JPH06109559A - Stress measuring apparatus - Google Patents

Abstract

PURPOSE:To detect a stress of an object to be measured made from a ferromagnetic material in a good S/N by utilizing a magnetic absorption method. CONSTITUTION:A bias coil 3 is driven by means of a first driving signal having a low frequency. A detecting coil 6 is disposed in the bias magnetic field and a first condenser is connected to the detecting coil 6 in parallel so as to be resonated by a second driving signal. A second condenser is connected in parallel to a balance coil 7 which is provided in separation from the object 1 and resonated in a frequency near that of the second driving signal. A bridge means 16 is constituted together with the respective parallel circuits. The bridge means 16 is driven by means of the second driving signal so that a stress is calculated on the basis of the first driving signal and the output of the bridge means 16. By improving a selection rate Q and using the bridge means 16, it is possible to measure a stress in a good S/N ratio, even utilizing the second driving signal with low frequency, thereby enabling the stress-measurement at a desired position in the depth direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stress measuring device for nondestructively measuring stress such as tensile stress or 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 the stress acting on an object to be detected, conventionally, a strain gauge is attached to the surface of the object to be detected. Such a prior art does not measure the stress actually acting on the detected object.

Other prior art is disclosed, for example, in US Pat. No. 3,792,348. This prior art utilizes the 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, the detection coil is driven at a high frequency, and stress is applied. Is detected by the magnetic absorption detecting means.

[0004]

In the prior art utilizing such a magnetic absorption phenomenon, the frequency for driving the detection coil is changed to a low band to measure the stress at a deep place of the object to be detected. , The S / N ratio deteriorates,
Therefore, it is not possible to measure the stress at a deep location of the object to be detected, and the detection coil can be driven only at a high frequency to measure only the stress near the surface of the object to be detected.

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

[0006]

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

Further, according to the present invention, the frequency of the first drive signal for driving the bias coil is the second for driving the bridge means.
It is characterized in that the value is 0.01 or less of the frequency of the drive signal.

Furthermore, the present invention is characterized in that the strength of the bias magnetic field is 10 times or more the strength of the magnetic field generated by the detection coil.

[0009]

According to the present invention, the bias coil is driven by the first signal generating source at a low frequency, and the first capacitor is connected in parallel to the detection coil provided in the bias magnetic field to form the first parallel circuit. The second parallel circuit is resonated by the second drive signal by connecting the second capacitor to the balanced coil provided apart from the detected object so as not to be affected by the detected object. In this way, a bridge is constructed with such first and second parallel circuits, and the stress of the detected object is calculated based on the first drive signal forming the bias magnetic field and the output of the bridge means. Therefore, the frequency of the second drive signal can be selected to be low, and the stress can be measured in a deep place of the detected object, that is, inside the detected object. Moreover, the detection coil and the balance coil are the first and second
A resonance circuit is formed by the capacitor, whereby the selectivity Q and the S / N ratio can be improved. Further, since the bridge means is used to obtain the output due to the difference in the inductance of the detection coil from the balanced coil, it is possible to improve the gain and thus the sensitivity.

In order to improve the S / N ratio and sensitivity as described above, the first coil for driving the bias coil is used.
The frequency of the drive signal is selected to be a low value of 0.01 times or less 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. Determined above.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an overall block diagram of an embodiment of the present invention. On a detected object 1 made of a ferromagnetic material whose stress is to be measured, for example, mild steel, a bias coil 3 wound around a U-shaped yoke 2 is provided with a first coil.
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 balance coil 7 is provided apart from the detected object 1 so as not to be affected by the detected object 1. The magnetic field due to these coils 6, 7 is designated by the reference numeral 9.

FIG. 3 shows a detection coil 6 and a balance coil 7.
3 is an electric circuit diagram showing an electric configuration related to FIG. A capacitor C1 and a variable 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. The second drive signal, which is, for example, a sine wave, from the second signal generation source 12 (see FIG. 1 described above) is given to the bridge means 16 from the line 13 via the variable resistors 14 and 15. The bridge means 16 is 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 the two connecting points 19 and 20 of the bridge means which face each other. This bridge means 16
The output of the bridge means 16 is derived from the remaining two connection points 21 and 22 facing each other. Bridge means 16
In, the diodes 23 and 24 are connected, and the capacitors 25 to 27 for removing high frequency noise are connected. Further, the first and second parallel circuits 10 and 11 and the resistor 1
A change-over switch 28 is provided which is switchable to the opposite polarity in relation to 7 and 18.

The frequency fH of the first signal source 4 applied to the bias coil 3 for forming the bias magnetic field is
For example, it is 200 to 500 Hz, and its 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 association with the bridge means 16 is 200 to 500 kHz.
And the magnetic field HRF generated by this is, for example, 10 to 20 [Oe]. Thus, the frequency of the first drive signal is selected to be 0.01 or less of the frequency of the second drive signal, and the strength HAC of the bias magnetic field is the strength of the magnetic field HRF generated by the detection coil 6. The object to be detected 1 can be magnetically saturated by a bias magnetic field of 10 times or more.

