Conceptual design of sacrificial sub-systems: failure flow decision functions

Abstract

This paper presents a method to conceptually model sacrificing non-critical sub-systems, or components, in a failure scenario to protect critical system functionality through a functional failure modeling technique. Understanding the potential benefits and drawbacks of choosing how a failure is directed in a system away from critical sub-systems and toward sub-systems that can be sacrificed to maintain core functionality can help system designers to design systems that are more likely to complete primary mission objectives despite failure events. Functional modeling techniques are often used during the early stage of conceptual design for complex systems to provide a better understanding of system architecture. A family of methods exists that focuses on the modeling of failure initiation and propagation within a functional model of a system. Modeling failure flow provides an opportunity to understand system failure propagation and inform system design iteration for improved survivability and robustness. Currently, the ability to model failure flow decision-making is missing from the family of function failure and flow methodologies. The failure flow decision function (FFDF) methodology presented in this paper enables system designers to model failure flow decision-making problems where functions and flows that are critical to system operation are protected through the sacrifice of less critical functions and flow exports. The sacrifice of less critical system functions and flows allows for mission critical functionality to be preserved, leading to a higher rate of mission objective completion. An example of FFDF application in a physical design is a non-critical peripheral piece of electrical hardware being sacrificed during an electrical surge condition to protect critical electronics necessary for the core functionality of the system. In this paper, a case study of the FFDF method is presented based on a Sojourner class Mars Exploration Rover (MER) platform.

Appendices

Appendix 1

Appendix 2

Flow type	Probability of passing failure downstream	Probability of passing failure upstream
Collectable energy	0.10	0.00
Electrical energy	0.40	0.02
Digital signal	0.50	0.02
Control signal	0.50	0.02
Positional information	0.47	0.00
Visual information	0.47	0.00
Rotational work	0.50	0.15
Translational work	0.50	0.15
Alignment work	0.25	0.15

Function	Probability of accepting failure flow
Accumulate energy	0.50
Store energy	0.12
Distribute electrical	0.24
Control magnitude electrical	0.20
Convert electrical to rotation	0.16
Transmit rotation	0.16
Convert rotation to translation	0.16
Direct command	0.44
Process signal	0.01
Store data	0.01
Record position	0.22
Record visual	0.22
Transmit data	0.44

Appendix 3

Index	Flow type
f1	Collectable energy
f2	Electrical energy
f3	Digital signal
f4	Position information
f5	Visual information
f6	Rotational work
f7	Translation work
f8	Steering work

Index	Function type	Index	Function type
1	Operating environment	26	Convert electric-to-rotation 7
2	Accumulate energy 1	27	Convert electric-to-rotation 8
3	Accumulate energy 2	28	Convert electric-to-rotation 9
4	Accumulate energy 3	29	Convert electric-to-rotation 10
5	Accumulate energy 4	30	Transmit rotation 1
6	Accumulate energy 5	31	Transmit rotation 2
7	Accumulate energy 6	32	Transmit rotation 3
8	Accumulate energy 7	33	Transmit rotation 4
9	Accumulate energy 8	34	Convert rotation-to-translation 1
10	Accumulate energy 9	35	Convert Rotation-to-Translation 2
11	Accumulate energy 10	36	Convert rotation-to-translation 3
12	Accumulate energy 11	37	Convert rotation-to-translation 4
13	Accumulate energy 12	38	Convert rotation-to-translation 5
14	Accumulate energy 13	39	Convert rotation-to-translation 6
15	Store energy 1	40	Direct command
16	Store energy 2	41	Process signal
17	Store energy 3	42	Process signal (digital)
18	Distribute electricity	43	Store data 1
19	Control magnitude electrical	44	Store data 2
20	Convert electric-to-rotation 1	45	Store data 3
21	Convert electric-to-rotation 2	46	Record position
22	Convert electric-to-rotation 3	47	Record visual 1
23	Convert electric-to-rotation 4	48	Record visual 2
24	Convert electric-to-rotation 5	49	Record visual 3
25	Convert electric-to-rotation 6	50	Transmit data (analogue)

Appendix 4