Fundamentals and Practices of Sensing Technologies
Keiji Taniguchi, Hon. Professor of
Dr. Masahiro Ueda, Honorary Professor, Faculty of Education and Regional Studies
Dr. Ningfeng Zeng, an Engineer of Sysmex Corporation
(A Global Medical
Dr. Kazuhiko Ishikawa, Assistant Professor
Faculty of Education and
[Editor’s Note: This paper is presented as Part VIII of a series from the new book “Fundamentals and Practices of Sensing Technologies”; subsequent chapters will be featured in upcoming issues of this Journal.]
Abstract for Chap. 4
Many measurement technologies by means of the light have been developed and used practically for many manufacturing industries. The principles of these technologies are very simple usually, which are based basically on a light attenuation due to absorption in the material and a light reflection due to scattering on the surface. Another principle is based on an interference of the laser light, which is successfully applied for a thickness measurement of the thin film and coating.
The practical application technologies using light attenuation were described in chapter 4, and those using light reflection were described in chapter 5. These technologies were all the results of our joint research works with many corporations.
The measuring technologies using light have, generally, many advantages as follows.
(1) The real-time measurement and monitoring can be realized because of the extremely high propagation velocity (m/s).
(2) The nondestructive measurement and detection can be realized since many light receiving elements with very high sensitivity are now in the market.
(3) The point measurement can be realized because the light can easily be focused into the very small area.
(4) The measurement without electro-magnetic noise can be realized.
(5) The measurement at a far distance, i.e., the remote sensing technology, can be realized, which is used successfully in many industrial plants such as a nuclear power plant, for an example.
(6) Optical path can easily be controlled by means of an optical fiber.
These advantages results in the followings in the practical plants.
(i) The measurements, which were hitherto done manually, can be replaced by these technologies, that is, these makes a reduction of labor.
(ii ) The technologies make high quality manufacturing.
(iii) The technologies economize the resources.
In this chapter, the purposes, the principles, the methods and results of these technologies are described. That is, a blood leakage sensor is described in 4.1, a dye color sensor in 4.2, a thickness sensor for polyethylene foam sheet in 4.3, a weight density sensor for row glass wool in 4.4, a rib form sensor for polyethylene sheet in 4.5, and a weight density sensor for glass wool pipe in 4.6.
4.1 Blood Leakage Sensor
Recently, high sensitivities are needed more and more in the field of medical electronics. One typical application is a blood leak sensing system used in an artificial dialyzer. A number of sensors such as a temperature sensor, flow rate sensor, pressure sensor, negative pressure sensor and bubble sensor are used as well as a blood leakage sensor in an artificial dialyzer. A number of dialyzing treatments are normally needed in any one week for a patient suffering from a kidney disorder, and therefore a blood leak could potentially cause fatal damage, even if leakage in each dialyzing treatment is a very small amount. For this reason numerous research projects have focused on a blood leak sensor1).
Most instruments used to detect blood are based on an optical non-intrusive method using an infrared ray or a visible ray from a semi-conductor laser or a diode. In the systems presently used for the dialyzing treatment, an alarm is triggered only when blood concentration exceeds a pre-set threshold level, and even then no quantitative data of the total amount of blood leakage is provided. Furthermore, patient's life is in danger during the treatment since the sensitivity of the sensor in current use is too low to detect a minor leakage. A patient can be released from the danger if a highly sensitive blood leak sensor is developed which gives a linear sensitivity with concentration in real time.
The main principle of the present method is based on the light attenuation theory. Laser light is attenuated by the presence of blood cells in the optical path in a container, and thus the measurement of the light attenuation gives the concentration of blood. The sensitivity and the accuracy may then depend on the sensitivity and the stability of the intensity measurement system, and on the fluctuations of the incident light, respectively. The sensitivity for blood detection of present ranges between 0.01 and 0.001 in weight %2)(weight ratio of blood to solution), which is too small for recent medical appliances.
The purpose of this study is to propose a simple method for the improvement of both the sensitivity and stability of blood concentration measurement, without using complicated equipment to stabilize the laser output, and to describe the construction of a practical sensor system.
4.1.2 Principles and Methods
Light intensity decreases by absorption or scattering when it propagates in a non-transparent medium. We can then determine the concentration or density of the medium by measuring the light attenuation. Since the intensity decreases in proportion to an optical path length through the medium (if the medium is homogeneous), the path length is extended by the multi-reflection technique, using a pair of side mirrors mounted on the test cell in order to enhance the sensitivity. The present technique is effective for a higher sensitivity, however the use of a narrow-beamed laser is necessary to avoid an over-enlarging beam size at the exit of the cell.
Figure 4.1 shows an experimental setup. Laser light from a semi-conductor laser is divided into two beams by a beam splitter, one is a reflected beam and the other a transmitted beam.
Fig. 4.1 Experimental setup of the sensor. (Numerical values in this figure are in mm ).
The reflected beam directly enters a light power meter S1 and its power is measured to provide the incident laser light level and is used for the normalization of the attenuated light. The transmitted beam enters a cell in which a test subject is filled and is led to a light power meter S2 after the multi-reflection on a pair of side mirrors inside the cell. The attenuated power is measured by the meter S2 and is divided by the power of the incident light given by S1. This normalization automatically eliminates errors due to the unknown fluctuations in the intensity of the incident beam. The multi-reflection extends the optical path length and thereby increases the sensitivity. A semi-conductor laser with a wavelength of 680nm has been chosen because it is reasonably priced and is easily available for practical use. A physiological salt solution mixed with blood was used as a test subject in order to evaluate the system sensitivity and stability. The light attenuation of the laser was caused by scattering rather than by absorption, since the laser light irradiates a red corpuscle.
The principle of the method is also shown by a schematic diagram in Fig. 4.2. Although the light attenuation due to light scattering in a weak solution has been discussed in detail in the literature3,4), Lambert’s law of simple estimation of attenuation has been used, whereby the light decreases exponentially with the path length in the medium. That is, the light
Fig. 4.2 Schematic diagram of the optics.
power Io at a distance x from the entrance is related to the incident light power Ii as
where is an absorption coefficient which depends only on the concentration or the density of the subject n to be measured. Applying Eq. (4.1) to the present method (see Fig. 4.2), the light power I2 received in the second sensor S2 can be related to the light power I1, received in the first sensor S1 as follows,
Where, T and T' are the transmissivities of the beam splitter and the test cell glass, respectively, r' and r are the reflexibilities of the beam splitter and the side mirrors in the cell, respectively, p is a normalized optical path length defined by p=x/L', where x shows a total optical path length. As is shown in Eq.(4.2), the normalized output light power I2/I1 is directly proportional to Io (x)/Ii. A logarithmic expression of Eq.(2) is given as,
An absorbance defined by log(I2/I1) is thus directly proportional to p. As shown in Fig. 4.3, the linearity between log(I2/I1) and p was determined experimentally. The gradient of the straight
Fig. 4.3 Effect of the optical path length, p, on the normalized output light power I2/I1 (in a logarithmic scale) for blood concentration, n.
line, , shows an apparent absorption coefficient , from which an absorption coefficient can be calculated as follows,
The multi-reflections technique does not allow us to set the incident angle of the laser beam to be normal to the mirrors in order to maintain its consecutive reflections. This requires the term in Eq.(4.4), which compensates for its effects on . In this setup, the value of p in the equation is usually larger than three and the magnitude of the effects of the shift is considered to be less than 3%.
