Whenever I turn on my Canon Pixma MX340 multi-function inkjet, I immediately hear motors whirring and I had always been curious: I haven't even tried to print anything yet. What is it doing? Now that I've retired the machine and slowly taking it apart, I want to get some information hopefully leading to some answers. I have attached an Arduino Nano to two photo interrupter sensors and the quadrature encoder reporting position of the paper feed motor gearbox assembly. The Nano watches those sensors and report on their status every 10 milliseconds to a serial port as a list of comma-separated values. Thanks to the "everything is a file" nature of Linux, I could run "cat /dev/ttyUSB0 > capture.csv" and I had a file I could import into Excel.

Here is the startup sequence that has long intrigued me, in the form of an Excel scatter chart. Horizontal axis is the time stamp. This graph covered about 9 seconds with a data point every 10ms, resulting in dots densely packed into a line. Vertical axis is the encoder counter value, with the encoder as a gray line maxing out at almost 32000 counts. (Getting a number for count per rotation is on the to-do list.)

The two photo interrupter sensors did not change state throughout startup.

Due to the way I wired up the quadrature decoder, positive values on on this counter correspond to rotating the paper feed backwards. I usually think of printing as advancing forwards on the Y-axis, so this is reversed from that I would have chosen if I had known. But now that it is connected, I don't care enough to fix this minor issue. If it starts causing problems I can flip the two wires and reverse their direction.

The fact this plot appears to be a series of straight lines tells me the motor does not vary its speed (significantly) once it is in motion.

Zooming on the initial set of movements, I see an acceleration curve before it reaches that speed, and there's a deceleration curve at the end of each movement. My eyes did not perceive acceleration and deceleration, so this exercise is already telling me things I couldn't pick up with my own eyes. I can also see the first two movements here are both backwards, but the second movement has a gentler slope reflecting a slower speed. Another difference I could not see with eyeballs.

Zooming in on a different part of the graph, this is the point of maximum backwards travel. On the big graph I saw a small bump on either side of the peak, now I see the motor decelerated to a stop before immediately accelerating back up in the same direction, only to decelerate to a stop again before going in the opposite direction. It then slowed down again to a stop before resuming in the same direction at a different speed. Both of these pauses are extremely short, only a few milliseconds if that.

What did that motion accomplish? In fact, what did any of these motions accomplish? I don't have answers yet, but I'm excited I'm peeling back at least the first layer of this onion. Next: power down standby sequence.

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

Captured CSV and Excel worksheets are included in the companion GitHub repository.