This invention relates to vector plotters, i.e., plotters which operate in a vector mode as opposed to the raster scan mode of dot matrix printers, and, more particularly, in a vector plotter having a plotter head mounted for relative movement over the surface of a plotting media in an X and Y coordinate system to plot vectors in any desired direction, to the improvement comprising a projecting dot-producing printhead carried by the plotter head at a printing distance above the surface of the plotting media.

Vector plotters and raster scan printers using projecting (printwire) dot-producing impact printheads are very different devices which operate from different computer drawing file outputs (e.g., HPGL plot files as opposed to raster scan print files) and have completely different mechanical and electrical architectures. Vector plotters can produce lines (vectors) in any desired direction by moving a carriage, carrying a line producing means (e.g. a pen), relative to a plotting media (e.g. paper) , simultaneously in an X and Y coordinate system, along the line being produced. During plotting the line producing means and plotting media experience significant changes in relative velocity due to stopping and starting accelerations with the relative velocity often never being constant at any time during production of a line.

Projecting dot-producing impact printers for raster printing have been commercially available for 20 years or more. During that time they have undergone continuous development leading to significant increases in printing rates, print quality, life, reliability and ease of use.

Printers using projecting dot-producing impact printheads are dot matrix printers using a raster scanning motion of the printhead relative to a printing media (e.g. paper) in which that relative motion is always in the same one of X and Y coordinates and is of substantially constant velocity while the printhead is printing dots to produce a matrix of dots representing desired indicia with relative motion in the other of the X and Y coordinates occurring only while printing of dots is not taking place. So-called raster scan plotters also operate in the same raster scan fashion, rather than in a vector mode.

Commercial pen-type vector plotters have from their introduction some 15 years ago, even to this day, suffered from significant problems of drafting rate, drafting quality, reliability and difficulties in set-up and operation. A typical vector plotter according to prior art techniques commonly accepted is shown in Figure 1 where it is generally indicated as 10. A sheet of paper or other plotting media 12 is gripped by pinching and driving rollers 14, 16, respectively, at its two side edges, a roller 16 being driven by a D.C. Servo motor 17. By movement of the driving rollers 16 under the control of a computer (not shown) the media 12 is moved forward and backward (out of and into the drawing figure as it is viewed) under a plotting head 18 to move the media 12 in one direction (e.g. X) of a two-dimensional coordinate system. The plotting head 18 is slidably mounted on a track 20 and is moved orthogonally to the X direction (e.g. in the Y direction or left and right as the figure is viewed) by a drive belt or cable 22 supported and driven by the two rollers 24, one of which is driven by a D.C. Servo motor 25. Thus, by combined movement of the media 12 and the plotting head 18, the printhead can be

moved along any desired path to any point on the plotting surface of the media 12 from any other point.

The actual creation of the plot on the media 12 is done by a writing instrument 26 carried by the plotting head 18. Most often, the writing instrument 26 is an ink pen. When using ink pens, to draw a different width line or to draw in a different color requires that the pen be changed. This is accomplished in one of two ways — either the plotting head 18 is moved off-plot to a pen- changing location and one pen in the printhead 18 is changed for another, often manually after a pause command; or, a turret holding multiple pens is carried by the plotting head 18 and rotated to a new pen position. Either approach results in lost plotting time and throughput. A moving turret, of course, has a high mass and, therefore, slows down the speed of movement and increases the reversal time of the printhead. Having a moving turret is also a complex design problem which will not be addressed in any particular depth herein. For the foregoing reasons, having the plotting head 18 go off-plot is the more common approach. The time to change pens is, of course, lost time.

Another time loss (wasting on average about 50% of the operating time of the plotter overall and much more during vector printing of text) in conventional vector plotters employing a writing instrument 26 as in Figure l is the raising and lowering of the writing instrument 26 when not actually drawing as the plotting head 18 or media 12 is moved to a new relative position. The writing instrument 26 is raised sufficiently to clear the media 12 and then moved to the next plotting position. At that point, it is lowered into contact with the media 12 before plotting resumes. The greater the number of vectors

(especially short vectors) comprising a plot, the greater the percentage of lost time (and throughput) expended in raising and lowering the pen.

Also, plotting speed is limited by the speed at which the writing instrument 26 can transfer its ink or other plotting material onto the medium 12. Problems of variations in ink supply or clogging of the pens together with limited maximum controlled ink flow rates also still exist. This, of course, is also a factor of the quality of the medium 12, the plotting material, and the transfer technique. A cheap "felt-tip" pen writing on a cheap paper will quickly beginning skipping and producing uneven line quality when the plotting speed is increased. Pens employing liquid inks and high quality metal or ceramic tips, on the other hand, can produce high quality plots at much higher speeds. Even with high quality pens sensitivity of the pens to choice of paper and limitations dictated by capillary feeds often dictate a lower plotting speed for satisfactory results; and limitations imposed on maximum acceleration and deceleration of pens by the pen itself and its friction contact with the paper (in normal operation the pen is accelerating or decelerating for a significant portion of its down time) also impose lower plotting rates. The problem is that the user quite often is unaware of these facts and will blame the manufacturer of a plotter for poor plotting quality when it is the materials used that are to blame. Thus, plotters may be produced or used by customers well below their possible maximum plotting speeds just to assure that such problems do not exist. Even today most plotters, offering up to 32 i.p.s. operating speed in the x and y coordinates, suggest that the coordinate speed be limited to 12 i.p.s. for quality drafting. In addition, line width control is

difficult with the use of pens as thickening tends to occur where the pen is lifted (stopped) and lowered (started) .

Set-up and operation of vector plotters require considerable expertise if the plotter is to operate up to its performance potential. These problems, stemming from the multitude of pens needed for different line widths and colors, have led to the use of multi-pen carousels (often involving multiple interchangeable carousels for a single drafting operation) , e.g. the production of patent drawings, with the attendant difficulties in pen placement/interchange in relation to software and physical association with the drafting paper. Not only does the sensitivity to choice of paper effect both drafting rate and quality, it also adds to the expertise needed to set¬ up and operate such plotters.

