Proceedings of Meetings on Acoustics Volume 19, 2013 http://acousticalsociety.org/ ICA 2013 Montreal Montreal, Canada 2-7 June 2013 Engineering Acoustics Session 1pEAb: Transduction, Transducers, and Energy Harvesting 1pEAb7. A new loudspeaker for low frequency radiation by linear motion type piezoelectric ultrasonic actuators Hiroya Saito*, Hirokazu NEGISHI, Juro OHGA, Ikuo OOHIRA, Kunio OISHI and Kazuaki Maeda *Corresponding author's address: Tokyo Univercity of Technology, Hachioji, 192-0982, Tokyo, Japan, hirosaito12@gmail.com The authors had proposed new direct-radiator loudspeaker constructions with a conventional paper cone radiator driven by Ultrasonic Motors (=USM), as a substitution for voice-coil motor. However, those models needed a revolution to linear motion conversion mechanism, and avoiding zero region non linearity, like class A amplifier. These complications came from the conventional USM, since it is a rotational and having zero region non-linearity inherently. Here, the authors would propose a new mechanism by using new ultrasonic linear actuators, called Longitudinal-Bending Multilayered Transducers with Independent Electrodes (=LBMTIE). The beauty of LBMTIE is linear and to control vertical motion and horizontal motion independently, hence zero region non-linearity avoided. Therefore it is possible to substitute the voice-coil motor directly, which avoid the complicated mechanisms mentioned above. In this LBMTIE driven loudspeaker, vertical movement voltage be fixed and horizontal voltage is driven by audio signal, like voice-coil motor. In addition, there is a big contrast against conventional voice-coil motor, which is a typical transducer; as its electrical input and sound pressure output are direct proportion each other. This is because LBMTIE driven loudspeaker may behave a sort of modulator, which is not direct proportion in between input electric power and output sound pressure level. Published by the Acoustical Society of America through the American Institute of Physics 2013 Acoustical Society of America [DOI: 10.1121/1.4800552] Received 21 Jan 2013; published 2 Jun 2013 Proceedings of Meetings on Acoustics, Vol. 19, 030026 (2013) Page 1
INTRODUCTION Personal computers or audio-video equipment, which are recently becoming prevalent, are required to include function of voice and music reproduction. Conventional audio reproduction systems for thus function are not sufficient to meet taste of recent users. Sound presence and very low frequency reproduction ability of them should be checked. Especially, low frequency characteristics of loudspeaker device must be improved. The conventional electrodynamic loudspeaker includes limitation of development in low frequency response, because driving force of electrodynamic transducer is induced indirectly, via an air gap. The authors propose a completely new loudspeaker construction by using piezoelectric ultrasonic actuators as a driver. The piezoelectric actuator is characterized by very high driving mechanical impedance because its mover contacts its stator tightly. THE LOUDSPEAKER BY ROTATIONAL TYPE PIEZOELECTRIC ULTRASONIC ACTUATOR The piezoelectric ultrasonic actuator was researched in the Union of Soviet Socialist Republics in the 1960s, and various types of it have been developed [1]. The piezoelectric ultrasonic actuator is being used for a driver of the autofocus mechanism of cameras, and position adjuster of a lens or a mirror for optical apparatus. The feature of a common piezoelectric ultrasonic actuator is -High response characteristic by direct drive. -Low speed and high torque, suitable for a direct controller. -Simple structure, suitable for miniature mechanism. -High controllability. These merits of the ultrasonic actuator are mainly due to a direct friction drive construction. FIGURE 1 shows a cut model of a rotationally type piezoelectric ultrasonic actuator. It is consisted by a circular metal mover and a metal ring stator. FIGURE 1. Structure of an ultrasonic actuator. Proceedings of Meetings on Acoustics, Vol. 19, 030026 (2013) Page 2
LOUDSPEAKER BY ROTATIONAL TYPE ULTRASONIC ACTUATOR FIGURE 2 shows a preliminary model of the loudspeaker examined by the authors, which is driven by a reciprocal motion of a rotational type ultrasonic actuator. As is described in FIGURE 3, this driving construction includes a conversion mechanism of rotation to linear motion. This experimental model induced too much wave distortion. It might be caused by friction characteristics of the rotation to linear motion conversion mechanism. In addition, combination of a spring and a sliding rail was less suitable to transfer driving force completely. FIGURE 2. The loudspeaker by rotation to linear conversion. FIGURE 3. Driving principle of the loudspeaker by rotation to linear conversion mechanism. LOUDSPEAKER BY LINEAR MOTION TYPE PIEZOELECTRIC ULTRASONIC ACTUATORS The authors developed a new loudspeaker with no conversion mechanism from rotational to linear motion. Linear motion piezoelectric ultrasonic actuator Driving principle of this motor is explained by FIGURE 4. This is one of predecessors in linear motion type piezoelectric ultrasonic actuator born in 1990 s, which configuration and motion are shown. Trajectory of the motion of tip top due to this vibration which is due to deformation of piezoelectric ceramic by an electric input, is on a small elliptical orbit, because the ceramic pear forms as a dual mode standing wave vibrator whose two vibrational modes are shown. One of intrinsic shortcoming for this model is linearity in near zero speed regions. As it has in active zone, which is source of sound distortion in loudspeaker. Proceedings of Meetings on Acoustics, Vol. 19, 030026 (2013) Page 3
