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Windmill-type ultrasonic micromotor fabricated by thermoplastic green machining process
Sensors and Actuators A 134 (2007) 519–524
Windmill-type ultrasonic micromotor fabricated by thermoplastic green machining process Chang-Bun Yoon a , Sung-Mi Lee a , Hyoun-Ee Kim a,∗ , Kyung-Woo Lee b a
b
School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea Kyungwon Ferrite Ind. Co., Ltd., 1260-4 Chungwang-dong, Shiheung-si, Kyonggi-do 429-450, Republic of Korea Received 31 January 2006; received in revised form 6 May 2006; accepted 30 May 2006 Available online 7 July 2006
Abstract A miniaturized windmill-type piezoelectric ultrasonic motor (USM) was developed using a thermoplastic green machining process. A thermoplastic body, consisting of 60 vol% piezoelectric material and 40 vol% thermoplastic binder, was CNC-machined to create windmill-type blades at the center of a green disk. After the binder burnout and sintering processes, the disk was poled in the thickness direction. By applying an AC field in the thickness direction, the inward blades at the center of the disk converted the radial displacements into tangential ones, causing the shaft at the center to rotate. When the USM stator with dimensions of 5.2 mm (diameter) × 0.6 mm (thickness) was operated in the principal radial vibration mode (260 kHz) at 20 Vp–p , the displacements of the blades in the x- and y-directions were 40 and 20 nm, respectively. The characteristics of the USM, monitored by using a non-contact method with a moment of inertia of 3.6 kg mm2 , consisted of a maximum torque of 22 N m, a maximum speed of 16.4 rad/s, and a maximum efficiency of 12%. © 2006 Elsevier B.V. All rights reserved. Keywords: Actuator; Ultrasonic motor; PZT
1. Introduction Piezoelectric ceramics have been used extensively for actuator and sensor applications [1]. Among the different types of piezoelectric ceramics, lead zirconate titanate (PZT) has attracted a great deal of attention, because it has excellent electromechanical properties, such as a high piezoelectric constant (dij ) and a high electromechanical coupling coefficient (kij ) [2,3]. Piezoelectric ultrasonic motors (USMs) fabricated using PZT-based materials have superior characteristics compared to conventional electromagnetic motors, such as their high torque at low speed, the absence of magnetic fields, the high resolution of the position control, and their compactness. They are good candidates for applications in precision micromechanical systems, such as medical devices, automation, and auto-focusing or zoom driving mechanisms for optical cameras [4,5]. Piezoelectric USMs generally consist of three basic components, viz. the rotor, stator, and housing units [6]. When an
∗
Corresponding author. E-mail address: [email protected] (H.-E. Kim).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.05.040
electric signal at the resonance frequency is applied to the stator, the microscopic vibration of the stator is converted into the macroscopic rotational motion of the rotor due to the friction between the stator and rotor. Most ultrasonic motors can be classified into two groups depending on the type of vibration wave applied to the stator, viz. the standing wave and traveling wave type [7,8]. Currently, traveling-wave ultrasonic motors (TWUMs) are widely used because of their excellent motor characteristics, which are achieved in spite of their size-compactness. However, TWUMs have some disadvantages, which are closely related to their geometric design. For example, annular-ring type TWUMs are divided into sectors poled in alternate directions. Therefore, the poling process is complicated and the sample is prone to be damaged during the poling process [9]. Furthermore, the opposite dilatation between neighboring piezoelectric elements causes serious fatigue problems for long-term applications. To solve these problems, many different types of ultrasonic motors utilizing standing waves or mixed bending mode have been proposed [10–13]. These actuators also showed excellent motor characteristics on the millimeter scale. Among these standing wave actuators, a motor with a metal–ceramic com-
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posite structure showed excellent performance in terms of its speed and torque [11]. In this motor, four inward metal blades were attached to a piezoceramic disk with a hole at the center. The blades were designed so as to convert the radial vibration of the piezoceramic disk into tangentional motion about a shaft at the center. In spite of its excellent performance, however, this windmill-type motor had the disadvantages of low productivity and reliability during the fabrication process. Such disadvantages are unavoidable as long as the motors are fabricated by cutting-and-bonding processes. In this paper, we present a new windmill-type motor composed solely of piezoceramic material. The stator for the motor was made using the thermoplastic green machining process. A thermoplastic disk, consisting of 60 vol% PZT-based ceramic and 40% binders, was machined to form four windmill-type blades at the center and subsequently sintered after binder burnout [14]. This thermoplastic green machining process was very effective in reducing the size and also in enhancing the reproducibility of the ultrasonic motor.
