A Tiny LEGO Rover

When I saw Bert van Dam's BEAM Walker I knew I had to build a tiny rover. The requirements:

Small! A frame no longer than 8 studs.
Powered, capable of steering to avoid obstacles.
At least one sensor to detect obstacles.
Not required to carry the RCX, but use as few tether wires as possible.

Drivetrain

Initially I planned to use one micromotor and a wheel design that would cause the rover to turn while backing up (ala mini-R2D2). While experimenting with swivel wheel designs I realized that a micromotor was going to be smaller than anything I could come up with to let a technic wheel pivot back and forth. Adding the second micromotor meant another wire tethered to the rover, but it would give me differential steering and a smaller footprint. Each wheel is attached by a technic axle 3 with stud and spaced out from the body using two half bushings, one of which is driven by a small white rubber band connected to the micromotor. Together with wheel diameter, this gears up the micromotor significantly to get some speed. I started with the next smaller size of technic wheel, but in the end more ground clearance and higher traction were more important.

Bumper

The tricky part turned out to be the bumper design. I wanted it to be small, but wide enough to hit anything in front of the rover. Because of the very low speed provided by the micromotors, it had to be very sensitive. The result is a transverse mounted switch, with a carefully tensioned bumper to trigger it:

The most critical part of the contruction is the bumper tensioner, which holds the bar mechanism up high enough that a tiny force from the front of the rover will activate it. The bumper is attached by a 5 stud long axle, and in the center of the frame there is a cam which strikes an inverted ramp brick:

The 2 axle with half bushing must be carefully adjusted so that the slightest pressure on the bumper triggers the switch. The half bushing on a 3 axle through the button's axle hole serves as a backstop which prevents the bumper arm from getting hung up on the button. That way the button's own internal spring can push the bumper back down.

Software

Since it uses differential steering and has a single front bumper, it works fine with the Scout's built in Forward/Avoid code (which I believe is default program 5 on the RCX).

Larger Pictures

Right Side
Left Side
Bottom