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A linear ultrasonic motor using (K0.5Na0.5)NbO3 based lead-free piezoelectric ceramics
Sensors and Actuators A 165 (2011) 410–414
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A linear ultrasonic motor using (K0.5 Na0.5 )NbO3 based lead-free piezoelectric ceramics Jiamei Jin a , Dandan Wan b , Ying Yang a,∗ , Qian Li b , Meng Zha b a b
Precision Driving Laboratory, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China College of Materials Science and Technology, Precision Driving Laboratory, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 17 February 2009 Received in revised form 29 October 2010 Accepted 30 October 2010 Available online 9 November 2010 Keywords: Lead-free Piezoelectric ceramics Ultrasonic motor Quadrate plate
a b s t r a c t A linear ultrasonic motor using lead-free piezoelectric ceramics equipped with quadrate plate transducers has been developed. Four lead free piezoelectric plates driving elements formed a multi-driving-end, producing a large thrust and output velocity. The design of the stator enables two operating modes with coincident frequency. A microscopic view suggests that the material particles at the top of four projections move in ellipses and drive the slider alternately via frictional forces to realize linear motion. The fabrication and characterization of the lead-free piezoelectric ceramics achieving the drive of stator were given in the context. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, linear ultrasonic motors (USMs) have drawn extensive attention due to their merits such as high force density, simple mechanical structure, low speed without additional gear or spindle mechanisms, noiseless operation, high holding forces without an energy supply, absence of magnetic fields, high dynamics and very good positioning accuracy [1,2]. In the normal case, flexural and longitudinal modes are combined to achieve an elliptic micromotion of the material particles on the surface. This micro-motion is converted to direct linear or rotation motion of slider. The ultrasonic motors in the former case are called linear ultrasonic motors. Various kinds of linear ultrasonic motors previously reported [3–8] were based on two different vibration modes. To obtain high amplitudes of the micro-motion and thus achieve high power density, the ultrasonic stator should be driven near its own eigen-frequency. Moreover, a slight frequency deviation would lead to undesirable disturbance of the elliptic motion. It is necessary to design the geometric construction of stator to make the eigen-frequencies of two different modes well-matched. On the other hand, the stators intermittently drive the rotors (or sliders) when the stators contact with the rotors, and the rotors (or sliders) will move further with inertia after the stators separate from the rotors. The duty cycle of the contact and the “flight” determines the output force and the velocity of motor. It is demonstrated that multi-stator or multi-driving-end
∗ Corresponding author. Tel.: +86 25 84891937. E-mail address: [email protected] (Y. Yang). 0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.10.017
of single-stator ultrasonic motor could provide large thrust and output velocity [9]. At present, most of the drive elements of USM are Pb(Zr,Ti)O3 (PZT) based ceramics. However, most used PZT ceramics contain about 60 wt.% or more lead, which is a typical heavy element with high toxicity. It is an urgent need for environmental protection to develop lead-free piezoelectric ceramics [10,11]. (K0.5 Na0.5 )NbO3 (KNN), particularly Li-, Sb-, and Ta-modified ones, has been considered a potential candidate for lead-free piezoelectric actuators because of their high piezoelectric properties [12,13]. In this paper, we designed and fabricated a novel linear USM, the stator of which employed a quadrate configuration. In quadrate sheet, for the existence of two vertical centre principal axes of inertia, coincident eigen-frequencies of two same shape modes in two orthogonal directions were obtained, which can effectively avoid frequency deviation. Additionally, the stator consisting of four piezoelectric plates formed a multi-driving-end, enhancing the thrust and output velocity. To explore the application possibility of lead-free piezoelectric ceramics, we manufactured an USM using self-synthesized (K0.44 Na0.52 Li0.04 )(Nb0.91 Sb0.04 Ta0.05 )O3 + x mol% MnO2 ceramics. The electrical properties as well as driving performance of the lead free piezoceramics were demonstrated. 2. USM design and fabrication The configuration of the stator is illustrated in Fig. 1a. The quadrate stator contains four piezoelectric plates and four projections, and their positions are determined by modal analysis, as shown in Fig. 1b. Here, B30 and B03 represent modes in x and y direc-
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411
Fig. 1. (a) Structure of the square stator. (b) Operating modal of the square stator. (c) Distribution of the piezoelectric ceramics. (d) Vibration mode analysis of the stator. (e) Harmonic analysis of the stator.
tions, respectively. Moreover, the subscripts indicate that there are three nodal lines in both x and y directions. In order to obtain a linear motion of slider in the x-direction, the projections should only move in x–z plane. In B03 mode, the four projections should only vibrate in the z-direction, so two of them were located at the crest and the other two at the trough. In case of B30 mode, the
four projections should contain motion component in both x and z directions, and thus they were fixed at the middle point between the crest (or trough) and nodal line to gain equal motion component in different directions. Furthermore, the distribution of piezoelectric ceramic plates is shown in Fig. 1c. The No. 1 and 4 ceramics located at the crest and trough of B03 vibration shape, which will
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J. Jin et al. / Sensors and Actuators A 165 (2011) 410–414
is understood that by the bonding connections between the stator and piezoelectric ceramics plates result in a decline of the stator’s stiffness.
