Field Inspection System of Aperture of Small Blind Holes in the Machining Process

摘要

A precise field inspection system of the aperture of small blind holes with capacitance sensors is presented in this paper. Based on the working principle of non-contact capacitance sensor, influence of the edge effect of gauge head and width of protecting ring are studied, one sighting capacitance sensor for measuring the aperture of the small blind holes or through holes are introduced. The system composed of one aperture measuring capacitance sensor, one measuring circuit and software. The metal workpiece, as one of the measuring electrodes, constitutes the capacitance sensor with the gauge head. The center of arbitrary cross-section of the small hole can be sighted when the capacitance generated by the sighting sensor is a minimum. This function insures the precision of the result. The measuring circuit can reduce the noise of the electric signals obtained by the sensor, and amplify the signals. Then the aperture of the blind hole can be compared with the standard size automatically, and the result is immediately displayed and saved in computer after data processing if the system is equipped a USB interface and Lab VIEW programme. One of the key techniques is the noise elimination function of measuring circuits. Furthermore, according to the curve of the temperature drift memorized by the software when the capacitance sensor works in a certain temperature for a long period of time, the temperature drift can be compensated by the software. Thus, the accuracy of the measurement result is assurable. In laboratory experiment, the measuring error of the workpiece, in which there are three experimental blind holes with the diameter of 3 millimeters, is 0.5 micron. The paper solves the problem how to measure the aperture of small holes, especially small blind holes. Being non-contact measurement, the system dispenses with distortion error compensations of gauge head or workpiece. With the help of current micro-machining technique, the gauge head can be made so small that it can measure the holes of several millimeters. The aperture measuring system with is portable, easy to manipulate, and has high measurement accuracy and fast response. It is a good method to solve the problem mentioned above.