How not to steer
October 26, 2010
I recently hacked some servos for fun, and wound up with a servo that could only turn 90 degrees. This seemed like a good candidate to use as a steering servo. I had never tried steering with a servo, but I'd seen it done. How hard could it be?
I started with my two servo-hacked, brainless gear motors with attached wheels, which I had hot glued together with a battery pack for testing.
Next I took the 90 degree servo and the front section of a broken toy car.
Simply connecting both gear motors straight to the battery, and the steering servo and front section to an R/C receiver, I had what looked like a very easy steerable platform.
Unfortunately, initial testing showed that no matter how I turned the front wheels, the little bot just pushed straight ahead.
I decided the problem was that the rear wheels were nice grippy rubber, while the front wheels were low-friction plastic. I added some electrical tape with the sticky side outwards to the front wheels.
Now surely it would work! And in fact it did work much better. Now my little robot could turn. Sort of. It wasn't perfect, and of course the electrical tape would pick up dust and dirt, and also would sometimes slip off the wheels. No matter! I had proved my point. More grip in the front and you can steer. I dug out some little rubber wheels I had lying around.
After adding these snazzy yellow wheels to the front, I figured I'd be all set. (It looks a bit like a tractor now, doesn't it?)
I hooked up the power and tested again. The results were... disappointing. The rubber wheels were pretty much as ineffective as the original plastic ones without the tape on them. Hmmm.
I thought maybe the side-to-side slop of the axle might be making the steering ineffective. So I added some bushings to use as spacers. You can see one bushing installed on each axle, and an unused one in front just to show you what they look like.
Oh, I also know that the two yellow wheels are different widths. I fixed this later, but it didn't make a difference. I also tried steering on different surfaces. It seemed to work a little bit better on wood floors, and even better on carpet. However, it still just wasn't reliable.
I finally realized that the problem was my front wheels were on a fixed axle. They could not rotate independently. So I came up with a new solution using some cut-off plastic wall anchors for mounts, and two bottle caps with wood screws for the wheels and axles.
Now that I had freely rotating independent front wheels, surely everything would be good. Right?
Of course not. The new wheels were plastic, and we were back to the problem that the didn't have enough grip to overcome the force of the two rear wheels driving forward. So, I added some "tires".
OK, I added some rubber bands, but on this scale it's the same thing.
This combination of freely rotating wheels and some grip on the front tires did the trick. I'll needs a more permanent solution, but this definately showed that my prototype worked.
The rubber bands fell off the left wheel at one point, and I could see that I could still turn to the right, but turning left became impossible. I tried again with more rubber bands on each wheel, and it worked acceptably well.
The video shows some of my tests. I hope you found this educational or entertaining, or both.