JP5157743B2 - X-ray thickness measuring device - Google Patents

Description

  The present invention irradiates a sheet member with radiation emitted from an X-ray tube having a first grid and a second grid, the tube current of which is controlled by the first grid voltage, and based on the transmission attenuation amount, the sheet The present invention relates to an X-ray thickness measuring device that measures the thickness (basis weight) of a member.

  FIG. 5 is a measurement principle diagram of a thickness (basis weight) meter. The basis weight meter is a detector that utilizes transmission absorption of radiation (β rays). This detection method is suitable for measuring the area weight of a general sheet member (plastic film, sheet, etc.) because the mass absorption coefficient of β-ray shows an almost constant absorption coefficient for any substance. .

A predetermined dose of radiation X1 emitted from the X-ray tube 2 is irradiated from the lower part of the sheet member 1, and the radiation X2 transmitted through the sheet member 1 is measured by the ionization chamber 3. When the measured value of the ionization chamber 3 is R, and the measured value of the ionization chamber 3 when there is no object to be measured is R0, R = R0 · exp (−μρt). Here, t is the thickness [m] of the sheet member 1, ρ is the density [g / m ³ ], and μ is the mass absorption coefficient (m ² / g).

Therefore, if μ is known, the basis weight tρ (g / m ³ ) is obtained by measuring the transmitted radiation dose, and from this, the thickness t can be measured. The thickness profile data of the sheet member 1 can be generated by moving the sheet member 1 in the P direction at a predetermined speed and scanning the basis weight unit in the direction orthogonal to P.

  FIG. 6 is a characteristic diagram showing the relationship between the thickness t of the sheet member 1 and the transmitted radiation dose R. An approximate characteristic curve F including intersection points C1, C2, and C3 of the transmitted radiation doses R1, R2, and R3 with respect to the sheet member having known thicknesses t1, t2, and t3 is generated, and an unknown thickness t is determined based on the characteristic curves. taking measurement.

  FIG. 7 is a functional block diagram showing a configuration example of a conventional X-ray thickness measuring apparatus. The same components as those described in the measurement principle diagram of the thickness (basis weight) meter in FIG. The X-ray tube 2 has a general triode configuration, and includes a heater 21, a cathode 22, a target 23, a first grid 24 for controlling tube current, and a second grid 25 for thermoelectron convergence.

  The heater 21 is connected to the fixed voltage source 4 and heats the cathode 22. The target 23 is connected to the high voltage source 5, and the radiation X <b> 1 is emitted through the target 23 by thermoelectrons emitted from the cathode 22 and colliding with the target 23. A fixed voltage source 6 is connected to the second grid 25.

  The tube current Ix flowing from the target 23 toward the cathode 22 flows through the tube current detection resistor 7 connected between the cathode 22 and the reference potential, is converted into a detection voltage Vx, and is input to the comparator 9 via the buffer 8. The

  The comparator 9 compares the detection voltage Vx of the tube current Ix with the tube current setting voltage Vi, and feedback-controls the variable voltage source 10 that supplies the first grid voltage Vg1 to the first grid 24 based on the deviation. By this feedback loop, the tube current Ix is controlled to a predetermined value determined by the tube current setting voltage Vi.

  As described above, Patent Document 1 discloses a known technique for controlling the tube current Ix at a constant value by operating the first grid voltage Vg1 and controlling the radiation dose X1 emitted from the X-ray tube 2. is there.

  FIG. 8 is a life curve of an X-ray tube having variations. The standard life curve is indicated by the solid line F. The first grid voltage Vg1, which maintains the tube current at a predetermined value, decreases by a predetermined curve from the standard initial value at the start of use of the X-ray tube, and becomes uncontrollable when the control allowance becomes zero at time T1, It exhibits VI characteristics that will end its life.

