Micro-hole Inspection System Using Low-Frequency Sound

Transcription

1 Journal of JSEM, Vol.14, Special Issue (2014) s105-s109 Copyright C 2014 JSEM 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 , Japan 2 Faculty of Engineering, Shinshu University, Nagano , 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 microhole. 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,, 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 microholes [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, P 1 is the pressure on the inlet side of the circular tube, and P 2 is the pressure on the outlet side. Q = πr 4 ( P 1 - P 2 ) / 8νL (1) Function generator AC voltmeter cylindrical metal casing (built-in loudspeaker and a microphone) Fig. 1 Micro-hole stamped in metal Fig. 2 Prototype inspection system -s105-

2 Y. NAGASU, K. EGUCHI, K. ITOH, M. OTANI and N. NAKAYAMA micro-hole sample Processed by EDM microphone AC voltmeter Φ=104.8μm Φ=100.6μm Fig. 5 Micro-hole formed by EDM loudspeaker function generator Φ=99.8μm Stamped hole Φ=104.8μm Fig. 3 Inspection system schematic flow rate passing through the micro-hole measures the pressure change inside the metal casing sound vibrates Fig. 4 Detail plan around microphone 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 P 1 - P 2. 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 Fig. 6 Micro-hole formed by metal stamping 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 V p-p and 10 to 90 Hz, respectively. 3.1 Different diameter samples The test samples had a thickness of 0.1 mm and the microhole 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 Metal Stamping Electric Discharge Machining Table 1 Different diameter samples Average Hole Volume 10-3 [mm 3 ] s106-

3 Journal of JSEM, Vol.14, Special Issue (2014) Φ=308μm Φ=297μm Fig. 7 Drilled micro-hole Φ=47μm 58 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 microhole 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 Fig. 8 Effects of speaker frequency Φ=47μm 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 microholes 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 Fig. 9 Effects of input voltage Results Figure 8 shows the effect of the loudspeaker frequency on the measured effective voltage for different hole Φ=47μm Table 2 Different thickness sample Thickness [mm] Average Aspect Ratio Fig. 10 Measured voltages for various microhole diameters Table 3 Difference in measured voltage Input Voltage [Vp-p] Measured Voltage Measured Voltage at φ108.3µm at φ46.9µm Difference 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 -s107-

4 Y. NAGASU, K. EGUCHI, K. ITOH, M. OTANI and N. NAKAYAMA t=0.3mm input voltages. However, the difference in the measured voltage between micro-holes of 47 µm and 108 µm in diameter is maximal for 10 V p-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 V p-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 microhole 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 µm). 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 V p-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 Fig. 11 Measured voltages for micro-hole aspect ratio 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 microholes 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 -s108-

5 Journal of JSEM, Vol.14, Special Issue (2014) 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), [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), [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), [4] Hata, S.: Trends and Future of Industrial Application of Image Processing Technologies, J. Soc. Precision Engineering, 67-6 (2001), [5] Hata, S.: Trends of Automatic Visual Inspection, J. Soc. Precision Engineering., 56-8 (1990), [6] Okamoto, S. and Yoshimura, K.: Automatic Visual Inspection of Soldered Parts on Printed Circuit Boards, J. Welding. Soc., 58-4 (1989), [7] Torigoe, I.: Measurement using audio-frequency sound, J. Acoust. Soc. Jpn., (2005), [8] Ishii, Y.: An acoustic volumeter, J. Instrument and Control Eng., 36-4 (1997), [9] Komiya, K.: Measurement of volume of old weights using an acoustic volumeter, Bull. Soc. Historical Metrology Jpn., 30-1 (2008), s109-