U.S. patent number 3,916,251 [Application Number 05/522,989] was granted by the patent office on 1975-10-28 for filament current regulator for rotating anode x-ray tubes.
This patent grant is currently assigned to CGR Medical Corporation. Invention is credited to Leo Hernandez, Charles L. Laughinghouse.
United States Patent |
3,916,251 |
Hernandez , et al. |
October 28, 1975 |
Filament current regulator for rotating anode X-ray tubes
Abstract
A system for controlling the filament power applied to a
rotating anode X-ray tube when used with a three phase generator. A
square-wave drive signal is applied to the filament from an
inverter synchronously operated in accordance with the 60Hz power
line. The amplitude of the square-wave output from the inverter is
varied in response to the operation of a series regulator
selectively controlled by the composite output of a summing
amplifier having inputs corresponding to filament current, desired
MA, and space charge compensation prior to excitation of the X-ray
tube but only to the actual MA compared against a reference
potential during excitation.
Inventors: |
Hernandez; Leo (Glen Burnie,
MD), Laughinghouse; Charles L. (Linthicum, MD) |
Assignee: |
CGR Medical Corporation
(Baltimore, MD)
|
Family
ID: |
24083205 |
Appl.
No.: |
05/522,989 |
Filed: |
November 11, 1974 |
Current U.S.
Class: |
378/110; 315/106;
315/308; 315/357 |
Current CPC
Class: |
H05G
1/34 (20130101) |
Current International
Class: |
H05G
1/34 (20060101); H05G 1/00 (20060101); H05B
039/04 () |
Field of
Search: |
;315/94,105,106,307,308,311,357 ;250/401-404,413,414,421 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mullins; James B.
Claims
Accordingly, we claim as our invention:
1. A system for controlling the filament power applied to an X-ray
tube having at least one filament and an anode, comprising in
combination:
a square-wave filament drive circuit, including means operated in
synchronism with the power line feeding the X-ray tube, coupled to
said filament and providing a controlled square-wave signal of
variable amplitude thereto for powering said at least one
filament;
control circuit means coupled to said drive circuit providing a
signal for controlling the amplitude of said square-wave, said
control circuit means including means for being selectively enabled
and operated in response to a first input signal coupled thereto
prior to excitation of said X-ray tube wherein X-rays are generated
thereby and to a second input signal coupled thereto after
initiation of excitation of said X-ray tube;
first circuit means including a signal summing point coupled to and
being responsive to signals representative of a desired anode
current, space charge compensation, and the filament current
flowing in said filament and providing a composite signal thereby,
said composite signal being said first input signal; and
second circuit means including error signal generator means coupled
to signals respectively representative of the actual anode current
and a predetermined reference signal and being operable to provide
an error signal output, said error signal being said second input
signal.
2. The system as defined by claim 1 wherein said filament drive
circuit includes an inverter circuit.
3. The system as defined by claim 2 wherein said drive circuit
additionally includes signal generator means responsive to a power
line frequency potential and providing a pulse train having a
frequency twice the power line frequency;
a binary circuit coupled to said pulse train providing a pair of
complementary square-wave output signals coupled to said inverter
circuit for operating said inverter circuit in synchronism with and
at the power line frequency.
4. The system as defined by claim 3 wherein said signal generator
means comprises a zero cross-over detector circuit, and
wherein said binary circuit comprises a flip-flop circuit.
5. The system as defined by claim 4 wherein said inverter circuit
includes an output circuit including means coupled to said control
circuit.
6. The system as defined by claim 5 wherein said output circuit
includes an output transformer having primary winding including a
center tap and a secondary winding, and
wherein said control circuit is coupled to said center tap of said
primary winding and said secondary winding is coupled to said at
least one filament of said X-ray tube for supplying filament power
thereto.
7. The system as defined by claim 6 wherein said control circuit
means comprises a series regulator coupled between a supply
potential adapted to power said inverter and said center tap of
said primary winding.
8. The system as defined by claim 1 wherein said means included in
said control circuit means comprises a regulator circuit connected
to said filament drive circuit, said regulator circuit additionally
including enabling circuit means adapted to receive a command
signal for rendering said regulator circuit operable prior to
excitation of said X-ray tube, and an input circuit selectively
coupled to said first and second input signal for controlling the
operation of said regulator circuit.
