The Department of Astronautics at the United States Air Force Academy is currently planning to fly an experiment in a NASA Get-Away-Special (GAS) canister. The experiment was named the flex beam experiment. The primary technical objective of the flex beam experiment is to measure the damping of a thin beam in the vacuum and zero G environment of space. By measuring the damping in space, it is hoped to determine the amount of damping the beam normally experiences due to the gravitational forces present on Earth. This will allow validation of models which predict the dynamics of thin beams in the space environment. The experiment will also allow the Academy to develop and improve its ability to perform experiments within the confines of a NASA GAS canister. Several experiments, of limited technical difficulty, were flown by the Academy. More complex experiments are currently planned and it is hoped to learn techniques with each space shuttle flight

Bubb, Keith W.

Lamberson, Steven E.

Lash, Thomas A.

English

NASA Technical Reports Server

N90-21088
UNITED STATES AIR FORCE ACADEMY
GET-AWAY-SPECIAL
FLEXIBLE BEAM EXPERIMENT
Captain Keith W. Bubb)
Major Steven E. Lamberson,
and Captain Thomas A. Lash
United States Air Force Academy
Colorado Springs) Colorado
NASA/DOD Control Structures Interaction Conference
I February 1989
PRECEDING PAGE BLANK NOT FILMED
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https://ntrs.nasa.gov/search.jsp?R=19900011772 2020-03-19T23:21:34+00:00Z
OBJECTIVES
The Department of Astronautics at the United States Air Force Academyis currently
planning to fly an experiment in a NASAGet-Away-Special (GAS)canister. The
experiment has been namedthe Flex Beamexperiment and is being conducted
jointly between th@ Department of Astronautics and Department of Engineering
Mechanics. The experiment will allow the Departments to achieve several goals,
both scientific and academic.
The primary technical objective of the Flex Beamexperiment is to measure the
damping of a thin beamin the vacuumand "zero G" environment of space. By
measuring the damping in space, we hope to determine the amount of damping the
beamnormally experlences due to the gravitational forces present on Earth. This
will allow us to validate models which predict the dynamics of thin beamsin the
space environment.
The experiment will also allow the Academyto develop and improve its ability to
perform experiments within the confines of a NASAGAScanister. Several
experiments, of limited technical difficulty, have been flown by the Academy.
More complex experiments are currently planned and we hope to learn techniques
with each SpaceShuttle flight. Finally, we try to maintain somelevel of student
involvement in our projects. This helps to motivate the cadets to strive for more
challenging goals than pure coursework can offer.
• Measure damping of thin beam in vacuum
• Validate models
Develop GAS experiment capabilities
46o
• Student involvement
GAS CANISTER IMPLEMENTATION
We plan to physically implement the flex beam experiment in the GAS canister by
using two thin stainless steel beams. One will be used as a control test
condition to verify integrity of the data acquisition system. This beam will be
put in a cantilever configuration, as the beam's natural frequency should not
change in the absence of gravity. The second beam, which is the one of interest,
will be put in a plnned-free configuration.
We will be measuring the motion of the beams with strain gauges placed along the
beams at the points of maximum expected moment. Strain gauges will also be placed
on the shim stock used to create the pinned end so the pendulum mode of the
pinned-free beam can be measured.
The beams will be placed within the vacuum of space while the experiment
electronics, data recorder, and batteries will be placed within a sealed container
and maintained at approximately I atmosphere.
• 2 thin stainless steel beams - 2" x 24" x 0.025"
• Strain gauges at points of expected maximum moment
• Beams in vacuum of space
• Controller, recorder, batteries in sealed containers (1 atm)
461
BEAMNATURALFREQUENCIES
This chart shows the expected results of our experiment. Weare planning to fly
two stainless steel beams, one in a cantilever orientation and one as a pinned-
free orientation. Wewill not be using a physical pin because of friction, but
instead are using thin pieces of shim stock material bonded to the stainless steel
beam. The frequency of the cantilever beam should not change based on the absence
of gravity. We do, however, expect the frequencies of the pinned-free beam to
change as shown on the figure.
Support Condition Mode Freq at 1 G
Cantilever 1 Bend 5.226 Hz
2 Bend 32,75 Hz
Pinned Free
Freq in Space
5.226 Hz
32.75 Hz
Pendulum 0.903 Hz 0.1 Hz
1 Bend 22,92 Hz 22.92 Hz
2 Bend 74.26 Hz 74.26 Hz
Note: An actual pin is not used due to friction/A ihin shimis used instead.
=
462 ?
BEAMSTRUCTURES
The cantilever and plnned-free beamstructures are shown in this chart. Also
shownare the strain gauge locations envisioned using the placement method
