MIRROR SCAN VELOCITY CONTROL

Abstract: Mirror scan control for driving a movable mirror of an interferometer in an infrared spectrophotometer, with constant scan velocity and accurate position determination, using a laser (18) emitting a two frequency beam, a standard Michelson interferometer (13) a reference signal synthetizer (54). A closed loop electronic servo control controls velocity through comparison of the signal (80) from detector (44) with a controlled frequency reference signal. A second loop is used for stabilizing the frequency difference of laser frequency components. Phase lock control techniques are used for locking the signal detected to a common reference signal also used by the laser servo control.

Description: - l - MIRROR SCAN VELOCITY CONTROL Field of the Invention The present invention pertains to spectrophotometric instrumentation utilizing infrared light to measure spectral absorbance characteristics of a sample material, and in particular to Fourier transform analysis of infrared absorbance characteristics utilizing an interferometer and a laser to obtain spectral data. Background of the Invention The Fourier transform infrared (FT-IR) spectrophotometer consists of two basic parts: (1) an optical system which includes an interferometer through which an infrared light beam is directed before passing the beam through a .sample, and (2) a dedicated computer which is used to analyze the spectral information contained in the light. issuing from the sample. The advantage in improved performance of the FT-IR spectrophotometer results from the use of the interferometer, rather than a grating or prism, to obtain variance in wavelength of the infrared beam applied to the sample to measure spectral characteristics. An interferometer permits measurement of the entire spectral profile of the sample, increasing accuracy of analysis, in a fraction of the time previously required. The operation of a Michelson interferometer as applied to FT-IR spectrophotometry to vary and analyzing infrared light wavelength passing through a sample is well known. The interferometer consists of two perpendicularly arranged optical paths, each having a reflector or mirror positioned at its end to reflect light traversing the path. The mirror of one path is fixed. The mirror of the other is longitudinally movable to increase or decrease the length of the light path. An infrared light beam entering the interferometer is optically split into two components by a beam splitter so that a separate component of the beam will traverse each optical path. After reflection of each light beam component and redirection along its respective path, the components are reco bined through the beam splitter to constructively and destructively interfere. The reconstructed beam is directed through a sample and focused onto a photodetector for measurement of intensity and intensity variance of the range of frequencies in the issuing beam. The intensity characteristic of any selected frequency of the reconstructed light beam depends in part on the difference in length of the optical paths over which the beam components travel. Generally, when the movable mirror is axially moved or scanned at a constant velocity, the intensity of an emerging light beam will modulate in a regular sinusoidal manner for any selected wavelength of light passing through the interferometer. A typical infrared light beam emerging from the interferometer is a complex mixture of modulated frequencies due to its polychromatic nature. After the infrared light beam has passed through a sample material, it can be detected to determine specific wavelengths of light which have been absorbed by the sample. This is accomplished by measuring change in the regular sinusoidal intensity pattern expected when the light beam leaves the interferometer. The measurement of the differences in the characteristics of sinusoidal patterns for each light wavelength composing the emerging beam indicates those wavelengths of light which are absorbed by the sample. Infrared light absorbance characteristics measured provide spectral data from which the matter comprising the sample can be determined. The output signal of the detector which measures the intensity modulation of the emerging light beam can be recorded at very precise intervals during scanning of the movable mirror, to produce a plot known as an interferogram. The inter erogram is a record of data points indicating the output signal produced by the infrared photodetector as a function of the difference in length of the optical paths traversed by the components of the infrared beam passing through the inter¬ ferometer. Successive scans of the sample are obtained and co-added to obtain an average interferogram having improved signal-to-noise characteristics. The average interferogram provides information and data relating to the spectral characteristics of the sample material. • After mathematical preparation, a Fourier transform calculation is performed on the interferogram to obtain a spectral fingerprint of the sample composition. The results are compared with known reference data to determine the composition of the sample. Most Fourier transform techniques require averaging of a large number of interferograms in order to obtain accurate results. As many as 32 to 50 scans of the movable mirror during which measurements are taken may be averaged. It is important that an interferogram be precisely reproducible in order to maintain accuracy in its averaging with other co-related interferograms. Since an interferogram is generated as a function of mirror position, more accuracy in the interferogram and the applied Fourier transformation will be obtained if more accuracy is obtained in the determination of mirror position at the time when the data points are measured to define the interferogram. To accomplish accuracy and reproducibility in generating an interferogram, both the rate of the data point measurement (sampling rate) and mirror velocity must be very precisely controlled. In other words, the exact position of the mirror must be determinable when the data point is measured. Modern systems accomplish sampling rate and mirror velocity control and/or mirror position measurement by passing a laser beam through the interferometer, concurrently with the infrared light beam. The laser beam is used to directly measure the movement and/or position of the movable mirror to accurately determine change in path length of the interferometer. Since the laser beam undergoes the same splitting by the beam, splitter and traverses the same change in optical path as the infrared light beam, the recombined laser beam exhibits a measurable monochromatic light wavelength displaying an intensity interference pattern containing information about the mirror scan velocity of the movable mirror. The intensity interference pattern also serves to indicate position of the mirror during a scan and to indicate and correlate the collection of data points at uniform intervals of mirror displacement. In a conventional system, when the movable mirror is moving at a constant velocity, a Doppler shift in frequency is generated in the component of the laser beam traversing the changing length optical path. When the Doppler shifted component is recombined with the component traversing the fixed length path, a modulated frequency beam exhibiting a measurable amplitude modulation or beat signal is produced, yielding a varying intensity or fringe pattern which may be analyzed to determine the mirror position and/or velocity. The beat signal is useful because the frequency of the laser beam produced by most lasers is much too high for measurement by common detectors. Conventional systems generally drive the movable mirror at a velocity which provides a 5 KHz amplitude modulation or beat signal frequency in the exiting beam. The beat signal frequency is equal to the magnitude of the Doppler shift in frequency because it equals the difference in frequency between the recombined light beam components. At increased mirror velocities, the beat signal frequency will increase providing increased resolution while at slower mirror velocities the beat signal frequency will