Measurement of wave propagation power flow in structures

Details Measurement of wave propagation power flow in structures Measurement of wave propagation power flow in structures

1986-08-22

1988-04-19

Myracle, Jerry W.

Measuring and testing

Vibration

By mechanical waves

73602, G01M 700

Patent

active

047381388

DESCRIPTION:

BRIEF SUMMARY
The invention relates to apparatus and methods for measuring the power flow associated with wave propagation, particularly though not exclusively for measuring vibration power flow through structures.
In attempting to control vibration in structures it is desirable to identify dominant paths of vibration transmission from sources (eg machines) through the structure to some point of interest. Consideration of only the amplitudes of vibration in a structure is of little help as stationary waves may cause large local vibration amplitudes without a significant power transmission. It is therefore convenient to introduce a concept of structural wave intensity, defined as the power flow per unit width of cross-section, which is measurable as a vector quantity at any given point.
In structural elements such as beams, where wave propagation is in one dimension only, the power flow crossing the total cross-sectional area is considered. The mechanical power transmitted can be given simply by the instantaneous product of the force and the velocity at the point of action. By repeating the intensity measurements at many positions on a structure it should be possible to build up a pattern representing paths of significant power flow.
Previous investigations have indicated that flexural waves are the most significant type of motion in the transmission of power through structures. Hence, intensity measurements can be greatly influenced by physical factors such as the size, material and shape of the structural elements.
The four accelerometer finite difference method is suitable for measuring the one dimensional power flow in beams. The method attempts to achieve maximum generally of application by estimating both the bending moment and the shear force components of power flow in the base. Under ideal conditions the dynamic range of the four accelerometer finite difference method would be infinite, however instrumentation tolerances have a deleterious effect and even moderate values of tolerances produce quite significant measurement errors.
In any practical structure, particularly where reverberent conditions may be expected, any measurement errors will be almost completely dominated by spurious results generated by stationary waves. The ability of a measuring system to provide an accurate measurement of the travelling wave component in the presence of a possibly much larger standing wave is termed its dynamic range. The systems susceptibility to these errors is governed by the instrumentation phase tolerances either in the transducers, the signal conditioning amplifiers or in the signal processing.
As broadband examination of flexural wave intensity is not very meaningful existing narrow band systems use filters, such as band-pass or switched banks of filters over one octave or a third of an octave. However, the inventor has found that intensity patterns change rapidly with changing frequency and even a third of an octave is too broad. Phase matching of the filters is also crucial as it governs the dynamic range and hence the usefulness of the system. Thus for any of the prior art structural wave intensity measuring systems to be capable of making accurate measurements, very narrow bandwidth filters with very close phase matching between channels must be used. To a large extent these two requirements are conflicting since filters with a rapid increase of attenuation outside the passaband imply a large number of singularities in their transfer functions. This in turn dictates that the phase shift around the cut-off and cut-on frequencies will be large, and hence the problem of obtaining accurate close phase matching becomes more acute. For the system to be useable over a range of frequencies it is necessary either to make these filers tunable or to use a bank of switched frequency filters. With tunable filters, however, satisfactory phase matching is virtually impossible while the use of a switched bank of filters increases the problems of construction and the maintenance of the same measurement bandwidth results in greate

REFERENCES:
patent: 4180811 (1979-12-01), Yoshimura et al.
patent: 4598588 (1986-07-01), Hanson
Herbertz, J. Measurement of Ultrasonic Power . . . Horns, Ultrasonics, vol. 14, No. 6, 1976, pp. 278-280, Great Britain.

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