Proceedings of the 15th IEEE International Conference on Nanotechnology July 27-30, 2015, Rome, Italy
Design and Simulation of MEMS Directional Microphone for Battle Damage Assessments L.Prashanth*,N. Manikantha Satyasri Vishal, Nikhil S.V,Vidyashree N,Nithya G, Veda Sandeep Nagaraja
S.L.Pinjare School of Electronics and Communication Engineering Reva University, Bengaluru, Karnataka, India - 560064
Nitte Meenakshi Institute of Technology, Center for Nanomaterials and MEMS, Department of Electronics and Communication Engineering, Govindapura, Gollahalli, Yelahanka, Bengaluru, Karnataka, India - 5600464 *email:
[email protected], Ph No: 919916692595 Abstract—MEMS Directional Microphone has found immense application in consumer products, medical applications and also defense applications. A directional microphone can be used in defense application where by the device can be mounted on an MAV and sent to enemy territory to record the conversation of the enemy and take strategic decisions based on the conversation. It can also be used in Battle Damage Assessment (BDA) applications where by the extent of damage because of the battle can be analyzed by finding the extent of pressure due to the attack and direction of the missile attack. In this paper the authors have designed and simulated a MEMS directional Microphone whose mechanical structure is inspired by the hearing organ of the parasitoid Ormia Ochracea. Eight different structures have been designed and simulated using COMSOL Multiphysics and Conventorware Turbo. On simulation the best frequency response for parallel plate structure is 15.7 kHz and for comb structure is 7.68 kHz. A detailed mathematical analysis has also been carried out where by the theoretical results have been compared with the simulated results. The MEMS directional microphone can be fabricated using SOIMUMPs or PolyMUMPs process for different structures designed in the paper. Keywords—MEMS, Directioal Microphone, Ormia Ochrarcea.
I. INTRODUCTION By tracking the direction of the incoming sound wave, the location of the sound source can be determined. Thus a MEMS directional microphone is a device that has the capability of detecting the direction of the incoming sound wave pressure falling on its diaphragm. Since MEMS directional microphone is designed in macro scale with excellent agreement of performance it finds immense application in defense, consumer products like mobile phones etc. The directional microphone has major advantage of a single device detecting the direction of incoming acoustic pressure thus replacing the need for an array of microphones for detecting directionality. The directional microphone designed in this paper is biologically inspired from the parasitoid fly Ormia ochracea’s auditory system. The Ormia ochracea fly shows remarkable ability to locate and track its host male Cricket for reproducing itself by tuning to its calling song at about 5 KHz, since the auditory organ of the fly are separated by only 520μm, the best interaural time difference (ITD) is 5.2μm and interaural intensity difference (IID) is less than 1db. However, theobtained ITD & IID at the mechanical response level are on
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the order of 50μs and 13 dB respectively. The key to this great amplification is due to an intertympanal bridge that couples the motions of two tympanal membranes [1]. In [2] the author proposed the development of a novel, biologically inspired acoustic sensor that is capable of detecting sound source with an accuracy of 2°. In [3] the author proposed the development of directional microphone by perforating the diaphragm using PolyMUMPs process. Perforations were used as an additional parameter to adjust the wing mass and increase the surface area, the capacitance measured was 110pF. In [4] the author compared the solid plate and perforated plate directional microphone using SOIMUMPs process. By comparison it’s observed that the modes of resonant frequency i.e. rocking and bending mode is better in the device with perforated plate than with solid plate. In this paper the authors have compared different sensing structure of MEMS directional microphone to obtain the directional microphone with better sensitivity and frequency responses which are biologically inspired by the auditory organ of the parasitoid fly Ormia ochracea and also mathematical analysis has carried out to validate the simulation results. There are mainly two structures; one is a parallel plate structure designed using PolyMUMPs process and the other uses interdigitated comb fingers in which the capacitance is measured between the comb fingers designed using SOIMUMPs process. The use of this approach opens up numerous design possibilities that have not been previously carried out. The improvement in sensitivity, frequency response and damping is observed. Detailed designs have beendeveloped with consideration given to a wide range of design parameters. The second section of the paper gives the design and implementation details of the MEMS structure. The third section provides the mathematical analysis of the structure. The fourth section gives detailed analysis of the simulation result. II. DESIGN AND IMPLEMENTATION In order to analyze the performance of MEMS directional microphone several methods have been adopted. The devices were designed and simulated using Conventorware 2010 tool to meet the desired specifications. Performance parameters such as sensitivity, frequency response and damping ratio were analyzed.
