Automatic print head spacing mechanism for ink jet printer

Abstract: An ink jet printer (10) having a print head (14) for directing an ink jet toward a sheet of paper (16) includes automatic print head spacing apparatus (80) that holds the print head apart from the sheet of paper (16) by a preselected distance. In a preferred embodiment, the printer (10) includes a rotatable drum (20) for supporting the sheet of paper (16) and media securing system (24) of a first predetermined thickness for securing the paper to the drum (20). The print head (14) is positioned at the preselected distance after a major surface (130) of the print head (14) engages and is pushed back by the media securing system (24).

Description: The present invention relates to ink jet computer printers and, in particular, to a mechanism for automatically providing a predetermined separation between an ink jet print head and a page on which an image is to be printed. Many computer printers, including some low resolution ink jet printers, scan a print head back and forth across a print medium (e.g., paper) to print graphics and text images thereon. Printing typically occurs while the print head is scanned in each direction, thereby employing relatively fast bidirectional printing. An ink jet printer projects microscopic ink droplets from the print head onto the paper to form a printed image. Since the print head is typically not in contact with the paper, the droplets are projected to the paper through air. Accordingly, there is a propagation time during which the droplets propagate from the print head to the paper. This propagation time is dependent upon the velocity at which the droplets are ejected from the print head and the distance between the print head and the paper. The print head is scanned across the paper at a scanning velocity. A droplet projected from the print head will have the scanning velocity in the direction the print head is being scanned. A droplet projected toward an image location on the paper must, therefore, be ejected from the print head at an ejection time that occurs before the print head is aligned with the image location. Nominally, the ejection time precedes the alignment of print head with the image location by about the propagation time of the droplet. When printing takes place in only one scan direction, all droplets are subjected to the same scanning velocity. As a result, the alignment of droplets ejected during successive scans is substantially independent of the propagation time of the droplets. In bidirectional printing, however, droplets are subjected to different scanning velocities during the successive scans in opposite directions. As a result, the alignment of droplets ejected during successive scans is dependent upon the propagation time of the droplets (i.e., the velocity at which the droplets are ejected from the print head and the distance between the print head and the paper). The velocity at which the droplets are ejected can be precisely controlled by the print head. Accordingly, the distance between the ink jet print head and the paper must be accurately maintained to provide adequate alignment of the droplets ejected during successive scans in opposite directions. In low speed, low resolution ink jet printers of the type presently available, the distance can be maintained automatically, for example, by a semi-rigid follower wheel that rolls across the paper with the print head as it is scanned. In some other printers, a manual adjustment cam of the type used on conventional typewriters allows a user to select the desired distance. These conventional spacing mechanisms do not provide spacing to within a tolerance adequate for high resolution ink jet printers. Such printers can form images with about 120 dots/cm and require that the distance between the print head and paper be maintained at within a tolerance of about ±0.025 mm. Moreover, such printers are sometimes adapted to print onto media having a wide range of thicknesses. It is known from PATENT ABSTRACTS OF JAPAN, Vol. 12, No. 257 (M-719), 1988 to provide a method of setting the position of a print head at a predetermined separation distance from an ink receiving medium in an ink jet printer, the printer having a print head assembly comprising the print head for directing an ink jet toward the ink receiving medium; media support means for supporting the ink receiving medium to receive the ink jet directed from the print head; media securing means of a first predetermined thickness for securing the ink receiving medium to the media support means; and print head spacing means coupled to the print head assembly for holding the print head apart from the ink receiving medium. The present invention is characterised by the steps of; (a) holding the print head assembly by action of the print head spacing means at a minimum separation from the media support means; (b) securing the ink receiving medium to the media support means by operation of the media securing means; (c) moving the media support means to a position at which the print head assembly is contacted by the media securing means and displaced thereby into a new position in which the separation between the print head assembly and the media support means is determined by the sum of the thickness of the ink receiving medium and the thickness of the media securing means; and (d) locking the print head assembly thereafter in said new position, whereby the thickness of the media support means corresponds to the predetermined separation distance between the print head and the ink receiving medium. According to a further aspect of the present invention there is disclosed an ink jet printer