Biomedical Engineering Reference
In-Depth Information
Thermoelectric Coefficients of Some Common Materials at 0
◦
C
TABLE 2-9
Thermoelectric Coefficient,
Thermoelectric Coefficient,
α
S
(
μ
VK
−
1
)
α
S
(
μ
VK
−
1
)
Material
Material
Aluminium
3.5
Nichrome
25
Antimony
47
Nickel
−
15
Bismuth
−
72
Platinum
0
Cadmium
7.5
Potassium
−
9
Carbon
3.0
Rhodium
6
Constantan
−
35
Selenium
900
Copper
6.5
Silicon
440
Germanium
300
Silver
6.5
Gold
6.5
Sodium
−
2
Iron
19
Tantalum
4.5
Lead
4
Tellurium
500
Mercury
0.6
Tungsten
7.5
Therefore, the temperature of the probe tip can be determined as
V
out
α
SA
−
α
SB
T
tip
=
T
ref
+
(2.74)
In practice, manufacturers generally provide calibration functions for their products. These
functions are usually high-order polynomials and are calibrated with respect to a certain
reference temperature. Suppose that the coefficients of the calibration polynomials are
a
0
,
a
1
,
a
2
,...,
a
n
. The temperature at the probe tip can then be related to the voltage
output as
a
2
V
out
+···+
a
n
V
out
T
tip
=
a
0
+
a
1
V
out
+
(2.75)
Note that equation (2.75) is effective only if the reference temperature,
T
Ref
, in the mea-
surement remains equal to the reference temperature specified on the data sheet.
Materials should have a high thermoelectric coefficient, low thermal conductivity,
and low resistivity. Unfortunately, as shown in Table 2-9, materials like gold, silver, and
copper that have low resistivity also have poor thermoelectric coefficients, whereas those
with high thermoelectric coefficients, like bismuth (Bi) and antimony (Sb), have high
resistivities (Fraden, 2003).
2.4.15 Tactile Sensing
Tactile feedback is one of the critically important aspects of any successful hand prosthesis.
In addition, it also plays a role in improving haptic feedback in remotely monitored medical
examinations and surgery (Pawluk, Son et al., 1998).
Tactile feedback relies on contact-based effects including contact stresses, slippage,
heat transfer, and hardness. These properties, in a grasped object, can be classified into
geometric and dynamometric types (Webster, 1999). Among the geometric properties are
presence, location in relation to the end-effector, shape, dimensions, and surface condi-
tions. Among the dynamometric parameters associated with grasping are force distribution,
slippage, elasticity, and hardness as well as friction.
Though tactile sensing requires sophisticated sensors, it is also reliant on the processes
through which the device interacts with the explored object. These include controlling
