A Portable Oscillograph With Unique Features

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  • A Portable Oscillograph With Unique FeaturesBY KIRK A. OPLINGER*

    Associate. A.T.EE.

    Synopsis.---This paper describes a simplifi ed portable oscillo- datmping instead of the uisual oil (lamsping. 7'hese galvantometersgraph having a nutmtiber of new optical and electrical features. 7'he are very rugged and have been built for recordiing frequencies a.s highoptical system, which cotnsists of a combination of cylindrical letnecs as 14,000 cycles per sec.with axes at right angles, is designed( to perinit sim ulta(neous vietivng 7'he oscillograph is entirely self-containedl and may be operatted1and photographintg. A continuous time axis for both the viewing from a 110-volt 80-cycle lighting circuit wvithouit aux.iliary attach-screen and filmt is secured by mc(rs of a snall, variable speed, re- ments. 7'he compactness and portability of the instrument cant bevolving mirror. scctn from its over-all dimensions which nre 8 X I 1 X 11 in.New galvanometers have been developed uwhich haveelectromnagnetic and from its totatl weieight, which is approximai( tely IS lb.

    INTRODUCTION plier or shunt resistor. The values marked on the panelD URING the past few years, numerous applications give the approximate d-c voltages and currents for aD have been opened to the oscillograph in connection deflection of one inch. Both galvanometers may be

    with radio broadcasting and the use of vacuum used for measurements of potentials up to 300 volts ortubes for industrial purposes. Outside the field of elec- currents up to 10 amp without the use of externaltrical engineering, the oscillograph has been used for resistors. The control switches have a stop to preventacoustical studies and for the measurement of vibra- the operator from accidently switching to the currenttions, accelerations, and pressures. In many of these side when connected to a voltage circuit. However, noapplications, the oscillograph has not only simplified damage will result to the instrument if this mistake isthe development work, but it has often furnished data made, since both the resistors and galvanometers arethat could be obtained in no other manner. protected by fuses.

    In designing an oscillograph for applications such asmentioned above, simplicity and ruggedness are of pri-mary importance, as the instrument will not generallybe used by an experienced oscillograph operator. Thesize of the oscillograph and the ease of making visualobservations are also important since the oscillographwill be used much the same as an instrument to makecomparative measurements of amplitude, phase, fre-quency, or wave shape. To meet all of these require-ments, an entirely new oscillograph, differing greatlyfrom previous designs, has been developed using a newtype of optical system and galvanometer. This newinstrument may be used as conveniently as an ordinaryvoltmeter or ammeter, and has other characteristicswhich will extend the range of application of oscillo-graphic instruments.

    DESCRIPTION

    A general view of the oscillograph is shown in Fig. 1.The instrument is entirely self-contained and may beoperated from a 110-volt 60-cycle lighting circuit with- Fi(;. 1 (IENEtAI VIEW OF (SC'llOG;AP1tout auxiliary attachments. The oscillograph is verycompact as shown by its over-all dimensions which are In the upper left hand corner of the oscillograph are8 X 1112 X 11 in. Its total weight is approximately two controls which permit adjusting the zero position of18 lb. each galvanometer. The "timing" knob in the oppositeA top view of the panel is shown in Fig. 2. Terminals corner of the panel varies the timing axis, as will be

    for the two galvanometers are located at the lower left explained later, and thus controls the number of cyclesand right hand corners of the panel. Each galvanom- visible on the viewing screen.eter has a single switch for selecting the desired multi- Both film and viewing screen are stationary and in

    *Researeh L,a)ratories,We~stinghoise Elee. & Alfg. C'o., E~astposition for use at all times. When it is desired to take

    littsburgh, I'a. a photograph of any recurrent phenomenon, it is onlyPresen1ted(at te summer coattehetio Aof the A.I.E.E., Chicago, necessary to press the "expose" button near the center

    Illinois, June 26-30, 1933. of the panel. This button opens a shutter to the camera,879

    33-90

  • 880 OPLINGEIR: A I'ORTABLE OSWILLOGRAPH WITH UtNIQUtE FEATURES Transactions A.l.E.E.