identified using an impedance analyzer (HP4194A, HewlettPackard, Palo Alto, CA). The displacements of the windmill blades in the x- and y-directions were measured as a function of frequency with a laser dopler vibrometer (OFV552, Polytec, Waldbronn, Germany). The electric power used to operate the USM at the resonance frequency was monitored with a twochannel digital real-time oscilloscope (TDS220, Tektronix, TX, USA) equipped with a current probe (N2775A, Agilent Technologies, Inc., CA, USA) and a passive probe (10073C, Agilent technologies, Inc.). The theoretical resonance frequency and displacements of the windmill-type stator were calculated based on finite element analyses performed with a commercial software package (ATILA, ISEN Recherche, Lille, France). 3. Results and discussion
2. Experimental procedure
The stator used for the piezoelectric USM (5.2 mm outer diameter and 0.6 mm thickness) with four windmill blades at the center was fabricated by the thermoplastic green machining process, as shown in Fig. 1(A). The composition of the piezoelectric material was Pb((Zn1/3 Nb2/3 )0.3 (Zr0.48 Ti0.52 )0.7 )O3 doped with
The windmill-type piezoelectric stator for the USM was fabricated using PZN-PZT powder doped with MnO2 (PZNPZT/Mn). The composition of the powder was Pb((Zn1/3 Nb2/3 )0.3 (Zr0.48 Ti0.52 )0.7 )O3 doped with 0.4 wt% MnO2 [15,16]. Highly pure PbO, ZnO, TiO2 , Nb2 O5 , ZrO2 and MnO2 (all purity 99.9%, Aldrich Chemical Co. Inc., Milwaukee, WI) were used as the starting materials. These powders were mixed by ball milling using zirconia balls and ethanol as media for 24 h. After ball milling, the mixture was dried on a hot plate and subsequently calcined at 850 ◦ C for 4 h. The calcined powder was then ball-milled again for 48 h. The 60 vol% piezoelectric powders were blended with 40 vol% thermoplastic binders (EEA 6182; Union Carbide, Danbury, CT) using a heated high-shear mixer (Jeong-sung Inc., Seoul, Korea) at 110 ◦ C. After compounding, the thermoplastic compound was warm-pressed at 110 ◦ C for 30 min with an applied load of 30 MPa and then extruded through a 24 mm × 1 mm orifice. The green body was machined using a mini-CNC machine (Modela; Roland DGA Corp., Japan) in accordance with a predetermined CAD design to make windmill-shape blades. Milling was conducted using a carbideendmill with a diameter of 0.5 mm at a rotating speed of 6500 rpm. The binders were completely removed in air by slowly heating the sample up to 500 ◦ C in order to prevent the formation of defects. Subsequently, the sample was sintered at 1050 ◦ C for 2 h in air at a heating rate of 5 ◦ C/min. In order to minimize the loss of PbO from the specimens during sintering, a PbO-rich atmosphere was maintained by placing an equimolar mixture of PbO and ZrO2 in the crucible. In order to measure the electromechanical properties of the sample, electrodes were formed on two faces by applying a thin silver paste, followed by heat treatment at 500 ◦ C for 30 min. Thereafter, the piezoelectric motor was poled in a silicone oil bath at room temperature by applying an electric field of 3 kV/mm for 30 min. The resonance frequency of the motor was
Fig. 1. (A) Optical photograph and (B) SEM micrograph of sintered windmilltype USM stator fabricated by thermoplastic green machining.
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0.4 wt% MnO2 (PZN-PZT/Mn), which was found previously to have good electromechanical properties [16]. The fracture surface of the specimen sintered at 1050 ◦ C showed that the specimen was almost fully dense with an average grain size of ∼2 m, as shown in Fig. 1(B). The microstructure was not very different from that of the bulk specimen prepared by drypressing, indicating that the binders were completely removed without any adverse effects on the densification of the ceramic. The piezoelectric properties of the PZN-PZT/Mn sintered at 1050 ◦ C were as follows: a piezoelectric coefficient (d33 ) of 330 pC/N, an electromechanical coefficient (kp ) of 0.60, a relative dielectric constant (KT ) of 1600, and a mechanical quality factor (Qm ) of 800. The setup used for the measurement of the properties of the windmill-type USM is shown in Fig. 2. The windmill-type stator was poled in the thickness direction and the rotor was placed at the center, as schematically shown in Fig. 2(A). When the sinusoidal electric field was applied to the piezoelectric stator, with its four inward blades placed 90◦ apart at the center, it successively shrank and expanded in the radial direction, pushing the rotor in the tangential direction. In order to measure the characteristics of the current USM by the transient response method, the speed of the rotor was reduced by increasing its mass. A ceramic disk placed on top of the rotor was used as the mass. The actual stator and rotor of the windmill-type USM are shown in Fig. 2(B). Both faces of the stator were electroded with silver paste.
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Fig. 2. Experimental setup for windmill-type motor. (A) Schematic illustration and (B) optical photograph.
Fig. 3. FEM simulation results showing the principle of operational under the application of a sinusoidal voltage: (A) 0 V, (B) +25 V, (C) 0 V, (D) −25 V at 264 kHz. The displacements indicated in these diagrams, amplified by a factor of 10,000, were calculated using 3D harmonic analyses.
