The invention relates to a method for calculating and regulating the
elongation of a moving material web, in particular for application in a web tension
regulating device disposed between a roll exchanger and a first printing unit of
a printing press.
Document DE 22 56 882 discloses such a method of regulating the elongation
of a moving material web which is unwound from a paper roll, thread through a web
tension regulating device and fed into the first printing unit of the printing press.
The web tension regulating device comprises a dancer roller which creates an upstream
tension on the paper web and which is located upstream of a drive roller which creates
a downstream tension on the paper web. The upstream tension and the upstream velocity
of the paper web are measured by the use of two separate rollers and the measured
values are used as input for a calculating unit. The downstream tension and the
downstream velocity of the paper web are measured by the use of a separate roller
and the measured values are used as input for the same calculating unit which subsequently
calculates a regulation value by use of the afore-mentioned input values. The regulation
value and an additional desired value of the web elongation are used as input for
a regulating unit which subsequently regulates the rotational speed of the drive
roller by means of regulating its drive motor.
Another method is known from WO-A-92/10419, in the case of which it
is pointed out that this method is applied at a point between the printing rollers
and the output rollers of a printing press.
The elongation of a material web which is subjected to certain processing
operations must be kept essentially constant, in order to allow accurate positioning
of the operations on the material web. For example, in multi-colour printing operation
accurate positioning of the various printing units relative to the moving material
web is essential for obtaining a uniform printing quality. It is known that the
modulus of elasticity of a paper web on a stock roll is not constant, but changes,
for example as the result of a varying moisture content. In the course of regulation
of the material web to produce constant tension, as is also known in the art, with
a changing modulus of elasticity the elongation ε will have to be adjusted
in an inversely proportional manner, in order to keep the web tension σ constant
(σ = E · ε).
The known method comprises measuring a length change from a reference
length of the material web, which change is caused by an imposed increase or decrease
in the web tension, and measuring the web tension before and after the change in
the web tension. This change in tension is imposed by means of an acceleration or
deceleration roller, which is disposed downstream of the printing cylinders (viewed
in the direction of movement of the material web). The elongation is calculated
from the measured data, the length change and the web tension, based on the assumption
that there is a linear correlation between the web tension and the elongation. If
necessary, the web tension is then adjusted in order to maintain the elongation
at the desired value. It is also disclosed that, instead of an additional acceleration
or deceleration roller, the supply rollers, printing rollers or output rollers can
be used to achieve the web tension change. However, this is not a preferred option,
on account of the moment of inertia of such rollers.
A disadvantage of this known method is that the measurement procedure
followed disrupts the movement of the material web, because a change in the web
tension is essential for performing the measurement, which change is achieved by
accelerating or decelerating the material web. Such a disruption of the movement
of the material web can result in a disruption of the subsequent processing operations,
for example a printing process. This disadvantage is partially overcome by carrying
out the acceleration or deceleration of the material web after it has been printed.
However, this has the disadvantage that a deviation from the desired elongation
cannot be established and corrected until after printing of the web.
Another disadvantage is that if the modulus of elasticity E of the
moving material web changes, said modulus must be determined again in order to permit
calculation of the elongation.
A further disadvantage of this known method of regulation is that
it is not possible to watch the elongation continuously, because the measurements
are made at different times, before and after an imposed change in the web tension
The object of the present invention is to provide a continuous method
for calculating and regulating an essentially constant elongation of a moving material
web wherein a disruption of the movement of the material web is avoided.
Another object of the invention is to design the regulating method
in such a way that variations in the web tension are filtered out, so that they
barely affect the regulating method.
The objects described above are achieved by the method of claim 1.
In the case of the method according to the invention, the unwound
length per period of time or the speed per period of time is measured upstream and
downstream of a drive roller. From these measured values the elongation can be calculated
using the general formula: ε = (v2 - v1) / v1
or ε = (l2 - l1) /l1, where index 2 indicates
the measuring position downstream of the drive roller and index 1 indicates the
measured position upstream of the drive roller. This calculated value of the elongation
is compared directly with the value of the desired elongation of the material web
without measuring the web tension. Moreover, it is not essential to know the exact
value of the modulus of elasticity in order to be able to carry out the regulation.
The web tension can then be adjusted in order to correct the measured elongation
to the desired value, if such a correction is necessary.
It has been found that the modulus of elasticity of a paper web from
a stock roll often decreases while it is being unwound, i. e. the outer paper layers
have a higher modulus of elasticity than the inner layers. In order to present such
a material web to a processing operation with a constant elongation, the tension
of the web will therefore have to be varied in a corresponding way. If the measured
elongation of the material web is higher than the desired value, the web tension
must be reduced. If the measured elongation is too low, the web tension must be
In order to ensure that the regulating method according to the invention
is not affected by rapid changes in the modulus of elasticity of the material web,
the length change or speed change is measured for certain intervals, and an average
elongation is calculated from this measurement for said intervals. The influence
of rapid changes in the modulus of elasticity is filtered out in this way. However,
if a great change is expected, for example after the stock roll has been exchanged,
the interval for which the elongation is calculated is preferably shortened, in
order to be able to anticipate the changing modulus of elasticity more quickly.
