Magnetic field sensors for the industrial automation

A ELSEVIER

Sc~q,r, Ind Actuators A 59 119q71 177 I,;2

Magnetic field sensors for the industrial automation T. R e i n i n g e r *. C . H a n i s c h FESTO KG R&D, Linlh~rg,~tn~x.,~'6, D-73734 I~dmNn, (f,'lm~olr

Abstract The simple noncontacting dett:ction of posili¢,n plays a major role in Ihe aultmation technique. Especially for the application in production plants, a high level of quality, reliability and durability is an absolute neo:xsity. In this environmem, the use of magnelic field sensors Ibr delecting the position ofa pernument magne! is at first glance a very ~,.el[ knt,,,vn and soked physical problem, but with respect to the economic reality it pla~:esmajor demands on existing inanufaeturing technologies. In developing magnetic switches, ol~e has to consider different interfaces in order to acluexe the important aim to avoid undelined switdting states under worst ease assumptions and tO minimize the Ovel-al[ sizt." (.it"lhe senso(, illchldtng the electronics, t, Iq97 Elsevier Science S.A. Keylrord~: idagnetic Sell,ors: Industrial automation: Svdtch dexelopmcnl

I. Introduction For industrial automation, the actuation with compressed air plays an important role because pnetmtatics has without doubt always b¢ol ~m ideal moduktr system Ibr all kinds of automation (ipplicalions in almost every field of technology, The components arc simple to assemble, versatile and flexible in use, easily convertible, re-usable, robust and relatively inexpensive. Rapid results can be expected thanks to easily-understandable teach-yourself technology ~e~d. last but not least., last amortization period adds to the dislinguishing features of pneumatics within automation concepts. Pneumatics as a modular system is constantly complemented and exp.'mded by new teclmologi,r~d deve!opments, such as servopneumatic ;ictnators, and units and comolete devices incorporating electronics, such ;~s valw tcrminah. These developments mean that systems bas,:d on pneumatics are awfilable in combination with mi:roprocessors, actuators and sensors Ibr the entire field of automnlion. The broad range of cylinders and valves, v.ith a closely-spaced choice of sizes is important in order t'.r provide economical solutions to the increasing complexity and variety of machines using pnenmat-' iC.~.

*Corrc~;pondin~ atnlnlor.Tel.: +-19 71t ~472,",~t:fax: +49 71r 3473613: e mail: l~536,162/,,.\~mpuscr',e,com 0924-4247 97 $17.II0 ~ 1997 El.~vicr ~i,:n,.:e S.A. All ruhts rc.~r'~,:d. Pil 50a24-4247[ 97 i0 ] 438-6

In lh~.~c machine'.; the knowledge of the position of moving paris is essential in controlling the timing of mechanical actuation Imtx~rtant ,solutions lbr this kind of problems are m~gne~ic switches and magnetic displacement sensors. Realizing such solutions successfully means matching magne!ic ~nsor technology with many boundary conditions at ttie interfaces bet~x'en the physical sensor and its environment in the application. This paper deals with the solution of the problems that arise at those interlaces.

2, Technological problem Figs. I and 2 illustrate the areas of application. The sir,lple~t one is the '.ransversal or rot~*tional movement of a, mass from position A to lx~sition B. Using pneumatics this is done by a compressed air driven pistion in a cylinder. "rhe direction and speed of the motion is controlled by either it digital or scrvo valve. Magnetic switches tbat are operated by permanent m~tgnets on the piston infide the cylinder provide the information of .,pccili~ positions for further process control. F o r plt,~::se free positioning [e-g- to mo',e the piston not just from one end of the cylinder to the other but to move it to an arbitrary position in between) one car, build up a closed !oop consisting for instance of a contactless magnetos(rio(lye displacement sensor, a servo valve, a cylinder and an electronic control unit (Fig. 2).

17S Magnetostrictive displacement sensors ;Ire deteeti:~g Ihc propagnlion delay time of an ultrasonic wave in n mugnetostri':tive tube created by the interchange (Wiedem;tnn ef[ecl) of u longitudinal Idue to the permunent magnet in the piston) and a cireuhtr magnetic field (due to a pulse current in the center wire of d;e tube). The accuracy of such sensors approaches some opticul displacement sensors. This outlook is intended to show thai. tile points discussed below can easily be :tdjusted to more eomplic~tcd magnetic sensors. Since in most applications simple m~gnetie switches either with reed coOl;letS or magnetoresi~tive sensor elemcnts-...~lre used the following lines of argumentation are basicly nle~lnt for this t) pe. To, use ;~ magnetic switch for detecting the position of U permlnlent magnet ill a piston is on first glance ;i ',cry '.yell known and solved physical problem, but with respect to tile economic reulity the development of such switches pluees many demtmds on the inlerl~ees und conditions which have Io be met. The geometric form of these switches usually ullovcs tbr the mechanical udapt~ltion or even integration at or in the cylinders. An ex~mple of ~ typicul sitnution is given in Fig. 3.

