Assessment Study of the State of the Art in Adaptive Control and its Applications to Aircraft Control

Summary Many papers relevant to reconfigurable flight control have appeared over the past fifteenyears. In general these have consisted of theoretical issues, simulation experiments, and in somecases, actual flight tests. Results indicate that reconfiguration of flight controls is certainly feasiblefor a wide class of failures.However many of the proposed procedures although quite attractive, need further analyticaland experimental studies for meaningful validation. Many procedures assume the availability offailure detection and identification logic that will supply adequately fast, the dynamicscorresponding to the failed aircraft. This in general implies that the failure detection and faultidentification logic must have access to all possible anticipated faults and the correspondingdynamical equations of motion. Unless some sort of explicit on line parameter identification isincluded, the computational demands could possibly be too excessive. This suggests the need forsome form of adaptive control, either by itself as the prime procedure for control reconfiguration orin conjunction with the failure detection logic.If explict or indirect adaptive control is used, then it is important that the identitied modelsbe such that the corresponding computed controls deliver adequate performance to the actual aircraft.Unknown changes in trim should be modelled, and parameter identification needs to be adequatelyinsensitive to noise and at the same time capable of tracking abrupt changes.If however, both failure detection and system parameter identification turn out to be too timeconsuming in an emergency situation, then the concepts of direct adaptive control should beconsidered. If direct model reference adaptive control is to be used (on a linear model) with stabilityassurances, then a positive real or passivity condition needs to be satisfied for all possibleconfigurations. This condition is often satisfied with a feedforward compensator around the plant.This compensator must be robustly designed such that the compensated plant satisfies the requiredpositive real conditions over all expected parameter values. Furthermore, with the feedforward onlyaround the plant, a nonzero (but bounded error) will exist in steady state between the plant and modeloutputs. This error can be removed by placing the compensator also in the reference model. Designof such a compensator should not be too difficult a problem since for flight control it is generallypossible to feedback all the system states.It is also important to note that multiple model based approaches are very attractive in termsof their potential speed of response to abrupt changes and/or failures. However unless some tuningis present, an extraordinary number of models may be required.In view of the advantages offered by direct adaptive control and multiple model based control,it is anticipated that the combination of these methods should be very effective for reconfigurable