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Ultrasonic micro-motor using miniature piezoelectric tube with diameter of 1.0 mm
Ultrasonics 44 (2006) e603–e606 www.elsevier.com/locate/ultras
Ultrasonic micro-motor using miniature piezoelectric tube with diameter of 1.0 mm Hui Zhang a, Shu-xiang Dong a,b, Shu-yi Zhang a,*, Tian-hua Wang a, Zhong-ning Zhang a, Li Fan a b
a Laboratory of Modern Acoustics, Institute of Acoustics, Nanjing University, Nanjing 210093, PR China Department of Material Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
Available online 5 June 2006
Abstract At the present moment, the smallest piezoelectric ultrasonic micro-motors utilizing miniature PZT piezoelectric ceramic tubes were developed. The motor consists of a PZT-metal composite tube stator, two steel rotors and a thin shaft that keeps the two rotors pressing on both ends of the stator elastically. The dimensions of the PZT tube are 1.0 mm in outer diameter, 0.6 mm in inner diameter and 5.0 mm in length. The diameter and total length of the assembled micro-motor is 1.0 mm and 8 mm (including an adjusting spring), respectively. The tube-type micro-motor is driven by two pairs of alternative voltages with phase shift 90 between the adjacent electrodes and operated in the first circular-bending vibration mode of the stator with the resonance frequency about 58 kHz. The experimental results show that the tube-type micro-motors have perfect performances: (i) high rotation frequency over 3000 rpm and (ii) large starting torque over 7.8 lN m under the conditions of the input voltage of 110 Vp–p and the resonance frequency. The micro-motor is well suitable for operating in micro-spaces, such as in intravascular, micro-robots and micro-craft applications. 2006 Elsevier B.V. All rights reserved. Keywords: Ultrasonic motor; Piezoelectric tube; Finite element method
1. Introduction Piezoelectric ultrasonic motors exhibit many advantages, such as high rotation torque, high compactness in size, simple construction, direct drive and none of electromagnetic interferences. The piezoelectric ultrasonic motor can generate a large output torque with very simple and minimal construction, and operate in very small space, such as medical catheters, micro-robot and micro-craft drivers. Recently, various types of ultrasonic micro-motors in millimeter sizes have been reported [1–6]. A piezoelectric cylindrical ultrasonic micro-motor with a stator 1.4 mm in diameter and 5 mm in length was fabricated by Morita et al. [3]. This micro-motor was based on a bending vibration mode and operated at a resonance frequency of 227 kHz. The maximum rotation frequency *
Corresponding author. fax: +86 25 83313690. E-mail address: [email protected] (S.-y. Zhang).
0041-624X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2006.05.064
was 680 rpm at 20 Vp–p driving voltage. The output torque was calculated to be 0.67 lN m from the transient response of the rotor. Dong et al. and Cagatay et al. reported separately ultrasonic micro-motors with piezoelectric tubes in diameters 1.5 mm and 1.6 mm, resonance frequencies 67 KHz and 130 kHz, stalled torques 45 lN m and 500 lN m, maximum rotation frequency 2000 rpm and 45 rad/s, respectively [5]. In 2001 [6], the micro-motor was reported which was fabricated by the piezoelectric ceramic cylindrical rod. The prototype motor was 1 mm in diameter and 5 mm in length. The resonance frequency is 32 kHz with one fixed boundary or 83 kHz with two free boundaries. The rotation frequency is near 1800 rpm and the stalled torque is 4 lN m. In this paper, to obtain the ultrasonic micro-motor with better functions and miniature structure, we present a new piezoelectric ultrasonic micromotor with the PZT tube which is 1 mm in outer diameter, 0.6 mm in inner diameter. In the micro-motor a PZT ceramic/metal composite tube is used as the stator instead of a
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titanium tube coated with PZT film or a simple PZT tube, as a PZT/metal composite stator can generate higher output torque, stronger driving effect. This type of motor with a thin diameter and long configuration is well-suited for small and long space applications. The output characteristics of prototype micro-motor and the resonance frequencies of the stator are measured at the first bending vibration mode. The experimental results show that the micro-motor has high output torque, fast rotation frequency. 2. Structure and principle of operation Fig. 1 shows the piezoelectric ultrasonic micro-motor structure. A piezoelectric ultrasonic micro-motor is designed using a PZT ceramic/brass composite tube as the stator. The rotor is a shaft on which two steel rotor caps are attached. The composite stator can generate wobbling motions at its ends by combining bending vibrations in two orthogonal directions of the tube. This ultrasonic micro-motor is a mode-rotation type, which utilizes the first bending vibration mode. With four electrical sources to each electrode, the bending vibration is generated. The phase difference is 90 between the adjacent electrodes. Please note that a traveling wave is propagated at the end surface of the stator. The rotor turns around by a frictional force. When four electric signals are applied on the outer silver electrodes, the micro-motor can operate smoothly. The driving direction is reversible by exchanging one pair of symmetry electrodes. Table 1 shows some of the materials property characteristics of the piezoelectric tube.