having a print head assembly with a print head for directing an ink jet toward the ink receiving medium, the printer comprising media support means for supporting the ink receiving medium to receive the ink jet directed from the print head; media securing means of a first predetermined thickness for securing the ink receiving medium to the media support means; and print head spacing means coupled to the print head assembly and operable to hold the print head apart from the media support means by a distance which is variable between a minimum separation and a maximum separation, the print head spacing means being operable to permit displacement of the print head assembly from the position of minimum separation to a new position at which the separation between the print head assembly and the media support means is determined by the sum of the thickness of the ink receiving medium and the thickness of the media securing means, and wherein the print head spacing means further comprises locking means operable to hold the print head assembly thereafter in said new position. The present invention thereby automatically provides a spacing mechanism that establishes a predetermined separation between the print medium (e.g. paper) and the print head and maintains the predetermined separation to within a tolerance that is compatible with high resolution ink jet printers. An advantage of the mechanism is that it is compatible with media having a wide range of thicknesses. In a preferred embodiment, the printer includes a rotatable drum for supporting the paper and a paper securing apparatus of predetermined thickness for securing the paper to the drum. During a spacing calibration process, the paper securing apparatus is pressed against the print head to push it away from the paper by the predetermined separation distance. The spacing mechanism secures the print head at the predetermined separation distance to within a tolerance acceptable for high resolution ink jet printing. The printer includes front and rear guide rails along which a carriage supporting the print head slidably moves across the paper, the front guide rail being positioned between the rear guide rail and the drum. The carriage has a fixed-length coupling to the front guide rail, and the spacing mechanism provides an extendable coupling that varies the distance between the rear guide rail and the carriage. Varying the length of the extendable coupling causes the carriage to pivot about the front guide rail and position the print head at the predetermined separation distance. Additional objects and advantages of the present invention will be apparent from the detailed description of a preferred embodiment thereof, which proceeds with reference to the accompanying drawings. Fig. 1 is a simplified isometric view of an ink jet printer that includes an automatic print head spacing apparatus of the present invention. Fig. 2 is a enlarged, fragmentary plan view showing the print head assembly in the printer of Fig. 1. Fig. 3 is an enlarged, isometric view of an automatic print head spacing apparatus of the present invention. Figs. 4A and 4B are respective front and side elevation views of a print head assembly and an associated print head spacing apparatus. Figs. 5A, 5B, and 5C are schematic side view diagrams showing the relationships between the print head and a print media support apparatus prior to, during, and immediately after a predetermined distance between them is automatically provided in accordance with the present invention. Fig. 1 shows a high resolution, full-color ink jet computer printer 10 having an ink jet print head assembly 12 that supports a print head 14 (Fig. 4B). Print head 14 preferably includes 96 orifices (not shown) from which ink droplets are ejected toward a print medium such as a sheet of paper 16. Printer 10 is capable of printing on a variety of print media including transparent films and labels. The following description refers specifically to paper 16 for purposes of simplicity. Paper 16 is mounted on the outer surface 18 of a media support means or drum 20. Paper 16 is fed through a pair of media feed rollers 22a and 22b and secured to surface 18 by a media securing means or system 24. Securing system 24 includes a media clamp 26 that receives and clamps the side margin of a leading end of the sheet of paper 16 against drum 20. Media clamp 26 slides into and remains stationary within a slot 28 in drum 20. A drum motor (not shown) slowly rotates drum 20 in a direction 34 about an axis 36 extending through the center and along the length of drum 20, thereby pulling the sheet of paper 16 through media feed rollers 22a and 22b and under a back tension blade 38 that is held under tension against surface 18 by a spring 40 (Fig. 5C). The sheet of paper 16 slides under and is held against surface 18 by back tension blade 38 as drum 20 rotates. A print head positioning system 50 includes a carriage 52 slidably mounted on a pair of spaced apart, parallel guide rails 54a and 54b and supporting print head assembly 12. A carriage drive belt 56 is attached to carriage 52 and held under tension by a pair of spaced apart belt pulleys 58a and 58b. A carriage motor 60 linked to pulley 58a drives carriage 52 in directions 62a and 62b along guide rails 54a and 54b. Carriage 52 includes bushings 64a and 64b (Fig. 4B) that allow carriage 52 to slide smoothly along rails 54a and 54b. While printing text or graphics images on the sheet of paper 16, the drum motor rotates drum 20 about axis 36 in incremental angular steps at about 1.5°/second. Simultaneously, carriage motor 60 drives carriage 52 along guide rails 54a and 54b at a speed of about 66 