DE19501795B4 - Optimize the printing speed of color thermal printers - Google Patents

Abstract

thermal printer with a print cycle for generating halftone images line by line, by a receiving medium and a selectively addressable printing elements provided printhead are movable relative to each other to substantially at the same time a complete one Line of pixels associated with each other at a print line speed to print, and by the pressure elements depending on the desired Density grades of an associated pixel for different lengths addressed are characterized by a drive control device that the Essentially sets print line speed to a value which is required to get the maximum image density of each Image obtained, and means that before printing a color plane Analyze image data for This color level required optimal print line speed calculate and according to the calculated optimal Print line speed this speed for the whole Adjust color level.

Description

The The invention relates to theme printers in general and to improvements on thermal printers for generating line halftone images in particular.

thermal printer this type are known per se. They are usually with one Roller or drum provided, which is a receiving medium and a dye-carrying Encoder medium moved past a thermal print head. The receiving medium exists mostly from normal or coated paper sheets, while the dye-bearing donor medium is formed as an endless belt. By acting the printhead on the dye-bearing donor medium becomes dye transferred to the receiving medium.

Of the Printhead usually consists of a linear array with close spaced pressure elements and extends in the pressure zone over the transfer drum. The individual heating elements can as electrical resistances accomplished be selectively switched to the temperature of the heating element on the for the transfer of the dye on the receiving medium required level. Instead, you can however, the printing elements also electrically from a linear array conductive elements used to generate thermal image information with a donor band functioning as a "resistance" interact. A printhead of this type is disclosed in US-A-4,800,399 disclosed.

In both cases is achieved by selective addressing of the printing elements in the printhead a whole line of image information printed simultaneously. Around To produce halftone images in which each pixel (pixel) of a Image line any degree of density from a variety of different Density levels may be the length of time during which the individual pressure elements are energized, changed, and either by change the width of the applied current pulse or by change the total number of current pulses during the printing of a Picture line given time interval. To create a full-color image will the entire printing cycle for repeated on the dye-bearing donor medium existing "color planes". Depending on execution The printer usually has three or four color planes.

The thus transferred from the dye-bearing donor medium to the receiving medium Dye quantity is dependent from the temperature, the pressure, the efficiency of the media and the Duration of the temperature and pressure.

In Thermal printers for Halftone images will be during printing a line, the receiving medium usually continuously moved relative to the printhead. This improves the print quality, because the pixels of adjacent image lines then merge. Furthermore this prevents sticking of the heating elements on the donor belt. The continuous Movement of the receiving media is usually effected by a precision stepper motor, the printing drum at a substantially constant speed moved so slowly that each Pixels in a line the largest gray level can accept (e.g., 256).

to To achieve a sufficiently high degree of density, the print line speed of Color thermal printers compared to other printers, for example Laser printers, preferably for black and white text or simple binary Graphics are used, relatively low being held.

The Print line speed is usually fixed and is chosen that she, if At some point in the picture high density is required, this requirement fulfill can. But if the transport speed of the receiving medium must be set to a value in which a line in each line Image with the highest density can be generated, of course, this has a limiting effect on the speed of printing in total.

ever However, the nature of the image to be printed is in one or more the available Color planes frequently No or very little dye required. In these cases is the print line speed slower than this to generate of the desired Picture would be necessary. In other words, the picture could in itself with a faster line speed and still be printed with the required density.

US-A-4 990 930 addresses the problem of increasing the productivity of a thermal printer. The patent discloses a method of increasing the speed at which the media is transported to the beginning of the next print line as soon as the pixel with the highest density in the current print line has been printed out. However, with the optimization of the speed of the printer achieved in this way, an acceleration and deceleration cycle must be accepted in virtually every print line. It would be desirable to have a system that increases the productivity of a thermal printer while maintaining a substantially constant media speed light. This would improve the accuracy of registration and reduce wear on the media transport mechanism of the printer.

