JP3217492B2 - Stress measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stress measuring apparatus for non-destructively measuring a stress such as a tensile stress or a compressive stress acting on an object to be detected made of a ferromagnetic material such as mild steel.

[0002]

2. Description of the Related Art In order to measure a stress acting on an object to be detected, conventionally, a strain gauge is attached to the surface of the object to be detected. Such prior art does not measure the stress actually acting on the detected object.

Another prior art is disclosed, for example, in US Pat. No. 3,792,348. This prior art utilizes a phenomenon of so-called magnetic absorption, in which a low-frequency bias magnetic field is applied to an object to be detected, a detection coil is provided in the bias magnetic field, and the detection coil is driven at a high frequency to generate a stress. Is detected by the magnetic absorption detecting means.

[0004]

In the prior art utilizing such a magnetic absorption phenomenon, the frequency at which the detection coil is driven is changed to lower the frequency and measure the stress at a deep place of the object to be detected. , The S / N ratio deteriorates,
Therefore, stress cannot be measured at a deep place of the detected object, and only the stress near the surface of the detected object can be measured by driving the detection coil only at a high frequency.

It is an object of the present invention to provide a stress measuring device capable of measuring a stress at a desired position of an object to be detected by improving an S / N ratio.

[0006]

According to the present invention, there is provided a bias coil for generating a bias magnetic field on a detection target made of a ferromagnetic material whose stress is to be measured, and a first drive signal for driving the bias coil at a low frequency. , A detection coil provided in a bias magnetic field, a balanced coil provided apart from the detected object so as not to be affected by the detected object, and a second drive having a frequency higher than the bias magnetic field. A second signal generation source for generating a signal and a detection coil are connected in parallel to form a first parallel circuit, and a first capacitor that resonates near the frequency of the second drive signal and a balancing coil are connected in parallel. A second parallel circuit, a second capacitor resonating around the frequency of the second drive signal, and a bridge together with the first parallel circuit and the second parallel circuit;
A bridge means for applying a second drive signal from the signal source between the two opposing connection points and deriving outputs from the remaining two opposing two connection points; and a first driving signal from the first signal generation source. Based on the drive signal and the output of the bridge means,
Means for calculating and calculating the stress of the detected object.

Further, according to the present invention, the frequency of the first drive signal for driving the bias coil is equal to the second drive signal for driving the bridge means.
The frequency of the driving signal is 0.01 or less.

Further, the present invention is characterized in that the strength of the bias magnetic field is at least ten times the strength of the magnetic field generated by the detection coil.

[0009]

According to the present invention, the bias coil is driven at a low frequency by the first signal generation source, and the first capacitor is connected in parallel to the detection coil provided in the bias magnetic field to form the first parallel circuit. Resonance is caused by the second drive signal, and a second capacitor is connected to a balanced coil provided apart from the detected object so as not to be affected by the detected object, and the second parallel circuit is resonated by the second drive signal. A bridge is formed together with the first and second parallel circuits, and the stress of the object to be detected is calculated and obtained based on the first drive signal forming the bias magnetic field and the output of the bridge means. Therefore, by selecting a low frequency of the second drive signal, it is possible to measure a stress in a deep place of the detected object, that is, inside the detected object. Moreover, the detection coil and the balance coil are composed of the first and second coils.
A resonance circuit is formed by the capacitor, whereby the selectivity Q can be improved, and the S / N ratio can be improved. Further, an output resulting from the difference in inductance between the detection coil and the balanced coil is obtained by using the bridge means, so that it is possible to improve the gain and hence the sensitivity.

[0010] In order to improve the S / N ratio and the sensitivity, the first driving of the bias coil is performed.
The frequency of the drive signal is selected to be lower than 0.01 times the frequency of the second drive signal for driving the bridge means, and the strength of the bias magnetic field is 10 times the strength of the magnetic field generated by the detection coil. Specified above.

[0011]

FIG. 1 is an overall block diagram of an embodiment of the present invention. A bias coil 3 wound around a U-shaped yoke 2 is provided on a detection object 1 made of a ferromagnetic material whose stress is to be measured, for example, mild steel.
The sine wave from the signal generation source 4 is power-amplified by the power amplification means 5 and driven by the first drive signal. The detection coil 6 and the balance coil 7 are provided in the bias magnetic field thus formed.

FIG. 2 is a simplified sectional view showing the detection coil 6 and the balance coil 7. These coils 6,7
Is wound around a core 8 perpendicular to the detected object 1. The balancing coil 7 is provided away from the detected object 1 so as not to be affected by the detected object 1. The magnetic field from these coils 6, 7 is indicated by reference numeral 9.