cm/second, and a printer controller 70 delivers print control signals to a control input 72 of print head 14. In response to the print control signals, print head 14 ejects ink droplets directed toward the sheet of paper 16 supported on surface 18 of drum 20. The ink is preferably of a hot melt type that is contained in and heated by an ink supply chamber 74 and an ink reservoir 76 of print head assembly 12. The print control signals are delivered to print head 14 while carriage 52 is driven in both directions 62a and 62b, thereby providing boustrophedon or bidirectional printing in which successive image lines are printed alternately in directions 62a and 62b. Printer 10 is capable of forming a full-page, full-color image at a resolution of about 120 dots/cm in about 2 minutes. With reference to Figs. 1, 2, and 3, carriage 52 includes an automatic print head spacing apparatus 80 that provides a predetermined separation distance 82 between print head 14 and the sheet of paper 16. Separation distance 82 must be accurately maintained so that a droplet directed to an image location on a line formed during a scan in one direction will be properly aligned with an adjacent image location on the line formed during the next succeeding scan in the opposite direction. At a resolution of 120 dots/cm, the nominal center-to-center spacing of droplets is 0.084 mm, and each droplet is positioned on the sheet of paper 16 within a tolerance of ± 0.038 mm. To achieve such performance, spacing apparatus 80 automatically positions print head 14 such that separation distance 82 is 0.8 mm ± 0.025 mm. Spacing apparatus 80 includes a mounting plate 84 with slotted apertures 86 through each of which a screw 88 extends to hold mounting plate 84 against ink supply chamber 74. A guide rail spacing element 90 supporting bushing 64a at one end is slidably secured to mounting plate 84 by a pair of parallel brackets 92a and 92b. An extension locking member 94 that is substantially perpendicular to spacing element 90 is slidably secured to mounting plate 84 by a pair of parallel, U-shaped brackets 96a and 96b. Locking member 94 includes a ramp section 98 that is frictionally engaged by an arcuate slider 100 (Fig. 4A) of spacing element 90. Slider 100 is held against ramp section 98 under pressure by a coil spring 102 that encircles a stem 104 of spacing element 90 and extends between bracket 92a and a shelf 106 on spacing element 90. Spring 102 has a free length of 3.49 cm ± 0.010 cm, and has a compression range of 1.24 - 1.94 cm over which respective forces of 1.5 - 1.66 lb ± 10% are exerted. A torsion spring 108 extends between mounting plate 84 and spacing member 90 to prevent the latter from twisting about its length. Twisting of spacing member 90 interrupts the frictional contact between slider 100 and ramp section 98, thereby causing slippage between them. Torsion spring 108 exerts a torque of 0.31 cm - lb on spacing member 90. Locking member 94 cooperates with spacing element 90 to space mounting plate 84 apart from guide rail 54a by varying distances. Spacing apparatus 80 functions, therefore, as an extendable coupling between guide rail 54a and print head assembly 12. Varying the length of the extendable coupling causes print head assembly 12 to pivot about guide rail 54b and change separation distance 82 between print head 14 paper 16. Preferably, the length of the extendable coupling is capable of varying by about 2 mm to provide a variation of about 1.25 mm in separation distance 82, thereby allowing printer 10 to accommodate paper 16 and other print media of thicknesses between 0.025 and 0.38 mm. The invention is not, however, limited to this range of media thicknesses. With reference to Fig. 4A, slider 100 is reset to a low portion 120 of ramp section 96 immediately prior to securing a sheet of paper 16 to drum 20. This provides a maximum distance between print head assembly 12 and guide rail 54a and, therefore, a minimum separation between print head 14 and surface 18 of drum 20. Slider 100 is set to low portion 120 by moving carriage 52 in direction 62a so that an arm extension 122 of locking member 94 contacts a locking member stop 124 positioned at one side of printer 10. Carriage 52 continues in direction 62a until locking member 94 is slid across mounting plate 84 so that slider 100 reaches low portion 120. With reference to Fig. 5A, print head 14 is positioned at the minimum distance from surface 18 of drum 20 after arm extension 122 resets slider 100 to low portion 120. Back tension blade 38 carries on its outer surface an embossed segment 126 that is spaced from print head 14 by about 0.13 mm, thereby allowing either of a pair of rigid print head safety ramps 128 to pass by embossed segment 126 without contacting it. Safety ramps 128 protect print head 14 from being damaged if print head assembly 12 inadvertently strikes an object as the assembly is scanned along guide rails 54a and 54b. With reference to Fig. 5B, a sheet of paper 16 (shown separated from surface 18 for purposes of clarity) secured by media clamp 26 is rotated under back tension blade 38 so that embossed segment 126 engages or is pressed against a major surface 130 of print head 14. As a result, print head assembly 12 is pushed back toward guide rail 54a such that mounting plate 84 is forced downward along spacing member 90. Simultaneously, slider 100 is raised off of ramp section 98. A spring 132 (shown in broken lines in Fig. 4A) extending from mounting plate 84 to the end of locking member 94 opposite arm extension 122 retracts when slider 100 is raised. Locking member 94 slides across mounting plate 84 until a higher portion of ramp section 98 re-engages