Machinedesign 1679 Mtc200 Control898 0 0

CNCs open up

July 1, 2000
The PC revolution continues its spread into manufacturing, and machine tool controls will never be the same. Neither will motion control.

A device that is still operating after ten years used to be something to brag about. This is no longer true for the industrial control. Be it a PC, PLC, or CNC, a control that old most likely can't communicate to a high-level manufacturing network. It certainly can't use the latest Windows-based software. It requires a specialist to upgrade. And it's still using old computing architectures and motion algorithms.

More controls are in this aged state then you might think; research firms put the number at about 60% of the world's CNCs. Until recently, the incentive to move to newer controls was small. But the influences of the Open Modular Architecture Control (OMAC) concept demanded by General Motors, the explosion of the Internet and intranets, and the requirement to access production and performance data from every device on the factory floor have given OEMs and end users reason to change.

From isolated island to on-line peripheral

Every controller has data on the operation of the device it directs. Because more than 95% of all CNCs are proprietary, however, engineers could not access the data without the control manufacturer's help. Obtaining different data meant returning to the control manufacturer to gain access.

To reduce this dependence on the manufacturer, one solution is to replace the old controls with something a bit more open. There are three options, each offering different degrees of openness. The two PC-based solutions have little effect on how motion is controlled. The software solution brings changes.

Engineers can keep the traditional CNC and add a PC to it to function as the front-end device. The CNC continues to run the machine and handle all motion control without any change. The personal computer, however, gives engineers access to Windows-based applications and third-party software for such functions as statistical process control, tool management, and interaction with manufacturing execution systems. Another advantage is that all such controls have common screen displays.

Access to machine data, however, is still limited unless efforts are taken to integrate the two controls. In addition, upgrades will be restricted to those doable on the personal computer. The machine tool supplier must handle CNC upgrades.

The next option is to replace the CNC with a PC and a motion card. Here, the motion card is really the machine control reduced to the size of a board. It either fits inside a personal computer or is placed near the drive component and connected to the personal computer through cables. The motion card uses fast, powerful 32-bit microprocessors, typically has one or more DSP chips on board, and a lot more memory. Motion control is relatively unchanged. The motion program is written on the PC, but the machine control handles all the number crunching and closes the servo loop.

Access to machine data is better because processors on the motion card are compatible with the personal computer's operating system. Also, the two controls share the same backplane or, in the case of the separate motion card, use standard interface drivers. Through the PC, the machine tool can connect to manufacturing networks to send data to corporate measurement and analysis systems.

Depending on the control manufacturer, initial investment with this configuration is less than traditional machine controls. Life cycle costs are also low and reconfiguration is easier because you don't need to purchase a new control every time an application needs different or additional features. In most PC-based systems, such features are often available as software or plug-in boards.

A disadvantage, however, is that some installations may require special skills available only from that manufacturer. Also, you may be limited to the manufacturer's stock for replacement or upgrades of the motion card.

Rethinking control

The most recent option is to use a PC and eliminate the motion card as well as its cost. Using a software operating system that duplicates the functions of a CNC, the personal computer handles the motion calculations, sends signals to the drives, receives data from the various interface cards and input and output modules, sends data back, and performs the other usual control functions.

However several manufacturers dispute the potential savings from eliminating the motion card. As they point out, the cost of one or more motion processors is small compared to the cost of the interface devices on the machine tool.

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This doesn't deter developers who are creating CNCs-on-a-disk. They maintain that motion cards aren't needed because they've redesigned machine control functions and made them more efficient.

In older controls, the system architecture, motion and control algorithms, and data handling techniques have not changed in several decades. Each part has inefficiencies that new techniques and designs eliminate. For example, newer control versions streamline real-time updates. The microprocessor does not need to update an operator screen at a rate faster than once every 10 msec, as done in older systems. The human eye cannot see changes at faster rates. Slowing the refresh rate frees up several CPU cycles for other tasks.

On-going research at universities and several motion-control companies has resulted in faster, better motion algorithms, which some developers are already implementing. New S-curve algorithms, for example, vary the voltage increases or alter the accel-decel curve to reduce stress on a motor.

Data collection techniques have improved too. Most old controls still collect hundreds of data points, each often the same as the last. Accumulating only the points that change, though, can free up storage as well as the number of CPU cycles devoted to the task. Data compression methods also aid control efficiency. The result of these and other techniques is a control architecture that can respond to signals that match drive response.

Most CNCs-on-a-disk handle three or four axes of motion. One version, however, lets the microprocessor control up to 10 machine axes, update the operator interface and input- output modules, and send data to other computing systems. Controllable motions include linear and circular interpolation, cutter radius compensation, velocity feed forward (look ahead), and feed rate override, all of which require millisecond, even microsecond, response.

CNCs-on-a-disk come in distributed or centralized formats. In the distributed design, software functions are spread among several processors in the personal computer. Most versions let engineers create motion programs on line.

The centralized version uses one microprocessor for all control functions including multi-axis motion. The developers of this version claim that a Pentium chip, for example, is plenty fast to upgrade the operator display screen, receive data from the various interfaced devices, calculate motion trajectories, and send signals to the axes drives. Early in the development of this version, the engineers tested a three-axis machining center version of the CNC software on a personal computer with one 486 DX266 processor. It handled the operator display and communication with a soft PLC. It also calculated PID tuning, executed diagnostic routines, and interpolated the drive data to create the trajectory for the machine tool. Performing all these functions, it still closed the servo loops on the axes in 1 msec. The motion portion of the program took up 30% of the processor cycles. Today's versions of this software closes the servo loop in less than 1/4 msec. Part programming is done off-line though.

Such changes in CNCs and motion control are really just the early effects of the PC revolution. For a preview of what will happen to machine controls, keep your eyes on the PLC industry.

Saving the iron

The iron of a machine tool rarely goes bad. Other parts, however, do. Though it might be worth it to refurbish or update machine ways, ballscrews, even motors and drives, it's not as easy to justify a new CNC on an old machine.

Software based CNCs, though, can offer new life to the old iron. Frequently, all that's needed is a few days, a PC, and a CNC on a disk, and the old iron can become almost new again. If it's just the PC and the software, installation can be as simple as wiring inputs and outputs and the E-stop function.

Engineers at Melling Tool, Jackson, Mich., for example, wanted to unload a 10 year old three-axis vertical machining center. Control problems made it usable only 20% of the time. When they were unable to find a buyer, they tried the OpenCNC software from MDSI, Ann Arbor, Mich. They purchased a PC from CompUSA and began the retrofit. Less than two weeks later, the machine was operating. Engineers removed most of the electrical cabinet, but they did not have to replace the drives and motors. The machine now operates at full capacity and its rapid traverse rate improved from 390 ipm to 590 ipm.

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