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A. PARALLEL PLATE STRUCTURE: Thefirst structure is a parallel plate structure as shown in figure 1 and 2, which is implemented using PolyMUMPs process. The sensor has top and bottom plates with the dimension of 1x2mm² each. The top plate replicates the structure of hearing organ of the fly. Figure 1(a) shows the parallel plate structure which has no contact to substrate and thus on the application of large pressure there is a possibility of the top plate touching the bottom plate. This is overcome in the structure in the figure 1(b) where the designincorporatesspringson all 4 edges connected to the substrate thus restricting the movement of top plate. Further analyses were carried out by introducing perforations on diaphragm as shown in figure 2 (a) & 2 (b).
III. MATHEMATICAL MODELLING 1] Parallel plate capacitance. The capacitance of fixed parallel plate capacitor is given in equation 1. C=
ௗ
……………. (1)
Where = permittivity of the material between the parallel plates (free plate permittivity 8.85xͳͲିଵଶ ܨൗ݉) A = plate area d = distance between top and bottom plate. For variable parallel plate capacitance, the movable plate moves normally to the fixed plate as defined by the Z coordinate. The capacitance for the movable capacitor is given by equation 2.
…………… (2) C= ௗି௭ 2] Interdigitated comb finger capacitance. Fig 1(a): Parallel plate structure.
Fig 1(b): Parallel plate structure with spring support.
Fig 1: parallel plate structure with solid wings.
Fig 2(a): perforated parallel plate structure
Fig 2(b): perforated parallel plate structure with springsupport
Fig 2: perforated parallel plate structure.
B. COMB DRIVE STRUCTURE:The second structure withcomb drive capacitance is as shown in the figure 3 and 4, which is implemented using SOIMUMPs process. The comb design consists of two set of interdigitated fingers, one set is attached to the solid wings that forms movable part of the structure, and other serves as fixed comb structure as shown in figure 3 (a).In figure 3(b) the analysis is carried out by introducing perforations on the wings. Further analysis was carried out by introducing asymmetry in the wing structure as shown in figure 4(a) and 4 (b).
C= 2n (z+z0) b.......... (3) d Where = permittivity of the material (8.85xͳͲିଵଶ ܨൗ݉ሻ n= number of comb fingers, b=comb finger width, z0=initial overlap, d=distance between comb fingers 3] Phase difference: The phase difference of a signal arriving at point is given by equation 3. ଶగ ο( ……………ݎ4) ο= Where f is the frequency of the wave, C is the speed of sound wave, οݎis the difference in the distance. As the distance to the position increases, ο ݎbecomes smaller and so does the phase differenceο. At a point “sufficiently” far from the source, the phase difference becomes negligible, and the pressure is estimated as the sum of the radiated pressures [5]. 4] Distance between source and device: Since distance depends on where the phase difference is considered negligible, the transition distance from the near field to the far field is an approximation given by equation 5. గ;
……………. (5) R= Where L is the maximum length of the source.
Fig 3(a): Comb drive structure with solid wings
Fig 3(a): Comb drive structure with perforated wings
Figure 3: Comb drive structure with symmetry wings.
Fig 4(a): Comb drive structure with solid wings
Fig 4(b): Comb drive structure with solid wings
Figure 4: Comb drive structure with asymmetry wings.