decrease. precision with this technique can be maintained to approximately one cycle in 5,000 ' to provide very accurate velocity and position information. In a conventional system, however, the movable mirror must be in motion to obtain a Doppler shift in frequency of the light beam traversing its path. Thus motion of the movable mirror is necessary to obtain a measurable beat signal frequency in the recombined light beam. To explain, when the movable mirror is stationary, component light beams traveling along adjacent paths of the interfereometer are recombined to form an identical frequency light beam since no Doppler frequency shift has been introduced in either component. The emerging recombined beam exhibits no intensity modulation and no beat signal. Thus when the mirror is not moving, there is no information contained in the emerging laser beam which can be used to determine mirror position or mirror velocity. This occurs at every instance that the movable, mirror is stopped, such as when it reaches the end of its-- scan and stops to proceed in the other direction. . Furthermore, in a conventional interferometric system the Doppler shift in frequency generated by a scanning mirror produces the same intensity modulation effect in the recombined light beam independent of the direction of mirror travel. For instance, a 5 KHz amplitude modulation or beat frequency can be obtained for travel of the mirror in either a forward or backward direction. Thus it is impossible to determine the direction of mirror travel from the emerging laser beam, even though the difference between optical paths may be increasing or decreasing. This shortcoming generally requires additional circuitry to obtain an indication of direction of mirror travel so that the exact position of the mirror may be determined at any given time. Moreover, in conventional systems the amplitude modulation or beat frequency of the emerging light beam becomes very difficult to measure as the velocity of a scanning mirror becomes very slow. For instance, for a 0.3 centimeter per second scan velocity, a beat frequency of 5 KHz is generated in the emerging light beam. However, if the mirror is driven at a scan velocity of 0.03 centimeters per second, the beat frequency is reduced to .5 KHz, which becomes very difficult to measure. Thus, as the scan velocity is decreased, the modulation frequency in the recombined light beam is decreased to a level which is difficult to measure with modern electronic detectors, reducing accuracy and resolution * An FT-IR spectrophotometer has limited resolution in sample identication determined by its ability to produce and reproduce accurate interferograms. The only part fundamental to the optical system is the movable mirror of the interferometer. This part greatly determines the accuracy with which a spectrophotometer can generate interferograms. The accuracy with which the spectrophotometer can analyze a sample is directly related to the accuracy and reproducibility of the interferogram and thus the ability of the instrument to control and determine the velocity and position of the movable mirror. The conventional use of a laser reference to control and determine the velocity and position of the movable mirror continues to suffer limited precision and control ability. Improvements in the precision with which mirror position can be measured and mirror velocity controlled will necessarily produce significant improvement in the accuracy with which an infrared spectrophotometer can analyze a sample substance. Summary of the Invention The present invention comprises an improved mirror scan control for driving a movable mirror of an interferometer in an infrared (IR) spectrophotometer, with a constant scan velocity, and for permitting more accurate determination of the position of the movable mirror throughout the scan range. In an IR spectrophotometer having a laser which generates a laser beam having two frequency components, a standard Michelson interferometer, and a reference signal source, an inventive closed loop electronic servo control provides control of mirror scan velocity through comparison of the signal derived from the laser beam passing through the interferometer with a controlled frequency reference signal. The laser is stabilized by a second closed loop electronic servo control which maintains the frequency difference between the tv/o frequency components of the laser beam constant. The mirror scan servo control in cooperation with the laser servo control employs phase-lock control techniques in displayed modulation or beat signal frequency of the resultant light beam permits a continuous information signal to be generated indicating the scan velocity and position of the movable mirror in the interferometer. The reference signal source generates a second reference signal having a frequency which is an adaptation of the first reference signal frequency applied to the laser servo control. The second reference signal is increased or decreased in frequency by a value corresponding to a calculated Doppler affected change in frequency caused when the movable mirror is scanned at a selected constant velocity. . The second reference signal is applied to the scan servo control and compared in phase with the intensity variation or beat frequency of the light beam leaving the interferometer, to generate a control signal for governing constant scan velocity of the movable mirror. The scan velocity of the mirror is corrected by application of the control signal until a phase lock is obtained between the second reference signal and a signal generated from ' intensity detection of the emerging beam from the interferometer, which exhibits the beat frequency characterized by the Doppler affected change in frequency. A phase lock between these signals maintains a constant velocity scan of the movable mirror, while making available a reference signal known to have information corresponding to mirror scan velocity and mirror position. A direction control is also provided to instruct the reference signal source to adjust the second reference signal upwardly or downwardly in frequency. Adjustment of the reference signal upwardly or downwardly in frequency determines the direction, in addition to the - velocity, in which the movable mirror will be driven by the scan servo control. With the movable mirror at a which a detected signal indicative of functional parameters of the movable mirror is locked to a common reference signal also used by the laser servo control. The mirror scan servo control accomplishes precise and stable control through analysis of intensity modulation in the heterodyned components of the laser beam. A laser beam having two frequency components of slightly different frequency is obtained by applying a magnetic field to a helium-neon laser. This phenomenon is well known and described as the Zeeman-splitting effect. The difference in frequencies between the components of the laser beam is stabilized at a selected difference by the laser servo control. This causes a continuous beat signal of constant frequency to be displayed by the intensity variation of the light beam dye to the heterodyne mixing of the differing frequency components. Stabilization is accomplished by locking the phase of a signal generated from intensity detection of the laser beam which exhibits the beat signal to a first reference signal generated by the reference signal source, having a frequency equal to the selected frequency difference between the components of the laser beam. The laser beam, having differing frequency components selectively separated, is directed through the interferometer. Each frequency component of the beam is combined with its opposing frequency component after traversing their respective optical paths in the interferometer, through optical polarization techniques. The resultant light beam exhibits an intensity modulation or beat signal frequency equal to the frequency difference of the components, plus or minus . a Doppler affected frequency change caused by the scanning of the movable mirror. The continually t-ff standstill, a beat frequency equal to the frequency difference between the frequency components