DE4300825A1 - X=ray tube current control - Google Patents

Abstract

X-ray generator has a control circuit for the X-ray tube current, where the heating current actual value at the X-ray tube (1) is registered and compared (6) with a nominal value. The output signal from the comparator (6) acts through a regulator (4) to bring it into line with the nominal value. In addition, the output signal from the comparator is passed to an active limit (10) to contain the heating current to a max. value which only occurs when the heating current actual value exceeds the max. value for a given duration.The control circuit has an integral regulator (11) with, at its input, a comparator (14) for the heating current actual value with max. value, and the output signal of a limit (12), for the output signal of the integral regulator (11).

Description

The heater is used to regulate the current of an X-ray tube current of the cathode of the x-ray tube varies, causing the Electron emission of the cathode is controlled. Since the Ka method reacts relatively sluggishly due to its heat capacity, the X-ray tube can be used to accelerate temperature changes for a short time with increased heating currents the. This overheating may depend on the amount of heating Stromes last only a short time, otherwise the filament of the Cathode is destroyed. So there must be a limitation of the Heating current of the X-ray tube take place depending on the Excessive heating current is delayed in the regulation and the heating current reaches its maximum permissible value limited. This heating current limitation should also be in the error take effect, z. B. in the event of anode voltage failure the x-ray tube, which means that no actual value for the x-ray tube Renstrom more is available and thus the controller for the X-ray tube current impermissibly high setpoints for the Heating current controller specifies.

The invention has for its object an Röntgenge nerator of the type mentioned so that Ab the X-ray tube current and thus the heating current from the setpoint by overheating the filament be quickly compensated for, with a sure limitation of the heating current to a maximum value.

This object is achieved by an X-ray gene generator with a control circuit for the X-ray tubes current in which the actual heating current value of the X-ray tube is recorded and compared in a comparator with a target value is, the output signal of the comparator heating Actual current value via a controller in the sense of an adjustment  influenced to the setpoint and the comparator additionally the output signal of an active limiter for limiting the heating current to a maximum value which only occurs when the heating current Actual value exceeds the maximum value for a certain duration steps.

The invention is based on one in the drawing illustrated embodiment explained in more detail. It demonstrate:

Fig. 1 shows an X-ray generator with a heating current control according to the invention, and

Fig. 2 is a circuit-technical detail of the heating current control of FIG. 1.

In Fig. 1, an X-ray tube 1 is shown, which has a Ka method 2 and an anode 3 . To control the roentgen genröhrenstromes the heating current for the cathode 2 is used as a manipulated variable and in a control circuit with a controller 4 , a downstream amplifier 5 , an upstream th comparator 6 and a measuring amplifier 7 for the heating current actual value signal supplied by egg nem encoder 8 ge regulates. At the input 9 of the comparator 6 there is a signal corresponding to the heating current setpoint I _hsoll .

To limit the heating current to a maximum value, an active limiter 10 with an integral controller 11 , a limiter 12 and a characteristic element 13 is provided. A comparator 14 is connected upstream of the integral controller 11 and receives a signal corresponding to the maximum value of the heating current I _hmax at the input 15 .

In normal operation, ie without exceeding the predetermined maximum value, I _{hmax is} greater than I _hist and the integral controller 11 thus integrates up to a minimum value specified by the limiter 12 . The characteristic member 13 suppresses all output values that are less than a predetermined threshold, so that the integral controller 11 has no intervention in the heating current control circuit 2 , 4 to 9 . The output signal of the characteristic element 13 is zero in this case.

If the actual heating current value I _{hist exceeds} the predetermined maximum value I _hmax , the integral controller 11 begins to integrate from the negative limiter value. As long as the output signal from the integral controller 11 is smaller than the threshold of the characteristic element 13 , the output signal of the characteristic element 13 remains at zero. The time until the threshold is reached depends on the one hand on the integrator time constant T _i and on the other hand on the difference between I _hmax and I _hist . The greater the difference, the shorter the time to reach the threshold. Exactly this behavior is desirable and corresponds to the fact that the filament of the cathode 2 may only be briefly exposed to the increased current load.

If the threshold of the characteristic element 13 is reached, the heating current control circuit 2 , 4 to 9 of the control circuit with the integral controller 11 and the target-actual comparator 14 is superimposed. Due to the intervention of the integral controller 11 , the setpoint value is reduced in the heating current control circuit 2 , 4 to 9 until I _{hist is} equal to I _hmax . This transition follows the time course of a PT ₁ element. The time constant of the PT ₁ behavior is predetermined by the amplification factor of the characteristic element 13 and the time constant of the integral controller 11 and thus adjustable.

Advantage of this arrangement is the fact that the heating current controller 2 , 4 to 9 is included in the limitation and thus z. B. the "overflow" of the integral part is prevented. In addition, a variation of the I _hmax value is easily possible, since this z. B. can be specified as a voltage.

In Fig. 2 a possible implementation of the limiter circuit with operational amplifiers is shown.

Claims (2)

1. X-ray generator with a control circuit for the X-ray tube current, in which the actual heating current value of the X-ray tube ( 1 ) is detected and compared in a comparator ( 6 ) with a desired value, the output signal of the comparator ( 6 ) the actual heating current value via a Controller ( 4 ) influenced in the sense of an adjustment to the setpoint and the comparator ( 6 ) additionally the output signal of an active limiter ( 10 ) for limiting the heating current to a maximum value is supplied, which only occurs when the actual heating current value exceeds the maximum value for a certain duration. 2. X-ray generator according to claim 1, which contains a control circuit with an integral controller ( 11 ) at its input a comparator ( 14 ) for the actual heating current value with the maximum value and the output signal of a limiter ( 12 ) for the output signal of the integral controller ( 11 ) This output signal is fed to a characteristic element ( 13 ) which is connected to the comparator ( 6 ) of the control circuit for the heating current.