A piezo driven flapping wing mechanism for micro air vehicles

Transcription

1 DOI.7/s TECHNICAL PAPER A piezo driven flapping wing mechanism for micro air vehicles Yuxin Peng Jie Cao,2 Li Liu 3 Haoyong Yu Received: 24 September 25 / Accepted: 8 December 25 Springer-Verlag Berlin Heidelberg 26 Abstract In this paper, a novel flapping wing mechanism driven by a linear actuator is proposed. The linear actuator consists of a piezoelectric element and a permanent magnet. Based on the principle of impact drive mechanism, the linear actuator can move a reciprocating linear motion with a long travel range, which can be directly converted to a flapping motion via a crank-slider mechanism. In comparison with conventional flapping wing mechanisms driven by a rotary motor with a crank-rocker mechanism, no gearbox is needed in our design. Therefore, the proposed flapping wing mechanism can be made with small volume and light weight. A prototype was designed and constructed and experiments were carried out to test the performance of the flapping wing mechanism. The experimental results confirm that the designed flapping wing mechanism can obtain a continuous flapping motion by moving the linear actuator reciprocatingly. Introduction Micro air vehicles (MAVs) are centimeter scale flying robots with a number of applications, including search, rescue, exploration, and reconnaissance (Karpelson et al. 28; Yan et al. 25; Anderson et al. 2; Conn et al. * Haoyong Yu bieyhy@nus.edu.sg 2 3 Department of Biomedical Engineering, National University of Singapore, Singapore 7575, Singapore School of Optoelectronics, Beijing Institute of Technology, Beijing 8, China School of Software Engineering, Chongqing University, Chongqing 433, China 26). Flapping wing MAVs (FWMAVs) are a class of new conceptual MAVs that take design cues from flying insects in order to achieve a small size (<3 5 cm), high maneuverability and super remote ability (Yan et al. 25). A FWMAV is expected to work in a variety of confined spaces such as urban canyons, caves, indoors, and jungles that large flying robots can t be serviceable. To achieve such tasks, FWMAVs must be designed with small volume and light weight (Karpelson et al. 28; Yan et al. 25; Anderson et al. 2; Conn et al. 26). The component most responsible for size and weight of FWMAVs is the actuation mechanism for flapping motion. Therefore, selection of an actuation scheme is one of the critical challenges in FWMAV design. Generally, there are two major categories for wing flapping actuators: rotary and linear (Anderson et al. 2; Conn et al. 26). The most popular rotary actuators used in FWMAVs are DC electric motors (Nguyen et al. 24; De Croon et al. 29; Conn et al. 27; Khan et al. 29). However, FWMAVs driven by DC electric motors always require gearboxes to reduce the motor speed to an appropriate level. Therefore, the additional gearboxes and smaller size limitation of the motor seriously hinder the potential for a reduction in scale for FWMAVs. To avoid size limitations of FWMAVs, linear actuators are thought to be a good choice due to their simple design and low weight (Anderson et al. 2; Conn et al. 26). The most popular linear actuators include electroactive polymers (EAP), shape memory alloy (SMA), solenoid, and piezoelectric elements (PZTs). However, an EAP actuator always requires large voltages (over V) and the power electronics for generating such a large voltage from a 5 V battery are large and heavy. SMAs and solenoids are limited by their low bandwidth and cannot operate fast enough to drive FWMAVs. Finally, PZTs are considered to be suitable for miniaturization of FWMAV

2 Wings Rotary actuator Thorax Crank-rocker mechanism Muscles Cross section cut-away of an insect thorax Simplified model Linear actuator Crank-slider mechanism Fig. Simplified model of the insect flight mechanism design due to their merits of small size, fast response, high bandwidth, and relatively large force output. However, the total displacement of a PZT is extremely small (several micrometers) and few FWMAVs using linear PZTs can be found in the literature. One the other hand, numerous methods have been proposed to amplify the small displacement of a PZT (Roberts 999; Ma et al. 24; Peng et al. 2, 23; Shimizu et al. 23; Higuchi et al. 99; Morita et al. 22). The most efficient driving methods are clamping