbing. Here are a few things about 8wd based on our (1251's) 8 wheel drive for 2008.

Pros:

1. Very difficult to get pushed side ways since there is more contact points.
2. If done right, no rocking back and forth.
3. Easier to adjust weight distribution in the design phase.
4. Using shorter loops of chain require less tension adjustments.
5. If done with 6 motors, very hard to be pushed in low gear and maintains high maneuverability at high speeds. (in our case 16.5 fps)

Cons:

1. Heavier base
2. Harder to manufacture (more time consuming)
3. If done with 6 motors, battery dies much faster.

My two cents:

I would recommend the 8 wheel drive to anyone if the have they weight and time to pull it off. Also like Drew said if you have the 4 wheels in the middle lower; you might as well make a 6WD. The best way to do it is to make your front/back wheels a little higher, so you always have 6 wheels on the ground. Make sire your 6 contact points are farther apart in width and closer in length, this will improve turning.

Also you don't need your wheels to be so wide as others have said this does not increase the force of friction. The number of contact points is more important.

27-10-2008 11:00

JVN

Re: pic: Concept 8wd Drivetrain

Long Rambling post... Beware. There may be some decent content. No promises.

There are a lot of people throwing out pros and cons for an 8WD that don't make a lot of sense to me. In particular, those people talking about robot turning and traction. Let's talk through this quickly. To me, there are several different configurations that should be discussed.

1. No wheels are dropped, all 4 are in-line.
2. The 3 of the wheels are dropped so only one pair is raised (front or back)
3. The middle 2 wheels are dropped, and the robot drives on these most of the time.
4. For fun... let's also talk about a "standard" 6WD with middle wheel drop.
5. aaaaand... we'll show a 6WD with NO wheel drop.

Since we're only talking about the effects of the 8WD itself, we don't need to discuss any of the "additional" characteristics of the drivetrain. Examples of some of these characteristics which have NO bearing on the PRO/CON of an 8WD system are: Motor usage, gearbox/transmission, wheel size, drivetrain configuration (widebody vs narrow), robot CG, overall robot weight, etc.
Though these characteristics are all VERY important to the overall drivetrain performance, let us assume "all other things equal" so we can have a good comparison.

First, we will talk about robot turning.
For reference, everyone should immediately go read this whitepaper and learn all the physics behind it:
http://www.chiefdelphi.com/media/papers/1443

Based on this paper, almost everything "cancels out". (If you don't believe me, read the whitepaper again). The main comparison we need to look at, is the length of the support polygon (as support polygon is the polygon you draw between the points where the robot touches the ground at any given time). If you took the time to do your HW and read the above whitepaper, and have a good understanding of the physics involved you will understand why.

As you can see in my attached image (excuse the crude drawing). we can compare the cases listed above pretty quickly.

Longer Support Polygon = more turning scrub = higher resistance to being spun, worse robot turning.

Shorter Support Polygon = less turning scrub = less resistance to being spun, better robot turning.

"But wait... I want to change something to affect this"
Well, we could change the traction material on some of the wheels to drastically alter the drivetrain characteristics, but remember we promised we would do an "all other things equal" comparison". (We promised, and we need to honor that promise.) Because of this "all other things equal" the above comparisons are pretty straightforward.

Next let's talk quickly about overall traction...
I am one of those people who does not believe "more contact patch" = "better traction". Call me old fashioned, but I think we're pretty darn close to a F= mu * N model for robot traction. (More contact patch DOES however result in reduced tread wear, which is nice.) However, if you're one of those crazy people who REALLY likes more grip on the floor, we just need to make one more comparison. "How many wheels are on the floor?" Take a peek at the attachment one more time and make this comparison, go ahead, I'll wait.

More considerations?
These are the straight-forward ones.

• 8WD makes it easier to build a "climbing robot" while still maintaining a low-ground clearance.
• 8WD has more parts which is by definition more complex to build, heavier, and probably less reliable than an "all other things equal" 6WD system.
• Depending on where the CG is, some of these configurations will "rock less" than others. This may be important to you. You can play with your support polygon to further affect this.

