JPH06323810A - Multiprobe displacement measuring apparatus - Google Patents

Abstract

PURPOSE:To allow simultaneous measurement of displacement for two measuring surfaces using an inexpensive and easily adjustable apparatus by distributing a laser light emitted from a semiconductor laser to two probes and then processing the returning interference lights individually. CONSTITUTION:The oscillation frequency of a semiconductor laser 12 is subjected to continuous modulation with a triangular modulation current Sa and distributed through an optical switch 26 to two probes 27A, 27B. The light (reference light) reflected on the end faces of the probes 27A, 27B has a frequency different from that of the light (object light) reflected on the measuring faces of objects 28A, 28B thus causing heterodyne interference. An interference signal is introduced through the optical switch 26 to a photodiode 30 in order to detect a signal Sc. The signal Sc is then subjected to time sharing at a multiplexer 32 to produce signals Se1, Se2, and BPFs 34A, 34B extract specific frequencies Sf1, Sf2 thus producing pulse signals Sg1, Sg2. The frequency differences between the signals Sg1, Sg2 and a pulse signal Sb' generated based on the modulation period of laser light are accumulated 44A, 44B thus measuring the moving distances of the objects 28A, 28B.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-probe displacement measuring device, and more particularly, it is possible to measure displacement of an object to be measured with high accuracy by using a semiconductor laser whose oscillation frequency can be linearly modulated by a driving current. The present invention relates to a heterodyne interferometric multi-probe displacement measuring device.

[0002]

2. Description of the Related Art Conventionally, a frequency modulation type heterodyne interferometric displacement measuring device has been known, for example, Japanese Patent Laid-Open No. 63-10.
1702 gazette. That is, this measuring device periodically changes the drive current applied to the semiconductor laser in a triangular wave shape to modulate the oscillation frequency and the emission intensity of the semiconductor laser. Then, this laser light is split into a reference light and an object light (light that is irradiated on the object to be measured) by a beam splitter, and the object light is emitted from the emission surface of the beam splitter to the measurement surface,
The reflected light is made incident again from the emission surface and is superimposed on the reference light. There is a time delay corresponding to the distance from the emission surface to the measurement surface between the object light and the reference light reflected by the measurement surface and returned, and their frequencies are different. Therefore, heterodyne interference occurs between the reference light and the object light, and the beat frequency thereof corresponds to the distance from the emission surface to the measurement surface. Therefore, the displacement of the measurement object is measured by counting the beat frequency.

However, since the above-mentioned conventional heterodyne interferometric displacement measuring device does not use an optical fiber in the optical path of the laser beam, it is easily affected by the environment and the size of the device becomes large due to the beam splitter, the mirror and the like. There's a problem. Further, since the measurement object is measured in a stationary state, the displacement of the moving measurement object cannot be measured in real time.

On the other hand, the applicant of the present invention has recently constructed a Fizeau-type interferometer using a common path as an optical path for most of the optical path by using an optical fiber and an optical fiber coupler to provide a compact device which is hardly affected by the measurement environment. In addition, we proposed a frequency modulation optical fiber displacement measuring device capable of measuring the displacement of a moving object in real time by utilizing the Doppler frequency shift due to the movement of the measurement surface (Japanese Patent Application No. 4-2114330). Description, Japanese Patent Application No. 4-
296057).

[0005]

By the way, in the case of the above displacement measuring device, the displacements of the two measuring surfaces cannot be measured at the same time. The present invention has been made in view of such circumstances, and a multi-device capable of simultaneously measuring the displacements of two measurement surfaces with one device and having a cost reduction and adjustment easier than the two devices. An object is to provide a probe displacement measuring device.

[0006]

