FR2542255A1 - Method for correcting registry deviations on printing devices - Google Patents

Abstract

When a registry deviation epsilon o has been measured from one station of a rotary printing machine to another, the length of travel of the web between the two stations is modified by a length greater than epsilon o, and then, without a stop, this modification is reversed so that the final modification is epsilon o when the registry deviation is cancelled.

Description

The present invention relates to a method for correcting registration differences on the printing devices of a band to be printed in motion, in particular printing presses, comprising a number of printing stations, each defining a printing line, in which, for a tracking difference measured at a first printing station, the length of the strip path is changed between the printing line of the first station and the printing line of a second printing station placed immediately upstream thereof, of a length greater than this measured gap
Printing presses have a number of printing stations, or groups, each comprising a printing or engraved transfer cylinder, or a rotary offset transfer transfer cylinder, for example, and a pressure roller, both defining a printing line. It is sometimes noted that the impressions of a station and the previous station, on a tape to be printed, preferably fed from a reel, are not in agreement or do not coincide. They are said to not spot each other and that there is a tracking difference.

 To remedy this defect, it is necessary to modify the length of the strip travel between the two printing lines of the stations concerned, for a determined point on the strip.

 For this purpose, it is possible either to offset, using differentials, the printing cylinder of the downstream station, by driving it in an appropriate manner in rotation from its normal angular setting, or and preferably, by directly modifying in appropriate manner the length of the web path between the two printing lines, using at least one register roller, or compensator.

 There is however a problem, insofar as, as soon as one realizes the locating deviation and that one actuates, for example, the register roller, the impressions locate again only after the scrolling of a certain portion of the tape to be printed, which is therefore lost. In other words, between the time of the corrective action and the time when the error is corrected, the tape still ran.

It will be noted, with reference to FIG. 1, that if the length of the strip is changed between the printing lines of the two stations in question, the difference in re
notes perage / CO, the tracking difference t, after the modification of length, varies as the strip passes, according to an exponential law as a function of the meters of strip
debited by the device downstream of the register roller, and this, if it is admitted that the modification of length of the course is instantaneous.

 It has already been proposed, in order to shorten this portion of lost tape, to take into account not only the measured tracking difference, but also its derivative, which amounts to modifying the length of the tape path by a length greater than l 'measured difference 5 or assuming the greater tracking error than it actually is.

 However, such a solution still has drawbacks. Indeed, either the tracking difference is effectively canceled but the portion of lost tape is still too long, or the tracking difference reverses and stabilizes, so in the other direction, and always according to a law exponential (see Figure 2).

 It has also already been proposed to also take into account the tension of the strip between the two printing stations. In this regard, and instead of acting on the register to modify the length of the tape path, and therefore necessarily also the tension, but to then let it recover with the tracking, the action on the register is continued for a certain time to maintain this tension. This amounts to modifying the identification not according to the exponential function mentioned above, but practically according to the tangent at the origin of the curve representative of this function (see Figure 3). This solution is also not satisfactory, always because of the lost tape length M.

 The present invention therefore aims to improve the correction method of the type mentioned above, in order to reduce the lost tape length as much as possible.

To this end, the present invention relates to a method for correcting registration differences on the printing devices of a band to be printed in motion, in particular printing presses, comprising a certain number of printing stations, each defining , a printing line, in which for a measured tracking difference t0, at a first printing station, the length of the strip path is modified, between the league>'printing of the pressman station and
gfor a determined point of the strip, the printing line of a second printing station, placed immediately upstream thereof, of a length su greater than this difference ò, characterized in that after having modified the length of the strip path in one direction, it is again modified in the other direction, until the final modification of the length of the path is equal to the measured tracking difference gO, l ' inversion of the modification of the length of the course being carried out so that the final modification tO of the course is carried out with the cancellation of the tracking error.

Thus, and thanks to the method of the invention, the tracking difference is canceled when the strip path is modified by a length equal to the tracking difference observed at the origin, so that, then, the tracking is maintained. In other words, this tracking difference is canceled according to the invention after the scrolling of a portion of lost tape of defined and reduced length. The process of the present invention will be better understood after having emphasized that, as long as the register is not in the position corresponding to the true original deviation and this deviation has not been canceled, the tracking deviation varies according to an exponential law on. However, as soon as the route has been changed from fO and
o that there is no longer a tracking error, then tracking is stabilized.

 In a preferred implementation of the method of the invention, the register is moved from its initial position, or the setting of the printing cylinder is varied at a maximum constant speed.

