Cryogenic refrigerator and method for controlling the same

Abstract

To provide a cryogenic refrigerator that can shorten cool-down time.SOLUTION: A cryogenic refrigerator 10 comprises a compressor 12, an expander 14 to be driven by an expander motor 42, an inverter 70 for controlling operational frequency of the expander motor 42, a high-pressure line 63 for supplying high-pressure working gas to the expander 14 from the compressor 12, a low-pressure line 64 for recovering low-pressure working gas to the compressor 12 from the expander 14, a pressure measurement part constituted so as to measure pressure of the high-pressure line 63 and pressure of the low-pressure line 64, or measure differential pressure between the high-pressure line 63 and low-pressure line 64, and a controller 110 for comparing the differential pressure between the high-pressure line 63 and the low-pressure line 64 with target pressure on the basis of output of the pressure measurement part, and controlling the inverter 70 so as to increase the operational frequency of the expander motor 42 when the differential pressure exceeds the target pressure.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a cryogenic refrigerator and a method for controlling a cryogenic refrigerator. [Background technology]

[0002] Cryogenic refrigerators are used to cool various objects such as superconducting devices, measuring instruments, and samples that are used in cryogenic environments. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 4-64869 Summary of the Invention [Problem to be solved by the invention]

[0004] To cool an object with a cryogenic freezer, the cryogenic freezer must first be started and cooled from an initial temperature, such as room temperature, to the desired cryogenic temperature. This is also called cooling down the cryogenic freezer. Since the cool-down is merely a preparation to start cooling the object, it is desirable for the time required to be as short as possible.

[0005] One exemplary objective of certain aspects of the present invention is to reduce the cool-down time of a cryogenic refrigerator. [Means for solving the problem]

[0006] According to one aspect of the present invention, a cryogenic refrigerator includes a compressor, an expander having a motor and driven by the motor, an inverter that controls the operating frequency of the motor, a high-pressure line connecting the compressor to the expander so as to supply high-pressure working gas from the compressor to the expander, a low-pressure line connecting the compressor to the expander so as to recover low-pressure working gas from the expander to the compressor, a pressure measurement unit configured to measure the pressure of the high-pressure line and the pressure of the low-pressure line, or to measure the differential pressure between the high-pressure line and the low-pressure line, and a controller that compares the differential pressure between the high-pressure line and the low-pressure line with a target pressure based on the output of the pressure measurement unit, and controls the inverter to increase the operating frequency of the motor if the differential pressure exceeds the target pressure.

[0007] According to an aspect of the present invention, there is provided a method for controlling a cryogenic refrigerator. The cryogenic refrigerator includes a compressor, an expander having a motor and driven by the motor, an inverter for controlling an operating frequency of the motor, a high-pressure line connecting the compressor to the expander so as to supply a high-pressure working gas from the compressor to the expander, and a low-pressure line connecting the compressor to the expander so as to recover a low-pressure working gas from the expander to the compressor. The method includes measuring a differential pressure between the high-pressure line and the low-pressure line, comparing the measured differential pressure between the high-pressure line and the low-pressure line with a target pressure, and controlling the inverter to increase the operating frequency of the motor when the differential pressure exceeds the target pressure.

[0008] Any combination of the above components or mutual substitution of the components or expressions of the present invention between methods, devices, systems, etc. are also valid as aspects of the present invention. Effect of the Invention

[0009] According to the present invention, the cool-down time of a cryogenic refrigerator can be shortened. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating a schematic diagram of a cryogenic refrigerator according to an embodiment; [Diagram 2]1 is a diagram illustrating a schematic diagram of a cryogenic refrigerator according to an embodiment; [Diagram 3] 4 is a flowchart illustrating a method for controlling a cryogenic refrigerator according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the description and drawings, the same or equivalent components, members, and processes are given the same reference numerals, and duplicated descriptions are omitted as appropriate. The scale and shape of each part shown in the drawings are set for convenience in order to facilitate the description, and are not to be interpreted as being limiting unless otherwise specified. The embodiments are merely examples, and do not limit the scope of the present invention in any way. All features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0012] 1 and 2 are schematic diagrams illustrating a cryogenic refrigerator 10 according to an embodiment. As an example, the cryogenic refrigerator 10 is a two-stage Gifford-McMahon (GM) refrigerator. In FIG. 1, a compressor 12 and an expander 14 that constitute the cryogenic refrigerator 10 are shown together with a control device 100, and in FIG. 2, the internal structure of the expander 14 of the cryogenic refrigerator 10 is shown.

