Variable-capacitance Micromotor Research Articles

A High-Speed Chip-Scale Rotary Polygon Optical Scanner Based on a Micromotor Integrated With a Micro-Polyhedron Mirror

Micro-opto-electro-mechanical systems (MOEMS) based mirrors with a suitable actuation range are often used for optical beam scanning applications. While several MOEMS scanners have been reported, they often have limitations with respect to their range of motion, field of view, mass, size and fabrication cost. In this work, we present a rotary micro-polygon scanner based on an electrostatic variable capacitance micromotor for laser beam steering applications. The miniaturized scanner consists of a hollow micro-polyhedron mirror with a base radius of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$200 {\mu }\text{m}$ </tex-math></inline-formula> and a height of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$400 {\mu }\text{m}$ </tex-math></inline-formula> . It is mounted on the rotor of a variable capacitance micromotor using an epoxy-based adhesive. The rotor has a diameter of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$600 {\mu }\text{m}$ </tex-math></inline-formula> and is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5 {\mu }\text{m}$ </tex-math></inline-formula> thick. The micromotor was fabricated using a commercial three-layer polysilicon surface micro-machining process (PolyMUMPs) and the micro-polyhedrons were fabricated using a commercial bulk micro-machining process (PiezeoMUMPs). After fabrication of the components, the micro-scanner was assembled and operated under atmospheric conditions using a three-phase square wave excitation ranging from 0 V to 200 V and achieved rotational speeds of up to 2100 rpm. An optical laser beam with a wavelength of 632 nm was reflected off the sidewall of the rotating micro-polyhedron, achieving an experimentally measured scanning angular range of 57.88°, an angular scan rate of 220 radians per second and a scan speed of 25,200 lines per minute or 420 lines per second with a 12 sided micro-mirror. The ratio of the output power over the input optical power is measured over a scanning range of 22°, which is limited by the size of the photo-detector, yielding a ratio spanning from 0.18 to 0.22. This rotary micro-scanner can provide high scanning speeds and a wide scanning field of view with a footprint of 1.25 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a thickness of 0.83 mm, which is suitable to implement chip-level high-performance micro-beam steering systems for various applications. [2022-0020]

Multiphysical computation of the structural bending in a bottom-drive VCM

Purpose This paper intends to lay a background knowledge towards the feasibility of developing a bottom-drive variable capacitance micromotor (VCM) using a surface micromachining process (SMP). The purpose of this paper is to determine the possibility of neglecting the bending of the rotor plates caused by the electrostatic normal forces when deploying a set of mechanical supports. Design/methodology/approach A multiphysics simulation approach is considered in order to analyse the coupled electromechanical effects in a steady state and to evaluate if the proposed geometries are useful to reduce the bending of the plates. Findings A surfaced micromachined bottom-drive VCM requires mechanical reinforcement in order to eliminate the risk of an electrical short circuit caused by the deformation in the rotor plates. The combination of an external supporting ring and anchored structural ribs on top of the rotor poles is sufficient to neglect the deformation in the poles of the rotor. Originality/value An original analysis with the objective of setting a background in the development of a bottom-drive electrostatic micromotor using a SMP is presented.

Rotation Control and Characterization of High-Speed Variable-Capacitance Micromotor Supported on Electrostatic Bearing

This paper presents the rotation control design and experimental performance of a microelectromechanical systems (MEMS) variable-capacitance motor where a free-spinning rotor is suspended and centered in an evacuated vacuum cavity by a contactless electrostatic bearing. The micromachined device is based on a glass/silicon/glass bonding structure, fabricated by bulk micromachining, driven by a three-phase electrostatic motor, and used as an angular rate gyroscope by spinning-up the rotor rate over 104 r/min. A closed-loop phase commutation scheme is proposed in our rotation design where three-phase drives are switched depending on one channel of the rotor's angular position. The design of the micromotor spin-up and constant-speed operation is described based on the proposed electronic commutation. Experimental results of the motor spin-up process under different vacuum settings, static, and dynamic characteristics of the constant-speed control loop together with scale factor of the spinning-rotor gyroscope are described for the device operated in vacuum. It is indicated that the rotor can be spun up to 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> r/min within 400 s, and up to 2.96 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> r/min in steady state under a drive voltage of 11.8 V. Measurement data in constant-speed control mode show that the standard deviation of the spin rate error is 0.07 r/min at a rated speed of 1.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> r/min.

