JP2647960B2 - Speed control device for moving mirror - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a speed controller for controlling the speed of a movable mirror constituting an interferometer.

[Conventional technology]

In a Fourier transform infrared spectrophotometer, a Michelson interferometer was used to divide the light from the light source into two light beams with a Michelson interferometer, and then superimposed again and led to a detector, and the interferogram obtained here Is Fourier-transformed to obtain a spectral transmission curve. In this case, an interferometer using a laser as a light source is used to monitor the movement of the movable mirror of the interferometer.

Since the laser light is monochromatic light, when the laser light is input to the interferometer, split into two light beams, reflected by the fixed mirror and the moving mirror, and combined, a cosine wave following the movement of the moving mirror is obtained.
Therefore, the movable mirror is controlled by using the cosine wave.

Conventionally, a method of detecting a position at which a cosine wave crosses a zero point, measuring a time interval between the zero points, and controlling the moving speed of the movable mirror according to the length thereof has been generally adopted. In other words, the cosine wave period is measured at the time interval between the zero points,
Based on this period, feedback control is performed so that the moving speed becomes the target speed.

Further, in the case of the above-mentioned method, if the speed becomes low and the time interval between the zero points becomes long, it is impossible to cope with the speed fluctuation in the middle of the zero points. Therefore, the Beckman method has been proposed as a method capable of performing high-accuracy speed control even at a low speed.

The Beckman method utilizes polarization based on the Zeeman effect, in which when a magnetic field is applied from around a laser oscillator, the frequency shifts and the laser beam is split into circularly polarized laser light in the opposite direction. In brief, the laser light having the frequency f ₀ has a shift of the frequency f ₁ due to the Zeeman effect in a magnetic field, and a laser light circularly polarized in the opposite direction can be obtained. Next, this is changed from circularly polarized light to vertical (f ₀ + f ₁ ) parallel (f ₀ −f ₁ ) linearly polarized light by a λ / 4 phase plate and introduced into the interferometer. In the moving mirror of the interferometer, a Doppler shift occurs such as vertical (f ₀ + f ₁ + f) parallel (f ₀ −f ₁ −f) corresponding to the frequency f (= 2v / λ) proportional to the moving speed v. . On the other hand, on the fixed mirror side, using a λ / 2 phase plate, vertical (f ₀ + f ₁ ) → (f ₀ −f ₁ ) parallel (f ₀ −f ₁ ) → (f ₀ + f ₁ ) Perform a parallel swap. Then, a vertical component is caused to interfere (multiply) through a polarizer for both, and the shift caused by the Zeeman effect and the frequency component of the speed of the moving mirror are obtained. From the detector. Furthermore, to obtain a signal of the reference signal to the output of the detector (2f ₁ + f ') multiplied by, dropped a higher frequency by passing the low-pass filter (f-f'). Then, the thus obtained (f-
By controlling the speed of the movable mirror so that f ') becomes zero, the moving speed of the movable mirror is set and controlled at the frequency f'.

As mentioned above, the Beckman method utilizes the Zeeman effect and polarization to increase the modulation frequency,
(2f ₁ + f) separation is performed. Therefore, control is possible even when the speed of the movable mirror is reduced, and reciprocation control is also possible.

[Problems to be solved by the invention]

However, the former method of measuring the cycle by detecting the zero point of the cosine wave has a problem that the loop gain cannot be increased because the feedback is delayed by a time corresponding to the cycle. Also, when the speed slows down,
The period between the zero points becomes long, and it is impossible to cope with the speed fluctuation in the middle of the zero points.

The latter Beckman method is excellent from a control point of view, but requires a large-scale device for applying a magnetic field or polarizing. In addition, in order to keep the amount of shift of the laser wavelength when a magnetic field is applied to the laser light constant,
There is a problem that a magnetic field having a high degree of stability must be applied, the control becomes large, and the apparatus requires a complicated and high-precision system.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problem, and an object of the present invention is to provide a moving mirror speed control device capable of controlling the speed of the moving mirror with high accuracy by a simple configuration.

