Laser gauge for measuring changes in the surface contour of a moving part

A gauge for measuring small changes in the contour of the surface of a moving part, such as a rotating cylinder or sliding flat surface, as the part passes a point, using a monochromatic source of coherent light is disclosed. The gauge is based on the fact that the size and rate of movement of the speckles of a speckle pattern (formed by coherent light focused onto a nonspecular moving surface) vary in accordance with whether or not the beam is actually focused on the surface. In the preferred form of the present invention, an expanded and collimated laser beam is focused by a focusing lens onto the surface of the moving part. The speckle pattern light reflected by the moving surface is received by one or more photodetectors. The photodetectors develop pulses at a rate determined by the rate of speckle movement. These pulses are applied to a signal processor that counts them, if they are above a predetermined level. Pulse counts are made over predetermined time periods; and, if speckle size, as well as speckle rate is used as the measurements basis, only pulses occurring simultaneously on the outputs of two photodetectors are counted. The pulse counts made during adjacent time periods are analyzed by a signal processor and, then, compared. The results of the comparisons are used to control the focusing lens such that the coherent beam is brought to, and maintained in, focus on the surface of the part. Focus exists when the pulse count is maintained at a maximum level. The focusing lens position is denoted by an indicating scale mechanism, which provides the displacement information needed to determine the size of changes in the contour of the surface that cause refocusing of the focusing lens.

BACKGROUND OF THE INVENTION 
This invention is directed to measuring instruments and, more particularly, 
to optical measuring gauges. 
The present invention was developed for use in monitoring the change in the 
contour of the surface of a part (e.g., elevational changes) while the 
part is being machined by a lathe or other machine tool, for example. 
Since the invention was developed for use in machine tool environments, it 
will be described and illustrated in such an environment. However, it is 
to be understood, and will be readily recognized by those skilled in the 
art and others, that the invention is also useful in other environments. 
In general, the invention is useful in any environment where it is 
desirable to detect small changes in the contour of a moving surface along 
an axis normal to the surface, without contacting the surface. For 
example, the invention can be used to detect the transverse movement 
(vibration) of the rotating shaft of a ship in order to determine when 
said transverse movement exceeds an acceptable level. The present 
invention can also be used to provide an automatic focusing device useful 
in the precise photography of a moving surface, for another example. 
In many environments, it is desirable to precisely measure the change in 
surface contour of a moving item or part without contacting the surface 
whose change is to be measured. Such a requirement is particularly 
important in environments where unwanted electrical grounds may occur if 
the sensing device contacts the moving surface and in environments where a 
contacting sensor may score or mar the moving surface. Such a requirement 
is also important in environments where the part is rough and moving 
rapidly, because contacting sensing elements will rapidly wear away and be 
destroyed. This latter problem is of particular significance in machine 
tool environments. 
In the past, small displacements in the surface contour of a rotating 
cylinder or a sliding, flat surface have been measured using capacitive or 
inductive transducers as the sensing element. These systems have the 
disadvantage that they are effective only over short distances. Thus, it 
is impossible to use them when the displacement range of the moving 
surface may vary over several inches, or when it is impossible to position 
such devices near the surface because of other objects or items. Moreover, 
in some environments inductive and capacitive devices cannot be used 
because it is impossible to shield them from the effects of extraneous 
magnetic and electrostatic fields located in the measuring vicinity, which 
act to distort the resultant information. 
In environments wherein a noncontacting sensor is required, and capacitive 
or inductive transducer sensors cannot be used, the prior art has proposed 
the use of optical gauges, many of which use interferometer principles. 
Optical gauges using interferometer principles have the disadvantage that 
they measure contour changes only, and not absolute values of surface 
contour. Such a measuring technique also has the disadvantage that any 
temporary interruption of the light transmission path during measuring 
will render the resultant measurement in error and, thus, useless. In 
addition, interferometric techniques require specular (highly polished) 
surfaces on parts to be measured, along with precise alignment prior to 
measurement. 
Other prior art optical gauges proposed for use in machine tool 
environments have mounted a retroreflector on the tool holder. Light is 
directed toward the retroreflector and reflected back to a sensor. The 
transmission time is measured and used to determine the position of the 
retroreflector with respect to the light source and/or the photodetector. 
However, this arrangement also has disadvantages. Specifically, prior art 
optical gauges using retroreflectors measure tool-holder position, not 
part position. While the information developed is related to changes in 
the surface of the moving part normal to the beam, the information is 
subject to error because it does not compensate for tool wear, tool holder 
wear, or any misalignment between the position of the retroreflector and 
the actual part. More specifically, retroreflector type optical gauges do 
not compensate for any mechanical errors present between the 
retroreflector and the part being machined, said errors being primarily 
caused by wear and misalignment. 
Therefore, it is an object of this invention to provide a new and improved 
optical measuring gauge. 
It is another object of this invention to provide a new and improved 
noncontacting surface contour measuring gauge. 
It is a further object of this invention to provide a new and improved 
optical gauge for measuring changes in the surface contour of a moving 
part. 
It is still another object of this invention to provide an optical gauge 
that measures the absolute value of surface contour changes. 
It is a still further object of this invention to provide an optical 
surface contour measurement gauge that makes measurements directly from 
the surface of a moving part. 