Research on the optimization method of top-drive variable-capacitance micromotors

As a class of important MEMS devices, electrostatic micromotors can be used as a platform for developing micropumps, microgyroscopes, micro propeller, and micro medical tools etc. The researches about the electrostatic micromotors in the past twenty years were mainly focused on the actuation principle and fabrication technics. In this paper, the optimal design method for top-drive variable capacitance electrostatic micromotors (TDVCM) is investigated and an analytical model for calculating the output torque of the TDVCM is presented. Using the analytical model, the design parameters of TDVCM can be optimized without the finite element analysis, such that the design procedure has better efficiency in calculation time. In order to verify the numerical calculation accuracy of the method proposed in this paper, the optimizing procedure of an 8/6 TDVCM with various kinds of simple excitation schemes is presented, and the optimization results are verified by the finite element analysis.

Optimization Shape of Variable-Capacitance Micromotor Using Seeker Optimization Algorithm

In the current paper, the optimization shape of a polysilicon variable-capacitance micromotor (VCM) was determined using the seeker optimization algorithm (SOA). The optimum goal of the algorithm was to find the maximum torque value and minimum ripple torque by varying the geometrical parameters. The optimization process was performed using a combination of SOA and the finite-element method (FEM). The fitness value was calculated via FEM analysis using COMSOL3.4, and SOA was realized by MATLAB7.4. The proposed method was applied to a VCM with eight and six poles at the stator and rotor, respectively. For comparison, this optimization was also performed using the genetic algorithm. The results show that the optimized micromotor using SOA had a higher torque value and lower torque ripple, indicating the validity of this methodology for VCM design.

Optimization of Variable-Capacitance Micromotor Using Genetic Algorithm

In this paper, the optimization shape of a polysilicon variable-capacitance micromotor (VCM) was determined using genetic algorithm (GA). The optimum goal of the algorithm was found for a maximum torque value and a minimum torque ripple, following changing the geometric parameters. The optimization process was carried out using a combination of GA and finite-element method (FEM). The fitness value was calculated by FEM analysis using COMSOL3.4, and the GA was realized by MATLAB7.4. The proposed method has been applied for the two case studies, and it has been also compared with successive sampling method. The results show that the optimized micromotor using GA had a higher torque value and a lower torque ripple, indicating the validity of this methodology for VCM design.

Experimental study of a variable-capacitance micromotor with electrostatic suspension

A variable-capacitance micromotor where the rotor is supported electrostatically in five degrees of freedom was designed, fabricated and tested in order to study the behavior of this electrostatic motor. The micromachined device is based on a glass/silicon/glass stack bonding structure, fabricated by bulk micromachining and initially operated in atmospheric environment. The analytical torque model is obtained by calculating the capacitances between different stator electrodes and the rotor. Capacitance values in the order of 10−13 pF and torque values in the order of 10−10 N m have been calculated from the motor geometry and attainable drive voltage. A dynamic model of the motor is proposed by further estimating the air-film damping effect in an effort to explain the experimental rotation measurements. Experimental results of starting voltage, continuous operation, switching response and electric bearing of the micromotor are presented and discussed. Preliminary measurements indicate that a rotor rotating speed of 73.3 r min−1 can be achieved at a drive voltage of 28.3 V, equivalent to a theoretical motive torque of 517 pN m. Starting voltage results obtained from experimental measurement are in agreement with the developed dynamic model.