and feeding mechanisms and friction drive mechanisms. In the clamping and feeding mechanisms represented by the inchworm drive method (Roberts 999; Ma et al. 24), the moving element is driven over a long travel range by repeating sets of clamping and feeding motions of a number of PZTs. However, the mechanism is hard to miniaturize. The discontinuous clamping operation and relatively large volume also reduce the actuation frequency of the mechanism. In comparison with the inchworm drive method, the friction drive mechanisms represented by the impact drive mechanism (IDM) has simpler construction, higher actuation frequency and much more compact size (Higuchi et al. 99; Peng et al. 2), which might be suitable for miniaturization of FWMAVs. The motivation of this research is to develop a novel flapping wing mechanism driven by a linear actuator so as to reduce the volume and weight of the mechanical components of FWMAVs. The proposed linear actuator is only composed of a PZT and a permanent magnet. Based on the principle of IDM, the actuator can be driven with a long travel range. Then, a crank-slider mechanism is used to convert the linear oscillation of the actuator into a flapping motion. Therefore, due to the small size of the actuator and no gearbox used in the mechanism, the flapping wing mechanism can be designed with a simple structure as well as a compact size. Design and principle of the flapping wing mechanism together with experimental results are presented. 2 Principle and design of the flapping wing mechanism 2. Design of the flapping wing mechanism A simplified model of the insect flight mechanism is shown in Fig.. The elevation and depression of the wing is produced by alternately contraction and relaxation of insect muscles, and wing rotation is induced at the wing hinge. Therefore, the biomimetic mechanism of insects can be simplified as a four-bar linkage. The most-used four-bar linkage variations in flapping wing mechanisms are crankrocker mechanisms and crank-slider mechanisms. Rotary actuator driven FWMAVs always use the former to transmit the rotary motion to a reciprocating flapping motion. However, the scheme requires additional gearboxes to reduce the actuator speed and therefore increases the volume and weight of the system. For simper construction, the crank-slider mechanism is employed in our design. A linear actuator is employed as the slider to generate reciprocating linear motion directly, which mimics the contraction and relaxation of insect muscles. Therefore, due to no gearbox used in the system, it is expected that that the flapping wing mechanism can be constructed with small volume and light weight. Figure 2 shows the schematic design of the flapping wing mechanism with a crank-slider mechanism. As shown in Fig. 2a, one slider is employed to drive the crank-slider mechanism with a reciprocating linear motion, rendering a flapping motion of the flapping wings. The geometry of the flapping mechanism is chosen by the expected displacement of the linear motion and the desired wing

3 Link Flapping wings L 3 Flapping wing PZT L 2 Slider (Movable frame) L Steel plate Slider Base Linear actuator Base Link Permanent magnet Fig. 2 Schematic design of the flapping wing mechanism. a Crankslider mechanism, b layout of the flapping wing mechanism motion. The length L of the slider is designed to be 9 mm, and the length L 2 of the link is 5 mm. The length of the transmission part of the flapping wing, L 3, is designed to be 3.5 mm. Figure 2b shows the design of the flapping wing mechanism. The whole mechanism consists of a movable frame, two links, two flapping wings, a base with a steel plate, and a linear actuator. The movable frame is employed as the slider to convert the linear actuator motion to a flapping motion of the flapping wings through the links. The flapping wings are designed with some holes in order to mount potential fabricated wings in the future. The steel plate is glued on a guide way of the base. The linear actuator, which is composed of a PZT and a permanent magnet, is attached to the steel plate by the magnetic force. Based on the principle of IDM, the linear actuator can generate a reciprocating linear motion along the guide way. The length of the guide way is design to be 4.63 mm, which corresponds to a maximum flapping stroke angle of 95 in the mechanism. 