Moment of inertia and turning performance

[icyplanetnhc]

Smooth turning has been a challenge for our robot this year. We noticed that unlike our 6-wheel tank drivetrain, several teams have been able to create 6WD with butter-smooth turning. We recognized that our off-center CG degraded our turning abilities. However, I feel that there should be another cause that adds to the "jumpiness" of our turns. My prime suspect is the moment of inertia, both about the turning axis and the middle wheels. Suppose that you have a robot with a fairly well-centered CG. Will large or small moment of inertia about the turning axis result in smoother turning performance?

[Peck]

The moment of inertia for the robot is a complex system. (Assuming from here a simple normal system) A low moment of inertia results in a lower rotational momentum and therefor a greater ability to start and stop turning. A high moment of inertia results in a higher rotational momentum and therefor a greater desire to keep turning at the same rate.

If you only want to have a consistent, smooth turn, a high moment of inertia is what you want BUT at the cost of responsiveness and control.

also keep in mind that your CG is only ideally placed when centered if your robot turns exactly around the CG.

Finally: the choppy turning may be because of your programing or your power transfer method.

[MrForbes]

Usually it's a matter of traction vs. center wheel drop. If you have a bit too much traction, or not enough center wheel drop, then it's kind of hard to turn.

[Brandon_L]

Originally Posted by squirrel

Usually it's a matter of traction vs. center wheel drop. If you have a bit too much traction, or not enough center wheel drop, then it's kind of hard to turn.

Last year we had problems with our 6 wheel kitbot drive. The center wheel wasn't dropped enough, so pretty much all 6 wheels were on the ground. It was extremely jumpy and hard to turn. We solved this in the offseason by switching to the grip wheels in the rear, powered, removing the middle wheels all together, and throwing free spinning omnis on the front. Worked great. And we learned our lesson, this year we have an 8wd that drives like a beast.

[BJT]

We usually do a 6wd with a 1/8 inch drop center and roughtop tread on all wheels. Our robots always turn smooth as silk but this years robot didn't turn well at all. I think our weight was too centered over the middle wheels. we were able to bolt a 7 pound weight to the back of the robot which completely cured it. With a 1/8 drop center those front wheels are really close to the carpet and I think you need to keep them from grabbing as much as possible for smooth turning.

[Andy A.]

The prime suspect for a 'jumpy' 6 wheel drivetrain would be a combination of traction materials and CG in relation to the center wheels. Also, it's possible for a small center drop to be overwhelmed by the frame flexing enough to allow the corners wheels to see more weight then is ideal.

However there are instances where turning problems can be related to control setup or PID loop tuning, and not just hardware. Are you using a PID loop on your drive train? Are you using one joystick drive? Two? A gamepad? Simple mistakes in the axis mixing or PID terms can result in really wonky turning behavior.

It can also be helpful, though not at all necessary, to use Jaguars on drive motors. They have slightly better performance that can help improve turning at low speeds. The victor/jaguar thing is still a touchy subject, I know, but this is one thing the jags really do have going for them.

[BJC]

Our team has done a lot of personal testing and evaluation on the top teams in the league and we have found that the CoG and tortional stiffness is generally what sets their robot's drivetrain apart from most others.

CoG, contrary to popular belief we have found that setting the CoG far to one side on a 6wd is best. A 6wd performs best when it is actually a 4wd with 2 extra wheels. Placing the CoG in the middle generally results in the turning point of the robot changing in the middle of the turn as the robot accelerates/decelerates and the outer wheels alernating touching the ground. This is especially important because a 6wd does not turn about the middle wheels but rather between the middle wheels and whichever pair of outer wheels happens to be touching the ground. So by limiting rocking as much as possible performace greatly increases.

Tortional stiffness is absolutely critical to a good 6wd. In 2008 we discovered that without a very stiff chassis the opposite outer wheels will touch down essentially creating a long wheelbase while turning. This will also seriously limits turning.

