Pattern Repeat Length Control System

Abstract

Digital inputs are fed to a programmed digital computer for a precise measurement and calculation of key line-speed parameters. The ratio of the web infeed speed measured at a point upstream of all processing actions relative to the embossing roll speed is the controlled variable. Digital speed recorders measure the web infeed speed and the embossing roll speed. The relationship between these two is automatically manipulated by the digital computer which reflect the process dynamics and describe the desired system response.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to a computer controlled pattern printing system and, more particularly, to a system wherein the repeat length of the pattern is accurately controlled.

2. Description of the Prior Art

Various systems are known at present for controlling repeat length. These systems characteristically monitor and control independently a number of different factors such as web tension, temperature, web speed, etc. On the basis of one or all of these factors, a control is effected which can be achieved to a maximum accuracy of about plus or minus 0.5 percent. Other systems are available which attempt to hold the sheet material or webs in a relatively stress-free state so that no stresses are developed within the sheet to cause variations in the repeat pattern.

With a number of the earlier designs placed on the semi-flexible webs which were used primarily as floor coverings, there was no great need to have exact precision in any repeat pattern. The patterns were such that when two sheets were placed side by side there was no need for registry between the patterns on adjacent sheets. However, at this time design requirements have now reached the stage that registry between adjacent sheets is necessary. Therefore, it is necessary that the repeat pattern length be accurately controlled so that when a 50 feet long web is cut in half and the two 25 feet pieces are placed side by side, the pattern along the adjacent edges will match for the whole 25 feet length of the sheet. Should there not be match, the aesthetic effect of the pattern is destroyed.

SUMMARY OF THE INVENTION

Control of the pattern repeat length is accomplished by controlling the speed of the embossing roller. The speed of the embossing roller is controlled based upon monitoring of two key features. There is monitoring of the web speed Xr.sub.0 upstream of the first processing roller at a position where the web is well behaved, that is, at the infeed end of the processing line. There is also the measuring of the embossing roll speed Xr. The ratio (Xr.sub.0 /Xr) of the upstream web speed to the embossing roll speed downstream is used to control the embossing roll speed. By the measuring of just the two above factors, a simplified system is provided. Digital means is used to measure the two factors to high precision (.apprxeq.0.02 percent) and to determine the ratio of them.

Pulse generators are connected to the embossing roll and a disc which engages the web upstream of the first process roller. Both embossing roll and web speed are developed by this means and fed to a digital computer. A control with a series of variables is used to read the ratio of web speed to embossing roll speed, to determine the ratio error, and then to control the ratio to maintain it at that point which provides proper uniform repeat pattern length.

BRIEF DESCRIPTION OF THE DRAWING

FIG. I is a schematic drawing of the processing apparatus; and

FIG. II is a simplified data-flow diagram for the computer control system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An existing line for the processing of a sheet flooring material was analyzed. The sheet forming material and its general mode of manufacture are set forth in U. S. Pat. No. 3,148,076. The smooth laminating roll of the patent has been replaced by an embossing roll as is conventional in the art. A mathematical model of the entire processing line was developed. The mathematical analysis involved the measuring of a number of different incremental speed factors along the web at positions between the various process stage. A series of process factors were incorporated within the mathematical analysis. On the basis of the different measurements and different factors, a mathematical expression was developed to define the manufacturing operation which will provide for controlled repeat length patterns. From the mathematical analysis and its application to the existing processing line, it was determined that one particular factor provided a control medium which was sensitive to all the other variable factors. The particular factor which could be used along to control the embossing roll feed is the relationship of the upstream web feed to the embossing roll speed downstream thereof. By the measuring of these two speeds and maintaining their proper ratio, the normal control system is considerably simplified in that it avoids the need to measure all of the many different factors which had been taken into account separately heretofore. However, digital means must be used to measure the two factors and to determine the ratio therebetween so that the embossing roll speed can be controlled to a very high degree of accuracy to achieve the uniform pattern repeat length.

