Moving coil, multiple energy print hammer system including a closed loop servo

Claim 1

A printing system having a moving coil hammer for use with an independently operable printing machine having a platen, a print element containing predetermined characters and a carriage for traversing said platen, said print hammer system comprising: means for creating a permanent magnetic field in the area of said hammer, means for generating a velocity command which is the velocity profile for the character to be printed, means for converting said velocity command into a current, a coil connected to said hammer and coupled to said current for moving said hammer against and away from said print element, a resistor in series with said coil for generating a voltage proportional to the voltage dropped by the internal resistance of said coil, an inductor in series with said coil for generating a voltage proportional to the voltage dropped by the inductance of said coil, means for determining the velocity of said hammer by calculating all resistance and inductance voltages in the coil circuit and subtracting said voltages from the total voltage applied by said means for converting, the remainder equaling the back emf voltage of the coil which is a voltage proportional to the hammer velocity, wherein said means for generating is responsive to said hammer velocity voltage determination to generate an updated velocity command, and wherein said means for determining further includes a variable gain amplifier to provide compensation for any changes in the resistance of said coil in the moving coil hammer due to changes in the temperature of said coil, thereby allowing the sensed velocity of the moving coil hammer to be independent of any changes in the resistance of the coil in the moving coil hammer.