In an X-ray thickness measuring device used as a sensor in a process control system in a plant, since it is greatly spread to the plant due to a failure stop, it is necessary to perform replacement maintenance before a predetermined time until the life of the X-ray tube is exhausted.
Japanese Patent Laid-Open No. 5-188018

  As shown in FIG. 8, the X-ray tube has a variation in the life curve due to the variation of the VI characteristic, and has a predetermined instrumental difference in the range from F ′ to F ″ with respect to the standard life curve F. The lifetime varies from T1 ′ to T1 ″. Also, variation in VI characteristics affects measurement accuracy.

  Since it is structurally difficult to suppress the instrumental difference in the VI characteristics of the X-ray tube, conventionally, a method of offsetting the instrumental error by shifting the control start voltage of the first grid voltage Vg1 for each X-ray tube has been adopted. .

  For this reason, the control start voltage of the first grid varies depending on the individual, which leads to the fact that the control cost of the first grid voltage Vg1 varies depending on the individual, resulting in a difference in the lifetime, and the standardization management of the lifetime Is hindering.

  Therefore, the replacement maintenance time of the X-ray tube is different individually, and the maintenance becomes extremely complicated. For this reason, maintenance is performed in which a safety margin is taken and replaced at the same time within a predetermined period of time, and a wasteful use of resources for exchanging still usable X-ray tubes occurs.

  The present invention has been made to solve the above-mentioned problems, and it is possible to normalize the control margin of the first grid voltage of the X-ray tube, suppress the difference in VI characteristics, and standardize the life prediction. The purpose is to realize an X-ray thickness measuring apparatus.

In order to achieve such a subject, the present invention has the following configuration.
(1) The sheet member has a first grid and a second grid, and the sheet member is irradiated with radiation emitted from an X-ray tube whose tube current is controlled by the first grid voltage, and based on the transmission attenuation amount. In an X-ray thickness measuring apparatus for measuring the thickness of
A variable voltage source connected to the second grid ;
An X-ray thickness measuring apparatus , wherein the voltage of the variable voltage source is set so that the first grid voltage becomes a standard initial value while the tube current is maintained at a predetermined value .

( 2 ) The apparatus according to ( 1 ), further comprising second grid voltage setting means for automatically setting the voltage of the variable voltage source so that the radiation dose emitted from the X-ray tube becomes a standard value. X-ray thickness measuring device.

( 3 ) The X-ray thickness measuring apparatus according to (1), further comprising display means for displaying a value of a radiation dose emitted from the X-ray tube.

According to the configuration of the present invention, the following effects can be expected.
(1) The second grid is connected to a variable voltage source, and by setting the voltage, the control start voltage of the first grid at a predetermined tube current is set to a standard initial value, thereby the first grid voltage Vg1 of the X-ray tube. The control allowance can be normalized to suppress instrumental differences in VI characteristics.

(2) The lifetime prediction of the X-ray tube can be standardized based on the time transition of the first grid voltage.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a functional block diagram showing an embodiment of an X-ray thickness measuring apparatus to which the present invention is applied. The same elements as those of the conventional apparatus described with reference to FIG.

The characteristic part of the present invention with respect to the conventional configuration is that a variable voltage source 100 connected to the second grid 25 is provided. The second grid 25 has a thermoelectron convergence function and is connected to the variable voltage source 100 . The present invention focuses on the fact that the VI characteristics of the X-ray tube can be adjusted within a predetermined range by setting this voltage.

  The embodiment of FIG. 1 shows the simplest adjustment method, and the voltage of the variable voltage source 100 is set by a manual operation by the operator 200. Prior to setting the voltage of the variable voltage source 100, the tube current setting voltage Vi is set so that the tube current Ix becomes a predetermined standard value. The first grid voltage Vg1 in this state varies depending on the X-ray tube.

  While checking the first grid voltage Vg1 with an appropriate voltage measuring means, the operator 200 completes the adjustment operation by a simple operation that is set by manual operation of the variable voltage source 100 so that the voltage Vg1 becomes the standard control start voltage. .

  FIG. 2 is a life characteristic diagram of an X-ray tube showing the concept of instrumental error adjustment according to the present invention. By adjusting the setting of the second grid voltage Vg2 of the X-ray tube having the life curves F ′ and F ″ having variations with respect to the standard life curve F, the control start voltage of the first grid voltage Vg1 is set to the standard initial value. Match.

