R. Ben Mrad

A model for voltage-to-displacement dynamics in piezoceramic actuators subject to dynamic-voltage excitations

A model that predicts the expansion of piezoceramic actuators when subject to dynamic-voltage excitations is developed as an extension of the classical Preisach model. The model is presented in a recursive form that is suitable for real-time implementation. The model uses measurements of the first-order reversal curves and the average rate of change of the input-voltage signal. The model is shown through experiments to offer high accuracy under voltage excitations covering a wide frequency band.

On the classical Preisach model for hysteresis in piezoceramic actuators

A classical Preisach model [Mathematical Models of Hysteresis, Springer, New York, 1991] is developed using a piezoceramic actuator in a stacked form. The classical Preisach model is shown to offer excellent modeling accuracy when the actuator is not subject to any load and is subject to an excitation voltage signal at a low frequency. The accuracy of the Preisach model is shown to increasingly deteriorate as the load being applied to the piezoceramic actuator is increased or the range of frequencies contained in the voltage excitation signal gets wider. The classical Preisach model remains, though, a good model for hysteresis in piezoceramic actuators in applications where the load fluctuation is relatively small and the range of frequencies of the voltage excitation is limited.

Design, Modeling, and Demonstration of a MEMS Repulsive-Force Out-of-Plane Electrostatic Micro Actuator

An analytical model is developed for a two-layer repulsive-force out-of-plane micro electrostatic actuator by using conformal mapping techniques. The model provides the means to establish the performance characteristics in terms of stroke and generated force of the actuator and is used to develop design and optimization rules for the actuator. Numerical simulations were conducted in order to verify the analytical model. A simple physical model is also presented that explains the mechanism for generating the repulsive force. A Multi-User-MEMS-Processes repulsive-force out-of-plane rotation micromirror is developed to experimentally verify the analytical model and to demonstrate the repulsive-force actuators capability of driving large-size rotation plates by using surface micromachining technology. Experimental measurements show that the repulsive-force rotation micromirror with a size of 312 mum times 312 mum achieved a mechanical rotation of 0deg-2.1deg at a dc driving voltage of 0-200 V. The micromirror achieved an open-loop settling time of 2.9 ms for a mechanical rotation of 2.3deg and an open-loop bandwidth of 150 Hz (-3 dB).

A discrete-time compensation algorithm for hysteresis in piezoceramic actuators

A discrete-time Preisach model that captures hysteresis in a piezoceramic actuator is developed. The model is implemented using a numerical technique that is based on first-order reversal functions and is presented in a recursive form that is amenable for real-time implementation. The first-order reversal functions are experimentally obtained using a piezoceramic actuator in a stacked form. The developed model shows good agreement with actual measured data. Two hysteresis compensation schemes based on the developed discrete-time Preisach model are also developed and used in order to obtain any desired linear voltage-to-displacement relationship. The ability of the first hysteresis compensator to lead to an arbitrary linear voltage-to-displacement relationship is shown through experimental tests under the condition that no-load is applied to the actuator and then a load typical of many piezoactuator applications is applied to the actuator. The second hysteresis compensation scheme is used as part of an open-loop tracking controller and is shown experimentally to lead to high tracking accuracy.

Repulsive-force out-of-plane large stroke translation micro electrostatic actuator

A repulsive-force out-of-plane large stroke translation micro electrostatic actuator is presented. A model of the actuator is presented and is used to relate the applied voltage to the out-of-plane displacement. Prototypes of the actuator are fabricated using the surface micromachining technology PolyMUMPs. The measured results show that the actuator can achieve a static out-of-plane translation of 86 µm for a driving voltage of 200 V. The measured static performance matches well with the results predicted by the model. The measured bandwidth (–3 dB) for the out-of-plane translation of the micro actuator is 80 and 200 Hz for input sinusoidal driving voltages varying in the ranges of 75–125 V and 125–175 V, respectively. The translation micro actuator can also achieve 2D rotation along any direction within a mechanical rotation range of ±1.5°. Vector display based on the actuator is also demonstrated.

