CNC's In The Fast Lane Powerful 32-bit computer numerical controls (CNCs) are driving machine tools to higher levels of productivity. But faster controls may pay even higher dividends in accuracy and ease of operation.

The 32-bit computer numerical control has arrived. This new generation of CNCs promises to make machine tools more productive thanks to substantially enhanced processing speed. But that's not all. These powerful new controls are taking the accuracy of the machine tool ever closer to its mechanical limitations--and sometimes beyond, with electronics that can compensate for inherent mechanical deficiencies.

The concept of a more powerful CNC is increasingly being embraced by the world's control builders. Mazak, GE Fanuc, General Numeric, Mitsubishi, Bosch and Cincinnati Milacron all now offer 32-bit processors in some models sold in the U.S. Although the various companies may have embodied differing design philosophies in their respective controls, the one trait they all share is dramatically improved processing speed. One builder reports that its 32-bit units process data up to ten times faster than its best 16-bit control. But computing speed doesn't mean much by itself. What's important is how these powerful computers are making machine tools-and the people who use them-more productive.

To understand how a faster computer can improve machine tool performance, it helps to have a general idea of how all the parts of a control system function together. These parts include the central processing unit (CPU), the electro/mechanical servo control systems for spindle and axis control, and the various machine, human and auxiliary interfaces. And especially important is the mechanism, or bus structure, by which information travels from one portion of the control system to another. These data "roadways" play an important role in the overall efficiency of a CNC.

What sets apart the 32-bit CPU is raw computing power. One measure of this prowess is the speed at which a single command block of three-axis contouring data can be processed. A block includes new coordinate values for each of the three axes. A typical 16-bit control will process a block in 50 to 60 milliseconds (thousandths of a second); a very good 16-bit CNC can reduce the time to 17 to 20 milliseconds. But a 32-bit control can process a block in a little as 2 milliseconds.

This improvement is important in two ways. First, the CPU's primary task of transforming the part program to position control signals for the axis servo drives happens much faster. This speed can serve to improve precision and increase feed rates in contouring motions. Second, the relative ease by which this primary task is accomplished enables the CPU to perform more tasks in a given time frame to reduce total cycle time.

Many people think of a CNC as a single "brain" for a machine tool. Most controls, in fact, have multiple processors that are dedicated to specific tasks and are capable of running multiple tasks when required. The main processor is concerned primarily with generating tool path data. Secondary processors handle functions such as background edit, shop-floor programming, communications with peripheral devices, and so on. Further, most of today's CNCs incorporate programmable logic controllers (PLCs) that can handle a wide range of sequencing functions including tool change logic, portions of probe control, some aspects of torque-controlled machining and so on.

For this "information factory" to function efficiently, there must be a fast and reliable means for passing information between the various control system components. Especially critical is data transfer in the servo loop; that is, the flow of control data to the axis servos and the flow of feedback data to the CPU.

Newer controls have significantly improved "digital data bus" structures. A bus refers to the circuitry by which information is passed between different system components, and sometimes between different functions on a single computer board. Though improved bus communications is not exclusive to 32-bit controls (as a design element, it's been one of the major contributors to improved speed in 16-bit units), the new computers are pushing bus design to higher levels of functionality. These improved information highways are a necessary part of faster CNC performance since slow computer response often is due to information traffic jams rather than inadequate processing power.

Fast In The Turn

High CNC processing speed is of particular advantage in contouring motions. Just a few years ago, this capability might have been considered a solution in search of a problem. But now that more shops are doing sophisticated contouring and that high speed machining is becoming practical, a real need is emerging for faster controls.

Contouring places the heaviest demand on a CNC. When interpolation is employed, each single point-to-point servo command is the result of complex calculations that are being made "on the fly", that is, as the tool is cutting. This real-time computing is further complicated when additional data--such as tool offsets, cutter diameter compensation, axis error compensation or workpiece offsets--must be factored into the calculation.