Frictionless Recording Torque MagnetometerA. A. Aldenkamp, C. P. Marks, and H. Zijlstra Citation: Review of Scientific Instruments 31, 544 (1960); doi: 10.1063/1.1931246 View online: http://dx.doi.org/10.1063/1.1931246 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/31/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Digital enhancement of torque magnetometer signals Rev. Sci. Instrum. 47, 938 (1976); 10.1063/1.1134777 Simple Recording Torque Magnetometer J. Appl. Phys. 31, S184 (1960); 10.1063/1.1984658 Improved Torque Magnetometer J. Appl. Phys. 29, 493 (1958); 10.1063/1.1723195 A Recording Torque Magnetometer Rev. Sci. Instrum. 21, 605 (1950); 10.1063/1.1745665 Some Uses of the Torque Magnetometer Rev. Sci. Instrum. 8, 56 (1937); 10.1063/1.1752236

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Til E REV IEW OF SC I E~T IF I C I ~ ST RI · l\"F.~TS VOLC ME 31. :\,C:\·I BER 5 MAY, I

Frictionless Recording Torque Magnetometer

A. A. AI.DI::::-a,A),ri' , C. p. ~I ARi.: S, k,\"D H. ZIJl.STRA .IfelallllrgictJl Laboratory, N. i/. Philips' Glm:i/ulII/}( IIj abrieken, Eindlwrcn, NC"lul!lllds

(Rrccived January 18. 1960; and in final form, February 29, 1960)

An instrument for measuring magnetic anisotropy by recording magnetic torque cu rves is descrihed . T he spt.'Cial construction of the transducer which converts the torque exert ed on the sample in to an elect ric signal makes it possible to avoid bearings , so that t he instrument is essentially free of fr iction. The instrume nt is opemted with commercially available elec tronic apparatus and is suitable for routine measuremen ts on large nu mbe rs of samples . The maximum sensi liv ity is 150 d em per cen timeter deflect ion of the recorder stylus.

INTRODUCTION

FOR the measurement of magnet ic anisotropy the torque magnetometer is a widely used instrument. I t meas­

ures the torque that is required to rot ate a magnetic sampl e in a magnetic field. This is of int erest fi rstly for determining the magnet ic anisotropy energy of a single crystal, which gives information aboul the fundam ental mechanism of magnetization. \ Veiss,t for ins tance made a study of pyr­rhotinc using a manually opera ted lorquemeter, with point by point readings.

A more technical application was found by GO~S2 who used the same type of instrument for measuring the degree of crystal orientation in textured polycrystallinc silicon­iron transformer steel. Rose3 found a relation between the magnetic anisotropy a nd the deep drawing properties of rolled steel sheet.

These technica l applicat ions provided a la rge number of speci mens to be tested, so, the need was Cdt for an auto­matically operating recording instrument as made by .i\lill er.~ His device consisted of a vertical shaft on jewel bearings to which the specimen was attached . The torque exert ed on the shaft was measured by a strain-gauge trans­ducer and recorded on a stripchart recorder. The maximum sensitivi ty was 1000 d em per centimeter deflection of the recorder. Our problem was to measure the a nisotropy of weak specimens, which requi red a higher sensit i\·ity. Therefore a new type of magnetometer ''''as designed In which friction was almost completely eliminated.

PRINCIPLE OF THE INSTRUMENT

T he sample is rota ted in a magnet ic field by a vertical motor-dri \'en sha ft. This shaft incorporates a transducer which is highly sensitive to twisting and shows a great resistance to bending. In this arrangement t he shaft be­tween the transducer a nd the sample does not requi re a bearing,

The transducer is built up of two circular disks attached to each other by flat springs (Fig. 1). If a torque is exerted

I P. Weiss, J. phys. radium 4. -l69 (1905) . ~~ . P. Goss, Trans. Am. Soc. Metals 23 , 511 (1935), 3 K. Rose, Materials & Methods 30, 62 (October, 1949). 4 D. S. )!Iiller, Rev . Sci. Instr. 21 , 605 ( t950) .

544

upon the sample point .B will be displaced with respect t point A. For small twisting angles this displacement will be di rectly proport ional to the twisting angle. The displace_ ment is measured by an inductive displacement meter which is connected to a direct-reading measuring bridge.'