Table 1 Material parameter data of the PZT tube Mechanical quality factor Coupling factor Piezoelectric charge coefficient Compliance Density Permittivity/ 1 kHz
Qm K31 d31 (1012 C/N) S E33 (1012 m/N) q (mg/mm3) e33/e0
80 0.33 185 20.7 7.8 1850
ical coupling factor k 2eff . The resonance frequency fr and anti-resonance frequency fa of the first bending mode for the micro-motor stator with the metal caps can be analyzed by the finite element method (FEM) codes ANSYS. The k 2eff of the stator can be estimated from the equation k 2eff ¼ ½1 ðfr =fa Þ2 1=2
ð1Þ k 2eff
Fig. 2 shows the fr and of the first bending mode which are predicted by the ANSYS and Eq. (1). The outer and inner diameters are held constant at 1.0 mm and 0.6 mm, respectively, the length of the piezoelectric tube is varied. When the length of the tube increases, the resonance frequency fr decreases. It can be seen that when the length of the tube is 4 mm, fr is below 100 kHz, as shown in Fig. 2(a). The small resonance frequency can improve vibration amplitude of the piezoelectric tube, so the torque of the micro-motor can be raised [5]. Fig. 2(b) revealed that k 2eff is strongly dependent on the length of the piezoelectric tube. When the length of the tube
3. Theoretical analysis of micro-motor High stator resonance frequencies significantly decreased the output torque, due to a decrease in the vibration amplitude of the stator. Calculations demonstrated that thin composite stator operating in the first bending vibration mode is potential way to decrease the resonance frequency as the motor is miniaturized. The resonance frequencies fr can be decreased further by attaching metal caps to the ends of the piezoelectric tube. Higher output torque can be expected from this type of stator construction [7]. The piezoelectric stator can be considered as a transducer. Consequently, the piezoelectric stator’s structure can be designed to have maximum effective electromechan-
Fig. 2(a). Relation of the resonance frequency and the length of the PZT tube.
Fig. 1. Structure of a micro-motor.
H. Zhang et al. / Ultrasonics 44 (2006) e603–e606
Fig. 2(b). Relation of the effective electromechanical coupling factor and the length of the PZT tube.
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Fig. 3. The photograph of the micro-motor.
increases, k 2eff increases, and the length of the tube is larger than 4 mm, k 2eff increases sharply. Larger values of the length of the tube are found to result in the enhanced k 2eff ; however, this also resulted in a significantly higher length of the micro-motor. Thus, a modest length of the tube is chosen for our prototype to optimize torque. The results show the PZT tube length should be 5 mm. The PZT tube with an outer diameter of 1.0 mm and a length of 5 mm is chosen, the parameters of the metal caps: density: 8.9 mg/mm3, Poisson’s ratio: 0.3, Young’s modulus: 9 · 1010 N/m2, outer diameter:1.2 mm, inner diameter: 0.3 mm, length: 0.6 mm. By the FEM, the fr and k 2eff at the first bending vibration mode for the stator is analyzed, the fr is 63 kHz, and k 2eff is 0.153. 4. Experimental results Fig. 4(a). Relation between rotation frequency and driving voltage.
