A velocity servo for continuous scan Fourier interference spectrometer of the double pass retroreflector type having two cat's eye retroreflectors is described. The servo uses an open loop, lead screw drive system for one retroreflector with compensation for any variations in speed of drive of the lead screw provided by sensing any variation in the rate of reference laser fringes, and producing an error signal from such variation used to compensate by energizing a moving coil actuator for the other retroreflector optical path, and energizing (through a highpass filter) piezoelectric actuators for the secondary mirrors of the retroreflectors

Schindler, R. A.

English

NASA Technical Reports Server

United States Patent [I91 P I 1  4,193,693 
[45] Mar. 18, 1980 Schindler 
[54] VELOCITY SERVO FOR CONTINUOUS 
SCAN FOURIER INTERFERENCE 
SPECTROMETER 
[76] Inventor: Robert A. Frosch, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Rudolf A. Schindier, 
Sierra Madre, Calif. 
[21] Appl. No.: 880,729 
[22] Filed: Feb. 24, 1978 
[51] Int. Cl.2 ................................................ GOlB 9/02 
[52] U.S. c1. .................................................... 356/346 
[58] Field of Search ......................................... 356/346 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,488,123 1/1970 Nichols ................................ 356/346 
3,535,041 10/1970 Webb ................................... 356/349 
3,809,481 5/1974 Schindler ............................. 356/346 
3,936,193 2/1976 Auth ..................................... 356/346 
Primary Examiner-Conrad J. Clark 
Attorney, Agent, or Firm-Monte F. Mott; John R. 
Manning; Paul F. McCaul 
1571 ABSTRACT 
A velocity servo for a continuous scan Fourier interfer- 
ence spectrometer of the double-pass retroreflector 
type having two “cat’s eye” retroreflectors uses an 
open-loop, lead-screw drive system for one retroreflec- 
tor with compensation for any variations in speed of 
drive of the lead screw provided by sensing any varia- 
tion in the rate of reference laser fringes, and producing 
an error signal from such variation used to compensate 
by energizing a moving-coil actuator for the other ret- 
roreflector optical path, and energizing (through a 
highpass filter) piezoelectric actuators for the second- 
ary mirrors of the retroreflectors 
9 Claims, 2 Drawing Figures 
DETECTOR 
(FIG. 2) 
https://ntrs.nasa.gov/search.jsp?R=19800012079 2020-03-21T02:28:58+00:00Z
U.S. Patent MU. 18, 1980 Sheet 1 of 2 4,193,693 
18 DETECTOR 
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DETECTOR 
(FIG. 2) 
FIG, I 
U.S. Patent MU. 18, 1980 Sheet 2 o f 2  
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4,193,693 
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4,193,693 
VELOCITY SERVO FOR CONTINUOUS SCAN 
FOURIER INTERFERENCE SPECTROMETER 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 USC 2457). 
BACKGROUND OF THE INVENTION 
This invention relates to interferometers utilizing 
double-pass cat’s-eye retroreflectors, and more particu- 
larly to an improvement in the drive of a continuous 
scan interferometer of the type shown in U.S. Pat. No. 
3,809,481. 
In an interferometer (interference spectrometer) of 
the double-pass type, a single fixed mirror is employed 
in the path of both split beams to cause them to double 
back through separate retroreflectors. Changes in 
length of one optical path relative to the other are 
achieved by linear displacement of one retroreflector 
using a motor driven lead screw for large, low fre- 
quency changes, a moving-coil actuator in the other 
optical path for smaller, midfrequency changes, and a 
piezoelectric actuator on at least one of the two optical 
paths for small, high frequency changes. An object of 
this invention is to simplify the scanning servo system 
and circuit design without degrading performance. 
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DESCRIPTION OF PREFERRED 
EMBODIMENTS 
Referring now to FIG. I, there is shown the optical 
system of an interferometer of the double-pass, cat’s-eye 
retroreflector type more fully described in the afore- 
mentioned U.S. Pat. No. 3,809,481. The basic organiza- 
tion and use of such a system is described in U.S. Pat. 
