U.S. patent number 3,733,484 [Application Number 04/872,061] was granted by the patent office on 1973-05-15 for control for electron microprobe.
This patent grant is currently assigned to Walter C. McCrone Associates, Inc.. Invention is credited to Michael A. Bayard.
United States Patent |
3,733,484 |
Bayard |
May 15, 1973 |
CONTROL FOR ELECTRON MICROPROBE
Abstract
An electron microprobe has a holder for a sample to be analyzed.
The holder is movable along an X-axis and a Y-axis, each at right
angles to the electron beam directed at the sample. Drives, each
including an electric motor, are operatively connected to the
holder to adjust the position of the holder along the respective
axis. There are also electron beam position controls for
positioning the beam in a path oriented as desired with respect to
the X and Y axes. An electron beam current control enables the
quantity of electrons in the beam to be adjusted. A third drive
includes an electric motor, and is connected to a reflector to
position the reflector with respect to the X-rays emitted from the
sample, and with respect to X-ray detectors. The controls and
drives are connected to a control matrix through which signals are
supplied to the respective controls and drives, as dictated by a
computer. Each of the controls and drives produces an analogue
feedback signal indicative of the setting thereof. These feedback
signals are sent to a selector matrix from which a specific signal
is selected, as dictated by the computer, with the selected signal
being delivered to the computer through an analogue to digital
converter. The X-ray detectors also produce signals which are
ultimately delivered to the computer either directly or through the
analogue to digital converter. By appropriately programming the
computer, the apparatus automatically analyzes the sample for
constituents as desired.
Inventors: |
Bayard; Michael A. (Chicago,
IL) |
Assignee: |
Walter C. McCrone Associates,
Inc. (Chicago, IL)
|
Family
ID: |
25358755 |
Appl.
No.: |
04/872,061 |
Filed: |
October 29, 1969 |
Current U.S.
Class: |
250/310; 318/592;
318/601; 318/640; 318/663 |
Current CPC
Class: |
G05D
3/20 (20130101); G05B 19/39 (20130101); H01J
37/256 (20130101); G05B 2219/42251 (20130101) |
Current International
Class: |
H01J
37/252 (20060101); H01J 37/256 (20060101); G05B
19/19 (20060101); G05B 19/39 (20060101); G05D
3/20 (20060101); G01n 023/22 () |
Field of
Search: |
;250/51.5,49.5PE
;318/640,601,592,594,663 ;340/172.5 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3103584 |
September 1963 |
Shapiro et al. |
3189805 |
June 1965 |
Poepsel et al. |
|
Primary Examiner: Lindquist; William F.
Claims
I claim:
1. In an electron microprobe apparatus for use with a source of
power for analyzing a sample mounted on a movable holder by
producing a secondary emission of X-rays from the sample by
impacting an electron beam thereupon and for use with computer
means for producing actuating signals and control signals and
having an informational input, wherein the apparatus includes a
cathode device for producing said electron beam, an X-axis beam
control device between the cathode device and the sample for
controlling the orientation of the beam along an X-axis transverse
thereto, a Y-axis beam control device between the cathode device
and the sample for controlling the orientation of the beam along a
Y-axis transverse thereto, an X-axis holder control device for
controlling the position of the holder along a first axis
transverse to the beam, a Y-axis holder control device for
controlling the position of the holder along a second axis
transverse to the beam, an X-ray detector, and an adjustable device
for picking up the secondary emission and separating it into
components and directing selected components to said detector,
the combination therewith of:
a plurality of power driven means connected to said devices for
controlling the setting of the respective devices and for producing
electrical feedback signals in analogue form representative of the
control settings of the devices;
control matrix means connected between the computer means and the
power driven means for directing actuating signals from the
computer means to the respective power driven means as dictated by
control signals from the computer means;
an analogue to digital converter having an analogue input and a
digital output, said digital output being connected to said
informational input of the computer means;
selector matrix means connected to said power driven means for
receiving said feedback signals, to said computer means for
receiving control signals and to the input of said converter for
selecting a feedback signal as dictated by a control signal from
the computer means and delivering said selected signal to said
converter so that said selected signal is delivered to said
computer means by said converter in digital form, said matrix means
comprising a plurality of switches each having an operating
element, a plurality of electric circuits, each circuit connecting
a respective switch and a respective device to the source of power,
said switches being controllable between a first state at which
each switch energizes the respective device through the respective
electric circuit and a second state at which said respective device
is deenergized, and means operatively connecting said each of said
switches to said computer means to actuate the operating element of
a respective switch as dictated by a control signal from the
