Making your robot drive easier
 

I have noticed that all teams who consistently win don't have a complicated design but seem to drive well. SO I sugested to the team we just make a robot that drives and drives well, this year. But I'm not quite sure whet else to do to make it drive easier. Other than making a really solid gearbox with no slop. And ramping the controls.

any ideas?

Re: Making your robot drive easier
 

less c.o.f. wheels so with that spiffy nice gear box in high gear you could turn very easily.. or make omni wheels! Yay! omni wheels

Re: Making your robot drive easier
 

Intuitive controls. Even if you have a complex drive system, if you have a simple/intuitive control system your drivers will be able to drive it.
And practice, lots of practice.

Here's an example (idea taken from some bots, controls from an arcade game):
A bot has four wheels. each has its own drive motor. Each can also spin a continuous 360 degrees (Can spin forever without reversing). Complex drive.
On the control side: 2 joysticks. One for the left, one for the right. Each side goes the direction the joystick is pointed. Up means forward, down means backwards, left means left, right means right. U.L. means 45 degrees to the left from forward. The farther from center, the faster it goes. Front/back wheels are linked in code.

This control system is simple, and follows a standard skid-steer setup (with some mods). however, you can translate (go left/right w/o rotating) by pointing the the joysticks that way. So basically, you can make your robot dance, or rotate while moving forward (advanced technique).

Granted, this sort of set-up would take lot's of practice. lots and lots.

Re: Making your robot drive easier
 

I'm going to disagree with PitBull a little bit...

I think the key to an easy-to-drive drivetrain is having the optimal lateral coefficient of friction (not the lowest). That is much easier said than done, but here is why I feel this way:

1) If you have too much lateral friction, your drivetrain will not turn well (or maybe not turn at all). This is obviously not desirable for many reasons that everyone knows, so I'm not going to explain further.


2) If you have too little lateral friction, the robot can become difficult to drive. Why? Inertia!

When your driver let's go of the stick, the robot will continue to turn. The amount of excess turn is inversely proporional to the amount of lateral friction you have. If you have too little friction, the robot becomes hard to control since the driver will let go of the stick where he/she want the robot to stop turning, and the robot will overshoot past the stopping point. This can be overcome with a lot of practice, but the whole point of this thread is to make a robot that doesn't require as much practice to drive.

Over the years, we have had robots that had very little lateral friction and I will definitely say that they were our most difficult robots to drive. Once we went to 4-wheel drive, the robots would stop turning the instant the driver let go of the stick - these robots were very easy to drive.

So, the moral of the story is: less friction is not always better. There is an optimal amount of friction - good luck achieving it.

Re: Making your robot drive easier
 

Make a really solid gearbox. I find that you don't need any more then 2 speeds. High and Low. Also you can have wheels with a high C.O.F but to make your bot turn easier you need to 6 wheel drive and lower the middle set of wheels about 5 thou lower then the other ones and there you have a killer drive system.

Re: Making your robot drive easier
 

SNIP
I would really like to hear more about this idea. Anybody make a six wheeled robot with all wheel drive??? I saw team 418 at the Lone Star Regional but didn't get an up close look at their drive. I was more impressed with their ball catcher and laminated wood biggens ball arms :ahh:

post some pics or PM me or send a link to your site with photos

Thanks,

next year. :)

APS

aschuetze@sbcglobal.net

Re: Making your robot drive easier
 

Team 233 has a 6-wheel all drive system. I'm assuming, when you say 6-wheel all drive, you simply mean all wheels are drive wheels.

Anyway, each side is linked to a joystick for tank driving. The middle wheels are lowered just a bit to give us the ability to turn well, and the friction of six wheels sitting on the ground makes us very hard to push sideways. I don't think our team will ever change from this 6-wheel setup (unless we get a radically different game from FIRST).

Re: Making your robot drive easier
 

Here's a link to the 47 robot who uses a sweet 6 wheel drive with chain drive on each side team 47's bot

and here is the 254/60 "bionic poof" with their 6 wheel drive with the middle ones lowered a little bit

i think, correct me if i am wrong, that 25 was one of the teams that most successfully implemented this last year and many teams have used similar ideas.

