Combined Feedback and Adaptive Feedforward Control for Tracking Applications in Atomic Force Microscopes

An atomic force microscope (AFM) can provide images with resolution at the atomic scale. The quality and speed of AFM images depend upon the overall dynamics of the AFM system. The behavior of AFMs varies considerably, and currently, the variability causes commercial AFMs to behave unreliably, which slows down and frustrates users. Since the time required to attain a quality AFM image is typically on the order of several minutes or more, substantial motivation exists to reduce the imaging time in AFMs. This thesis discusses combining feedback control with feedforward control for improved speed and performance in tracking applications for AFMs. In particular, two model-inverse-based feedforward architectures are examined. Initial combined feedforward/feedback experimental results show improvement over the feedback-only tracking results, but, being model-inverse-based, these controllers struggle in the presence of system model variation and/or uncertainty. As a result, tracking performance degrades as trajectories vary from the conditions with which the model was identified. To correct for this, a dual-adaptive feedforward algorithm is introduced that adapts on parameters in two feedforward filters. This partnership of adaptive feedforward controllers improved experimental tracking results and robustness to model uncertainties resulting in repeatable high-performance and high-speed tracking throughout the range of the device

Improvement in the Imaging Performance of Atomic Force Microscopy: A Survey

Nanotechnology is the branch of science which deals with the manipulation of matters at an extremely high resolution down to the atomic level. In recent years, atomic force microscopy (AFM) has proven to be extremely versatile as an investigative tool in this field. The imaging performance of AFMs is hindered by: 1) the complex behavior of piezo materials, such as vibrations due to the lightly damped low-frequency resonant modes, inherent hysteresis, and creep nonlinearities; 2) the cross-coupling effect caused by the piezoelectric tube scanner (PTS); 3) the limited bandwidth of the probe; 4) the limitations of the conventional raster scanning method using a triangular reference signal; 5) the limited bandwidth of the proportional-integral controllers used in AFMs; 6) the offset, noise, and limited sensitivity of position sensors and photodetectors; and 7) the limited sampling rate of the AFM's measurement unit. Due to these limitations, an AFM has a high spatial but low temporal resolution, i.e., its imaging is slow, e.g., an image frame of a living cell takes up to 120 s, which means that rapid biological processes that occur in seconds cannot be studied using commercially available AFMs. There is a need to perform fast scans using an AFM with nanoscale accuracy. This paper presents a survey of the literature, presents an overview of a few emerging innovative solutions in AFM imaging, and proposes future research directions.This work was supported in part by the Australian Research Council (ARC) under Grant FL11010002 and Grant DP160101121 and the UNSW Canberra under a Rector's Visiting Fellowshi

A new scanning method for fast atomic force microscopy

In recent years, the atomic force microscope (AFM) has become an important tool in nanotechnology research. It was first conceived to generate 3-D images of conducting as well as nonconducting surfaces with a high degree of accuracy. Presently, it is also being used in applications that involve manipulation of material surfaces at a nanoscale. In this paper, we describe a new scanning method for fast atomic force microscopy. In this technique, the sample is scanned in a spiral pattern instead of the well-established raster pattern. A constant angular velocity spiral scan can be produced by applying single frequency cosine and sine signals with slowly varying amplitudes to the x-axis and y -axis of AFM nanopositioner, respectively. The use of single-frequency input signals allows the scanner to move at high speeds without exciting the mechanical resonance of the device. Alternatively, the frequency of the sinusoidal set points can be varied to maintain a constant linear velocity (CLV) while a spiral trajectory is being traced. Thus, producing a CLV spiral. These scan methods can be incorporated into most modern AFMs with minimal effort since they can be implemented in software using the existing hardware. Experimental results obtained by implementing the method on a commercial AFM indicate that high-quality images can be generated at scan frequencies well beyond the raster scans