    and at the same time, places an overvoltage on the tion of the lenses L1 and L2. The other dimension ofoscillograph lamp. Provision has been made to use the image is determined by the height of the slit andeither a standard cut film holder or pack with 21 X3X4- the magnification of the lenses L3 or L, which focus onin. film in a manner similar to that of an ordinary the film and screen respectively.camera. To secure a continuous time axis, the number of faces

    OPTICAL SYSTEM on the revolving mirror, and the angles subtended bythe film and viewing screen, have been chosen so thatSimultaneous viewing and photographing is made there is always a spot of light entering on both the film

    possible by the optical system shown in Fig. 3. This and screen just as the previous spot is leaving. Thissystem also gives an optical multiplication of the arrangement makes it possible to study transientgalvanometer deflection, thereby making it possible to phenomena without the possibility of a transient occur-obtain the equivalent of a long optical lever in a short ring at a time when the screen is dark.

    Directly above the viewing screen is a spherical lens,so placed that it gives an enlarged virtual image whichis comparable in size to the image on the film. Withoutthis lens, the image does not appear equally bright at allpoints on the screen. The lens corrects for this and also

    Pi(. 2-0)sc iLLGA)( tAI'll PANE

    #,. 'l.0CR

    PI. I INSIDE VIEW OF O6>LOR

    .. Fi(_. 3-OPTCL gives greatly increased brilliancy which is sufficient forSYSTEM viewing even in a brightly lighted room. If desired, the

    viewing lens with its screen may be removed and re-space. Referring to Fig. 3, the slit is illuminated by a placed by a large curved screen which is shown mountedstandard 6-volt 32-cp automobile headlight lamp whose on the lid in Fig. 1. This arrangement makes it possiblefilament is imaged on the galvanometer mirror by a to trace wave formns and to use the oscillograph in asmall condensing lens. The lenses L, and L2 are cylin- darkened room for such purposes as classroom demon-drical with their axes at right angles to the axes of the strations before a group of students.cyli-ndrical lenses L3 and L.,. Optical multiplication is An inside view of the oscillograph is given in Fig. 4,obtained by means of the lenses Li and L2. The lens showing the arrangement of the different parts of theL, gives a reduced image of the slit directly in front of optical system. The small revolving mirror is driventhe lens L2. This image, together with any motion by the friction between a wheel on the mirror shaft andimparted to it by the galvanometer mirror, is then en- a face plate mounted on the end of the motor shaft.larged by the lens L2 on to the film. The size of the The speed of the mirror, and hence the timing on theimage on the film, in the vertical direction, is therefore screen, is varied by sliding the motor back and forth tofixed by the width of the slit and the overall magnifica- change the driving radius.

  • September 1933 OPLINGER: A PORTABLE OSCILLOGCRAPH WITH UNIQUE FEATUItES 88I

    GALVANOMETERS tion in the net armature stiffness, and this reductionThe galvanometers used in the oscillograph are as must be less than the mechanical stiffness of the arma-

    shown in Fig. 5. They are of the moving iron type with ture, if the armature is to remain stable in the gap.a balanced armature., This design results in a very This latter requirement was a serious limitation in therugged construction which can withstand large over- design of the balanced diaphragm receiver where theloads without damage and is particularly suitable for net diaphragm stiffness must be fairly low in order thatportable work. These galvanometers were originally the unit could cover the desired frequency range. How-developed for sound recording systems and have proved ever, in the case of the galvanometer, the armature stiff-to be very satisfactory for this purpose.2 The arrange-ment of the armature, poles, and coils is shown in Fig. 6.Permanent magnets of cobalt steel are used to supply asteady flux in the four air gaps. When current flows inthe coils, the flux in one pair of diagonally opposite gapsis increased, while the flux in the other pair is decreased,resulting in a force couple which tends to rotate the FIG. 6-BALANCED ARMATUREl CONSTRUCTIONarmature about its vertical axis. The restoring forcefor the armature is furnished by its support. ness must necessarily be relatively high in order toThe construction of the armature and method of obtain a high resonance frequency and therefore, a large

