-s105-

Micro-hole Inspection System Using Low-Frequency Sound

Yoshinori NAGASU

1, 2, Kakumasa EGUCHI

1, Kazunori ITOH

2, Makoto OTANI

2 and Noboru NAKAYAMA

2

1 Nagano Prefecture General Industrial Technology Center, Okaya 394-0084, Japan

2 Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan

(Received 22 November 2013; received in revised form 14 April 2014; accepted 19 April 2014)

Abstract

This paper presents a method for inspecting micro-holes

with diameters of less than 100 µm by using audible sound.

Fluid flow through micro-holes affects the performance of

a product such as CO gas-detection sensor components,

microphone filters, and fuel-injection plates. Therefore, if

the flow rate of fluid passing through micro-holes could be

accurately assessed by means of an inspection system, it

would be possible to guarantee the performance of the

product.

The prototype inspection system used in this study

consists of an AC voltmeter, a function generator and a

cylindrical metal casing containing a small built-in

loudspeaker and a microphone. When a loudspeaker in a

sealed casing is driven by a low-frequency sine wave, an

excited sound vibrates the air inside the metal casing. The

developed system measures the change in pressure at the

microphone as the sound wave passes through a micro-

hole. It is therefore the same as measuring the flow rate of

a fluid passing through a micro-hole. This system is able to

detect a 5-µm diameter difference in a micro-hole.

As the flow rate through a micro-hole can be easily

measured, this inspection method is considered to be

applicable to micro-hole components intended for use in

flow control. Measurement time takes about 1 s after a

standard has been set, which is faster than existing

measurement methods.

Key words

Micro-hole, Diameter, Aspect Ratio, Low-frequency

Sound, Pressure

1. Introduction

Automotive fuel-injection plates, gas-detection sensors,

and high-performance microphones all have components

containing micro-holes that are used to control flow rate.

In order to guarantee their performance, the shape of these

micro-holes needs to be precisely realized in such

components. Scanning electron microscope (SEM)

photomicrographs of a micro-hole are shown in Fig. 1.

Micro-hole shapes can be inspected using conventional

inspection methods such as an optical microscope and

contact 3D scanning [1-3]. However, these methods cannot

accurately predict the flow rate of a fluid nor are they fast

enough for the 100%-inspection that is often required on a

line producing these components. Image-processing

technology can realize high-speed inspection of micro-

holes [4-6], however cannot inspect their shape. For

products that require flow assurance of a gas passing

through holes, such as automotive fuel-injection plates and

gas-detection sensors, the shape of the hole affects the

performance of the product. Therefore, a new inspection

method that is simpler and faster is required. A possible

solution is measurement using sound. For example,

acoustic impedance measurement utilizing audible-range

sound is used for volume and surface area measurements

of an object in a closed volume [7-9]. In this study, we

propose a fast, highly accurate inspection method using

audible sound for micro-holes that are mass-produced by

metal stamping. This paper presents details of the proposed

method and validates its performance.

2. Methodology Flow through micro-holes affects the performance of a

product. Therefore, if the flow rate of fluid passing through

micro-holes could be accurately assessed by means of an

inspection system, it would be possible to guarantee the

performance of the product.

Flow through a circular tube of radius r and length L is as

shown in Eq. (1) by the Hagen-Poiseuille equation. In this

equation, Q is the flow through micro-holes, ν is the

coefficient of viscosity, P1 is the pressure on the inlet side

of the circular tube, and P2 is the pressure on the outlet side.

Q = πr4

( P1 - P2 ) / 8νL (1)

AC voltmeter

cylindrical metal casing

(built-in loudspeaker and a microphone)

Function

generator

Fig. 2 Prototype inspection system Fig. 1 Micro-hole stamped in metal

Journal of JSEM, Vol.14, Special Issue (2014) s105-s109Copyright Ⓒ 2014 JSEM

-s106-

Flow through a circular tube as determined by Eq. (1) is

seen to change according to hole radius r, hole depth L and

pressure gradient P1 - P2. In the prototype system, a

microphone is used to detect the change in pressure of an

sound wave as it passes through a hole. The external

appearance of a prototype inspection system is shown in

Fig. 2, and a schematic is shown in Fig. 3.

The inspection system consists of an AC voltmeter (VT-

181, TEXIO Technology Co.), a function generator

(DF1906, NF Co.) and a cylindrical metal casing with a

small built-in loudspeaker (SW070WA02-01, Wavecor

Ltd.) and a microphone (ATH-C500M, Audio Technica

Co.). A micro-hole component is placed on top of the

metal casing as shown in Fig. 4. The outer diameter of

the test container is 120 mm and the height is 150 mm.