The measured data of the light power I1 and I2 are processed by a data processing system, shown in Fig. 4.4. The continuous analog signals of I1 and I2 are sampled, digitized and stored
in a computer where the operation of I2/I1 is completed. Results are given in the form of curves after smoothing. The resolution of the data acquisition system is 12 bits and the sampling rate is about 800/s. One data point in the figure is made by averaging 50 measured data, and then requires 63 ms sampling time.
Fig. 4.4 Block diagram for the data acquisition system.
4.1.3 Experimental Results and Discussions
Before taking experimental data, measurements were made in order to determine the system parameters of the optical components, such as the transmissivity (T and T'), and the reflection coefficient, (r and r'). The measurements gave T=0.93, T'=0.90, r=0.90 and r'=0.04. This gives us T2T'/(r'r)=21.6, which is in good agreement with the experimental data obtained at p=0 in Fig. 4.3.
First, the effects of the optical path length were investigated. Experiments were carried out by changing only the optical path length p, keeping the concentration and laser output constant. Second, a series of experiments were conducted for various concentrations and laser outputs. Results obtained are summarized in Figs. 4.3 and 4.5. The normalized output light power I2/I1 in the logarithmic scale decreases linearly with the normalized optical path length p as shown in Fig. 4.3, as well as with the concentration n, as shown in Fig. 4.5. The straight lines in Figs. 4.3 and 4.5 correspond to each normalized optical path length and concentration, respectively, which were obtained by the least squares method. It can be seen from these two figures that the normalized output power decreases almost exponentially with the product of the normalized optical path length p and the concentration n. Combining this fact with Eq.(4.2) implies that the absorption coefficient is to be proportional to n.
Fig. 4.5 Effect of the blood concentration, n, on the normalized output light power I2/I1 (in a logarithmic scale) for various optical path length, p.
The absorption coefficient for each concentration n is shown in Fig. 4.6, which is determined from the gradient of the straight lines in Fig. 4.3.
As seen in this figure, the absorption coefficient increases linearly with concentration n as discussed above, and can be expressed as follows,
where c is a constant independent of concentration n and optical path length p. n0 is the absorption coefficient of the solution without blood and is estimated experimentally to be n0=3.5510-4/mm, as seen in Fig. 4.6. The constant c can also be determined experimentally by the gradient of the straight line in Fig. 4.6 and is calculated as 36.4/(mmconcentration in volume). It depends only on the interaction between blood and the laser, and therefore the wavelength of the laser plays an important role in determining c.
Fig. 4.6 Effect of the blood concentration, n, on the absorption coefficient, .
When red laser light is used, as in this case, red corpuscles scatter the red light; the constant c is mainly determined not by absorption but by scattering. If a blue laser is used, on the other hand, the attenuation is caused by absorption. Combining Eq. (4.5) with Lambert's law (Eq.(4.2)) results in the following Lambert-Beer's law5) which is applicable for a diluted solution, as in our case,
The sensitivity is determined in practice by dividing an increment of the normalized output laser power by the corresponding increment of the blood concentration (n) in a solution, i.e.,
Then the relative sensitivity K divided by S(1) gives,
The sensitivity for p=1 corresponds to one of the conventional sensors. It is apparent that the sensitivity of the present method is p times higher than that of conventional sensors, although there is an advantage with the conventional sensors in that they do not require an expensive laser (an economical laser diode is quite enough for conventional sensors since the path length is shorter).
The normalized optical path length p can be multiplied by repeating the reflection on the pair of side mirrors, if necessary, to increase the sensitivity. However, the maximum sensitivity is limited by a cross sectional area of the laser beam after the multi-reflections because the laser beams should not overlap each other on the side mirrors. The maximum sensitivity can then be given by pmax=W'/d, where W' is a width of the side mirrors as shown in Fig. 4.1, and d the laser beam diameter.
The normalized output power I2/I1 is completely independent of the fluctuations of the laser output power, as can be seen in Eqs. (4.2) and (4.3). This means that a laser can be used without any complicated stabilizing equipment or compensator for the light source, and is another distinguishing feature of the present system, in addition to its high sensitivity. The independence of I2/I1 from the laser fluctuations has been confirmed by varying the laser output over an intentionally wide range. The result is shown in Fig. 4.7. The normalized output laser power is almost constant over the wide range of the laser output I/Imax where Imax is the maximum output power of the laser.
Fig. 4.7 Fluctuations of the normalized output power, I2/I1. (The laser output power, I, is normalized by the maximum output power, IMAX.)
The cell configuration used in this study is rectangular as shown in Fig. 4.1. However, a cylindrical cell, as shown in Fig. 4.8, may be more practical from the view point of commercial production. With this configuration, star-shape multi-reflections can be applied and then both the inlet and outlet of the laser lights can access the same window.
Fig. 4.8 A proposed cylindrical configuration for a practical sensor.
In conclusions, the following results were obtained.
(1) The sensitivity of the sensor for blood leak detection has been improved by the use of a beam splitter and a pair of side mirrors, which gives us about a few tens of times higher sensitivity than conventional sensors on the market.
(2) Further, the fluctuations of the laser power are completely compensated only by using a beam splitter.
(3) The practical sensor with the cylindrical cell has been proposed.
4.2 Dye Color Sensor
In a dyeing process, it is the most important to dye a cloth with consistent color. The tinting power of dye, however, depends on its concentration, the material to be dyed, the temperature of the dye, and other parameters. The concentration of each color dye in a dye mixture decreases with time. The color dyes consumed must be replenished instantaneously to maintain the same tint. A realtime measurement of a dye concentration is then essential for this purpose.6)
A typical method now used practically is spectrum analysis using a high power light source. This method, however, is expensive and takes tens minutes to produce each measurement because a precise measurement requires a high spectral sensitivity and further dilution of the dye. A real-time measurement is thus practically impossible.
The principle of a real-time optical sensor for measuring dye concentration and the result of preliminary experiments using a semiconductor laser have been reported previously.7) In this section, the flexibility of a sensor system is discussed from the viewpoint of practical use.
