can cup to help grasp tools, too. It had to be functional
enough to work with a variety of tools and other devices,
and rugged enough to survive the space environment. Plus,
it had to be extremely compact (much like a human hand,
wrist, and arm).
The Willow Garage PR2
The PR2 from Willow Garage in Menlo Park, CA is a
most unique robot. The drawing in Figure 7 shows just
how much of the volume of the PR2 is dedicated to the
arm structures. You might notice that the PR2 does not
have multi-fingered hands like the Robonaut but instead
uses a more standard robot parallel jaw claw. This ‘hand’
design doesn’t make the robot any less functional for its
intended uses. On a recent visit to Willow Garage, I
managed to get some close-up views of PR2’s arm — truly
the most unique humanoid robot design I’ve ever seen.
Instead of the typical shoulder joint protruding from the top
of a chest cavity, Willow Garage employed two rotating
cylindrical structures on each side of the robot upon which
the arm shoulder joints and arm assemblies are mounted.
Figure 8 shows the view looking downward at the top of a
right arm. The parallel jaw claw is visible as well as the
beautifully machined weight compensating mechanisms.
The PR2 has two 7 DOF arms with a range of motion
similar to that of a human to pick up and manipulate
common household objects. Each arm has pan and tilt
shoulder joints, upper arm roll joints, elbow flexion joints,
forearm roll joints, wrist flexion joints, and gripper roll
joints. Each arm carries a single DOF gripper capable of
grasping most small objects up to 1.7 Kg, and is outfitted
with a pressure sensor array on the gripper tips. The arm’s
four main DOFs are back-drivable so when the arm
encounters an immovable object, the force of the
interaction drives the motors back and the arm gives way.
This allows the robot to operate in unstructured
environments. The spine of the PR2 can telescope upward
to extend its reach from objects on the ground to objects
on countertops.
Final Thoughts
Robot manipulators have come a long way since the
early Oak Ridge hot cell robot arms. Industrial robot
manufacturers have seen the need to extend their product
line into multi-arm, multi-axis manipulator systems such as
the new Yaskawa Motoman SDA5D shown playing with
some LEGO blocks in Figure 9. There are many applications
where two multi-axis arms are far better than a standard
industrial robot arm. Factory assembly lines, space rovers,
undersea ROVs, tourist submarines, and robots in the home
will all soon have manual dexterity and the manipulative
ability of humans. SV
Tom Carroll can be reached at TWCarroll@aol.com.
Robot Manipulators
FIGURE 7. PR2 drawing.
FIGURE 8. Top of PR2’s arm.
FIGURE 9.
Motoman
SDA5D.
SERVO 09.2010 81
