ARMAX Modal Parameter Identification for Structures Excited with Piezoceramic Actuators

Experimental methods used to determine the dynamic behaviour of helicopter structures can be broadly categorised as either ground testing methods or in-flight testing methods. A major limitation of ground testing is that dynamic properties determined from a grounded helicopter structure are usually significantly influenced by the boundary conditions. Therefore, it is desirable to carry out testing while the helicopter is in flight. A requirement of many testing methods is that the structure be excited with a known force, and in this study the use of multiple piezoceramic actuators to excite a structure is investigated. A multiple-input multiple-output autoregressive moving average with exogenous excitation (ARMAX) model is used for modal parameter estimation for the case where some excitation sources are not measured. The performance of this estimation technique is demonstrated with an experimental study involving a cantilever aluminium beam. Modal parameter estimates obtained from the ARMAX model are compared with results obtained from single-input single-output modal analysis with curve fitting of frequency response functions. The ARMAX estimation algorithm accurately estimates modal parameters; however, the effectiveness of piezoceramic actuators is dependent on the modal deformation in the actuator contact area. Accurate estimates of modal parameters are obtained for cases where over 200% unmeasured periodic excitations are present.