PRINT HEAD DRIVE ASSEMBLY

Abstract: A printer drive assembly is disclosed which corrects for positional error by reducing or eliminating mechanical interfaces due to multiple gears in the drive system. Tooth structures and shapes are changed and spacing is arranged such that camming between teeth is substantially eliminated and rotational movement of a first gear by an integral number of rotations causes linear motion of the print head within the printer by one print line.

Description: TITLE: PRINT HEAD DRIVE ASSEMBLY BACKGROUND OF THE INVENTION Field Of the Invention The present invention relates generally to an apparatus and method for driving a print head within a printer, and, more particularly, to an apparatus and method for translating rotational motion of a motor to linear motion of a print head with reduced positional error. Description Of The Prior Art Most conventional printers print one line at a time onto a print medium. Consecutively printed lines together form a two-dimensional image. For this type of a printer to operate, there must be relative motion established between the print head, which is printing the line of the image, and the print medium. In this way, the print head prints one line of the image and then the print head or the print medium is moved to a predetermined position relative to the other to allow the print head to print the next consecutive line. When a positional error occurs in the relative motion between the print head and the print medium, the positional error is seen as a flaw in the image. These positional errors can occur in numerous ways. One example of how positional error occurs is by using multiple gears in the drive system. In a drive assembly which moves a print head over a fixed print medium, there are often multiple gears used to drive the print head. That is, the motor drives a first gear which in turn drives a second gear which could possibly drive additional gears. Such multiple gearing can be used for various reasons including, compactness, large reduction in speed, and efficiency, for example. The last gear in the system is connected to a pulley which may drives a wire. The wire is connected to the print head and rotation of the gears then pulls the wire and moves the print head. However, each gear in the system risks introduction of play into the movement of the print head. That is, each gear, or an other mechanical element, has a tolerance associated with it when it is manufactured. The tolerances are cumulative for an overall positional error. Thus, multiple gears add to a greater positional error. Conventional gear elements have triangularly-shaped teeth, known in the art as "involute" teeth. Involute teeth are used for many reasons including the fact that the interaction between two gears having involute teeth produces a smooth mesh due to the receptive shape. That is, the tolerance in manufacturing of the gears can be less while still properly interdigitating the teeth of multiple gears. Further, if the teeth are perfectly shaped and spaced, perfect speed reduction is achieved. However, with involute teeth, the force placed on the gear by the motor translates to a force on the teeth of one gear against the teeth of another gear. The interaction of the mating teeth is that of one angled surface pressing against another. Thus, as the force is exerted, the driving gear tends to ride up the other due to a camming action. That is, since both teeth are angled and there is a force on one being transferred to the other, there is a tendency for the tooth exerting the force to ride up the slope of the other tooth. If a gear rides up the teeth of another gear causing the centers of the gears to be displaced, then there is additional positional error being introduced in the print head drive assembly. Also, if the involute is not perfectly shaped, additional positional error results. Further, when the gears are fabricated there is generally little thought with regard to the gear size as it relates to lines of movement, that is, how much movement in the slow-scan direction occurs due to one rotation of the gear. This is due to the fact that most conventional printing systems include an encoder on the motor which is used to precisely measure rotational motion of the motor. The assumption is then that a predetermined amount of rotational motion, as measured by the encoder, translates into a predetermined amount of linear motion in the print head. However, a problem known in the art as "drunkenness" can introduce further positional error. If a line movement is by more or less than an integral number of rotations then the drunkenness will introduce the positional error. SUMMARY OF THE INVENTION The invention provides a print head driver assembly and a method associated therewith. The print head driver assembly is for use in a printer which prints consecutive lines of an image on a print medium to form the image thereon. The printer uses a print head which has relative motion between the print head and the print medium. In the preferred embodiment, the relative motion is that of the print head being driven by a motor such that the print head moves one line at a time over the print medium. To accomplish such motion, a worm gear is attached to the motor. The worm gear has worm gear teeth which are spaced such that an integral number, N of rotations of the worm gear corresponds to the distance between consecutive lines of