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A novel MEMS omnidirectional inertial switch with flexible electrodes
Sensors and Actuators A 212 (2014) 93–101
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A novel MEMS omnidirectional inertial switch with flexible electrodes Xi Zhanwen a,∗ , Zhang Ping a , Nie Weirong a , Du Liqun b , Cao Yun a a b
School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 24 October 2013 Received in revised form 17 February 2014 Accepted 24 February 2014 Available online 14 March 2014 Keywords: Omnidirectional inertial switch MEMS Flexible electrode Enhanced contact effect
a b s t r a c t A novel MEMS omnidirectional inertial switch was designed, simulated and fabricated. The switch is composed of three main parts, the proof mass, the axial flexible electrode and four radial flexible electrodes. It introduces the flexible electrodes to form a dual mass–spring system. The switch has omnidirectional sensitivities in a half sphere. When any acceleration, over the threshold value in radial or/and axial direction, acts to the switch, the switch will turn on. Also, the switch has the enhanced contact effect. Dynamic simulation results based on FEM confirm that the contact effect is improved by this new design compared to that of traditional inertial switch. The contact duration is prolonged under the shock loading, and the bouncing effect is alleviated. The switch has a 6-layer structure, which is manufactured based on non-silicon surface micromachining technology. The tests have been done and the results coincide with that of the simulation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction MEMS inertial switches, also known as to shock sensors or threshold accelerometers, have great potential to be widely used in toys, accessories, automotive, military weapons and industrial applications. That is due to their smaller size, lower cost, less power consumption, more functionality and better performance than conventional mechanical ones. Furthermore, the inertial switches have ability of avoiding electromagnetic interference in applications [1,2]. The MEMS omnidirectional inertial switch usually has a structure with a mass–spring system, where the proof mass served as the movable electrode and is suspended by surrounding springs [3] .There is also other structure with a proof mass suspended by central springs [4–6]. The schematic diagram of these conventional designs is shown in Fig. 1. In Fig. 1, when the switch responses to a shock acceleration, the movable electrode will contact with stationary electrode in rigid mode. The contact-bouncing effect would be inevitable and the switch-on time is transient (usually less than 10 s)[7,8]. The poor contact-bouncing effect and the short switch-on time make it difficult for signal processing and weaken the reliability of the switch [9,10]. The movable contact point [1,7], squeeze film effect [1], the carbon nanotube (CNT)contact pad [11], electrostatic force [12], have been adopted to
∗ Corresponding author. Tel.: +86 2584303068. E-mail address: [email protected] (X. Zhanwen). http://dx.doi.org/10.1016/j.sna.2014.02.035 0924-4247/© 2014 Elsevier B.V. All rights reserved.
the inertial micro-switches to prolong the contact duration and eliminate contact-bouncing effect, but these inertial switches were limited to a single axis of sensing. The present work proposed a novel design of the MEMS omnidirectional inertial switch with four radial and an axial flexible electrodes shown in Fig. 2. By introducing this flexible contact mechanism, the switch forms a dual mass–spring system, which gives the improved switch characteristics such as omnidirectional sensitivities, reduced contact-bouncing effect and prolonged switch-on time. 2. The inertial switch design and simulation 2.1. Design and working principle The structure of the omnidirectional inertial switch is illustrated in Fig. 3. It senses acceleration in hemisphere (both in-plane and out-of-plane) with the single proof mass. The proof mass thickness t is designed to be much larger than the spring thickness t1 in order to minimize the area coverage while enabling the desired radial and axial direction sensitivity. Four radial flexible electrodes supported by spring are symmetrical around the proof mass. Each one has the gap of d1 from the proof mass. The circle loop supported by an elastic cross beam is used as the axial flexible electrode. It has a distance of d2 from the proof mass, which also helps to enhance the contact effect in axial direction. The use of flexible electrodes is to prolong the switch-on time and make the switch-on state more stable.
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Fig. 1. A schematic diagram of conventional inertial switch.
Fig. 2. A schematic diagram of the novel inertial switch.
Apart from the proof mass–spring system (m1 , k1 ), the radial flexible electrodes are suspended by spring instead of being rigidly fixed on the substrate. The axial flexible electrode is supported by elastic cross beam. The radial and axial flexible electrodes have contact gap d1 and d2 from the proof mass, respectively. For example, as for an acceleration acting along the +Z axis, the working principle is demonstrated by the dynamic process and corresponding switch state, as shown in Fig. 4. Fig. 4(a) shows the action process of a conventional inertial MEMS switch with the fixed electrode. Fig. 4(b) explains the action process of the novel inertial MEMS switch with flexible electrodes. In Fig. 4(b), (1) The proof mass moves towards one of flexible electrodes due to the acceleration. (2) When the acceleration exceeds the threshold value, the displacement reaches d2 , the proof mass contacts the flexible electrode and the switch is turned on. (3) The proof mass keeps moving on with the flexible electrode, therefore the switch-on state is held on for a longer time. (4) After the disappearance of the acceleration, the proof mass and the flexible electrode rebound, the switch is turned off, until the proof mass is separated from the flexible electrode. (5) Finally, the proof mass and the flexible electrode restore to the equilibrium position after all the energy is dissipated by free vibration. As for an acceleration acting along other directions in hemisphere, the proof mass moves towards the flexible electrodes in radial and/or axial direction, the switch state is similar to aforementioned cases. The inertial switch has radial and axial flexible electrodes, and it is able to provide identifiable direction information according to the identified electrode position. When acceleration is applied on the switch in the sensitive directions, the proof mass will be subjected to the inertial acceleration a(t) in the opposite direction. Take radial direction motion as example, the responding motion can be expressed with the motion equation as: m1 x¨ + c x˙ + k1 x + m1 a(t) = 0
(1)
where m1 is the mass of the proof mass, x is the displacement in radial direction, c presents the damping coefficient, k1 is the system stiffness and a(t) is the applied acceleration component in radial direction and is a function of time t. The acceleration a(t) applied to the switch in practical work is similar to a half-sine pulse with amplitude a0 and duration t0 (about 1 ms). To simplify the solving process, c is neglected [9]. By solving Eq. (1), we have the displacement x(t) of the proof mass. Fig. 3. Structural sketch map of the proposed inertial switch. (a) A conventional inertial MEMS switch with the fixed electrodes (b) A inertial MEMS switch with flexible electrodes.
