Aerospace and Electronic Systems Magazine April 2017 - 24
Electromechanical Actuator Fan Failure Analysis and Safety-Critical Design
Fault tree analysis on Case 4 (dual-fan).
the fans running in opposite directions has the advantage of reducing the vortex loss because the second fan can "pick up" the air in
the opposite direction. If both fans are running at the same direction, the second fan has to "catch up" with the very high vortex
speed (75∼100 m/s) in order to accelerate the moving air and this
causes internal losses to the dual-fan. Blade shape design is not
within the scope of this article and is the subject of a future article. The reason for seven and five blades is because an identical
number of blades for both fans may lead to resonance in case they
need to run simultaneously. The two fans are used one at a time. If
one of the fans fails, the redundant one will be switched on immediately. The fan is structurally integrated with the EMA motor but
is powered and controlled independently. The fan speed can also
be controlled based on the temperature of the EMA motor, thus
saving fan power. Figure 10 shows the fault tree analysis on Case
4 (dual-fan) and it can be seen the failure probability is 4 × 10−10 if
FTA method is used.
In short, dual-fan by using single 50,000-hour lifespan fan
is designed. With dual-fan, an overall fault probability 4 × 10−10
(= 2.5 × 109 hours) can be achieved. If a nonoperation factor 2
is considered, which means a single 50,000 hours' fan can hold
100,000 hours' lifespan (only work in half time), the overall fault
probability is about 1/100,000 = 1 × 10−5. By using two fans, a
fault probability 1 × 10−10 is reached. In addition, in previous faulttolerant cases: dual-winding, dual-bearing, if one wants to improve
the reliability of winding or bearing, it will incur high cost because
the dual-winding and dual-bearing need new research and development investment. And the results are not attractive. It cannot improve the EMA's overall fan reliability. Since a fan weighs only
150 to 170 g, it is attractive to use dual-fan to improve the safety
hours from ∼O(5 × 104) up to ∼O(2.5 × 109) hours without incurring heavy weight and high cost.
fault-tolerant cases: dual bearings, dual windings, and dual
fans to improve the safety requirement from ∼O(5 × 104) up to
∼O(2.5 × 109) hours without incurring heavy weight and high
The performance characteristics of a high-performance fan
(Ametek) under a wide range of ambient pressure and rotational speed have been studied. It was shown that with wider fan
blades, the fan performance improves considerably. A sevenblade and five-blade dual-fan design was introduced, with each
fan rotating on its own independent shaft in opposite directions.
For the failure probability improvement, our approach is to analyze failure modes and mechanisms effects on brushless direct
current fans taking into consideration of the expected application conditions. After summarizing the possible failure causes
and failure modes of BLDC fan by focusing on each failure
mechanism, the life expectancy of fan ball bearings based on
major failure mechanisms of lubricant deterioration was calculated and compared to the literature. The advantages and disadvantages of the four fault-tolerant cases are discussed and
it is shown the approach of using dual fans can significantly
improve fan reliability. This demonstrates the importance of
selecting a suitable dual-fan design to fulfill the aircraft EMA
Future work for a fan will involve identifying the actual life
cycle profile experiment testing and health state of a fan based on
sensor data. A loop test will be conducted on the seven- and fiveblade designs in conjunction with computational fluid dynamics
analysis. By in-situ monitoring failure precursors and dynamically
predicting remaining useful life of a fan during an accelerated life
test, the test time and cost required for fan reliability assessment
can be significantly reduced.
This research is sponsored by Air Force Research Laboratory via
a subcontract with UDRI, Inc., under agreement number FA865012-D-2224/0002. The U.S. Government is authorized to reproduce
and distribute reprints for Governmental purposes notwithstanding
any copyright notation thereon.
CONCLUSIONS AND RECOMMENDATIONS
The fault probability approach for fan reliability, including
hardware analysis, is discussed. This study compares the three
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