JPS6145312A - Control method for thickness of plastic sheet or the like - Google Patents

Abstract

PURPOSE:To attain an optimum control action and to improve the offset accuracy by stopping regularly the working of a scan type thickness gauge at the prescribed positions for a fixed period of time to perform measurement of thickness of a plastic sheet, etc. and setting a blind sector of a proportional integrating action input to the thickness deviation. CONSTITUTION:A plastic sheet 14 is extruded through a T-die 12 of an extrusion molding machine 10 and molded in the prescribed thickness by a molding roll 16. A scan type thickness gauge 22 consisting of a pair of a signal source 18 and a detector 20 set on a conveying path of the sheet 14, and a scanning motor 24 is controlled by external controllers 26-34 consisting of a speed setting unit 26, etc. In this case, the working of the gauge 22 is stopped periodically at the prescribed positions for a fixed period of time by a timer 50 incorporated to a computer system 42. Then the sampling mode is switched to an observation mode of a fixed position for execution of continuous measuring actions. Thus a blind sector optimum to the present conditions is set. If the thickness variance of the sheet 14 is recorded, the lip distance of the die 12 is controlled to control the sheet thickness is its width direction.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to a control method for adjusting the average thickness in the delivery direction of a plastic sheet or film (hereinafter simply referred to as a sheet) formed by an extrusion molding machine.

[Prior art and its problems]

Generally, the average thickness control method for this type of sheet is based on the sheet feeding direction (Machine Direction).
Thickness control (referred to as MD control) for the width of the sheet (on) and thickness control (referred to as CD profile control) in the cross direction of the sheet are known. The MD control is performed simultaneously with the CD profile control using a scanning thickness gauge.

Furthermore, this type of sheet forming process has the following characteristics.

+1) The dead time (detection delay) is very large compared to the time constant.

(2) Loft changes (replacement of molds, new filling of molding material, etc.) are often performed, and operating conditions are inconsistent.

(3) Relatively gradual thickness fluctuations occur due to filter clogging at the extruder outlet, material properties, changes in environmental conditions, etc.

(/1) If we strictly examine the change in yL over time, we will find that it is quite complex, with short (medium) period, long period components, and 1
It is shown as a composite form of the error components of the thickness gauge and thickness gauge (see Figure 5).

(5) Short (medium) jhjJ in thickness change over time
The surging force of the extruder is mainly caused by the discharge fluctuation (surging force), but some also occur in the cooling and stretching processes in the downstream region, and their behavior causes subtle changes depending on the screw or line speed.

(6) A relatively unstable non-twin phenomenon exists between the extruder mold (generally T-die) outlet and the cooling roll, causing meandering etc. in the extrusion molding seam 1-(S). (See Figure 6).
This Ne Tsukuin too! , varies depending on material properties, molding temperature, air gap, sheet take-up speed, etc.

However, MD control mainly controls the above item (3), and is performed by feedback of the screw speed of the extruder or the sheet forming line speed. In addition, since MD control has the characteristics of the above item <1), sampling PI (proportional integral) control is generally adopted for control operation (see Figure 7), but when it becomes more advanced, Smith's dead time compensation law is added (see Figure 8). The circuit shown in FIG. 8 can also be expressed as shown in FIG. 9 by equivalent conversion. Note that in the circuit configurations shown in FIGS. 7 to 9, the contents of the simplified symbols are as follows.

Sv: Set value P■: Proportional integral operation M■: Manipulated amount
P■: Measured quantity of the controlled object G(S): Transfer function S Nira plus operator excluding dead time L: Dead time Also, as the speed, that is, dead time changes with the above control, the line speed is measured. Automatic correction of deviations in the transfer function is also performed (referred to as automatic retune).

The control method using the above-mentioned "sampling PI, dead time compensation, and automatic chain" (see Figures 8 and 9) is quite advanced, and the sheet thickness offset (steady deviation)
However, item (2) above is very good in terms of minimizing the amount of dirt.

(4) or (6), etc.), it lacks stability and tends to cause minute cycling in synchronization with the sampling period (see Figures 10 and 4).

Moreover, the spectrum obtained by frequency analysis of a general thickness measurement signal is shown in Fig. 1). In Figure 1), the peaks of the spectrum each indicate the amplitude of components generated in the extruder, the neck-in section, or the cooling/stretching process.

Components in the high frequency range that are not subject to MD control are considerably removed by a low-pass filter (time constant adjustment circuit) of the thickness gauge or by the thickness averaging process, but cannot be completely processed. Not only can these residual deviations not be controlled, but if the period of a specific component approaches the sampling period, the 10th
This may also result in increased fluctuations, as shown by the dashed line in the figure. Therefore, the above-mentioned PI control and dead time compensation method are not necessarily effective means for solving such problems.

[Purpose of the invention]

An object of the present invention is to provide a method for controlling the thickness of a plastic sheet, etc., which can mold a product with the most stable thickness in a thickness control system in the sheet feeding direction of a plastic sheet, etc. using an extrusion molding machine. .

