Thermal transfer printer ink ribbon-changing carousel apparatus and method

A thermal transfer color printer (100) utilizes a single print head (102) positioned outside a carousel (104) of monochrome ink ribbons (106) each stretched between a pair of spaced-apart spools (108, 110) mounted around the periphery of the carousel. A spool spacing (112) provides sufficient clearance for the thermal print head to be positioned between to spools to contact the ribbon. A media drum (116) is positioned coaxially within the carousel. The carousel has a media opening (122) through which a print medium (118) enters and exits a medium clamp (144) attached to the media drum. To print a particular color, the carousel is rotated such that the media opening is adjacent to a media supply, and a print medium is guided from the media supply into the medium clamp. The media drum is rotated to close the medium clamp and position a leading edge (138) of the print medium adjacent to the print head. The print head is moved against the ribbon, and the print head is electrically driven by a printer controller (240) to selectively transfer ink to the print medium in a predetermined pattern. To print a different color, the carousel is indexed to a selected ribbon position, the media drum is rotated to position the leading edge of the print medium as before, and the printing process is repeated for the newly selected ribbon color. When printing is completed, the carousel is indexed to the initial position, and the print medium is guided from the medium clamp into the exit path.

TECHNICAL FIELD 
This invention relates to thermal transfer color printing (e.g., thermal 
wax transfer, dye diffusion thermal transfer, or the like) and more 
particularly to an apparatus and a method for indexing separate monochrome 
colored ink ribbons into a printing position adjacent to a thermal print 
head. 
BACKGROUND OF THE INVENTION 
Thermal transfer printing involves the controlled transfer of an ink (e.g., 
a colorant dispersed in a wax-based material) from a carrier such as a 
polymer ribbon onto a print medium surface. A thermal transfer printer 
having a print head with a large number of independently activatable 
heating elements per unit of length is one prior art apparatus employed 
for this purpose. The ribbon is placed within the printer such that the 
carrier side is adjacent to the heating elements, and the ink side is 
adjacent to a print media support upon which a print medium rests during 
printing. 
To print an image, the print head contacts the ribbon, and ink is 
transferred to particular locations on the print medium surface when 
predetermined combinations of heating elements are activated adjacent to 
those image-forming locations. The ribbon is locally heated by the heating 
elements to a temperature at or above the melting point of the ink. In 
this manner, an amount of ink softens and adheres to the print medium at 
the predetermined locations to form the image. 
Color images are printed with a ribbon that typically includes separate 
panels of differently colored inks such as the subtractive primary colors, 
yellow, magenta, and cyan. Color printing is accomplished by sequential 
passes of the print medium past the print head, each pass selectively 
transferring different colored inks at predetermined times. 
Thermal printing ribbons are available with a single yellow, magenta, cyan, 
or black ink panel (a monochrome ribbon); repeating sets of yellow, 
magenta, and cyan ink panels; or repeating sets of yellow, magenta, cyan, 
and black panels (multicolor ribbons). Ribbons are typically supplied on 
spools that have encoded end caps to communicate to the printer whether 
the ribbon includes one, three, or four panels. Multicolor ribbons 
typically have an encoding stripe running along one edge of the length of 
the ribbon to communicate panel location and panel color data to the 
printer. Obviously, monochrome ribbons are easier to manufacture, do not 
require the encoding stripe, and are therefore less costly. 
Therefore it would be preferable to use multiple monochrome ribbons rather 
than a multicolor ribbon for thermal transfer color printing. Other 
workers have tried using multiple monochrome ribbons, each having an 
associated thermal print head, for transferring ink from each ribbon to a 
print medium proper registration of images transferred from each of the 
different ribbons requires precise alignment of the print heads and 
accurate positional indexing of the print medium relative to the print 
heads. Unfortunately, the resulting mechanism is bulky and relatively 
complex, the electronics drivers are replicated four times, and a higher 
capacity power supply is required resulting in a cost that offsets any 
savings associated with using the monochrome ribbons. 
To improve multi-pass color printing registration, some printers clamp an 
edge of the print medium to a media drum and wrap the print medium around 
the drum for printing. Such an arrangement provides accurate control of 
print medium positioning relative to the print head(s). 
