Showing papers on "Capacitive sensing published in 1999"

A three-axis micromachined accelerometer with a CMOS position-sense interface and digital offset-trim electronics

Abstract: This paper describes a three-axis accelerometer implemented in a surface-micromachining technology with integrated CMOS. The accelerometer measures changes in a capacitive half-bridge to detect deflections of a proof mass, which result from acceleration input. The half-bridge is connected to a fully differential position-sense interface, the output of which is used for one-bit force feedback. By enclosing the proof mass in a one-bit feedback loop, simultaneous force balancing and analog-to-digital conversion are achieved. On-chip digital offset-trim electronics enable compensation of random offset in the electronic interface. Analytical performance calculations are shown to accurately model device behaviour. The fabricated single-chip accelerometer measures 4/spl times/4 mm/sup 2/, draws 27 mA from a 5-V supply, and has a dynamic range of 84, 81, and 70 dB along the x-, y-, and z-axes, respectively.

Touch confirming touchscreen utilizing plural touch sensors

Abstract: A method and apparatus for discriminating against false touches in a touchscreen system is provided. The system is designed to confirm a touch registered by one touch sensor with another touch sensor, preferably of a different sensor type, prior to acting upon the touch (i.e., sending touch coordinates to the operating system). If the touch registered by the first touch sensor is not confirmed by the second touch sensor, the touch is invalidated. Thus the strengths of one type of sensor are used to overcome the deficiencies of another type of sensor. In one aspect, the secondary touch sensor comprises a force sensor to discriminate between true and false touches on other types of touch sensors, such as contaminants on optical and surface acoustic wave sensors, noise or weak signals on capacitive sensors, etc. The force sensor may be a simple one-element system that merely indicates that a touch has occurred or a multi-element system that can provide confirming or supplementary coordinate data. In another aspect, a capacitive sensor is used to confirm or veto touch data from optical, surface acoustic wave, and force sensors. As is the case with the secondary force sensor, a secondary capacitive sensor may be a simple discrete type or capable of providing touch coordinates in its own right. In a specific embodiment, one in which no touch overlay is used on a CRT monitor, the secondary touch sensor may employ the resistive coating on the surface of the CRT in combination with a current monitoring circuit that measures the amplitude of the electromagnetic noise signal coupled to the resistive coating. In this application when the screen is touched by a grounded object, the detected signal amplitude change exceeds a preset threshold thus indicating a valid touch.

Nanoindentation and contact stiffness measurement using force modulation with a capacitive load-displacement transducer.

Abstract: We have implemented a force modulation technique for nanoindentation using a three-plate capacitive load-displacement transducer. The stiffness sensitivity of the instrument is ∼0.1 N/m. We show that the sensitivity of this instrument is sufficient to detect long-range surface forces and to locate the surface of a specimen. The low spring mass (236 mg), spring constant (116 N/m), and damping coefficient (0.008 Ns/m) of the transducer allows measurement of the damping losses for nanoscale contacts. We present the experimental technique, important specimen mounting information, and system calibration for nanomechanical property measurement.

Touch mode capacitive pressure sensors

Abstract: Touch mode capacitive pressure sensors offer better performance in industrial applications than other devices. In touch mode operation, the diaphragm of the capacitive pressure sensor touches the substrate structure in operation range. The advantages of this mode of operation are near-linear output characteristics, large over-range pressure and robust structure that make it capable to withstand harsh industrial field environment. The principle, design, and characteristics of touch mode capacitive pressure sensors using various materials and technologies are discussed in this paper.

