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A novel doppler based ultrasonic surface roughness measurement

Surface topography is of great importance in specifying the function of a surface. A significant proportion of component failure starts at the surface due to either an isolated manufacturing discontinuity or gradual deterioration of the surface quality. Typical, these problems of lower surface integrity, lead to in service stress corrosion and fatigue failure. The most important parameter describing surface integrity is surface roughness. In the manufacturing industry, surface must be within certain limits of roughness. Therefore, measuring surface roughness is vital to quality control of machining workpiece, especially if it is non-contact method compared to the direct conventional method; which uses stylus type devices. In this paper, a novel Doppler based ultrasonic sensor has been proposed for high-precision surface roughness measurement. Despite conventional non-contact ultrasonic techniques which use the ultrasonic travel time to measure the distance of the undertest surface from the sensor to describe the roughness parameters, the proposed sensor implements the Doppler effect on the continuously emitted ultrasonic bursts for surface roughness measurement. When an object is ready to undergo roughness testing, it is mounted on a motor rotating at a constant velocity of ω. The device comprises an ultrasonic transmitter emitting sound pulses travel across to the undertest object. The sonic wave is reflected diffusely on the object where it is separated into many weak sounds, a few of which are received by the receiver. The Doppler effect causes the frequency of the received signal to be shifted with respect to the roughing depth and its shape. The relationship between the rate of change of the frequency of the received signal and the peak-to-valley height and the roughing shape is mathematically analyzed. After amplification, the received signal is applied to a high-pass filter and therefore to a rectifier stage. Of its frequency response, the circuit provides an output voltage related to the frequency of the received signal. Analysis of the output voltage by a processor allows the parameters of the surface roughness to be calculated. Since the sonic speed changes in various environmental situations, it may cause some errors in transit-time based sensors. Using Doppler effect, these problems are avoided and therefore a more precise measurement is achieved. Also the continuous ultrasonic emission allows the processor to sample the surface at any point where needed. The proposed method also can be applied for vibration analysis of the rotating fans. The fabrication of a prototype sensor and experimental verification of the analytical results are reported. The experimental results agree well with the theory.