Software basics for a robotic-cell story.
"If you watch our demonstration cell,' says Gary C Schatz, Manager, advanced manufacturing planning, Turbine Components Plant, Westinghouse Electric Corp, Winston-Salem, NC, "you'd probably say you had seen the same thing in Japan three years ago. But if something should go wrong, then our system would demonstrate capability you've never seen before. It can correct for its own mistakes.
"And we've added the ability to change rules and add to the data base by editing, not stringing wire. We've recognized that we don't know what's coming next, so we are prepared for unexpected variables.'
Schatz says that most automated factories and FMS cells are not nearly as sophisticated as the publicity leads you to believe. Even some really big names in the business do nothing more than tie together various machine tools with conveyors, automatic loaders, and robots, using simple hard-wired connections between them. If anything goes wrong, they can't react intelligently--except to stop. Most FMS setups are serial in concept, but are hard wired in practice.
Communications: hard wired versus software
Hard wiring is both simple and complex. It's simple because much of the control is still basic limit-switch technology. That is, when Machine A is finished with the workpiece, it snaps a switch to signal Machine B to take over. Most of the control still resides in a local NC, CNC, PC, or other control at each individual machine tool.
But a hard-wired FMS system is complex, because each machine must be wired to all other machines in the cell or group. If you add a new machine, you must wire it to all the other machines. No on machine or computer has overall control.
Software communications, on the other hand, are software driven. Such systems are easy to hook up and easy to diagram. Instead of connecting each machine to all other machines, you wire each machine to a central computer-- and change the software program. The computer does all the communicating, and much of the control, for all the machines. If you add a new machine to the cell, just wire two lines to the computer (send and return).
If something goes wrong with a serial system, the computer can come up with a repair solution. That's the big difference. It does more than just issue start, stop, and operation-finished signals.
But, of course, programming is a bear.
The goal of engineers under the direction of Gary Schatz and J Isasi is to develop a cell that will process 1000 workpieces without manual input or intervention --strictly unmanned. At this writing, the GFM forging machine is doing fine under its own CNC operation, pounding or swaging hot bars from a rotary-hearth furnace into preforms for turbine blades. (Sometimes this is called preforging.) The machine is remarkable in itself. Even more spectacular will be software control of the entire cell, including two robots, the furnace, forging machine, and related inspection and second-operation stations.
All of the machines are in place, the computer works well, and the robots are ready for action. Then why isn't the system working to full capacity under full software control? Very simply, there's a language barrier--even snobbery between the different levels of machine controls. For example, the controls for the two robots have much lower intelligence than the CNC units driving the machine tools in the cell. The robots in fact will try to move in the same space, colliding with each other, unless given counterinstructions by the central computer.
Putting it another way, no matter how good the programmer of a conventional cell is, once the cell becomes complex, he or she cannot possibly cope with all potential interactions. This is especially true when machines or part geometries change. Programming for a bigger preform, for example, could cause a robot collision if the robot must reach further in space. With a smarter programming language and simulation software, the computer can warn the operator that danger lurks.
The computer can certainly keep track of such mundane details as robot positioning, but in the new setup, it must also constantly translate from computer language to robort language. It must also translate from Pascal to machine-control language, ladder logic, or any other language used by the individual machines in the cell.
Beyond this, the central computer acts as a cell interface unit (CIU) that controls the destiny of the cell. It must know everything that is going on in the cell, so it can make command decisions. The sophistication of control is similar to that of a full CNC compared to an older NC machine control. Old NC is nothing more than a simple tape reader with discrete logic control, whereas CNC is programmable logic control. In the Westinghouse setup, the various parts of the FMS cell communicate to the CIU in the same way the various parts of a single CNC machine tool communicate to the control's central mainframe computer.
This takes much computing time and tremendous programming skill. And, of course, it's the programming that takes the most time and still needs perfecting. The software, being developed by Carnegie-Mellon University, must rune the system smoothly and be easy to change and reprogram for new operations and new equipment. When perfected, it will help control a variety of manufacturing cells, and it will communicate with other management-system programs being developed in the Robotics Institute at Carnegie-Mellon. The programs will manage at a factory level, and the new language will use a data base with a set of rules, operating with a set of grammatical constraints.
Accounting for failure
One reason for special languages, say the Westinghouse engineers, is that even in the simplest cells using Pascal, Fortran, or APT, a single failure of a tool or machine component will stop the program. This is because the languages are procedural in nature. Although the cell operating program may be very large, it is still not a language interpreter. At any random statement in the program, the proposed task is critically dependent on the previous tasks. If these previous tasks were not carried out because of some machine or program error, then the particular task in the statement of interest may be adversely affected or not enacted. For example, in a "worst case,' a billet could be dropped at a random time and place in the cell.
In an old system, everything would stop. With the new software, however, the cell could keep right on running, assuming the billet didn't roll over a power cable and sever it! The rule-based language sets up program segments that will operate only when certain preconditions already defined become true. The segments require only that certain conditions be true, not that they happen in a certain sequential order. In a sense, the new language can nandle several tasks at once, whereas conventional languages do only one thing at a time, and in an exact sequence.
Photo: Jerry Colyer at the command post. He says, "Interconnecting CNC machine tools is more than matching RS-232 plugs and sockets. These merely match voltages; they do not guarantee any sort of communication at all! Traditional cells put in hard stops to prevent accidents; we try to put the stops in software.'
Photo: Elements of the cell shown here are the GFM chuck at left, Machining Systems cropper-stampers to cut off the tip, and one of the two robots. Items not shown include a noncontact vision-based gage station, and the F&D rotary-hearth furnace
Photo: The GFM forge uses two hammers and controlled rotation of the chuck to forge a red-hot cylinder into a turbine-blade preform. The machine has its own CNC unit that communicates with the rest of the cell through a central computer. Engineers had to overcome the lack of a common EIA language before communications could proceed. An industry standard is badly needed before future development of largescale systems.
Photo: Layout of the GFM swaging or forging cell.
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|Title Annotation:||Computer-Integrated Manufacturing|
|Author:||Miller, Paul C.|
|Publication:||Tooling & Production|
|Date:||Sep 1, 1984|
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