Electrostatic micromotor and its reliability
Introduction
A microactuator is the key device for MEMS to perform physical functions. In microactuators, the popular actuation methods include electrostatic, magnetic, piezoelectric, thermal, shape memory, and other technologies [1], [2]. Each actuation principle has its own advantages and disadvantages. The choice and optimization should be made according to the requirements of applications and one category of actuators is the micromotor [3], [4]. At present, there are many kinds of micromotors, such as the electrostatic micromotor, ultrasonic micromotor, electromagnetic micromotor, resonant micromotor and biology micromotor.
Generally speaking, the electrostatic micromotors are suited for several lower torque, high-speed applications such as microsensors, microactuators, optical switches and data storage media [5]. Moreover, they are more suitable to perform tasks, which can be completed within a chip [6]. The successful electrostatic micromotors have been based on the various principle including corona, variable capacitance, harmonic drive, vibration, change induction etc. [7]. The electrostatic micromotors contain: top-drive motor, side-drive motor, wobble motor, center-pin motor, flange motor [8], linear stepper motor [9], ultrasonic motor [10], double stator axial-drive variable capacitance motor [11], out-rotor motor [12], induction motor [13], and shuffle motor [14]. In addition, there are many methods for the driving of electrostatic micromotors [15]. Table 1 lists the various electrostatic micromotors. Micromotors have not been widely used in industrial applications but are in a developmental stage, which suggests a near-future explosion of applications. For example, biomedical applications, drug delivery systems [16], surgical tools [17], probe [18], and resting in fluid for lubrication [19], are considered very promising. Micromotors can be used in optical systems in integrated circuits (ICs) for various purposes [20], [21]. The application range of micromotors extends from systems for the maintenance of fine tubes in power plants to inspection of blood vessels in the human body. Ultrasonic intravascular systems are based on the use of catheters [16]. Isabelle Dufour et al. investigated the possibility of using an electrostatic micromotor for an intravascular echographic system. To overcome the drawback, putting a micromotor in the front of the endoscope to drive a triple prism seems to be an appropriate alternative [22].
Therefore, it is necessary to investigate the reliability of electrostatic micromotors in MEMS. The paper presents an analysis methodology to study the design for micromotor reliability, as shown in Fig. 1.
Section snippets
Failure modes and mechanisms
Reliability of micromotors is a very young and important field. A failure is said to occur when a micromotor or its system no longer performs the required functions under the special conditions within the stated period of time. There have two main failures: irreversible failures and degradation failures. Failure modes usually refer to observable effects and failure mechanisms, and there are the processes directly causing the observable failure modes. Table 2 shows the common micromotor failure
Studying the failure mechanisms
Experimental and numerical methods can be used to study the friction and wear issues in micromotors.
Selecting materials
The primary performance and reliability metrics considered are the actuation voltage, speed of actuation, actuation force, stored energy, electrical resistivity, mechanical quality factor, and resistance to fracture, friction, fatigue, shock, and stiction. The materials properties governing these parameters are the Young’s modulus, density, fracture strength, intrinsic residual stress, resistivity, and intrinsic material damping [49]. Srikar and Spearing [49] plotted the relationship between
Conclusions
The electrostatic micromotor technologies are still in the initial stage of the development, particularly in its applications and reliability. Our knowledge of the physics of failure mechanisms in the micro-domain is still very limited. Currently there is hardly any dedicated equipment for the study of micromotors reliability. The future of electrostatic micromotors will be marked by new actuation principles, design methods and tools, modeling and simulation methods, control systems,
Acknowledgments
The authors are grateful to Dr. K. X. Wei and H. Huang for their valuable suggestions. This project is supported by National Outstanding Youth Foundation of People’s Republic of China under Grant No. 10325209.
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