Referring now to FIG. I, there is shown the overall processing system. A conventional infeed accumulator 2 is used to collect a substantial length of web material from a roll of web material. The infeed accumulator provides for a continuous operation. Material is rapidly fed off one roll of material into the accumulator. During the time that a second roll is being positioned and spliced to the first roll, the total processing apparatus will be pulling collected material from the accumulator. This will provide for constant flow of web material through the processing apparatus.

The web material 4 is fed from the accumulator by appropriate roll structures to a horizontal plane where the processing of the web material begins. A hopper 6 with a conveyer 8 deposits chips of plastic material onto the upper surface 10 of the web material. Vibrators 12 then vibrate the web to secure a uniform coating and distribution of the chips across the web material. The web material with the chips then passes to a heating station 14. The chips are heated to soften them such that they will adhere to the web and at the same time take the embossing which is going to be provided by the embossing roll 16. The embossing roll 16 is backed up by an appropriate back-up roll 18. From the point where the chips are placed on the web until the web and chips pass through the embossing roll 16, there is performed what is called the upstream processing 20.

After the upstream processing 20 is performed, the embossed sheet then passes to downstream processing such as a cooling area 22 where the sheet is permitted to cool by being passed over a series of rollers in a cooling atmosphere. The web than passes on down past point 24 through a series of further processing steps. These further processing steps can include the application of further material to fill in the embossed areas with a clear coat, the application of heat and further laminating pressures, etc., until such time as the sheet is returned to a cooled state which will permit the sheet to then be placed upon a roll in its finished state.

The invention herein is directed primarily to the provision of a pulse generator 26 which is positioned ahead of the upstream processing 20 to determine the line speed (Xr.sub.o of the web in its unprocessed state. Here the web is at ambient temperature and uniform tension and has been unaffected by any processing operation. The web speed is measured and fed into a digital ratio measurement and control system 28. At the same time, a second pulse generator 30 is connected to the embossing roll to provide readings of the surface speed (Xr) thereof. The data from element 30 is fed to the digital ratio measurement and control system 28 which operates to compute the ratio (Xr.sub.0 /Xr) and to then compare it with a set point to determine whether the embossing roll speed should be changed. If it is determined that the embossing roll speed should be varied, the digital control provides a control signal to the embossing roll differential 32 to vary its speed. The signal controlled differential may be a conventional differential such as the "Specon Draw Transmission " of Fairchild Hiller. A motor 34 drives the embossing roll 16 through the differential 32.

The digital computer, used to compute the ratio (Xr.sub.o /Xr ) and to use that ratio in comparison with various preset parameters, can be any type of conventional digital computer. Specific operations have been carried out by the use of the Hewlett Packard Computer No. HP-2114 B used in conjunction with the digital recorder HP-5050B (used to store preset parameters). The computer and recorder system has seven adjustable parameters. The following is a list of the parameters, their range and the typical value at which they were actually set during a normal control cycle.

Parameter Value __________________________________________________________________________ a Set Point (.1 to .9999) .9968 b Response Coeff. (.1 to 10) .80 c Dead Band Range (0 to .001) .0004 d Motor Volt. Coeff. (.1 to 1.0) .50 e Alarm Limit (.001 to 0.003) .0012 f Max Delta R (.001 to .003) .0010 g System Cycle Coeff. (.5 50) .5 __________________________________________________________________________

These parameters and their relationship to one another are indicated in FIG. II.

The set point a is the control point at which the ratio of upstream web speed Xr.sub.0 to embossing roll speed Xr should be held. The above values for parameter a correspond to those employed for the production of a product set forth in the above-described patent and processed in the manner of the above-described process. Controlled repeat length is secured when the ratio (Xr.sub.o /Xr) of upstream web speed to embossing roll speed equals .9968. The set point a is the desired ratio of the two speeds and the point at which you wish to establish control.

