Computer Controlled Systems: Theory and Design

Pull-in instability as an inherently nonlinear and crucial effect continues to become increasingly important for the design of electrostatic MEMS and NEMS devices and ever more interesting scientifically. This review reports not only the overview of the pull-in phenomenon in electrostatically actuated MEMS and NEMS devices, but also the physical principles that have enabled fundamental insights into the pull-in instability as well as pull-in induced failures. Pull-in governing equations and conditions to characterize and predict the static, dynamic and resonant pull-in behaviors are summarized. Specifically, we have described and discussed on various state-of-the-art approaches for extending the travel range, controlling the pull-in instability and further enhancing the performance of MEMS and NEMS devices with electrostatic actuation and sensing. A number of recent activities and achievements methods for control of torsional electrostatic micromirrors are introduced. The on-going development in pull-in applications that are being used to develop a fundamental understanding of pull-in instability from negative to positive influences is included and highlighted. Future research trends and challenges are further outlined.

Electrostatic pull-in instability in MEMS/NEMS: A review

In this paper, adaptive control is presented for a class of single-degree-of-freedom (1DOF) electrostatic microactuator systems which can be actively driven bidirectionally. The control objective is to track a reference trajectory within the air gap without knowledge of the plant parameters. Both full-state feedback and output feedback schemes are developed, the latter being motivated by practical difficulties in measuring velocity of the moving plate. For the full-state feedback scheme, the system is transformed to the parametric strict feedback form, for which adaptive backstepping is performed to achieve asymptotic output tracking. Analogously, the output feedback design involved transformation to the parametric output feedback form, followed by the use of adaptive observer backstepping to achieve asymptotic output tracking. To prevent contact between the movable and fixed electrodes, special barrier functions are employed in Lyapunov synthesis. All closed-loop signals are ensured to be bounded. Extensive simulation studies illustrate the performance of the proposed control.

Adaptive Control of Electrostatic Microactuators With Bidirectional Drive

The feedforward compensation of nonlinearities, i.e., hysteresis and creep, and unwanted vibrations in micromanipulators is presented in this paper. The aim is to improve the general performances of piezocantilevers dedicated to micromanipulation/microassembly tasks. While hysteresis is attenuated using the Prandtl-Ishlinskii inverse model, a new method is proposed to decrease the creep phenomenon. As no model inversion is used, the proposed method is simple and easy to implement. Finally, we employ an input shaping technique to reduce the vibration of the piezocantilevers. The experimental results show the efficiency of the feedforward techniques and their convenience to the micromanipulation/microassembly requirements.

https://hal.archives-ouvertes.fr/hal-00436606/document

Complete Open Loop Control of Hysteretic, Creeped, and Oscillating Piezoelectric Cantilevers

From a controls point of view, micro electromechanical systems (MEMS) can be driven in an open-loop and closed-loop fashion. Commonly, these devices are driven open-loop by applying simple input signals. If these input signals become more complex by being derived from the system dynamics, we call such control techniques pre-shaped open-loop driving. The ultimate step for improving precision and speed of response is the introduction of feedback, e.g. closed-loop control. Unlike macro mechanical systems, where the implementation of the feedback is relatively simple, in the MEMS case the feedback design is quite problematic, due to the limited availability of sensor data, the presence of sensor dynamics and noise, and the typically fast actuator dynamics. Furthermore, a performance comparison between open-loop and closed-loop control strategies has not been properly explored for MEMS devices. The purpose of this paper is to present experimental results obtained using both open- and closed-loop strategies and to address the comparative issues of driving and control for MEMS devices. An optical MEMS switching device is used for this study. Based on these experimental results, as well as computer simulations, we point out advantages and disadvantages of the different control strategies, address the problems that distinguish MEMS driving systems from their macro counterparts, and discuss criteria to choose a suitable control driving strategy.

https://nocweba.ntu.edu.sg/laq_mems/journal papers/2005/open-loop versus closed-loop control of mem devices choices and issues.pdf

Open-loop versus closed-loop control of MEMS devices: choices and issues

An analysis is performed of a MEMS optical switch to determine a dynamical model containing electrical, mechanical, and optical components. The model is compared to experimental results and shows a good match, aside from the damping term which is difficult to analyze. Improved parameters are determined by system identification from the actual experimental data. The improved model is used to design a controller that has two parts, a feedforward part and a feedback part. The feedforward portion, not normalized in MEMS control, is shown to be instrumental in obtaining fast optical switching with minimal overshoot.

