I was ultimately was unsuccessful at achieving controlled pressure between pencil and paper using a force sensitive resistor (FSR). 

The first challenge is that the FSR has to be compressed between two contact surfaces to show a change in resistance. I went about making this work by placing the FSR between a driver cart (i.e. directly connected to z-axis motor lead screw) and a driven cart (i.e. cart that the pencil cart is mounted to) on the linear z-axis. Downward movement of the lead screw cart compresses the FSR at the pencil cart when the pencil graphite makes contact with the paper. The main issue is that whenever there is any small upward movement to decrease pressure between pencil and paper, the pencil cart tends to separate from the lead screw cart and removes all compression at the FSR. Such large changes in resistance for such small changes in movement resulted in oscillations around the pressure control point.

Challenge two is that the weight of the pencil cart works against achieving a low pressure pencil marking. The weight of the pencil cart has to be overcome before compression at the FSR will occur. The result was that the lightest pencil marking possible was closer to what is generally a medium pencil marking value that can be achieved by hand. This essentially truncated the value gamut of the pencil by about fifty percent.

The next strategy is to control pressure between pencil in paper by measuring and controlling to the displacement of a spring. A compression spring and linear potentiometer will be mounted between the lead screw and pencil carts. Displacement will be measured by change in resistance of the potentiometer. A spring constant will be selected such that a small pressure difference will correlate to a relatively large displacement (and resistance).