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Miniature tubular centrifugal piezoelectric pump utilizing wobbling motion
Sensors and Actuators A 157 (2010) 322–327
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Miniature tubular centrifugal piezoelectric pump utilizing wobbling motion Yu Ting Ma, Fan Rang Kong, Cheng Liang Pan, Qi Zhang, Zhi Hua Feng ∗ Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei, Anhui 230026, China
a r t i c l e
i n f o
Article history: Received 8 July 2009 Received in revised form 22 October 2009 Accepted 29 November 2009 Available online 4 December 2009 Keywords: Piezoelectric Pump Centrifugal Wobbling
a b s t r a c t A new kind of piezoelectric pump utilizing centrifugal force directly is introduced in this paper. It uses a metal tube to act as both the wobbling part and the liquid channel. Several piezoelectric elements attaching to the metal tube are used as the actuators to push the metal tube to wobble at its resonant frequency. The centrifugal force generated by wobbling motion will force the liquid in the metal tube out. The dynamic characteristics of the pump are studied. A prototype pump is fabricated by four piezoelectric tubes and a steel tube with an outer diameter of 1.0 mm, an inner diameter of 0.7 mm and a length of 80 mm. Experimental results show that the system can pump tap water at a flow rate of 7.7 ml/min under a backpressure of 2.0 kPa. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Piezopumps have been considerably developed over the past several years. They have many excellent abilities such as liquid handling in small and precise volume, providing a high actuation force with a relative fast mechanical response at high operation frequency, and miniaturization in a small size, etc. [1–3]. Therefore, they are able to serve in chemical, medical, and biomedical applications with great scientific and commercial potential [4–6]. The most reported piezoelectric pumps are reciprocating displacement piezoelectric pumps such as diaphragm pumps and stack driven piston type pumps. Harald van Lintel et al. first reported a two-valve, single-chamber reciprocating displacement piezopump driven by the lateral strain of the piezoelectric disk diaphragm in 1988 [7]. Mauck and Lynch reported a hydraulic pump utilizing a piezoelectric stack actuator to drive a piston in a cylinder in 1999 [8]. In order to enable the liquid to flow in a particular direction, these pumps usually need one-way flow mechanisms at the inlet and outlet such as check valves. But the traditional check valves operate at relatively low frequencies of less than 200 Hz, making it difficult to reconcile with vibration of piezoelectric elements under high frequency. By use of MEMS technology, new types of check valves were produced [9–11]. They have excellent high frequency response, but the technology is still in research, and MEMS manufacturing is complex, high cost, not practical and popular nowadays. Some valve-less piezoelectric pumps make use of novel port structure, such as the nozzle/diffuser structure [12],
∗ Corresponding author. Tel.: +86 551 3607894; fax: +86 551 3607894. E-mail address: [email protected] (Z.H. Feng). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.11.035
valvular conduits structure [13], shaped vortex structure [14], Yshaped port [15], variable flow resistance ports [16], etc. Their properties was less affected by frequency and piezoelectric pumps working at a high frequency often use this structure, but it depends on the overall flow difference in and out of the port in a cycle to realize flow in one direction, so it is not really a one-way flow port. The utilization of chamber volume change is not complete. Other types of piezoelectric pumps also appear, such as a novel integrated peristaltic pump reported by Bu et al. in 2003 [17], and a traveling wave pump developed by Bar-Cohen and Chang in 2001 [18]. Although the performances of these pumps are promoted, their structures and control modes tend to be complicated. Improvements and new inventions of piezoelectric pumps are still expected. To avoid some problems existing in previous piezoelectric pumps, a novel pump utilizing centrifugal force as the direct driving power is presented, and it has advantages of simple structure, convenient control, stable output and high frequency operation. 2. The principle and character of the pump The schematic configuration of the proposed piezoelectric pump is shown in Fig. 1. A long metal tube setting in the center of the structure acts as a vibrating component and liquid channel. Four piezoelectric elements used as actuators are adhered to the metal tube. The common end of the piezoelectric elements is fastened to a basement and the other end of the metal tube can move freely. The piezoelectric elements are arranged and excited so that they can push and pull the metal tube to make it wobble. If the driving voltages are at the bending resonant frequency of the system, the wobbling amplitude of the metal tube will be amplified greatly. In this case, the liquid in the metal tube will obtain a centrifugal
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323
Fig. 2. A part of the metal tube is taken as a Bernoulli–Euler beam.
precision. On one hand, it is necessary to meet the requirements of low driving voltage and large flows (greater than 1 ml/min); on the other hand, a small flow (less than 1 l/min) has to be studied and fabricated. The piezoelectric pump proposed can be large or small to satisfy different applications, and multi-parameter design can meet various practical needs. It has considerable capacity for development. 3. Dynamic analysis of the pump
Fig. 1. The structure of the centrifugal pump using wobbling motion of a metal tube to push the liquid out.
