US2875407A - Electrical instrument of the differential galvanometer type - Google Patents

Description

Fella. 4, 1959 N. D. LAWLESS ET AL 42,75,461? ELECTRICAL INSTRUMENT OF THE DIFFERENTIAL GALVANOMETER TYPE Filed Aug. 3, 1956 n iv: v y 1 v z W V fix/Q5 ATTORNEY United States Patent ELECTRICAL INSTRUMENT or THE DIFFER- .ENTIAL GALVANQMETER TYPE Application August 3, 1956, Serial No. 601,935

1 Claim. or. 324-140 This invention relates to electrical instruments and more particularly to electrical instruments of the differential galvanometer type such as used in indicating liquid level particularly in gasoline tanks of automotive vehicles.

The present invention is an improvement over the type of apparatus disclosed in the United States Patent 2,139,385, granted December 6, 1938, in the names of Bruce H. Schwarze and Robert H. Bigler.

In instruments of the type herein considered the magnetic flux from magnet coils acts upon a moving element such as an armature. Between a coil of the instrument and the armature a coil end piece of iron is generally interposed. This end piece may be arcuate or flat but heretofore acts with the armature to define an air gap which often varies in length as the armature is rotated. Because of non-uniform magnetic field distribution due to the variable air gap, the effect of the flux on the armature is erratic and the operation of the instrument is not as satisfactory as would be desired.

It has now been found that the use of a cupped end piece provides a stronger and more uniform acting instrument because the air gap between the end piece and the armature may be shortened from that heretofore experienced as necessary and that the shortened gap may be made more uniform or substantially uniform regardless of changes in the angular position of the armature with respect to the coil. An increase in the ease of calibration and assembly also results so that a more easily read dial is permissible and stable operation of the instrument is assured.

Accordingly, an object of the present invention is to provide an improved electrical instrument facilitating ease in proper assembly and utilizing an armature and a coil for producing a flux in such a manner as to give a uniform operational effect.

A feature of the invention is an improved electrical in strument utilizing two coils acting on an armature and in which at least one of the coils is provided with an end piece having a cupped symmetrical surface facing the armature.

This and other important features of the invention will now be described in detail in the specification and then pointed out more particularly in the appended claim.

In the drawings:

Fig. l is a front elevation of an instrument in which the present invention is embodied, the dial being removed;

Fig. 2 is a bottom view of the instrument shown in Fig. 1;

Fig. 3 is a sectional viewitaken through one of the coils in Fig. 1 and is drawn to an enlarged scale; and

Fig. 4 is a perspective view of an end piece employed in the structure of Figs. 1, 2 and 3 and is drawn to the same scale as Fig. 3.

The instrument as depicted in the drawings is very similar in many respects to that disclosed in the United States Patent 2,139,385 mentioned above. It comprises 2 a brass frame 10 to which a dial 12 is attached by means of rivets 14 and 16. A pointer 18 has an intermediate portion 18' which extends through an arcuate slot (not shown )in the dial 12 and is fixed to rotate with a fiat iron armature 20 and a balance weight 22 fixed to a shaft 24. The shaft 24 is journaled on the frame 10 in the customary manner as in bearings such as the bearing 25. Two coils 26 and 28 are fixed to the frame 10 by means of two L-shaped brackets 30 and 32, respectively. The coils 26 and 28 are so arranged that their axes are di rected toward the armature 20 and its supporting shaft 24. The circuitry is such that a constant current passes through the wire of the coil 26 and a variable current flows through the wire of the coil 28 as is conventional. Terminals 31 and 33 are provided as necessary connections through an insulating strip 35.

In the instant disclosure, the core 26 is of conventional construction and has a cylindrical and fiat end piece 36 facing the armature 20. The coil 28 is also of conventional construction in that it is provided with a spool core serving to support it, but a free end of this core bears a shoulder portion 40 at one end for retaining a special end piece or conical washer 42. This washer is apertured at 43 to receive a reduced portion of the core spool and is so placed in the assembly that it is spaced from the coiled wire 44 of the coil 28 by a metal washer 46 and a wire retaining paper disc 29. The cupped and symmetrical surface 48 of the washer 42 is so placed in the completed assembly as to face the armature 20.

It is to be understood that upon operation. of the instru ment bypassing current through the coils 2'6 and 28, the strength of the fields from the two coils influences the iron armature 20 and determines the position of the pointer 18. This is the principle of operation disclosed in the Patent 2,139,385, heretofore referred to. The flux generated by the coil 28 will be affected by the end piece 42 so that a substantially uniform air gap is provided between the armature 20 and the end piece as the pointer 18 moves through its scale of readings of the instrument and regardless of the axial positioning of the end piece.

If the end piece 42 were of arcuate and elongated construction, it would be necessary to maintain or adjust its rotative position on the coil axis so that it would be in the same plane as the armature. With the instant construction, however, no attention need be paid to this alignrnent as the end piece will be positioned properly regardless of inadvertent rotation on the coil axis.

The drawings show an instrument in which only one coil is provided with this special end piece giving the uniform air gap, but it is to be appreciated that either or both coils 26 and 28 may be provided with such end' pieces without departing from the present invention. The surface 48 on the end piece is preferably made conical but it could be spherical. In any event, it should be substantially symmetrical in order to gain the desired effect.

It has been found that in the use of the cupped end piece 42, even though used in combination with only one coil, the pointer movement is stronger than heretofore experienced with similar instruments of the same general type. The pointer movement is less erratic; in other words, more stable in the pointer operation, and calibration of the instrument is easily performed for there are no conditions under which the pointer is inclined to jump unduly.

The theory which forms the basis of the present invention may be stated in another way and that is that it concerns the maintenance of the substantially constant length air gap between the iron end piece in the circuit and the moving element (the gage armature). The ideal magnetic circuit, insofar as the amount of magnetic flux is concerned, would have a closed iron path (no air gap).

Patented Feb. 24,1959

Ijoweyer, in order to have an air gap must be introduced into the iron path. In some prior constructions, a particular scale position of the pointer and armature wcTuld give the longest air gap. A movement of the pointer in either direction from that position'would shorten the air gap. The particular position of the pointer. in such a situation would, therefore, be unstable since a magnetic circuit constantly tends to shortenits air gap. In using the present invention the air gap is approximately the same when the pointer has movedthrough its full arcuate range as when in any intermediate position and there is no tendency for the armature and pointer to jump. We claim:

An electrical instrument including a frame, a flat armature pivoted on said frame for rotation in its own plane, two coils with corresponding cores supported on said the magnetic circuit do work,

frame with their axes directed toward said armature and lying in planes parallel with the plane of said armature, an end piece fixed to one end of each of said Cores and cooperating with the said afmature to define an air gap, and one of said end pieces having a cupped surface facing the edge of said armature in defining its air gap, and said cores, armature and end pieces being of magnetic material. 7

References Cited in the file of this patent UNITED STATES PATENTS 1,220,587 Beede Mar. 27, '1917 1,306,624 Porter June 10, 1919 2,040,060 Middleton May 5, 1936 2,391,992 Malone Ian. 1, 1946

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