3. Piezoelectric ceramics preparation
Fig. 2. (a) Operating principle of linear USM. (b) Prototype of linear USM.
excite B03 mode. In contrast, the No. 2 and 3 ceramics located at the crest and trough of B30 vibration shape, which will excite B30 mode. Each of the two pieces of piezoelectric ceramic located in diagonal direction was poled in reverse directions. When a driving voltage is applied to the two diagonal plates, a bending vibration is generated in one-direction, owing to the expanding of one piezoelectric ceramic plate whereas contracting the other. Two pairs of such piezoceramic plates were assembled orthogonally on the stator. Thus, two orthogonal bending vibration modes could be excited, as shown in Fig. 1d. A square phosphor bronze plate 20 × 20 × 2 mm3 in dimension was prepared for the stator, and each projection was 2 × 2 × 2 mm3 in dimension. The piezoceramics poled in thickness direction were cut into 10 × 6 × 1 mm3 and glued to stators using epoxy resin. ANSYS software analysis demonstrates that the modal frequencies of B30 and B03 are 55.935 kHz and 55.927 kHz, respectively. By applying two sine drives to the two pairs of the piezoceramic plates (phase shifted by 90◦ ), a vibration can be excited at each projection. The top of the projections moves in ellipses, but in different phases. ANSYS analysis is also used to illustrate the ellipse movement of the top, as shown in Fig. 1e. This square stator could operate in its B30 and B03 bending modes at coincident frequency, thus, it has the potential as a new slider drive mechanism. Fig. 2a illustrates the construction and working principle of the quadrate linear ultrasonic motor. The stator’s projections were elastically pressed together in order to ensure the frictional contact with the slider. Then the slider would move linearly via frictional force when it contacted with the projections. The four projections will drive the slider alternatively in one vibration cycle of the stator. It is proved that the way of operation will enable a large thrust and a quick velocity. A grating with a resolution of 40 nm was fixed on the slider in order to investigate the velocity and displacement of the motor, as shown in Fig. 2b. The resonant modes of the assembled stator were measured using a Doppler Laser Vibrometer OFV056 (Polytec). A notable B03 bending mode with resonant frequency of 52.5 kHz was found and the resonant frequency of B30 bending mode was 52.38 kHz. It is worth noticing that the two resonant frequencies are a little bit lower than theoretical values. It
(K0.44 Na0.52 Li0.04 )(Nb0.91 Sb0.04 Ta0.05 )O3 + x mol% MnO2 (x = 0, 0.25, 0.5, 0.75, 1.0) ceramics were prepared by a conventional solidstate fabrication technique. The compounds of K2 CO3 , Na2 CO3 , Nb2 O5 , Li2 CO3 , Sb2 O3 , Ta2 O5 in stoichiometric ratio were mixed for 24 h in a agate jar with zirconia balls. Then, the slurry was dried and calcined at 900 ◦ C for 4 h. The calcined powders were ball-milled with different ratios of MnO2 for 24 h, dried and then 2 wt.% PVA aqueous solution was added as a binder. Disk samples of 25 mm diameter were subsequently pressed uniaxially at 200 MPa, followed by normal sintering at 1025–1125 ◦ C for 4 h. The disks were cut into 10 × 6 × 1 mm3 by a diamond. After silver electrodes firing and poling in a 120 ◦ C silicone oil bath at 3 kV/mm DC voltage for 30 min, the disks and plates were used to test the electrical properties and driving performance, respectively. All of the samples were aged more than one day. Fig. 3a shows the XRD patterns of the KNLNST + x mol% MnO2 ceramics, with 0.0 ≤ x ≤ 1.0. The phase structures were pure perovskite for all the specimens, and no second phase could be certified. The fluctuation of peak splits at around 45◦ was an effective evidence for Mn4+ ions entering the crystal lattice, which anastomosed well with following analysis. The values of dielectric constant (εr ), dielectric loss (tan ı) at 1 kHz, piezoelectric coefficient (d33 ), mechanical quality factor (Qm ) and planar coupling coefficient (kp ) as a function of MnO2 content are shown in Fig. 3b for the samples sintering at each optimal temperature. The dielectric constant was 696 for the x = 0 sample, and it slightly increased by doping a spot of MnO2 , however, it decreased with excessive addition of MnO2 . Because of low melting point, the addition of MnO2 could improve the densification, which not only inhibited the volatilization of alkaline oxides, but