4.2.2 Principle and Method
Figure 4.9 shows the principle of the method. Laser light containing the three base colors
Fig. 4.9 Optical principle of the method. A part of the incident light of three primary colors is
absorbed in a dye mixture consisting of the three primary dye colors.
is guided into a measuring cell. Part of the light is absorbed in the cell and the rest passes through the cell and is received on a photodiode. The intensity of the transmitted light can be expressed by Beer's law,8) if the dye is diluted, as
Where Ii represents the power of the incident laser light, Io the output power of the transmitted laser light, K is an absorption coefficient, d is the cell width. The overall concentration, n, includes the concentration of a dye, nx, and a reduced concentration, no, taking account of the optical glass of the cell walls and additives for dyeing, etc. As is shown in Eq. (4.9), the absorbance defined by log(Io/Ii) is in direct proportion to nd and the absorption coefficient can then be given as an absolute value of a gradient of the straight line absorption curve.
A dye can be mixed with three base colors, i.e, red, yellow and blue base dyes. Unknown parameters to be solved are thus the dye concentration of these three base colors. Three pieces of information are therefore essential for the solution. These can be obtained from the transmitted light power of three base colors. Light of wavelength R=670 nm from a semiconductor laser and wavelengths of G=515 nm and B=458 nm from a multiline Ar ion laser are used for the light sources.
The absorption coefficient depends mainly on the wavelength of the light (R, G, B ) and the dye color C(R, Y, B ). The absorption coefficient has then nine components:
The dye toning mixed with three base color dyes involves no chemical reactions. The light attenuation is then proportional to the product of each attenuation by the individual color dyes. The superposition principle for light attenuation, therefore, can be applied to the present case where many color dyes are mixed together. The assumption will be realized as discussed in section 4.1. The transmitted light powers through the mixed dye are expressed as follows from Eqs. (4.9) and (4.10):
The concentration of each dye color can then be obtained as,
The concentration sensitivity of the sensor, in other words, the resolving power of the concentration, is defined by a small change of concentration due to a small change of the light power. It can, therefore, be given as,
the negative signs in Eqs. (4.12) and (4.13) imply that an increase of light
power corresponds to a decrease of the concentration. Figure 4.10 shows the
optical arrangement and the system of the sensor. Any wavelength of laser light
can be selected by properly choosing the shielding plates
Fig. 4.10 Optical arrangement and sensor system.
cell is focused on a photodiode and converted to an electric signal. The signal is amplified, sampled and digitized in 12 bits. The maximum sampling
frequency of the data acquisition is limited to about 1 kHz by the A/D converter. The digitized signal is used in Eq. (4.12) to calculate the concentration and the results are displayed on the monitor. All the data used in this calculation are obtained by averaging by 80 samples.
The dyes used in the experiments are color indication numbers of C.I. Reactive Red 112, C.I. Reactive Yellow 15 and C.I. Reactive Black 5. These are all ionized in water.
4.2.3 Results and Discussions
The output power of laser light was measured for a small change of the concentration. Figure 4.11 shows an example with a laser light of wavelength R = 670 nm and blue color dyes. As seen in this figure, the logarithm of output laser power decreases almost linearly
Fig. 4.11 Output power of the laser light with a wavelength R =670 nm passing through a dye solution with various concentrations of blue dye.
with an increase of dye concentration in a limited range of concentration. This indicates the validity of the Lambert-Beer's law expressed in Eq. (4.9). The slope, -Kd, has been calculated by the method of the least-squares fit. As seen in the figure, the absolute value of the slope, Kd decreases slightly as the concentration becomes high (the range of practical use). The small change of slope would be one of the major reasons for error of this method.
The value of the absorption coefficient in this case was calculated as K(R,CB)=7.28 (g/l)-1cm-1 for d=0.55 cm. Similar experiments were run for all combinations of the three base color lights and dyes. Table 4.1 shows all the data for K. The value changes on a large scale, depending on the combination of laser light and dye colors. This enables us to increase the sensitivity of the method to a level which is high enough for practical use.
Table4.1 Absorption coefficients for all combinations of three basic lights and dyes.
Using the data in table 4.1 in Eq. (4.12), we obtain an expression for each concentration,
An experiment of increasing each color dye successively was carried out to confirm the reliability of the method. The dye concentration in the measuring cell filled with water was successively increased by droplets of high concentration dye. The output of the laser light was measured for each droplet and was used for the calculation of the dye concentration. Figure 4.12 shows an example of the calculated concentration for blue dye. As shown in this figure, the calculated concentration of blue color dye changes only by the droplets of blue color dye. The values of the change by both the droplets are about 0.009 and 0.016 g/l, respectively, which are in good agreement with the real change of 0.013 g/l.
Fig4.12 Calculated concentration of blue dye for droplets of each color of dye.
The resolving power of the concentration, shown in Eq. (4.13), can also be obtained by using the data in table 1, as follows,
Thus the resolving power depends directly on the resolution of the data acquisition system for the output light power, Io/Io, which is given by the A/D converter if the resolving power of the photodiode is sufficient. The total range of light output is between 1 and 256, that is, the resolving power is 1/256, when the A/D converter with 8 bits is used. As an example, if the light power changes for three base color of light are (Io/Io)R = 0.01, (Io/Io)G = 0.1 and (Io/Io)B = 0.1 , the resolving power of the concentration are calculated as n R = -0.788 mg/l, n Y = -2.90 mg/l and n B = -2.49 mg/l. These are in good agreement to the experimental result in Fig. 4.12.
Firstly, the validity of the superposition principle for the light attenuation is discussed as follows. The reliability of this method is based on the superposition principle for the light attenuation. This can be considered to be valid from the fact that the dye mixing involves no chemical reaction. This has been examined experimentally. Figure 4.13 shows an example of light attenuation through the mixed dyes. The absorption coefficient K(R,CR) is almost independent of the blue dye. This validates the adaptation of the superposition principle. A small change of K(R R, C R) will yield an error as discussed in the next section.
Fig. 4.13 Output power of the laser light with a wavelength R =670 nm passing through a
dye solution with various concentrations of red color dye added with and without
blue color dye: (a) without blue color dye, (b) with blue color dye of n B =0.0625 g/l,
(c) n B =0.125 g/l, and (d) n B =0.345 g/l.
Secondly, the error due to a small change of absorption coefficient is discussed as follows. In figure 4.12, a difference between true and calculated values of concentration and also the concentration change of blue dye due to the gradual addition of red dye and yellow dye, are sources of error. The former was found to be about 3 mg/l and the latter about 1 mg/l. Three major sources of error may be considered. The first results from light fluctuation due to ambient light. However, the ratio of the intensity of ambient light to the laser light has been made less than 1/1000 by shielding the light receiving system. The value is less than the sensitivity and can be neglected. The second factor results from non-linearity between log(Io/Ii) and (n) in Eqs. (4.9) or (4.11), as shown in figure 4.11, when the concentration becomes high. We can, however, make the apparent concentration low enough by using a thin cell width as discussed in section 4.2.3 A. The last results from a small change of the absorption coefficient due to the mixing of dye as described in section 4.1. This will be a substantial problem for this method. A small change of concentration [n] due to a small change of absorption coefficient [K] can be calculated from Eq. (4.12),
where [K]-1 shows an inverse of the matrix [K]. Thus, the change of dye concentration may be the same order of the change of the absorption coefficient.