It is a still further object of this invention to provide an optical gauge 
suitable for use in a machine tool environment for measuring changes in 
the contour of the surface of a moving part, as the part moves past a 
point. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a monochromatic coherent light 
beam, such as a laser beam, is collimated and focused, by a focusing lens, 
onto the nonspecular surface of a moving part, such as a part being 
machined by a lathe or other machine tool, for examples. The focusing lens 
is mounted for movement in a direction normal to the surface of the moving 
part. A portion of the light reflected back along the axis of the beam by 
the moving surface is optically collected and directed toward the light 
sensitive surface of one or more photodetectors such that a speckle 
pattern is formed at the detection plane of the photodetector(s). The 
speckle pattern causes the photodetector(s) to develop pulses at a rate 
related to the rate of movement of the speckles creating the speckle 
pattern. The pulse output of the photodetector(s) is processed by a signal 
processor to provide control signals that are used to control the position 
of the focusing lens. More specifically, the control signals are used to 
control the direction and distance of lens movement necessary to maintain 
the light beam focused on the moving surface. This result is accomplished 
by the signal processor counting the pulses produced by the 
photodetector(s) over predetermined intervals and comparing adjacent pulse 
counts. The results of the comparisons are used to move the focusing lens 
in a direction that causes the pulse output of the photodetector(s) to be 
at a maximum. 
In the preferred form of the present invention, the focusing lens is 
mounted on a carriage driven back and forth by a stepping motor. Mounted 
on the carriage is an indicator that coacts with a scale mounted in a 
fixed position along an axis lying parallel to the direction of lens 
movement. This arrangement allows the position of the focusing lens to be 
rapidly determined by reading the position of the indicator. 
Alternatively, the position of the indicator can be detected by a suitable 
position detector and the resultant information used in an overall control 
system to control the position of the tool being used to machine the part. 
The gauge of the present invention is based on the phenomena of speckled 
reflection of coherent light from a rough surface. Coherent light, such as 
the light produced by a laser, reflecting from a rough (nonspecular) 
surface, rather than producing a well-defined spot of light, produces 
instead a pattern of randomly shaped spots of light, known as a speckled 
pattern. The average size of the randomly shaped spots of light or 
speckles in a given reflection vary in accordance with whether or not the 
incident beam is focused on the reflecting surface. The average speckle 
size is a maximum value when the focus of the incident beam coincides with 
the reflecting surface and drops off sharply if the illuminating system is 
slightly defocused. Also, if the reflecting surface is in motion, the 
speckle pattern constantly changes as the light beam focuses on different 
parts of the moving surface. The rate of change in the speckle pattern 
(speckle rate) is related to the rate of motion of the surface and is also 
dependent upon whether or not the incident beam is focused on the 
reflecting surface. In a typical machine tool application, for example, a 
part being machined in a lathe, the rate of motion of the surface will be 
constant and, therefore, the speckle rate will be solely dependent upon 
the focus of the light beam, with the speckle rate being maximum when the 
laser beam is focused on the surface of the moving part. 
The actual configuration of a gauge formed in accordance with the present 
invention will depend upon whether or not the error signal is to be based 
solely on speckle rate variations, or is to be based on a combination of 
speckle rate variation and speckle size variation. The preferred 
configuration is based on speckle rate and speckle size. If speckle rate 
and speckle size variations are both used, the reflected light is directed 
toward two spaced photodetectors. If speckle rate alone is used, reflected 
light is directed toward a single photodetector. In either case, the 
photodetector outputs are pulse trains that are analyzed by a signal 
processor and the result used to control the direction of movement of the 
focusing lens, as discussed above. 
In the case of a two photodetector embodiment of the invention, the 
distance between the two photodetectors is such that the photodetectors 
will receive reflected light from the same speckle source when the speckle 
is average in size. The actual distance is not critical; normally, it is 
of the order of a few millimeters. In this embodiment of the invention, 
the signals from the two photodetectors are analyzed by a correlator that 
develops cross-correlation function between the two signals by ANDing the 
signals together to determine when the signals are coincident. 
Statistically, the probability is that both detectors will be 
coincidentally illuminated 50 percent of the time, even though the 
illumination is produced by different speckles. This is the case when the 
system is far out of focus, since speckle size is small. As the in-focus 
condition is approached, speckle size increases and the two detectors are 
more and more often overlapped by the same speckle. Since the mean speckle 
size increases and reaches a maximum at the in-focus condition, the cross 
correlation function peaks at the in-focus position. Thus, the output is a 
pulse train whose rate is related to the focus state of the light beam. 
The single photodetector embodiment also produces a pulse train related 
solely to speckle rate, i.e., no coincidental determination is made. 
Regardless of whether one or two photodetectors are used, preferably, the 
resultant pulse trains are applied to a binary counter during adjacent 
predetermined fixed periods of time. At the end of each period, the 
contents of the counter are transferred to a first register and the 
counter is cleared. At the time data is transferred from the counter to 
the first register, count data in the first register is transferred to a 
second register. Thus, the first and second registers store pulse counts 
for immediately adjacent periods of time. The contents of the two 
registers, representing the present and prior pulse counts, are 
continuously compared and the result used to control the direction of 
focusing lens movement. That is, each comparison showing a first register 
value larger than, or equal to, the preceding value (as stored in the 
second register), causes the focusing lens to move in one direction while 
a comparison showing a decrease in pulse counts causes focusing lens 
movement in the opposite direction. The rate or speed of focusing lens 
movement is controlled by multiplying the output of the first register 
with an intensity signal derived from the output of one photodetector. The 
result of the multiplication is used in an inverse manner to control the 
output of a voltage controlled oscillator. Thus, the frequency of the 
pulses produced by the VCO decreases as focus is approached. The pulses 
produced by the VCO, of course, cause rotation of the shaft of the 
stepping motor used to control the position of the carriage on which the 
focusing lens is mounted. These pulses are applied directly or inverted 
before being applied to the stepping motor in accordance with the results 
of the comparison, whereby the results of the comparison control the 
direction of movement of the focusing lens. 