also promoted the pore shrinkage and grain growth. For the poled ceramics, the increased grain size makes domain reorientation easier and promotes the domain wall motion, which could increase the effective dielectric coefficient. Moreover, the substitution of Mn4+ ion in the crystal lattice would change the phase content and resultantly affect (εr ). However, high dosage of MnO2 dopant may deteriorate the sintering behavior, causing a low dielectric constant, furthermore, the greater variation in the phase compositions could also have influence on (εr ). Loss tangent decreased from 0.057 to 0.0401 by adding MnO2 into KNLNST ceramics because of the improvement in density. In addition, the oxygen vacancies induced by the substitution of Mn4+ ions in B-site could pin ferroelectric domain walls [14,15]. As a consequence, the motion of domain walls under the external field would be markedly restricted, resulting in a low tan ı. It seems that Qm increased simply with the increment of MnO2 content. Actually, the Qm value is determined by both the grain size and the hardening effect owing to the substitution of Mn4+ ions. From the fracture surface microstructure, fine grains were really observed in MnO2 -doped KNLNST ceramics, which could optimize Qm in some degree. It is noticeable that the ion radius of Mn4+ (0.053 nm) is similar to those of Nb5+ (0.064 nm), Sb5+ (0.060 nm) and Ta5+ (0.064 nm), but is much smaller than those of alkaline ions. Thus, it can be preliminarily inferred that Mn ions may occupy the B-site based on the tolerance factor of perovskite structure, that could be further confirmed by the temperature dependence of (εr ) and tan ı for pure KNLNST and KNLNST + 0.5 mol% MnO2 samples under 10 kHz, as shown in Fig. 3c. The graph reveals clearly that TC , which represents ferroelectric tetragonal phase to paraelectric cubic phase transition temperature, increased from 320 to 400 ◦ C
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413
Fig. 3. (a) XRD patterns of the KNLNST + x mol% MnO2 ceramics sintered at each optimal temperature. (b) Variations in (εr ), tan ı, d33 , Qm and kp values of the KNLNST + x mol% MnO2 ceramics with 0.0 ≤ x ≤ 1.0 sintered at each optimal temperature. (c) Temperature dependence of the dielectric constant and dielectric loss for pure KNLNST and KNLNST + 0.5 mol% MnO2 samples at 10 kHz.
with MnO2 addition. Meanwhile, the TO–T , which expresses the orthorhombic to tetragonal polymorphic phase transition temperature, also increased from room temperature to ∼80 ◦ C. Although the change mechanism of TC and TO–T is very complicated, as it is correlative to several factors, such as ion radius, coordination number, electronegativity and spin configuration of electrons. However, it can be still considered that Mn4+ ions substitute in B-site, increasing TC as well as TO–T and behaving as an acceptor by generating oxygen vacancies which can pin the domain wall motion leading to an enhancement of Qm . The kp varies in complex, originating from the competition between the sintered state including grain size as well as relative density and the transformation of the phase compositions. The d33 values of the MnO2 -modified KNLNST ceramics were much lower than that of the pure KNLNST ceramic. The change may be understood by the substitution of Mn4+ ions in Bsite, which would not only shift the chemical composition, but also induce hardening effect. With overall consideration of the electric properties, the proper 0.5 mol% MnO2 -modified KNLNST piezoceramics of εr = 723, tan ı = 0.051, Qm = 72, kp = 41.8% and d33 = 186 pC/N were prepared as drive elements of the USM.
then the slider moved on 47 m, as shown in Fig. 4. The positioning accuracy was not ideal as we expected, which might be owing to the excessive assembly clearance. Thus, an improvement in assembly technology will probably increase the positioning accuracy. KNLNST piezoceramic plates were used to determine the main performance characteristics of the prototype. Table 1 shows the electrical properties of the two kinds of ceramics and their corresponding output parameters. When a pair of drive voltages of
4. Results and discussion To investigate positioning accuracy of the linear USM, a pair of drive voltages of 300 Vpp (phase shifted by 90◦ ) were applied to the stator and test frequency was selected at 54 kHz. When the slider glides over a distance of 23 mm, the driving power was shut off, and
Fig. 4. Positioning accuracy of linear USM.
