A method for nonlinear modification and toughness enhancement control of pressure difference in liquefied natural gas (LNG) tanks

Abstract

This invention discloses a nonlinear modification-based robustness enhancement control method for differential pressure in liquefied natural gas (LNG) tanks. The method includes establishing a mathematical model of the differential pressure control system for the insulation layer of the LNG tanks; obtaining a nominal model from the model; obtaining a mirror-mapped object from the nominal model using mirror mapping technology; designing a positive feedback controller based on the mirror-mapped object using a first-order closed-loop gain shaping algorithm; modifying the output of the positive feedback controller using nonlinear modification technology; obtaining a control signal based on system error and the modified output of the positive feedback controller; designing a switching control strategy; switching between the positive and negative feedback controllers based on the switching control strategy; and designing a step-by-step target tracking strategy. This step-by-step target tracking strategy is used to switch the tracking target and obtain control signals. Conventional differential pressure control is achieved by executing the control signals via a vent valve. This invention enhances the robustness, stability, and reliability of the ship's control system.

Description

Technical Field

[0001] This invention relates to the field of ship control system technology, and in particular to a nonlinear modification and toughening control method for pressure differential in liquefied natural gas (LNG) tanks. Background Technology

[0002] Maritime transport is the most important mode of transportation, with the majority of bulk cargo between countries being transported by sea. As countries advocate for the efficient use of clean energy, liquefied natural gas (LNG) shipping is gradually becoming an important part of maritime transport. Because maritime transport of LNG involves certain risks, the pressure control of its storage tanks must be highly robust to ensure safe maritime transport.

[0003] For ship control systems, malfunctions such as reversed wiring may occur due to human error. Many advanced sensors can directly output digital signals, and advanced ship control systems can directly receive various sensor signals continuously fed into the network. After processing, these signals are used as input signals for control algorithms. These measurement signals may temporarily change from positive to negative due to human factors or unexpected interference, thus leading to insufficient resilience and reduced accuracy of the ship control system. Summary of the Invention

[0004] This invention provides a method for nonlinear modification and toughness enhancement control of pressure difference in liquefied natural gas (LNG) tanks to overcome the aforementioned technical problems.

[0005] A method for nonlinear modification and toughness enhancement control of pressure difference in liquefied natural gas (LNG) tanks, comprising:

[0006] Step 1: Establish a mathematical model for the differential pressure control system of the liquefied natural gas (LNG) tank insulation layer. This mathematical model is unstable and exhibits time-delay characteristics.

[0007] Step 2: Obtain the nominal model from the mathematical model of the differential pressure control system of the LNG tank insulation layer, and obtain the mirror-mapped object from the nominal model using mirror mapping technology.

[0008] Step 3: Based on the mirror mapping object, a positive feedback controller is designed using a first-order closed-loop gain shaping algorithm.

[0009] Step 4: Modify the output of the positive feedback controller based on nonlinear modification techniques.

[0010] Step 5: Obtain the control signal based on the system error and the modified output of the positive feedback controller.

[0011] Step Six: Design a switching control strategy. Based on the switching control strategy, switch between the positive feedback controller and the negative feedback controller. The positive feedback controller is used to control the control signal when the measurement signal is reversed. Design a step-by-step target tracking strategy. This strategy is used to obtain the reverse connection time period corresponding to the measurement signal reversal. During the reverse connection time period, the tracking target of the positive feedback controller is adjusted from target 1 before the measurement signal reversal setting to target 2. The control signal is obtained according to target 2 corresponding to the positive feedback controller. After a period of reversal, target 2 corresponding to the positive feedback controller is returned to target 1, and the control signal is obtained according to target 1 corresponding to the positive feedback controller.

[0012] Step 7: Implement conventional differential pressure control by executing control signals through the vent valves in the insulation layer of the liquefied natural gas tank.

[0013] Preferably, the mathematical model of the differential pressure control system for the insulation layer of the liquefied natural gas tanker is shown in formula (1).

