JPS61148313A - Acoustic type micrometer - Google Patents

Abstract

PURPOSE:To enable measurement of very small distance without any air pressure equipment, by arranging a sound tube having a measuring end at the position separated at a very small distance from the surface of an object to be measured, an acoustic resonance system containing the tube and a sound source for driving the system. CONSTITUTION:A measuring end 2 of an acoustic tube 1 is placed at the position separated from an object 3 to be measured at a small distance through a gap (d) and the measurement of the distance given by the gap (d) utilizes that the sound impedance at the measuring end 2 varies sensitively with the distance. By an acoustic resonance system containing the acoustic tube 1, changes in the impedance are converted into a resonance frequency thereof and changes in the sound pressure therein to detect a very small distance. So to speak, a relatively large sound source 4 of the output acoustic impedance is driven by a sinusoidal wave voltage from an oscillator 5 and the sound pressure returned via the gap (d) is received with a microphone 6 and amplified 7. Then, the sinusoidal wave voltage is converted to DC output E with a rectifying/smoothing circuit 8 to be shown on a display meter 9 and thus, the value of very small distance (d) can be determined.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is an acoustic method that utilizes the fact that when an object to be measured is placed facing one end of an acoustic tube, the acoustic impedance of the tube end changes sharply depending on the distance from the object surface. It is related to the minute displacement meter of the formula.

As a means of measuring minute distances to the surface of an object without contact,
Conventionally, pneumatic micrometers and meters have been widely used. This takes advantage of the fact that the flow resistance of a nozzle that continuously jets air changes depending on the distance from the object surface directly facing the nozzle. Usually, changes in this distance are caused by changes in air flow rate or nozzle back pressure. is detected as a change in

This pneumatic micrometer measures the dimensional difference between a manufactured article and a standard master article. It is industrially important as one of the so-called comparative measuring instruments. However, pneumatic micrometers require air pressure supply equipment, so they cannot be easily used everywhere, and furthermore, maintenance such as replacing air filters and draining drains is time-consuming. Additionally, because air is jetted out from the nozzle at high speed using pressures ranging from a fraction of an atmosphere to several atmospheres, there is also the drawback that small or light objects may move and cannot be measured.

The present invention provides an acoustic micrometer that does not have the disadvantages of conventional methods by utilizing a new method of utilizing changes in acoustic impedance at the end of an acoustic tube. Unlike flow resistance, acoustic impedance is generally expressed as a complex number consisting of a real part and an imaginary part. Furthermore, the acoustic impedance and acoustic tube characteristics are generally a function of frequency. Therefore, the physical phenomena in the acoustic micrometer of the present invention are more complex than those in the pneumatic micrometer. However, from another point of view, this means that more diverse measurement means can be used, thereby producing various unique effects.

That is, the first object of the present invention is to provide a portable micrometer that does not require pneumatic equipment as in conventional systems. This is because everything, including the earphones and speakers used as sound sources, is electrically driven and can be powered by batteries if necessary.

The second objective is to provide a micrometer that requires no maintenance. This is because parts that require maintenance, such as air filters, and narrow nozzle holes that are prone to clogging are not used.

The third purpose is to provide a micrometer that applies almost no force to the object to be measured.The pressure amplitude of the sound used in the micrometer of the present invention is at most 10 mm of water column.
(approximately 100P&), and since it is an alternating pressure, the static force applied to the object to be measured is extremely small, and for example, it is possible to measure minute displacements of the liquid surface with almost no disturbance.

The fourth object is to provide a micrometer that can perform two measurement modes, fine and coarse, at the same time using a wooden acoustic tube. This is a unique effect caused by the simultaneous coexistence of a plurality of resonance states with different frequencies in the sound WV.

A fifth object is to provide a micrometer that compensates for the effects of changes in air viscosity caused by changes in temperature. This is because the temperature at that time can be determined from the resonant frequency of the acoustic tube, and this is also a unique effect of using sound.

