Design and fabrication of the trapezoidal electrostatic comb-Drive actuator

Vietnam Journal of Mechanics, VAST, Vol. 34, No. 4 (2012), pp. 261 – 269 DESIGN AND FABRICATION OF THE TRAPEZOIDAL ELECTROSTATIC COMB-DRIVE ACTUATOR Pham Hong Phuc1, Dinh Khac Toan1, Dang Bao Lam1, Nguyen Tuan Khoa1, Nguyen Tien Dung2 1Hanoi University of Science and Technology, Vietnam 2Thai Nguyen University of Technology, Vietnam Abstract. This paper reports the design, fabrication and characterization process of the trapezoidal Electrostatic Comb-drive Actuator (ECA) with the slope angle α = 2◦. Together with the trapezoidal ones, the rectangular ECA with identical dimension was also designed and fabricated for comparison purpose. In order to reduce calculating de- viation, the fringing effect was also taken into consider while carrying out theoretical analysis. The obtained results pointed out the fact, that the trapezoidal ECA excels the rectangular ones with the same numbers of teeth in electrostatic force and displacement generation, while requires relatively low driving voltage. But it is also observed that with higher driving voltage (larger than 50 V), the trapezoidal ECA starts to lose its stability (the lateral pull-in phenomenon occurs). Keyword: electrostatic comb-drive actuator. 1. INTRODUCTION A rectangular electrostatic comb-drive actuator (ECA) was firstly introduced by Tang et al. [1]. While working, the fabrication errors can cause imbalance of the forces acting on the electrodes, which can lead to destabilization of the rectangular ECA [2]. Impact of destabilization increases with a thinner finger thickness and lower stiffness of the beam of the actuator. On the other hand, for overcoming this problem, choosing larger size of the electrodes could also lead to bulky dimension of the whole system. In [3], Johan B. C. Engelen et al.mentioned about the optimized shape of the ECAs. After optimization, the ECA can generate larger and constant electrostatic force. With the same number of fingers in a length unit, the ECA with optimized teeth can work with relatively high stability, good vibration damping ability, and can produce the forces, which is 1.8 larger than those generated by the normal rectangular ECA. Since the first research of an ECA by Tang et al [1], there have been a large number of applications of this type of actuators [4]. One of them was a linear motor [5] using the linear ECAs, the other one was a rotational motor [6, 7] with the curved ECA. The ECAs can also be found in the micro transportation and assembling systems [8, 9, 10]. In those applications, the actuating members in the ECA are tangential electrostatic forces, which are relatively small. The normal electrostatic forces, which are much larger, can also 262 Pham Hong Phuc, Dinh Khac Toan, Dang Bao Lam, Nguyen Tuan Khoa, Nguyen Tien Dung be used e.g. in the gap-closing actuator (GCA) [5], etc. However, those actuators using the normal electrostatic force have to deal with a disadvantage that the normal force could rapidly decrease when the distance between the fingers of ECA becomes larger. The rectangular ECAs using tangential forces have the simple structure, simple fabrication process, and high stability, but they are still accompanied with low overall performance characteristics such as small force at relatively high driving voltage and large dimension. To overcome those disadvantages, the authors present the ECA with the trapezoidal finger, in which the combination of the tangential and normal electrostatic forces will be used as a driving force. 2. THE TRAPEZOIDAL ECA 2.1. ECA design Fig. 1 shows the configuration of the trapezoidal ECA with following parameters: slope angle α = 2◦, initial gap between two opposite finger d, overlap a, tooth thickness b and tooth length lc. We know that when the slope angle α increases, the electrostatic b d a lcα Fig. 1. Configuration of the trapezoidal ECA forces acting on the fingers also increase, but the gained displacement will decrease, and vice versa. While the trapezoidal ECA is working, the overlap becomes larger, the gap d becomes smaller, and therefore, the acting forces in the ECA increase simultaneously. 