Micro Stirling Mechanical Cryo-cooler teardown by Fraser

[Fraser]

In the past I have been asked whether I could do a Stirling Cryo-Cooler from a cooled thermal camera. The problem with this request is that such cryo-coolers are not common and if you have one that works, you generally do not dismantle it as such renders the cooler unuseable.

One of my cooled cameras that was awaiting my assessment is a FLIR SC3000 Quantum Well LWIR camera. Sadly that cameras cooler proved to be dead and it had clearly been dismantled by the previous owner. This presented me with the opportunity to fulfill fellow forum members requests for a teardown of the cooler 

Before we start, a little about these Stirling mechanical cryo-coolers. Some thermal imaging sensors and arrays require cooling to very low temperatures in order to work as an imaging sensor. When I say "low temperatures" we are dealing with similar temperatures to that of Liquid Nitrogen. Many thermal cameras operate their Stirling Coolers at 77K (-196 Celsius) but some operate at different temperatures. This QWIP unit operates at 70K (-203 Celsius). These are the low temperatures needed to lower the internal noise of a semiconductor thermal detector to a level where it can detect the thermal energy found in thermal imaging camera systems.  At room temperature such a detector would be swamped with its own thermal noise. There are special semiconductor detectors that are also cooled but can operate satisfactorily at "only" -70 Celsius but they tend to use Peltier Stack coolers and not a Stirling mechanical cooler so will not be covered here. The Micro Stirling Mechanical Cryo-cooler is a precision piece of mechanical engineering that has an operation al life before it requires a service. The predicted operational life of a rotary Micro Cro-Cooler used to be around 2000 hours. This has improved over the years and a 10,000 hour life is not unusual now. Servicing a Micro Cryo-Cooler, such as we are discussing here, is a specialist task that requires the correct service tools, techniques, spare parts and Ultra Pure Helium gas (NOT Helium balloon gas!). This results in Cryo-Cooler servicing being expensive and difficult to source. A cryo-cooler service can easily cost $5000 per cooler.

The cryo-cooler that we are going to see in this thread is a Rotary type but there is another type that is called a Linear Cryo-cooler. There are also split cooler systems. I will not go into the different types of Cryo-Cooler here as there is plenty of information available on the topic via Google. Are such Cryo-Coolers still made and used ? Yes they are. High performance thermal imaging systems often use a Stirling Cryo-cooler to produce low noise imagery. You will also find such Micro Stirling Cryo-coolers flying on satellites where they are used in all manner of applications including Imaging, Sensor arrays and specialist science systems that require operation at very low stable temperatures.

OK, enough of the background, on with the pictures. I have included some images that show the design of a Micro Stirling Cryo-cooler to help readers understand the content of the images. 

[Fraser]

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So what was wrong with the SC3000 Micro Stirling Cryo-cooler ?

Tests showed that the brushless motor controller/driver has failed. This means that the field coil is not receiving the correct drive but the whole integrated coil and controller PCB can be replaced without breaching the coolers Helium gas seals. Sadly someone decided that opening the coolers sealed casing and releasing the Ultra High Purity (UHP) Helium was justified. The moment the seals were breached, the cooler effectively became scrap. The failed controller and drive PCB is potted into the top of the motors field coil housing and so would not be a simple repair. The QWIP detector end of the assembly also has a very unhealthy rattle so something has broken loose inside. This Integrated QWIP detector and Cryo-cooler assembly is beyond economic repair. Hence the teardown.

In the provided images the reader will see that the motor shaft incorporates a couple of offset sections that drive the Compressor Piston and Displacer connecting rods. It looks a little like piston connecting rods on the crank shaft of an Internal Combustion Engine. The offset sections of the motors drive shaft provide the correct phase relationship between the compressor Piston and Displacer movements for the Stirling Cooling effect. Ultra High Purity Helium is used as the gas within the coolers sealed casing as it it greatly improves performance and will not freeze at 77K. Any air contamination of the Helium Gas fill can lead to 'piston lock' as a result of freezing.

The cryo-cooler that I have disassembled for this thread showed signs of high run time hours so it was likely nearing the end of its operational life when it eventually suffered an electronics failure. There is solid material contamination within the motor magnet area and this may be the result of the high run time hours or the previous disassembly. If it is due to run-time hours, something was wearing and releasing black material into the ultra clean environs of the Compressor Piston and Displacer   I have another Micro Stirling Cryo-Cooler that sounds like its has bolts rattling around inside it when running. I suspect a bearing or connecting rod pin failure. Such a failure would likely distribute solid material around the ultra clean areas of the cooler and lead to its self destruction over time. Machines suffer from friction that casuses wear so it is not surprise to see a Rotary Stirling Cooler suffering as a result of its lack of liquid lubrication.

I trust that this teardown has been of interest to those who know of the technology but have not seen the internal workings of a unit before.

[coppercone2]

what are the highest tolerances required for the parts in the cryo cooler?
and a university particle physics department with a accelerator might be a source of the UHP helium gas, I have seen big walls of filters / objects that had something about high helium purity on it (but the requirement for the feed gas was already some purer form of helium).

