# An Optimal Stable Selective Model Inversion for Nonminimum-phase Systems

## TL;DR

This paper introduces an optimal stable model inversion method for nonminimum-phase systems, balancing model accuracy and robustness in feedback control through a multi-objective H infinity approach.

## Contribution

It presents a novel inversion algorithm that handles unstable zeros and nonminimum-phase systems, ensuring causality, stability, and robustness in feedback control.

## Key findings

- Effective in motion control applications
- Balances model accuracy and robustness
- Validated on high-order systems

## Abstract

Stably inverting a dynamic system model is the foundation of numerous servo designs. Existing inversion techniques have provided accurate model approximations that are often highly effective in feedforward controls. However, when the inverse is implemented in a feedback system, additional considerations are needed for assuring causality, closed-loop stability, and robustness. In pursuit of bridging the gap between the best model matching and a robust feedback performance under closed-loop constraints, this paper provides a modern review of frequency-domain model inversion techniques and a new treatment of unstable zeros. We provide first a pole-zero-map-based intuitive inverse tuning for motion control systems. Then for general nonminimum-phase and unstable systems, we propose an optimal inversion algorithm that can attain model accuracy at the frequency regions of interest and meanwhile constrain noise amplification elsewhere to guarantee system robustness. The design goals are achieved by a multi-objective H infinity formulation and all-pass factorization that consider model matching, causality of transfer functions, frequency-domain gain constraints, and factorization of unstable system modes in a unified scheme. The proposed algorithm is validated on motion control systems and complex high-order systems.

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Source: https://tomesphere.com/paper/1903.09054