3, Inler[aces We would like to think of the development or u m;,gnelie switch in terms of interfuces: an internal interface between physical ~;ensor und the necessary electronics und so to speak the external interfaces of mechanical adaptation to tile ;~iven geometry, the required sensitivity lbr the magnetic fields available and environmental conditions. Depending on the point of viev, there can be overlaps between what we call intern:tl and external intcrl'~ce. But s;r,ce the emphazis will be on mcetit~g the demzmds pislon

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of the right sCilsitivJtyof a switch we will combine internal geometric tolerances with tile ovendl ~nld we will neglect any dependence of the sensitivity ,',f the switd~ on materials tlsed within the switch or the internal electronics since ttiis c;m be avoided by proper design. At Ihe level or the internal interface the necessary electronics are derived rt'onl the spceilicutions of lhe physical ~nsor used and tile definitions of the electric data the device hu:; to meet later in its use. Since the tirst of Jnntmr)' 1996 electronic devices hct'ee to meet the Europe=in regulation for lilt CE sign. It is true that this is a requirement which ha~ to he :mributed to the cxlernul interface environment but it has u h~rgc impact on the testing and the &sign of the device itsell'. Very citrcl'tll consi.J,:r~t~iotl has to be paid to these Eurolm~tn st'n'J;h.lr{J~ ~,",:,?rwise it might take several redesigns before the producl pa:-;ses the tests.

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'File just mentioned points are basic general prerequisites lbr a sensor product. If a magnetic switch is considered for a product range of pneumatic cyllndcrs one has at lirst to decide on the !e',el of magnetic field which shall be available at the sensor element whi,:h is going to be nsed its it switch. Since this held can be geared within wide limits by choice of geometry and type of magnetic material this point will be discussed in detail. The magnetic licld .q:fi.,,i:ivity of the switch is ttsu:dly influenced by ,.'nvirotlmental conditions such as the temperatu,e range in ',~hich the switch is to Ix used. It also might he influenced by vibration or mechanical impact. And linally one has to consider the influence of all tolerances (geometric tolerances, tolermces of tht: magnetic materials, tolerances of the scnsititvity of tile sensor elemenl) on the swilching function in order to guarantee a clear function under worst case cc,nditions and to avoid undetined switching states.

4. Matching of intt,rfaccs hi order to optimize tile interfaces l~etxwen Sensm'- Magnet- Design

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i~'ed very carefully, and tbeoretica!iy. This can be done experimentally by measuring the componcnt,~ of the magnetic lield at the place ~',here the se:,sor ,;,,;'itches or theoretically by calcnlating the lleld distribution in the vicinity ~1' the switching location. To find general field conditions for proper switching we investigated the switching lield dependence on the geometry of the permanent magnet. One hinds, as one would expect, that the ibr magnetic rings ~ith a large diameter the lieid situation nece:,~ry for switching is comparable to the homogenous case. For the dc:elopmcnt ,:,f sin,diet components; we ha','e found a .;';'ong depcnden.:e of the switching licld to the overall size of the d::.,ign (sensor-magnet-ferromagnetic parts). In order to characterize this field one hits to know this dependency very clearly. In Fig. 4 an example l\',r this complex t',¢haxiour is shown in the case of I~rromagnetic parts in the vicinity. The diagram in the lower part of the figure sketches tile principle change of the switching Ikid ;xith the global geometry of the permanent magnet compared to tile situation in a homogeneos magnetic lield. Witb this data base linduding the knowledge of the temperature dependence of the sensiti',ity of the snitch) one can de, elopa magnetic s~itch for pneumatic c?liaders.

to achieve tile aim of clear ~mctioning we have to take title accotlnt ,l~c folh~t~mg ~'esponsiblc

boundary conditions: o .switching.lield range of the physical sensor element and the final sensorproduct • diMtulce end lOIClkllR'£'S betv~een pelnlanent magnet and sensor element • magnetk' pro/~ertit,:: and tolerances • the demanded tcmperatttre range, and as the case might require • additional fi'rrontat, tlelh' parts in the vtcmn.v of the sensor and tile permanent magnet • possible interferences by exlerattl nlegncti~ ./ic,'ds. ~. 1. St,,sor-~h'si.~,

Usually the dependence of tile sensor output on I!1¢ orientation of the physical sensor in the magnetic lield and of the lleld strength in it homogenous magnetic tleld are known tYom data sheets at the beginning of the. Tolerances due to tolerances of the nrmufactoring process during sensor prodt~ction hate to he tested experimentally. One of the nl;dn difficulties iu usif~gtiaese data arises form the tact that for pneumatic cylinders permanent magnetic ring magnets are used. This means that we cannot regaJd the magnetic hi¢l.:s .'is homoget:ous Therelbre the field situation necess~try tbr a proper switching of a given sensor element has to he character-

