Development of a new displacement-measuring ultrasonic sensor based on astigmatic focus error detection

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In-process measurement is generally considered to be the most functional method for obtaining high-dimensional accuracy during cutting and grinding operations, with contact type sizing devices being commonly used when such accuracy is required. Optical techniques are not suitable for use in these operations due to wet conditions arising from lubricant application.In contrast, corresponding ultrasonic techniques can be effectively applied when fully immersed in a liquid, though another problem occurs regarding measurement accuracy of conventional ultrasonic displacement sensors which employ the pulse-echo method. The resolution is, typically, 2-20mum, which is not adequate to meet desired in-process measurement standards.This drawback in measurement resolution led us to develop an entirely new type of ultrasonic displacement measurement method, i.e., we built the first-ever displacement-measuring ultrasonicsensor whose operation is based on the principle of astigmatic focus error detection. Basically, the sensor incorporates a cylindrical acoustic lens that detects a change in distance between an acoustic objective lens and target surface converting it to an equivalent change in sound pressure distribution which is measured by a quadrant type ultrasonic detector. Here, we describe in detail the measuring principle of this new method and the basic analysis applied in the design method, after which results of experimental evaluations are presented demonstrating method/system suitability to meet its intended application as an in-process measurement device possessing high-dimensional accuracy during cutting or grinding operations.