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Claims  |
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What is claimed is:
1. A method of automatically focusing an optical instrument onto a sample
surface, comprising the steps of:
(a) moving an objective of the optical instrument to a position between the
sample surface and a fringe window at a first speed by operating a
position sensor to sense when the objective is a predetermined safe
distance from the sample surface and turning off a motor moving the
objective in response to such sensing;
(b) moving the objective away from the sample surface at a second speed;
(c) illuminating the sample surface with broad bank light through an
interferometer in the optical instrument, splitting off a part of a
resulting interference beam, and sensing intensities of the split off part
of the interference beam by means of a first detector;
(d) stopping movement of the objective in response to sensing of a change
in a variable which is a function of sensed intensities, the change
indicating presence of fringes in the interference beam;
(e) moving the objective in the fringe window at a third speed, sensing a
change in the variable, and recording values of the variable being
produced in response to the first detector, and determining and storing a
maximum value of the variable;
(f) moving the objective in the fringe window at a fourth speed, sensing
intensities of signals being produced by the first detector, and comparing
corresponding values of the variable to the stored maximum value;
(g) stopping the objective in response to the comparing when a value of the
variable being produced by the first detector is equal to a preselected
percentage of the maximum value.
2. The method of claim 1 including operating a capacitive probe to sense
the position of the sample surface.
3. The method of claim 1 including operating an inductive probe to sense
the position of the objective relative to the sample surface.
4. The method of claim 1 including producing a light beam incident at a
selected angle to the sample surface, and detecting the presence of a
resulting beam reflected from the sample surface by means of a detector
when the objective is the selected distance from the sample surface, and
turning off the motor in response to the detecting of the reflected beam.
5. An apparatus for automatically focusing an optical instrument onto a
sample surface, comprising in combination:
(a) an objective of the optical instrument;
(b) means for moving the objective to a position between the sample surface
and a fringe window at a first speed, wherein the moving means includes
means for moving the objective of the optical instrument to the position
between the sample surface and the fringe window by operating a position
sensor to sense when the objective is a predetermined safe distance from
the sample surface and means for turning off a motor moving the objective
in response to such sensing when the objective is the predetermined safe
distance from the sample surface;
(c) means for moving the objective away from the sample surface at a second
speed;
(d) means for illuminating the sample surface with broad band light through
an interferometer in the optical instrument, splitting off a part of a
resulting interference beam;
(e) a first detector;
(f) means for sensing intensities of the split off part of the interference
beam by means of the first detector;
(g) means for stopping movement of the objective in response to sensing of
a change in a variable which is a function of sensed intensities, the
change indicating presence of fringes in the interference beam;
(h) means for moving the objective in a fringe window at a third speed and
sensing a resulting change in the variable;
(i) means for recording values of the variable being produced in response
to the first detector;
(j) means for determining and storing a maximum value of the variable;
(k) means for moving the objective in the fringe window at a fourth speed
and sensing intensities of signals being produced by the first detector;
(l) means for comparing corresponding values of the variable to the stored
maximum value; and
(m) means for stopping the objective in response to the comparing when a
value of the variable being produced by the first detector is equal to a
preselected percentage of the maximum value.
6. The apparatus of claim 5 including a capacitive probe positioned to
sense the position of the sample surface.
7. The apparatus of claim 5 including an inductive probe positioned to
sense the position of the objective relative to the sample surface.
8. The apparatus of claim 5 including means for producing a light beam
incident at a selected angle to the sample surface, and detecting the
presence of a resulting beam reflected from the sample surface by means of
a detector when the objective is the selected distance from the sample
surface, and means for turning off the motor in response to the detecting
of the reflected beam.
9. A method of automatically focusing an optical instrument onto a sample
surface, comprising the steps of:
(a) moving an objective of the optical instrument to a position between the
sample surface and a fringe window at a first speed;
(b) moving the objective away from the sample surface at a second speed;
(c) illuminating the sample surface with broad band light through an
interferometer in the optical instrument, splitting off a part of a
resulting interference beam, and sensing intensities of the split off part
of the interference beam by means of a first detector;
(d) stopping movement of the objective in response to sensing of a change
in a variable which is a function of sensed intensities, the change
indicating presence of fringes in the interference beam;
(e) moving the objective in the fringe window at a third speed, sensing a
change in the variable, and recording values of the variable being
produced in response to the first detector, and determining and storing a
maximum value of the variable;
(f) moving the objective in the fringe window at a fourth speed, sensing
intensities of signals being produced by the first detector, and comparing
corresponding values of the variable to the stored maximum value;
(g) stopping the objective in response to the comparing when a value of the
variable being produced by the first detector is equal to a preselected
percentage of the maximum value.