[0013] 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 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, which enables the cryogenic refrigerator 10 to provide the desired cryogenic cooling. The expander 14 is also called a cold head. The working gas, also called a refrigerant gas, is typically helium gas, although other suitable gases may be used. For ease of understanding, the flow direction of the working gas is indicated by arrows in FIG. 1.

[0014] Generally, 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 both significantly higher than atmospheric pressure, and can be referred to as the first high pressure and the second high pressure, respectively. For convenience of explanation, the first high pressure and the second high pressure are also simply referred to as the high pressure and the low pressure, respectively. 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 ease of understanding, the flow direction of the working gas is indicated by an arrow.

[0015] The expander 14 includes a refrigerator cylinder 16 and a displacer assembly 18. The refrigerator cylinder 16 guides the linear reciprocating motion of the displacer assembly 18, and forms an expansion chamber (32, 34) for the working gas between the refrigerator cylinder 16 and the displacer assembly 18. The expander 14 also includes a pressure switching valve 40 that determines the timing at which the working gas is sucked into the expansion chamber and the timing at which the working gas is exhausted from the expansion chamber.

[0016] In this document, in order to explain the positional relationship between the components of the cryogenic refrigerator 10, for convenience, the side closer to the top dead center of the axial reciprocating motion of the displacer will be referred to as "top" and the side closer to the bottom dead center as "bottom." The top dead center is the position of the displacer where the volume of the expansion space is maximum, and the bottom dead center is the position of the displacer where the volume of the expansion space is minimum. During operation of the cryogenic refrigerator 10, a temperature gradient occurs in which the temperature decreases from the top to the bottom in the axial direction, so the top side can also be called the high temperature side and the bottom side the low temperature side.

[0017] The refrigerator cylinder 16 has a first cylinder 16a and a second cylinder 16b. The first cylinder 16a and the second cylinder 16b are, for example, members having a cylindrical shape, and the second cylinder 16b has a smaller diameter than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are arranged coaxially, and the lower end of the first cylinder 16a is rigidly connected to the upper end of the second cylinder 16b.

[0018] The displacer assembly 18 includes a first displacer 18a and a second displacer 18b that are connected to each other and move together. The first displacer 18a and the second displacer 18b are, for example, cylindrical members, and the second displacer 18b has a smaller diameter than the first displacer 18a. The first displacer 18a and the second displacer 18b are arranged coaxially.

[0019] 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.

[0020] As shown in Fig. 2, the first displacer 18a accommodates the first regenerator 26. The first regenerator 26 is formed by filling a tubular main body of the first displacer 18a with a wire mesh such as copper or other suitable first regenerator material. The upper and lower lids of the first displacer 18a may be provided as separate members from the main body of the first displacer 18a, and the upper and lower lids of the first displacer 18a may be fixed to the main body by suitable means such as fastening or welding, thereby accommodating the first regenerator material in the first displacer 18a.

[0021] Similarly, the second displacer 18b accommodates the second regenerator 28. The second regenerator 28 is formed by filling the cylindrical main body of the second displacer 18b with a non-magnetic regenerator material such as bismuth, a magnetic regenerator material such as HoCu2, or other suitable second regenerator material. The second regenerator material may be formed in a granular shape. The upper and lower lids of the second displacer 18b may be provided as separate members from the main body of the second displacer 18b, and the lower lid of the upper lid of the second displacer 18b may be fixed to the main body by suitable means such as fastening or welding, thereby accommodating the second regenerator material in the second displacer 18b.

[0022] 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. For heat exchange with a desired object or medium to be cooled by the cryogenic refrigerator 10, the expander 14 includes a first cooling stage 33 and a second cooling stage 35. The room temperature chamber 30 is formed between the upper cover of the first displacer 18a and the upper part of the first cylinder 16a. The first expansion chamber 32 is formed between the lower cover of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is formed between the lower cover of the second displacer 18b and the second cooling stage 35. The first cooling stage 33 is fixed to the lower part of the first cylinder 16a so as to surround the first expansion chamber 32, and the second cooling stage 35 is fixed to the lower part of the second cylinder 16b so as to surround the second expansion chamber 34.

