Electrostatic Transducer Research Articles

A Novel MEMS Speaker With Peripheral Electrostatic Actuation

We design and fabricate electrostatically actuated micro-speakers with circular diaphragms and peripheral electrode configuration. The novel electrode configuration mitigates the squeeze film damping and increases pull-in voltage when compared with the conventional parallel plate electrostatic transducer design. Finite element analysis shows that under dynamic conditions, we can achieve diaphragm deflections as much as 80 times the gap between the electrodes because of the proposed design. We compare the conventional design of electrostatically actuated audio speakers – where typical electrode separation gaps are around 60 to $100~{\mu }\text{m}$ and actuation voltages are in the range of 10 to 200 volts – with the proposed peripheral electrode design with an electrode separation gap of $1~ {\mu }\text{m}$ and actuation voltage ≤100 volts. The SPL obtained is comparable to the existing designs and ranges from 40 to 60 dB at 1 kHz at 1 cm with different sizes of the micro-speakers. The efficiency achieved with the proposed design is 100−4% at resonance which is at least two orders higher than the typically observed value for electrostatic micro-speakers. [2020-0134]

A MEMS nanoindenter with an integrated AFM cantilever gripper for nanomechanical characterization of compliant materials

This work presents the development of a MEMS nanoindenter that uses exchangeable AFM probes for quasi-static nanomechanical characterization of compliant and ultra-compliant materials. While the electrostatic micro-force transducer of the MEMS nanoindenter provides a maximum indentation depth up to 9.5 µm with a maximum output force of 600 µN, experimental investigations reveal that it can achieve a depth and force resolution better than 4 pm Hz−1/2 and 0.3 nN Hz−1/2, in air for f≥ 1 Hz. A passive AFM probe gripper is integrated into the MEMS nanoindenter, allowing the nanoindenter to utilize various AFM probes as an indenter for material testing. A proof-of-principle experimental setup has been built to investigate the performance of the MEMS nanoindenter prototype. In proof-of-principle experiments, the prototype with a clamped diamond AFM probe successfully identified an atomic step (∼0.31 nm) within a Si < 111 > ultraflat sample using the scanning probe microscopy mode. The nanomechanical measurement capability of the MEMS nanoindenter prototype has been verified by means of measurements of reference polymer samples using a silicon AFM probe and by means of measurements of the elastic properties of a PDMS sample using a spherical diamond-coated AFM probe. Owing to its compact and low-cost but high-resolution capacitive readout system, this MEMS nanoindenter head can be further applied for in-situ quantitative nanomechanical measurements in AFMs and SEMs.

Lateral pull-in instability of electrostatic MEMS transducers employing repulsive force

We report on the lateral pull-in in capacitive MEMS transducers that employ a repulsive electrostatic force. The moving element in this system undergoes motion in two dimensions. A two degree-of-freedom mathematical model is developed to investigate the pull-in quantitatively. The nonlinear electrostatic force, which is a vector function of two spatial coordinates, is determined by calculating the potential energy of the system using a boundary element approach. The equilibrium points are found by numerically solving the nonlinear coupled static equations. A stability analysis reveals that depending on the values of the lateral and transverse stiffness, the system undergoes different bifurcations when the voltage on the side electrodes is considered as the control parameter. Three-dimensional bifurcation diagrams are presented and discussed to elucidate the nonlinear nature of the system. The results establish important criteria for designing MEMS transducers with reliable and robust performance.

Characterization of a parametric resonance based capacitive ultrasonic transducer in air for acoustic power transfer and sensing

Capacitive transducers typically require a DC bias or a pre-charged electret to operate, rendering them non-ideal for applications such as energy harvesting and wireless power transfer. Parametric resonance based capacitive transducers offer an alternative to traditional capacitive transducers as they can be operated without the need for a bias voltage or a pre-charged electret. In this paper, we experimentally validate a 1D lumped parameter model and characterize a CPUT operating in air at 50 kHz for ultrasonic power transfer and sensing applications. This particular CPUT is able to recover 40.5 μW of power at an efficiency of 0.32% without any DC bias when excited by a 50 kHz piezoelectric transducer. The application of the CPUT as a highly sensitive acoustic sensor is explored by making use of active electrical circuits that reduce the resistance of the system. Finally, the capability of the CPUT as a highly directional acoustic sensor is presented with the electrostatic transducer demonstrating a directivity of ±1° when operated as a CPUT as compared to ±13° when operated as a conventional biased receiver. These results set the stage for the application of CPUT as a multi-functional acoustic transducer.