Sixth, it is to provide a micrometer with a simple structure and low manufacturing cost.In the acoustic micrometer of the present invention, except for the measurement end of the acoustic tube, there are no parts that require precision machining. There are no electronic circuits, and the electronic circuit can be easily constructed using commercially available parts. For these reasons, the micrometer of the present invention can be manufactured at a fraction of the cost of conventional pneumatic micrometers.

In FIG. 1, l is an acoustic tube of length L and internal cross-sectional area S, and two flange 2 are attached to the measuring end. Reference numeral 3 denotes an object to be measured, whose surface directly faces the flange 2 with a gap of distance d in between. It is driven by a wave voltage gs(t)=Esg""t(l). where t represents time, ω represents angular frequency, and horse has a complex amplitude t representing both the amplitude and phase of the sine wave. Also, 1=J:T. 6 is a microphone of a sound pressure detector installed near the base of the acoustic tube 1, and its output is amplified by an amplifier 7 to obtain etrL(t) = E, g"" (2
) becomes a sine wave voltage. where E is the complex amplitude, which is generally a function of ω. 8 is a rectifying and smoothing circuit; e.

DC output E with a magnitude proportional to the amplitude IE'ml of (f)
will occur. 9 is a meter that displays E.

Now, with the gap distance d constant and ω changed, the frequency characteristic G (ω) = E? Taking Fl/Es(3), if d is very small and the measurement end is almost closed, the acoustic tube l will be in a state where both ends are closed, and if the speed of sound in the air is C, then ω2 = πc/l
(4) and its integer multiple ω, and as shown in Figure 2(a), 1G(ω)1 exhibits resonance peaks at those frequencies (secondary and higher resonance peaks are not shown). ), if d is slightly increased, Fig. 2(b)
As in, the resonance peak mentioned above almost disappears. When d is roughly increased, the acoustic tube l is closed at one end,
The other end becomes open, and as shown in FIG. 2(C), a resonance peak occurs at ω, = πc/21 (5) and an odd multiple of ω. ω in the above equation is 1/2 of ω2 in equation (4), but in reality, due to the influence of the flange at the open end, ω1 becomes ω2/
It will be slightly smaller than 2. Note that the peak at 0 corresponds to third-order resonance, and is 3ω (resonance peaks of 5th and higher orders are not shown).

Therefore, by fixing the frequency of the oscillator 5 to ω=1 and increasing d from 0, as shown by the solid line in Fig. 3(a),
The output E of the rectifying and smoothing circuit rapidly decreases as d increases, and when d becomes several hundredths of a millimeter, E becomes almost O. That is, d can be determined from the magnitude of E with high sensitivity. Next, if we fix ω = ω and further increase d, a resonance state with the measurement end open will occur when d is several tens of hundredths of a millimeter away, as shown in Figure 3 (b). As shown by the solid line, E increases rapidly as d increases, and this portion can also be used to measure d. However, the rate of increase in E in this case is slower than in the case where ω=%.

That is, in the embodiment described here, there are two measurement modes, one of which is a high-sensitivity mode in which the measurement end is brought close to the object surface and the acoustic tube is in a resonant state with the measurement end closed.
The other is a low-sensitivity mode in which the measurement end is placed relatively far from the object surface and the acoustic tube is used in a resonant state with the measurement end open. However, even though the sensitivity is low, it is a comparison with the first mode, and the sensitivity is sufficient for a micrometer. A minute displacement meter can be realized using either of the above two modes. The dotted line in Figure 3 shows the characteristics when the flange 2 is made larger, and the sensitivity as a minute displacement meter is:
This shows that the size of the flange and the thickness of the pipe can be changed depending on the throat.

A more quantitative explanation of what has been said above is as follows.

The volume velocity of air entering and exiting is my(t) =
Consider the acoustic impedance of the gap Z) = PI/U, as %ejbrt. The acoustic tube 1 is not limited to a circular cross section, but here it is a circular tube with an inner radius a, and the flange 2 may also be of any shape, but it is a circular tube with a radius of a. is attached concentrically to the tip of the acoustic tube 1, the above glue can be expressed as follows.

