Note: Descriptions are shown in the official language in which they were submitted.
1 1 RCA 75,934
CONSTANT POTENTIAL HIGH VOLTAGE GENERATOR
This invention relates to an improved high voltage
generator such as may be used for an x-ray tube. More
particularly, the invention relates to a circuit for
stabilizing the high voltage for reducing fluctuations in
x~ray energy production.
In order to perform computed tomography (CT)
scanning, an x-ray tube, which includes an anode and a
cathode, is rotated about and directs x-radiation through a
patient to an array of x-ray intensity detectors. The
signals produced by the detectors are translated to binary
form and the latter are processed to provide the data
employed to produce a density mapping of a cross-sectional
area of the patient. To generate the x-rays, electrons from
the cathode are accelerated to the anode by the force
exerted on them by an electric field between these two
electrodes. ThiS field is proportional to the voltage
between those electrodes. Fluctuations in the voltage
result in fluctuations of the electric field and therefore
fluctuations in the speed with which the electrons strike
the anode. Electrons striking the anode transfer their
kinetic energy to x-radiation energy, and to heat energy
which is dissipated by the anode. Variations in electron
kinetic energy result in variations of x-ray energy emitted
by the x-rày tube and this, of course, is highly undesirable
because it results in an inaccurate mapping of the patient
cross section.
A prior art constant potential generation
apparatus for x-ray apparatus is disclosed in the United
States Patent No. 3,325,645 entitled "X~~ay Tube System with
Voltage and Current Control Means." It includes an
alternating high voltage source whose output is coarsely
adjusted then rectified, and then smoothed or filtered with
a capacitor. The filtered voltage is transmitted to the
x-ray tube cathode and operates as the accelerating potential
for electrons in the x-ray tube. In that apparatus, fine
adjustment in the accelerating ~otential is achieved by a
feedback control circuit which modifies the unfiltered high
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1 2 ~CA 75,934
tension voltage in response to the filtered voltage
appearing at the x-ray tube. Any tendency of the filtered
voltage to increase results in a change in the impedance of
the feedback control circuit in a sense which tends to
decrease the unfiltered voltage and vice versa.
Ilhile achieving substantial commercial success,
especially in the application for which it was designed,
namely x-ray diffraction, the prior art apparatus disclosed
in U.S. Patent 3,325,645 has some disadvantages when apPlied
to CT. It employs a filtering capacitor to smooth a
pulsating signal. Such a capacitor charges to many thousands
of volts and can retain a substantial amount of stored
energy. In CT applications this stored energy can present a
safety hazard to both service personnel and to the x-ray
tube. Another disadvant~ge is the limited range of feedback
control. If the unfiltered voltage from a transformer
secondary in the prior art circuit is outside of a certain
range, the feedback control circuit no longer stabilizes the
accelerating potential. For stabilization beyond this
range, a primary transformer control circuit is used to
return to the proper range the output from the secondary
transformer. This adds complexity to the stabiliza~ion
circuit.
In a system embodying the invention which is
suitable, for example, for~producing the high voltage needed
for an x-ray tube, means are provided for producing first
and second varying d.c. voltages, the first such voltage
having an average potential which is relatively positive
with respect to a point of reference potential and the
second haviny an average potential which is relatively
negative with respect to said point of reference potential.
The difference between these voltages is sensed to produce
~5 a control voltage proportional to the difference. This
control voltage is compared with a reference voltage level
indicative of a desired constant difference in potential
between the first and second vaxying d~c. voltages~ Means
are provided, responsive to the error signal, for changing
the values of the first and second voltages in a sense to
1 3 RCA 75,934
reduce the error signal to zero and thereby to maintain the
difference between the first and second voltages at a
constant value.
In the drawing:
FIGURE l is a schematic showing of a CT scanning
unit;
FIGURE 2 is a rear elevational view of a CT
machine wlth its housing removed;
FIGURE 3 is a sectional view of the CT scanner of
FIGURE 3 along the line 3-3 of FIGURE 2;
FIGURE 4 is a diagram mostly in block form of an
improved x-ray potential generator embodying the invention;
FIGURE 5 is a more detailed sehematic diagram of
FIGURE 4.
