ECE 477 Group 6
Digital Systems Senior Design Project
Homework 9 Reliability and Safety Analysis
Spring 2005
Introduction
The Arbuckle’s Cat Feeder is a commercial product that integrates a
microcontroller with an RFID system. Additional peripherals include an RPG switch, an
LCD display, a reed relay circuit, a feeder system, and an Ethernet module. The RFID
reader detects an RFID tag from a cat and sends the information to the microcontroller.
The microcontroller then decides on whether to dispense a user selected amount of food
out of the feeder if necessary. The user interacts with the Cat Feeder device by inputting
with the RPG switch while the LCD displays the status of the device. Administrator
functions to the cat feeder can also be initialized locally or remotely (via Ethernet) with
the device.
The microcontroller is the heart of the design and the functionality of the Cat
Feeder relies heavily on this control unit. If the microcontroller fails to work correctly,
the entire product will be of no use. Furthermore, since the device acquires the use of
external components such as the LCD and RPG, it is crucial to ensure that these devices
work properly to maintain the reliability and safety of the Cat Feeder. The following
report focuses on the reliability analysis of the product.
Reliability Analysis
With the aid of the Military Handbook for Reliability Prediction of Electronic
Equipment and Designing for Reliability, Maintainability, and Safety, the reliability
analysis can be approximated accurately. For the reliability model for components, the
predicted number of failures for 106 hours of operation and MTTF (Mean Time to Failure)
for each component can be calculated.
The calculations need to be precise since failure in any of the components could
be dangerous to cats and young children using the device. Our goal is not only to
promote customer satisfaction to the functionality and reliability of our product but also
to prevent any unnecessary injuries from occurring. This reliability analysis will look
into the following components which are believed to be most likely to fail:
-> Motorola MC9S12NE64 Microcontroller
-> Grayhill 61C11Rotary Pulse Generator Switch
-> Relay Circuit
-> Micrel 4690 Step-down Regulator
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ECE 477 Group 6
Digital Systems Senior Design Project
Homework 9 Reliability and Safety Analysis
Spring 2005
Motorola MC9S12NE64 Microcontroller
The failure rate per 106 hours is characterized by the equation:
λp = (C1πT + C2πE) × πQ × πL
λp = ( (0.28 * 16) + (0.032 * 2) ) * 1 * 1.2 = 5.4528
MTTF = 1 / (5.4528 * 10^-6) = 183392 hours
Parameter
C1
Value
0.28
Justification
The Motorola MC9S12NE64 is a 16-bit microprocessor
(MIL-HDBK-217F, section 5-1)
Assume junction temperature < 100 Celsius
(MIL-HDBK-217F, section 5-8)
There are 112 pins
(MIL-HDBK-217F, section 5-9)
Environment is “Ground Fixed”
(MIL-HDBK-217F, section 5-10)
Assumed Class B
(MIL-HDBK-217F, section 5-10)
Product in production < 1.5 years
(MIL-HDBK-217F, section 5-10)
πT
16
C2
0.032
πE
2.0
πQ
1.0
πL
1.2
p
MTTF
5.4528
183392 hours
Table 1.1
Microcontroller p and MTTF
Grayhill 61C11 Rotary Pulse Generator Switch
The failure rate per 106 hours is characterized by the equation:
λp = λb πcyc πL πE
λp = 0.02 * 1 * 1.48 * 3.0 = 0.0888
MTTF = 1 / (0.0888 * 10^-6) = 11261261 hours
Parameter
λb
Value
0.02
Justification
Lower Quality
(MIL-HDBK-217F, section 14-3)
< 1 cycle per hour
(MIL-HDBK-217F, section 14-3)
Stress is resistive at 0.5
(MIL-HDBK-217F, section 14-3)
Environment is "Ground Fixed"
(MIL-HDBK-217F, section 14-3)
πcyc
1
πL
1.48
πE
3.0
p
MTTF