AC impedance and state-of-charge analysis of a sealed lithium-ion rechargeable battery

Abstract: A 7.2V, 1.25 Ah sealed lithium-ion rechargeable battery has been studied for estimating its state-of-charge (SOC) by AC impedance. The dispersion of impedance data over the frequency range between 100 kHz and 25 mHz comprises an inductive part and two capacitive parts. As the inductive behaviour of the battery is attributed to the porous nature of the electrodes, only the capacitive components have been examined. The data obtained at several SOC values of the battery have been analyzed by a non-linear least-squares fitting procedure. The presence of two depressed semicircles in the capacitive region of the Nyquist plots necessitated the use of an electrical equivalent circuit containing constant phase elements instead of capacitances. The impedance parameters corresponding to the low-frequency semicircle have been found useful for predicting the SOC of the battery, mainly because the magnitude of these parameters and their variations are more significant than those of the high-frequency semicircle. The frequency maximum (f(max)) of the semicircle, the resistive component (Z') corresponding to f(max), the phase angle (phi) in the 5.0 Hz-0.1 Hz frequency range, the equivalent series resistance (R-s) and the equivalent series capacitance (C-s) have been identified as suitable parameters for predicting the SOC values of the lithium-ion battery.

Circuit and method for eliminating surface currents on metals

Abstract: A two dimensional periodic pattern of capacitive and inductive elements (12, 14) defined in the surface of a metal sheet are provided by a plurality of conductive patches (62) each connected to a conductive back plane sheet (30) between which an insulating dielectric (26) is disposed. The elements act to suppress surface currents in the surface defined by them. In particular, the array forms a ground plane mesh (24) for use in combination with an antenna. The performance of the ground plane mesh is characterized by a frequency band within which no substantial surface currents are able to propagate along the ground plane mesh. Use of such a ground plane in aircraft or other metallic vehicles thereby prevents radiation from the antenna from propagating along the metallic skin of the aircraft or vehicle. The surface also reflects electromagnetic waves without the phase shift that occurs on a normal metal surface.

Low-noise MEMS vibration sensor for geophysical applications

Abstract: The need exists for high-sensitivity, low-noise vibration sensors for various applications, such as geophysical data collection, tracking vehicles, intrusion detectors, and underwater pressure gradient detection. In general, these sensors differ from classical accelerometers in that they require no direct current response, but must have a very low noise floor over a required bandwidth. Theory indicates a capacitive micromachined silicon vibration sensor can have a noise floor on the order of 100 ng//spl radic/Hz over 1 kHz bandwidth, while reducing size and weight tenfold compared to existing magnetic geophones. With early prototypes, we have demonstrated Brownian-limited noise floor at 1.0 /spl mu/g/Hz, orders of magnitude more sensitive than surface micromachined devices such as the industry standard ADXL05.

An RC load model of parallel and series-parallel resonant DC-DC converters with capacitive output filter

Abstract: A novel analytical methodology is proposed and applied to investigate the steady-state processes in voltage-fed parallel and series-parallel resonant DC-DC power converters with a capacitive output filter. In this methodology, the rectifier, output capacitor and load are replaced by an equivalent circuit which includes a capacitor and resistor connected in parallel. Excellent agreement was obtained when comparing numerical values calculated by the proposed model to cycle-by-cycle SPICE simulation and to the numerical results of earlier studies.

Theory and analysis of electrode size optimization for capacitive microfabricated ultrasonic transducers

Abstract: Theoretical analysis and computer simulations of capacitive microfabricated ultrasonic transducers indicate that device performance can be optimized through judicious patterning of electrodes. The conceptual basis of the analysis is that electrostatic force should be applied only where it is most effective, such as at the center of a circular membrane. If breakdown mechanisms are ignored, an infinitesimally small electrode with an infinite bias voltage results in the optimal transducer. A more realistic design example compares the 3-dB bandwidths of a fully metalized transducer and a partially metalized transducer, each tuned with a lossless Butterworth network. It is found that the bandwidth of the optimally metalized device is twice that of the fully metalized device.

A 600-dpi capacitive fingerprint sensor chip and image-synthesis technique

Abstract: This paper examines the possibility of a low-cost, high-resolution fingerprint sensor chip. The test chip is composed of 64/spl times/256 sensing cells (chip size: 2.7/spl times/10.8 mm/sup 2/). A new detection circuit of charge sharing is proposed, which eliminates the influences of internal parasitic capacitances. Thus, the reduced sensing-capacitor size enables a high resolution of 600 dpi, even using a conventional 0.6 /spl mu/m CMOS process. The partial fingerprint images captured are synthesized into a full fingerprint image with an image-synthesis algorithm. The problems and possibilities of this image-synthesis technique are also analyzed and discussed.

