# Enhanced Design of a Soft Thin-Film Vibrotactile Actuator Based on PVC Gel
## 4 Experiments and Results
Figure 9a,b show the results when the proposed actuator was operated by the square wave input and the sinusoidal input, respectively. The measured resonant frequencies are almost the same (about 160 Hz), whereas the measured resonant peaks are different from each other (0.816 g for square wave and 0.632 g for sinusoidal wave). The vibration amplitude under the square wave input is bigger than the sinusoidal input in an entire frequency range (0 to 300 Hz). The relationship between output acceleration and input voltage is an important factor. So, we measured output acceleration as a function of input voltage. The same experimental environment (Figure 7) consisting of a function generator, an accelerometer, a high voltage amplifier, and an oscilloscope was used for measuring the force-voltage relationship of the proposed actuator. The applied voltage was altered in the range of 0 V to 1000 V at 100 V intervals and the input frequency was fixed at 1 Hz. The output force increases monotonically with the amplitude of voltage input. Figure 10 shows that the measured output force was increased from 0 g (0 V) to 0.416 g (1000 V) for the input with a frequency of 1 Hz. In order to show that the proposed actuator can be operated under bending condition, we measured the displacement using a laser displacement sensor (Keyence LK-G15, Nevada, NV, USA), and data was gathered by using a signal conditioner (Bruel & Kjaer Pulse analyzer, Bruel & Kjaer,  
Figure 10: Voltage–acceleration relationship of the proposed structure based PVC gel actuator. Figure 9: Acceleration behaviors when the proposed actuator was operated by the square wave input (**a**), and the sinusoidal input (**b**), respectively. Naerum, Denmark) with a PC. During measurements, an AC voltage of square wave was applied to the actuator by a function generator (Agilent 33220A, Agilent Technologies, Folsom, California, CA, USA) that was connected to a high voltage power amplifier (Trek 10/40A-HS, TREK, New York, NY, USA). We prepared four bending samples and one flat sample. The bending radius of the prepared bending samples were adjusted to 27.5, 22.5, 17.5, and 12.5 mm, which were denoted as 27.5, 22.5, 17.5, and 12.5 r, respectively. Figure 11 is the results of the prepared samples' displacements according to the bending radius. The displacement gradually decreased with decreasing bending radius. The measured displacement decreased from 42.75 um (Flat) to 41.4 um (27.5 r) for the input with voltage 1000 V. It also decreased from 41.4 um to 24.81 um with decreasing bending radius from 27.5 r to 12.5 r  
The perceptual strength of vibration that users feel may be lower with our soft actuator than other heavier actuators when providing vibrations of the same amplitude. Furthermore, the contact area of our actuator on the skin varies during stimulation due to the membrane's flexibility, whereas other solid contacts maintain the same contact area. For these reasons, we conducted the same perceptual experiment as [19] to evaluate the range of perceived vibrotactile intensity that our soft actuator can provide in comparison to a traditional rigid actuator in a perceptual experiment. In this experiment, two vibrotactile actuators were used. One actuator was the proposed actuator (for reference stimulus), and the other actuator used to create comparison stimuli was a mechanical mini-shaker (model 4810, Bruel & Kjaer, Naerum, Denmark); dynamic mass 18 g) with an amplifier (model 2709, Bruel & Kjaer, Naerum, Denmark). The vibratory stimulus (with sinusoidal waveform), which is one of four frequencies (80, 120, 160, and 200 Hz), was randomly selected and was conveyed to subjects. For each frequency, we provided the reference stimulus to users using our soft actuator with the maximum amplitude (1 kV\({}_{\text{P-P}}\)) in a sinusoidal waveform. Sinusoidal comparison stimuli were provided by the mini-shaker at the same frequency, but with varying amplitudes.