By an exciter system care must be taken to insure the

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Unformatted text preview: l, road inputs to automobiles, and environmental inputs to proposed large space structures). For these cases, (5-41) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + care must be taken not to confuse poles that are system resonances with those that exist in the output spectrum due to inputs. In general, the poles of the response include those of the frequency response and of the input spectrum. Therefore, if the force is not measured, it is not possible without some prior knowledge about the input to determine if the poles of the response are truly system characteristics. If no poles or zeros exist in the force spectrum in the frequency range of interest, then any poles in the response in this range must be a result of the system characteristics. Obviously, when the excitation can be measured, it is prudent to do so. 5.3.2 Terminology and Nomenclature Historically, a number of terminology and nomenclature issues have not been rigorously defined when excitation methods have been described. The following terminology is important to the explanation of different excitation methods together with the associated digital signal processing requirements. Signal Type - Signal type refers to the basic form of the signal, such as random, impact, sinusoidal or chirp. Frequency Shaping - Frequency shaping refers to any frequency domain constraint or characteristic that is applied to the specific signal type. With respect to random excitation, a common frequency shaping is pseudo random. Other frequency shaping is commonly applied to sinusoids and chirps via the rate at which the change of frequency and/or amplitude occurs. Impact excitation is commonly frequency shaped by controlling the tip characteristic of the hammer. Delay Blocks - The number of contiguous blocks of excitation that take place without the associated input and output data being acquired are referred to as the delay blocks (N d ). This is normally associated with a excitation technique that is periodic in nature. The delay blocks are needed in order to give the transient response to any start or change in the periodic excitation to decay out of the response signal(s) so that both the input(s) and output(s) are periodic with respect to any observation period (T). It is this requirement that makes swept sinusoidal excitation methods (analog swept or digitally stepped) so time consuming, particularly on lightly (5-42) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + damped systems. Each delay block is equal in length to the observation period (T) and the number of delay blocks is normally chosen as an integer. The number of delay blocks does not have to be an integer for all excitation methods but, for the purposes of this paper and in common usage, is normally chosen as an integer. The delay blocks are not recorded and are not used in the estimation of the FRFs. Capture Blocks - The number of capture blocks refers to the number of contiguous blocks of time data (excitation (input...
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