Microstrip directional couplers with ideal performance via single-element compensation

Abstract: Microstrip directional couplers suffer from poor directivity because of inhomogeneous dielectric, i.e., partly dielectric substrate, partly air. It is possible to compensate for this poor performance by introducing a single lumped capacitor or inductor at the edges or center of the coupled region. No attempt at a theoretical design of these couplers has been made in the literature. This paper fills the void by presenting an accurate approach to the design of microstrip directional couplers with ideal match or high directivity of both, using a single capacitive of inductive compensation. The method is valid for tight and loosely coupled structures. The method is validated via design and experimental results.

A sensitive differential capacitance to voltage converter for sensor applications

Abstract: There is a need for capacitance to voltage converters (CVC's) for differential capacitive sensors like pressure sensors and accelerometers which can measure both statically and dynamically. A suitable CVC is described in this paper. The CVC proposed is based on a symmetrical structure containing two half ac bridges, is intrinsically immune to parasitic capacitances and resistances, is capable of detecting capacitance changes from dc up to at least 10 kHz, is able to handle both single and differential capacitances, and can easily be realized with discrete components. Its sensitivity is very high: detectable capacitance changes of the order of 2 ppm of the nominal value (24 aF with respect to a nominal capacitance of 12 pF) result in a measured output voltage of 1.5 mV. However, due to drift the absolute accuracy and resolution of the CVC is limited to 3.5 ppm. A differential accelerometer for biomedical purposes was connected to the CVC and showed a sensitivity of 4 V/g. The measured rms output voltage noise in the frequency range of 2-50 Hz is 750 /spl mu/V, resulting in a signal to noise ratio of 75 dB at an acceleration of 1 g in the frequency range of 2-50 Hz.

A micromachined pressure/flow-sensor

Abstract: The micromechanical equivalent of a differential pressure flow-sensor, well known in macro mechanics, is discussed. Two separate pressure sensors are used for the device, enabling to measure both, pressure as well as volume flow-rate. An integrated sensor with capacitive read-out as well as a hybrid, piezo-resistive variant is made. The fabrication processes are described, using silicon and glass processing techniques. Based on the sensor layout, equations are derived to describe the sensor behavior both statically as well as dynamically. With the derived equations, the working range of the sensor and the thermal and time stability is estimated. The computed results of the stationary behavior are verified with the measured data. A good similarity in linearity of the pressure/flow relation is found. The computed hydraulic resistance, however, differs from the measured value for water with 21%. This difference can be explained by the high sensitivity of the resistance to the resistor channel cross-section parameter in combination with the difference between the rounded etched shape and the rectangular approximation. From fluid dynamics simulations, a working range bandwidth of about 1 kHz is expected. Thermal influences on the sensor signal due to viscosity changes are in the order of 2% flow signal variation per Kelvin. From these results, it can be concluded that the sensor can be used as a low cost, low power consuming flow and pressure-sensing device, for clean fluids without particles and without the tendency to coat the channel walls. If a high accuracy is wanted, an accurate temperature sensing or controlling system is needed.

Capacitive detection scheme for space accelerometers applications

Abstract: This paper presents the design and performance of an ultra-sensitive position sensor dedicated to future space accelerometers. The electromechanical transducer is based on a capacitive scheme and aims at detecting the motion of a proof-mass in an electrostatic accelerometer. A new design called “area variation capacitance” is investigated to minimize back-action forces exerted on the proof mass. The resolution of that position sensor is a few tenths of picometer in the 0–1 Hz bandwidth. The position is digitized with a sigma–delta converter to feed the control laws of the accelerometer.