Thirdly, the adaptability for practical use is discussed as follows. Figure 4.14 shows an example of the output laser power for an extremely high absorption coefficient. The measurable range of the concentration is between 0 and 0.63 g/l in this case. However, we can measure higher concentrations by using a thinner cell width d, because the absorption a coefficient K is the slope of a straight line between ln(Io/Ii) and (nd) as seen in Eq. (4.9). That
Fig. 4.14 Output power of the laser light with a wavelength B =458 nm passing through a
dye solution with various concentrations of blue color dye.
is, we can, apparently, make the concentration low by using thin cell width. As an example, if a concentration range between 0 and 3 g/l is required, it can, theoretically, be achieved only by making the cell width about d=1 mm. This will, however, cause an error due to a concentration fluctuation in such a thin cell width. One useful method to solve this problem may be to incorporate the light emitter and receiver in one unit as a sensor head using an optical fiber, as shown in Fig. 4.15. Practically only the sensor head is immersed in the dye solution.
Fig. 4.15 Illustration of a proposed optical sensor head.
Lastly, the optimum wavelength of the laser light is discussed as follows. Three base colors of lights with the wavelengths R = 670 nm, G = 515 nm and B = 458 nm were used for the light sources in this study. However, another wavelength may also be used effectively. A large difference in absorbance, defined by log(Io/Ii), for each wavelength and dye color is desirable for high sensitivity. Figure 4.16 shows the absorption spectrum for three base color dyes. It may be concluded from Fig. 4.16 that the desirable wavelengths of the laser lights are 430 nm, 460 nm 510 nm, 540 nm and 580 nm. Any three wavelengths in these five wavelengths, e.g., 460 nm, 510 nm and 580 nm, will be sufficient for determining three unknown parameters as discussed in this paper.
Fig. 4.16 Absorption spectra of three base color dyes.
In conclusions, the following results were obtained.
(1) The sensor system can be used effectively for monitoring or detecting a small change of dye concentration.
(2) The concentration sensitivity of the method was about a few mg/l and may be satisfactory for dyeing machines presently on the market.
4.3 Thickness Sensor for Polyethylene Foam Sheet
Numerous optical sensors have recently been developed in response to the fact that small-size semiconductor lasers have become commercially available at a reasonable cost. 9) Although the application of these sensors and lasers involves a wide range of laws and theories in physics, Lambert's law is one of the most frequently used to estimate light attenuation. The law was successfully employed in a high sensitivity optical blood leakage detection system10) and also in a dye concentration measurement system11).
In the present study, light attenuation has been used to measure the thickness of sheets of polyethylene foam and polystyrene. Polyethylene is widely used for a variety of products such as bath mats, packing sheets for shipping and heat insulators for building comfort. Polystyrene is mainly used for cups and hot food containers. The thickness of the foam sheet is important to the makers from the viewpoint of standardization of manufacture. The major difficulty in thickness measurement of foam sheets is caused by the heterogeneous cell structure of the foam, which yields enormous fluctuations in the measured data depending on the measurement technique and on the sampling locations. An averaging method has been adopted for solving this difficulty and enables us to determine the thickness with good accuracy. A complete measuring system has also been constructed in order to demonstrate the suitability of the present method for industrial use.
4.3.2 Principle and Method
Figure 4.17 shows a fundamental illustration of a light attenuation process. Polyethylene
or polystyrene foam consists of small cells filled with air. The light is scattered at each cell wall as it passes through the foam. The transmitted light intensity, It, is then given by the multiplication of the transmittance, Tk, on each cell surface in the measuring optical path, as follows12):
Where Ii is the input light intensity and N the number of the cell surface along the optical path. In particular, T1 shows the transmittance on the surface of the foam sheet. The transmittance, Tk(k=1,2, N), and the total number, N, are then a function of an irradiated position x, as shown in Fig. 4.17(a). Thus, the output, i.e., transmitted light intensity, depends on the position.
Fig. 4.17 Light attenuation in a foam sheet from the microscopic viewpoint (a), and from the macroscopic viewpoint (b).
We expect, however, that the output light intensity after passing through an optical path d, Id, follows Lambert's law5) if a sample is uniform, as given by
where is an absorption coefficient. This is an approximate expression for light attenuation form a macroscopic viewpoint. That is, the output intensity, Id, is considered to be the mean value of It (x) for many sampling points around the measuring point.
As is shown in the following section, the mean values of T(x), <T(x)>, at each measuring point x, are approximately equal and we can then assume:
Using Eqs. (4.17) and (4.18), and the relation given in Eq. (4.19), we obtain the following relation:
Equation (4.18) gives us information on the thickness and Eq. (4.20) on the cell density, N/d or the cell transmittance.
Figure 4.18 shows the experimental set-up for a measurement. Laser light of wavelength 670 nm from a semiconductor laser passes through a polyethylene sheet. The transmitted light diverges by scattering in the foam and is focused on a light meter. The light power, Id, is processed by the data acquisition system shown schematically in Fig. 4.19.
Fig. 4.18 Fundamental optics of the experimental set-up.
Fig. 4.19 Block diagram of the data acquisition and processing system.
The continuous analog signals of Id are sampled, digitized and stored in a computer memory. Results are given in the form of curves after smoothing. The resolution of the data acquisition system is 12 bits and the sampling rate is about 400Hz.
Figure 4.20 shows a particular experimental set-up to measure the effect of cell structure on the transmittance, e.g., light attenuation. The polyethylene foam sheet is placed between a pair of transparent optical glass plates, slightly inclined to the normal to the laser beam. The thickness of the sandwiched foam sheet, d, is changed by compressing the glass plate. The optical path length through the sheet foam is thus changed by the compression of the glass plate or its inclination.
Fig. 4.20 Optics to measure the effects of compression and inclination of a semi-transparent foam sheet.
4.3.3 Results and Discussion
Figure 4.21 shows an example of the transmitted light power at each measuring point. As shown, each data point differs to a great extent from the mean value, the range of which
reaches 70%. This is attributed to the difference of total transmittance, T(x), along each optical path, as shown in Fig. 4.17(a), T1 may be the major factor. The difference can, however, be reduced by smoothing, i.e., averaging. Two technical methods, both of which are based on the same principle, are available for this smoothing. One is to enlarge the spot size of the laser irradiation. This method, however, necessitates the use of a large focusing lens and further to cut an ambient light off to a very small amount. The other method, used in this study, is to make many measurements around a small region.
Fig. 4.21 Transmitted light power at each point. () d1=2.76 mm; () d2=5.58 mm; () Mean
value of 3.3510-3 for d2=5.58 mm; (-) Mean value of 3.2410-2 for d1=2.76 mm.
The smoothing effect is shown in Fig. 4.22. As shown, smoothing over 80 points restricts the fluctuation of the mean value within a range of 1%. All the data except one example given in Fig. 4.21 were averaged over 80 points.
