JPH06307840A - Optical surface roughness meter - Google Patents

Abstract

PURPOSE:To contactlessly measure the surface roughness of a surface to be measured which operates at a high feed speed such as an online process for metal rolling by using a sensed error signal as a focal error signal for servo motions through calculation till the high frequency band. CONSTITUTION:The emitted beam of light from a semiconductor laser 1 is passed through an optical system 2 and condenced by an objective lens 4, and the reflected light by a surface to be measured 20 is two divided so as to generate photo-sensor outputs 61a, 61b. The result is fed to a wide band focal error calculation amplifier 17, and signals from DC to high frequency range are emitted which include a focal error signal 12 for servo motions and a surface unevenness signal. The high frequency of this output is cut 30 to turn into an error signal 12 and further subjected to phase compensation 22 and amplification 23 so that an objective lens actuator 3 is driven, and the focus position of the objective lens 4 is put in pursuit after the mean level of the surface unevenness of the surface to be measured 20. Accordingly a fine surface unevevenness of a steel plate is sensed as a focal error from the focus position and included in the surface unevenness signal. By a BPF 29 this signal is divergently emitted in a roughness signal 51 and waviness signal 52. This enables contactlessly measuring the surface roughness having a high feed speed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention applies non-contact surface roughness (Ra value) and waviness (Wca value) of surfaces used online in metal rolling and plating processes by applying optical technology and tracking servo technology. Regarding a measuring device.

[0002]

2. Description of the Related Art It is desired that a non-contact method is used to measure the surface roughness and unevenness of a steel sheet or the like used on-line during a metal rolling or plating process. An optical surface roughness meter using light is known as a method capable of measuring the surface in a non-contact manner. For this optical surface roughness meter, roughly divided into two methods are known. One is a device that utilizes the scattering of light due to the unevenness of the surface, and the other is a device that applies the focus error detection optical system that is used in the pickup of an optical disk device.

A method utilizing light scattering is described in Reference Document 1 (C
AMP-ISIJ Vol. 5 (1992) 520), the photodetectors are arranged in an array of the relationship between the reflected light whose surface is considered to be a mirror surface and the angle and intensity of the light scattered at an angle from this reflected light. Since the output of is calculated based on a certain relational expression, it takes some time to collect and calculate the data. Further, it is well known that the surface roughness meter of this type is necessarily large in scale. Therefore, this method is not suitable for simply measuring the surface roughness. Further, with this method, it is difficult to measure waviness with a long period among the surface irregularities.

On the other hand, the one using the focus error detecting optical system uses an optical system and a servo having the same structure as the optical pickup of the optical disk device. For example, the method disclosed in Reference Document 2 (Sensor Technology, Special Issue, 1992, p. 114) applies a servo to keep a constant distance between a solid surface and an objective lens, and the objective lens corresponding to the unevenness of the surface. This is a method of measuring the change in position of the device using a differential transformer. Here, this conventional method will be described in detail.

As shown in FIG. 11, the optical pickup 5 is
It comprises a semiconductor laser 1, an objective lens 4, an objective lens actuator 3, and an optical system 2 which separates reflected light and emitted light and includes focus error detection. Objective lens actuator 3
Is a device for converting a servo electric signal into a mechanical movement of the objective lens 4. In FIG. 11, the light emitted from the semiconductor laser 1 passes through the optical system 2 and is condensed by the objective lens 4 at one point with space. Further, in FIG. 12, light is reflected by a surface to be measured (a solid surface such as a steel plate in the present invention), enters the objective lens 4 again, is reflected by the beam splitter 6, and is a lens 7 which is an optical element for detecting a focus error. Then, the light beam is split into two light beams through the prism 8 and is detected by the two-split photodetector 9.