FIG. 3 shows a detection coil 6 and a balance coil 7.
FIG. 3 is an electric circuit diagram showing an electric configuration related to FIG. A capacitor C1 and a variable-capacitance capacitor C2 are connected in parallel to the detection coil 6 to form a first parallel circuit 10. Similarly, a capacitor C3 and a variable capacitor C4 are connected in parallel to the balanced coil 7 to form another second parallel circuit 11. A second drive signal, for example, a sine wave, from the second signal source 12 (see FIG. 1 described above) is provided to the bridge means 16 from the line 13 via the variable resistors 14 and 15. The bridging means 16 includes a first
And the second parallel circuits 10 and 11 together with the resistors 17 and 1
8 constitutes a bridge, and the second drive signals from the variable resistors 14 and 15 are given to two opposing connection points 19 and 20 of the bridge means. This bridge means 16
The output of the bridging means 16 is derived from the remaining two opposing connection points 21 and 22. Bridge means 16
, Diodes 23 and 24 are connected, and capacitors 25 to 27 for removing high-frequency noise are connected. Furthermore, the first and second parallel circuits 10 and 11 and the resistor 1
A changeover switch 28 that can be switched to the opposite polarity is provided in association with 7 and 18.

The frequency fH of the first signal source 4 applied to the bias coil 3 for forming a bias magnetic field is
For example, the frequency is 200 to 500 Hz, and the bias magnetic field HAC is 100 to 200 [Oe]. The frequency fRF of the second drive signal from the second signal source 12 provided in connection with the bridge means 16 is 200 to 500 kHz.
The magnetic field HRF generated thereby is, for example, 10 to 20 [Oe]. As described above, the frequency of the first drive signal is selected to be equal to or less than 0.01 of the frequency of the second drive signal, and the strength HAC of the bias magnetic field is determined by the strength of the magnetic field HRF generated by the detection coil 6. The detection target object 1 can be magnetically saturated by the bias magnetic field.

FIG. 4 shows a detected object 1 made of a ferromagnetic material.
3 shows a state in which a bias magnetic field of a low frequency is applied by the bias coil 3 and a magnetic field of a high frequency is applied by the detection coil 6. Thus, the low-frequency bias magnetic field HAC and the high-frequency magnetic field HRF are superimposed on the detection target object 1.

FIG. 5 shows a hysteresis loop of the object 1 to be detected. A current, which is a sine wave first drive signal indicated by reference numeral 29, is supplied from the first signal generation source 4 to the bias coil 3 via the power amplifier circuit 5, thereby forming a hysteresis loop indicated by reference numeral 30. can get. The AC magnetic permeability μRF of the detection object 6 at the frequency of the second drive signal of the detection target object 1 is represented by Expression 1.

[0017]

(Equation 1)

Here, the magnetic field generated by the detection coil 6 is HRF as described above, and the magnetic flux density is BRF.

FIG. 6 is a graph showing the relationship between the bias magnetic field HAC generated by the bias coil 3 and the AC magnetic permeability μRF. Reference numerals 1 to 5 used in FIG. 6 individually correspond to reference numerals 1 to 5 used in FIG. This indicates that the AC permeability μRF is a function of the low frequency bias magnetic field HAC.
Therefore, the change ΔZ of the impedance Z of the detection coil 6
, It can be understood that the impedance change ΔZ is a function of the AC magnetic permeability μRF.

[0020]

## EQU00002 ## Z + .DELTA.Z = (R0 + .DELTA.R) + j.omega. (L0 + .DELTA.L) Here, R0 indicates the resistance of the detection coil 6, and L0 indicates its inductance. ω represents the angular frequency of the second drive signal. ΔR and ΔL are the changes in resistance and inductance due to the change in AC magnetic permeability μRF.

[0021]

ΔR = KR1 · GR (μRF) ^1/2 −KR2 = f (μRF) = f (μRF (HAC))

[0022]

ΔL = KL1 · GL1 (μRF) ^1/2 + KL2-GL2 = g (μRF) = g (μRF (HAC)) where K is a material constant peculiar to the detection target object 1, and G is detection. The geometric constant of the material associated with coil 6, R,
R1; L1 indicates a value related to the detection coil 6;
L2 indicates a value related to the balanced coil 7.
f and g indicate functions.

The output derived from the bridging means 16 via the line 32 corresponds to the change ΔZ of the impedance Z shown in the equation (2). Therefore, from this impedance change ΔZ, according to the equations (3) and (4), The AC permeability μRF can be determined. AC permeability μRF is
This corresponds to the tensile stress and the compressive stress acting on the detected object 1. A Lissajous figure is obtained by applying a stress to a specimen made of the same material as that of the object 1 to be measured, such as aluminum, for example, and the stress and the Lissajous figure are stored in a memory in correspondence with the stress. By comparing a Lissajous figure obtained when a stress is applied to the detected object 1 with the stored Lissajous figure, and examining the corresponding stored stress when the Lissajous figure matches. ,
The stress applied to the detected object 1 can be known.

The output of the bridging means 16 is filtered from line 32 by a filter 33 as shown in FIG.
Only the frequency component of the drive signal is extracted, amplified by an amplifier circuit 34, a Lissajous figure is obtained by an oscilloscope 35, converted into a digital value by an analog / digital converter 36, and provided to a processing circuit 37. The analog-to-digital converter 36 is also supplied with the first drive signal from the first signal generation source 4 and is converted into a digital signal.
The processing circuit 37 is realized by, for example, a computer, and includes a memory 38, a printer 39, and a visual display means 40. The type of stress acting on the detection target object 1, that is, the tension or compression, and the value of the stress, etc. Is calculated.