9. The system as defined by claim 8 wherein said control circuit
means additionally includes first switch means adapted to couple
said summing point to said input circuit and being normally in a
closed circuit condition prior to excitation of said X-ray tube but
being in an open circuit condition after initiation of excitation
of said X-ray tube, and
second switch means adapted to couple said second circuit means to
said input circuit and being normally in an open circuit condition
prior to excitation of said X-ray tube but being in a closed
circuit condition after initiation of excitation of said X-ray
tube.
10. The system as defined by claim 9 wherein said first and second
switch means connect to a common circuit junction and additionally
including signal amplifier means having input means coupled to said
common junction and output means coupled to said input circuit of
said regulator means.
11. The system as defined by claim 10 wherein said first and second
switch means comprise semiconductor switch means having a pair of
signal conducting electrodes and a control electrode and wherein a
like signal electrode of each semiconductor switch is coupled to
said common junction.
12. The system as defined by claim 11 wherein said first and second
semiconductor switch respectively comprises first and second field
effect transistors having source, drain, and gate electrodes and
wherein the drain electrodes of both transistors are connected to
said common junction and wherein the source electrode of said first
transistor is connected to said summing point and said source
electrode of said second transistor is connected to the output of
said error signal generator means.
13. The system as defined by claim 12 wherein said control circuit
means additionally includes an exposure gate control circuit
coupled to the gate electrodes of said first and second field
effect transistor, being operable to provide a gate control signal
to said first field effect transistor prior to excitation of said
X-ray tube for rendering said transistor conductive while providing
a gate control signal to said second field effect transistor to
render said transistor non-conductive but upon initiation of
excitation of said X-ray tube, coupling a gate control signal to
said second field effect transistor for rendering it conductive
while coupling a gate control signal to said first field effect
transistor for rendering it non-conductive.
14. The system as defined by claim 12 wherein said gate control
circuit also includes timer circuit means triggered upon initiation
of excitation of said X-ray tube and being operable to initiate the
generation of said gate control signals to cause said first and
second field effect transistor to switch respective operating
states for a predetermined time and thereafter return to their
normal operating states.
15. The system as defined by claim 1 and additionally including a
filament current sensor circuit providing a signal corresponding to
the RMS value of the actual filament current flowing in said at
least one filament;
circuit means providing a signal indicative of a safe current limit
for said at least one filament, and
comparator circuit means coupled to said last-mentioned filament
current signal and safe current limit signal and providing an
output signal rendering said control circuit means and said drive
circuit inoperative when the signal corresponding to the actual
filament current exceeds a said safe current limit signal.
16. The system as defined by claim 15 wherein said means included
in said filament drive circuit includes an inverter circuit which
is operable to generate said square-wave signal; and
wherein said means included in said control circuit means comprises
a series regulator and circuit interrupter means coupled between
said inverter circuit and a supply potential, said circuit
interrupter means being operable in accordance with said output
signal from said comparator to open and disconnect said supply
potential from said inverter when the actual filament current
exceeds a predetermined safe limit.
17. The system as defined by claim 1 wherein said square-wave
filament drive circuit comprises:
a transformer having a primary winding connected to a power line
and a secondary winding;
a zero cross-over detector coupled to said secondary winding
providing output pulses having a frequency twice the power line
frequency;
a pulse shaping circuit coupled to said detector for providing
trigger pulses adapted to trigger a multivibrator;
a bi-stable multivibrator coupled to the output of said pulse
shaper and being triggered by the output pulses thereof to provide
a first and second complementary square-wave output;
first and second driver circuit means respectively coupled to said
first and second square-wave output, and
an inverter circuit having first and second inputs respectively
coupled to said first and second driver circuit and being
alternately driven by said first and second square-wave output and
having an output circuit including an output transformer having a
primary and secondary winding wherein said secondary winding
provides a bi-polar square-wave output signal for powering said at
least one filament.
18. The system as defined by claim 17 wherein said zero cross-over
detector includes a light emitting diode located within optical
coupler means and also including therein photosensitive transistor
means optically coupled to said diode and providing output pulse
signals at a frequency twice the power line frequency.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to X-ray apparatus and more particularly to
a system which is adapted to eliminate excessive ripple in the KV
waveform of a three phase generator used in connection with a
rotating anode X-ray tube.