[0014]

[0015] Among them, P e The pressure difference between the primary and secondary insulation layers is given by denoted as ...

[0016] Preferably, the nominal model is as shown in formula (2).

[0017]

[0018] Where G0(s) is the nominal model, K0 represents the open-loop gain of the system, T0 represents the time constant of the closed-loop system, and s represents the Laplace operator.

[0019] Preferably, the positive feedback controller is based on formula (3).

[0020]

[0021] Where u' is the controller output, e is the system error, u is the controller adjusted output, K0 represents the system open-loop gain, T0 represents the closed-loop system time constant, s represents the Laplace operator, and T1 is the time constant.

[0022] Preferably, the switching between the positive feedback controller and the negative feedback controller based on the switching control strategy is performed according to formula (4).

[0023]

[0024] In the formula K *K' represents the controller that switches the idling law, and K' represents the positive feedback controller.

[0025] Preferably, the adjustment of the tracking target of the positive feedback controller during the reverse connection time period from target 1 before the measurement signal reverse connection setting to target 2 is performed according to formula (5).

[0026]

[0027] In the formula P r P represents the target for controlling and tracking the differential pressure of the liquid tank insulation layer. r1 With P r2 They are tracking target 1 and tracking target 2, respectively. r1 <P r2 ; t represents the simulation duration, t0 represents the start time of the reverse connection of the sensor measurement signal, and t1 is the time point after the measurement signal is reversed. The reverse connection time period is composed of t0 and t1, and t0 < t1.

[0028] This invention provides a nonlinear modification-based robustness enhancement control method for differential pressure in liquefied natural gas (LNG) tanks, improving system resilience. By designing a switching control between conventional negative feedback control laws and redundant positive feedback control laws, fault-tolerant control is successfully achieved for reversed sensor measurement signals in the LNG tank insulation layer differential pressure control system without altering existing hardware. Furthermore, to address the reduced tolerance to model perturbations after control law switching, a step-by-step target tracking strategy is designed to enhance system robustness. By employing a mirror mapping method, the unstable LNG tank insulation layer differential pressure system is mapped to a stable system, further enhancing robustness. This improvement helps address the challenges of LNG carriers in various transportation environments, improving system stability and reliability. The ingenious design of a closed-loop gain shaping algorithm successfully reduces the model perturbation effects caused by pure time delay. This allows the controller to better adapt to system changes, improving the robustness of the control system to external disturbances and ensuring effective pressure control of the LNG carrier. The introduction of S-function nonlinear modification technology effectively reduces the amplitude of the controller output, thereby improving system energy efficiency. This invention helps to save nitrogen production in the pressure control system of liquefied natural gas (LNG) tanks and reduce wear on the relief valve, thereby enabling safe transportation of LNG. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a flowchart of the method of the present invention;

[0031] Figure 2 This is a simulation block diagram of the control system for the switching control strategy of this invention;

[0032] Figure 3 This is a simulation block diagram of the step-by-step target tracking strategy control system of the present invention;

[0033] Figure 4 This is the system switching control output when the perturbation of the model in this invention is reduced;

[0034] Figure 5 This is the system output of the present invention when there is model perturbation. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Figure 1 This is a flowchart of the method of the present invention, as shown below. Figure 1 As shown, the method in this embodiment may include:

[0037] Step 1: Establish a mathematical model for the differential pressure control system of the liquefied natural gas (LNG) tank insulation layer. This mathematical model is unstable and exhibits time-delay characteristics.

[0038] Step 2: Obtain the nominal model from the mathematical model of the differential pressure control system of the LNG tank insulation layer, and obtain the mirror-mapped object from the nominal model using mirror mapping technology.

[0039] Step 3: Based on the mirror mapping object, a positive feedback controller is designed using a first-order closed-loop gain shaping algorithm.

[0040] Step 4: Modify the output of the positive feedback controller based on nonlinear modification techniques.

[0041] Step 5: Obtain the control signal based on the system error and the modified output of the positive feedback controller.

