CA1261035A - Leading edge detection in self-correcting printer-verifiers - Google Patents

Abstract

ABSTRACT

Indicia having a requirement for a high degree of precision is printed onto a sheet and is scanned or read automatically shortly thereafter. Electronic control signals result from a comparison of the actually printed indicia with a pre-defined quality standard, and are used selectively to adjust command control signals to the printer to correct indicia printed thereafter. The leading edge of sheets to which the indicia is to be printed is precisely located by a light absorption detection or sensing mechanism which controls a sheet advance motor. The indicia printed, preferably bar code, is read by an optical bar code scanner traversing the printed code, The scanner is controlled by a scanner microprocessor control, and the printer is controlled by a printer microprocessor control which controls a comparison of the scanned code with a set quality standard to result in output signals to control the printer microprocessor control in a feedback loop to increase or to decrease the time of voltage application to printing elements in a thermal printer.

Description

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:t .1 Ihe present invention relates to ~le art of . 5 printing, and more Farticularly to leading edge detec-tion ;~ I; useful in apparatus for printing an~ verifying precision cf , bar codes, and correctlng subsequent pr mts of bar codes on ~ ~ labels and shee-ts.

i !l ~. I() ll 2. Description o~ thc Prior ~rt:

~ ar codes in various forms are now familiar as identifying and pricing indicia for various commodities and material, and are used to record pricing information and material llandling and locating information on commodities I_ and products in applications as varied as grocery store 18 clleckou:s and inventory control. ~ar codes typically 1 comprise vertical bars of dark, non-reflective lines of ~() varying widths interspacPd by higllly light reflective spaces 21 oE varying widths. ~ group of such bars and spaces, 22 representing a "symbol" is scanned by optical scanners 23 reflecting light off the bars and spaces to photodetectors

2~l which distinguish the difference betwesn the dark bars and the rsflective spaces, and of their relative widths.
~ '~0 ~_ To clistinguisll between the relative widths of bars and of spaces, proccssing of the informatioll from tlls ~!) scanlled code must distinguish the time of the scanner's pass : 3() over of the ~ars and spaces. Usually, great tolerances must 3l be afforded to the differcnt, varying widths in order to :1~
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1 accominodate a substantial variability in the speed or ~elocity of the optical scanller, and to accurately :~ distinguish between bars, or non-refleCtiVe areas and l spaces, or highly reflcctive areas.
, (; It is greatly desired to reduce the widths of the _ bars and spaces generally conceded to tolerances without sacrificing accuracy in the optical scanning, in order to !) increase the information in a given space and to increase 1~ the reliability of the lnformation read-out in the scanning 1~ procedure. For example, it has been shown to print an 1~ alignment mark to trigqer the commencement of an optical bar 13 code scanning at a precise point, with the objective of l;~ reducing the symbol space, as is shown in U~S. Patent No.
Ir) 4,349,7~1 to ~obart et al., where the alignment mark is scanned contemporaneously with the printing so as to 1~ position the scanner at the center of the bar code, and thus 1~ increase the accuracy in the scanning of the code.
1'.) "~() Thermal printers have been known and preferred in 21 many applications where non-impact printing is a desired 2~ goal~ Thermal printers are characterized by the controlled 23 application of precise amounts of heat, either by a print 2~ element head or pixel to a paper sensitive to the 2~ temperature to produce an optically readable print, or to a 2G thermally sensitive ribbon adjacent the paper to be printed, 2/ in which by the application of heat to the ribbon, optically 2~ readable material is melted onto the paper. In such ~!) thermally sensitive printing, precision in -the temperature

3~ of the printing elements is important. Methods and 3l ayparstus-s for s sing th- temperat~re ~nd correcLin~ or , ' ~26~(33~;;
-control.ling the temperature -to the print head in response to the sensed temperature have been de~cribed, for example, in the dot . matri~ printer controls of U.S. Paten-t No. ~,577,1~7 to Brennan, Jr. and U.S. Patent Mo. 4,449,03~ to McClure, et al.
i While greatly improving the quality of thermal printing, it is ~till de~ired to have bar code printers automatically positioning the edge of label~ to be printed in such a printing and verifying printer~ It i~ de~ired further to have Q printer with the self-correcting and contemporaneous verification features in a non-impact printer~ This may be done in conjunction with an apparatuo which self-correct~ the precise widths of the bars and space~ in a bar code during the printing proce~s in response to a contemporaneous read-out of the immediately preceding printed code, and potentially independent of temperature ~ensing. It may also be done in conjunction with a bar code printer having contemporaneous read-out and verification of the printed code, and automatic rejection of code which does not fit within pre-defined width dimension tolerance~ of preci~ion for the bars and spaces.

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The invention relate~ to a method of determining in a printer the leading edge of each of a series of successively po~itioned labels through the printer into pooition adjacent printing elements within the printer. The method comprises the steps of: moving each label succes~ively through a first light path comprised of a first light source and a first photodetector capable of detecting light through the label; producing a first

4 -kh/,~, -~2~35 electrical signal. representative of the change in opacity when the label i9 moved through the fir~t light path; further moving the label through a second light path comprised of a second light cource and a second photodetector capable of detecting light through the label; producing a second electrical signal representative of the change in opacity when the label i~ moved through the second light path; comparing the fir~t electrical signal with the second electrical signal and producing from this comparison a digital initiate oignal for initiating an operation routine in a control procescor, whereby a print operation routine is initiated operating the printer to print indicia on the label when the label is positioned adjacent the printing elements.

This method may be used with a non-impact bar code printer as later described comprising a thermal printer combined with an optical scanning system for reading indicia, such as bar kh/

l cocle imprintecl on Labels, longitudi~ally extended sheets and 2 tl~e lil;e.
:1 1 In operation ~ longitudinally extended sheet nonrEllly hav~ng r gumlled labels mounted on the sheet ii~ advanced -through a (; tllermal printer capable of applying heat directly to a - thermally sensitive paper, or of aE~plying heat to burn or ~X melt thermally sensitive ribbon onto t~le slleet. The leading !l e~lge of the labels on the extended slleet is sensed by two 1~1 successive-y positioned photodetecting paths sensing the ]I clegree of opacity of the paper passing through tlle light 1~ path. Differential voltaye signals resulting rom the ]~ initial passing of tlle leading edge througll tlle first, but 11 not the second of the detectors generates a differential signal from a differential amplifier, whicll signal is shaped I(; and amplified to form a digital signal to a print control 1~ processor for advancing the label on tlle eYtended slleet lX tllrougll the tllerlnal prlnter, and for initiating the print I!) cycles at tlle correct time when the label is properly 2() positioned ju~taposed the printing elements of the tllermal 21 printer, '-)2 2X A transport assembly holding a paper path tray and ~l drive rollers is capable of movement in order to present 2a easy access to the print area and to the area tllrough which ~(; the paper must be initially fedO When rnoved into the print 2/ position, the transport assembly can be latched into place, X and the print head can be varia~ly positioned so as to apply !1 more or less pressure against tlle sheets to be printed.
control panel and readout display provide operational ~1 accessibility to the functioning of the printer and verifier.

