Software basics for a robotic-cell story “If you watch our demonstration cell,’ says Gary CSchatz, Manager, advanced manufacturing planning, Turbine ComponentsPlant, Westinghouse Electric Corp, Winston-Salem, NC, “you’dprobably say you had seen the same thing in Japan three years ago. Butif something should go wrong, then our system would demonstratecapability you’ve never seen before. It can correct for its ownmistakes. “And we’ve added the ability to change rules and add tothe data base by editing, not stringing wire. We’ve recognizedthat we don’t know what’s coming next, so we are prepared forunexpected variables.
‘ Schatz says that most automated factories and FMS cells are notnearly as sophisticated as the publicity leads you to believe. Evensome really big names in the business do nothing more than tie togethervarious machine tools with conveyors, automatic loaders, and robots,using simple hard-wired connections between them. If anything goeswrong, they can’t react intelligently–except to stop. Most FMSsetups 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 becausemuch of the control is still basic limit-switch technology. That is,when Machine A is finished with the workpiece, it snaps a switch tosignal Machine B to take over.
Most of the control still resides in alocal NC, CNC, PC, or other control at each individual machine tool. But a hard-wired FMS system is complex, because each machine mustbe wired to all other machines in the cell or group. If you add a newmachine, you must wire it to all the other machines. No on machine orcomputer has overall control.
Software communications, on the other hand, are software driven.Such systems are easy to hook up and easy to diagram. Instead ofconnecting each machine to all other machines, you wire each machine toa central computer– and change the software program.
The computer doesall 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 thecomputer (send and return). If something goes wrong with a serial system, the computer can comeup with a repair solution.
That’s the big difference. It doesmore 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 JIsasi is to develop a cell that will process 1000 workpieces withoutmanual input or intervention –strictly unmanned. At this writing, theGFM forging machine is doing fine under its own CNC operation, poundingor swaging hot bars from a rotary-hearth furnace into preforms forturbine blades.
(Sometimes this is called preforging.) The machine isremarkable in itself. Even more spectacular will be software control ofthe entire cell, including two robots, the furnace, forging machine, andrelated inspection and second-operation stations. Dueling robots All of the machines are in place, the computer works well, and therobots are ready for action. Then why isn’t the system working tofull capacity under full software control? Very simply, there’s alanguage barrier–even snobbery between the different levels of machinecontrols. For example, the controls for the two robots have much lowerintelligence than the CNC units driving the machine tools in the cell.
The robots in fact will try to move in the same space, colliding witheach other, unless given counterinstructions by the central computer. Putting it another way, no matter how good the programmer of aconventional cell is, once the cell becomes complex, he or she cannotpossibly cope with all potential interactions. This is especially truewhen machines or part geometries change. Programming for a biggerpreform, for example, could cause a robot collision if the robot mustreach further in space. With a smarter programming language andsimulation software, the computer can warn the operator that dangerlurks. The computer can certainly keep track of such mundane details asrobot positioning, but in the new setup, it must also constantlytranslate from computer language to robort language. It must alsotranslate from Pascal to machine-control language, ladder logic, or anyother 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 everythingthat is going on in the cell, so it can make command decisions. Thesophistication of control is similar to that of a full CNC compared toan older NC machine control. Old NC is nothing more than a simple tapereader with discrete logic control, whereas CNC is programmable logic control.
In the Westinghouse setup, the various parts of the FMS cellcommunicate to the CIU in the same way the various parts of a single CNCmachine tool communicate to the control’s central mainframecomputer. This takes much computing time and tremendous programming skill.And, of course, it’s the programming that takes the most time andstill needs perfecting. The software, being developed byCarnegie-Mellon University, must rune the system smoothly and be easy tochange and reprogram for new operations and new equipment. Whenperfected, it will help control a variety of manufacturing cells, and itwill communicate with other management-system programs being developedin the Robotics Institute at Carnegie-Mellon. The programs will manageat a factory level, and the new language will use a data base with a setof 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, asingle failure of a tool or machine component will stop the program.This is because the languages are procedural in nature.
Although thecell operating program may be very large, it is still not a languageinterpreter. At any random statement in the program, the proposed taskis critically dependent on the previous tasks. If these previous taskswere not carried out because of some machine or program error, then theparticular task in the statement of interest may be adversely affectedor not enacted. For example, in a “worst case,’ a billetcould 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 billetdidn’t roll over a power cable and sever it! The rule-basedlanguage sets up program segments that will operate only when certainpreconditions already defined become true. The segments require onlythat certain conditions be true, not that they happen in a certainsequential order. In a sense, the new language can nandle several tasksat once, whereas conventional languages do only one thing at a time, andin an exact sequence.
Photo: Jerry Colyer at the command post. He says,”Interconnecting CNC machine tools is more than matching RS-232plugs and sockets. These merely match voltages; they do not guaranteeany sort of communication at all! Traditional cells put in hard stopsto 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 thetwo robots.
Items not shown include a noncontact vision-based gagestation, and the F&D rotary-hearth furnace Photo: The GFM forge uses two hammers and controlled rotation ofthe chuck to forge a red-hot cylinder into a turbine-blade preform. Themachine has its own CNC unit that communicates with the rest of the cellthrough a central computer. Engineers had to overcome the lack of acommon EIA language before communications could proceed. An industrystandard is badly needed before future development of largescalesystems. Photo: Layout of the GFM swaging or forging cell.