of the laser beam will be exhibited in the exiting heterodyne beam. A scan of the movable mirror in one direction will generate an increase in the beat frequency due to the Dopper affected frequency change, while scan of the movable mirror in the other direction will generate a decrease in the beat frequency due to the same Doppler effect. Therefore, by adjusting the reference signal upwardly or downwardly in frequency from the standstill beat frequency found in the laser beam and the generated heterodyne signal therefrom, the movable mirror can be directed to scan in a forward or rearward direction. W τ ith use of a two-f equency laser which yields a continually modulated light beam emerging from the interferometer, the direction of the mirror scan can be easily determined and directed by an upward or downward frequency shift ' in the reference signal frequency characterizing the laser beam. The inventive scan servo control comprises a three-tier electronic control circuit which generates the scan control signal applied to the movable mirror drive electronics. A first control circuit tier generates a position error signal which denotes a change in mirror position required during the scan to place the mirror in a theoretically correct position at any given point in time if a constant scan velocity of the mirror is maintained. This is accomplished by comparing the detected signal emerging from the interferometer containing information of mirror position through the Doppler affected frequency change in the beat signal frequency, with a reference signal generated by the reference signal source which contains information determining theoretical mirror position with respect to ti-me. A second control circuit tier generates an instantaneous velocity error signal which denotes a difference in mirror scan velocity required at any given point, in time throughout a scan, to bring the mirror to a theoretically desired velocity at which the scanning mirror should remain constant. This is accomplished by comparing the detected signal• emerging from the interferometer containing information of mirror velocity through the Doppler affected frequency change in the beat signal frequency, with the output signal of a tracking voltage controlled oscillator circuit. This velocity error signal indicating a rate of change of position of the scanning mirror from theoretical, more quickly responds to differences in mirror movement from the constant velocity scan desired. This signal thus provides anticipatory control of velocity to position the mirror at the theoretically perfect location relative to time, to decrease reaction time of the scan servo control, improving accuracy and performance and maintaining constant velocity scanning of the mirror. Additionally, the signal permits a damping effect to the position error signal to guard against overdriving the mirror when a position error is determined. A third control circuit tier compensates for error found in the detected signal upon which mirror scan control is governed, which is introduced by inadequate control of the frequency difference between the component frequency modes of the Zeeraan split laser beam. Because this frequency difference is the basis for the beat signal frequency, and thus provides the basis of measurement of the mirror velocity and position, an inconsistency, or error, found in the frequency difference between the components can result in an erroneous scan control signal to be generated. Thus \2 compensation for actual error measured in frequency difference removes a source of error introduced in generating the scan control signal. The position error, velocity error, and laser compensation signals are summed and provided with selected gain and filtering. The summation signal is applied to the movable mirror drive electronics to direct mirror scan. The scan servo control can provide a constant velocity mirror scan in either direction along the entire range of mirror movement. This is accomplished by the direct phase-lock control using a constant frequency reference signal applied to the scan control for each direction of mirror movement. Advantageously, this permits samples to be accurately measured during forward and backward scans of the movable mirror, substantially reducing the time required to obtain sufficient data for accurate analysis techniques. Furthermore, accurate determination of the position of the movable mirror can be made during either forward or rearward scans by simply counting increasing or decreasing steps in comparative signal phase with techniques commonly known. As the movable mirror is scanned, the number of cycles through which the phasing of the signal representing the emerging laser beam advances or retards relative to the reference signal, provides a measure of the distal position of the movable mirror within the scan range. Thus by summing the number of cycles shifted between the compared signals, the exact position and change of" position of the movable mirror within its scan range can be easily determined for every sample scanned during which data points are measured. The improved scan control presented overcomes the deficiencies in prior art systems by providing continuous and precise means for determining the position of the mirror, and means for controlling the mirror scan velocity. The scan control eliminates the need for a separate optical system to re-establish position of the mirror after each scan reversal. The increased resolution in obtaining interferogram data points using this control servo system yields more accurate analysis of the sample material by the spectrophotometer. Due to the continuous information signal provided by the two ■ frequency laser beams and the highly accurate means for position control, it is only necessary to calibrate the spectrophotometer one time for each scan series, thus reducing the complexity and amount of necessary data to obtain an accurate sample analysis. Description of the Drawings Figure 1 is a schematic drawing of an interferometric portion of an infrared spectrophotometer and in particular of the Mirror Scan Servo Control which comprises the present invention; Figure 2 is a schematic representation of the interferometric portion of the spectrophotometer depicting the polarization relationship of the individual component frequency modes of the two-frequency laser beam as the beam passes through the interferometer; Figure 3 is a schematic drawing of the Mirror Position Control included in the mirror scan servo control; Figure 4 is a schematic drawing of the Mirror Velocity Control included in the mirror scan servo control ; Figure 5 is a schematic drawing of the Laser Signal Stability compensator included in the mirror scan servo control. Best Mode of the Invention The interferometer of a Fourier transform infrared (FT-IR) spectrophotometer is described with reference to Figures 1 and la. A Michelson inter¬ ferometer is depicted which comprises a beam splitter 10 .positioned to distribute portions of an incident light beam along each of two perpendicular optical paths 11 and 13. The beam splitter 10 receives a laser beam 16 from a magnetically influenced laser 18, and an infrared light beam, whose boundary is indicated by lines 20, generated by an infrared light source 22. Generally the infrared beam 20 is reflected and collimated by a non-planar mirror 24 for entry into the interferometer, while the laser beam 16 is directly applied to the beam splitter 10 through an opening 26 centrally located in the non-planar mirror 24. The beam splitter 10 directs a first component of each light oea 16 and 20, along a first fixed length optical path 11, which is bounded by a mirror 12. The light beams 16 and 20 are reflected by the mirror 12 to return along the optical path 11 to the " beam splitter 10. A second component of each of the light beams 16 and 20 is directed by the beam splitter 10 along a second optical path 13 which is bounded by a movable mirror 14. The movable mirror 14 is longitudinally movable with respect to the optical path 13, to change the length of the optical path within the selected scan range, indicated by arrow 15. The movable mirror 14 is driven by mirror