Showing papers on "Capacitive sensing published in 1993"

Capacitive sensors: When and how to use them☆

Abstract: Capacitive sensors for the detection of mechanical quantities all rely on a displacement measurement. The movement of a suspended electrode with respect to a fixed electrode establishes a changing capacitor value between the electrodes. This effect can be measured and if the mechanical quantity controls the movable electrode, a sensor is realized. Since the value of the capacitor is directly related to its size, and a small capacitor means high noise susceptibility, capacitive sensors should be as large as possible. Capacitive pressure sensors have been developed with success for industrial applications, where large membrane sizes are not a critical issue. However, most centres of expertise in silicon sensors show an interest in exploiting silicon technology to produce capacitive pressure sensors as well. From the above, this miniaturization trend appears to be an unsound idea. On the other hand, the principle of capacitive sensors allows the realization of measuring systems with so far unknown performance. Indeed, the capacitive sensor reveals distinct advantages when compared to its piezoresistive counterpart: high sensitivity, low power consumption, better temperature performance, less sensitive to drift, etc. Nevertheless, only a minor fraction of the market for pressure sensors is taken up by capacitive-type sensors. When observing the characteristics of capacitive sensors, it may seem surprising to encounter so few devices in real-world applications. The reasons for the lack in breakthrough can be found in the design complexity and the requirements for a matched sensing circuit. This paper will extensively discuss the justification of the choice for this research effort, and will elaborate on the techniques to fabricate the devices based on electronic manufacturing procedures. Basically, silicon capacitive sensors differ from piezosensors in that they measure the displacement of the membrane, and not its stress! This has important implications on the final assembled device: less package-induced problems can thus be expected. However, a far more important issue is their extremely high sensitivity, together with a low power consumption. These issues make them especially attractive in biomedical implant devices, or in other telemetry applications, where power is not randomly available. So far, this is the only field of success for these sensors. However, due to the interesting detection principle, new fields of application emerge, offering unique and superior performance when compared to available sensors. Uniaxial accelerometers are a good example, where extremely high cross-sensitivity reduction can be obtained.

Computer mouse or keyboard input device utilizing capacitive sensors

Abstract: A computer input device for use as a computer mouse or keyboard comprises a thin, insulating surface covering an array of electrodes. Such electrodes are arranged in a grid pattern and can be connected in columns and rows. Each column and row is connected to circuitry for measuring the capacitance seen by each column and row. The position of an object, such as a finger or handheld stylus, with respect to the array is determined from the centroid of such capacitance values, which is calculated in a microcontroller. For applications in which the input device is used as a mouse, the microcontroller forwards position change information to the computer. For applications in which the input device is used as a keyboard, the microcomputer identifies a key from the position of the touching object and forwards such key identity to the computer.

Verification of frequency scaling laws for capacitive radio‐frequency discharges using two‐dimensional simulations*

Abstract: Weakly ionized processing plasmas are studied in two dimensions using a bounded particle‐in‐cell (PIC) simulation code with a Monte Carlo collision (MCC) package. The MCC package models the collisions between charged and neutral particles, which are needed to obtain a self‐sustained plasma and the proper electron and ion energy loss mechanisms. A two‐dimensional capacitive radio‐frequency (rf) discharge is investigated in detail. Simple frequency scaling laws for predicting the behavior of some plasma parameters are derived and then compared with simulation results, finding good agreements. It is found that as the drive frequency increases, the sheath width decreases, and the bulk plasma becomes more uniform, leading to a reduction of the ion angular spread at the target and an improvement of ion dose uniformity at the driven electrode.

Digital liquid level sensing apparatus

Abstract: A digital liquid level sensing apparatus for detecting variations in the dielectric of a substance being sensed. The apparatus includes a capacitive element array including a plurality of individual (i.e., segmented) input plates positioned along an axis of measurement of a fluid to be detected. The array also includes a common output plate having a length sufficient to span the entire accumulated length of the input plates. A controller sequentially applies DC excitation pulses to the input plates which cause a series of output currents to be coupled onto the output plate. The output currents are input to a current-to-voltage amplifier which generates a series of corresponding analog output voltages. The analog output voltages are then input to a peak voltage detector circuit to generate a series of peak voltage signals representative of the magnitudes of the analog output voltages. The controller converts each of the peak voltage signals into a corresponding digital value and stores each of the digital values in an on-board memory. The controller then sequentially compares each of the values against at least one predetermined reference value indicative of an output produced by an input plate disposed in air until a predetermined difference is detected between the reference value and any one of the stored digital values. This indicates a predetermined difference in the dielectric, thus indicating that a corresponding input plate is at least partially submerged in fluid.

