Abstract:
The present invention provides an interferometer and the like, that is capable of ensuring the speed stability of the movable mirror while achieving the speed up of the reciprocal movement of the moving mirror and suppressing the increment of the maximum instantaneous thrust force required for the turning back as much as possible. The interferometer includes a moving mirror, a movement mechanism for reciprocating the moving mirror, a movement control part for controlling the movement mechanism and allowing the moving mirror to be reciprocated at a constant speed, and a measurement part for measuring a position of the moving mirror. The movement control part is adapted to receive target position data indicating a target position of the moving mirror, and control the movement mechanism to bring the measurement position of the moving mirror measured by the measurement part close to the target position indicated by the target position data.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to JP Application No. 2014-227166, filed Nov. 7, 2014, the disclosure of which is incorporated in its entirety by reference herein. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an interferometer using, for example, an infrared light source, a spectrophotometer using the interferometer and a control method for the interferometer. 
       BACKGROUND ART 
       [0003]    An interferometer used, for example, in a spectroscopic unit of a Fourier-transform infrared spectroscopy (referred to as “FTIR” hereinafter) is intended that, light emitted from an infrared light source is split into two light beams. One of the two light beams is reflected by a fixed mirror, while the other is reflected by a reciprocal moving mirror. Then, these light beams are synthesized again to thereby generate an interference wave by an optical path difference. 
         [0004]    In this FTIR, the intensity of the synthesized light by this interferometer draws sine curves every different wavelength, when the moving mirror is continuously moved. The synthesized light in this interferometer is spectrally dispersed through Fast Fourier Transform (FFT) to thereby obtain the light intensity of every wavelength (spectrum). Thus, it is adapted to analyze a composition ratio and concentration of a measuring object. 
         [0005]    At this time, it is required to measure the interference waveform with accuracy conformable to the FFT operation in this interferometer. It is indispensably necessary to observe a position of the moving mirror with high accuracy. 
         [0006]    Thus, the moving mirror is equipped on a predetermined movement mechanism. The moving mirror is configured to be reciprocated at a constant speed in a predetermined section by controlling the movement mechanism with a movement control part. 
         [0007]    The movement mechanism is configured of, for example, a cart on which the moving mirror is mounted, a linear guide for supporting the cart reciprocally, and a linear actuator such as a voice coil motor for reciprocating the cart, and the like. 
         [0008]    The movement control part is adapted to feedback-control the position of the cart (and the moving mirror). The movement control part is followed by measuring the position of the cart by a position sensor such as a laser displacement measurement system, and generating control signal to the linear actuator so that the measurement position follow to a target position. Since this target position is reciprocated at a constant speed, it is set to depict a triangular waveform (also, referred to as “target position triangular waveform” hereinafter), when a horizontal axis represents a time as shown in  FIG. 1 . 
         [0009]    In such a system, although a constant speed reciprocal movement is targeted, it is necessary that the cart and the moving mirror should be turned back, while being decelerated and accelerated in the around of an apex of the target position triangular waveform at which a forward movement and a backward movement are switched. Therefore, there occurs a turn-back section in which the constant speed cannot be maintained as shown in  FIG. 1 . 
         [0010]    Meanwhile, in recent years, there arises an increased demand for shortening a measurement time, and it becomes necessary to speed up the reciprocal movement of the mirror accordingly. 
         [0011]    However, if the reciprocal movement is speeded up, a thrust force at a timing of turn-back is significantly increased. There is concerned a problem that an excessive load is applied to the movement mechanism of the mirror, which results in short life thereof and increasing frequency of breakdowns. For example, If two or three times faster than conventional one is required, but the maximum instantaneous thrust force required for the turning back of the mirror also becomes two or three accordingly. Therefore, deformation of the voice coil constituting the movement mechanism and wear of a steel ball of the linear guide become remarkable, results in negative influence on the product lifetime are caused and the breakdowns become easily occur. 
         [0012]    Further, since the time required for turning back the cart becomes shorter in accordance with the faster speed, the time elapsed from the turned of the cart up to the movement at a constant speed becomes also shorter. It results in that speed stability of the cart is deteriorated, but also the measurement accuracy is deteriorated. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0013]    Therefore, the present invention has been made in order to solve the above problems. An essential subject thereof is to provide an interferometer and the like capable of ensuring the speed stability of the movable mirror while achieving the speed up of the reciprocal movement of the moving mirror and suppressing the increment of the maximum instantaneous thrust force required for the turning back as much as possible. 
