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Design and Analysis of a Decoupled XY MEMS Microgripper with Integrated Dual-Axis Actuation and Force Sensing*
Proceedings of the 20th World The International Federation of Congress Automatic Control Proceedings of the 20th9-14, World The International Federation of Congress Automatic Control Toulouse, France, July 2017 Available online at www.sciencedirect.com The International of Automatic Control Toulouse, France,Federation July 9-14, 2017 Toulouse, France, July 9-14, 2017
ScienceDirect
PapersOnLine 50-1 (2017) 808–813 DesignIFACand Analysis of a Decoupled XY Design and Analysis of a Decoupled XY Design Analysis of with a Decoupled XY MEMSand Microgripper Integrated MEMS Microgripper with Integrated ⋆ MEMS Microgripper with Integrated Dual-Axis Actuation and Force Sensing Dual-Axis Actuation and Force Sensing ⋆⋆ Dual-Axis Actuation and Force Sensing ∗ ∗∗
Sijie Yang ∗ , Qingsong Xu ∗∗ Sijie Yang ∗∗ , Qingsong Xu ∗∗ Sijie Yang , Qingsong Xu ∗∗ ∗ University of Macau, Avenida da Universidade, Taipa, Macau, China ∗ ∗ University of Macau, Avenida da Universidade, Taipa, Macau, China (e-mail: [email protected]). ∗ University of Macau, Avenida da Universidade, Taipa, Macau, China ∗∗ (e-mail: [email protected]). University of Macau, Avenida da Universidade, Taipa, Macau, ∗∗ (e-mail: [email protected]). ∗∗ University of Macau, [email protected]) da Universidade, Taipa, Macau, China (e-mail: ∗∗ University of Macau, [email protected]) da Universidade, Taipa, Macau, China (e-mail: China (e-mail: [email protected]) Abstract: This paper presents the design, analysis, and simulation study of a novel microAbstract: This paper themicrogipper design, analysis, anddecoupled simulation study of aactuators novel microelectromechanical systempresents (MEMS) with two electrostatic and Abstract: This paper presents themicrogipper design, analysis, anddecoupled simulation study of aactuators novel microelectromechanical system (MEMS) with two electrostatic two capacitive force sensors dedicated to the testing in compressive and shear directionsand of electromechanical system (MEMS) microgipper with two decoupled electrostatic and two samples. capacitiveThe force sensors dedicated the testing and shearactuators directions of soft gripper mechanism is to designed basedinoncompressive the two-prismatic-prismatic (2-PP) two capacitiveThe force sensors dedicatedis to the testing and shear directions of soft samples. gripper mechanism designed basedinoncompressive the two-prismatic-prismatic (2-PP) parallel structure and folded leaf flexure guiding mechanism. Owing to the design of decoupled soft samples. The gripper mechanism is designed based on the two-prismatic-prismatic (2-PP) parallel structure andthe folded leaf flexure guiding mechanism. Owing to the design decoupled actuation structure, translations of the two gripping arms are parallel, whichofensures the parallel structure andthe folded leaf flexure the guiding mechanism. Owing to the design ofensures decoupled actuation structure, translations twoobject gripping arms are parallel, whichAt the reaction force between two tips and of grasped in sole gripping direction. the same actuation structure, thetwo translations of the twoobject gripping armsgripping are parallel, whichAtensures the reaction force between tips and grasped in sole direction. the same time, this structure also offers a good decoupling property. The analytical model is verified by reaction force between two tips and grasped object in sole gripping direction. At the same time, thisout structure also offers a good decoupling property. analytical is verifiedand by carrying finite element analysis (FEA) simulation study,The which confirmsmodel the feasibility time, thisout structure also offers a good decoupling property. analytical is verifiedand by carrying finite analysis simulation study,The which confirmsmodel the process feasibility effectiveness of theelement proposed design.(FEA) The fabrication procedure by SOIMUMPs is also carrying out of finite element analysis (FEA) simulationprocedure study, which confirms the process feasibility and effectiveness the proposed design. The fabrication by SOIMUMPs is also given, and the microgripper will be fabricated for future experimental study. effectiveness of microgripper the proposedwill design. The fabrication procedure by SOIMUMPs process is also given, and the be fabricated for future experimental study. given, the(International microgripper will be of fabricated futureHosting experimental study. © 2017,and IFAC Federation Automaticfor Control) by Elsevier Ltd. All rights reserved. Keywords: MEMS gripper, 2-DOF microgripper, 2-PP parallel mechanism, FEA simulation, Keywords: MEMS gripper, 2-DOF microgripper, 2-PP parallel mechanism, FEA simulation, SOIMUMPs fabrication process. Keywords: MEMS gripper, 2-DOF microgripper, 2-PP parallel mechanism, FEA simulation, SOIMUMPs fabrication process. SOIMUMPs fabrication process. 1. INTRODUCTION low power consumption, no hysteresis, and fast response 1. INTRODUCTION low power consumption, hysteresis, and fast response speed are also suitable forno MEMS gripper application (Xu 1. INTRODUCTION low power consumption, no hysteresis, and fast response speed are also suitable for MEMS gripper application (Xu With the increasingly widespread development on mi- (2015)). speed are also suitable for MEMS gripper application (Xu (2015)). With the increasingly widespread development on microassembly and biomedical fields, various micro- and (2015)). With the increasingly widespread development on and mi- To avoid damaging the samples, various sensors are often croassembly and objects biomedical fields, micronanometer scaled require to bevarious manipulated safety To avoid in damaging the samples, various sensors are often installed the microgrippers, cooperating with actuating croassembly and biomedical fields, various microand nanometer scaled require manipulated safety To avoid in damaging the samples, various sensors are often and accurately. Asobjects a crucial tool to to be execute the micromainstalled the microgrippers, cooperating with actuating arm to form a completely integrated microgripper system, nanometer scaled objects require to be manipulated safety and accurately. a crucial tool to execute the employed microma- installed in the microgrippers, cooperating with actuating nipulation tasks,As MEMS microgripper is usually arm to form a completely integrated microgripper as reported in the literature, e.g., Chen et al system, (2008). and accurately. As a crucial tool to execute the micromanipulation tasks, MEMS is usually employed arm to form aincompletely integrated to pick-deliver-place tinymicrogripper objects including bio-samples as reported the methods, literature, e.g., microgripper Chen al system, (2008). Concerning sensing there are fouretmain types nipulation tasks, MEMS microgripper is usually employed to pick-deliver-place including bio-samples reported sensing in the methods, literature,there e.g., are Chen etmain al (2008). (Volland et al (2002)),tiny hair objects organization (Xu (2015)), and as Concerning four types of sensing schemes which are usually applied in MEMS to pick-deliver-place tiny objects including bio-samples (Volland et al (2002)), hair organization (Xu (2015)), and sensing which methods, there areapplied four main types soft material (Keekyoung et al (2008)). The goal of this Concerning of sensing schemes are usually in MEMS applications. Electro-thermal