JP2972511B2 - Method for measuring thickness of foamed polyethylene sheet by laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the thickness of a foamed polyethylene sheet by means of a laser in which the thickness is measured with high accuracy without causing damage to an object to be measured without contact with the laser. .

[0002]

2. Description of the Related Art Conventionally, foamed polyethylene sheets such as packing of electric appliances and precision equipment, lining of automobiles, dew-prevention sheets for buildings, bath mats and the like are extruded by an inflation method, a T-die method, etc., and are several meters / second. It is transported at a high speed and taken up by a take-up roll.However, uneven thickness occurs in the thickness direction due to various factors during extrusion molding. The measurement is carried out by sandwiching the foamed polyethylene sheet on either side in the width direction.However, in such a measuring means, the error becomes extremely large due to the force applied when the clamping gauge is clamped, or It is possible to measure both sides of the polyethylene sheet, but it is not possible to measure near the center in the width direction. It had a disadvantage that damage the Nshito itself.

[0003]

The present invention utilizes the fact that the intensity of transmitted light attenuates in proportion to the thickness of a foamed polyethylene sheet, automatically measures the thickness in a non-contact manner, and saves labor. In addition, the measurement accuracy is improved by instantaneously measuring the attenuation many times, and the measured change is fed back to the extruder, which enables the automation of thickness adjustment by a foamed polyethylene sheet by laser. It is intended to provide both a method for measuring the thickness of a thin film and its adjustment.

[0004]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of measurement errors and damages based on the prior art, the present invention applies a laser beam from a fixed laser oscillator to a foamed polyethylene sheet conveyed at a high speed after extrusion molding. When the transmitted light is condensed through a lens and the transmitted light intensity of the transmitted light is detected by a photodetector, the detected portion is periodically shifted at a predetermined sampling cycle so as to sequentially displace the detection portion in the transport direction. By measuring a number of times, and averaging this data and comparing and calculating the incident light intensity of the laser light, the thickness of the expanded polyethylene sheet is measured by measuring the thickness of the expanded polyethylene sheet. It is an object of the present invention to provide a method of measuring a thickness to eliminate the above-mentioned drawbacks.

[0005]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings . As shown in FIGS. 3 and 4, reference numeral 1 denotes a thickness measuring device for a foamed polyethylene sheet W according to the present invention. An apparatus 1 is an extruder 2 for continuously extruding a foamed polyethylene sheet W by an inflation method, a T-die method, or the like.
And a take-up roll 3 that winds the foamed polyethylene sheet W into a roll, and provided during a transport process, and is provided below the laser oscillator 4 provided above the foamed polyethylene sheet W and below. It comprises a light receiving section 5.

[0006] The foamed polyethylene sheet W having a width of several meters and a thickness d of 1 to 10 mm is wound at a transport speed of several meters per second.

As the laser oscillator 4, a semiconductor laser having a relatively low output so as not to damage the foamed polyethylene sheet W and having a laser beam diameter of about 1 to 2 mm is used.

Further, a plurality of laser oscillators 4 and light-receiving portions 5 arranged in an upper and lower pair are arranged in the width direction of the foamed polyethylene sheet W, and it is impossible to measure the deviation of the thickness d in the width direction of the foamed polyethylene sheet W. ing.

Further, as shown in FIG. 1, the light receiving section 5 receives the laser light from the laser
When passing through, the original directivity is lost because it is complicatedly scattered by the internal complicated small cell structure, and when passing through the foamed polyethylene sheet W, it is scattered in the same manner as light from a point light source, For this purpose, a lens 7 for collecting the scattered laser light on the light receiving surface of the photodetector 6 such as a CCD sensor or a photodiode is provided.

The data of the transmitted light intensity Io from the photodetector 6 is input to a computer (not shown) for performing a sequential calculation process and the like, and the incident light intensity Ii of the laser light from the laser oscillator 4 and a predetermined value. Transmitted light intensity I transmitted periodically through foamed polyethylene sheet W at sampling period T
The average value obtained by averaging o many times is compared to obtain a thickness d, and a die driving unit and a nozzle (not shown) for adjusting the thickness in the extruder 2 are obtained based on the obtained change in the thickness d. ) Is given a predetermined input signal.

Further , as shown in FIG. 5, reference numeral 8 denotes a guide mechanism for restricting vertical movement of the foamed polyethylene sheet W in the thickness direction during the conveying process. The guide rollers 9 and 9a made of a transparent acrylic material having no laser light attenuation are disposed immediately below and at an interval approximately 5 mm wider than the regular thickness of the foamed polyethylene sheet W.