5] Quality factor: The quality factor Q of an oscillating device is defined as ratio between resonance frequency divided by the frequency response of half power points as given by equation 6. Q=Ȧres………… (6) Ȧ+- Ȧí Where Ȧres is the angular frequency at the maximum power Ȧ+ and Ȧí are the angular frequencies above and belowȦres. at which the power amplitude is half of its maximumvalue.If Q value is large it is considered as sharp resonance. If Q value is small it is considered as broad resonance
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1) Symmetric solid wings structure: a) Bending mode: Ȧres = 7688 Hz ,Ȧ+ = 8000Hz, Ȧ- = 7000Hz Q = 6.92 b) Rocking mode: Ȧres = 5585Hz , Ȧ+ = 6000 Hz , Ȧ- = 4500Hz Q = 3.72 Thus Bending mode as sharp resonance curve. 2) Asymmetric solid wings structure: a) Bending mode: Ȧres = 7450 Hz ,Ȧ+ = 7600Hz, Ȧ- = 6500Hz Q = 6.77 b) Rocking mode: Ȧres = 4369Hz , Ȧ+ = 4800 Hz , Ȧ- = 3500Hz Q = 3.36 Here Rocking mode as sharp resonance curve. The quality factor obtained theoretically for symmetric solid wings structure bending mode as sharper resonance curve and it is in agreement with the simulated results. IV. SIMULATION RESULTS. The two structures of Directional microphone 1) parallel plate structure and 2) interdigitated comb structures were designed and simulated using CONVENTOWARE
Fig 8 (b): Bendingmode
Fig 8 (a): Rocking mode :13.75Khz
Fig 8 (c): Freq response graph
Fig 8: frequency response of perforated parallel plate structure with spring support.
The displacement of perforated parallel plate structures are 112μ m and 58.53μm respectively. It can be seen that as the mass of the structure decreases as in the case of the perforated wing structure, the displacement increases for the same amount of pressure. The frequency response of the perforated parallel plate structure with and without spring is as shown in figure 7 and 8 The next sets of structures have been simulated using SOIMUMPs process and the same is shown in figure 9(a) and 9 (b). These structures use comb drive for their capacitive sensing.
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Fig5 (a): Rocking mode: 3.525 KHz
Fig 5 (b): Bending mode: 3.436 KHz
Fig 9 (a): Rocking mode: 5.585KHz
Fig 5(c):freqresponse graph
Fig 9 (b): Rocking mode: 7.688 KHz
Fig9(c):freq response graph
Fig 9: frequency response of symmetric solid wings structure
Fig5: frequency response of parallel plate structure
Fig 6 (a): Rocking mode: 7.895 KHz
Fig 6 (b): Bending mode: 9.127 KHz
Fig6(c): freqresponse graph
Fig10 (a): Rocking mode: 5.132 KHz
Fig 6: frequency response of parallel plate structure with spring support.
Fig 10: frequency response of symmetric perforated wings structure
The maximum displacement for parallel plate structure is 25.73μm when 1Pa pressure is applied on the left wing. The parallel plate structure with spring support has a displacement of 3.169μm on applying the same amount of pressure thus constraining the displacement. The frequency response of the parallel plate structure with and without spring is as shown in figure 5 and 6.The structures in figure 5 were further analyzed by introducing perforations as shown in figure 7.
Fig 7 (a): Rocking mode:6.695 KHz
Fig7 (b): Bending mode : 5.147Khz
Fig10 (b): Bending mode: 7.593 KHz
The displacements of directional microphone with comb structure are as follows. Symmetric solid wings structure gives a maximum displacement of 3.37μm when 1Pa pressure is applied on the left wing. Perforations are introduced to reduce the damping effect observed in solid wings. This structure gives a maximum displacement of 3.46μm.The structures in figure 9 were further analyzed by introducing asymmetries in the structure by varying wings width as shown in figure 11.