2. Description of the Prior Art
Circuits for controlling the filament of an X-ray tube are well
known. For example, U.S. Pat. No. 3,567,995, Lauritzen, discloses a
controlled DC filament-cathode circuit for an X-ray tube. U.S. Pat.
No. 2,494,218, Weisglass, discloses control circuitry for the
filament of an X-ray tube wherein silicon controlled rectifiers and
"mag-amp" devices are utilized. U.S. Pat. No. 3,766,391, Siedband,
et al. discloses among other things a circuit arrangement for TRIAC
control of the RMS value of the filament current.
It has been observed, however, that when a three phase generator
including a high voltage transformer and full wave rectifier is
coupled across the anode and filament of an X-ray tube, KV ripple
is seriously affected by the type of filament drive employed
particularly when operating with X-ray tube currents in the region
of 1,000 milliamperes (ma) or above. Tests conducted utilizing pure
sine-waves, sine-waves whose conduction angle is controlled, direct
current and square waves for driving the filament indicate that the
KV waveform ripple content was excessive for both types of
sine-wave drive while being substantially reduced and/or eliminated
with a DC or a square-wave drive. Inasmuch as it has been found to
be impractical to use direct current for driving the filament of an
X-ray tube, the present invention contemplates the use of a
controlled square-wave drive for overcoming the aforementioned
ripple problem.
SUMMARY
Briefly, the subject invention is directed to the concept of
supplying the filament of an X-ray tube by means of a square wave
signal voltage generated by an inverter circuit operated in
accordance with the output from a bistable circuit which in turn is
controlled by a zero cross-over detector circuit responsive to the
60 Hz power line voltage. The output of the inverter, moreover, is
controlled by means of a series regulator operated in accordance
with the output of a summing amplifier selectively having applied
thereto signals corresponding to the actual filament current,
actual MA, desired or "preset" MA, and space charge compensation.
An exposure gate circuit, moreover, is included whereby the
aforementioned filament current, preset MA and space charge
compensation signals are summed together at the input of a summing
amplifier prior to an X-ray exposure but wherein only a signal
corresponding to the actual MA signal compared against a fixed
reference is applied to the summing amplifier during an actual
exposure. Additionally, protective circuitry is also included which
if the filament current is too high for the filament (spot size)
utilized causes a circuit breaker to trip, deactivating the
inverter through the series regulator and thereby removing the
filament drive from the X-ray tube.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an electrical block diagram illustrative of the preferred
embodiment of the subject invention;
FIG. 2 is an electrical schematic diagram illustrative of a
three-phase high voltage generator adapted to excite a rotating
anode X-ray tube utilized in connection with the subject
invention;
FIGS. 3a and 3b are electrical schematic diagrams illustrative of
the preferred embodiment of the subject invention;
FIG. 4 is a set of waveforms illustrative of pure sine-wave type
excitation of an X-ray filament;
FIG. 5 is a set of waveforms illustrative of sine-wave excitation
of an X-ray filament wherein the conduction angle of the controller
is controlled;
FIG. 6 is a waveform illustrative of DC excitation of an X-ray tube
filament; and
FIG. 7 is illustrative of square-wave excitation of an X-ray tube
filament in accordance with the subject invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings and more particularly to FIG. 1,
reference numeral 10 designates a rotating anode X-ray tube having
a filament 12 and an anode 14 coupled to a three-phase high voltage
generator 16 shown in detail in FIG. 2. The X-ray tube 10 normally
contains at least two filaments depending upon spot size desired,
however, only one is shown for purposes of illustration in the
present specification.
Referring now to FIG. 2, the three-phasee generator 16 is shown
comprising a high voltage three-phase transformer 18 having a three
phase Y connected primary circuit 19 connected to terminals 20, 22
and 24 to which are connected a three-phase 60Hz power line. The
secondary circuit of the transformer 18 includes a three phase Y
connected secondary circuit 26 and a delta (.DELTA.) connected
secondary circuit 28. The Y connected secondary circuit 26 is
connected to a three phase full wave rectifier circuit 30 having a
negative polarity (-) output terminal 32 and a positive polarity
(+) output terminal 34. The delta connected secondary circuit 28 is
connected to a three phase full wave rectifier circuit 36 having a
negative (-) output terminal 38 and a positive (+) output terminal
40.