the image, where N can be any integer greater than zero including one. In mechanical communication with the worm gear is a drive gear. The drive gear is also in mechanical communication with the print head such that movement of the drive gear causes linear motion of the print head. The drive gear has drive gear teeth which interdigitate with the worm gear teeth such that rotation of the worm gear moves the print head by the predetermined distance. Thus, movement of the worm gear by an integral number of rotations causes the drive gear to rotate by one tooth, moving the drive gear a predetermined amount, which, in turn, moves the print head by the distance between consecutive lines in the image. In the preferred embodiment, the interaction between the gears, the worm gear and the drive gear in this case, is enhanced by using square-toothed gears. When force is exerted on the worm gear to drive the drive gear, the lateral surfaces of the square teeth meet. Since the lateral surfaces of the teeth are substantially perpendicular to a tangent of the gear, the camming action is substantially eliminated, and lateral deflection of the worm does not result in "lost motion." In further aspects, the invention provides methods in accord with the apparatus described above. The aforementioned and other aspects of the invention are evident in the drawings and in the description that follows. BRIEF DESCRIPTION OF THE DRAWINGS The structure and operation of the invention, together with other objects and advantages thereof, may best be understood by reading the detailed description to follow in connection with the drawings in which: Fig. 1 shows an overhead block diagrammatic view of a printer incorporating the invention; Fig. 2 shows an overhead block diagrammatic view of a second embodiment of the invention; Fig. 3 shows the interaction between gear teeth in one embodiment of the invention; Fig. 4 shows an alternative embodiment of the gear teeth where square teeth are used; and Fig. 5 shows another alternative embodiment of the gear teeth where saw- shaped teeth are used. DESCRIPTION OF THE PREFERRED EMBODIMENTS While the present invention retains utility within a wide variety of printing devices and may be embodied in several different forms, it is advantageously employed in connection with an optical printer. Though this is the form of the preferred embodiment and will be described as such, this embodiment should be considered as illustrative and not as restrictive. Further, although particular gears and gear systems are described, one skilled in the art will realize that other types of gears or drive systems, such as direct drive for example, can be substituted for the illustrated gears and gear systems without detriment to the invention. Referring now to Fig. 1, there is shown .an overhead block diagrammatic view of an optical printer 10. Optical printer 10 utilizes a print head 12 to print on a print medium 14. Since the printer in this example is optical, print medium 14 is photosensitive and is preferably an instant photographic film. In this embodiment of optical printer 10, print head 12 is driven linearly by a wire 16 which is attached to print head 12 via a connector 18. Wire 16 is driven by a drive gear 20. Wire 16 is wrapped around a first pulley 22 which is mechanically connected to drive gear 20. Wire 16 also passes around a second pulley 24 disposed on an opposite end of optical printer 10. Wire 16 is substantially aligned with a slow-scan axis of optical printer 10. In this way, rotational motion of drive gear 20 is translated into linear motion of wire 16, due to the fact that the wire is being driven around first pulley 22 and second pulley 24, resulting in linear motion of print head 12. To achieve the rotation of drive gear 20, a worm gear 26 mechanically interacts with drive gear 20. Worm gear 26 is disposed on a shaft of a motor 28. As motor 28 rotates, worm gear 26 likewise rotates. Worm gear 26 then drives drive gear 20 resulting in linear motion of print head 12. Although worm gear 26 provides the precise movement of teeth preferred for the invention, those skilled in the art will realize that numerous types of drive systems can be used to achieve the desired reduction and use of a worm gear in the gear system is simply meant to be illustrative. Also disposed on the shaft of motor 28 is an encoder 30. As is well known in the art, an encoder is generally a disk having a plurality of apertures disposed radially along a periphery of the encoder. An optical device (not shown) then monitors rotation of the encoder by projecting a light through the apertures and counting how many times that light is interrupted. Since the apertures are disposed at known intervals, a position and velocity of the shaft can be measured through encoder 30. In this embodiment, worm gear 26 is substantially coaxial with the shaft of motor 28 such that they share a worm gear axis 32. Worm gear 26 is fabricated such that worm gear axis 32 is substantially perpendicular to a transverse axis 34 which passes through a center of drive gear 20. Preferably, transverse axis 34 is at an angle with respect to fast scan direction 36 such that the motor fits inside the printer. This mechanical interaction allows for the reduction of gears and, therefore, reduction of positional error in print head 12. Referring now to Fig. 2, optical printer 10 is shown having a similar configuration to that