(Summary of the invention) The method for controlling the thickness of a plastic sheet, etc. according to the present invention includes:
A scanning thickness gauge is used to measure the profile of a plastic sheet, etc. to be extruded by moving a measuring head equipped with a signal source and a detector back and forth in the width direction of the sheet, etc., which is continuously transported. In the method for controlling the thickness of the sheet, etc. in the feeding direction of the sheet, the scanning thickness gauge is stopped at a predetermined position for a certain period of time, and the thickness of the sheet, etc. is measured by fixed point observation, and the thickness measured in advance is measured. A feature of the present invention is that a dead zone for PI operation input is set according to the σ deviation width.

In the thickness control method described above, the dead zone of the PI operation input can be calculated by multiplying the maximum amplitude value accompanying jI height fluctuation within a predetermined measurement time by a correction coefficient.

Further, the dead zone of the PI operation input can also be calculated by performing frequency analysis due to thickness variation using fast Fourier transform or the like, calculating the maximum amplitude value above a specific frequency, and multiplying this by a correction coefficient.

Furthermore, in the present invention, the PI operation input in the dead zone can be set to have a predetermined proportional relationship to the thickness deviation.

[Embodiments of the invention]

Next, an embodiment of the method for controlling the thickness of a plastic sheet or the like according to the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a block diagram of a control system that implements the control method of the present invention. That is, in FIG.
indicates an extrusion molding machine, and the T installed on this extrusion molding machine
A predetermined plastic sheet 14 is extruded from the die 12 and molded to a predetermined thickness by a forming roll 16. A part of the conveyance path of the plastic sheet 14 formed in this way is equipped with a transmissive scanning type measuring head, for example, comprising a pair of signal sources 18 and a detector 20 that reciprocate in the width direction of the plastic sheet 14. A total of 22 is provided. In addition, this thickness gauge 22.

A scanning motor 24 is provided to reciprocate and scan the measurement head in the width direction of the sheet 14, and this scanning motor 24 is controlled by an external controller to be described later.

However, the external controller that controls the scanning motor 24 includes a speed setter 26, a motor controller 28, a position setter 30, and a converter that inputs the measurement head position signal Xs obtained from the thickness gauge 22. It is composed of a comparator 32 and a comparator 34.

Therefore, in the comparator 34, the feed bank position signal Xs of the measurement head 1 obtained via the converter 32 is
and a position signal for setting the turning point of the measuring head set by the position setting device 30, and the signal obtained by the comparison calculator 34 and the speed setting signal from the speed setting device 26 are sent to the motor controller. 28 and predetermined motor control signal C.
s to the scanning motor 24.

Incidentally, the thickness signal t3 of the sheet 14 is taken out from the thickness gauge 22 together with the measurement gutter position signal Xs. This thickness signal t5, conversion amplifier 36, time constant 1)i
The position data and the thickness data are stored and held in correspondence with each other in the computer system 42 via the I-regulator 38 and the A/D converter 40. The computer system 42 shown in FIG. 1 is composed of a CPU 44, an internal storage device 46, peripheral devices 48, and a timer 50.

Further, the computer system 42 includes the extrusion molding machine 1
0 for the extrusion screw drive morph 52 and the forming roll drive morph 54, respectively, with a motor controller 56.
One-loop direct control system (Dir
ect DigiLal Control System
) to form.

In the control system configured in this way, the thickness gauge 2
2 is filtering of short-period fluctuation components of the thickness gauge itself (especially when using β rays), filtering of high-frequency components that enter from the outside, and unevenness in the thickness of the sheet 14 (fine irregularities that are meaningless for die adjustment). In order to achieve smoothing of the components), the detector 20 is provided with a response delay (represented by a time constant Tc).

In this case, the measurement profile will cause distortion and phase shift between the true value and the previous measurement value and the return measurement value, but for this type of phase shift, the average value of the return measurement data should be used. It is solved by

In addition, the computer system 4
2, when thickness unevenness in the width direction of the sheet 14 is recorded,
As a means for adjusting this, for example, the lip gap m of the T-die 12 is adjusted using a slide lip or a choke bar.

That is, the pattern of the flow path gap formed by the slide lip or choke bar and the fixing member (fixed lip) opposing it is manually or 84] by automatically pushing and pulling to adjust the thickness of the sheet 14 formed by the T-die 12 in the width direction.

However, the present invention was made based on the knowledge that the average thickness of the sheet 14 is most stable under specific ranges and conditions, and the above-mentioned The operation of the scanning thickness gauge 22 is stopped at a predetermined position for a certain period of time periodically or according to a predetermined rule by a timer 50 built into the computer system 42, and the scanning thickness gauge 22 is continuously operated by switching from one point to a fixed point observation using a zamp printer. This method is characterized by taking measurements and setting the optimal dead zone for the current situation based on these measurement data.

