###### Abstract
Moreover, this is a noncontact method [5] and the liftoff (the air gap between the surface of the electrodes and specimen) can be optimized, leading to an applicable method in the detection of corrosion under insulation (CUI), including the detection of large surface features in metals and therefore the presence of even small amounts of trust [10]. This technique is especially useful in defence where composites are extensively used for both equipment and strengthening structures [11]. These features of coplanar capacitive sensor make it an attractive option for applications in NDT [6], material characterization [8], and imaging [12]. The principle of this technique involves placing two (or more) electrodes over the surface of the sample and then applying an AC voltage between them. This system acts as a capacitor whose altering in capacitance indicates variations in the internal structure (such as the presence of a defect) [13]. Normally, in a regular capacitor, the plates are parallel. When a voltage is applied between these plates, they produce a uniform electric field distribution. This electric field does not restrict itself to the area between the electrodes when the electrodes are gradually open, but extends to a wider space and forms a fringing field [9]. This field expands into the specimen for NDT. Scanning a set of electrodes over the surface of the sample and measuring variations in stored charges for a given voltage creates a map of the changes in the electrical properties within the specimen [7]. The fringing field between the driving and sensing electrodes becomes predominant when the electrodes open to a coplanar plane. This kind of sensor is named a coplanar sensor in literature [9]. Figure 1 shows a schematic diagram showing how the electric field distribution changes when the two capacitor electrodes change from a parallel-plate to a coplanar so that the final part produces a fringing field [7]. The coplanar electrodes can be scanned over the surface of a large mechanical structure to investigate the physical properties of the specimen and obtain information. Such as some other NDT methods, the capacitive technique is sensitive to liftoff, which refers to the air gap between the surface of the coplanar electrodes and specimen which causes a decrease in the capacitance between the electrodes and hence a reduction in output voltage. In addition, it reduces the extent of penetration of the electric field into the sample which leads to a decrease in the depth of penetration [9]. The lift-ff should therefore be as small as possible to achieve greater signal strength and sensing depth (especially for investigating dielectric material) [14] with no need to use couplant materials. The sensor manufacturing steps include material selection for electrodes, the insulation layer of the surface of the electrodes, the sensor substrate, and eventually the selection of a production technique. The electrodes are made of materials with high electrical conductance, such as copper. A thin insulation layer (with a few micrometres in thickness) is usually placed on the surface of the electrode to prevent direct contact of the electrodes with the specimen and to protect the electrode surface against scratches. The thickness of the insulation layer and the sensor substrate influence the electric field strength and depth of penetration, therefore, need to be optimized. Several manufacturing techniques of the capacitive sensor, including microelectromechanical systems (MEMS) [15], printed circuit boards (PCB) and manual production can be selected depending on sensor dimensions and costs [9]. To evaluate the sensor performance, it is required to explain the general design parameters. Therefore, in this paper, firstly the design factors such as the geometry of the electrodes, the number of electrodes and their arrangement, shielding plate and guard electrode of the capacitive sensor are presented.