0.0888
11261261 hours
Table 1.2
RPG p and MTTF
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ECE 477 Group 6
Digital Systems Senior Design Project
Homework 9 Reliability and Safety Analysis
Reed Relay Circuit
The failure rate per 106 hours is characterized by the equation:
λp = λb λQ πE
λp = 0.5 * 4.0 * 3.0 = 6
MTTF = 1 / (6 * 10^-6) = 1666667 hours
Parameter
λb
Value
0.50
Justification
Solid State Time Delay
(MIL-HDBK-217F, section 13-2)
Quality Factor
(MIL-HDBK-217F, section 13-2)
Environment is "Ground Fixed"
(MIL-HDBK-217F, section 13-2)
λQ
4.0
πE
3.0
p
MTTF
6.0000
1666667 hours
Table 1.3
Relay p and MTTF
Micrel 4690 Power Step-down Regulator
The failure rate per 106 hours is characterized by the equation:
λp = (C1πT + C2πE) × πQ × πL
λp = ( (0.02 * 45) + (0.0023 * 2) ) * 1.0 * 1.2 = 1.08552
MTTF = 1 / (1.08552 * 10^-6) = 921217 hours
Parameter
C1
πT
C2
πE
πQ
πL
p
MTTF
Value
0.02
Justification
Digital
(MIL-HDBK-217F, section 5-1)
Assume junction temperature < 120 Celsius
45
(MIL-HDBK-217F, section 5-8)
0.0023 There are 7 pins
(MIL-HDBK-217F, section 5-9)
Environment is "Ground Fixed"
2.0
(MIL-HDBK-217F, section 5-10)
Assumed Class B
1.0
(MIL-HDBK-217F, section 5-10)
Product in production < 1.5 years
1.2
(MIL-HDBK-217F, section 5-10)
1.08552
921217 hours
Table 1.4
5V-to-3.3V Step-down Regulator p and MTTF
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Spring 2005
ECE 477 Group 6
Digital Systems Senior Design Project
Homework 9 Reliability and Safety Analysis
Spring 2005
Conclusions
Component
MC9S12NE64
Grayhill 61C11 RPG
Reed Relay Circuit
Micrel 4690 Power Regulator
Table 1.5
p
5.4528
0.0888
6.0000
1.08552
MTTF
183392 hours
11261261 hours
1666667 hours
921217 hours
Comparison of p and MTTF
By observing Table 1.5, it can be seen that the MC9S12NE64 and Reed Relay
circuit have the greatest likelihood to fail in the device. For the microcontroller, the
MTTF value of 183392 hours is probably due to the maximum junction temperature that
is estimated to operate at a maximum of 100 Celsius. The Reed Relay circuit and Micrel
4690 Power Regulator also have increased risks of failing.
Since the junction
temperature is probably overestimated, the component lifetime or MTTF should be a
little longer. Nonetheless, the Cat Feeder can improve on the heat issue by adding heat
sinks to a number of locations in the design.
FMECA (Failure Mode, Effects, and Criticality Analysis)
Functional Blocks
Letter Functional Block
MC9S12NE64
A
B
C
5-to-3.3V Regulator
and RS232
Transceiver
Relay Circuit
D
E
RFID
F
Ergos Cat Feeder
RPG, LCD, IR
Description
The Motorola MC9S12NE64 is the main control unit of
the product. It interfaces with all other components in
the design.
Converts a 5 VDC input to a 3.3 VDC output. Since
some components use 5 V while others use 3.3V, this
needs to be carefully checked.
Power MOSFET switch that turns on auger in Feeder
Tank to dispense food.
RFID system to identify cats based on RFID tags
IR detects the level of food in the feeder, the LCD and
RPG are used by the user to check the status of the
device or modify changes.
Cat Feeder system used to dispense food.
Table 1.6
Functional Blocks
4
ECE 477 Group 6
Criticality
Criticality
Low
High
Digital Systems Senior Design Project
Homework 9 Reliability and Safety Analysis
Spring 2005
Failure Effect
Maximum Probability
10-13 < λ < 10-10
Limited or broken functionality of
specific components in product.
No serious injuries caused.
Customer Dissatisfaction.
λ >= 10-10
Product becomes unsafe for users.
Potential for injuries.
Table 1.7
Criticality Definitions
FMECA Worksheet – ECE477 Group 6
Failure Failure
Possible
No.