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Fig. 5. Transient response of the motor at 60 Vp–p (21.2 Vrms ) with an applied load in the form of a ceramic disk with a mass moment of inertia of 3.6 kg mm2 (diameter 38 mm, mass 20 g). The transient speed was fitted to an exponential function.
Fig. 4. (A) Electrical impedance spectra of windmill-type USM stator at 0.5 Vp–p and (B) the displacement of the blade in the directions (shown in Fig. 3(A)) at ±10 V (20 Vp–p ) as a function of the electric frequency.
The principle of operation of the windmill-type piezoelectric USM stator are shown in Fig. 3. These figures show the results of a FEM simulation (×10,000 factor) performed with the application of a sinusoidal AC field, which were obtained from 3D harmonic analyses conducted using a commercial program (ATILA). Before the application of the electric field, the rotor was tightly fitted at the center of the stator, as shown in Fig. 3(A). When an electric field parallel to the poling direction was applied to the piezoelectric stator, the stator and the space between the blades shrank, as shown in Fig. 3(B). As a result of these movements, the rotor at the center was pushed in the tangential direction. After passing the zero field (Fig. 3(C)), when a reverse field with respect to the poling direction was applied, the stator was expanded and, at the same time, the space between the blades became larger, as shown in Fig. 3(D). At this stage, the rotor with the cone-shaped tip was pushed deeper into the stator. In this way, the microscopic movements of the stator were converted into the macroscopic rotational motion of the rotor by the friction between the stator and pressed rotor. The electromechanical characteristics of the piezoelectric stator are shown in Fig. 4. The impedance of the stator as a function of the frequency under 0.5 Vp–p is shown in Fig. 4(A).
The resonance frequency of the USM stator without a load was found to be 262 kHz. The displacements of the windmill-type blades in the x- and y-directions (as shown in Fig. 3(A)) were measured as a function of frequency using the laser vibrometer at ±10 V (20 Vp–p ), as shown in Fig. 4(B). The maximum displacements in the x- and y-directions were about 40 and 20 nm, respectively, at the resonance frequency (260 kHz). These directional vibrations push the rotor in the tangential direction. The performance of the current USM was evaluated using a non-contact method based on the following equations [17]: t (1) Ω = Ω0 1 − exp − τr dΩ dt T (N m)Ω (rad/s) 100 η (%) = Pin (W) T =J
(2) (3)
where Ω is the angular velocity, t the time, Ω0 the steady-state speed, and τ r is the rise time constant. In Eqs. (2) and (3), T is the torque that drives the load, J the moment of inertia, η the efficiency, and Pin is the input electric power. In this method, a disk whose moment of inertia is known (J = 1/2mr2 ) is placed onto the motor, and the transient speed is obtained as a function of time (Eq. (1)) by starting the motor. More explicitly, the angular acceleration of the motor is calculated by taking the derivative of the measured speed. The transient torque is calculated by multiplying the angular acceleration by the moment of inertia of the total load (Eq. (2)). The instantaneous output mechanical power (Pout ) is calculated by multiplying the torque (T) by the angular velocity (Ω). The efficiency can be calculated by dividing the output mechanical power by the input electric power. The measured transient response of the motor in the loaded state is shown in Fig. 5. In the non-loaded state, the maximum speed of the USM was more than 3000 rpm at
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References
Fig. 6. Torque, speed, and efficiency of windmill-type ultrasonic motor measured by the transient response method.
60 Vp–p . The speed decreased linearly in proportion to the load. In the loaded state with a moment of inertia of 3.6 kg mm2 , the measured speed–time relation curve is shown in Fig. 5. This curve showed saturated behavior with a maximum speed of 16.4 rad/s for a duration of 10 s with an electric configuration of 60 Vp–p (21.2 Vrms ) and 100 mAp–p (35 mArms ). An exponential curve was fitted to the transient speed of the rotor based on Eq. (1). The values of τ r and Ω0 were estimated to be 3 s and 16.4 rad/s, respectively, using the trial and error method. The torque and efficiency were calculated based on Eqs. (2) and (3), respectively, and are shown in Fig. 6. The maximum torque was 22 N m at the minimum speed, while the maximum speed was 16.4 rad/s at the minimum torque. The maximum efficiency was 12% at a torque of 11 N m and a speed of 8.3 rad/s. Since the efficiency was dependent not only on the stator but also on the other mechanical units, such as the rotor design and ball-bearings, a higher efficiency would be expected if the design of rotor and stator is improved. Therefore, this windmilltype USM motor is potentially very reproducible, compact and economical, because the stator is composed of a single ceramic component made by the thermoplastic green machining process.