Fig. 4.22 Relation between the transmitted light power fluctuation (%) and the reduced
Fig. 4.23 shows an example of the relation between transmitted light power and foam thickness. It can be seen that the transmitted light power decreases exponentially with the
Fig. 4.23 Relation between the averaged light power and the foam thickness d.
thickness. A straight line can be obtained by the method of a least mean square fit. The gradient of the straight line gives the absorption coefficient, , in Eq. (4.18). The value depends only on the property of the material used. A value of the thickness, d, has to be known for a determination of absorption coefficient, , and should be measured by another method, e.g., the gage method.
In applying the method practically, the absorption coefficient, , has to be determined repeatedly for each product.
The effect of the cell structure on the transmitted light power was examined using the optical arrangement shown in Fig. 4.20, where the optical path length is changed. The number of cell surfaces remained constant when we compressed the foam sheet and so the transmitted light power may be assumed unchanged.
Figure 4.24 shows an example of the effect of sheet compression on the transmitted light power. As expected, the transmitted light power was almost the same for each compressed foam sheet, i.e., d=3.7 mm and d=3.2 mm. This implies that each cell surface has an equal
Fig. 4.24 Transmitted light power under squeezing. () Normal foam sheet; (), ()
squeezed foam sheets. Each datum is an averaged value over 80 points in the
vicinity of each other.
transmittance and that the total transmittance through the optical path depends on the transmittance of the material and the number of cell surfaces. That is, the assumption in Eq. (4.19) is justified.
The optical path length increases when the normal of the foam sheet is inclined to the optical axis, and then the transmitted light decreases. In this case, the foam thickness, d, has to be replaced by (d/cos-1), as shown in Fig. 4.20. Figure 4.25 shows the effects on the transmitted light power, where the abscissa shows reduced inclination, 1/cos -1. This figure also implies the validation of Eq. (4.19).
The accuracy of this method depends on the linearity between the thickness and the decrease in the transmitted light power, as shown in Eq. (4.18). The light fluctuation due to ambient light introduces errors. It is very difficult to discriminate laser light from ambient light, but it is easy to reduce the effect to ambient light. Two methods are available. One is to use a pulsed laser of high peak power, but this is too expensive for our purpose. The other is to cover the laser light receiving system with a proper cylinder case. This can reduce the ambient light power below 0.05% of the maximum power of the transmitted laser light.
Fig. 4.25 Effect of inclination on the transmitted light power. The ordinate shows the light
power normalized by the one for =0 and the abscissa the reduced inclination
4.3.4 Practical Applications
In practical foam production, molten polyethylene in an extruding machine is squeezed out from a circular arc air nozzle and is formed to a sheet. The circular nozzle is divided into a few small portions, each of which has a nozzle aperture which can be controlled independently. The speed of output of the sheet is about 1-2 m/s and its width is about 2 m. The usual gage method of picking up contact elements is therefore difficult. Furthermore, it can only measure the thickness at both edges of the sheet. On the contrary, the present method may solve the above disadvantages simultaneously. Figure 4.26 shows an example of the practical optical set-up for the measurement. One optical sensor is desirable for each nozzle, where a sensor signal is processed to control the nozzle aperture. All the sensor signals from each sensor are processed on the computer simultaneously. The thickness of each portion along the width can thus be adjusted to the same thickness.
Fig. 4.26 Optical system for practical use.
Figure 4.27 shows an example of the results. The sampling rate of this system was about
Fig. 4.27 Thickness of the polyethylene foam sheet measured in the manufacturing process. Each datum was plotted at intervals of 3 m and total length was 300 m.
400 Hz. The production speed of the foam sheet was 1.5 m/s. As discussed above, one datum was given as the average of 80 points and then the average along a 0.3(=1.5*80/400) m length. The spot size of the laser was about 5 mm in diameter at an irradiated surface and then each sampling point slightly overlapped each other. The deviation of the thickness was found to be within 0.01 mm over 90 m from this figure. The deviations on other portions in the width direction were kept within this range.
Figure 4.28 shows another example of the measurement. This is a result given by the system developed for the precise measurement for a food container. The foam sheet for a food container was produced with relatively slow speed of about 10 cm/s. Each sampling point was
Fig. 4.28 Thickness of the polyethylene foam sheet measured in the manufacturing process.
overlapped close together. That is, each datum was not an average over totally different points but over locally common points. The result by the usual gage method is also shown for the comparison. Both results agreed quite well within a deviation of 20m. From the result, the error of the method in this study is supposed to be within a few tens of micrometers.
In conclusions, the following results were obtained.
(1) The light attenuation was used in this measurement, and averaging enabled us to determine the thickness precisely in spite of large fluctuations in measured data due to the heterogeneous structure of the sheets.
(2) Automatic thickness control can be accomplished in the manufacturing of these sheets.
(3) The error of this measurement is a few tens micrometers.
4.4 Weight Density Sensor for Row Glass Wool
The glass has recently been used more and more, in particular owing to the advancement of manufacturing technique of high purity and glass fiber. An optical fiber in a communication service is the representative example. The other examples are processed goods of glass wool such as a dust proof paper, separator to insulate electric current in a battery and heat-resisting and sound-resisting mats used mainly in a car and a building. An efficiency of the finished goods, i.e., homogeneity in particular, depends on a manufacturing process, mainly on a weight density of raw glass wool.
The weight density may be measured by an attenuation of a sound wave and electromagnetic waves such as microwave and light wave through the raw glass wool. A source of the microwave will, however, be rather expensive for an industrial use. The sound wave will have a troublesome problem of noise when it is used in the factory. That is, a signal obtained by the sound wave of proper frequency for the measurement will include rather large noise produced in the factory. The noise will usually include all over the frequency range and we can not distinguish between signal and noise. On the contrary, we can, easily screen the noise light from the signal light.
We have previously reported a new method to monitor the thickness of a semitransparent foam sheet in realtime using a laser light13). The principle of the method is based on the light attenuation through the foam sheet. The method can also be applied to glass wool weight density since the thickness is directly proportional to the weight density if the foam sheet is homogeneous. It was very simple and was found to be effectively applied for industrial use. It has, however, a drawback to scan the laser beam to examine all the area because an intensity of the laser is rather small. The raw glass wool usually produced is rather thick and then light attenuation and divergence through it become rather large. It, therefore, requires a focusing lens for a transmitted laser light due to a divergence through the glass wool. This prevents use of laser light as a light source and photodiode as a light receiver.
In this section, we propose a practical method to overcome these drawbacks, that is, high intensity white light is used as a light source and a large scale solar cell as a light receiver.
4.4.2 Method and System
The fundamental principle of the method is based on the light attenuation theory12,13). But the method in this paper is different from the previous one in two points from a practical point of view. One is to use a white light with high intensity instead of laser light. This enables us to use solar cell as a light sensor and the method to be effectively applicable for high attenuation object. The other is to use a solar cell as a light sensitive sensor instead of photodiode. The solar cell, in itself, is not a light sensitive sensor but a converter from a light power to an electric power. The solar cell has a large dimension as compared to the photodiode. It makes, therefore, a spatial averaging over the cell dimension without scanning the area. It can, further, measure a rather diverged light through the thick semitransparent object such as glass wool without focusing lens. The solar cell has, however, such disadvantages as a low sensitivity to the light and incomplete characteristics in each cell. The low sensitivity can be solved by using an intense light source and a large scale cell. The incomplete characteristics can be compensated by using an amplifier to each solar cell independently as in Fig. 4.29.