x(t) =
a0 ωn2 − ω02
sin ω0 t −
ω0 sin ωn t ωn
Fig. 4. Comparison of the dynamic process and switch state between (a) and (b).
(2)
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Table 1 Geometric parameters of the switch. Component
Geometric parameter
Value (m)
Proof mass
Radius r Thickness t Width w1 /w2
455 100 18/20
Thickness t1 /t2 Meander width b1 /b2 Connector radius r1 /r2 Circle loop outer radius R3 Inner radius r3 Thickness t3 Cross beam length l4 Width w4 Thickness t4 d1
40/40 600/330 31/20 94 80 30 330 50 30 25
d2
30
Proof mass spring/radial flexible electrode spring
Axial flexible electrode
Distance to radial flexible electrodes Distance to axial flexible electrode Fig. 5. The theoretical curve and simulation results of threshold acceleration ath versus ωn .
Table 2 The displacement of proof mass in radial and axial direction (unit: m).
where
ˇ
ω0 = , ωn = t0
0◦
k1 m1
(3)
when the maximum value of x(t) is equal to d1 (d1 = x0 ) and ω0 < ωn < 5ω0 , the corresponding applied acceleration is threshold acceleration ath. ath = x0
˛
ωn2 − ω02
90◦ 75◦ 60◦ 45◦ 30◦ 15◦ 0◦
15◦
30◦
45◦
Radial
Axial
Radial
Axial
Radial
Axial
Radial
Axial
32.0 31.0 27.8 22.7 16.0 8.3 0.0
0.0 10.0 19.4 27.5 33.8 37.8 39.3
32.0 31.0 27.8 22.7 16.1 8.3 0.0
0.0 10.0 19.4 27.5 33.8 37.8 39.3
32.2 30.9 27.8 22.7 16.0 8.2 0.0
0.0 10.0 19.5 27.6 33.8 37.8 39.3
32.1 30.9 27.8 22.7 16.0 8.3 0.0
0.0 10.1 19.5 27.6 33.9 37.8 39.3
(sin(2ω0 /(ωn + ω0 )) − (ω0 /ωn ) sin(2ωn /(ωn + ω0 ))) (4) 2.2. Dynamic simulation on the inertial switch
The threshold acceleration ath is related to gap d1 (d1 = x0 ) and ωn = k1 /m1 . When ω0 < ωn < 5ω0 , as ωn increases, the threshold acceleration increases gradually. By changing the mass of the proof mass, different values of ωn are obtained, so the switch with different ωn can be simulated. The threshold acceleration obtained by simulation is consistent with the theoretical results, as shown in Fig. 5. When the displacement of the proof mass in radial or axial direction is equal to the gap d1 or d2 , respectively, the threshold acceleration ath of the switch in radial or axial direction would be obtained. The mass–spring system stiffness is different in radial and axial direction, so it is needed to adjust gaps d1 and d2 . Therefore the consistent threshold acceleration in radial and axial direction could be obtained. The switch has only axial and radial flexible electrodes, so the exactly same threshold acceleration is impossible at any direction in hemisphere for the omnidirectional switch. The threshold acceleration of the proposed omnidirectional switch can be calculated using the parameters. And it can be confirmed by applying different amplitude loads in radial and axial direction.
The geometric parameters used in the simulation are listed in Table 1. The switch model is built in ANSYS. A full simulation model is employed. The material is nickel, where the Young’s modulus is 180 GPa and Poisson’s ratio is 0.3 [13]. Then the model is meshed and the grid of contact area is made to have high quality. Finally, the displacements of outer end of the springs and cross beams are constrained to be zero at all degrees of freedom.
2.2.1. Modal analysis The spring–mass systems of designed device structures are selected for the finite element modal analysis. And simulation modal results of the switch are shown in Fig. 6. The first four modal frequencies are 1961.7 Hz, 2101.1 Hz, 2101.1 Hz, 5281 Hz, respectively. The first mode shape is in the vertical direction, the second and third mode shape is in the horizontal direction. The fourth mode is far away from the first three modes.
Fig. 6. The first four natural frequencies of the spring-mass system.
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Fig. 8. The maximum displacement of the mass under 450 g in 3-D space.
Fig. 7. Pulse acceleration curve and different direction in hemisphere.
