MODELING AND ANALYSIS OF
MANUFACTURING SYSTEMS
Session 7
FLEXIBLE
MANUFACTURING
SYSTEMS
E. Gutierrez-Miravete
Spring 2001
DEFINITION
A FLEXIBLE MANUFACTURING
SYSTEM (FMS) IS A SET OF
NUMERICALLY CONTROLLED
MACHINE TOOLS AND SUPPORTING
WORKSTATIONS CONNECTED BY AN
AUTOMATED MATERIAL HANDLING
SYSTEM AND CONTROLLED BY A
CENTRAL COMPUTER
ELEMENTS OF FMS
• AUTOMATICALLY
REPROGRAMMABLE MACHINES.
• AUTOMATED TOOL DELIVERY AND
CHANGING
• AUTOMATED MATERIAL HANDLING
• COORDINATED CONTROL
FMS FEATURES
• MANY PART TYPES CAN BE LOADED
• PARTS CAN ARRIVE AT MACHINES IN
ANY SEQUENCE
• PARTS IDENTIFIED BY CODES
• MANY MACHINES CAN BE INCLUDED
• SMALL FMS LEAD TO FLEXIBLE
CELLS
FMS FEATURES
• EXPENSIVE TO IMPLEMENT BUT
SAVINGS CAN BE SIGNIFICANT
• FLOOR SPACE REDUCIBLE BY 1/3
• EQUIPMENT UTILIZATION UP TO 85%
OR MORE
• DETAILED PRODUCTION SEQUENCE
NOT NEEDED WELL IN ADVANCE
FMS FEATURES
• REDUCED VARIABLE COSTS AND
THROUGHPUT TIME LEAD TO
ENHANCED MANUFACTURING
COMPETITIVENESS
• ELIMINATION OF STARTUP CYCLES
LEAD TO STANDARIZED
PERFORMANCE
• MODULAR DESIGN
FMS FEATURES
•
•
•
•
REDUCED DIRECT LABOR COSTS
THREE SHIFTS READILY FEASIBLE
IDEAL FOR JIT
CAN EASILY BE TURNED OVER TO
NEW SET OF PRODUCTS IF THE NEED
ARISES
MANUFACTURING
FLEXIBILITY
• BASIC
– MACHINE (VARIETY OF OPERATIONS)
– MATERIAL HANDLING (PART MOBILITY
AND PLACEMENT)
– OPERATION (VARIETY OF OPERATIONS
PRODUCING SAME PART FEATURES)
MANUFACTURING
FLEXIBILITY
• SYSTEM
– PROCESS (VARIETY OF PARTS
PRODUCIBLE WITH SAME SETUP)
– ROUTING (ABILITY TO USE DIFFERENT
MACHINES UNDER SAME SETUP)
– PRODUCT (CHANGEOVER)
– VOLUME (PRODUCTION LEVEL)
– EXPANSION (ADDED CAPACITY)
MANUFACTURING
FLEXIBILITY
• AGGREGATED
– PROGRAM (UNATTENDED RUNNING)
– PRODUCTION (RANGES OF PARTS,
PRODUCTS, PROCESSES, VOLUME,
EXPANSION)
– MARKET (COMBINATION OF PRODUCT,
PROCESS, VOLUME AND EXPANSION)
COMMENTS
DOES FLEXIBILITY REMOVE
VARIABILITY FROM THE
SYSTEM?
NO, BUT IT ENABLES IT TO
PERFORM EFFECTIVELY
COMMENTS
KEY ISSUE
CAN A SYSTEM BE DESIGNED WHICH
IS USEFUL OVER A SUFFICIENT TIME
HORIZON, PART MIX AND SMALL
CHANGEOVER TIMES SO AS TO
OFFER AN ALTERNATIVE TO
SIMULTANEOUS PRODUCTION OF
MEDIUM VOLUME PART TYPES?
