U.S. patent number 3,695,771 [Application Number 05/061,580] was granted by the patent office on 1972-10-03 for method and apparatus for inspecting surfaces.
Invention is credited to Andras M. Bardos.
United States Patent |
3,695,771 |
Bardos |
October 3, 1972 |
METHOD AND APPARATUS FOR INSPECTING SURFACES
Abstract
Method and apparatus for inspecting the surface of a workpiece
for the presence of burrs. A focused laser beam is swept across the
surface of the workpiece while a photodetector positioned at an
acute angle to the workpiece surface senses reflections produced by
the beam. The photodetector, when positioned in the plane swept by
the beam, observes reflections from the tip of the burr and from
the portion of the path on the surface which is not obscured by the
burr. The length of the path portion from which reflections are
obscured by the burr is directly proportional to the height of the
burr. Suitable logic and measuring networks calibrated to the sweep
velocity on the workpiece and triggered by the reflections are
capable of determining the length of the obscured portion and
therefore the height of the burr.
Inventors: |
Bardos; Andras M. (Bristol,
CT) |
Family
ID: |
22036703 |
Appl.
No.: |
05/061,580 |
Filed: |
August 6, 1970 |
Current U.S.
Class: |
356/446; 250/548;
356/237.2; 250/559.06; 250/559.48 |
Current CPC
Class: |
G01B
11/0608 (20130101); G01B 11/303 (20130101) |
Current International
Class: |
G01B
11/30 (20060101); G01B 11/06 (20060101); G01n
021/32 () |
Field of
Search: |
;356/210,237
;250/219DF |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wibert; Ronald L.
Assistant Examiner: Chew, II; Orville B.
Claims
What is claimed is:
1. Apparatus for detecting burrs on a surface comprising:
means for generating an illuminating beam;
means connected to the generating means for scanning the beam at a
known rate along a path across the surface to be inspected;
illumination sensing means for detecting diffuse illumination
reflected from the path on the surface swept by the illuminating
beam, said illumination sensing means having a directionally
sensitive axis in a plane scanned by said illuminating beam and at
an acute angle to the surface scanned by said illuminating beam
whereby light from said illuminating beam may be obscured from said
sensing means in a region on said surface behind a burr on the side
of the burr removed from said sensing means during part of the
scanning of said beam across said surface; and
measuring means connected to the illumination sensing means and
responsive to the signals from the sensing means for measuring the
interval between successive reflections detected by said sensing
means from the path scanned by the beam.
2. The detecting apparatus of claim 1 wherein:
the measuring means includes a level detecting means connected to
the illumination sensing means and having a first preselected
threshold level rendering the measuring means selectively
responsive to reflected illumination.
3. The detecting apparatus of claim 2 wherein:
the measuring means has a second preselected threshold level less
than the first threshold level of the level detecting means;
and
the measuring means includes switching means connected with the
level detecting means for rendering the measuring means operative
at either the first or the second threshold level.
4. The detecting apparatus of claim 3 wherein:
the switching means is connected to the output of the level
detecting means and responsive to the output of the level detecting
means for rendering the measuring means operative at the second
threshold level as a function of the output of the level detecting
means.
5. The detecting apparatus of claim 1 wherein:
the beam generating means is a laser beam generator.
6. The detecting apparatus of claim 1 wherein:
the scanning means scans the beam at a known rate from the edge of
the surface to the path along the surface;
the illumination sensing means is a photodetector positioned behind
the beam with respect to the direction of scanning and with the
directionally sensitive axis directed toward the scanning beam.
7. The method of inspecting a surface for burrs comprising the
steps of:
generating an illuminating beam;
sweeping the beam across a surface to be inspected at a known rate
along a path on the surface;
detecting, in a plane swept by said beam and at an acute angle with
respect to the surface, illumination diffusely reflected from the
path on the surface swept by the beam whereby light from said
illuminating beam may be obscured from detection at said acute
angle in a region on said surface behind a burr during part of the
sweeping of the beam across the surface; and
measuring the interval between successive detected reflections from
the path.