System dynamics approach to user independence in high speed AFM

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 135-146).As progress in molecular biology and nanotechnology continues, demand for rapid and high quality image acquisition has increased to the point where the limitations of atomic force microscopes (AFM) become impediments to further discovery. Many biological processes of interest occur on time scales faster than the observation capability of conventional AFMs, which are typically limited to imaging rates on the order of minutes. Imaging at faster scan rates excite resonances in the mechanical scanner that can distort the image, thereby preventing higher speed imaging. Although traditional robust feedforward controllers and input shaping have proven effective at minimizing the influence of scanner distortions, the lack of direct measurement and use of model-based controllers has required disassembling the microscope to access lateral motion with external sensors in order to perform a full system identification experiment, which places excessive demands on routine microscope operators. This work represents a new way to characterize the lateral scanner dynamics without addition of lateral sensors, and shape the commanded input signals in such a way that disturbing dynamics are not excited in an automatic and user-independent manner. Scanner coupling between the lateral and out-of-plane directions is exploited and used to build a minimal model of the scanner that is also sufficient to describe the source of the disturbances. This model informs the design of an online input shaper used to suppress components of the high speed command signals. The method presented is distinct from alternate approaches in that neither an information-complete system identification experiment, nor microscope modification are required. This approach has enabled an increase in the scan rates of unmodified commercial AFMs from 1-4 lines/second to over 100 lines/second and has been successfully applied to a custom-built high speed AFM, unlocking scan rates of over 1,600 lines/second. Images from this high speed AFM have been taken at more than 10 frames/second. Additionally, bulky optical components for sensing cantilever deflection and low bandwidth actuators constrain the AFM's potential observations, and the increasing instrument complexity requires operators skilled in optical alignment and controller tuning. Recent progress in MEMS fabrication has allowed the development of a new type of AFM cantilever with an integrated sensor and actuator. Such a fully instrumented cantilever enables direct measurement and actuation of the cantilever motion and interaction with the sample, eliminating the need for microscope operators to align the bulky optical components. This technology is expected to not only allow for high speed imaging but also the miniaturization of AFMs and expand their use to new experimental environments. Based on the complexity of these integrated MEMS devices, a thorough understanding of their behavior and a specialized controls approach is needed to guide non-expert users in their operation and extract high performance. The intrinsic properties of such MEMS cantilevers are investigated, and a combined approach is developed for sensing and control, optimized for high speed detection and actuation.by Daniel J. Burns.Ph.D

Image processing techniques for high-speed atomic force microscopy

Atomic force microscopy (AFM) is a powerful tool for imaging topography or other characteristics of sample surfaces at nanometer-scale spatial resolution by recording the interaction of a sharp probe with the surface. Dispute its excellent spatial resolution, one of the enduring challenges in AFM imaging is its poor temporal resolution relative to the rate of dynamics in many systems of interest. This has led to a large research effort on the development of high-speed AFM (HS-AFM). Most of these efforts focus on mechanical improvement and control algorithm design. This dissertation investigates a complementary HS-AFM approach based on the idea of undersampling which aims at increasing the imaging rate of the instrument by reducing the number of pixels in the sample surface that need to be acquired to create a high-quality image. The first part of this work focuses on the reconstruction of images sub-sampled according to a scheme known as μ path patterns. These patterns consist of randomly placed short and disjoint scans and are designed specifically for fast, efficient, and consistent data acquisition in AFM. We compare compressive sensing (CS) reconstruction methods with inpainting methods on recovering μ-path undersampled images. The results illustrate that the reconstruction quality depends on the choice of reconstruction methods and the sample under study, with CS generally producing a superior result for samples with sparse frequency content and inpainting performing better for samples with information limited to low frequencies. Motivated by the comparison, a basis pursuit vertical variation (BPVV) method, combing CS and inpainting, is proposed. Based on single image reconstruction results, we also extend our analysis to the problem of multiple AFM frames, in which higher overall video reconstruction quality is achieved by pixel sharing among different frames. The second part of the thesis considers patterns for sub-sampling in AFM. The allocation of measurements plays an important role in producing accurate reconstructions of the sample surface. We analyze the expected image reconstruction error using a greedy CS algorithm of our design, termed simplified matching pursuit (SMP), and propose a Monte Carlo-based strategy to create μ-path patterns that minimize the expected error. Because these μ path patterns involve a collection of disjoint scan paths, they require the tip of the instrument to be repeatedly lifted from and re-engaged to the surface. In many cases, the re-engagements make up a significant portion of the total data acquisition time. We therefore extend our Monte Carlo design strategy to find continuous scan patterns that minimize the reconstruction error without requiring the tip to be lifted from the surface. For the final part of the work, we provide a hardware demonstration on a commercial AFM. We describe hardware implementation details and image a calibration grating using the proposed μ-path and continuous scan patterns. The sample surface is reconstructed from acquired data using CS and inpainting methods. The recovered image quality and achievable imaging rate are compared to full raster-scans of the sample. The experimental results show that the proposed scanning combining with reconstruction methods can produce higher image quality with less imaging time

