Dynamic mirror alignment device for the interferometer of an infrared spectrometer

An interferometer mirror such as may be used in an FTIR spectrometer is mounted to a mirror alignment device which allows alignment of the mirror during operation of the interferometer. The alignment device includes a base, a mirror support to which the mirror is mounted, and means for mounting the mirror support to the base to allow resilient pivoting of the mirror about an initial position around two orthogonal axes when force is applied to the mirror support. Two drive coils of square configuration are mounted around the periphery of the mirror support. Each drive coil has lower coil sections along two opposite quadrants and higher coil sections, with the two drive coils being mounted to the mirror supports so that the lower sections of each are in adjacent quadrants. A magnetic field, such as that provided by permanent magnets, is applied to the lower sections of each coil while the upper sections of each coil are outside the magnetic field. When current is supplied in one direction to a drive coil, the current interacts with the magnetic field passing through the lower sections of the coil to tilt the mirror support about one axis of rotation in one direction, whereas current supplied to the coil in the opposite direction will tilt the magnet support in the opposite direction about the axis. In this manner, current can be supplied to the two coils as necessary to hold the mirror support and thus the mirror attached to it in a desired orientation.

FIELD OF THE INVENTION 
This invention relates generally to the field of optical systems and 
particularly to the optics used in the interferometers of Fourier 
transform infrared spectrometers. 
BACKGROUND OF THE INVENTION 
Fourier transform infrared (FTIR) spectrometers are utilized in the 
analysis of chemical compounds. In these instruments, a beam of infrared 
radiation having a band of infrared wavelengths is passed into an 
interferometer, typically a Michelson interferometer, and is modulated 
before being passed through the compound to be analyzed and then to a 
detector. The interferometer modulates the radiation received by it to 
provide an output beam in which a narrow range of infrared wavelengths is 
typically reduced or enhanced in intensity, with the affected range of 
wavelengths changing periodically over time. The time correlated output 
data from the detector is analyzed by Fourier transformation to derive 
information on the characteristics of the sample through which the beam 
was passed. 
In the typical Michelson interferometer used in such FTIR spectrometers, 
the input beam is received by a beam splitter which partially passes the 
beam through to a moving mirror and partially reflects the beam to a fixed 
mirror, or vice versa, and the reflected beams are recombined at the beam 
splitter to yield the output beam. The relative position of the moving 
mirror with respect to the beam splitter and fixed mirror will determine 
which wavelengths constructively and destructively interfere when the 
beams from the two mirrors are recombined at the beam splitter. The 
movement of the moving mirror toward and away from the beam splitter 
results in the scanning of the constructively and destructively 
interfering wave lengths across a desired band of infrared wave lengths. 
Examples of such Michelson interferometer systems in FTIR instruments are 
shown in U.S. Pat. Nos. 4,799,001 and 4,847,878. 
It is critical in the design of FTIR instruments that the surfaces of the 
fixed mirror and the moving mirror be accurately held orthogonal to each 
other. Mirror position accuracy is crucial because deviations in the 
mirror alignment produce small errors in the time domain interferogram 
which may translate into large errors in the frequency domain spectrum. In 
a typical interferometer, mirror deviations larger than one wavelength of 
the received radiation beam are considered significant and can seriously 
degrade the quality of the instrument. 
Static alignment of the mirrors of the interferometer is typically 
accomplished by means of differential screws at the back of the mirror 
which are manually adjusted to align the mirror to a desired position as 
perfectly as possible. This is a time consuming procedure requiring skill 
and experience, and adds to manufacturing expense and to field service 
costs because realignment in the field is often required. 
Efforts have been made to eliminate the need to manually align the 
interferometer mirrors. Automatic static alignment at least relieves the 
user from performing time consuming realignments. For example, stepper 
motors have been used to carry out automatically the manual alignment 
procedure described above. Such devices typically use a digital computer 
which aids in the alignment of the static mirror at periodic service 
intervals. A disadvantage of this approach is the slow speed, large size, 
high cost, and high precision bearings required for the alignment 
mechanism. To attempt to adjust the moving or fixed mirror dynamically to 
compensate for the tilting of the moving mirror as it moves on its bearing 
requires more speed that can be readily obtained with a mechanism using 
lead screws and stepper motors. Another approach has been to use 
piezoelectric positioners to align dynamically the tilt of the mirrors. 