[1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, New York, US, 1971, pp. 271–280. [2] K. Uchino, Ferroelectric Devices, Marcel Dekker, Inc., New York, US, 2000, pp. 74–89. [3] Y. Xu, Ferroelectric Materials and Their Application, North-Holland, Amsterdam, Netherlands, 1991, pp. 101–159. [4] T. Sashida, T. Kenjo, An Introduction to Ultrasonic Motors, Clarendon Press, Oxford, UK, 1993, pp. 17–24. [5] K. Uchino, Piezoelectric Actuators and Ultrasonic Motors, Kluwer Academic Publishers, Boston, US, 1997, pp. 265–273. [6] B. Koc, S. Cagatay, K. Uchino, A piezoelectric motor using two orthogonal bending modes of a hollow cylinder, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 49 (4) (2002) 495–500. [7] S. Cagatay, B. Koc, K. Uchino, A 1.6 mm, metal tube ultrasonic motor, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 50 (7) (2003) 782–786. [8] S. Dong, S.P. Lim, K.H. Lee, J. Zhang, L.C. Lim, K. Uchino, Piezoelectric ultrasonic micromotor with 1.5 mm diameter, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 50 (4) (2003) 361–367. [9] L. Petit, R. Briot, L. Lebrun, P. Gonnard, A piezomotor using longitudinal actuators, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 45 (2) (1998) 277–284. [10] T. Morita, M.K. Kurosawa, T. Higuchi, A cylindrical micro-ultrasonic motor (stator transducer size: 1.4 mm in diameter and 5.0 mm long), Ultrasonics 38 (2000) 33–36. [11] B. Koc, P. Bouchilloux, K. Uchino, Piezoelectric micromotor using a metal–ceramic composite structure, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 47 (4) (2000) 836–843. [12] T. Morita, Miniature piezoelectric motors, Sens. Actuators A 103 (2003) 291–300. [13] C.-B. Yoon, G.-T. Park, H.-E. Kim, J. Ryu, Piezoelectric ultrasonic micro-motor by co-extrusion process, Sens. Actuators A 121 (2005) 515– 519. [14] Y.-H. Koh, C.-B. Yoon, S.-M. Lee, H.-E. Kim, Thermoplastic green machining for the fabrication of a piezoelectric ceramic/polymer composite with 2–2 connectivity, J. Am. Ceram. Soc. 88 (4) (2005) 1060– 1063. [15] S.-M. Lee, C.-B. Yoon, S.-H. Lee, H.-E. Kim, Effect of lead zinc niobate addition on sintering behavior and piezoelectric properties of lead zirconate titanate ceramic, J. Mater. Res. 19 (9) (2004) 2553–2556. [16] S.-M. Lee, C.-B. Yoon, S.-H. Lee, K.-W. Lee, H.-E. Kim, Low-temperature sintering of PZT-PZN piezoelectric ceramics doped with MnO2 , J. Eur. Ceram. Soc, in press. [17] K. Nakamura, M. Kurosawa, H. Kurebayashi, S. Ueba, An estimation load characteristics of an ultrasonic motor by measuring transient response, IEEE Trans. Ultrason. Ferroelect. Freq. Control 38 (1991) 481–485.
Biographies 4. Summary and conclusions A windmill-type ultrasonic motor was fabricated using a thermoplastic green machining process. A thermoplastic green body consisting of 60 vol% piezoelectric material and 40 vol% thermoplastic binder was machined to create four windmill-type blades at the center of the disk. The sintered windmill-type USM stator with a diameter of 5.2 mm and a thickness of 0.6 mm had a radial resonance frequency of 260 kHz, at which the windmill blade showed displacements of 40 nm (x-direction) and 20 nm (y-direction) at 20 Vp–p . In the non-loaded state, the maximum speed of the USM was more than 3000 rpm at 60 Vp–p . When the moment of inertia was 3.6 kg mm2 , the maximum torque, maximum speed and maximum efficiency were, respectively, 22 N m, 16.4 rad/s and 12%.
Chang-Bun Yoon received his BS and MS degrees in 2000 and in 2002, respectively, from School of Materials Science and Engineering at the Seoul National University, where he is currently a PhD candidate. His research interests include the areas of piezoelectric ceramics and composites for ultrasonic motors and actuators. Sung-Mi Lee received her BS degree in Department of Materials Science and Engineering at Hong-Ik University in Korea in 2003. Since 2003, she is in the School of Materials Science and Engineering at Seoul National University for PhD degree. She is investigating on processing and properties of piezoelectric PZN-PZT materials. Hyoun-Ee Kim is a professor of the School of Materials Science and Engineering at the Seoul National University since 1991. Before joining to the SNU, he worked as a research scientist in Metals and Ceramic Division at the Oak Ridge National Lab in USA. He received his BS degree from Seoul National University in Korea in 1981 and PhD degree in ceramic engineering from the Ohio State University in 1987. His research interests include processing and charac-
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terization of piezoelectric ceramics and films. Utilization of the piezoelectric materials for ultrasonic motors and transducers are also included in his research field. Kyung-Woo Lee is a chief executor of R&D Center at the Kyungwon Ferrite Industry Co., Ltd., since 1998. Before joining to the Kyungwon, he worked as
a research scientist in Thin Films Technology Division at the Korea Institute of Science and Technology (KIST). He received his MS degree in electrical engineering from Yonsei University (Korea) in 1981. His research interests include processing and characterization of ceramics, sensors and actuators using piezoelectric effect.