It cannot be denied that today's commercial pen-type vector plotters are greatly superior or the first commercial units. However, the problems asserted above are still significant and are the problems addressed by the inventors of the present invention.

A number of attempts have been made to overcome the limitations of the use of pens in vector plotters by the substitution of ink jet printheads, which unlike projecting dot-producing impact printheads are considered non-impact printheads (see Jerome L. Johnson, "Principles of Non-Impact Printing, A Unified Description of All Non- Impact Printing Technologies Based On Fundamental Principles", Palatino Press, Irvine, CA, 1986). These attempts have met with significant problems and, although the patent literature includes a significant number of proposals with respect to such a combination, no practical and effective ink jet vector plotter has been marketed

commercially. The reasons for the problems encountered with the use of ink jets in vector plotters are many and diverse. They include:

1) limited maximum acceleration for effective operation (approximately 3 g) of ink jet printheads compared to that normally encountered in vector plotters (4 g or more) ;

2) problems associated with causing the jetted ink to hit the printing media at a desired spot when a vector being plotted is changing direction;

3) the problem of different colors requiring different ink jet printheads;

4) a sensitivity to media which is consistent with the sensitivity of vector plotter pens; and 5) the problem of retaining ink in a desired portion of the reservoir during acceleration and deceleration.

These problems have so far rendered an ink jet printhead an unsatisfactory substitute for pens to overcome the problems of vector plotters herein discussed.

Wherefore, it is an object of this invention to provide a vector plotter wherein the problems associated with a writing instrument that must be raised and lowered are eliminated. It is another object of this invention to provide a vector plotter wherein the problems associated with a writing instrument that must transfer a plotting material to the plotting media through a dragging transfer motion are eliminated. It is still another object of this invention to provide a vector plotter wherein the problems associated with slow pen change times are eliminated.

It is yet another object of this invention to provide a vector plotter of substantially conventional design wherein the problems associated with changing instruments to change line width are eliminated. The inventors discovered that the above and other objects may be achieved in accordance with the invention by a vector plotter operating in vector mode plotting (as opposed to raster scan printing mode) utilizing a projecting dot-producing impact printhead carried by the plotter at a printing distance from the surface of the plotting media (and the platen) , despite initial thinking that projecting, dot-producing impact printheads in vector mode plotting would not be as fast as existing pen plotters, or that they could produce plots of quality as good as those produced by existing pen plotters, and despite the formidable problems discussed below which initially or during development weighed against adoption of projecting, dot-producing impact printheads for vector mode plotting. It is another object of this invention to provide such a vector plotter which fulfills the aforementioned objects and which, other than for structure cooperating with the projecting dot-producing impact printhead such as the platen and control of relative movement between the ribbon and the printhead, is substantially of conventional design and uses already existing plotter software.

In implementing this last object, the inventors have identified a number of problems in the utilization of impact printheads in vector plotters. These include l) the need to provide an adequately constant printing distance between the printhead and the platen throughout the length of the platen traversed by the printhead during plotting operations in the light of the

limited projection of the printwire of an impact printhead to produce a printed dot on a printing media supported by the platen;

2) the need to address potential limitations on print ribbon life where vectors are printed utilizing dots produced by a single printwire;

3) the avoidance of line fade in a vector being produced by an impact printhead as a result of insufficient relative motion between a print ribbon and the impact printhead;

4) the production of constant dot spacing in the production of a vector by an impact printhead in the light of the substantial variations in plotting velocity between the printhead and print media which occurs in vector plotters;

5) the achievement of constant density lines where different lines are produced by printwires of different diameters;

6) removal of unwanted debris from a print ribbon during use; and

7) providing for a multi-color operation utilizing an impact printhead in an efficient manner.

The problem associated with providing a constant printing distance facilitating use of a projecting dot- producing impact printhead is exacerbated by the fact that the carriage traverse relative to the platen of many vector plotters is substantially greater (often 36 inches or more) than exists in dot matrix printers.

It is yet another object of the this invention to provide practical and economical solutions to aforementioned problems of the utilization of an impact printhead in a vector plotter.

Summarv:

According to the invention there is provided a vector plotter for plotting a vector in any desired direction in an X and Y coordinate system on a plotting media, comprising a plotter head, said plotter head and said media being mounted for simultaneous relative movement in said X and Y coordinate system to move said plotter head relative to said media along said vector while said plotter head plots said vector on the plotting media, a projecting dot-producing impact printhead, carried by said plotter head at a constant printing distance from a surface of the plotting media which repeatedly projects at least one printwire to impact the plotting media to produce said vector; and logic means within said vector plotter to control said plotter head movement and operation of said printhead to produce said vector. Description of the Drawings:

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a simplified drawing of a prior art vector plotter employing a pen on the printhead;

Figure 2 is a simplified drawing of a vector plotter of the general type shown in Figure 1 modified according to the present invention in its most basic embodiment;

Figure 3 is a simplified drawing of a preferred configuration of the printhead of the present invention employing multiple dot-producing printheads, respectively, to produce dots of different diameters; Figure 4 is a simplified drawing of a vector plotter of the general type shown in Figure 1 modified according to the present invention in a preferred embodiment

incorporating dynamic rebound assistance to reduce the turn-around time of the printhead;

Figure 5 is a prior art ribbon which can be employed in a vector plotter according to the present invention shown adapted to be controlled according to the present invention;

Figure 6 is a simplified drawing showing how a vector plotter according to the present invention can be modified to produce color plots according to an alternate embodiment thereof;

Figure 7 is a novel graphite-based ribbon according to the present invention which can be employed in the vector plotter of the present invention to produce erasable drawings; Figure 8 is an enlarged cross-section through the ribbon of Figure 8 in the area designated as VIII;

Figures 9a and b are simplified drawings of alternative printwire organization in a multiple printwire printhead according to the present invention; Figure 10 is a simplified drawing of printwire organization in a multiple printwire printhead according to the present invention in another embodiment;

Figure 11 is a simplified drawing of printwire organization in a multiple printwire printhead according to the present invention in a hybrid embodiment;

