Video measuring system for defining location orthogonally

Apparatus for determining the location of a physical feature, such as a hole in a panel, relative to two coordinate axes by optically scanning across the panel and hole from a reference while measuring the distance of the hole from the reference and the length of the hole chord for each of a plurality of scans, then adding half the chord length to the respective distance to the hole edge and averaging the results for the plurality of scans. Scan lines used for the hole edge and chord measurement can be selected from among a plurality of scan lines having a predetermined chord length. The measuring apparatus can also be readily adjusted to accommodate a range of feature dimensions.

Copyright International Business Machines Corporation 1984. The 
specification of this patent document contains material to which a claim 
of copyright is made. The copyright owner, assignee herein, has no 
objection to the duplication of the specification but reserves all other 
copyright rights whatsoever. 
BACKGROUND OF THE INVENTION 
This invention relates generally to apparatus for optically measuring 
dimensions with video equipment and, more particularly, to such apparatus 
as used for automatically and accurately measuring dimensions of small 
features along two orthogonal axes. 
During the manufacture of a printed circuit panel, a large number of 
through holes or vias is drilled by a multiple spindle drilling machine 
that is relatively moved from one drilling location to another over the 
panel. Dimensional accuracy of the hole placement must be assured and the 
panel must be carefully inspected to ensure that the drilling machine has 
been accurately positioned. Manual inspection of even a few selected holes 
makes the cost prohibitive. Accordingly, an automated system of 
measurement is required. 
The automated optical measurement of relatively small dimensions is 
well-known. Most systems use a moving beam of light that traverses the 
surface of an object and changes reflectivity as contrasting features are 
encountered. An example of this method is United Kingdom Patent 
Specification No. 1,404,311 in which pulses are gated to an accumulator 
during the time light is reflected from a contrasting feature in the path 
traced by a small beam. The accumulated pulses become a measure of the 
feature dimension when the beam velocity is known or when scale marks are 
sensed during movement. In the case of holes or openings, one solution has 
been to gate pulses from a fixed frequency oscillator to an integrator or 
accumulator during the time a relatively moving light beam passes through 
the opening or impinges on a grid work of opaque lines during the 
movement. Again, the velocity of the beam is uniform and known to enable 
conversion to a dimension. Examples of this latter technique are taught in 
U.S. Pat. Nos. 2,447,024 and 3,546,671. 
The holes drilled in printed circuit panels are becoming smaller in 
diameter as drilling capabilities are improved to enable more circuits and 
their vias to be placed within a unit area. In addition, the panels are 
being laminated with more layers so that the aspect ratio, length to 
diameter, of the hole is increasing. These factors make the usual 
approaches of light transmittance through the hole or direct reflectance 
for gating a counter unacceptable because the sensors are inaccurate or 
incapable at the smaller dimensions. A system using a vidicon camera and 
magnifying lens, such as that described in U.S. Pat. No. 3,551,052, is 
necessary to scan the drilled opening. This latter reference, however, is 
inadequate in determining the location of an opening with respect to a 
reference and cannot provide a dimensional output as a pulse number since 
it responds with only a single pulse during each traversal of a 
contrasting area. 
In order to obtain accurate measurement of location, several scans are 
required to assure that irregularities at the edges of the opening are 
minimized in influence and that measurements can be taken with a high 
degree of accuracy, such as the distance of a reference to the edge of the 
opening or the distance across the opening on a particular scan line. The 
techniques heretofore known have been unable to perform these required 
measurements, thereby preventing the desired automating of hole 
inspections. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is a primary object of this invention to provide an optical digital 
scanning system by which the distance of features of an object from a 
reference can be determined. 
Another important object of this invention is to provide an optical 
scanning system that enables the location of an object feature to be 
determined relative to a reference by averaging the distance of a 
plurality of measurements obtained during a plurality of linear scans of 
the object and feature. 
Yet another important object of this invention is to provide a video 
scanning system in which features on an object can be inspected as to 
location by selected linear scans of the object and computation of 
distances and averages thereof from a reference. 
The foregoing objects are attained in accordance with the invention by 
providing video scanning means effective to scan an object a plurality of 
times in successive scans at a known velocity and sense the boundaries of 
contrasting features with the scan lines effective to gate pulses of fixed 
frequency into accumulator means in proportion to the time the scanning 
beam is sensing a particular feature and thereby convert the length of the 
scanned feature to a digital value. The scan lines are selected as to 
those effective for gating the pulses by either feature dimension or 
desired frequency of scan. The accumulated digital values can be combined 
after processing by computation means to provide distance values from a 
reference and deviations from an average. 
The measuring system uses high-speed video sensing and includes optics to 
change magnification to vary the image size examined by the video scanning 
means and thus advantageously accommodate object features of varying sizes 
such as those encountered in drilled holes. Chord lengths of the holes are 
readily selected by setting minimum accumulator values for gated pulses. 
The use of multiple accumulators allows separate measurements and thus 
more versatility. 