Capacitive proximity sensors

Abstract: Capacitive proximity sensors (10) are disclosed that can be fabricated in different shapes, sizes and materials used in a wide variety of applications. Each such sensor includes a sensing electrode (11) and a guard electrode (12). The sensor electrode and guard electrode can be parallel conductors separated by an insulating layer (13), such as a double-sided circuit board shaped to suit the application. One such sensor is transparent for use in conjunction with an optical bar-code scanner. Another sensor is annular in shape for use in conjunction with a telephone transmitter. The sensing electrode and the guard electrode are driven in unison by an RF signal (21). The proximity of an object to the sensor is observed by detecting (in 24) changes in the RF current flowing through the sensing electrode caused by the proximity of an object to the sensing electrode.

Self-testable micro-accelerometer and method

Abstract: An apparatus and method of operation for a self-testable accelerometer having a housing, a diaphragm for detecting movement in response to an applied force, a mass on the diaphragm, at least one capacitive plate attached to the housing and disposed such that a potential difference between the capacitive plate and the mass causes a movement of the mass, means for producing a potential difference between the mass and the capacitive plate, and at least one piezoresistive element attached to the housing and the diaphragm for sensing movement of the mass. The accelerometer's housing comprises a frame, a base and a cap with air gaps providing squeeze film damping and mechanical stops for inhibiting movements of the mass. The combination of the capability to electrostatically produce a movement of the mass and the piezoresistive sensing of that movement allows testing and calibration of the accelerometer at any stage in its manufacture or deployment without the need for external equipment such as shakers.

Radiofrequency capacitive heaters: the effect of coupling medium resistivity on power absorption along a mouse leg.

Abstract: A capacitive radiofrequency source in conjunction with a temperature-controlled electromagnetic coupling medium has the potential of delivering uniform heating distributions in a mouse leg for experimental studies to investigate the use of hyperthermia as a treatment for cancer. The system has been adopted by a number of groups who have confirmed that uniform temperatures can be achieved in the presence of blood flow along a one-dimensional line which extends between the plates across the leg. In this paper, a simple mathematical model is presented and verified experimentally to demonstrate that parallel-plate capacitive radiofrequency heaters produce an inherent absorbed power distribution along the leg which is determined by the impedances across the loads. Hence, the thicker or thinner regions of the leg can be preferentially heated by using a coupling medium with lower or higher salinity, respectively. Uniform power absorption along the mouse leg required that the coupling medium had equivalent electrical properties to those of tissue.

Capacitive patient presence monitor

Abstract: A capacitive sensor pad includes a foam plastic pad and heavy aluminum foil plates located to the opposite side thereof. The plates are adhesively bonded to the inner surfaces of an outer cover. The capacitor of the pad is connected in circuit with an oscillator and produces a frequency-related output. A ripple counter establishes a frequency-related output proportional to the capacitance. A microprocessor reads the counter output and a plurality of samples are averaged to establish a reference base and the true weight affect of the patient on the sensing pad. Other factors which might effect the signal are readily attended to by programmed compensation. Each subsequent sample is averaged and compared with the reference base. If within a permitted range, the latest and current signal is averaged with the reference base and establishes a new base, and continuously tracks changes in the sensing system. A selected change in a selected time delay system by requiring a series of alarm states or immediately if advance warning is desired, actuates an alert or alarm system, which requires positive resetting to terminate the alarm system. The system is positively reset to return to normal position monitoring. The system may be set to automatically reset the alarm system after an alarm condition is established and then removed by the continuous tracking of the patient movement.

