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BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates generally to a non-contacting, electro-optical
measuring system for mapping the contoured surface of an object being
tested, such as the contour of a helicopter rotor blade, and more
particularly to a system of this type which makes use of an
electro-optical triangulation rangefinder assembly to effect accurate and
rapid contour measurements.
2. Prior Art
Though the invention is applicable to contour measurement of any
three-dimensional surface without making contact with the surface being
measured, we shall for purposes of explanation describe the invention in
connection with contour measurement of helicopter rotor blade surfaces.
Such blades are relatively massive and present particular problems which
are solved by the present invention.
The performance of a helicopter and its operating life are greatly affected
by the extent to which the rotor blades adhere to predetermined contour
design requirements. If the manufactured blade deviates significantly
therefrom the rotating blade is subject to vibration which not only
degrades the performance of the vehicle in flight, but also reduces the
life of the blade and the gear box associated therewith.
The manufacture of many precision parts such as instrumentation components
and various shaped turbine and propeller blades, entails close control of
and the ability to determine the contour of various surfaces on these
parts. Conventional gauging systems for this purpose make use of contact
probes that physically engage the surface under observation to carry out
the required contour measurement. To a large degree, the accuracy of these
measurements depends upon the stability of the mechanical structure which
serves as a reference, for any twisting, bending or settling of the
structure that takes place after the measuring gauge is calibrated or in
the course of measurement will adversely affect the accuracy of the
reading.
When the object being measured is a helicopter blade, contact probes must
cover over 40 feet in the span direction and 48 inches in the chord
direction with a position accuracy of better than plus or minus 0.001
inches under normal shop conditions. The cost and complexity of a
mechanical X-Y carriage for orienting a contour measuring probe with this
order of accuracy is exceptionally high. If multiprobes are used, the
relative position of one to the other must be known and held to better
than plus or minus 0.001 inches. This too is very expensive to realize in
practice. Moreover, contact probes are subject to wear and must be
replaced at fairly frequent intervals.
Another practical drawback of conventional contact-type contour gauging
systems is that they afford little, if any, flexibility in operation. Once
the probes are positioned for a certain type of blade, before any other
blade configuration can be measured, a costly probe rearrangement is
entailed.
To effect contour mapping of shaped objects, it is also known to use
non-contact, electro-optical systems such as those described in the Waters
U.S. Pat. No. 3,909,131; the Lowrey et al. U.S. Pat. No. 3,986,774; the
Erb U.S. Pat. No. 3,671,126 and the Zoot et al. U.S. Pat. No. 3,679,307.
These prior patents describe electro-optical techniques for surface
gauging and represent noncontacting optical concepts predicated on
geometrical relationships between a source of radiation and a detector.
The above-identified Waters patent is of particular interest; for the
invention disclosed in this patent is based on the principle that as a
focused spot of radiation from a laser source is moved in discrete
increments across the surface being gauged, contour variations can be
measured by triangulation, this being accomplished by electronically
following the position of an image of the spot on a detector array. Two
modes of operation are disclosed by Waters. In one mode, the object being
tested is translated until the imaged spot is driven through a null
position. In the other mode, it is the detector that is translated
relative to the object.
In a non-contacting system of the Waters type operating in the mode in
which the contoured object is physically displaced with respect to the
triangulation system, this translation results in shifting masses which
act to flex and distort the system, thereby giving rise to significant
measurement errors, particularly when contour mapping large surfaces such
as helicopter rotor blades.
Alternately, when Waters translates his detector, this translation
generates optical and mechanical errors in rotational angle which are
difficult to calibrate or control. These drawbacks are acknowledged by
Waters who recommends object translation as "a superior operational mode
from an accuracy consideration" (column 7, lines 24 and 25).
Also of interest are the following prior art patents relating to
non-contact electro-optical measuring systems: Nordqvist, U.S. Pat. No.
3,655,990; Foster et al., U.S. Pat. No. 3,858,983; Zanoni, U.S. Pat. No.
