Page 212 - Mechatronics for Safety, Security and Dependability in a New Era

Ch41-I044963.fm Page 196 Tuesday, August 1, 2006 3:54 PM
Ch41-I044963.fm
196 Page 196 Tuesday, August 1, 2006 3:54 PM
196
REAL-TIME SCHEDULING PROCESSES OF HOLONS

Real-time Scheduling of Holons

New real-time scheduling process of the individual holons is proposed to select a suitable combination of the
resource holons and the job holons which can carry out the machining processes in the next time period. The
resource holons and the job holons mean here the equipment carrying out the machining processes and the
work-pieces to be machined, respectively.

At the time t when some machining processes are finished, and some resource holons and job holons become
'idling' status, all the 'idling' holons select their machining schedules in the next time period. The real-time
scheduling processes consist of following five steps.

(1) Collection of status data
The individual 'idling' holons firstly gather the status data from the other holons.
(2) Selection of candidate holons
The individual 'idling' holons select all the candidate holons for the machining processes in the next time period.
(3) Evaluation of objective function values of individual holons
The individual 'idling' holons evaluate the objective function values for the cases where a holon selects candidate
holons for the next machining process.
(4) Generation of all pareto optimal combinations based on objective functions of individual holons
The individual holons send the selected candidates and their objective function values to the coordination holon.
The coordination holon generates all pareto optimal combinations of the job holons and the resource holons which
can carry out the machining processes in the next time period, based on their objective function values. The pareto
optimal combinations means that there are no feasible combination which will improve the objective function
value of one holon without degrading the objective function value of at least one another holon [7].
(5) Determination of suitable combination based on objective functions of whole HMS
The coordination holon selects a most suitable combination of the job holons and the resource holons from the
pareto optimal combinations, from the view point of the objective functions of the whole HMS.

Evaluation of Objective Functions of Individual Holons

The objective functions of the individual holons were proposed in the previous research [6], as shown in Table 1.
The individual holons have one of the objective functions. The objective functions are evaluated by referring to the
following technological information representing the machining process and machining capability of all the job
holons and the resource holons.
Ms,: M i machining process of the job holon i (i= 1, •••,«), (k=\, ••',/?).
Rjhn'. m-\h candidate of resource holon, which can carry out the machining process MR (m=\, "\f}-
Tih n: Machining time in the case where the resource holon _/?«,„ carries out the machining process Mn,
W{. Waiting time until the job holon i becomes idle if it is under machining status.
AQk'- Required machining accuracy of machining process M,% It is assumed that the machining accuracy is
represented by the levels of accuracy indicated by 1,2, and 3, which mean rough, medium high, and high accuracy,
individually.

The individual resource holons have the following technological information representing the machining
capability of the resource holons for the machining process M*-

W m: Waiting time until the resource holon R^ becomes idle if it is under machining status.
Qkn,: Machining accuracy in the case where the resource holon R jkm carries out the machining process M ik.
jfo,, is also represented by the levels of 1,2 and 3.
n Machining cost in the case where the resource holon R^,, carries out the machining process Afe.