2.2. Edge effect (fringing effect) When two plates are applied by V -voltage between (one plate is positive and the other is negative) as shown in Fig. 2, an electric field is created between them. The electric field lines seem linear in the center and non-linear toward the edge of the capacitor. This phenomenon changes its capacitance, therefore, it varies forces applying on two plates. According to Guo Zhanshe et al. [11], the capacitance C can be expressed as: C = abεε0 d + bεε0 pi { 1 + ln [ 1 + pia d + ln ( 1 + pia d )]} + aεε0 pi { 1 + ln [ 1 + pib d + ln ( 1 + pib d )]} (1) Design and fabrication of the trapezoidal electrostatic comb-drive actuator 263 where d is the gap between two fingers, a is the overlap of those and b is the thickness of a comb finger. Fig. 2. Electric field between two parallel plates Fig. 3a presents the force analysis in the well-known rectangular ECA, in which the acting tangential force Fy and normal force Fx can be expressed as below: Fx = 1 2 ∂C (d) ∂d V 2, Fy = 1 2 ∂C (a) ∂a V 2 (2) 2.3. Force analysis in the trapezoidal ECA In Fig. 3b, the initial position of a point on the surface of trapezoidal tooth is at A0. After applying a driving voltage, the movable tooth is pulled in y-direction, thus, it Fixed combs Movable combs α b) Forces in the trapezoidal ECA Fig. 3. Force analysis in the comb actuator is moved a distance of ∆y, and the point A0 travels to the point A. Therefore, the gap d and the overlap a are calculated as: d = d0 −∆y sinα, a = a0 +∆y cosα (3) 264 Pham Hong Phuc, Dinh Khac Toan, Dang Bao Lam, Nguyen Tuan Khoa, Nguyen Tien Dung As the rectangular ECA, the trapezoidal ECA is acted by the tangential force Ft and the normal force Fn. Those are calculated in Eq. (2), where the gap d and the overlap a are defined by Eq. (3). Fig. 3b shows that the force Ft is separated into Ftx in x-direction and Fty in y-direction, and similarly, Fn is separated into Fnx and Fny . Total forces acting on the one side of the finger in x and y direction can be calculated as below: ∑ Fy = Fty + Fny, ∑ Fx = Fnx − Ftx (4) Or ∑ Fy = Ft cosα+ Fn sinα and ∑ Fx = Fn cosα− Ft sinα (5) Where parameters of the trapezoidal ECA were chosen as follows: the initial overlap a0=5 µm, the initial gap d0=2 µm, the angle α=2 ◦. The dimensions of the rectangular ECA are intentionally chosen as the same as those of the trapezoidal ECA. From Eqs. (1), (2) and (4), the relation between the forces acting in the ECA and the moved distance ∆y can be established in Fig. 4a. The ratio of the force in the trapezoidal ECA to the force in the rectangular ECA is shown in Fig. 4b. y(µm) F (N) (a) Force - displacement relation y(µm) Ftrapezoidal/Frectangular (b) The ratio of forces Fig. 4. Effects of the ratio R/h (m = 1, n = 6, k = k1 = 1, T = 300 K, ξ∗ = 0.1, L/R = 1) It is observed that when ∆y increases, the force in the rectangular ECA decreases, but on the contrary, the force in the trapezoidal ECA increases rapidly. The ratio of two forces also increases as presented in Fig. 4b and this ratio approximately equals 1.55 when ∆y = 10 µm. Design and fabrication of the trapezoidal electrostatic comb-drive actuator 265 2.4. Displacement in the trapezoidal ECA Fig. 5a shows the designed trapezoidal ECA which following parameters: the length of the beam l = 700 µm, the rectangular section bw×h = 4×30 µm 2, number of movable fingers n = 134, E = 169 GPa is Young’s modulus of single crystalline silicon. Fig. 5b presents the calculation diagram, where the forces acting on beam nF t are replaced by a (a) Configuration of the trapezoidal ECA (b) Calculation diagram Fig. 5. Force calculation in the trapezoidal ECA spring with a stiffness of k. The stiffness k is defined as: k = 4Ehb3w l3 (6) Replacing parameters in Eq. (6) by their values, we get the value of beam stiffness k = 3.7845 µN/µm. Because the viscous drag force and friction force are negligible while the ECA is working, the equation of force balance of the actuator can be presented as below: nFt (∆y)− k∆y = 0 (7) Table 1. Calculated displacements of the trapezoidal ECA Voltage(V) 20 30 40 50 Displacement (µm) 1,25 2,95 5,69 10,57 We use the numerical method to solve Eq. (7) with two steps: Firstly estimating initial roots by geometry method, and then using Newton-Raphson method to solve Eq. (7) to get value of displacement of the trapezoidal ECA. The results are shown in Tab. 1. 