Moment of inertia and turning performance

[Nathan Streeter]

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.

[Ether]

Originally Posted by Nathan Streeter

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

[DampRobot]

Our robot was symmetrical this year. Well, not exactly symmetrical, but so much so that our driver has trouble telling one end of the robot from the other. We are using 6wd off a CIM and a FP a side. All 6 wheels are 6in performance wheels with blue nitrite tread, and the center wheels are dropped about an eighth of an inch.

We turn very easily, at least much more easily than most 6wd robots with all traction wheels. However, if you watched our robot play, you would say that it turned very jerkily. Why? The driver and the programming.

[JesseK]

This is merely an explanation for why a LOW c.g. matters.

Consider a c.g. that's off-center to the right and is 18" off the ground (really high for FRC). When moving forward, that c.g. creates a moving inertia. During a sweeping left-hand turn while moving, that inertia becomes a centrifugal force that attempts to lift the left side of the robot. The higher the c.g., the more leverage the force has to lift the left side. This centrifugal force is easily noticable on robots that do high-speed 0-radius turns in place. It's like an off-balanced top.

This lifting causes a major traction loss, even if it only lifts a few tenths of an inch. Realize that with even a few degrees of lateral lift, a wheel that makes flat contact with the field (such as the KOP wheels) will lose nearly all of its traction. Rounder wheels, like the AM pneumatic wheels, will only lose some of their traction. This traction loss is why high c.g. robots cannot do sweeping turns like the more agile robots. A good picture of this concept is on page 41 of this slide deck: http://www.chiefdelphi.com/media/papers/2597

[Nathan Streeter]

Originally Posted by Ether

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

I'm assuming that you are implying that you do often want a drive train that turns with negligible resistance, and that they can be controllable?

In the "real world" (i.e. with cars, airplanes, segways, etc.) you're definitely right - you basically always want no mechanical resistance in the drive mechanism. As a result, you have a drive mechanism that is capable of being quite responsive. However, if that mechanism is controlled carelessly, it could result in a vehicle that responds easily to accidental maneuvers. These potential side-effects are mitigated by control mechanisms with resistance or "feedback" (such as the yoke or stick on a plane), control motions that aren't inherently twitchy (two hands on a reasonably sized steering wheel), software damping of otherwise twitchy motions (a Wii uses some of this, I conjecture), and/or by training the manipulator to use the responsive mechanism advantageously (fighter pilot).

These are all better ways of getting good controlability out of an otherwise difficult-to-control system; however, I made several unspoken assumptions when I thought what I wrote. I figured teams would be working with control mechanisms that aren't perfect (the current joysticks at least have a little more resistance than the old ones...), that they'd be using control motions that aren't always easy to make steady (unsupported arms on two different joysticks), that most teams wouldn't be using software to damp out twitchiness (perhaps some teams do), and that teams wouldn't be requiring a level of responsiveness that could only be acquired with a resistance-free mechanism.

From our own team's experience, our drivers had a hard time getting accustomed to our very low-resistance drive trains... but we ended up with robots that could drive fairly well by compensating. We didn't want all the responsiveness we had, so we used software to map the distance from the neutral axes exponentially, so that small motions would be less significant. I consider this raising the tolerances, rather than "damping out twitchiness..." which I presume would be feasible but significantly more difficult, and perhaps not even preferable. We also used software so that when the driver hit a button they could limit the robot to half (or quarter) power for fine motions, such as placing a tube on a peg last year or lining up for a shot in 2010. Then we got the drivers to practice a lot.

So, in the FRC robot world, my experience has been that a hyper-sensitive robot is not preferable and that a good way to limit the sensitivity is to leave a little resistance in the system (i.e. put grippy tires at the corners rather than omnis); however, some teams may think otherwise. Perhaps your team disagrees... I could imagine some teams preferring hyper-sensitive robots that are driven well solely by the skill and practice of their drivers...

I apologize for my non-absolute statement that could easily be misinterpreted.

[Ether]

I think your expanded explanation makes a lot of sense.

[Alan Anderson]

Originally Posted by Ether

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

Have you ever tried to turn an active Segway by manhandling it instead of using the steering control? It has plenty of resistance. Granted, it's mostly due to software controlling the wheels and not to strictly mechanical tendencies.

I think it would be better to say that a Segway has very little wheel scrub to worry about.

[Al Skierkiewicz]

Originally Posted by Alan Anderson

Have you ever tried to turn an active Segway by manhandling it instead of using the steering control? It has plenty of resistance. Granted, it's mostly due to software controlling the wheels and not to strictly mechanical tendencies.

I think it would be better to say that a Segway has very little wheel scrub to worry about.

Alan,
Even though the Segway is disabled there is something electrically that is working on the motors, akin to 'brake' mode. When you turn it off completely, it acts as you would expect.