Here, η is the viscosity coefficient of air, and ρ is the density of air. This approximation formula holds true within the range of ωρd2/η<40 (7), and in the case of the aforementioned low sensitivity mode with the measurement end open, it can be handled by this formula (6). In addition, in the case of high sensitivity mode with the measurement end closed, ωρd2/η< l (8), but in this case, the imaginary part of equation (6) can be ignored and %'F (6η/ πdJ)In Cb/ a) (
9) may be approximated.

Next, the sound pressure inside the root of the acoustic tube l is p<t> ==
p , J'de, and the volume velocity at that point is U(
t) = UeJ"', and considering the acoustic impedance Z = P/U when looking from the measurement end from the root to the tip, it is generally expressed as where k is the wave number = ω/c (11) It is.

Assuming that we are now in a resonant state with the measurement end open, ω = ω.

Then, from equation (5), kL=π/2, so the above equation becomes? = Cp c/S)"/!1 (12). Substituting equation (6) into 2. of this equation, lz and l become smaller as d increases, and therefore 1.?1 increases. Considering that the output acoustic impedance of the sound source 4 is large and therefore E71 is almost constant regardless of the magnitude of lzI, the above (zl
The change in d corresponds almost directly to the change in E due to d shown in FIG. 3(b).

In equation (6), if d = ψ, E becomes O, but in reality, E does not become infinite. This is because there is sound attenuation inside the acoustic tube 1, and the output of the sound source 4. This is due to the fact that acoustic impedance is actually finite. Also d
If becomes too large, the approximation of equation (6) itself will no longer hold true.

In the case of resonance with the measurement end closed, if ωmiω2, kl=π from equation (4), so equation (10) becomes Z=Z, (13),
In this case, Z is expressed by equation (9), so Z is l /
It changes in proportion to d'. Therefore, E is also L/
It will change in proportion to dj.

However, even if d=o, as shown in Figure 3(a),
In reality, E does not become infinitely large, and this is due to reasons such as sound attenuation inside the acoustic tube 1, as in the case where the measurement end is open.

In the above explanation, we have explained that the lowest order resonance frequency of the acoustic tube is used, but as mentioned above, the resonance with the measurement end open occurs at an angular frequency that is an odd multiple of ω, and the resonance with the measurement end closed occurs at an angular frequency that is an integer multiple of ω2, and d can be measured in the same manner as above using any of these resonance frequencies. If ω is increased, the range of equations (7) or (8) may be exceeded, and the approximation of equations (6) or (9) may no longer hold, but in that case, the acoustic impedance of the gap now It is only expressed in a more complicated format, but there is no change in the essential phenomenon that the change in d causes a sharp change in the pores, and therefore the resonance state within the acoustic tube. The reason for connecting it to the base of the sound tube is that whether the measurement end is open or closed, the base of the acoustic tube l is always the location where the sound pressure is maximum, so we are willing to accept a reduction in the microphone output. If not, you can connect the microphone elsewhere.

In equation (9), if the flange radius is increased, In
(b/a) increases, and the ratio of change in E to change in d becomes gradual, as shown by the dotted line in FIG. 3(a). In other words, the sensitivity as a minute displacement meter becomes low.

The situation is the same for equation (6). On the other hand, if b is made smaller, the sensitivity becomes higher, but if the flange is removed as one of the extreme shapes, the outer diameter 2b of the acoustic tube 1 corresponds to the flange outer diameter, and the tube wall thickness (b - The slope corresponds to the width of the flange. Although such a structure is a preferable form because of its low manufacturing cost, the sensitivity may become too high depending on the purpose of use.

In such a case, as shown in Figure 4(a), cutting all or part of the end face of the measuring end diagonally at an angle α reduces the sensitivity in the high-sensitivity mode of closing the measuring end. It is effective for This α is usually a fraction of a fraction. Similarly, it is also effective to make one or more small holes 10 in the tube wall near the measurement end, as shown in FIG. 4(b).