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Table 1 Electrical properties and output parameters of the piezoceramic plates. Material
d33 (pC N−1 )
kp (%)
Loss (%)
Qm
εr
TC (◦ C)
SpeedMax (mm s−1 )
PowerMax (N)
PZT-8 KNLNST + 0.5 mol% MnO2
238 185
57 42
0.3 5.0
800 72
1200 725
300 400
195 20
5 0.5
150–200 Vpp (phase shifted by 90◦ ) were applied to the stator, the traveling speed of the slider was 10–20 mm/s for 0.5 mol% MnO2 modified KNLNST piezoceramics driven, which was much lower than that of 120–195 mm/s for PZT-8 [9]. By applying weight to the rotor, the maximum load (driving force) of the motor was ∼0.5 N for 0.5 mol% MnO2 -modified KNLNST drived USM, that was only one tenth of 5 N for that of PZT-8. The unsatisfied driving performance of MnO2 -modified KNLNST piezoceramics may come down to their lower Qm and higher tan ı comparing to those of PZT-based piezoceramics. Fortunately, it should be noted that some equipment such as fine needle aspiration biopsy and other micro-medical treatment equipment, does not require large force, therefore it is feasible to replace PZT-based piezoceramics by lead-free piezoceramics in the similar occasions for harmlessness to human beings. 5. Conclusion A linear ultrasonic motor using a lead-free quadrate plate transducers has been developed. The frequencies of two operating modes are well-matched since the stator has been designed with exact dimension and two pairs of piezoelectric ceramic plates were placed in two orthogonal directions. The driving elements, four KNN-based piezoelectric plates, have formed a multi-driving-end, enabling a large thrust and output velocity. The lead-free MnO2 -modified KNLNST piezoceramics were synthesized and investigated. Although the present used lead-free piezoceramic does not produce as large force and torque as PZT-based piezoceramics in serving as the driving elements of resonant actuators, it is possible to apply the lead-free piezoceramics in occasions which need less force and torque but are concerned more about the human safety and environmental friendliness. Acknowledgements This study was supported by National Natural Science Foundation of China (50735002, 10874090, and 50775109), NFSC-
Guangdong Joint Fund (U0934004) and NUAA Research Funding No. NJ2010013. References [1] C.S. Zhao, Ultrasonic Motors Technologies and Applications, Science Press, Beijing, 2007. [2] B. Watson, J. Friend, L. Yeo, Piezoelectric ultrasonic micro/milli-scale actuators, Sens. Actuators A 152 (2009) 219–233. [3] H.H. Gu, S.Y. Zhang, L.P. Cheng, D. Ma, F.M. Zhou, Z.J. Chen, X.J. Shui, Study on non-contact linear motors driven by surface acoustic waves, Sens. Actuators A 155 (2009) 163–167. [4] T. Kanda, A. Makino, T. Ono, K. Suzumori, T. Morita, M.K. Kurosawa, A micro ultrasonic motor using a micro-machined cylindrical bulk PZT transducer, Sens. Actuators A 127 (2006) 131–138. [5] J.T. Leinvuo, S.A. Wilson, R.W. Whatmore, M.G. Cain, A new flextensional piezoelectric ultrasonic motor—design, fabrication and characterization, Sens. Actuators A 133 (2007) 141–151. [6] C.B. Yoon, G.T. Park, H.E. Kima, J. Ryu, Piezoelectric ultrasonic motor by co-extrusion process, Sens. Actuators A 121 (2005) 515– 519. [7] T. Senjyu, T. Kashigawi, K. Uezato, Position control of ultrasonic motors using MRAC and dead-zone compensation with fuzzy inference, IEEE Trans. Power Electr. 17 (2002) 265–272. [8] T. Hemsel, J. Wallaschek, Survey of the present state of the art of piezoelectric linear motors, Ultrasonics 38 (2000) 37–40. [9] J.M. Jin, C.S. Zhao, Linear ultrasonic motor using quadrate plate transducer, Front. Mech. Eng. China 4 (2009) 88–91. [10] T. Takenaka, H. Nagata, Current status and prospects of lead-free piezoelectric ceramics, J. Eur. Ceram. Soc. 25 (2005) 2693–2700. [11] T. Takenaka, H. Nagata, Y. Hiruma, Current developments and prospective of lead-free piezoelectric ceramics, Jpn. J. Appl. Phys. 47 (2008) 3787– 3801. [12] F. Rubio-Marcos, P. Ochoa, J.F. Fernandez, Sintering and properties of leadfree (K, Na, Li)(Nb, Ta, Sb)O3 ceramics, J. Eur. Ceram. Soc. 27 (2007) 4125– 4129. [13] E.Z. Li, H. Kakemoto, S. Wada, T. Tsurumi, Effects of manganese addition on piezoelectric properties of the (K, Na, Li) (Nb, Ta, Sb) O3 lead-jpne ceramics, J. Ceram. Soc. Jpn. 115 (2007) 250–253. [14] D.M. Lin, K.W. Kwok, H.L.W. Chan, Effects of MnO2 on the microstructure and electrical properties of 0.94(K0.5 Na0.5 )NbO3- 0.06Ba(Zr0.05 Ti0.95 )O3 leadfree ceramics, Mater. Chem. Phys. 109 (2008) 455–458. [15] C.W. Ahn, S. Nahm, M. Karmarkar, D. Viehland, D.H. Kang, K.S. Bae, S. Priya, Effect of CuO and MnO2 on sintering temperature, microstructure, and piezoelectric properties of 0.95(K0.5 Na0.5 )NbO3- 0.05BaTiO3 ceramics, Mater. Lett. 62 (2008) 3594–3596.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A linear ultrasonic motor using (K0.5 Na0.5 )NbO3 based lead-free piezoelectric ceramics Jiamei Jin a , Dandan Wan b , Ying Yang a,∗ , Qian Li b , Meng Zha b a b
Precision Driving Laboratory, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China College of Materials Science and Technology, Precision Driving Laboratory, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 17 February 2009 Received in revised form 29 October 2010 Accepted 30 October 2010 Available online 9 November 2010 Keywords: Lead-free Piezoelectric ceramics Ultrasonic motor Quadrate plate
