Jian-jun Qu

Mechanical and electrical characteristics of traveling wave ultrasonic motors during the whole life of friction material

It is difficult to evaluate the characteristics of traveling wave ultrasonic motors (TWUSMs) during the whole life of friction material (FM) because the wear of FM is quite slow. To solve this problem, an accelerated life test method was proposed and the mechanical and electrical characteristics of TWUSMs in the whole life of FM were analyzed systematically. The results show that the mechanical characteristics including speed and efficiency have a decline trend after an ascent with the increase of FM thickness, especially when the load torque is great. When the load torque is greater than 0.4 Nm, the optimum range of FM thickness is 0.10–0.20 mm. When the residual FM thickness is less than 0.17 mm, holding torque is greater than stall torque. Electrical characteristics of TWUSM show monotonicity in the whole life of FM. Therefore, the variations of electrical characteristics could be the wear indicator of FM. Based on experiment analysis, mechanism of interactions and couplings in the energy transfer process of the TWUSM system was explored. The energy transfer process of TWUSM was divided into four subprocesses. The wear of FM is in the intermediate subprocess, which has influence on both output performance and drive circuit of TWUSM. The four subprocesses interact and couple with each other closely. Consequently, the mechanical part and electrical part of TWUSM should be not isolated, and not only the forward action, but also the backward action in energy transfer process should be considered for a more realistic theoretical model of TWUSM.

Driving characteristics of ultrasonic motor with different application mode of friction material

Friction material is one of the most important parts for ultrasonic motor. The characteristics of ultrasonic motor were affected directly by application modes of friction material. Two friction couples were prepared to simulate the contact behavior on two traveling wave ultrasonic motors. One friction couple is composed of rotor with friction material layer and metal stator; the other friction pair is composed of metal rotor and stator with friction material layer. Impedance characteristics of the stator were measured by impedance analyzer. Based on the two traveling wave ultrasonic motors, the variations of friction drive and wear properties of the two ultrasonic motors were studied. The pre and post surface morphology and surface roughness of contact interface of the two friction couples were investigated by metalloscope and JB-4C precision roughness instrument, respectively. The wear mechanism of contact interface was analyzed. The results showed that the resonance frequency of stator with friction material layer decreases slightly. The motor with friction material layer stator obtains bigger stall torque. Ploughing wear characteristic was the dominant wear mechanism and wear of contact interface of the motor with friction material layer rotor was more severe than the motor with friction material layer stator.

Numerical simulation of contact state of ultrasonic motor with gradient friction material

Friction material is one of the most important parts for ultrasonic motor. Due to the special principle of ultrasonic motor, the wear of friction material is inevitable. Output characteristics of ultrasonic motor depend on the contact state between stator and rotor to a great extent. However, the contact state will be changed by wear of friction material, and the properties stability of ultrasonic motor also will be declined. Consequently, attentions of study on friction material with special properties, which can maintain the properties of ultrasonic motor, are attracted. In this paper, a new type of gradient friction material, which normal elastic modulus gradient declined from surface to rotor substrate, was introduced and a simplified contact model of ultrasonic motor was established. The effects of wear of gradient friction material on contact state of ultrasonic motor were studied based on the contact model. Evolution trend of contact state of ultrasonic motor with wear of gradient friction material and homogeneous friction material were compared and analyzed. The results shown that better stability of contact state, stability of output characteristics and longer service life of ultrasonic motor would be obtained by using this type of gradient friction material. Thus, a new field is presented for choosing friction material for ultrasonic motor.

Design and analysis of a disc-type of traveling wave ultrasonic motor with fixing inner side

Using B13 vibrating mode and fixing inner side of stator, a new disc-type of traveling wave ultrasonic motor (TWUSM) was designed. Finite element method was introduced to simulate the vibrating mode of stator, considering structure factors like outer diameter of stator, numbers and height of stator teeth. The variation curves of related vibration parameters were calculated, such as amplitude of stator teeth, resonant frequency, vibrating speed elliptical locus of particles in contact surface and so on. Furthermore, a prototype of 40 disc-type of TWUSM was fabricated. And the driving characteristics were investigated. The results showed that this kind of TWUSM had a simple structure, light weight, high rotational speed, and a certain torque as well.

