*3.3.2. Experimental results*

A set of photos showing sensor prototype, experimental setup and sensor vibrating under the action of alternating electric field force is shown in **Figure 17**. The sensor shown in **Figure 17(a)** has a 40-mm long cantilever beam with a natural frequency at 50 Hz. The beam length depends on the power frequency and the hardness of coating material, varying from 39 to 40 mm. The frequency responses of the sensor can be investigated with a frequency-swept shaker. A photo in **Figure 17(a)** shows the sensor vibrating under a 50-Hz, 5-kV electric field environment **Figure 17(b)** is a photo of the experimental setup.

**Figure 17.** A set of photos on sensor (a) and experimental setup (b).

**Figure 18.** Test for investigating frequency response of sensor with a shaker (a) and results (b).

**Figure 18(b)** shows two test results on the frequency response distributions of the sensor in two orthogonal directions, the front direction and lateral direction, obtained with a frequencyswept shaker as schematically shown in **Figure 18(a)**. This experiment was to investigate the natural frequency of the sensor, which was designed initially at 60 Hz. The beam length in this case was 39.2 mm. From the test results, clearly, this sensor only has a natural frequency at 60 Hz in both orthogonal directions in the frequency range of 0–100 Hz.

With the experimental setup shown in **Figure 15(c)** as well as in **Figure 17(b)**, we carried out several experiments for investigating sensor performances as well as for verifying the feasibility of the proposed sensor with respect to electric field sensing.

**Figure 19(a)** shows a measured electric field signal (inset graph) and its frequency distribution from 0 to 100 Hz. In this measurement, the sensor was placed between two electrodes separated by 10 cm, which were connected to 60-Hz, 3000-V voltage. **Figure 19(b)** shows a group of measured RMS voltages of signal amplitude under different AC voltages, varying from 0 to 5 kV. The test arrangement is schematically delineated in **Figure 19(b)**. From these results above, clearly, the 60-Hz sinusoid wave, as a typical vibration signal, really reflects a power-frequency electric field force acting on the sensor, and the RMS voltage of signal amplitude is proportional to the applied electric field strength in a relation = V/D.

**Figure 19.** (a) Measured electric field signal waveform at 60-Hz power frequency (inset graph) and its FFT, and (b) relationship between measured RMS voltage and imposed AC voltage.