From the foregoing summary, it will be appreciated that a new and improved 
optical gauge for measuring changes in the contour of the surface of a 
moving part along an axis normal to the surface is disclosed. The gauge of 
the invention is noncontacting and ideally suitable for use in a machine 
tool environment. The moving part may be rotating or sliding. In either 
case, the coherent beam intersects the moving surface along an axis normal 
to the surface at the point of impingement. Because the invention is 
optical, rather than capacitive or inductive, it does not have the 
disadvantages of capacitive and inductive noncontacting arrangements, 
discussed above. Moreover, because the invention does not use an 
interferrometer technique, it does not have the disadvantage of prior art 
optical systems using interferrometer techniques. Also, because the 
invention does not use a retroreflector mounted on a tool located adjacent 
to a part, but rather uses the surface of the part directly, it does not 
have the disadvantages of retroreflector type optical systems, also 
discussed above. Thus, any error due to tool holder wear, machine wear, or 
other misalignment are not present. As noted above, the invention is not 
only useful in machine tool environments, it is also useful in other 
environments wherein it is desired to maintain a particular item, such as 
a focusing lens, at a particular position with respect to a moving 
surface; or, where it is desired to detect changes in the contour or 
position of a moving surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a diagram of a preferred embodiment of a laser gauge measurement 
system formed in accordance with the invention and comprises: an optical 
system 10; a translation stage 12; and a detecting and motor control 
system 14. The optical system 10 includes a pair of redirecting planar 
mirrors (or prisms) 22 and 24. Light from a coherent light source 20, 
which may take the form of a laser, is received by the first redirecting 
mirror 22 and directed toward the second redirecting mirror 24. The second 
redirecting mirror 24 directs the laser light toward an expanding lens 26. 
The expanding lens 26 expands the laser light and directs it toward a 
collecting lens 28. Light collected by the collecting lens 28 is focused 
by a focusing lens 30 onto the surface of a part 32, illustrated as a 
rotating cylinder. The optical axis 45 of the focusing lens is normal to 
the surface of the part 32 at the point of impingement. 
The translation stage 12 includes a stepping motor 44, a carriage 46 and an 
indicator scale 48. The shaft of the stepping motor 44 is attached to the 
carriage 46 by a suitable rotary to linear conversion mechanism (such as a 
turnscrew mounted in a threaded follower) such that movement of the shaft 
rotationally in one direction or the other cuases corresponding linear 
movement of the carriage 46 parallel to the optical axis 45 of the 
focusing lens 30, as indicated by the arrow 47. The focusing lens 30 is 
mounted on the carriage 46. Thus, movement of the carriage 46 causes the 
focusing lens 30 to be moved back and forth, along the optical axis 45 of 
the focusing lens. Thus, the focusing lens is moved along a line 
orthogonal or normal to the point of impingement of the light focused by 
the focusing lens 30 onto the surface of the part 32. The scale 48 is 
mounted adjacent to the carriage 46 so as to cooperate with an indicator 
49 mounted on the carriage 46. Thus, as the carriage 46 is moved back and 
forth in the manner herein described, the position of the carriage and, 
thus, the focusing lens 30, is readily determined by determining the 
position of the indicator 49 with respect to the scale 48. 
The detecting and motor control system 14 includes a narrow mirror 34 (the 
long side of which is illustrated in FIG. 1) mounted transversely along 
the optical axis 45 of the focusing lens 30, between the collimating lens 
28 and the focusing lens. The mirror may be one-way so that light passes 
through the mirror when coming from the laser but is reflected when 
approaching the mirror from the opposite direction. Or the mirror may be 
narrow enough not to have an undue effect on light moving toward the part. 
The detecting and motor control system 14 also includes one or more 
photodetectors 36, a signal processor 40 and a motor control 42. The 
photodetector(s) 36 is mounted so as to detect light reflected by the thin 
mirror 34. The output of the photodetector(s) is connected to the signal 
processor 40. The signal processor analyzes the output of the 
photodetector(s) in the manner hereinafter described and, in accordance 
therewith, produces two control signals, denoted direction and step. The 
direction and step signals are applied to the motor control 42. In 
accordance therewith, the motor control 42 applies step control pulses to 
the stepper motor 44. 