Copending U.S. Pat. Ser. No. 5,305,020 of Gibbons et al. Apr. 19, 1994 for 
A THERMAL TRANSFER PRINTER HAVING MEDIA PRE-COAT SELECTION APATUS AND 
METHODS, assigned to the assignee of this application, describes such a 
drum and medium clamp arrangement for use with a multicolor thermal 
transfer ribbon. Referring to FIG. 1, a thermal transfer printer 10 
(hereafter "printer 10") is shown that includes a drum 12 that receives a 
print medium 14A from a media tray 16. (Print medium 14 is shown in 
printer 10 at three locations designated by a letter suffix, i.e., 14A, 
14B, or 14C.) A leading edge 18 of print medium 14A is fed to a medium 
clamp 20 that secures print medium 14B to drum 12, which then rotates in a 
direction indicated by arrow 22 to wrap print medium 14B around drum 12. 
Printer 10 also includes a multicolor thermal transfer ribbon 24 suspended 
between a supply spool 26 and a take-up spool 28. Take-up spool 28 is 
driven in a direction indicated by arrow 30 with a torque sufficient to 
feed ribbon 24 through a nip formed between drum 12 and a thermal print 
head 32 at a rate determined by the rotation of drum 12. 
The type of ribbon 24 (black, three, or four panels) is encoded by hub 
length into a left hub 42 and a right hub 43 on supply spool 26. Hubs 42 
and 43 are each of a normal or extended length and selectively activate a 
left microswitch 44 and/or a right microswitch 45 as listed in Table 1. 
The states of microswitches 44 and 45 are sensed by a printer controller 
46. 
TABLE 1 
______________________________________ 
RIBBON TYPE LEFT SWITCH RIGHT SWITCH 
______________________________________ 
Black On On 
Three panel Off Off 
Four panel On Off 
______________________________________ 
Ribbon 24 further includes an opaque encoding stripe 48 having a coded 
marker 50 at location indicating the boundaries between panels 34, 36, 38, 
and 40. Coded marker 50 typically is a series of transparent stripes 
detectable by a photosensor array 52 mounted adjacent to thermal print 
head 32. The number of stripes in each coded marker 50 indicates to 
printer controller 46 which 15 of panels 34, 36, 38, or 40 is aligned with 
thermal print head 32. 
In operation, printer 10 receives a print job at a data communications 
interface 54. The print job is transferred to a system bus 55 that is in 
communication with printer controller 46, a processor 56, and a memory 58. 
Processor 56 processes data and commands contained in the print job and 
transmits control and printing data to printer controller 46. Processor 56 
executes the printer driver stored in memory 58 and exchanges data with a 
PostScript.RTM. interpreter. 
After the print job is interpreted by processor 56 and stored as yellow, 
magenta, and cyan image data in memory 58, printer controller 46 causes 
print medium 14A to feed from media tray 16 to medium clamp 20 on drum 12. 
Medium clamp 20 is activated, and drum 12 is caused to rotate such that 
leading edge 18 of print medium 14B is just past the nip between drum 12 
and thermal print head 32. Ribbon 24 is moved by take-up spool 28 until a 
coded marker 50 is detected by photosensor array 52 indicating that a 
yellow panel 36 is positioned under thermal print head 32. Drum 12 is 
rotated one revolution, and yellow panel 36 is moved through the nip while 
all the image data stored in memory 58 simultaneously drives thermal print 
head 32, thereby thermally transferring a yellow image to print medium 14B 
and advancing ribbon 24 such that magenta panel 38 is in the nip. Drum 12 
is rotated a second revolution, and magenta panel 38 is moved through the 
nip while the magenta image data stored in memory 58 simultaneously drives 
thermal print head 32, thereby thermally transferring a magenta image to 
print medium 14B and advancing ribbon 24 such that cyan panel 40 is in the 
nip. The sequence is repeated for the cyan image data until a full color 
image is transferred and registered on print medium 14B. Drum 12 reverses 
and rotates in the direction of an arrow 60, releasing medium clamp 20, 
and feeding print medium 14C from printer 10 by means of a conventional 
exit path mechanism (not shown). 
