## 4 Experimental Evaluations
### Experimental Results of Actuator Performance  
Following the actuator characterization study above, the actuators' vibrotactile performance was experimentally evaluated to study the effect of the electrode configurations and to compare the performance of the proposed dual-electrode ERA with that of the single-electrode ERA. To properly activate the actuators, a total of three different input signals were generated by the controller, which can be visualized in Figure 6. When powering the single-electrode actuator, a positively offset sine input signal was used, as seen in Figure 6a. However, when using the dual-electrode ERA, this waveform could not be inputted into both electrodes without causing interference. To ensure there was no interference of electrostatic forces generated between the two electrodes, they needed to have proper input signals and configuration. Thus, half period sine waves, seen in Figure 6b, were inputted into each electrode with a phase difference relative to one another. These inputs could be used simultaneously to activate both electrodes without any interference. As seen in Figure 7, the dual-electrode actuator's resonant frequency was slightly higher than that of the single-electrode actuator because of the variations in their manufacturing discussed earlier. The figure further shows that the dual-electrode actuator produced a higher maximum acceleration of 14.32 g-forces at its resonant peak, compared to the single-electrode actuator which produced a maximum of 8.27 g-forces at its resonant frequency. Thus, the dual-electrode ERA with both electrodes activated produced a significantly greater vibration intensity compared to the single-electrode ERA, producing over 6 g-forces more, or a 73% increase, at their maxima. The results indicate that the dual-electrode configuration with proper control can substantially increase vibration intensities in a comparable actuator with a single electrode. The dual-electrode ERA was studied to investigate the effect of electrode activations and the impact that the hole in the top electrode had on its performance. In the dual-electrode ERA, the two electrodes can be activated individually or simultaneously with the input signals at a phase difference. An inevitable hole was made in the top electrode to allow an accelerometer to be mounted directly onto the actuator's mass. Placing the accelerometer directly onto the mass was highly important in this study to properly evaluate vibration performance. The hole made in the top electrode resulted in an approximate 20% reduction in the electrode area. Based on the fact that the attraction force produced by electrostatic plate actuators changes proportionally with the electrode area, the hole in the top electrode was expected to lower performance. Thus, the effect that the hole had on the performance was examined as a part of this study. Note that, in real-world applications, the hole is not necessary, so the performance of the actuator would not be compromised by such an alteration. Figure 8 shows the experimental results of the dual-electrode ERA operated on various electrode activation conditions. The actuator generated the strongest vibrations when both electrodes were  
Figure 6: Input signals used in the experimental setup: (**a**) single-electrode ERA offset sine input; (**b**) dual-electrode ERA upper and lower electrode inputs. activated, producing a maximum of 14.32 g-forces compared to the top and bottom's 4.97 g-forces and 7.62 g-forces, respectively. When only the bottom electrode was activated, which had no hole, the actuator performed similarly to the single-electrode actuator, confirming that the manufacturing variations did not cause a significant difference between the actuators. As previously hypothesized, the top electrode with a hole performed worse than the bottom electrode. The top electrode generated a maximum of 4.97 g-forces compared to the bottom electrode's maximum of 7.62 g-forces: 35% lower. This finding is consistent with our hypothesis when fabricating the electrode.