drive electronics 28 directing a linear motor, which is a commercially available element manufactured by Systems Magnetic Co. and available under Part No. ES-11269. The second components of each of the light beams 16 and 20 are reflected from the movable ' mirror 14 to return along optical path 13 to the beam splitter 10, where they are recombined with the first components of the light beams 16 and 20, respectively, returning along, the first optical' path 11. The recombination of the first and second components of the laser beam 16 form a heterodyne beam 30 containing information ' of the velocity and position of the movable mirror 14 through intensity modulation caused by interference of the differing frequencies exhibited by the first and second components. The differing frequencies of the first and second components are due in part to the Doppler effected change in frequency caused in" the light component traversing the changing length optical path. The recombined components of infrared beams 20 form a heterodyned beam 32 in which frequencies are modulated at a characteristic rate to provide a detectable frequency range of infrared light which can be applied to the sample material for analysis. The recombined laser and infrared light beams, 30 and 32, respectively, are directed along an exit path 33 of the interferometer in which a reflector 34 similar to reflector 24, is positioned. The reflector 34 receives the collimated infrared beam 32 and reflects and focuses the beam on a sample chamber 36. The infrared beam 32 passes through the sample chamber 36 to a third mirror 38 and reflects therefrom to focus on an infrared photodetector 40. The photodetector 40 receives the intensity and frequency modulated infrared beam which has . been modified by the absorbtion characteristics of the sample material through which it passes. The modulation 15 in the light beam is detected to produce an electrical inf rmation signal proportional to the beam modulation which is used to generate an interferogram. The recombined laser beam 30 passes from the interferometer through an opening 42 in the mirror 34. Light beam 30 is directed to a detector 44. Electrical signal 45 produced by detector 44 is used to obtain a measure of the intensity modulation, i.e., the beat signal frequency, which the light beam 30 exhibits. The detector 44 may simply comprise a single photodetector centrally positioned in the emerging laser beam 30 to detect intensity modulation therein. The produced signal 45 are applied to a Mirror Scan Servo Control . 50 to generate a mirror scan drive signal 52. The drive signal 52 is applied to the mirror dr.ive.electronics 28 to control the velocity and direction of movement of the movable mirror 14. The He-Ne laser 10 is magnetically influenced to produce a laser beam 16 having two component frequency modes separated by a measurable frequency αiference, each having opposing circular polarization. The differing frequencies and polarizations of the components are used to obtain a continuous information signal i-n the Heterodyne laser beams 30 entering and leaving the interferometer. Referring to Figure 2, the laser beam 16 possessing the two component frequency modes is passed through a quarter wave plate 15 before entering the interferometer. The quarter wave plate 15 converts each of the circularly polarized components into a pair of linearly polarized components. For visualization one of the linearly polarized components exists in a plane parallel with the drawing, as shown by the bars 17, and has a frequency f ] _. The second linearly polarized component exists in a plane perpendicular to the drawing, as shown by the dots 19, and has a frequency 2 * This phenomena results from linear polarization of the respective circular polarizations of the light beam components. Thus, since the light beam directed into the interferometer consists of two components each having an individual frequency and circular polarization, they are clearly distinguishable from one another by polarization techniques to provide two independent optical signals in the laser beam. The first component 21 of the laser beam 16 reflected along the fixed length optical path 11, passes through a second quarter wave plate 23, reflects from the mirror 12, and passes again through the quarter wave plate 23 to return to the beam splitter 10. Passing the first component 21 of the beam 16 twice through the quarter wave plate 23 acts to rotate the polarizations of each component frequency mode through an angle of 90° about the axis of the beam 16. For example, the first frequency component 17 having frequency fτ_ which was vertically polarized upon entering the fixed optical path 11 shown by dots 19 returns to the beam splitter 10 from the optical path 11 with a horizontal polarization as shown by bars 17'. Similarly, the second frequency component having frequency £2 which was horizontally polarized entering the fixed length optical path 11 shown by bars 17 returns to he beam splitter 10 from the optical path 11 with a vertical polarization, shown by dots 19 ' . The second component 25 of the laser beam 16 which passes through the beam splitter 10 and along optical path 13, is reflected from the moving mirror 14 without change in polarization. Each of the component frequency modes of the second components 25 of the laser 4 -8 beam may, however, be changed in frequency by a value L £ Doppler which is caused by a Doppler effect produced by movement of the movable mirror 14. Thus the frequency component having frequency ι changes in frequency to f]_ x Δf Doppler, and the frequency component having frequency f 2 changes in frequency to f ~ xi Doppler. Since only unidire.ctionally polarized compone'nts of a light beam can combine through the beam splitter 10, the frequency component of the laser beam having frequency fτ_ which has traversed the first optical path 11 and which has been changed in polarization by 90° r will reco bine with the frequency component of the laser beam having frequency f 2 ± Δ Doppler which has traversed the second optical path 13 due to their like polarizations. A recombined component of a light beam in one polarization plane 27 will thus exhibit a frequency of fτ_ - (f ± Δf Doppler). The recombined component of a light beam in the perpendicular polarization plane 29 exhibits a frequency of (f ] _ ± Δf Doppler) - f 2 . The light beam components in polarization planes 27 and 29 having orthogonal polarization are directed from the interferometer through a polarizer 31 whicn filters, i.e. removes one of the two polarized components. Thus, the detector 44 will receive a light beam having uniplanar polarization and having a frequency which is modulated by the combination of the differing frequency components of the laser beam having traversed different optical paths, respectively, where one may have a varying Doppler shift in frequency Δf Doppler introduced. It should be noted that a Doppler shift in frequency Δf Doppler is introduced to the light beam components only when the movable mirror 14 is moving. When the movable mirror 14 is held stationary, there is no Doppler effect generated. Thus, when the mirror 14 is stationary the frequency component having frequency fτ_ traversing the first optical path 11 will recombine at the beam splitter 10 with the same polarization frequency component having differing frequency f 2 traversing the second optical path 13, to yield a heterodyne frequency light beam which exhibits an amplitude (intensity) modulation, or beat signal having a frequency equal to the diffrence between the component frequencies, i.e., f ] _ - f 2 . If frequency f ] _ equals f 2 as in prior art systems, there is no beat signal generated. Where frequency fτ_ differs from f 2 , as with the Zeeman split component frequency modes used in the present invention, a continuous beat signal is generated whose frequency, i.e. beat frequency, is modulated by Doppler shifts Δf Doppler in the frequency of one component. Thus, the photodetector 44 will receive a light beam having a measurable and continuous beat signal and when the frequency equals f j _ - f , it can be determined that the mirror is stationary.. Due to the