Control of single- and dual-probe atomic force microscopy

“Atomic force microscope (AFM) is one of the important and versatile tools available in the field of nanotechnology. It is a type of probe-based microscopy wherein an atomically sharp tip, mounted on the free end of a microcantilever, probes the surface of interest to generate 3D topographical images with nanoscale resolution. An integral part of the AFM is the feedback controller that regulates the probe deflection in the presence of surface height changes, enabling the control action to be used for generating topographical image of the sample. Besides sensing, the probe can also be used as a mechanical actuator to manipulate nanoparticles and fabricate nanoscale structures. Despite its capabilities, AFM is not considered user-friendly because imaging is slow, and fabrication operations are laborious and often performed in open-loop, i.e. without any monitoring mechanism. This dissertation is composed of two journal articles which aim to address prominent AFM challenges using feedback control strategies. First article proposes a novel control design methodology based on repetitive control technique to accurately track AFM samples. Theoretical and experimental results demonstrate that incorporating a model of the general sample topography in the control design leads to superior tracking in AFM. Second article introduces a novel dual-probe AFM (DP-AFM) design that has two independent probes. Such a setup provides an opportunity to implement process control strategies where one probe can be used to perform one of the many AFM operations while the other probe can provide feedback by imaging the process. To demonstrate this capability, an application involving real-time plowing depth control where plow depth is controlled with nanometer-level accuracy is also presented”--Abstract, page iv

Nanopositionnement 3D à base de mesure à courant tunnel et piezo-actionnement

The objective of this thesis was to elaborate high performance control strategies and their real-time validation on a tunneling current-based 3D nanopositioning system developed in GIPSA-lab. The thesis lies in the domain of micro-/nano mechatronic systems (MEMS) focused on applications of fast and precise positioning and scanning tunneling microscopy (STM). More precisely, the aim is to position the metallic tunneling tip (like in STM) over the metallic surface using piezoelectric actuators in X, Y and Z directions and actuated micro-cantilever (like in Atomic Force Microscope AFM), electrostatically driven in Z direction, with high precision, over possibly high bandwidth. However, the presence of different adverse effects appearing at such small scale (e.g. measurement noise, nonlinearities of different nature, cross-couplings, vibrations) strongly affect the overall performance of the 3D system. Therefore a high performance control is needed. To that end, a novel 3D model of the system has been developed and appropriate control methods for such a system have been elaborated. First the focus is on horizontal X and Y directions. The nonlinear hysteresis and creep effects exhibited by piezoelectric actuators have been compensated and a comparison between different compensation methods is provided. Modern SISO and MIMO robust control methods are next used to reduce high frequency effects of piezo vibration and cross-couplings between X and Y axes. Next, the horizontal motion is combined with the vertical one (Z axis) with tunneling current and micro-cantilever control. Illustrative experimental results for 3D nanopositioning of tunneling tip, as well as simulation results for surface topography reconstruction and multi-mode cantilever positioning, are finally given.L'objectif de la thèse est l'élaboration de lois de commande de haute performance et leur validation en temps réel sur une plateforme expérimentale 3D de nano-positionnement à base de courant à effet tunnel, développée au laboratoire GIPSA-lab. Elle s'inscrit donc dans le cadre des systèmes micro-/nano-mécatronique (MEMS), et de la commande. Plus précisément, le principal enjeu considéré est de positionner la pointe métallique à effet tunnel (comme en microscopie à effet tunnel STM) contre la surface métallique en utilisant des actionneurs piézoélectriques en X, Y et Z et un micro-levier (comme en microscopie à force atomique AFM) actionné électrostatiquement en Z avec une grande précision et une bande passante élevée. Cependant, la présence de différents effets indésirables apparaissant à cette petite échelle (comme le bruit de mesure, des non-linéarités de natures différentes, les couplages, les vibrations) affectent fortement la performance globale du système 3D. En conséquence, une commande de haute performance est nécessaire. Pour cela, un nouveau modèle 3D du système a été développé et des méthodes de contrôle appropriées pour un tel système ont été élaborées. Tout d'abord l'accent est mis sur de positionnement selon les axes X et Y. Les effets d'hystérésis et de fluage non linéaires présents dans les actionneurs piézoélectriques ont été compensés et une comparaison entre les différentes méthodes de compensation est effectuée. Des techniques modernes de commande robuste SISO et MIMO sont ensuite utilisées pour réduire les effets des vibrations piézoélectriques et des couplages entre les axes X et Y. Le mouvement horizontal est alors combiné avec le mouvement vertical (Axe Z) et une commande du courant tunnel et du micro-levier. Des résultats expérimentaux illustrent le nano positionnement 3D de la pointe, et des résultats de simulation pour la reconstruction de la topographie de la surface ainsi que le positionnement du micro-levier à base d'un modèle multi-modes