As mentioned above, this embodiment has a high-sensitivity mode with the measurement end closed and a low-sensitivity mode with the measurement end open, but these two modes are not used by switching, but by utilizing the difference in resonance frequency. They can coexist simultaneously in one acoustic tube. Figure 5 shows such an embodiment, where 5' is a sine wave oscillator with an angular frequency of ω, 5'' is a sine wave oscillator with an angular frequency of ω2, and the sum of these two sine waves is transmitted through the earphone. Assume that the input voltage e to the sound source 4 is e.There are sounds of two frequencies coexisting inside the acoustic tube 1, which are detected by the microphone 6 and amplified by the amplifier 7, resulting in the microphone output eR. .11' and 11'' are narrowband filters with center angular frequencies ω and %, respectively, and e.

is separated into a component with an angular frequency ω and a component with an angular frequency ω2 and sent to rectifying and smoothing circuits 8' and 8'' to obtain DC outputs E and E2 having magnitudes proportional to the amplitudes of the respective components.

Here, looking at how E and E2 change with changes in the gap distance #d, we can see that when the first d of the eel is sufficiently large, the acoustic tube 1 is in a resonant state with the measurement end open, and the output E
, is large and E2 is small. As the measuring end approaches the object to be measured, E gradually decreases, and E7,
Both E2 have shifted to a small state. As d is gradually decreased, the acoustic tube 1 enters a resonant state with the measurement end closed, and E remains small, but E2 increases, and d=o.
And it becomes maximum. That is, according to this embodiment, the distance between the measuring end of the wooden acoustic tube and the object to be measured can be distinguished in three states. This is useful, for example, in a positioning servo device, where an object is moved at high speed while it is away from the target, and when it approaches a certain distance, it is moved at low speed while accurately measuring the distance.

The bandpass filters l 1' and ll'' simply have an angular frequency ω
It not only helps to separate the components of , and %, but also helps reduce the influence of external noise and improve measurement accuracy.
Inside the acoustic tube l, the sound pressure level is 1 in the resonance state.
Since a strong sound field of about 30 dB (average effective value of about 80 Pa) is formed, there is almost no influence of external noise under normal usage conditions. However, if there is particularly large external noise, it may enter the acoustic tube from the measurement end and affect the output of the rectifying and smoothing circuit. In such a case, even when driving the acoustic tube with a single sine wave as shown in Figure 1, a bandpass filter is inserted between the amplifier 7 and the rectifying and smoothing circuit 8 to detect components near the frequency of the sine wave. It is effective to reduce the influence of the above-mentioned noise by allowing only the noise to pass through.

In the embodiments described above, the sound tubes l were all straight pipes made of one piece of wood, but the sound tubes may be curved as long as the sound waves travel one-dimensionally through them. There is no problem even if the line branches into two or more lines from the middle, and FIG. 6 shows such an embodiment. In this case, the object to be measured 3 is cylindrical, and in order to measure its diameter, measurement ends having arcuate flanges 2' and 2'' are arranged at regular intervals across the cylindrical object. Then, the acoustic tube l is divided into branch tubes 1' and // from the middle,
Each reaches the measurement end mentioned above. In this case, the resonant frequency is from the sound source 4 to the flange 2' or 2'.
□Since it is determined by the total length of the pipe up to the fixed end, the lengths of branch pipes 1' and 1 must be equal to each other, and l
Let the internal cross-sectional area of 1' be S, and that of 1' and 1 be S, respectively.
, and S2, it is desirable to set S = S, + 52 (14) If S and S, + 52 are too different, sound reflection will occur at the branch point of the acoustic tube.

During resonance, the sound pressure inside the acoustic tube is not large enough, resulting in a decrease in measurement sensitivity.