slider 100, thereby securing slider 100 in place. As a result, print head 14 is secured in place with a separation distance 82 established by the combined thicknesses of embossed segment 126 on back tension blade 38, media clamp 26, and the sheet of paper 16 or other print media. Spring 132 has a free length 4.35 cm ± 0.013 cm, and has an extension range of 6.6 - 8.6 cm over which respective forces of 15 - 25 gm ± 10% are exerted. Fig. 5C shows print head 14 spaced apart from paper 16 by separation distance 82 after media clamp 26 is rotated past back tension blade 38. In the preferred embodiment, media clamp 26 and back tension blade 38 have respective thicknesses 138 and 140 of 0.30 mm, and embossed segment 126 has a thickness 142 of 0.20 mm. Printer 10 is capable of receiving paper 16 and other print media with a thickness 144 of between 0.025 and 0.38 mm. Accordingly, the predetermined separation distance 82 between print head 14 and paper 16 is the sum of thicknesses 138, 140, and 142, which is about 0.80 mm ± 0.025 mm. To prevent it from damaging or misaligning print head 14, embossed segment 126 engages surface 130 of print head 14 at a preselected location at which there is a reduced possibility of damaging print head 14. The preselected location is above the positions of the print head orifices (indicated by arrow 146 in Figs. 5A, 5B, and 5C) and behind which location there are no ink-carrying manifolds. Moreover, the preselected location and embossed segment 126 are located at about the middle of the lengths of print head 14 and back tension blade 38, respectively. These locations minimize the variations in separation 82 caused by slight skewing of print head 14 on carriage 52 or by guide rails 54a and 54b not being parallel to axis 36 of drum 20. It will be obvious to those having skill in the art that many changes may be made in the above-described details of the preferred embodiment of the present invention without departing from the underlying principles thereof. For example, an alternative to arm extension 122 and locking member stop 124 for resetting slider 100 could include a solenoid-controlled rotating cam held under tension by a spring. Moreover, an alternative to slider 100 and ramp section 98 for securing spacing member 90 in place could include a tensioned pivoting locking arm through which a spacing member could freely slide in only one direction.

Claims: A method of setting the position of a print head (14) at a predetermined separation distance (82) from an ink receiving medium in an ink jet printer, the printer having a print head assembly (12) comprising the print head (14) for directing an ink jet toward the ink receiving medium (16); media support means (20) for supporting the ink receiving medium (16) to receive the ink jet directed from the print head (14); media securing means (24) of a first predetermined thickness for securing the ink receiving medium (16) to the media support means (20); and print head spacing means (80) coupled to the print head assembly (12) for holding the print head apart from the ink receiving medium (16); the method being characterised by comprising the steps of; (a) holding the print head assembly by action of the print head spacing means at a minimum separation (142) from the media support means; (b) securing the ink receiving medium to the media support means by operation of the media securing means; (c) moving the media support means to a position at which the print head assembly is contacted by the media securing means and displaced thereby into a new position in which the separation between the print head assembly and the media support means is determined by the sum of the thickness of the ink receiving medium and the thickness of the media securing means; and (d) locking the print head assembly thereafter in said new position, whereby the thickness of the media support means corresponds to the predetermined separation distance between the print head and the ink receiving medium. A method as claimed in claim 1 wherein the surface of the print head assembly (12) that engages the media securing means (24) includes a major surface of the print head (14). A method as claimed in claim 1 and wherein the printer comprises first and second guide rails (54b, 54a) along which the print head assembly (12) is slidably carried, the print head assembly (12) having a fixed slidable coupling (52) to the first guide rail (54b) and the print head spacing means (80) including an extendable coupling (80) of variable length between the second guide rail (54a) and the print head assembly (12) for holding the print head (14) apart from the ink receiving medium (16) by the preselected distance, the step of displacing the print head assembly comprising varying the length of the extendable coupling (80) to cause the print head (14) to pivot about the first guide rail (54b) and position the print head (14) at the preselected distance. A method as claimed in claim 3 wherein the first guide rail (54b) is positioned between the second guide rail (54a) and the media support means (20). A method as claimed in claim 3 or claim 4 wherein the extendable coupling (90) includes a mounting plate (84) fixedly mounted to the print head assembly (12) and slidably supporting a guide rail spacing member (90), the method including the step of displacing the print head assembly by spacing the mounting plate (84) apart from the second guide rail (54a) by varying distances. A method as claimed in claim 5 wherein the extendable coupling (80) includes an extension locking member (94) that engages and holds in place the guide rail spacing member (90), thereby to hold the print head (14) at the