continual intensity modulation or beat signal exhibited by the emerging light beam 30, even when the movable mirror 14 is stationary, an information signal can be continually produced which permits velocity control and position measurement of the mirror 14 throughout the scan range. Control of the mirror scan velocity and duration is easily accomplished by monitoring the increase or decrease in the beat signal frequency exhibited by the emerging light beam 30. A difference in frequency from the £ - f 2 frequency difference exhibited by the combined component frequency modes of the laser beam when the mirror is stationary is indicative of mirror velocity. By determining whether the beat signal frequency is increased or decreased from the frequency fτ_ f 2 displayed at mirror standstill, the direction of 2-G mirror travel an be determined. When the movable mirror 14 is moving at a constant velocity scan, the Doppler shift Δf Doppler introduced into the beat signal frequency of the light beam will be constant. It is this beat signal ' frequency which the present mirror scan servo -control uses to control mirror scan velocity. The magnetically influenced laser 18 is stabilized to provide a constant separation between the frequenciess of the component frequency modes, i.e., fη_ - - y . This maintains a constant beat signal (intensity modulation) frequency in the laser beam 16. The stabilized beat signal frequency increases the accuracy of analysis of the mirror velocity and position. An exactly predictable bea signal frequency permits accurate measurement of a Doppler shift frequency .Δf Doppler to determine whether a constant velocity scan of the movable mirror 14 is being performed, and to generate a control signal to velocity errors determined. Referring to Figure 1, a basic element of the Mirror Scan Servo Control is a reference signal synthesizer 54. The signal synthesizer 54 generates a reference signal 56, which is applied to a laser servo control 58 to stabilize the difference in frequencies between the component frequency modes of the laser beam 16. The stabilized difference in frequency between the frequency components of the laser beam 16 provides the constant intensity modulation or beat signal when the components of beam 16 are combined in the laser beam entering the interferometer. The reference signal 56 may also be applied to a dedicated computer (not shown) as a reference the beat signal frequency (laser beam intensity modulation) is required for data measurement analysis. The signal synthesizer 5.4 also generates a second reference signal 60 which exhibits the frequency of the first reference signal 56 with a selected upward or downward frequency shift Δf theoretical. The upward or downward direction of frequency shift is determined by a forward/reverse input signal 62 applied to the signal synthesizer by a directional control 64. The magnitude of the frequency shift Δf theoretical is determined by a programmable scan speed selector 66. The second reference signal 60 is applied to the mirror scan servo control 50 as an absolute reference for control of the scan velocity of the movable mirror 14. The reference signal synthesizer 54 comprises digital electronic circuitry generally known in the art for generating shifted frequency and variable frequency signals. For example, the frequency synthesizer 54 may include a crystal oscillator which generates a frequency stabilized signal having a uniform periodic waveform. The output signal of the crystal oscillator may be varied in frequency, for example, by a divider or multiplier circuit to provide reference signals 56 and 60. The frequency of the reference signal 56 may be selected within a range of frequencies. The range is determined by the differences in frequency that may be exhibited between the component frequency modes.of laser beam 16 through variance of laser operating parameters. For a magnetically influenced He-Ne laser the range is generally 100 to 1500 KHz. It is preferable to select a frequency at a lower level in this range to provide a more useful signal aαapted for use with currently available digital electronic components. The reference signal 56, for example, is preferably controlled at a frequency of 250 KHz. This may be obtained by selecting a crystal oscillator of the correct frequency or by using a number of known frequency generators, such as one having a multiplier/divider circuit applied to a higher frequency crystal oscillator. The 250 KHz frequency selected for reference signal 56 determines the exact frequency difference fτ_ - f desired between the component frequency modes of the laser beam 16. This permits use of a phase-locked control loop to stabilize the frequency difference which results in a stable intensity modulation, or beat signal freσuency equal to the 250 KHz frequency of the reference signal. This stable 250 KHz beat, signal frequency is easily measurable in the energizing laser beam by known electronic devices. It provides a useful information signal which can be used to determine the velocity and position of the movable mirror 14, and reference interferogram data measurements. The frequency of the second reference signal 60 is based upon the selected frequency of the first reference signal 56. The reference signal 60 is thus based upon tne 250 KHz frequency of the first reference signal and is modified in frequency upwardly or downwardly by a selected value equal to Δf theoretical which is desired or caused by movement of the mirror 14. The absolute value of the frequency modification is programmable into the reference signal synthesizer. Various circuits and techniques for obtaining a selected frequency shift in a signal are well known to those skilled in the art pertaining to signal trequency synthesizers . The reference signal 60 is preferably modified in frequency by 5 KHz, upwardly or downwardly, to provide a 245 KHz or a 255 KHz signal as determined by the directional control 64. The change in frequency which is added to or subtracted from the 250 KHz frequency of the first reference signal 56, can be applied to the mirror scan servo control 50 to accurately determine the scan velocity of the movable mirror 14 throughout the scan range. The selected 5 KHz frequency shift is utilized to drive the movable mirror 14 at a constant velocity of approximately 0.3 centimeters per second, in either forward or background direction, throughout the scan range. The directional control 64 can comprise an up/down counter array for summing the changes in phase between the beat signal generated from the heterodyned component frequencies (controlled to 250 KHz) and the frequency of the intensity modulation detected from the heterodyned laser beam emerging from the interferometer (250 KHz ± Doppler frequency shift). By maintaining a count of the number of phasing changes occurring between these signals, the distance that the movable mirror 14 has traversed in a scan can be determined. For example, the up-dov/n counter array may have an up-count input responsive to the signal 68 to generate an increasing count for each cycle of the signal detected trom the entering laser beam and a down-count input responsive to the signal 74 to generate a decreasing signal detected from the intensity modulation of the emerging laser beam. Consequently, the up-down counter array maintains counts of the number of phase changes occurring between these signals. The number. of phase changes counted is compared to the known number of phase changes which must be made during the total length scan. A typical scan range of 2 centimeters exhibits approximately 6.3 x 10 changes in phase as the movable mirror 14 moves through a scan. By determining the number of phase changes which movement of the mirror 14 has caused within the known total number of phase changes occurring within the scan range, the directional control determines the position of the mirror within the scan range and when the end of a selected scan is reached by the mirror 14. When the mirror 14 has reached the end of the useful scan range, i.e., a selected number of phase shifts, the directional control produces a forward/reverse signal 62 which is supplied to the reference signal