The volume of air between the microphone and the

speaker is about 48 mL, and that between the

microphone and the micro-hole is about 0.5 mL.

When a loudspeaker in a sealed casing is driven by a

low-frequency sine wave, an excited sound vibrates the

air inside the metal casing. If the vibration of the speaker

is large, the pressure change inside the metal casing

increases. The microphone detects the pressure change

inside the metal casing. Here, the detected pressure

varies according to the volume of a micro-hole. Thus,

micro-holes can be inspected by analyzing the

differences between the measured values. This system

measures the change in pressure at the microphone when a

sound wave passes through a hole. This is effectively the

same as measuring the flow rate of a fluid passing through

the micro-hole.

3. Validation of the Prototype Inspection System

The prototype inspection system was validated using

micro-hole test samples with micro-holes of various

diameters and depths. By inspecting samples with different

hole diameters however uniform thickness, it is possible to

evaluate whether a difference in the diameter of a hole can

be detected. Likewise, by inspecting samples with a

different thickness however constant hole diameter, it is

possible to evaluate whether a difference in the depth of a

hole can be detected. The appropriate driving voltage and

output frequency of the speaker required to output the

audio signal were experimentally determined and found to

be 5.0 to 10 Vp-p and 10 to 90 Hz, respectively.

3.1 Different diameter samples

The test samples had a thickness of 0.1 mm and the micro-

hole diameter ranged from 50 to 110 µm. Table 1 shows

the front, back, average diameters and hole volumes of the

test samples. There are two basic ways of producing a

hole: electrical discharge machining (EDM) and metal

stamping. The micro-hole diameter was measured using a

non-contact type coordinate measuring machine (NH3-SP,

Mitaka Kohki Co.). The diameter was measured by

loud-

speaker

microphone

micro-hole

sample

AC voltmeter

function

generator

Fig. 3 Inspection system schematic

measures the pressure

change inside the metal

casing

flow rate passing through

the micro-hole

sound vibrates

Fig. 4 Detail plan around microphone

100μm

Front side

Φ=104.8μm

Backside

Φ=100.6μm

Processed by EDM

Fig. 5 Micro-hole formed by EDM

Front side

Φ=99.8μm

100μm

Backside

Φ=104.8μm

Stamped hole

Fig. 6 Micro-hole formed by metal stamping

Table 1 Different diameter samples

Front side

Diameter [µm]

Backside

Diameter [µm]

Average

Diameter

[µm]

Hole

Volume

×10-3

[mm3

]

Metal

Stamping

99.8 104.8 102.3 0.82

Electric

Discharge

Machining

112.4 104.2 108.3 0.92

104.8 100.6 102.7 0.83

101.2 96.0 98.6 0.76

97.6 90.0 93.8 0.69

94.2 88.4 91.3 0.65

79.0 74.2 76.6 0.46

67.8 72.4 70.1 0.39

60.6 56.2 58.4 0.27

48.2 45.6 46.9 0.17

Y. NAGASU, K. EGUCHI, K. ITOH, M. OTANI and N. NAKAYAMA

-s107-

approximating the micro-hole shape to a circle by applying

least square fitting.

The shape of the micro-hole differs depending on the

hole-formation method used. In the case of EDM, the

diameter on the front side is approximately 5 µm larger

than that on the backside (Fig. 5). This is due to stagnant

powder around the EDM electrode. In the metal-stamping

method, the diameter of a micro-hole on the backside is 5

µm larger than that on the front side (Fig. 6). Stamped

holes have a four-layer structure that includes a shear drop,

sheared surface, fractured surface, and burring. The micro-

hole diameter on the backside increases due to the effects

of burring. The cross-sectional shape is not straight either,

so the average of diameters on the front side and backside

was chosen as the representative micro-hole diameter of

the test samples.

3.2 Different thickness samples

The test samples have a diameter of 0.3 mm and the

thickness ranges from 0.3 to 1.9 mm. The aspect ratio of

hole depth and hole diameter is from 1.1 to 6.3. The micro-

holes were drilled by a compound jig borer. Table 2 shows

the thickness, diameter of the front side, diameter of the

backside and aspect ratio of the test samples. Figure 7

shows an image of the front side and backside of the holes

drilled. There is a maximum difference in hole diameter of

about 30 µm for the front side and the backside.

