In order to validate this data procedure and estimation method, we carried out experiment on the ROMAC stability test rig as depicted in Fig. ef{fig2:subfig:a}. The fluid and magnetic bearing configurations were in cite{cloud2007stability}, and during the excitation process, the temperature of inlet oil was 37 ${}^{circ}mathrm{C}$. The shaft rotates at 3500 rpm, and the actuator #2 that is located at the node 47 in the FE model excited this test rig at six frequencies (55 Hz to 80 Hz by step of 5 Hz). The excitation force is expressed in Eq.( ef{eq32}). egin{equation}label{eq32}
m{F}{
m{ = }}left( {egin{array}{*{20}{c}}
{{F_{ex}}}\
{{F_{ey}}} end{array}} ight) = {F_e}left( {egin{array}{*{20}{c}}
{Xcosleft( {omega t} ight)}\ {Ysin left( {omega t} ight)} end{array}} ight) end{equation} The coefficients of X and Y are used to adjust the amplitude of the excitation force in 푋 and 푌 directions.By adjusting the excitation force according to Table ef{tab10}, we did multiple excitations for every excitation frequency to obtain high quality estimation in Eq.( ef{eq16}) and ( ef{eq17}). …show more content…
egin{table}[htbp] small caption{Comparison between measured and predicted rotor free-free natural frequencies}label{tab10} centering egin{tabular}{lcc} oprule Excitation method& X& Y\ midrule Excitation in X& 1.0& 0\ Excitation in Y& 0& 1.0\ Forward and backward& 1.0& $pm$1.0\ Cross-coupled excitation 1& 1.0& $pm$0.5\ Cross-coupled excitation 2& $pm$0.5& 1.0\ ottomrule end{tabular} end{table} In the test, vibrations at five sections (Nodes 6, 13, 44, 56 and 62) were measured, and the data of four sections (Nodes 6, 13, 56 and 62) were used to identify the coefficients of bearing by using this interpolation-iteration method. Moreover, the location of the sensors is shown in Fig. ef{fig11}, and the sensors at node 62 are supported by portable magnetic base. All the measured vibrations and excitation magnetic force were transferred to the same coordinate that is plotted in Fig. ef{fig11}. egin{figure}[htbp] centering includegraphics[width=5cm]{Fig11} caption{Sensor positions} label{fig11} end{figure} subsection{Experiment results} Figures ef{fig12} and ef{fig13} show the estimated force coefficients of bearings with test data. Figure ef{fig12} depicts the estimated principle coefficients of the two bearings. With the increase of the excitation frequency, the principle stiffness coefficients of the left bearing remain nearly invariant, while those of the right bearing decrease a little. The principle damping coefficients of the left bearing slightly decrease with the excitation frequency, and those of the right bearing almost keep horizontal except for a small projection at 70Hz. Moreover, the principle stiffness $Kxx$ and $Kyy$ of the left bearing are larger than those of right bearing, and $Cxx$ and $Cyy$ of the left bearing are smaller than those of the right bearing. For each bearing, $Kxx$ and $Kyy$ are very close, and so are the …show more content…
ef{fig15}. The first forward log decrement remain nearly invariant in the excitation frequency range, while the first backward frequency drops a little.
egin{figure}[htbp] centering includegraphics[width=8cm]{Fig15.pdf} caption{Mode paramenters} label{fig15} end{figure} Using the sine-swept excitation method, we did both backward and forward excitaions for the test rig, and Fig. ef{fig16:subfig} shows the full spectrum waterfall for the left bearing (Node 10). Using the idenitification method [Li], the first mode parameters are estimated (1B: f=85.3Hz, LogDec=0.726; 1F: 86.3Hz, LogDec=0.442). By comparing the prdicted and measured first forward mode parameters, we can drow conclusion that the force coeffients of bearings are reasonably