discussed earlier. Strain gauge locations are designated as A, B, or C and will
be referred to later in this presentation.
IF - L_
A
Cantilever Pinned-Free
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ACTUATORS
The purpose of the actuators is to begin the vibration of the beams. Due to space
and hardware limitations, we are planning on a maximumdeflection of I inch.
Regardless of the maximumdeflection, we are interested in accurately knowing the
deflection and being able to consistently repeat the deflection.
Another prime criteria for the actuators, as with all of our hardware, is low
power consumption. Part of the problem with our first flight of this experiment
was the lack of power once the experiment began operation. Cold soak of the
batteries has been determined as the cause of failure with the dampers in the
first flight.
• Begin vibration of beams
• Known deflection - maximum of 1"
• Repeatable
Low power consumption
!!
±
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ACTUATORPOSSIBILITIES
Three types of actuators are currently under consideration. The first type we
have experimented with is an electromagnet, which was used to pull the beamsto
start the vibration. This method adequately pulls the beamsbut we have so far
experienced major problems with residual magnetism. This residual significantly
affects the motion of the thin beams.
A mechanical striker was used on the first flight of our experiment, but we are
not sure that we can implement one which will provide a known, repeatable
deflection within the space and budget constraints we have. Another problem with
a striker, assuming it operates in an "impulse" input fashion, is synchronization.
The pinned-free beammust be deflected at the free end and part way up the beam
simultaneously if we wish to achieve a symmetrical input (which we do). Again,
our concern is the ability to develop high precision strikers in our budget.
The final option we will investigate is a push-pull rod concept. This would act
muchas a linear solenoid but must provide the ability to quickly retract the rods
to let the vibration begin. This method also has synchronization concerns as well
as the potential problem of limited maximumbeamdisplacement.
• Electromagnets
m
m
Meet objectives
Residual magnetism problems
• Mechanical striker
m Simple to implement
Synchronization difficulty
• "Push-pull" rods
w Space qualified solenoids - cost
Maximum displacement limited
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DAMPERS
The dampers are used to hold the free end of each beamfixed during all non-
experiment times. In addition, they will be used to stop any remaining vibration
in the beamsbetween experiment samples.
Like the actuators, low power consumption is a requirement for the dampers. We
are searching for on/off damperswhich will movewhenpower is applied and hold
their position once power is removed. Thus far, the linear solenoids we have
tested will not hold their position unless power is continuously applied. This
was the cause of failure in our last flight and we are not willing to accept this
problem again.
In our search for dampers, Wemayonc@again arrive at a "home grown" solution by
designing our own mechanismsrather than purchasing something off the shelf. Our
other concerns to date have been the cost of vacuumrated solenoids and the
limited linear displacement available on the solenoids we have found.
• Hold free end of beam during non-experiment times
Stopvibrations between experiment Samples
Space qualified solenoids
Cost
m Maximum displacement of 1"
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SEQUENCEOF OPERATIONS
For those not familiar with GASoperations, a brief statement mayhelp. OnceNASA
has closed the relays, everything else in the experiment must be self contained.
This meansthat the sequenceof operations must be programmedto begin once the
main power relay is closed.
For our experiment, we will repeat the test 20 times. Wehave a 45 minute time
limit for data recording because of the data recorder we are using. Therefore, we
want to allow about 2 minutes of vibration data for each sample, with a total of
20 samples. The remaining 5 minutes will allow 15 seconds for the dampers to
close, stop the vibrations, and use the actuators to start the vibrations again.
At the end of this sequencethe damperswill remain closed, the recorder will shut
down, and NASAwill open the main power relay to the canister.
NASA relay closed - activate experiment
Start Recorder
• Open dampers to release free ends of beams
• Activate vibration actuators
• Wait 1 minute
• Close dampers to stop remaining vibration
Repeat vibration tests for 40 minutes
Stop Recorder
• NASA relay opened (experiment shutdown)
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DATAREDUCTION
This chart shows the strain gauge locations planned. By designing the strain
gauge circuit in a simplified wheatstone bridge arrangement, we are able to
achieve compensation for thermal, axial, and torsional distortions which might
otherwise appear in the data. Each strain gauge location planned will actually