previously described. However, in this embodiment, motor 28 is oriented such that worm gear axis 32 is substantially parallel to fast scan direction 36. This orientation is achieved by utilizing a first gear 40 and a second gear 42. First gear 40 is disposed on a shaft of motor 28. Second gear 42 is then disposed on a shaft of worm gear 26. Gears 40 and 42 have an M-to-1 relationship between their teeth, where M is an integer greater than zero. As long as this relationship is maintained, gears 40 and 42 mechanically interact so as to drive the worm gear as previously described. To ensure a proper interface, a spring 44 is disposed in mechanical contact with second gear 42 to keep the gear from vibrating along the axis putting a constant bias against second gear 42 to thrust worm gear 26 against a fixed reference surface. This is necessary since there is a counter force against worm gear 26 when worm gear is driving drive gear 20. Also shown is a reverse gear 46. Generally speaking, print head 12 is biased toward an initial position at one end of print medium 14 by a spring (not shown). Thus, a return motion can be achieved simply by releasing the rotational force on drive gear 20 and allowing the spring that is driving the print head towards its initial position to move print head 12. In this embodiment, however, a reverse gear 46 is shown. Reverse gear 46 has a structure similar to that of drive gear 20 wherein a second wire 48 is disposed in mechanical communication with reverse gear 46. Reverse gear 46 can be engaged using any of numerous mechanisms well known in the art. For example, reverse gear 46 can normally be in a position disengaged from the drive gear and only engages when necessary to drive print head 12 in a reverse direction. Once engaged, a mechanical interaction between reverse gear 46 and drive gear 20 occurs. This interaction causes rotation of reverse gear 46 moving reverse wire 48 thereby driving print head 12 linearly in a reverse direction. A reversal of a motor attached to worm gear 26 can also be used to return the print head at a higher rate of speed than would an elastic device, such as a spring for example. The speed can be in excess of that used in the forward, or slow-scan, direction. An additional advantage with using a reversal of a motor, is that fewer parts are required. It also provides a controlled return of the print head or a monitoring of the position of the print head by an encoder. Referring now to Fig. 3, there is shown a motor 28 having encoder 30 disposed on a shaft 51. Friction provided by loading the print head down to guide rails (not shown) by means such as springs (not shown), loads drive gear 20, which in turn loads worm gear 26 into motor 28. The friction maintains a substantially constant force against worm gear 26 to maintain engagement of worm gear 26 with drive gear 20. Worm gear teeth 52 are shown interdigitating with drive gear teeth 54. A typical substantially triangular-shape tooth configuration is shown in this embodiment. Due to the limited number of mechanical interactions, (i.e., drive gear 20 being used and tuning a set number of teeth for each N rotations), most of the positional error that is normally found in a line printer has been eliminated. Encoder 30 can be used to monitor velocity of the motor by counting the number of apertures in the encoder that pass through the optical detector per unit time. This velocity information can then be used to with a feedback loop to motor 28, and its associated circuitry, to make corrections and ensure proper print head speed. In this way, encoder 30 can be used to obtain a substantially constant velocity. When gears are designed in accordance with the invention, encoder 30 has another function. Encoder 30 can be made to monitor and control a positional change of print head 12. This is done in the invention by making the teeth on drive gear 20 and worm gear 26 spaced such that an integral number of teeth correspond substantially to that of the spacing between consecutive lines in the image. In the simplest example, the integral number would be one tooth. Movement of drive gear 20 by one tooth then results in a corresponding linear movement of print head 12 by one print line in the slow scan direction. Thus, encoder 30 can simply be used to monitor an amount of rotational movement in shaft 51 that corresponds to one movement of the tooth in drive gear 20. In one implementation of the invention, encoder 30 now has need for only one aperture in encoder disk 30 and thus when the aperture is rotated one complete revolution, then one tooth movement of drive gear 20 has been achieved thereby moving print head 12 by one print line. Precision in the system has thereby increased. One skilled in the art will realize that this example works for all integral numbers of teeth per print line in that precision correspondence can be maintained. Referring now to Fig. 4, an alternative gear structure is shown where an alternative worm gear 60 is shown driving an alternative drive gear 62. In this embodiment, square worm teeth 64 interdigitate with square drive teeth 66. Since the lateral surfaces of teeth 64 and 66 are substantially flat and perpendicular to a tangent of gears 60 and 62, the camming action of an angled tooth as previously described does not occur. Since the angled surfaces are replaced by parallel substantially vertical surfaces, there is minimal tendency for