One Another attempt to increase a printer's productivity will be disclosed in US-A-S 210 547. In this case, information about the Size of one Receive media sheets and the size and position the image of a printer control circuit to be printed on the sheet from an external device as inputs. If the picture gaps contains the media transport speed is increased at these locations. Thereby will the for the total printing required Time reduced. Although this method increases the performance of a Printer, however, is only for the areas of the receiving media sheet are applicable in which no image information must be printed. As a result, the overall effect on the productivity of the printer limited.

Further, on the DE 4 203 023 A1 which shows a printing method for a color video printer in which a maximum value of a heating degree is determined for a largest value of a line data of a line memory and a printing cycle for the line data is ended when the maximum value of the heating degree is reached irrespective of a total heat degree of number for the line. In the EP 0 180 911 A1 In turn, there is described a control method for a color printer in which movement of a print medium toward and away from a printhead has a higher speed than movement during the printing operation.

The The present invention therefore has the object of printing speed of thermal printers of the type described for generating halftone images with high resolution without a significant loss of print quality.

According to the invention this Task in a device having the features of claim 1, as well as a method with the features of claim 7 solved. preferred Embodiments of the invention will become apparent from the dependent claims.

Corresponding A feature of the invention is the printing speed for each color plane changed so, that she for the biggest in this Color level required density sufficient. In this way, the for the Total printing required Time to the lowest possible Duration shortened.

to increase the printing speed are in a preferred embodiment the invention analyzes the image data before expressing a color plane, around for the Color level required optimal print line speed through Calculation to determine. When initiating printing, the Print line speed for set the entire color level to this optimal value.

at For example, this data analysis can be at any point be determined in the color plane required largest density. After determination of this value is by calculation or at hand a search table determines the best print line speed.

To A feature of the invention is a thermal printer with a printing cycle for generating halftone images line by line, wherein (1) a receiving medium and a printhead provided with selectively addressable printing elements are movable relative to each other to substantially simultaneously a complete Line of pixels associated with each other at a print line speed to print and in which (2) the printing elements in dependence from the desired ones Density grades of an associated pixel for different lengths addressed are provided with a drive control device which controls the print line speed substantially to a value that is required to the maximum image density of each image just to receive.

To Another feature of the invention belongs to the thermal printer further a drive that (1) the receiving medium at high speed through the printing station transported to a first image line to be printed Receive section of the receiving medium with the printhead (2) the transport speed of the receiving medium subsequently slows down until the specified print line speed is reached is.

One Another feature of the invention provides that the thermal printer with a Drive is provided, which the receiving medium at high speed transported by the printing station after the last one to be printed Leave the image line aligned with the print head position Has.

Of the Thermal printer according to the invention is further characterized in that by repeating the entire Print cycle in multiple color planes a full-color image is generated and the print line speed between each print cycle is adjustable so that in each printing level the maximum needed Image density is achieved.

The analysis of the image data can take place at one of several locations in the image chain. To For example, software resident in a computer controlling the printer, such as the computer's print driver, could be used. The computer would report the optimization information to the printer before printing a color plane. Instead, the printer could also analyze the image data itself upon receipt from the host computer. There are printers that have a software level known as a "raster image processor" that would be suitable.

The Invention will be dargestellen in the following with reference to a in the drawing embodiment explained in more detail.

It demonstrate

1 a schematic view of a suitable for generating color images in a dye-receiving medium according to the invention suitable thermal printer,

2 a perspective schematic representation of several, in the printhead of in 1 shown printer used heating elements,

3 a partial section of a typical dye-carrying donor medium and

4a . 4b and 4c a logic flow diagram illustrating the optimization of the printing speed according to a preferred embodiment of the invention.

1 shows a thermal printer 10 according to a preferred embodiment of the invention with a cover mechanism 12 to which several main components are attached. These main components include a printhead positioning arm 14 , a printhead assembly 16 and a supply reel 18 and a take-up spool 20 for the dye-bearing donor medium. The main support structure 22 the printer includes a pressure roller assembly 24 , a transport device 26 for the dye-receiving medium and a reservoir 28 for the dye-receiving medium. The thermal printer 10 is here in operating position with closed lid mechanism 12 shown.