consist of this bridge arrangement. Wewill record time histories of the strain
at each gauge location by meansof an analog data recorder. The expected
resolution of the data is I microstrain.
Strain Gauge Locations Compensation Achieved
Thermal
Axial
_ _._ Torsion
Simplified Wtieatstone Bridge
Time histories will be obtained at each strain location
Expected Resolution: 1
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RESOLUTION
This chart indicates the peak moment,peak strain, and percent of resolution
available for strain location. The information is broken out by beamtype, strain
location, and vibration mode.
Cantilever Beam
Mode Location
1 Bend A
2 Bend A
Peak Moment Peak Strain*
5.713 in-lb 76,2
1.07 in-lb 14.3
Pinned-Free Beam
Mode Location
Pendulum B
1 Bend B
1 Bend C
* Values are in microstrains
Peak Moment PQak Strain*
0.013 in-lb 207.9
0.051 in-lb 817
18.54 in-lb 247.5
469
VACUUMOPERATIONS
The requirements we face for operating part of our GAScanister experiment in a
vacuumare indicated here. Most of these are expected kinds of requirements, but
all of them are new for our department because of the limited experience we have
with hardware in space. Muchof our time is being spent searching our suppliers
for the equipment. Another block of time is being spent and will be spent
learning just what these requirements translate to in terms of actual hardware.
These difficulties are the price we pay to becomeexperienced in space operations
and we expect this Flex Beamexperiment to teach us a lot about GAScanister
experiment design as well.
Space Qualified Parts
m
m
Electrical Connectors
Solenoids and Acuators
Maintain Pressure for Electronics
m
Sequencer / Controller
Analog Tape Recorder
Power Supply
i ,
i,
Difficulties
- Source for- Solenoids and actuators
- Material selection! gaskets, glue
470
PRE-FLIGHT ESTING
All good scientific experiments involve large amounts of testing, some with actual
hardware, some with simulation. We plan to perform extensive hardware testing
prior to turning our experiment over to NASA for flight. We want to insure our
hardware is reliable, our expected results are reasonable, and our reason for
space flight is valid.
The tests are aimed at determining the amount of atmospheric damping present at
various atmospheric pressures. To accomplish this, we will test the system at a
constant temperature with various atmospheric pressures. Testing will also be
done to determine any temperature dependent damping the stainless steel beams may
possess. Testing at cold temperatures will also provide important reliability
information about our experiment hardware.
Test Description
Ground Test - 1 Atmosphere
Objective
System Checkout - Benchmark
Ground Test -
Reduced pressures to near
vacuum (constant Temp)
Reduced temperatures to
-35 C (constant pressure)
Pendulum mode -
Damping vs Air Pressure
Damping vs Temperature
471
SCHEDULEMILESTONES
The schedule for this experiment is shownbelow. If the actuator and damper
mechanismsare determined by March 1989, hardware assembly should be complete
by May or June 1989. This is only possible if we locate readily available off
the shelf equipment. Hermetic electrical connectors currently look like the
problem area in terms of cost and availability.
The schedule allows for testing throughout the summerwith data reduction
following. Oncewe are convinced of the validity of our test data and rationale
for flight, we will inform NASAthat we are ready for launch. From that time on,
it becomesa waiting game.
June 1989
September 1989
October 1989
?????
??? + 2 months
??? + 3 months
Hardware Assembly Complete
Pre-Flight Testing complete
Test Anaiysis Complete
Shuttle Flight
Flight Data Analysis Complete
Report Complete and Available
472
FOLLOW-ONRESEARCH
The Departments of Engineering Mechanics and Astronautics are currently working
together on a six-foot hanging bea_ experiment. Someof the experiment goals deal
with validating predicted modesof vibration and examining the effects of actuator
placement on the beam's dynamics. These will both be accomplished using simple
open loop configurations.
The goals also include closing the loop on the actuators and validating the closed
loop performance of the vibrating beamwith the pendulummodeinvolved (since the
beamis under the gravitational attraction of Earth). The experimenters would
then like to project what closed loop performance might be achieved whenthe
pendulummodeis not present, as in space.
Finally, we hope to eventually develop a GAScanister implementation of this
experiment with actuators operating in closed loop to try and verify the earlier
projections.
Six- foot Hanging Beam Experiment
• Open Loop
- Validate Predicted Modes (including Pendulum)
- Examine Effects of Actuator Placement onDynamics
• Closed Loop
- Validate Closed Loop Performance with Pendulum
Mode
- Project Closed Loop Performance without Pendulum
Mode
• Develop GAS canister Implementation of experiment
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United States Air Force Academy get-away-special flexible beam experiment

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