[0042] Step Six: Design a switching control strategy. Based on the switching control strategy, switch between the positive feedback controller and the negative feedback controller. The positive feedback controller is used to control the control signal when the measurement signal is reversed. Design a step-by-step target tracking strategy. This strategy is used to obtain the reverse connection time period corresponding to the measurement signal reversal. During the reverse connection time period, the tracking target of the positive feedback controller is adjusted from target 1 before the measurement signal reversal setting to target 2. The control signal is obtained according to target 2 corresponding to the positive feedback controller. After a period of reversal, target 2 corresponding to the positive feedback controller is returned to target 1, and the control signal is obtained according to target 1 corresponding to the positive feedback controller.

[0043] Step 7: Implement conventional differential pressure control by executing control signals through the vent valves in the insulation layer of the liquefied natural gas tank.

[0044] Based on the above scheme, this invention provides a nonlinear modification-based robustness enhancement control method for LNG carrier tank pressure differential, improving system robustness. By designing a switching control between conventional negative feedback control law and redundant positive feedback control law, fault-tolerant control is successfully achieved for reversed sensor measurement signals in the LNG carrier tank insulation layer pressure differential control system without changing the existing hardware. Furthermore, to address the reduced tolerance of the control system to model perturbations after switching control laws, a step-by-step target tracking strategy is designed to enhance system robustness. By employing a mirror mapping method, the unstable system of LNG carrier tank insulation layer pressure differential is mapped to a stable system, thereby enhancing system robustness. This improvement helps address the challenges of LNG carriers in different transportation environments, improving system stability and reliability. Through the ingenious design of a closed-loop gain shaping algorithm, the influence of model perturbations caused by pure time delay is successfully reduced. This allows the controller to better adapt to system changes, improving the robustness of the control system to external disturbances and ensuring effective pressure control of the LNG carrier. The introduction of S-function nonlinear modification technology effectively reduces the amplitude of the controller output, thereby improving system energy efficiency. This invention helps to save nitrogen production in the pressure control system of liquefied natural gas (LNG) tanks and reduce wear on the relief valve, thereby enabling safe transportation of LNG.

[0045] Specifically, this embodiment provides detailed information on the control method, including:

[0046] One important function of the insulation layer in an LNG carrier's cargo system is to ensure the integrity of the cargo tank's shape by maintaining the pressure difference between the primary and secondary insulation spaces. This is typically accomplished by filling the insulation spaces with inert nitrogen or extracting nitrogen from the insulation spaces.

[0047] The pressure difference model of the insulation layer of the LNG ship's liquid tank is shown in Equation (1).

[0048]

[0049] In the formula, P e The pressure difference between the primary and secondary insulation layers and These are the first and second derivatives of the pressure difference between the primary and secondary insulation layers, respectively. The density of nitrogen gas (kg / m³) 3 ), g is the acceleration due to gravity, F r κ represents the damping force under non-ideal conditions, and u represents the intake flow rate. κ is a constant determined by the properties of the insulating layer material. The forces acting on the intake flow are mainly viscous force, inertial force, buoyancy, and gravity. The ratio of inertial force to viscous force is the Reynolds number. Since the characteristic parameter of a circular intake is the intake diameter d, the Reynolds number can be expressed in the form of equation (2).

[0050]

[0051] In the formula, u0 is the initial intake flow rate, and v is the kinematic viscosity coefficient. The ratio of inertial force to buoyancy is the Froude number, as shown in formula (3).

[0052]

[0053] In the formula, ρ0 is the initial density of the intake air, ρ a This refers to air density.

[0054] For the GTT NO.96 type LNG carrier, the normal gauge pressure of the main insulation space is 0.4kPa to 0.6kPa, and the normal gauge pressure of the secondary insulation layer is slightly lower than that of the main insulation layer, at 0.2kPa to 0.4kPa. This pressure difference of 0.2kPa ensures that the pressure of the liquid tank, the main insulation layer and the secondary insulation layer decreases sequentially, which is beneficial to protecting the main wall and the secondary wall. The safety valve setting is set at 1.0kPa.