The resonant frequency of the acoustic tube is proportional to the sound speed C within the acoustic tube, as shown in equation (4) or equation (5). And the speed of sound changes depending on the temperature of the air.

Therefore, the oscillation frequencies of oscillators 5 in FIGS. 1 and 6, and oscillators 5' and 5'' in FIG. , and automatically adjusts the oscillation frequency of these oscillators using the signal.However, as described below, even without the above measures, the output of the rectifying and smoothing circuit of the device can be It is also possible to use a method that is independent of changes in resonance frequency due to temperature.

In FIG. 7, 12 is a band noise generator, and the other parts are exactly the same as in FIG. 1. In this case, the signal es(t) given to the sound source 4 from the signal es(t) is a noise voltage, and has a uniform power spectrum Φ(ω) within a bandwidth of W, as shown in FIG.

For example, in the high-sensitivity mode with the measurement end closed, if the frequency characteristic from this es(t) to the microphone output e, 1I(t) is G(ω), then W is slightly larger than the bandwidth of G(ω). , Φ(ω) are set to completely cover G(ω). Then, the root mean square value of the microphone output becomes e:(t)1, so even if the resonant frequency 1 changes slightly depending on the temperature, as long as G(ω) does not go out of the band with a width of W, the above e:(t) will not change. Therefore, the output E of the rectifying and smoothing circuit is also insensitive to changes in ω2 due to temperature. When the gap distance d changes, IG(ω)(
The magnitude of changes sharply and e'Q<t> changes,
The fact that E changes accordingly is the same as when driving an acoustic tube with a sine wave.

FIG. 9 also shows an embodiment that deals with changes in resonance frequency due to temperature. 13 is a phase detector, and voltage controlled oscillator 14
Together with this, they constitute a 7A lock loop (PLL). That is, 13 generates a DC output proportional to the phase difference between the microphone output e, I and the output e9 of 14, so that eS has a constant phase difference with respect to 14.
control the oscillation frequency. The output e3 of 14 is also the sound source 4
The resulting sound pressure inside the sound tube 1 is detected by the microphone 6, and amplified by the amplifier 7 to become the aforementioned microphone output. In short, 13.
14.4, l, and 6.7 constitute one oscillation loop, whose oscillation frequency is mainly dominated by the resonance frequency of the acoustic tube l, and the phase difference between ~ and e is adjusted appropriately by the phase detector 13. By doing so, the above-mentioned oscillation frequency is always made to match this resonant frequency. According to this method, when the resonant frequency of the acoustic tube 1 changes due to a change in temperature, the frequency of the sound generated from the sound source 4 changes accordingly, and measurements are always performed at the peak frequency of the resonant frequency characteristic. .

The device shown in Figure 9 not only eliminates the influence of changes in resonance frequency due to temperature, but also actively utilizes this change in resonance frequency to eliminate changes in microphone sensitivity and air viscosity coefficient due to temperature. All impacts can be compensated. As shown in Fig. 10, the oscillation angular frequency generated from the sound source 4 increases from evening to night due to the rise in temperature, and at the same time, due to changes in microphone sensitivity, the microphone output el amplitude increases from IE to 1≦lX. Suppose we did. Here, if el and l are input to the rectifying and smoothing circuit 8 after passing through the compensation filter 15, and the frequency characteristic H(ω) of 15 is set as shown in FIG.
IE above)? The increase in Fl is IH(ωz)l −IH
(ω1) It can be compensated by a decrease of 1. The temperature
□If IEml decreases when it increases,
IH(ω)1 may be set as shown by the dotted line in FIG.

In short, with this device, temperature information is included in the oscillation frequency itself, and this information can be used to perform temperature compensation without using a separate temperature detector such as a thermistor. One specific means of compensation is to use a filter as mentioned above, but for example, the oscillation frequency can be counted and measured with a counter, and the final digital output can be corrected by digital calculation based on the result. , various known methods are applicable.