Figure 12 is a diagrammatic cross-sectional elevation on section line 12-12 of Figure 14 illustrating a ribbon transverse oscillation mechanism for use on a carriage of the present invention; Figure 13 is a diagrammatic elevation similar to that of Figure 12 with the ribbon oscillating mechanism shown in a ribbon changing position;

Figure 14 is a front elevation of the mechanism of Figure 12;

Figure 15 is a plan view of the mechanism illustrated in Figure 12 with certain elements omitted; Figure 16 is a diagrammatic illustration of an interior portion of a cassette according to one embodiment of the present invention incorporating means for removing undesired debris from a ribbon as it returns to the cassette from the carriage; Figure 17 is a diagrammatic cross-section along section line 17-17 of Figure 16;

Figure 18 is a front elevation of a plotter frame with a guide bar/platen assembly mounted thereto by a 3 point mounting system in accordance with one embodiment of the invention;

Figure 19 is a diagrammatic sectional elevation taken on section 19-19 of Figure 18; and

Figure 20 is a diagrammatic sectional elevation taken on section 20-20 of Figure 18. Description of the Preferred Embodiments:

The critical portions of a vector plotter according to the present invention in its most basic embodiment are shown in simplified form in Figure 2. The vector plotter 10' has the pen-holding portions of the plotting head 18 replaced by a projecting dot-producing impact printhead 28. Most noticeably, whereas the prior art vector plotter 10 of Figure 1 produces its "vectors" (from dots to extended lines) by contacting the surface of the plotting media 12 with a plotting tip of the pen carried thereby, the dot-producing printhead 28 of this invention essentially "projects" its dots by impact through a ribbon. In the detailed description of the several embodiments which follows hereinafter, an impact dot-

printer of the type employed in so-called "dot-matrix" printers will be used as the primary and preferred example.

As shown in Figure 2, the dot-producing printhead 28 is carried by the plotting head 18 with the printing face 30 thereof closely adjacent and parallel to the plotting media 12 as the plotting head 18 moves back and forth along its support track 20. A ribbon 32 containing the plotting material (such as ink) is disposed under the printing face 30. Thus, as the plotting head 18 moves across the plotting media 12 and platen 21, there is nothing to be raised and lowered, as necessary to meet the stated objects. Any place where plotting is to take place, a conventional activation signal is input to the printhead 28 on line 34 causing the tips of the dot- producing printwires or pins (not shown) to emerge from the printing face 30, strike the ribbon 32, and transfer a dot of the plotting material to the media. It is preferred that the ribbon 32 be contained in a removable and replaceable cassette 36 such as that depicted in Figure 5. Novel cassettes according to this invention will be addressed later herein. It should also be noted that the construction and operation of dot-matrix type printheads wherein a printing pin is driven from the printing face 30 of the printhead 28 by magnetic drivers, or the like, is well known to those skilled in the art. Since conventional vector plotters come in many sizes, styles, and modes of construction, those things necessary to remove the pen holder, mount the printhead 28, and mount the ribbon cassette 36 therein will vary from plotter to plotter; however, the accomplishment thereof is something that can be accomplished by those skilled in the art without undue experimentation. Wherefore, such

details are eliminated herefrom in the interest of simplicity and the avoidance of redundancy. Thus, the drawings and descriptions hereof are limited to providing details of those aspects which are at the points of novelty of the invention. Having thus addressed the present invention in its most basic form wherein the pen, pen holder, and pen raising and lowering mechanism of a conventional vector plotter have been removed from the plotting head 18 and replaced by the dot-producing printhead 28, we will now direct our attention to various additional modifications which can be made to further improve the capabilities and performance of a vector plotter according to the present invention.

Turning first with particularity to Figure 3, shown therein is a preferred approach to the printhead 28 whereby the printhead 28 can produce various dot sizes and line widths. In other words, this is the modification which eliminates the time losses associated with carrying a turret on the plotter head 18 or moving the plotter head 18 off-plot to change pens. To accomplish this, the printhead 28 actually comprises multiple printheads 28' (i.e. single printwire drivers labelled "HEAD #1", "HEAD #2", "HEAD #3", and "HEAD #4", respectively). In a tested embodiment, there were four printheads 28' as shown wherein each printhead 28' was separately addressable through in the input line 34. Each printhead 28' produces a different sized dot (i.e. its dot-printing printwire is of a different diameter) . Thus, to change line/dot width, the controlling computer merely has to address a different printhead 28'. As can be appreciated, there is virtually no lost time whatsoever. As those skilled in the art will readily recognize and appreciate, however, the logic 38 providing the X,Y positional signals to the driving

rollers 16 and the plotter head 18 must include logic to adjust the position for the offset of the selected printhead 28'. This logic can be incorporated into the computer program driving the plotter 10' or can be incorporated into the plotter 10' itself (the preferred approach) so that standard programs such as those performing CAD/CAM functions can function normally and have the fact that the plotter 10' is non-pen oriented be transparent to them. Thus, a CAD program, for example, can output a vector from a first point X^Y-- _, to a second point X Ϊ ₂ ^w i ^{tn a} given "pen" number and the above- described correctional logic incorporated into the plotter 10' will automatically adjust the positions throughout the drawing of the vector for the offset of the selected pen (i.e. printhead 28').

Another preferred modification to the "standard" vector plotter 10 of Figure 1 to achieve improved performance according to the novel aspects of the present invention is shown in Figure 4. As described in detail with respect to dot matrix printers in U.S. Patents 4,889,438 and 4,948,280 which are assigned to the common assignee of this application, there are techniques available for rapidly reversing the direction of a printing head moving in one direction. By adapting those techniques to the plotting head 18 of the vector plotter 10" as depicted in Figure 4, direction reversal of the plotter head 18 and the printhead 28 it carries can be greatly improved. One technique of that above-described application which is particularly applicable to the construction of a "standard" vector plotter modified to incorporate the dot-printing improvements of this invention is to mount tape gripping ballistic reversers 40 at the two ends of travel of the plotting head 18. The

drive tape 22 carrying the plotting head 18 passes through jaws contained in the reversers 40. Upon receiving a direction reversal command, the control logic 38 sends a command to the reverser 40 opposite the direction in which the plotting head 18 is presently moving. The jaws in the reverser 40 immediately grip the tape 22 stopping the movement of the plotting head 18 against the inertial force tending to keep it moving in its present direction. A spring member within the reverser 40 absorbs the inertial force and then rebounds to snap the plotting head 18 back in the opposite (i.e. reversed) direction. Thus, the time of reversal at any point along the path of travel of the plotting head 18 in either direction is greatly reduced, thereby improving the overall throughput of the plotter 10".

