Hello everyone,
We are working on an off-season project with our FIRST team and we have run into a snag with our six wheel drive train. Attempting to turn produces violent shaking of the chassis. I understand why the problem is happening (forces perpendicular to the rolling action of the wheels), but does anyone have any ideas on how to fix this.
Video evidence: http://www.youtube.com/watch?v=u0X_5iQXbZw
Thanks
Patrick
When looking at how your robot will turn draw a picture of your wheelbase from the top. Then draw a circle from the center of your robot through all your wheels, this is the path your wheels take as the robot turns. If the wheelbase is longer than it is wide your outer wheels will literally be dragged sideways while you are turning. This makes for very slow and inefficent turning (if at all).
Your problems:
You are using pneumatic wheels. While they make it more difficult to be pushed they also make it more difficult for YOU to turn. A higher Coefficent of Friction (CoF) results in more “stick” between your wheels and the carpet. This makes it that much more difficult for your wheels to drag themselves sideways. The “jumping” you see in the video is your outer wheels literally hopping up and down in order to drag themselves around as you turn. Additionally, pneumatic wheels conform to the carpet which means that even if you drop the center wheel some (see #2) the outside wheels can still touch the carpet providing drag.
Drop the center wheel 1/8" to 1/4". This takes what would otherwise be a very long drivebase and basically cuts it in half. If only 4 of the 6 wheels are touching the ground and those wheels form a rectangle from the top which is wider than it is long then the wheels will be traveling in mostly the forward direction while the turning is taking place. Many teams who do this bias the weight of the robot far forward of backward so that the majority of the time the robot is only of 4 of the wheels only momentarity “rocking” onto the other two wheels during deceleration/ stopping/ quick turns. The last thing regarding dropped center 6/8 wheel drives is that they require a stiff chassis. The forces acting on a chassis while turning want to pull down the opposite outside corner wheels of the chassis. Fun enough, these are the exact wheels that you DON’T want to touch the ground while turning and will dirastically reduce your ability to turn as the chassis momentarily has a long wheelbase. If you have a flexible chassis to counteract this you need to significantly lower the middle wheel (the AndyMark kitbot is 1/4") in order to maintain the floor clearence of the outer wheels when turning fast.
(Note: a dropped center 6wd also requires the middle wheel to be in the center of the two outer wheels so turning is the same going forward and backward.)
I hope that answers your question.
Regards, Bryan
So theoretically, if I was able to drop the center wheel enough so that only 4 wheels were touching the ground, would the wobbling/shuffling stop?
It should. I’ve been on a team that used 6WD drop center with 6 pneumatics. After a couple days or so of that, we changed to 4 pneumatics on the “heavy” side of the robot and 2 AM wheels on the “light” side. There are other teams with a similar starting setup that haven’t had that issue.
Your other easy option would be to keep the drivetrain flat but replace two end wheels with omni wheels. This would reduce the sideways scrub on that end of the robot.
I might suggest playing around with both solutions; whichever one works best for a given robot may vary.
You could also try talking to team 25 about how they brute-force a 6WD with 6 pneumatic tires to turn without hopping all over the place.
Having only 2 wheels dragging as opposed to 4 will reduce shaking. You just get a little rocking as a trade-off.
General solutions for reducing the turning friction, which is presumably causing your problem:
Reducing the friction by decreasing the wheelbase length
-a. Moving the outside wheels towards the centre wheels
-b. Dropping the centre wheel (effectively halves the wheelbase if only 4 contact the ground at a time)
Reducing the friction directly
-a. Reduce the width of the wheels (doesn’t work too well with pneumatic tires)
-b. Use different outside wheels with lower coefficients of friction (omni wheels are a very-low friction option)
I personally think 1b and 2b would be the most effective, as has been advocated by other posters.
A potentially easy fix: try increasing the pressure in the middle tires and lowering the pressure in the four corner tires. Not sure if it will help at all but it’s a quick test, and might have the same effect as “dropping” the two center wheels.
