On Duty Cycle Concept in Reliability

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On Duty Cycle Concept in Reliability - Definitions, Pitfalls, and Clarifications
By Frank Sun, Ph.D.
Product Reliability Engineering
HGST, a Western Digital company
For ASQ Reliability Division Webinar
August 14, 2014

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On Duty Cycle Concept in Reliability

  1. 1. © 2014 HGST, a Western Digital company On Duty Cycle Concept in Reliability - Definitions, Pitfalls, and Clarifications By Frank Sun, Ph.D. Product Reliability Engineering HGST, a Western Digital company For ASQ Reliability Division Webinar August 14, 2014
  2. 2. © 2014 HGST, a Western Digital company HGST Product Segment Overview CinemaStar Desktop 3.5-inch Deskstar series Enterprise 2.5/3.5-inch Ultrastar series Solid State Drives (SSD) Endurastar Mobile 2.5-inch Travelstar series LifeStudio and G-Technology Personal Storage HGST provides digital storage that delivers pioneering technology and QUALITY that customer can count on.
  3. 3. © 2014 HGST, a Western Digital company 3 Acknowledgements  This presentation is based on the paper co-authored with Mr. Steve Lock from HGST and Dr. Wendai Wang from Throatec Corporation: • The 19th ISSAT International Conference on Reliability and Quality in Design, August 5-7, 2013, Honolulu, Hawaii, U.S.A. • International Journal of Reliability, Quality and Safety Engineering, Vol. 20, No. 5 (2013) 1350017 (16 pages).  The authors deeply appreciate the insightful discussion with and inputs from their colleagues and friends, especially • Mr. Raul Gonzalez, Director of Testing and Reliability Engineering from HGST, • Dr. Guangbin Yang from Chrysler, • Dr. Mingxiao Jiang from Medtronics, • Mr. Jiliang Zhang from Lab 126, and • Dr. Wei Huang from Loral, etc.  Also we thank the reviewers for their valuable time and constructive comments and suggestions.
  4. 4. © 2014 HGST, a Western Digital company 4 Table of Contents  Introduction  The Diverse Manifestation of Duty Cycle in Various Industries  Time-Based Duty Cycle Definition  Stress-Condition-Based Duty Cycle Definition  How Duty Cycle is Used in Reliability Calculation  Conclusions and Recommendations
  5. 5. © 2014 HGST, a Western Digital company 5 Introduction  In real world, products (systems, equipment, or components) may not operate continuously during their mission time, or may be subjected to workloads greater or lesser than the rated capacity.
  6. 6. © 2014 HGST, a Western Digital company 6 Introduction – cont.  To reflect product field operational usage effects, a factor called the Duty Cycle (DC) is often used.  Duty cycle is one of the most frequently used, and yet one of the most ambiguous concepts. It can mean different things for different people, different industries, or different applications.  For better communication among people and industries, there is an increasing need to provide an overview and clarification regarding the definition and standardization of duty cycle in terms of their intended perspectives.
  7. 7. © 2014 HGST, a Western Digital company 7 The Diverse Manifestation of DC in Various Industries Industries Duty Cycle Manifestations Remarks or Examples Aerospace industry Mission profiles in different phases (taxiing, taking-off, cruising, approaching, landing, etc.) in terms of time, thrust, speed, temperature, altitude, etc. Automotive industry In a narrow sense: the percent of time a system or component is operated; In a broad sense: duty cycle is often used to describe the usage profile over time or event. An automotive fuel injector may be open for 80ms to inject fuel into the engine, and then close for 20ms: DC= 80%. Hard disk drive industry Percent of time that product is powered on, is busy, is running at maximum rated data transfer capability, etc. Medical device industry The programming sessions during the day and/or at night when using implantable programmable devices, or the recharge cycle when using a rechargeable device. The term "usage" is also used in some companies Power generation industry Percent of time spent in different states, such as Service hours, Reserved Shutdown hours, Planned Outage hours and Unplanned Outage hours in different states in service. • Cycling Rate = Number of Starts / Service Hours • Service Factor = Service Hours / Period Hours • Average Load (Power Output) Print and copy Industry The number of copies or prints that the device can reliably produce on a monthly basis. Radio frequency (RF) field Also called the duty factor, is the measure of the fraction of the time radar is transmitting. Satellite industries How much an equipment is really used. Typically the battery duty cycle is about 100% at rocket launch, and 25% once in orbit. Semiconductor equipment industry Equipment Utilization is commonly used as a kind of duty cycle equivalence • Operational Utilization = Productive Time / Operations Time • Total Utilization = Productive Time / Total Time Solar energy industry Similar to power generation and semiconductor industry % of sunny days, % of rainy days, Sun rising/setting times, daily ambient temperature profile, wind profile, sun light spectrum, clouding frequency and durations, and etc.