a b s t r a c t A linear ultrasonic motor using lead-free piezoelectric ceramics equipped with quadrate plate transducers has been developed. Four lead free piezoelectric plates driving elements formed a multi-driving-end, producing a large thrust and output velocity. The design of the stator enables two operating modes with coincident frequency. A microscopic view suggests that the material particles at the top of four projections move in ellipses and drive the slider alternately via frictional forces to realize linear motion. The fabrication and characterization of the lead-free piezoelectric ceramics achieving the drive of stator were given in the context. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, linear ultrasonic motors (USMs) have drawn extensive attention due to their merits such as high force density, simple mechanical structure, low speed without additional gear or spindle mechanisms, noiseless operation, high holding forces without an energy supply, absence of magnetic fields, high dynamics and very good positioning accuracy [1,2]. In the normal case, flexural and longitudinal modes are combined to achieve an elliptic micromotion of the material particles on the surface. This micro-motion is converted to direct linear or rotation motion of slider. The ultrasonic motors in the former case are called linear ultrasonic motors. Various kinds of linear ultrasonic motors previously reported [3–8] were based on two different vibration modes. To obtain high amplitudes of the micro-motion and thus achieve high power density, the ultrasonic stator should be driven near its own eigen-frequency. Moreover, a slight frequency deviation would lead to undesirable disturbance of the elliptic motion. It is necessary to design the geometric construction of stator to make the eigen-frequencies of two different modes well-matched. On the other hand, the stators intermittently drive the rotors (or sliders) when the stators contact with the rotors, and the rotors (or sliders) will move further with inertia after the stators separate from the rotors. The duty cycle of the contact and the “flight” determines the output force and the velocity of motor. It is demonstrated that multi-stator or multi-driving-end
∗ Corresponding author. Tel.: +86 25 84891937. E-mail address: [email protected] (Y. Yang). 0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.10.017
of single-stator ultrasonic motor could provide large thrust and output velocity [9]. At present, most of the drive elements of USM are Pb(Zr,Ti)O3 (PZT) based ceramics. However, most used PZT ceramics contain about 60 wt.% or more lead, which is a typical heavy element with high toxicity. It is an urgent need for environmental protection to develop lead-free piezoelectric ceramics [10,11]. (K0.5 Na0.5 )NbO3 (KNN), particularly Li-, Sb-, and Ta-modified ones, has been considered a potential candidate for lead-free piezoelectric actuators because of their high piezoelectric properties [12,13]. In this paper, we designed and fabricated a novel linear USM, the stator of which employed a quadrate configuration. In quadrate sheet, for the existence of two vertical centre principal axes of inertia, coincident eigen-frequencies of two same shape modes in two orthogonal directions were obtained, which can effectively avoid frequency deviation. Additionally, the stator consisting of four piezoelectric plates formed a multi-driving-end, enhancing the thrust and output velocity. To explore the application possibility of lead-free piezoelectric ceramics, we manufactured an USM using self-synthesized (K0.44 Na0.52 Li0.04 )(Nb0.91 Sb0.04 Ta0.05 )O3 + x mol% MnO2 ceramics. The electrical properties as well as driving performance of the lead free piezoceramics were demonstrated. 2. USM design and fabrication The configuration of the stator is illustrated in Fig. 1a. The quadrate stator contains four piezoelectric plates and four projections, and their positions are determined by modal analysis, as shown in Fig. 1b. Here, B30 and B03 represent modes in x and y direc-
J. Jin et al. / Sensors and Actuators A 165 (2011) 410–414
411
Fig. 1. (a) Structure of the square stator. (b) Operating modal of the square stator. (c) Distribution of the piezoelectric ceramics. (d) Vibration mode analysis of the stator. (e) Harmonic analysis of the stator.
tions, respectively. Moreover, the subscripts indicate that there are three nodal lines in both x and y directions. In order to obtain a linear motion of slider in the x-direction, the projections should only move in x–z plane. In B03 mode, the four projections should only vibrate in the z-direction, so two of them were located at the crest and the other two at the trough. In case of B30 mode, the
four projections should contain motion component in both x and z directions, and thus they were fixed at the middle point between the crest (or trough) and nodal line to gain equal motion component in different directions. Furthermore, the distribution of piezoelectric ceramic plates is shown in Fig. 1c. The No. 1 and 4 ceramics located at the crest and trough of B03 vibration shape, which will
412
J. Jin et al. / Sensors and Actuators A 165 (2011) 410–414
is understood that by the bonding connections between the stator and piezoelectric ceramics plates result in a decline of the stator’s stiffness.
