Discounted MPC and infinite-horizon optimal control under plant-model mismatch: Stability and suboptimality
Pith reviewed 2026-05-10 16:50 UTC · model grok-4.3
The pith
Model predictive control using a surrogate model guarantees exponential stability and suboptimality under proportional plant-model mismatch, uniformly in the horizon length.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is that under continuity of the surrogate model and a cost-controllability assumption, plant-model mismatch bounded proportionally to states and controls ensures exponential stability of the closed loop for MPC and infinite-horizon problems, while also providing a suboptimality bound that recovers the surrogate optimal cost, with robustness independent of the horizon length.
What carries the argument
The proportional mismatch bounds that preserve the equilibrium at the origin, analyzed within a unified quadratic-cost framework for finite and infinite horizons.
If this is right
- Stability and suboptimality guarantees are independent of the prediction horizon length.
- Discounting allows handling larger mismatches while maintaining stability.
- The achieved cost approaches the surrogate optimum as mismatch vanishes.
- Both discounted and undiscounted cases are covered by the same analysis.
Where Pith is reading between the lines
- This implies that simplified models can be used reliably in control design if their error scales with signal amplitude.
- The uniformity in horizon suggests benefits for long-horizon planning without increased robustness requirements.
- Extensions might consider time-varying mismatch bounds or nonlinear costs under similar assumptions.
Load-bearing premise
Plant-model mismatch is bounded proportionally to the norms of the state and control, ensuring the origin remains an equilibrium for the real plant.
What would settle it
Observing that the closed-loop system diverges or converges sub-exponentially for some initial condition despite the mismatch satisfying the proportional bound would falsify the stability result.
Figures
read the original abstract
We study closed-loop stability and suboptimality for MPC and infinite-horizon optimal control solved using a surrogate model that differs from the real plant. We employ a unified framework based on quadratic costs to analyze both finite- and infinite-horizon problems, encompassing discounted and undiscounted scenarios alike. Plant-model mismatch bounds proportional to states and controls are assumed, under which the origin remains an equilibrium. Under continuity of the model and cost-controllability, exponential stability of the closed loop can be guaranteed. Furthermore, we give a suboptimality bound for the closed-loop cost recovering the optimal cost of the surrogate. The results reveal a tradeoff between horizon length, discounting and plant-model mismatch. The robustness guarantees are uniform over the horizon length, meaning that larger horizons do not require successively smaller plant-model mismatch.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript develops a unified framework for analyzing closed-loop stability and suboptimality of both finite-horizon discounted MPC and infinite-horizon optimal control when the controller is designed using a surrogate model that differs from the true plant. It assumes quadratic stage costs and plant-model mismatch bounds that are linear in the state and input (ensuring the origin remains an equilibrium for both systems). Under the standing assumptions of model continuity and cost-controllability, the paper proves exponential stability of the closed loop and derives an explicit suboptimality bound showing that the closed-loop cost recovers the optimal cost of the surrogate. The results identify trade-offs among prediction horizon, discount factor, and mismatch size, with the distinctive claim that all robustness guarantees are uniform with respect to the horizon length.
Significance. If the derivations hold, the contribution is meaningful for robust MPC theory. The uniformity of the stability and suboptimality bounds with respect to horizon length is a practically relevant feature that removes the usual requirement of successively tighter mismatch tolerances for longer horizons. The unified treatment of discounted/undiscounted and finite/infinite-horizon cases under a single set of quadratic-cost assumptions broadens the applicability of the results to a range of uncertain systems.
minor comments (3)
- [Abstract] The abstract and introduction would benefit from a brief statement of the precise mismatch bound (e.g., the constants multiplying ||x|| and ||u||) so that readers can immediately assess the strength of the uniformity claim.
- [Introduction] Notation distinguishing the true plant trajectory from the surrogate-model trajectory should be introduced consistently at the first use (currently the distinction appears only after several paragraphs).
- [Abstract] The statement that the robustness guarantees are 'uniform over the horizon length' would be clearer if accompanied by an explicit remark on whether the constants in the exponential decay rate and suboptimality bound are independent of N.
Simulated Author's Rebuttal
We thank the referee for the positive review, accurate summary of our contributions, and recommendation for minor revision. The emphasis on the uniformity of the robustness guarantees with respect to horizon length is particularly appreciated, as this is a central feature of the framework.
Circularity Check
No significant circularity identified
full rationale
The paper's derivation chain rests on standard assumptions (continuity of the model, cost-controllability, and plant-model mismatch bounds linear in states/controls that preserve the origin as equilibrium) together with Lyapunov-based stability arguments and suboptimality estimates. These are externally verifiable conditions from the MPC literature and do not reduce by construction to fitted parameters, self-definitions, or load-bearing self-citations whose validity depends on the present work. The claimed uniformity of robustness bounds over horizon length follows directly from the stated hypotheses without circular renaming or ansatz smuggling.
Axiom & Free-Parameter Ledger
axioms (3)
- domain assumption Continuity of the model
- domain assumption Cost-controllability
- domain assumption Plant-model mismatch bounded proportionally to states and controls
Reference graph
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