The photodetector outputs 61a and 61b become the focus error signal 12 for servo in the focus error operational amplifier 11. An example of the output characteristic is shown in FIG. In FIG. 13, the horizontal axis is the focus error and the vertical axis is the electrical output (voltage).
In the example shown in FIG. 13, the measurable range is from −10 μm to +10 μm. For example, if the output voltage is +10 μm, 10
If the noise voltage at that time is 10 mV, SN
The ratio is 1000, and the surface unevenness is 100 μm of 10 μm.
It is possible to measure up to 0.01 μm, which is 1/0. The frequency characteristic of the focus error operational amplifier 11 in the prior art is set so as to satisfy the frequency band required for servo, so that it is only up to a high frequency range of several 100 KHz (see the case of c in FIG. 2). This bandwidth is mainly determined by the bandwidth of the operational amplifier used in the focus error operational amplifier 11. Since wide band operational amplifiers are expensive, there was no reason to use unnecessarily wide band operational amplifiers.
Further, since it is not desirable to amplify to an unnecessary high frequency region from the viewpoint of noise, the band is often limited by a circuit even if an operational amplifier having a wide frequency band is used.

When the output of the focus error operational amplifier 11 is used as an error signal to configure a servo according to the servo block diagram of FIG.
If the servo focus error signal 12 is not present at the focus position and is not zero, it is amplified according to the servo block diagram of FIG. 14, an electric signal is given to the objective lens actuator 3, and the objective lens 4 is supplied until the error becomes zero. To move. Therefore, the focus position of the objective lens 4 is always on the surface to be measured as shown in FIG. 15, and the objective lens 4 is at a constant distance from the surface to be measured, and its position change or movement corresponds to the surface unevenness. By measuring the positional change of the lens 4, the surface irregularities can be measured. By appropriately processing the signal of the surface irregularities, the values of the surface roughness and waviness defined by JIS can be obtained.

When the servo is operating, the focus position is always on the surface to be measured, and the servo focus error signal is always zero. To measure the objective lens position, the actual objective lens position may be read by the differential transformer 22, or it may be added to a mechanism or device for physically moving the objective lens as described in JP-A-59-27207. You may measure a current value and a voltage value. The read movement amount of the objective lens is output by the signal processing device 25 as a value such as an Ra value defined as the roughness.

[0009]

However, in the conventional method described here, the feed rate is several millimeters / second (reference document 2), and in the rolling process, for example, 1500 m / min. Could not be used as is for online measurement of such high speed objects. The reason is that the focus position of light accurately follows the surface irregularities of the surface to be measured, or the movement of the surface irregularities of the surface to be measured is made to correspond to the movement of the objective lens. Measurement is possible only within the range. Therefore, the feed rate of the steel sheet or the like is limited. Since the feed rate is higher than this limit in the online process such as metal rolling and plating process, the surface roughness meter to which the focus error detection optical system is applied cannot be used in the conventional technique. Therefore, in these applications, the method utilizing the scattering of light has been adopted, but there is a problem that the apparatus becomes large in scale as described above.

Therefore, the present invention provides a surface roughness meter capable of dealing with a surface to be measured having a high feed rate so that the surface roughness and unevenness used on-line in metal rolling and plating processes can be measured in a non-contact manner. It is provided.

[0011]

Since the optical surface roughness meter of the present invention is an on-line device, it is a non-contact optical type, and the servo is operated, but the focus position of the objective lens is uneven on the surface. The servo band is set so as to have a constant distance with respect to the average level, and the deviation from the average due to the unevenness of the surface to be measured is detected as a high frequency component of the focus error sensor to measure the unevenness of the surface.

[0012]

[Function] In the rolling process and the like, the steel plate is, for example, 1500 m /
If the unevenness on the surface repeats peaks and valleys at intervals of 30 μm, for example, the unevenness of the surface is 800,000 times when 25 m / sec is divided by 30 μm. / Sec, that is, a frequency of 0.8 MHz. In addition, a waviness with a longer cycle than that of the surface of the steel plate is, for example, 50.
If it is mm, the signal component of the swell is 5 m at 25 m / sec.
It becomes 5 KHz divided by 0 mm. On the other hand, if it is considered that the surface runout due to the movement of the steel sheet depending on the distance between the rolling rolls is a node of the surface runout at the roller portion such as the pinch roller, it may be assumed that it is at most several tens Hz. Here, for example, if the roll interval is 5 m, the wavelength is doubled to 10 m, and the frequency is 2.5 Hz. here,
We used the relationship that the product of wavelength and frequency equals velocity.