FIG. 7 shows a magnetic absorption signal which is a Lissajous waveform of the oscilloscope 35 obtained at two points opposite to each other when the object 1 to be detected is a blade of a compressor when the blade is bent. I have. The amplitude of this waveform is
This is the amplitude of the magnetic absorption signal. 7 (1) to 7 (7) are as shown in Table 1.

[0026]

[Table 1]

FIG. 8 shows the experimental results of the present inventor, showing the waveform of the magnetic absorption signal when a stress is applied to the cold-rolled nickel wire. 8 (1) to 8
The stress of (8) is as shown in Table 2.

[0028]

[Table 2]

It is understood from the waveforms shown in FIGS. 7 and 8 that there is a correlation between the stress acting on the detected object 1 and its AC magnetic permeability μAC.

FIG. 9 is a graph showing the experimental results of the present inventor. When the object to be detected 1 is a nickel-plated aluminum rod having an outer diameter of 1/8 inch, the magnetic absorption signal obtained from the bridge means 16 to the line 32 depends on the stress of the object to be detected 1 Obtained as indicated at 41. On the other hand, when a strain gauge is attached to the same object to be detected, an experimental result obtained from the strain gauge is indicated by a line 42. Thus, according to the magnetic absorption signal of the present invention, the change in the level of the magnetic absorption signal with respect to the stress change is substantially constant over a wider range than the stress,
Therefore, it is understood that the stress is measured with high accuracy over a wide range.

[0031]

As described above, according to the present invention, the bias coil is driven at a low frequency by the first drive signal from the first signal source, and the detection coil is provided in the bias magnetic field. A first capacitor is connected in parallel to resonate near the frequency of the second drive signal, and a second capacitor is connected to a balanced coil provided apart from the detected object so as not to be affected by the detected object. (2) Resonating near the frequency of the drive signal, thus forming the bridge means, which is driven by the second drive signal to output the first drive signal from the first signal source and the output of the bridge means. Since the stress of the object to be detected is calculated and calculated based on the above, the Q of the first and second parallel circuits constituting the resonance circuit is increased, thereby improving the S / N ratio. To, also it is possible to improve the sensitivity by using a bridge unit. By changing the frequency of the second drive signal in this way, it is possible to measure the stress at a desired position in the depth direction of the detected object.

[Brief description of the drawings]

FIG. 1 is an overall block diagram of an embodiment of the present invention.

FIG. 2 is a simplified cross-sectional view showing a detection coil 6 and a balance coil 7.

FIG. 3 is an electric circuit diagram showing an electric configuration related to a detection coil 6 and a balance coil 7;

FIG. 4 is a diagram showing a magnetic field formed by a bias coil 3 and a detection coil 6;

FIG. 5 is a diagram showing a hysteresis loop of the detected object 1 showing an experimental result of the present inventor.

FIG. 6 is a graph showing the experimental results of the present inventor, and shows that the AC magnetic permeability μRF is displaced by the bias magnetic field HAC.

FIG. 7 is a graph showing a Lissajous waveform of a magnetic absorption signal showing experimental results of the present inventor.

FIG. 8 is a diagram showing a Lissajous waveform showing an experimental result of the present inventor.

FIG. 9 is a graph showing the polarity of the magnetic absorption signal corresponding to the stress showing the experimental results of the present inventor.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 object to be detected 2 yoke 3 bias coil 4 first signal source 5 power amplifier circuit 6 detection coil 7 balanced coil 8 core 10 first parallel circuit 11 second parallel circuit 12 second signal source 16 bridge means 17, 18 resistance 19,20 A pair of opposing connection points 21,22 A pair of remaining opposing connection points

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-22524 (JP, A) JP-A-5-203503 (JP, A) WO 89/1613 (WO, A1) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01L 1/00 G01L 1/12

Claims (3)

(57) [Claims] 1. A bias coil for generating a bias magnetic field on an object to be detected made of a ferromagnetic material whose stress is to be measured, and a first signal generator for generating a first drive signal for driving the bias coil at a low frequency. A detection coil provided in the bias magnetic field, a balanced coil provided apart from the detected object so as not to be affected by the detected object, and a second signal generation for generating a second drive signal having a higher frequency than the bias magnetic field A first capacitor connected in parallel to the source and the detection coil to form a first parallel circuit, a first capacitor resonating near the frequency of the second drive signal, and a second parallel circuit connected in parallel to the balanced coil;
A second capacitor that resonates in the vicinity of the frequency of the second drive signal, and a bridge formed with the first parallel circuit and the second parallel circuit, between the two connection points where the second drive signal from the second signal generation source faces A bridge means for deriving outputs from the two remaining two connection points provided; a first drive signal from the first signal source; and an output of the bridge means, the stress of the object to be detected. And a means for calculating and calculating the stress. 2. The stress measurement according to claim 1, wherein the frequency of the first drive signal for driving the bias coil is 0.01 or less of the frequency of the second drive signal for driving the bridge means. apparatus. 3. The stress measuring device according to claim 1, wherein the intensity of the bias magnetic field is at least ten times the intensity of the magnetic field generated by the detection coil.