Piezoelectric sensor device and a method for detecting change in electric constants using the device

Abstract

A piezoelectric sensor device has a piezoelectric body vibrator consisting of the piezoelectric body which is sandwiched by a pair of electrodes, a power source which applies a voltage to the piezoelectric body vibrator so as to get excited for vibration, means for monitoring electric constants to detect changes in electric constants accompanied by vibration of the piezoelectric body. The change in electric constants in the piezoelectric body is detected as a change in frequency for vibration of the piezoelectric body corresponding to an electric constant under the determined conditions. The piezoelectric sensor device has a means to obtain the frequency value from frequencies at not less than two points giving a determined electric constant. According to the piezoelectric sensor device the dispersion in measured values due to change in vibration aptness of vibration system and the polarization state of piezoelectric body vibrator can be made smaller.

Description

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT[0001] This invention relates to a sensor device using a piezoelectric body vibrator used for measuring features of fluid in terms of the coefficient of viscosity, specific gravity, and density, etc.[0002] When those products being manufactured, to be used or sold in form of fluid such as chemicals, foods, lubricating oil, and car wax, etc. are placed under control on their manufacturing process and guaranteed in terms of their performance, it is important to measure the features of fluid in terms of the coefficient of viscosity, specific gravity, and density, etc. In recent years, in order to measure the features of such fluids, it is proposed to make use of a piezoelectric body vibrator, and in this regard, for example, in Japanese Patent Application Laid-Open No. 8-201265, a device for measuring coefficient of viscosity of fluids comprising a piezoelectric body vibrator consisting of piezoelectric body sandwiched by electrodes,...

AI Technical Summary

Problems solved by technology

As described above, the value of frequency f.sub.max giving the maximum value of .theta. almost remains unchanged, and therefore, it is possible to take for consideration a method where this f.sub.max is measured, which, however, results in larger errors since the value of .theta. remains almost unchanged before and after f.sub.max.

In addition, the piezoelectric body vibrator is left subject to aging (being left alone at a room temperature or higher temperature under the Curie point) for several hours to several days to stabilize the polarization state after an electric field with the level higher than the coercive field has been applied for comparatively long period at a temperature close to the Curie point as polarization processing at its forming, the piezoelectric body vibrator to be used for such a sensor device is extraordinarily highly sensitive, but on the other hand, maintains poor stability in many cases, and therefore even if aging described above is provided, the change in polarization state (deterioration in polarization state, depolarization) is apt to occur due to application of stress, changes according to lapse of time, etc.

Although it is preferred, however, to conduct measurement always polarizing in this way in terms of holding the polarization state constant, there are problems where resolution capability gets worse with vibration to be controlled or durability of a piezoelectric body vibrator is adversely influenced since a voltage is always applied to the piezoelectric body vibrator.

However, measurement should take place preferably during the period between immediately after polarization processing and 60 seconds thereafter, and further preferably during the period between 0.1 second to three seconds after polarization processing since it will be practically inconvenient if too much time is required from completion of polarization processing until when measurement takes place, which could lead to lengthening the measurement time period.

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