The response coefficient b is a gain control feature which is utilized to optimize the control system response. The response coefficient b is used in effect as an overall gain control for the system so that the system functions with a fairly even constant response.

The dead band range c represents the range of ratios deviating from the set point a in which no error correction is made. The purpose here is to eliminate the need for making very minor adjustments.

The motor voltage coefficient d is the voltage adjustment feature which is used to establish the speed of change of the control motor (part of differential) for the embossing roll 16. It is a setting of the magnitude of the correction which will be made to the control motor.

The alarm limit e is the range beyond which the deviations between the computed ratio and the set point will set off a visual and audible signal to direct the controller's attention to an excessive error. Even though the apparatus will compensate for errors, it is necessary that an operator have his attention directed to the fact that substantial errors are occurring.

The maximum delta R, parameter f, represents the maximum incremental step which can be taken during each cycle of operation to alter a ratio outside of the dead band back to a ratio within the dead band range. For a given error, there will be a certain desired change in the ratio. The limit of this change can be varied by varying parameter f, the maximum delta R.

Finally, the system cycle coefficient g sets up the cycle time for the system. During each cycle, the system will take the necessary measurements, compute the ratio, take into account the parameters a - f, and provide a correction if necessary before it will then repeat its cycle. The system provides an inverse relationship between the system cycle time g and the line speed Xr.sub.o to provide a calculation period corresponding to uniform web length.

FIG. II sets forth the operational data-flow diagram of the ratio measurement and control unit 28 for controlling the repeat length and indicates the relationship between the various parameters. The two digital devices 26 and 30, FIG. I, are basically tachometers including pulse generators, providing pulse signals indicative of the speed of Xr.sub.0 and Xr. The computer computes the ratio Xr.sub.o /Xr of the two speeds. The cycle time g for the system determines the time period for ratio measurement and error correction. A typical system cycle time is 10 seconds for a fixed line speed at normal operating levels. The measured ratio Xr.sub.o /Xr is compared with the set point reading a. If the two values are the same, the cycle is aborted and a new cycle comparison is run within the time set by the system cycle time. If there is a difference between the set point a and the measured ratio Xr.sub.o/X r, it is compared with the dead band range c. If the difference is within the dead band range, no corrections will be made, and the cycle will abort. If the difference is outside the dead band range c, then an error correction will be made. A large difference (determined by parameter ewill operate the alarm circuit. The amount of correction is calculated based upon the deviation from the set point. The maximum delta R, parameter f, limits the error correction made during each cycle to some maximum value. Operating conditions may be such that a false large error signal could be received. Rather than to permit a large speed change to be made in a single cycle, maximum delta R holds the correction signal to a reasonable limit. If the error had been real and there had been a substantial error in the system, the next recycle would pick the error up again, and a constant recycling would gradually bring the system back into control. In the above case, the control step sizes taken to gain control would be based upon the limit determined maximum delta R. The control signal determines the running time of the correction motor and is dependent upon the motor voltage coefficient setting d. The correction is made and the cycle terminates with a new cycle starting based upon the system cycle coefficient g. A continuing check and correction of the system is provided with the above controls. A digital system is particularly useful because of its high speed of response and its precision control capability.

Claims

What is claimed is:

1. In a method for controlling the repeat pattern length on embossed materials, involving the steps of providing upstream processing to the web before the embossing step, embossing the web material, and providing downstream processing to the web of material, the improvement comprising: sensing the line speed of the web prior to the upstream processing of the web, sensing the embossing roll speed during the embossing operation, and maintaining the ratio of web speed to embossing roll speed within a preset range to provide a product at the end of the downstream processing, which product has the repeat patterns of the embossing in a controlled repeat length.

2. In the method of claim 1, the further step of comparing the sensed speed ratio with a fixed reference ratio and automatically providing control signals to an embossing roll drive to keep the ratio within the preset range.

3. In the method of claim 2, the further step of controlling the size of the control signal within preset limits.