i lZ61035 ~¦ IntegraL ~/ith the printer is an optical scanner 2 positiolled to transversely scall the sheet subsequel-t to the ~ printing operation. 'rhe scallner and printer are controlled 1 l~y a master control processor, ill turn controlling a scanller control processor and a print control processor. ~y use of Il stepper motors, two drive rollers advance the sheet so tllat _ tl~e area, such as the label to be imprinted is precisely ~ postiolled juxta~osed printing elemellts of a thermal line !) printer. The printing elements or pixels on the print head 1() are driven by print head drivers controlled by the print Il control processor in the control system. Data is input ~o a 12 master control processor from a dis)~ terminal or otller 11 computer, and a label descripticn bufFer is created so that 1~1 the master control processor can set up the print control r~ processor.
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1- ~fter printiny, the printed sheet is scanned by an 1~ optical scanner driven by a scanner stepper drive motor l!) transversely from one side to the other side of the printed 2(~ sheet. In a first pass, the optical scanner establishes a 21 reflectivit~ tllreshold by determining peaks or averages of 22 reflectivity in the printed code. In the reverse pass, the 2:1 widths of the individual bars and spaces are measured, and 21 the measurement dimensions sent through a scan head driver ~a and interface to a scanner control processor, thence to a ')(; measured data buffer or memory. The scanner is limited in ~¦ its transverse pass by a right limit switch and a left limit ~>,~ switch each comprising a light path in a photodetector ')!~ circuit interrupted by flanges on the scanner head assembly.
~() Rt the left limit switch, the direction of the scanner .31 assembly on its path defined by the scanner cable is reverse~ t the right limit switch, the scar-ner asseIl1~1y ~ stops in a nestil)g position with the reading head reading a X calibration plaque for calibrating the detection circuit of I the scanner.
-(i The master control processor has established in a _ verified print quality buffer tile standard widths which the '3 individual bars and spaces of the printed code should have, !) and compares the actuaLly measured dimensions of the bars I() and spaces with the standard. A deviation between the II actually measured dimensiol1s and the standard dimensions is 12 established, ~nd recorded in a print deviation buffer or l:I memory. Averages are taken of the deviations, and if these II averages exceed a predetermined tolerance limit, a Iru correction signal is generated to modify the time pulse a~
I(; the print head drive siqnal selection in the print control I- processor to increase or decrease the voltage application time for the individual print elements for the bars in the 15) symbol found to be intolerably deviant from the standard.
~() ~I The control system tl1rough a master control 2~ processor can generate signals to actuate a "void" stamp 2:3 solenoid to brand the rejected label, and to actuate other 21 alarm indicators as may be desired. Further, a command is 2u generated to cause the printer to re-print the rejected 2~ label or page.

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l)5) Other novel features which are believed to be :3() characteristic of the invention, both as to organization and 3~1 method of operation, together with further objects and 3~1 advantages thereof, will be better understood from the : .:
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' following descriptioll considered in connection with the accompanying drawings in which a preferred embodiment of the invention is illustrated by way of example. It is to be understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as the definition of limits of the invention.
BRIEF DESCRIPTION OF` THE DRAWINGS
Fig. 1 is a perspective view of a printer and verifier used in conjunction with the present invention;
Fig. 2 is another perspective view of the printer and verifier of Fig. 1, having certain elements removed and others cut-away for clarity;
Fig. 3 is a detailed perspective view of the scanning head and printing elements of the embodiment of Fig.

1 ;
Fig. 4 is a side elevational, cross-section view of the optical scanning head of Figs. 2 and 3;
Fig. 5 is a perspective view of the print head;

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l~ig. 6 ia a detail0d eide glevAtion view of -ths pre~ure ~etting arnl of the embodiment of Fig. l;
Fig. 7, appearing on the fourth nl~eet of drawing~ a front elevation view of the ~procket and l~bel edge detector of -the invention;
Fig 8, appearing on the fourth ~heet of drQwings, i~ a per~pective view of the ~procket and label edg0 detector of Fig. 7, showing the interior detector element3 and tha overall caaing in broken line for clarity;
- Fig. 9 ie an electrical ~chematic of tha aprocket and label edge detector circult of the preferred embodiment;
Fig. 10 i~ a detailed eida eleva~tion view of the "void"
~olenoid and linkage of the embodiment of Fi~ure l;
Fig. 11 ia a perapective view of the acanning head and printing elemente of the preferred embodiment in an alternative mode of opera-tion;
Fig. 12 i~ a block diagram of the operation of the embodiment including the present invention;

Fig. 13 is A block ~chematic diagram of the control ~y~tem ;
O Fig. 14 is a flow diagram of the processes m~luding -the present invention;

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~L2f5~L~35 Fig. 1~, appeariny on the fifth sheet of drawings, is a pa~l diagram of the optical scanning path;
Fig~ 16 is a flow diagram of the processes of the correction signal generation; and Figs. 17, 18 and 19 are map diagrams of memory buffers within the process control embodiment. Fig. 18 appears on the eigh-th sheet of drawings.

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I DESCnlPTION OP l~llE PREFE~RED EMBoDIMr~NT

:3 Initially referring to FIGS. 1 and 2 of the drawings, a self-correcting printer and verifler 10 for _ printing indicia, such as a bar code 12 on a longitudinally extended sheet 1~ is depicted in perspective view. The 7 sheet 14 travels in the direction of arrows 16 -through the 8 printer-verifier 10 from a row or a fanfolded stack of edge ~1 connected sheets forming an indeterminate length of paper, 10 often times sprocketed on the linear edges for advancement 11 by sprocket wheels. The paper may be thermally sensitive 12 paper of the kind which, when a certain level of heat is 13 applied adjacent the paper will form marking or printing on l4 the paper. ~s will be explained in greater detail below, other types of paper may also be used, such as an ex.tellded lG sheet of backing having a series of successive gummed labels 17 mounted on it.
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l'~ The printer and verifier 10 is comprised of printer chassis sidewalls 17 and a chassis base 19 in effect 21 housing many of the operative elements, and providing 22 mounting for substantially all of the alements. To the 23 front of the printer 10, a control and display panel 20 2-1 including a read-out digital display 22 is shown.