Windmill-type ultrasonic micromotor fabricated by thermoplastic green machining process Chang-Bun Yoon a , Sung-Mi Lee a , Hyoun-Ee Kim a,∗ , Kyung-Woo Lee b a
b
School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea Kyungwon Ferrite Ind. Co., Ltd., 1260-4 Chungwang-dong, Shiheung-si, Kyonggi-do 429-450, Republic of Korea Received 31 January 2006; received in revised form 6 May 2006; accepted 30 May 2006 Available online 7 July 2006
Abstract A miniaturized windmill-type piezoelectric ultrasonic motor (USM) was developed using a thermoplastic green machining process. A thermoplastic body, consisting of 60 vol% piezoelectric material and 40 vol% thermoplastic binder, was CNC-machined to create windmill-type blades at the center of a green disk. After the binder burnout and sintering processes, the disk was poled in the thickness direction. By applying an AC field in the thickness direction, the inward blades at the center of the disk converted the radial displacements into tangential ones, causing the shaft at the center to rotate. When the USM stator with dimensions of 5.2 mm (diameter) × 0.6 mm (thickness) was operated in the principal radial vibration mode (260 kHz) at 20 Vp–p , the displacements of the blades in the x- and y-directions were 40 and 20 nm, respectively. The characteristics of the USM, monitored by using a non-contact method with a moment of inertia of 3.6 kg mm2 , consisted of a maximum torque of 22 N m, a maximum speed of 16.4 rad/s, and a maximum efficiency of 12%. © 2006 Elsevier B.V. All rights reserved. Keywords: Actuator; Ultrasonic motor; PZT
1. Introduction Piezoelectric ceramics have been used extensively for actuator and sensor applications [1]. Among the different types of piezoelectric ceramics, lead zirconate titanate (PZT) has attracted a great deal of attention, because it has excellent electromechanical properties, such as a high piezoelectric constant (dij ) and a high electromechanical coupling coefficient (kij ) [2,3]. Piezoelectric ultrasonic motors (USMs) fabricated using PZT-based materials have superior characteristics compared to conventional electromagnetic motors, such as their high torque at low speed, the absence of magnetic fields, the high resolution of the position control, and their compactness. They are good candidates for applications in precision micromechanical systems, such as medical devices, automation, and auto-focusing or zoom driving mechanisms for optical cameras [4,5]. Piezoelectric USMs generally consist of three basic components, viz. the rotor, stator, and housing units [6]. When an
∗
Corresponding author. E-mail address: [email protected] (H.-E. Kim).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.05.040
electric signal at the resonance frequency is applied to the stator, the microscopic vibration of the stator is converted into the macroscopic rotational motion of the rotor due to the friction between the stator and rotor. Most ultrasonic motors can be classified into two groups depending on the type of vibration wave applied to the stator, viz. the standing wave and traveling wave type [7,8]. Currently, traveling-wave ultrasonic motors (TWUMs) are widely used because of their excellent motor characteristics, which are achieved in spite of their size-compactness. However, TWUMs have some disadvantages, which are closely related to their geometric design. For example, annular-ring type TWUMs are divided into sectors poled in alternate directions. Therefore, the poling process is complicated and the sample is prone to be damaged during the poling process [9]. Furthermore, the opposite dilatation between neighboring piezoelectric elements causes serious fatigue problems for long-term applications. To solve these problems, many different types of ultrasonic motors utilizing standing waves or mixed bending mode have been proposed [10–13]. These actuators also showed excellent motor characteristics on the millimeter scale. Among these standing wave actuators, a motor with a metal–ceramic com-
520
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posite structure showed excellent performance in terms of its speed and torque [11]. In this motor, four inward metal blades were attached to a piezoceramic disk with a hole at the center. The blades were designed so as to convert the radial vibration of the piezoceramic disk into tangentional motion about a shaft at the center. In spite of its excellent performance, however, this windmill-type motor had the disadvantages of low productivity and reliability during the fabrication process. Such disadvantages are unavoidable as long as the motors are fabricated by cutting-and-bonding processes. In this paper, we present a new windmill-type motor composed solely of piezoceramic material. The stator for the motor was made using the thermoplastic green machining process. A thermoplastic disk, consisting of 60 vol% PZT-based ceramic and 40% binders, was machined to form four windmill-type blades at the center and subsequently sintered after binder burnout [14]. This thermoplastic green machining process was very effective in reducing the size and also in enhancing the reproducibility of the ultrasonic motor.