Electric field imaging

Abstract: The physical user interface is an increasingly significant factor limiting the effectiveness of our interactions with and through technology. This thesis introduces Electric Field Imaging, a new physical channel and inference framework for machine perception of human action. Though electric field sensing is an important sensory modality for several species of fish, it has not been seriously explored as a channel for machine perception. Technological applications of field sensing, from the Theremin to the capacitive elevator button, have been limited to simple proximity detection tasks. This thesis presents a solution to the inverse problem of inferring geometrical information about the configuration and motion of the human body from electric field measurements. It also presents simple, inexpensive hardware and signal processing techniques for makin the field measurements, and several new applications of electric field sensing. The signal processing contribution includes synchronous undersampling, a narrowband, phase sensitive detection technique that is well matched to the capabilities of contemporary microcontrollers. In hardware, the primary contributions are the School of Fish, a scalable network of microcontroller-based transceive electrodes, and the LazyFish, a small footprint integrated sensing board. Connecting n School of Fish electrodes results in an array capable of making heterodyne measurements of any or all n(n – 1) off-diagonal entries in the capacitance matrix. The LazyFish uses synchronous undersampling to provide up to 8 high signal-to-noise homodyne measurements in a very small package. The inverse electrostatics portion of the thesis presents a fast, general method for extracting geometrical information about the configuration and motion of the human body from field measurements. The method is based on the Sphere Expansion, a novel fast method for generating approximate solutions to the Laplace equation. Finally, the thesis describes a variety of applications of electric field sensing, many enabled by the small footprint of the LazyFish. To demonstrate the School of Fish hardware and the Sphere Expansion inversion method, the thesis presents 3 dimensional position and orientation tracking of two hands. 1 (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.) 1Please see the URL http://www.media.mit.edu/people/jrs/thesis.html for video clips, code, and other information related to this thesis.

Precision positioning using a microfabricated electrostatic actuator

Abstract: This paper presents a microfabricated actuator designed for high precision servo-positioning in a magnetic hard disk drive. The device is actuated using electrostatic force generated with parallel-plate capacitive electrodes. The displacement of these electrodes is measured using a dedicated capacitive sensing interface, allowing closed-loop control to be used to extend the servo bandwidth. Using the sensing electronics and a simple phase-lead compensator, a prototype device was used to actuate a 1.6 mg ceramic slider over a 1.2 kHz bandwidth. Using optical position measurements, the same actuator was used to achieve a 2.5 kHz bandwidth.

Modeling of touch mode capacitive sensors and diaphragms

Abstract: The understanding of deflection, stress and strain of thin diaphragms with clamped edges under various loads including when the diaphragm touches the substrate is of great importance for designing and fabricating sensors and actuators. This article reports the finite element analysis (FEA) of these diaphragms for computer-aided design of touch mode capacitive pressure sensors. The advantages of the touch mode operation are good linearity in touched range, mechanically robust and large overload protection. These simulation results predict the change of the performance when the device parameters are varied and can be served as a design tool to arrive at a set of desired performance. The results also are useful for designing actuators with diaphragms.

Capacitive micromachined ultrasonic transducer arrays with reduced cross-coupling

Abstract: There is described a capacitive micromachined ultrasonic transducer array which is configured to minimize the excitation and propagation of plate waves traveling in the substrate and ultrasonic waves propagating at the interface between the array surface and the immersion fluid

Evaluation of MEMS capacitive accelerometers

Abstract: Capacitive based microelectromechanical systems (MEMS) accelerometers are devices that measure acceleration based on a change in capacitance due to a moving plate or sensing element. These devices have been implemented in many commercial applications, such as automobile air bags, navigation, and instrumentation. These devices have been employed in these and many other applications because they generally offer more sensitivity (more mV/g) and more resolution than similar piezoresistive accelerometers. For most commercial applications, the maximum g-sensing level (MGSL) employed in capacitive accelerometers is 500 g/sub n/. However, in many applications, there can be high-frequency components to an acceleration profile that are much higher than the MGSL of an accelerometer. For example, in vibration monitoring of a hard drive, the peak acceleration can be as high as 10 kg/sub n/. The response and recovery times of an accelerometer to such shock over range are important in many critical applications. In this article, three commercial MEMS-based capacitive accelerometers (Silicon Designs, Inc. 1220, Analog Devices ADXL, 181-1000, and Endevco 7290A-100) are evaluated below and above their respective MGSLs. The output of these devices is compared to that of an Endevco piezoresistive 7270-A accelerometer and an Endevco 2270 comparison standard accelerometer. The emphasis on this investigation is to determine the response of these devices to high-g shock levels and to evaluate their failure modes.