sensor reveals some merits (Volland et al (2002)), hair organization (Xu (2015)), and soft material (Keekyoung et al (2008)). The goal of this of sensing schemes which are sensor usuallyreveals applied in MEMS work is to design a novel 2-DOF MEMS-based microgripapplications. Electro-thermal some merits compact size, simple structure, and good resolution. soft material (Keekyoung et al (2008)). The goal of this like work is tointegrated design a novel 2-DOF microgripapplications. Electro-thermal sensor reveals some merits per with actuators andMEMS-based sensors. like compact size, simple structure, good resolution. While the limitations of this sensor lieand in the sensitivity to work is to design a novel 2-DOF MEMS-based microgripper with integrated actuators and sensors. like compact size, simple structure, and good resolution. thetemperature limitations of this sensorconsumption. lie in the sensitivity to ambient and energy Piezoelecper with integrated actuators sensors. Regarding the driving scheme,and different types of actuators While While thetemperature limitations of this sensorconsumption. lie in the sensitivity to ambient and energy Piezoelectric sensor has a wide bandwidth and flexibility. However, Regarding the driving scheme, different types of actuators can be adopted to actuate MEMS grippers. For example, ambient temperature and energy consumption. PiezoelecRegarding the driving scheme, different types of actuators tric sensor has a wide bandwidth and flexibility. However, manufacturing problem is a big issue of this type of can be adopted actuator to actuate MEMS grippers. For example, electro-thermal exhibits some advantages of large the sensor has a wide bandwidth andissue flexibility. can be adopted actuator to actuate MEMS grippers. For example, the manufacturing problem a big of thisHowever, typebut of electro-thermal exhibits advantages of worklarge tric sensor. Pizeoresistive sensor ishas a high bandwidth, force and low input voltage. But some the relatively high the manufacturing problem is a big issue of this typebut of electro-thermal actuator exhibits some advantages of large sensor. Pizeoresistive sensor has a high bandwidth, the large footprint and complex fabrication process are force and low input voltage. But the relatively high working temperature restricts its application in some situations. sensor. Pizeoresistive sensor has a high bandwidth, but force and low input voltage. But the relatively high worklarge footprint complex as fabrication are unwanted in practice.and In addition, comparedprocess with other ing temperature restricts its application in some situations. Another commonly used actuator in MEMS area is shape the the large footprint and complex as fabrication processother are ing temperature restricts its application in some situations. in practice. In addition, compared types of sensors, capacitive sensor (Bazaz et with al (2011)) Another used in MEMS area is shape memory commonly alloy, which hasactuator high energy density and large unwanted unwanted in practice. In addition, as (Bazaz compared with other Another commonly used actuator in MEMS areaand is shape types of sensors, capacitive sensor et al (2011)) memory alloy, which has high energy density large provide fast response speed, low noise disturbance, stroke. While the high power consumption and hysteresis can of sensors, capacitivespeed, sensorlow (Bazaz etdisturbance, al (2011)) memoryWhile alloy,the which has highconsumption energy density large types fast response high provide bandwidth, and simple physical noise structure. Hence, stroke. high power and and hysteresis effect are two main problems. As for piezoelectric actuator, can can provide fast response speed, low noise disturbance, stroke. While the high power consumption and hysteresis bandwidth, anddesigned simple physical structure. Hence, capacitive sensors are for the microgripper in this effect are two problems. for piezoelectric actuator, although the main bandwidth andAsresponse speed are good high high bandwidth, anddesigned simple physical structure. Hence, effect are two main problems. for piezoelectric actuator, sensors are for the microgripper in this paper. although thecomplex bandwidth andAs response speed are enough, the material process complicates thegood fab- capacitive capacitive sensors are designed for the microgripper in this although thecomplex bandwidth andprocess response speed are good paper. enough, the material complicates the fabrication. In this work, electrostatic actuator is selected for paper. Mechanism structure is another important factor which enough, the complex material process complicates the fabrication. Ininthis work, electrostatic is selected for Mechanism structure is another important factor which actuation both axes, because itactuator has simple structure needs to be considered in the device design. Different rication. In this work, electrostatic actuator is selected for actuation in both axes, becauseMeanwhile, it has simple structure is another important factor which and it is easy to manufacture. the merits of Mechanism needs to bestructure considered in theneeds device Different constructions satisfy different of design. the device. For actuation in both axes, because it has simple structure and it is easy to manufacture. Meanwhile, the merits of needs to be considered in the device design. Different satisfy different needs(DOF) of the grippers device. For ⋆ example, multi-degree of freedom are and it work is easy manufacture. theScience meritsand of constructions This wastosupported in partMeanwhile, by the Macao constructions satisfy different needs(DOF) of the grippers device. For ⋆ This work was supported in part by the Macao Science and example, multi-degree of freedom are proposed in previous work by Muntwyler et al (2010) Technology Development Fund under Grant 090/2015/A3 ⋆ This work was supported in part by the Macao Science and example, multi-degree of freedom (DOF) grippers are proposed in previous work by Muntwyler et al (2010) Technology Development Fund under Grant 090/2015/A3 and 024/2014/A1. and Qu et al. (2015). For testing the compressive and proposed in previous work by Muntwyler et al (2010) Technology Development Fund under Grant 090/2015/A3 and ⋆⋆ 024/2014/A1. and et al. For testing the compressive and Corresponding author: Q. Xu (Phone: +853 8822 4278; Fax: +853 shearQu forces of (2015). micro-objects, the grippers are actuated 024/2014/A1. and Qu et al. (2015). For testing the compressive and ⋆⋆ Corresponding author: Q. Xu (Phone: +853 8822 4278; Fax: +853 8822 2426; E-mail: [email protected]). shear forces of micro-objects, the grippers are actuated ⋆⋆Corresponding author: Q. Xu (Phone: +853 8822 4278; Fax: +853 shear forces of micro-objects, the grippers are actuated 8822 2426; E-mail: [email protected]). 8822 2426; E-mail: [email protected]). Copyright © 2017, 2017 IFAC 831Hosting by Elsevier Ltd. All rights reserved. 2405-8963 © IFAC (International Federation of Automatic Control) Copyright © 2017 IFAC 831 Peer review under responsibility of International Federation of Automatic Copyright © 2017 IFAC 831Control. 10.1016/j.ifacol.2017.08.144
Proceedings of the 20th IFAC World Congress Sijie Yang et al. / IFAC PapersOnLine 50-1 (2017) 808–813 Toulouse, France, July 9-14, 2017
actuating arm
809
sensing arm
lr VOLTAGE
q
lf Fixed pads
actuating lateral comb drive
lp
lsy
m
lsx
FX
sensing transverse comb drive
Ft
Fx
Fig. 2. The diagram of working principle for comb-drive electrostatic actuator.
