Measuring and testing – Vibration – Sensing apparatus
Patent
1989-12-15
1991-11-12
Chapman, John E.
Measuring and testing
Vibration
Sensing apparatus
310329, 73517R, G01P 1108
Patent
active
050637827
DESCRIPTION:
BRIEF SUMMARY
This invention relates to low frequency accelerometers, and associated control circuits, and in particular to piezoelectric low frequency accelerometers and their associated filter and amplifier circuits.
In many applications it is important in the construction of piezoelectric accelerometers to keep production costs as low as possible while at the same time endeavoring to attain as high as possible electrical output in relation to the acceleration induced into the piezolelectric elements involved.
In practice in relation to the construction of piezoelectric accelerometers for use at low frequencies it is also necessary to maximize the source capacity since it has been found that the provision of high source capacitance facilitates simplification of the design of systems, installations or the like in which such accelerometer are utilized.
In known arrangements for the production of acceptable accelerometers using piezoelectric material in the so-called radial mode of strain application, the piezoelectric material is subjected to compressive or tensile forces along directions radially directed of the mounting thereof from a support.
With such known arrangements it is a common practice, to deposit a layer of piezoelectric material onto a support element of the requisite mechanical properties.
This support element can, if desired, be formed by a further piezo electric material.
To maximize the sensitivity of the device it is conventional practice to arrange for the line of zero strain of the resulting accelerometer to lie along the junction between the piezoelectric element(s) and the associated support.
It is convenient to note that, in general, piezoelectric materials are operationally reversible in that an applied mechanical strain will produce an electrical output and in that any electrical signal input will produce a related mechanical strain effect in the piezoelectric material. In addition, piezoelectric materials are pyroelectric since any change in the temperature of the piezoelectric material will produce a corresponding electrical output.
In this connection it has been found that relatively small changes in the ambient temperature cause production of electrical outputs related to the temperature change and which are sufficiently large enough to cause spurious readings from the accelerometer.
It is an object of the present invention to provide an accelerometer which is particularly suitable for use at low frequencies, and in which the significance of ambient temperature changes upon the output of an accelerometer is reduced.
The frequency range for accelerometer applications contemplated for the present invention is from 0.1 Hz to 500 Hz. Within this range it is desirable that the lower limit should be selectively variable at least within the range 0.1 Hz to 20 Hz. In the case of the upper range limit such selective adjustment should be at least possible between 10 Hz and 500 Hz. Also it is required that the source impedance should be less than 1000 Ohms, and that any associated circuitry should be able rapidly to recover from overloads of up to, for example, in applications of accelerometers to the monitoring of the performance of a motor vehicle suspension system, 3 g for accelerometers mounted to the body of a vehicle and up to 50 g in applications where the accelerometers are mounted to the wheels of a vehicle. In so far as the question of ambient temperature conditions is concerned the accelerometer needs to be capable of operating over a temperature range of -55 to +125 degrees Celsius.
A further object of the invention is to provide an accelerometer in which the effects of the inherent pyroelectric characteristics of the piezo electric material are eliminated or are at least reduced. It is a still further object of the present invention to provide a band pass filtering and amplifying circuit which is able to operate with a low voltage supply of, for example, 5 volts, which is suitable for use with low frequency piezo electric accelerometers and which is able to produce from the output of the
REFERENCES:
patent: 3940974 (1976-03-01), Taylor
patent: 4374472 (1983-02-01), Nishimura
patent: 4473768 (1984-09-01), Kerr et al.
patent: 4658650 (1987-04-01), Yorinaga et al.
Chapman John E.
Reynolds Locke
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