Another embodiment of the guide mechanism 8 is as follows.
As shown in FIG. 6, a suction guide 10 for sucking the foamed polyethylene sheet W and restricting the vertical displacement is provided other than in the optical path between the laser oscillator 4 and the light receiving section 5, or vice versa. It is sufficient if it blows out and prevents lifting by its force.

Next, a method for measuring the thickness of the foamed polyethylene sheet by the laser according to the present invention will be described. The foamed polyethylene sheet W formed by the extruder 2 is conveyed at a speed of several meters / second and taken up by the winding roll 3. When a laser beam having a predetermined beam diameter and a constant incident light intensity Ii is irradiated from the laser oscillator 4 to the foamed polyethylene sheet W being wound and conveyed, the foamed polyethylene sheet W The light passes through the foamed polyethylene sheet W while being scattered by the complicated small cell structure in the W (see FIG. 2) .

Next, the transmitted light transmitted through the foamed polyethylene sheet W is condensed by a lens 7 disposed below the foamed polyethylene sheet W, and is incident on a light receiving surface of a photodetector 6. The data converted into the electric signal by 6 is measured as the transmitted light intensity Io.

In order to measure the transmitted light intensity Io many times by shifting the position in the transport direction of the foamed polyethylene sheet W, the transmitted light intensity Io is measured many times at a predetermined sampling period T. An average value obtained by averaging such data by a computer is compared with data of the incident light intensity Ii to obtain a thickness d.

If the beam diameter of the laser beam is R and the conveying speed of the foamed polyethylene sheet W is V, the sampling period T needs to satisfy the condition of T ≧ R / V because the measurement areas do not overlap. T ≒ (1 to 1)
5) It is preferable to set to (R / V).

For example, if R = 2 mm and V = 1 m / sec, T ≧ 2 msec, and the minimum measurement which is the length in the transport direction of the foamed polyethylene sheet W when the average of 40 measurements is measured as one measurement value. The area is 80 mm and the minimum time required for it is 80 ms.

By detecting the transmitted light intensity Io at a plurality of locations in the width direction of the foamed polyethylene sheet W, the deviation of the thickness d in the width direction can be measured.

The transmitted light intensity Io changes due to the fluctuation of the incident light intensity Ii and causes a measurement error. Therefore, the transmitted light intensity Io is normalized by the incident light intensity Ii so as to eliminate such an influence, that is, Io ₁ =
Defined by Io / Ii.

However, if the incident light intensity Ii is always constant, there is no need to normalize at all.

However, even if standardized as described above, since the inside of the foamed polyethylene sheet W is formed by a complicated small cell structure, the laser light is attenuated due to complicated scattering there, so that In the measurement,
As shown in FIG. 7, the transmitted light intensity Io ₁ standardized for the same thickness has a very large fluctuation range at all at random, so it is necessary to perform averaging a number of times to remove the influence. is there.

FIG. 7 shows a foamed polyethylene sheet W
The two types of thickness d, the mark ● is d1 = 2.76 mm, and the mark × is d2
= The transmitted light standard intensity distribution diagram based on the experimental data at each arbitrary measurement point for 5.58Mm, the vertical axis represents the transmitted light standard intensity Io _1, the horizontal axis is the number indicating each arbitrary measurement point position Pi (i = 1, 2, 3,... N), but the actual measurement is performed at the position of 200 points. Here, 40 measurement points arbitrarily extracted for each thickness are shown. ing.

[0023] The horizontal solid line d1 = average value of the transmitted light standard intensity Io ₁ normalized with respect to 2.76mm in FIG Av1 = 0.0
325, the horizontal dotted line indicates an average value Av2 = 0.0030 of the transmitted light standard intensity Io ₁ normalized with respect to d2 = 5.58mm.

Then, the sum of the squares of the differences obtained by subtracting the average value from the measured values of the transmitted light standard intensity Io _{1 at} the above-mentioned 200 individual measurement points is averaged, and the average is further squared. The deviation S is S1 for the thicknesses d1 and d2, respectively.
= 0.0092 and S2 = 0.0006.

[0025] Further, the average value Av1, Av2 average variation rate epsilon = S / Av1 or Av2 of the transmitted light standard intensity Io ₁ normalized by the transmitted light standard intensity Io ₁ The sample standard deviation S, the thickness d
Ε1 = 0.28 and ε2 = 0.20 for 1 and d2, respectively, which are considerably large values especially when the thickness d is small. Therefore, averaging is indispensable for accurate measurement of the thickness. It has been found.

