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Artificial Intelligence and Soft Computing: Behavioral ... - Arteimi.info

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12.5 Multi-agent Planning<br />

The least distributed form of multi-agent planner decomposes the goal into<br />

sub-goals <strong>and</strong> assigns them to the other agents. Generally the decomposed<br />

sub-goals should be mutually independent. The decomposition problem is<br />

similar with that a single-agent planning. The allocation of sub-problems to<br />

different agents is made through a knowledge about the agents. In other<br />

words, the allocator agent must know: which plan which agent can execute<br />

more efficiently. When all the agents are identical, the allocator (master)<br />

should consider load balancing of the slave agents, so that the overall goal is<br />

executed at the earliest. If the allocated tasks are dependent, synchronization<br />

among the slaves is necessary. In single-agent planning dependencies are<br />

h<strong>and</strong>led during creation of the plan. However in multi-agent planning, since<br />

the goal is distributed, an unpredictable amount of time may be required by an<br />

agent; consequently the dependencies among the tasks are lost. Proper<br />

synchronization from each slave to the master, or among the slaves, is<br />

required for the execution of the independent plans.<br />

The next section will present a special type of scheduling problems,<br />

where a strategy-based planning serves the timing conflict among the agents.<br />

Indeed, it is not a multi-agent planning, as the agents (machines) do not<br />

participate in the planning process. It is to be noted that in a multi-agent<br />

planning the agents must be active planners. The task co-ordination problem<br />

among robots is an ideal example of such planning. For instance, two robots<br />

have to carry a large board inside a room filled with many obstacles. How will<br />

they plan interactively to transfer the object from one location to another? In<br />

this wide book, we do not have much scope to provide a solution to this<br />

problem. Interested readers, however, may attempt to solve it by assuming<br />

that both the robots have common knowledge of their world <strong>and</strong> one robot can<br />

sense the difficulty the other robot is facing in transferring the board.<br />

12.6 The Flowshop Scheduling Problem<br />

In flowshop scheduling there exists n different jobs, each of which has to be<br />

processed through a sequence of m different machines. The time allotment for<br />

a given job k to a machine, say, Mi is constant. To illustrate this problem,<br />

consider three machines, M1, M2 <strong>and</strong> M3, <strong>and</strong> the jobs J1 <strong>and</strong> J2 have to pass<br />

through these machines in sequence (see fig.12.17)<br />

The time requirement for processing the part of a job Ji in all three<br />

machines is supplied. One has to determine the job schedules (J1 < J2) or (J2 <<br />

J1), where Jk < Jl for k, l ∈ (1,2) denotes that job k is to be executed prior to<br />

job l, so that the completion time of the last job on last machine M3 is<br />

minimized. This time is often referred to as make-span [3].

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