10. An apparatus for automatically focusing an optical instrument onto a
sample surface, comprising in combination:
(a) an objective of the optical instrument;
(b) means for moving the objective to a position between the sample surface
and a fringe window at a first speed;
(c) means for moving the objective away from the sample surface at a second
speed;
(d) means for illuminating the sample surface with broad band light through
an interferometer in the optical instrument, splitting off a part of a
resulting interference beam;
(e) a first detector;
(f) means for sensing intensities of the split off part of the interference
beam by means of the first detector;
(g) means for stopping movement of the objective in response to sensing of
a change in a variable which is a function of sensed intensities, the
change indicating presence of fringes in the interference beam;
(h) means for moving the objective in a fringe window at a third speed and
sensing a resulting change in the variable;
(i) means for recording values of the variable being produced in response
to the first detector;
(j) means for determining and storing a maximum value of the variable;
(k) means for moving the objective in the fringe window at a fourth speed
and sensing intensities of signals being produced by the first detector;
(l) means for comparing corresponding values of the variable to the stored
maximum value; and
(m) means for stopping the objective in response to the comparing when a
value of the variable being produced by the first detector is equal to a
preselected percentage of the maximum value. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The invention relates to automatic focusing systems for microscopes,
especially interference microscopes, and more particularly to automatic
focusing systems which directly detect the presence of interference
fringes to determine the degree of focus of the optical system.
In a typical interference microscope, light from a source, typically a
laser, is split by appropriate means to travel down a reference path and a
sample path. The reference path and sample path differ in that the
reference path is focused on a reflective reference mirror and the sample
path is focused on the sample to be measured. Light reflected from the
reference mirror and light reflected from the sample interfere to create
an interference pattern which is detected by a photodetector array. The
resulting signals are analyzed using various well known interferometric
techniques to determine the topography of the sample surface. Accurate
data can be obtained from an interference microscope only if the "focus
error" is minimal.
All presently known interference microscopes are manually "randomly"
focused by a human operator who must adjust the axial position of the
interference microscope objective until a sample surface visually appears
to be "in focus". Unfortunately, the interference pattern is only present
over a narrow axial range, typically a few microns, near the "ideal" focus
range of the microscope objective relative to the sample surface. This
makes it difficult to focus an interference microscope.
This narrow axial range where fringes are visible is very difficult for a
human operator to detect as he or she "adjusts" the microscope objective
through the focus range of the microscope, due to limitations in the
sampling rate of the average human eye. Furthermore, human reflex rate
limitations limit the ability to stop movement of the microscope objective
once visual observation of interference fringes has occurred. This is
especially true for low power microscope objectives. For low power
microscope objectives, the depth of focus is so large that images which
appear to the human eye to be well focused in fact may not be accurately
focused. That is, when considering an interference microscope for lower
magnifications, the test surface may appear to be in focus to the eye of
the observer over a fairly large range of movement of the microscope
objective, but the fringe pattern will be visible only within an axial
range of a few microns.
It sometimes requires a human operator a long time (e.g., many minutes) to
achieve accurate focusing of a typical interference microscope being
utilized to observe the surface of a typical sample. After the
interference microscope is finally focused, then disturbing it, for
example to insert or remove a filter, or to cause lateral movement of an
X-Y stage to observe an adjacent area of the sample surface, often results
in loss of focus. The operator therefore may need to refocus each observed
area of the sample. It is not an uncommon occurrence for a manual focusing
adjustment by a relatively inexperienced operator to result in "crashing"
the microscope objective into the sample surface, possibly destroying
both.
Many automatic focusing devices for various optical systems are known.
Most, such as the automatic focusing devices disclosed in U.S. Pat. Nos.
4,600,832, 4,333,007, 4,385,839, 3,798,449, and 4,207,461, require that
discernable features of the sample be present. The contrast of the images
of such features is utilized utilize auxiliary optical sources and
additional apertures to obtain focus information. One reference, U.S. Pat.
