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Research paper
A pneumatic actuator based on vibration friction reduction with bending/longitudinal vibration mode Han Gao a,b , Michaël De Volder b , Tinghai Cheng c , Gang Bao a,∗ , Dominiek Reynaerts b a b c
Harbin Institute of Technology, 150080 Harbin, China Katholieke Universiteit Leuven, 3001 Leuven, Belgium Changchun University of Technology, 130000 Changchun, China
a r t i c l e
i n f o
Article history: Received 10 July 2016 Received in revised form 22 September 2016 Accepted 27 October 2016 Available online xxx Keywords: Pneumatic actuator Vibration Friction reduction Bending vibration Longitudinal vibration
a b s t r a c t Piston–cylinder pneumatic actuators are widely applied in various fields of automation and robotics. The sealing rings comprised in these actuators unfortunately introduce friction and affect the positioning accuracy and output force. In this work, piezoelectric actuators are built in the pneumatic actuators to introduce vibrations with lower friction force. The friction reduction effect is compared between the bending vibration mode at a resonant frequency of 1.272 kHz and the longitudinal vibration mode at a frequency of 12.133 kHz. The pneumatic actuator has a bore diameter of 6.4 mm and a stroke of 13 mm. A static/dynamic friction force measurement system is established and the test results show a maximum 66.7% reduction of stiction force and a 50.8% reduction of dynamic friction force in bending vibration mode. And the friction reduction effect also happens in the longitudinal vibration mode, with a 47.4% reduction of stiction force and a 29.7% reduction of dynamic friction force. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Pneumatic actuators are commonly applied in automation and robotics [1–4] due to their high force, relatively low manufacturing cost, clean operating conditions and high reliability [5–7]. The seals used in these devices plays a non-negligible role in the overall performance of the actuators. Typical pneumatic actuators comprise rubber rings to limit leakage, but they introduce non-linear friction and stick-slip phenomenon when the cylinder operates at a low speed [8,9]. This limits for instance the positioning accuracy of piston-cylinder pneumatic actuators. There have been multiple methods to reduce friction force in piston-cylinder pneumatic actuators. For instance, dithering techniques were developed where a low amplitude and relatively high frequency periodic velocity signal is superimposed to the servo valve to improve pneumatic actuator friction characteristics [10,11]. Lubricants are improved, for instance by the addition of nano-particles [12]. Further, a new type of low friction cylinder using externally pressurized air bearings was developed to improve the friction characteristics of cylinder [13]. A multi-lobed seal was also developed for better friction characteristics in pneumatic actu-
∗ Corresponding author. Present address: Yikuang 2, 2F building, 231, Harbin, China. E-mail address: [email protected] (G. Bao).
ators [14]. Finally, new liquid seal technologies using ferrofluids [15] or surface tension [16] were developed. Alternatively, a method of reducing friction force by vibrations has attracted much attention in recent years [17,18]. It was observed that vibrating of one or two surfaces in contact dramatically alters their friction coefficient [19]. The experimental results of friction reduction induced by vibrations have been validated by a series of valuable investigations [20–25]. Most of them have some similar conclusions: the friction reduction has the best effect at resonant frequency vibration; a larger vibration amplitude produces a smaller friction force; and the vibrations from all directions to the sliding surface have friction reduction effect. Similarly, the outside barrel of pneumatic cylinder was excited by ultrasonic transducer to reduce the friction. The frictional measurements revealed a 40% reduction at 31.65 kHz [26]. Afterwards, a longitudinal vibration mode pneumatic actuator was developed by screwing a transducer onto one end of the cylinder [27]. Then, a piezo-stack was integrated onto the piston of pneumatic actuator to generate a longitudinal vibration along piston movement direction [28]. In the following research, a design which integrates two piezoelectric stacks in a self-made pneumatic cylinder is proposed. This actuator with a bore diameter of 5 mm and a stroke of 10 mm showed a 52% reduction from the original friction force at a driving frequency of 18.29 kHz with a bending vibration mode [29].
http://dx.doi.org/10.1016/j.sna.2016.10.039 0924-4247/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Gao, et al., A pneumatic actuator based on vibration friction reduction with bending/longitudinal vibration mode, Sens. Actuators A: Phys. (2016), http://dx.doi.org/10.1016/j.sna.2016.10.039
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Fig. 1. Schematic of the pneumatic actuator prototype (front view and top view).
In this paper, the design of a dual-mode pneumatic actuator is firstly presented, followed by the description of the static/dynamic friction measurement system and the operation process. Next, tribological characteristics measurements are performed and the discussion is shown on the results. 2. Actuator structure and measurement system The pneumatic actuator prototype developed in this paper mainly consists of a cylinder (aluminum) with a bore diameter of 6.4 mm, a seal house and end cover(brass), a piston and rod (rod diameter: 3 mm), two sealing rings (TRELLEBORG, NBR, OD: 6.46 mm, ID: 2.9 mm), two piezo stacks (PIEZOMECHANIK, size: 5 × 5 × 9 mm3 , max stroke: 9 m, R-frequency: 100 kHz), a flexure hinge (steel), a clamper (steel), and a pre-load bolt (steel). The H-shaped flexure hinge is used to accommodate misalignment between the piezoelectric stacks and the cylinder. This avoids stress concentration, and hence improves the lifetime. The piezo stacks are preloaded by a bolt through the cylinder, the H-stock, and the clamper. As can be seen from Fig. 1, two flat parallel surfaces are manufactured on the cylinder body, and the distance between the two flat surfaces is the same as the thickness dimension of the clamper in order for an easier assembly to the piezo stacks’ preloading. For a better view of the prototype, the separated parts and the assembly pictures are shown in Fig. 2. It has been shown from the previous references that pneumatic actuators with different vibration modes have been investigated separately, but few scholars have ever tried several modes in a single prototype. This is also one of the targets of the current research. The actuator design used in this research is using two piezo-stacks located at the base of the cylinder. As is illustrated in Fig. 3(a), a bending vibration mode as calculated by finite element simulations for the prototype can be gained when the two piezo-stacks are driven with a phase difference of , thus one of the stacks will expand while the other is shrinking. Consequently, a first order bending mode vibration happens as the clamper is fixed by four bolts, and the cylinder can be seen as a cantilever beam. In this case, the rubber rings comprised in the cylinder has a vibration
Fig. 2. (a) Each part of the prototype (before assembling); (b) Picture of the prototype assembly.