JVN's editorial:
Yes, I would use an 8WD, but probably only in a situation where we needed to climb a step or something and I couldn't make a 6WD climb it elegantly. To me, there just isn't any compelling reason to go this direction for a flat field. The 6WD designs I've played with have a reasonable amount of turning scrub, and turn just fine (I don't need a longer or shorter support polygon, I'm happy with the balance I have).

Honestly, I love my 6WD for a robot which requires "max pushing force". If we ever had a game where pushing wasn't required, I would consider doing a 2 Traction + 2 Omni drivetrain or a 2 Omni + 2 Traction + 2 Omni drivetrain. (These configurations would have great handling with max stability and still reasonable pushing force.) Heck... I might even do 6WD anyways. The important thing, is that I didn't do a swerve drive.

Remember to ALWAYS use physics in engineering discussions. There is really no room for "feelings" in this sort of thing. I don't care how you feel about an 8WD, or what you "think" might happen. I only care about your physical justifications for how and why things happen.

For goodness sakes... READ THIS WHITEPAPER:
http://www.chiefdelphi.com/media/papers/1443

If anyone has any questions about this, feel free to ask.

Good Luck!
-John

27-10-2008 12:04

sdcantrell56

Re: pic: Concept 8wd Drivetrain

I too agree that we are close to the ideal equation for traction. The main reason for the wider wheels on my drivetrain is for the reduced tread wear not for increased traction. I would like to not have to change treads frequently so that is where the 1.5" width came from. Also yes if the game does not include much climbing or if the object to be climbed is not particularly steep then a 6wd layout will most likely be used.

One thing about this particular design is that the overall weight gain from adding 2 extra wheels is not particularly large (>3lbs) and the drivetrain has an inherent cool factor. If we have the extra weight, the cool factor certainly outweighs anything else assuming all things else are equal. Im confident we can run 2 extra chains and wheels without a problem. Something about laser cutting everything tends to have everything line up perfectly.

27-10-2008 13:10

gorrilla

Re: pic: Concept 8wd Drivetrain

make sure you think throug hall aspects of your descision or else you may be regretting you choice

and cool factor should not be considered

i saw plenty of teams that had cool robots but dident preform very well

27-10-2008 15:35

sdcantrell56

Re: pic: Concept 8wd Drivetrain

O believe me, we always think through our design. And I seriously doubt there would be anything to regret about an 8wd drivetrain as long as it's executed correctly which of course it will be.

The idea that a design should not be done because it is cool is a ridiculous notion. We had arguably one of the coolest designs this year and I will challenge anyone to argue that it was not brutally effective. We did win the Xerox creativity award at the Championships for it as well as were one of the top pure scoring robots in the world.

This idea that cool is not something to be considered kind of stifles innovative design for the sake of being safe. I caution all teams not to become to scared of a design to give it a try. Of course carefully design it, and thoroughly test it, but by all means try something different.

27-10-2008 17:30

Daniel_LaFleur

Re: pic: Concept 8wd Drivetrain

I'm not so sure I agree with this.

The goal of FIRST is to inspire. If the 'cool factor' inspires then the goal has been achieved.

27-10-2008 18:13

sdcantrell56

Re: pic: Concept 8wd Drivetrain

Precisely!!! That is indeed the purpose of FIRST.

27-10-2008 18:24

JVN

Re: pic: Concept 8wd Drivetrain

Yes. This is 100% correct. However whether or not "cool factor" inspires varies greatly from team-to-team and year-to-year.

Our team uses a weighted-objectives-table (WOT) to help determine what strategies and designs we want. "Cool Factor" is typically weighted significantly less than "efficiency", "elegance", and "effectiveness". This is a clear quantitative assessment of what our team values.

Excuse the expression, but that is just how we roll. Your mileage may vary.

-John

27-10-2008 18:36

Cory

Re: pic: Concept 8wd Drivetrain

I'm with John, but I'd argue that if you have an elegant robot it's probably pretty darn cool.

27-10-2008 18:43

sdcantrell56

Re: pic: Concept 8wd Drivetrain

Elegant is probably what I was looking for. Elegance is what I strive for in all of my designs, and nothing says elegance to me like a highly efficient, and light weight 8 wheeled drivetrain that uses the bare minimum of metal and still is stronger than most metal framed robots.

