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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a device and method for damping mechanical
vibrations of printing presses.
The drive train of a printing press with the parts connected thereto, such
as cylinders and rollers, for example, constitutes a system having
dynamics which are determined by spring constants, moments of inertia,
rotating and oscillating masses, and so forth. The rotating parts of this
driven drive train can be excited to vibrations due to the following
effects: angle-dependent effects, i.e. synchronous oscillations, recurring
over one rotation and effects which do not recur periodically with one
rotation, are to be considered as distinct. Recurring load moment
deviations, such as are generated, for example, by cam transmissions or by
the failure of single or n-revolution gears are to be counted with the
angle-dependent, i.e., synchronous, vibrations. Aperiodic or non-cyclical
vibrations recurring with one rotation, i.e., asynchronous vibrations, can
be produced, for example, by periodic excitations deviating from the
rotational frequency. They occur, for example, when belts are used, due to
vibrator shock or stroke, or due to errors or failure of half-rotation
gears. Aperiodic noise phenomena, such as from ink separating from paper
or effects of paper pulling, for example, cause asynchronous vibrations.
Furthermore, vibrations can be produced in the system due to parameter
deviations which, in comparison with the sheet travel, exhibit a slight
change in velocity (for example, oil temperature deviations, which have an
effect upon basic friction).
Many angle-synchronous disturbance have a high excitation energy. The
periodic vibration shapes over one rotation which result therefrom do not,
however, have any noticeable effect upon the printing quality with respect
to ghosting in the printed image, because the rotating parts of the
printing press always assume the same angular position at the instant of
paper transfer or acceptance. Asynchronous disturbances become noticeable,
however, in the printing quality. They cause ghosting because the angular
position of the rotating parts of the printing press is subject to
deviations during sheet transfer. The effect of these disturbances becomes
noticeable due to their most often low excitation energy essentially when
characteristic or natural frequencies of the printing press are excited,
wherein the damping is low. The relatively slow parameter deviations
mentioned hereinbefore have no effect upon the ghosting.
It has become known heretofore, for the purpose of reducing mechanical
vibrations, to reinforce the side walls of the printing press and/or to
install reinforced gears as well as other reinforced components. These
measures are expensive, increase the weight of the printing press and do
not always produce the desired results.
It is accordingly an object of the invention to provide a device and a
method for damping mechanical vibrations of printing presses which
improves the printing quality.
SUMMARY OF THE INVENTION
With the foregoing and other objects in view, there is provided, in
accordance with one aspect of the invention, a device for damping
mechanical vibrations of a printing press having rotating parts,
comprising at least one actuating member assigned to the rotating parts of
the printing press for applying adjusting forces thereto, and at least one
vibration pick-up operatively connected to the actuating member for
controlling the actuating member so that the adjusting forces applied by
the actuating member damp the mechanical vibrations.
With the device according to the invention, preferably asynchronous
disturbances are opposed and subdued. Because these disturbances have
considerably low excitation energies, they can be damped by means of
relatively low adjustment forces (in comparison with the total drive
power). The vibrations are detected, in accordance with the invention, by
at least one vibration pick-up. Data determined by the vibration pick-up
are evaluated and result in the activation or control of an actuation
member which is embodied as an active adjusting member. The adjusting
forces applied by the actuating member act in opposition to the forces
exciting the vibrations, so that a damping is set or introduced. Ghosting
is prevented by the damping of the asynchronous disturbances, so that the
printing quality is improved.
In accordance with another feature of the invention, the actuating member
is formed as a controllable eddy-current brake. This brake is activated or
controlled in accordance with the excitation frequency and thus engages
actively in the entire system, thereby eliminating the asynchronous
vibrations.
In accordance with a further feature of the invention, the printing press
has at least one drive motor, and the function of the actuating member is
embodied in the drive motor.
In accordance with an alternative feature of the invention, the printing
press has at least one drive motor, and the actuating member is formed by
an additional motor.