Modeling and Design Optimization of a Rotational Soft Robotic System Driven by Double Cone Dielectric Elastomer Actuators.

Dielectric elastomers (DEs) consist of highly compliant electrostatic transducers which can be operated as actuators, by converting an applied high voltage into motion, and as sensors, since capacitive changes can be related to displacement information. Due to large achievable deformation (on the order of 100%) and high flexibility, DEs appear as highly suitable for the design of soft robotic systems. An important requirement for robotic systems is the possibility of generating a multi degree-of-freedom (MDOF) actuation. By means of DE technology, a controllable motion along several directions can be made possible by combining different membrane actuators in protagonist-antagonist configurations, as well as by designing electrode patterns which allow independent activation of different sections of a single membrane. However, despite several concepts of DE soft robots have been presented in the recent literature, up to date there is still a lack of systematic studies targeted at optimizing the design of the system. To properly understand how different parameters influence the complex motion of DE soft robots, this paper presents an experimental study on how geometry scaling affects the performance of a specific MDOF actuator configuration. The system under investigation consists of two cone DE membranes rigidly connected along the outer diameter, and pre-compressed out-of-plane against each other via a rigid spacer. The electrodes of both membranes are partitioned in four sections that can be activated separately, thus allowing the desired MDOF actuation feature. Different prototypes are assembled and tested to study the influence of the inner radius as well as the length of the rigid spacer on the achievable motion range. For the first experimental study presented here, we focus our analysis on a single actuation variable, i.e., the rotation of the rigid spacer about a fixed axis. A physics-based model is then developed and validated based on the collected experimental measurements. A model-based investigation is subsequently performed, with the aim of studying the influence of the regarded parameters on the rotation angle. Finally, based on the results of the performed study, a model-based optimization of the prototype geometry is performed.

Influence of Electrostatic Forces on the Vibrational Characteristics of Resonators for Coriolis Vibratory Gyroscopes.

The Coriolis Vibratory Gyroscopes are a type of sensors that measure angular velocities through the Coriolis effect. The resonator is the critical component of the CVGs, the vibrational characteristics of which, including the resonant frequency, frequency mismatch, Q factor, and Q factor asymmetry, have a great influence on the performance of CVG. The frequency mismatch and Q factor of the resonator, in particular, directly determine the precision and drift characteristics of the gyroscope. Although the frequency mismatch and Q factor are natural properties of the resonator, they can change with external conditions, such as temperature, pressure, and external forces. In this paper, the influence of electrostatic forces on the vibrational characteristics of the fused silica cylindrical resonator is investigated. Experiments were performed on a fused silica cylindrical resonator coated with Cr/Au films. It was shown that the resonant frequency, frequency mismatch, and the decay time slightly decreased with electrostatic forces, while the decay time split increased. Lower capacitive gaps and larger applied voltages resulted in lower frequency mismatch and lower decay time. This phenomenon was theoretically analyzed, and the variation trends of results were consistent with the theoretical analysis. This study indicates that, for fused silica cylindrical resonator with electrostatic transduction, the electrostatic influence on the Q factor and frequency, although small, should be considered when designing the capacitive gap and choosing bias voltages.

