Patent Application
20090308269
The invention relates generally to dampening spray systems in offset printing processes and, more specifically, to formation and supply of drive signals to spray dampening valves in the dampening spray systems.
In the offset printing process, a small amount of a dampening solution, i.e., water with certain additives, is supplied to the offset plate, which then comes in contact with the inking rollers, the ink adhering to the image on the plate and the dampening solution adhering to the other portions of the plate. The quantity and placement of the dampening solution must be varied for different types and densities of ink, variations in printing densities and ink coverage, and press speed. Control of the application of the dampening fluid is particularly important in four-color process, where variations will affect color. If too little fluid is applied, printing will occur in areas where none is desired. If too much fluid is applied, printing may not occur in some areas, or image density may drop.
Various systems for dampening the plate cylinder of an offset printing apparatus are in use today. One such system employs dampening rollers, which rotate partially within an open trough containing dampening fluid. The dampening rollers bear directly or indirectly against the plate cylinder, thereby supplying a film of dampening fluid to the plate cylinder. This system, however, suffers from a number of inherent disadvantages from the standpoint of both operation and maintenance. From the operational standpoint, the system is too imprecise and difficult to control. Frequently, too much or too little solution is applied to the plate roller, or at least to certain areas of the plate roller, reducing the printing quality.
Another known dampening system eliminates the open fluid container and the immersed dampening roll, and replaces them with a closed system which pumps dampening fluid as a spray onto a dampening roll train for application to the plate cylinder. In such a spray dampening system, the dampening fluid is sprayed onto the press rollers by means of a linear array of spray nozzles with the spray patterns of the individual nozzles merging to form a continuous composite spray pattern across the surface of the press roller. It is important in obtaining proper dampening that the distribution of dampening fluid be as uniform as possible. There should be no starved areas where the amount of dampening fluid is substantially less than the other areas on the surface of the roller, and the overlapping of the adjacent individual spray patterns should be minimized so that there is little or no excessive dampening fluid applied to any portion of the dampening roller.
Various attempts have been made at adjusting the amount of dampening fluid applied to the dampening roll. In known dampening systems nozzles fluctuate between open and closed positions to regulate the amount of dampening fluid applied to the rolls, nozzles are pulsed on and off with the pulse width and/or frequency being adjusted, and dampening fluid is delivered through alternate laterally adjacent nozzles, etc.
In this context it has been shown that it is basically more cost-effective to administer the dampening agent in short bursts of low volumes than to supply larger volumes of dampening agent in one go. In practice, however, the options for shortening the opening times of conventionally marketed valves are limited, each nozzle having an associated valve. Even valve elements with very small masses cannot be moved speedily or precisely enough to reduce the opening and closing times to under 10 or even 5 milliseconds. With cycle times that short, the valve elements are unable to move into their full opening and closing positions.
In general valves are preferably solenoid or servo type valves each including a reciprocal plunger or other valve flow controller moveable by its valve drive element. As is well known in the art, when the plunger reciprocates up under the direction of the valve drive element, pressurized dampening fluid present in the supply line passes under the valve plunger and out the nozzle. Conversely, when the plunger is shifted downward by the valve drive element, the plunger interrupts the flow of dampening fluid from the supply line. The valve drive elements are preferably solenoid type operators but may also be voice coil, piezo, polymer activated, or servo type drive elements.
When the solenoid type operators are used, residual magnetism develops over time and can degrade operation of the valves. When the valves are unipolar the same magnetic field is generated. For example, if the coil always generates the same magnetic field polarity for the pole pieces, over time each piece will take on some degree of residual magnetism.
The energy required to actuate the valve diminishes due to the residual magnetism. The state of the residual magnetism is unknown and cannot be predicted. Thus the characteristics of the valve change over time in a non-predictable manner. Appropriate compensation is not practical. For example, a control algorithm cannot be constructed that will accurately predict this behavior.
The residual magnetism can be affected by other parameters, such as, external shock forces.
One embodiment of the present method and apparatus encompasses an apparatus. In this embodiment the apparatus may comprise: a printing unit having a plurality of spraybar assemblies; each of the spraybar assemblies having a respective plurality of spray valves; a controller that outputs respective predetermined control signals of a plurality of control signals for the plurality of spray valves in each of the spraybar assembly; and each control signal of the plurality of control signals having a series of positive and negative pulses; wherein residual magnetism is substantially minimized in the spray valves by the occurrence of positive and negative pulses.
Another embodiment of the present method and apparatus encompasses a method. This embodiment of the method may comprise: providing at least one spraybar assembly in a printing unit, the spraybar assembly having a plurality of spray valves; outputting respective predetermined control signals of a plurality of control signals for the plurality of spray valves in the at least one spraybar assembly; and forming each control signal of the plurality of control signals with a series of positive and negative pulses that substantially minimizes residual magnetism in the spray valves by the occurrence of positive and negative pulses.
Features of the embodiments will become apparent from the description, the claims, and the accompanying drawings in which:
In the offset printing process, a small amount of a dampening solution, for example water with certain additives, is sprayed onto the offset plate by a plurality of solenoid valves.
