A novel method combining the nuclear norm minimization (NNM) and continuous-time (CT) subspace identification method (CSIM) is proposed to identify the CT Hammerstein model with little priori information. The nuclear norm minimization, which is the heuristic convex relaxation of the minimum rank constraint, is applied to the CT subspace identification method, for the purpose of improving the robustness and accuracy of identification. The proposed method can perform the identification well without the priori information about the Hammerstein model, which not only reduces the complexity of the identification problem but also broaden its applications. The nonlinear block of Hammerstein model is approximated with the pseudospectral method, which replaces the nonlinear function with Lagrange basis functions. A typical numerical example is presented to verify the NNMCSI method and the identification results are compared with the refined instrumental variable method.
I. INTRODUCTIONThe identification of block-oriented nonlinear systems has been an active research area for the last several decades. A block-oriented nonlinear system represents a nonlinear dynamical system as a combination of linear dynamic systems and static nonlinear blocks, mainly including Hammerstein [1-3], Wiener [4-6], Hammerstein-Wiener [7-10] and Wiener-Hammerstein systems . A typical CT Hammerstein system consists of a static nonlinear block and a CT linear time-invariant (LTI) system. A comprehensive overview of nonlinear Hammerstein system identification can be found in Giri and Bai [11]. There are many identification methods of the block-oriented nonlinear systems developed over the last decades, such as the over-parameterization method [12], the stochastic method , the separable least squares method [11], the blind method [13], the frequency domain method [14], the subspace method [15] and the iterative or recursive method [1-3, 6-8]. Unfortunately, the majority of the work focuses on the discrete-time cases, and only a little work is contributed to the CT Hammerstein system [10, 16, 17]. Mingxiang Dai is with Science and Technology on Transient