[0023] The first regenerator 26 is connected to the room temperature chamber 30 through a working gas passage 36a formed in the upper lid of the first displacer 18a, and is connected to the first expansion chamber 32 through a working gas passage 36b formed in the lower lid of the first displacer 18a. The second regenerator 28 is connected to the first regenerator 26 through a working gas passage 36c formed from the lower lid of the first displacer 18a to the upper lid of the second displacer 18b. The second regenerator 28 is also connected to the second expansion chamber 34 through a working gas passage 36d formed in the lower lid of the second displacer 18b.

[0024] A first seal 38a and a second seal 38b may be provided so that the working gas flow between the first expansion chamber 32, the second expansion chamber 34 and the room temperature chamber 30 is guided to the first regenerator 26 and the second regenerator 28, not through the clearance between the refrigerator cylinder 16 and the displacer assembly 18. The first seal 38a may be attached to an upper cover of the first displacer 18a so as to be disposed between the first displacer 18a and the first cylinder 16a. The second seal 38b may be attached to an upper cover of the second displacer 18b so as to be disposed between the second displacer 18b and the second cylinder 16b.

[0025] 1, the expander 14 includes a refrigerator housing 20 that houses a pressure switching valve 40. The refrigerator housing 20 is coupled to the refrigerator cylinder 16, thereby forming an airtight container that houses the pressure switching valve 40 and the displacer assembly 18.

[0026] 2, the pressure switching valve 40 includes a high-pressure valve 40a and a low-pressure valve 40b, and is configured to generate periodic pressure fluctuations in the refrigerator cylinder 16. The working gas discharge port of the compressor 12 is connected to the room-temperature chamber 30 via the high-pressure valve 40a, and the working gas intake port 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 selectively and alternately open and close (i.e., when one is open, the other is closed).

[0027] The pressure switching valve 40 may take the form of a rotary valve. That is, the pressure switching valve 40 may be configured so that the high pressure valve 40a and the low pressure valve 40b are alternately opened and closed by the rotational sliding of a valve disc relative to a stationary valve body. In this case, the expander motor 42 may be connected to the pressure switching valve 40 so as to rotate the valve disc of the pressure switching valve 40. For example, the pressure switching valve 40 is arranged so that the valve rotation axis is coaxial with the rotation axis of the expander motor 42.

[0028] Alternatively, the high pressure valve 40a and the low pressure valve 40b may each be a valve that can be controlled individually, in which case the pressure switching valve 40 does not need to be connected to the expander motor .

[0029] The expander motor 42 is connected to a displacer drive shaft 44 via a motion conversion mechanism 43 such as a Scotch yoke mechanism. The expander motor 42 is attached to the refrigerator housing 20. The motion conversion mechanism 43 is accommodated in the refrigerator housing 20, similar to the pressure switching valve 40. 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 into the room temperature chamber 30 and is fixed to the upper cover of the first displacer 18a. The rotation of the expander motor 42 is converted into an axial reciprocating motion of the displacer drive shaft 44 by the motion conversion mechanism 43, and the displacer assembly 18 reciprocates linearly in the axial direction within the refrigerator cylinder 16.

[0030] The expander motor 42 is, for example, a permanent magnet type motor driven by three-phase AC. The operating frequency of the expander motor 42 is controlled by the inverter 70. The expander motor 42 can operate at a rotation speed according to the operating frequency of the expander motor 42, which corresponds to the output frequency of the inverter 70. As an example, the output frequency of the inverter 70 can vary in the range of 30 Hz to 100 Hz, or in the range of 40 Hz to 70 Hz.

[0031] The expander motor 42 and the inverter 70 are supplied with power from an external power source 80 such as a commercial power source (three-phase AC power source). Note that the expander motor 42 and the inverter 70 may be connected to and supplied with power from the external power source 80 via the compressor 12, for example. In this case, the compressor 12 may be regarded as the power source for the expander motor 42 and the inverter 70.

[0032] The expander 14 may also include a temperature sensor 46 that measures the temperature of the second cooling stage 35 (and / or the first cooling stage 33) and outputs a measured temperature signal indicative of the measured temperature.

