Stability Testing and Analysis of a PMAD DC Test Bed for the Space Station Freedom
ABSTRACT The Power Management and Distribution (PMAD) DC Test Bed at the NASA Lewis Research Center is introduced. Its usefulness to the Space Station Freedom Electrical Power System (EPS) development and design are discussed in context of verifying system stability. Stability criteria developed by Middlebrook and Cuk are discussed as they apply to constant power DC to DC converters exhibiting negative input impedance at low frequencies. The utility-type Secondary Subsystem is presented and each component is described. The instrumentation used to measure input and output impedance under load is defined. Test results obtained from input and output impedance measurements of test bed components are presented. It is shown that the PMAD DC Test Bed Secondary Subsystem meets the Middlebrook stability criterion for certain loading conditions. INTRODUCTION The Space Station Freedom (SSF) Electrical Power System (EPS) is a multi-kilowatt DC power system. The eventual goal is to provide 56kW (75kW peak) of power through six (6) 9.3kW (12.5kW peak) channels. Each channel is comprised of a source subsystem, a 16OVdc (nominal) primary distribution system, and a 120Vdc secondary distribution system. The source subsystem generates and regulates the 16OVdc primary power. The primary distribution power is converted to 120Vdc by DC to DC Converters (DDCUs). The secondary distribution system then distributes the 120Vdc power to the SSF users. A Power Management and Distribution (PMAD) DC Test Bed has been developed at the NASA Lewis Research Center to provide early component and systems level data to support the development of flight hardware and EPS design. The PMAD DC Test Bed consists of breadboard hardware, configured as a single EPS channel, from power sources to user loads[l]. The Space Station Freedom Electrical Power System is unique in many ways. It will be the first space-based power system that will not have clearly defined loading conditions. Over the life of the power system hundreds
' .... " O_ erview and Evolutiofiof the LeRC PMAD DC Testbed
Sincet_ beginning of the Space Station FreedomProgram (SSFP),theLewis Research Center (LcRC) has bccn activelyinvolvedin thedevelopment of electrical powersystemtestbedstosupportofthe overalldesign effort. Throughoutthistime, the SSFP Programhas changed thedesignbaselinenumerous times,however, the testbed efforthasendeavored to trackthcsc changes. Beginning in August1989with thebaseliningof an all DC System,a testbed was dcvclopedwhich supportedthisdesignbaseline. However, about the time of the Test Bed's ComplctioninDecember 1990, the SSFP was again going throughanother design scrub known as Restructure. This paper describes the LeRC PMAD DC Test Bed and highlights the changes that have taken place in the Test Bed configuration and design resulting from the SSFP Restructure Exercise in December 1990. These changes have principally included the reduction of primary power channel size with an accompanying reduction in the size of various power processing elements. In addition to the scrubbing of the channel size, a substantial reduction was made in the amount of flight software with the subsequent migration of these functions to ground control centers. The impact of these changes on the design of the power hardware, the controller algorithms and the control software along with a description of their current status is presented. An overview of the testing that has been conducted with the test bed during the last year is also highlighted. These tests include investigations of stability and source impedance, primary and secondary fault protection, and performance of a rotary utility transfer device. Finally, information is presented on the evolution of the test bed to support the verification and operational phases of the Space Station Freedom Program in light of these restructure scrubs.
New approach to vision-based BGA package inspection
Ball grid array (BGA) techniques have developed very fast due to the advantages of more I/O ports, small size, and so on. In this paper, a vision-based algorithm for BGA package inspection is proposed to accurately estimate center and diameters of solder-balls of BGA packages, so that defects of BGA packages, such as missing or extra balls, oversize or undersize balls, over-height or under-height balls, and pitch can be detected for ensuring the quality of BGA packages.