In order to achieve the above object, the present invention comprises a semiconductor laser and a triangular wave generating means for inputting a triangular wave modulation current having a predetermined period T _s (frequency f _s ) to the semiconductor laser, An optical switch, a photoelectric conversion element, optical means for guiding laser light oscillated from the semiconductor laser to the optical switch, and guiding light returning via the optical switch to the photoelectric conversion element, and the optical switch. First and second optical fibers to which laser light is alternately applied via, and a distal end of the first optical fiber,
A first probe for reflecting a part of the light on the emission end face and transmitting the rest for parallel light or focusing to irradiate the first measurement surface, and the second probe is disposed at the tip of the second optical fiber,
A second probe for reflecting a part of the light on the emission end face and transmitting the rest for transmitting parallel light or condensing the second measurement surface and a signal from the photoelectric conversion element are input, and the input signal is input. And a switching control means for switching the optical switch and the multiplexer for each of the gradually increasing period and the gradually decreasing period of the triangular wave modulation current, respectively. And first and second frequency components having a predetermined bandwidth centered at a frequency nf _s that is n times the frequency f _s of the triangular wave, respectively, from the output signals of the first and second output terminals of the multiplexer. And a pulse signal having a frequency obtained by multiplying the frequencies of the output waveforms of the first and second bandpass filters by m times, respectively. And a pulse signal of a frequency obtained by multiplying the same frequency as the frequency nf _s by m times in synchronization with the cycle T _s of the triangular wave modulation current output from the triangular wave generating means. Output the third
Pulse output means and the first of the pulse signals output from the first and third pulse output means, respectively.
First counter for integrating the difference in frequency generated by the relative movement of the lens and the first measurement surface, and the second counter in the pulse signals respectively output from the second and third pulse output means. And a second counter that integrates a difference in frequency generated due to relative movement between the lens and the second measurement surface.

[0007]

According to the present invention, the oscillation frequency of the semiconductor laser is continuously modulated by the triangular wave modulation current, and the laser light emitted from the semiconductor laser is alternately distributed to the first and second probes via the optical switch. The interference light returning from the first and second probes via the optical switch is converted into an electric signal by the photoelectric conversion element, and this electric signal is passed through the multiplexer to the interference signal on the first probe side and the second signal. The interference signal on the probe side is processed separately. As a result, the displacement of two measurement surfaces can be simultaneously measured with one device, and the cost can be significantly reduced as compared with the two devices. Further, the wavelength of the semiconductor laser needs to be calibrated only once, and the measurement performance at the two locations can be made the same.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of a multiprobe displacement measuring device according to the present invention will be described in detail below with reference to the accompanying drawings. Figure 1
FIG. 2 is a block diagram showing an embodiment of a multi-probe displacement measuring device according to the present invention, and FIGS. 2 (A) to 2 (H) show signals output from the respective parts of FIG. 1 when a triangular wave modulation current is used. It is a waveform diagram.

As shown in FIG. 1, this multi-probe displacement measuring apparatus mainly includes a triangular wave generator 10, a semiconductor laser 12, a collimator lens 14, an objective lens 16, an optical fiber coupler 18, optical fibers 21 to 25, and an optical switch 26. , Probe 27A, probe 27B, photodiode 30, multiplexer 32, bandpass filter 34
A, 34B, waveform shapers 36, 38A, 38B, frequency multiplication circuits 40, 42A, 42B, gate circuits 43A, 4
3B and up / down counters 44A and 44B.

The triangular wave generator 10 has a triangular wave modulation current Sa (FIG. 2) having a period T _S (frequency f _s , angular frequency ω _s ).
(A)) is output to the semiconductor laser 12 and the waveform shaper 36. The waveform shaper 36 converts the triangular wave modulation current Sa input from the triangular wave generator 10 into a rectangular wave signal Sb (FIG. 2 (B)).
And outputs the signal Sb to the optical switch 26, the multiplexer 32, and the frequency multiplication circuit 40. The signal Sb is at a high level while the triangular wave modulation current Sa is gradually increasing, and is at a low level when the triangular wave modulating current Sa is gradually decreasing.

The oscillation frequency and the emission intensity of the semiconductor laser 12 are modulated by the input triangular wave modulation current Sa.
The semiconductor laser 12 has a heating and / or cooling element 13
A and a temperature sensor 13B are provided, and the temperature controller 13 controls the heating and / or cooling element 13A so that the temperature detected by the temperature sensor 13B becomes a constant value. Thereby, the wavelength change due to the temperature change of the semiconductor laser 12 is suppressed and the measurement accuracy is improved.

The modulated laser light output from the semiconductor laser 12 is focused on the tip of the optical fiber 21 via the collimator lens 14 and the objective lens 16. The laser light incident on the optical fiber 21 is guided to the optical switch 26 via the optical fiber coupler 18 and the optical fiber 22. The optical switch 26 optically connects either one of the optical fibers 23 and 24 to the optical fiber 22 and switches alternately according to the signal Sb from the waveform shaper 36. That is, when the signal Sb is at high level, the optical fibers 22 and 23 are connected to each other, and the signal Sb
Is low, the optical fibers 22 and 24 are connected.

