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Re: 11/4 SEDSAT Update(possible spurious data in current sensor



At 7:21 AM -0700 11/4/98, Dennis Ray Wingo wrote:
>To All
>
>I have been going through the telemetry from various people and have found
>the following anomaly.  The main current sensor occasionally comes up with
>the number 187611 as it is reading current.  This is a very strange number
>as it is outside the bounds of the A/D converter on the bird. (The
>conveter is an Analog Devices AD 7874 four channel 12 bit device which
>gives a range of -2048 to + 2047 hex in the range of -10 Vdc to + 10 Vdc).
>Question to chris bond.  Would an erroneous number like that effect the
>software calculations of AMPS IN BATT?  I am nearly certain that this is
>not an artifact of transmission as this number only crops up in the
>current reading on the main sensor and no where else is it apparant.  I
>noticed this in the WOD file for either orbit 14 or 16 and also in the
>orbit 138 data gathered by one of the amateur radio operators.
>
You should be able to check whether or not it is corrupting the AHI by
looking at the sequence of AHI readings. They don't track precisely
one-to-one since the data is not sent as recorded, but only at longer
intervals. If the reading is only for one second then it might cause
(roughly) at 200k mAs error, or a small fraction of an Ah. If it happens
rarely, this will not be a big error. If it happens regularly it will be a
problem.

Chris and I talked about some potential issues in the apparent differences
in the impled current from the AHI readings versus the recorded main bus
currents. I don't know if this has been conclusively resolved. One
explanation is that the satellite sends its current sample, which can be
effected by the PTT being on to send the data, even though in the
intervening time PTT is off. Hence main current and AHI readings shouldn't
necessarily track.

It is worth noting that the AHI algorithms were carefully tested in the lab
with the real hardware. By measurement it was found that AHI was within 5%
of actual power levels in the batteries (based on discharge and charge
measurements with the lab equipment). This doesn't mean that something else
is going on, only that the AHI algorithms did perform very consistently in
the lab.

>If this is the case then a software filter will be needed that rejects any
>number outside the normal range of current readings.  Chris I don't
>remember but what is the max scale for the current reading.  If I remember
>it is 0.001 ohm resistor in series with the main current.  The resulting
>voltage drop is then amplified by a factor of 100.  If this is the case
>then the measure is 2 volts per amp with a max of five amps recordable.
>
Outlier rejection or tracking is good software practice in any case.


Mark Maier


Dr. Mark W. Maier				maier@ece.uah.edu
Associate Professor				205-890-6642
Elec. and Computer Engineering			205-890-6803 FAX
University of Alabama in Huntsville		KF4YGR
Huntsville, AL  35899		http://eb-p5.eb.uah.edu/~maier

Stereo Image Compression	SEDSAT-1	Systems Architecting



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