Ultra-low temperature refrigerator and method for starting the same

Abstract

The present invention provides an ultra-low temperature refrigerator that detects the completion of initial cooling at a low price. The ultra-low temperature refrigerator (10) comprises: an expander (14) having a cooling stage; an exhaust temperature sensor (48) that measures the exhaust temperature of the expander (14) and outputs an exhaust temperature signal (T2) indicating the measured exhaust temperature; and a controller (110) that compares the measured exhaust temperature with a reference temperature based on the exhaust temperature signal (T2) during the execution of initial cooling of cooling the cooling stage from an initial temperature to an ultra-low temperature, and terminates the initial cooling when the temperature difference between the measured exhaust temperature and the reference temperature is within a reference range.

Description

[0001] This application claims priority based on Japanese Patent Application No. 2020-186513 filed on November 9, 2020. The entire contents of the Japanese Patent Application are incorporated herein by reference. Technical Field

[0002] The invention relates to an ultra-low temperature refrigerator and a method for starting the ultra-low temperature refrigerator. Background Art

[0003] Cryogenic refrigerators are used to cool various objects such as superconducting equipment, measuring equipment, and samples used in ultra-low temperature environments. In order to use a cryogenic refrigerator to cool an object, first, the cryogenic refrigerator needs to be started and cooled from an initial temperature such as room temperature to a target ultra-low temperature. Such initial cooling of a cryogenic refrigerator is also called temperature reduction.

[0004] Patent Document 1: Japanese Patent Application Publication No. 2014-169813

[0005] In a typical cryogenic refrigerator, in order to know that the initial cooling is completed, a temperature sensor is installed at the part cooled to a cryogenic temperature, and the temperature measured by the temperature sensor is monitored. However, a temperature sensor capable of measuring a cryogenic temperature is relatively expensive. Summary of the invention

[0006] One of the exemplary purposes of an embodiment of the present invention is to provide a completed ultra-low temperature refrigerator for detecting initial cooling at a low price.

[0007] According to one embodiment of the present invention, an ultra-low temperature refrigerator comprises: an expander having a cooling stage; an exhaust temperature sensor, which measures the exhaust temperature of the expander and outputs an exhaust temperature signal representing the measured exhaust temperature; and a controller, which compares the measured exhaust temperature with a reference temperature based on the exhaust temperature signal during the execution of initial cooling of the cooling stage from an initial temperature to an ultra-low temperature, and terminates the initial cooling when the temperature difference between the measured exhaust temperature and the reference temperature is within a reference range.

[0008] According to one embodiment of the present invention, a startup method for an ultra-low temperature refrigerator comprises the following steps: executing initial cooling of a cooling stage of an expander from an initial temperature to an ultra-low temperature; during the execution of the initial cooling, measuring the exhaust temperature of the expander; and during the execution of the initial cooling, comparing the measured exhaust temperature with a reference temperature, and ending the initial cooling when the temperature difference between the measured exhaust temperature and the reference temperature is within a reference range.

[0009] According to one embodiment of the present invention, an ultra-low temperature refrigerator comprises: an expander having a cooling stage; a high-pressure pipeline connected to the expander, through which a working gas sucked into the expander flows; a low-pressure pipeline connected to the expander, through which a working gas discharged from the expander flows; and a pressure sensor for measuring the pressure of the high-pressure pipeline or the pressure of the low-pressure pipeline; and a controller for determining whether the initial cooling is completed based on either the pressure of the high-pressure pipeline or the pressure of the low-pressure pipeline measured by the pressure sensor during the execution of initial cooling of the cooling stage from an initial temperature to an ultra-low temperature.

[0010] According to one embodiment of the present invention, a startup method for an ultra-low temperature refrigerator comprises the following steps: executing initial cooling of a cooling stage of an expander from an initial temperature to an ultra-low temperature; during the execution of the initial cooling, measuring the pressure of the working gas sucked into the expander or the pressure of the working gas discharged from the expander; and during the execution of the initial cooling, determining whether the initial cooling is completed based on either the measured pressure of the working gas sucked into the expander or the measured pressure of the working gas discharged from the expander.

[0011] In addition, any combination of the above-mentioned constituent elements or any method, apparatus, system, etc. in which constituent elements and expressions of the present invention are interchangeably replaced may also be practiced as additional embodiments of the present invention.

[0012] According to the present invention, a cryogenic refrigerator capable of detecting completion of initial cooling can be provided at a low price. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a diagram schematically showing a cryogenic refrigerator according to an embodiment.

[0014] Figure 2 It is a diagram schematically showing a cryogenic refrigerator according to an embodiment.

[0015] Figure 3 This is a graph showing an example of changes in the exhaust gas temperature and the intake gas temperature of the expander during initial cooling of the cryogenic refrigerator according to the embodiment.

[0016] Figure 4 This is a flowchart for explaining a method for starting up a cryogenic refrigerator according to an embodiment.

[0017] Figure 5 This is a graph showing an example of changes in pressure of the high-pressure line and the low-pressure line during initial cooling of the cryogenic refrigerator according to the embodiment.

[0018] Figure 6 This is a flowchart for explaining a method for starting up a cryogenic refrigerator according to an embodiment.

[0019] In the figure: 10-ultra-low temperature refrigerator, 14-expander, 46-intake temperature sensor, 48-exhaust temperature sensor, 63-high pressure pipeline, 64-low pressure pipeline, 70-inverter, 110-controller, T1-intake temperature signal, T2-exhaust temperature signal. DETAILED DESCRIPTION

[0020] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description and the accompanying drawings, identical or equivalent components, parts, and processes are marked with the same symbols, and repeated descriptions are appropriately omitted. For the convenience of description, the scale and shape of each part are appropriately set in each of the accompanying drawings, and unless otherwise specified, they are not interpreted as limiting. The embodiments are examples and do not limit the scope of the present invention in any way. All features or combinations thereof described in the embodiments are not necessarily the essence of the invention.

[0021] Figure 1 and Figure 2 1 is a diagram schematically showing a cryogenic refrigerator 10 according to an embodiment. As an example, the cryogenic refrigerator 10 is a two-stage Gifford-McMahon (GM) refrigerator. Figure 1 The compressor 12, the expander 14 and the control device 100 constituting the ultra-low temperature refrigerator 10 are schematically shown in FIG. Figure 2 2 shows the internal structure of the expander 14 of the cryogenic refrigerator 10 .

[0022] The compressor 12 is configured to recover the working gas of the cryogenic refrigerator 10 from the expander 14, increase the pressure of the recovered working gas, and then supply the working gas to the expander 14 again. The compressor 12 and the expander 14 form a refrigeration cycle of the cryogenic refrigerator 10, so that the cryogenic refrigerator 10 can provide the desired cryogenic cooling. The expander 14 is also called a cold head. The working gas is also called a refrigerant gas, which is usually helium, but other suitable gases can also be used. For ease of understanding, in Figure 1 The arrows in the figure indicate the flow direction of the working gas.