3,768,910; Zanoni, U.S. Pat. No. 3,847,485 and Zanoni, U.S. Pat. No.
3,907,439.
SUMMARY OF INVENTION
In view of the foregoing, the primary object of the present invention is to
provide a low-cost and relatively simple non-contacting electro-optical
system adapted to automatically measure the surface contours of helicopter
rotor blades and other shaped objects, which measurement is carried out at
high speed and with a high order of accuracy.
More particularly, an object of this invention is to provide a system of
the above type in which a contoured surface is mapped by means of an
electro-optical rangefinder assembly operating on the triangulation
principle, the assembly being mounted on a carriage that is movable in a
stepwise manner with respect to an object being measured which is fixedly
held at a stable position by a fixture.
Also an object of the invention is to provide a carriage-mounted
rangefinder assembly constituted by a pivoted laser beam illuminator and a
pivoted automatic tracker mounted for rotation at spaced pivot points on a
carriage beam, the line extending between points forming a triangulation
baseline. The major variables are the two measured angles, one between the
optical axis of the illuminator and the baseline, and the other between
the optical axis of the tracker and the baseline.
An important aspect of the invention resides in the fact that the
illuminator and tracker are balanced so that their centers of gravity lie
on their axes of rotation. As a consequence, one encounters no mechanical
distortion of the contour measuring system due to shifting mass centroids
as the illuminator and tracker are rotated.
A salient feature of a system according to the invention is that it is
capable of functioning in a factory or engineering environment to perform
contour measurement on relatively large and massive objects such as
helicopter blades, with a speed and accuracy that represents a notable
advance over prior techniques. Another advantage of the invention is its
inherent flexibility; for the same system may be used to effect contour
measurement of different types of blades without rearrangement--for all
that is required when a different type of blade is to be automatically
contour-measured is operator control or software reprogramming.
Still another object of the invention is to provide a noncontact
electro-optical automatic contouring system in which flexure and
distortion in the translating carriage structure supporting the
rangefinder assembly is corrected by means of straight calibration bars
whose mountings are mechanically independent of the rangefinder. The
mechanical structure of the present system is less elaborate and costly
than structures heretofore required, in that the requirements for
dimensional stability are reduced to a considerable degree due to the
self-calibration feature.
Yet another object of the invention is to provide a two-stage tracker angle
readout in which the tracker angle used in calculating the contour is the
algebraic sum of an angle measured electromechanically over an angular
range of .+-.30.degree. to an accuracy of one second and an angle measured
electro-optically over an angular range of .+-.0.1.degree. to an accuracy
of one second, thereby affording both fine and coarse measurements.
Briefly stated, in a system in accordance with the invention when arranged
to effect contour measurement of a helicopter rotor blade, the blade is
supported in a stable position by a fixture which holds the blade with its
leading and trailing edges placed between upper and lower flat calibration
bars.
In order to carry out contour-measurement concurrently on both contoured
sides of the fixture-supported helicopter blade, a gantry-like carriage is
provided which rides on rails that straddle the fixture, the carriage
including a pair of vertical beams on opposing sides of the blade and
equi-spaced from the chord thereof.
Supported at corresponding positions on these beams are two like
rangefinder assemblies, each assembly being constituted by a pivoted laser
beam illuminator and a pivoted automatic tracker mounted for rotation on
the associated beam on spaced pivot points, the line extending between the
pivot points representing a triangulation baseline. The illuminator and
tracker are each counterbalanced to maintain fixed centroids.
The carriage is caused to step incrementally along the rails from one end
of the blade to the other. At each carriage step, the rangefinder
assemblies are both activated, to cause each illuminator to swing through
a sector whereby the laser beam spot impinging on the blade scans across
the related surface from the leading to the trailing edge thereof. Means
associated with each assembly act to determine the changing angles assumed
by the illuminator and those assumed by the tracker in the course of a
scan, these angular values being fed to a computer in which the baseline
value is stored, the computer calculating the changing location of the
spot by triangulation.