FIGURE 1 shows a eomputed tomography system 10
used for examining the internal strueture of a patient.
The unit eomprises a scanning unit 12, couch 16, signal
processor 20 and imager 22. The seanning unit 12 is mounted
to the floor and remains stationary relative to the patient.
The seanning unit includes a housing 13 which covers x-ray
apparatus including an x~ray tube which rotates for CT
scanninq. The housing 13 is formed with an aperture 14.
The couch 16 is movably mounted and is operative to position
the patient ~ithin the aperture 14 for x-ray scanning. The
- signal processor 20 and the imager 22 are electrically
connected to the scanning unit. The scanning unit obtains
x-ray intensity da~a and sends this intensity data to the
signal processor. The x-ray intensity data is then
proeessed by the signal processor to obtain information
concerning the relative densities of a patient eross-section
of interest. This density data is transferred to the imager
where the doctor can view the relative density information
on a viewing screen.
Referring to FIGURES 2 and 3, an x-ray tube
support and manipulating assembly comprising a stationary
detector scanning unit is shown generally at 50. The
assembly 50 includes a housing and frame structure 51. A
pair of spindle bearings 52 are carried bv the housing and
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1 4 RCA 75,934
frame structure 51, (see FIGURE 2). A tubular spindle 54 is
journalèd in the bearings 52. The spindle 54 delineates a
patient receiving opening 55 (corresponding to 14 of FIG. 1).
An x-ray tube assembly 58 (see FIGURE 3) is fixed
to the tubular spindle for orbital rotation about an axis 56
of the spindle 54 and the opening 55. The x-ray tube
assembly includes an x^-ray tube indicated by the dotted line
60, a collimator shown diagrammatically at 61, and other
known and conventional components of an x-ray tube assembly
of the type used in CT studies.
The tube support and manipulating assembly 50
shown in FIGURES 2 and 3 is of a maehine of the stationary
detector type. For clarity of illustration, and because the
detector array is now known in the art, the annular deteetor
array which is around the orbital path of the x-ray tube
assembly 58 is not shown except in a fragmentary schematic
way at 62 in FIGU~E 3.
In use, the x-ray tube is orbited about the axis
56 over a range of orbital motion over a path of suffieient
length to accelerate the tube to its full speed for a 360
scan, and to decelerate through an additional orbital path
which is long enough to permit the tube to be smoothly
brought to a stop. The orbital motion is first in one
direction and then the other. Expressed another way, the
tube may be moved in a clockwise direction to perform one
study and then counterclockwise during a following scan to
perform the next study.
A drive for this orbital motion is shown
schematically and it includes an annular motor 64 which is
conneeted to the spindle 54. The drive shown is for
schematic illustration only. Any of the known and
com~ercially accepted drive systems ean be employed.
3~ Four flexible eonduits or eable 68 are eonnected
to the x-ray tube assembly 58. These cables include
conductors for supplying electron accelerating potential for
the x-ray tube, for collimator and filter adjustment, and
sueh other power requirements the tube assembly may have.
The cables 68 extend from the x-ray tube through the opening
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1 5 RCA 75,934
65 where they are adjacent the spindle 54 and into a cable
delivery opening 69 (see FIGU~E 3).
In a CAT system embodying the invention, the
accelerating potential is supplied by a voltage generator
embodying a stabili~ation circuit. The stabilization
circuit shown in block form in FIG. 4 and in schematic form
in FIG. 5 maintains the potential at a highly constant level
thereby insuring the x~radiation is of a constant mean
energy level.
~ eferring now to FIGURE 4, a high voltage
generator for an x-ray tube 112 (corresponding to tube 60 of
FIG. 3) is shown schematically at 110. The x-ray tube 112
has an anode 114 and cathode 116 electrically connected to
the generator 110. An input 120 transmits a potential to
the anode 114, and a second input 122 to the cathode 116.
