Intraocular Pressure Measuring System Based on

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Energy Procedia

Energy Procedia 00 (2011)16000–000 Energy Procedia (2012) 1288 – 1294 www.elsevier.com/locate/procedia

2012 International Conference on Future Energy, Environment, and Materials

Intraocular Pressure Measuring System Based on Piezoelectric Bimorph Yue Tong a, zhengrong Xiab , * a,b

Department of Physics, Huainan Normal University,Huainan, 232001, China

Abstract Based on a method of measuring intraocular pressure by the effect of piezoelectric bimorphs, a testing system for intraocular pressure is proposed. In this paper, we discuss the measuring principle, the measuring circuit; Pig eye is used as the research object, and a relationship between the output current of bimorph and the internal pressure of eyeball is established through experiment. The experimental result shows that the internal pressure of eyeball and the output current of intraocular bimorph have relatively good linear relationship within the range of human intraocular pressure, the linear relationship expression between intraocular pressure and current can be attained by using linear fitting method. The fitting function expression is used as calculation basis, the known intraocular pressure of simulation eyeball is measured, and measurement results show that the error is relatively small, which proves that this method is practical and feasible in a certain extent. © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer] Keywords: Sensors, Intraocular pressure, Piezoelectric Bimorphs, Curve Fitting

Introduction Intraocular pressure value is an important basis in the diagnosis of glaucoma. The main measuring methods of intraocular pressure are tonometer method, liquid manometer, and finger-press method in clinic. At present, the commonly used tonometers are Goldmann applanation tonometer, Proview tonometer, Tono-Pen tonometer, Perkins hand-held tonometer, non-contact tonometer and so on. As this type of products has relatively accurate monitoring results, they are widely accepted by many ophthalmologists. However, these products also have some shortcomings, which are mainly presented in the aspects like expensive cost, complicated operating process, high treatment cost for patients and so on, so they are inconvenient to be promoted to community medicine and family users. In order to solve this problem, we design a kind of intraocular pressure measuring system in which the piezoelectric bimorph is * Yue Tong. Tel.: +8605546883360; fax: +8605546863551. E-mail address: [email protected].

1876-6102 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society. doi:10.1016/j.egypro.2012.01.206

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used as a sensor, this system has simple structure, low-cost, easy operating process and higher accuracy, so this intraocular pressure measuring device is suitable for family. 1. Structure and working principle of piezoelectric bimorph sensors 1.1. Piezoelectric bimorph sensorse Piezoelectric bimorph is the commonly used device in micro-electro mechanical system, and it is mostly used in micro-force detection, acceleration detection, biochemical detection, data storage and so on[1-3]. Its structure is shown in Figure 1, the double chips are combined by two piezoelectric patches with the same polarization direction. The inner electrode with negligible thickness is in the middle of the two patches, and it leads the wire into one pole of the driving power; the electrodes on the surface and under the surface are linked together, and then they are connected to the other pole of power. If alternating sine wave voltage is applied between the upper and lower electrodes of piezoelectric layer, when the applied voltage is in the same direction of the polarization, the piezoelectric ceramic transducer will extend because of the converse piezoelectric effect, conversely, it will shrink. At this moment, the generated exciting force will vary periodically according to the simple harmonic motion under the control of alternating sine wave voltage, which will force piezoelectric bimorph to do equivalent periodic twoway side telescopic vibration[4-7]. Assume that the length, width and height of bimorph are l , w , and h respectively, acting force F is applied on the surface of piezoelectric bimorph, the surface of piezoelectric patch will have voltage output based on the direct piezoelectric effect. If the deflection of bimorph cantilever is w( x) ，the computational expression of the strain

εx

of any point on the bimorph that is − h / 2 away from the

middle and the curvature radius Rρ on the middle of beam surface is shown as following：

ε x = h / 2 Rρ 2

2

In the formula, Rρ = dx / d w , the thickness of piezoelectric bimorph is usually relatively small, when

w( x) is comparatively small, the strain ε x of bimorph that is − h / 2 away from the middle can be

used to express the strain of beam. Suppose that the electronic field is distributed evenly among the bimorph, the electric field of any point on the piezoelectric bimorph can be obtained according to the piezoelectric equation:

−h31h ⋅ Es = h31ε x =

d 2w 2dx 2

（1）

h31 is the piezoelectric stiffness constant of piezoelectric material. The voltage of any point on the

piezoelectric patch can be worked out by Formula (1) :

h2d 2 w V= 2hEs = −h31 ⋅ 2dx 2

（2）

The actual output voltage of piezoelectric bimorph can be obtained by finding integral on Formula (2):

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Yue Tong andname zhengrong XiaProcedia / Energy00 Procedia 16 (2012) 1288 – 1294 Author / Energy (2011) 000–000 w l 2 w 0 − 2

∫ ∫ Vdxdy

Vout =

wl

h h 2 dw h2 =31 ( )= − h31 ⋅ θ l dx x =l l

（3） x =l

θ is the angle of beam. If a force is applied on the middle of beam end（which is shown in Figure 1), the end corner is as following according to the beam bending theory.