2.2 Driving principle of the flapping wing mechanism The driving principle of the flapping wing mechanism can be illustrated in Fig. 3. As shown in Fig. 3a, the linear actuator is driven based on the principle of IDM. In the forward motion, the PZT is driven by a saw-tooth wave voltage of slow increase and rapid decrease. When the PZT expands slowly, the movable frame moves forward while the permanent magnet keeps stationary due to the static friction. When the PZT shrinks quickly to generate an impulsive force and the permanent magnet gets the momentum of the movable frame. At this time, the inertial force generated by the movable frame exceeds the maximum static friction force of the permanent magnet. Consequently, the permanent magnet moves forward against static friction. By repeating these steps, the movable frame can move forward in infinite distance continuously. The backward motion can be obtained by reversing the sequence of extension and contraction of the PZT. Therefore, reciprocating linear motion can be obtained by the linear actuator based on IDM, which can be subsequently converted to a flapping motion of the flapping wing mechanism. The drive characteristics of the linear actuator are dependent on the parameters of the saw-tooth voltage applied to the PZT, including the driving frequency, amplitude and duty ratio defined as Duty ratio = t T % where T and t are the cycle and the rise time of the sawtooth voltage waveform, respectively (see Fig. 3b). A constructed prototype of the flapping wing mechanism is shown in Fig. 4a. A PZT with a size of 4.5 mm (L) 3.5 mm (W) 5 mm (H) is employed to connect the movable frame and the permanent magnet. The size of the permanent magnet is 4 mm (L) 4 mm (W).5 mm (H). A steel plate, which is made of SUS43, is equipped to the guide way of the base. By utilizing the magnetic force due to the permanent magnet, the linear actuator can be held by the base with a constant contact force. The other parts of the flapping wing mechanism are made of plastic by 3D printing. Figure 4b shows the weight breakdown of the flapping wing mechanism in gram and percentage. It can be seen that the flapping wing mechanism has a small size and a weight as light as.34 g. ()

4 Forward motion: Backward motion: Linear actuator (i) Initial position (i) Initial position (ii) Slow extension (ii) Slow contraction (iii) Rapid contraction (iii) Rapid extension Slow increase Rapid decrease Rapid increase Slow decrease t T Forward motion Backward motion Fig. 3 Driving principle of the flapping wing mechanism. a Driving principle of the linear actuator based on IDM, b applied voltage to the PZT 3 Experiments In order to investigate the frequency response of the linear actuator, an experimental setup was established as shown in Fig. 5. The movable frame was fixed by a jig. A sine wave with a peak-to-peak voltage of 5 V was swept to the PZT from Hz to 5 khz. The amplitude of PZT displacement was measured by an optical fiber displacement sensor (MTI instruments, MTI-2). Figure 6 shows the dynamic characteristics of the linear actuator. The resonant frequency of the driving unit was found to be approximately 9.2 khz, which indicated that the linear actuator should be operated at a lower frequency. To investigate the performance of the flapping wing mechanism, an experimental setup was established as shown in Fig. 7. The base of the mechanism was fixed by a jig. Since the flapping angle of the mechanism could be calculated by the linear displacement of the movable frame, only the linear displacement of the movable frame was measured in our experiments. A thin mirror was fixed on the movable frame as the target for displacement measurement. Therefore, the linear displacement of the movable frame could be measured by a laser displacement sensor (Keyence LK-H52). To investigate the relationship between the frequency of the PZT voltage and the mean velocity of the movable frame, the amplitude of the voltage was set to 3 V and the duty ratios were set to 8 and 2 %, respectively. It can be seen from Fig. 8 that it was not until 3 khz that the movable frame began to move. It was thought that the frequency <3 khz was not enough to overcome the static friction force between the permanent magnet and the steel plate. The movable frame began to move when the driving frequency became larger than 3 khz. It indicates that the impact force generated by the PZT became large enough to move the permanent magnet. It can be seen that the

