Constant potential H-V generator

An x-ray tube voltage generator with automatic stabilization circuitry is disclosed. The generator includes a source of pulsating direct current voltage such as from a rectified 3 phase transformer. This pulsating voltage is supplied to the cathode and anode of an x-ray tube and forms an accelerating potential for electrons within that tube. The accelerating potential is stabilized with a feedback signal which is provided by a feedback network. The network includes an error signal generator which compares an instantaneous accelerating potential with a preferred reference accelerating potential and generates an error function. This error function is transmitted to a control tube grid which in turn causes the voltage difference between x-ray tube cathode and anode to stabilize and thereby reduce the error function. In this way stabilized accelerating potentials are realized and uniform x-ray energy distributions produced.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
This invention relates to an improved high voltage generator for providing 
energy to a computed tomography (CT) scanning x-ray tube. More 
particularly, the apparatus includes a high voltage stabilization circuit 
for reducing fluctuations in x-ray energy production. 
2. Prior Art 
In order to generate x-radiation for CT scanning, an x-ray tube including 
an anode and cathode electrode is positioned in close proximity to a 
patient and x-radiation from that tube is transmitted through the patient 
to an array of x-ray intensity detectors. The intensity data is used in a 
reconstruction algorithm to provide a density mapping of a patient 
cross-sectional area. To generate the x-rays, electrons from the cathode 
are accelerated to the anode due to the force exerted on them by an 
electric field between the two electrodes. This field is proportional to 
an electric potential difference applied between those electrodes. 
Fluctuations in the voltage separation between cathode and anode result in 
fluctuations of the electric field and therefore fluctuations in the speed 
with which the electrons strike the anode material. As the electrons 
strike the anode, their kinetic energy is transferred to x-radiation 
energy and heat energy which is dissipated by the anode. Variations in 
electron kinetic energy result in variations of x-ray energy emitted by 
the x-ray tube. It is apparent, therefore, that variations or fluctuations 
in the electrical potential difference between the cathode and the anode 
result in fluctuations or variations in the energy of x-radiation emitted 
by the x-ray tube. 
The reconstruction of a cross-sectional density mapping by a CT scanner 
requires accurate intensity information regarding the x-radiation which 
was passed through the patient. To reconstruct these images it is 
advantageous to have non-varying accelerating potentials across the x-ray 
tube to achieve stable x-ray generation and therefore accurate 
reconstruction images. To generate x-rays the accelerating potential is 
provided by a high voltage generator which must provide stable voltages of 
the order of one hundred thousand volts. 
One prior art constant potential generation technique is disclosed in the 
U.S. Pat. No. 3,325,645 entitled "X-Ray Tube System With Voltage and 
Current Control Means" which has been assigned to the assignee of the 
present invention. The apparatus embodied in that prior art disclosure 
includes an alternating high voltage source whose output is, first 
coarsely adjusted, rectified and then smoothed or filtered with a 
capacitor. This filtered signal is then transmitted to an x-ray tube 
cathode and provides the accelerating potential for electrons in the x-ray 
tube. 
Fine adjustment in the accelerating potential is achieved by a feedback 
control circuit which modulates the amplitude of the signal reaching the 
filtering capacitor. By modifying the unfiltered high tension voltage in 
response to the voltage appearing at the x-ray tube the accelerating 
potential is stabilized. An increase in the filtered signal results in a 
change in the impedance of the feedback control circuit which tends to 
reduce the unfiltered signal. Conversely a decrease in the filtered signal 
results in a change in the impedance of the feedback circuit which tends 
to add or increase that unfiltered signal. This negative feedback control 
reduced variations in accelerating potential by reducing the ripple in the 
filtered high voltage signal. 
While achieving substantial commercial success especially in the 
application for which it was designed, x-ray diffraction, the prior art 
apparatus disclosed in U.S. Pat. No. 3,325,645 contains some 
disadvantageous features when applied to CT. One such feature is the 
utilization of the filtering capacitor to smooth a pulsating signal. Such 
a capacitor charges to a voltage of 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. 
The prior art voltage generator has a limited range of feedback control. If 
the unfiltered voltage from a transformer secondary in the prior art 
circuit deviated from a certain range the feedback control circuit could 
no longer stabilize the accelerating potential. For stabilization beyond 
this limited effective range a primary transformer control circuit is 
required to return to the proper range the output from the secondary 
transformer to the filter capacitor. This limitation added complexity to 
the stabilization circuit. Finally, the commercial embodiment of the '645 
patent operated at too low an accelerating potential and current range for 
optimum CT use. 