I should also throw it out there cory that your team is one that I strive to learn from and to eventually be like. Your designs are truly beautiful.

27-10-2008 18:48

billbo911

Re: pic: Concept 8wd Drivetrain

Any chance you would want to share your WOT? Noob teams, and even those with years of experience, could learn a lot from a simple tool like that.


BTW, no need to apologies for that expression.

27-10-2008 20:02

AdamHeard

Re: pic: Concept 8wd Drivetrain

In terms of design, the goal should be to create the most competitive product for the customer (the customer being FIRST).

27-10-2008 21:27

Daniel_LaFleur

Re: pic: Concept 8wd Drivetrain

While I agree with what you are saying here (If you've ever looked at the last 7 years of robots my teams have built you'll definately see 'industrial machine' written all over them ), I just wanted to point out that the goals of FIRST isn't to win, it's to inspire.

And while winning does tend to inspire others, it's not the only way to inspire ... and we as mentors need to keep reminding ourselves of that (we mentors tend to be a competitive lot )

27-10-2008 21:30

Daniel_LaFleur

Re: pic: Concept 8wd Drivetrain

Again I'll disagree here.

Our goal should be to inspire the students and help change the culture of the areas we live in. And our competitive product is the future of these students ... not some piece of whirring metal.

We're not here to win a regional or any other 'event'. We're here to win a future for these kids. If a team wants to build a 8 wheel drive chassis, and are inspired by doing that, then we should encourage them to achieve that goal and let them understand the design strengths and weaknesses of doing just that.

27-10-2008 21:53

sdcantrell56

Re: pic: Concept 8wd Drivetrain

From all of this discussion, the only true weakness I see to an 8 wheel drivetrain is the added weight. However, with careful planning and material selection, not to mention smaller wheels and sprockets for the same ramp climbing ability, even that weakness can be negated. The extra machining and assembly are so small that it really is not a problem.

27-10-2008 21:54

gorrilla

Re: pic: Concept 8wd Drivetrain

thats great even though i am not totally convinced that it would be a better preformer than a six wheel drive system except for climbing

27-10-2008 22:11

sdcantrell56

Re: pic: Concept 8wd Drivetrain

Fortunately I am convinced and have faith in our ability to effectively connect an additional pair of wheels.

27-10-2008 22:27

R.C.

Re: pic: Concept 8wd Drivetrain

Well said and Dean Kamen did say that this program is not about the winning, it is about the learning and the future of students. Inspiration can come from anywhere: crazy robots (the pink team for me), old videos, random ideas, etc... I think that it is wonderful that a team is going through with an eight wheel drive and best of luck.

27-10-2008 23:10

JVN

Re: pic: Concept 8wd Drivetrain

I bet you can connect 10 more pairs of wheels, 20 pairs even. The question is, why would you?

Without justification (benefit for the albeit minor cost) my team would not do it. If you find that justification, great. I'm just surprised that people would argue "we can, therefore we should". The fact that it is easy doesn't mean that it is smart.

-John

27-10-2008 23:15

s_forbes

Re: pic: Concept 8wd Drivetrain

People ask me that all the time when I ride my unicycle...

27-10-2008 23:42

sdcantrell56

Re: pic: Concept 8wd Drivetrain

The justification to have 8 wheels is for the added ability of climbing an obstacle without having to raise the cg. I completely agree if the field ends up being completely flat, or has a minor obstacle then I am sure we will be going with a 6wd design as usual and probably direct drive the center wheel. This drivetrain is merely one that we would go with if we need to climb obstacles. Who knows what the actual game will be.

28-10-2008 11:30

Andrew Schreiber

Re: pic: Concept 8wd Drivetrain

This may be something you need to ask 703...
http://www.chiefdelphi.com/media/photos/27823

http://www.chiefdelphi.com/media/photos/26312

And I can attest to how SCARY that drive train was. RUSH had their arm sheared off at the base when the decided we needed to get out of their way in 2007. Granted, their bot weighed the MAX legal amount that year but no one shoved them and the time they climbed our ramp they ripped our graphics clean off.

