2. Flexible Manufacturing Systems (FMS)
2.1 Overview
2.1.1 Distinguishing characteristics,
• An automatic materials handling subsystem links machines in the system and provides for automatic interchange of workpieces in each machine
• Automatic continuous cycling of individual machines
• Complete control of the manufacturing system by the host computer
• Lightly manned, or possibly unmanned
• Characteristics of application,
Medium product mix
Medium production volume
Allows fast changeover on products
• Various measures of flexibility,
Able to deal with slightly, or greatly mixed parts.
Variations allowed in parts mix
Routing flexibility to alternate machines
Volume flexibility
Design change flexibility
• Major historical developments,
Weaving Looms with paper tapes,
NC machines with paper tapes
Hard wired NC machines
Computer controlled NC machines (CNC)
Direct Numerical Control (DNC)
• Components of FMS Systems,
Robotics
Material Handling / Transport
Machines
Manual / Automated Assembly Cells
Computers
Controllers
Software
Networks
Interfacing
Monitoring equipment
• Humans are not without function in an FMS cell,
loading and unloading workparts to and from the system
changing tools and settings
equipment maintenance and repair
• Computers provide essential support in a workcell for,
CNC: Computer Numerical Control
DNC: Direct Numerical Control of all the machine tools in the FMS. Both CNC and DNC functions can be incorporated into a single FMS.
Computer control of the materials handling system
Monitoring: collection of production related data such as piece counts, tool changes, and machine utilization
Supervisory control: functions related to production control, traffic control, tool control, and so on.
• FMS systems are intended to solve the following problems,
Production of families of workparts, often based on group technology
Random launching of workparts into system is OK, because setup time is reduced with FMS.
Reduced manufacturing lead time: this is possible because FMS has organization, and fast setup.
Reduced work in process
Increased machine utilization
Reduced direct and indirect labor
Better management control
• The most common problems in an FMS are,
Scheduled maintenance
Scheduled tool changeovers
Tooling problems (failures and adjustments)
Electrical Failures
Mechanical Problems (e.g., oil leaks)
• Implementation Strategies,
find and identify a champion (someone who will push for automation)
spend time to educate workers and engineers on FMS
invest in the planning stages
look at others in industry
use employee involvement from the start
install in stages: don’t try to implement all at once
• Things to Avoid when making a decision for FMS,
ignore impact on upstream and downstream operations
allow the FMS to become the driving force in strategy
believe the vendor will solve the problem
base decisions solely on financials
ignore employee input to the process
try to implement all at once (if possible)
• Justification of FMS,
consider “BIG” picture
determine key problems that must be solved
highlight areas that will be impacted in enterprise
determine kind of flexibility needed
determine what kind of FMS to use
look at FMS impacts
consider implementation cost based on above
• Factors to consider in FMS decision,
volume of product
previous experience of company with FMS
product mix
scheduling / production mixes
extent of information system usage in organization (eg. MRP)
use of CAD/CAM at the front end.
availability of process planning and process data
Process planning is only part of CIM, and cannot stand alone.
2.1.2 General Concepts
• Manufacturing requires computers for two functions,
Information Processing: This is characterized by programs that can operate in a batch mode.
Control: These programs must analyze sensory information, and control devices while observing time constraints.
• A CIM system is made up of Interfaced and Networked Computers. The general structure is hierarchical,
• The plant computers tend to drive the orders in the factory.
• The plant floor computers focus on departmental control. In particular,
synchronization of processes.
downloading data, programs, etc., for process control.
analysis of results (e.g., inspection results).
• Process control computers are local to machines to control the specifics of the individual processes. Some of their attributes are,
program storage and execution (e.g., NC Code),
sensor analysis,
actuator control,
process modeling,
observe time constraints (real time control).
• The diagram shows how the characteristics of the computers must change as different functions are handled.
• To perform information processing and control functions, each computer requires connections,
Stand alone: No connections to other computers, often requires a user interface.
Interfaced: Uses a single connection between two computers. This is characterized by serial interfaces such as RS-232 and RS-422.
Networked: A single connection allows connections to more than one other computer. May also have shared files and databases.
• Types of common interfaces,
RS-232 (and other RS standards) are usually run at speeds of 2400 to 9600 baud, but they are very dependable.