Such positioners are also typically large and expensive, and require high 
voltage (e.g., 1000 volts) drive levels. The power supplies required for 
such high voltages also create undesirable operating hazards as well as 
being relatively expensive. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an interferometer mirror such as 
the fixed mirror of a Michelson interferometer in an FTIR spectrometer is 
mounted so that it can be tilted about two orthogonal axes. Magnetic 
positioning means are provided between the mirror support which holds the 
mirror and a base to drive the tilting of the mirror about one or both of 
the axes utilizing a control current pass through coils in the magnetic 
control units. By allowing such control over the tilt of the mirror in a 
relatively rapid and responsive manner, the mirror can be aligned 
dynamically when the interferometer is in operation to maintain alignment 
despite shifts in the position of the moving mirror as it translates back 
and forth on its bearing. The present invention provides a light-weight, 
inexpensive and rapidly responsive mirror adjustment system which allows 
accurate adjustment of mirror position at the speed required to 
dynamically align it during such operation. 
The dynamic mirror alignment device of the present invention includes a 
base, a mirror support of nonmagnetic material to which the mirror is 
mounted, or in appropriate circumstances, the mirror itself where it acts 
as its own support, and means for mounting the mirror support to the base 
to allow pivoting of the mirror about an initial position when force is 
applied to the mirror support. The mounting preferably includes a pivot 
mount comprising a bearing ball which is received in a central pivot 
pocket on the backside of the mirror support so that the mirror mount can 
pivot about two orthogonal axes. Preferably, springs or other resilient 
means are provided between the base and the mirror support at peripheral 
positions spaced away from the central pivot mount to urge the mirror 
support back to a normal position. 
Two drive coils are mounted around the periphery of the preferably square 
or rectangular mirror support. Each drive coil has straight coil sections 
along two opposite quadrants which are at a lower position, and thus 
closer to the base, and coil sections along the opposite quadrants which 
are at a higher position. The two drive coils are mounted so that the 
lower sections of each occupy adjacent quadrants. Means are provided on 
the base for applying a magnetic field to the lower sections of each of 
the drive coils to intersect and pass through the coils, preferably 
substantially parallel to the axes about which the mirror support is 
mounted for tilting. The means for providing the magnetic field can 
comprise permanent magnets mounted to the base which have a slot formed 
therein into which the lower sections of the drive coils fit, with the 
poles of the permanent magnets being arranged such that the lines of 
magnetic flux pass across the slots in the permanent magnets. 
By applying a current through one of the drive coils in one direction of 
current flow, the mirror support and the mirror attached to it will be 
tilted about one axis in relation to the current through the coil. 
Reversing the current flow through that coil changes the direction of 
tilt. Similarly, a current can be applied to the other drive coil to tilt 
the mirror about the other axis. 
Preferably, damping means are provided to damp the motion of the mirror 
about its axes to minimize overshoot and vibration. Such damping can be 
implemented in the preferred construction of the device by providing a 
damping fluid in the slots of the magnets through which the lower coil 
sections move as the mirror support is tilted. Preferably, the fluid 
filling the slots is a ferromagnetic fluid which will serve to further 
concentrate the lines of magnetic flux within the slots. Other types of 
damping mechanisms may also be utilized. 
The resilient spring return mechanisms may be implemented utilizing coil 
springs which are mounted between the base and the mirror support with 
adjustment screws allowing the initial compression of the springs to be 
adjusted. In this manner, an initial manual alignment of the starting 
position of the mirror support can be accomplished. 
Further objects, features and advantages of the invention will be apparent 
from the following detailed description when taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the drawings, the dynamic mirror alignment device of the 
present invention is shown generally at 10 in FIG. 1 in top plan view, and 
is illustratively shown having a base 12 to which is mounted a mirror 
support 13. The fixed mirror 14 of the interferometer is mounted onto the 
mirror support 13 as described further below. It is understood that the 
fixed mirror 14 may be any of the various forms employed typically in 
infrared spectrometers, and, in appropriate cases, the construction of the 
mirror support may be incorporated directly into the mirror 14 so that 
they form an integral unit. Front and back coil supports 16 are mounted to 
the front and back edges of the mirror support 13, which is preferably 
formed as a substantially square plate of a non-ferromagnetic material 
such as aluminium, and side coil supports 17 are mounted to the sides of 
the mirror support 13 and aligned orthogonal to the coil supports 16. As 
explained further below, the coil supports 16 are aligned with permanent 
magnets 19 mounted on the base 12, and the side magnets supports 17 are 
aligned with permanent magnets 20 mounted on the base 12. 