preselected distance. A method as claimed in claim 6 wherein the guide rail spacing member (90) is slid during the displacement step in a first direction along the mounting plate (84) and the extension locking member (94) includes a cross arm (122) supported by the mounting plate (84) and in frictional engagement with the guide rail spacing member (90), the cross arm (122) being slid across the mounting plate (84) in a direction transverse to the first direction. A method as claimed in claim 6 wherein the guide rail spacing member (90) is slid during the displacement step in a first direction along the mounting plate (84) and the extension locking member (94) includes a cross arm (122) supported by the mounting plate (84) and having a ramp segment (98) in frictional engagement with the guide rail spacing member (90), the cross arm (122) being slid across the mounting plate (84) in a direction substantially perpendicular to the first direction. A method as claimed in any preceding claim wherein a leading edge of the ink received medium is secured by a media clamp (26) and a back tension blade (38) engages a major surface of the ink receiving medium (16). A method as claimed in claim 9 wherein the surface of the print head assembly is engaged by a spacing element (126) of the back tension blade (38). An ink jet printer having a print head assembly (12) with a print head (14) for directing an ink jet toward the ink receiving medium (16), the printer comprising media support means (20) for supporting the ink receiving medium (16) to receive the ink jet directed from the print head (14); media securing means (24) of a first predetermined thickness for securing the ink receiving medium (16) to the media support means (20); and print head spacing means (80) coupled to the print head assembly (12) and operable to hold the print head apart from the media support means by a distance which is variable between a minimum separation and a maximum separation, the print head spacing means being operable to permit displacement of the print head assembly from the positon of minimum separation to a new position at which the separation between the print head assembly and the media support means is determined by the sum of the thickness of the ink receiving medium and the thickness of the media securing means, and wherein the print head spacing means further comprises locking means (94) operable to hold the print head assembly thereafter in said new position. A printer as claimed in claim 11 wherein the surface of the print head assembly (12) that engages the media securing means (24) includes a major surface of the print head (14). A printer as claimed in claim 11 and comprising first and second guide rails (54b, 54a) along which the print head assembly (12) is slidably carried, the print head assembly (12) having a fixed slidable coupling (52) to the first guide rail (54b) and the print head spacing means (80) including an extendable coupling (80) of variable length between the second guide rail (54a) and the print head assembly (12) for holding the print head (14) apart from the ink receiving medium (16) by the preselected distance, whereby varying the length of the extendable coupling (80) causes the print head (14) to pivot about the first guide rail (54b) and position the print head (14) at the preselected distance. A printer as claimed in claim 13 wherein the first guide rail (54b) is positioned between the second guide rail (54a) and the media support means (20). A printer as claimed in claim 13 or claim 14 wherein the extendable coupling (90) includes a mounting plate (84) fixedly mounted to the print head assembly (12) and slidably supporting a guide rail spacing member (90) for spacing the mounting plate (84) apart from the second guide rail (54a) by varying distances. A printer as claimed in claim 15 wherein the locking means includes an extension locking member (94) that engages and holds in place the guide rail spacing member (90), thereby to hold the print head (14) at the preselected distance. A printer as claimed in claim 16 wherein the guide rail spacing member (90) is slidable in a first direction along the mounting plate (84) and the extension locking member (94) includes a cross arm (122) supported by the mounting plate (84) and in frictional engagement with the guide rail spacing member (90), the cross arm (122) being slidable across the mounting plate (84) in a direction transverse to the first direction. A printer as claimed in claim 16 wherein the guide rail spacing member (90) is slidable in a first direction along the mounting plate (84) and the extension locking member (94) includes a cross arm (122) supported by the mounting plate (84) and having a ramp segment (98) in frictional engagement with the guide rail spacing member (90), the cross arm (122) being slidable across the mounting plate (84) in a direction substantially perpendicular to the first direction. A printer as claimed in any of claims 11 to 18 wherein the media securing means (24) includes a media clamp (26) for securing a leading edge of the ink receiving medium (16) and a back tension blade (38) that engages a major surface of the ink receiving medium (16). A printer as claimed in claim 19 wherein the back tension blade (38) includes a spacing element (126) that engages the surface of the print head assembly. element (126) of the back tension blade (38).

Inventors: CARPENTER CLAYTON W, HANKS DAVID W, MACLANE DONALD B, VAN HORNE ARTHUR C, CARPENTER, CLAYTON W., HANKS, DAVID W., MACLANE, DONALD B., VAN HORNE, ARTHUR C.
Applicants: TEKTRONIX INC, TEKTRONIX, INC.
Application year: 1992
Main classification: B41J25
IPC codes: B41J13, B41J25, B41J2