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What digital AC drives bring to machining.

What digital AC drives bring to machining

Recent advances in machine-tool drive technology have brought significant improvements to machining--higher accuracies, higher speeds, and greater overall machining consistency. A key contributor was the development and refinement of digital AC drives.

Once the best servo-parameter values for a specific machine have been found, they can now be keyed into other similar machines or replacement drives as simply as exchanging a PROM or downloading software. This close tailoring of drive characteristics to a machine's capabilities also yields faster acceleration/deceleration rates and improved response to load changes. The control circuit's digital structure enchances flexibility--the number of variables in the control loop--and the ability to tackle difficult and higher-performance applications.

Finally, fewer components in the control loop mean improved reliability, and--with no analog-to-digital conversion required--much sharper accuracy. The bottom-line result is that users can expect four-year mean-time-between failures for digital systems that include a CNC, digital servos, encoder feedback, and a digital machine spindle.

Counting the digital benefits

Today, integrated digital AC-drive systems merge digital-AC benefits directly into the CNC. Higher speeds of up to 2400 ipm at a resolution of 10 microinches are now possible. More precise acceleration and deceleration (acc/dec) avoids machine stress while optimizing response. Linear acc/dec offers fast acceleration (achieving programmed speed in 30 msec with a low-inertia motor); bell-curve acc/dec enables a soft start if the machine cannot tolerate faster acceleration.

An array of digital features offer these benefits:

* Velocity feed forward. This reduces large following errors or lags during high-speed machining to produce sharper corners with faster cutting cycles, for example. * Friction compensation. This prohibits overshoot when overcoming a machine's static friction. * Backlash acceleration. This prevents protrusions in the slide or tool path during circular or arc cutting. * Rigid tapping. This permits tapping speeds up to 4000 rpm and much faster cycle times using less expensive solid or rigid toolholders.

Soon, high-speed digital systems will have adaptive control--either in the form of a self-tuning regulator or model reference. The self-tuning regulator calculates the system parameters and adjusts the control to accommodate them. It has been tested in highly repetitive applications, such as turning oval parts where the following error has been reduced by a factor of 100. The reference model, on the other hand, compares the actual system response with a reference model and makes the proper adjustments to keep the system in tune with the model.

Is high tech too high?

A Harvard University study showed less than 11% of all operating machine tools in the US are computer controlled, a rate far below that of either Europe or Japan.

One reason is financial: CNC-based machine tools are more expensive than their manual counterparts. Another stumbling block is the advanced sophistication of CNC machines. They require a certain level of expertise, and the lack of training programs for workers hurts, particularly in single-factory companies.

However, solutions are becoming available that address these concerns. For example, to help users over the financial hurdle, GE Fanuc Automation and GE Capital have teamed up to form a unit to lease machine tools retrofitted with GE Fanuc CNCs--an arrangement that improves cash flow.

On the scene since CNCs first became fully programmable, ladder language remains the favorite, although various higher-level languages--PASCAL and C--exist to perform interlocking, tool-changing, material-transfer, and other machine-control functions. Despite slight differences in the ladder languages offered by each CNC manufacturer, once a user has mastered one version, it is much easier to learn others.

Transfer-line turnaround

As more and more standalone CNC machine tools are put into use in automotive plants, resistance to them is melting as the capability to program and maintain them grows. This change of attitude is particularly apparent on transfer lines in engine and transmission plants.

Transfer lines link several machines with a synchronous material-handling system. Although single-axis drives are used primarily, the use of multiaxis systems is increasing as the industry attempts to improve machining productivity and boost performance by tightening dimensional tolerances on key power-train components. Transfer lines are being designed for lower volumes and higher model mixes, and therefore require quick part program (or motion profile) modification. Also, shorter product life cycles require lines with designed-in modularity, something well suited to the distributed-control nature of CNCs.

New levels of precision

CNC and servo technology--specifically integrated digital systems--can satisfy these objectives. An example is the single-axis slide on a transfer line. CNCs and servos add value to the part where it's needed: at the cutting tool.

The ability to adjust acceleration parameters lets the user command the slide to move and accelerate as fast as the machine will allow and helps reduce cycle time. Instead of commanding the slide from "move" to a "hard stop" position and limiting motor torque, current CNC technology can position the slide precisely. Cuts are made no more or less than needed when users can avail themselves of external position feedback and accuracies of 0.0001". If the physical reference varies from part to part, the probe function simplifies the job by referencing the feed move from a probe touch-off point on each part.

Machining inaccuracies--such as ballscrew slop when it reverses direction or inconsistencies in screw pitch--may surface when users crank up the gain in the servo. Backlash compensation (or reverse error) and pitch-error compensation can eliminate such inaccuracies. Also, overshoot functions monitor and reduce this problem, and produce cleaner hole bottoms and further reduce cycle times. Holes can be tapped at higher cutting speeds with the rigid tapping feature, and the velocity feed-forward function reduces lag error.

The CNC and servo can also reduce nonmachining time. Because the absolute encoder-feedback function remembers slide position, it eliminates rehoming operations with every line stoppage.

Finally, zone-signal output can segment the motion envelope into eight zones. With this data, the slide control can enable other functions--such as tool change or part indexing--even when the tool is not in the home position. Zone data also helps avoid damaging collisions and reduces dependence on limit switches.

There are several CNC options to accommodate manufacture of a variety of parts on the same line. Dozens of motion profiles (externally selectable by BCD input) can be stored onboard. A built-in custom macro permits adapting one basic profile to numerous similar parts in the same family.

Thus, CNCs and digital servo systems can make a big difference where more speed, flexibility, and throughput are needed. The US auto industry would do well to take full advantage of these benefits.

PHOTO : Three features that reduce the number of limit switches to improve throughput and reduce cycle times: 1) Limit-switchless reference provides the three position references shown, plus two end-travel limits. 2) Absolute position feedback remembers positions during power off. 3) Zone-signal output divides the motion envelope into up to eight position zones.

PHOTO : Dual-spindle machining station, part of an automotive-transmission transfer line. All single-axis stations will use GE Fanuc Power Mate CNCs.

Cosmo Mirra GE Fanuc Automation North America Inc Charlottesville, VA
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Mirra, Cosmo
Publication:Tooling & Production
Date:Jul 1, 1991
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