  8. 8. © 2014 HGST, a Western Digital company 8 General Classifications  No matter how each industry defines its own duty cycle terminology, in general, the duty cycle definitions can be classified into two major categories:  Time-based Duty Cycle  Stress-condition-based Duty Cycle
  9. 9. © 2014 HGST, a Western Digital company 9 Time-based Duty Cycle - Definition  The proportion of time during which a component, device, or system is in active state, or being operated: DCT =  / T where DCT is the time-based duty cycle, τ is the duration that the function is active, Τ is the period of the function.
  10. 10. © 2014 HGST, a Western Digital company 10 Time-based Duty Cycle - Examples Time-Based Duty Cycle Product Example Comments Power-On-Hour Duty Cycle: the percent of time the product is powered on. Operation-Time Duty Cycle: the percent of the time during which the product is conducting functional activities (being operational) Hard Disk Drive (HDD) Neither the “Power-On-Hour Duty Cycle” nor the “Operation- Time Duty Cycle” has the visibility of workload effects. A product under 100% “Power-On-Hour Duty Cycle”, or under 100% “Operation-Time Duty Cycle” can be always active but under loaded, and “enjoying an easy life”. Smart Phone Battery Smart phone battery life not only depends on how long the phone is turned on each day, but also depends on what, how many, and how intense the energy consuming programs, features, and apps are running in the background. Copy Machine A copy machine in a small home business is turned on all the time (100% POH duty cycle) but only makes a few copies a day (or a week), while another copy machine in a copy center is also powered on 24 hours a day but making hundreds or thousands pages of copies a day. These two copy machines would definitely have very different life expectancies.  In summary, the time-based duty cycle may give people misleading indication for the above scenarios, and is certainly not enough to quantify the reliability impact.
  11. 11. © 2014 HGST, a Western Digital company 11 Stress-Condition-Based Duty Cycle 1 - Definition  Load-Intensity-Based Duty Cycle Definition: Second)orMinute,Hour,Day,Month,Year,(inDurationOperation OperationduringProductatoApplied(Stress)LoadingTotal OperationofTimeUnitper ProductatoApplied(Stress)LoadAverage ValueAbsoluteinCycleDutyBased-Intensity-Load   IntensityLoadAchievableMaximum OperationofTimeUnitperApplied(Stress)LoadingTotalActual (% )PercentageinCycleDutyBased-Intensity-Load   Typically applies to usage with cumulative damage effects.
  12. 12. © 2014 HGST, a Western Digital company 12 Stress-Condition-Based Duty Cycle 1 - Example HDD (Operational) Load-Intensity-based Duty Cycle Load Factor Duty Cycle Read-Based Actual Data Read per Unit Time of Drive Operation Write-Based Actual Data Written per Unit Time of Drive Operation Read-Write-Based Actual Data Read/Write per Unit Time of Drive Operation (= Data Transfer Duty Cycle) Seek-Based Actual Seeks per Unit Time of Drive Operation Start/Stop-Based Actual Starts/Stops per Unit Time of Drive Operation Load/Unload-based Actual Loads/Unloads per Unit Time of Drive Operation
  13. 13. © 2014 HGST, a Western Digital company 13 Stress-Condition-Based Duty Cycle 2 - Background  Mission-Profile-Based Duty Cycle Example: Altitude profile of a space truck during its mission to ISS and return
  14. 14. © 2014 HGST, a Western Digital company 14 Stress-Condition-Based Duty Cycle 2 - Background  Mission-Profile-Based Duty Cycle Example: A predicted network workload profile during 24-hour period
  15. 15. © 2014 HGST, a Western Digital company 15 Stress-Condition-Based Duty Cycle 2 - Definition  Load-Intensity-Based Duty Cycle Definition:  In addition to the RMS value, there are numerous other attributes that can be used to characterize the duty cycle for a stress profile (especially when it is periodic in nature), such as • peak-to-peak amplitude • semi amplitude (half peak-to-peak amplitude) • peak amplitude • average (mean) value • frequency of stress changes.             2 1 2)( 12 1 T T dttV TT MissionWholetheduringStressAppliedofValueRMS CycleDutyBased-Profile-Mission
  16. 16. © 2014 HGST, a Western Digital company 16 How Duty Cycle is Used in Reliability Calculation );.;( ).;.;( );( DutyCycleStressEnvtR StressOpStressEnvtR itionStressCondtR    Reliability is the probability that an item can perform its intended function for a specified time interval under stated conditions.