There is no difference between the block diagram shown as the prior art in FIG. 14 and the present invention shown in FIG. 1 in the servo loop itself, and the output of the photodetector is in the high frequency band (where the surface roughness signal can be obtained). (In the example, from 500 Hz, which is one tenth of 5 KHz, to 8 MHz, which is ten times 0.8 MHz)
Using a calculator capable of calculating the focus error, the output signal of this calculator is processed into a roughness signal and a swell signal. The servo mechanically moves the objective lens so that the amount of error becomes 1 / gain, but the error remaining even after the servo is operated is known as a steady deviation.

FIG. 3 (a) shows the vibration of the steel sheet, and FIG. 3 (b) shows the change over time of the vibration as the horizontal axis distance, and at the same time, shows the movement of the servo-driven objective lens by the broken line. It shows with. By the way, according to the technology of the optical disk device,
The band of the focus servo of the objective lens is approximately several KHz, and it is assumed here that it is 3 KHz (case a in FIG. 2). Then, a gain of 60 dB or more is usually obtained in the low frequency range, and a steady-state deviation of 1 μm or less can be expected by following a surface deflection of about 1 mm. Below servo gain is 60d
In the case of B or more, it is assumed that there is no steady deviation, but the effect will be described finally. Here, FIG. 2 shows the frequency characteristics of each part of the servo system, where a is the servo gain frequency characteristics, b is the gain frequency characteristics of the wide band focus error operational amplifier of the present invention, and c is the focus error of the prior art. The gain frequency characteristic of the arithmetic unit, d is the bandpass filter frequency characteristic for separating the roughness signal and the swell signal in the embodiment of the present invention, and e is each signal (A: surface deviation of steel plate, B: steel plate surface deviation). It shows the frequency distribution of undulation and C: roughness, respectively.

In the case of e in FIG. 2, fine irregularities on the surface having a frequency spectrum represented by C are in the frequency region where the servo gain is 0 dB or less, so the servo does not work and the objective lens does not respond. That is, the objective lens can follow the surface runout due to the vibration of the steel plate,
It cannot follow the fine unevenness of the steel plate. In the example of FIG. 2, there is a frequency region where the servo gain is about 10 dB against the waviness of the steel plate, and the effect of the servo should not be ignored at all, but the servo gain is 0 dB in the main frequency distribution of the waviness, It is assumed that the servo has no effect and the movement of the objective lens cannot follow the waviness of the steel plate.

The state described above is shown in FIG. Figure 3
In (a), fine unevenness is repeated at a cycle of 30 μm, and 50
A model of the surface of a steel plate that undulates in mm cycles is shown, but it is drawn that the movement of the objective lens shown by the broken line does not respond to undulations and fine irregularities. The objective lens is always at a position apart from the average level of surface irregularities by a constant distance (working distance WD). This average level is the focal position of the light emitted from the objective lens, and the difference between the distance of the average level and the reflecting surface of the light having fine irregularities is detected as a focus error. In the prior art, the servo was applied so that the focus is always on the surface to be measured as described above, but in the present invention, the virtual position of the average level of the surface waviness and fine irregularities is the focus position.

Since the photodetector normally responds up to a frequency of several tens of MHz, the roughness signal of the surface to be measured can be obtained by error-calculating and amplifying the output of the photodetector up to a high frequency. For example, a specific example will be described. As mentioned in the first assumption, the roughness signal (related to the Ra value) is 0.
Since the component is centered at 8 MHz, the roughness signal can be detected by considering the phase change and having a band from 80 KHz to 8 MHz. In addition, the waviness of the steel plate surface (Wca
Since it is centered on 5 KHz to detect (related to value),
Similarly, the band may be set to 500 Hz to 50 KHz.
Therefore, the focus error operational amplifier 17 may be an operational amplifier that operates up to a high frequency range. However, in the present invention, since it is necessary to configure a servo, the frequency characteristic of the focus error operational amplifier 17 also needs a direct current component, and a wide band from direct current to 8 MHz is used as in the case of b in FIG.

