JPH038688B2 - - Google Patents

Description

Detailed Description of the Invention The present invention uses an electromagnetic ultrasonic generator to propagate ultrasonic waves from a material to be measured, detects reflected ultrasonic waves from the back side on the front side, and detects the reflected ultrasonic waves based on the detection time of the reflected ultrasonic waves. This invention relates to a method for measuring the thickness of a material to be measured.

As a method of measuring the thickness of hot steel materials, etc., an electromagnetic ultrasonic generator is used to propagate ultrasonic waves from the surface of the material to be measured, and the ultrasonic waves that are reflected back and forth between the front and back surfaces are detected on the front side. A method is known in which the thickness D is determined by substituting the detection times of successive reflected waves among the detection times at the time into the following equation (1).

D=(T _o+1 −T _o )・V/2 ...(1) However, Tn: Time from the start of ultrasonic propagation to detection of the nth reflected ultrasonic wave T _o+1 : Time of start of ultrasonic propagation Time from to detection of the n+1th reflected ultrasound V: Sound velocity in the material to be measured If there are defects or impurities in the material to be measured other than the ultrasound reflected from the back surface, the reflection from them Ultrasonic waves are also detected and can be confused with reflected waves from the back surface, so the threshold value is set to a level lower than the output value of the reflected ultrasound waves from the back surface, but higher than the output value of the reflected ultrasound waves from defects or impurities. The thickness is measured by setting a dead zone between the transmitted waves and the received waves that do not require detection. However, even in such cases, high-power reflected ultrasound waves other than the reflected ultrasound waves from the back surface may be detected exceeding the threshold, or the dead zone may have a margin due to fluctuations in the thickness of the material being measured. Therefore, waves other than the reflected ultrasonic waves from the back surface may be detected outside the dead zone, that is, reflected waves from defects near the back surface, etc., and in this case, the The error was larger when the thickness was thinner than when it was thicker. In other words, since the dead zone is set taking into account the degree of separation between signals, the distance from the back surface of the defective part where the reflected wave can be detected is not much different even for thin objects than for thick objects, resulting in relatively low accuracy. It is.
Therefore, even if a threshold value and a dead zone are set and measured, it cannot be said that this alone is sufficient.

The present invention has been made in view of the above circumstances, and in addition to setting a threshold value as in the conventional method, a dead zone is set for each of a plurality of reflected waves used for measurement, and furthermore, from the start of ultrasonic propagation to the reflection from the back surface. By performing correction calculations based on the fact that the time it takes to detect an ultrasonic wave is an integer multiple of one round trip time for any reflected wave, thickness measurement can be performed with high precision, especially when the thickness of the material to be measured is thin. The purpose is to provide a method that can measure the

The thickness measuring method according to the present invention propagates electromagnetic ultrasonic waves from the surface of the material to be measured and detects the reflected ultrasonic waves from the back side on the front side. Each time a reflected ultrasound exceeding a threshold is detected, a predetermined time is set as a dead zone, and the time from the start of ultrasound propagation of the detected reflected ultrasound is T _o (n is a subscript indicating the number of reflections on the back surface). This time is used to calculate the thickness D using the following formula.

D=T _o・V/2・N or D=(T _o +T _o+1 +…+T _o+n・V/2・(m+1)・
(N+m/2) However, V: Sound velocity in the material to be measured N: Correction value for the number of reflections n on the back surface (= ^* [T _o /T _o+1 −T _o ]) ^* [ ]: Values in [ ] A symbol representing an integer value closest to a numerical value m: Natural number Next, the present invention will be specifically explained based on the drawings.
FIG. 1 is a schematic diagram showing the implementation state of the present invention,
In the figure, 1 indicates an electromagnetic ultrasonic wave generator. The electromagnetic ultrasonic transmitter 1 generates a magnetic field parallel to the surface of the steel pipe P and generates an induced current on the surface of the steel pipe P.
It detects the current on the surface, and a steel pipe P is transported in the axial direction below it.

