The conventional extended Kalman filter (EKF) approach is only, applicable when the information of external inputs to structures is available. Some improved methodologies with different complexities have been proposed in the last decade, but previous approaches based solely on acceleration measurements are inherently unstable which leads to drifts in the estimated unknown inputs and structural displacements. Although regularization schemes or post signal processing can be used to treat the drifts, they are not suitable for the real-time identification of structural systems and unknown inputs. In this paper, it is aimed to directly extend the conventional EKF for real-time simultaneous identification of structural systems and unknown inputs. Based on the procedures of the conventional EKF, an extended Kalman filter with unknown inputs (EKF-UI) is directly derived. Moreover, data fusion of partially measured displacement and acceleration responses is applied to prevent in real time the previous drifts in the estimated structural displacements and unknown inputs. Several numerical examples are used to demonstrate the effectiveness of the proposed EKF-UI for real-time identification of linear or nonlinear structural systems and unknown external excitations.
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