Circuit for direct control of anode current in X-ray tubes with unipolar or bipolar power supplies by automatic adjustment of grid current

Abstract

A circuit for the purpose of directly controlling the anode current of a cold cathode X-ray tube by automatically adjusting the grid current (IG) of the X-ray tube, with a feedback signal proportional to the anode current, said control being performed by a closed loop comprising: a closed-loop control circuit (CLR1) that generates an amplified error signal (SMOD) that is the amplified difference between a signal proportional to the anode current (IA) and the anode's required current (IDEM); a digital sequencer circuit that converts the received signal into several digital control signals; a grid current control circuit that receives the digital control signals from the digital sequencer circuit, the grid current control circuit comprising at least one set formed by an inverter (INV) and optionally a converter (DC-DC CONV); and a voltage-to-current adapter transformer (TR) whose rectified output provides the grid intensity (IG) necessary to obtain the required anode intensity (IA).The circuit avoids the need to calibrate and / or characterize the anode current (IA) vs. grid current (IG) curve, maintaining the X-ray tube in perfect condition throughout its lifetime.

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION The present invention, as the title suggests, is directed to a circuit for directly controlling the anode current of a cold cathode X-ray tube by automatically adjusting the grid current.

[0002] The present invention is characterized by the special design and configuration of all elements that are part of the control circuit, making it possible to control the current of the X-ray anode (IA) by a closed loop control that automatically adjusts the grid current (IG) of the X-ray tube using a feedback signal that is directly proportional to the anode current (IA).

[0003] This avoids the need to calibrate and / or characterize the anode current (IA) versus grid current (IG) curve, which is a fundamental parameter that must be controlled with very high precision since it determines the amount of x-ray photons emitted from the anode tube.

[0004] Due to this circuit characteristic, degradation of the X-ray tube due to daily use is compensated for by the closed control loop of the circuit of the present invention, thereby achieving greater accuracy and stability of the emitted radiation over the life of the X-ray tube without periodic recalibration for changes in the X-ray tube characteristics due to such degradation.

[0005] The present invention therefore relates to the scope of X-ray devices, in particular cold cathode devices. [Background technology]

[0006] It is known in the art that the energy of the X-ray radiation depends on the kV applied between the anode and cathode, and the radiation dose depends on the anode current and exposure time.

[0007] Currently, what are called cold cathode or ambient temperature cathode X-ray tubes have been developed to distinguish them from thermionic emission tubes, which require an incandescent filament to heat the cathode and emit electrons toward the anode. In this new cold cathode technology, electron emission is controlled by a cathode (at room temperature) made of a series of carbon nanotubes, with a grid interposed between the cathode and the anode. A high enough electric field is applied between the grid and the cathode to extract electrons, which are then attracted and accelerated toward the anode. X-ray photons are emitted by electrons that strike the anode, and they leave the anode with an energy determined by the voltage applied between the anode and the cathode. The electric field between the grid and the cathode must be controlled to control the number of electrons that strike the anode.

[0008] Figure 1 is a schematic diagram of a cold cathode X-ray device in which the X-ray tube has a monopolar power supply, comprising an anode (A) and a cathode (K) formed from a series of carbon nanotubes and positioned opposite the anode (A). A grid (G) is interposed between the anode (A) and the cathode (K). A first loop is formed by the anode (A), cathode (K), and the power supply (KV) between them. Figure 1a shows the same tube, but in this case a bipolar power supply, with half the voltage supplying the anode (A) and the other half supplying the cathode (K). The centers of both power supplies are grounded to reduce the stress on the power supplies themselves and the X-ray tube's insulation relative to ground. There is also a second loop formed by the grid (G), cathode (K), and power supply (V) that can be controlled by voltage or current. Both loops have a common portion, with current IA flowing through the first loop and current IG flowing through the second loop, and current IA+IG flowing through the common portion.

[0009] Figure 2 is a graph showing that the anode current (IA) varies linearly with the grid current (IG). Figure 3 shows that controlling the anode current (IA) via the grid voltage is very complicated because the relationship between the anode current (IA) and the grid current (IG) with respect to the grid voltage (VG) is exponential.

