The present invention relates in general to dot matrix printing and, more particularly, it relates to assemblies of electromagnet print-wire actuators, such assemblies being referred to herein as cartridges.
In dot matrix printing, a plurality of electromagnets or solenoids are used to drive individual print wires, upon actuation, into the printing medium, typically an inked ribbon, to create a dot impression on the paper. Characters are created with a linear array of print wires, vertically arranged, which may be 6 to 9 wires high, by selectively actuating print wires 4 to 6 times as the paper moves horizontally one character width. A very common "matrix" for printing one character is a rectangle 7 wires high by 5 wires wide. While print heads including a full 35 wire array are known, it is much more common to use a single array of 7 wires that is actuated 5 times to form a character. This is so, because as can readily be appreciated, a printing head with a 35 wire array is necessarily large and cumbersome, and in it the duty factor (the percentage of use for a given wire) varies from very high for some wires to very low for others, leading to uneven wear problems. A seven-wire head has much lower mass, and the duty factor is much more even.
In the early days of dot-matrix printing, the preferred actuators were linear solenoids, e.g. a cylindrical, spring-mounted armature surrounded by a coil and having a print wire secured in one end thereof. The energy of actuation was used to drive the print wire and tension a spring for the return stroke. A print head having seven such solenoids might have one directly on the print axis and the six others distributed therearound on a mounting surface that was spherical. While the print wires were thus all of substantially the same length, one was straight and the rest were curved. It was apparent that a substantial portion of the available energy was dissapated as friction (heat) in guiding the curved wires.
To reduce frictional losses, a whole generation of print heads was designed that had the print wire driven by a pivoted armature to which it was attached at right angles. With this arrangement, only the tip of the armature was near the print axis, the coils were placed in a circular array around it, and wire curvature was greatly reduced.
Prior workers have labored ingeniously to reduce the mass and increase the speed of dot matrix print heads, but certain limits are inherent. First, there is the finite rise and die-away time described by the hysteresis curve of any electromagnet. Since many dot matrix printers are designed to print multiple copies, delivered print wire power is also a design factor. There are also practical limits to the speed of carriage movement. In this connection, the development of electronics allowing the head to print in both directions has been of great assistance to overall speed, inasmuch as this eliminates "dead" carriage return time. But carriages moving at high acceleration curves tend to malfunction frequently, are expensive to build, and are noisy.