And John, if the students on the team decided they wanted to do an 8 wheel drive just because it would be fun to do and they would learn something would you do it? I realize FIRST is about inspiring but is it not also about teaching? My personal opinion is that if the students say they want to do something after a mentor has explained to them the problems then it is not our job to over ride their decision, instead we should support them. Yes it may fail but then we have a good lesson and can show them how to learn from mistakes. If it works, we learned something. "Because we can" may not be a good reason but, "Because we want to" is.

28-10-2008 11:47

sdcantrell56

Re: pic: Concept 8wd Drivetrain

As well as asking 703 how about we ask 25 or 1270 both of which no one wanted to get in a pushing match with. I also believe both teams were extremely successful in 2007 with 8wd. Also If we can pull off 8wd without sacrificing anything else then why not do it. It does have advantages over other drivetrains, which assuming we can execute it properly will help us be more successful.

28-10-2008 12:17

JVN

Re: pic: Concept 8wd Drivetrain

Does anyone have any quantitative measurements of their performance, or are we basing everything off anecdotal evidence (as usual)?

Yep, 703 was great at pushing in 2007. They had an incredible defensive machine. Yet... I bet that NO ONE made any quantitative measurement of it's performance. Here is what I would looooooooove to see:

• Step 1 - Measure it's linear pulling force while on carpet (use a fish-scale).
• Step 2 - Remove all but the 4 outer wheels.
• Step 3 - Throw the extra wheels on top of the frame, so the overall robot weight doesn't change.
• Step 4 - Measure it's linear pulling force again, while on carpet.

I speculate that these numbers will be extremely similar. Within 5%.
Maybe I'm wrong. I would love to be wrong. I would LOVE for someone to prove me wrong with some solid engineering analysis beyond the usual "703 pushed really hard, they pushed against truck town and everyone knows that nobody pushes truck town..."

Anyone? Anyone? I'd be genuinely happy being proven wrong. I don't have the time myself to do the kind of testing I'm talking about.

Absolutely. If the students decided they wanted to do something "just because they want to" even if it doesn't pass our cost-benefit analysis, I would need to respect that decision.

However I'm chuckling to myself, because I don't really believe our students would ever force me to back up that statement. I guess I'm lucky that our school district's values, our sponsor's values, team's values, our student's values and my values are all pretty much in line.

Then again... maybe it is a pied-pipe type of thing...
Moral of the story, you'd have difficulty finding a Robowrangler who has a bad experience, and our program keeps our sponsors, parents, school, and community happy; I can't really ask for anything more.

-John

28-10-2008 12:46

JesseK

Re: pic: Concept 8wd Drivetrain

I think it boils down to the same thing everything else in life (such as politics, relationships, etc) does: some things you really just have to try yourself. Sure, physicists can scream that surface contact area has nothing to do with dynamic friction forces, but that's hard for some people to visualize. You can throw an equation at them as a proof, but they may find it hard to see the effects of equations balancing themselves when certain values go up or down. You can even give them fantastic spreadsheets that show the numbers going up and down based upon input values, but the end result will be the same.

I decided I wanted to do a c-channel H frame for our prototype and refused to use anything but the CNC to mill out the wheel holes. Sure, there were simpler ways, more efficient materials, and simpler designs (though I'll argue that one after I post the design). But the point is, now that it's all said and done I truly understand why the manufacturing process using c-channel is complicated and will seek to improve it.

Let's not forget, we're all supposed to be stubborn engineers who take nothing for granted and will argue to the end about an idea until proven right or wrong... correct???

28-10-2008 13:17

Andrew Schreiber

Re: pic: Concept 8wd Drivetrain

John, I was by no means suggesting that you wouldnt take students into account, merely showing a valid reason to do something like this. I apologize if my statement was taken that way.

Now, about the evidence, after reading the threads about their drive train their goal was to get the benefits of a tread without using tread, did they meet their goals? For that answer you would most definitely have to get one of them. From my perspective they achieved their goals, 1) eliminate the ability of breaking a tread if pushed sideways. 2) They had an incredibly low cg and still did not bottom out climbing obstacles. They proved that a large number of wheels gives you those benefits. They also proved that the draw back was complexity and weight. Did they gain any additional pushing force out of it? Probably not. Would they have been able to shove us around without 14 wheels? Most likely, but the fact remains that they DID gain an advantage from doing that many drive wheels.

