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 Salih ÖZTOP

FMS Loading Problem2/26 Context 1) Introduction 2) Analysis of Loading Problem 3) Literature Review

FMS Loading Problem3/26 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

FMS Loading Problem4/26 1. Introduction First article: Short-term production planning problems in FMSs, Stecke and Solberg (1981) Few of them focused on loading problem

FMS Loading Problem5/26 2. Analysis of Loading Problem Elements affecting the loading problemElements affecting the loading problem –Characteristics of the FMS –Characteristics of the plant where FMS operates –Production Planning Hierarchy (interface of loading module with upper and lower level of the management hierarchy)

FMS Loading Problem6/26 Characteristics of the FMS 2.1 Characteristics of the FMS MachinesMachines Control SystemControl System ToolsTools Tool Handling SystemTool Handling System Parts, Pallets and FixturesParts, Pallets and Fixtures

FMS Loading Problem7/26 Machines Machines M1: First FMSs: different machines with different capabilities New FMSs: identical machines Reasons for this evaluation: -More versatile machining centers due to increased number of control axes, increased spindle power and speed. Parallel machine FMSs vs. General FMSs M2:Grouping some identical machines Ex. For higher accuracy assign jobs to this group. M3: Under tight part position tolerances, two or more operations are done on the same machine

FMS Loading Problem8/26 Control System Control System Control System types: A Supervisor controls and coordinates different machines, CNCs and PLCsA Supervisor controls and coordinates different machines, CNCs and PLCs Computerized Numerical Controls (CNCs) controls specific deviceComputerized Numerical Controls (CNCs) controls specific device Programmable Logic Controllers (PLCs) controls specific deviceProgrammable Logic Controllers (PLCs) controls specific device

FMS Loading Problem9/26 Control System Control System Lack of standardization in those controllers causes some problems: -Integration of hardwares of different companies is difficult -Integrating FMS with other systems in the same company -Changing control hierarchy -Integrating new sensors in an existing control system Some projects continue to standardize control systems such as OSACA,OMAC-TEAMAPI,OSEC...

FMS Loading Problem10/26 Control System Control System Since part programs in CNCs are seen as a whole indivisible program, following problems arise. P1: All tools required in the program are requested at the beginning P2: A tool assigned to a part program is busy for the whole part program duration P3: Whole part program should be executed on the same machine P4: Sequence of operations are given P5: No alternative operation is considered

FMS Loading Problem11/26 System Tools and Tool Handling System T1: Tool life issues: Tools subject to wear and need to be reconditioned T2: Number of tool copies Expensive, only few copies of that tool type is available TH1: Tool magazine capacity Finite capacity. Large tool magazine size  high tool seek time  high spindle idle time TH2: Tool transport system Automatic tool transport system that can exchange the tools in the tool magazines of different machines and tool store.

FMS Loading Problem12/ Parts, pallets, and Fixtures Parts should be positioned correctly before machining operations and transpotation activities therefore they require some fixtures and pallets Parts are mounted on fixtures and fixtures are mounted on pallets

FMS Loading Problem13/ Parts, pallets, and Fixtures F1: Part Loading In general parts are manually loaded to the fixtures But now automated load/unload robots are present. F2: Fixture Cycle Time After usage of the fixture, it can need to be cleaned or inspected before going back to the load/unload station. F3: Parts requiring more than one fixture To change the position of the part, new fixtures are needed. Single fixture is not enough to complete all the machine work on that part F4: More than one part type on the same fixture When required part amount is small, one fixture type can be used for different parts

FMS Loading Problem14/26 Characteristics of the plant where FMS operates 2.2 Characteristics of the plant where FMS operates E1: Characteristics of the Shifts FMSs are automated, but they require people for loading/unloading, monitoring, placing parts to fixtures etc. Unpersonned shifts are an important constraint E2: Tool Room Management Tool room give service to all of the plant not just the FMS. E3: Preventive Maintenance In preventive maintenance times whole FMS or some of its machines can be unavailable. E4: Downstream Assembly Operations If the parts produced in FMS are components of other assembled products, then the part ratios should be considered as a constraint.

FMS Loading Problem15/ Production Planning Hierarchy Loading model must fit into the production-planning hierarchy The characteristics of the model affected by the higher or lower levels of the hierarchy

FMS Loading Problem16/ Higher Level H1: Requests are handled periodically or continuously Generally periodic handling of request is used but in small firms continuous case is mostly used due to the dynamic structure of this firms H2: Due Dates/ Release Dates 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 higher level, loading problem should be modeled differently H3: Priorities In real life some parts have higher priority than the others. Ex. Parts of strategic clients Contracts can cause some priority restrictions

FMS Loading Problem17/ Lower Level Lower levels are affected by the loading level decisions. Most of the details of the lower levels are not very important L1:Unforeseen Events Unforeseen events of little importance such as tool breakages are dealt at lower levels. If the problem causes a capacity decrease then a new loading plan is needed quickly. L2: Limits of the Lower Levels Lower levels can create constraints that are not directly related to FMS but related to the management software. Ex. FMS includes a tool transport system but the management software cannot handle this tool scheduling.

FMS Loading Problem18/ Literature Review Articles formalize the loading problem as -Mixed integer programming problem -0-1 programming problem In solution cases they slightly modify the models with relaxation and suggest some heuristics to overcome the computational problems

FMS Loading Problem19/ Types of Loading Problems in Literature Parallel(identical) vs. General FMSs Parallel systems are special form of general case.Since there are several parallel FMSs, they take interest of researchers

FMS Loading Problem20/ Tool Management Strategy 1) Batching (Tool Dominant) Most common one Configuration of each tool magazine is frozen for a given length of time. In that period machines can only use the tools at the tool magazine 2) Flexible (Part Dominant) Continuos evaluation of the tool magazine configuration. 3) Hybrid Continuous evaluation with some limitations

FMS Loading Problem21/ Tool Management Strategy

FMS Loading Problem22/ Objective Functions 1) Directly connected to the firm’s goals Cost minimization Flow time/WIP minimization Minimization of number of late part types Minimization of makespan 2) Not connected to the firm’s goals Workload balance among work centers Load minimization of some sub systems Maximization of number of alternative routings

FMS Loading Problem23/ Objective Functions

FMS Loading Problem24/ Constraints 1) Assignment Constraints They limit the assignment of parts to machines due to technological constraints and to reduce the computational complexity by reducing the solution space 2) Capacity Constraints Tool magazine capacity, limited number of fixtures/pallets, limited number tools for each tool type, available machining time 3) Management Constraints Workload balancing among machines Due date restrictions Part priorities

FMS Loading Problem25/ Constraints

FMS Loading Problem26/26 4. Future Research Directions 1) Test Problems To compare different algorithms to similar problems a common test problem data collection should be formed 2) Clarify the characteristics of production planning hierarchy 3) Due date requirements 4) Unforeseen events Can make the loading planning impractical 5) Tool life management with unpersonned shifts 6) Integrate production and process planning Same part can be obtained by different operations 7) Tool selection and operation sequence relaxation Same job can be done by another tool and with different operation sequences

FMS Loading Problem27/26 5. Questions & Answers Thank you