[0033] 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 installed in the compressor housing 58 as a working gas discharge port of the compressor 12, and the low-pressure gas inlet 51 is installed in the compressor housing 58 as a working gas intake 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 intake 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 referred to as a compressor unit.

[0034] The compressor body 57 is configured to compress the working gas drawn in from its intake port and discharge it from its discharge port. The compressor body 57 may be, for example, a scroll type, a rotary type, or other pump that pressurizes the working gas. In this embodiment, the compressor body 57 is configured to discharge a fixed constant flow rate of the working gas. Alternatively, the compressor body 57 may be configured to vary the flow rate of the working gas to be discharged. The compressor body 57 may also be referred to as a compression capsule.

[0035] The first pressure sensor 54 is disposed in 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 representing the measured pressure. The second pressure sensor 55 is disposed in 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 representing the measured pressure. Therefore, the first pressure sensor 54 and the second pressure sensor 55 can also be called a high pressure sensor and a low pressure sensor, respectively. In this specification, either the first pressure sensor 54 or the second pressure sensor 55 or both of them may be collectively referred to simply as a "pressure sensor".

[0036] The bypass line 56 connects the high pressure line 52 to the low pressure line 53 so as to return the working gas from the high pressure line 52 to the low pressure line 53 by bypassing the expander 14. The bypass line 56 is provided with a relief valve 60 for opening and closing the bypass line 56 or for controlling the flow rate of the working gas flowing through the bypass line 56. The relief valve 60 is configured to open when a differential pressure equal to or greater than a set pressure acts between its inlet and outlet. The relief valve 60 may be an on-off valve or a flow control valve, and may be, for example, a solenoid valve. The set pressure can be set appropriately based on the designer's empirical knowledge or on experiments, simulations, etc. performed by the designer. This makes it possible to prevent the differential pressure between the high pressure line 63 and the low pressure line 64 from exceeding the set pressure and becoming excessive.

[0037] 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 compare a measured differential pressure between the high pressure line 63 and the low pressure line 64 with a set pressure, and control the relief valve 60 to open the relief valve 60 when the measured differential pressure is equal to or greater than the set pressure, and close the relief valve 60 when the measured differential pressure is less than the set pressure. The control device 100 may obtain the measured differential pressure between the high pressure line 63 and the low pressure line 64 based on a first measured pressure signal P1 from the first pressure sensor 54 and a second measured pressure signal P2 from the second pressure sensor 55. As another example, the relief valve 60 may be configured to operate as a so-called safety valve, i.e., may be mechanically opened when a differential pressure equal to or greater than a set pressure acts between the inlet and outlet.

[0038] The compressor 12 may have various other components. For example, the high-pressure flow path 52 may be provided with an oil separator, an adsorber, etc. The low-pressure flow path 53 may be provided with a storage tank and other components. The compressor 12 may also be provided with an oil circulation system that cools the compressor body 57 with oil, a cooling system that cools the oil, etc.

[0039] The cryogenic refrigerator 10 also includes a gas line 62 for circulating the working gas between the compressor 12 and the expander 14. The gas line 62 includes a high-pressure line 63 connecting the compressor 12 to the expander 14 so as to supply the working gas from the compressor 12 to the expander 14, and a low-pressure line 64 connecting the compressor 12 to the expander 14 so as to recover the working gas from the expander 14 to the compressor 12. The refrigerator housing 20 of the expander 14 is provided with a high-pressure gas inlet 22 and a low-pressure gas outlet 24. The high-pressure gas inlet 22 is connected to the high-pressure gas outlet 50 by a high-pressure pipe 65, and the low-pressure gas outlet 24 is connected to the low-pressure gas inlet 51 by a low-pressure pipe 66. The high-pressure line 63 is composed of the high-pressure pipe 65 and the high-pressure flow path 52, and the low-pressure line 64 is composed of the low-pressure pipe 66 and the low-pressure flow path 53. The bypass line 56 may be considered to be part of the gas line 62. The bypass line 56 connects the high pressure line 63 to the low pressure line 64 so as to return the working gas from the high pressure line 63 to the low pressure line 64 , bypassing the expander 14 .