Printer 10 properly registers high-resolution color images with a 
relatively simple and inexpensive mechanism. However, if even one dot of 
ink is transferred from a single color panel to form an image, ribbon 24 
must be advanced to a new starting position before printing a subsequent 
image. This wastes almost three ribbon panels (four if ribbon 24 has a 
black panel) and requires considerable time to advance ribbon 24 to the 
new starting position. 
U.S. Pat. Ser. No. 4,778,290 issued Oct. 18, 1988 for a PRINTER FOR 
PRINTING OF A FULL LINE IN SEVERAL COLORS BY INTERCHANGEABLE RIBBON 
CARTRIDGES describes a thermal printer having a single print head and a 
drum and medium clamp arrangement that provides multi-pass color 
registration of ink images transferred from multiple monochrome ribbons to 
a print medium wrapped around the drum. A carousel of multiple monochrome 
ribbon cartridges indexes an appropriate ribbon to a printing position 
adjacent to the drum. The print head, mounted within the carousel, is 
moved to form a nip between the print head and the drum through which the 
print medium and the ribbon are frictionally drawn by rotation of the 
drum. When printing is not desired, the print head is withdrawn a few 
millimeters to remove the ribbon-driving friction, thereby conserving 
ribbon material. 
The ribbon carousel allows use of monochrome ribbons, improves ribbon 
utilization, and reduces the time required before a new image can be 
printed. 
However, there are problems associated with mounting the thermal print head 
inside the carousel. Commercially available thermal print heads are 
physically large and dissipate a considerable amount of heat. Therefore, 
the carousel must provide print head clearance between each pair of ribbon 
spools, space for a print head heat sink, some heat removal means from 
inside the carousel, and a mechanism for moving the print head into 
contact with the ribbon. Such requirements result in an unduly large and 
massive ribbon carousel that requires expensive drum-to-ribbon-to-print 
head alignment structures and a complex print head moving mechanism. 
Therefore, what is needed is a method and apparatus for thermal transfer 
printing with multiple monochrome ribbons and a commercially available 
print head but without the mechanical complexity previously associated 
with print head heat removal, print head movement, and 
drum-to-ribbon-to-print head alignment. 
SUMMARY OF THE INVENTION 
An object of this invention is, therefore, to provide an improved 
monochrome ribbon changing apparatus and method for improving ribbon 
utilization in a compact thermal transfer color printer. 
Another object of the present invention is to provide an improved 
monochrome ribbon changing apparatus and method that readily allows 
positioning and cooling a commercially available thermal print head. 
A further object of the present invention is to provide an improved 
monochrome ribbon changing apparatus and method that incorporates a simple 
print medium feed and exit path mechanism. 
Still another object of the present invention is to provide an improved 
monochrome ribbon changing apparatus and method that provides inherently 
simple drum-to-ribbon-to-print head alignment. 
A thermal transfer color printer, according to this invention, utilizes a 
single print head positioned outside a carousel of monochrome ink ribbons 
each stretched between a pair of spaced-apart spools mounted around the 
periphery of the carousel. The spool spacing provides sufficient clearance 
for a commercially available thermal print head to be moved into the space 
between the spools to contact the ribbon. A media drum is positioned 
coaxially within the carousel. The carousel has a media opening through 
which a print medium enters and exits a medium clamp that is attached to 
the media drum. To print a particular color, the carousel is rotated such 
that the media opening is adjacent to a media supply, and a print medium 
is guided from the media supply into the medium clamp. The media drum is 
rotated to close the medium clamp and position a leading edge of the print 
medium adjacent to the print head. The print head is moved against the 
ribbon, and the print head is electrically driven to selectively transfer 
ink to the print medium in a predetermined pattern. To print a different 
color, the carousel is indexed to a newly selected ribbon position, the 
media drum is rotated to position the leading edge of the print medium as 
before, and the printing process is repeated for the selected ribbon 
color. When printing is completed, the carousel is indexed to the initial 
printing position, and the print medium is guided from the medium clamp 
into the exit path. 