       Solution to Problem 
       [0014]    That is, an interferometer according to the present invention includes: a moving mirror; a movement mechanism for reciprocating the moving mirror; movement control part for controlling the movement mechanism and allowing the moving mirror to be reciprocated at a constant speed; and a measurement part for measuring a position of the moving mirror. In this configuration, the movement control part is adapted to receive target position data indicating a target position of the moving mirror, and control the movement mechanism so as to bring the measurement position of the moving mirror measured by the measurement part close to the target position indicated by the target position data. A time-varying waveform of the target position indicated by the target position data is formed into a trapezoidal waveform by horizontally cutting apex portions of the triangular waveform, or is further formed into a smooth wave by rounding away the corner points of the trapezoidal waveform, while a graph waveform in the case where a vertical axis represents the target position and a horizontal axis represents a time. 
         [0015]    In this interferometer of the present invention, since the time-varying waveform of the target position forms the trapezoidal waveform or forms a shape of the trapezoidal waveform with its corner portions smoothly changed, a timing of starting deceleration becomes before the apex of the triangular waveform indicating the conventional target position. Thus, the time required for the turn-back operation can be longer than that in the conventional interferometer. Therefore, the speed stability after acceleration is improved, and not only the speed stability of the moving mirror can be ensured even if the reciprocation speed of the moving mirror is made faster, but also the performance related to the measurement accuracy of the interferometer can be maintained. 
         [0016]    Moreover, in the conventional case, since the target position is the apex of the triangular waveform at the timing of turn-back operation, the target position is rapidly changed, and an excessive load is applied to the movement mechanism by the turn-back operation. Whereas, according to the present invention, since the time-varying waveform of the target position forms a trapezoidal waveform or forms a shape by smoothly changing the corner portions of the trapezoidal waveform and the change at the timing of turn-back operation is made moderate, the load applied to the movement mechanism can be decreased, and short-life and increment of breakdown frequency on the movement mechanism can be prevented in spite of faster reciprocation speed of the moving mirror. Particularly, this effect becomes remarkable in the case where the corner portions of the trapezoidal waveform of the time-varying waveform of the target position is made smooth. 
         [0017]    As a more specific embodiment, there can be exemplified a configuration that, the movement control part further includes a target position data generation part for generating target position data. The target position data generation part calculates the target position based on an original target position having a triangular waveform shaped time-varying waveform and outputs the target position data indicating the target position. 
         [0018]    With this configuration, only by inputting original target position data indicating an original target position of a triangular waveform shaped temporally varying waveform to the target position data generation part, it is possible to generate the target position data of a time-varying waveform forming a trapezoidal waveform by horizontally cutting apex portions of a triangular waveform, or forming a shape of the trapezoidal waveform with its corner portions smoothly changed to thereby easily generate the target position data. 
         [0019]    As an embodiment to have a particularly remarkable effect of the present invention, there can be exemplified a configuration follows. The target position data generation part includes: a first-stage reference governor part for receiving the original target position data indicating the original target position, and calculating an intermediate target position of a time-varying waveform that forms a trapezoidal waveform by cutting apex portions of the triangular waveform indicated by the original target position data; and a second-stage reference governor part for calculating the target position having a waveform of the trapezoidal waveform with its corner portions smoothly changed by applying a process of low-pass filter to the time-varying waveform of the intermediate target position. 
         [0020]    With this configuration, the first-stage reference governor part is possible to calculate an intermediate target position of a time-varying waveform of a trapezoidal wave formed by horizontally cutting the apex of the triangular wave. And the second-stage reference governor part is also possible to easily generate a target position having a time-varying waveform of the trapezoidal waveform of the intermediate target position with its corner portions smoothly changed. 
         [0021]    Also, a spectrophotometer using the above interferometer is one of the present invention. 
         [0022]    With this spectrophotometer, the speed stability of the moving mirror can be ensured, even if the reciprocation speed of the moving mirror is intended to be faster to shorten the measurement time. Therefore, it is possible to obtain data with high reliability. 
         [0023]    Further, a control program of the above interferometer is a program for controlling the interferometer which includes: a moving mirror; a movement mechanism for reciprocating the moving mirror; movement control part for controlling the movement mechanism and allowing the moving mirror to be reciprocated at a constant speed; and a measurement part for measuring a position of the moving mirror. The control program exhibits functions as the movement control part adapted to receiving target position data indicating a target position of the moving mirror, and controlling the movement mechanism so as to bring the measurement position of the moving mirror measured by the measurement part follows the target position indicated by the target position data, and setting a time-varying waveform of the target position indicated by the target position data so as to be formed into a trapezoidal waveform, from a triangular waveform by horizontally cutting apex portions of the triangular waveform, or be further formed into a smooth waveform by rounding away the corner points of the trapezoidal waveform. 