[0040] Therefore, the working gas recovered from the expander 14 to the compressor 12 passes from the low-pressure gas outlet 24 of the expander 14 through the low-pressure piping 66 to enter the low-pressure gas inlet 51 of the compressor 12, then passes through the low-pressure flow path 53 to return to the compressor main body 57, and is compressed and pressurized by the compressor main body 57. The working gas supplied from the compressor 12 to the expander 14 passes from the compressor main body 57 through the high-pressure flow path 52 to exit the high-pressure gas outlet 50 of the compressor 12, and then passes through the high-pressure piping 65 and the high-pressure gas inlet 22 of the expander 14 to be supplied to the expander 14.

[0041] When the pressure difference between the high-pressure line 63 and the low-pressure line 64 exceeds the set pressure of the relief valve 60 and the relief valve 60 is open, a part of the working gas flowing through the high-pressure line 63 is diverted from the high-pressure flow path 52 to the bypass line 56. Because the bypass line 56 merges with the low-pressure flow path 53, the working gas bypasses the expander 14 and flows back to the compressor body 57, and the pressure difference between the high-pressure line 63 and the low-pressure line 64 becomes small. If the pressure difference falls below the set pressure of the relief valve 60, the relief valve 60 closes and the flow of working gas from the high-pressure line 63 to the low-pressure line 64 through the bypass line 56 is blocked.

[0042] 1, the control device 100 that controls the cryogenic refrigerator 10 includes a controller 110 that controls the inverter 70. The controller 110 is electrically connected to the first pressure sensor 54 and the second pressure sensor 55 to obtain a first measured pressure signal P1 and a second measured pressure signal P2. The controller 110 is also electrically connected to the temperature sensor 46 to obtain a measured temperature signal from the temperature sensor 46.

[0043] As will be described in more detail below, the controller 110 compares the differential pressure between the high pressure line 63 and the low pressure line 64 with a target pressure based on the first measured pressure signal P1 and the second measured pressure signal P2, and controls the inverter 70 to increase the operating frequency of the expander motor 42 when the differential pressure exceeds the target pressure, and to decrease the operating frequency of the expander motor 42 when the differential pressure is below the target pressure.

[0044] In the illustrated example, the control device 100 is provided separately from the compressor 12 and the expander 14 and connected to them, but this is not limited thereto. The control device 100 may be mounted on the compressor 12. The control device 100 may be provided on the expander 14, for example, by being mounted on the expander motor 42. Alternatively, the controller 110 and the inverter 70 may be installed separately, for example, the controller 110 is mounted on the compressor 12 and the inverter 70 is mounted on the expander 14.

[0045] The control device 100 is realized as a hardware configuration by elements and circuits such as a computer CPU and memory, and as a software configuration by a computer program, etc., but is depicted as functional blocks realized by the cooperation of these in Fig. 1. Those skilled in the art will understand that these functional blocks can be realized in various forms by combining hardware and software.

[0046] When the compressor 12 and the expander motor 42 are operated, the cryogenic refrigerator 10 generates periodic volume fluctuations and pressure fluctuations of the working gas synchronized therewith in the first expansion chamber 32 and the second expansion chamber 34. Typically, in the intake stroke, the low pressure valve 40b is closed and the high pressure valve 40a is opened, so that the high pressure working gas flows from the compressor 12 through the high pressure valve 40a into the room temperature chamber 30, is supplied to the first expansion chamber 32 through the first regenerator 26, and is supplied to the second expansion chamber 34 through the second regenerator 28. In this way, the first expansion chamber 32 and the second expansion chamber 34 are pressurized from low pressure to high pressure. At this time, the displacer assembly 18 is moved upward from the bottom dead center to the top dead center, and the volumes of the first expansion chamber 32 and the second expansion chamber 34 are increased. When the high pressure valve 40a is closed, the intake stroke ends.