What teams need to take from this discussion, in my opinion, is that there are situations in which 8wheel drive would be effective, there are times when 3 wheel swerve would be effective. You wouldn't try to tighten a bolt with a hammer would you? Always weigh the costs with the benefits and use the appropriate tool for the job.

28-10-2008 14:09

AndyB

Re: pic: Concept 8wd Drivetrain

I think John's point was that you can't tell if having the extra 8 wheels made any difference. If they were just as effective with 6 wheels, why use 8 more.

They would have had VERY similar traction, and in that game, 14 wheels could handle the ramps just as well as a 6wd could. I don't really understand your argument. If they didn't gain any pushing power, but gained weight and complexity, how is that an advantage?

The only advantages I see are played into tread not breaking (not really a big deal if attached properly, nor worth the weight), and CG (wheels aren't the only way to get your CG lowered). I would argue that you have more risk in more wheels than tread breaking risk.

28-10-2008 14:15

gorrilla

Re: pic: Concept 8wd Drivetrain

exactly my thinking

28-10-2008 14:37

Andrew Schreiber

Re: pic: Concept 8wd Drivetrain

First off, I am NOT an engineer, so I can only offer my observations and my basic understanding of Physics. I apologize for this shortcoming on my part and am working towards fixing it

Now, their decision was to go with wheels instead of treads. They felt that they wanted to use many small wheels linked together with gears instead of chain. Were these decision what you would have done? Maybe not, but they chose them anyway, why? Instead of trying to explain it myself I will link you to two posts:
http://www.chiefdelphi.com/forums/sh...1&postcount=35
http://www.chiefdelphi.com/forums/sh...9&postcount=36

I do not feel that 14 wheel drive is necessary, they probably could have done just as well with 8 wheel or even 6 wheel drive. But they are probably a good person to ask WHY one would go with more than 6 wheels.

28-10-2008 14:55

AndyB

Re: pic: Concept 8wd Drivetrain

My argument is not why they did 14wd, I'm only asking you to reconsider this statement. Because apparently, I'm missing your point.

28-10-2008 15:00

JVN

Re: pic: Concept 8wd Drivetrain

Right... I agree. I'm not questioning 703's design process, or the results of their design process (again, it was a great machine...)

What I AM questioning, is why we let people get away with loosely referencing a machine like this as proof that "more wheels = more pushing force" or a similar argument. ESPECIALLY if "the physics" doesn't support such an argument.

Anecdotal non-quantitative arguments are driving me batty... That was my point. I'm past the point where I'm debating the merits of an 8WD.

-John

28-10-2008 15:02

Andrew Schreiber

Re: pic: Concept 8wd Drivetrain

Incorrect wording on my part, They gained what they perceived as an advantage. They did 14 wd because they thought they gained something by it. Thats all I'm trying to say.

Also, John, I also mentioned they had a weight advantage over many other teams. (For anyone who doesn't recall, 2006 had weight classes where a shorter robot could weigh more) I never said the wheels were what gave them additional traction, in fact I know that they cannot. Force due to friction is equal to normal force (in this case mass * gravity) times the coefficient of friction. So for anyone thinking I was saying that increasing surface area increases friction I am by no means saying that. I am saying 703 had an impressive drive train that was very good defensively.

And as for letting people get away with anecdotal non-quantitative arguments, I would hardly say you are letting me get away with it. Some people, myself one of them, don't know the physics behind things, we can only comment based on our observations. Now if we read things into those observations that are not true I would hope that we could be corrected and would be willing to learn.

28-10-2008 16:49

B.Johnston

Re: pic: Concept 8wd Drivetrain

Not to stir the waters here, but, there are a few benefits to having a few more parts and or a little more weight via an 8 wheel drive that havent been discussed yet.

The 2008 1680 design was 8 wheel drive with a 4 in line (no drop) profile.

Each side comprised 2 separate modules of:
1 - 2.5inch cim motors with a 3.57 reduction (14:50 gearing)
1 - 6inch "First Wheel" direct driven in the middle front or rear position and
1 - 6inch AM Omni Wheel at the outside corners driven by 25 chain from the directly connected center wheels.

