The AutoGraph project is about exploring what it takes to automate the process of drawing on paper using a graphite pencil.
Graphite pencils are a very dynamic drawing tool. Applying a small force to a pencil against paper will result in a light marking. Pressing hard will produce a dark marking. Additionally, each marking wears away and changes the geometry of the graphite tip.
The challenge is that graphite marking behaviors are not easy to predict and control. Automating the drawing process with graphite pencils will require new tools and methods.
Humans use pressure sense, muscle memory, and rapid visual feedback to make consistent pencil markings on paper. It follows then that my first thought on how to automate this process is to use a force sensor, a three axis position control gantry, and a camera.
It may be possible to indirectly predict force applied between the graphite tip and paper based on a known position of the graphite tip relative to the paper. The challenging part of this approach would be knowing where the tip of graphite is even when every distance traveled on paper wears away some of the tip. The graphite tip wear behavior would have to be studied well enough to be able to make good predictions of how much tip wear occurs per unit length the pencil travels on paper. I imagine graphite tip wear increases with increased force between the pencil and paper as well.
Fixing the orientation of the pencil relative to the paper and using a force sensitive resistor is going to be the first pencil force measurement method I try. It seems to be a simpler approach than tracking the position of the graphite tip.
Interestingly, I found a force sensitive resistor that is used in stylus pens for purchase online. It has 5 to 500g sensitivity and shows near linear 1/resistance response to force input.