X-RAY TUBE MONITORING

Abstract

A power transfer and monitoring system for an X-ray tube includes a transformer including a primary coil and a secondary coil, a current supply that supplies a sinusoidal current to the transformer, and a current calculation unit which measures the primary current of the transformer, and synthesises the transformer magnetising current, and which to subtract the synthesised transformer magnetising current from the primary current to generate a value for the filament current.

Claims

1. A power transfer and monitoring system for an X-ray tube, including: a transformer including a primary coil and a secondary coil; a current supply that supplies a sinusoidal current to the transformer; a current calculation unit which measures the primary current of the transformer, and synthesises the transformer magnetising current, and which to subtract the synthesised transformer magnetising current from the primary current to generate a value for the filament current.

2. A power transfer and monitoring system of claim 1, wherein there is included a voltage calculation unit which measures the voltage at the regulator output, and using the calculated filament current, to calculate the filament voltage.

3. A power transfer and monitoring system of claim 1, wherein there is included a resonant circuit which converts a high voltage supply waveform into the sinusoidal current supplied to the primary coil.

4. A power transfer and monitoring system according to claim 1, wherein there is provided a rectification circuit between the secondary coil and any X-ray tube connected to the secondary circuit.

5. A power transfer and monitoring system according to claim 4, wherein the rectification circuit includes two diodes connected to terminal legs of the secondary coil to supply a first contact of an X-ray tube, and a middle leg of the secondary coil to supply a second contact of the X-ray tube.

Description

[0012] The invention will now be described, by way of example, with reference to the drawings, of which

[0013] FIG. 1 is a diagrammatic representation of the monitoring system according to an embodiment of the invention;

[0014] FIG. 2 is a diagrammatic representation of an approximate equivalent circuit of the system of FIG. 1;

[0015] FIG. 3 is a plot of load current of a dummy load against 2 the monitor output of the system of FIG. 1

[0016] FIG. 4 is a plot of the filament voltage against monitor output of the system of FIG. 1

[0017] 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

[0018] FIG. 6 is a gain plot of NDI 225 FB Tube obtained from measurements using the system of FIG. 1.

[0019] Referring to FIG. 1, a regulator supplies a dc voltage to a full bridge switching converter. The frequency of the full bridge converter is adjusted until resonance is achieved between C_RES and the leakage inductance of the transformer, so that a sinusoidal current of a single frequency is generated, that is, having no harmonic frequencies.

[0020] As the transformer primary current is sinusoidal it means that the secondary current is also sinusoidal. The transformer secondary voltage is full wave rectified and smoothed before being applied to the filament.

[0021] 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 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 subtracted from primary current to allow determination of the secondary and hence filament current.

[0022] The transformer primary current is measured using a current transformer terminated in resistor R terminate. 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 primary current, yielding a signal which is proportional to secondary, and hence filament current. The resulting filament current monitor is then used in a voltage scaling and subtraction unit to yield the filament voltage. 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: [0023] I.sub.LoadI.sub.Secondary [0024] I.sub.PrimaryI.sub.Secondary+I.sub.Magnetising [0025] I.sub.MonI.sub.PrimaryI.sub.Magnetising

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.