identified using an impedance analyzer (HP4194A, HewlettPackard, Palo Alto, CA). The displacements of the windmill blades in the x- and y-directions were measured as a function of frequency with a laser dopler vibrometer (OFV552, Polytec, Waldbronn, Germany). The electric power used to operate the USM at the resonance frequency was monitored with a twochannel digital real-time oscilloscope (TDS220, Tektronix, TX, USA) equipped with a current probe (N2775A, Agilent Technologies, Inc., CA, USA) and a passive probe (10073C, Agilent technologies, Inc.). The theoretical resonance frequency and displacements of the windmill-type stator were calculated based on finite element analyses performed with a commercial software package (ATILA, ISEN Recherche, Lille, France). 3. Results and discussion
2. Experimental procedure
The stator used for the piezoelectric USM (5.2 mm outer diameter and 0.6 mm thickness) with four windmill blades at the center was fabricated by the thermoplastic green machining process, as shown in Fig. 1(A). The composition of the piezoelectric material was Pb((Zn1/3 Nb2/3 )0.3 (Zr0.48 Ti0.52 )0.7 )O3 doped with
The windmill-type piezoelectric stator for the USM was fabricated using PZN-PZT powder doped with MnO2 (PZNPZT/Mn). The composition of the powder was Pb((Zn1/3 Nb2/3 )0.3 (Zr0.48 Ti0.52 )0.7 )O3 doped with 0.4 wt% MnO2 [15,16]. Highly pure PbO, ZnO, TiO2 , Nb2 O5 , ZrO2 and MnO2 (all purity 99.9%, Aldrich Chemical Co. Inc., Milwaukee, WI) were used as the starting materials. These powders were mixed by ball milling using zirconia balls and ethanol as media for 24 h. After ball milling, the mixture was dried on a hot plate and subsequently calcined at 850 ◦ C for 4 h. The calcined powder was then ball-milled again for 48 h. The 60 vol% piezoelectric powders were blended with 40 vol% thermoplastic binders (EEA 6182; Union Carbide, Danbury, CT) using a heated high-shear mixer (Jeong-sung Inc., Seoul, Korea) at 110 ◦ C. After compounding, the thermoplastic compound was warm-pressed at 110 ◦ C for 30 min with an applied load of 30 MPa and then extruded through a 24 mm × 1 mm orifice. The green body was machined using a mini-CNC machine (Modela; Roland DGA Corp., Japan) in accordance with a predetermined CAD design to make windmill-shape blades. Milling was conducted using a carbideendmill with a diameter of 0.5 mm at a rotating speed of 6500 rpm. The binders were completely removed in air by slowly heating the sample up to 500 ◦ C in order to prevent the formation of defects. Subsequently, the sample was sintered at 1050 ◦ C for 2 h in air at a heating rate of 5 ◦ C/min. In order to minimize the loss of PbO from the specimens during sintering, a PbO-rich atmosphere was maintained by placing an equimolar mixture of PbO and ZrO2 in the crucible. In order to measure the electromechanical properties of the sample, electrodes were formed on two faces by applying a thin silver paste, followed by heat treatment at 500 ◦ C for 30 min. Thereafter, the piezoelectric motor was poled in a silicone oil bath at room temperature by applying an electric field of 3 kV/mm for 30 min. The resonance frequency of the motor was
Fig. 1. (A) Optical photograph and (B) SEM micrograph of sintered windmilltype USM stator fabricated by thermoplastic green machining.
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0.4 wt% MnO2 (PZN-PZT/Mn), which was found previously to have good electromechanical properties [16]. The fracture surface of the specimen sintered at 1050 ◦ C showed that the specimen was almost fully dense with an average grain size of ∼2 m, as shown in Fig. 1(B). The microstructure was not very different from that of the bulk specimen prepared by drypressing, indicating that the binders were completely removed without any adverse effects on the densification of the ceramic. The piezoelectric properties of the PZN-PZT/Mn sintered at 1050 ◦ C were as follows: a piezoelectric coefficient (d33 ) of 330 pC/N, an electromechanical coefficient (kp ) of 0.60, a relative dielectric constant (KT ) of 1600, and a mechanical quality factor (Qm ) of 800. The setup used for the measurement of the properties of the windmill-type USM is shown in Fig. 2. The windmill-type stator was poled in the thickness direction and the rotor was placed at the center, as schematically shown in Fig. 2(A). When the sinusoidal electric field was applied to the piezoelectric stator, with its four inward blades placed 90◦ apart at the center, it successively shrank and expanded in the radial direction, pushing the rotor in the tangential direction. In order to measure the characteristics of the current USM by the transient response method, the speed of the rotor was reduced by increasing its mass. A ceramic disk placed on top of the rotor was used as the mass. The actual stator and rotor of the windmill-type USM are shown in Fig. 2(B). Both faces of the stator were electroded with silver paste.
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Fig. 2. Experimental setup for windmill-type motor. (A) Schematic illustration and (B) optical photograph.
Fig. 3. FEM simulation results showing the principle of operational under the application of a sinusoidal voltage: (A) 0 V, (B) +25 V, (C) 0 V, (D) −25 V at 264 kHz. The displacements indicated in these diagrams, amplified by a factor of 10,000, were calculated using 3D harmonic analyses.
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Fig. 5. Transient response of the motor at 60 Vp–p (21.2 Vrms ) with an applied load in the form of a ceramic disk with a mass moment of inertia of 3.6 kg mm2 (diameter 38 mm, mass 20 g). The transient speed was fitted to an exponential function.
