Measuring the dimensions of bodies of complex shape using an optoacoustic ranging method

Abstract

A description is given of an automatic apparatus for measuring the dimensions of objects of complex geometrical shape using an optoacoustic ranging method. The results are given of measuring the dimensions of a test object, a steam-turbine blade, and a shoemaker's last. An accuracy of better than ++_ 50 #m was achieved.Means of measuring the dimensions of bodies of complex geometrical shape are at present being used more and more in various branches of the national economy. The principal branches in which such devices are employed are automobile engineering (bodywork and its components), aircraft engineering (streamlined components, cabin glazing), the shoemaking industry (lasts), orthopedics (prosthetic and orthotic devices), power engineering (turbine blades), tool production (dies), computer production (dies), restoration work (replicas), design.The authors have developed and manufactured an apparatus designed to measure objects such as steam-turbine blades. The principal requirements imposed on the apparatus during its development were as follows: the measurements must be made automatically and without mechanical contact with the object, maximum overall dimensions of the investigated components of 1000 x 100 x 100 mm, weight below 40 kg, time to measure one component less than 5 rain, measurement accuracy better than +0.1 rnm.In creating such an apparatus the authors had to correctly choose a noncontacting method for measuring the dimensions. Only two methods were considered as having the specified measurement accuracy. These were either to use different versions of the photoelectric method [I] or to use the optoacoustic ranging method [2,3]. It is well known that the photoelectric methods, based on analyzing light beams scattered by the surface of the object, are characterized by high accuracy (an error of _< 5 t~m) with a short time required to obtain the results. However, serious disadvantages include a small working range of measurement (1-10 ram), a complex electronic processing circuit, and also an absence of Russian-made measurement heads operating in accordance with this principle.The optoacoustic ranging method consists of recording acoustic pulses in air which arise when laser radiation interacts with a surface, and measuring the time intervals required for the sound to propagate from the object to a microphone. This method provides the specified accuracy and has a working distance range of 10-1000 mm. Since the processing electronic circuit is quite simple and the measuring part is capable of being made in practically any laboratory, the authors decided on the optoacoustic ranging method when they were ~eveloping the apparatus.The kinematic scheme of the interchangeable component and measuring part was dictated by the measured object and was conventional (rotation and vertical motion of the object, movement of the measuring part of the apparatus in a horizontal plane). The radiation source generating the acoustic pulses took the form of a laser of 1.06 t~m wavelength, pulse duration 10 -8 sec, and...