Double-driven shield capacitive type proximity sensor

Abstract: A capacity type proximity sensor comprised of a capacitance type sensor, a capacitance type reference, and two independent and mutually opposing driven shields respectively adjacent the sensor and reference and which are coupled in an electrical bridge circuit configuration and driven by a single frequency crystal controlled oscillator. The bridge circuit additionally includes a pair of fixed electrical impedance elements which form adjacent arms of the bridge and which comprise either a pair of precision resistances or capacitors. Detection of bridge unbalance provides an indication of the mutual proximity between an object and the sensor. Drift compensation is also utilized to improve performance and thus increase sensor range and sensitivity.

Liquid transfer device for an analysis unit

Abstract: Liquid transfer device for an analysis unit with a capacitive liquid level detector (10) for detecting the dipping of a liquid transfer needle (5) into an analysis liquid (4). More reliable operation, in particular a better ratio between useful signal and noise signals, is achieved by the fact that the liquid transfer needle (5) is part of a coaxial electrode arrangement (11). The coaxial electrode arrangement includes, in addition to the liquid transfer needle (5), at least one coaxial electrode (12, 13) surrounding the needle and insulating from it and a detection circuit (17). This detection circuit detects a change in capacitance between the electrodes (5, 13) of the liquid level detector (10), and includes an isolation amplifier circuit (23) to whose input (23a) and output (23b) two adjacent electrodes (5, 12) of the coaxial electrode arrangement (11) are connected as signal electrode and compensating electrode. As a result of this structure, no voltage difference occurs between the signal electrode and the compensating electrode.

Capacitive sensing system and technique

Abstract: An arrangement senses a liquid level by monitoring the peak capacitance formed between a probe and the liquid as the probe approaches or departs from the liquid. In an automated system, a computer controls the position of a pipette probe for aspirating or dispensing liquid in the vessel. The charge developed via the capacitance on the probe is coupled to a capacitive sensor circuit which provides a peak detector with an amplified signal representing the peak capacitance between the probe and the liquid. This amplified signal is detected by a peak-capacitance discrimination circuit, the output of which is monitored by the computer for determining the precise position of the probe with respect to the liquid surface level. Other aspects of the present invention include a negative-static elimination circuit coupled to the output of a detector latch and a positive-static elimination circuit coupled to the input of the detector latch.

System for detecting human presence in hazardous situations

Abstract: The disclosure relates to a system for the detection of human presence in hazardous situations near moving machinery; appropriately shaped electrodes are placed in an area to be safeguarded and a radio-frequency signal of appropriate frequency and voltage is applied on the electrodes creating an alternating electric field around them. A bridge circuit is used to measure the capacitive impedance created by the electrodes and a dissipation factor is calculated from the real and imaginary parts of the impedance. If a foreign object is introduced into the electric field, the characteristics of the dielectric are modified creating a change of the value of the dissipation factor. The dissipation of energy in the high frequency region is much greater if this foreign object is a biological tissue as compared to other types of materials, such as conductors or insulators. Thus, the system detects human presence by noting the increase of the dissipation factor above a threshold value, and the movement of the appropriate machine part can then be stopped, thereby eliminating the danger of injury to the person.

Micro-machined resonator oscillator

Abstract: A micro-miniature resonator-oscillator is disclosed. Due to the miniaturization of the resonator-oscillator, oscillation frequencies of one MHz and higher are utilized. A thickness-mode quartz resonator housed in a micro-machined silicon package and operated as a "telemetered sensor beacon" that is, a digital, self-powered, remote, parameter measuring-transmitter in the FM-band. The resonator design uses trapped energy principles and temperature dependence methodology through crystal orientation control, with operation in the 20-100 MHz range. High volume batch-processing manufacturing is utilized, with package and resonator assembly at the wafer level. Unique design features include squeeze-film damping for robust vibration and shock performance, capacitive coupling through micro-machined diaphragms allowing resonator excitation at the package exterior, circuit integration and extremely small (0.1 in. square) dimensioning. A family of micro-miniature sensor beacons is also disclosed with widespread applications as bio-medical sensors, vehicle status monitors and high-volume animal identification and health sensors. The sensor family allows measurement of temperatures, chemicals, acceleration and pressure. A microphone and clock realization is also available.