Alternatively the rotary ballistic rebound reversal embodiments of the aforesaid patent application could be used in conjunction with the drive motor (25) for the head drive tape 22. In addition, similar energy storage/return devices could be used in conjunction with drive motor (17) for the rollers 14, 16 which drive the media 12 in the X direction.

Another possible modification to the present invention which provides the ability to print in multiple colors is shown in Figure 6. In this case, the ribbon 32 comprises parallel bands 42 of colored ink as the plotting material within the ribbon 32. For example, one might choose to employ the three basic colors, for additive or subtractive color production, and black as the bands 42. In that way, full color plots can be produced by multiply placing dots at various positions according to techniques well known by those in the printing arts. To accomplish the multiple color plotting desired, shifting apparatus 44

connected to the control logic 38 is connected to the ribbon 32 to shift it forward and backward (down and up as the figure is viewed) as indicated by the arrow 46 so as to place the selected color band 42 under the presently active printhead 28'.

Erasable (e.g. pencil) drawings can be produced by employing the unique ribbon cassette 36' of this invention as depicted in Figure 7. The cassette 36' has a folded ribbon 32' within a plastic case 48 as with the prior art cassette 36 of Figure 5. Likewise, as with the prior art cassette 36, there is a drive roller 50 with accompanying pinch roller which pulls the ribbon 32' across the exposed printing area 52. In the cassette 36', however, the ribbon 32' preferably has a flexible base strip 54 made of paper of between .001" and .0015" in thickness or polyester of between .001" and .002" in thickness as these materials seem to give better performance with respect to the method for transfer and retention of the graphite compound to be described which is employed therein. Of the two, polyester is preferred because of its greater durability. As shown in the enlarged cross-sectional drawing of Figure 8, the printing side (i.e. the bottom) 56 of the base strip 54 is coated with a gripping layer (approximately one-half mil) 58 of silica in the form of silicate. For greater durability, the silica of the layer 58 could be replaced with alumina. The gripping layer 58 is then impregnated with a clay/graphite compound 60 of the usual type for such printing purpose?.

The most novel aspect of the cassette 36' is that the clay/graphite compound 60 is continually replenished as it is used in a manner which assures that the proper amount is available for printing; but, excesses or deficiencies are eliminated. To accomplish this, there is a block 62

of the clay/graphite compound 60 attached to one end of a torsion spring 64. The other end of the torsion spring 64 is attached to the case 48 at 66. The block 62 is pressed against the printing side 56 of the ribbon 32' in a convenient place such as on the periphery of the drive roller 50 as shown by the force of the torsion spring 64. A polyurethane foam pad 68 mounted on a spring 70 is positioned as shown to brush off loose particles of the compound 60. The materials and construction as shown have the performance characteristic that only that amount of the compound 60 needed to replace that used is transferred to the ribbon 32'. This if for the following reason. The drag force (D) on the ribbon 32' by the block 62 is equal to ≤ ₈ N where N is equal to the spring force of the torsion spring 64 and ≤ _g is the kinetic friction coefficient. Note, however, that ≤ ₈ declines as the gripping layer 58 is impregnated with graphite (a very low frictional material often employed for dry lubrication purposes) . Therefore, less torque is required to move the ribbon 32' through the pinching motion of the block 62 against the ribbon 32' and, correspondingly, less graphite is required if the plotting rate is relatively low. This means that graphite is added only to areas of the gripping layer 58 that have been depleted. As can be appreciated, to operate as described above, the torsion spring 64 must provide enough force (N) to cause transfer of the graphite compound 60 in those areas of the gripping layer 58 where some is needed due to depletion and must low enough to not transfer in those areas where the kinetic friction coefficient ≤ _B has been reduced due to the sufficient presence of the graphite compound 60.

Another consideration relative to the ribbon cassette 36 for preferred operation is driving of the ribbon 32 by

the drive roller 50. As depicted in Figure 5, it is usual in prior art dot matrix printers to have such ribbon cassettes connected to a motor drive 72 which moves the ribbon 32 across the printing area 52 of the cassette 36 so that the dot matrix printhead employed is constantly striking a new area of the ribbon 32 and so that the ink in the ribbon 32 is uniformly consumed. This does not present a problem as a dot matrix printer only moves its printhead across the paper (and thereby across the ribbon 32) in a fixed direction and at a constant rate. Thus, the ribbon 32 can be driven in a direction opposite the direction of printhead movement to accomplish the above- described objectives. Such an installation and driving of the cassette 36 in the vector plotter 10' of this invention will work for its intended purpose; however, improved performance and ribbon life can be achieved with the following modifications made to the method of driving and controlling the ribbon 32. The problem is that the plotter head 18 of a vector plotter 10' can move bi- directionally, remain stationary while printing, and move at different speeds. Thus, it is possible for the printhead 28 while plotting (i.e. printing dots) to track the movement of the ribbon 32 and repeatedly strike the same spot of the ribbon 32 thereby causing ink depletion, loss of line quality, and possible permanent local damage to the ribbon 32. Since the control logic 38 knows what the printhead 28 is doing, it is possible for it to control the motor drive 72 in a manner which will assure that the foregoing does not take place by adjusting the speed and/or direction of movement of the ribbon 32. Thus, it is preferred that, as depicted in Figure 5, the motor drive 72 be controlled by the logic 38 in the above- described manner.