The torque of the drive motor is able to be influenced or affected, for
example, by means of suitable components of the power electronics
depending upon or in accordance with the data determined by the vibration
pick-up, so that the drive motor per se also performs the function of the
actuating member and serves to reduce the vibrations. In this regard, a
double function accrues to the drive motor, because it supplies drive
power, on the one hand, and serves to damp vibrations, on the other hand.
In a corresponding manner, an additional motor can be provided in the
drive string or train of the printing press and can be suitably activated
or controlled to reduce the vibrations.
In accordance with an added feature of the invention, there is provided a
vibration-damping control system having a control circuit to which the
vibration pick-up and the actuating member are connected. Due to suitable
construction of the control circuit, assurance is always provided that
occurring, preferably asynchronous vibrations, will be controlled down to
zero, which can result in the achievement of an optimal damping.
In accordance with an additional feature of the invention, the actuating
member is controllable by the control system so that only aperiodic or
asynchronous vibrations occurring with rotations of the rotating parts of
the printing press are damped.
In accordance with yet another feature of the invention, the printing press
has a string of drives extending therethrough, and including a plurality
of the vibration pick-ups respectively distributed at a plurality of
locations along the length of the string of drives.
In accordance with an alternative feature of the invention, the printing
press has a string of drives extending therethrough, and including a
plurality of the actuating members respectively distributed at a plurality
of locations along the length of the string of drives.
In accordance with a combination of both of the alternative features of the
invention, pluralities of both the vibration pick-ups and the actuating
members are, respectively, distributed at a plurality of locations along
the length of the string of drives.
In accordance with a concomitant aspect of the invention, there is provided
a method of damping printing quality-reducing mechanical vibrations in a
stock-guiding system of a printing press, which comprises picking up
vibrations from the stock-guiding system as measured values, processing
the measured values to produce adjusting forces, and applying the
adjusting forces to the stock-guiding system of the printing press so as
to damp the vibrations.
Preferably, aperiodic or asynchronous vibrations occurring with the
rotations of the rotating parts of the printing press are detected or
picked up and damped.
Other features which are considered as characteristic for the invention are
set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a
device and method for damping mechanical vibrations of printing presses,
it is nevertheless not intended to be limited to the details shown, since
various modifications and structural changes may be made therein without
departing from the spirit of the invention and within the scope and range
of equivalents of the claims.
The construction and method of operation of the invention, however,
together with additional objects and advantages thereof will be best
understood from the following description of specific embodiments when
read in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic and schematic view of a printing press provided
with a device for damping vibrations in accordance with the invention; and
FIG. 2 is a block and schematic diagram of a control device or system
forming part of the invention.
FIG. 3a is a block diagram of a vibration detection device connected to the
drive train;
FIG. 3b is a diagram showing pulse trains generated by the vibration
detection device of FIG. 3a;
FIG. 4 is a signal processor for processing the pulse trains generated by
the vibration detection device, connected with a fast Fourier transform
converter;
FIG. 5 shows signal forms as generated by the signal processor of FIG. 4;
FIG. 6 shows an arrangement wherein the vibrations sensed from the drive
trains are processed and inverted and fed back in opposite phase to the
drive motor, and having a harmonic vibrations;
FIG. 7 shows an arrangement wherein the vibrations sensed from the drive
train are processed and fed to an eddy current brake coupled to the drive
motor and drive train; and having a harmonic selection arrangement for
suppressing some or all harmonics with an inhibiting gate and oscillator;
FIG. 8 shows an arrangement for suppressing all vibrations in the drive
train, based on an eddy current brake controlled by an inverting
calibrating amplifier having an input receiving the drive train vibrations
picked up from the drive train and processed.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings and, first, particularly to FIG. 1 thereof,
there is shown therein a printing press 1 having a plurality of printing
units 2 each identified by a suffix 1-6, namely six printing units 2.sub.1
-2.sub.6 in the illustrated embodiment. Each printing unit 2 has a
plurality of cylinders and rollers, of which, in the interest of clarity,
only a few thereof are shown in FIG. 1.