This gave us.

1. A long support polygon.
2. Short overhangs for obstacles.
3. No wheels dropped, all 4 in-line traction.
4. A short effective wheel base for tank pivot turns.
5. A moderate increase in resisting being spun vs the (6 Omni-Wheel-Omni combo.

And new to this disscussion ... Fault Tolerance.

Each side could maintain some motive force with the following failures

1. 1 chain = 75% wheels driving (2 Wheels - 1 Omni).
2. 2 chains = 50% wheels driving (2 Wheels).
3. 1 Transmission = 50% wheels driving (1 Wheel - 1 Omni).
4. 2 Transmissions = 0% wheels driving (Okay Dead in the water here).

Granted Multi speed transmissions may have some mechanical advantage here.
But there's no reason you couldn't have different mechanical ratio's and electronically gear the drive pairs.

Imagine...

the rear module geared down for pushing,

the front module geared for speed,

under load (pushing/acceleration)the rear pair could receive max current and the front would operate at a reduced level...
until the rear has reached its "slip" limit...
the current drive would then be redirected to the front (still at a loaded condition)...
this would pull the now unloaded rears to an rpm above that which they have torque at.
Presto.. two separate velocity profiles, no gear changes.

Something to consider.

PS 3.57:1 as a final drive ratio was a little fast last year (34ft/sec)

28-10-2008 17:04

AdamHeard

Re: pic: Concept 8wd Drivetrain

I believe Fault tolerance is something that can be built into any drive train, and isn't a distinct advantage to an 8 wheel.

28-10-2008 17:44

B.Johnston

Re: pic: Concept 8wd Drivetrain

It can be...

But here it was done by reducing the number of parts.

The second driven gear was added (per side).

And 2 sprockets and a chain were eliminated (per side).

The 8 wheels design inherently lent itself to being efficiently divided up into 4 identical modules.

28-10-2008 19:31

sdcantrell56

Re: pic: Concept 8wd Drivetrain

This is one of the reasons why I have gravitated toward 8wd is the redundancy of the chain. Unless 2 chains break on each side of my drivetrain, there will always be at least one center wheel plus its outer wheel powered. Also once again this drivetrain was designed for a game with climbing obstacles. I did not design it to drastically increase traction over a 6wd although I believe the traction would be increased by a very small amount given the use of roughtop. Also I like the use of 8wd because there will be less rock than a 6wd. Finally, treadwear will be reduced. These advantages in my mind outweigh the disadvantage of slightly more complexity and about a 2.5lb weight gain.

If the game is a perfectly flat field than chances are very good that we will go with a 6wd version of this wooden drivetrain.

28-10-2008 19:43

Cory

Re: pic: Concept 8wd Drivetrain

This is also true of any direct drive to the center wheel 6wd robot.

I'm always curious why so many people cite the desire to have a robot that can continue driving through multiple chain/sprocket/etc failures. In the hundreds of matches my teams have competed through, as well as our collaborative partners, I have only ever witnessed two chains break-in both cases it was a sprocket failure.

If your drivetrain is well designed and the chains are properly tensioned, the odds of losing a chain, let alone more than one chain are nearly negligible.

28-10-2008 20:05

sdcantrell56

Re: pic: Concept 8wd Drivetrain

I do agree with you and in fact with all of the teams I have been a part of, I have never had a chain break. The first year we had multiple chains come off of sprockets, but ever actually had one break. I guess I am always worrisome and because of this like to design redundancy into the system. I do agree though it is probably unnecessary as long as the chain is properly aligned and tensioned there is really a very minimal chance of breaking, even with #25 chain.

28-10-2008 22:17

Triple B

Re: pic: Concept 8wd Drivetrain

"just say no" to chain tensioners.
i think maybe a 7wd base, lighter than 8wd and 1 wheel cooler than 6wd.
and the "coolness factor" is one of thee most important design features.
mike d

28-10-2008 22:29

vivek16

Re: pic: Concept 8wd Drivetrain

Yes, I bet our 5wd last year could give you guys a run for your money.

