Photos of Kearfott Ring Laser Gyro-Based Inertial Reference Units

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These are photos of Kearfott navigational Inertial Reference Units (IRUs) or parts of them that use HeNe laser-based Ring Laser Gyroscopes (RLGs). Only two samples of one model at present.

Note that the terms Inertial Reference Unit (IRU) and Inertial Measurement Unit (IMU) may be used interchangeably. The Kearfott Website uses IMU; the label on the actual device has IRU. Go figure. ;-)

Kearfott Inertial Reference Unit K600A374-01

This is a complete system, probably intended for "Sea Navigation" - e.g., in submarines, larger UAVs, and the like. For ships, the Z axis would be largely wasted. ;-) Exactly what the difference is compared to "Land Nav" systems is probably more in the interfaces and other capabilities than the specific functions. The primary physical diffference appears to be of the paint color in the Kearfott brochures: Land Navs are black; Sea Navs are biege. I suppose that black doesn't show dust and dirt like a light color would. ;-)

           

Kearfott Inertial Reference Unit: Outside (left), Inside Labeled (center), Inertial Sensor Assembly, Platform (ISAP, right)

For much more information on this IMU, go to Kearfott K600A374-01 IMU with Monolithic Block Triple RLG.

Kearfott Inertial Reference Unit K600A374-01 #1

(The photos in the first Web album are courtesy of Ismael Tremblay.)

See Kearfott Inertial Measurement Unit Assembly K600A374-01 #1 (Web Album, 25 photos).

The RLUs in this IRU are interesting because all three axes are fabricated in a single block of special low expansion glass or similar material. The accelerometers are totally separate. At present not much else is known other than that the RLGs do power up, at least briefly.

The last pic shows an ideal application for this IRU. ;-)

Kearfott Inertial Reference Unit K600A374-01 #2

But first, here are some short videos of what happens with the RLG Block when the IRU is powered. Click on the small image to see the corresponding video.

                   

Overall Setup for Powered ISAP RLG Block Photos (left) and Closeups

These videos show the naked RLG Block from IRU #1 tethered to the power supply and electronics from IRU #2. Minor rearrangement and camera angle variations similar to the next three are used for all the powered photos in the Web Album, below. The last one has the RLG Block oriented in approximate agreement with the schematic derived from the Kearfott patent. However, even with the thing in-hand and powered, it's virtually impossible to figure out the discharge and beam paths due to the contoured exterior of the block.

The clicks near the beginning and end of the videos are from the outlet strip power switch - on and off. Around the mid-point of the video, the RLG Block glows brightly three times in succession accompanied by a continuous audible whine (likely from the dither PZT). This behavior is 100 percent repeatable and IRUs #1 and #2 do exactly the same thing, so it's very likely a feature, not a bug. The assumption is that there needs to be some handshaking with whatever the IRU is connected to for it to remain active. The indicator LED does stay lit but that is the only remaining sign of life. So these videos get rather boring after almost no time at all.....

And now for the good stuff. ;-)

See Kearfott Inertial Measurement Unit Assembly K600A374-01 #2 (Web Album, ~96 photos so far).

Description of the Web Album photos for #2:

• Overall views (Front, Right, Front_Right, Right, Back_Right, Back, Back_Left, Left, Front_Left): These are virtually identical in appaerance to the sample on the Kearfott Website in the year 2025 under "Sea Nav IMUs".

• Closeups of the five military-style circular connectors:
  • J1 (66 pins): Control / Signal / Information.
  • J2 (15 pins): Power input. 28 VDC nominal, 30 W max. 24 VDC OK. +Vin: Pins A,B; -Vin: Pins M,N. Case GND: Pin P. No other pins used.
  • J3 (13 pins): ????
  • J4 (22 pins): ????
  • J5 (13 pins): ????

  (J3 and J5 have the same pin configuration but are keyed differently.)

• Label: This has both the numbers as described above.

• Indicator: This is also visible in the lower right corner of the Label photo. Originally I thought it was some sort of fancy optical sensor because it does NOT look like a photodiode or LED. But the mystery has been mostly solved after powering the unit multiple times for the photo shoots and actually paying attention to what it does, which would appear to simply be a high priced mil-spec LED in a hermetic package no doubt costing the U.S. Taxpayer BIG BUCKS. It turns on bright green a couple seconds after power is applied to the IRU. Whether it is simply a fancy Power LED or something more sophisticated is not known.

• Caution Label: Like we didn't know. ;-)

• Inside Top: This is what greets you after removing the cover. It's a very clean layout with only 3 or 4 major subassemblies (depending on how they are counted): Main PCB Assembly (top of photo, mostly hidden), "Inertial Sensor Assembly, Platform" (ISAP, the round can on elastomer mounts, lower left) - the heart with the ring laser gyro block and accelerometers; and the power input, conditioning, and interface modules (lower right).

• Main PCB in Situ: What's visible is the top PCB; a similar size board is mounted on the other side of the aluminum frame.