Figure 4.29 shows a whole system used in a manufacturing plant. Four high intensity white light sources with each output of 500 W were used. These were placed at a distance about 800 mm from a top of the glass wool to illuminate it uniformly as possible as we can. Sixteen solar cells of each dimension 90mm230mm were used as the light receivers. These were placed at about 30mm from the bottom of the glass wool to receive the transmitted light directly and at the same time to avoid an attachment of the glass wool to the cells.
Fig. 4.29 Illustration of the system.
The outputs of each solar cell are amplified independently to compensate the output characteristics of each solar cell and the lack of uniformity of irradiated light intensity on the glass wool. That is, all the outputs of each solar cell are adjusted by each amplifier to show the same value under the same weight density. These amplified signals are then digitized by an A/D converter, averaged by a computer and displayed on the monitor as shown in Fig.4.29. Sampling time ts of the data acquisition is about ts= 0.1s and a processed data is obtained at an interval Td= 1s in this case. That is, one data is obtained as a 10 times averaging ( n = Td/ts, n; times for averaging). The data can, however, be obtained at an arbitrary time interval by the computer control. The production speed v of the glass wool is about v = 250mm/s and then the processed data on the monitor is a mean value for an area of 90mm250mm(= 25mm10). Strictly speaking, the averaging area is not so as discussed in 4.43.
Lamber's law5), which gives a relation between input Ii and output Io light intensity, can be expressed as follows,
where D shows the glass wool weight density of the corresponding area and k an absorption coefficient which depends on a property of the material. Thus, a natural logarithms of the light intensity ratio is in direct proportion to the weight density. The weight density of a practical product has nearly a constant value, D=Dc, as shown in Fig. 4.29. In this case, a small increment in density, Dc, may causes a small decrease in output light intensity, Ioc, using a first approximation as follows,
This shows that a small increment in the weight density from about a constant density Dc is in direct proportion to a small decrease in the output light intensity Ioc. The error of this approximation is only 2% for Dc0.2, since e0.2-(1+0.2) = 0.021. We use Eq. (4.22) in a practical application of this system.
4.4.3 Result and Discussion
The system has
successfully been used in a manufacturing plant of Japan Inorganic Chemistry
Co. Ltd. at Yuki Factory,
Figure 4.30 shows an example of the light intensity for one channel (by optical method) and the measured weight density of the corresponding area (by direct method). Thus, the tendency between both values by optical and direct methods with time flight is found to be coincide perfectly with each other. All the data for 16 channels have similar tendency.
Fig. 4.30 Weight density of the raw glass wool by both direct and optical methods.
Figure 4.31 shows both change rates by optical and direct methods in Fig. 4.30 to discuss the validity of this method. The optical change rate is different about 10% from the direct one.
This may be considered as an error of this optical method. It is not, however, necessarily so since a cutting the raw glass wool into pieces is very difficult and then the measured value itself includes some errors. The set point of this weight density is 500 g/m2 in manufacturing plant. The averaged values, however, is 547 g/m2 which is larger about 10% than that of the set point. This is indispensable to assure the finished good's efficiency, i.e., adiabatic and sound-proofing effectiveness. It was further found that the change rate of optical value is always smaller than that of measured value. This is unavoidable to this method which uses large scale photo-receiver for measuring a light intensity through a moving object.
Fig. 4.31 Change rate of the weight density by both direct and optical methods. Optical values are reversed.
Figure 4.32 shows this principle. A piece of raw glass wool used for both measuring methods has a dimension 9cm25cm. The solar cell having a dimension of 9cm23cm receives transmitted light corresponding to that area on the glass wool. The glass wool moves
Fig. 4.32 Principle of the received light intensity on the large scale solar cell from the measuring area of the raw glass wool.
25mm between each data sampling and 25cm between each processed data since it is obtained by a ten times averaging of the sampling data. In the central region of the piece, i.e., regions 5 and 6 of sampling time for averaging, the solar cell receives 9/10 of the transmitted light from the measuring piece. But it receives only a 5/10 from the measuring piece and a remaining part from the adjacent piece as shown in this figure. The optical value, therefore,
shows the results obtained from wider region than that of measuring piece. This is a reason for the difference between measured and optical change rates. The difference may be reduced slightly by taking a weighted mean in place of an arithmetic mean used in this experiment. The method of a weighted mean can not, however, settle the problem radically because the measured region for both methods of optical and direct measurement is essentially different.
Errors within plus or minus 10 % or so exist in our optical system as seen in Fig. 4.31. It can be, however, decreased in accordance with 1/(N)1/2 from the theory of error13), where N is an averaging times. To do this effectively, a smaller size photo-receiver than that used in this system must be used for the same measuring area and the sampling time of data acquisition must be reduced considerably. This can effectively make N large without the essential problem caused by using large photo-receiver. However, this inversely leaves a strong point of using large scale solar cell described in chapter 1. In conclusion, we must find out a meeting point between them.
The optical method in this study can only measure a relative value of the weight density. The correction is required to obtain an absolute value from the optical value. For this purpose, the absorption coefficient k in equation (4.21) has to be determined from some light outputs for some known weight densities, for examples, 400 ,500 and 600 g/m2.
In conclusions, the following results were obtained.
(1) The optical method using a white light and a solar cell as a light source and light receiver can be used successfully for the weight density measurement of raw glass wool in real time.
(2) The mean error of the method is about 10%.
(3) The system has now been successfully used in a manufacturing plant to find out inferior goods of more or less than 25% weight density. It can further be applied for production process.
4.5 Rib Form Sensor for Polyethylene Sheet
Polyethylene sheet has been widely used in industry. For example, it has been used in a battery as an impregnation material for sulfuric acid. For such applications, ribs are usually constructed on the sheet surface to control the amount of sulfuric acid; the amount of electricity generated and the life time of the battery are determined by the size of the rib. The size of each rib, i.e., width and height, and the separation between ribs have been measured by microscopic observation in off-line. It is very laborious and time-consuming process.
Recently sensors were introduced to this problem.14~16) They are costly, however, and difficult to use in a manufacturing plant in real time; they can only measure a surface contour of the sample in off-line.
In this section, we propose an optical method and present a measuring system for the size of rib, the separation between each rib, and the thickness of the polyethylene sheet.
4.5.2 Method and System
Figure 4.33 shows a photograph of polyethylene sheet used in a battery, and Fig. 4.34 shows a cross-sectional scheme of it. The sheet size is usually about 12cm in width, about
Fig. 4.33 Photograph of the polyethylene sheet with ribs on the surface.
