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#1
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CMG 3 to be offline for tests
From this morning's DPC:
CMG #3 will be taken offline for 4 days starting wednesday for tests. This will leave the station with only 2 CMGs, with the expectation that they will saturate more often and require russian segment thrusters to desaturate. Mike Foale commented that taking the CMG offline would automatically send attitude control to the russian segment, but ground then explained something (which I missed) to the effect that the US segment would remain in control but that the CMGs would saturate more quickly. Does anyone know if the software automatically switches to russian segment attitude control if there are fewer than 3 active CMGs ? (i.e. for those tests, do they have to bypass that part of software) Would failure of CMG system still leave the US segment software in control of attitude control (using its GPS) with the US segment then commanding russian thrusters, or would it be considered "failed" and the russian segment be wholly responsible for attitude control ? |
#2
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CMG 3 to be offline for tests
Small correction - the CMGs will not saturate more often - in fact
they shouldn't saturate at all. The software will automatically request a desaturation if the momentum exceeds a software threshold (50% for attitude hold controllers and 90% for momentum management controllers). If the Minimum ORUS Available PUI (program unique identifier) registers "NO" (meaning less than 2 CMGs on-line and/or no source for attitude & rate information) then the US GNC will relinquish control and the Russian Segment will take over. The US Segment can continue to use GPS and RGAs to provide Pointing & Support information, but once the Minimum ORUs flag says no, the Warning message "Loss of CMG Attitude Control" goes into alarm and the Russian Segment becomes solely responsible for control of the vehicle. On Tue, 13 Apr 2004 04:34:08 -0400, John Doe wrote: From this morning's DPC: CMG #3 will be taken offline for 4 days starting wednesday for tests. This will leave the station with only 2 CMGs, with the expectation that they will saturate more often and require russian segment thrusters to desaturate. Mike Foale commented that taking the CMG offline would automatically send attitude control to the russian segment, but ground then explained something (which I missed) to the effect that the US segment would remain in control but that the CMGs would saturate more quickly. Does anyone know if the software automatically switches to russian segment attitude control if there are fewer than 3 active CMGs ? (i.e. for those tests, do they have to bypass that part of software) Would failure of CMG system still leave the US segment software in control of attitude control (using its GPS) with the US segment then commanding russian thrusters, or would it be considered "failed" and the russian segment be wholly responsible for attitude control ? |
#3
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CMG 3 to be offline for tests
capbrit wrote:
Small correction - the CMGs will not saturate more often - in fact they shouldn't saturate at all. The software will automatically request a desaturation if the momentum exceeds a software threshold (50% for attitude hold controllers and 90% for momentum management controllers). Is it correct to state that with 2 instead of 3 CMGs, russians thrusters will fire more often ? The US Segment can continue to use GPS and RGAs to provide Pointing & Support information, but once the Minimum ORUs flag says no, the Warning message "Loss of CMG Attitude Control" goes into alarm and the Russian Segment becomes solely responsible for control of the vehicle. Does the Russian segment have the ability to draw readings from the US GPS/RGAs to help it determine current attitude/Position of station ? Or does it operat 100% independantly from US segment when it has been given control ? |
#4
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CMG 3 to be offline for tests
On Tue, 13 Apr 2004 20:40:43 -0400, John Doe wrote:
Only during attitude holds. During Momentum Management (the majority of the time), the momentum level hovers around 18% - drop to 2 CMGs and that becomes around 40% - far below the 90% threshold, so no thrusters are fired. The Russians use a strapped down inertial navigation system called BINS. They determine a base quaternion for the vehicle and propagate from that using a gyroscope called GIVUS to measure vehicle rates and a suite of attitude sensors (star cameras, sun sensors, magnetometers, and infrared horizon sensors). If GIVUS fails, there is a Russian backup gyro called Ort. Ort, however, has a short lifetime, so the Russians will switch to US segment data from GPS and the RGAs. Similarly, if there are problems with GPS or the RGAs, the US segment can use Russian navigation data. An important thing to understand is that regardless of whom is controlling the vehicle's attitude, both segments require valid navigation data so that the communications antennae and solar arrays will know where to point. Is it correct to state that with 2 instead of 3 CMGs, russians thrusters will fire more often ? Does the Russian segment have the ability to draw readings from the US GPS/RGAs to help it determine current attitude/Position of station ? Or does it operat 100% independantly from US segment when it has been given control ? |
#5
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CMG 3 to be offline for tests
capbrit wrote excellent description of attitude control.
Many thanks. |
#6
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CMG 3 to be offline for tests
"capbrit" wrote GOOD STUFF capbrit, please visit often. you can also reach me directly via joberg at houston dot rr dot com |
#7
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CMG 3 to be offline for tests
Additional info from this morning's DPC.
They had have a few control problems with CMG #2 lately, and Houston explained to Foale that during the 4 day test, it is possible that #2 might fail (temporarily), at which point control would fall back to the russian segment, with at least 3 alarms/warnings issued. |
#8
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CMG 3 to be offline for tests
Yes, #2 has been having LOCs (losses of comm). If that happens while
there are only two active CMGs, the following messages will appear: CMG 2 Failed (suppressed caution, no tone) Loss of CMG Attitude Control (Warning) RS Autorecovery Initiated Once the RS Autorecovery completes and they take control, the last two messages will go out of alarm. On Wed, 14 Apr 2004 04:39:36 -0400, John Doe wrote: Additional info from this morning's DPC. They had have a few control problems with CMG #2 lately, and Houston explained to Foale that during the 4 day test, it is possible that #2 might fail (temporarily), at which point control would fall back to the russian segment, with at least 3 alarms/warnings issued. |
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