# A Disc-Type High Speed Rotary Ultrasonic Motor with Internal Contact Tenth
## 5 USM's Output Performance Test  
An experimental investigation was performed, to validate the operating principle and to assess output performance of the motor (Figure 11). A motor prototype was set up into a holder and excited by a single channel of sinusoidal excitation. A laser tachometer was used to record the rotation speed of the motor, when input voltage and frequency changed. Figure 12 shows the relationship between motor driving frequency and rotation speed, under different input voltage. It is apparent that the prototype produces the best output performance under the frequency of 98.5 kHz, which verifies the ideal driving performance of in-plane B\({}_{03}\) mode. The maximum unloaded rotational speed of the motor reached 5172 rpm when 30 V was applied. The ultrasonic motor is generally used in intermittent working situations due to its friction driving principle. Long-term continuous operation of the USM will bring serious friction and wear, which will result in a sudden drop of the performance or even cause it to stop working. The present design is aimed to be used in continuous operation in high rotation speed scenarios, so it is important to reduce the friction and wear, and effectively prolong the working life of the USM. Figure 13a shows the initial surface state of the titanium contact teeth without a protective layer. Figure 13b shows the wear of the contact teeth after six hours of continuous testing, and it is noticed that the teeth surface is seriously worn. To alleviate the performance degradation caused by friction and wear,  
Figure 10: (**a**) Laser measurement of the stator vibration. (**b**) Magnitude of the motor contact point. (**c**) Magnitude–frequency characteristics of the stator. (**d**) Laser measurement setup. an alumina ceramic cover was pasted on the contact teeth, to extend the service life of the USM. Figure 13c shows the initial surface state of alumina covered titanium teeth before wear, and Figure 13d shows the surface condition of alumina covered titanium teeth after six hours of continuous testing. By comparing the two experimental results, the stator covered with alumina is more wear-resistant than that with no coatings. Moreover, the alumina-alumina contact interface has superior mechanical transmission efficiency than the alumina-titanium contact interface. Figure 14 shows rotation speed changes of the USM versus the operating time between parts with and without alumina coatings. It is indicated that the USM with alumina coated stator teeth has a larger initial output speed of 3321 RPM and a much lower output speed drop of 13%, while the corresponding values of the compared USM which did not have alumina coated stator teeth are 2240 rpm and 88%, respectively. Therefore, the alumina coating layer plays an important role in improving the output performance and working life of the designed ultrasonic motor. Figure 11: Prototype and experimental setup of the ultrasonic motor (USM). Figure 12: The relationship between driving frequency and rotation speed under different input voltages. Figure 15a shows the response of the unloaded rotational speed, with different preload force. It is found that when the preload is 0.5 N, the USM obtains the highest rotational speed. Figure 16 shows the setup of the stalling torque measurement. The shaft of the USM passes through two bearings which are supported by a holder and connects to a pulley. The diameter of the pulley is 6.4 mm, and the relation between input voltage and stalling torque is shown in Figure 15b. Maximum stalling torque is about 0.35 mN\(\cdot\)m when driving voltage reaches 30 V\({}_{\text{PP}}\). The power efficiency of it can be calculated by the following equation after getting the needed parameters:  
\[\eta=\frac{nM_{b}}{9550u_{in}} \tag{8}\]  
where \(\eta\), \(u_{in}\), \(i_{in}\), \(n\) and \(M_{b}\) refer to power efficiency, input voltage, input current, USM's rotation speed and stalling torque, respectively.