O.M. El Rifai

Coupling in piezoelectric tube scanners used in scanning probe microscopes

A new model for tube scanners used in scanning probe microscopes (SPM), and particularly in atomic force microscopes (AFM), is presented. The model captures the coupling between motion in different axes as well as a bending motion due to a supposedly pure extension of the tube. In addition, the effect of coupling on the AFM cantilever dynamics is presented in a revised version of our model in El Rifai, and Youcef-Toumi, (2000). It is shown that due to coupling, the bending mode becomes observable from the AFM cantilever deflection sensor output. This is contrary to the ideal uncoupled case. Consequently, to avoid exciting the first bending mode, a large reduction in feedback bandwidth is required (a factor of 35 for the commercial AFM under consideration). As a result, ringing might occur during scanning at relatively low scan speeds, few Hertz, which will introduce artifacts in the image. Furthermore, in scanning a 4 /spl mu/m step, an estimated change of 12% will result in the steady state probe-sample force between the top and bottom of the step. This adversely affects the scanner calibration for large heights, adding to the nonlinear sensitivity of the piezoelectric material. The results presented within are supported by experimental data.

A Model-based Self-tuning Controller forThermoacoustic Instability

Abstract Active Control of thermoacoustic instabilities in continuous premixed combustion processes is being increasingly investigated for operating at lean low NOx conditions. Recently, we have developed a model-based approach for active control design which accounts for the underlying acoustics, heat release dynamics, and sensor and actuator dynamics. While this model captures a number of the dominant dynamic features of a premixed laminar combustor, there are a number of uncertainties associated with it as well. In this paper, we study the sensitivity of this model with respect to parametric uncertainties, and the efficacy of a fixed control design for suppressing pressure oscillations. We show that under certain conditions, the fixed controller is inadequate and present a self-tuning controller which is capable of delivering the desired performance in the presence of these uncertainties. The controller proposed is based on a rigorous analytical foundation, and is shown through simulation results to le...

Dynamics of contact-mode atomic force microscopes

The rapid development of the atomic force microscope (AFM) has been motivated by a continuous unfolding of new applications and fundamental research areas utilizing AFM technology. As a result, there is a demand for better understanding of its dynamics and control to cope up with application requirements. This work presents a dynamic model for an AFM including its scanner, cantilever, probe-sample interaction force, and a simple sliding friction model. A description of possible sources of disturbances and noises are also given. Using a PI controller, simulations for constant-speed contact-mode scanning are presented. Simulation results were capable of reproducing experimental observations which are also presented. The results demonstrate the high sensitivity of AFM images to scan parameters and illustrate some potential artifacts that can result if parameters are not properly selected.

Modeling and Control of AFM-based Nano-manipulation Systems

This paper develops a model and control scheme for nano-manipulation systems based on atomic force microscopes (AFM). The model includes the micro-cantilevers and piezotube actuators coupled dynamics. An identification-based controller is proposed for piezotube scanner positioning accounting for the piezotubes nonlinear sensitivity and axes coupling. A novel robust adaptive controller is developed to compensate for large parametric uncertainties including time varying and switching parameters due to probe-surface contacts as well as time varying and impulsive forces due to contact and impact. Discussions and simulations are presented for typical nano-manipulation tasks.

Design and control of atomic force microscopes

Models for two atomic force microscope (AFM) designs were presented, namely, cantilever-on-scanner and sample-on-scanner design. It was found that coupling between scanners bending and extension motion is present in both designs, making the scanner bending mode observable from the output. As a result, the ultimate feedback bandwidth is limited by the lower frequency bending mode in contrast to being ideally by the extension mode. Simulation and experimental data provided insight into pole-zero flipping and changes in the system dynamics as a function of force set point and input amplitude. Closed loop performance under integral and PID control were compared. It was found that PID control offers lower bandwidth over integral control if high-frequency roll-off and step response overshoot constraints were to be met. In addition, integral controller has a single gain compared 3 for PID, which makes it easier for parameter tuning by AFM users.

