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Description  |
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FIELD OF THE INVENTION
The invention relates to a method of producing electron beam diffraction
patterns in which an object to be examined is irradiated by an electron
beam and the electrons of the electron beam, diffracted by the object, are
imaged by means of an integrating image device or an integrating image
material, particularly a photographic layer, the examined object being one
which is liable to be substantially altered by the electron beam, when
that beam is accelerated by a pre-determined voltage to the energy density
necessary to produce an image.
The invention also relates to an apparatus for carrying out the method
mentioned above, the apparatus comprising an electron beam device for
producing a focussed electron beam, an object slide for locating the
object to be examined in the path of the focussed electron beam in a plane
running transversely, particularly perpendicularly, to the electron beam,
and an integrating image device or an integrating image material,
particularly a photographic layer, for imaging the electrons of the
electron beam, diffracted by the object, the focussed electron beam and
the object being movable relative to each other and/or the apparatus
comprising an electrically controllable electron beam interruption or
swing-out device.
BACKGROUND OF THE INVENTION
It has been known for a long time to produce electron beam diffraction
patterns to determine the structure of crystals or polycrystalline
material. The interferences which occur when electron beams pass through
crystals or through polycrystalline materials or are reflected in
crystals, particularly Laue interferences in monocrystals and
Debye-Scherrer rings in polycrystalline materials, allow more exact
conclusions concerning the structural composition of the examined
materials.
Whereas the examination of the structure of inorganic, particularly mineral
materials, using electron beam diffraction patterns has produced genuine
results concerning the crystalline structure of these materials,
particular difficulties have arisen in this respect during the examination
by electron beam diffraction of the structure of organic and biological
substances or, generally speaking, during the examination of natural and
synthetic materials which have a periodic structure and which are
relatively sensitive and have a relatively poor heat-conductivity. This is
because considerable structural changes occur as a result of irradiating
these substances with electrons of the energy densities necessary to
produce an image. These structural changes are referred to below as beam
damage. Much has been written in technological literature about undesired
structural changes of this type, particularly by L. Reimer in the
periodical "Lab. Invest." 14, 1082 (1965), by K. Stenn and G. F. Bahr in
the periodical "J. Ultrastruct. Res." 31, 526 (1970), by R. M. Glaeser in
the book "Physical Aspects of Electron Microscopy and Microbeam Analysis",
page 205, published by Wiley and Sons New York 1975, and edited by B. M.
Siegel and D. R. Beaman, and by W. Baumeister, M. Hahn and U. P. Fringeli
in "Zeitschrift fur Naturforschung" 31c 746 (1976).
As has been experimentally established by G. Siegel in "Zeitschrift fur
Naturforschung" 27a, 325 (1972), the crystalline arrangement of a paraffin
crystal is considerably altered at a current density of the electron beam
of 8 electrons per square Angstrom at an accelerating voltage of the
electron beam of 100 kV. Similar experiments in adenosine crystals, which
R. M. Glaeser described in the periodical "J. Ultrastruct. Res." 36, 466
(1971), have shown that the crystalline arrangement of an adenosine
crystal is almost completely destroyed at a current density of the
electron beam of 6 electrons per square Angstrom at an accelerating
voltage of 80 kV.
The problem arising in respect of these results in the production of
electron beam diffraction patterns using photographic films becomes clear
when it is considered that a current density of one electron per
.mu.m.sup.2 is necessary to blacken a photographic film using electrons.
This means that with a resolution of 5 Angstrom, which corresponds to a
magnification .times.100,000, it is necessary to irradiate the object,
i.e., the organic or biological substance which is to be examined, with
100 electrons per square Angstrom when the primary electron beam has a
diameter in the order of 1 .mu.m in the object plane.
Because of this problem it has hitherto been impossible to obtain electron
beam diffraction patterns except for relatively insensitive substances,
without producing noticeable changes of the original structure of the
substances. Thus, as a result of the previously mentioned beam damage,
experiments involving examining sensitive substances by means of electron
beam diffraction patterns do not allow precise conclusions concerning the
original unchanged structure of those substances.