synthesizer 54 to change the frequency of the second reference signal 60 from an increased value to a decreased value of Δf theoretical or from-the decreased value to an increased value, whichever is opposing. The change of the second reference signal frequency instructs the mirror scan servo drive to change the scan direction of the movable mirror 14. The design of the up/down counter array described as comprising the directional control is generally known to those skilled in the .art. The up/down counter array is a standard digital technique and may be constructed with Motorola CMOS 4029 counters as described in related Motorola data sheets. A signal 68 having the frequency characterizing the beat signal of the laser beam, is obtained for the directional control 64 from the output of a pnotodetector 70 detecting a rearwardly directed portion 72 of the laser beam. Signal 68 is the same as the signal obtained from the laser for the laser servo control 58. A signal 74 having a characteristic frequency of the heterodyned components of the laser beam 30 is obtained from the signal produced by a detector 44 which detects the intensity changes in the laser beam 30 leaving an interferometer. The laser stabilization control 58 stabilizes the- difference in frequency f- j _ - f 2 between the component frequency modes of the laser beam. The laser servo conrol 58 accomplishes stabilization by a phase-lock electronic control loop technique. The phase lock control loop is closed by supplying a correction signal to frequency adjusting elements of the laser 18, which accurately control the difference in frequency exhibited in the laser beam. By applying phase-lock control to the laser utilizing the stable 250 KHz reference signal 56, the beat signal frequency found in the laser beam is stabilized at 250 KHz. Since the beat signal frequency is equal to the difference in frequency between the frequency components, the frequency difference between components of the laser beam entering the interferometer is also stabilized at 250 KHz. The circuit and operation of the laser servo control 58 is described in a copending patent application for a Laser Stabilization Control Means, invented by Wyntjes and Hersher, Serial Number 472,538 and filed on March 5, 1983. Thus, the beat signal frequency of the laser beam 16 entering the interferometer is stabilized at a known value of 250 KHz. The laser beam 30 exiting the inter erometer will also exhibit a beat signal with a frequency of 250 KHz when the movable mirror 14 is stationary, i.e. there is no Doppler frequency cnange introduced. The accurate control of the beat signal frequency exhibited by the laser beam through the interferometer provides a highly accurate measurement tool for indicating the position of the movable mirror 14 and for controlling velocity of mirror scan. It is of great advantage that due to the mixing of the two component frequency modes of the laser beam the beat signal in the laser beam is present continually, independent of mirror motion. Mirror, position and velocity control is accomplished by the following measurements. A measurement of the number of phase changes occurring between the beat signal frequency of the beam leaving the interferometer and the beat signal frequency exhibited by the laser beam entering the interferometer provides accurate means of determination of mirror displacement along the mirror scan range. This is accomplished by standard digital counting techniques in the directional control 64. A measurement of the difference in frequency between the beat signal freσuency exhibited by the laser beam leaving the intefero eter and the beat signal frequency of the laser beam entering the interferometer, provides accurate indication of velocity of the scanning mirror. This is accomplisned by phase comparison of the respective signals. By using the frequency difference detected , scan velocity of the mirror can be maintained constant. The mirror scan servo control system comprises two phase locked electronic control loops for generating a mirror scan control signal 52. Scan control 50 supplies the scan control signal 52 to mirror drive electronics (not shown) for controlling the movement of mirror 14. The scan control 50 is shown schematically in Figure 1. Referring to this figure, the scan control 50 receives an electrical signal 80 produced by the detector 44 in response to intensity fluctuation exhibited in the emerging laser beam 30 which the detector 44 receives. The signal 80 provides information of the intensity modulation, or beat signal frequency, and phasing of the heterodyne light beam emerging from the interferometer. The signal 80 is an electrical signal whose voltage oscillates with a frequency equal to the intensity variation of that portion of the heterodyne beam 30 it measures. The detector 44 is generally positioned centrally with reference to the cross section of the beam 44 so that the portion of the light beam detected will clearly define the investigated beat signal. The detector signal 30 is applied to a first phase locked servo control loop referred to as the mirror position control 81 and to a second phase locked servo control loop referred to as the mirror velocity control 86. The mirror position control 84 and the mirror velocity control 86 cooperate to provide a highly efficient mirror scanning control signal 52 for driving the movable mirror 14. The detector signal 80 is applied to a frequency divider 88 which divides its frequency by a value of 16 prior to its application of the mirror position control 84, as indicated by 81. Similarly, a reference signal 60 generated.by the reference signal synthesizer 54 is applied to the mirror position control 34 to provide information of ideal mirror position and • - • locity. The reference signal exhibits a frequency of 250 KHz plus or minus the expected Doppler shift in frequency Δf, which summed values are also divided by a value of 16 to proportionately reduce frequency. Frequency is reduced in both the detector signal 80 and the reference signal 60 to permit an increased range of position comparison using the mirror position control 84. This is explained in that each cycle of the signals applied to the mirror position control 84 is equal to 16 times the actual frequency and tnus the actual phase of the applied reference and detected signals. Thus by using for error detection signals which are expanded by 16 times, the rangt over which correction may be -Trccomplisned is nn.ancea by 16 times. The electronics of the mirror position control b4 is schematically shown in Figure 3. The mirror position control comprises a phase comparator 90 which receives the detector signal 31, divided by 16, from the frequency divider 88. Comparator 90 also receives the reference signal 6U which has been divided by 16 from the reference signal s ntnesizer 54. The phase comparator compares the phases of the received signals, and produces an output signal 92 which is proportional to tne difference in phase between them. The output signal 92 prererablv increases or decreases in voltage in response to phase difference. Signal 92 is thus proportional to a difference in frequency between the input signals. The output signal 92 is a position error signal whose voltage is changed in relation to the difference in mirror position from that* theoretically desired, at any given time during mirror scan, due to comparison with the reference signal that provides the theoretically correct information. This signal can thus be used as a correction signal for position of the movable mirror 14. ~ The phase detector 90 is a commercially available device manufactured by Motorola, Inc. and available as Part No. MC14046B. Further information regarding the phase detector may be obtained from Motorola CMOS Data Book on page 7-124. The output signal 92 of the phase detector 90 (position error signal) is modified to a desired voltage level by a selected resistive network 93 and is provided with capacitance filtering 94, such as by .01 mic capacitor to provide selected averaging of peaks in the generated signal. The position error signal 72 is then applied to selected gain and filter electronics 95 as shown in Figure 