KEYWORDS: Systems modeling, Systems engineering, Thirty Meter Telescope, Monte Carlo methods, Error analysis, Observatories, Reverse modeling, Model-based design, Adaptive optics, Wavefronts
The OpenSE Cookbook is an open-sourced collection of patterns, procedures, and best practices targeted for systems engineers who seek guidance on applying model-based and executable systems engineering (MBSE) using SysML. Its content has emerged from the system level modeling effort on the European Framework Program 6 (FP6) and the Thirty Meter Telescope (TMT). The TMT MBSE approach applied the Executable Systems Engineering Method (ESEM) and the open-source Engineering Environment (OpenMBEE) to specify, analyze, and verify requirements of TMT’s Alignment and Phasing System (APS) and the Narrow Field Infrared Adaptive Optics System (NFIRAOS). In these applications, implicit dependencies are made explicit in a formal model through the use of ESEM, OpenMBEE, and SysML modeling constructs. The value proposition for applying this MBSE approach was to establish precise requirements and fine-grained traceability to system designs, and to verify key requirements beginning early in development. The integration of ESEM and the OpenMBEE tooling infrastructure (providing linked-data and web-operability) is a significant added value for the MBSE approach. The APS is responsible for the overall pre-adaptive optics wavefront quality, using starlight to measure wavefront errors and align the TMT optics. In the formally integrated and executable SysML model, simulations are performed to analyze the impact of changed requirements and verify specified constraints for various operational scenarios.
The APS team used several modeling patterns to capture information such as the requirements, the operational scenarios, involved subsystems and their interaction points, the estimated or required time durations, and the mass and power consumption. Adaptive optics systems are designed to sense real-time atmospheric turbulence and correct the telescope’s optical beam to remove its effect. The system model for the adaptive optics operational modes was developed to capture sequence behaviors and operational scenarios to run Monte-Carlo simulations for verifying acquisition time, observing efficiency, and operational behavior requirements. The model is particularly useful for investigating the effect of parallelization, identifying interface issues, and re-ordering sequence acquisition tasks. A former version of the Cookbook (which is now updated to MBSE challenges, goals, and lessons learned) included modeling guidelines and conventions for all system aspects, hierarchy levels, and views, which were developed during for the Active Phasing Experiment (APE), an opto-mechatronical system technology demonstrator for the Extremely Large Telescope (ELT). The Cookbook utilizes the above mentioned system models as real-world case-studies to demonstrate and document the applications of the recipes, providing also instructional examples and addressing the available tooling support. The Cookbook is accompanied by a number of SysML models and aodel libraries which facilitate model authoring and maintenance. The Cookbook covers the different aspects of Systems Engineering such as management of Requirements, Design (behavior and structure), Interfaces, Interdisciplinary Integration, Analysis, Trade Studies, and Technical Resources. This paper presents the background, motivation, architecture, and highlights some key content of the Cookbook. For example, interface management, error budget management, requirements verification, Monte Carlo driven analysis, and timing analysis of operational scenarios. The paper discusses how the capabilities of OpenMBEE contributed significantly to the adoption of executable systems engineering.
The Extremely Large Telescope[1] (ELT) is a 39 meters optical telescope under construction at an altitude of about 3000m in the Chilean Atacama desert. The optical design is based on a novel five-mirror scheme and incorporates adaptive optics mirrors. The primary mirror consists of 798 segments, each 1.4 meters wide. The architecture of the control system[2] is split in layers and in a high number of subsystems/components developed by different parties. This implies a high number of interfaces that must be designed and maintained under configuration control, to ensure a flawless integration of the different parts. Having interfaces (and data) definitions in a flexible central place allows us to extract several different artifacts (for example Interface Control Documents (ICDs), Interface Definition Language (IDL) files, tabular spreadsheets, help files, other generated code formats like code stubs or state machine implementations). In this paper, we explain how selecting a graphical modeling language like SysML and using graphical and tabular editing features made available by state of the art modeling tools presents a number of advantages with respect to other solutions like spreadsheets, a relational database, or a custom textual DSL. Still, using standard export/import formats (EMF XMI), we do not bind ourselves to a specific vendor. We describe the workflow that we have identified for the definition of interfaces, what artifacts we want to automatically produce and why. We also describe what technologies we are using to reach these objectives. A key aspect of this work is the selection of interface design patterns that are formal enough to allow automatic generation of the artifacts and, at the same time, pragmatic and simple to gain acceptance from all users and not incur in overhead.
