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Technical Information
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   Piezoactuators

Technical Papers
High-Speed Precision for 3-D Images

Long Range Motion with Nanometer Precision

Piezoresistive Sensors

Understanding Noise at the Nanometer Scale

Nanopositioning Resolution

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PicoQ™ Precision

Newly developed tests show that Mad City Labs closed loop nanopositioning systems are capable of providing picometer level positioning. Position noise, the lower limit on controllable motions, can be thought of as the true positioning resolution of a nanopositioning system. Position noise is the sum of all unwanted (but unavoidable) noise in the complete system - the electronic control system as well as the position sensors inside the stage.

While the overall concept is simple, actual experimental determination of picometer level position noise requires an indirect approach. Mad City Labs' test procedure uses the nanopositioner's own internal position sensors to provide the necessary noise data. Since the internal piezoresistive position sensors provide analog signals which track the real-time position of the stage, it is possible to compare a calibrated motion created by a reference signal to the general background resulting from system noise. An FFT plot of the position sensor signal clearly shows a spike which matches the frequency of the sinewave reference input surrounded by low amplitude background noise which is the summation of all system noise sources. Since the amplitude of the reference signal is known, the amplitude of the position noise can be easily computed.

Linearity

The accuracy and reproducibility of any piezoactuator driven nanopositioning stage depends primarily on the linearity and sensitivity of the stage. The linearity and sensitivity of a stage are different depending on the operation mode, open or closed loop.

When operated in the open loop mode, the hysteresis and creep of the piezoactuator determine the non-linearity, which is typically 8% or more. This non-linearity is due to uncorrected hysteresis in the piezoactuator.

Linearity is greatly improved when operating in closed loop mode. In closed loop operation, a control circuit compares the input signal to a signal from a position sensitive detector. The control circuit continuously adjusts the driver voltage to ensure that the input signal matches the position signal.

The linearity of a nanopositioning stage, when operated in closed loop mode, depends on the linearity of the position sensitive detector - not the piezoactuator. Mad City Labs nanopositioning stages use semiconductor piezoresistive networks for position sensing. These detectors have a linearity of better than 0.05%. The closed loop linearity of a Mad City Labs nanopositioning stage is shown below. The linearity matches that of the sensors and is better than 0.05%.