8. The method of inspecting according to claim 7 wherein:
the step of measuring includes measuring the time interval between
a reflection from a burr on the surface and a subsequent reflection
from the path.
9. The method of inspecting according to claim 7 wherein:
the step of generating includes generating a focused laser
beam.
10. The method of inspecting according to claim 7 wherein:
the step of measuring comprises measuring the time interval between
successive detected reflections above preselected threshold levels,
the threshold level for the first of the reflections being greater
than the threshold level for the succeeding reflection.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of inspection equipment and is
more particularly directed to equipment employed for inspecting
machined workpieces for the presence of burrs.
2. Description of the Prior Art
In the process of metal working, a workpiece may be subjected to
multiple cutting operations to form various contours on the
exterior of the workpiece. Various cutting tools such as hobbing
machines, milling cutters and lathes may be employed to generate
the desired configurations. At the intersection of the surfaces
formed by the cutting tools, it is very possible that burrs will be
produced as a result of the cutting action of various tools. Burrs
may be found in the middle of a machined surface as well. Burrs are
undesirable since they may interfer with close tolerances generated
by the cutting tools and also because they are hazardous to
personnel. Burrs are jagged and sharp and may cause injury to the
person or clothing which comes in contact with the burr.
It is particularly important, where close tolerance work is
involved, to know precisely whether a burr is present and possibly
even the size of the burr. The presence of any burr whatsoever may
be detrimental to the intended use of certain workpieces and may
require special grinding operations if the burr must be moved. The
size of the burr is important in instances where tolerances are
critical.
Accordingly, it is desirable to be able to inspect a workpiece for
the presence of a burr and to obtain some knowledge concerning the
size of the burr. Unfortunately, burrs are generally quite small
and difficult to measure due to their irregularity. Furthermore, if
the surface of the workpiece to be inspected is extensive, the task
of examining the surface for burrs may be time consuming and
expensive. It is accordingly desirable to have an instrument which
can automatically examine a workpiece surface for the presence of
burrs and determine the height of such burrs where their presence
is indicated.
It is, therefore, an object of the present invention to disclose a
method and apparatus by which a burr on a workpiece can be
detected.
It is a further object of the present invention to disclose
apparatus which is capable of determining the height of a burr on a
workpiece.
It is still a further object of the present invention to disclose
apparatus which employs a focused laser beam to provide an accurate
measurement of the height of a burr.
SUMMARY OF THE INVENTION
The novel apparatus and method by which a burr can be detected and
measured employs a focused laser beam which is scanned across the
surface of a workpiece at points where burrs might be expected. A
photodetector is positioned at one side of the workpiece and behind
the laser beam as it is scanned at a known rate. The photodetector
is positioned with a sensitive axis directed toward the surface of
the workpiece being examined in the plane swept by the beam and
elevated at an acute angle above the workpiece surface. As the
laser beam is scanned across a burr and on the adjacent workpiece
surface, the photodetector senses reflections of the laser beam as
the beam traverses the tip of the burr and reflections from the
workpiece surface which are not obscured by the burr in the field
of view of the photodetector. For example, the region immediately
behind the burr with respect to the detector is hidden from the
detector and no reflections will be sensed by the detector until
the scanning beam emerges from the obscured region. As the beam
emerges, the detector again receives reflections from the path on
the surface scanned by the beam. Of course, as is apparent from the
drawings, it is the diffuse reflection from the surface of the
workpiece which is sensed by the detector, and references herein to
reflections from the workpiece surface will be understood to be
such diffuse reflections. If the velocity at which the beam scans
the workpiece is known, the length of the scanning path obscured by
the burr can be determined from an examination of the time interval
between reflections sensed by the detector at the tip of the burr
and the exit of the beam from the obscured region. Since the length
of the scanning path obscured is directly proportional to the
height of the burr, the height of the burr can be determined from
the time interval.