COMMENTS
THE PART TYPES ASSIGNED TO THE
FMS SHOULD HAVE SUFFICIENT
PRODUCTION VOLUMES TO MAKE
AUTOMATION ATTRACTIVE BUT
INSUFFICIENT TO JUSTIFY
DEDICATED PRODUCTION LINES
ORIGINS OF FMS
• LINK LINES (1960’S)
• NC MACHINES AND
CONVEYORS
• BATCH PROCESSING
ACRONYMS
•
•
•
•
•
•
FMS
NC
DNC
CNC
AGV
JIT
FMS PRIORITIES
• MEETING DUE DATES
• MAXIMIZING MACHINE
UTILIZATION
• MINIMIZE THROUGHPUT
TIMES
• MINIMIZE WIP LEVELS
FMS COMPONENTS
•
•
•
•
MACHINES
PART MOVEMENT SYSTEMS
SUPPORTING WORKSTATIONS
SYSTEM CONTROLLER
MACHINES
• PRISMATIC VS ROTATIONAL
PARTS
• HORIZONTAL MACHINING
CENTERS (HMC) AND HEAD
INDEXERS (HI)
• TOOL MAGAZINES AND
AUTOMATIC TOOL CHANGERS
PART MOVEMENT
• CONVEYORS
• TOW CARTS
• RAIL CARTS
• AGV’S
SUPPORTING
WORKSTATIONS
• LOAD/UNLOAD STATIONS
• AUTOMATIC PART WASHERS
• COORDINATE MEASURING
MACHINES
CONTROLLER
• COMPUTER
• WORKER (ATTENDANT)
• TRACKING SYSTEM FOR
– PARTS
– MACHINES
PLANNING AND CONTROL
HIERARCHY
DECISION MAKING PROCESS
– WHICH INFORMATION SHOULD
BE COMMUNICATED?
– HOW DO SYSTEM
COMPONENTS COMMUNICATE?
COMPONENTS OF THE
MANUFACTURING FACILITY
– FACILITY
– SHOP
– CELL
– WORKSTATION
– EQUIPMENT
MULTILEVEL CONTROL
HIERARCHY
• TREE STRUCTURE OF THE
HIERARCHY
• INFORMATION FLOWS ONLY
BETWEEN ADJACENT LAYERS
• EACH LEVEL HAS ITS OWN
PLANNING HORIZON AND DECISION
TYPES
• Fig. 5.5 and Table 5.1 , p. 133
GENERIC CONTROL MODEL
• GENERIC CONTROL STRUCTURE USED
TO ACCOMPLISH PLANNING,
EXECUTION AND FEEDBACK
• COMMANDS ARE RECEIVED FROM THE
NEXT HIGHER LEVEL AND TASKS ARE
BROKEN INTO SUBTASKS
• SUBTASKS ARE ASSIGNED TO
COMPONENTS AT NEXT LOWER LEVEL
GENERIC CONTROL MODEL
• SUBTASK MONITORING PERFORMED
THROUGH RECEIPT OF STATUS
FEEDBACK FROM LOWER LEVEL
• TASK STATUS INFORMATION RELAYED
TO NEXT HIGHER LEVEL
• EACH CONTROLLER HAS A
PRODUCTION MANAGER RECEIVING
COMMANDS AND SCHEDULING TASKS
GENERIC CONTROL MODEL
• QUEUE MANAGER MAINTAINED FOR
EACH LOWER LEVEL COMPONENTS TO
MANAGE ASSIGNED SUBTASKS
• DISPATCH MANAGER RECEIVES
DISPATCH ORDERS AND MANAGES
SUBTASK EXECUTION FOR EACH QUEUE
MANAGER
• Fig. 5.6, p. 134
BASIC STEPS IN DECISION
HIERARCHY
• LONG TERM PLANNING OR SYSTEM
DESIGN (PART TYPES & EQUIPMENT
SELECTION)
• MEDIUM RANGE PLANNING OR
SETUP (DAILY DECISIONS ABOUT
PARTS & TOOLING)
• SHORT TERM OPERATION
(SCHEDULING & CONTROL)
SYSTEM DESIGN
• PROBLEM: SELECTING SYSTEM SIZE,
HARDWARE, SOFTWARE AND PARTS
FOR THE FMS
• SIZE & SCOPE ARE SELECTED
ACCORDING TO CORPORATE
STRATEGY
• HARDWARE & SOFTWARE SELECTED
TO FIT SCOPE
SYSTEM DESIGN
• PART SELECTION IS DONE
ACCORDING TO AN ECONOMIC
CRITERION & STRATEGIC
CONSIDERATIONS
• KNAPSACK PROBLEM: LOAD THE
FMS TO MAXIMIZE SAVINGS
SUBJECT TO FMS CAPACITY
KNAPSACK PROBLEM
P = PRODUCTIVE TIME PER PERIOD