2.2.2. The displacement response of proof mass By applying half sine acceleration pulse with the 450 g amplitude and duration 1.0 ms, transient simulation has been done at different direction in hemisphere, as shown in Fig. 7. The displacements of the proof mass in radial and axial direction are listed in Table 2. Fig. 8 shows a simulated displacement distribution of the proof mass in the hemisphere. Table 2 shows that the displacements of proof mass in radial direction are related to the included angle between the acceleration and the Z-axis. As a result, the omnidirectional switch will have uniform threshold acceleration in the radial directions. The maximum displacements of proof mass in radial and axial direction are 32 m and 39 m, respectively. When d1 = 25 m, the threshold acceleration in radial direction is 355 g, and when d2 = 30 m, the threshold acceleration in the axial direction is 350 g. Meanwhile, the proof mass contacts slightly the flexible electrodes, but it is not enough to trigger the switch on-state signal. Only if the input acceleration is greater than the threshold we can make sure of switch onstate. 2.2.3. The omnidirectional sensitivity of the switch Fig. 3 shows that the MEMS omnidirectional switch has radial and axial flexible electrodes, the radial and axial flexible electrodes
have a contact gap d1 and d2 from the proof mass, respectively. Ideally, the movement of the proof mass is translational motion, the contact of the proof mass with the flexible electrode occurs when the displacement component in the radial or axial direction is greater than the gap. In XOZ plane, the maximum displacement curve of proof mass approximates arc circular (Fig. 8) under the same acceleration. Therefore, it is impossible to use the exactly same acceleration value as a switch threshold acceleration in any direction. The acceleration threshold in X, Z direction are 355 g and 350 g, respectively, corresponding to the fabricated inertial switch with the gap values of d1 = 25 m and d2 = 30 m. Only the acceleration from the Z direction with the value of 350 g can make the switch closed in the Z direction. When the acceleration is 400 g respectively within the scope of X, Z axis angle of 29◦ , the switch can be closed with a closed blind angle of 32◦ . The switch will close under the acceleration of more than 499 g in any direction in the XOZ plane, and there is no closed blind angle. Fig. 9(a) shows that the switch is closed only in Z-direction, as 350 g acting in different directions. Fig. 9(b) shows that it has closed blind corner in XOZ plane, as 400 g acting in different directions. Fig. 9(c) shows that it can be closed in any direction in the xoz plane, as 499 g acting in different directions. For the design of MEMS omnidirectional inertial switch, the threshold acceleration can be defined as radial and axial directional threshold acceleration respectively. When any acceleration
Fig. 9. The states of the switch contacted between the proof mass and flexible electrodes.
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Fig. 10. Simulated displacement–time curves of the proof mass in the switch under 450 g half-sine acceleration in radial and axial direction.
component in radial or/and axial direction applied to the omnidirectional switch is over the threshold acceleration, the switch will turn on. It indicates that the switch has omnidirectional sensitivity. 2.2.4. The switch on-state duration When the proof mass is induced by the acceleration, the pair of switch electrodes move to contact, forming the electrical connection. As the two electrodes get closer, the flexible electrode is moving with the proof mass until the two electrodes are separated. Resultantly, the electrical on-time becomes much longer compared to the conventional inertial micro switches based on two rigid electrodes depicted in Fig. 4(a) and (b). In order to clearly illustrate the dynamic impact process, the over-threshold acceleration, 450 g, was applied to the switch along its radial and axial direction. Fig. 10 shows the simulated displacement–time curves of the proof mass in radial and axial direction. As a comparison, the contact duration of the radial flexible electrodes can prolong to 45 s in Fig. 10(b), which is much longer than the electrode being rigidly fixed on substrate in
Fig. 10(a). The contact duration of the axial flexible electrode was prolonged to 33 s in Fig. 10(d), which is much longer than that in Fig. 10(c). The stiffness coefficient k2 of the radial flexible electrode also influences the contact-effect, either larger or smaller value will lead to undesirable results. Theoretical formula of the stiffness of micro spring was deduced, which has been proven that this analytical calculation method and finite element simulation can match to better than 1% [14,15]. By reducing the width of the spring, we can get a range of different spring stiffness coefficient. When the stiffness coefficient k2 is too smaller, such as 147.98 N/m (the width is 6 m), the proof mass and the flexible electrodes will have contact effect, the flexible electrodes will bounce, causing discontinuous contact between the two electrodes, shown in Fig. 11(a). When the stiffness coefficient k2 becomes larger, such as 15312.35 N/m (the width is 26 m), the displacement of the flexible electrode becomes smaller and the contact duration is shorter, shown in Fig. 11 (c). Taking into account the fabrication process, the spring width designed is 20 m, as shown in Fig. 11 (b), which eliminates the bounce
Fig. 11. The contact duration in different stiffness of the radial flexible electrode.
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Fig. 12. Dependence of the dynamic properties of the switch on contact time versus the stiffness.
Fig. 14. SEM image of the omnidirectional inertial switch.
effect when the proof mass touches the flexible electrode and has extended contact duration. Fig. 12 shows the relationship between k2 and contact duration.
proof mass, axial flexible electrodes, radial flexible electrodes, support springs and anchors, three times sputtering of Cu seed layers and releasing of the three-dimensional suspended structure. The fabrication details of the omnidirectional inertial switch are shown in Fig. 13. As electrical conductive layer, Cu seed layers were sputtered with the thickness about 200 nm. In order to release the three-dimensional suspended structure, inorganic acid boiling was used to remove the SU-8 sacrificial layer. The SEM image of the switch with a size of 2.8 mm × 2.8 mm × 210 m and minimum linewidth 15 m is shown in Fig. 14. Because the omnidirectional inertial switch is a six-layer device and was fabricated by six times
3. The fabrication of the inertial switch The omnidirectional inertial switch was fabricated based on non-silicon surface micromachining technology with multiple SU-8 photoresist sacrificial layers on a single wafer. The basic micromachining technologies used herein were six times lithography of thick SU-8 photoresist, six times Ni electroforming of
Fig. 13. The fabrication details of the omnidirectional inertial switch.
