JP4453920B2 - Touch probe contact detection method and apparatus - Google Patents

Description

  The present invention relates to a touch probe contact detection method and apparatus for detecting contact between a stylus part and a measurement object in a three-dimensional measuring instrument, an ultra-precise displacement measuring instrument, and a shape measuring instrument used in the fields of precision and ultra-precision measurement. .

  Conventionally, touch probes are used in contact displacement measuring instruments. As a touch probe contact detection method, for example, there is a method in which a displacement caused by contact between a measurement object and a probe tip is measured, or a contact is mechanically opened and closed by the displacement. However, in these cases, since the detection resolution and the force required for opening and closing the contact, that is, the measurement pressure, are related to each other, the detection to lower the measurement pressure to reduce the damage to the measurement object is not possible. There is a problem that if the measurement pressure is increased in order to avoid the problem, the measurement object is damaged. In addition, a set of detection mechanisms for each measurement direction is often required, which increases the probe shape. Such disadvantages significantly affect the measurement results as the object to be measured becomes smaller. Further, in principle, the stylus part is designed so as to be in contact with the measurement surface of the measurement object perpendicularly, and there is a problem that when the inclination of the measurement surface increases, a measurement error occurs or measurement becomes impossible.

  Therefore, in Patent Document 1, etc., the stylus of the touch probe is vibrated and the vibration waveform is detected, and the contact is made by detecting the change in the vibration waveform when the stylus contacts the object to be measured. Judging. The change in the vibration waveform captures the change in the resonance frequency and the change in the amplitude. Further, in Patent Document 2, the stylus is vibrated at the natural frequency of the stylus and the amplitude is increased to increase the sensitivity, which is different from the natural frequency (basic frequency and its harmonic components) of the housing. This reduces the influence of disturbance.

On the other hand, in Patent Document 3, an alternating voltage is applied between the tip of the stylus and the measured side, and the resulting second harmonic vibration of the stylus is detected to detect surface potential, charge, dielectric constant, etc. A non-contact electric force probe microscope operating method has been proposed. This pre-measures the relationship between the distance between the sample and the probe and the second harmonic vibration amplitude of the leaf spring, and thereby the second harmonic vibration of the leaf spring that becomes the appropriate distance between the sample and the probe. After setting the amplitude setting value and bringing the probe close to the measurement point on the sample and reaching this setting value, after collecting several types of electrical information, the probe is moved away from the sample and the next sample on the sample is collected. It moves to a measurement point, collects electrical information by the same operation, and measures the shape of the sample.
Japanese Patent Laid-Open No. 48-60653 Japanese Patent No. 2625364 JP-A-8-220110

  However, in Patent Documents 1 and 2, it is necessary to increase the contact pressure in order to increase the sensitivity of the amplitude change. In addition, since the amplitude changes depending on the contact angle between the stylus and the object to be measured, the sensitivity decreases when the contact angle decreases, and the problem that the amplitude change cannot be stably captured over a large angle range still remains. Remain. In order to reduce the influence of noise and increase sensitivity, it is better to set the excitation frequency to the natural frequency of the probe. However, it becomes difficult to reduce the contact pressure at the natural frequency, so it is difficult to reduce the contact pressure. There was a problem that it was difficult to set arbitrarily.

  Patent Document 3 measures the value of electrical harmonics generated between the object to be measured and the probe in a non-contact manner. The distance between the object to be measured and the probe and the harmonics are measured. Therefore, there is a problem that adjustment is very troublesome and lacks in convenience as in a contact type.

  In view of such conventional problems, the problem of the present invention is not to cause harmful damage even when the measurement pressure is low and the measurement target is an optical surface. To provide a touch probe contact detection method and apparatus capable of reliably detecting contact between a stylus part and a measurement object even when tilted at an angle.