The inductive d isplacement meter is e~sentially a differ enlial transformer which has usually a movable ferromag .. netic core. For our purpose we have constructed an air cored t ra nsformer with a movable central coil. This leadS 10 the same result and avoids int erference of static stra tields on the transducer. The sensitivity is so high that a. di::;placement. of a few microns corresponds to full-scale deflection. This means that the d ifference of rotation angle bel ween t he mot or-driven shaft a nd t he sample-bearing sha ft is negligible wit h respect to the absolut e rotation angle of the motor-driven ~haft. \Ve may say, therefore, that we are measuring the turque exerted on the sa mple by a rotat ion that is indica ted by the rot ation of the motor .. driven shaft. The broad flat springs give a high rigidity aga inst. bending.

. motordri vrn I shaft

----

"

II sampl~

FIG. I. Principle or the (ransducer.

~ Philips displacement mete r, type PR 9300.

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R ECO RDI NG MAGXE T O ME I E R 545

The ou tpu t of th t: measuring bridge is connected to a recording pOlentiometer: fi The char t movement is sy n­

·zed wi th the rot at ion of the lorquemeter. chronl

CONSTRUCTION DETAILS (FIG. 2)

The tr;l !l:,ducC'f is bui lt up of l\vO circular di::5ks (1) and (2) made of PCf:,pex, which arc attached to each other with Ihree lIat phu,ph or-bronze , prings (3). These springs have recisely the S lIllC length of 40 mm , width of 10 rom, a nd ~ickness of O..l mOl . They are soldered on little plates (4) which arc bolt('d on the Perspex disks. T o the upper disk (I) are fi xed t he t\\"o stationary coils (5) of a differen t ia l tranSfOTi11 l' r. Between these coils is the movable coil (6) which is a tt ached to the lower disk (2). These three coils have c(IUai di mensions, with an outer diameter of 16 rom, aD inner dia meter of 5 mm, and a length of 5 mm. The out er coils each con~i s t of 1450 turns of O. to-mm enameled copper wire and the inner coil of 680 turn~ of O.l S-mm enamelecl copper wire. The cen tral coil (6) can be adjusted by a screw (7) to correct the differentia l transformer for

asymmetry. The tran:-ducer is enclosed in a copper shielding can (8)

fitted to the upper disk (I). A ilexih le it'ad connects the differential lran:;former via

" 9

12········ 10 - •

3 ··

G ·· 4·

13

· 14

15

... . ...... ····· 1

···· ····· 8 ················ 5

.. ........... ·· ·· ···· 7

···_··2

FIr. . 2. Construct ional dt:tails of the torque magnetometer. -I Philips milli volt recorder, type PR 2200 A/ 21 A.

r\

. \ I \ ,

I

1\ 1\ I , \

\. \ ~

\ • \1 Ii , 1\

FIG. 3. Record ing of the magnet ic torque exerted on the calibrati ng sample by a fidd of 5000 oe for 360" rOla lion.

Ihe uppt.:r d isk to the measuring bridge. T wo of the flat ~ prings are used as an electrical connec tion for the movable coil .

The driving mechanism must meet high demands a ~

regards freedom of vibration a nd slack. A worm ,yhed (9) rotates the vertical sha ft: ( 10) att ached to the upper d isk (I ).

T he sha ft (10) turns on ba ll bearings. The worm (11) is pressed against the worm wh eel (10) by a ftal spring (12) and is driven by a synchronous motor (13) via a reducti on­gear (1-1). The worm can be li fted from the worm whet! hy a handle (15), so tha t aft er each opera tion the torque-meter call be relurned to it s starting position by ha nd.

Great. care had to be exercised in earthing I he ded .ronic equipment. After this precaution a sensitiv ity of 150 d em per centimeter deviation of the recorder was ob ta ined. The measuring bridge ' '''as provided wi th a calibra ted a t lcnu­ut er, so that the device could be adapted to sampl es \vith high anisotropy .

The sensitivity could have been magnified electronically, but was limited by clastic aftereffect of the transducer. Improvement might be possible, by clamping the springs (3) instead of soldering them and by replacing the Perspcx by another material. This was not undertaken by th e authors as the instrument already met their requirements.

It was found to be important no t to cut the flat spri ngs, but to etch them ou t of the sheet.

CALIBRATION AND LINEARITY

The torque-meter was calibra ted by means of a cylindrical bar of pure nickel with a length of 1 i mm and a diameter of 1 mm. This sample shows a magnetic shape anisotropy

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546 ALDE :-I K AMP , ~'I ARKS , A i\ D Z IJLSTR A

which gives risc to a torque L as a fUIlClion of the angle 0 betwt:en the ax is of tht:: nickel ba r and the magnetic fi eld direction.

