Closed loop software control of the MIDEX power system

MIDEX (Medium Class Explorer) is NASA´s medium class series of scientific spacecraft. The MIDEX power system was designed to be adaptable to multiple missions, and will be used on the Microwave Anisotropy Probe (MAP), Earth Orbiter-1 and with modifications, on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. The power system design achieves its flexibility in part by containing mission unique modifications in software and firmware. Historically at Goddard Space Flight Center, the power system control loop was closed with analog hardware. In the MIDEX power system design, the outer system control loop is implemented with microprocessor based software. While the ease of adjustability in software allows the design to accommodate Low Earth Orbit (LEO), GEO (Geosynchronous Orbit) or libration point full sun missions, as well as any solar cell or battery technology, digital control poses a number of challenges. This paper describes the performance of the system´s inner analog and outer digital control loops. A number of challenges were encountered in implementing Goddard´s first spacecraft to use a digital power system control. These challenges and the solutions are addressed