US3711776A

US3711776A - Core-magnet type instrument having linear response characteristic

Info

Publication number: US3711776A
Application number: US00089687A
Authority: US
Inventors: T Inami; Y Takizawa
Original assignee: Canon Inc; Canon Electronics Inc
Current assignee: Canon Inc; Canon Electronics Inc
Priority date: 1969-11-17
Filing date: 1970-11-16
Publication date: 1973-01-16
Anticipated expiration: 1990-01-16
Also published as: DE2056323A1; GB1336313A

Abstract

A core magnet type instrument which renders a linear response characteristic comprises a magnet core, a yoke surrounding and spaced apart from the magnet core to form an air gap therebetween, and a moving coil movable in the air gap and having a pointer. The magnet core is shaped into an ellipse to improve a response characteristic of the instrument.

Description

United States Patent 91 Inami et al.

[ 1 Jan. 16, 1973 CORE-MAGNET TYPE INSTRUMENT HAVING LINEAR RESPONSE CHARACTERISTIC Inventors: Tetsuzo Inami; Yoshiyuki Takizawa,

both of Chichibu, Japan Assignees: Canon Kaliushiki Kaisha, Tokyo; Canon Denshi Kabushiki Kaisha, Chichibu-shi, Japan Filed: Nov. 16, 1970 Appl. No.: 89,687

Foreign Application Priority Data Nov. 17, 1969 Japan ..44 91947 US. Cl. ..324/151 A, 324/132 Int. Cl. ..G0ll l/16, G011 l5/l0 Field of Search .324/132, 154, 151, 151 A;

[56] References Cited UNITED STATES PATENTS 3,005,952 .l0/l96l Basingeru ..3'24/151 R OTHER PUBLICATIONS .Bunn et al. Wireless World, December, 1960, pp.

Primary Examiner-Alfred E. Smith Attorney-Ward, McElhannon, Brooks & Fitzpatrick [57] ABSTRACT 5 Claims, 21 Drawing Figures PRIOR ART HQ 4 PATENTEDJAH 16 I973 Q 3.711.?76

sum 1 0P4 FIG. l B

PRIOR ART I 6 -29 I 'FIG. 2 FIG. 3

pmorz ART ART B(GAUSS) B(GAUSS) VmoR ART FIG 6 I FIG, 7 PRlOQ Al? y B(GAUSS) 5 v5 4 more ART PATENTEIJJAH 16 I975 3.711.776

sum 3 OF 4 FIG. l3 FIG; l4

8(GAUSS) i MAGNETIZING DIRECTION CORE-MAGNET TYPE INSTRUMENT HAVING LINEAR RESPONSE CHARACTERISTIC The present invention relates to a core-magnet type instrument having a linear response characteristic and more particularly to a core-magnet type instrument used as ammeter, voltmeter and the like.

In general, in the core-magnet type instrument used as ammeter, voltmeter and the like a moving coil is so arranged as to rotate around an internal magnet which is shaped into a cylindrical form at least over an angle of rotation of the moving coil, and the current or voltage to be measured is applied to the moving coil so that the latter may be deflected through an angle which may be read through a pointer attached to the moving coil. In the instrument of the type described, it is essential that the moving coil responds linearly over a wide range of the variable to be measured.

In this specification, the linear response means that the moving coil rotates in linear proportion to the variable applied to the moving coil. Therefore when the pointer is attached to the moving coil, the angle of deflection of the pointer is in proportion to the variable being measured so that the scale may be equidistantly graduated. For example when the instrument of the type described is used as an exposure meter for camera, it is desired that the instrument has a linear deflection angle characteristic over a wide range of brightness because the exposure error may be reduced to the minimum throughout the measuring range. In addition, the design of the instrument is much facilitated. For example the exposure meter may be easily coupled to other instrument. Furthermore the interchangeability of the graduated scale of the instrument may be attained so that the universal graduated scales may be employed in various instruments. In consequence it will be no longer necessary to provide specific graduated scales for individual instrument.

In practice, however, it is extremely difficult for the instrument of the type described to provide a linear response characteristic as will be described in more detail hereinafter. In brief, in a moving-coil type instrument the relation between the current to be measured and an angle of deflection 9 of the moving coil is given by the following relation:

where B Flux density in the air gap between the internal magnet and the yoke;

b =.width of the moving coil;

1 length of the moving coil in the air gap between the internal magnet and the yoke;

n number of turns of the moving coil; and

1== coefficient of control of control spring. Since I, b, n and 1- are constant, the angle of deflection becomes in proportion to the current i to be measured when B is made constant. However, it is extremely difficult to obtain the uniform or constant flux density B throughout a wide range of angle of deflection because when the internal magnet is magnetized in a predetermined direction, the flux density is maximum in that direction while in other directions the flux density is varied.

