Gernot Bachler
Motion Research & Development
B&R Industrial Automation Corp.
Roswell, Ga.

For more than 40 years, CNCs have been synonymous with high-precision machine tools. And the prime goal of CNCs has been ensuring the cutting head precisely follows a programmed toolpath. On the other hand, there is also an increasing demand for production machines to operate as fast as possible using PLC process signals. This, combined with morecomplex demands on individual machines, is blurring the previously clear definition of "classic" CNC machines.

For instance, industrial robots are being used more and more frequently for production tasks ranging from simple parts handling to welding complex components. Unlike CNCs, the main focus is not geometric path consistency. Instead, decisive factors are precise and repeated movements from one position to another, as well as dynamics of the move itself.

Typically, machine manufacturers try to avoid separate controllers for different functions on a production machine. The preferred approach is a complete automation platform that handles drives, robot movements, CNC path control, visualization, and communication networks, among other tasks.

In the past, this wasn't a realistic option for machine manufacturers. Traditional CNC and robotic controllers were "closed" in the truest sense of the word — the systems could not readily communicate with each other. When a manufacturing process required CNCs and PLCs, the machine builder had to introduce complicated mechanisms to access the required data and synchronize actions between the two.

A new approach makes the CNC path controller an integral part of the automation system. PLC and CNC tasks run simultaneously and are completely synchronized. This can improve performance, simplify control architecture, and help reduce the costs for system integration, troubleshooting, and maintenance.

B&R terms the integrated control system a Soft CNC. It is embedded in the PLC's real-time operating system and can handle even the most complex tasks. For instance, the Soft CNC is used in such applications as classic milling machines, plasma and water-jet cutters, soldering machines, and sealant-dispensing equipment.

FLEXIBLE SOLUTION

What makes the control so versatile? For one, CNC cycle times of 400 µsec allow submicron path precision. Positioning commands transfer without jitter via Ethernet Powerlink to the appropriate drives. Furthermore, an almost unlimited number of I/Os can be added as needed.

Better yet, automation algorithms can be encapsulated in the application layer rather than built directly into the CNC core, as is typically the case with other controls. The traditional approach brings two major disadvantages. First, the machine builder must specify technical details and provide this information to the controller manufacturer. This often means giving away proprietary know-how. And functions integrated in the CNC are available to the machine builder's competitors as well.

The Soft CNC, in contrast, lets a machine builder introduce its own method to control, for instance, a plasma nozzle's height and adjust its programmed axis position. There is no restriction on sensor types or height-control algorithms, as is traditionally the case.

In addition, the flexible system architecture and a large number of built-in, preprogrammed functions permit customizing machines to meet specific requirements. Part programs and movement procedures follow the DIN 66025 Standard. Basic control features include interpolation, dwell time, plane selection, mirroring, and cutting-diameter compensation, as well as zero-point offset, absolute and relative coordinates, and feed-rate definition, to name a few. Additional, more-advanced functions also aid programming and versatility.

Correction functions. Typical examples include free rotation of a working plane in 3D space and skew correction for machine axes. The latter helps when it is mechanically impossible or considerably difficult to square the coordinate axes. The Soft CNC distorts the part program so the actual path matches the path in an ideally squared system.

This lets engineers use the CNC system's advantages to control robots. For example, it can replace a simple series of straight lines with free-form curves, and the integrated look-ahead function ensures an optimal path speed. This architecture is especially suited for applications that need path precision as well as simple pointto-point movements.