Mode
Causes
A1
Output = External short
0V
circuit, failure
of any
components,
connections in
PCB board.
A2
Output > Power regulator
5V
failure.
Failure Effects
The MCU is
not powered
up.
Method of Criticality
Remarks
Detection
Observation Low
The cat feeder
will not be
powered up.
Damage to
MCU, power
regulator, IR
Observation High
The Cat Feeder
will overheat
significantly.
Observation High
The Cat Feeder
components
can source or
sink to much
current.
The
microcontroller
has no access
or control over
the Ergos
Feeder auger
used to
dispense food.
B1
Output
not =
3.3V
Power regulator Damage to
failure. PCB
Relay circuit.
connections.
C1
Ergos
Feeder
does not
dispense
food
when
MCU
asserts
auger
device
RFID
not
scanning
tags
Malfunction in
microcontroller,
Malfunction in
auger motor,
4N25
optocoupler
failure.
Damage to
Observation Low
power
regulator, IR,
Relay Circuit,
and
Microcontroller
Malfunction in
RFID device.
RFID tag
Observation Low
information
cannot be
identified in the
device.
D1
5
No cats can be
identified by
the device.
ECE 477 Group 6
Digital Systems Senior Design Project
Homework 9 Reliability and Safety Analysis
Spring 2005
E1
RPG
does not
input
data
Malfunction in
RPG, software
bugs, PCB
connections.
The RPG will
not serve as a
functional
input.
Observation Low
The user
cannot give a
selection input.
E2
LCD
does not
display
data
The status of
the device
cannot be
displayed.
Observation Low
E3
IR does
not
detect
the
correct
portion
of food
Ergos
Cat
Feeder
does not
dispense
food at
correct
amounts
SCI port
failure,
Malfunction in
LCD, software
bugs
IR positioned at
less optimal
locations,
malfunction in
IR, main
program
software bugs.
Auger Motor
malfunctioned,
Software bugs,
failure in
microcontroller,
IR not working,
Relay circuit
not working.
Table 1.8
The
microcontroller
has no
knowledge of
the amount of
food left in the
container.
Auger cannot
dispense at
correct portion
or time.
Observation Low
The user
cannot know
the settings
and status of
the cat feeder.
The device
does not know
the correct
amount of food
to dispense.
F1
Observation Low
FMCEA Worksheet
6
An incorrect
amount or no
amount of cat
food will be
dispensed
when a cat
approaches.
ECE 477 Group 6
Digital Systems Senior Design Project
Homework 9 Reliability and Safety Analysis
Spring 2005
List of References
[1] U.S. Department of Defense, Reliability Prediction of Electronic Equipment,
MIL-HDBK-217F
http://shay.ecn.purdue.edu/~dsml/ece477/Homework/Spr2005/Mil-Hdbk-217F.pdf
[2] George Novacek, Designing for Reliability, Maintainability, and Safety, Circuit
Cellar December 2000.
http://shay.ecn.purdue.edu/~dsml/ece477/Notes/PDF/4-Mod10_ref.pdf
[3] Freescale MC9S12NE64
http://shay.ecn.purdue.edu/~477grp6/Documents/MC9S12NE64V1.pdf
[4] Texas Instrument s Low Frequency RFID Evaluation Kit RI-K2A-001A
http://shay.ecn.purdue.edu/~477grp6/Documents/RI-K2A-001A.PDF
[5] Sipex SP3223 intelligent +3V to +5.5V RS-232 Transceivers datasheet
http://www.sipex.com/products/pdf/SP3223_3243E.pdf
[6] Micrel 4690 Switching regulator datasheet
http://www.micrel.com/_PDF/Eval-Board/mic4690_eb.pdf
[7] Grayhill RPG 61B15 datasheet
http://embrace.grayhill.com/embrace/IMAGES/PDF/I-19-20.pdf
[8] CrystalFontz 634 20x4 LCD
http://shay.ecn.purdue.edu/~477grp6/Documents/634full.pdf
[9] LM339 comparator datasheet
http://www.fairchildsemi.com/ds/LM/LM339.pdf
[10] 4N25 Optocoupler datasheet
http://www.fairchildsemi.com/ds/4N/4N25.pdf
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