Linear motors take off

As everyone is learning, linear motors can be the most efficient way to achieve linear motion. However, these motors are not going to replace every rotary-to-linear mechanical system. There are some advantages with mechanical systems, such as those attained with gear boxes or geared speed reducers, that linear motors don't provide.

Cost is another factor. The cost of the two main types of linear motors, inductive and permanent magnet, is two to three times more than rotary motors of equivalent horsepower. In certain applications, though, the total life cost of a linear motor is on par with that of a precision rotary-to-linear solution. Here, linear motors have a distinct advantage, offering unmatched speed, accuracy, and smooth motion.

The need for speed

For applications that need very fast linear motion, engineers are turning to permanent-magnet linear motors. Most laser-cutting machines, for example, use linear motors because they enable these tools to reach cutting speeds of 3,000 ipm. You'll find linear motors at the heart of many aluminum-cutting machines for the same reason. With these motors, the machines can cut faster than is possible with a conventional rotary motor and ball screw system, up to 1,500 ipm versus 300 to 500 ipm.

"The biggest advantage machine tools gain, though," says Anwar Chitayat, president of Anorad, a manufacturer of linear motors in Hauppauge, N.Y., "is not the cutting speed. It's the speed at which they can move to a position. In many cases, like when you're drilling holes, you've got to move from point to point. The actual drilling time may not be that much, but the time it takes to get there, and to start drilling, is important. Manufacturers want that time to be as small as possible."

With acceleration rates of 1.5 g or more, linear motors ramp up to full speed fast. They spend as little as 15% of their time accelerating and decelerating. The rest of the time they move at full speed. Depending on the motor, positioning speeds range from 0.0001 ips (0.006 ipm) to 100 ips (6,000 ipm). This is one of the reasons linear motors are turning up in transfer line equipment.

Semiconductor manufacturers also appreciate these fast positioning speeds. "Aside from rapid component insertion, reducing the time it takes to move to a component insertion point, as well as the time between assembly stations, means more production for chip makers," says Cliff Kirk of Northern Magnetics, Santa Clarita, Calif., now a subsidiary of Baldor Electric Co. "For semiconductor manufacture, these speeds provide a payback in only one month."

Acceleration does more than provide fast ramp ups, though. In metal cutting, it also affects programmed feed rates. "The minimum radius curve an axis can handle is determined by the acceleration capability of that axis," says Preston Miller, CNC manager, drive products, GE Fanuc. "The greater the acceleration, the smaller radius curvature the axis can make. In contour machining, this determines the maximum programmed feed rate, provided other process parameters do not influence it."

Linear motors can attain such speeds because there is little or no friction nor mechanical linkage between the motor and the load to slow the motor components down. "There is nothing to hinder the coil. You get clean motion just by going through the magnetic field," says Ed Novak, product manager, Drive Components Div., Aerotech. A linear motor's maximum speed is limited primarily by the mechanical constraints of the machine.

When accuracy counts

Not only do linear motors run fast, they position and cut accurately at these speeds, to a fraction of a mil in some cases.

There are two main types of accuracy with linear motors. Static accuracy is a measure of how close a motor stops to a specified position. "This is the main driver for the semiconductor industry," says John Floresta, director of engineering, Kollmorgen, "even more than speed." A positioning head on a chip insertion machine, for example, may have to be accurate to less than a micron for proper component placement.

A machine tool making a contour cut demonstrates dynamic accuracy, or precise positioning during motion. Machine tools using linear motors can cut a part to within 3 to 5 mm, versus 25 mm with traditional rotary motors. This results in parts cut closer to net shape, which often means subsequent finishing operations can be eliminated.

Linear motors achieve such dynamic accuracies because they permit more gain, resulting in less servo lag in a system. Gain, which is usually expressed as in./min/mil in the machining industry, is limited by the number of mechanical linkages in a machine tool. Most machine tools have a gain of 1.8 in./min/mil, assuming a cutting speed of 300 ipm and a servo lag of 0.0033 in. It's possible to achieve a higher effective gain of 90 in./min/mil with advanced servo algorithms available in many digital controls, feedforward factors, and so on. Linear motors, however, remove all the mechanical connections. This gives them a gain of 5.4 in./min/mil, which translates into a servo lag of 0.0008 in. When combined with servo algorithms in a digital control, the effective gain can be raised to 360 in./min/mil.

The linear motor has a servo bandwidth limit that is a combination of the number of coils, the length and size of the drive wires, and the PWM frequency of the amplifier. But this limit is much higher than the mechanical limitations found on most machines or motion equipment.

Eliminating the rough spots

When equipment must deliver smooth motion, a linear motor is often the best choice as the prime mover.

Jitter with linear motors is minimal, or nonexistent if air bearings separate the motor components. Velocity ripple can vary by as little as ± 0.01%. Thus, a load pushed by a linear motor may vary along the linear path by as little as 1 to 5 nm. For semiconductor manufacturing, such smooth motion is crucial when a machine is etching a transistor path on a chip.

Linear motors attain such smoothness through the arrangement of the magnets and coils. In general, small magnets placed close together generate consistent magnetic flux density for smoother motion. The tradeoff, however, is cost. A less costly alternative has larger magnets, spaced a bit further apart and letting the control algorithms in a digital drive manage the magnetic flux vectors.

Another contributor to smoothness is the material surrounding the coils. Ferromagnetic materials, for example, make linear motors bumpier. Iron-core motors are subject to cogging, which occurs when eddy currents interact with the magnetic force. "Cogging is always a factor in these motors," says Miller. "But there are techniques that can keep it to less than 5% of continuous force. For example, servo algorithms in the control can reduce its effects. Another solution is to reduce cogging in the initial motor design. For example, you can place magnets to lessen this force. What you try to do with the magnets is create a sinusoidal flux vector. So even on ironcore motors, you can get a sinusoidal flux vector as well as a sinusoidal commutation for smooth operation."

If cogging can't be tolerated, you can always choose an ironless motor, where the coil windings are wrapped in a non-magnetic material such as epoxy. You won't get as much force as with an iron-core motor, however. If the application needs high force as well as smooth motion, one solution is to use several ironless motors together to reach the required force.