Fig. 1. The schematic sketch of the MEMS microgripper with parameter symbols. by different actuation schemes in both x- and y-axes along with integrated sensors to detect the force in both vertical and horizontal directions. However, the aforementioned two grippers have the problem of motion coupling. The generated actuating forces in the two axes of the arm are not independent, which influences the accuracy of the output displacement for the gripping jaw. In addition, majority of the existing grippers grasp objects by rotating the gripper arms. This process causes the reaction force between gripper tips and grasped objects. When the gripped objects have a circular and smooth surface, they are easily pushed out and slip out of the gripper arms with the generated force (Keoschkerjan (2002)). To this end, we present, in this paper, the design of a new decoupled dual-axis actuation MEMS gripper with integrated force sensing capabilities based on the methodology of pseudo-rigid-body (PRB) model. The aforementioned issue is overcome by a new structure design with parallel movement of the gripping tip. The remaining parts of the paper are arranged as follows. The gripper design is illustrated in section 2. Finite element analysis (FEA) simulation is introduced in section 3 to verify the feasibility of analytical model. Section 4 describes the gripper fabrication process. Section 5 concludes this paper. 2. DESIGN OF THE GRIPPER A schematic diagram of the microgripper is shown in Fig. 1. This gripper consists of a left actuating arm and a right sensing arm. As for the actuating part, two comb-drive electrostatic actuators are nested inside decoupled XYmotion stage. Each actuator is supported by four folded leaf flexures (FLF), which are connected to the move-body and fixed by two pads. The gripper tips have an initial gap of 100 µm. The comb-drive actuator along x-axis pushes the left arm to grasp objects. Another actuator along yaxis pushes the left arm slightly higher than the right arm to conduct the shear testing. 832
It is notable that the actuation arm is composed of two prismatic-prismatic (PP) parallel structure. In micromotion scale, with the help of this configuration, the movement of the actuation tip is approximately parallel. Thus, the aforementioned problem caused by reaction force can be overcome. In addition, the sensing data can also be more accurate. The sensing arm is linked to two pairs of parallelogram flexures in x- and y-axes, which are connected with two comb-drive capacitive sensors and organized perpendicular to each other. They have the capabilities of testing compression and shear forces generated in the sample object, respectively. For more accurate test of the sensing force in two directions, the parallelogram flexures act as a guiding mechanism to transmit the force from actuation arm to sensing arm. 2.1 Actuator Design Electrostatic actuator is commonly used in MEMS domain. Owing to its simple structure, fast response speed, and low energy consumption, it has been widely accepted in microgripper actuation scheme. In general, according to the construction, electrostatic actuators can be classified into two types, namely, transverse comb-drive and lateral comb-drive. In this paper, lateral comb-drive is employed to produce a larger displacement to drive the gripper. Electrostatic lateral-comb actuator is shown in Fig. 2. The driving forces of the two identical actuators are expressed by: εtV 2 N Fx = Fy = (1) q where ε = 8.85 × 10−12 C 2 /(N m2 ) is the permittivity of air, q is the gap width between two adjacent plates, t is the thickness of gripper, V is the driving voltage, and N represents the number of comb-tooth pairs. 2.2 Design of Folded Leaf Flexure In order to connect the actuating part to fixed pads, leaf flexures are introduced in this work. Traditional straight leaf flexures are slender (Amjad et al (2008); Piriyanont et al (2013)). Hence, by using this type of long straight leaf flexure, it is prone to cause the load effect problem and enlarge the footprint of the device size. On the other
Proceedings of the 20th IFAC World Congress 810 Sijie Yang et al. / IFAC PapersOnLine 50-1 (2017) 808–813 Toulouse, France, July 9-14, 2017
layer 3
ac layer 1
layer 3
g2
layer 2
Fg layer 1
X
layer 2
Gain
Vout
F
Fig. 3. Schematic sketch of the folded leaf flexure and its deformation.
leaf flexure
g1
ac MS3110
Fig. 5. Schematic diagram of the working principle for capacitive sensor.
O1 lr
lr
Θ
lb
O2 F
Fig. 4. PRB model of a parallelogram flexure. hand, slender leaf flexure has another drawback of stress stiffening. That is, due to the compressive stress occurring in leaf flexure, the transverse stiffness will increase gradually. In this work, the folded leaf flexure (FLF) is introduced to solve the aforementioned problem. By folding the flexures in parallel, not only the physical size of microgripper is reduced, but also the stress stiffening is mitigated to a large extent. It also plays a role of translation guiding mechanism, which guides the force through a pure translational motion in one direction. Taking 3-layer flexures as an example, the structure schematic of FLFs is shown in Fig. 3. Hence, one can get the equations below: F lf2 M lf − =0 (2) 2EI EI F lf3 M lf2 − =χ (3) 3EI 2EI 3 where E is Young’s modulus of Silicon, I = bh 12 is the moment of inertia, F and M are the axial force and moment of force, and lf is the length of folded flexure. By substituting equation (2) to equation (3), the displacement of one single flexure can be solved as follows: F lf3 χ= (4) 12EI The stiffness of the folded flexure can be calculated as follows: Ebh3 F = Kf = (5) Nχ N lf3 where N = 3 in this case design. 2.3 Mechanism Structure Design To realize the two perpendicular axes of pure translational movement for the gripping arm, the two-prismaticprismatic (2-PP) parallel structure as introduced by Xu 833
(2012) is adopted in this work. The actuating mechanism is designed based on pseudo-rigid-body (PRB) (Pei et al (2010)), which is illustrated in Fig. 4. Assume that the flexures experience bending deformation only. As a result, each flexure of the module exhibits a 1-DOF rotational compliance. While the remaining parts of this module are rigid. Hence, the output displacement of each movable body has the same compliant deformation, which mainly arises from the four compound parallelogram flexures. The PRB analytical model of one flexure is shown in Fig. 4, where O1 and O2 act as two pivots. lb is the characteristic radius. Referring to Howell (2001), the torsional spring constant can be expressed by: 2γKΘEI Kc = (6) lr where γ = 0.85 is the characteristic radius factor, and KΘ = 2.65 is a constant. Based on PRB model, the translational stiffness Kr of the compound parallelogram flexure can be expressed as: Kc Kr = 2 (7) lb Hence, the output displacement can be calculated as follows: Kt 0 d1 Fx = (8) 0 Kt d2 Fy where Kt is the total stiffness in one working axis. d1 and d2 are the output displacements of the two actuators, respectively. The output displacement of the actuating arm in both x and y axes can be computed by: Ft Ft d1 = d2 = = (9) Kt Kf + Kr where Ft = Fx = Fy . 2.4 Sensor Design The working principle of the capacitive sensor is shown in Fig. 5. Taking the capacitive sensor for compressive force testing as an example, the sensor design process is presented in this section. Assume that only the compressive action produces the force. When an object is clamped by the two gripper tips,
Proceedings of the 20th IFAC World Congress Sijie Yang et al. / IFAC PapersOnLine 50-1 (2017) 808–813 Toulouse, France, July 9-14, 2017
811
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 6. FEA static simulation results of the actuation arm. (a)–(c) A force of 100 µN is applied on movable plate in x-axis: (a) total deformation, (b) x-axis deformation, (c) y-axis deformation; (d)–(f) A force of 100 µN is applied on movable plate in y-axis: (d) total deformation, (e) y-axis deformation, (f) x-axis deformation. Ksy = Kp + Ky =
2Etwp3 4Etws3 + 3 lp3 lsy
(11)
The gripping force Fsx and Fsy , which are generated in x and y axes, can be computed as: Fsx = Ksx × Dx
(a)
(13) Fsy = Ksy × Dy where Dx and Dy represent the displacements of right tip in x and y axes, respectively.
(b)
Fig. 7. FEA static simulation result of the sensing arm. the gripping force will be transmitted from the actuating tip to sensing tip in horizontal direction. During the force transmission by two parallelogram flexures, the movable plates which are fixed by four leaf flexures will move forward along the force transmission direction. The two distances between the two parallel plates of the capacitive sensor will change, respectively. Thus, the capacitive difference of the sensor will change simultaneously. The total stiffness of the right sensing arm in x-axis can be expressed as: Ksx = Kp + Kx =
2Etwp3 4Etws3 + 3 lp3 lsx
(12)
(10)
Similarly, the total stiffness of the sensing arm in y-axis can be calculated as follows: 834
The voltage Vout of output signal is generated by a converter chip (e.g., MS3110 produced by MicroSensors, Inc.), which converts capacitive change to voltage variation. That is, C1 − C2 Vout ∝ G × (14) Cf with A A A A C1 = ε( + ), C2 = ε( + ) (15) g1 − δ g2 + δ g1 + δ g2 − δ where G and Cf values can be tuned according to specific situation. A represents the plate area. g1 and g2 are two gaps of the parallel capacitive plates. The output voltage and the deformation δ is proportional (Beyeler et al (2007)). 3. FEA SIMULATION STUDY In this section, FEA simulation study is carried out to evaluate the performance of the designed microgripper. To
Proceedings of the 20th IFAC World Congress 812 Sijie Yang et al. / IFAC PapersOnLine 50-1 (2017) 808–813 Toulouse, France, July 9-14, 2017
Table 1. Main parameters of the microgripper Parameter length of folded flexure length of sensing flexure 1 length of sensing flexure 2 length of rotation flexure length of parallelogram flexure thickness of device layer width of actuating flexure width of folded flexure width of sensing flexure width of rotation flexure width of parallelogram flexure gap of teeth length of teeth overleap gap 1 of capacitive plates gap 2 of capacitive plates number of teeth paris
Symbol lf lsx lsy lr lp t wa wf ws wr wp q m g1 g2 N
Value 600 400 450 1200 450 30 7 7 5 7 7 5 30 5 20 736
Unit µm µm µm µm µm µm µm µm µm µm µm µm µm µm µm µm
(a)
(b)
Fig. 8. FEA modal simulation results of the actuation part.