To overcome this problem there has been proposed a method for attaching the compensators to the internal magnet in the direction of magnetization so that the variation in flux density through an angle may be prevented. However, this method cannot overcome the problem completely as will be described in more detail hereinafter. In addition, the step of attaching the compensators to the internal magnet is added in the manufacturing process and the desired characteristics will not be attained unless the compensators are attached to the internal magnet without any gap therebetween. This presents the serious problem in the manufacture. Furthermore, the compensators are employed in order to reduce the flux density in the direction of the magnetization so that the flux density as a whole is reduced, resulting in the low sensitivity of the instrument.

It is therefore one of the objects of the present invention to provide an instrument having a linear response characteristic which may eliminate the defect encountered in the prior art instrument.

It is another object of the present invention to provide an instrument having a linear response characteristic in which the configuration of the internal magnet is made into an elliptical form at least over an angle of rotation of the moving coil.

It is a further object of the present invention to provide a magnet core or internal magnet which is made by sinterring a ferromagnetic material such as barium ferrite and has a linear response characteristic.

The present invention will become more apparent from the following description of the preferred embodiments thereof taken in conjunction with the accompanying drawing in which:

FIG. 1(A) is a top view of the prior art instrument having a moving coil and a circular or cylindrical magnet core or internal magnet;

FIG. 1(B) is a side view partly in section of the instrument shown in FIG. I-(i);

FIGS. 2 and 3 are graphs showing the flux density distribution of the instrument shown in FIG. 1;

FIG. 4 is a graph illustrating the relation between the current and the angle of deflection in the instrument shown in FIG. ll;

FIG. 5 is a cross section of the improved prior art instrument;

FIGS. 6 and 7 are graphs showing the flux density distribution of the prior art instrument shown in FIG. 5;

FIG. 8 is a graph illustrating the relation between the current and the angle of deflection in the prior art instrument shown in FIG. 5',

FIGS. 9 and 10 are graphs for explanation of the principle of the present invention;

FIG. 11 is a top view of an instrument incorporating the internal magnet or core in accordance with the present invention;

FIGS. 12 and 13 are graphs showing the flux density distribution of the instrument shown in FIG. 11;

FIG. 14 is a graph illustrating the relation between the current and the angle of deflection in the instrument shown in FIG. 1 ll;

FIG. 15 is a diagram illustrating the relative position of the internal magnet of the instrument shown in FIG. 1 1 with respect to its moving coil; and

FIGS. 16 through 20 are graphs for explanation of the two examples of the present invention.

The prior art instrument will be described with reference to FIGS. 1-4. Referring first to FIG. 1, reference numeral 1 designates an internal magnet; 2, a

moving coil which has a plurality of turns of windings securely held in position by means of a suitable adhesive agent and is pivoted for rotation around the internal magnet 1', 3, a yoke; 4, a pointer made integral with the moving coil 3; 5 and 5', a pair of vertically spaced apart brackets for pivoting the moving coil therebetween; 6 and 6', control spring retaining arms fixed to the

brackets

5 and 5 respectively; 7, a O-adjustment arm extending from the retaining arm 6'; and 8, spiral control springs loaded between the retaining arms 6 and 6' and the moving coil 2. It should be noted that the spiral springs are insulatively fixed to the retaining arms 6 and 6' (the insulators are not shown) and the current to be measured flows through the moving coil 2.

When the internal magnet l is magnetized in the direction X in FIG. 1(B), the distribution of the flux density between the internal magnet I and the yoke 3 is indicated as shown in FIGS. 2 and 3. In FIG. 2, the origin 0 is the center of the internal magnet 1; the X- axis is the direction of magnetization; and the Y-axis is at a right angle relative to the X-axis. The angle 0 is an angle of inclination relative to the X-axis. As viewed from FIG. 2, the distribution of the flux density is in the form of two circles and the flux density B in the direction at an angle 6 relative to the X-axis may be represented by the length between the center 0 and the intersection of the straight directed line at an angle 0 with the circumference of the flux density distribution circle. From FIG. 2, the relation between the angle 0 and the density B may be plotted as shown in FIG. 3, and may be given by the following equation:

B=Bmcos0 where Bm flux density at 6 O, that is along the X- axis. The relation between the current to be measured and the angle of deflection in case the internal magnet 1 having the flux density distribution characteristic as described above may be illustrated in FIG. 4. FIG. 4 shows the relation between the current indicated by i% (the maximum current that the instrument can measure is I00 percent) and the angle of deflection 0 of the pointer 4.