       Advantageous Effects of Invention 
       [0024]    According to the interferometer, the spectrophotometer using the interferometer and control program of the interferometer of the present invention, while achieving the speed up of the reciprocal movement of the moving mirror, the increment of the maximum instantaneous thrust force required for the turning-back is suppressed as much as possible. Further, it allows to maintain the life performance, prevent a breakdown, and it is possible to ensure the speed stability of the moving mirror and prevent deterioration of the measurement accuracy. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a graph showing a relationship between a target position of a conventional moving mirror and a time; 
           [0026]      FIG. 2  is a schematic diagram showing a spectrophotometer and an interferometer in the present embodiment; 
           [0027]      FIG. 3  is a block diagram showing a target position data generation part in the present embodiment; 
           [0028]      FIG. 4  is a graph showing a time-varying waveform of an intermediate target position generated by a first-stage reference governor part in the present embodiment; 
           [0029]      FIG. 5  is a graph showing a time-varying waveform of a target position generated by a second-stage reference governor part in the present embodiment; 
           [0030]      FIG. 6 ( a )  is an enlarged view of a portion A in  FIG. 5 ; 
           [0031]      FIG. 6 ( b )  is an enlarged view of a portion A in  FIG. 5 ; 
           [0032]      FIG. 7 ( a )  is a graph when a vertical axis represents a position and speed of the moving mirror and a horizontal axis represents a time in the conventional interferometer; 
           [0033]      FIG. 7 ( b )  is a graph when a vertical axis represents a position and speed of the moving mirror and a horizontal axis represents a time in the interferometer of the present embodiment; and 
           [0034]      FIG. 8  is a schematic diagram showing a spectrophotometer and an interferometer in another embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0035]    The following describes an embodiment of the present invention referring to the accompanying drawings. 
         [0036]    A spectrophotometer  1  of the present embodiment is, for example, a FTIR, and as shown in  FIG. 2 . An interference wave depicting different sine curves for each wavelength generated by an interferometer  2  is incident to a measurement sample placed on a sample stage  3 . The light transmitted through the measurement sample is dispersed through Fast Fourier Transform (FFT) in an analyzing part  4  to thereby obtain the light intensity of every wavelength (spectrum). Thus, it is adapted to analyze a composition ratio and concentration of the measurement sample. 
         [0037]    As a configuration of generating an interference wave depicting different sine curves for every wavelength, the interferometer  2  of the present embodiment splits light that emitted from an infrared light source  5  into two light beams through a beam splitter  6 . Then, while one of the light beams is reflected by a fixed mirror  7 , the other of the light beams is reflected by a moving mirror  8  which is reciprocated by a movement mechanism  9 . At this time, since the moving mirror  8  is continuously moved by the movement mechanism  9 , the interference wave obtained by synthesizing the light beam reflected by the fixed mirror  7  and the light beam reflected by the moving mirror  8  depicts different sine curves for every wavelength. 
         [0038]    The movement mechanism  9  includes a linear guide  9   a  placed in parallel on one of optical axes which are split by a beam splitter  6 , a cart  9   c  moving on the linear guide  9   a , and a linear actuator  9   b  such as a voice coil motor for reciprocating the cart  9   c . The moving mirror  8  is mounted on this cart  9   c  so as to intercept the optical axis. 
         [0039]    Thus, a moving speed of the cart  9   c  is varied based on a current value flowing to the linear actuator  9   b , and a moving direction of the cart  9   c  is changed depending on a flowing direction of the current. 
         [0040]    Note that the linear actuator  9   b  is not limited to the voice coil motor and any may be used so long as it reciprocates the cart  9   c.    
         [0041]    A measurement part  10  for measuring a position of the moving mirror  8  which is moved by the movement mechanism  9  is a position sensor. The measurement part  10  includes, for example, a measuring light source  10   a  and a light receiving part  10   b  for receiving the light. This position sensor can be an example of measuring a position of the moving mirror  8 , utilizing the fact that a frequency of the light from the measuring light source  10   a  is varied by Doppler effect occurring at a time of reflecting the light by the moving mirror  8 . Then, the measurement part  10  transmits actual position data indicating a measured measurement position to a control mechanism  11 . 
         [0042]    Note that, in addition to the position sensor mentioned above, a velocity sensor or an acceleration sensor, or the like can be also used as the measurement part  10 . 