force large enough to overcome the resistance such as gravity and viscous damping, and be forced out of the metal tube. The metal tube should be full of liquid at first to provide an initial force. The pump contains two main driving modes: (1) piezoelectric elements produce bending vibration in one direction and drive the metal tube to swing back and forth correspondingly. (2) Two pairs of piezoelectric elements are vertically distributed. They produce bending vibrations of the same amplitude and frequency with 90◦ phase difference, driving the metal tube to swing in a circle. In general, piezoelectric elements are driven by voltages to produce stretch and shrink deformation, causing the metal tube to vibrate, and the vibration drives the liquid to flow out due to the centrifugal force. This pump is different from the previous chamber, peristaltic and traveling wave piezoelectric pumps. It does not need any flow valve to control the liquid flow direction or special inlet and outlet, also does not need complex driving circuit. Liquid flows stably, and it is particularly well suited to high-frequency pumping. A check valve can be used to prevent liquid from flowing backward in gravity when the pump stopped running, thereby avoiding to inject liquid into the metal tube fully again. With the help of check valve, the pump needs only once initialization during intermittent running, greatly simplifying the operating procedures. As the check valve keeps open all the time during pumping, it has no influence on the working frequency of the pump. This pump can make use of different pump structures and driving modes to get the swinging or circular movement of the metal tube. The number of piezoelectric elements can be single or multiple and piezoelectric elements also have a variety of forms, such as tube, circular film, and rectangle sheet. For different forms of driving modes, metal tube can be single or arrays. When piezoelectric tubes are chosen for driving elements, their outer electrodes are linked together and grounded, and driving voltages are applied on the inner electrodes. This structure enhances safety in operation. It generates little electromagnetic interference to surrounding circuits and also has good ability of preventing external disturbance. The number of piezoelectric tubes can be adjusted according to the size of metal tube. According to the applications of the pumps, research on piezoelectric pumps moves toward low power consumption and high
The pump usually works at its first bending resonant frequency because the vibrating amplitude of the metal tube is greatly amplified at this moment. Since the vibration amplitude of the actuating part of the pump is very small and it can be regarded as clamped, the vibration of the part of the metal tube that stretches out of the piezoelectric elements is considered. According to theory of vibration, the transverse vibration of a slender tube can be dealt as a Bernoulli–Euler beam as shown in Fig. 2, and the result will be a little higher than the actual value. There are three resonant frequencies to be explained: One is the resonant frequency of the metal tube without liquid and denoted as fs01. The second one is the resonant frequency of the metal tube when it is full of liquid but no flow occurs and denoted as fs02. The third one is that of the metal tube with liquid flowing out and denoted as fs03. 1. When there is no liquid in the metal tube, the free transverse vibration equation of the beam is [19]: ∂2 ∂z 2
Ym Iy
∂2 x ∂z 2
+ m Am
∂2 x =0 ∂t 2
(1)
where Ym refers to the Young’s modulus of the metal tube, m is the density of the metal tube, Am is the cross-section area of the metal tube, Iy is the moment of inertia of the cross-section with respect to axis y and Iy = (Ro4 − Ri 4 )/4, with Ro and Ri are the outer and inner radius of the metal tube. Substituting the boundary condition of a cantilever beam into the equation above, the first bending resonant frequency can be got as 3.5156 fs01 = 2
Ym Iy m Am L04
,
(2)
where L0 is the length of the part of the metal tube considered. 2. When the metal tube is full of liquid but no flow occurs, the free transverse vibration equation of the beam is [19]: ∂2 ∂z 2
∂2 x Ym Iy 2 ∂z
+ (m Am + l Al )
∂2 x =0 ∂t 2
(3)
Similarly, the first bending resonant frequency can be got as: 3.5156 fs02 = 2
Ym Iy (m Am + l Al )L04
,
(4)
where l is the density of the liquid, Al is the cross-section area of the liquid segment. 3. When the metal tube is filled with liquid flowing out at a velocity of V, the free transverse vibration equation of the beam is
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∂2 ∂z 2
Ym Iy
∂2 x ∂z 2
+ l Al V 2
+ (m Am + l Al )
∂2 x ∂2 x + 2l Al V 2 ∂z∂t ∂z
∂2 x =0 ∂t 2
(5)
where the four terms are the stiffness force of the structure, the centrifugal and gyroscopic forces of the flowing liquid, and the inertial force of the structure and flowing liquid, respectively. Suppose the dimensionless variables are defined as =
x , L
=
=
z , L
U=
Ym Iy (l Al + m Am )L4
l Al VL, Ym Iy
=
force is analyzed and shown in Fig. 3, where dL is the length of the micro-segment of the liquid, L denotes the position of the microsegment. ı is the vibration amplitude of the micro-segment; dF is the centrifugal force acting on the micro-segment, which can be divided into two components tangential and perpendicular to the metal tube, dF and dF . is the angle that the metal tube bends. So the following relationship can be got:
l Al , l Al + m Am
·t
(6)
dF = dF sin dF = dm · ω2 · ı
(10)
dm = l · Al · dL ω = 2f
(11)
where dm is the mass of the micro-segment of the liquid, f is the frequency of the metal tube’s wobbling motion in a circle. So the pressure that pushes the liquid caused by centrifugal force is
Eq. (5) may be written in a dimensionless form: P=
∂2 ∂2 ∂4 ∂2 + U 2 2 + 2 U + =0 4 ∂∂ ∂ ∂ ∂ 2
dF sin Al
(7)
N
ϕr ()qr ()
(8)
r=1
where ϕr () = cos r − cosh r + r (sin r − sinh r ), r = (sin r − sinh r )/(cos r + cosh r ), (r = 1, 2, ..., N) is the eigenfunction of the cantilever beam. Substituting Eq. (8) into (7), and multiplying the resulting equation by the orthogonality ϕs , finally the equation is obtained as N
4r
1
ϕr ϕs d
qr + U 2
0
r=1
+2
1
0
U 0
1
∂ϕr ϕs d ∂
∂qr + ∂
∂2 ϕr ∂ 2
dx ·x = dL
l ω2 x · (12)
A high working frequency f and a large vibration amplitude x(L0 ) will bring a high pressure P. But, as the stress of the metal tube material is limited, the vibration amplitude of the metal tube should also be limited, so does the pressure. For a certain pump, the maximum actuating pressure determined by the maximum stress of the metal tube material can be deduced as follows. The first bending mode shape of the metal tube is [19]: X(z) = C[cos ˇ1 z − cosh ˇ1 z + r1 (sin ˇ1 z − sinh ˇ1 z)]
(13)
where r1 = (sin ˇ1 L0 − sinh ˇ1 L0 )/(cos ˇ1 L0 + cosh ˇ1 L0 ) ˇ1 L0 = 1.875
ϕs dqr
1
ϕr ϕs d 0
l ω2 dL ·
dx = 22 f 2 l x2 (L0 )
To discrete Eq. (7) with Galerkin’s method, suppose: (, ) =
=
∂2 qr ∂ 2
C is the constant to be determined. So the curvature of the metal tube is
=0
(9)
with qr () = A exp(iωr ) We considered the first vibration mode, so s = 1, and also set N = 5 to obtain enough accuracy. The variations in the resonant frequencies with the flow velocity can be obtained from the numerical solutions of Eq. (9). If the metal tube works at the mode of swinging in a circle, the pressure that pushes the liquid out can be calculated as follows. Taking an arbitrary small segment from the liquid, its actuating
Cˇ1 2 [−cos ˇ1 z − cosh ˇ1 z + r1 (−sin ˇ1 z − sinh ˇ1 z)] [1 + C 2 ˇ 2 [−sin ˇ z − sinh ˇ z + r (cos ˇ z − cosh ˇ z)]2 ]3/2
k(z) =
1
1
1
1
1
(14)
1
which has the maximum value at the location of z = 0. As max = Ym · Ro · k(0) ≤ [ ]
(15)
so Ym · Ro · 2 ·
1.8752 L02
· C ≤ [ ]
(16)
where [ ] is the maximum stress that the metal tube can withstand before it is broken. The largest vibration amplitude at the tip can be determined as Xmax (L0 ) =
[ ]L02 3.5156Ro Ym
(17)
When the metal tube is full of liquid and no flowing occurs, Pmax =
Fig. 3. Calculation of the pressure that pushes the liquid out.
