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ABSTRACT: A novel and simple approach to optical wavelength measurement is presented in
this paper. The working principle is demonstrated using a tunable waveguide
micro ring resonator and single photodiode. The initial calibration is done
with a set of known wavelengths and resonator tunings. The combined spectral
sensitivity function of the resonator and photodiode at each tuning voltage was
modeled by a neural network. For determining the unknown wavelengths, the
resonator was tuned with a set of heating voltages and the corresponding
photodiode signals are collected. The unknown wavelength was estimated, based
on the collected photodiode signals, the calibrated neural networks, and an
optimization algorithm. The wavelength estimate method provides a high spectral
precision of about 8 pm (5*10^(-6) at 1550 nm) in the wavelength range between
1549 nm to 1553 nm. A higher precision of 5 pm (3*10^(-6)) is achieved in the
range between 1550.3 nm to 1550.8 nm, which is a factor of five improved
compared to a simple lookup of data. The importance of our approach is that it
strongly simplifies the optical system and enables optical integration. The
approach is also of general importance, because it may be applicable to all
wavelength monitoring devices which show an adjustable wavelength response.
04/2013;
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[hide abstract]
ABSTRACT: The use of bone mineral density as a surrogate to diagnose bone fracture risk in individuals is of limited value. However, there is growing evidence that information on trabecular microarchitecture can improve the assessment of fracture risk. One current strategy is to exploit finite element analysis (FEA) applied to 3D image data of several mm-sized trabecular bone structures obtained from non-invasive imaging modalities for the prediction of apparent mechanical properties. However, there is a lack of FE damage models, based on solid experimental facts, which are needed to validate such approaches and to provide criteria marking elastic-plastic deformation transitions as well as microdamage initiation and accumulation. In this communication, we present a strategy that could elegantly lead to future damage models for FEA: direct measurements of local strains involved in microdamage initiation and plastic deformation in single trabeculae. We use digital image correlation to link stress whitening in bone, reported to be correlated to microdamage, to quantitative local strain values. Our results show that the whitening zones, i.e. damage formation, in the presented loading case of a three-point bending test correlate best with areas of elevated tensile strains oriented parallel to the long axis of the samples. The average local strains along this axis were determined to be (1.6±0.9)% at whitening onset and (12±4)% just prior to failure. Overall, our data suggest that damage initiation in trabecular bone is asymmetric in tension and compression, with failure originating and propagating over a large range of tensile strains.
Journal of the mechanical behavior of biomedical materials. 05/2011; 4(4):523-34.
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ABSTRACT: In many scientific and medical applications, such as laser systems and microscopes, wavefront-sensor-less (WFSless) adaptive optics (AO) systems are used to improve the laser beam quality or the image resolution by correcting the wavefront aberration in the optical path. The lack of direct wavefront measurement in WFSless AO systems imposes a challenge to achieve efficient aberration correction. This paper presents an aberration correction approach for WFSlss AO systems based on the model of the WFSless AO system and a small number of intensity measurements, where the model is identified from the input-output data of the WFSless AO system by black-box identification. This approach is validated in an experimental setup with 20 static aberrations having Kolmogorov spatial distributions. By correcting N=9 Zernike modes (N is the number of aberration modes), an intensity improvement from 49% of the maximum value to 89% has been achieved in average based on N+5=14 intensity measurements. With the worst initial intensity, an improvement from 17% of the maximum value to 86% has been achieved based on N+4=13 intensity measurements.
Optics Express 11/2010; 18(23):24070-84. · 3.59 Impact Factor
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ABSTRACT: In many scientific and medical applications wavefront-sensorless adaptive optics (AO) systems are used to correct the wavefront aberration by optimizing a certain target parameter, which is nonlinear with respect to the control signal to the deformable mirror (DM). Hysteresis is the most common nonlinearity of DMs, which can be corrected if the information about the hysteresis behavior is present. We report a general approach to extract hysteresis from the nonlinear behavior of the adaptive optical system, with the illustration of a Foucault knife test, where the voltage-intensity relationship consists of both hysteresis and some memoryless nonlinearity. The hysteresis extracted here can be used for modeling and linearization of the AO system.
Optics Letters 02/2009; 34(1):61-3. · 3.40 Impact Factor
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ABSTRACT: A new mechanical scanner design for a high-speed atomic force microscope (AFM) is presented and discussed in terms of modeling and control. The positioning range of this scanner is 13 mum in the X- and Y-directions and 4.3 mum in the vertical direction. The lowest resonance frequency of this scanner is above 22 kHz. This paper is focused on the vertical direction of the scanner, being the crucial axis of motion with the highest precision and bandwidth requirements for gentle imaging with the AFM. A second- and a fourth-order mathematical model of the scanner are derived that allow new insights into important design parameters. Proportional-integral (Pl)-feedback control of the high-speed scanner is discussed and the performance of the new AFM is demonstrated by imaging a calibration grating and a biological sample at 8 frames/s.