         [0043]    The control mechanism  11  is intended to control the movement mechanism  9 . It is structurally a so-called computer circuit having a CPU, an internal memory, an I/O buffer circuit, an AD converter and the like. Thus, by operating in accordance with a control program stored in a predetermined region of the internal memory, the CPU and peripheral equipment are cooperated to thereby exhibit a function as a movement control part  11   a  as shown in  FIG. 3 . 
         [0044]    The movement control part  11   a  is intended to control the movement mechanism  9  by generating a control signal so as to move the moving mirror  8  to a target position and sending the control signal to the movement mechanism  9 . More specifically, the movement control part  11   a  includes a deviation calculating part  16  for calculating a deviation between a target position of the moving mirror  8  and a measurement position measured by the measurement part  10 , and a computing part  17  for calculating the control signal so as to bring the measurement position close to the target position by reducing the deviation. 
         [0045]    This target position is generated using a target position data generation part  12 . The target position data generation part  12  is intended to generate target position data indicating a target position and includes a first-stage reference governor part  13  for calculating an intermediate target position from an original target position, and a second-stage reference governor part  14  for calculating the target position from the intermediate target position. 
         [0046]    In the present embodiment, the original target position is stored in an original target position data storage part  15 . When representing this original target position in a graph of a vertical axis representing a position and a horizontal axis representing a time, a time-varying waveform thereof becomes a symmetrical waveform of a triangular waveform. 
         [0047]    As shown in  FIG. 4 , the first-stage reference governor part  13  is intended to acquire the original target position data indicating the original target position from the original target position data storage part  15 , and cut apex portions of the triangular wave of the temporally varying waveform of the acquired original target position to thereby calculate the intermediate target position having a temporally varying waveform of a trapezoidal waveform. Specifically, the first-stage reference governor part  13  cuts apex portions of the triangular wave to maintain the section of a specific length that a moving mirror  8  moves at a constant speed in order to perform a measurement with an interferometer. That is, the first-stage reference governor part  13  removes the apex of the triangular waveform by cutting the range that lies between the peak of the apex and the constant speed section before and after the apex. 
         [0048]    As shown in  FIG. 5 , second-stage reference governor part  14  is intended to calculate the target position with the corner portions of the trapezoidal waveform of the time-varying waveform smoothly changed by applying a process of a low-pass filter to the time-varying waveform of the intermediate target position that is calculated by the first-stage reference governor part  13 . At this time, a FIR filter or an IIR filter can be used as the low-pass filter. 
         [0049]    Further, the time-varying waveform of the target position calculated by applying a process of a low-pass filter may be a trapezoidal waveform of the intermediate target position with its corner portions smoothly changed while its horizontal portion remaining as shown in  FIG. 6( a ) , or may be the same while its horizontal portion not remaining as shown in  FIG. 6( b ) . 
         [0050]    In the above target position data generation part  12 , the time-varying waveform of the target position indicated by the target position data via the first-stage reference governor part  13  and the second-stage reference governor part  14  represents a trapezoidal waveform formed by horizontally cutting apex portions of a triangular wave with the corner portions of the trapezoidal waveform smoothly changed, as shown in  FIG. 7( b ) . It is understood that, by making the time-varying waveform of the target position as shown in  FIG. 7( b ) , a timing of starting deceleration of the moving mirror  8  is made faster than in the conventional interferometer shown in  FIG. 7( a ) , and the time required for turning-back the moving mirror  8  can be increased. Moreover, it is understood that a predetermined constant speed section is provided as to the speed of the moving mirror  8  controlled on the basis of the target position as described above. 
         [0051]    The interferometer  2  of the present embodiment configured as described above has remarkable effects as follows. 
         [0052]    That is, in the interferometer  2  of the present embodiment, the time-varying waveform of the target position is a trapezoidal waveform with its corner portions smoothly changed. Then, the timing of starting deceleration is before the apex of the triangular waveform which is a conventional target position to thereby obtain a longer time required for the turn-back operation than in the conventional interferometer. Therefore, the speed stability after acceleration is improved, and even if the reciprocation speed of the moving mirror  8  is intended to be made faster, the speed stability of the moving mirror  8  can be ensured and the performance related to the measurement accuracy of the interferometer  2  can be maintained accordingly. 
         [0053]    Further, since the time-varying waveform of the target position is a trapezoidal waveform or a shape with its corner portions smoothly changed to thereby moderate the change at the timing of turning-back operation, an excessive force is suppressed from being applied to the movement mechanism  9 . Then, it is possible to prevent short life and increment of breakdown frequency of the movement mechanism  9  in spite of speed up in reciprocating the moving mirror  8 . 
