Piezoresistive Accelerometer Research Articles

Analysis on twin-mass structure for a piezoresistive accelerometer

A novel mechanical structure for a piezoresistive silicon accelerometer is described and analysed. This structure consists of two seismic masses instead of one and five beams instead of the two or four in existing designs. The single sign and uniform profile of the stress at the central beam makes it ideal for locating sensing elements. Analytical analyses show that no lateral sensitivity can be caused by an acceleration in the beam direction and the sensitivity caused by a lateral acceleration vertical to the beam direction is also eliminated to the first order of approximation by a symmetrical design of the sensing elements. Both the normal sensitivity and basic resonance frequency of the structure are in between those of the cantilever beam structure and the double-side-supported structure, but the figure of merit, i.e., the sensitivity-frequency product, is higher for the two-mass structure than for the other two structures.

Damping of piezoresistive silicon accelerometers

The two most difficult steps in the fabrication of a silicon micromachined accelerometer are the stress-free mounting of the sensing element and its critical damping. This paper reports on the response of piezoresistive accelerometers encapsulated in a stainless steel housing filled with either oil or air. The viscosity of the chosen damping materials as well as the spacing between the seismic mass of the sensing element and the housing mainly determine the dynamic characteristics of the device. Results on oil-damped in comparison to undamped and air-damped sensors are discussed.

A smart accelerometer with on-chip electronics fabricated by a commercial CMOS process

Piezoresistive accelerometers with a monolithically integrated operational amplifier were produced, the fabrication process based on a commercial 3 μm CMOS process. The mechanical structures were realized using wet anisotropic etching of silicon with KOH and the electrochemical etch-stop at p—n junctions. Measurements show that the integration of these necessary micromachining process steps into the IC process do not influence the parameters of the electronic devices. Also, the parameters of the mechanical structures are comparable to discrete devices. The realization of application-specific smart mechanical sensors and actuators using a standard CMOS process is now possible.

High Bandwidth Orientation Measurement and Control Based on Complementary Filtering

When sufficiently small and cheap enough, sensor systems for the fast and accurate, direct measurement of the orientation of a robot relative to its environment offer a large potential for enhancing the safety and efficiency in various robotic applications. Examples include precision assembly, safety supervision and run-away detection, or attitude control of mobile systems. This paper presents an estimation system capable of angle measurements of up to tens of Hertz while consisting of relatively small, cheap and lightweight sensors: an allowably slow inclinometer and two accelerometers. Based on careful analysis and simulations of the estimation system, design criteria are proposed for the tuning of the third-order complementary filters as a function of the characteristic sensor parameters. The performance of the analog and the digital filter design is then experimentally verified for two different types of micromechanical accelerometers: a high performance capacitive accelerometer and a piezoresistive accelerometer. The overall high-bandwidth estimation system exhibits small phase loss over a wide range of frequencies with good robustness against noise and external impacts. The successful application of this estimation system is finally demonstrated in the closed-loop control of an inverted pendulum.

Characterization and modelling of silicon piezoresistive accelerometers fabricated by a bipolar-compatible process

This paper reports on the development of silicon piezoresistive accelerometers, fabricated using an industrial bipolar-compatible process. Three different types of accelerometers are presented. Their measured sensitivities and resonance frequencies are in good agreement with those calculated from analytical formulae and finite-element modelling.

Accelerometer systems with built-in testing

The ability to test sensors in the field without removing them from service is increasingly being sought as a way to assure ultimate reliability of the sensor subsystem. A piezoresistive accelerometer is presented which provides such a feature. Two issues in ultra-high reliability applications, such as safe-and-arming, are whether the accelerometer is free and working and whether the device is broken. A unique solution to these questions has been designed and implemented in a piezoresistive accelerometer; this approach allows the testing of the device by electrostatic deflection of the mass. This represents a new class of sensors which are ‘Smart’ sensors in that they can accommodate recalibration in the field, even though they may not contain circuitry on the sensor to allow storage of revised calibration information. Other examples of these new ‘Smart’ sensors are magnetic field sensors with integrated current loops. A number of key advantages result from this testable configuration for the accelerometer. The spring constants of the device may vary from unit to unit or over temperature as do the piezoresistive coefficients over temperature; however, as long as the voltage and initial separation gap are held constant for the electrostatic self-test, the output will be proportional to a given acceleration. Applications of the self-testing technique are in temperature compensation, testability and uni-directional force—balance applications.

Accelerometer systems with self-testable features

In recent years, substantial effort has been devoted to the design and fabrication of a new class of silicon sensors, the accelerometer. A number of companies have been working in the area to produce, for the first time, an accelerometer that is substantially more cost effective and with higher performance than previously possible. Careful electromechanical design and micromachining process development has allowed silicon accelerometers to be fabricated in volume. Two questions that arise in ultra-high reliability applications, such as safe-and-arming, are whether the aceelerometer is free and working and whether the device is broken. A unique solution to these questions has been designed and implemented in a piezoresistive accelerometer; this approach allows the device to be tested by electrostatic deflection of the mass. A number of key advantages result from this configuration. Even though the spring constants of the device may vary from unit to unit or over temperature, and even though the piezoresistive coefficients vary over temperature, as long as the voltage and initial separation gap are held constant, the output will be proportional to a given acceleration. Applications for the self-testing technique are in temperature compensation, testability and uni-directional force-balance applications. This approach of building testability into the sensor bridges the gap between the open-loop sensors now in production and the much more complex closed-loop force-balance devices.