Edit: I forgot to add, an 8wd has mathmatically much better turning characteristics than a 6wd provided the wheels are properly spaced.

But don't take my word for it, if you don't believe me run some experiments.
Regards, Bryan

[Brandon Holley]

Building on what Bryan said...

CoG is a very overlooked thing in 6WD/8WD design. Its important to really consider where the approximate CoG of your robot will be as its being designed. This becomes especially true as you begin to build taller structures on top of your drive system. Weight higher up will tend to amplify the issues you are seeing with turning performance.

-Brando

[Jared Russell]

Torsional stiffness and a low CoG are absolutely critical to smooth turning in a dropped-center, skid steer drivetrain.

[icyplanetnhc]

Originally Posted by BJC

Our team has done a lot of personal testing and evaluation on the top teams in the league and we have found that the CoG and tortional stiffness is generally what sets their robot's drivetrain apart from most others.

CoG, contrary to popular belief we have found that setting the CoG far to one side on a 6wd is best. A 6wd performs best when it is actually a 4wd with 2 extra wheels. Placing the CoG in the middle generally results in the turning point of the robot changing in the middle of the turn as the robot accelerates/decelerates and the outer wheels alernating touching the ground. This is especially important because a 6wd does not turn about the middle wheels but rather between the middle wheels and whichever pair of outer wheels happens to be touching the ground. So by limiting rocking as much as possible performace greatly increases.

Tortional stiffness is absolutely critical to a good 6wd. In 2008 we discovered that without a very stiff chassis the opposite outer wheels will touch down essentially creating a long wheelbase while turning. This will also seriously limits turning.

Edit: I forgot to add, an 8wd has mathmatically much better turning characteristics than a 6wd provided the wheels are properly spaced.

But don't take my word for it, if you don't believe me run some experiments.
Regards, Bryan

Yes, our 8WD drive from last year worked well for the most part. It had difficulty in an off-season game because the softer carpet allowed our front and rear wheels to make contact with the ground. We tried to alleviate that problem this year by increasing the drop of our center wheel.

With regards to method of using a 6WD as a short-base 4WD with 2 additional wheels, this is something that we've considered last year, but I believe we chose against it because it might've made aiming on one side of the robot, either the scoring grid or minibot tower, difficult (this was before we chose to use "alignment legs" at the bottom of the robot). In addition, if our robot's turning axis is biased to one side, other robots may have a large moment arm to turn our robot.

[Nathan Streeter]

Originally Posted by icyplanetnhc

With regards to method of using a 6WD as a short-base 4WD with 2 additional wheels, this is something that we've considered last year, but I believe we chose against it because it might've made aiming on one side of the robot, either the scoring grid or minibot tower, difficult (this was before we chose to use "alignment legs" at the bottom of the robot). In addition, if our robot's turning axis is biased to one side, other robots may have a large moment arm to turn our robot.

While it would make aiming on one side of the robot consistently harder since the point of rotation would be farther from one end of the robot, it isn't actually very different from having the "standard" drop-center 6WD... The drop-center 6WD is, to a degree, just two short 4WDs that are "randomly" switched between. From my impression, the drop-center 6WD rarely actually turns around its center wheels, but is usually turning around a point between one of the sets of 4WD. I would wager that most drop-center 6WDs are actually easier to spin than most "4WD+2WD" drive trains. Since the weight in a 4+2 is more firmly placed on four of the wheels, it seems like it might be more resistant to turning; however, that is conjecture rather than experimentally-verified fact...

[BJC]

Originally Posted by icyplanetnhc

Yes, our 8WD drive from last year worked well for the most part. It had difficulty in an off-season game because the softer carpet allowed our front and rear wheels to make contact with the ground. We tried to alleviate that problem this year by increasing the drop of our center wheel.

