Design and Simulation of Capacitive Type Comb-Drive Accelerometer to Detect Heart Beat Frequency

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Design and Simulation of Capacitive Type Comb-Drive Accelerometer to Detect Heart Beat Frequency1 P. Ashok Kumar1, G.K.S. Prakash Raaju1, K. Srinivasa Rao2,a 1 – Department of ECE, KL University, Green Fields, Andhra Pradesh, India 2 – Professor and Head of Micro Electronic Research Group, Department of ECE, KL University, Green Fields, Andhra Pradesh, India a – ashok09411@gmail.com, drksrao@kluniversity.in DOI 10.2412/mmse.20.62.334 provided by Seo4U.link

Keywords: inertial sensors, MEMS accelerometer, sensitivity capacitive comb drive, proof mass.

ABSTRACT. MEMS Accelerometers are one of the most common inertial sensors, a dynamic sensor capable of a vast range of sensing. The basic principle of operation behind the MEMS accelerometer is the displacement of a small proof mass etched into the silicon surface of the integrated circuit and suspended by small beams. MEMS accelerometers in Structural Health Monitoring systems can be able to measure the range of acceleration ±4g with increasing sensitivity and decreasing cross sensitivity. In this paper, we have designed and simulated the MEMS capacitive comb drive accelerometer with respect to the vibrational frequency of the chest wall. The design consists of a proof mass (4000µm×6200µm) with movable electrodes (2500µm×200µm), fixed beam of electrodes (2500µm×200µm) and a cantilever beam as a spring (1000µm×2000µm). The Device operates in dynamic mode and simulated using COMSOL MULTIPHYSICS 5.2 version software. The output total displacement at 20 Hz is 1.52E-10 µm and at 40 Hz is 1.45E9µm, acceleration at 20 Hz is 9.63E-7 m/s2 and at 40 Hz is 9.19E-6 m/s2 is measured. The simulated results are correlated to the modeling results at the range of frequency 20 Hz – 40 Hz.

Introduction. According to the survey of World Health Organization on 22 September 2016 “Global Hearts”, a new initiative from the World Health Organization (WHO) and partners launched on the margins of the UN General Assembly, aims to beat back the global threat of cardiovascular disease, including heart attacks and strokes - the world’s leading cause of death. The cardiovascular diseases generally have no symptoms and it can be identified by chest pain or shortness of breath. The diagnosis of heart diseases is often done by observing the heart beats. These heart beats can be monitored by different structural health monitoring systems. Structural Health Monitoring (SHM) systems collect and analyze information about a civil structure so that indications of a structure distress can be identified early. The existing SHMs Electrocardiogram (ECG), Ballistocardiogram (BCG) Phonocardiogram (PCG), Ultrasound cardiogram (UCG) are costly devices, contains more number of components and is difficult to handle for long time ambulatory monitoring of heart functions and its motions.

Miniaturization of Bio-medical sensors has increased importance of Microsystems technology in medical applications. Bio-MEMS sensors play an important role in heart rate monitoring. The optical sensors [1] based on the blood flow, the pressure sensors [2] which transform the vibrations (due to pumping of blood by heart) into electrical signals and body worn sensors using mechanical transducers [3] are the existing designs for monitoring the heart rate. Recent research has shown that accelerometer sensors can be used to reliably detect some physical activity types when tested on small datasets. The physical mechanisms underlying MEMS accelerometer include piezoelectric, piezoresistive, electromagnetic, electrostatic, ferroelectric, optical, tunneling, thermal and capacitive. The advantages of the MEMS capacitive accelerometer are high sensitivity, low temperature sensitivity,

1

© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

good DC response and noise performance, low power dissipation, low drift and reduced parasitic due to single integration chip In this paper, the MEMS inertial sensor called capacitive type comb-drive accelerometer is designed to monitor heart beat frequency with increasing sensitivity (Âą4g) and decreasing cross-sensitivity.

The accelerometer is placed on the chest wall a slight variation on the chest surface due to heart beat allows visualizing the heart motion for assisting and understanding heart function. The device produces linearity at the heart beat frequency range.

Theory. MEMS accelerometers are the electromechanical devices used to measure acceleration forces, such forces may be static like continuous force of gravity or dynamic to sense movement of vibrations. The proposed MEMS capacitive comb-drive accelerometer has the sensitivity range Âą4g.

Initially the proposed accelerometer has the nominal capacitance when there is no acceleration (a=0).

