(19)
(11)EP 0 129 308 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
27.12.1989 Bulletin 1989/52

(21)Application number: 84302613.9

(22)Date of filing:  17.04.1984
(51)International Patent Classification (IPC)4G01L 1/08, G01G 7/02

(54)

Electromagnetic force-transducer

Elektromagnetischer Kraftwandler

Transducteur de force électromagnétique


(84)Designated Contracting States:
CH DE GB LI

(30)Priority: 10.06.1983 JP 104729/83

(43)Date of publication of application:
27.12.1984 Bulletin 1984/52

(73)Proprietor: SHIMADZU CORPORATION
Nakagyo-ku Kyoto 604 (JP)

(72)Inventors:
  • Kawara, Toshio
    Shibayama Yamashina-ku Kyoto, 607 (JP)
  • Shibahara, Yoshihumi
    Moriyama-shi Shiga, 524-01 (JP)

(74)Representative: Smith, Philip Antony et al
REDDIE & GROSE 16 Theobalds Road
London WC1X 8PL
London WC1X 8PL (GB)


(56)References cited: : 
CH-A- 529 998
DE-A- 2 819 451
CH-A- 529 999
GB-A- 2 076 543
  
      
    Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


    Description

    Background of the invention



    [0001] The present invention relates to an electromagnetic force-transducer, and more particularly to an electromagnetic force-transducer suitable for use in an electronic balance.

    [0002] Electromagnetic force-transducers, particularly those used in electronic balances, comprise a fixed magnetic circuit including a permanent magnet, an inner magnetic path and an outer magnetic path with a gap between the inner and outer magnetic paths across which a magnetic field acts to generate axial force. An example of such electromagnetic force-transducers is disclosed in the UK Patent Application GB-2 076 543A.

    [0003] Fig. 1 shows a cross-sectional view of a conventional electromagnetic force-transducer. In the figure a permanent magnet 1, a pole piece 2 and a flat-bottomed cylindrical yoke 3 constitute a magnetic circuit. The cylindrical yoke 3 has, on its inner surface, an annular prominence providing a pole piece section 3a which faces the pole piece 2 with an annular magnetic gap left therebetween. In the annular magnetic gap there is positioned an electromagnetic coil 4 wound around a coil frame 5 kept movable in a vertical direction coaxially with the magnet 1, the pole piece 2 and the cylindrical yoke 3. The coil frame 5 is to be provided with a force-transmitting means (not shown in the figure) connected to a force-loading portion (for instance, the weighing tray of a weighing balance) where a force to be balanced is loaded. The electromagnetic coil 4, supplied with a current, produces an electromagnetic force. With the current controlled suitably the electromagnetic force is enabled just to balance a force externally loaded to the coil 4. The magnitude of the electromagnetic force is obtained from measuring the current flowing in the coil 4. Therefore, it is desirable for the electromagnetic force to be exactly proportional to the current. However, the linearity between the current and the produced electromagnetic force is violated by the effect of the magnetic field which the coil 4 itself makes on the permanent magnet 1. A compensating coil 6 wound around the permanent magnet 1 is for cancelling the field made by the coil 4. The compensating coil 6, which does not contribute to the electromagnetic force production at all, is an obstacle to designing the force-transducer to be short in height. Further it makes the electric circuit complex.

    [0004] On the other hand another type electromagnetic force-transducer is proposed, for instance, in the Japanese Laid-Open Utility Model Application No. 55-164519. In this model the area of the electromagnetic coil plane being divided into two halves bounded by a diameter, two magnetic fields directed oppositely to each other are applied respectively to one of the two halves and to the other half in the direction perpendicular to the coil plane so as to make the coil produce an electromagnetic force with its direction lying in the coil plane. With this manner of field application the effect of the magnetic field made by the electromagnetic coil is cancelled throughout the entire magnetic circuit so as to enable the produced electromagnetic force to be proportional to the current flowing in the coil. The force-transducer of this type, however, has a disadvantage that the effective circumferential length of the electromagnetic coil is reduced by a factor of 2/n.

