Ali Bazaei

High-speed Lissajous-scan atomic force microscopy: Scan pattern planning and control design issues

Tracking of triangular or sawtooth waveforms is a major difficulty for achieving high-speed operation in many scanning applications such as scanning probe microscopy. Such non-smooth waveforms contain high order harmonics of the scan frequency that can excite mechanical resonant modes of the positioning system, limiting the scan range and bandwidth. Hence, fast raster scanning often leads to image distortion. This paper proposes analysis and design methodologies for a nonlinear and smooth closed curve, known as Lissajous pattern, which allows much faster operations compared to the ordinary scan patterns. A simple closed-form measure is formulated for the image resolution of the Lissajous pattern. This enables us to systematically determine the scan parameters. Using internal model controllers (IMC), this non-raster scan method is implemented on a commercial atomic force microscope driven by a low resonance frequency positioning stage. To reduce the tracking errors due to actuator nonlinearities, higher order harmonic oscillators are included in the IMC controllers. This results in significant improvement compared to the traditional IMC method. It is shown that the proposed IMC controller achieves much better tracking performances compared to integral controllers when the noise rejection performances is a concern.

Tracking of Triangular References Using Signal Transformation for Control of a Novel AFM Scanner Stage

In this paper, we design feedback controllers for lateral and transversal axes of an atomic force microscope (AFM) piezoelectric tube scanner. The controllers are constrained to keep the standard deviation of the measurement noise fed back to the displacement output around 0.13 nm. It is shown that the incorporation of appropriate inner loops provides disturbance rejection capabilities and robustness against dc gain uncertainties, two requirements for satisfactory operation of signal transformation method. Simulations and experiments show significant improvement of steady-state tracking error with signal transformation, while limiting the projected measurement noise.

Design, Modeling, and Control of a Micromachined Nanopositioner With Integrated Electrothermal Actuation and Sensing

In this paper, a real-time feedback control of a novel micromachined one-degree-of-freedom thermal nanopositioner with on-chip electrothermal position sensors is presented. The actuation works based on thermal expansion of silicon beams. The sensing mechanism works based on measuring the difference between the electrical resistances of two electrically biased identical silicon beams. The difference increases with displacement, as the heat conductance of the sensor beams varies oppositely with position, resulting in different beam temperatures and resistances. The sensor pair is operated in differential mode to reduce low-frequency drift. The nanopositioner has a nonlinear static input-output characteristic. An open-loop controller is first designed and implemented. It is experimentally shown that uncertainties and sensor drift result in an unacceptable nanopositioner performance. Hence, feedback control methods are necessary for accurate nanopositioning. A closed-loop feedback control system is designed using a proportional-integral controller together with the nonlinear compensator used for the open-loop control system. The closed-loop system provides an acceptable and robust tracking performance for a wide range of set point values. For triangular reference tracking, which is needed in raster-scanned scanning probe microscopy, the tracking performance of the closed-loop system is further improved by incorporating a feedforward controller.

A Micromachined Nanopositioner With On-Chip Electrothermal Actuation and Sensing

This letter describes the design of a micromachined nanopositioner with thermal actuation and sensing capabilities in a single chip. The positioner has a dynamic range of 14.4 m, and the sensor drift is 8.9 nm over 2000 s with a differential sensing scheme. The on-chip displacement sensing enables a feedback control capability. A proportional-integral feedback controller is designed and implemented digitally. The closed-loop step response results show a positioning resolution of 7.9 nm and a time constant of 1.6 ms.

Video-Rate Lissajous-Scan Atomic Force Microscopy

Raster scanning is common in atomic force microscopy (AFM). The nonsmooth raster waveform contains high-frequency content that can excite mechanical resonances of an AFM nanopositioner during a fast scan, causing severe distortions in the resulting image. The mainstream approach to avoid scan-induced vibrations in video-rate AFM is to employ a high-bandwidth nanopositioner with the first lateral resonance frequency above 20 kHz. In this paper, video-rate scanning on a nanopositioner with 11.3-kHz resonance frequency is reported using a smooth Lissajous scan pattern. The Lissajous trajectory is constructed by tracking two sinusoidal waveforms on the lateral axes of the nanopositioner. By combining an analog integral resonant controller (IRC) with an internal model controller, 1- and 2-kHz single tone set-points were successfully tracked. High-quality time lapsed AFM images of a calibration grating recorded at 9 and 18 frames/s without noticeable image distortions are reported.

