4.2 Application of FTA method

From the composition and configuration of the Fe₂O₃ gas sensor and its use in the field, Fault Tree is constructed and analyzed.

From the analyses, heater temperature (heater voltage), vibration, humidity and reduction gas are effective factors for deteriorating the sensor life. Therefore, we examined the effect of these factors in a preparatory experiments.

　

4.3 Experiments

1) Measuring circuit. Measuring circuit used in these experiments is shown in Fig. 18.

　

Fig. 18 Measuring Circuit

　

2) Influence of heater temperature (applied voltage). The result of the output voltage with time is shown in table 5. In the experiment heater voltage was applied to the sensor stepwise from 6 to 9 volts and we had the abrupt change in output voltage at 9 volts.

Measuring heater voltage is 5 volts.

　

Table 5 The influence of heater temperature

　

3) Influence of humidity. The result of humidity with time is shown in table 6. In the experiment the heater voltage (5volts) was applied intermittently.

4) Influence of vibration and reduction gas. We could not have any changes in outpupt voltage in these tests and still continue them.

　

Table 6 Influence of humidity

The data in the table 6 is the output voltage of sensor samples (volts).

　

5. Conclusions

　

(1) To develop an accelerated testing for one failure mode, FTA is a useful tool. The procedure using this metod for developing accelerated testing is suggested.

(2) The above procedure was applied to develop the accelerated testing for the thermister temperature sensor by unraveling the failed machanism effectively and identified that "-10℃〜80℃, 95%RH, Temperature Cycling with Humidity" test is appropriate by the above mechanism and Design of Experiment.

(3) The acceleration factor reached 105 times to the field use.

(4) This procedure is useful and applied to develop accelerated testing of gas sensors.

(5) As for the V₀ Characteristic estimated factors of life failure in Fig. 17 were examined in the preparatory experiments. Among four factors We had the changes in V₀ in two factors.

　

References

　

1 Zimei SU, Takeshi Hashimoto, "Some Considerations on Reliability Tests of Gas Sensors", Proceedings of The 4th International Conference on Reliability Maintainability and Safety (ICRMS '99), Shanghai, China, 1999

2. Jianying GUO, "Evaluation Method of Environment Test for Products", 21st Reliability and Maintainability Symposium, 1991

3. Jianying GUO, Wang MEI, Lei QI "Reliability Research of MgCr204-TiO₂ Ceramic Humidity Sensor", Proceedings of '92 Harbin・China-Japan Scientific Symposium on Electronic Sensing Technology, Scientific Symposium Sensing, 1992

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