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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
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