International Business Machines Corporation

George Melnyk

Robert F Jessen

James F Driftmyer

A pair of ribbon spools are independently operated by respective motors to rotate in one direction until the ribbon end and then in the opposite direction, and so forth. Spools alternately function as feed and take-up spools and motors as a drive and drag motors, respectively To eliminate excess drag motor cogging, a ribbon tension control system uses dynamic braking of the drag motor, and includes drag circuitry with rectifier bridge circuitry and a constant current sink settable for drawing predefined selectable levels of current from the windings of the drag motor. The motors are polyphase, permanent magnet, stepper motors which, when turned by the ribbon, generate alternating back emf signals in their windings. A full-wave rectifier bridge is connected to each winding of the drag motor with bridge outputs connected to a common sink which is an emitter follower circuit whose current level is set and maintained by an operational amplifier. The current level of the emitter follower circuit is set by adjusting the reference voltage of the amplifier using a resistive voltage divider network connected to a binary switch controlled by a microprocessor with a memory. The microprocessor monitors the back emf signals, periodically adjusts the rotational speed of the drive motor depending on distribution of ribbon, and adjusts the sink based on drag value tables stored in memory.

Ribbon tension control with dynamic braking and variable current sink

A coil is connected to a voltage source by a switch enabled by a drive signal of fixed duration. Drive current in a coil is terminated by a chopper circuit if it reaches a predetermined peak level before the end of preselected rise time interval. The chopper circuit is inhibited from operation until the end of the rise time interval during which time the drive current is allowed to decay. The chopper circuit is enabled at the end of the rise time interval for chopping the drive current at an average peak current value for the duration of the preselected time. The total amount of energy supplied to the actuator is adjusted by allowing the current to decay if the peak current is reached before the end of the predetermined rise time interval and then by chopping the current at a predetermined switching rate for the duration of the preselected time of the drive signal. Short circuit protection is also provided during the predetermined rise time interval.

Chopper drive control circuit

Apparatus for sensing the position of an electrically conductive printing band in which the band moves at varying depths within the gap of an inductor in an inductor-capacitor resonance circuit that is periodically energized to induce eddy currents in the band. The oscillatory ringing resulting from pulsing the tank circuit decays at a rate that varies with the magnitude of the induced eddy current and, hence, the depth of the band in the gap. By counting the number of voltage excursions across the tank circuit resulting from an excitation that exceed a predetermined threshold during decay, the position of the band can be deduced. Corrective positioning may thereafter be undertaken.

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Endicott, NY, United States of America

George Melnyk