This scanning operation, which takes place simultaneously on both sides of
the blade, is repeated at every carriage step from one end of the object
to the other, thereby mapping the entire contour of both surfaces. At the
beginning and end of each contour scan, the rangefinder assemblies are
calibrated by contouring the flat calibration bars located adjacent the
leading and trailing edges of the blades, so as to introduce a correction
factor taking into account any mechanical distortion of the rangefinder
position resulting from carriage movement along the rails.
OUTLINE OF DRAWINGS
For a better understanding of the invention as well as other objects and
further features thereof, reference is made to the following detailed
description to be read in conjunction with the accompanying drawings,
wherein:
FIG. 1 is a perspective view of a non-contacting electro-optical contour
measuring system in accordance with the invention, the system being
arranged to carry out contour mapping of opposing surfaces of a helicopter
rotor blade by means of a pair of rangefinder assemblies;
FIG. 2 is an enlarged perspective view of one of the rangefinder
assemblies;
FIG. 3 shows a portion of a helicopter rotor blade to be tested;
FIG. 4 is a schematic diagram of the measuring system;
FIG. 5 is a block diagram of the measuring system; and
FIG. 6 is a block diagram of the fine angle tracker.
DETAILED DESCRIPTION OF INVENTION
System Structure
Referring now to FIG. 1, there is shown a fixture for supporting a
helicopter rotor blade 10 lengthwise at a stable position suitable for
contour mapping by a non-contacting electro-optical measuring system in
accordance with the invention, which system includes a pair of
triangulation rangefinder assemblies, one for each side of the blade.
Rotor blade 10, by way of example, has a length of 40 feet and a chord of
48 inches. As best seen in FIG. 3, chord C is a straight line joining the
leading edge L of blade 10 which has an airfoil shape, to the trailing
edge T thereof. The blade possesses contoured front and rear surfaces
S.sub.1 and S.sub.2 whose contours are to be mapped to determine whether
they conform to predetermined design requirements and, if not, the extent
to which they deviate therefrom.
The fixture for supporting the blade lengthwise is constituted by a
frame-shaped bed or base 11 which rests on or is bolted to the floor and
is level therewith, the base including a pair of parallel girders 11A and
11B. Placed between these girders and mounted at spaced positions on
transverse ties 12 which bridge the girders are a series of uprights 13A,
13B, 13C etc.
Each upright, such as upright 13C, is provided with a pair of spaced upper
and lower cantilever arms 14 and 15 whose ends are equipped with clamps
14C and 15C adapted to grip the trailing and leading edges, respectively,
of the blade. The number of uprights and their spacing are such as to
securely support the blade being tested so that its position is stationary
and remains so in the course of testing.
Also supported by the upper cantilever arms 14 of the uprights is a
horizontally-extending calibration bar 16 having a narrow flat horizontal
edge, the bar extending the full length of the blade. Supported below the
lower cantilever arms 15 of the uprights on upright bracket 17 is a
horizontally-extending calibration bar 18 also having a similar edge.
Thus the fixture locates the blade between two reference bars which
establish the coordinate system for contour measurement. In practice,
therefore, the means to position the electro-optical rangefinder need not
be controlled, say, to plus or minus 0.001 inches relative to the rotor
blade, the only requirement being that its position be known to plus or
minus 0.001 inches as determined by scanning the reference bars.
Since the series of uprights optically obscure side S.sub.2 of the blade,
the uprights are made in a narrow width consistent with structural
requirements to minimize this effect and to maximize the exposure of side
S.sub.2 to the electro-optical rangefinder which scans this side.
In practice, base 11 may be made in girder sections of suitable stock
length, so that the base is readily transportable and can be installed
without difficulty. Standard couplers are used to join the girder section
together, means being also provided for levelling the entire length of the
base.
The optical sensor of the contour measuring system requires a transporting
mechanism which moves the sensor spanwise along the blade being measured.
This transporting mechanism is constituted by a gantry-type carriage,
generally designated by numeral 20, which rides on a pair of rails 21A and
21B resting on base girders 11A and 11B.