Referring now back to FIG. 1, the negative output terminal 32 of
the generator 16 is connected to the filament 12 of the X-ray tube
10 while the positive output terminal 40 is connected to the anode
14. The positive terminal 34 is connected to a point of reference
potential hereinafter referred to as "ground" while the negative
terminal 38 is connected to one side of a variable resistance 42
located in a calibration or "preset" section 44. The opposite side
of the resistance 42 is returned to ground and provides a return
path for the X-ray anode tube current (MA). Accordingly, the
resistance 42 constitutes what is referred to in the art as a "drop
wire resistor" the voltage across which constitutes a signal
indicative of the actual anode current or MA of the X-ray tube.
This signal constitutes one operational input signal for the
control circuitry comprising the subject invention.
The filament 12 of the X-ray tube 10 is connected to the secondary
winding of a filament transformer 46 coupled to inverter circuit
output transformer 48 whose primary winding 50 includes a center
tap 52 which is connected to the output of a series type regulator
circuit 54. The end terminals of the primary winding 50 are
connected to a DC to AC inverter circuit 56 which includes suitable
driver circuits which are operated in accordance with the
square-wave outputs of a binary divide by two (.div. 2) flip-flop
circuit 58. The flip-flop circuit 58 in turn is driven by the
output of a pulse shaper 60 which is adapted to provide suitable
trigger pulses at a 120Hz rate in accordance with the output of a
zero cross-over detector 62. The cross-over detector 62 is coupled
to the 60Hz power line through the secondary winding 64 of a
transformer 66 whose primary winding is coupled to a single phase
60Hz power line.
The series regulator 54 is adapted to control the amplitude of the
square-wave output from the inverter 56 as it appears across the
secondary winding 51 of the output transformer 48 in accordance
with the output of a summing amplifier 68 whose input prior to
X-ray tube excitation is connected to a voltage summing point 70 by
means of a first but normally closed electrically operated switch
device 72 which connects to circuit junction 69. During actual
exposure, however, the switch 72 is opened and a second but
normally open switch 74 is closed coupling a voltage indicative of
the actual MA to the circuit junction 69. Operation of the switches
72 and 74 is provided by an exposure gate control circuit 76 whose
input is responsive to the voltage across "drop wire resistor" 42.
The voltage summing point 70 is adapted to continuously receive
three signal voltage corresponding, respectively, to the RMS value
of the filament current, the desired or preset MA, and space charge
compensation. The latter signal is a function of both the desired
MA and selected high voltage or KV.
The signal voltage proportional to the RMS value of the filament
current is provided by a filament current sensor circuit 78 which
is coupled to the secondary winding 79 of a transformer 80 whose
primary winding 81 is coupled in series to ground with the
secondary winding 51 of the inverter output transformer 48. The MA
preset or voltage proportional to the desired MA is provided by
means of a resistance voltage divider circuit provided by a
variable resistor 82 connected in series to a fixed resistor 84.
The upper end terminal of the variable resistor 82 is connected to
a reference potential of positive polarity while the lower end of
the fixed resistor 84 is connected to ground. The common connection
85 between resistances 82 and 84 is connected to the summing point
70. The space charge compensation signal voltage is provided by a
space charge function generator 86 having a variable resistor 88
coupled thereto whose value is proportional to the desired MA as
well as a voltage input proportional to desired KV which as noted
above is the high voltage used to excite the X-ray tube 10 from the
three-phase generator 16. This signal is provided by a digital to
analog converter 90 which receives a digital word from a KV select
keyboard and logic section 92. The signal voltage corresponding to
actual MA applied to junction 69 during X-ray exposure, however,
constitutes an error signal derived from the comparison of the
voltage across the dropwire resistor 42 and a negative polarity
reference voltage applied to a comparator amplifier 94 at terminal
95.
The series regulator 54 while being controlled by the output of the
summing amplifier 68 is adapted to be operated only when a supply
voltage (+24.sup.v) applied to terminal 96 is coupled thereto
through a circuit breaker 98 and a control voltage is applied to
terminal 100. The control voltage applied to terminal 100 comprises
a "boost command" signal from one of two switches, not shown, which
must be closed sequentially in order to make an X-ray exposure. The
boost command switch is actuated just prior (1.0 second or more) to
the operation of the X-ray exposure switch in order to bring the
rotating anode 14 up to speed and to bring the filament 12 up to
temperature before applying the high voltage.
