A precision cryostat temperature control method based on linear auto-disturbance rejection

Abstract

The present invention discloses a precision low-temperature thermostat temperature control method based on linear auto-disturbance rejection, establishes a heat transfer physical model for a general low-temperature thermostat, and provides an auto-disturbance rejection control method suitable for the low-temperature thermostat in combination with the physical model, which can effectively improve the anti-disturbance capability of the parameters, reduce the overshoot of the temperature control, and achieve rapid control of the target temperature. The present invention designs an extended state observer based on the linear auto-disturbance rejection control method, improves the dynamic performance and steady-state performance of the low-temperature thermostat, solves the problems of large overshoot, small applicable temperature range of parameters, and poor anti-disturbance capability of conventional PI or PID control methods, enables the low-temperature thermostat to achieve a wider range of temperature regulation, improves temperature control efficiency, and saves experimental time.

Description

Technical Field

[0001] The present invention relates to the technical field of temperature control, and in particular to a temperature control method of a precision low-temperature thermostat based on linear auto-disturbance rejection. Background Art

[0002] With the development of cryogenic refrigeration technology, cryostat based on closed-cycle refrigerators has been gradually developed and applied, such as reproducing the low temperature fixed point defined by the International Temperature Scale and providing a high-quality thermal environment for thermodynamic temperature measurement. The temperature control of the current cryostat mainly adopts the PID control algorithm. Although this method can obtain a relatively ideal control accuracy at a specified temperature point through parameter adjustment, the PID control parameters have a small applicable temperature range because the cryostat is a nonlinear system. When the experimental conditions such as ambient temperature, specific heat capacity of experimental samples, and vacuum degree of the thermostat undergo unknown changes, the control parameters need to be manually adjusted by the operator based on engineering experience, which will greatly reduce the anti-interference ability of the temperature control and the operating efficiency of the cryostat.

[0003] The active disturbance rejection control method is a technology evolved from PID control. Its core idea is to regard the internal uncertainty and external disturbance of the system as total disturbance, eliminate the disturbance through observer design, and then simplify the system into a standard form. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention aims to provide a temperature control method for a precision cryostat based on linear auto-disturbance rejection.

[0005] In order to achieve the above object, the present invention adopts the following technical solution:

[0006] A temperature control method for a precision cryostat based on linear auto-disturbance rejection specifically comprises the following steps:

[0007] S1, remember P h is the heating power of the heater on the cryostat insulation shield, M and C a are the mass and specific heat capacity of the thermal insulation screen and its internal components, T a0 and T a are the initial temperature and current actual temperature of the insulation screen, respectively, q r For the heat flux between the insulation shield and the secondary cold head caused by the temperature fluctuation of the refrigerator, the relationship between the power of the heater on the insulation shield of the cryostat and the insulation shield temperature increment is established as follows:

[0008]

[0009] Among them, H as is the heat transfer coefficient between the insulation screen and the secondary screen; A a is the surface area of ​​the insulation screen, The influence of the temperature fluctuation of the refrigerator on the temperature stability of the insulation screen is expressed as follows: a =(T a -T a0 );

[0010] After Laplace transformation, the temperature change model of the insulation screen is obtained:

[0011]

[0012] s is the transformation parameter of Laplace transformation;

[0013] S2. Constructing a thermal insulation screen temperature control system: The thermal insulation screen temperature control system includes a controller, a constant current source, a heater, a thermometer and a controlled object, wherein the controlled object is the thermal insulation screen of the low-temperature thermostat and its internal components, and the thermometer is connected to the temperature sensor on the thermal insulation screen; the controller calculates the control value based on the difference between the actual temperature of the thermal insulation screen obtained by the thermometer and the reference temperature target value, and applies the control value to the heater on the thermal insulation screen in the form of current through the constant current source, and the temperature change of the heater acts on the controlled object, thereby causing the temperature change of the thermal insulation screen;

[0014] S3, based on the temperature change model of the adiabatic screen, let ΔT a =y=x1,P h =u, let f represent the total disturbance inside and outside the temperature control system, if f is differentiable, is its first-order differential, then Based on the principle of linear active disturbance rejection control, the extended state equation is established:

[0015]

[0016]

[0017] Then the extended state observer of the temperature control system is set as:

[0018]

[0019]