5 .5 Amplitude µm khz Frequency khz Fig. 6 Dynamic characteristics of the linear actuator.2 (9%).6 (5%).6 (2%). (7%).27 (2%).26 (9%).37 (28%) Movable frame Base PZT Permanent magnet Links Flapping wings Pins Fig. 4 Fabricated flapping wing mechanism. a Prototype of the flapping wing mechanism, b weight breakdown of flapping wing mechanism in gram and percentage Fig. 7 Experimental setup for the flapping wing mechanism Movable frame Displacement Optical fiber displacement sensor 6 4 8% 2% PZT Permanent magnet Fig. 5 Experimental setup for the frequency response of the linear actuator velocities in the forward motion and backward motion of the movable frame were almost the same, which could be used for generating a reciprocating linear motion for the movable frame. The maximum velocities were observed to be 3.8 and 3.6 mm/s at 7 khz when the duty ratios were set to 8 and 2 %, respectively. Figure 9 shows the relationship between the amplitude of the PZT voltage and the velocity of the movable frame. The driving frequency was set as 7 khz and the duty ratio was set to 8 %. It was observed that when the voltage was lower than 6 V, the movable frame didn t move because the impact force of the PZT was Velocity mm/s Frequency khz Fig. 8 Relationship between frequency of the PZT voltage and velocity of the movable frame at different duty ratios insufficient to drive the movable frame at low voltage. When the voltage was increased from 6 to 6 V, the velocity increased linearly with the increasing of the amplitude of driving voltage, and reached the maximum velocity of 8.3 mm/s at 6 V. The results can be used for controlling the velocity of the movable frame and the

6 .5 Velocity mm/s Voltage V Fig. 9 Relationship between amplitude of the PZT voltage and velocity of the movable frame Velocity mm/s Displacement mm Displacement mm Time s Duty ratio % Fig. Relationship between duty ratio of the PZT voltage and velocity of the movable frame corresponding flapping frequency of the flapping wing mechanism. The relationship between the duty ratio of the PZT voltage and the velocity of the movable frame was also investigated when the driving frequency was set to 7 khz and the voltage was set to 6 V. As shown in Fig., a symmetric curve was obtained with respect to the center that was at the duty ratio of 5 %. It can be seen that the velocity of the movable frame was approximately zero when the duty ratio was 5 %, which means that flapping wing mechanism remained stationary. The movable frame moved to the positive direction when the duty ratio became >5 % and reached the maximum velocity of.4 mm/s when the duty ratio was 7 %. On the other hand, the movable frame moved to the opposite direction when the duty ratio was <5 % and reached the maximum velocity of.4 mm/s when the duty ratio was 3 %. The results can be utilized for controlling the moving direction of the movable frame. It was also observed that the velocity decreased when the duty ratio became >7 or <3 %, which was due to the relatively large distortions Time s Fig. Reciprocating linear motion of the movable frame. a Driven by a -cycle voltage with the duty ratio of 7 % and a -cycle voltage with the duty ratio of 3 % alternately, b driven by a 2- cycle voltage with the duty ratio of 7 % and a 2-cycle voltage with the duty ratio of 3 % alternately of the PZT displacement waveforms when the duty ratio reached a certain value. Finally, the flapping motion of the flapping wing mechanism was investigated when a reciprocating linear motion was induced by the linear actuator. As shown in Fig. a, the applied voltage of the PZT was set to 6 V and the frequency was set to 7 khz. To achieve a reciprocating linear motion, the linear actuator was driven by cycles of a saw-tooth voltage with the duty ratio of 7 %, followed by cycles of a saw-tooth voltage with the duty ratio of 3 %. By repeating these operations, the movable frame can be driven with a reciprocating linear motion, which can be converted to a flapping motion of the flapping wings. It was observed that the amplitude of the periodic reciprocating linear motion was.68 mm, which could be converted to a maximum flapping angle of 26 in our design. The flapping frequency was calculated to be 8.5 Hz.

