Sumeet S. Aphale

Design, Identification, and Control of a Flexure-Based XY Stage for Fast Nanoscale Positioning

The design, identification, and control of a novel, flexure-based, piezoelectric stack-actuated XY nanopositioning stage are presented in this paper. The main goal of the design is to combine the ability to scan over a relatively large range (25times25 mum) with high scanning speed. Consequently, the stage is designed to have its first dominant mode at 2.7 kHz. Cross-coupling between the two axes is kept to -35 dB, low enough to utilize single-input--single-output control strategies for tracking. Finite-element analysis (FEA) is used during the design process to analyze the mechanical resonance frequencies, travel range, and cross-coupling between the X- and Y-axes of the stage. Nonlinearities such as hysteresis are present in such stages. These effects, which exist due to the use of piezoelectric stacks for actuation, are minimized using charge actuation. The integral resonant control method is applied in conjunction with feedforward inversion technique to achieve high-speed and accurate scanning performances, up to 400 Hz.

Integral resonant control of collocated smart structures

This paper introduces integral resonant control, IRC, a simple, robust and well-performing technique for vibration control in smart structures with collocated sensors and actuators. By adding a direct feed-through to a collocated system, the transfer function can be modified from containing resonant poles followed by interlaced zeros, to zeros followed by interlaced poles. It is shown that this modification permits the direct application of integral feedback and results in good performance and stability margins. By slightly increasing the controller complexity from first to second order, low-frequency gain can be curtailed, alleviating problems due to unnecessarily high controller gain below the first mode. Experimental application to a piezoelectric laminate cantilever beam demonstrates up to 24 dB modal amplitude reduction over the first eight modes.

A New Method for Robust Damping and Tracking Control of Scanning Probe Microscope Positioning Stages

This paper demonstrates a simple second-order controller that eliminates scan-induced oscillation and provides integral tracking action. The controller can be retrofitted to any scanning probe microscope with position sensors by implementing a simple digital controller or operational amplifier circuit. The controller is demonstrated to improve the tracking bandwidth of an NT-MDT scanning probe microscope from 15 Hz (with an integral controller) to 490 Hz while simultaneously improving gain-margin from 2 to 7 dB. The penalty on sensor induced positioning noise is minimal. A unique benefit of the proposed control scheme is the performance and stability robustness with respect to variations in resonance frequency. This is demonstrated experimentally by a change in resonance frequency from 934 to 140 Hz. This change does not compromise stability or significantly degrade performance. For the scanning probe microscope considered in this paper, the noise is marginally increased from 0.30 to 0.39 nm rms. Open and closed-loop experimental images of a calibration standard are reported at speeds of 1, 10, and 31 lines per second (with a scanner resonance frequency of 290 Hz). Compared with traditional integral controllers, the proposed controller provides a bandwidth improvement of greater than 10 times. This allows faster imaging and less tracking lag at low speeds.

Minimizing Scanning Errors in Piezoelectric Stack-Actuated Nanopositioning Platforms

Piezoelectric stack-actuated parallel-kinematic nanopositioning platforms are widely used in nanopositioning applications. These platforms have a dominant first resonant mode at relatively low frequencies, typically in the hundreds of hertz. Furthermore, piezoelectric stacks used for actuation have inherent nonlinearities such as hysteresis and creep. These problems result in a typically low-grade positioning performance. Closed-loop control algorithms have shown the potential to eliminate these problems and achieve robust, repeatable nanopositioning. Using closed-loop noise profile as a performance criterion, three commonly used damping controllers, positive position feedback, polynomial-based pole placement, and resonant control are compared for their suitability in nanopositioning applications. The polynomial-based pole placement controller is chosen as the most suitable of the three. Consequently, the polynomial-based control design to damp the resonant mode of the platform is combined with an integrator to produce raster scans of large areas. A scanning resolution of approximately 8 nm, over a 100 mum times 100 mum area is achieved.

Integral Resonant Control for Vibration Damping and Precise Tip-Positioning of a Single-Link Flexible Manipulator

In this paper, we propose a control design method for single-link flexible manipulators. The proposed technique is based on the integral resonant control (IRC) scheme. The controller consists of two nested feedback loops. The inner loop controls the joint angle and makes the system robust to joint friction. The outer loop, which is based on the IRC technique, damps the vibration and makes the system robust to the unmodeled dynamics (spill-over) and resonance frequency variations due to changes in the payload. The objectives of this work are: 1) to demonstrate the advantages of IRC, which is a high-performance controller design methodology for flexible structures with collocated actuator-sensor pairs and 2) to illustrate its capability of achieving precise end-point (tip) positioning with effective vibration suppression when applied to a typical flexible manipulator. The theoretical formulation of the proposed control scheme, a detailed stability analysis and experimental results obtained on a flexible manipulator are presented.