  In the present invention, there is provided a touch probe contact detection method comprising a stylus portion that is sine-wave vibrated at a constant excitation frequency (f), and a sensor unit that measures and outputs a vibration waveform of the stylus portion. When the vibration waveform output by the sensor unit is converted into a frequency distribution, and a value of a predetermined harmonic (n × f) that is an integer (n) times the excitation frequency is equal to or greater than a predetermined threshold value, The above-described problem has been solved by providing a touch probe contact detection method for determining that the touch probe has come into contact with an object to be measured.

  That is, the frequency characteristic of the stylus that vibrates with a correct sine wave having the frequency f is a line spectrum having only the frequency component f. When the stylus comes into contact with the measurement object (object to be measured), a significant harmonic component n × f appears in the frequency characteristics before a noticeable change in amplitude appears. The present invention utilizes this property. In the conventional device that detects changes in amplitude, the amount of change in amplitude at the time of contact starts from zero and gradually increases. Therefore, the degree of amplitude change to be detected is gradual and a detection error is likely to occur. However, in the present invention, since the occurrence of vibrations with completely different frequencies, ie, the generation of harmonics, is detected, it is easy to detect and there are few errors. Further, since the restriction on the contact angle between the object to be measured and the stylus is loose, the detection sensitivity can be maintained even on an inclined surface. In order to avoid the influence of noise or the like, a predetermined threshold is provided to detect the occurrence of vibration.

  In the invention described in claim 2, the predetermined harmonic is a harmonic (2 × f) that is twice the excitation frequency (f). This is because the amplitude of the double harmonic becomes the largest. Furthermore, in the invention described in claim 3, the excitation frequency (f) is different from the natural frequency of the stylus part. This is because when the frequency is the same as the natural frequency, the amplitude of the stylus increases and the sensitivity increases, but the contact pressure increases and harmonics are easily generated as disturbance. Further, since the excitation frequency can be arbitrarily selected without being limited to the natural frequency, a desired measurement pressure can be obtained.

  Such a touch probe detection method can be implemented by the following apparatus. That is, in the invention described in claim 4, the stylus that can contact the object to be measured, the vibrator that vibrates the stylus at a constant excitation frequency (f), and the vibration waveform of the stylus part. A touch probe contact detection device comprising: a frequency analyzer for obtaining a frequency distribution from a vibration waveform output by the sensor unit; and an integer (n) of excitation frequencies of the frequency analysis. Touch probe contact detection comprising: an output device that outputs a predetermined double harmonic (n × f); and a comparator that outputs an ON-OFF signal by comparing an output from the output device with a predetermined threshold value Providing equipment.

  In a fifth aspect of the invention, the touch probe contact detection device may be configured such that the predetermined harmonic is a harmonic (2 × f) that is twice the excitation frequency (f). Furthermore, in the invention described in claim 6, it is preferable that the excitation frequency (f) is a touch probe contact detection device in which the natural frequency of the stylus part is different.

  In the present invention, since the generation of harmonics is detected to determine the contact of the stylus with the object to be measured, the sensitivity is improved. In addition, since the detection sensitivity can be maintained even on an inclined surface, it is possible to measure almost the entire area of the hemisphere, and it is possible to measure a curved portion that is small, such as a microlens. Furthermore, since the generation of harmonics is detected by the threshold value, the influence of noise is small, and contact can be reliably detected.

  In the inventions according to claims 2 and 5, the sensitivity is improved by setting the harmonic to a harmonic (2 × f) that is twice the excitation frequency (f). Further, in the inventions according to claims 3 and 6, the excitation frequency (f) can be made different from the natural frequency of the stylus part and the contact pressure can be reduced, so that the surface to be measured can be less damaged. It became a thing.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a conceptual diagram of a system of a touch probe contact detection device according to an embodiment of the present invention. As shown in FIG. 1, in the touch probe contact detection device 1 of the present invention, a leaf spring 4 is fixed perpendicularly to a moving body 3 that can be moved back and forth with respect to an object 2 to be measured. The plate surfaces 4a and 4b of the plate spring 4 are directed in the moving direction of the moving body, and the plate spring is attached so as to vibrate in the moving direction of the moving body. A rod-like probe (stylus) 5 having a sharpened tip 5a is fixed perpendicularly to a plate surface 4b on the measured object 2 side of a plate spring to form a so-called cantilever 4 '. A piezoelectric element (vibrator) 6 is attached to the opposite side of the probe 5 with the leaf spring 4 interposed therebetween so that the displacement direction is perpendicular to the plate surface 4a of the leaf spring. Further, a sensor unit 7 that measures and outputs the vibration waveform of the leaf spring 4 is attached to the leaf surface 4 b of the leaf spring in the vicinity of the probe 5 or the piezoelectric element 6. A sine wave function generator (FFT analyzer or the like) 9 is connected to the piezoelectric element 6 via an amplifier 8, and a sine wave signal (excitation frequency (f)) given from the sine wave function generator is passed through the piezoelectric element. Thus, the probe 5 is configured to vibrate sinusoidally.

