How much belt does a Makelangelo need?

Jenelle wrote in on the live chat to ask: “Where is the bottom of the machine? is it the bottom of the board?”

The machine height and machine width affect your belt length, but your belt length also affects the machine height. It works like this:

The setup

This is the Makelangelo you know and love, in my front window. There are two motor mounts in the top corners, each with a pulley that pulls the belt. The control box in the top center controls the motors. The belts pull the pen holder around, and a servo on the pen holder lifts the pen off the glass. The water bottles act as a counterweight and keep the belt from slipping on the motor pulleys.  (In this photo the wire to the servo is hanging, disconnected.)

The control box currently has an Adafruit Motor Shield v2 on there because I was making it go faster. I put my laptop on a box below the window, and use the “flip for glass” command in the software so text in the pictures makes sense from outside house. I’m using water soluble markers. I can draw an image in a few minutes, wipe it off, and do another. This suction cup system would also work great on a whiteboard.

PS: Coding with a view > coding facing a wall.

The Measurements

Now I’ve drawn informative lines. A picture’s worth a thousand words, right? The distance between the vertical green lines is the machine width.  The distance between the red horizontal lines is the maximum height.

…but here it gets a little complicated. If either water bottle hits the windowsill it will have a ripple effect on the pen holder and the rest of the drawing. It’s also true that the pen holder and the bottle can’t hit the motor mount. That means the shortest possible belt is when the bottle is almost at the bottom and the pen holder is almost hitting the motor mount on the same side. I’ve drawn a yellow arc that starts one-bottle-height up from window sill. The yellow line that goes from the arc to the corner is the length of the shortest possible belt. This is only for one of the belts! The movement is limited by both belts and can’t reach any of the yellow zone at the bottom of the rectangle.

On the recommended 3’x4′ (914.4mm x 1219.2mm) board I would use ~4’6″ (1.5m) of belt on each side. One 3m piece of timing belt cut in half is perfect.

There is another….

If you want to draw all the way to the bottom corners then you will need more belt than the minimum. There is a catch! You won’t be able to draw all the way in the top corners because the bottle would hit the windowsill.  (In the picture above I’ve highlighted the no-go zone.)

You can get around this a couple of ways:

• Tell the computer that your machine is (height – bottle) tall. On the recommended 3’x4′ (914.4mm x 1219.2mm) board I tell the computer the size is 3’x3′ (914.4mm each side).  In this strategy the starting point for all drawings is 1’6″ down from the top middle of the board, and the drawings never reach the bottom of the board.
• Tell the computer that your machine is (height) tall, then make sure your paper is inside the top limit.
• Make room for the bottle to drop lower: mount your makelangelo on a board on the wall
• Make a pulley system for the bottles so they move less and pull more belt. I… I only just thought that one up.

These limitations are some of the reasons why you won’t see a Makelangelo that’s short and wide.

Final thoughts

The taller your picture gets, the less pressure the pen has on the wall, board, or window.  I don’t have any clever ideas how to improve that one.  If you do, please share it in the R&D forums.

The green lines are measured from here

to here

I just got approved for the Bay Area Maker Faire in San Mateo, California May 16 & 17, 2015. Come see all the wild things people have been building, play with the Makelangelo, and get a special coupon code probably worth more than your fair entry pass, just for saying ‘awongalema’.