The electromagnet of the transmitting/receiving section 1 includes an iron core 2 having a C-shaped cross section and excitation coils 3a and 3b installed in the middle part thereof.
The opening 2a sandwiched between the opposing tips of the iron core 2 faces the steel pipe P, and a DC power source (not shown) is connected to the excitation coils 3a and 3b. Therefore, the end of the opening 2a of the iron core 2 serves as a magnetic pole and applies an axial DC magnetic field to the portion of the steel pipe P facing the opening 2a. A ring-shaped transmitting coil 4a and a ring-shaped receiving coil 4b having a larger diameter than the ring-shaped transmitting coil 4a are concentrically assembled in the opening 2a, and are arranged with their axial lengths aligned with the radial direction of the steel pipe P. has been done. A pulse current generating circuit 6 is connected to the transmitting coil 4a, and a pulse current based on a trigger signal from the synchronous pulse generating circuit 5 is applied.

The DC magnetic field generated by the excitation coils 3a and 3b and the pulsed current generate electromagnetic ultrasonic waves, which is known per se. In other words, when an axial DC magnetic field is applied to the portion directly below the coils 4a and 4b of the steel pipe P by the excitation coils 3a and 3b, and a pulse current is applied to the transmitter coil 4a, the magnetic flux in the radial direction of the steel pipe P changes, and this magnetic Eddy currents are generated on the surface of the steel pipe P as the speed changes. Lenz force is generated by this eddy current and the previously applied DC magnetic field in a direction parallel to the surface of the steel pipe P, resulting in strain that changes in a direction perpendicular to the surface of the steel pipe P (Fleming's left-hand rule). The strain is generated and the strain propagates in a direction perpendicular to the surface of the steel pipe P. That is, longitudinal ultrasonic waves are generated from the surface of the steel pipe P. This ultrasonic wave propagates inside the steel pipe P and is reflected on the inner circumferential surface of the steel pipe P, and this reflected ultrasonic wave is transmitted to the receiving coil 4 by the process opposite to that described above (Fleming's right-hand rule).
b is detected as an induced voltage generated by an eddy current.

The terminal voltage of the receiving coil 4b is applied to an amplifier 7, where it is amplified, and the amplified signal is subjected to envelope detection by a synchronous detector 8 (FIG. 2A) and applied to a time difference measuring circuit 9.

The time difference measuring circuit 9 is the n-th, n+1-th, n+
Each time from the trigger signal of the second reflected wave
It measures T _o , T _o+1 , T _o+2 , etc., and its configuration is as follows. That is, FIG. 3 shows a block diagram of the time difference measuring circuit 9. As shown in FIG. When the trigger signal from the synchronizing pulse generating circuit 5 is applied to the counter 95, timer 96 and timer 97, the counter 95 is reset by the signal and starts counting clock pulses from the clock pulse generating circuit 99. [Figure 2 C]. At the same time, the timer 96 starts measuring a set time, for example, the time required for the ultrasonic wave to travel back and forth approximately n-2 times between the front and back surfaces of the steel pipe P [Fig. 2 (d)]
Outputs a high level signal. This signal is applied to the negative logic input terminal of AND gate 91 via OR gate 98. The output of the synchronous detector 8 is given to the other terminal of the AND gate 91. While the timer 96 is outputting a high level signal, the AND gate 91 is closed, and the output of the synchronous detector 8 is not input to the comparator 93 at the subsequent stage. In other words, this period becomes a dead zone. Also, the timer 97 is reset. When the timer 96 reaches the set time, the timer 96 output becomes low level.
AND gate 91 opens and the signal from synchronous detector 8 is applied to comparator 93. The comparator 93 is given a predetermined level voltage as a threshold from the threshold setting device 92, and when the input signal is higher than the threshold, a high level signal is output and sent to the one shot multi 94. It will be done. The one-shot multi 94 is triggered by this signal and emits a short-time pulse signal. [Figure 2 b]. This pulse signal is given to timer 96 and timer 97,
The timer 96 is reset and starts counting again, and the timer 97 starts counting for a time slightly shorter (approximately 80%) than the time it takes for the ultrasonic waves to make one round trip between the front and back surfaces of the steel pipe P [Fig. 2 H] level signal
Output to AND gate 91. Therefore, the output of the synchronous detector 8 during this period is not input to the comparator 93, and becomes a dead zone. On the other hand, the pulse signal output by the one-shot multi 94 is given to the arithmetic circuit 11 via the interface 10, and the arithmetic circuit 11 at that point transfers the count value of the counter 95 to the interface 10.
Load via. This count value corresponds to the time T _o+1 required for the ultrasonic wave to make n-1 round trips.