A High-Bandwidth MEMS Nanopositioner for On-Chip AFM: Design, Characterization, and Control

We report the design, characterization, and control of a high-bandwidth microelectromechanical systems (MEMS) nanopositioner for on-chip atomic force microscopy (AFM). For the fabrication, a commercially available process based on silicon-on-insulator is used. The device consists of a scan table, moved in the x-y plane by two sets of electrostatic comb actuators, capable of generating strokes in excess of ±5 μm. The first resonance frequencies of the nanopositioner are approximately 4.4 and 5.3 kHz in lateral directions. Electrothermal sensors are used to measure the displacement of the scan table. To enable fast scans, a dynamic model of the system is identified and used to design a feedback controller that damps the oscillatory behavior of the device. The nanopositioner is tested as the scanning stage of an AFM to perform high-speed scans.

Design and Analysis of Nonuniformly Shaped Heaters for Improved MEMS-Based Electrothermal Displacement Sensing

Conventional heaters used in microelectromechanical systems (MEMS) electrothermal displacement sensors typically feature a uniform cross section, which results in a nonuniform temperature profile. In this paper, electrothermal sensors with a shaped beam profile are introduced, with simulation results showing that a much flatter temperature distribution is achieved across the length of the heater. The proposed sensor design is implemented as the displacement sensor for a MEMS nanopositioner together with a more conventional electrothermal sensor design for comparative purposes. Experimental testing indicates that the shaped profile significantly improves upon the conventional sensor design in a number of areas, including sensitivity, linearity, and noise performance.

Analysis of Nonlinear Phenomena in a Thermal Micro-Actuator With a Built-In Thermal Position Sensor

An analysis of nonlinear effects associated with a chevron thermal micro-actuator with a built in thermal position sensor under static conditions is presented in this paper. The nonlinearities present in both actuator and sensor are studied. The phenomena considered for the sensor include: thermal coupling from actuator to sensor and temperature dependence of electrical resistivity. Those considered for the actuator include: non-uniform spatial distribution of temperature in arms, temperature dependency of thermal expansion coefficient, deviation of arm shape from straight line due to physical constraints, and temperature dependence of electrical resistivity.

Displacement Sensing With Silicon Flexures in MEMS Nanopositioners

We report a novel piezoresistive microelectromechanical system (MEMS) differential displacement sensing technique with a minimal footprint realized through a standard MEMS fabrication process, whereby no additional doping is required to build the piezoresistors. The design is based on configuring a pair of suspension beams attached to a movable stage so that they experience opposite axial forces when the stage moves. The resulting difference between the beam resistances is transduced into a sensor output voltage using a halfbridge readout circuit and differential amplifier. Compared with a single piezoresistive flexure sensor, the design approximately achieves 2, 22, and 200 times improvement in sensitivity, linearity, and resolution, respectively, with 1.5-nm resolution over a large travel range exceeding 12 μm.

Improved dynamic performance of wind energy conversion system by UPFC

There is a continuously growing demand for wind power generation capacity. This situation forces the revision of the grid codes requirements, to remain connected during grid faults, i.e., to ride through the faults, and contribute to system stability during fault condition. In a typical fault condition, the voltage at the Point of Common Coupling (PCC) drops below 80% immediately and the rotor speed of induction generators becomes unstable. In this paper, Unified Power Flow Controller (UPFC) is used to improve the low voltage ride- through (LVRT) of wind energy conversion system (WECS) and to damp the rotor speed oscillations of induction generator under fault conditions. By controlling the UPFC as a virtual inductor, we aims to increase the voltage at the terminals of the wind energy conversion system (WECS) and thereby mitigate the destabilizing electrical torque and power during the fault. The WECS is considered as a fixed-speed system, equipped with a squirrel-cage induction generator. The simulation results show that UPFC can improve the LVRT and rotor stability of the WECS.