Any physics/mechanics experts out there?

Stryker · Post by **Stryker** » Mon Apr 04, 2005 10:32 am

I'm working on a robot, made completely out of legos, for a informal construction competition with another guy. The specs are as follows:

I'm using 2 lego RCX units, capable of being programmed in a C-like language, with 3 sensor inputs and 3 motor outputs apiece. They can communicate with each other by IR. However, the other guy only has 1 RCX and 1 Scout controller, which only has 2 motor outs, so the rules limit me to the use of 5 motors.

The contest is to build a car that is 4-wheel-drive and 4-wheel steer. I have the rear drive/steer mechanism done, but it uses 3 of my 5 motors--one for each wheel and one to turn. I need to make the front wheels be steerable, in the same fashion as a front-wheel-drive car, preferably--using only 2 motors. This means I'll have to set up one motor to control the turning of the two front wheels and one to power them. How should I do this? It's not exactly easy powering the wheels because of the angles involved. I'm using a deformable rectangle shape to turn. If necessary I'll post a paint pic of it. I have a differential gear available.

Anyone have any clues as to how to make this happen?

Question 2: Will rubber, when heated up to about 120+ degrees F, melt enough to allow it to be bonded onto wood? (This project is for another robot that needs rubber on its claws to enable it to grip things. The robot is constructed mostly from PVC and plywood, with some sheet aluminum here and there for strength).

DCrazy · Post by **DCrazy** » Mon Apr 04, 2005 12:21 pm

How about using a basic Technic-like steering mechanism and using the differential? There's a piece that looks like this: _/\/\/\/\/\_ which works nicely for rack and pinion steering setups.
Top view (schematic)
Front view (slightly different)

The problem, I guess, is hooking your wheels up to a differential if they have the capability to rotate.

suicide eddie · Post by **suicide eddie** » Mon Apr 04, 2005 1:11 pm

how about using heat-shrink plastic or rubber, available from any good electronics/model shop

JMEaT · Post by **JMEaT** » Mon Apr 04, 2005 2:09 pm

I was in a tech club back in high school and we had to do the same thing. God I hated that software...

DCrazy · Post by **DCrazy** » Mon Apr 04, 2005 5:15 pm

What software, Interactive C? It's gotten a lot better over the past few years. In fact, KIPR has released a new version for the Gameboy Advance-based controller. We (my school's BotBall robotics team) are gonna have a field day with that thing.

JMEaT · Post by **JMEaT** » Mon Apr 04, 2005 8:14 pm

I don't recall the name. It reminded me more of BASIC than C. It was DOS based and had a pink boarder with white text.

snoopy · Post by **snoopy** » Mon Apr 04, 2005 9:05 pm

You can write the code yourself- that's what most of the guys at my school opt to do on the freshman lego RCX project that most of them do.

I'd suggest kinda what people are saying, have a pull/push rod to steer the tires, both connected to a common rack. For the driving, use u-joints (2 per wheel) to join your differential to the wheels... I'd have the drive motor mounted above the differential (simply b/c up is generally free space), and the rack for steering mounted behind them.... I'm not sure how packaging is looking for you... but there's got to be a way to do it.... maybe upsize your chassis a little bit.

Stryker · Post by **Stryker** » Tue Apr 05, 2005 7:14 am

Well, I think I've got it figured out. Thanks for the help guys! It appears that, out of almost 1,000 parts per Mindstorms kit I have, and over 1,000 other miscellaneous parts, I have NO U-joints. Freaky. At any rate, I've decided that I don't need to control motors individually--rear/left, for example, generally won't need to travel at a different speed or direction than front/left. This being a four-wheel-drive, four-wheel-steer car, if I want the car to turn normally, therefore requiring a different motor speed per wheel, I can make the calculations required for the turn then just slow both motors on one side of the car down.

Therefore, I'm going to simply wire the front and back motors on the same sides of the car together. I have 6 motors available, but I can only use 5 ports for the competition. This means that if I wire the right motors together and the left motors together I'll actually come out ahead by one motor port. This also means I can use the same design for both the front linkage and the rear linkage, greatly simplifying design.

I'll post pics when I get it done! Thanks again.

DCrazy · Post by **DCrazy** » Tue Apr 05, 2005 7:18 am

Stryker · Post by **Stryker** » Fri Apr 08, 2005 7:10 pm

Sorry for the resurrection, but I figured it'd be better just to post an update here:

Construction is mostly complete! The only things left to do are to attach the 2 RCX units and wire the whole thing up. No small task, mind you--keeping the wires out of the way of all moving parts could prove interesting, to say the least.

Pics are here!

Basic Top View
Robot inside top of mindstorms box (for perspective)
Slightly blurred view of the turning motor gearbox
Showing off independent steering setups (penny included for perspective)

All images are 1600x1200, so watch out if you have a small screen res, it might take you a day or two to get to all parts of the picture. Each image is around 744 KB.

I was running out of parts towards the end of construction, thus the somewhat odd/varied assortment of parts on each side of the robot. It took probably 1,000+ parts to get this far, and I still have to mount the RCX units.

The robot weighs in at somewhere around 3 pounds (without RCX units), and both pairs of wheels have an approximate turning maximum of 35-40 degrees. Not sure if you can see it in the pics or not, but there's a nice, heavy pair of long beams running along the length of the thing in the center; that keeps the middle from sagging and provides a lot of structural stability. This thing shouldn't break under any force it can be reasonable expected to bear.

Edit: Typical--screwed up the links.