Zero turn mower style steering trial

[Brewer4377]

Between the holidays we are going to build a prototype chassis similar to a zero turn mower and control it with a left joystick and a right joystick. When we put the unit through its paces in some trials, I want to make sure that we give it a thorough challenge. Any recommendations of particular maneuvers that will demonstrate either a strength or a weakness would be greatly appreciated.

Thanks,
Paul

[MrBasse]

You used to see a lot of caster style robots back when I was in high school, but they haven't been too common lately. They went out of favor and there are a lot of strong opinions as to why. A search will turn up a lot of results, although eh are skewed with the influx of swerve designs comin out lately. So look at old posts.

I'd say give it a shot and see if you like it. Push it around with another drive base, drive over bumps, try running up a ramp, try fine steering adjustment to aim at a target. The biggest concern I would have would be a hard turn when the wheels are pointed in the wrong direction while weighted. We have that issue with our robot cart and it requires some thought to eliminate forcing your wheels to go straight.

All of this assumes you plan casters for two wheels and two driven wheels though.

[smart1]

Back in 06' and prior we used castors. The robot won't turn consistently because the castor wheels are always turning around, which to be fair in teleop isn't that big of a deal because you can correct the steering but in auto it is. I also know alot about castor wheels from 13 years of driving an electric wheelchair, I always have to correct my steering. You could use omni wheels on the front instead of castors to get rid of the unpredictability. I would advise against that even because you get pushed easily.

[pribusin]

Our Ultimate Ascent bot had an unintentional zero-turn drive with 2 omni wheels at the front and four traction wheels near the back. Even though all 6 were powered, the omnis provided no sideways stability if another bot pushed us at the front. Steering was also more difficult than with a standard center-dropped 6-wheel drive. We quickly changed our design after we ran it for the first time.

Definitely test it to see for yourself but make sure you test a front impact scenario when other bots try to push your nose sideways.

[Brewer4377]

Thanks for the good feed back so far.

At the moment, the plan is to try a left joystick and a right joystick, just like the zero turn mowers themselves. Will this be easy for the new drivers to grasp? If it is difficult to master, what would you recommend?

Thanks,
Paul

[IKE]

If you plan to go that route, look around for discussions on "driving straight". Typically the "zero turn" style drives are diffi ult to drive fast and straight. If you are not driving the front wheels, using the 2009 hard plastic wheels on carpet may be a good choice as well. They are very slick, but predictable on carpet so you do not get the caster wheel shift when changing directions.
Overall, this configuration tends to be limited for FRC. Having non powered wheels puts you at a disadvantage in a pushing match or if you need to stand your ground. The lack of naturally going straight makes many autonomous routines more difficult.

[pribusin]

Left/Right Joystick is the most common approach. In the past we've also tried a single joystick with a twist handle. The twist handle would control rotation while forwards and backwards motion would control forward and reverse drive speed. We came up with an algorithm to mix the two so you could drive and steer. It was quite effective. It was used on a meccanum drive base but should work well for yours also. The downside to the traditional two joystick tank drive is that directions reverse when the robot is driving towards you and that takes practice to master.

Good luck

[MrBasse]

I feel like you'll get a lot of west side Michigan advice on this one...

One of our drivers insisted that he had to have two joysticks to drive this upcoming season. I told them to try it. The task I gave them was to drive down a 60' long and 10' wide hallway with one joystick then with two and check the difference. One joystick was nearly half the time and they didn't hit the walls once. With two they hit the wall at least ten times until I was almost ready to make them stop.

Some kids just get a control system and adapt to it quickly. Others won't. Again, I'm not recommending one way or another. The sample code is already out there, just load it up and see what you like better. We had no idea until we tried it.

Same as a game pad, I had thought that kids would be super used to that and so much better with one. But the accuracy just isn't there compared to the long travel of a joystick. But we never would have known unless we tried it.

[artdutra04]

Originally Posted by MrBasse

You used to see a lot of caster style robots back when I was in high school, but they haven't been too common lately. They went out of favor and there are a lot of strong opinions as to why. A search will turn up a lot of results, although eh are skewed with the influx of swerve designs comin out lately. So look at old posts.

It's not just opinions - every unpowered caster you have on your robot is mathematically robbing you of traction, causing you to both lose pushing matches and being easier to be pushed during defense.

2WD with casters used to be really popular in FRC a decade+ ago before there were were good kitbots and COTS sprockets, wheels, omni-wheels, etc. When you used to have to buy steel sprockets from SDP/SI, McMaster, Small Parts, and/or Martin you used to have to have a lathe and mill to make them usable for live and dead axle applications.

Teams that didn't have these machine shop resources could often make a 2WD work, but couldn't really tackle anything more. So they threw two casters on their robot to allow it to drive around. Swerve hasn't replaced casters, kitbots and COTS parts have made them obsolete.

[BBray_T1296]

Every pound of your robot not on a drive wheel is a pound less applying to the F=uN equation used to determine maximum pushing traction

[dradel]

They do make these which would do away with caster swing.
http://www.mcmaster.com/#standard-ca...nsfers/=v7gx7i

[Andrew Schreiber]

Originally Posted by dradel

They do make these which would do away with caster swing.
http://www.mcmaster.com/#standard-ca...nsfers/=v7gx7i

Which are equally terrible for the reasons Art stated.

[dradel]

Was simply stating would do away with caster swing. Didn't say I supported using them.