Probes 27A and 27B are provided at the tips of the optical fibers 23 and 24, respectively. These probes 27A and 27B are for reflecting a part of the light on the emission end face and transmitting the rest for parallel light or focusing to irradiate the measurement surface, and are configured by, for example, a SELFOC lens.

Now, between the end face reflected light (reference light) of the probe 27A and the measurement face reflected light (object light) which is emitted from the probe 27A and reflected by the measurement face of the object 28A to be measured and then incident on the probe 27A again. Has a time delay corresponding to the distance D ₁ between the emission end surface of the probe 27A and the measurement surface of the measurement object 28A, and the frequencies of the two are different. Therefore, heterodyne interference occurs between the reference light and the object light.

Similarly, between the reference light of the probe 27B and the object light emitted from the probe 27B, reflected by the measurement surface of the measurement object 28B and incident on the probe 27B again, the emission end surface of the probe 27B and the measurement object are measured. There is a time delay corresponding to the distance D _{2 from} the measurement surface of 28B, and heterodyne interference occurs. The interference signals returned from the probes 27A and 27B via the optical fibers 23 and 24 are the optical switch 26, the optical fiber 22, and the optical fiber coupler 18.
And is guided through the optical fiber 25 and detected by a photodiode 30 installed at the tip of the optical fiber 25. FIG. 2C shows a waveform example of the signal Sc detected by the photodiode 30.

The signal Sc from the photodiode 30 is applied to the multiplexer 32. Multiplexer 32
Is added with a signal Sb from a waveform shaper 36, and the multiplexer 32 time-divides the signal Sc by this signal Sb in synchronization with the switching by the optical switch 26, and a signal corresponding to the interference light from the probe 27A. Se ₁ (Fig. 2
(D)) is output to the bandpass filter 34A, and the signal Se ₂ (FIG. 2) corresponding to the interference light from the probe 27B is output.
(E)) is output to the bandpass filter 34B.

The bandpass filters 34A and 34B are
The components of f _s are extracted from the input signals Se ₁ and Se _1, respectively, and the extracted signals Sf ₁ and Sf ₂ (see FIG.
(F), (G))) to the waveform shapers 38A and 38B. The waveform shapers 38A and 38B convert the input signals Sf ₁ and Sf ₂ into rectangular wave signals Sg ₁ and Sg _{2 respectively.}
(FIG. 2 (H), (I)), and the frequency multiplication circuit 42
A and 42B respectively multiply the frequencies of the signals Sg ₁ and Sg _{2 so} as to have a preset multiplication factor, and the multiplied signals (pulse signals) Sg ₁ ′ and Sg ₂ ′ are input to the gate circuits 43A and 43B. Output to ₁ (Figure 2 (K)
~ (M)).

On the other hand, to the input G ₂ of the gate circuits 43A and 43B, the signal Sb from the waveform shaper 36 is passed through the frequency multiplication circuit 40 to the frequency multiplication circuits 42A and 43B.
A signal (pulse signal) Sb '(FIG. 2 (J)) multiplied by the same magnification as is added. The gate circuit 43A is input and the pulse signal Sg ₁ 'of the input G ₁ G ₂ pulse signal S
A new pulse signal is generated according to the frequency difference of b ', and the up input U or the down input D of the up / down counter 44A is generated.
Output to. Accordingly, the up-down counter 44A will be integrating the difference frequency between the 'pulse signal Sg ₁ and' pulse signal Sb. Similarly, the gate circuit 43B is input and the pulse signal Sg ₂ 'inputs G ₁ G ₂ pulse signal S
A new pulse signal is generated according to the frequency difference of b ', and the up input U or the down input D of the up / down counter 44A is generated.
Output to. Accordingly, the up-down counter 44B will be integrating the difference between the frequency of 'a pulse signal Sg _2' pulse signal Sb and.