In the prior art, the frequency characteristic of the focus error operational amplifier 11 is at most 1 as shown in the case of c in FIG.
It was 00 KHz. Since the 8MHz band is unnecessary for the servo system, the high frequency cut filter 30 is 100KHz.
The servo system is configured as shown in FIG. The servo system configuration such as the phase compensation method is the same as that of the conventional technique. The output of the focus error calculation device 27 has a bandwidth of 80 KHz to 8 MHz and 500 Hz to 50 Hz.
A bandpass filter 29 for dividing into a KHz band is connected, and respective filter outputs 51 and 52 become a roughness signal and a swell signal.

Although the steady-state deviation has been described as zero, it is not actually zero. The above-mentioned steady-state deviation is 1 μm or less only in the frequency component of 100 Hz or less where the servo gain is sufficiently high, and since the gain is small for frequencies higher than this, tracking may be insufficient and the error may be large. In the case of e in FIG. 2, A is the vibration spectrum of the steel plate, and the fundamental wave of 2.5 Hz and its harmonics are several tens Hz.
However, if the frequency exceeds 100 Hz, the vibration component of the steel sheet becomes extremely small, and it is a negligible amount including the steady deviation. At higher frequencies, the vibration spectrum of the steel sheet does not exist, but only the signal to be detected. The non-zero steady-state deviation occurs because the servo gain is finite, and there is no problem if this amount is always constant. If the stationary deviation has a frequency, it should be the frequency characteristic of surface wobbling, so it should be cut by the filter and not included in the roughness signal or undulation signal. There is no particular problem except for the relationship.

In the above example, the steel plate is, for example, 1500 m /
It is assumed that the movement is made in minutes, but in another example, it may be 150 m / sec. Assuming the other assumptions are the same, the surface irregularities have a frequency of 80,000 times / sec, that is, 80 KHz.
The swell signal component is 0.5 KHz. The surface runout associated with the movement of the steel plate is 0.25 Hz. In this case, there is a large overlap between the servo band and the band of the swell signal, so it may be necessary to correct the swell signal. The low-frequency portion of the swell signal is at a position where the servo gain is sufficiently high at 50 Hz, which is one-tenth of 500 Hz, and the signal near this frequency decreases due to the action of the servo. Therefore, it is necessary to correct the amplitude of the swell signal having a frequency characteristic opposite to the servo gain frequency characteristic. In FIG. 4, “f” shows the amplification frequency characteristic for the correction.

[0021]

EXAMPLE An example of the optical surface roughness meter of the present invention will be described below. Although FIG. 1 shows a configuration example according to one embodiment, a conventional optical system is used. In that case, the two-divided photodetector 9 has a sufficiently wide frequency band. The outputs 61a and 61b are guided to the wide band focus error operational amplifier 17, and output a DC to high frequency region signal including the servo focus error signal signal 12 and the surface unevenness signal. The surface unevenness signal is a signal including a roughness signal and a waviness signal. The output of the wide band focus error calculation / amplification device 27 has its high frequency cut by the high-frequency cut filter 30 and is converted into the servo focus error signal 12. However, the high frequency cut filter may be included in the phase compensator 22.

Further amplified by the phase compensator 22 and the power amplifier 23, the objective lens actuator 3 is driven,
The focus position of the objective lens 4 is made to follow the average level of the surface unevenness of the steel plate. Therefore, the fine surface unevenness of the steel sheet is detected as a focus error with the focus position and included in the surface unevenness signal. The output signal from the wide band focus error calculation / amplification device 27 is branched, and the bandpass filter 29 outputs the roughness signal 5
1 and the swell signal 52 can be obtained.

FIG. 5 shows a configuration example according to another embodiment of the present invention, which is a wide band focus error operational amplifier 27 of FIG.
And the bandpass filter 29 between the linearity compensator 2
8 is inserted. The usefulness of the linearity compensator 28 will be described below. FIG. 7 shows the linearity of focus error detection and the distortion of detection due to the offset caused by the steady deviation. The cause of the offset is not only the steady-state deviation but also the electrical offset of the operational amplifier due to a temperature change or the like. FIG. 7A shows the detected distortion when there is no offset, and FIG. 7B shows the detected distortion when there is only e offset. This distortion occurs when the linearity of the focus error detection signal is not good.