AVAILABLE ON BOTTLENECK FMS
RESOURCE
pi = TIME PER PERIOD REQUIRED FOR
PART i
si = SAVINGS PER PERIOD IF PART TYPE
i
KNAPSACK PROBLEM
maximize i
si Xi
subject to
i p i < P
SOLVING THE KNAPSACK
PROBLEM
•
•
•
•
GREEDY HEURISTIC
Example 5.1, p. 136
OPTIMIZATION
Example 5.2, p. 138
SYSTEM SETUP
• ASSIGNMENT OF OPERATIONS AND
ACCOMPANYING TOOLING TO
MACHINES
• PART SELECTION PROBLEM: BATCH
FORMATION
• LOADING PROBLEM: SEQUENCING
AND ROUTING OF PARTS
PART SELECTION
• GOAL: PLACE REQUIRED PARTS INTO
COMPATIBLE BATCHES SUCH THAT
• EACH BATCH USES ALL MACHINES
• REQUIRE A LIMITED NUMBER OF
TOOLS ON EACH MACHINE
• HAVE SIMILAR DUE DATES FOR
PARTS IN THE BACTH
PART SELECTION
• GREEDY HEURISTIC: FORM BATCHES
BY ARRANGING PART ORDERS BY DUE
DATES
• PART ORDERS ARE SEQUENTIALLY
ADDED TO CURRENT BATCH
WITHOUT VIOLATING CONSTRAINTS
• BATCH IS THEN READY FOR LOADING
• Example 5.3, p. 140
Part Selection as a Mixed-Integer
Program
• Time phased set of part orders Dit for part
i in time t
• Time available in machine j , Pj
• Time required by product i in machine j
pij
• Number of parts of type i made in time t
xit
• Number of tool slots in machine j , Kj
Part Selection as a Mixed-Integer
Program
• Number of tool slots required by tool l in
machine j , klj
• Set of tools l required on machine j to
produce part i , l j(i)
• Holding cost per period for part i hi
• Formulation: p. 142
Part Selection as a Mixed-Integer
Program
• Goal: Minimize inventory cost while
meeting due dates
• Example 5.4 , p. 142
Incremental Part Selection
• Several part types in process at any time
• System operates almost continuously
• Goal: Minimize makespan to complete all
available part orders
• Procedure: Minimize idle time by balancing
work loads subject to part demand and tool
magazine capacity
• Formulation: p. 144
LOADING PROBLEM
• BATCH TO BE PROCESSED IS KNOWN
• OBJECTIVES REQUIRED
• LOADING SOLUTION MUST BE
ROBUST AND FLEXIBLE
• SOLUTION METHODOLOGIES
– MATHEMATICAL PROGRAMMING (p.145)
– HEURISTIC APPROACHES (p. 148)
LOADING PROBLEM:
HEURISTIC APPROACH
• PHASE I : ASSIGN OPERATIONS TO
MACHINE TYPES
• PHASE II:
– OPERATIONS COMBINED INTO
CLUSTERS TO REDUCE TRANSFERS
– MACHINE GROUPS FORMED
– OPERATIONS AND TOOLS ASSIGNED
TO GROUPS
SCHEDULING AND
CONTROL
• BASIC PROBLEM AREAS
– SEQUENCING AND TIMING OF PART
RELEASES TO THE SYSTEM
– SETTING OF INTERNAL PRIORITIES IN
THE SYSTEM
– ABILITY OF SYSTEM TO TAKE
CORRECTIVE ACTION WHEN
COMPONENTS FAIL
Flexible Assembly Systems
• For the combination of raw materials and
components into products with functional
characteristics.
• Automated vs manned systems
• Example: Vibratory bowl feeders and vision
systems
• Role of Design for Assembly