FIG. 4 shows an object to be detected 1 made of a ferromagnetic material.
Shows a state in which the bias coil 3 applies a low-frequency bias magnetic field, and the detection coil 6 applies a high-frequency magnetic field. In this way, the low-frequency bias magnetic field HAC and the high-frequency magnetic field HRF are superimposed on the detected object 1.

FIG. 5 shows a hysteresis loop of the detected object 1. A current, which is a sinusoidal first drive signal indicated by reference numeral 29, is applied to the bias coil 3 from the first signal generation source 4 via the power amplification circuit 5, whereby a hysteresis loop indicated by reference numeral 30 is generated. can get. The AC magnetic permeability μRF at the frequency of the second drive signal of the object to be detected 1 by the detection coil 6 is represented by the following formula 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 permeability μRF. Reference numerals 1 to 5 used in FIG. 6 individually correspond to reference numerals 1 to 5 used in FIG. From this, it can be seen 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 seen that the impedance change ΔZ is a function of the AC permeability μRF.

[0020]

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

[0021]

## EQU00003 ## .DELTA.R = KR1.GR (.mu.RF) ^{1 / 2-} KR2 = f (.mu.RF) = f (.mu.RF (HAC))

[0022]

ΔL = KL1 · GL1 (μRF) ^1/2 + KL2-GL2 = g (μRF) = g (μRF (HAC)) where K is a substance constant peculiar to the object to be detected 1 and G is the detected value. Is the geometric constant of the material associated with coil 6, R,
R1; L1 represents a value related to the detection coil 6, R2,
L2 indicates a value related to the balanced coil 7.
f and g indicate functions.

The output derived from the bridge means 16 via the line 32 corresponds to the variation ΔZ of the impedance Z shown by the equation 2, and therefore from this impedance variation ΔZ, according to the equations 3 and 4, The AC magnetic permeability μRF can be obtained. AC permeability μRF is
It corresponds to the tensile stress and the compressive stress acting on the detected object 1. A stress is applied to a sample piece made of the same material as the detected object 1 whose stress is to be measured, for example, aluminum, to obtain a Lissajous figure, and the stress and the Lissajous figure are stored in a memory in correspondence with each other. By comparing the Lissajous figure obtained when a stress is applied to the detected object 1 with the stored Lissajous figure, and examining the stored corresponding stress when the Lissajous figure matches ,
The stress acting on the detected object 1 can be known.

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

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 detected object 1 is a compressor blade when the blade is bent. There is. The amplitude of this waveform is
It is the magnetic absorption signal amplitude. The stresses in FIGS. 7 (1) to 7 (7) are as shown in Table 1.

[0026]

[Table 1]

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

[0028]

[Table 2]

From the waveforms shown in FIGS. 7 and 8, it is understood that there is a correlation between the stress acting on the object to be detected 1 and its AC 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 and its outer diameter is ⅛ inch, the magnetic absorption signal obtained from the bridge means 16 to the line 32 is a line depending on the stress of the object to be detected 1. As shown 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 shown 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 in a range wider 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 generating source, and the detection coil is provided in this bias magnetic field. The first capacitor is connected in parallel so as to resonate near the frequency of the second drive signal, and the second capacitor is connected to the balance coil provided apart from the object to be detected so as not to be affected by the object to be detected. The bridge means is configured so as to resonate near the frequency of the two drive signals, and the bridge means is driven by the second drive signal to generate 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 based on this, the Q of the first and second parallel circuits forming 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 manner, it becomes possible to measure the stress at a desired position in the depth direction of the detected object.

[Brief description of 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 electrical configuration related to a detection coil 6 and a balance coil 7.

FIG. 4 is a diagram showing magnetic fields formed by a bias coil 3 and a detection coil 6, respectively.

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

FIG. 6 is a graph showing an experimental result of the present inventor, showing that the AC permeability μRF is displaced to the bias magnetic field HAC.

FIG. 7 is a graph showing a Lissajous waveform of a magnetic absorption signal showing an experimental result 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, which shows the experimental result of the present inventors.

[Explanation of symbols]

 1 Detected Object 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 connecting points 21,22 A pair of remaining opposing connecting points

Claims (3)

[Claims] 1. A bias coil for generating a bias magnetic field, and a first signal source for generating a first drive signal for driving the bias coil at a low frequency, to a detected object made of a ferromagnetic material whose stress is to be measured. A detection coil provided in the bias magnetic field, a balance coil provided away from the detection object so as not to be influenced by the detection object, and a second signal generation for generating a second drive signal having a frequency higher than the bias magnetic field A source and a detection coil, which are connected in parallel to form a first parallel circuit, and a first capacitor which resonates near the frequency of the second drive signal, and a balance coil, which is connected in parallel to form a second parallel circuit,
A bridge is configured with a second capacitor that resonates near the frequency of the second drive signal, a first parallel circuit and a second parallel circuit, and between two connection points where the second drive signal from the second signal source opposes. Based on the given bridge means for deriving outputs from the remaining two opposing connection points, the first drive signal from the first signal source, and the output of the bridge means, the stress of the object to be detected is And a means for calculating and determining. 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 measurement device according to claim 1, wherein the strength of the bias magnetic field is 10 times or more the strength of the magnetic field generated by the detection coil.