Capacitive Based Pure Bending Strain Sensor For Knee Replacement Monitoring

Shashikant Verma 1, Mandeep 2, Anurekha Sharma 3, Jaideep Gupta 4

1., 4. Mechanical Engineering Department, NIT, Kurukshetra, Haryana, India 2., 3. Electronics Science Department, Kurukshetra University, Kurukshetra, Haryana, India

Computational Methods: Sensor was modeled in two different ways:

First, using analytic function and giving prescribed displacement to the

sensor. Analytic function was defined which represent the deflection of cantilever test

substrate and its value was stored in a variable. This variable was used to give

prescribed displacement to the sensor.

Second method involves modeling both sensor and cantilever test substrate

together and load was applied at free end of the beam.

Electromechanics module and stationary study was used to compute the model.

Geometry of sensor and cantilever test substrate:

Fabrication: Sensor was virtually fabricated in Intellisuite 8.8.

Sensor was fabricated on silicon(100) substrate of thickness .310mm and layer of

SiO2 was deposited by dry oxidation.

Silicon was etched using RIE (Reactive Ion Etching) .

Thin layer of metal of thickness .2µm was sputtered on etched silicon.

Glass was taken as another substrate of thickness .5µm and a thin layer of metal

(gold) is sputtered on glass.

Both substrate are bonded using Pyrex bonding.

The fabrication process flow was shown below:

Taking Si(100) substrate

Etch Si using RIE (Reactive Ion Etching) etch

Sputtering metal on etched Si

Taking Glass substrate

Sputtering metal on glass substrate

Bonding of both Si and glass substrate

using Pyrex bonding

Results: Simulated and calculated capacitance for different design:

Conclusions: The small initial gap provides sufficient nominal capacitance to

avoid sensitivity losses due to parasitic capacitance. It is also clear from the

observation that there is less difference between calculated and simulated value

of capacitance for design 2 and 3 as compared to 1st . The sensitivity of design 3

is very high as compare to other design.

References:

1. J.-T. Lin et al. / Sensors and Actuators A 138, 276-287, (2007)

2. E.Procter, J.T. Strong, Capacitive Strain Gauge, Elsevier, 301-303, (1992)

3. K.I. Arshak, D. Collins, F. Ansari, New high gauge factor thick film transducer

based on a capacitor configuration, Int. J. Electron, 387-399, (1994)

Introduction: MEMS (Microelectromechanical system) capacitive based pure

bending strain sensor is presented for use in monitoring the progress in healing of the

knee after injury or after knee replacement. The sensor is designed and simulated in

COMSOL multiphysics 4.4b . The cantilever structure of sensor is composed of two

parallel plates with narrow gap between them and conjoint end. The sensor is

mounted on the cantilever, which responds to the strain. The mechanism of sensing

is based on the concept of change in capacitance due to change in gap between

capacitor plates.

•It utilizes a variable gap configuration comprised of silicon and glass beams that are

bonded at one end and open at the opposing end.

•The bottom silicon plate was fixed to the bending test structure. As the structure

bends, the bottom plate conforms to the structure and moves away from the straight

top plate.

•The gap therefore increases and change in capacitance takes

place.

•Three different designs were modelled corresponding to three

different metal coverage area with different initial gap of 3µm,

6µm and 7.4µm.

Fig.1 Capacitive sensor

Fig.2 33% metal

coverage

Fig.3 67%metal

coverage

Fig.4 100% metal

coverage

Length(mm) Width(mm) Height(mm)

Cantilever test substrate(Si, Steel)

56 46 5

Silicon base 9 2 .307, .304, .3026

Silicon anchor 1.5 2 .003, .006, .0074

Top Borosilate glass 9 2 .5

Fig. 10 Displacement produced in

sensor placed on loaded beam (load

(373.503N) corresponding to 1st strain

at 100% metal coverage with 3µm

initial gap is applied at free end).

Fig. 11 Displacement produced in

sensor as a result of prescribed

displacement when only sensor is

modelled at same condition as that for

Fig. 10.

Table 2. Capacitance with 3µm gap Fig. 6 Graph between capacitance vs. strain

with 3µm initial gap

Table 3. Capacitance with 6µm gap Fig. 7 Graph between capacitance vs. strain

with 6µm initial gap

Table 4. Capacitance with 7.4 µm gap Fig. 8 Graph between capacitance vs. strain

with 7.4µm initial gap

Average sensitivity of sensor over a range of 0µε to 1930µε for nine permutation is

shown in table 5;

Initial gap(mm) Result Design 1(100%) Design 2(67%) Design 3(33%)

.003 Calculated (C) 379.29 409.07 436.15

Simulated (C) 402.60 420.14 439.79

.006 Calculated (C) 295.14 338.08 376.66

Simulated (C) 338.56 367.37 398.20

.0074 Calculated (C) 268.08 314.74 354.33

Simulated (C) 315.33 344.52 377.64

Table 5. Average sensitivity over range of 0µε to 1930µε

Table 1. Dimension of cantilever test substrate and

sensor

Future work: To study the effect of material of substrate on the sensitivity of

sensor as if the substrate was of polyimide then there was unpredictable

displacement of cantilever.

Fig. 5 Process Flow