IEEE Transactions on Control Systems Technology 10/2007; · 1.77 Impact Factor
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G. Schitter
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ABSTRACT: This article reviews mechanical design and control of atomic force microscopes (AFM) with a special emphasis on high-speed imaging. The mechanical design and the control system determine the achievable imaging speed of the AFM. To enable AFM imaging at video-rates, imaging speed - and thus system performance - has to be increased by at least two orders of magnitude relative to today's commercial AFMs. Methods and results presented in this paper demonstrate how this can be achieved.
American Control Conference, 2007. ACC '07; 08/2007
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ABSTRACT: The topography of freshly fractured bovine and human bone surfaces was determined by the use of atomic force microscopy (AFM). Fracture surfaces from both kinds of samples exhibited complex landscapes formed by hydroxyapatite mineral platelets with lateral dimensions ranging from ∼90 nm × 60 nm to ∼20 nm × 20 nm. Novel AFM techniques were used to study these fracture surfaces during various chemical treatments. Significant topographical changes were observed following exposure to aqueous solutions of ethylenediaminetetraacetic acid (EDTA) or highly concentrated sodium fluoride (NaF). Both treatments resulted in the apparent loss of the hydroxyapatite mineral platelets on a timescale of a few seconds. Collagen fibrils situated beneath the overlying mineral platelets were clearly exposed and could be resolved with high spatial resolution in the acquired AFM images. Time-dependent mass loss experiments revealed that the applied agents (NaF or EDTA) had very different resulting effects. Despite the fact that the two treatments exhibited nearly identical results following examination by AFM, bulk bone samples treated with EDTA exhibited a ∼70% mass loss after 72 h, whereas for the NaF-treated samples, the mass loss was only of the order of ∼10%. These results support those obtained from previous mechanical testing experiments, suggesting that enhanced formation of superficial fluoroapatite dramatically weakens the protein-hydroxyapatite interfaces. Additionally, we discovered that treatment with aqueous solutions of NaF resulted in the effective extraction of noncollagenous proteins from bone powder.
Nanotechnology 04/2007; 18(13):135102. · 3.98 Impact Factor
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ABSTRACT: A new scanner design for a high-speed atomic force microscope (AFM) is presented and discussed in terms of modeling and control. The lowest resonance frequency of this scanner is above 22 kHz. The X and Y scan ranges are 13 micrometers and the Z range is 4.3 micrometers. The focus of this contribution is on the vertical positioning direction of the scanner, being the crucial axis of motion with the highest bandwidth and precision requirements for gentle imaging with the atomic force microscope. A mathematical model of the scanner dynamics is presented that will enable more accurate topography measurements with the high-speed AFM system
American Control Conference, 2006; 07/2006
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Advanced Intelligent Mechatronics. Proceedings, 2005 IEEE/ASME International Conference on; 02/2005
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ABSTRACT: The dynamic behavior of the piezoelectric tube scanner limits the imaging rate in atomic force microscopy (AFM). In order to compensate for the lateral dynamics of the scanning piezo a model based open-loop controller is implemented into a commercial AFM system. Additionally, our new control strategy employing a model-based two-degrees-of-freedom controller improves the performance in the vertical direction, which is important for high-speed topographical imaging. The combination of both controllers in lateral and vertical direction compensates the three-dimensional dynamics of the AFM system and reduces artifacts that are induced by the systems dynamic behavior at high scan rates. We demonstrate this improvement by comparing the performance of the model-based controlled AFM to the uncompensated and standard PI-controlled system when imaging pUC 18 plasmid DNA in air as well as in a liquid environment.
Ultramicroscopy 09/2004; 100(3-4):253-7. · 2.47 Impact Factor
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ABSTRACT: Fast and precise positioning is a basic requirement for nanotechnology applications. Many scanning-probe microscopes (SPM) use a piezoelectric tube scanner for actuation with nanometer resolution in all three spatial directions. Due to the dynamics of the actuator, the imaging speed of the SPM is limited. By applying model-based open-loop control, the dynamic behavior of the scanner can be compensated, reducing the displacement error, topographical artifacts, modulation of the interaction force, and modulation of the relative tip-sample velocity. The open-loop controlled system enables imaging of up to 125-μm-sized samples at a line scan rate of 122 Hz, which is about 15 times faster than the commercial system.