When the cylindrical inner magnet is employed, the deflection angle characteristic is deviated from the theoretical curve indicated by the dotted line in FIG. 4. That is the non-linear curve in FIG. 4 is in the form ofS which intersects with the theoretical curve at 0 0, at an intermediate point of the range of the effective angle of deflection and at another end of the range thereof.

One of the prior art improvements which is intended to eliminate the above described defect (the nonlinearlity of the curve of the angle of deflection) is illustrated in FIG. 5. The compensators 10 are affixed to the internal magnet or core 11 in the direction of magnetization. The compensators 10 are made of a soft iron magnetic material. Because of the reluctance of the compensators l0 interposed between the yoke 13 and the internal magnet 11, the flux density B in the air gap between the yoke 13 and the internal magnet 11 is reduced as shown in FIGS. 6 and 7. As viewed from FIG. 7 the flux density B in the directions adjacent to 0 0 is greatly reduced as compared with the case only the cylindrical internal magnet l is used as shown in FIG. I. In consequence, the current vs. angle of deflection characteristic may have the linearlity almost along the range of effective angle of deflection as shown in FIG. 8. However, the manufacture of the core assembly having the ideal linear response encounters difficulty because it is greatly affected by various factors such as the thickness of the compensators, the methods for affixing them to the internal magnet and so on. In addition the provision of the compensators will result in the decrease in sensitivity of the instrument.

According to the present invention, the internal magnet is shaped in the form of an ellipse in accordance with the theoretical equations to be described in more detail hereinafter in order that the magnetic flux linked across the moving coil may be made uniform or constant all times in the all range of the angle of deflection of the moving coil when the internal magnet or core is magnetized in a predetermined direction.

Next the principle of the present invention will be described in detail hereinafter. As shown in FIG. 9, the center of a circular magnet core is the origin 0; the direction of magnetization (toward the N pole) is in the X-axis; and the Y-axis is at a right angle relative to the X-axis. The point P is a point arbitrarily selected outside of the circular magnet core while the point B is a point arbitrarily selected along the peripheral edge of the circular magnet core. (fi= l (the distance between the center of the magnet core and the inner edge of the yoke), T37 rd), r,,, and m, is the strength of magnetic pole at the point of magnetization A. Then the strength of magnetic pole m4; at the point B angularly spaced apart from the point A by 4) is given as follows:

o (I) Since the magnetic pole is at infinity, the magnetic potential at the point P is given by From Eq. (2) it is seen that the strength of magnetic pole must be varied in order that the magnetic potential along the locus of the point P whose distance from the center 0 is maintained constant may be made equal independently of the angle 0.

The coordinate system employed in FIG. 10 is the same as in FIG. 9. The distance OA from the center 0 to the peripheral edge A angularly spaced apart from the X-axis by 0 is assumed to be r in order that the magnetic potential along the locus of the point P whose distance from the center of the magnet core 0 is I may be made equal within the range of :01. Then the magnetic potential U, at the point P in FIG. 10 and the magnetic potential U, at the point P are U',,= Ua+ V, 4 where V, and V, are the magnetic potentials at angles except 0 and a. Since V, is approximately equal to V,,, Eqs. (3) and (4) are made equal when U Ua On the other hand, U and Ua .are given by That is the problem is to obtain r for making Eqs. (5) and (6) equal.

m a (8) Therefore, substituting Eqs. (7) and (8) in Eqs. (5) and (6), and making U U we have m, cosa cosO/l-r r /r,,,=cos ell-r Hence,

r9 =l(C/1+C) where C= r,,,/l r,, cosa/cos 0 the origin 0 and the intersection between the curve a and the side edge of the moving coil is not used in a strict sense. Therefore the actual effective angle of This means that the flux density curve must have a flat top within an angle of 150 20:).

It is seen that except the configuration of the magnet core the designs of the moving coil, the control springs,

the pointer and so on may be made as with the case of the conventional instruments having 'the moving coils. When the pointer is fixed to the coil, the uniformly graduated scale may be employed.

