Fig. 1. Schematic diagram of ECT system

Fig. 2. Schematic diagram of cross-section of traditional sensor

Fig. 3. Traditional sensor capacitance measurement. (a) Capacitive coupling; (b) front end circuit model

Fig. 4. Schematic diagram of cross-section of differential sensor

Fig. 5. Differential sensor capacitance measurement. (a) Differential sensor capacitive coupling; (b) differential sensor equivalent measurement circuit

Fig. 6. 3D model of new sensor

Fig. 7. Capacitance acquisition circuit of differential electrode

Fig. 8. Switch strategy. (a) Excitation mode; (b) acquisition mode; (c) grounding mode

Fig. 9. Stepping motor control sequence diagram

Fig. 10. Schematic diagram of stepping motor drive

Fig. 11. ECT sensor with controllable electrode spacing. (a) Front view of the new sensor; (b) differential electrode structure

Fig. 12. Physical field model and parameters of 60 mm pipeline

Fig. 13. Reconstruction effect of 60 mm pipeline image

Fig. 14. Reconstruction effect of 80 mm pipeline image

Fig. 15. Reconstruction effect of 100 mm pipeline image

Fig. 16. Comparison of SNR data. (a) Traditional sensors; (b) pole spacing controllable sensor

Fig. 17. Actual experimental image reconstruction effect