Accelerating Adaptive Systems via Normalized Parameter Estimation Laws
Mohammad Boveiri, Mohammad Khosravi, Peyman Mohajerin Esfahani
TL;DR
This work addresses slow convergence in adaptive control when persistent excitation is unavailable by introducing normalized parameter estimation laws that enforce $\|x(t)\|_2^{2/r} \in \mathcal{L}_1$ for a chosen $r \ge 1$. The approach leverages Lyapunov-based normalization to accelerate adaptation, extends to multidimensional systems with matched and unmatched uncertainties via the vanishing degree concept, and further incorporates momentum for higher-order schemes. Theoretical results demonstrate boundedness, convergence, and the $\mathcal{L}_1$-property of the $r$-th root of the state norm, with infinite vanishing degree in many common settings allowing large $r$. Numerical experiments across scalar, multi-dimensional, and robotic examples validate faster convergence and reduced control effort compared to standard methods. The framework integrates with any CLF-based certainty-equivalence controller and avoids persistent excitation, time-varying gains, or prior knowledge of parameter values, offering practical improvements for adaptive control tasks.
Abstract
In this paper, we propose a new class of parameter estimation laws for adaptive systems, called \emph{normalized parameter estimation laws}. A key feature of these estimation laws is that they accelerate the convergence of the system state, $\mathit{x(t)}$, to the origin. We quantify this improvement by showing that our estimation laws guarantee finite integrability of the $\mathit{r}$-th root of the squared norm of the system state, i.e., \( \mathit{\|x(t)\|}_2^{2/\mathit{r}} \in \mathcal{L}_1, \) where $\mathit{r} \geq 1$ is a pre-specified parameter that, for a broad class of systems, can be chosen arbitrarily large. In contrast, standard Lyapunov-based estimation laws only guarantee integrability of $\mathit{\|x(t)\|}_2^2$ (i.e., $\mathit{r} = 1$). We motivate our method by showing that, for large values of $r$, this guarantee serves as a sparsity-promoting mechanism in the time domain, meaning that it penalizes prolonged signal duration and slow decay, thereby promoting faster convergence of $\mathit{x(t)}$. The proposed estimation laws do not rely on time-varying or high adaptation gains and do not require persistent excitation. Moreover, they can be applied to systems with matched and unmatched uncertainties, regardless of their dynamic structure, as long as a control Lyapunov function (CLF) exists. Finally, they are compatible with any CLF-based certainty equivalence controllers. We further develop higher-order extensions of our estimation laws by incorporating momentum into the estimation dynamics. We illustrate the performance improvements achieved with the proposed scheme through various numerical experiments.