US 3694785 A
A temperature compensating differential transformer is provided including a yoke of magnetic material having two cavities therein. First and second bobbins, each carrying a primary and secondary winding thereabout, are provided in the cavities in axial alignment. The primary winding is formed of two coils connected in parallel. One of the primary coils is formed, in part, of a wire having an extremely low temperature coefficient of resistance. The other primary winding coil and the remainder of the first primary winding coil is formed of the same material as the secondary winding coils. A magnetic core is positioned along the bobbin axis to magnetically couple the primary and secondary windings.
Description (OCR text may contain errors)
United States Patent Chass  TEMPERATURE COMPENSATING DIFFERENTIAL TRANSFORMER  Inventor: Jacob Chass, Rego Park, NY.
 Assignee: Pickering & Co. Inc., Plainview, Long Island, NY.
 Filed: Feb. 22, 1972  Appl. No.: 228,078
 US. Cl ..336/136, 336/179  Int. Cl ..H0lf 15/16, H01f2l/06  Field of Search ..336/179, 136, 130, 180
 References Cited UNITED STATES PATENTS 3,376,533 4/1968 Chass ..336/l3 6 3,308,411 3/1967 Roshala ..336/l79 X 3,308,412 3/1967 Curtis et al. ..336/l79 X 1,891,481 12/1932 Scofield ..336/179 3,287,680 11/1966 Houpt et al ..336/179 [451 Sept. 26, 1972 3,546,648 12/1970 Chass ..336/1 36 Primary Examiner-Thomas J. Kozma AttorneyDavid S. Kane eta].
[5 7] ABSTRACT A temperature compensating differential transformer is provided including a yoke of magnetic material having two cavities therein. First and second bobbins, each carrying a primary and secondary winding thereabout, are provided in the cavities in axial alignment. The primary winding is formed of two coils connected in parallel. One of the primary coils is formed, in part, of a wire having an extremely low temperature coefficient of resistance. The other primary winding coil and the remainder of the first primary winding coil is formed of the same material as the secondary winding coils. A magnetic core is positioned along the bobbin axis to magnetically couple the primary and secondary windings.
5 Claims, 3 Drawing Figures TEMPERATURE COMPENSATING DIFFERENTIAL TRANSFORMER BACKGROUND OF THE INVENTION Differential transformers, such as that described in US. Pat. No. 3,376,533 are commonly used as displacement transducers. The input to such transformers is a mechanical displacement of the transformer core and the transformer output is an AC voltage proportional to the mechanical displacement.
An error which affects the accuracy of absolute displacement measurement by means of such a differential transformer results from temperature variations since the resistance of the transformer windings and hence the output voltage of the transformer is temperature dependent. This error becomes particularly important where a differential transformer must operate in a temperature changing environment. In that case, unless means of temperature compensation are provided, different output voltages will result from the same displacement under differing temperature conditions.
In view of the above, it is the principal object of the present invention to provide a differential transformer adapted to be utilized to compensate for temperature SUMMARY OF THE INVENTION The above and other beneficial objects and advantages are attained in accordance with the present invention by providing a differential transformer for making temperature sensitivity adjustments for another differential transformer, the adjusting transformer comprising ayoke of magnetic material, including a base member and three spaced apart flanges extending transverse to the base member. First and second bobbins of nonmagnetic, nonconducting material are seated in axial alignment in the spaces defined between the flange members. A primary and a secondary winding are wound about each of the bobbins, each of the windings being formed of two coils, one coil about each of the bobbins. The primary winding coils are connected to one another in parallel and the secondary windings are connected to one another in series.
One of the primary winding coils is formed, in part, of a material having an extremely low temperature coefficient of resistance. The other primary winding coil, the remainder of the first primary coil and the secondary winding coils are formed of conventional nonmagnetic wire.
A bore, axially aligned with the bobbins extends through the flange separating the bobbins. A core of magnetic material extends through the flange bore partially into each of the bobbins. The core is longitudinally shiftable within the bore and means are provided for locking the core at a desired location.
The present temperature compensating differential transformer is designed to be cascaded to the output of a linear variable differential transformer and, by properly choosing the extent of the nontemperature varying portion of primary coils, the contribution of temperature change to the total error of the linear variable differential transformer can be reduced significantly and virtually eliminated so as to make the combined differential transformer temperature independent.
BRIEF DESCRIPTION OF THE DRAWINGS In the associated drawings:
FIG. 1 is an end view of the differential transformer in accordance with the present invention;
FIG. 2 is a side elevational sectional view taken along reference lines 2-2 of FIG. 1 in the direction indicated by the arrows; and
FIG. 3 is a circuit diagram of the present differential transformer shown cascaded to a linear variable differential transformer the output of which is to be temperature compensated.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present differential transformer is illustrated in the accompanying drawing wherein similar components bear the same reference numeral throughout the several views. Reference is first made to FIGS. 1 and 2 wherein a differential transformer 10, constructed in accordance with the teachings of the present invention is shown comprising an elongated yoke 12 and a pair of bobbins l4 and 16. The yoke 12 is formed of a highly permeable magnetic material such as I-IY-MU-80 and consists of a base 18, a first end flange 20, a second end flange 22, and an intermediate flange 24 positioned between the end flanges. Each of the flanges 20, 22 and 24 extends generally transverse to the plane of base 18 with the intermediate flange spaced equidistant from the end flanges to form two slots or cavities, each cavity defined between the intermediate flange and one of the end flanges.
