JPS6213359Y2 - - Google Patents

Description

[Detailed explanation of the idea] This invention relates to an X-ray tube current measuring circuit.

The high-voltage circuit of an X-ray device includes the capacitance of the high-voltage cable (approximately 250 pF/m) and the stray capacitance of the high-voltage transformer. Therefore, in general, when detecting tube current at the neutral point on the secondary side of a high-voltage transformer, a charge current for the above capacity flows at the time of startup, so
The tube current signal is transformed as shown in FIG. 1, resulting in a so-called "popout". This "pop-out" varies depending on the length of the high-voltage cable, but the height is 2 to 5 times the steady value, and the period is 1
It will be about ~2msec. Therefore, if this signal is used as a tube current signal as it is, a large error will occur, so conventionally, while this "pop-out" is occurring, a gate signal created in another circuit is used to gate the "pop-out" signal. During this period, no measurement is made, and after that, the amount for the period during which no measurements were taken is added to the tube current value measured using this tube current signal. Therefore, the circuit configuration was complicated.

The object of the present invention is to provide an X-ray tube current measurement circuit that is extremely simple and does not require signals from other parts, and can eliminate errors caused by charge current due to stray capacitance, etc. purpose.

An embodiment of the present invention will be described below with reference to FIG. In Figure 2, the high voltage generated in the secondary windings 1 and 2 of the high voltage transformer is transferred to the rectifier circuit 3.
1 and 32, and then applied between the cathode and anode of the X-ray tube 4. An X-ray tube current detection resistor 5 is connected to the neutral point of this high voltage circuit, and the voltage generated at one end of this resistor 5 is sent to an operational amplifier 6 as a tube current signal. The tube current signal amplified in this way is sent to the non-inverting input terminal of the operational amplifier 11 via a voltage dividing circuit made up of resistors 8 and 9 and a variable resistor 10, and is also sent to the differentiating circuit 7. This differentiating circuit 7 consists of a capacitor 71 and a resistor 72,
This differential output passes through an input resistor 12 to an operational amplifier 1.
1 is sent to the inverting input terminal. The resistor 13 is a feedback resistor. The output of the operational amplifier 11 is sent to a voltage-frequency converter 14, and the pulses generated here are counted by a preset counter 15.

The tube current signal (signal at point b) output from the operational amplifier 6 has a "jump" due to the charge current at the time of rise, as shown by the dashed line b in FIG. Since this signal is differentiated by the differentiating circuit 7, the inverted waveform of the differentiated output (signal at point a) becomes as shown by the broken line a in FIG. Since the signal at point a is applied to the inverting input terminal of the operational amplifier 11, it can be considered that the inverted signal is added. Therefore, if the constant in the differentiating circuit 7 is appropriately selected in relation to the stray capacitance of the high voltage circuit, it is possible to cancel the charge current.
In one example tested by the present inventor, when using a high voltage cable of about 12 m, the capacitor 71 was 0.08μ
When F and resistance 72 are set to kΩ, the charge current can be canceled.

When the charge current is canceled in this way, the output of the operational amplifier 11 becomes as shown by the solid line c in FIG. 3, indicating an accurate tube current value that is not affected by stray capacitance. Therefore, voltage-frequency converter 1
4 outputs pulses with a frequency proportional to the input voltage, and since the preset counter 15 counts these pulses, the total counted value is the product of the tube current value and the imaging time, that is, corresponds to the mAs value. By presetting the count value corresponding to the desired mAs value in the preset counter 15, the output can be output from the preset counter 15 when all the count values reach the preset value. It can be used as
Furthermore, by dividing this total count value by the imaging time at that time, the tube current (mA) value can be obtained.

Note that in the above example, the tube current signal is directly differentiated and cancellation is performed using this differentiated output, but another circuit generates a rectangular wave, etc., and differentiates this, and the differentiated output is inverted by the operational amplifier 11. It may be configured to cancel in addition to the input terminal.

As described above with respect to the embodiments, according to the present invention, the influence of stray capacitance existing in a high voltage circuit can be removed with an extremely simple circuit, and the X-ray tube current can be accurately measured. Moreover, it is inexpensive because it does not require a gate circuit or a gate signal from another source. Particularly when the imaging time is short, the proportion of time in which the charge current is flowing increases, so canceling this has an extremely large effect in practice. It is also possible to cancel pulse-like noise superimposed on the tube current signal.

[Brief explanation of the drawing]

Fig. 1 is a time chart showing the waveform of the tube current signal, Fig. 2 is a block diagram showing an embodiment of the present invention, and Fig. 3 is a time chart showing the voltage waveform at points a, b, and c in Fig. 2. It is. 1, 2...Secondary winding of high voltage transformer, 31, 32
... Rectifier circuit, 4 ... Set counter.

Claims (1)

[Scope of utility model registration request] The tube current signal detected at the neutral point of the high voltage circuit for applying high voltage to the X-ray tube is input to the differentiator circuit,
Subtract this differential output from the original tube current signal,
An X-ray tube current measuring circuit characterized in that a tube current is measured using a signal after subtraction.