Moheimani, S. O. Reza

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Reza Moheimani holds the James Von Ehr Distinguished Chair in Science and Technology and is Professor of Mechanical Engineering and Computer Science. "An important aspect of Dr. Moheimani’s research is concerned with microelectromechanical systems, also known as MEMS or micro-machines." His other research interests include:

  • Control Systems
  • Mechatronics
  • Nanotechnology

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Recent Submissions

Now showing 1 - 5 of 5
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    Scanning Tunneling Microscope Control: A Self-Tuning PI Controller Based on Online Local Barrier Height Estimation
    (Institute of Electrical and Electronics Engineers Inc.) Tajaddodianfar, F.; Moheimani, S. O. Reza; Randall, J. N.; 0000-0002-1225-4126 (Moheimani, SOR); 298210 (Moheimani, SOR); Moheimani, S. O. Reza
    We identify the dynamics of a scanning tunneling microscope (STM) in closed loop and show that the plant dc gain is proportional to the square root of local barrier height (LBH), a quantum mechanical property of the sample and/or tip that affects the tunneling current. We demonstrate that during a scan, the LBH may undergo significant variations and this can adversely affect the closed-loop stability if the controller parameters remain fixed. Feedback instabilities increase the risk of tip-sample crash in STMs. In order to improve the closed-loop performance, we estimate the LBH, on the fly, and use that to adaptively tune the proportional-integral (PI) controller parameters. Experimental results obtained with the self-tuning PI controller confirm the improved STM performance compared to the conventional fixed-gain PI controller. Additional experiments confirm effectiveness of the proposed method in extending the tip lifetime by lowering the chance of a tip/sample crash. ©2018 IEEE
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    Internal Model Control of Cycloid Trajectory for Video-Rate AFM Imaging with a SOI-MEMS Nanopositioner
    (Institute of Electrical and Electronics Engineers Inc.) Alipour, Afshin; Nikooienejad, Nikooienejad; Maroufi, Mohammad; Moheimani, S. O. Reza; 0000-0002-1225-4126 (Moheimani, SOR); 298210 (Moheimani, SOR); Alipour, Afshin; Nikooienejad, Nikooienejad; Maroufi, Mohammad; Moheimani, S. O. Reza
    We demonstrate high-performance tracking of a cycloid trajectory for video-rate atomic force microscopy imaging by employing internal model control. To acquire sequential images using cycloid scanning, the stage needs to follow a slow periodic triangular wave superimposed on a sinusoidal signal along one axis with the remaining axis tracking a purely sinusoidal signal. The sharp turnarounds in the triangular signal result in a large tracking error. We utilize a trapezoidal signal to address this issue. To obtain high-precision positioning, the controller comprises the internal model of harmonic waveforms and the ramp signal plus additional integrator to compensate for stage nonlinearities. The controller is implemented on a two degree of freedom microelectromechanical system nanopositioner and operated at scan frequencies ranging from 500 Hz to 2580 Hz in a window size of 5 µm by 10 µm. While the pitch size of the trajectory is set to be 46 nm, the RMS value of tracking error remains below 7 nm. The highest scan rate of 20 frames per second is achieved at f =2580 Hz with the maximum transient tracking error of 15 nm.
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    An Adjustable-Stiffness MEMS Force Sensor: Design, Characterization, and Control
    (Elsevier Ltd. All Rights Reserved.) Maroufi, Mohammad; Alemansour, Hamed; Bulut Coskun, M.; Reza Moheimani, S. O.; Maroufi, Mohammad; Alemansour, Hamed; Bulut Coskun, M.; Reza Moheimani, S. O.
    This paper presents a novel one-degree-of-freedom microelectromechanical systems (MEMS) force sensor. The high-bandwidth device contains on-chip sensing and actuation mechanisms, enabling open- and closed-loop modalities. An active compliance mechanism is incorporated to render the device more conducive to characterization of soft samples. When operated in closed loop, the adjustable stiffness enables the sensor to attain a larger dynamic range and minimize the nonlinearities originating from flexures. Analytical models are employed to design and calibrate the sensor. In open loop, the sensing resolution of 23.3 nN within a bandwidth of 2.35 kHz and a full-scale range of ± 42.6 µ N are experimentally obtained. The resolution is enhanced to 9.3 nN by employing an active compliance mechanism. When operated in closed loop, a resolution of 12.9 nN is achieved within a dynamic range of 71.2 dB and a sensing bandwidth of 3.6 kHz is demonstrated. The sensor performance is tested by obtaining the stiffness of an atomic force microscope probe and measuring the force produced by a self-actuated piezoelectric microcantilever.
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    Q Control of an Active AFM Cantilever with Differential Sensing Configuration
    (Institute of Electrical and Electronics Engineers Inc.) Coskun, M. Bulut; Alemansour, Hamed; Fowler, Anthony G.; Maroufi, Mohammad; Moheimani, S. O. Reza; 0000-0001-5349-6319 (Coskun, MB); 0000-0002-5219-5781 (Fowler, AG); 0000-0001-7127-5026 (Maroufi, M); 0000-0002-1225-4126 (Moheimani, SOR); 298210 (Moheimani, SOR); Coskun, M. Bulut; Alemansour, Hamed; Fowler, Anthony G.; Maroufi, Mohammad; Moheimani, S. O. Reza
    Microcantilevers featuring separate built-in actuation and displacement sensing capabilities allow effective and simple implementation of control methods, opening a pathway to achieving higher scan speeds in tapping-mode atomic force microscopy. Such active cantilevers are a significant milestone to eventually obtain video-rate on-chip atomic force microscopes (AFMs) that can even surpass the functionality and imaging speed of their macroscale counterparts at a significantly lower cost. In this brief, we present an active AFM cantilever with an on-chip actuator and two built-in displacement sensors, designed to be integrated into on-chip AFMs. The common feedthrough problem present in this type of architecture is addressed by a differential sensing configuration, and the revealed dynamics are used for the system identification. A positive position feedback controller is designed to actively tailor the Q factor of the cantilever. The imaging performance of the microcantilever with and without Q control is compared by attenuating the cantilever's Q factor from 177 to 15 using the feedback loop. A common artifact in high-speed scans, the parachuting effect, is mitigated, rendering higher imaging speeds achievable.
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    On the Effect of Local Barrier Height in Scanning Tunneling Microscopy: Measurement Methods and Control Implications
    (American Institute of Physics Inc, 2018-10-24) Tajaddodianfar, Farid; Moheimani, S. O. Reza; Owen, J.; Randall, J. N.; 0000-0002-6135-1993 (Tajaddodianfar, F); Tajaddodianfar, Farid; Moheimani, S. O. Reza
    A common cause of tip-sample crashes in a Scanning Tunneling Microscope (STM) operating in constant current mode is the poor performance of its feedback control system. We show that there is a direct link between the Local Barrier Height (LBH) and robustness of the feedback control loop. A method known as the "gap modulation method" was proposed in the early STM studies for estimating the LBH. We show that the obtained measurements are affected by controller parameters and propose an alternative method which we prove to produce LBH measurements independent of the controller dynamics. We use the obtained LBH estimation to continuously update the gains of a STM proportional-integral (PI) controller and show that while tuning the PI gains, the closed-loop system tolerates larger variations of LBH without experiencing instability. We report experimental results, conducted on two STM scanners, to establish the efficiency of the proposed PI tuning approach. Improved feedback stability is believed to help in avoiding the tip/sample crash in STMs.

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