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For a given nlagnctic s~itch o,1,2 has to dimension the permanent magnetic (rig for new c dindcrs. Matching the interface betx~ecn tile magnet and tile design of tile mechanical construction including tile mechanical lixtur¢ t;f the magnetic switch to tile cylinder is importam in order to optimize the ncccessary field distrihulion and the magnetic stability of the pemlanent maguet in its working point. For this lask FEM calcula'ions provide a vet,: good tool. Fig. 5 displays a three dimensional dement plot of a linite element model of a cylinder for which the geometry and the magnetic material of the magnet had to bc determined. The ring in the center of the drawing sho~s the penuaacnt m;~gnel. The t~o sticks above the permanent magnet indicate the Ix~ssiblet'x~sitionsof the sensor inside a cylinder groove. All the other parts shown, are ferronmgnctic parts in the vicinity like monnting pla,es or screws. They have to be considered. because they influence the magnetic lield distribution. With such a simulation tool one is able t~ oetermine the magnetic licid distribntiou for all l',r,ssible tolerances of geometry, Icmpcraturc. sv~itch sensitivity, magnetic prepcrtics at the future Iteration of Ibe magnetic switch. 4.3. :lb.Jgnct-.gt'tt~or The methods and possibilities described in the last two section yield the tools Ik~r the dimensioning of a

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Geometry of Magnet (a.u,) Fi':". 4. hllluenc¢ of I'erronl~lgn,21J¢paris on Ih,2 field db.tfibutJon alld the corre~pondill7 stlilt'ilin~ lleitt dep¢lld¢li~ oI1 ilk' 'global size of the ~,eollldD in principle. permancz~t magnet Ibr a given magnetic s~fitch and vice versa. The detcrrMnatiou of a criterion for the magnetic iield necessary to switch a certain magnetic switch its u function t~l" its sensitivity makes it possible to calculate the dimensions of a pcrmenent magnet Ibr u specific applicalion amt to choo~ie the right magnetic muterM. Thi~, kind of caclulation not only gives a staten3ent on the magnetic stability of the pt:rm~mcnt ma~.net in the gi,,cn design but ~Jso ~ result whether the magnetic switch will work property -..even under wors! case assumpticms. In terms of product developmcnl Olll~ sh~)rtens the time of development because with '(his IesuJ{ one is able to order u specitic permanent magnetic

material with lhe exact knowledge of lhe geometric djmc¢~sions and tolcrunccs for t11¢ application, This redtlces the Iria[ and error approach of (l~¢ past. As ::aid hdbre, mulching the iulerl'uce between the l'(lagnet and the sensor means we must lake into acCOLlnt '¢vOl'St CUSEClznditions for the ,~eneration of the magnetic lield distributions and compare these with the possible range ol sensitivities of a certain type of magnetic switch. This is illusm|ted further by Fig. 6. It shows a:/. i) typical lield distribulion o1" Lhe Iield comporicnt parallel to the axis or the ring Illa~nct ill ~l certain ma.m)et-scns,,,r distance. It also cc,ntaius the maximum and the minimum 1i¢1d distt'ibutions for the worst case assumptions.

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5. Aims We just described possible tools Dr the ,
Because the sensor elements under consideration here only detect the absolute value of the magnetic field one has to avoid undefined switching situations. ]-hat means the lowest admissible switching lield must be higher than the inaximnm field in the side hands of the maximum field distribution, and the highest admissible switching field must bc Io~er than the maximuo~ lield of the minimum field distribution,

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6. Cnnchzsions We h~e sketched the houndar3 conditions and interfaces to be matched during the dc'.elopment of a magnetic s~dtch tov,'ards a final product for the industrial automation industry. It starts with a rather ','.'ellknown physical sensor element such as tt MR chip, a Hall element or even a reed contact and gods its far its for instance meeting CE standards. We have mentioo,~ the difficulties which go beyoi~d ',lie study of parameters mlhtcncing the working of a physical sensor. We hope Ih~tt by doing so ~c can add at facet which is a basis for bridging industrial research alctivities with the work done in research laboralorics.

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7;umlas R(,mi~gt'r earned his doctoral degree in physics tit the Max-Planck-lnstitut fi+irMctallforschung, Institut t'{ir Physik in Stuttgart, Germany. in 1990. He mainly worked on the magrtetization processes lind domain structures in nanocystalline and amorpfious I'erromt~gnets at the slime inswute till Iq92. In 1993 he joined the R&D Department of FIRST() where he is, among other things, responsible For magnetic materials in sensors and actuators. Chri.w~ph Hani~ch earned his doctoral degree in physics at the Institute of Applied Physics of the University of DCtsseldorf. Germany, in 1984. with a thesis on ferromagnetic domains in Fe-Si single crystal picture frames. Before he joined the R&D Departement of

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FESTO in 1988 he worked at the MPA Stuttgart, Germany, on hydrogen technology and a magnetic testing proceedure for defects in steel components. He

is now responsible for tile basic research at FESTO with main interest in ~nsor technology and smart m~terials.