Contains reports on two research projects.Bell Telephone Laboratories Incorporate

Howland, Bradford H.

Howland, Howard C.

Proll, A. F.

DSpace@MIT

XIII. NEUROPHYSIOLOGYAcademic and Research StaffProf. J. E. Brown Dr. E. R. Gruberg B. HowlandProf. J. Y. Lettvin Dr. S. A. Raymond Janet MacIverDr. E. M. Ettienne Rosalyn V. StirlingGraduate StudentsR. E. Greenblatt M. Lurie J. F. NolteI. D. Hentall K. J. Muller R. S. StephensonJ. E. Lisman Susan B. UdinA. FIBER OPTIC PROXIMITY PROBE OF NOVEL DESIGNThe fiber optic proximity probe is an important device for the measurement, withoutcontact, of small mechanical displacements. '2 In an earlier report, we have shownthat such a device can also be adapted to the determination, with considerable accu-racy, of the location of the focal plane of a lens. 3 ' 4 The usual form of such a devicemakes use of a bifurcated, randomly intermixed fiber optic light guide, as follows: Twosets of approximately equal numbers of small diameter glass light conducting fibers arefitted with terminations at their proximal ends so as to couple to a light source and aphotocell. These sets of fibers are now joined, that is, randomly woven together, andshare a common termination at their distal end. The polished, flat distal terminationforms the proximity probe, which faces a first-surface mirror attached to the movingobject, located a few thousandths of an inch away. The distance from the probe to themirror is seen to determine the degree of spreading of the beams of emergent lightbefore reentering the fiber bundle. The result is that the light received by the photocellis a sensitive monotonically increasing nonlinear function of the separation of the probeand the mirror.The commercial success of this idea is due to the choice of a random intermixing inpreference to a more ideally ordered array of the elements of the two sets of fibers thatcannot, at present, be manufactured economically. We have tested several such ran-domized bifurcated light guides and found that some operate exceedingly well. Unfortu-nately, the required degree of random intermixing is not always achieved, and especiallywith fiber diameters smaller than 3 mils it is very difficult in the assembly process toeffect complete randomization -the separate sets of fibers understandably tend toremain together in large clumps. Tests at our workbench with 3-mil fibers confirm theextremely taxing nature of the randomization operation, which, lacking some sort ofcomputer control must remain an art rather than a science. It is for these reasons thatThis work was supported in part by a grant from Bell Telephone LaboratoriesIncorporated.QPR No. 101 265(XIII. NEUROPHYSIOLOGY)we seek a different solution to the problem.The method that we describe here should make it possible, in principle, to use anordinary fiber optic light guide comprised of fibers of any practical size as a prox-imity probe. The method depends on the possibility of exciting two different modesof transmission in each fiber of the bundle depending on whether the light, as it ismultiply reflected, spirals down the fiber in a clockwise, or counterclockwise direc-tion, as illustrated in Fig. XIII-1. Here we show a single fiber of the bundle, whichN'/ NSECTION A-AFig. XIII-1. Arrangement of illumination, semicircular mask, andlight-guiding fiber to illustrate spiral modes of lighttransmission through a fiber.has been fitted at the proximal end with a semicircular aperture mask, oriented asshown, and obliquely illuminated from the direction N'N. In the cross section AA weshow in an end view several successive reflections of an entering ray, as it passes downtne fiber. The point to note is that the axis of the fiber lies always to the left of the rayon its downward passage,and this must remain the case even following several thousandsof internal reflections that occur in each foot of light guide.Now let us suppose that there is a mirror in contact with the far end of the fiberwhich reflects the light directly back. In the view of cross section AA, this light willappear to spiral in the same sense (counterclockwise), but when it emerges from thefiber, under the assumption that it is not stopped by the aperture mask, it will emergeQPR No. 101 266(XIII. NEUROPHYSIOLOGY)in a direction which is, on the average, directed away from the light source. Theseemergent rays will form a cone of illumination, with maximum intensity in the direc-tion MM', and minimum in the direction NN'.Next, we consider the action of a bundle of light-transmitting fibers, each of whichis equipped with its own semicircular mask, as shown in Fig. XIII-2. We have includeda simple