Looked like you need to send it through like 10 different canisters to get it cleaner.

I would not be surprised if they had additional surface finish steps in something like that, I doubt its just ground. Like burnishing or electro polishing.

[Fraser]

I regret that I have no inside knowledge on these coolers and their tolerances. The UHP Helium was a challenge for me to obtain when I went searching for it from BOC. They would not even entertain supplying it to me and the cost was high for small amounts anyway. The Helium pressure inside the cooler is around 40 Atmospheres (~588 psi) and you need specialist equipment to purge the air out of the cooler, fill it with UHP Helium and then insert the Helium seal and grub screw, all without disconnecting from the coolers single fill port. I soon gave up on the idea of refilling my old Micro Cryo Coolers with UHP Helium. 

[Spirit532]

what are the highest tolerances required for the parts in the cryo cooler?

H6/h7 or thereabouts for the motion parts, minus the crankshaft. Gas dynamic bearings(running in helium, obviously) in operation, until it crashes and starts rubbing(like in this teardown).

I doubt its just ground. Like burnishing or electro polishing.

Fine ground or lapped(honed), depends on which part.

Looked like you need to send it through like 10 different canisters to get it cleaner.

You just buy UHP 6.0 helium from a supplier. Cleaning it is quite difficult, way cheaper to just buy. UHP isn't too expensive.

I soon gave up on the idea of refilling my old Micro Cryo Coolers with UHP Helium. 

Should've sent them to me

[MrSheep]

Hey Fraser,

Can you unscrew the screw on the side of the cryocooler that is used for filling the pressurized helium. Just curious to see what kind of screw/ seal they use. I believe the port is here circled in pink (see attached).

Thanks

[Fraser]

Sadly not possible at the moment. The camera is rebuilt and buried in the spares donor store.
From memory, the Helium gas fill hole is nothing exciting. Just a flat sealing seat at the bottom of the threaded hole with a indium seal that is crushed by the flat faced threaded screw slug. No special seal recesses or strange sealing surface profiles. When it comes down to it, the is is just a flat faced machine screw crushing a ‘donut’ shaped seal against a sealing flange. The actual gas fill hole at the bottom of the threaded orifice is very small. Less than a mm in diameter.

Fraser

[MrSheep]

Ah yeah that is understandable. yeah I figure it was something like that. I am guessing it is something similar to this seal just with a smaller plug:
https://hydrogen.wsu.edu/2016/02/09/making-a-cryogenics-compatible-o-ring-seal/

Maybe one day when one of my rotory ones stop cooling. I'll do a tear down. I do have a few units with the Ricor K508 in them. To service them if I recall was around $5500. Not terrible considering the cameras that use the cooler cost a lot more. $50-100k. I think the FPA is probably the most expensive part in a cooled camera system.

I might disassemble my other Non-working Amber Radiance 1 cryocooler. It has a plastic screw over the actual fill port on the Hughes Cryocooler. This particular unit rattles (metallic sound) a little bit on startup but quiets down after a few minutes. I not entirely sure there is damage or it's just like that because of a lack of helium.

[MrSheep]

Also do you have any idea what kind of solder they used on the housing and vacuum pinch off tube? I wonder if they used some kind of soldering iron rather than a torch that you would use when brazing copper pipes.

[MrSheep]

I have a similar motor controller/stator assembly I took apart. In order to get to the electronics you need to first remove the rubber seal. And then with tweezers and a flathead screwdriver u pry the black metal ring to release the coil. I will show some pics in a bit when I can get to a computer.

I believe the controller/stator assembly you have is of a K508 variant if not the same. Coercive Systems still manufactures the Ricor K535's so I bet they had collaboration with Ricor in the past with K508-like cryocoolers.

[Fraser]

MrSheep,

Thank you for this helpful insight into disassembly of the field coil assembly. I still like learning about these clever little heat pumps. The way that the field coils and controller is outside the Helium containment and acting on a magnetic armature through the metal casing is very clever. Such avoids the need for multiple gas tight seals on the connections to the field coil controller and likely reduces contamination of the Helium by coil/related outgassing. Of course it also means a failed coil or driver may be changed without having to break the seals on the Helium containment.

I have the Stirling cooler out of an AGEMA 900 series cooled scanning camera that forum member Ultrapurple (Giles, RIP) gave to me. I have always intended to mount that cooler on a wooden mount as an ornament. I love fine engineering and these Stirling coolers are things of beauty to me. It is such a pity that they can fail at any time due to gas leakage or ‘hours run’ related wear. That is why I like the Linear type of coolers with their simpler mechanism and potentially much longer lifespan.

Fraser

[MrSheep]

Yes very clever design indeed. The outside stator interacts with the hermetically sealed permanent magnet shaft on the inside. Great for maintenance purposes because you can replace the stator without disturbing the cooler itself. This design with the electronics on the inside of the K508 reminds me of my Thales RM3. The Thales is a little more advance since it has a serial port that communicates with the DSP. You can program the cooler set point. However I do prefer the old analog version such as the K508 and other linear cooler controllers where you can set the set point temperature via a potentiometer. No need for fancy proprietary software just a flat head screw driver. By removing the small rectangular metal plate (as seen in the photo). You can adjust the potentiometer to whatever set point u want.