2~r3 2G A paper transport assembly 24 is shown movably 27 mounted to the chassis sîdewalls 17 on rails 26 to move 2.~ forward and rearwardly in the direction of double arrow 28.
2() The rails 26 are connected to the sidewalls 17 by chassis 30 mounts 25. The transport assembly 24 may be secured in the l 1 2!61(~35 I orward or operating pOSitiorl by a convellient latchilly .~ system, not shown wi-icll is released by latch lever 30.
;l l A print head platen 32 is mounted to tlle transport r asse~bly 24 for driving the paper 14 in the paper path, (; whl.ch will be explained in greater detail shortly. Mounted _ between the sidewalls 17, AS depicted in FIG. 1 is a roll 34 of thermal printing ribbon 35 which is fed from a spool ~ about which it is wrapped, to the take-up roll 36 for the printing ribbon, wrapped around spool 38.

~2 The ribbon take-up roll spool 38 i5 held between 13 tlle sidewalls 17 on one side by a ribbon take-up roll 14 mandrel or cone 40, man~pulated on the outside of the wall 17 by mandrel knob 42, and on the other side oE the IG spool 38 by a mandrel or cone capable of being driven by 1~ shaft 44. Shaft 44 is driven by a ribbon tension servo 46.
IX The motor current to the servo 46 i5 controlled by a sisnal 1~ responsive to a sensing of the tension of the printing 2() ribbon 35, as more fully explained with reference to 2l FIG. 11, below.

23 The feed roll 34 of prin~ing ribbon 35 is 2~i similarly held between the sidewalls 17. On one side, as seen in FIG. 1, the ribbon mandrel or cone 48 engages the 2G roll spcol, and allows for r~latively frec rotation of ~he 27 roll 34.
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2~) ~ solenoid 50 is connected by solenoid linkages 52 to drive a "void" stamp, as will be explained in greater 31 ~etail below.
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3L~6~l(33~5 1 ~ print heacl pressure settiny arm 54 can be rotated about its spindle, and fixed by set screw 5G at .~ various set notches 58 for controlling the pressure of the 4 print head 90 (FIG. 2~ against the paper 14, as explained ~) further below.
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7 Rn optical head encoder 60 is mounted on a common ~ shaft with a right optical head cable pulley 62 for !) providing accurate location information of the scanning 10 head 100, again described in greater detail below.

Il 12 A pillCh roller lever 66 referring to FIGS. 1 and 13 2, operates as a cam for pinch roller 68 to pinch firmly 1~l p~per 14 between it and a paper tension drive roller 70 operably positioned in the transport assembly 24.
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17 In FIG. 2, the sheet advancing system is depicted 18 in greater detail. ~ toothed timing belt 72 drives a 1(~ s~ocketwheel 74 which is axially connected to the paper 20 tension drive roller 70. The timing belt 72 is driven by 21 gear 76, which is connected to the gear 78. The gear 78 is 22 driven by the paper drive motor 80 through gearing 82 23 connected to the shaft of paper drive motor 80. Gear 78 is 24 axially connected to the print head platen 32 to provide the main driving force for the paper 14. An adjustable tension 2G roller 84 is provided to control the tension of the toothed 27 belt 72.
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2(J A paper path tray ~8 is provided to guide the 3~ paper 14 along a proper paper path.

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1 Print head qO, compriSit~g a print llead circuit 2 board 92 holding the print elemel~ts 94 (FIG. 3~ and a heat 3 sink ~6 is positioned in operable and printing relationship ,~ to the paper 14 opposi.te a vertical back wall of the r) tray 88.
(i Asprocket asld label edge detector system 98 is depicted at a point at the beginning of the paper path where I) paper 14 entcrs into the paper transport assembly 24.

I] ~ scanning head 100 is movably positioned to read 12 the bar code 12 printed on the paper 14 b5~ traversing the l3 lateral width of the paper 14 in the direction of arrows 1~ 102. As better seen in FIG. 3, the scanning head 100 comprises an optical slide assembly 101 slidably mounted on IG rail 103 and stabilized by slider assist guide ]0~. ~n 1/ optical head cable 105 is connected to the optical slide 18 assembly 101, and is looped around head cable pulley ~2 at 1~ the ri.ght end (as viewed from the front), and head cable pulley 106 at the left end. Tensioning pulleys 107 are 21 provided to main-tain the head cable 105. The p~lley 106 is 22 mounked to the shaft of the scanner stepper drive motor 108.

24 An optical head or block 109 is mounted within the optical slide assembly 101.
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27 Limit switches 110, 111 are positioned at each end 2g of the scanner traverse path 102. Each limit swi-tch 110,111 2(~ comprises a v~rtical light path, having a light emitting diode (LED) light source and a photodetector. The light 31 path is positioned to be interrupted by one or the other of ~32 ~ 3~

1 t~le flanges 112, 113 flaring from the optical slide assembly 2 101- The controller for tlle optical slide assemble receives 3 the signal resulting from the interruption of the light path in either o~ t~le limit switches 110, 111, and stops the traverse of the scanner head 100. In the case of the left G limit switch 111, the scanner head traverse is reversed, and 7 the scanner head 100 commences a selective reverse scan of 8 the printed matter. The scanner head 100 upon return to tlle ~ right side stops upon interrupting, by its right flange 112 10 the limit switch 110, in its nestiny position.

12 A caLibration plaque 115 is provided having both a 13 standard darkness area 116 and a standard reflectivity area 14 117. The dark area 116 is designed to reflect only two percent (2~) of ligllt focused on it, while tlle highly IG reflective area 117 should reflect approximately 80~ of 17 light focused upon it~ The calibration plaque 115 can be 1~ used, when the scanning head 100 is in its nesting position, 1(J to test or verify the operation of tlle light path and of the scanning and reading circuits. Additionally, the light 21 reflectivity sensitivity of the scanner's photodetectors can 22 be calibrated from the reflective plaque 117.

24 A ribbon strip plate 118 is provided with 2~ articulated tensioning arm 119 to guide the thermal printing 2G rib~on 35, if used, from the paper 14 after a printing step.
27 The tensioning arm 119 can provide, further, a signal to 2~ control the direct current ribbon tension servo 46, as will 21~ be explained more fuLly with reference to EIG. 11.