         [0054]    Furthermore, only by inputting the original target position data indicating the original target position of a time-varying waveform of a triangular waveform, it is possible to generate the target position data of the time-varying waveform of a trapezoidal waveform formed by horizontally cutting apex portions of a triangular waveform or a trapezoidal waveform with its corner portions smoothly changed, and the target position data can be easily generated. 
         [0055]    Next, a spectrophotometer  100  of another embodiment will be described. Note that the same parts as in the description of the spectrophotometer  1  are designated by the same reference numerals and the description thereof is omitted. 
         [0056]      FIG. 8  is a schematic view showing the spectrophotometer  100  in the present embodiment. The spectrophotometer  100  in the present embodiment includes: an interferometer  2 ; a gas cell  30  for accommodating a measurement sample; a detector  40  for detecting light transmitted through the gas cell  30 ; and a measurement part  50  for measuring a concentration of a measurement sample by Fourier-transforming a signal intensity of the light detected by the detector  40 . Note that, a light source  5  of the interferometer  2  is adapted to emit the light of an infrared wavelength region. Further, as a measurement sample, for example, exhaust gas of an automobile and the like may be used. 
         [0057]    The gas cell  30  is intended to allow interference light generated by the interferometer  2  to be transmitted through a sample within the cell. It is formed in a housing shape. This gas cell  30  is provided with an incident window to be incident the interference light generated by the interferometer  2 , and an exit window to transmit the interference light. 
         [0058]    The detector  40  is intended to detect a signal intensity of the light that has been transmitted through the sample within the gas cell  30 . For example, a semiconductor type sensor and a pyroelectric type sensor are used therefor. 
         [0059]    The measurement part  50  is intended to separate light intensities of respective wavenumber components through Fourier transform of the intensity signals of the light detected by the detector  40 . Then, the concentration of the sample is intended to be measured based on the Lambert-Beer Law that the absorbance and concentration of each wavenumber are proportional. 
         [0060]    The present invention should not be limited to the configurations described above. 
         [0061]    Although the target position data generation part is provided in the above embodiments, the target position data may be generated without using the target position data generation part. In this case, the target position data is generated by, for example, inputting a target position on a table or using an equation that represents the target position. In the case where the target position is generated without using the target position data generation part, it is possible to generate a target position of a time-varying waveform of a desired shape. 
         [0062]    Further, the target position data generation part may be configured of only the first-stage reference governor part. Thus, although the time-varying waveform of the target position becomes a trapezoidal waveform, even in this case, since the timing of starting deceleration becomes before the apex of the triangular waveform of the conventional target position. Then, the time required for re-acceleration becomes shorter and the speed stability after acceleration can be maintained. 
         [0063]    Note that the control mechanism may include a stability diagnostic part for calculating a speed stability index indicating speed stability of the movement mechanism (i.e., moving mirror). 
         [0064]    In the case where this stability diagnostic part determines that the measurement position measured by the measurement part has reached a constant speed movement start position, the speed stability index is calculated using the actual position data transmitted from the measurement part in a predetermined section from the constant speed start position. 
         [0065]    This speed stability index is expressed by the following equation. 
         [0000]    
       
         
           
             Vo 
             = 
             
               
                 Vstdevp 
                 Vavr 
               
               × 
               n 
             
           
         
       
     
         [0066]    Where Vstdevp is a standard deviation of the speed of the movement mechanism, Vavr is an average value of the movement mechanism, n is a natural number. 
         [0067]    In this embodiment, n=3 is used. 
         [0068]    By providing the stability diagnostic part in this way, the speed stability of the movement mechanism (i.e., moving mirror) can be evaluated. Therefore, in the case where the time-varying waveform of the target position is formed in a shape of a trapezoidal waveform with the apex portions of the triangular waveform horizontally cut and the corner portions thereof smoothly changed, it is possible to confirm whether or not the speed stability of the movement mechanism (i.e., moving mirror) can be ensured, specifically, whether or not the performance related to the measurement accuracy of the interferometer can be maintained. 
         [0069]    Moreover, it is not always necessary for the first-stage reference governor part to horizontally cut the apex portions of the triangular waveform, it may be also configured to cut the apex portions obliquely. 
         [0070]    In addition, various modifications of the embodiments can be made without departing from the spirit of the present invention. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           2  . . . Interferometer 
           8  . . . Moving mirror 
           9  . . . Movement mechanism 
           10  . . . Measurement part 
           11   a  . . . Movement control part