Jl du Bois

Modeling and testing of a semiactive hydraulic damper in periodic working regimes

This paper presents analytical and experimental studies of a modified semiactive hydraulic damper operating in periodic working regimes. This work was completed as a part of the Rotor Embedded Actuator Control Technology project sponsored by the Technology Strategy Board in the United Kingdom. The damper tested is based on modifications to an industrially employed helicopter damper. The work presented covers the relevant aspects of the model development, damper modification, test planning, and analysis, as well as a model-simulation correlation study. The model of the damper directly reflects the actual hydraulic modification, enabling a semiactive mode of operation. Pressure-flow representations used in the damper modeling are based on a novel testing methodology using triangular piston excitation waveforms. A specific test structure based on varying the relative phase difference between two harmonic input signals is used to assess the damper properties in periodic working regimes. The test configuration with a base harmonic piston excitation combined with a harmonic modulation at a frequency 3 times higher than the base frequency leads to significant changes in the second and fourth harmonic components of the resulting periodic damper forces. A successful model-simulation correlation study suggests that a simple one-state dynamic model of this damper features good predictive properties, equally applicable in the extended simulation contexts.

Application of cepstrum prewhitening on non-stationary signals

In the field of vibration based condition monitoring a trusted symptom of a defective bearing is the observation of peaks, at characteristic frequencies, in the squared envelope spectrum (SES). If a machine is operating in a varying speed regime the SES is computed on the order tracked signal, i.e. the signal resampled at constant angular increments, and the SES can still be used for diagnostic. Despite its versatility a common problem with the SES is that peaks from other sources of vibrations, as for instance gears, can prevent the diagnosis of a defective bearing. Therefore pre-processing techniques are applied to the vibrational signal before the computation of the SES to enhance the signal from the bearings. Among these techniques cepstral pre-whitening (CPW) has gained much attention offering a remarkable capability of eliminating, in a blind way, both harmonics and modulation side-bands of the unwanted components. In the case of a varying speed regime the usual procedure consists of three steps: order track the signal, calculate the CPW, evaluate the SES. In this paper on the contrary the CPW is applied before the step of order tracking; therefore the proposed approach is: CPW the raw time signal, order tracking, evaluation of the SES. The remarkable observation is that for this approach the cepstrum does not present peaks at characteristic quefrencies, being the raw signal acquired in a varying speed regime. However this paper shows by means of numerical simulations and analysis of experimental data, that with the proposed methodology the masking components coming from the gears are suppressed and the signal from the defective bearing is enhanced.