[0055] First, a mathematical model is established, including step one: establishing a mathematical model for the differential pressure control system of the liquefied natural gas (LNG) tank insulation layer. The mathematical model of the differential pressure control system of the LNG tank insulation layer is an unstable model with time delay characteristics. The mathematical model of the differential pressure control system of the LNG tank insulation layer is shown in formula (4).

[0056]

[0057] Among them, P e The pressure difference between the primary and secondary insulation layers is given by denoted as ...

[0058] Specifically, the mathematical model of the pressure-to-flow rate transfer function after linearization near the equilibrium point can be expressed as equation (4). This model is used to represent the system input (intake flow rate u) and output (pressure difference P between primary and secondary insulation layers). e The relationship between ) and the gauge pressure P at the equilibrium point. e0 =2.0 mbar, initial flow rate u0 = 42 m 3 / h. For a certain LNG carrier, its parameters are T0 = 0.78K0 = 3.25τ = 0.25.

[0059] Step 2: Obtain the nominal model from the mathematical model of the differential pressure control system of the liquid tank insulation layer of the liquefied natural gas ship. That is, if the time delay is not considered, the nominal model is as shown in formula (5).

[0060]

[0061] Where G0(s) is the nominal model, K0 represents the open-loop gain of the system, T0 represents the time constant of the closed-loop system, and s represents the Laplace operator.

[0062] Equation (5) indicates an unstable controlled object because it has a pole s = T0 in the right half-plane. The mirror mapping method is an indirect robust controller design method for unstable systems. Its essence is to design a robust controller for a stable system after mirror mapping of unstable poles about the imaginary axis and root locus shaping. Finally, it is proven that the designed robust controller still has good robust performance for unstable systems before mirror mapping and root locus shaping. Based on the mirror mapping technique, the mirror mapping object is obtained from the nominal model. Let the mirror mapping object of the nominal model be G(s). Using the mirror mapping object of Equation (6), a robust controller can be designed using the closed-loop gain shaping algorithm.

[0063]

[0064] For unstable controlled objects, a controller is designed using its mirror mapping G, and a robust controller K is designed using a first-order closed-loop gain shaping algorithm. The closed-loop transfer function of the system can be replaced by a pre-designed first-order inertial system.

[0065]

[0066] In the formula, T1 is the time constant of the closed-loop system, and its value is approximately equal to the reciprocal of the bandwidth frequency of the closed-loop system (T1 = 0.2s in the final simulation experiment). Substituting equation (6) into equation (7), then

[0067]

[0068] Since the closed-loop gain shaping algorithm is a robust control algorithm, Equation (8) is essentially equivalent to a robust PD controller.

[0069] Based on the mirror mapping object, a positive feedback controller is designed using a first-order closed-loop gain shaping algorithm. That is, the controller represented by the ratio of the final output u of the controller to the system error e can be expressed by equation (9).

[0070]

[0071] Where u' is the controller output, e is the system error, u is the controller adjusted output, K0 represents the system open-loop gain, T0 represents the closed-loop system time constant, s represents the Laplace operator, and T1 is the time constant.

[0072] Step 4: Modify the output of the positive feedback controller using nonlinear modification techniques. To improve the controller output performance, a nonlinear modification using an S-function is applied. The S-function is set to (1-exp(-1.5u')) / (1+exp(-1.5u')), where u' is the output of the robust PD controller.

[0073] Step 5: Obtain the control signal based on the system error and the modified output of the positive feedback controller.

[0074] Step 6: Design the switching control strategy. The simulation block diagram of the switching control strategy control system is shown below. Figure 2 As shown, a switching control strategy is used to switch between a positive feedback controller and a negative feedback controller. The positive feedback controller is used to control the control signal when the measurement signal is reversed.

[0075] The switching between the positive feedback controller and the negative feedback controller based on the switching control strategy is performed according to formula (10).

[0076]

[0077] In the formula K * K' represents the controller that switches the idling law, and K' represents the positive feedback controller.

