ISS Double-Gimbaled CMG Subsystem Design using the Agile Development Method

Ravi Inampudi
2016 AIAA Modeling and Simulation Technologies Conference   unpublished
This paper presents an evolutionary approach in simulating a cluster of 4 Control Moment Gyros (CMG) on the International Space Station (ISS) using a common sense approach (the agile development method) for concurrent mathematical modeling and simulation of the CMG subsystem. This simulation is part of Training systems for the 21st Century simulator which will provide training for crew members, instructors, and flight controllers. The basic idea of how the CMGs on the space station are used for
more » ... tation are used for its non-propulsive attitude control is briefly explained to set up the context for simulating a CMG subsystem. Next different reference frames and the detailed equations of motion (EOM) for multiple double-gimbal variable-speed control moment gyroscopes (DGVs) are presented. Fixing some of the terms in the EOM becomes the special case EOM for ISS's double-gimbaled fixed speed CMGs. CMG simulation development using the agile development method is presented in which customer's requirements and solutions evolve through iterative analysis, design, coding, unit testing and acceptance testing. At the end of the iteration a set of features implemented in that iteration are demonstrated to the flight controllers thus creating a short feedback loop and helping in creating adaptive development cycles. The unified modeling language (UML) tool is used in illustrating the user stories, class designs and sequence diagrams. This incremental development approach of mathematical modeling and simulating the CMG subsystem involved the development team and the customer early on, thus improving the quality of the working CMG system in each iteration and helping the team to accurately predict the cost, schedule and delivery of the software. * Lead Software Engineer, Training systems for the 21st Century (TS21), Lockheed Martin, Houston, Texas, 77058, AIAA Member 1 OF 18 AMERICAN INSTITUTE OF AERONAUTICS AND ASTRONAUTICS all Guidance, Navigation and Control (GNC), including propulsive and CMG attitude control. The ADCO works in partnership with Russian controllers to manage the station's orientation, controlled by the on-board Motion Control Systems. They also plan and calculate future orientations and maneuvers for the station. 2 The ISS GNC subsystem for TS21 has two sensor models and an actuator model. The sensors are the Rate-Gyro Assembly (RGA) and the Space Integrated Global Positioning System/Inertial Navigation System (SIGI). The SIGI is broken down into a GPS model and an accelerometer model. The actuator model needed for ISS attitude control is a cluster of four CMGs. The Russian segment (RS) is considered an external model and interacts with the TS21 GNC models through the 1553 telemetry buses. The CMGs operate as momentum storage devices that exchange momentum with the ISS, through induced gyroscopic torques. The CMGs are non-propulsive actuators that provide continuous attitude control in the microgravity environment. Control moment gyros create moments through gimbaling of constant rotation rate momentum wheels. Each wheel is gimbaled to the commanded pointing direction using inner and outer gimbals. The gimbaling of all four wheels is done symmetrically, until all are aligned, at which point desaturation is required to reinitialize the CMGs to their initial state. A minimum of two CMGs are required for attitude control. The magnitude of the induced torque is proportional to the rate of change of the direction of angular momentum vector. 4 TS21 goal for building ISS simulation is to create a working, reliable software incrementally that is flexible, maintainable and reusable. As TS21 is a developmental and non-sequential project with multiple subsystems, agile development process is used to quickly and reliably develop the CMG subsystem simulation. This natural process also facilitates to concurrently work with engineers, managers and the customer. Agile development is a set of software development methods in which requirements and solutions evolve through collaboration between self-organizing, cross-functional teams. It promotes adaptive planning, evolutionary development, early delivery, continuous improvement, and encourages rapid and flexible response to change. 5, 6 A few ways of developing better software using the agile method are 1. Have a self-organized and motivated team. 2. Working software is a more useful measure. 3. Involve customer feedback on a regular and frequent basis. 4. Respond to change and continuous development. The basic idea is to start with something simple and small that works, and build the software incrementally by implementing a set of high priority features in a set period of time. For instance, pick a simple feature, describe the feature in a sentence, understand the underlying model (business, engineering or mathematical), and write short code to implement the feature. To write production code and test it at the same time, one widely used practice in agile development methodologies is the test driven development (TDD) strategy. The key rule of TDD is summarized as "test twice, code once," which refers to this three-step procedure involved in any code change: 7 1. Write a test of the new code and see it fail. 2. Write the new code, which does the simplest thing that could possibly work. 3. Verify that the test succeed, and re-factor the code. These three basic steps form the TDD cycle. The production code and the test cases independently evolve together and this practice decouples the test cases as well as the production code modules from each other. The key element of TDD are the unit test frameworks and such frameworks can contribute to every stage of software design, development, testing and debugging. Furthermore, the principles of object-oriented design play an important and powerful role in writing flexible (decoupled), testable, maintainable and reusable software. This paper presents the mathematical modeling, software design and implementation of a cluster of 4 control moment gyros (CMG) on the space station (ISS). The introduction section briefly explained the CMG subsystem as well as the agile development method. How the actual CMGs on the space station are used is then briefly explained to set up the context for simulating a CMG simulation. Next, different refer-
doi:10.2514/6.2016-1433 fatcat:kdmq7bw635fhjmhythhoxuo2hy