  17. 17. © 2014 HGST, a Western Digital company 17 How Duty Cycle is Used in Reliability Calculation           t etR )(  Mission time, “t”, is affected by time- based duty cycle  Characteristics life, “”, is affected by stress-condition-based duty cycle  For the sake of illustration, assume that a product lifetime follows a Weibull distribution with the following reliability function  Mission time or effective operation time, “t”, will be a function of time-based duty cycle  Weibull shape parameter, “β”, and Weibull scale parameter, “”, will be functions of stress-condition-based duty cycle.  Shape parameter, “”, is affected by stress-condition-based duty cycle
  18. 18. © 2014 HGST, a Western Digital company 18 How Duty Cycle is Used in Reliability Calculation - cont.  The effective operation time “t” will be affected by time- based duty cycle as follows: t = (Calendar Time Duration of Product Mission)  (Time-based Operational Duty Cycle)  Weibull shape parameter, β, is an indicator of product failure mechanism:  if the applied stress conditions are beyond product design limits, then β will experience statistically significant change;  Otherwise, β will remain the same theoretically.  Weibull scale parameter, , that represents product inherent characteristic life, is definitely affected by the applied stress level: (Stress-condition-based Duty Cycle 2) = (Stress-condition-based Duty Cycle 1)  AF(Stress-condition-based Duty Cycle 1 vs 2)
  19. 19. © 2014 HGST, a Western Digital company 19 How Duty Cycle is Used in Reliability Calculation - cont.  For example, when workload and temperature are the two dominating failure-governing factors of a mission-profile-based duty cycle; i.e., the Combination Model or the so-called Erying Relationship can be used to estimate their joint acceleration factor where AF = acceleration factor, L1 = mean/median workload of duty cycle 1, L2 = mean/median workload of duty cycle 2, T1 = mean/median absolute temperature of duty cycle 1, T2 = mean/median absolute temperature of duty cycle 2, n = the exponent, an indication of failure sensitivity to applied workload, EA = the activation energy, in eV, an indication of failure sensitivity to applied temperature, K = the Boltzman’s constant, 8.617385x10-5 eV/ºK.               21 11 1 2 TTK En A e L L AF    )(),()(),()( 21 tttVtVtV TempWorkload 
  20. 20. © 2014 HGST, a Western Digital company 20 How Duty Cycle is Used in Reliability Calculation - cont.  Note that equation on previous page is only valid given that the stress levels under use and accelerated mission profiles, TU versus TA and LU versus LA, are given and fixed.  However, since these stress levels, especially those at use conditions, TU and LU, are random variables (while the test conditions are typically accelerated but under control), the more accurate expression of acceleration factor should be the following double integral with respect to TU and LU: where f (LU, TU) is the joint probability density function of stress levels at use conditions, LU and TU.                   U U AU A T L UUUU TTK En U A dTdLTLfe L L AF ),( 11
  21. 21. © 2014 HGST, a Western Digital company 21 How Duty Cycle is Used in Reliability Calculation - cont. (Example: Ea=0.4 eV; n=0.7) A sample plot of HDD acceleration factor as a function of temperature and data transfer duty cycle (workload) where 100% duty cycle is defined at 180 MB/Sec. 0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% AccelerationFactor,AF Data Transfer Duty Cycle, % Acceleration Factor vs Data Transfer Duty Cycle & Temp with Respect to the Reference Temp = 35C and Duty Cycle =20% 30 35 40 45 50 55 60 65 70