The foregoing and other objects, features and advantages of the invention 
will be apparent from the following, more particular, description of a 
preferred embodiment of the invention as illustrated in the accompanying 
drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the measuring apparatus comprises generally a table 
assembly 10 movable along the Y axis carrying a circuit panel 11 having 
therein drilled holes 12 that are viewed by a pair of overhead video 
cameras 13, 14 supported for movement along an X axis on a movable 
carriage assembly 15. Table 16 is mounted for reciprocating motion on 
guide rods 17 supported in blocks 18. A motor 19 and lead screw 20 can be 
selectively energized to drive the table to a desired position along the Y 
axis. Magnitude and direction of motion along that axis is indicated by 
signals in well-known phase quadrature from a photosensitive optical 
transducer 21 sensing opaque marks on linear scale 22 secured to table 16. 
Circuit panel 11 is held in position on the table by locating vee blocks 
23. 
Drilled holes 12 and panel 11 are viewed by a pair of identical video 
cameras 13, 14 each oriented along one of the orthogonal X and Y axes to 
provide images of a selected hole that are displaced 90.degree. from each 
other. Commercially available video cameras may be used such as Model 
LSV-1.5/DBA-1 from Sierra Scientific Corp. in Mountain View, Calif. The 
cameras are supported on a carriage 30 that is movable along the X axis on 
guide rod 31, supported in hanger blocks 32, only one of which is shown. 
Camera carriage 30 is moved on guide rod 31 by a selectively operable 
motor and lead screw, not shown, similar to that for table 16. A linear 
scale 33 is sensed by photosensitive optical transducer 34 to provide 
output signals in phase quadrature that can be processed in a well-known 
manner to indicate magnitude and direction of motion. A selected hole 35 
to be viewed is illuminated by light from a source, not shown, delivered 
through bundles 36 of optical fibers. The selected hole is viewed by 
cameras 13 and 14 through a collimating lens 37, a beam splitter 38 and 
mirror 39 and through respective magnifying lenses 40 and 41. Lens 37 is 
supported in a cylindrical housing or quill 42 that is moved vertically 
from a retracted position into contact with panel 11 to assure proper 
focus when video camera images are to be generated. 
The inspection system shown is under the control of a larger computer 
system for achieving automatic inspection procedures. The larger system 
may include one or more central processing units for controlling 
positioning of the table, cameras shutters and lens focusing among other 
operations. The individual hole calculations, selection of useful data, 
and averaging of data is under the control of a microprocessor 60, to be 
subsequently described. 
Inspection of selected drilled holes is undertaken to determine the 
coordinate locations of their centers with respect to a home position. 
This is done initially by placing panel 11 on table 16 and moving the 
table to a "home" position that is a mark on one of locator blocks 23. All 
other locator blocks can also be checked. The video measuring apparatus is 
adjusted to provide a 0--0 coordinate location for the home position at 
the monitor screen center by counting pixels to its midpoint. Thereafter, 
the motion of the table and camera carriage assemblies 10 and 15 can be 
measured by the photosensitive transducers 21, 34 and respective scales 
22, 33. 
A hole whose diameter and location are to be determined is imaged by both 
orthogonally oriented video cameras oriented so that the measurements can 
be orthogonally defined. The measurements can be done simultaneously with 
the described arrangement to achieve efficiency. Since both camera systems 
operate in a similar manner, only the system for the X axis will be 
described in detail. 
An image of hole 35 (FIG. 1) as seen by vidicon camera 13 and seen in a 
monitor, not shown, is shown in FIG. 2a. The camera is typically a 
non-interlacing type that has 525 scan lines of video in a frame. The scan 
lines 45 each start from the left and move right across the field of view. 
The size of the imaged drilled hole is determined by the magnifying lens 
40 for the X axis and this enables a range of hole diameters to be scanned 
merely by changing the magnification. In the measuring system of the 
invention, the location of the center of hole 35 is defined with respect 
to the left edge 46 of the frame as seen in FIG. 2a. This requires that 
the distance from the left frame edge to the center of the hole has to be 
measured. The distance that the hole center lies from the frame edge 46 is 
measured by gating fixed frequency pulses, such as from an oscillator to 
two different accumulators or counters during the generation of a scan 
line. A vidicon operating at 15 Hz completes a scan line each 126 usec. 
Since the scan line velocity is a known value and the pulse frequency is 
fixed, the accumulator values represent distance from the time each 
accumulator is gated until blocked. 
A diagram of a circuit and microprocessor for determining the distance from 
the reference edge to a hole edge and the center of the hole is shown in 
FIG. 3. The microprocessor shown is a mode1 6800 available from the 
Motorola Corporation at Schaumburg, Illinois. The circuit uses multiple 
inspection scans that produce signal levels shown in FIGS. 2b and 2c as 
gating signals to control the accumulation of pulses representing 
distance. In FIG. 2b, the waveform shown represents that of a single 
horizontal scan line 47 across the approximate midpoint of the monitor 
view of a portion of the circuit panel and drilled hole 35. The negative 
excursion 48 represents the horizontal synchronizing pulse for the scan 
line; signal level 49 represents a dark surface; and level 50 represents a 
reflective or light surface. Thus, the scan line trace, after 
synchronization, rises to level 50 indicating a light or reflective panel 
surface, then later falls to a dark level 49 as the hole edge is 
encountered where it remains until rising again after leaving the hole and 
again sensing panel surface. These levels are used to gate oscillator 
pulses into different counters. A further gating signal is shown in FIG. 
