## 5 Experiments on Prototype Motors
### Test of Mechanical Characteristics  
A system was built to test the mechanical characteristics of the motors, as shown in Figure 14. The motor driver was composed of a TMS320F28335 DSP main processor with peripheral circuits, including a power module, signal processing module, two-phase power amplifier module, impedance matching module, and feedback signal acquisition module. The driver controlled the motor speed by frequency modulation, and the driving frequency was 22-30 kHz. The input voltage was 24 V DC and the output voltage was 350-450 V\({}_{\mathrm{PP}}\) AC. The motor could realize millisecond start-stop and forward-reverse motion. The load device provided a load change of 0-3 N-m in 3 s. The mechanical characteristics of 70 HA and 70 HB were measured and compared with those of the USM-60 motor (Shinsei Industrial Co., Ltd., Tokyo, Japan), which is the most representative traveling wave motor currently available; this is shown in Figure 15, where \(\beta^{\prime}\) is the measured amplitude-velocity ratio. From Figure 15, the \(\beta^{\prime}\) value of USM-60 is 4.17, which is the minimum; that of 70 HA is 5.88, and that of 70 HB is 18.37, which is the maximum. This means that the mechanical characteristics were changed from soft to hard. Compared with 70 HA, the maximum speed of 70 HB was reduced from 65 rpm to 50 rpm and the maximum stall torque was increased from 1.2 N\(\cdot\)m to 2.4 N\(\cdot\)m. The operating frequency of 70 HA is in the range 50.5-53.5 kHz, and the maximum efficiency is only roughly 10% at 50% of the maximum stall torque. The operating frequency of 70 HB is in the range 25-28 kHz, and the maximum efficiency is roughly 27% at 55% of the maximum stall torque. Overall, the maximum speed of the improved 70 HB dropped by 23.1%, the maximum stall torque increased by 100%, and the maximum efficiency increased by 170%. In addition, we compared the 70 HB with the same size motor in Ref. [11]. It can be seen that as the driving frequency decreases, the output speed and torque of both motors increase and peak near the resonance point. The output performance indicators drop sharply when the driving frequency is below the resonance point. Although the efficiency of 70 HB is lower than that of the reference motor, the power consumption of 70 HB is also relatively lower. The maximum input power of the reference motor is 70 W, while that of 70 HB is only 26 W, as shown in Figure 15b,c. In summary, it is fully demonstrated that the design of 70 HB's low-speed-high-torque characteristics was successful. The measured vibration parameters were substituted into Equations (4) and (8) of Section 2. The preload was set to 550 N and the average dynamic friction coefficient was set to 0.2. Thus, the limiting speed and the limiting stall torque of the motor could be calculated and were found to be slightly larger than the measured values, as shown in Tables 4 and 5. In summary, the simulation and calculation results play an important role in designing and improving the motor. Figure 14: System for testing the mechanical characteristics of TRUMs: (**a**) a principle block diagram of the test system and (**b**) a photograph of the test system. Because preload has a great influence on the output performance of the motor, it was necessary to analyze the influence of preload on the performance of the prototype. By adjusting the preload nut when assembling the motor, the mechanical output characteristics of the motor with different preload levels could be tested. The experiments showed that the no-load speed and stall torque of 70 HA and 70 HB were smaller when the preload was small (about 200 N). With increasing preload, the no-load speed and stall torque of both motors increased rapidly and tended to be flat.