The potential difference between these two inputs causes
electrons emitted by the cathode to accelerate toward the
anode. I~hen the accelerated electrons strike the anode
their kinetic energy of motion is transformed into heat and
x-radiation energy 118.
To provide a voltage differential to the anode and
cathode, the generator 110 includes a source of alternating
voltage which in the embodiment shown comprises two high
voltage transformers secondary circuits 130, 132. One
transformer secondary 130 produces a signal which is
rectified and sent to the x-ray tube anode, the second
transformer secondary 132 produces a second signal which is
also rectified and sent to the x-ray tube cathode. Although
the outputs from these secondaries are rectified, they are
not filtered and therefore comprise pulsating d.c. signals~
In the preferred embodiment the output from a first
secondary 130 is rectified to provide a voltage above ground
potential and the other output is rectified to produce an
output below ground potential. Although the secondary
transformers are driven by the same primary, the existence
of other components within the circuitry produces a phase
shift between the two pulsating signals and therefore the
; 40 signals are not symmetric above and below ground. Due to
1 6 ~CA 75,93
this factor, the voltage waveform representing the
anode-to-cathode potential is an irregularly shaped,
pulsating waveform. It is the function of the remainder of
the circuitry within the generator 110 to smooth and
stabilize this waveform so that while the voltage at the
anode and that at the cathode may each vary relative to
ground, their potential difference remains constant so that
the electrons accelerated in the x-ray -tube have a stable
average kinetic energy and therefore the x-radiation
produced has a stabilized mean value.
To achieve this stabilized energy, the generator
110 includes a reference voltage generator 134, an error
voltage genera-tor 136, and a feedback circuit responsive to
the output 13B from the error generator~ The reference
generator when it is on, produces a constant reference
voltage level at 135 which is proportional to a desired high
voltage potential difference between cathode and anode of
the x-ray tube. This reference signal is sent to an error
generator 136 which compares the reference signal 135 with
a control signal input 137 produced by differential circuit
221. The control signal 137 is proportional to the
potential difference appearing across the anode and cathode
~5 of the x-ray tube. But for the advantageous operation of
the feedback loop system of the generator 110 this control
signal 137 would comprise a pulsating potential proportional
to the voltage difference supplied by the transformer
secondaries. Due to the advantageous operation, however,
of the feedback technique embodied by the invention, the
pulsating output from these secondaries is modified and the
accelerating potential across the tube anode and cathode is
stabilized at the reference value.
In an embodiment where two transformers'
secondaries provide the accelerating potential difference to
the x-ray tube, two feedback portions 140, 142 are required
in the generator 110. Each portion includes a control tube
(triodesl44 and 146, respectively) with a control grid 148,
150. The error signal 138 is sent through each of the
. 40 feedback portions and modifies the control tube grid voltage
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1 7 RCA 75,934
in such a way that the signal sent to the x-ray tube
electrodes 114, 116 from the transformer secondaries is
modified and the potential difference appearing across the
tube is stabilized at the constant reference value.
As seen in FIGURE 4, each control tube 144, 146
comprises a portion of an electrical path between the
secondary transformers 130, 132 and electrical ground 152.
By modifying the voltage on the grids 148, 150 of the
control tubes the impedance these control tubes presents
is altered in such a way that the error signal 138 will
be minimized. The change in control tube impedance is
transmitted as a signal having two components to the x-ray
tube electrodes. Two shunt paths 154, 156 (shown in FIG. 5
as ~C networks) bypass the transformer secondaries and
transmit the a.c. component resulting from the change in
control tube i~pedance to the x-ray tube cathode/anode 114,
116. These shunt paths represent a low impedance path for
alternating current signals generated through control of the
control grids 148, 150 and allow these a.c. signals to
moderate fluctuations in accelerating potential. There is
- also a d.c. component corresponding to the voltage drop
across the triodes 144 and 146 which affects the voltages
applied to the anode 114 and cathode 116. Thus, the voltage
voltages at the anode and cathode are a function both of the
output of the transformer secondaries and of the a.c. and
d.c. components of the voltage developed in the feedback
paths.
