Space Acceleration Measurement
System II (SAMS-II) |
The Space Acceleration Measurement System-II, or SAMS-II,
detects vibrations present while the space station is operation. Vibrations
exist on the space station for a variety of reasons: equipment operation,
structural motion, crew movement, and thermal expansion are but a few. No
matter how one designs a space station, some residual vibration will always
be present. The mere presence of vibrations is not a problem, but when scientists
try to study theoretical effects of removing gravity through on-orbit experiments,
the vibration tends to spoil their experiments
Requirements
The researchers within the science disciplines that will conduct
initial investigations onboard the International Space Station have a difficult
jobthey are designing experiments to be conducted onboard a vehicle
which itself is being designed and built. SAMS-II has asked these same researchers
to define their requirements for the acceleration measurement system that
will support them. In conjunction with the input gathered and the input
from users of the shuttle-based SAMS instruments, a generalized system was
defined which would support the entire set of users meeting their initial
measurement needs as well as those expected to become realized as the space
station becomes operational and research experience develops.
Simply put, the SAMS-II must measure the acceleration environment
for multiple payloads conducting research in space throughout the lifetime
of the International Space Station. It must accurately acquire this data
within the resources available and constraints imposed by the space station,
and provide this information to the ultimate user, the scientist, in various
formats, all within a timely manner. An Experiment Support Requirements
Document and the System Requirements Document for SAMS-II provide additional
insight into the specific items which SAMS-II is challenged to accomplish.
Capabilities of SAMS-II
SAMS-II has developed a distributed architecture design that
results in a measurement system that is expandable, upgradable, and deployable
onboard the International Space Station. Multiple Remote Triaxial Sensor
(RTS) systems can be deployed near the payloads requiring direct measurements
of the acceleration environment. A controller, initially consisting of a
space station-derived laptop, ties the independent RTS systems together
on-orbit and provides a single point communication link to the SAMS-II Ground
Operations Equipment where data are received for distribution to users.
The project relies on the infrastructure of the space station
similar to the manner in which a researcher relies on the capabilities of
his laboratory to conduct scientific investigations. In this manner, duplication
of resources is avoided when possible, and the needs of users are met while
minimizing the complexity and cost of developing the SAMS-II. The space
station network will serve as the communication means to move data from
source to receiver. A distributed deployment of sensors will serve multiple
users, but only when each user requires a sensor. Users will be able to
modify acquisition parameters in a manner as if they "owned" the
hardware by properly selecting the frequency range of the measurement, thereby
minimizing the burden on the network traffic load. Converting onboard data
compensation into engineering units is implemented to provide a future capability
of providing a direct feedback to payloads and crew of the current environment.
A centralized controller system will provide a means to keep track of data
acquired, status of each unit's health, and allow for simplified enhancements
to the systems by both modular software and hardware upgrades.
 In an effort
to initiate research early while the space station is still under construction,
it was realized that the early users would only be provided with a subset
of the complete capabilities planned for ultimate use by future space station
researchers. The early deployment of an interim Control Unit (ICU) follows
suit, and will result in a two-phased capability to be executed, initially
providing the limited number of initial users core measurement services.
Ultimately, the ICU will be upgraded to a full-fledged Control Unit (CU).
This second unit will allow onboard data analysis and direct feedback of
information to payloads, thereby allowing onboard control of experimental
parameters, with the hope of optimizing the experiment time on the station.
The CU deployment is timed to coincide with the initial operations of the
facility class rack systems being developed. When these facilities are deployed,
it is expected that more measurements will be required and additional SAMS-II
performance features will be desired. Throughout this deployment of interim
and final control capability, the core element of SAMS-II will be the RTS
system. This system comprises sensor heads and a supporting electronics
unit, which are installed in multiple locations throughout the space station.
Each system is capable of measuring from 0.01 Hz up beyond 300 Hz. This
wide-band region is known as the vibratory, or g-jitter regime. Amplitudes
are expected from 1 micro-g up to as high as 10 milli-g. The RTS heads are
capable of measuring across this range and beyond, should there be a higher
amplitude transient disturbance taking place. Multiple RTS systems are deployed
at various locations throughout the space station; specifically to support
the ongoing microgravity research program experiments.
A complimentary measurement instrument, the Microgravity Acceleration
Measurement System (MAMS) is in development to measure the frequency range
from 0-0.01 Hz, in the "quasi-steady" frequency regime. MAMS is
capable of measuring the atmospheric drag and residual acceleration (deceleration)
of the space station due to rotation about its center of mass. The amplitudes
MAMS will measure are expected to be on the order of 0.01 micro-g to 1 milli-g.
Interim Control Unit Capabilities
The Interim Control Unit acts as a system traffic cop for
numerous RTS systems that may be operating at any one time. The ICU provides
a singular location in which the software required to operate an RTS is
housed. Whenever an RTS is powered up, the ICU recognizes the RTS, instructs
it across the space station network to configure itself according to preprogrammed
settings, and allows the RTS to initiate measurements for payload customer.