  An output signal from the sensor unit 7 is input to a personal computer (hereinafter referred to as “PC”) 12 via an amplifier 10 and an analog-digital converter (hereinafter referred to as “A / D converter”) 11. The output signal from the sensor unit 7 input to the PC is subjected to frequency analysis (Fourier transform) by the program, and the intensity of the harmonic (2 × f) twice the excitation frequency f is output by frequency analysis. An ON-OFF signal is internally output in comparison with a set threshold value. Instead of the PC, a frequency analyzer that converts the vibration waveform output by the sensor unit into a frequency distribution, and an output that outputs the intensity of a predetermined harmonic (2 × f) that is twice the excitation frequency of the frequency analysis. It goes without saying that a comparator and a comparator that outputs an ON-OFF signal by comparing the output from the output device with a predetermined threshold value may be provided.

  The moving table 3 moves on the guide surface 20a of the base 20, and a moving table moving device 21 is attached between the base and the moving table. In addition, a linear scale 22 is attached between the moving table 3 and the base 20 so that the distance between the moving table and the base can be measured. The moving device 21 is made into a piezoelectric element, and a voltage is applied from a PC through a digital / analog converter (hereinafter referred to as “D / A converter”) 23 and an amplifier 24 to enable displacement of several μm to several hundred μm. Yes. Further, an output signal from the linear scale 22 is fed back to the PC via an amplifier 25 and an A / D converter 26, and feedback control using the linear scale of the mobile device is performed in the PC.

  For example, the method of using such a touch probe contact detection device is as follows. The sine wave signal output from the sine wave function generator 9 is input to the vibrator 6 through the amplifier 8 to vibrate the leaf spring 4 in a cantilever manner. The vibration of the leaf spring 4 is read by the sensor unit 7 and taken into the PC 12 via the amplifier 10 and the A / D converter 11, and the frequency analysis is performed in the PC. The value of the second harmonic of the excitation frequency f is a predetermined value. Monitor if it is above threshold. On the other hand, a step signal is output from the PC 12, applied to the moving device (piezoelectric element) 21 through the D / A converter 23 and the amplifier 24, and the moving table 3 is brought closer to the measured object 2 side. At the same time, the displacement of the moving table 3 is read by the linear scale 22 and fed back to the PC via the amplifier 25 and the A / D converter 26. When the value of the second harmonic of the excitation frequency f becomes a predetermined threshold value or more, it is determined that the probe 5 is in contact with the DUT 2, and the position is measured by reading the value of the linear scale 22 at that time. Do.

  In the present embodiment, harmonics when touched will be described in detail. 2 shows a frequency analysis result in a non-contact state when the excitation frequency of the vibrator 6 is set to 200 Hz using the above-described apparatus, and FIG. 3 shows a frequency analysis result in a contact state. As shown in FIG. 2, although there is noise over the entire frequency, a peak appears at a frequency of 200 Hz in a non-contact state, but there is no particular peak at other frequencies. In contrast, as shown in FIG. 3, it can be seen that in the contact state, there is a peak at a frequency of 200 Hz, and a small peak is generated at 400 Hz, which is a second harmonic of the excitation frequency. If there is no noise, it is only necessary to determine whether or not there is a peak at 400 Hz. However, since there is actually noise, it can be seen that it is preferable to determine a contact / non-contact by setting a threshold value.