The cylinders and rollers of the printing press 1 form a printing material
or stock-guiding system. Each printing unit 2 is driven by a respective
drive motor M.sub.1, M.sub.3, M.sub.5, M.sub.n, M.sub.n+2, M.sub.n+4, such
as an electric motor. Respective conventional vibration pick-ups or
receivers S.sub.2, S.sub.3, S.sub.4, S.sub.5, S.sub.n, S.sub.n+2,
S.sub.n+4 are assigned to the individual printing units 2 and are
connected to a control device or system 3.
Each motor M.sub.n has a dedicated motor control 2.sub.c of conventional
construction, which controls the power output of the respective motor
under control of a respective motor control output 2.sub.d of the control
system 3.
Various different embodiments of the invention are actually illustrated in
FIG. 1 and are explained hereinafter in greater detail, however, it will
be apparent that additional non-illustrated embodiments also fall within
the range of the invention.
The printing unit 2, lying farthest to the left-hand side of FIG. 1 has a
drive motor M1 which acts upon a drive string, particularly via a
conventional gear train, illustrated symbolically as a dashed line D, of
the printing press 1. A vibration pick-up S2 which is connected to the
control system 3 is assigned to one of the rollers or cylinders of the
aforementioned printing unit 2. A conventional operative connection
2.sub.a exists between the printing unit 2 and each of the drive motors
M.sub.1 -M.sub.n+4.
The device according to the invention of the instant application operates
in the following manner:
The vibration pick-up S.sub.2 senses, via data lead 2b, the occurrence of
vibrations in the appertaining printing unit 2, and transmits
corresponding data to the control system 3 which performs an evaluation
thereof. In particular, aperiodic vibrations are detected or determined
and a control value is formed which has a reactive effect upon, i.e., is
fed back to the drive motor M.sub.1 via the motor control 2.sub.c, or
directly to the motor M.sub.n as described in more details below. The
drive motor M.sub.1 thus forms an actuating member B.sub.1. Control of the
motor M.sub.1 is effected in such a manner as not to exert a constant
driving torque, but rather, due to the particular control by the control
device 3, additional adjusting forces are applied which have an opposing
effect upon the asynchronous vibrations, so that altogether a damping of
the vibrations is produced.
The second printing unit 2.sub.2 from the left-hand side of FIG. 1 has an
arrangement corresponding for the most part to that of the aforediscussed
printing unit 2, disposed farthest to the left-hand side of the figure,
but is different, however, in that more than one vibration pick-up, namely
two vibration pick-ups S.sub.3 and S.sub.4, are provided, which determine
the vibrations at different locations of the drive string or train and
feed the respective data via control leads 3a, 3b to the control system 3.
A drive motor, namely the motor M3, accordingly, serves simultaneously as
an actuating member B.sub.3 and a vibration damping member.
The third printing unit 2.sub.3 from the left-hand side of FIG. 1 is
provided with yet another embodiment of the vibration-damping device
according to the invention. Only one vibration pick-up S.sub.5 is provided
which is connected to the control system 3. In this embodiment, however,
two elements are provided as operating members for applying adjusting
forces, namely a drive motor M.sub.5 acting as an actuating member
B.sub.5, and an additional motor M.sub.(5), acting as an actuating member
B.sub.(5), at another location of the drive train. Both of the motors
M.sub.5 and M.sub.(5), are so controlled by the control system 3 that the
moments transmitted thereby are set in accordance with the required
driving power and also with respect to the damping of the asynchronous
vibrations.
The fourth printing unit 24 from the left-hand side of FIG. 1 has an
embodiment of the vibration-damping device according to the invention
which conforms to the aforedescribed embodiment thereof in the printing
unit 2, located farthest to the left-hand side of the figure, with the
exception that the control system 3 additionally controls a conventional
eddy current brake W to act upon the appertaining drive train of the
respective printing unit 2 so as to damp the occurring vibrations.