[0047] In the exhaust stroke, the high-pressure valve 40a is closed and the low-pressure valve 40b is opened, so that the high-pressure first expansion chamber 32 and the second expansion chamber 34 are opened to the low-pressure working gas intake port of the compressor 12, and the working gas expands in the first expansion chamber 32 and the second expansion chamber 34, and the low-pressure working gas is discharged from the first expansion chamber 32 and the second expansion chamber 34 to the room temperature chamber 30 through the first regenerator 26 and the second regenerator 28. At this time, the displacer assembly 18 is moved downward from the top dead center to the bottom dead center, and the volumes of the first expansion chamber 32 and the second expansion chamber 34 are reduced. The working gas is collected from the expander 14 to the compressor 12 through the low-pressure valve 40b. When the low-pressure valve 40b is closed, the exhaust stroke is completed.

[0048] In this manner, a refrigeration cycle such as a GM cycle is configured, and the first cooling stage 33 and the second cooling stage 35 are cooled to a desired cryogenic temperature. The first cooling stage 33 can be cooled to a first cooling temperature in the range of, for example, about 20 K to about 40 K. The second cooling stage 35 can be cooled to a second cooling temperature (for example, about 1 K to about 4 K) lower than the first cooling temperature.

[0049] The cryogenic refrigerator 10 can perform a normal operation and a cool-down operation prior to the normal operation. The cool-down operation is an operation mode in which the cryogenic refrigerator 10 is rapidly cooled from room temperature to a cryogenic temperature when the cryogenic refrigerator 10 is started, and the normal operation is an operation mode in which the cryogenic refrigerator 10 maintains a state cooled to a cryogenic temperature by the cool-down operation. The cryogenic refrigerator 10 is cooled to a standard cooling temperature by the cool-down operation, and is maintained within a cryogenic temperature allowable range including the standard cooling temperature in the normal operation. The standard cooling temperature varies depending on the application and settings of the cryogenic refrigerator 10, but is typically about 4.2K or less for cooling applications of superconducting devices, for example. In some other cooling applications, the standard cooling temperature may be, for example, about 10K to 20K, or less than 10K. The switching from the cool-down operation to the normal operation may be controlled by the control device 100. For example, the control device 100 may compare the measured temperature of the second cooling stage 35 (and / or the first cooling stage 33) with the above-mentioned standard cooling temperature based on the measured temperature signal from the temperature sensor 46, and perform a cool-down operation if the measured temperature is higher than the standard cooling temperature, and transition from the cool-down operation to steady-state operation if the measured temperature is equal to or lower than the standard cooling temperature.

[0050] 3 is a flowchart illustrating a method for controlling the cryogenic refrigerator 10 according to the embodiment. This method is repeatedly executed at a predetermined cycle by the controller 110 during operation of the cryogenic refrigerator 10. This method can also be called accelerated cooling of the cryogenic refrigerator 10, and this accelerated cooling is executed at least during cool-down operation.

[0051] 3, in this method, first, it is determined whether or not the current operation mode of the cryogenic refrigerator 10 is the cool-down operation (S10). The controller 110 may determine whether or not the current operation mode is the cool-down operation based on information indicating the current operation mode of the cryogenic refrigerator 10. The information indicating the current operation mode may be stored in the controller 110 as a result of a process of switching between the cool-down operation and the normal operation.

[0052] When the current operation mode is cool-down operation (Y in S10), the differential pressure between the high pressure line 63 and the low pressure line 64 is measured (S12). This differential pressure is measured using a pressure measurement unit of the cryogenic refrigerator 10, for example, the first pressure sensor 54 and the second pressure sensor 55 as described above. The controller 110 obtains the measured differential pressure ΔPM between the high pressure line 63 and the low pressure line 64 from the first measured pressure signal P1 and the second measured pressure signal P2.

[0053] Next, the measured differential pressure ΔPM between the high pressure line 63 and the low pressure line 64 is compared with the target pressure PT (S14). The value of the target pressure PT is set to a pressure value smaller than the above-mentioned set pressure at which the relief valve 60 opens. However, the target pressure PT is set to a pressure value as close as possible to the set pressure, and for example, the difference between the target pressure PT and the set pressure may be within 0.1 MPa. The target pressure PT may be smaller than the set pressure by, for example, 0.03 MPa to 0.07 MPa, for example, 0.05 MPa. When the set pressure of the relief valve 60 is, for example, 1.6 MPa, the target pressure PT may be smaller than, for example, 1.6 MPa and equal to or greater than 1.5 MPa. The target pressure PT may be in the range of, for example, 1.53 MPa to 1.57 MPa, for example, 1.55 MPa. The target pressure PT can be appropriately set based on the designer's empirical knowledge or experiments or simulations by the designer. The target pressure PT is input in advance to the controller 110 by a user of the cryogenic refrigerator 10 or is automatically set by the controller 110 and stored in the controller 110.

