US 5,490,638 A · Ribbon tension control with dynamic braking and variable current sink

Abstract

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.

Claims17

Dependent claims

A ribbon drive system including:

a pair of ribbon spools and a pair of motors individually operable to drive said spools,
each said motor being alternately operable to drive their respective ribbon spools whereby one motor is a drive motor when the other motor is a driven motor feeding ribbon connected between the spools,
said driven motor generating alternating back emf signals in windings of said driven motor in response to motion of said ribbon by said drive motor, and
dynamic electrical braking means for producing braking torque by said driven motor for applying tension to said ribbon, comprising:
bridge circuit rectifying means connected to said windings of said driven motor for rectifying said back emf signals,
current sink circuit means communicating with the rectifying circuit means for receiving the rectified output of said bridge rectifying circuit means and for maintaining a constant current from windings of the motors at a selected level, and
control means for selectively setting the level of current drawn by said current sink from said rectifying means in said current sink circuit means for controlling the level of braking torque produced by said driven motor.

4 more elements

A ribbon tension controller for controlling the tension of a ribbon as it is transferred from a rotating supply spool to a rotating take-up spool, comprising:

alternating current generator means coupled to said supply spool for producing braking torque to resist the rotation of said supply spool,
control means for producing a digital ribbon tension control signal related to the distribution of the amount of ribbon between said spools, and
dynamic electrical braking means for dynamically varying the braking torque produced by said generator means, including:
bridge circuit means electrically coupled to said generator means for rectifying alternating back emf signals produced in the winding of said generator means in response to rotation of said supply spool and to current sink circuit means for receiving the rectified output of said bridge circuit means and for maintaining a constant current drawn from the windings at a selected level, and
control means responsive to said digital tension control signal for selectively setting the level of current drawn by said current sink circuit means for controlling the tension on said ribbon.

2 more elements

Dependencies are inferred from the claim text.

Ribbon tension control with dynamic braking and variable current sink

Life of this patent

Ribbon tension control with dynamic braking and variable current sink

Life of this patent

Abstract

Claims17