-Vivek

p.s. no kidding, PM me if you want more details about the 5wd

28-10-2008 22:34

sdcantrell56

Re: pic: Concept 8wd Drivetrain

Mike please tell me how you used #25 chain without tensioners? I think your drivetrains are my absolute favorite in all of FIRST. I love the use of triangular bearing blocks to allow for easy wheel removal.

28-10-2008 23:11

B.Johnston

Re: pic: Concept 8wd Drivetrain

One way to avoid tensioners is design.

Not that you can see the chain in the pic I attached.

We:

1. used sprockets with an even number of teeth.
2. spaced them apart a multiple of the chain pitch (#25 = 1/4")
   our axle CL to CL = 10"
3. changed the chain loop when it stretched to sag more than 3/8"

And to answer Cory:
Here's why I like redundancy and fault tolerance.

2006

2nd match of the Greater Toronto Regional (Finals)

We're(1680) up on 1114 by 1 match

30 seconds in our driven transfer sprocket shatters kinking the chain.

We spin in circles for the rest of the match.

now even 1 -1 against our sister triplets (this was an amazing tank system)

Change the broken sprocket with a replacement (from 1114 BTW)

Third match in autonomous we throw the chain...

The rest is history.

With redundancy and fault tolerance you shouldn't be susceptible to these sort of events.

Those who fail to learn from history will repeat it.

29-10-2008 00:24

AdamHeard

Re: pic: Concept 8wd Drivetrain

Yes, redundancy and fault tolerance are good, no one is disputing that.

What people are disputing is putting too much emphasis on it rather than what the problem truly is.

If you are throwing chains or breaking sprockets in a drivetrain, the issue isn't that you don't have enough redundancy built in; the issue is something is causing you to throw chains and/or break sprockets. Sure, adding a lot of fault tolerance is a way to reduce the negative effects of the problem, but I would rather have a system that didn't break as much period.

Also, when it comes to choosing numbers of wheels in a drive, fault tolerance should not be a factor. Fault tolerance is something that results from good design.

29-10-2008 01:00

sdcantrell56

Re: pic: Concept 8wd Drivetrain

I completely agree that if it is a design flaw that is causing the failure, then creating redundancy to lessen the impact is the absolute wrong way to do things. A team should build a drivetrain, and the entire robot so that all that needs to be done once it is assembled is tightening the chains. This should also be a simple task.

29-10-2008 04:27

B.Johnston

Re: pic: Concept 8wd Drivetrain

This is going to be a long post so please bear with me.

I agree with you and adam above that there is no substitute for good design.

I believe that good design includes the analysis of the functional requirements of the task you are performing.

This includes appropriately matching component properties in a system.

I think this is where my emphasis on fault tolerance and redundancy is being misunderstood.

For me the meaning of fault tolerance is the ability to recover (perhaps not fully) from an unanticipated occurrance.

For example if you have a tank track system and your tread is stripped off your robot because opponents have discovered that when you are pushed sideways they can be broken.

A fault tolerant design would, perhaps allow for you to drive with no track on(rather than being immobilized because your drive is sitting on a skid plate or bogies).

An example of redundancy does not neccessarily include the use of a second system which fully duplicates the function of the primary system either.

An arm mechanism driven by 2 #25 chains operating in parallel to support the functionality needed rather than a single heavier #35 chain would provide a redundant system capable of withstanding the failure of one chain and sprocket system (accidental entanglement with another robot) if done properly.

Here full function may not be possible but some function would remain.

Neither would be an example of poor design, they are both designed to mitigate the outcome of failures that while not wholly unexpected are somewhat probable.

To design a system so "strong" that it never breaks is not inherently good design.

If we apply enough force to a wheel we can break traction and start it spinning.

This force needed would be our design target (plus a moderate margin) for the strength of the components in the system.

To use components which far exceed this constraint is not good design.

You would carry a combination of penalties in weight, cost, bulk, parasitic losses etc that while nothing broke, something was affected along the way.

A function that couldn't be included, a speed that couldn't be reached, an incline that couldn't be climbed etc.

While I don't advocate either redundacy or fault tolerance as a substitute for good design they can be used as part of a strategy which maximizes the outcome despite the circumstances.