The device senses the acceleration when it is placed on the chest wall which is transferred to the proof mass through the flexible elements such as springs. The Proof mass and movable comb fingers move along the direction of body force while fixed comb fingers remains stationary. The movement develops the small parallel plate capacitors which are added up to get effective capacitance.

Proposed Design Structure. Figure.1 illustrates the MEMS comb drive capacitive accelerometer that can be used prototype design of Multiphysics model. The comb drive structure consists of proof mass with interdigitated fingers extending from the frame. The proof mass is suspended from the fixed beams with two cantilever springs. The capacitor plates are arranged such that there are two types of plates: fixed and movable plates as shown in Fig.1. When the proof mass vibrates with the input vibrations then the movable plates are also moves developing a capacitance with the given voltage of 5V across the plates.

Fig. 1. 3D view of MEMS comb-drive accelerometer structure.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Table 1. Geometries of the Design.

S.No

Component

Dimensions

1

Proof mass

6200Âľm Ă— 4000Âľm Ă—500Âľm

2

Electrodes (fixed and moving)

2500Âľm Ă— 200Âľm Ă—500Âľm

3

No.fo electrodes

6

4

Overlapping area of electrodes

1Âľm

5

Cantilever spring

200 Âľm Ă— 30 Âľm Ă— 500 Âľm

The proof mass can be derived from is đ?‘€=đ??´đ?œŒh= 9.87Ă—10-5 đ?‘˜đ?‘”, where đ??´ the total area is obtained from the structure layout, h is the structural thickness and đ?œŒ is the density of Silicon 2.33Ă—103đ?‘˜đ?‘”/đ?‘š3. The spring values calculated from cantilever beams and the spring approximated constant đ??ž=1.557 đ?‘ /đ?‘š .

The resonance frequency value is 20Hz obtained from the equation.

1

đ?‘˜

đ?‘“đ?‘&#x; = 2đ?œ‹ √đ?‘š

(1)

Other important parameters Area moment of Inertia and estimating the damping effect of the system are the most important steps in the analysis and design process of the proposed design.

Table 2. Premilinary parameters to design the comb drive accelerometer.

S. No

Parameters

Value

1.

Young’s modulus of silicon (E)

160GPa

2.

Proof mass (ms)

3

Spring constant (K)

1.557

4.

Frequency of heart vibrations

20 Hz

5.

Natural frequency (ω)

125.6 rad/sec

6.

Area moment of inertia (I)

1.04Ă—10-14 m4

7.

Damping coefficient (B)

8.

Damping ratio (Ń”)

9.87Ă—10-5 kg

1.59Ă—10-9 N-s/m 6.4Ă—10-8

Principle of operation. Accelerometer measures the vibrations on the chest wall driven at the resonant frequency by the inplane and Outplane sensing. The vast majority of accelerometers have in MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

common a mechanical sensing element consisting of a proof mass attached by a mechanical suspension system to a reference frame. The accelerometer can thus be modeled by a second order massDamper-spring system as shown in Fig. 2.

Fig. 2. Mass-Damper-Spring system.

According to Newton’s second law any external inertial forces due to acceleration displace the supporting frame relative to the proof mass. This in turn applies a force F = ma on the spring with m being the mass of the proof mass and a being the acceleration. The spring is deflected until its elastic force equals the forced produced by the acceleration. In the first order force acting on the spring is proportional to its displacement F=kx.

Hence, đ?‘Ž=

đ?‘š đ?‘˜

đ?‘Ľ

(2)

where a is the acceleration produced by force; m is proof mass; k is the spring constant; x is the displacement of the proof mass.

Initially when there is no acceleration i.e, a = 0, then there is no displacement of the proof mass and electrodes attached to it hence there will be the nominal capacitance about 5.1 pF. When the device is placed on the chest wall and senses any external acceleration changes the displacement of proof mass the changes can be monitored along each axis and then analyzed to provide the information of the heart motion occurring.

Simulation. The accelerometer is modeled in 3D using COMSOL Multiphysics, the simulation and results are carried out in this software. The simulations are necessary to find the variation of the comb drive capacitance with respect to the displacement with given input acceleration. The simulations are carried out using the physics Electrostatics and solid mechanics in MEMS module. Stationary and frequency domain analysis is done for capacitance and displacement results.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Results and Analysis.