The manner in which the control system or device 3 functions is described
hereinafter in greater detail, with regard to FIG. 2:
The vibration pick-ups S.sub.n are connected to a fast Fourier transform
device 4 for fast Fourier transformation (FFT) which is a component of the
control system 3. Devices for performing Fourier analysis are well known
in technical spectrum analysis, see e.g. Van Nostrand Scientific
Encyclopedia pg. 2064-2067. The fast Fourier transform is simply anyone of
several fast converging versions of the conventional Fourier transform.
Incremental rotary-angle transmitters for the rotary angle .phi. of drive
train D are suitable as the vibration pick-ups Sn and are coupled with
respective driven cylinders of the printing press 1.
FIG. 3a shows diagrammatically a set of conventional pulse generators each
having a faceted mirror wheel 21a, 21b, illuminated by respective light
transmitters LT.sub.1, LT.sub.2, which are coupled by a pulsing light beam
to respective light receivers LR1, LR.sub.2, which generate pulse trains
P1, P.sub.2. The mirror wheels 21a, 21b are mechanically coupled by a
section of the drive train D.
Furthermore, a controller 5 is provided in the control system 3 and is
connected to the FFT device 4. In a practical realization of the invention
there may be provided an FFT device 4 for each vibration sensor S.sub.n
and a corresponding controller 5 for each FFT device 4 connected to a
respective drive motor control 2c. Alternatively, a single set of an FFT
device 4 and a controller 5 may be shared by a common multiplexing
arrangement serving several drive motors M.sub.n and vibration pick-ups
S.sub.n. The controller 5 has an output connected to the drive motors
M.sub.n. In the FFT device 4, the rotary angle .phi..sup.n (t) is analyzed
or broken down into spectral component of respective frequencies
.omega..sub.i, vibration amplitude A.sub.i at .omega..sub.i and phase
position .phi..sub.i at .omega..sub.i wherein i represents the ordinal
number for the harmonic present in the vibration. With the aid of the
controller 5, the frequencies critical for the operation of the printing
press 1 are selected, for example, the frequencies at which the inherent
or natural frequencies of the press are excited. Then, a correction factor
K.sub.i, respectively, for the i-th harmonic is applied to the amplitude
A.sub.1. The controller 5 calculates an adjustment value for the torque M
of a drive motor M.sub.n from the vibration value K.sub.i
.multidot.A.sub.i .multidot.sin (.omega..sub.i t+K.sub.i
.omega..phi..sub.i), as shown in more detail in connection with FIG. 3a,
3b and FIG. 4.
In FIG. 3a a driving pulse wheel 21a is connected to a point of the drive
train D, advantageously at a point near the drive shaft of a respective
drive motor M.sub.n. A driven pulse wheel 21b is connected to a point of
the drive train D further away from the drive motor, so that the elastic
deformation of the intervening section D, of the drive train D is subject
to a small angular deflection or pulse angle .omega..sub.t, seen in FIG.
3b as the phase angle between the two pulse trains P1 and P2 from
respective light receivers LR.sub.1 and LR.sub.2 in FIG. 3a. It follows
that the deflection angle .omega..sub.t is a function of both time and the
moment of torque difference present at the two locations on the drive
train to which the respective pulse wheels 21a and 21b are attached.
It will also be readily appreciated that due to the elasticity in the
section D' of the drive train D and the rotating masses angular
oscillations in the phase angle .omega..sub.t occur when the driven
elements are subject to periodically and aperiodically occurring loads.
Such loads occur in printing machines, for example, in driving of sheet
grippers and vibrating ink rollers. It is one of the objects of the
invention to analyze the phase angle .omega..sub.t in order to identify
elements that cause the angular oscillations, and it is a further object
to apply the phase angle .omega..sub.t so as to dampen rotary oscillations
in the drive train of the printing machine, as will be described in more
detail below.
