The present invention relates generally to ink jet printers and, more particularly, to a deflection electrode arrangement for an ink jet printer which provides for correction of any misalignment between the deflection electrode and the drop catchers.
As shown in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis, ink jet printers of the type to which the present invention is directed include a print head which generates a plurality of jet drop streams which are directed at a print receiving medium. The jet drop streams are arranged in one or more rows. Deflection electrodes are positioned adjacent each of the jet drop streams for selectively charging drops in the streams. The drop streams thereafter pass through an electrical deflection field which separates the drops into two or more sets of drop trajectories. As shown in the Mathis patent, the deflection fields for a two-row printer may be produced by positioning a deflection electrode between the rows of jet drop streams and supplying a deflection potential to the electrode, which potential tends to deflect charged drops in the streams outward, away from the electrode. Electrically grounded drop catchers are positioned outwardly from the drop streams such that charged drops are deflected to the catchers and are thereby prevented from striking the print receiving medium. As shown in U.S. Pat. No. 4,085,409, issued Apr. 18, 1978, to Suresh C. Paranjpe, by selectively applying a plurality of charge levels to the drops in the jet drop streams, the drops may be deflected in each stream sufficiently such that they strike the catcher, or deflected to a lesser degree, such that they strike the print receiving medium at various ones of a number of print positions. Prior art deflection electrodes, such as shown in the Mathis patent, typically comprise a thin strip or ribbon of electrically conductive material, such as stainless steel. With such deflection electrodes, the deflection potential presented by the electrode is substantially constant along the length of the electrode.
The deflection field experienced by each of the drops in the jet drop streams is a function of the voltage differential between the deflection electrode and a catcher and also of the spacing between the electrode and the catcher, since E=V/d, where E is electrical field strength, V is potential differential, and d is the spacing between the deflection electrode and the catcher. In order for charged drops in the jet drop streams to be deflected outwardly by a uniform deflection distance, the electric deflection field must be uniform along the row of jet drop streams. As a consequence, great care has been taken to assure that the deflection electrode and each of the catchers are properly aligned and that the spacing between the deflection electrode and each catcher is accurate and uniform along the entire row of jet drop streams. Thus extreme care has been necessitated in positioning and aligning the catchers with respect to the deflection electrode after servicing of a printer.
It is seen, therefore, that there is a need for an ink jet printer in which alignment between the deflection electrode and the catchers is not critical to proper deflection of charged drops in the jet drop streams.