Optimization Shape of Variable Capacitance Micromotor Using Differential Evolution Algorithm

A new method for optimum shape design of variable capacitance micromotor (VCM) using Differential Evolution (DE), a stochastic search algorithm, is presented. In this optimization exercise, the objective function aims to maximize torque value and minimize the torque ripple, where the geometric parameters are considered to be the variables. The optimization process is carried out using a combination of DE algorithm and FEM analysis. Fitness value is calculated by FEM analysis using COMSOL3.4, and the DE algorithm is realized by MATLAB7.4. The proposed method is applied to a VCM with 8 poles at the stator and 6 poles at the rotor. The results show that the optimized micromotor using DE algorithm had higher torque value and lower torque ripple, indicating the validity of this methodology for VCM design.

Closed-loop control of a long-range micropositioner using integrated photodiode sensors

Closed-loop control of an electrostatically actuated micropositioner using integrated feedback sensors is reported. A photodiode-based position sensing mechanism has been incorporated into a variable-capacitance micromotor supported on microball bearings with a range of 4 mm and a resolution of 120 μm. Accurate and reliable positioning has been demonstrated using a proportional control law and the effect of the proportionality constant has been investigated for various stepping distances and actuation voltages. A minimum settling time of 0.1 s was achieved for a 1 mm step at 150 V. Closed-loop excitation has enabled sustained synchronous motion and a maximum speed of 20.4 mm/s. This is a noted improvement compared to open-loop excitation, which exhibits erratic motion and a maximum speed of 7.2 mm/s. Using feedback control, two critical functions for robust positioning have been demonstrated; the micropositioner can establish a necessary frame of reference and autonomously respond to arbitrary disturbances. The closed-loop position control system presented in this work illustrates the feasibility and functionality of smart microsystems using integrated feedback sensors.

Design, Fabrication, and Characterization of a Rotary Micromotor Supported on Microball Bearings

We report the design, fabrication, and characterization of a rotary micromotor supported on microball bearings. This is the first demonstration of a rotary micromachine with a robust mechanical support provided by microball-bearing technology. A six-phase bottom-drive variable-capacitance micromotor (Phi = 14 mm) is designed and simulated using the finite-element (FE) method. The stator and the rotor are fabricated separately on silicon substrates and assembled with the microballs. Three layers of low-k benzocyclobutene polymer, two layers of gold, and a silicon microball housing are fabricated on the stator. Microball housing and salient structures (poles) are etched in the rotor and are coated with a silicon carbide film that reduces the friction without which the operation was not possible. A top angular velocity of 517 r/min, corresponding to the linear tip velocity of 324 mm/s, is measured at plusmn150-V and 800-Hz excitation. This is 44 times higher than the velocity previously demonstrated for linear micromotors supported on the microball bearings. A noncontact method is developed to extract the torque and the bearing coefficient of friction through dynamic response measurements. The torque is indirectly measured to be -5.62 plusmn 0.5 muN ldr m at plusmn150-V excitation which is comparable with the FE simulation results predicting -6.75 muN ldr m. The maximum output mechanical power at plusmn150 V and 517 r/min was calculated to be 307 muW. The bearing coefficient of friction is measured to be 0.02 plusmn 0.002 which is in good agreement with the previously reported values. The rotary micromotor developed in this paper is a platform technology for centrifugal micropumps used for fuel-delivery and cooling applications.

Scale-dependent analysis of sliding wear between the bushing and the ground plane in micromotors

Among the problems in microelectromechanical systems, friction and wear are the most concerned ones. In this paper, a model to predict the sliding wear between the rotor bushing and the substrate plane in a variable capacitance micromotor is presented. The model is based on the Archard’s wear law and takes into account the scale effects, including the effects of such surface forces as meniscus force due to humidity and the electrostatic force as a result of the capacitance coupling between the rotor and substrate, as well as the scale effects of material properties, which become scale dependent in microscale. The wear process and durability of the hemispherical bushing are investigated and discussed. Simulation results show that they are largely affected by these effects, and it is indicated that sufficient attention should be taken in the design and fabrication of micromotors on the scaling effects.

