AO:3.1 OVERVIEW
AO:3.1.1 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 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
- Design change flexibility
Major historical developments,
- Weaving Looms with paper tapes,
- NC machines with paper tapes
- Computer controlled NC machines (CNC)
- Direct Numerical Control (DNC)
Components of FMS Systems,
- Material Handling / Transport
- Manual / Automated Assembly Cells
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 tool changeovers
- Tooling problems (failures and adjustments)
- 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,
- 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
- consider implementation cost based on above
Factors to consider in FMS decision,
- previous experience of company with FMS
- 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.
AO:3.1.2 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),
- 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.