Dynamics of atomic force microscopes: experiments and simulations

This paper focuses on how AFM in-contact dynamics change with amplitude of disturbance, i.e. sample surface topography, and force set-point. The models presented by El Rifai et al. (2000, 2001) were used to compare simulations to experimental results. The models were able to reproduce experimental behavior observed in frequency response data and AFM images. Three main effects were observed, namely, changes in the DC gain, in modal damping, and in the pole-zero pattern. The results were analyzed and explanations were provided. These large changes in the dynamics of the AFM impose a challenge on its feedback system. It requires a controller that provides robust performance to ensure high fidelity of data and reduce image artifacts. Consequently, the selection of scan parameters should be tied-in to controller parameters to achieve potentially higher levels of performance.

On robust adaptive switched control

In this paper, an algorithm for robust adaptive control design for a class of single-input-single-output (SISO) switched linear systems is developed. The control scheme guarantees robust exponential stability with respect to any bounded parametric uncertainty and bounded disturbance without requiring a priori knowledge of such bounds. Furthermore, switched system stability is preserved independent of switching speed for plant/controller parameter switching. The problem reduces to an analysis of an exponentially stable system driven by piecewise continuous inputs due to plant and controller parameter switching. This system is a modification of the closed loop error dynamics in standard adaptive control systems, through modifying the adaptation law. The results are illustrated for model reference adaptive control of a SISO linear plant.

On automating atomic force microscopes: an adaptive control approach

The atomic force microscope (AFM) requires the user to manually tune controller gains and scan parameters in a trial and error fashion for different sample cantilever combinations. In this paper, steps towards automating this process are presented. Modeling and experimental results are shown revealing the structure of AFM dynamics and how it is impacted by different choices of scan and controller parameters. A robust adaptive controller is designed to address these issues. The performance of the designed adaptive controller is verified by simulating scanning experiments. The adaptive controller eliminates the need for the user to manually tune controller gains for different sample cantilever combinations and compensates for uncertainties arising from the user choice of different scan parameters. In addition, a substantial reduction in contact force and retrace line setpoint error can be achieved with the adaptive controller in comparison with a well-tuned integral controller.

In-contact dynamics of atomic force microscopes

Demonstrates and explains the great impact scan parameters and cantilever properties have on the dynamics of atomic force microscopes (AFM), and hence its performance. Results show that when operating in air and in-contact with hard samples, modal damping is a strong function of contact force set-point and amplitude of disturbance, i.e. sample surface topography. Small amplitudes and large set-points result in lower damping. In addition, a large contact force can result in damage to the sample and increase friction force between probe and sample. Further, cantilevers with long probes result in pole-zero flipping possibly due to the compliance of the probe being comparable to that of the cantilever. These large changes in the dynamics of the AFM impose a challenge on its feedback system. It requires a controller that provides robust performance to ensure high-fidelity of data and reduce image artifacts. Consequently, the selection of scan parameters should be tied-in to controller parameters to potentially achieve higher level of performance.

On factors affecting the performance of atomic force microscopes in contact-mode

The importance of the atomic force microscope (AFM), in nano-technology, demands understanding of factors affecting AFM performance and image quality. Through a simulation study, the impact of scanning parameters on AFM images acquired in contact-mode has been investigated. The performance and limitations of commercial AFM controllers was also discussed. Based on the generated images, it is seen that high speed scans may require a large force set-point to maintain probe-sample contact at the expense of higher surface deformation, contact area, and friction force. Hence, reducing resolution and increasing image distortion. The sensitivity of AFM images to scan and controller parameters, invites the development of a systematic procedure for parameter selection for a given sample and operating environment. This would greatly reduce the current guess work in parameter selection and improve image repeatability. Currently, commercial control systems do limit the capabilities of the AFM in terms of speed and data repeatability. A good controller should be able to compensate for the large uncertainty due to different cantilevers and samples that may be used, nonlinearities, and system resonances. In addition, it should provide a tight tolerance on sample surface tracking both during transient and at steady state operation. This would enable robust high-speed operation and allow small or negative force set-point to be used.