90
3 Physical Theory of RFID System Physical Anti-Collision
In the range of 0.3 ~ 3 mm, all the reading distances are almost the same. Therefore,
the test chamber’s thickness has little influence on the performance of RFID tag.
Small errors can be ignored in practical applications. For the convenience, 3 mm
thick plastic box is selected in the proposed system.
The tag is placed in the middle of the test chamber (8 cm (λ/4)). The ambient
temperature is 22.4 °C. Based on the practical application, we assume that the NaCl
solution from 0.01 mol/L to 2 mol/L is placed in the experiment box, and the humidifier sprays the solution into the experiment box. When the hygrometer shows that the
humidity reaches 80%, the humidifier stops humidifying and starts the RFID dynamic
ranging system. The maximum recognition distance of UHF RFID is within 10 m
[36], so the experimental data is within a reasonable range. For each situation, three
experimental measurements were carried out and the results were expressed as the
mean of the three measurements. d 1 , d 2 , d 3 represent the experimental data of three
measurements. d represents the mean of the three measurements. Reading range in
salt mist environment is shown in Table 3.8. Average reading distance that varies
with salt mist environment concentration is shown in Fig. 3.20.
The measurement results have roughly consistency with the simulation results
in Fig. 3.20. As the increase of salt mist environment concentration, the dynamic
reading distance of RFID tags decrease. Minor errors are caused by errors in the
test chamber and the RFID system. When the tag antenna is placed in a conductive
environment, the gain pattern and antenna efficiency are reduced due to boundary
conditions [37]. As shown in Eq. (3.49), increase in conductivity results in a decrease
in the reading distance. Because the tag antenna is placed in a salt mist environment,
the electromagnetic waves emitted by the antenna eddy currents will result in a
decrease in the electromagnetic energy absorbed and emitted by the tag antenna.
Eventually, the feedback electromagnetic wave energy received by the antenna is
reduced resulting in a reduction in the read distance.
The experimental data is fitted to the function after trying various functions. The
curve matching the power function has the best effect, which is consistent with
the result of the formula. The fitting function is shown in Fig. 3.21. The various
parameters of the fitting function are as follows:
The fitting function uses the Allometric2 model of the power function, and the
equation is as follows:
y = a + bx
c
(3.50)
The numerical ranges of the three parameters are respectively a = 5.22788 ±
0.06528, b = −2.25495 ± 0.07286, c = 0.60844 ± 0.02511. According to statistical
theory analysis, the closer the correlation coefficient R 2 is to 1. The smaller the
residual sum of squares is, the better the fitting effect of the model is. The correlation
coefficient of this fitting function is R
2
= 0.99659. The residual sum of squares is
0.05605. Residual sum of squares is 0.05605. The fitting effect of Eq. (3.50) is good.
Then, the absolute error and relative error are calculated by Eqs. (3.51) and (3.52).
S=|y 1 − y 2 |
(3.51)
3 Physical Theory of RFID System Physical Anti-Collision
In the range of 0.3 ~ 3 mm, all the reading distances are almost the same. Therefore,
the test chamber’s thickness has little influence on the performance of RFID tag.
Small errors can be ignored in practical applications. For the convenience, 3 mm
thick plastic box is selected in the proposed system.
The tag is placed in the middle of the test chamber (8 cm (λ/4)). The ambient
temperature is 22.4 °C. Based on the practical application, we assume that the NaCl
solution from 0.01 mol/L to 2 mol/L is placed in the experiment box, and the humidifier sprays the solution into the experiment box. When the hygrometer shows that the
humidity reaches 80%, the humidifier stops humidifying and starts the RFID dynamic
ranging system. The maximum recognition distance of UHF RFID is within 10 m
[36], so the experimental data is within a reasonable range. For each situation, three
experimental measurements were carried out and the results were expressed as the
mean of the three measurements. d 1 , d 2 , d 3 represent the experimental data of three
measurements. d represents the mean of the three measurements. Reading range in
salt mist environment is shown in Table 3.8. Average reading distance that varies
with salt mist environment concentration is shown in Fig. 3.20.
The measurement results have roughly consistency with the simulation results
in Fig. 3.20. As the increase of salt mist environment concentration, the dynamic
reading distance of RFID tags decrease. Minor errors are caused by errors in the
test chamber and the RFID system. When the tag antenna is placed in a conductive
environment, the gain pattern and antenna efficiency are reduced due to boundary
conditions [37]. As shown in Eq. (3.49), increase in conductivity results in a decrease
in the reading distance. Because the tag antenna is placed in a salt mist environment,
the electromagnetic waves emitted by the antenna eddy currents will result in a
decrease in the electromagnetic energy absorbed and emitted by the tag antenna.
Eventually, the feedback electromagnetic wave energy received by the antenna is
reduced resulting in a reduction in the read distance.
The experimental data is fitted to the function after trying various functions. The
curve matching the power function has the best effect, which is consistent with
the result of the formula. The fitting function is shown in Fig. 3.21. The various
parameters of the fitting function are as follows:
The fitting function uses the Allometric2 model of the power function, and the
equation is as follows:
y = a + bx
c
(3.50)
The numerical ranges of the three parameters are respectively a = 5.22788 ±
0.06528, b = −2.25495 ± 0.07286, c = 0.60844 ± 0.02511. According to statistical
theory analysis, the closer the correlation coefficient R 2 is to 1. The smaller the
residual sum of squares is, the better the fitting effect of the model is. The correlation
coefficient of this fitting function is R
2
= 0.99659. The residual sum of squares is
0.05605. Residual sum of squares is 0.05605. The fitting effect of Eq. (3.50) is good.
Then, the absolute error and relative error are calculated by Eqs. (3.51) and (3.52).
S=|y 1 − y 2 |
(3.51)