For fabricating the micro-motor, choosing the PZT tube with 1 mm in outer diameter, 0.6 mm in inner diameter and 5.0 mm in length, including the adjusting spring and rotor, the total length of the motor is 8 mm. The micro-motor is shown in Fig. 3. Using the HP impedance frequency analyzer, the resonance frequencies are measured. The resonance frequency of the micro-motor stator is 58.5 kHz, which is slightly lower than the free stator’s resonance frequency 61 kHz. The speed and torque of the motor was significantly dependent on the surface condition of the rotors and the pressing force loaded to the rotors, which are contacted to the motor stator via a pressing force with a fine spring. The torque was measured using a weight attached to the side of the rotor via a soft, fine string. The maximum weight pulled on micro-motor (whose rotor has a diameter of 1.2 mm) is 1.3 g. Accordingly, the maximum torque produced by the motor can be estimated as 7.8 lN m under the condition of driving frequency 58.5 kHz and 110 Vp–p. Fig. 4 shows the dependence of the rotation frequency on
Fig. 4(b). Relation between rotation frequency and driving frequency.
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driving voltage under the condition of driving frequency 60 kHz. Fig. 4(a) shows the rotation frequency versus the driving voltage. A linear dependence can be seen, with a rotation frequency of 3300 rpm at 110 Vp–p. Fig. 4(b) shows the rotation frequency versus driving frequency for micro-motor using 90 Vp–p. It is can be seen that the motor exhibits a maximum of 3500 rpm at a driving frequency equaling to fr. At lower and higher driving frequencies, the rotation frequency decreases. The experimental results show that the micro-motor exhibits good performances, it can be taken as the ultrasonic driver in the situations requiring micro or slimness space. 5. Conclusions Utilizing the piezoelectric tube with 1 mm in outer diameter, a tube type micro-motor is fabricated. Two metal caps are used for optimizing the output performances of the micro-motor. The experimental results show that the rotation frequency is over 3000 rpm and the starting torque can come up to 7.8 lN m. By comparison with the micromotors with similar structure and dimension, our micromotor faster rotation frequency and higher output torque. The micro-motor designed by the piezoelectric tube has the thin and long configuration, it is suitable for operating
as the driver in the micro and thin space, such as microrobots, micro-crafts and micro medical equipments, etc. Acknowledgement This work is supported by National Natural Scinence Foundation of China under No. 10374051. References [1] T. Sashida, An Introduction to Ultrasonic Motors, Clarendon press, Oxford, 1993. [2] P. Muralt, M. Kohli, T. Maeder, Fabrication and characterization of PZT thin-film vibrators for micromotors, Sens. Actuators 48 (1995) 157–165. [3] T. Morita, M. Kurosawa, T. Higuchi, A cylindrical micro ultrasonic motor utilizing PZT thin film, Sens. Actuators 83 (2000) 225–230. [4] B. Koc, P. Bouchilloux, K. Uchino, Piezoelectric micromotor using a metal-ceramic composite structure, IEEE Trans. UFFC 47 (2000) 836– 843. [5] S.X. Dong, S.P. Lim, K.H. Lee, J.D. Zhang, L.C. Lim, Piezoelectric ultrasonic micromotor with 1.5 mm diameter, IEEE Trans. UFFC 50 (2000) 361–366. [6] K. Zhang, T.Y. Zhou, H. Wang, S.M. Yuan, Z. Zhao, K.L. Jiang, G.Z. Bai, Study on piezoelectric cylinder micro ultrasonic motor with 1 mm diameter, Acta. Acustica 29 (2004) 258–261. [7] J. Haener, Formulas for the frequencies including higher frequencies of uniform cantilever and free-free beams with additional masses at the ends, J. Appl. Mech. 25 (1958) 412.
