A Review of Different Approaches to the FMS Loading Problem A Review of Different Approaches to the FMS Loading Problem A. Grieco, Q. Semeraro, T. Tolio

A Review of Different A Review of Different Approaches to the FMS Approaches to the FMS

Loading ProblemLoading ProblemA. Grieco, Q. Semeraro, T. Tolio

Salih ÖZTOP20202801

07.04.2003

07.04.2003 FMS Loading Problem 2/26

Context

1) Introduction2) Analysis of Loading Problem3) Literature Review


1. Introduction

Aim of the paper• Address the different factors affecting the

loading problem in Flexible Manufacturing Systems(FMS)

• Survey previous studies addressing some of these problems

• Give some future research directions


1. Introduction

• First article: Short-term production planning problems in FMSs, Stecke and Solberg (1981)

• Few of them focused on loading problem


2. Analysis of Loading Problem

• Elements affecting the loading problemElements affecting the loading problem – Characteristics of the FMSCharacteristics of the FMS– Characteristics of the plant where FMS Characteristics of the plant where FMS

operatesoperates– Production Planning Hierarchy (interface of Production Planning Hierarchy (interface of

loading module with upper and lower level of loading module with upper and lower level of the management hierarchy)the management hierarchy)


2.1 Characteristics of the FMSCharacteristics of the FMS

• MachinesMachines

• Control SystemControl System

• ToolsTools

• Tool Handling SystemTool Handling System

• Parts, Pallets and FixturesParts, Pallets and Fixtures


2.1.1 MachinesMachines

M1: M1: First FMSs: different machines with different capabilitiesFirst FMSs: different machines with different capabilitiesNew FMSs: identical machinesNew FMSs: identical machines

Reasons for this evaluation:Reasons for this evaluation:-More versatile machining centers due to increased number of -More versatile machining centers due to increased number of control axes, increased spindle power and speed.control axes, increased spindle power and speed.Parallel machine FMSs vs. General FMSsParallel machine FMSs vs. General FMSsM2:M2: Grouping some identical machines Grouping some identical machines Ex. For higher accuracy assign jobs to this group.Ex. For higher accuracy assign jobs to this group.M3:M3: Under tight part position tolerances, two or more Under tight part position tolerances, two or more operations are done on the same machineoperations are done on the same machine


2.1.2 Control SystemControl System

Control System types:Control System types:•A A SupervisorSupervisor controls and coordinates controls and coordinates different machines, CNCs and PLCsdifferent machines, CNCs and PLCs•Computerized Numerical ControlsComputerized Numerical Controls (CNCs) (CNCs) controls specific devicecontrols specific device•Programmable Logic ControllersProgrammable Logic Controllers (PLCs) (PLCs) controls specific devicecontrols specific device



Lack of standardization in those controllers causes some Lack of standardization in those controllers causes some problems:problems:

-Integration of hardwares of different companies is difficult-Integration of hardwares of different companies is difficult-Integrating FMS with other systems in the same company-Integrating FMS with other systems in the same company-Changing control hierarchy-Changing control hierarchy-Integrating new sensors in an existing control system-Integrating new sensors in an existing control system

Some projects continue to standardize control systems such as Some projects continue to standardize control systems such as OSACA,OMAC-TEAMAPI,OSEC...OSACA,OMAC-TEAMAPI,OSEC...



Since part programs in CNCs are seen as a whole indivisible Since part programs in CNCs are seen as a whole indivisible program, following problems arise.program, following problems arise.P1: P1: All tools required in the program are requested at the All tools required in the program are requested at the beginningbeginningP2:P2: A tool assigned to a part program is busy for the whole part A tool assigned to a part program is busy for the whole part program durationprogram durationP3:P3: Whole part program should be executed on the same Whole part program should be executed on the same machinemachineP4:P4: Sequence of operations are given Sequence of operations are givenP5:P5: No alternative operation No alternative operation isis considered considered


2.1.3 Tools and Tool Handling System System

T1: Tool life issues:T1: Tool life issues:Tools subject to wear and need to be reconditionedTools subject to wear and need to be reconditionedT2: Number of tool copiesT2: Number of tool copies Expensive, only few copies of that tool type is availableExpensive, only few copies of that tool type is availableTH1: Tool magazine capacityTH1: Tool magazine capacityFinite capacity. Large tool magazine size Finite capacity. Large tool magazine size high tool seek high tool seek timetimehigh spindle idle timehigh spindle idle timeTH2: Tool transport systemTH2: Tool transport systemAutomatic tool transport system that can exchange the tools in Automatic tool transport system that can exchange the tools in the tool magazines of different machines and tool store.the tool magazines of different machines and tool store.