4. Results

Figure 8 shows the effect of the loudspeaker frequency

on the measured effective voltage for different hole

volumes. Here, the input voltage was 10 Vp-p. The average

hole diameters are indicated on the curves. It can be seen

that the measured voltage increased with frequency.

However, the lowest frequency of 10 Hz yielded the

largest dependence of the voltage on the hole volume.

Figure 9 shows the relation between input voltage to

loudspeaker and the measured effective voltages. The input

voltage values were 10, 7.5, and 5.0 Vp-p 5.0 Vp-p and the

frequency was set to 10 Hz. It can be seen that the

measured effective voltage changed in proportion to the

Table 2 Different thickness sample

Thickness

[mm]

Front side

Diameter

[µm]

Backside

Diameter

[µm]

Average

Diameter

[µm]

Aspect

Ratio

0.34 299 308 303 1.1

0.66 315 299 306 2.2

0.91 296 309 302 3.0

1.27 297 308 302 4.2

1.56 301 310 305 5.1

1.88 325 296 311 6.0

Table 3 Difference in measured voltage

Input

Voltage

[Vp-p]

Measured Voltage

at φ108.3µm

[mV]

Measured Voltage

at φ46.9µm

[mV]

Difference

[mV]

5.0 0.24 0.15 0.09

7.5 0.31 0.19 0.12

10 0.38 0.22 0.16

Φ=47μm

5870

77

91

94

99

103

108

Fig. 10 Measured voltages for various micro-

hole diameters

Φ=47μm58 70 77 91 94 99 103 108

Fig. 8 Effects of speaker frequency

Φ=47μm5870

77

9194

99103

108

Fig. 9 Effects of input voltage

Front side

Φ=297μm

Backside

Φ=308μm

100μm

Fig. 7 Drilled micro-hole

Journal of JSEM, Vol.14, Special Issue (2014)

-s108-

input voltages. However, the difference in the measured

voltage between micro-holes of 47 µm and 108 µm in

diameter is maximal for 10 Vp-p input voltage (Table 3).

This result shows that the inspection resolution can be

improved by increasing the input voltage to the

loudspeaker.

Figure 10 shows the dependence of the effective voltage

on the micro-hole volume for an input voltage of 10 Vp-p

and a frequency of 10 Hz. The average hole diameters are

indicated on the curve. The measured voltage clearly

increases with micro-hole diameter; a 5 µm difference

yields a 20 µV difference in voltage, although the change

in voltage becomes smaller as the diameter decreases to

below 80 µm or so. When three measurements per micro-

hole were made, the difference between the measured

voltage was found to be less than ±2 µV.

The two test samples have the same average micro-hole

diameter (approximately 102-103µm). Each was processed

by the metal-stamping method and the EDM method. The

two samples have different front side and backside

diameters, however neither show a significant difference in

measured effective voltage.

Figure 11 shows the relation between the aspect ratio of

a micro-hole and the measured effective voltages. The

input voltage is set to 10 Vp-p and the frequency is set to 10

Hz. The thicknesses are presented along the curve. The

results demonstrate that the measured voltage decreases as

the micro-hole depth increases; a 0.3-mm difference in

micro-hole depth yields a 0.2-mV difference in voltage in

this case. The influence of micro-hole diameters that are different

on the front side and the backside was then examined. The

same measurements were performed with the same test

samples installed the other way up. The results showed no

significant difference in measured voltage between the

original and inverted installations, indicating that the

proposed method is capable of guaranteeing the flow rate

through a micro-hole. However, if the difference in

diameters between the front side and backside is larger, the

effect on measured voltage is unknown.

5. Discussion

In Eq. 1, the flow rate passing through the hole was

shown to vary due to the pressure difference between the

inlet and outlet, hole depth and hole diameter. The results

in Fig. 9 show that the flow rate passing through the holes

is increased by a high input voltage. This is believed to be

because the difference between the atmospheric pressure is

increased because the pressure in the cylindrical casing is

higher. This makes it possible to detect a smaller hole

diameter difference due to the measured voltages being

increased in the case of high flow.

It was also found, according to the results in Fig. 8, Fig.

10 and Fig. 11, that the measurement resolution is

increased at a low frequency. This is thought to be because

lower-frequency sound waves can more easily pass

through a micro-hole due to diffraction. Therefore, in order

to improve the measurement resolution, it is desirable to

set a higher input voltage and a lower frequency.