θ

x =1

= Fl 2 / 2 EI

（4）

The following formula can be obtained by taking Formula (4) into Formula (3)

Vout = Fh31h 2l / 2 EI

（5）

Formula (5) shows that the output voltage of bimorph is proportional to its applied force under the assumption of small deformation and elasticity. At this moment, a generated exciting force will vary periodically according to the simple harmonic motion under the control of alternating sine wave voltage, and the function relation of its input and output voltage is as following: U = U o sin ωt

(6)

In the formula, U is the output excitation voltage of piezoelectric bimorph, U o is the input excitation voltage, t is the time, ωt is the frequency of excitation angle. When the measurand is applied on the bimorph, the vibration state (amplitude and frequency) of bimorph will change correspondingly, and its output voltage will also change correspondingly, there is a certain relationship between this variable and the measurand. The measurand can be obtained by detecting vibration changes of the bomorph. Piezoelectric Bimorph Shell

R1

Driving signal

U out

Vent

Pressure d Simulated eye

Fig.1 Piezoelectric Bimorph

Fig.2 The diagram of Piezoelectric Bimorph Sensors

1.2. Establishment of the relationship between intraocular pressure and measuring signal. Figure 2 is the structure diagram of the piezoelectric bimorph sensor. A vibration unit is formed by the piezoelectric bimorph and the contact lever. The electrodes on the two sides of piezoelectric bimorph are respectively connected to two sine driving signals that have 1800 phase difference, the leading-out terminal of intermediate metal is connected to the ground through the sampling resistance R11, and it is also used as the output unit of the sensor. Both sides of piezoelectric bimorph are fixed on the base by compressing tightly to both sides with two groups of elastic block, while a contact lever is attached firmly in the middle of the piezoelectric bimorph f, this contact lever contacts with the measured object directly by stretching out of the base. Under the influence of AC signal, the piezoelectric bimorph will produce


resonance in the direction which is perpendicular to its surface, and the contact lever will also have the same vibration. When the contact lever contacts the measured object, the object will vibrate together with the bimorph. When the constant AC voltage is maintained, the current flowing in the circuit will change according toits vibration frequency. The output signal of bimorph is non-linear[6], and multi-group data of the relation between the circuit flowing in piezoelectric bimorph and the input signal frequency in a certain frequency range can be measured when different pressure values of the measured object are already known. Curve fitting is applied to the optical data in a certain frequency after analyzing the linearity of each group, and then a fitted formula of current and pressure can be drawn. The known pressure value is set in the range that the human eye may have, the corresponding intraocular pressure value can be obtained from this formula if the current value is detected. 2. Measuring system drive and detection circuit design Measuring system consists of power module, piezoelectric ceramic vibration driving module, piezoelectric bimorph sensor module, signal input module and so on. the system block diagram is shown in Figure 3:

Fig. 3 The system block diagram

Fig.4 The circuit diagram of power supply

2.1. power module Power supply circuit is shown in Figure 4, and the mode of dual power supply is adopted. The voltage of positive and negative 15V that generated from converting chip voltage 7815 and 7915 is adopted, this voltage provides power amplifier with TDA1521 and at the same time provides SCM system with 5V DC voltage that is converted through 7805. 2.2. Piezoelectric ceramic vibration driving module The sine wave with wide application and flexible transition performance is adopted as the driving signal source through the analysis of the electrical characteristics of piezoelectric bimorph. The driving circuit is shown in Figure 5, and the original signal comes into two LM 324 operational amplifiers through the protection process. One is used in the non-inverting amplification of preceding phase, and another one is used in the inverting amplification of preceding phase, the signal of every point on the two signal channels has same value and inverted phase. Piezoelectric ceramic is equivalent to capacitive load, as the dielectric constant of PZT material is relatively large, the capacitance C of piezoelectric ceramic is also relatively large. RC circuit is formed by the capacitance C and the output resistance R of driving circuit, which will affect the dynamic characteristics of piezoelectric ceramic. In order to adapt to the application of the dynamic characteristics

