## 3 Experiment and Result  
As we mentioned before, two voltage input signals were used for creating electrotactile beat (ETB) in the proposed module. Let us consider a case where the frequency of a first voltage input signal is set as the resonant frequency and the frequency of another voltage input is almost same (not exactly same) as the resonant frequency. In this case, the vibration amplitude created from the proposed ETB module will dramatically increases. To investigate the resonant frequency of the proposed ETB module, we observed the acceleration behavior. During this experiment, one voltage input was applied to electrodes 2 and 3. Figure 6 shows the experimental setup consisting of a function generator (Protek 9305, Protek, Incheon, Korea), an accelerometer (Charge Accelerometer type 4393, Bruel&Kjaer, Naerum, Denmark), a high voltage amplifier (Trek 10/40A-HS, TREK, New York, NY, USA) and an oscilloscope (MSO/DPO 2000, Tektronix, Beaverton, OR, USA). The function generator was connected to a high voltage amplifier whose gain is 1000. We attached a mass of 100 g on the proposed ETB module and put the accelerometer on the mass of 100 g. AC voltage input (sine wave) of 1 V\({}_{\mathrm{PP}}\) was created by the function generator and then was amplified by the high voltage amplifier in order to be conveyed to the proposed ETB module. The vibration force, which was created by the proposed ETB module, was measured by the accelerometer and the measured vibration force was displayed on the oscilloscope. In this experiment, the vibration force was obtained as a function of input frequency in a range of 1 Hz to 250 Hz. Figure 5: The designed ETB module. (**a**): Constructed portable ETB platform consisting of a control part, a circuit part and an operation part. (**b**): Magnified image of the circuit part. Figure 7 shows the acceleration behavior of the proposed ETB module as function of input frequency. The resonant peak in the proposed ETB module are shown at 90 Hz. The amplitude of the vibration is reached about 2.6 g (=9.8 m/s\({}^{2}\)) at 90 Hz. As we mentioned before, the two voltage inputs are applied to the proposed module to create electric beats which will be used for stimulating human skin. To investigate the haptic behavior of the proposed ETB module, we constructed an experimental environment consisting of a frequency controller, two high voltage amplifiers (Trek 10/40A-HS, Trek, Lockport, NY, USA), an accelerometer (Charge Accelerometer type 4393, Bruel & Kjaer, Naerum, Denmark) and an oscilloscope (MSO/DPO 2000, Tektronix, Beaverton, OR, USA) as shown in Figure 8. In the experiment, the reference frequency was changed from 60 Hz to 150 Hz at 30 Hz interval (60 Hz, 90 Hz, 120 Hz and 150 Hz) and another frequency for generating the beat was slightly different from the reference frequency. This experiment was conducted with 12 frequency sets consisting of two AC sinusoidal input voltages (1 kV) having slightly different frequencies, such as 60 Hz + 65 Hz, 60 Hz + 70 Hz, 60 Hz + 80 Hz, 90 Hz + 95 Hz, 90 Hz + 100 Hz, 90 Hz + 110 Hz, 120 Hz + 125 Hz, 120 Hz + 130 Hz, 120 Hz + 140 Hz, 150 Hz + 155 Hz, 150 Hz + 160 Hz, 150 Hz + 170 Hz. To create electrotactile beat, two voltage inputs having slightly different operating frequencies were generated the frequency controller. Two voltage inputs were applied to terminals 2 and 3 in the ETB module after being amplified by the high voltage amplifier and the ground signal was applied to the terminal 1. It is generally known that the electric field makes electric attraction force between two parallel electrodes and the amplitude of the electrostatic attraction force depends on the amplitude of the electric field. After attaching an  
Figure 6: The experimental setup for measuring the resonant frequency of the proposed ETB module. Figure 7: Vibrational behavior of the proposed ETB module as function of frequency.