Let's Make Robots!

Sh*tBot

 
Supposed to be a basic four-wheel platform

My "nineteenth" attempt to make a four-wheel robot using Lego Mindstorms nxt. All ended in failure as I couldn't get any of them to turn. This was a kind of articulated tractor-trailer effort, but still no good. I've rturned to two wheel / skid-steering for now.

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The weight of the NXT brick is trying to turn on a point behind the drive wheels, where it might work better if the point is between them. Check out Ben Hur, a lego chariot-like robot that places the twist in between. Weight further behind would resist turning as you've seen. Steve Hassenplug has had some other twisty lego robots.

Thanks for the Ben Hur link - loads of good ideas on that site ;)

Mike

From the sounds of it you've tried the "bobcat" approach without success. You could use a trick that Frits used with his wall racers and put electrical tape on the wheels so they dont grip as much otherwise you may need to resort to a more traditional 4 wheel steering. I'm suprised this one didn't steer unless you had a solid axle in the rear as you really need the rear wheels to go at different speeds in corners.

Unless you specifically wan't this to be an all lego bot because it was your Xmas present then might I suggest some hybridization. use some cable ties to strap some small caster wheels on the back like ratbot.

If you want all lego and have enough wheels then 6wheels are easier to skid steer than 4 because you can have the middle wheels slightly lower so that the bot pivots on them as long as it is reasonably well balanced.

The rear wheels were on independent stub axles so could turn freely. I tried a (Lego) caster under the articulation between tractor and trailer - strictly making it a five-wheel robot - to prevent it coming apart under stress.

I'm not against hybridization, but have been raiding the kids' lego mountain to give myself some sort of discipline to work within.

I don't think skid steering is the way to go. When I try again, I will probably start with differential drive to the front wheels and a pivoting rear axle or maybe steerable rear wheels.

The major impediment with nxt (or perhaps it is just my inexperience) is that the motors are very large in relationship to both wheel diameters and the structural components (beams, plates, etc) making it hard to integrate the motors into the overall structure. The lack of fixing points on the motors also doesn't help.

Mike

Mount the motors elsewhere and use a drive train to deliver power.

Seemes like a good construction. More pictures please.
I've also tried to make a robot like that but unsuccessful.
Are going to try again and see if it works.
Are going to take some ideas from you if it is OK.

 /Emil Hemdal (SWE)

It's gone now, so no more pictures. I still want to create a four-wheel robot so will try again at a future date.

Mike

how did this one fail? it actually looks promising in design but the pivot point might be too far back for it to effectivly turn.   You could try putting the motors forward of the wheels and shift the pivot point closer to the wheels as well.

How did it fail? When it detected an object ahead, it just carried straight on. Now that I think about it, I think the reason was that I tried to turn like skid steering - ie by reversing one of the wheels. What I should have done was accelerate the other wheel.

Mike

yeah, skid stearing isn't for every kind of robot.    :)