Carriage 20 includes a pair of parallel vertical beams 22A and 22B whose
top ends are bridged by a cross beam 23, the lower end of beam 22A being
joined to a two-wheel truck 24 riding on rail 21A. The lower end of beam
22B is joined to a single wheel truck (not shown) which rides on rail 21B,
thereby providing a stable, three-point suspension.
The length of the fixture base and of the rails thereon are at least 50
feet so that carriage 20 can travel beyond and clear the 40-foot length
rotor blade held on the fixture at either end of the blade to a degree
sufficient to allow loading and unloading of blades to be tested.
As best seen in FIG. 2, pivotally mounted on a pad 25 secured to vertical
beam 22A is a first rangefinder assembly I including laser beam
illuminator 26 which is rotatable through sufficient angles to scan the
surface S.sub.1 for the entire chord C; that is, from the leading to the
trailing edge of the blade. Pivotally mounted on a pad 27 secured to beam
22A at a position above that of pad 25 is a tracker 28 which is rotatable
through sufficient angles to intercept the illuminator beam reflected from
surface S.sub.1 of the blade. The straight line extending between the
pivot point or axis of illuminator 26 and the pivot point of tracker 28
represents the baseline of the triangulation rangefinder. In one practical
embodiment, this baseline is 40 inches long.
Laser beam illuminator 26 preferably generates a 0.001 inch by 0.100 inch
line image on the surface of the rotor blade, which image is hereafter
referred to as the laser beam spot. In operation, illuminator 26 is
rotated about its pivot axis; and as the illuminator swings, the spot
travels along the surface in the chord direction. Tracker 28 captures the
beam reflected from the spot position in its field of view and tracks it
as it travels across the chord.
The structure of the illuminator and the structure of the tracker are both
counterbalanced to minimize shifting of masses on the supporting frame of
the measuring equipment, thereby improving measuring accuracy and reducing
the cost and complexity of the supporting frame. By counterbalancing is
meant that the structure of the tracker and that of the illuminator have a
fixed mass distribution such that they are balanced in the inactive state
with respect to either side of the center of gravity. There are only two
small masses which shift in the course of operation, these being one
focusing lens in the illuminator which is motor-driven, and one in the
tracker which is motor-driven. These masses shift only a few inches while
making measurements and are continuously counterbalanced, so that there is
no net shift.
Mounted on beam 22B at positions corresponding to illuminator 26 and
tracker 28 of the first rangefinder assembly I is a second and identical
rangefinder assembly II for contour-measuring the S.sub.2 side of the
blade. The second assembly is constituted by a pivotally-mounted laser
beam illuminator 26b' and a tracker 28' whose structure and function are
the same as those of illuminator 26 and tracker 28 of the first assembly.
Carriage 20 is motor-driven, as by means of an endless belt 29 operatively
coupled to beam 21B, to advance the carriage and the rangefinder
assemblies borne thereby in incremental steps from one end of the blade
being measured to the other. When the carriage occupies a given step, a
scanning action is carried out concurrently on opposing sides of the blade
by rangefinders I and II to determine the contours of the blade sides.
Thus, as shown in FIG. 3, at the first step, chord C is scanned at one end
of blade 10, then the carriage is incrementally advanced, and at the next
step Chord C.sub.1 is scanned, this being repeated at chord C.sub.2 and so
on until the chord at the other end of the blade is reached and both sides
of the blades are contour mapped.
Because the leading edge L of the air-foil shaped blade has a bullet-nosed
formation and is inaccessible to the tracker, it cannot directly view the
surface thereof. To facilitate tracking of the leading edge, a mirror 30
is fixed to the rangefinder assembly at a position below the leading edge
L of the blade. The mirror is oriented to intercept the line-of-sight of
the tracker and to redirect it to the leading edge of the blade. The
mirror is positioned so that the area around the leading edge is fully
visible to the tracker, starting from the area where the chordal plane
intersects the rotor surface, and extending back along the rotor surface
to the area which is directly visible to the tracker without the use of
the mirror.