simplify the simulation process, electrostatic comb drive and capacitive sensor are not taken into account. Besides, the actuating and sensing parts are simulated, respectively. Actually, this does not influence the simulation results. ANSYS software is used for the static and modal simulation studies. The main parameters of the gripper are list in Table 1. 3.1 Static Simulation
(a)
(b)
Fig. 9. FEA modal simulation results of the sensing part.
As for the actuation part, four fixed pads are set as fixed supports when conducting the FEA simulation. Two forces of 100 µN, which are generated by electrostatic actuator, are applied at the end of the movable plates in x and y axes, respectively. The simulation results are shown in Fig. 6. The total displacements of the left tip are depicted in Fig. 6(a) and (d), which are 15.054 µm and 14.653 µm, respectively. Besides, the results of the decoupling capability analysis of the gripper’s actuating part are illustrated in Fig. 6(b) and (c) for x-axis and Fig. 6(e) and (f) for y-axis, respectively. It is clear to see that the cross-axis difference in x-axis direction are 15.05 µm and 0.1 µm along x and y axes, respectively. In view of the decoupling property in y-axis direction, 14.654 µm and 0.04 µm displacements appear in x- and y-axis, respectively. Hence, the decoupling property for the left part of the microgripper can be calculated as 0.6% and 0.2% in x and y directions, respectively. Moreover, simulation results of the right sensing arm are demonstrated in Fig. 7. Fig. 7(a) and (b) illustrate the force testing in two different axes, respectively. By applying two forces of 20 µN on the top area and grasping area of sensing tip, the displacements in these two directions can be obtained. Then, the stiffnesses Ksx and Ksy of the right arm in x and y axes can be computed, respectively. In order to have a clear comparison, the results of analytical model and FEA simulation are listed in Table 2. And the property of decoupling is shown in Table 3 3.2 Modal Analysis Simulation
Table 2. Comparison between analytical model and FEA simulation results Symbol Ktx Kty Ksx Ksy
analytical 6.59 6.59 73.5 62
simulation 6.64 6.82 74.1 66.7
error 0.7% 3.4% 0.8% 7%
unit µN/µm µN/µm µN/µm µN/µm
Table 3. Result of decoupling property for actuating part Direction x-axis y-axis
Cross-axis error 0.6% 0.2%
left actuating part of the micro-gripper. According to the results, the two directional movements of the gripper arm appear in two frequencies of 1575.8 Hz and 1577.7 Hz, respectively. The modal simulation of the right sensing part is also conducted by FEA. The result is shown in Fig. 9, which indicates the two directional motions of the sensing arm in x and y axes. The corresponding resonant frequencies are 8742 Hz and 9340.2 Hz, respectively. The simulation result shows that the stiffness of the sensing arm is much higher than that of the actuation arm. Thus, the sensing arm will undergo a smaller deformation than the actuation arm when the object is grasped. In addition, the relatively high natural frequency reveals a fine bandwidth performance of the microgripper. 4. FABRICATION PROCESS OF THE GRIPPER
Modal analysis is conducted to illustrate the dynamic performance of the designed gripper structure. Fig. 8 reveals the first two continuous natural frequencies of the 835
The commercial program of MUMPs (Multi-Users MEMS Processes) supplies a cost-effiecient and a proof-ofconcept standard way for EMS product manufacture.
Proceedings of the 20th IFAC World Congress Sijie Yang et al. / IFAC PapersOnLine 50-1 (2017) 808–813 Toulouse, France, July 9-14, 2017
SiO2 layer
device layer
a
Handle layer
a stable performance, and it is easy to be controlled in future experiments when it is fabricated by SOIMUMPs method in the future.
Contact layer
d
REFERENCES
SiO2 layer b
e
813
Support layer
c
Fig. 10. Fabrication process of the microgripper Three approaches are widely used in MEMS fabrication, namely, PolyMUMPs, MetalMUMPs, and SOIMUMPs. In this work, SOIMUMPs is selected for the silicon-oninsulator (SOI) micromachining process. The fabrication process has been designed as the following steps: a. The device layer of silicon-on-insulator wafer is 50 µm, handle layer is 400 µm, and the SiO2 interlayer is 2 µm for substrate manufacture. b. The SiO2 layer of 1.5 µm was deposited on the bottom side by using thermal oxidation, and the layer is patterned by lithographlical. RIE (reactive ion etching) ion etching is used for remaining oxide layer remove. c. The backside is etched by DRIE (deep reactive ion etching). When the 200 µm of the handle layer is etched, the SiO2 patterned in step b is removed. And then, the remaining handle layer of 200 µm is etched by DRIE. The SiO2 interlayer is etched by RIE. d. A 250 nm aluminum layer is deposited on wafer top side by sputtering, and patterned to form a contact layer. e. The structure with comb actuators and capacitive sensors is etched by DRIE on the top side. And then, it is transported to the support layer which is called dummy wafer. In the future work, the gripper prototype will be fabricated for experimental study. 5. CONCLUSION This paper presents a novel design of an MEMS microgripper with two integrated decoupled orthogonal lateral electrostatic comb drives and two capacitive force sensors for testing the compressive and shear force of microscale samples. With the design of a 2-PP parallel structure, the decoupling property of the actuating part exhibits a good performance which is verified by FEA simulation results. Besides, the relatively low stiffness of the actuating part reflects the larger output displacement generated with the actuator. On the other hand, the sensing arm is linked to two parallelogram flexures which act as a guiding mechanism, and more accurate sensing result can be obtained by this design. Modal simulation analysis has been carried out, which shows that the microgripper has
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Volland. BE, Heerlein. H and Rangelow. IW. Electrostatically driven microgripper. Microelectronic Engineering, vol. 61, pp. 1015–1023, 2002. Xu. Q. Design, fabrication, and testing of an MEMS microgripper with dual-axis force sensor. IEEE Sensors Journal, vol. 15, pp. 6017–6026, 2015. Keekyoung. K, Xinyu. L, Yong. Z, Ji. C, Xiaoyu. W and Yu. S. Mechanical characterization of polymeric microcapsules using a force-feedback MEMS microgripper. Proc. of 2008 30th Annual Int. Conf. of IEEE Engineering in Medicine and Biology Society, pp. 1845–1848, 2008. Chen. T, Chen. L, Sun. L, Wang. J and Li. X. A sidewall piezoresistive force sensor used in a MEMS gripper. Proc. of Int. Conf. on Intelligent Robotics and Applications, pp. 207–216, 2008. Bazaz. S. A, Khan. F, and Shakoor. R. I. Design, simulation and testing of electrostatic SOI MUMPs based microgripper integrated with capacitive contact sensor. Sens. Actuators A, Phys, vol. 167, pp. 44–53, 2011. Pei. X, Yu. J, Zong. G and Bi. S. An effective pseudo-rigidbody method for beam-based compliant mechanisms. Precision Engineering, vol. 34, pp. 634–639, 2010. Muntwyler. S, Kratochvil. B. E, Beyeler. F, and Nelson. B. J. Monolithically integrated two-axis microtensile tester for the mechanical characterization of microscopic samples. Journal of Microelectromechanical Systems, vol. 19, pp. 1223–1233, 2010. Qu. J, Zhang. W, Jung. A, Silva-Da. C. S, Liu.X. A MEMS microgripper with two-axis actuators and force sensors for microscale mechanical characterization of soft materials. Proc. of 2015 IEEE Int. Conf. on Automation Science and Engineering (CASE), pp. 1620– 1625, 2015. Keoschkerjan. R and Wurmus. H. A novel microgripper with parallel movement of gripping arms. Proc. of 8th Int. Conf. on New Actuators, pp. 321–324, 2002. Amjad. K, Bazaz. S .A and Lai. Y. Design of an electrostatic MEMS microgripper system integrated with force sensor. Proc. of 2008 Int. Conf. on Microelectronics, pp. 236–239, 2008. Piriyanont. B, Moheimani. S.R and Bazaei. A. Design and control of a MEMS micro-gripper with integrated electro-thermal force sensor. Proc. of 2013 3rd Australian Control Conf. (AUCC), pp. 479–484, 2013. Xu. Q. Mechanism design and analysis of a novel 2-DOF compliant modular microgripper. Proc. of 2012 7th IEEE Conf. on Industrial Electronics and Applications (ICIEA), pp. 1966–1971, 2012. Howell, Larry. L. Compliant mechanisms. John Wiley & Sons, 35–47, 2001. Beyeler. F, Neild. A, Oberti. S, Bell. D. J, Sun. Y, Dual. J and Nelson. B. J. Monolithically fabricated microgripper with integrated force sensor for manipulating microobjects and biological cells aligned in an ultrasonic field. Journal of Microelectromechanical Systems, vol. 16, pp. 7–15, 2007.