X. Zhanwen et al. / Sensors and Actuators A 212 (2014) 93–101
Fig. 15. Shock test devices for micro-switch prototypes.
Fig. 16. Shock test results of micro-switch under the acceleration (a) 380 g, (b) 450 g in X-axis direction.
Fig. 17. Shock test results of micro-switch in the direction of (a) 45◦ with the X-axis direction in the XOY plane, (b) 45◦ with the Z-axis direction in the XOZ plane.
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Fig. 18. Shock test results of micro-switch under the acceleration (a) 450 g, (b) 500 g in Z-axis direction.
Ni electroplating, the main difficulty met in fabrication process was low bonding strength among electroforming deposit layers.
4. Testing and results analysis The fabricated inertial switch prototype was fixed on test board with a standard accelerometer (CA-YD-180) with sensitivity of 5.292 mV/g and a voltage division circuit with DC supply voltage 3 V and load resistance 200 . A data acquisition system was adopted to capture the acceleration data when the switch had output voltage, as shown in Fig. 15. The different testing angle between switch and acceleration was achieved by a special fixture. Half-sine Accelerations with different amplitude and pulse width can be obtained by adjusting the shock table parameters. Fig. 16 shows the results of shock test, where different acceleration amplitudes in X-axis direction are applied to the switch. When the acceleration amplitude is about 380 g, 450 g, the contact duration of the switch is about 40 s, 60 s, respectively, as show in Fig. 16. Also, a half-sine acceleration with amplitude 450 g and duration 1 ms was applied to the switch in different shock directions, such as 45◦ with the X-axis direction in the XOY plane, Z-axis direction and 45◦ with the Z-axis direction in the XOZ plane. The test results are shown in Fig. 17. It is shown in Figs. 16–18 that the switch can be reliably turned on in radial direction, axial direction and a certain direction with the axial direction under the applied half-sine acceleration with amplitude 450 g and duration 1 ms, the switch with flexible electrode is very beneficial for improving contact effect and prolonging the contact duration. However, there is a little deviation on its contact duration compared with the simulation results. This is because of fabrication etching technology error. When the spring width is smaller, the stiffness decreases obviously and the mass displacement increases, which causes increase of the contact duration. So it is considered as the main possible reason for the deviation between the test and simulation results. When higher acceleration is applied to the switch in Z-axis direction, the contact duration of the switch decreases inversely as shown in Fig. 18(a) and (b), which is due to the bounce phenomenon previously mentioned. Based on the measured parameters of the switch prototype, by modifying the simulation model with the thickness of circle loop decreased to 22 m, the width of cross beam decreased to 42 m, and the spring width of proof mass and radial electrode decreased to 14 m and 18 m, respectively, the test results coincide with the
simulation results. It is necessary to further optimize the structural parameters to improving performance of the switch. 5. Conclusion The omnidirectional inertial switch has been realized in this design by integration of single proof mass and flexible electrodes. The flexible electrodes were utilized for omnidirectional inertial switch to extend the contact duration and therefore to obtain reliable and stable output signals. When the proof mass collides with the flexible stationary electrode, it keeps moving to the flexible electrodes. Therefore the switch-on state is held on for a longer time, significantly prolonging the contact duration. When the acceleration component in radial or/and axial direction applied to the omnidirectional switch is over the threshold acceleration, the switch will turn on. It indicates that the switch has omnidirectional sensitivities. The tests verify the simulation results. Acknowledgments This work is supported by National Natural Science Foundation of China (51075057). References [1] T. Matsunaga, M. Esashi, Acceleration switch with extended holding time using squeeze film effect for side airbag systems, Sens. Actuators A: Phys. 100 (2002) 10–17. [2] J. Zhao, J. Jia, H. Wang, W. Li, A novel threshold accelerometer with postbuckling structures for airbag restraint systems, IEEE Sens. J. 7 (2007) 1102–1109. [3] C.H. Robinson, Omnidirectional microscale impact switch, 2004, US 6,765,160. [4] D.J. Jean, O. MD, MEMS multi-directional shock sensor, 2007, US 7,159, 442. [5] D.S. Greywall, MEMS-based inertial switch, 2007, US 7,218,193. [6] G.L. Smith, O. MD, Triaxial MEMS acceleration switch, 2012, US 8,237,521. [7] H.G. Cai, G.F. Ding, Z.Q. Yang, Z.J. Su, J.S. Zhou, H. Wang, Design, simulation and fabrication of a novel contact-enhanced MEMS inertial switch with a movable contact point, J. Micromech. Microeng. 18 (2008) 115033. [8] Z. Yang, G. Ding, H. Cai, X. Xu, H. Wang, X. Zhao, Analysis and elimination of the ‘skip contact’ phenomenon in an inertial micro-switch for prolonging its contact time, J. Micromech. Microeng. 19 (2009) 045017. [9] W. Ma, Y. Zohar, M. Wong, Design and characterization of inertia-activated electrical micro-switches fabricated and packaged using low-temperature photoresist molded metal-electroplating technology, J. Micromech. Microeng. 13 (2003) 892–899. [10] Z.Q. Yang, G.F. Ding, H.G. Cai, X.L. Zhao, A MEMS inertial switch with bridge-type elastic fixed electrode for long duration contact, IEEE Trans. Electron Devices 55 (2008) 2492–2497.
