Skid steer mobile platform - details and opinions wanted.

vaclav_sal

I just run into “skid steer cheap Chinese platform” of what apparently is a four wheeled platform which is controlled by “tank” style - stopping wheels on one side from rotating or rotating them in opposite direction.
Unfortunately even after googgling for it I still do not know what makes it tick.
Apparently there are encoders ( optical?) in each wheel (?)

I would love to hear from you robotic guys some details, if possible, or where to get it.
Being a total ignorant about this technology – is $50 shipped a “cheap” ( advertized as cheap) price or not?

What would be a reasonable “payload” of this platform – including batteries and control electronic?
Initially I am looking into “automatic recharge” capability and simple object avoidance, nothing too fancy.

Eventually I would like to be able to recognize small toys ( leggos , blocks, marbles etc. ) scattered throughout the house and to collect them.
“Mobile” grand kids under 2 years do not do that no matter what the bait is.
Thanks for reading.
Vaclav

GordonMcComb

URL for this platform?

Without seeing it, it's hard to say what its capacity is. Depending on its size and the robustness of the motors it may be capable of transporting a few ounces, or maybe a hundred pounds. Hard to know.

Having encoders on a 4WD slip-steer base is fairly pointless, except maybe to keep it going in more or less a straight line. There's no hope of knowing how much the vehicle has turned with slip-steer.

$50 for four motors, four wheels, and the chassis isn't bad. Sounds like it may be the platform from DAGU or DFRobot (both are from small factories in China). A pointer to the one you're looking at would be helpful. DAGU also has a much more expensive and capable 4WD. We're talking $300-$400 for that one.

-- Gordon

vaclav_sal

This is where I started
http://arduino.cc/forum/index.php?topic=52481.0

Here is another similar stuff - it has some answers for me.
http://www.alibaba.com/product-gs/433681431/Arduino_4WD_aluminum_mobile_robot_platform.html

I still do nont see any encoder, maybe I was dreaming or missed it.

GordonMcComb

That's looks like the DAGU Mr. Basic base, but equipped with four drive motors rather than the usual two. RobotShop has the two-motor version for about $30, and as the motors and wheels are about $10 for the two, that brings it up to $50.

IF the poster put encoders on it they're not all that useful because of the severe slip in turns. You can see it in the video. It would only be useful to help keep the robot from detracking slightly in straight forward or backward movement.

There's nothing in the Arduino code that he posted for reading encoders. It's just the usual "robot tai chi" exercise to demonstrate motion routines. Besides that, the basic Arduino doesn't have enough counters or hardware interrupts to reliably deal with four encoders (it could do two, but with a lot of interrupt servicing).

DFRobot offers one similar to your Alibaba find:

http://www.robotshop.com/dfrobot-4wd-arduino-mobile-platform-3.html

I don't have direct experience with this base, but the wheels look a bit "grippy" for use on super-polished floors. The lower the durometer of the wheels, the more traction they'll have on smooth, hard surfaces. That makes these kinds of 4WD drives "chatter" when they turn. They're best for more slippery surfaces, like carpet.

Semi-flexible plastic (as opposed to rubber) wheels work well on these kinds of bases.

My sideline company Budget Robotics used to build and sell a servo-driven 4WD unit that, in my opinion, had the right combination of wheel placement, wheel base dimensions, and tire hardness. I spend weeks trying different geometries to come up with the best approach.

Here's someone else's picture of the a project they did with the robot base:

http://plaza.ufl.edu/dhaddad/TRON2.htm

Sadly, my source of wheels dried up (they were pulled from surplus imported toys from China), but if you can find ones like it, your robot will be better able to travel over varied surfaces, even polished ones that make rubber wheels stick like glue.

-- Gordon

vaclav_sal

Thanks Gordon,
It looks like Mr Basic is a good start for me and my 7 years old grandson who is getting too "old" for Leggos!
I have no idea why I though it had 4 motors and encoders. It is not needed with all the drive gears and as you said there was nothing in the code to support it anyway.

Funny you had mention in your article Seattle Robotic. Used to live near Seattle and the name rings a bell. Are they still in business? It seem like 20 or so years ago. Was your TRON a commercial success? It seems that video recognition is unheard of in commercial ventures.

I volunteer to make peanut butter here and the outfit just got new optical sorting mashine. Mainly to throw away the peanuts skins after roasting.
It does not seems to work well, but it is fast.The main "gotcha" is they went from flame roasting to hot air process and the flame used to burn off lot of the skin. Now there is more skin but it looks same color as roasted peanut.


Back to Mr Basic.
I hope it does come as a kit, it must have 50 screws and nuts in the base!
Kid would enjoy building it, no doubt.

I better get busy building something cool for it, the kid would get bored watching it spin around aimlessly.
Thanks for all.
Vaclav


PS technical question - some of these mobile platforms use servos and some just motors. Unless there is a position feedback from the servo, why would servo be needed?

GordonMcComb

The article I posted was written by someone else, who happened to get one of the kits. The name of the platform was Rigel -- that's a bright navigation star in the constellation of Orion (it's the blue one to the lower right). For a while I was naming some of my robot bases after important navigation stars, especially if they had been mentioned in the old Star Trek TV show (yes, I'm a Trekker).

When R/C servos are used for pushing the robot around the floor they've been modified for full rotation. Parallax has some that are already modified:

http://www.parallax.com/tabid/768/txtSearch/900-00008/List/0/SortField/4/Default.aspx

The documentation on that page explains more how they work.

-- Gordon