direction perpendicular to the motion of the piston. Inversely, a longitudinal vibration mode for the prototype is easily to be generated as the piezo stacks are excited with the same phase, as is shown in Fig. 3(b). And the rubber rings of the actuator have a vibration direction parallel to that of the piston motion. The frictional experiments in this paper are carried out on a tribometer, which is able to perform accurate displacement and instantaneous friction force measurements. The measurement system is mainly consisted of a voice coil linear motor (Unite Precision Mechanical Technology VCAR0436-373, with Elmo controller), a force sensor (ZHONGNUO, ZLBS-50N, resolution: 0.025 N), a laser displacement sensor (ZSY, ZLDS100-500-125, resolution: 1 m), a data acquisition card (National Instruments, USB-6211), a vibration power supply (QD-8C), a flexible coupling, a mounting block and the prototype. As is shown in Fig. 4, the functional elements of the apparatus are decoupled by utilizing a self-made flexible coupling screwed between the force sensor and the piston rod, in order to have a better elimination for the misaligned error. The motion of the piston is generated by the voice coil linear motor which allows to generate a frictionless and stable motion. The active coil of the linear actuator, which acts as the movement source is fixed to a connecting part which can slide back and forth on a guide rail, and the force sensor fixed between the connecting part and the piston rod to record the friction force. The experimental setup picture is shown in Fig. 5.
Fig. 3. (a) Bending vibration mode of the pneumatic cylinder model; (b) Longitudinal vibration mode of the pneumatic cylinder model.
Please cite this article in press as: H. Gao, et al., A pneumatic actuator based on vibration friction reduction with bending/longitudinal vibration mode, Sens. Actuators A: Phys. (2016), http://dx.doi.org/10.1016/j.sna.2016.10.039
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Fig. 4. 3D model of the experimental setup (the amplified figure indicates the structure of the flexible coupling and the prototype mounting).
Fig. 5. Picture of the experimental setup.
The displacement and force signals are recorded using a data acquisition system based on LabVIEW, and the driving signals for the piezoelectric stacks are generated by a self-developed vibration power supply. 3. Test results and discussion The actual vibration modes and corresponding vibration amplitudes are important for friction reduction. A wide bandwidth dual channel scanning vibrometer (POLYTEC: PSV-400-M2) is utilized to detect the vibration shape and vibration velocity/amplitude. Fig. 6(a) shows the first order bending vibration mode of the pro-
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totype, when two piezo stacks vibrate in anti-phase at a resonant frequency of 1.272 kHz. The inset picture shows the vibration velocity amplitude distribution at the location of end cover. Similarly, a resonant frequency of 12.133 kHz is got while two piezo stacks vibration in the same phase, and the longitudinal vibration mode is obviously found when the laser scanning surface locates on the disc-surface of the end cover, as is illustrated in Fig. 6(b). Fig. 7(a) shows the vibration amplitude as a function of the applied voltage which reaches 30 m at an input voltage peak-peak value of 60 V for the bending vibration mode, while the vibration amplitude of the measured longitudinal vibration surface reaches about 0.6 m at the same voltage. The obvious difference is due to the vibration amplitude magnification function in bending mode of the cylinder body at a much lower vibration frequency. The experimental setup consists of two regulators (SMC, IR2020-02BG, output pressure: 0.005-0.8 MPa), two air tanks (SMC, VBAT20, capacity: 20 L), a precision pressure gauge (YOKOGAWA, MT120, range: 700 kPa, resolution: 0.01 kPa), a laser displacement sensor (ZSY, ZLDS100-500-125), and a force sensor (ZHONGNUO, ZLBS-50N). First, the stiction force is measured by detecting the lowest required pressure to move the piston, as is shown in Fig. 8(a). The minimum air pressure measured by the precision pressure gauge to motivate the piston into a macroscopic movement here is the minimum starting pressure. So the product between the air pressure and the sectional area of the piston can be seen as the stiction force. Afterwards, a series of vibration amplitudes are tried to investigate the influence of vibration amplitude on the stiction force. Each test is repeated by at least five times, and the results are listed in Fig. 9 for both vibration modes. As shown in Fig. 9, the stiction force decreased up to 33% by the increasing of the vibration amplitude. These measurements are in accordance with previous studies [22,30,31]. This friction reduction phenomenon with bending vibration can be interpretted as near-field acoustic levitation [32]. There is a narrow gap between the sealing-ring and the seal house, which is hardly be seen by macroscopic view. A homogenization distribution of normal force between the friction couples happens and an acoustic levitation is generated when the bending vibration is introduced onto the actuator body. Compared to the bending vibration mode, the force reduction ratio is smaller in longitudinal vibration mode, and the stiction force can be reduced to 52.6% of its original value. Next, the output force measurements of the pneumatic actuator are carried out using the connection shown in Fig. 8(b). The measurement results are shown in Fig. 10 at different vibration amplitudes and an overpressure Pn of 0.1 MPa. Fig. 11 shows the measurement relation between output force changes and vibration amplitude at a series of air pressures.
Fig. 6. (a) The measured first order bending vibration mode of the prototype with a resonant frequency of 1.272 kHz, when the phase difference of two piezo stacks is ; (b) The longitudinal vibration mode of the prototype with a resonant frequency of 12.133 kHz, when the phase difference is zero.
