Autonomous Laser Pressure Fluctuation Meter
Article received for editing 28.03.2019
Article accepted for publication 15.04.2019
The use of laser-interference measuring devices for pressure variations  in scientific research has made it possible to accumulate much experience with such devices. Modifications were even made using different sources of radiation and device layout . But a number of negative features in the operation of these devices was also revealed. Among them are as follows: 1) large geometrical dimensions and mass, which leads to inconvenience of operation and instability of the interference pattern, 2) the effect of changes in the outside temperature on the interferometer readings, 3) inability to work autonomously, 5) inability to connect external equipment.
In connection with these drawbacks, based on the design of the laser meter of hydrosphere pressure variations, a new device was created: an autonomous laser pressure fluctuation meter (ALPFM), Fig. 1.
The principle of action, tested in many instruments of the facility, based on the Michelson interferometer, constructed according to a modified scheme of unequal-arm type, is applied here. The optical-mechanical scheme of the device and the principle of its operation are shown schematically in Fig. 2
A helium-neon frequency-stabilized laser MellesGriot, a compact model, is used as a radiation source.
One “arm” of the interferometer is a reference. A beam that propagates along the other “arm” passes through a mirror mounted on a membrane in the lid of the device. The outer side of the membrane 3 is in contact with water and this beam is thus measuring. The spatial convergence of both beams falling onto the dividing plate 6 (after passing through their optical paths) makes it possible to obtain an interference pattern of variable brightness caused by the change in the path difference of the beams. The change in brightness is recorded by the photodetector 2 of the registration system 7, which generates a control signal to compensate for the difference in the path of the beams. The same signal is output and it is also fed to one of the piezoceramic elements 5 to return the interference to the extremum. A “test” (or search) signal is fed to the second element, which is a harmonic oscillation, providing the correct system of extreme regulation.
The instrument uses a hydrostatic pressure compensation system. It is necessary to equalize the pressure on both sides of the membrane in order to bring it to the neutral position before measurements. When the device is immersed, a solenoid valve is opened upon command, which passes air from a special rubber container into a small volume chamber between the membrane and the main body space, separated by a transparent pressure window 9. When the diving is finished, the valve closes before the measurements start. When the device is lifted, the pressure from the compensation chamber is relieved.
The optical bench is made of stainless steel, which, together with the reinforcement ribs located on both sides and steel stretch marks on the upper side, allows for greater rigidity of the structure. The radiation source is located under the optical bench, from where the beam through the hole is output using a periscopic system of mirrors. The power supply units of the device are located in a separate volume from the optical-mechanical part inside the device case.
It is necessary to mention the temperature meters among the additional equipment of the hardware part. Temperature sensors based on a DS18B20 digital thermometer are installed on the optical bench of the interferometer and on the outside of the instrument (in a thin-walled probe rod located in the area of the membrane). Measuring the temperature inside the instrument is necessary, since temperature variations can introduce a significant error in the readings of the non-equal interferometer. The resolution of the sensor when using a 12-bit analogue-to-digital converter (ADC) board is 0.0625 °C.
For a rough measurement of outboard pressure (e. g., to determine the depth of installation of the device), it is possible to install a pressure sensor. For this purpose, there is a pressure-tight fitting in the cover of the device for installing an overpressure strain meter D0.4-T. The maximum measured pressure of the sensor is 0.8 MPa, which corresponds to an immersion depth of ~ 80 m, the resolution is 90 Pa, i. e. 0.01% of measured pressure range. If it is necessary to work at a depth of more than 80 m, it is possible to quickly replace the pressure sensor. For matching the sensor with the measuring and recording parts of the equipment, a measuring amplifier with a variable gain in the range of 5 ÷ 4001 is used.
On the cover of the device there is an air-tight connector for additional equipment. This is a universal interface with a separate power supply and data line. In particular, the meter can work in conjunction with the ECO FL fluorometer, designed to determine the biological characteristics of water, in particular, the content of chlorophyll-a. This sensor allows for measurements in the range of 0 ÷ 125 µg / l with a resolution of 0.02 µg / l. Depth sensors, hydrophones, a radio module for telemetry and sending preliminary data, and other devices can also be connected.
