I've wrapped up my second pass exploring the paper feed motor of my retired Canon Pixma MX340 multi-function inkjet. Capturing data from its quadrature encoder reporting rotation motion had to be done while the system was still in a running state, and I think it's the last useful data I could extract at my current skill level. Thus this marks the transition point between phase 2 and 3 of my original teardown plan. I will switch focus from electronic probing of phase 2 to mechanical disassembly like in phase 1. The main difference is while I still prefer to keep things in a running state for as long as I can, maintaining functionality is now less critical.

The most obvious target for starting phase 3 is the print carriage assembly, or "the X-axis" in my mind. Sitting on top of all remaining mechanical bits, it carries the two ink cartridges ("C" for colors cyan/magenta/yellow and "B" for black) with their integrated print heads across the printing area as well as a print head maintenance/parking area to the right. After seeing how the paper feed motor was linked to many other mechanisms in addition to feeding paper, I had expected to see similar clever multi-use of the X-axis motor and surprisingly didn't find additional gears.

Speaking of that paper feed motor, I've taken my first step to unraveling its mysteries: it actuates a "parking brake" for the print carriage: A small white plastic pawl that extends (left picture) against the print carriage edge to keep it in its parked position, and retracts (right picture) to allow print carriage movement.

This pawl is connected to a shaft in the paper feed motor gearbox, but not rigidly. So when the pawl reaches one endpoint or another, the underlying shaft continues turning. This will loosen as the system wears, but (1) this geometry can tolerate a lot of wear and still do its job and (2) it has outlasted the service life of this machine so it was evidently good enough.

This pawl was retracted by the first 1800 encoder count rotation when the system powered up, and it was extended with the final 1800 encoder count rotation when the system goes into standby. There are many additional 1800 moves as the printer runs, so I doubt this parking pawl was the sole purpose of 1800 rotations. Certainly a useful related functionality, though.

Once carriage moves beyond its parked position, this pawl will no longer limit carriage movement. It will still move along with its loosely-attached shaft. If it should try to extend, though, it will hit either the flat back of the carriage or empty air depending on position within its full range of motion.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.