Please cite this article in press as: H. Gao, et al., A pneumatic actuator based on vibration friction reduction with bending/longitudinal vibration mode, Sens. Actuators A: Phys. (2016), http://dx.doi.org/10.1016/j.sna.2016.10.039
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Fig. 7. (a) The relation between the vibration amplitude and the input voltage peak-peak value at the bending vibration mode; (b) The relation between the vibration amplitude and the input voltage peak-peak value at the longitudinal vibration mode.
Fig. 8. Schematic of the pneumatic system (a) The measurement system for stiction force; (b) The measurement system for output force; (c) The measurement system for dynamic force.
Fig. 9. The experiment results to reveal the relation between minimum starting pressure/stiction force and vibration amplitude. (a) The results for bending vibration mode; (b) The results for longitudinal vibration mode.
Fig. 10. The influence of vibration amplitude on the pneumatic actuator output force at a no-rod chamber air pressure Pn of 0.1 MPa. (a) The measurement results at bending vibration mode; (b) The measurement results at longitudinal vibration mode.
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Fig. 11. The influence of vibration amplitude on the pneumatic actuator output force changes at different air pressures. (a) The measurement results at bending vibration mode; (b) The measurement results at longitudinal vibration mode.
Fig. 13. The dynamic friction force as a function of the displacement in a period of motion. The outer circle indicates the one with piezo off, while the inner one indicates piezo on (frequency: 1.272 kHz, vibration amplitude: 30.54 m, air pressure: 0 MPa). Fig. 12. The recorded displacement of the piston and the dynamic friction force when the piezo is on or off (frequency: 1.272 kHz, vibration amplitude: 30.54 m, air pressure: 0 MPa).
It can be seen from the above results that there is an increasing trend for the output force as the vibration amplitude is larger. But unfortunately, the increased output force value is quite limited, which is only about 1 N, regardless of the air pressure. And the force improvement is even smaller at the longitudinal vibration mode. We can deduce that the added force comes from the reduced stiction force originated from the vibration. There is no macroscopical movement for the piston so there is no dynamic friction, which is also to be considered. Fig. 12 shows an example of the dynamic friction experiment process at a frequency of 1.272 kHz and vibration amplitude of 30.54 m without extra air pressure. The displacement of the piston and the dynamic friction force is recorded by corresponding sensors when the piezo is on or off. It is evident that the friction has a magnifying effect when the vibration disappears. An arbitrary period of motion is chosen and the forcedisplacement curves with piezo on and off are described in Fig. 13. It can be seen that the circle goes smaller and has a flattening trend. Given there is a frequent motion direction shift for the piston when the dynamic friction study is carried out. So it is crucial to keep the air pressures of the two chambers to be constant for a more accurate force measurement. Two air tanks are used to provide constant and stable air pressures to both chambers of the pneumatic actuator, as is shown in Fig. 8(c). Thus, the air tanks (20 L for each) which have a capacity of 50,000 times larger than the volume of the
Table 1 The air pressures of two chambers for a force balance. Pr is for the pressure in rod chamber, and Pn is for the pressure in no-rod chamber. No.
1
2
3
4
5
6
Pr (MPa) Pn (MPa)
0.1 0.07803
0.2 0.15606
0.3 0.23409
0.4 0.31212
0.5 0.39015
0.6 0.46818
pneumatic actuator are utilized with regulated pressure to keep the piston in a force balance condition. As described above, the cylinder has a bore diameter of 6.4 mm and the piston rod has a diameter of 3 mm. The respective pressure values are listed in Table 1. As can be seen in Fig. 14, the dynamic friction force is decreasing as the vibration amplitude increases. And the larger the air pressure is, the bigger the friction force becomes. Similarly, the friction reduction effect of the longitudinal vibration mode is not as good as that of the bending mode. The pneumatic system and the its actuating theory involves a series of complicated parameters, and nonlinear factors such as gas compressibility, humidity changes, rubber friction and wear influence each other. A variable of friction reduction ratio is defined here for an evaluation level as mentioned in [31,33,34]. It is a ratio between reduced friction force and the original value, as shown in Eq. (1). ϕ=
F0 -F F0
(1)
where F0 is the original friction, F is the friction in the presence of vibrations. Thus, the larger the ratio is, the larger the friction
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Fig. 14. The influence of vibration amplitude on dynamic friction force (peak-peak value) at different air pressure. (a) The results at bending vibration mode; (b) The results at longitudinal vibration mode.
Fig. 15. Calculated friction reduction ratio in function of the air pressure. (a) The value at bending vibration mode; (b) The value at longitudinal mode.
reduction effect is. Fig. 15 presents the test results for the friction reduction ratio in function of the air pressure. We can see from the results that the friction reduction becomes weaker as the air pressure goes higher. For the bending vibration mode, the dynamic friction force can only be reduced by about 35% at 0.6 MPa, while the value is able to reach 50% without an extra air pressure. It reveals that the vibration induced friction reduction is best to be used in pneumatic actuators at a lower pressure, at least aiming at the prototype in this research. Similarly, the effect at longitudinal vibration mode is not as good as that at bending mode and the friction reduction ratio varies between 14% and 30%. The friction reduction difference between the two vibration modes is likely due to the vibration amplitude difference. It could also depend on the vibration induced friction reduction mechanism, which is to be investigated further. Actually, the ratio difference between each vibration direction in Cartesian coordinates has been found and analyzed in [19,35]. And a more comprehensive and convincing explanation when the vibration direction is perpendicular to the contact surface is to be investigated in the future. The friction reduction condition is affected by the ratio between the vibration speed and sliding speed when the vibration and the macroscopic sliding have the same direction. Consequently, it is crucial to have investigations on the influence of friction reduction effect on the speed ratio when this pneumatic actuator works at the longitudinal vibration mode. The vibration speed peak value of the end cover surface can be calculated by |vv | = |2f0 A|, and the vibration speed vv has a value of 45 mm/s when the piezo-stacks work at a frequency of 12.133 kHz and a vibration amplitude of 0.6 m. The linear motor controller is
Fig. 16. The relation between dynamic friction peak-peak force and relative sliding speed with and without vibrations.
set at a series of reciprocating motion, whose mean sliding speed varies from 4.17 mm/s to 52.1 mm/s. The measured dynamic friction peak-peak force with relative sliding speed is illustrated in Fig. 16 with and without vibrations. It can be revealed that the dynamic friction force of the prototype increases with the relative sliding speed both when the piezo-stacks are on and off. The friction force remains stable when the sliding speed is above 40 mm/s without vibration, and the friction force with vibrations (amplitude: 0.6 m) approaches to the value of non-vibration force. The friction reduction phenomenon
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would like to focus on optimizing the pneumatic actuator design, and better understanding of the friction reduction mechanism. The use of low cost piezo actuators will also be investigated for the commercial viability of this technology.
