EP1120250B1 - Transport system for a printing press - Google Patents

Abstract

A printing-material transport system for a printing machine, having at least one printing-material guide surface and a device for producing an air cushion between the guide surface and the printing material, comprising at least one sensor arrangement for registering the spaced distance between the printing material and the guide surface, and a control device for controlling the thickness of the air cushion, based upon the registered distance, so that the spaced distance registered by the sensor arrangement comes to lie within a desired range.

Description

The present invention relates to a printing material transport system for a printing press, as shown in DE-A-197 30 042 known. Such transport systems are used to convey a substrate, in particular individual sheets of a substrate, from a printing unit of a multicolor printing press to the next, to turn double-sided to be printed sheets, or store finished printed sheets.

Due to the fast working cycles of modern printing machines, such transport systems must be able to transport the printing material at speeds of several tens of kilometers per hour without any loss of freshly printed ink. In order to protect the color, it is desirable to hold the substrate at most with grippers at its edges, but otherwise to be able to transport substantially contactlessly. There are therefore transport systems have been developed in which the substrate is transported along a guide surface and between the guide surface and the printing material, an air cushion is generated, which is intended to avoid contacts between the two. This does not always succeed with satisfactory security. If the substrate has a low stiffness, it tends to flutter during rapid transport, and thus can come into contact with the guide surface. If the substrate has a high rigidity, this can lead to contact with the guide surface at curved points of the transport path.

An apparatus for controlling a sheet guide in a sheet-fed press according to DE-A-197 30 042 contains a cylinder with a lateral surface, actuators for blowing air and a distance sensor. The lateral surface is closed, wherein the blowing air and the distance sensor are arranged outside the cylinder and are directed to a sheet which rests on the lateral surface.

In the DE-A-195 49 351 there is disclosed an arc control apparatus when fed to a machine in which a capacitive sensor is disposed above the plane of an investment table. The sensor responds to a change in the dielectric when sheets are fed one or more times.

In a device for conveying sheets after the DE-A-4 328 445 Sensors are used to detect flapping movements.

Since the behavior of the transported substrate depends not only on its rigidity, but also on the transport speed, and in the case of an arcuate substrate also on its format, it is extremely expensive to determine values for the strength of the air cushion, the in all conceivable conditions of use ensure satisfactory protection against greasing.

To solve this problem, according to the present invention, a printing material transport system for a printing machine with at least one Bedruckstoffleitfläche and means for generating an air cushion between the guide surface and the printing material is proposed, which has the features of claim 1. Advantageous embodiments emerge from the subclaims.

As a means for generating the air cushion serve a positive pressure source and connected thereto, arranged on the guide surface air outlet openings. The guide surface can be designed in sheet form in the form of sheets and plates, wherein the guide surface for the pneumatic distance control of a sheet blowing and / or Saugluftöffnungen. The air cushion or supporting air cushion may also be between ironing or pipes, wherein the narrow in relation to the arc surface surfaces are part of said guide surface.

The sensor arrangement comprises a capacitive sensor arranged on the guide surface. Such a sensor should be as small as possible, in particular as flat as possible, so that it can be arranged on the guide surface without disturbing the transport of the substrate.
Such a sensor can essentially be made up of thin metallic and insulating foils glued onto the guide surface.

In a planar arrangement, the sensor preferably has a measuring electrode and a shielding electrode surrounding the measuring electrode and insulated therefrom. These electrodes can each be acted upon by a control circuit with AC signals, wherein the phase and amplitude of the two AC signals relative to each other are controlled by the control circuit so that the electric field of the measuring electrode is up to a typical measurement distance substantially perpendicular to the measuring electrode. As a result, a substantially constant from the measuring electrode to the measuring location strength of the electric field and consequently a linear characteristic of the sensor is ensured.

When transport systems for sheet-fed presses usually gripper bridges are used, which extend substantially across the width of the printing press, hold a sheet to be conveyed at its leading edge and pull behind him. In order to avoid detecting the gripper bridges when detecting the distance of the sheet and thereby falsifying the detection result, it is preferable to provide a timing circuit which receives a synchronization signal coupled to the printing cycle of the printing press and ensures that no distance detection takes place if any Gripper bridge is located in the detection range of the sensor.

Further features and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the accompanying figures.

Figur 1

einen schematischen Schnitt durch das letzte Druckwerk und den Ausleger einer Schön- und Widerdruckmaschine oder einer Schöndruckmaschine, und

Figur 2:

ein Detail aus Figur 1.

