Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2000-06-06
2002-06-04
Kwok, Helen (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C310S329000, C310S333000
Reexamination Certificate
active
06397677
ABSTRACT:
BACKGROUND AND SUMMARY OF INVENTION
The present invention relates generally to accelerometers employing piezoelectric materials, and more specifically to shear piezoelectric sensors responsive to acceleration or vibration.
The acceleration experienced by a rotating member or structure is often a very important parameter to consider during a system design. For example, an automobile crash imparts tremendous energy to the occupants and significant rotational inertia is also present. Mechanical structures deform dynamically at resonant frequencies and the resulting stresses can cause tremendous damage.
A Finite Element Analysis (FEA) is typically employed to form a mathematical model of the system. This analysis relates the deformation at one surface of a discrete elemental section to the surface deformation at opposing and adjacent elemental surfaces by an appropriate stress strain relationship. As surface displacements and rotations are considered in the analysis where each of them represents a degree of freedom of the system. Attachments such as welds, joints, bolts or the like can introduce significant error into the FEA model because the required stiffness estimates are generated from engineering judgement and empirical data. Therefore, a dynamic measurement or analysis must be performed when the results may have critical consequences.
An experimental study such as the above FEA is typically performed using only linear accelerometers. A spatially narrow array provides a means to estimate rotations; however, measuring rotations still presents a great deal of difficulty at interfaces such bolted joints. These interfaces often have large relative rotation but vary minimal linear displacement and therefore a method of measuring rotational acceleration is very important. Unfortunately, measuring this dynamic rotational data has not been straightforward due to the lack of convenient and accurate rotational accelerometers.
A variety of techniques have been attempted which use a pair of spatially separated, sensitivity-matched linear accelerometers to estimate and determine rotational acceleration. When linear accelerometers are located on a fixture at a prescribed distance apart, the output signal difference between them is used to estimate rotational acceleration. However, a problematic fact is the prevailing levels of output signal generated by the translational movement tends to overshadow those due to rotational motions. This makes the differencing operations above liable to serious error. See, D. J. Ewens, Modal Testing: Theory and Practice; Research Studies Press Limited 1984.
It is the purpose of the present invention to provide a transducer element or accelerometer which allows the accurate measurement of angular acceleration which may be suitable for vehicle impact testing, though not limited solely for this.
Manufacturers of accelerometers have more control over the sensitivity matching process and can incorporate technologies which have the qualities required by the design constraints of a rotational accelerometer. The design of an accelerometer always involves the optimization of a parameter compromise; thus, there is not a single accelerometer that fulfills all realms of acceleration measurement.
Application specific designs are tailored for their best fit into the field of interest. For example, Experimental Modal Analysis (EMA) is a field of study which predominately incorporates a sensor well-suited for low frequency ranges (less than 1000 Hz) and controlled environmental conditions. A piezoelectric bimorph is perhaps best for this set of conditions.
A bimorph is formed from two piezoelectric plates which are inversely polarized, then sandwiched and fused together, then sliced to form a rectangle. The piezoelectric element of a bimorph serves as a seismic mass since it is mounted in a manner showing flexure when exposed to acceleration. When the bimorph is packaged in a cantilever beam arrangement, the rectangular shape results in an extremely flexible seismic system, in its sensitive axis, as compared to the two orthogonal directions defining planes transverse thereto. Even though the seismic system is not extremely stiff, as is typical to most accelerometers, the obtainable frequency response is well-suited for EMA.
The typical bimorph piezoelectric accelerometer is symmetric about a central fulcrum. Any rotation about this central fulcrum generates equal magnitude, but inverted, charges from each of the symmetric beams and therefore a self-cancellation occurs. Central rotations flex the symmetric halves in opposite directions, while linear acceleration creates similar bending on both sides of the fulcrum. When the beams are arranged to have opposite polarity, however, the charges then sum to provide an output proportional to angular acceleration about the fulcrum. Such a system is shown, for example, in U.S. Pat. No. 4,996,878.
The prior art beam-type accelerometer, as disclosed in U.S. Pat. No. 4,996,878, was capable of detecting rotational acceleration. The beam-type design required electrical contacts or leads interconnecting each the stressed faces of the beams. Each of these leads was connected to a miniature charge amplifier and a miniature multiconductor cable. The low impedance voltage outputs from each of the beams are then connected to a remote signal conditioner, which has facility for powering the sensor's internal electronics and processing the independent signals from the faces of the beams. Additionally, the prior art required precision adjustable potentiometers to precisely fine-tune the sensitivity of the signal from each face in order to create an exact match to its counterpart. One summing amplifier and one differencing amplifier would then provide as output the sums and differences of the sensed charges. These outputs would be proportional to the linear and angular acceleration sensed. Thus, the prior art required post-processing electronics in order to obtain readable figures. The addition of these post-processing components necessarily introduces numerous aspects of error into the design.
As alluded to above, the bimorph beam-type accelerometer required precise sensitivity matching of the beams. This is not a simple task because even a minor difference in sensitivity can introduce excessive error. It has been shown that an error in sensitivity matching as small as one fourth of one percent can contribute 12.3% error in the computed rotational acceleration even on a simple cantilever beam structure. See, Shumin Li, David L. Brown, Armin Sietz, Milte Lally's “The Development of Six-Axis Arrayed Transducer” Proceedings International Modal Analysis Conf. 1994. Therefore, it is clear that producing an accurate rotational accelerometer from commonly available commercial hardware is very difficult, yet of vital importance in design.
Moreover, bimorph beams necessarily involve fusing two distinct crystals together with a type of epoxy. The introduction of this epoxy into the crystalline structure, or even a slight glitch in the epoxy attaching the beams to the fulcrum, will also introduce another possibility of error, and may further require frequent recalibration.
There are several other shortcomings to the beam-type bimorph design. The beam-type bimorph design is not well-suited for applications outside of EMA. A preferred design for other higher frequency or higher-impact applications, such as in crash testing, is in the shear-type accelerometer. For higher-frequency applications, accelerometers utilizing the shear principle have some special advantages over beam-type bimorph accelerometers.
The shear-type rotational accelerometer is assembled in much the same way as the bimorph beam-type accelerometer. A pair of accelerometers are aligned with polarities reversed such that their axes of sensitivity are parallel and spaced-apart. A first axis is thereby formed to be parallel to, and running between, the axes of sensitivity. A rigid connection extends between the two accelerometers, and thereby defines a second axis that runs perpendicu
Insalaco Michael D.
Kinsley Norton
Barnes & Thornburg
Kistler Instrument Corporation
Kwok Helen
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