Realisierung der Steuerungs-/Regelungsalgorithmen mittels FPGA für ein hochauflösendes und schnelles Rasterkraftmikroskop mit aktivem Cantilever

Die Entwicklungen des Rasterkraftmikroskops (AFM: atomic force microscope) betrafen alle seine Komponenten, angefangen von Kraftsensoren, Regelungstechniken, Materialien, und Ausrüstung bis zu den Betriebsmoden. Die meisten dieser Entwicklungen haben das Ziel, die Auflösung des AFM zu verbessern und seine Geschwindigkeit zu erhöhen. Meine Doktorarbeit versucht, zur Entwicklung der Rasterkraftmikroskopie dadurch beizutragen, dass neue Steuerungs- und Regelungsmethoden für das AFM-System mit dem selbstaktuierten piezoresistiven Cantilever (Aktiver Cantilever) als Kraftsensor entworfen und auf Basis der "Field Programmable Gate Array" (FPGA) implementiert werden. In dieser Arbeit wird die Performanz des AFM-Systems mit aktivem Cantilever in der "Geschwindigkeit-Auflösung-Ebene" verbessert. Dafür werden digitale Regelungs- und Steuerungsalgorithmen mit hohem Durchsatz für das AFM-System entworfen und auf FPGA implementiert. Eine Methode wird im Rahmen dieser Arbeit für die automatische Annäherung der Sonde in Richtung der Oberfläche der Probe in einer sehr schnellen und sicheren Art und Weise entwickelt. Die schnelle Annäherung führt zu Verbesserung der AFM-Produktivität besonders bei den Anwendungen, die eine Wiederholung des Annäherungsprozesses während der gleichen Sitzung erfordern (step and image). Rückkoppelungsregelung und Vorwärtsregelung auf Basis eines FPGA werden untersucht, entworfen und implementiert, um die Auswirkungen der Hysterese und Vibrationen des Scanners (Positioniersystem) zu kompensieren. Es wird gezeigt, dass sich durch die Normierung der Hysterese-Kurven die Komplexität des Hysterese-Modells und dadurch des inversen Modells der Hysterese stark reduzieren lässt. Ein neues alternatives Verfahren zur Charakterisierung der Hysterese mittels des AFM-Amplitudenbilds wird erläutert. Der Scanner wird als lineares System höherer Ordnung betrachtet und identifiziert. Ein digitaler Kompensator wird zum Unterdrücken der Scanner-Vibration entwickelt und im FPGA implementiert. Die Scan-Trajektorie in den XY-Richtungen hat einen signifikanten Einfluss auf die Wahl der Steuerungsarchitektur und die erreichbare Scan-Geschwindigkeit. Dafür werden verschiedene Methoden ("Input-Shaper", sinusförmiges und spirales Scannen) in dieser Arbeit implementiert. Zusätzlich werden eine nichtlineare Erfassungsmethode des AFM-Bildes und eine Phasenkorrektur verwendet, um die Verzerrung des AFM-Bildes aufgrund der nicht-linearen Scansignale zu vermeiden. In dieser Arbeit wird eine neue Struktur des digitalen Lock-In entwickelt, der die Amplitude und Phase der Cantilever-Schwingung sehr schnell ermitteln kann. Die Detektionszeit ist kleiner als eine Schwingungsperiode. Die Regelung für ein "Z-Scanner-lose-AFM" wird entworfen und auf FPGA implementiert. In diesem System wird der TMA (Thermomechanischer Aktuator) des aktiven Cantilevers anstatt des Z-Piezoaktuator verwendet, um die Topographie der gescannten Oberfläche zu verfolgen. Dieses Prinzip wird ausgenutzt, um ein AFM-System mit einem aktiven Cantilever-Array (4 Cantilever) zum parallelen Scannen einer großen Oberfläche (0.5mm x 0.2mm) zu entwickeln. Als effektive Scan-Geschwindigkeit werden 5,6 mm/s erreicht. Im Rahmen dieser Arbeit wird eine neuartige adaptive Variante zur Erhöhung der Scangeschwindigkeit entwickelt. Diese Methode verhindert ebenfalls effektiv das Sättigungsproblem, das beim Scannen der Oberflächen-Topographien mit hohem Aspekt-Verhältnis entsteht. Verglichen mit einem Standard-Regler ermöglicht der adaptive Regler eine deutlich höhere Stufenauflösung bei vergleichbaren Scanraten. Daher wird das Scannen vier- bis sechsmal schneller bei gleicher Abbildungsqualität. Beim Scannen mit einer kleinen Kraft gibt es sogar eine bis zu 17-fache Verbesserung.The developments relating to the atomic force microscope (AFM) involved all its components, from the force sensors, control techniques, materials and equipment to its operating modes. The majority of these developments have the aim of improving the resolution of the AFM and enhancing its speed. My doctoral thesis endeavours to contribute to the development of atomic force microscopy by designing new control methods for the AFM system with the self-actuating piezoresistive cantilever (active cantilever) as a force sensor and implementing them based on the “Field Programmable Gate Array” (FPGA). In this thesis, the performance of the AFM system with active cantilever is improved in the “speed-resolution plane”. Digital open-loop and closed-loop control algorithms with high throughput are designed for the AFM system and implemented on the FPGA. During the course of this doctoral thesis, a method is designed for the automatic approach of the probe towards the surface of the sample in a very fast and safe manner. A rapid approach leads to improved AFM productivity particularly with applications that the approach process to be repeated within the same session (step and image). The thesis studies, develops and implements open-loop and closed-loop control based on an FPGA to compensate for the effects of the hysteresis and vibrations of the scanner (positioning system). It shows that normalisation of the hysteresis curves allows the complexity of the hysteresis model and thus the inverse model of the hysteresis to be significantly reduced. A new alternative method for characterization of the hysteresis by means of the AFM amplitude image is explained. The scanner is viewed and identified as a higher-order linear system. A digital compensator is developed to suppress scanner vibration and is implemented in the FPGA. The scan trajectory in the XY direction has a significant influence on the choice of the control architecture and the achievable scan speed. Various methods (“Input Shaper“, sinusoidal and spiral scanning) are implemented in this thesis to achieve this. In addition, a non-linear method for recording the AFM image and phase correction are used to avoid the distortion of the AFM image due to the non-linear scan signals. This thesis develops a new digital lock-In structure, which can very rapidly detect the amplitude and phase of the cantilever oscillation. The detection time is less than one oscillation period. The control for a “Z scanner-less AFM” are developed and implemented on the FPGA. This system uses the TMA (Thermomechanical Actuator) of the active cantilever instead of the Z-piezo actuator to track the topography of the scanned surface. This principle is used to develop an AFM system with an active cantilever array (4 cantilevers) for the parallel scanning of a large surface (0.5 mm x 0.2 mm). 5.6 mm/s are arrived at as the effective scanning speed. This thesis also develops an innovative adaptive version for increasing scanning speed. This method also effectively prevents the saturation problem, which occurs when scanning the surface topographies with a high aspect ratio. Compared to a standard controller, the adaptive controller allows a much higher step resolution of at comparable scan rates. Therefore, the scanning becomes four to six times faster with the same image quality. When scanning with a small force, there is even an up to 17-fold improvement

