An optical system monitors the angular position of a rotating scanning mirror to indicate the effective start and end of each scan. At a certain angular position, a ray of energy transmitted to the mirror is reflected a plurality of times between the reflectors associated with the optical system and the line on the mirror parallel to the axis, and then to a detector to sense that angular position. A single optical system may be arranged to sense a plurality of different angular positions for each revolution of the mirror

Cline, R. W.

Lansing, J. C., Jr.

English

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https://ntrs.nasa.gov/search.jsp?R=19740013191 2020-03-17T14:51:19+00:00Z
United States Patent
Fletcher et al.
[ i i ] 3,802,779
[45] Apr. 9, 1974
[54] METHOD AND APPARATUS FOR
OPTICALLY MONITORING THE ANGULAR
POSITION OF A ROTATING MIRROR
[76] Inventors: James C. Fletcher, Administrator of
the National Aeronautics and Space
Administration with respect to an
invention of; Jack C. Lansing, Jr.,
1616 Loma St., Santa Barbara,
Calif. 93103; Richard W. Cline,
7321 Padova Dr., Goleta, Calif.
[22] Filed: June 6, 1972
[21] Appl. No.: 260,241
[52] U.S. Cl. :... 356/152, 250/231 SE, 350/299
[51 ] Int. Cl : GOlb 11/26
[58] Field of Search..... ;... 356/138, 149, 152;
250/235,231 SE; 350/299
[56] References Cited
UNITED STATES PATENTS
3,708,231 1/1973 Walters 356/152
Primary Examiner—Maynard R. Wilbur
Assistant Examiner—S. C. Buczinski
Attorney, Agent, or Firm—R. F. Kampf; John R.
Manning
[57] ABSTRACT
An optical system monitors the angular position of a
rotating scanning mirror to indicate the effective start
and end of each scan. At a certain angular position, a
ray of energy transmitted to the mirror is reflected a
plurality of times between the reflectors associated
with the optical system and the line on the mirror par-
allel to the axis thereof, and then to a detector to
sense that angular position. A single optical system
may be arranged to sense a plurality of different angu-
lar positions for each revolution of the mirror.
10 Claims, 3 Drawing Figures
'ATENTEOAPR 91974
SHlET 1 OF 2
3.802,779
9a
23
FIG.i
PATENTEDAPB 919/4 3.802,779
SKLEI a OF 2
17
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kl,
V — -Ifl
2,3,6,7 FIRST
PLANE
V SECOND
,4,5,8 PLANE
FIG. 2
23
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12-
22 21
9o-
20
13
x- 5, ^7 6
10
- I I rv 7
- . ; ; 3,802,779
-•• • ' •'' ' ''- 1 - ' - ' . • .. . '2
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METHOD AND APPARATUS FOR OPTICALLY slight vertical inclination of the system causes the ray
MONITORING THE ANGULAR POSITION OF A bundles to "walk" thereby clearing the image from the
ROTATING MIRROR mirror system.
Thus, it is an object of this invention to provide a new
ORIGIN OF THE INVENTION 5 and improved method and apparatus for monitoring
the angular position of a rotating mirror which is more
The invention described herein was niade in the per- sensitive, more rapid, functional at multiple angles and-
formance of work under a NASA contract and is sub- /or has a high degree of precision.
ject to the provisions of Section 305 of the National It is another object of this invention to provide a con-
Aeronautics and Space Act of 1958, Public Law 10 cept of faceted folding mirror design that creates an ef-
85-568 (72 STAT 435; 42 U.S.C. 2457). fective multiplication of the angular motion of an opti-
BACKGROUND OF THE INVENTION ^ u- , c.u- . -jIt is another object of this invention to provide a con-
This invention relates to optical monitoring of the an- 'cept for a faceted folding mirror that provides for de-
gular position of a rotating mirror. More particularly, 15 tection at several angular positions.
it relates .to a method and apparatus for determining It is another object of this invention to provide a high
certain angular positions of a rotating scanning mirror. frequency multiple reflection monitoring method and
apparatus for a rotating mirror.