3. Piezoelectric ceramics preparation
Fig. 2. (a) Operating principle of linear USM. (b) Prototype of linear USM.
excite B03 mode. In contrast, the No. 2 and 3 ceramics located at the crest and trough of B30 vibration shape, which will excite B30 mode. Each of the two pieces of piezoelectric ceramic located in diagonal direction was poled in reverse directions. When a driving voltage is applied to the two diagonal plates, a bending vibration is generated in one-direction, owing to the expanding of one piezoelectric ceramic plate whereas contracting the other. Two pairs of such piezoceramic plates were assembled orthogonally on the stator. Thus, two orthogonal bending vibration modes could be excited, as shown in Fig. 1d. A square phosphor bronze plate 20 × 20 × 2 mm3 in dimension was prepared for the stator, and each projection was 2 × 2 × 2 mm3 in dimension. The piezoceramics poled in thickness direction were cut into 10 × 6 × 1 mm3 and glued to stators using epoxy resin. ANSYS software analysis demonstrates that the modal frequencies of B30 and B03 are 55.935 kHz and 55.927 kHz, respectively. By applying two sine drives to the two pairs of the piezoceramic plates (phase shifted by 90◦ ), a vibration can be excited at each projection. The top of the projections moves in ellipses, but in different phases. ANSYS analysis is also used to illustrate the ellipse movement of the top, as shown in Fig. 1e. This square stator could operate in its B30 and B03 bending modes at coincident frequency, thus, it has the potential as a new slider drive mechanism. Fig. 2a illustrates the construction and working principle of the quadrate linear ultrasonic motor. The stator’s projections were elastically pressed together in order to ensure the frictional contact with the slider. Then the slider would move linearly via frictional force when it contacted with the projections. The four projections will drive the slider alternatively in one vibration cycle of the stator. It is proved that the way of operation will enable a large thrust and a quick velocity. A grating with a resolution of 40 nm was fixed on the slider in order to investigate the velocity and displacement of the motor, as shown in Fig. 2b. The resonant modes of the assembled stator were measured using a Doppler Laser Vibrometer OFV056 (Polytec). A notable B03 bending mode with resonant frequency of 52.5 kHz was found and the resonant frequency of B30 bending mode was 52.38 kHz. It is worth noticing that the two resonant frequencies are a little bit lower than theoretical values. It
(K0.44 Na0.52 Li0.04 )(Nb0.91 Sb0.04 Ta0.05 )O3 + x mol% MnO2 (x = 0, 0.25, 0.5, 0.75, 1.0) ceramics were prepared by a conventional solidstate fabrication technique. The compounds of K2 CO3 , Na2 CO3 , Nb2 O5 , Li2 CO3 , Sb2 O3 , Ta2 O5 in stoichiometric ratio were mixed for 24 h in a agate jar with zirconia balls. Then, the slurry was dried and calcined at 900 ◦ C for 4 h. The calcined powders were ball-milled with different ratios of MnO2 for 24 h, dried and then 2 wt.% PVA aqueous solution was added as a binder. Disk samples of 25 mm diameter were subsequently pressed uniaxially at 200 MPa, followed by normal sintering at 1025–1125 ◦ C for 4 h. The disks were cut into 10 × 6 × 1 mm3 by a diamond. After silver electrodes firing and poling in a 120 ◦ C silicone oil bath at 3 kV/mm DC voltage for 30 min, the disks and plates were used to test the electrical properties and driving performance, respectively. All of the samples were aged more than one day. Fig. 3a shows the XRD patterns of the KNLNST + x mol% MnO2 ceramics, with 0.0 ≤ x ≤ 1.0. The phase structures were pure perovskite for all the specimens, and no second phase could be certified. The fluctuation of peak splits at around 45◦ was an effective evidence for Mn4+ ions entering the crystal lattice, which anastomosed well with following analysis. The values of dielectric constant (εr ), dielectric loss (tan ı) at 1 kHz, piezoelectric coefficient (d33 ), mechanical quality factor (Qm ) and planar coupling coefficient (kp ) as a function of MnO2 content are shown in Fig. 3b for the samples sintering at each optimal temperature. The dielectric constant was 696 for the x = 0 sample, and it slightly increased by doping a spot of MnO2 , however, it decreased with excessive addition of MnO2 . Because of low melting point, the addition of MnO2 could improve the densification, which not only inhibited the volatilization of alkaline oxides, but