7a shows the actual surface, b shows the output characteristics of the detection, and c in FIG.
And d are detected waveforms. If the offset is constant, there is no problem in detecting the roughness, but if the offset changes with time, the absolute value of the output or the like becomes uncertain. In order to reduce this uncertainty, a linearity compensator 28 is required as shown in FIG. The linearity compensator 28 has a relationship between the input voltage and the amplification degree as shown in FIG.
As shown in FIG.
As shown in (c), it is converted into a focus error signal with rich linearity. Therefore, with the configuration of FIG. 5, a reliable roughness signal can be obtained even if there is some offset due to a steady deviation or the like.

The emitted light of the semiconductor laser is kept at a constant light intensity by the semiconductor laser driving device 26. However, since the light reflectance changes locally on steel plates and the like, it is necessary to determine whether the change in the signal due to the reflectance change or the focus error is actually large simply by making the semiconductor laser light intensity constant. I can't. In the conventional method, since the servo is configured to make the error zero, even if the reflected light intensity fluctuates to some extent, no change is brought to the measurement result, and conventionally, there are many driving methods in which the output of the semiconductor laser 1 is constant. It was In the present invention, the absolute value of the error signal cannot be trusted unless the reflected light has a constant intensity. Therefore, in the present invention, it is necessary to keep the light intensity constant in a form that also incorporates a local change in the light reflectance. For that purpose, it is necessary to adopt a semiconductor laser driving method for detecting the reflected light intensity by providing another optical detector 10 (FIG. 8) in the optical pickup in addition to the two-divided light detection 9 and making the reflected light intensity constant. is there.

In FIG. 8, the light reflected by the surface to be measured of the steel plate 20 is partially reflected by the half mirror 13 placed in the optical system and detected by the photodetector 10. Light detector 10
Output is proportional to the intensity of the reflected light from the surface to be measured, so that the reflected light output is amplified by the photodetector output amplifying device 31 and fed back to the semiconductor laser driving device 26. Further, in FIG. 8, the optical system is simplified by using the addition signal of the two two-divided detectors 9 for monitoring the intensity of the reflected light from the surface to be measured. The output of the two-divided detector 9 is once passed through a buffer 19 and divided into an adder and a subtractor. The buffer 19 is passed through to compensate that the wide band focus error operational amplifier 17 and the wide band addition operational amplifier 18 operate independently.

In order to keep the light intensity of the semiconductor laser constant, it may not be enough to adjust the amount of current applied to the semiconductor laser. It is well known that the intensity of emitted light of a semiconductor laser greatly changes when there is returned light, and it causes light intensity noise. Since the frequency component of the noise of the emitted light spreads to 10 MHz or more, the frequency region overlaps the roughness signal, which is extremely harmful. Therefore, it is necessary to devise to reduce the return light noise.

To reduce the return light, there is a well-known method of combining a polarization beam splitter and a quarter-wave plate. FIG. 10 shows an example in which the configuration of FIG. 9 and the quarter wavelength plate 14 and the polarization beam splitter 15 are used in combination in order to reduce the return light to the semiconductor laser 1. In the configuration of FIG. 10, the noise of the semiconductor laser 1 is reduced, so that the measurement can be performed with higher accuracy. It is not necessary to use the semiconductor laser 1 as the optical pickup, and an optical pickup using another laser may be used as long as the size is not limited. Also, the focus error detection optical system is not limited to the method shown in FIG. 11, and many other methods are known, and which of these known methods is to be selected is a matter of design technical scope. It does not limit the invention. For example, it is expected that a high performance online surface roughness meter will be obtained by combining the optical system described in JP-B-4-7804 with the present invention.

The frequency band of the wide band focus error operational amplifier 27 described in the above assumption is an example, and there is no reason to limit the band width from DC to 8 MHz. Since the target roughness of assembling and the line feed speed of the steel plate differ depending on the type of the steel plate, the wide band focus error operational amplifier 27 and the band pass filter 29 corresponding to the respective purposes are provided.
Must be designed. For example, if the fine cycle of the surface irregularities is 3 μm, which is one-tenth of the explanation example, the frequency band of the wide band focus error operational amplifier 27 must be 80 MHz. Although the bandwidth of the signal is 10 times that of the signal in the example of the present invention, the magnification may be lower or higher depending on the uneven distribution.