computer means;
a beam control device connected to said cathode device for
controlling the emission of electrons from the cathode device and
one of said power driven means connected to said emission control
device to control said emission; and
a sample current measuring device for producing an output signal
indicative of the current flow from the sample to ground, said
current measuring device being connected to said selector matrix
means;
said X-ray detector including a ratemeter and a scaler each
producing output signals, said ratemeter being connected to said
selector matrix means to deliver its output signal thereto, said
ratemeter being connected to said computer means to deliver its
output signal thereto;
each of the X-axis holder control device and the Y-axis holder
control device including a rotatable threaded shaft which rotates a
given number of turns to traverse the holder through its maximum
range of movement; and
each of the power driven means connected to said holder control
devices comprising:
an electric motor connected to the control matrix means to be
operated in accordance with signals from the control matrix,
means operatively connecting said motor to said shaft for driving
the shaft in accordance with the operation of the motor,
a first potentiometer having a contact movable within a given range
on the potentiometer, said contact being connected to said selector
matrix means,
means operatively connecting said movable contact and said shaft
for moving the movable contact within said range on said
potentiometer in response to said shaft rotating said given number
of turns,
second potentiometer means having contact means movable
repetitively through a given range on the second potentiometer,
said contact means being connected to said selector matrix
means,
gearing means operatively connecting said movable contact means of
the second potentiometer means and said shaft for moving said
contact means through a greater portion of its range in response to
a given rotation of the shaft when the portion of the range of the
first potentiometer traversed by the contact thereof in response to
the same given rotation of the shaft, and
means connecting said potentiometer and potentiometer means to said
source of power for imposing a voltage across the potentiometer and
the potentiometer means,
whereby voltages are obtained at said contact and said contact
means which together are indicative of the holder position along
the respective axis.
2. In the combination as set forth in claim 1, wherein at least one
of said control devices comprises a control potentiometer having a
movable slider; and
said power driven means connected to said one control device
comprises:
an electric motor connected to said control matrix to be operated
in accordance with signals from the control matrix,
means connecting said motor to said slider for positioning said
slider in response to operation of the motor;
a feedback potentiometer having a movable pickoff member, said
movable pickoff member being connected to said selector matrix,
means connecting said motor and said pickoff member for positioning
the pickoff member in response to the operation of the motor,
and
means connecting said feedback potentiometer to said source of
power for imposing a voltage across the feedback potentiometer,
whereby a voltage is obtained at said pickoff member indicative of
the position of said slider.
3. In a microprobe apparatus for analyzing a sample mounted on a
movable holder by producing a secondary emission from the sample by
impacting a primary beam thereupon and for use with computer means
for producing actuating signals and control signals and having an
informational input, wherein the apparatus includes an emitter
device for producing said primary beam, an X-axis beam control
device between the emitter device and the sample for controlling
the orientation of the beam along an X-axis transverse thereto, a
Y-axis beam control device between the emitter device and the
sample for controlling the orientation of the beam along a Y-axis
transverse thereto, an X-axis holder control device for controlling
the position of the holder along a first axis transverse to the
beam, a Y-axis holder control device for controlling the position
of the holder along a second axis transverse to the beam, a
secondary emission detector, and an adjustable device for picking
up the secondary emission and separating it into components and
directing selected components to said detector,
the combination therewith of:
a plurality of power driven means connected to said devices for
controlling the respective devices and for producing analogue
feedback signals representative of the control settings of the
devices;
control matrix means connected between the computer means and the
power driven means for directing actuating signals from the
computer means to the respective power driven means as dictated by
control signals from the computer means;
an analogue to digital converter having an analogue input and a
digital output, said digital output being connected to said
informational input of the computer means; and
selector matrix means connected to said power driven means for
receiving said feedback signals, to said computer means for
receiving control signals and to the input of said converter for
selecting a feedback signal as dictated by a control signal from
the computer means and delivering said selected signal to said
