This document discusses duty cycle concepts in reliability engineering. It begins with definitions of time-based and stress-condition-based duty cycles. Time-based duty cycle is the proportion of time a system is active, while stress-condition-based duty cycle considers the level of stress applied. The document then discusses how duty cycle manifests differently across various industries and how it is used to calculate reliability, with duty cycle affecting mission time, failure mechanisms, and characteristic life. Examples are provided for hard disk drives to illustrate the effects of duty cycle on acceleration factors and mean time to failure.

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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 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. © 2014 HGST, a Western Digital company 27
Thanks !

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)