How it works:

• Force control between the graphite and paper. Force control is achieved using a series elastic actuator design. A non-contact hall effect position sensor is used to measure the displacement of a compression spring. A hobby servo motor (with the internal potentiometer hacked for absolute position sense) is commanded to compress the spring against the graphite according to the targeted force. The control strategy is nothing fancy (i.e. no custom PID loop). The system just tries to minimize the error between the current spring displacement and the commanded spring displacement. Making the system minimize error as fast as possible is critical because it has a huge impact on drawing quality. The faster the system can respond, the higher print resolution it can achieve at higher xy machine feed rates.

• Automated graphite dispensing. A photogate sensor is triggered when the graphite wears down and the pencil gets too close to the paper. The system then commands a feed hold at the controller to pause the xy machine. While paused, the servo motor moves the mechanical pencil to up a position that depresses the top of the pencil. The system returns to the last commanded force. Finally, the system commands a cycle start to resume the drawing.

Component selection:

• Pencil control motor. Needs to be as fast as possible. Forces are very low so unlikely to be constrained by force capabilities. Needs absolute position sense.
• Spring position sensor. Needs to be non-contact so it's resistance does not need to be overcome to compress the spring. Less resistance means the system can draw fainter lines. Achieving fainter lines is critical to maximizing contrast between faint and dark lines of a drawing.
• Pencil. Needs to be a high quality mechanical pencil. Low quality mechanical pencils have less of a hold on the graphite. The result is that the graphite will slip in the pencil mechanism at low forces. The low slip force limits the maximum force that can be applied between the paper and pencil. A lower force limits how dark the darkest pencil marking that can be. Achieving darker lines is critical to maximizing contrast between faint and dark lines of a drawing.
• Linear guide. Needs to be low mass and have low resistance to motion. Higher mass and higher resistance limit how faint of a marking the system can achieve.
• Compression spring. The compression spring sits between the pencil control motor and pencil. It's selection is very fragile and has huge impacts on the faintest and darkest markings that the system can achieve. At the low end of displacement, the compression spring should just barely be able to overcome the resistance force of the linear guide. At the high end, the spring should compress to a force that just barely starts to cause the graphite to push too hard into the paper. Any deviation from these guidelines will result in less contrast in the drawing - faintest markings will be darker and/or darkest markings will be fainter.