No. 4,620,089, uses an interference pattern that is present when the
object is in focus. This system requires differential sensing of the
interference pattern by means of two optical paths and two detectors.
Another prior art technique involves sensing focus error. Focus error
sensing techniques usually require using a laser as the light source.
There clearly is an unmet need for an automatic focusing apparatus for an
optical system, especially for an interference microscope, that avoids the
large amount of time often required for manual focusing of interference
microscopes. There also is an unmet need for an automatic focusing system,
especially for an interference microscope, which can accurately and
repeatedly focus the optical system or interference microscope to thereby
eliminate human focusing errors which are inherent in present manual
focusing techniques.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to reduce the time needed to
focus an optical system using manual focusing techniques.
It is another object of the invention to eliminate focusing errors inherent
in present manual techniques for focusing interference microscopes.
It is another object of the invention to avoid the need to manually refocus
an interference microscope between measurements of adjacent areas of the
same sample surface.
It is another object of the invention to provide an economical, simple
apparatus and technique for automation of surface topography measurements
of a large number of areas of a sample surface using an interference
microscope.
Briefly described, and in accordance with one embodiment thereof, the
invention provides a system and technique for automatically focusing an
optical system by rapidly moving an objective of the optical system to a
position between a sample surface and a fringe window at a first speed by
operating a position sensor to sense when the objective is a predetermined
distance from the sample surface and turning off a motor moving the
objective, illuminating the sample surface with a broad band beam from a
light source through an interferometer, splitting off a part of a
resulting interference beam, moving the objective away from the sample
surface at a second speed, sensing intensities of the split part of the
interference beam by means of a first detector, stopping movement of the
objective in response to sensing of a change in a variable which is a
function of sensed intensities from the first detector representing
presence of fringes in the interference beam, moving the objective in the
fringe window at a third speed, sensing intensities of the split part of
the interference beam by means of the first detector and recording values
of the variable being produced in response to the first detector,
determining and storing a maximum value of the variable, moving the
objective in the fringe window at a fourth speed, sensing intensities of
signals being produced by the first detector, comparing corresponding
values of the variable to the stored maximum value, and stopping the
objective in response to the comparing when a value of the variable being
produced by the first detector is equal to a preselected percentage of the
maximum value. In the described embodiment, the second speed causes the
objective to coast or overshoot beyond the fringe window. The third speed
is substantially less than the second speed, so overshooting of the
objective at the third speed is much less than at the second speed and a
substantially larger number of intensities produced by the first detector
are sampled. The fourth speed is substantially less than the third speed,
and overshooting of the objective for the corresponding transition is
minimal. In the described embodiment, the optical system includes an
interference microscope. A portion of the interference beam is directed to
a CCD camera which, in conjunction with suitable scanning and
analog-to-digital conversion circuitry, inputs a digital representation of
the fringe pattern corresponding to the sample surface into a computer
which performs an interferometric analysis thereon. In one embodiment a
retractable element is inserted in the interferometer to prevent light
from reaching the reference mirror of the interferometer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of an interference microscope including the automatic
focusing system of the present invention.
FIG. 1A is a diagram illustrating a preferred embodiment of the sample
proximity sensor included in FIG. 1.
FIG. 2 is a diagram showing output signals produced by the point detector
in FIG. 1 for different light sources.
FIG. 3 is a diagram of a modified Mirau interferometer including a
retractable element to prevent light from reaching the reference mirror to
allow the interference microscope of FIG. 1 to be used as an ordinary
microscope with automatic focusing capability.
FIG. 3A is a plan view diagram of the retractable light blocking element in
FIG. 3.
FIG. 4 is a diagram useful in describing a sequence of operations during
the coarse focus and fine focus procedures of the present invention.
FIG. 5 is an expanded waveform of the point detector output signal during
the procedure explained with reference to FIG. 4.
FIGS. 6 and 6A constitute a flowchart of the program executed by the
processor in FIG. 1 to carry out the coarse focusing and fine focusing
procedures of the invention.