The circuit breaker 98, however, is adapted to be tripped, i.e.
opened, by means of the solenoid 102 which in turn is adapted to be
energized by means of a filament current protection circuit 104.
The current protection circuit 104 is coupled to a filament spot
size selection circuit 106 since the tube 10 actually includes at
least two filaments for different focal spot sizes and a current
limit selection circuit 108. Circuit 104 is adapted to compare the
actual filament current from the sensor circuit 78 with a signal
from 108 corresponding to the maximum allowable current for a
selected filament and thus prevent inadvertent damage to the X-ray
tube 10.
Considering now the invention in greater detail, reference is made
to a schematic diagram of the preferred embodiment as illustrated
in FIGS. 3a and 3b. The single phase 60Hz line voltage appearing
across the secondary winding 64 of the transformer 66 as shown in
FIG. 2, is applied to terminals 110 and 112. Since the center tap
of the secondary winding 64 is grounded, the line voltage appearing
at terminals 110 and 112 are mutually 180.degree. out of phase with
respect to one another. Terminals 110 and 112 are coupled to a
common junction point 114 by means of the half-wave rectifier
semiconductor diodes 116 and 118. Connected between junction 114
and ground is a transient suppressor 120 comprised of a metal oxide
varistor well known in the art. The zero cross-over detector 62 is
coupled between junction 114 and ground by means of a resistor 122.
The zero cross-over detector 62 is comprised of a light emitting
diode 124 which is optically coupled to a Darlington transistor
circuit including a photo transistor 126 and transistor 127. As
60Hz line voltage is applied, the signal appearing at circuit
junction 128 constitutes pulses having a frequency of 120Hz. These
pulse signals are next fed to the pulse shaper 60 which comprises a
signal amplifier which is adapted to provide substantially
square-wave output pulses having respective pulse widths in the
order of 200 microseconds in relation to the 8.3 millisecond
period. The 120Hz pulse train output from the pulse shaper 60 is
fed to the C input of flip-flop circuit 58 which constitutes a J-K
flip-flop. It acts as a divide by two (.div. 2) binary counter
providing 60Hz complementary square-wave outputs at the Q and Q
output terminals. The 60Hz square wave from the Q output of the
flip-flop 58 is fed to first driver amplifier comprising transistor
130 whose collector electrode is coupled to the base of transistor
132 forming one half of the inverter circuit 56. In a similar
manner, the Q output from the flip-flop 58 is coupled to a second
driver comprising transistor 134 which is adapted to be coupled to
the base of transistor 136 in the other half of the inverter 56.
The emitters of transistors 132 and 136 are returned to ground
through a common resistor 138 while their respective collectors are
coupled to opposite ends of the primary winding 50 of the output
transformer 48. The collector supply voltage for the transistors
132 and 136 in the inverter is applied through the primary
winding's center tap 52 which is connected back to a positive
supply potential (+24.sup.v) applied to terminal 96 through the
circuit breaker 98 and transistor 140 in the series regulator 54.
Whereas the respective square-wave signals applied to the base of
transistors 132 and 136 comprise unipolar complementary
square-waves separated in time by a half a period, the inverter
circuit 56 operates to provide a bi-polar square-wave as shown by
FIG. 7 on the secondary winding 51 of transformer 48 which bi-polar
square-wave signal is fed to the filament 12 of the X-ray tube 10
by means of the filament transformer 46 (FIG. 1).
