Barry,
Insufficient testing. They have been able to duplicate the problem in the
lab, using another Mars Lander. The problem occurs some percentage of the
time. Other times it does not.
Scott
-----Original Message-----
From: [email protected] [SMTP:[email protected]]
Sent: Thursday, March 30, 2000 7:05 PM
To: Lacey,Scott; Tony J. O'Hara
Cc: [email protected]
Subject: RE: Mars Lander EMC problem?
Scott and Tony,
That means the sensor was unable to tell the leg jolt from the
landing shock.
Did the NASA report explain why the sensor couldn't implement its
function
properly? Was it another design mistake omitting transition unit of
propelling force from Pound to Kilogram?
Barry Ma
[email protected]
---------- Original Text ----------
From: "Tony J. O'Hara" <[email protected]>, on 3/30/00 11:41
AM:
>Tony,
I don't think it was actually EMC. The report I saw on CNN said the
legs
opened with a "jolt", fooling the sensors which were supposed to cut
the
retro rockets when they detected the shock of landing.
Scott Lacey<
It looks like Scott Lacey is right, thank you! I made the assumption
that
"spurious signal" was electrically generated. It appears it was
mechanically or magnetically generated! Is a Hall effect sensor
"microphonic"?
Well, I guess another lesson for me not to jump to conclusions
before all
the data is available!
Tony
Colorado
Below is an excerpt from the complete NASA report.
<<Premature shutdown of descent engines.
PLAUSIBLE. A magnetic sensor is provided in each of the three
landing legs
to sense touchdown when the lander contacts the surface, initiating
the
shutdown of the descent engines. Data from MPL engineering
development unit
deployment tests, MPL flight unit deployment tests, and Mars 2001
deployment tests showed that a spurious touchdown indication occurs
in the
Hall Effect touchdown sensor during landing leg deployment (while
the
lander is connected to the parachute). The software logic accepts
this
transient signal as a valid touchdown event if it persists for two
consecutive readings of the sensor. The tests showed that most of
the
transient signals at leg deployment are indeed long enough to be
accepted
as valid events, therefore, it is almost a certainty that at least
one of
the three would have generated a spurious touchdown indication that
the
software accepted as valid. The software - intended to ignore
touchdown
indications prior to the enabling of the touchdown sensing logic -
was not
properly implemented, and the spurious touchdown indication was
retained.
The touchdown sensing logic is enabled at 40 meters altitude, and
the
software would have issued a descent engine thrust termination at
this time
in response to a (spurious) touchdown indication.
MOST PROBABLE CAUSE OF LOSS OF MISSION>>
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