[0078] The design of the hierarchical target tracking strategy and the simulation block diagram of the hierarchical target tracking strategy control algorithm system are shown below. Figure 3 As shown, the step-by-step target tracking strategy is used to obtain the reverse connection time period corresponding to the reverse connection of the measurement signal, and adjust the tracking target of the positive feedback controller during the reverse connection time period from target 1 before the measurement signal reverse connection setting to target 2. The control signal is obtained according to target 2 corresponding to the positive feedback controller. The adjustment of the tracking target of the positive feedback controller during the reverse connection time period from target 1 before the measurement signal reverse connection setting to target 2 is performed according to formula (11).

[0079]

[0080] In the formula Pr P represents the target for controlling and tracking the differential pressure of the liquid tank insulation layer. r1 With P r2 They are tracking target 1 and tracking target 2, respectively. Typically, P... r1 <P r2 ; t represents the simulation duration, t0 represents the start time of the reverse connection of the sensor measurement signal, and t1 is the time point after the measurement signal is reversed. The reverse connection time period is composed of t0 and t1, and t0 < t1.

[0081] After a period of reverse connection, the target 2 corresponding to the positive feedback controller is returned to target 1, and the control signal is obtained according to target 1 corresponding to the positive feedback controller. Here, the period of reverse connection refers to the time caused by human factors or unexpected interference.

[0082] Step 7: Implement conventional differential pressure control by executing control signals through the vent valves in the insulation layer of the liquefied natural gas tank.

[0083] Specifically, this embodiment provides a proof and analysis of the effect of the nonlinear modification function on system stability, including,

[0084] e -1.5u' Expand using Taylor series and retain down to the first order: e -λu' ≈1-λu', then

[0085]

[0086] When u' is not too large, then

[0087]

[0088] When λ = 1.5

[0089]

[0090] Let ω = 0.75, and then proceed with the following theoretical analysis.

[0091] Impact on system steady state: Equation (6) is the model after mirror mapping of the hydraulic control system, with the insulation layer pressure difference set as a step signal and an amplitude of P. r According to the Laplace transform final value theorem, the steady-state output P of the system is:

[0092]

[0093] Therefore, the steady-state error of the system is 0, and the control algorithm designed in this embodiment does not have any additional impact on the steady state of the system.

[0094] Impact on system dynamic performance: From system input P r The transfer function to the system output P is:

[0095]

[0096] Given ω < 1, according to the closed-loop gain shaping theory, the open-loop frequency response GK' of the system satisfies the requirements of high gain at low frequencies and low gain at high frequencies. Therefore, in the low-frequency range, equation (16) is similar to the closed-loop transfer function of the standard feedback system. In comparison, the addition of ω has little impact on the dynamic performance of the system.

[0097] Impact on control output: From system input P r The transfer function to the controller output v is

[0098]

[0099] Similar to the analysis of equation (16), the numerator of equation (17) decreases more significantly than the denominator, therefore the addition of ω will reduce the control output.

[0100] When the measured signal changes from positive to negative for various reasons, the control system is equivalent to changing from a negative feedback control system to a positive feedback control system. Equation (18) gives the equivalent transformation between the positive feedback control method and the negative feedback control method. It is equivalent to multiplying the controller of the original negative feedback control method by -1 and then multiplying the system output by -1, thus realizing the equivalent transformation between negative feedback and positive feedback.

[0101] By finding the transfer function of the closed-loop control system, we can know that:

[0102]

[0103] The left side of equation (18) is the transfer function of the negative feedback control system, and the right side is the transfer function of the positive feedback control system. When the positive feedback controller K” = -K’, the output of the positive feedback system differs from the output of the negative feedback system by a negative sign. The formula on the right side of equation (18) is used for the switching control scheme when the measured signal is negative.

