Michael S. Holmes

Life-extending control of fossil fuel power plants

The objective of life-extending control is to achieve a trade-off between structural durability and dynamic performance. This paper focuses on structural durability of the main steam header under load following to illustrate how the life-extending control of fossil fuel power plants can be achieved via feedforward/feedback. This concept is potentially applicable to both new and aging power plants under a variety of operational modes such as hot start-up, scheduled shutdown, and load following where the plant power is rapidly maneuvered to meet the varying load demand. The feedforward control policy is synthesized via nonlinear optimization of a multi-objective cost functional of dynamic performance and service life under the constraints of actuator saturation, operational limitations, and allowable structural damage, including thermomechanical fatigue and plastic deformation. A linear robust feedback control law that is superimposed on the feedforward sequence is synthesized based on induced L2-norm techniques. The results of simulation experiments are presented to demonstrate that the proposed feedforward/feedback control policy is potentially capable of ramping the plant power up at a rate of 10% of the full load per minute while maintaining the specified performance and satisfying the damage constraints.

Fuzzy Damage Mitigating Control of Mechanical Structures

This paper presents the architecture and synthesis of a damage mitigating control system for mechanical structures where the objective is to achieve high performance with increased reliability, availability, component durability, and maintainability. The proposed control system has a two-tier structure. In the lower tier a linear sampled-data controller tracks a reference trajectory vector while the upper tier contains a fuzzy-logic-based damage controller which makes a trade-off between system performance and the damage in critical components. The synthesis procedure is demonstrated by simulation experiments on the model of a reusable rocket engine. The simulation results explore the feasibility of automatically regulating the damage/ performance trade-off in a real-time setting.

Fuzzy damage-mitigating control of a fossil power plant

Presents the architecture and synthesis of a damage-mitigating control system where the objective is to achieve high performance with increased reliability, availability, component durability, and maintainability. The proposed control system has a two-tier structure. In the lower tier, a linear robust sampled-data controller tracks a reference trajectory vector while the upper tier contains a fuzzy-logic-based damage controller that makes a tradeoff between system dynamic performance and structural durability in critical component(s). The synthesis procedure is demonstrated on the model of a commercial-scale fossil-fueled power plant under load-following operation. Simulation experiments are designed to explore the feasibility of real-time fuzzy damage-mitigating control in fossil power plants, and the results show that substantial gain in structural durability of a critical component can be achieved with no significant loss of performance.

Life-extending control of mechanical structures: experimental verification of the concept

The concept of life-extending control is built upon the two disciplines of Systems Science and Mechanics of Materials, and its goal is to achieve an optimized trade-off between dynamic performance and structural durability of the plant under control. Experimental and simulation results reported in recent publications show that a life extending control system can substantially reduce the structural damage accumulated in critical components with no significant loss of plant performance. This enhancement of structural durability is accomplished via nonlinear optimization to generate a sequence of open-loop commands that maneuver the plant from a known initial state, along a prescribed trajectory, close to the final desired-state subject to constraints on the damage rate and accumulation in critical components. This paper presents a methodology for analytical development of a robust feedforward-feedback control policy for life extension and high performance of mechanical structures. The concept of life-extending control is experimentally verified in a laboratory testbed which is a two-degree-of-freedom (2DOF) mechanical system excited by a computer-controlled shaker table. Test results demonstrate that the fatigue life of test specimens can be substantially extended with no appreciable degradation in the dynamic performance of the mechanical system.

Fuzzy damage mitigating control of mechanical structures

Presents the architecture and synthesis of a damage mitigating control system where the objective is to achieve high performance with increased reliability, availability, component durability, and maintainability. The proposed control system has a two-tier structure. In the lower tier a linear sampled-data controller tracks a reference trajectory vector while the upper tier contains a fuzzy-logic-based damage controller which makes a trade-off between system performance and the damage in critical plant components. The synthesis procedure is demonstrated by simulation experiments on the model of a reusable rocket engine. The simulation results explore the feasibility of automatically regulating the damage/performance trade-off in a real-time setting.