[0023] In addition, usually, the pressure of the working gas supplied from the compressor 12 to the expander 14 and the pressure of the working gas recovered from the expander 14 to the compressor 12 are much higher than the atmospheric pressure, and can be respectively referred to as the first high pressure and the second high pressure. For the convenience of explanation, the first high pressure and the second high pressure are respectively referred to as high pressure and low pressure. Typically, the high pressure is, for example, 2 to 3 MPa. The low pressure is, for example, 0.5 to 1.5 MPa, for example, about 0.8 MPa. For the convenience of understanding, the flow direction of the working gas is indicated by an arrow.

[0024] The expander 14 includes a refrigerator cylinder 16 and a displacer assembly 18. The refrigerator cylinder 16 guides the displacer assembly 18 to perform linear reciprocating motion, and expansion chambers 32 and 34 of the working gas are formed between the refrigerator cylinder 16 and the displacer assembly 18. In addition, the expander 14 includes a pressure switching valve 40 that determines the start time of suction for causing the working gas to flow into the expansion chamber and the start time of exhaust for exhausting the working gas from the expansion chamber.

[0025] In this specification, in order to facilitate the description of the positional relationship between the components of the cryogenic refrigerator 10, the side close to the top dead center of the axial reciprocating movement of the displacer is marked as "up", and the side close to the bottom dead center is marked as "down". The top dead center is the position of the displacer when the volume of the expansion space becomes the maximum, and the bottom dead center is the position of the displacer when the volume of the expansion space becomes the minimum. When the cryogenic refrigerator 10 is in operation, a temperature gradient is generated in which the temperature decreases from the top to the bottom in the axial direction, so the upper side can also be called the high temperature side, and the lower side can be called the low temperature side.

[0026] The refrigerator cylinder 16 includes a first cylinder 16a and a second cylinder 16b. For example, the first cylinder 16a and the second cylinder 16b are cylindrical components, and the diameter of the second cylinder 16b is smaller than the diameter of the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are coaxially arranged, and the lower end of the first cylinder 16a is rigidly connected to the upper end of the second cylinder 16b.

[0027] The displacer assembly 18 includes a first displacer 18a and a second displacer 18b connected to each other, and they move integrally. As an example, the first displacer 18a and the second displacer 18b are cylindrical components, and the diameter of the second displacer 18b is smaller than the diameter of the first displacer 18a. The first displacer 18a and the second displacer 18b are coaxially arranged.

[0028] The first displacer 18a is accommodated in the first cylinder 16a, and the second displacer 18b is accommodated in the second cylinder 16b. The first displacer 18a can reciprocate in the axial direction along the first cylinder 16a, and the second displacer 18b can reciprocate in the axial direction along the second cylinder 16b.

[0029] like Figure 2 As shown, the first displacer 18a contains the first cold storage device 26. The first cold storage device 26 is formed by filling a metal mesh such as copper or other appropriate first cold storage material in the cylindrical main body of the first displacer 18a. The upper cover and the lower cover of the first displacer 18a may be components different from the main body of the first displacer 18a, and the upper cover and the lower cover of the first displacer 18a may be fixed to the main body by appropriate methods such as fastening and welding, thereby allowing the first cold storage material to be contained in the first displacer 18a.

[0030] Similarly, the second displacer 18b contains the second cold storage device 28. The second cold storage device 28 is formed by filling the cylindrical main body of the second displacer 18b with a non-magnetic cold storage material such as bismuth, a magnetic cold storage material such as HoCu2, or other appropriate second cold storage material. The second cold storage material may be granular. The upper cover and the lower cover of the second displacer 18b may be components different from the main body of the second displacer 18b, and the upper cover and the lower cover of the second displacer 18b may be fixed to the main body by appropriate methods such as fastening and welding, thereby allowing the second cold storage material to be contained in the second displacer 18b.

[0031] The displacer assembly 18 forms a room temperature chamber 30, a first expansion chamber 32, and a second expansion chamber 34 inside the refrigerator cylinder 16. In order to perform heat exchange with a desired object or medium to be cooled by the ultra-low temperature refrigerator 10, the expander 14 has a first cooling stage 33 and a second cooling stage 35. The room temperature chamber 30 is formed between the upper cover portion of the first displacer 18a and the upper portion of the first cylinder 16a. The first expansion chamber 32 is formed between the lower cover portion of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is formed between the lower cover portion of the second displacer 18b and the second cooling stage 35. The first cooling stage 33 is fixed to the lower portion of the first cylinder 16a in a manner surrounding the first expansion chamber 32, and the second cooling stage 35 is fixed to the lower portion of the second cylinder 16b in a manner surrounding the second expansion chamber 34.

[0032] The first regenerator 26 is connected to the room temperature chamber 30 through a working gas flow path 36a formed in the upper cover of the first displacer 18a, and is connected to the first expansion chamber 32 through a working gas flow path 36b formed in the lower cover of the first displacer 18a. The second regenerator 28 is connected to the first regenerator 26 through a working gas flow path 36c formed from the lower cover of the first displacer 18a to the upper cover of the second displacer 18b. In addition, the second regenerator 28 is connected to the second expansion chamber 34 through a working gas flow path 36d formed in the lower cover of the second displacer 18b.

[0033] In order to allow the working gas flowing between the first expansion chamber 32, the second expansion chamber 34 and the room temperature chamber 30 to be introduced into the first cold storage device 26 and the second cold storage device 28 without being introduced into the gap between the refrigerator cylinder 16 and the displacer assembly 18, a first seal 38a and a second seal 38b may be provided. The first seal 38a may be installed on the upper cover portion of the first displacer 18a in a manner arranged between the first displacer 18a and the first cylinder 16a. The second seal 38b may be installed on the upper cover portion of the second displacer 18b in a manner arranged between the second displacer 18b and the second cylinder 16b.

[0034] like Figure 1As shown, the expander 14 includes a refrigerator housing 20 that accommodates a pressure switching valve 40. The refrigerator housing 20 is combined with the refrigerator cylinder 16 to form an airtight container that accommodates the pressure switching valve 40 and the displacer assembly 18.

[0035] like Figure 2 As shown, the pressure switching valve 40 is configured to include a high-pressure valve 40a and a low-pressure valve 40b and to generate periodic pressure fluctuations in the refrigerator cylinder 16. The working gas outlet of the compressor 12 is connected to the room temperature chamber 30 via the high-pressure valve 40a, and the working gas intake of the compressor 12 is connected to the room temperature chamber 30 via the low-pressure valve 40b. The high-pressure valve 40a and the low-pressure valve 40b are configured to be selectively and alternately opened and closed (that is, when one is opened, the other is closed).

