802 Dang Bao Lam, Dinh Khac Toan, Nguyen Tuan Khoa, Pham Hong Phuc, Vu Ngoc Hung VCM2012 Mô tơ quay hai chiều sử dụng kết hợp các bộ chấp hành tĩnh điện - nhiệt điện A novel bi-directional micro rotational motor based on a combination of the electrostatic and electrothermal actuators Dang Bao Lam, Dinh Khac Toan, Nguyen Tuan Khoa, Pham Hong Phuc, Vu Ngoc Hung Hanoi University of Science and Technology Email: baolam@mail.hut.edu.vn Abstract: This paper presents a novel design of a bi-directional micro rotational motor (MRM) driven by eight driving electrostatic comb-drive actuator (ECA) and four anti-reverse Chevron electrothermal actuators (CEA). To rotate clockwise, the motor utilizes four ECAs and two anti-reverse CEAs. Two pairs of centrally symmetrical CEAs can be used for changing the rotating direction of the MRM as well as for anti-reverse purpose. Simple design with the standard electrostatic and electrothermal actuators located inside of the rotor ring allows the MRM can relatively easily transmit rotating motion to other micro mechanisms. Another advantage is that the MRM can be fabricated with only one single mask. Acquired calculated and simulated results points out that the proposed model of MRM could run properly and is ready for the next steps – fabrication and test of the micro rotational motor. Tóm tắt: Trong bài báo này các tác giả đề xuất phương án thiết kế micro mô tơ quay hai chiều được dẫn động bằng tám bộ chấp hành kiểu tĩnh điện răng lược và bốn bộ chấp hành nhiệt dạng chữ V. Bốn trong số tám bộ chấp hành tĩnh điện dùng để dẫn động vành rô tơ quay thuận chiều kim đồng hồ, số còn lại được sử dụng khi đổi chiều quay. Bằng cách bố trí thích hợp theo từng cặp, các bộ chấp hành nhiệt dạng chữ V đóng vai trò cơ cấu chống đảo, đồng thời cho phép động cơ có khả năng đổi chiều quay. Với thiết kế đơn giản cùng các bộ chấp hành tĩnh điện và nhiệt tiêu chuẩn được bố trí nằm trong vành rô tơ, mô tơ có ưu điểm là chỉ cần một mặt nạ để chế tạo cũng như có thể dễ dàng tích hợp với các cơ cấu bên ngoài để truyền chuyển động. Các kết quả tính toán và mô phỏng thu được cho thấy micro mô tơ được thiết kế có thể hoạt động tốt, đủ điều kiện để tiếp tục chế tạo, đo đạc và thử nghiệm. Keywords: micro rotational motor, electrostatic actuator, electrothermal actuator Abbreviation: MRM Micro Rotational Motor ECA Electrostatic Comb-drive Actuator CEA Chevron Electrothermal Actuator GCA Gap-Closing Actuator 1. Introduction Micro motors play an vital role in the micro robot systems as well as in the other MEMS devices such as micropump, microcooler, micro total analysis system (TAS), etc. The micro motors can be mainly clasiffied into electrostatic, electrothermal, piezoelectric and electromagnetic according to the actuation method. Among them, the motors with electrothermal actuators consume more power than electrostatic and electromagnetic ones, but they provide higher forces and need lower driving voltages [1, 2]. The motors with piezoelectric actuators have relatively higher energy efficiency but are more difficult to fabricate [3]. The electrostatic motors are not only more efficient than those with the electrothermal or electromagnetic actuators but also are easier to fabricate and integrate with other silicon structures than piezoelectric actuators [4]. To create rotating motion, Garcia and Sniegowski [5, 6] arranged electrostatic comb actuators in two perpendicular X-drive and Y- drive sets, so that linear motion from actuators can be transmitted to a rotor through linkage mechanisms. The inchworm mode of operation is also widely applied in design of rotational micro motors. Sammoura presented an inchworm motor using four electrostatic gap- closing actuators (GCA) to rotate a free moving rotor [7]. With the same actuation method, Kim S.C. and Kim S.H. demonstrated a rotary motor [8], in which the rotor is wrapped by two Tuyển tập công trình Hội nghị Cơ điện tử toàn quốc lần thứ 6 803 Mã bài: 171 opposing belts which are connected to piezoelectric actuators. Geisberger et al. used four electrothermal bent-beam actuators with contact shoes and brakes to generate rotation of a rotor [9]. The majority of those motors have their actuators arranged outside of the rotor, hence this configuration can cause difficulties with transmission rotary movement from the motors to the driven mechanisms. To overcome this shortcoming, the authors presented a rotational motor, in which four electrostatic comb-drive actuators (ECA) and ratchet mechanisms are used to rotate an outer rotor ring [10]. However, the motor can only generate uni-directional movement. In this paper, the authors propose a novel design of a bi-directional rotational micro motor with eight driving ECAs and four anti-reverse electrothermal actuators (ETA). Working in pairs as a directional switch, the ETAs help the motor to rotate clockwise and counter- clockwise. 