Next, the thickness d of the expanded polyethylene sheet W
There processing of obtaining the average value of the transmitted light standard intensity Io ₁ of 40 points in each arbitrary measurement point for six foamed polyethylene sheet W up to 1 to 8 mm, becomes substantially linear as shown in FIG. 8, which foam It was found that the attenuation of the laser light in the polyethylene sheet W followed Lambert's law.

As a result, according to Lambert's law, if the incident light intensity is Ii, the transmitted light intensity is Io, and the transmittance at the surface is T, Io = TIi EXP (-αd) or LOG (Io / Ii) = (- αLOG e) d + LOG T, and the absorbance defined by LOG (Io / Ii) is proportional to the thickness d of the foamed polyethylene sheet W.

Here, (-αLOG e) is a constant determined by the cell structure inside the foamed polyethylene sheet W, and is found to be -0.38 mm ^{-1 when} determined by the method of least squares.

[0029] Also, the percentage change of the transmitted light standard intensity Io ₁ by the number of the averaging, as shown in FIG. 9, the two thickness d of the foamed polyethylene sheet W, ● mark d1 = 2.7
6mm, x mark is within about 7% for 10 times for d2 = 5.58mm, about 5% for 20 times, about 4% for 40 times, and about 1% for 100 times It became.

Here, FIG. 9 shows that the thickness d1 = 2.76 mm, d
The relationship between the variation ratio of the average value Av of the standardized transmitted light intensity Io ₁ and the number N of sample points for the foamed polyethylene sheet W of 2 = 5.58 mm and the number N of sample points are shown, and the vertical axis indicates the transmitted light standard intensity Io _1.
N samples are arbitrarily taken out of all the samples (N = 200) of the measured values, and the average value Av _{N = n} and the average value A of all the samples
The absolute value of the difference from v _{N = 200} is the variation ratio of the transmitted light intensity normalized by the average value Av _{N = 200} of all the samples close to the true value | Av _{N = n} −Av _{N = 200} | / Av _{N = 200} , and the horizontal axis shows a value of 1 / N ^1/2 .

These relations are substantially linear as shown in the drawing, that is, the variation ratio of the transmitted light is proportional to 1 / N ^1/2 , and is therefore a random error according to a Gaussian distribution. Although the example is shown for two types of samples having different values of d, the regression lines obtained by the least squares method are indistinguishable from each other on the figure.

Next, the relationship between the small transmitted light intensity ΔIo and the thickness Δd accompanying it is ΔIo / Io ≒ −α × Δd, considering the first-order approximation from the above-mentioned Lambert's law. D1 = 2.76mm, d2 = 5.58m
m, the thickness variation ratio Δd / d of the expanded polyethylene sheet W.
, The following results were obtained.

[0033]

Further, since the foamed polyethylene sheet W is conveyed at a high speed, when it moves up and down in the optical axis direction of the laser beam,
It is conceivable that a change in the amount of light incident on the lens 7, that is, the amount of light incident on the photodetector 6, affects the thickness variation ratio Δd / d, which is a measurement error in thickness measurement. The measurement time required for obtaining one average value is T ≧ 2 if R = 2 mm and V = 1 m / sec.
m seconds, and the measurement was performed in a very short time, such as 80 msec for an average of 40 times and 400 msec for an average of 200 times, and the vertical vibration of the foamed polyethylene sheet W was generally about several seconds. During the processing, the measurement can be performed in a state where the foamed polyethylene sheet W is fixed at one of the upper and lower positions at a fixed position, and thus the measurement error is not affected at all.

As for the fluctuation of the light quantity when the foamed polyethylene sheet W moves up and down in the optical axis direction of the laser beam, as shown in FIG. the distance to 7 a, if the diameter of the lens 7 and D, Ω = (D / a ) 2/16 next, when from the transmitted light which is a point light source, the lens 7 from a point source of Z = 0 Let E (0) be the light amount and E (Δa) be the light amount from Z = Δa: E (Δa) / E (0) = (1−Δa / a) ⁻² ≒ (1 + 2)
Δa / a) or [E (Δa) −E (0)] / E (0) = 2Δa /
When the light source approaches the lens 7 by Δa, the light amount approximately increases by 2Δa / a times the original value, and decreases by the same amount when the light source moves away from the lens 7.

However, in this embodiment, the vertical movement of the foamed polyethylene sheet W is restricted by increasing the distance between the guide rollers 9 and 9a in the guide mechanism 8 by about 5 mm from the regular thickness of the foamed polyethylene sheet W. variation can be suppressed to about 5mm at the maximum value, in order to approximately 3% and equivalent range of fluctuation average of 40 times a fluctuation ratio of the transmitted light standard intensity Io ₁ for this, a from the above formula 1
It may be set to 50 mm.

