Fault Tree Analysis of the HERMES CubeSat Felix Bidner

What is FTA? A form of failure analysis / safety engineering Takes the form of a logic block diagram Brief history - Developed in 1960’s at Bell Laboratories - Adopted by aerospace industry HERMES

What is FTA? Deductive Approach - Define an undesired state/event in the system and deduce what could lead to that state Component in B fails Subsystem B fails System A fails Analysis begins at the undesired state HERMES

What FTA Is Not Not meant to identify all possible failure modes of the system Only describes the logic leading to a defined top event Often complemented by inductive analysis such as Failure Modes and Effects Analysis (FMEA) Not inherently quantitative but can be adapted to be so If individual event probabilities are known, the probability of the top event can be determined HERMES

Why Use It? Identify relationships between subsystems Determine weaknesses in the system Prioritize weaknesses by their severity Prevention & Diagnostics HERMES

Fault Tree Symbols Events Basic event Undeveloped Event HERMES

Fault Tree Symbols Logic Gates Priority AND AND Exclusive OR OR Voting OR AND OR Transfer Symbol - HERMES

Sample Fault Tree Construction HERMES

Building a Fault Tree Identify a top event Assign boundaries to the analysis Scope Level of Detail Perform FTA HERMES

Applying FTA to the HERMES CubeSat

HERMES Overview Standard 1U CubeSat as specified by California Polytechnic State University 1 kg, 10 x 10 x 10 cm Inexpensive to construct and launch Primary objective is to test high-speed communications in S-band HERMES

Divide into Subsystems ADCS Attitude Determination & Control CDH Command & Data Handling HSCOM High Speed Communications EPS Power PCOM Primary Communications Structures & Thermal HERMES

First Iteration of FTA One top event chosen for each subsystem Rough outlines of fault trees developed for each top event What components depend on multiple subsystems? Antenna Software Solar Panels Structures & Thermal HERMES

Further Modifications How can we detect events from the fault tree? Check appropriate sensors for anomalous readings Cross-reference with Telemetry Dictionary Single points of failure A fault event such that the entire system may fail Denoted by Y/N Sensor Subsystem Minimum Value (Raw) Maximum Value (Raw) Average (Raw) MAGX ADCS 1320 1400 1343 MAGY 1468 1546 1486 MAGZ 1222 1293 1241 MAGREF 1317 1365 1333 SOLAR_CURRENT_XP EPS 4009 1928 SOLAR_CURRENT_XM 658 3967 1944 SOLAR_CURRENT_YP 1156 4032 3993 SOLAR_CURRENT_YM 1943 3855 1981 SOLAR_CURRENT_ZP 433 3598 2926 SOLAR_CURRENT_ZM 369 3694 1931 HERMES

EPS Fault Tree HERMES

Antenna Fault Tree HERMES

Solar Panels Fault Tree HERMES

Putting the FTA to Use Monitoring satellite status Diagnostic Tool Useful for monitoring time-dependent events (i.e. battery charge) Allows mission operators to foresee possible failures and act accordingly Diagnostic Tool If a failure event does occur, its causes may be identified through the FTA HERMES

References & Acknowledgments Stamatelatos, Michael, and William Vesely. Fault Tree Handbook with Aerospace Applications. NASA, Aug. 2002. Web. 29 Mar. 2010. <http://www.hq.nasa.gov/office/codeq/doctree/fthb.pdf>.  Long, Allen R. Beauty and the Beast - Use and Abuse of the Fault Tree as a Tool. Tech. Fault-tree.net. Web. 29 Mar. 2010. <http://www.fault- tree.net/papers/long-beauty-and-beast.pdf>. "Fault Tree Analysis (FTA, System Analysis) Basics." Reliability Engineering, Reliability Theory and Reliability Data Analysis and Modeling Resources for Reliability Engineers. Weibull.com. Web. 29 Mar. 2010. Thanks to Trevor Schlieper and Katie Brissenden for their contributions to the FTA. HERMES

CDH Fault Tree HERMES

HSCOM Fault Tree HERMES

PCOM Fault Tree HERMES

ADCS Fault Tree HERMES

Structural/Thermal Fault Tree HERMES