1 [ ]2 (Ro4 − Ri 4 )l · · 8 [m (Ro 2 − Ri 2 ) + l Ri 2 ]Ro 2 Ym
(18)
The result suggests that, the length of the part of the metal tube considered has no influence on the maximum actuating pressure. This is a rough estimation without considering the character of the piezoelectric actuator. The stress exerted on the piezoelectric actuator can be decreased to a small value by certain structure design of the pump. For example, the two ends of each piezoelectric element are enclosed with two metal couplers, as Fig. 4(a) shows. The couplers increase the area of the exerted force, so disperse the stress exerted on the piezoelements.
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325
Fig. 4. (a) The mechanical structure of a prototype pump. (b) The photograph of the prototype pump.
4. An experimental prototype pump A prototype pump was fabricated with four piezoelectric tubes and a stainless steel tube, as Fig. 4(a) shows. The PZT tubes which were used as actuators, have an outer diameter of 1 mm, a wall thickness of 0.2 mm, and a length of 23 mm. The inner surface of each piezoelectric tube was covered with electric glue (Epo-Tek H20e) serving as one electrode and the outer surface was electroplated with gold serving as another electrode. The two ends of each piezoelectric tube were enclosed with two short stainless steel couplers to ensure that piezoelectric tubes make good contact with the middle steel tube. The steel couplers have an outer diameter of 1.3 mm, an inner diameter of 1 mm, and a total length of 5 mm. The piezoelectric tubes were poled in thermostatic silicone oil of 80 ◦ C with an electric field intensity of 2000 V/mm, lasting for 20 min. Two piezoelectric tubes are poled radially inward, another two are poled radially outward. The stainless steel tube in the middle has an outer diameter of 1 mm, an inner diameter of 0.7 mm, and a length of 80 mm (from the surface of the basement to its tip). The interface of the steel tube and the couplers were covered with epoxy adhesive (DP460) to link them firmly, insuring that four piezoelectric tubes are arranged in the same axial direction, uniformly surrounding the steel tube. Two piezoelectric tubes poled reversely are regarded as one phase and posited oppositely. The common ends of the piezoelectric tubes and steel tube were fastened to a steel basement. A plastic pipe linked to the fixed end of the steel tube is used as liquid channel. Fig. 4(b) shows a photograph of the pump. In this work, piezoelectric tubes made of PZT-5A1 are supplied by the Smart Materials Company which has the following charge coefficients: d33 = 440 pC/N, d31 = −185 pC/N. The inner electrodes of the four tubes were connected to driving voltages and the common outside electrode were grounded. When the two phases are driven by sinusoidal voltages with a 90◦ phase shift to each other, a wobbling motion can be generated and the tip of steel tube moves around a circle. The first bending vibration mode is used in the experiment. With four inner electrodes linked parallel together, the pump’s frequency characters were measured with an impedance analyzer (GW8101, Taiwan), as shown in Fig. 5. It appears that the resonant frequency of the pump with no liquid is about 254 Hz. According to Eq. (2), when Ym = 210 GPa, m = 7800 kg/m3 , L0 = 60 mm, Ro = 0.5 mm, Ri = 0.35 mm, then fs01 = 246 Hz. According to Eq. (4), when l = 1000 kg/m3 , then fs02 = 232 Hz. When there is flowing
Fig. 5. Frequency characters of the pump with no liquid in it.
liquid in the steel tube, according to the calculation result from Eq. (9), the relationship between the first bending resonant frequency and the flow velocity were shown in Fig. 6. As the flow velocity increases, the resonant frequency decreases. In summary, fs01 > fs02 > fs03. In order to obtain a stable pumping, the driving frequency is chosen a little higher than fs03. Suppose that the steel tube full of liquid vibrates at a frequency of fs0 and no liquid flows out of the steel tube. The pressure produced by the centrifugal force must be equal to the backpressure at this moment. If a disturbance occurs, for example, which makes the height of the liquid in the steel tube decline slightly, the resonant frequency of the system fs03 will increase. As fs0 is a little higher than fs03, the vibration amplitude will increase, and the liquid in the steel tube will be forced to move upwards to compensate the drop of the liquid. The character of the pump is experimentally tested and the schema of the setup is shown in Fig. 7. The working liquid was tap
Fig. 6. Fundamental resonant frequency as a function of flow velocity.
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Fig. 7. Experimental setup for flow rate and backpressure measurement
water. Two sinusoidal voltages with a 90◦ phase shift were generated by a multi-way synchronized waveform generator, and then were magnified by a two-way voltage amplifier to drive the four piezoelectric tubes. Digital oscilloscope (Tektronix, TDS 2012) was used to monitor the driving signals. A sleeve made of absorbent tissue was put around the free tip of steel tube to absorb the output water. Then the incremental mass of the tissue measured by an electronic balance was divided by the time interval to obtain the average flow rate. A plastic pipe with an inner diameter of 2.5 mm and 1000 mm in length was connected to the inlet of the steel tube. The backpressure comes from the height difference between the outlet and the top surface of the water in which the plastic pipe is immersed, and can be adjusted by changing the height of the water tank relative to the outlet. First, the frequency character of the prototype pump was investigated. The driving voltage and the backpressure were set at constant values of 120 Vp-p and 2.0 kPa. The relationship between flow rate of the pump and exciting frequency is shown in Fig. 8. It illustrates that the piezoelectric pump could work at a frequency ranging from 228.9 Hz to 238.3 Hz, and the flow rate reaches the
Fig. 8. Flow rate at different driving frequencies, under 120 Vp-p driving voltage and 2.0 kPa backpressure. A maximum flow rate of 7.7 ml/min occurs at 232 Hz.
Fig. 9. The relationship between the flow rate and backpressure at different driving voltages shows a good linearity.
Fig. 10. The pattern of the liquid throwing out from the tip of steel tube: (a) rotating mode and (b) back-and-forth mode.