IEEE Transactions on Control Systems Technology 06/2004; · 1.77 Impact Factor
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ABSTRACT: The performance of an atomic force microscope (AFM) is improved substantially by utilizing modern model-based control methods in comparison to a standard proportional-integral (PI) controlled AFM system. We present the design and implementation of a two-degree-of-freedom (2DOF)-controller to accomplish topography measurements at high scan-rates with reduced measurement error. An H∞-controller operates the AFM system in a closed loop while a model-based feedforward controller tracks the scanner to the last recorded scan-line. Experimental results compare the actual performance of the standard PI-controlled AFM and the 2DOF controlled system. The new controller reduces the control error considerably and enables imaging at higher speeds and at weaker tip-sample interaction forces.
Asian Journal of Control 05/2004; 6(2):156 - 163. · 1.03 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Utilizing modern model-based control methods improves the performance of an atomic force microscope (AFM) substantially when compared to the state of the art commercial realizations. The design and implementation of a two-degree-of-freedom (2 DOF) controller on a commercial AFM system is presented enabling topography measurements on the nano-scale at higher scan rates with reduced measurement error. The closed-loop operation of the AFM system is performed by an H<sub>∞</sub>-controller, while the scanner is simultaneously tracked to the last scan line by a model-based feedforward controller. Experimental results obtained at 15 Hz line-scan rate exhibit a maximum control error reduced by a factor of about 6 in comparison with the commercial system.
American Control Conference, 2003. Proceedings of the 2003; 07/2003
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[show abstract]
[hide abstract]
ABSTRACT: The imaging speed of atomic force microscopy (AFM) is limited due to the dynamics of the piezo scanner along the scanning direction. This article presents the identification and open-loop control of a piezoelectric tube scanner to enable fast imaging. By applying a model based open-loop control the dynamic behaviour of the piezo tube can be compensated. The lateral displacement error is reduced and topographical artifacts, additional cantilever deflection, modulation of the tip–sample interaction force, and modulation of the relative tip–sample velocity vanish. The open-loop controlled AFM scanner enables imaging at a line scan rate of 122 Hz, which is about 15 times faster than standard systems. 2003 Elsevier Science B.V. All rights reserved.
Microelectronic Engineering 01/2003; 6768:938-944. · 1.56 Impact Factor
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[hide abstract]
ABSTRACT: This paper presents a method for cancelling mechanical vibrations in scanning probe microscopes by recording the vibrations with an auxiliary distance sensor and subsequently removing them from the topography signal. For the experimental verification, the auxiliary sensor was mounted in a commercial atomic force microscope (AFM) side by side with the probe tip. Combination of the signals from the AFM and distance sensor converts the microscope into a differential instrument, allowing for subtraction of the vibration-induced noise up to the control bandwidth of the AFM system. Imaging with sub-nanometre resolution in a noisy environment was demonstrated.
Nanotechnology 09/2002; 13(5):663. · 3.98 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We identify the dynamics of an atomic force microscope (AFM) in order to design a feedback controller that enables faster image acquisition at reduced imaging error compared to the now generally employed proportional integral differential (PID) controllers. First, a force model for the tip-sample interaction in an AFM is used to show that the dynamic behavior of the cantilever working in contact mode can be neglected for control purposes due to the relatively small oscillation amplitude of the cantilever in response to a defined topography step. Consequently, the dynamic behavior of the AFM system can be reduced to the behavior of the piezoelectric scanner making the design of a model based controller for the AFM possible. Second, a black box identification of the scanner of a commercial AFM (Nanoscope IIIa, Digital Instruments) is performed using subspace methods. Identification yields a mathematical model of the scanner which allows us to design a new controller utilizing H∞ theory. Finally, this controller is implemented on an existing AFM and operated in contact mode. We demonstrate that such an H∞-controlled AFM system, while scanning at rates five times faster than conventional PID-controlled systems, operates with reduced measurement error and allows scanning at lower forces.
Review of Scientific Instruments. 01/2001; 72:3320.
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[show abstract]
[hide abstract]
ABSTRACT: The use of bone mineral density as a surrogate to diagnose bone fracture risk in individuals is of limited value. However, there is growing evidence that information on trabecular microarchitecture can improve the assessment of fracture risk. One current strategy is to exploit finite element analysis (FEA) applied to 3D image data of several mm-sized trabecular bone structures obtained from non-invasive imaging modalities for the prediction of apparent mechanical properties. However, there is a lack of FE damage models, based on solid experimental facts, which are needed to validate such approaches and to provide criteria marking elastic–plastic deformation transitions as well as microdamage initiation and accumulation. In this communication, we present a strategy that could elegantly lead to future damage models for FEA: direct measurements of local strains involved in microdamage initiation and plastic deformation in single trabeculae. We use digital image correlation to link stress whitening in bone, reported to be correlated to microdamage, to quantitative local strain values. Our results show that the whitening zones, i.e. damage formation, in the presented loading case of a three-point bending test correlate best with areas of elevated tensile strains oriented parallel to the long axis of the samples. The average local strains along this axis were determined to be (1.6±0.9)% at whitening onset and (12±4)% just prior to failure. Overall, our data suggest that damage initiation in trabecular bone is asymmetric in tension and compression, with failure originating and propagating over a large range of tensile strains.
