the H-bridge C2 Package failure rate .0056 Based on equation from MIL-Hnbk-217f page 5-14 [1] for SMT with 16 pins πE Environment factor 4 Handbooks value for mobile devices πQ Quality factor 10 Assumed from the notes that this is the value to use, most likely the value is too large πL Learning factor 1 Number used for devices older than 2 years in production λP Failure rate per 10^6 hours 6.024 MTTF Mean Time To Failure 166002.656 Approximately 19 years to a failure for one device CTS CB3 HCMOS/TTL Clock Oscillator [5] Model Equation: Parameter name Description Value Comments λb Base failure rate .027 Based on the [1] for devices with 20 MHz frequency πQ Quality factor 2.1 Based on devices with a non MIL-SPEC πE Environment factor 10 Handbooks value for mobile devices λP Failure rate per 10^6 hours .567 Based on a quartz oscillator calculation MTTF Mean Time To Failure 1763668.43 Approximately 201 years to a failure for one device The overall system of this project is reliable. All of the parts have a reliability within the same scale of 10^-6 failures per hour. Also, the system will not be a highly bought solution; therefore a slightly higher failure rate is acceptable. The best way to improve reliability of the design would be to buy military parts, which would reduce each by a factor of 5. This could be an acceptable increase in price because the niche for the market of this product is so small, so someone buying this would be willing to pay more. Failure Mode, Effects, and Criticality Analysis (FMECA)
Appendix A: Schematic Functional Blocks Figure 1 - Microcontroller Subsection Figure 2 – Fuel Gauge Subsection Figure 3 – User I/O LEDs Figure 4 – User I/O Input Buttons Figure 5 – User I/O Reset Button Figure 6 – Power Subsection Figure 7 – Motor Controller Subsection Appendix B: FMECA Worksheet Sub-blocks Microcontroller Fuel Gauge User I/O Power Motor controller Failure No. Failure Mode Possible Causes Failure Effects Method of Detection Criticality Remarks
A1 Pin output stuck at ‘1’ Over voltage to the microchip Short circuit in LEDs always off, invalid USB communication Observation – Inspect pins, overheated High This assumes that multiple pins are stuck at ‘1’ A2 Pin output stuck at ‘0’ Under voltage to microchip, microchip “burned” out LEDs stuck on, fuel gauge does not turn on, unable to control motors Observation – All LEDs stuck on, probing pins Low This could be 1 or more pins stuck at ‘0’ A3 Failure of A/D channel(s) Chip defect, software malfunction, bad reference lines, noisy power lines Erratic movement of the robot Observation – Robot does not move through the maze correctly High B1 Battery reading discharging to quickly Rsense shorted, Cf shorted Misinformation to the user about the battery state Observation – battery indicator drops quickly Low B2 Battery reading not discharging Rsense open circuited Thinks the battery is not discharging, no power to board Observation – robot is not running Low B3 Battery discharging at too slow a rate Cf open circuited, R16/R17 open circuited Misinformation to user about the battery state Observation – Battery discharges at an slower rate than reality Low
C1 Microcontoller stops working R(7-12) or R14/15 short circuits Robots stops running Observation – Microcontroller stops working, probing the resistors Low This assumes the micro will stop working from too much current to the micro from the I/O C2 User input fails R13/14/15 open circuits User buttons are floating Observations – button presses are erratic Low C3 LEDs don’t turn on R(7-12) open circuits No information to user Observation – LEDs don’t work Low D1 No power to circuit Cout_reg, Cpow5d, Cin_reg, Cinbulk, Cdpowin short circuited, linear regulator dead Short to battery, no power to circuit Kills the battery quickly by drawing to much current High D2 5V Power rail > 5V Linear regulator failed Damage to components on PCB Observation – regulator too hot, smoke High D3 5V Power rail < 5V Linear regulator failed Robot doesn’t work Observation – smell or probing circuit High
D4 7.4 V Power rail > 7.4 V Overcharging/Failure of battery Damage to all components Observation – bloated battery/battery leakage, etc. High D5 7.4 V Power rail < 7.4 V Battery level too low, battery “died” Robot will not turn on, or is unstable Observation – Robot does not work Low E1 Erractic behavior of output pins of digital isolator Decoupling capacitors shorted Unable to control robot Observation – Robot will move erratically High E2 No output to motors Digital isolators failed, H-bridge failed, Cd(1/2)_iso(1/2) open circuited Motors will not run Observation – robot does not move Low