A visual line quality degradation or fading can occur as a result of insufficient relative motion between the ribbon and the plotting media and/or the ribbon and the dot-producing printhead. Both film and fabric ribbon manufacturers generally specify a minimum number of continuous impacts in a given location if a depletion of the ink supply at that location is to be avoided. During non-printing time, there is some degree of redistribution of ink by capillary and mechanical action in the depleted area. This limitation is often expressed in units of maximum dot impression overlap with the range generally involving a dot overlap of one half to one sixth depending on quality considerations.

In order to avoid a problem it is necessary to maintain minimum velocity differential between the dot- producing printhead in the direction of ribbon motion and the velocity of the ribbon itself. This differential depends upon the printwire diameter, the desired dot overlap, the refire rate of the printhead, the velocity differential (ΔV min) = the dot overlap multiplied by dot size multiplied by maximum printhead frequency (fmax) . Thus for a dot overlap of one third with a dot size of .014 inches and a printhead refire rate of 3,000 hz, ΔV min = 1/3 x .014 x 3,000 = 14 inches per second. At a result of this it can be seen that in order to avoid fade problem the velocity of the printhead parallel to the movement of and relative to the ribbon should always be at least 14 inches per second. To achieve this desired differential the ribbon transport speed can be adjusted to maintain the differential, the carriage speed can be adjusted to maintain the differential or both of these can be adjusted to maintain the differential. Alternatively, the ribbon transport speed can be arranged always to be

higher than the maximum carriage Y coordinate speed by an amount exceeding the minimum required velocity differential.

In order to extend ribbon life in accordance with one embodiment of the present invention, the ribbon is oscillated transversely of its transport direction in order that the printwire(s) , during vector producing plotting, impact a desired portion of ribbon with a single color ribbon. This may be the entire width of the ribbon while with a multi-color ribbon, involving longitudinally extending parallel different colored ink strips, the amplitude of the oscillations may be limited to the width of a single color with the relative position of the printwire(s) and the ribbon being adjusted when a color change is desired as is described elsewhere in this application. Such a construction is illustrated in Figures 12, 13, 14 and 15. In these figures a carriage 80 carrying four dot producing printheads 28, respectively carrying four printwires 74 of four different diameters, is supported by three rollers 82 and a longitudinally extending guide bar 84 (not shown in Figure 15) , located relative to a platen 86 (not shown in Figure 15) , located for the movement of the carriage 80 in the direction of the Y coordinate at a fixed constant printing distance above the platen. A guide plate 88 fixedly attached to the carriage 80 extends over the platen 86, with the printing medium (not shown) , when present, being disposed between the guide plate 88 and the platen 86. The guide plate 88 supports ribbon 32 at a location spaced from the printing medium and is provided with openings (not shown) through which the printwires 74, when driven to produce a dot on the printing medium, move the ribbon to impact the printing medium to produce the dot. In this illustration

the ribbon 32 is a multi-color ribbon having four longitudinally extending strips of different colors.

A ribbon support 90 is slidingly supported on the carriage 80 by guide grooves 92 for reciprocal movement normal to the Y coordinate (as defined by the longitudinal extension of the guide bar 84) . This reciprocal movement is produced by a stepper motor 94 mounted on the carriage 80 and connected to the ribbon support 90 by a drive shaft 96 which is fixedly attached to the ribbon support 90. The shaft 96 is driven by the stepper motor to produce the reciprocal motion by virtue of a nut and screw transmission (not shown) within the stepper motor 94.

To produce ribbon life extending oscillation of the ribbon normal to its direction of transport, the stepper motor is operated to move the ribbon support 90 as indicated by arrows 98 to oscillate a desired width of the ribbon 32 under the printwire 74. In the embodiment illustrated, this oscillation would simply move the ribbon repetitively under the printwires 74 in order that they impact the full width of one of the strips of colored ink. As previously mentioned, should a single color ink ribbon be in use, the oscillation preferably would move the full width of the ribbon repetitively in an oscillatory manner under the printwires. As shown in Figure 12, the ribbon is at its furthest extension, during the oscillation, out from the carriage. The ribbon would move during oscillation from this position toward the stepper motor 94. The stepper motor 94 would, of course, in multi-color printing be arranged to locate the desired color strip of the ribbon under the printwires 74 when printing of that color is desired and then produce the desired oscillation of the ribbon within the bounds of that color strip. It will be appreciated that the width of the ribbon relative

to the printwire diameter in these illustrations is not to scale as the printwires would in fact be much smaller relative to the ribbon width than that illustrated. The disproportionate dimensions are used for the purposes of clarity of explanation.

When it is desired to remove a ribbon from or replace a ribbon in the ribbon support 90, the ribbon support 90 is moved by the stepper motor 94 to the far right position illustrated in Figure 13 in which ribbon engaging latches 100 can be pivoted to the position shown in Figure 13 to permit removal and placement of a ribbon from and into the ribbon support 90. These latches can only be pivoted into the open position shown in Figure 13 when the ribbon support is fully extended as shown in Figure 13. The latches are prevented from this pivoting movement in other positions of the ribbon support 90 by virtue of their engagement with the carriage 80 as can be seen from the front elevation of Figure 14 in which a portion of the latches underly the structure of the carriage to prevent the pivotal motion except when the latches are in the fully extended position of Figure 13.

With reference particularly to Figure 14 the guide bar 84 and platen 86 are seen supported by a plotter frame member 102 which carries a ribbon guide roller 104 positioned to guide the ribbon from cassette (not shown) to the ribbon support 90 of the carriage 80 through which it is guided between the printwires 74 and the guide plate 88 with the guidance involving rollers 106, guide surfaces 108 and ribbon edge locating guide surface (not shown) all supported on the ribbon support 90 and its latches 100.

At the side of the carriage 80 remote from the guide roller 104 the ribbon leaves the ribbon support 90 to pass

over further guide rollers (not shown) as it returns to the cassette (not shown) .

