Method and apparatus for maintaining constant drop size mass in thermal ink jet printers

DETAILED DESCRIPTION It is the object of the present invention to provide an improved thermal ink jet printer which maintains a substantially constant spot size in ink droplets ejected from the printhead while printing.
It is another object of the invention to maintain the spot size of the ink droplets ejected from the printhead constant during a printing mode by periodically measuring the mass of a predetermined plurality of ink droplets between each printed copy.
In the present invention, a thermal ink jet printer with a printhead of the type having an ink supply manifold and a plurality of parallel ink channels with each channel having a nozzle and a heating element, is mounted on a translatable carriage and has a rotatable cylindrical platen.
The printhead is mounted on a carriage which confronts and is reciprocatingly translated along the platen.
An oscillator circuit is mounted inside the platen and includes a resonant vibratory device on which the ink droplets ejected from the printhead nozzles by electrical driving pulses selectively applied to the heating elements, are received and the mass thereof is measured.
The resonant vibratory device such as a piezoelectric sensor or a quartz crystal, provides the means for measuring the mass of ink droplets deposited thereon.
The difference in frequency before and after drop deposition is proportional to the droplet mass.
The frequency change of the sensor is measured to provide a signal representative of the mass of the droplet and comparison of this signal to a desired value generates a comparison signal in the printer controller that adjusts the driving pulses which expel the ink droplets from the printer, so that the drop mass and therefore the droplet size is maintained constant.
A more complete understanding of the present invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, wherein like parts have the same index numerals.