Once the ICU receives the data from the RTS, it is checked for completeness,
and combined with other sensor data and hardware performance data for subsequent
downlink to the ground.
The ICU consists of a ThinkPad laptop from the International
Space Station pool of flight grade Portable Computer Systems. To communicate
with the RTS systems, the ICU and its SAMS-II specific software operates
across the Ethernet network known as the Medium Rate Link in a two-way communication
mode similar to that used to network computers together between offices.
This laptop is integrated within an International Subrack Interface Standard
(ISIS) drawer with SAMS-II power and cooling subsystems. Although not expected
to be needed for baseline operation, the ICU will have available within
the laptop sufficient hard disk capacity to store up to 10 hours of data
from five sensor heads running at their maximum frequency range. Such a
backup approach is being made available in case downlink services are interrupted,
but on-orbit research is able to continue while services are restored.
Remote Triaxial Sensor Capabilities
The SAMS-II RTS systems consist of two primary building block
elements: the RTS-Sensor Enclosure (SE) and the RTS-Electronics Enclosure
(EE). Each subsystem serves a distinct role in the measurement of microgravity
acceleration onboard the International Space Station. The RTS-SE is mounted
as close as possible to the experiment being supported, as it contains the
digitizers that convert the analog signals at the source to minimize the
influence of measurement noise. Integrated temperature transducers in the
sensors also allow for temperature compensation of the data when the data
are applied against temperature calibration curves. Precise alignment between
the three sensors provides a triaxial measurement of the environment as
well.
Each EE provides power and command signals to up to two SE
units and receives the digital acceleration data. The EE can conduct the
compensation of the data for temperature and alignment effects, and can
convert the data directly to engineering units. The EE also serves as the
network interface to the control unit across the Ethernet on the space station.
Generally, the EE hardware is embedded within a science rack at the time
of its initial launch, and an RTS cable is routed to a more accessible SE
which is likely to be installed onto a particular science payload when the
payload is brought up on orbit.
A custom designed RTS will be provided to support the Low
Temperature Microgravity Physics Facility which will be conducting fundamental
physics research outside the shirt-sleeve laboratory of the space station.
This facility and the custom RTS will be located on the Japanese Experiment
Module-Exposed Facility.
RTS systems are in production, and are being deployed to users
as their system development schedule requires them.
Operations and Data Analysis
The Principal Investigator Microgravity Services (PIMS) Project,
part of the Acceleration Measurement Program at NASA Glenn, provides the
science community with usable acceleration information from data collected
by various measurement instruments. Services have focused on responses to
specific data requests from principal investigators and a generation of
general acceleration environment summary reports for each microgravity mission
supported by SAMS and the Orbital Acceleration Research Experiment (OARE).
The role of PIMS during the space station era will evolve
from supporting individual science missions on short duration carriers such
as the shuttle and sounding rockets to an ongoing activity ranging from
new user education, vehicle characterization for future user knowledge,
and interpretation of events affecting the acceleration environment onboard
the space station. Reports still will be generated, but they will take on
a perspective of a status rather than a completed mission. Many lessons
from the early involvement on the Mir space station with a SAMS unit are
being applied to the plans for acceleration measurement support of the International
Space Station users.
Customers
The primary focus of the SAMS-II instrument is to support
the microgravity science disciplines conducting research onboard the space
station. These disciplines are Biotechnology, Combustion, Fluids Science,
Fundamental Physics, and Materials Science. Each discipline is developing
a general purpose facility to conduct modular research as well as initiating
research using small, self-contained experiments which are housed in the
EXPRESS Racks available on orbit and within the Microgravity Science Glovebox
system.
Secondary users may request Acceleration Measurement support
on the space station by contacting SAMS-II and developing a specific user
agreement for services. Depending on the desire of the non-microgravity
customer to collect data, a series of options may be made available. Options
may range from sharing existing data, specifically deploying a single-use
sensor, or development of a full blown custom system for long-term use.
History of Shuttle-Based Measurements
In the mid-1980's. the NASA Microgravity Research Division
directed the Glenn Research Center Microgravity Science Division to develop
a Space Acceleration Measurement System. SAMS first flew as a general-purpose
measurement system in June 1991. Subsequently, it has successfully supported
nearly every major microgravity science mission conducted onboard the Shuttle,
including international cooperative flights. As of August 1998, on-orbit
acceleration data have been acquired using SAMS during 20 shuttle missions.
In addition, a SAMS unit on the space station Mir collected data during
four years of service. This system was launched to Mir in 1994 and collected
data in support of the Shuttle-Mir Science Program experiments. Other measurement
systems have been developed and flown on shuttles for specific applications
which are similar yet complimentary to SAMS such as the OARE. Following
the acquisition of OARE in 1993, the system supported seven microgravity
research flights. A free flyer version of SAMS (SAMS-FF) has been developed
and deployed onboard a sounding rocket, supporting its first science experiment
in 1997.
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