[0010] Figure 4 shows the circuit diagram for controlling the anode current (IA). The anode is grounded via the power supply (VAG), the grid (G) is directly grounded, and the cathode (K) is grounded via the power supply (Vsup) and a MOSFET switch. A voltage drop (Vcont) occurs between them and is controlled by the cathode current demand signal (IKDemand).

[0011] In the control method shown in FIG. 4, the following relationship is satisfied: VGK=Vsup-Vcont VAK=VAG+VGK VAK=VAG+Vsup-Vcont

[0012] This control scheme provides linear control of the cathode current (IK), but is very complex and requires MOSFETs operating at several thousand volts, which results in a very large source-drain voltage drop.

[0013] Another drawback of this type of control is that the voltage between the anode and cathode of the X-ray tube depends on the grid voltage, which varies with the age of the X-ray tube, and directly affects the energy of the emitted radiation.

[0014] Figures 5 and 5a show the control of X-ray exposure by controlling the grid current while maintaining independent voltages between the anode and cathode (VAK monopolar or bipolar). However, to obtain the desired anode current, this grid current must be known in advance. The grid current, determined by a previous calibration or the IA vs. IG curve shown in Figure 2, can vary with daily use of the X-ray tube and even within the same X-ray tube due to manufacturing tolerances. The advantage of this type of control is that if the required IG current is known, it can be preloaded into the DC-DC converter before the start of X-ray exposure, reducing the start of X-ray exposure by several microseconds. This time reduction is essential for accurate timing of scanning techniques and X-ray exposures, for example, in a production line where X-ray subjects move along a conveyor belt. Considering that this time is not critical for over 90% of radiological applications, it is preferable to focus on accuracy, stability, and reduced downtime during new recalibrations.

[0015] The present invention therefore aims to develop a circuit for directly controlling the anode current of a cold cathode X-ray tube by automatic adjustment of the grid current (i.e., without the need for prior calibration or characterization of said current). Furthermore, since the control circuit powering the grid is isolated from the circuit powering the circuit between the anode and the cathode, it is possible to establish a constant voltage VAK (monopolar or bipolar) between the anode and the cathode independently of the grid control, resulting in a control circuit as described below. Summary of the Invention

[0016] The object of the invention is essentially contained in the independent claims, with different embodiments being contained in the dependent claims.

[0017] The present invention is directed to a circuit for directly controlling the anode current (IA) in an X-ray exposure by a feedback signal proportional to the anode current, designed to automatically control the grid current (IG) of the X-ray tube to maintain an accurate and stable required anode current (IA) (IDEM), which ultimately contributes to the number of X-ray photons emitted.

[0018] The anode current (IA) is controlled by a circuit that automatically supplies the grid current (IG) through a closed-loop control, said circuit comprising: a closed-loop control circuit for comparing a signal proportional to the anode current (IA) with a demand signal for the anode current (IDEM) and obtaining a signal that amplifies the difference between the signal proportional to the anode current and the demand signal for the anode current; a digital sequencer circuit connected to the closed-loop control for receiving the signal obtained by amplifying the difference between the signal proportional to the anode current and the required anode current, and converting the received signal into a plurality of digital control signals. a grid current control circuit receiving the digital control signal from the digital sequencer circuit, the grid current control circuit comprising at least one inverter and optionally a DC-DC converter, the digital control signal being applied to the DC-DC converter and the inverter through an adapter transformer and a rectifier at its output to obtain a grid current that provides an anode current substantially equal to the required current;

[0019] Preferably, but not exclusively, the closed loop control circuit comprises a comparator having a first input receiving a signal proportional to the anode current (IFB) and a second input being an anode current demand signal (IDEM). The two signals are compared and amplified with a gain G(s) to provide at its output a signal which is the amplified difference between the signal proportional to the anode current and the anode demand current.