2. Configuration and working principle F. 2 Movement-transmission mechanism Initial position Working position 13 14 10 11 F.1 Configuration of the first bi - directional rotational micro motor 10 12 11 Elastic point 7 804 Dang Bao Lam, Dinh Khac Toan, Nguyen Tuan Khoa, Pham Hong Phuc, Vu Ngoc Hung VCM2012 Figure 1 shows the configuration of the bi- directional MRM. This configuration consists of four driving units (1), (2), (3), (4), which are responsible for generating clockwise or counter- clockwise motion of the outer rotor ring (9); and four anti-reverse units (5), (6), (7), (8), which prevent reverse movement of the rotor ring. Each driving unit contains right ECA (10), left ECA (11) and the movement-transmission mechanism (12) (F. 2). The movement-transmission mechanism (12) help to overcome the gap between teeth of the outer ring (9) and the driving rack (13), which is inherently unavoidable during D-RIE fabrication process (F. 2a) After fabrication, the anti-gap structure (14) will be pushed upward until the two levers are engaged and locked at the working position (F. 2b). The springs will assure an always-contact status between the teeth. To make the outer ring (9) rotate clockwise, a voltage is applied to the chevron electro-thermal actuators in the anti-reverse units (5) and (7) and set the units in “ON” status. In this state, the beam of the unit is pushed upward, the triangular teeth-shape head of the beam is engaged to the outer ring. This arrangement prevents the rotor ring (9) from moving anti- clockwise but because of the elastic point at the base of the beam, it can still move in clockwise direction. When a periodic voltage is applied between fixed electrode and movable electrode of the right ECA (10), the movable parts of the ECA will rotate to the right. Through movement-transmission mechanisms (12), a rotational movement of the ECA is transmitted to the outer ring (9) and makes it rotate clockwise. When voltage decreases to zero, the beams with comb fingers and the movement- transmission mechanisms will move back to the initial position due to an elastic force. The outer ring cannot go backward because of the anti- reverse unit. The rotational motion in counter-clockwise direction can be achieved by deactivation of the right ECAs (10) in the driving units and the anti-reverse units (5), (7) and at the same time, set the units (6) and (8) into “ON” status and applying a periodic voltage in the left ECAs (11). 3. Force analysis and Simulation F. 3 Clockwise rotation of the MRM 5 12 9 10 F a F f1 F f2 F ar F. 4 Force analysis (a) driving period F el F es Q F sp F f F N (b) returning period Tuyển tập công trình Hội nghị Cơ điện tử toàn quốc lần thứ 6 805 Mã bài: 171 The force analysis of the MRM is separated into analysis for driving and returning period, respectively. In those periods, we have to count on the following forces: - F es :electrostatic force - F el : electrostatic force of the ECA beam - F a : force of driving rack acting on the rotor ring - F ar : force of anti-reverse mechanism acting on the rotor ring - F f1 , F f2 : friction forces in the driving period - F sp : maximal elastic force of the spring when the teeth start to disengage after sliding - F f : friction force in the returning period - F N : normal force a) Driving period (when the driving tooth pushes the outer ratchet ring) There are four driving ECAs and two anti- reverse units participating in action. The rotor ring can only rotate if the following condition is satisfied (F. 4a): M 0 es el ar 4[(F F ) h] 2.h.F (sin f.cos ) 0 ar es el F (sin f.cos ) F F 2 ar es el F (sin f.cos ) F F 2 (1) Where f = 0.38 is the static friction coefficient between the silicon–silicon contact surfaces, h = 6m is the height of the tooth, α = 60 0 is a slope angle of the tooth. The forces F es , F el and F