摘要:

Evaluation of the inherent noise levels of high‐responsivity sensors is critical for good design but this area is often treated casually until testing reveals a problem. Careful noise analysis early in the design process can save time, effort, and much frustration and reveal options for better performance. Once the sensor is fabricated, careful measurement of its noise can uncover deficiencies in the design or construction. In fact, serious examination of sensor noise can often reveal more about the fundamental workings of the sensor than can measurement of its transduction response. The usual assumption that the preamplifier dominates the noise of a sensor system, while sometimes true over limited bands, often leads either to suboptimal performance or to unrealistic expectations. This paper contains a discussion of noise resulting from thermal‐equilibrium agitation of mechanical elements, internal Johnson noise, equilibrium and non‐equilibrium shot noise, 1/f noise, stress‐induced noise in piezoceramics, various optical noise sources in fiber sensors, and preamplifier voltage and current noise. In addition, several measurement techniques are presented. These include effective isolation techniques for sub‐nano‐g resolution in ordinary laboratory spaces; coherence measurement; use of resistors as primary noise sources; and evaluation of preamplifier noise. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50331

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Acoustic velocity sensors, ‘‘geophones,’’ are used for geophysical exploration in large numbers worldwide. Information is presented to aid potential users in other fields to evaluate the geophone as a viable sensor. Past history and present design are covered in a series of slides and drawings. Intrinsic noise, frequency bandwidth, and dynamic range of typical production geophones are characterized. Some successful non‐geophysical applications are described. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50356

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Microfabricated accelerometers have been developed for a wide variety of applications; however, the principal commercial focus has been on signal detection in the milli‐g to tens or hundreds of g accelerations. The development of a microfabricated device to detect accelerations in the 10 to 100 nano‐g range is a substantial technological challenge because of the conflict between the required increase in mass (and reduction in suspension stiffness) and the small volume. In an underwater sensor, designed to be nearly neutrally buoyant, there are additional restrictions on the packaging of the sensor with regard to overall density, resistance to hydrostatic pressure, and flexibility of power and signal leads. The design goal of this project is to demonstrate a two‐axis sensor in an 8 cm3(and 8 gram) package capable of immersion to 600 meters. The sensor must have a self noise below 100 nano‐g per root hertz from 5 to 1000 Hz. Several of these requirements have been demonstrated with an accelerometer structure based on electron tunneling and microfabricated from single‐crystal silicon. The electron‐tunneling transduction mechanism provides an inherently large transduction constant (although at the expense of requiring closed‐loop control) and is readily adapted to batch fabrication in silicon. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50357

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Design equations, based on Ralph Woollett’s 1960 report [‘‘The Flexural Disk Transducer,’’ U.S. Navy Underwater Sound Laboratory Research Report No. 490], are presented for a bimorph accelerometer. Figures‐of‐merit are compared for PZT‐4, PZT‐5A, PZT‐5H, PZT‐8 piezoceramics, and PVDF‐TrFE copolymer. Neutrally buoyant, spherical and cylindrical accelerometer configurations can be designed to meet bandwidth, sensitivity, and depth requirements. Experimental results for PZT‐8 bimorphs indicate that simply‐supported edge conditions are easily achievable. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50358

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The most common method of directly measuring the vibration of a fluid‐loaded structure is through the use of accelerometers mounted on the surface. When that surface consists of a low density, highly‐compliant material it is necessary to take steps to insure that the sensor does not influence the motion of the surface. This can be accomplished with the use of small, neutrally‐buoyant accelerometers. However, if low frequency measurements are desired, where the acceleration is small, the signal‐to‐noise ratio obtained with small, low sensitivity accelerometers may not be acceptable. To address the problem of low‐frequency measurements of a submerged compliant surface the Underwater Sound Reference Department of the Naval Undersea Warfare Center (NUWC/USRD) has developed a class of neutrally‐buoyant capacitive displacement sensors. A displacement sensor requires that the mass‐spring system, which constitutes the detector, be operated above the resonance frequency, resulting in the mass being inertial. Thus, only the dynamic mass of these sensors needs to be equal to the mass of the displaced fluid so as to not alter the mass of the surface to be measured. These sensors are intended for use in the Anechoic Tank Facility at NUWC/USRD and must be capable of operating at hydrostatic pressures of 4.1 MPa (600 psi) and over a temperature range of −4 to 40 °C, while being rigid enough so that they neither affect nor are sensitive to the incident acoustic field. Since the sensor responds to the displacement of the surface, instead of the acceleration, these sensors are ideally suited for low frequency measurements. Both their design as well as experimental results will be presented. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50359