Since the undesired structural changes are attributable to the fact that
the energy which is supplied to the examined organic and biological
substances by the electron beam cannot be dissipated to the surroundings
fast enough by these substances, there is a possibility of obtaining
electron beam diffraction patterns of these sensitive substances by a
method involving cooling the substances to a very low temperature during
their examination, that is to a temperature of 4.degree. K. using liquid
helium. Such a method, using a 400 kV-electron microscope comprising a
superconductive objective lens, has developed by the laboratory of Siemens
AG, Munich and has been reported by J. Dietrich, F. Fox, E. Knapek, G.
Lefrane, R. Nachtrieb R. Weyl and H. Zerbst in the periodical
"Ultramicroscopy" 2, 241 (1977). However, for this purpose a high degree
of technical complexity is required.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and an apparatus
for producing electron beam diffraction patterns, wherein sensitive
natural and synthetic materials having a periodic structure, particularly
organic and biological substances, can be examined without substantial
beam damage occurring, and by means of which, therefore, the original
structure of the examined substances are substantially unchanged, so that
the electron beam diffraction patterns allow reliable conclusions
concerning the genuine structure of these substances. It is a further
object of the invention to provide a method and apparatus for examining
sensitive materials by electron beam diffraction, avoiding the use of very
low temperature cooling of the examined object.
According to the invention there is provided a method of producing electron
beam diffraction patterns, in which an object to be examined is irradiated
by means of an electron beam and the electrons of the electron beam,
diffracted by the object, are imaged using an integrating image apparatus
or an integrating image material, the object being examined being one
which is liable to be substantially altered by an electron beam when that
beam is accelerated to a pre-determined voltage to the energy density
which is necessary for the production of an image, wherein at least one
region of the object corresponding in size to the cross section of the
electron beam is irradiated intermittently using an electron beam
accelerated by the pre-determined voltage, which electron beam has an
electron density which is lower than that at which the object is
substantially altered, and wherein irradiation is carried out for such a
period of time that the electron density of the electron beam, integrated
during this time, is at least equal to the electron density necessary to
produce an image.
Expressed in simple terms, in the method of the invention the sensitive
object is not irradiated for a short period of time, as is the
conventional practice, but is irradiated intermittently over a period of a
few hours at one or more points by an electron beam of very low beam
current density. As a result of this, the electron energy required to
produce an image of the diffracted electron beam is acquired during the
course of time.
According to a preferred embodiment of the method of the invention, a
pre-determined continuous surface of the object is scanned once or a
plurality of times with the electron beam, the electron beam being allowed
to pass in a pre-determined pattern over the continuous surface of the
object. The intermittent irradiation is therefore achieved in this case by
virtue of the fact that the electron beam scans successive individual
points of the surface and optionally returns once or a plurality of times
to the individual points. By this scanning method, the object can be
scanned in a surface-covering manner by the electron beam.
Although a wide variety of scanning patterns can be used, for example a
spiral-form scanning, or a scanning consisting of concentric circles,
squares or rectangles, the method of the invention is preferably
implemented by allowing the electron beam to pass over the continuous
surface of the object in a meandering or continuous raster pattern. This
takes advantage of the fact that conventional object slides are
displaceable in two directions perpendicular to each other, and also
avoids any need to interrupt the electron beam during irradiation.
In principle, scanning the object using the electron beam can be effected
by allowing electron beam to travel and keeping the object stationary or
by keeping the electron beam stationary and allowing the object to move.
In available electron microscopes generally used to produce electron beam
diffraction patterns, considerable modification would be necessary to
allow the beam to travel, and it is preferred to implement the method of
the invention by moving the object through a stationary electron beam in
pre-determined preferably meandering pattern, transversely preferably
perpendicularly to the electron beam.
In all commercial electron microscopes there is the possibility of moving
the object by adjusting the microscope stage and this is effected using
two manual spindles in two directions perpendicular to each other. In a
meandering scanning, one spindle of the microscope stage adjustment is
passed from one adjusted stop to another adjusted stop and is halted
there. Then the other spindle performs a defined longitudinal step and
then the first spindle is rotated in the reverse rotational direction to
such an extent that the microscope state returns to the starting point.
This procedure is continued until the other spindle which performs the
small movement steps, has itself reached its pre-adjusted final position.
Here, the rotational direction of this other spindle is immediately
reversed and the total scanning procedure now returns, as just described,
but in the opposite direction. The privileged direction of scanning, i.e.
the direction in which the longer paths of the meandering pattern run, can
be rotated through 90.degree. C. if required, for which purpose the
spindle drives of the microscope stage adjustment are easily exchanged.
