System Reliability Analysis. Introduction:

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Bruno Cross
5 years ago
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1 System Reliability Analysis. Introduction: The project under consideration is a shopping cart that follows a user around a store. It does this by listening for an ultrasonic signal and triangulating the location based on voltage levels of its 3 different sensors. We thus need a transmitter that fits on the user s hand, and is self-contained, and 3 Ultrasonic receivers for triangulation. We also have functionality to avoid obstacles through IR proximity sensors, but we have used digital sensors that trigger at a particular level, and as such do not need any separate amplification circuitry for them. We also need to be able to follow the user, and in doing so, need to drive motors. To do this we have a PWM signal driving an opti decoupler. To allow for sufficient current, we have a BJT switch that sources current directly the batteries. Thus, the major functional blocks are Ultrasonic receivers, Ultrasonic Transmitters, Obstacle Detection IR, Motor Driving circuitry, and Power Circuitry. Reliability of the overall system will be dependent on the reliability of these individual blocks, and will roll-up into a system level score. Since there is no redundancy in place, in this revision, the failure rates will add up at the system level. While the overall product is safe, there is a possibility of injury inherent with moving objects, and this will be defined as in the FMECA. The failure effect is collision, and this could happen with a few failure modes, specifily of the motors or the IR detection circuitry. Any other failure mode is non-criti, leading to the overall safety of the shopping cart system.

2 Reliability analysis: For this section, the following high-failure rate or criti components were selected Microcontroller Atmel ATMega32L, Linear power chip Max 663, Opto-Isolator H11A1, switching BJT These are parts that would operate at higher temperatures (switching BJT, Max663), or are criti to system availability (ATMega32L). Reliability culations for these components follow: 1. Microcontroller ATMega32L λ P = [C 1 * П T + C 2 * П E ] * П Q * П L [1] C 1 die complexity (8 bit) = 0.14 П T temperature coefficient (T < 100 o C ) = 1.5 C 2 Pin constant (40 pins) = П E Environmental Constant (Ground Mobile) = 4.0 П Q Quality Factor (Commercial, Unknown screening) = 10. П L Life Constant (In production for about a year) = 1.5 Thus: λ P = [0.14 * * 4] * 10 * 1.5 λ P = 4.29 failures per 10 6 MTTF = 2.331E5 2. Linear Power chip Max663 λ P = [C 1 * П T + C 2 * П E ] * П Q * П L [1] C 1 die complexity (linear CMOS, transistors) = П T temperature coefficient (T < 100 o C ) = 16 C 2 Pin constant (8 pins) = П E Environmental Constant (Ground Mobile) = 4.0 П Q Quality Factor (Commercial, Unknown screening) = 10. П L Life Constant (In production for 2 years at least) = 1

3 Thus: λ P = [0.020 * * 4] * 10 * 1 λ P = failures per 10 6 MTTF = 2.998E5 3. Opto-Isolator H11A1 λ P = λ B * П T * П Q * П E [2] λ B Base failure rate (photodiode Output, Single Device) = П T temperature coefficient (T < 100 o C ) = 6.6 П E Environmental Constant (Ground Mobile) = 8.0 П Q Quality Factor (Plastic DIP) = 8. Thus: λ P = * 6.6 * 8 * 8 λ P = failures per 10 6 MTTF = 4.304E5 4. Switching Transistor BJT 2N3704. λ P = λ B * П T * П A * П R * П S * П Q * П E [3] λ B Base failure rate (NPN) = П T temperature coefficient (T < 100 o C ) = 4.2 П A Application Factor (switching) = 0.70 П R Power Factor (P r < 1W ) = 1.0 П S Stress Factor (0 < V S < 0.3) = 0.11 П Q Quality Factor (Plastic DIP) = 8. П E Environmental Constant (Ground Mobile) = 9.0

