Swerve Concept

[AdamHeard]

Originally Posted by Greg Woelki

It would be advisable to go further on the speed side of the motor curve than one would be with the reduction that I used in my calculation, hence a minimum of a quarter of a second. I suppose that half a second would be more reasonable in this set up.

I'm not sure where you're getting your numbers at all.

The two wheels spaced far apart will take substantially more torque to turn, there is no way they will get BETTER performance than current optimized narrow centered wheel swerves.

[Greg Woelki]

Originally Posted by AdamHeard

I'm not sure where you're getting your numbers at all.

The two wheels spaced far apart will take substantially more torque to turn, there is no way they will get BETTER performance than current optimized narrow centered wheel swerves.

In this post, OP said a BAG is used for pivoting

I estimated that around 80in*lbs would be needed to turn that two-wheel module. A BAG motor has a stall torque of 3.5in*lbs, so, while more reduction would be better, I went with 100:1 in my calculation. Free speed of a BAG motor is 14000 rpm, so 140rpm with the reduction and 108rpm under the load. That works out to 0.56 seconds per rotation, or a bit over 0.25 seconds for 180 degrees.

So, while it may take more torque to pivot and while one may want to run the motor further on the speed side of its curve, that is where I got my numbers.

[Jefferson]

Originally Posted by Greg Woelki

[url="http://content.vexrobotics.com/vexpro/pdf/217-3197-Drawing-20140115.PDF"]Having a wheel on each side will significantly increase the torque required for pivoting and it will probably take a minimum of a quarter of a second or so to rotate a module 180 degrees.

Why would you ever rotate a wheel more than 90 degrees at once?

[Tyler2517]

Originally Posted by Jefferson

Why would you ever rotate a wheel more than 90 degrees at once?

I was about to ask this.

[Greg Woelki]

Originally Posted by Jefferson

Why would you ever rotate a wheel more than 90 degrees at once?

You wouldn't, that was just my picturing of the module rotating, not really applicable.

[jimbo493]

Originally Posted by Greg Woelki

You wouldn't.

We will to calibrate the steering, but only then

[RyanShoff]

For reference, last year we used Bag motors with 90:1 versa planetary gearboxes (10x3x3) for steering with no issues. We had no further reduction after the Versas.

[magnets]

Originally Posted by Jefferson

Why would you ever rotate a wheel more than 90 degrees at once?

If you don't want to switch the direction of your drive motor, like team 118 did in 2008.

[Gdeaver]

A swerve control algorithm can always run the drive motors one direction and check to see which way to turn. Least distance. Or the algorithm can look for the shortest distance with motor reversal. Both will work. We have done the latter. This does put a stress mechanical and electrically on the module. The first has increased response times. Now say you have a CVT solution for swerve, but the response is asymmetric on the direction of the drive motors. Works really good counter clockwise, terrible clockwise shaft end. Do you accept greater than 90 degree steering solutions for CVT or is the steering response more important? That's what I've asked our team.

[nathannfm]

Originally Posted by jimbo493

2 Wheels allow for more traction

Originally Posted by Greg Woelki

Well keep in mind that, while more wheels will keep the tread pattern from wearing down as quickly, doubling the number of wheels will give only a very minor improvement in traction since, while it is simplified, friction very closely equals N*mu.

Originally Posted by Bryce Paputa

On a deformable surface like carpet, the n mu approximation isn't a valid assumption, he'd pretty much need to test it himself with his wheel setup in order to figure out how friction would be effected. With a rough top tread I would think you would get more friction

Originally Posted by Greg Woelki

Do you know of any quantitative data that teams have collected about this with different wheels/treads? I have been unable to find any.

MOE just did this test last week. Our test rig was a simple 4 wheel cart with the wheels locked in place. We used 4 inch AM Performance Wheels (1 inch wide) with riveted on blue nitrile*. The cart was first weighed down to 127lb total and dragged with a force gauge. It slipped when the gauge read ~108lb. The test was then repeated with more weight (192lb total) and slipped at ~160lb. We took this to mean that the relationship between weight and force needed to slip was relatively linear as the % increase in force needed to pull was about the same. We then added 4 more identical wheels to the same 4 axles and locked them in place effectively creating a 2 inch wide, 4 inch diameter wheel. The cart was pulled a third time and the reading was ~180lb. This shows that (at least for a 4 inch diameter wheel with blue nitrile) wheel width will certainly give you more traction but it is not anywhere close to 1:1. More like 100% increase in width = 13% increase in traction, so it's worth is still subject to some debate.

We opted to go with the force gauge test because we felt it was safer, easier and more accurate than the ramp test (the cart tended to fall forward off the ramp before it slipped)

*not new, slight wear.