The location of the mirror surface must be determined relative to the
triangulation rangefinder, in order to make contour measurements by
tracking through the mirror, and in order to relate these contour
measurements to those made by tracking the rotor surface directly. A
preferred method for determining the mirror surface location uses
illuminator angle and tracker angle measurements made on two points on the
rotor surface which are visible to the tracker both directly and through
the mirror. The unique mirror position which is consistent with these four
pairs of angle measurements can then be determined by standard algebraic
methods.
Alternatively, a mirror can be used to redirect the flux from the
illuminator onto the rotor surface, without deflecting the tracker's line
of sight. Two or more mirrors may be used for access to curved surfaces
which are not directly visible to either the illuminator or the tracker.
System Operation
Referring now to FIGS. 4 and 5, it will be seen that rangefinder assemblies
I and II of the contour measuring system are borne by means 22A and 22B,
each assembly having an illuminator and a tracker in operative relation to
a respective side of rotor blade 10. Since the two assemblies are
identical, only the motors and other devices associated with rangefinder
assembly I will be described in this section.
The incremental stepping of carriage 20 to advance the rangefinder
assemblies through successive chords C, C.sub.1, C.sub.2 etc. from the
front end to the rear end of the blade 10 being tested is effected by a
carriage stepping motor 31 which drives belt 29. Motor 31 operates in
conjunction with an encoder 32 that provides a signal indicative of the
carriage step position. This signal is supplied to a computer 33 which
governs the operation of the system and also acts in response to signals
supplied thereto by the rangefinder assemblies to carry out computations
based on the sensed values in order to effect measurements by
triangulation.
In practice, computer 33 may be a general purpose microcomputer with
sufficient speed and memory to handle all of the control functions of the
contour measurement system as well as the necessary computational and
report-generating functions. Use may be made of a dual floppy-disc system
which is associated with the microcomputer to provide ample nonvolatile
storage for measurement programs, blade specification data and general
purpose utility programs. Two independent disc drivers are supplied so
that discs may be copied for back-up to lessen the risk of downtime due to
hardware failure.
The angular position of illuminator 26 of rangefinder assembly I is
controlled by a scan motor 34 and that of automatic tracker 28 by a scan
motor 35 included in a servo system. One form of servo system suitable for
automatically controlling a tracker is disclosed in the above-identified
Nordqvist patent whose disclosure is incorporated herein.
Illuminator 26 includes a focusing lens 36 controlled by a focus motor 37
whose position is sensed by an encoder 37A. Tracker 28 includes a focusing
lens 38 controlled by a focus motor 39 whose position is sensed by an
encoder 39A. As the laser illuminator pivots, its beam is maintained in
focus on the surface of the rotor blade by a closed-loop control system
which acts to focus the output lens of the illuminator, a similar focusing
control system being included in the pivoting tracker.
The changing angular position of illuminator 26 in the course of a scan is
sensed by an illuminator scan-sensor or encoder 40, and that of the
tracker by a tracker scan-sensor or encoder 41. These changing angular
values are fed to computer 33 wherein the known value of the baseline is
stored. One commercially-available angle sensor suitable for these
purposes is sold under the "Inductosyn" trademark.
The length of the baseline varies slightly as the illuminator angle and the
tracker angle change. This baseline variation is due to the fact that the
optical axes of the illuminator and tracker do not, in general, exactly
intersect their corresponding rotation axes, due to constructional
tolerances in making and assembling their components.
The illuminator and the tracker could, in principle, be designed
incorporating adjustment mechanisms which would permit alignment of their
components so that their optical axes would exactly intersect their
rotation axes. However, a preferred system design is to measure and record
the residual misalignments of these axes during system calibration, and
then to calculate the small baseline corrections which are required as
part of the data reduction program used in contour measure.
In order to contour the surface, illuminator 26 pivots so that the laser
spot travels across that chord (C, C.sub.1, C.sub.2, C.sub.3 etc.) which
is in registration with the existing step position at an angular rate of
about 3.degree. per second. And as as the illuminator pivots, tracker 28
is caused to swing to track the laser spot under closed loop servo
control, this being accomplished in practice to a "coarse" accuracy of
0.050". An open-loop sensor located in the tracker (to be hereinafter
described) determines the position of the laser spot to a "fine" accuracy
of 0.0005".