Y.T. Ma et al. / Sensors and Actuators A 157 (2010) 322–327
highest value of 7.7 ml/min at a frequency of 232 Hz. The small flow velocity cause little change of the resonant frequencies within the working range, so fs03 almost equals to fs02. Then, the influence of backpressure and driving voltage amplitude on flow rate were measured at a certain frequency. The driving voltage frequency was kept at a constant value of 233.2 Hz (a little higher than fs03 = 232 Hz). The driving voltage varied from 60 Vp-p to 120 Vp-p with step of 15 Vp-p . Fig. 9 shows the changing trend of the flow rate. Every point was got by averaging two readings. The flow rate is nearly inversely proportional to the backpressure and proportional to the applied voltage amplitude. At an applied voltage of 120 Vp-p and a frequency of 233.2 Hz, the flow rate and backpressure reach their maximum values of 8.6 ml/min (with a backpressure of near zero) and 5.4 kPa (with a flow rate of near zero), respectively. From the Eq. (12), the highest backpressure that the pump supports can be deduced. When ω = (2 × 233.2) rad/s, x(L0 ) = 2.3 mm, = 1000 kg/m3 , then P = 5.7 kPa, which is coincident to the experimental result. The top view of the output liquid is shown in Fig. 10(a). The trace of the outlet of the steel tube is almost a circle and the water was thrown out from its circumferential edge. The pattern of jetting water using another mode (back-and-forth) is as shown in Fig. 10(b). 5. Conclusion A design for a centrifugal piezoelectric pump was presented. The piezoelectric pump is simple and easy to fabricate. It possesses some outstanding characters such as no sliding parts and so is supposed to have higher efficiency and long working life; it has no severe quality demand on the pumping liquid as no precision valve or micro-channel are needed. A variety of piezoelectric pumps may be derived from this structure since different number of diversified piezoelectric elements can be used in multiple ways. Check valve and closed-loop feedback control circuitry can be used to enhance stable and continuous working of the pump. The dynamic characteristics of the pump are studied. A prototype pump was made and tested. The actuating PZT tubes worked at the first bending vibration mode. The flow rate is proportional to the driving amplitudes and inversely proportional to the backpressure. At an applied voltage of 120 Vp-p and a frequency of 233.2 Hz, the flow rate and backpressure reach their maximum values of 8.6 ml/min and 5.4 kPa, respectively. At present, many countries in the world are carrying out researches on piezoelectric pumps, the majority of which are piezoelectric volumetric pump. The centrifugal piezoelectric pump has not been reported and it is in the lead. With improvement of manufacturing of the piezoelectric elements, both the micro-pump and medium sized pumps can be generated based on this structure. The micro-pump based on this structure may be used in small parts cleaning and adhesive spraying. Micro-pumps using carbon nanotubes may be made based on this structure. Small and medium-sized pumps based on this structure can be used in decorative fountains. Of course, the pump has shortcomings. For example, liquid with high Reynolds number is hard to pump. But a higher resonant frequency and larger amplitude can be obtained to overcome the high damping of dense liquid by proper design of the piezoelectric pump’s structure. Future work will focus on the influence of character parameters of piezoelectric actuator and working liquid on the pump. By optimization, a piezoelectric pump with more remarkable performance may be fabricated.
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Acknowledgement This work was partially supported by the Graduate Innovation Research Fund of USTC. References [1] P. Woias, Micropumps-past, progress and future prospects, Sens. Actuators B: Chem. 105 (1) (2005) 28–38. [2] D.J. Laser, J.G. Santiago, A review of micropumps, J. Micromech. Microeng. 14 (6) (2004) 35–64. [3] N.T. Nguyen, X.Y. Huang, T.K. Chuan, MEMS-micropumps: a review, J. Fluids Eng. Trans. ASME 124 (2) (2002) 384–392. [4] D.A. Henderson, Novel piezo motor enables positive displacement microfluidic pump, NSTI Nanotechnol. Conf. Trade Show Technol. Proc. 3 (2007) 272–275. [5] S.B. Choi, J.K. Yoo, M.S. Cho, Y.S. Lee, Position control of a cylinder system using a piezoactuator-driven pump, Mechatronics 15 (2) (2005) 239–249. [6] E.G. Chapman, S.L. Herdic, C.A. Keller, C.S. Lynch, Development of miniaturized piezo-hydraulic pumps, Proc. SPIE Int. Soc. Opt. Eng. 5762 (2005) 299–310. [7] H.T.G. van Lintel, F.C.M. vande Pol, S. Bouwstra, Piezoelectric micropump based on micromachining of silicon, Sens. Actuators 15 (2) (1988) 153–167. [8] L. Mauck, C.S. Lynch, Piezoelectric hydraulic pump, Proc. SPIE Int. Soc. Opt. Eng. 3668 (2) (1999) 844–852. [9] M. Seong, K.P. Mohanchandra, Y. Lin, G.P. Carman, Development of a high flowrate/high operating frequency nitinol MEMS valve, Proc. SPIE Int. Soc. Opt. Eng. 6932 (2008) 69322F. [10] A.M. Cardenas-Valencia, J. Dlutowski, J. Bumgarner, C. Munoz, W. Wang, R. Popuri, L. Langebrake, Development of various designs of low-power, MEMS valves for fluidic applications, Sens. Actuators A: Phys. 136 (1) (2007) 374–384. [11] I. Chakraborty, W.C. Tang, D.P. Bame, T.K. Tang, MEMS micro-valve for space applications, Sens. Actuators A: Phys. 83 (1) (2000) 188–193. [12] E. Stemme, G. Stemme, A valveless diffuser/nozzle-based fluid pump, Sens. Actuators A: Phys. 39 (2) (1993) 159–167. [13] F.K. Foster, R.L. Bardell, M.A. Afromowitz, N.R. Sharma, A. Blanchard, Design, fabrication and testing of fixed-valve micro-pumps, ASME Fluids Eng. Div. Publ. FED 234 (1995) 39–44. [14] I. Izzo, D. Accoto, A. Menciassi, L. Schmitt, P. Dario, Modeling and experimental validation of a piezoelectric micropump with novel no-moving-part valves, Sens. Actuators A: Phys. 133 (1) (2007) 128–140. [15] J.H. Zhang, Y.L. Li, Q.X. Xia, Research on the valve-less piezoelectric pump with Y-shape pipes, Front. Mech. Eng. China 2 (2) (2007) 144–151. [16] S. Matsumoto, A. Klein, R. Maeda, Development of bi-directional valve-less micropump for liquid, Proc. IEEE Micro. Electro. Mech. Syst. MEMS (1999) 141–146. [17] M. Bu, T. Melvin, G. Ensell, J.S. Wilkinson, A.R.G. Evans, Design and theoretical evaluation of a novel microfluidic device to be used for PCR, J. Micromech. Microeng. 13 (4) (2003) S125–S130. [18] Y. Bar-Cohen, Z. Chang, Piezoelectrically actuated miniature peristaltic pump, Proc. SPIE Int. Soc. Opt. Eng. 4327 (2001) 425–432. [19] J.P. Den Hartog, Mechanical Vibrations, Dover Publications, New York, 1985, Chap.4, p. 148.
