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# Control of Print Hammer Flight Time

IP.com Disclosure Number: IPCOM000043260D
Original Publication Date: 1984-Aug-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 2 page(s) / 61K

IBM

## Related People

Ward, ED: AUTHOR [+2]

## Abstract

A print hammer is moved into engagement with a platen by applying a current for a selected period of time to a coil which surrounds the hammer. The current is turned off prior to the hammer striking the platen so that there is a free flight time of the hammer. The voltage, E, across the hammer coil is defined by the equation (Image Omitted) where L is the inductance in the coil, Le is the eddy current inductance, Ke is the back emf coefficient, x is the distance of travel of the hammer, i is the current in the coil, and R is the resistance of the coil. When the current in the coil is turned off, the first term of the equation goes to zero; this is indicated in Fig. 1. This is when the print hammer begins its free flight.

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Control of Print Hammer Flight Time

A print hammer is moved into engagement with a platen by applying a current for a selected period of time to a coil which surrounds the hammer. The current is turned off prior to the hammer striking the platen so that there is a free flight time of the hammer. The voltage, E, across the hammer coil is defined by the equation

(Image Omitted)

where L is the inductance in the coil, Le is the eddy current inductance, Ke is the back emf coefficient, x is the distance of travel of the hammer, i is the current in the coil, and R is the resistance of the coil. When the current in the coil is turned off, the first term of the equation goes to zero; this is indicated in Fig. 1. This is when the print hammer begins its free flight. The hammer has a residual magnetism that causes the back emf,

(Image Omitted)

not to go to zero when the current in the coil goes to zero. As the residual magnetism increases, Ke increases when the current in the coil is zero. When the hammer strikes the platen, the term dx changes sign because of the hammer reversing direction. This creates a voltage spike ranging from 70-150 millivolts, as indicated in Fig. 1. The block diagram of Fig. 2 shows a circuit for amplifying this voltage spike and using it to produce an impact signal. When an impact signal is produced from the circuit of Fig. 2, it is supplied to a processor (Fig. 3) to be utilized to determine whether the hammer is striking the platen too early or too late. The voltage from the coil is s...