Contains reports on one research project.Bell Telephone Laboratories (Grant

Howland, Bradford H.

Weibel, Frank W.

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XVI. NEUROPHYSIOLOGYAcademic and Research StaffProf. Jerome Y. Lettvin Dr. Edward R. Gruberg Dr. Stephen A. RaymondDr. Michael H. Brill Dr. Donald C. Quick Dr. Stephen G. WVaxmanGraduate StudentsIan D. Hentall Eric Newman XWilliam MI. SaidelBradford Howland Louis L. Odette Donald W. SchoendorferLynette L. Linden Marylou V. SolbrigA. A NEW CLASS OF PRECISION MECHANISMS FOR OPTICALRESEARCHBell Telephone Laboratories (Grant)Bradford Howland, Frank W. Weibel1. IntroductionIn the course of perfecting a new type of x-y filar micrometer, we have come upona form of construction for precision mechanisms that is adaptable in a wide range ofapplications. In particular, we have designed two forms of microscope measuringstage, a precision variable hexagonal aperture, several adjustable slit mechanisms,and an angular interpolating turntable with a potential of arc-second accuracy. In allof these designs we utilize a compact form of construction in which various plates,driven by micrometer screws and separated by steel balls running in linear or curvi-linear V-grooves, are sandwiched together to form a precision mechanical assembly.2. Precision x-y Filar MicrometerA filar micrometer is a measuring instrument that operates in the focal plane of theeyepiece of a compound microscope. For applications to printed circuits, and perhapsto cell biology, it is useful to be able to measure simultaneously in both x and y coor-dinates. Figure XVI-1 shows a model that illustrates the mechanical principle of ournew form of x-y movement. The device is symmetric in x and y coordinates, the x-ycoordinate system of the moving stage is at 450 to the parallel axes of the stationarymicrometers, and the motion generated by each micrometer screw, in effect, is geareddown by a factor of \-2:1. In this crude form, the device performs very poorly, sinceit is not possible to find low friction materials that will maintain the required mechan-ical accuracy. For a second model we used flanged miniature ball bearings, runningin milled slots. This model performed quite accurately, but in its fabrication an exces-sive amount of precision machining was required.PR No. 118 245Fig. XVI-1.Model illustrating the principle of the newx-y stage in the filar micrometer.TOP COVER Eo oTOP DRIVING DPLATE-mFig. XVI-2.Prototype model of the new x-y filarmicrometer.7XMAGNIFIERRETICLE HOLDERBOTTOM DRIVINGPLATEBOTTOMFig. XVI-3.COVER * AParts and assembly of the new filar micrometer. Fivemachined plates, separated by balls running in linearV-grooves form the sandwich construction. Solid andopen lines indicate V-races on the top and bottom platesurfaces.PR No. 118%&2C=:) C246(XVI. NEUROPHYSIOLOGY)Figure XVI-2 is a photograph of our third model, in which we used the new sandwichconstruction. Figure XVI-3 shows in detail the arrangement of the five sandwich platesthat form the assembly, and the grooves and slits in each of the individual plates. Thetwo cover plates are stationary, and have V-grooves milled parallel to the micrometeraxes. The adjacent pairs of inner plates are driven by separate micrometer screwsthat have narrow shafts and hemispherical end tips staggered in the vertical plane. Theinnermost plate, which holds the reticle (a glass plate with precision fiducial marks),is supported at opposite corners by pairs of balls running in V-grooves at 450 to theparallel axes of the micrometers and engaging similarly oriented V-races in the adja-cent plates. The reticle is a rectangular array of fine optical crosses, spaced exactly0.0707" apart along each of the 450 diagonals. Thus four turns of either micrometerscrew (0. 100" motion) translates the measuring array of the reticle into itself.To locate any object in the field of view of the eyepiece and reticle, it is only neces-sary to move each micrometer screw by an amount that is never more than four turns,thereby positioning one of the set of numbered crosses over the target. The measuredaccuracy in this model of the device is 0. 0002", corresponding to a perceived error of0. 0014" with a 7X magnifier, or approximately 0. 5 arc min at a conventional viewingdistance. We judge this to be satisfactory. Furthermore, tests with a measuringmicroscope indicate that there is less than 0. 