A thin-film polysulfone-based capacitive-type relative-humidity sensor

Abstract: A capacitive type relative humidity sensor utilizing newly developed cross-linked fluorinated polyimide has been fabricated using thin film technology and evaluated for commercial building and factory applications. The sensor showed a good initial performance with less than 1% r.h. hysteresis and a 0 ± 0.1% r.h./°C temperature coefficient of capacitance along with good stability especially under high temperature and high humidity conditions. Also, because of the inherent chemical resistance of the polyimide, good sensor stability when subjected to volatile environments has been demonstrated.

Finite-element modelling and characterization of a silicon condenser microphone with a highly perforated backplate

Abstract: A new method to simulate the sensitivity and frequency response of capacitive microphones has been developed. Finite-element analysis is used in combination with equivalent circuits to evaluate mechanical, electrostatic and acoustic effects. The simulation method has been applied to a condenser microphone in monocrystalline silicon. The design has a thin and highly perforated backplate in combination with a small air gap of 2 μm. Sensitivities in the range 1–5 mV/Pa have been recorded with a bias voltage between 5 and 13 V. The device has a first resonance at 27 kHz. The measured results are in good agreement with the results obtained with the new simulation method.

Capacitive electrode system for detecting open solder joints in printed circuit assemblies

Abstract: Disclosed is a system that determines whether input and output leads of semiconductor components are present and properly soldered to a printed circuit board. The system includes a signal source which is connected to a wiring trace on the printed circuit board, which is soldered to the lead being tested. A capacitive test probe is placed on top of the component and connected to a capacitance measuring device. The signal source signal is capacitively coupled through the lead of the integrated circuit package being tested to the capacitive test probe, so if a predetermined capacitance is measured by the capacitance measuring device, the lead is connected to the circuit assembly. As the capacitances being measured are small, the capacitive test probe may include an amplifier, a shield or a buffer circuit to reduce stray capacitance.

Capacitive pressure sensor having circuitry for eliminating stray capacitance

Abstract: A pressure sensor measures pressure by measuring capacitance between two capacitive plates. Pressure change appears as a change in capacitance. Stray capacitance interferes with this measurement. The stray capacitance arises between the capacitor plates and surrounding material. Circuitry reduces stray capacitance by maintaining substantially no potential difference between a capacitor plate and surrounding material responsible for stray capacitance.

Printed circuit board having integrated decoupling capacitive core with discrete elements

Abstract: A printed circuit board laminate is disclosed having a high capacitance power distribution core. The power distribution core comprises a pair of conductive plates electrically connected to an array of high capacitance core tiles, separated by a compliant dielectric filler. The resulting capacitance of the power distribution core is sufficient to eliminate the need for decoupling capacitors on a typical printed circuit board. Separate power supply areas of variable decoupling capacitance can be formed for mounted integrated circuits with different power supply requirements. A method for manufacturing such board laminates is also disclosed that is compatible with standard printed circuit board assembly technology.

High sensitivity CMOS humidity sensors with on-chip absolute capacitance measurement system

Abstract: In this paper we report a new approach for an integrated humidity sensor fabricated using an industrial CMOS process with a measurement technique for evaluating on-chip floating capacitances. The sensor is based on a capacitive element covered with polyimide as humidity sensitive material. The circuit produces an output current which is precisely related to the absolute value of the capacitance and is insensitive to parasitic capacitances to ground, offsets and charge injection from switches, which enhances the sensitivity of the device. The response to relative humidity (r.h.) is highly linear and a maximum sensitivity of 0.9% (%r.h.) −1 is achieved, i.e. six times better than our previous CMOS humidity sensors ( Sensors and Actuators A, 25–27 (1991) 509–512). The hysteresis is reduced to less than 1% r.h. and the temperature coefficient to less than 0.1% r.h. °C −1 . Cross-sensitivity to flow of humid air is below 2% r.h. for flow rates up to 15 m/s. Our measurement technique can be used for evaluating on-chip capacitive microsensors, whenever exact knowledge of the device capacitances is crucial.