Mixed H 2 /L 1 control with low order controllers: a linear matrix inequality approach

This paper addresses the problem of designing stabilizing controllers that minimize the /spl Hscr//sub 2/ norm of a certain closed-loop transfer function while maintaining the /spl Lscr//sub 1/ norm of a different transfer function below a prespecified level. This problem arises in the context of rejecting both stochastic as well as bounded persistent disturbances. Alternatively, in a robust control framework it can be thought as the problem of designing a controller that achieves good nominal /spl Hscr//sub 2/ performance, while at the same time, guaranteeing stability against unmodeled dynamics with bounded induced /spl Lscr//sub /spl infin// norm. The main result of this paper shows that, for the state feedback case, a suboptimal static feedback controller can be synthesized by a two stage process involving a finite-dimensional convex optimization problem and a line-search.

Nonlinear Life-Extending Control of a Rocket Engine

ROCKET engine has a number of critical components that operate close to mechanical design limits. These components often typify behavior of the remaining components and hence are indicators of the effective service life of a reusable rocket engine. Fatigue damage in the turbine blades is one of the most serious causes for engine failure. This Note focuses on the conceptual development of a nonlinear life-extending control system for rocket engines via damage mitigation in both the fuel (H2) and oxidizer (O2) turbine blades. The fundamental concept of life-extending control (LEC) was introduced by Lorenzo and Merrill. 1 Subsequently, a growing body of literature has emerged for feedforward 2,3 and feedback 4 control of rocket engines for life extension. Whereas the LEC technology was developed initially for rocket engines, it has broad applications for other systems such as fossil-fueled power plants 5 and mechanical structures, 6 where both dynamic performance and structural durability are critical issues. ThedesignapproachpresentedinthisNoteisdifferentfrompreviousworkinthesensethatthisapproachallowsadaptationoftheLEC featuretoaugmentaconventionalperformancecontrollerofarocket engine. Unlike the previously reported design approaches, 2,3,5 the proposed technique does not require an optimal feedforward control sequence, which is sensitive to plant modeling uncertainties and variations in the initial conditions. Furthermore, for other control applications such as military aircraft, the life extension feature ofthecontrolsystemcanbeactivatedordeactivatedattheoperator’ s discretion.

Life extending controller design for reusable rocket engines

The goal of life extending control (LEC) is to enhance structural durability of complex mechanical systems, such as aircraft, spacecraft, and energy conversion devices, without incurring any significant loss of performance. This paper presents a concept of robust life-extending controller design for reusable rocket engines, similar to the Space Shuttle Main Engine (SSME), via damage mitigation in both fuel and oxidiser turbines while achieving the required performance for transient responses of the main combustion chamber pressure and the oxidant/fuel mixture ratio. The design procedure makes use of a combination of linear robust control synthesis and nonlinear optimisation techniques. Results of simulation experiments on the model of a reusable rocket engine are presented to this effect.

Fuzzy damage-mitigating control of a fossil-fueled power plant

Presents the architecture and synthesis of a damage-mitigating control system where the objective is to achieve high performance with increased reliability, availability, component durability, and maintainability. The proposed control system has a two-tier structure. In the lower tier, a linear sampled-data controller tracks a reference trajectory vector while the upper tier contains a fuzzy-logic-based damage controller which makes a trade-off between system dynamic performance and structural damage in critical component(s). The synthesis procedure is demonstrated on the model of a commercial-scale fossil-fueled power plant. Simulation results are presented which demonstrate the trade-off between plant dynamic performance and structural damage in a critical component.

Life extending control of a reusable rocket engine

The goal of life extending control in reusable rocket engines is to achieve high performance without overstraining the mechanical structure; and the major benefit is an increase in structural durability with no significant loss of performance. This paper investigates the feasibility of a decision and control system for life extension and performance enhancement of a reusable rocket engine, such as the Space Shuttle Main Engine (SSME). Creep damage in the coolant channel ligament in the main thrust chamber is controlled while engine performance is maximized. For open loop control of up-thrust transients of the rocket engine, an optimal feedforward policy has been synthesized based on an integrated model of plant, structural and damage dynamics. Optimization is based on the integrated model of plant, structural and damage dynamics under creep damage constraints in the critical plant component, the coolant channel ligament in the main thrust chamber. The results demonstrate the potential of life extension of reusable rocket engines via damage mitigating control. The concept of life extending control, as presented in this paper, is not restricted to rocket engines; it can be applied to any system where structural durability is an important issue.