3.1 Thermodynamic Analysis of Physical Anti-Collision …
61
(1) Heat conduction: The basic law of heat conduction is given by [9]
φ = −kA
∂t
∂n
(3.1)
where φ is heat flux, k is heat conductivity coefficient and
∂t
∂n
is normal temperature
gradient.
From Eq. (3.1), the direction of the temperature gradient is opposite to the one of
heat flow. Thermal conductivity presents the ability of a material’s heat conduction
and the main factors influencing the thermal conductivity are the kinds of material,
temperature, and so on.
(2) Heat convection is defined by the Newton cooling law [10]:
φ=Ah c (t w − t f )
(3.2)
where h c is the coefficient of convective heat transfer, which presents the transferred
heat for 1 °C on per area, A is the area of a solid’s surface, t w is the temperature of
fluid, and t f is the temperature of the solid’s surface.
(3) Heat radiation is given by [11]
A
0
+
R
0
+
D
0
= α+β + γ = 1
(3.3)
where 0 is the radiation power fall on the tag, A is the absorbed one, R is the
reflected one, D is the penetrated one, α is absorptivity, β is reflectivity and γ is
transmittance.
The value of absorptivity, reflectivity, and transmittance are related to the tag’s
nature and will change with environment temperature and the tag’s radiation
wavelength. For most materials, thermal radiation is not easy to penetrate:
α+β=1
( 3 . 4 )
(2) Reading distance of RFID system
In practical application, RFID tags are attached to the surface of targets and generate
induction current to send data due to the electromagnetic field transmitted by reader
antennas. Then, the readers detect and decode the backscatter signal of tags. Eventually, the readers send the data of tags to the background processor and the RFID
system achieves the purpose of automatic identification of goods.
Reading range is an important characteristic parameter of passive RFID tags. The
power density of an electromagnetic wave incident on the RFID-tag antenna in free
space is given by [12]
61
(1) Heat conduction: The basic law of heat conduction is given by [9]
φ = −kA
∂t
∂n
(3.1)
where φ is heat flux, k is heat conductivity coefficient and
∂t
∂n
is normal temperature
gradient.
From Eq. (3.1), the direction of the temperature gradient is opposite to the one of
heat flow. Thermal conductivity presents the ability of a material’s heat conduction
and the main factors influencing the thermal conductivity are the kinds of material,
temperature, and so on.
(2) Heat convection is defined by the Newton cooling law [10]:
φ=Ah c (t w − t f )
(3.2)
where h c is the coefficient of convective heat transfer, which presents the transferred
heat for 1 °C on per area, A is the area of a solid’s surface, t w is the temperature of
fluid, and t f is the temperature of the solid’s surface.
(3) Heat radiation is given by [11]
A
0
+
R
0
+
D
0
= α+β + γ = 1
(3.3)
where 0 is the radiation power fall on the tag, A is the absorbed one, R is the
reflected one, D is the penetrated one, α is absorptivity, β is reflectivity and γ is
transmittance.
The value of absorptivity, reflectivity, and transmittance are related to the tag’s
nature and will change with environment temperature and the tag’s radiation
wavelength. For most materials, thermal radiation is not easy to penetrate:
α+β=1
( 3 . 4 )
(2) Reading distance of RFID system
In practical application, RFID tags are attached to the surface of targets and generate
induction current to send data due to the electromagnetic field transmitted by reader
antennas. Then, the readers detect and decode the backscatter signal of tags. Eventually, the readers send the data of tags to the background processor and the RFID
system achieves the purpose of automatic identification of goods.
Reading range is an important characteristic parameter of passive RFID tags. The
power density of an electromagnetic wave incident on the RFID-tag antenna in free
space is given by [12]