In a s trong magnetic field L is determined by

1 L =-(.r, -.r,, )J'V sin28,

2

in which }lJ, = the demagnetizing fa ctor perpendicular to the bar ax is, XJI = the demagnetizing fac tor parallel to the bar ax is, I =saturation magne tization, V= volume of the bar.

The va lues of .Vi and .V .. GIn be derivt:d from calcula­tions of \Varmuth ,i who det ermined the demagnetizing fa ctors of cyl indric,t l bars of va rious dimensiona l ratios and magnetic susceptibil ities. In a. s trong magnetic field the

.. K. Warmuth, .\rch. Elcktrotcch. 33, i -l-i l (939).

nl~: li EVIEW OF SC I E. :-.;rT IF IC J :\" STRCME~TS

susceptihility of the sample in question may be conside zero, so that for .\o"l- .VII the value of 6.0 is found.

W ith 1= 485 gauss and V= 1.33XlO- ' COl' we find maximum torque

L " ,,= 9400 d COl.

The torq ue-curve was measured at a fi eld s trength 5000 oc and is given in Fig . 3. Th e maximum deviation · 9.8 em which leads to II sensit ivity of 960 d cm per cen meter deflection.

The linearity of the transducer was checked by clamp· a calibrated torsion fiber between the :-ample holder and adju::itable rotating clamp, so that a predetermined tor could be exer ted on t he torque-meter. The rt'lation betw the deflection on the recorder and the t' xert ed torque mea~url'd up to 10000 d em and no deviation from lineari wa ::; observed in this range .

\ ·OLt; ME .l l . Nt: ~IB ER 5

Lightweight Suntracker for Balloon Applications

M. L. S HECHET·

COllt air Scimtijic Resmrc" ["(Iboralory, Sail f)i~go, CilHorn ilJ

(Received July 30, 1959; and in fina l form, February 29,19(0)

A lightweight inexpensive sun tracker for balloon applications is desc ribed. The entire tr:lcking system including powt:r supplies (or a 12-hr flight weighs l.i6 kg. The system has demonstr:lted in the laboratory a capabil ity of pointing a t.42 -kg load at the sun wi thin ± O.12°. A very simple and reliable acquisit ion ann t racking techn ique i~ employed which uses no vacuum lubes or t ransistors.

INTRODUCTION

SO?\IE phases of upper atmospheric scientific experi­mentation have betn hampered in the past because of

the lack of an economical lightweight balloon-borne sun­tracking system of moderate accuracy. As a result of a req uin:ment generated by the Upper Atmospheric P hysics Program of the Convair Scientific Research Laboratory in Sa n Diego, a n economica l lightweight sys tem for pointi ng inst rumenta tion at the sun has been built and tested. En­vironmental a nd life t est s of the prototype werc success­fu lly performed and the lirs l Right took place in February, 1959. An ea rly balloon failure prevenled complete eva lua­tion of the sun tracker performa nce under Bight conditions, however all d,lta \vhich were obtained relative to the sun­tracker indicated reliable a nd effective operation of the uni t .

S Ullt rackers used on earl ier balloon AightS1.2 have been 100 ht:avy and too expen~ive for the Convair application.

• Xow wilh Cubic Corporation, Sa n Diego. Californ ia. I ).[artin and Barbam Schwarzschild , Sci. 200, No.5, p. 52 (:\"a.)',

1959). 2 H. D. Edwards, A. Goddard , Jr., ), 1. J U7,,1. , T. )'1aher, and

F. Speck; Rev. Sci. Inst r. 27, 381 (1956).

The very low weight of the unit desnibed he re was of ticular importance s ince the requirt'ments of the expeq. ment included balloon float a lli ludes in excess of 37 km. O ne of t he carlier sys tems with comparable capabilities had" weight of the order of .17 kg.

The sun tracker described here weighs 1. i6 kg and is capahle of pointing the load to an accuracy of ±0.12~, The ,veight of the load can he as great as 4-.5 kg without basic modification of l he apparatus alt hough a 1A2-kg load was used during the system tests. The ~ys t em is inten' to operate for 12 hr al a ltitudes a bove 36 km. .!

A programer is included in t he apparatus so that the iJt!' s trumcntat ion can be pointed away periodically from t

sun 10 provide background and cal ibrat ion readings. The ~ystem is discussed firs t as a. basic suntracker which

may be used for divcr:-c pllrpose~. Aft t:: r the basic system has bc:t"n described and explained , t hose additions to ba :-oic unit which adapt it for the pa rl icular l'xpcrimentatC discussed .

BASIC DESIGN CONCEPTS

In order to minimize weight and power rcquiremen~5, ' th l.." ~untracker mechanica l configuration is a rranged

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