  22. 22. © 2014 HGST, a Western Digital company 22 How Duty Cycle is Used in Reliability Calculation - cont. (Example: Ea=0.4 eV; n=0.7) A sample plot of HDD steady-state MTTF as a function of temperature and data transfer duty cycle (workload) where 100% duty cycle is defined at 180 MB/Sec. 0 500,000 1,000,000 1,500,000 2,000,000 2,500,000 3,000,000 3,500,000 4,000,000 4,500,000 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% MTTF,hr Data Transfer Duty Cycle, % MTTF vs Data TransferDuty Cycle & Temp 30 35 40 45 50 55 60 65 70
  23. 23. © 2014 HGST, a Western Digital company 23 How Duty Cycle is Used in Reliability Calculation - cont. (Example: Ea=0.4 eV; n=0.7) A sample plot of HDD steady-state AFR as a function of temperature and data transfer duty cycle (workload) where 100% duty cycle is defined at 180 MB/Sec. 0.0000% 1.0000% 2.0000% 3.0000% 4.0000% 5.0000% 6.0000% 7.0000% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% AFR,%/year Data Transfer Duty Cycle, % AFR with POH=8760 Hrvs Data TransferDuty Cycle & Temp 30 35 40 45 50 55 60 65 70
  24. 24. © 2014 HGST, a Western Digital company 24 How Duty Cycle is Used in Reliability Calculation - cont. (Example: Ea=0.4 eV; n=0.7) A sample plot of HDD one-year mission reliability as a function of temperature and data transfer duty cycle (workload) where 100% duty cycle is defined at 180 MB/Sec. 93.0000% 94.0000% 95.0000% 96.0000% 97.0000% 98.0000% 99.0000% 100.0000% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% R(T),%/year Data Transfer Duty Cycle, % R(T)with T=8760 Hr vs Data TransferDuty Cycle & Temp 30 35 40 45 50 55 60 65 70
  25. 25. © 2014 HGST, a Western Digital company 25 Conclusions and Recommendations  Duty Cycle is one of the most frequently used terminologies in the modern industry, yet its definition and use have been loose, ambiguous, subtle, and many times misleading.  It’s particularly important to reliability engineering, since it forms a foundation for product reliability specifications, for reliability evaluation and assessment, and for reliability testing.  The authors of this paper proposed a definition system that classifies the existing duty cycle into the following major and sub- categories: • Time-based duty cycle » a. Power-on-hour duty cycle » b. Operation-time duty cycle • Stress-condition-based duty cycle » a. Workload-intensity-based duty cycle » b. Mission-profile-based duty cycle  Their definitions and applications in reliability assessment are discussed and illustrated with examples.
  26. 26. © 2014 HGST, a Western Digital company 26 Conclusions and Recommendations – cont.  Generally speaking, the time-based duty cycle affects the product effective operating time, and the stress-condition-based duty cycle affects both failure mechanism and product inherent characteristics life.  To facilitate effective and responsible communications, it is suggested any statement that involves the use of duty cycle, such as a claim of “2 million-hour MTTF at 100% duty cycle”, would clearly specify what exactly it is meant, such as • % of the time that a product is powered-on • % of the time that a product is operating • ratio of actual workload vs. rated capacity • mission-profile based stress (vector) function  It is hoped that from the extensive coverage and discussion of this paper, the readers get a systematic view and clarification regarding the definition and use of duty cycle.
  27. 27. © 2014 HGST, a Western Digital company 27 Thanks !
  28. 28. © 2014 HGST, a Western Digital company 28 DC Interpretation - An HDD Example  In hard disk drive (HDD) industry, when somebody states that his product operates at 100% duty cycle, it could be interpreted in different ways: • the product is powered-on 24×7 (24 hours a day and 7 days a week) • the product is always doing something (no idle) when it’s powered on, such as seeking, writing, or reading • the product is operating at the maximum rated data transfer capability, in megabytes per second (MB/Sec) • the product is operating at the same workload intensity as a reference level, such as that at reliability demonstration test (RDT)

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