.~2 ` ' ~2~3Si I ~ "void" stamp or mar~er 122 comprises an inl~
.~ pad 124, and is mounted to move i~l the direction of arrow 126 to engage the paper 14, as will be more fully e~plained with respect to FIG, 10. ~ thermistor 120 is r) moullted to the heat sink 96 for use i31 temperature G stabilization of the print head 90. Temperature control by sensing the heat sink temperature i5 not part of the preferred embodiment, but is sho~n for convenience.
() In FIG. 4, the optical head or block 109 is shown 11 in cross-section elevation. ~ light source mounting 12 block 128 houses three light emitting diodes (LED's) 13 providing light. Lens 132 is movably set within the ligl-t 14 path 133. The lens may be fixed in any conventional manner as by a set screw, for example. The block 109 comprises, in 1G addition, an aperture 134 for controlling flair of the 1/ light ~ithin the path 133. A photodetector 136 is at the 1~ end of the light path 133 to detect the contrast between the 1~ reflective and non~reflective surfaces scanned. The light 2() source mounting block 128 is press-fit onto the optical 21 block 109 in order to align the LED's 130 to the sheet 14 to 22 be scanned, and to provide access to the lens 132 for ~3 movement in order to focus the image along the light 21 path 133. The number of LED's is not crucial so long as a sufficient number of them is used to difuse the 2G illumination and make the light reflected independent of 27 paper peculiarities, such as grain.
2g 2J In ~IG. 5, the print head 90 i5 depicted. The print head circuit board 92 contains the printing elements ~2 or pixels 94. In the preferred embodiment, 1728 pixeLs are l lil~early arranged along a sillg~e line at a distance of 0.5 2 mils apart, each pixel having a width of approximately 0.5 3 mils. A heat sink 96 comprising a metal block is mounted to ~] tlle circuit board 9~ having a maximum area of surface-to-sUXfaCe contact therebetween in order to have the (; greatest possible heat transfer. ~ thermistor 120 ~FIG. 3) 7 may be used to monitor the temperature of the heat sink 96, 8 and in turn the printing elements 94. A cover 95 is shown ~ cover.ing the circuit board 92.

~] In FIG. 6, the details of the print head pressure l~ setting arrangement are depicted in cross-section elevation 13 ~ print head pressure setting arm 54 rotates about axle 141 l4 connecting the arm 54 to the cam 140, which engages a cover plate for the print head 90. ~hen the arm 54 is rotated in lG the direction of arrow 142, the printing end of the print 11 head 90 is moved in the direction of arrow 144 into a 18 greater pressure contact with the sheet 14. The arm 54 can lV be fixed in position by turniny set screw 56 into any of the notches 58, and thereby maintaining the desired pressure of 21 the print head 90 against the sheet 14.

23 In FIGS. 7 and 8, the operation of thesprocket and 2~ label edge detector system is shown. The linear edges of the longitudinally extended sheet 14 pass between light 2~ sources, comprising LED's 148 and photodetectors 150 shown 27 in broken line in ~IG. 8. The edge detectors 98 can be 28 moved horizontally along the directions of arrow 152 and 2~ fixed to the chassis frame 154 in relation ~o the sheet 14 by bolts 156.

.~2 lZl~i~0;15 I I l'llotodetectors 150 detect 1iqh~ fran tlle source 148 ~rou~h ~e sheet 14.
ever ~-le op~city of the sheet ch~lges, such as wllen the edge of 3 a ~abel 158 passes therethrough~ tlle voltage level of the l photodetector 150 will alter. In its quiescent state, tlle .) voltage received by amplifiers 160 and 162 from the G photodetectors 150, representatively shown as points 2 and 3 / in ~IG. 9, will be relatively comparable~ and after 8 amplification will provide comparable inputs to difference 9 amplifier 164. Comparator 164 will provide a low output to amplifier 166 until tlle inputs of amplifier 164 are Il different. Upon detecting a difference of at least a 12 magnitude determined by divider 165, amplifier 164 gives a 1~ lligh output which is amplified by amplifier 166. ~mplifier 1~l lhG forms the signal more into a digital pulse, which after forming by diodes 167 is amplified to S volts by amplifier ~ 168 to produce a signal to tlle ribbon and paper drive feed 17 drivers and interface 222 with the Print Control Processor 1~ (~CP) 210 ~FIG. 14). A control signal controlling the paper 1~ drive motor 80 thus i5 generated. In the control system, tlle paper stepper drive motor 80 then advances sheet 14 a 21 predetermined distance calculated as the distance betwee 22 the edge detector g8 and the point of engagement by the 23 printing elements 94 to the sheet 14.

2~ In operation, when an edge of a portion of 2~ slleet 14 having different opacity, such as label 158, first 27 interrupts the light path from light source 148 to the 2~ detector 150, the first amplifier 160 receives a different 29 voltage input representative of th~ different opacity, causing in turn a high output signal from differential 31 amplifier 162. Tllis output signal is eliminated when the ~2 voltage to amplifiers 162, 164 is changed by a certain ~2~
1 interrupts the light pa~h from light source 148 to the detector 150, the first amplifier 160 receives a different :~ voLtage input representative of the different opacity, 4 causing in turn a high output signal from differential

5 amplifier 162. This output signal is eliminated when the (; voltage to amplifiers 162, 164 1s changed by a certain amount predetermined by divider 165, representing detection by the second photodetector 150 of the same leading edge of (.) different opacity, such as the leading edge of label 158.
1~ The output signal of differential amplifier 164 is then low.
11 The signal briefly generated is formed through 12 amplifiers 166, 168 to result in a digital start signal to 13 the print control processor to commence printing at a 14 precisely clocked time subsequent to the receipt of the signal, calculated to be the precise time that the edge, or l(; some set distance beyond the edge of label 15B is advanced by the paper drive motor 80 to the paper path position 18 opposite the printing elements 94.
I!) In FIG. 10, the solenoid 50 operating the "void"

21 stamp is depicted. The rod of the solenoid 50 operates 22 through linkages 52 to rotate the arm 121 of the stamp 122 23 in the direction of arrow 126 so that the ink pad 124 will 24 engage or stamp impressions upon the paper 14. The solenoid 50 is actuated at any time that the processor 2G controls determine that a bar code deviates from standard or 27 acceptable tolerances, so as to reject the bar code so printed, whether it is a portion of a longitudinally ~(~ extended sheet 14, or a particular label mounted on such a sheet. It is denominated as "void" stamp because it is 32 contempl ed that the word "void" will be iormed on the ink .'...
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`' :' ' ~ s 1 pad for stamp printilly. ~ny otller symbol, of course, coul~
2 be formed on the pad 12~ to impri~lt w~atever indicia might 3 ~e desired.

_ In FIG. 11, an alternative mode of operating -the (; preferred embodiment of the invention is shown. The _ preferred embodiment of an invention will operate equally 8 using the print head 90, whether the paper 14 is comprised () of a thermally sensitive paper which will produce an 1() imprinted indicia upon the application of heat from the 11 printing elements 94, or may be ordinary paper against which l2 a thermally responsive ribbon 35 is heated so as to -transfer 13 to or melt on the paper 14 a dark imprint when the thermal 1~ ribbon 35 is heated by the pixels, of the heating elements 94. In FIG. 11, the printing by use o:E a thermal l~ ribbon 3S is depicted. The ribbon 35 is rolled off oE
1/ ribbon roll 34 onto take-up roll 36 wound around its l~ core 38. The path of the ribbon 35 is guided around ribbon ID guide 172. Platen 32 drives the sheet 14, and consequently drives the ribbon 35 which is adjacent to and held in 21 friction contact with it by virtue of the pressure set by 22 the print head 90. The path of the ribbon 35 is then 23 stripped from sheet 14 by ribbon strip plate 118 and over 2~1 articulated tensioning arm 119.