Biographies Yuting Ma received her BE degree from Hefei University of Technology, Anhui, China, in 2006. Currently, she is studying for her PhD degree in precision engineering at the University of Science and Technology of China. She has particular interests in piezoelectric materials for novel actuators and sensors. Fanrang Kong was born in 1951. He received his BE from Tsinghua University in 1978, ME from Nanjing University of Aeronautics and Astronautics in 1983 and PhD degrees from Huazhong University of Science and Technology in 1990, respectively. Since 1998, he has been a Professor in the Department of Precision Machinery and Precision Instrumentation of USTC. His research interests include smart-material actuators, status monitoring and faults diagnosis. Chengliang Pan received his BE degree from the University of Science and Technology of China (USTC) in 2005. He is currently taking up his PhD degree in the Department of Precision Machinery and Precision Instrumentation at USTC. He is focused on the piezoelectric actuators and applications. Qi Zhang received his BE degree from the University of Science and Technology of China (USTC) in 2006. He is currently studying for his PhD degree in the Department of Precision Machinery and Precision Instrumentation at USTC. He is working on the design of novel piezoelectric motors and flexible actuators. Zhihua Feng was born in 1964. He received his BE, ME and PhD degrees from the University of Science and Technology of China (USTC) in 1987, 1990, and 2005, respectively. He is now a Professor in the Department of Precision Machinery and Precision Instrumentation, USTC. His research interests include smart actuators and sensors, energy harvesting, power electronics, and robotics.
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Miniature tubular centrifugal piezoelectric pump utilizing wobbling motion Yu Ting Ma, Fan Rang Kong, Cheng Liang Pan, Qi Zhang, Zhi Hua Feng ∗ Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei, Anhui 230026, China
a r t i c l e
i n f o
Article history: Received 8 July 2009 Received in revised form 22 October 2009 Accepted 29 November 2009 Available online 4 December 2009 Keywords: Piezoelectric Pump Centrifugal Wobbling
a b s t r a c t A new kind of piezoelectric pump utilizing centrifugal force directly is introduced in this paper. It uses a metal tube to act as both the wobbling part and the liquid channel. Several piezoelectric elements attaching to the metal tube are used as the actuators to push the metal tube to wobble at its resonant frequency. The centrifugal force generated by wobbling motion will force the liquid in the metal tube out. The dynamic characteristics of the pump are studied. A prototype pump is fabricated by four piezoelectric tubes and a steel tube with an outer diameter of 1.0 mm, an inner diameter of 0.7 mm and a length of 80 mm. Experimental results show that the system can pump tap water at a flow rate of 7.7 ml/min under a backpressure of 2.0 kPa. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Piezopumps have been considerably developed over the past several years. They have many excellent abilities such as liquid handling in small and precise volume, providing a high actuation force with a relative fast mechanical response at high operation frequency, and miniaturization in a small size, etc. [1–3]. Therefore, they are able to serve in chemical, medical, and biomedical applications with great scientific and commercial potential [4–6]. The most reported piezoelectric pumps are reciprocating displacement piezoelectric pumps such as diaphragm pumps and stack driven piston type pumps. Harald van Lintel et al. first reported a two-valve, single-chamber reciprocating displacement piezopump driven by the lateral strain of the piezoelectric disk diaphragm in 1988 [7]. Mauck and Lynch reported a hydraulic pump utilizing a piezoelectric stack actuator to drive a piston in a cylinder in 1999 [8]. In order to enable the liquid to flow in a particular direction, these pumps usually need one-way flow mechanisms at the inlet and outlet such as check valves. But the traditional check valves operate at relatively low frequencies of less than 200 Hz, making it difficult to reconcile with vibration of piezoelectric elements under high frequency. By use of MEMS technology, new types of check valves were produced [9–11]. They have excellent high frequency response, but the technology is still in research, and MEMS manufacturing is complex, high cost, not practical and popular nowadays. Some valve-less piezoelectric pumps make use of novel port structure, such as the nozzle/diffuser structure [12],
∗ Corresponding author. Tel.: +86 551 3607894; fax: +86 551 3607894. E-mail address: [email protected] (Z.H. Feng). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.11.035
valvular conduits structure [13], shaped vortex structure [14], Yshaped port [15], variable flow resistance ports [16], etc. Their properties was less affected by frequency and piezoelectric pumps working at a high frequency often use this structure, but it depends on the overall flow difference in and out of the port in a cycle to realize flow in one direction, so it is not really a one-way flow port. The utilization of chamber volume change is not complete. Other types of piezoelectric pumps also appear, such as a novel integrated peristaltic pump reported by Bu et al. in 2003 [17], and a traveling wave pump developed by Bar-Cohen and Chang in 2001 [18]. Although the performances of these pumps are promoted, their structures and control modes tend to be complicated. Improvements and new inventions of piezoelectric pumps are still expected. To avoid some problems existing in previous piezoelectric pumps, a novel pump utilizing centrifugal force as the direct driving power is presented, and it has advantages of simple structure, convenient control, stable output and high frequency operation. 2. The principle and character of the pump The schematic configuration of the proposed piezoelectric pump is shown in Fig. 1. A long metal tube setting in the center of the structure acts as a vibrating component and liquid channel. Four piezoelectric elements used as actuators are adhered to the metal tube. The common end of the piezoelectric elements is fastened to a basement and the other end of the metal tube can move freely. The piezoelectric elements are arranged and excited so that they can push and pull the metal tube to make it wobble. If the driving voltages are at the bending resonant frequency of the system, the wobbling amplitude of the metal tube will be amplified greatly. In this case, the liquid in the metal tube will obtain a centrifugal
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Fig. 2. A part of the metal tube is taken as a Bernoulli–Euler beam.
