## 3 Experiment and Result
accelerometer (Charge Accelerometer type 4393, Bruel & Kjaer, Narum, Denmark) onto the center of the contact plate of the ETB module, acceleration values are measured and recorded. Figure 9 shows the haptic behavior of the proposed ETB module as function of two input frequencies. The scale of the X-axis was also set as 1 s. As we expected, the results show that the measured acceleration signal has two frequencies which will be used for two mechanoreceptors at the same time. Let us take 90 Hz + 95 Hz as an example where we can easily find two vibration signals (one frequency is 5 Hz and the other is 92.5 Hz). It means that the proposed module can simulate not only Meissner corpuscle but also Pacinian corpuscle simultaneously. Also, the result shows the largest amplitude of vibration at a reference frequency of 90 Hz. In order to show that the proposed ETB module can create vibrotactile force sufficiently strong for transferring haptic information to users, we first measured the absolute vibrotactile threshold through a simple experiment and then compared the vibrational force obtained from the proposed ETB module and the absolute threshold. The experiment was conducted with 10 persons whose average age is 26.9 years. We depicted the absolute vibrotactile threshold (colored in red) in Figure 9. The vibrotactile strength obtained from the proposed ETB module is stronger than the absolute vibrotactile threshold. Therefore, acceleration created from the proposed ETB module is sufficiently strong for transferring haptic information to users. Figure 8: Experimental environment to investigate haptic behavior of the proposed ETB module. Figure 9: Vibration amplitudes of the ETB module. In order to show that the proposed ETB module can create vibrotactile force sufficiently strong for transferring haptic information to users, we first measured the absolute vibrotactile threshold through a simple experiment and then compared the vibrational force obtained from the proposed ETB module and the absolute threshold. The experiment was conducted with 10 persons whose average age is 26.9 years. All of them were between 25 and 33 years old. Eight subjects were right-handed, and the others were left-handed, as determined by self-reports. All subjects do not have any sensorimotor abnormalities and they were paid after the experiment. Each subject was seated and instructed to touch the proposed ETB module with his/her index finger. After that, the subjects were presented with beat vibrations generated by the ETB module. We provided 12 vibration samples to each subject. Each sample is a beat vibration obtained from two input voltages having slightly different frequencies (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). In each trial, three 1-s long vibrational stimuli were delivered to the user and there was a 250 ms pause between stimuli as shown in Figure 10. Among the three 1-s long vibration stimuli, one randomly selected interval contained a test stimulus and the other two contained no signal. After feeling the three 1-s long vibration stimuli, he/she found the test stimulus by pressing a corresponding keypad (1, 2 or 3). During this experiment, we set an amplitude of the initial stimulus as higher than a threshold which was found in pilot experiments and then we gradually reduced its amplitude. The subjects worn a headphone to remove sound noises from the proposed ETB module. Beep sound was conveyed to the subject via the headphone to inform the start and end of each trial to the subjects. We depicted the absolute vibrotactile threshold (colored in red) in Figure 9. The vibrotactile strength obtained from the proposed ETB module is stronger than the absolute vibrotactile threshold. Therefore, vibrational force created from the proposed ETB module is sufficiently strong for transferring haptic information to users.