The cover mechanism is on a support shaft 32 in the main support structure 22 of the printer. After releasing a lock 34 can the lid mechanism 12 be opened by turning. The printhead positioning arm 14 is over a support shaft 36 with the lid mechanism 12 connected.

One includes normal print cycle loading the dye-receiving medium, printing out Information on the dye-receiving medium and the ejection of the finished expression. All of these processes are described in US-A-5 176 458 Completely described. The disclosure of said patent is hereby incorporated by reference by reference to a part of this specification explained, so here a brief description of the embodiment shown in the drawing the thermal printer is enough.

The print cycle begins with a loading phase during which the printhead assembly 16 enters a loading position, advances a sheet of dye-receiving medium into a printing position, and printhead assembly 16 is brought into the required position for printing. The printhead assembly 16 and the printhead positioning arm 14 initially in the eject position, ie at the highest point in the range that the printhead positioning arm 14 can drive through. A first cam 38 is brought into a loading position between the ejection position and the printing position. One on the printhead positioning arm 14 attached first cam follower 40 keeps in touch with the first cam 38 upright. The printhead positioning arm 14 turns around his support shaft 36 in a loading position.

A pen reception opening 42 on the printhead positioning arm 14 limits the possibility of movement of a first pen 44 the printhead assembly 16 on movements along an axis which is perpendicular in this embodiment of the invention. In the loading position, the first pin touches 44 one side of the pin receiving recess 42 and an unillustrated set screw for adjusting the point at which the printhead positioning arm 14 the first pen 44 touches when the arm 14 moved from the printing position to other positions, and to change the point in the movement of the printhead Positionierarms, on which a second cam follower 46 a second cam surface 47 touched.

When retracting the printhead positioning arm 14 in a loading position also moves the printhead assembly 16 in their loading position. The second cam follower 46 and the cam surface 47 allow for a tilting movement of the printhead assembly 16 in the direction of the pressure roller assembly 24 until a guide 48 the pressure roller assembly 24 touched. If the printhead assembly 16 She has reached her loading position, she is from the pressure roller assembly 24 moved away.

A sheet of dye-receiving medium 50 moves into a leadership 52 for the dye-receiving medium and then moves to egg a curved path to a gap between the printhead assembly 16 and the pressure roller assembly 24 , Upon entering this gap, the dye-receiving medium contacts a dye-bearing donor medium 54 and then becomes the transport mechanism 26 for the dye-bearing medium and an associated pinch roller. In the embodiment of the invention described here, the dye-receiving medium is present in leaf form. However, dye-receiving media in roll form may also be used. When the dye-receiving medium 50 from the transport device 26 has been detected safely, moves the printhead assembly 16 to the pressure roller assembly 24 to the dye-bearing donor medium 54 and the dye-receiving medium 50 for thermal printing stacked against a pressure roller assembly 24 to press.

Upon completion of the loading phase, the printer enters 10 in a print phase while the printhead assembly 16 the dye-bearing donor medium 54 and the dye-receiving medium in the printing roll assembly 24 press and print information on the dye-receiving medium.

When printing is complete, the printer enters 10 in an ejection phase during which the printhead assembly is removed from the platen assembly and the finished print is ejected from the printer. When the printhead assembly becomes 16 is in the ejection position, the dye-receiving medium between the transport device 26 and a pinch roller 55 captured to the finished printout from the thermal printer 10 to transport. After completing the ejection phase of the print cycle, the printer is ready for another print cycle.

As in 2 is the printhead of the printhead assembly 16 with a variety of heating elements 56 , For example, electrical resistors, provided against the dye-bearing donor medium 54 are pressed to press this against the dye-receiving medium. By closing one of a variety of switches 58 becomes the associated heating element 56 connected to a voltage source V _S. The amount of dye transferred depends on the length of time during which the switch 58 closed is.