Evaluation of MEMS capacitive accelerometers

Abstract: Capacitive based microelectromechanical systems (MEMS) accelerometers are devices that measure acceleration based on a change in capacitance due to a moving plate or sensing element. These devices have been implemented in many commercial applications, such as automobile air bags, navigation, and instrumentation. These devices have been employed in these and many other applications because they generally offer more sensitivity (more mV/g) and more resolution than similar piezoresistive accelerometers. For most commercial applications, the maximum g-sensing level (MGSL) employed in capacitive accelerometers is 500 g/sub n/. However, in many applications, there can be high-frequency components to an acceleration profile that are much higher than the MGSL of an accelerometer. For example, in vibration monitoring of a hard drive, the peak acceleration can be as high as 10 kg/sub n/. The response and recovery times of an accelerometer to such shock over range are important in many critical applications. In this article, three commercial MEMS-based capacitive accelerometers (Silicon Designs, Inc. 1220, Analog Devices ADXL, 181-1000, and Endevco 7290A-100) are evaluated below and above their respective MGSLs. The output of these devices is compared to that of an Endevco piezoresistive 7270-A accelerometer and an Endevco 2270 comparison standard accelerometer. The emphasis on this investigation is to determine the response of these devices to high-g shock levels and to evaluate their failure modes.

Method of fabricating silicon capacitive sensor

Abstract: The sensors include buried electrical feedthrough (112b) to provide an electrical connection into a sealed silicon cavity (108). The buried feedthrough consists of a conductor (112b) in a shallow groove (106) in a substrate (102), communicating between the sensing cavity (108) and an external contact area (110). The sensor designs also feature a method for forming a silicon-to-silicon fusion bond (SFB) wherein at least one of the two surfaces (152, 252) has to be a rough silicon surface unsuitable for good SFB joints because it was bonded heavily boron-doped by means of diffusion. The method of this invention includes preparing each doped surface (152, 252) for SFB by polishing the surface with a Chemical-Mechanical Polishing (CMP) process. The sensor designs can also include optional reference capacitors (141, 241) on the same chip (100, 200) as the sensing capacitor (140, 240).

Rotating sputter magnetron assembly

Abstract: A magnetron especially advantageous for low-pressure plasma sputtering or sustained self-sputtering having reduced area but full target coverage. The magnetron includes an outer pole face surrounding an inner pole face with a gap therebetween. The outer pole of the magnetron of the invention is smaller than that of a circular magnetron similarly extending from the center to the periphery of the target. Different shapes include a racetrack, an ellipse, an egg shape, a triangle, and a triangle with an arc conforming to the target periphery. The small shape allows high power densities to be applied to the area of the target actually being sputtered. Preferably, the magnetic flux produced by the outer pole is greater than that produced by the inner pole. The asymmetry provides several advantages in high-density plasma sputtering. The invention allows sustained self-sputtering of copper and allows sputtering of aluminum, titanium, and other metal at reduced pressures down to at least 0.1 milliTorr. However, at least for titanium, bottom coverage is improved for higher chamber pressures. For some metals, the pedestal bearing the wafer should be RF biased to a limited degree. The invention allows ionization fractions of the metal of 20% and greater with only the use of capacitive power coupling and can produce bottom coverage of greater than 25% in a hole having an aspect ratio of 5.

