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BACKGROUND OF THE INVENTION
The field of the invention is high precision probes that measure the
distance to the surface of a test object. More particularly, the invention
relates to high precision laser based measuring systems and associated
optics and electronics to perform such distance determinations relating to
the surface of a test object.
A preferred embodiment of the device is designed to be used with currently
manufactured Coordinate Measuring Machines (CMM's). Coordinate Measuring
Machines are used by many manufacturers, worldwide, to precisely determine
if manufactured parts are in tolerance. The location of faces on parts,
holes, screw threads, etc. can be precisely determined via the use of
Coordinate Measuring Machines. Such machines usually have probes which
must contact the surface of a part to be tested. A leading manufacturer of
ruby tipped mechanical probes which physically contact test surfaces is
Renishaw Electrical, Ltd. an English company.
In operation, the tip of the sensor is moved around in space by attaching
it to the arms of a Coordinate Measuring Machine. The arms allow free
travel of the sensor in all three dimensions and contain encoders which
allow one to precisely determine the exact position of the probe tip in
space. Typical resolution for a Coordinate Measuring Machine is 0.00001"
or alternately 0.1 mil (2.5 micrometers).
Coordinate measurement is accomplished as follows. The three-dimensional
part is mounted onto the measurement table. The arm of the Coordinate
Measuring Machine is moved so that the tip of the contact probe comes down
and touches the surface of the part. When the part is touched, the tip of
the probe is deflected slightly and opens an electrical contact. When the
contact is broken, encoders in the Coordinate Measuring Machine arms are
electronically locked so that they maintain their readings even if the arm
overshoots. A computer then reads the three axis coordinates where the
part surface was encountered. By this means, the exact location of one
point on an object, the X, Y, Z coordinate of the point in space, can be
determined. Subsequent points on the part are measured in a similar manner
until a sufficient number of points are measured, and the critical
parameters of the part have been determined, to see if it is in tolerance.
A disadvantage of the contacting probe tip is that it does not work for
flexible parts such as thin metal pieces, plastics, any kind of liquid, or
other soft, deformable materials such as a foam product or clay, which
could be deformed via the measurement process.
An additional disadvantage of the contacting method is that the probe tips
typically have a small ruby sphere attached to the end. The diameter of
the sphere is well known, so that its radius can be compensated for in the
measurement process but, nonetheless, the radius of this sphere is
inherently large (with 0.050" being an example). This means that an object
with very fine detail cannot be measured by standard probe tips, because
the probe tip is too large to reach into small features of complex
objects. If smaller probe tips are made in order to extend into small
crevices and similar areas, then the pressure from a sharpened tip that is
exerted onto a part will be such as to severely dent or distort the part
at the measurement point, giving false readings.
Contacting probes have another disadvantage. Because they mechanically
contact the part, they have to be retracted away from the part before they
can be moved laterally so that the probe tip is not dragged along the part
surface.
All these disadvantages are overcome by the use of the disclosed
laser-based, non-contact sensor system.
SUMMARY OF THE INVENTION
The instant invention is designed to be physically plug compatible with
currently manufactured Coordinate Measuring Machines and currently
manufactured contact probes. The invention produces an optical trigger
signal which, when received by the coordinate measuring machine, is
electrically equivalent to the contact of the piece by the ruby tipped
mechanical probe.
The sensing mechanism is produced by a laser light source, such as a laser
diode, and one or more lenses which focus the light on the surface of the
object.
A receiving lens detects the spot of laser light and focuses it in a manner
to scan a solid state light detector pair. As the reflected light is
focused on the first detector of the pair, an "in range" signal is
generated by associated electronics and an audio-visual indication is
generated for the operator, such as the lighting of an LED, to alert the
operator that the trigger point or measurement point is about to be
reached. Such an expedient is not possible with mechanical probes. As the
focused light falls equally on the two detectors of the detector pair, or
in a predetermined proportion, the trigger point has been reached and a
signal is generated to the CMM that the coordinate measurement should be
taken and a primary function of the invention has been achieved.
In the event of manual operation of the Coordinate Measuring Machine, where
overshoot of the measurement point is possible, light focused on the
second detector of the detector pair continues to indicate that the sensor
head is still "in range" of the measurement position. As the focused light
beam passes from the second detector of the detector pair, the in range
light is extinguished so that the operator does not contact the surface.