0001" coupling between the axes of thisdevice.The new filar micrometer using sandwich construction has many advantages: (a) thedevice is symmetrical in x and y motions, (b) neither micrometer moves relative tothe observer, (c) there is a significant gearing down of the motion of the micrometerscrews, (d) the device utilizes rolling contact in all moving elements save for themicrometer screw, and hence friction, stiction, and hysteresis errors are minimaland the mechanism proper can operate without lubrication, and (e) the fabrication dif-ficulties for the separate elements are less than those in an x-y stage of conventionaldesign and comparable precision.3. Precision x-y Measuring Microscope StageFigure XVI-4 shows a precision measuring microscope stage, similar in design tothe filar micrometer. The nonintegral 1/-2 ratio by which the stage operates necessi-tates the use of special pitch micrometer screws such as are used in certain precisiontooling. In this application, the sandwich construction has the following advantages:(a) low profile, (b) large optical aperture, (c) ease of adaptability to electronic read-out or drive, since both micrometers are stationary, (d) low inertial effects, since allmoving parts are lightweight (i. e. , the stage need not be clamped when the microscopeis transported), and (e) ease of sealing the entire mechanism against intrusion of dirtPR No. 118 247(XVI. NEUROPHYSIOLOGY)Fig. XVI-4. Prototype model of a measuring microscope stageutilizing the new sandwich construction. Specialmicrometer screws with \f times normal pitchratios would be used in production instruments.and abrasives. The disadvantages include a need for special pitch micrometer screws,and the limited range of travel of the measuring stage, which is 0. 7" in each direction.4. Precision Geared-Down Microscope StageThe element limiting the accuracy of a measuring microscope of conventional designis the micrometer screw, which rarely exceeds 0. 0001" (or 2 Jm in a metric microm-eter) least count. The resolution of an ordinary 20X microscope objective is 0. 5 FIm,and the repeatability of settings is generally better. Thus we may contemplate the advan-tages of an x-y micrometer stage in which the motion of each of the micrometer screwsis "geared down" by an additional factor of, say, 5:1 or 10:1. Such a design can easily berealized with sandwich construction, by using V-grooves inclined at precisely determinedangles. Such a stage would be useful in very accurate measurements over a moderate-sized field, as in a semiconducting chip. The advantage of such a direct measurementstage over the filar micrometer is that errors arising from uncertainties in the magni-fication of the microscope become irrelevant.It has been pointed out that the sandwich construction that we have described can beutilized in a more conventional way to construct an x-y stage in which the micrometersare mounted at right angles, and the stage moves in 1:1 ratio with the micrometerscrews. Such a construction has already been anticipated.15. Precision Variable Hexagonal ApertureThe versatility of sandwich construction is further illustrated in Fig. XVI-5a wherewe show a novel form of variable hexagonal aperture, and in Fig. XVI-5b which showsPR No. 118 248(XVI. NEUROPHYSIOLOGY)Fig. XVI-5. (a) Overview of the variable hexagonal aperture assembly. Themicrometer screw drives an intermediate plate, which inturn, by means of balls running in appropriately oriented V-grooves, drives each of the blade assemblies symmetrically.(b) Enlarged view showing the arrangement of the three movingblades. The key element of the assembly is the hermaphro-ditic blade moving in from the left.the somewhat unusual arrangement of the three converging blades, at least one of whichmust be hermaphroditic (i. e., composed of two opposing elements). It will also oper-ate with three hermaphroditic blades. The device has four linearly moving parts, plusthe micrometer screw, and requires 18 steel balls. The micrometer screw drives asecondary plate (not visible) that, by means of inclined ball races, drives the individualblades, thereby opening the aperture. The micrometer setting thus directly determinesthe radius of the circumscribing circle. With this design the maximum aperture size is1.1" and the minimum is less than 0. 005", which corresponds to a 220:1 range. Further-more, the edges of the blades that define the hexagon are coplanar, and the centrationof the hexagon is accurate to 0. 