measurement test apparatus whereby we are able to adjust the location of themirror, here the polished end surface of a micrometer shaft, relative to the polished,flat distal end of the fiber bundle. The source S provides oblique illumination, as before.KS RMrxFig. XIII-2. Arrangement of fiber bundle, photographic aperture mask,light sources, photocells, and movable mirrored surface toillustrate the method of proximity detection.Two photocells, R and S, monitor the intensities of the cone of illumination exiting thefiber bundle in the directions MM' and NN'. Note that the distal surface of the bundleshown in cross section BB' is unmasked; furthermore, we do not postulate any coherentordered relation between fibers at the two ends of the bundle.QPR No. 101 267(XIII. NEUROPHYSIOLOGY)The operation of the method now becomes clear: If the mirrored end surface of themicrometer is in contact with the distal end of the fiber bundle, light spirals directlyback through each fiber in such a way as to emerge predominately in the direction MM',regestering on photocell R, as before. If we now move the mirror a short distance fromthe end of the bundle, at least some of the light will exit from the fibers and re-enterother fibers so as to spiral back with the opposite sense as before, thereby causing somefinite illumination to exit in the direction NN' and register on photocell S. In the limitingcase, the light intensities returning by each spiral mode of transmission will be equal.The ratio of light intensities recorded by photocells S and R is thus seen to be a sensitivemeasure of the distance from the surface of our probe to the mirrored surface that itopposes.This new method (which we have not had a chance to test) has several possible advan-tages over the randomly intermixed fiber optic probe: (a) There will be less uncertaintyin the preparation of the critical element, which is the collective semicircular aperturemask. The task of preparing this mask is within the capabilities of apparatus routinelyavailable for microelectronics fabrication. (b) It should be possible to extend the sen-sitivity of the method with the use of smaller glass fibers than are practical with therandomly intermixed light guide. (c) Photographs of the cross section of fiber opticlight guides show a high degree of ordering of the individual fibers, which tend toarrange themselves in hexagonal groups. Thus, we may expect that random differencesin the fabrication of the original bundle will not manifest themselves as large differencesin performance of the unit as a whole.B. HowlandReferences1. C. D. Kissinger, "Fiber Optic Proximity Probe," U. S. Patent Spec. No. 3, 327, 584(June 27, 1967).2. W. R. Moore, "Foundations of Mechanical Accuracy" (Moore Special Tool Com-pany, Inc., Bridgeport, Conn., 1970).3. B. Howland and A. F. Proll, "Apparatus for the Accurate Determination of FlangeFocal Distance," Quarterly Progress Report No. 92, Research Laboratory of Elec-tronics, M.I.T., January 15, 1969, pp. 441-445.4. B. Howland and A. F. Proll, "Apparatus for the Accurate Determination of FlangeFocal Distance," Appl. Opt. 9, 1247-1249 (1970).B. KINEMATIC THRUST BEARING, PART IIIn a previous report, I we described a novel form of thrust bearing which gavepromise of improved accuracy and the possibility of correction of periodic errorswhen used in conjunction with worm gear drives. We shall now report a new versionQPR No. 101 268(XIII. NEUROPHYSIOLOGY)of the kinematic thrust bearing which is simpler to construct and assemble, andhas the added advantage that the configuration is tractable to mathematical analysis,using a computer.In Fig. XIII-3 we show a cross-sectional drawing of the device, which requires twosets of balls of two sizes. The outer race is constructed in two pieces, with pro-vision for decentration of the outer segment by means of 3 set screws, in the mannerof a three-jaw lathe chuck. Not visible in this drawing is the small eccentricity of theinner race, which fits on the shaft with a hole drilled 4 mils from its geometric center.Additional conventional ball bearings cause the shaft to run concentric with the innersection of the outer race.Fig. XIII-3. Cross section of kinematic thrust bearing for the case N = 6.In Fig. XIII-4 we show the finished prototype model of the bearing with 6 pairs ofballs, together with a gauge block and electronic indicator head that permits us tomeasure the axial displacement of the shaft to 5 microinches. (Not shown is a smallQPR No. 101 269(XIII. NEUROPHYSIOLOGY)Fig. XIII-4. Photograph of kinematic thrust bearing with test apparatusto measure axial cam motion.electric motor and gear