Life prediction model of friction material for traveling wave ultrasonic motor

Traveling wave ultrasonic motor (TWUSM) relies on friction force for torque transmission. Wear often exists on friction material (FM) inevitably, and largely affects life of TWUSM. A new life prediction model of FM for TWUSM was proposed. Compared with standing wave USM, wear of TWUSM FM was resulted from scratching of traveling wave crests on stator. Considering motion of a particle in contact interface between stator and rotor, visco-elastic polymer contact model was used to calculate normal pressure distribution and relative sliding speed, which were introduced to traditional wear law of Archard. In the model, special relation between time and distance for traveling wave was used to analyze contact intervals. The change of thickness of FM during service which made decrease of preload was also taken into account. Parameters of common disc-type TWUSM were used to compute life prediction curve, and meanwhile the experimental investigation was carried out. The life prediction result found out to be consistent with the test.

An equivalent wear method of friction material for ultrasonic motor

As one of key materials for improving the performance of ultrasonic motor, the wear of friction material plays an important role in contact friction properties and output performance of ultrasonic motors. However, the wear loss of friction material is small in the friction system of ultrasonic motor. If effects of wear of friction material on properties of ultrasonic motor studied with a real motor, the experiment process were time-consuming and low efficiency and influenced by lots of known or unknown factors. In this paper, based on operation principle and wear characteristics of ultrasonic motor, an equivalent wear experimental method which using reduction of thickness of friction material equivalent to wear of friction material was proposed. And the relationships between wear of friction material and performance of ultrasonic motor were established. Validity of the method was carried out on one 60 traveling wave ultrasonic motor. This research could provide a new method for studying performance and life of ultrasonic motor.

Design principle and experiment of gradient friction material for ultrasonic motor

As the key contact material part in ultrasonic motor (USM) system, friction materials play a very important role in USM by their influence on its efficiency, speed, torque, etc. Due to the special driving principle, contact and output properties of USM are influenced strongly by wear of friction material. Therefore, studies on special and high performance of friction material for USM are essential. Researches showed that the stability of contact and output of USM could be improved by using friction material with special gradient properties. In this paper, one kind of gradient coating friction material was made according to design principle of gradient friction material for USM. The gradient friction material is combined by three coating layers. Its elastic modulus declines from surface layer to bottom layer with decrease of whole thickness of gradient friction material. The contact state and drive properties of USM were measured between gradient friction material and one kind of homogeneous friction material. Compared to homogeneous friction material, USM with gradient friction material can obtain more stable of contact state and drive properties. The result could provide experimental guidance for design and choosing of friction material for USM.

The effect of high-frequency transverse vibration on sliding friction force in vacuum

A novel tribometer was developed which was used to test the sliding friction force between PTFE-based composite and bronze with or without high-frequency transverse vibrations in atmosphere, low vacuum of 1×103Pa, medium vacuum of 2Pa and high vacuum of 2.0×10−3Pa. The effect of driving voltage, normal pressure, sliding velocity and environmental pressure on sliding friction force was studied. The results show that the sliding friction force with high-frequency transverse vibrations slows down as the driving voltage increases. The effect of sliding velocity on sliding friction with high-frequency transverse vibrations is great. With the increase of sliding velocity, the reduction of sliding friction by high-frequency transverse vibrations declines. The reduction of sliding friction with transverse vibrations is also affected by environmental pressure. With the increase of vacuum, the reduction of sliding friction by transverse vibrations decreases a little. The sliding friction coefficient with transverse vibrations becomes bigger as normal lpressure increases.