Bobbin 14 is positioned between end flange 20 and intermediate flange 24. Bobbin 16 is positioned between intermediate flange 24 and end flange 22. The bobbins are each formed of a nonmagnetic, nonconductive material, such as a plastic or ceramic. First and second secondary coils 26 and 28 of the same electrically conductive insulated wire are respectively wound about bobbins 14 and 16 and are connected to each other in series aiding relationship. First and second primary coils 30 and 32 of electrically conductive insulated wire are wound over the entire length of the first and second secondary coils and insulated therefrom. The primary coils are connected to each other in parallel opposition arrangement. Primary coil 32 is formed of the same insulated wire as secondary coils 26 and 28 which is a conventional copper or aluminum wire. Primary coil 30 is formed, in part, of a wire 31 of low temperature coefficient of resistance such as cupron or bobbins l4 and 16. Elongated rod or core 36 is positioned within bore 34 and has portions extending into each of the bobbin bores. Core 36 is formed of the same magnetic material as yoke 12. End 38 of core 36 is provided with a screwdriver tip receiving slot 40 which is accessible through the bore in end flange 22 so that the position of the core may be altered with a screwdriver passing through flange 22. The-core may then be locked in position by set screw 42 which extends through flange 24. Core 36 is used to adjust the sensitivity of the output of differential transformer 10.
As previously mentioned, primary coil 32 and portion 29 of primary coil 30 are formed of the same copper or aluminum wire as secondary coils 26 and 28 and thus, the resistance of coil 32 and portion 29 of coil 30 will vary with a change in temperature in the same manner that the resistance of the secondary coils will change with temperature. The remainder 29 of primary coil 30 is formed of a material, such as cupron or manganin wire, having an extremely low temperature coefficient of resistance so that its resistance will not change with the temperature. By properly choosing the ratio of the temperature and nontemperature varying portions of primary coil 30, the desired temperature compensating effect may be attained as will be discussed forthwith.
, If the total coil 30 (that is, the combined resistance of coils 29 and 31) is equal to the resistance of coil 32 at room temperature, and the value of the combined resistance is substantially greater than the resistance of coil 32 (by a factor of approximately 100 for satisfactory results) then, all other things being equal, the output of the differential transformer will gain with a temperature increase. Conversely, if thevalue of the combined resistance is less than the resistance of coil 32 (again by a factor of approximately 100 for satisfactory results) the output of the differential transformer will decrease witha temperature increase. Thus, by properlyselecting the ratio of turns of coils 29 and 31 of coil 30, differential transformer may be used to provide any desired temperature compensating scale factor.
As was previously mentioned, a principal use for the differential transformer'of the present invention is to alter the temperature sensitivity of the output of another differential transformer which, in turn, is used to determine and measure displacement. A typical differential transformer with which the present differential transformer may be utilized, is that illustrated and described in U.S. pat. No. 3,376,533 which, as set forth in the referenced patent, may be used to determine position as a function of output voltage. Since the measuring transformer is an extremely sensitive instrument, the transformer coils cannot readily be formed of cupron or manganin wire without introducing relatively high noise levels which would adversely affect the sensitivity of the transformer. Accordingly, a temperature compensating device such as that of the present invention must be used with the measuring transformer where temperature variations are an adverse factor to be considered.
In FIG. 3, a schematic-representation of the differential transformer 50 of U.S. Pat. No. 3,376,533 is illustrated cascaded to the differential transformer 10 of the present invention. As described in that patent, the output voltage measured across terminals 52 and 54 is a linear function of the displacement of core 56 when an input AC current is applied across primary winding 58. When the present differential transformer is cascaded to differential transformer 50, the input to the present difierential transformer comprises the output of the measuring transformer 50. The output of the present transformer is the output of the measuring transformer adjusted by a temperature factor introduced by virtue of the relative values of coil 30 and resistor 31. Thus, by properly choosing the ratio of turns between coil portions 29 and 31 and hence varying the resistance value of coil 30, differential transformer 10 may be made to compensate for, overcompensate for, or undercompensate for temperature variations experienced by the measuring transformer. To this end, the adjusting transformer 10, must, obviously, experience the same temperature variations as the measuring transformer 50. in this regard, it is convenient to package both the measuring and adjusting transformers in a single envelope. In addition to the temperature compensation available with differential transformer 10, a sensitivity scale factor may be introduced by suitably positioning core 36 of transformer 10 and by the ratio of the number of turns of the secondary winding to the number of turns of the primary winding of the transformer.
Thus, in accordance with the above, a differential transformer is provided which is adapted to provide temperature compensation for a linear measuring differential transformer. Although only one embodiment of the differential transformer has been specifically illustrated and described herein, it is to be understood that minor variations may be made from the described embodiment without departing from the spirit and scope of the present invention asdefined in the appended claims.
Having thus described the invention, what is claimed 1. A differential transformer comprising: a yoke of magnetic material; first and second bobbins in axial alignment seated within said yoke, said bobbins each being formed of a nonmagnetic, nonconductive material; a core of magnetic material disposed along the longitudinal axis of said bobbins; first and second secondary coils of electrically conductive wire wound about said first and second bobbins respectively and connected to one another; first and second primary coils of electrically conductive wire wound about said first and second bobbins respectively coaxial with the secondary coils and electrically insulated therefrom, said primary coils being connected to one another, and one of said primary coils being formed, at least in part, of a wire material of extremely low temperature coefficient of resistance relative to the other primary coil and both secondary coils.
2. The differential transformer in accordance with claim 1 wherein said other primary coil and both secondary coils are formed of identical wire.
3. The differential transformer in accordance with claim 1 wherein said one primary coil includes a number of turns of wire of low temperature coefficient of resistance connected in series with a number of turns of the same wire as said other primary coil.
4. The differential transformer in accordance with claim 1 wherein said secondary coils are connected to each other in series aiding relationship and said primary coils are connected to each other in parallel opposition relationship.
5. The invention in accordance with claim 1 wherein said core is movable along the longitudinal axis of said bobbin and further comprising means for adjusting the position of said core.