The mirror alignment device of the present invention allows dynamic 
reorientation of the mirror 14 in two degrees of freedom--rotation about 
two orthogonal axes 22 and 23 which preferably bisect the mirror 14 in 
each direction so that the mirror is symmetrical about these axes and so 
that the axes intersect at a point 24 at the center of the mirror. The 
axis 22 may be denoted the roll axis and the axis 23 may be denoted the 
pitch axis. Preferably, the two axes 22 and 23 line in the same plane, 
which may be but does not necessarily have to be within the mirror 14. For 
example, the plane defined by the lines 22 and 23 may lie below the mirror 
and pass through the mirror support 13 without affecting the operation of 
the mirror alignment device. As generally shown in the perspective view of 
FIG. 2, in which the mirror 14 is removed from the device for purposes of 
clarifying the illustration, the coil supports 16 extend down into slots 
25 formed in the magnets 19, and the bottom portions of the coil supports 
17 extend down into slots 26 formed in the magnets 20. 
The driving force for rotating the mirror support 13 and the magnet 14 
about the axes of rotation is provided by two orthogonal driving coils 28 
and 34, both preferably formed of multiple turns of wire conductor, which 
extend around the periphery of the mirror support 13 within the coil 
supports 16 and 17. The form of the preferred driving coils 28 and 34 is 
best shown in FIG. 3 which illustrates the two coils separated from one 
another and from the surrounding support structure to better show the 
shape of the coils. The driving coil 28 is formed in a substantially 
saddle shape having two straight lower coil sections 29 and, orthogonal 
thereto, to upper coil sections 30, with terminal wires 31 leading from 
the coil for connection to a power supply (not shown) which provides 
driving current to the coils. The lower coil portions 29 extend down into 
the slots 26 of the magnets 20, being shown in dash lines in FIG. 3 to 
illustrate the relationship between the coil 28 and magnet 20. The lower 
portions 29 of the coil 28 are held within the coil support 17, while the 
upper sections 30 of the coil 28 are held within the coil supports 16. The 
coil 34 is formed similarly to the coil 28, although oriented orthogonally 
thereto, having straight lower coil sections 35, upper coil sections 36 
and terminal wires 37. The lower sections 35 of the coil 34 extend into 
the slots 25 of the magnets 19, with the magnets 19 being shown in dashed 
lines in FIG. 3 for illustrative purposes. The lower sections 35 of the 
coil 34 are supported by the coil support 16 whereas the upper sections 36 
of the coil 34 are supported by the orthogonally oriented coil support 17. 
The straight coil sections 27 are thus substantially perpendicular to the 
straight coil sections 35. Drive current is provided to the coil 34 via 
the terminal wires 37 from an adjustable current source (not shown) which 
may be of any well known type of modulable power supply that can provide 
drive current to the coils. 
The operation of the drive coils can be illustrated with respect to the 
views of FIGS. 2 and 3. The permanent magnets 20 are formed so that the 
north and south poles of these magnets face each other across the slot 26 
so that magnetic lines of flux extend substantially directly across the 
slot. Consequently, when current is applied to the terminals 31 of the 
coil 28 in one direction, the current flowing through one of the lower 
sections 29 of the coil 28 will interact with the magnetic field in the 
slot 26 to provide an upward drive force on that portion of the coil in 
accordance with the relation F=I.times.B, where F is the force vector 
acting on the wires of the coil, I is the current vector, B is the 
magnetic field vector and x represents the cross product. The (e.g., 
upward) force on the coil section is transmitted to the mirror support 13. 
The current flowing in the opposite direction in the other lower section 
29 will interact with the magnetic flux in the slot 26 in the magnet 20 
(which is oriented in the same direction as the opposite magnet 20) to 
produce a force on the coil section which is the opposite direction from 
the force on the first coil section. Thus, a torque will be applied to the 
mirror support 13 which will tend to rotate the mirror support and about 
the roll axis 22. Changing the direction of current flowing into the 
terminals 31 will result in a torque being applied to rotate the magnet 
support 13 and magnet 14 in the opposite direction about the axis 22. 