[0036] The pressure switching valve 40 may also be in the form of a rotary valve. That is, the pressure switching valve 40 may be configured to alternately open and close the high-pressure valve 40a and the low-pressure valve 40b by the rotational sliding of the valve disc relative to the stationary valve body. In this case, the expander motor 42 may be connected to the pressure switching valve 40 to rotate the valve disc of the pressure switching valve 40. For example, the pressure switching valve 40 is configured in such a way that the valve rotation axis and the rotation axis of the expander motor 42 become coaxial.

[0037] Alternatively, the high-pressure valve 40 a and the low-pressure valve 40 b may be valves that can be controlled separately. In this case, the pressure switching valve 40 may not be connected to the expander motor 42 .

[0038] The expander motor 42 is connected to the displacer drive shaft 44 via a motion conversion mechanism 43 such as a scotch yoke mechanism. The expander motor 42 is mounted on the refrigerator housing 20. Like the pressure switching valve 40, the motion conversion mechanism 43 is housed in the refrigerator housing 20. The motion conversion mechanism 43 converts the rotational motion output by the expander motor 42 into a linear reciprocating motion of the displacer drive shaft 44. The displacer drive shaft 44 extends from the motion conversion mechanism 43 toward the room temperature chamber 30 and is fixed to the upper cover portion of the first displacer 18a. The rotation of the expander motor 42 is converted by the motion conversion mechanism 43 into an axial reciprocating motion of the displacer drive shaft 44, and the displacer assembly 18 performs a linear reciprocating motion along the axial direction in the refrigerator cylinder 16.

[0039] The expander motor 42 is, for example, a permanent magnet motor driven by three-phase alternating current. The operating frequency of the expander motor 42 is controlled by the inverter 70. The expander motor 42 can operate at a rotational speed corresponding to the operating frequency of the expander motor 42. As an example, the output frequency of the inverter 70 (i.e., the operating frequency of the expander motor 42) can be changed within the range of 30 Hz to 100 Hz or within the range of 40 Hz to 70 Hz.

[0040] The expander motor 42 and the inverter 70 are powered by an external power source 80 such as a commercial power source (three-phase AC power source). In addition, the expander motor 42 and the inverter 70 can be connected to the external power source 80 via the compressor 12, for example, to obtain power. In this case, the compressor 12 can be regarded as the power source of the expander motor 42 and the inverter 70.

[0041] The compressor 12 includes a high-pressure gas outlet 50, a low-pressure gas inlet 51, a high-pressure flow path 52, a low-pressure flow path 53, a first pressure sensor 54, a second pressure sensor 55, a bypass line 56, a compressor body 57, and a compressor housing 58. The high-pressure gas outlet 50 is provided in the compressor housing 58 as a working gas discharge port of the compressor 12, and the low-pressure gas inlet 51 is provided in the compressor housing 58 as a working gas suction port of the compressor 12. The high-pressure flow path 52 connects the discharge port of the compressor body 57 to the high-pressure gas outlet 50, and the low-pressure flow path 53 connects the low-pressure gas inlet 51 to the suction port of the compressor body 57. The compressor housing 58 accommodates the high-pressure flow path 52, the low-pressure flow path 53, the first pressure sensor 54, the second pressure sensor 55, the bypass line 56, and the compressor body 57. The compressor 12 is also called a compressor unit.

[0042] The compressor body 57 is configured to compress the working gas sucked from its suction port and discharge it from the discharge port. The compressor body 57 can be, for example, a vortex type, a rotary type, or other pump that increases the pressure of the working gas. In this embodiment, the compressor body 57 is configured to discharge a fixed and constant flow rate of the working gas. Alternatively, the compressor body 57 can also be configured to be able to change the flow rate of the discharged working gas. The compressor body 57 is also called a compression chamber.

[0043] The first pressure sensor 54 is arranged on the high-pressure flow path 52 to measure the pressure of the working gas flowing through the high-pressure flow path 52. The first pressure sensor 54 is configured to output a first measured pressure signal P1 indicating the measured pressure. The second pressure sensor 55 is arranged on the low-pressure flow path 53 to measure the pressure of the working gas flowing through the low-pressure flow path 53. The second pressure sensor 55 is configured to output a second measured pressure signal P2 indicating the measured pressure. Therefore, the first pressure sensor 54 and the second pressure sensor 55 may also be referred to as a high-pressure sensor and a low-pressure sensor, respectively. In addition, in this specification, sometimes either the first pressure sensor 54 or the second pressure sensor 55 is referred to as a "pressure sensor", or both are collectively referred to as a "pressure sensor".

[0044] The bypass line 56 connects the high-pressure flow path 52 to the low-pressure flow path 53 so that the working gas bypasses the expander 14 and flows back from the high-pressure flow path 52 to the low-pressure flow path 53. A relief valve 60 is provided on the bypass line 56, which is used to open and close the bypass line 56 or to control the flow rate of the working gas flowing through the bypass line 56. The relief valve 60 is configured to be opened when a pressure difference above the set pressure acts between its inlet and outlet. The relief valve 60 can be an on-off valve or a flow control valve, for example, a solenoid valve. The set pressure can be appropriately set based on the designer's experience or experiments or simulation tests conducted by the designer. In this way, the pressure difference between the high-pressure line 63 and the low-pressure line 64 can be prevented from exceeding the set pressure and becoming too large. In addition, the pressure of the high-pressure line 63 can be prevented from becoming too large.

[0045] As an example, the relief valve 60 may be opened and closed under the control of the control device 100. The control device 100 may control the relief valve 60 in the following manner, that is, the measured pressure difference between the high pressure line 63 and the low pressure line 64 is compared with the set pressure, and the relief valve 60 is opened when the measured pressure difference is greater than the set pressure, and the relief valve 60 is closed when the measured pressure difference is less than the set pressure. The control device 100 may obtain the measured pressure difference between the high pressure line 63 and the low pressure line 64 based on the first measured pressure signal P1 from the first pressure sensor 54 and the second measured pressure signal P2 from the second pressure sensor 55. Alternatively, the control device 100 may also control the relief valve 60 in the following manner, that is, the measured pressure of the high pressure line 63 is compared with the upper limit pressure based on the first measured pressure signal P1, and the relief valve 60 is opened when the measured pressure is greater than the upper limit pressure, and the relief valve 60 is closed when the measured pressure is less than the upper limit pressure. As another example, the relief valve 60 may be configured to operate as a so-called safety valve, that is, to be mechanically opened when a pressure difference greater than a set pressure acts between the inlet and outlet.

[0046] In addition, the compressor 12 may also have various other components. For example, an oil separator and an adsorber may be provided on the high-pressure flow path 52. A storage tank and other components may also be provided on the low-pressure flow path 53. Furthermore, the compressor 12 may also be provided with an oil circulation system for cooling the compressor body 57 with oil and a cooling system for cooling the oil.

