Mad City Labs Catalog of Nanopositioning Systems ...

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 and 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.
Position noise plot for the X-axis of a Nano-LP100 nanopositioning
system. The reference sinewave input is 3.5nm peakto-peak
at 7 Hz, the noise fl oor is 0.03nm (30 picometers).
Linearity
The accuracy and reproducibility of any piezo actuator
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 piezo actuator 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 the 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
piezo actuator. 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 measured
linearity matches that of the sensors and is better than
0.05%.
Linearity of Piezo Resistive Sensor
0
0 20 40 60 80 100
Position (microns)
Measured non-linearity is less than 0.05%. Nonlinearity
correction techniques such as look up tables and high order
polynomial fi ts are not needed to produce highly linear motion
with Mad City Labs systems.
www.madcitylabs.com sales@madcitylabs.com phone: 608-298-0855 fax: 608-298-9525
Non Linearity (%)
0.07
0.06
0.05
0.04
0.03
0.02
0.01
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