Now with reference of Figures 16 and 17 an embodiment of the present invention in which the ribbon cassette is provided with a cleaning station, for removing unwanted debris from both surfaces of the ribbon, incorporated in an area of the cassette through which the ribbon returns to the cassette. This cassette is of the "harmonic " format in which ribbon in the cassette is stored in an accordion-like manner. With reference to these figures, a cassette 110 includes a ribbon storage area 112 in which a ribbon 32 is stored in accordion-like style. Integrally formed with the housing of the cassette and incorporated within that housing is a ribbon cleaning station 113 through which ribbon returning to the cassette passes to the storage area. The cleaning station includes a guide roller 114 for the incoming ribbon and an interior guide roller 116 to guide the ribbon from the cleaning station to the storage area. As the ribbon 32 passes through the cleaning station it is contacted on opposite faces by spaced polyurethane low density foam pads 118 supported on a pad support structure forming a part of the housing of the cassette. The pads 118 are so positioned that the ribbon must follow a slightly serpentine path past the pads in order to ensure good cleaning contact of the ribbon with the pads. The pads present square faces to the ribbon with these faces being oriented so that each transverse section of the ribbon first contacts a corner of each pad and then subsequent portions of each pad of ever increasing width are contacted so that the angled edges of the pad tend to move debris sideways off of the ribbon to fall into an unused area of the cleaning station 113.

The dot-producing printhead 28 prints individual dots while the vector plotter 10' prints vectors (i.e. lines) of various lengths. A single dot is, of course, the shortest vector possible whether it is produced with a pen or a dot-producing printhead. A vector plotter also produces purely vertical vectors, purely horizontal vectors, and angled vectors between the two. Thus, the printhead 28 of the vector plotter 10' of this invention must be able to do the same. It must also be able to produce vectors of different line widths as it duplicates the printing action of different pen tip widths. These objectives can be accomplished with a single dot-producing printhead employing dot overlapping techniques adapted from dot matrix printer technology. There are several different configurations of the printhead 28 (and its printwire-driving printhead 28') , however, which have been found to provide much better line quality, speed of plotting, and versatility in this regard. For this reason, they are preferred over the single dot printhead. They will now be described with particularity. The first is the four-pin printhead 28 of Figure 3. The layout of the printwires (i.e. printing pins) 74 an embodiment thereof is depicted in Figure 9a. The printwires 74 are equally spaced horizontally and vertically (for ease of positional adjustment by the logic 38) and comprise 0.25mm, 0.35mm, 0.5mm and 0.7mm dot-producing diameters, respectively (meaning that, because of ribbon-induced increase of the dot-produced, the printwires 74 are 0.2mm, 0.3mm, 0.45mm and 0.65mm in diameter, respectively). In this arrangement, line widths of 0.25mm, 0.35mm, 0.5mm and 0.7mm are made by the choice of the printhead 28' (i.e. printwire 74) making the dots and line quality is a function of dot overlap. Dot overlap is, of course a

function of the refire rate of the printheads 28' and the speed of relative movement of the printing printhead 28' over the media 12. Figure 9b illustrates a variation in which a four-pin layout of printwires of similar diameters (to those of Fig. 9a) are disposed in straight line alignment.

This latter aspect brings up another advantage of the dot-oriented vector plotter 10' of this invention over prior art vector plotters employing a pen, or the like, as the vector-drawing device. Having different plotting speeds for "draft" and "finished" work is generally a virtual impossibility as the cheap felt-tip type pens normally employed for draft work are usually the most prone to skipping, etc. at higher speeds. Also, most plotters are made to work at the fastest speed that will produce good line quality without skipping if proper plotting materials are employed. By employing a printhead adapted from dot matrix printer technology in a vector plotter, one can also have a vector plotter which can be operated at two or more speeds (e.g. draft and finished) with only minor degradation in line quality between speeds. This is because the concept of "skipping" as with a pen (wherein large portions of vectors may fail to be generated) is not of concern. Rather, the overlap distance between the generated dots simply increases as the relative speed of the printhead 28 over the surface of the media 12 increases.

Another possible printwire/printhead arrangement is shown in Figure 10. This is a nine-pin head employing three triad arrays of three printwires 74 each. By employing multiple printwires 74 of like diameter in combination, increases in the printing and plotting capability of the vector plotter 10' become possible. As

can be seen from the drawing, the printing ends of the printwires 74 of each of the triads is disposed at the angles of an equilateral right triangle. Also, the size of the triangle is the same for each of the three printwire diameters. This, of course, is to simplify the positioning of the printhead by having a constant off-set between the three triads.

A hybrid printhead employing raster and vector capabilities as depicted in Figure 11 would improve the performance capability of the plotter 10' even more. The applications programs employing such a plotter would, of course, have to be modified to take advantage of its hybrid capability as will be recognized by those skilled in the art inasmuch as text, shading, and hatching, for example, would no longer be generated by vectors; but rather, by raster technology such as that employed in dot matrix printers to form various type fonts. As depicted in Figure 11, such a hybrid raster/vector printhead would employ the four-wire arrangement of Figure 9 at a variant thereof (e.g. four aligned wires) as the "vector" portion at 74 in combination with a twelve-wire, or a variant of such, (sharing one, preferably the smallest, printwire 74) raster printhead at 76. Such a hybrid printhead would be effective for printing text characters, and the like, principally and optimally in a horizontal direction only and, thus, its use would be limited to those applications plotting programs which could take advantage of its increased speed with those limitations. In the alternative, such a hybrid printhead could be mounted for rotation to put the raster printwires 74 (at 76) perpendicular to the direction of printing (under the control of the logic 38), i.e. the raster scanning direction, regardless of what that direction happened to

be. Again, the input commands to the logic 38 as contained in the vector plotter for producing these various "patterns" (i.e. text, hatching, shading, etc.) would be in the form of non-vector commands which would cause the vector plotter 10' to produce the desired pattern in a raster-scanning fashion. This would be the case whether it was one printwire 74 producing a dashed or dotted line in one scan pass, the same one printwire 74 producing text, hatching, or the like in multiple passes, or multiple printwires 74 producing them in one or multiple passes, depending on the area to be plotted with the "pattern".

