Launching into linear motors

May 1, 2000
It takes a strong supporting staff, including good mechanical design and all-digital amplifiers, to make linear motors shine

The economics of operating a machine tool are simple: with each cycle, whether it’s the pass of a milling bit or plunge of a drill, the owner gains or loses money. The goal, of course, is to optimize the gain per cycle, while running more and more cycles in shorter and shorter periods of time.

Lately, machine-tool builders have been relying on technology to stay in the race. With recent improvements in motor controls and drives, feed rates of up to 20 m/min (13 ips) are now common. But what’s fast today is likely to be too slow tomorrow.

To take the next step, machine-tool builders must be ready to integrate today’s cutting-edge technology, namely linear motors, digital drives, and advanced control algorithms. The following analysis exploring the technological issues associated with linear motors can serve as the launch pad.

Need for speed

The objective of machine-tool builders is to sell machines. Today this means designing either a cost-competitive machine or a premium machine capable of higher output, tighter tolerances, and better finish quality. In the latter case, the money saved by the user, in achieving fewer and faster production steps, will offset the additional cost of the machine.

Suppose your company, like many, decides to produce the higher-end machine. As the designer, your task essentially will be to reduce total machining time while concurrently increasing accuracy. For the most part, the design objectives are already determined for you: O

bjective 1: Improve cutting speed. This means optimizing speed in the feed axes as well as in the spindles and cutting tools.

Objective 2: Reduce noncutting time. The feed axes must be capable of high traverse rates and accelerations (accel/decel) especially for short moves.

Objective 3: Improve accuracy at high speed. To maintain accuracy during highspeed machining, you’ll have to find ways to minimize the trade-offs between speed and precision. Accuracies of about 4 μm (0.0001 in) are the norm.

Behind the times

Advances in computer technology have made an impact on machine control. Today’s digital controllers now incorporate multiple processors, dedicated DSPs (digital signal processors), and 64- bit RISC (reduced instruction set computer) chips at comparable or even lower prices than controls manufactured just a few years ago.

Drive technology has also improved rapidly thanks to recent developments that push amplifiers to their utmost limits while compensating for inherent shortcomings. Advanced algorithms reduce following error; intelligent gain control improves precision at high speed; and acceleration and backlash compensation improve path accuracy for cutting circles and arcs.

But silicon and software can only go so far. If machine tools are expected to get better, mechanical technology must improve as well. Unfortunately, the mechanics of machine tools haven’t changed much over the last 50 to 60 years, except for minor improvements in motors and drive screws.

Consider a conventional stage drive, consisting of a motor, gear reducer and/or coupling, drive screw and nut, and support bearings including bearing blocks and retainers. Each of these elements affect the machine tool and limit its performance. Motors, for example, are limited in maximum rotation speed as defined by the speed/torque curve (torque drops as speed increases).

Motor shafts likewise have limitations, stemming from windup twist under heavy load and acceleration. In some cases, this deflection is severe enough to cause position errors.

Gear reducers add unnecessary inertia to the system as well as backlash, noise, and “gear slop” that increases with wear. Plus, they are only partially efficient. Direct motor couplings, though more efficient, suffer from windup distortion, backlash, and hysteresis.

The effects of drive screws are even more complicated. Designers need to be aware of critical rotation speeds, mechanical backlash, windup, pitch-cycle errors, friction, alignment errors, and length limitations.

Drive screws also present an undesired inertia that’s transmitted directly back to the motor. And the faster the system, the greater the inertia because it takes a larger screw to prevent oscillations. This, in turn, necessitates a larger motor, increasing inertia even further, which can severely degrade system responsiveness, acceleration, and bandwidth.

What’s more, steel, the usual ball screw material, has a long vibration decay time. This increases settling time, the time it takes a motion axis to settle within tolerance after completing a move. In some cases, settling time can exceed the duration of the move itself. Because many processes, such as drilling and precision boring, cannot begin until the axis is in position, systems with high settling times can spend most of their time doing nothing.

Linear advantages

One way to get around mechanical drive restrictions is by using linear motors. Though not new, linear motors are becoming more and more widespread in machine-tool applications, and for good reason.

Linear motors consist of two noncontact elements linked by an electromagnetic field. One element, a series of magnets, usually attaches to the machine base, while the other element, a set of coils wound around a steel laminate core, attaches to the moving slide. In this way, the slide is engaged and propelled directly by an electromagnetic force.

One advantage of such an arrangement is that there’s no mechanical connection, so there’s no mechanical hysteresis or pitch-cycle error to be concerned with. And because there are no load-bearing parts in contact, there’s no wear and, thus, no need for periodic maintenance. What’s more, accuracy depends entirely on the quality of the feedback system and stage bearings, so it’s easier to control.

When driven by digital servo systems, linear motors can achieve unprecedented stiffness and bandwidth. Their slides react almost instantaneously to position and force commands, limited only by the speed of the electromagnetic field. Time constants of 1 msec or less are common, minimizing following error and maximizing precision. In some milling and contouring machines, the first pass is so smooth that secondary operations are unnecessary.

Linear motor systems are also efficient in terms of power transmission because there are no intervening elements to waste energy. And unlike ball screws, they are not limited in travel.

Stiff stuff

Perhaps the best measure of servo drive/transmission system performance is dynamic stiffness. This is defined as how well the system maintains position in response to an impulse load, and can be calculated by dividing the measured impulse load by the deflection distance. In general, greater bandwidth and higher gain mean greater dynamic stiffness.

Dynamic stiffness must be considered not only for dc levels, but also for frequencies (vibrations) produced by external forces such as spindles and cutters, typically 10 to 40 Hz. It must also account for the mechanics of the system.

To realize the benefits of linear motors, the machine structure itself must be stiff and able to quickly damp unwanted vibrations. Cast-iron meehanite has proven time and again the material of choice, but newer epoxy composites are a viable alternative where complex machine bases or lighter weight are needed.

On the electronic side, besides using an all-digital drive, it helps to close servo loops in the main processor rather than in the servo amplifier. This allows the use of advanced servo algorithms, featuring bell-shaped accel/decel profiles, velocity feedforward, unexpected load detection, digital tuning, fast sample times (1 msec), and look-ahead block processing. Such algorithms also eliminate axis hesitation upon reversal (backlash, mechanical and servo) and provide a tight match between part processing requirements and servo response.

The net effect of being able to tune the servo system right down to the way it accelerates (jerk control) allows a much higher bandwidth (2:1) over conventional drive systems and a significant (25 X) improvement over analog drives.

There’s no question that with linear motors, and all-digital drives, machine tools perform many tasks better. They produce parts with higher accuracy, tighter tolerances, and at higher feed rates, while reducing nonmachining time through high accel/decel rates.

According to tests, machine tools using linear motors, digital drives, and highspeed (40,000 rpm) spindles can reduce total cycle time by nearly 60%. With no “ringing” at the end of each move, a linear motor doesn’t waste time waiting around for motion axes to settle. All of which translate to higher productivity.

Masatoyo Sogabe is manager of the Fanuc Servo Lab, Linear Motor Development Div., Japan.

Chuck Weidner is product manager for motors at GE Fanuc, Charlottesville, Va.

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