It is to be noted that the inverter circuit 56 becomes inoperative
when either the boost command voltage is absent or upon the opening
of the circuit breaker 98 which removes the +24 volts from the
center tap 52 of the primary winding 50 of the output transformer
48. The collector-emitter junction of transistor 140 of the series
regulator 54 provides a series impedance with the center tap 52 to
vary the magnitude of the supply potential applied to the inverter
transistors 132 and 134. The series regulator circuit 54
additionally includes transistors 142, 144 and 146. Transistors 142
and 144 operate to render the regulator circuit selectively
operative and inoperative while transistor 146 is adapted to
control the level of conduction and accordingly the series
impedance value exhibited by the transistor 140. The emitter of
transistor 144 is coupled to supply voltage (+40.sup.v) terminal
148 which is also coupled to the collector of transistor 142
through the Zener diode 150 and resistor 152. When a boost command
control signal (+24.sup.v) is applied to the base of transistor
142, transistor 144 is turned on to render transistor 140
conductive, which in turn permits inverter circuit 56 to become
operative and generate the bi-polar square-wave fed to the X-ray
tube filament 12 shown in FIG. 1. The amplitude of the bi-polar
square-wave applied to the filament 12, however, is determined by
the voltage applied to the base of transistor 146 from the output
of the summing amplifier 68.
The summing amplifier is comprised of an amplifier of known design,
having a feedback resistor 154 coupled from the output to its
negative input terminal which is also common to circuit junction 69
while the positive input terminal has a feedback voltage applied
thereto by means of fixed resistors 156 and 158, and capacitor 162
which resistors and capacitor are coupled to the collector of
transistor 146 through capacitor 162. The negative (-) input of the
amplifier 68 accordingly constitutes the signal input and is
adapted to receive the algebraic summation or composite of the
desired (preset) MA signal, the RMS value of the filament current
signal, and the space charge compensation signal prior to exposure
while only the signal proportional to the actual MA compared
against a reference voltage during the X-ray exposure. This was
shown to be accomplished by means of the normally closed switch 72
and the normally open switch 74 shown in FIG. 1. These switches in
actuality comprise field effect transistor (FET) switches wherein
the drain and source of FET switch 72 respectively are connected
between circuit juntion 69 and the summing point 70. The drain of
FET switch 74 is connected to circuit junction 69 while the source
is connected to the output of the error amplifier 94 used to
compare the actual MA signal appearing across resistance 42 (FIG.
1) and the reference voltage (-15.sup.v) applied to terminal 95.
The gate of FET switch 74 is connected to the collector of
transistor 162 which forms part of the gate control circuit 76
while the gate of FET switch 74 is connected by means of junction
165 to the collector of transistor 166 which is also part of the
gate control circuit 76 as well as the base of transistor 164. The
gate control circuit 76 is additionally comprised of a timer
circuit 168, amplifier 170, and transistors 172, 174 and 176.
In the configuration shown, FET switch 72 is normally conductive
such that prior to excitation of the X-ray tube 10, the summing
point 70 is directly connected to circuit junction 69 which is also
common to the input to the summing amplifier 68. During an actual
exposure, however, FET switch 72 is rendered non-conductive,
disconnecting summing point 70 from the summing amplifier input;
however, FET switch 74 is rendered conductive, thereby coupling the
output of the error amplifier 94 to circuit junction 69. This is
accomplished in the following manner. Transistor 166 is biased such
that it is normally non-conductive bringing circuit junction 165
substantially to +24.sup.v. Since junction 165 is coupled to the
base of transistor 164 through resistor 178, this transistor is
conductive. Accordingly, the gate of FET switch 72 has the negative
supply potential (-15.sup.v) applied thereto, rendering it
conductive, whereas the gate of FET switch 74 being at +24.sup.v,
remains non-conductive. Upon the excitation of the X-ray tube 10 by
the high voltage generator 16 (FIG. 1), a negative polarity voltage
proportional to the actual MA appearing across the variable
resistor 42 shown in FIG. 1, appears at terminal 180. This voltage
is coupled to the positive (+) input of amplifier 94 by means of
resistor 179 which is also common to circuit junction 182. A
transient suppressor 181 and a pair of Zener diodes are also
coupled to ground on either side of resistor 179. The MA signal
appearing at circuit junction 182 is coupled to pin 2 of the timer
168 by means of amplifier 170 and transistor 172 which triggers the
timer 168 whereupon a signal appears at pin 1. The timer output
signal is coupled to the emitter of transistor 174, which operates
to render transistor 166 conductive, thereby causing circuit
junction 165 to fall to the negative supply potential, i.e.
-15.sup.v. This then causes FET switch 74 to become conductive,
coupling the output of the error amplifier 94 to the input of the
summing amplifier 68. At the same time, transistor 164 becomes
nonconductive, rendering FET switch 72 non-conductive, thereby
disconnecting the summing point 70 from circuit junction 69.
