Problem Statement

The major challenge for SysML is to add value for systems engineering of interdisciplinary projects. For the demonstration of the feasibility of using SysML during the systems engineering process of real-world complex systems, such a system shall be modeled. The Active Phasing Experiment (APE), a project of the ESO, is chosen as a case study which fulfills these aspects.

Object of Case Study

The Active Phasing Experiment (APE) represents a technology evaluation breadboard for large telescopes. The essential purpose of the APE experiment is to explore, integrate, and validate active wave front control schemes and technologies for a European Giant Optical Telescope (EGOT). This includes the evaluation and comparison of the performance of different types of wave front sensors in the laboratory and on the sky on the one hand, and the integration of the control of a segmented aperture control into an already existing active system and driving both the active system and the control of the segments on the other hand. APE is close to completion and deployment in an operational environment. APE will be deployed in the lab, standalone, but also in an already existing telescope.

Why using SysML for APE?

APE, as any complex system, has a large number of functional, performance, physical and interface requirements which have to be satisfied. This implies the need for a professional requirements engineering and management during the project. This is the first application of SysML during the development.

APE has about 50 high-level system requirements. The control system has also about 50 requirements, complemented by 150 Use Cases.

APE consists of various elements, like wheels, translation stages, lenses, detectors, (segmented) mirrors, light sources, an interferometer, sensors and actuators (19 small axes, 10 TCCDs, 11 other devices, 183 actuators for segmented mirror). The control system alone consists of 12 computing nodes. These elements offer all kinds of optical, mechanical, electronic and software interfaces, both system internal and external to other systems. Their management alone is very challenging for the systems engineering team. Besides these challenges, which apply for many complex systems, APE has some other aspects:

The most noticeable challenge of APE is the highly demanding optical layout, which is a unique challenge for every optical system. The highly sensitive system requires a consistent coordinate system of the various parts to ensure a correct optical path. Apart from this it also challenges the control, since there are several open and closed loop systems required. A significant amount of data is produced by image processing data flows. Since APE will be deployed in the lab and in an already existing telescope, slightly different functional aspects are active depending on the deployment mode. Therefore different interfaces to existing systems are needed.

We believe that SysML will contribute significantly to tackle these challenges of APE. Another fact about the nature of the project seems important: Being a collaboration of institutes which are geographically distributed, this poses a further challenge on the systems engineering discipline.