By comparison with a pen carrying carriage, it is more important in a vector plotter using a projecting dot- producing printwire printhead that the distance from the carriage carrying the printhead to the platen supporting the drawing media be maintained at a substantially constant printing distance. The criticality results from the limited projection distance of a printwire during a printing of a dot by comparison with the greater available movement from a pen as it is raised from or lowered on to the plotting media. It is especially important that the construction of the guide bar 84 and its support over the platen 86 be controlled and arranged in such a manner that the printing distance will be maintained at a sufficiently constant magnitude even if the vector plotter is supported on an uneven floor with the consequent possible distortion or flexing of its frame. Such a support arrangement is illustrated in Figures 18, 19 and 20. With reference to the drawing, the vector plotter frame 120 supports a guide bar/platen assembly 122 by only means of a 3 point mounting 124, the 3 points 123 being non-aligned with two of the points 123 being adjacent one end 126 of the

assembly and with the other point 123 being disposed adjacent the other end 128 of the assembly whereby the 3 points 123 lie in a substantially horizontal plane.

The assembly 122 is resiliently held to a cross member 130 of the frame 120 adjacent the 3 points 123 by mounting bolts 132 which are fixedly attached to the assembly 122 and which pass through the cross member 130 to elastomeric resilient damper 134 which are held captive by boltheads of the bolts 132 to resiliently bias the 3 contact points 123 against the cross member 130 while committing some distortion of frame 120 to occur without an associated distortion of the assembly 122. The three point mounting provides the only support of the assembly 122 by the frame 120. The guide bar is mounted to the assembly 122 by means of five eccentrics 136 evenly spaced along the guide bar and supported by a strong back 134 extending the length of the assembly 122. It will be noted that the guide bar 84 is omitted from Figure 18 and that only two of eccentric mounting holes of the strong back are shown in Figure 18. Eccentrics 136 may be turned in their associated mounting holes 138 to deform the guide bar 84 into accurate alignment with the platen 86 in such a manner that the distance between these is sufficiently constant throughout the full traverse of carriage 80 along the guide bar 84 over the platen 86. Once alignment is achieved the eccentrics are locked into place by lock screws 140.

The provision of a separate assembly 122 allows that assembly to be completely assembled, aligned and tested as an independent unit before being mounted by the 3 point mounting 124 to the plotter frame and the independent nature of this assembly ensures that alignment of the guide bar and the platen will be retained even if some

twisting or bending of the plotter frame takes place. The separate nature of the assembly 122 allows the complete unit to be tested for parallelism, tolerances, gap adjustment, etc. in a much easier, cheaper manner than would be possible within the complete structure. By using such an arrangement, the platen and the guide bar may be extruded aluminum pieces which may be hard anodized before assembly and adjustment.

The term "logic 38" is a general term which can include the programming of a computer driving the plotter 10' as well as control logic contained within the plotter 10' itself.

The logic 38 should provide firing signals to the printheads 28' so that a given quality of line is printed regardless of the speed, angle, or curvature of the line (vector) . Control of the dots should be such as to space the dots such that specified end dimensions are held to plus or minus 0.0005 inches as a worst case for horizontal and vertical lines and plus or minus 0.0007 inches as a worst case for diagonal lines approaching 45 degrees, which will provide the same eleven types of lines currently provided by conventional vector plotters as well as provide for the expansion to a much greater variety, if desired. Additionally, the logic 38 should not allow the relative movement of the plotter head 18 over the media 12 to exceed the maximum refire rate of the printheads 28'.

An important aspect of controlling a vector plotter having a projecting dot-producing impact printhead is the control of the operation of the printhead and its motion across the printing medium to achieve a constant dot pitch and a constant pressure per square inch of impact of different diameter pins thereby to produce a constant density of line.

With appropriate inputs from existing hardware such as encoders, appropriate software can perform the following functions.

1) Give firing signals to the printhead so that a given quality line (dot spacing) is printed regardless of the speed, angle, or curvature of the plot. The software and logic control spaces the dots so that specified end dimensions are held to plus or minus 0.0005 inches. At present, the vector plotter of the present invention simply prints solid lines with dotted or dashed lines made the same way they are now, i.e. by stopping at the end of each dot or dash and starting at the beginning of the next one. This takes more time than is actually needed with an impact printhead. As an option, circuitry/software can provide the same 11 types of lines currently provided, without stopping at the end or beginning of each dot or dash, as well as providing for the expansion to a much greater variety, if desired.

2) Provide for various line widths of 5 widths of 0.18, 0.25, 0.35, 0.5, & 0.7 mm. respectively. However, only 4 print modules can be used in the present design at any one time. The print modules could identify themselves by means of a different value resistor for each line width that the electronics will read so that the electronics/software will know the position in the carriage of each print module. When the user calls for a given line width, the electronics will connect the proper print module driver and software will set the proper current and strobe length for each line width. All line widths take different currents and strobe length.

3) To control dot spacing software will not move the carriage or paper so that the vectored speed of the printhead over the paper exceeds the maximum refire rate

of the printhead, which varies by line width. There are two ways that the dot spacing can be set, i.e. using either software or hardware.

Currently, hardware adjusts the firing so that a uniform dot spacing is maintained independent of the variable speed, during starting and stopping, or the angle or curvature of the line. In the case of lines along to the X or Y coordinates, the hardware fires dots every plurality of encoder ticks that matches the dot spacing specified during the acceleration or deceleration period. When the velocity of the line drawing approaches (or is a constant speed) the dot firing switches to firing on a time basis, instead of encoder ticks. This produces a much higher quality line, to the eye, than firing from the encoder. (It appears that the eye can pick out the slightest irregularities in the encoder ticks.)

In the case of a diagonal or angled line, the encoder is used that represents the biggest incremental value (this indicates that the line is not more than 45 degrees from the encoder axis used) and the number of encoder ticks will be shortened so that the specified dot spacing will be maintained. An encoder is present to detect movement along each of the x and y coordinates.

In the case of diagonal or curved lines, hardware adjust the dot spacing as measured from one of the encoders "on the fly" by the ratio of two measurements which are proportional Vx and Vy. The arc tangent of the instantaneous ratio gives the instantaneous angle of the curve or diagonal line. By using a lookup table, the number of encoder ticks between dots to maintain the desired dot spacing is determined.

