| Piezoelectric ceramic actuators with large stroke and low generative force are studied for constructing a new type of air acoustic transducer for low frequency active noise control. The electromechanical performance of cantilever piezoelectric bimorph, unimorph and RAINBOW actuators are studied, both theoretically experimentally. Bimorph has the best electromechanical properties among all the bending actuators. By choosing a suitable thickness ratio of passive layer and piezoelectric layer in unimorph and RAINBOW actuators, maximum tip deflection, blocking force and electromechanical coupling coefficient can be obtained. In high temperature reduction processing of RAINBOW actuator, oxygen diffusion in ceramic layer is the controlling process and a parabolic law is followed between reduction thickness and reaction time in the temperature of 975{dollar}spcirc{dollar}C to 1085{dollar}spcirc{dollar}C. A resonant method is developed for the determination of Young's modulus of the reduced layer of RAINBOW actuator, which is used for further optimal design of RAINBOW actuators. Nonlinear piezoelectric and elastic response contribute significantly to the actuator performance. Two new types of piezoelectric actuators, namely cantilever shear mode actuator and multilayer pseudo-shear mode actuator are developed. In shear mode actuator, large bending deflection is induced by electric field due to its mechanical boundary condition. Multilayer pseudo-shear actuator directly utilizes piezoelectric transverse d{dollar}sb{lcub}31{rcub}{dollar} mode, in which a stack of pre-poled rectangular PZT ceramic sheets are bonded together at alternate ends in such a way that the displacement of the top layer is the sum of all the following layers thus large displacement can be achieved. Pseudo-shear actuator demonstrates larger generative force than bending actuators. A new air acoustic transducer panel, with flextensional displacement amplification mechanism is developed, in which the piezoelectric actuators are used as driving elements. A curved diaphragm which acts as a mechanical transformer is capped on the top of actuators, and it vibrates perpendicular to diaphragm surface when driven by actuators. So the diaphragm functions as an acoustic radiation source. The transducer can generate more than 80 dB sound pressure in the frequency range of 200 Hz to 800 Hz, which is sufficient for low frequency active noise control. |