When the set time of the timer 97 has elapsed, its output becomes low level and the AND gate 91 opens.
The output of the synchronous detector 8 is input to a comparator 93. Then, when the nth reflected wave exceeds the threshold value, the one-shot multi 94 is triggered and reads the count value of the counter 95 at that time. This count value is the time T _o required for the ultrasonic wave to reciprocate n times.
corresponds to

Similarly, the required number of data T _o+1 , T _o+2 ...
Load. Note that during this time, the output of the timer 96 remains at a low level.

Next, the contents of the calculation in the calculation circuit 11 will be explained. First, subtract T _o+1 − _{T o} to find the time it takes for the ultrasonic wave to make one round trip between the front and back surfaces of the steel pipe P, and then
Perform division of T _o /(T _o+1 − _{T o} ). This value is the number of ultrasound reflections. The denominator of this above division is T _o
−It is better not to use T _o-1 . This is because in some cases, as shown in FIG. 4, the n-1th reflected wave output may not be detected correctly because it overlaps with the dead zone time of the timer 96.

Now, as mentioned above, n = T _o / (T _{o +1} - _{T o} ), but if, for example, the timer 97 is set too long, or the threshold value is set too low, If the reflected wave from a defective part is detected at a point close to the reflected wave from the above T _o /
(T _o+1 +T _o ) value is not an integer value n.

Therefore, in the present invention, if T _o /(T _o+1 ) is not an integer value, it is corrected to become an integer value to eliminate measurement error factors. That is, the correction value N for the number of reflections n on the back surface expressed by the following equation (2) is used instead of n.

N= ^* [T _o /T _o+1 −T _o ] ...(2) However, ^* [ ]: symbol that is the closest integer value to the value in [ ], and D=T _o・V/2・N ...(3) Calculate the thickness D of the steel pipe P using equation (3). This D
The value is displayed on the display 12 and recorded on the recorder 13.

Note that T _o is not limited to the above example; timer 9
It is also possible to rely on any detection signal after the second one after the dead zone in No. 6 is released.

Next, another method of the present invention will be explained. In this method, the thickness D is determined using the following equation (4) instead of the above-mentioned equation (3).

D=(T _o +T _o+1 +...T _o+n・V/2・(m+1)・(N+
m/2)...(4) However, m is a natural number and corresponds to the number of reflected signals used for calculation - 1. To explain this method in detail, for example, the first three data T _o , T _{o +1} , Using T _o+2 , T _o , T _o+1
N value obtained by substituting into equation (2) and T _o , T _o+1 , T _o+2
The thickness D is calculated by substituting and into equation (4). The obtained D value is obtained by calculating the average time of T _o , T _{o +1} , and T _{o +2} , that is, the average time of the nth, n+1, and n+2 reflected ultrasound waves, and apparently corresponding to this time. The value obtained by further averaging using the number of round trips N+1 [=(T _o +T _o+1 +T _o+2 )・V/2・3・(N
+1)] and is accurate because error factors are eliminated by introducing the N value.

Note that the present invention is not limited to measuring the thickness of steel pipes, but can also measure plate-shaped ones. Furthermore, as a device for applying a DC magnetic field parallel to the surface of the material to be measured, not only the electromagnet as described above but also a permanent magnet may be used. Also, although separate transmitting coils and receiving coils are provided, one coil may be used for both transmitting and receiving purposes.
Furthermore, it goes without saying that a plurality of devices configured in this manner can be used to simultaneously measure required locations.

Next, the effects of the present invention will be explained. The threshold value is set to a level lower than the output value of the reflected ultrasonic waves from the back side, and the dead zone is set so that each time the reflected ultrasonic waves exceed the threshold value, the ultrasonic waves inside the material to be measured are set to 1.
A period corresponding to 80% of the reciprocating time was set, and the circumferential surface of the steel pipe was measured over its entire length using the method of the present invention.

Figure 5 is a graph showing the measurement results with length (m) on the horizontal axis and thickness (mm) of the steel pipe on the vertical axis, and each reflected ultrasonic wave is detected for comparison. In each case, the T _o value obtained without setting a dead zone and
Results calculated by substituting T _o+1 into equation (1) (Comparative Example 1)
And in the same way, after each reflected wave is detected, T _o+1 , T _o , T _o+1 , T _o+2 ,... are obtained without providing a dead zone.
The results (Comparative Example 2) calculated by substituting the values into the following equation (5) are also shown.