converter so that said selected signal is delivered to said
computer means by said converter in digital form;
each of the X-axis holder control device and the Y-axis holder
control device includes a rotatable threaded shaft which rotates a
given number of turns to traverse the holder through its maximum
range of movement; and
each of the power driven means connected to said holder control
devices comprises:
an electric motor connected to the control matrix means to be
operated in accordance with signals from the control matrix;
means operatively connecting said motor to said shaft for driving
the shaft in accordance with the operation of the motor;
a first potentiometer having a contact movable within a given range
on the potentiometer, said contact being connected to said selector
matrix means,
means operatively connecting said movable contact and said shaft
for moving the movable contact within said range on said
potentiometer in response to said shaft rotating said given number
of turns,
second potentiometer means having contact means movable
repetitively through a given range on the second potentiometer,
said contact means being connected to said selector matrix
means,
gearing means operatively connecting said movable contact means of
the second potentiometer means and said shaft for moving said
contact means through a greater portion of its range in response to
a given rotation of the shaft than the portion of the range of the
first potentiometer traversed by the contact thereof in response to
the same given rotation of the shaft, and
means connecting said potentiometer and potentiometer means to a
source of power for imposing a voltage across the potentiometer and
the potentiometer means,
whereby voltages are obtained at said contact and said contact
means which together are indicative of the holder position along
the respective axis.
4. In the combination as set forth in claim 3, wherein
at least one of said control devices comprises a control
potentiometer having a movable slider; and
said power driven means connected to said one control device
comprises:
an electric motor connected to said control matrix to be operated
in accordance with signals from the control matrix,
means connecting said motor to said slider for positioning said
slider in response to operation of the motor,
a feedback potentiometer having a movable pickoff member, said
movable pickoff member being connected to said selector matrix,
means connecting said motor and said pickoff member for positioning
the pickoff member in response to the operation of the motor,
and
means connecting said feedback potentiometer to said source of
power for imposing a voltage across the feedback potentiometer,
whereby a voltage is obtained at said pickoff member indicative of
the position of said slider.
5. In the combination as set forth in claim 4, wherein said matrix
means comprises a plurality of switches each having an operating
element, a plurality of electric circuits, each circuit connecting
a respective switch and a respective device to the source of power,
said switches being controllable between a first state at which
each switch energizes the respective device through the respective
electric circuit and a second state at which said respective device
is deenergized, and means operatively connecting said each of said
switches to said computer means to actuate the operating element of
a respective switch as dictated by a control signal from the
computer means.
6. In the combination as set forth in claim 4, wherein
said apparatus is an electron microprobe, with said primary beam
being an electron beam, said apparatus has a cathode for emitting
said beam, said apparatus includes a beam control device connected
to said cathode for controlling the emission of electrons from the
cathode and one of said power driven means connected to said
emission control device to control said emission, said apparatus
has a sample current measuring device for producing an output
signal indicative of the current flow from the sample to ground,
said current measuring device being connected to said selector
matrix means, said secondary emission is X-rays, said secondary
emission detector is an X-ray detector including a ratemeter and a
scaler each producing output signals, said ratemeter being
connected to said selector matrix means to deliver its output
signal thereto, said ratemeter being connected to said computer
means to deliver its output signal thereto.
7. In the combination as set forth in claim 3, wherein
said apparatus is an electron microprobe, with said primary beam
being an electron beam, said apparatus has a cathode for emitting
said beam, said apparatus includes a beam control device connected
to said cathode for controlling the emission of electrons from the
cathode and one of said power driven means connected to said
emission control device to control said emission, said apparatus
has a sample current measuring device for producing an output
signal indicative of the current flow from the sample to ground,
said current measuring device being connected to said selector
matrix means, said secondary emission is X-rays, said secondary
emission detector is an X-ray detector including a ratemeter and a
scaler each producing output signals, said ratemeter being
connected to said selector matrix means to deliver its output
signal thereto, said ratemeter being connected to said computer
means to deliver its output signal thereto.