As an illustration, assume that the voltage
difference between the cathode and anode of the x-ray tube
is smaller than an optimum value. That is, the desired
accelerating potential is greater than the instantaneous
actual accelerating potential appearing across that tube.
3~ i~hen this condition exists the x-ray beams emitted by the
tube have an average energy less than an optimum desired
value. Under these circumstances control signal 137 will
tend to decrease in value to level lower than that of
reference signal 134 and the error generator 136 will cause
an error output signal 138 to be sent to the grids 148, 150.
1 8 RC~ 75,~34
In actual operation, it is not the error signal 138 but an
amplified signal which is used to control the voltage on the
two grids. This amplification is achieved by sending the
signal through two compensating amplifiers 158, 160 and then
through two grid drivers 162, 164. (The signal sent to grid
driver 162 passes through an isolation circuit which
includes means for translating the output of 158 to light,
and means for translating the light signal back to a
voltage, as will be discussed later.) The voltage output on
the grid drivers will modify the voltage on the grids 148,
150 to increase the x-ray tube anode/cathode voltage
differential to its desired constant value to thereby
increase the control signal 137 so that it again equals the
reference signal 135 to thereby reduce the error signal 137
to zero. If as was postulated the anode to cathode voltage
differential is to be increased, the potential drop across
the control tubes 144, 146 must be decreased through
modification or adjustment of the grid potentials. The
decrease in the voltage drop across tube 144, ~lhich is a
negative voltage drop between the secondary 130 and ground,
results in the anode 114 becoming more positive relative to
ground. The decreased voltage drop across tube 146, which
is a positive voltage drop between the secondary 132 and
ground, resul-ts in the cathode 122 becoming more negative
relative to ground.
It should be appreciated that the grid potential
does not stabilize at an optimum value and that instead the
system operates in a dynamic feedback mode. The rectified
outputs from the transformer secondaries are pulsating
voltages so that the grid drivers 162, 164 must continually
adjust as the error signal generated in the error generator
changes. The feedback circuitry responds quickly enough to
the pulsating d.c. voltage to achieve voltage stabilization
(substantially constant anode 114-to--cathode 116 voltage).
This stabilization requires no filtering capacitors and is
satisfactory for accurate computed tomography x-ray
generation.
The tl,~o control tubes 144, 146 included in the
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1 9 RCA 75,934
feedback portions 140, 142 perform similar functions yet due
to the opposite polarity of the cathode and anode x-ray tube
potentials, the two tubes are configured differently. One
control tube 14~ has its anode essentially grounded and the
second 146 tube has its cathode very close to ground. The
opposed (non-grounded) electrodes are many thousand volts
removed from ground with the grounded anode tube having its
filament well below ground and the grounded filament tube
having its anode well above ground.
To control the flow of electrons in the control
tubes, the control grid voltage must be held in a range
near the filament voltage. For the grounded filament tube
146 this constraint presents no problem. Its grid 150
potential is maintained slightly below ground and may be
increased to a value of approximately 150 volts negative.
Modification of this voltage by the grid driver 146 modifies
the flow of electrons in that -tube 146 and therefore
modifies the impedance between ground and the cathode of the
x-ray tube.
The constraint on the other grid 148 presents
control problems since that grid must be maintained at a
potential on the order of the non-ground filament potential
which is approximately 10,000 volts below ground. The
problem presented is to send a control function proportional
to the error signal to a control grid 148 which is
maintained at a potential of approximately 10,000 volts.
Electrical coupling between the high voltage grid and the
low voltage error siynal would result in voltage spikes,
arcing, and current flows of unsuitable magnitude.
Avoidance of these problems has been achieved by the
inclusion of an electrical isolation circuit portion 166
interposed between the error signal compensation amplifier
158 and the high potential grid driver 162.