Additional objects and advantages of this invention will be apparent from 
the following detailed description of a preferred embodiment thereof which 
proceeds with reference to the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 2 shows a thermal transfer color printer 100 (hereafter "printer 100") 
utilizing a single commercially available thermal print head assembly 102 
positioned outside a carousel 104 that holds multiple monochrome ink 
ribbons 106. Thermal print head assembly 102 is preferably a commercially 
available type such as model no. KST-219-12MPL2-SH manufactured by Kyocera 
of Kyoto, Japan. Each of ribbons 106 are suspended between a supply spool 
108 and a take-up spool 110 that are spaced apart around the periphery of 
carousel 104. Spools 108 and 110 are preferably about 2.5 centimeters in 
diameter and have a spool-to-spool spacing 112 of about 5.1 centimeters, 
sufficient to provide clearance for moving thermal print head assembly 102 
in a direction indicated by arrows 114 between a retracted position (shown 
in solid lines) and a ribbon-contacting position (shown in dashed lines). 
A media carrier, such as a media drum 116 is positioned coaxially within 
carousel 104. Media drum 116 has a diameter of about 12 centimeters, a 
size sufficient to hold A4-size media, and is coated with rubber to 
enhance media-to-drum dimensional stability and thermal transferability of 
ink to a print medium 118. In this configuration, media drum 116 also 
serves as a printing platen. 
Alternative media carriers may include a series of rollers that transport a 
web of print media material. The rollers may be positioned within carousel 
104 in a number of configuration. For example, three rollers having their 
rotational axes arranged in a triangular configuration in which one of the 
rollers also serves as a printing platen. 
Both carousel 104 and media drum 116 rotate about a common axis of rotation 
120 to maintain proper alignment of ribbons 106, print medium 118, and 
media drum 116 relative to thermal print head assembly 102. The overall 
diameter of carousel 104, preferably about 19 centimeters, is determined 
by the diameter of media drum 116, spools 108 and 110, and slight 
additional clearances for ribbon and print medium guides. The relative 
positionings of media drum 116, carousel 104, spools 108 and 110, and 
thermal print head assembly 102 provide a compact, relatively simple, 
inherently aligned, and thermally sound structure for printer 100. 
Carousel 104 includes a media opening 122 through which print medium 118 
enters and exits media drum 116. Media opening 122 includes a feed roller 
126, an idler roller 128, and an exit roller 130 that are geared together 
and commonly driven in the contra-rotating directions indicated by arrows 
132 by a drive gear 134. A media drive motor 136 rotates drive gear 134 
that is positioned to mesh with feed roller 126 when carousel 104 is 
indexed to the black printing position shown in FIG. 2. 
A leading edge 138 of print medium 118 is fed by conventional means into a 
nip formed between feed roller 126 and idler roller 128. Carousel 104 
includes a feed guide 140 and a media guide 142 that direct leading edge 
138 toward a medium clamp 144 on media drum 116. 
Medium clamp 144 is opened by rotating media drum 116 with a drum motor 146 
and drum drive belt 148 in a counter-clockwise ("CCW") direction until a 
clamp lever arm 150 contacts a clamp actuator 152 attached to carousel 
104. Clamp lever arm 150 preferably extends from an end of medium clamp 
144 to prevent interference with structures such as thermal print head 
assembly 102. After medium clamp 144 receives leading edge 138, a slight 
clockwise ("CW") rotation of media drum 116 allows medium clamp 144 to 
close. A spring 154 biases medium clamp 144 toward the closed position 
with a force sufficient to secure leading edge 138 of print medium 118. 
Clamp actuator 152 is biased toward media drum 116 by a spring 156 that 
allows clamp lever arm 150 to pass unimpeded under clamp actuator 152 when 
media drum 116 rotates in the CW direction. Once leading edge 138 is 
secured in medium clamp 144, media drum 116 rotates CW until print medium 
118 clears media opening 122 and wraps around media drum 116. Print medium 
118 is secured against media drum 116 by conventional bale rollers (not 
shown) that are attached to carousel 104. 
To print a particular color image on print medium 118, carousel 104 is 
rotated CW by carousel motor 158 and carousel drive belt 160 until the 
appropriate ribbon 106 is indexed adjacent to thermal print head assembly 
102. If black is the desired color, carousel 104 requires no rotation, a 
feature that facilitates relatively high-speed black printing, such as 
textual printing. 
