- Eliminating Vibration in the Nano-World
As in the macro-world, solutions to vibration problems range from simply increasing dwell time to allow settling, to using an algorithm in the controller to nullify resonance.
Until recently, photonics engineers, among others, had been powerless
against settling-time bottlenecks in industrial processes that require submicron-scale
precision. Their only effective ways to address fixturing and structural resonance
excited by process motion had been either to slow down processes by using less aggressive
motion profiles or to insert dwell times to allow mechanical damping to settle things
down by converting any mechanical resonance into heat.
However, as process tolerances get tighter —
especially with nanoscale applications such as those involving microelectromechanical
systems (MEMS) — the time required for adequate settling grows exponentially
(see sidebar on page 62). Another approach then becomes necessary to optimize the
process. In such cases, the solution often lies in the control strategy employed
with the system controller. More precisely, it may lie in the use of a control algorithm
designed specifically to address motion control of systems with unwanted dynamics
such as vibration.
Working with a control algorithm called Input
Shaping, Polytec PI engineers have improved throughput by up to three orders of
magnitude in certain motion-control applications, such as those required in the
manufacture of fiber Bragg gratings, by eliminating settling dwells and allowing
more aggressive motion profiles. The technique also can help improve the actuation
speed for optical MEMS. Originally developed at Massachusetts Institute of Technology
in Cambridge and commercialized by Convolve Inc. in New York, it is an integrated
option for Polytec’s digital piezo-positioning controllers.
Although the software code supporting
the algorithm is complex, the concept it encapsulates is fairly simple. The software
developers based the algorithm on the fact that, within almost any system prone
to vibration, a motion transient will further excite the vibration. In most systems,
scientists can characterize such vibration by measuring one or more of the frequencies
excited by the motion transient. Based on this information, it is possible to generate
a modified command signal to move the system at the maximum rate that does not excite
vibrations. This theory holds true whether the vibration is an element of a production
process used to manufacture photonic components or simply a side effect of movement
by a component such as a MEMS device.
Consider the manufacture of fiber Bragg
gratings. Each is simply a diffraction grating fashioned deep within a fiber or
waveguide through interferometric optical exposure. To improve the spectral characteristics
of the finite-length grating, the grating transitions must be apodized through nanometer-precision
scanning motions of optics in the exposure apparatus. This demanding process produces
a simple, compact, rugged, entirely passive device capable of precisely isolating
a single wavelength.
However, as service providers begin
to demand that a fiber carry more and more channels, the selectivity of the gratings
must keep pace. Inaccuracies from vibrations of structures throughout the manufacturing
tooling begin to influence the outcome more dramatically. Not only must the positioners
in the tooling perform to nanometer-scale levels, but a method also must be in place
to suppress all unwanted motions in other components. Air-isolation tables address
ambient disturbances, but resonances driven by the scanning can still take hundreds
of milliseconds to damp out — an unacceptable throughput penalty.
Optimizing MEMS motion
Using the control strategy patented by Convolve
engineers, it is possible to nullify structural resonances from manufacturing tooling
such as the closed-loop piezo stage used for high-throughput, nanometer-scale mask
position modulation. Benefits can include higher process throughput as well as optimized
grating fidelity for narrower channel width and improved system efficiency (Figure
Figure 1. In the fiber Bragg grating manufacturing process, vibration of
tooling components in
response to rapid actuation of high-throughput nanopositioners can
efficiency, as indicated by quasi-sinusoidal phase-mask dither
A resonance nullification strategy smooths the waveform significantly.
of Dover Instruments.
In the case of optical MEMS devices
such as switches, cross connects, mirrors and attenuators, the compactness of the
Input Shaping algorithm and its robustness relative to unit-to-unit variations make
it practical for embedding into device controllers. The goals of this control scheme
• Optimization of MEMS elemental positioning times.
• Elimination of settling intervals in open- and closed-loop actuation.
• Prevention of parasitic excitation of adjacent elements in MEMS arrays.
• Bandwidth increase (gain boosting) in closed-loop actuation, with the goal of eliminating overshoot and ringing issues in underdamped actuation.
One way to examine the benefits of such a control
strategy involves inspection with a laser Doppler vibrometer (Figure 2). In recent
testing, scientists studied vibrometer displacement and spectral measurements before
and after implementing the algorithm in series with the position-command signal
for an open-loop MEMS switching/scanning element. In this example, the implementation
process was a simple one, requiring a straightforward measurement of the resonant
characteristics of the MEMS device with the vibrometer. This also helped to define
the real-time operating parameters for the algorithm. The outcome was the elimination
of resonances of each micro-optical element, regardless of actuation profile, as
well as elimination of the resonant reaction of neighboring MEMS elements.
Figure 2. Without a control scheme to nullify resonance, open-loop
actuation of a MEMS
micromirror inherently has a significant ringing problem, which could
lead to long
settling intervals and transient channel-crosstalk issues associated
with the square-wave
input. Courtesy of Applied MEMS Inc.
In closed-loop actuations, it may also
be possible to significantly boost proportional and integral gains, with the control
scheme addressing the overshoot and oscillation about the terminal position that
are otherwise characteristic of overboosted and underdamped applications. This can
further improve the switching throughput of many devices compared with critical
damping. Even better, the elimination of resonant behavior may eliminate the need
for closed-loop actuation in the first place, which could produce a significant
Regardless of whether the vibration
problem is with the manufacturing process or a MEMS component, it is important to
note that there is a fundamental difference between resonance nullification with
the algorithm and active damping, alignment stabilization or other after-the-fact
compensation of unwanted resonant behavior. In those situations, compensation occurs
after the device is observed to have begun ringing, with the aim of counteracting
the unwanted motion or extracting energy from the resonance and converting it into
heat or storing it elsewhere in the system. These all take time.
The advantage of resonance nullification
through an algorithm in the system controller is that it prevents energy from entering
the resonant mode. Long-term, besides offering benefits to photonic component manufacturing
processes, this control scheme has the potential to further address the uniquely
nonlinear actuation physics of specific MEMS designs.
The author would like to thank Mark Tanquary of
Convolve Inc. and Eric Lawrence and David Oliver of Polytec PI for their assistance
in this research.
About the author
Scott Jordan is director of NanoAutomation products
for Polytec PI Inc. in San Jose, Calif.
Feeding off the Motion Transient
There is no way around the fact that vibrational physics translates into problematic process
throughput as device tolerances tighten. One common solution to vibration damping
is simply to increase dwell times to allow things to settle down. The problem is
that, the tighter the tolerances, the longer it takes for adequate settling. After
a motion, the amplitude of the resonant ringing of each element in a structure scales
as e—t/—τ, where τ is the time constant for each element’s resonant
characteristics. For structures with damping characteristics typical of precision-motion
subassemblies, τ ~ (ωnξ)—1 where ωn is the resonant
angular frequency and ξ is the damping ratio for the resonance. The parameter
is commonly defined as the ratio of the damping for the resonance vs. critical damping
(ξ = C/Cc) and varies from 0 (no damping) to 1 (critical damping).
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