REG NASA-LLIS-0392-1995 Lessons Learned - Galileo AACS Excessive Memory Keep Alive (MKA) Current Draw.pdf

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1、Lessons Learned Entry: 0392Lesson Info:a71 Lesson Number: 0392a71 Lesson Date: 1995-02-24a71 Submitting Organization: JPLa71 Submitted by: J.C. MarrSubject: Galileo AACS Excessive Memory Keep Alive (MKA) Current Draw Abstract: Excessive Memory Keep Alive (MKA) current draw in the Galileo AACS was tr

2、aced to 41 sneak paths between the MKA power supply and the memorys unpowered interface power supply. Look for potential sneak paths through interfaces between powered and unpowered portions of a subsystem. Address these interfaces at CDR and in the ECR approval process.Description of Driving Event:

3、 Memory Keep Alive (MKA) current draw in excess of the maximum specification was observed in the Galileo Attitude and Articulation Control Subsystem (AACS) on the spacecraft in the Spacecraft Assembly Facility (SAF) prior to the scheduled 1986 launch. This occurred again during subsystem test of mod

4、ifications made to the AACS in preparation for the Galileo VEEGA Mission.Excessive MKA current draw had the potential of endangering the integrity of both AACS memories by pulling the MKA voltage below the minimum required for memory retention. It also had the potential for causing loss of contents

5、of both AACS memories by blowing the MKA supply fuses in the power subsystem.Investigation led to the discovery of forty-one (41) sneak paths between the 10 volt MKA supply and the memorys unpowered 5 volt TTL interface supply. These sneak paths resulted in the 5 volt supply voltage being raised suf

6、ficiently to partially turn on many of the forty-one 78L12 interface drivers such that they were in an intermediate logic state, allowing each to draw as much as 30 milliamps of current. This provided the potential to draw as much as three times the maximum specified AACS MKA current.Reference(s): P

7、FR # 45982 and #50424.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Lesson(s) Learned: Sneak paths between subsystems or between powered and unpowered portions of a subsystem can result in excessive current draw.Recommendation(s): 1. The cognizant

8、engineer must pay particularly careful attention to interfaces between powered and unpowered portions of his subsystem or between his subsystem and other subsystems to allow early detection and correction of potential sneak paths. These interfaces should be singled out for special attention during t

9、he design and development of all subsystems.2. Interfaces between powered and unpowered portions of subsystem circuitry should be specifically addressed at Critical Design Reviews (CDR) and in the Engineering Change Request (ECR) approval process.Evidence of Recurrence Control Effectiveness: N/ADocu

10、ments Related to Lesson: N/AMission Directorate(s): N/AAdditional Key Phrase(s): a71 Flight Equipmenta71 Hardwarea71 Safety & Mission Assurancea71 SpacecraftAdditional Info: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 1988-10-20a71 Approval Name: Carol Dumaina71 Approval Organization: 125-204a71 Approval Phone Number: 818-354-8242Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-