Acknowledgements This project was funded by the China Scholarship Council (CSC). The authors would like to thank Yansong Guo and Eddy Smets for manufacturing the prototype We also thank Prof. Weishan Chen for the vibrometer and Dr. Xiaohui Yang for the vibration measurement.
References Fig. 17. Influence of the speed ratio on friction ratio.
fades away above a sliding speed of 50 mm/s, which is closed to the value of vibration speed (45 mm/s). The relation of the friction ratio (friction with vibrations divided by that without vibrations) and speed ratio (vibration speed divided by sliding speed) is studied and shown in Fig. 17. There is almost no friction reduction (friction ratio is 1) when the speed ratio is below 1. The friction reduction phenomenon appears when the speed ratio is above 1 and it has a decreasing trend when the ratio increases. Some similar conclusions were drawn in the past researches [19,20], and V. C. Kumar et al. explained this decreasing phenomenon by calculating the average friction force with a high frequency reciprocating vibration. So it can be concluded that there is a limitation for the sliding speed of the actuator prototype when it works at the longitudinal vibration mode. For instance, the friction reduction only appears when the sliding speed is smaller than the critical speed 45 mm/s for this vibration system. Two vibration modes are to be compared for the further potential application: when the prototype is excited at 1.272 kHz with a bending mode, the magnifying vibration amplitude due to the cylinder structure is much larger than the other mode. Consequently, a smaller voltage or power is enough for the bending vibration mode if the same friction reduction effect is to be accomplished from the energy conservation aspect. In a specific circumstance which is sensitive to low frequency vibration, the longitudinal mode may be preferable. 4. Conclusions and outlook A dual-mode piston type pneumatic actuator with integrated piezo stacks was developed in this research. The piezo actuators were driven at a resonant frequency of 1.272 kHz to generate a first order bending vibration mode, or driven at 12.133 kHz to get a longitudinal vibration mode for reducing friction force. The vibration had a positive effect in reducing the stiction force, but the improvement of the actuator output force is not remarkable in presence of vibrations. In the dynamic friction measurement, a positive correlation between the friction force and the vibration amplitude was found for both modes. But it was found the increasing air pressure had an effect of reducing the friction reduction phenomena. Both of the vibration modes had a function of improving the friction characteristics of the pneumatic actuator by different levels but there is a limitation of sliding speed for friction reduction when the actuator works at longitudinal vibration mode. Overall, we envisage the development of low friction pneumatic actuators to be particularly important in applications requiring high precision position control [36], as well as in miniature piston-cylinder actuators which often suffer from high friction [37,38]. In the future research, the authors
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Biographies
Han Gao was born in Harbin, China, in 1986. He received his mechanical engineering master degree from Harbin Institute of Technology in 2011. From 2012 to 2014, he was a visiting researcher in the Micro and Precision Engineering (MPE), Katholieke Universiteit Leuven, financially supported by China Scholarship Council. He is now a Ph.D student in Harbin Institute of Technology. His research interests include piezoelectric components, pneumatic technology, and tribological experiment systems.
Michaël De Volder was born in Antwerp, Belgium, in 1978. He received the M.S. and Ph.D. degrees in mechanical engineering the K. U. Leuven, Belgium, in 2002 and 2007, respectively. In 2005, he was a visiting researcher with the Precision and Intelligence Laboratory, Tokyo Institute of Technology. In 2008, he was a visiting postdoctoral researcher at the Massachusetts Institute of Technology, and the University of Michigan. Currently, he is working at University of Cambridge, United Kingdom. He was the recipient of the Iwan Akerman Award in 2008, the Robert M. Caddell prize in 2010 and the prize of the research council K. U. Tinghai Cheng was born in Mudanjiang, China, in 1983. He received the M.S. and Ph.D. degrees in mechanical engineering in Harbin Institute of Technology, China, in 2008 and 2013, respectively. Since 2013, he is an associate professor at Changchun University of Technology, China. His research interests include piezoelectric actuator and piezoelectric energy harvesting, etc.
Gang Bao was born in 1960. He received the M.S. and Ph.D. degrees in mechanical engineering in Harbin Institute of Technology, China, in 1986 and 1992, respectively. and became associate professor at Harbin Institute of Technology in 1995. Since 2004 he is a full professor. He is the dean of the Fluid Control and Automation of Harbin Institute of Technology from 2002 to 2011. He is also the dean of Pneumatic Center till now. His research activities are pneumatic components and systems, physical simulation experiment systems, and virtual assembly technology. He is a committee member of Chinese Mechanical Engineering Society and a trustee of SMC Educational Foundation. Dominiek Reynaerts was born in Tienen, Belgium, in 1963. He received his mechanical engineering degree from the Katholieke Universiteit Leuven, Belgium, in 1986. He obtained his Ph.D. degree in mechanical engineering, also from Katholieke Universiteit Leuven, Belgium, in 1995 and became assistant professor at the Katholieke Universiteit Leuven in 1997. Since 2007 he is a full professor. He is a chairman of the Dept. of Mechanical Engineering of the Katholieke Universiteit Leuven since 2008. His research activities are manufacturing and machine design with focus on precision engineering and micromechanical systems. He is a member of IEEE and Euspen.