2G The tensioning arm 119 can provide a movement to a 27 potentiometer to geslerate a correspondingly varying voltage 2g as a control to the ribbon tension servo 46, and thus to 2'~ increase or decrease the motor current to the direc-t current servo 46. The servo 46 then keeps a rotary axle pressure on 31 the core 3B isl order to keep the ribbon 35 taut and to 32 prevent fouling with the optical scanning block 109 and ~1 ~LZ6~:113S

I light mounting block 128. The path through which ribbon 35 2 traverses i3 kept clear o~ the stamp 122 and its operational 3 space, also.
'1 ~IG. 12 is a block diagram which diagramatically G shows the interrelationships between the major elements / comprising the preferred embodiment. The self-correcting 8 printer and verifier l9n includes a printer 192 for printing (~ indicia, such as bar code symbols on a print medium such as longitudinally extended sheet 14. Sheet 14 may include 11 successive pages or may include successive spaced labels 158 12 [FIGS. 7 and 8] removably affixed to th~ sheet 14 Eor 13 imprinting thereon. Thus successive pages or labels of 14 sheet may have corresponding successive fields of indicia printed ther~on. The printed indicia 12 is read by a l(i scanner 194 including a reading element 196. The 1; printer 192 is controlled by a control system 200 by control 18 signals directed along path 202. Printer feedback l~J signals may provide some information along path 204 to the control system 200 to assist in controlling the printer 192 21 in its primary printing operations. The control system 200 22 provides control signals along path 208 to control and 23 operate the scanner 194 to scan printed indicia at a time 2~ subsequent to its imprinting on sheet 14. Scanner 194 also 2~ produces scanner feedback signals applied to control 2~ system 200 along path 206 which signals include a scanner 27 output signal representative of of the scanned printed 2~ indicia and also may include encoded scanner position 2~ signals which gives control system 200 information as to the 30 position along the lîne at which thQ scanner is reading.
31 The control system 200 receives the scanner feedback signals 32 along path 206, and uses the scanner output signal to modify ... .
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1 he printer control siynals applied to the printer alony 2 path 202 to modify, in turn, the characteristics of the 3 printed indicia 12.

It should be noted that the control system 200 G controls the stepper motor 80 in its incremental advancement of sheet 14. The control system 200 will have received the edge or label detector sensing signal sensing the position 9 of an edge, for example, and by sensing the incremental advances of the sheet 14, will have precise information of the location of an edge, including information of whether a 1~ label is located juxtaposed the print head. In situations 13 where the print operation creates its own printed edges or 1~ borders, the control system 200 will have the number of lS incremental advances beyond the end of the last edge so as 1~ to be able to locate precisely the beginning of the next 17 succeeding label's leading edge.
~8 1~J In general, the printer 192 may or may not 2~ contribute printer feedback signals along path 204 to the 21 master control system 200. If a thermal printer i5 22 utilized, where the temperature of the printing elements are 23 crucial, a temperature feedback signal may be returned which 2~ is generated by a thern~istor mounted on the heat sink for the thermal print head and may be used to control energy 2~ applied to the printing elements, for example, by providing 27 information for controlling the amplitude of a voltage pulse 2~ driving the printing elelnent. Such temperature feedback ~!) control is well known in the prior art but is extremely slow and delayed in operation because of the very large thermal ~1 time lags in a thermal print head~

~2 ~6~

1 In instarlces of printing on a series of spaced, removable labels mounted on sheet 14, the printer 192 may 3 use a label eclge detector to generate label edge feedback ,~ signals at the leading edge of such labels to communicate to the control system 200 the precise location of the label, so (; that the control system 200 can generate print commands ~hen I the label is correctly positioned in relation to the 8 printing elements.

(~3 It can be appreciated, that thermistor generated 11 temperature feedback signals may not be utilized when the 12 printer 192 is not a thermal printer, for example if it is a 13 laser printer. Similarly, label edge detector feedback may 14 not be utilized if the printer is not printing on sheet carried labels or similar sections.

I(i 1/ In the preferred embodiment of the invention, 1~ which utilizes a dot matrix thermal printer, temperature 1tJ feedback control of drive pulse energy content is supplemented or replaced by a much faster acting control 21 method which scans printed indicia to determine dimensional 22 deviation of the printed indicia from predetermined desired 23 dimensions and quickly alters energy content of drive pulses 21 to the print elements in accordance witll such dimensional deviation to reduce the dimensional deviation of subsequent 2~ printed indicia. This is a quick acting system in which 2/ imprinted dot size is increased or decreased in accordance 2~ with the alterations of drive pulse energy content to 2~ thereby increase or decrease the dimensions of the printed indicia.

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11/1~/~4 I The scanner may include as its reading head 196 an optical scanning head of the preferred embodiment. The :) sc~nner output signal produced by reading head 196 4 represents precisely the dimensions along the scan path of tile scanned printed indicia, such as bar code 12. The (i control system 200 operates upon the feed back scanner output signals to compute dimensional deviation of the scanned printed indicia from predetermined desired ~) dimensions, which are stored in control system 200. The 1() computed deviation is used to correct or modify the printer 1l drive signals applied to the printer along path 202 from 12 control system 200. Thus, the control system 200 can 13 command the printer to increase or to decrease the elemental 1~1 dot sizes to thereby increase or decrease the width of the bars in the code 12, in response to the magnitude of the lG dimensional deviation of the previously printed code 12 from 1/ its predefined desired dimensions. The computed deviation 1~ can also be used by the control system to control a "void"
llJ stamp, and to apply command signals along path 202 to 2U reprint a voided label, page or other print field rejected 21 ~y the void stamp. The control system 200 also operates to 22 control the sheet advance drivel and also the thermal ribbon 23 driver if thermal ribbon is used, and controls the selection 2~1 of those print head elements which are to receive print head drive signals~
2G .
27 In FIG. 13, a block schematic of the control 2~ system 200 is shown. The control system 200 includes a 2J master control processor ~MCP) and a print control processor 3~ (PCP) 210~ which share a common random access memory (RAM) 3l 212. The MCP 20~ receives commands from a scanner control 3~ processor (SCP) 214, which has i.ts own RAM 216. The SCP 214 ~ ~.6~
t is operatively connected to the gcan head drivers and 2 interface 218 from which emits the scanner drive motor 3 control, and throucJh which the SCP 214 receives information from the scanning head and the right and left limit signals r) from the limit switches. The scanner position encoder, such (; as the optic~l head encoder, generates signals interfaced b~
the scan head interface 218 to the SCP 214.