[0047] Furthermore, the cryogenic refrigerator 10 includes a gas pipeline 62 for circulating the working gas between the compressor 12 and the expander 14. The gas pipeline 62 includes: a high-pressure pipeline 63 that connects the compressor 12 to the expander 14 so that the working gas is supplied from the compressor 12 to the expander 14; and a low-pressure pipeline 64 that connects the compressor 12 to the expander 14 so that the working gas is recovered from the expander 14 to the compressor 12. Therefore, the working gas sucked into the expander 14 flows in the high-pressure pipeline 63, and the working gas discharged from the expander 14 flows in the low-pressure pipeline 64.

[0048] A high-pressure gas inlet 22 and a low-pressure gas outlet 24 are provided on the refrigerator housing 20 of the expander 14. The high-pressure gas inlet 22 is provided on the refrigerator housing 20 and serves as a working gas suction port of the expander 14, and the low-pressure gas outlet 24 is provided on the refrigerator housing 20 and serves as a working gas discharge port of the expander 14. Figure 1 As shown, a high-pressure side connecting pipe 25a having a high-pressure gas inlet 22 at the front end and a low-pressure side connecting pipe 25b having a low-pressure gas outlet 24 at the front end can extend from the refrigerator housing 20. The high-pressure side connecting pipe 25a and the low-pressure side connecting pipe 25b are, for example, rigid pipes, but can also be flexible pipes.

[0049] The high-pressure gas inlet 22 of the expander 14 is connected to the high-pressure gas outlet 50 of the compressor 12 via the high-pressure pipe 65. The low-pressure gas outlet 24 of the expander 14 is connected to the low-pressure gas inlet 51 of the compressor 12 via the low-pressure pipe 66. The high-pressure pipeline 63 is composed of the high-pressure pipe 65 and the high-pressure flow path 52, and the low-pressure pipeline 64 is composed of the low-pressure pipe 66 and the low-pressure flow path 53. The high-pressure pipe 65 and the low-pressure pipe 66 are, for example, flexible pipes, but may also be rigid pipes.

[0050] The bypass line 56 in the compressor 12 may also be regarded as a part of the gas line 62. The bypass line 56 connects the high-pressure line 63 to the low-pressure line 64 so that the working gas bypasses the expander 14 and flows back from the high-pressure line 63 to the low-pressure line 64.

[0051] Therefore, the working gas recovered from the expander 14 to the compressor 12 enters the low-pressure gas inlet 51 of the compressor 12 from the low-pressure gas outlet 24 of the expander 14 through the low-pressure pipe 66, and then returns to the compressor body 57 through the low-pressure flow path 53, and is compressed and pressurized by the compressor body 57. The working gas supplied from the compressor 12 to the expander 14 is discharged from the compressor body 57 through the high-pressure flow path 52 from the high-pressure gas outlet 50 of the compressor 12, and then supplied to the expander 14 through the high-pressure pipe 65 and the high-pressure gas inlet 22 of the expander 14.

[0052] When the pressure of the high-pressure pipeline 63 exceeds the upper limit pressure, the relief valve 60 is opened, and a part of the working gas flowing through the high-pressure pipeline 63 is diverted from the high-pressure flow path 52 to the bypass pipeline 56. The bypass pipeline 56 merges with the low-pressure flow path 53, so the working gas bypasses the expander 14 and flows back to the compressor body 57, and the pressure of the high-pressure pipeline 63 decreases. If the pressure of the high-pressure pipeline 63 becomes lower than the upper limit pressure, the relief valve 60 is closed, and the flow of the working gas from the high-pressure pipeline 63 to the low-pressure pipeline 64 through the bypass pipeline 56 is cut off. Similarly, the relief valve 60 can also adjust the pressure difference between the high-pressure pipeline 63 and the low-pressure pipeline 64 so that it does not exceed the set pressure.

[0053] The expander 14 is provided with an intake air temperature sensor 46 and an exhaust air temperature sensor 48. The intake air temperature sensor 46 is configured to measure the temperature of the working gas supplied to the expander 14 (i.e., the intake air temperature), and output an intake air temperature signal T1 indicating the measured intake air temperature. The exhaust air temperature sensor 48 is configured to measure the temperature of the working gas discharged from the expander 14 (i.e., the exhaust air temperature), and output an exhaust air temperature signal T2 indicating the measured exhaust air temperature.

[0054] The intake temperature sensor 46 is, for example, disposed on the high-pressure gas inlet 22 of the expander 14, and the exhaust temperature sensor 48 is, for example, disposed on the low-pressure gas outlet 24 of the expander 14. However, the intake temperature sensor 46 is not limited to its location as long as it can measure the temperature of the working gas sucked into the expander 14. For example, the intake temperature sensor 46 can be disposed on the inside or outside surface of the high-pressure side connecting pipe 25a, between the high-pressure gas inlet 22 and the high-pressure piping 65, or on the inside or outside surface of the high-pressure piping 65. Similarly, the exhaust temperature sensor 48 is not limited to its location as long as it can measure the temperature of the working gas discharged from the expander 14. For example, the exhaust temperature sensor 48 can be disposed on the inside or outside surface of the low-pressure side connecting pipe 25b, between the low-pressure gas outlet 24 and the low-pressure piping 66, or on the inside or outside surface of the low-pressure piping 66.

[0055] In this way, the intake temperature sensor 46 and the exhaust temperature sensor 48 are both arranged in the non-cooling part of the ultra-low temperature refrigerator 10. These temperature sensors are not limited to being arranged on the expander 14, and can also be arranged on the gas pipeline 62. However, in order to avoid being affected by the ambient temperature change (such as cooling) and to accurately measure the temperature of the working gas sucked into the expander 14 and the temperature of the working gas discharged from the expander 14, the intake temperature sensor 46 and the exhaust temperature sensor 48 are preferably arranged on the expander 14 or near it instead of being arranged on the compressor 12.

[0056] like Figure 1As shown, the control device 100 for controlling the cryogenic refrigerator 10 includes a controller 110 for controlling the inverter 70. The controller 110 is electrically connected to the intake air temperature sensor 46 and the exhaust air temperature sensor 48, thereby obtaining an intake air temperature signal T1 and an exhaust air temperature signal T2. In addition, the controller 110 is electrically connected to the first pressure sensor 54 and the second pressure sensor 55, thereby obtaining a first measured pressure signal P1 and a second measured pressure signal P2.

[0057] In the illustrated example, the control device 100 is provided separately from the compressor 12 and the expander 14 and connected thereto, but the present invention is not limited thereto. The control device 100 may also be mounted on the compressor 12. The control device 100 may also be provided on the expander 14 (for example, mounted on the expander motor 42, etc.). Alternatively, the controller 110 and the inverter 70 may also be provided separately, for example, the controller 110 may be mounted on the compressor 12 and the inverter 70 may be mounted on the expander 14, etc.