Development of novel high-performance six-axis magnetically levitated instruments for nanoscale applications

This dissertation presents two novel 6-axis magnetic-levitation (maglev) stages that are capable of nanoscale positioning. These stages have very simple and compact structure that is advantageous to meet requirements in the next-generation nanomanufacturing. The 6-axis motion generation is accomplished by the minimum number of actuators and sensors. The first-generation maglev stage is capable of generating translation of 300 ??m in x, y and z, and rotation of 3 mrad about the three orthogonal axes. The stage demonstrates position resolution better than 5 nm rms and position noise less than 2 nm rms. It has a light moving-part mass of 0.2126 kg. The total power consumption by all the actuators is only around a watt. Experimental results show that the stage can carry, orient, and precisely position an additional payload as heavy as 0.3 kg. The second-generation maglev stage is capable of positioning at the resolution of a few nanometers over a planar travel range of several millimeters. A novel actuation scheme was developed for the compact design of this stage that enables 6-axis force generation with just 3permanent-magnet pieces. Electromagnetic forces were calculated and experimentally verified. The complete design and construction of the second-generation maglev stage was performed. All the mechanical part and assembly fixtures were designed and fabricated at the mechanical engineering machine shop. The single moving part is modeled as a pure mass due to the negligible effect of the magnetic spring and damping. Classical as well as advanced controllers were designed and implemented for closed-loop feedback control. A nonlinear model of the force was developed and applied to cancel the nonlinearity of the actuators over the large travel range. Various experiments were conducted to test positioning, loading, and vibration-isolation capabilities. This maglev stage has a moving-part mass of 0.267 kg. Its position resolution is 4 nm over a travel range of 5 ?? 5 mm in the x-y plane. Its actuators are designed to carry and precisely position an additional payload of 2 kg. Its potential applications include semiconductor manufacturing, micro-fabrication and assembly, nanoscale profiling, and nano-indentation