Metho d s to I mp rove Piezo Dyna m ics

The dynamic behavior of apiezo positioning system depends on factors includingthe system’s resonant frequen-cy, the position sensor and thecontroller used. Simple con-troller designs limit the usable closed-loop tracking band-width of a piezoelectric systemto 1/10 of the system’s reso-nant frequency. PI offers con-trollers that significantly in- crease piezo actuator systemdynamics (see table). Two ofthe methods are described below; additional informationis available on request.

Piezo actuators Piezo Actuators
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was developedbased on research at theMassachusetts Institute ofTechnology and commercial-ized by Convolve, Inc.(www.convolve.com). It is anoption in several PI digitalpiezo controllers.

Nanopositioning & Scanning Systems Nanopositioning & Scanning Systems Active Optics / Steering Mirrors Active Optics / Steering Mirrors Tutorium: Nanoposi- tionieren mit Piezos Tutorial: Piezo- electrics in Positioning Capacitive Industrial position sensors Capacitive Position Sensors

InputShaping

Piezo Drivers & Nano- positioning controllers Piezo Drivers & Nano- positioning Controllers
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Signal processor. There the sophisticated signal process-ing algorithms assure thatnone of the undesired reso-nances in the system or its aux- illary components is excited.Because the processor is out-side the servo-loop, it works inopen-loop operation as well.The result: the fastest possible motion, with settling within a time equal one period of thelowest resonant frequency.InputShaping

Hexapods / Micropositioning Hexapods / Micropositioning Photonics Alignment Solutions Photonics Alignment Solutions
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StopsStructural Ringing Caused byHigh-Throughput Motion

Motion controllers Motion Controllers Ceramic Linear motors & Stages Ceramic Linear Motors & Stages

A patented, real-time, feedfor-ward technology calledInputShaping

Index Index
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nullifies reso- nances both inside and outsidethe servo-loop and virtually eliminates the settling phase. The procedure requires deter- mination of all critical resonantfrequencies in the system. Anon-contact instrument like a Polytec Laser Doppler Vibro-meter is especially well-suitedfor such measurements. Thevalues, most importantly theresonant frequency of the sam- ple on the platform, are thenfed into the InputShaping

Fig. 33. InputShaping ® eliminates therecoil-driven resonant reaction of loadsand neighboring components due to rapid nanopositioner actuation. Top: Polytec Laser Vibrometer reveals the resonant behavior of an undampedfixture when the nanomechanism isstepped. Bottom: Same setup, same step, but with InputShaping ® . Structural ringingis eliminated. With no excitation ofvibration in the moved components, the target position is attained in a time smaller than one period of the resonant frequency.

Various Methods to Improve Piezo Dynamics

MethodGoals FeedforwardReduce phase difference between output and input (tracking error) Signal preshaping (software)Increase operating frequency of the system, correct amplitude and phase response. Two learning phases required; only for periodic signals.Adaptive preshaping (hardware)Increase operating frequency of the system, correct amplitude and phaseresponse. No learning phase, but settling phase required; only for periodic signals. Linearization (digital, in INDUSTRIAL DSP)Compensate for piezo hysteresis and creep effectsInputShaping
® Cancel recoil-generated ringing, whether inside or outside the servo-loop. Reduce the settling time. Closed- and open-loop. Dynamic Digital Linearization (DDL)Increase operating frequency of the system, correct amplitude and phase response. Integrated in digital controller. No external metrology necessary, for periodic signals only.

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