· Hansel & Gretel

August 2005
                      

Just like Hansel and Gretel, this robot leaves a trail of breadcrumbs (LEGO bricks) to be able to retrace its steps.

At first glance, this robot looks pretty complex. And indeed, it is actually not one big robot, but much rather an assembly composed of many smaller subsystems.
But first, a little explaination of what this bot does. It drives straight for a random amount of time (approx. 5 secs). Then it randomly chooses a direction (left or right) and turns that way. But to later be able to return to the starting point, it lays a 2x2 LEGO brick before making a turn. The brick's color specifies the direction (black is left, white is right). After four (or any number of) turns, the entire vehicle turns around 180° degrees and makes its way back to where it came from.

On this shot you can see the large "thing" on the front of the robot is raised. To understand this, you must know in greater detail what the robot does. Everytime it needs to drop a brick, a deployer on the top of this construct moves to pull one brick of the desired color out of its reservoir (the two towers). It then falls through the deployment tube onto the floor. The tube is then raised using pneumatics. This is especially important when the robot begins its "return home" procedure, as the tube would block the bricks it laid earlier preventing them to be detected by the light sensor. You can see the sensor between the green beams that help in collecting the bricks (first picture).

This top view reveals the deployment mechanism. The bricks are aligned inside the reservoirs, the two "holes" behind the 4x8 plates with the little lights facing rear, next to the yellow liftarms. Look at the next photo to see the same deployer without the reservoirs.
By the way: The deployer was called Hansel, while the driving base's name is Gretel.

The deployment tube is the large hole inbetween the deployer "arm". And that arm works like this:
The round bricks prevent the bricks in the reservoirs from escaping, at the same time not blocking the arm as it would happen with normal square bricks. When needed, the arm slides so that the space between the two black barrels falls under the reservoir. Exactly one brick falls there, all others remain inside. This one brick is then slided towards the deployment tube.
The touch sensor on the bottom of the picture (=> back of the robot) activates when the deployer arm is centered.

Another touch sensor closes when the deployer is fully raised. The raise/lower movement is controlled by a pneumatic cylinder, which in turn is triggered by a pneumatic valve and powered by a pneumatic compressor (see next two pictures).
One special thing about this picture: Look at the 1x2 brick with the LEGO logo. This is a pretty old piece and I have no idea where it came from, but I think it looks pretty good on a robot! :-)

The pneumatic valve switch is an exact replica of the one shown here by Ben Williamson. It has a decent reaction time, enough power and is precise thanks to its touch sensor (that is closed when the switch is centered).

The compressor is also taken from an existing design, this time it's the Technic Double-Acting Compressor by Ralph Hempel.

Either I'm no longer creative, or the solutions found on the Internet are way better than mine. I think that's why also the driving base of the bot is not my own creation. The dual differential drive shown on Alex and Michael Whetmore's site is able to drive (almost) perfectly straigt, while still allowing on-the-spot turning.
It uses only two wheels, but instead of a caster I decided to use two 2x2 inverted tiles in the front of the robot to reduce friction without losing precision.

The bottom of the driving base reveals the light sensor used to find the bricks, and a rotation sensor used to gain some precision while turning (especially on the 180° turn, where a little deviation can lead to a fatal result, if the robot misses the bricks due to cummulative errors.).

The NQC programs for this robot can be found here. This .zip-archive contains three files:

• rcxauto.nqc - RCX module (automatic)
  Four turns to random direction everytime
• rcxlrrl.nqc - RCX module (predefined)
  Four turns in predefined pattern (Left-Right-Right-Left)
• scout.nqc - Scout module

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