Kevin J. Melcher

Development of a Numerical Model for High-Temperature Shape Memory Alloys

A new thermomechanical hysteresis model for a high-temperature shape memory alloy (HTSMA) actuator material is presented. The proposed Brinson–Preisach model is capable of predicting the strain output of a tensile-loaded HTSMA when excited by arbitrary temperature–stress inputs for the purpose of actuator and control design. Quasistatic generalized Preisach hysteresis models available in the literature require large sets of experimental data for model identification at a particular operating point, and substantially more data for multiple operating points. The minor loop algorithm is an alternate approach to common Preisach methods that is better suited for research-stage alloys, such as recently developed HTSMAs, for which a complete identification database is not yet available. A detailed description of the minor loop hysteresis algorithm is presented in this paper and a methodology for determination of model parameters is introduced. The algorithm is assembled together with a modified form of the one-dimensional Brinson constitutive equation to provide a continuous thermomechanical response even within the characteristically wide detwinning region of the HTSMA. The computationally efficient algorithm is shown to demonstrate each of the unique characteristics of Preisach minor loop hysteresis over the usable actuation range in high-stress, high-temperature applications.

A Study on the Requirements for Fast Active Turbine Tip Clearance Control Systems

This paper addresses the requirements of a control system for active turbine tip clearance control in a generic commercial turbofan engine through design and analysis. The control objective is to articulate the shroud in the high pressure turbine section in order to maintain a certain clearance set point given several possible engine transient events. The system must also exhibit reasonable robustness to modeling uncertainties and reasonable noise rejection properties. Two actuators were chosen to fulfill such a requirement, both of which possess different levels of technological readiness: electrohydraulic servovalves and piezoelectric stacks. Identification of design constraints, desired actuator parameters, and actuator limitations are addressed in depth; all of which are intimately tied with the hardware and controller design process. Analytical demonstrations of the performance and robustness characteristics of the two axisymmetric LQG clearance control systems are presented. Takeoff simulation results show that both actuators are capable of maintaining the clearance within acceptable bounds and demonstrate robustness to parameter uncertainty. The present model-based control strategy was employed to demonstrate the tradeoff between performance, control effort, and robustness and to implement optimal state estimation in a noisy engine environment with intent to eliminate ad hoc methods for designing reliable control systems.

System-Level Design of a Shape Memory Alloy Actuator for Active Clearance Control in the High-Pressure Turbine

*† ‡ This paper describes results of a numerical analysis evaluating the feasibility of hightemperature shape memory alloys (HTSMA) for active clearance control actuation in the high-pressure turbine section of a modern turbofan engine. The prototype actuator concept considered here consists of parallel HTSMA wires attached to the shroud that is located on the exterior of the turbine case. A transient model of an HTSMA actuator was used to evaluate active clearance control at various operating points in a test bed aircraft engine simulation. For the engine under consideration, each actuator must be designed to counteract loads from 380 to 2000 lbf and displace at least 0.033 inches. Design results show that an actuator comprised of 10 wires 2 inches in length is adequate for control at critical engine operating points and still exhibit acceptable failsafe operability and cycle life. A proportional-integral-derivative (PID) controller with integrator windup protection was implemented to control clearance amidst engine transients during a normal mission. Simulation results show that the control system exhibits minimal variability in clearance control performance across the operating envelope. The final actuator design is sufficiently small to fit within the limited space outside the high-pressure turbine case and is shown to consume only small amounts of bleed air to adequately regulate temperature.

Application of Diagnostic Analysis Tools to the Ares I Thrust Vector Control System

The NASA Ares I Crew Launch Vehicle is being designed to send astronauts into Earth orbit in support of missions to the International Space Station, to the moon, and beyond. The launch vehicle is an in-line two-stage rocket with the crew vehicle, Orion, on top of the stack. The Ares I is undergoing design and development utilizing commercial-off-the-shelf tools and hardware when applicable, along with cutting edge launch technologies and state-of-theart design and development techniques to ensure a safe, reliable, cost-effective space transportation system. In support of the vehicle’s design and development, the Ares Functional Fault Analysis group was tasked to develop an Ares Vehicle Diagnostic Model (AVDM) and to demonstrate the capability of that model to support failure-related analyses and design integration. The AVDM is a directed graph model of failure effect propagation paths within the vehicle architecture and is a comprehensive representation of the system’s failure space behavior. The AVDM is intended to support system engineering activities during the design process, and to provide diagnostic support throughout the development and deployment of the Ares I Launch Vehicle. During the Ares I design phase, the AVDM has been demonstrated to be valuable in the systems engineering process for assessing the completeness of schematics, and improving quality of various system design documents and analyses. The AVDM, along with supporting tools, has provided detection and fault isolation information to determine which components meet the diagnostic requirements for launch pad replacement and to assess system response to off-nominal conditions. One important component of the AVDM is the Upper Stage (US) Thrust Vector Control (TVC) diagnostic model—a representation of the failure space of the US TVC subsystem. This paper first presents an overview of the AVDM, its development approach, and the software used to implement the model and conduct diagnostic analysis. It then uses the US TVC diagnostic model to illustrate details of the development, implementation, analysis, and verification processes. Finally, the paper describes how the AVDM model can impact both design and