[0020] In a possible non-limiting embodiment, the grid current (IG) control circuit preferably has a connection to a supply voltage (Vsup) which is connected to a DC-DC converter (DC-DC CONV), also known as a "step-down converter", which is further connected to an inverter whose output is connected to a transformer, the output of which is connected to a rectifier, the output of which is connected to the grid (G) and to ground.

[0021] It should be noted that the DC-DC converter and the inverter can be combined into a single inverter with a different topology than that described, controlled by voltage and / or current, and achieve similar results as described in this invention. The same applies to closed loop control, which can be analog or digital, with similar results.

[0022] In one possible embodiment, the digital sequencer circuit, upon receiving an exposure command (exp), generates two signals: a first signal (S1) to the DC-DC converter and a second group of control signals (S2) to the inverter (INV).

[0023] In a preferred, non-limiting embodiment, the DC-DC converter comprises a capacitor in parallel with a first switch and a first diode, a coil (L) and a return diode in parallel with the assembly formed by the first switch and coil.

[0024] The inverter is formed of a bridge including a second switch, a third switch, a fourth switch, and a fifth switch, the second switch and the fifth switch being in series to form a first branch, the third switch and the fourth switch being in series to form a second branch, and both branches being parallel to each other.

[0025] Each branch is connected at its midpoint to a transformer, the output of which is connected to a rectifier bridge to obtain the required grid current, and the bottom ends of both branches of the inverter and the DC-DC converter are connected to ground.

[0026] The described control circuit allows for direct control of the anode current by controlling the grid power supply using a transformer TR, which can provide grid voltages of 15 kV or more if required. The grid control circuit is independent of the anode-cathode circuit, so the VAK voltage is not affected by the grid control. Since the VAK (monopolar or bipolar) voltage is related to the emitted radiation (photon) energy, the total X-ray dose is much more accurate and stable.

[0027] The direct anode current control circuit has the very important advantage that it can be the basis for either stationary or portable radiological equipment, and is fully applicable to equipment with tomography and / or tomosynthesis, with one or more independently controlled X-ray tubes. First, controlling the grid does not change the anode-cathode voltage at all. Second, the grid control circuitry can provide 15KV or more if required. And third, the instrument maintains optimal accuracy throughout its lifetime without requiring calibration, characterization and / or maintenance.

[0028] Unless otherwise defined, all technical and scientific elements used herein have the meaning commonly understood by one of ordinary skill in the art to which this invention belongs. In the practice of the present invention, methods and materials similar or equivalent to those described herein can be used.

[0029] In the present specification and claims, the use of "comprises" and similar expressions does not exclude other technical features, additional elements, components or steps. For those skilled in the art, other objects, advantages and features of the present invention will be inferred in part from the description herein, and will be apparent in part from the practice of the invention. [Brief explanation of the drawings]

[0030] To complement the description given herein and to aid in a better understanding of the features of the present invention, a series of drawings are attached hereto as a part thereof, in accordance with a preferred practical embodiment, in which the following illustrative and non-limiting figures are shown: [Figure 1] FIG. 1 is a schematic diagram of a cold cathode X-ray device in which the X-ray tube is powered by a monopolar power supply. [Figure 1a] FIG. 1a shows a similar cold cathode X-ray device in which the X-ray tube is powered by a bipolar power supply. [Figure 2] FIG. 2 is a graph showing that the anode current (IA) varies linearly with the grid current (IG). [Figure 3] FIG. 3 is a diagram showing the relationship between the anode current (IA) and the grid current (IG) with respect to the grid voltage (VG). [Figure 4] FIG. 4 is a schematic diagram of a prior art circuit for controlling the anode current (IA) via the cathode current (IK), which affects the voltage (VAK) between the anode and cathode (and thereby the energy of the emitted radiation). [Figure 5] FIG. 5 shows a unipolar power supply for a circuit that controls X-ray exposure via grid current (IG), which requires current characterization and / or calibration to obtain the required anode current (IA). [Figure 5a] FIG. 5a shows a bipolar power supply for a circuit that controls X-ray exposure via grid current (IG), which requires current characterization and / or calibration to obtain the required anode current (IA). [Figure 6] FIG. 6 is a block diagram of the controls used for an embodiment of the present invention with a monopolar power supply for an x-ray tube that directly controls the anode current (IA) through automatic control of the grid current (IG). [Figure 6a] FIG. 6a is a block diagram illustrating a similar X-ray tube in which power is supplied to the X-ray tube by a bipolar power supply. [Figure 7] 7 shows a non-limiting preferred embodiment of the grid current control circuit, in which the DC-DC converter consists of a capacitor (C) in parallel with a first switch (Q1) and a diode (D1), an inductor (L), and a return diode (D2) in parallel with the assembly formed by the first switch (Q1) and the inductor (L). The output of the DC-DC converter powers an inverter formed by transistors Q2, Q3, Q4, and Q5, which in turn provides high frequency AC current to transformer TR1. The output of TR1 is rectified and provides DC current to the grid of the X-ray tube. [Figure 8] FIG. 8 is a diagram illustrating an example of an operation cycle of a digital sequencer operating under analog closed-loop control before, during, and at the end of X-ray irradiation. [Figure 9] FIG. 9 shows a possible embodiment of a current control circuit with a pulse width modulator PWM, which starts from an analog control input (SMOD) and acts directly and digitally on the DC-DC converter. DETAILED DESCRIPTION OF THE INVENTION