    [0005] An object of the present invention is to provide an electromagnetic force-transducer which produces an electromagnetic force proportional to a supplied current without employing a compensating coil.

    [0006] Another object of the present invention is to provide an electromagnetic force-transducer which can be designed to be short in height.

    [0007] According to this invention the electromagnetic force-transducer is characterised in that a second coil, axially-spaced from the first, is disposed in a second gap in which the magnetic field is in the opposite direction, the coils being so connected that the current flow is in opposite senses in the two coils whereby both coils are acted upon by a force in the same axial direction while their magnetic fields are opposed.

    [0008] In such a construction the electromagnetic coil consisting of the two windings does not have any resultant influence on the static-magnetic field provided by the magnetic circuit, because the fields made by the two windings are oppositely directed to offset each other along the entire magnetic circuit.

    Brief description of the drawings



    [0009] The present invention will further be described in detail with the accompanying drawings, in which:

    Fig. 1 shows a cross-sectional view of a conventional electromagnetic force-transducer;

    Fig. 2 shows a cross-sectional view of an electromagnetic force-transducer embodying the present invention;

    Fig. 3 illustrates a directional relation between the coil current and the static-magnetic field provided by the magnetic circuit in the above embodiment;

    Fig. 4 illustrates the magnetic field made by the electromagnetic coil in the above embodiment;

    Fig. 5 shows a perspective view of the outer magnetic yoke used in the magnetic circuit in another embodiment of the present invention; and

    Fig. 6 shows a cross-sectional view of the whole magnetic circuit in which the outer magnetic yoke shown in Fig. 5 is used.


    Detailed description of the invention



    [0010] In Fig. 2, which shows an embodiment of-the present invention, a disc-shaped permanent magnet 11, an upper and a lower pole piece 12 and 13 fixed respectively to the upper and the lower surface of the magnet 11, and a cylindrical outer magnetic yoke 15 surrounding the magnet 11 and the pole pieces 12, 13 constitute a magnetic circuit. The cylindrical outer yoke 15 has, on its inner surface, two prominences providing two annular magnetic pole piece sections 15a and 15b facing the upper pole piece 12 and the lower pole piece 13, respectively. In the annular magnetic gaps made between the annular pole piece section 15a and the upper pole piece 12 and between the annular pole piece section 15b and the lower pole piece 13, there are respectively positioned a winding 16 and another winding 17, which are wound on a common coil-frame 18 and constitute an entire electromagnetic coil. The coil frame 18 is connected with a force-transmitting means 19 and kept movable in a vertical direction coaxially with the above elements constituting the magnetic circuit. The entire magnetic circuit is fixed on a common support 14 which is made of a non-magnetic material.

    [0011] In such a construction of the electromagnetic force-transducer, the disc-shaped permanent magnet 11 is magnetized downward in the vertical direction, that is, in the thickness direction of the same, so that the static-magnetic flux in the magnetic circuit can schematically be pictured as shown with lines 20 in Fig. 3, intersecting the winding 16 in the direction from outside to inside and the winding 17 in the direction from inside to outside. Therefore, if the windings 16 and 17 are supplied with the currents in the directions shown respectively by dots and crosses, they both produce electromagnetic forces directed upward, as is shown by arrows 21 and 22. The support 14 (not shown in Fig. 3. See Fig. 2) of the magnetic circuit does not affect the symmetric pattern of the static-magnetic flux shown in Fig. 3, because the support 14 is made of a non-magnetic material as is described above. In the present invention, further, the static-magnetic field in the magnetic circuit is not affected by the field which the electromagnetic coil makes thereon, because the two windings 16, 17 consituting the electromagnetic coil are current-supplied in the directions opposite to each other so that the magnetic fields produced by both of the windings 16 and 17 offset each other at the center of the coil. Consequently, the electromagnetic force is always proportional to the current. Fig. 4 illustrates a magnetic flux pattern shown by the electromagnetic coil made up of two windings 16 and 17.