Capacitive and inductive coupling connector

Abstract: A coupling connector relies on inductive and capacitive coupling over a length L c to "connect" signals from a first circuit card to a second circuit card. The coupling connector includes a flex circuit which in a connected state is compressed to provide a coupling length L c between the first and second circuit cards. A typical application is for coupling cards to a backplane having a plurality of parallel, signal carrying tracks. The near end of each coupling track is input to a receiver having an input impedance equal to the characteristic impedance of the coupling track. The far end of the coupling track is terminated by a resistor whose value equals the characteristic impedance of the coupling track.

Capacitive moisture sensor.

Abstract: A capacitive moisture sensor includes insulator means (22); capacitance means including a sensing capacitor having as plurality of spaced capacitive sensor conductors (14) mounted with the insulator means for exposure to the atmosphere; and first (26, 28) and second electrodes (44, 46) mounted with the insulator means remote from the spaced capacitive sensor conductors; means for applying a periodic input current across the first and second electrodes; and means for detecting a change in capacitance between the first and second electrodes indicative of moisture bridging at least two of the capacitive sensor conductors.

Capacitance sensors for measurement of phase volume fraction in two-phase pipelines

Abstract: The effect of phase distribution (flow pattern) is considered for determining the volume fraction in two-phase pipelines. Experiments were performed with a variety of capacitance sensor designs. It was found that for a well-agitated mixture, two flow patterns occur for different compositions of the mixture, resulting in a discontinuous calibration curve. Sensitivity loss was found when the continuous phase was electrically conducting. This results in operational difficulties in applications to oil wells. It is concluded that capacitive sensors are practical only when the continuous phase is nonconductive or as a low electrical conductivity. >

Voltage compensating delay element

Abstract: A signal-delaying capacitive circuit applied to a node in a microcircuit device is immunized against the variation of the supply voltage by a metal-oxide semiconductor connected in series between the node and the signal-delaying capacitive circuit. The gate of tile semiconductor is biased with a voltage signal proportional to the supply voltage, whereby the resistance of the semiconductor is increased as the supply voltage decreases; thus, isolating the capacitive circuit from the node and reducing the delay.

Multi-coplanar capacitor for electrical connector

Abstract: A multi-coplanar, multi-layer capacitor array for use as an RFI filter associated with an electrical connector module having a pattern of pin and socket connections includes a multi-layer board structure of super imposed layers including ground plane layers and capacitor layers interleaved with one another. An array of apertures extend vertically through each of the layers forming a grid having a center-to-center spacing and pattern congruent with the apertures in the desired connector module and are configured to receive the conductor pins. The ground plane layers include a continuous conductive layer on top of a dielectric layer with clearance holes surrounding the apertures so as to be electrically insulated from the connector pins and the capacitors. The capacitor plane layers further include a plurality of defined coplanar individual separate capacitor areas encompassing a plurality of terminal pins but connected to one, the rest being provided with clearance holes, thereby associating each pin with an unique capacitive plane. The separate capacitors provide RFI filtering for the signals on each pin.

In situ method of forming a bypass capacitor element internally within a capacitive PCB

Abstract: An in situ method for forming a bypass capacitor element internally within a PCB comprising the steps of arranging one or more uncured dielectric sheets with conductive foils on opposite sides thereof and laminating the conductive foils to the dielectric sheet simultaneously as the PCB is formed by a final lamination step, the conductive foils preferably being laminated to another layer of the PCB prior to their arrangement adjacent the dielectric sheet or sheets, the dielectric foils even more preferably being initially laminated to additional dielectric sheets in order to form multiple bypass capacitive elements as a compound subassembly within the PCB. A number of different dielectric materials and resins are disclosed for forming the capacitor element. A dielectric component in the capacitor element preferably includes dielectric material and thermally responsive material, the thermally responsive material either forming a carrier for the dielectric material or formed as two separate sheets on opposite sides of a sheet of the dielectric material.

High‐resolution capacitive pressure gauges

Abstract: Design, construction, methods of measurement, and uses of high‐resolution capacitive pressure gauges in low temperature applications are reviewed. Pressure gauges for use from 10−3 to over 107 Pa, with a resolution of as high as 1×10−8 are described. Many uses that have been made of these gauges are mentioned with emphasis on 3He melting pressure thermometry.