The two examples of the instruments A and B based Thus the length of the magnetic core r9 at an angle of "P the Principle explained above will be described 0 may be derived from Eq. (10). In addition, it is seen from Eq. (10) that the radius r in the direction of 0 of the magnetic core may be determined in combination with the radius r,,, of the magnetic core to be manufactured, the inner radius of the yoke I and the effective angle of deflection of the pointer.

The configuration of the magnet core which satisfies Eq. (10) is substantially an ellipse designated by 21 in FIG. 11 and the distribution of the magnetic flux density is shown in FIGS. 12 and 13. In FIG. 12 the flux density B is the vector length between the origin 0 and the curve a and the flux density in the direction of magnetization becomes more uniform as compared with with reference to FIGS. 16-20 hereinafter.

(I) Instrument A Diameter of magnet core: 10 mm 5 Radius of yoke: l2.8 mm

Effective angle B of deflection: 60

2 0: (angle not used because of the of moving coil): 40

(II) Instrument B Diameter of magnet core: 10 mm Radius of yoke: 12.8 mm Effective angle B 75 the circular magnet core indicated by the broken line b.

is exactly in proportion to the angle of deflection of the pointer.

The circular or cylindrical magnet cores were previously prepared for both of the instruments A and B and the distances from the center of the cylindrical core tothe peripheral edge were calculated as shown in Table I.

6 0 10 20 30 40 S0 A 4.46 4.48 4.57 4.65 4.80 5.00 B 4.10 4.12 4.19 4.31 4.48 4.71 5.00

Table I: Distances from the centers of the cores to their peripheries to satisfy Eq. (10), unit mm. The configurations of the magnet cores for the instruments A and B shaped in accordance with Table 1 are illustrated in FIG. 16 by c and d respectively. The flux density distributions are shown in Tables 2 and 3 and in FIGS. 17 and 18 respectively.

'raiaie iQinsTnun/ian r 1; a=50 The reason why the effective range of angle of deflection B is smaller than the angle a will be at once noted from FIG. 15. Since the moving coil has the width w as described above, the angle -y between the center line of the moving coil and the line connecting [B 10 Gauss] TABLE 3.INS'IRUMENT 13;a=60

113x10 Gauss] where 0 an angle between the reference line passing 65 through Nos. 1, 2 and 3 designate the cores provided in a similar manner. FIGS. 17 and 18 illustrate the average values of these three cores.

The current vs. angle of deflection characteristics of said magnet core is magnetized in a single predetermined direction relative to said yoke; and

the configuration of said magnet core satisfies the following equation at least within an angle of rotathe instruments A and B are illustrated in Table 4 and 5 5 tion of said moving coil:

and FIGS. 19 and 20 respectively, from which it is seen .1,

that the ideal linear proportionality is attained. r =1 C/l C i $4131.18 4.-1NsTRUMEN i .4; 60 meter 7' No.1[l(percent 8.87 17.28 25.43 33.85 41.9 50.32 58.46 66. 52 74. 66 86. 78 91. 86 100 N0. 21(percent 9.49 18.34 26.73 35.02 43.32 51.52 59.54 67. 37 75.48 8.34 92.16 100 NO. 31(percent 8.75 17.5 25.9 33.93 41.6 49.37 57.85 66.07 74.1 82.5 91.5 100 I 1.1618 5. 1N s'112b114E NT B; 75 166661 No.1l(percent 7.05 13.97 20.7 27.3 34.3 40.83 47.31 53. 78 60.3 67.0 73.02 79.06 85.72 92.7 No. 2{i(perc9nt 7.48 14.6 21.24 28.1 34. 77 41.1 47.97 54.66 61.41 68.04 74. 17 80.26 86.34 93.0 100 'No. 31(percent 65 14 66 21.47 28.07 34.52 41. 33 47.86 54.45 60.98 67.4 74. 03 80.34 86.66 92. 97 100 FIGS. 19 and 20 illustrate the average values of the where C =r,,,/lr,,, cos X/cosO three cores (Nos. l-3) when incorporated in the same instrument r length of a directed [me at an angle of 0 with The magnet core in accordance with the present in- 25 respect to the single dlfecnon P mag'netlzaflon vention may be produced by molding a ferromagnetic from h center rotation of Said moving coll to material such as barium ferrite in a mold having a cavih p p F of P magnet core; ty shaped in accordance with, for example,Table 1. Al- FHQE BQH 9f5.' l .}9 ternatively, suitable magnets may be machined into a a (effeqwe angle of deflectfon movmg desired shape 30 coll plus twlce the angle of moving coil width); and

