# Enhanced Design of a Soft Thin-Film Vibrotactile Actuator Based on PVC Gel
## 4 Experiments and Results  
The stiffness of the PVC gel is an important factor in the proposed actuator. Hence, we investigated the stress-strain behavior which is associated with the stiffness of the PVC gel. To investigate the stress-strain relationship of the PVC gel, uniaxial tensile tests of the PVC gel was carried out using dumbbell-shaped specimen, which was cut out from a flat drop-cast film. The test was conducted using a universal testing machine (UTM, H5 KT, Tinius Olsen, PA, USA) according to ASTM D638 type V with a crosshead speed of 50 mm/min at room temperature. Figure 6 shows the stress-strain relationship of the PVC gel. We can see that the stress increases as the strain increases. We prepared two soft PVC gel based vibrotactile actuators (one is the proposed actuator and the other is the previously developed actuator), and we also prepared the other soft actuators based on polyurethane [19] and cellulose acetate [20] in order to investigate their haptic behaviors and to compare them. The prepared four actuators have the same size but have different structure. Figure 7 shows the experimental environment 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, Oregon, OR, USA). A mass of 100 g was attached on each prepared vibrotactile actuator. A function generator, which creates 0 to 1 V\({}_{\text{PP}}\) square AC input voltage, was connected to a high voltage amplifier (having a gain of 1000) and the amplified voltage was applied to each vibrotactile  
Figure 5: Perspective view of the gold-sputtered polyimide film; (**a**) Initial state of the actuation part; (**b**) Compressed state of the actuation part. Figure 6: Mechanical property of the PVC gel. actuator. An accelerometer was put on the mass of 100 g for measuring vibration force. The measured acceleration was displayed on an oscilloscope. In this experiment, the vibration force was obtained as a function of input frequency in a range of 0 Hz to 300 Hz. We compared the proposed actuator and three other soft vibrotactile actuators. Figure 8 shows the haptic results of four soft vibrotactile actuators as a function of the input frequency. Figure 8a shows the result of the proposed actuator with new design. We plotted the haptic results of the proposed vibrotactile actuator and the other vibrotactile actuators (the conventional design, cellulose acetate based actuator, and polyurethane based actuator) for easy comparison in Figure 8b. It can be seen that the vibration amplitude of the proposed vibrotactile actuator is higher than the other vibrotactile actuators in the wide frequency range. In particular, the vibration amplitude of the proposed actuator at the resonant frequency increased to about 0.816 g (g = 9.8 m/s\({}^{2}\)). Due to the design for concentrating most of electric energy on the ridges of the PVC gel, we can increase the vibration amplitude of the proposed actuator. The applied input voltage waveform affects the haptic behavior of vibrotactile actuators. So, we conducted another experiment to investigate the haptic behaviors of the proposed vibrotactile actuator according to the input waveforms. In this experiment, we applied two input voltage waveforms (one is square wave and the other one is sinusoidal wave) to the proposed actuator and measured the output acceleration behaviors. The same experimental environment as Figure 7 was used for this  
Figure 8: Vibration amplitudes of the four soft vibrotactile actuators. (**a**) Vibration amplitude of the proposed design; (**b**) vibrotactile amplitudes of the four actuators (the proposed design, the conventional design, the polyurethane based actuator, and the cellulose acetate based actuator). Figure 7: Experimental environment to investigate haptic behavior of the soft thin-film vibrotactile actuator based on wave-shaped PVC gel. experiment.