With regards to method of using a 6WD as a short-base 4WD with 2 additional wheels, this is something that we've considered last year, but I believe we chose against it because it might've made aiming on one side of the robot, either the scoring grid or minibot tower, difficult (this was before we chose to use "alignment legs" at the bottom of the robot). In addition, if our robot's turning axis is biased to one side, other robots may have a large moment arm to turn our robot.

Drop definatly is a huge part of the equation. Too much and you rock back and forth, to little and the wheel scrub makes it difficult to turn. This is again why tortional stiffness is SO important. With a flexable chassis you end up with dropped wheels while turning and raised wheels when your not: the worst of both worlds.

As to not wanting to be turned sideways, it seems to be mostly a non-issue compaired to the performance gains, you don't notice the powerhouse teams that use 6-8wds year after year getting pushed around very much. (and when you think about having the CoG on top of the middle wheels in the 6wd wouldn't having a changing point of rotation actually make it harder to line up to something?)

Interesting thread
Bryan

[Michael Corsetto]

Originally Posted by BJC

Drop definatly is a huge part of the equation. Too much and you rock back and forth, to little and the wheel scrub makes it difficult to turn. This is again why tortional stiffness is SO important. With a flexable chassis you end up with dropped wheels while turning and raised wheels when your not: the worst of both worlds.

Any advice on how to maximize torsional stiffness in a frame? Just beef up the gussets? Add a second frame layer a few inches up? I know this is kind of a vague question, considering all of the different frame construction methods, just wondering how teams typically negate this (if at all).

Some of my own observations:

Welded tube frames are always rock solid in my experience. 1662 uses 1"x1.5", 1/8" wall tubing welded frame. No flex whatsoever. This is consistent with many WCD around California.

1678 uses 8020 extrusion frames, and they flex like mad. Maybe it was just the gussets we used, but I have a feeling that the 8020 beams are less able to resist torsional forces. (If I remembered everything I learned in mechanics of materials 4 years ago I might be able to figure it out... ) Last year we had to create a pyramid strut system to our arm apex in order to remove the torsion experience while turning.

How is the kitbot frame's torsional stiffness? I don't have much experience with it.

Awesome thread, all this talk about frame flex has got me thinking!

-Mike

[IKE]

Originally Posted by Michael Corsetto

How is the kitbot frame's torsional stiffness? I don't have much experience with it.

Awesome thread, all this talk about frame flex has got me thinking!

-Mike

Kit bot=Noodle frame. It does have a fair amount of rock, so it works out well.
1114's Kitbot on Steroids improves this using the robust base-plate.

Super structure design can make a frame stiff.
Welding can make a frame stiffer.
Gussets can make a frame stiffer.
Additional rivets at attachment locations.
Gluing.
In general, Triangulation beats Boxification (I made the last one up).

There are lots of ways that can make a frame stiffer, but many do not without some attention to detail.

If you are doing a sheet metal chassis, making a "model" out of posterboard can be very helpful. The posterboard is much less stiff than metal, but the general weak spots should be the same locations.

If you like doing FEA, you can apply moments in opposite directionon the center of the outer rails. Moments on the order of 180 to 240 in*lbs is a good place to start. Look at the displacements relative towhere the wheels would be. Displacements on the order of 1/16th of an inch or more are important. If you think about the "pile" of the carpet. A light touch to fully supported (60-75 lbs) is on the order of 1/8".

If someone was interested in doing some testing, you could anchor 3 corners of a frame onto something rigid (say very stiff table or multiple pieces of plywood). You could then apply weight to the 3rd corner and measure displacement . Trying different gussets, bracket, welding... and repeating the experiement would make for a very nice paper and possibly a cool sciencefair project.

[JesseK]

Originally Posted by IKE

... and repeating the experiement would make for a very nice paper and possibly a cool sciencefair project.

Personally I like to drive the full-weight robot at full speed into a brick wall while it has bumpers on. Driving it in at various angles so the corners hit also helps.

Why? Usually a frame stays square "just fine" until an impact. Then things can get bent out of shape.