[0058] The control device 100 may be implemented in terms of hardware structure by components or circuits represented by a CPU or memory of a computer, and in terms of software structure by a computer program, etc., but Figure 1 In the embodiment, the functional blocks are appropriately described as being implemented by their cooperation. It should be understood by those skilled in the art that these functional blocks can be implemented in various forms by a combination of hardware and software.

[0059] The ultra-low temperature refrigerator 10 generates periodic volume changes and pressure changes of the working gas synchronized therewith in the first expansion chamber 32 and the second expansion chamber 34 when the compressor 12 and the expander motor 42 are in operation. Typically, in the air intake process, by closing the low-pressure valve 40b and opening the high-pressure valve 40a, the high-pressure working gas flows from the compressor 12 through the high-pressure valve 40a into the room temperature chamber 30, and is supplied to the first expansion chamber 32 through the first cold storage device 26, and then supplied to the second expansion chamber 34 through the second cold storage device 28. As a result, the first expansion chamber 32 and the second expansion chamber 34 are increased from low pressure to high pressure. At this time, the displacer assembly 18 moves from the lower dead center toward the upper dead center, and the volumes of the first expansion chamber 32 and the second expansion chamber 34 increase. If the high-pressure valve 40a is closed, the air intake process ends.

[0060] In the exhaust process, by closing the high-pressure valve 40a and opening the low-pressure valve 40b, the high-pressure first expansion chamber 32 and the second expansion chamber 34 are connected to the low-pressure working gas suction port of the compressor 12, so that the working gas expands in the first expansion chamber 32 and the second expansion chamber 34, and as a result, the low-pressure working gas is discharged from the first expansion chamber 32 and the second expansion chamber 34 toward the room temperature chamber 30 through the first regenerator 26 and the second regenerator 28. At this time, the displacer assembly 18 moves from the top dead center to the bottom dead center, and the volume of the first expansion chamber 32 and the second expansion chamber 34 decreases. The working gas is recovered from the expander 14 through the low-pressure valve 40b to the compressor 12. If the low-pressure valve 40b is closed, the exhaust process is completed.

[0061] Thus, a refrigeration cycle (e.g., GM cycle, etc.) is formed, and the first cooling stage 33 and the second cooling stage 35 are cooled to a desired ultra-low temperature. The first cooling stage 33 can be cooled to a first cooling temperature (e.g., in the range of about 20K to about 40K), for example. The second cooling stage 35 can be cooled to a second cooling temperature (e.g., about 1K to about 4K) lower than the first cooling temperature.

[0062] The cryogenic refrigerator 10 can perform steady-state operation and cooling operation before steady-state operation. Cooling operation is an operation mode in which the cryogenic refrigerator 10 is rapidly cooled from the initial temperature to a cryogenic temperature when the cryogenic refrigerator 10 is started, and steady-state operation is an operation mode in which the cryogenic refrigerator 10 is maintained in a state of being cooled to a cryogenic temperature by cooling operation. The initial temperature may be ambient temperature (e.g., room temperature). The cryogenic refrigerator 10 is cooled to a standard cooling temperature by cooling operation, and is maintained within an allowable temperature range of a cryogenic temperature including the standard cooling temperature in steady-state operation. The standard cooling temperature varies according to the purpose and setting of the cryogenic refrigerator 10. For example, in the cooling application of a superconducting device, a typical standard cooling temperature is about 4.2K or less. In other cooling applications, the standard cooling temperature may be, for example, about 10K to 20K, or about 10K or less. The switching from cooling operation to steady-state operation may be controlled by the control device 100. As described above, cooling may also be referred to as initial cooling.

[0063] Initial cooling is nothing more than a preparatory stage for starting to cool an object using a cryogenic refrigerator, so it is desirable that the time required is as short as possible. Therefore, "accelerated cooling" is sometimes used. In accelerated cooling, the operating frequency of the expander motor 42 is controlled by the inverter 70 to, for example, a frequency higher than the power frequency of the external power supply 80. The increase in the operating frequency of the expander motor 42 is equivalent to an increase in the number of refrigeration cycles of the cryogenic refrigerator 10 per unit time, so the refrigeration capacity of the cryogenic refrigerator 10 can be increased. Therefore, by accelerating cooling, the cooling time of the cryogenic refrigerator 10 can be shortened.

[0064] At first glance, if the expander motor 42 is driven at a high operating frequency even in the steady-state operation after the initial cooling, the cryogenic refrigerator 10 can continue to exert a high refrigeration capacity. However, this is not actually the case. If the expander motor 42 is driven at a high operating frequency, the refrigeration capacity may sometimes decrease and become insufficient.

[0065] The working gas flow rate required by the expander 14 in order to make the ultra-low temperature refrigerator 10 output the specified refrigeration capacity is related to the density change of the working gas that depends on the cooling temperature. Therefore, the higher the temperature, the less the flow rate can be. Therefore, when the discharge flow rate of the compressor 12 is constant, the higher the cooling temperature, the more excess working gas flow will become, and the greater the pressure difference between the high-pressure pipeline 63 and the low-pressure pipeline 64 will become. In order to avoid excessive pressure difference, as described above, the working gas can flow back through the bypass pipeline 56. Therefore, in the initial cooling where the temperature is higher than the steady-state operation (especially at the beginning of the initial cooling), a large amount of excess gas will flow back through the bypass pipeline 56 and will be wasted. In accelerated cooling, the excess gas that flows back is reduced by increasing the flow rate of the working gas used in the expander 14, thereby effectively utilizing the discharge flow rate of the compressor 12.

[0066] Therefore, it is possible to consider installing an automatic control function on the ultra-low temperature refrigerator, that is, to set a temperature sensor on the cooling stage (for example, the second cooling stage 35) of the ultra-low temperature refrigerator 10, and use the temperature sensor to monitor the measured temperature to detect the completion of the initial cooling, and then end the initial cooling (and accelerated cooling) and switch to steady-state cooling.

[0067] However, to achieve this function, a temperature sensor capable of measuring ultra-low temperatures is required, and such temperature sensors are relatively expensive. If the ultra-low temperature temperature sensor is not installed in the ultra-low temperature refrigerator in order to avoid cost increases, the user of the ultra-low temperature refrigerator needs to determine the completion of the initial cooling and manually end the accelerated cooling, which makes the operation cumbersome. Alternatively, the use of accelerated cooling has to be abandoned, which limits the reduction of the cooling time.

