Min-Kuang Wu

Damage-Mitigating Control of Mechanical Systems: Part I—Conceptual Development and Model Formulation

A major goal in the control of complex mechanical systems such as advanced aircraft, spacecraft, and power plants is to achieve high performance with increased reliability, availability, component durability, and maintainability. The current state-of-the-art in control systems synthesis focuses on improving performance and diagnostic capabilities under constraints that often do not adequately represent the dynamic properties of the materials. The reason is that the traditional design is based upon the assumption of conventional materials with invariant characteristics. In view of high performance requirements and availability of improved materials, the lack of appropriate knowledge about the properties of these materials will lead to either less than achievable performance due to overly conservative design, or over-straining of the structure leading to unexpected failures and drastic reduction of the service life. The key idea of the research reported in this paper is that a significant improvement in service life could be achieved by a small reduction in the system dynamic performance. The concept of damage mitigation is introduced and a continuous-time model of fatigue damage dynamics is formulated in this paper which is the first part of a two-part paper. The second part which is a companion paper presents synthesis of an open loop control policy and the results of simulation experiments for transient operations of a reusable rocket engine.

Damage-Mitigating Control of Mechanical Systems: Part II—Formulation of an Optimal Policy and Simulation

The objective of damage-mitigating control introduced in the first part of this two-part paper is to achieve high performance without overstraining the mechanical structures. The major benefit is an increase in the functional life of critical plant components along with enhanced safety, operational reliability, and availability. Specifically, a methodology for modeling fatigue damage has been developed as an augmentation to control and diagnostics of complex dynamic processes such as advanced aircraft, spacecraft, and power plants. In this paper which is the second part, an optimal control policy is formulated via nonlinear programming under specified constraints of the damage rate and accumulated damage. The results of simulation experiments for upthrust transient operations of a reusable rocket engine are presented to demonstrate efficacy of the damage-mitigating control concept.

Damage-mitigating control of a reusable rocket engine

The goal of damage mitigating control in reusable rocket engines is to achieve high performance with increased durability of mechanical structures such that functional lives of the critical components are increased. The major benefit is an increase in structural durability with no significant loss of performance. This report investigates the feasibility of damage mitigating control of reusable rocket engines. Phenomenological models of creep and thermo-mechanical fatigue damage have been formulated in the state-variable setting such that these models can be combined with the plant model of a reusable rocket engine, such as the Space Shuttle Main Engine (SSME), for synthesizing an optimal control policy. Specifically, a creep damage model of the main thrust chamber wall is analytically derived based on the theories of sandwich beam and viscoplasticity. This model characterizes progressive bulging-out and incremental thinning of the coolant channel ligament leading to its eventual failure by tensile rupture. The objective is to generate a closed form solution of the wall thin-out phenomenon in real time where the ligament geometry is continuously updated to account for the resulting deformation. The results are in agreement with those obtained from the finite element analyses and experimental observation for both Oxygen Free High Conductivity (OFHC) copper and a copper-zerconium-silver alloy called NARloy-Z. Due to its computational efficiency, this damage model is suitable for on-line applications of life prediction and damage mitigating control, and also permits parametric studies for off-line synthesis of damage mitigating control systems. The results are presented to demonstrate the potential of life extension of reusable rocket engines via damage mitigating control. The control system has also been simulated on a testbed to observe how the damage at different critical points can be traded off without any significant loss of engine performance. The research work reported here is built upon concepts derived from the disciplines of Controls, Thermo-fluids, Structures, and Materials. The concept of damage mitigation, as presented in this report, is not restricted to control of rocket engines. It can be applied to any system where structural durability is an important issue.

Fatigue damage control of mechanical systems

This paper presents the concept and architecture of a fatigue damage control system for mechanical structures. In contrast to the conventional cycle-based approach, fatigue damage is represented via nonlinear differential equations with respect to time in the state-variable setting. This damage model is compatible with the dynamic model of the plant, i.e. the process under operation and control, and the instantaneous damage rate depends on the current level of accumulated damage. The objective here is to achieve an optimized trade-off between dynamic performance and structural durability of the plant. This interdisciplinary effort requires augmentation of the system-theoretic techniques for decision making and control with governing equations and inequality constraints representing the fatigue damage properties of structural materials. The major challenge in the reported work is to characterize the fatigue damage generation process in mechanical structures and then utilize this information for synthesizing algorithms of performance optimization, robust control and risk assessment for plant operation.

Damage-mitigating control of space propulsion systems for high performance and extended life

A major goal in the control of complex mechanical system such as spacecraft rocket engines advanced aircraft, and power plants is to achieve high performance with increased reliability, component durability, and maintainability. The current practice of decision and control systems synthesis focuses on improving performance and diagnostic capabilities under constraints that often do not adequately represent the materials degradation. In view of the high performance requirements of the system and availability of improved materials, the lack of appropriate knowledge about the properties of these materials will lead to either less than achievable performance due to overly conservative design, or over-straining of the structure leading to unexpected failures and drastic reduction of the service life. The key idea in this report is that a significant improvement in service life could be achieved by a small reduction in the system dynamic performance. The major task is to characterize the damage generation process, and then utilize this information in a mathematical form to synthesize a control law that would meet the system requirements and simultaneously satisfy the constraints that are imposed by the material and structural properties of the critical components. The concept of damage mitigation is introduced for control of mechanical systems to achieve high performance with a prolonged life span. A model of fatigue damage dynamics is formulated in the continuous-time setting, instead of a cycle-based representation, for direct application to control systems synthesis. An optimal control policy is then formulated via nonlinear programming under specified constraints of the damage rate and accumulated damage. The results of simulation experiments for the transient upthrust of a bipropellant rocket engine are presented to demonstrate efficacy of the damage-mitigating control concept.

Single input cerebellar model articulation controller (CMAC) based maximum power point tracking for photovoltaic system

In this paper, a single-input cerebellar model articulation controller (CMAC)-based maximum power point tracking (MPPT) for PV system is proposed. As a type of neural network based controller with simple computation that results in fast learning, it is more suitable for hardware implementation. The single-input CMAC control system adopts two learning stages. During off-line learning stage the CMAC controller learns about the control surface of single input fuzzy logic controller (S-FLC). At the end of this stage, the CMAC controller is capable to approximate and imitate the behavior of S-FLC. Then, an on-line learning follows this process to improve the system stability. The linear interpolation is also used to improve its performance. Simulation results show that the proposed method can be used effectively to track the MPP of solar panel, provides fast response due to temperature and solar irradiation changing and has good performance at steady state.