Show it:

FIG. 1

a schematic section through the last printing unit and the boom of a perfecting machine or a letterpress machine, and

FIG. 2:

a detail of Figure 1.

The structure of the printing unit of Figure 1 is substantially known and need not be described in detail. Transfer drums 1,2 take over a sheet to be printed from an upstream printing unit and pass it to a counter-pressure cylinder 3. On this the sheet passes through a gap between impression cylinder 3 and blanket cylinder 4, in which it is printed and then transferred to a transport system, the majority comprises guided on endless chains 5 gripper bars 6. The chains 5 run in Figure 1 clockwise, and transport printed sheets each on its underside hanging to a delivery stack 8. A working with infrared or UV radiation dryer 7 is arranged on the transport system so that its radiation on the last printed Top of each bow drops.

In order to ensure an exact and uniform guidance of each sheet on the transport system, a Bedruckstoffleitfläche 10 is arranged in the form of a sheet-metal equipped with nozzles on the lower strand of the chains 5, which prevents uncontrolled dropping a sheet down. The nozzle-forming perforations of the guide surface 10 open onto compressed air boxes 11, of which two are present in the system shown in FIG. These are supplied by a pump 12 via control valves 13 with compressed air. The degree of opening of the control valves 13 can be controlled by an electronic control circuit, which is not shown in Figure 1 and whose operation will be discussed later.

Figure 2 shows in detail a section of the guide surface 10. The guide surface 10 consists of a flat sheet with perforations formed therein 9 (transport nozzles), which open onto a mounted under the guide surface compressed air box 11.

Between the two perforations 9 shown on the guide surface 10, an insulating plastic film 20 of a few microns thickness is applied, the three formed of a metal foil, mutually insulated, concentric electrodes 14,15,16 carries. The innermost of the three electrodes represents a measuring electrode 14. It is in each case annularly surrounded by the shield electrode 15 and the ground electrode 16. In addition to concentric or annular formation of the electrodes 14, 15, 16, shielding electrodes 15 and ground electrodes, other shapes such. B. rectangular shapes, can be used. The thickness of the electrodes is only a few micrometers. The three electrodes 14, 15, 16 are each connected to outputs of a control circuit 17 via plated-through holes 21 insulated against the guide surface 10, which is mounted on a printed circuit board 18 on the underside of the guide surface 10 within the compressed air box 11.

gegeben, wobei A die Oberfläche der Kondensatorplatten und d ihren Abstand bezeichnet.A printing material sheet 19 drawn away from the gripper bridges 6 via the guide surface 10, together with the measuring electrode 14, forms a capacitor whose capacitance depends on the distance between the sheet 19 and the measuring electrode 14. For an ideal plate capacitor, this dependence is given by the formula $$\mathrm{C}\mathrm{=}{\mathrm{\epsilon \; \epsilon }}_{\mathrm{0}}\mathrm{A}\mathrm{/}\mathrm{d}$$

where A denotes the surface of the capacitor plates and d denotes their distance.

In order to ensure that this simple relationship also applies to the capacitor present here, it is necessary that the electric field between the measuring electrode 14 and the arc 19 induced by the application of an electrical voltage to the measuring electrode 14 be similar to that of an ideal plate capacitor, that is, it must be substantially parallel and perpendicular to the surface of the measuring electrode 14. To achieve this, the shield electrode 15 is provided. The two electrodes are each acted upon by the control circuit 17 with two alternating voltage signals, which are largely identical in amplitude and phase. This has the consequence that the unavoidable divergence of the field lines at the edge of the plates, which is unavoidable in the case of a real plate capacitor, is restricted to the field of the shield electrode 15, whereas that of the Measuring electrode 14 outgoing field lines extend virtually parallel to the sheet 19.

The earth electrode 16, which surrounds the shield electrode 15 in a ring-shaped manner as shown in FIG. 2, can also be dispensed with if the electrically conductive guide surface 10 is held at ground potential in its place.

wobei hier A die Fläche der Messelektrode 14 ist.The control circuit generates a first AC signal by impressing an alternating current of predetermined magnitude and frequency to the measuring electrode 14. The voltage amplitude of this signal is proportional to the reactance Xc of the plate capacitor. $$\begin{array}{cc}{X}_c& =\frac{1}{i\omega C}\hfill \\ & =\frac{1}{i\omega {\epsilon }_0{\epsilon }_R}\frac{d}{A}.\hfill \end{array}$$

where A is the area of the measuring electrode 14.

A second AC voltage signal applied to the shield electrode 15 is generated by the control circuit 17 by means of a voltage follower from the first AC signal.