While different printing operations may require fewer or more rollers than shown in
The spray dampening control system, generally designated 15, includes one or more solenoid-operated spray nozzle bars, one shown as reference character 16. In the printing operation, spray bar 16 may include, for example, four solenoid-operated spray nozzles 18a, 18b, 18c, 18d, spaced apart from each other and from the inking roller 12, such that the spray jet emitted from the nozzles uniformly covers the surface of such roller. For the sake of clarity the inking system for applying the oil base ink to the inking roller 12 is not shown. Herein the solenoid-operated spray nozzle is also referred to in general as a valve.
Each solenoid controlled spray nozzle may have an input plumbed to a liquid supply line 20. The electrically controlled nozzles 18a, 18b, 18c, 18d may be, for example, of a normally off type where the application of an electrical signal opens the nozzle valve to allow the pressurized liquid, damping fluid, to be sprayed for the duration of the signal. Ideally electrically controlled nozzles of this type, being either fully on or off, provide a constant angle of spray when activated and thus maintain a constant area of coverage even though the volume of liquid sprayed may be reduced. With this arrangement the volume of liquid sprayed is increased either with an increase in frequency, or by an increase in the duration of the electrical signal. A main liquid supply conduit 22 provides dampening fluid at a pressure, for example, of about 40-90 pounds to the supply lines 20 through respective shut off valves 24 and filters 26.
Typically, there is an array of 4, 6, 8, etc valves. The common lead going out to each of the valves may have its own unique return lead. The leads may be on printed circuit boards, specific wiring, or any other means of carrying electrical current and signals.
When the valves are unipolar the same magnetic field is generated. As described above, if the coil always generates the same magnetic field polarity for the pole pieces, over time each of the components will take on some degree of residual magnetism.
In one embodiment each control signal of the plurality of control signals may have a series of positive pulses interleaved with a series of negative pulses. Each negative pulse may be a mirror image of each positive pulse. In another embodiment each control signal of the plurality of control signals may have at least one positive pulse followed by at least one negative pulse. For example, two positive pulses may be followed by two negative pulses, etc., or two positive pulses may be followed by one negative pulse followed by one positive pulse, and followed by two negative pulses, etc. Thus, the control signal may be formed of groups of positive pulses that alternate with groups of negative pulses. The combination of positive and negative pulses combine to substantially cancel out the residual magnetism of an associated valve such that residual magnetism is substantially eliminated in the valves.
In the system depicted in
Operation of the valves in the present embodiments also exhibits the hysteresis loop.
The following are the stages of pulse N.
1. The device will begin at to at the origin of the B-H curve.
2. Phase A is applied to magnetically pre-charge the valve. The magnitude of phase A current is not enough to open the valve. However, it moves toward “a” on the B-H curve.
3. Beginning at t1 and during phase B, full current is applied, the valve opens, and moves to “a” on the B-H curve.
4. Beginning at t2 and during phase C, the valve is held open at lower current. The valve moves toward “b” on the B-H curve.
5. Beginning at t3 and during phase D, the intent is to close the valve. The current is reversed in order to dissipate the magnetic field created by phases A, B and C. Also, this will partially negate some of residual magnetism established by phases A, B and C. The valve moves past “b” on the B-H curve and stops somewhere between “b” and “c”. If phase D did not exist, the valve would stop at “b”. The full cycle ends at t4, in anticipation of the next cycle.
The following are the stages of Pulse N+1.
6. Phase E is equal but opposite to phase A. It is assumed to be greater than any residual magnetism. It moves the valve state past “c” and toward “d” on the B-H curve. It is not enough to open the valve.
7. Phase F opens the valve and moves it to “d” on the B-H curve.
8. Phase G moves the valve between “d” and “e” on the B-H curve.
9. Phase H leaves the valve between “e” and “f” on the B-H curve (absence of phase H would leave the valve at “e”).
For a next Pulse N+2 (not shown), a first phase will move the valve past “f”, and leave it somewhere between “f” and “a”. Each pulse will move the valve state across the H axis, negating residual magnetism with every pulse. If the precharge phases A and B are not present, residual magnetism would have to be overcome when trying to move the valve, which will generate error. The precharge in the alternating format negates the residual magnetism before the “motion” phase (B and F) is delivered.
Furthermore, the control signal may be a voltage waveform rather than a current waveform. In this embodiment a bipolar driver may be used as a current controller, which is programmable. This allows for having a peak current, a middle current, and a lesser current. The peak current opens, the middle current stabilizes, and the lesser current holds until time to release.
It is to be understood that various other waveshapes, some of which can be very complex, may be used according to the present method and apparatus. For example, the drive signal may be a voltage square wave, a current square wave, and a multi-step, multiphase voltage or current waveform.
The present method and apparatus may be incorporated in a many other applications than just those depicted in this disclosure.
The steps or operations described herein are just some examples of the present method and apparatus. There may be many variations to these steps or operations without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted, or modified.
Although embodiments of the present method and apparatus have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.
