Method and apparatus for controlling a multi-nozzle ink jet printhead

A multi-nozzle ink jet print head having:

• a plurality of nozzles 21 divided into more than one group 22 of nozzles with each group of nozzles having more than one nozzle;
• a piezoelectric actuator 2 for causing streams of ink emitted from the nozzles to be broken up into individual droplets;
• a plurality of charge electrodes 23 divided into more than one group 24 of charge electrodes, each group of charge electrodes having a respective charge controller 25 positioned for each group of nozzles;
• means 11 for generating a modulation waveform to operate the piezoelectric actuator 2 to cause droplets to be generated in each stream;
• means 26 for operating the charge controllers to supply a charge signal waveform to the charge electrodes and charging droplets in the streams;
• means 27 for setting the phase relationship of the charge signal waveform relative to the modulation waveform,
• adjusting means 28 for adjusting the phase relationship of the charge signal waveform relative to thc modulation waveform during the printing process when droplets do not require to be printed, said adjusting means operating independently for each group of charge electrodes, by:
• operating the charge controller to apply a DC voltage simultaneously to all the charge electrodes in the group to charge all the droplets to prevent printing;
• operating the charge controller to apply a pulse signal waveform to the charge electrode controller, to reduce the amplitude of the DC voltage periodically and temporarily to a level below that of the DC voltage but still sufficient to cause droplets to be deflected to avoid printing;
• operating the charge controller to sense the aggregate level of charge applied to the droplets and generating signals representative thereof;
• determining the phase relationship of the pulse signal waveform relative to thc modulation waveform from the generating signals;
• and,
• advancing the pulse signal waveform relative to the modulation waveform or, delaying the pulse signal waveform relative to the modulation waveform to reset the phase relationship of the pulse signal waveform relative to the modulation waveform if the pulse signal waveform is advanced relative to the modulation waveform.