One controller does it all

Aug. 23, 2007
Bundling CNC and PlC functions simplifies machine control.

p>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.

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.

Advanced programming functions allow the use of elements from high-level languages, such as loops, conditional statements, and branches. Other capabilities include, for example, tangential transition arcs, acceleration/deceleration ramps, and automatic tangential-axis functions. Data from PLC application programs are exchanged over a powerful interface, so users can even control program execution in real time. Thus, settings such as tool radius and end points of path sections can be changed while the program is being executed, because data are available synchronous to the path. Additional features include:

Dynamic functions. Influencing dynamic characteristics such as path speed, acceleration, and jerk is especially critical when using path control in certain cases. The Soft CNC's ability to change these characteristics during runtime or automatically adjust them according to the path curvature is essential for some applications.

The controls also include cutterdiameter compensation. Invalid intersections, notches, and peaks do not necessarily require a stop along the path. In fact, on a plasmacutting machine stops could heavily damage the workpiece. Instead, the Soft CNC automatically makes needed corrections according to user specifications.

Also, an integrated look-ahead ensures a consistently optimized path speed. This is especially important when a particular path would tend to have frequent acceleration changes and cause machine vibration.

Error compensation. Sometimes it is necessary to cheat physics a little in order to improve the process. Thus, it can be helpful to intentionally overshoot dynamic-axis limits. For example, cutting machines automatically align the cutting tool tangentially to the path. When a transition between two path elements creates too large an angle and the tool is moving too fast, the tool might not align quickly enough. To stay within preset limits, the path speed would have to be reduced but this would reduce the quality of the cut. Thus, it is better to overshoot the limit values for the tangential axis. The temporary position error has much less of an influence on cut quality than a speed reduction.

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.

The control also handles axiscorrection functions such as spindle pitch error and backlash compensation. These result in excellent path precision, even on machines that are mechanically less precise.

All these functions are useful beyond the traditional CNC area because, in general, a CNC system offers advantages of better path precision and potentially higher path speeds due to built-in lookahead functions. Another advantage could be the programming interface. CNC programs that use ISO Standard G codes are rather simple, letting operators familiar with CNC terminology program robots without special training in vendorspecific programming languages.

The Soft CNC also has interfaces that let it identify the kinematic description of a mechanical structure: reverse and forward transformations. Forward transformation calculates position and orientation in space using the robot's joint angles; reverse transformation is the opposite, calculating joint angles based on position in space.

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.

The Soft CNC supports transformation functions for six-axis articulated-arm robots and Scara systems. Programming is the same as with CNCs; however, there are now six significant axes. X, Y, and Z define position in space, and A, B, and C define orientation (for example, as an Eularian angle). In the future, other languages should also be available for programming robot movements.

B&R Industrial Automation Corp., br‑

The Soft CNC is embedded in the PLC's realtime operating system, meeting demands for complex machining and robotic handling tasks. This concept opens the door to innovative machine designs.

Using CNC systems to control robots offers advantages such as better path precision and higher speeds, due to built-in look-ahead functions. It can also simplify programming.

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