STEREOSCOPIC ENGINEERING
By K. W. HARDS
Issued by British Information Services Reproduced from New Scientist (18th November 1965) with kind permission of the author and the publishers
If development work now in hand realizes its early promise, the photogrammetry techniques now widely used for the aerial surveys may emerge as a major tool for programming complex three-dimensional metalworking operations under numerical control. Other branches of engineering requiring measurements on large, inaccessible or moving structures could benefit, too.
Photogrammetry is an optical method of measuring objects in three dimensions from stereoscopic pairs of photographs. Accuracy is within one six-thousandth of the object-to-camera distance at which the photographs were taken. The technique was developed mainly for aerial survey work where a series of over-lapping photographs are taken from an aircraft. From these overlapping photographs, diapositives are produced for mounting on the tilting tables of the stereoscopic machine.
When viewed through a binocular microscope the photographs appear to the observer as a three-dimensional model. The tables can be adjusted to align the photographs in space to reproduce exactly the camera's positions when the photographs were taken. The optical system is linked with a plotting mechanism so that sections or plans up to ten times the size of the diapositives can be drawn as the stereoscopic image is followed with a three-dimensional reference mark. Alternatively, the automatic read-out of the co-ordinates can be used.
Exactly the same procedures are used for engineering measurements, except that a pair of ground photographs are taken by specially developed precision cameras mounted on a fixed base, or by photo-theodo-lites set up at the ends of a predetermined base length. Fairey Surveys of Maidenhead are pioneering the commercial application of photogrammetry to engineering problems. And, as is so often the case with advanced metalworking techniques, the motor industry is showing most initial interest in their activities. They see the opportunity to shorten the "lead time" between completion of the clay model of a new car and the availability of jigs, fixtures, and tools for making the actual vehicle.
The clay model is designed and styled artistically, so its curves and shapes do not follow a mathematical law. Thus the dimensional information needed by the production engineer to make the tools can be obtained only by physically measuring the model and recording literally thousands of co-ordinate points in three dimensions. With photogrammetry, this tedious procedure is replaced by taking overlapping photographs of the model, using two cameras mounted a fixed distance apart (Fig. 1). Diapositives of these photographs are then mounted in a stereoscopic plotting machine (Fig. 2), and as the 3-D image created is plotted optically, the co-ordinates in all three dimensions are typed out automatically. The technique completes in days a task that normally takes weeks.
Dimensional data obtained from the plotting machine can be used to prepare magnetic or punched-paper tape for use in one of two ways: either to control a machine tool cutting the punches and dies used in shaping the bodywork or to control a machine making the templates used in established methods of toolmaking.
Unfortunately, with the first method at the present stage of development, the finished tools may err by as much as 0.04 inches from the size required. Much of this difference would arise from random errors on the original model and, hopefully, these could be eliminated by smoothing off the curve in the computer as the cutting path is being programmed.
This device, however, does not minimize errors from the photogrammetric process itself, which could be unacceptable to the motor industry if they have an adverse effect on the "light line" of the finished body. But this has yet to be established and would certainly not affect the technique's use for the production of tools for vacuum cleaner castings, food mixers and other household hardware, and so on. Here, absolute accuracy is not so critical so long as all parts are forevermore made the same size. And once the co-ordinates have been recorded on tape this is guaranteed. Where its accuracy is acceptable, photogram-metry offers a very real prospect of speeding up three-dimensional programming.
Should insufficient accuracy prevent the direct transfer of manufacturing information to the machine tool, the motor industry is still likely to use photo-grammetry, by integrating it with conventional methods for tool manufacture. In the second method under development, dimensional data are recorded in the same way as before but are then used to make the sectional templates traditionally used in the toolroom to ensure that finished dies are the same shape and size as the wooden model. A numerically controlled machine is again used, this time working in two dimensions only, but an opportunity is afforded to finish the curve by hand, smoothing out any unacceptable errors.
Assistance in programming the three-dimensional machining work is not photogrammetry's only potential use to engineering it has two other special advantages. First, no direct contact with the subject being measured is required: and secondly, the subject can be "stopped" in motion to measure parts or assemblies under actual working conditions.
This second advantage arises naturally from the first, and it was this ability to measure structures Instantly in otherwise inaccessible places that first started the Fairey team's development of their topo-graphical services for more commercial applications. The Berkeley Nuclear Laboratories had a problem in measuring deformations on the surface of a one-thirteenth scale model of a reactor pressure vessel after It had undergone tests. The coordinates of a large number of points on the surface of the spherical model had to be determined, before and after the tests, With reference to an origin outside the sphere itself.
A telescope mounted on a lathe bed intersecting the points. to be coordinated had originally been used. Taking the measurements, and making the trigonometrical calculations, took a long time to complete. Photogrammetry engineers set up a photo-theodolite on the laboratory floor about 8 feet from the model, made exposures from two camera positions and measured the resulting diapositives on a Wild Autograph A8 plotting instrument Fig3. ). They took only two weeks to deliver the results.
Photogrammetry is now being explored for use on large microwave aerials, to check the accuracy of construction, and to measure deformations under wind loads or expansion caused by temperature changes. Less exotic pieces of civil engineering, including buildings, can be checked in the same way, both in the course of construction and subsequently for sub-sidence or other deterioration. Twin photo-theodolites arranged for simultaneous electric firing are used to photograph very large structures.
Photogrammetry has already found many other applications in the measurement of aircraft altitude, for example, and for checking navigational and inertial instruments. Satellite and missile tracking, and lunar mapping are others. Measurements of the growth and movement of cloud formations, and analysis of clear air turbulence, have been made in this way. In the laboratory, wind tunnel measurements of model attitudes, and hydrological model tests to determine flow conditions and consequent silting or erosion, have used photogrammetry. It has even proved successful for tasks as far apart as determining the shape of yacht sails, and work-study measurements. The list will surely grow.