Automatic measurement of micro-deviations in periodic structures

Optical and electronic techniques have been combined in a precise automatic instrument which rapidly measures and simultaneously records small deviations from the ideal in structures intended to be perfectly periodic. The instrument was primarily developed for measuring pitch uniformity in twt helices where the location of each helical turn must be determined to an accuracy of about ±2 microns. It is, however, readily adaptable for measuring other periodic structures such as ladder lines, grids, and precision screws. The speed and accuracy with which measurements are made have opened up new possibilities in fabrication control. For example, it is now practicable, for the first time, to precisely measure every helix destined for traveling-wave tube use at several different stages in its processing. Where each individual helix measurement formerly required nearly 2 man-days, it is now accomplished in less than 5 minutes. The instrument utilizes a beam of light and an optical grating system to obtain a very accurate scale of distance. The location, with respect to this distance scale, of the salient petiole features in the structure being measured is established through the interception of a second light beam by the structure. Electronic circuitry automatically stores and processes the position information and feeds the processed information, in the form of deviations from the corresponding ideal structure, to a pen recorder.