US3742441A

US3742441A - Seismometer

Info

Publication number: US3742441A
Application number: US00022622A
Authority: US
Inventors: T Riley
Original assignee: Mark Products Inc
Current assignee: Shawcor Ltd
Priority date: 1970-03-25
Filing date: 1970-03-25
Publication date: 1973-06-26
Anticipated expiration: 1990-06-26

Abstract

The moving element of the seismometer is supported by spring spiders, the outer portions of which are stiffened by stiffening members integrally attached to the outer portions. By stiffening the outer portions of the spiders, the distortion of the output signal that was due to the transverse or radial bending of the outer portion of the springs as the springs flexed is substantially reduced.

Description

United States Patent 1 Riley [45] June 26, 1973 SEISMOMETER 3,451,040 6/ 1969 Johnson ..340/17 75] Inventor: Travis E. Riley, Houston, Tex 3,412,376 1 Johnson 7 [73] Assignee: Mark Products, Inc., Houston, Tex. Primary Examiner-Benjamin A. Borchelt Assistant Examiner-R. Kinberg [22] flied March 1970 Attorney-Hyer, Eickenroht, Thompson & Turner [211 Appl. No.: 22,622

[57] ABSTRACT [521 US. Cl ..340/17 The moving element o he seismometer is supported 51 Int. Cl. ..G0lv 1/16 by spring p the outer portions of which are stif- [58] Field of Search ..340/17 felled by stiffening members integrally attached to the outer portions. By stiffening the outer portions of the spiders, the distortion of the output signal that was [56] References Cited due to the transverse or radial bending of the outer UNITED STATES PATENTS portion of the springs as the springs flexed is substan- 2,748,370 5/1956 Baltosser ..340/17 3,239,804 3/ 1966 Elskamp et al. ..340/l7 15 Claims, 5 Drawing Figures SEISMOMETER This invention relates to seismometers, generally, and in particular to electromagnetic seismometers.

Designers and manufacturers of electromagnetic seismometers, or geophones, as they are often called, work to reduce to a minimum or eliminate distortion of the output signal. It has been determined that one source of distortion is the circular springs, usually called spring spiders, that connect the two elements of the geophone that move relative to each other to produce the output signal. These springs are usually etched from a sheet of material of generally uniform 7 thickness and have outer and inner or central portions connected by spring arms. It has been found that the outer and central portions of such spring spiders will flex 'or bend transversely as the spring arms flex, if these portions of the spring are not held against such movement by the manner in which they are attached to the supporting and moving elements of the geophone. It is desirable, however, that the two elements connected together by the spring spiders be able to rotate on their axes relative to each other. T permit this, either the outer or.central portions of the spring spiders have a low frictional engagement with the element to which it is. connected and such a connection does not prevent the transverse or radial bending that produces the distortion discussed above. One reason that this flexing or transverse bending of either of the outer or central portions of the spider causes distortion is because it is unpredictable. It is unpredictable because the degree of freedom for such flexing that is allowed by each connection is impractical to determine with any accuracy.

This results, primarily, because of the machining tolerances necessary to produce the parts at a reasonable and competitive cost. Another unpredictable factor is the resistance offered to such bending by the drag of the edge of the spring spider against the element of the geophone to which it is connected.

Therefore, it is an object of this invention to provide a seismometer of the spring spider type described above that has substantially no distortion of its output signal due to transverse bending of the outer and central portions of the spring spiders and yet allows relatively free rotation between at least one of the relatively movable elements and the spring spider.

It is another object of this invention to provide an electromagnetic seismometer having a spring spider that connects the two elements thereof so they can move axially relative to each other to produce the desired signal and allows at least one of the elements to rotate relative to the spring, yet the outer and central portions of the spring have substantially no transverse or radial bending.

It is another object of this invention to provide a springspider for a seismometer that has substantially no transverse bending or flexing in its outer portion even though the element connected to the outer portion can rotate relative to the outer portion of the spring with little frictional resistance.

It is another object of this invention to provide a spring spider for a seismometer that has an outer portion that is stiffer than the remainder of the spider to thereby reduce distortion of the output signal of the seismometer due to transverse bending of the outer portion.