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A metallic glass accelerometer has been developed for use as an underwater sound velocity sensor. The device uses the metallic glass material Metglas 2605SC which has been processed to achieve a virgin coupling coefficient of 0.96. The mechanical to electrical conversion is based on the detection of the change in the inductance of the device as a result of bending motion. The detection method uses a carrier frequency signal which is amplitude modulated by the received signal. This scheme was originally described by Wun‐Fogle, Savage and Clark [‘‘Sensitive wide frequency range magnetostrictive strain gauge,’’ Sensors and Actuators, 1_2_, 323–331 (1987)]. The bender is in the form of a three layered laminate with a closed magnetic path window frame structure. The theory of operation along with measured and calculated results are presented for a prototype element with approximate dimensions 1.5×1.0×0.1 inches. Calculated and measured results agree for a reduced effective coupling coefficient of 0.72 and operation with a carrier field intensity of 0.87 Oe and carrier frequency of 20 kHz. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50333

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A (3,1) drive piezofilm vibration sensor is introduced. Operated above the lumped element resonance frequency of 600 Hz, the sensor delivers a voltage signal proportional to displacement over the frequency range of 2 kHz to 8 kHz. It is anticipated that the sensor response is flat above 8 kHz, but calibration has not been performed at higher frequencies. The sensor is very sensitive, detecting acoustic displacements as small as 10−5nm. Because of its simple design the sensor is robust and easy to assemble. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50334

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A crossed dipole array provides a directional receiving capability in a relatively small sensor package and is therefore very attractive for many applications in acoustics. Particle velocity measurements on two axes perpendicular to each other are required to provide the dipole signals. These can be obtained directly using particle velocity sensors or via simple transfer functions using acceleration and displacement sensors. Also, the derivative of the acoustic pressure with respect to space provides a signal proportional to the particle acceleration and gives rise to the pressure gradient sensor. Each of these sensors has strengths and drawbacks depending on the frequency regime of interest, the noise background, and whether a point or a line configuration of dipole sensors is desired. In this paper, the performance of acceleration sensors is addressed using a sensor concept developed at DREA. These sensors exploit bending stresses in a cantilever beam of piezoelectric material to obtain wide bandwidth and high sensitivity. Models which predict the acceleration sensitivity, pressure sensitivity, and natural frequency for this type of sensor are described. Experimental results obtained using several different versions of these sensors are presented and compared with theory. The predicted performance of acceleration sensors are compared with that of pressure gradient arrays and particle velocity sensors. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50335

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Until recently, accelerometer manufacturing appeared to be a reasonably mature field. But, this situation changed rapidly when researchers began to build miniature accelerometers using micron scale lithographic techniques developed for producing integrated circuits. Several micro‐ electro‐mechanical systems (MEMS) accelerometers are now available commercially. The MEMS devices are attractive because they are relatively inexpensive to produce and they include electronic circuits to perform a variety control and signal processing functions on the same chip. How does the performance of these new devices compare to their older and larger competitors? The physics of the scaling laws suggests that performance should decrease with size. The MEMS technology may be well positioned to take advantage of new, small‐scale sensing and actuating methods and, in the process, MEMS fabricated accelerometers may avoid or overcome the engineering limitations of older generation devices by using high precision micro‐machining, arrays of sensors, on‐chip temperature control circuitry, etc. This study compares the performance and physical characteristics of micro‐machined and conventional accelerometers. We review the physical operating principles and describe the basic scaling laws and other factors that ultimately limit accelerometer performance. Then we tabulate and discuss the current performance and characteristics of diverse types of commercial accelerometers. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50336

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Acoustic and vibration measurements were conducted at the Naval Undersea Warfare Center’s Seneca Lake Facility to investigate theinsitusignal response of a linear array of velocity sensors (sensors that measure either acoustic particle acceleration, velocity, or displacement have generically been denoted asvelocitysensors) on a coating. The coating used at Seneca Lake consisted of air‐voided elastomeric tiles with an overall coating thickness of approximately 3 inches. The accelerometer array and coating were mounted on the Seneca Lake Hull Fixture, which measures 33 feet lengthwise with an arc length of 20 feet. The fixture weighs approximately 30 tons. Specifically, measurements ofinsitusensitivity, velocity reduction, reflection gain, array beam response, and equivalent planewave self‐noise levels are presented. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50337

出版商:AIP    

年代:1996

数据来源: AIP