Fig. 4. (A) Electrical impedance spectra of windmill-type USM stator at 0.5 Vp–p and (B) the displacement of the blade in the directions (shown in Fig. 3(A)) at ±10 V (20 Vp–p ) as a function of the electric frequency.
The principle of operation of the windmill-type piezoelectric USM stator are shown in Fig. 3. These figures show the results of a FEM simulation (×10,000 factor) performed with the application of a sinusoidal AC field, which were obtained from 3D harmonic analyses conducted using a commercial program (ATILA). Before the application of the electric field, the rotor was tightly fitted at the center of the stator, as shown in Fig. 3(A). When an electric field parallel to the poling direction was applied to the piezoelectric stator, the stator and the space between the blades shrank, as shown in Fig. 3(B). As a result of these movements, the rotor at the center was pushed in the tangential direction. After passing the zero field (Fig. 3(C)), when a reverse field with respect to the poling direction was applied, the stator was expanded and, at the same time, the space between the blades became larger, as shown in Fig. 3(D). At this stage, the rotor with the cone-shaped tip was pushed deeper into the stator. In this way, the microscopic movements of the stator were converted into the macroscopic rotational motion of the rotor by the friction between the stator and pressed rotor. The electromechanical characteristics of the piezoelectric stator are shown in Fig. 4. The impedance of the stator as a function of the frequency under 0.5 Vp–p is shown in Fig. 4(A).
The resonance frequency of the USM stator without a load was found to be 262 kHz. The displacements of the windmill-type blades in the x- and y-directions (as shown in Fig. 3(A)) were measured as a function of frequency using the laser vibrometer at ±10 V (20 Vp–p ), as shown in Fig. 4(B). The maximum displacements in the x- and y-directions were about 40 and 20 nm, respectively, at the resonance frequency (260 kHz). These directional vibrations push the rotor in the tangential direction. The performance of the current USM was evaluated using a non-contact method based on the following equations [17]: t (1) Ω = Ω0 1 − exp − τr dΩ dt T (N m)Ω (rad/s) 100 η (%) = Pin (W) T =J
(2) (3)
where Ω is the angular velocity, t the time, Ω0 the steady-state speed, and τ r is the rise time constant. In Eqs. (2) and (3), T is the torque that drives the load, J the moment of inertia, η the efficiency, and Pin is the input electric power. In this method, a disk whose moment of inertia is known (J = 1/2mr2 ) is placed onto the motor, and the transient speed is obtained as a function of time (Eq. (1)) by starting the motor. More explicitly, the angular acceleration of the motor is calculated by taking the derivative of the measured speed. The transient torque is calculated by multiplying the angular acceleration by the moment of inertia of the total load (Eq. (2)). The instantaneous output mechanical power (Pout ) is calculated by multiplying the torque (T) by the angular velocity (Ω). The efficiency can be calculated by dividing the output mechanical power by the input electric power. The measured transient response of the motor in the loaded state is shown in Fig. 5. In the non-loaded state, the maximum speed of the USM was more than 3000 rpm at
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References
Fig. 6. Torque, speed, and efficiency of windmill-type ultrasonic motor measured by the transient response method.
60 Vp–p . The speed decreased linearly in proportion to the load. In the loaded state with a moment of inertia of 3.6 kg mm2 , the measured speed–time relation curve is shown in Fig. 5. This curve showed saturated behavior with a maximum speed of 16.4 rad/s for a duration of 10 s with an electric configuration of 60 Vp–p (21.2 Vrms ) and 100 mAp–p (35 mArms ). An exponential curve was fitted to the transient speed of the rotor based on Eq. (1). The values of τ r and Ω0 were estimated to be 3 s and 16.4 rad/s, respectively, using the trial and error method. The torque and efficiency were calculated based on Eqs. (2) and (3), respectively, and are shown in Fig. 6. The maximum torque was 22 N m at the minimum speed, while the maximum speed was 16.4 rad/s at the minimum torque. The maximum efficiency was 12% at a torque of 11 N m and a speed of 8.3 rad/s. Since the efficiency was dependent not only on the stator but also on the other mechanical units, such as the rotor design and ball-bearings, a higher efficiency would be expected if the design of rotor and stator is improved. Therefore, this windmilltype USM motor is potentially very reproducible, compact and economical, because the stator is composed of a single ceramic component made by the thermoplastic green machining process.