precision. On one hand, it is necessary to meet the requirements of low driving voltage and large flows (greater than 1 ml/min); on the other hand, a small flow (less than 1 l/min) has to be studied and fabricated. The piezoelectric pump proposed can be large or small to satisfy different applications, and multi-parameter design can meet various practical needs. It has considerable capacity for development. 3. Dynamic analysis of the pump
Fig. 1. The structure of the centrifugal pump using wobbling motion of a metal tube to push the liquid out.
force large enough to overcome the resistance such as gravity and viscous damping, and be forced out of the metal tube. The metal tube should be full of liquid at first to provide an initial force. The pump contains two main driving modes: (1) piezoelectric elements produce bending vibration in one direction and drive the metal tube to swing back and forth correspondingly. (2) Two pairs of piezoelectric elements are vertically distributed. They produce bending vibrations of the same amplitude and frequency with 90◦ phase difference, driving the metal tube to swing in a circle. In general, piezoelectric elements are driven by voltages to produce stretch and shrink deformation, causing the metal tube to vibrate, and the vibration drives the liquid to flow out due to the centrifugal force. This pump is different from the previous chamber, peristaltic and traveling wave piezoelectric pumps. It does not need any flow valve to control the liquid flow direction or special inlet and outlet, also does not need complex driving circuit. Liquid flows stably, and it is particularly well suited to high-frequency pumping. A check valve can be used to prevent liquid from flowing backward in gravity when the pump stopped running, thereby avoiding to inject liquid into the metal tube fully again. With the help of check valve, the pump needs only once initialization during intermittent running, greatly simplifying the operating procedures. As the check valve keeps open all the time during pumping, it has no influence on the working frequency of the pump. This pump can make use of different pump structures and driving modes to get the swinging or circular movement of the metal tube. The number of piezoelectric elements can be single or multiple and piezoelectric elements also have a variety of forms, such as tube, circular film, and rectangle sheet. For different forms of driving modes, metal tube can be single or arrays. When piezoelectric tubes are chosen for driving elements, their outer electrodes are linked together and grounded, and driving voltages are applied on the inner electrodes. This structure enhances safety in operation. It generates little electromagnetic interference to surrounding circuits and also has good ability of preventing external disturbance. The number of piezoelectric tubes can be adjusted according to the size of metal tube. According to the applications of the pumps, research on piezoelectric pumps moves toward low power consumption and high
The pump usually works at its first bending resonant frequency because the vibrating amplitude of the metal tube is greatly amplified at this moment. Since the vibration amplitude of the actuating part of the pump is very small and it can be regarded as clamped, the vibration of the part of the metal tube that stretches out of the piezoelectric elements is considered. According to theory of vibration, the transverse vibration of a slender tube can be dealt as a Bernoulli–Euler beam as shown in Fig. 2, and the result will be a little higher than the actual value. There are three resonant frequencies to be explained: One is the resonant frequency of the metal tube without liquid and denoted as fs01. The second one is the resonant frequency of the metal tube when it is full of liquid but no flow occurs and denoted as fs02. The third one is that of the metal tube with liquid flowing out and denoted as fs03. 1. When there is no liquid in the metal tube, the free transverse vibration equation of the beam is [19]: ∂2 ∂z 2
Ym Iy
∂2 x ∂z 2
+ m Am
∂2 x =0 ∂t 2
(1)
where Ym refers to the Young’s modulus of the metal tube, m is the density of the metal tube, Am is the cross-section area of the metal tube, Iy is the moment of inertia of the cross-section with respect to axis y and Iy = (Ro4 − Ri 4 )/4, with Ro and Ri are the outer and inner radius of the metal tube. Substituting the boundary condition of a cantilever beam into the equation above, the first bending resonant frequency can be got as 3.5156 fs01 = 2
Ym Iy m Am L04
,
(2)
where L0 is the length of the part of the metal tube considered. 2. When the metal tube is full of liquid but no flow occurs, the free transverse vibration equation of the beam is [19]: ∂2 ∂z 2
∂2 x Ym Iy 2 ∂z
+ (m Am + l Al )
∂2 x =0 ∂t 2
(3)
Similarly, the first bending resonant frequency can be got as: 3.5156 fs02 = 2
Ym Iy (m Am + l Al )L04
,
(4)
where l is the density of the liquid, Al is the cross-section area of the liquid segment. 3. When the metal tube is filled with liquid flowing out at a velocity of V, the free transverse vibration equation of the beam is
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∂2 ∂z 2
Ym Iy
∂2 x ∂z 2
+ l Al V 2
+ (m Am + l Al )
∂2 x ∂2 x + 2l Al V 2 ∂z∂t ∂z
∂2 x =0 ∂t 2
(5)
where the four terms are the stiffness force of the structure, the centrifugal and gyroscopic forces of the flowing liquid, and the inertial force of the structure and flowing liquid, respectively. Suppose the dimensionless variables are defined as =
x , L
=
=
z , L
U=
Ym Iy (l Al + m Am )L4
l Al VL, Ym Iy
=
force is analyzed and shown in Fig. 3, where dL is the length of the micro-segment of the liquid, L denotes the position of the microsegment. ı is the vibration amplitude of the micro-segment; dF is the centrifugal force acting on the micro-segment, which can be divided into two components tangential and perpendicular to the metal tube, dF and dF . is the angle that the metal tube bends. So the following relationship can be got:
l Al , l Al + m Am
·t
(6)
dF = dF sin dF = dm · ω2 · ı
(10)
dm = l · Al · dL ω = 2f
(11)
where dm is the mass of the micro-segment of the liquid, f is the frequency of the metal tube’s wobbling motion in a circle. So the pressure that pushes the liquid caused by centrifugal force is
Eq. (5) may be written in a dimensionless form: P=
∂2 ∂2 ∂4 ∂2 + U 2 2 + 2 U + =0 4 ∂∂ ∂ ∂ ∂ 2
dF sin Al
(7)
N
ϕr ()qr ()
(8)
r=1
where ϕr () = cos r − cosh r + r (sin r − sinh r ), r = (sin r − sinh r )/(cos r + cosh r ), (r = 1, 2, ..., N) is the eigenfunction of the cantilever beam. Substituting Eq. (8) into (7), and multiplying the resulting equation by the orthogonality ϕs , finally the equation is obtained as N