This is easily possible due to their similar construction.
However, the method of the invention can also be implemented by keeping the
object and the electron beam stationary relative to each other and
periodically interrupting the impingement of the electron beam on one and
the same point of the object. For this purpose, the electron beam is
preferably periodically swing out of the region of the object, which can
be easily effected in any commercially available electron microscope, as
electron microscopes generally have an electrically controllable swing-out
mechanism for the electron beam.
An apparatus for implementing the method involving relative movement
between the object to be examined and the focussed electron beam, has an
adjusting device which enables a pre-determined continuous surface of the
object to be scanned once or a plurality of times with the electron beam,
which device allows the electron beam to pass over the continuous surface
in a pre-determined pattern.
The adjusting device is preferably such as to allow the electron beam to
travel over the surface in the meandering pattern mentioned above.
If it is intended to move the object slide and keep the electron beam
stationary, the apparatus comprises an object slide adjusting device for
moving the object slide in a transverse, particularly perpendicular,
movement plane relative to the focussed electron beam. This object slide
adjusting device comprises, as indicated above a first shaft, as a result
of the rotation of which the object slide is adjustable in the movement
plane via a first driving connection in a first coordinate direction, for
example in the x-direction, and also a second shaft, as a result of the
rotation of which the object slide is adjustable in the movement plane via
a second driving connection in a second coordinate direction, for example
the y-direction. A first driving device is coupled to the first shaft to
drive the first shaft automatically and a second driving device is coupled
to the second shaft to drive the second shaft automatically. A first
sensor device is coupled to the first shaft to determine the respective
position of the object slide in the first coordinate direction and a
second sensor device is coupled to the second shaft to determine the
respective position of the object slide in the second coordinate
direction. A control device is connected to both the driving and sensor
devices to effect programmed control of both driving devices dependent on
signals from both sensor devices.
The driving devices can each comprise an electromotor which is coupled to
the respective shaft of the object slide adjusting device via a gear and a
slip friction clutch.
The electromotor can also be coupled to a tacho-generator and, together
with this generator, it can be connected to a motor control circuit
keeping the velocity or the electromotor constant and switching over the
direction of rotation of the electromotor.
To enable the object slide to be occasionally adjusted manually if
required, although however the adjustment caused thereby is perceived by
the sensor device, the driving devices can be decoupled from the
respective shaft independent of the sensor devices.
An apparatus to implement the method of the invention in which the electron
beam and the object to be examined are maintained stationary relative to
each other, comprises an electrically controllable electron beam
interruption or swing-out device, and further comprises an interval
tracing circuit, coupled to the electron beam interruption-or swing-out
device, for periodically interrupting or swinging the focussed electron
beam out of the region of the object. The interval tracing circuit
comprises a first control circuit for adjusting the length of time of the
interruption-or swing-out period during which the focussed electron beam
is respectively interrupted or swun out, and a second control circuit for
adjusting the length of time of the irradiation period, during which a
pre-determined point of the object is irradiated by the focussed electron
beam. In this manner, the swing-in time of the electron beam or the
irradiation time and the swing-out time of the electron beam (the dark or
recovery time) are adjustable in wide limits independent of each other and
their total determines the period duration of scanning a pre-determined
point of the object using the electron beam.