As rangefinder assemblies I and II are advanced by the carriage
step-by-step to carry out contour measurements on either side of the rotor
blade along the full length thereof, the 50-foot long rails on which the
carriage travels will tend to settle and twist and thereby slightly alter
the rangefinder orientation relative to the blade. To correct for the
resultant errors, the rangefinders are calibrated by cotouring the two
flat reference bars 16 and 18 adjacent the leading and trailing edges of
the blade on the holding fixture. The angular orientation of the pivot
axis is monitored by an auto collimator connected rigidly to the pivot
axis, the auto collimator looking at a fixed target on the bladeholding
fixture.
In practice, the laser illuminator may be constituted by a helium-neon
laser source 42 emitting 5 milliwatts of polarized light at 0.6328 micron
wavelength in a 0.8 mm diameter beam with a 1 milliradian beam divergence.
The resultant light path is folded by flat mirrors M.sub.1 and M.sub.2
(see FIG. 2) for system compactness, and its diameter is expanded sixty
times by an optical beam expander 43 in order to reduce its divergence
from 1000 microradians to 25 microradians. In the direction parallel to
the span axis, a hundred times larger beam divergence of 2500 microradians
is generated by a cylindrical lens incorporated in the beam expander. The
expander output is focused on the rotor surface by a servoed objective
lens 36 so that it generates a 0.001 inch by 0.100 inch line image.
The use of a spot having a line formation rather than a round formation is
useful in contouring rough, somewhat cylindrical surfaces, such as the
fiberglass surface of a helicopter rotor blade. To this end, the
illuminator beam expander incorporates an anamorphic lens which spreads
out the spot of light into a 0.001.times.0.100 inch line parallel to the
longitudinal axis of the rotor blade. This lens interacts with the rotor
surface to decohere the laser illumination, thereby reducing the tracker's
speckle noise to a negligible level. In practice, the amount of light
collected by the tracker when contouring a dark surface can be well under
one microwatt. It is important, therefore, to be able to distinguish this
significant light from all ambient or stray illumination and thereby
prevent the system output from being distorted in the typical factory
environment in which rotor blades undergo contour measurement. To this
end, the system in accordance with the invention incorporates three
distinct kinds of filtering which, taken together, render the
electrooptical system substantially immune to stray illumination that
would otherwise bias the contour reading.
The first filter is temporal in nature, this being accomplished by a
modulator 44 interposed in the optical path between laser 42 and beam
expander 43, the beam being modulated at a repetition rate of 1,000 Hz.
The tracker output is synchronously demodulated to be optimally sensitive
to the modulated incoming flux and to reject the 120 and 60 Hz frequencies
commonly encountered in a-c powered factory light sources.
The second filter is spectral in character, in that the illuminator light
output is concentrated at a spectral wavelength (0.6328 microns in the
case of radiation from a helium-neon laser). The tracker output is
correspondingly optically filtered to restrict its response to a narrow
bandwidth (i.e., .+-.0.005 microns in the helicopter rotor contouring
system), which encompasses the selected spectral wavelength and rejects
other wavelengths. The third filter is spatial, and to this end the
tracker is masked to receive radiation only from a field view .+-.1/4 inch
around its aim point.
In the present system, the tracker angle used in contour calculation is the
algebraic sum of two angles, one being a coarse angle measured
electromechanically by scan position encoder 41 over an angular range of
.+-.30.degree. to an accuracy of one second of arc. This measurement may
be effected with a commercially-available angle sensor known as an
Inductosyn.
The other angle measurement used in contour calculation is a fine angle
measurement carried out electro-optically by a fine tracker 45 over an
angular range of .+-.0.1.degree. to an accuracy of one second of arc. This
two-stage angle readout for the tracker tolerates leads or lags of several
minutes of arc in the tracker's optical axis, making it possible to use a
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