[0020] in, C = [1 0], D = [0]; By simplifying the parameters through pole configuration, we can obtain l1=2w, l2=w 2 , where w is the observer bandwidth; the extended state observer is used to track the total disturbance f. When the total external disturbance f to the temperature control system is bounded, the extended state observer is proven to converge; if the extended state observer estimates f can reliably track f, then the control law Bring in the temperature control system to get At this time, the temperature control system is simplified into an integrator, and the proportional link is used to control the temperature control system; let u0 = k p (ry), k p is the adjustable coefficient, r(s) is the reference signal, and the closed-loop transfer function of the temperature control system is:

[0021]

[0022] In the controller, the input and output signals of the controlled object are calculated by the extended state observer to obtain the estimated values ​​of the output and total disturbance of the controlled object. When the total disturbance of the temperature control system can be tracked by the extended state observer, 1 / b0, G(s) and the extended state observer can be equivalent to an integrator, using the proportional link k p The integrator can be controlled.

[0023] Furthermore, b0 is identified by step response and by changing k p The value of can realize the adjustment of the tracking performance of the temperature control system; when k p When and b0 are determined, the parameter setting can be completed by determining the parameter of w. w mainly affects the speed of the observer tracking the target value or disturbance f and its anti-interference ability. p When and w are both greater than 0, the controller is stable.

[0024] The beneficial effects of the present invention are as follows: the present invention provides a heat transfer physical model of a general low-temperature thermostat, and in combination with the physical model provides a self-disturbance rejection control method suitable for the low-temperature thermostat, which can effectively improve the adaptability and anti-interference ability of parameters and reduce the overshoot of temperature control, thereby achieving rapid control of the target temperature.

[0025] Specifically, the present invention designs an extended state observer based on the linear self-disturbance rejection control method, which improves the dynamic performance and steady-state performance of the low-temperature thermostat, solves the problems of large overshoot, small parameter applicable temperature range, poor anti-interference ability, etc. of conventional PI or PID control methods, enables the low-temperature thermostat to achieve temperature regulation in a wider range, improve temperature control efficiency, and save experimental time. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a structural schematic diagram of an existing low-temperature thermostat;

[0027] Figure 2 A schematic diagram of a temperature variation model of an insulation screen and its transfer function in an embodiment of the present invention;

[0028] Figure 3 This is a flow chart of the temperature control system of the thermal insulation screen in an embodiment of the present invention;

[0029] Figure 4This is a schematic diagram of the control principle of the adiabatic screen based on linear auto-disturbance rejection in an embodiment of the present invention;

[0030] Figure 5 This is a simulation diagram showing the effect of parameter w on tracking speed and system anti-interference performance in an embodiment of the present invention;

[0031] Figure 6 This is a comparison diagram of the adaptability of the controller under different loads in an embodiment of the present invention;

[0032] Figure 7 This is a comparison chart of the tracking of different temperature target values ​​by the controller in an embodiment of the present invention. DETAILED DESCRIPTION

[0033] The present invention will be further described below in conjunction with the accompanying drawings. It should be noted that this embodiment is based on the technical solution and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to this embodiment.

[0034] like Figure 1 As shown, the cryostat consists of a vacuum chamber 1, a primary protective shield 2, a secondary protective shield 3, an insulation shield 4, a heater module 5, a temperature sensor 6, a secondary cold head 7, a primary cold head 8 and a sample stage 9, which provide a highly stable thermal environment for the sample to be tested. The sample to be tested can be placed on the sample stage. The vacuum chamber 1 and the primary protective shield 2 are generally made of aluminum, stainless steel and other materials, and are mainly used to provide an external vacuum environment. The primary cold head 8 is in contact with the primary protective shield 2. The secondary protective shield 3 is sealed with indium wire to provide an internal vacuum environment for the sample to be tested, and can be filled with inert gases such as helium to assist the refrigerator to improve the cooling efficiency. The internal vacuum degree can reach 10 -6 Pa, the secondary cold head 7 contacts the secondary protective shield 3, so that the internal ambient temperature of the thermostat can be reduced to about 10K, and the first heater module and temperature sensor are arranged outside the secondary protective shield 3 to provide primary temperature control. The second heater module and temperature sensor are arranged outside the insulation shield 4 to achieve secondary temperature precision control, and a fully sealed structure is adopted to finally provide a quasi-adiabatic isothermal environment for the sample under test. The precision low-temperature thermostat can be used for low-temperature thermometer calibration and the reproduction of the triple points of equilibrium hydrogen, neon, oxygen and argon.

[0035] With respect to the above-mentioned cryostat, this embodiment provides a precise cryostat temperature control method based on linear auto-disturbance rejection.