As a consequence, the laser triggered optical probe is similar in function
to a mechanical contacting probe, except that it never needs to touch the
part in order to give a trigger signal. The additional capabilty of giving
an in-range signal tells the operator that he is getting close to the part
surface.
Additionally, the spot size that the laser beam is focused to is only
approximately 0.001 inches. Therefore, a very small probe point is used
and very complex, small detail objects can be probed. The laser beam, of
course, can also be moved laterally across the surface without any drag,
which provides alternative means of detecting contoured surfaces. This
ability can also at least double the data gathering rate of a Coordinate
Measuring Machine. Additionally, the laser beam will cause no perturbation
to the surface via the measurement process; hence it is suitable on soft,
deformable materials such as clay, thin sheets and the like.
The response speed of the laser base sensor is also extremely fast and the
system is accurate. Response speed is approximately 100 microseconds for a
trigger signal and the accuracy is in the 0.00001 inch range.
These and other objects and advantages of the invention will become
apparent to those skilled in the art upon a review of the Description of
the Preferred Embodiment of the Invention and the accompanying drawings
and claims.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of the optical system, the light source and
the detector pair which senses the position of the object in order to
generate the trigger signal.
FIG. 2 is a similar mechanical schematic which shows a slightly different
optical arrangement to enhance the compactness of the sensor head.
FIG. 3 is a block diagram of the electronics of one embodiment of the
invention.
FIG. 4, comprising FIGS. 4a-4e, show electronic circuits used in the
invention. FIGS. 4a and 4b are the minimum normally required for an
operational device, the remaining circuits are enhancements for different
features and functions of the invention.
FIGS. 5a and 5b, set forth a logic table identifying the operation of a
preferred embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Shown in FIG. 1 is a mechanical schematic of one form of the invention. In
a preferred embodiment for use with current Coordinate Measuring Machines
(CMM) a measurement is initiated by receipt of an arming signal from the
CMM indicating its computer is ready to make a measurement. Although its
predominant emission is infrared, the laser 20 diode also preferably emits
light in the visable spectrum so the target is visible to the operator.
The light is focused by the source lens 24, which can be a three element
lens of conventional manufacture, onto the surface of the object 30 to be
tested establishing a focused point of light approximately one thousandth
of an inch in diameter. The light is reflected from the object 30 surface
and imaged with a receiving lens 34 onto a detector pair 38a,b located
within the sensor head 15. The two detectors 38a,b must be extremely close
together and, in fact, are conventionally fabricated by companies such as
United Detector Technology on a single piece of silicon with a separation
of approximately 0.005."
Referring to FIG. 1, the ray which passes through the center of a lens 34
is not deviated. Using this principle, if the object 30 is too far away,
the light entering the receiver optical system 34 will be focused on a
position which is beyond detector segment No. 1, 38a; therefore, very
little light will fall on either detector 38. In this condition, there
will be no in range signal and the sensor 15 will remain inactive. The
laser 20 power may be adjusted up to maximum intensity as the detector 38a
tries to receive a signal.
As the sensor 15 moves closer to the object 30, the image of the laser spot
moves such that a fair amount of light first falls on detector No. 1, 38a.
When this detector 38a has a sufficient amount of light focused on it, the
"in range" signal will begin. As the sensor 15 moves continually closer to
the object 30, the image of the laser spot moves across detector No. 1,
and approaches the junction between detectors No. 1 and No. 2, 38a, b.
When the detector 38 signals are equal (that is equal amounts of energy are
falling on detector No. 1 and detector No. 2 in the pair), the trigger
point is reached and the CMM receives the signal to store the coordinate
measurement.
A number of expedients are used, or can be used, to address environmental
effects on the probe's, 15 accuracy. For example, visable light is
preferably emitted by the laser 20 so that the operator can see the target
spot. However, the predominate energy emitted is preferably in the near
infrared and a filter 40 is preferably used which blocks all visable light
so that the sensor 38 is blind to ambient illumination. In conditions
where incandescent ambient lighting is used, rather than fluorescent which
is preferred, or if the object 30 surface is grossly out of focus so that
the noise in the system yields nearly equal signals on the two detectors
38a,b, it is not necessary to have equal signals on the detectors 38a and
b to have the trigger point, which might be triggered by equal amounts of
incandescent light, or noise, falling on the detectors, 38. Any other
fixed signal ratio can be chosen by adjustment of the reference potential
of the operational amplifiers referred to in connection with FIG. 4.