drive that enables rotation of the bearing with minimumdisturbing forces.)The parts for this model were somewhat inexpertly machined on an old South Bendlathe, from stainless steel, and were then polished with fine emery paper. The ballswere 10 microinch sphericity grade from Industrial Tektonics, made of high-speedsteel. After adjustment of the screws for minimum axial runout, we measured an axialmotion of between ±10 and ±15 microinches displacement, which is far better than theQPR No. 101 270(XIII. NEUROPHYSIOLOGY)accuracy of the machined elements of the bearing. This result we take to be encouraging.The cooperative action of the elements of this bearing is such as to cause the shaftto run smoothly as the inner shaft rotates, with the possibility of a very nearlysinusoidal axial camming motion, which is adjustable in phase and magnitude by meansof the screws that center the outer race. We have verified this conjecture bydecentering the outer race 20 mils, and measuring the axial motion, which when plottedas a function of shaft angles within the accuracy of the measurement was a sinusoidalmotion having peak-to-peak amplitude of approximately 2 mils.A second attack on the problem was performing a numerical computer analysis ofthe behavior of the device with different eccentricities, phases of rotation of the shaft,phasing of the balls, and so forth. To make this task feasible, we decided to changethe shape of the two segments of the outer races to spherical, rather than conical. Theresult of this is that the track of the centers of the outer balls is always a circle, dis-placed and inclined to the plane of the inner balls, and very slightly diminished insize with large eccentricities. Since the machined model is made of cones thatare tangent to the spheres in the mathematical model, the differences in performancefor small relative eccentricities is expected to be minimal.With this mathematical model, we wrote a computer program that successivelyapproximates the state of the model as follows: First, we choose a trial value for axialposition of the inner race. Next, we place the first ball at an arbitrarily chosen starting-90 0o  900 1800 270' 3600 4500/ =0.000A A =0.001a=0.003U-0.0010 -Fig. XIII-5. Computed axial cam action for different degrees of decenteringof the outer raceway.QPR No. 101 271(XIII. NEUROPHYSIOLOGY)phase, and then, again using a simple iterative scheme, fit the balls in place aroundthe circles, adding, for good measure, an (n+l) t h ball. Now, the position of this super-numerary ball is compared with the position of the starting ball, and adjustments aremade in an outer loop to iteratively adjust the axial distances so that, after a numberof iterations the (n+l) t h and the first balls coincide to the limit set by an error criterion.The axial position is then determined for a new set of input conditions.With this program, we have been able to verify that the axial motion of our computermodel (based on spherical races) is indeed similar to that which we observe with ourphysical model.In Fig. XIII-5 we show the computer results for the case of an inner race eccen-tricity of 4 mils. We have plotted here the axial displacement as a function of shaftangle for various amounts of decentration of the outer race, with values in mils markedon the curves. The phase of this displacement is determined by the direction inwhich the outer race is perturbed. The shape of these curves is sinusoidal, with max-imum value of harmonic distortion of 0. 2%. Furthermore, even with these large dis-placements, the errors in axial displacement caused by random changes of phasing of-8the balls amount to less than 10 - 8 in. displacement. Further experiments will berequired to establish the optimum proportions for the design of this device.NOTE: This is a report of a cooperative research project between the ResearchLaboratory of Electronics and the M. I. T. Lincoln Laboratory. We wish to acknowledgethe programming assistance of Frederick S. Zimnoch of Lincoln Laboratory.B. Howland, H. C. Howland, A. F. Proll[Bradford Howland is an M. I. T. Lincoln Laboratory Staff Member. Howard C. Howlandis an Assistant Professor at Cornell University, in the Division of Biological Sciences.Arthur F. Proll is now with the Northeastern Tool Company, Haverhill, Massachusetts.]References1. B. Howland and H. C. Howland, Quarterly Progress Report No. 98, Research Lab-oratory of Electronics, M. I. T., July 15, 1970, pp. 215-216.QPR No. 101 272

Neurophysiology

http://dspace.mit.edu/bitstream/1721.1/56179/1/RLE_QPR_101_XIII.pdf

Neurophysiology

Abstract

Similar works

Full text

Available Versions

DSpace@MIT