A scan mirror for a multi-spectral scanner rotates at Other objects and the advantages of the present in-
approximately 300° per second. It is necessary to deter- 20 veritiofi will become apparent from the detailed de-
mine the position of the scan mirror to an accuracy of scription to follow, together with the accompanying
two seconds of arc. Due to small uncertainties in th'e drawings.
indiSteff erfeSvJ s?aTt BRIEF DESCRIPTION OF THE DRAWINGS
and end of each scan. Existing devices for monitoring 25 There follows a detailed description of preferred em-
the angular position of rotating members are either too bodiments of the invention to be read together with the
slow or not sufficiently precise. accompanying drawings.
SUMMARY OF THE INVENTION FIG. l isa diagrammatic view of a preferred embodi-
• ... : ment of the invention.
Thus, it is a'purpose of the present invention to pro- 30 FIG. 2 is an optical schematic view viewing the sys-
vide a new and improved method and apparatus for tern of FIG. 1 from above.
monitoring the angular position of a rotating mirror FIG. 3 is an optical schematic view, viewing the sys-
rapidly with a high degree of precision. teni of FIG. 1 from side elevation.
. » ™™>-ED ptaOUMlON <* THE PREFERRED
light beam is transmitted to the mirror and caused to EMBODIMENTS
reflect off of the mirror a plurality of tjmes, always Referring now to the drawings, like elements are
along a line on the surface of the mirror which is paral- represented by like numerals throughout the several
lei to the axis of rotation thereof, the ray finally being views.
received by a detector. . In the preferred embodiment, three angular positions
In a preferred embodiment the device uses a three- 0,, 02, and 03 are sensed. At 0, the rotating- mirror will
faceted folding mirror to accomplish an optical path cause the energy bundle to reflect at the three zones
eight times , the separation between the rotating mirror identified as A on the folding mirror IS in FIG. 1. At
and the position monitor device. The arrangement de- 02, the three zones B are employed and at position 03
scribed senses the angular position three times each 45 the three zones C are employed.
re volution, e.g., at angular positions, 0,, 02 and 03. Simi- Referring to the figures, there is shown a scanning|ar|y, a device employing a five-faceted folding mirror mirror 9 and an imaginary vertical line 9a. This line
would sample the rotational position five times each may be the axis of rotation of the scanning mirror 9 or
revolution. The positional information can be com- the axis may be spaced from but parallel to this line 9a.
bined with timing data to determine angular rate. Continuous incoherent radiation is emitted from the
In the preferred embodiment, a precise angular posi- gallium arsenide (GaAs) diode 10. Energy is reflected
tion 0 of the rotating mirror will cause the energy bun- by a suitable plane folding mirror 11 to a primary mir-
dle to reflect at a certain three zones on the plane fold- ror 12 and then through a meniscus lens 13 to the line
ing mirrors. At position 02 the energy bundle will reflect 9a on the mirror. This emitted ray is designated by the
at three other zones and at position 03 at three other numeral 1 in all of the figures. As is evident from FIG.
zones. 3, this ray is inclined upwardly by the angle X.
The use of multiple reflections from the scanning The monitoring device is intended to sense certain
mirror achieves a substantial increase in the scan mir- precise angular locations of the mirror. The rotating
ror angular detection sensitivity. In a simple design with scanning mirror has an angular scan interval of approx-
a single reflection having separation from the moving imately 5.8 degrees. That is, the mirror could be pre-
mirror to the detector of d, the linear displacement S cisely perpendicular to the vertical plane including the
caused by rotation through A 0 is 5 = 2d 0. With the sys- ray 1 , or it could be turned about its axis in either direc-
tem illustrated the displacement S — 32 d A0 or a 16 tion by approximately 2.9°. When turned by said 2.9°
times increase in available resolution. Although sys- clockwise, as viewed in FIG. 2, it is at position 0, which
terns utilizing multiple reflections may have been em- is illustrated in the figures and whereat zones A are
ployed before, the concept of multiple reflection being used. When perpendicular to the vertical plane with ray
used at multiple positions is believed to be new. The 1 it will be at 02 using zones B and when at 2.9° in the
3,802,779
20
counterclockwise direction it will be at 83 using zones
G. In the illustrated position 6t, the ray 1 upon reaching
the line 9a is reflected not directly back toward the
emitter in the same plane, but rather to one side thereof
through the angle Y which equals 5.8°. The first de- 5
fleeted ray, indicated by the numeral 2 in the figures,
in addition to being deflected through the horizontal
angle Y is deflected upwardly through the vertical
angle X as shown most clearly in FIG. 3. The vertical
plane passing through the ray section 2 and through the 10
line 9a will be referred to hereinafter as the first plane,
and the vertical plane passing through the ray 1 and the
line 9a will be referred to hereinafter as the second
plane.