In all of the above embodiments, the absolute value of the acoustic impedance Z when looking at the acoustic tube from the sound source is the maximum at resonance, but the resonance of the acoustic tube is the minimum at the time of resonance. There are some things. The embodiment described below utilizes this resonance.

In FIG. 11, reference numeral 4 denotes a sound source with relatively low output acoustic impedance, such as a speaker, which generates a sinusoidal sound pressure p(t) inside a container 23 for distributing sound pressure. Measuring sound tubes 1', LTR and dummy sound tubes 21 are branched from 23. Flanges 2', 21 are attached to the tips of 1', 1'', which connect objects to be measured 3', 3, respectively. ``i is L facing directly. However, the internal cross-sectional areas of these acoustic tubes do not have to be equal to each other. Let's now explain the high-sensitivity mode in which the measurement end is used in a nearly closed state. In this case, the measurement end is the location where the sound pressure is maximum in the resonance state. The microphones 6' and 6" are placed close to the measuring end to detect the sound pressures a(f) and p2(f) at the measuring end.The outputs of these microphones are amplified by amplifiers 7' and 7", respectively. are rectified by 'rectifying and smoothing circuits 8' and 8'', and the magnitude of each is determined by the meter 9.
', 9''.

On the other hand, at the tip of the dummy acoustic tube 21, a small hole 22 having an acoustic impedance approximately the same size as the gap at the measurement end of the measurement acoustic tube is provided. The sound pressure p(t) behind this small hole is detected by the microphone 16 and converted into a DC voltage proportional to the amplitude of the sound pressure by the amplifier 17 and the rectifying and smoothing circuit 18. Control the amplification factor. The output of amplifier 17 is also
The sound source 4 is driven through a phase shift circuit 19 and an amplifier 20.

By appropriately setting the phase shift of 19, this loop self-oscillates and is controlled so that the amplitude of the sound pressure p,<t) is constant. The oscillation frequency at this time is not only the resonant frequency of the dummy acoustic tube 21, but also equal to the resonant frequency of the acoustic tubes 1' and l''.

Sound pressure applied from the container 23 to each acoustic tube p<t, =
Pijwt and the sound pressure p at the tip of each acoustic tube,
(t) = P, ej"' (i = 1.2.3
>Tono relationship 1*Tsuki LL It is expressed as follows.

Here, S represents the internal cross-sectional area of each acoustic tube, and Z,
is the acoustic impedance of each tube end, and in the case of 1' and 1, it is the impedance of the gap as given by equation (9), and in the case of the dummy acoustic tube 21, it is the impedance of the gap at the small hole 2.
If the shape of 2 is a cylinder with a circular cross section of radius r and height h, then Z3" = 877 h/πa" (1
7) How can it be expressed approximately?

The absolute value of P in equation (16) is maximum because ω is (5)
This is a case where ω given by the formula and its odd multiple are taken, and this and S are P-=-j Z-P/ Cp c/S-)
(18) At this time, the acoustic input impedance seen from the root to the tip of the acoustic tube is the minimum, and the maximum acoustic power is injected from the container 23 into each acoustic tube. When the gap distance between the flange and the object to be measured becomes smaller, the acoustic impedance Z or Z2 becomes ρc/
It becomes much larger than St or ρc/5z, and the bow or bow increases rapidly in proportion to it.

By amplifying and rectifying this as described above, the gap distance can be measured with high sensitivity.

A feature of this embodiment is that a plurality of acoustic tubes can be easily driven simultaneously with one sound source, and when one of the acoustic tubes is used as a dummy, various unique effects can be produced. In other words, the dummy acoustic tube not only keeps the oscillation frequency of the sound source equal to the resonant frequency of the acoustic tube, but also maintains the amplitude of the pressure P□ at its tip constant. If the temperature coefficients of are equal and at the same temperature, the effect of temperature change will be canceled by the dummy and the measuring sound tube. and the influence of changes in air density ρ due to atmospheric pressure.
By terminating the tip of the dummy acoustic tube with an acoustic impedance similar to that of the gap to be measured, they cancel each other out.The device shown in Figure 11 is a low-sensitivity device that uses the measurement end in an almost open state. It can also be applied to modes as is. However, in this case, in the acoustic tube in a resonant state, the distance of 1/2 from the measurement end is the location where the sound pressure is maximum, so it is a good idea to connect the microphone near that location.

