Electrostatic Feedback Research Articles

Electrical Control of Effective Mass, Damping, and Stiffness of MEMS Devices

We propose the use of electrostatic force feedback to control the effective mass, damping, or stiffness of micro electro mechanical system (MEMS). Our analysis suggests that if feedback force is proportional to sensed displacement, velocity, or acceleration of a MEMS proof mass, then feedback can be used to increase or decrease the apparent stiffness, damping, or mass of the system. Such feedback might be used to compensate the process variations, packaging stress, thermal drift, and damping. Prior efforts by others include position or velocity-based feedback for modifying frequency, bandwidth, quality factor, or sensitivity of resonators. However, we present a comprehensive means of controlling the response of MEMS. We develop an analytical steady-state MEMS model that includes feedback forces and circuit delay, and we develop a stability model. Our analytical models are verified using numerical simulations that include circuit delay, electrical feedback delay, and noise. Our results support the realization toward performance-on-demand MEMS (PODMEMS).

An assistive haptic interface for appearance-based indoor navigation

Computer vision remains an under-exploited technology for assistive devices. Here, we propose a navigation technique using low-resolution images from wearable or hand-held cameras to identify landmarks that are indicative of a user’s position along crowdsourced paths. We test the components of a system that is able to provide blindfolded users with information about location via tactile feedback. We assess the accuracy of vision-based localisation by making comparisons with estimates of location derived from both a recent SLAM-based algorithm and from indoor surveying equipment. We evaluate the precision and reliability by which location information can be conveyed to human subjects by analysing their ability to infer position from electrostatic feedback in the form of textural (haptic) cues on a tablet device. Finally, we describe a relatively lightweight systems architecture that enables images to be captured and location results to be served back to the haptic device based on journey information from multiple users and devices.

39-3: An Electrostatic Haptic Display with a Projected Capacitive Touch Screen

This paper shows that 1 MΩ resistance of an electrostatic electrode can be used to commercialize the electrostatic tactile technology. The DC offset method induces electrostatic tactile feedback best among 3 kinds of modulation methods to reduce operational voltages, and reduces 39.9% of them in comparison with the conventional method.

Real-Time Operation and Characterization of a High-Performance Time-Based Accelerometer

An accelerometer based on the electrostatic pull-in time of a microstructure is presented in this paper. The device uses a parallel-plate overdamped microstructure, and real-time control operation is performed using a field programmable gate array and high precision digital-to-analog converters. Both open-loop and closed-loop measurements are presented. The low noise is a key feature of this approach, which is limited only by the mechanical–thermal noise of the microstructure used, 2 $\mu \text{g}/\surd $ Hz as shown in the open-loop results (3 $\mu \text{g}/\surd $ Hz in closed-loop operation). The time readout method has extremely high-resolution capabilities. The pull-in time sensitivity can be adjusted up to 1.6 $\mu \text{s}/\mu \text{g}$ , and the electrostatic feedback voltage sensitivity is 61.3 $\text{V}^{2}$ /g. The closed-loop control allows operation of the accelerometer in a much larger range than in open-loop (±500 mg have been achieved) and the linearity is greatly improved ( $\mu \text{g}$ has been measured over 45 h in open loop and 250 $\mu \text{g}$ in closed loop. [2015-0074]

Harmonic Distortion Analysis for Switched-Capacitor Electromechanical Sigma–Delta Modulators

As the conversions between the sensing element and interface in the feedback loop and forward path are nonlinear, harmonic distortions appear in the output spectrum, which will decrease the signal-to-noise and distortion ratio. Nonlinear distortions are critical for a high-resolution electromechanical sigma–delta ( $\Sigma \Delta $ ) modulator. However, there exists no detailed analysis approach to derive harmonic distortion results in the output signal for electromechanical $\Sigma \Delta $ modulators. In this paper, we employ a nonlinear op-amp dc gain model to derive the nonlinear displacement to voltage conversion in the forward path, and the nonlinear electrostatic feedback force on the proof mass is also computed. Based on a linear approximation of the modulator in the back end, the harmonic distortion model in the output spectrum of the proposed fifth-order electromechanical $\Sigma \Delta $ modulator is derived as a function of system parameters. The proposed nonlinear distortion models are verified by simulation results and experimental results.