Variable-capacitance micromotor with levitated diamagnetic rotor

A novel micromotor is described, in which the gear-shaped pyrolytic graphite rotor is levitated, rotated and constrained by the combination of static magnetic, electrostatic forces and torques. Two concentric ring-shaped permanent magnets are presented, which are used for realising the static magnetic field. By the microelectromechanical systems process, the metal electrodes, pads and SU-8 column post were fabricated on the silicon substrate above the magnets. The stator was assembled by the silicon and the magnets. The three-phase axial variable-capacitance motor principle was applied for driving the rotor. The rotor is stably levitated to a height of about 0.70 mm over the magnets. When a 30 V driving voltage was added, the rotor speed was over 10 rpm in atmosphere.

Dynamic characterization of a linear electrostatic micromotor supported on microball bearings

We report the dynamic characterization of a six-phase, bottom-drive, linear, variable-capacitance micromotor (B-LVCM) supported on microball bearings. The B-LVCM was fabricated and tested using a new test-bed. The motion of the micromotor was captured using low- and high-speed video cameras, tracked using image processing software, and analyzed to obtain the instantaneous velocity (11 mm/s), acceleration (1.9 m/s 2), and net force (0.19 mN) (all in amplitude) from the position data. The predicted (simulated) and measured net forces at different applied voltages are in good agreement. The micromotor was then modeled as a mass–spring–dashpot system and the lumped parameters were extracted from position measurement. This characterization methodology provides an understanding of the dynamic behavior of both the variable-capacitance micromachines and the microball bearings on which they are supported.

Microball-bearing-supported electrostatic micromachines with polymer dielectric films for electromechanical power conversion

This paper presents our latest research activities toward the development of electrostatic micromotors/microgenerators with a microball-bearing support mechanism and benzocyclobutene (BCB) low-k polymer insulating layers. The primary applications of these devices are long-range, high-speed micropositioning, high-speed micro pumping and micro power generation. In this paper, we present the development of the first generation of microball-bearing-supported micromachines. This device is a 6-phase, bottom-drive, linear, variable-capacitance micromotor. The design and fabrication of the linear micromotor, and characterization of the motor capacitance, force and motion in 3-phase and 6-phase excitation modes are presented. The micromotor consists of a silicon stator, a silicon slider and four stainless-steel microballs. The aligning force profile of the micromotor was extracted from simulated and measured capacitances of all phases. An average total aligning force of 0.27 mN with a maximum of 0.41 mN at 100 V dc was measured. The ac operation of the micromotor was verified by applying square-wave voltages and characterizing the slider motion. An average slider speed of 7.32 mm s−1 at 40 Hz and 120 V P-P was reached without losing the synchronization. The design, fabrication and characterization methods presented in this paper can be used as a technology platform for developing rotary micromachines.

Friction and wear study of the hemispherical rotor bushing in a variable capacitance micromotor

Friction and wear are the most serious problems for micromotors in microelectromechanical systems (MEMS). In the paper, a linear-sliding wear model of the contact between the rotor bushing and the ground plane is presented to describe the wear and a corresponding simplified method is proposed to simulate the wearing process. The effects of geometry parameters, material properties and applied operating conditions on the evolution of dimensional and volumetric wear rates and frictional torques are explored for normally loaded rotating rotor bushing sliding on the ground plane. A hemispherical-bushing-on-ground-plane configuration finite element model (FEM) is set up and the implementation of the contact task based on ANSYS and the contact element approach is introduced to provide the numerical simulations acted as a guide to solution of the contact problems in micromotors. Numerical simulations and results of the wear rates, frictional torques, contact stresses and contact pressure are studied and the effects of roughness, material properties, geometry parameters and FEM mesh of the bushing and the ground plane are discussed. It is indicated that the nonlinear effects cannot be ignored and the results should not be used to predict the absolute wear lifetime whereas surface engineering, lower wear materials and rational designs for micromotors in MEMS should be applied to bring the friction and wear behaviors into the acceptable regimes.