Please cite this article in press as: H. Gao, et al., A pneumatic actuator based on vibration friction reduction with bending/longitudinal vibration mode, Sens. Actuators A: Phys. (2016), http://dx.doi.org/10.1016/j.sna.2016.10.039
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Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Research paper
A pneumatic actuator based on vibration friction reduction with bending/longitudinal vibration mode Han Gao a,b , Michaël De Volder b , Tinghai Cheng c , Gang Bao a,∗ , Dominiek Reynaerts b a b c
Harbin Institute of Technology, 150080 Harbin, China Katholieke Universiteit Leuven, 3001 Leuven, Belgium Changchun University of Technology, 130000 Changchun, China
a r t i c l e
i n f o
Article history: Received 10 July 2016 Received in revised form 22 September 2016 Accepted 27 October 2016 Available online xxx Keywords: Pneumatic actuator Vibration Friction reduction Bending vibration Longitudinal vibration
a b s t r a c t Piston–cylinder pneumatic actuators are widely applied in various fields of automation and robotics. The sealing rings comprised in these actuators unfortunately introduce friction and affect the positioning accuracy and output force. In this work, piezoelectric actuators are built in the pneumatic actuators to introduce vibrations with lower friction force. The friction reduction effect is compared between the bending vibration mode at a resonant frequency of 1.272 kHz and the longitudinal vibration mode at a frequency of 12.133 kHz. The pneumatic actuator has a bore diameter of 6.4 mm and a stroke of 13 mm. A static/dynamic friction force measurement system is established and the test results show a maximum 66.7% reduction of stiction force and a 50.8% reduction of dynamic friction force in bending vibration mode. And the friction reduction effect also happens in the longitudinal vibration mode, with a 47.4% reduction of stiction force and a 29.7% reduction of dynamic friction force. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Pneumatic actuators are commonly applied in automation and robotics [1–4] due to their high force, relatively low manufacturing cost, clean operating conditions and high reliability [5–7]. The seals used in these devices plays a non-negligible role in the overall performance of the actuators. Typical pneumatic actuators comprise rubber rings to limit leakage, but they introduce non-linear friction and stick-slip phenomenon when the cylinder operates at a low speed [8,9]. This limits for instance the positioning accuracy of piston-cylinder pneumatic actuators. There have been multiple methods to reduce friction force in piston-cylinder pneumatic actuators. For instance, dithering techniques were developed where a low amplitude and relatively high frequency periodic velocity signal is superimposed to the servo valve to improve pneumatic actuator friction characteristics [10,11]. Lubricants are improved, for instance by the addition of nano-particles [12]. Further, a new type of low friction cylinder using externally pressurized air bearings was developed to improve the friction characteristics of cylinder [13]. A multi-lobed seal was also developed for better friction characteristics in pneumatic actu-
∗ Corresponding author. Present address: Yikuang 2, 2F building, 231, Harbin, China. E-mail address: [email protected] (G. Bao).
ators [14]. Finally, new liquid seal technologies using ferrofluids [15] or surface tension [16] were developed. Alternatively, a method of reducing friction force by vibrations has attracted much attention in recent years [17,18]. It was observed that vibrating of one or two surfaces in contact dramatically alters their friction coefficient [19]. The experimental results of friction reduction induced by vibrations have been validated by a series of valuable investigations [20–25]. Most of them have some similar conclusions: the friction reduction has the best effect at resonant frequency vibration; a larger vibration amplitude produces a smaller friction force; and the vibrations from all directions to the sliding surface have friction reduction effect. Similarly, the outside barrel of pneumatic cylinder was excited by ultrasonic transducer to reduce the friction. The frictional measurements revealed a 40% reduction at 31.65 kHz [26]. Afterwards, a longitudinal vibration mode pneumatic actuator was developed by screwing a transducer onto one end of the cylinder [27]. Then, a piezo-stack was integrated onto the piston of pneumatic actuator to generate a longitudinal vibration along piston movement direction [28]. In the following research, a design which integrates two piezoelectric stacks in a self-made pneumatic cylinder is proposed. This actuator with a bore diameter of 5 mm and a stroke of 10 mm showed a 52% reduction from the original friction force at a driving frequency of 18.29 kHz with a bending vibration mode [29].
http://dx.doi.org/10.1016/j.sna.2016.10.039 0924-4247/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Gao, et al., A pneumatic actuator based on vibration friction reduction with bending/longitudinal vibration mode, Sens. Actuators A: Phys. (2016), http://dx.doi.org/10.1016/j.sna.2016.10.039
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Fig. 1. Schematic of the pneumatic actuator prototype (front view and top view).
In this paper, the design of a dual-mode pneumatic actuator is firstly presented, followed by the description of the static/dynamic friction measurement system and the operation process. Next, tribological characteristics measurements are performed and the discussion is shown on the results. 2. Actuator structure and measurement system The pneumatic actuator prototype developed in this paper mainly consists of a cylinder (aluminum) with a bore diameter of 6.4 mm, a seal house and end cover(brass), a piston and rod (rod diameter: 3 mm), two sealing rings (TRELLEBORG, NBR, OD: 6.46 mm, ID: 2.9 mm), two piezo stacks (PIEZOMECHANIK, size: 5 × 5 × 9 mm3 , max stroke: 9 m, R-frequency: 100 kHz), a flexure hinge (steel), a clamper (steel), and a pre-load bolt (steel). The H-shaped flexure hinge is used to accommodate misalignment between the piezoelectric stacks and the cylinder. This avoids stress concentration, and hence improves the lifetime. The piezo stacks are preloaded by a bolt through the cylinder, the H-stock, and the clamper. As can be seen from Fig. 1, two flat parallel surfaces are manufactured on the cylinder body, and the distance between the two flat surfaces is the same as the thickness dimension of the clamper in order for an easier assembly to the piezo stacks’ preloading. For a better view of the prototype, the separated parts and the assembly pictures are shown in Fig. 2. It has been shown from the previous references that pneumatic actuators with different vibration modes have been investigated separately, but few scholars have ever tried several modes in a single prototype. This is also one of the targets of the current research. The actuator design used in this research is using two piezo-stacks located at the base of the cylinder. As is illustrated in Fig. 3(a), a bending vibration mode as calculated by finite element simulations for the prototype can be gained when the two piezo-stacks are driven with a phase difference of , thus one of the stacks will expand while the other is shrinking. Consequently, a first order bending mode vibration happens as the clamper is fixed by four bolts, and the cylinder can be seen as a cantilever beam. In this case, the rubber rings comprised in the cylinder has a vibration
Fig. 2. (a) Each part of the prototype (before assembling); (b) Picture of the prototype assembly.