FIG. 7 is a diagram of a point detector having a selectable effective
diameter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, automatic focusing interference microscope 1 includes
a Mirau interferometer 2 having a reference mirror 2A on a glass plate 2B,
and a beamsplitter 2C. A microscope objective 4 is supported above a test
surface 3 by a piezoelectric transducer (PZT) 5. PZT 5 is supported by a
frame 2D of Mirau interferometer 2. Mirau interferometer 2 is supported by
the frame of microscope section 1A. The vertical or Z position of
microscope section 1A is controlled by a motor 60, which is connected by a
mechanical link 60A to microscope section 1A. Test surface 3 is supported
on an X-Y stage 66. Movement of X-Y stage 66 in the X direction is
controlled by motor 61, which is connected by a mechanical link 61A to X-Y
stage 66. Movement of X-Y stage 66 in the Y direction is controlled by
motor 62, which is connected by a mechanical link 62A to X-Y stage 66. The
position of microscope objective 4 is precisely controlled by PZT 5 in
response to three PZT driver signals 59 produced by computer 30. Motors
60, 61, and 62 are controlled by signals 63, 64, and 65, respectively,
produced by motor controller circuitry 58.
A white light source, which can be a typical quartz halogen lamp, directs
white light through a typical commercially available illumination assembly
7, which directs the white light beam onto an ordinary beamsplitter 9.
Beamsplitter 9 reflects the white light beam 10 into the upper end of
microscope objective 4.
The beams reflected from the reference mirror 2A and the sample surface 3
then pass back up through microscope objective 4, upward through
beamsplitter 9, through a collimating lens 31, and through a
multilayer-coated beamsplitter 12 which deflects approximately 30 percent
of the interference beam 20 into eyepiece assembly 13. Most of the
interference beam 20 continues upward through imaging lens 11 to
beamsplitter 14. Part of the interference beam 20 is reflected by
beamsplitter 14 onto a fast (compared to the human eye or a CCD array)
point detector 15, which is connected to a preamplifier 19A. In the
prototype embodiment constructed, point detector 15 is a single
photodiode. The output of preamplifier 19A is connected by signal
conductor 16 to the microcontroller 28. Microcontroller 28 can be an Intel
8098. Block 68 includes an X-Y "joystick controller" and a Z joystick
controller by means of which an operator can manually control motors 60,
61, and 62 to control the lateral position of X-Y stage and the vertical
position of microscope section 1A relative to sample surface 3, and is
connected by suitable conductors 68A to inputs of microcontroller 28.
Microcontroller 28 communicates by bi-directional bus 28A with computer
30, which can be a Hewlett-Packard 330 desk-top computer in combination
with a commercially available WYKO PMI (Phase Measuring Interface).
Microcontroller 28 generates outputs 57 which control motor controller
circuitry 58. Microcontroller 28 and motor controller circuitry 58
actually are included in the above WYKO PMI unit.
Microcontroller 28 monitors position feedback information from motors 60,
61, and 62 and also monitors the intensity signals on conductor 16 to
control positioning of microscope section 1A by Z axis motor 60 and the
positioning of stage 66 by X and Y axis motors 61 and 62. During normal
operation (i.e., manual operation), microcontroller 28 continually checks
to determine if an autofocus command has been initiated by depressing an
"autofocus" button (not shown). If no such command has been received,
microcontroller 28 updates and reacts to the joystick and motor position
inputs every 600 microseconds by appropriately controlling Z axis motor
60. The Z axis motor 60 is disabled whenever an autofocus command is
received by microcontroller 28. It should be noted that the autofocus
command can be initiated either by pressing the autofocus button on the Z
joystick in block 68 or it can be initiated by the computer 30. Once a
focus command has been received, microcontroller 28 stops updating the
joystick status and begins updating the intensity measurements from
conductor 16 and the Z motor position feedback information. This operation
continues until the focusing of microscope 1 has been accomplished, as
subsequently explained.
In response to the autofocus command, microcontroller 28 first checks to
determine if interference microscope 1 is already "in focus". This is
accomplished by slowly moving microscope section 1A along Z axis 26 and
monitoring the sensed intensities for large variations that indicate the
presence of fringes. If the presence of fringes is detected,
microcontroller 28 continues movement of microscope section 1A through a
fringe window 55 (FIG. 4) and then stops. Once the microscope objective
has passed through fringe window 55, the microcontroller 28 increases its
update interrupt rate to once every 200 microseconds.
