4WD Turning Difficulties

[JVN]

Originally Posted by Don Wright

To be completely honest, this didn't look as bad as I thought it would...

Is that video on real Regolith?

Our testing (with a stock kitbot weighted 150 lbs and a stock goal) shows it is worse than this video would lead you to believe.

[James Tonthat]

Yeah, we got the same results agreeing with John. We had to spin our wheels for a good 5 seconds to get our bot to rotate (without trailer), on Glasteel FPR.

Edit: To be fair, our robot wasn't loaded to full weight, it was at about 35-40 lbs.

[Ian Curtis]

Originally Posted by jtkellertx

Yeah, we got the same results agreeing with John. We had to spin our wheels for a good 5 seconds to get our bot to rotate (without trailer), on Glasteel FPR.

Edit: To be fair, our robot wasn't loaded to full weight, it was at about 35-40 lbs.

While we didn't use exact weights, we drove a 4 CIM AM Shifter in Low Gear (with a long drive base) at about 30 pounds and about 180 pounds (yes, I know this is 30 pounds more than an actual robot!). While noticeable, the difference in turning ability was nothing to write home about.

[pfreivald]

Four-wheel tank drive on a ~140lb robot seems about as easy to turn and maneuver with as a car towing a small trailer on a very icy road... Unreliable at anything approaching speed, but something you can definitely live with -- especially if there are no ditches into which one might slide, and the intactness of one's body is not on the line.

I didn't notice that skid steering was any better than the steering of an actual car on 1/4" of ice... and I had a chance to test that out in the school parking lot just this past Wednesday! (We had a snow day because of an ice storm, so I did a little 'research' with my car.)

Patrick

[kramarczyk]

I took a few minutes to update the spreadsheet from the beginning of the thread to include the forces from the trailer in the FBD. At present I still do not have the physical properties (i.e. an adequate trailer) to validate the spreadsheet, but it does seem to agree with what I have seen in videos from other teams. If some folks concur that the physics are correct I'll put it up in the white papers. Let me know if it is worthwhile.

[SWIM]

Originally Posted by iCurtis

While we didn't use exact weights, we drove a 4 CIM AM Shifter in Low Gear (with a long drive base) at about 30 pounds and about 180 pounds (yes, I know this is 30 pounds more than an actual robot!). While noticeable, the difference in turning ability was nothing to write home about.

a = f/m
friction force = normal force * mu
normal force = mass * gravity

a = ( mass * gravity * mu ) / mass

a = mu * gravity


so, a 30lb robot and a 180lb robot should be able to change direction equally fast.

perhaps the 30lbs and the 180lbs were distributed differently, changing the moment of the center of gravity?

[ZakuAce]

Has anyone seriously considered a robot with two wheels in the middle as the drive wheels and having it pivot? We're thinking it would get good traction, as all the weight (well most of it) would be on the two drive wheels, and the trailer would help balance out the wheels to keep the center of gravity in the middle. Also, the simplicity factor would allow us to spend more time making a way to score.

[SWIM]

Originally Posted by ZakuAce

Has anyone seriously considered a robot with two wheels in the middle as the drive wheels and having it pivot? We're thinking it would get good traction, as all the weight (well most of it) would be on the two drive wheels, and the trailer would help balance out the wheels to keep the center of gravity in the middle. Also, the simplicity factor would allow us to spend more time making a way to score.

I've seen that discussed on here before, I think that the concensus was that the hitch allows too much vertical movement for that to be feasible. You'd probably rock the hitch up and down enough for your team to get called on the rule that states the hitch needs to be a certain height

[Daniel_LaFleur]

Originally Posted by ZakuAce

Has anyone seriously considered a robot with two wheels in the middle as the drive wheels and having it pivot? We're thinking it would get good traction, as all the weight (well most of it) would be on the two drive wheels, and the trailer would help balance out the wheels to keep the center of gravity in the middle. Also, the simplicity factor would allow us to spend more time making a way to score.

This would give you no more traction than a 4WD robot as the downward force (max ~150lbs) is the same.

