Abstract: In an ink jet printer, drops are generated at a test frequency which is a harmonic of the drop generation rate for printing. If an error in velocity is detected at the test frequency, a coarse correction velocity is made to bring the correct number of drops within the range of one-half the wavelength at the nth drop location relative to a drop detector. Drops are then generated at the printing frequency and a fine correction in the velocity is made if a velocity error is detected.

Abstract: A servo system for adjusting the velocity of ink drops in a magnetic ink jet printer has a coarse control loop for making coarse increment adjustments to the pump which supplies ink under pressure to a nozzle and a fine control loop for making fine increment adjustments to the pressure of said pump. The velocity of the drops is determined by a pair of drop sensors located one drop wavelength apart at a fixed distance from the drop generation point. The direction of any phase error between pairs of drops is used to generate direction control signals applied to the coarse control loop until a reversal in phase error occurs. A toggle control allows the coarse control loop to make coarse adjustments in pump pressure around a null point velocity before switching direction control signals to the fine control loop.