Objective quantification of tremor in conscious unrestrained rats, exemplified with 5-hydroxytryptaminemediated tremor

To accomplish an objective quantification of tremor in conscious unrestrained rats, a registration device based on accelerometry was developed. Tremor intensity was continuously recorded by a small piezoresistive accelerometer (Egal 125-10D, Entran Devices) mounted on the back of the freely moving rat. The accelerometer was connected to a Grass Polygraph, where the analog signal was quantified as arbitrary sol| marks/ min by a 7P10 integrating unit. The integrated signals were then further analyzed by a desk-top computer (Luxor ABC-80). By analyzing tremor response to central serotonergic system activation, the reproducibility as well as the advantages and limitations of this recording system were demonstrated.

Design optimization for cantilever-type accelerometers

A procedure is presented for the optimization of cantilever-type piezoresistive and capacitive accelerometer structures with respect to high sensitivity for a given bandwidth, or with respect to large bandwidth when the sensitivity is given. Products of suitable powers of sensitivity S and the resonant frequency ω are shown to be the characteristic quantities to be maximized, since they can be chosen to be independent of at least one design parameter that is used later to make a trade-off between sensitivity and resonant frequency. The maximization of the power products automatically yields an optimum for the direct bandwidth—sensitivity product ω S. For piezoresistive accelerometers, small cantilevers with short end masses of the largest possible weight are found to be optimal. In the case of capacitive sensors, the most decisive factor is the spacing between the cantilever and the counterelectrode, which should be as small as possible.

Hand-arm vibration, part II: Vibration transmission characteristics of the hand and arm

A method for measuring the vibration levels due to vibration induced into the hand that were transmitted to specified locations on the hand and arm is presented. This method employed the use of subminiature clamped piezo-resistive accelerometers which weigh less than 0·3 g. The results of the investigation implied that vibration at frequencis above 100 Hz that was directed into the hand and fingers was isolated primarily to the hands and fingers. These results also indicated that large relative motions could possibly exist between adjacent tissues in the hand and fingers and across bone joints.

Dynamic loading of containment during blowdown: review of experimental data from marviken and brunsbüttel

This paper deals with the problem of the dynamic loading of the pressure suppression type containment due to pressure fluctuations which are induced during blowdown as a result of steam condensation in the suppression pool. It reviews the experimental data on pressure fluctuations and on the corresponding dynamic response of the structures submerged in the water charge. Two series of containment response experiments were performed on real pressure suppression systems. One series of measurements was performed within the framework of relief valve vent tests in the Brunsbüttel Power Plant (Federal Republic of Germany) in 1974. Another series of measurements was carried out within the framework of the blowdown tests performed at the Marviken Power Plant (Sweden) in 1972–1973. For the measurements of pressure fluctuations in the suppression pool strain gage-based pressure transducers were used. The dynamic response of the loaded structure was measured by strain gages or by piezoresistive (in some cases piezoelectric) accelerometers. The signal was amplified by carrier-frequency amplifiers (or loading amplifiers). The amplified signals were recorded on magnetic tape (FM recording). A two-step evaluation procedure was used. In the first step the time history plots of the variables directly measured (such as pressure, bending strain and acceleration) were plotted and interpreted. In the second evaluation step the cross-power spectral density function of different signal pairs was calculated, plotted and interpreted. For evaluation of the fluctuating pressure field in the suppression pool the combination was chosen of a reference pressure signal (measured at a fixed position called ‘reference position’) with a second pressure signal (measured at a different, arbitrary position) in most cases to perform the cross-spectra analysis. The set of cross-spectral density functions obtained by analyzing all signal combinations chosen was used to determine the power spectra, RMS amplitudes and phases of the corresponding pressure waves impinging on the structure submerged in the suppression pool. For the evaluation of pressure field—structural response relations one pressure signal was combined with one response signal (bending strain or acceleration) to give the input for cross-spectral analysis. By the combination of two response signals (bending strain—bending strain or acceleration—acceleration) for cross-spectral analysis the modal shapes of the responding structure was determined.

Radiation Effects on Piezoresistive Accelerometers

This paper describes the results of combined neutron and gamma ray irradiation tests conducted on two makes of piezoresistive accelerometers. Units were irradiated to total neutron doses of 7 X 1013, 5 X 1014, and 5 X 1015neutronscm2. One make, Bytrex, Inc. Model AV 1000, operated satisfactorily at all doses.

Structural and operational characteristics of piezo-transistors and allied devices

The results of a broad investigation are reported of device aspects of the ASE (anisotropic stress effect in p: n junctions) ranging from some basic studies to its exploitation in micro-miniature integrated transducers. Newly found manifestations of the ASE include changes in the negative resistance regions of four-layer diodes and avalanche transistors, and large reductions of input and output impedances with relatively little change in g m in a field effect transistor when stress is applied to the gate junction. Relaxation effects with time constants of the order of minutes have been observed with the application of large stresses (≳ 10 11 dyn cm −2). The existence of the ASE has been demonstrated up to stress frequencies of the order of 1·5 Mc. The fabrication of small basic piezo-transistor elements and their incorporation in phonograph pickups, pressure transducers, and accelerometers is described. The evaluation of the latter devices yielded a figure of merit (sensitivity × bandwidth/weight) of 140,000 which exceeds by about three orders of magnitude that of piezoresistive semiconductor strain gage accelerometers. Noise levels were equivalent to signals of the order of 0·05 G. High temperature sensitivity of piezo-transistors and some hysteresis phenomena constitute some of the problem areas in the further development of these devices. Efforts to develop integrated piezo-junction transducers with digital output are described. The best result so far was a sensitivity of the order of 30 c/dyn −1 at about 100 kc of microcircuit oscillators incorporating a piezo-transistor.