IoT Network Quality/Reliability

Transcription

1 IoT Network Quality/Reliability IEEE PHM June 19, 2017 Byung K. Yi, Dr. of Sci. Executive V.P. & CTO, InterDigital Communications, Inc Louis Kerofsky, PhD. Director of Partner Development InterDigital Communications, Inc.

2 Purpose: Provide information and directions for IoT Network Developer which would help him/her to put Engineering Quality and Reliability concepts at the early stage of IoT system development. It starts with fundamentals of Reliability Theory To the end, it provides design goals and guidelines for those, performance, safety, maintenance and cost factors which together form the working elements of reliability engineering, system engineering, quality engineering, and most importantly cost effectiveness. An effective quality/reliability engineering program begins with recognition that the Quality/Reliability is a function of the design parameters as well as a life cycle cost. 2

3 Reliability: Quality in the time domain Classical definition: The probability than a device will perform the intended functions satisfactorily for a specific period of time under the stated set of use condition In order for an item to be reliable, it must do more that meet an initial factory performance or quality specification, it must also operate satisfactorily for an acceptable period of time in the field for which it is intended. Reliability elements: probability, performance requirements, time and use conditions. Def: The Reliability of a component at time t, Say R(t), is defined as R(t)=P[T>t], where T is life length of the component R is reliability function 3

4 Reliability: Quality in the time domain Equally, the probability that that the component dose not fail during the interval [0,t] In terms of the pdf of T, say f, we have R t = t ft s ds In terms of cdf of T R(t) = 1 P T t = 1 F t Def: The failure Rate z(t) (sometimes called hazard fn): z t = f(t) = f(t) 1 F(t) R(t), defined for F(t)<1 Interpret z(t) is the conditional probability P(t T t + t T > t) 4

5 Reliability: Quality in the time domain P(t T t + t T > t): Probability that item will fail during the next t time units given the item is functioning at time t, P t T t + t T t Where t ξ t + t = P(t T t+ t) P(T t) = t+ t t f s ds R(t) = t f(ξ) R(t) tf(ξ)/r(t) It represents the proportion of the items that will fail between t ξ t + t. 5

6 Terminology: Probability: Quantitative term which expresses the likelihood of an event s occurrence (or non occurrence) as a value between 0 and 1 Performance: Criteria which is clearly describe or define what is considered to the satisfactory operation Time: The measure of period during which one can expect satisfactory performance Use condition: The environmental conditions which one expects an item to function. 6

7 Which Rolex has better Quality/Reliability? 7

8 Life Characteristic Curve Failure Rate(λ) Tb Tw Increasing Age(Hours/ Cycles) 8

9 Stress vs Strength plots 9

10 The Weibull Distribution f X ( x) x 1 x exp x 0 : the shape parameter : the scale parameter 0, , 1 : Exponential Distribution 2 : Rayleigh Distribution 2.5 : Approximately Lognormal Distribution 3.6 : Approximately Normal Distribution 10

11 I. Infant Mortality period An initial development stage exhibits a high failure rate ( infant Mortality, burn-in or debugging period: Product Conformance Test, ATP, Integration Testing ) Tb Should be coincident with a product launch and the failure rate at time Tb should be a design element e.g.: Manufacturability 11

12 II. Useful Life Period The device population reaches its lowest failure rate level, which is marked by a constant failure rate(exponential failure distribution) The period between Tb and Tw is the most significant period for Quality/ Reliability prediction and assessment activities 12

13 III. Wear-out Period The final life period occurs when the item population reaches the point where the failure rate starts to increase noticeably (Tw) Deterioration of the design strength of device as a consequence operation and exposure to environment stresses. ex.: flex cable, RF system including antenna, display, battery life, and cracking plastics 13

14 Exponential failure rate R(t) = e λt : reliability (probability of success) R(t) = is the probability that the item will operate without failure for the time period, t λ: is the device failure rate MTBF: =1/λ 0 = t E e λt dt = 1 λ f(t): Exponential Distribution 0 = t E 0 = dt t f t λt tλe λt dt=t e e λt dt =1/λ 14