Above the lens 13 the plane folding mirror 15 has 15
three separate reflecting surfaces, 16, 17, and 18, all
facing the mirror 9. The zones A, B and C are shown
on these surfaces 16, 17 and 18. All three reflecting
surfaces are vertical, the reflecting surface 16 being
perpendicular to the said second plane and the surface
17 being perpendicular to the first plane. The reflecting
surface 18 is not utilized at the precise position 0, of the
apparatus as shown in the views. However, if the mirror
9 were positioned at 83 at the opposite (counterclock- „
wise as viewed in FIG. 2) extreme of its scan interval, ,
the reflected ray section 2 would be directed to surface
18 rather than surface 17 and the said first plane would
be perpendicular to the surface of 18 rather than the
surface of 17, whereby zones C would be used..
 30
As is evident from the figures, after the ray section 2
is reflected by the>surface 17, the ray follows the fol-
lowing path; ray section 3 within the first plane to the
line 9a, ray section 4 to the mirror 16 within the second
plane, ray section 5 back to the line 9a also within the 35
second plane, ray section 6 back to the mirror 17
within the first plane, ray section 7 back to the line 9a
in the first plane, and finally ray section 8 in the second
plane to the detector. As is evident from FIGS. 2 and
3, the reflections off the line 9a of the mirror alternates 40
the sections back and forth between the two planes
while the reflecting surfaces 16 and 17 reflect the ray
sections back to the mirror within the same planes. All
reflected rays are inclined upwardly by the angle X. In
angular position 02, zones B are used. Here the consec- 45
utive reflected rays "walk" up each time by an angle X,
but they are not concurrently reflected sideways by the
angle Y.
Referring again to the illustrated position 6,, detected
ray section 8 is received through a filter 20 by a plane 50
convex lens 21 having a 30° prism thereon, this lens
splitting the received ray, the two portions thereof
being deflected by the diagonal reflector 22 to a pair of
deflectors 23.
While it is a main feature of the present invention
that at any one angular position of the mirror there be
at least two reflections off of the line 9a of the mirror,
it will be evident that there need not be precisely eight
ray sections as shown in the figures. For example, re-
flecting surface 16 could be omitted and the detector
lowered to receive the ray section 4. Alternately, the
reflecting surfaces 16, 17 and/or 18 could be made
much taller and the detecting means raised vertically so
that following the ray section 8, the ray could be de-
 65
fleeted again back to the mirror for at least two addi-
tional reflections off of the mirror before reaching the
detecting means.
In practice, as the mirror rotates, each one of the po-
sitions #,, 02, and 03 would be sensed in succession.
Although the invention is not limited to the precise
angles shown herein, in one preferred arrangement, the
angle X would be approximately 1° to 2°.
The present apparatus could also operate in a DC
mode.
To electrically process the error signal, two silicon
photodiode detectors are employed with bias voltage in
opposite' directions. The output from the detectors is
summed with critical detection occurring at the zero
crossing between the positive going signal and the neg-
ative going signal. Electrical signal processing includes
gating and triggering so as to have extremely low false
alarm probability.
Although the invention has been described in consid-
erable detail with respect to the preferred embodi-
ments thereof, it will be apparent that the invention is
capable of numerous modifications and variations ap-
parent to those skilled in the art without departing from
the spirit and scope of the invention.