The isolation portion 166 comprises a frequency
modulated receiver 169 interconnected through a light pipe
to a frequency modulated driver 167. The error signal is
sent to the frequency modulation driver 167 which transforms
the voltage signal into a frequency modulated signal. The
frequency modulated signal is transmitted through the
light pipe to the frequency modulated receiver which decodes
the frequency modulated information and returns it into
the form of an electrical voltage signal. The light pipe
is, of course, an electrical insulator and therefore the
high potential on the grid 148 does not affect the low
potential portions of the generator 110. The coding and
decoding of information through the electrical isolation
portion 166 is achieved by amplitude modulating with a
160 kilocycle frequency modulated subcarrier, a light
beam signal. Techniques for modifying this signal in
such a way as to carry the error signal information are
known. One optical coupling system capable of performing
such functions is commercially available under the tradename
or trademark Burr-Brown. That system comprises a model
3712T transmitter, a model 3712R receiver and fiber optic
coupling.
Connected to the non-grounded electrodes of
each control tube are two voltage dividers 168, 170.
These dividers function in helping maintain the two control
tubes 144, 146 within a dynamic range of operation. Two
outputs 172, 174 from the dividers 168, 170 are transmitted
to a summing or balancing amplifier 176. This amplifier
176 receives these two signals and produces an output
proportional to their algebraic sum. As appreciated by
those skilled in the art, the output 172 from one voltage
divider 168 is a signal proportional to the output on
the non-grounded filament of the grounded anode control
tube 144. The output 174 from the other divider is propor-
tional to the voltage appearing at the anode of the grounded
filament control tube 146. To maintain the differential in
voltage across the cathode and anode of the x-ray tube these
values need not be equal, but to insure control tube
operation is maintained in a dynamic range of operation
(i.e. neither tube goes into saturation or cutoff) an output
178 from the SUmMing amplifier 176 is used to modify the
error signal 138 emitted by the error generator. This
modification maintains each nn-grounded control tube
electrode at approximately the same absolute voltage from
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1 11 RCA 75,934
ground and -thereby maintains the control tube in an
effective operating range to dynarnically control x-ray tube
potential differences. sut for the utilization of this
balancing or su~ming amplifier 176 it is possible that while
the potential difference across the x-ray tube cathode and
anode 114, 116 would be maintained at a relatively stable
value, one control tube voltage drop would be substantially
less than the other and at some time the feedback
stabilization circuitry would fail due to either cutoff or
saturation of one or the other of the control tubes.
Operation of the summing or balancing amplifier in the
feedback loop maintains the voltage drop across each control
tube at ap~roximately equal absolute values. Modification
of the voltage on the tube control grids 148, 150 continues
to maintain the difference in potential across the x-ray
tube at a constant level.
Figure 5 is a detailed schematic of the system
shown in Figure 4. As noted above a voltage from two inputs
120, 122 appears across the anode 114 and cathode 116 of an
x-ray tube 112. Electrons are emitted from the cathode 116
in response to a current flow generated by a filament supply
210. They accelerate across the x-ray tube, strike the
anode 114, and x-radiation is emitted.
The high voltage is provided by two secondary
transformers 130, 132. One secondary 130 is configured in a
wye format and the second secondary 132 is configured in a
delta format. Outputs from the delta and the wye secondary
windings are rectified by a number of diodes. Diodes 212a-c
and 214a-c serve to rectify the output from the wye
transformer secondary and a second set of diodes 216a-c and
218a-c serve to rectify the output from the delta
transformer. Were it not for the feedback operation of the
present generator, the outputs from these rectified
transformer secondaries would be pulsating DC potentials
and would provide an irregular pulsating accelerating
potential to the x-ray tube.
The feedback correction circuit includes a high
voltage divider 220 which reduces in magnitude the high
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1 12 ~CA 75,934
voltages appearing at the cathode and anode of the x-ray
tube. These smaller magnitude voltages are suitable for use
in the feedback POrtiOnS of the x-ray stabilization
generator. The high voltage divider 200 comprises a first
222 and a second 224 voltage divider which reduce the high
input from the anode and cathode by a factor of 10,000.
