RU2519833C2 - Calibration method of piezoelectric accelerometer at lower frequencies, and device for its implementation - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to measurement equipment, and namely to methods and devices for determination of sensitivity of piezoelectric accelerometers at lower frequencies. The essence of a calibration method of a piezoelectric accelerometer at low frequencies consists in the fact that the accelerometer is turned in the earth's gravitational field by means of a turning platform and accelerometer output voltage is measured by means of a measuring circuit; with that, first, the accelerometer is installed on the turning platform with its sensitivity axis in a vertical plane at any angle to horizontal axis; centre of mass of an inertia element of the accelerometer is aligned with the rotation axis, thus changing the rotation frequency; the accelerometer is turned through an angle of more than 360° at each frequency; maximum values of output signals are determined at each frequency, as per which conversion coefficients are determined to build up amplitude-frequency response of the accelerometer in the range of low frequencies. The turning facility includes a base, on which a platform is installed by means of rotation supports, which consists of a shaft and a head piece having a horizontal platform to attach the test accelerometer; with that, the head piece is installed with possibility of being moved in the plane perpendicular to the shaft axis; to end surfaces of the shaft there applied is a coordinate network for fixation of their mutual position in the interface plane.

EFFECT: reducing calibration error caused by action of centrifugal forces.

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Description

The invention relates to the field of measurement technology, in particular to methods and devices for determining the sensitivity of piezoelectric accelerometers at low frequencies.

The frequency range of vibration acceleration, which is fractions, units of hertz, is very informative for assessing the technical condition of industrial facilities, analysis of seismic processes and other types of mechanical disturbances. The change in the sensitivity of the measuring instruments in the indicated frequency range (amplitude-frequency characteristic of the accelerometer) ultimately affects the accuracy of the measurement of acceleration, therefore, it is necessary to determine it for each accelerometer in order to obtain the actual sensitivity values.

A known method and device for calibrating accelerometers in vibration mode using electrodynamic vibration stands. For example, a calibrator of type 4294 from Bruhl & Kj (r (Denmark) reproduces at a fixed frequency of 159.2 Hz a vibrational acceleration of 1g with an error of ± 3%. The vibration test bench for type 4290 calibration of the same company reproduces 0.1g vibrational acceleration in the frequency range 200 Hz - 20 kHz with an error of ± 11% [Bruel & Kjer, Product Catalog June 2004]. The disadvantage of this method is the inability to ensure the calibration of accelerometers at low frequencies and a significant error in the reproduction of the acceleration unit.

Also known is a calibration method and device implemented in the calibration stand APS 113-AB of SPEKTRA (Germany) with linear movement of the platform on which the accelerometer is mounted. The stand provides vibrational displacement parameters: frequency 0.2 Hz, acceleration 0.013 g, displacement range - 0.158 m. Accordingly, for a frequency of 1 Hz - 0.32 g acceleration, range - 0.158 m [APS Dynamics, Products & Services, 2008].

The disadvantage of this method of calibration is the low level of acceleration reproduced by the bench - hundredths of "g", the large swing of the table, which introduces an additional error in the calibration due to the spurious signal from the piezo accelerometer due to the pyroelectric effect due to friction of the calibrated accelerometer against air.

In installations with linear displacement of the platform, the manifestation of the pyroelectric effect in piezoelectric elements from the friction of the accelerometer against air can be characterized (according to the theory of boundary layers) by a quadratic dependence on the displacement rate.

For oscillatory motion, the dependence is known:

$$V=2\pi fS\text{(one)}$$

where: V is the displacement velocity (m / s);

f is the frequency (Hz);

S - displacement amplitude (m).

Then the electric charge from the pyroelectric effect on the electrodes of the piezoelectric element will have the following proportionality:

$$Q~A{(2\pi fS)}^2\text{(2)}$$

where: Q - electric charge (C);

f is the frequency (Hz);

S is the displacement amplitude (m);

A is the coefficient of pyrosensitivity.