Description
BACKGROUND OF THE INVENTION
Electron microprobes are employed to make composition analyses of
various samples, i.e. small areas of a metal surface, occlusions in
a polished section, small areas in thin sections, free particles
mounted on a substrate, etc. Under the prior art practices, these
analyses require constant attention by a qualified operator. Once
the raw data is obtained by the operator, the operator must spend a
substantial amount of time reducing the raw data to useful
analytical answers. Following these practices, a skilled operator
cannot be expected to make complete quantitative analysis of more
than about three to four samples per day. The electron microprobe
is an expensive piece of equipment and the cost of the time of a
skilled operator is not insubstantial. Thus, not only is the amount
of analytical output relatively small, but the cost per analysis is
relatively high.
Utilizing the present invention, the operator need only work about
an hour a day to do 10 to a hundred times as much work as he
previously accomplished in 8 hours. The operator can analyze 32
samples per 8 hour day using the spectrometers of the microprobe,
and can analyze as many as 400 a day using the nondispersive
detector. Furthermore, during an eight hour day during which
analyses are being made, perhaps seven-eighths of the operator's
time can be devoted to activities other than overseeing the
analyses that are taking place. It is possible for the apparatus to
be set up in the evening to carry out analyses overnight. In the
morning when the operator returns to work, the results of those
analyses (the raw data which has been reduced to useful analytical
answers) is waiting for him.
SUMMARY OF THE INVENTION
The present invention relates to a unique combination of a
microprobe and controls therefor which will automatically carry out
analyses of samples and produce refined analytical answers.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic representation of an electron microprobe
having the automatic control apparatus of the present
invention;
FIG. 2 is a schematic drawing of the control and feedback apparatus
for one of the sample holder drives; and
FIG. 3 is a partial schematic diagram illustrating the control and
feedback for one of the electron beam positioning controls.
DESCRIPTION OF SPECIFIC EMBODIMENT
Essentially, an electron microprobe comprises a cathode 15 from
which an electron beam 16 is emitted, with the electrons being
driven towards a sample 17. The impact of the electrons on the
sample produce a secondary emission of X-rays which are indicated
on FIG. 1 by the dot-dash line 18. The X-rays are picked up and
directed by a reflector 19 to X-ray detectors 20 incorporated in a
probe 21.
The position at which the beam 16 impacts on the sample 17 is
controlled in two ways. A relatively coarse control is obtained by
reason of the fact that the holder 24, on which the sample 17 is
mounted, is movable along an X-axis and along a Y-axis, both at
right angles to the line of beam 16. As an aid to illustration,
various axes are identified herein as X and Y axes, but these are
solely to facilitate understanding of the apparatus. Thus, as
indicated by arrow 25, holder 24 is movable along an X-axis. It is
also movable along a Y-axis as indicated by arrow 26. The fine
control of the position at which the beam strikes the sample is
obtained by using magnetic fields to align the position of the
electron beam 16. Thus, there are a pair of deflecting coils 27
positioned to produce a magnetic field, when energized, to align
the beam 16 along the X-axis. There are also similar coils 28 which
produce a magnetic field for aligning the beam along the
Y-axis.
The angle at which the X-ray beam 18 will be emitted from the
sample 17 depends upon the element upon which the beam impacts.
Thus, for one element being struck by the electrons, the X-rays
emitted therefrom will be in the form of an inverted cone having a
first specific central angle; while, for another element, the
central angle of the cone will be a second specific angle. The
illustrated line 18 from the sample 17 to the reflector 19 is but
one line extending from the apex of one of the cones and there are
a multitude of similar lines at other positions about that one
cone. The reflector 19 is movable with respect to these cones, as
indicated by the arrow 29, so that it will pick up and direct to
probe 21 X-rays of the cone having a particular central angle.
Thus, for a specific setting of the position of reflector 19, it
will be known that the probe is receiving X-rays produced by a
particular element at the point of impact on sample 17.
The quantity of that element present at the particular point will
be roughly indicated by the current flowing between the sample and
ground, and can be measured by a sample current detecting means 32.
In effect, this detector is nothing more than a refined current
measuring apparatus (e.g. meter). A more accurate measurement of
the quantity of the element present is obtained by measuring the
quantity of X-rays along path 18 by means of the X-ray detectors
20. These detectors take two forms, often simultaneously
incorporated in a single probe; namely, (1) a ratemeter and (2) a
scaler. The scaler produces a digital output signal in relation to
time. The ratemeter produces a voltage signal. Both, of course,
have values indicative of the quantity of X-rays received.
