4WD Turning Difficulties

[dlavery]

For all those that are making early design decisions based on driving experiments involving driving Robots on the "Regolith" flooring without a Trailer attached, I would urge exercising a LOT of caution. The driving/handling characteristics of a Robot with a Trailer are COMPLETELY different than a Robot without a Trailer. Learning how the Trailer affects the performance and agility of the entire vehicle system is critically important. And once you understand those effects, learning how to use them to your advantage is equally important. And don't make a decision too early - a driver with about an hour of practice will learn how to use the Trailer to spin the Robot through turns and pivots with a lot more agility than may have originally been anticipated.

As one benchmark, after a bit of practice I was able to take a standard kit-bot system with trailer attached and run it from one end of the Crater (starting with the Trailer touching the Alliance Station Wall) to the far end, execute a 180-degree turn and make contact with the far wall, and run back to the starting wall in about 12 seconds. This was repeatable several times.

-dave



.

[Brandon Holley]

Originally Posted by dlavery

As one benchmark, after a bit of practice I was able to take a standard kit-bot system with trailer attached and run it from one end of the Crater (starting with the Trailer touching the Alliance Station Wall) to the far end, execute a 180-degree turn and make contact with the far wall, and run back to the starting wall in about 12 seconds. This was repeatable several times.

-dave



.

Dave,
Is it possible you have a career ahead of you in some kind of R/C drifting competitions?????

[Don Wright]

http://www.youtube.com/watch?v=03tYPM1Bw4c

Videographic evidence of what "standard" 4wd bots will turn like. Bot is weighted down (with about 60-80lbs on the bot and some unknown amount on the trailer).

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

[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.