When the timer circuit 168 is triggered upon the X-ray tube
excitation, an external visual type EXPOSURE indicator 184 e.g. an
indicator light is also energized. After a predetermined time delay
determined by the circuit elements shown generally by reference
numeral 186, the timer again reverts back to its normal state and
an END OF EXPOSURE indicator 188 connected to pin 13 is energized.
Considering the purpose of transistor 176 shown coupled to
transistor 174, it has its base connected to terminal 190 which is
adapted to receive an inhibit type control signal provided during a
system calibrate procedure wherein the system presets which
constitutes, for example, the variable resistors 82 and 88 shown in
FIG. 1 and located in the calibrator module 44 are set prior to
making an X-ray exposure. Transistor 176 thereby inhibits any
change from the normal operating states of FET switches 72 and 74,
except during actual X-ray tube excitation.
The actual MA signal applied to the error amplifier 94 from
terminal 180 has a negative reference voltage (-15.sup.v) applied
to its negative input terminal by means of terminal 192 and a fixed
resistor 194. The amplifier 94 itself consists of a comparator or
error amplifier having a feedback resistor 196 coupled between the
output and the negative input terminal. Thus the output voltage of
the error amplifier 94 comprises the error or difference voltage
between the reference voltage and the voltage appearing at terminal
180. This error signal is coupled to circuit junction 69 by means
of resistor 197.
Considering now in greater detail the circuitry supplying the three
operational signal inputs to the summing point 70, which as noted
before, comprises the desired or preset MA signal, the signal
corresponding to the RMS value of the filament current, and the
space charge compensation signal, the voltage proportional to the
preset MA which appears at junction 85 shown in FIG. 1, is provided
by coupling the variable resistor 82 to terminal 198. A summing
resistor 200 is connected between circuit junction 85 and the
summing point 70. A capacitor 202 is coupled between terminal 198
and ground to provide a desired filtering effect. The voltage
corresponding to the RMS value of the actual filament current is
coupled to summing point 70 by means of the summing resistor 204.
This signal voltage is developed by the filament current sense
circuit 78 which includes an integrator consisting of an
operational amplifier 206 having feedback means 208 and whose
negative input is connected to the DC output of a semiconductor
diode bridge rectifier 210 coupled to the secondary winding 79 of
the transformer 80. A transient suppressor 212 is also connected
across the secondary winding 79. Regarding the space charge
compensation signal voltage, it is coupled to summing point 70 by
means of summing resistor 214. This voltage is developed by the
space charge function generator circuit 86 shown in FIG. 1. This
circuit, however, is comprised of a pair of series connected
operational amplifiers 216 and 218, whose output is connected to
one input (-) of amplifier 220. The negative (-) and positive (+)
inputs of amplifier 216 are respectively coupled to terminals 222
and 224 across which an analog voltage proportional to the selected
KV is applied from the digital to analog converter 90 shown in FIG.
1. This voltage is amplified and applied to the negative input
terminal of amplifier 220 by means of the resistor 226. The
negative input terminal of amplifier 220 also has a negative
reference potential (-15.sup.v) connected to terminal 228. This
reference voltage is coupled thereto by means of the fixed
resistors 230 and 232, whose common connection is connected to a
Zener diode 234. Additionally, across the negative input terminal
and output terminal of amplifier 220, is connected a pair of
circuit terminals 236 and 238 to which is coupled the variable
resistance 88 shown in FIG. 1, which is also proportional to
desired MA.
Referring now to the filament current protection circuit 104 of
FIG. 1 in greater detail, it consists of an amplifier 240 coupled
to the output of the integrator amplifier 208 included in a
filament current sensor circuit 78. The output of the amplifier 240
is coupled to one input (+) of a comparator amplifier 242 whose
other input (-) is coupled to a pair of FET switches 244 and 246
which are selectively closed depending on which X-ray tube filament
for desired spot size is used. The gate of FET switch 244 is
connected to terminal 248 which is adapted to be connected to the
filament selector (spot selector) circuit 106 shown in FIG. 1. In a
like manner, the FET switch 246 is connected to terminal 250, which
also connects to the selection circuit 106. Thus depending upon the
spot size selected, either FET switch 244 or 246 is rendered
conductive, which couples a voltage from either potentiometer 252
or 254 in the current limit selection circuit 108. The
potentiometers are calibrated to correspond to current limit
settings for the selected filament, so that depending upon the spot
size selected, a voltage from either potentiometer 252 or 254 is
coupled to the closed FET switch 244 or 246 to the other input of
the comparator amplifier 242, which if the selected reference
voltage is exceeded by the output of amplifier 240, transistor 256
is rendered conductive, whereupon the solenoid 102 will activate
the circuit breaker 98, causing the inverter to become inoperative
and remove the filament drive from the X-ray tube.