By the way, when the measurement object 28A is stopped, the frequency of the pulse signal Sg ₁ 'output from the frequency multiplication circuit 42A is the frequency of the reference pulse signal Sb' output from the frequency multiplication circuit 40. And the count value of the up / down counter 44A does not change,
When the measurement object 28A moves, the frequency of the signal Sg ₁ output from the waveform shaper 38A (the pulse signal Sg ₁ ′ output from the frequency multiplication circuit 42A) changes due to the Doppler frequency shift. That is, the measurement target 28A is the probe 2
When moving in the direction away from 7A, pulse signal S
The frequency of g ₁ ′ increases (see FIG. 2 (K)), and when the measurement object 28A moves toward the probe 27A, the frequency of the pulse signal Sg ₁ ′ decreases (FIG. 2).
(See (L)).

Therefore, the pulse signal Sb 'and the pulse signal S
The difference in the number of pulses g ₁ ′ corresponds to the moving speed and the moving direction of the measuring object 28A, and the integrated value of the difference in the number of pulses corresponds to the moving amount of the measuring object 28A. Accordingly, the moving distance of the measurement target 28A can be measured based on the increase / decrease amount of the count value of the up / down counter 44A when the measurement target 28A is moved from a certain position to another position.

Similarly, the moving distance of the measuring object 28B can be measured, but the measuring object 28B is the probe 27B.
The frequency of the pulse signal Sg ₂ ′ decreases as it moves away from the measurement target 28 (see FIG. 2 (M)).
When B moves toward the probe 27B, the frequency of the pulse signal Sg ₂ ′ increases, and the pulse signal Sg ₂ ′ increases.
The opposite is true for g ₁ ′. Therefore, in order to make the characteristics of the probes 27A and 27B (the changing direction of the up / down counter) the same, the up input U and the down input D of the up / down counters 44A and 44B to which the pulse signals from the gate circuits 43A and 43B are input are the same. It is the opposite.

Next, the signals in each of the above parts are expressed by the following equations. The signals Se ₁ (t) and Se ₂ (t) separated by the multiplexer 32 are expressed by the following equation: Se ₁ (t) = A · cos (ω _b1 t + φ _b1 ) ... (1) (0 ≦ t <T _S / 2) Se ₂ (t) = A · cos (ω _b2 t −φ _b2 ) ... (2) (T _S / 2 ≦ t <T _S ). Here, the angular frequencies ω _b1 and ω _b2 and the phases φ _b1 and φ _b2 of the beat signal in the equations (1) and (2) are respectively expressed by the following equation: ω _b1 = 4 (Δω / T _S ) · τ ₁ = 4 (Δω / T _S ) · 2π (2D ₁ / C) (3) ω _b2 = 4 (Δω / T _S ) · τ ₂ = 4 (Δω / T _S ) / 2π (2D ₂ / C) (4) φ _b1 = 2π (2D ₁ / λ ₀ ) ... (5) φ _b2 = 2π (2D ₂ / λ ₀ ) ... (6) In Expressions (3) to (6), Δω is the angular frequency modulation width as shown in FIG. 3, τ ₁ is the time delay between the reference light of the probe 27A and the object light, and τ ₂ is the probe 27B.
Is the time delay between the reference light and the object light, and ω ₀ and λ ₀ are the angular frequency and wavelength of the semiconductor laser 12 when driven by the bias current i ₀ .

The bandpass filters 34A and 34A are also provided.
Signal Sf ₁ output from the B (t) and Sf ₂ (t) _{is, Sf 1 (t) = B} · cos (ω S t + φ b1) ... (7) Sf 2 (t) = B · cos (ω _S t −φ _b2 ) ... (8)

As is clear from the equations (7) and (8), the signs of the phase φ _b2 and the phase φ _b1 are opposite. In addition, Sf ₁ (t),
By measuring the phase changes Δφ _b1 and Δφ _b2 of Sf ₂ (t), the displacements ΔD ₁ and ΔD _{2 of the} measurement surface can be obtained. In this embodiment, the output signal of the photodiode 30 is supplied to the multiplexer 3
The two signals Se ₁ and Se ₂ are distributed according to ₂ , but the present invention is not limited to this, and an optical switch is separately provided in the optical fiber 25, and it is distributed to two interference lights at the stage of the interference light, and thus distributed. Each interference light may be photoelectrically converted by two photodiodes.

[0025]

As described above, according to the multi-probe displacement measuring device according to the present invention, one device can simultaneously perform two measurements.
It is possible to measure the displacement of one measuring surface and 2
The cost can be significantly reduced compared to a single device. Further, there is an advantage that the wavelength of the semiconductor laser needs to be calibrated only once, and the adjustment becomes easy.