    support is shown in Fig. 7. It will be noted that the steady flux may be used to secure maximum sensitivity.support has a 90 deg twist at the top which places the The galvanometers, as used in the oscillograph, arearmature at right angles to the flat, tapered stem. This designed for twenty ohms impedance and have a sen-construction gives the desired torsional stiffness for the sitivity of 0.10 amp d-c per inch deflection. Thisarmature, and at the same time, furnishes a high bend- sensitivity depends, of course, upon the frequencying stiffness to keep the armature centered in the air range, the above sensitivity being for a galvanometer

    with a 5,000 cycles per second response. The frequencyrange may be extended simply by increasing the tor-

    i - _ l l i ll l _ sional stiffness of the armature support and galvan-ometers of this type have been used for recordingfrequencies as high as 14,000 cycles per second. Galvan-ometers of this range, however, have much lower

    FIG. 5O-4VING IRON TYPE CIALVANOMETER sensitivity. In contrast to other types of galvanometers,the moving iron type becomes more rugged as the

    gap. The mirror is mounted on a short stem extending frequency range is extended.above the armature. It is desirable to have this mirror The damping of a galvanometer is also an importantlarge, since it is one of the limiting factors in the amount factor in determining frequency response. If there isof light that is available on the oscillograph viewing only a slight amount of damping, deflections near thescreen and film. On the other hand, the mirror size resonance frequency will be greatly magnified, since thecannot be increased indefinitely, since its inertia mustbe comparable to that of the armature for optimumoverall performance. The mirror shown in Fig. 7 is1/8X5/32 in. and has approximately fifteen times thearea of an ordinary oscillograph galvanometer mirror.

    In a galvanometer of this type, theforce factor, orforceper unit current, is a measure of the sensitivity. Sincethis force factor varies directly with the flux density inthe air gap it is desirable to have as large a steady fluxas possible. With the balanced armature type of con- FIG. 7-GALVANOMETERstruction, this is possible without saturating the arma- ARMATURE AND SUPPORTture, since the flux passes directly across the gap, anddoes not travel the length of the armature. However, galvanometer sensitivity is much greater for frequenciesan increase in steady flux results in a magnetic reduc- in this region. No matter how high this resonance fre-

    1. This construction was suggested by C. R. Hanna who has quency is placed, there is always the probability thatpublished the fundamentals of design, for moving iron systems, high frequency components of the applied wave willin his paper "Design of Telephone Receivers for Loud Speaking occur in the range of resonance. To prevent accentua-Purposes," Proc. I.R?.E., 1925. p. 437-460.tinothscmpet,tersoseftegavom2. "The Mitchell Recording Camera Equipped Interchange- tion of these components, the response of the galvanom-ably for Variable Area and Variable Density Souind Recording," eter at resonance must be reduced by adequate damp-C. R. lianna, Trans. Soc. Motion Pic. Engrs, v. 13, 1929, p. 312-6. ing. With the correct amount of damping, a galvanom-

  • 882 OPLINGER: A PORTABLE OSCILLOGRAPH WITH UNIQUE FEATURES Transactions A.I.E.E.

    eter will have practically uniform sensitivity throughout to cover the voltage steps, and is arranged so that onits entire frequency range. This damping should remain the lowest voltage tap, there is still sufficient resistanceconstant regardless of time or temperature, so that there to give essentially a constant current to the shuntedis no change in calibration. In most galvanometers, it galvanometer circuit.is necessary to use some form of mechanical damping,such as oil or rubber, but with the moving iron type it CONCLUSIONShas been possible to get sufficient electromagnetic damp- The portable oscillograph, which has been described,ing to approach the above requirements closely. If a was designed to meet a large variety of general engi-

    neering applications. The following unique featuresmake this instrument especially adaptable to such

    089.. E - applications.

    0.7. 4 ll l l l l l | ; l l _1. Simultaneous viewing and photographing.06 -- -_ _ _ _ _ _ _ lA ___ 2. A continuous time axis furnished by a small revolving mirror

    0.5_ _ ____lLa_ _the speed of which is variable.