B.T.W. The 2006 sprocket that fractured was a 19 tooth #35 martin sprocket capable of absorbing about twice the power that our double 2.5 cim/andymark 2 speed drive could deliver. It fractured at an inclusion near the keyway due to an internal flaw in its structure. The other sister drive trains were good enough to win three regionals that year.

30-10-2008 04:06

AdamHeard

Re: pic: Concept 8wd Drivetrain

Bruce, I realize I got too wrapped up in my point, and was really ambiguous as to what I said applies to.

I mainly was referring to the teams that think they need all this redundancy because they break 5 sprockets a regional, or a chain snaps every other match.

You're right, fault tolerance is something that really should be designed in (and with good design, can be done pretty elegantly as many robots prove) and not ignored.

No hard feelings, this is all just some good technical discussion

30-10-2008 11:06

Aren_Hill

Re: pic: Concept 8wd Drivetrain

We had 7 runs of #25 chain on the drivetrain alone this year (swerve)
and not once did any of those 7 come off.

I agree completely with Cory on the fact chains aren't a common failure point unless you make them by mistreating them.

30-10-2008 11:47

sdcantrell56

Re: pic: Concept 8wd Drivetrain

Yeah I must agree I have never actually witnessed a robot I was involved with breaking chain. It can definitely happen but for the most part as long as it is tensioned and aligned there's just not enough force to break the actual chain.

I'm currently working on a new version that is 6wd with a plywood frame and the center wheel is directly driven by the transmission shaft. Hopefully I'lll have it up within a week or so.

31-10-2008 02:19

cbale2000

Re: pic: Concept 8wd Drivetrain

I should check the search page for posts about 703 more often, but alas at least I'm only a few days late.

I heard someone was looking for info on powerful drive trains? I'd be happy to answer some questions if there are still any about our drive systems (I noticed a few a page or so up)

That could be very easily arranged if you're still interested, we've still got that bot laying around and (for the most part) working.

31-10-2008 11:34

sdcantrell56

Re: pic: Concept 8wd Drivetrain

A test of the 703 bot would be very interesting. Let's see if all those wheels help traction at all.

03-11-2008 17:51

Joachim

Re: pic: Concept 8wd Drivetrain

The standard physics rule for both static and dynamic friction (Amonton's or Amonton/Coulomb laws) is that friction is the product of the load and the appropriate static or dynamic coefficient for the materials in contact, and is independent of the contact area (see, for example, http://hyperphysics.phy-astr.gsu.edu...frict.html#fri [Article on friction at the Hyperphysics site of Georgia State]).

But this model works well only for some materials under some conditions--and rubber is not one of the materials.


Here are a couple of literature references:

(1) http://www.springerlink.com/content/n30715161g635138/ (abstract of a book chapter entitled "The Influence of Contact Pressure on the Dynamic Friction Coefficient in Cylindrical Rubber-Metal Contact Geometries") which states in part: "As it is commonly know[n], classic Coulomb’s and Amonton’s friction laws, which mainly establish that the friction coefficient is independent of the area of contact, are proven to be not valid in the case of rubber-like materials."

(2) http://www3.interscience.wiley.com/c...69929/ABSTRACT (abstract of an article entitled "Analytical and experimental investigation of the static friction regime for rubber-rigid ball contact") which states in part: "The parameters of the static friction regime in terms of static friction force . . . are investigated for a rubber ball/metal flat configuration. . . . The coefficient of static friction decreases significantly by increasing the normal load."

Where robot weight is fixed and rubber or rubber-like drive contact surfaces are used, increasing the drive contact surface area--such as by more wheels, wider tires, larger radius tires, and/or belts or treads with many bogies--gives lower contact pressure, and often increases the available traction (friction), static and dynamic.

Of course this does not prove, for a given drivetrain on FRC carpet, that more wheels equals more traction. But there is a recognized effect that appears under the right conditions (and a physics-based model to characterize it is explained to some degree in the article text of the second reference, for those who can access it). So it is at least rationally conceivable that the proposed 8 wheels would give more traction.

25-11-2008 23:05

JVN

Re: pic: Concept 8wd Drivetrain

A month later:
http://www.chiefdelphi.com/forums/sh...575#post777575

-John