Design and simulation of eyeball pressure sensor

Number of pages: 151 File Format: word File Code: 32268
Year: 2011 University Degree: Master's degree Category: Electronic Engineering
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    Dissertation for Master's degree (M.SC)

    Treatment: Electronics

    Chapter 1: Outline

    1-1 Introduction

    The main problem of the research is the design and analysis of the capacitive pressure sensor based on MEMS technology and the investigation of how much the sensitivity of the sensor can be increased to measure eyeball pressure?

    In this research, the capacitive pressure sensor based on MEMS technology is investigated to measure intraocular pressure. This sensor is implanted inside the eyes of glaucoma patients so that the intraocular pressure [1] can be continuously observed.

    MEMS stands for Micro Electro Mechanical Systems. MEMS technology is used to create small integrated parts such as microsensors or systems that integrate mechanical and electrical components together. Pressure sensors made using MEMS technology have small size, low cost and excellent performance. The size of the manufactured components starts from a few micrometers and extends to a few millimeters [1].

     

     

    Capacitive pressure sensor[2] is a common mechanism for a pressure sensor. These types of sensors consist of two parallel electrode plates. The upper electrode plate of the capacitor is called the diaphragm and shows great sensitivity to external pressure. By applying external pressure, the diaphragm is deformed, the distance between the two electrodes decreases, and as a result, the capacity of the capacitor increases.

    The capacitor plates are such that one electrode plate is fixed (lower electrode[3]) while the diaphragm moves relative to it. According to the formula (1-1), increasing or decreasing the space between the two plates leads to a change in the capacitance.

    ? is the dielectric constant of the material between the diaphragm and the bottom plate, A is the effective area of ??the capacitor electrodes and d is the distance between them.

    (images are available in the main file)

    In this research, we investigate the capacitive pressure sensor that can be implanted in the eye using MEMS technology. This intraocular sensor is implanted in glaucoma patients to monitor eye pressure and helps doctors to better treat glaucoma and have a more complete history of intraocular pressure (IOP). In eye pressure sensors, the pressure applied to the diaphragm of the sensor is done through the eye fluid (aqueum) and creates a displacement of several microns in the diaphragm. Measuring eyeball pressure is very important for patients suffering from glaucoma. Glaucoma is usually caused by increased intraocular pressure and poor functioning of the eye. Glaucoma usually has no symptoms and the patient may go to the border of blindness. This is while the patient does not experience pain or significant symptoms [2].

    The introduced pressure sensor is one of the three components used in the eye pressure measurement system. The second component is the data collection and processing unit, and the third component is the central database that stores all recorded information (Figure 2-1). The primary circuit (information processing unit) and the secondary circuit (sensor) communicate with each other through selfie coupling. The primary circuit constantly generates a periodic signal and sends it to the secondary circuit. The sent signal stimulates the secondary circuit and the result of this stimulation returns to the primary circuit and changes the characteristics of the primary circuit. Measuring the frequency response of the periodic signal in the primary circuit gives information about the capacitive capacity of the sensor and finally the pressure applied to the diaphragm. (The capacitive capacity of the sensor is directly related to the pressure applied to the sensor.)

    How the data collection unit and the central database unit are connected with the implanted sensor

    In this project, we will focus on the first phase of designing the eye pressure capacitive sensor.The second phase includes the manufacturing of the part and the third phase includes the design of the DAP unit and the database, which is part of the future plans.

    The most important issue that is of great importance is the sensitivity of the sensor. If the diaphragm does not have an acceptable sensitivity, it cannot be moved well, and as a result, the capacity of the capacitor will change slightly.

    The important parameters for the design of the pressure sensor are:

    Capacity capacity

    Shape of the diaphragm

    Amount Diaphragm deviation

    Aperture thickness

    Air gap size

    Materials used for the diaphragm

    Ultimately, the goal is to increase the sensitivity of the pressure sensor so that by applying the least pressure, we have the most changes in the sensor diaphragm.

     

     

     

     

     

     

     

     

     

    1-2 Research Objectives

    This project deals with the design and simulation of capacitive eye pressure sensor using MEMS technology, which has higher sensitivity compared to its similar samples. Considering the introduction of important parameters for the design of the pressure sensor and since the characteristics of the displacement of the diaphragm have the greatest impact on the behavior of the sensor, it is necessary to have a detailed analysis of the displacement and the factors affecting the diaphragm. Therefore, the research objectives can be set in order to improve these parameters, including diaphragm thickness, diaphragm material, and diaphragm sensitivity. In this research, the most important goal is to increase the sensitivity of the pressure sensor. To have a large movement in the diaphragm of the sensor by applying the least pressure.

    This problem can be done by reducing the stress and stiffness of the diaphragm. Stress reduction is associated with limitations and problems during construction. In order to increase the sensitivity, we try to reduce the dependence of the diaphragm on the body in order to reduce the effect of stress on the diaphragm and finally the diaphragm can be more sensitive to pressure. The design of the part is associated with the simulation and optimization of materials. 1-3 The importance of the research topic and the motivation for choosing it. In this research, an attempt is made to measure the eyeball pressure of patients suffering from glaucoma by designing a microsensor. Since glaucoma usually has no symptoms, this method helps doctors to find better information about the pressure of the eyeball.

