US2887877A - Method of testing electromechanical filters - Google Patents

Description

May 26, 1959 R. L. S'HARMA METHOD OF TESTING ELECTROMECHANICAL FILTERS Filed Aug. 51, 1954 SIGNAL GENERATOR ATTORNEY Electromechanical filters such as desci-ibed in Patent No. 2,656,516 are becoming of increasing importance. to a the electrical industry for the reason that they I are very selective and obtain substantially better resultsithanelectrical components.

Techniques of manufacturing suchv filters have-been described in the copending patent applications ofFrank r :1

C. Wallace entitled Assembly Jig, Serial No. 253,575, filed October 29, 1951, now Patent No. 2,716,178; End Wire Frequency Measuring and Cutting Jig, Serial No. 276,398, filed March 13, 1952, now Patent No. 2,690,803; Disc Holding Frequency and Measuring Device, Serial No. 284,082, filed April 24, 1952, now Patent No. 2,734,379; Disc Sander, Serial No. 292,277, filed June 7, 1952, now Patent No. 2,715,802; and patent on Disc Edge Deburring Machine, Patent No. 2,650,458, issued September 1, 1953, to Frank C. Wallace. The general problem presented, however, is to obtain a plurality of discs which are tuned to a desired frequency and connect them by coupling wires with the proper response characteristics. Each end disc is connected to an end wire which is tuned for the desired results.

In practice the discs are tuned separately before being assembled. Then the end discs are tuned with the end wires before the filter is assembled. However, even though great care is taken in the assembly of the parts, it sometimes happens that after the filter has been assembled the over-all response is bad. It may be that an end wire or perhaps one of the discs is out of tune, but it is very diflicult to determine which part is out of tune. This present invention relates to a method for determining the response of various portions of the mechanical filters so as to allow a determination to be made as to which disc or Wire is out of tune.

Further objects, features, and advantages of the invention will become apparent from the following description and claims when read in view of the drawings, in which:

Figure 1 is a side view of the damping tool of this invention,

Figure 2 is a perspective view of the invention,

Figure 3 illustrates the damping tool in use with a mechanical filter,

Figure 4 illustrates a modification of the invention, and

Figure 5 is a partial sectional view of Figure 4.

Figure 1 illustrates the damping tool of this invention, and it comprises a handle 10 which might be made of a suitable plastic, for example, into which is inserted a blade 11. The blade 11 may be made of metal, or of any other suitable material.

A spring clamp 12 is attached to the handle 10 at its center and has a pair of legs, 13 and 14, for clamping the discs of the mechanical filter. The width of the legs 13 and 14 is great enough to span two discs in an electromechanical filter.

In use, the handle 10 is used to place the legs 13 and 14 over a pair of discs 16 and 17 forming a part of a mechanical filter assembly. It is to be noted that a number of other discs, 18 and 19, may be in the filter assembly.

2,887,877 Patented May 26, 1959 The discs are connected by coupling wires 21. The end disc 16 has a driving wire 22 attached thereto, which may =be connected to a magnetostrictive driving means.

Before placing the clamp over the discs 16 and 17 a liquid placed on both sides of the blade 11 so that when the blade passes between the discs 16 and 17 a thin layer of liquid will exist between each disc and the blade.

The blade 11 is centered with respect to discs 16 and 17. It has been observed that the discs 16 and 17 will be completely damped out by the liquid 23 between them and the blade.

This may be explained as follows:

Suppose that the liquid is water, and that the thickness of water between each side of the blade and the disc is designated by 11.. Assume that the disc 16 is vibrating and that the blade 11 is not. There will be a steady motion of water under pressure between the two fixed plates.

The equation for the state of equilibrium is bz Ba; (1)

Where u is the velocity of liquid particles, a is the coefficient of viscosity of the liquid, p is the pressure, x is the distance along an axis parallel to the plane of the discs, and z is the distance along the axis transverse to the plane of the discs.

Solving Equation 1, which is a partial differential equation of the second order, may be accomplished as follows:

Solving for A and B when z is variable,

l 2 'F P2 'uz(h ham or L am-. am (4) Further simplification results in is the damping factor R which appears in the equation for forced vibration as follows T a a; z t+Rs- 0 4 w 1 and r is the variable radius on the circular plate, so

When R is made very large, the vibrations can be completely damped. From (7) it is obvious that R can be made very large by having h" very small (of the order of .005 inch in our case).

The usefulness of this tool (1) With this positive damping technique, it is now possible to tune the end wire to the desired frequency very easily. The filter may be assembled and the damping tool placed over the first two discs to dampen them. The end wire may then be tuned because it is isolated dynamically from the discs.

(2) After the end wire is tuned the tool may be moved so that it engages the second and third discs to dampen them. Then the response of the end wire and first disc may be measured and adjusted until correct.

The response is measured by magnetostrictive means including a variable frequency signal generator 29 that is controlled by knob 30. A coupling coil 28 surrounds driving wire 22 and a biasing magnet 31 is mounted adjacent coil 28. Wire 22 is made of magnetostrictive material and is excited by alternatingcurrents in coil 28.

A coupling coil 26 is connected to a meter 27 and indicates the amplitude of vibration in wire 22 induced by coil 28. A biasing magnet 32 is mounted adjacent coil 26.

The disc may be tuned by removing a part with a small grinder and the end wire may be tuned by cutting it to the correct length. The tool may be moved from disc to disc until the entire assembly has been tested and corrected.

At times it becomes desirable to tune the end wire of an end disc and an end wire assembly while disassembled from the filter. This may be accomplished with the modification shown in Figures 4 and 5. A block 35 is formed With a hole 36 into which the disc 37 may be received. A driving wire 38 extends upwardly from disc 37. A clip 39 extends from block 35 to engage the disc. Liquid 41 is placed in the hole 36 and dampens disc 37 as previously described. The coils 26 and 28 and associated driving and indicating circuitry are not shown but are the same as in Figure 3. The frequency of the end wire may then be determined.

I claim:

A method of successively measuring at a desired frequency the response of individual elements of an electromechanical filter that has a plurality of serially connected resonant elements comprising, applying a damping tool to said filter for damping a resonant element which, in relation to a source for inducing mechanical vibrations of said desired frequency to said elements, succeeds that one of said elements of which the response is to be measured, said damping tool having a surface in close proximity to the surface of said succeeding resonant element for maintaining a thin layer of liquid between the surface of said succeeding resonant element and the adjacent surface of said damping tool whereby said succeeding element is dynamically isolated from said one element that is connected to said vibration source, measuring the amplitude of the vibrations induced into said one element, and tuning said one element to obtain the desired response at said desired frequency.

References Cited in the file of this patent UNITED STATES PATENTS 1,431,868 Bedell Oct. 10, 1922 1,562,543 Cox Nov. 24, 1925 1,942,323 Blodgett Jan. 2, 1934 2,439,739 Hussman Apr. 13, 1948 2,526,211 Erickson Oct. 17, 1950 2,692,681 Douglas Oct. 26, 1954

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