IEEE Robotics & Automation Magazine - June 2021 - 33

averaged across the valid orientations at that point. These
averaged metrics appear in Figure 11 at their commanded
locations in the workspace. As seen in the characterization
experiments, the limited xy manipulation range leads to high
translation error at points that are farther from the z-axis.
Subsequently, a higher slip metric and rotation error are
expected at these points because the contact triangle deforms
more for higher x and y values and ineffectually transmits
contact forces to the object. In comparison, for points closer
to the z-axis, the orientation capability is superior, and the
slip metric and translation error diminish. Thus, the most
effective manipulation strategy with this hand might be to
orient objects immediately after a grasp so as to achieve
smaller rotation errors and then carry out any xy translations
since this motion is relatively more likely to drop an object
and introduce errors.
Teleoperation Experiments on WAM Robot Arm
In addition to commanding the hand open-loop, it is desirable
to incorporate real-time feedback to improve control.
Since it was not the goal of this work to develop a complex
control scheme for the hand but rather to present the
mechanical design, teleoperation was used as a proxy for an
eventual closed-loop controller, incorporating feedback
from a user who could observe the grasped object's actual
position and orientation. To provide an intuitive interface, a
3Dconnexion SpaceNavigator 3D mouse was used to control
the Stewart Hand mounted on a Barrett WAM robot
arm. The mouse outputs a 6-DoF signal that was directly
used to command the desired pose of the grasped object. A
shape block sorting toy (as detailed in the following) was
used to emulate a potential peg-in-hole insertion task-a
common class of manipulation problems for industrial
assembly tasks.
The cube box was placed at a tilted
angle next to the shape block to be
inserted. The hand-mounted on the
WAM arm-started at a position facing
vertically down above the block.
Once the hand grasped the block, a
preset arm trajectory was executed
that roughly positioned the object
above the appropriate hole on the
cube box. This preset trajectory did
not change the orientation of the hand
and was determined by visually aligning
the palm of the Stewart Hand
(while it was not grasping any object)
with the center of the target shape
hole. Once the preset WAM trajectory
was carried out and the grasped block
was roughly above the target hole, the
teleoperator used the 3D mouse to
orient and translate the block until it
was partially inserted into the hole.
Throughout this step, the teleoperator
30
20
10
-10
-20
-30
-30 -20 -10
x (mm)
Figure 10. The envelopes for the actual xy ranges of motion at five input grasping torque
values, including the envelope of a fixed platform replacing the fingertips. The actual
workspace areas normalized by the commanded area of these manipulation envelopes
are shown for each of the grasping motor torques and the fixed platform.
JUNE 2021 * IEEE ROBOTICS & AUTOMATION MAGAZINE *
33
10 20 30
could observe the block to obtain visual feedback about its
actual pose. Once the shape block was partially inserted, the
hand was commanded to release the block, dropping it into
the hole. Since the shape blocks were untethered and lightweight,
they tended to arbitrarily translate/rotate slightly during
the grasping action. It was the responsibility of the
operator to counteract this misalignment during the manipulation
stage, employing the 6-DoF dexterity of the hand for
both reorientation and insertion.
This experiment was carried out with four different
shapes (Figure 12), further demonstrating the hand's ability
to adapt to irregular object geometries through the differential
mechanism. For each of the shapes, the cube box was
placed at a different tilt angle to add complexity to the task,
on top of the misalignment introduced at the grasping step,
as previously described. The holes in the cube box had only
a small amount of clearance relative to the corresponding
shape blocks. Thus, the task required the precise alignment
of the shape block and the hole before insertion was possible.
All four shapes were successfully inserted into their
respective holes. For more complex object shapes, such as
the blue star, the teleoperator iteratively tested the block's
alignment by attempting to insert it in the hole, reorienting,
and then trying again. While the operator relied on visual
feedback during this process, future work might seek to
implement this strategy using the fingertip slip metric to
ascertain whether the object is coming into contact with the
environment, based on how the grasping triangle changes
shape as the fingers slide along the object surface. This
experiment demonstrated that the performance of the Stewart
Hand can be further augmented by closing the feedback
loop, as exemplified by simple and effective human teleoperation
for a peg insertion task with a variety of shape blocks.
Commanded
Fixed Platform
Actual (300 N⋅mm)
Actual (440 N⋅mm)
Actual (570 N⋅mm)
Actual (710 N⋅mm)
Actual (840 N⋅mm)
Actual/Commanded
Workspace Areas for
Grasping Torques
Fixed Platform 0.717
300 N⋅mm 0.202
440 N⋅mm 0.235
570 N⋅mm 0.239
710 N⋅mm 0.24
840 N⋅mm 0.159
y (mm)
IEEE Robotics & Automation Magazine - June 2021

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