It is another object of this invention to provide a spring spider for a seismometer that causes little, if any, distortion in the output signal of the seismometer due to irregular and unpredictable transverse bending of either the central or outer portion of the spider spring, whichever is connected to an element so as to allow the element to rotate on its axis relative to the spring.

These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached drawings and appended claims.

In the drawings! FIG. 1 is a vertical sectional view through the preferred embodiment of the electromagnetic seismometer or geophone of this invention;

FIG. 2 is a plan view of one of the spring spiders used with the geophone of FIG. 1;

FIG. 3 is a cross sectional view of the spring spider of FIG. 2 taken along line 3-3;

FIG. 4 is a partial sectional view on an enlarged scale of the coil-mass assembly of the seismometer of FIG. 1 with the two supporting spring spiders assembled therewith; and

FIG. 5 is a view on an enlarged scale of the portion of FIG. 4 within circle 5.

Electromagnetic seismometers or geophones of the type to which this invention relates have two elements that move relative to each other due to earth tremors. One element is in engagement with and moved by the earth. The other resists the movement due to its inertia. This results in relative movement between the two elements and produces an output signal that is proportional to the relative movement imparted. The inertial member or element is supported by one or more springs, which in turn are supported by and connected to the other element. The springs yield and allow the desired relative movement, when the geophone or seismometer is subjected to an earth tremor.

In the seismometer shown in the drawings, the inertial element is coil-mass assembly 12. As shown in FIG. 4, it includes coaxial cylindrical members or spools l4 and 16. These members are spaced apart by insulation ring 17. Annular outer shell 18 holds the spools and insulation ring in assembled condition. Each

spool

14 and 16 has an annular groove on the exterior thereof in which is

wound coils

19 and 20, respectively.

Positioned inside annular coil-mass assembly 12, and in axial alignment with the opening therethrough, is the other element of the geophone, permanent magnet assembly 22. This assembly includes a permanent magnet, having

pole pieces

23 and 24, that provide a magnetic field within which coils l9 and 20'are located.

The permanent magnet assembly is held against movement relative to the housing of the seismometer by

spacer rings

25 and 26, which are positioned at opposite ends of the permanent magnet assembly and between

end plates

27 and 28 of the housing. These end plates combine with cylindrical housing member 29 to completely enclose the permanent magnet assembly and the coil-mass assembly of the seismometer, the two relatively moving elements.

The coil-mass assembly is supported on

spring spiders

32 and 34, which allow the coil-mass assembly to move along its longitudinal axis relative to permanent magnet assembly 22.

Spring spiders

32 and 34 are identical in the embodiment shown; therefore, only one will be described in detail. As shown in FIG. 2, spring spider 32 includes inner or central portion 35 and outer portion 36, both of which are annular. Arcuate

spring arms

37, 38, and 39 connect the central and outer annular portions of the spring and allow the portions to move axially relative to each other by bending the spring arms as shown in FIG. 3. Actually, FIG. 3 shows the amount of preform placed in the spring prior to its being assembled in the seismometer. In other words, the outer and inner or central annular portions of the spring are displaced axially relative to each other before the spring is heat treated. The distance these portions are axially displaced is determined by the spring rate of the spring and the weight of the coil-mass assembly. Preferably, when the weight of the coil-mass assembly is placed on the outer annular portion of the spring, it will deflect the spring until it is substantially flat again.

These springs are etched out of sheet material, usually, and then trimmed by punch presses to the desired outer and inner configuration. In the embodiment shown, central annular portion 35 of spring 32 is provided with projection or tab 40. This tab engages a groove (not shown) in spacer ring 25 and holds the spring spider from rotating relative to the spacer ring. The spring is mounted on spacer ring 25 by clamping inner annular portion 35 of the spring between spacer ring 25 and clamping ring 41 (FIG. 1). These two rings are forced together to clamp spring portion 35 between them by

end plates

27 and 28. In the same manner, central portion 42 of spring spider 34 is clamped between spacer ring 26 and the lower surface of permanent magnet assembly 22. The surfaces between which

central portion

35 and 42 of the spring spiders are clamped are wide enough to effectively hold these portions of the springs from bending transversely as the spring arms are flexed.