No. 3,535,024. Briefly, the system can be used to deter- 
mined the amplitude of radiation components of various 
optical frequencies with high precision. Such a determi- 
nation is made by detecting the amplitude of an interfer- 
ence fringe through a photo-detector IO and measuring 
the amplitude for continuously scanned optical path 
differences in the interferometer. The components of 
various frequencies in the radiation are determined by 
utilizing a Fourier transformation of the output ampli- 
tude of the detector versus optical path difference. 
Basically, the apparatus is comprised of a beam-split- 
ter plate 11 having a half-silvered front surface 12 at one 
end and a full silvered back surface 153 on the other end 
of the plate. The half-silvered surface is broad enough 
to receive and split a beam from an unknown source as 
well as a beam from a source 14 of known wavelength. 
The full silvered mirror is also broad enough to receive 
and deflect the split beams from the unknown source, 
and the known source passing through a first retrore- 
flector 15, but not through a second retroreflector 86. A 
fixed mirror E7 receives the split beams through both 
retroreflectors and doubles them back for recombina- 
tion at the half-silvered surface 12. The combined beams 
SUMMARY OF THE INVENTION of the unknown wavelength are deflected by a mirror 
18 into the photodetector BO. The recombined beam of 
In accordance with the present invention, the lead the known wavelength is directed into a separate photo- 
screw for moving a retroreflector in a continuous scan 35 detector 19. 
Fourier interference spectrometer of the double-pass A small mirror 20 is used to fold the input laser beam 
type having two ’‘Cat’s eye’’ retroreflectors is driven path into the interferometer, and a mirror 21 is used to 
Open loop to Scan ContinuouslY from one extreme Path fold one of the split beam paths, to permit the retrore- 
difference to another using a synchronous motor ener- flector 16 to be oriented parallel to the actuator 15 for 
gized from an AC current source of substantially con- 40 more compact packaging of the instrument. An 
stant frequency to move one retroreflector for changes tional benefit of using the mirror 21 is that it provides 
in Path length while sensing any variation in speed of for making small low frequency changes in the optical 
the lead screw as a variation in the rate of reference path of the moving retroreflector to compensate for 
laser fringes from a laser beam Passing through the deviation of the speed of a lead-screw 22 driven by a 
interference spectrometer into a separate detector. Any 45 synchronous motor 23. A moving-coil actuator is 
deviation from a predetermined fringe rate directly employed to move the mirror zn at low frequencies of 5 
related to the desired lead-screw speed produces an to 500 HZ. Alternatively, the moving-coil actuator 
error signal that is applied to a moving-coil actuator to could be used to move the retr,oreflector 15, but it is 
change the path length through the other retroreflector preferred to move the nairror 21 at the frequencies of 5 
in a compensating direction for correction in variations 50 to 5W HZ becallse of its smaller mass. Secondary mir- 
in the low frequency range of 5 to 500 Hz, and to a rors 1% and 1 6 ~  may both be mounted on piezoelectric 
piezoelectrically driven secondary mirror for at least actuators 25 and 26 in order to alter the path lengths of 
one of the two retroreflectors for variations with high the split beams foor compensation of the lead-screw 
frequency above 500 Hz. Variations in the very low speed at high frequencies above 500 Hz. Variations of 
frequency range of less than 5Hz will not occur in a 55 the lead-screw speed bellow 5 Ppz will not occur because 
properly designed motor driven lead screw using a it is driven by a synchronous motor, or a stepping mo- 
synchronous motor. tor, energized by a sinusoidal signal of substantially 
The novel features that are considered characteristic constant frequency from a source 27. A stepping motor 
of this invention are set forth with particularity in the is basically a form of a synchronous motor; a stepping 
appended claims. The invention will best be understood 60 motor i s  characterized by having many poles to obtain 
from the following description when read in connection small angular increments and by low inductance wind- 
with the accompanying drawings. ings. 