Improvement of a classical electrodeposition process for electrostatic variable-capacitance, side-drive micromotor fabrication

An improvement of a classical electrodeposition process through a photoresist mold for electrostatic side-drive micromotor fabrication is proposed. A step of selective and quasi-conformal deposition is used to reduce the air gap and bearing clearance below the value allowed by the maximum photoresist aspect ratio. This will allow the fabrication of variable-capacitance micromotors with better operation characteristics.

3-D computer aided analysis of the "Berkeley" electrostatic micromotor

The basic ideas of IC-processed electrostatics are reviewed. The three-dimensional field model of a variable-capacitance electrostatic micromotor is then presented and the underlying analysis problem is solved by means of the finite element method. The micromotor, acting as a position actuator, is investigated at steady state and particular attention is paid in the calculation of the torque-angle curve featuring the device. The influence of stator geometry on the electromechanical performance is also investigated and design considerations are accordingly drawn. Finally, the step-transient response of the actuator is simulated. >

Coupled electrostatic and mechanical FEA of a micromotor

The electrostatic forces occurring in a novel double stator axial-drive variable capacitance micromotor (VCM) are studied as a function of rotor-stator overlap, applied voltage, rotor support morphology, and rotor thickness. Analytical equations are developed using parallel plate assumptions, and results are compared with those obtained with 3D Finite Element Analysis (FEA) for tangential, axial, and radial electrostatic forces. The influence of the axial forces on the rotor deflections are studied using iterative indirect coupled field analysis, where the axial forces obtained from the electrostatic 3D FE model are iteratively applied to a structural FE model until stable rotor deflections are obtained. It was found that the axial forces, taking the rotor deflection into account, are twice as high as those obtained by analytical evaluation neglecting rotor deflections and about 70 times higher than the radial forces at a typical operating voltage of 100 V. Inclusion of bushing supports results in lower axial forces and decreases the influence of rotor tilt. Tangential forces likely to be exerted on the rotor at start-up are also examined and compared with analytical predictions. The study demonstrates that FEA provides more accurate results than analytical equations due to the geometry and field simplifications assumed in the latter. >

Study on si electrostatic and electroquasistatic micromotors

An electrostatic variable-capacitance micromotor and an electroquasistatic induction micromotor are discussed. Both the micromotors, with rotors of 160 μm diameter, have been fabricated by the surface micromachining of polysilicon. In order to reduce the friction in the micromotors, the application of a gas-lubricated bearing is proposed. An aerodynamic in-pumping spiral grooved thrust microbearing with a groove width of 4 μm and a groove depth of 300 nm has been designed and used in the micromotors.

A perturbation method for calculating the capacitance of electrostatic motors

Recent advances in micromachining technology have made possible the fabrication of a number of electrostatic devices, including variable capacitance micromotors. These variable capacitance micromotors employ one or more sets of conducting plates that act as capacitors. An accurate estimate of the capacitance between these sets of plates for various configurations is necessary to design and simulate such devices. Capacitance calculations can be performed using a variety of methods, including finite element techniques. An efficient method of calculating the capacitance is to reduce the problem to a set of integral equations that are then approximated by a set of algebraic equations. This method was previously used to calculate the capacitance of a linear micromotor with an infinite set of conducting lands on both the rotor and the stator, which introduces spatial periodicity that greatly simplifies the problem. Recent micromotor designs, however, employ only a few sets of conducting lands per phase, which makes it unclear if the previous results can be applied. In this paper, a new perturbation method for solving the integral equations and thereby estimating the capacitance is presented. This method does not make the assumption of infinite sets of conducting lands to introduce spatial periodicity. The perturbation method is computationally efficient and can be used for any design, including those that use very few repeated sets of conducting lands. The results obtained by this method are compared with the previous results which include experimental data. Finally, the method is extended to rotary micromotors and to three dimensional problems.