FIG. 3 shows thermal print head assembly 102 moved into contact with ribbon 
106 to form a nip between an array of heater elements 200 and media drum 
116 through which ribbon 106 and print medium 118 are pulled by CW 
rotation of media drum 116. 
Thermal print head assembly 102 is preferably of media width, about 5 
centimeters, and is fabricated on a 3.53 centimeter wide ceramic substrate 
having 0.5 centimeter high heater array drivers 202 attached to one 
surface and a heat sink 204 attached to the other surface thereof. Thermal 
print head assembly 102 is positioned such that heater array drivers clear 
ribbon 106, ribbon supply spool 108, and ribbon take-up spool 110. 
Positioning thermal print head assembly 102 outside the volume enclosed by 
carousel 104 promotes heat dissipation from heat sink 204 and allows 
implementation of a simple print head positioner 206. 
The precise rotational index position of carousel 104 is sensed by a 
microswitch 208 that is actuated by a bump 210 on carousel 104. Other 
bumps are positioned around the periphery of carousel 104 at appropriate 
indexing locations for each of ribbons 106. Carousel motor 158 is stopped 
in response to microswitch 208 to properly index ribbons 106 relative to 
thermal print head assembly 102. 
Absolute encoding of index position may alternately be provided by a 
photosensor array that straddles a rim on carousel 104 and senses a unique 
hole pattern dedicated to each index position. Ribbon colors may also 
encoded by means such as ribbon spool hub length encoding as described in 
copending Patent application Ser. No. 07/994,383. Absolute encoding of 
both index position and ribbon color allows an intelligent controller to 
achieve color printing with a minimum of carousel 104 indexing. 
A properly indexed ribbon 106 causes a gear 212 on take-up spool 110 to 
engage a ribbon drive gear 214 that is geared to a DC drive motor 216. The 
rotational torque of DC drive motor 216 characteristically decreases as 
the rotational velocity increases, thereby assisting ribbon 106 through 
the nip while winding used ribbon on take-up spool 110. The rotational 
torque characteristic of DC drive motor 216 also compensates for the 
relative proportion of ribbon 106 distributed between supply spool 108 and 
take-up spool 110. 
A pair of drag gears 218 resist rotation of supply spool 108 and take-up 
spool 110 so that there is sufficient ribbon tension to prevent ribbons 
106 from wrinkling and causing wax transfer-preventing creases from 
forming. Pairs of drag gears 218 are associated with each of ribbons 102 
to positionally stabilize any of ribbons 106 not indexed to the printing 
position. An optional DC motor and gear may be engaged to supply ribbon 
108 when it is in the printing index position to supply controllable 
amounts of drag beyond that available from drag gears 218. 
Thermal print head assembly 102 is moved between the ribbon-contacting and 
retracted positions by print head positioner 206. In a first embodiment, a 
head positioning motor 220 rotates a cam 222 by means of gear 224. 
Predetermined rotational positions of cam 222 are encoded in gear 224 by 
holes 226 that are sensed by a photosensor array 228. Holes 226 are 
positioned to correspond to the retracted, ribbon-contacting, and a 
slightly retracted position of thermal print head assembly 102. Head 
positioning motor 220 is controlled in response to photosensor array 228 
sensing the appropriate one of holes 226. 
A follower 230 rides on cam 222 to translate the rotational positions of 
cam 222 into linear positions of a shaft 232 that slides in a bearing 234 
and is attached to thermal print head assembly 102. Springs 236 bias 
thermal print head assembly 102 away from media drum 116 with a force 
sufficient to cause follower 230 to ride on cam 222. When thermal print 
head assembly 102 is in the ribbon-contacting position, a spring 238 
compressed between follower 230 and shaft 232 biases thermal print head 
assembly 102 against ribbon 106 with a force suitable for printing on 
print medium 118 and for advancing ribbon 106 through the nip. 
