Cell Integration into a Manufacturing System

Cell Integration into a Manufacturing System

Integrating the cell into a larger manufacturing system is basically a step up the system hierarchy.Now the problems become ones concerning the cell as a whole (or, to use the earlier lexicon, a module). This purpose of this section is not to provide a detailed set of guidelines for the system design but to consider the most important features of the system and to discuss the relation ship between the cells and the system.

System Definition

The system should be usable as a building block for a complete factory. Like the cells beneath it, the system will be a module with a defined input and output. It will depend on the next higher level of the hierarchy for certain services such as inventory, engineering support, and scheduling.

System Capability

Initially, the manufacturing system may consist of just one or two cells but with expansion and flexibility in mind the initial design should at least have provisions for incorporating the following features:

· The system will monitor material flow within its boundaries. Whether the parts are moved manually or automatically from one cell to the next, the system should be able to keep track them.

· Information as well as materials will flow from cell to cell. The system will be responsible for coordinating the information passage. For example, when a batch of parts travels from one cell to another it is accompanied by descriptive information. This is done so that the destination cell knows how many parts are in the batch, what their description is, what their orientation is, and so on. At first, information giving the part description and orientation may not be used since the cell program will assume a particular orientation for a particular part type. As cells become more sophisticated they will assume less. Instead, they will rely on their sensors, aided by the information accompanying the parts as they enter the cell.

· The manufacturing system will store and maintain programs associated with producing the families of parts. In particular:

o The system level computer- will be responsible for maintaining therepertoire of part programs used by CNC machines in the cells.

o Likewise, the cell hosts require a variety of instructions from the system. The sequence programs, for example, for a given part will come from the system level computer.

· The system will be responsible for maintaining the statistics for the cells and machine tools. These are the usage, maintenance, and history statistics that are necessary for autonomous operation.

· One of the more important functions of the system level supervisors should be the capability to gracefully degrade the system as individual components fail. The system should also be able to recover from failure when the components are repaired.

The system may be a very large and versatile collection of cells, but it is not factory. In particular, the following operations lie beyond the scope of the manufacturing system:

· The retrieval and initial processing of raw materials is done outside the system. For example, the bar stock used for the family of parts discussed in section 3 is usually delivered in 20 foot long sections. Handling these sections and cutting them to a more manageable size are operations that lie outside the manufacturing system.

· Inventory and its control are handled outside of the manufacturing system. In-process parts are controlled by the system, but storage of the finished parts and storage of the bar stock and bar stock sections are controlled at the factory level.

· Maintenance functions required in the factory are not part of the system. These include the maintenance of the machines, robots, and computers, and also of the software required to run them.

· The CAD/CAM (Computer Aided Design / Computer Aided Manufacturing) system mentioned in section 4.4.2 will reside in computers outside the manufacturing system. The manufacturing system computer will store parts programs and sequence programs but will not be used for designing parts and generating CNC programs. Figure 4-1 schematically shows the relationship between the manufacturing system and the operations listed above.

Modifications at the Cell Level

The modifications to a machining cell that transform it from a solitary, developmental cell into a member of a manufacturing system are not difficult. The design for the cell specifies input and output for the cell, and these are services that the manufacturing system provides to the cell. The cell has been designed so that virtually no modifications in the cell equipment are required. In fact, the performance demanded of individual machines in a cell may reduce as the number of cells in a manufacturing system increases. Redundancy makes each piece of equipment less crucial to the function of the system as a whole.

The main modification of the cell will be to the cell host. A communication channel between the manufacturing system and the cell host must be established. The communication channel will be via the local network of the factory and the cell host must be given the software and the hardware to use it. The cell host will receive its instructions and programs from the manufacturing system computer instead of its own on-line storage.

System Components

The most important components of the manufacturing system are the cells that it comprises, but some additional components will be needed to tie the cells together:

Inter - cell materials handling devices

The devices responsible for transporting components between the cells in a manufacturing system can take many forms. The traditional conveyor-based

systems are expensive and difficult to maintain. The newer developments based on robot trucks, for example, also require a large capital investment. The success of the system depends most on implementing the cells correctly. Achieving a completely unmanned material flow is less important, and initially we allow the handling of materials between the cells to be manual. To use robot trucks requires not only the very high initial expense of the carts and their guiding apparatus, but also the development of an intelligent, automated system for controlling them, Sensors are required to keep track of where the individual carts are at any given time, and what parts they are carrying. Batches of parts will enter or leave the cells only at occasional intervals, so the autonomous nature of the cell and manufacturing system is not too compromised.