SUMMARY OF THE INVENTION 
A system utilizing the present invention overcomes disadvantages in prior 
art high voltage generators by utilization of a differential feedback 
circuit which maintains the electrical potential difference between anode 
and cathode of an x-ray tube at a stabilized value. The invention provides 
a range of dynamic control over this electron accelerating potential large 
enough to obviate the need for filtering capacitors. No primary 
transformer control is needed during a CT exposure. A primary transformer 
control is applied 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. 
As is conventional the invention uses a source of alternating current in 
the form of a three phase transformer which generates an alternating 
current signal. This signal is rectified and transferred to an electrode 
on the x-ray tube and provides an accelerating voltage to cause electron 
flow to the tube anode. If this rectified pulsating signal were used 
directly to accelerate electrons from the cathode, these electrons would 
accelerate according to the pulsating potential from the three phase 
transformer and generate non-uniform x-rays when striking the anode. 
A system of the present invention, however, causes the accelerating 
potentials to be modified in such a way that uniformity in accelerating 
potential is achieved. A signal proportional to the actual accelerating 
potentional on the x-ray tube is compared with a reference signal which is 
proportional to a desired or optimum accelerating potential. As the 
reference signal is compared to the actual accelerating potential, an 
error signal is generated and used as a feedback signal which controls 
modification of the x-ray tube accelerating potential. A control tube in a 
feedback control circuit provides the means for stabilizing the 
accelerating potential. 
The tube includes a control grid which forms a portion of the feedback 
circuit. When the grid potential is modified in response to the error 
signal, electron flow in the control tube is modified and as a result the 
accelerating potential appearing at the x-ray tube electrode is changed. 
This negative feedback allows an unfiltered signal to be used to 
accelerate electrons in the x-ray tube. The dynamic range of the control 
tube is large enough to effectively modulate the output of the rectified 
alternating current. No capacitors are needed and thus no charge build up 
occurs which might present a danger to service personnel or to the x-ray 
tube. 
In one embodiment of the invention the source of alternating voltage 
comprises two three phase transformer secondaries with full wave 
rectification for providing two pulsating DC 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 are energized by the same 
primary but their outputs are not symmetric about ground potential. 
Instead, one pulsating DC 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. 
As the two pulsating DC voltages are sent to the anode and cathode of the 
x-ray tube the difference between them is sensed and a signal proportional 
to this difference is generated. It is this signal which is compared to a 
reference signal for generation of an error signal. Once this error signal 
has been generated, it is transmitted to two control tubes connected to 
the x-ray electrodes. One tube forms a variable impedance path between the 
x-ray tube cathode and ground and the other control tube provides a second 
variable impedance path between the x-ray anode and ground. Modification 
of the voltages on the control grids of these tubes modifies the control 
tube impedance, thereby varying the voltage appearing at the x-ray tube 
anode and cathode. The impedance on these two tubes is changed in such a 
way that the difference in voltage across the x-ray tube is stabilized. 
In the preferred embodiment of the invention one control tube is connected 
in a standard cathode grounded configuration and the other control tube 
has its anode grounded with its control grid and cathode at a voltage on 
the order of 10,000 volts negative. Since fluctuations in the control grid 
voltage are used to vary the control tube impedance, the invention must 
provide a way to accurately transmit the error signal to this high voltage 
level and modify the grid potential. To accomplish this function an 
electrical isolation coupling is provided. 
The isolation coupling sends the error signal to the high voltage grid by a 
frequency modulated transmission system utilizing an optical coupler. The 
error signal is transmitted to a frequency modulation driver which sends 
the information contained in the signal to a high voltage frequency 
modulation receiver which is tied to a high common potential, which in one 
embodiment is the cathode potential of the grounded anode tube. The error 
information is again converted to a voltage and used to modulate the 
grounded anode control tube grid. In this way a relatively low voltage 
error signal can be used to modulate a control grid at a very high 
potential without disrupting the low voltage portions of the feedback 
circuitry. 