• Connector PCB in Situ: At least for some of the connectors.

• Top Main PCB: Some LSI parts but mostly smaller ICs. The black polka-dot rectangular affair at the right is the high voltage power supply for the RLGs and possibly the dither PZT.

• Bottom Main PCB: More chips including a Texas Instruments TMS320 DSP. I would guess that the black painted-over 32 pin packages contain the firmware. ;-)

• Power Connector Module Inside: This is what is behind J2. It shows the very simple but almost impossible to follow wiring due to the use of all white insulation. ;-)

• Power Filters: These appear to be RFI-type filters for the input power. (Look up their Martek part numbers!) Also visible is the Power Connector Module.

• DC Power Converter: This is another readily identifiable Martek module. It is a DC-DC converter that takes 28 VDC (nominal) and provides +5 VDC and ±15 VDC for all the electronics. The spec is actually 14 to 40 V in, so fortunately it will run happily off 24 VDC, which is a lot more common for powering on the bench with a fixed supply. ;-)

• Interface PCB: Function of this PCB is unknown but it has a bunch of opto-isolators on it so it must have something to do with interacing in a nasty electrical environment.

• RLG HV Connector: This is really just a white wire with high voltage insulation and a female contact secured with a black plastic cap for the HeNe lasers inside the RLG block and possibly the dither PZT. It has approximately -600 VDC on it with respect to the case starting a few seconds after the IRU is powered. There is enough high quality capacitance somewhere such that enough charge may still be present hours after powering down to result in a mildly shocking experience. ;-)

• ISAP in Situ: Another view.

• ISAP with Mounts: Front, Right Side, Back, Left Side, Top, Bottom. The upper cable is for the RLG, the lower cable is for the accelerometers. The +HV socket for the RLG is also visible.

• ISAP Label: And S/N 670. ;-)

• Inertial Measurement Unit Labeled: And an interior view with major assemblies annotated. ;-)

• ISAP Housing, Mounts, Shroud: Exposing the cylinder with the RLG block and accelerometers.

• ISAP Shroud: The purpose of the Shroud is not entirely obvious as it is not structural. But it is made of a thin soft ferrous material which is perhaps Mu-metal, and thus may be a serving as a magnetic shield. It is conceivable that even weak magnetic fields could affect the behavior of the lasers in the RLG at the margins of precision. If that isn't its function, it must be to make disassembly more annoying. ;( ;-)

• ISAP Shroud Label: The wording of the label on the shroud is strange (and probably unrelated to the shroud itself): "KEARFOTT CORPORATION IS THE INITIAL TRANSFERER OF THIS PRODUCT, WHICH CONTAINS THORIUM LICENSED UNDER 10 CFR 40.13(c)(i)(ii)". It's even stranger looking up the definition of 10 CFR 40.13. ;-) 10 CFR 40 is part of the "Code of Federal Regulations" dealing with nuclear energy. ;-) Part 13 states that the company is exempt from regulations because there is very little of the stuff present. Thorium is a weak Alpha emitter so a trace amount may have been placed inside the RLG block to assist starting of the HeNe lasers. Any Alpha particles would be totally blocked by the block (no pun intended intentionally) so forget your Gieger counter. ;-)

• ISAP RLG PCB Top: The RLG monolithic block is below this PCB. The wires in the 3 ribbon cables (5, 6, 7 wires each) and 3 sets of individual orange, yellow, and green wires need to be carefully unsoldered to remove the PCB. That's a total of 27 wires and they are a lot thinner and closer together than they appear in the photos. In fact everything in the "can" (which is around 8 cm or just over 3-1/8 inches in diameter) is actually much smaller than it appears in the photos. ;( ;-) Detaching (and re-attaching) the wires requires a steady hand, super fine tip soldering iron, and Mark II upgraded eyeballs (or a good magnifier or microscope).

• ISAP Accelerometers with PCB: The three accelerometer units are attached to a metal frame and wired to their respective PCBs.

• ISAP Accelerometer Interface ICs: A fancy part for each one. ;-) This may be custom as the only hits via a Web search return info on an older Kearfott IMU.

• ISAP RLG Block in Situ1: The visible portion of the RLG block exposed when the accelerometer assembly is removed. Three of the six mirrors are visible with their PZTs for cavity length control. The purpose of the what appears to be Home Depot Fiberglas foam is not known. It's very soft and fluffy and not likely to provide much damping or thermal isolation.

• ISAP_RLG_Block_Insulation1: After removal. I hope the specific locations of each clump were recorded. ;-)

• ISAP_RLG_Block_in_Situ2: And with the foam removed. When the RLG lasers are powered there is around 500 VDC on the central pillar as well as the small test-point at the 1'oclock position, which is actually connected directly to the pillar. Go figure. ;-) I wanted to take photos of RLG Block powered in situ but for some reason, neither the RLG discharges or PZT drive will come on with the cover in place even if not attached. There is just a brief faint "weep" and it then apparently aborts. ;( ;-)

• ISAP_RLG_Block_Cover1: Removing this is more than a matter of three screws. There are 27 teeny wires that need to be carefully unsoldered. And everything is must smaller and more closely spaced than it appears in the photos.