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PapersOnLine 50-1 (2017) 808–813 DesignIFACand Analysis of a Decoupled XY Design and Analysis of a Decoupled XY Design Analysis of with a Decoupled XY MEMSand Microgripper Integrated MEMS Microgripper with Integrated ⋆ MEMS Microgripper with Integrated Dual-Axis Actuation and Force Sensing Dual-Axis Actuation and Force Sensing ⋆⋆ Dual-Axis Actuation and Force Sensing ∗ ∗∗
Sijie Yang ∗ , Qingsong Xu ∗∗ Sijie Yang ∗∗ , Qingsong Xu ∗∗ Sijie Yang , Qingsong Xu ∗∗ ∗ University of Macau, Avenida da Universidade, Taipa, Macau, China ∗ ∗ University of Macau, Avenida da Universidade, Taipa, Macau, China (e-mail: [email protected]). ∗ University of Macau, Avenida da Universidade, Taipa, Macau, China ∗∗ (e-mail: [email protected]). University of Macau, Avenida da Universidade, Taipa, Macau, ∗∗ (e-mail: [email protected]). ∗∗ University of Macau, [email protected]) da Universidade, Taipa, Macau, China (e-mail: ∗∗ University of Macau, [email protected]) da Universidade, Taipa, Macau, China (e-mail: China (e-mail: [email protected]) Abstract: This paper presents the design, analysis, and simulation study of a novel microAbstract: This paper themicrogipper design, analysis, anddecoupled simulation study of aactuators novel microelectromechanical systempresents (MEMS) with two electrostatic and Abstract: This paper presents themicrogipper design, analysis, anddecoupled simulation study of aactuators novel microelectromechanical system (MEMS) with two electrostatic two capacitive force sensors dedicated to the testing in compressive and shear directionsand of electromechanical system (MEMS) microgipper with two decoupled electrostatic and two samples. capacitiveThe force sensors dedicated the testing and shearactuators directions of soft gripper mechanism is to designed basedinoncompressive the two-prismatic-prismatic (2-PP) two capacitiveThe force sensors dedicatedis to the testing and shear directions of soft samples. gripper mechanism designed basedinoncompressive the two-prismatic-prismatic (2-PP) parallel structure and folded leaf flexure guiding mechanism. Owing to the design of decoupled soft samples. The gripper mechanism is designed based on the two-prismatic-prismatic (2-PP) parallel structure andthe folded leaf flexure guiding mechanism. Owing to the design decoupled actuation structure, translations of the two gripping arms are parallel, whichofensures the parallel structure andthe folded leaf flexure the guiding mechanism. Owing to the design ofensures decoupled actuation structure, translations twoobject gripping arms are parallel, whichAt the reaction force between two tips and of grasped in sole gripping direction. the same actuation structure, thetwo translations of the twoobject gripping armsgripping are parallel, whichAtensures the reaction force between tips and grasped in sole direction. the same time, this structure also offers a good decoupling property. The analytical model is verified by reaction force between two tips and grasped object in sole gripping direction. At the same time, thisout structure also offers a good decoupling property. analytical is verifiedand by carrying finite element analysis (FEA) simulation study,The which confirmsmodel the feasibility time, thisout structure also offers a good decoupling property. analytical is verifiedand by carrying finite analysis simulation study,The which confirmsmodel the process feasibility effectiveness of theelement proposed design.(FEA) The fabrication procedure by SOIMUMPs is also carrying out of finite element analysis (FEA) simulationprocedure study, which confirms the process feasibility and effectiveness the proposed design. The fabrication by SOIMUMPs is also given, and the microgripper will be fabricated for future experimental study. effectiveness of microgripper the proposedwill design. The fabrication procedure by SOIMUMPs process is also given, and the be fabricated for future experimental study. given, the(International microgripper will be of fabricated futureHosting experimental study. © 2017,and IFAC Federation Automaticfor Control) by Elsevier Ltd. All rights reserved. Keywords: MEMS gripper, 2-DOF microgripper, 2-PP parallel mechanism, FEA simulation, Keywords: MEMS gripper, 2-DOF microgripper, 2-PP parallel mechanism, FEA simulation, SOIMUMPs fabrication process. Keywords: MEMS gripper, 2-DOF microgripper, 2-PP parallel mechanism, FEA simulation, SOIMUMPs fabrication process. SOIMUMPs fabrication process. 1. INTRODUCTION low power consumption, no hysteresis, and fast response 1. INTRODUCTION low power consumption, hysteresis, and fast response speed are also suitable forno MEMS gripper application (Xu 1. INTRODUCTION low power consumption, no hysteresis, and fast response speed are also suitable for MEMS gripper application (Xu With the increasingly widespread development on mi- (2015)). speed are also suitable for MEMS gripper application (Xu (2015)). With the increasingly widespread development on microassembly and biomedical fields, various micro- and (2015)). With the increasingly widespread development on and mi- To avoid damaging the samples, various sensors are often croassembly and objects biomedical fields, micronanometer scaled require to bevarious manipulated safety To avoid in damaging the samples, various sensors are often installed the microgrippers, cooperating with actuating croassembly and biomedical fields, various microand nanometer scaled require manipulated safety To avoid in damaging the samples, various sensors are often and accurately. Asobjects a crucial tool to to be execute the micromainstalled the microgrippers, cooperating with actuating arm to form a completely integrated microgripper system, nanometer scaled objects require to be manipulated safety and accurately. a crucial tool to execute the employed microma- installed in the microgrippers, cooperating with actuating nipulation tasks,As MEMS microgripper is usually arm to form a completely integrated microgripper as reported in the literature, e.g., Chen et al system, (2008). and accurately. As a crucial tool to execute the micromanipulation tasks, MEMS is usually employed arm to form aincompletely integrated to pick-deliver-place tinymicrogripper objects including bio-samples as reported the methods, literature, e.g., microgripper Chen al system, (2008). Concerning sensing there are fouretmain types nipulation tasks, MEMS microgripper is usually employed to pick-deliver-place including bio-samples reported sensing in the methods, literature,there e.g., are Chen etmain al (2008). (Volland et al (2002)),tiny hair objects organization (Xu (2015)), and as Concerning four types of sensing schemes which are usually applied in MEMS to pick-deliver-place tiny objects including bio-samples (Volland et al (2002)), hair organization (Xu (2015)), and sensing which methods, there areapplied four main types soft material (Keekyoung et al (2008)). The goal of this Concerning of sensing schemes are usually in MEMS applications. Electro-thermal sensor reveals some merits (Volland et al (2002)), hair organization (Xu (2015)), and soft material (Keekyoung et al (2008)). The goal of this of sensing schemes which are sensor usuallyreveals applied in