In the two control tube embodiment of the invention, a wide range of 
control is made possible through a second feedback circuit which 
continuously maintains the control tubes in an active region of operation 
and prevents either tube from either saturating or going into a cutoff 
state of operation. To achieve this function, the non-grounded electrodes 
of the two control tubes are monitored to determine what voltages appear 
at these non-grounded electrodes. A control signal proportional to these 
voltages is generated and sent to a summing amplifier in a separate 
portion of the feedback circuit. The output from this summing amplifier is 
added to the error signal before the combined signal is transmitted to the 
control grid of one control tube. Should the biasing of one or the other 
of the control tubes deviate from an optimum operating range, this second 
feedback portion of the circuit will tend to modify the voltage appearing 
at the control tube non-grounded electrode and return the tube to its 
proper operating range. Once the voltage on this non-grounded electrode 
control tube is modified the system automatically adjusts the voltage on 
the second control tube. 
From the above it is apparent that one object of the present invention is 
to provide a stabilized high voltage accelerating potential difference 
between the anode and cathode of an x-ray tube. One advantageous feature 
of the differential feedback control embodied in the invention is the wide 
dynamic range of the system which allows for stabilization without the 
utilization of filtering capacitors. This lack of potentially dangerous 
filtering capacitors is one obvious advantage the system has over the 
prior art. 
A second objective of the present invention is to provide a differential 
control circuit wherein a control tube's operating characteristics are 
monitored and controlled to provide x-ray generating potential without the 
need for a circuit to continuously control the voltage input to the system 
and appearing across the transformer primary. 
A further feature of the invention is the utilization of electrical 
isolation circuitry which allows a low voltage error signal to be 
transmitted to a high electrical potential control grid by the use of an 
optically coupled circuit. The electrical isolation is achieved through a 
frequency modulated transmitter and receiver arrangement which 
advantageously utilizes an optically coupled rather than an electrically 
coupled portion of the feedback loop. 
Other features and advantages of the invention will become better 
understood when the description of a preferred embodiment of the invention 
is considered in conjunction with the accompanying drawings.

PREFERRED EMBODIMENT OF THE INVENTION 
FIG. 1 shows a computed tomography system 10 used for examining the 
internal structure of a patient. The unit comprises a scanning unit 12, 
couch 16, signal processor 20 and imager 22. The scanning unit 12 is 
mounted to a floor and remains stationary relative to the patient. The 
scanning unit includes a housing 13 which covers x-ray apparatus including 
an x-ray tube which rotates for CT scanning. The housing 13 is constructed 
to include an aperture 14. The couch 16 is movably mounted and is 
operative to position the patient within 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 
data and sends this intensity data to the signal processor. The x-ray 
intensity data is then processed by the signal processor to obtain 
information concerning the relative densities of a patient cross-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 FIGS. 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 by the housing and frame structure 51, 
(see FIG. 2). A tubular spindle 54 is journaled in the bearings 52. The 
spindle 54 delineates a patient receiving opening 55. When the scanner is 
in use, a patient is supported on a stretcher with portions of the 
patients body disposed within the opening 55. 
An x-ray tube assembly 58 (see FIG. 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 FIGS. 2 and 3 is of 
a machine of the stationary detector type. For clarity of illustration, 
and because the detector array is now known in the art, the annular 
detector 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 FIG. 3. 
In use, the x-ray tube is orbited about the axis 56 over a range of orbital 
motion over a path of sufficient length to accelerate the tube to its full 
speed for a study, a 360.degree. scan and deceleration 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 a range 
in a clockwise direction and then counterclockwise on the next study. 
A drive for this orbital motion is shown schematically and it includes an 
annular motor 64 which is connected to the spindle 54. The drive shown is 
for schematic illustration only. Any of the known and commercially 
accepted drive systems can be employed. 
Four flexible conduits or cables 68 are connected 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 such other power requirements the tube assembly may have. 
The cables 68 extend from the x-ray tube through the opening 65 where they 
are adjacent the spindle 54 and into a cable delivery opening 69 (see FIG. 
3). 
The accelerating potential is supplied by a voltage generator embodying a 
stabilization circuit. The stabilization circuit maintains the potential 
at a highly constant level thereby insuring the x-radiation is of a 
constant mean energy level. 