Study of self-calibrating MEMS accelerometers

Micro-electromechanical System(MEMS) accelerometers are widely used in a number of inertial navigation systems and vibration detection system thanks to their small size, low cost and low power consumption. In order to improve their performance, the accelerometers have been designed to compensate the zero-bias caused by process variations. A new method of self-calibration sensitivity applies a self-test structure to simulate standard acceleration; depending on the standard and real-time values of the accelerometer’s output and by adjustment of the time division feedback, the scale factor of capacitive accelerometers can be flexibly adjusted to achieve sensitivity in self-calibrating MEMS accelerometers. Moreover, this research also uses the following: a PID feedback structure to improve the stability of the closed-loop system; a correlated double sampling (CDS) circuit to attenuate noise, which can eliminate zero drift caused by offset voltage of the pre-amplifier; a time division multiplexing electrostatic force feedback circuit to achieve the operation of a closed-loop micro-accelerometer. The structure can completely avoid electrostatic feedback coupling with a capacitance change detection circuit, which can also improve the bandwidth and stability of the accelerometer. By means of capacitance compensation array the zero-bias performance of accelerometers can be improved. The bias stability of the accelerometer can be reduced from 173mg to 31mg by testing.

Performance of a Sensitive Micromachined Accelerometer With an Electrostatically Suspended Proof Mass

A three-axis micromachined accelerometer with a proof mass suspended electrostatically in six degrees of freedom was designed and tested to evaluate its performance of this sensitive sensor for potential microgravity space applications. The device is based on a glass/silicon/glass bonding structure, fabricated by bulk micromachining process, and operated with force-balance technology. The motion of the proof mass with respect to each side is fully servo-controlled by capacitive position sensing and electrostatic force feedback. The design and simulation of multiaxis suspension control loops are presented based on the stiffness requirements for different full-scale ranges. The ground test of this sensitive accelerometer is facilitated by setting the vertical axis at a relatively high measurement range to counteract the one-g gravity, whereas the range in two lateral axes can be set as low as possible to achieve high sensitivity. Detailed experimental results of electrostatic suspension, threeaxis accelerometer, and its cross-axis sensitivity are presented with the device operated initially in an atmospheric environment. The preliminary test results of a prototype accelerometer show that a sensitivity up to 688.8 V/g and a noise density down to 3 μg/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> can be achieved by setting an extremely low full-scale range of ±2.90 mg. The results also show that much different stiffness levels in the design of three-axis suspension is a major source of cross coupling effects in the prototype accelerometer.

A high-order closed-loop ΣΔ interface for micro-machined accelerometer sensor

This paper presents a closed-loop interface for micro-machined accelerometer sensor based on a force-feedback sigma-delta loop. A high-order closed-loop accelerometer interface circuit is investigated, and a time-shared multiplexing electrostatic feedback technique is used to eliminate feedthrough between feedback signals and pick-up charge signals. A low noise capacitance detection circuit is proposed with correlated-double-sampling technique to eliminate the 1/f noise and offset of operational amplifier. The interface is fabricated in a standard 0.5 μm CMOS process and the active circuit area is about 13 mm2. The chip consumes 20 mW from a 5 V supply with a sampling clock of 250 kHz. The average noise floor of the digital accelerometer is about ?115 dBV/Hz1/2 over a 1.5 kHz bandwidth. Corresponding to a sensitivity of 302 mV/g, the measured resolution of the accelerometer system is about 6 μg/Hz1/2. The nonlinearity is 0.085 %. The figure of merit shows that the proposed sigma-delta interface achieves a good performance.

Interface circuit of sigma-delta accelerometer with on-chip-test function

A fifth-order fully differential interface circuit (IC) with on-chip-test function is presented to improve the noise performance for micromechanical sigma-delta (Σ-Δ) accelerometer. The proposed on-chip-test technique for Σ-Δ accelerometers avoids a shaker table applying a sinusoidal signal as the simulated acceleration which involves distortion itself. An electrostatic force feedback linearization circuit is presented to reduce the harmonic distortion resulting in a larger dynamic range (DR). The post-simulation results show that the electrostatic force feedback linearization circuit decreases the harmonic distortion effectively and the proposed on-chip-test technique achieves 98 dB third-order harmonic distortion detection, and the nonlinearity of the proposed circuit is 0.02%.

Nonlinearity Analysis of Closed-Loop ΣΔ Micro-Mechanical Accelerometer

A high-order interface circuit based on sigma-delta (ΣΔ) modulation is advantageous to enhance the resolution and reduce the quantization noise of micro-accelerometer, while the performance is restricted by the effect of nonlinearity of op-amp, electrostatic force feedback and quantizer. A fifth-order closed-loop ΣΔ capacitive accelerometer is proposed in this paper and a theoretic nonlinearity of the accelerometer was investigated. The nonlinear behavioral models based on SIMULINK are given in the paper. The simulation and test result are presented through optimization based on the nonlinearity analysis at last.

