IEEE Robotics & Automation Magazine - June 2023 - 107

process, the wooden pin was partially inserted into the hole, and
the height of the wooden pin was lowered. Therefore, the position
of the finger that was pressing the wooden pin was lowered
as well. That is, by observing changes in the finger position, it is
possible to determine whether the hole position has been found.
Changes in finger height at the moment in which the
hole position is identified are displayed in the graph in Figure
10(e). In this graph, the finger height decreases when the
hole position is identified. When the hole position is identified,
the search operation is stopped, and the assembly is
completed by pressing in the direction of insertion of the
wooden pin. At this time, the pressing force is higher than
that when performing a blind search because an appropriate
pressing force is required for the blind search, and a higher
force is required to complete the assembly. Therefore, the
height of the wooden pin is lower than that at the moment
when the hole position is found.
SELECTION OF ROBOT POSTURE IN ASSEMBLY
The robot posture significantly influences the quality of
assembly. A robot's ability to perform a task depends on its
posture [20]. For example, when a robot is in a stretched posture,
it can be difficult to apply additional force by using its
ability to bear its own weight. Therefore, we configured the
system such that the assembly position and the robot posture
can be changed using a manipulation station. Therefore, we
changed the assembly position and selected a good posture
for assembly by considering the robot's dynamics.
In Figure 11, the posture used in the competition is compared
with other postures in the screw-bolting task. Posture 1
was the posture used in competition, and the manipulation station
was rotated by -135°. Posture 2 was another posture that
could be used to perform assembly without interference, and
the manipulation station was rotated by 0°.
We considered effective stiffness and manipulability as
indices to evaluate posture, and the equations are as follows:
KJ ,KJxyz
= i
^h~
det JJT
where Kxyz and Ki
-1 T 1
= ^h ,
(4)
(5)
are the effective stiffness and joint stiffness,
respectively. J is a Jacobian matrix, and ~ denotes
manipulability.
We verified the effective stiffness of the robot to ensure
that it did not shake during the bolting task, and the results
indicated that the stiffness of the screw-bolting robot was
higher in posture 1. The frame-handling robot was used to fix
the frame in the bolting task. In posture 2, the y-directional
stiffness of the frame-handling robot was high, but its manipulability
was 0, resulting in a bad posture. In this manner, we
selected the assembly posture by using an index that evaluates
the robot posture within the possible assembly position,
Posture 1
Posture 2
Frame-Handling Robot
Frame-Handling Robot
Effective
Stiffness
Ellipsoid
Effective
Stiffness
Ellipsoid
Screw-Bolting Robot
Screw-Bolting Robot
FIGURE 11. A simulation of the robot posture based on the angle of rotation of a manipulation station. We compared effective stiffness
and manipulability in an assembly posture and proceeded with the assembly work in a good posture.
JUNE 2023 IEEE ROBOTICS & AUTOMATION MAGAZINE
107

IEEE Robotics & Automation Magazine - June 2023

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