Advantageously, the length of the strip path is modified in one direction during the travel of a portion of strip of length m. and in the other direction during the scrolling of a portion of strip of length (mO-mi), mO and mi being given by the system of equations
ekm = 2 ekmi 1
2m. - mo = K where e is the base of the natural logarithms and 1s and K of the constants.

Preferably - m and mi are expressed in meters
oi

t étant l'écart de repérage initial mesuré,
o exprimé en mm,
étant la vitesse de la bande exprimée en -1
m.s
de l'action
V étant la vitesse linéairedu registre, ou
la vitesse périphérique du décalage angulaire
du cylindre imprimeur, exprimée en mm.s
L'invention sera mieux comprise à la lecture de la description suivante du procédé de l'invention, en référence aux dessins annexés, sur lesquels
- la figure 1 représente la variation de l'écart de repérage selon un procédé classique de l'art antérieur, en mo-difiant la longueur du parcours de bande d'une longueur égale à l'écart de repérage initial
- la figure 2 représente la variation de l'écart de repérage selon un autre procédé de l'art antérieur, perfectionnant celui illustré sur la figure 1, en tenant compte de la dérivée de l'écart
- la figure 3 représente la variation de l'écart de repérage selon un autre procédé de l'art antérieur, perfectionn ant celui illustré sur la figure l en tenant compte de la tension de bande
- la figure 4areprésente la loi de déplacement du rouleau registre, selon le procédé de l'invention, en fonction du défilement de la bande, et
- la figure 4b représente de l'écart de repérage selon le procédé de l'invention.L being the strip length between the two printing lines, expressed in meters,

t being the initial tracking error measured,
o expressed in mm,
being the speed of the strip expressed in -1
ms
of action
V being the linear speed of the register, or
the peripheral speed of the angular offset
of the printing cylinder, expressed in mm.s
The invention will be better understood on reading the following description of the process of the invention, with reference to the accompanying drawings, in which
- Figure 1 shows the variation of the tracking difference according to a conventional method of the prior art, by modifying the length of the strip path by a length equal to the initial tracking difference
- Figure 2 shows the variation of the tracking difference according to another method of the prior art, perfecting that illustrated in Figure 1, taking into account the derivative of the difference
- Figure 3 shows the variation of the tracking difference according to another method of the prior art, perfecting that illustrated in Figure l taking into account the tape tension
FIG. 4 represents the law of displacement of the register roller, according to the method of the invention, as a function of the movement of the strip, and
- Figure 4b shows the tracking difference according to the method of the invention.

 We will not describe here the printing stations of the press on which the tracking differences of a band to be printed in movement, preferably elastic, are eliminated by a process which will however be described.

 We will not describe either the moven, s of variation of for a determined point of this band the length of the band path / that is to say the register roller (s), with their drive servomotor, nor the differentials printing cylinders which one might want to use in the absence of register rolls to vary the length of the strip path between a first printing station and a second printing station located immediately upstream of the first.

 Reference will be made in the description which follows to a modification of the length of the strip path by a register roller, but the method finds its application just as well in the event of variation of the setting of the printing cylinders.

 We will mention more particularly the tracking difference on a strip running on a printing press, but the method applies to any printing device of any strip.

 Finally, the tape in question can be cut from a reel, but this is not a process limitation in this case.

 The operator of the press having therefore noted a tracking difference between two adjacent printing stations and measured this difference, of a length #o (figure 4b), he starts the roller servomotor at maximum speed V register to drive it in a direction beyond the position rO in which it modifies the length of the strip path of t (FIG. 4a), in the species represented by increasing it. Thus the register roller is driven to an intermediate position ri until the strip has passed since the start of the correction, of a length mi which will be defined later.

 During this scrolling of strip of length mid, the tracking difference varied from g0 to t i (FIG. 4b).

It will be noted that the law of variation of the tracking error t, from ò to., Is such that the slope of the derivative
oi
increases, with, in #i, ------- a relatively strong derivative slope. The reason for this character of the
law during - the movement of the register is due to the fact that it does not reach the position ri, nor even the position rO, instantaneously, as was assumed with reference to Figures 1, 2 and 3. The Because of the steep slope of the law at the end of the movement of the register is due to the fact that it moves at full speed.

 It will also be noted that the larger this initial difference, the steeper the slope.

 As soon as the register has reached the position ri, and without marking the slightest stopping time, its training is always reversed at maximum speed V, which in this case reduces the length of the strip path to replace it in the position rO defined above (FIG. 4a), at the instant when the strip has passed, since the start of the correction, of a length mO (FIG. 4a), which will be defined below, and where the tracking difference 'is canceled (Figure 4b).