    The normal level of intraocular pressure is around 16 mmHg. Pressure over 22 mmHg is normally high, and pressure between 45-50 mmHg will be very dangerous. There are different techniques to measure intraocular pressure. It is possible to measure eye pressure using conventional techniques such as Goldman's tonometer. The use of this technique is still used in ophthalmologists' offices, but this technique is highly dependent on the thickness of the cornea. On the other hand, intraocular pressure is constantly changing during the day. This problem causes many glaucoma patients to gradually lose their vision, even though they are under controlled observation. Therefore, the need for continuous intraocular pressure measurement seems essential [3].

    The sensor investigated in this project is a part of the passive capacitive pressure sensor for intraocular pressure measurement. Therefore, by optimally designing the capacitive pressure sensor and increasing its sensitivity, a more accurate measurement of eye pressure can be provided for doctors.

    Abstract

    In this thesis design and simulation of MEMS capacitive pressure sensor is presented. This sensor will be implanted in the eye of Glaucoma patients to monitor intraocular pressure (IOP) on a continuous basis.

  • Contents & References of Design and simulation of eyeball pressure sensor

    List:

    Chapter 1: Overview of the plan. 1

    1-1 Introduction. 1

    1-2 research objectives. 5

    1-3 The importance of the research topic and the motivation for choosing it. 5

    1-4 research hypotheses. 6

    1-5 limitations and research problems. 7

    1-6 Project Structure 7

    Chapter Two: An Overview of Studies 8

    Introduction. 8

    2-1 Introduction of MEMS. 9

    2-2 MEMS transducers. 10

    2-3 pressure sensors. 10

    2-3-1 Piezoelectric pressure sensors. 10

    2-3-2 piezo resistance pressure sensors. 11

    2-3-3 Capacitive pressure sensors. 11

    2-3-3-1 Reasons for using the capacitive pressure sensor. 12

    2-4 eye structure. 13

    2-4-1 eyelid. 14

    2-4-2 conjunctiva. 14

    2-4-3 cornea. 14

    2-4-4 iris and pupil. 15

    2-4-5 front room. 16

    2-4-6 lenses. 16

    2-4-7 vitreous. 17

    2-4-8 retina. 17

    2-4-9 sclera. 18

    2-4-10 optic nerve. 18

    2-4-11 eye muscles. 18

    2-5 What is glaucoma? 18

    2-5-1 Primary open-angle glaucoma. 19

    2-5-2 acute angle-closure glaucoma. 19

    2-6 Conventional techniques for measuring intraocular pressure. 21

    2-6-1 Goldman applanation tonometer. 21

    2-6-2 Non Contact Tonometry (NCT: Non Contact Tonometry) 23

    2-6-3 Tonopen (Tonopen) 24

    2-6-4 Dynamic Contour Tonometry (DCT) 24

    2-7 Need for continuous intraocular pressure measurement. 25

    8-2 Techniques for continuous measurement of intraocular pressure. 26

    2-8-1 Measurement techniques by wired sensors. 27

    2-8-2 induction coupling tachometer. 29

    2-8-2-1 Disabled device. 30

    2-8-2-2 active device. 41

    2-9 discussion. 42

    Chapter three: Research method. 43

    Introduction. 43

    3-1 Design of MEMS capacitive pressure sensors. 43

    3-2 Flat diaphragm modeling. 45

    3-3 Checking the structure of the capacitive pressure sensor. 48

    3-3-1 mechanical sensitivity of the diaphragm. 49

    3-3-2 sensor sensitivity. 50

    3-3-3 Selecting the working area for the eye pressure sensor. 51

    3-4 checking the capacitance. 52

    3-5 Bending analysis of a thin plate. 53

    3-5-1 Checking the basic equations of thin plates with small displacement. 54

    3-5-2 Review of boundary conditions. 56

    3-5-3 displacement of thin plate under uniform external pressure. 57

    3-6 Calculating the capacitance of the capacitive pressure sensor. 69

    3-7 finite element analysis. 70

    3-7-1 Fixed four-sided square diaphragm. 70

    3-7-2 grooved square aperture. 71

    Chapter four: simulation results. 73

    Introduction. 73

    4-1 Diaphragm simulation. 73

    4-1-1 Mathematical results. 74

    4-1-2 diaphragm stress effect. 74

    4-1-3 Effect of aperture size. 75

    4-1-4 Effect of diaphragm thickness. 75

    4-1-5 mechanical sensitivity of the diaphragm. 76

    4-1-6 finite element simulation results. 78

    4-2 Simulation of eye pressure sensor structure. 86

    4-2-1 Effect of design parameters on static and dynamic behavior of eye pressure sensor. 87

    4-2-2 Pauline voltage check for square diaphragm structure. 87

    4-2-3 Capacitance of eye pressure sensor. 90

    4-2-4 stress distribution on the diaphragm. 92

    4-2-5 Frequency response of capacitive eye pressure sensor. 93

    4-3 Using a polysilicon diaphragm to increase the sensitivity of the eye pressure sensor. 95

    4-4 Investigation of eye pressure sensor with grooved polysilicon diaphragm. 99

    4-5 Comparison of polysilicon and p++si pressure sensors in clamped mode. 102

    4-6 Comparison of polysilicon and p++si pressure sensors with grooved diaphragm. 111

    4-7 Comparison of capacitive pressure sensor with polysilicon diaphragm in clamped and grooved mode. 119

    Chapter five: conclusions and suggestions. 128

    5-1 Conclusion. 128

    5-2 suggestions. 131

    References. 132

     

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Design and simulation of eyeball pressure sensor