FIG. 4 shows an electronic circuit that processes the pulse trains P1 and
P2 so as to generate the harmonics, i.e. spectral components of the
function .omega.t. A flip-flop FF 22 receives the pulses P1 at a set input
S, which sets the flip-flop at a trailing edge of each pulse of pulse
train P1. A next following trailing edge of a pulse of pulse trains P2
resets the flip-flop. Pulse wheels 2/a and 2/b are preferably set so that
pulse train P2 trails the pulse train P1 by a time distance .omega..sub.t,
which is always a positive and is a function of the elastic angular
deviation between pulse trains P1 and P2, the output Q of FF is set for a
duration .omega.t which is equal to the instantaneous phase shift between
pulse trains P1 and P2. A ramp generator 23 is at its clock input C
activated by inverter 28, which starts the ramp, and its input RP is kept
active for the duration of a logic high at output Q of flip-flop FF, i.e.
during the varying times .omega..sub.t1, .omega..sub.t2, .omega..sub.ti,
etc. The ramp generator 23 delivers at its output R a pulse which has an
amplitude equal to the varying durations of pulse .omega..sub.t . The ramp
generator output has the form of a triangle with an amplitude proportional
with the time .omega..sub.t1, as shown in FIG. 4, and is entered at input
SI of a sample and hold circuit 24 of conventional construction. The
sample and hold circuit 24 is enabled at input E by the output Q of
flip-flop 22, and holds the magnitude of pulse wt for the duration of a
complete cycle of pulse train P1, until it is again enabled at the
following trailing edge of pulse train P1. The output of the sample and
hold circuit becomes a staircase function as shown in FIG. 5, line a. The
sample and hold output is connected to an input of a low-pass filter 26
(LP), which smoothes out the discontinuities of the staircase function, to
deliver at its output L a smooth signal SM. as shown in FIG. 5b. The
signal SM is in condition to be delivered to the fast Fourier transform
circuit 4 (FFT), which at its output FT delivers the harmonics of the time
varying function .omega..sub.t, shown as K.sub.i .multidot.A.sub.i
.multidot.sin (.omega..sub.i t+K.sub.i .omega..phi..sub.i), as described
above. The FFT circuit requires for its operation various clock signals CL
used for sampling the input signal SM. By proper selection of these clock
signals, certain harmonics of the input signal SM can be selected and used
to suppress the vibrations of the drive train as described in more detail
below.
In one embodiment of the invention the output signal SM from the LP filter
26 is used as a feed-back signal in the drive motor circuit to dampen the
oscillations represented by the function SM representing the instantaneous
value of the vibration .omega..sub.t.
In one particular embodiment shown in FIG. 6, certain harmonics may be
selected to damp those harmonics found to be especially undesirable.
In FIG. 6 the output of the FFT 4 is connected with an analog gate, shown
symbolically as a field-effect transistor 29, having its control gate
connected to an oscillator 31 set to a harmonic selection frequency for
the particular harmonic or harmonics that are not to be suppressed. The
selected harmonic(s) are connected to a minus input of a summing circuit
35, having a minus input connected to the gate 29, and a plus input
connected directly to the output of signal processor 39. The output of
summing circuit 35 is connected to an input of the controller 5, which has
an output connected to an inverting feedback circuit 32, which inverts the
signal(s) to be suppressed and calibrates it to the proper level before it
is connected to a plus input b of another summing circuit 33. The summing
circuit 33 has a plus input for receiving a motor power set control
voltage. The output of summing circuit 33 controls via motor control
circuit 2c the nominal power to be delivered by the motor M.sub.n to the
drive train D and the vibration signals(s) to be suppressed. Due to the
inversion and calibration performed in feed-back circuit 32, the unwanted
signal is suppressed at the input to the motor M.sub.n. Calibration is
performed by means of resistors R1, R2 in a local feedback loop of OP-amp
35.
FIG. 7 shows another arrangement briefly described above, wherein the
controller 5, receives the signal to be suppressed from a summing circuit
35, which has a plus input receiving the main vibration signal and a minus
input receiving the harmonics not to be suppressed, as in FIG. 6. The
output of summing circuit 35 is connected to the input of controller 5.