An actual spectrometer will have various other components which are
inconsequential so far as the explanation of the present invention
is concerned. For example, the electron beam 16 will be in an
enclosure in which there is a relatively low air pressure
maintained so as not to interfere with the movement of the electron
beam; various provisions will be made for accelerating the electron
beam; etc. Also, in the conventional spectrometer, there are three
probes 21. Each probe is associated with a respective reflector 19,
individually movable. These are each positioned at different radial
positions about the axis of beam 16 and thus pick up various
portions of the cones of emitted X-rays. The purpose of using
three, rather than only one illustrated, is merely to permit
simultaneous examination for different elements at the point of
impact of the electron beam. So far as the present invention is
concerned, a description of the apparatus in connection with one of
these probes and its associated reflector will fully explain the
manner in which the invention is employed in connection with each
of the two others.
The exact manner in which the holder 24 is movably mounted and
traversed is not consequential. The mechanism for doing this
includes a lead screw 33 (threaded rod) operatively connected to
holder 24 so that a rotation of the lead screw in one direction or
the other will move holder 24 in one direction or the other along
the X-axis. Similarly, a lead screw 34 is operatively connected so
that its rotation will move the holder along the Y-axis. A third
lead screw 35 is operatively connected to reflector 19 so that its
rotation will move the reflector to various positions transversely
to the various cones of the X-rays to obtain the desired
positioning of the reflector to reflect the X-rays in a particular
cone from a specific element.
In the present invention an X-axis drive apparatus, generally 36,
is connected to lead screw 33. A Y-axis drive apparatus, generally
37, is connected to lead screw 34, and a reflector drive apparatus,
generally 38, is connected to lead screw 35. These drives 36, 37,
38 position the respective lead screws in response to input signals
on lines 41, 42 and 43 respectively. Each drive apparatus includes
a mechanism (hereinafter described) for producing feedback signals
indicative of the then position of the drive. These feedback
signals appear on output lines 44-46 respectively.
A beam control apparatus, generally 49, is connected to deflecting
coils 28. A similar beam control apparatus, generally 50, is
connected to deflecting coils 27. Each of these controls adjusts
the current flow through the respective coils so as to obtain the
desired positioning of the electron beam 16. The control of this
current by controls 49 and 50 is dictated by a signal on input
lines 51 and 52 respectively. Each control apparatus also includes
means for providing a feedback signal indicative of the current
control setting. These feedback signals appear on output lines 53
and 54 respectively. Similarly, there is a beam current control
apparatus, generally 56, operatively connected to cathode 15 to
regulate the magnitude of electron beam 16. This magnitude is
dictated by an input signal on line 57. The setting of the control
is indicated by a feedback signal on output line 58.
Each of the input lines 41, 42, 43, 51, 52, 57 are connected to a
control matrix, generally 60, which produces signals to the
respective apparatus as dictated by a computer, generally 61.
Through line 59, computer 61 dictates what signal and to which
apparatus the control matrix 60 shall send a signal. The feedback
signals on lines 44, 45, 46, 53, 54, 58 are sent to a selector
matrix, generally 62. The selector matrix also receives a signal
from the sample current means 32 over output line 63. It receives a
voltage signal from the ratemeter in probe 21 over output line 64.
Under the dictation of computer 61, as indicated by line 65, the
selector matrix sends the feedback signal from one of these
components from which is receives feedback signals to an analogue
to digital converter (sometimes called a digital voltmeter)
generally 66, as indicated by connecting line 67. The converter
changes the analogue feedback signal it receives to a digital
signal and sends it to a computer on line 68. The digital signal
from the scaler in probe 21 is delivered directly to the computer
61 on line 69. The probe also includes a timer in conjunction with
the scaler and the time signal therefrom is sent to the computer 61
on line 70, whereby the computer has the time base against which
the scaler digital signal is read. Lines 68, 69 and 70 are
informational inputs for the computer.
As illustrative of each of the lead screw drive devices 36, 37, 38,
and the matrixes connected thereto, FIG. 2 illustrates the
structure and operation of the X-axis drive 36. This drive
apparatus is divided into two components one of which (36a) does
the driving, and the other of which (36b) provides the feedback
signal.