The output from these two voltage dividers is transmitted to
two power amplifiers 226, 228.
Two outputs 230, 232 leave the high voltage
divider 220 and are transferred through a second pair of
power amplifiers 234, 236. These two outputs form inputs to
a differential amplifier 238. The output 137 from the
differential error amplifier is a signal proportional to the
absolute voltage difference between the high voltage
appearing between the cathode and anode. In the preferred
embodiment shown in Figure 5 a voltage separation of 20 kv
produces an output 137 from the differential amplifier 238
of one volt.
As noted previously, a reference generator 134
provides a reference voltage level at output 135 proportional
to a desired accelerating potential. In the preferred
embodiment of the invention a one volt signal appears at the
output 135 for each 20,000 volts of desired accelerating
potential. The output 135 is generated by a reference input
233 and two amplifiers 235, 237. The input 233 is
transmitted to the output 135 only during a patient exposure.
A switch 239 completes the circuit during e~posure and at
other times completes a connection to a -15 volt power
supply.
The two outputs 135, 137 from the reference
generator 134 and the differential amplifier 238 respectively
are sumrned by a summing or error generator 136. IE the
instantaneous voltage appearing between the cathode and
anode of the x-ray tube is equal to the desired accelerating
potential, the output from the error generator will be zero
volts. A difference between the actual instantaneous
accelerating potential and the desired or reference signal
produces either a positive or negative voltage output from
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1 13 ~CA 75,~34
the error generator 136 which is used to modify the grid
potentials on the two control tubes.
A plurality of operational amplifiers 240-242 are
included which transmit the error signal to a cathode grid
driver 164 which modifies voltages appearing upon the
grounded filament tube control grid. Other amplifiers 243,
244 transmit the error signal to a frequency modulated
driver 167 which in turn transmits the error signal to the
isolated portion of the circuit 166. These operational
amplifiers 240-244 are inserted to maintain the proper power
transfer and also to maintain circuit stability. Without
these amplifiers it is possible that under varying feedback
conditions the circuitry might go into oscillation and
disrupt functioning of the system.
An output 2 4 6 f rom the amplifier 244 forms an
input to the frequency modulated driver 167. This driver
converts the error signal which has been in the form of a
voltage into a frequency modulated signal which can be
conveniently transferred to an isolated portion 166 by
optically coupled circuitry such as a light pipe. ThQ
frequency modulated signal is received by a receiver 169
which reconverts the frequency modulated signal into a
voltage signal and transmits it through two amplifiers to
the anode grid driver 162. Both anode and cathode grid
drivers comprise amplifiers with gains of approximately 150
and a dynamic range of approximately 170 volts. By
modulating the voltage output from the two grid drivers it
is possible to change the control tube impedances and
therefore the voltage drop across these two control tubes.
This modification in control tube impedance results in a
voltage signal appearing at two outputs 248, 250 on the
nongrounded electrodes of the two control tubes. Due to the
presence of a shunt path 154, 156 between these points and
the x-ray tube anode and cathode respectively this modulated
signal appearing on the nongrounded electrodes of the two
control tubes is transmitted to the cathode and anode of the
x-ray tube. In this way modifications in the control
voltage on the control tube grids directly modifies the
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1 14 RCA 75,934
voltage separation appearing between the cathode and the
anode of the x-ray tube and by proper modulation of this
control voltage the voltage separation appearing across
these two electrodes is maintained at a steady or constant
value.
Since it is desirable not only to maintain constant
the voltage separation of the x-ray tuke but also to
maintain each control tube in a dynamic range of operation,
the nongrounded electrode voltages are also adjusted to
insure that they are always at approximately the same
absolute voltage level. It is important to fix the control
tubes in a dynamic range of operation so that the maximum
possible control over x-ray tube accelerating potentials is
achieved. The voltage appearing at the nongrounded
electrodes of the two tubes is monitored and a nonequality
in their absolute value (note: one is approximately 10,000
volts above ground and one 10,000 volts below ground)
results in a control signal modifying the error signal
transmitted to the cathode and anode grid drivers.