Referring now to FIG. 4, a high voltage generator for an x-ray tube 112 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. When 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 DC 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 signals are not symmetric above and 
below ground. Due to this factor, an irregular shaped, pulsating potential 
difference is supplied between the cathode and anode of the x-ray tube. It 
is the function of the remainder of the circuitry within the generator 110 
to smooth and stabilize this potential difference in order 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 further comprises a 
reference generator 134, an error generator 136, and responsive to the 
output 138 from the error generator. The reference generator produces a 
voltage output or signal 135 which is proportional to a desired potential 
difference between cathode and anode of the x-ray tube. This reference 
signal 135 is a constant value voltage and is proportional to the proper 
accelerating potential to produce a desired average or mean fan beam x-ray 
radiation energy. This signal is sent to an error generator 136 which 
compares the signal 135 with an input 137 which is proportional to the 
potential difference appearing across the anode and cathode of the x-ray 
tube. But for the advantageous operation of the feedback loop system of 
the generator 110 this input 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. 
The output from the error generator 136 is a voltage signal which is 
proportional to the difference between the optimum or reference input 135 
and the actual potential difference appearing across the x-ray tube. This 
output 138 is an error signal and is used as an input to a in such a way 
that the potential difference between anode and cathode of the x-ray tube 
is stabilized even though the output from the transformer secondaries 
remains a pulsating d.c. signal. 
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 144, 146 with a control grid 148, 150. The error signal 138 
is sent through each of the feedback portions and modifies the control 
tube grid voltage 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 FIG. 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. To insure that a change in 
control tubes impedance is transmitted as a signal to the x-ray tube 
electrodes, two shunt paths 154, 156 by-pass the transformer secondaries 
and transmit signals resulting from the change in control tube impedance 
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 AC signals to 
moderate fluctuations in accelerating potential. The voltage appearing at 
either electrode therefore comprises two portions; the output from a 
transformer secondary and the control signal passing through the shunt 
path from the control tube. 
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. When this condition 
exists the x-ray beams emitted by the tube have an average energy less 
than an optimum desired value. Under these circumstances comparison of the 
two signals 135, 137 entering the error generator 136 will cause an error 
output signal 138 to be sent to the grids 148, 150. 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 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 and thereby bring this value in 
conformity with an optimized or reference voltage. 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. 
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 DC voltage to achieve 
voltage stabilization. This stabilization requires no filtering 
compacitors and is satisfactory for accurate computed tomography x-ray 
generation. 
The two control tubes 144, 146 included in the 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 144 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 164 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 signal 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 from 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 
proportional 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 
non-grounded control tube electrode at approximately the same absolute 
voltage from ground and thereby maintains the control tube in an effective 
operating range to dynamically control x-ray tube potential differences. 
But for the utilization of this balancing or summing 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 approximately equal absolute 
values. Modification of the voltage on the tube control grids 248, 150 
continues to maintain the difference in potential across the x-ray tube at 
a constant level. 
Turning now to FIG. 5, a detailed schematic of the system shown broadly in 
FIG. 4 is presented. 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 necessary to accelerate electrons to a sufficiently 
large kinetic energy to create x-radiation. This 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 attached to these outputs. 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 
produces lower voltage signals representing the high voltages appearing at 
the cathode and anode of the x-ray tube. These smaller magnitude voltages 
are more suitable to serve as control functions in the feedback portions 
of the x-ray stabilization generator. The high voltage divider 220 
comprises a first 222 and second 224 voltage divider which reduces 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 from the differential 
error amplifier 137 is a signal proportional to the absolute voltage 
difference between the high voltage appearing between the cathode and 
anode. In a preferred embodiment shown in FIG. 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 an output 135 
proportional to a preferred or 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 exposures 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 summed by a summing or error 
generator 136. If the instantaneous voltage appearing across 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 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 246 from 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. The 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 PG,21 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 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 anode-to-cathode 
voltage of the x-ray tube 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 voltage 
value from zero potential. 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 grids and send a signal proportional to 
these voltages to a summing amplifier 176. If the two voltages are equal 
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 such a way that the cathode grid driver 164 will 
modify the grid potential in response to both the error signal and the 
balance signal. The change is so as to again produce equality between the 
nongrounded electrodes of the control tubes and maintain them in a dynamic 
range of operation. 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 transmit the signal from the voltage dividers 
168, 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 
k.OMEGA. resistors. 
The balance portion also includes a switch 270 which disables the balance 
signal 178. When 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 from zero 
potential. This switch is used for testing and aligning purposes. 
Each control tube circuit further includes a larger 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 resistors 
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 (FIG. 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 
utilized in the preferred circuit. 
Although a preferred embodiment has been described, 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.