Biomedical Engineering Reference
In-Depth Information
4.1 Endoscopic Force Sensor: Sensor Design
The structure of the sensor explained in this section consists of three layers. The top layer
is made of micro-machined silicon, which has a rigid tooth-like structure similar to that
shown in Figure 4.1. The bottom layer of the grasper consists of a flat Plexiglas substrate.
Sandwiched between the Plexiglas and silicon is a 25 μm thick PVDF film. The two
flat surfaces of the PVDF film are patterned aluminum electrodes that provide electrical
output to an amplifier. The top side consists of four strips of aluminum electrode and
the bottom side has a single common electrode covering the entire area occupied by the
top four electrode strips. The areas of intersection between the top and bottom electrodes
form the active piezoelectric sensors. Four output signals are derived from the device.
The top exposed portion of the silicon is anisotropically etched to form four tooth-like
structures at 3 mm intervals. A 6 mm thick Plexiglas plate was prepared as a substrate
for the sensor. The PVDF film is attached between the silicon and the substrate by
employing electrically nonconductive glue in such a way that the four sensing elements
on the PVDF film are firmly affixed directly below the four tooth-like structures on the
silicon. A detailed drawing of the device is shown in Figure 4.2. A cross-sectional view of
the sensor is illustrated in Figure 4.3. When force is applied to any point on the surface
of the silicon, the stress in the PVDF results in a polarization charge at each surface.
The amplitude of the signal is proportional to the magnitude of the applied force, and
differences between the slopes of the four output signals indicate any localized position
of the applied force.
4.1.1 Modeling
The silicon is modeled as a uniform beam of finite length located on an elastic foundation
(such as PVDF/Plexiglas substrate). When a concentrated load
P
(point force) is applied
at
x=a
(e.g., point C in Figure 4.4) on a beam, the lateral load is transmitted to the
substrate by the shear force which, at any point, is given in [12].
EI
d
4
v
d
x
4
=
Q
(4.1)
Figure 4.1
Endoscopic graspers [11] (©
Microline Surgical)