  The overall dimensions of the cover cylinder are 3-1/4 inches in diameter (not including the small protrusions) and 5-3/4 inches tall. The RLG Block itself is around 2-1/2 inches at its maximum width. So this is really so much smaller than it appears in the photos and videos! ;-)

• ISAP_RLG_Block_Top1-2: Two views almost straight down.

• ISAP_RLG_Block_Down1-6: Six Views at roughly a 45 degree angle.

• ISAP_RLG_Block_Side1-6: Six views at roughly a 15 degree angle.

• ISAP_RLG_Block_Close1-5: Five closeup views of the interior attempting to show details of the pillar area, but the block gets in the way distorting everything within it.

  Two of these shots show what appear to be coils of magnet wire near the bottom, specific purpose unknown, though a guess would be that they are part of the pulsed starter for the six HeNe laser discharges. With a supply voltage of 600 V and "tube" discharge voltage of 500 V, the current would be around 0.33 mA though the 300K ohm ballast resistor for each discharge path, which is consistent with measurements made on other similar RLGs. There is probably no actual current regulator, only a transistor or MOSFET switch to disable the discharges when the RLGs are not running since the 600 VDC supply remains on constantly.

• ISAP_RLG_Block_Not_Powered, Powered, and Dark_Powered: Several sets of photos from various orientations and distances showing the RLG Block in room lighting, with the discharge powered, and the same in the dark. However, being tethered to the HV and RLG cables severely limits options for creative photo composition. ;-)

  The first set of three shots shows the overall setup for taking these photos with the naked ISAP RLG Block next to its clothed buddy inside the IMU.

  Since this IRU is not connected to anything except power, it does not stay on for more than a few seconds. This is not thought to be a fault since unit #1 does the same thing. So it's a feature, not a bug. At least that is the assumption: Its microbrain expects some sort of hand-shaking or acknowledgement and quits when this does not occur. Further, the RLG Block is only powered three (3) times for a second or so and then shuts off until the next power cycle. So the photos of the RLG Block with the discharge glowing required applying power to the IMU with the camera already focused and its shutter mode set to "Repeat". ;-)

• ISAP_RLG_Block_Central_Pillar1: This runs through the entire block top to bottom and plays some role in powering the HeNe discharges as there is around -500 V with respect to the IMU case on both the pillar itself and the little test-point at the 1 o'clock position when the lasers are lit. (The test-point is electrically connected to the pillar, strange!) That would be consistent with the expected value of the "tube" discharge voltage and the Central Pillar may indeed be the common cathode for all six of the discharges. However, though there is a well insulated wire from the HV Connector leading into the pillar, there is no continuity to it detectable using a DMM, or even by applying a few dozen VDC from an adjustable power supply. Since the HV Connector receives approximately -600 VDC (as measured) when the IRU is powered, ~100 V is lost to circuitry, which among other things probably controls power to the HeNe discharges.

• ISAP_RLG_Block_Ballast1: The blue resistors standing up are one of three pairs that provide the main ballast for the HeNe discharges. They are around 300K ohms each. Their common point is connected to the other two pairs. The blue resistor laying down along with the diode are believed to be related but exactly how is not clear.

• ISAP_RLG_Block_Tip-off1: This is where the air gets sucked out and the HeNe gas mixture gets sucked in. ;-) Unlike some other RLG designs, it is NOT a cathode. The glob covering it is to protect fragile humans as the metal is very sharp after it is pinched off with several tons of force forming a cold weld.

• ISAP_RLG_Block_Mirror+PZT1-3: These are closeups of the three mirrors that are dithered for cavity length control. These shots are in no particular order.

• ISAP_RLG_Block_Mirror+Detector1-3: These are closeups of the three mirrors which have the interferometers combining the clockwise and counter-clockwise beams and detectors for each of the three axes. They are in no particular order.

• ISAP_RLG_Block_Schematic1: This diagram is derived from the Kearfott patent and shows the arrangement of the mirrors and beam paths inside the RLG Block. The leg length of the discharge paths in the RLG Block is between 1.5 and 1.75 inches - it's difficult to measure precisely or even if the geometry is a precise square. With 4 legs, the result is a total ring cavity length of between 6 and 7 inches for an enclosed area of between 2.25 and 3.0625 square inches. Therefore, the sensitivity is higher than that of the Honeywell GG1320, which has a two inch leg length but only 3 legs for a 6 inch ring cavity length with an enclosed area of 1.73 square inches..

• ISAP_RLG_Block_Oriented_Similar_to_Schematic1: The photo was rotated so that the locations of the mirrors are very approximately like those in the schematic. More or less. ;-)