Instrumentation & Measurement Magazine 26-1 - 59

Fig. 1. Coupled electromagnetic models of resonant systems used in seawater.
are susceptible to proximity effects. Because the magnetic
permeability of the induction and excitation coil core is easily
affected by temperature, the inductance of the induction
and excitation coil changes, resulting in measurement errors
[7]. In addition, inductively coupled sensors also have the disadvantages
of a weak output signal, low sensitivity, and large
temperature coefficient.
In 2007, Marin Soljacic et al. first proposed using LC resonant
coils for wireless energy transmission in the mid-range
of energy, thus introducing the concept of magnetic resonance
[8]. In 2014, Awai et al. conducted a systematic study on the
penetration ability of magnetic resonance systems in seawater.
They discovered the influence of seawater of different depths
in a fixed radius container on the coil's quality factor, resonant
frequency, and system transmission efficiency. They proposed
using a double-layer spiral coil to limit the electric field distribution
and reduce the eddy current loss [9]. In 2016, Xiong and
Dong performed conductivity measurements on a solution
with an LC passive coil. They found that when the passive coil
is in resonance, electromagnetic induction coupling between
the detection coil and the solution can be enhanced. In addition,
they significantly improved the detection sensitivity and
solution measurement distance [10].
This study proposes a magnetic coupling resonance-type
seawater conductivity measurement system with a dual-coil
structure. The passive transceiver coil is composed of two
detection coils and capacitive elements. The measurement
system consisting of passive transceiver coils is placed in seawater.
The seawater conductivity measurement is realized by
analyzing the relationship between the transmission loss of the
resonant magnetic field between the passive transceiver coils
and the seawater conductivity.
Working Principle
Model Analysis
The dual-coil magnetic coupling resonance seawater conductivity
measurement system is shown in Fig. 1. It consists of a
detection coil, a receiving coil, and seawater. The detection coil
and the receiving coil are composed of the coil inductance and
capacitance, respectively. The detection coil was previously
February 2023
ture in Fig. 1 is equivalent to the circuit shown in Fig. 2. Us
connected to the signal
transmission circuit, and
the receiving coil was connected
to the load resistance
after signal processing.
When the measurement
system is operated
in seawater, the changing
magnetic field generates
an induced electric field
between the two coils. The
seawater to be measured
is equivalent to the series
connection of inductance
and resistance. The strucis
the
excitation voltage, and I, Z, L, R and C represent the loop current,
equivalent complex impedance, equivalent inductance,
equivalent resistance, and capacitance, respectively.
In (1)-(4), seawater is equivalent to R0
the self-resonant frequencies of the coils L1
and L0
coil's resistances are R1 and R2
under seawater. RL is the load resistance. M12
and L2
ductance between the detection and receiving coils. M10
seawater.
1
. Assuming that
are equal, the
. Rrad is the radiation resistance
is the mutual inis
the
mutual inductance between the detection coil and seawater.
M20
2
is the mutual inductance between the receiving coil and
Zʹ and Zʹ are the purely resistive values of the two coils.
Since the receiving coil has little effect on seawater, the value of
M20
is not considered here.
11


Z R R jL1
1
rad  
Z j L R 
22 L
rad
0 00
Z R R R jL2
According to Kirchhoff's law:
1
10
 j M Z0

 j M12 00


Z I2
10
1
   
jC
 2
jC
 1
(2)
(3)
(1)
 Z j M j M I Us

12
1
 ∙   
    
I
    
     
    
0 
(4)
Fig. 2. Equivalent circuit for the combination of resonant system and seawater.
IEEE Instrumentation & Measurement Magazine
59
Instrumentation & Measurement Magazine 26-1

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