ECE 477 Digital Systems Senior Design Project Spring 2008
Homework 11: Reliability and Safety Analysis
Due: Friday, April 4, at NOON
Team Code Name: _RoboRubik______Group No. _11___
Team Member Completing This Homework: __Erik Carron______
e-mail Address of Team Member: _jcarron______@ purdue.edu
Evaluation:
SCORE
/DESCRIPTION
10 /Excellent – among the best papers submitted for this assignment. Very few corrections needed for version submitted in Final Report.
9 /Very good – all requirements aptly met. Minor additions/corrections needed for version submitted in Final Report.
8 /Good – all requirements considered and addressed. Several noteworthy additions/corrections needed for version submitted in Final Report.
7 /Average – all requirements basically met, but some revisions in content should be made for the version submitted in the Final Report.
6 /Marginal – all requirements met at a nominal level. Significant revisions in content should be made for the version submitted in the Final Report.
* /Below the passing threshold – major revisions required to meet report requirements at a nominal level. Revise and resubmit.
* Resubmissions are due within one week of the date of return, and will be awarded a score of “6” provided all report requirements have been met at a nominal level.
Comments:
1.0 Introduction
RoboRubik is a digitalized Rubik’s Cube. It uses pushbuttons, LEDs, and an embedded web server to interface to the user. There will be two modes. One that the user can directly modify the cube by pushing the pushbuttons, and another which will aid the user in solving a Rubik’s Cube after inputting the state of the cube through the website. Since RoboRubik is basically a toy, there are very little high criticality concerns with it. The only possible way the user can be harmed from the RoboRubik is by the brightness of the LEDs and a possible battery acid leakage. Both are very unlikely to happen, but possible. For reliability, there are very little major components that can fail. These components do not work at a high temperature which makes them less likely to fail. The components that will be investigated are the microcontroller, the WiPort, the voltage regulator, and the LED drivers.
2.0 Reliability Analysis
For the reliability analysis, we decided to investigate the microcontroller, the WiPort, the voltage regulator, and the LED driver. All these components play a major role in the functionality of the RoboRubik and they all operate at a temperature above room temperature. The MIL-HDBK-217F military handbook was used to calculate the reliability of each of these components. The coefficients selected and the reliability values are shown in table 1.0.
Reliability CalculationsComponent / C1 / C2 / ∏T / ∏E / ∏Q / ∏L / λ / MTTF / Model
Micro / 0.28 / 0.019 / 0.98 / 5.0 / 1.0 / 1.0 / 0.3694 / 2.7070 / Microprocessor
WiPort / 0.29 / 0.017 / 0.98 / 5.0 / 1.0 / 1.0 / 0.3692 / 2.7086 / MOS Logic Array
Voltage Reg. / 0.01 / .0016 / 7.0 / 5.0 / 1.0 / 1.0 / 0.078 / 12.8205 / Bipolar Linear Array
LED Driver / 0.02 / .011 / 0.98 / 5.0 / 1.0 / 1.0 / 0.0746 / 13.4048 / MOS logic Array
Reference: MIL-HDBK-217F λ is in number failures per million hours
Table 1.0
Reliability Coefficients and Calculations
As you can see, the microprocessor and the WiPort are the most vulnerable. They both have about 2.7 years between failures for every one million products. The other two have about 13 years between failures. This is acceptable for the RoboRubik because there is pretty much no high criticality. There were several assumptions made when calculating these values. The first was that all these products have been in production for over two years. Another was that each device had a worse case scenario of having an operating temperature of 85˚C. The Environment Factor was not exactly known so the average of 5 was used. The Quality Factor was also not exactly known and neither was the class the components fell into so a value of 1 was used. As far as the complexity of the components, the microprocessor used the 16-bit value. The WiPort used the 30,000 to 60,000 MOS logic array. The Voltage Regulator used the 1 to 100 bipolar linear gates. The LED driver used the 101-1000 MOS logic array. The package failure rate was calculated with the nonhermetic package and the number of pins for each component. An easy way to improve these reliability factors is to decrease the operating temperatures. The temperatures are proportional to the amount of current each component draws so by decreasing the current, the temperature goes down. This can be done by efficiently strobing the LED drivers. Another way to improve operating temperature is by making sure the cube has enough air circulation to dissipate heat.
3.0 Failure Mode, Effects, and Criticality Analysis (FMECA)
There are very little high criticality failures for the RoboRubik. As mentioned before the only possibility to harm the user is by the LEDs being too bright and the battery leaking acid. There are many possibilities for low criticality failures such as any one the major components failing. There are four functional blocks that are investigated for failure. They are the microcontroller block, the WiPort block, the voltage regulator block, and the LED driver block.
4.0 Summary
The RoboRubik has a high reliability rate for being a toy. There is very little possibility to harm the user but possibility to unsatisfy the user due to functionality failure.
List of References
[1] MIL-HDBK-217F, United States Department of Defense, 2 January 1990.
[2] “MC9S12C Family User Guide” [Online document], [cited 2008 Feb 22], Available
HTTP:
http://cobweb.ecn.purdue.edu/~477grp11/files/datasheets/Microcontroller%20MC9S12C32
MFA25%20-%20Freescale.pdf
[3] “Lantronix WiPort Integration Guide” [Online document], [cited 2008 Feb 22], Available
HTTP: http://www.lantronix.com/pdf/WiPort_IG.pdf
[4] “Allegro A6279 Datasheet” [Online document], [cited 2008 Feb 22] Available
HTTP: http://www.allegromicro.com/en/Products/Part_Numbers/6278/6278.pdf
[5] “Fairchild KA378R33 Datasheet“[Online document], [cited 2008 Feb 22] Available
HTTP: http://www.fairchildsemi.com/ds/KA%2FKA378R33.pdf
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ECE 477 Digital Systems Senior Design Project Spring 2008
Appendix A: Schematic Functional Blocks
Figure 1 Microcontroller Block A
Figure 2 WiPort Block B
Figure 3 Voltage Regulator Block C
Figure 4 LED Driver Block D
Appendix B: FEMCA Worksheet
A1 / No power / Bypass capacitor may have shorted. / The microcontroller does not work and therefore the RoboRubik does not work / Observation / Low
A2 / Odd functionality / Oscillator circuit may have shorts, possible pin shorts/breaks. Not enough power. / RoboRubik does not function the way it should if at all / Observation / Low
A3 / Does not work with power / IC may be fried / The RoboRubik does not work at all / Observation / Low
B1 / No Website / The WiPort is not powered due to bypass capacitor short. IC may be fried. / The user is unable to use the website and solver aid mode. / Observation / Low / WiPort is very complicated and can have many things go wrong internally
C1 / No Output / The IC may be fried / Nothing is powered in the RoboRubik / Observation and measurement / Low
C2 / Power Short / There is a short between the power and ground / Nothing is powered. The voltage regulator and battery gets very hot. Possible battery leakage / Observation / High
D1 / Partial LEDs / Some of the pins are fried. / RoboRubik has a few LEDs not working / Observation / Low
D2 / Bright LEDs / The IC is fried / LEDs get really bright and hurt retinas of user. / Observation / High / Bad
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