In operation, the light emitted by the coherent light source 20 is 
redirected by the first and second redirecting mirrors 22 and 24, expanded 
by the expanding lens 26 and collimated by the collimating lens 28. The 
collimated light is focused by the focusing lens 30 onto the surface of 
the moving part 32. The moving part reflects light, in the form of small 
spots of light or speckles, back along the optical axis 45. The reflected 
light is received by the focusing lens 30 and, thus, by the narrow mirror 
34. The mirror 34 directs the light onto the photosensitive surface of the 
photodetector(s). The position of the narrow mirror 34 and the 
photodetector(s) is such that a speckle pattern is formed at the 
photosensitive surface of the photodetector(s). As herein described, a 
speckle pattern is formed when coherent light is reflected from a 
nonspecular (rough) surface. The speckles forming the speckle pattern 
cause pulses to occur across the output of the photodetector(s). The 
output of the photodetector(s), as more fully hereinafter described, is 
processed by the signal processor and controls the nature of the direction 
signals and the production of step pulses. The step pulses and direction 
signal cause the motor control 42 to control the movement (stepping) of 
the shaft of the stepping motor 44. As long as the incident light is 
exactly focused on the surface of the moving part 32, no step pulses 
occur. However, when an out-of-focus condition develops, as a result, for 
example, of a tool cutting away a portion of the surface of the part 32, 
the signal processor 44 produces one or more step pulses. The step pulses 
via the motor control cause the shaft of the stepping motor 44 to rotate 
in the direction denoted by the direction signal, i.e., clockwise or 
counterclockwise. The direction is such that the focusing lens is moved 
toward focus. The focusing lens 30 is moved until the laser beam is again 
exactly focused on the surface of the part 32. The change in position of 
the focusing lens 30 is denoted by the change in position of the indicator 
49 with respect to the scale 48. Thus, the change in position of the 
indicator or the net change in stepping motor pulses (times a scale 
factor) denotes the change in surface contour of the part. 
The invention is based on the fact that light reflected from a nonspecular 
(rough) surface will form a speckle pattern, i.e., a pattern of light and 
dark areas, the light areas or spots forming speckles. The speckles are 
largest at the image plane of the focusing lens when the beam is in focus. 
That is, when the focal point of the focusing lens 30 coincides with the 
surface of the part 32, the speckle size is a maximum at the image plane 
of the focusing lens 30, which is the plane at which the photodetector(s) 
36 are located. When the focal point of the focusing lens 30 is not 
exactly focused on the surface of the part 32, the size of the speckles 
decreases. In addition to this speckle size phenomena, the speckle rate, 
i.e., the number of speckles impinging on the photodetector(s) per unit of 
time increases as a result of decreasing beam diameter at the focus 
position. That is, at focus, the size of the laser beam incident on the 
surface of the part is at its smallest and a larger number of speckles are 
detected. This result occurs because, at focus, a larger number of beam 
diameters are crossed per unit time by the moving target surface. Because 
a larger number of beam diameters are crossed, the speckle rate increases, 
whereby the number of pulses developed across the output of the 
photodetector(s) increases. Thus, the photodetector pulse rate is related 
to the focus position of the focusing lens 30. The signal processor uses 
the pulse rate information to produce the step and direction signals in 
the manner hereinafter described. 
FIG. 2 is a block diagram illustrating a signal processor formed in 
accordance with the invention for use when two photodetectors are mounted 
so as to detect the rate and size of the speckles forming a speckle 
pattern. That is, the signal processor illustrated in FIG. 2 is 
operatively based on both the speckle rate and size phenomena discussed 
above, as opposed to only being based only on speckle rate change 
phenomena. 
The signal processor illustrated in FIG. 2 includes: an input circuit 50; a 
data storage subsystem 60; a step control circuit 70; a timing circuit 80; 
and, a direction control circuit 90. The input circuit 50 is a cross 
correlator circuit that includes: first and second operational amplifiers 
51A and 51B; first and second preamplifiers 53A and 53B; first and second 
potentiometers 54A and 54B; first and second comparators 55A and 55B; and, 
a three-input AND gate 56. 
One photodetector 36A is connected across the input of the first 
operational amplifier 51A and the other photodetector 36B is connected 
across the input of the second operational amplifier 51B. The output of 
the first operational amplifier 51A is connected to the input of the first 
preamplifier 53A and the output of the first preamplifier 53A is connected 
to one input of the first comparator 55A. Similarly, the output of the 
second operational amplifier 51B is connected to the input of the second 
preamplifier 53B and the output of the second preamplifier 53B is 
connected to one input of the second comparator 55B. Potentiometers 54A 
and 54B are each connected between a voltage source designated +V and 
ground. The sliding contact of the first potentiometer 54A is connected to 
the second input of the first comparator 55A and the sliding contact of 
the second potentiometer 54B is connected to the second input of the 
second comparator 55B. The outputs of the first and second comparators are 
each connected to one input of the AND gate 56. The third input of the AND 
gate 56 is an enable input, which is connected to receive an output of the 
timing circuit 80 produced in the manner hereinafter described. 
In operation, the operational amplifiers 51A and 51B amplify the pulses 
developed when speckle light impinges on the photosensitive surface of the 
photodetectors. The amplified pulses are applied to the inputs of the 
related first or second comparator 55A and 55B. If the amplitude of a 
received pulse is above the detection level set by an adjustable arm of 
the related potentiometer 54A or 54B, the comparator produces a pulse. If 
both comparators produce pulses during overlapping time periods, and if 
the AND gate is enabled, a pulse occurs on the output of the AND gate. 
Thus, the input circuit forms a cross-correlator, which requires that 
light above a preset level of intensity simultaneously impinge on the 
photodetectors 36A and 36B, in order for a pulse to be produced. While the 
amplifiers 51A and 51B are illustrated as DC coupled to their respective 
photodetectors, they can be AC coupled, if desired. If AC coupled, the 
speckle created signal will settle to a mean value, whereby a zero 
threshold level can be conveniently used to detect pulses, i.e., the 
potentiometers 54A and 54B can be set to ground level. On the other hand, 
if the amplifiers are DC coupled, as shown, the threshold value can be 
readily varied, whereby acceptable suitable speckle pulse count rates can 
be set by adjusting the level of the voltage applied by the potentiometers 
to their respective comparators. 