KEYWORDS: Systems modeling, Systems engineering, Control systems, Telescopes, Control systems design, Astronomy, Instrument modeling, Interfaces, Wavefronts, Visual process modeling
Model Based Systems Engineering (MBSE) is an emerging field of systems engineering for which the System Modeling Language (SysML) is a key enabler for descriptive, prescriptive and predictive models. This paper surveys some of the capabilities, expectations and peculiarities of tools-assisted MBSE experienced in real-life astronomical projects. The examples range in depth and scope across a wide spectrum of applications (for example documentation, requirements, analysis, trade studies) and purposes (addressing a particular development need, or accompanying a project throughout many - if not all - its lifecycle phases, fostering reuse and minimizing ambiguity). From the beginnings of the Active Phasing Experiment, through VLT instrumentation, VLTI infrastructure, Telescope Control System for the E-ELT, until Wavefront Control for the E-ELT, we show how stepwise refinements of tools, processes and methods have provided tangible benefits to customary system engineering activities like requirement flow-down, design trade studies, interfaces definition, and validation, by means of a variety of approaches (like Model Checking, Simulation, Model Transformation) and methodologies (like OOSEM, State Analysis)
ESO is currently in the final phase of the standardization process for PC-based Programmable Logical Controllers (PLCs) as the new platform for the development of control systems for future VLT/VLTI instruments. The standard solution used until now consists of a Local Control Unit (LCU), a VME-based system having a CPU and commercial and proprietary boards. This system includes several layers of software and many thousands of lines of code developed and maintained in house. LCUs have been used for several years as the interface to control instrument functions but now are being replaced by commercial off-the-shelf (COTS) systems based on BECKHOFF Embedded PCs and the EtherCAT fieldbus. ESO is working on the completion of the software framework that enables a seamless integration into the VLT control system in order to be ready to support upcoming instruments like ESPRESSO and ERIS, that will be the first fully VLT compliant instruments using the new standard. The technology evaluation and standardization process has been a long and combined effort of various engineering disciplines like electronics, control and software, working together to define a solution that meets the requirements and minimizes the impact on the observatory operations and maintenance. This paper presents the challenges of the standardization process and the steps involved in such a change. It provides a technical overview of how industrial standards like EtherCAT, OPC-UA, PLCOpen MC and TwinCAT can be used to replace LCU features in various areas like software engineering and programming languages, motion control, time synchronization and astronomical tracking.
KEYWORDS: Systems modeling, Control systems, Telescopes, Systems engineering, Safety, Performance modeling, Sensors, Reverse modeling, Large telescopes, Mirrors
Large telescopes pose a continuous challenge to systems engineering due to their complexity in terms of requirements,
operational modes, long duty lifetime, interfaces and number of components. A multitude of decisions must be taken
throughout the life cycle of a new system, and a prime means of coping with complexity and uncertainty is using models
as one decision aid. The potential of descriptive models based on the OMG Systems Modeling Language (OMG
SysMLTM) is examined in different areas: building a comprehensive model serves as the basis for subsequent activities of
soliciting and review for requirements, analysis and design alike. Furthermore a model is an effective communication
instrument against misinterpretation pitfalls which are typical of cross disciplinary activities when using natural language
only or free-format diagrams. Modeling the essential characteristics of the system, like interfaces, system structure and
its behavior, are important system level issues which are addressed. Also shown is how to use a model as an analysis tool
to describe the relationships among disturbances, opto-mechanical effects and control decisions and to refine the control
use cases. Considerations on the scalability of the model structure and organization, its impact on the development
process, the relation to document-centric structures, style and usage guidelines and the required tool chain are presented.
The attention to proper requirement analysis and maintenance is growing in modern astronomical undertakings. The increasing degree of complexity that current and future generations of projects have reached requires substantial system engineering efforts and the usage of all available technology to keep project development under control. One such technology is a tool which helps managing relationships between deliverables at various development stages,
and across functional subsystems and disciplines as different as software, mechanics, optics and electronics. The immediate benefits are traceability and the possibility to do impact analysis. An industrially proven tool for requirements management is presented together with the first results across some projects at ESO and a cost/benefit analysis of its usage. Experience gathered so far shows that the extensibility and configurability of the tool from one hand, and integration with common documentation formats and standards on the other, make it appear as a promising solution for even small scale system development.
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