We claim:
1. A device for monitoring the angular position of a
rotating mirror comprising:
means for emitting a ray of energy to a first point on
the mirror such that the ray is reflected therefrom,
a reflecting means for receiving the said reflected ray
and for reflecting it at least one additional time
back to the mirror to a point thereon, which point
forms a line with the first said point, the line being
parallel to the axis of rotation of the mirror, the
said reflections of the ray to and from the reflecting
means thus lying in a common plane,
detection means for detecting the said ray after the
last reflection off of the said line of the mirror, said
detection means positioned to receive and detect at
one location the last reflected ray only when the
mirror is at certain of a plurality of angular posi-
tions relative to its axis qf rotation,
and wherein said emitter, said reflecting means and
said mirror are arranged such that all reflections of
the ray between the emitted and the last deflected
:
 ray are inclined vertically by an angle X.
2. A device according to claim 1, wherein, in at least
one of said angular positions the said reflections from
the rays to and from the reflecting means lie in a first
common plane, the said emitted and last reflected ray
lie in a second common plane, the said first and second
common plane intersecting at the said line on the mir-
ror and forming an angle Y between them, and wherein
each time the ray is deflected off of the said mirror, it
is deflected about said angle Y from one of said planes
to the other.
3. A device according to claim 2, including a further
reflecting means for receiving certain deflected rays
from the line of the mirror in the second plane and for
reflecting such deflected rays back to the mirror and
into the first plane;
the device positioned to reflect the emitted ray at
least seven times at each angular position to form
eight straight ray sections, in sequence, between
and inclusive of the emitted and detected sec-
tions, said eight sections including:
a first ray section being the said emitted ray, second
and third ray sections from the line of the mirror to
the said reflecting means and back to the line of the
3,802,779
mirror, fourth and.fifth ray sections from the line
of the mirror to the said further reflecting means
and back to the line of the mirror, sixth and seventh
ray sections from the line of the mirror back to the
said reflecting means and back to the line of the 5
mirror, and the eighth ray section being the last
said ray from the line of the mirror to the detection
means, and wherein, in those angular positions in
which the ray sections lie in said first and second
planes, said first, fourth, fifth and eighth ray sec- 10
tions lie in the said second plane and the said sec-
ond, third, sixth and seventh ray sections lying in
the first plane.
4. A device according to claim 3, said reflector
means and said further reflector means comprising a 15
plane folding mirror having a first reflector surface par-
allel to the first plane and constituting said reflector
means, and a second reflector surface parallel to the
second plane and constituting said further reflector
means, and said means for emitting and detecting the 20
light ray located on opposite sides of the second reflect-
ing surface in said second plane.
5. A device according to claim 4, said emitting means
comprising a high frequency GaAs emitter, said detec-
tor means including a prism arranged to split the said 25
last ray into two sections and including a detector de-
vice for detecting each section.
6. A device according to claim 1, said emitting means
comprising a GaAs emitter.
7. A device according to claim 2, wherein the angle 30
between the said first and second planes is 5.8°.
8. A method for monitoring the angular position of
a rotating mirror comprising:
transmitting an energy ray to a line on the mirror,
which line is.parallel to the axis of rotation of the
mirror, said ray being inclined relative to the said
mirror line at an angle X and forming with said mir-
ror line a common plane,
reflecting this ray from the mirror line at least one ad-
ditional time back to the mirror line, the reflections
of the ray to and from the reflecting means thus
lying in a common plane,
detecting the said 'ray at one location after the last re-
flection off of said mirror line only when the mirror
is at certain ones of a plurality of angular positions
relative to the axis of rotation of the mirror, thus
detecting said angular positions,
and each of the reflected rays between the emitted
and the last said ray, inclusive, being inclined at an
angle X relative to a plane passing perpendicularly
to the axis of rotation.
9. The method of claim 8, in which in at least one of
said certain angular positions the said common plane of
the rays from the mirror to the reflecting means and
back to the mirror form a first plane, the said emitted
ray and the said last reflected ray forming a second
plane, the two planes intersecting at said mirror line.
10. The method of claim 9 including receiving the ray
when first deflected back into the second plane, and re-
deflecting this ray back to the line of the mirror, and
hence into the first plane away from and back to the
mirror and then from the mirror back into the second
plane in the same manner as the first said deflection of
the ray.
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Method and apparatus for optically monitoring the angular position of a rotating mirror

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