2.1.4 Parts, pallets, and Fixtures

Parts should be positioned correctly before machining Parts should be positioned correctly before machining operations and transpotation activities therefore they operations and transpotation activities therefore they require some fixtures and palletsrequire some fixtures and pallets

Parts are mounted on fixtures and fixtures are mounted on Parts are mounted on fixtures and fixtures are mounted on palletspallets


2.1.4 Parts, pallets, and Fixtures

F1: Part LoadingF1: Part LoadingIn general parts are manually loaded to the fixturesIn general parts are manually loaded to the fixturesBut now automated load/unload robots are present.But now automated load/unload robots are present.F2: Fixture Cycle TimeF2: Fixture Cycle TimeAfter usage of the fixture, it can need to be cleaned or After usage of the fixture, it can need to be cleaned or inspected before going back to the load/unload station.inspected before going back to the load/unload station.F3: Parts requiring more than one fixtureF3: Parts requiring more than one fixtureTo change the position of the part, new fixtures are To change the position of the part, new fixtures are needed. Single fixture is not enough to complete all the needed. Single fixture is not enough to complete all the machine work on that partmachine work on that partF4: More than one part type on the same fixtureF4: More than one part type on the same fixtureWhen required part amount is small, one fixture type can When required part amount is small, one fixture type can be used for different partsbe used for different parts


2.2 Characteristics of the plant where FMS operatesCharacteristics of the plant where FMS operates

E1: Characteristics of the ShiftsE1: Characteristics of the ShiftsFMSs are automated, but they require people for FMSs are automated, but they require people for loading/unloading, monitoring, placing parts to fixtures etc. loading/unloading, monitoring, placing parts to fixtures etc. Unpersonned shifts are an important constraintUnpersonned shifts are an important constraintE2: Tool Room ManagementE2: Tool Room ManagementTool room give service to all of the plant not just the FMS.Tool room give service to all of the plant not just the FMS.E3: Preventive MaintenanceE3: Preventive MaintenanceIn preventive maintenance times whole FMS or some of its In preventive maintenance times whole FMS or some of its machines can be unavailable.machines can be unavailable.E4: Downstream Assembly OperationsE4: Downstream Assembly OperationsIf the parts produced in FMS are components of other If the parts produced in FMS are components of other assembled products, then the part ratios should be assembled products, then the part ratios should be considered as a constraint. considered as a constraint.


2.3 Production Planning HierarchyLoading model must fit into the production-planning hierarchy

The characteristics of the model affected by the higher or lower levels of the hierarchy


2.3.1 Higher LevelH1: Requests are handled periodically or continuouslyH1: Requests are handled periodically or continuouslyGenerally periodic handling of request is used but in small Generally periodic handling of request is used but in small firms continuous case is mostly used due to the dynamic firms continuous case is mostly used due to the dynamic structure of this firmsstructure of this firmsH2: Due Dates/ Release DatesH2: Due Dates/ Release DatesIf due dates are decided at the higher levels, they are used If due dates are decided at the higher levels, they are used as an input to the loading level. If they are not decided at as an input to the loading level. If they are not decided at higher level, loading problem should be modeled differentlyhigher level, loading problem should be modeled differentlyH3: PrioritiesH3: PrioritiesIn real life some parts have higher priority than the others. In real life some parts have higher priority than the others. Ex. Parts of strategic clientsEx. Parts of strategic clientsContracts can cause some priority restrictionsContracts can cause some priority restrictions


2.3.2 Lower LevelLower levels are affected by the loading level decisions.Lower levels are affected by the loading level decisions.Most of the details of the lower levels are not very importantMost of the details of the lower levels are not very important

L1:Unforeseen EventsL1:Unforeseen EventsUnforeseen events of little importance such as tool Unforeseen events of little importance such as tool breakages are dealt at lower levels. If the problem causes a breakages are dealt at lower levels. If the problem causes a capacity decrease then a new loading plan is needed quickly.capacity decrease then a new loading plan is needed quickly.