20cm in length, and few tenths mm in thickness; ribs have been constructed on it. Each rib size is a few tenths mm in width and a few tenths mm in height, and the separation of it is about 10 mm. The purpose of this study is to measure the following parameters concerning
Fig. 4.34 Cross sectional view of the sheet.
the polyethylene sheet; (i) the whole width W, (ii) the thickness t, (iii) the rib separation S, (iv) the rib width w, and (v) the rib height h (see Fig. 4.34).
Figure 4.35 shows a principle of the method, which is based on the fact that a probing light intensity, i.e., a transmitted light intensity, is linearly proportional to the gap between the shielding plate and the polyethylene sheet with a rib. A light sheet was used as a probing light.
Fig. 4.35 Principle of the method.
A semiconductor laser was used to form a thin light sheet, which was essential for measuring a small width precisely.17) In this case, the width of the laser sheet should be small enough compared to the width of the rib in order to obtain a sharp edge of the rib, which determines the measuring precision of the width. A light sheet of about 1/10 mm in width was used in this experiment. A silicon photodiode was used as a light receiver. The height and width of the rib, the separation between each rib, and also the thickness of the polyethylene sheet can then be measured by scanning a sheet laser as shown in this figure. In this experiment, a light source and a light receiver were fixed and a sample was mounted on the stepping motor as shown in figure 4.37.
Figure 4.36 shows a block diagram of the entire system, and figure 4.37 shows a photograph of the equipment made for the experiment. The laser light intensity received on the silicon photodiode was amplified to a level acceptable for A/D input, in this case 5 V, and was digitized by a 12-bit A/D converter, i.e., 1~4096 steps. The digitized intensity was then used for the calculation of rib size, separation between ribs, and thickness of the polyethylene sheet.
Fig. 4.36 Block diagram of the whole system.
Fig. 4.37 Equipment of the system.
A control signal for driving the stepping motor was about 100 Hz with a rectangular wave and was synchronized with the received laser signal to measure the widths of the rib and polyethylene sheet, and the separation between ribs. One cycle of the rectangular wave drives the motor 1/100 mm and then the positioning precision of this measurement is 1/100 mm. Then 120 seconds are required to scan the entire sheet with the width of 120 mm. The sampling frequency of data acquisition is about 1 kHz; 10 data are obtained within one cycle and a mean value of these 10 data is used as a measured value of the received light intensity. This averaging will reduce the measuring error due to a fluctuation of the scattered light on the surface of polyethylene sheet. Figure 4.39 shows these measured values at each position.
The error comes from a change in the received light intensity due to laser output, and dust on the light source and light receiver when the system is used in the factory. The problem could, practically, be solved as follows. The measured intensity I is normalized by light intensity I0 through a constant gap g0, in this experiment g0=0.5 mm; the normalized light intensity, I/I0, is used as data. The calculation was made by means of software.
4.5.3 Experimental Results and Discussions
The intensity of a laser beam usually has a Gaussian distribution in a radial direction. This causes a measurement error since a linear relation between the transmitted light intensity I, and a gap g, is obtained under condition that the intensity distribution along the laser sheet is constant. This can be solved by using a central portion of the laser beam as a probing light. Figure 4.38 shows the relation between the transmitted light intensity I, and the gap g. As shown in this figure, the transmitted light intensity linearly increases with an
Fig. 4.38 Relation between transmitted light intensity I and a gap g.
increase of the gap.
Figure 4.39 shows received light intensity versus position on the polyethylene sheet. Each datum is a mean value of 10 sampling data as mentioned in section 4.5.2. In this figure, the top and bottom were inverted to give the thickness of the polyethylene sheet and the height of the rib directly.
Fig. 4.39 Transmitted light intensity across the sheet surface. In this figure, the top and
bottom of the light intensity are inverted to give a thickness directly.
A separation s, a rib’s width w, a sheet’s thickness t, and a rib’s height h are obtained from this figure. The problem is, however, caused on the determination of s and w because the intensity distribution is not rectangular but sine-like configuration. These values can, however, be determined by the following two methods. One is to find the position of an abrupt change of the transmitted light intensity. That is, the width can be measured as a distance between the positions of an abrupt decrease and the abrupt increase of the intensity due to a rib. The other is to find out the position of the threshold light intensity; the space, in which a light intensity is below the threshold level, is the rib's width. It seems that the latter method is simple to use. However, it was found from the experiments that the former method has three main advantages over the latter method. Firstly, it is easy to find out the abrupt change of the light intensity automatically by the use of software; on the contrary, it is difficult to find out the threshold level automatically for all kinds of polyethylene sheet. Secondly, the latter method involved an error due to the large change of the base thickness; it was indistinguishable between ribs and base, in other words, threshold level can not be determined. However, the former method has no error due to such change. Finally, the width obtained by microscopic observation agreed better to the one by the former method than that by the latter method; the width obtained by the latter method was about 10 % smaller than that by microscopic observation. Thus the former method was used in this experiment.
The thickness of the polyethylene sheet t, and the height of the rib h, can be determined by the amount of the intensity decrease. Thus, all the information on the sheet can be obtained automatically within 120 seconds.
Table 4.2 shows these values obtained from figure 4.39. Each symbol of s1, s2, h1, h2, w1 and w2 is expressed with suffix number k, l, m and n which correspond to each dimension from left to right as shown in Fig. 4.34.
Table 4.2 Results for the widths of polyethylene sheet and rib, W and w, the thickness of polyethylene sheet t, the separation s, and the height of the rib h, which are shown schematically in figure 4.34.
In conclusions, the following results were obtained.
(1) The system consists of a sensor head including laser and silicon photo diode, a scanning system of the laser head, and a data processing system.
(2) The precision of the measurement is about 1/100 mm and the time required for the measurement is about 120 seconds. The system has already been placed in operation in a manufacturing plant.
4.6 Weight Density Sensor for Glass Wool Pipe
The glass wool and fiber have recently seen widespread applications. They are dust-proof papers in semiconductor factories, separators to insulate electric current in batteries, heat- and sound-resisting mats employed in automobiles and buildings. The efficiency of the finished goods depends mainly on the uniformity of weight density of raw glass wool when it is used in mats, fiber diameter when it is used in filters, and thickness when it is used in pipes. We have developed an optical system for monitoring the weight density of raw glass wool 18,19) and the thickness of a semi-transparent foam sheet11,13) by passing light through them. Further, we have developed a system for measuring the mean diameter of optical fiber by reflected light20,21). A light interacting with the measured object can be used directly in all these methods since the objects are semi-transparent and roughly constitute a flat plane.
Another example of glass wool application will be a pipe for protecting against heat and cold of a metal pipe through which water, steam and other fluids flow. The deviation of the pipe's thickness, i.e., an eccentricity, will be most important for a practical use. The eccentricity has hitherto been measured by CCD camera. This method has two distinct disadvantages. Firstly, it can only measure eccentricity at the edge and not at the center region of the pipes. Secondly, measurement errors are produced by a lack of clarity due to broken pieces of the glass wool. The technique in this paper for measuring eccentricity of glass wool pipe is simple and efficient, and further it does not possess the disadvantages inherent with CCD camera method.