• Types of Common Networks,
IEEE-488 connects a small number of computers (up to 32) at speeds from .5 Mbits/sec to 8 Mbits/sec. The devices must all be with a few meters of one another.
Ethernet: connects a large number of computers (up to 1024) at speeds of up to 10 Mbits/sec., covering distances of km. These networks are LAN’s, but bridges may be used to connect them to other LAN’s to make a WAN.
• Types of Modern Computers,
Mainframes: Used for a high throughput of data (from disks and programs). These are ideal for large business applications with multiple users, running many programs at once.
Workstations (replacing Mini Computers): have multiprocessing abilities of Mainframe, but are not suited to a limited number of users.
Micro-processors, small computers with simple operating systems (like PC’s with msdos) well suited to control. Most computerized machines use a micro-processor architecture.
2.2 Computer Communications to Support FMS
2.2.1 Basic Computer control functions
• NC part program storage
• Distribution of the part programs to the individual machine tools
include post processing for specific machine formats
• Production control
decisions of part mix, rates, inputs to parts of system
considers data like,
desired production rate for parts per day
number of raw workparts available
number of applicable pallets
routes pallets to load/unload area
gives instructions to operator about desired parts via a data entry unit (DEU)
• Traffic Control
Regulates materials transfer system to ensure parts are moved between desired workstations. This may be problematic, depending upon the transportation system used. Can act like a railway switch operator.
• Shuttle Control
moves parts between stations and main conveyor
coordinates actions between machines, and primary handling system
• Work handling system monitoring,
monitor both work parts, and pallets.
• Tool Control
tracks tools at each station (and reroute parts if necessary, or notifies operator to install parts)
tool life monitoring, and operator orders for replacement
• System performance monitoring and Reporting
Generate management reports
2.2.2 Data Files Required in a FMS system,
• Part program file
numerical control files for each part, and for each machine which may make a part
• Routing File
A list of machines which the parts must be routed through for completion, including alternates in some cases.
• Part Production File
Production parameters for each workpart
allowances for in-process inventory
etc.
used for production control
• Pallet reference file
a record of which parts a specific pallet is fixtured for
each pallet is individually identified
• Station Tool File
a file for each workstation identifies,
codes of cutting tools at station
used for tool control purposes
• Tool-life file
keeps tool-life value for each tool in the system for comparison with maximum value.
2.2.3 System reports generated by an FMS system,
• Utilization Reports
summarize individual, and group efficiencies for stations.
• Production Reports
daily and weekly reports of parts produced in the FMS
• Status Reports
a snapshot of present conditions in the FMS system for,
etc
• Tool Reports
list of tools at (or needed at) a workstation
tool life status reports
• A Graphical Depiction of a Workstation Controller
2.3 The Future of FMS
• FMS systems which deliver directly into warehouse, and do not require labor
• The use of robots that have vision, and tactile sensing to replace human labor
• Technology will make 100% inspection feasible. Thus making faster process adjustment possible.
• Computer diagnosis will improve estimation of machine failure, and guide work crews repairing failures.
• International coordination and control of manufacturing facilities.
• Customers have completely custom orders made immediately, and to exact specifications, and at a lower cost
• Networks will tend to eliminate the barriers caused by international borders
• Standards will be developed which make installation of a new machine trivial
• Networking between manufacturers and suppliers will streamline the inventory problems
• Marketing will be reduced, as customer desires are met individually, and therefore do not need to be anticipated by research.
• Finished goods inventories will fall as individual consumer needs are met directly.
• Better management software, hardware, and fixturing techniques will push machine utilization towards 100%
• The task of Design and Process Planning will become highly automated, therefore reducing wasted time on repetitious design, and discovering careless mistakes.
• Simplification of systems overall: MRP, MPCS, etc.
• More front end simulation
• Computing power increases: more sophisticated tools
2.4 Poblems
Problem 2.1 What is concurrent (parallel) processing and why is it important for workcell control?
Answer 2.1 to allow equipment to do other tasks while one machine is processing
Problem 2.2 What is meant by the term “Device Driver”?
Answer 2.2 a piece of software that allows connections to a specific piece of hardware