MEMS work is to design a novel 2-DOF MEMS-based microgripapplications. Electro-thermal some merits compact size, simple structure, and good resolution. soft material (Keekyoung et al (2008)). The goal of this like work is tointegrated design a novel 2-DOF microgripapplications. Electro-thermal sensor reveals some merits per with actuators andMEMS-based sensors. like compact size, simple structure, good resolution. While the limitations of this sensor lieand in the sensitivity to work is to design a novel 2-DOF MEMS-based microgripper with integrated actuators and sensors. like compact size, simple structure, and good resolution. thetemperature limitations of this sensorconsumption. lie in the sensitivity to ambient and energy Piezoelecper with integrated actuators sensors. Regarding the driving scheme,and different types of actuators While While thetemperature limitations of this sensorconsumption. lie in the sensitivity to ambient and energy Piezoelectric sensor has a wide bandwidth and flexibility. However, Regarding the driving scheme, different types of actuators can be adopted to actuate MEMS grippers. For example, ambient temperature and energy consumption. PiezoelecRegarding the driving scheme, different types of actuators tric sensor has a wide bandwidth and flexibility. However, manufacturing problem is a big issue of this type of can be adopted actuator to actuate MEMS grippers. For example, electro-thermal exhibits some advantages of large the sensor has a wide bandwidth andissue flexibility. can be adopted actuator to actuate MEMS grippers. For example, the manufacturing problem a big of thisHowever, typebut of electro-thermal exhibits advantages of worklarge tric sensor. Pizeoresistive sensor ishas a high bandwidth, force and low input voltage. But some the relatively high the manufacturing problem is a big issue of this typebut of electro-thermal actuator exhibits some advantages of large sensor. Pizeoresistive sensor has a high bandwidth, the large footprint and complex fabrication process are force and low input voltage. But the relatively high working temperature restricts its application in some situations. sensor. Pizeoresistive sensor has a high bandwidth, but force and low input voltage. But the relatively high worklarge footprint complex as fabrication are unwanted in practice.and In addition, comparedprocess with other ing temperature restricts its application in some situations. Another commonly used actuator in MEMS area is shape the the large footprint and complex as fabrication processother are ing temperature restricts its application in some situations. in practice. In addition, compared types of sensors, capacitive sensor (Bazaz et with al (2011)) Another used in MEMS area is shape memory commonly alloy, which hasactuator high energy density and large unwanted unwanted in practice. In addition, as (Bazaz compared with other Another commonly used actuator in MEMS areaand is shape types of sensors, capacitive sensor et al (2011)) memory alloy, which has high energy density large provide fast response speed, low noise disturbance, stroke. While the high power consumption and hysteresis can of sensors, capacitivespeed, sensorlow (Bazaz etdisturbance, al (2011)) memoryWhile alloy,the which has highconsumption energy density large types fast response high provide bandwidth, and simple physical noise structure. Hence, stroke. high power and and hysteresis effect are two main problems. As for piezoelectric actuator, can can provide fast response speed, low noise disturbance, stroke. While the high power consumption and hysteresis bandwidth, anddesigned simple physical structure. Hence, capacitive sensors are for the microgripper in this effect are two problems. for piezoelectric actuator, although the main bandwidth andAsresponse speed are good high high bandwidth, anddesigned simple physical structure. Hence, effect are two main problems. for piezoelectric actuator, sensors are for the microgripper in this paper. although thecomplex bandwidth andAs response speed are enough, the material process complicates thegood fab- capacitive capacitive sensors are designed for the microgripper in this although thecomplex bandwidth andprocess response speed are good paper. enough, the material complicates the fabrication. In this work, electrostatic actuator is selected for paper. Mechanism structure is another important factor which enough, the complex material process complicates the fabrication. Ininthis work, electrostatic is selected for Mechanism structure is another important factor which actuation both axes, because itactuator has simple structure needs to be considered in the device design. Different rication. In this work, electrostatic actuator is selected for actuation in both axes, becauseMeanwhile, it has simple structure is another important factor which and it is easy to manufacture. the merits of Mechanism needs to bestructure considered in theneeds device Different constructions satisfy different of design. the device. For actuation in both axes, because it has simple structure and it is easy to manufacture. Meanwhile, the merits of needs to be considered in the device design. Different satisfy different needs(DOF) of the grippers device. For ⋆ example, multi-degree of freedom are and it work is easy manufacture. theScience meritsand of constructions This wastosupported in partMeanwhile, by the Macao constructions satisfy different needs(DOF) of the grippers device. For ⋆ This work was supported in part by the Macao Science and example, multi-degree of freedom are proposed in previous work by Muntwyler et al (2010) Technology Development Fund under Grant 090/2015/A3 ⋆ This work was supported in part by the Macao Science and example, multi-degree of freedom (DOF) grippers are proposed in previous work by Muntwyler et al (2010) Technology Development Fund under Grant 090/2015/A3 and 024/2014/A1. and Qu et al. (2015). For testing the compressive and proposed in previous work by Muntwyler et al (2010) Technology Development Fund under Grant 090/2015/A3 and ⋆⋆ 024/2014/A1. and et al. For testing the compressive and Corresponding author: Q. Xu (Phone: +853 8822 4278; Fax: +853 shearQu forces of (2015). micro-objects, the grippers are actuated 024/2014/A1. and Qu et al. (2015). For testing the compressive and ⋆⋆ Corresponding author: Q. Xu (Phone: +853 8822 4278; Fax: +853 8822 2426; E-mail: [email protected]). shear forces of micro-objects, the grippers are actuated ⋆⋆Corresponding author: Q. Xu (Phone: +853 8822 4278; Fax: +853 shear forces of micro-objects, the grippers are actuated 8822 2426; E-mail: [email protected]). 8822 2426; E-mail: [email protected]). Copyright © 2017, 2017 IFAC 831Hosting by Elsevier Ltd. All rights reserved. 2405-8963 © IFAC (International Federation of Automatic Control) Copyright © 2017 IFAC 831 Peer review under responsibility of International Federation of Automatic Copyright © 2017 IFAC 831Control. 10.1016/j.ifacol.2017.08.144
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actuating arm
809
sensing arm
lr VOLTAGE
q
lf Fixed pads
actuating lateral comb drive
lp
lsy
m
lsx
FX
sensing transverse comb drive
Ft
Fx
Fig. 2. The diagram of working principle for comb-drive electrostatic actuator.
