Category : Physical Sensors
J.K.
Reynolds1, D.A. Catling2, R. Blue3,
N.Maluf4,
and T.W.
Kenny1
1Stanford University, 2NASA Ames, 3JPL,
and 4Lucas NovaSensor
NASA, through the New Millennium Program (NMP), has initiated a series of
space missions intended to stimulate the infusion of new technologies into
the Space Program. Among the missions selected as platforms for this infusion
is a novel spacecraft for Mars. This paper will report on selection, packaging,
and calibration of off-the-shelf micromachined pressure sensors for measurements
on Mars.
The Deep Space-2 Microprobe will be released during the orbital insertion
of Pathfinder 2, and will enter the Mars atmosphere, where it will be slowed
to a velocity of 200 m/s. Upon impact with the surface,
a "Penetrator"
will separate from the remainder of the spacecraft, and travel approximately
1m into the surface, as illustrated in Fig. 1. The remaining "Aftbody" will
come to rest on the surface. The penetrator is designed to survive this
deployment, and will utilize onboard instrumentation to detect moisture in
the surrounding soil and measure other thermo-mechanical properties. The
Aftbody will include a communications system suitable for transmission to
an orbiting spacecraft, a wiring harness to the penetrator, some solar cells,
and a small suite of meteorological instrumentation. Sensors for atmospheric
pressure and temperature will be included in this suite. During impact, the
aftbody is expected to experience deceleration of order 100,000 g with a
very wide spectral distribution.
Absolute measurement of atmospheric pressure is made difficult by the following
environmental factors on Mars : The pressure is about 1% of earth atmospheric
pressure, and changes by as much as a factor of 3 from day to night. The
atmospheric temperature can be expected to change from nearly 300K in full
daylight to less then 200K at night. The total power available on the spacecraft
is severely limited, so temperature regulation is not possible. Perhaps most
importantly, the sensor must not suffer offset or sensitivity errors due
to the violent nature of the deployment. The intended
packaging scheme for this application is illustrated
in Fig. 2. Of equal importance is the fact that the
instrumentation must be delivered for integration
on the spacecraft no later than 7/97.
Space-rated pressure sensors are not available for such a mission. We have
decided to pursue the following approach : We have noted that the performance
of commercial micromachined pressure sensors are not appropriate for this
mission. However, we intend to select and individually calibrate a set of
Novasensor pressure sensors. The temperature coefficients of such devices
are large, but should be very stable, and we expect that careful calibration
will allow us to eliminate errors well enough to measure atmospheric pressure
on Mars to an accuracy of 0.1% over the 2-week lifetime of the mission. The
use of such pressure sensors will demonstrate 2 important issues. (1)
Micromachined silicon pressure sensors can be packaged to survive violent
deployment, and are therefore suitable for almost any space applications.
(2) Off-the-shelf MEMS devices are not generally intended for such
high-performance applications, but careful
instrumentation design and precise calibration can
enable their use.
This paper will describe the measurement of atmospheric pressure on Mars.
A description of the design and testing of the instrument package will be
presented later. Calibration data for Lucas Novasensor pressure sensors will
also be presented. This data will illustrate the potential high performance
of off-the-shelf piezoresistive pressure sensors which are carefully instrumented
and individually-calibrated.
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