GB2205178A

GB2205178A - Offset printing press operating method

Info

Publication number: GB2205178A
Application number: GB08812333A
Authority: GB
Inventors: Willi Jeschke; Hugo Rambausek
Original assignee: Heidelberger Druckmaschinen AG
Current assignee: Heidelberger Druckmaschinen AG
Priority date: 1987-05-27
Filing date: 1988-05-25
Publication date: 1988-11-30
Also published as: GB8812333D0; DE3717904A1; FR2615791A1; FR2615791B1; DE3717904C2; JPS63309449A

Description

b - 1 - 2205178 1 1 1

DESCRIPTION

OFFSET PRINTING PRESS OPERATING METHOD This invention relates to a method of controlling the printing process performed in an offset printing machine.

In offset printing, the quality of the printed product is principally determined by three variables: ink flow water flow and paper supply. However, it is also influenced by iurther variables, e.g. temperature, speed, paper quality, and water composition (alcohol concentration). If, in offset printing, the quality-affecting variables are examined with a view to the printing process being controlled, one recognizes that this process analysis points to what may be called multi-variable control, i.e. indicates that the individual process systems are not independent of each other, but rather exert a certain influence on each other. There are thus a series of control variables which have to be set to respective reference values (desired values) for the purpose of achieving a certain desired printing quality. Consequently, a plurality of control loops with associated sensors and actuators are required. On account of the interdependence of the various individual process systems, an alteration in an actuating variable in one of the control loops may indeed have an effect on a control variable of another control loop, or on a plurality of control variables of other control loops.

The "multi-variable control" described above is difficult to deal with mathematically; one is liable to have to employ complicated equations which require large computer capacities for automatic printing 4 '.

i control and costly software for computer-aided process control. Aside from this complexity, difficulty may be encountered with respect to the stability of such a multi-variable control, which particularly susceptible to interterence. In view of the above-mentioned difficulties, printers have up to now restricted themselves to controlling the printing process manually, on the basis of visual impressicas. The pressman intervenes according to his feeling for the printing process, i.e. on the basis of his slazill and exDerience. This method, however, may have the following disadvantages: (1) often there is unnecessary or excessive intervention in the printing process, thus causing unnecessarily great fluctuations in quality; and (2) there is a need for highly qualified workers who are able to perform the desired manual control supremely well; however, such workers are rare, and moreover command high wages. In the case of manual control, corrective interventions are often made too late, particularly with printing machines running at higher and higher speeds. Besides his visual impressions, which are evaluated subjectively or "intuitively", the pressman obtains print quality data with the aid oi measuring equipment. The best known measuring device is the "dens itometer", with the aid of which colour densities may be determined, e.g. in measuring fields provided on the printed product solely for this purpose. It is often expedient for such measuring to be carried out "off- line", i.e. on a special measuring table separate from the printing machine. Very often, sample prints are withdrawn from the running printing machine, typically at intervals

1 v 1 le- - 1 1 -U of about one hundred prints, and checked in order to gain a sufficient survey of the uniformity of the printing quality. This technique, however, is very time-consuming.

It has been suggested that a quality check with the aid of a densitometer should be made within the printing machine, so that the printing process is not affected. However, due to the high speed of printLig, such a check, employ;.ng a densitometer which is equipped with only one sensor for scanning the weasuring fields, caDnot be carried out on one single sheet; rather must it be effected on a plurality of successive sheets. however, this means that the measuring result obtained loses some of its indicative value. The use of a plurality of simultaneously measuring sensors, on the other hand, causes problems because of the inevitable differences in the characteristics of the various individual sensors.

Regardless of whether measuring is carried out "off-line" or "on-line" (i. e. within the machine), the responsible operator must, at various times, intervene in the printing process, in order to make the necessary corrections. Consequently, the operator has had to be present all the time.

It is therefore an object of the invention to provide a method of controlling a printing process performed in an offset printing machine which permits simple and low-cost control, without the operator having to monitor the printing process all the time.

According to the present invention, there is provided a method of controlling a printing process performed in an offset printing machine, comprising the steps of:

1 (a) setting and coordinating the variables influencing the printing process (ink density, amount of dampening medium, printing speed, temperature and/or one or isore other variables); (b) controlling a predetermined one of sali-d variables (particularly ink density) chosen as a control variable of a main control loop, by means of a variable chosen as an actuating var.,.,,).ble (particularly ink quantity); and adjusting the remaining variables, so as to obtain constancy with reppect to the reference values arrived at in "(a)", by means of auxiliary control loops which are discoupled at least to a substantial extent, said variables being used in said main control loop as disturbance variables. In the method of the invention, the disturbance variables are thus eliminated, by adjusting them to the respective constant values. In doing so,.one confines oneself to a certain value of the respective disturbance variable, without controlling the associated reference value. The disturbance variables of the main control loop are thus constant, so that it is not necessary for the main control loop to react to continual modifications. The individual variables influencing the printing process have to be determined, either manually or automatically, only once. Thereafter said variables serve as preset values. One of the variables influencing the printing process is chosen as the control variable of the main control loop. In most cases, the ink density is chosen as the control variable, owing to its predominant role in the printing process. All other variables belong to appropriately set-up auxiliary control loops which have to be discoupled 7 t 7 S from each other at least to a substantial extent. Discoupling the auxiliary control loops used for the individual disturbance variables requires at least an optimum (-.oordination of theú.r respective reference values. In this connection, an "optimum coordination" means a state which involves no modification, or the slightest possible modification, of another control variable when modifying to a limited e,., .tent the actuating value for a discoupling variable to be controlled.

