Precision
Epoxy Products
" Your
Complete Epoxy Source "
a division of :
Rock Art, Ltd.
"
Professionals of Epoxy Applications " ™
4279 Midway Drive
Douglasville, Georgia 30134
Phone : (770) 489-0340
Rock Art's Float Unit
is used for testing and confirming the criteria of acceptance on our Aerospace Epoxy Test
Beds. Pictured is the simulator floating on our Epoxy Test Bed at
Rensselaer Polytechnic Institute in
Troy, NY.
The consistency and accuracy required
over a large area makes the time and tools needed to measure one of our Aerospace
Epoxy Test Beds very expensive. In reality however, no matter how the surface is
measured, what really matters is how your Robotic Spacecraft Simulator floats
on the epoxy floor. An ideal test bed surface provides for little to no
drifting of the float unit at any location on the test bed surface. If the float unit
sits perfectly still when it is floating above a flat surface on a cushion of
air in a state of weightlessness, then there is no doubt that the surface is
perfectly level. The sophisticated Robotic Spacecraft Simulators being deigned
and used on our epoxy test beds can detect when the unit is drifting and
activate air thrusters to correct movement and stabilize the unit. This
however is not a desirable scenario; the more the unit has to correct itself
because of an imperfect test bed surface, the faster the self contained air supply of
the unit is depleted which in turn reduces the duration of test time. The
test bed surface not only needs to be level, it also needs to be smooth and
nonporous. To further reduce air consumption, the test bed should allow the
float unit to function with a minimal air gap between the test bed surface and
the air bearing itself. This equates to a reduced flow of air through the air
bearings resulting in a longer test time.
When we completed the Epoxy Test Bed for USC, they were hoping to float
their simulators using only 20 psi of air. Upon initial testing they had to
use 40 psi of air through the air bearings for the unit to float properly. Then after consulting with the Air Bearing manufacturer,
they were told that with the size of the bearings being used and the weight
of the simulator, they should expect to be using 80 psi of air. The
manufacturer was quite amazed the USC units were
floating properly with only 50% of the air flow recommended. Pictured
is the actual first test of a USC Simulator.
Before we completed the Epoxy Test Bed at Georgia Tech, a granite table top
surface was being used as the test bed surface. The simulator's lower unit
had to
use 60 psi of air to create a large enough air gap between the air bearings and the
granite surface to function properly. During their first test on our Epoxy
Test Bed, it was discovered they would be able to properly float their
simulator using only 10 psi of air. The excitement in the lab upon this
discovery resembled mission control after a successful space launch. The
self contained air supply would be lasting six times longer than before
simply by installing the proper test bed surface. Pictured is the actual
first test of the GT Simulator's lower unit.
The evolution of Floating Robotic Spacecraft
experimentation has come along way in a relatively short period of time. As
you can see from the simulators pictured, no two are built the same. There are
however common parallels that should be considered and incorporated into the
construction of any Robotic Spacecraft Simulator. Before the availability of the Epoxy Test Bed
from Precision Epoxy, any excessive drifting, turning of the float unit or the
spinning of the unit during straight path movements and other irregularities
during flight simulation was easily
explained by potential flaws with the test bed surface. With the introduction
of our test bed surface, indications are that some of these problems with
flight simulation are caused by design flaws in the float units themselves.
Corrections in design of your float unit will eliminate having to deal with such
problems that distract from the time and resources being spent on the actual
research being pursued. We addressed the three most obvious design flaws with the
robotic float units we have seen and designed our own unit to function with consistency
and dependability every time. Granted our float unit is as basic as can be and
does not have a computer, GPS acquisition, thrusters, gyroscopes, reaction
wheel, cameras or
any of the hardware used on a Robotic Spacecraft using one of our
Epoxy Test Beds. Our float unit is however, the first stage or carriage
vehicle of any design that will carry additional accessories. No matter what the final
vehicle ends up becoming or weather the design uses 3 air bearings at 120
degree intervals or 4 air bearings at all 4 corners of a square unit, the
following three design principles need to be employed for the float unit to function with consistency
and dependability. Pictured is a first generation Spacecraft Simulator on our
Epoxy Test Bed at the Naval Postgraduate School's Spacecraft Robotics Lab in
Monterey, California.
1) The first design consideration is the balance
of the float unit. The total weight of the unit should be balanced so that it
is distributed evenly to all the air bearings. Our float unit was constructed
with careful placement of each component. Each component was machined to be of
identical size and weight to any  matching
component. In addition the size, weight and placement of
each individual component were considered for proper balance. After construction was
completed, the unit was scaled.
Scaling is a procedure used in the Motorsports Industry as
apart of set-up to prepare a race vehicle for racing at a particular race
track. The vehicle is rolled onto a perfectly level surface called a Set-up
Surface Plate. Then four identical electronic scales are placed under each of
the four tires. The total weight of all four scales equals the total weight of
the vehicle. The weight distribution from front to rear, left to right and
cross weight (front corner to opposite rear corner) percentages can also be
determined and adjusted. The idea is to distribute the weight of the vehicle to all
four corners (to each of the four tires) in such a manner to achieve a perfect
balance that suits a race track's characteristics; this allows for optimal
handling and maximum speed. A float unit simulator is floating on a perfect
surface and in order for it to track in a true direction, the unit needs to be
balanced. To accomplish a perfect balance we placed our float unit on the
perfectly level Epoxy Test Bed with each air bearing resting on one of three
identical scales. At this point the balance of the float unit can be
determined and counter weight added as needed so that one third of
the total weight of the unit is supported by each of the three air bearings.