[0104] To verify the effectiveness of the control law designed in this invention, simulation experiments were conducted using Matlab. A switching control was designed at 50 seconds. During program debugging, it was found that the control effect was relatively ideal with both standalone negative and standalone positive feedback. However, when switching back to negative feedback at 50 seconds, the control system diverged, which was inconsistent with the initial assumption. However, reducing the pure time delay of the model from 0.25 seconds to 0.15 seconds allowed for switching control between the two algorithms, as shown below. Figure 4 As shown, this indicates that the jitter generated during system control switching reduces the robustness of the entire control system to model perturbation energy, thus weakening the resilience of the control system.

[0105] Another phenomenon observed during simulation experiments was that a pure time lag of 0.25s could be tolerated if the setpoint was lowered to 1.5. Therefore, lowering the setpoint after switching, stabilizing for 20 seconds, and then raising it back to the original setpoint might be a feasible solution. Based on this phenomenon, a new simulation system was designed, incorporating a step-by-step target tracking strategy. During the switching control at 50s, the setpoint was lowered to 1, and then raised to 2 after 20s, implementing the switching control step-by-step. The simulation results are as follows. Figure 5 As shown. From Figure 5 As can be seen, the control law designed in this invention has an overshoot of less than 25% and a settling time of 2s when there is a 0.25s pure time delay model perturbation.

[0106] Based on the above experimental results, and after improvements to the pressure control of liquefied natural gas (LNG) tanks, and in comparison with existing research, the beneficial effects of this invention are summarized in the following three points:

[0107] (1) Improved System Resilience: By designing a switching control algorithm between conventional negative feedback control law and redundant positive feedback control law, fault-tolerant control was successfully achieved for the reversed sensor measurement signals in the differential pressure control system of the LNG tank insulation layer without changing the existing hardware. Furthermore, to address the reduced tolerance of the control system to model perturbations after switching control laws, a step-by-step target tracking strategy (i.e., ...) was designed. Figure 3 As shown, during the 50s switching control, the setpoint is lowered to 1, and then raised to 2 after 20s, gradually implementing the switching control, thereby enhancing system robustness. Typically, to ensure the normal operation of a liquefied natural gas (LNG) ship's systems, numerous sensors are deployed on board, and this improvement helps to prevent accidents caused by sensor malfunctions.

[0108] (2) Robust Controller Design: By employing a mirror mapping method, the unstable system of the LNG carrier's liquid tank insulation layer pressure difference is mapped into a stable system, thereby enhancing the system's robustness. This improvement helps address the challenges of LNG carriers under different transportation environments, improving the system's stability and reliability. Through the ingenious design of a closed-loop gain shaping algorithm, the model perturbation effect caused by pure time delay is successfully reduced. This allows the controller to better adapt to system changes, improving the robustness of the control system to external disturbances and ensuring effective control of the LNG carrier's pressure.

[0109] (3) Application of nonlinear modification technology: The introduction of S-function nonlinear modification technology effectively reduces the amplitude of the controller output, thereby improving the system's energy efficiency. This invention helps to save nitrogen production in the liquefied natural gas (LNG) tank pressure control system and reduce the wear of the relief valve, thus achieving safe transportation of LNG.

[0110] Overall beneficial effects:

[0111] This invention provides a nonlinear modification-based robustness enhancement control method for differential pressure in liquefied natural gas (LNG) tanks, improving system resilience. By designing a switching control between conventional negative feedback control laws and redundant positive feedback control laws, fault-tolerant control is successfully achieved for reversed sensor measurement signals in the LNG tank insulation layer differential pressure control system without altering existing hardware. Furthermore, to address the reduced tolerance to model perturbations after control law switching, a step-by-step target tracking strategy is designed to enhance system robustness. By employing a mirror mapping method, the unstable LNG tank insulation layer differential pressure system is mapped to a stable system, further enhancing robustness. This improvement helps address the challenges of LNG carriers in various transportation environments, improving system stability and reliability. The ingenious design of a closed-loop gain shaping algorithm successfully reduces the model perturbation effects caused by pure time delay. This allows the controller to better adapt to system changes, improving the robustness of the control system to external disturbances and ensuring effective pressure control of the LNG carrier. The introduction of S-function nonlinear modification technology effectively reduces the amplitude of the controller output, thereby improving system energy efficiency. This technology helps save nitrogen production in the LNG tank pressure control system of LNG carriers and reduces wear on the relief valve, thereby enabling safe transportation of LNG.