Fig 7(c): freq response graph
Fig 7: frequency response of perforated parallel plate structure
Fig11 (a): Rocking mode: 4.369 KHz
Fig11 (b): Bending mode: 7.450 KHz
Fig11(c): freq response graph
Fig 11: frequency response of asymmetric solid wings structure
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The asymmetric solid wings structure gives a maximum displacement of 5.50μm when 1Pa pressure is applied on the left win and perforated wings give a maximum displacement of 5.67μm for same pressure. Table 1: comparison of frequency responses Structure
Rocking
mode
Bending
(KHz)
mode (KHz)
Parallel plate structure
3.525
3.436
Parallel plate structure with spring
7.895
9.127
Perforated parallel plate structure
6.695
5.147
Perforated parallel plate structure
13.75
15.75
5.585
7.688
Asymmetric solid wings comb structure
4.369
7.450
Perforated Symmetric solid wings
5.132
7.593
comb structure
Table 1 gives frequency responses of the structure thus depending on application different structures can be used. Table 2 gives the comparison of sensitivity. On comparison of the results it can be seen that the best sensitivity is got in the structure which has unperforated wings with springs attached to it. The structure was designed using the POLYMUMPs process. V.CONCLUSION In the paper the authors have designed and simulated numerous structures of the MEMS directional microphone. The structure of the Directional microphone takes inspiration from the hearing organ of the Ormia Ochracea parasitoid. The paper contains mainly two structures. The first structure uses the concept of parallel plate capacitance which can be fabricated using POLYMUMPs process and the second one uses the comb drive structure which uses SOIMUMPs process. The paper gives details of the mathematical analysis of the quality factor and also the simulation results. On comparing the simulation results it was observed that the structure with unperforated wings with springs attached to it gave the best sensitivity. This structure was designed using the POLYMUMPs process.
with spring Symmetric solid wings comb structure
comb structure
Table 2: comparison of the sensitivities Structure
Parallel plate structure Parallel plate structure with spring Perforated parallel plate Perforated parallel plate structure with spring Symmetric solid wings comb structure Asymmetric solid wings comb structure Perforated Symmetric solid wings comb structure Perforated Asymmetric solid wings
Capacitance (pF) (Left wing)
Capacitance (pF) (Right wing)
25.73
-8.871
-8.376
0.495
3.169
0.0631
4.955
4.891
Displacement (μm)
C (pF)
112
0.176
-1.554
0.173
58.53
-0.965
-0.795
0.17
2.7E00
9.343E-02
9.637E-02
0.003
1.3E-01
9.445E-02
7.892E-02
0.015
3.4668
8.939E-02
8.851E-02
0.0087
4.5417
9.711E-02
9.799E-02
0.0088
ACKNOWLEDGEMENT The authors would like to thank NPMASS for setting up the National MEMS Design Center at NMIT and professors at CeNSEIISc for giving technical guidance as and when required. Also the authors would like to thank the management of NMIT for setting up the Center for Nanomaterials and MEMS where this work was carried out. REFERENCE [1] H.J Liu , M. Yu ”Fly Ear inspired miniature Directional Microphone Modeling and experimental study “,.proceeding of ASME ,International Mechanical Engineering congress and exposition IMECE(2009). [2] Su. Q, R.N Miles, M.G Weinstein, R.A Milles,L.Tan,W.Cui, “Response of a biologically inspired MEM differential microphone diaphragm”, proceedings of the SPIE Aerospace 2002,Orlando FI,pp.4743-15. [3] Touse,Michael,”Design fabrication and characterization of a MEMS directional microphone”,Master’s Thesis, Monterey,CA: Naval postgraduate school, June 2011 [4] Norbahrin Muammad, “Characterization of MEMS a Directional microphone with solid and perforated wings”, june 2009 [5]Dimitrios Chatzopoulous, “Modeling the performance of MEMS based Directional microphone”, December 2008 [6]Veda Sandeep Nagaraja ,S.L.Pinjare,”Design and simulation of MEMS Directional Microphone”, ISSS International Conference July 2014.
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