Re: Making your robot drive easier
 

We had a 6-wheeled robot this year...I don't know whether it was six-wheel drive though. It took us a few days to get the middle-axle lowering set the right way, but it helped a LOT. Oh I should tell you that we used some spiffy donated tracks, and it made us look like an R/C tank out there.

Robo pics and videos are here: http://www.sau53.org/net9/First/media.htm
You may also see the team at PARC in May. I wish I could go, but I'm going to Washington D.C. that weekend :-C

Re: Making your robot drive easier
 

i posted a pic of our 6 wheel drive train. theres some stuff in the way, but you can still see most of it.

http://www.chiefdelphi.com/forums/pi...gle&picid=7644

Re: Making your robot drive easier
 

Watch your speed. I saw some teams build robots that were incredibly fast, but I can just imagine the nightmare those drivers go through whenever they try to do something rather than getting from point A to B.

Re: Making your robot drive easier
 

feedback - PID control loop on the steering

using the default code to let the position of the joysticks control how much voltage (pwm) is thrown at the drive motors is as crude a control method as you can get (well, there is one thats worse, only having a GO/STOP switch :^)

no matter what your drivetrain is if you put yaw rate and/or linear speed sensors on the bot, and use feedback, you can remove all the nonlinearitiy, inertia, friction.... from the control system and MAKE the bot do exactly what the driver is commanding at any given instant.

another idea is to add a 'jog' function to your control system - I posted stuff about this before - let me search and see if I can find it...

yep,heres the thread - BTW it contains several other simple ideas on how to make your bot better:

http://www.chiefdelphi.com/forums/sh...&highlight=jog

Re: Making your robot drive easier
 

Let me elaborate a little. We used the brecoflex tracks this season. They have and awesome amount of friction (cof ?). The problem we had was we could turn because they spread out on the floor 26" and was only 21" wide. To solve the we put a larger we wheel in the center reducing the amount of track on the ground by about 45% in either direction. This gave us just the right amount of track on the ground to turn and allowed us to move alittle faster.

If you have any questions about it PM me or IM me.

-Pat

Re: Making your robot drive easier
 

if you are really looking for a good team's drive that uses 6 wheels, check out team 42. for the past 4 years they have gone with the same drive, and its really worked out well for them.

Re: Making your robot drive easier
 

Best way to drive well.......... have a great driver! A great driver can overcome and drive system.

Re: Making your robot drive easier
 

we did six wheel direct drive, with lowered center wheel.

here it is without any end-efffects:

http://www.chiefdelphi.com/forums/pictures.php?s=&action=single&picid=6516&direction =DESC&sort=date&perrow=4&trows=3&quiet=Verbose

Re: Making your robot drive easier
 

Roger, where those red plates powdercoated? or just painted? where did you get those wheels btw?

Re: Making your robot drive easier
 

plates are powdercoated, and the wheels are 2" x 8.5" skyway bead-lok modified using a dremel and a belt sander.

Re: Making your robot drive easier
 

You could aso check out team 25. They have had a great 6 wheel drive system for the last two years.

Re: Making your robot drive easier
 

Well, while we're on the subject of Home Depot cart type drive systems (you know, 6 wheels with center ones lower), 980 and 599 had them too.

Re: Making your robot drive easier
 

Question about "Home Depot Lumber cart Drive-train"
Is the lowering of the center wheels as significant as is with the lumber carts? They have a definate see-saw effect and are truely always driving on just four wheels. Center set plus either set on the corners.

Or is it just enough to lower the contact patch on the corner wheels so that the frictional forces are reduced such that it can turn and not dance around corners?

A post early on indicated approximately 5 thousandths of an inch lower. Is that consistant with other six wheel 'bots? How much did you lower the center wheels?

Thanks a bunch for the posts and pics :]

APS

Re: Making your robot drive easier
 

All you need is 1/4 of an inch maybe less, This allowed us to actually TURN with 12.5" wheels (allot of bots couldn't turn that didn't have a sixth wheel and still had 12 inch wheels) , this way if we decided not to hang we could stilll go around the field and stuff.
PID is pretty cool we haven't tried it but it is way better because then you control the amount of acceleration during turning not simply the voltage ... this works esp good on arms where as you go straight up the lever arm is significantly lower then at 90 degs so less voltage is req'd for the same amount of acceleration...the computer compensates...so the driver doesn't have to jerk the stick back and forth

Re: Making your robot drive easier
 

I don't see how the 6 wheel drive would be optimal. If the center two wheels are lower, then only 4 wheels are touching at one time. If this is true, then you actually have power going to the two wheels that aren't on the ground simply being thrown out the window. Why not have a 4 wheel drive system with the wheels very close together. Am I missing something here?