As explained above, preferably one of the elements of the geophone can rotate relative to the other. The connection described above will effectively hold the central portions of the spiders from radial or transverse bending, but it will also create a substantial friction force that will resist relative rotational movement between the permanent magnet assembly and the spiders. One common manner of connecting a coilmass assembly to the outer portions of spring spiders is shown in the drawing. In this type of connection the outer annular portions of the spring spiders, such as outer annular portion 36, extend into annular grooves, such as

grooves

44 and 45, in the coil-mass assembly. The outer portions of the springs are held in these grooves by

snap rings

46 and 47, which extend into annular

snap ring grooves

48 and 49 located adjacent the spring grooves. There is usually some clearance between the springs and the snap rings and between these members and the grooves in which they are located. This clearance is desired to make the assembly of the springs and snap rings easier. In addition, this clearance is desired so that the coil-mass assembly can rotate freely relative to the spring spiders. Not being rigidly confined or restrained, the outer annular portions will tend to bend transversely as the spring arms are flexed by the movement of the coil-mass assembly. The amount of transverse bending varies with each spring for many reasons, including the clearance discussed above. This is one cause of distortion in the output signal of the seismometer, as explained above.

Therefore, in accordance with this invention, stiffening member 50 is integrally attached to outer portion 36 of spring spider 32 to reduce, if not substantially eliminate, the transverse flexing of this portion of the spring by increasing its thickness to substantially increase its stiffness and resistance to such bending. In the embodiment shown, stiffening member '50 comprises an annular ring substantially of the same thickness as the outer annular portion of the spring spider. The ring is integrally connected to the spider by spot welding it to the spider at spaced points indicated with the number 51. The stiffening member could be attached in other ways, such as by glueing or soldering.

In one embodiment the spring had a uniform thickness of 0.0045 and its outer portion was stiffened sufficiently to substantially eliminate distortion due to transverse bending by a stiffening member having a thickness of 0.004. Preferably, the stiffening member is about as thick as the portion of the spring to which it is attached. If it drops below about one-half the spring thickness, it doesnt add as much stiffness as this portion of the spring should have. A stiffening member above one and a half times as thick as the spring is probably above the point of diminishing return and would add little if any more stiffness than would such a member between 1 and 1% times as thick as the spring.

Another advantage of employing thickening member 50 is that it avoids the possibility of the outer portions of the spiders, such as portion 36 of spider 32, from working their way into the spaces between the snap rings and the bottom of the snap ring grooves. This is particularly likely where machining tolerances are at their maximum and the gap between the snap ring and the bottom of the snap ring groove is a maximum. Should this occur, the coil-mass assembly would not be co-axial with the permanent magnet assembly and further distortion would probably be introduced into the output signal. By increasing the thickness of the outer ring through the use of the stiffening member, the possibility of this occurring is eliminated.

From the foregoing description of one embodiment of this invention by way of example, it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the apparatus and structure.

The invention having been described, what is claimed is:

1. A seismometer comprising first and second elements movable relatively coaxially to provide an output signal due to their relative movement and means yieldingly suspending one of said elements for said movement relative to the other, said suspending means comprising a spring spider having an inner portion and an outer portion connected together by a plurality of spring arms, said outer and inner portions being connected to the first and second elements, respectively, with the connection between the spring spider-and one of said elements being such that said element and spring spider can rotate relatively around said axis of relative movement of the elements, and a stiffening member integrally attached to said portion of the spring spider that is connected to said one element to permit said relative rotation to increase the thickness of this portion and its ability to resist bending as the spring arms bend due to relative movement of said elements.

2. The seismometer of claim 1 in which said stiffening member is an annular member having an outside and inside diameter about the same as the portion of the spider to which it is attached.

3. The seismometer of claim 1 in which the stiffening member has a thickness substantially the same as the spider.

4. The seismometer of claim 3 in which the stiffening member is welded to the portion to which it is attached.

5. The seismometer of claim 1 in which the outer portion of the spider and the stifiening member are annular and said spider is connected to said first element by the outer portion and attached stiffening member being positioned in an annular groove in the first element.