The measuring and logic network employed for determining the
obscured path portion includes a level detector having a selected
threshold which starts a timer. The threshold of the level detector
is selected so that only the flash obtained from the tip of the
burr initiates timing. A reduced threshold in the circuitry is
subsequently established so that less intense reflections from the
flat surface of the workpiece can be detected as the beam emerges
from the obscured region.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention with its numerous objects and advantages will
be better understood by reference to the following drawings in
which the same elements are identified by the same reference
numeral throughout the several FIGS.
FIG. 1 is an elementary representation of the system which employs
a focused laser beam to measure the height of a burr on a
workpiece.
FIG. 2 is a plot of the signals produced in the photodetector
employed by the burr detecting and measuring apparatus.
FIG. 3 is a detailed block diagram of the burr detecting and
measuring system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a elementary representation of the burr detection
apparatus showing the relationship of the scanning laser beam L,
the workpiece W which is to be examined for the presence of a burr
B, the photodetector 10 which senses light from the laser beam L
reflected off of the burr and the flat surfaces of the workpiece W
and the measuring and logic circuits 11 which control the beam scan
and determine burr height.
The detector 10 has a photosensitive element which senses light
radiated along the sensitive axis 12 positioned at an acute angle
.alpha. with respect to the upper surface of the workpiece W. From
the positional relationship of the detector 10 and the workpiece W,
it will be noted that the detector 10 cannot receive reflections
from the surface of the workpiece within the burr "shadow"
indicated by the shaded region in FIG. 1. It should be understood
that the term "shadow" refers to that region behind the burr with
respect to the detector 10 which is obscured from the field of view
of detector 10 due to the burr B.
From the geometry in FIG. 1, it will be evident that the length of
the scanning path which falls within the "shadow" will be equal to
v.DELTA.t, where v represents the scanning rate of the beam across
the surface of the workpiece and .DELTA.t represents the time
required for the beam to scan from the tip of burr B to the end of
the obscured region. It is assumed that the beam scans at a
constant rate to the right in FIG. 1 as would be the case where
scanning was produced by a saw tooth generator. It will be noted
that the burr height, h.sub.b, is directly proportional to the
length of the obscured portion of the scanning path. The
relationship between burr height and the obscured portion is
represented by equation 1 as follows:
(1) h.sub.b = (v .DELTA.t)tan .alpha.
From the above equation 1 it will be immediately recognized that by
scanning the beam L at a known velocity or rate established by a
saw tooth generator, the height of the burr can be determined
simply by measuring the time interval during which the
photodetector observes no reflections from the workpiece W. The
photodetector 10 is accordingly employed for this purpose.
Reference to FIG. 2 reveals the signals produced by the
photodetector 10 as the laser beam L is scanned across the burr B
at the edge of the workpiece and then onto the surface of the
workpiece W. As the beam L sweeps over the tip of the burr at time
t.sub.1, represented in FIGS. 1 and 2, the detector 10 receives a
flash of reflected light which produces the pulse P. The leading
edge of the pulse P is substantially vertical due to The
instantaneous flash of the light reflected off the tip of the burr
B. As the beam passes over the burr, the reflected energy decays
toward the null indicating that the beam has passed into the
"shadow" at the right-hand side of the burr B in FIG. 1. As the
beam continues through the "shadow," no signal is received or
sensed by the detector 10 since the burr obscures any illumination
reflected from the impingement point of the beam within the
"shadow." As the laser beam L emerges from the shadow at time
t.sub.2, the detector 10 again receives reflected illumination from
the scanning path of the beam on the surface of workpiece W.
It will be noted that the level of the illumination provided from
the generally flat surface of the workpiece is substantially less
than the peak value of pulse P. This is due to the fact that the
tip of the burr B produces a substantially stronger reflection of
the laser beam L than the illumination reflected from the flat
surface of workpiece W. If the laser beam L is scanned across a
burr and onto the surface of the workpiece to be examined, the
initial flash of reflected light from the burr may be used to
trigger a timer as the beam traverses the tip of the burr. The
emergence of the laser beam from the obscured portion of the
scanning path can also be detected by photodetector 10 to stop the
timer triggered by the pulse P. As indicated in FIGS. 1 and 2, the
timer would be started at time t.sub.1 and would be stopped at time
t.sub.2. It follows that
(2) .DELTA.t = t.sub.2 - t.sub.1
As seen from equation 1 .DELTA.t is directly proportional to the
burr height h.sub.b and therefore the measured time interval may be
used as a direct measurement of burr height h.sub.b.