[0068] In this regard, the inventors have discovered other simple and inexpensive methods for detecting the completion of cooling without using a temperature sensor provided in the ultra-low temperature section. In an embodiment, as described later, the controller 110 is configured to compare the measured exhaust temperature with the reference temperature based on the exhaust temperature signal T2 during the execution of the initial cooling of the cooling stage from the initial temperature to the ultra-low temperature, and terminate the initial cooling when the temperature difference between the measured exhaust temperature and the reference temperature is within the reference range. The controller 110 may also be configured to use the measured intake temperature as the reference temperature based on the intake temperature signal T1. The controller 110 may also be configured to control the inverter 70 so that the operating frequency of the expander motor 42 decreases when the initial cooling is terminated.

[0069] Figure 3 This is a graph showing an example of changes in the exhaust gas temperature and the intake gas temperature of the expander 14 during initial cooling of the cryogenic refrigerator according to the embodiment. Figure 3 The temperature changes shown are obtained through experiments. Figure 3 The upper part of the diagram shows the exhaust gas temperature and the intake gas temperature of the expander 14 measured by the exhaust gas temperature sensor 48 and the intake gas temperature sensor 46. For ease of understanding, the cooling temperature of the second cooling stage 35 is also measured and shown in FIG. Figure 3 In addition, at this time, the bypass line 56 of the compressor 12 is controlled so that the pressure difference between the high-pressure line 63 and the low-pressure line 64 is constant.

[0070] like Figure 3 As shown in FIG. 1 , at the moment of starting the ultra-low temperature refrigerator 10 (time 0), the exhaust temperature and the intake temperature are both the ambient temperature (e.g., about 25° C.). The exhaust temperature rises to a certain maximum temperature (e.g., about 42° C.) after the initial cooling begins, and then slowly drops to the ambient temperature. The intake temperature is roughly equal to the ambient temperature during the initial cooling period. In this way, when the ambient temperature remains unchanged, the intake temperature also becomes constant corresponding to the ambient temperature.

[0071] As can be seen from the graph, the temperature difference between the exhaust temperature and the intake air temperature increases immediately after the initial cooling begins, but the temperature difference gradually decreases as the second cooling stage 35 is cooled. At time A (about 46 minutes in this example) when the second cooling stage 35 is cooled to the above-mentioned standard cooling temperature (for example, about 4K), the temperature difference between the exhaust temperature and the intake air temperature decreases to about 7°C. As time goes on, the temperature difference between the exhaust temperature and the intake air temperature eventually falls within a few degrees Celsius (for example, within 5°C or within 3°C). Therefore, it can be considered that the initial cooling ends at time A, or at time B (in this example, the time when the temperature difference becomes within 5°C) when the temperature difference between the exhaust temperature and the intake air temperature further decreases.

[0072] Figure 4 It is a flowchart for illustrating the method for starting the ultra-low temperature refrigerator 10 involved in the embodiment. This method is executed by the controller 110 when starting the ultra-low temperature refrigerator 10. If the ultra-low temperature refrigerator 10 is started, initial cooling (S10) begins. At this time, the controller 110 can perform accelerated cooling, or control the inverter 70 to make the operating frequency of the expander motor 42 higher than that in steady-state operation. The operating frequency of the expander motor 42 in the initial cooling can be higher than the input frequency (for example, 50 Hz or 60 Hz) flowing from the external power supply 80 to the inverter 70.

[0073] The exhaust gas temperature and intake air temperature of the expander 14 are measured (S12). As described above, the intake air temperature sensor 46 and the exhaust gas temperature sensor 48 are used for measurement. The controller 110 obtains the measured intake air temperature of the expander 14 based on the intake air temperature signal T1, and obtains the measured exhaust air temperature of the expander 14 based on the exhaust gas temperature signal T2.

[0074] The measured exhaust temperature is compared with the reference temperature (S14). In this embodiment, the measured intake temperature is used as the reference temperature. The controller 110 determines whether the temperature difference between the measured exhaust temperature and the reference temperature is within the reference range. If the measured exhaust temperature is much greater than the reference temperature, resulting in the temperature difference between the measured exhaust temperature and the reference temperature exceeding the reference range ((i) of S14), the initial cooling is continued, and then the exhaust temperature and the intake temperature of the expander 14 are measured again (S12), and the measured exhaust temperature is compared with the reference temperature (S14).

[0075] Here, the reference range may be, for example, within a few degrees Celsius (e.g., within 5°C). The reference range may be appropriately set based on the designer's experience or experiments or simulation tests performed by the designer. The reference range is input to the controller 110 in advance by the user of the cryogenic refrigerator 10 or is preset in the controller 110 and stored in the controller 110.

[0076] If the temperature difference between the measured exhaust temperature and the reference temperature is within the reference range ((ii) of S14), the controller 110 ends the initial cooling and switches the ultra-low temperature refrigerator 10 to steady-state operation (S16). In the case of performing accelerated cooling, the controller 110 controls the inverter 70 to reduce the operating frequency of the expander motor 42 (S18). That is, the controller 110 controls the inverter 70 to change the operating frequency of the expander motor 42 from the first value for initial cooling to the second value for steady-state operation. The second operating frequency value for steady-state operation is less than the first operating frequency value for initial cooling, for example, equal to or lower than the input frequency (for example, 50 Hz or 60 Hz) flowing from the external power supply 80 to the inverter 70.

[0077] As a result, the cryogenic refrigerator 10 ends initial cooling and starts steady-state operation. The object to be cooled, which is thermally connected to the second cooling stage 35, can be cooled to a target ultra-low temperature and used at an ultra-low temperature.

[0078] Therefore, according to the ultra-low temperature refrigerator 10 involved in the embodiment, during the execution of initial cooling, it is not necessary to measure the temperature of the ultra-low temperature cooling part (for example, the second cooling stage 35), and the completion of initial cooling can be detected based on the measured exhaust temperature and the measured intake temperature of the expander 14. Since the intake temperature sensor 46 and the exhaust temperature sensor 48 are arranged in the non-cooling part of the ultra-low temperature refrigerator 10, a general temperature sensor for measuring the temperature near room temperature can be used. There is no need to use a temperature sensor for ultra-low temperature measurement that is more expensive than this temperature sensor. Therefore, an ultra-low temperature refrigerator that detects the completion of initial cooling can be provided at a low price.

[0079] The measured intake temperature is used as a reference temperature for comparison with the measured exhaust temperature. Assuming that the ambient temperature of the cryogenic refrigerator 10 changes, the influence will be manifested in both the exhaust temperature and the intake temperature. According to the embodiment, since the temperature difference between the measured exhaust temperature and the measured intake temperature is used, the influence of the change in the ambient temperature can be offset.

[0080] In addition, it is not necessary to use the measured intake air temperature as the reference temperature. In one embodiment, the controller 110 may use the measured exhaust air temperature at the start of initial cooling as the reference temperature. In this case, the ultra-low temperature refrigerator 10 may not have the intake air temperature sensor 46, and in step S12, only the exhaust air temperature may be measured using the exhaust air temperature sensor 48. Furthermore, if the value of the ambient temperature can be obtained, the controller 110 may use the ambient temperature value as the reference temperature. The controller 110 may also use a temperature value (which may be a fixed value) representing the ambient temperature as the reference temperature.

