Improved residual mode separation for finite-dimensional control of PDEs: Application to the Euler–Bernoulli beam

We consider a simply-supported Euler–Bernoulli beam with viscous and Kelvin–Voigt damping. Our objective is to attenuate the effect of an unknown distributed disturbance using one piezoelectric actuator. We show how to design a state-feedback controller based on a finite number of dominating modes that guarantees that the L² gain is not greater than a given value. If the remaining (infinitely many) modes are simply ignored, the calculated L² gain is wrong. This happens because of the spillover phenomenon that occurs when the effect of the control on truncated modes is not accounted for in the feedback design. We propose a simple modification of the H_∞ cost that prevents spillover. The key idea is to treat the control as a disturbance in the truncated modes and find the corresponding L² gains using the bounded real lemma. These L² gains are added to the control weight in the H_∞ cost for the dominating modes, which prevents spillover. A numerical simulation of an aluminum beam with realistic parameters demonstrates the effectiveness of the proposed method. The presented approach is applicable to other types of PDEs, such as the heat, wave, and Kuramoto–Sivashinsky equations, as well as their semilinear versions. The proposed method gives a Lyapunov functional that can also be used for guaranteed cost control, regional stability analysis, and input-to-state stability.