FIG. 12 shows an example using a Helmholtz resonator.

Reference numeral 4 denotes a sound source with a relatively large output acoustic impedance, such as an earphone, which drives the container 24 at a volume velocity of U(t)=UJ''.The resulting sound pressure inside the container 24 p(
f) =7'g/''' is detected by the microphone 6, and its output electrical signal is amplified by the amplifier 7 and becomes an input to the phase detector 13. 13 is a voltage controlled oscillator 14
The output e is used as another input, and the 14 oscillation frequencies are controlled so that the phase difference between eR and eR is constant. Output e
s also drives the sound source 4. As in the case of FIG. 9, these constitute an oscillation loop by the PLL and the acoustic resonance system, and the oscillation frequency of the loop is equal to the resonance frequency of the Helmholtz resonator consisting of the container 24 and the acoustic tube l.

The container 24 is different from the container 23 in FIG. 11 which is a mere sound pressure distributor, and in this Helmholtz resonator, c=v, 7γF, (111
), where 4 is the internal volume of the container 24, the bow is the atmospheric pressure, and γ is the specific heat ratio of air. Further, the length L of the acoustic tube 1 is sufficiently small compared to the wavelength of the sound used, and the air inside thereof acts as an acoustic inertance Lt==ρL/S (20). In addition, the air in the gap of thickness d sandwiched between the flange 2 and the object to be measured 3 acts as acoustic inertance, but the value is that the acoustic tube l is a circular tube with an inner radius α, and the flange 2 is a circular tube with an inner radius α. If it is a circle, then from the imaginary part of equation (6), L = (3ρ15πd)In (b/a) (2f
), the overall acoustic inertance L is the sum of the above two values, but now, assuming that the length L of the acoustic tube 1 has effectively increased to L, L = Lt + L, = ρL/S (2
2), and if the resistance of the tube wall of the acoustic tube l is ignored, the acoustic resistance R becomes only the resistance of the gap, and its value is calculated from the real part of equation (6) as R=R,= (Bη/ πd3) In (b/a)(2
3).

In the Helmholtz resonator, the above-mentioned elements constitute one secondary parallel resonant system. The acoustic impedance Z=P/U when looking at the container 24 from the sound source 4 is as follows. In the above equation, ω is the resonance angular frequency and q = l / nτ = c σTv (25), where C is c=J leek', / p (2B),
This is nothing but the speed of sound in air, and ζ is a damping coefficient, ζ==(R/2)JvAL (27).

This device using a Helmholtz resonator is used only when the end of the acoustic tube l is close to open.

Therefore, it is in the state of ζ(1. Under this condition, ω=ω
Then, equation (24) is approximated as Z', L/CR (28). Since R is inversely proportional to the male as shown in equation (23), Z changes sensitively depending on the magnitude of d. Considering that the output acoustic impedance of the sound source 4 is large, the above change in Z directly changes the sound pressure p.
This is a change in the size of . That is, as the gap distance d is gradually increased from O, the sound pressure p rapidly increases when the end of the acoustic tube reaches a distance that can be considered open. This sound pressure change is detected by the microphone 6.

In the same way as in the previous embodiments, this is displayed as a change in the distance 9d.

Another use of the Helmholtz resonator is the gear distance ad
This method uses the resonant frequency change due to (21
), the effective length l' of the air acoustic tube in the gap changes. In the device of FIG. 12, the oscillation frequency is equal to the resonant frequency ω.

However, when d is gradually increased, ω, near the point where the sound pressure starts to increase rapidly,
increases. This frequency change is measured, for example, by counting the output e of the voltage controlled oscillator 14 with a counter (not shown), and is displayed as a change in d.

