The number of parts with micro-cavities increases with the development of hi-tech industries, such as aerospace industry, and auto industry. The proper measurement of these micro-cavities becomes a “bottleneck” for further development of these hi-tech industries, and this justifies the necessity of developing a good type of micro-cavity measuring equipment and method.
How to precisely measure the dimensions of a micro-cavity is a new problem for the measurement field and the solution of this problem involves many key techniques, and what we have to do is to minimize the conflicts among accuracy, range and depth. A wider range can be achieved at a deeper depth using a contact measuring method and a longer measurement arm, while the accuracy will decrease with the increasing length of measurement arm. Professor Zhang Guoxiong with Tianjin University invented a 3D measuring sensor with a diaphragm to solve the problem. In the 3D measuring sensor, a capacitance sensor is used to detect the distortion of the diaphragm, and an elasticity-dimension-chain is used to compensate the distortion of the diaphragm. The sensor can be used to fulfill the task of measuring a diameter of 0.2 mm, at an aspect ratio of 30:1, and an uncertainty better than 1 μm. In comparison with the contact method, non-contact optical methods can be used to fulfil the task of measuring holes of small diameter at higher accuracy, but these methods can be used to measure the surface of a hole only at a very shallow depth and cannot be used to obtain the inside information of a hole because of the bulky size of an optical system. What's more, the measurement accuracy may drop for such small defects as burrs in the hole. The vibroscanning method can be used to measure the diameter of a micro-hole by detecting the changes in voltage caused by the displacement of the probe from the wall of an object. But the measurement accuracy is subject to the influence of contaminants, and the probe may be broken for excessive vibration. So the minimum diameter and depth achievable with this method can not be large enough, even when a twin-stylus or high-aspect-ratio probe is used.
With the development of optical fiber technique, some novel methods, which use optical fibers as sensors to measure the dimensions of a micro-cavity, have been developed. However, it is almost impossible to use these methods to measure a micro-hole with a diameter of less than 1 mm.
During 1997˜1998, a new measuring method, which uses a single optical fiber and a CCD image processing technique, is developed by Physikalisch-Technische Bundesanstalt (PTB) and Tianjin University. This method can be used to fulfill the task of measuring micro-cavities, in which an illuminated micro-sphere is imaged on a CCD camera as an object, and the lateral displacement of the sensor is transformed into a longitudinal displacement of the micro-sphere, and indicated in image signal brightness captured through the CCD camera. However, the image from the CCD camera is so weak that it is not suitable for image processing because most of the light enters the micro-sphere through the optical fiber can not enter the optic imaging system. Consequently the measurement depth is very shallow because of the shading effect of the wall of the micro-hole, and the method is no longer usable if the aspect-ratio is greater than 10:1. When the micro-sphere get in contact with the wall of the micro-hole, the image from the CCD camera will not be changed because the depth of field of the optical system is fixed, and so, the sensitivity and accuracy achievable with this method are not good enough, either.