6. The seismometer of claim 5 in which the spring spider is held in said groove by a removable snap ring.

7. The seismometer of claim 1 in which the stiffening member has a thickness between 7% and 1% times the thickness of the portion of the spring spider to which it is attached.

8' A seismometer comprising first and second relatively movable elements providing an output signal due to their relative movement, said first element being annular with said second element positioned in the opening therein for movement relative to said first element axially of said opening, means yieldingly suspending one of said elements for said movement relative to the other, said suspending means comprising a spring spider having a central portion and an outer portion connected together with a plurality of spring arms, said outer and central portions being connected to the first and second elements, respectively, and a stiffening member integrally attached to the outer portion of the spring spider to increase its thickness and its ability to resist bending as the spring arms bend due to relative movement of said elements.

9. The seismometer of claim 8 in which said stiffening member is an annular member having an outside and inside diameter about the same as the outer portion of the spider.

10. The seismometer of claim 8 in which the stifiening member has a thickness substantially the same as the spider.

11. The seismometer of claim 10 in which the stiffening member is welded to the outer portion.

12. The seismometer of claim 8 in which the outer portion of the spider and the stiffening member are annular and said spider is connected to said first element -by the outer portion and attached stiffening member being positioned in an annular groove in the first ele ment.

13. The seismometer of claim 12 in which the spring spider is held in said groove by a removable snap ring.

14. The seismometer of claim 8 in which the stiffening member has a thickness between and 1% times the thickness of the outer portion of the spider.

15. An electromagnetic seismometer, comprising a permanent magnet assembly, a coil-mass assembly, and a spring suspension system suspending one of the permanent magnet assembly and the coil-mass assembly for movement relative to the other, said spring system having at least one spring spider, said spider having a central portion and an outer annular portion connected together with a plurality of curved spring arms, one of said assemblies having an annular groove to receive the outer annular portion of the spider for transmitting axial motion of said assembly to the spider, and means for connecting said other assembly to said central portion of the spring spider, said spider being characterized by its outer annular portion having a thickness substantially greater than the rest of the spider so the outer annular portion is stiffer and will resist transverse bending as the relative movement of the assemblies flex the spring arms.

Claims

1. A seismometer comprising first and second elements movable relatively coaxially to provide an output signal due to their relative movement and means yieldingly suspending one of said elements for said movement relative to the other, said suspending means comprising a spring spider having an inner portion and an outer portion connected together by a plurality of spring arms, said outer and inner portions being connected to the first and second elements, respectively, with the connection between the spring spider and one of said elements being such that said element and spring spider can rotate relatively around said axis of relative movement of the elements, and a stiffening member integrally attached to said portion of the spring spider that is connected to said one element to permit said relative rotation to increase the thickness of this portion and its ability to resist bending as the spring arms bend due to relative movement of said elements.

5. The seismometer of claim 1 in which the outer portion of the spider and the stiffening member are annular and said spider is connected to said first element by the outer portion and attached stiffening member being positioned in an annular groove in the first element.

7. The seismometer of claim 1 in which the stiffening member has a thickness between 1/2 and 1 1/2 times the thickness of the portion of the spring spider to which it is attached.

8. A seismometer comprising first and second relatively movable elements providing an output signal due to their relative movement, said first element being annular with said second element positioned in the opening therein for movement relative to said first element axially of said opening, means yieldingly suspending one of said elements for said movement relative to the other, said suspending means comprisiNg a spring spider having a central portion and an outer portion connected together with a plurality of spring arms, said outer and central portions being connected to the first and second elements, respectively, and a stiffening member integrally attached to the outer portion of the spring spider to increase its thickness and its ability to resist bending as the spring arms bend due to relative movement of said elements.

10. The seismometer of claim 8 in which the stiffening member has a thickness substantially the same as the spider.

12. The seismometer of claim 8 in which the outer portion of the spider and the stiffening member are annular and said spider is connected to said first element by the outer portion and attached stiffening member being positioned in an annular groove in the first element.

14. The seismometer of claim 8 in which the stiffening member has a thickness between 1/2 and 1 1/2 times the thickness of the outer portion of the spider.