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Unread 04-01-2012, 09:39 PM
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Moment of inertia and turning performance

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?
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Last edited by icyplanetnhc : 04-02-2012 at 05:27 PM.
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Unread 04-01-2012, 09:51 PM
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Re: Moment of inertia and turning performance

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.
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Last edited by Peck : 04-01-2012 at 09:58 PM.
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Unread 04-01-2012, 09:51 PM
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Re: Moment of inertia and turning performance

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.
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Unread 04-01-2012, 09:51 PM
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Re: Moment of inertia and turning performance

Quote:
Originally Posted by icyplanetnhc View Post
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?
My guess is moment of inertia plays a very minor role.

The biggest factors affecting turning for a drop-center 6WD are

- amount of drop of the center wheels

- wheel tread material

- type of flooring

- wheelbase/trackwidth ratio

- location of center of mass


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Unread 04-01-2012, 09:52 PM
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Re: Moment of inertia and turning performance

For wheels having similar traction, wheelbase length is the primary factor in how easily a robot turns. The two typical ways to eliminate bouncy turns on a 6-wheel drivebase are to 1) get the center of mass directly over slightly-lowered center wheels, and 2) use omniwheels at the corners to remove friction in the sideways direction.

Last edited by Alan Anderson : 04-02-2012 at 08:39 AM. Reason: specify 6-wheel drivebase
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Unread 04-01-2012, 10:09 PM
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Re: Moment of inertia and turning performance

No lie. At last year's inspection, we weighed in a tiny bit over the weight limit. Me, wanting to tidy the robot, trimmed all the ends of the zipties we used and, miraculously, we weighed in exactly at 120 pounds!

We used a lot of zip ties that year....
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Unread 04-01-2012, 10:13 PM
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Re: Moment of inertia and turning performance

Quote:
Originally Posted by SuperNerd256 View Post
No lie. At last year's inspection, we weighed in a tiny bit over the weight limit. Me, wanting to tidy the robot, trimmed all the ends of the zipties we used and, miraculously, we weighed in exactly at 120 pounds!

We used a lot of zip ties that year....
Wrong thread maybe Andrew?

Lol at your story anyway
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Unread 04-02-2012, 01:22 AM
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Re: Moment of inertia and turning performance

Quote:
Originally Posted by squirrel View Post
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.
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Unread 04-02-2012, 01:47 AM
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Re: Moment of inertia and turning performance

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.
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Unread 04-02-2012, 10:26 AM
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Re: Moment of inertia and turning performance

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.
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Unread 04-02-2012, 10:59 AM
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Re: Moment of inertia and turning performance

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.
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Last edited by BJC : 04-02-2012 at 11:08 AM.
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Unread 04-02-2012, 11:28 AM
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Re: Moment of inertia and turning performance

From watching a few of your matches on youtube, the symptoms (rocking "hops" when turning, with size of rock roughly proportional to speed of turn) seem very much like those of a drivetrain with too much resistance to turning, the result of having too much traction at the corners of a drivebase with a long wheelbase and narrow track.

Some ways to reduce this issue:
- Reduce traction at corners by:
* Making corner tires less grippy (slick wheels, or something less extreme)
* Making corner tires move sideways easily (90 degree omnis)
* Reducing weight on corners (increasing "rocker" and/or stiffening frame so that sag doesn't eliminate "rocker")
* Reducing weight on corner wheels by centering weight over center wheels
- Shorten wheelbase and/or widen track to get a footprint more advantageous for turning

I wouldn't recommend a rocker greater than 1/8"... If putting the center wheels 1/8" lower isn't sufficient, look at your frame sag. Then consider the lateral grippiness of your corner tires and where your CoG is.

Note that you don't want the robot to turn without any resistance, as it'd be nearly uncontrollable; however, you don't want so much resistance that you're hopping at even fairly low-speed turns... Some teams prefer just using omnis at the corners, while others prefer using rocker, or a more advantageous wheelbase/track ratio. Personally, I prefer a combination of the rocker, CoG, and wheelbase/track ratio to provide a drive base that goes straight naturally but still turns well and won't be spun around by a defender.
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Unread 04-02-2012, 11:28 AM
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Re: Moment of inertia and turning performance

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
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Unread 04-02-2012, 11:44 AM
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Re: Moment of inertia and turning performance

Torsional stiffness and a low CoG are absolutely critical to smooth turning in a dropped-center, skid steer drivetrain.
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Unread 04-02-2012, 11:57 AM
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Re: Moment of inertia and turning performance

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Originally Posted by Nathan Streeter View Post
Note that you don't want the robot to turn without any resistance, as it'd be nearly uncontrollable;
Segways turn without any* resistance.

A 6WD with omnis at all four corners is like a Segway with training wheels.


*well, for all practical purposes
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