A Fast and Low-cost Measuring System for Electrical Impedance Tomography

Electrical impedance tomography (EIT) embodies a nondestructive, noninvasive method to estimate the conductivity distribution inside an object via measuring the surface voltage under simultaneous feeding with alternating current. The accuracy and speed of voltage measurement in the individual electrodes is a critical stage of the reconstruction as the most common, 16 electrode configuration requires 208 measuring cycles, including the current source switching. The paper discusses a designing procedure to produce a measuring card for an experimental tomograph, emphasizing measurement speed and accuracy as well as manufacturing costs reduction. Thus, the presented comparison of the card with various solutions relying on commercially available devices considers not only technical parameters but also overall cost. Importantly, the design approach consists in utilizing the MOSFET technology for galvanically isolated switching of the voltmeter or DAC electrodes and also current source switchovers. Each electrode is connected via 4 bidirectional switches (photocoupler) that replace mechanical relays. The employed Toshiba TLP3545A coupler exhibits the following characteristics: max open switch voltage - 60 V; max control LED current - 3 mA; max current when closed - 4 A; max resistance when closed - 60 mΩ insulation voltage - 2.5 kV; min/max closed time 0.5/5 ms; min/max open time - 0.1/0.5 ms. All the parameters fundamentally determine the properties of the measuring card. The number of switches per electrode was chosen such that the card enables the user to connect the supply signal and the relevant measuring system to arbitrary dual polarity electrodes, facilitating simple measurement in adjacent, cross, polar, pseudo-polar, and other configurations.