[0036] The temperature control of the cryostat is mainly achieved by the coarse adjustment of the secondary protection screen and the fine adjustment of the insulation screen. The secondary protection screen provides a temperature control basis for the insulation screen. The primary cold head and the secondary cold head mainly exchange heat with the secondary protection screen and the insulation screen respectively through heat conduction. Since the insulation screen is in direct contact with the sample platform, the temperature control accuracy of the insulation screen directly affects the temperature stability of the experimental platform of the sample being tested. According to the first law of thermodynamics, the amount of heat added by the insulation screen is equal to the increase in the internal energy of the insulation screen and its internal components, as well as the heat leakage from the insulation screen to the secondary protection screen.

[0037] Based on the above process, the relationship between the power of the heater on the insulation screen and the temperature increment of the insulation screen is derived as follows:

[0038]

[0039] Among them, P h is the heating power of the heater on the insulation screen, M and C a are the mass and specific heat capacity of the thermal insulation screen and its internal components, T a0 and T a are the initial temperature and current actual temperature of the insulation screen, respectively, and q is the heat flux between the insulation screen and the secondary protection screen and the secondary cold head. r is the heat flow between the insulation screen and the secondary cold head caused by the temperature fluctuation of the refrigerator, then equation (1) can be expressed as:

[0040]

[0041] Among them, H as A is the heat transfer coefficient between the insulation screen and the secondary screen, which is mainly affected by the vacuum degree; a is the surface area of ​​the insulation screen, The influence of the temperature fluctuation of the refrigerator on the temperature stability of the insulation screen is expressed as follows: a =(T a -T a0 ), the temperature change model of the insulation screen after Laplace transformation of equation (2) is:

[0042]

[0043] From formula (3), the transfer function model G(s) of the adiabatic screen temperature control system can be obtained as follows: Figure 2 As shown, the insulation screen temperature control system is constructed as Figure 3 shown.

[0044] The insulation screen temperature control system includes a controller, a constant current source, a heater, a thermometer and a controlled object, wherein the controlled object is the insulation screen and internal components of the low temperature thermostat, and the thermometer is connected to the temperature sensor on the insulation screen, and the temperature measurement range generally meets 10K-300K, and the temperature measurement accuracy can reach 0.2mK. The controller calculates the control value based on the difference between the actual temperature of the insulation screen obtained by the thermometer and the reference temperature target value, and applies the current to the heater on the insulation screen through the precision constant current source. The temperature change of the heater acts on the controlled object, thereby causing the temperature change of the insulation screen.

[0045] Based on the temperature variation model of the insulation screen in formula (3), let ΔT a =y=x1,P h =u, let f represent the total disturbance inside and outside the temperature control system, if f is differentiable, is its first-order differential, then Based on the principle of linear active disturbance rejection control, the extended state equation is established:

[0046]

[0047] Then the extended state observer of the temperature control system is set as:

[0048]

[0049] in, C=[1 0],D=[0]. By simplifying the parameters through pole configuration, we can obtain l1=2w, l2=w 2 , where w is the observer bandwidth. The extended state observer is used to track the total disturbance f. When the total external disturbance f to the temperature control system is bounded, the extended state observer is proven to converge. If the extended state observer estimates f can reliably track f, then the control law Bring in the temperature control system to get At this time, the temperature control system can be simplified into an integrator, and the proportional link is used to control the temperature control system. Let u0 = k p (ry), k p is the adjustable coefficient, s is the transformation parameter of Laplace change, r(s) is the reference signal, then the closed-loop transfer function of the temperature control system is:

[0050]

[0051] The controller based on linear anti-disturbance control in the temperature control system is as follows: Figure 4As shown in the figure, after the input and output signals of the controlled object are calculated by the extended state observer, the estimated values ​​of the output and total disturbance of the controlled object are obtained. When the total disturbance of the temperature control system can be tracked by the extended state observer, 1 / b0, G(s) and the extended state observer can be equivalent to an integrator, using the proportional link k p The integrator can be controlled.

[0052] b0 can be obtained by system identification through step response, and by changing k p The value of k can adjust the system tracking performance. p It mainly affects the response speed of the system. p When and b0 are determined, the parameter setting can be completed by determining the parameters of w. w mainly affects the speed of the extended state observer tracking the target value or disturbance f and its anti-interference ability, such as Figure 5 As shown, when k p When and w are both greater than 0, the controller is stable, where Figure 5 (a) in the figure is the effect of w on the target temperature tracking. Figure 5 (b) in the figure is the influence of w on the system's anti-interference ability.