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[14] Z.Q. Yang, G.F. Ding, H. Wang, H.G. Cai, X.L. Zhao, Modeling, simulation and characterization of a micromachined acceleration switch with anti-stiction raised strips on the substrate, IEEE Trans. Compon. Packag. Manufac. Technol. 1 (2011) 1195–1204. [15] G.K. Fedder, Simulation of microelectromechanical systems (Ph.D. thesis), Dept. Electr. Eng. & Comput. Sci., Univ. California at Berkeley, Berkeley, 1994.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A novel MEMS omnidirectional inertial switch with flexible electrodes Xi Zhanwen a,∗ , Zhang Ping a , Nie Weirong a , Du Liqun b , Cao Yun a a b
School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 24 October 2013 Received in revised form 17 February 2014 Accepted 24 February 2014 Available online 14 March 2014 Keywords: Omnidirectional inertial switch MEMS Flexible electrode Enhanced contact effect
a b s t r a c t A novel MEMS omnidirectional inertial switch was designed, simulated and fabricated. The switch is composed of three main parts, the proof mass, the axial flexible electrode and four radial flexible electrodes. It introduces the flexible electrodes to form a dual mass–spring system. The switch has omnidirectional sensitivities in a half sphere. When any acceleration, over the threshold value in radial or/and axial direction, acts to the switch, the switch will turn on. Also, the switch has the enhanced contact effect. Dynamic simulation results based on FEM confirm that the contact effect is improved by this new design compared to that of traditional inertial switch. The contact duration is prolonged under the shock loading, and the bouncing effect is alleviated. The switch has a 6-layer structure, which is manufactured based on non-silicon surface micromachining technology. The tests have been done and the results coincide with that of the simulation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction MEMS inertial switches, also known as to shock sensors or threshold accelerometers, have great potential to be widely used in toys, accessories, automotive, military weapons and industrial applications. That is due to their smaller size, lower cost, less power consumption, more functionality and better performance than conventional mechanical ones. Furthermore, the inertial switches have ability of avoiding electromagnetic interference in applications [1,2]. The MEMS omnidirectional inertial switch usually has a structure with a mass–spring system, where the proof mass served as the movable electrode and is suspended by surrounding springs [3] .There is also other structure with a proof mass suspended by central springs [4–6]. The schematic diagram of these conventional designs is shown in Fig. 1. In Fig. 1, when the switch responses to a shock acceleration, the movable electrode will contact with stationary electrode in rigid mode. The contact-bouncing effect would be inevitable and the switch-on time is transient (usually less than 10 s)[7,8]. The poor contact-bouncing effect and the short switch-on time make it difficult for signal processing and weaken the reliability of the switch [9,10]. The movable contact point [1,7], squeeze film effect [1], the carbon nanotube (CNT)contact pad [11], electrostatic force [12], have been adopted to
∗ Corresponding author. Tel.: +86 2584303068. E-mail address: [email protected] (X. Zhanwen). http://dx.doi.org/10.1016/j.sna.2014.02.035 0924-4247/© 2014 Elsevier B.V. All rights reserved.
the inertial micro-switches to prolong the contact duration and eliminate contact-bouncing effect, but these inertial switches were limited to a single axis of sensing. The present work proposed a novel design of the MEMS omnidirectional inertial switch with four radial and an axial flexible electrodes shown in Fig. 2. By introducing this flexible contact mechanism, the switch forms a dual mass–spring system, which gives the improved switch characteristics such as omnidirectional sensitivities, reduced contact-bouncing effect and prolonged switch-on time. 2. The inertial switch design and simulation 2.1. Design and working principle The structure of the omnidirectional inertial switch is illustrated in Fig. 3. It senses acceleration in hemisphere (both in-plane and out-of-plane) with the single proof mass. The proof mass thickness t is designed to be much larger than the spring thickness t1 in order to minimize the area coverage while enabling the desired radial and axial direction sensitivity. Four radial flexible electrodes supported by spring are symmetrical around the proof mass. Each one has the gap of d1 from the proof mass. The circle loop supported by an elastic cross beam is used as the axial flexible electrode. It has a distance of d2 from the proof mass, which also helps to enhance the contact effect in axial direction. The use of flexible electrodes is to prolong the switch-on time and make the switch-on state more stable.
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Fig. 1. A schematic diagram of conventional inertial switch.
Fig. 2. A schematic diagram of the novel inertial switch.
Apart from the proof mass–spring system (m1 , k1 ), the radial flexible electrodes are suspended by spring instead of being rigidly fixed on the substrate. The axial flexible electrode is supported by elastic cross beam. The radial and axial flexible electrodes have contact gap d1 and d2 from the proof mass, respectively. For example, as for an acceleration acting along the +Z axis, the working principle is demonstrated by the dynamic process and corresponding switch state, as shown in Fig. 4. Fig. 4(a) shows the action process of a conventional inertial MEMS switch with the fixed electrode. Fig. 4(b) explains the action process of the novel inertial MEMS switch with flexible electrodes. In Fig. 4(b), (1) The proof mass moves towards one of flexible electrodes due to the acceleration. (2) When the acceleration exceeds the threshold value, the displacement reaches d2 , the proof mass contacts the flexible electrode and the switch is turned on. (3) The proof mass keeps moving on with the flexible electrode, therefore the switch-on state is held on for a longer time. (4) After the disappearance of the acceleration, the proof mass and the flexible electrode rebound, the switch is turned off, until the proof mass is separated from the flexible electrode. (5) Finally, the proof mass and the flexible electrode restore to the equilibrium position after all the energy is dissipated by free vibration. As for an acceleration acting along other directions in hemisphere, the proof mass moves towards the flexible electrodes in radial and/or axial direction, the switch state is similar to aforementioned cases. The inertial switch has radial and axial flexible electrodes, and it is able to provide identifiable direction information according to the identified electrode position. When acceleration is applied on the switch in the sensitive directions, the proof mass will be subjected to the inertial acceleration a(t) in the opposite direction. Take radial direction motion as example, the responding motion can be expressed with the motion equation as: m1 x¨ + c x˙ + k1 x + m1 a(t) = 0
(1)
where m1 is the mass of the proof mass, x is the displacement in radial direction, c presents the damping coefficient, k1 is the system stiffness and a(t) is the applied acceleration component in radial direction and is a function of time t. The acceleration a(t) applied to the switch in practical work is similar to a half-sine pulse with amplitude a0 and duration t0 (about 1 ms). To simplify the solving process, c is neglected [9]. By solving Eq. (1), we have the displacement x(t) of the proof mass. Fig. 3. Structural sketch map of the proposed inertial switch. (a) A conventional inertial MEMS switch with the fixed electrodes (b) A inertial MEMS switch with flexible electrodes.
