Offset-Free Robust Nonlinear Control Using Data-Driven Model: A Nonlinear Multi-Model Computationally Efficient Approach

TL;DR

This paper introduces a data-driven, offset-free robust nonlinear model predictive control scheme using symbolic regression to explicitly model uncertainties, improving safety margins and disturbance rejection in nonlinear systems.

Contribution

It develops a novel multi-model NMPC approach with symbolic regression-based surrogate models for robustness, applicable to multiple operating zones, and validated on a complex electric pump system.

Findings

Maintains disturbance tracking and rejection.

Increases safety margins and eliminates violations.

Achieves real-time performance with multiple models.

Abstract

Robust model predictive control (MPC) aims to preserve performance under model-plant mismatch, yet robust formulations for nonlinear MPC (NMPC) with data-driven surrogates remain limited. This work proposes an offset-free robust NMPC scheme based on symbolic regression (SR). Using a compact NARX structure, we identify interpretable surrogate models that explicitly represent epistemic (structural) uncertainty at the operating-zone level, and we enforce robustness by embedding these models as hard constraints. gest margins. The single-zone variant (RNMPC$_{SZ}$) was investigated. Synthetic data were generated within one operating zone to identify SR models that are embedded as constraints, while the nominal predictor remains fixed, and the multi-zone variant (RNMPC$_{MZ}$), zone-specific SR models from multiple operating zones are jointly enforced in the constraint set; at each set-point change, the nominal predictor is re-scheduled to the SR model of the newly active zone. In both cases, robustness is induced by the intersection of the admissible sets defined by the enforced SR models, without modifying the nominal cost or introducing ancillary tube dynamics. The approach was validated on a simulated pilot-scale electric submersible pump (ESP) system with pronounced nonlinearities and dynamically varying safety envelopes (downthrust and upthrust). RNMPC$_{SZ}$ and RNMPC$_{MZ}$ maintained disturbance tracking and rejection, and by intersecting models in the constraints, they increased margins and eliminated violations (especially near the upthrust), with a slight increase in settling time. Including up to four models per zone did not increase the time per iteration, maintaining real-time viability; RNMPC$_{MZ}$ presented the lar