The manufacturing system supervisory computer.

The supervisory computer for the manufacturing system will fill much the same role for the system that the cell host computer does for the cell. A few of the more important chores performed by the system computer are listed below:

· Schedule the flow of parts between the cells within the manufacturing system.

· Direct the flow of information concerning the parts.

· Store the CNC part programs (for both the CNC machine tools and the robots) in an on-line storage facility.

· Maintain a database that relates particular parts to the programs required to make them.

· Supply the part programs to the cell host when a new part is sent to a cell.

· The manufacturing system computer must also provide the cell host with its operating software. If the cell hosts need software support each time they are booted, then the system computer will be responsible for that too.

· The statistics arising from the individual cells will need to be compiled, compressed, and analyzed by the manufacturing system computer.

· The software for gracefully degrading the system as one or more cells experience problems will reside in the system computer.

The computer to accomplish these functions will need to be a fairly substantial machine. A Digital VAX 11/780, or comparable equipment, should be sufficient for most systems. The computer will be running some advanced software, so the same caveats apply to this system that apply to the cell host, ie. it should run a good operating system that can support program and system development.

If there is only one cell, a single computer could act as both the cell host and the manufacturing system supervisor but there are some arguments against doing this. First, the entire production floor will become dependent on a single device. Although reliable, today’s computer technology docs fail now and again. Such a failure could have serious effects on the system productivity. Second, future expansion will require a separation of computing power (one machine will not economically support the whole load). Many headaches can be avoided by separating the two supervisors (cell and manufacturing system) from the beginning. And finally, the separated supervisors will be more marketable as separate products. A good cell host with the proper software is a product not available on today’s market. It should be.

Network links.

The system computer will communicate to the cells over a factory wide network. The technology for Local Area Networks (LANS) is developing rapidly. Most computer manufacturers are building some sort of network or interface to an established network. The manufacturing system, the cells and the factory should capitalize on the new LAN technology.

The Institute of Electrical and Electronic Engineers (IEEE) has established a Committee to propose a standard for LANs. That committee, the so-called 802 committee is now promulgating three preliminary standards. The third of those, IEEE - 8 0 2 - 3 , is essentially compatible with the network known as Ethernet . The effect of the IEEE proposal will be primarily felt within this country, but the technical press is mentioning Ethernet as the de facto standard for LANS. In addition to being accepted by the industry, Ethernet is a reasonable networking system. It was developed by Xerox in about 1972 and is now being marketed by Xerox, Dec, and Intel (primarily). The factory is a harsh environment for computer networks, and at least one consortium has implemented a fiber -optic version of the Ethernet. The optical nature of the transmission medium (the ‘ether’) is much less susceptible to electromagnetic interference than is the original coax cable.

The hardware and transmission protocols are only part of the networking system, though. The software to support the LAN at each computer must also exist. Typically, the software to support a complex system lags the hardware implementations (a truism of software engineering). The software for the Ethernet is well advanced and will probably be available by the time anyone wants to implement a system such as this.

Another possibility is to use one of the systems which is marketed by a single vendor. DECnet and IBM’s SNA (Systems Network Architecture) are two examples. The major problem with such an approach is the restriction to one vendor’s computers for the whole system, or the special development of interfacing software. Given the wide variety of computers within the manufacturing system and the even wider variety of tasks, there is much to be said for using a network system that will be a US standard.

The last factor for consideration here is the way the LAN will adapt to the growth of the manufacturing system. The small manufacturer will probably not invest in a full-blown CAD system until the price drops considerably. Nonetheless, the original choice for the LAN should not later restrict the choice of CAD systems. Similarly, machine tool makers will be implementing links toLANS at some point in the future.

When this occurs, the serial links in the manufacturing cells may be replaced with a faster LAN scheme, provided that the LAN originally chosen for the manufacturing system is compatible with what the machine tool builders offer. The adoption of a US standard is advisable both cases.