L2: Limits of the Lower LevelsL2: Limits of the Lower LevelsLower levels can create constraints that are not directly Lower levels can create constraints that are not directly related to FMS but related to the management software.related to FMS but related to the management software.Ex. FMS includes a tool transport system but the Ex. FMS includes a tool transport system but the management software cannot handle this tool scheduling.management software cannot handle this tool scheduling.


3.1 Literature ReviewArticles formalize the loading problem asArticles formalize the loading problem as

-Mixed integer programming problem-Mixed integer programming problem-0-1 programming problem-0-1 programming problem

In solution cases they slightly modify the models with In solution cases they slightly modify the models with relaxation and suggest some heuristics to overcome the relaxation and suggest some heuristics to overcome the computational problemscomputational problems


3.2 Types of Loading Problems in Literature

Parallel(identical) vs. General FMSsParallel(identical) vs. General FMSsParallel systems are special form of general case.Since there Parallel systems are special form of general case.Since there are several parallel FMSs, they take interest of researchersare several parallel FMSs, they take interest of researchers


3.2.2 Tool Management Strategy

1) Batching (Tool Dominant)1) Batching (Tool Dominant)Most common oneMost common oneConfiguration of each tool magazine is frozen for a given Configuration of each tool magazine is frozen for a given length of time. In that period machines can only use the tools length of time. In that period machines can only use the tools at the tool magazineat the tool magazine2) Flexible (Part Dominant)2) Flexible (Part Dominant)Continuos evaluation of the tool magazine configuration.Continuos evaluation of the tool magazine configuration.3) Hybrid3) HybridContinuous evaluation with some limitationsContinuous evaluation with some limitations


3.2.2 Tool Management Strategy


3.3.1 Objective Functions

1) Directly connected to the firm’s goals1) Directly connected to the firm’s goalsCost minimizationCost minimizationFlow time/WIP minimizationFlow time/WIP minimizationMinimization of number of late part typesMinimization of number of late part typesMinimization of makespanMinimization of makespan

2) Not connected to the firm’s goals2) Not connected to the firm’s goalsWorkload balance among work centersWorkload balance among work centersLoad minimization of some sub systemsLoad minimization of some sub systemsMaximization of number of alternative routingsMaximization of number of alternative routings


3.3.1 Objective Functions


3.3.2 Constraints

1) Assignment Constraints1) Assignment ConstraintsThey limit the assignment of parts to machines due to They limit the assignment of parts to machines due to technological constraints and to reduce the computational technological constraints and to reduce the computational complexity by reducing the solution spacecomplexity by reducing the solution space2) Capacity Constraints2) Capacity ConstraintsTool magazine capacity, limited number of fixtures/pallets, Tool magazine capacity, limited number of fixtures/pallets, limited number tools for each tool type, available machining limited number tools for each tool type, available machining timetime3) Management Constraints3) Management ConstraintsWorkload balancing among machinesWorkload balancing among machinesDue date restrictionsDue date restrictionsPart prioritiesPart priorities


3.3.2 Constraints


4. Future Research Directions1) Test Problems1) Test ProblemsTo compare different algorithms to similar problems a To compare different algorithms to similar problems a common test problem data collection should be formedcommon test problem data collection should be formed2) Clarify the characteristics of production planning 2) Clarify the characteristics of production planning hierarchyhierarchy3) Due date requirements3) Due date requirements4) Unforeseen events4) Unforeseen eventsCan make the loading planning impracticalCan make the loading planning impractical5) Tool life management with unpersonned shifts5) Tool life management with unpersonned shifts6) Integrate production and process planning 6) Integrate production and process planning Same part Same part can be obtained by different operationscan be obtained by different operations7) Tool selection and operation sequence relaxation7) Tool selection and operation sequence relaxationSame job can be done by another tool and with different Same job can be done by another tool and with different operation sequencesoperation sequences


5. Questions & Answers

Thank you

Documents

A Review of Different Approaches to the FMS Loading Problem A Review of Different Approaches to the FMS Loading Problem A. Grieco, Q. Semeraro, T. Tolio