In the two basic embodiments of the apparatus of the invention described
above, i.e. the embodiment in which a relative movement between object and
electron beam takes place, and the embodiment in which object and electron
beam are maintained stationary to each other, it is possible to provide a
timewise controlled disconnection device, coupled to the electron beam
device and also to the control device or to the interval tracing circuit,
to interrupt the focussed electron beam or to swing the beam out of the
region of the object and optionally to disconnect the driving devices.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS
The advantages and features of the invention mentioned above and also other
advantages and features are explained in the following in more detail with
reference to the accompanying drawings, in which:
FIG. 1 is a schematic illustration of an electron microscope involving a
particularly preferred embodiment of the apparatus according to the
invention, whereby the method of the invention can be implemented to
produce electron beam diffraction patterns;
FIG. 2 is a lateral front view of a driving and sensor device coupled to a
shaft of an object adjusting device of the electron microscope;
FIG. 3 is a block circuit diagram of a particularly preferred embodiment of
an apparatus according to the invention;
FIG. 4 is a block circuit diagram of a motor regulation-and control circuit
for one of the two shafts of the object adjusting device for the object
slide of an electron microscope;
FIG. 5 is a circuit diagram showing more detail of the regulation of the
rotational velocity of the electromotor of the driving device according to
FIG. 2;
FIG. 6 shows a more detailed embodiment of a circuit for the control of
both driving devices for both shafts of an object slide adjusting device;
FIG. 7 is a circuit diagram of a timewise controlled disconnection device
and an interval tracing circuit for periodically swinging the focussed
electron beam out of the region of the object to be examined; and
FIG. 8 is a schematic illustration of a top plan view of the object and the
scanning path of a meandering scanning of the object using an electron
beam.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows a conventional electron microscope 1 which is generally used
with a corresponding beam path for diffraction, while the imaging lenses
are disconnected to produce electron beam diffraction patterns. The upper
part of the microscope comprises a casing 2 which can be evacuated. Within
the casing is an electron beam device 3 for producing a focussed electron
beam 4.
The electron beam device 3 substantially includes, in conventional manner,
an electron source 5, a pin diaphragm-form anode 6, a first condenser 7, a
second condenser 8, a diaphragm 9 provided in the region of the second
condenser 8, and a deflecting system 10.
It should be noted at this point that the electron beam system of an
electron microscope does not necessarily have to be used as the electron
beam device 3, but that in principle any other electron beam device is
suitable provided it allows the production of a focussed electron beam
having a low beam current density of less than a few electrons per square
Angstrom at accelerating voltages of 50 kV and more, preferably beam
current densities of less than one electron per square Augstrom. Beam
current densities of less than 0.1 electron per square Angstrom are
particularly preferred, for example a beam current density of
2.5.times.10.sup.-2 electrons per Angstrom square.
The apparatus shown in FIG. 1 also comprises an object slide 11 as is
usually used in electron microscopes. This object slide 11, on which the
object 12 to be examined (see also FIG. 8) is located over an opening 13
through which the electrons diffracted by the object 12 can pass,
comprises a microscope stage 14 which is movable in a plane running
transversely, preferably perpendicularly, to the electron beam 4. This
plane is the plane of the drawing in FIG. 8.
In order to move the object slide 11 or the microscope stage 14, an object
slide adjusting device 15 is provided. The device 15 comprises a first
shaft A, as a result of the rotation of which the microscope stage 14 is
adjustable in the movement plane in a first coordinate direction, namely
in the x-direction (see FIG. 8). The object slide adjusting device 15 also
comprises a second shaft B, as a result of the rotation of which the
microscope stage 14 is adjustable in the movement plane in a second
coordinate direction, which is preferably perpendicular to the first
coordinate direction, namely in the y-direction (see FIG. 8). The first
and second shafts A and B are referred to below as microscope stage
adjusting spindles or more shortly, as spindles.
The shafts A and B are each connected to the object slide 11 via a driving
connection formed in conventional manner, whereby each of these driving
connections substantially comprises a rack 16 and toothed wheels 17 by
which the rotational movement of both shafts A and B is transformed into a
translation movement of the microscope stage 14. The shafts A and B are
guided through the casing 2 by means of vacuum-tight glands 18 to the
outside and are there mounted in bearing brackets 19 and can be turned
manually by handles 20.
At a suitable distance beneath the microscope stage 14, there is an
integrating image device or an integrating image material 21, preferably
in the form of a photographic film or a photographic plate, upon which an
electron beam diffraction pattern is produced using the electrons of the
electron beam 4 diffracted by the object 12.
In order for the microscope stage 14 to be adjusted automatically so that a
continuous surface 22 (see FIG. 8) is scanned once or a plurality of times
by the electron beam 4 in a meandering pattern 23, a first driving device
24 is coupled to the first shaft A, which drives this shaft automatically.
Correspondingly, a second driving device 25 is coupled to the second shaft
B to drive this shaft automatically. There is also a first sensor device
26 coupled to the first shaft A to determine the respective position of
the microscope stage 14 in the x-direction, and a second sensor device 27
coupled to the second shaft B to determine the respective position of the
object slide in the y-direction. These sensor devices 26 and 27 can for
example be mechanical registers, their reading values respectively stating
the position of the microscope stage 14, based on an arbitrary origin, in
the x-direction and the y-direction.