[0030]

As described above, according to the present invention,
Since the surface roughness can be measured in a non-contact manner with a simple device using an optical pickup, an inexpensive optical surface roughness meter can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example according to an embodiment of an optical surface roughness meter of the present invention.

FIG. 2 is a diagram showing an example of frequency characteristics of each part of a servo system.

3A and 3B schematically show the movements of a measurement surface and an objective lens in the present invention, where FIG. 3A shows the surface runout of a steel plate, and FIG. 3B shows how the objective lens follows the surface runout of a steel plate. Further, (c) is a diagram showing a state in which the objective lens does not follow the waviness and fine surface irregularities of the steel sheet.

FIG. 4 is a diagram showing an example of gain-frequency characteristics of a swell signal bandpass filter.

FIG. 5 is a diagram showing a configuration example of an optical surface roughness meter of the present invention.

6A and 6B show characteristics of a linearity compensator, FIG. 6A shows a focus error signal, FIG. 6B shows an input voltage / output voltage characteristic of the linearity compensator, and FIG. 6C shows a corrected focus error signal. It is a figure.

FIG. 7 is a diagram showing detection distortion due to an offset in a focus error signal.

FIG. 8 is a diagram showing a configuration example according to another embodiment of the optical surface roughness meter of the present invention.

FIG. 9 is a diagram showing another configuration example of the present invention.

FIG. 10 is a diagram showing another configuration example of the present invention.

FIG. 11 is a diagram showing an example of a conventional optical pickup.

FIG. 12 is a diagram showing an example of an optical system that separates reflected light and emitted light and includes focus error detection.

FIG. 13 is a diagram showing a relationship between a focus error and a voltage output in focus error detection.

FIG. 14 is a configuration diagram of a servo in a conventional method.

FIG. 15 is a diagram schematically showing the movements of the measurement surface and the objective lens in the prior art.

[Explanation of symbols]

1 semiconductor laser 2 optical system that separates reflected light and emitted light and includes focus error detection 3 objective lens actuator 4 objective lens 5 optical pickup 6 beam splitter 7 lens 8 prism 9 2 split photodetector 10 photodetector 11 focus error calculation Amplifier 12 Servo focus error signal 13 Half mirror 14 Quarter wave plate 15 Polarizing beam splitter 17 Wide band focus error operational amplifier 18 Wide band addition operational amplifier 19 High frequency buffer 20 Measured surface such as steel plate 21 Focus error operation device 22 Phase compensation Device 23 Power amplification device 24 Working transformer 25 Signal processing device 26 Semiconductor laser drive device 27 Wide band focus error calculation device 28 Linear compensation device 29 Band pass filter 30 High frequency cut filter 31 Photodetector output amplifier 42 Rolling roll 51 Roughness signal 52 waviness signals 61a, 61b photodetector output

Claims (4)

[Claims] 1. A laser light source, an objective lens for irradiating and condensing light emitted from the laser light source onto a measurement surface to be measured, an objective lens actuator for driving the objective lens, and a surface to be measured. An optical system that captures the reflected light and separates the light emitted from the laser light source to detect an error in the distance between the surface to be measured and the focus of the light, a photodetector, the focus position of the objective lens, and the In an optical surface roughness meter, which comprises a servo system that makes the difference in distance from the surface to be measured zero, the frequency band of the device that calculates the photodetector output as an error signal is the frequency of the servo system. A wide band focus error calculation amplification device capable of calculating up to a frequency band higher than the band, the output of the wide band focus error calculation amplification device is used as a servo focus error signal, and the output is further branched to obtain a table. Optical surface roughness meter is characterized in that connected to the band-pass filter which outputs the unevenness signal. 2. The optical surface roughness meter according to claim 1, further comprising a linearity compensator for linearizing an output of a device for calculating a focus error signal from the photodetector output. Optical surface roughness meter. 3. The optical surface roughness meter according to claim 1, wherein a quarter-wave plate and a polarization beam splitter are arranged in combination in the optical system. 4. A photodetector for detecting reflected light, an amplification device for the photodetector, and a light source driving device having the output of the amplification device for feeding back the intensity of light reflected and returned to be constant. The optical surface roughness meter according to claim 1, wherein the optical surface roughness meter is provided.