[0081] Furthermore, according to the embodiment, the operating frequency of the expander motor 42 is reduced when the initial cooling is completed, so that the reduction in the refrigeration capacity of the cryogenic refrigerator 10 in the steady-state operation can be suppressed.

[0082] In the above embodiment, accelerated cooling is performed in the initial cooling. However, accelerated cooling is not necessary. In one embodiment, the operating frequency of the expander motor 42 in the initial cooling can be equal to the input frequency (e.g., 50 Hz or 60 Hz) flowing from the external power supply 80 to the inverter 70, and when switching from the initial cooling to the steady-state operation, the controller 110 can control the inverter 70 to reduce the operating frequency of the expander motor 42.

[0083] In addition, the operating frequency of the expander motor 42 may also be adjusted during initial cooling or steady-state operation. For example, the controller 110 may control the operating frequency of the expander motor 42 within a first range during initial cooling, and control the operating frequency of the expander motor 42 within a second range during steady-state operation. The second range may be an operating frequency lower than the first range. The first range may be greater than the input frequency to the inverter 70, and the second range may be equal to or lower than the input frequency to the inverter 70.

[0084] In addition, the completion of the initial cooling may be detected based on the pressure of the gas pipeline 62 instead of the exhaust gas temperature of the expander 14. Next, this embodiment will be described.

[0085] Figure 5 : This is a graph showing an example of changes in the pressure of the high-pressure line 63 and the low-pressure line 64 during initial cooling of the cryogenic refrigerator according to the embodiment. Figure 5 The pressure changes shown are obtained experimentally. Figure 4 The upper part of the diagram shows the pressures of the high pressure line 63 and the low pressure line 64 measured by the first pressure sensor 54 and the second pressure sensor 55. For ease of understanding, the cooling temperature of the second cooling stage 35 is also measured and shown in FIG. Figure 5 In addition, at this time, the bypass line 56 of the compressor 12 is controlled so that the pressure difference between the high-pressure line 63 and the low-pressure line 64 is constant.

[0086] like Figure 5 As shown, at the moment of starting the ultra-low temperature refrigerator 10 (time 0), the pressure of the high-pressure pipeline 63 is about 2.5 MPa, and the pressure of the low-pressure pipeline 64 is about 0.7 MPa. In the initial cooling, until the second cooling stage 35 is cooled to about 20K, the pressures of the high-pressure pipeline 63 and the low-pressure pipeline 64 are respectively kept substantially constant. With further cooling, the second cooling stage 35 is cooled from about 20K to about 4K, and roughly synchronously therewith, the pressure of the high-pressure pipeline 63 drops from about 2.5 MPa to about 2.3 MPa, and the pressure of the low-pressure pipeline 64 drops from about 0.7 MPa to about 0.5 MPa.

[0087] This pressure drop is based on the relationship between the temperature and density of the helium gas used as the working gas. When the helium gas is in the temperature range of about 20 to 30K, the density is significantly increased compared to other temperature ranges. Therefore, when the second cooling stage 35 is cooled to about 20 to 30K, the density of the helium gas increases in the expansion chamber of the expander 14, and thus the helium gas is absorbed from the gas line 62 to the expansion chamber. As a result, as the gas line 62 is cooled from a high temperature higher than the temperature range to a low temperature, the pressure of the gas line 62 decreases.

[0088] Therefore, the initial cooling can be considered to be completed at the time C when the change in the measured pressure of the high-pressure line 63 or the measured pressure of the low-pressure line 64 exceeds the threshold and expands (in this example, when the pressure drop exceeds 0.1 MPa, for example).

[0089] Figure 6 This is a flowchart for explaining a method for starting the cryogenic refrigerator 10 according to the embodiment. This method is executed by the controller 110 when starting the cryogenic refrigerator 10. When the cryogenic refrigerator 10 is started, initial cooling starts (S10).

[0090] The pressure of the high-pressure pipeline 63 or the pressure of the low-pressure pipeline 64 is measured (S22). As described above, the measurement is performed using the first pressure sensor 54 or the second pressure sensor 55. The controller 110 can obtain the pressure of the high-pressure pipeline 63 based on the first measured pressure signal P1, and obtain the pressure of the low-pressure pipeline 64 based on the second measured pressure signal P2.

[0091] The change in the measured pressure is compared with the pressure threshold (S24). The controller 110 calculates the change in the measured pressure and compares the calculated pressure change with the pressure threshold. The change in the measured pressure may be, for example, the change in the current measured pressure relative to the measured pressure obtained at the start of initial cooling. If the change in the measured pressure is small and the pressure change is lower than the pressure threshold ((i) of S24), the initial cooling is continued, and then the pressure is measured again (S22), and the change in the measured pressure is compared with the pressure threshold (S24).

[0092] Here, the controller 110 can calculate the moving average of the measured pressure, calculate the change in the moving average, and compare the pressure change with the pressure threshold. The pressure threshold is, for example, about 0.1 MPa. The pressure threshold can be appropriately set based on the designer's experience or experiments and simulation tests conducted by the designer.

[0093] If the measured pressure change is higher than the pressure threshold value ((ii) of S24), the controller 110 ends the initial cooling and switches the cryogenic refrigerator 10 to steady-state operation (S16). When performing accelerated cooling, the controller 110 controls the inverter 70 to reduce the operating frequency of the expander motor 42 (S18). In this way, the cryogenic refrigerator 10 ends the initial cooling and starts steady-state operation.

[0094] In this way, the controller 110 determines whether the initial cooling is completed based on the pressure of the high-pressure pipeline 63 or the pressure of the low-pressure pipeline 64 measured by the pressure sensor during the initial cooling. Therefore, according to the ultra-low temperature refrigerator 10 involved in the embodiment, during the execution of the initial cooling, it is not necessary to measure the temperature of the ultra-low temperature cooling unit (for example, the second cooling stage 35), and the completion of the initial cooling can be detected based on the pressure of the high-pressure pipeline 63 or the pressure of the low-pressure pipeline 64.

[0095] In addition, the controller 110 may determine whether the initial cooling is completed based on the pressure of the high-pressure line 63 and determine whether the initial cooling is completed based on the pressure of the low-pressure line 64. If at least one of the determination based on the pressure of the high-pressure line 63 and the determination based on the pressure of the low-pressure line 64 (preferably both) indicates that the initial cooling is completed, the controller 110 may end the initial cooling and switch to steady-state operation.