It would reduce your force required to turn (scrubbing) significantly.

I would be concerned that when you are fully turned (60+degrees difference between your robot and your trailer) you could possibly tip enough to touch the floor (breaking the rules) or tiping outside the envelope.

Also, I believe the robot needs to fit within the sizing box without being supported by the sizing box, and a 2 wheel bot may not do that on it's own.

[Doug Leppard]

Originally Posted by ZakuAce

Has anyone seriously considered a robot with two wheels in the middle as the drive wheels and having it pivot? We're thinking it would get good traction, as all the weight (well most of it) would be on the two drive wheels, and the trailer would help balance out the wheels to keep the center of gravity in the middle. Also, the simplicity factor would allow us to spend more time making a way to score.

We considred it for awhile but figured we could not use the hitch to keep us balanced and it was too risky. But we are trying to keep the CG over the rear wheels as much as possible and trying to achieve about the same thing.

[ZakuAce]

we were thinking of using more wheels to support our robot at the front and back when it pivots. The mentors were thinking that even though the robot may be resting on an un-powered wheel at some point, the amount of force that is placed on those wheels will be negligible, as most of the force will still be on the center drive wheels. I guess it is more like the 6 wheel drive with the lowered center drive wheels, only we'll use 5 (2 drive, 2 in front on each side of our ball gatherer, and 1 in the back)

EDIT: So... the trailer hitch can never change from that height? Well that could be a problem. I suppose we could have the rear supporting wheel powered and at the same height as the main drive wheels, and have it programmed to run only when moving foreword and backword. I really want to keep the number of drive motors as low as possible.

I'll bring up this idea that was really good, but ultimately shot down because of complexity.

One of the mentors brought up an idea of a three wheeled system. it would work similar to a crab drive, but has a huge difference. The base that the wheels are attached to would be able to rotate, or rather, the robot would rotate around it. The wheels move, then the entire body moves around the base to face the direction of movement. It seemed really stable and a great way to keep the same weight distribution on all wheels, but it was rather complicated and could interfere with a ball gathering mechanism.

[Erin Rapacki]

To this general topic about 4WD: I designed the wheel base to be wider than it is long, the closest being... make sure the insides of the wheels make a square. This way the wheels don't have to slip along an arc across a surface while it's turning.

[writchie]

Originally Posted by Doug Leppard

We considred it for awhile but figured we could not use the hitch to keep us balanced and it was too risky. But we are trying to keep the CG over the rear wheels as much as possible and trying to achieve about the same thing.

From the drawings of the swivel and other parts it appears that the design intent is to create a rigid attachment without freedom to rotate about the trailer wheels.

The hitch pin is a close fit. The close fit requires that the hole in the swivel have a vertical axis. The same is true for the holes in the c-channel. The 0.385 of gap is, IMHO, intended to allow for the tolerances in the height of the hitch swivel (when level) and the robot c channel (when level). I would not expect the swivel to ride much up and down on the pin. Of course I could be wrong and only testing will reveal how rigid the attachment is in practice.

[ZakuAce]

Originally Posted by writchie

From the drawings of the swivel and other parts it appears that the design intent is to create a rigid attachment without freedom to rotate about the trailer wheels.

The hitch pin is a close fit. The close fit requires that the hole in the swivel have a vertical axis. The same is true for the holes in the c-channel. The 0.385 of gap is, IMHO, intended to allow for the tolerances in the height of the hitch swivel (when level) and the robot c channel (when level). I would not expect the swivel to ride much up and down on the pin. Of course I could be wrong and only testing will reveal how rigid the attachment is in practice.

I was thinking the same thing.

Also, I talked with the mentors today, and one of them brought up a really good point. If this was illegal, in theory, then suspension could be called illegal, since it changes the height of the robot, and could potentially make the hitch higher or lower than 2-13/16 inches from the ground. So I think we're sticking with our current plan. We're only going to have the height of the drive wheels and the wheels it will pivot onto on the front and back a 1/16 of an inch or less difference.

[Chris Fultz]

this video appears to be on a school type tile floor, which is different from the field material.