The 18th and 19th centuries were a Golden Age of mechanical invention, with the automaton being the most sophisticated type machine. Many of the most famous machines looked like people and they performed very human tasks. Some could write and draw, others could play music using a real instrument customized for their use.
Most of these machines stored the program to control their movement using carefully cut cams. A rod would trace the shape of the cam as the cam was rotated. The rise and fall of the rod as the cam rotated was translated into motion of the hand or other mechanism.
After seeing the movie "Hugo" I decided to create my own automaton using only LEGO parts. Of course LEGO could not have cams designed to makes specific letters as part of its standard inventory so I required a new mechanism to store the pen stroke program.
Like all of my machines, it does not use computers, motors, or electronics. It is a hand cranked gear mechanism.
The Automaton is composed of three main elements. The plotter which moves the pen and paper to actually write the message, encoded pen stroke program stored on a series of "chains", and the reader which decodes the program chain into the required pen strokes for the plotter.
The Plotter is where ink is applied to paper under control of the machine. It draws by moving the paper in the X (left/right) dimension and the pen in the Y (up/down) and Z (lift/lower) dimensions.
A small marker pen rides in a cart over the paper. An axle connected by gear to the cart can move it from one side of the paper to the other across the Y dimension. This handles up/down pen strokes. There are small rubberized wheels that can roll the paper forward and backwards, i.e. a paper-feed. This movement through the X dimension handles left/right pen strokes. There is also a simple mechanism to raise and lower the pen through the Z dimension as required to separate letters or other items being drawn. Simultaneous movement in X and Y dimensions produces diagonal lines.
There are two compatible LEGO link pieces that can be used to form a chain. A narrow link frequently used like bicycle chain and a wide link often utilized as caterpillar track. These chain links are used to encode binary values in a program chain. A narrow link indicates "true" or "do something" and a wide link "false" or "do nothing".
The machine has five different chains that program the movements of the pen and paper. Two chains cooperate to move the pen up/down, two chains move the paper left/right, and a final chain lifts/lowers the pen.
The chain is fed, very much like film in a projector, through the machine to a reader. There is a tensioner which improves the reliability of chain movement. Behind the machine are gears, which appear empty in the photographs. These gears are used to accommodate chains longer than the currently loaded message.
In the example below it's easy to note the down stroke of the 'L' followed by the base stroke to the right. The letter "E" is interesting because it has several back strokes made with the pen lifted. Narrow links indicate a movement in the labeled direction. Simultaneous strokes in two dimensions form a diagonal line. Lift/lower movement always occurs before the matching pen stroke.
The chain is read using a reciprocating fork mechanism. The fork descends to the current link of the message program chain. A narrow link under the fork allows it to make a full down stroke which engages a ratchet. A wide link under the fork blocks the fork tines preventing a full down stroke. The ratchet does not engage.
If the ratchet was engaged, the up stroke will rotate a wheel 1/6th of a revolution performing some action. If not engaged, the upstroke does nothing.
After the upstroke clears the chain, the chain will automatically be advanced to the next link so that the following pen stroke can be read.
For two way motion, i.e. pen left/right or paper forward/backwards, two complementary mechanisms share a single axle. One chain reader can turn the axle 1/6th of a turn clockwise, the opposite chain reader can turn the axle 1/6th of a turn counter-clockwise. Together they cooperate in controlling one dimension of pen movement. This allows none, left, or right movement. Of course both chains indicating an opposite movement at the same time is an error and could damage the machine.
Pen lift/lower only requires a single chain. A narrow link rotates the axle 1/6th of a turn which will toggle the pen lift status. If the pen was low, it will be raised. If raised, it will be lowered.
Much like in a film projector, a Geneva drive mechanism is used to convert continuous rotational movement from the crank into stepped movement of the chain at the appropriate times. The Geneva drive keeps the chain solidly locked in place when it's not being advanced.