x(t) =
a0 ωn2 − ω02
sin ω0 t −
ω0 sin ωn t ωn
Fig. 4. Comparison of the dynamic process and switch state between (a) and (b).
(2)
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Table 1 Geometric parameters of the switch. Component
Geometric parameter
Value (m)
Proof mass
Radius r Thickness t Width w1 /w2
455 100 18/20
Thickness t1 /t2 Meander width b1 /b2 Connector radius r1 /r2 Circle loop outer radius R3 Inner radius r3 Thickness t3 Cross beam length l4 Width w4 Thickness t4 d1
40/40 600/330 31/20 94 80 30 330 50 30 25
d2
30
Proof mass spring/radial flexible electrode spring
Axial flexible electrode
Distance to radial flexible electrodes Distance to axial flexible electrode Fig. 5. The theoretical curve and simulation results of threshold acceleration ath versus ωn .
Table 2 The displacement of proof mass in radial and axial direction (unit: m).
where
ˇ
ω0 = , ωn = t0
0◦
k1 m1
(3)
when the maximum value of x(t) is equal to d1 (d1 = x0 ) and ω0 < ωn < 5ω0 , the corresponding applied acceleration is threshold acceleration ath. ath = x0
˛
ωn2 − ω02
90◦ 75◦ 60◦ 45◦ 30◦ 15◦ 0◦
15◦
30◦
45◦
Radial
Axial
Radial
Axial
Radial
Axial
Radial
Axial
32.0 31.0 27.8 22.7 16.0 8.3 0.0
0.0 10.0 19.4 27.5 33.8 37.8 39.3
32.0 31.0 27.8 22.7 16.1 8.3 0.0
0.0 10.0 19.4 27.5 33.8 37.8 39.3
32.2 30.9 27.8 22.7 16.0 8.2 0.0
0.0 10.0 19.5 27.6 33.8 37.8 39.3
32.1 30.9 27.8 22.7 16.0 8.3 0.0
0.0 10.1 19.5 27.6 33.9 37.8 39.3
(sin(2ω0 /(ωn + ω0 )) − (ω0 /ωn ) sin(2ωn /(ωn + ω0 ))) (4) 2.2. Dynamic simulation on the inertial switch
The threshold acceleration ath is related to gap d1 (d1 = x0 ) and ωn = k1 /m1 . When ω0 < ωn < 5ω0 , as ωn increases, the threshold acceleration increases gradually. By changing the mass of the proof mass, different values of ωn are obtained, so the switch with different ωn can be simulated. The threshold acceleration obtained by simulation is consistent with the theoretical results, as shown in Fig. 5. When the displacement of the proof mass in radial or axial direction is equal to the gap d1 or d2 , respectively, the threshold acceleration ath of the switch in radial or axial direction would be obtained. The mass–spring system stiffness is different in radial and axial direction, so it is needed to adjust gaps d1 and d2 . Therefore the consistent threshold acceleration in radial and axial direction could be obtained. The switch has only axial and radial flexible electrodes, so the exactly same threshold acceleration is impossible at any direction in hemisphere for the omnidirectional switch. The threshold acceleration of the proposed omnidirectional switch can be calculated using the parameters. And it can be confirmed by applying different amplitude loads in radial and axial direction.
The geometric parameters used in the simulation are listed in Table 1. The switch model is built in ANSYS. A full simulation model is employed. The material is nickel, where the Young’s modulus is 180 GPa and Poisson’s ratio is 0.3 [13]. Then the model is meshed and the grid of contact area is made to have high quality. Finally, the displacements of outer end of the springs and cross beams are constrained to be zero at all degrees of freedom.
2.2.1. Modal analysis The spring–mass systems of designed device structures are selected for the finite element modal analysis. And simulation modal results of the switch are shown in Fig. 6. The first four modal frequencies are 1961.7 Hz, 2101.1 Hz, 2101.1 Hz, 5281 Hz, respectively. The first mode shape is in the vertical direction, the second and third mode shape is in the horizontal direction. The fourth mode is far away from the first three modes.
Fig. 6. The first four natural frequencies of the spring-mass system.
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Fig. 8. The maximum displacement of the mass under 450 g in 3-D space.
Fig. 7. Pulse acceleration curve and different direction in hemisphere.