Re: Making your robot drive easier
 

1. The energy isn't being thrown out the window, because usually the efficiency of a wheel not on the ground is very high. Very little energy is being "lost". There isn't any real voodoo involved, the torque from the gearbox is just transmitted to the wheels under load (the ones touching the ground), and is mostly being turned into pushing force. The inefficiencies of this design aren't any greater or less than a normal gearbox. It is just as easy to make a high efficiency 6WD base, as a high efficiency 4WD base (which is still a tricky thing to accomplish).
Striving for high efficiency is something we've spent a lot of time looking at. Check out any drive team 65 has built. Those pesky Huskies sure know what they're doing. Being able to take the power from the motor, and put most of it to the floor is a huge advantage. ;)




2. Putting 4 wheels close together would work. Except, it is highly unstable, and the robot is vulnerable to flipping (especially if it has a high CG). Think about it. When the robot comes to an abrupt stop, the CG of the robot tries to rotate around the furthest forward point touching the ground. The further forward this point, the less likely the robot is to flip over forwards.


6WD, with the middle wheel dropped allows for excellent turning, while still utilizing high traction wheels for ALL contact with the ground (which maximizes pushing force). It also allows for a looong wheelbase, which as mentioned above increases robot stability.


Problems with this design?
Well, another set of wheels is more weight. The means of driving these wheels is... more weight and more complexity in the drive. Also if the distance the middle wheel is dropped is "too much" then the robot will rock back and forth during driving and "waddle" across the field. 6WD can ve VERY VERY elegant when done correctly, but there are of course pitfalls to overcome. 60/254 overcame them very, very nicely. ;)


I'm a fan,
John

Re: Making your robot drive easier
 

38 used a 6 wheel drive system for the first time this year. I think we used 6" pneumatic skyway wheels, the middle pair about 3/16" lower than the other two. Last year we used 8" pneumatic tires in a 4 wheel drive setup, and danced and hobbled more than we turned... too much lateral friction.

I think one advantage of a 6 wheel system is to have the weight of the robot pivot about an axis in the middle, rather than distribute it to all 4 wheels in contact with the ground at the extremes of the chasis. It seems to help quite a bit with traction, while keeping a low enough cof to turn without dancing. It's sort of cool watching it turn- if you drive one side and leave the other idling, the robot will pivot perfectly about the undriven side's center wheel. This compared to our 4 wheel system, where if one side is drive, the robot will coast in one direction, rather than pivot.

I have to say though, I love 25's entire drive- no chain! Gears the whole way through, to all 6 wheels!

Re: Making your robot drive easier
 

6 wheel drive is cool and all, but...

Why use 6 wheels when you could get away with 2?

2 wheel center drive with skids at 4 corners makes for an extreamly quick and easy to drive bot. Plus the simplicity and light weight, and you've got one honey of a drive train done and running in a week, and plenty of time and weight left over for doohickeys.

If you want to do a 2 wheel drive you pretty much throw out any form of climbing abilty. But, if you're going after balls or want speed, manuverbilty, simplicity and rock solid stabilty (two wheelers almost never get tipped over) all in one, 2 wheel drives are the way to go.

Get the right balence, and its like driving a spunky sports car vs a truck with locked axles.

-Andy A.

Re: Making your robot drive easier
 

Pushing Force!

If you are using 2WD with 4 skids on the corners, you have at most 6 points of contact with the ground, and for most operation at least 4 points of contact. (Sometimes it balances on just 2 wheels, but not in the cases I'll be describing).

This means, there is weight resting on non-powered wheels (casters, skids, whatever). Why is this bad?

Simply:
Pushing Force = (Normal Force of Robot resting on Drive Wheels) * (Wheel coefficient of Friction)

By putting weight on "Dead wheels" you are greatly limiting your pushing force. If you have 4 points of contact, but only 2 of them are powered (i.e. 2 wheels, 2 casters) you are only resting 1/2 your robot weight on the drivewheels, and only utilizing HALF of your potential pushing force. (Yes, this is a simplification assuming weight is evenly distributed over all wheels, but it's still a valid point).