    1 1 -4__1 3. A new type of galvanometer with electro-magnetic damping-1 _-T:T_==__T_ X _ and high frequency response.

    02 _ _ _ _ _ ===______ l == __7i _ 4. An optical system which magnifies the galvanometer de-F ___-__ ___ flections and which give a brilliant trace that can be observed in

    -l_I===-_ _ -_1i_ a brightly lighted room.7068O 00 2 3 4 5 6 7.91000 2 34 54 ;7 800C 5. A simple method of taking photographs similar to an ordi-

    FRE QUE NCYI

    FIG. 8-GALVANOMETER FREQUENCY RESPONSE nary camera.6. A large viewing screen for making tracings or for givingdemonstrations to a group of persons.

    strong magnetic field is used, the motion of the armature 7. Simplicity, compactness, and ruggedness comparable withat resonance will generate a back-voltage which will the average electrical indicating instrument.oppose the applied voltage to prevent any large de- Although this oscillograph is a research develop-flections. The effectiveness of this back voltage is ment product, it will be made available in the neardetermined by the total resistance of the circuit. future in a form that will differ only in minor detailsA typical response curve for the galvanometer used from the instrument described.

    in the oscillograph is shown in Fig. 8. Although the The author wishes to acknowledge his indebtednessdamping obtainable with electromagnetic systems as to C. R. Hanna for many valuable suggestions in con-used in this galvanometer is somewhat less than the nection with the above development, to W. 0. Osbondesired value, it should be emphasized again that it is who designed one of the earlier types of galvanometers,

    and to S. Sentipal for working out many details in theD.-1~I 5ALME-R 7mechanical design. Credit is also due other members of

    the research laboratories and shop organization whoFIG. 9-DIAGRAM OF have made contributions to this development.

    GALVANOMETER CIRCUIT

    Discussionpractically independent of temperature, and the fre- Everett S. Lee: Mr. Oplinger's paper describing a portablequency response characteristic is therefore more satis- oscillograph makes it appropriate at this time for us to look bothfactory than that of an oil damped galvanometer which backward and forward in the oscillograph art. The earliestat low temperatures may be over-damped and at higher design of oscillograph which we could find was that of Blondel'stemperatures under-damped. in 1891, just 42 years ago. A later design was of 1900, and one

    In order to use a single galvanometer for making by Duddell of 1898.ieosarrckit The first electromagnetic oscillograph of the General Electricboth current and voltage measurements, the circuit Companv was produced in 1904. For years thi,s oscillograph wasshown in Fig. 9 was devised for use in the oscillograph. quite generally used until the advent of the permanent magnetThis circuit makes the galvanometer response inde- galvanometer oscillographs a few years ago. Such a galvanometerpendent of the resistance of the external circuit and also is completely self-contained and is the element for all of theseprovides a simple arrangement for making tap connec- oscillographs, the popular 6-element oscillograph, the 2-element

    * . . . . . Y=rl ~~~~~~~~~withlarge tracing screen, and the portable 2-element oscillograph.tionson thshut andmultplierresitors. When The portable 2-element design has been a most popular oscillo-making current measurements, a small resistance hay- gra,ph for the pa,st three years, having been described before theing a number of taps is permanently shunted across the Institute May 7, 1930.1 This is for viewi.ng recording phenomena,galvanometer. The taps give the desired current range with photography of the same obtained with the addition of theand the value of the shunt is such as to give the desired film holder, and for photographing transients with the continuous

    drive film holder using films 30 in. long at a variety of film speeds.frequency response. It can be shown that this response The development of this continuous drive film holder as an at-is the same for any tap position. For making voltage tachment to this porta,ble oscillogra,ph has considerably extendedmeasurements, another resistor is connected in series

    1. See A Newu Portable OscejUograph, C:. M1. Hathaway, A.IE.E. JL.,with the current circuit. This latter resistance has taps Aug 19.30, voL. 49, NO. 5, PP. 646-649.