It is pointed out here that the photodetectors may take on any one of 
various well-known forms, including photodiodes (illustrated), photo 
transistors, charge coupled devices, etc. 
The photodetectors 36A and 36B are spaced from one another such that they 
will simultaneously receive light reflected from the same speckle source 
when the speckle is average in size and the focusing lens is focused on 
the part 32. The actual spacing is not critical; normally, it will be in 
the order of a few millimeters. In this regard, statistically, the 
probability is that both photodetectors will be coincidentally illuminated 
50 percent of the time, even though the illumination is produced by 
different speckles, because speckles and dark areas are about the same 
size. This is the situation that exists when the focusing lens is far out 
of focus and speckle size is small. As focus is approached, speckle size 
increases and the two photodetectors 36A and 36B more and more often 
receive light from the same speckle. Maximum coincidence occurs at focus. 
Thus, the rate of the pulses developed on the output of the AND gate 56 is 
a maximum at focus. The actual pulse rate increase is actually a result of 
two phenomena--an increase in the absolute level of speckle rate and an 
increase in the rate of coincidence of light from the same speckle being 
received by both photodetectors. 
The data storage subsystem 60 comprises: a counter 61; and, first and 
second registers 62 and 63. The output of the AND gate 56 is connected to 
the count input of the counter 61. The counter 61 produces a parallel 
output denoting the number of pulses counted. The output of the counter is 
applied to the input of the first register 62 and the output of the first 
register 62 is applied to the input of the second register 63. The clear 
input of the counter 61 is connected so as to clear receive a signal from 
the timing circuit 80, produced in the manner hereinafter described. The 
enable inputs of the first and second registers 62 and 63 are connected to 
receive enable signals produced by the timing circuit, also produced in 
the manner hereinafter described. 
In operation, after being cleared, the counter 61 counts the pulses 
occurring on the output of the AND gate 56. After a predetermined time 
period the first register is enabled and the count value contained in the 
counter is transferred to the first register 62. At the same time any 
count stored in the first register is transferred to the second register 
63. Thereafter the counter 61 is cleared and the cycle is repeated. Thus, 
the first and second registers always contain count values made during the 
last and the immediately preceding time periods, respectively. These count 
values are compared by the direction control circuit 90 in the manner 
hereinafter described and control the nature of the direction signal. 
The timing circuit 80 comprises: an SR flip-flop 81; first and second 
one-shots 82 and 84; and, a timer 86. The output of the timer 86 is 
connected to the S (set) input of the SR flip-flop 81. The Q output of the 
SR flip-flop is connected to the enable input of the AND gate 56 of the 
input circuit 50. The Q output of the SR flip-flop is also connected to 
the input of the first one-shot 82. The output of the first one-shot 82 is 
connected to the enable inputs of the first and second registers 62 and 63 
of the data storage subsystem 60 and to the input of the second one-shot 
84. The output of the second one-shot 84 is connected to the clear input 
of the counter 61 of the data storage subsystem and to the R (reset) 
inputs of the SR flip-flop 81 and the timer 86. 
Initially, the SR flip-flop and the timer 86 are reset when the second 
one-shot 84 is fired. When reset, the SR flip-flop enables the AND gate 
56. The pulses formed on the output of the AND gate 56 of the input 
circuit 50 are then counted by the counter 61 in the manner previously 
described. After a predetermined period of time, the timer 86 produces an 
output pulse that sets the SR flip-flop 81. When this occurs, the Q output 
of the SR flip-flop goes low and the AND gate 56 of the input circuit 50 
is disabled. At the same time, the high-low shift of the Q output of the 
SR flip-flop 81 fires the first one-shot 82. When the first one-shot 82 is 
fired, the first and second registers 62 and 63 are enabled. Thus, the 
first register 62 reads the count value stored in the counter 61. At the 
same time, the second register 63 reads the count value stored in the 
first register 62. (If necessary, a time delay may be included between the 
output of the first one-shot and the first register to allow the second 
register to read the count value stored in the first register prior to 
that value changing as a result of the first register reading the count 
value stored in the counter.) After a predetermined period of time, the 
output of the first one-shot terminates, i.e., returns low. This shift 
causes the second one-shot to fire. When the second one-shot fires, the 
counter 61 is cleared, and the SR flip-flop and the timer are reset. 
Thereafter, the cycle is repeated. 
The step control circuit 70 includes: an integrator 71; a multiplier 72; a 
digital-to-analog (D/A) converter 74; and a voltage controlled oscillator 
(VCO) 76. The output of the first preamplifier 53A of the input circuit 50 
is connected to the input of the integrator 71. The output of the 
integrator 71, which is an analog signal, is applied to one input of the 
multiplier 72. The parallel data output of the first register 62 is 
applied to the input of the D/A converter 74 and the output of the D/A 
converter 74, which is also an analog signal, is applied to the second 
input of the multiplier 72. The output of the multiplier 72 is used to 
control the rate of production of pulses by the VCO 76, in an inverse 
manner. That is, as the output of the multiplier 72 goes up in value, the 
number of pulses produced by the VCO 76 per unit time goes down. The VCO 
output pulses, which are denoted step pulses, are applied to the step 
input of the motor control 42. Each time a VCO pulse occurs, the motor 
control causes the shaft of the stepping motor to step. The direction of 
rotation (e.g., clockwise or counterclockwise) is controlled by the 
direction signal produced by the direction control 90 in the manner 
hereinafter described. 