2.5. Simulation of displacements of the trapezoidal ECA The rectangular and trapezoidal ECA are simulated in multi-field (electric and mechanical fields) using FEA method (Finite Element Analysis). Fig. 6 shows a result from simulation of the trapezoidal ECA working at voltage of 50 V and with the number 266 Pham Hong Phuc, Dinh Khac Toan, Dang Bao Lam, Nguyen Tuan Khoa, Nguyen Tien Dung Fig. 6. Displacement of the trapezoidal ECA at 50 V (11 comb fingers) Table 2. Simulated displacements of the trapezoidal ECA Voltage(V) 20 30 40 50 Displacement (µm) 1,32 3,16 6,15 10,81 of comb fingers n =11. Simulation results with the driving voltages ranged from 20 V to 50 V are presented in Tab. 2. 3. FABRICATION AND CHARACTERIZATION 3.1. Fabrication The ECAs has been fabricated by using SOI wafer with the thicknesses of the device layer, buried SiO2 layer, and silicon substrate were 30 µm, 4 µm and 450 µm, respectively. (a) Rectangular teeth (b) Trapezoidal teeth Fig. 7. SEM images of the rectangular and trapezoidal ECAs The rectangular and trapezoidal ECAs after fabrication are shown in Fig. 7a and 7b, respectively. Fig. 8 shows the video-captured of the rectangular (8a) and trapezoidal (8b) Design and fabrication of the trapezoidal electrostatic comb-drive actuator 267 (a) Rectangular ECA (b) Trapezoidal ECA Fig. 8. The video-captured image when the ECAs working at 30V ECA. It is clearly observed that at the same driving voltage (Vpp=50 V), the displacements of the trapezoidal one is larger than those of the rectangular ECA. To perform the operational test, the ECAs were driven by square wave periodic voltage ranged from 20 V to 50 V. Fig. 9 shows the video-captured photograph taken (a) Displacement - Driving voltage (b) Deviation between calculated and measured data Fig. 9. Displacement of trapezoidal ECA when measuring displacements of the rectangular and trapezoidal ECAs at the voltage of 30 V. Tab. 3 shows the measured displacements at 20, 30, 40 and 50 V, respectively. Table 3. Measured displacements (µm) of the rectangular and trapezoidal ECAs Finger/Voltage (V) 20 30 40 50 Rectangular 1 3 4 6 Trapezoidal 1.5 3.5 7 10.5 It is also observed that when driving voltage was increased to 60 V, the lateral pull-in phenomenon occurs in the trapezoidal ECA. 268 Pham Hong Phuc, Dinh Khac Toan, Dang Bao Lam, Nguyen Tuan Khoa, Nguyen Tien Dung 3.2. Characterization Fig. 9a shows the relation between the calculated, simulated and measured displace- ments and the driving voltage of the trapezoidal ECA. With increased value of the driving voltage, the value of displacement also increases and a kind of quadratic relation is created. The largest deviation between the calculated data and the measured ones is close to 8% at the voltage of 50 V as shown in Fig. 9b. (a) T ra p e z o id a l / R e c ta n g u la r (b) Fig. 10. Relation between measured displacements and driving voltage (a); the displacement ratio (b) of the rectangular and trapezoidal ECAs The graph comparing measured displacements in the rectangular and trapezoidal ECAs is shown in Fig. 10. At low voltages, there is almost no deviation between both types of ECAs. When driving voltage increases (from 40 V to 50 V), the measured displacements of the trapezoidal ECA are much larger than those of the rectangular ones. At voltage of 50 V, this ratio reaches the value of 1.62. 4. CONCLUSION This paper has presented the design, fabrication and testing of both types of the ECAs, i.e. rectangular and trapezoidal. In comparison with the classical rectangular ECAs, the trapezoidal ones have several advantages and also minor disadvantage as below: 1. With the same teeth number, counted in the same length unit, the forces generated in the trapezoidal ECA is larger than those in the rectangular one. The attracting force in the movement direction increases 1.1 times initially, and 1.55 times at displacement of 10 µm. 2. The obtained results show that displacement of the trapezoidal ECA is larger the those of the rectangular ECA. The largest deviation (62%) was observed at voltage of 50 V. On the other hand, the measured values of displacement are smaller than the theoretically calculated and simulated ones, which can be explained by measurement and fabrication errors. Design and fabrication of the trapezoidal electrostatic comb-drive actuator 269 3. When driving voltage increased to 60 V, the lateral pull-in phenomenon occured because of the unbalanced state of the force components in x-direction. 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