Model Validation and Analysis Using Feedforward Control Flight Test Data

Abstract

The German Aerospace Center (DLR) operates the ACT/FHS research helicopter with a full model-based control system operating from hover to 120 kn forward flight including take-off and landing as a key element of the ALLFlight project. The used feedforward controller is based on linear model inversion of identified local models, therefore the overall feedforward controller performance is highly dependent on the accuracy of the used inverted models. While several approaches exist to validate model accuracy in time and frequency domain, a validation method for model inversion purposes is rarely treated in literature. In this paper a flight test campaign that has been conducted to analyze model validity of the feedforward controller is presented and evaluated. A new model validation method using the feedforward controller to follow a reference command is discussed. Measured deviations from the commanded references are evaluated in time domain to highlight the model subsystems that are not precisely represented by the linear models. The obtained results are compared with a validation method from system identification. Model extensions are proposed to increase the model accuracy for feedforward controller usage.