Excellent point, I point you to one of IKE’s threads for more information on frame flex and its effect. http://www.chiefdelphi.com/forums/showthread.php?t=68307
Edit: And because it hasn’t been posted yet. WARNING MATH AHEAD! (This is where I need a smiley that is a big warning light) http://www.chiefdelphi.com/media/papers/1443
Somewhat contrary to my post, flex isn’t always a bad thing though. It can make it tougher to turn, but it also can make it tougher to BE turned. You do want rigid rails along your chain paths though to make sure chains do not jump.
The nature of the kit 6x6 will give you a chassis with pretty good rail stiffness, and a pretty soft torsion to it. This will want to cause opposite corners to dig in when you are turning (bad), but also causes the same corners to dig in when someone is trying to push you around (this can be good if you are trying to hold your ground). You could probably do a pretty good master’s thesis on torsionally rigidity and its effects of dynamic behaviour. I know I have read a dozen or so SAE papers on the subject.
Again though the most important aspect is to understand your functional objectives and priorities. Initially we ranked our mobility objectives as:
Miving is given.
Ability to go over bumps
Ability to hold ground while shooting
Ability to maneuver well
Ability to move long distances quickly
This year we knew we wanted to clear the back row, and move to the midfield, thus the high priority on going over the bump. (In theory you can have 5 balls in a line that the opponent cannot stop you from scoring. That plus hanging equals 7 points which could win most matches.) In 2006, we were pushed around while trying to shoot, thus the hgih priority on holding ground. After the first competition, we found that more defense was played on the goals and balls than robot to robot and thus re-prioritized maneuverability over holding ground. Thus we changed from grippy wheels to the eventual migration to Omnis in the rear (we also needed less rock for ball collector improvements).
Understanding your objectives/priorities/assumptions helps you make decisions when you need to make improvements.
Amount of Rock, which wheels, and how you construct your frame are secondary to how you want it to ideally behave, and where you will make compromises. We consider these design attributes and/or tuning elements used to get the behaviour you want.
(P.S. I really liked the architecture of 201. If we didn’t have all the experience and capability with 6x6, I would have pushed hard for an identical architecture. With a little more power, it would have been even more lethal.)
This is how 254 and 968 have driven their robots for the last 2 years.
On 971, we do the same thing, except we use a steering wheel instead of a joystick for turning.
It’s nice for the exact same reason that Jason described, you have decoupled the steering from the throttle. I don’t know if this is common or not, but we decouple our steering from our throttle in software as well. Our steering wheel controls the radius of our turns, not the turn power. I’ve found that this makes it a lot more accurate. If you like how your current turn trajectory is going, and just want to go slower, all you have to do is let off the throttle, compared to letting off the throttle and the steering power. Fiddling with this kind of stuff is a great summer project. It’s amazing how much more responsive good code can make a robot feel.
As a bonus, read up on IBM’s Trackpoint mouse. There’s a pretty cool feature that it has that helps with robot handling.
what does it take to make a great 6 or 8 wheel tank drive?
Chain/Belt management
Unless you plan to go gear to gear, a la frc25…then you need to plan out how to properly tension your chains. The West Coast chassis and its derivatives all have dedicated chain tensioning designed in.
Force at the Carpet
Great turning requires the proper amount of force at the carpet. Drop center can effectively shorten your wheelbase and help your cause, but without enough force at the carpet you’ll get poor turning. Do some quick calculations or use the JVN whitepaper to understand how wheel diameter and subtle gear ratio adjustments can lead to huge leaps in the force available for turning.
Suspension or Flex
Chassis flex or dedicated suspension setups may be the secret to amazing 6wd and 8wd chassis designs. 254 and 968 deploy lots of drop and small diameter large force generating wheels to get the job done…they are also careful CG managers. But it seems to me that the sheet metal bots and those who have special suspension features tucked inside also demonstrate excellent turning prowess. frc118 has deployed suspension in the last two seasons…I’ve heard nothing but positive things.
Hey, everyone who contributed, thanks a ton! I look forward to putting this information to use!
Also, thanks (primarily) Jason, for the recommendation regarding programming and control.
Actually, one of the many things that an all-traction wheel 6WD or 8WD has going for it is that it tends to be far more robust to flex, uneven terrain, or damage than those with alternating omni wheels, all omni wheels, swerve drive, or all mecanum wheels. As long as at least one wheel on each side is in contact with the ground, you can still drive straight and turn (though how well you turn and about which point you turn depends very much on which wheels are touching). We had rounds where we took significant frame damage or lost one or more chains and were still able to drive and score effectively (with an all-traction, drop center 8WD).
Obviously, things always work better with a pristine, square chassis, but the drop off isn’t as bad as it would be with other drive types.
Here’s my requirements for a good drivetrain (take it for what it’s worth):
1> It must be controllable – Having a robot that can go 20’/sec doesn’t help if it’s uncontrollable.
2> It must satisfy the games objective and teams strategy – Being able to push a tank across the field does no good if there is no tank to push. Look at the game and its requirements and design for that.
3> The controls must be intuitive – The driver will be ineffective if he has to constantly look at his controls and not the playing field.
4> It must be reliable – It makes no difference if you have the greatest crab/swerve/omni/14-wheel drive if it is constantly breaking. The kitbot drivetrain is reliable enough to take you where you need to go.
5> It must be maintainable – Things happen, and even the most reliable drivetrain can be damaged. If you cannot fix it during the competition (IE fast) then you need to rethink some of what you are doing. Unmaintainable leads to uninspiring.
You’ll note that I did not discuss type of drivetrain, this is because that is very game specific. The best drivetrain is the one that accomplishes the job at hand, regardless of its configuration.