## 4 Numerical Simulation and Experimental Work
### Experimental Results  
Figure 8 illustrates the experimental arrangement used for the capacitive technique experiments presented in this work. The experiments use a capacitance setup that is composed of a single pair of back-to-back triangular electrodes made of copper plates as shown in Figure 3. A shielding plate is placed on its top side to ensure that the radiated electric field emanates predominantly in the direction towards the specimen and to provide protection against the effect of the ambient stray fields on the output results. In addition, a guard electrode is applied to increase the penetration depth of the sensor. One of the electrodes is used as a driver electrode and the other one as a receiver. The driving electrode was excited by a sinusoidal voltage of 10 V amplitude peak-to-peak at a frequency of 100 kHz and the shielding plate and guard electrode are connected to the ground potential. To generate an electric field, one of the plates is employed as a transmitter and connected to Ectane. Ectane is a multitechnology, powerful and compact instrument for generating and analyzing signals. The frequency range of this instrument is 5 Hz up to 10 MHz and the generator output is up to 20 V, peak to peak. A differential amplifier is placed inside Ectane to process the received signal successively. The output signal from the Ectane is a simple DC voltage level that is proportional to the instantaneous value of the dielectric property of the material averaged over the field distribution within the material.  
The hidden depth of the defect is also a factor affecting the coplanar capacitive sensor performance. To investigate the effects of it in nonconducting specimens, a set of experiments were carried out on a 6.5 mm thick Plexiglas plate. The plate contains three subsurface defects of 12 mm diameter located at different depths (0.5 mm, 1.0 mm, and 1.5 mm). Figure 9a illustrates the schematic diagram of the sample. The coplanar CI probe was scanned over the Face (A) (the face without the defects) using a two-axis scanning system (\(x\)-\(y\) table). In this scan, the coplanar capacitive sensor was held 1.0 mm off the surface of the specimen (a constant liftoff = 1.0 mm) and scanned over a 40 mm \(\times\) 160 mm area with a 0.2 mm step in the \(x\)-axis and 1.0 mm in the \(y\)-axis. The speed of the probe was set to 10 mm/s.  
Figure 9b is the image obtained by scanning the coplanar capacitive sensor over the surface of the Plexiglas sample. The technique easily detected these defects, and this is due to the different dielectric properties between the material of the specimen (Plexiglas) and the defect (air). It can be seen from the result, the defect at 0.5 mm depth, which is the shallowest defect in the Plexiglas specimen, appeared as a darker colour, especially a darker red colour which is representing the highest output voltage in this result. Likewise, the other two defects appeared as a lighter area, depending on their depth representing lower output voltage values. This result is in good qualitative agreement with the result of the simulations as seen in Figure 7.  
Figure 8: (**a**) The experimental arrangement used for the capacitive imaging and (**b**) typical configuration: Ectane with the capacitive sensor.  
It can be inferred that when a specimen is placed under the coplanar capacitive electrodes, most of the electric field from the sensor penetrates into the specimen and then ends on the sensing electrode. The dielectric properties of the specimen and defects influence the electric field distribution and, hence, on the electric potential of the sensing electrode. Therefore, an existing defect in the specimen will change the electric field pattern, which leads to the changing in the detectable signal of the sensing electrode. It is worth mentioning the depth of penetration is determined mainly by the geometry of the sensor, the electric properties of the specimen, and the liftoff.