Towards the Analogy of Electrostatic and Electromagnetic Transducers

Electromechanical, or mechatronic, transducers exchange energy between the mechanical and electrical domain via electrostatic and electromagnetic transduction principles, which are the basis for actuators and sensors. This paper reveals the simple analogy between electrostatic and electromagnetic transducers. Summarizing the well-known Gauß, Ampère’s, Ohm’s, Chua’s, Newton’s, Hook’s and the damping laws into a signal-flow diagram with linearized coefficients, the model shows the transducers reciprocity and physical behavior of damped spring-mass mechatronic systems. The presented modeling approach simplifies the derivation of transfer functions, the pull-in phenomenon, and the coupling factors for the design of feedback methods for mechatronic systems. The analogies are shown by comparing an electrostatic and an electromagnetic parallel-plate actuator, represented by a torsional MEMS scanning mirror and a hybrid reluctance fast steering mirror, respectively. The paper discusses the actuators performance and compares the transducer coefficients and the intrinsic stiffness regarding pull-in.

An electret‐based thermoacoustic‐electrostatic power generator

Sections PDFPDF Tools Share Summary This study reports a new concept for power generation from thermal energy, which integrates a thermoacoustic engine (TAE) with a contact‐free electret‐based electrostatic transducer. The TAE converts thermal energy into high‐intensity acoustic energy, while the electret‐based electrostatic transducer converts the generated acoustic energy into electricity. The experiments demonstrate the feasibility and potential of the proposed electret‐based thermoacoustic‐electrostatic power generator (TAEPG). The dynamic response of the electrostatic transducer and energy conversion inside the TAE are further investigated using a lumped element model and a frequency‐domain reduced‐order network model. Good agreement is achieved between experimental measurements and theoretical predictions. Furthermore, a parametric study is performed to study the effect of key parameters including the external heating power, air gap, and resistive load on the performance of the TAEPG. Results show that an open‐circuit voltage amplitude of 4.7 V is produced at a closed‐end pressure amplitude of 480 Pa in the experiment, and it is estimated that 25.2% of the acoustic power generated by the TAE has been extracted by the electret‐based electrostatic transducer. In this case, the maximum electric power output is measured to be 2.91 μW at a resistive load of around 2.2 MΩ. By increasing the external heating power, the TAEPG can produce a maximum voltage amplitude of 8 V. This work shows that the proposed concept has great potential for developing miniature heat‐driven power generators.

Thin-film bidirectional transducers for haptic wearables

We have designed, fabricated, and characterized a flexible electrostatic transducer (FET) for potential use as a wearable haptic actuator. This transducer both generates motion through the vibration of a curved electrode and measures the displacement of the same curved electrode to act as a displacement sensor. The transducer was analyzed through theory, simulation, and experiment to determine its displacement and sensing performance. It was found that a transducer with a footprint of 25 mm × 12.5 mm was able to generate displacements between 0.1 mm and 3.2 mm of displacement when operated at voltages between 25 V and 150 V. It was also capable of audible actuation at frequencies anywhere between 0.01 Hz and 10 kHz, including at target frequencies relevant to haptic communication. The transducer was able to sense changes to its shape through a change of capacitance, with a signal change of around 10% during maximum displacement. This sensing allows for both bidirectional communication and automatic displacement control through self-sensing. Simulation and theoretical results provide insight into the mechanism of actuation for the actuation and sensing systems, including predictions of displacement for a given voltage. User studies were also conducted, where it was found that the transducer generated perceivable and comfortable vibrations at both 26 Hz and 260 Hz, with 260 Hz being perceivable at lower amplitudes. The present work describes and characterizes a promising transducer for haptic communication.

A CMOS-Integrated MEMS Platform for Frequency Stable Resonators-Part I: Fabrication, Implementation, and Characterization