4r
1
ϕr ϕs d
qr + U 2
0
r=1
+2
1
0
U 0
1
∂ϕr ϕs d ∂
∂qr + ∂
∂2 ϕr ∂ 2
dx ·x = dL
l ω2 x · (12)
A high working frequency f and a large vibration amplitude x(L0 ) will bring a high pressure P. But, as the stress of the metal tube material is limited, the vibration amplitude of the metal tube should also be limited, so does the pressure. For a certain pump, the maximum actuating pressure determined by the maximum stress of the metal tube material can be deduced as follows. The first bending mode shape of the metal tube is [19]: X(z) = C[cos ˇ1 z − cosh ˇ1 z + r1 (sin ˇ1 z − sinh ˇ1 z)]
(13)
where r1 = (sin ˇ1 L0 − sinh ˇ1 L0 )/(cos ˇ1 L0 + cosh ˇ1 L0 ) ˇ1 L0 = 1.875
ϕs dqr
1
ϕr ϕs d 0
l ω2 dL ·
dx = 22 f 2 l x2 (L0 )
To discrete Eq. (7) with Galerkin’s method, suppose: (, ) =
=
∂2 qr ∂ 2
C is the constant to be determined. So the curvature of the metal tube is
=0
(9)
with qr () = A exp(iωr ) We considered the first vibration mode, so s = 1, and also set N = 5 to obtain enough accuracy. The variations in the resonant frequencies with the flow velocity can be obtained from the numerical solutions of Eq. (9). If the metal tube works at the mode of swinging in a circle, the pressure that pushes the liquid out can be calculated as follows. Taking an arbitrary small segment from the liquid, its actuating
Cˇ1 2 [−cos ˇ1 z − cosh ˇ1 z + r1 (−sin ˇ1 z − sinh ˇ1 z)] [1 + C 2 ˇ 2 [−sin ˇ z − sinh ˇ z + r (cos ˇ z − cosh ˇ z)]2 ]3/2
k(z) =
1
1
1
1
1
(14)
1
which has the maximum value at the location of z = 0. As max = Ym · Ro · k(0) ≤ [ ]
(15)
so Ym · Ro · 2 ·
1.8752 L02
· C ≤ [ ]
(16)
where [ ] is the maximum stress that the metal tube can withstand before it is broken. The largest vibration amplitude at the tip can be determined as Xmax (L0 ) =
[ ]L02 3.5156Ro Ym
(17)
When the metal tube is full of liquid and no flowing occurs, Pmax =
Fig. 3. Calculation of the pressure that pushes the liquid out.
1 [ ]2 (Ro4 − Ri 4 )l · · 8 [m (Ro 2 − Ri 2 ) + l Ri 2 ]Ro 2 Ym
(18)
The result suggests that, the length of the part of the metal tube considered has no influence on the maximum actuating pressure. This is a rough estimation without considering the character of the piezoelectric actuator. The stress exerted on the piezoelectric actuator can be decreased to a small value by certain structure design of the pump. For example, the two ends of each piezoelectric element are enclosed with two metal couplers, as Fig. 4(a) shows. The couplers increase the area of the exerted force, so disperse the stress exerted on the piezoelements.
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325
Fig. 4. (a) The mechanical structure of a prototype pump. (b) The photograph of the prototype pump.
4. An experimental prototype pump A prototype pump was fabricated with four piezoelectric tubes and a stainless steel tube, as Fig. 4(a) shows. The PZT tubes which were used as actuators, have an outer diameter of 1 mm, a wall thickness of 0.2 mm, and a length of 23 mm. The inner surface of each piezoelectric tube was covered with electric glue (Epo-Tek H20e) serving as one electrode and the outer surface was electroplated with gold serving as another electrode. The two ends of each piezoelectric tube were enclosed with two short stainless steel couplers to ensure that piezoelectric tubes make good contact with the middle steel tube. The steel couplers have an outer diameter of 1.3 mm, an inner diameter of 1 mm, and a total length of 5 mm. The piezoelectric tubes were poled in thermostatic silicone oil of 80 ◦ C with an electric field intensity of 2000 V/mm, lasting for 20 min. Two piezoelectric tubes are poled radially inward, another two are poled radially outward. The stainless steel tube in the middle has an outer diameter of 1 mm, an inner diameter of 0.7 mm, and a length of 80 mm (from the surface of the basement to its tip). The interface of the steel tube and the couplers were covered with epoxy adhesive (DP460) to link them firmly, insuring that four piezoelectric tubes are arranged in the same axial direction, uniformly surrounding the steel tube. Two piezoelectric tubes poled reversely are regarded as one phase and posited oppositely. The common ends of the piezoelectric tubes and steel tube were fastened to a steel basement. A plastic pipe linked to the fixed end of the steel tube is used as liquid channel. Fig. 4(b) shows a photograph of the pump. In this work, piezoelectric tubes made of PZT-5A1 are supplied by the Smart Materials Company which has the following charge coefficients: d33 = 440 pC/N, d31 = −185 pC/N. The inner electrodes of the four tubes were connected to driving voltages and the common outside electrode were grounded. When the two phases are driven by sinusoidal voltages with a 90◦ phase shift to each other, a wobbling motion can be generated and the tip of steel tube moves around a circle. The first bending vibration mode is used in the experiment. With four inner electrodes linked parallel together, the pump’s frequency characters were measured with an impedance analyzer (GW8101, Taiwan), as shown in Fig. 5. It appears that the resonant frequency of the pump with no liquid is about 254 Hz. According to Eq. (2), when Ym = 210 GPa, m = 7800 kg/m3 , L0 = 60 mm, Ro = 0.5 mm, Ri = 0.35 mm, then fs01 = 246 Hz. According to Eq. (4), when l = 1000 kg/m3 , then fs02 = 232 Hz. When there is flowing
Fig. 5. Frequency characters of the pump with no liquid in it.
liquid in the steel tube, according to the calculation result from Eq. (9), the relationship between the first bending resonant frequency and the flow velocity were shown in Fig. 6. As the flow velocity increases, the resonant frequency decreases. In summary, fs01 > fs02 > fs03. In order to obtain a stable pumping, the driving frequency is chosen a little higher than fs03. Suppose that the steel tube full of liquid vibrates at a frequency of fs0 and no liquid flows out of the steel tube. The pressure produced by the centrifugal force must be equal to the backpressure at this moment. If a disturbance occurs, for example, which makes the height of the liquid in the steel tube decline slightly, the resonant frequency of the system fs03 will increase. As fs0 is a little higher than fs03, the vibration amplitude will increase, and the liquid in the steel tube will be forced to move upwards to compensate the drop of the liquid. The character of the pump is experimentally tested and the schema of the setup is shown in Fig. 7. The working liquid was tap
Fig. 6. Fundamental resonant frequency as a function of flow velocity.
