IEEE Robotics & Automation Magazine - June 2023 - 102

GENERATION OF PDDL COMMANDS
A PDDL is a standard language for efficiently defining a
robot's action plan, and it consists of objects and predicates
[12]. In furniture assembly, robots repeatedly execute the
assembly process with set motions. The repetitive motions of
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the robot should be effectively defined, and the assembly
object should be accurately marked. For this purpose, the
PDDL was used to simplify the instructions and accurately
transmit assembly information. In this study, commands
transmitted to the robot were generated from the analyzed
connection tree by using a modified
form of the PDDL.
The modified PDDL consisted of an
Work Cell
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object, including its posture and position,
and a predicate classified into three
actions. The object represents the name
of the frame part with individual identity.
The posture, a component, is displayed
by dividing the posture in the 3D
space when viewed from the reference
frame direction. The position indicates
the hole label at the site where assembly
occurs. The predicate indicates the
action to be performed and consists of a
place command to move the frame part,
an assembly command to assemble two
frame parts, and a connector command
to join a connector part to the frame part.
In the connection relationship tree
FIGURE 4. The structure of the generated connection relationship tree of the Stefan chair.
The AI-based algorithm expands the connection relationship tree structure by obtaining
additional information from the assembly instruction manual.
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created as discussed in the section
" Connection Relationship Tree Inference, "
the connection relationships
of all frames were divided into layers
based on the top node. Therefore,
the PDDL command is generated by
searching from the upper layer to the
lower layer (see Figure 5). If there was
a connector part in the child frame part,
the connector command was executed
first, and then the assembly command
was executed. Commands at the same
layer level took precedence over commands
where the robot arm could
assemble quickly based on the top
frame coordinate system. Finally, the
generated PDDL command was transmitted
to the robot system to perform
assembly work.
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FIGURE 5. An example of a generated PDDL command. The AI algorithm developed herein
generates an assembly sequence from the connection relationship tree.
10 IEEE ROBOTICS & AUTOMATION MAGAZINE JUNE 2023
2
THE ROBOT SYSTEM ASSEMBLING
FURNITURE
In this section, we introduce the
robot's system and the technology
used for furniture assembly. The specifications
of the assembly system used
in the competition and the overall
assembly process are explained. Moreover,
we introduce multifingered grippers
and the manipulation station,
which are key technologies for successful
assembly. The multifingered

IEEE Robotics & Automation Magazine - June 2023

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