    [0012] Further, the present invention can be embodied with the cylindrical outer yoke divided into some yoke pieces 15' along generatrices, as is shown in perspective in Fig. 5. The whole magnetic circuit of this embodiment is cross-sectionally shown in Fig. 6, in which reference numbers 11',12' and 13' respectively indicate a permanent magnet, an outer pole piece and a lower pole piece similar to those used in the embodiment shown in Fig. 2. In this embodiment the magnetic circuit support 14' not only is of non-magnetic material but also has a thermal expansion coefficient smaller than those of the materials used in the magnetic circuit. Such a construction of the magnetic circuit enables a thermal expansion and contraction at the magnetic gaps to be decreased, and therefore, results in decreasing an error due to temperature variations.

    [0013] Another embodiment of the present invention can be executed by replacing the disc-shaped permanent magnet 11 in the embodiment shown in Fig. 2 with a non-magnetized magnetic material and, at the same time, by constituting the annular pole piece sections 15a and 15b with annular permanent magnets magnetized radially. This embodiment can be further modified by constituting the whole or a part of the cylindrical outer yoke with a cylindrical permanent magnet magnetized in the direction parallel to the cylinder- axis instead of displacing the annular pole piece sections 15a and 15b with the annular permanent magnets.


    Claims

    1. An electromagnetic force transducer comprising a fixed magnetic circuit including a permanent magnet (11), an inner magnetic path (11, 12,13) and an outer magnetic path (15) with a gap (15a-12) between the inner and outer magnetic paths across which a magnetic field acts to generate axial force on a current-carrying coil (16) disposed in the gap, characterized in that a second coil (17), axially-spaced from the first (16), is disposed in a second gap (15b-13) in which the magnetic field is in the opposite direction, the coils being so connected that the current flow is in opposite senses in the two coils whereby both coils are acted upon by a force in the same axial direction while their magnetic fields are opposed.
     
    2. An electromagnetic force-transducer as defined in claim 1, wherein said permanent magnet (11) is included in said inner magnetic path (11, 12, 13).
     
    3. An electromagnetic force-transducer as defined in claim 1, wherein said permanent magnet is included in said outer magnetic path (15).
     
    4. An electromagnetic force-transducer as defined in claim 1 or 2, wherein said inner magnetic path (11, 12, 13) is made by stacking a lower pole piece (13), a plate-shaped permanent magnet (11) magnetized in its thickness direction and an upper pole piece (13), and wherein said outer magnetic path (15) is made of a cylindrical magnetic yoke (15) surrounding said inner magnetic path (11, 12, 13) coaxially, said cylindrical yoke (15) having as pole pieces an upper inward protrusion (15a) and a lower inward protrusion (15b) facing said upper pole piece (12) and said lower pole piece (13), respectively.
     
    5. An electromagnetic force-transducer as defined in claim 1 or 2, wherein said inner magnetic path (11, 12, 13) is made of a lower pole piece (13), a magnetic core piece (11) and an upper pole piece (12), all being stacked successively or made in one body, and wherein said outer magnetic path (15) is made cylindrical surrounding said inner magnetic path (11, 12, 13) coaxially and consists of an upper and a lower cylindrical magnetic yoke and a ring-shaped permanent magnet interposed between said upper and lower cylindrical magnetic yokes, said upper and lower cylindrical magnetic yokes having respective inward protrusions corresponding respectively with the upper and lower pole pieces.
     
    6. An electromagnetic force transducer as defined in claim 4 or 5, wherein said outer magnetic path (15) forming a cylinder is split into longitudinal pieces (15') by a plurality of slits provided along generatrices of the cylinder.
     