Saito et al. FIGURE 4. Driving principle of composite vibration type ultrasonic actuator. FIGURE 5 shows the piezoelectric ultrasonic actuator driver construction developed by Dr. M. Takano et al. in 2006. Left hand drawing describes structure of multilayer piezoelectric ceramic driver, whereas right hand shows its photograph. Unlike the predecessor mentioned above, electrode for longitudinal vibration and bending vibration is independent each other. Then the difference of their characteristics will be explained in followings. FIGURE 5. Linear motion type piezoelectric ultrasonic actuator. FIGURE 6 shows that small rotational motion of the finger tip is induced by a composite of longitudinal vibration (L1) and bending vibration (B2) at the same near resonant frequency, of the piezoelectric ceramic rod having four divided electrodes. Its resonant frequencies of longitudinal extension vibration and transverse bending vibration of the ceramic are designed to be close frequency proximity. (b) B2 mode (a) L1 mode FIGURE 6. (a) L1 mode. (b) B2 mode. Proceedings of Meetings on Acoustics, Vol. 19, 030026 (2013) Page 4
FIGURE 7 describes relationship between applied voltage and velocity of the mover. Input-output linearity is excellent even very low input voltage region. Unlike the predecessor mentioned above, this model is very suitable for driving loudspeaker without distortion. FIGURE 7. Velocity characteristics. Driving circuit The multilayer piezoelectric actuator used here requires two sorts of ac voltages. One of them shall be modulated by audio signal as DSB(Double Side Band) wave form. FIGURE 8 describes a block diagram of the driving circuit developed by the authors. Four power amplifiers drive B2 and L1 electrodes by BTL connections. Signal for B2 is modulated by a multiplexer. Phase of signal for L1 is shifted by 90 degree to the signal for B2. FIGURE 8. Driving circuit block diagram. FIGURE 9 shows DSB signal waveform applied to B2 electrode. Proceedings of Meetings on Acoustics, Vol. 19, 030026 (2013) Page 5
FIGURE 9. DSB waveform. FIGURE 10 shows waveform to electrodes of 0 degree and 90 degree. FIGURE 10. Phase shift in 90 degree. EXPERIMENTAL MODEL The authors are examining a few experimental models. FIGURE 11 shows the first model of them. Left hand is schematic diagram and right one is photo. It is a direct radiator loudspeaker including 6 ultrasonic actuators to drive a paper cone, 46 cm in diameter. Output sound of this model still includes audio signal waveform distortion. The reason behind is assumed that the rigidity of slider support system is not enough, then one of such improvement is on going, which is explained in following. FIGURE 11. First experimental model FIGURE 12 shows the second experimental model the authors are developing now. This model is characterized by: The actuator chips are applied vertically. Contact point positions are adjustable. Proceedings of Meetings on Acoustics, Vol. 19, 030026 (2013) Page 6
FIGURE 12. Second experimental model As this model is still under construction, arrival of test motors and others are awaited. CONCLUSION In this paper, new loudspeakers using the linear ultrasonic actuators were proposed. Still it is in examination stage, yet the driving electronic circuit system was completed. The experimental results and characteristic of the newest model will be presented at the Congress. ACKNOWLEDGMENTS The authors wish to thank Prof. K. Nakamura of Tokyo Institute of Technology and Dr. M. Takano of Industrial Research Institute of Ishikawa for their valuable supports. REFERENCES 1. T. Kenjo, T. Sashida Introduction to ultrasonic motors, Sogo Denshi Pub. (Tokyo, Japan, 1991) [in Japanese]. 2. M. Takano, M. Takimoto, and K. Nakamura Electrode design of multilayered piezoelectric transducers for longitudinal-bending ultrasonic actuators, Acoust. Sci. & Tech. Vol. 32, No. 3, pp100-108 (2011). 3. J. Ohga Sound system with wideband piezoelectric rectangular loudspeakers using a tuck shaped PVDF bimorph, Proc. acoustics 2008, pp. 4573-4578 (Paris, France. June 29-July 4, 2008). 4. J. Ohga, R. Suzuki, K. Ishikawa, H. Negishi, I. Oohira, K. Maeda, H. Kubota Loudspeaker for low frequency signal driven by four piezoelectric ultrasonic motors, AES 132nd Conversion, No.0040 (Apr. 2012). 5. J. Ohga, Variety of electroacoustical devices by piezoelectric materials Is the loudspeaker without magnet nor coilpractical? -, Fundamentals Review, IEICE, 1, 4, pp. 46-61 (2008) [in Japanese]. Proceedings of Meetings on Acoustics, Vol. 19, 030026 (2013) Page 7