1, after which the signal is applied to a differential amplifier 100. The detector signal 80 is also applied to the mirror velocity control 86. The mirror velocity control 86, illustrated schematically i ' n Figure 4, comprises a tracking voltage controlled oscillator circuit. Specifically, a phase comparator 96 is provided which receives the detector signal 80 and the output signal 97 of the voltage controlled oscillator to generate an output signal 98. The output signal 98 of the pnase comparator is provided with a selected voltage range through resistive network 99 and capacitance filtering 101. Signal 98 is then applied to the input of the voltage controlled oscillator 102. By applying the output signal 98 of the phase comparator 96 to the input of the voltage controlled oscillator 102, the oscillator is driven to generate an output 97 whose frequency is controlled by the phase comparator output 98. The oscillator output 97 is thus driven to a selected frequency value matching the detector signal frequency, which frequency value varies the phase comparator output 98 to define an electronic control loop. By using the output signal 98 of the phase comparator, an analog varied voltage control signal is provided which can be used as a velocity error signal. The velocity error signal indicates the rate of change of position of the movable mirror from that theoretically desired. The voltage of the signal 98 is thus changed in relation to the difference in actual mirror velocity from that theoretically desired at any given point in time. The signal thus provides an anticipatory control signal for any change in mirror velocity. This signal being faster in response to errors in mirror position and scan velocity acts to damp the control effects of the position error signal 92 generated by the mirror position control 84. The phase comparator 96 provided in the mirror velocity control similarly compares of the signals 80 and 97 applied, and produces an output signal which- is proportional to the difference in phase between them. The phase detector 96 is a commercially available device manufactured by Motorola, Inc. and available as Part No. MC14046B. E'urther information regarding the phase detector may be obtained as previously indicated. The voltage controlled oscillator 102, and its application as a tracking voltage controlled oscillator circuit, is also a commercially available device manufactured by Motorola, Inc. and incorporated in Part No. C14046B. Further information regarding this device may similarly be obtained rro the Motorola CMOS Data 3oo . The output signal 98 of phase comparator 96 is also modified by selected gain and filter electronics after which it is applied to differential amplifier 103. . . The position error signal 92, and the velocity error signal generated by the mirror position control 84 and the mirror velocity control 86, respectively, are applied to the differential amplifier 100 with proper polarity so that differential amplifier 100 acts to sum the position of velocity error signals. The summed signal, applied as the mirror control signal 52 and thus contains information of position error of the movable mirror at any point in time and acts also an anticipatory signal for velocity error of the mirror scan velocity, signal 52 is applied to the mirror drive electronics. A laser signal stability compensator 108 is provided in the mirror scan servo control to compensate the position error signal 92 generated by the mirror position control 34 for inaccuracies and errors in stabilization of the difference in frequencies between the component frequency modes of the laser beam. It is important to correct for these inaccuracies and .errors because the difference in frequency is essentially the basis of intensity modulation in a light beam 30 upon which the detected signal 30 is generated. Referring to Figure 5, the laser signal stability compensator 108 comprises a phase comparator 110 to which a reference and a laser detector signals 56 and 68, respectively, are applied. The reference signal 56 is generated by the reference signal synthesizer 54 and exhibits a stabilized frequency of 250 KHz, which frequency is tne difference in frequencies at which the frequency components ■ of the laser beam should be continually separated. The laser detector signal 63 is generated by detecting the intensity modulation, i.e., beat signal frequency, of the light beam emitted from the laser and ideally should also . have a frequency of 250 KHz. However, generally it will include a freαuencv error due to inaccuracies in control of the laser referred to as E L . The phase comparator 110 compares the phases of the applied signals 56 and 68, and produces an output signal 112 which is proportional to the difference in phase between them. The output signal 112 of phase comparator 110 thus will be a signal whose voltage is proportional to the frequency error found in the detector signal applied, i.e. E L . The output signal 112 of the phase comparator 110 is provided with a selected voltage adjustment, such as by variable rheostat 113, to generate a laser error signal which is used to compensate the position error signal 92 for errors in the laser frequency difference. The laser error signal 112 is applied and summed with the position error signal 92 prior to application of the position error signal to its resoective gain and filter electronics. When the interferometer is in operation and the mova-ole mirror 14 is scanned, the mirror scan servo control 50 continually compares the phase relationships of the detected signal 80, the reference signals 56 and 60, the detected laser signal 68 to generate a mirror scan control signal 52 which contains information of errors in mirror position, scan velocity, and laser frequency control. Control signal 52 can be used by the mirror drive electronics to incrementally increase or decrease the scan velocity of the movable mirror 14 to oring the scan to a theoretically desired parameter. The mirror scanning control signal 52 will be incrementally adjusted to bring the phasing relationships described for the detected and referencing signals into an ideal phase locked control to theoretically provide accurate constant velocity scan of the movable mirror. When a lock in phase of obtained the mirror scan control signal 52 will stabilize to indicate a perfect constant velocity scan of the movable mirror to the mirror drive electronics. The selected scan velocity can be easily cnanged by programming the reference signal synthesizer ">! to generate a reference signal having a different frequency base. The different frequency reference signals 56 and 60 generated will be phase locked to the m .-sured intensity modulation of the beam to drive the movable mirror 14 at a different scan velocity, thus producing a Doppler frequency shift equivalent to the theoretical frequency shift selected in the reference signals. As the movable mirror 14 reaches an end of scan, the direction control 64 instructs the reference signal synthesizer 54 to change the frequency in the reference signals from an increased to a decreased value, or vice versa. The change in sign of the reference frequency shift requires the Doppler shift in frequency produced by the movable mirror 14 to change in sign, for example, from a +5 KHz to a -5 KHz. This requires the mirror to change the direction of scan. Therefore, phase comparison in the mirror scan servo control 50 produces a mirror scan control signal 52 which instructs the mirror drive electronics to scan the mirror 14 in an opposing direction. Bidirectional scan velocity control is accomplished by the mirror scan servo control 50 without additional circuitry to the scanned circuit described. Since the reference signals 56 and 60, and the beat signal frequency measurable in the heterodyne light beam 30 are continuously displayed, the mirror scan velocity and position control may be maintained througnout the directional change of the mirror scan. This affords greater assurance of accuracy and reduces the number of scans required to generate data for an interferogram. The usable portion of each mirror scan during v/hich control is maintained is substantially increased.