Thus, the error in the thickness measurement is the sum of the variation ratio of the transmitted light standard intensity Io _{1 and} the variation ratio of the light amount when the foamed polyethylene sheet W moves up and down in the optical axis direction of the laser beam. The maximum error is (0.03 + 0.0.
03) = 0.06.

If the other structure of the guide mechanism 8 for regulating the vertical movement of the foamed polyethylene sheet W, the suction guide 10, the compressed air Ar, and the like can reduce the variation of Δa, the measurement is further performed. The accuracy is improved.

[0039]

In short, the present invention relates to a foamed polyethylene sheet W formed by an extruder 2 and scattering transmitted light .
In the thickness measuring method, a laser beam from a laser oscillator 4 is irradiated on a foamed polyethylene sheet W being conveyed, and transmitted light transmitted through the foamed polyethylene sheet W is condensed by a lens 7 so as to form a photodetector 6. And the transmitted light intensity Io detected by the photodetector 6 is periodically measured a number of times, and the average value of the data is calculated. The average value and the incident light intensity Ii of the laser beam are calculated. The thickness d of the foamed polyethylene sheet W is measured by comparing with the above. Therefore, in the measurement of a single position, averaging should be performed irrespective of a very large measurement error due to the structure of the foamed polyethylene sheet W. Absorbance LOG (Io /
The Ii), Ru can be made proportional to the thickness d. or,
The sheet to be measured scatters transmitted light.
Scattered light and disturbance due to laser light from a point light source
Laser can be measured without the influence of light, and laser
-Condensed through the lens 7 for the scattering state that hinders the directivity of light, so that the transmitted light intensity Io scattered and transmitted by the foamed polyethylene sheet W is all collected on the photodetector 6. Therefore, the measurement accuracy of the thickness d obtained from the transmitted light intensity Io and the incident light intensity Ii is about 6%.
It can be measured with high accuracy within.

Therefore, the thickness change of the foamed polyethylene sheet W can be measured in a non-contact and continuous manner with high accuracy, so that not only the thickness measurement itself can be saved, but also the thickness measurement can be performed. By processing the electrical signal indicating the change and feeding it back to the extruder 2, it can be automated that the thickness adjustment is manually performed.

Since the transmitted light intensity Io is detected at a plurality of locations in the width direction of the foamed polyethylene sheet W, in addition to the above-described effects, it is possible to measure the central portion of the foamed polyethylene sheet W in the width direction. The practical effect is enormous.

[Brief description of the drawings]

FIG. 1 is a schematic view of a thickness measuring device for a method for measuring the thickness of a foamed polyethylene sheet according to the present invention.

FIG. 2 is a schematic view of the internal structure of a foamed polyethylene sheet.

FIG. 3 is a schematic perspective view showing an arrangement position of the same thickness measuring device.

FIG. 4 is a schematic view of an apparatus for producing a foamed polyethylene sheet.

FIG. 5 is a front view of a guide mechanism in the thickness measuring device.

FIG. 6 is a front view showing another embodiment of the present invention.

FIG. 7 is a distribution diagram of transmitted light intensity at each measurement point.

FIG. 8 is a graph showing the relationship between the transmitted light standard intensity averaged 40 times and the thickness of the foamed polyethylene sheet.

FIG. 9 is a graph showing the relationship between the variation rate of the average value of the transmitted light standard intensity and the number of sample points.

FIG. 10 is a conceptual diagram of a light quantity fluctuation with respect to a vertical displacement of a foamed polyethylene sheet.

[Explanation of symbols]

 2 Extruder 4 Laser oscillator 6 Photodetector 7 Lens

Claims (2)

(57) [Claims] Claims: 1. An extruder , which is formed by an extruder to scatter transmitted light.
A method for measuring the thickness of a foamed polyethylene sheet to be
Irradiate the laser beam from the laser oscillator to the foamed polyethylene sheet being conveyed, the transmitted light passing through the foamed polyethylene sheet is condensed by a lens and input to the photodetector,
The transmitted light intensity detected by the photodetector is periodically measured a number of times, the average value of the data is calculated, and the average value is compared with the incident light intensity of the laser light to form a bubble. A method for measuring the thickness of a foamed polyethylene sheet using a laser, comprising measuring the thickness of the polyethylene sheet. 2. The method for measuring the thickness of a foamed polyethylene sheet according to claim 1, wherein the transmitted light intensity is detected at a plurality of locations in the width direction of the foamed polyethylene sheet.