Furthermore, since most of the optical path of the laser beam is composed of the optical fiber and the optical fiber coupler, it is possible to measure a large displacement with high accuracy and high resolution, which is hardly affected by the measurement environment, and to make the apparatus compact. be able to. Moreover, since the Doppler frequency shift due to the movement of the measurement surface is used, the displacement of the moving object can be measured in real time.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a multi-probe displacement measuring device according to the present invention.

2A to 2M are signal waveform diagrams output from the respective units of FIG. 1 when a triangular wave modulation current is used.

FIG. 3 is a waveform diagram showing oscillation angular frequencies and changes in emission intensity of object light and reference light when a triangular wave modulation current is used.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Triangular wave generator 12 ... Semiconductor laser 13 ... Temperature controller 13A ... Heating and / or cooling element 13B ... Temperature sensor 14 ... Collimator lens 16 ... Objective lens 18 ... Optical fiber coupler 21, 22, 23, 24, 25 ... Optical fiber 27A, 27B ... Probes 28A, 28B ... Measurement object 30 ... Photodiodes 34A, 34B ... Bandpass filters 36, 38A, 38B ... Waveform shapers 40, 42A, 42B ... Frequency multiplication circuits 43A, 43B ... Gate circuits 44A, 44B ... Up Down counter

Claims (3)

[Claims] 1. A semiconductor laser, a triangular wave generating means for inputting a triangular wave modulation current having a predetermined period T _s (frequency f _s ) to the semiconductor laser, an optical switch, a photoelectric conversion element, and the semiconductor laser oscillated from the semiconductor laser. Optical means for guiding the laser light to the optical switch and for guiding the light returning through the optical switch to the photoelectric conversion element; and first and second laser light which is alternately applied via the optical switch. An optical fiber and a first optical fiber, which is disposed at the tip of the first optical fiber and reflects a part of the light on the emission end face and transmits the rest of the light through parallel light or converging light to illuminate the first measurement surface. And a second optical fiber, which is disposed at the tip of the second optical fiber and reflects a part of the light on the emission end face and transmits the rest of the parallel light or condenses the second measurement surface. With probe
A multiplexer for inputting a signal from the photoelectric conversion element and alternately switching the input signal to a first output terminal and a second output terminal for output, and an optical switch and a multiplexer for gradually increasing and gradually decreasing the triangular wave modulation current, respectively. Switching control means for switching the optical switch and the multiplexer for each period of time, and a frequency nf _s that is n times the frequency f _s of the triangular wave from the output signals of the first and second output terminals of the multiplexer as the center frequency. First and second bandpass filters for extracting frequency components of a predetermined bandwidth, and first for outputting a pulse signal having a frequency obtained by multiplying the frequencies of the output waveforms of the first and second bandpass filters by m times, respectively. And second pulse output means, and in synchronization with the cycle T _s of the triangular wave modulation current output from the triangular wave generating means, The same frequency as the frequency nf _s is set to m
Third pulse output means for outputting a pulse signal having a doubled frequency, and the relative relationship between the first lens and the first measurement surface in the pulse signals respectively output from the first and third pulse output means. A first counter that integrates a difference in frequency generated by movement, and a relative relationship between the second lens and the second measurement surface in pulse signals output from the second and third pulse output means, respectively. A multi-probe displacement measuring device, comprising: a second counter that integrates a frequency difference generated by movement. 2. The optical means comprises third, fourth and fifth optical fibers and an optical fiber coupler, and the laser light oscillated from the semiconductor laser is supplied to the first optical fiber and the optical fiber coupler. And the light returning to the optical switch via the fourth optical fiber and the optical switch to the photoelectric conversion element via the fourth optical fiber, the optical fiber coupler and the fifth optical fiber. The multi-probe displacement measuring device according to claim 1, which is derived. 3. The first and second photoelectric conversion elements which output signals to the first and second bandpass filters, instead of the photoelectric conversion elements and the multiplexer, and the fifth photoelectric conversion element.
The second optical switch for alternately outputting the light from the optical fiber of claim 1 to the first and second photoelectric conversion elements in synchronism with the optical switch, and the multi-probe according to claim 2. Displacement measuring device.