An optimisation method for the over-zero switching scheme

Due to the presence of a capacitive component on the Electrical Impedance Tomography (EIT)'s excitation and measurement interface, the Over-Zero Switching (OZS) scheme was proposed previously to eliminate the effect of transient time, increase the speed of data acquisition and improve the accuracy. The switching timing in relation to the sinusoidal excitation plays an important role in the OZS scheme. However, the correct setting of switching timing is normally based on trial-by-error, which has experienced a difficulty in the system set-up process. This paper presents an optimisation method for OZS based on the analysis of the electrical impedance of a sensor's interface. A simple equivalent RC circuit is utilised to simulate the sensor's excitation interface. By analysing the transient process of the RC circuit, the relationship between the optimal OZS timing and the phase angle of the equivalent impedance is established. In practice, the OZS timing and the impedance phase angle can be calculated from online measurements within a one quarter sinusoidal period. The experimental results show that the optimisation method is able to set the optimal OZS timing for a process medium with different electrical impedance property in EIT applications. (c) 2012 Elsevier Ltd. All rights reserved.