Identification and adjustment of the position and attitude for the electrostatic accelerometer's proof mass

In an electrostatic force feedback accelerometer (EFFA11EFFA: electrostatic force feedback accelerometer.) system, nonlinearities in the transfer function between the input acceleration and the output voltage signal are mainly brought in by the detection circuit, demodulation circuit, and unsatisfactory closed-loop position of the proof mass. Closed-loop control can overcome the first two effect factors but remains ineffective to the last. The traditional way to adjust the closed-loop position is rolling the EFFA on the turntable, and then experientially adjusting according to the output feedback voltages to make the proof mass gradually approach the ideal closed-loop position by repeated times. This paper proposes a novel method which can simplify the complex process and save the turntable. According to the feedback voltages changing with the varying preload-voltage, the relative closed-loop position of the proof mass can be calculated and the proof mass can be adjusted to any required positions including the ideal one. The whole test process turns to be automatical and quantitative by using digital controllers such as DSP. Applying this novel way to the electrostatic levitation accelerometer which cannot be placed on the turntable because of its extremely low range, the problem of position adjustment can be solved.

Analysis and fabrication of a novel MEMS pendulum angular accelerometer with electrostatic actuator feedback

A new type of sensor to directly detect angular acceleration is essential for inertial and control technology. The above interest motivates us to propose a novel micro electromechanical system (MEMS) pendulum angular accelerometer with electrostatic actuator feedback. It adopts a proof pendulum with optimized moment of inertia, suspended to dual anchors by a pair of torsion spring beams, as sensing component. A pair of electrodes are designed as differential capacitors to detect the torsional angular of pendulum, then measure input angular acceleration in sensing axis. Another pair of electrodes are designed as electrostatic actuators for feedback control loop. The structure and operating principle of the MEMS angular accelerometer are introduced. Then, the structure kinetics analysis and signal detecting scheme based on differential capacitors are provided in detail, and the sensitivity and resolution of sensor are derived. Compared with the other MEMS angular accelerometers, the proof pendulum with optimized moment of inertia improves sensitivity and resolution of sensor. The electrostatic actuators feedback loop optimizes the dynamic capability and nonlinearity characteristic. The sensor is fabricated by MEMS fabrication technology. The ANSYS simulation and test results prove the validity of the theoretical analyses. The MEMS angular accelerometer can be used in industrial robots and aircraft by further implementing the signal processing electrocircuit.

Force Rebalance Controller Synthesis for a Micromachined Vibratory Gyroscope Based on Sensitivity Margin Specifications

This paper presents a design method of force rebalance control for the sense mode of a micromachined vibratory gyroscope with multiobjective. The control strategy is mainly based on constraining sensitivity margin specifications via numerical optimization approach, which embeds the sense mode in a closed-loop system to possess more robust performance. This paper also provides a quantitative methodology of configuring the system parameters to realize the functional electrostatic force feedback control for the microgyroscope in the case of nonideal coupling and external angular disturbance. Theoretical results predicted by the control loop are shown to be in close agreement with the experimental results using a practical microgyroscope, which indicated a satisfactory performance of the proposed control algorithm. The maximums of the sensitivity function and complementary sensitivity function are measured to be 4.5 and 1 dB, resulting in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GM</i> ≥ 7.84 dB and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PM</i> ≥ 35°. The gyroscope achieves a scale factor of 7.1 mV/deg/s with nonlinearity of 0.2% which is improved by one order of magnitude compared with that of the open loop. The bandwidth is extended to about 98 Hz from 30 Hz in the open loop. The quadrature error and temperature stability of the scale factor are reduced from - 10.33 to -59.22 dB and from 4858 to 646 ppm/ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C with the force rebalance control, respectively.

An Electrostatic Force Feedback Approach for Extending the Bandwidth of MEMS Vibratory Gyroscope

This paper proposed an effective approach for extending the bandwidth of MEMS vibratory gyroscope by employing the electrostatic force feedback control. The mathematical model for the bandwidth is established through the dynamic model of the gyroscope, which indicates that the bandwidth of the sensor depends on the difference between the resonant frequencies of the two working modes. The system bandwidth can be enlarged by utilizing electrostatic force rebalance control to null Coriolis force caused by external angular rate which can also improve the performance of the transient response. Simulation results forecast a satisfactory performance of the control system with suggested control method.

High resolution interface circuit for closed-loop accelerometer

This paper reports a low noise switched-capacitor CMOS interface circuit for the closed-loop operation of a capacitive accelerometer. The time division multiplexing of the same electrode is adopted to avoid the strong feedthrough between capacitance sensing and electrostatic force feedback. A PID controller is designed to ensure the stability and dynamic response of a high Q closed-loop accelerometer with a vacuum package. The architecture only requires single ended operational amplifiers, transmission gates and capacitors. Test results show that a full scale acceleration of ±3 g, non-linearity of 0.05% and signal bandwidth of 1000 Hz are achieved. The complete module operates from a ±5 V supply and has a measured sensitivity of 1.2 V/g with a noise of floor of in closed-loop. The chip is fabricated in the 2 μm two-metal and two-poly n-well CMOS process with an area of 15.2 mm2. These results prove that this circuit is suitable for high performance micro-accelerometer applications like seismic detection and oil exploration.