This paper introduces the comprehensive design concepts, procedure, and post-process flow of a robust manufacturing platform for complementary metal-oxide- semiconductor microelectromechanical systems (CMOS-MEMS) resonant transducers with enhanced frequency stability and capacitive-transduction efficiency. Thanks to a sandwich-stacking configuration of the interconnection metal line (TiN/AlCu/TiN) existing in standard CMOS technology, a deep-submicron TiN-to- TiN gap is successfully demonstrated by removing the central AlCu alloy layer through the proposed post-fabrication platform, thus realizing a novel titanium nitride composite (TiN-C) structure. Compared with the traditional release approaches utilized in the general post-CMOS fabrication, the proposed TiN composite transducer with TiN-to-TiN gap can simultaneously attain (i) a 400nm air gap for effective electrostatic transduction, (ii) adequate material selection and arrangement for near-zero frequencytemperature coefficient, and (iii) conductive electrodes to overcome frequency drift induced by dielectric charging during capacitive operation. In Part I of this study, the design guideline, layout rules, and detailed post-fabrication flow are addressed. The fabrication results of each post-process step are also exhibited. Numerical analysis, experiment results, and further discussions in terms of frequency stability for resonant transducers will be covered in Part II. [2018-0230]

On the Lateral Instability Analysis of MEMS Comb-Drive Electrostatic Transducers.

This paper investigates the lateral pull-in effect of an in-plane overlap-varying transducer. The instability is induced by the translational and rotational displacements. Based on the principle of virtual work, the equilibrium conditions of force and moment in lateral directions are derived. The analytical solutions of the critical voltage, at which the pull-in phenomenon occurs, are developed when considering only the translational stiffness or only the rotational stiffness of the mechanical spring. The critical voltage in a general case is numerically determined by using nonlinear optimization techniques, taking into account the combined effect of translation and rotation. The influences of possible translational offsets and angular deviations to the critical voltage are modeled and numerically analyzed. The investigation is then expanded for the first time to anti-phase operation mode and Bennet’s doubler configuration of the two transducers.

Capacitive silicon micro-electromechanical resonator for enhanced photoacoustic spectroscopy

Photoacoustic spectroscopy (PAS) has been increasingly applied to detect gas traces in many applications. Gas absorption is detected through the excitation of a mechanical transducer, actuated by the acoustic pressure generated after optical absorption. PAS is potentially the best method to achieve some selective, sensitive, compact, and reliable sensors. However, the main limitation comes from the use of some mechanical transducers which are not specifically designed for this application. Great interest for realizing efficient devices with specific characteristics led us to study microelectromechanical systems (MEMS). Silicon is the core material of this technology. It offers high performances in terms of quality factor and residual stress and is an attractive alternative to conventional acoustic transducers. MEMS are widely used as transducers, and electrostatic transduction is a well-established method. In this work, we describe mechanical resonators fabricated on a silicon-on-insulator (SOI) wafer to be used as acoustic transducers in PAS. The performances of the developed devices are strictly linked to their mechanical properties and viscous damping. Their sensitivity is evaluated through an experimental setup; we achieved to detect methane and ethylene using a distributed feedback (DFB) laser diode and a DFB-QCL (Quantum Cascade Laser) emitting at 1.6 μm and 11 μm, respectively. By demonstrating stable and reproducible detection, this work opens the way to a concept of compact gas sensors based on tunable diode laser absorption spectroscopy and capacitive silicon microelectromechanical resonators.

Fabrication and characterisation of microscale hemispherical shell resonator with diamond electrodes on the Si substrate

A self-aligned fabrication method is employed for microscale hemispherical shell resonator with integrated diamond electrodes on a silicon (Si) substrate. The millimetre-scale three-dimensional (3D) hemispherical shell resonator, with integrated electrostatic excitation and capacitive detection transducers for full control of the resonator, is fabricated on the Si substrate by employing a microelectromechanical systems process. The eight integrated diamond electrodes evenly distributed outside the hemispherical shell are fabricated using a self-aligned method, which ensures uniform capacitive gap along the whole circumference. The Boron-doped polycrystalline diamond, a material with low thermo-elastic damping and low surface losses to reach the high quality, is used as the millimetre-scale 3D hemispherical shell resonator with integrated electrodes. The fabrication process enables the production of almost perfect hemisphere mould (<2% roundness variation) with an average surface roughness of 3 nm. Vibration modes and frequencies are carried out by a scanning laser Doppler vibrometer; the elliptical mode frequency is determined to be at 81.3 kHz, with the two M = 2 degenerate modes having a relative frequency mismatch of 0.86%. This process can be further optimised to fabricate high-quality and full-symmetric microscale hemispherical resonator gyroscopes in batch.