In operation, the D/A converter provides a main control voltage that causes 
the output of the VCO to increase when the count value is low and decrease 
when the count value becomes high. As previously discussed high count 
values occur when the focusing lens is at or near the focus position. 
Thus, the rate of the pulses produced by the VCO decreases as focus is 
approached. The integrator output 71 provides damping to prevent 
oscillation about the focus position. More specifically, the integrator 
voltage increases the absolute magnitude of the voltage by an increasing 
multiplication factor as focus is approached because the rate of pulses 
occurring on the output of the first preamplifier increases as focus is 
approached. The increased level of the integrator output, because it 
increases the main control voltage produced by the D/A converter causes 
increased damping (reduced number of step pulses) as focus is approached. 
At focus, preferably, the VCO produces no step pulses so as to avoid 
focusing lens oscillation. 
The direction control circuit 90 comprises: a digital comparator 91; a two 
input AND gate 93; and, a D flip-flop 95. The digital comparator 91 has 
two parallel data inputs designated A and B. The A input is connected to 
the output of the first register 62 and the B input is connected to the 
output of the second register 63. The output of the comparator, denoted 
A&lt;B, is applied to one input of the AND gate 93. The A&lt;B output is low 
when A is greater than B (or A equals B) and high when A is less than B. 
The output of the second one-shot 84 is applied to the second input of the 
AND gate 93. The output of the AND gate 93 is applied to the clock input 
of the D flip-flop 95. The Q output of the D flip-flop 95 is applied to 
the direction input of the motor control 42. The Q output of the D 
flip-flop 95 is applied to the D input of the D flip-flop 95. 
In operation, as will be recognized from the foregoing description, the 
value of A is the count value made during the last count period. The value 
of B is the immediately preceding count value. Anytime the lens is 
approaching focus, regardless of the direction of approach, the A&lt;B output 
of the comparator 91 is low because the output of the first register is 
greater than the output of the second register. Anytime the lens moves 
away from focus the A&lt;B output is high because the output of the second 
register will be higher than the output of the first register. 
Specifically, as will be recognized from the foregoing discussion, the 
number of pulses counted is a maximum at focus. Thus, anytime the latest 
count value increases with respect to the previous count value, focus is 
being approached. This result occurs regardless of which direction focus 
is approached from. More specifically, as illustrated in FIG. 1, the 
focusing lens 30, can approach focus from the direction of the collimating 
lens 28 or from the direction of the moving surface 32. The A&lt;B output is 
low as long as focus is being approached, regardless of the approach 
direction. Contrariwise, anytime the focusing lens moves away from focus, 
A becomes less than B. When this situation occurs, the A&lt;B output of the 
comparator 91 shifts from low to high whereby the two input AND gate 93 is 
enabled. 
When the AND gate 93 is enabled, it applies the next pulse occurring on the 
output of the second one-shot 84 to the clock input of the D flip-flop 95. 
Since the input of the D flip-flop is the Q output of the D flip-flop, 
each one-shot pulse passed by the AND gate 93 causes the Q and Q outputs 
of the D flip-flop to switch states. The high/low state of the Q output of 
the D flip-flop controls the direction of step motor stepping by 
controlling whether or not the step pulses are applied directly to the 
stepping motor or inverted prior to their application to the stepping 
motor. More specifically, as will be readily understood by those familiar 
with stepping motors, positive step pulses cause rotation of the stepping 
motor shaft in one direction (e.g., clockwise) and negative step pulses 
cause rotation in the opposite direction (e.g., counter-clockwise). As a 
result, when the Q output of the D flip-flop is high positive pulses may 
be applied to the stepping motor and, when the Q output is low negative 
pulses may be applied or vice versa. Thus, if the D flip-flop was 
previously in a state whereby the step pulses caused the stepping motor 44 
to move the focusing lens 30 toward the part 32 and the lens passes 
through focus, the next second one-shot clock pulse will cause a reversal 
of the Q output of the D flip-flop (because A will not be less than B). As 
a result, the next step pulse will cause the stepping motor 44 to move in 
the opposite direction, i.e., back toward focus. As noted, preferably, at 
focus the step (VCO) pulses will cease. 
In summary, the input circuit 50 illustrated in FIG. 2 is a 
cross-correlator that produces an output when both photodetectors are 
coincidentially illuminated. The pulses produced as a result of the 
coincidental detection of light are counted by the counter 61. At 
predetermined equal intervals, the count value is shifted first to the 
first register 62 and, then, to the second register 63. The outputs of the 
two registers are continuously compared by the comparator 91 and the 
result used to control the application of one-shot pulses to the D 
flip-flop 95. The application of a one-shot pulse to the D flip-flop 95 
causes its outputs to shift states, whereby further step pulses applied to 
the stepping motor by the motor control 42 are shifted in polarity from 
their previous polarity. As a result, the direction of movement of the 
shaft of the stepping motor reverses. In order to avoid stochastic hunting 
around the focal point, i.e., continuous shifting back and forth, damping 
is provided by the integrator 71, as previously discussed. Without 
damping, lock-in at the in-focus position would never be achieved. Should 
a surface contour change be suddenly introduced, such as by a portion of 
the surface of the moving part 32 being removed by a machine tool, the 
output of the multiplier will immediately reduce in magnitude, whereby the 
VCO will start to produce pulses. As a result, the focusing lens will move 
in search of a new focus position. If the direction of movement starts out 
wrong, the direction will reverse as soon as A is found to be less than B. 