While updating the intensities received on conductor 16 every 200
microseconds, microcontroller 28 causes the microscope section 1A to pass
back through fringe window 55 very slowly and measures an "amplitude
profile" of the sample surface 3 through the entire width of fringe window
55, updating the maximum measured intensity as objective 4 moves through
the fringe window. Once the amplitude profile of the sample surface 3 and
the fringe window has been obtained, microcontroller 28 moves the
microscope objective 4 back through the fringe window again, continually
sampling the intensities on conductor 16 until the measured intensity
reaches approximately 99 percent of the maximum intensity previously
obtained. Microscope 1 then is focused, and microcontroller 28 returns to
the above described manual mode of operation until the autofocus button is
again depressed.
If the microscope objective 4 is not initially within fringe window 55,
microcontroller 28 instead executes a coarse focus algorithm. During the
coarse focus algorithm, microcontroller 28 updates the Z axis motor
position and the intensity information on conductor 16 every 600
microseconds, while microcontroller 28 rapidly moves the microscope
section 1A down to the memory lock location 34 of FIG. 4. Then
microcontroller 28 samples the intensity signal on conductor 16 once every
200 microseconds while it moves microscope section 1A up at the maximum
rate at which presence of fringes can be reliably detected, and continues
until large amplitude variations in the intensity signal on conductor 16
indicates presence of fringes. Then the fine focusing technique described
above is performed.
A narrow bandpass filter 17 is positioned between beamsplitter 14 and a
camera including a CCD (Charge Coupled Device) array 18 and camera
scanning electronics 19. A portion of the interference beam 20 passing
through beamsplitter 14 also passes through filter 17 before impinging on
CCD detector array 18. The signals produced by detector array 18 represent
the profile of sample surface 3, and are scanned by camera scanning
electronics in block 19, which produce amplified signals 27 that are
digitized and input to computer 30 for suitable processing in accordance
with the needs of the user.
The reference path and sample path in Mirau interferometer 2 are
essentially identical except that the reference path is focused on mirror
or reference surface 2 and the sample path is focused on the test surface
3. Numeral 26 designates the vertical axis of interference microscope 1.
The diameter of the point detector needs to be sufficiently small that its
output signal is heavily influenced by either the presence or absence of a
fringe of the interference pattern.
The waveforms shown in FIG. 2 represent waveforms produced on conductor 16
from the point detector 15 in response to different sources as the
microscope moves through its focus range. Waveform 21 shows the point
detector output for the laser source (with less than 1 nanometer spectral
bandwidth). This waveform has essentially no envelope. Waveform 22 shows
the output of point detector 15 as a function of microscope objective
position for a filtered white light source with 40 nanometers spectral
bandwidth. The peak points of waveform 22 represent the ideal focus
position of the microscope objective 4. Finally, waveform 23 shows the
output of point detector 15 as a function of microscope objective position
if light source 6 is a white light source. It is seen that this response
is very peaked, and identification of the ideal microscope objective focus
point is much more distinct than for waveforms 21 or 22. Since for a
narrow band source such as the laser the signal modulation envelope is
very broadband, and ideal focus is not easily achieved, and the described
embodiments of the invention operate best for broad band spectral sources
in which the peak signal is clearly indicative of best focus.
The image formed by a properly focused interference microscope typically
consists of a pattern of light and dark alternating interference fringes.
The number of fringes in the orientation of the fringes across the image
plane are dependent on the relative tilt between the sample surface and
the reference surface. Interference microscopes are assembled such that
the brightest fringe occurs at "best focus", i.e., within the depth of
focus of the microscope objective.
Referring to FIGS. 4 and 5, the first step in operating the automatic
focusing microscope 1 after test sample 3 has been positioned for a
measurement is to "manually" move (i.e., by means of the Z joystick in
block 68) the microscope objective 4 to a location that is known to be
closer to the surface of sample 3 than the ideal focus point. More
specifically, the operator sets and stores a position in the memory of
microcontroller 28 referred to as the "memory lock position" by manually
moving microscope objective 4 closer to sample 3 than the known focus
distance and activating a "set memory lock" button (not shown).
The next step is to press an "autofocus" button (not shown). Briefly, the
first step in the procedure is to determine if the microscope is already
at the best focus position. If it is not, a coarse focus procedure is
initiated to first move the microscope objective rapidly until the
presence of interference fringes is detected. Then a fine focus procedure
is initiated to slowly, precisely locate the microscope objective at the
best focus position.
If the microscope objective 4 is not within the "fringe window" 55 (FIG.