Considering the subject invention now from an operational
viewpoint, an operator selects a desired KV on the keyboard 92 and
makes selected presets on the calibrator section 44 by means of the
variable resistances 42, 82 and 88. As noted above, in making an
X-ray exposure, an operator must actuate two switches sequentially.
The first switch applies a supply voltage to terminal 100 of the
series regulator 54 which enables the regulator to permit the
inverter to supply a square-wave drive to the X-ray tube filament
10, in order to bring the filament up to temperature as well as
bring the rotating anode 14 up to speed. The signal corresponding
to the filament current, the desired MA, and the space charge
compensation is summed at the summing point 70 and fed through the
FET switch 72 to the series regulator via the summing amplifier 68,
at which time the conductivity of transistor 140 is controlled
accordingly. In accordance with the conductivity variation of the
series regulator resistor 140, the voltage applied to the center
tap 52 of the primary winding 50 of the inverter output transformer
48 causes the amplitude of the bi-polar square-wave to vary, it
being borne in mind that the inverter is being driven in accordance
with the 60Hz line voltage coupled to the zero cross-over detector
62 and the flip-flop 58 which is used to trigger the inverter. Once
the filament 12 has been brought up to temperature, usually 1.0
second or more after actuation of the boost command switch, the
exposure switch can then be actuated, at which time the high
voltage transformer assembly or generator 16 shown in FIG. 2 is
actuated. The anode current or actual MA develops a voltage across
variable resistor 42, which voltage is coupled through the
amplifier 170 to trigger the timer 168, causing the exposure gate
control circuit 76 to open FET switch 72 and close FET switch 74,
at which time the output of the error amplifier 94 is the only
input to the summing amplifier 68, and thus is the only operational
signal used to control the series regulator 54 for controlling the
amplitude of the filament drive supplied by the inverter circuit
56. In the event that the filament current as sensed by the
transformer 80 and the sensor circuit 78 is in excess of a preset
limit as established by the potentiometers 252 and 254, depending
upon the spot size selected, the filament protection circuit 104
will activate the circuit breaker 98, causing the inverter circuit
56 to become inoperative and thus remove all filament voltage from
the X-ray tube.
Thus what has been shown and described is a squarewave filament
drive circuit and regulator therefor which is particularly adapted
for use with a three-phase high voltage generator for a rotating
anode X-ray tube. The advantage of the square-wave drive
immediately becomes evident with reference to FIGS. 4 through 7,
inclusive. FIG. 4, for example, illustrates the power variation for
a sine-wave filament voltage drive. Curve 258, for example, is
illustrative of a sine-wave of filament voltage at a line frequency
of 60Hz. Since power is proportional to the square of the voltage,
curve 260 depicts the power variation with time and thus contains a
ripple varying at a 120Hz rate. Even where a discontinuous
sine-wave of voltage such as shown in FIG. 5 and denoted by curve
262 is applied for example, by means of a triac or other apparatus,
the power curve 264 still exhibits a 120Hz variation which can be
readily observed and has been found to be objectionable for a
three-phase generator system. FIG. 6 discloses that with a direct
current applied to the filament, the power variation 265 is
constant with respect to time, thus exhibiting none of the
objectionable ripple referred to. However, as already noted, it has
been found to be impractical to use DC on the filament of an X-ray
tube. The same ripple free effect nevertheless can be obtained by
use of a square-wave drive such as shown in FIG. 7 wherein curve
266 corresponds to the filament drive appearing at the output or
secondary winding 51 of the inverter output transformer 48 and
wherein curve 268 denotes the power variation with time. Thus
precise and adequate control of filament current of an X-ray tube
is maintained in an improved manner by the embodiment set forth in
the foregoing specification.
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