It will be very hard to build a float unit that is exactly balanced without
the use of counter weight. With proper planning and placement of components,
the counter weight needed can be minimal. The float unit will need to be
scaled every time a component is changed or added. Simply by changing the
regulator valve on the air tank of our float unit from the one used in the
original design to a smaller regulator, the balance of the unit was changed.
Although the location used to place the counter weight remained the same, it
required an increase from 82 grams to 136 grams. The overall weight of the unit
remained the same. Equal weight distribution to each air bearing assures that the unit will have
a true float path every time.
2) The second design consideration is to assure
an equal flow of air to each of the air bearings. When it was
noticed that some of the the Spacecraft Simulator Float Units we saw tested
had the air bearings plumbed
in line from the air source to bearing number one, to bearing number two and
then to bearing number 3 it immediately raised a red flag. The numerous
negative scenarios possible with this type of plumbing system made it
obvious that an uneven flow of air through each of the three air bearings could be
encountered. Not having the identical flow of air through each bearing means
that the air gap between the bearings and the test bed surface will not be
the same. This will alter the weight distribution and throw the unit out of
balance diminishing the possibility of a true flight path. Furthermore,
should air flow be reduced through one bearing, the air gap could be reduced
enough to cause that bearing to drag or hook the test bed surface. The only
correction for this is to increase the pounds per square inch of air flow
through the entire system resulting in decreased test time. By proper design
of the air line plumbing system, negative scenarios can be reduced or
eliminated and valuable time saved by not having to deal with the potential
problems that will present themselves with a poorly designed plumbing
system.
The design of the plumbing system for our float unit consist of a single air
line from the air tank regulator to the filter staging area. Another single
line runs from the filter out connection to a custom designed air manifold
located in the center of the float unit's 3/16 inch aircraft aluminum base
plate platform and at an equal distance from each of the three bearings.
Each air line from the manifold to each air bearing is identical in size
and length. This design, unlike the in-line plumbing design, assures an
equal flow of air to each air bearing. This air line design also allows for
easy and positive isolation of any problems that can occur to the particular
air bearing at fault. A clogged or dirty air bearing, an air leak from a
cracked or damaged line or fitting and so on can be quickly located and
corrected. Pictured is the under side of our float unit showing air manifold
and plumbing to bearings.
The first stage or carriage platform of our basic float
unit is now ready to have additional stages added as needed to accommodate
additional hardware. Stages can be added at any size, shape or height
required as long as they are positioned with the additional weight being
balanced properly. We are currently using 40 mm bearings supplied by
New Way Air Bearings in Aston, PA. The 40 mm bearing is rated by the
manufacturer to handle a weight of up to 50 lbs at 60 psi. With a three air
bearing carriage system, our unit should float up to 150 lbs dead weight.
Our unit is currently at 26.5 lbs total weight which allows for additional
add-ons of 123.5 lbs if needed with the existing air bearings. New Way
offers air bearings up to 200 mm as well as all the mounting hardware.
3) The third design
consideration is the choice of air to be used in the float unit's compressed
air storage tank. Normal compressed air will need an extensive filtering
system to remove moisture, oil residue and any other contaminants that may
clog or damage the air bearings. This can be a major problem causing
unwanted down time.
We decided on CO 2 for our unit because of its purity
which in turn allows for a minimal inline filtration system. The likelihood
of the air bearings being clogged reducing the air flow is also reduced. There
is a wide
selection of CO 2 tank designs and sizes available that can be found in various industries
from the Paint Ball market to the Beverage Industry. These tank choices give
more flexibility with the float unit's design. Easy availability to
have our tanks refilled was also a consideration.
This paper was posted to share the
information we have learned through our experience over the years. We have
designed and installed many perfectly flat and level floor plate surfaces of
all sizes and for all types of uses giving us the insight for the suggestions
contained herein. Furthermore, because our surfaces over large areas are so perfect, that to measure
them for accuracy by any other means than the float unit validation, is
neither a
practical nor reliable method.
We would like to thank New Way Air
Bearings for all their assistance and
for the outstanding products they
provided for our Float Unit project.
Contact information:
New Way Air Bearings
50 McDonald Blvd.
Aston, PA 19014
Phone: 610-494-6700
Fax: 610-494-0911
Web: www.newwayairbearings.com
A source for custom built turn-key Robotic Spacecraft Simulators or custom
components is Guidance Dynamics Corporation. We have worked on several
projects with Eric Rasmussen, President of Guidance Dynamics and his staff and
have witnessed his units ranging from 3 degree-of-freedom to 6
degree-of-freedom in operation. The Products they provide are very impressive.
For more information visit their web-site at www.guidancedynamics.com
.
Contact information:
Eric Rasmussen
Guidance Dynamics Corporation
4685 East Industrial Street
Suite 3A
Simi Valley, CA 93063
Phone: 805-582-0567
Web: www.guidancedynamics.com
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