Two voltage dividers 168, 170 sample the voltage
at the nongrounded electrodes of the two control tubes and
send a signal proportional to these voltages to a summing
amplifier 176. If the two voltages are equal tof the same
absolute magnitude) then the output 178 from the summing
amplifier is zero volts and the error signal appearing at
a junction 180 within the feedback circuit is unmodified.
If, however, the two voltages appearing at the nongrounded
electrodes are unequal, a signal 178 modifies the error
signal sent to the cathode grid driver in sense to cause the
- anode voltage of triode 146 again to become equal, in
absolute value, to the cathode voltage of triode 144. Thus,
a type of double feedback circuit is arranged to maintain
the voltage or accelerating potential across the cathode and
anode of the x-ray tube at a constant value, and to maintain
the control tubes in a dynamic range of operation to achieve
maximum control over the accelerating potential.
The balance portion of the circuit includes two
amplifiers 260, 262. These are buffer amplifiers and
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1 15 RCA 75,934
transmit the signal from the voltage dividers 16~, 170 and
transmit those signals to a summing junction 264. Also
connected to the output of the voltage dividers 168, 170
are two zener diodes 266, 268. These protect the amplifiers
260, 262 from large voltage spikes should either divider 168,
170 have an open circuit in its 25 kQ resistors.
The balance portion also includes a switch 270
which disables the balance signal 178. I~hen disabled, the
constant potential between x-ray tube cathode and anode is
maintained but the control tubes' nongrounded electrodes
are no longer maintained at the same potential relative to
ground. This switch is used for testing and aligning
purposes.
Each control tube circuit further includes a large
resistor 270, 272 connected between an x-ray tube electrode
and ground. This resistor helps bias the control tubes even
at low x-ray tube currents. With the resistor 270, 272 in
the circuit, the current passing through the control tubes
is equal to the current passing through these large biasing
resistors plus the current flowing through the x-ray tube.
Other circuits within the system 10 monitor x-ray tube
current and modify that current as changes are made in the
desired current selection. To accurately monitor the x-ray
tube current, two outputs 256, 258 are transmitted to other
circuitry not shown in the diagrams. These outputs are
combined into one signal proportional to tube current and
used to control the output of the filament supply 210.
In the detailed schematic (Figure 5) preferred
values for capacitors and resistors have been given but the
high voltage stabilization could be achieved using other
component values. A model #6423F control tube is utilizea
in the preferred circuit.
Although a preferred embodiment has been d~scribed,
it should be appreciated that design modifications could be
incorporated without departing from the spirit or scope of
the invention as set forth in the appended claims.
A system utilizing the present invention maintains
the electrical potential difference between anode and
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1 16 ~CA 75,g34
cathode of an x-ray tube at a stabilized value over a large
range without the need for filtering capacitors. Further,
no primary transformer control is needed during a CT
exposure. A primary transformer control may be employed
prior to the exposure. Stabilized high voltage is then
maintained by the dynamic control which is flexible enough
-to take into account normal power line fluctuations.
In the embodiment of the invention described, two
three-phase transformer secondaries with full wave
rectification are employed for providing two pulsating d.c.
voltages. One of these voltages is above ground and the
other below ground and it is the difference in these
voltages which appears across the cathode and anode of the
x-ray tube. Both secondary transformers may be energized by
the same primary but their outputs need not be symmetric
abGut ground otential. In one embodiment, one pulsating
d.c. signal leads the other so that the voltage difference
between the two signals has a periodicity of 12 cycles per
one primary energization cycle. As noted above this
periodicity would adversely affect x-ray generation but for
operation of the differential feedback control featured in
the preferred embodiment of the invention.
3S