There is also a control device 30 connected to both the driving and sensor
devices 24, 25 and 26, 27 via electrical circuits 28 and 29 (the first
circuit is partially obscured in FIG. 1 because it passes behind the
casing 2), to effect programmed control of the driving devices 24 and 25
dependent on signals from the sensor devices 26 and 27.
As can be seen from FIG. 1, but more clearly from FIG. 2, a respective
axially extending pinion 31 is attached in a rotation-fast manner the
shafts A and B to couple the driving devices 24, 25 and the sensor devices
26, 27. A driving toothed wheel 33 positioned in a rotation-fast manner on
the output shaft 32 of the driving device 24 or 25 cooperates with this
pinion. Correspondingly, a toothed wheel 36 positioned in a rotation-fast
manner on the input shaft 34 of the mechanical register 35 of the relevant
sensor device 26 or 27 respectively cooperates with the same pinion 31.
The pinion 31 is displaceable in the axial direction of shaft A or B, to
such an extent, as is indicated in FIG. 2 by dashed lines, that the
driving toothed wheel 33 can be brought into a non-meshing position with
the pinion 31, but the toothed wheel 36 still remains in a meshing
position with the pinion 31. The result of this is that the relation
between the rotating position of shafts A and B or between the position of
the microscope stage 14 and the revolution counters 35 of the sensor
devices 26 and 27 is always achieved if the shafts A and B are adjusted
manually using the handles 20 and the driving devices 24 and 25 are
unoperated.
The driving and sensor devices for both shafts A and B are constructed
identically so that it will suffice to explain the structure of the
driving and sensor device for shaft A using FIG. 2. The driving device 24
or 25 and the relevant sensor device 26 or 27 are located on a common
frame 37 which is attachable to the respective bearing bracket 19 of the
shaft A or B by means of screws, which are not shown, so that the
apparatus according to the invention can be installed by simple assembly
on to an available electron microscope without necessitating substantial
constructional modification to the electron microscope.
Each of two driving devices 24 and 25 comprises an electromotor 38 which is
connected to the output shaft 32 via a step-down gear 39 and a slip
friction clutch 40. A tacho-generator 41 is also fitted on to the
electromotor 38 to control the rotational speed.
In order to achieve a continuous movement, a direct current motor is
preferably used as the electromotor 38, which has a particular advantage
over a steping motor in that its drive requires slightly less expensive
electronic circuitry. In an exemplary case, a motor having a nominal
voltage of 4.5 V and a nominal speed of 6000 rotations/min was selected.
Since the speed variation required in this case was 7:30 which is
relatively high, a speed control of the electromotor 38 was provided via
the tacho-generator 41. The tachogenerator 41 had an output voltage of 1.4
V/1000 rotation/minute. For the gear, there was used one having a gear
reduction of 54.6:1. With the nominal speed of the electromotor 38 to 6000
rotations/min and the selected gear, the rotational speed of the shafts A
and B was 110 rotations/min. However, only rotational speed s of the
shafts A and B of 20/0.33=60 rotations per minute should be achieved.
Consequently, the electromotor 38 was only run at half the nominal speed,
so that sufficient acceleration and power reserves are available. The
acceleration constant of the electromotor 38 using the tacho-generator 41
was 20 ms and was therefore neglibile.
The slip friction clutch 40 between the electromotor 38 or the step-down
gear 39 on one side and the driving toothed wheel 32 is used to prevent
damage in case a mechanical stoppage occurs as a result of a malfunction.
It should also be mentioned that the pinion 31 on the shafts A and B must
be run out sufficiently long because the shafts move up and down when they
rotate.
The sensor devices 26 and 27 each comprise a reflection light barrier
arrangement 42 and a mechanical revolution counter 35. This reflection
light barrier arrangement 42 is actuated by the revolution counter 35 in
the manner described below following and transmits control signals to the
control device 30 as a result of which electro motor 38 is controlled as
is described in more detail below with reference to FIGS. 4 and 6.