Ultrasonic Sonar Object and Range Detection Measurement Display using HC-SR04 Sensor on Arduino ATMEGA 2560

Ultrasonic sonar object and range detection measurement using Hc-sr04 display can be use in every application. Ultrasonic sonar popular to use due to low cost, availability, no radiation for human applied in more industry such as medicine, robotic, automation. Ultrasonic sensor is a measurement device that consist of two transducer one for transmitting an ultrasonic wave and the other for receiving the reflective wave. We use HC-SR04 ultrasonic sensor can detect lower range from 1cm to 2.5 meter with precession about 0.1 cm and frequency up to 40Khz. The target must be proper orientation and perpendicular to the direction propagation of pulse. The amplitude of receive signal decrease depend on the medium and the distance between transmitter and the target. The transmitter converts electrostatic energy from a vibrating membrane to an ultrasonic waveform whilst the receiver converts the reflected ultrasonic waveform back into electrical energy. This electrical energy combine with motor servo to see the angle of sweeping and ultrasonic waveform using arduino atmega 2560 then be interpreted by a computer display in two dimension for measurement angle and distance of object. Conversion between electrical energy to an ultrasonic waveform use electrostatic transducer or normally we call piezoelectric transducer .

Acoustic emission system for a bat robot

One of the more difficult challenges in producing a biomimetic bat robot is replicating the bats' biosonar emission capabilities. For mimicking horseshoe bats (Rhinolophidae), this means creating a high-power acoustic source that can also properly illuminate the noseleaf structure over a bandwidth of approximately 20 kHz. Computer tomography (CT) scans were used to inform an accurate noseleaf model which was then abstracted from life using three-dimensional mesh software. A waveguide was designed to direct sound from two electrostatic transducers into a small nozzle, creating a point source. Numerical acoustic simulations were used to optimize these waveguide shapes to obtain a good compromise between point source behavior, high output amplitudes, and minimal internal reverberation. Physical geometries were procedurally generated to create a three-dimensional design space with parameters eccentricity, length and convexity. Data from each waveguide were analyzed for volume, multidirectionality, and echoing. A second round of simulations was executed to determine the effect of nozzle width on the output. Findings indicated that waveguide convexity and eccentricity played primarily into echoing and increased length reduced output volume. As nozzle diameter increased, the output volume increased, but sound became unidirectional. Future research may include development of alternative emission systems without a need for waveguides.

Bat-board: An integrated electronic system for a robotic bat

The biosonar of horseshoe bats combines the standard elements of any sonar system, i.e., pulse generation and echo processing, with a peripheral dynamics provided by deforming baffle shapes for ultrasonic emission ("noseleaves") and reception (ears). Realizing a “bat robot” to reproduce this biological system requires the implementation of three main electronic functionalities: power distribution, analog signal conditioning, and robot control. To meet the size constraints of the bat robot, the work presented here has designed/implemented a “Bat Board” that integrates all three of these functionalities within a single circuit board. The power distribution component of the Bat Board supplies the high voltage amplifier, the air compressors/valves for the pneumatic actuators in the noseleaf/the ears of the sonar head, and the data acquisition systems. The high voltage signal amplification component was designed to power the electrostatic transducers that were selected for their broad-band output. To combat interferences originating from the power distribution and the digital control signals, different PCB layout paradigms were implemented in the design process such as separating ground planes and the extensive use of bypass capacitors. The result of this efforts has been a single PCB implementation with a very compact footprint and the potential for further miniaturization.