So, here is the blunt (however opinionated ;)) truth:
If you use 2WD, you better not plan on winning any pushing contests.
Physics is against you, and it's not just a polite suggestion -- It's the law!

John

Re: Making your robot drive easier
 

Who said you have to push anything? It's all about driving simplicity and ease here. When it comes down the straight out driving ease and manuverbilty (which is a big component of ease), 2 center wheels are just a dream. Coupled with a PID loop, and getting the bot into tight spaces and making real precise movements is a breeze.

I've driven both types, and if the game calls for climbing or pushing absoulty go for 6 wheels. But, if you want to go after balls or otherwise be quick, 2 wheels are much easier to drive at high speed then any other setup I've seen.

And mark my words, one day FIRST will put out a game that encourages speed and manuverbilty over bulldozing.

Re: Making your robot drive easier
 

2WD with four skid plates will be more maneuverable than most 6WD systems, this much is true. But, when 6WD is implemented very carefully, it will only be slightly behind a 2WD setup, the loss of agility shouldn't be overwhelming. On the other hand the difference in pushing power between 6WD and 2WD is rather substantial.

I'd be willing to give up a small amount of turning ability for a large amount of pushing power any day. Give a little, get a lot. The decision seems simple.

Also, it's hard for me to envision a game where pushing power isn't rewarded in any sort of way. Then again, I'm sure I said this before the 2001 season...:o

Re: Making your robot drive easier
 

When that day comes, the teams with 6WD will have both. :)

It's all about tradeoffs.

6WD is more complex, but has many MANY more benefits.
Check out the 254/60 bot.

John

Re: Making your robot drive easier
 

Team RUSH has used that same wheel set up in past years, and we used it again this season.

Re: Making your robot drive easier
 

This is determinable (spelling?) based on the robot and your best judgement. Our robot used tracks as previously mentioned and we increased the size of them 4 one hundreths. It really depend on the situation (length and wheel size)

I also see a debate about manueverability of two wheel and six wheel drive. From my point of view tracks are a simple solution...as long as they are used in moderation. We found you can have too much of a good thing. We have found that by having a caster with a track extending 1/2 to 3/4 of our base we have extremely good handling and manuverability and lots of power. This is the first year we've gotten away from the caster and managed fine.

Just some thoughts.


-Pat

Re: Making your robot drive easier
 

I couldn't agree more.

For years we had trouble tripping breakers while turning with a 4WD base due to too much lateral friction. Last summer we started experimenting with the 6WD base, and when we finally got to drive it we found it turned TOO well. Our center wheel is offset 3/16", and when we turned at full speed, the robot would basically turn on the 2 center wheels. The other wheels were still in contact, but they had very little weight on them so there was very little friction preventing the robot from turning. In high gear our robot goes about 15 ft/s, and when we turned the robot flew around at more than 1 rev/s.

We liked the extra manueverability, but it was too difficult to control with a standard 2 joystick tank drive. We ended up using a steering wheel and a throttle. See this thread for more information about how we implemented the steering wheel.

Another problem was going straight. Less lateral friction meant it was easier for the robot to turn even when one side was only slightly more powerful than the other. In our case the flip-flopped drill motors were our biggest problem, but even things like worn down treads or slightly more friction on one side could cause the robot to veer off to one side if you powered the motors at the same voltage. 980 had the same problem as us. We (254/60) solved it using prox sensors which gave us about 54 counts/rev and allowed us to adjust the voltage being sent to one side to keep it going straight. 980 used a gyro (the ADXRS150EB) in basically the same way. We were going to use a gyro too, but Analog Devices ran out of them.

I don't like the 2WD idea because it would be way too manueverable. You need some friction out there on the ends of your robot to provide some torque to slow you down while turning. And if you try to solve it by adding more friction to the skid-plates then you just lose power when you want to go straight. You can try to solve it using software and sensors (and in fact I'm probably going to work on something over the summer to use sensors to have the robot maintain a certain orientation), but it's not going to be easy.

We plan on sticking with the 6WD at least for the next couple of years. Sure there are alternatives out there (treads, swerve, holonomic), but as John said 6WD is a great combination of strength and agility.