Linear motors aid high-volume manufacture of fiberoptic devices

Oct. 1, 2000
Just as computer memory, processor speed, and capabilities have greatly surpassed the assumptions of 20 years ago, so too has fiberoptic communications technology expanded at an accelerated rate.

High velocity, acceleration, accuracy, and an inherently noncontact design, make linear motors the choice for high-precision, high-volume fiberoptic alignment.

Mike Formica

Just as computer memory, processor speed, and capabilities have greatly surpassed the assumptions of 20 years ago, so too has fiberoptic communications technology expanded at an accelerated rate. Despite many advances, however, there are still major challenges to overcome—including the mass-production of photonic devices. Technology that was spawned in the laboratory must now be made practical for high-volume production. Following the lead of the semiconductor industry, optoelectronics manufacturers must now wrestle with challenges such as throughput, cost of ownership, yield, and time-to-market.

FIGURE 1. The FiberAlign 130 is used for fiber-to-fiber, fiber-to-laser diode, and fiber-to-waveguide alignments. The system above shows a typical fiber-to-fiber alignment application.

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Precision motion is an important part of the hardware used in mass production of optoelectronic devices. The three basic stage-drive technologies used in fiberoptic manufacturing are linear motors, ball screws, and piezo devices. Of the three, the linear motor—already the standard in the high-precision wafer-inspection and fabrication machines used in the semiconductor industry—has advantages for the optoelectronics fabrication industry as well.

A common use of positioning devices is the high-precision alignment of a fiber end to a device such as a laser diode or waveguide. While there are several techniques for accomplishing this, the common element is the ability to move the fiber in extremely small increments. To do this, high-precision linear stages are used to provide small incremental motions (see Fig. 1). While all three stage-drive technologies are capable of making extremely small step sizes, the linear motor provides the best overall performance.

Ball-screw technology
For years ball-screw-based positioning systems have been the norm in optoelectronics manufacturing. The technology is well understood, they provide good performance, and are relatively reliable. The basic technology consists of several components including a rotary motor (stepper, DC servo, or brushless servo), a coupling, a ball screw, a nut, and end bearings (see Fig. 2). The rotation of the motor is converted into linear motion by spinning the shaft of the screw through a coupling. The nut is fixed by the stage carriage, causing both to translate along the screw as the screw rotates. While suitable for laboratory use, ball screws are not ideal for high-throughput production and are rarely found on ultrahigh-precision machines.

Since a ball screw is inherently a friction-based device, it is subject to wear over time. Even with regular maintenance, ball screws will ultimately wear out and need to be replaced. As they age, their performance characteristics change, typically resulting in higher backlash, poorer accuracy, and increased vibration. High duty cycles result in significant heat, causing the screw to expand and degrading short-term system accuracy.

FIGURE 2. In a stage driven by a ball screw, a ball nut attached to the stage rides on and is propelled by the screw.

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Despite attempts to compensate for the inherent deficiencies of a screw-based mechanism (using linear encoders and zero-backlash nuts, for example) ball screws are used less as system accuracy and resolution increase. When a rotary encoder is used, it is located on the rear shaft of the motor. Fundamentally, it measures the amount the motor moves, but is not an accurate indication of linear stage motion. Several sources of error (such as coupling compliance, screw backlash, screw windup, and runout) can cause the actual motion of the load to deviate from what the encoder records.

While a ball-screw system may also use a linear encoder, the friction of the screw and compliance of the coupling makes extremely small step sizes impossible. In these cases, the encoder thinks it has moved, but the stage has effectively been bound up and does not make any true motion. A subsequent step may release the stage, which will then move both steps at once. The result is an unpredictable movement from step to step. When the step sizes are on the order of a micron, this effect is negligible, but as resolution increases to the submicron and nanometer realms, the effect can be devastating.

Piezo positioners
Piezo-driven devices are rapidly gaining popularity due to their ability to make extremely small motions. Unlike a ball screw, a piezo is a frictionless device and does not suffer from many of the same limitations. When voltage is applied to a piezo crystal it expands, creating linear motion. The expansion is very small relative to the size of the crystal, so incremental motion on the order of nanometers can be achieved. This advantage is also the source of the device's single biggest drawback: limited travel range. Typically, piezos are stacked to create a device capable of motion on the order of 10 to 100 µm. To create a larger motion requires an impractically large stack.

Often, a piezo positioner is used in conjunction with a ball screw stage. The ball-screw stage supplies the large travel required for loading and unloading, and the piezo is used to provide the ultraprecise step motion. While this arrangement can lead to a functional system, it is cumbersome to implement. Precise mechanical alignment is required of both devices to ensure that they operate along the same axis of motion. The system still does not offer high absolute accuracy, as the piezo location is initially dictated by the ball screw and is therefore subject to all of the ball-screw's errors. Not only does the total axis count increase, making software control more complicated, but in most cases the piezo controller and ball-screw controller come from different manufacturers and have different control syntaxes.

Linear motors
A simple solution is the brushless linear servomotor. A linear motor operates on the same principle as a rotary motor, where an energized motor coil interacts with a permanent magnet to create relative motion. In a rotary motor this interaction creates a torque, while in a linear motor it creates a linear force. The linear motor can have either a moving coil or moving magnet design. In both cases, the moving element is directly mounted to the stage carriage, eliminating the complex drive train associated with a ball-screw system.

FIGURE 3. Windings and magnets of a linear motor interact with each other to provide relative motion. Here, the windings move and the magnets are fixed; linear motors can also be designed with moving magnets and fixed windings.

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Because the linear motor is a completely noncontact device, there is zero friction and no cogging. Exceptionally smooth velocity control is achieved and systems are capable of extremely small, precise step sizes. The feedback device—either a noncontact linear encoder or a laser interferometer—dictates the fundamental resolution of a linear motor stage. Typically used in high-precision applications, such as fiber Bragg grating manufacturing, a laser interferometer is capable of resolution in the subnanometer range. Linear encoder systems typically offer a resolution of 4 to 20 nm.

The noncontact nature of the linear motor also means there are no parts to wear and no maintenance. Unlike a screw-based stage system, the performance of a stage driven by a linear motor doesn't change over time, making it the ideal solution for low-maintenance, full-scale production. One of the most important criteria in a production tool is throughput. Linear motors perform well in this area, as they are capable of high velocities and accelerations. A typical high-speed linear-motor-based pick-and-place system, for example, may have a top velocity of 5 m/s and an acceleration of 5 g. While this is more than would be required in most optoelectronic applications, the linear motor nonetheless has no equal in terms of speed and acceleration. The higher precision of a linear-motor-based stage system also contributes to throughput.

In fiber-alignment applications, the optimum fiber position is determined by finding the peak of a profile of coupled optical power as a function of position. Any variation in step size will cause the motion system to hunt for peak power in an inefficient manner. The absolute accuracy of step size associated with a linear motor allows it to find peak power significantly faster and more reliably than either a ball screw or piezo device. Not only is throughput increased, but parts quality is also improved.

The next-generation fiberoptic communications technology being developed will require equally sophisticated production tools. Linear-motor technology, considered by some to be a mere novelty years ago, is quickly becoming the only viable solution for many high-performance photonics-manufacturing applications.

MIKE FORMICA is director of product marketing at Aerotech, Inc., 101 Zeta Drive, Pittsburgh, PA 15238; e-mail: [email protected].

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