Power transfer and monitoring devices for X-ray tubes

Abstract

A power transfer and monitoring device for an X-ray tube may include: an X-ray filament; a transformer including a primary coil and a secondary coil, wherein the secondary coil of the transformer includes a first leg, a second leg, and a middle leg; a current supply configured to supply a sinusoidal current to the primary coil of the transformer; and a calculation unit configured to measure a primary current of the transformer, configured to determine a synthesized transformer magnetizing current, and configured to subtract the synthesized transformer magnetizing current from the primary current of the transformer to determine a value of filament current through the X-ray filament. The first and second legs of the secondary coil of the transformer alternately supply current to a first end of the X-ray filament. The middle leg of the secondary coil of the transformer supplies current to a second end of the X-ray filament.

Claims

1. A power transfer and monitoring device for an X-ray tube, the power transfer and monitoring device comprising: an X-ray filament; a transformer comprising a primary coil and a secondary coil, wherein the secondary coil of the transformer comprises a first leg, a second leg, and a middle leg; a current supply configured to supply a sinusoidal current to the primary coil of the transformer, which causes a sinusoidal current in the secondary coil; and a calculation unit configured to measure a primary current of the transformer, configured to determine a synthesized transformer magnetizing current, and configured to subtract the synthesized transformer magnetizing current from the primary current of the transformer to determine a value of filament current through the X-ray filament; wherein the first leg and the second leg of the secondary coil of the transformer supply current to a first end of the X-ray filament, wherein the middle leg of the secondary coil of the transformer supplies current to a second end of the X-ray filament, and wherein a rectification circuit is included between the secondary coil of the transformer and the X-ray filament, with a capacitor between the rectification circuit and the X-ray filament.

2. The power transfer and monitoring device of claim 1, further comprising: a regulator configured to supply direct current (DC) voltage to the current supply; wherein the calculation unit is further configured to measure the DC voltage from the regulator, and using the value of the filament current through the X-ray filament, to calculate a filament voltage.

3. The power transfer and monitoring device of claim 1, wherein the current supply comprises a resonant circuit configured to convert a supply waveform into the sinusoidal current supplied to the primary coil of the transformer.

4. The power transfer and monitoring device of claim 1, wherein the rectification circuit comprises a first diode connected to the first leg of the secondary coil of the transformer, a second diode connected to the second leg of the secondary coil of the transformer.

5. The power transfer and monitoring device of claim 1, wherein the calculation unit is further configured to calculate a filament voltage using the value of the filament current through the X-ray filament.

6. The power transfer and monitoring device of claim 1, further comprising: a regulator configured to supply direct current (DC) voltage to the current supply.

7. The power transfer and monitoring device of claim 1, wherein the capacitor is in parallel with an output of the rectification circuit.

8. The power transfer and monitoring device of claim 1, wherein the capacitor is in parallel with the X-ray filament.

9. The power transfer and monitoring device of claim 1, wherein the rectification circuit comprises a first diode connected between the first leg of the secondary coil of the transformer and the first end of the X-ray filament, a second diode connected between the second leg of the secondary coil of the transformer and the first end of the X-ray filament, and the middle leg of the secondary coil of the transformer connected to the second end of the X-ray filament.

10. The power transfer and monitoring device of claim 3, wherein the resonant circuit comprises a capacitor.

11. The power transfer and monitoring device of claim 6, wherein the calculation unit is further configured to measure the DC voltage from the regulator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described, by way of example, with reference to the drawings, of which

(2) FIG. 1 is a diagrammatic representation of the monitoring system according to an embodiment of the invention;

(3) FIG. 2 is a diagrammatic representation of an approximate equivalent circuit of the system of FIG. 1;

(4) FIG. 3 is a plot of load current of a dummy load against 2× the monitor output of the system of FIG. 1;

(5) FIG. 4 is a plot of the filament voltage against monitor output of the system of FIG. 1;

(6) FIG. 5 is a plot of the filament characteristic of a Thales THX 225 WA using a GX225 X-ray generator calculated from measurements using the system of FIG. 1; and

(7) FIG. 6 is a gain plot of NDI 225 FB Tube obtained from measurements using the system of FIG. 1.