The output of controller 5 is connected via an inverting calibrating power
amplifier 34 to an eddy current brake W, which is mechanically coupled to
the drive motor M.sub.n. It follows that the eddy current brake W may be
an integral part of the motor M.sub.n, or it could be coupled to a
suitable point of the drive D. The power amplifier 36 has an external
control loop with a control potentiameter 36 for calibrating the amount of
braking power to be applied to the eddy current brake W. In FIG. 7 the
power output to be delivered by the motor M.sub.n to the drive train D is
controlled by a motor power set input to the motor control 2c, while the
amount of damping to be impressed on the motor by the eddy current brake W
is adjusted by potentiometer 36. FIG. 7 shows the eddy current brake as
having an electromagnet 37 magnetically coupled to an armature 38 of the
eddy current brake in well known manner. The electromagnet is powered by
the inverting calibrating amplifier 34. In the arrangement according to
FIG. 7 it is contemplated that the electromagnet 37 during normal
operation is biased with a certain amount of constant current flowing
through the power amplifier 34, so that the eddy current brake W presents
a constant drag on the motor M.sub.n. In case a sudden loading is applied
to the drive train D, causing a vibration that is sensed by the vibration
pickup Sn, the vibration signal is processed in signal processor 39, the
FFT 4, the control 5, and the inverting calibrating amplifier 34 as
described above, and the electromagnet 37 will modulate the drag on the
motor M.sub.n in opposite phase of the vibrations, so as to counteract the
vibrations. In other words, if a momentary increase in the load on the
drive train is encountered, the motor M.sub.n will momentarily encountered
increased drag. However, the sensor Sn will detect the increased load as
an increased torque in the drive train, and the constant drag on the motor
M.sub.n will be momentarily relieved due to momentary reduction in the
drag due to the resulting reduction in the current in the electromagnet
37. As a result the motor will be momentarily relieved and apply
correspondingly more torque to the drive train, thereby maintaining
substantially constant speed of the drive train, assuming that the
calibration amplifier 34 is properly calibrated by the potentiometer 36.
It will be readily understood from FIG. 7 that the sensor Sn, signal
processing circuit 39, the FFT 4, the controller 5, the calibrating
inverting amplifier 34 and the electromagnet 37 with the eddy current
brake W together form a stabilizing negative feedback system that will
counteract the vibrations and those harmonics of the vibrations, selected
by the oscillator 31 and the gating transistor 29.
In cases wherein it is desired to damp all vibrations in the drive train D,
it is advantageous to use the signal LM directly as it appears at the
output of low pass filter 26 in FIG. 4, without the use of an FFT and
harmonic selection circuit 29, 31, 35 as shown in the arrangement in FIG.
8. Again, vibrations can be controlled by acting directly on the drive
power applied to the motor or by acting on an eddy current brake coupled
to the motor, or to the drive train. FIG. 8 again shows a version of the
invention using an eddy current brake W with a biasing electromagnet 37.
The arrangement is similar to that shown in FIG. 7, except the signal
processing drives the eddy current brake W directly from the output of the
low-pass filter 26 via the inverting calibrating amplifier 34. The eddy
current brake W may be realized as a secondary drive motor M.sub.5 (FIG.
1) being operated in a manner as shown in FIG. 6.
The use of an eddy current brake or a smaller second drive motor has the
advantage that the eddy current brake or the smaller motor can be
controlled more rapidly than a large drive motor with its larger inertial
masses and larger power consumption.
Again in FIG. 8 the eddy current brake may be replaced or supplemented by
the motor drive control arrangement 2c that acts directly on the drive
power to the motor M.sub.n in the manner as shown in FIG. 6.
It is believed to be clear from the foregoing that the aforedescribed
embodiments of the vibration-damping device according to the invention may
be considered to be only examples which may be installed in a printing
press in any desired combination and also, to a broad or wide extent, with
a plurality of vibration pick-ups and/or actuating members.
It should be understood that the vibration pickup S.sub.n may have pulse
wheels using different sensing methods than optical, i.e. electromagnetic,
electrostatic, mechanical and others, as found to be most effective in a
given environment.
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