Micro-hole Inspection System Using Low-Frequency Sound

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.