NXT RC rock crawler.

[mattallen37]

Here is something I built to use the new "big" wheels I got. They don't look all that big anymore...

Mechanical characteristics:
It is four wheel drive. Because of the difference in the wheel diameters, I use different gear ratios. According to my logic and math, the gear ratios should be very well matched for the wheels. I can lock/unlock both differentials using driving rings. It has 5 NXT motors (4 for driving, and one for steering), and one PF medium motor (to lock/unlock the rear differential). I have to manually lock/unlock the front differential. I used an interesting design for the front "rocker". It tries to keep level. It is not like using a single point for the tilting, it uses multiple crossed beams, causing it to behave in a very desirable manner.

Program information.
It is remote controlled using the PSP-Nx and a playstation 2 wireless controller. I used RS485 for the "master" (front NXT) to tell the slave what to do. The master controls the front drive motors and the steering motor (it uses APR for the steering ). The slave controls the two rear drive motors, and the differential locking motor. I implemented my ramp up/down motor control for "smooth" driving (I have control over the ramp rate with the controller).

Here are some pictures of it:

The front assembly:

Note, there is a 3 attachment limit per post, so I plan to make another post (or two) for some more pictures.

[mattallen37]

Here is a picture of the rear gearing, and differential locking driving ring.

Here is a picture of the front gearing, and the differential locking driving ring.

Here is a picture of the back, showing the rear differential locking mechanism the motor uses to lock/unlock the differential.

[mattallen37]

Here is a picture of one of the front wheels resting on an NXT:

Here is a picture of it with the other front wheel on the NXT:

This picture shows it resting on two NXTs:

The vehicle is powerful enough to drive up the wall until it is totally vertical (coming from an angle, so the front wheel "grabs" the wall). It can drive up and over a 3-5 inch (10-16L) vertical stone side with no trouble. It can roam across a pile of 1" wood cubes.

[timpattinson]

cool Maybe you should add more clearance so it can go over bigger objects
-Tim

[mattallen37]

Well, the front is only 2L below the axle, and the back is only 1L (2 and 4L from the ground). I can't get much more clearance than that. If I had bigger wheels...

[hergipotter]

Looks very cool!
Can you make a video of it driving over some obstacles?

[aswin0]

Looking at the pictures I think the name Brick Crawler might be more appropriate

Anyway, it's a cool robot.

[sidneys1]

Pretty nifty! You've inspired me to dig my NXT out from under six months of dust and build something

Have a nice day,
~Sidneys1

[doc222]

That sideways pendular is cool, it supports alot. One of the hardest things with NXT and any TT or 4x4 is the large motors. I have looked at so many on utube i think I lost my site. There are people that even run events for trial trucks, Rock Crawlers and just 4x4 in Lego most, about all run PF some are still wire draggers(very old school).

One trick to gain height that works well, is a drop axle. Since this means a second stubaxle below the main output drive it is best to gear reduce here as well to reduce gear/drivetrail lash. In lego most seems to use the 3:1-2:1+-( 8t to 24t- 12t to 20) in R crawls/ trials where speed is not the main need but power to climb trough a rocky area. It will get you up about 2 bricks not much but works alot better.

There are two things to think about when talking ground clearing chassi and Axles/transaxles. If you are jumping say logs chassi height is important since you dont want to high center the body, the axles will clear any logs. now if you climb over that log with one tire at a time now axle hieght is important.

A video would be cool, nothing better than seeing Lego trucks climbing through rocks and rolling over. There some Utube videos of Lego trucks rolling down hills that are very cool though very hard on Lego. The slow motion ones are very cool watching the truck fall apart roll by roll.

Cool project,

regards

Doc

[mattallen37]

Thanks for all the replies!

Even if I did do an axle drop, that would only get me clearance from the motor assembly to the ground; the axle case would have the same clearance. I guess that could be better, but that would certainly add weight, complexity, and possibly more bulk than it would be worth. This was actually more of an experiment to mess with some ideas I had, than to build a rock crawler. If I do someday want to build a lego rock crawler (to be a rock crawler, not just to test some ideas), I will have the pictures and experience of this for inspiration.

Some ideas for a future design:

• A differential frame (front and rear rockers, with the frame being the average of the tilt) for much more flexibility. Possibly achieving close to a full 90 degrees.
  Full RC locking of drive differentials (front and rear).
  Gearing down more after the differential (hopefully with the same sized wheels, so the same gear ratios).
  Four wheel steering.
  Tie-rods on the front and back of each axle (for more support).

I don't really have a good place to do a video. As you can tell by my YouTube videos I am also far from being a good camera-man.