Journal of the Mechanical Behavior of Biomedical Materials.
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P. J. Thurner,
B. Erickson,
Z. Schriock,
J. Langan,
J. Scott,
M. Zhao,
G. E. Fantner,
P. Turner,
J. H. Kindt, G. Schitter,
P K Hansma
[show abstract]
[hide abstract]
ABSTRACT: The mechanical properties of healthy and diseased bone tissue are extensively studied in mechanical tests. Most of this research is motivated by the immense costs of health care and social impacts due to osteoporosis in post-menopausal women and the aged. Osteoporosis results in bone loss and change of trabecular architecture, causing a decrease in bone strength. To address the problem of assessing local failure behavior of bone, we combined mechanical compression testing of trabecular bone samples with high-speed photography. In this exploratory study, we investigated healthy, osteoarthritic, and osteoporotic human vertebral trabecular bone compressed at high strain rates simulating conditions experienced in individuals during falls. Apparent strains were found to translate to a broad range of local strains. Moreover, strained trabeculae were seen to whiten with increasing strain. We hypothesize that the effect seen is due to microcrack formation in these areas, similar to stress whitening seen in synthetic polymers. From the results of a motion energy filter applied to the recorded movies, we saw that the whitened areas are, presumably, also of high deformation. We believe that this method will allow further insights into bone failure mechanisms, and help toward a better understanding of the processes involved in bone failure.
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P. J. Thurner,
B. Erikson,
Z. Schriock,
J. Langan,
J. Scott,
M. Zhao,
G. E. Fantner,
P. Turner,
J. H. Kindt, G. Schitter,
P K Hansma
[show abstract]
[hide abstract]
ABSTRACT: The mechanical properties of healthy and diseased bone tissue are extensively studied in mechanical tests. Most of this research is motivated by the immense costs of health care and social impacts due to osteoporosis in post-menopausal women and the aged. Osteoporosis results in bone loss and change of trabecular architecture, causing a decrease in bone strength. To address the problem of assessing local failure behavior of bone, we combined mechanical compression testing of trabecular bone samples with high-speed photography. In this exploratory study, we investigated healthy, osteoarthritic, and osteoporotic human vertebral trabecular bone compressed at high strain rates simulating conditions experienced in individuals during falls. Apparent strains were found to translate to a broad range of local strains. Moreover, strained trabeculae were seen to whiten with increasing strain. We hypothesize that the effect seen is due to microcrack formation in these areas, similar to stress whitening seen in synthetic polymers. From the results of a motion energy filter applied to the recorded movies, we saw that the whitened areas are, presumably, also of high deformation. We believe that this method will allow further insights into bone failure mechanisms, and help toward a better understanding of the processes involved in bone failure.
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P. J. Thurner,
B. Erickson,
Z. Schriock,
J. Langan,
J. Scott,
M. Zhao,
J. C. Weaver,
G. E. Fantner,
P. Turner,
J. H. Kindt, G. Schitter,
D E Morse,
P K Hansma
[show abstract]
[hide abstract]
ABSTRACT: The mechanical properties of healthy and diseased bone tissue were extensively studied in mechanical tests. Most of this research was motivated by the immense costs of health care and social impacts due to osteoporosis in post-menopausal women and the aged. Osteoporosis results in bone loss and change of trabecular architecture, causing a decrease in bone strength. To address the problem of assessing local failure behavior of bone, we combined mechanical compression testing of trabecular bone samples with high-speed photography. In this exploratory study, we investigated healthy, osteoarthritic, and osteoporotic human vertebral trabecular bone compressed at high strain rates. Apparent strains were found to transfer into to a broad range of local strains. Strained trabeculae were seen to whiten with increasing strain. Comparison of whitened regions seen in high-speed photography sequences with scanning electron micrographs showed that the observed whitening was due to the formation of microcracks. From the results of a motion energy filter applied to the recorded movies, we saw that the whitened areas are, presumably, also areas of high deformation. In summary, high-speed photography allows the detection of microdamage in real time, leading toward a better understanding of the local processes involved in bone failure.
