The impedance flow sensors have usually been considered as the competitive components in the multi-flow measurement, primarily due to their advantages of fast response, high dynamic range and sensitivity, non-invasive and/or non-intrusive measurements. In this review, the impedance sensors have been categorized into two categories, i.e. , the probes for local/average parameter measurement and sensor arrays for distributed parameter measurement. Maxwell’s equations and boundary conditions have been referred as a starting point to understand the measuring principle of impedance sensors, from which the sensitivity distribution can be attained. The sensitivity distribution can be utilized to characterize the sensor and employed as an objective function in optimizing the sensor geometry. The EMA models usually offer a way to relate the impedance measurements to individual phase fraction, which have been critically reviewed. In addition, the application range, spatial and temporal resolutions, advantages and disadvantages of the sensor arrays, i.e. , the wire mesh sensor, field focusing sensor, electrical resistance and capacitance tomography sensors have also been provided. Finally, the future trends of impedance sensor development have been briefly discussed.

Eddy current (EC) testing has become one of the most common techniques for measuring metallic planar structures in various industrial scenarios such as infrastructures, automotive, manufacturing, and chemical engineering. There has been significant progress in measuring the geometry, electromagnetic properties, and defects of metallic planar structures based on electromagnetic principles. In this review, we summarize recent developments in EC computational models, systems, algorithms, and measurement approaches for planar structures. First, the computational models including analytical models, numerical methods, and plate property estimation algorithms are introduced. Subsequently, the impedance measurement system and probes are presented. In plate measurements, sensor signals are sensitive to probe lift-off, and various algorithms for reducing the lift-off effect are reviewed. These approaches can be used for measureing thickness and electromagnetic properties. Furthermore, defect detection for metallic plates is also discussed.

The eddy-current testing technique has been extensively explored for the measurement of metallic pipes in various industrial processes. In this study, a simplified analytical model of metallic pipes encircled by a coaxial sensor is derived according to the asymptotic forms of modified Bessel functions. The original Dodd and Deeds (DD) model is thus approximated by an elementary function describing the relationship between the measured inductance spectrum and the pipe properties. Due to the simple nature of the elementary function, the Jacobian matrix can be evaluated much more efficiently. Accordingly, a fast Newton-Raphson method-based optimization approach is developed. The proposed method has been evaluated by comparing with the numerical simulation of finite element method (FEM) and experimental measurements which indicate its effectiveness for the measurement of metallic pipe properties.

The electrical capacitance tomography (ECT) technique has been extensively studied for real-time tomographic imaging of various industrial processes. Due to the ‘soft-fieldߣ and under-determined problems in the image reconstruction, the tomographic images of ECT are usually low in the spatial resolution when compared with its radioactive counterparts, i.e. r-ray tomography and computed tomography. This paper presents an ECT sensor that consists of 8 external electrodes and 8 internal electrodes, with enhanced image quality at the center of imaging region and better accuracy on phase fraction measurement. The internal electrodes are placed on cross planes inside the investigated vessel. In this way, the signal-to-noise ratio of measurement channels as well as image quality are improved. Numerical simulations show that the proposed sensor can achieve better reconstructed images than a conventional 12-electrode sensor. In addition, the phase fraction calculation using ECT data can also benefit from the internal electrodes and corresponding measurements, which is an important parameter for multiphase flow processes.

Steel microstructure determines the mechanical properties of steel such as hardness and tensile strength. To monitor steel microstructure evolution during thermal processing such as hot rolling, online measurement of microstructure is vital to steel quality control. Previous studies using multi-frequency eddy-current testing to monitor the change of electromagnetic properties and infer steel microstructures have achieved remarkable success, yet due to the high-frequency excitation signal, the inspected thickness of steel microstructures is constrained on the surface due to the skin effect. This study introduces the pulsed eddy-current (PEC) method to characterise the electromagnetic property profile along the depth of thick steel plates. The low-frequency square wave signal enables PEC to reflect multi-layer characteristics in a single measurement with good penetration ability. An explicit analytical equation for calculating the time domain PEC response is derived for the first time which shows advantages in terms of speed and gaining in-depth physical insights. A custom PEC system has been designed and built with a TMR sensor as the receiver. Theoretical and numerical analysis are performed. A time domain intersection point on the Tau curve is found to be a useful indication of the properties of a planar structure. Experimental results on layered planar structures of various electromagnetic properties are analysed and validated.