    7. An electromagnetic force-transducer defined in any one of the preceding claims, wherein said inner magnetic path (11, 12, 13) and said outer magnetic path (15) are fixed on a non-magnetic supporting means (14,14') the thermal expansion coefficient of which is smaller than those of the materials constituting said inner and said outer magnetic path.
     


    Ansprüche

    1. Elektromagnetischer Kraftwandler mit einem festen magnetischen Kreis, zu dem ein Dauermagnet (11), ein innerer Magnetweg (11, 12, 13) und ein äußerer Magnetweg (15) gehören, wobei zwischen dem inneren und dem äußeren Magnetweg ein Spalt (15a-12) vorhanden ist, durch den ein Magnetfeld wirkt, um auf eine in dem Spalt angeordnete Spule (16) eine axiale Kraft aufzubringen, dadurch gekennzeichnet, daß eine zweite Spule (17) in einem axialen Abstand von der ersten (16) in einem zweiten Spalt (15b-13) angeordnet ist, in welcher das Magnetfeld die entgegengesetzte Richtung aufweist, wobei die Spulen derart geschaltet sind, daß der Strom in beiden Spulen in entgegengesetzten Richtungen fließt, so daß auf beide Spulen eine Kraft in der gleichen axialen Richtung wirkt, während ihre Magnetfelder einander entgegengesetzt sind.
     
    2. Elektromagnetischer Kraftwandler nach Anspruch 1, dadurch gekennzeichnet, daß der genannten Dauermagnet (11) in dem genannten inneren Magnetweg (11, 12, 13) angeordnet ist.
     
    3. Elektromagnetischer Kraftwandler nach Anspruch 1, dadurch gekennzeichnet, daß der genannte Dauermagnet in dem genannten äußeren Magnetweg (15) angeordnet ist.
     
    4. Elektromagnetischer Kraftwandler nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß der genannte innere Magnetweg (11, 12, 13) dadurch erzeugt ist, daß ein unteres Polstück (13), ein in seiner Dickenrichtung magnetisierter plattenförmiger Dauermagnet (11) und ein oberes Polstück (13) übereinander gestapelt sind und daß der genannte äußere Magnetweg (15) aus einem zylindrischen Magnetjoch (15) besteht, das den genannten inneren Magnetweg (11, 12, 13) koaxial umgibt, wobei das zylindrische Joch (15) als Polstück einen oberen, nach innen ragenden Vorsprung (15a) und einen unteren, nach innen ragenden Vorsprung (15b) aufweist, die dem genannten oberen Polstück (12) bzw. dem genannten unteren Polstück (13) zugwandt sind.
     
    5. Electromagnetischer Kraftwandler nach Anspruch 1 oder 3, dadurch gekennzeichnet, daß der genannte innere Magnetweg (11, 12, 13) aus einem unteren Polstück (13), einem Magnetkernstück (11) und einem oberen Polstück (12) besteht, wobei alle Teile aufeinander gestapelt oder als einheitlicher Körper ausgeführt sind, und daß der genannte äußere Magnetweg (15) zylindrisch ausgebildet ist und den inneren Magnetweg (11, 12, 13) koaxial umgibt, wobei er aus einem oberen und einem unteren zylindrischen Magnetjoch und einem ringförmigen Dauermagneten besteht, welche letzterer zwischen dem oberen und dem unteren zylindrischen Magnetjoch angeordnet ist, wobei das obere und das untere zylindrische Magnetjoch jeweils nach innen ragende Vorsprünge aufweisen, die den oberen und unteren Polstücken entsprechen.
     
    6. Elektromagnetischer Kraftwandler nach Anspruch 4 oder 5, dadurch gekennzeichnet, daß der genannte äußere Magnetweg (15), der einen Zylinder bildet, durch mehrere entlang Mantellinien des Zylinders ausausgebildeter Spalte in Längsabschnitte (15') unterteilt ist.
     
