When PCs Control Machines

June 1, 2000
Motherboard or motion card: This is one of the fundamental decisions for specifying CNC on a personal computer platform.

Gary Corey
CamSoft Corp.
Lake Elsinore, Calif.


The Thermwood router installed at Huck Store Fixture Co., Carson City, Nev., now runs under the direction of a CamSoft Corp. CNC system. Based on motion card and PC hardware, it handles multiple operations so the router can use full 4 X 8-ft sheets of wood or laminates.

Time was when the idea of doing CNC on a personal computer was heretical. Those days have long passed, but it is interesting that some people still fundamentally misunderstand this area. CNC on a PC is not just a slightly more complicated example of motion control via computer. Yet that is the way some still view such tasks. Too bad, because developers who don't grasp a few of the intricacies in this discipline are likely to find themselves in a tough spot.

The idea behind PC-based CNC is to eliminate the need for tracing wires from a programmable logic controller or writing ladder logic. The computer becomes the PLC and does the logic. The first point to understand is that there are two different philosophies for handling CNC tasks on a PC. One is to use the main CPU and motherboard circuitry to control the servoloop. The other is to install a motion-control card which essentially takes over servocontrol.

Some people believe that using the PC's CPU for servocontrol saves money and that it provides relief from having to search for replacement parts, such as motion-control boards, in the event of a hardware failure. The reality is, however, that there are still a lot of add-ons to purchase — d/a (digital-to-analog) converter cards, encoder feedback cards, and digital I/O to name a few. All in all, using the motherboard to close the servoloop doesn't greatly simplify the hardware and still raises the specter of hunting down replacement parts.

The well-known Blue Screen of Death may provide the most compelling argument for PC-based systems with motion cards. All Windows systems have an unfortunate reputation for freezing up. When the main CPU is controlling the servo, a seized operating system can effectively open the feedback loop. The result: Machinery operators may find out first hand whether the emergency travel-limit switches actually work.

Systems that employ a motion card for servo-control use the host CPU strictly for user inter-face chores and card programming. This lets them handle such freeze ups more gracefully. Standard practice is to equip the motion boards with watchdog timers that fire every 60 msec. The timers periodically ask the main CPU to say something. If there is no response, the motion card shuts down in an orderly fashion.

Competitive issues among suppliers taking the motion-card approach tend to focus on the degree of sophistication that the software displays.

Several suppliers make motion cards with enough horsepower to handle CNC, and numerous integrators build systems around these boards. The fastest cards can update a servoaxis every 62.5 µsec which makes possible feedrates during cutting of up to 122,000 ipm. The most capable cards carry dual 32-bit digital signal processors that usher in high-performance features — cutting a 3D profile on one axis, say, while changing a tool for another. The use of a second processor also produces the fastest block-to-block cutting speeds.

Software sophistication is what differentiates these systems. Ease of programming is not a strong point for most boards that come straight from a hardware supplier. Many of them provide a graphic user interface, but it tends to have a general-purpose nature. It is helpful for telling the board how to control point-to-point operations as in, say, a robot arm. It tends to be less helpful in constructing routines that push a tool through work material along some contour.

Most board makers look at the latter situation as a special case of motion control. They usually deal with it by programming their hardware with a high-level language such as Basic or C. Creating CNC routines is left to the end user or a knowledgeable systems integrator.

Fortunately, it is now possible to find PC-based software that has been developed for CNC retrofitters and OEMs. It lets them create a graphical user interface for a machine tool without requiring the skills of a C, C++, or VB programmer. End users can customize an application and create buttons, slider bars, and other well-known graphical objects that talk directly to the motion card and produce needed actions.

A point to note is that this software works on a variety of motion cards, digital I/O cards, and standard PCs. These systems give users a choice for maintenance and repair. The nonproprietary hardware involved can come from local sources.

Modern software for CNC also tends to give end users numerous choices for cutting a part. Besides traditional G-code programming, CAD-to-motion or picture-to-part features can turn graphics into motion without postprocessing or having to write G code. These packages can dis-play solid-model graphics depicting real-time cutting on the screen. Conversational programming methods are available as well, where a question and answer format prompts users for information, with full multimedia photographs and text-to-speech.

Finally, good PC-based controllers can offer simplified diagnostics and servicing. Simple commands referenced by terminal strip number or descriptive names let installers diagnose, modify, and test the logic before they begin physical wiring. Also possible is remote I/O logic diagnostics via modem. Technicians and maintenance personnel may remotely control the machine via any number of serial connection protocols. If a motion board goes bad, use of terminal strips make it possible to unplug the cable that connects the machine wiring to the computer without disturbing the wiring.

Last year, Huck Store Fixture Co. had pretty much decided that its mid-1980s vintage CNC controller had to go. "It was hard to program because you had to type in all the G code, and it wouldn't do any math for you," explains Production Manager Steven Bibby. The manufacturer of store fixtures has since changed the CNC control for its Thermwood router over to a PC-based system with a built-in CAD/CAM package. "Now we just take dxf files of our drawings and put them into the CNC and have it run," says Bibby.

The production facility, in Carson City, Nev., employs a controller and software from CamSoft Corp., Lake Elsinore, Calif. The package handles two air drills on the router. It also lets the fixture maker nest more parts and use full 4 8-ft sheets of wood or laminates, eliminating a beam-saw operation. The ability to drill holes for adjustable shelves on the same program as a dovetail eliminates a line-bore operation as well.

Huck figures the PC-based package has let CNC productivity climb some 20%. One reason is that an operator can use a customized handheld pendant and walk around the machine to set up parts, rather than being stuck at the control itself. Programming time is down dramatically because most part programs come directly from CAD-system-generated dxf files, and because they can be written elsewhere and downloaded into the tool, thereby boosting machine cutting time.

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