Ink Jet Print Head Mounting Mechanism

The present invention relates to ink jet printing apparatus of the type having insertable print cartridges carried by a carriage and more particularly to apparatus for accurately aligning said print cartridges with respect to said carriage.

There are known drop on demand ink jet printer systems in which a print head carriage assembly, which supports a print head, traverses the print head across the width of a print medium in line printing operation. Between lines, the print receiving medium is advanced to prepare for the next line printing sequence. One well known approach is to construct the print head as a unitary, disposable print cartridge which contains an ink supply, drop generating structures, and electrical connections adapted for electrically coupling the print head to the printer which provides drop generating energy in response to print information signals generated by the ink jet printer.

Heretofore, prior art ink jet printers have typically controlled up to five degrees of freedom of the position of the print head with respect to the carriage. Prior art designs have employed various combinations of pad to pad, pin and slot, and knife edge to detent surface alignment schemes. See for example U.S. Pat. No. 4,736,213 issued Apr. 5, 1988, where a cartridge having two alignment pins and a detent surface cooperates with a carriage having two slots and a knife edge to align the cartridge to the carriage. Some of the prior art schemes result in over constraining the cartridge in one or more degrees of freedom, thereby introducing strain and possible distortion of the cartridge, whereas other schemes as noted above underconstrain the cartridge introducing unreliability in the carriage to cartridge interface in one or more degrees of freedom. As the number of ink jets and hence the number of electrical connections to the print head has grown in recent years, it has become increasingly important to precisely locate and exactly constrain the print head with respect to the cartridge. The term exactly constrain as employed herein means to fix each of the three directions x, y and z, and each of the three possible rotations about these directions (all six possible degrees of freedom) without overconstraining any one of the degrees of freedom, thereby avoiding induced stress at the interface. Furthermore, it is desirable to make the ink jet apparatus as small as possible to be portable. Since pads or location planes occurring on adjacent mounting surfaces require considerable area, it is desirable to avoid the use of pad to pad location features.

It is an object of the present invention to provide an improved mounting scheme for an ink jet print head in a carriage of an ink jet printer that avoids the problems noted above.

The object is achieved according to the present invention by providing a system for exactly constraining the ink jet print head in the carriage including a first alignment feature between the print head and the carriage for uniquely defining a point, a second alignment feature between the print head and the carriage for uniquely determining a line through the point, and a third alignment feature between the print head and the carriage for uniquely determining a rotation about the line. In a preferred mode of carrying out the invention, the first alignment feature is a generally spherical boss on the carriage that cooperates with a conical indentation on the print head to define a point; the second alignment feature is a generally spherical boss on the carriage that cooperates with a trough on the print head to define a line through the point, and the third alignment feature is a generally spherical boss on the carriage that cooperates with a flat on the print head to define a rotation about the line. In the preferred embodiment, mating planar electrical contacts are located on the carriage and the print head in a plane defined by the three locating features, and a cam is provided on the carriage for urging the print head into engagement with the carriage alignment features.

FIG. 1 is a perspective view of an inkjet print head cartridge, illustrating the principle of operation of the exact constraint alignment system of the present invention, and;

FIG. 2 is a perspective view of an ink jet print head cartridge and carriage according to the present invention.

The principles of operation of the present invention will first be described with reference to FIG. 1, where there is shown three spheres 10, 12 and 14, arranged in a triangular pattern and representing three alignment features which may be provided on a part to be aligned with an ink jet print head cartridge, generally designated 16. Cartridge 16 defines three alignment features for cooperating with the three spherical alignment features: a conical depression 18; a trough 20 having sloping sides; and a recessed flat 22. When sphere 10 is moved into engagement with conical depression 18, a point is defined e.g. the center of sphere 10. Assuming that spheres 10, 12 and 14 are rigidly connected, when sphere 12 is seated into trough 20, a line is defined e.g. a line 24 between the centers of sphere 10 and 12. Finally, when sphere 14 is brought into contact with recessed flat 22, a rotation .alpha. about line 24 is uniquely defined and all six degrees of freedom between an object bearing the spherical alignment features and the cartridge 16 are exactly constrained. It should be noted that there is no overconstraint in this system, i.e. a surface bearing the alignment spheres 10, 12 and 14 would not experience any stress due to the constraints.

Turning now to FIG. 2, an ink jet print head and carriage employing the exact constraint alignment system according to the present invention are shown. The print head generally designated 16 includes a body 28 containing an ink reservoir, an ink drop generator 30 connected to the ink reservoir, and a generally planar electrical contact pad 32 (shown in FIG. 1) for connecting the ink drop generator 30 to the electronics of the ink jet printer (not shown). As shown in FIG. 1, one side of the ink jet print head 16 defines three alignment features arranged in a triangular pattern, a conical depression 18, a trough 20 with sloping sides, and a recessed flat 22.

A carriage, generally designated 33, includes a cradle 34 (shown partially broken away) for receiving the print head 16. On one wall 36 of the cradle 34 is a planar array of electrical contacts 38 adapted to mate with the contacts 32 on the print head 16. Three generally spherical alignment features 40, 42, and 44 are provided on the wall 36 of the cradle 34. A pair of cams 48 is mounted in an opposite wall 46 of the cradle 34. When the print head 16 is inserted into the cradle 34, the cams are actuated by a lever 50 to urge the print head into engagement with the electrical contacts 38 and the alignment features on wall 36.

In operation, carriage 34 is moved along a pair of shafts 52 and 54 by a motor 56 that drives a cable 58 connected to the carriage 34. A flexible electrical connector 60 provides electrical connection to the contacts 38 in the carriage to the ink jet printer electronics (not shown).

Preferably, the generally spherical alignment feature 40 on wall 36 is a spherical ball boss and alignment features 42 and 44 are generally spherically tipped conical bosses that can be molded or otherwise formed in wall 36. Alternatively, the alignment features 40, 42 and 44 may all comprise spherical bosses, and may be formed for example by embedding ball bearings in the surface of wall 36. It will be appreciated that any one or more of the alignment features can be reversed from the preferred arrangement shown i.e., the spherical alignment feature 40 may be provided on the ink jet print head 56, and the conical depression, trough, or flat provided on the carriage 34.

The present invention is advantageous in that an ink jet print head in precisely aligned in a carriage without over or under constraint, resulting is improved drop placement accuracy and exact control of print quality, improved registration of the electrical contacts in the print head and carriage, no excessive force or stress required for the print head to seek the constraints, simple insertion to one mounting plane, and 100% interchangeability between print heads.