[0096] The controller 110 may determine whether the initial cooling is completed based on the temperature and determine whether the initial cooling is completed based on the pressure. The determination based on the temperature may be, for example, a reference Figure 3 and Figure 4 The determination based on the comparison between the measured exhaust gas temperature and the reference temperature is described. The determination based on the pressure can be, for example, a reference Figure 5 and Figure 6 The determination based on the comparison of the measured pressure change amount and the pressure threshold is described. If at least one of the determination based on temperature and the determination based on pressure (preferably both) indicates that the initial cooling is completed, the controller 110 can end the initial cooling and switch to steady-state operation.

[0097] As described above, if the relief valve 60 is opened and closed under the control of the control device 100, different set pressures can be used in the initial cooling and steady-state operation. The control device 100 can control the relief valve 60 in the following manner, that is, in the initial cooling, the measured pressure difference between the high-pressure pipeline 63 and the low-pressure pipeline 64 is compared with the set pressure for the initial cooling, and the relief valve 60 is opened if the measured pressure difference is greater than the set pressure, and the relief valve 60 is closed if the measured pressure difference is less than the set pressure. In addition, the control device 100 can also control the relief valve 60 in the following manner, that is, in the steady-state operation, the measured pressure difference between the high-pressure pipeline 63 and the low-pressure pipeline 64 is compared with the set pressure for the steady-state operation, and the relief valve 60 is opened if the measured pressure difference is greater than the set pressure, and the relief valve 60 is closed if the measured pressure difference is less than the set pressure. Similarly, regarding the upper limit pressure determined for the high-pressure pipeline 63, different upper limit pressures can be used in the initial cooling and steady-state operation.

[0098] The pressure sensors such as the first pressure sensor 54 and the second pressure sensor 55 do not necessarily need to be installed on the compressor 12, and may be installed on any location (e.g., the gas pipeline 62 or the expander 14) where the pressure can be measured. For example, the first pressure sensor 54 may be installed on any location of the high-pressure pipeline 63, and the second pressure sensor 55 may be installed on any location of the low-pressure pipeline 64. Similarly, the bypass pipeline 56 and the relief valve 60 do not necessarily need to be installed on the compressor 12, and may be arranged outside the compressor 12 and connected to the high-pressure pipeline 63 and the low-pressure pipeline 64.

[0099] In the above embodiment, the cryogenic refrigerator 10 is described as a two-stage GM refrigerator, but the invention is not limited thereto. The cryogenic refrigerator 10 may be a single-stage or multi-stage GM refrigerator, or may be another type of cryogenic refrigerator, for example, a GM pulse tube refrigerator having an expander motor for driving an expander.

[0100] The present invention has been described above based on the embodiments. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and various design changes and variations are possible, and such variations are also within the scope of the present invention.

Claims

1. A cryogenic refrigerator, characterized in that: have: an expander having a cooling stage; an exhaust temperature sensor for measuring the exhaust temperature of the expander and outputting an exhaust temperature signal representing the measured exhaust temperature; and a controller that compares the measured exhaust temperature with a reference temperature based on the exhaust temperature signal during initial cooling of the cooling stage from an initial temperature to a very low temperature, and terminates the initial cooling when a temperature difference between the measured exhaust temperature and the reference temperature is within a reference range, During the execution of the initial cooling, the exhaust gas temperature changes as follows: it rises from the temperature at the start time of the initial cooling to a maximum temperature, and then starts to decrease from the maximum temperature. The controller ends the initial cooling if the temperature difference is within the reference range while the changing exhaust temperature is decreasing.

2. The ultra-low temperature refrigerator according to claim 1, characterized in that: further comprising an intake air temperature sensor for measuring an intake air temperature of the expander and outputting an intake air temperature signal indicating the measured intake air temperature, The controller uses the measured intake air temperature as the reference temperature based on the intake air temperature signal.

3. The ultra-low temperature refrigerator according to claim 1 or 2, characterized in that: Also available: a high-pressure pipeline connected to the expander, through which flows the working gas sucked into the expander; a low-pressure pipeline connected to the expander, through which the working gas exhausted from the expander flows; and A pressure sensor is used to measure the pressure of the high-pressure pipeline or the low-pressure pipeline. The controller determines whether the initial cooling is completed based on any one of the pressure of the high-pressure line or the pressure of the low-pressure line measured by the pressure sensor during the initial cooling.

4. The ultra-low temperature refrigerator according to claim 1 or 2, characterized in that: A frequency converter is further provided, the frequency converter controlling the operating frequency of a motor driving the expander, The controller controls the inverter to reduce the operating frequency of the motor when the initial cooling is finished.

5. The ultra-low temperature refrigerator according to claim 1 or 2, characterized in that: A bypass line is further provided for connecting a high-pressure line through which the working gas sucked into the expander flows to a low-pressure line through which the working gas exhausted from the expander flows so that the working gas bypasses the expander.

6. A method for starting a cryogenic refrigerator, characterized in that: The following steps are required: performing initial cooling of cooling a cooling stage of the expander from an initial temperature to a very low temperature; During the initial cooling, measuring the exhaust temperature of the expander; and During the execution of the initial cooling, the measured exhaust temperature is compared with a reference temperature, and the initial cooling is terminated when the temperature difference between the measured exhaust temperature and the reference temperature is within a reference range. During the execution of the initial cooling, the exhaust gas temperature changes as follows: it rises from the temperature at the start time of the initial cooling to a maximum temperature, and then starts to decrease from the maximum temperature. When the changing exhaust gas temperature is decreasing, if the temperature difference is within the reference range, the initial cooling is terminated.

7. A cryogenic refrigerator, characterized in that: have: an expander having a cooling stage; a high-pressure pipeline connected to the expander, through which flows the working gas sucked into the expander; a low-pressure pipeline connected to the expander, through which the working gas exhausted from the expander flows; A pressure sensor, for measuring the pressure of the high-pressure pipeline or the pressure of the low-pressure pipeline; a controller that, during the execution of initial cooling of cooling the cooling stage from an initial temperature to a super-low temperature, determines whether the initial cooling is completed based on any one of a pressure drop amount of the high-pressure pipeline or a pressure drop amount of the low-pressure pipeline measured by the pressure sensor; and The bypass line is controlled to connect the high-pressure line to the low-pressure line so that the working gas bypasses the expander, and to keep the pressure difference between the high-pressure line and the low-pressure line constant during the initial cooling.

8. A method for starting a cryogenic refrigerator, characterized in that: The following steps are required: performing initial cooling of cooling a cooling stage of the expander from an initial temperature to a very low temperature; During the initial cooling, measuring the pressure of the working gas sucked into the expander or the pressure of the working gas exhausted from the expander; and During the execution of the initial cooling, it is determined whether the initial cooling is completed based on any one of a measured pressure drop of the working gas sucked into the expander or a measured pressure drop of the working gas discharged from the expander, The step of performing the initial cooling includes causing the working gas to reflux by bypassing the expander so that a pressure difference between a pressure of the working gas sucked into the expander and a pressure of the working gas discharged from the expander becomes constant.

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