Bruno Borovic

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

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.

Control of a MEMS optical switch

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.

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

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.

Photonic MEMS devices : design, fabrication and control

MEMS Optical Switches and Systems J. Li and A. Q. Liu Design of MEMS Optical Switches J. Li and A. Q. Liu MEMS Thermo-Optic Switches J. Li and T. Zhong PHC Microresonators Dynamic Modulation Devices S. T. Hwee Gee and A.Q. Liu MEMS Variable Optical Attenuators X. Zhang, H. Cai, and A.Q. Liu MEMS Discretely Tunable Lasers X. Zhang and H. Cai MEMS Continuously Tunable Lasers X. Zhang and A. Q. Liu MEMS Injection-Locked Lasers X. Zhang and A. Q. Liu Deep Etching Fabrication Process J. Li and A. Q. Liu Deep Submicron Photonic Bandgap Crystal Fabrication Processes S. H. G. Teo and A. Q. Liu Control Strategies for Electrostatic MEMS Devices B. Borovic and F. L. Lewis Control of Optical Devices B. Borovic and F. L. Lewis

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

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 devices 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.

Open vs. Closed-Loop Control of the MEMS Electrostatic Comb Drive

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SCADA in hydropower plants

The purpose of this paper is to address the issue of driving the very small scale (1.5 times 1 mm) comb drive (actuator). Two approaches are available, input signal shaping approach that drives the comb drive in open loop, and a closed loop approach. Position is measured by optical sensor. We compare the two approaches using both experimental and simulation results. First, the description, model, and experimental setup of the actual device are given. It is followed by simple parameter identification. Input shaping open-loop techniques are addressed first. It is followed by closed-loop approach. Finally, the comparison between two approaches, sensitivity issues, and reflections on the design of the device itself are given

Behaviour-Based Vision-Guided Teleoperated Mems Probestation

This paper presents the development of a dynamical model of a MEMS based optical switch. The model is verified using experimental results. An inherent weakness in the use of the model for control design is the unavailability of position information at all times due to the saturated output characteristics. To counteract this problem, a nonlinear observer is designed for the saturated output system to estimate the states (both inside and outside of the saturation region) for dynamic control of the switch. The nonlinear observer is used to synthesize feedback linearization nonlinear control. The central idea of the feedback linearization control is to compensate for the nonlinearity in the system dynamics. Simulation results show that the proposed nonlinear observer and controller design approaches have not only good tracking ability and convergence but also extremely good parameter variation robustness

Experimentally verified procedure for determining dynamical model of the ETM MEMS structures

For the purpose of improving the supervisory control in a hydropower plant, the SCADA application was built during the general overhaul of the Miljacka hydropower plant on the river Krka in Croatia. The lowest level of the execution of the controlling algorithms is on the SIEMENS PLCs S7-400. The SCADA application is the top of the supervisory control structure. The control application is in a form of the enclosed logical parts of the process displayed on the individual control panels used for the supervisory control. The communication of the SCADA used with PLCs is realised by the communication server Applicom PC 2000 ETH, using SINEC H1 (industrial Ethernet) protocol via fibre optics. The SCADA is operated using the National Instruments programming package LOOKOUT on the operating system Windows NT 4.0.