In the work of the interferometer, a system is used to record the interference meter of pressure variations  with some modifications. The changes affected the improvement of the characteristics of the digital-to-analogue converter (DAC), the 14-bit model is now installed. The registration system uses the principle of retaining interference at the maximum brightness level. This is provided by the working body of the registration system – compensating piezoceramics, which moves its unfixed end with a fixed mirror, at a constant speed, keeping the interference at the extremum. To determine the direction of displacement from the extremum position, the second piezoelectric ceramics introduces an artificial harmonic perturbation into the optical signal, with a frequency of 100 kHz – a “buildup” signal, which is a test or a search signal. For the considered system with the constant speed of movement of the feedback body (compensating piezoceramics) between the maximum speed ν of movement of the recording system, the frequency f of the test (search) signal, the wavelength λ of laser radiation and the digit capacity N DAC can be described as follows:
From (1) it follows that with increasing digit capacity, it is necessary to increase the frequency of the search signal. In the developed device, the frequency of the search signal is 100 kHz, which is four times higher than in previously created systems. Provided that the dynamic range of the amplifier controlling the working body of the recording system is not narrower than that of the DAC, the maximum measurement accuracy of the displacements of the diaphragm of the pressure meters is 0.75 λ / (2N–1) or 0.06 nm.
The pressure recorded by the pressure variation meter of the hydrosphere, equipped with the described recording system, can be calculated using the formula describing the behaviour of the flat membrane fixed along the edges :
Here, Δl is the membrane displacement; h is the membrane thickness; E is Young's modulus; σ is the Poisson's ratio; R is the radius of the membrane. Membranes made of stainless-steel sheet with a thickness of 0.1; 0.5; 1; 2 mm. The membrane with a thickness of 1 mm was used in the tests. Substituting the following values into (2): R = 5 cm, h = 1 mm, E = 2.11011 N / m2, σ = 0.25 and Δl = 0.06 nm, we get that the pressure meter resolution is P = 11.5 MPa. At the same time, in terms of frequency characteristics, the system is able to record pressure variations in the frequency range from the lowest (close to zero) to 1 000 Hz.
The transfer of the instrument readings to the coast station via a cable line is convenient only for small distances from the coast, with an available infrastructure, a coastline without steep slopes with free access to the coastline. All this greatly complicates the installation and operation of the system, and taking measurements at distances of more than 500 m from the coast becomes a difficult task in practice. Therefore, the possibility of autonomous operation of the laser-interference complex is a necessity. However, as practical experience has shown, the transition to a completely autonomous operation with the placement of registration means and power sources in the instrument case was not advisable. Worsening the achieved characteristics of the weight and size of the device means the complexity of operation, the need to use larger boats and lifting mechanisms. Furthermore, when carrying out measurements near the coastline, the use of a cable line is justified by the absence of restrictions on energy consumption, the duration of operation time, the ability to instantly receive any amount of information recorded by the sensors, and the simplification of telemetry. In order to simultaneously preserve these advantages and ensure the possibility of working both with and without a cable line, an autonomation container was developed (Fig. 3).
The container has batteries (lithium-ion 4S electric power battery with a capacity of 7965 Wh), blocks for matching and stabilizing electrical circuit parameters, as well as a microcomputer with a solid-state drive for recording information from the device. The container is connected to the pressure connector of the device, which is usually used for the shore cable. The autonomous operation time of the device is more than 6 days, which, taking into account the significance of energy consumption by the gas laser, the recording system, pressure and temperature meters, and communication systems operating at high frequencies, is a very good indicator. Successful battery life tests were conducted for 145 hours. Fig. 4 shows a plot of the recording, demonstrating pressure fluctuations during wind disturbances. The ordinate is the voltage at the output of the recording system, which is proportional to the measured pressure. Conversion factor 0,25 V / Pa. The abscissa is time. Installation depth of the device is 12 m.
The digital temperature sensors installed in the device allow the adjustment of the interferometer readings taking into account temperature variations. This is important because a change in the temperature inside the instrument leads to a change in the length of the reference “arm” due to thermal expansion of the interferometer parts. An external sensor provides information on the temperature field outside the instrument. In this model of the device, it is necessary to adjust the output signal of the interferometer detection system by –3.305 V for each step of measuring the thermal sensor, i. e. by 0.0625 °C. The voltage value was chosen empirically during the tests. The function of correcting the interferometer reading was included in the measurement processing software.
The tests of the developed ALPFM were successful and the device can be used for scientific research. The use of this equipment allows to solve the problems for the study of amplitude-phase variations of pressure fluctuations, temperature tours and other parameters in the hydrosphere in a wide frequency range.
This work was partially funded by RFBR grant No. 18-05-80011_Hazardous phenomena.