Among the tasks of C6A are a) to allocate production among the works in the division based on forecasts of product demand, b)
2. Refer to the preceding section for identification of the nota- nota-tion used
149
-We may further divide C
6A into three temporal layers as follows:
C~A:
Responsible for annual plan at the division level.3 1 3 3 months
T6A = year; T6A =
3 = production allocation over the period of the year u6A
x6A3 = experience over the past quarter in meeting the production plan, changes in demand forecast C2
6A: Responsibl~ for quarterly plan
=
3 months; T2production allocation for the quarter
experience over the past month in meeting the plan,
..
C1
6A: Responsible for monthly plan
1 month;
T~A =
1 week (or less)11 l~ li
(u6A' u
9A ,·
~.),
where u6A denotes the allocation of received orders to i th works of Division A to be processed during current month.information of order received (specifications, due dates, etc.), current state of works (back-log, slab inventory, etc.), mill performance.
In addition to u6d
=
u6A)' the outputs of C6A include:1=
various actions required in securing supplies of of the required raw materials, etc.=
information transmitted to company level concern-ing performance of the division, e.g. aggregated production capabilities .The data processing function may include the averaging of
pro-duction figures, trend analyses on market demand, order projections, etc. A sudden and significant change in any of the input f~ctors
may trigger an "on-demand" reassessment of the plan in order to
150
-accomodatethe new circumstances (e. g. m.s.j:or unscheduled shut-down of a facility, labor strike, severe quality control prob-lem). Alternately, the plan may be only modified in an ad hoc fashion to resolve a current problem pending reassessment at the regular per~od.
The adaptation function is incorpo~ated in the following ways:
a) Models used for determining production capacities, track times, performance standards, etc. are reviewed annually and updated
according to
(i) evaluation of previous year's results, and
(ii) known changes in equipment, operating practice or other relevant conditions.
b) Models used for prediction of aggregated values (i.e. based on probability distribution models of product mix, equipment break-downs, quality rejects, etc.) may also be updated annually based on observations of the previous period plus consideration of any tangible factors e~pected to affect the distributions.
I At the works level, the tasks of say, C51 are to transform the current monthly plan (defined by
u~~)
into, successively, monthly and weekly production plans for Works no.l, leading to the pre-paration of the weekly schedule. Thus, the control function3
2 I
-C
S1
may be considered in three layers: C51, C51,C51 corresponding to the monthly plan, weekly plan and weekly schedule, respectively.In general, C51 is responsible for coordinating the conflicting requirements of the various shops and mills in the works. One element of this coordination is the grouping and sequencing of orders to maximize overall productivity of the works.
The control output of
SI that is directed to' the Thus,
u~~
may denote the schedule_ 151_
of heats required of the oxygen
converters~ u~~
may refer to the set of orders to be processed by the Hot Strip Mill during the current period~ etc.Looking at the next lower level~ C4a~ C4b~ C
4c '"
denote the control systems for the various shops and mills in Works no.l.Each ofthese controllers may be expanded into two or more temporal
layers~ depending on the nature of the decision-making and control function3. ~.~ general~ each controller will have re-sponsibility for coordinating the activities of its infimal operational and process control functions~ e.g. C4c would have among its tasks the coordination of. the Reheat Furnace schedule and operations with the Rolling Mill schedule and operations ,-through the selection of the rolling sequence~ the slab temper-ature leaving the furnace~ etc.
Finally~ we may identify for each of the controllers in the hierarchy the functions and operations that correspond to the various components of the functional control hierarchy e.g. Information Processing~ Implementation and Adaptation.
4.5 Review of Hierarchical Control Concepts and Approach
The purpose of this section is to relate to some of the antecedents of the hierarchical approach described in this chapter and to
identify some alternative hierarchical structures that have been advanced. This review represents only a small sampling of developments in the field and is not meant to be complete.
Selected references are identified by superscript and listed at the end of this section.
Most of the underlying concepts and some of the terminology pre-sented here have their origins in the pioneering work of Mesarovic and his
groupl~2
which develops a conceptual and analyticalfoundation for hierarchical structures and multilevel coordination theory. Many of our modifications were prompted by pragmatic
aspects of the application of the theory to industrial systems, e.g. more explicit concern for on-line implementation and the effects of disturbance inputs, focus on achieving feasible, suboptimal performance objectives as opposed to a "mathematical"
optimum, etc.
The bulk of the literature in the field is oriented to decompo-sition and multilevel coordination theory and its application
t~ optimization and mathematical programming problems of various kinds. There are a .2ry large number of references on the
subject; we cite here only three books representative of the work in this area.
3 ,4,5
The application to scheduling problems have particular relevance to steel mill operations and some of the coordination tasks defined for the production control sy~temmay well make use of these techniques.
A closely related area of application of coordination theory is in handling the dual problem of optimization and identification (e.g. of parameters of the model). We cite again only one of several references in this area
6
and note that this problem, too, is relevant to our field of study, e.g. with respect to . adaptation of models and some prediction algorithms.
The essential features of the functional multilayer hierarchy were first described by Lefkowitz! Applications of the hierarchical structure to control of process systems are discussed further by Findeisen
8 .
Extension of the multilayer concept to discrete manufacturing type systems is presented by Lefkowitz and Scfloeffler
9.
This work mo-tivated some modifications in the conceptual structure of the mul-tilayer control system; specifically, making more explicit the role of a common data base, information processing function, and the provisions for contingency event monitoring and control.An immediate precursor to the formulation of the temporal multi-layer hierarchy is'the work of Donoghue.IO
The basic contribu-tion here was the organizacontribu-tion of the decision-making and control functions according to the relative time scales of the required
ac 153 ac
-tions. ImporGan~ extensions to the concepts ana structuring of
. ' k' 11 t ' 1 1
the temporal hlerarchy were made by Chellust ln, par leu ar y with respect to its association with the planning and scheduling functions of the steel production system and also with respect to the consideration of model ~ggregation relative to the hierar-chy.
Williams has provided major inputs to the field of hierarchical
t 1 1, t " 1 2 .
con ro as appled 0 lndustrlal systems. He lS ·presently dtrecting a large ~esearch project at Purdue University, in joint collaboration with the major U.S. steel producing com-panies, on "Systems Engineering of Hierarchy Computer Control Systems for Large Steel Manufacturing Complexes". 13
In his general formulation of the hierarchy, four levels are distinguished:
Levell: Specialized Dedicated Digital Control Level 2: Direct Digital Control
Level
3:
Supervisory Control Level4:
Management InformationThis structure is similar, in part, to the functional hierarchy
.
shown in Fig.4.3;
with, however, a stronger orientation to thetechnological level of control and to the practical aspects of digital computer implementation. An extension of this general
formula-tion leads to a structuring of the overall plant producformula-tion con-trol system as shown in Fig.
4.8
(taken from Reference 13), which is being considered in the current Purdue study.There are some strong similarities with the conceptual formulation of the hierarchy that we've presented here. There are also dif-.
ferences e.g. the Purdue version of the control hierarchy has more;
detail with respect to the technological level o~ control, and is more oriented to the practical aspects of implementation of the system through digital computer hardware and software. There is also more explicit concern with the communications aspects of the problem and man-machine interfaces.