Similarly, current applied to the terminals 37 of the coil 34 will cause 
interaction of the current flowing in the lower sections 35 of the coil 
with the magnetic flux within the slots 25 of the magnets 19 to apply a 
torque to the mirror support 13 to tend to rotate the mirror support and 
mirror in one direction or the other about the pitch axis 23, depending on 
the direction of current flow through the coil. 
The current carrying wires of each lower section of the coils 28 and 39 
preferably are straight and parallel to the appropriate one of the axes 22 
or 23, and the magnetic field through the straight lower sections of each 
coil is preferably parallel to the other axis and thus perpendicular to 
the wires of the coil section through which the field passes. It is seen 
that if the field is perpendicular to the wires of the lower coil section 
and parallel to the non-rotating axis (and thus parallel to the wires of 
the lower or active coil section of the other coil), a current through the 
coil will result in a torque only about the desired axis of rotation and 
not about the other axis. 
The upper or return sections 30 (of the coil 28) and 36 (of the coil 19) 
are above the magnets 19 and 20, respectively, and have substantially no 
flux passing therethrough from the magnets 19 and 20 since most of the 
flux from these magnets is confined to the slots in the magnets. Thus, a 
current flowing in the coil 28 will impose substantially no or very little 
torque about the pitch axis 23 and current flowing in the coil 34 will 
apply substantially no torque about the roll axis 22. 
Further details of the various parts of the device 10 are illustrated in 
the views of FIGS. 4-7. FIG. 4 is a top view of the device essentially as 
shown in FIG. 2 in which the mirror 14 and the magnets 19 and 20 have been 
removed. The top surface of the base 12, which is preferably formed of a 
non-ferromagnetic material, such as aluminum, is milled down in portions 
40 where the magnets 19 will be mounted. Slots 42 are milled in the top 
surface of the base 12 to accommodate the magnets 20. Holes 43 are formed 
at four peripheral portions of the mirror support 13 near the corners and 
have a bevelled indentation about their top openings. Clips 45 are held 
within the bevelled portions 44 and connect to tension springs 46 which, 
as shown in the cross-sectional view of FIG. 7, extend downwardly through 
holes 48 in the base 12 to connecting clips 49 at the bottom of the base 
12. 
As best illustrated in the cross-sectional view of FIG. 6, the mirror 
support 13 is preferably mounted for pivoting about the axes 22 and 23 on 
a central pivot mount composed of a bearing ball 51 which is received in a 
central pivot pocket 52 on the underside of the mirror support plate 13. 
The pivot pocket 52 may be formed as a substantially conical depression in 
the bottom of the mirror support. As also illustrated in FIG. 6, the 
mirror 14 is mounted to the top surface of the mirror support 13 by 
connectors 54. The bearing ball 51 is supported in a desired position by a 
mounting post 56 which is threadingly engaged by a set screw 57 which is 
screwed into a hole in the base 12. The tension springs 46 tend to 
resiliently restore the mirror support 13 to an initial position if it is 
displaced rotationally from that initial position about the bearing ball 
pivot 51. 
To allow adjustment of the initial or "home" position of the mirror 
support, four compression springs 60 are mounted between the back surface 
of the mirror support 13 and threaded studs 61 which thread through 
threaded opening 62 in the base 12. The four compression springs 60 are 
oriented at four corners around the central balls 51 to allow the pressure 
applied by these springs to be adjusted so that the mirror support and 
mirror are displaced about either of the two axes 22 or 23 to a desired 
position. Each of the studs 61 has a slot 64 formed in its bottom surface 
to allow a screw driver to be used to adjust the position of the studs. 
As is best shown in the cross-sectional view of FIG. 7, each of the coil 
supports 16 (and each of the coil supports 17, which are formed 
identically thereto) may include an inner plate 71 and an outer plate 72 
which are connected together and to the edge of the mirror support 13 by 
spacer studs 75 which also hold plates apart by a desired spacing. The 
plates 71 and 72 may be formed of a suitable plastic, e.g., polystyrene. 