[0031] Preferred embodiments of the invention proposed are described below with reference to the drawings.

[0032] 1 to 5 and 5a illustrate prior art X-ray irradiation control.

[0033] Figure 6, Figure 6a shows a block diagram of a direct anode current control circuit (IA). The anode (A) is connected to a power supply (VAK with a unipolar or bipolar power supply) that is grounded via an anode current (IA) meter, and the cathode (K) is also grounded. Meanwhile, the grid (G) is connected to the anode current through a closed control loop, which in a preferred, non-limiting embodiment, is The device comprises a PID-like or equivalent closed loop control circuit (CLR1), preferably but not limited to, having a first input receiving a signal from a current (IFB) proportional to the anode current (IA) and a second input being a signal (IDEM) desired from the anode current. The two signals are compared and amplified by a gain amplifier G(s) which outputs an amplified error signal (SMOD), which is the amplified difference between the signal proportional to the anode current and the anode desired current (IDEM).

[0034] The digital sequencer circuit has a first input which is the output from the closed loop control, specifically the amplified error signal (SMOD), and a second input which is a signal (EXP) instructing X-ray exposure. Upon receiving the X-ray exposure signal (EXP), the digital sequencer generates a first output (S1) which controls the DC-DC converter by a PWM signal, and a second set of signals (S2) which sequence the operation of the inverter (INV).

[0035] The grid current (IG) control circuit preferably has a connection to a power supply (Vsup) which is connected to a DC-DC converter (DC-DC CONV), known as a "step-down converter", but not limited to this, which is further connected to an inverter (INV) whose output is connected to a transformer (TR), whose output is connected to a rectifier (RECT), whose output is connected to the grid (G) and ground.

[0036] In Figure 6, the power supply is a monopolar power supply from a power supply (VAK), with the anode (A) connected to the grounded power supply (VAK) via a shunt that measures the anode current IA, and the cathode (K) also grounded.

[0037] In Figure 6a, a bipolar power supply is shown, where the anode (A) is connected to a grounded power supply (VAK / 2) via a shunt that measures the anode current (IA), and the cathode (K) is connected to another grounded power supply (VAK / 2).

[0038] FIG. 7 shows a preferred embodiment of the elements of the grid current control circuit described above, where the DC-DC converter consists of a capacitor (C) in parallel with a first switch (Q1) and a diode (D1), an inductor (L) and a return diode (D2) in parallel with the assembly formed by the first switch (Q1) and inductor (L).

[0039] The inverter is formed of a bridge comprising a second switch (Q2), a third switch (Q3), a fourth switch (Q4) and a fifth switch (Q5), where the second switch (Q2) and the fifth switch (Q5) are in series and form a first branch, and the third switch (Q3) and the fourth switch (Q4) are in series and form a second branch, and the two branches are parallel to each other.