~) Information supplied through interface 218 from the scanning head includes a scanner output signal Il representative vf the reflectence of scanned printed bars 12 and spaces and thereby representative of the precise width 13 of the printed bars, which information is prepared by the 1~ SCP 214 and transferred to the MCP 208 for comparison with prestored desired reflectances and dimensions for the lG scanned bars and spaces. The MCP 208 from such comparisons 1/ computes an averaqed dimensional deviation for the scanned 1~ bars which it makes available to the PCP 210 for altering a 1~ print time control number, i.e. the Dot Time Count Reference number (FIG. 16), which is used to generate a print control 21 signal whose time duration is proportional to the Dot Time 22 Count Reference number. Tlle print control signal thus is 23 used to dri~e, i.e. to switch on the print head drivers 220 2~ to set the time of the voltage pulse driving the printing elements.
2~
27 In the present thermal line printer, the print 2X head drivers each comprise a switching circuit which is ~(j switched "on" to pass current through a circuit which includeS a corresponding printing element. Each head driver 31 compriSe5 such a 5witching circuit in associa~ion with a 3~ corresponding input gate and bistable memory element.

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1 Bistable memory element is setable to a "1" or a "O" value ,~ and controls the yate to pass or not to pass an applied :3 print control signal through it to the switching circuit, ~l selectably to drive thereby the switching circuit for the r~ duration of the print control signal to switch voltage to (i the printing element. Thus, overall the entire set of head drivers can be considered as a register of memory elements each selectably controlling whether the corresponding switch ~) can be actuated by the print control signal pulse in 1() accordance with the "1" or "O", i.e., print or non-print, Il values set into and stored in the "register."
1~ The print control signal is thus an actuating 13 signal determining the "on" time of the application of 1;l current to the printing element, causing the printing element to heat. The printillg element is in close l(; proximity, and in some instances in contact with the print 1- medium, such as paper. Where thermally sensitive paper is 1~ used, the printing element, when heated, will cause the lD thermally sensitive paper to darken or to create an image 2~ comparable in shape to the printing element. The more heat 2] that is generated during the print cycle, the more area on 22 the paper i5 darkened. Similarly, in those thermal printing 23 operations which employ a thermal ribbon, the thermal ribbon 2i is moved or advanced adjacent to the paper to be imprinted, ,~ and the printing element is positioned opposite the thermal 2G ribbon from the paper. When the printing element is heated 27 by the application of voltage, adjacent coating of the 2~ thermal ribbon is melted by the lleat of the printing ~(J element, and tral!sferred onto the paper to create the darkened image. Tile longer that the voltage is applied to 31 the printing element, the more o~ the ribbon coating 3') I acljacent the printing element will be melted and transEerred 2 onto the paper.

Thus, it may be appreciated that the widtll of the r, line created by one or more adjacent printing elements can G be widened or narrowed by the corresponding increase or I decrease in the amount of time that the printing element is B being heated by the application of voltage in the driving t~ circuit. Consequently, the application oE a longer or 1() greater time duration print control signal pulse to the 11 switch which switches curre~t through the print element will 12 correspondingly increase the width of the line printed by 13 that printing element. Similarly, the width of the 14 imprinted line can be narrowed by correspondingly decreasing the print control signal pulse duration.
lG
1l The print control processor 210 also generates 18 those signals necessary to control the sheet drive motor 80 l~J which controls not only the advancement of the sheet, but also the advancement o~ the ribbon in those applications 21 where thermal ribbon is used. The signals to drive the 22 sheet drive motor ~0 are interfaced through the ribbon and 23 paper drive feed drivers and interface 222. Through the 2~1 samè interface 222, signals from the sprocket and label edge detector are received by the PCP 210, which generates the 2B signals necessary to advance thë sheet and the thermal 27 ribbon to position thP labels or other portion of the sheet 2~ to be printed in juxtaposition with the print head before 2~) signals from the PCP 210 actuate the print head drivers 220.

:)l The ~ICP 208, on the basis of the comparison of the :~2 widths of the actually printed bars and their spaceS with ~'' .
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~ ~%6~ 3~

1 the predefined or predetermined measured widths, and of the 2 print deviation resulting from this comparison, determines 3 wllether the deviation is within or not within tolerances.
~1 In the event that the devlation is not within tolerances, a command flows to the alarm/Yoider drivers 224 to generate a G signal to the "void" stamp solenoid, and to such other alarm indications as may be employed, such an alarm light, alarm sounds and the like.

(~) FIGS. 14 ~nd 16 present a flow diagram of the 11 operations of the preferred embodiment of -the present 12 invention. In FIG. 19~ the operation begins with power-up 13 which activates a start print operation or routine in the 1~ MCP 208. In the start print operation, various counters and flag bits within the processors are initialized.
11;
1/ At the conclusion of the start print operation, 18 the load print operation 234 is initiated within the MCP
1J 208. The print data can come from a disk terminal 236 or from another computer. The data will include definitions of 21 the symbols and indicia to be printed, and quality 22 definitions against which printing is to be verified and to 23 which later printing is to be corrected. At the conclusion 24 of the load print operation 234, the prepare print buffer 2~ operation 238 is ~initiated in the MCP 208. In this 2G operation, the page/label description buffer 290 is loaded 27 with the data defining the symbols and indicia to be 28 printed.
2'J
At the conclusion o~ the load print operation 234, 3l the print set-up operation 242 is initiated in the MCP 208.
32 It sho~ld noted at this point that, except ~or lo~ding :, :

