21-09-2008, 09:12 AM
Integrating the cell
Integrating the cell into a larger manufacturing system is basically a step up the system hierarchy of figure 3-5.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 relationship between the cells and the system.
5.1. 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.
5.2. 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 the repertoire 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.