Modeling of dry stiction in micro electro-mechanical systems (MEMS)

Stiction, a term commonly used in micro electro-mechanical systems (MEMS) to refer to adhesion, is a major failure mode in MEMS. Undesirable stiction, which results from contact between surfaces, can severely compromise the reliability of MEMS. In this paper, a model is developed to predict the dry stiction between uncharged micro parts in MEMS. In dry stiction the interacting surfaces are assumed to be either hydrophobic or placed in a dry environment. In this condition the van der Waals (vdW) and asperity deformation forces are dominant. Here a model is developed for the vdW force between rough micro surfaces, and the new model is combined with a newly developed multiple asperity point model for the elastic/plastic deformation of rough surfaces in contact to solve the equilibrium condition of the forces. This in turn will yield the equilibrium distance between micro surfaces, using which the apparent work of adhesion can be found. The theory result is compared with the available experimental data from the literature. The developed model can be easily used for design purposes. If the topographic data and material constants are known, the desirable adhesion parameters can be quickly found from the model.

Guidance-Based On-Line Robot Motion Planning for the Interception of Mobile Targets in Dynamic Environments

This paper presents a novel method for the interception of moving targets in the presence of obstacles. The proposed method provides simultaneous positional interception and velocity matching of the target moving in a dynamic environment with static and/or mobile obstacles. An acceleration command for the autonomous robot (i.e., interceptor) is first obtained from a rendezvous-guidance technique that takes into account the kinematic and dynamic limitations of the interceptor, but not the motion of the obstacles. This command is subsequently augmented, though only when necessary, in order to avoid those obstacles that are about to interfere with the time-optimal motion of the interceptor. The augmenter acceleration command is obtained in our work through a modified cell-decomposition method. Extensive simulation and experimental results have clearly demonstrated the efficiency of the proposed interception method, tangibly better than other existing obstacle-avoidance methods.

An Overview of Electrospray Applications in MEMS and Microfluidic Systems

Integrating electrospray into microelectromechanical systems (MEMS) and microfluidic systems supports applications in diverse fields from biotechnology to aerospace. Electrospray also functions as a production tool, allowing for novel methods of MEMS fabrication. This review covers the three most significant applications of electrospray in MEMS and microfluidic systems technology: 1) as an integral part of a microfluidic device, most notably electrospray emitters for coupling a microfluidic chip to a mass spectrometer; 2) as a method for fabricating and manufacturing MEMS; and 3) for micropropulsion in aerospace applications using MEMS-based emitters. Perspectives on future research directions and opportunities are provided.

Development of a High-Performance Microelectrostatic Repulsive-Force Rotation Actuator

A high-performance microelectrostatic repulsive-force rotation actuator is developed and tested. A model of the actuator is also developed and used for design optimization. The model is developed using a hybrid approach that combines analytical analysis with numerical simulations. Expressions and a systematic method are developed based on this model for selecting the parameters of the actuator in order to achieve the maximum stroke for given operating conditions and geometrical parameters. An expression for the finger width leading to generating a maximum torque within the actuator was derived. A method was then developed to optimize the finger length and finger width of the actuator to achieve the maximum stroke. The design-optimization process proposed was used to design a repulsive-force rotation actuator which was fabricated using a standard multiuser surface micromachining process. The performance of the optimized actuator showed an improvement of more than 100% in comparison with a nonoptimized design which was fabricated using the same process and which had the same size, the same suspension spring stiffness, and was subject to the same driving voltage. The optimized actuator can rotate a 312 ¿m × 312 ¿m micromirror out of plane by 4.4° at a driving voltage of 200 V, while the nonoptimized actuator could only rotate a micromirror of the same size by 2.1°.

Modeling of Elastic/Plastic Contact Between Nominally Flat Rough Surfaces Using an n-Point Asperity Model

The asperities of rough surfaces have long been considered to be points higher than their immediate neighbors. Based on this concept, theories were developed for quantitatively understanding the elastic and plastic nature of contact between rough surfaces. Recently it has been recognized that the above model for asperities is inadequate. Consequently, all the models that have been constructed based on that model are inadequate too. In this paper, based on a newly developed multiple-point asperity model, the elastic and plastic contact problem between nominally flat surfaces is reformulated. This leads to finding the deformed area, and load produced by the contact. The model is developed for the general form of isotropic rough surfaces with arbitrary height distribution and autocorrelation function (ACF). The microcontact areas generated by each asperity contact are considered to be circles. The Gaussian distribution of heights and exponential ACF are considered as a benchmark to compare the results of the new model with the existing models. Using results from numerical models developed by other groups, the new model is validated.