15 Exponential Model derived [1/3] N o : devices are tested N s : devices survive the test after time t N F : devices failed the test after time t R t R t dr t dt = N S N O = = N O N F N O = 1 dn F N O dt N S N S +N F = 1 F t = f i t f i t = failure density function, i.e. The probability that a failure will occur in the next time increment dt 15

16 Exponential Model derived [2/3] In general, it can be assumed the hazard rate of a device remain constant over practical intervals of time z i t = λ i = f i t R i t = dr i t dt t R i MBTF = න 0 R i t dt = න e λit dt 0 dr i t dt + λr i t = 0 MBTF = 1 λ i e λ it 0 R i t = e λ it MBTF = 1 λ i 16

17 Exponential Model derived [3/3] The hazard rate z(t) is defined as the ratio of functional failure rate to the fractional surviving quantity, that is, number of the original population still operation at time t or simply the conditional probability of failure N s : devices survive the test after time t z t = f(t) R(t) = f(t) λe λt z t = t 1 F(t) 1 0 λe λs ds z t = f(t) 1 0 t f(s) ds z t = λe λt 1 + e λs 0 t f t = λe λt z t = λe λt 1+e λt 1 = λ 17

18 Serial equipment configuration Reliability of the series configuration is the product of the reliabilities of the individual: R s (t)=r 1 (t).r 2 (t)..r n (t) 18 R 1 (t) R 2 (t) R n (t) n i i T MTBF ] exp[ ) ( 1 ) ( n i i t t t t n i t t e e e e e t R n n i

19 Design to cost The meaning of design to cost is: Maximize performance within unit cost goals and Minimize CRM cost to minimize life cycle costs 19

20 Noritak: Kano s model Relationship between customer satisfaction and quality 20

21 Basic level of quality The quality that customers assume the product would have. They take for granted. e.g.: Voice Quality.; RF Performance, sms/mms expected level of quality: The expected quality that customers explicitly consider. e.g.: Battery life, image quality Exciting level of Quality The exciting level of quality comes from innovation the customer receives more than they expected e.g.: Touch screen To be a leader in a industry, constant innovations are required 21

22 Quality Function Development (QFD)s [1/2] Benchmarking involves research into the best practices at the process level. Benchmarking should never be the primary strategy for improvement Competitive analysis is an approach to goal setting or industry standard. Competitive analysis virtually guarantees second rate quality. e.g. Apple i-phone, mechanical watch Long term quality goal setting is turned out to be not to set them at all or ad hoc approach e.g. Crisis management 22

23 Quality Function Development (QFD)s [2/2] QFD is a customer driven process for product planning, development and deploy 4 phases of QFD Organization phase Descriptive phase Breakthrough phase Implementation Phase 23

24 Conclusions: How to design a Reliable IoT Physical System System with Inherent Graceful Degradation feature One particular data point could be interpolated thru other points in the group (Special Redundancy) D1 D3 D2 Gateway G1 Let s assume if 2 out of three worked, we consider Group G1 is a successful measure Assuming Reliability of D is RD, then R G1 A = R 3 D R D1 2 (1 R D1 ) 24

25 Conclusions: How to design a Reliable IoT Physical System Reliability function scaled through Duty Cycle Factor Device is measuring temperature 10 times a day with operating 1 sec each time Duty Cycle Factor D SF = Heavy Duty Cycle/Continuous measurement and/or Industrial IoT application requires different Redundant Parallel system 25

26 Conclusions: How to design a Reliable IoT Physical System IoT reliability could be considered as two separate elements: Devices and Data Device is inherently unreliable, uncertain, error prone in IoT environment dynamics Challenges would be how to introduce the notion of the Guaranteed Quality of Information (GQoI) based on the services provided. Current design should be future proven for all dynamic changes of the environment in which the system operates 26

27 Thanks! 27

28 Appendix: One Equation good to have in your pocket For the continuous random process, 0 E X = 0 1 F x dx F x dx F(x) x 28