The measured AC voltage amplitude is compared in the control circuit 17 with a limit value. If the limit is exceeded, this means that the sheet 19 of the guide surface 10 is closer than allowed. In such a case, controlled by the control circuit 17, the power of the pump 12 or the opening degree of the respective compressed air box supplying valve 13 is increased to the air cushion formed by the exiting from the perforations 9 compressed air cushion between the guide surface 10 and the sheet 19 reinforce and thus bring it to a greater distance. Conversely, the thickness of the air bag is reduced when the measured distance exceeds a second limit.

It can preferably be provided that the arrangement is designed as a two-point control system, that is, the measured value is compared with a limit value. If the limit is reached or exceeded, the air cushion is increased or decreased accordingly. Alternatively, it is also possible to perform the operation by means of a controller with a predetermined setpoint. This represents a particularly high-quality solution. As a setpoint, the distance X of the arch 19 from the guide surface 10 is defined. The deviation from the setpoint position is determined as ± ΔX. The actuator of the controller is preferably adjusted in proportion to the deviation ± ΔX.

Alternatively, the control circuit 17 may impress an AC voltage of predetermined frequency and amplitude and measure the magnitude of the resulting AC current. Decisive for the detection is that values of the voltage and current, either given or measured, are present, which allow the conclusion on the reactance of the capacitor.

With the capacitive measuring principle of the sensor arrangement from FIG. 2, distance measuring values with a frequency in the kilohertz range can be obtained. This makes it possible, in the passage of a single sheet 19 in front of the sensor, to record hundreds of measured values which make it possible to detect a rapid flapping motion of the sheet 19.

Since the sensor of Figure 2 is essentially composed of only two layers of thin films and therefore has a thickness in the range of millimeter fractions, it does not appreciably affect the air flow ratios between the guide surface and a sheet 19 transported above. Such a sensor arrangement can therefore also be retrofitted into already existing transport systems for printing machines without difficulty.

According to a preferred embodiment of the invention, the control circuit 17 is coupled to an angle encoder, which is connected to any, synchronous to the machine cycle drum the printing press can be mounted and the control circuit 17 provides a synchronization signal. This allows the control circuit 17 to inhibit the recording of distance measured values whenever a gripper bridge 6 passes the sensor, which would falsify the measurement results. In the same way, the measured value recording can be suppressed if no sheet 19 is in front of the sensor. For this purpose, it is necessary that the control circuit 17 in addition to the synchronization signal information about the length of each processed sheet 19 receives. This temporary suppression of the measured value recording facilitates subsequent processing of the measured values; in particular a characteristic value formation z. B. low-pass filtering from the digestion over the average distance between the printing material sheet 19 and guide surface 10 can be obtained.

Sensor arrangements of the type described with reference to Figure 2 may be provided in a transport system of a printing press at various locations to locally regulate the thickness of the air cushion there. They can be used not only on the boom, as shown in Figure 1, but also during transport of the printing material between two printing units of a machine, or in the sheet turner a printing press.

Claims (5)

1. Printing material transport system for a printing press, including
   at least one printing material guide surface and means for generating an air cushion between the guide surface (10) and the printing material (19),
   at least one sensor arrangement for registering the distance between printing material (19) and guide surface (10), and
   control means (17) that are capable of controlling the strength of the air cushion based on the detected distance in such a way that the distance detected by the sensor arrangement is within a desired range,
   characterized in
   that the means for generating the air cushion include an excess pressure source (12) and air outlet openings (9) arranged on the guide surface (10) and connected to the excess pressure source (12), and
   that the sensor arrangement comprises a capacitive sensor (14-18, 20) arranged on the guide surface (10).
2. Printing material transport system according to claim 1,
   characterized in
   that the sensor is constructed of metallic and insulating foils (20).
3. Printing material transport system according to claim 1,
   characterized in
   that the sensor includes a measuring electrode (14) and a shielding electrode (15) in a planar arrangement, the shielding electrode (15) surrounding and insulating against the measuring electrode (14).
4. Printing material transport system according to claim 3,
   characterized in
   that the sensor arrangement includes a control circuit (17) for applying a first alternating voltage signal to the measuring electrode (14) and for registering the reactance of a capacitor formed by the measuring electrode (14) and the printing material (19) located opposite the measuring electrode (14).
5. Printing material transport system according to one of the preceding claims,
   characterized in
   that the sensor arrangement (14-18, 20) comprises a time control circuit (17) that is equipped in such a way that it receives a synchronisation signal coupled to the operating cycle of the printing press and ensures that the distance between printing material (19) and guide surface (10) is only registered during part of each cycle of the printing press.

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