5/14/12 OUTLINE. A Situa(on Summary Approaches for Describing A Relevant System System Models Influence Diagrams Quan(ta(ve Modelling

Transcription

1 OUTLINE A Situa(on Summary Approaches for Describing A Relevant System System Models Influence Diagrams Quan(ta(ve Modelling 1

2 Systems Modeling We will apply the systems concepts and thinking discussed before to a real- life problema(c situa(on Define a relevant system for it. Several cases problema(c situa(ons discussed System modelling: The process of conceptualizing a system of interest. This involves two steps: Describing a problem situa(on and a relevant system A Situation Summary The first step when approaching a problem situa(on is to familiarize yourself with: the situa(on Its processes and structures The people involved Their aims and desires Rela(onship between them The hierarchy or power structure The resources available The data sources and informa(on 2

3 A Problem Situation To get a feel for anything you discovered and seems relevant for describing the problem situa(on. Rich Picture Diagram is a useful tool for describing a problem situa(on Rich Picture Diagram A cartoon- like summary of everything the observer knows about the situa(on studied Diagram or concept - is not a system descrip(on Only one approach of making a situa(on summary 3

4 4

5 Telat Kompak Selfish Care Karakter Tmip 2010 A Kri(s Krea(f Cemberut Tidak serius 5

6 Rich Picture: A Piece of Sky is Missing Problema(c Situa(on The person looking at the situa(on The structural elements The process elements The rela(onships between structure and process 6

7 Rich Picture Diagram Guidelines: Elements of structure (All components of the situa(on are rela(vely stable - e.g., all physical aspects) Elements of process (All aspects that undergo change - e.g., ac(vi(es, flow and processing of materials) Rela(onship between structure and process and between process (how does the structure affect condi(on of process; one process affects other processes) 7

8 Rich Picture Diagram For human ac(vity systems: A rich picture should be included hard facts and so+ facts. The rich picture is never finished Common mistake: Each item pictured needs to be connected one or more other items (Note that rich picture is not a system descrip(on) The uses of rich picture: An ideal vehicle for communica(ng with other people about complex and problema(c situa(on Approaches for describing a relevant system A system descrip(on consists in specifying the transforma(on process(es) of the system the boundary of the system the components and subsystems and the structure the inputs and the outputs (desired & undesired outputs) Two Approaches for describing a relevant system: Structural Approach Process Approach 8

9 Structural Approach A typical structure usually found for situa(on considered, e.g., a wai(ng line structure. The situa(on is well understood Using a known basic structure allows fast progress toward a complete system descrip(on A Queuing System 9

10 Process Approach No assump(on about the possible system structure The observed processes and rela(onships are used to discover a good structure A good star(ng point is to determine from whose standpoint to view the system and then define the prime transforma(on process. Rules help in iden(fying the components, inputs and outputs of the system. Rules Any aspect that affects the systems, but in turn is not significantly affected by it, is an input from the environment system. This covers all external controls. Any aspect that is directly or indirectly affected or controlled by the system, but turn does not affected any other aspect of the system, is a system output. 10

11 Any en(ty that is either part of the system s structure or its transforma(on process(es) is a component of the system, unless it is an output of the system. Similarly, if the system or any of its components affects an en(ty or exercises control over it, then that en(ty is a component, unless it is a system output. Any aspect that does not affect the system, or is not affected by it, or not part of its structure or transforma(on process is irrelevant, and can be ignored System Models The ac(vity of building a model is referred to as systems modeling. A model may be iconic, symbolic, or analogous Symbolic Models: Representa(on of the rela(onships between various en((es or concept by means of symbols. 11

12 Mathematical Models Rela(onships between various en((es are expressed in the form of mathema(cal expressions: Func(on, Equa(on, and Inequali(es Example: Case Study C [Murthy, et.al. (1990)] Op(mal Produc(on 12

13 Case 1: The Ozone Hole A system for how the ozone layer in the stratosphere is gradually destroyed through the con(nued release of CFSs into the atmosphere, allowing a greater frac(on of the sun s uv- rays to reach the earth s surface. Input: Process: Component Structure: Output: 13

14 Case 2: Lubricating Oil Division (LOD) A situa(on summary Iden(fying the problem A system relevant Influence diagram The Narrow System:[LOD] A system for the replenishment and stock control of packaged finished goods which keeps the total opera*ng costs for the LOD as low as possible while maintaining the current level of customer service 14

15 Relevant System:[LOD] To use the typical inventory/produc(on control structure for defining the system But the problem situa(on includes aspects not normally found in such a structure e.g. rules to meet big and small customer orders Use the Process Approach Relevant System:[LOD] Big customer order is met by scheduling a special produc(on run (is it outside the narrow system?) This aspect affects the inventory/produc(on control system of LOD Cutoff point is considered as one of the two decision variables. What type of control open loop control (the customer order classifica(on input to the system) 15

16 Identifying the system aspects See Table 5-1. The easiest way is to start out by lis(ng The control inputs The performance outputs Any other system outputs, Then followed by structure, process, component of the system Two poten(al inputs breakdown in the opera(on of the refinery and produc(on capaci(es, are ignored (why?) InPluence Diagrams (ID) Insigheul for bringing out the transforma(on process of the system in term of the structural and causal rela(onships between systems components. Depicts the influence rela(onship Between the inputs into a system and its component (the system variables) Between the component of the system, and Between the component and the outputs of the system, including performance measures (these influence rela(onship can usually be measured in quan(ta(ve terms) 16

17 Approach To Draw An ID Control and other inputs State variables System outputs (in a logical sequence) For example, the customer order pafern (an input) & cutoff point (a decision) determine order paferns (met from stock or special produc(on run) Each customer order pafern determines the annual volume by spec- prod- run (AVPR) or annual volume met from stock (AVS). Approach To Draw An ID AVPR the annual number of setups Together with the cost of produc(on setup the annual setup cost for special produc(on run, and so forth. 17

18 Figure 5-1: OR/MS methodology F O R M U L A T I O N M O D E L I N G Summarizing problem situation 1 FORWARD LINKAGES Identifying 2 problem for analysis Decribing 3 relevant PROJECT PROPOSAL system Building a 4 mathematical model 1 Finding 5 preferred solution 6 Validation & performance testing Analyzing 7 sensitivity PROJECT REPORT of solution I M P L E M E N T A T O N Planning 8 Implementatio n 9 Establishing control over solution 10 Implementing solution 11 Following up solution use BACKWARD LINKAGES DOCUMENTATION & USER'S MANUAL SOLUTION AUDIT 18