also promoted the pore shrinkage and grain growth. For the poled ceramics, the increased grain size makes domain reorientation easier and promotes the domain wall motion, which could increase the effective dielectric coefficient. Moreover, the substitution of Mn4+ ion in the crystal lattice would change the phase content and resultantly affect (εr ). However, high dosage of MnO2 dopant may deteriorate the sintering behavior, causing a low dielectric constant, furthermore, the greater variation in the phase compositions could also have influence on (εr ). Loss tangent decreased from 0.057 to 0.0401 by adding MnO2 into KNLNST ceramics because of the improvement in density. In addition, the oxygen vacancies induced by the substitution of Mn4+ ions in B-site could pin ferroelectric domain walls [14,15]. As a consequence, the motion of domain walls under the external field would be markedly restricted, resulting in a low tan ı. It seems that Qm increased simply with the increment of MnO2 content. Actually, the Qm value is determined by both the grain size and the hardening effect owing to the substitution of Mn4+ ions. From the fracture surface microstructure, fine grains were really observed in MnO2 -doped KNLNST ceramics, which could optimize Qm in some degree. It is noticeable that the ion radius of Mn4+ (0.053 nm) is similar to those of Nb5+ (0.064 nm), Sb5+ (0.060 nm) and Ta5+ (0.064 nm), but is much smaller than those of alkaline ions. Thus, it can be preliminarily inferred that Mn ions may occupy the B-site based on the tolerance factor of perovskite structure, that could be further confirmed by the temperature dependence of (εr ) and tan ı for pure KNLNST and KNLNST + 0.5 mol% MnO2 samples under 10 kHz, as shown in Fig. 3c. The graph reveals clearly that TC , which represents ferroelectric tetragonal phase to paraelectric cubic phase transition temperature, increased from 320 to 400 ◦ C
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Fig. 3. (a) XRD patterns of the KNLNST + x mol% MnO2 ceramics sintered at each optimal temperature. (b) Variations in (εr ), tan ı, d33 , Qm and kp values of the KNLNST + x mol% MnO2 ceramics with 0.0 ≤ x ≤ 1.0 sintered at each optimal temperature. (c) Temperature dependence of the dielectric constant and dielectric loss for pure KNLNST and KNLNST + 0.5 mol% MnO2 samples at 10 kHz.
with MnO2 addition. Meanwhile, the TO–T , which expresses the orthorhombic to tetragonal polymorphic phase transition temperature, also increased from room temperature to ∼80 ◦ C. Although the change mechanism of TC and TO–T is very complicated, as it is correlative to several factors, such as ion radius, coordination number, electronegativity and spin configuration of electrons. However, it can be still considered that Mn4+ ions substitute in B-site, increasing TC as well as TO–T and behaving as an acceptor by generating oxygen vacancies which can pin the domain wall motion leading to an enhancement of Qm . The kp varies in complex, originating from the competition between the sintered state including grain size as well as relative density and the transformation of the phase compositions. The d33 values of the MnO2 -modified KNLNST ceramics were much lower than that of the pure KNLNST ceramic. The change may be understood by the substitution of Mn4+ ions in Bsite, which would not only shift the chemical composition, but also induce hardening effect. With overall consideration of the electric properties, the proper 0.5 mol% MnO2 -modified KNLNST piezoceramics of εr = 723, tan ı = 0.051, Qm = 72, kp = 41.8% and d33 = 186 pC/N were prepared as drive elements of the USM.
then the slider moved on 47 m, as shown in Fig. 4. The positioning accuracy was not ideal as we expected, which might be owing to the excessive assembly clearance. Thus, an improvement in assembly technology will probably increase the positioning accuracy. KNLNST piezoceramic plates were used to determine the main performance characteristics of the prototype. Table 1 shows the electrical properties of the two kinds of ceramics and their corresponding output parameters. When a pair of drive voltages of
4. Results and discussion To investigate positioning accuracy of the linear USM, a pair of drive voltages of 300 Vpp (phase shifted by 90◦ ) were applied to the stator and test frequency was selected at 54 kHz. When the slider glides over a distance of 23 mm, the driving power was shut off, and
Fig. 4. Positioning accuracy of linear USM.