ar can be calculated as follow: 2 0 es 0 n. . .b F .V g (2) el beam F k .d (3) ar ar F k . (4) Noting that n 68 is number of the movable comb fingers in each ECA, b 30µm is the thickness of a comb finger, g 0 = 2µm is the gap between two fingers, and 0 are permittivity of air and vacuum, respectively. V is the driving voltage. k beam is the stiffness of the ECA beam and in case the teeth of the outer ring are driven by driving rack, k ar is the stiffness of the anti- reverse beam, is the displacement of head of anti-reverse mechanism. Also, the displacement of the driving ratchet rack must satisfy condition below: d i.p g Where d is the displacement of driving rack, integer number i = 1, 2 , p = 10µm is the pitch of teeth and g = 2µm is an initial gap in the stopper system. Consequently, the applied voltage must be satisfied the following formula: ar 0 beam 0 F (sin f.cos ) g .(k .d ) 2 V n. . .b (5) Using Ansys, with simulating force F sim = 1 (N), we can calculate the stiffness of the anti- reverse beam (F.5) and the stiffness of the ECA beam (F.6): F. 5 Displacement of the anti-reverse beam F sim F. 6 Displacement of the ECA beam 806 Dang Bao Lam, Dinh Khac Toan, Nguyen Tuan Khoa, Pham Hong Phuc, Vu Ngoc Hung VCM2012 ar 1 k 2,33 N / m 0,43 ; beam 1 k 5,26 N / m 0,19 If i=1, by substitution into equation (5) we have the minimum driving voltage V min = 55.27 (V). And if i=2 then V min = 81.43 (V). b) Returning period (when driving tooth slides against the outer ratchet ring to the initial position) In the returning period, under influence of the elastic force of the beam, the driving rack moved back to the initial position and tightly acted on driven outer rotor ring. The interactive forces between them are illustrated in figure 4b. Simulated results point out that the force generated in the thermal CEA is much larger than electrostatic forces acting on the MRM, and from this reason we can assume when the anti- reverse units (5) and (7) are in the “ON” position, the anti-reverse lever only swings around an elastic point of the lever. The maximal elastic force of the spring when the ratchet teeth start to disengage after sliding F sp can be calculated as below: sp sp F k .h Where k s = 3.8 µN.µm -1 is the stiffness of the spring in y-direction, h = 6 µm is the height of the tooth. The component force Q in y-direction will compress the spring and thus causes free-sliding between two teeth. el N el F Q F .con F .sin .cos .sin 2 2 Condition for the driving tooth can go back to the initial position: sp Q F Or beam sp 1 k .(i.p g) .sin2 k .h 2 (6) From (6), we can clearly see that only when i=2, this condition is satisfied. In other words, in order to guarantee the outer ring can rotate, the minimum driving voltage must be V min = 81.43 (V). The graph illustrated in figure 8 shows the relation between step displacements of the MRM and driving voltage. 4. Conclusion This paper has presented the design of a bi- directional micro rotational motor (MRM) driven by eight driving electrostatic comb-drive actuator (ECA) and four anti-reverse Chevron electrothermal actuators (CEA). By alternately using four ECAs and two anti-reverse CEAs, the motor can move in two directions, i.e. clockwise and anti-clockwise. Simple design gives the MRM the possibility of convenient transmission of rotating movement to the other micro mechanisms. The designed micro motor can be applied in precise micro transmission, micro robot systems or the other MEMS devices, such as a micro gearing system, micro-pump etc. Acquired results show that the proposed model is ready for the next steps – fabrication and test of the micro rotational motor. ACKNOWLEDGMENTS This work is supported by the National Potential Project Program, Ministry of Science and Technology of Vietnam (Code KC.03.TN01/11-15). REFERENCES [1] Baker S. Michael, Plass A. Richard, Headley J. Thomas, Walraven A. Jeremy – Final Report: Compliant Thermo-Mechanical MEMS Actuators, Sandia Report (2004), SAND2004-6635. [2] S. De Cristofaro et al – Electromagnetic wobble micromotor for microrobots actuation, Sensors and Actuators A 161 (2010) 234–244. F. 8 MRM displacement – driving voltage relation 0 5 10 15 20 25 30 35 40 45 40V 50V 60V 70V 80V 90V 100V Step displacement (µm) Voltage (V) F. 7 Displacement of the spring Tuyển tập công trình Hội nghị Cơ điện tử toàn quốc lần thứ 