DETAILED DESCRIPTION

(8) Referring to FIG. 1, power transfer and monitoring device 10 has regulator 100 supplying a direct current (DC) voltage to full bridge switching converter 102. The frequency of full bridge switching converter 102 is adjusted until resonance is achieved between C_Res 104 and the leakage inductance of transformer 106, so that a sinusoidal current of a single frequency is generated, that is, having no harmonic frequencies. Transformer 106 comprises primary coil 105 and secondary coil 107. Primary coil 105 includes first leg 105a and second leg 105b, while secondary coil 107 includes first leg 107a, second leg 107b, and middle leg 107c.

(9) As the transformer primary current is sinusoidal, it means that the transformer secondary current also is sinusoidal. The transformer secondary voltage is full wave rectified using, for example, rectification circuit 108, and smoothed using, for example, capacitor C_output 110, before being applied to the first and second ends 101a, 101b of the X-ray filament R_Load 103.

(10) Large leakage inductances are associated with transformers with a large isolation tolerance; to reduce the effects of leakage inductance, a sinusoidal current drive is used. The full bridge resonant converter allows a square wave voltage drive to be used to provide a sinusoidal current drive. The current wave-shape contains very few harmonics, and therefore there is a fixed relationship between the DC filament current and the transformer secondary current. If then the transformer primary current were measured, this would be proportional to transformer secondary current if it wasn't for the transformer magnetising current giving an error. To correct for this error, the transformer magnetising current is determined (or “synthesised”) and is subtracted from the transformer primary current to allow determination of the transformer secondary current and, hence, the filament current.

(11) The transformer primary current is measured using a current transformer terminated in resistor R_terminate 112. From the primary voltage, a signal which is proportional to the transformer magnetising current is produced. This is subtracted from the signal proportional to the transformer primary current in synthesis and subtraction unit 114, yielding a signal which is proportional to the transformer secondary current and, hence, the filament current. The resulting filament current monitor is then used in voltage scaling and subtraction unit 116 to yield the filament voltage. A calculation unit comprises synthesis and subtraction unit 114 and voltage scaling and subtraction unit 116. The relationships between I.sub.Load and I.sub.Secondary, I.sub.Primary, I.sub.Secondary and I.sub.Magnetising, and I.sub.Monitor and I.sub.primary and I.sub.Magnetising are as follows:
I.sub.Load∝I.sub.Secondary
I.sub.Primary∝I.sub.Secondary+I.sub.Magnetising
I.sub.Mon∝I.sub.Primary−I.sub.Magnetising

(12) The system model approximates to a voltage generator in series with a diode and resistor as shown in FIG. 2. If the voltage drop across the diode is known, and the value of R-cct is known together with the filament current, then by measuring VDC, the voltage across the filament can be deduced. To obtain these values, a measurement is carried out on the filament before EHT is applied. The filament characteristic is first plotted. Then, EHT is applied, and by calibrating the system back to the zero High Voltage measurement, a matching plot of the filament characteristic is produced.

(13) Once this calibration has been done once for a given filament, the voltage can then be measured continuously through the life of the filament. From this a continuous gain plot of the X-ray tube can be produced through to the life of the filament.

(14) As a filament wears, the core becomes thinner, until the point where it is destroyed. As the core becomes thinner, the filament resistance increases. If a measurement is taken at the start of life, and the filament is monitored throughout its life, the end of life can be predicted as the filament resistance starts to increase rapidly approaching failure.

(15) The accuracy of using this system to measure the filament current and voltage has been demonstrated by experiment. The actual current of a dummy load against measured I mon output is shown plotted in FIG. 3, showing that the measured I mon output does indeed accurately reflect the load current. FIG. 4 shows the actual load voltage plotted against the V mon output, where it can be seen that the actual and measured voltages correspond reasonably closely.

(16) FIG. 5 shows a filament characteristic; the squares line shows measurements that have been done directly (No EHT applied), whereas the circular-dot line shows the characteristic measured using the monitor outputs with EHT applied. FIG. 6 shows a gain plot of an X-ray tube produced with EHT applied, and using the monitor outputs.