the driving tooth to ride up the other tooth. Thus, a change in force on the drive system that usually occurs, with one tooth riding up another tooth, is decreased. Moreover, the decrease of the change in force minimizes lateral defection of the worm gear and thus minimizes any resulting lost motion of the print head. The constant friction applied by the spring, as earlier described, in combination with the decrease of varying forces due to the shape of the teeth further minimizes lost motion of the print head in the system. Fig. 5 shows an alternative tooth shape for worm gear 60' and drive gear 62'. In this implementation, peaked worm teeth 68 interdigitate with peaked drive teeth 70. Since the lateral surface engaged when drive gear 62' is moving the print head are substantially flat and perpendicular to the tangent of gears 60' and 62', the camming action of an angled tooth as previously described does not effectively occur. Since the angular engaged surface is replaced with a more vertical surface, there is minimal tendency for the driving tooth to ride up the other tooth. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims: What is claimed is: 1. A drive mechanism for use in a printer which uses a motor to drive a print head to print consecutive lines of an image on a medium where the consecutive lines are separated by a distance, the drive mechanism comprising a first gear in mechanical communication with the motor and the print head, the first gear having a circumference such that an integral number of revolutions of the first gear moves the print head by the distance between the consecutive lines of the image. 2. The drive mechanism according to claim 1 wherein the first gear has first gear teeth, the drive mechanism further comprising a second gear in mechanical communication with the first gear and the print head, the second gear having second gear teeth adapted to interdigitate with the first gear teeth such that rotation of the first gear moves the print head by the distance. 3. The drive mechanism according to claim 2 wherein the first gear teeth and the second ear teeth have a substantially square shape. 4. The drive mechanism according to claim 2 wherein the first gear teeth and the second ear teeth have a substantially saw-toothed shape. 5. The drive mechanism according to claim 2 wherein the second gear has a second circumference and rotation of the second gear moves the print head by the distance between the consecutive lines of the image. 6. The drive mechanism according to claim 2 wherein the second gear has gear teeth have a substantially constant spacing between the gear teeth. 7. The drive mechanism according to claim 1 wherein the first gear is a worm gear. 8. The drive mechanism according to claim 1 further comprising .an encoder, the encoder rotating at a known rate with respect to linear movement of the print head to govern color registration. 9.. The drive mechanism according to claim 1 wherein a surface of the first gear teeth and a surface of the second gear engage so as minimize lost motion of the print head. 10. The drive mechanism according to claim 1 wherein the first gear provides a substantially constant load on the motor to minimize lost motion. 11. The drive mechanism according to claim 1 wherein the integral number of revolutions minimizes the effect of a change in drive velocity on the distance. 12. The drive mechanism according to claim 1 further comprising an elastic device in mechanical communication with the first gear to minimize vibration in the first gear. 13. The drive mechanism according to claim 1 further comprising reversing means for driving the print head in a reverse direction once the photosensitive medium has been exposed. 14. The drive mechanism according to claim 11 wherein the reversing means drives the print head at a rate faster than a rate which the print head moves in a forward direction. 15. A printer that uses a print head to print consecutive lines of an image at a predetermined interval onto a photosensitive medium, the printer comprising: drive means for generating a rotational drive force, the drive means being disposed within the printer and having a rotational period; and translation means in mechanical communication with the drive means for translating the rotational drive force into linear motion of the print head such that the print head moves the predetermined interval in the rotational period. 16. The printer according to claim 15 wherein the translation means comprises: first gear means in mechanical communication with the drive means for developing a drive force; and second gear means for responding to the drive force by moving the print head by the predetermined interval. 17. The printer according to claim 16 wherein the first gear means and the second gear means each have teeth adapted to interdigitate, the teeth being structured such that substantially vertical edges of the teeth communicate between the first gear means and the second gear means. 18. The printer according to claim 16 wherein the first gear means and the second gear means each have teeth adapted to interdigitate, the teeth on the first gear being spaced apart a distance such that the print head moves the predetermined interval.

Inventors: JOHNSON BRUCE K, JOHNSON, BRUCE, K.
Applicants: POLAROID CORP, POLAROID CORPORATION
Application year: 1998
Main classification: B41J19
IPC codes: B41J2, B41J13, B41J19, B41J23