Control arrangement for microelectromechanical devices and systems

Abstract: A microelectromechanical system in which, operationally independent of the movable element and the component for moving it, a microelectromechanical sensor senses the position of the movable element. The microelectromechanical sensor adjoins the movable element, at least in part, and may be a strain gage, or a gage of a capacitive, piezoelectric, piezoresistive, or pressure type, among others. The resulting signal is fed back to control the component for moving the movable element. In an array of movable elements and sensors, the signal of each sensor is specific to one movable element.

Micromachined low frequency rocking accelerometer with capacitive pickoff

Abstract: A micro electro mechanical sensor that uses capacitive readout electronics. The sensor involves a micromachined low frequency rocking accelerometer with capacitive pickoff fabricated by deep reactive ion etching. The accelerometer includes a central silicon proof mass, is suspended by a thin polysilicon tether, and has a moving electrode (capacitor plate or interdigitated fingers) located at each end the proof mass. During movement (acceleration), the tethered mass moves relative to the surrounding packaging, for example, and this defection is measured capacitively by a plate capacitor or interdigitated finger capacitor, having the cooperating fixed electrode (capacitor plate or interdigitated fingers) positioned on the packaging, for example. The micromachined rocking accelerometer has a low frequency (<500 Hz), high sensitivity (μG), with minimal power usage. The capacitors are connected to a power supply (battery) and to sensor interface electronics, which may include an analog to digital (A/D) converter, logic, RF communication link, antenna, etc. The sensor (accelerometer) may be, for example, packaged along with the interface electronics and a communication system in a 2″×2″×2″ cube. The proof mass may be asymmetric or symmetric. Additional actuating capacitive plates may be used for feedback control which gives a greater dynamic range.

Capacitive type pressure sensor

Abstract: By sealing a diaphragm with less processes and lower cost and reducing deformation due to remaining stress, a stable and highly reliable pressure sensor construction is proposed. The pressure sensor is low in measurement error and small in floating capacitance and leakage current and good in characteristic. As a means to attain the above object, a polycrystalline silicon diaphragm is sealed with a silicon oxide film deposited through a LPCVD method and then completely covered. The diaphragm is placed on a surface of a semiconductor substrate with a nearly constant gap of 0.15 to 1.3 μm, and has difference-in-grade constructions of a deformation reducing means due to remaining stress.

Vortex flow sensor

Abstract: The sensor (1) is provided by a capacitive turbulence sensor (3) positioned in a measuring pipe within the turbulence current generated by a baffle plate inserted in the flow path, for capacitive measurement of the pressure variations The turbulence sensor fits through a sealed opening in the mantle surface of the measuring pipe, covered by a membrane (33) with an attached sensor flag (31) on one side and a capacitive electrode device (34) on the opposite side

A capacitive CO2 sensor system with suppression of the humidity interference

Abstract: This paper presents a capacitive CO 2 sensor system with an effective numerical suppression of the humidity interference. The system consists of an heatable interdigital capacitor (IDC) with a CO 2 indicator coating (heteropolysiloxane), a capacitive humidity sensor and (optional) a temperature sensor. The CO 2 measurement range has a high resolution between 100 and 3000 ppm CO 2 and covers with a lower resolution 100–10000 ppm CO 2 (twice TLV). These specifications follow the needs of indoor air quality monitoring (e.g., for mechanical ventilation). The software algorithm for the humidity interference rejection can be scaled down to a microcontroller (i.e., 8051 family). A long term study results in a lifetime of the CO 2 sensors of more than 8 months.

A lateral capacitive CMOS accelerometer with structural curl compensation

Abstract: We present successful experimental results from the first lateral capacitive accelerometer to be designed and manufactured in a conventional CMOS process. Compatibility with conventional CMOS provides advantages of low cost, high yield and fast prototyping that should be transferable to any CMOS foundry. A fully differential capacitive-bridge interface which cannot be realized in polysilicon technology is designed and implemented. Out-of-plane curling associated with the composite structural layers is compensated to first order through a curl matching technique. The prototype accelerometer has a measured sensitivity of 1.2 mV/g and a 0.5 mg//spl radic/Hz noise floor at the output of the sensing element.