The drive component 36a comprises a reversible synchronous motor
73, the output shaft of which is connected to a brake 74. The motor
shaft is also connected to a clutch 75 and then through a two-speed
transmission 76 to lead screw 33. The brake is employed to obtain
the quick stopping of the output shaft when the motor 73 is
deenergized. The clutch 75 is employed to connect and disconnect
the motor from the transmission 76. The transmission 76 can be set
to obtain rapid rotation or a comparatively slow rotation of lead
screw 33 for either a quick traversal of holder 24 or a slow
traversal thereof.
The control matrix 60 comprises a plurality of substantially
identical control segments, only one of which 60a is illustrated in
FIG. 2. The number of such control segments that are employed will
depend upon the number of control functions to be achieved. In
actual embodiments of the present invention, nine such control
segments are used providing a total of 81 input signals to various
devices. Not all of these input signals are actually used,
however.
The control matrix segment 60a comprises four bistable flip-flops
80-83, which receive control signals from computer 61 through lines
59a - 59d respectively. There is also a clock 89 which receives a
signal from the computer over line 59e. Line 91 carries signals
from clock 89 to each of the flip-flops and to the computer. The
output signals from the flip-flops proceed to decoder 92 over lines
93-96. Decoder 92 has nine output lines 98-106, each leading to an
amplifier 107. The nine amplifiers have nine output lines 110-118
leading to the solenoids 120a - 128a of relays which also include
switches 120b - 128b actuated by the respective solenoid.
Preferably, these are reed relays, but various forms of relays or
solid state switches could be employed as an alternative. The input
line 41, illustrated in FIG. 1, is actually made up of a number of
separate lines (41a - 41f in FIG. 2) corresponding to the number of
functions or types of actuations incorporated in the drive
apparatus 36. Lines 41a and 41b are connected to switches 120b and
121b respectively to energize the motor for forward or reverse
operation respectively. The switches are also connected to one side
of the source of alternating current 130, the other side being
grounded. Line 41c is connected to switch 122b; line 41d is
connected to switch 123b; line 41e is connected to switch 124b; and
line 41f is connected to switch 125b. The other side of switches
122b - 125b are connected to a direct current source 131. Thus, for
example, when switch 122b is closed brake 74 will be energized from
source 131; etc.
A signal on line 59a to flip-flop 80 is indicative of the number 1;
a signal on line 59b to flip-flop 82 is indicative of the number 2;
a signal on line 59c to flip-flop 82 is indicative of the number 4;
and a signal on line 59d to flip-flop 83 is indicative of the
number 8. If two or more flip-flops simultaneously receive signals
from the computer, their respective numbers are added by decoder 92
to produce a signal on the line that equals the sum of the
flip-flop numbers. Thus, for example, if brake 74 were to be
energized, coincident signals would be received by flip-flops 80
and 81. At the appropriate time clock 89 would be actuated to tell
flip-flops 80 and 81 to release their respective signals to decoder
92 and to clear themselves. The simultaneous signals on lines 93
and 94 would cause decoder 92 to produce an output signal on line
100 indicative of the sum 3. Through the respective amplifier 107,
solenoid 122a would be energized to close switch 122b and energize
brake 74.
For purposes of simplification, all of these relays have been shown
as single acting relays. In some instances, however, they would be
of the latching type with two coils, one of which was energized to
latch the relay switch closed and the other of which was energized
to unlatch the relay and open the switch. The respective coils
would be connected to separate outputs on decoder 92 (through
respective amplifiers) to be actuated independently when signals
were received from the decoder.
Like the control matrix 60, the selector matrix 62 is divided into
a number of segments, one of which 62a is illustrated in FIG. 2.
Again, there are four outputs 65a - 65d to the four flip-flops
139-142 respectively. A clock 144 receives a signal from the
computer over line 65e and delivers a signal to the flip-flops and
to the computer over line 146. Each flip-flop has an output line to
decoder 147, the four lines being numbered 148-150 respectively.
There are nine output lines 155-163 from decoder 147 to nine
amplifiers 107. In turn the nine amplifiers 107 have nine output
lines 165-173 to the nine relay solenoids 175a - 183a. Each
solenoid is part of a relay including switches 175b - 183b
respectively. One side of each of the switches is connected to line
67. The other side of each of the switches is connected to receive
one of the analogue feedback signals from a control apparatus.