D=T _o・V/2 _o ...(5) This figure shows that the thickness measurement value is stable in the case of the present invention, and does not vary greatly locally as in Comparative Example 1 and Comparative Example 2. This shows that there is no influence of reflected ultrasound waves from other than the back surface. Also, the number of reflections n observed at the tube end of Comparative Example 2
It can be seen that abnormalities in the thickness measurement values due to errors in the above are also eliminated. Therefore, for these reasons, the values measured by the present invention during the manufacturing process of pipes etc. can be used as control information.

As detailed above, the present invention sets a threshold value and a dead zone to improve detection accuracy, and furthermore, it is necessary to set a threshold value and a dead zone in order to improve detection accuracy, and furthermore, it is assumed that the time from the start of ultrasonic propagation to the point of detection of reflected ultrasonic waves from the back side is an integral multiple of one round trip time. Since the thickness is determined by using this method, it has an excellent effect of accurately measuring the thickness even when the thickness of the material to be measured is thin. Furthermore, as shown in equation (3) or (4), the present invention calculates using the time T _o from the start of ultrasonic propagation to the time of detection of reflected ultrasonic waves. The accuracy is higher than that using a time difference, for example, T _o −T _o-1 . That is, to explain based on equation (3), the error included in T _o and T _o-1 is expressed as ΔT _o ,
Assuming ΔT _{o -1} , the error included in the thickness is ΔT _o /2N. On the other hand, when using the time difference between successive reflected ultrasound waves, the thickness is expressed as D = (T _o - T _o-1 V/2, so the maximum error value is ΔT _o + ΔT considering the positive and negative signs. It becomes _o-1 /2.

In this way, the present invention can significantly reduce errors compared to the measuring force method that uses time differences.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the implementation state of the present invention, Figure 2 is a schematic diagram showing the implementation state of the present invention.
3 is a block diagram of the time difference measuring circuit of the device used in the present invention, and 4.
The figure is an explanatory diagram of an indistinguishable signal due to a dead zone, and FIG. 5 is a graph showing measurement results according to the method of the present invention. P...Steel pipe, 1...Electromagnetic ultrasonic transmitter/receiver, 9...
...Time difference measurement circuit, 11...Arithmetic circuit.

Claims (1)

[Claims] 1. Propagating electromagnetic ultrasonic waves from the surface of a material to be measured,
When detecting reflected ultrasonic waves from the back side on the front side, a threshold of a predetermined level is set, and each time a reflected ultrasonic wave exceeding the threshold is detected, a predetermined time is set as a dead zone, and the detection is performed. Time T _o from the start of ultrasound propagation of the reflected ultrasound
(where n is the subscript indicating the number of reflections on the back surface), N= ^* [T _o /T _o+1 −T _o ] However, ^* [ ]: The symbol that is the closest integer value to the number in [ ] A method for measuring thickness, comprising: calculating a correction value N for the number of reflections n on the back surface expressed as n, and calculating thickness D using the following formula using this N value and the time. D=T _o・V/2・N However, V: Sound velocity in the material to be measured 2 The above-mentioned time T _o is related to the first and subsequent reflected ultrasonic waves detected after the dead zone related to the transmitted wave. A method for measuring thickness according to claim 1. 3 Propagate electromagnetic ultrasonic waves from the surface of the material to be measured,
When detecting reflected ultrasonic waves from the back side on the front side, a threshold of a predetermined level is set, and each time a reflected ultrasonic wave exceeding the threshold is detected, a predetermined time is set as a dead zone, and the detection is performed. Time T _o from the start of ultrasound propagation of the reflected ultrasound
(where n is the subscript indicating the number of reflections on the back surface), N= ^* [T _o /T _o+1 −T _o ] However, ^* [ ]: The symbol that is the closest integer value to the number in [ ] A method for measuring thickness, comprising: calculating a correction value N for the number of reflections n on the back surface expressed as n, and calculating thickness D using the following formula using this N value and the time. D=(T _o +T _o+1 +…+T _o+n・V/2・(m+1)・
(N+m/2) However, V: Sound velocity in the material to be measured m: Natural number 4 The above time T _o is related to the first and subsequent reflected ultrasounds among the reflected ultrasounds detected after the dead zone related to the transmitted wave. A method for measuring thickness according to claim 3.