Preliminary Evaluation of an Active Clearance Control System Concept

*† ‡ § ¶ §§ Reducing blade tip clearances through active tip clearance control in the high pressure turbine can lead to significant reductions in emissions and specific fuel consumption as well as dramatic improvements in operating efficiency and increased service life. Current engines employ scheduled cooling of the outer case flanges to reduce high pressure turbine tip clearances during cruise conditions. These systems have relatively slow response and do not use clearance measurement, thereby forcing cold build clearances to set the minimum clearances at extreme operating conditions (e.g., takeoff, reburst) and not allowing cruise clearances to be minimized due to the possibility of throttle transients (e.g., step change in altitude). In an effort to improve upon current thermal methods, a first generation mechanically-actuated active clearance control (ACC) system has been designed and fabricated. The system utilizes independent actuators, a segmented shroud structure, and clearance measurement feedback to provide fast and precise active clearance control throughout engine operation. Ambient temperature performance tests of this first generation ACC system assessed individual seal component leakage rates and both static and dynamic overall system leakage rates. The ability of the nine electric stepper motors to control the position of the seal carriers in both open- and closed-loop control modes for single and multiple cycles was investigated. The ability of the system to follow simulated engine clearance transients in closed-loop mode showed the system was able to track clearances to within a tight tolerance (≤ 0.001 in. error).

Modeling the Dynamics of Supersonic Inlet/Gas-Turbine Engine Systems for Large- Amplitude High-Frequency Disturbances

This paper reviews the development of a simulation tool to predict the dynamic response of a coupled supersonic inlet/engine system to disturbances resulting in large-amplitude, high-frequency events. These events include inlet unstart and compressor surge and stall resulting from destabilizing conditions within the system. The motivation for development of this tool originated from the need to better understand the dynamic interaction of a mixed compression inlet and a gas turbine engine, and to predict system performance and operability for design studies. The tool consists of a ID Euler model of a supersonic inlet and a 2D Euler mean-line compression-system model coupled by means of a message-passing library. Two mixed compression inlet types were modeled, one was an axisymmetric inlet, and the other was a 2D inlet. Results, presented herein, show that the coupled system model is capable of predicting the occurrence of inlet unstart and compressor surge/stall resulting from a variety of disturbances. One of the strengths of this model is its ability to predict compressor surge and stall resulting from inlet circumferential distortion.

Extended Testability Analysis Tool User Guide

This paper provides an overview of the analysis and reporting capabilities of the Extended Testability Analysis (ETA) Tool which is currently being prepared for release by researchers at the NASA Glenn Research Center. The ETA Tool is a software tool that augments the analysis and reporting capabilities of a commercial-off-the-shelf (COTS) testability analysis software package. An initial diagnostic assessment is performed by the COTS software using a qualitative, directed-graph model of the system being analyzed. The testability analysis from the COTS software provides failure effect detection and fault isolation metrics, and generates a dependency matrix that correlates the system failure modes with the tests available to detect those failure modes. The ETA Tool accesses system design information captured within the COTS-based diagnostic model, along with testability analysis output from the COTS software, and creates a series of six reports for various system engineering needs. The ETA Tool also allows the user to perform additional studies on the testability analysis results, by determining the detection sensitivity to the loss of certain sensors or tests. The ETA Tool was developed to support the NASA Ares I Crew Launch Vehicle design and development. The Ares Functional Fault Analysis (FFA) group was tasked to develop a diagnostic model that would become part of the Ares ground-based diagnostic system. The FFA group selected the COTS software to build a collection of subsystem models. These models were developed independently, while adhering to a set of FFA project-defined modeling conventions. The subsystem models were subsequently integrated into a vehicle-level diagnostic model. The diagnostic models have proven to be valuable system engineering tools, providing consistency in the verification of system engineering requirements and of results from various design studies. In addition to being requested by the Upper Stage Thrust Vector Control Design Team for off-nominal analysis and analytical verification of recoverable fault requirements, analysis reports from the ETA Tool have been requested by several Ares System Engineering groups, including: Ascent Risk Analysis, Launch Commit Criteria and Ground Logistics and Supportability.