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Fig. 7. Experimental setup for flow rate and backpressure measurement
water. Two sinusoidal voltages with a 90◦ phase shift were generated by a multi-way synchronized waveform generator, and then were magnified by a two-way voltage amplifier to drive the four piezoelectric tubes. Digital oscilloscope (Tektronix, TDS 2012) was used to monitor the driving signals. A sleeve made of absorbent tissue was put around the free tip of steel tube to absorb the output water. Then the incremental mass of the tissue measured by an electronic balance was divided by the time interval to obtain the average flow rate. A plastic pipe with an inner diameter of 2.5 mm and 1000 mm in length was connected to the inlet of the steel tube. The backpressure comes from the height difference between the outlet and the top surface of the water in which the plastic pipe is immersed, and can be adjusted by changing the height of the water tank relative to the outlet. First, the frequency character of the prototype pump was investigated. The driving voltage and the backpressure were set at constant values of 120 Vp-p and 2.0 kPa. The relationship between flow rate of the pump and exciting frequency is shown in Fig. 8. It illustrates that the piezoelectric pump could work at a frequency ranging from 228.9 Hz to 238.3 Hz, and the flow rate reaches the
Fig. 8. Flow rate at different driving frequencies, under 120 Vp-p driving voltage and 2.0 kPa backpressure. A maximum flow rate of 7.7 ml/min occurs at 232 Hz.
Fig. 9. The relationship between the flow rate and backpressure at different driving voltages shows a good linearity.
Fig. 10. The pattern of the liquid throwing out from the tip of steel tube: (a) rotating mode and (b) back-and-forth mode.
Y.T. Ma et al. / Sensors and Actuators A 157 (2010) 322–327
highest value of 7.7 ml/min at a frequency of 232 Hz. The small flow velocity cause little change of the resonant frequencies within the working range, so fs03 almost equals to fs02. Then, the influence of backpressure and driving voltage amplitude on flow rate were measured at a certain frequency. The driving voltage frequency was kept at a constant value of 233.2 Hz (a little higher than fs03 = 232 Hz). The driving voltage varied from 60 Vp-p to 120 Vp-p with step of 15 Vp-p . Fig. 9 shows the changing trend of the flow rate. Every point was got by averaging two readings. The flow rate is nearly inversely proportional to the backpressure and proportional to the applied voltage amplitude. At an applied voltage of 120 Vp-p and a frequency of 233.2 Hz, the flow rate and backpressure reach their maximum values of 8.6 ml/min (with a backpressure of near zero) and 5.4 kPa (with a flow rate of near zero), respectively. From the Eq. (12), the highest backpressure that the pump supports can be deduced. When ω = (2 × 233.2) rad/s, x(L0 ) = 2.3 mm, = 1000 kg/m3 , then P = 5.7 kPa, which is coincident to the experimental result. The top view of the output liquid is shown in Fig. 10(a). The trace of the outlet of the steel tube is almost a circle and the water was thrown out from its circumferential edge. The pattern of jetting water using another mode (back-and-forth) is as shown in Fig. 10(b). 5. Conclusion A design for a centrifugal piezoelectric pump was presented. The piezoelectric pump is simple and easy to fabricate. It possesses some outstanding characters such as no sliding parts and so is supposed to have higher efficiency and long working life; it has no severe quality demand on the pumping liquid as no precision valve or micro-channel are needed. A variety of piezoelectric pumps may be derived from this structure since different number of diversified piezoelectric elements can be used in multiple ways. Check valve and closed-loop feedback control circuitry can be used to enhance stable and continuous working of the pump. The dynamic characteristics of the pump are studied. A prototype pump was made and tested. The actuating PZT tubes worked at the first bending vibration mode. The flow rate is proportional to the driving amplitudes and inversely proportional to the backpressure. At an applied voltage of 120 Vp-p and a frequency of 233.2 Hz, the flow rate and backpressure reach their maximum values of 8.6 ml/min and 5.4 kPa, respectively. At present, many countries in the world are carrying out researches on piezoelectric pumps, the majority of which are piezoelectric volumetric pump. The centrifugal piezoelectric pump has not been reported and it is in the lead. With improvement of manufacturing of the piezoelectric elements, both the micro-pump and medium sized pumps can be generated based on this structure. The micro-pump based on this structure may be used in small parts cleaning and adhesive spraying. Micro-pumps using carbon nanotubes may be made based on this structure. Small and medium-sized pumps based on this structure can be used in decorative fountains. Of course, the pump has shortcomings. For example, liquid with high Reynolds number is hard to pump. But a higher resonant frequency and larger amplitude can be obtained to overcome the high damping of dense liquid by proper design of the piezoelectric pump’s structure. Future work will focus on the influence of character parameters of piezoelectric actuator and working liquid on the pump. By optimization, a piezoelectric pump with more remarkable performance may be fabricated.
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Biographies Yuting Ma received her BE degree from Hefei University of Technology, Anhui, China, in 2006. Currently, she is studying for her PhD degree in precision engineering at the University of Science and Technology of China. She has particular interests in piezoelectric materials for novel actuators and sensors. Fanrang Kong was born in 1951. He received his BE from Tsinghua University in 1978, ME from Nanjing University of Aeronautics and Astronautics in 1983 and PhD degrees from Huazhong University of Science and Technology in 1990, respectively. Since 1998, he has been a Professor in the Department of Precision Machinery and Precision Instrumentation of USTC. His research interests include smart-material actuators, status monitoring and faults diagnosis. Chengliang Pan received his BE degree from the University of Science and Technology of China (USTC) in 2005. He is currently taking up his PhD degree in the Department of Precision Machinery and Precision Instrumentation at USTC. He is focused on the piezoelectric actuators and applications. Qi Zhang received his BE degree from the University of Science and Technology of China (USTC) in 2006. He is currently studying for his PhD degree in the Department of Precision Machinery and Precision Instrumentation at USTC. He is working on the design of novel piezoelectric motors and flexible actuators. Zhihua Feng was born in 1964. He received his BE, ME and PhD degrees from the University of Science and Technology of China (USTC) in 1987, 1990, and 2005, respectively. He is now a Professor in the Department of Precision Machinery and Precision Instrumentation, USTC. His research interests include smart actuators and sensors, energy harvesting, power electronics, and robotics.