[1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, New York, US, 1971, pp. 271–280. [2] K. Uchino, Ferroelectric Devices, Marcel Dekker, Inc., New York, US, 2000, pp. 74–89. [3] Y. Xu, Ferroelectric Materials and Their Application, North-Holland, Amsterdam, Netherlands, 1991, pp. 101–159. [4] T. Sashida, T. Kenjo, An Introduction to Ultrasonic Motors, Clarendon Press, Oxford, UK, 1993, pp. 17–24. [5] K. Uchino, Piezoelectric Actuators and Ultrasonic Motors, Kluwer Academic Publishers, Boston, US, 1997, pp. 265–273. [6] B. Koc, S. Cagatay, K. Uchino, A piezoelectric motor using two orthogonal bending modes of a hollow cylinder, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 49 (4) (2002) 495–500. [7] S. Cagatay, B. Koc, K. Uchino, A 1.6 mm, metal tube ultrasonic motor, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 50 (7) (2003) 782–786. [8] S. Dong, S.P. Lim, K.H. Lee, J. Zhang, L.C. Lim, K. Uchino, Piezoelectric ultrasonic micromotor with 1.5 mm diameter, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 50 (4) (2003) 361–367. [9] L. Petit, R. Briot, L. Lebrun, P. Gonnard, A piezomotor using longitudinal actuators, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 45 (2) (1998) 277–284. [10] T. Morita, M.K. Kurosawa, T. Higuchi, A cylindrical micro-ultrasonic motor (stator transducer size: 1.4 mm in diameter and 5.0 mm long), Ultrasonics 38 (2000) 33–36. [11] B. Koc, P. Bouchilloux, K. Uchino, Piezoelectric micromotor using a metal–ceramic composite structure, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 47 (4) (2000) 836–843. [12] T. Morita, Miniature piezoelectric motors, Sens. Actuators A 103 (2003) 291–300. [13] C.-B. Yoon, G.-T. Park, H.-E. Kim, J. Ryu, Piezoelectric ultrasonic micro-motor by co-extrusion process, Sens. Actuators A 121 (2005) 515– 519. [14] Y.-H. Koh, C.-B. Yoon, S.-M. Lee, H.-E. Kim, Thermoplastic green machining for the fabrication of a piezoelectric ceramic/polymer composite with 2–2 connectivity, J. Am. Ceram. Soc. 88 (4) (2005) 1060– 1063. [15] S.-M. Lee, C.-B. Yoon, S.-H. Lee, H.-E. Kim, Effect of lead zinc niobate addition on sintering behavior and piezoelectric properties of lead zirconate titanate ceramic, J. Mater. Res. 19 (9) (2004) 2553–2556. [16] S.-M. Lee, C.-B. Yoon, S.-H. Lee, K.-W. Lee, H.-E. Kim, Low-temperature sintering of PZT-PZN piezoelectric ceramics doped with MnO2 , J. Eur. Ceram. Soc, in press. [17] K. Nakamura, M. Kurosawa, H. Kurebayashi, S. Ueba, An estimation load characteristics of an ultrasonic motor by measuring transient response, IEEE Trans. Ultrason. Ferroelect. Freq. Control 38 (1991) 481–485.
Biographies 4. Summary and conclusions A windmill-type ultrasonic motor was fabricated using a thermoplastic green machining process. A thermoplastic green body consisting of 60 vol% piezoelectric material and 40 vol% thermoplastic binder was machined to create four windmill-type blades at the center of the disk. The sintered windmill-type USM stator with a diameter of 5.2 mm and a thickness of 0.6 mm had a radial resonance frequency of 260 kHz, at which the windmill blade showed displacements of 40 nm (x-direction) and 20 nm (y-direction) at 20 Vp–p . In the non-loaded state, the maximum speed of the USM was more than 3000 rpm at 60 Vp–p . When the moment of inertia was 3.6 kg mm2 , the maximum torque, maximum speed and maximum efficiency were, respectively, 22 N m, 16.4 rad/s and 12%.
Chang-Bun Yoon received his BS and MS degrees in 2000 and in 2002, respectively, from School of Materials Science and Engineering at the Seoul National University, where he is currently a PhD candidate. His research interests include the areas of piezoelectric ceramics and composites for ultrasonic motors and actuators. Sung-Mi Lee received her BS degree in Department of Materials Science and Engineering at Hong-Ik University in Korea in 2003. Since 2003, she is in the School of Materials Science and Engineering at Seoul National University for PhD degree. She is investigating on processing and properties of piezoelectric PZN-PZT materials. Hyoun-Ee Kim is a professor of the School of Materials Science and Engineering at the Seoul National University since 1991. Before joining to the SNU, he worked as a research scientist in Metals and Ceramic Division at the Oak Ridge National Lab in USA. He received his BS degree from Seoul National University in Korea in 1981 and PhD degree in ceramic engineering from the Ohio State University in 1987. His research interests include processing and charac-
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terization of piezoelectric ceramics and films. Utilization of the piezoelectric materials for ultrasonic motors and transducers are also included in his research field. Kyung-Woo Lee is a chief executor of R&D Center at the Kyungwon Ferrite Industry Co., Ltd., since 1998. Before joining to the Kyungwon, he worked as
a research scientist in Thin Films Technology Division at the Korea Institute of Science and Technology (KIST). He received his MS degree in electrical engineering from Yonsei University (Korea) in 1981. His research interests include processing and characterization of ceramics, sensors and actuators using piezoelectric effect.