In this section, a practical system for detecting the eccentricity of a glass wool pipe has been developed optically by a hybrid method. The thickness of the pipe is measured by the contact method and the displacement of a contact head is measured by an optical method.
4.6.2 Principle and Method
Figure 4.40 shows the photographs of the glass wool pipe and Fig. 4.41 the schema of the pipe's cross section. The pipe usually has an eccentricity, i.e., a deviation of the thickness. This caused a serious problem at a junction when it was used to cover the iron pipe in practice. An unacceptable pipe having too large an eccentricity should then be rejected. The
eccentricity of the glass wool pipe can, usually, be defined as,
where tmax is the maximum thickness of the glass wool pipe, tmin the minimum thickness and ta the mean thickness, or defined thickness. By the method described in this paper, the eccentricity can be measured in all directions as the pipe rotates in a 360 arc, i.e., 1 cycle.
Fig. 4.40 Photographs of physical glass wool pipe, (a) without a cut, and (b) with a cut.
Fig. 4.41 Definition of eccentricity.
The cross section of the glass wool pipe was usually unclear due to the chips in cutting. Further, the pipe itself was too soft for the thickness to be accurately measured. These make the CCD camera method difficult. It is thus necessary to use the contact method by loading a constant pressure on the surface for an accurate measurement of the pipe's thickness. For this purpose we developed a hybrid sensor system consisting of two sensor heads; a contact head and a light head.
Figure 4.42 shows the hybrid sensor system. Figure 4.42(a) shows a whole view of the system and Fig. 4.42(b) an enlarged view of the sensor. Both ends of the glass wool pipe are held on the pipe support. The contact head with a rotator is placed on the pipe as shown in Fig. 4.42 and imposes a constant pressure on the pipe, which enables an accurate
Fig. 4.42 Schema of the hybrid sensor system consisting of two sensor heads; (a) whole view of the sensor system, and (b) an enlarged view of the contact and optical heads.
measurement. The measurement can be done at any position in the axial direction of the pipe. The CCD camera method can only measure the eccentricity of the cross section at the end. As the pipe rotates, the contact head will be displaced up and down according to the thickness of the glass wool pipe. A sheet-like laser light was used to measure the displacement of the contact head. That is, the light sensor can accurately detect the displacement since the light intensity is in proportion to the pipe's thickness. The dusts on a laser and a photo receiver cause a measurement error when the system is used in a practical plant. The problem will be solved by using the windows on them and by wiping off the dusts periodically. Another solution for the problem will be to normalize the light intensity through the gap in each measurement by the light intensity through a constant gap.
In practical applications, pipes having too large an eccentricity should be removed. Figure 4.43 shows an electronic circuit for this purpose. An analog signal for the thickness will be obtained on the amplifier, as shown in a cut. A signal for the quality can, finally, be obtained by this logic circuit. That is, the signal for the inferior goods, i.e., the pipe having lager eccentricity than the pre-defined one, en, is expressed by "NG" and the one for superior goods by "OK". The signal of "NG" is obtained if the analog signal at any time during a rotational cycle only exceeds the range between VH and VL, both of which are determined by en. The logic circuit is reset in every rotational cycle and the next reading starts. The value of en is usually 10~20%, which is determined according to the mean thickness of the pipe.
Only the signals of "NG" and "OK" will be required for the practical applications. The analog signal for all the directions, i.e., over a 360 arc, will, however, be shown in this paper to examine the eccentricity in detail.
Fig. 4.43 Electronic circuit to measure the thickness of the glass wool pipe without a straight
cut for judging its quality as acceptable or unacceptable.
4.6.3 Results and Discussions
Preliminary experiments were carried out using pipes without a cut. Fig. 4.44(a) shows a representative result for a pipe with small eccentricity i.e., for superior quality, and (b) with
Fig. 4.44 Thickness of the glass wool pipe without straight cut: (a) with small eccentricity, and (b) with large eccentricity.
large eccentricity i.e., for inferior goods. T shows a period for 1 cycle and was about 1.6 second in all the experiments. In this case, the pre-defined eccentricity was en=0.13.
The glass wool pipe is usually 1~2m in length and should be cut in that direction when it is used for covering iron pipe, vinyl pipe and other types. In practical applications, it is difficult to measure pipe thickness previous to cutting.
Figure 4.45 shows the result for a pipe with the cut which is practically applied. A decrease in pipe thickness occurs suddenly at the edge of the cut as shown in this figure, and this will cause an error when judging for superior or inferior quality. A misjudgment occurs only at the thin region as shown in figure 4.45(b) and does not occur at the thick region as shown in figure 4.45(a).
The degree of misjudgment was, however, negligibly small in the context of practical use when the open area of the cut was rather small. When the open area was larger than about 20 arc, a misjudgment arose. This problem can, however, be solved by using another photo light sensor to detect the cut.
Fig. 4.45 Thickness of the glass wool pipe with a cut: (a) the cut exists at a thick region, and (b) the cut exists at a thin region, which yields misjudgment.
Figure 4.46 shows an electronic circuit designed for this purpose. The sensor signal to detect the cut, E2, can successfully be used to suppress the sudden change of E1 at the cut by means of the sample holding circuit. Then, the logic circuit will accurately judge the quality of the goods. That is, the circuit will display "NG" when the eccentricity is over the defined one, en, and "OK" when it is smaller than en.
Fig. 4.46 Electronic circuit with a sample holding circuit to compensate an error due to the straight cut. Connections A, B, and C in this circuit correspond to A, B, and C in Fig. 4.43.
Figure 4.47 shows the result obtained by this sample holding circuit. The sudden change in thickness at the cut has been kept at a constant value and thus the circuit in figure 4.46 will make an accurate judgment for practical use.
Fig. 4.47 Suppression of the sudden decrease in thickness at the cut by using the sample holding circuit shown in Fig. 4.46.
The time for a rotational cycle, i.e., period T, was about 1.6 second in this experiment. The minimum time necessary for the measurement was, practically, about 1 s; when the minimum time was shorter than 1 s, the contact head vibrated as the glass wool pipe rotated and misjudgment occurred. However, eccentricity measurements may be achieved at higher pipe rotational speed if the dynamic responses of the contact head and glass wool pipe are understood. The time of 1 s, was, however, short enough in the practical application.
Thus, the signal processing system allows implementation of on-line process monitoring and alarm warning signals for unacceptable pipe eccentricity during manufacturing.
In conclusions, the following results were obtained.
(1) A practical system for measurement of the thickness of glass wool pipes has been developed by using a hybrid sensor system consisting of a contact sensor and an optical sensor.
(2) The sensor system has two main merits in its high accuracy and high speed, and in its simplicity of construction. The system has now been successfully applied to a practical situation.
[Chapter 5 will be presented in the upcoming July-August 2010 issue of this Journal.]
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