001" over the range of aperture sizes. This device is anextension of the well-known variable square aperture assemblies that are used exten-sively in optics and in biology in the form of a rat guillotine.The variable hexagonal aperture is more accurate and has a larger range of adjust-ment than the conventional leaf aperture that is used in camera shutters, and the hex-agonal opening more closely approximates a circle than does the equally accuratevariable square aperture. It can be shown that the hexagon is the highest order poly-gon that can be so synthesized. Moreover, it need not be a regular hexagon; the anglesand the drive ratios may be varied to effect an approximation to an ellipse, which mayfind use in anamorphic systems. Such an ellipticity adjustment can be appended to thepresent device, with the aid of a second micrometer and an additional drive plate.6. Precision One-Degree Interpolating TurntableA need exists for a simplified design of an angular turntable to provide interpolationPR No. 118 249(XVI. NEUROPHYSIOLOGY)between the steps of the Ultradex.Z Two forms of this ultraprecision angular indexingdevice exist: one steps in one-degree increments, the other, the 1440, steps in quarter-degree increments. 3 Each exhibits angular accuracies of the order of 0. 1 arc second,or 0. 5 microradian.Figure XVI-6 is a photograph illustrating the construction of the interpolating turn-table, which utilizes sandwich construction with both linear and circular concentric V-grooves. The operation of this device depends on two effects: the essential equivalenceof the angle and its sine for angles less than 10, and the use of a bearing formed by ballsrunning in sectors of concentric V-grooves of large radii, whereby the center of rota-tion of the bearing is located considerably outside the physical boundary of the device.The effective lever arm by which linear motion is translated into angular rotation isthe distance from the center of the coaxial V-grooves to the bearing axis of the turn-table. Thus, for example, when using a metric micrometer, and an equivalent radiusof 57. 295 cm (~22. 557"), each centimeter of travel corresponds to 1", and the devicereads directly in increments of 0. 00020 (0.72 arc sec).Fig. XVI-6. Construction of the plus and minus one-degree angular interpolatingturntable. Plates A and B are separated by four balls, running inmatched concentric V-grooves having approximately 15" and 19"radii. Plate pairs B and C, and C and D are separated by steel ballsrunning in orthogonal sets of linear matched V-grooves. The angularrotation of the top plate is determined by the setting of the precisionmicrometer screw; each small division (0. 0001") corresponds to1. 2 arc sec, and 1" equals 0. 300".We have determined that the performance of the crudely machined (student-made)model shown in Fig. XVI-6, which has a sensitivity of 1. 2 arc sec per 0. 0001" leastdivision, is as follows: accuracy ±10 arc sec; repeatability of setting ±1. 0 arc secaverage; hysteresis (with reversal of micrometer motion) 1.5 arc seconds. In view ofthe known inaccuracies in machining this model, these data, especially the repeatabilityand hysteresis error, are encouraging. A second model made from hardened and groundsteel is being fabricated. Consultations with machine tool experts indicate that the accu-racy of this device is most likely to be limited by the accuracy of grinding and lappingPR No. 118 250(XVI. NEUROPHYSIOLOGY)of the concentric V-grooves.7. SummaryThe sandwich construction described here, utilizing a multiplicity of parallel plates,driven by micrometer screws and constrained by balls running in linear or curvilinearV-grooves, can be used to form a variety of precision mechanisms that have applicationsin optics and mechanics. Several such devices have already been built and tested, andtheir performance has been generally satisfactory.The models of devices shown in Figs. XVI-3, XVI-4, and XVI-5 were constructedin the machine shop of Group 63-69 of M. I. T. Lincoln Laboratory, which is operatedwith support from the U. S. Air Force, and we are especially indebted to the machineshop staff for their expertise and helpful suggestions. We are grateful also toF. Williams Sarles, Jr. of the same laboratory, for his support of this research.Frank W. Weibel is a member of M. I. T. Lincoln Laboratory, which is supported inpart by the U. S. Department of the Air Force.References1. Naum G. Basin, Alexei V. Vysotsky, Anatoly P. Kurochkin, and Ura J. Okun, UnitedStates Patent Office 3, 829, 978. WORKTABLE FOR POSITIONING WORKPIECES INMEASURING DEVICES TO CHECK DIMENSIONS. Patented August 20, 1974.2. Registered Trade Mark, The A. A. Gage Company, Detroit, Michigan.3. See Wayne K. Moore, "Foundations of Mechanical Accuracy," Moore Special ToolCompany, Bridgeport, Connecticut, 1972.PR No. 118 251

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