Bezvesilna O. M., doctor of technical sciences, professor

Tkachuk A. H., PhD, associate professor

National Technical University of Ukraine "Kyiv Polytechnic Institute named after Igor Sikorsky", Ukraine

Zhytomyr state technological University, Ukraine

THREE-AXIS LOW-FREQUENCY PIEZOELECTRIC SENSOR

Studying the parameters of the Earth's gravitational field – the gravity acceleration g (gravity) and its anomalies Δg required in geodesy, geophysics, geodynamics and oceanography. Information about the Earth's gravitational field is used in seismology (earthquake prediction), aviation and space equipment (correction of the inertial navigation systems of missiles, planes, orbits of the spacecraft), for the study of geodynamic phenomena, the implementation of the tasks of engineering Geology, archaeology, cartography and the like [1]. On the aircraft measured Δg in remote areas of the Earth at a speed greater than the speed of ground-based measurements. For this purpose, use the aviation gravimetric system (AGS), the sensitive element of which is gravimeter.

Today there are many types of gravimeters AGS, the principle of which is based on different physical phenomena. One of the most accurate gravimeters is piezoelectric (PG).

At the Department of instrumentation in National Technical University of Ukraine "Kyiv Polytechnic Institute named after Igor Sikorsky" and at the Department automation and computer-integrated technologies in Zhytomyr state technological University under the guidance of Professor Bezvesilna O. been developing new types of PG – dual-channel PG and three-coordinate PG, which can be used as part of the AGS, and in the stabilizer arms as sensing elements to measure acceleration [2-3].

Existing gravimeters measure only the gravity g_z along the vertical axis Oz, what are the components of gravity g_x and g_y along the axes Ox and Oy are equal to zero because of their small size [2]. However, to achieve the accuracy of measurement of gravity high for 1 mGal, the above components must be taken into account. For example, if g_x=g_y=0,9 mGal [4], then the module of these accelerations will be equal to:

As you can see, the neglect of g_x and g_y leads to the emergence of significant errors, which is unacceptable.

Improving the accuracy of measurement in three-axis low-frequency gravimeter is assured by the fact that for each measurement axis Oz and Ox, Oy, the gravity is mounted the sensing element Az, Ax, Ay, done with two channels, each with one equipped by piezoelectric elements that are identical (Fig. 1). Inertial mass is attached to the bottom of piezoelectric element one channel and to the top of piezoelectric element of the second channel. The piezoelectric element of the first channel of each sensing element operates on the basis of tensile deformation, and the piezoelectric element of the second channel – compressing deformation.

Piezoelectric elements have a natural frequency equal to the frequency of intersection of the spectral density of the useful signal of gravity signal and a primary interference of the vertical acceleration of the aircraft.

Output electrical signals of the piezoelectric elements both channels all sensing elements are received at the inputs of operational amplifiers that, in addition to strengthen perform and function the summation of the signals from the inputs of BCOM.

In BCOM will be carried out the necessary calculations to determine the value of the full vector and the module of the gravity.

So, by using three sensing elements Az, Ax, Ay it is possible to measure the complete vector of gravity: and not just one component, as in the prototype.

Fig.1. Three-axis low-frequency piezoelectric sensor

In new sensor what is offered at each axis measurement set the sensing element is made with two channels, each with piezoelectric element 1 and 2 along the axis Oz, 3 and 4 along the axis Ox, 5 and 6 on the axis Oy.

Inertial mass 7, 8 and 9 attached to the bottom of piezoelectric elements 1, 3 and 5 of one channel and the top of piezoelectric elements 2, 4 and 6 of the second channel.

Outputs of piezoelectric elements both channels of the three sensing elements are connected to the inputs of operational amplifiers 10, 11 and 12, the outputs of which are connected to inputs of BCOM 13.

The piezoelectric elements of both channels of the three sensing elements Az, Ax, Ay, which are located along the axes Oz , Ox and Oy are acting gravity , and respectively. Also on each measuring axis acting a vertical acceleration of the aircraft and instrumental errors from the influence of the residual non-identical structures of the same piezoelectric elements and masses from the influence of changes in temperature, humidity and pressure of the external environment.

If you use three dual-channel piezoelectric elements Az, Ax, Ay, the algorithm of two-channel measurement setup described in [6], we get the inputs BCOM 13 three signals, which contain double values , and in which there is no value of vertical acceleration of the aircraft and instrumental errors from the influence of the residual non-identical structures of the same piezoelectric elements and masses from the influence of changes in temperature, humidity and pressure of the external environment. These signals are summed and amplified in the operational amplifiers ОP10, OP11, ОP12 and served in BCOM 13, where the determining:

- complete vector of gravity: ;

- full acceleration module gravity:

,

where - masses 7,8,9; - piezoelectric constant.

Therefore, the accuracy of three-axis low-frequency piezoelectric sensor of aviation gravimetric system is higher than available gravimeters.

References:

1. Bezvesilna О. Corrected gyrocompass synthesis as a system with changeable structure for aviation gravimetric system with piezoelectric gravimeter / O. Bezvesilna, A. Tkachuk // Aviation (Vilnius, Lithuania). – 2014. – №18(3). – Р.134-140.

2. Bezvesilna О. Design of piezoelectric gravimeter for automated aviation gravimetric system / O. Bezvesilna, I. Korobiichuk, A. Tkachuk, R. Szewczyk, M. Nowicki // Journal of Automation, Mobile Robotics & Intelligent Systems. –2016. – Vol. 10, Issue 1. – P. 43-47.

3. Bezvesilna О. Two-channel MEMS gravimeter for the automated aircraft gravimetric system / O. Bezvesilna, I. Korobiichuk, A. Tkachuk, T. Chilchenko // Recent advanced in systems, Control and Information Technology. –2017. – Vol. 5. – P. 481-487.

4. Bezvesilna О. Piezoelectric Gravimeter of the Aviation Gravimetric System / O. Bezvesilna, I. Korobiichuk, A. Tkachuk, R. Szewczyk, M. Nowicki // Advances in Intelligent Systems and Computing. – 2016. – №440. – Р.753-761.