In one embodiment, the revolution counter 35 was a three-figure register of
the type provided for example in tape recorders for counting the length of
the tape; a revolution counter of this type has a resolution of 1/10
revolution. Corresponding to the three-figure revolution counter 35, the
reflection light barrier apparatus 42 comprises three reflection light
barriers which are operated upon two pre-determined reading values of the
revolution counter 35 and respectively switch over the direction of
rotation of the electromotor 38 via the control device 30 so that the
electromotor 38 therefore always turns the coupled shaft A or B to and fro
between two rotational positions determined by the pre-determined reading
values of the revolution counter 35. For example, reflecting elements,
e.g. circular white discs of 4 mm in diameter can be positioned on the
counting discs 43 A, 44 A and 45 A of the revolution counter 35 for shaft
A (see FIG. 6) and also on the counting discs 43 B, 44 B and 45 B of the
revolution counter 35 for shaft B in the positions 00.0 and 20.0 on the
circumference of these counting discs, while the remaining part of the
circumference of these counting discs can be blackened. These reflecting
elements 46 which have been drawn in exaggerated form in FIG. 6 for
clarity of illustration, operate the relevant three reflection light
barriers of the respective reflection light barrier arrangement 42 in a
manner which is described in more detail below, and emit corresponding
electrical control signals. If the three reflection light barriers of the
sensor device 26 simultaneously register three reflecting elements 46 for
the shaft A, then their movement is stopped and the shaft B is initiated
to perform a timewise limited rotation. If then the three reflection light
barriers of the reflection light barrier arrangement 42 belonging to shaft
B simultaneously register three reflecting elements 46, then shaft B is
stopped and its direction of rotation is immediately reversed.
The block circuit diagram shown in FIG. 4 will now be discussed in detail.
FIG. 4 only shows the regulation and control circuit for shaft A, which
runs continuously from stop to stop, i.e. operates the longer path
distances 23a of the meandering movement (see FIG. 8). The regulation and
control circuit for shaft B which operates shorter distances 23b of the
meandering movement, is in principle, constructed identically.
A timing element 47, which is triggered by the revolution counter 35 of
shaft A gives the duration for the respective operation of shaft B. A
control system 48 for maintaining the rotational velocity of the motor 38
constant comprises a proportional-integral-controller 49 which is also
referred to below as a PI-controller, and whose integral part, which is
formed by a capacitor 50 (see FIG. 5) in a feedback circuit 51 of the
control amplifier 52, is short-circuited when resting (by the switch 53 in
FIG. 5), to prevent very small offset-voltages (i.e. input error voltages)
of the control amplifier 52 from starting the electromotor 38. The
controlled quantity substantially achieves the supply voltage-potential
with a large nominal-actual value deviation at the output of the
controller. To avoid this overloading the electromotor 38, a limiting
circuit 54 for the motor operating voltage is provided in the feedback of
the control amplifier 52. A power amplifier 55 is connected before the
electromotor 38. This power amplifier 55, as shown in FIG. 5, consists of
an integrated operational amplifier 56 followed by transistor power stage
57.
Since the voltage emitted by the tachogenerator 41 pulsates corresponding
to the internal number of poles and the speed of the tachogenerator, the
voltage has to be smoothed by means of a low-pass filter 58 which
preferably has a time constant of 10 ms, before the control difference of
the nominal-value is formed in the control amplifier 52. The nominal value
for rotational velocity is adjusted via a potentiometer 59. The
potentiometer 59 is supplied with a voltage whose polarity is determined
by a rotational direction logic 60. In FIG. 6, the rotational direction
logic for shaft A is marked 60A and the rotational direction logic for
shaft B is marked 60B. Attention is directed at this point to the fact
that if A or B is respectively added to a reference numeral this signifies
that this is a component or a unit which is provided to operate shaft A or
B, although this addition, as can be seen from the preceding description,
is not provided in the cases in which a distinction is not necessarily
required. The disconnection for the standstill is effected using a chopper
transistor 61 having a very low residual voltage.
FIG. 4 shows that between the output of the revolution counter 35 and the
input of the timing element 47 on the one hand and the PI-controller 49
and disconnection device 62 for the standstill of the electromotor 38 on
the other hand, there is provided an interlocking logic 63 which is also
connected to a time disconnecting device 64. A preferred embodiment of the
device 64 is shown in detail in the left part of FIG. 7. The nominal
value-input for the speed of the electromotor 38 is effected at 65, via a
pole reversal circuit 66 (also see FIG. 5). A line passes at 67 to the
regulation and control circuit for shaft B.