After the trigger signal is given, the arming light 42 is subsequently
extinguished (by CMM software) at the point where the two signals are
equal or in the predetermined proportion. If the sensor head 15 then moves
too close to the object 30, the image falls predominantly on detector No.
2, 38b. While the trigger has been given, because the sensor is too close,
the in range signal 44 remains on. If the object moves much too close, the
light falls off detector No. 2, 38b, and the in-range signal 44 is
extinguished.
The laser 20 power is maintained such that a desirable signal level falls
on the detector pair 38. The light may be turned down for extremely shiny
objects, which would put too much light into the detector 38, and turned
up for dull, diffusely reflecting or absorbing objects, which do not
return much light to the detector 38.
A cylindrical lens 45 may optionally be added to the sensor 15 which will
serve to make the illumination pattern a small stripe with a 10 to 1
aspect ratio instead of a point on the object 30. It is preferable that
the stripe be in the direction so that its long axis is vertical relative
to the schematics in the drawings. The advantage of doing this is that one
can do a small amount of "averaging" over the test surface and in this way
can compensate for some of the microstructure in the surface, scratches or
other surface roughness, which is always a factor which limits accuracy in
laser-based, non-contact range sensors. If the line is fairly short, then
the sensor 15 is still measuring a small area on the object 30. Therefore,
a compromise can be made on the length of a line produced by the
cylindrical lens 45. A size of approximately 0.010" in length of the line
segment (with the laser beam then being 1 by 10 mils) would be a
reasonable size.
The entire sensor head 15 is mounted via a rod 50 which is axially aligned
with the laser beam. Three indicator lights 42-44 are on the sensor head.
One light 43 is a "laser on" indication which is required by federal law;
another light 42 is the red "armed" signal; and the third light 44 is the
green "in-range" signal.
Electrically, the sensor can be made pin compatible with a Renishaw
mechanical triggering probe by maintaining equivalent voltages for the
arming signal to the sensor 15 and the trigger output.
FIG. 2 shows a second mechanical schematic of an alternative embodiment of
the physical layout of the laser source 20 and detectors 38 and optic
system. Using the arrangement shown in FIG. 2, a compact sensor head 15
can be constructed using two focusing lenses 34,35 and a reflecting mirror
52 so that the entire optical system can be included in a housing portion
which is less than 11/2 by 3 inches. The principal of operation and
mounting of the compact sensor head 15 is substantially the same as that
described for the first embodiment.
While a 45.degree. angle has been shown as the angle for the receiving lens
34 to maximize the rate of scanning of the focused layer beam past the
detector pair 38, it will be understood that other angles may be employed
to achieve the objects of the invention.
FIG. 3 shows a block diagram of electronics for the sensor system. The
detectors 38 are a central feature. If a sufficient signal is present on
one of the detectors 38, the in-range threshold 60 is achieved and the
"in-range" signal 62 is given.
The laser power is also set with a laser power control circuit 64 to give a
satisfactory signal from the detector pair 38. A comparison is made of the
power focused on the detectors 38, and if these signals are equal, then
the trigger signal 75 is given by a comparator circuit 68 which may be
latched with a latch 70 as discussed below. If the sensor is armed via
software, then the internally latched trigger signal can be logically
combined with the arming signal 72 to extinguish the red arm/triggered
light.
FIG. 4 shows the schematic diagrams for the optical trigger. FIG. 4a is the
circuit which controls the laser diode 20 power output. The schematic
diagram of FIG. 4b is the circuitry which receives power from the dual
photo detector 38 and generates the trigger signal 75 and the in-range and
out-of-range signals 62.
Referring to FIG. 4a, the power level circuitry uses as its feedback signal
the output from diode D-1, which is packaged as part of the laser diode
20. This signal is amplified by a transconductance amplifier, operational
amplifier No. 101, which is then fed into an integrator, operational
amplifier 102. The output of the integrator is then buffered and amplified
by an operational amplifier 103 and transistor T-1, to provide a
controlled current to the laser diode D-2. This current is determined by
the reference level adjusted on potentiometer R-1, which is compared to
the signal from the operational amplifier 101, and is proportional to the
amount of light emitted by the laser diode D-2. If the signal either
increases or falls off in intensity, an error signal is generated on the
output of the integrator, operational amplifier 102, which is in the
opposite direction of the increase or decrease of light current and,
therefore, corrects the condition back to the set level, which is adjusted
on potentiometer R-1. A negative voltage regulator 120 is used to provide
a bias voltage for the photodiode D-1 and to enable operation of this
circuitry from a single 12-volt supply.
