Robot Manipulators
maneuvering items far too dangerous for a human
to handle.
Robot Manipulator
Design Requirements
As I’ve mentioned, there are widely diverse
scenarios that require robotic manipulation.
Environments can range from a vacuum to extreme
pressure to extreme cold to extreme heat. Earth’s
twin — the planet Venus — is not a particularly
pleasant place for a space robot and certainly not
for a human being. Venus is about as close to hell
as anyone could ever imagine. With a surface
pressure of almost as high as at the Deepwater
Horizon site (at 1,323 pounds per square inch) and
a temperature of 870 degrees Fahrenheit, if you or
your spacecraft weren’t squashed or burned, the
sulfuric acid rains would certainly remove all of
your skin in quick order.
In comparison, the bottom of the sea may not
be as hot as the surface of Venus (unless an ROV is
exploring a ‘black smoker’ volcanic vent) but the pressure
can exceed 16,000 psi in extreme depths and actually be
below freezing. Add in corrosive salt water that can ruin
electrical and electronic systems in seconds and you have
an extreme environment for deep sea explorations.
Designing a robot arm to just work on Earth is hard
enough. Early robot arms frequently used hydraulic cylinders
to provide enough force for the intended factory
applications. Anyone who has ever used hydraulic systems
invariably learns that they always manage to spring a leak
at some time. Non-robotic uses such as heavy construction
equipment that have booms, large road scraping blades,
scoops, and similar motions that require a lot of force use
high pressure hydraulic cylinders filled with oil. No matter
how secure the seals, the oil under several thousands of psi
of pressure leaks out and manages to collect dirt on every
moving part. Early robots always seemed to be dirty.
Reversing the environment from thousands of internal
psi trying to escape hydraulic cylinders, joints, and hoses, to
extreme external ocean pressure trying to get into the
mechanisms, electronics, and motors of deep sea robotic
manipulators, it’s clear you have a serious design hurdle.
Designers of deep-diving ROVs with sophisticated
manipulator arms have taken two different approaches to
sealing the internal parts of the arms. One was to use
hydraulic cylinders to move the arms and end-effectors in
much the same manner as terrestrial applications (such as
Caterpillar tractor blade cylinders). Some designs started to
fail as external pressures actually were higher than the
internal pressure, and salt water intrusion through failing
seals caused corrosion and eventual failure.
The other approach was to forgo failure-prone high
pressure seals and fill the interior with inert oil that is noncompressible, then use a flexible bladder to separate the
sea water from the oil. This way, the interior of the motors
FIGURE 3. Master/slave manipulators.
and mechanisms are kept at the same ambient pressure as
the outside ocean water. The oil-filled brushless AC or DC
motors driving standard gear trains and mechanisms
operate in the same manner as similar mechanisms on the
surface. Rotating shaft seals do not require the same
pressure integrity as they would under a much greater
differential pressure. Electrical power and control lines still
have to traverse from extreme pressure to minimal pressure,
but these seals do not have moving components. Plus,
they’re easier to design and build. Lighting, certain types of
FIGURE 4. NASA Robonaut.
SERVO 09.2010 79
