Vibrations et chocs mécaniques, et leur surveillance — Vocabulaire.
free impedance
ratio of the applied excitation complex force to the resulting complex velocity with all other connection points of the system free, I.e. having zero restraining forces
NOTE I Historically, often no distinction has been made between blocked impedance and free impedance. Caution should, therefore, be exercised in interpreting published data.
NOTE 2 Free impedance is the arithmetic reciprocal of a single element of the mobility matrix, While experimentally determined free Impedances could be assembled into a matrix, this matrix would be quite different from the blocked impedance matrix resulting from mathematical modelling of the structure and, therefore, would not conform to the requirements for using mechanical impedance in an overall theoretical analysis of the system.
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blocked Impedance
impedance at the input when all output degrees of freedom are connected to a load of infinite mechanical impedance
NOTE 1 Blocked impedance is the frequency-response function formed by the ratio of the phasor of the blocking or driving-point force response at point i. to the phasor of the applied excitation velocity at polntj, with all other measurement points on the structure ‘blocked, I.e. constrained to have zero velocity. All forces and moments required to fully constrain all points of interest on the stn,cture need to be measured in order to obtain a valid blocked impedance matrix.
NOTE 2 Any changes in the number of measurement points or their location will change the blocked impedances at all measurement points.
NOTE 3 The primary usefulness of blocked impedance is in the mathematical modelling of a structure using lumped mass, stiffness and damping elements or finite element techniques. When combining or companng such mathematical models with experimental mobility data, it is necessary to convert the analytical blocked impedance matrix into a mobility matrix or vice versa.
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frequency-response function
frequency-dependent ratio of the motion-response Fourier transform to the Fourier transform of the excitation force of a linear system
NOTE 1 Excitation can be harmonic, random or transient functions of time. The test results obtained with one type of excitation can thus be used for predicting the response of the system to any other type of excitation.
NOTE 2 Motion may be expressed in terms of velocity, acceleration or displacernent the corresponding frequency- response function designations are mobility. accelerance and dynamic compliance or impedance, effective (i.e. apparent) mass and dynamic stiffness, respectively (see Table 1).
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mobility
mechanical mobility
complex ratio of the velocity, taken at a point in a mechanical system, to the force, taken at the same or another point in the system.ISO 2041-2009 pdf free download.