2.2.2. The displacement response of proof mass By applying half sine acceleration pulse with the 450 g amplitude and duration 1.0 ms, transient simulation has been done at different direction in hemisphere, as shown in Fig. 7. The displacements of the proof mass in radial and axial direction are listed in Table 2. Fig. 8 shows a simulated displacement distribution of the proof mass in the hemisphere. Table 2 shows that the displacements of proof mass in radial direction are related to the included angle between the acceleration and the Z-axis. As a result, the omnidirectional switch will have uniform threshold acceleration in the radial directions. The maximum displacements of proof mass in radial and axial direction are 32 m and 39 m, respectively. When d1 = 25 m, the threshold acceleration in radial direction is 355 g, and when d2 = 30 m, the threshold acceleration in the axial direction is 350 g. Meanwhile, the proof mass contacts slightly the flexible electrodes, but it is not enough to trigger the switch on-state signal. Only if the input acceleration is greater than the threshold we can make sure of switch onstate. 2.2.3. The omnidirectional sensitivity of the switch Fig. 3 shows that the MEMS omnidirectional switch has radial and axial flexible electrodes, the radial and axial flexible electrodes
have a contact gap d1 and d2 from the proof mass, respectively. Ideally, the movement of the proof mass is translational motion, the contact of the proof mass with the flexible electrode occurs when the displacement component in the radial or axial direction is greater than the gap. In XOZ plane, the maximum displacement curve of proof mass approximates arc circular (Fig. 8) under the same acceleration. Therefore, it is impossible to use the exactly same acceleration value as a switch threshold acceleration in any direction. The acceleration threshold in X, Z direction are 355 g and 350 g, respectively, corresponding to the fabricated inertial switch with the gap values of d1 = 25 m and d2 = 30 m. Only the acceleration from the Z direction with the value of 350 g can make the switch closed in the Z direction. When the acceleration is 400 g respectively within the scope of X, Z axis angle of 29◦ , the switch can be closed with a closed blind angle of 32◦ . The switch will close under the acceleration of more than 499 g in any direction in the XOZ plane, and there is no closed blind angle. Fig. 9(a) shows that the switch is closed only in Z-direction, as 350 g acting in different directions. Fig. 9(b) shows that it has closed blind corner in XOZ plane, as 400 g acting in different directions. Fig. 9(c) shows that it can be closed in any direction in the xoz plane, as 499 g acting in different directions. For the design of MEMS omnidirectional inertial switch, the threshold acceleration can be defined as radial and axial directional threshold acceleration respectively. When any acceleration
Fig. 9. The states of the switch contacted between the proof mass and flexible electrodes.
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Fig. 10. Simulated displacement–time curves of the proof mass in the switch under 450 g half-sine acceleration in radial and axial direction.
component in radial or/and axial direction applied to the omnidirectional switch is over the threshold acceleration, the switch will turn on. It indicates that the switch has omnidirectional sensitivity. 2.2.4. The switch on-state duration When the proof mass is induced by the acceleration, the pair of switch electrodes move to contact, forming the electrical connection. As the two electrodes get closer, the flexible electrode is moving with the proof mass until the two electrodes are separated. Resultantly, the electrical on-time becomes much longer compared to the conventional inertial micro switches based on two rigid electrodes depicted in Fig. 4(a) and (b). In order to clearly illustrate the dynamic impact process, the over-threshold acceleration, 450 g, was applied to the switch along its radial and axial direction. Fig. 10 shows the simulated displacement–time curves of the proof mass in radial and axial direction. As a comparison, the contact duration of the radial flexible electrodes can prolong to 45 s in Fig. 10(b), which is much longer than the electrode being rigidly fixed on substrate in
Fig. 10(a). The contact duration of the axial flexible electrode was prolonged to 33 s in Fig. 10(d), which is much longer than that in Fig. 10(c). The stiffness coefficient k2 of the radial flexible electrode also influences the contact-effect, either larger or smaller value will lead to undesirable results. Theoretical formula of the stiffness of micro spring was deduced, which has been proven that this analytical calculation method and finite element simulation can match to better than 1% [14,15]. By reducing the width of the spring, we can get a range of different spring stiffness coefficient. When the stiffness coefficient k2 is too smaller, such as 147.98 N/m (the width is 6 m), the proof mass and the flexible electrodes will have contact effect, the flexible electrodes will bounce, causing discontinuous contact between the two electrodes, shown in Fig. 11(a). When the stiffness coefficient k2 becomes larger, such as 15312.35 N/m (the width is 26 m), the displacement of the flexible electrode becomes smaller and the contact duration is shorter, shown in Fig. 11 (c). Taking into account the fabrication process, the spring width designed is 20 m, as shown in Fig. 11 (b), which eliminates the bounce
Fig. 11. The contact duration in different stiffness of the radial flexible electrode.
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Fig. 12. Dependence of the dynamic properties of the switch on contact time versus the stiffness.
Fig. 14. SEM image of the omnidirectional inertial switch.
effect when the proof mass touches the flexible electrode and has extended contact duration. Fig. 12 shows the relationship between k2 and contact duration.
proof mass, axial flexible electrodes, radial flexible electrodes, support springs and anchors, three times sputtering of Cu seed layers and releasing of the three-dimensional suspended structure. The fabrication details of the omnidirectional inertial switch are shown in Fig. 13. As electrical conductive layer, Cu seed layers were sputtered with the thickness about 200 nm. In order to release the three-dimensional suspended structure, inorganic acid boiling was used to remove the SU-8 sacrificial layer. The SEM image of the switch with a size of 2.8 mm × 2.8 mm × 210 m and minimum linewidth 15 m is shown in Fig. 14. Because the omnidirectional inertial switch is a six-layer device and was fabricated by six times
3. The fabrication of the inertial switch The omnidirectional inertial switch was fabricated based on non-silicon surface micromachining technology with multiple SU-8 photoresist sacrificial layers on a single wafer. The basic micromachining technologies used herein were six times lithography of thick SU-8 photoresist, six times Ni electroforming of
Fig. 13. The fabrication details of the omnidirectional inertial switch.