    7. Elektromagnetischer Kraftwandler nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß der genannte innere Magnetweg (11, 12, 13) und der genannte äußere Magnetweg (15) auf einer nichtmagnetischen Unterlage (14, 14') angeordnet sind, deren Wärmeausdehnungskoeffizient kleiner ist als diejenigen der Materialien, die den inneren und den äußeren Magnetweg bilden.
     


    Revendications

    1. Un transducteur de force électromagnétique comprenant un circuit magnétique fixe comportant un aimant permanent (11), un trajet magnétique intérieur (11, 12, 13) et un trajet magnétique extérieur (15) avec un entrefer (15a-12) entre les trajets magnétiques intérieur et extérieur en travers dual un champ magnétique agit pour engendrer une force axiale sur une bobine (16) traversée par le courant, disposée dans l'intervalle, caractérisé en ce qu'une deuxième bobine (17), espacée axialement de la première (16), est disposée dans un deuxième entrefer (15b-13) dans lequel le champ magnétique est dans le sens opposé, les bobines étant reliées de telle façon que le flux de courant se fait dans des sens opposés dans les deux bobines de sorte que la force appliquée à chacune des deux bobines est dans la même direction axiale alors que leurs champs magnétiques sont opposés.
     
    2. Un transducteur de force électromagnétique selon la revendication 1, dans lequel ledit aimant permanent (11) est compris dans ledit trajet magnétique intérieur (11, 12, 13).
     
    3. Un transducteur de force électromagnétique selon la revendication 1, dans lequel ledit aimant permanent est compris dans ledit trajet magnétique extérieur (15).
     
    4. Un transducteur de force électromagnétique selon la revendication 1 ou 2, dans lequel ledit trajet magnétique intérieur (11, 12, 13) est réalisé en empilant une pièce polaire inférieure (13), un aimant permanent en forme de plaque (11) magnétisé dans la direction de son épaisseur et une pièce polaire supérieure (13), et dans lequel ledit trajet magnétique extérieur (15) est constitué arceau magnétique cylindrique (15) entourant ledit trajet magnétique intérieur (11, 12, 13) de façon coaxiale, ledit arceau cylindrique (15) possédant comme pièces polaires une saillie supérieure intérieure (15a) et une saillie inférieure intérieure (15b) faisant face à ladite pièce polaire supérieure (12) et à ladite pièce polaire inférieure (13), respectivement.
     
    5. Un transducteur de force électromagnétique selon la revendication 1 ou 3, dans lequel ledit trajet magnétique intérieur (11, 12, 13) est constitué d'une pièce polaire inférieure (13), d'une pièce de noyau magnétique (11) et d'une pièce polaire supérieure (12), toutes empilées successivement ou réalisées en un corps, et dans lequel ledit trajet magnétique extérieur (15) est réalisé cylindrique entourant ledit trajet magnétique intérieur (11, 12, 13) de façon coaxiale et consiste en un arceau magnétique cylindrique supérieure et un arceau magnétique cylindrique inférieur et un aimant permanent annulaire interposé entre lesdits arceaux magnétiques cylindriques supérieure et inférieur, lesdits arceaux magnétiques cylindriques supérieur et inférieur possédant des saillies intérieures respectives correspondant respectivement avec les pièces polaires supérieure et inférieure.
     
    6. Un transducteur de force électromagnétique selon la revendication 4 ou 5, dans lequel ledit trajet magnétique (15) formant un cylindre est divisé en pièces longitudinales (15') par plusieurs fentes ménagées le long de génératrices du cylindre.
     
    7. Un transducteur de force électromagnétique selon dans l'une quelconque des revendications précédentes, dans lequel ledit trajet magnétique intérieur (11, 12, 13) et ledit trajet magnétique extérieur (15) sont fixés sur un des moyens de support non magnétique (14, 14') dont le coefficient de dilatation thermique est inférieur à celui des matières constituant lesdits trajets magnétiques intérieur et extérieur.
     




    Drawing