[0040] Each branch is connected to a transformer (TR1) at its midpoint, the output of which is connected to a rectifier bridge from which grid current (IG) is obtained, and the lower ends of both branches of the inverter and the DC-DC converter are grounded.

[0041] Figure 8 illustrates the operation of the digital sequencer at different clock cycles (C1 to C12), the state of each switch (Q1, Q2, Q3, Q4, Q5), the irradiation instruction input (EXP), and the error signal, which in this example is an amplified analog signal (SMOD).

[0042] In cycle (C1), there is no illumination instruction, so the five transistors (Q1, Q2, Q3, Q4, Q5) are off.

[0043] In cycle (C2), an exposure command (exp) is received and transistor (Q1) starts modulating according to the amplified error signal (SMOD) which is compared with the sawtooth signal (DS) generated by the digital sequencer itself in synchronization with the clock signal (CLOCK). At the same time, an instruction is given to turn on transistor (Q2) and its diagonal (Q4), while keeping transistor (Q3) and its diagonal (Q5) off.

[0044] In cycle C3, transistor Q1 continues to modulate as in the previous cycle, i.e. controlled by signals SMOD and DS. Now transistor Q2 and its diagonal Q4 are turned off, and the opposite diagonal Q3 and Q5 are turned on, generating an AC signal to power transformer TR, rectifying its output and providing the grid DC current IG required to achieve the required value of the anode current.

[0045] From cycles (C2) to (C11), the same control of transistor (Q1) continues, i.e., the diagonal alternation of the inverter is also maintained: when (Q2) and (Q4) are on, transistors (Q3) and (Q5) are off, and in the next cycle the conduction states of the four transistors of the inverter are reversed.

[0046] Finally (in this example, in cycle (C12)), the signal (EXP) gives the command to end the irradiation. At this time, the five transistors (Q1, Q2, Q3, Q4, Q5) are turned off simultaneously in an instant. At that moment, the current circulation to the grid stops and the X-ray irradiation is instantly turned off. The energy stored in the inductance (L) is transferred to the power supply (Vsup) via diodes (D1) and (D2), and everything returns to the initial state of cycle (C1).

[0047] Finally, Figure 9 shows the circuit that converts the amplified analog error signal (SMOD) into a digital signal that controls the transistor (Q1) and thus the output current (ISUP) of the DC-DC converter. As can be seen, it comprises a positive-edge triggered flip-flop (FF1) with an activation input connected to an AND gate (AND1) receiving a clock signal (CLOCK) and an exposure start signal (EXP), and a reset signal (RES) connected to a comparator (COMP1) receiving the amplified error signal (SMOD) and the instantaneous value of the sawtooth signal (DS). When both inputs to the gate (AND1) are "1", the flip-flop (FF1) activates its output (Q) to turn on the transistor (Q1). When the value of the amplified error signal (SMOD) and the instantaneous value of the sawtooth signal (DS) are reached and / or exceeded, the comparator (COMP1) resets the output (Q) of (FF1) to "0" and turns off the transistor (Q1) until it is turned on again in the next clock cycle (CLOCK).

[0048] It is worth noting that although the ripple in the grid current (IG) is directly proportional to the anode current, it has no effect on the dose control of the X-ray exposure, since dose control depends on the amount of photons generated at the tube anode, i.e. the integral of the anode current over the exposure time (internationally known as mAs). Conversely, the ripple in the voltage (VAK) is of great importance, as it generates photons of different energies and therefore has a very significant direct effect on the emitted dose.

[0049] Although the essence of the present invention and its implementation method have been fully described, it is hereby declared that the present invention also falls within the scope of protection of the present invention in other embodiments that differ in detail from those exemplified, as long as the basic principle thereof is not changed, altered or modified.