A Survey of Modeling and Control Techniques for Micro- and Nanoelectromechanical Systems

In the current times, microelectromechanical systems and nanoelectromechanical systems form a major interdisciplinary area of research involving science, engineering, and technology. A lot of work has been reported in the area of modeling and control of these devices, with the aim of better understanding their behavior and improving their performance. This paper presents a review of the emerging advances in the modeling and control of these micro- and nanoscale devices and converges on the exciting research in on-chip control , with a mechatronics and controls perspective and concludes by projecting future trends.

A mathematical approach to Integral Resonant Control of second-order systems

Systems with colocated sensor-actuator pairs exhibit an interesting property of pole-zero interlacing. Integral Resonant Control (IRC) exploits this property to result in superior damping performance over multiple resonant modes by prescribing an adequate feed-through term to swap the pole-zero interlacing to a zero-pole one - thus enabling a simple integral feedback controller to add substantial damping to the system. Over the past few years, the IRC has proved extremely popular and versatile and has been applied to damp the resonance in a variety of systems. So far, a simulation-based manual search has been used to determine the three main parameters of the IRC scheme namely: (i) feed-through term, d, (ii) integral gain, k and (iii) resulting damping, ζ. In this paper, a full quantification of the effect of feed-through term on second-order colocated systems as well as a mathematical formulation for the relation between the feed-through term, integral gain and achievable damping are presented. These results add to the current understanding regarding the behaviour of colocated systems and facilitate the IRC design for a specified damping.Systems with colocated sensor-actuator pairs exhibit the interesting property of pole-zero interlacing. Integral resonance control (IRC) exploits this property by changing the pole-zero interlacing to zero-pole interlacing. The unique phase response of this class of systems enables a simple integral feedback controller to add substantial damping. Over the past few years, IRC has proven to be extremely versatile and has been applied to a wide variety of systems whose dominating dynamics of interest can be accurately modeled by second-order transfer functions. To date, a manual approach has been employed to determine the parameters of the IRC scheme, namely the feed-through term and the integral gain. In this paper, the relationship between the feed-through term, integral gain, and achievable damping is derived analytically for undamped/lightly damped second-order systems. The relationship between damping controller and an outer servo loop is also derived. These results add to the current understanding of colocated systems and automate the design of IRC controllers with a specified damping and tracking bandwidth. The presented results are applied to design and implement a damping and tracking controller for a piezoelectric nanopositioning stage.

Butterworth Pattern-based Simultaneous Damping and Tracking Controller Designs for Nanopositioning Systems

The Butterworth filter is known to have maximally flat response. Incidentally, the same response is desired in precise positioning systems. This paper presents a method for obtaining a closed-loop Butterworth filter pattern using common control schemes for positioning applications, i.e. Integral Resonant Control (IRC), Positive Position Feedback (PPF), and Positive Velocity and Position Feedback (PVPF). Simulations show a significant increase in bandwidth over traditional design methods and verify the desired pole placement is achieved. Experiments are performed using a two-axis serial kinematic nanopositioning stage. The results show a significant improvement in bandwidth and increased positioning accuracy, specifically at the turn-around point. This allows a greater portion of the scan to be used and improved positioning accuracy at high scanning speeds.

Optimal Integral Force Feedback and Structured PI Tracking Control: Application for High Speed Confocal Microscopy.

Abstract In this paper, an improvement to Integral Force Feedback (IFF) damping control is proposed. The main limitation of Integral Force Feedback is that the maximum modal damping depends on the systems parameters. Hence, for some system achievable damping is insignificant. The proposed improvement allows any arbitrary damping ratio to be achieved for a system by introducing a new feed-through term in the system. To achieve displacement tracking, one technique is to enclose the system in an integral feedback loop. However, the bandwidth is limited due to the effects of an additional pole in the damping loop. The proposed Structured PI controller is parameterised so that it contains a zero that cancel the additional pole. Experiment was conducted on a commercial objective lens positioner. The results show an exceptional tracking and damping performance and the systems insensitivity to changes in resonance frequency. The maximum bandwidth achievable with a commercial PID controller is 26.1 Hz. In contrast, with the proposed method, the bandwidth is increased to 255 Hz.

A new robust damping and tracking controller for SPM positioning stages

This paper demonstrates a simple second-order controller that eliminates scan-induced oscillation and provides integral tracking action. The controller can be retrofitted to any scanning probe microscope with position sensors by implementing a simple digital controller or op-amp circuit. The controller is demonstrated to improve the tracking bandwidth of an NT-MDT scanning probe microscope from 15 Hz (with an integral controller) to 490 Hz while simultaneously improving gain-margin from 2 dB to 7 dB. The penalty on sensor induced positioning noise is minimal. For the Scanning Probe Microscope considered in this paper, the noise is marginally increased from 0.30 nm RMS to 0.39 nm RMS. Open- and closed-loop experimental images of a calibration standard are reported at speeds of 1 and 10 lines per second (with a scanner resonance frequency of 290 Hz). Compared to traditional integral or PID controllers, the proposed controller provides a bandwidth improvement of approximately ten times. This allows faster imaging and less tracking lag at low speeds.