In many CIJ printer manuals and papers about CIJ printing it is described that CIJ printers use a closed loop system to monitor droplet breakup. It works by charging some of the droplets with a test signal, measuring the charge of these droplets and analyzing which one got the best charge from the test signal to choose the right timing for droplet charging.

Until lately, I often read that, but didn't fully understand how exactly the printers do that.

Great thanks to @Paulo Campos for helping me understand how Phase Detection works.

Here is my description of it:

A CIJ printer works by using a piezo for breaking up an ink stream into equal-sized droplets. These droplets get charged to a certain voltage level when they fly through a charge electrode and get deflected out of the stream's flight path according to their charge when they pass by a high voltage deflection plate. According to their deflection angle they hit the surface at a lower or higher position of the vertical line that the printhead can reach from it's position.

For droplet charging the right timing is crucial. The charging pulse needs to take place at a certain moment in the droplet formation process.

That's where phase detection comes into play:

The droplet formation process is based on the piezo's vibration which is based on the piezo's drive signal which is a high frequency sine wave. That means that different stages of drop formation occur at different phase angles of the sine wave. To find out at which phase angle of the sine wave the droplets receive the most charge, a phase detection test is used.

The phase detection test signal is applied to the charge electrode whenever the charge electrode is not used for the charging of droplets that are meant for printing.

The phase testing signal is a burst of 16 pulses that cover 0 to 360 degrees in 22,5-degree steps of the piezo signal's sine wave so that each pulse starts at another phase of the sine wave.

This is done to charge 16 successive droplets with different timings/phase settings which will later be read by a phase detection circuit and the printer controller which then can select the best timing/phase setting to get the best possible droplet charging.

In contrast to print charging, test charging usually uses a low negative voltage instead of a high positive voltage to make sure the ink droplets that are used for testing hit the gutter instead of the printing target and to make it easier for the printer controller to differentiate between both types of charge signals.

A positive voltage leads to negatively charged droplets and a negative voltage leads to positively charged droplets. With that, the printer controller sees the positively charged phase detection droplets as positive spikes and the negatively charged printing droplets as negative spikes and can easily differentiate between them. The duration of the measured spikes is dependent on the time of flight of the ink droplets - It shows how long the ink droplet is present to the sensor.

The signal that gets read from the phase detection sensor usually has the shape of a hedgehog, with the highest charge droplets in the middle and the lowest charge droplets on the outside.

The printer controller then selects the phase setting with the highest charge on its droplet and applies it to the charge pulse when printing.

And that's basically it.

It's worth mentioning, that every changing parameter like ink pressure, viscosity, conductivity, dirt in the nozzle and environmental conditions has more or less effect on breakup or charging, so it's likely that the best phase for charging doesn't stay the same during printing what makes a phase detection function essential for operation.

It's also very useful to have the signal from the charged ink droplets on an oscilloscope screen while adjusting parameters on the printer to see which effect they have on breakup and charging.

To date, I could successfully generate the phase testing signal and just recently I could make some progress on the feedback signal reading.

More about this in the next Log.

Thank you for your interest in my project :)