The Case Study of Automotive Airbag Reliability

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  • Topic: Transducer, Airbag, Microelectromechanical systems
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  • Published : October 31, 2012
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The case st udy of A ut omot i ve Ai r bag Reli abil it y

(Page 252)

(for comprehensive hints to solve the case study, refer to Page 217-220) Q# 1: What is the role of the dual actuators in the mechanical air bag system? Describe the effect of having only one.
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The role of dual actuators is to activate the mechanism of the airbag.

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These dual actuators are the two shafts that move off the edge of spring-loaded firing pins where the pins then stab dual primers, igniting the enhancer.

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The actuators are designed in parallel in the system to provide better reliability. One of the actuator acts as a primary actuator and the second actuator acts as a redundancy or backup actuator in the mechanical air bag system in a case the system fails.

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The effect of having only one actuator eventually will reduce the reliability of the mechanical air bag system. (to prove this, we can calculate the reliability of the AMS Sensor Block Diagram in Exhibit 6.24 by: Firstly, by having two parallel actuators (Reliability = 99.97%) and the latter by having only one actuator (Reliability = 98.52%))

Q# 2: Compute the reliabilities of each system. What conclusions do the data suggest?
Reliability of All Mechanical Sensor (AMS) Air Bag System (see Exhibit 6.24) For simplicity, two parallel actuators are computed as one actuator by applying Eq. 6.3 on Page 219.
R=.985266
RAC = .999783

Actuator
=

Actuator

Actuator
R=.985266
The reliability for two parallel Actuators is
RAC = 1 - (1 - 0.985266)(1 - 0.985266)
RAC = 0.999783
Then calculate the reliability of AMS Air Bag System (RAMS) as (0.999993)(0.999783)(0.999986)(0.999986)(0.999994)(0.999975)(0.999999) RAMS = 0.999716  99.97% (see Equation 6.2 on Page 218)

Page 1 of 3

Reliability of Electromechanical Sensor System (EMS) Air Bag System Similar idea is applied to calculate the reliability of EMS. First compute the reliability of the parallel system of Battery and capacitor (apply Eq. 6.3). The reliability of Arm sensor, sensor #1 and#2 is assumed to be 0.999239, hence no need to calculate the reliability of the two parallel sensors. The reliability of Battery & Capacitor (RBC)

R=.998318
RBC=.999997

Battery
=

Bat + Cap

Capacitor
R=.998318

RBC = 1 - (1 - 0.998318)(1 - 0.998318)
RBC = 0.999997
The reliability of EMS Air Bag System (REMS)
Assumption: the reliability of the serial system from Squib to Cover is computed as 0.999892.
REMS = (0.999239)(0.999997)(0.957124)(0.982782)(0.999892)
REMS = 0.939824  93.98% (apply Equation 6.2)

Reliability of Electronic Sensor System (ES) Air Bag System
In the similar fashion, to determine the reliability of ES, the ideas in the two previous AMS and EMS are also adopted. The reliability of Battery & diodes and Start/run switch and DC/DC converter, Capacitor & diode is assumed as to be 0.998919 (They are seen as one sub system with one reliability). The reliability of ES Air Bag System (RES)

RES = (0.998919)(0.998019)(0.996439)(0.999905)(0.994625)(0.999574) (0.974324)(0.982386)(0.988661)(0.991532)(0.999782)(0.999788) RES = 0.926202  92.62% (apply Equation 6.2)

Page 2 of 3

Based on the reliabilities of the three types of sensors in the air bag system, the AMS has the highest reliability as 99.97% and followed by EMS (93.98%) and ES (92.62%) respectively. These results show that the design of the airbag reliability has to be improved as two manufacturers set a reliability goal as at least 99.99%.

Q#3: The following table lists some engineering calculations of system reliabilities for each type of system over time when repairability is taken into account. Plot these data on a graph. What do the data suggest?

System Reliability for Automotive Airbag
1.000000

Reliability

0.999500
0.999000
0.998500
0.998000
0.997500
AMS
EMS
ES

5
0.999844
0.99987
0.99919

10
0.999716
0.999759
0.998494

15
0.999588
0.999648
0.997799

17
0.999537
0.999604...
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