# Flexible Vibrotactile Actuator Based on Dielectric Elastomer for Smart Handheld Devices
## 3 Experiments and Results
### Haptic Performance of the Proposed Actuator  
To find the best weight ratio of PDMS to Ecoflex, we prepared six dielectric layer samples as mentioned above and fabricated six corresponding vibrotactile actuators. The  
Figure 5: Mechanical properties of the dielectric layer samples. Figure 6: Experimental environment used to investigate the dielectric properties of the dielectric layer samples. Figure 7: Dielectric properties of the dielectric layer samples. haptic performance of each vibrotactile actuator was quantitatively verified using the experimental environment shown in Figure 8. We attached 100 g of mass to the proposed actuator, and then we placed an accelerometer (2.4 g, charge accelerometer type 4393, Bruel & Kjaer, Virum, Denmark) on the upper surface of the mass. The AC input voltage generated from the function generator (Protek 9305, GSInstech, Incheon, Korea) was amplified (1000 times) by the high-voltage amplifier (Trek 10/40A-HS, Trek Inc., New York, NY, USA). Subsequently, the amplified voltage was applied to an actuator. While applying the amplified input signal, we measured the vibration using an accelerometer and displayed the measured vibration on an oscilloscope (TDS1012B, Tektronix, Shanghai, China). In the first experiment, we investigated the frequency response of the proposed actuator by sweeping the frequency of the input signal from 1 Hz to 300 Hz using a fixed peak-to-peak input voltage of 2 kV (Figure 9a). As the amount of Ecoflex increases in the actuator, the maximum amplitude of the measured vibration increases. However, when the ratio of PDMS to Ecoflex exceeded 1:5, the maximum amplitude decreased. Therefore, the dielectl\({}_{1:5}\)-based vibrotactile actuator achieved the best performance among the prepared actuators. Additionally, the haptic performance of the proposed actuator according to the amplitude of the input voltage is as important as its frequency response. In a second experiment, we measured the haptic behavior of the proposed actuator as a function of the amplitude of the input voltage at a fixed frequency of 150 Hz (resonant frequency) (Figure 9b). The vibration strength is proportional to the amplitude of the input voltage. Moreover, the dielectl\({}_{1:5}\) based vibrotactile actuator has the highest performance among the prepared vibrotactile actuators. In order to verify the reproducibility of the proposed actuator, we prepared five actuators based on the dielectl\({}_{1:5}\), which shows the best haptic performance. We observed the haptic performance of the prepared five actuators as a function of the input frequency from 1 Hz to 300 Hz at the fixed amplitude (2 kV\({}_{\text{pp}}\)) (Figure 9c). Moreover, we investigated their haptic performance as a function of the input voltage at the resonant frequency (Figure 9d). As can be observed from Figure 9e,f, even though there are some discrepancies in the results of the vibrational force's variation, the discrepancies are smaller than the just-noticeable difference (16%) [21], which is a differential threshold for the vibrational force that a human can reliably discriminate. It means that there is little variability due to the manufacturing of the actuator. Figure 8: Experimental setup used to investigate the haptic behavior of the proposed actuator. Figure 9: Haptic performance of the proposed actuator: (**a**) vibration output acceleration of the actuator samples according to input frequency; (**b**) vibration output acceleration of the actuator samples according to voltage; (**c**) haptic performance of the prepared five actuators based on dielect\({}_{1.5}\) as a function of input frequency; and (**d**) haptic performance of the prepared five actuators based on dielect\({}_{1.5}\) as a function of input voltage; (**e**) discrepancies among the prepared actuators in frequency response; (**f**) discrepancies among the prepared actuators in voltage response; (**g**) response time of the proposed actuator based on dielect\({}_{1.5}\). The response time is also an important factor in haptic performance.