In the space between the plates 71 and 72 above the stud 75, the wires of 
the upper coil section 30 of the coil 28 are wound. In the space between 
the plates 71 and 72 beneath the spacer stud 75, the wires of the lower 
coil section 35 of the coil 34 are wound. The lower section 35 between the 
plates 71 and 72 extends into the slot 25 between the poles of the magnet 
19 A ferromagnetic fluid 77 may be held within the slot 25. The fluid 77 
may serve the dual purposes of restraining the magnetic lines of flux 
between the two pole portions 78 and 79 of the magnet 19, and providing 
fluid damping of the motion of the mirror since the fluid 77 will tend to 
slow vibrational movement of the coil support 16 in contact with the 
fluid. The coil support 17 and the coil sections 29 and 36 which are held 
within them, are formed in identically the same manner as described above 
for the coil supports 16, and similar damping fluid may be used in the 
slots 26 of the permanent magnets 20. 
The dynamic mirror alignment mechanism of the present invention is mounted 
in place in an interferometer and adjusted in the following manner. First, 
the base is affixed to a portion of the mounting frame of the 
interferometer (not shown), such as by firmly securing the base 12 of the 
device 10 to the frame by passing bolts through the mounting holes 80 
formed at the ends of the base. The relative rotational position of the 
face of the mirror 14, defining the initial or home position of the mirror 
with respect to the axes of rotation 22 and 23, is adjusted by the 
operator by turning the compression spring studs 64 one way or the other, 
to turn them in or out with respect to the base, until the desired home 
position of the mirror is obtained. Current drive amplifiers are then 
connected to the coils 28 and 34 to provide current to the terminal wires 
31 and 37 in the proper direction to drive the mirror in rotation about 
one or both of the axes 22 or 23. The precise control of the positioning 
of the mirror utilizing feedback information obtained optically may be 
done in any desired manner to provide the feedback signals which control 
the current into the coils 28 and 34. 
The mirror support 13 is preferably formed of a non-ferromagnetic material 
such as aluminum. The magnets may be formed of various permanent magnet 
materials, preferably a high energy product magnet such as neodymium iron 
boride (NdFeB), and the magnet can be formed in a U-shape as shown, or of 
two separated plates of magnets forming the two legs of the "U" with a 
spacer in between the legs at the bottom, with the top and bottom of each 
of the two legs forming north or south poles and with the north and south 
poles opposed to one another. Alternatively, the magnets may be formed of 
wound wire coils, with or without ferromagnetic pole pieces, to provide 
the magnetic field into which the lower coil sections 29 and 35 extend. 
The coil supports 16 and 17 may be formed integrally with the coils, for 
example, by forming the coils on an amature and embedding the coils in a 
hardenably epoxy to form the coil supports which may then be attached to 
the mirror support 13. The main qualification for the magnets 19 and 20 is 
that they be constructed so that they apply magnetic flux to the lower 
sections 29 and 35 of the coils 28 and 34, respectively, in the proper 
direction, and not to the upper portions 30 and 36, respectively, of these 
coils. 
An alternative form for the coils 28 and 34 is shown at 90 in FIG. 8. The 
coil 90 is supplied with current through terminal wires 91 and has two 
individual coil sections 93 and 94 with a connecting line 95 joining them. 
The coils 93 and 94 are wound of multiple turns of wire around, for 
example, the coils supports 16 and 17. The coils 93 and 94 have straight 
lower sections 96 and 97 which extend into the slots of the magnets 19 or 
20, and straight upper sections 98 and 99 which are outside of the field 
of the magnets 19 and 20. The connector 95 carries current from one of the 
coils to the other. The coils 90 function in the same manner as the coils 
28 and 34 described above. 
It should be understood that the position of the coils 28 and 34 and the 
magnets 19 and 20 may be interchanged in the present invention. That is, 
the magnets 19 and 20 may be mounted on the mirror support 13, and the 
coil supports 16 and 17 and the coils 28 and 34 may be mounted to the base 
12. Otherwise, the device will operate in the same manner as described 
above. 
Although specific constructions and embodiments of the present invention 
have been shown and described for illustrative purposes, it is understood 
that the invention is not so limited, and that it encompasses all modified 
forms thereof as come within the scope of the following claims.