During printing, thermal print head assembly 102 is electrically driven by 
a conventional printer controller 240 (FIG. 2) that selectively heats 
elements of heater array 200, thereby melting and transferring wax-based 
ink carried in ribbon 106 to print medium 118. During periods of no 
printing, head positioning motor 220 drives gear 224 until the appropriate 
hole 226 is sensed by photosensor array 228, thereby positioning thermal 
print head assembly 102 slightly away from ribbon 106. The ribbon pulling 
force provided by media drum 116 and DC drive motor 216 is removed, 
thereby conserving ribbon 106. Periods of no printing can be as brief as 
the time required to rotate media drum 116 the equivalent of a single 
printed line. 
FIG. 4 shows a preferred lead screw-actuated embodiment of print head 
positioner 206. Thermal print head assembly 102 is moved between the 
ribbon-contacting position shown and retracted positions by a head 
positioning motor 260 having a threaded core that rotates around and 
linearly actuates a lead screw 262 that is fixedly attached to a 
compression plate 266 which is biased away from thermal print head 
assembly 102 by a compression spring 268. Predetermined linear positions 
of a collar 270 on threaded shaft 266 are sensed by microswitches 272 and 
274. The predetermined linear positions of collar 270 correspond to the 
ribbon-contacting head position shown being sensed by microswitch 272, a 
slightly retracted head position which is sensed when collar 270 moves 
away from microswitch 272, and a retracted head position (shown in dashed 
lines) being sensed by microswitch 274. Head positioning motor 260 is 
controlled in response to microswitches 272 and 274 sensing the 
appropriate predetermined positions of collar 270. 
When thermal print head assembly 102 is in the ribbon-contacting position, 
compression spring 268 is compressed between compression plate 266 and 
heat sink 204 with a force suitable for printing on print medium 118 and 
for advancing ribbon 106 through the nip. 
Thermal print head 102 is secured at one end of a swing arm 278 that 
rotates around a pivot bearing 280 located at the other end of swing arm 
278. 
FIG. 5 is an isometric view of head positioner 206 showing that swing arm 
278 and compression spring 268 preferably comprise two pieces. Swing arms 
278 are attached by conventional fasteners 290 to each end of thermal 
print head 102. Lead screw 262 is fixedly attached to compression plate 
266 by an E-ring 292. Compression springs 268 are conventionally secured 
between compression plate 266 and heat sink 204 by pins 294, E-rings 296, 
and spring keepers (not shown). Vertical displacement of compression plate 
266 is restricted by blades 298 that protrude through clearance holes 300 
in swing arms 278. 
Referring again to FIG. 2, printing with a different one of ribbons 106 
entails first moving thermal print head assembly 102 to the retracted 
position with print head positioner 206. Carousel 104 is then indexed to 
the desired ribbon position, media drum 116 is rotated CW to position 
leading edge 138 of print medium 118 to the initial printing position, 
thermal print head assembly 102 is moved to the ribbon-contacting 
position, and the printing process is repeated for the newly selected 
ribbon color. 
When printing is completed, thermal print head assembly 102 is moved to the 
retracted position, carousel 104 is rotated CW to the black ribbon index 
position, and media drum 116 is rotated CCW to open the medium clamp, 
while print medium 118 (shown in dashed lines) is pulled between media 
guide 142 and an exit guide 250 by idler roller 128 and exit roller 130. 
Print medium 118 is subsequently delivered into a conventional exit path 
(not shown). 
Alternative embodiments of portions of this invention can include using 
other than yellow, magenta, cyan, and black ribbons; other than a 
drum-type print media carrier; greater or fewer than four ribbons; use of 
more than one black ribbon; use of a media pre-coat ribbon; spool 
hub-length encoding of ribbon colors; print head positioning by a 
solenoid, escapement, or lever arms; and use of indexing sensors other 
than microswitch and photosensor array types. Skilled workers will also 
understand the applicability of this invention to a variety of printing 
technologies including thermal wax transfer and dye diffusion thermal 
transfer printing. 
It will be obvious to skilled workers that many changes may be made to the 
details of the above-described embodiments of this invention without 
departing from the underlying principles thereof. Accordingly, it will be 
appreciated that this invention is also applicable to printing 
applications other than those found in the field of thermal transfer 
printing. The scope of the present invention should be determined, 
therefore, only by the following claims.