The web tension is corrected initially by adjusting the force exerted
on the material web without changing the speed of the drive roller. To this end,
the pressure is adjusted in a dancer cylinder which sets the force exerted on the
material web by a dancer roller disposed downstream of the drive roller to filter
out web tension variations.
Such a dancer roller is also advantageously held in a centre position
by adjusting the speed of the drive roller.
The method according to the invention in fact comprises a double regulation:
a first regulation of the web tension, based on the measured and desired elongation,
and subsequently a regulation of the speed of the drive roller in order to produce
the desired correction.
The regulating method according to the invention is used in a web
tension regulating device placed upstream of a printing press. This means that correction
of the elongation is possible by adjusting the web tension prior to the printing
The invention also relates to a device for conveying a material web
with an essentially constant elongation according to claim 5.
Such a device, also called the infeed, is used to make an accurate
adjustment of the characteristics of a material web coming from a stock roll in
a roll exchanger or the like, and subsequently to present said material web to following
The means for adjusting the web tension are a dancer roller disposed
between the drive roller and measuring device, and a dancer cylinder for correcting
the force exerted on the material web by the dancer roller.
A regulating device for correcting the position of the dancer roller
in the device according to the invention is also present, in order to prevent the
dancer cylinder from filling up or emptying. This regulating device adjusts the
speed of the drive roller.
The invention will now be explained with reference to the drawing,
- Fig. 1
- is a diagrammatic illustration of a preferred embodiment of a device according
to the invention; and
- Fig. 2
- is a graph of the elongation, web tension and modulus of elasticity as a function
Fig. 1 shows the so-called infeed 1, which is disposed between a roll
exchanger and a printing press (neither of which is shown). A material web 2 is
passed from the roll exchanger over a drive roller 3 to the first printing station
of a printing press, as indicated diagrammatically by arrows. A nip roller 4 ensures
that the material web is pressed against the drive roller 3. Said drive roller 3
is provided with a motor 5, which is connected to regulator 6, which controls the
speed of the motor 5 and consequently that of the drive roller 3. This speed control
can be carried out on the basis of the current, voltage or frequency supplied to
the motor. Disposed upstream and downstream of the drive roller 3 are recorders
7 and 8 which measure the speed of the material web 2 upstream and downstream of
the drive roller 3. Said recorders 7, 8 pass on the measured values by way of lines
9 and 10 respectively to a computing and regulating unit 11 which calculates the
elongation. The desired elongation, often a value determined by the user in practice,
is stored in the regulating unit 11. A control signal for a transducer 12 is calculated
by comparing the actual elongation and the desired elongation. The transducer 12
regulates the pressure in a dancer cylinder 13 of a dancer roller 14. The dancer
roller 14 exerts a force on the material web 2, as a result of which the web tension
is set. If the measured elongation is higher than the desired elongation, the pressure
in the dancer cylinder 13 will be reduced, in order to lower the web tension, and
vice versa. The dancer roller 14 is connected by means of an arm 15 to an eccentric
16. The dancer roller 14 is consequently movable through a certain angle, the position
shown in the figure being a centre position. If the pressure in the cylinder 13
is changed in order to change the web tension of the material web 2, this takes
the dancer roller 14 out of its centre position. This change of position of the
dancer roller 14 is recorded by means of a measuring element 17 and passed on to
regulating unit 18. In order to return the dancer roller 14 to its centre position,
the current speed of the material web 2 measured with a press tachometer 19 and
the measured position deviation are converted to a desired drive speed of the drive
roller 3 while maintaining the web tension at a constant value. The calculated value
of the drive speed is passed on to the motor 5 by way of the motor regulator 6.
With this system the elongation added to the material web 2 by the
drive roller 3 is maintained at a constant value. This is illustrated further in
Fig. 2, in which the curves of the elongation ε, the modulus of elasticity
E and the web tension σ are plotted as a function of time t. The jumps in
modulus of elasticity and the web tension σ indicate, for example, a change-over
from one stock roll to another. This shows the virtually constant value of the elongation
which is obtained by a continuous adjustment of the web tension to the changing
modulus of elasticity.
By integrating the measurements made by the recorders over the measuring
periods, higher frequencies of disruption in the web tension or modulus of elasticity
are filtered out without adverse effects on the regulation. This means that the
high frequency disruptions are filtered out, while the relatively slowly varying
modulus of elasticity of the material web is followed accurately. In order to maintain
the elongation of the material web at a constant value when there is a splice, the
duration of the measuring periods can be shortened in the period around the splice.
Of course, it is also possible using the same dancer construction
as that described above to measure the web tension difference over the drive roller
by measuring the web tension simultaneously both upstream and downstream of the
drive roller or measuring the couple on the motor of the drive roller. The modulus
of elasticity can be determined continuously in this way. The desired web tension
which must be imposed on the paper web by the dancer, combined with the desired
(set) elongation, can be calculated from this value.
As shown further in Fig. 1, the device can also be provided with a
switch, in order to permit regulation at constant web tension. In that case the
recorders 7 and 8 and the computing and regulating unit are put out of action. Instead
of the control signal based on the elongation, a control signal based on the desired
web tension is passed on to the transducer, following which the regulation of the
web tension by means of the dancer construction is carried out in the manner described