Ultrasonic micro-motor using miniature piezoelectric tube with diameter of 1.0 mm Hui Zhang a, Shu-xiang Dong a,b, Shu-yi Zhang a,*, Tian-hua Wang a, Zhong-ning Zhang a, Li Fan a b
a Laboratory of Modern Acoustics, Institute of Acoustics, Nanjing University, Nanjing 210093, PR China Department of Material Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
Available online 5 June 2006
Abstract At the present moment, the smallest piezoelectric ultrasonic micro-motors utilizing miniature PZT piezoelectric ceramic tubes were developed. The motor consists of a PZT-metal composite tube stator, two steel rotors and a thin shaft that keeps the two rotors pressing on both ends of the stator elastically. The dimensions of the PZT tube are 1.0 mm in outer diameter, 0.6 mm in inner diameter and 5.0 mm in length. The diameter and total length of the assembled micro-motor is 1.0 mm and 8 mm (including an adjusting spring), respectively. The tube-type micro-motor is driven by two pairs of alternative voltages with phase shift 90 between the adjacent electrodes and operated in the first circular-bending vibration mode of the stator with the resonance frequency about 58 kHz. The experimental results show that the tube-type micro-motors have perfect performances: (i) high rotation frequency over 3000 rpm and (ii) large starting torque over 7.8 lN m under the conditions of the input voltage of 110 Vp–p and the resonance frequency. The micro-motor is well suitable for operating in micro-spaces, such as in intravascular, micro-robots and micro-craft applications. 2006 Elsevier B.V. All rights reserved. Keywords: Ultrasonic motor; Piezoelectric tube; Finite element method
1. Introduction Piezoelectric ultrasonic motors exhibit many advantages, such as high rotation torque, high compactness in size, simple construction, direct drive and none of electromagnetic interferences. The piezoelectric ultrasonic motor can generate a large output torque with very simple and minimal construction, and operate in very small space, such as medical catheters, micro-robot and micro-craft drivers. Recently, various types of ultrasonic micro-motors in millimeter sizes have been reported [1–6]. A piezoelectric cylindrical ultrasonic micro-motor with a stator 1.4 mm in diameter and 5 mm in length was fabricated by Morita et al. [3]. This micro-motor was based on a bending vibration mode and operated at a resonance frequency of 227 kHz. The maximum rotation frequency *
Corresponding author. fax: +86 25 83313690. E-mail address: [email protected] (S.-y. Zhang).
0041-624X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2006.05.064
was 680 rpm at 20 Vp–p driving voltage. The output torque was calculated to be 0.67 lN m from the transient response of the rotor. Dong et al. and Cagatay et al. reported separately ultrasonic micro-motors with piezoelectric tubes in diameters 1.5 mm and 1.6 mm, resonance frequencies 67 KHz and 130 kHz, stalled torques 45 lN m and 500 lN m, maximum rotation frequency 2000 rpm and 45 rad/s, respectively [5]. In 2001 [6], the micro-motor was reported which was fabricated by the piezoelectric ceramic cylindrical rod. The prototype motor was 1 mm in diameter and 5 mm in length. The resonance frequency is 32 kHz with one fixed boundary or 83 kHz with two free boundaries. The rotation frequency is near 1800 rpm and the stalled torque is 4 lN m. In this paper, to obtain the ultrasonic micro-motor with better functions and miniature structure, we present a new piezoelectric ultrasonic micromotor with the PZT tube which is 1 mm in outer diameter, 0.6 mm in inner diameter. In the micro-motor a PZT ceramic/metal composite tube is used as the stator instead of a
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H. Zhang et al. / Ultrasonics 44 (2006) e603–e606
titanium tube coated with PZT film or a simple PZT tube, as a PZT/metal composite stator can generate higher output torque, stronger driving effect. This type of motor with a thin diameter and long configuration is well-suited for small and long space applications. The output characteristics of prototype micro-motor and the resonance frequencies of the stator are measured at the first bending vibration mode. The experimental results show that the micro-motor has high output torque, fast rotation frequency. 2. Structure and principle of operation Fig. 1 shows the piezoelectric ultrasonic micro-motor structure. A piezoelectric ultrasonic micro-motor is designed using a PZT ceramic/brass composite tube as the stator. The rotor is a shaft on which two steel rotor caps are attached. The composite stator can generate wobbling motions at its ends by combining bending vibrations in two orthogonal directions of the tube. This ultrasonic micro-motor is a mode-rotation type, which utilizes the first bending vibration mode. With four electrical sources to each electrode, the bending vibration is generated. The phase difference is 90 between the adjacent electrodes. Please note that a traveling wave is propagated at the end surface of the stator. The rotor turns around by a frictional force. When four electric signals are applied on the outer silver electrodes, the micro-motor can operate smoothly. The driving direction is reversible by exchanging one pair of symmetry electrodes. Table 1 shows some of the materials property characteristics of the piezoelectric tube.
