Figure 4. Astronaut in WETF.
very carefully. NASA drew up a detailed
list of requirements for a system to
handle payloads on the space shuttle.
Having heard a bit about some of
Canada’s aerospace expertise, the then
NASA Administrator, Thomas Paine went
to our northern neighbors in 1974 to ask
for their assistance in building part of the
entirely-new space transportation system
based on a reusable space shuttle. Little
did NASA, we at Rockwell, or our
Canadian counterparts realize just how
expensive something reusable would end
up costing to develop and build.
The Canadian government worked
with the National Research Council to
determine the best approach. Spar
Aerospace Ltd., a Canadian company
based in Weston, Ontario, was chosen
after determining that the manipulator
arm system was the most appropriate
part of the shuttle project to undertake.
Spar designed, developed, tested, and
built the RMS after winning the NASA
contract. The terms of the contract
specified that Canada would pay for the
development and construction of the
initial Canadarm flight hardware. NASA
also contracted to buy at least three
more Canadarms as part of the deal.
Also in the contract was stated that
Canada would have preferential access to
Figure 5. End Effector.
88 SERVO 01.2007
shuttle launch-es as part of its
developing
space program.
Spar was later
acquired by
Richmond, B.C.
Canada-based
MacDonald
Dettwiler &
Associates Ltd. in 1999 and formed into
MD Robotics.
Figure 3. RMS on
Air Bearing Floor.
Spar Aerospace’s
Difficult Task
One might assume that a large
robot arm would be fairly simple to
design and build. Take a couple of light
weight carbon fiber composite tubes
23 feet long and attach a two axis
shoulder joint, a single axis elbow joint,
and a three axis wrist/end effector,
attach it to a good location on the
shuttle, and you have a space robot
arm. Toss in a bit of industrial robot
technology and couple that with a ‘
typical’ crane and you’re set to go. Right!
Gravity, or the lack thereof, a vacuum, and extreme temperatures are just a
few of the obstacles that make the design
and construction of a 50-foot robot for
space tasks difficult. One might think that
the zero gravity environment would allow
the program to use a weaker type of system. That’s true, to a point. On Earth, the
arm cannot even lift itself off the floor,
but the shuttle’s RMS cannot use the
usual several hundred horsepower diesel
engine that you might find on a ‘cherry
picker’ or solid boom crane, or even the
equivalent with an electric motor. A
winch cable arrangement can’t be used,
as there is no gravity pulling ‘down’ on a
boom to keep the cables taut.
NASA had to use a jointed stiff
‘arm’ with powered joints. Hydraulics or
pneumatics certainly could not be used.
The joint mechanisms had to be quite
precise with virtually no gear slop. A
one-degree slop in the shoulder joint
could cause a 10-1/2 inch loss of accuracy at the end effector. This is unacceptable when the positioning of payloads in
space sometimes require sub-millimeter
movement in three axes. Unlike on the
ground where gravity can hold a boom
in one position, the micro-gravity space
environment would allow a RMS boom
with mechanical slop to bounce back
and forth like a clock’s pendulum.
Precision and
Reliability
Gear slop is sometimes minimized
or eliminated by using two gears that
act as a single gear. The two gears are
spring-loaded so they push in opposite
directions and effectively enlarge each
tooth just enough so that the gear that
is driving them sees no slop at all. This
cannot be used on the RMS arm, as the
payload mass is far too large, so precision machining was required. NASA
specified type 302 stainless steel alloy as
the material for the gears, but, when the
machined gears were heat-treated for
strength, they shrank a few micrometers
— too much to pass inspection. Since the
gears were too hard to be machined
after heat-treating, Spar had to calculate
the shrinkage and machine them a bit
larger to fit specs after the treating. It
worked. Lubrication of mechanisms in a
vacuum under extreme heat and cold
conditions was also solved.
After solving literally thousands of
similar problems, Spar had to then test
the RMS system under predicted space
conditions. Temperature extremes,
vibration, duty cycles, and even excess
gravity loading are easy to simulate on
the ground, but not the lack of gravity.
Despite what some SciFi movies may
show, there is no way to produce zero or
micro-gravity on Earth. The RMS arm is
too weak to even lift itself off the floor in
Earth’s gravity so zero G must be attained
in some manner for successful testing.
You can take NASA’s converted KC135
‘Vomit Comet’ tanker and simulate 25
second bursts of zero gravity in parabolic
dives (followed by 35 seconds of 2 Gs —
over and over again), or go to space.
Since the latter was too expensive
and the other offered test periods that
were too short, Spar used what several
aerospace companies and NASA have
used for years — an air bearing floor. We
did this at Rockwell when we tested two
spacecraft closing in to dock in space.
Think of an air hockey table in reverse.
Instead of air entering from holes in a flat
floor to allow pucks to float about on a
cushion of air, drive air into the pucks