Apparatus for measuring ice distribution profiles

Abstract: The present invention is an improved ice sensor which is particularly effective in measuring and quantifying non-uniform, heterogeneous ice typically found on aircraft leading edges and top wing surfaces. In one embodiment, the ice sensor comprises a plurality of surface mounted capacitive sensors, each having a different electrode spacing. These sensors measure ice thickness by measuring the changes in capacitance of the flush electrode elements due to the presence of ice or water. Electronic guarding techniques are employed to minimize baseline and parasitic capacitances so as to decrease the noise level and thus increase the signal to noise ratio. Importantly, the use of guard electrodes for selective capacitive sensors also enables distributed capacitive measurements to be made over large or complex areas, independent of temperature or location, due to the capability of manipulating the electric field lines associated with the capacitive sensors.

Humidity sensors using chemically modified polymeric materials

Abstract: Various chemical modifications of polymers are proposed to meet the requirement for use in two types of humidity sensors, i.e. the resistive type and the capacitive type. Hydrophilic polymers are adequate to prepare the resistive type sensor, but they must be modified in order to be insoluble in water. Three methods, i.e. graft polymerization, cross-linking and IPN formation, have been proposed. For the capacitive type sensor, the cross-linking of hydrophobic polymers has been proposed to modify them so they will not deform in the vapors of organic solvents.

Sensor having a membrane as part of an electromechanical resonance circuit forming receiver and transmitter converter with interdigital structures spaced apart from one another

Abstract: Sensor provided with a thin membrane, capable of storing electrical charges, whose surface is in contact with the chemical, biological, and/or other physical parameters to be measured. The membrane is part of an electromechanical resonance circuit, operating with propagating acoustic waves. Provision is made opposite the membrane for converters, equipped with interdigital structures, to act as transmitter and receiver for the propagating acoustic plate waves. Each of the converters is equipped with interdigital structures that act as narrow band, filters. The transmitter converter and the receiver converter is arranged with a distance between them. The signal from the receiver converter is detected capacitively. A suitable electronic control and evaluation circuit or feedback is provided in addition between the output signal of the receiver converter and the transmitter converter. Finally, a DC voltage is applied between the membrane on the one hand and the transmitter converter and the receiver converter on the other. The voltage has an alternating voltage superimposed on it in the transmitter converter. The generation of the acoustic plate waves in transmitter converter is produced solely on the basis of electrostatic attractive force or excitation between interdigital structures of the transmitter converter and the membrane. The output signal of the receiver converter is additionally decoupled in a capacitive manner in such fashion that the vibration amplitude of the acoustic propagating plate waves is measured in the receiver converter by the capacitor formed by interdigital structures of the receiver converter and the membrane. Finally, the membrane and interdigital structures of the transmitter converter on the transmitter side and the membrane and interdigital structures of the receiver converter on the receiver side are arranged spaced apart from one another by a spatial distance in the form of a gap. They have no mechanically solid connection with one another. A dielectric in the form of a narrow gap is therefore formed such that movement of the membrane toward the interdigital structures is possible in the dielectric.

Capacitive liquid level sensor having phase detecting circuitry

Abstract: A capacitive liquid level sensor having phase detecting circuitry incorporates a capacitive sensor array, an oscillator generating an input signal, a switching circuit sequentially coupling the input signal to the input plates of the capacitive sensor, one at a time, a pair of phase lock loop circuits (PLLs) each having voltage controlled oscillators (VCOs), a second switching circuit selectively coupling the output plates of the capacitive sensor array, in sequential fashion, to the PLLs, and an exclusive OR-gate (XOR) receiving the outputs of the VCOs from each of the PLLs. The XOR generates a first logic level signal when a difference in the phase of two signals from any two adjacent output plates indicates that a phase difference is present. A second logic signal is generated if no phase difference is detected. The signals are perfectly in phase when any two adjacent output plates are either submerged in fluid or both disposed in air. The phases of the output signals differ, however, when the two adjacent output plates are such that one is submerged in fluid while the other is at least partially disposed in air. Signal conditioning means including a pair of D flip flops and a resistive ladder coupled through a switching network provide an output signal to an optional display device for indicating a corresponding fluid level within a fluid reservoir within which the capacitive sensor array is disposed. In an alternative embodiment an integrator is included for determining the composition of the fluid being sensed.