The regulation and control circuit for shaft A, shown in FIG. 4, functions
so that when the revolution counter 35 has reached one of the two
pre-determined reading values, a signal from the allocated reflection
light barrier arrangement 42 is transmitted to the interlocking logic 63
which thereupon stops the electromotor 38 via the disconnection device 62
and the PI-controller 49. This signal passes simultaneously to the
rotational direction logic 60 so that it reverses the direction of
rotation of the electromotor 38 via the pole reversal circuit 66. This
signal also passes to the timing element 47 whence it is transmitted to
the regulation and control circuit for shaft B via the line 67, where it
causes a pre-determined rotational step of shaft B, which corresponds to a
short path course 23b (see FIG. 8) of the meandering path 23. When this
step has been performed, the motor 38 can effect another new longer path
course 23a via shaft A.
Attention is directed here to the fact that FIG. 4 represents a basic block
circuit diagram, whereby FIGS. 5 and 6 illustrate more detailed circuitry
including means for the production of breaking signals (not shown in FIG.
4), from which certain deviations are produced.
The more detailed embodiment of a control circuit and its function will now
be explained with reference to FIG. 5 and FIG. 6.
As has already been described above, the reflection light barriers
distinguish reflecting elements 46, for example white circles, on a dark
base. For this purpose, each of the reflection light barriers comprises a
light emitting diode 68 which throws infrared light on the circumference
of the black counting disc 43A, 43B or 44A, 44B or 45A, 45B which is
allocated thereto. A respective phototransistor 69 is positioned suitably
for each light-emitting diode 68, each phototransistor 69 being connected
to a second transistor 70 to form a current-amplifying Darlington Pair.
The phototransistor does not receive a signal when the black circumference
surface of the counting disc absorbs the infrared light, and it does
receive an infrared light signal when this is reflected by a reflecting
element 46. For this purpose, each light-emitting diode 68 and its
corresponding phototransistor 69 are positioned at an angle to one
another, at the point of intersection of which the circumferential surface
of the relevant counting disc is located. Achieving a good discrimination
depends on the blank circumferential surface of the counting discs
effectively absorbing the infrared light, for which purpose the surface
can be coated, for example with photoresist, and on the reflection
elements 46 reflecting effectively.
A switching transistor 71 and a Schmitt-trigger 72 form an exact logic 0 to
logic 1 transistion, from the current changes caused by the respective
phototransistor 69. Three of these light barriers each belong to a
revolution counter whose logic signals are connected to a NAND-gate 73A or
73B. If three reflecting elements 46 appear simultaneously this
corresponds, as already explained above, to one of the two pre-determined
reading values of the revolution counter 35 for the shaft A or shaft B.
Consequently, a voltage jump from logic 1 (approximately 5 V), to logic 0
(approximately 0 V) occurs at the output of the NAND-gate 73A or 73B when
the relevant revolution counter 39 has reached one of the two
pre-determined reading values. This output 74A or 74B is simultaneously
the input of the corresponding rotational direction logic 60A or 60B. As a
result of the above mentioned voltage jump, the flip-flop 75A or 75B
present at the input of the rotational direction logic is switched over,
whereby a signal is emitted via the outputs 76A, 77A or 76B, 77B of the
rotational direction logic 60A or 60B, which signal causes a change in the
direction of rotation of the relevant electromotor 38 for the shaft A or
shaft B.
Simultaneously, when the above mentioned voltage jump occurs at the output
74A, the timing element 47 also connected to this output is activated,
which determines the respective operating time of shaft B, so that the
electromotor 38 performs a defined rotation of, for example 27.degree.,
for shaft B. This corresponds to a short distance 23b (see FIG. 8) of the
meandering path 23. Meanwhile, at output 78A at which a start or stop
signal for the electromotor appears which drives shaft A, a signal
stopping the electromotor is present. The signal which causes the
electromotor to operate the electromotor driving shaft B appears at output
78B which is the start and stop output for this electromotor.
There is also a second timing element 80 connected behind the first timing
element 47 which emits a delayed braking signal at the output 79A and 79B.
A braking signal for the electromotor 38 is emitted at the output 79A at a
given time, which electromotor drives shaft A, while at output 79B at a
given time, a braking signal for the electromotor 38 appears which motor
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