  In addition, the same measurement was performed with a contact angle of 45 degrees in an oblique direction. FIG. 4 shows a frequency analysis result in a contact state at a contact angle of 45 degrees. As shown in FIG. 4, although the noise is slightly larger than in FIG. 3 described above, it can be seen that almost the same harmonics are generated at the time of contact. Thus, according to this invention, it turns out that the influence of a contact angle can also be made very small.

  As described above, according to the present embodiment, it is referred to as the generation of harmonics, not to determine contact / non-contact by measuring a change in a measurement value that is constantly generated, such as a displacement of the amplitude of a measurement waveform. Since contact / non-contact is determined based on qualitative changes, it can be seen that the determination with high accuracy with little influence of noise and less error can be made, and the contact angle can be covered in a wide range.

Next, the measurement pressure of the present invention will be described. In the vibration caused by the cantilever as described above, when the vibration amplitude a, the vibration frequency f, the vibration mass m, and the measurement pressure N are given,
N = a · (2πf) ² · m Therefore, the measurement pressure N can be adjusted by the vibration frequency f.
If the frequency is the natural frequency fn, the spring constant k of the leaf spring is
Since (2πfn) ² = k / m,
N = a · k.

  Thus, when the natural frequency is used, the measurement pressure is determined by the vibration amplitude and the spring constant of the leaf spring. As described in claims 3 and 6, in the present invention, the excitation frequency is not limited to the natural frequency, and it is possible to easily determine contact / non-contact even if any frequency is selected. Contact / non-contact can be determined at a lower measured pressure.

  In the embodiment of the present invention, an example using a leaf spring or the like has been described, but other vibration bodies and vibration directions may be used. Needless to say, various functions such as a function generator, a vibrator, and a measuring element can be used and applied.

It is a conceptual diagram of the system of the contact detection apparatus of the touch probe which shows embodiment of this invention. It is a frequency analysis result in a non-contact state when the excitation frequency of the vibrator is 200 Hz. It is a frequency analysis result in a contact state when the excitation frequency of the vibrator is 200 Hz. It is a frequency analysis result in a contact state when the excitation frequency of the vibrator is 200 Hz and the contact angle is 45 degrees.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Touch probe contact detection apparatus 2 Object to be measured 5 Touch probe 5a Touch probe 6 Vibrator 7 Sensor unit 12 PC, frequency analyzer, output device, comparator f Excitation frequency

Claims (6)

A touch probe contact detection method comprising: a stylus portion that is sine-wave vibrated at a constant excitation frequency (f); and a sensor unit that measures and outputs a vibration waveform of the stylus portion. The frequency distribution of the output vibration waveform is obtained, and the touch probe is measured when the value of a predetermined harmonic (n × f) that is an integer (n) times the excitation frequency exceeds a predetermined threshold value. A method for detecting contact of a touch probe, comprising: determining that the object has been touched. The touch detection method according to claim 1, wherein the predetermined harmonic is a harmonic (2 × f) that is twice the excitation frequency (f). The touch probe contact detection method according to claim 1, wherein the excitation frequency (f) is different from a natural frequency of the stylus part. A stylus capable of contacting the object to be measured; a vibrator that vibrates the stylus with a sine wave at a constant excitation frequency (f); and a sensor unit that measures and outputs a vibration waveform of the stylus part. A touch probe contact detection device comprising: a frequency analyzer for obtaining a frequency distribution of a vibration waveform output by the sensor unit; and a predetermined harmonic (n × f) that is an integer (n) times the excitation frequency of the frequency analysis. ), And a comparator that compares the output from the output device with a predetermined threshold value and outputs an ON-OFF signal. The touch detection device according to claim 4, wherein the predetermined harmonic is a harmonic (2 × f) that is twice the excitation frequency (f). 6. The touch probe contact detection device according to claim 4, wherein the excitation frequency (f) is different from the natural frequency of the stylus part.

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