In FIG. 2, that portion of the drive apparatus 36 that provides the
feedback signal is identified as 36b. This includes a potentiometer
185 having a slider or pick-off 186. Slider 186 is coupled to lead
screw 33 so that the slider is moved along the potentiometer
resistance in accordance with the rotation of the lead screw.
Assume that the lead screw 33 is effective in moving holder 24
through the length D of the lead screw, then slider 186 is coupled
so that it moves the full length of the resistance of potentiometer
185 during the course of the effective operation of the lead screw
through the dimension D. Thus, for example, if it takes 270 turns
of the lead screw 33 to move the holder 24 through the dimension D,
then slider 186 moves one two-hundred-seventieth of the length of
the resistance of the potentiometer 185 during each rotation of the
lead screw. A wire 188 connects one side of the potentiometer
resistance to a direct current source of electric power (e.g. 131).
The other side of the potentiometer and the source of power are
grounded. Thus a voltage will be obtained at slider 186 (in
relation to ground), which is indicative of the position of the
slider on the potentiometer, and thus indicative of the position of
the holder 24 along the lead screw 33.
To obtain even more accurate resolution of the position of the
holder along the lead screw, a second potentiometer 190 has a
slider 191 which is coupled through the gearing arrangement 192 to
the lead screw 33. Potentiometer 190 is capable of continuous
rotation and the slider 191 thereof is geared to lead screw 33 in a
way such that it turns much more rapidly in relation to the
rotation of lead screw 33 than is the movement of slider 186 for a
given amount of rotation of the lead screw. For example, slider 191
could rotate through two revolutions of the potentiometer
resistance for each revolution of lead screw 33. A continuously
rotating potentiometer does have a gap in its path of movement, at
which gap the slider is not in contact with the potentiometer
resistance. To overcome this problem in actual practice, two such
potentiometers 190 are ganged together but aligned so that their
gaps are offset from each other. Thus it is possible to avoid a
loss of signal at the gap. For the purposes of simplifying the
illustration, only a single potentiometer 190 is shown in FIG.
2.
Again, wire 188 connects to one side of potentiometer resistance
190 so that a voltage is impressed across the potentiometer. Thus
the voltage appearing at line 44b (in relation to ground) is
indicative of the position of slider 191 on the resistance.
Utilizing this arrangement, an accuracy in one part in four
thousand as to the position of lead screw 43 is obtained without
having to use gearing connecting the potentiometers to the lead
screw which has an accuracy greater than one percent. In an actual
embodiment, the potentiometer 185 was a 10-turn potentiometer and
was gear connected to lead screw 33.
To "read" the position of lead screw 33, the computer first sends
out a signal to flip-flop 139. Upon the actuation of clock 144, a
signal indicating the number 1 is received by decoder 147. In turn,
the decoder sends out a signal on its number 1 line, i.e. line 155.
Solenoid 175 is thereby energized to close switch 175b. Over line
67 a voltage is obtained by the converter 66 indicative of the
position of the slider 186 on the "coarse" potentiometer 185.
Converter 66 changes this voltage signal into a digital signal and
sends it to computer 61 over line 68. Next the computer reads the
"fine" potentiometer 190. To do this, a signal is sent by the
computer over line 65b to flip-flop 140. In turn, the decoder 147
sends a signal over lines 156 and 166 to solenoid 176a. This
results in a closing of switch 176b. Now a voltage is obtained at
line 67 (switch 175b having opened in the meantime), indicative of
the position of the slider 191 on its resistance.
FIG. 3 illustrates the construction and operation of the X-axis
beam control apparatus 50 in relation to the computer. The same
arrangement would be used for the Y-axis beam control 49, and a
similar arrangement for the electron beam current control 56.
The beam control apparatus 50 includes a potentiometer 200 having a
slider 201 connected to motor 202 through a gear box 203. A
comparable (or actually the same one) gear box 204 connects a
slider 205 of potentiometer 206 to motor 202. The resistance of
both of the potentiometers are connected to a suitable source of
direct current, e.g. source 131, so that a voltage is impressed
thereacross. Thus the voltage at the sliders 201 and 205 will vary,
depending upon the position of the slider on the potentiometer
resistance.