[Means for Solving the Problems] For this purpose, the present invention is an apparatus for controlling the speed of a moving mirror constituting an interferometer, wherein a detecting means for detecting an amplitude modulation signal from the interferometer input with laser light, A differential signal generating means for generating two differential frequency signals having different phases from the frequency of the amplitude modulation signal and the frequency of the reference signal, and differentiating one of the two differential frequency signals. A control signal generating means for generating a control signal for multiplication with the other, and a driving means for controlling a driving voltage of the movable mirror according to the sign of the control signal are provided.

[Action]

In the speed control apparatus for a moving mirror according to the present invention, an amplitude modulation signal corresponding to the moving speed of the moving mirror is obtained from the detecting means, and two difference frequency signals having different phases are generated from the frequency of this signal and the frequency of the reference signal. When one of the two difference frequency signals is multiplied by the other, the positive and negative polarities differ depending on the magnitude of the speed determined from the frequency of the reference signal and the moving speed of the movable mirror. By controlling the voltage, the moving speed of the movable mirror can be made a speed determined by the frequency of the reference signal.

〔Example〕

 Hereinafter, embodiments will be described with reference to the drawings.

FIG. 1 is a diagram showing one embodiment of a speed control device for a moving mirror according to the present invention, wherein 1 is a linear motor, 2 is a moving mirror, 3
Is a fixed light, 4 is a semi-transparent mirror, 5 is a laser oscillator, 6 is a detector, 7 is a reference signal generator, 8 is a difference frequency signal generation circuit,
9 denotes a differentiating circuit, 10 denotes a multiplying circuit, and 11 denotes a driving circuit.

In FIG. 1, assuming that the moving speed of the movable mirror 2 is v and the wavelength of the laser beam is λ, the detector 6 obtains a signal having a frequency f of f = 2v / λ. The difference frequency signal generating circuit 8 outputs two difference frequency signals whose phases are shifted by 90 ° C. from the output signal of the detector 6 and a reference signal having a frequency f ′ corresponding to the set speed of the movable mirror 2. −f ′) (f−f ′) (π / 2), for example, can be configured with a multiplier and a low-pass filter, and can be configured with a heterodyne lock-in amplifier or a general lock-in. It can also be configured with an amplifier. Then, one of the two difference frequency signal outputs of the difference frequency signal generation circuit 8 passes through the differentiating circuit 9, and the two are multiplied by a multiplication circuit 10.

For example, in the difference frequency signal generation circuit 8 using a lock-in amplifier, a signal O ₁ = cos ｛2π (ff ′) t + φ｝ O ₂ = cos ｛2π (ff ′) with a phase relationship shifted by π / 2. Since t + φ + π / 2｝ is obtained, when these signals are compared, the phase relationship is reversed depending on whether the moving speed (f) of the movable mirror is higher or lower than the set speed (f ′). Therefore, the present invention provides a signal
_One of the methods is to perform the operation of shifting the phase of one of O ₁ and O ₂ by π / 2, and to take a correlation with the other. One of the methods is to differentiate _one signal O ₁ and perform multiplication. It is.

First, differentiating the signal O ₁ gives Also, the signal O ₂ is given by O ₂ = −sin {2π (ff ′) t + φ}. Becomes When such an operation is performed, as is apparent from the equation, π (ff ′) of the first term becomes a DC component, and f becomes f ′
The sign is inverted depending on whether it is larger or smaller. That is, assuming that the speed v 'determined by the frequency f' of the reference signal is v '= f'λ / 2, the speed v of the movable mirror depends on whether it is faster or slower than v' determined by the frequency f 'of the reference signal. The positive and negative polarities of the first term change. Since the positive and negative polarities differ depending on the direction of the 90 ° phase shift between the two difference frequencies, the positive and negative signs and the control direction for increasing or decreasing the speed of the movable mirror are determined according to the circuit.