What may be called an applied disturbancevalue may contribute to the stabilization of the above-mentioned discoupled loops. In this case, the influencing disturbance variable is simply measured and the measuring signal is used to determine the actuating variable for the control loop which maintains the reference value of the control variable. Preferably, the ink quantity is chosen as the actuating variable of the main control loop. However, in the method ot the invention it is also possible to take (e.g.) the thickness of the dampening medium film as the control variable of the main control loop; in this case the water quantity would represent the actuating variable. It is particularly advisable that the control variable of the main control loop should be a predominant variable in the printing process.

In the method ot the invention, it is appropriate to aim at a state in which the extent ot coupling among the individual control loops is as low as possible, i.e. at one in which their mutual influence is very weak. Once this state is obtained, complicated monitoring measurements of the ink density can be-dispensed with as long as the process is remaining stable. In this state, only the J -rj- auxiliary control loops for the disturbance variables are still functioning.

A method in accordance with the invention may utilise a system for monitoring the modifying of the actuating variables ot the auxiliary control loops. Such a monitoring system may be so arranged that step "(a)" comprising setting and coordinating the variables influencing the printing process is repeated if a modification of at least one accuating variable of an auxiliary control loop exceeds a permissible tolerance range, or if there is a modification within a certain control deviation. Making the control variables of the auxiliary control loops (disturbance variables) visible, as such, does not provide the desired solution, because said variables are adjusted to constant values. however, during the monitoring of the actuating variables of the auxiliary control loops, a process instability is likely to appear it very considerable modifications occur. Provided certain control deviations are permitted, information on the stability of the process may also be derived from the modifications of said control deviations. The occurrence of such modifications implies that obviously the boundary conditions, which cannot be detected, have been modified. If such a tendency of a respective control variable, or a very considerable modification of an actuating variable of an auxiliary control loop, is recognized, then it is appropriate for the main control loop (typically employing the ink density) to be reactivated, and, according to the circumstances, for a corrected new and optimum state of equilibrium to be obtained. This can be done by checking the setting of the variables influencing the printing process. In the event of new reterence values having ty Z1 1 -1 \_) to be assigned to the disturbance variables, the reference values serve as preset values for the auxiliary control loops, during subsequent printing control.

In the method of the inveation, sample sheets for densitometer measuring or another printed product checking operation may be taken it a modification of an actuating variable exceeds a certain tolerance range, or ii thFre is a modification of a control deviation. It may be presumed that such modifl-cations will be very rare, because, given a reasonably robust construction, the relevant machine components, e.g. its inking unit, dampening unit, power transmission and sheet transfer means, are so designed that their static and dynamic modes of behaviour can be reproduced at any time. One is thus dealing with components which are permanent constituents of, or which are "immanent in", the system; once the characteristics of said components are determined, they permit an optimization with respect to the presetting of the reference values for the auxiliary control loops. Consequently, a machine operator has to intervene only very seldom when the method of the invention is in use.

It is not necessary to measure a respective disturbance variable itself, as a control variable. It is rather possible to select, for at least one of the control systems in which the disturbance variables are employed, an auxiliary control variable which depends functionally on the respective disturbance variable, and to adjust said auxiliary control variable to a constant value. For example, the viscosity of an ink can be ascertained by measuring its temperature. Again, instead of measuring the amount of water, it is possible to - I,,- - 1 d ascertain the speed of the pan roller. What is important is that the functional relationship between the control variable of the auxiliary control loop and the auxiliary control variable should be known.

In implementing the present method of controlling a printing process performed in an offset printing machine, the first step ("(a)") is to set and coordinate the variables influencing the printing process. It will be understood thal, the "setting" can be performed manually, or by means of remote conrroll ryslems, or again by providing for the presetting of reference variables from a central control console. In this connection, provision may also be made for automatic.outputs of quality data and machine condition data. The logic operations involving these process variables, however, are executed by the operator or pressman, so that the quality of the print will depend, to a decisive degree, on the capabilities of the operator or pressman. The setting includes the startup of the printing machine, and the resetting. The work of the pressman is facilitated by densitometer measuring, in addition to the output of machine condition data, e.g. speed, temperature, contact pressure and water quantity data. The complexity of the setting operation is reflected by the interconnections between the individual variables influencing the printing process; thus the modific ation of one variable also has effects on other variables, and, when there is occasion for intervening in the printing process, it may be necessary to reset a variety of variables influencing the printing process.

In this description, the term "multi-variable control" is used, and this has reference to the point that the individual process systems tor the printing

1 Al 11 9 'Ij J process (e.g. those for the ink density, quantity of dampening medium, temperature and speed) are not independent of each other but rather are interrelated to ea--h other.