The detector 38 circuitry of FIG. 4b, which is used to actually measure the
trigger 75 and range 62 signals, utilizes two transconductance amplifiers,
operational amplifiers 104 and 105. The outputs from these amplifiers
104,105 are filtered through a low-pass filter made up of resistors R-2
and R-3 and capacitors C-1 and C-2. These two filtered signals are fed to
the inputs of a differential comparator, operational amplifier 106. The
output of comparator operational amplifier 106 changes state as the
relative magnitude of its two input voltages from operational amplifiers
104 and 105 change due to a shifting of the incoming light on the
photodiode pair 38a,b. The output of the comparator operational amplifier
106 drives an open collector transistor T-2, either directly as shown in
FIG. 4b or through a latch 70 as shown in FIG. 4c, which provides the
trigger signal 75 to the Coordinate Measuring Machine.
Operational amplifier 107 looks at the arithmetic average of the outputs
from the two photodiodes 35a,b in the photodiode pair 38. This average
voltage, corresponding to the amount of light on the photodiode pair 38,
is compared to a reference level and used to generate the in-range and
out-of-range signals 62. The out-of-range signal 62 drives the base of
transistor T-3, which in turn drives the base of transistor T-4, to
inhibit the strobe input of comparator operational amplifier 106. This
inhibiting is done to disable the trigger signal when the sensor 15 is out
of range and there is no appreciable amount of light falling on the
photodector pair 38a,b.
Shown in FIG. 4c, is a comparator 109 used for power adjustment of the
laser 20. Since the surface that is being detected can vary from very dark
to very shiny, power adjustment of the laser beam may be necessary if
different objects or an object having varing degrees of reflectivity if
being measured.
The power adjustment circuit consists of an operational amplifier 109 which
is connected between the output of operational amplifier 105, which
measures the output of the first detector 38a of the detector pair 38, and
is connected to the reference voltage to the power circuit 64 for the
laser 20. As the sensor 15 circuit is armed or activated, the first
detector 38a will not detect the laser 20 beam and thus the power
adjustment circuit 64 will boost the laser power up to make sure that a
dark surface is not being detected. If too much signal is detected because
the laser power has been increased too much or if a very reflective
surface is being sensed, the reverse process takes place and the laser
power is turned down. In this manner the system can be optomized for the
most accurate detection.
Shown in FIG. 4e is a CMM arming circuit for the sensor. In the circuit of
FIG. 4e only a visual indication is utilized which is received from the
CMM. In this circuit the signal from the CMM, which indicates it is ready
to take a measurement, is connected through a transistor T5 which, when
on, lights a red LED 42 to tell the operator to take the measurement.
A latching circuit 70 shown in FIG. 4d can be used to provide an internal
trigger signal ITS, which is latched in the "on state" to make a
measurement until the probe 15 enters zone "C" of FIG. 5a. As soon as the
"too far" zones, C and D are reached the event trigger signal will be
latched until the probe returns to zone B and the sensor system resets.
The internal latch, therefore, maintains the trigger signal until the
sensor 15 has moved out of range away from the object 30 to reset the
latch for a second measurement. This latch is used to differentiate
between out-of-range signals which are too close rather than too far. For
example, after the trigger event has occurred, if the sensor 15 is moved
even closer to the object 30, the received beam will become predominantly
on detector No. 2 38b which will provide an indication of in-range, but
with the latch 70 the circuit will remember the fact that the trigger has
occurred. If the sensor 15 continues to move towards the object 30, the
received laser beam will pass detector No. 2 38b and an out-of-range
indication would be present. In this condition with the latch circuit, the
sensor will know if it is too close or too far and the operator will know
that the last event that happened is that the sensor 15 was too close to
the object 30.