6 807 Mã bài: 171 [3] Takeshi Morita – Miniature piezoelectric motors, Sensors and Actuators A 103 (2003) 291–300. [4] Richard Yeh, Seth Hollar, and Kristofer S. J. Pister - Single Mask, Large Force, and Large Displacement Electrostatic Linear Inchworm Motors, J. Microelectromechanical System, vol. 11, No. 4 (2002), 330-336. [5] E I Garcia and J J Sniegowski - Surface Micromachined Microengine as the Driver for Micromechanical Gears, Transducers’95, Stockholm, Sweden (1995), pp 365-368. [6] J J Sniegowski and E I Garcia - Surface- Micromachined Gear Trains Driven by an On-Chip Electrostatic Microengine, IEEE Electron Device Letters, Vol. 17, No. 7, (1996), pp 366-368. [7] Firas N. Sammoura- Novel Rotary Inchworm Motor, Robotics.eecs.berkeley.edu/~pister/245/projec t/Sammoura.pdf [8] S -C Kim and S H Kim - Precise Rotary Motor by Inchworm Motion Using Dual Wrap Belt, Review of Scientific and Instrument, Vol. 70, No. 5, (1999), pp 2546-2550. [9] A Geisberger, D Kadylak and M Ellis - A silicon electrothermal rotational micro motor measuring one cubic millimeter, Journal of Micromechanics and Microengineering, 16 (2006), pp.1943–1950. [10] Phuc Hong Pham, Dzung Viet Dao, Lam Bao Dang, S. Sugiyama - Single mask,simple structure micro rotational motor driven by electrostatic comb-drive actuators, Journal of Micromechanics and Microengineering, Vol. 22, No. 1 (2012). Dang Bao Lam received the MSc. degree in Mechanics of Machinery from Hanoi University of Science and Technology in 2004. He has been doing his PhD thesis on micro robot systems at the Hanoi University of Science and Technology. Since 2001, he has been a lecturer at the School of Mechanical Engineering, HUST. His research interests include: the micro robot system, micro motors, micro mechanisms, dynamics of machines and robotics. Dinh Khac Toan received the MSc. degree in MEMS technology from Hanoi University of Science and Technology in 2011. Since 2009, he has been a lecturer at the School of Mechanical Engineering, HUST. His research interests include: micro motors, micro actuators, machine design. Nguyen Tuan Khoa received the MSc. degree in MEMS technology from Hanoi University of Science and Technology. Since 2009, he has been a lecturer at the School of Mechanical Engineering, HUST. His research interests include: the micro mechanisms driven by electrothermal or electrostatic actuators. Pham Hong Phuc received the MSc. degree in Mechanical Engineering from Hanoi University of Science and Technology in 2002, and the PhD. degree in MEMS technology from Ritsumeikan University, Japan in 2007. He has been a lecturer in Hanoi University of Science and Technology and a leader of MEMS researching group at the Department of Design of Machinery and Robotics. His research interests include: the micro motors, micro robot systems and micro mechanisms. Vu Ngoc Hung received the BSE. degree in Semiconductor Physics from Kishinev University in 1979, and the PhD. degree in Solid Physics from the University of Science and Technology in 1992. He has been a senior lecturer at the Hanoi University of Science and Technology from 1980 and a professor/senior researcher in the Iternational Training Institute for Materials Science, HUST since 1992. His research interests include: the micro-electromechanical systems, QCM nano materials. Vu Ngoc Hung received the BSE. degree in Semiconductor Physics from Kishinev University in 1979, and the PhD. degree in Solid Physics from University of Science and Technology in 1992. He has been a senior lecturer at the Hanoi University of Science and Technology from 1980 and a professor/senior researcher in the Iternational Training Institute for Materials Science, HUST. His research interests include: the micro-electromechanical systems, QCM nano materials. . Dang Bao Lam, Dinh Khac Toan, Nguyen Tuan Khoa, Pham Hong Phuc, Vu Ngoc Hung VCM2 012 Mô tơ quay hai chiều sử dụng kết hợp các bộ chấp hành tĩnh điện - nhiệt điện A novel bi-directional micro. mô tơ quay hai chiều được dẫn động bằng tám bộ chấp hành kiểu tĩnh điện răng lược và bốn bộ chấp hành nhiệt dạng chữ V. Bốn trong số tám bộ chấp hành tĩnh điện dùng để dẫn động vành rô tơ quay. hành tĩnh điện dùng để dẫn động vành rô tơ quay thuận chiều kim đồng hồ, số còn lại được sử dụng khi đổi chiều quay. Bằng cách bố trí thích hợp theo từng cặp, các bộ chấp hành nhiệt dạng chữ V