초전도 양자 간섭 소자(SQUID) 조셉슨 접합 자속 양자화 & 1,000Hz 0-Alloc C# 초저잡음 뇌자도(MEG) 계측 서보 엔진

using System;
using System.Runtime.CompilerServices;

namespace ZeliDesk.Stem.Physics.Quantum
{
    /// 

    /// 초전도 양자 간섭 소자(SQUID) 초저잡음 뇌자도(MEG) FLL 서보 제어 구조체 (0-Alloc C#)
    /// 
    public ref struct SquidMegServoEngine
    {
        public const double FluxQuantum = 2.067833848e-15; // Wb (h / 2e)
        
        private readonly double _criticalCurrentIc;   // A
        private readonly double _shuntResistanceR;     // Ohm
        private readonly double _mutualInductanceMf;   // H
        private readonly double _feedbackResistanceRf; // Ohm
        private readonly double _kp;                   // 비례 제어 게인
        private readonly double _ki;                   // 적분 제어 게인
        private readonly double _dt;                   // 제어 주기 (s)
        
        private double _feedbackCurrent;              // A
        private double _integralError;                // V*s
        private double _estimatedFieldFemtoTesla;     // fT
        private double _pickupCoilArea;               // m^2

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public SquidMegServoEngine(
            double criticalCurrentIc,
            double shuntResistanceR,
            double mutualInductanceMf,
            double feedbackResistanceRf,
            double pickupCoilArea,
            double kp,
            double ki,
            double dt = 0.001)
        {
            _criticalCurrentIc = criticalCurrentIc;
            _shuntResistanceR = shuntResistanceR;
            _mutualInductanceMf = mutualInductanceMf;
            _feedbackResistanceRf = feedbackResistanceRf;
            _pickupCoilArea = pickupCoilArea;
            _kp = kp;
            _ki = ki;
            _dt = dt;
            _feedbackCurrent = 0.0;
            _integralError = 0.0;
            _estimatedFieldFemtoTesla = 0.0;
        }

        /// 

        /// SQUID 양단 전압 신호를 입력받아 FLL 피드백 전류를 튜닝하고 뇌자도(fT)를 복원 (1,000Hz 루프)
        /// 
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void UpdateServoStep(double measuredSQUIDVoltage, out double feedbackVoltageOut, out double bFieldFemtoTesla)
        {
            // 최대 감도 작동점(0.25 Phi0) 기준 전압 오차 연산
            double operatingPointVoltage = _criticalCurrentIc * _shuntResistanceR * 0.70710678; // V_ref
            double error = measuredSQUIDVoltage - operatingPointVoltage;

            // PI 제어기를 통한 보상 귀환 전류 계산
            _integralError += error * _dt;
            if (_integralError > 1e-3) _integralError = 1e-3;
            else if (_integralError < -1e-3) _integralError = -1e-3;

            double compensationDeltaI = (_kp * error) + (_ki * _integralError);
            _feedbackCurrent += compensationDeltaI * _dt;

            // 귀환 코일이 생성한 상쇄 자속 Phi_f = M_f * I_f
            double appliedFeedbackFlux = _mutualInductanceMf * _feedbackCurrent;

            // 검출된 순수 생체 자기 선속 밀도 B = Phi / Area (펨토테슬라 단위 변환)
            double fieldTesla = appliedFeedbackFlux / _pickupCoilArea;
            _estimatedFieldFemtoTesla = fieldTesla * 1e15;

            // 피드백 전압 출력 V_out = R_f * I_f
            feedbackVoltageOut = _feedbackResistanceRf * _feedbackCurrent;
            bFieldFemtoTesla = _estimatedFieldFemtoTesla;
        }

        public double EstimatedFieldFemtoTesla => _estimatedFieldFemtoTesla;
        public double FeedbackCurrent => _feedbackCurrent;
    }
}