Abu Sebastian

High bandwidth nano-positioner: A robust control approach

This article presents the design, identification, and control of a nano-positioning device suited to image biological samples as part of an atomic force microscope. The device is actuated by a piezoelectric stack and its motion is sensed by a linear variable differential transformer. It is demonstrated that the conventional proportional-integral control architecture does not meet the bandwidth requirements for positioning. The design and implementation of an H∞ controller demonstrates substantial improvements in the positioning speed and precision, while eliminating the undesirable nonlinear effects of the actuator. The characterization of the resulting device in terms of bandwidth, resolution, and repeatability provided illustrates the effectiveness of the modern robust control paradigm.

Transient-signal-based sample-detection in atomic force microscopy

In typical dynamic mode operation of atomic force microscopes, steady state signals like amplitude and phase are used for detection and imaging of material. In these methods, the resolution and bandwidth are dictated by the quality factor (Q) of the cantilever. In this letter, we present a methodology that exploits the deflection signal during the transient of the cantilever motion. The principle overcomes the fundamental limitations on the trade off between resolution and bandwidth present in existing methods and makes it independent of the quality factor. Experimental results provided corroborate the theoretical development.

Robust control approach to atomic force microscopy

The imaging problem using an atomic force microscope (AFM) is addressed in the framework of modern robust control. Subsequently stacked /spl Hscr//sub /spl infin// and Glover McFarlane controllers are designed for high bandwidth and robustness. It is postulated that the sample profile can be accurately imaged without building explicit observers. Experimental results substantiate this claim.

Harmonic analysis based modeling of tapping-mode AFM

In this paper we use harmonic balance and averaging techniques to analyze the tapping mode dynamics of the atomic force microscope (AFM). A model for the cantilever sample interaction is developed. Experimental results show that the analysis and the model predict the behavior of the tapping cantilever.

Thermally driven non-contact atomic force microscopy

In this letter a thermally driven frequency modulated atomic force microscopy (FM-AFM) technique is developed. Thermal fluctuations of the cantilever are employed to estimate the cantilever’s equivalent resonant frequency. The corresponding cantilever oscillations are the smallest possible at a given temperature. Related experiments that establish the feasibility of thermally driven FM-AFM in ambient room conditions have achieved tip-sample separations less than 2nm with long term separation stability (>30min). Employing this method a narrowband 250Hz modulation of the tip-sample separation was detected with a vertical resolution of 0.25A in a 0.4Hz bandwidth. The corresponding estimated force sensitivity is 7 fN. In all experiments the cantilever tip was maintained in the attractive regime of the tip-sample interactions. This demonstrates a thermally driven non-contact mode operation of AFM. It also provides a limits of performance study of small amplitude FM-AFM methods.

H/sub /spl infin// loop shaping design for nano-positioning

This article presents the identification and control of a nano-positioning device. The device consists of two stages, which enable two-dimensional positioning. Each stage is actuated by piezo-electric stacks and its motion is sensed by a linear variable differential transformer (LVDT). A 2 /spl times/ 2 transfer function has been identified to describe the device. In this paper the main limitations to nano-positioning have been overcome through control. Feedback laws have been designed to address the undesirable effects of hysteresis and creep, which are significant in the open loop implementation, and to meet the steady state tracking and bandwidth requirements of nano-positioning. Great emphasis has been placed on robustness, which leads to a system that can withstand the diverse conditions where it will be used and does not necessitate tuning, as is the case with the existing designs. Accordingly, Glover-McFarlane H/sub /spl infin// loop shaping controllers have been employed to robustify existing non-model based designs. The merits of these designs along with the experimental results obtained by using them have been presented.

An observer based sample detection scheme for atomic force microscopy

In dynamic mode operation of atomic force microscopes steady state signals like amplitude and phase are typically used for the detection and imaging of sample. Due to the high quality factor of the micro-cantilever probe the corresponding methods are inherently slow. In this paper we present a novel methodology for fast interrogation of sample that exploits the transient signals. A novel method is introduced for the detection of small time scale tip-sample interactions. Simulations and experiments show that the method results in significant increase in bandwidth and resolution as compared to the steady state data based methods.

Design, identification and control of a fast nanopositioning device

This paper presents the design, identification and control of a nanopositioning device. The device is actuated by a piezoelectric stack and its motion is sensed by a linear variable differential transformer (LVDT). A fourth order single input single output model has been identified to describe its dynamics. It is demonstrated that PI control law does not meet the bandwidth requirements for positioning. This motivated the design and implementation of an H/sub /spl infin// controller which demonstrates substantial improvements in the positioning speed and precision besides eliminating the undesirable nonlinear effects of the actuator. The characterization of the device in terms of bandwidth, resolution, and repeatability is also shown.

Control of the nanopositioning devices

We present a paradigm which prescribes a procedure for a systematic design, analysis and development of nanopositioning devices. In this effort, we have used many tools from modern control theory to model devices, to quantify device resolution, bandwidth, range, and robustness, and to tackle undesirable nonlinear effects such as hysteresis and creep. The implementation of this procedure for the simultaneous achievement of robustness, high precision, and high bandwidth objectives is presented. Emphasis is placed on the robustness aspects that make the nanopositioner operable in diverse need for tuning that is present paradigm are demonstrated operating conditions thus alleviating the need for tuning that is present in existing designs. The merits of the through experimental results.

System tools applied to micro-cantilever based devices

Micro-cantilever based devices can be used to investigate and manipulate matter at atomic scales. Taking the case study of atomic force microscope (AFM) we demonstrate the power of system tools in the analysis of micro-cantilever based devices. They capture important characteristics and predict inherent limitations in the operation of these devices. Such a systems approach is shown to complement the physical studies performed on these devices. Tractable models are developed for the AFM operating in tapping-mode. For the interrogation of samples, it is also imperative that sample positioning should be done with high precision and at high speeds. This broadband nanopositioning problem is shown to fit into the modern robust control framework. This is illustrated by the design, identification and control of such a positioning device.