[0054] The controller 110 compares the measured differential pressure ΔPM with the target pressure PT and outputs the comparison result indicating the magnitude relationship between the two. That is, the comparison result by the controller 110 indicates one of the following three states: (i) the measured differential pressure ΔPM is greater than the target pressure PT, (ii) the measured differential pressure ΔPM is less than the target pressure PT, or (iii) the measured differential pressure ΔPM is equal to the target pressure PT.

[0055] The inverter 70 is controlled based on the comparison result by the controller 110, and the operation frequency of the expander motor 42 is controlled according to the output frequency of the inverter 70. Specifically, (i) when the measured differential pressure ΔPM is greater than the target pressure PT, the controller 110 controls the inverter 70 to increase the operation frequency of the expander motor 42 (S16). (ii) when the measured differential pressure ΔPM is smaller than the target pressure PT, the controller 110 controls the inverter 70 to decrease the operation frequency of the expander motor 42 (S18). (iii) When the measured differential pressure ΔPM is equal to the target pressure PT, there is no need to increase or decrease the operation frequency of the expander motor 42, so the controller 110 controls the inverter 70 to maintain the current operation frequency. Note that the case (iii) may be included in either (i) or (ii).

[0056] Alternatively, the target pressure PT may be different between when the operating frequency of the expander motor 42 is increased and when it is decreased. For example, when the measured differential pressure ΔPM is greater than the first target pressure PT1, the controller 110 may control the inverter 70 to increase the operating frequency of the expander motor 42. When the measured differential pressure ΔPM is smaller than the second target pressure PT2, the controller 110 may control the inverter 70 to decrease the operating frequency of the expander motor 42. The second target pressure PT2 may be smaller than the first target pressure PT1. When the measured differential pressure ΔPM is between the first target pressure PT1 and the second target pressure PT2, the controller 110 may control the inverter 70 to maintain the current operating frequency of the expander motor 42.

[0057] When increasing or decreasing the operation frequency of the expander motor 42, the controller 110 may increase or decrease the operation frequency by a predetermined amount from the current operation frequency value of the expander motor 42. However, when the operation frequency is to be increased and the current operation frequency value has already reached the upper limit value, the controller 110 may maintain the operation frequency at the upper limit value without increasing the operation frequency. For example, when the possible range of the operation frequency of the expander motor 42 is 30 Hz to 100 Hz and the current value is already the upper limit value of 100 Hz, the controller 110 does not further increase the operation frequency from 100 Hz but maintains it at 100 Hz. Similarly, when the operation frequency is to be decreased and the current operation frequency value has already reached the lower limit value, the controller 110 may maintain the operation frequency at the lower limit value without decreasing the operation frequency.

[0058] Alternatively, the controller 110 may control the inverter 70 to adjust the operating frequency of the expander motor 42 (e.g., by feedback control such as PID control) so as to minimize the deviation of the measured differential pressure ΔPM from the target pressure PT (which may be the first target pressure PT1 or the second target pressure PT2). In this manner, the controller 110 may control the inverter 70 to compare the differential pressure between the high pressure line 63 and the low pressure line 64 with the target pressure, and to increase the operating frequency of the expander motor 42 when the differential pressure exceeds the target pressure, and to decrease the operating frequency of the expander motor 42 when the differential pressure is below the target pressure.

[0059] In this manner, when the measured differential pressure ΔPM exceeds the target pressure PT, the operating frequency of the expander motor 42 is increased. When the operating frequency increases, the flow rate of working gas used for cooling in the expander 14 increases, so if the discharge flow rate of the compressor 12 is constant (or if it fluctuates, it is sufficiently small), the measured differential pressure ΔPM decreases and approaches the target pressure PT or falls below the target pressure PT. Also, when the measured differential pressure ΔPM falls below the target pressure PT, the operating frequency of the expander motor 42 is decreased. Since the flow rate of working gas used in the expander 14 decreases, the measured differential pressure ΔPM increases and approaches the target pressure PT or exceeds the target pressure PT.