From the foregoing it is seen that when the magnet rm length of a Q F at f g of X i core is shaped into an elliptical form in accordance respect to the dkrecnon of ,magnetlzatlon f sand with the present invention, the ideal linear proporcenfer of rotalnon of Sand movmg coll to the tionality may be attained between the current i and the Penphery core angle of deflection 0. In consequence, the prior art 35 Sald core bemg fi respect to Sam yolfe compensators attached to the core may be eliminated. member and Send gap foYmed P f f The magnet cores in accordance with the present in- Y and sa1d core bemamamtamed vention may be produced easily by a suitable process umform magmt1C field over angle of rota such as sinterring without the problems of the dimen- H v V I sional tolerances and eccentricity. Furthermore, the in- 40 2. A core magnet type instrument according to claim struments incorporating the magnet cores in ac- 1 wherein said magnet core is shaped substantially into cordance with the present invention may be manufacan ellipse; and said moving coil has a side whose length tured without increasing the assembly steps and the like is at least equal to the major axis of said ellipse. as In the case of the prior art instrument. In addition, A core magnet p instrument according to claim when the mstliumem 'P magnet 45 1 wherein said yoke member is made of a cylindrical f F P mvemlon Same magnet material and assembled in unitary construction W ththe prior art instrument, the magnet ic flux density with said magnet com distributionmay be more effectively utilized without 4 A t any loss. This advantage is essentially remarkable in W core type msirumem accordmg to 9 case of the large sized instrument 50 1 wherein said core magnet IS a magnet made by sinter- What is claimed is: ring barium ferrite.

l. A core magnet type instrument having a linear 5. A magnet core type instrument according to claim response characteristic, comprising a magnet core, a 1 wherein a pointer is fixed to said moving coil and is yoke member spaced apart from said core so as to form deflected equiangularly when the current flOWS through an air gap therebetween and a moving coil movable in 55 Sai m ing Coil. said air gap: characterized in that 4 =1 I UNHED STATES PATENT GFFKQE 9F CREQTEQN I Patent No. 3,711, 776 7 Y I Dated January 15 1213 7 I lnveniofls) TE'PSUZO INAMI et a1 It is. certified that error appears. in' the above-identified patent arid that said Letters Patent vare herebycorrectd as s'hownbelow:

' Column 3, line 5; for "3" to-read 2-;

Column 8,.TABLE line 4 from'."8.34" to read --83.4--;

Signed and sealed this 10th ay of July 1973.

(SEAL) Atte st EDWARD MQFLETCHERQJR; Rne y Attesting Officer A ti Commissioner of Patents"

Claims

1. A core magnet type instrument having a linear response characteristic, comprising a magnet core, a yoke member spaced apart from said core so as to form an air gap therebetween and a moving coil movable in said air gap: characterized in that said magnet core is magnetized in a single predetermined direction relative to said yoke; and the configuration of said magnet core satisfies the following equation at least within an angle of rotation of said moving coil: r l C/1 + C where C rm/l - rm . cos X/cos theta r length of a directed line at an angle of theta with respect to the single direction of magnetization from the center of rotation of said moving coil to the periphery of said magnet core; l inner radius of said yoke; Alpha 1/2 (effective angle of deflection of said moving coil plus twice the angle of moving coil width); and rm length of a directed line at an angle of X with respect to the direction of magnetization from said center of rotation of said moving coil to the periphery of said magnet core, said core being fixed with respect to said yoke member and said air gap formed between said yoke member and said core being maintained with a uniform magnetic field over said angle of rotation.

2. A core magnet type instrument according to claim 1 wherein said magnet core is shaped substantially into an ellipse; and said moving coil has a side whose length is at least equal to the major axis of said ellipse.

3. A core magnet type instrument according to claim 1 wherein said yoke member is made of a cylindrical magnet material and assembled in unitary construction with said magnet core.

4. A magnet core type instrument according to claim 1 wherein said core magnet is a magnet made by sinterring barium ferrite.

5. A magnet core type instrument according to claim 1 wherein a pointer is fixed to said moving coil and is deflected equiangularly when the current flows through said moving coil.