[0112] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for nonlinear modification and toughness enhancement control of pressure difference in liquefied natural gas (LNG) tanks, characterized in that, include, Step 1: Establish a mathematical model for the differential pressure control system of the liquefied natural gas (LNG) tank insulation layer. This mathematical model is unstable and exhibits time-delay characteristics. Step 2: Obtain the nominal model from the mathematical model of the differential pressure control system of the LNG tank insulation layer, and obtain the mirror-mapped object from the nominal model using mirror mapping technology. Step 3: Based on the mirror mapping object, a positive feedback controller is designed using a first-order closed-loop gain shaping algorithm. Step 4: Modify the output of the positive feedback controller based on nonlinear modification techniques. Step 5: Obtain the control signal based on the system error and the modified output of the positive feedback controller. Step Six: Design a switching control strategy. Based on the switching control strategy, switch between the positive feedback controller and the negative feedback controller. The positive feedback controller is used to control the control signal when the measurement signal is reversed. Design a step-by-step target tracking strategy. This strategy is used to obtain the reverse connection time period corresponding to the measurement signal reversal. During the reverse connection time period, the tracking target of the positive feedback controller is adjusted from target 1 before the measurement signal reversal setting to target 2. The control signal is obtained according to target 2 corresponding to the positive feedback controller. After a period of reversal, target 2 corresponding to the positive feedback controller is returned to target 1, and the control signal is obtained according to target 1 corresponding to the positive feedback controller. Step 7: Implement conventional differential pressure control by executing control signals through the vent valves in the insulation layer of the liquefied natural gas tank.

2. The method for nonlinear modification and toughness enhancement control of pressure difference in liquefied natural gas (LNG) tanks according to claim 1, characterized in that, The mathematical model for the differential pressure control system of the liquid tank insulation layer of the liquefied natural gas ship is shown in formula (1). Among them, P e The pressure difference between the primary and secondary insulation layers is given by denoted as ...

3. The method for nonlinear modification and toughness enhancement control of pressure difference in liquefied natural gas (LNG) tanks according to claim 1, characterized in that, The nominal model is shown in formula (2). Where G0(s) is the nominal model, K0 represents the open-loop gain of the system, T0 represents the time constant of the closed-loop system, and s represents the Laplace operator.

4. The method for nonlinear modification and toughness enhancement control of pressure difference in liquefied natural gas (LNG) tanks according to claim 1, characterized in that, The positive feedback controller is defined by formula (3). Where u' is the controller output, e is the system error, u is the controller adjusted output, K0 represents the system open-loop gain, T0 represents the closed-loop system time constant, s represents the Laplace operator, and T1 is the time constant.

5. The method for nonlinear modification and toughness enhancement control of pressure difference in liquefied natural gas (LNG) tanks according to claim 1, characterized in that, The switching between the positive feedback controller and the negative feedback controller based on the switching control strategy is performed according to formula (4). In the formula K * K' represents the controller that switches the idling law, and K' represents the positive feedback controller.

6. The method for nonlinear modification and toughness enhancement control of pressure difference in liquefied natural gas (LNG) tanks according to claim 1, characterized in that, The adjustment of the tracking target of the positive feedback controller during the reverse connection period from target 1 before the measurement signal reverse connection setting to target 2 is carried out according to formula (5). In the formula P r P represents the target for controlling and tracking the differential pressure of the liquid tank insulation layer. r1 With P r2 They are tracking target 1 and tracking target 2, respectively. r1 <P r2 ; t represents the simulation duration, t0 represents the start time of the reverse connection of the sensor measurement signal, and t1 is the time point after the measurement signal is reversed. The reverse connection time period is composed of t0 and t1, and t0 < t1.

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