The drive circuit 11 controls the moving mirror 2 to the speed of v 'by changing the voltage of the linear motor 1 of the moving mirror 2 according to the sign of the output of the multiplying circuit 10 and accordingly to the sign of the sign.

The drive circuit 11 controls the speed of the movable mirror 2 by changing the voltage of the linear motor 2, and a multiplication circuit 19
When the voltage applied to the linear motor 1 is changed according to whether the output of the first term obtained from the above is positive or negative, the speed v
Can be controlled to a speed v ′ determined by the frequency f ′ of the reference signal.

FIG. 2 is a diagram showing a configuration example of a difference frequency signal generation circuit using a lock-in amplifier, wherein 21 is a preamplifier, 22 is a low-pass filter, 23 is a synthesis circuit, 24 is an oscillator, 25 is a band-pass filter, and 26 and 27 is a low-pass amplifier, 28 is a vector computer, and 29 to 31 are multiplication circuits. This difference frequency signal generation circuit outputs an output signal from the laser interferometer,
That is, a signal of a frequency f corresponding to the moving speed v of the movable mirror is supplied from the detector 6 to the preamplifier 21, and a sine wave signal of a frequency f 'corresponding to the set speed v' of the movable mirror is synthesized.
, The signals O ₁ and O ₂ whose phase relations shown in the above equation are shifted by π / ₂ are generated and output from the low-pass amplifiers 26 and 27. If the frequency f 'of the signal at the set speed v' is about 5 kHz, for example,
A signal having a frequency f _i as high as about Hz is generated. The signal is synthesized with a signal having a frequency f ′ by a synthesis circuit 23, and further multiplied by an output of a laser interferometer having a frequency f by a multiplication circuit 29 ( ± f + f '+ f _i ), and the subsequent multiplication circuit
In 30 and 31 from _{(-f + f '+ f i} ) And the high-frequency components are dropped by the low-amplifiers 26 and 27. In the illustrated example, the heterodyne method of once converting to a high frequency and removing the sideband is shown, but a general lock-in amplifier that directly multiplies the detector signal by the reference signal is used. The same output can be obtained by using the same.

It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible. For example, in the above-described embodiment, the configuration for controlling the speed of the moving mirror of the Michelson interferometer has been described, but it goes without saying that the present invention can be similarly applied to other interferometers as long as they are two-beam interferometers. In addition, instead of the interferometer used for the Fourier transform infrared spectrophotometer, the moving mirror of the interferometer is attached to another object whose moving speed is to be controlled,
It may be applied to control the movement of the object.

[Effects of the Invention] As is apparent from the above description, according to the present invention, two difference frequency signals having different phases are generated from the frequency of the signal corresponding to the moving speed of the movable mirror and the frequency of the reference signal, and the differential signal is generated. Since the driving voltage of the movable mirror is controlled by performing a multiplication operation, the movable mirror can be controlled with a high difference frequency with high accuracy without using a complicated configuration or circuit unlike the conventional method.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of a moving object speed control method according to the present invention, and FIG. 2 is a diagram showing a configuration example of a difference frequency signal generation circuit using a lock-in amplifier. 1 ... linear motor, 2 ... moving mirror, 3 ... fixed mirror, 4
…… Semi-transparent mirror, 5 …… Laser oscillator, 6 …… Detector, 7…
... Reference signal generator, 8 ... Difference frequency signal generation circuit, 9 ...
... differentiation circuit, 10 ... multiplication circuit, 11 ... drive circuit.

Claims (1)

(57) [Claims] An apparatus for detecting and controlling a speed signal of a movable mirror by inputting a laser beam to an interferometer, a detecting means for detecting an amplitude modulation signal from the interferometer, a reference for generating a constant frequency signal Signal generating means for generating two difference frequency signals having different phases from the frequency of the amplitude modulation signal and the frequency of the reference signal; differentiating one of the two difference frequency signals and multiplying the other by the other; A speed control device for a moving mirror, comprising: a control signal generating means for generating the driving signal; and a driving means for controlling a driving voltage of the moving mirror according to a sign of the control signal.