Control of a MEMS optical switch

Comb drives inherently suffer from electromechanical instability called lateral pull-in, side pull-in or, sometimes, lateral instability. Although fabricated to be perfectly symmetrical, the actuator’s comb structure is always unbalanced, causing adjacent finger electrodes to contact each other when voltage–deflection conditions are favorable. Lateral instability decreases the active traveling range of the actuator, and the problem is typically approached by improving the mechanical design of the suspension. In this paper, a novel approach to counteracting the pull-in phenomenon is proposed. It is shown that the pull-in problem can be successfully counteracted by introducing active feedback steering of the lateral motion. In order to do this, however, the actuator must be controllable in the lateral direction, and lateral deflection measurements need to be available. It is shown herein how to accomplish this. The experimentally verified dynamic model of the comb drive is extended with a lateral motion model. The lateral part of the model is verified through experimental results and finite element analysis and is hypothetically extended to accommodate both sensor and actuator functionalities for lateral movement. A set of simulations is performed to illustrate the improved traveling range gained by the controller.

https://nocweba.ntu.edu.sg/laq_mems/journal%20papers/2006/The%20lateral%20instability%20problem%20in%20electrostatic%20comb%20drive%20actuators.pdf

The lateral instability problem in electrostatic comb drive actuators: modeling and feedback control

A method is presented for determining lumped dynamical models of thermal microelectromechanical systems (MEMS) devices for purposes of feedback control. As a case study, an electrothermal actuator is used. The physical properties and a set of assumptions are used to determine the basic structure of the dynamical model, which requires the development of the electrical, thermal, and mechanical dynamics. The importance of temperature-dependent parameters is emphasized for dynamical modeling for purposes of feedback control. To confront temperature dependence in a practical yet effective manner, an average temperature is introduced to preserve the energy balance inside the structure. This allows the development of a practical method that combines structure of the model, through the average body temperature, with finite element analysis (FEA) in novel way to perform system identification and identify the unknown parameters. The result is a lumped dynamical model of a MEMS device that can be used for the design of feedback control systems. We compare computer simulated results using the dynamical model with experimental behavior of the actual device to show that our procedure indeed generates an accurate model. This dynamical model is intended for further synthesis of driving signal and control system but also gives a qualitative insight into the relationship between device's geometry and its behavior. The method enables fast development of the model by conducting relatively few static FEA and is verifiable with dynamic experimental results even when temperature measurements are not available. $hfillhbox[1406]$

Method for Determining a Dynamical State&#8211;Space Model for Control of Thermal MEMS Devices

This paper highlights some microelectromechanical systems (MEMS) control issues and provides an overview of MEMS control.

/pdf/control-issues-for-microelectromechanical-systems-3fgttr6emb.pdf

B. Borovic

Papers

Open-loop versus closed-loop control of MEMS devices: choices and issues

Control of a MEMS optical switch

The lateral instability problem in electrostatic comb drive actuators: modeling and feedback control

Method for Determining a Dynamical State–Space Model for Control of Thermal MEMS Devices

Control issues for microelectromechanical systems

B. Borovic

Papers

Open-loop versus closed-loop control of MEMS devices: choices and issues

Control of a MEMS optical switch

The lateral instability problem in electrostatic comb drive actuators: modeling and feedback control

Method for Determining a Dynamical State&#8211;Space Model for Control of Thermal MEMS Devices

Control issues for microelectromechanical systems

Method for Determining a Dynamical State–Space Model for Control of Thermal MEMS Devices