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of piezoelectric ceramic, dual power complementary push-pull power amplifier TDA1521A is used as a core device to fabricate power amplifying circuit. This power amplifying circuit has fewer external components and needs no debugging, etc., and it is a recognized 2 × 15W power amplifying circuit with minimum external components. The driving signal that is amplified by the power amplifying circuit is sent into the pulse step-up transformer, the turn ratio of preceding phase and backward phase of the pulse step-up transformer is 1:12, the preceding phase is a group of sine input, the backward phase is two groups of positive and negative phase output; the form of common tap in backward tap is adopted in pulse step-up transformer, and the backward phase has inverting function. Thus, the sine signal that is output from power amplifier changes into two reversed high-pressure driving signals by flowing through the pulse step-up transformer, and this signal can be directly used to drive piezoelectric bimorph.

Fig.5 Driving Circuit of Piezoelectric Ceramic

Fig.6 Signal acquisition circuit

1.3. signal input module Sampling circuit is shown in Figure 6. The output current signal of piezoelectric bimorph forms differential pressure through R11, and two LM324 are used to constitute a differential amplifier, to amplify the voltage differences between the two sides of R11.The output voltage signal of piezoelectric bimorph can be used directly in the SCM system for data processing after flowing through full-wave rectifier and low-pass filter circuit. 3. Experimental analysis As the physiological parameters of pig eye are basically the same with that of human eye, the eyeball of pig is used as experimental subject. The eyeball is fixed with the cornea facing up, and the container with 0.9% standard saline is connected to vitreous body by inserting into the optic nerve on the back end of pig eye with an injection needle. The height of container is changed to generate differential pressure between the container and the eyeball of pig, under the hydraulic pressure that is within the normal human intraocular pressure range (11-21mmHg, that is 149 to 286 mm water column), slightly contacting the corneal surface of pig eye with contact clever, loading signal source of a certain voltage and changing the frequency f of signal source, in order to facilitate the generation of different output signals under various frequencies. Many "differential pressure - current" curves are drawn to these datum (Figure 7), in

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which the current can be translated into measurable voltage for calculation through sampling resistance R11 . It can be seen from the data analysis that the frequency - current curve linearity is relatively better when the frequency of input signal is between 600-700Hz, and the curve at f = 670 Hz has the highest linearity. At this frequency, the origin software is adopted to conduct linear fit on the "differential pressure - current" curve, the fitted curve is shown in Figure 8, and the fitted equation is: Y = −18567.86627 + 9649.54789 X In the formula, Y is the pressure and X is the current.

2.00

700

f=600Hz f=610Hz f=620Hz f=630Hz f=640Hz f=650Hz f=660Hz f=670Hz f=680Hz f=690Hz f=700Hz

1.95 1.90 1.85 1.80

500 400 300

Y=A+B*X A=-18567.86627 B=9649.54789 R=0.99374 SD=29.56105

200 100

1.75

pressure difference (mm H2O)

1.70 -100

600

pressure difference (mm H2O)

2.05

800

Current (mA)

2.10

（6）

0

100

200

300

400

Fig.7 " Pressure - current" curve

500

600

700

800

0

Current (mA) -100 1.92

1.93

1.94

1.95

1.96

1.97

1.98

1.99

2.00

Fig.8 "Intraocular pressure - current" fitting curve

4. Conclusions (1) This paper proposes a technical program using the inverse piezoelectric effect of piezoelectric ceramic to measure the intraocular pressure, and produces an intraocular pressure measuring system under the laboratory conditions; pig eye is used as the research object, and a relationship between the output current of bimorph and the internal pressure of eyeball is established through experiment. The experimental result shows that the internal pressure of eyeball and the output current of intraocular bimorph have relatively good linear relationship within the range of human intraocular pressure, the linear relationship expression between intraocular pressure and current can be attained by using linear fitting method (functional relation). (2) The experimental result of intraocular pressure measurement based on piezoelectric bimorph is closely related to the current measurement accuracy, the quality of piezoelectric bimorph and so on. The fitting function expression is used as calculation basis, the known intraocular pressure of simulation eyeball is measured, and measurement results show that the error is relatively small, which proves that this method is practical and feasible in a certain extent. (3) The device proposes a new detection method, which is characterized by simple operating process, better practical applicability and so on. It has significant meaning in medical research and clinical application.

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