The dye-bearing donor medium 54 consists of a leader and a repeating array of dye fields. The dye detectors may follow one another directly, as shown in the drawing, or be spaced apart. As in 3 As shown, each row consists of a yellow, a magenta and a cyan-blue dye field. For printing out a color plane on the dye-receiving medium 28 Of course, only one row is used at a time.

By "dye" herein is meant a colored substance that is transferred from the dye-bearing donor medium to a dye-receiving medium as a function of the application of energy by the individual elements of the print head. In the printhead shown here are the heating elements 56 designed as electrical resistors. However, other energy sources, such as a laser diode array and individual lasers have also been used for this purpose, which are also suitable for the implementation of this invention. After forming a color plane on the dye-receiving medium, this is referred to as "printout".

As shown in the drawing, the dye-donating medium becomes two LEDs 60 and 62 illuminated from above. The LED 60 emits yellow light, the LED 62 Red light. Two photosensors "A" and "B" located under the dye-bearing donor medium receive light that passes through the dye-bearing donor medium. The photo sensors "A" and "B" produce a signal for the beginning of a row and for each individual color patch in that row. A detailed description of this signaling is given to S. Stephenson co-pending US Pat. 4,710,781, the disclosure of which is hereby incorporated by reference into a part of this patent specification. Those skilled in the art will appreciate that other known devices can be used to signal the particular beginning of a series of dye fields, see, for example, US Pat. 4,893,951.

As in 4a . 4b and 4c As shown, the flow of a preferred method according to the invention begins with the generation of image data by the block 70 by using, for example, a scanner, a computer program for making drawings, an electronic camera, an imager, etc. When a computer is used, the operator can modify, edit, or combine image data to produce the image to be printed.

According to the color sequence for which the printer is designed, the computer divides the print data into different color planes, for example yellow, magenta and cyan or yellow, magenta, cyan and black (logic function block 72 ). When transferring the data for one color level to the printer, various counters and flags are initially set to zero. These include, inter alia, counters and flags for the number of the first line, the number of the last line, the largest density "D _MAX " and the number of the current line (logic function block 74 ). The block 76 increases the current line count. The logic function block 78 resets the counter for the current pixel (PIXEL _n ) to zero and a flag for "PIXEL USED" to NO.

The logic function block 80 increases the pixel count. The logic decision block 82 decides whether the PIXEL _{n should be} printed. If the density of PIXEL _{n has} a value other than zero, the logic function block sets 84 the flag for "PIXEL USED" to YES. Thereupon the logic decision block compares 86 the density of the PIXEL _n with the value D _MAX . If PIXEL _{n is} greater than D _MAX , then the logic function block updates 88 the value of D _MAX to match the density of PIXEL _N.

If the current line is the first line in which a pixel whose density has a value other than zero occurs, the flag for the first line becomes as in 4a . 4b and 4c shown by the logic blocks 90 and 92 updated to bring it up to the current line level. After updating, the flag for the first line is not changed until the end of the color plane.

If the logic decision block 94 determines that more pixels are to be checked in the current print line, the process is repeated until the last pixel of the line has been checked. After examining the last pixel of a line, this line is called the LAST LINE if any pixel in that line has a density whose value is not zero. See logic blocks 96 and 98 ,

If the logic decision block 100 determines that the current print line is not the last line of the color plane, the process for the next print line is repeated. Otherwise, the color plane can now be printed.

The fastest print line speed that can be used for each color plane is calculated using the value DMAX, as this value indicates the pixel that needs to be energized the longest. When you create color correction tables for thermal printers, you usually start from a specific print line speed. Therefore, the generation of new table values is preferably based on the print line speed determined on the basis of the value D _MAX .

One sheet to be printed with a color plane image of a receiving medium can be transported through the printing station at high speed, until the area of the leaf, that of the flag for the FIRST LINE should receive specific image, aligned with the printhead is. However, the color plane is not printed until the sheet is up the calculated fastest line speed slows down has been.