The encoder is used that instantaneously has the higher velocity and an instantaneous angle of 0 to 45

degrees. At 45 degrees, switching from one encode to the other occurs. During slow down for the end of an arc the correct length is maintained by adjusting the last dot spacing. The fact that the last dot has a shorter spacing is not as noticeable as the broadening of the line that occurs with a pen when the velocity is slow.

To implement this desirable function the pins are fired by either the carriage (X axis) or paper feed encoder (Y axis) . The carriage encoder is used for all lines within 45 degrees of the carriage motion and the paper feed encoder is used for angles greater than 45 degrees. Thus the desired dot spacing is maintained to within plus or minus 0.0007 (at the worst) for 45 degree lines and plus and minus 0.0005 for horizontal or vertical lines.

The hardware measures the instantaneous angle by measuring the number of encoder ticks, in a known short period of time, of the carriage and paper encoders. By taking the ratio of these encoder ticks and using a look- up table, we can determine the number of encoder ticks between pin fires to get a uniform desired spacing between dots at any instantaneous line angle.

Dots are fired by a counter that counts quadrature encoder ticks. The number of counts is adjusted for the instantaneous angle of the line.

There are other more complicated ways of ending the line at the correct point. The line length is not usually an even multiple of the dot spacing but usually has a remainder. In order to make the line come out to an exact length, one of several other more complicated methods could be used:

1) Make "n" uniform dots where "n" equals the line length divided by dot spacing and make one more dot that

has a spacing equal to "r", the remainder. "n" and "r" would ideally be supplied by the software, or if necessary, be calculated by the hardware. In most cases, this method is probably the most convenient method to use. 2) A theoretically better job involves arranging for the hardware to increase the spacing of "r" dots so that the exact length is realized. While this method is a little better for straight lines, it is impossible to use in the case of curved lines. Solid, dotted, dashed, or various almost unlimited combinations of dot dashed lines are possible. Currently, there are 11 different line types provided to the customer. On all but solid lines, software will give pen up or down commands to regulate where lines or dots and spaces exist. In doing this, software can adjust the positions as it does so now to get proportional lines. There will be no need of a pause or for pen up or down delays at the start and the end of each line. There is no need for stopping at the start and end of each dot, dash, or line.

The print module takes about 270 microseconds to hit the media and make a dot after it is commanded to do so electronically. The time of flight varies with line width. At 32 ips carriage speed, it takes almost 9 mils (>0.2mm) of translation for the pin to make a dot after it is fired. However, when we start a line and are standing still, we make a dot and then start accelerating, at say 4 Gs. We want to fire the next dot after moving, say 10 mils which occurs in 3.6 milliseconds, at which time we are moving at 5.6 ips which puts the dot at 11.5 mils instead of 10 mils. We want the next dot at 20 mils, which occurs at 5.1 milliseconds and where the speed will be 7.9 ips which puts the dot at 22.1 mils, a separation

of 10.5 mils. We want the next dot at 30 mils, which occurs at 6.2 milliseconds and where the speed is 9.6 ips which puts the dot at 32.6 mils, a separation of 10.5 mils. We want the next dot at 40 mils, which occurs at 7.2 millisecond and where the speed is 11.1 ips which puts the dot at 43 mils, a separation of 10.4 mils. The separation keeps approaching 10 mils as the speed approaches 32 ips where we switch over to firing on time intervals. As we slow down, the dots tend to get closer together by a fraction of a mil instead of further apart. These variations are just marginal as to whether they should be compensated or not. Actually, we should set the dot spacing a little closer at the beginning and end of the lines due to the fact that the eye notices the beginning and end of the line more carefully as this is where the most information occurs. Thus dot spacing can be allowed to increase in the middle of a line, by allowing vector speed to exceed the maximum speed for constant dot spacing at maximum refire rate, by a reasonable margin without readily apparent line degradation.

As a net result, there is no need to compensate for the time of flight of the print module, unless it is to make the dot spacing a little closer at the beginning and end of the line.

With software control of dot spacing, we would still use the encoder ticks to fire the print module on the acceleration and deceleration parts of the line. We would use the encoder that was closer in angle to the instantaneous angle to fire the print module. Since the plotter electronics always knows the instantaneous angle of the line being plotted, this can steer us to a look up

table to tell us the number of ticks to use for refire during this phase.

When we refire with time during the constant speed phase, we do not need to use the lookup table. Tests conducted have established that plotting speeds about twice as fast a present pen-oriented vector plotters are achieved by the plotter of the present invention. A substantial portion of this increase is due to the elimination of the pen-up, pen-down motions of the prior art plotters. Plot quality is also superior to that of present, prior art pen plotters. By changing the driving software to take advantage of capabilities built into the control logic 38 where that logic is incorporated into the vector plotter of this invention, even greater improvements in speed can be expected. For example, by specifying a line type and a beginning and ending point to a "smart" plotter according to the present invention instead of outputting a series of short vectors with stops between each for pen up and down motions as in the present approach, the plotter of the present invention could produce a dashed line in one continuous motion of the plotter head 18 across the media 12 from the starting position to the ending position.

While the embodiments of the present invention have been described, by way of example, with particular constructions it will be appreciated that a number of variations of those embodiments could be adopted by a man skilled in the art without the need for undue experimentation and without departing from the concepts of the present invention. For example, the relative oscillation and transverse movement of the ribbon relative to the printhead could be achieved by maintaining the ribbon still and traversing the head itself, constant

pressure per square inch for different diameter pins can be achieved by controlling current magnitude and pulse width of the energization of the relevant printhead, a reel to reel ribbon system could be used instead of a cartridge, the vector plotter could be of a type in which the head moves in both the X and Y coordinates with the printing medium remaining stationary relative to the structure of the plotter (the ribbon is such could be replaced by an ink carrying sheet, much like carbon paper) and vice-a-versa.

A significant advantage of the basic concept of the present invention is that the use of a projecting dot- producing impact printhead in a vector plotter allows the existing plotter software to be utilized.