As noted above, the invention can use a single photodetector sensing 
arrangement, as opposed to a dual photodetector sensing arrangement, if 
coincidence information is not needed or not desired. A modification of 
the input circuit using a single photodetector input is illustrated in 
FIG. 3. This input circuit depends solely on the phenomena that pulse rate 
increases at focus because a larger number of beam diameters are crossed 
per unit time by the moving surface of the moving part of focus. This 
input circuit does not depend at all on the phenomena that speckle size 
increases as focus is approached, except to the extent that increased 
speckle size may increase the magnitude of photodetector pulses and, thus, 
make them detectable where they would not be detectable when the focusing 
lens is not focused. 
Basically, the input circuit illustrated in FIG. 3 is identical to the 
input circuit of FIG. 2, except that one photodetector channel is 
eliminated. Thus, the input circuit illustrated in FIG. 3 comprises a 
single photodetector channel including: an operational amplifier 51C; a 
preamplifier 52C; a potentiometer 54C; and, a comparator 55C; FIG. 3 also 
includes a two input AND gate 56C. The output of a single photodetector 
36C is connected across the inputs of the operational amplifier 51C. The 
output of the operational amplifier 51C is connected to the input of the 
preamplifier 51C; and, the output of the preamplifier 51C is connected to 
one input of the comparator 55C. The other input of the comparator 55C is 
connected to the sliding element of the potentiometer 54C, which is 
connected between a voltage source designated +V and ground. The output of 
the comparator is connected to one input of the AND gate. An enable signal 
produced by a timing circuit 80 in the manner previously described is 
applied to the second input of the AND gate 56C. The output of the 
preamplifier 52C is connected to the integrator 71. The output of the AND 
gate 56C is connected to the counter 61. 
In operation, as the moving part rotates and reflects the laser beam, the 
speckles of the resulting speckle pattern illuminate the photodetector 
36C. This illumination causes the output of the operational amplifier to 
create speckle related pulses. The pulses are amplified by the 
preamplifier 52C and compared with the setting of the potentiometer 54C. 
If a pulse is of adequate magnitude, a corresponding pulse occurs on the 
output of the comparator 55C. Assuming the AND gate 56C is enabled, when a 
pulse occurs on the output of the comparator, the pulse is applied to the 
counter and counted. Since the counter and other elements of an overall 
signal processor using a single photodetector are identical to the dual 
photodetector arrangement illustrated in FIG. 2, and function in the 
manner discussed above they will not be further discussed here. The end 
result, of course, is the same, i.e., the focusing lens is moved to a 
focusing position and remains there until an out-of-focus situation 
occurs. When such a situation occurs, the focusing lens is automatically 
refocused. The change in the position of the focusing lens is related to 
the change in surface contour and is determinable by reading the position 
of the pointer on the scale or by counting stepping motor pulses with an 
up/down counter. 
FIG. 4 is a block diagram of an alternative embodiment of an input circuit 
suitable for use in the signal process illustrated in FIG. 2. As with the 
input circuit illustrated in FIG. 2, the input circuit illustrated in FIG. 
4 is a cross-correlator and is used in combination with a pair of 
photodetectors mounted so as to simultaneously receive light from the same 
speckle when the system is in focus. However, the specific nature of the 
input circuit is substantially different. 
The input circuit illustrated in FIG. 4 includes: first and second 
operational amplifiers 101A and 101B; first and second blanking one-shots 
103A and 103B; first and second coincidence one-shots 105A and 105B; and, 
a three-input AND gate 107. The first operational amplifier 101A receives 
the signal occurring across the output of a first photodetector 36D and 
the output of the first operational amplifier 101A is connected to the 
input of the first blanking one shot 103A. The output of the first 
blanking one-shot 103A is connected to the input of the first coincidence 
one-shot 105A. The output of the first coincidence one-shot 105A is 
connected to one input of the AND gate 107. Similarly, the input of the 
second operational amplifier 101B receives the signal occurring across the 
output of a second photodetector 36E. The output of the second operational 
amplifier 101B is connected to the input of the second blanking one-shot 
103B. The output of the second blanking one-shot 103B is connected to the 
input of the second coincidence one-shot 105B. The output of the second 
coincidence one-shot 105B is connected to the second input of the AND gate 
107. The third input of the AND gate 107 is connected to the enable output 
of a timing circuit of the type illustrated in FIG. 2 and previously 
described. 
In operation, when one or the other, or both of the operational amplifiers 
receive an input signal of suitable magnitude their respective blanking 
one-shots are fired. Firing of the blanking one-shots cause the related 
coincidence one-shots to be fired. The blanking one-shots have a 
relatively long time period, when compared to the time periods of the 
coincidents one-shots, as illustrated in FIG. 5 and hereinafter described. 
The first line of FIG. 5 illustrates the output of the blanking one-shot 
103A, the second line illustrates the output of the first coincidence 
pulse one-shot 105A. The third line illustrates the output of the second 
blanking one-shot 103B and the fourth line illustrates the output of the 
second coincidence pulse one-shot 105B. The fifth line illustrates the 
output of the AND gate 107 for the conditions depicted directly above. In 
this regard, the left side of FIG. 5 illustrates a set of conditions, 
occurring between t.sub.0 and t.sub.5 that create a pulse on the output of 
the AND gate 107 and the right side illustrates a set of conditions 
occurring between t.sub.6 and t.sub.11 that do not create a pulse, even 
though both photodetectors receive speckle light. 