direction perpendicular to the motion of the piston. Inversely, a longitudinal vibration mode for the prototype is easily to be generated as the piezo stacks are excited with the same phase, as is shown in Fig. 3(b). And the rubber rings of the actuator have a vibration direction parallel to that of the piston motion. The frictional experiments in this paper are carried out on a tribometer, which is able to perform accurate displacement and instantaneous friction force measurements. The measurement system is mainly consisted of a voice coil linear motor (Unite Precision Mechanical Technology VCAR0436-373, with Elmo controller), a force sensor (ZHONGNUO, ZLBS-50N, resolution: 0.025 N), a laser displacement sensor (ZSY, ZLDS100-500-125, resolution: 1 m), a data acquisition card (National Instruments, USB-6211), a vibration power supply (QD-8C), a flexible coupling, a mounting block and the prototype. As is shown in Fig. 4, the functional elements of the apparatus are decoupled by utilizing a self-made flexible coupling screwed between the force sensor and the piston rod, in order to have a better elimination for the misaligned error. The motion of the piston is generated by the voice coil linear motor which allows to generate a frictionless and stable motion. The active coil of the linear actuator, which acts as the movement source is fixed to a connecting part which can slide back and forth on a guide rail, and the force sensor fixed between the connecting part and the piston rod to record the friction force. The experimental setup picture is shown in Fig. 5.
Fig. 3. (a) Bending vibration mode of the pneumatic cylinder model; (b) Longitudinal vibration mode of the pneumatic cylinder model.
Please cite this article in press as: H. Gao, et al., A pneumatic actuator based on vibration friction reduction with bending/longitudinal vibration mode, Sens. Actuators A: Phys. (2016), http://dx.doi.org/10.1016/j.sna.2016.10.039
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Fig. 4. 3D model of the experimental setup (the amplified figure indicates the structure of the flexible coupling and the prototype mounting).
Fig. 5. Picture of the experimental setup.
The displacement and force signals are recorded using a data acquisition system based on LabVIEW, and the driving signals for the piezoelectric stacks are generated by a self-developed vibration power supply. 3. Test results and discussion The actual vibration modes and corresponding vibration amplitudes are important for friction reduction. A wide bandwidth dual channel scanning vibrometer (POLYTEC: PSV-400-M2) is utilized to detect the vibration shape and vibration velocity/amplitude. Fig. 6(a) shows the first order bending vibration mode of the pro-
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totype, when two piezo stacks vibrate in anti-phase at a resonant frequency of 1.272 kHz. The inset picture shows the vibration velocity amplitude distribution at the location of end cover. Similarly, a resonant frequency of 12.133 kHz is got while two piezo stacks vibration in the same phase, and the longitudinal vibration mode is obviously found when the laser scanning surface locates on the disc-surface of the end cover, as is illustrated in Fig. 6(b). Fig. 7(a) shows the vibration amplitude as a function of the applied voltage which reaches 30 m at an input voltage peak-peak value of 60 V for the bending vibration mode, while the vibration amplitude of the measured longitudinal vibration surface reaches about 0.6 m at the same voltage. The obvious difference is due to the vibration amplitude magnification function in bending mode of the cylinder body at a much lower vibration frequency. The experimental setup consists of two regulators (SMC, IR2020-02BG, output pressure: 0.005-0.8 MPa), two air tanks (SMC, VBAT20, capacity: 20 L), a precision pressure gauge (YOKOGAWA, MT120, range: 700 kPa, resolution: 0.01 kPa), a laser displacement sensor (ZSY, ZLDS100-500-125), and a force sensor (ZHONGNUO, ZLBS-50N). First, the stiction force is measured by detecting the lowest required pressure to move the piston, as is shown in Fig. 8(a). The minimum air pressure measured by the precision pressure gauge to motivate the piston into a macroscopic movement here is the minimum starting pressure. So the product between the air pressure and the sectional area of the piston can be seen as the stiction force. Afterwards, a series of vibration amplitudes are tried to investigate the influence of vibration amplitude on the stiction force. Each test is repeated by at least five times, and the results are listed in Fig. 9 for both vibration modes. As shown in Fig. 9, the stiction force decreased up to 33% by the increasing of the vibration amplitude. These measurements are in accordance with previous studies [22,30,31]. This friction reduction phenomenon with bending vibration can be interpretted as near-field acoustic levitation [32]. There is a narrow gap between the sealing-ring and the seal house, which is hardly be seen by macroscopic view. A homogenization distribution of normal force between the friction couples happens and an acoustic levitation is generated when the bending vibration is introduced onto the actuator body. Compared to the bending vibration mode, the force reduction ratio is smaller in longitudinal vibration mode, and the stiction force can be reduced to 52.6% of its original value. Next, the output force measurements of the pneumatic actuator are carried out using the connection shown in Fig. 8(b). The measurement results are shown in Fig. 10 at different vibration amplitudes and an overpressure Pn of 0.1 MPa. Fig. 11 shows the measurement relation between output force changes and vibration amplitude at a series of air pressures.
Fig. 6. (a) The measured first order bending vibration mode of the prototype with a resonant frequency of 1.272 kHz, when the phase difference of two piezo stacks is ; (b) The longitudinal vibration mode of the prototype with a resonant frequency of 12.133 kHz, when the phase difference is zero.