Low vibration rectification in a closed-loop, in-plane MEMS device

A method for a geometry of a lateral comb drive for an in-plane, electrostatic force feedback, closed-loop, micromachined accelerometer or closed-loop Coriolis rate gyroscope device, or closed-loop capacitive pressure or force measuring device. When vibration is applied to the device, the error in the time-average output, which is vibration rectification error, due to this input vibration is minimized or eliminated. The geometry resulting from practice of the present invention is space-efficient because drive force is maximized while vibration rectification is minimized or eliminated.

Active Vibration Control and Isolation for Micromachined Devices

Some harsh environments contain high frequency, high amplitude mechanical vibrations. Unfortunately some very useful components, such as micro-electro-mechanical systems (MEMS) gyroscopes, can be very sensitive to these high frequency mechanical vibrations. Passive micromachined silicon low-pass filter structures (spring-mass-damper) have been demonstrated in recent years. However, the performance of these filter structures is typically limited by low damping. This is especially true if operated in low pressure environments, which is often the optimal operating environment for the attached device that requires vibration isolation. An active micromachined vibration isolator can be realized by combining a state sensor, and electrostatic actuator and feedback electronics with the passive isolator. Using this approach, a prototype active micromachined vibration isolator is realized and used to decrease the filter Q from approximately 135 to approximately 60, when evaluated in a low pressure environment. The physical size of these active isolators is suitable for use in or as packaging for sensitive electronic and MEMS devices, such as MEMS vibratory gyros.

Development of a Prototype Miniature Silicon Microgyroscope

A miniature vacuum-packaged silicon microgyroscope (SMG) with symmetrical and decoupled structure was designed to prevent unintended coupling between drive and sense modes. To ensure high resonant stability and strong disturbance resisting capacity, a self-oscillating closed-loop circuit including an automatic gain control (AGC) loop based on electrostatic force feedback is adopted in drive mode, while, dual-channel decomposition and reconstruction closed loops are applied in sense mode. Moreover, the temperature effect on its zero bias was characterized experimentally and a practical compensation method is given. The testing results demonstrate that the useful signal and quadrature signal will not interact with each other because their phases are decoupled. Under a scale factor condition of 9.6 mV/°/s, in full measurement range of ± 300 deg/s, the zero bias stability reaches 15°/h with worse-case nonlinearity of 400 ppm, and the temperature variation trend of the SMG bias is thus largely eliminated, so that the maximum bias value is reduced to one tenth of the original after compensation from -40 °C to 80 °C.

Diffraction based optical MEMS microphones and accelerometers with active electrostatic force feedback

Diffraction-based optical displacement detection method and its use in low noise micromachined microphones have been shown earlier. [Hall et al., J. Acoust. Soc. Am. 118, 3000-3009 (2005), Garcia et al., J. Acoust. Soc. Am. 121, 3155 (2007)]. In these devices, the integrated electrostatic port of the sensor is uncoupled from the integrated optical sensing. This structure enables one to use this port for sensitivity tuning, self characterization, and active control to adjust the device dynamics. Given that the displacement noise of integrated optical sensor is below the thermal-mechanical noise of the mechanical structure, one can implement force feedback methods such as active Q-control, or adjust device stiffness without adding substantial noise to the system. We implemented micromachined optical microphones and accelerometers with integrated optoelectronics integrated in a 1.5mm3 volume. We present experimental results on force feedback Q-control of low noise omnidirectional, and biomimetic directional optical microphones, as well as adjusting the stiffness of accelerometers to improve their frequency response. [Work supported by NIH Grant 5R01DC005762-03 and the Catalyst Foundation.]

Force feedback linearization for higher-order electromechanical sigma–delta modulators

A higher-order electromechanical sigma–delta modulator can greatly improve the signal-to-noise ratio compared with a second-order loop that only uses the sensing element as a loop filter. However, the electrostatic force feedback on the proof mass is inherently nonlinear, which will produce harmonics in the output spectrum and limits the total signal-to-noise and distortion ratio. High performance inertial sensors, which use sigma–delta modulators as a closed-loop control system, have strict requirements on the output signal distortion. In this paper, nonlinear effects from the force feedback and pick-off circuits are analysed and a strategy for force feedback linearization is put forward which can considerably improve the signal-to-noise and distortion ratio. A PCB prototype of a fifth-order electromechanical modulator with a bulk micromachined accelerometer was used to demonstrate the concept.