 We therefore had the register roller do a pseudo round trip.

 It will be noted that the curve representative of the variation in the tracking difference # (FIG. 4b) has an inflection point for m = mi, which the skilled person will easily appreciate since the register is brought closer to his position. starting point, and that between points m - m. and m = mO it always evolves according to an exponential law.

 For m = mO, the register is in the position where it has modifies the length of the strip path by a length equal to é: and the tracking difference is zero, so that; from this moment, or for the following strip lengths, the tracking is maintained.

 Let us now approach the lost mi and mO lengths of tape.

e est la base des logarithmes népériens , dont la valeur est égale à 2,718,
L est la longueur de bande comprise entre les deux lignes d'impression des deux postes concernés, exprimée en mètres
(m). Elle est constante, à la variationde longueur près, qui est du second ordre et donc négligeable.
The values of mi and m are given by the system
io following two equations with two unknowns

e is the base of natural logarithms, whose value is equal to 2.718,
L is the strip length between the two printing lines of the two stations concerned, expressed in meters
(m). It is constant, except for the variation in length, which is second order and therefore negligible.

is the initial tracking error measured, expressed in milli
o meters (mm).

 tr is the given speed of the strip expressed in meters per second (m.sl).

V is the given linear speed of the action of the register ex
awarded in millimeters per second (mm.s1).

 All these parameters are therefore constant and known either by measurement or by construction. From these parameters, we can therefore solve the above system of equations, for example using a computer or a microprocessor, to automatically correct the tracking difference.

où a, b et c sont des constantes et K le rapport

The Applicant has further found that, approximately, mO is given by the formula

where a, b and c are constants and K the ratio

Advantageously, b is the length L defined above and a and c are constants for example equal to 2 and 0.2, respectively,
as for mr. it is given by the formula

 It will be noted that if, contrary to the method described above, the damper had been driven still beyond the position ri, we would have caused, on the one hand, an inversion of the tracking difference and, on the other hand , a cancellation of this difference according to a still exponential law, and that if we had inverted the drive of the register before it reaches the position ri, the reference difference, which certainly would not have been reversed, would have always canceled after an exponential change.

 Finally, the process described above has been illustrated with tracking differences in one direction. The method applies of course just as well to the correction of tracking differences in the other direction. In other words, the centralization of the register in position r. can result in both an increase in the length of the strip path, as in the example shown, and a decrease in this length.

 The process obviously applies to all positions on the press.

 Of course, one might also want to modify the length of the strip path by other means than those which have been envisaged above, and in particular, for example, by moving the whole of a printing station.

Claims (8)

Claims  l, A method of correcting registration differences on the printing devices of a band to be printed in motion, in particular printing presses, comprising a certain number of printing stations defining, each, a line of printing, in which, for a tracking difference (#o) measured at a first printing station, one for a determined point on the strip, modifies the length of the strip path, / between the printing line of the first station and the printing line of a second printing station disposed immediately upstream thereof, of a length greater than this difference òE ca characterized by the fact that after having modified the length of the strip path in one direction (r.), we modify it again in the other direction (rO), until the final modification of the length of the course is equal to the measured tracking difference tO, the inversion (ri ) of the modification of the length of the course being carried out so that the final modification tO of the course is performed with -the cancellation of the tracking deviation.  2. Method according to claim 1, in which the inversion (ri) of the modification of the length of the strip path takes place without stopping time.  3. Method according to one of claims l and 2, wherein said modification is carried out at maximum speed.  4. Method according to one of claims 2 and 3, wherein the length of the strip path is changed in one direction during the travel of a portion of strip of length m. and in the other direction during the scrolling of a portion of strip of length (m0-m), mO and mi being given by the system of equations

 2 m. - m = K  i o where e is the base of natural logarithms and k and K of constants.  5. Method according to claim 4, in which - mO and mi are expressed in meters

 the length of the tape run.  V being the speed of means of variation  in m.s  tr being the speed of the strip expressed -1  expressed in mm,  being the initial tracking error measured,

L being the strip length between the two printing lines expressed in meters,  6. The method of claim 5, wherein mO and m. are given by approximate formulas

 where a, b, c are constants.  7. Method according to one of claims 1 to 6, wherein the length of the strip path is varied directly using at least one register roller.  8. Method according to one of claims 1 to 6, wherein the length of the strip path is varied by varying the angular setting of a printing cylinder.