The experimental results show that the proposed method

is capable of detecting a difference in micro-hole diameter

from the measured voltages. Specifically, for 100-µm

diameter micro-holes having a thickness of 100-µm, which

were the parameters adopted in this study, the proposed

method is able to detect a 5-µm difference in micro-hole

diameter.

The results in Fig. 10 show that the measured voltage is

correlated with the volume of a micro-hole, not the

diameter or cross-sectional shape. As the micro-hole

volume is proportional to the flow rate through the holes,

the proposed method is suitable for inspection of flow

control parts. The results in Fig. 11 show that differences in hole depth

are also detectable, although the resolution is only about

0.1 mm, which is significantly lower than that for the hole

diameter.

The time required to measure the flow rate through a

micro-hole is about 1 s, after a micro-hole components has

been set which is faster than existing measurement

methods.

The coefficient of the viscosity of a fluid is proportional

to temperature, however, the influence of this on the

measurements in this study were not confirmed. It is

believed that in order to obtain a stable measurement result,

it is necessary to keep the measurement environment as

constant as possible.

Further study is needed with regard to the shape of the

metal casing. There is a possibility that the accuracy of the

measurement can be improved by optimizing the volume

of the container. In particular, the detection performance of

the microphone might be improved by reducing the

volume between the micro-holes and the microphone. In

addition, by selecting speakers and a microphone that

better match low frequencies, the stability of the

measurement results may be increased.

6. Conclusion

This paper proposed a method for inspecting micro-

holes with diameters of less than 100 µm by using audible

sound. This system is able to detect a 5-µm diameter

difference in a micro-hole.

The experimental results indicate that a flow rate

through a micro-hole correlates with a voltage measured

by a microphone installed in the inspection system. The

t=0.3mm

0.7

0.9

1.3

1.6

1.9

Fig. 11 Measured voltages for micro-hole aspect ratio

Y. NAGASU, K. EGUCHI, K. ITOH, M. OTANI and N. NAKAYAMA

-s109-

flow rate passing through the holes is increased by a high

input voltage. The measurement resolution is increased at a

low frequency. Thus, it is expected that the proposed

method can be used as an effective tool for determining the

diameter of micro-holes.

Acknowledgement

This work was supported by A-STEP FS-Stage for

Exploratory Research, Grant Number AS231Z02722B, by

the Japan Science and Technology Agency. We are

indebted for the guidance we received from Hikaru

Yamagishi, Ryouichi Arai, Kazuyuki Kamijo and Takeo

Chigono on how to operate the measuring and processing

equipment used in this study. The advice and comments

given by Seiichi Kudo, Eimatsu Sakagami and Hideaki

Miyasaka were a great help in my reseach. We also

received generous support from Micro-Technology

Division of Misuzu Industries Co. in the form of the

provision of the measurement samples.

References

[1] Youkaichiya, M., Funabashi, T. and Takaya, Y.: The

high precision measurement of the minute shape by the

UA3P series, Proc. Autumn Meet. Jpn. Soc. Precision

Eng. (2011), 878-879.

[2] Murakami, H., Katsuki, A., Onikura, H., Sajima, T. and

Kondo, E.: Development of a system for 3-D micro

metrology using an optical fiber probe, Proc. Spring

Meet. Jpn. Soc. Precision Eng. (2011), 811-812.

[3] Murakami, H., Katsuki, A., Onikura, H., Sajima, T.,

Kawagoishi, N. and Kondo, E.: Development of a

system for measuring micro hole accuracy using an

optical fiber probe, J. Advanced Mechanical Design,

Systems, and Manufacturing 4 (2010), 995-1004.

[4] Hata, S.: Trends and Future of Industrial Application of

Image Processing Technologies, J. Soc. Precision

Engineering, 67-6 (2001), 879-882.

[5] Hata, S.: Trends of Automatic Visual Inspection, J. Soc.

Precision Engineering., 56-8 (1990), 1366-1370.

[6] Okamoto, S. and Yoshimura, K.: Automatic Visual

Inspection of Soldered Parts on Printed Circuit Boards, J.

Welding. Soc., 58-4 (1989), 300-304.

[7] Torigoe, I.: Measurement using audio-frequency sound,

J. Acoust. Soc. Jpn., 61-11 (2005), 653-658.

[8] Ishii, Y.: An acoustic volumeter, J. Instrument and

Control Eng., 36-4 (1997), 288-291.

[9] Komiya, K.: Measurement of volume of old weights

using an acoustic volumeter, Bull. Soc. Historical

Metrology Jpn., 30-1 (2008), 55-58.

Journal of JSEM, Vol.14, Special Issue (2014)