7 A 2-cycle voltage with the duty ratio of 7 % and a 2-cycle voltage with the duty ratio of 3 % were also applied to the PZT alternately. The applied voltage of the PZT was set to 6 V and the frequency was set to 7 khz, which could obtain a maximum velocity of the linear actuator. The reciprocating linear motion of the movable frame is shown in Fig. b. The amplitude of the periodic reciprocating linear motion was observed to be.35 mm, which corresponded to a maximum flapping angle of 4 in our mechanism. In addition, due to larger displacement generated by the linear actuator, the flapping frequency reduced to 4.25 Hz correspondingly. It should be noted that higher flapping frequency could be obtained by improving the amplitude of the applied voltage. 4 Conclusions In this research, a novel piezo-driven flapping wing mechanism for micro air vehicles was proposed and developed, with the goal of achieving light weight in a small size. The flapping wing mechanism could be driven by a linear actuator which consisted of a PZT and a permanent magnet. Based on the principle of IDM, the linear actuator could move with a long range of reciprocating linear motion, which could be converted to a continuous flapping motion via a crank-slider mechanism. Due to the simple transmission without any gearboxes, the flapping wing mechanism was made with light weight and small volume. The performance of the flapping wing mechanism has been also investigated experimentally. It was confirmed that the linear actuator could achieve the maximum velocity of.4 and.4 mm/s at 7 khz and 6 V when the duty ratios were set to 7 and 3 %, respectively. Finally, the flapping motion performance of the flapping wing mechanism was investigated when a number of cycles of voltage with the duty ratios of 7 and 3 % were applied to the linear actuator alternately. The experimental results verified the feasibility of the proposed flapping wing mechanism. Further miniaturization of the flapping wing mechanism as well as experimental verification after mounting fabricated wings will be carried out as our future work. References In: Proceedings of the 49th AIAA aerospace sciences meeting, 2. pp Conn A, Burgess S, Hyde R, Ling CS (26) From natural flyers to the mechanical realization of a flapping wing micro air vehicle. In: IEEE international conference on robotics and biomimetics, 26. ROBIO 6, 26. IEEE, pp Conn A, Burgess S, Ling C (27) Design of a parallel crank-rocker flapping mechanism for insect-inspired micro air vehicles. Proc Inst Mech Eng C J Mech Eng Sci 22:2 222 De Croon GCHE, De Clercq KME, Ruijsink R, Remes B, De Wagter C (29) Design, aerodynamics, and vision-based control of the DelFly. Int J Micro Air Veh :7 97 Higuchi T, Yamagata Y, Furutani K, Kudoh K (99) Precise positioning mechanism utilizing rapid deformations of piezoelectric elements. In: Micro electro mechanical systems, 99. Proceedings, an investigation of micro structures, sensors, actuators, machines and robots. IEEE, 99. IEEE, pp Karpelson M, Wei GY, Wood RJ (28) A review of actuation and power electronics options for flapping-wing robotic insects. In: IEEE international conference on robotics and automation, 28. ICRA 28. IEEE, pp Khan Z, Steelman K, Agrawal S (29) Development of insect thorax based flapping mechanism. In: IEEE international conference on robotics and automation, 29. ICRA 9, 29. IEEE, pp Ma L, Xiao J, Zhou S, Sun L (24) A piezoelectric inchworm actuator of linear type using symmetrical lever amplification. Proc Inst Mech Eng N J Nanoeng Nanosyst Morita T, Yoshida R, Okamoto Y, Higuchi T (22) Three DOF parallel link mechanism utilizing smooth impact drive mechanism. Precis Eng 26: Nguyen QV, Chan WL, Debiasi M (24) Design, fabrication, and performance test of a hovering-based flapping-wing micro air vehicle capable of sustained and controlled flight. In: IMAV 24: international micro air vehicle conference and competition 24, pp 8 25 Peng Y, Kaneko J, Arai Y, Shimizu Y, Gao W, Okamoto K, Chiba M, Aisawa S (2) A linear micro-stage with a long stroke for precision positioning of micro-objects. Nanotechnol Precis Eng 9: Peng Y, Ito S, Sakurai Y, Shimizu Y, Gao W (23) Construction and verification of a linear-rotary microstage with a millimeter-scale range. Int J Precis Eng Manuf 4: Roberts D (999) Development of a linear piezoelectric motor based on the inchworm model. In: Proceedings of the 999 Symposium on smart structures and materials, 999. International society for optics and photonics, pp Shimizu Y, Peng Y, Kaneko J, Azuma T, Ito S, Gao W, Lu T-F (23) Design and construction of the motion mechanism of an XY micro-stage for precision positioning. Sens Actuators A Phys 2: Yan X, Qi M, Lin L (25) Self-lifting artificial insect wings via electrostatic flapping actuators. In: Micro electro mechanical systems (MEMS) 25 28th IEEE international conference on, 25. IEEE, pp Anderson ML, Sladek NJ, Cobb RG (2) Design, fabrication, and testing of an insect-sized MAV wing flapping mechanism.