Design and Implementation of a Bistable Force/Acceleration Sensing Device Considering Fabrication Tolerances

We report on a design methodology, fabrication, and characterization of a bistable actuator operated by a parallel-plate electrostatic transducer. The operational principle of the device, which can serve as a bifurcation-based contact-free force/acceleration sensor, is based on a pull-in voltage monitoring. Bistability appears due to the interplay between the hardening nonlinearity of straightened curved beams used as the compliant suspensions and softening nonlinearity of the electrostatic force directed along the beams. Careful mapping of the parameters at the design stage resulted in reduced device sensitivity to fabrication tolerances and allowed to ensure bistability, necessary for the functionality of the sensor. The devices were fabricated from silicon on insulator substrates using deep reactive ion etching and bistability was demonstrated experimentally. Acceleration was emulated using electrostatic force and sensitivity of 0.173 V/g was registered in experiments. Good agreement between the experimental results and the model predictions was observed. [2018-0031]

Optimal power, power limit and damping of vibration based piezoelectric power harvesters

Power harvesting describes the process of acquiring the ambient energy surrounding a system and converting it into usable electrical energy. Much of the work over the past two decades has focused on the conversion of ambient vibration energy sources using piezoelectric, electromagnetic and electrostatic transduction. Attempts were made to obtain a general model that could be applied to any transduction mechanism. Of the most interest is an electromagnetic generator model that was used by many researchers to model piezoelectric power harvesters. Two major results from the model are the power limit expression and the equal relationship between the electrically induced damping and the mechanical damping to reach the power limit. However, piezoelectric power harvesters cannot be accurately modeled by this electromagnetic model due to the essential difference in physics. There have also been attempts to obtain the power limit expression based on piezoelectric relationships, but they either neglect the piezoelectric backward coupling to the structure, or assume the power limit occurs at the resonance of the system. This paper obtains the power limit expression based on the piezoelectric coupled equations without those assumptions. In addition, the relationship between the electrically induced damping and mechanical damping at the power limit is studied. Furthermore, a closed-form criterion is derived and proposed to define strongly and weakly coupling power harvesters, whose differences in power characteristics are explained through analytical and numerical analysis. While most of the discussion is focused on linear power harvesters connected to a resistive circuit, the aim of this paper is to provide a comprehensive and deep understanding of this simple configuration, answers to important questions, and a starting point to develop a more general theory on power harvesters because similar system characteristics are observed in power harvesters with more complexities.

Frequency domain analysis of droplet-based electrostatic transducers

Squeezing a water droplet between two electrodes can generate a potential difference by converting some of the mechanical energy in vibrations into electrical energy. By utilizing the high capacitance inherent to electric double layers, and the surface charging at a polymer/water interface, we demonstrate a sensor that generates up to 892 mV peak-to-peak between 1 and 100 Hz, in response to a 250 μm deformation. This frequency response is described and explained using a linearized model in which the interfacial charge acts as the priming voltage, removing the need for external charging normally required in capacitive generators. The model suggests how to design the cell for maximum power output and provides an intuitive understanding of the high pass nature of the sensor. It successfully predicts the point of maximum power transfer.

Fused Silica Micro Shell Resonator With T-Shape Masses for Gyroscopic Application

This paper presents a novel micro shell resonator (MSR) with T-shape masses based on fused silica using out-of-plane electrode structures. Eight circular-distributed T-shape masses are designed along the rim of resonator shell to improve transduction efficiency, including drive efficiency and detection efficiency. The dynamic parameters and transduction efficiency are calculated and optimized with finite element method, revealing 3.76 times improvement in drive efficiency, 4.65 times improvement in detection efficiency, and 17.81 times increase in mechanical sensitivity. In addition, it is feasible to trim the frequency by adding or removing mass on the T-shape masses. The key feature of the process is based on micro blow-torching process and whirling platform, which form the resonator structure with smooth roughness and good structure uniformity. Femtosecond laser ablation is used to release the T-shape masses for good symmetry and high processing quality. Then, metalized MSR with T-shape masses is assembled on a glass substrate. Electrostatic transduction is used to detect spatial deformation of resonators by out-of-plane electrodes, which reveals a frequency mismatch of 0.175% at 6904.4 Hz and 6916.5 Hz with quality factors of 20.39 k ( $\tau =0.94$ s) and $Q = 36.94$ k ( $\tau =1.7\text{s}$ ). [2017-0086]