4), the microcontroller 28 drives the Z axis motor 60 at a "fast down"
speed (i.e., roughly 2500 microns per second) to move the microscope
section 1A to the memory lock position 34. Next, the coarse focus routine
executed by the microcontroller 28 generates signals that move the
microscope section 1A (which is quite massive and has a large momentum) at
a speed of approximately 13 percent of the "fast down" speed, as indicated
by arrow 36 in FIG. 4. This transition 36 continues until microcontroller
28 detects the presence of the interference pattern by recognizing that
the signal on conductor 16 is varying sinusoidally, as in region 23A of
FIG. 2 rather than monotonically as indicated by numeral 23B in waveform
23. (If the presence of interference fringes is not detected within a
preselected time period, the procedure is halted and the coarse focus
procedure is attempted again, and generates an error signal if fringes are
not detected.)
When a sinusoidal output is detected from point detector 15, the microscope
objective 4 then is known to be in the fringe window 55. Microcontroller
28 generates a command that turns off the control signal 63 to Z axis
motor 60 at "position 1", indicated by horizontal dotted line 42 in FIG.
4. The momentum of the microscope section 1A causes it to continue to
"coast" upward, as indicated by transition arrow 37 in FIG. 4. Microscope
section 1A "overshoots" fringe window 55, and comes to a stop at "position
2" designed by horizontal dotted line 44 in FIG. 4. If the momentum of
microscope section 1A does not carry it above fringe window 55,
microcontroller 28 takes over and causes transition 37 to continue to
ensure that microscope section 1A does not stop until the objective 4
passes beyond fringe window 55.
Next, after a suitable pause, microcontroller 28 generates new values of
control signal 63, reversing the direction of Z axis motor 60 and causing
it to lower microscope section 1A at a "slow" speed which is approximately
0.7 percent of the "fast down" speed, as indicated by transition arrow 38
in FIG. 4. The downward transition 38 of microscope section 1A continues
until fringes are detected (in the manner described above), which occurs
at "position 3", designated by horizontal dotted line 45, and immediately
turns off motor 60 at that point. The momentum of microscope section 1A
and motor 60 cause microscope section 1A to continue to coast downward
from dotted line 45, as indicated by transition arrow 39, to a resting
point beyond the lower edge of fringe window 55, referred to as "position
4" and designated by horizontal dotted line 46. If the momentum of
microscope section 1A does not carry microscope objective 4 beyond the
lower edge of fringe window 55, microcontroller 28 takes over and causes Z
axis motor 60 to carry it beyond fringe window 55. After a suitable pause
at position 46, microcontroller 28 generates new signals 63 to again turn
on motor 60 to raise microscope section 1A at the same speed as transition
38, causing microscope section 1A to undergo a very slow upward transition
40.
During transition 38 microcontroller 28 samples intensities of interference
beam 20 sensed by point detector 15 at a relatively large number of Z axis
locations (i.e., roughly every 0.1 microns), as indicated by points 70 in
section 38A of the waveform in FIG. 5. The waveform in FIG. 5 represents
the signal on conductor 16, and includes section 36,37 corresponding to
transitions 36 and 37, section 38A corresponding to transition 38, section
40A corresponding to transition 40 in FIG. 4.
During transitions 38 and 39, microcontroller 28 continually updates the
maximum intensity sensed on conductor 16 and stores it. After a suitable
pause at level 46, microcontroller 28 then reverses Z axis motor 60,
causing microscope section 1A to move upward, as indicated by transition
arrow 40. During transition 40, microcontroller 28 continually samples the
intensities of many points on section 40A of the waveform on conductor 16,
as indicated in FIG. 5. The sample points on section 40A are designated by
numeral 71 in FIG. 5. Microcontroller 28 compares each of these
intensities to the maximum intensity stored during transition 38,39, and
stops motor 60 when the intensity is equal to 99 percent of the maximum,
at position 6 indicated by horizontal dotted line 48.
When motor 60 is stopped, microscope section 1A is moving so slowly that
there is no appreciable overshoot. The 99 percent threshold level has been
selected to account for electrical noise that might be present on the
signals on conductor 16 and reduce the likelihood that the maximum
intensity stored during transition 38,39 contains a large enough noise
component that microcontroller 28 cannot find as large a peak in section
40A of the waveform of FIG. 5 as the noise-containing maximum intensity
value previously stored during transition 38A.
Then CCD detector array 18 and came | | |