Displacement analysis. When the device is placed on chest wall, the input acceleration is produced due to vibrations of the chest wall. The total displacement at different frequencies is observed.

Fig. 5. Maximum and minimum displacement with input acceleration.

Table 3. The total displacement at different heart beat frequencies.

Acceleration (m/s2)

Frequency

Displacement

9.63e-7

20

1.52e-10

2.36e-6

25

3.73e-10

4.95e-6

30

7.82e-10

7.61e-6

35

1.20e-9

9.19e-6

40

1.45e-9

The values of displacement at the frequency range of 20Hz – 40Hz is obtained and plotted against the acceleration.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

1.60E-09

Displacement (m)

1.40E-09 1.20E-09 1.00E-09 8.00E-10 6.00E-10

4.00E-10 2.00E-10 0.00E+00 0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

acceleration (m/s^2)

Fig. 6. Acceleration vs Displacement.

The graph illustrates the displacement of the electrodes is directly proportional to the applied acceleration at the range of 20Hz to 40Hz frequency.

Capacitance analysis. Theoretically, the capacitance can be calculated with equation C = (2єNsLsHx)/d02

(3)

where є = є0єr (Permittivity of medium); Ns – No. of sensing electrodes; Ls – length of the sensing electrode; H – thickness of the sensing electrode; X – displacement of the electrodes; d0 – nominal distance between electrodes.

The practical capacitance can be calculated using the following energy relation 1

đ??¸ = 2 đ??śđ?‘‰ 2

(4)

The capacitance can be easily measured when the electrostatic energy is known with given 5V potential difference between moving and fixed electrodes.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 7. Capacitance 20 Hz and 40 Hz frequencies respectively.

Table 5. The total Capacitance at different heart beat frequencies.

Acceleration

Frequency

Capacitance (pF)

(Hz)

Theoretical capacitance (pF)

(m/s2) 9.63e-7

20

0.51

0.12

2.36e-6

25

1.26

0.91

4.95e-6

30

2.65

2.01

7.61e-6

35

4.08

3.63

9.19e-6

40

4.93

4.13

The values of capacitance with change in displacement is obtained and plotted against the acceleration.

6

capacitance (pF)

5

Practical Capacitance (pF)

4 3

Theortical capacitance (pF)

2 1

0 0.00E+00 2.00E-06 4.00E-06 6.00E-06 8.00E-06 1.00E-05 acceleration (m/s^2)

Fig. 8. Acceleration vs Capacitance.

The graph illustrates the linear increase of capacitance from 20Hz to 40Hz with the input accelerations. The increase in capacitance is due to decrease of distance between parallel plates of electrodes.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Summary. In this paper proposed design of comb-drive accelerometer is carried out with suitable capacitive sensing technique. Hence the accelerometer is capacitive type comb-drive accelerometer.

The mathematical modeling is done for to find the mass, spring constant and other preliminary parameters. The device is modeled at the resonant frequency 20Hz hence the device senses the acceleration only between 20Hz – 40Hz (maximum frequency of heart under stationary condition). The displacement and capacitance values are obtained by simulating the accelerometer in COMSOL Multiphysics software. By obtaining the simulated results it is concluded that when the displacement is in between 1.52e-10 to 1.45e-9 range then the person is in normal heart beat frequency range 20Hz – 40Hz otherwise the person is affected with Tachycardia or Bardycardia.

Acknowledgement. The authors would like to thank National MEMS Designing Center (NMDC) supported by NPMASS, IISc Bangalore.

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[3] Domenico Zito, Domenico Pepe, Bruno Neri, “Werable System-on-a-chip pulse radar sensors for Health Care: System Overview”, Advanced Information Networking and Applications Workshops, 2007, AINAW '07. 21st International Conference on Year: 2007, Volume: 2.

[4] Siram Sai Krishna, Nuti Venkata Subrahmanya Ayyappa Sai, Dr.K.Srinivasa Rao Alain Beliveau, (1999) Design and Simulation of MEMS-based Piezoelectric Accelerometer.. Evaluation of MEMS Capacitive Accelerometers. IEEE Explore, 1-9.

[5] B. S.Kavitha, S. B. (n.d.). Capacitive Accelerometer Characteristics Study. IEEE Explore, 1.

[6] Ndu Osonwanne, J. V. (2010). MEMS Comb Drive Reduction Deyond Minimum Feature Size: A Computational Study. IEEE Explore, 1-5.

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