Turning first to a discussion of the left side of FIG. 5, at t.sub.0, light 
is detected by the photodetector 36D connected to the first operational 
amplifier 101A. As a result, both the first blanking one-shot 105A and the 
first coincidence pulse 105A are fired (triggered) at t.sub.0. The first 
blanking one-shot pulse is present from t.sub.0 to t.sub.4, whereby any 
further pulses occurring on the output of first operational amplifier 101A 
will have no effect, i.e., a pulse occurring on the output of the first 
operational amplifier between t.sub.0 and t.sub.4 will not fire the first 
blanking one-shot because it is in a fired (high output) state. At 
t.sub.2, the output of the first coincidence one-shot terminates. However, 
prior to t.sub.2, at t.sub.1, the second operational amplifier 101B 
receives an output from its related photodetector 36E, whereby the second 
blanking one-shot 103B and the second coincidence one-shots 103B and 105B 
are fired. Since t.sub.1 occurs before t.sub.2, between t.sub.1 and 
t.sub.2, both of the coincidence pulse inputs to the AND gate 107 are 
high. Assuming the AND gate is enabled, a pulse occurs on its output. This 
pulse terminates at t.sub.2, when the output of the first coincidence 
one-shot 103A terminates. Thereafter at t.sub.3 the output of the second 
coincidence pulse one-shot terminates. At t.sub.4 and t.sub.5 the outputs 
of the blanking one-shots, respectively, terminate and the system has 
completed a cycle of operation. 
On the right side of FIG. 5, the first blanking one-shot 103A is fired at 
t.sub.6 and remains fired until t.sub.10. From t.sub.6 to t.sub.7, the 
first coincidence one-shot 105A is fired. At t.sub.8, which occurs after 
t.sub.7, the second operational amplifier fires the second blanking 
one-shot 103B and the second coincidence one-shot 105B. Since t.sub.8 
occurs after the termination of the pulse on the output of the first 
coincidence one-shot 105A, the second blanking one-shot 103B and the 
second coincidence one-shot 105B are fired too late for pulse coincidence 
to occur on the coincidence one-shot inputs of the AND gate 107. Thus, no 
pulse occurs on the output of the AND gate and the counter is not 
incremented. 
In summary, FIG. 4 illustrates a correlation type input circuit that 
provides the same general result as the input circuit illustrated in FIG. 
2 and previously described, albeit in a different manner. The result is a 
series of pulses whose rate depends on speckle rate and speckle size, as 
opposed to speckle rate alone. Speckle size has an effect because, as 
previously discussed, illumination coincidence increases as speckle size 
increases, and speckle size increases as focus is approached. 
It should be noted that while the surface of the part 32 is moving, the 
speed of movement does not have to be known for the invention to operate. 
Similarly, the focal length of the lenses does not have to be known. These 
items do not have to be known because the optical system and processing 
electronics parameters do not enter into the calibration of the overall 
system. Rather, the only calibrated parts are the mechanical position of 
the carriage with respect to the indicator, and the level of the pulses to 
be recognized (counted). 
It will be appreciated from the foregoing discussion that a new and 
improved gauge for measuring the dimensional variations of the surface of 
a moving part is provided by the invention. It has been found that contour 
changes in the approximate 10.sup.-4 range can be detected. The gauge of 
the present invention utilizes the change in characteristics (size and 
ratio), of the speckle reflection of laser light from a rough surface to 
provide an indication of contour variation. The invention provides a 
mechanism wherein light is directed toward a reflecting surface and the 
focus condition of the light is the condition that is detected. When the 
light is not focused on the reflecting surface, electronic signals are 
used to control a stepping motor that moves a focusing lens toward focus. 
The distance between the focusing lens and the surface is indicated by 
counting stepping motor pulses or on a scale which can be calibrated in 
terms of contour variation. Because the actual surface of the part, as 
opposed to a tool or other surface is utilized, machine wear, tool wear 
and misalignments are eliminated as sources of error in the measurement. 
Moreover, the gauge is a noncontacting gauge, whereby, disadvantages 
created by contacting instruments are not present. Also, the gauge can be 
mounted a significant distance from the surface of the part. 
While preferred embodiments of the invention have been illustrated and 
described, as will be appreciated by those skilled in the art and others 
various changes can be made therein without departing from the spirit and 
scope of the invention. For example, rather than usuing a visually 
readable gauge, an electronic position sensor can be used to sense the 
changes in position of the focusing lens and, thus, dimensional 
variations. The sensed position information can be used to control an 
electronic display, or used to control the position of a cutting tool, if 
desired. Also, coherent light sources, other than a laser can be utilized, 
if desired, even though a laser is preferred. Moreover, as noted, while 
the invention was developed primarily for use in machine tool gauging, it 
can be used in other environments. In general, the invention is useful in 
many environments wherein the contour variations of a moving surface, with 
respect to a position normal to the moving surface, are to be determined. 
Thus, the invention can be used in combination with a focusing device for 
a high speed camera system designed to take photographs of a moving 
surface. Moreover, while a stepping motor is the preferred mechanism for 
moving the focusing lens, other types of linear moving devices, including 
hydraulic and pneumatic devices can be utilized, if desired. Hence, within 
the scope of the appendant claims, the invention can be practiced 
otherwise than as specifically described herein.