If the marked LAST LINE has been reached and printed out is, the receive media sheet is either transported back so that the next Color level can be printed, or ejected as a finished expression, if the last color level has already been printed. The time, which is normally required to use the receiver sheet with the low print line speed from the last one to be printed To transport line to the trailing edge of the sheet is so saved. This increases the productivity of a printer elevated.

The Invention is described in detail here with reference to a specific embodiment Service. It is understood, however, that the invention by nature and scope deviations and changes across from quite possible in this example.

3

a

movement direction of the carrier

b

Farbstoffeldreihe

c

leader

4a

70

Image data generation

72

color sequence

74

FIRST LINE = zero

LATEST LINE = zero

D _MAX = 0

Line count = Zero

76

Line count increase

78

PIXEL _n = 0

PIXEL used = no

80

PIXEL count increase

4b

82

Density PIXEL _n = 0?

84

PIXEL used = yes

86

Density PIXEL _n > D _MAX ?

88

D _MAX = Density PIXEL _n

90

FIRST ROW = ZERO?

92

FIRST ROW = current line

4c

94

last PIXEL in the line?

96

PIXEL USED = YES?

98

LATEST ROW = current line

100

is this the last print line of the color plane?

d

color plane print

e

latest Color plane?

f

leaf at high speed back to the beginning

porting

G

manufacture Eject the expression

Claims (8)

Thermal printer with a line-by-line printing cycle Producing halftone images by using a receiving medium and a selectively addressable printing elements provided printhead relative to each other are movable at substantially the same time a complete line pixels associated with each other at a print line speed to print, and by the pressure elements depending on the desired Density grades of an assigned pixel can be addressed for different lengths of time, characterized by a drive control device that controls the print line speed substantially to a value that is required to obtain the maximum image density of the respective image, and Means analyzing image data before printing a color plane, the for This color level required optimal print line speed calculate and according to the calculated optimal print line speed this speed for the whole Adjust color level. Thermal printer according to claim 2, characterized that means are provided, which the receiving medium at high speed through the printing station until one of the first to be printed Image line to be received section of the receiving medium with the Printhead is aligned, and the transport speed then the receiving medium slow down until the specified print line speed is reached. Thermal printer according to claim 2, characterized that means are provided, which the receiving medium at high speed through the printing station after the last one to be printed Leave the image line aligned with the print head position Has. Thermal printer according to claim 1, characterized that the Addressing duration for the printing elements changeable is by the width of an applied to the printing elements electrical Impulses changed becomes. Thermal printer according to claim 1, characterized that by Repeat the entire print cycle in multiple color planes Full color image is generated. Thermal printer according to claim 5, characterized that means are provided, which the receiving medium at high speed through the printing station until one of the first to be printed Image line to be received section of the receiving medium with the Printhead is aligned, and the transport speed then the receiving medium slow down until the specified print line speed is reached, and that means are provided, which the receiving medium at high speed transport back through the printing station, after the last line of a color layer to print with its print head has left aligned position so that the next color plane can be received, without that full receiver sheet must be moved past the print head. Method for generating halftone images line by line, wherein a receiving medium and a selectively addressable printing elements provided printhead are movable relative to each other to substantially at the same time a complete line pixels associated with each other at a print line speed to print, and the printing elements for different lengths of time depending from the desired ones Density grades of an associated pixel are selectively addressed, characterized in that the Print line speed is set to a value substantially which is required to the maximum image density of each image to obtain image data before printing a color plane and the for This color level required optimal print line speed will calculate and according to the calculated optimal print line speed this speed for the whole Color level is set. Method according to claim 7, characterized in that that this Receiving medium at high speed through the printing station is transported to the first image line to be printed Receiving section of the receiving medium aligned with the printhead is, and then slows down the transport speed of the receiving medium until the specified print line speed is reached.