Tech Briefs Magazine - May 2022 - PIT-6

Cryocooled Infrared Imaging Systems
100%
" Bathtub " Curve
MTTF / MTBF (Before)
Infant
Mortality
Region
Random Failure Region
Time
Figure 2. Relationship between MTBF, MTTF, Failure Rate, and Cumulative Failure Distribution
MTBF (Between)
1
Wearout Failure Region
50%
fective displacer, leads to a larger cold
finger. Pulse tubes are also prone to orientation
sensitivity, particularly in
high-G environments. These drawbacks
have prevented pulse tube coolers from
being used in most tactical applications,
despite the lifetime advantage.
Paired with a non-contacting flexure
bearing compressor, pulse tube coolers
can achieve lifetimes greater than
100,000 hours.
Cryocooler Reliability and
Misconceptions
Reliability for cooled camera modules
and cryocoolers is often reported
in terms of either mean time between
The FLIR Neutrino SX8 CZ (Image: Teledyne FLIR)
ability if stabilization is needed. Generation
of acoustic noise is also a potential
issue.
Linear cryocoolers have become the
most common cooler type for newer systems
due to their longer lifetimes and
lower vibration. However, the linear cryocooler
mechanism does typically drive
slightly larger size for a given cooling capacity
and slightly longer cooldown
times. Because they are tuned to operate
near resonance, linear coolers are limited
to a single operating frequency. Input
power can be modulated through the
amplitude of the compressor pistons, but
the inability to increase frequency during
cool-down results in longer cool-down
times than what can be achieved using
rotary coolers. For modern linear coolers,
lifetimes of 20,000 to 30,000 hours
are becoming expected.
6
The pulse tube cooler is a special
type of linear Stirling cooler in which
the moving Stirling displacer in the
cold finger is replaced with an expander
containing no moving parts. A column
of gas replaces the physical Stirling
displacer. This " gas piston " is
paired with a stationary regenerator
and a phase shifting mechanism to control
the mass flow relative to the oscillatory
pressure. This phase control mechanism
is more sensitive to operating
conditions than the pneumatic tuning
of the standard linear Stirling cooler,
leading to lower efficiency at operating
points away from the design point.
Cooldown time is especially impacted
and is generally significantly longer
than other coolers of similar capacities.
The separation of the regenerator from
the pulse tube, which becomes the effailures
(MTBF) or mean time to failure
(MTTF). While the terms sound similar,
there are several important distinctions.
Generally, MTBF is applied to repairable
systems while MTTF is used for systems
which cannot be repaired. More
importantly, the two metrics are defined
differently and do not compare directly
to one another. To add further ambiguity,
sometimes MTTF is referred to as
mean time before failure (MTBF).
The MTBF (between) metric assumes
a constant rate of failures for a component,
which is a good assumption for
components not subject to mechanical
wear. It is equivalent to the inverse of
the random failure rate of the component
and is often defined as (# of hours
operational)/(# of failures) for a population
of interest. MTBF was often used
historically for cryocoolers and cooled
IR cameras. For modern cryocoolers,
the failure rate prior to wear out is generally
very low and the MTBF is, therefore,
very high. Because it does not account
for mechanical wear out of the
cryocooler and is only focused on the
random failure rate prior to wear out,
the calculated MTBF of a cryocooler
will often significantly exceed its MTTF
or expected lifetime.
#
(
) =
1
(
)
Cryocooler lifetime estimates are often
presented in terms of MTTF and calculated
using Weibull statistics. The
most-used Weibull distribution has two
parameters, a shape parameter indicative
of the amount of wear in the system
and a lifetime parameter representing
the point at which 63 percent of the population
will have failed. The two-paramePhotonics
& Imaging Technology, May 2022
=
#
PIT Imaging Feature 0522_1.indd 6
Cov
ToC
4/18/22 12:25 PM
Failure Rate
Cumulative Failure Distribution
http://info.hotims.com/82321-809

Tech Briefs Magazine - May 2022

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