Measuring and testing – Dynamometers – Responsive to force
Reexamination Certificate
1998-03-12
2001-05-22
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Dynamometers
Responsive to force
Reexamination Certificate
active
06234032
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a method and apparatus for measuring a load applied to an elastic body on the basis of deformation of the elastic body caused by the applied load, and more particularly, to a method and apparatus for making an accurate load measurement by eliminating the influence of deformation of an elastic body comprised of a piezoelectric element on a measured object or a measured load.
2. Relevant Arts
Most load measuring devices are designed to make a load measurement based on a deformation amount of an elastic body observed when a load is applied thereto. A spring scale, a typical load measuring device, measures a load by utilizing the relation that the elongation of a spring is proportional to a load applied to the spring. A load cell which is another type of load measuring device is comprised of an elastic body, having a small elastic constant, and a strain gauge affixed to the elastic body. The load cell is adapted to measure a load applied to the elastic body by utilizing a change in resistance of the strain gauge caused by deformation of the strain gauge generated by the applied load.
In a load measurement based on deformation of an elastic body, the deformation of the elastic body generally has no substantial influence on a measured object or a measured load (a load to be measured). For example, in the case of measuring the weight of a measured object with use of a spring scale, a spring is elongated when the measured object is suspended from the spring, so that the distance between the measured object and the center of the earth decreases. In a strict sense, therefore, the gravitational force acting on the measured object increases. However, the elongation of the spring is extremely small and hardly affects the accuracy of load measurement.
In some cases, however, the deformation of the elastic body may affect a measured object or a measured load. For instance, in the case of using a load cell to measure a load applied to a wall from a body fixed to the wall when the body is heated, a load generated by the heat expansion of the body and applied to the wall causes the elastic body of the load cell to be deformed. The deformation of the elastic body changes a state of the measured object, resulting in an inaccurate measurement and a meaningless measurement.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and apparatus for accurately measuring a load applied to a piezoelectric element, by eliminating the influence of deformation of the piezoelectric element on a measured object or a measured load and by eliminating a measurement error caused by the hysteresis characteristic of the piezoelectric element.
A load measuring method according to one aspect of the present invention comprises the steps of: (a) applying a voltage to a first piezoelectric element, the voltage serving to suppress a deformation which is to be naturally generated in the first piezoelectric element to which a load is applied; (b) applying the same voltage as the voltage applied to the first piezoelectric element to a second piezoelectric element which has the same characteristic as that of the first piezoelectric element; and (c) measuring the load applied to the first piezoelectric element based on an amount of deformation of the second piezoelectric element caused by the applied voltage.
With the load measuring method of the present invention, when a load is applied to the first piezoelectric element so that the first piezoelectric element is going to be deformed, a voltage for suppressing a deformation of the first piezoelectric element is applied to the first piezoelectric element. During the load measurement, the deformation of the first piezoelectric element is reduced to substantially zero by the applied voltage, so that the influence of the deformation on the measured object or the measured load is eliminated. The voltage applied to the first piezoelectric element takes a value corresponding to the load applied to the first piezoelectric element.
However, for the following reason, it is difficult to carry out a meaningful load measurement based on the applied voltage value which reduces the deformation of the first piezoelectric element to zero. In respect of deformation-vs.-applied voltage characteristic, the first piezoelectric element has a hysteresis characteristic which is represented by a hysteresis loop comprised of two curved lines, as in the case of ordinary piezoelectric elements. Upon application of a voltage, therefore, the first piezoelectric element is driven along a corresponding one of these curved lines of the hysteresis loop, which line is determined depending on a history of voltage application to the first piezoelectric element. Thus, the applied voltage required to reduce the deformation of the first piezoelectric element to zero takes one of two different values, depending on the voltage application history. In addition, the difference between these voltage values is large enough to hinder an accurate load measurement. This indicates that a meaningful load measurement cannot be achieved based on the applied voltage which reduces the deformation of the first piezoelectric element to zero. In other words, a difficulty is encountered in accurately measuring the load applied to the first piezoelectric element, while eliminating the influence of the deformation of the piezoelectric element on the measured object or the measured load.
To remove such a difficulty, the load measuring method of this invention comprises the step (b) of applying a voltage to a second piezoelectric element, in addition to the step (a) of reducing the deformation of the first piezoelectric element to zero. The second piezoelectric element has the same characteristic as that of the first piezoelectric element, and has the same voltage application history as that of the first piezoelectric element since it is applied with the same voltage as that applied to the first piezoelectric element. When the voltage is applied to the second piezoelectric element in the step (b), therefore, the second piezoelectric element is deformed in the same amount and in an opposite direction as those of deformation to be naturally generated in the first piezoelectric element which is applied with the load. In the step (c), the load applied to the first piezoelectric element is measured based on the amount of deformation of the second piezoelectric element.
Unlike a case where an attempt is made to measure the load based on the voltage which is applied to the first piezoelectric element to reduce the amount of the piezoelectric element to zero, the method of this invention, which includes the step (b) for converting the applied voltage into a corresponding amount of deformation, is enabled to remove a measurement error caused by the hysteresis characteristic of the piezoelectric element. Therefore, an accurate load measurement can be achieved, which eliminates the influence of deformation of the first piezoelectric element on a measured object or the measured load and a measurement error caused by the hysteresis characteristic of the piezoelectric element.
A load measuring apparatus according to another aspect of the present invention comprises a first piezoelectric element arranged to be applied with a load and a voltage and adapted to be deformed when at least one of the load and the voltage is applied thereto; first strain gauge means affixed to the first piezoelectric element for generating an output indicative of an amount of deformation of the first piezoelectric element; a second piezoelectric element having the same characteristic as that of the first piezoelectric element and adapted to be deformed when a voltage is applied thereto; second strain gauge means affixed to the second piezoelectric element for generating an output indicative of an amount of deformation of the second piezoelectric element; feedback control means for feedback-controlling the voltage applied to the first piezoelectric element so as to cau
Fuller Benjamin R.
Japan Tobacco Inc.
Thompson Jewel V.
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