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1 some specialized information and data relating to the 2 quality definitions and standards for the printing, the 3 operation is conventional for printers, which customarily ~1 have operations loading page data into processor memory for ~r~ colltroling the printillg operations.
(; In the pxint-set operation, the quality definition (lata is formatted into the verify print quality buffer 258 8 for a complete page or label of printed information. The l~ cletail of the verify print quality buffer 258 can be seen in the map diagram of FIG. 17. The buffer comprises a 32 bit 11 word memory storing the quality standards against which the 12 printed symbols are to be compared~ In the first word is 13 placed a Light Maximum Standard, i.e., a value representing 14 the minimum reflectance at which and above whicll an unprinted area will be accepted as a space. In the first 1G word also i5 stored a Light Minimum Standard, i.e., a value ]~ representing the maximum reflectance at which and below 1~ which a printed area wili be accepted as a bar. Data 11~ representative of the minimum contrast is also placed in -the first word. The contrast is defined as the Light ~laximum 21 minus the Light Minimum, and in the verify print yuality 22 buffer the minimum acceptable contrast of the standards for 23 these data is stored. Further, a minimum print contrast 21 signal (PCS) is placed. The PCS is defined as the quotient of the light maximum minus light minimum (or, the contrast) 2~ divided by the light maximum. For each bar and each space 27 in the symbol, a word i9 dedicated. In the least 28 significant digits, the nominal dimension is stored. Filing 2J out ~he word, the maximum positive and negative tolerances are placed for each bar and space. The end of the symbol is signified by a word of zeros, and thereafter the same 32 information is recorded for the second and successive symbols. Tlle end of the buEfer is signified by two zero 2 words.
~3 ~1 In the print set-up operation the print dot matrix buffer 243 is also prepared. ~11 data necessary to specify G the printing elements to be energized and those not to be energized are placed in the print dot matrix buffer 243 for ~ a complete page, label or other field of printed () information. In the thermal printer of the preferred 1() embodiment, a printed row is made up of 1,728 dots across 11 the width of the sheet 14, each of these dots being printed 12 or not printed by its corresponding printing element in 13 accordance with the requirements of the symbol or line to be 1~ printed. All the information required for printing such a printed row, and for printing all successive printed rows on 1~) a page or label, is stored in the print dot matrix buffer 243 by the MCP 208 during the print set-up operation 242.
1~ Each print or dot row is stored in the buffer 243 as a 1~ corresponding row of "l"s and "O"s, i.e. mapped into the binary memory cells of the words of the print dot matrix 21 buffer 243. In successive print operations, each successive 2~ dot row is read out of the print dot matrix buffer 243 and 23 copied through an intermediate dot row buffer into the 2~ corresponding bistable memory element of each print head driver, so that the array of print head drivers is switched, 2G upon actuation by the print control signal, "on" or "off" in 27 accordance with the contents, i.e. the "l"s and "O"s of the 28 corresponding row of the print dot matrix buffer 243.
~(~) In many printers, the printing is made by a 31 rectangular dot matrix of dots which are either activated or 32 energized, or not in order to form a symbol on the printed 5 ~ e~. ~he pre~erred ~ r~ ~ f the present invention, uses a thermal line printer where the matrix i5, in ef~ect, 3 a horizontal row of printing elements against which the 1 sheet is moved vertically.

G As an example, for bar code printing in the print 7 set-up operation, the MCP 208 may register into the print dot matrix buffer 243 signals to determine which of the ~ print control drivers, and consequently which of the printing elements 94 are to be turned "on" during the print 11 cycle to create the lines in the bar code, and which of the 12 printing elements are to be left "off" to create spaces 13 between the bars. The resultant picture formed on the 1~ printed page is the result of printing element dots arranged in a matrix of dots and spaces, i.e., a rectangular array of lG rows and columns of dots. The pattern of which of the 1~ particular dots are to be energized and thus heated, and 1~ which ones are to be left "off" in order to form the desired l(~ image on the printed paper, are mapped into the binary 2~ memory cells of the print dot matrix buffer 243.

22 At the conclusion of the print set-up operation 23 242, the print operation 24~ is initiated in the PCP 210.
2~ The print dot matrix buffer 243 is available to the PCP 210, and from such data the sheet drive motor 80 is incremented, 2G and selection signals are provided to establish which of the 27 printing elements are to be enerqized for each step of the 2~ stepper motor 80. The symbols then are printed on the sheet 2~ l4. Further, the print operation initiates the scanning operations, as indicated by the connection 1-1 in the 3_ diagram ~f FIG 4. 32 ~ ;, . ' ',', ~2~;~a0~i 1 The scanning operations are initiated by the 2 initiation of the start scan operation 250 in the MCP 208.
3 Similar to the start print operation, the start scan ~1 operation initialiæes counters and flag bits. At the r) conclusion of the start scan operation 250, the scan control G operation 252 is initia~ed in the SCP 21~. In scan control, the operation of the scanning or reading of the printed sheet or labels is timely initiated. When the printed (J symbols are moved into proper scanning alignment for the scanning head, the scanning head is moved in a double 11 traverse across the printed line of symbols, for example, 12 bars and spaces. The control command initiates the scanner 13 stepper drive motor to move the scanning head from its 14 quiescent or nesting position on the right of the scan path, and the scanning head lO0 reads the printed line from right IG to left in a first traverse or pass, as representatively 17 shown in FIG. 15 of the drawings. The first pass will lS appear to the sheet 14 to be at a slight downward slope to 1~ the left as the sheet 14 will be moved during the pass. The movement of the sheet 14 will be, for one traverse in the 2i preferred embodiment approximately 0.25 inch, and therefore 22 the double pass will be approximately 0.5 inch, so that the 23 entire scanning operation will be accomplished within the 21 pass-by of the printed line. W~len the scanning head 100 interrupts the light path of the left limit switch 111, the 2G scanning is reversed in its traverse to~pass from the left 27 back to the right, along the lower traverse or pass line as 2~ diagramatically shown in FI~. 15. The optical head encoder 2!) 60 presents information to the SCP ~14 during scan control operation 252 so that the scan read starts at the correct 31 line or lateral position when the scanning head 100 is at 3'~ the symbol to be verified~

~12~

1 In the first traverse pass, the scanning head 2 output signal provides a measure of the reflectance of the ,~ bars and spaces, and SCP 214 places the measured Light ~1 Maximum reflectance (i.e., the maximum reflectance of the r) spaces) and the Light Minimum reflectance (i.e., the minimum (; reflectance of the,bars) into the measured data buffer 256, / the details of which are seen more clearly in FIG. 18. In X the reverse traverse from left to right, the transitions of !) the output signal are utilized to determine the vertical 1() boundaries of bars and spaces, and thus the precise 1~ dimensions of each bar and space. In the scan control 12 operation, the width of each bar and of each space for each 13 symbol is representatively placed in the measured data 1~ buffer 256. A word in the buffer is dedicated for each bar and each space, as well as for the measured Light Maximum 1~ and Light Minimum. The end of the symbol is signified in 17 the buffer 256 by a word of zeros with a "1" in the most 18 significant digit. The end of the buffer is signified by 1~ two successive words of zeros with a "1" in the most significant digit~ as is shown in FIG. 18.
21 At the conclusion of scan control operation 25~, 22 the completed measured data of buffer 258 are made available 23 to the MCP 208, by being copied thereto through the SCP 214, 2~1 and the verify operation 260 is initiated in the MCP 208.
In the verify operation, the MCP 208 compares the measured 2G clata with the data in the verify print quality buffer 258, 27 and computes deviations o~ the bar and space dimensions, 28 Light Maximums and Light Minimums, all read by the scanning 2!) head on the one hand, irom the corresponding desired bar and space dimensions and reflectance standards established in 31 the verify print quality buffer 258 on the other hand. The 32 completed deviation information is stored in the print deviation buffer 262 aeen mor~ clearly in detail in ~IG. l9 : .