Propulsion Health Management System Development for Affordable and Reliable Operation of Space Exploration Systems

The constraints of future Exploration Missions will require unique integrated system health management capabilities throughout the mission. An ambitious launch schedule, human-rating requirements, long quiescent periods, limited human access for repair or replacement, and long communication delays, all require an integrated approach to health management that can span distinct, yet interdependent vehicle subsystems, anticipate failure states, provide autonomous remediation and support the Exploration Mission from beginning to end. Propulsion is a critical part of any space exploration mission, and monitoring the health of the propulsion system is an integral part of assuring mission safety and success. Health management is a somewhat ubiquitous technology that encompasses a large spectrum of physical components and logical processes. For this reason, it is essential to develop a systematic plan for propulsion health management system development. This paper provides a high-level perspective of propulsion health management systems, and describes a logical approach for the future planning and early development that are crucial to planned space exploration programs. It also presents an overall approach, or roadmap, for propulsion health management system development and a discussion of the associated roadblocks and challenges.

Systematic Sensor Selection Strategy: Development and Implementation Software Tool

This paper provides an overview of the Systematic Sensor Selection Strategy (S4) and introduces a software package that is being prepared for release to support development and implementation of S4 for user-defined applications. S4 was developed to identify an optimal subset of sensors from a larger pool of candidate sensors based on the inferred performance of the sensors in a diagnostic system. Selecting the proper sensors for an aerospace system is important for ensuring adequate measurement coverage that satisfies operational, maintenance, performance, and system diagnostic criteria. In particular, data acquired from the available sensor suite provides the foundation upon which any health management system is based, and directly impacts the diagnostic performance that can be achieved. A systematic approach is desired to perform sensor selection from a system-level perspective as opposed to performing decisions in an ad hoc or heuristic fashion. S4 was developed to meet this challenge. S4 identifies an optimal set of sensors for system fault diagnosis while taking conflicting constraints and objectives into consideration. S4 can be described as a general architecture structured to accommodate application-specific components and requirements. It identifies optimal sensor suite solutions by utilizing a user-defined merit function with a combinational optimization process. An S4 software package and user guide are currently under development to provide a tutorial for those wishing to implement the S4 approach. As an example application, the S4 software package includes a diagnostic system for an aircraft engine simulation called the Commercial Modular Aeropropulsion System Simulator. While the example application focuses on optimally selecting sensors based on their contribution to diagnostic performance, it could be easily adapted to support sensor optimization for operations, maintenance, parameter estimation, or other user-defined goals.

Functional Fault Model Development Process to Support Design Analysis and Operational Assessment

A functional fault model (FFM) is an abstract representation of the failure space of a given system. As such, it simulates the propagation of failure effects along paths between the origin of the system failure modes and points within the system capable of observing the failure effects. As a result, FFMs may be used to diagnose the presence of failures in the modeled system. FFMs necessarily contain a significant amount of information about the design, operations, and failure modes and effects. One of the important benefits of FFMs is that they may be qualitative, rather than quantitative and, as a result, may be implemented early in the design process when there is more potential to positively impact the system design. FFMs may therefore be developed and matured throughout the monitored systems design process and may subsequently be used to provide real-time diagnostic assessments that support system operations. This paper provides an overview of a generalized NASA process that is being used to develop and apply FFMs. FFM technology has been evolving for more than 25 years. The FFM development process presented in this paper was refined during NASAs Ares I, Space Launch System, and Ground Systems Development and Operations programs (i.e., from about 2007 to the present). Process refinement took place as new modeling, analysis, and verification tools were created to enhance FFM capabilities. In this paper, standard elements of a model development process (i.e., knowledge acquisition, conceptual design, implementation & verification, and application) are described within the context of FFMs. Further, newer tools and analytical capabilities that may benefit the broader systems engineering process are identified and briefly described. The discussion is intended as a high-level guide for future FFM modelers.