Measuring and testing – Vibration – Resonance – frequency – or amplitude study
Reexamination Certificate
2000-08-09
2003-01-28
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
Resonance, frequency, or amplitude study
C073S662000, C073S514290, C310S312000, C310S365000, C310S366000, C310S369000
Reexamination Certificate
active
06510738
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to device and method for measurement, and more particularly to device and method for measuring vibration.
BACKGROUND OF THE INVENTION
Conventionally, sensors include two primary categories such as the point sensor and the distributed sensor. For the former, it usually has bandwidth limitation as a result of its structural frequency response. The latter, such as the one disclosed in U.S. Pat. No. 4,868,447, includes the mode sensor and is closely related to the measured structural body.
Since the measurement of acceleration or acceleration rate is a very important information in vibration of the structure system, the accelerometer is directly put into measuring the acceleration at present. Accelerometers normally arc of piezoelectric type or capacitance type. The former relates to an interaction between mechanical energy and electrical energy, which was found in 1880 by Curie brothers, i.e. Pierre Curie and Jacques Curie upon studying the relation between pyroelectric phenomenon and crystal symmetry. (Cady, 1964)
Curie brothers found that when the tourmaline is applied with stress, one can obtain the charge from the surface thereof. They subsequently found such phenomenon in a series of materials, e.g. zinc blende, calamine, boracite, sodium chlorate, quartz and Rochelle salt. The electrical polarization phenomenon produced with the deformation of such materials is called piezoelectricity.
Common piezoelectrical materials generally have three categories, i.e. natural crystal, piezoelectric ceramics and piezoelectric polymer. Since it is very difficult to make a distributed sensor through a natural crystal, e.g. quartz, only piezoelectric ceramics and piezoelectric polymer will be discussed here. Because piezoelectric ceramics, e.g. lead zirconate titanate (PZT) has higher coupling factor and dielectric constant, it is suitable for use in the driving device requiring a large driving force.
Current methods for measuring the acceleration rate are generally achieved by modifying the interface circuit of the accelerometer as disclosed in U.S. Pat. No. 5,521,772. The system response and the bandwidth of the conventional point sensor, no matter whether piezoelectric accelerometers or capacitance accelerometers are concerned, are limited by the frequency response of the sensor structure. In other words, the influence of the electronic circuit on the performance, the bandwidth, gain and phase angle of the conventional accelerometer are not only influenced by the interface electronic circuit but also primarily controlled by structural design of the sensor. General basic requirements of an excellent accelerometer are as follows: high electromechanical conversion efficiency, larger dynamic range, broader bandwidth response, higher stability, light weight, low transverse sensitivity and low environmental sensitivities in respect of factors including temperature, humidity and electromagnetic interference. It is not easy for the point sensor mentioned above, which is subjected to limitations of structural characteristics of its matching sensor, to meet with all design requirements at the same time.
To take the piezoelectric accelerometer as an example, after the piezoelectric material is applied with the accelerometer since 1960, the piezoelectric accelerometer has been extensively put into use in various fields. Generally speaking, apparent advantages of adopting the piezoelectric material as the sensing element of accelerometer include: light weight, small bulk, high reliability and self generation without external power supply necessary for the capacitance accelerometer. The bandwidth limitation of the accelerometer is primarily dominated by the sensor structure. Accelerometers using the piezoelectric material as the sensing element have the following featured designs:
1) Compression design: As shown in
FIG. 1
, the base
11
, i.e. the sensor structure, supporting thereon the piezoelectric material
12
, i.e. the sensing element, is connected to the external cover
13
to generate the acceleration signal through compressed deformation of the piezoelectric material;
2) Cantilever design: As shown in
FIG. 2
, the base
21
serves as the fixed end of the cantilever beam
22
formed by the piezoelectric material, whose free end is fixed to a mass
23
for increasing the acceleration response. The acceleration signal is measured through the strain generated by vibration of the cantilever beam
22
;
3) Shear design: As shown in
FIG. 3
, it includes a base
31
, a piezoelectric material
32
and an added mass
33
. The signal is generated through the shear strain of the piezoelectric material;
4) Single ended compression design: As shown in
FIG. 4
, the piezoelectric material
42
on the base
41
is stacked with a mass
43
to be independent of the external cover
44
for dealing with the influence of the sonic vibration;
5) Mushroom design: As shown in
FIG. 5
, the base
51
attaches thereon a beam
52
which has two free ends and is coated thereon with a piezoelectric material
53
serving as the sensing element.
Basic structural designs of all the above piezoelectric accelerometers are point sensors which measure the acceleration by the electronic signal generated when the strain of the piezoelectric material is subjected to change. No matter whichever sensor design is concerned, purposes of superior dynamic response and broadened system bandwidth are always sought. The bottleneck of the conventional point accelerometer is that the integral system bandwidth of the accelerometer is limited by the resonant mode of the sensor structure. If a better bandwidth is sought by raising the first resonant mode frequency, the accelerometer will have a poor dynamic response to the low frequency. If the structural stiffness of the accelerometer structure is reduced to increase the dynamic response to lower frequencies, the bandwidth of the accelerometer will be lowered.
Alternatively, if the acceleration signal of a specific frequency is to be measured, the conventional method is to provide a filter with the electronic circuit for attenuating the accelerometer signal of undesirable bandwidths. Since this filter modulates signal on the time domain, it must follow the causality. According to the Bode Gain Phase Theorem, phase change of the original acceleration signal will be introduced so that there will be an extreme difference between the measured acceleration signal and the real acceleration signal, which requires an additional compensation to calibrate the measured signal. It is not wonderful in use.
It is therefore tried by the Applicant to deal with the above situations encountered in the prior art.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a vibration measuring device for accurate measurement in a specific bandwidth.
It is further an object of the present invention to provide a vibration measuring device for selective accurate measurement in a frequency range.
It is further an object of the present invention to provide a vibration measuring device utilizing a feedback control loop to moderate the deformation of sensing structure.
It is still an object of the present invention to provide an accelerometer having an increased bandwidth and a reducted bulk.
It is additional an object of the present invention to provide a point distributed piezoelectric sensor by gathering together advantages of distributed sensor and point sensor.
It is yet an object of the present invention to provide a sensor of spatial filtering function having the modulated gain without the influence on the phase angle.
It is furthermore an object of the present invention to provide a spatial filter which will not result in any unnecessary phase lag but has a freely selective bandwidth.
It is again an object of the present invention to provide a modal sensor which can lift the bandwidth limitation resulting from the sensing structure resonance through the manufacture of the higher modal sensor and raising the bandwidth to an even h
Hsiao Wen-Hsin
Lee Chih-Kung
Lin Chih-Ting
Shih Hsueh-Ching
Knobbe Martens Olson & Bear LLP
Miller Rose M.
National Science Council
Williams Hezron
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