[0053] Implementation Effect

[0054] The cryostat often has different loads during operation, such as changes in vacuum and system specific heat capacity, and samples with different heat capacities placed on the sample stage. as and b0 values, simulate the changes in thermostat vacuum and insulation screen specific heat capacity, and compare the adaptability of Linear Active Disturbance Rejection Control (LADRC) and PI control parameters under different load conditions, such as Figure 6 As shown in Figure 2, it can be seen that LADRC has better adaptability to load changes when the parameters remain unchanged, and it has better adaptability to load changes when the k is appropriate. p Under the parameter settings of b0, b1 and w, it has the characteristics of fast dynamic tracking speed and good stability. Figure 7 It can be seen from the simulation that when the temperature reference target value changes, the LADRC parameters are more adaptable and the overshoot is smaller when the controller parameters remain unchanged, which can avoid frequent parameter debugging and additional cooling process caused by temperature overshoot, reduce human intervention and improve system operation efficiency.

[0055] For those skilled in the art, various corresponding changes and modifications can be made according to the above technical solutions and concepts, and all of these changes and modifications should be included in the protection scope of the claims of the present invention.

Claims

1. A precision cryostat temperature control method based on linear auto-disturbance rejection, characterized in that: The specific steps include: S1, remember P h is the heating power of the heater on the cryostat insulation shield, M and C a are the mass and specific heat capacity of the thermal insulation screen and its internal components, T a0 and T a are the initial temperature and current actual temperature of the insulation screen, respectively, q r For the heat flux between the insulation shield and the secondary cold head caused by the temperature fluctuation of the refrigerator, the relationship between the power of the heater on the insulation shield of the cryostat and the insulation shield temperature increment is established as follows: Among them, H as is the heat transfer coefficient between the insulation screen and the secondary screen; A a is the surface area of ​​the insulation screen, The influence of the temperature fluctuation of the refrigerator on the temperature stability of the insulation screen is expressed as follows: a =(T a -T a0 ); After Laplace transformation, the temperature change model of the insulation screen is obtained: s is the transformation parameter of Laplace transformation; S2. Constructing a thermal insulation screen temperature control system: The thermal insulation screen temperature control system includes a controller, a constant current source, a heater, a thermometer and a controlled object, wherein the controlled object is the thermal insulation screen of the low-temperature thermostat and its internal components, and the thermometer is connected to the temperature sensor on the thermal insulation screen; the controller calculates the control value based on the difference between the actual temperature of the thermal insulation screen obtained by the thermometer and the reference temperature target value, and applies the control value to the heater on the thermal insulation screen in the form of current through the constant current source, and the temperature change of the heater acts on the controlled object, thereby causing the temperature change of the thermal insulation screen; S3, based on the temperature change model of the adiabatic screen, let ΔT a =y=x1,P h =u, let f represent the total disturbance inside and outside the temperature control system, if f is differentiable, is its first-order differential, then Based on the principle of linear active disturbance rejection control, the extended state equation is established: Then the extended state observer of the temperature control system is set as: in, C = [1 0], D = [0]; By simplifying the parameters through pole configuration, we can obtain l1=2w, l2=w 2 , where w is the observer bandwidth; the extended state observer is used to track the total disturbance f. When the total external disturbance f to the temperature control system is bounded, the extended state observer is proven to converge; if the extended state observer estimates f can reliably track f, then the control law Bring in the temperature control system to get At this time, the temperature control system is simplified into an integrator, and the proportional link is used to control the temperature control system; let u0 = k p (ry), k p is the adjustable coefficient, r(s) is the reference signal, and the closed-loop transfer function of the temperature control system is: In the controller, the input and output signals of the controlled object are calculated by the extended state observer to obtain the estimated values ​​of the output and total disturbance of the controlled object. When the total disturbance of the temperature control system can be tracked by the extended state observer, 1 / b0, G(s) and the extended state observer can be equivalent to an integrator, using the proportional link k p The integrator can be controlled.

2. The method according to claim 1, characterized in that b0 is identified by step response and by changing k p The value of can realize the adjustment of the tracking performance of the temperature control system; when k p When and b0 are determined, the parameter setting can be completed by determining the parameter of w. w mainly affects the speed of the observer tracking the target value or disturbance f and its anti-interference ability. p When and w are both greater than 0, the controller is stable.

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