It is seen from (2) that, ceteris paribus, the parasitic electric charge induced from the manifestation of the pyroelectric effect will be proportional to the square of the bias amplitude.

According to (2), the pyroelectric charge during calibration at facilities with linear displacement of the platform for f = 1 Hz and S = 0.079 m will be Q ~ 0.25A.

A known method of calibrating piezoelectric accelerometers by turning in the gravitational field of the earth and a device for its implementation (see AC No. 1295344 from 01/14/85, published in BI No. 9 from 03/03/87). The method consists in turning the accelerometer in the Earth’s gravitational field from a horizontal platform from the horizontal position of its sensitivity axis by a predetermined angle relative to the horizon plane and measuring the change in the output voltage of the piezo accelerometer at an additional electric capacitance connected to its output using a measuring circuit. The output of the accelerometer is short-circuited, with the start of rotation it is opened and connected to the measuring circuit, and the increment of the output voltage of the accelerometer is measured during rotation by a predetermined angle less than the time constant of the measuring circuit. In this method, there is no device for reproducing the acceleration unit and, as a result, an additional error. The absolute value of the acceleration of gravity (≈9.8 m / s ² ) in relation to a specific geographical point of the Earth is known with very great accuracy, therefore, the calibration error will be formed only by means of measuring electrical quantities.

The above method and device are the closest in technical essence to the claimed method and device, and therefore are selected as a prototype.

The disadvantages of the above method and device are:

- the sensitivity of the graduated accelerometer to centrifugal forces during rotation;

- the inability to determine the sensitivity of the accelerometer in the low frequency range.

The level of influence of centrifugal forces according to the above method can be estimated from the known dependence:

$$\alpha ={(2\pi f)}^2\text{L (3)}$$

where: α is the acceleration (m / s ² );

f- rotation frequency (Hz);

L is the shoulder from the axis of rotation to the center of mass of the inertial element of the accelerometer (m).

Even with very small overall parameters of the accelerometer and dimensions of the turntable realizing L≈0.05 m, according to (3), for a frequency f = 0.5 Hz, a force corresponding to the acceleration α≈0.5 m / will act on the piezoelectric element of the accelerometer s ² , and for the frequency f = 1 Hz, the acceleration α≈2 m / s ² , which introduces an error with respect to the acceleration of gravity, respectively 5% and 20%, which is transferred accordingly to the calculation of the sensitivity of the accelerometer.

The technical problem to be solved is the creation of a more accurate method for calibrating a piezoelectric accelerometer in the field of acceleration of gravity at low frequencies and a device for its implementation.

Achievable technical result is to reduce the calibration error caused by the action of centrifugal forces.

To achieve a technical result, in the method of calibrating piezoelectric accelerometers at low frequencies, namely that the accelerometer is rotated in the Earth's gravitational field using a rotary platform and the output voltage of the accelerometer is measured using a measuring circuit, it is new that the accelerometer is pre-installed on the rotary platform with its sensitivity axis in the vertical plane at any angle to the horizontal axis, combine the center of mass of the inertial element of the accelerometer ra with the axis of rotation, changing the rotational frequency, the accelerometer is rotated through an angle of more than 360 ° at each frequency, determining maximum values of the output signals on each of the frequencies at which the transform coefficients is determined to construct the accelerometer amplitude-frequency characteristics at low frequencies.

To achieve a technical result in a rotary installation containing a base on which the platform is mounted by means of rotation supports, the new one is that the platform consists of a shaft and a nozzle, which has a horizontal platform for attaching the accelerometer to be tested, while the nozzle is mounted for movement in the plane, perpendicular to the axis of the shaft, a coordinate grid is applied on the end surface of the shaft to fix their relative position in the interface plane.

A new set of essential features in the claimed method and device allows to increase the accuracy of the calibration of the accelerometer by reducing the calibration error caused by the action of centrifugal forces, and practically remove the restrictions on the implementation of the minimum calibration frequency.

The method is implemented by the device shown in figure 1.