4 Thus: λ P = * 4.2 * 0.70 * 1 * 0.11 * 8 * 9 λ P = failures per 10 6 MTTF = 58.14E6 These analyzed parts have λ P not in the 1E-9 range, but in the 1E-6 region. This is three orders of magnitude higher than the never region, and is significant. A major factor in this unreliability is the assumed temperature maximum of 100 o C. This is a very conservative approximation, and decreasing this by ventilation and heat-sinks would increase the reliability by at least an order of magnitude. Making the manufacturer run burn-in tests on the parts and tests group 1 [4] of the screen/tests would help increase reliability again by around an order of magnitude, bringing failure rates to 1E-8. FMECA: The circuit is divided into 5 major blocks for consideration for the FMECA. These are A Ultrasonic Transmitter B. Ultrasonic Receiver C. Obstacle Detection IR D. Motor Drive Circuitry E. Power Circuitry. There are two levels of critiity -criti, which will not lead to injury, and which could potentially lead to personal injury. The non-criti level needs to have a failure rate of at least <1E-7 while the criti path needs at least <1E-9. Each of these blocks has failure modes associated with them, leading to the following FMECA: Fail ure No. A1 A2 Failure Mode 555 output Duty cycle <> 50% + 5% No power to 555 timer Possible Causes Shorted resistors or capacitors that provide the oscillator reference to the 555 Open switches that control logic flow, dead battery, bad power chip (MAX663) Failure Effects No output ultrasonic signal No output ultrasonic signal Method of Detection it y Not Not Remarks to default stop state, since no signal received at microcontroller. to default stop state, since no signal received at microcontroller.

5 A3 A4 A5 B1 B2 No power to transmitter output transmitter No control over functionality Only noise on input ATD channel Constant 0V output to microcontroll er Bad 555 timer chip 555 timer providing bad filter, failing Max663 providing erratic power, bad switches randomly shorting. Bad switches Bad transducer, functioning op-amps Shorted output RC filter/envelope detector resistor or capacitor, or non functioning op-amps, or main board power chip bad or shorted noise suppression capacitors. No output ultrasonic signal Unpredictable behavior. Possible towards user. Constant signal being transmitted No appreciable output to Microcontroller ATD No appreciable output to Microcontroller ATD and deceleration, and turning of shopping cart. Stuck in normal follow mode of operation Not to default stop state, since no signal received at microcontroller. System could accelerate to maintain fixed signal strength, and if IR not working, might cause collision. Nuisance, but not a danger. Will be stuck in normal operation, and will not accept the Stop or Approach commands to default stop state. to default stop state.

6 B3 B4 C1 C2 C3 Noise on ATD channel. Constant high (3.3V) voltage to ATD Constant High input to IRQ channel Constant Low input to IRQ channel Inputs to IRQ Frayed wires connecting receiver boards to main boards, open noise suppression capacitors, envelope detector and rectifier circuit components bad Shorted 3.3V line to ultrasonic receiver input ATD channel Defective/dirt y IR sensor, Inverter chip not working, pull-up resistor open Defective IR sensor, Power supply failure, Pull up resistor shorted to VCC, inverter chip not functioning. Dirty/Defectiv e IR sensor, failing power supply Unpredictable behavior. Possible towards user No changing signal strength implying correct distance/speed being maintained No obstacle avoidance detection capability. No obstacle avoidance capability Effects Unpredictable. and deceleration, and turning of shopping cart. Cart will at constant speed in straight line avoid obstacles, and will cause collision, because it constantly detects an object s presence. Behavior. critic al System could accelerate to maintain fixed signal strength, and if IR not working, might cause collision. No change in speed or direction, and if IR not working, can cause collision. Collision imminent Cart will hang

7 D1 D2 Motor Stuck Off Motor stuck On Open switching BJT not sinking current, non functioning opto-isolator not transmitting PWM signals to motor. Fused optoisolator shorted to power, mechani Motor defect E1 0V output Short on the resistors and capacitors providing the biasing control E2 Incorrect output voltages, out of tolerance Failure of any resistor or capacitor generating the voltage on the chips. Stationary cart. Microcontroller pins may become damaged, cart will not stop System unavailable system behavior, inadequate amplification of ultrasonics, damage to microcontroller. Cart will continuously. behavior critic al critic al No control over speed or direction of ment collision imminent. List of References: [1] MIL-HDBK-217F Section 5.1, Microcontrollers and CMOS logic [2] MIL-HDBK-217F Section 6.11, Opto-Isolator [3] MIL-HDBK-217F Section 6.3, Switching BJT s [4] MIL-HDBK-217F Section 5.10, Testing/Screening. [5] Atmel Corporation, AVR ATMega32/ATMega32L Microcontroller Datasheet [6] Jameco and Digikey for looking up part details for choosing reliability parameter values

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