# Characteristic Analysis and Experiment of Adaptive Fiber Optic Current Sensor Technology
## 5 Adaptive Technology of Fiber Optic Current Sensor
### Real-Time Signal Processing System  
The output signal is processed to obtain the magneto-optical correction signal \(Y_{1}\) and a fiber optic AC signal \(Y_{2}\). Then \(Y_{1}\) enters the magnetic change detecting unit, and \(Y_{2}\) enters the temperature fluctuation detecting unit. The detecting unit outputs a warning signal to the signal selection unit, which subsequently determines the proportionality coefficient for signal correction, and a current is  
Figure 8: Optical system of the adaptive fiber optic current sensor. Figure 9: Position of the measured current and two pieces of magneto-optical glass. output through the filtering unit. The structure of the real-time signal processing system is shown in Figure 10.  
(1) Primary signal processing  
The coefficient adjustment make correction signal 1 and correction signal 2 be equal. The two signals are subtracted to obtain a magneto-optical compensatory correction signal:  
\[Y_{1}=u_{G1}-\frac{K_{1}}{1-K_{1}-K_{2}}u_{G2} \tag{18}\]  
Substituting Equations (15) and (16) into Equation (17):  
\[Y_{1}=4K_{1}P_{0}\theta_{1}=M_{1}i \tag{19}\]  
according to Equation (13), where  
\[M_{1}=\frac{4}{\pi}K_{1}P_{0}V\text{arctan}(\frac{L}{2h}) \tag{20}\]  
The magneto-optical compensatory correction signal \(Y_{1}\) obtained from the primary signal processing unit does not include a DC component and is proportional to the measured current. Signal subtraction will weaken the external interference effect when there is a certain degree of external magnetic interference, thus two magneto-optical glasses are used for compensation. The rotation angles along the directions of the two beams are opposite, thus the magnetic field change caused by current changes will double after subtraction. The correction coefficient between the fiber signal and the magneto-optical signal is defined as follows:  
\[Y_{2}=u_{F}-\frac{K_{2}}{2K_{1}}(u_{G1}+\frac{K_{1}}{1-K_{1}-K_{2}}u_{G2}) \tag{21}\]  
Substituting the corresponding output signal yields an AC optical fiber signal:  
\[Y_{2}=2K_{2}P_{0}\theta_{2}=M_{2}i \tag{22}\]  
where  
\[M_{2}=2K_{2}P_{0}NV \tag{23}\]  
The AC optical fiber signal \(Y_{2}\) does not contain a DC component. \(Y_{2}\) is only proportional to the measured current after the light source and the sensing fiber parameters are determined. Figure 10: Real-time signal processing system in the adaptive fiber optic current sensor. (2) Detecting magnetic field changes  
Magnetic field changes are calculated using the compensatory correction signal \(Y_{1}\) obtained from the primary signal processing unit:  
Figure 9:0  
where \(Y_{1m}\) is the \(m^{\prime}\)th peak AC value in \(Y_{1}\). The magnetic change warning value is defined as \(Set_{B}\). \(P_{1m}>Set_{B}\) indicates that the magnetic field has changed, and the detection unit outputs \(W_{B}=1\). The measured current will contain waveform distortion when the power system changes from the steady state to the transient state and the signal analysis method can be used to detect the moment of mutation. By observing whether waveforms \(Y_{1}\) and \(Y_{2}\) are equivalent, one can determine whether the change is caused by external magnetic field interference or the input current itself. If waveform distortion is present in both, then the change is caused by the measured current. If only \(Y_{1}\) is distorted and \(Y_{2}\) is unchanged, the magnetic field change is due to external magnetic interference. At this time, the compensatory correction signal is locked in order to maintain the fiber optic current sensor output. \(W_{B}=0\) when \(P_{1m}\leq Set_{B}\). (3) Detecting temperature fluctuations  
Temperature fluctuations are calculated using the fiber optic current sensor output signal \(Y_{2}\):  
Figure 9:1  
where \(Y_{2m}\) is the \(m^{\prime}\)th peak AC value in \(Y_{2}\). The temperature fluctuation warning value \(Set_{T}\) is set in order to output a corresponding warning signal in the presence of temperature fluctuations.