1.  Field of the Invention
The present invention relates to an ink jet printer, and more particularly, to an ink jet printer with a compensation function for malfunctioning nozzles. 
2.  Description of the Prior Art
In general, ink jet printers comprise one or more ink jet printing heads, and each ink jet printing head comprises a plurality of nozzles.  The ink jet printers utilize the nozzles to jet ink to print documents.  However, due to a demand by users to increase print quality, the ink jet printers must have increased printing resolution.  So, the size of the nozzles must be reduced, but this increases a probability of foreign materials building up on the nozzles. 
Please refer to FIG.  1 and FIG.  2.  FIG.  1 is a diagram of a prior art ink jet printer 10.  FIG.  2 is a printing diagram of the ink jet printer 10 shown in FIG.  1, when operating properly.  The ink jet printer 10 comprises an ink jet printing head 12.  The ink jet printing head 12 moves back and forth along an ink jet track 13 to print a document 11.  The ink jet printing head 12 comprises a plurality of nozzles 14a and 14b for jetting ink needed when printing to form a plurality of printing strips 16 and 19 on the document 11.  Each printing strip 16 or 19 is made of a plurality of ink jet spots 18. 
The ink jet printer 10 uses a popular printing method called shingling.  The shingling printing method is performed by using N-printing routes to print a printing area.  In the first printing route, only 1/N nozzles of total nozzles are used to jet ink.  In the second printing route, another 1/N nozzles of total nozzles are used to print.  This is repeated to perform N-printing routes to finish printing the printing area with nozzles at different positions.  As shown in FIG.  2, in the first printing route of the ink jet printing head 12, only half of the nozzles 14a, shown in FIG.  2, are used to form the printing strips 16.  In the second printing route, the other half of the nozzles 14b are used to form the printing strips 19.  The nozzles 14a or 14b are used separately, at different positions, to form the interlaced printing strips 16 and 19 to finish the printing operation.  Again, FIG.  2 is only a preferred embodiment of shingling printing.  Actually, there are many methods of shingling printing, but this printing method is obvious to those skilled in the art, so further details are omitted. 
Please refer to FIG.  3.  FIG.  3 is a printing diagram of the ink jet printer 10 shown in FIG.  2, when having a malfunctioning nozzle 15.  In general, nozzles are often damaged by some reason (such as blockage by a foreign matter), so that they cannot jet ink properly.  As shown in FIG.  3, when the ink jet printing head 12 comprises a malfunctioning nozzle 15, because the malfunctioning nozzle 15 cannot jet ink properly, if printing with the above method, there is a blank space 17 formed on the document 11, and the printing quality is affected. 
Please refer to FIG.  4.  FIG.  4 is another printing diagram of a prior art ink jet printer when operating properly.  The ink jet printer 20 also comprises an ink jet printing head 22, and the printing head comprises a plurality of nozzles 24 to jet ink to form a plurality of printing strips 26 on a document 21.  The printing method used in the ink jet printer 20 is different from the above printing method of shingling, and it is a printing method of area printing.  The ink jet printing head 22 executes a plurality of printing processes to form an area by moving.  It prints an area each time to form a plurality of printing areas 29 on the document 21 to finish the printing job of the entire document.  In every printing process, the relative position between the ink jet printing head 22 and the document 21 changes.  As shown in FIG.  4, labels I, II, and III of FIG.  4 represent the changes of positions of the ink jet printing head 22 relative to the document 21 in three successive printing processes.  Therefore, by changing the relative positions, the ink jet printing head 22 is capable of performing printing at different positions on the document 21. 
Please refer to FIG.  5.  FIG.  5 is a printing diagram of the ink jet printer 20 shown in FIG.  4, when having a malfunctioning nozzle 25.  As shown in FIG.  5, the ink jet printing head 22 comprises a malfunctioning nozzle 25.  When the ink jet printing head 22 prints on the document 21, because the malfunctioning nozzle 25 cannot jet ink properly, each printing area 29 formed by the ink jet printing head 22 has a blank space.  Therefore, when printing by the method of area printing, the printing quality of the ink jet printer 20 is also affected, because of the existence of the malfunctioning nozzle 25. 
It is therefore a primary objective of the claimed invention to provide an ink jet printer with a compensation function for malfunctioning nozzles to solve the above mentioned problems. 
According to claimed invention, the ink jet printer comprises an ink jet printing head having a plurality of nozzles.  The ink jet printer also has an ink jet compensation controlling device for driving the ink jet printing head, so that the nozzles are capable of forming a plurality of linear printing strips on the document, each printing strip comprising a plurality of points.  The ink jet printer also has a detecting device for detecting a working status of the nozzles.  When the nozzles work normally, the ink jet compensation controlling device uses the nozzles to form the printing strips.  If the detecting device detects at least one malfunctioning nozzle in the nozzles, the ink jet compensation controlling device selects a group of normal nozzles according to the position of the malfunctioning nozzle.  The group of normal nozzles comprises a plurality of normal nozzles that excludes the malfunctioning nozzle.  The detecting device then controls the group of normal nozzles to form printing strips on the document that correspond to strips that would normally have been printed by the malfunctioning nozzle. 
It is an advantage of the claimed invention that the ink jet printer uses other nozzles operating properly to perform the printing operation of the document when there is a malfunctioning nozzle in the ink jet printing head of the ink jet printer. 
These and other objectives and advantages of the claimed invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.