2c where a counter enable signal reaches an activating level at the 
termination of horizontal synchronizing pulse 47 in FIG. 2b. 
In FIG. 3, an oscillator 52 of fixed frequency, such as 10 Mhz, has its 
output connected as one input to AND gate 53 and one input to AND gate 54. 
When AND 53 is conditioned by a counter enable signal from FIG. 2c, 
indicating that a scan line has started, pulses will be transmitted to 
counter 55. Counter 55 will accumulate pulses until the transition in the 
light level to the dark level or from level 50 to level 49 on the trace in 
FIG. 2b. At that transition at terminal 56, a set signal to latch 57 will 
lock the counter value in the latch. This value represents the distance 
from the left side 46 of the image in FIG. 2a to the left edge of hole 35. 
The second input to AND 54 is from an inverted vidicon output and is 
effective from the stable horizontal synchronization pulse 48 at the dark 
levels 49 to condition AND gate 54 so that oscillator pulses are directed 
to counter 58. During the time that the dark level is present at AND 54, 
counter 58 will accumulate pulses and, as the scan line senses the far 
edge of the hole, the dark level signal will terminate signaling computer 
60 so that counter 58 contains a value representative of a chord length of 
hole 35. The dark-to-light signal causes the microprocessor 60 to initiate 
a comparison of the value in counter 58 with a preset value. 
During the generation of scan lines, it is desirable that only selected 
scan lines be considered for measurement. At the conclusion of a scan 
line, the accumulated count in counter 58 is compared at compare circuit 
59 with a preset value from microprocessor 60. If the accumulated value of 
counter 58 is less than the preset value, a reset signal is transmitted 
from compare circuit 59 to both counter 58 and latch 57. This action 
erases the accumulated values in counter 58 and latch 57 so that the scan 
line data is not used. A value that is too small in counter 58 indicates 
either that the scan line did not encounter a hole 35 in FIG. 2a or that 
the chord portion across the hole was too short to be of interest. Thus, 
the preset value is a technique of screening acceptable scan lines through 
the setting of minimum lengths. 
If the accumulated count in counter 58 is equal to or greater than the 
preset value in comparator 59, a signal is sent to microprocessor 60 to 
store the value of latch 57 and the value of counter 58 for that 
particular scan line within the microprocessor for later computation, such 
as adding the value of the latch 57 to half the value of the counter 58 to 
determine, for that scan line, the distance of the X-coordinate of the 
center of the hole 35 from the frame edge 46 of the video frame. Counters 
55 and 58 and latch 57 are then reset for the next scan line. The 525 scan 
lines provided by the vidicon are in excess of the number required to make 
a reliable measurement. Therefore, the microprocessor is programmed to use 
one out of any desired number such as 5 or 10 scan lines, and is 
programmed to add the value of the latch 57 to half the value of the 
counter 58 only for each of the selected ones of said scan lines 45 
resulting in a number of values which may be averaged by the 
microprocessor to significantly improve the determination of the distance 
of the X-coordinate of the center of the hole 35 from the frame edge 46 of 
the video frame. The vertical and horizontal sync pulses are used for 
counting frames and scan lines. This affords a technique of reasonably 
limiting the number of scan lines to be reviewed. As mentioned previously, 
the preset value placed by the microprocessor 60 in compare circuit 59 can 
be nearly any desired value. One technique of determining this value is 
shown in FIG. 4 and is to determine the theoretical lengths of chords 
between the intersections of a pair of diameters with the circumference 
when oriented at 45.degree. with a third diameter parallel to the scan 
lines. The resulting value can be used as a minimum chord length and 
changed, of course, for different sizes of holes. 
The invention offers significant improvement in determining the location of 
the center of a hole by using a plurality of linear scans to sense the 
hole edge. Selection and use of a predetermined number of scans, such as 
sixteen, permits the averaging of results that provide greater accuracy. A 
deviation of the computed hole center location from the average for each 
of the plurality of chords is easily calculated, and those values widely 
varying or beyond established limits can be discarded. Irregular hole 
edges can be detected in this manner to prevent erroneous conclusions. 
A computer data flow diagram is shown in FIG. 5 that describes the video 
portion of the inspection process. Sixteen chords are illustrated as the 
total used for a measurement. The number, however, can be changed to other 
values, as desired. 
As mentioned above, the automatic hole inspection process is controlled by 
a general purpose computing system. The microprocessor is a part of and 
communicates with this system for the computation of specific hole 
locations data. A preferred method of implementing the inspection system 
is shown in FIGS. 6-12; other techniques of the implementation can readily 
be devised by others skilled in the art. Source code by which the 
inspection system is preferably enabled is shown in appendix A. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made therein without departing from the spirit and scope of the invention. 
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