The latch circuit shown in FIG. 4d, utilizing D-latches, 201 and 202, is
provided to remember the trigger event after the sensor 15 goes out of
range in the too close position. This is accomplished by setting latch 201
upon the trigger event and then using latch 202 as a one shot to clear the
first latch 201 when the sensor 15 again enters the correct range, the too
far condition. This circuitry then takes the latched signal and uses that
with the corresponding combinational logic to arrive at the condition that
is required by the CMM user. This condition requires that transistor T2 be
closed or on during the time that the sensor 15 is out of range from the
too far condition and also in range before the trigger event occurs. The
condition is logically determined by the two AND gates 206,207 and the OR
gate 209 which drive the base of transistor T2. The circuitry also
provides a zero signal or open collector signal to the CMM system after a
trigger has occurred and any time the sensor 15 is in or out of range,
when the sensor is too close to the part 30 being measured.
Finally, with an AND gate (not shown) connected to the internal latch
signal and the arming signal complete fail-safe operability of the sensor
15 can be assured since the trigger event will not take place until after
(1) the internal latch has conditioned the circuit to take a measurement,
and (2) an arming signal from the CMM has been received.
A final embodiment of the invention would use neither the internal latching
signal nor the arming signal and could be used in a scanning mode for
detailed placement work of, for example, surface mounted microchips on a
substrate. For larger measurements of objects having a profile of greater
than 1/10th of an inch, the in-range signal 62 can be detected with the
range set above the bottom surface being detected and having the profiled
surfaces "in-range". For microchips and other measurements of less than
1/10th of an inch, the in-range signal can be established at the bottom
surface and the profile of the microchips be detected by the output of the
trigger signal 75. These and other varied uses of the invention will
become obvious to those skilled in the art upon a consideration of the
structure and operation of the invention.
In operation, the sensor 15 works as follows. The sensor 15 is mounted on
the arm of a Coordinate Measuring Machine as a replacement to the
contacting probe. The laser probe emits a laser beam which comes to focus
approximately 1" outside the sensor housing. This laser beam falls on the
surface of the part 30 and light is reflected back toward the receiver
optics 34. As the part surface is approached, the CMM first gets an
indication from the sensor 15 that it is within a short distance of the
trigger point, for example, approximately one tenth of an inch. This is
referred to as the "in-range" signal 62, meaning "you're close." This
signal is not available with a mechanical contacting probe. The operator
can then slow his rate of approach toward the surface, in the same manner
as computer-driven Coordinate Measuring Machines, which automatically
approach the part surface after they have been taught where the surface
should be.
The "in-range" condition is indicated by an electrical signal 62 and a
green light 44 indicating that the operator is close to the proper
position. An additional red light 42 will already be on which indicates
that the probe is armed and ready to take readings, but has not made
contact with the surface. On most Coordinate Measuring Machines, the
arming signal for the red light 42 is supplied via hardware and software
resident in the Coordinate Measuring Machine.
When the sensor 15 is in range and gets closer to the part surface, at some
precisely prescribed distance, which is approximately one inch from the
sensor disclosed, an electrical trigger will indicate that a precise range
has been crossed and detected. This signal is sent to the Coordinate
Measuring Machine which will then electrically lock the encoders on the
CMM arms and turn the red light 42 off on the sensor head 15 indicating
that a trigger has occurred. The signal, therefore, is the equivalent of
breaking or opening the contact in a contacting probe.
If one overshoots (that is moves too close toward the part), the trigger
signal will remain low, indicating the equivalent condition to the contact
being broken for a mechanical probe. When one goes sufficiently close to
the part, the in-range signal 62 will also go off. For the current sensor,
this occurs when the trigger point is overshot by approximately one-tenth
of an inch.
FIG. 5, is a diagram of the logic signals involved in the trigger operation
which are discussed above. Four distinct zones are utilized as shown in
FIG. 5a. Zone A is when the probe is too high or too far away from the
trigger point. Zone B is where the signal is falling on detector No. 1 and
the sensor is in range for a measurement, but still too high. The trigger
point is in the middle. Zone C is where light would be impinging primarily
on detector No. 2 and the probe is too close to the object. Zone D is when
the sensor is much too close.
The various logic levels for the various electronic signals and lights are
indicated in the logic diagram of FIG. 5b.
Having described a specific embodiment of our laser probe and a number of
modifications and variations in both structure and operation of the probe,
it will be apparent to those skilled in the art that many and various
changes and modifications can be made to the specific embodiment described
to achieve various of the objectives of the invention. All such
modifications and variations which fall within the scope of the appended
claims are within the intendment of the invention.
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