Information Technology Reference
In-Depth Information
method
(det(open(
D
),
I
,
C
,percept),
(door(
D
)),
% precondition
[approach(
D
), sense(open(
D
), percept)],
% subtasks
50).
% cost
Fig. 2.
Example HTN method for an acquisition task
open. Every method has an expected cost that describes how expensive it is to perform
a task as described by the method. In this example, the cost is “hard-coded”, but it is
also possible to calculate a situation dependent cost.
Knowledge acquisition tasks enable the planner to reason about possible knowl-
edge acquisitions, since they describe (1) what knowledge acquisitions are possible
under what conditions, (2) how expensive it is to acquire information from a specific
knowledge source, and (3) how to perform a knowledge acquisition task.
It might be possible that the same information can be acquired from different exter-
nal knowledge sources, and the expected cost to acquire the same information can be
completely different for each source. Thus, in order to acquire additional instances for
each literal of a set of open-ended literals, a planner needs to decide for each literal
from which knowledge source it should try to acquire an additional instance. The result
of this decision process is called a
knowledge acquisition scheme
. A knowledge acqui-
sition scheme is a set of tuples
(
l,ks
)
where
l
is a literal and
ks
is an external knowl-
edge source. It represents one possible combination of trying to acquire a non-derivable
instance for each open-ended literal by an adequate knowledge source. For example,
{
(on table(bobs mug),percept),(white coffee(bob),hri(bob))
}
represents the fact that the query
on table(bobs mug)?
should be answered by
perception and the query
white coffee(bob)?
should be submitted to Bob.
Formally a knowledge acquisition scheme is defined as follows:
Definition 1 (Knowledge Acquisition Scheme).
Let
st
be a statement that is possibly-
derivable with respect to
D
M
and the set of open-ended literals
L
x
=
1
≤i≤n
{
l
i
}
.
Moreover let
KS
be the set of knowledge sources. A set
kas
:=
1
≤i≤n
{
(
l
i
,k
i
)
}
(
k
i
∈KS
)
is called a
knowledge acquisition scheme
for
st
w.r.t.
D
M
.If
L
X
=
∅
,then
the corresponding knowledge acquisition scheme is also
∅
.
However, a knowledge acquisition scheme is only helpful for an agent if it is actually
able to perform the corresponding knowledge acquisition tasks. For example, if a robot
in principle is not able to find out whether a door is open, then the planner does not have
to consider method instance 2 and 3 for the situation illustrated by Fig. 1. A knowledge
acquisition scheme for which all necessary knowledge acquisition tasks can be possi-
bly performed by the agent is called
possibly-acquirable
and more formally defined as
follows:
Definition 2 (Possibly-acquirable).
An acquisition
(
l,ks
)
is called
possibly-acquirable
w.r.t. to a domain Model
D
M
iff there is an applicable or possibly-applicable planning
step (i.e., a method or an operator) for the knowledge acquisition task
det
(
l,I,C,ks
)
such that
I
are all derivable instances of
l
w.r.t.
D
M
and
C
is the context. Moreover, a