As soon as a aheet-ied printing machine has been made ready for a production run, the logically connected control loops are "divided" or separated, Bo that there are a main control loop and a plurality oi auxiliary contro' loops. A predominant variable which influences the printing process decisively is taken as the control variable of the main control. loop. This may for example be the ink density. In this case, preferably the ink quantity is chosen as the actuating variable of the main control loop.

The auxiliary control loops are systems to which the other variables influencing the printing process, e.g. amount of dampening medium, printing speed, and temperature, are assigned. In the method of the invention, the control variables of the auxiliary control loops represent the disturbance variables of the main control loop, said disturbance variables being adjusted so as to obtain constancy with respect to the reference values arrived at in step "(a)". Consequently, the disturbance variables of the main control loop do not, during the printing process, depart from the values arrived at in the setting step, so that an optimum state of equilibrium is secured.

Thus a situation is obtained in which the degree of coupling among the individual control loops is remarkably low. The mutual influencing of the control loops is weak, so that the stability of the entire system is high.

The control variables of the auiliary control loops can themselves be evaluated and processed for 4 b /- - .U the purpose of thei.r adjastmjzit to constant values. however, it is within the scope of the invention not to measure said control variables themselves, but to use auxiliary control variables which make it possible to intluence a control variable serving as a disturbance variable, provided that the functional relationship between control variable and auxiliary control variable is known. It is possible, for example, to determine the viscosity of the printing ink (representing a control variable) by measuring the temperature of the printing ink (representing an auxiliary control variable), provided that the functional relationship between the temperature and the viscosity of the printing ink is known. by virtue of the adjustment of the disturbance variables to constant values, the printing process can be maintained over a remarkably long period of time without the occurrence of instabilities. In particular, it is not necessary to check, e.g. with the aid of a densitometer, sample sheets which have been withdrawn from the machine every fifty or one hundred sheets, because, under stable process control, the printed product does not normally exhibit great tluctuations. however, if the process does lose some of its stability, e.g. because one of the variables deviates, in the course of time, then a check is to be recommended. Incipient instability of the process may be recognized by the observation of modifications of the actuating variables of the auxiliary control loops. As indicated earlier, making the control variables themselves visible does not provide a solution, because said control variables are adjusted to constant values. It one of the actuating variables of an auxiliary control loop A 3 kR j - 1 tu 1 I - 1 1 - departs from a given tolerance range, tor example, then desirably a siZnal is emitted urging the pressman to check the process. It the condition is accepted tiiat-. certain control deviations for Cae control variables of Clie auxiliary control loops are permitted, information on the state of the printing process can also be derived from modifications of said control deviations. Modifications of the ccatrol deviations can 4a.ndicate that some conditions have evidently changed; in this case,, too, a signal desirably will urge the pressman to cheek the printing process.

it, after such a check, the pressman concludes that a new equilibrium will have to be obtained, then new reference values are assigned to the. disturbance variables, by repeating the step of setting and coordinating the variables which influence the printing process, and these new reference values are supplied as new control variables to the auxiliary control loops for utilisation in the subsequent continuation of the printing process, said new reference values being kept constant.

It may be concluded that this invention thus provides a method of controlling a printing process performed in a she et-fed offset printing machine which makes continual monitoring unnecessary, and which, moreover, -requires a quality check (e.g. densitometer measuring) only it a signal is generated whereby modifications of boundary conditions are indicated.

It will be understood that the invention has been described above purely by way of example, and that various modifications of detail can be made within the ambit of the invention.

Claims

1. A method of controlling a printing process performed in an offset printing machi.ie, comprising the steps of: (a) setting and coordinating the variables influencing the printing process (ink density, amount of dampening medium, printing speed, temperature and/or one or more other variables); (b) controlling a predetermined one of said vartables (p&rticularly ink density) cbosen ne as control variable of a main control loop, by means of a variable chosen as an actuating variable (particularly ink quantity); and (c) adjusting the remaining variables, so as to obtain constancy with respect to the reference values arrived at in "(a)", by means of auxiliary control loops which are discoupled at least to a substantial extent, said variables being used in said main control loop as disturbance variables.

2. A method according to claim 1, which utilises a system monitoring the modifying of said actuating variables of said auxiliary control loops.

3. A method according to claim 1 or 2, wherein step "(a)" comprising setting and coordinating the variables influencing the printing process is repeated it a modification of at least one actuating variable of an auxiliary control loop exceeds a permissible tolerance range, or it there is a modification within a certain control deviation.

4. A method according to any of claims 1 to 33%. wherein sample sheets for densitometer measuring or another printed product checking operation are taken it a modification of an actuating variable exceeds a f 11 4.) 1 Y a certalln tolerance range, or It the.--e is a modification of a control deviation.

5. A method according to any of claims 1 to 4, wherein, for at. '9.east one of the control systems in which the disturbance variab3es are employed, an auxiliary control variable functionally depending on the respective disturbance variable is selected and adjusted to a constant value.

0 Published 1988 at The Patent Office, State House, 66,11 High H01born, London WCIR 4TP. Further copies may be obtained from The Patent Office, Wes Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1/87.