Fig. 1. The schematic sketch of the MEMS microgripper with parameter symbols. by different actuation schemes in both x- and y-axes along with integrated sensors to detect the force in both vertical and horizontal directions. However, the aforementioned two grippers have the problem of motion coupling. The generated actuating forces in the two axes of the arm are not independent, which influences the accuracy of the output displacement for the gripping jaw. In addition, majority of the existing grippers grasp objects by rotating the gripper arms. This process causes the reaction force between gripper tips and grasped objects. When the gripped objects have a circular and smooth surface, they are easily pushed out and slip out of the gripper arms with the generated force (Keoschkerjan (2002)). To this end, we present, in this paper, the design of a new decoupled dual-axis actuation MEMS gripper with integrated force sensing capabilities based on the methodology of pseudo-rigid-body (PRB) model. The aforementioned issue is overcome by a new structure design with parallel movement of the gripping tip. The remaining parts of the paper are arranged as follows. The gripper design is illustrated in section 2. Finite element analysis (FEA) simulation is introduced in section 3 to verify the feasibility of analytical model. Section 4 describes the gripper fabrication process. Section 5 concludes this paper. 2. DESIGN OF THE GRIPPER A schematic diagram of the microgripper is shown in Fig. 1. This gripper consists of a left actuating arm and a right sensing arm. As for the actuating part, two comb-drive electrostatic actuators are nested inside decoupled XYmotion stage. Each actuator is supported by four folded leaf flexures (FLF), which are connected to the move-body and fixed by two pads. The gripper tips have an initial gap of 100 µm. The comb-drive actuator along x-axis pushes the left arm to grasp objects. Another actuator along yaxis pushes the left arm slightly higher than the right arm to conduct the shear testing. 832
It is notable that the actuation arm is composed of two prismatic-prismatic (PP) parallel structure. In micromotion scale, with the help of this configuration, the movement of the actuation tip is approximately parallel. Thus, the aforementioned problem caused by reaction force can be overcome. In addition, the sensing data can also be more accurate. The sensing arm is linked to two pairs of parallelogram flexures in x- and y-axes, which are connected with two comb-drive capacitive sensors and organized perpendicular to each other. They have the capabilities of testing compression and shear forces generated in the sample object, respectively. For more accurate test of the sensing force in two directions, the parallelogram flexures act as a guiding mechanism to transmit the force from actuation arm to sensing arm. 2.1 Actuator Design Electrostatic actuator is commonly used in MEMS domain. Owing to its simple structure, fast response speed, and low energy consumption, it has been widely accepted in microgripper actuation scheme. In general, according to the construction, electrostatic actuators can be classified into two types, namely, transverse comb-drive and lateral comb-drive. In this paper, lateral comb-drive is employed to produce a larger displacement to drive the gripper. Electrostatic lateral-comb actuator is shown in Fig. 2. The driving forces of the two identical actuators are expressed by: εtV 2 N Fx = Fy = (1) q where ε = 8.85 × 10−12 C 2 /(N m2 ) is the permittivity of air, q is the gap width between two adjacent plates, t is the thickness of gripper, V is the driving voltage, and N represents the number of comb-tooth pairs. 2.2 Design of Folded Leaf Flexure In order to connect the actuating part to fixed pads, leaf flexures are introduced in this work. Traditional straight leaf flexures are slender (Amjad et al (2008); Piriyanont et al (2013)). Hence, by using this type of long straight leaf flexure, it is prone to cause the load effect problem and enlarge the footprint of the device size. On the other
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layer 3
ac layer 1
layer 3
g2
layer 2
Fg layer 1
X
layer 2
Gain
Vout
F
Fig. 3. Schematic sketch of the folded leaf flexure and its deformation.
leaf flexure
g1
ac MS3110
Fig. 5. Schematic diagram of the working principle for capacitive sensor.
O1 lr
lr
Θ
lb
O2 F
Fig. 4. PRB model of a parallelogram flexure. hand, slender leaf flexure has another drawback of stress stiffening. That is, due to the compressive stress occurring in leaf flexure, the transverse stiffness will increase gradually. In this work, the folded leaf flexure (FLF) is introduced to solve the aforementioned problem. By folding the flexures in parallel, not only the physical size of microgripper is reduced, but also the stress stiffening is mitigated to a large extent. It also plays a role of translation guiding mechanism, which guides the force through a pure translational motion in one direction. Taking 3-layer flexures as an example, the structure schematic of FLFs is shown in Fig. 3. Hence, one can get the equations below: F lf2 M lf − =0 (2) 2EI EI F lf3 M lf2 − =χ (3) 3EI 2EI 3 where E is Young’s modulus of Silicon, I = bh 12 is the moment of inertia, F and M are the axial force and moment of force, and lf is the length of folded flexure. By substituting equation (2) to equation (3), the displacement of one single flexure can be solved as follows: F lf3 χ= (4) 12EI The stiffness of the folded flexure can be calculated as follows: Ebh3 F = Kf = (5) Nχ N lf3 where N = 3 in this case design. 2.3 Mechanism Structure Design To realize the two perpendicular axes of pure translational movement for the gripping arm, the two-prismaticprismatic (2-PP) parallel structure as introduced by Xu 833
(2012) is adopted in this work. The actuating mechanism is designed based on pseudo-rigid-body (PRB) (Pei et al (2010)), which is illustrated in Fig. 4. Assume that the flexures experience bending deformation only. As a result, each flexure of the module exhibits a 1-DOF rotational compliance. While the remaining parts of this module are rigid. Hence, the output displacement of each movable body has the same compliant deformation, which mainly arises from the four compound parallelogram flexures. The PRB analytical model of one flexure is shown in Fig. 4, where O1 and O2 act as two pivots. lb is the characteristic radius. Referring to Howell (2001), the torsional spring constant can be expressed by: 2γKΘEI Kc = (6) lr where γ = 0.85 is the characteristic radius factor, and KΘ = 2.65 is a constant. Based on PRB model, the translational stiffness Kr of the compound parallelogram flexure can be expressed as: Kc Kr = 2 (7) lb Hence, the output displacement can be calculated as follows: Kt 0 d1 Fx = (8) 0 Kt d2 Fy where Kt is the total stiffness in one working axis. d1 and d2 are the output displacements of the two actuators, respectively. The output displacement of the actuating arm in both x and y axes can be computed by: Ft Ft d1 = d2 = = (9) Kt Kf + Kr where Ft = Fx = Fy . 2.4 Sensor Design The working principle of the capacitive sensor is shown in Fig. 5. Taking the capacitive sensor for compressive force testing as an example, the sensor design process is presented in this section. Assume that only the compressive action produces the force. When an object is clamped by the two gripper tips,
Proceedings of the 20th IFAC World Congress Sijie Yang et al. / IFAC PapersOnLine 50-1 (2017) 808–813 Toulouse, France, July 9-14, 2017
811
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 6. FEA static simulation results of the actuation arm. (a)–(c) A force of 100 µN is applied on movable plate in x-axis: (a) total deformation, (b) x-axis deformation, (c) y-axis deformation; (d)–(f) A force of 100 µN is applied on movable plate in y-axis: (d) total deformation, (e) y-axis deformation, (f) x-axis deformation. Ksy = Kp + Ky =
2Etwp3 4Etws3 + 3 lp3 lsy
(11)
The gripping force Fsx and Fsy , which are generated in x and y axes, can be computed as: Fsx = Ksx × Dx
(a)
(13) Fsy = Ksy × Dy where Dx and Dy represent the displacements of right tip in x and y axes, respectively.