  As a result, the VI characteristics of F ′ and F ″ can be matched with the standard characteristic F, the control margin of the first grid voltage Vg1 of the X-ray tube is normalized, the instrumental error of the VI characteristics is suppressed, and the life prediction is standardized. Is possible.

  FIG. 3 is a functional block diagram showing a main configuration of another embodiment of an X-ray thickness measuring apparatus to which the present invention is applied. In the embodiment of FIG. 1, it is assumed that the radiation emitted from the X-ray tube is controlled to a predetermined value by controlling the tube current to a predetermined value.

  In the embodiment of FIG. 3, the radiation dose X1 emitted from the X-ray tube 2 is measured with the ionization chamber 300 in a state where the first grid voltage Vg1 is maintained at the standard initial value, and the measured value R becomes the standard value. As described above, the second grid voltage setting means 400 for automatically setting the voltage of the variable voltage source 100 is provided.

  The second grid voltage setting means 400 is an R / V converter 401 that converts the measurement value R of the ionization chamber 300 into a voltage value Vr, compares the voltage value Vr with the standard radiation dose setting value Vs, and is variable based on the deviation. It comprises a comparator 402 that feedback controls the voltage of the voltage source 100.

  FIG. 4 is a functional block diagram showing a main configuration of still another embodiment of the X-ray thickness measuring apparatus to which the present invention is applied. In this embodiment, the radiation amount X1 emitted from the X-ray tube 2 is measured by the ionization chamber 300 while the first grid voltage Vg1 is maintained at the standard initial value, and the voltage value Vr of the R / V converter 401 is determined. Displayed on the display means 403.

The displayed voltage value Vr is monitored by the operator 200, and the voltage of the variable voltage source 100 that provides the second grid voltage Vg2 is set by manual operation so that the voltage value Vr becomes the standard value Vs.

  The X-ray tube to which the present invention is applied may be any of a transmission type, a reflection type, an open type, and a sealed type as long as it is an X-ray tube having a second grid. The radiation emission form may be any of an X-ray tube that transmits radiation through the target and an X-ray tube that generates radiation by reflecting from the target.

It is a functional block diagram which shows one Embodiment of the X-ray-type thickness measuring apparatus to which this invention is applied. It is a life curve of the X-ray tube which shows the concept of instrument difference adjustment by this invention. It is a functional block diagram which shows the principal part structure of other embodiment of the X-ray-type thickness measuring apparatus to which this invention is applied. It is a functional block diagram which shows the principal part structure of further another embodiment of the X-ray-type thickness measuring apparatus to which this invention is applied. It is a measurement principle figure of a thickness (basis weight) meter. It is a characteristic view which shows the relationship between the thickness of a sheet member, and a transmitted radiation dose. It is a functional block diagram which shows the structural example of the conventional X-ray-type thickness measuring apparatus. It is a life curve of an X-ray tube with variation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sheet member 2 X-ray tube 21 Heater 22 Cathode 23 Target 24 1st grid 25 2nd grid 3 Ionization chamber 4 Heater power supply 5 Target power supply 7 Tube current detection resistor 8 Buffer 9 Comparator 10 Variable voltage source 100 Variable voltage source 200 Operator

Claims (3)

A sheet member is irradiated with radiation emitted from an X-ray tube having a first grid and a second grid, the tube current of which is controlled by the first grid voltage, and the thickness of the sheet member is based on the transmission attenuation amount. In an X-ray thickness measuring apparatus for measuring
A variable voltage source connected to the second grid ;
An X-ray thickness measuring apparatus , wherein the voltage of the variable voltage source is set so that the first grid voltage becomes a standard initial value while the tube current is maintained at a predetermined value . The X-ray system according to claim 1, further comprising second grid voltage setting means for automatically setting the voltage of the variable voltage source so that the radiation dose emitted from the X-ray tube becomes a standard value. Thickness measuring device. The X-ray thickness measuring apparatus according to claim 1, further comprising display means for displaying a value of a radiation dose emitted from the X-ray tube.

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