Tech Briefs Magazine - May 2022 - PIT-5

Integral Rotary Cryocooler
Split Linear Cryocooler
Linear
Compression Space
Compressor Piston
Mechanical
Linkages
Controlling
Motion and
Phase
Expansion Space
Compression Space
Coil
Moving
Magnet
Rotary Motor
and Crankshaft
Displacer and
Regenerator
(Heat Exchanger)
Figure 1. Schematics of Rotary and Linear Cryocoolers
Dewar: The Dewar is a vacuum package
containing the FPA, cold-shield (defines
the f/number and prevents stray
light), and the cold-filter (determines
the wavelengths of the photons at the detector).
The electrical signals between
the ROIC and the camera electronics are
provided via hermetically sealed feedthroughs.
Faster f/numbers increase
performance, but also increase the size
of the optics.
Cryocooler: Mechanical cryocoolers
provide extended duration cooling to
the IR FPA. Modern designs integrate directly
to the Dewar as part of an Integrated
Dewar Cooler Assembly (IDCA).
They are also integrated with and controlled
by camera electronics, generally
through a dedicated cooler control electronics
module.
Camera Electronics: The camera electronics
may include multiple printed circuit
board assemblies (PCBAs) to accomplish
the following functionality:
* Sensor interface electronics include
power, clocks, and timing logic to the
FPA and, if necessary, digitization of
the FPA outputs.
* User interface electronics include signal-processing
for noise filtering, imMechanical
Spring
Figure
1. Schematics of Rotary and Linear Cryocoolers
age enhancement, operational logic,
and camera functions, including the
overarching command and control interface,
and video output standards for
the user interface.
* Cooler controller electronics control
the cooler operation to cool down and
maintain the FPA/Dewar temperature
chosen to optimize power and image
quality.
* Optics controller electronics control
the opto-mechanical lens assembly,
maintain continuously athermalized
focus over zoom, and support the user
interface.
Continuous Zoom (CZ) Optics: A CZ optic
can be integrated to provide the final
imaging solution. The CZ opto-mechanical
lens assembly includes the optical
assembly, mechanical packaging, focus
and zoom motors, and temperature sensors.
Cryocoolers
Past and Present:
Rotary and Linear Types
Stirling cryocoolers for tactical applications
are almost always rotary or linear
types. Rotary coolers use a crankshaft attached
to their moving piston and displacer.
This arrangement allows precise
control of the relative phase angle of
these components but introduces side
forces from the crankshaft linkages that
can impact lifetime and reliability. In
contrast, linear coolers are driven by
voice-coil type actuators with minimal
side forces and that rely on pneumatic
drive and tuning to control the phase of
the displacer relative to the compressor.
Rotary coolers were the most common
cooler type in early cooled sensors.
Due to its precise, mechanical phase
control mechanism and the fact that the
phase does not vary with frequency
(RPM), rotary coolers have historically
had higher efficiency and faster cooldown
times than linear coolers. The
drawback for these coolers has traditionally
been lifetime due to the increased
seal wear caused by the side forces generated
by their mechanical crankshaft
linkages. However, modern designs have
greatly improved reliability with some
manufacturers claiming lifetimes of
15,000 to 30,000 hours. Another drawback
for rotary coolers is exported vibration
from its unbalanced crank mechanisms,
which can cause jitter for sensitive
systems and, in turn, can lead to an increased
SWaP footprint and lower reliPiston
Piston
Linear
Motor
Compressor
Pneumatic
Expander
Centering
Spring
Displacer and
Regenerator
(Heat Exchanger)
Expansion Space
Photonics & Imaging Technology, May 2022
5
PIT Imaging Feature 0522_1.indd 5
Cov
ToC
4/18/22 12:13 PM
http://info.hotims.com/82321-809

Tech Briefs Magazine - May 2022

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