MEASURING DATA ACQUISITION HARDWARE FOR ELECTRICAL IMPEDANCE TOMOGRAPHY

. This article presents the design of the device with active electrodes to examine the flood embankment. There was discussed the method of determining the conductivity. The presented solution was based on electrical impedance tomography. There was described the concept of active electrodes and measuring equipment for data acquisition. Electrical impedance tomography, which is based on measuring potential difference, can be used to calculate conductivity. The problem depends on the fact that every material has unique conductance.


System with Active Electrodes
Electrical impedance tomography (EIT) is known that the inverse problem is nonlinear and highly ill-posed [1,[8][9][10]. The problem is the low level of measured values which should be measured quite accurately and in a very short time. EIT involves placing electrodes on the examined object. The two electrodes are connected to AC power and the voltage drop is measured on others [3,7]. Then, power supply is connected to the next two electrodes, and measuring steps are repeated until each electrode is connected to power supply. Figures 1 and 2 present electrodes and model measurements of the flood embankments.  Figure 2 shows a plan view of an electrode with a straight front, with cross-section. Active electrode is designed to be placed in the ground, and its aim is to introduce current and measuring potential differences. The electrode consists of two parts: an active electrode and a quick rotating, which enables the galvanic connection of the active electrode portion of the electric wire. The active part of the electrode is made of rod 1, the tapered tip 2 and 3 lesions according to the drawing, which allow you to quickly pivot connection with the second part of the electrode. The electrode is located in the insulation 4. The second element electrodes -quick swivel consists of six parts: the cylindrical cap comprising a connector 7, the housing 8 of the spring 9, the tab 11, the locking ring spring 10, and connecting pins 12.

EIT measurement device
The measurement in electrical impedance tomography involves placing electrodes on the examined object. This solution allows to obtain a conductivity distribution within the test object, and this consequently allows a distinction between materials of different conductivity. In carrying out further measurements over time and comparing them with previous we can observe the changes occurring in the study area [4]. Figure 3 presents the electrode arrangement for one projection angle [2,5,6].
The resolution of the image depends on the number of electrodes used and the accuracy of measured signals. To enhance the resolution, the number of electrodes should be increased. Then the number of necessary measurements will increase, which will result in greater demand for memory [11][12][13][14]. Figure 4 shows an exemplary result of the reconstruction of measurements on the circular container with 16 electrodes. The container was filled with water, in which was an empty glass bottle. Its position is marked with light green color. After achieving good with results, there was created a prototype of our device (Fig. 5). Its aim is to verify the repeatability of test results by eliminating laboratory equipment, and to validate the use of simple and cheap electronics the structure of the EIT.
The asset of such a device is to put the generator and the multiplexer on a small single circuit board, which allows mobility and enhances work comfort. Another advantage of this device is that the power supply voltage is 12V, and the maximum current is 60 mA, this reduces the energy requirements for carrying out image reconstruction of the test area. The prototype device is controlled from a program written in LabView. The device allows for measurement of voltage drops for all angles of projection and save them to a file. The measuring device includes a program written in LabView environment, microcontroller evaluation board STM32 F4 Discovery, board with the generator and multiplexers. The general scheme of cooperation between all the components of the measurement system is shown in Figure 6.     E. Active electrode (Fig. 12):  measurement of potential differences on the individual electrodes,  chip LTC 2495,  I2C communication,  manual addressing -27 addresses,  12V power supply,  stabilizing voltage,  signaling work LEDs  RJ-45 connector.

Results
Achieved test results of the geometrical model were satisfactory. The flood embankment model was applied with water on the upstream side. Water line was near to the last 16 electrodes. The water started to percolate into the soil of the embankment model. Measurements of the potential were performed after 30 minutes. Measurements of the potential were performed at different angles of projection whereby the information needed to determine an approximate distribution of conductivity inside the object is obtained. The measurement system (flood embankment model, device and active electrodes) was presented in Figure 14. Figure 15 shows the image reconstruction with real data taken 15 minutes after flooding.

Conclusion
In this paper, there was presented the construction of the device with active electrodes to examine the flood embankment. The prepared solution was based on electrical impedance tomography. The device parameters are correctly assumed According to our analysis and design, it is possible to build a small system EIT system to monitor flood embankments. A new nondestructive device was tested on the laboratory model. The flood embankment model was applied with water on the upstream side. The test results for prototype device were promising.