The figure 2 presents a typical amplitude-frequency characteristic of the accelerometer, which highlighted the low-frequency region in which this method is implemented.

The rotary installation comprises a base 1 on which a platform 3 is mounted by means of rotation supports 2, which consists of a shaft 4 and a nozzle 5 having a horizontal platform 6 for mounting the accelerometer under test 7, while the nozzle 5 is mounted for movement in a plane perpendicular to the shaft axis, coordinate grid 8 is applied on the shaft end surface to fix their relative position in the interface plane.

The connection of the nozzle with the shaft from the side of its flat part is made with the possibility of their relative displacement (for example, using magnet 9), which allows you to move the accelerometer fixed on the horizontal platform of the nozzle and to combine the center of mass of the inertial element 10 of the accelerometer with the axis of rotation of the shaft 11. This is realized the accelerometer is insensitive to centrifugal forces and increases the accuracy of calibration. The method is implemented as follows.

A method for calibrating a piezoelectric accelerometer in the Earth’s gravitational field at low frequencies is that the accelerometer is mounted on a horizontal platform of the nozzle, the center of mass of the inertial element 10 of the piezoelectric accelerometer is aligned with the axis of rotation 11 of the platform 3, after which the shaft and accelerometer 7 are continuously rotated by an angle of more than 360 degrees at each required frequency, determine the maximum values of the output signals at each of the frequencies by which the conversion coefficients for eniya accelerometer amplitude-frequency characteristics at low frequencies.

Evaluation of the possible influence of the pyroelectric effect in the proposed method from the rotation of the accelerometer in the gravitational field of the Earth can be performed using the well-known dependence

$$V=2\pi fS\text{(four)}$$

where: V is the linear velocity of rotation around the circle (m / s);

f is the frequency (Hz);

R is the radius of rotation (m).

For an accelerometer with a height dimension of ≈3 cm, the radius R of the most remote part of the structure will be ≈1.5 cm, then, according to (2) and (4), for f = 1 Hz:

$$Q~A{(2\pi fR)}^2\approx \text{0}\text{, 0088A (5)}$$

It can be seen that for the same frequency (f = 1 Hz), the generated electric charge when the accelerometer is rotated in the gravitational field of the earth when combined with the axis of rotation of the center of mass of the inertial element, compared with installations with linear platform displacement (Q ~ 0.25A ), almost 20 times less.

The proposed method for calibrating piezoelectric accelerometers and a device for its implementation at low frequencies can reduce the calibration error caused by the action of centrifugal forces, as well as provide, changing the frequency of rotation, the determination of the amplitude-frequency characteristics.

In comparison with the method implemented in the Spektra APS 113-AB calibration bench, the claimed method allows to practically remove the restrictions on the minimum frequency response of the frequency response, as well as to eliminate the additional error from reproducing the set acceleration value.

A prototype was made, which confirmed the practical implementation of the claimed method and device.

Claims (2)

1. A method for calibrating a piezoelectric accelerometer at low frequencies, namely, that the accelerometer is rotated in the Earth's gravitational field using a rotary platform and the output voltage of the accelerometer is measured using a measuring circuit, characterized in that the accelerometer is pre-installed on the rotary platform with its sensitivity axis in vertical plane at any angle to the horizontal axis, combine the center of mass of the inertial element of the accelerometer with the axis of rotation, changing the frequency of rotation turn the accelerometer by an angle of more than 360 ° at each frequency, determine the maximum values of the output signals at each of the frequencies, which determine the conversion coefficients for constructing the amplitude-frequency characteristics of the accelerometer in the low-frequency region. 2. A rotary installation comprising a base on which a platform is mounted by means of rotation supports, characterized in that the platform consists of a shaft and a nozzle having a horizontal platform for mounting the accelerometer under test, while the nozzle is mounted to move in a plane perpendicular to the shaft axis on the end surfaces of the shaft is marked with a coordinate grid to fix their relative position in the interface plane.

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