To accelerate the motor and lead screw at the start of 
a scan, the drive frequency ofthe source 27 is gradually BRIEF DESCRIPTJoN OF THE 
FIG. 1 is a schematic diagram ofthe optical system of 65 brought up to the required scan speed. Theriafter the 
an interferometer of the double-pass retroreflector type. motor is driven at an accurate, icrystal controlled fre- 
FIG. 2 is a schematic diagram of the improved scan quency. That, along with a precision ground antiback- 
drive system of the present invention. lash ballscrew and ballnut assembly in the lead screw, 
4.193,693 
3 
ensures a high degree of velocity stability in a low fre- 
quency sense. If compensation is necessary for any devi- 
ation of the lead-screw speed, it can be provided 
through the moving-coil actuator 24 and piezoelectric 
actuators 25 and 26. 
To monitor the speed of the retroreflector 16, Le., to 
monitor the rate at which the path length difference is 
varied, the light of known wavelength from the source 
14 is split, passed through the retroreflectors, and dou- 
bled back to recombine at the half-silvered surface 12. 
As the path length difference varies, fringes are pro- 
duced in the recombined light of known wavelength. 
These fringes occur at a predetermined rate, e.g. 800 
kHz, at the desired lead-screw speed. The photo-detec- 
tor 19 is connected to a conventional FM detector 
(FIG. 2) which provides an output linearly proportional 
to lead-screw speed over a range 700 to 900 kHz. By 
introducing a proper offset voltage in the detector, the 
output may be made zero at 800 kHz, positive at 900 
kHz and negative at 700 kHz in order to provide com- 
pensation in the proper direction for any deviation, both 
over and under the desired scan rate. 
Referring now to FIG. 2, the output of the FM detec- 
tor 28, is directed through a filter 20 which passes fre- 
quencies above about 5 Hz. The output of the filter is 
then applied through a switch 30 to a servo loop for the 
moving-coil actuator 24 and, via a highpass filter 31 and 
drive amplifier 32, to one of both piezoelectric actuators 
25 and 26. If both, one is connected with a polarity 
opposite the other so that they move in opposite direc- 
tions in response to any fringe rate deviations that occur 
at frequencies greater than 500 Hz. The filter 31 blocks 
deviations below 500 Wz. 
The moving-coil actuator 24 is comprised of a mov- 
ing coil 24a attached to a yoke 24b. The mirror 21 is 
attached to the yoke. A stationary permanent magnet 
24c completes the moving-coil actuator (except for 
bearings or flexures not shown) to constrain the mirror 
to move with its reflecting surface always at the same 
reflecting angle. A velocity pickup coil 42 on the yoke 
24b provides a velocity feedback signal through an 
amplifier 33 to a summing circuit 34. An amplifier 35 
then drives the moving coil 24u with the difference 
between the output of the detector 28 and the velocity 
feedback signal. 
The AC drive source which energizes the synchro- 
nous motor 23 is controlled in direction by direction 
control switches 36 and 37. Upon momentarily closing 
switch 36, the AC drive source 27 is latched for drive in 
a forward direction, and upon momentarily closing 
switch 37, the AC drive source is latched for drive in 
the reverse direction. These two direction control 
switches are applied to a direction control circuit 38 
which, in its simplest form, is a latching (set-reset) flip- 
flop. In the forward direction, the circuit is set to pro- 
vide a zero output signal. The switch 30 is then in the 
position shown to connect the output of the filter 29 to 
the compensation drive circuits. When the reverse 
switch is momentarily closed, the circuit 38 is reset to 
provide a positive output signal which, through an 
amplifier 39, energizes a relay 40 to connect the switch 
30 to an inverter 41, thereby reversing the sense (polar- 
ity) of the signal being applied to the compensation 
circuit for motion of the lead screw in the opposite 
direction. A relay switch is shown for simplicity. In 
practice an electronic switch would be used instead. 