(b)
Fig. 7. FEA static simulation result of the sensing arm. the gripping force will be transmitted from the actuating tip to sensing tip in horizontal direction. During the force transmission by two parallelogram flexures, the movable plates which are fixed by four leaf flexures will move forward along the force transmission direction. The two distances between the two parallel plates of the capacitive sensor will change, respectively. Thus, the capacitive difference of the sensor will change simultaneously. The total stiffness of the right sensing arm in x-axis can be expressed as: Ksx = Kp + Kx =
2Etwp3 4Etws3 + 3 lp3 lsx
(12)
(10)
Similarly, the total stiffness of the sensing arm in y-axis can be calculated as follows: 834
The voltage Vout of output signal is generated by a converter chip (e.g., MS3110 produced by MicroSensors, Inc.), which converts capacitive change to voltage variation. That is, C1 − C2 Vout ∝ G × (14) Cf with A A A A C1 = ε( + ), C2 = ε( + ) (15) g1 − δ g2 + δ g1 + δ g2 − δ where G and Cf values can be tuned according to specific situation. A represents the plate area. g1 and g2 are two gaps of the parallel capacitive plates. The output voltage and the deformation δ is proportional (Beyeler et al (2007)). 3. FEA SIMULATION STUDY In this section, FEA simulation study is carried out to evaluate the performance of the designed microgripper. To
Proceedings of the 20th IFAC World Congress 812 Sijie Yang et al. / IFAC PapersOnLine 50-1 (2017) 808–813 Toulouse, France, July 9-14, 2017
Table 1. Main parameters of the microgripper Parameter length of folded flexure length of sensing flexure 1 length of sensing flexure 2 length of rotation flexure length of parallelogram flexure thickness of device layer width of actuating flexure width of folded flexure width of sensing flexure width of rotation flexure width of parallelogram flexure gap of teeth length of teeth overleap gap 1 of capacitive plates gap 2 of capacitive plates number of teeth paris
Symbol lf lsx lsy lr lp t wa wf ws wr wp q m g1 g2 N
Value 600 400 450 1200 450 30 7 7 5 7 7 5 30 5 20 736
Unit µm µm µm µm µm µm µm µm µm µm µm µm µm µm µm µm
(a)
(b)
Fig. 8. FEA modal simulation results of the actuation part.
simplify the simulation process, electrostatic comb drive and capacitive sensor are not taken into account. Besides, the actuating and sensing parts are simulated, respectively. Actually, this does not influence the simulation results. ANSYS software is used for the static and modal simulation studies. The main parameters of the gripper are list in Table 1. 3.1 Static Simulation
(a)
(b)
Fig. 9. FEA modal simulation results of the sensing part.
As for the actuation part, four fixed pads are set as fixed supports when conducting the FEA simulation. Two forces of 100 µN, which are generated by electrostatic actuator, are applied at the end of the movable plates in x and y axes, respectively. The simulation results are shown in Fig. 6. The total displacements of the left tip are depicted in Fig. 6(a) and (d), which are 15.054 µm and 14.653 µm, respectively. Besides, the results of the decoupling capability analysis of the gripper’s actuating part are illustrated in Fig. 6(b) and (c) for x-axis and Fig. 6(e) and (f) for y-axis, respectively. It is clear to see that the cross-axis difference in x-axis direction are 15.05 µm and 0.1 µm along x and y axes, respectively. In view of the decoupling property in y-axis direction, 14.654 µm and 0.04 µm displacements appear in x- and y-axis, respectively. Hence, the decoupling property for the left part of the microgripper can be calculated as 0.6% and 0.2% in x and y directions, respectively. Moreover, simulation results of the right sensing arm are demonstrated in Fig. 7. Fig. 7(a) and (b) illustrate the force testing in two different axes, respectively. By applying two forces of 20 µN on the top area and grasping area of sensing tip, the displacements in these two directions can be obtained. Then, the stiffnesses Ksx and Ksy of the right arm in x and y axes can be computed, respectively. In order to have a clear comparison, the results of analytical model and FEA simulation are listed in Table 2. And the property of decoupling is shown in Table 3 3.2 Modal Analysis Simulation
Table 2. Comparison between analytical model and FEA simulation results Symbol Ktx Kty Ksx Ksy
analytical 6.59 6.59 73.5 62
simulation 6.64 6.82 74.1 66.7
error 0.7% 3.4% 0.8% 7%
unit µN/µm µN/µm µN/µm µN/µm
Table 3. Result of decoupling property for actuating part Direction x-axis y-axis
Cross-axis error 0.6% 0.2%
left actuating part of the micro-gripper. According to the results, the two directional movements of the gripper arm appear in two frequencies of 1575.8 Hz and 1577.7 Hz, respectively. The modal simulation of the right sensing part is also conducted by FEA. The result is shown in Fig. 9, which indicates the two directional motions of the sensing arm in x and y axes. The corresponding resonant frequencies are 8742 Hz and 9340.2 Hz, respectively. The simulation result shows that the stiffness of the sensing arm is much higher than that of the actuation arm. Thus, the sensing arm will undergo a smaller deformation than the actuation arm when the object is grasped. In addition, the relatively high natural frequency reveals a fine bandwidth performance of the microgripper. 4. FABRICATION PROCESS OF THE GRIPPER
Modal analysis is conducted to illustrate the dynamic performance of the designed gripper structure. Fig. 8 reveals the first two continuous natural frequencies of the 835
The commercial program of MUMPs (Multi-Users MEMS Processes) supplies a cost-effiecient and a proof-ofconcept standard way for EMS product manufacture.
Proceedings of the 20th IFAC World Congress Sijie Yang et al. / IFAC PapersOnLine 50-1 (2017) 808–813 Toulouse, France, July 9-14, 2017
SiO2 layer
device layer
a
Handle layer
a stable performance, and it is easy to be controlled in future experiments when it is fabricated by SOIMUMPs method in the future.
Contact layer
d
REFERENCES
SiO2 layer b
e
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Support layer
c
Fig. 10. Fabrication process of the microgripper Three approaches are widely used in MEMS fabrication, namely, PolyMUMPs, MetalMUMPs, and SOIMUMPs. In this work, SOIMUMPs is selected for the silicon-oninsulator (SOI) micromachining process. The fabrication process has been designed as the following steps: a. The device layer of silicon-on-insulator wafer is 50 µm, handle layer is 400 µm, and the SiO2 interlayer is 2 µm for substrate manufacture. b. The SiO2 layer of 1.5 µm was deposited on the bottom side by using thermal oxidation, and the layer is patterned by lithographlical. RIE (reactive ion etching) ion etching is used for remaining oxide layer remove. c. The backside is etched by DRIE (deep reactive ion etching). When the 200 µm of the handle layer is etched, the SiO2 patterned in step b is removed. And then, the remaining handle layer of 200 µm is etched by DRIE. The SiO2 interlayer is etched by RIE. d. A 250 nm aluminum layer is deposited on wafer top side by sputtering, and patterned to form a contact layer. e. The structure with comb actuators and capacitive sensors is etched by DRIE on the top side. And then, it is transported to the support layer which is called dummy wafer. In the future work, the gripper prototype will be fabricated for experimental study. 5. CONCLUSION This paper presents a novel design of an MEMS microgripper with two integrated decoupled orthogonal lateral electrostatic comb drives and two capacitive force sensors for testing the compressive and shear force of microscale samples. With the design of a 2-PP parallel structure, the decoupling property of the actuating part exhibits a good performance which is verified by FEA simulation results. Besides, the relatively low stiffness of the actuating part reflects the larger output displacement generated with the actuator. On the other hand, the sensing arm is linked to two parallelogram flexures which act as a guiding mechanism, and more accurate sensing result can be obtained by this design. Modal simulation analysis has been carried out, which shows that the microgripper has
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