The measuring and logic circuits by which the scanning and timing
functions can be performed are revealed in detail in FIG. 3.
The laser beam L is produced by the beam generator 14 which may be
a laser which produces a focused beam of coherent light. A saw
tooth sweep generator 16 is shown schematically connected to the
beam generator 14 to cyclically scan the beam at a known rate from
left to right as seen in FIG. 3. The beam would therefore cross
over the edge containing the burr and move onto the upper surface
of the workpiece W.
It will be noted that the photodetector 10 is positioned generally
behind the beam L as the beam sweeps from left to right so that the
time relationship of the pulses received by the detector is as
shown in FIG. 2. The sensitive axis 12 of the photodetector 10 will
lie generally in a plane which is swept by the beam L.
As noted above with respect to FIG. 2, the initial pulse of light
reflected off the tip of the burr B is substantially larger than
the generally steady reflected light level received from the
surface of the workpiece W due to the beam L or other light
sources. Since the peak value of the pulse is generally higher than
reflections from the flat surface of the workpiece W, it is
possible to establish a threshold level in the measuring circuitry
which exceeds the general level of reflected light at the workpiece
surface and therefore prevents triggering of a timer unless a burr
does exist at the edge of the workpiece W. In other words, unless
the initial reflection from the workpiece W is a large pulse
produced by a burr, the timer will not be triggered. For this
purpose a level detector 18 having an elevated signal sensing
level, indicated by the line s in FIG. 2, is used as a gating
mechanism for timer starting pulses in conjunction with a bistable
flip-flop 20 and AND gate 22. The output of level detector 18 is
connected to the "set" input of flip-flop 20 to turn the flip-flop
on and the output of the flip-flop is connected to AND gate 22. The
simultaneous appearance at AND gate 22 of a pulse directly from
detector 10 and also from flip-flop 20 triggers AND gate 22 and
produces an output pulse which drives shaping amplifier 24 into
saturation. It is, of course, assumed that the components are solid
state components which respond so rapidly that the leading edge of
the pulse P in FIG. 2 will correspond substantially with the
leading edge of the pulse from AND gate 22 and saturation of
amplifier 24. The saturation of amplifier 24 excites single shot
multivibrator 25 and produces a narrow pulse which is conducted to
the "set" input of bistable flip-flop 26. At the same time and for
reasons to be described hereinafter, the pulse is also conducted to
a two-bit counter 28. The pulse received by flip-flop 26 turns the
flip-flop on which produces a signal at one of the inputs to AND
gate 30. The second input to AND gate 30 is received from a clock
pulse generator 32. Pulse generator 32 may be a multivibrator which
produces a continuous train of clock pulses at a pre-establish rate
substantially greater than the sweep frequency of generator 16.
With flip-flop 26 turned on, the output of AND gate 30 is a
reproduction of the clock pulses from generator 32 and these pulses
are fed to counter 34. Counter 34 may be a standard digital counter
connected to a resetable display device 35.
Summarizing briefly, it is apparent that the leading edge of the
pulse P shown in FIG. 2 triggers AND gates 22 and 30 to allow clock
pulses from generator 32 to operate counter 34 and display device
35. In addition, the initial pulse turns bistable flip-flop 20 on
so that subsequent reflections received by photodetector 10 and
above the threshold level of the detector 10 and AND gate 20 will
pass directly to amplifier 24 whether the higher threshold of level
detector 18 is reached or not. The initial pulse also is received
by two-bit counter 28.
As the laser beam L continues to scan over the burr B and into the
shadow region, the signal output of detector 10 drops back to the
null condition as indicated in FIG. 2. This assumes that the
ambient light level is so low that the detector is not responsive.