25 Uses Skyway Bead Lock wheels with a tread pattern cut into them.
There’s really not much of a secret to how they turn, their wheelbase isn’t all that long, only about 25"-27" or so, which is right about the same as the track width of their drive.
If you were to theoretically drop the center wheels your turning would significantly better. However, in order to maintain that drop with pneumatic wheels you probably need to meet a couple standards:
a: You need a very very tortionally stiff chassis to pull off a 6wd with all pneumatic wheels. That sort of drivetrain would is very difficult to make and maintain and is usually not merited by the game. I would suggest that you entirely change out your wheel type to roughtop or wedgetop tread. If you really want to use pneumatic wheels I would at least change one or both of the two outside sets of wheels to a lower tration variety. If you keep the pneumatic wheels on the outside you may still have a hard time turning because the middle wheel compresses negating the drop and allowing all 6 of your wheels to touch (resulting in basically what you have now). This can be worked around but you really don’t lose a whole lot of ground holding abilitiy by ridding yourself of at least 2 of the pneumatic wheels.
b: If you do end up staying with pneumatic wheels Keep the wheels fully inflated and at a constant pressure. The best way to do this is to have a designated air pump which is set to what ever pressure you deem best for your tires, then only use that pump on your tires so they are always the same pressure. Otherwise maintaining the wheel diameter and middle wheel drop can be very difficult. (This is one of the reasons most teams prefer fixed diameter wheels such as the ones AndyMArk sells.)
In your situation I would drop the middle wheels 1/4" and change all 6 wheels to the KoP “sticky” wheels (with the darker gray rubber tires). If you do that you will find that the chassis will turn and handle well in high speed manuvers.
Not to get all design preachy, however, when you are designing in the future keep in mind that the goal of the drivetrain is to get you where you need to go. In order to do that you don’t need a lot of traction. You DO need to turn though. The trade off then seems fairly obvious. Besides, no one is going to bother to push you around unless you can drive normally and play the game successfully. (which involves a lot of turning)
Best of Luck, Bryan
25 has used 6 and 8 wheel drives using bead-lok wheels, and are very well known for it. Never pneumatic tires.
All 6? You really only need to change the corners to the AM wheels. Maybe only one end. Maybe none at all, if the drop works out.
Changing all 6 to AM (or KoP, there isn’t a difference) is way overkill for a simple turning problem like this one.
Okay, since this is the offseason and your really looking to expand your knowledge why not treat this as an experiment in design. Change one thing at a time systematically and record the effects.
By changing one thing at a time you can clearly see the effects each factor has on this drivetrain.
Based on what I saw in the video you are using an 80/20 chassis which is highly modular and should allow for tons of adjustments. While I can’t clearly see how your axles are mounted or your axles design for that matter, I’ll assume that you are running a “dead axle” in some form of a bearing block mounted to the 80/20.
Here’s a list of things to change, one thing at a time first, then combining.
-Placement of weight on the chassis (CG)
-Wheelbase/Wheel spacing
-“Drop” Distance of Center Wheel
-Tire Pressue
-Wheel Type
See what makes the robot handle the way that you and your team want.
Depending on the game and your teams’ goals for the robot drive train, a robot that bounces a bit as it turns isn’t necessarily a huge drawback.
You now have one pneumatic 6WD design in your “toolkit”. Play with it - try some of the things suggested in this thread - and you will have another.
That is simply what I would do because I’m not a big fan of pneumatic wheels. Are there other ways to do it? yes. I quoted myself earlier in the same post saying that changing only one or both of the outside wheels would be sufficent.
It’s all a matter of personal preference.
The following whitepaper provides the physis/math basis for the suggestions that everyone’s been giving. http://www.chiefdelphi.com/media/papers/1443
Try eight, with the middle dropped. Should be more stable, since you’re rocking back and forth less often.