JPS5939877B2 - X-ray tube filament temperature control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an X-ray tube filament temperature control method having a feedback circuit for maintaining a tube current at a set tube current value in an X-ray imaging apparatus. The purpose of this invention is to mutually change the transfer characteristics of the feedback circuit for heating and cooling the filament.

In order to always obtain a stable X-ray dose in an X-ray imaging apparatus, it is essential to keep the tube current of the X-ray tube constant, and on the other hand, the tube current largely depends on the filament humidity.

Therefore, in general, in a tube current control feedback system, when the tube current decreases below the set tube current value, the filament is heated to promote an increase in humidity, and when the tube current increases, the filament is cooled to lower the temperature, but when the filament is heated, Changes in humidity depend on the energy supplied to the filament (supplied power), and as the supplied energy increases, the temperature rise curve rises rapidly.However, changes in humidity when cooling the filament depend on the heat loss (radiation, heat conduction) of the filament. etc.), and the rate of change in humidity is different between the two.

In view of this point, the present invention changes the transfer characteristics of the feedback circuit to an optimal value when heating the filament and when cooling the filament,
This allows for the most appropriate filament temperature control and tube current control.

In the drawing, 1 is an AC power supply, 2 is an emission switch, 3 is a high voltage generation circuit, 4 is a high voltage transformer, 5 is a current voltage converter, 6 is an X-ray tube, 7 is a filament transformer, 8
9 is a filament drive circuit, 9 is a tube current error detection circuit, 10 is a Miller integration circuit, 11 is a preheat circuit, 9
11 constitute a feedback circuit 12.

Further, the tube current error detection circuit 9 includes a comparator A1 and a reference voltage source E.
1, the tube current fed back from the X-ray tube 6 side after being converted into voltage by the current-voltage converter 5 is compared with the reference voltage source E, and the input voltage of the comparator A1 (corresponding to the tube current) is compared with the reference voltage source E. It is configured to supply a negative output to the Miller integration circuit 10 when the voltage (1) is higher than the reference voltage, and a positive output when vice versa.

The Miller integrating circuit 10 has a series connection body of a forward direction diode D1 and an input resistor R1, and a series connection body of a reverse direction diode D2 and an input resistance R2. An integrating capacitor C1 and an amplifier A2 are connected here.

The preheating circuit 11 includes a preheating reference voltage source E2 and an adder A3.
The output of the Miller integration circuit is added to this reference voltage, and the added output is input to the filament drive circuit 8 to vary the primary side voltage of the filament transformer 7.

The switch SW1 in the tube current error detection circuit 11 is linked to the emission switch 2, and only when the emission switch 2 is turned on, the reference voltage source E0 is connected to the comparator A.
It is set to connect to 1.

When the emission switch 2 is turned on, a high voltage is applied to the X-ray tube 6 by a well-known circuit operation via the circuits 3, 4.5, and a tube current flows.

This tube current is converted into a voltage corresponding to the tube current value by the current-voltage converter 5 as described above, and then led to the tube current error detection circuit 9.

In the comparator A1, a comparison is made with the input voltage corresponding to the actual tube current using the reference voltage source E1 corresponding to the set tube current, and if the input voltage is higher than the reference voltage (i.e. the tube current is higher than the set tube current) If the input voltage is lower than the reference voltage (i.e., the tube current is smaller than the set tube current value), a negative output is sent to the Miller integration circuit 10. is sent to the Miller integration circuit 10.

In the Miller integration circuit 10, when the input signal is positive, current flows to the forward direction diode D1 side, and when the input signal is negative, current flows to the reverse diode D2 side with an integration constant determined by the resistor R1 and capacitor C0. flows, and a required integral operation is performed with an integral constant determined by resistor R2 and capacitor C1.

In other words, the integration constant of the Miller integration circuit 10 is automatically switched depending on whether the input signal is positive or negative.

This Miller integration circuit output is sent to the preheat circuit 11 and added to the reference voltage source E2.

If the output obtained by the tube current error detection circuit 9 is negative and therefore the actual tube current value is lower than the set tube current value, the filament drive circuit 8 is activated to the primary side of the filament transformer 7 by the addition output. The voltage is increased to heat the filament and increase the tube current toward the set value.

Further, the output obtained from the tube current error detection circuit 9 is positive,
Therefore, when the actual tube current value is higher than the set tube current value, the output of the Miller integral circuit works in a subtractive manner in the preheat circuit 11, and the filament drive circuit 8 suppresses the primary side voltage of the filament transformer 7 to cool the filament. Decrease the tube current toward the set value.

Through such circuit operation, the tube current of the X-ray tube 6 is always maintained at the set value.

The present invention is used as described above, and in the feedback circuit including the tube current error detection circuit 9, the Miller integration circuit 10, and the preheat circuit 11, the integral constant of the Miller integration circuit 10, that is, the feedback system The transfer characteristics are varied according to the positive and negative conditions of the output of the tube current error detection circuit 9, and are changed when the filament is heated to increase the tube current and when the filament is cooled to decrease the tube current. There is.

Therefore, the integration factor determined by resistor R2 and capacitor C1 should be set to the optimal value for heating the filament, and the integration factor determined by resistor R1 and capacitor C1 should be set to the optimal value for cooling the filament. For example, the feedback system transmission characteristics change when the filament is heated and when the filament is cooled, so that the characteristics when the filament is heated and cooled are maintained appropriately.

Therefore, the present invention is useful in a device that controls the filament temperature to keep the tube current constant and obtain a stable X-ray dose because it can be controlled to match the optimum temperature change rate during filament heating and cooling. be.

In other words, in the conventional feedback circuit, the transfer characteristics of the feedback circuit were determined without considering the difference in the rate of change in filament humidity between heating and cooling the filament, which was mentioned at the beginning. It was supposed to worsen the temperature characteristics.

According to the present invention, such inconveniences can be eliminated.

[Brief explanation of drawings]

The drawing is an electric circuit showing one embodiment of the present invention. 6...X-ray tube, 12...Feedback circuit.

Claims (1)

[Claims] 1. In a feedback circuit that compares the tube current of the X-ray tube with a set tube current and controls the temperature of the filament based on this comparison value, the transfer characteristics of this feedback circuit are made different between when the filament is heated and when the filament is cooled. An X-ray tube filament temperature control method.