Automatic control of machines often requires a signal proportional to the rod extension of a hydraulic cylinder. This rod extension can be measured in real time by tracking changes in the vibration modal properties of the cylinder. In particular, the natural frequency of a certain transverse mode is shown to be a quadratic function of the cylinder extension.;A free-free hydraulic cylinder was vibrated using a specially designed shaker. Acceleration signals of the cylinder were processed using several methods to compute the extension. The Fast Fourier Transform (FFT) method could not produce accurate results for a moving cylinder due to the interrelation of resolution and sample time inherent in the method. The filter-bank and dual-filter power-ratio methods also had disadvantages that ruled out their practical application to the cylinder.;Burg's Maximum Entropy method produces the power spectral density (PSD) function that estimates the cylinder extension reasonably well. The method works well in systems with little or no uncertainty. However, application on a real cylinder resulted in unacceptably noisy output. Bayesian inference was finally used to reduce the noise to acceptable levels.
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