Claims

1. 1. A direct control circuit of the anode current of an X-ray tube with automatic adjustment of the grid current, wherein the anode (A) is connected to a power supply (VAK) or (VAK / 2) which is grounded via an anode current meter, and the cathode (K) is also grounded, while the grid (G) is connected to said anode current in a closed control loop, said control loop in the illustrated preferred non-limiting embodiment comprising: a closed-loop control (CLR1) circuit having a comparator for comparing a signal proportional to the anode current (IA) with an anode current demand signal (IDEM), and for obtaining an amplified error signal (SMOD) by amplifying the difference between the signal proportional to the anode current (IA) and the anode demand current (IDEM); a digital sequencer circuit connected to the closed loop control circuit (CLR1) for receiving the amplified error signal (SMOD) and converting the received signal into various digital control signals; a grid current control circuit that receives the digital control signal from the digital sequencer circuit and includes at least one set formed of an inverter (INV) and optionally a converter (DC-DC CONV), the digital control signal is applied to the converter (DC-DC CONV) and the inverter (INV), and the inverter (INV) supplies a transformer that supplies a rectified output to a grid current (IG) so that an anode current substantially equal to a required current (IDEM) is obtained;

2. 2. A control circuit according to claim 1, characterized in that the closed loop control circuit (CLR1) further comprises a gain amplifier G(s) arranged at the output of the comparator.

3. 3. A control circuit according to claim 1 or 2, characterized in that the grid current (IG) control circuit is further connected to a supply voltage (Vsup) connected to an inverter assembly (INV) and optionally to a DC-DC converter (CONV DC-DC), the output of the inverter (Inv) being connected to a transformer (TR) and the output of the transformer (TR) being connected to a rectifier (RECT) whose output is connected to the grid and to ground.

4. 4. A control circuit according to claim 3, characterized in that the DC-DC converter is constituted by a capacitor (C) in parallel with a first switch (Q1) and a diode (D1), while it is constituted by a coil (L) and a return diode (D2) in parallel with the assembly formed by the first switch (Q1) and the coil (L); the inverter is formed by a bridge with a second switch (Q2), a third switch (Q3), a fourth switch (Q4) and a fifth switch (Q5), the second switch (Q2) and the fifth switch (Q5) being connected in series to form a first branch, and the third switch (Q3) and the fourth switch (Q4) being connected in series to form a second branch, the two branches being parallel to each other, each branch being connected at its middle to a transformer (TR1), the output of the transformer (TR1) being connected to a rectifier bridge from which the grid current (IG) is obtained, and the lower ends of both branches of the inverter and the DC-DC converter are grounded.

5. The control circuit according to claims 1 and 3, characterized in that the converter (DC-DC) and the inverter (INV) are integrated into a single inverter and are voltage and / or current controlled to obtain very similar results.

6. A control circuit according to any of the preceding claims, characterized in that the digital sequencer circuit, upon receiving an exposure command (EXP), generates two signals: a first signal (S1) for the conversion (CONV DC-DC) and a second group of control signals (S2) for the inverter (INV).

7. 7. A control circuit according to claim 6, wherein the digital sequencer circuit comprises a flip-flop (FF1) triggered by a positive edge, the flip-flop having an activation input connected to an AND gate (AND1) to which a clock signal (CLOCK) and an exposure start signal (EXP) are input, and a reset signal (RES) connected to a comparator (COMP1) to which the amplified error signal (SMOD) and the instantaneous value of the sawtooth signal (DS) are input, the flip-flop (FF1) activates its output (Q) when both inputs to the gate (AND1) are "1", and when the value of the amplified error signal (SMOD) and the instantaneous value of the sawtooth signal (DS) are reached and / or exceeded in the comparator (COMP1), the output becomes "0" and restarts the output (Q) of (FF1) until it is turned on again in the next clock cycle (CLOCK).

8. A control circuit according to any of the preceding claims, characterized in that the power supply is a monopolar power supply from a power supply (VAK), the anode (A) is connected to the power supply (VAK) which is grounded via a shunt which measures the anode current IA, and the cathode (K) is also grounded.

9. An X-ray irradiation control circuit using a grid current according to any one of claims 1 to 7, characterized in that the power supply is a bipolar power supply, the anode (A) being connected to a grounded power supply (VAK / 2) via a shunt that measures the anode current (IA), and the cathode (K) being connected to another grounded power supply (VAK / 2).

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