Biomedical Engineering Reference
In-Depth Information
voltage ratio of the rigid and compliant cylinders in each sensing element. This voltage
ratio can be used as a criterion to distinguish between different materials with varying
softness [1, 3]. In order to calibrate the system, a set of reference soft objects (with
known softness number) were used. In the next experiment, the standard ASTM D2240
test block kit (Instron Co.) with standard softness values (32.4, 41.9, 52.0, 61.3, 71.1,
81.1, and 88.8 Shore A) was used to measure the voltage ratios associated with each
material. The resulting voltage ratio and the softness of the material were saved in a
lookup table.
The voltage ratio measured for each unknown soft material was then compared to the
existing reference voltage ratios on the lookup table. The softness value corresponding to
the calculated voltage ratio was extracted and considered as the softness of the object
which was in touch with the sensing element. This softness value was then scaled and
converted into a grayscale value and displayed on the monitor. The complete image for
the two tactile arrays which was derived in this way, consisted of eight cells arranged in
two rows and four columns. Each cell in this 2
×
4 image was proportional to a sensing
element in the two sensor arrays.
In step 6, the grayscale value of each cell is obtained using following relationship:
[
I
]
=
(
[
σ
]
/α) K
(8.2)
where matrices [
I
]and[
σ
] represent the intensity and softness, respectively. For this
study, [
I
]and[
σ
] are defined as below:
I
U
1
I
U
2
I
U
3
I
U
4
[
I
]
=
I
L
1
I
L
2
I
L
3
I
L
4
σ
U
1
σ
U
2
σ
U
3
σ
U
4
[
σ
]
=
σ
L
1
σ
L
2
σ
L
3
σ
L
4
where
σ
ij
is the softness for each sensing element obtained from the lookup table in
step 5 and [
I
ij
] is the grayscale value of the softness display corresponding to the same
sensing element.
K
is a coefficient which determines the number of grayscale levels on
the display. We considered 256 grayscale levels (8 bit depth) for displaying the tactile
image, that is,
K
= 256. Symbol
is the softness sensitivity coefficient which enables us
to show different ranges of softness on the display. We considered four different ranges
for softness: Very Soft (
α
α
= 5Shore A), Soft (
α
= 20Shore A), Medium (
α
= 40Shore A)
and Hard (
= 100Shore A).
Figure 8.8a shows the resulting 2
α
4 image when the grasper is in touch with an
object. The two right-hand sensors of the grasper (
U
4
and
L
4
) are engaged as indicated in
Figure 8.8b. Furthermore, to create a smooth transition between the columns and rows, an
interpolation between the grayscale values of the adjacent columns and rows is necessary.
Evidently, increasing the number of cells in both directions enhances the image quality.
In step 7, a linear interpolation is performed to increase the number of columns from
four to
M
. The resulting '2
×
M
' matrix, [
G
] is shown in Equation 8.3:
×
G
U
1
G
U
2
G
U
3
···
G
U
M
[
G
]
=
(8.3)
G
L
1
G
L
2
G
L
3
···
G
L
M
