Research On Three- Feet Rotary Piezoelectric Actuator Using Bending Hybrid Vibrations

In the design of humanoid robot, the electrical motor servo system of its joint has the requirement such as low loss, high power density, light weight, single power supply, high dynamic performance and high reliability. At present, most of the rotary joints for the robots use the electromagnetic motor as the joint drive. And a common problem of the electromagnetic motor is that it is difficult to solve the contradiction between the low velocity, high torque and small size. The appearance of the piezoelectric actuator which has the different drive mechanism and different structure from the traditional electromagnetic drive provides a kind of ideal actuator for the direct drive joint of the robot. The merits such as reduction of the weight for the piezoelectric actuator, compact structure and high response speed make it favored by many scholars. After the research for more than 30 years, considerable development has been made on the piezoelectric actuator. Based on the vibration excitation of the converse piezoelectric effect, the piezoelectric actuator converts electrical energy into mechanical energy to inspire the proposed vibration modes and the corresponding trajectory. Then to achieve the drive between the stator and rotor of the piezoelectric actuator. Nowadays, piezoelectric actuators have been divided into various categories according to the differences of the structure and the mechanism.In this paper, design of piezoelectric actuator adopts the foot driving type, which has the characteristic of large output and high speed. The advantages make it a good choice for the driving system of the robot joint. And also, it shows the advantage of the piezoelectric actuator in the field of direct drive. The actuator proposed in this paper adopts the bonded type to meet the requirement of small size. The proposed actuator adopts d31 working mode of the piezoelectric element. Each driving foot vibrates under two orthogonal first bending vibration modes, which are excited by the orthogonal piezoelectric ceramics. Then the elliptical trajectory of the driving foot is superimposed by the modes. All the stators are fix to the base by the fixtures and drive the rotor to accomplish the rotation of the rotor. Three points can determine a plane, and this can provide good contact between the driving feet and the rotor. Thus, high drive efficiency can be achieved.On the basis of the theory and analysis, this paper firstly shows the design for the structure of one driving foot which can also called a piezoelectric tra nsducer. With the finite element modeling of the stator, the modal analysis is taken to analyze the stator, and the harmonic response analysis is taken using the frequency achieved by the modal mode. Then the relationship between the structural parameters and both the resonant frequency and the electromechanical coupling factor of the driving foot can be obtained by the simulation. The optimization of the structural parameters can be done by the data obtained from the simulation. And then, the trajectory of the driving foot is obtained by transient analysis, which can judge the feasibility of the actuator. Depend on the result of the analysis above, the structural framework of the actuator is designed, machined and assembled.In the experiments, the impedance- frequency characteristic of the piezoelectric actuator is test firstly to achieve its electrical parameters. Then the impedance matching parameters are obtained by the calculation of the electrical parameters. Using Laser Doppler vibrometer to test the vibration characteristic of the stator, and the resonance frequency and the corresponding vibration mode of the stator can be achieved. The mechanical output characteristic of piezoelectric actuator is tested, the maximum rotation speed of the actuator is 53.3rpm, and the maximum torque is 0.027N?m. At last, the mechanical output characteristic test and the temperature- time test for a single stator is taken, the maximum output force is 29.3N, and the maximum speed is 159.6mm/s, the temperature of the stator has a rise from 26.7-27.1°C to 32.3-36.3°C after the access of the drive signal.