Most FPA's have two diodes. One for the cryocooler controller one for the FPA and its processor (usually an FPGA). U can then see the FPA temp in software depending what type of system u have. I was actually able to fix a cooler in one of my cameras that didn't activate the fpa because the cooler set point was too high. After a few hours of research and disassembly I was able to locate the cooler controller and adjust the set point potentiometer.

Anyways attached is a picture of my older K508 Stator I am trying to repair. This is what it looks like fully disassembled. There is an outer O-ring u can remove and an inner o-ring which is under another metal ring. Everything comes out easily as it is lubed up with white thermal grease. (Just remember to apply it if it is a working unit and u want to reassemble it).

[ArsenioDev]

To service them if I recall was around $5500. Not terrible considering the cameras that use the cooler cost a lot more. $50-100k.

Who services them? my crew has several that may need service and I'd like to know for reference

[MrSheep]

To service them if I recall was around $5500. Not terrible considering the cameras that use the cooler cost a lot more. $50-100k.

Who services them? my crew has several that may need service and I'd like to know for reference

The main companies (FLIR, American Infrared Solutions (AIRS)) as well as the original manufactures of the coolers (Ricor, Thales, FLIR). What type do you have?
I am still working on my own setup. Hopefully I will get to do it in-house in the future. A lot of the tools to do it are proprietary so it's a lot of R and D on my part. A fun learning process though.

[MrSheep]

(Adding this again accidentally deleted post )
Here are more views of the control board. I love the use of pin sockets. Better for diagnostic and repairability. (The Thales RM3 stator has Dupont pins which are soldered)

So the purple wire is standby, the grey wire is shutdown, green is unknown. gold wires are for the FPA temp diode (anode and cathode). And a ground and power for the cooler.
You don't need the purple, grey, and green wire really. To test a cooler of this sort you only need power/ground. as well as the gold shielded diode wires. You can short the diode wires to make the cooler run.

This way you can make it work as a desk model by adding a switch to the diode wires. Or even more fun would be to use a potentiometer to trick the cooler into a steady state. You will witness it run in full rpm as well as set point rpm. I would use a precision pot like a blue bourns one. You just need to find the specific resistance value to find the set point. The way the temp sense lines work is that they provide a constant current to the FPA diode. When the temp changes the voltage at these wires change. Once they reach a certain value. (ex 1.045V) the cooler goes into steady state.

[samuel_hayden]

To service them if I recall was around $5500. Not terrible considering the cameras that use the cooler cost a lot more. $50-100k.

Who services them? my crew has several that may need service and I'd like to know for reference

The main companies (FLIR, American Infrared Solutions (AIRS)) as well as the original manufactures of the coolers (Ricor, Thales, FLIR). What type do you have?
I am still working on my own setup. Hopefully I will get to do it in-house in the future. A lot of the tools to do it are proprietary so it's a lot of R and D on my part. A fun learning process though.

Ricor, L3, Magnavox, FLIR, and a couple unknown units (haven't looked at the camera assemblies yet -- Tamam units)

[Fraser]

An interesting document from FLIR discussing cooler reliability.....

https://www.flir.co.uk/discover/cores-components/understanding-cryocooled-system-reliability/

PDF version attached.

Fraser

[Fraser]

I just purchased two non-working FLIR MIRV cooled MWIR surveillance scopes (Military use) that were made in 2007. It is a calculated risk as the coolers could both be dead. I suspect they contain the well known FLIR MC-3LP low power rotary cooler. If so, those coolers have an MTBF of 12,000 hours. Not bad for a rotary cooler. The price was right and I can still sell them to military FLIR scope collectors if they prove to be BER.

Fraser

[MrSheep]

Very cool. Yeah I have the MC3 micro cooler in one of my cameras. Always interested in its cold finger dimensions (never dared to open it since it is still working haha), so if yours doesn't work it would be helpful to see if a more readily available cooler such as the Ricor K508 would fit. Do you have a Datasheet or model for the scopes?

[Fraser]

Yes, I have the datasheet for the FLIR MIRV (Mini IR Viewer) and the units I bought come with the user manual 
Not much available to download from the old FLIR website using Wayback machine due to the nature of the devices.

The MIRV is a version of the FLIR See Spot III that was used with target laser designators operating at 1.06um. The See Spot III is sensitive to 4.5-4.8um and 1.06um wavelengths, so a bit unusual. The MIRV is thankfully a standard MWIR 3-5um unit though. I attach the datasheets for the MIRV, See Spot III and SNIPIR sniper scope versions of this unit.

I suspect that the MIRV contains either the MC3 or MC3LP cooler but cannot be certain as they are similar. The MC3 has a slightly lower MTBF at 8,000 hours so I am am hoping they used the MC3LP in the MIRV.

Fraser