Although particular embodiments of the invention 
have been described and illustrated herein, it is recog 
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nized that modifications and variations may readily 
occur to those skilled in the art. For example, the mov- 
ing-coil actuator could be applied to the retroreflector 
15 instead of the mirror 24. It is therefore intended that 
the claims be interpreted to cover such modifications 
and variations. 
I claim: 
1. In a continuous scan Fourier interference spec- 
trometer of the double-pass type having two cat's-eye 
retroreflectors and an open-loop, lead-screw drive sys- 
tem for one retroreflector to continuously vary the 
length of one optical path relative to another optical 
path through the other retroreflector, the combination 
comprising a source of a reference beam of known 
wavelength, said reference beam being directed 
through said spectrometer, means for detecting the rate 
of interference fringes of said reference beam as said one 
retroreflector is driven to vary the length of said one 
optical path, and for producing an error signal propor- 
tional to deviation of said rate with respect to a prede- 
termined rate, and means responsive to said error signal 
for compensating any deviation of the rate of variation 
of the path length difference between said one optical 
path relative to the other optical path from said prede- 
termined rate as said lead-screw drive system continues 
to vary the length of one optical path relative to the 
other. 
2. The combination of claim 1 including an element 
attached to a moving-coil actuator for altering the 
length of one optical path and wherein said compensa- 
tion means is comprised of low-frequency response 
means for driving said moving-coil actuator in response 
to said error signal. 
3. The combination of claim 2 including piezoelectric 
elements attached to mirrors in said retroreflectors for 
altering the optical path lengths therethrough in re- 
sponse to a signal, and wherein said compensation 
means is further comprised of high-frequency response 
means for driving said piezoelectric elements in re- 
sponse to said error signal. 
4. The combination of claim 2 wherein said element 
attached to said moving-coil actuator is comprised of a 
plane mirror in the path of one of said two retroreflec- 
tors positioned to alter the direction of the optical path 
through a predetermined angle, and said moving-coil 
actuator moves said mirror with its reflecting surface 
always at the same reflecting angle. 
5. The combination of claim 4 wherein said angle is 
90". 
6. In a velocity servo for a continuous scan Fourier 
interference spectrometer of the double-pass retrore- 
flector type having two retroreflectors and an open- 
loop, lead-screw drive means for moving one retrore- 
flector continuously from one extreme position to an- 
other at a desired rate, apparatus for compensating for 
any variations in speed of said one retroreflector com- 
prising: a reference beam of known wavelength di- 
rected through said interference spectrometer, means 
for detecting intr rference fringes of said reference beam 
directed through said interference spectrometer and for 
producing an output signal linearly proportional to the 
speed of said one retroreflector that is zero for a de- 
tected rate corresponding to said desired rate, of one 
polarity for a detected rate greater than said desired 
rate, and of opposite polarity for a detected rate less 
than said desired rate, and moving-coil means respon- 
sive to said output signal for altering the optical path 
length through one of said retroreflectors in response to 
4,193,693 
5 
said output signal, thereby compensating for any devia- 
tion of said lead-screw drive means from said desired 
rate. 
7. Apparatus as defined in claim 6 including means for 
detecting the velocity of said moving-coil means and for 5 ing coil 
producing a velocity feedback signal proportional to 
velocity detected, and means subtracting said velocity 
feedback signal from said output signal. 
quency response means in the light path of at least one IO in a direction perpendicular to its reflected surface. 
put signal to said high frequency response means for 
compensation at frequencies higher than can be corn- 
pensated by said moving-coil actuator. 
wherein said yI1ov- 
is attached to a plane mirror in the path 
of said one of said retroreflectors positioned to alter the 
direction of the light path through a predetermined 
9. Apparatus as defined in claim 
8. Apparatus as defined in claim 7 including high-fre- angle, and said moving Coil actuator moves said mirror 
retroreflector, and a high pass filter coupling said out- * * * * *  
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Velocity servo for continuous scan Fourier interference spectrometer

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