That is, in the present invention, a dead zone setting operation is added to the control operation described above, and this dead zone is automatically corrected according to the state of the molding process, thereby optimizing the control operation to always produce the best sheet. 14 can be molded.

Therefore, although there is no specific method for calculating the dead zone, if the dead zone is made too wide, an offset will occur, so it is desirable to use the minimum allowable value according to the disturbance.
For example, in general, the maximum amplitude value within the measurement time is multiplied by a correction coefficient. Alternatively, strict frequency analysis is performed using F, FT (fast Fourier transform), etc. to find the maximum amplitude value above a specific frequency, and this is multiplied by a correction coefficient. method etc. are suitably adopted. That is, this dead zone can be easily calculated and set in the computer system 42 based on internally recorded data.

The control system for the P-th control based on the dead zone setting described above is shown in FIG. In addition, in Fig. 2, e
is the deviation of the thickness of the sheet 14 measured by the thickness gauge 22 from the set value, e' is the PI operation input, PB is the proportional band, and T!
indicate the integration time, respectively. In the embodiment shown in FIG. 2, P! The level of the motion input e' is set to zero. However, as shown in FIG. 3, the level of the PI operation input e' in the dead zone is
It is also possible to have a fixed proportional relationship with respect to

〔Effect of the invention〕

As is clear from the embodiments described above, according to the present invention,
The operation of the scanning thickness gauge is periodically stopped at a predetermined position for a certain period of time, and measurement is performed by fixed-point observation, and the maximum amplitude value within a certain measurement time is multiplied by a correction coefficient to calculate P for the thickness deviation.
By setting the dead zone of the motion input, unnecessary PI motion input caused by disturbance etc. is removed, thereby optimizing the control motion of the extrusion molding machine and achieving thickness control without reducing offset accuracy and with extremely little variation. It is possible to produce stable plastic sheets.

Although the preferred embodiments of the present invention have been described above, it goes without saying that various design changes can be made without departing from the spirit of the present invention.

[Brief explanation of drawings]

Fig. 1 is a system configuration diagram showing an embodiment of a control system that implements the method for controlling the thickness of plastic sheets, etc. according to the present invention, and Fig. 2 shows the basic configuration of the control system that is the main part of the control method of the present invention. FIG. 3 is an operating characteristic diagram showing an example of the operating state of the control system of the control method of the present invention, and FIG.
The figure is a control characteristic diagram achieved by the control method of the present invention, FIG. 5 is a thickness characteristic diagram by a conventional thickness control method, and FIG. Figures 7 to 9 are block circuit diagrams showing control systems for implementing the conventional thickness control method, Figure 10 is a control characteristic diagram for the conventional thickness control method, and Figure 1) is a diagram for the conventional thickness control method. FIG. 3 is a frequency distribution characteristic diagram in the control method. 100. Extrusion molding fi 12. , T-tie 140
．． Plastic sheet 160. Forming roll 188. Signal source 200. Detector 220. thickness gauge
246. Scanning motor 260. speed setting device
280. Motor controller 3010 position setting device 3
20. Converter 340. Himame operator 366. Conversion amplifier 3800 time constant regulator 40. , A/D converter 426. Computer system 44. , C.P.I.J.
460. Internal storage device 4809 peripheral equipment +jj5
0,, timer 520. Screw drive morph 54.
8 Forming roll drive motor. 56.58. , motor controller patent applicant Toshiba Machine Co., Ltd. FIG, 5 FIG, 8 FIG, 6 FIG, 9

Claims (4)

[Claims] (1) Scanning type thickness that measures the profile of a plastic sheet, etc. to be extruded by moving a measurement head equipped with a signal source and a detector back and forth in the width direction of the sheet, etc., which is continuously transported. In the method of controlling the thickness of the sheet, etc. in the feeding direction using a thickness gauge, the scanning thickness gauge is stopped at a predetermined position for a certain period of time, and the thickness of the sheet, etc. is measured by fixed point observation, and the thickness of the sheet, etc. is measured in advance. A method for controlling the thickness of a plastic sheet, etc., characterized in that a dead zone of a PI operation input is set according to a deviation width of the thickness. (2) In the thickness control method according to claim 1, the dead zone of the PI operation input is calculated by multiplying the maximum amplitude value accompanying thickness fluctuation within a predetermined measurement time by a correction coefficient. etc. thickness control method. (3) In the thickness control method described in claim 1, the dead zone of the PI operation input is determined by performing frequency analysis accompanying thickness fluctuations using fast Fourier transform, etc., and calculating the maximum amplitude value above a specific frequency. A method for controlling the thickness of plastic sheets, etc., which consists of calculating by multiplying by a correction coefficient. (4) In the thickness control method according to any one of claims 1 to 3, the PI operation input in the dead zone is set to have a predetermined proportional relationship with respect to the thickness deviation. Method for controlling the thickness of plastic sheets, etc.