Table 1 Material parameter data of the PZT tube Mechanical quality factor Coupling factor Piezoelectric charge coefficient Compliance Density Permittivity/ 1 kHz
Qm K31 d31 (1012 C/N) S E33 (1012 m/N) q (mg/mm3) e33/e0
80 0.33 185 20.7 7.8 1850
ical coupling factor k 2eff . The resonance frequency fr and anti-resonance frequency fa of the first bending mode for the micro-motor stator with the metal caps can be analyzed by the finite element method (FEM) codes ANSYS. The k 2eff of the stator can be estimated from the equation k 2eff ¼ ½1 ðfr =fa Þ2 1=2
ð1Þ k 2eff
Fig. 2 shows the fr and of the first bending mode which are predicted by the ANSYS and Eq. (1). The outer and inner diameters are held constant at 1.0 mm and 0.6 mm, respectively, the length of the piezoelectric tube is varied. When the length of the tube increases, the resonance frequency fr decreases. It can be seen that when the length of the tube is 4 mm, fr is below 100 kHz, as shown in Fig. 2(a). The small resonance frequency can improve vibration amplitude of the piezoelectric tube, so the torque of the micro-motor can be raised [5]. Fig. 2(b) revealed that k 2eff is strongly dependent on the length of the piezoelectric tube. When the length of the tube
3. Theoretical analysis of micro-motor High stator resonance frequencies significantly decreased the output torque, due to a decrease in the vibration amplitude of the stator. Calculations demonstrated that thin composite stator operating in the first bending vibration mode is potential way to decrease the resonance frequency as the motor is miniaturized. The resonance frequencies fr can be decreased further by attaching metal caps to the ends of the piezoelectric tube. Higher output torque can be expected from this type of stator construction [7]. The piezoelectric stator can be considered as a transducer. Consequently, the piezoelectric stator’s structure can be designed to have maximum effective electromechan-
Fig. 2(a). Relation of the resonance frequency and the length of the PZT tube.
Fig. 1. Structure of a micro-motor.
H. Zhang et al. / Ultrasonics 44 (2006) e603–e606
Fig. 2(b). Relation of the effective electromechanical coupling factor and the length of the PZT tube.
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Fig. 3. The photograph of the micro-motor.
increases, k 2eff increases, and the length of the tube is larger than 4 mm, k 2eff increases sharply. Larger values of the length of the tube are found to result in the enhanced k 2eff ; however, this also resulted in a significantly higher length of the micro-motor. Thus, a modest length of the tube is chosen for our prototype to optimize torque. The results show the PZT tube length should be 5 mm. The PZT tube with an outer diameter of 1.0 mm and a length of 5 mm is chosen, the parameters of the metal caps: density: 8.9 mg/mm3, Poisson’s ratio: 0.3, Young’s modulus: 9 · 1010 N/m2, outer diameter:1.2 mm, inner diameter: 0.3 mm, length: 0.6 mm. By the FEM, the fr and k 2eff at the first bending vibration mode for the stator is analyzed, the fr is 63 kHz, and k 2eff is 0.153. 4. Experimental results Fig. 4(a). Relation between rotation frequency and driving voltage.