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Fig. 7. (a) The relation between the vibration amplitude and the input voltage peak-peak value at the bending vibration mode; (b) The relation between the vibration amplitude and the input voltage peak-peak value at the longitudinal vibration mode.
Fig. 8. Schematic of the pneumatic system (a) The measurement system for stiction force; (b) The measurement system for output force; (c) The measurement system for dynamic force.
Fig. 9. The experiment results to reveal the relation between minimum starting pressure/stiction force and vibration amplitude. (a) The results for bending vibration mode; (b) The results for longitudinal vibration mode.
Fig. 10. The influence of vibration amplitude on the pneumatic actuator output force at a no-rod chamber air pressure Pn of 0.1 MPa. (a) The measurement results at bending vibration mode; (b) The measurement results at longitudinal vibration mode.
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Fig. 11. The influence of vibration amplitude on the pneumatic actuator output force changes at different air pressures. (a) The measurement results at bending vibration mode; (b) The measurement results at longitudinal vibration mode.
Fig. 13. The dynamic friction force as a function of the displacement in a period of motion. The outer circle indicates the one with piezo off, while the inner one indicates piezo on (frequency: 1.272 kHz, vibration amplitude: 30.54 m, air pressure: 0 MPa). Fig. 12. The recorded displacement of the piston and the dynamic friction force when the piezo is on or off (frequency: 1.272 kHz, vibration amplitude: 30.54 m, air pressure: 0 MPa).
It can be seen from the above results that there is an increasing trend for the output force as the vibration amplitude is larger. But unfortunately, the increased output force value is quite limited, which is only about 1 N, regardless of the air pressure. And the force improvement is even smaller at the longitudinal vibration mode. We can deduce that the added force comes from the reduced stiction force originated from the vibration. There is no macroscopical movement for the piston so there is no dynamic friction, which is also to be considered. Fig. 12 shows an example of the dynamic friction experiment process at a frequency of 1.272 kHz and vibration amplitude of 30.54 m without extra air pressure. The displacement of the piston and the dynamic friction force is recorded by corresponding sensors when the piezo is on or off. It is evident that the friction has a magnifying effect when the vibration disappears. An arbitrary period of motion is chosen and the forcedisplacement curves with piezo on and off are described in Fig. 13. It can be seen that the circle goes smaller and has a flattening trend. Given there is a frequent motion direction shift for the piston when the dynamic friction study is carried out. So it is crucial to keep the air pressures of the two chambers to be constant for a more accurate force measurement. Two air tanks are used to provide constant and stable air pressures to both chambers of the pneumatic actuator, as is shown in Fig. 8(c). Thus, the air tanks (20 L for each) which have a capacity of 50,000 times larger than the volume of the
Table 1 The air pressures of two chambers for a force balance. Pr is for the pressure in rod chamber, and Pn is for the pressure in no-rod chamber. No.
1
2
3
4
5
6
Pr (MPa) Pn (MPa)
0.1 0.07803
0.2 0.15606
0.3 0.23409
0.4 0.31212
0.5 0.39015
0.6 0.46818
pneumatic actuator are utilized with regulated pressure to keep the piston in a force balance condition. As described above, the cylinder has a bore diameter of 6.4 mm and the piston rod has a diameter of 3 mm. The respective pressure values are listed in Table 1. As can be seen in Fig. 14, the dynamic friction force is decreasing as the vibration amplitude increases. And the larger the air pressure is, the bigger the friction force becomes. Similarly, the friction reduction effect of the longitudinal vibration mode is not as good as that of the bending mode. The pneumatic system and the its actuating theory involves a series of complicated parameters, and nonlinear factors such as gas compressibility, humidity changes, rubber friction and wear influence each other. A variable of friction reduction ratio is defined here for an evaluation level as mentioned in [31,33,34]. It is a ratio between reduced friction force and the original value, as shown in Eq. (1). ϕ=
F0 -F F0
(1)
where F0 is the original friction, F is the friction in the presence of vibrations. Thus, the larger the ratio is, the larger the friction
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Fig. 14. The influence of vibration amplitude on dynamic friction force (peak-peak value) at different air pressure. (a) The results at bending vibration mode; (b) The results at longitudinal vibration mode.
Fig. 15. Calculated friction reduction ratio in function of the air pressure. (a) The value at bending vibration mode; (b) The value at longitudinal mode.
reduction effect is. Fig. 15 presents the test results for the friction reduction ratio in function of the air pressure. We can see from the results that the friction reduction becomes weaker as the air pressure goes higher. For the bending vibration mode, the dynamic friction force can only be reduced by about 35% at 0.6 MPa, while the value is able to reach 50% without an extra air pressure. It reveals that the vibration induced friction reduction is best to be used in pneumatic actuators at a lower pressure, at least aiming at the prototype in this research. Similarly, the effect at longitudinal vibration mode is not as good as that at bending mode and the friction reduction ratio varies between 14% and 30%. The friction reduction difference between the two vibration modes is likely due to the vibration amplitude difference. It could also depend on the vibration induced friction reduction mechanism, which is to be investigated further. Actually, the ratio difference between each vibration direction in Cartesian coordinates has been found and analyzed in [19,35]. And a more comprehensive and convincing explanation when the vibration direction is perpendicular to the contact surface is to be investigated in the future. The friction reduction condition is affected by the ratio between the vibration speed and sliding speed when the vibration and the macroscopic sliding have the same direction. Consequently, it is crucial to have investigations on the influence of friction reduction effect on the speed ratio when this pneumatic actuator works at the longitudinal vibration mode. The vibration speed peak value of the end cover surface can be calculated by |vv | = |2f0 A|, and the vibration speed vv has a value of 45 mm/s when the piezo-stacks work at a frequency of 12.133 kHz and a vibration amplitude of 0.6 m. The linear motor controller is
Fig. 16. The relation between dynamic friction peak-peak force and relative sliding speed with and without vibrations.
set at a series of reciprocating motion, whose mean sliding speed varies from 4.17 mm/s to 52.1 mm/s. The measured dynamic friction peak-peak force with relative sliding speed is illustrated in Fig. 16 with and without vibrations. It can be revealed that the dynamic friction force of the prototype increases with the relative sliding speed both when the piezo-stacks are on and off. The friction force remains stable when the sliding speed is above 40 mm/s without vibration, and the friction force with vibrations (amplitude: 0.6 m) approaches to the value of non-vibration force. The friction reduction phenomenon
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would like to focus on optimizing the pneumatic actuator design, and better understanding of the friction reduction mechanism. The use of low cost piezo actuators will also be investigated for the commercial viability of this technology.
