# A High-Thrust Screw-Type Piezoelectric Ultrasonic Motor with Three-Wavelength Exciting Mode
## 4 Fabrication and Measurement
### Results and Discussion  
Figure 9 shows the frequency characteristics of the proposed screw-type piezoelectric ultrasonic motor under a load of 4 N. When the peak-to-peak voltage is 120 V\({}_{\text{P-P}}\), the output velocity of the motor increases with the increased excitation frequency until it reaches 0.96 mm/s at the frequency of 28.9 kHz, and then it drops quickly. The results indicate that the prototype works steadily and continuously when the frequencies are from 28.7 to 29.1 kHz. The maximum output velocity was gained at the resonant frequency of 28.9 kHz. Compared with the simulation and tested results, there is also a slight deviation that may be caused by fabrication and assembly errors, measurement error, and so on. Thus, the exciting frequency of the motor of 28.9 kHz is chosen as the input frequency parameter in subsequent experiments.  
Figure 10 plots the voltage characteristics of the screw-type piezoelectric ultrasonic motor. The output velocity of the motor is tested at the resonant frequency of 28.9 kHz, as shown in Figure 10a. The peak-to-peak voltages are from 40V\({}_{\text{P-P}}\) to 120V\({}_{\text{P-P}}\) and the phase shift of the excitation signals is 90\({}^{\circ}\). The output velocity increases obviously with the increase of driving voltage and a well linear relationship can be observed. The results indicate that the output velocities are 0.2 mm/s and 0.97 mm/s when peak-to-peak voltages are40V\({}_{\text{P-P}}\) and 120V\({}_{\text{P-P}}\). Figure 10b shows that the relationship  
Figure 8: Schematic drawing of the experiment system.  
Figure 9: Frequency characteristics of the screw-type piezoelectric ultrasonic motor.  
between the peak-to-peak voltages and maximum thrust. The curve of the maximum thrust follows a linear increased tendency with the increased peak-to-peak voltage, but is not an ideal straight line. The maximum thrusts are 17.8 N and 50.8 N under the peak-to-peak driving voltage of 40V\({}_{\text{P-P}}\) and 120V\({}_{\text{P-P}}\), respectively.  
Figure 11 shows the load characteristics curve of the screw-type piezoelectric ultrasonic motor when the peak-to-peak voltage, excitation frequency, and phase shift are120V\({}_{\text{P-P}}\), 28.9 kHz and 90\({}^{\circ}\), respectively. The output velocity rapidly decreases with the increasing load, and the motor cannot work normally when the load reaches 50.4 N. The output power curve of the actuator is also observed in Figure 11. The output power increases with the increasing load until it reaches 14.56 mW at the load of 39 N, and then the output power of the motor decreases rapidly. The force density of the motor is 247.8 N/kg, compared with the force density of 149.8 N/kg of the screw-type ultrasonic motor under the high exciting voltage of 250 V\({}_{\text{P-P}}\) for 34.5 kHz in the reference [27].Thus, the proposed motor excited by the three-wavelength exciting mode can meet the specific conditions that require high thrust as well as a compact structure.  
Figure 11: Load characteristics of the screw-type piezoelectric ultrasonic motor.  
Figure 10: Voltage characteristics of the screw-type piezoelectric ultrasonic motor: (**a**) the relationship between the driving voltages and the output velocities; (**b**) the relationship between the driving voltages and the maximum thrust.