At time t.sub.2, the laser beam L emerges from the "shadow" in FIG.
1 and produces the detector signal shown in FIG. 2. The level of
the signal at the time t.sub.2 is the general signal level of
reflections from the flat surface of workpiece W due to the beam
and is also sufficient to saturate amplifier 24 as the signal is
transmitted directly through AND gate 22 from detector 10. The
saturated output of amplifier 24 fires single shot multivibrator 25
which again pulses two-bit counter 28. The second stage output of
counter 28 is turned on and is connected to the reset input of
bistable flip-flop 26. Flip-flop 26 is shut off by the second stage
count and gates circuit 30 off to terminate pulsing of counter 34.
Since the response of the components is substantially simultaneous,
counter 34 is shut off at time t.sub.2. The count on the display
device 35 at this point is therefore the difference between time
t.sub.1 and time t.sub.2 equal to .DELTA.t.
Since the display on device 35 is equal to .DELTA.t the display is
also proportional to burr height, h.sub.b, as indicated by equation
1. Of course, the count itself may be used as a direct reading of
the burr height in an arbitrary set of units.
Since it is desirable to examine the workpiece along an entire
edge, the workpiece can be mounted on a table moving transversely
of the beam sweep path while the sweep generator 16 continuously
sweeps the laser beam L back and forth across the edge of the
workpiece under examination. With such a continuous operation, it
is necessary to reset the components in the measuring and logic
circuit 11 at the end of each sweep cycle. For this purpose,
differentiation circuit 36 receives the saw tooth sweep signal and
triggers the resetting pulse generator 38 at the termination of
each sweep cycle. Pulse generator 38 may be a single shot
multivibrator which, in response to the leading edge of the pulse
from differentiation circuit 36, generates a single output pulse
having a relative short pulse width. The output pulse from
generator 38 is applied to the "reset" input terminals of bistable
flip-flop circuits 20 and 26. It will be noted that flip-flop 26
may already have been shut off by the second stage output of
counter 28 at time t.sub.2 ; however, the pulse from generator 38
is also supplied to the reset input to insure that the flip-flop 26
has been reset for a subsequent sweep of the beam L.
The pulse from generator 38 is additionally conducted to the
two-bit counter 28 and display device 35. In each case, the pulse
resets the counts to zero for a subsequent sweep of the beam. With
the flip-flops 20 and 26 of the counters 28 and 34 reset, the
system is returned to its initial condition in preparation for a
subsequent sweep of the beam.
It will be realized that from the above circuitry, the beam will
continuously sweep back and forth across the edge of the workpiece
W and produce a count representive of the height of the burr B at
points along the edge. The sweep rate of generator 16 is low
compared to the pulse rate of generator 32 and permits the count on
display 35 to be observed or recorded. It may be desirable to
record the count on a strip chart recorder and at the same time
record information indicative of the location of the burr along the
workpiece under examination. It will also be understood that the
burrs being detected may be displaced inwardly from a sharp,
burr-free edge of the workpiece and the system will operate in
substantially the same manner as described above.
While a preferred embodiment of the burr detecting equipment has
been described and shown in the drawings, it will be understood
that various modifications and substitutions can be made to the
specific components without departing from the spirit of the
invention. For example, it will be readily apparent to those
skilled in the art that the beam may be caused to sweep from right
to left as seen in FIGS. 1 and 3 and the time .DELTA.t would not be
varied. Of course, appropriate alterations of the measuring and
logic circuit would be made to trigger and cut off the timer or
counter 34 in response to the signals produced by the output of
detector 10 in reverse order to that presented in FIG. 2. It may be
feasible in certain instances to hold the beam stationary and
translate the workpiece cyclically under the beam. Although a laser
beam is shown and described as the scanning beam, other light beams
such as that from a zirconium arc lamp may be used with similar
results. In general, the laser beam is preferred because a laser
produces a bright, small spot approaching a point source which is
desired for accuracy. The present invention, therefore, has been
described in a preferred embodiment by way of illustration rather
than limitation.
* * * * *