## 4 Experimental Investigation of the Actuator
More detailed measurements of the planar velocity were made when electrodes located in segments Planar\({}_{2}\) and Planar\({}_{3}\) were excited. Comparison of the measured maximum and minimum velocities at different payloads are given in Figure 15. It can be seen that velocities have minor differences at different payloads when the voltage of 80 V\({}_{\text{P-p}}\) and 200 V\({}_{\text{P-p}}\) is applied. The difference value does not exceed 9.49%. It shows that the actuator is able to provide stable planar motion characteristics in different directions at different payload values. The highest velocities of 8.8 mm/s, 8.6 mm/s, and 8.3 mm/s were obtained when a payload of 55.68 g and voltage of 80 V\({}_{\text{P-p}}\) were applied on corresponding electrodes (Figure 15a). On the other hand, the highest velocities of 19.8 mm/s, 18.8 mm/s, and 19.6 mm/s were obtained when the voltage of 200 V\({}_{\text{P-p}}\) was applied, and a payload of 55.68 g was used. Video S1 was included to supplementary materials in order to represent planar motion of robot. Angular velocity of the spherical payload was measured at different values of payload and excitation voltage values. Three different electrodes located in the segments Rotary\({}_{1}\)-Rotary\({}_{3}\) were affected by harmonic voltage in the range of 80 V\({}_{\text{P-p}}\) to 200 V\({}_{\text{P-p}}\). Firstly, angular velocities generated by contact located on segment Rotary\({}_{1}\) were investigated. The payloads values were the same as in previous measurements. The results are shown in Figure 16.  
The lowest angular speed of 4.8 RPM, 8.5 RPM, and 12.4 RPM was obtained at 80 V\({}_{\text{P-p}}\) when preload of 12.6 g, 25.1 g, and 55.68 g was applied. The highest angular speed values of 14.9 RPM, 24.7 RPM, and 31.3 RPM were obtained at 200 V\({}_{\text{P-p}}\) at the same preload values. Also, it can be seen that angular speed is almost linearly increased when excitation voltage is increasing. Angular speed also depends on payload, i.e., the higher payload causes higher friction force and angular speed, respectively. Measurements of the angular velocity were also made when electrodes located in segments Rotary\({}_{2}\) and Rotary\({}_{3}\) were excited. A summary of the results is given in Figure 10:0  
It can be seen that angular speeds of spherical payloads generated by different segments at 80 V\({}_{\text{P-p}}\) and 200 V\({}_{\text{P-p}}\) excitation signals are similar, and the difference does not exceed 15.6%. The lowest angular speeds were obtained when 80 V\({}_{\text{P-p}}\) excitation signal and payload of 12.6 g were applied to the actuator. The speeds reached values of 4.7 RPM, 5.1 RPM, and 4.3 RPM when electrode located in segment Rotary\({}_{1}\), Rotary\({}_{2}\), and Rotary\({}_{3}\) is excited, respectively. On the other hand, the highest angular speeds were obtained at a voltage of 200 V\({}_{\text{P-p}}\) and payload of 55.68 g. The speeds reached values of 31.3 RPM, 29.8 RPM, 30.9 RPM from Rotary\({}_{1}\), Rotary\({}_{2}\), and Rotary\({}_{3}\) segments, respectively. The difference between angular velocities does not exceed 5%. It shows that the actuator can provide stable angular motion while different excitation signals and payloads are applied. Video S2 was included to supplementary materials in order to represent angular motion of payload. Figure 10:1 Measured rotational velocity at the different preload when segment Rotary\({}_{1}\) is excited. Figure 10:2 Comparison of the measured linear velocity of the actuator at the different preloads when the voltage of 80 V\({}_{\text{P-P}}\) (**a**) and 200 V\({}_{\text{P-P}}\) (**b**) is applied.