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Fig. 15. Shock test devices for micro-switch prototypes.
Fig. 16. Shock test results of micro-switch under the acceleration (a) 380 g, (b) 450 g in X-axis direction.
Fig. 17. Shock test results of micro-switch in the direction of (a) 45◦ with the X-axis direction in the XOY plane, (b) 45◦ with the Z-axis direction in the XOZ plane.
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Fig. 18. Shock test results of micro-switch under the acceleration (a) 450 g, (b) 500 g in Z-axis direction.
Ni electroplating, the main difficulty met in fabrication process was low bonding strength among electroforming deposit layers.
4. Testing and results analysis The fabricated inertial switch prototype was fixed on test board with a standard accelerometer (CA-YD-180) with sensitivity of 5.292 mV/g and a voltage division circuit with DC supply voltage 3 V and load resistance 200 . A data acquisition system was adopted to capture the acceleration data when the switch had output voltage, as shown in Fig. 15. The different testing angle between switch and acceleration was achieved by a special fixture. Half-sine Accelerations with different amplitude and pulse width can be obtained by adjusting the shock table parameters. Fig. 16 shows the results of shock test, where different acceleration amplitudes in X-axis direction are applied to the switch. When the acceleration amplitude is about 380 g, 450 g, the contact duration of the switch is about 40 s, 60 s, respectively, as show in Fig. 16. Also, a half-sine acceleration with amplitude 450 g and duration 1 ms was applied to the switch in different shock directions, such as 45◦ with the X-axis direction in the XOY plane, Z-axis direction and 45◦ with the Z-axis direction in the XOZ plane. The test results are shown in Fig. 17. It is shown in Figs. 16–18 that the switch can be reliably turned on in radial direction, axial direction and a certain direction with the axial direction under the applied half-sine acceleration with amplitude 450 g and duration 1 ms, the switch with flexible electrode is very beneficial for improving contact effect and prolonging the contact duration. However, there is a little deviation on its contact duration compared with the simulation results. This is because of fabrication etching technology error. When the spring width is smaller, the stiffness decreases obviously and the mass displacement increases, which causes increase of the contact duration. So it is considered as the main possible reason for the deviation between the test and simulation results. When higher acceleration is applied to the switch in Z-axis direction, the contact duration of the switch decreases inversely as shown in Fig. 18(a) and (b), which is due to the bounce phenomenon previously mentioned. Based on the measured parameters of the switch prototype, by modifying the simulation model with the thickness of circle loop decreased to 22 m, the width of cross beam decreased to 42 m, and the spring width of proof mass and radial electrode decreased to 14 m and 18 m, respectively, the test results coincide with the
simulation results. It is necessary to further optimize the structural parameters to improving performance of the switch. 5. Conclusion The omnidirectional inertial switch has been realized in this design by integration of single proof mass and flexible electrodes. The flexible electrodes were utilized for omnidirectional inertial switch to extend the contact duration and therefore to obtain reliable and stable output signals. When the proof mass collides with the flexible stationary electrode, it keeps moving to the flexible electrodes. Therefore the switch-on state is held on for a longer time, significantly prolonging the contact duration. When the acceleration component in radial or/and axial direction applied to the omnidirectional switch is over the threshold acceleration, the switch will turn on. It indicates that the switch has omnidirectional sensitivities. The tests verify the simulation results. Acknowledgments This work is supported by National Natural Science Foundation of China (51075057). References [1] T. Matsunaga, M. Esashi, Acceleration switch with extended holding time using squeeze film effect for side airbag systems, Sens. Actuators A: Phys. 100 (2002) 10–17. [2] J. Zhao, J. Jia, H. Wang, W. Li, A novel threshold accelerometer with postbuckling structures for airbag restraint systems, IEEE Sens. J. 7 (2007) 1102–1109. [3] C.H. Robinson, Omnidirectional microscale impact switch, 2004, US 6,765,160. [4] D.J. Jean, O. MD, MEMS multi-directional shock sensor, 2007, US 7,159, 442. [5] D.S. Greywall, MEMS-based inertial switch, 2007, US 7,218,193. [6] G.L. Smith, O. MD, Triaxial MEMS acceleration switch, 2012, US 8,237,521. [7] H.G. Cai, G.F. Ding, Z.Q. Yang, Z.J. Su, J.S. Zhou, H. Wang, Design, simulation and fabrication of a novel contact-enhanced MEMS inertial switch with a movable contact point, J. Micromech. Microeng. 18 (2008) 115033. [8] Z. Yang, G. Ding, H. Cai, X. Xu, H. Wang, X. Zhao, Analysis and elimination of the ‘skip contact’ phenomenon in an inertial micro-switch for prolonging its contact time, J. Micromech. Microeng. 19 (2009) 045017. [9] W. Ma, Y. Zohar, M. Wong, Design and characterization of inertia-activated electrical micro-switches fabricated and packaged using low-temperature photoresist molded metal-electroplating technology, J. Micromech. Microeng. 13 (2003) 892–899. [10] Z.Q. Yang, G.F. Ding, H.G. Cai, X.L. Zhao, A MEMS inertial switch with bridge-type elastic fixed electrode for long duration contact, IEEE Trans. Electron Devices 55 (2008) 2492–2497.
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