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.

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.

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.