Claims: What is claimed is:. 1. A closed loop servo control for controlling the movement of a movable mirror in an interferometer used in spectroscopic measurement of a sample material, comprising: first means for producing a laser beam having more than one frequency component to produce a continuous modulation frequency from which movement of said movable mirror may be determined; second means for producing a reference signal having a frequency characteristic of a theoretical modulation frequency of said laser beam, which indicates a constant rate of movement of said movable mirror; third means for detecting the modulation frequency of said laser beam to obtain an electrical signal characteristic of said modulation frequency; fourth means receving said reference signal from said second means and said electrical signal from said third means, for comparing the phase between said reference signal and said electrical signal and producing a position error signal responsive to the difference in phase between said signals, and a velocity error signal responsive to the rate of change of mirror position with respect to time; and control means responsive to said error signal for controlling movement of said movaDle mirror, to obtain a laser beam from said interferometer having a modulation frequency which, when detected by said third means, produces an electrical signal having a phase which is locked in phase with said reference signal, to determine position velocity movement of said movable mirror. 2. The servo control of claim 1, wherein said first means includes a laser influenced by a magnetic field to obtain a laser beam having a plurality of component frequency modes of differing frequency, which component frequency modes produce a laser beam having a continual modulation frequency. 3. The servo control of claim 1, wherein said third means comprises at least one photodetector responsive to the modulation frequency of said laser beam. 4. The servo control of claim 1, wherein said fourth means comprises: a first phase detector, which generates an error signal proportional to the differences in phase between said reference signal from said second means and said electrical signal from said third means ; a second phase detector which generates an error signal proportional to a rate of change with respect to time from said electrical signal from said third means; and means for summing said error signals to produce a control signal containing information of mirror position and velocity. 5. The closed loop servo control cf claim 1 wherein said reference frequency produced by said second means is equal to a desired modulation frequency of said heterodyne laser beam, which indicates a constant rate of movement of said movable mirror. 6. A closed loop servo control for controlling the movement of a movable mirror in an interferometer used in spectroscopic measurement of a sample material comprising: first means for producing a heterodyne laser beam having a continuous amplitude modulation frequency from which the position and rate of movement of the movable mirror may be determined; second means for producing a first reference signal having a frequency characteristic of a desired modulation frequency of said heterodyne laser beam, and a second reference signal having a frequency characteristic of a desired modulation frequency which indicates a constant rate of movement of said movable mirror; third means for detecting said modulation frequency of said laser beam leaving said interferometer to obtain an electrical signal characteristic of said modulation frequency; output means receiving said reference signals from said second means and said electrical signal from said third means, responsive to the frequencies of said signals and producing an output signal having a first charactristic indicating error in mirror position at any point in time, and a second characteristic indicating error in velocity of said mirror at any point in time proportional to said difference; and drive means for driving the movable mirror responsive to said output signal to increase or decrease the rate at which said movable mirror is oveti. A mirror scan servo control for bidirectionally driving a movable mirror of an interferometer used in spectroscopic measurement, at constant velocity scan comprising: means for generating a laser beam having a characteristic modulation frequency and directing said beam through said interferometer to receive a change in modulation frequency in response to a doppler effect generated by movement of the movable mirror, and exiting said beam from said interferometer with a modulation frequency characteristic of the rate of scan of said movable mirror; a reference signal generator producing a pair of constant frequency reference signals, a first signal having a frequency greater than said modulation frequency of said laser bean and a second signal having a frequency less than said modulation frequency of said laser beam; means for detecting said laser beam leaving said interf rometer to produce an electrical signal having a frequency proportional to the modulation frequency displayed by said laser beam; a phase comparator receiving said reference signal and said electrical signal, the detector producing an output signal having a voltage proportional to the phase difference between said reference signal and said electrical signal, which indicates position error of said mirror, a tracking voltage controlled oscillator receiving said electrical signal and providing a phase-lock control loop to generate an output signal which indicates velocity error of said mirror during a scan, and drive means for bidirectionally driving said movable mirror responsive to said drive signal, said drive means responding to said output signals of said phase comparator and said tracking voltage controlled oscillator to drive said mirror in said laser bean as generated by the doppler effect of the moving mirror, to bring the phase of the modulation frequency into phase with the phase of said reference siσnal. A closed loop servo control for controlling the movement of a movable mirror in an interferometer used for spectroscopic measurement of a sample material, comprising: first means for producing a heterodyne laser beam having a continuous modulation frequency from which movement of said movable mirror may be determined; second means for producing a reference signal having a frequency characteristic of a desired modulation frequency of said heterodyne laser beam, which indicates a constant rate of movement of said movable mirror; third means for detecting said modulation freσuency of said laser beam to obtain an electrical signal characteristic of said modulation frequency; fourth means receiving said reference signal from said second means and said electrical signal from said third means, for comparing the phase between said reference signal and said electrical signal and producing an error signal responsive to the difference in frequency between said signals; fifth means for receiving said electrical signal from said third means to compare phase with a controlled frequency signal responsive to the output of said phase comparison and producing an error signal responsive to the difference in frequency between said signals; sixth means for generating an error signal responsive to changes in said moduation frequency of said laser" beam, and control means responsive to said error signals for controlling the rate of movement of said movable mirror, and to incrementally adjust said rate to obtain a phase lock control loop from which movement of said mirror may be determined. 3 S 9. The servo control of claim 7 wherein said second means is responsive to a direction signal to prouce an increase or decrease in frequency of said reference signal and additionally comprising means for determining the position of said movable mirror within a range of movement, and for producing a direction signal. 10. The servo control of claim 15 wherein said second means additionally comprises means for changing the frequency of said reference signal a selected value, to increase or decrease said reference signal relative to said continuous modulation frequency of said laser beam to obtain bidirectional control of said movable mirror. f

Inventors: BROWN JAMES R, COBY AUGUST D, VAN VLIET JAMES G, BROWN, JAMES, R., COBY, AUGUST, D., VAN VLIET, JAMES, G.
Applicants: BECKMAN INSTRUMENTS INC, BECKMAN INSTRUMENTS, INC.
Application year: 1985
Main classification: G01J3
IPC codes: G01J3