For fabricating the micro-motor, choosing the PZT tube with 1 mm in outer diameter, 0.6 mm in inner diameter and 5.0 mm in length, including the adjusting spring and rotor, the total length of the motor is 8 mm. The micro-motor is shown in Fig. 3. Using the HP impedance frequency analyzer, the resonance frequencies are measured. The resonance frequency of the micro-motor stator is 58.5 kHz, which is slightly lower than the free stator’s resonance frequency 61 kHz. The speed and torque of the motor was significantly dependent on the surface condition of the rotors and the pressing force loaded to the rotors, which are contacted to the motor stator via a pressing force with a fine spring. The torque was measured using a weight attached to the side of the rotor via a soft, fine string. The maximum weight pulled on micro-motor (whose rotor has a diameter of 1.2 mm) is 1.3 g. Accordingly, the maximum torque produced by the motor can be estimated as 7.8 lN m under the condition of driving frequency 58.5 kHz and 110 Vp–p. Fig. 4 shows the dependence of the rotation frequency on
Fig. 4(b). Relation between rotation frequency and driving frequency.
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H. Zhang et al. / Ultrasonics 44 (2006) e603–e606
driving voltage under the condition of driving frequency 60 kHz. Fig. 4(a) shows the rotation frequency versus the driving voltage. A linear dependence can be seen, with a rotation frequency of 3300 rpm at 110 Vp–p. Fig. 4(b) shows the rotation frequency versus driving frequency for micro-motor using 90 Vp–p. It is can be seen that the motor exhibits a maximum of 3500 rpm at a driving frequency equaling to fr. At lower and higher driving frequencies, the rotation frequency decreases. The experimental results show that the micro-motor exhibits good performances, it can be taken as the ultrasonic driver in the situations requiring micro or slimness space. 5. Conclusions Utilizing the piezoelectric tube with 1 mm in outer diameter, a tube type micro-motor is fabricated. Two metal caps are used for optimizing the output performances of the micro-motor. The experimental results show that the rotation frequency is over 3000 rpm and the starting torque can come up to 7.8 lN m. By comparison with the micromotors with similar structure and dimension, our micromotor faster rotation frequency and higher output torque. The micro-motor designed by the piezoelectric tube has the thin and long configuration, it is suitable for operating
as the driver in the micro and thin space, such as microrobots, micro-crafts and micro medical equipments, etc. Acknowledgement This work is supported by National Natural Scinence Foundation of China under No. 10374051. References [1] T. Sashida, An Introduction to Ultrasonic Motors, Clarendon press, Oxford, 1993. [2] P. Muralt, M. Kohli, T. Maeder, Fabrication and characterization of PZT thin-film vibrators for micromotors, Sens. Actuators 48 (1995) 157–165. [3] T. Morita, M. Kurosawa, T. Higuchi, A cylindrical micro ultrasonic motor utilizing PZT thin film, Sens. Actuators 83 (2000) 225–230. [4] B. Koc, P. Bouchilloux, K. Uchino, Piezoelectric micromotor using a metal-ceramic composite structure, IEEE Trans. UFFC 47 (2000) 836– 843. [5] S.X. Dong, S.P. Lim, K.H. Lee, J.D. Zhang, L.C. Lim, Piezoelectric ultrasonic micromotor with 1.5 mm diameter, IEEE Trans. UFFC 50 (2000) 361–366. [6] K. Zhang, T.Y. Zhou, H. Wang, S.M. Yuan, Z. Zhao, K.L. Jiang, G.Z. Bai, Study on piezoelectric cylinder micro ultrasonic motor with 1 mm diameter, Acta. Acustica 29 (2004) 258–261. [7] J. Haener, Formulas for the frequencies including higher frequencies of uniform cantilever and free-free beams with additional masses at the ends, J. Appl. Mech. 25 (1958) 412.