~ 3~

l of the drawings. For each symbol, the average of deviations 2 of all the bars ls computed and placed in the first word 3 dedicated to the symbol, with a reservation of the most significant digit for purposes that will be discussed below.
In the second word dedicated to that symbol, is placed a count of the number of bars and spaces in a symbol which are 7 out of tolerance specification with respect to the corresponding nominal dimensions. Further, in the second (~ word, if any o~ the PCS, Light Maximum or Light Minimuln values are outside the bounds set in the verify print 1~ quality buffer 258, corresponding out~of-bound indicating 12 flags are placed in the, respectively, most, next most and 13 third most significant digits. In the third word dedicated 1~1 to the symbol, a computed count is placed of the number of bars and spaces which exceed the tolerance by a lG predetermined gross amount, which may be a multiple of the 1~ normal tolerance.
1~
19 At the completion of the verify operation 260, the page/label scan completion operation 266 is initiated in the 21 MCP 208. In the page/label scan completion step 266, the 22 average bar, space and light contrast deviations for an 23 entire line as placed in the print deviation buffer 262 are 2~1 seen (FIG. 19). If all bits in the first and second words 2r, for all symbols of a page or label are "0", a normal print 2~ command is transmitted through line or connection 2-2, shown 27 in FIG. 14, to initiate the prepare print buffer 238 for i-ts 2~ next page or label in ths ordered course. However, if any 2!) bit in words one or two are "1", that is if any bar, space or light contrast measurement exceeds the predetermined 3l tolerance limit, a command is given in conjunction with 3~) operation 238, after completing the page or label, to I reprint the page or label which in the initial printing .~ exceeded the deviation tolerances preset into the verify 3 print quality buffer 258. If the deviations are within 1 predetermined tolerances, a normal print instruction can be 5 given in the connection 2-2 so that the printing operation G~ following after operation 238 may proceed as set forth ~ above. Additionally, in the event that the width dimensions 8 of the bars of a bar code of a particular page or label (~ exceed that of predetermined tolerances, the page/label scan completion step includes a command to the "void" stamp 11 solenoid to actuate an imprint upon the label as "void," and 12 to actuate visual and audio alarms as may be desired. If 13 any bi-t in the third word indicates by a "1" a "gross"
~ deviation, a command is given to stop all printing and to alert operators to the yross error.
11) 1/ The print deviation buffer 262 is made available 1~ to the PCP 210, which in its print operation 244 can 1~ generate commands to self-correct the time duration of the voltage applied to the individual printing elements, so as 21 to tend to reduce print bar and space width deviations, as 2') will be explained in greater detail below.

2-~ In FIG. 16, the interaction of the print deviation measurements from the buffer 262 in the print operation 244 2G in the PCP 210 to achieve the self-correction of the 27 printing is diagramatically set forth. The print step 244 2l~ is initiated at the completion of the print set-up operation 2!J 244 in the MCP 208. The bar width deviations from the print 3~) deviation buffer 262 are summed and averaged for a full 31 line, and this averaye resultant bar deviation for the line 3') is scaled 270 to put the count in compatible form to that -' ~IL2~

~ ready iD use In the PCP for generating the print control 2 signals driving the print head drivers 220. As the flow :3 diagram indicates, during the start print operation, a i flag A 272 is set at zero, so that for a first flow sequence, i.e. each first line, the scaled bar deviation 270 (; is subtracted from the dot time count standard -to produce a 7 dot time count reference 274. The flag A 272 is then set at 8 "1" and a counter A is set to zero. The negative or comple-() mental value of the dot time count reference is stored in counter A. After counter A receives a negative time count Il reference value, counter ~ is incremented by a fast clock 12 until it overflows, during which time a positive print 13 control pulse is sent to the print head drivers 220 by the l~l PCP 210 in the print step 244 of FIG. 14. When the counter ~5 A has overflowed, the voltage "on" pulse 278 to the print IG head drivers is terminated. In subsequent line cycles, when 1~ the flag A 272 is "1", the scaled bar deviation is sub-l8 tracted from the old dot time count reference to produce a 1(~) new dot time count reference 275 for storing in counter A.
Thus it can be seen that the dot time count reference is a 21 print time control number or value generated by the print 2') operation 244 of the PCP that controls the time duration oE
23 the print control slgnal. The dot time count reference is 2~1 updated and altered in aGcordance with the calculated dimen-2r~ sional deviations so as to tend to reduce these deviations.

27 ~he foregoing detailed description of our 28 inventiOn and a preferred embodiment, both as to apparatus 2(J and a5 to method, is illustrative of specific embodiments only. It is to be understoodt however, that additional 31 embodimentS may be perceived by those skilled ln the art.
~ The embodimentS described herein, together with those :

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I .ndditional embodim~nts ~re considered to be within the scope ~ c the prl~9el~t ~nventloll.

I~

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2(1) 22 . `

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Claims (4)

1. THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
   PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
   
   l. A method of determining in a printer the leading edge of each of a series of successively positioned labels through the printer into position adjacent printing elements within the printer, comprising the steps of:
   a. moving each label successively through a first light path comprised of a first light source and a first photodetector capable of detecting light through said label;
   b. producing a first electrical signal representative of the change in opacity when the label is moved through the first light path;
   c. further moving the label through a second light path comprised of a second light source and a second photodetector capable of detecting light through said label;
   d. producing a second electrical signal representative of the change in opacity when the label is moved through the second light paths;
   e. comparing the first electrical signal with the second electrical signal and producing from this comparison a digital initiate signal for initiating an operation routine in a control processor, whereby a print operation routine is initiated operating the printer to print indicia on said label when said label is positioned adjacent said printing elements.
2. 2. The method of Claim 1 further comprising the steps of incrementally advancing the individual labels from the first and second light paths a predetermined distance along a label path to said position adjacent said printing elements by a stepper motor operably connected to said labels to advance said labels along said label path.
3. 3. The method of Claim 1 wherein each of said labels is positioned upon a longitudinally extended carrier sheet which is advanced along a sheet travel path by an incrementally advancing motor means operably engaging said said longitudinally extended sheet for advancing said sheet having labels positioned thereupon, and wherein said first and said second light paths comprise photodetectors capable of detecting light through said carrier sheet and said labels, and wherein said first and said second electrical signals so produced are signals representative of a change in opacity of the carrier sheet without the label thereupon and the carrier sheet with the label thereupon.
4. 4 The method of Claim 1 where, after each of the steps of producing the first and the second electrical signals, said electrical signals are not compared in the comparing step unless each of said electrical signals exceeds a predetermined minimum voltage representative of a minimum change in opacity.
   
   The method of Claim 1 further comprising the steps of:
   a. electronically scanning said indicia printed on each said label;
   b. comparing the scanned indicia with a pre-defined standard of quality for said indicia;
   c. producing a print control signal proportionally related to any deviation between said printed indicia and said standard of quality;
   d. applying said print control signal to the printing elements and controlling the printing elements to make a correction in the printing step tending to reduce and deviation between said printed indicia and said standard of quality.