Acknowledgements This project was funded by the China Scholarship Council (CSC). The authors would like to thank Yansong Guo and Eddy Smets for manufacturing the prototype We also thank Prof. Weishan Chen for the vibrometer and Dr. Xiaohui Yang for the vibration measurement.
References Fig. 17. Influence of the speed ratio on friction ratio.
fades away above a sliding speed of 50 mm/s, which is closed to the value of vibration speed (45 mm/s). The relation of the friction ratio (friction with vibrations divided by that without vibrations) and speed ratio (vibration speed divided by sliding speed) is studied and shown in Fig. 17. There is almost no friction reduction (friction ratio is 1) when the speed ratio is below 1. The friction reduction phenomenon appears when the speed ratio is above 1 and it has a decreasing trend when the ratio increases. Some similar conclusions were drawn in the past researches [19,20], and V. C. Kumar et al. explained this decreasing phenomenon by calculating the average friction force with a high frequency reciprocating vibration. So it can be concluded that there is a limitation for the sliding speed of the actuator prototype when it works at the longitudinal vibration mode. For instance, the friction reduction only appears when the sliding speed is smaller than the critical speed 45 mm/s for this vibration system. Two vibration modes are to be compared for the further potential application: when the prototype is excited at 1.272 kHz with a bending mode, the magnifying vibration amplitude due to the cylinder structure is much larger than the other mode. Consequently, a smaller voltage or power is enough for the bending vibration mode if the same friction reduction effect is to be accomplished from the energy conservation aspect. In a specific circumstance which is sensitive to low frequency vibration, the longitudinal mode may be preferable. 4. Conclusions and outlook A dual-mode piston type pneumatic actuator with integrated piezo stacks was developed in this research. The piezo actuators were driven at a resonant frequency of 1.272 kHz to generate a first order bending vibration mode, or driven at 12.133 kHz to get a longitudinal vibration mode for reducing friction force. The vibration had a positive effect in reducing the stiction force, but the improvement of the actuator output force is not remarkable in presence of vibrations. In the dynamic friction measurement, a positive correlation between the friction force and the vibration amplitude was found for both modes. But it was found the increasing air pressure had an effect of reducing the friction reduction phenomena. Both of the vibration modes had a function of improving the friction characteristics of the pneumatic actuator by different levels but there is a limitation of sliding speed for friction reduction when the actuator works at longitudinal vibration mode. Overall, we envisage the development of low friction pneumatic actuators to be particularly important in applications requiring high precision position control [36], as well as in miniature piston-cylinder actuators which often suffer from high friction [37,38]. In the future research, the authors
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Biographies
Han Gao was born in Harbin, China, in 1986. He received his mechanical engineering master degree from Harbin Institute of Technology in 2011. From 2012 to 2014, he was a visiting researcher in the Micro and Precision Engineering (MPE), Katholieke Universiteit Leuven, financially supported by China Scholarship Council. He is now a Ph.D student in Harbin Institute of Technology. His research interests include piezoelectric components, pneumatic technology, and tribological experiment systems.
Michaël De Volder was born in Antwerp, Belgium, in 1978. He received the M.S. and Ph.D. degrees in mechanical engineering the K. U. Leuven, Belgium, in 2002 and 2007, respectively. In 2005, he was a visiting researcher with the Precision and Intelligence Laboratory, Tokyo Institute of Technology. In 2008, he was a visiting postdoctoral researcher at the Massachusetts Institute of Technology, and the University of Michigan. Currently, he is working at University of Cambridge, United Kingdom. He was the recipient of the Iwan Akerman Award in 2008, the Robert M. Caddell prize in 2010 and the prize of the research council K. U. Tinghai Cheng was born in Mudanjiang, China, in 1983. He received the M.S. and Ph.D. degrees in mechanical engineering in Harbin Institute of Technology, China, in 2008 and 2013, respectively. Since 2013, he is an associate professor at Changchun University of Technology, China. His research interests include piezoelectric actuator and piezoelectric energy harvesting, etc.
Gang Bao was born in 1960. He received the M.S. and Ph.D. degrees in mechanical engineering in Harbin Institute of Technology, China, in 1986 and 1992, respectively. and became associate professor at Harbin Institute of Technology in 1995. Since 2004 he is a full professor. He is the dean of the Fluid Control and Automation of Harbin Institute of Technology from 2002 to 2011. He is also the dean of Pneumatic Center till now. His research activities are pneumatic components and systems, physical simulation experiment systems, and virtual assembly technology. He is a committee member of Chinese Mechanical Engineering Society and a trustee of SMC Educational Foundation. Dominiek Reynaerts was born in Tienen, Belgium, in 1963. He received his mechanical engineering degree from the Katholieke Universiteit Leuven, Belgium, in 1986. He obtained his Ph.D. degree in mechanical engineering, also from Katholieke Universiteit Leuven, Belgium, in 1995 and became assistant professor at the Katholieke Universiteit Leuven in 1997. Since 2007 he is a full professor. He is a chairman of the Dept. of Mechanical Engineering of the Katholieke Universiteit Leuven since 2008. His research activities are manufacturing and machine design with focus on precision engineering and micromechanical systems. He is a member of IEEE and Euspen.
Please cite this article in press as: H. Gao, et al., A pneumatic actuator based on vibration friction reduction with bending/longitudinal vibration mode, Sens. Actuators A: Phys. (2016), http://dx.doi.org/10.1016/j.sna.2016.10.039