The problem here is the change in ω due to temperature. One method of compensating for this effect is to measure the temperature inside the container 24 using, for example, a thermistor, and use that value to correct the count value of the counter by digital calculation. Another method is to use another dummy device that is exactly the same as the device shown in FIG. 12, which oscillates while keeping the gap distance constant. Then, using a reversible counter, first count the oscillation signals from the measurement device in the forward direction for a certain period of time, then count the oscillation signals from the dummy device in the reverse direction for the same period of time, and calculate the difference between the oscillation frequencies of the two. be measured. There are two devices.

If they are in the same temperature environment, this method can compensate for the effects of temperature changes and extract only the frequency changes due to changes in the gap distance d of the measuring device.

Still another method is to appropriately frequency-multiply the oscillation signal of the dummy device and use this as a clock pulse to measure the period of the oscillation signal of the measurement device.

This method has the advantage that, as a general feature of period measurement using a counter, the measurement time is short, and therefore the number of measurements per unit time can be increased.

In summary, the essence of the present invention is the phenomenon that when the measuring end of an acoustic tube is placed at a position separated by a small gap from the object to be measured, the acoustic impedance of the measuring end changes sharply depending on the distance of the gap. is used to measure the gap distance. The above change in acoustic impedance is used for both its real and imaginary parts. Then, an acoustic resonance system including this acoustic tube is constructed, and the change in the acoustic impedance is converted into a change in the resonance frequency or the magnitude of the sound pressure therein, and detected. As a result, various unique effects not seen in conventional devices, as mentioned at the beginning, are produced.

[Brief explanation of drawings]

Fig. 1 shows an example of the present invention, Fig. 2 shows the frequency characteristics of the device shown in Fig. 1, Fig. 3 shows the change in sound pressure depending on the gap distance, and Fig. 4 shows the shape of an acoustic tube without a flange. example, fifth
The figure shows an example in which two measurement modes coexist in one acoustic tube, FIG. 6 shows an example in which a branched acoustic tube is used, FIG. 7 shows an example in which noise is used as the sound source signal, and FIG. The figure shows the noise power spectrum of the sound source signal and the resonant frequency characteristics of the acoustic system, Fig. 9 shows an example of using an oscillation loop including the acoustic system, and Fig. 10 shows an explanation of temperature compensation in the device shown in Fig. 9. Fig. 11 is an embodiment in which a plurality of acoustic tubes are driven by one sound source, and Fig. 12 is an embodiment in which a Helmholtz resonator is used. flange, 3-111 object to be measured, 4-1-- sound source, 5-111 oscillator, 6-- sound pressure detector such as microphone,
7---Amplifier, 8-111 rectifier and smoothing circuit, 9 display meter, 10---1 small hole, l l', 11''-111 band filter, L2---1 noise generator , 13-111 phase detector, 14--m-voltage controlled oscillator, 15-111 compensation filter, 16-- sound pressure detector such as microphone, 17-- amplifier, 18- 7- rectifying and smoothing circuit, 19-- one phase shift circuit, 20-- one variable amplification factor amplifier, 21-- dummy acoustic tube, 22-- one small hole,
23---Container for sound pressure distribution, 24---Container for Helmholtz resonator, c---One speed of sound, η---Viscosity coefficient of air, ρ---Density of air, No.-111 Internal volume of container, P ---1 atmospheric pressure, γ --- Constant pressure constant volume specific heat ratio of air.

Claims (1)

[Claims] Detects an acoustic tube having a measurement end at a position a short distance from the surface of the object to be measured, an acoustic resonance system including this acoustic tube, a sound source that drives this acoustic resonance system, and sound pressure in the acoustic resonance system. and a sound pressure detector that detects the minute distance by detecting a change in the magnitude of the sound pressure in the acoustic resonance system or a change in the resonant frequency of the acoustic resonance system caused by a change in the minute distance. formula micrometer.