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Table 1 Electrical properties and output parameters of the piezoceramic plates. Material
d33 (pC N−1 )
kp (%)
Loss (%)
Qm
εr
TC (◦ C)
SpeedMax (mm s−1 )
PowerMax (N)
PZT-8 KNLNST + 0.5 mol% MnO2
238 185
57 42
0.3 5.0
800 72
1200 725
300 400
195 20
5 0.5
150–200 Vpp (phase shifted by 90◦ ) were applied to the stator, the traveling speed of the slider was 10–20 mm/s for 0.5 mol% MnO2 modified KNLNST piezoceramics driven, which was much lower than that of 120–195 mm/s for PZT-8 [9]. By applying weight to the rotor, the maximum load (driving force) of the motor was ∼0.5 N for 0.5 mol% MnO2 -modified KNLNST drived USM, that was only one tenth of 5 N for that of PZT-8. The unsatisfied driving performance of MnO2 -modified KNLNST piezoceramics may come down to their lower Qm and higher tan ı comparing to those of PZT-based piezoceramics. Fortunately, it should be noted that some equipment such as fine needle aspiration biopsy and other micro-medical treatment equipment, does not require large force, therefore it is feasible to replace PZT-based piezoceramics by lead-free piezoceramics in the similar occasions for harmlessness to human beings. 5. Conclusion A linear ultrasonic motor using a lead-free quadrate plate transducers has been developed. The frequencies of two operating modes are well-matched since the stator has been designed with exact dimension and two pairs of piezoelectric ceramic plates were placed in two orthogonal directions. The driving elements, four KNN-based piezoelectric plates, have formed a multi-driving-end, enabling a large thrust and output velocity. The lead-free MnO2 -modified KNLNST piezoceramics were synthesized and investigated. Although the present used lead-free piezoceramic does not produce as large force and torque as PZT-based piezoceramics in serving as the driving elements of resonant actuators, it is possible to apply the lead-free piezoceramics in occasions which need less force and torque but are concerned more about the human safety and environmental friendliness. Acknowledgements This study was supported by National Natural Science Foundation of China (50735002, 10874090, and 50775109), NFSC-
Guangdong Joint Fund (U0934004) and NUAA Research Funding No. NJ2010013. References [1] C.S. Zhao, Ultrasonic Motors Technologies and Applications, Science Press, Beijing, 2007. [2] B. Watson, J. Friend, L. Yeo, Piezoelectric ultrasonic micro/milli-scale actuators, Sens. Actuators A 152 (2009) 219–233. [3] H.H. Gu, S.Y. Zhang, L.P. Cheng, D. Ma, F.M. Zhou, Z.J. Chen, X.J. Shui, Study on non-contact linear motors driven by surface acoustic waves, Sens. Actuators A 155 (2009) 163–167. [4] T. Kanda, A. Makino, T. Ono, K. Suzumori, T. Morita, M.K. Kurosawa, A micro ultrasonic motor using a micro-machined cylindrical bulk PZT transducer, Sens. Actuators A 127 (2006) 131–138. [5] J.T. Leinvuo, S.A. Wilson, R.W. Whatmore, M.G. Cain, A new flextensional piezoelectric ultrasonic motor—design, fabrication and characterization, Sens. Actuators A 133 (2007) 141–151. [6] C.B. Yoon, G.T. Park, H.E. Kima, J. Ryu, Piezoelectric ultrasonic motor by co-extrusion process, Sens. Actuators A 121 (2005) 515– 519. [7] T. Senjyu, T. Kashigawi, K. Uezato, Position control of ultrasonic motors using MRAC and dead-zone compensation with fuzzy inference, IEEE Trans. Power Electr. 17 (2002) 265–272. [8] T. Hemsel, J. Wallaschek, Survey of the present state of the art of piezoelectric linear motors, Ultrasonics 38 (2000) 37–40. [9] J.M. Jin, C.S. Zhao, Linear ultrasonic motor using quadrate plate transducer, Front. Mech. Eng. China 4 (2009) 88–91. [10] T. Takenaka, H. Nagata, Current status and prospects of lead-free piezoelectric ceramics, J. Eur. Ceram. Soc. 25 (2005) 2693–2700. [11] T. Takenaka, H. Nagata, Y. Hiruma, Current developments and prospective of lead-free piezoelectric ceramics, Jpn. J. Appl. Phys. 47 (2008) 3787– 3801. [12] F. Rubio-Marcos, P. Ochoa, J.F. Fernandez, Sintering and properties of leadfree (K, Na, Li)(Nb, Ta, Sb)O3 ceramics, J. Eur. Ceram. Soc. 27 (2007) 4125– 4129. [13] E.Z. Li, H. Kakemoto, S. Wada, T. Tsurumi, Effects of manganese addition on piezoelectric properties of the (K, Na, Li) (Nb, Ta, Sb) O3 lead-jpne ceramics, J. Ceram. Soc. Jpn. 115 (2007) 250–253. [14] D.M. Lin, K.W. Kwok, H.L.W. Chan, Effects of MnO2 on the microstructure and electrical properties of 0.94(K0.5 Na0.5 )NbO3- 0.06Ba(Zr0.05 Ti0.95 )O3 leadfree ceramics, Mater. Chem. Phys. 109 (2008) 455–458. [15] C.W. Ahn, S. Nahm, M. Karmarkar, D. Viehland, D.H. Kang, K.S. Bae, S. Priya, Effect of CuO and MnO2 on sintering temperature, microstructure, and piezoelectric properties of 0.95(K0.5 Na0.5 )NbO3- 0.05BaTiO3 ceramics, Mater. Lett. 62 (2008) 3594–3596.