Slider 201 is connected through an amplifier 210 to the deflection
coils 27. Amplifier 210 adjusts the magnitude of the current flow
through coils 27 in relation to the input potential of the
amplifier as picked off by slider 201. Slider 205 is connected by
line 54 to switch 183b of the selector matrix section 62a. Lines
52a and 52b connect the reversible motor 202 to switches 126b and
127b respectively of control matrix section 60a. Thus motor 202 is
energized from source 130 in a forward or reverse direction,
depending upon the closing of switches 126b and 127b. If, for
example, slider 201 is to be moved in one direction along its
resistance by closing switch 126b, a signal indicative of the
number 7 is sent from the computer to the flip-flops (i.e. signals
sent out by the computer on lines 59a, 59b and 59c). Upon the clock
89 directing that these signals be transmitted to the decoder 92,
signals are sent to the decoder on lines 93, 94 and 95. In response
thereto, the decoder 92 sends out a signal on line 104, through the
respective amplifier 107 and through line 116 to solenoid 126a.
This closes switch 126b to energize motor 202 for rotation in the
desired direction.
In response to the actuation of the motor moving slider 201, a
corresponding movement of slider 205 results. Thus, a potential
will be obtained on line 54 indicative of the position of sliders
205 and 201 on their respective resistances (and corresponding to
the current flow through coils 27). To read this signal the
computer sends out signals to the decoder 147 which sends a signal
on its number 9 line to energize relay solenoid 183a. The voltage
picked off by slider 205 then is transmitted through closed switch
183b to converter 66. Converter 66 sends a corresponding signal in
digital form to computer 61.
Assume that it was desired to see what transition elements were
located at a particular area on a sample and then to find all other
areas of the sample that were similar in composition over a 2
.times. 2 millimeter area with a resolution of ten micrometers. The
initial area of the sample would have been initially selected by an
individual looking at the sample through an optical microscope.
This microscope would have a holder identical to 24 so that the
coordinate would be the same for both holders when the sample was
transferred from one to the other. Assume that the initially
selected area had coordinates of X-1999 and Y-2999. The program (in
english) to the computer would be substantially as follows:
Store X position of 1999.
Read in the actual X position on X-axis drive 36b.
Match to the X position in store.
If different, move the drive (lead screw 33) fast if the difference
is greater than 10 units, or slow if the difference is less than 10
units, until a match is obtained with the X in the store.
Repeat the foregoing for Y position.
Analyze for the various transition elements by moving the reflector
19 to the required position for each and reading in and storing the
results from each position as detected by the probe 21.
As to each of these, recall the reflector position for the
particular element. Move the reflector to that position. Read the
feedback from the reflector drive apparatus. If match is obtained,
then read the ratemeter reading from probe 21 and store. Then move
the reflector 19 to the position for obtaining a background reading
on the same element. Check the feedback signal from the reflector
drive apparatus 38 to make sure that the reflector is in the
correct position. If match is obtained, read the ratemeter reading
from probe 21 and subtract from the ratemeter reading obtained for
the element as initially read. Store the difference. Repeat the
same procedure for each of the other elements.
Recall the readings for each of the intensities and if any are
above a predetermined minimum, store the element and quantity.
Step the holder in the X direction 10 micrometers. Read the
feedback to assure that the new position is correct.
Reposition reflector 19 for each of the elements, and take
ratemeter readings (as outlined above). Store the readings for only
those positions where the element readings match.
Repeat, in a plurality of steps, the X movements of 10 micrometers
and make the same readings at each stop until a total travel of 2
millimeters has been accomplished.
Step the Y-axis drive 37 ten micrometers. Read in the feedback
signal from drive 37 to make sure that the right position has been
obtained.
Now repeat the stepping of the X-axis movements and take readings
at each stop (as previously outlined) until a 2 millimeter travel
has been achieved.
Repeat the last two actions until the total coverage over the
entire 2 millimeter area (in 10 micrometer increments) has been
achieved.
Type out each position in readout 216 where a match was obtained
corresponding to the readings of the initially selected
position.
Various refinements, not illustrated, can be incorporated into the
apparatus. Such as, for example, limit switches on the movement of
the holder in relation to the lead screws so that exceeding the
desired limit of movement will shut down the apparatus. A further
refinement would be to read the actual electron beam current and if
it dropped below a given minimum, as for example due to filament
burnout, the apparatus would be shut down.
* * * * *