[0060] In this embodiment, if the current operation mode is not the cool-down operation (for example, the current operation mode is the steady operation) (N of S10), the controller 110 does not execute accelerated cooling. If the current operation mode is the steady operation, the operation frequency of the expander motor 42 may be fixed to a constant value, for example, the input frequency from the external power supply 80 to the inverter 70 or a value lower than this. Alternatively, if the current operation mode is the steady operation, the temperature adjustment control of the cryogenic refrigerator 10 may be executed, and for example, the inverter 70 may be controlled so as to adjust the operation frequency of the expander motor 42 based on the measured temperature signal from the temperature sensor 46 (for example, by feedback control such as PID control) so as to minimize the deviation of the measured temperature from a target temperature (for example, the above-mentioned standard cooling temperature).

[0061] Incidentally, if the measured differential pressure ΔPM increases and exceeds the set pressure of the relief valve 60 during operation of the cryogenic refrigerator 10, the relief valve 60 opens and the excess working gas is returned through the bypass line 56, and this returned working gas does not contribute to cryogenic cooling in the expander 14. The flow rate of working gas required by the expander 14 for the cryogenic refrigerator 10 to output a predetermined refrigeration capacity correlates with the density change of the working gas depending on the cooling temperature, so the higher the temperature, the smaller the flow rate of working gas may be. Therefore, assuming that the discharge flow rate of the compressor 12 is constant, the higher the cooling temperature, the larger the excess working gas flow rate becomes, and the larger the measured differential pressure ΔPM tends to be. Therefore, particularly at the start-up of the cryogenic refrigerator 10, that is, during the cool-down operation, a large amount of excess gas may be returned through the bypass line 56 and be wasted.

[0062] In contrast, according to the accelerated cooling of the cryogenic refrigerator 10 of the embodiment, when the measured differential pressure ΔPM exceeds the target pressure PT, the operating frequency of the expander motor 42 is increased, and the excess working gas flow rate can be used for cryogenic cooling in the expander 14. Since an increase in the operating frequency of the expander motor 42 corresponds to an increase in the number of refrigeration cycles per unit time of the cryogenic refrigerator 10, the refrigeration capacity of the cryogenic refrigerator 10 can be increased. Since the excess working gas flow rate increases during cool-down operation, this accelerated cooling is suitable for increasing the refrigeration capacity of the cryogenic refrigerator 10 during cool-down operation. Therefore, according to the embodiment, the cool-down time of the cryogenic refrigerator 10 can be shortened.

[0063] Generally, the refrigeration capacity of the cryogenic refrigerator 10 can vary in proportion to the measured differential pressure ΔPM, so a decrease in the measured differential pressure ΔPM can result in a decrease in the refrigeration capacity. However, according to the embodiment, when the measured differential pressure ΔPM falls below the target pressure PT, the operating frequency of the expander motor 42 is reduced. This reduces the flow rate of the working gas used by the expander 14, and the measured differential pressure ΔPM is restored, making it possible to prevent or mitigate a decrease in the refrigeration capacity.

[0064] Furthermore, in this embodiment, the target pressure PT is set to a pressure value smaller than the set pressure of the relief valve 60. In this way, when the measured differential pressure ΔPM increases, the target pressure PT is reached before the set pressure of the relief valve 60 is reached, so that the operating frequency of the expander motor 42 can be increased to increase the refrigeration capacity of the cryogenic refrigerator 10 without opening the relief valve 60 (i.e., without wastefully returning excess gas).

[0065] Furthermore, the difference between the target pressure PT and the set pressure is within 0.1 MPa. In this way, the pressure difference between the high pressure line 63 and the low pressure line 64 can be kept as high as possible with the relief valve 60 closed, and the refrigeration capacity of the cryogenic refrigerator 10 can be increased.

[0066] In the above embodiment, the accelerated cooling of the cryogenic refrigerator 10 is performed during cool-down operation. However, the accelerated cooling may be performed not only during cool-down operation but also during steady operation. When accelerated cooling is performed during steady operation, step S10 in the flow shown in FIG. 3, that is, the step of determining the current operation mode of the cryogenic refrigerator 10, may be omitted.

[0067] Alternatively, the controller 110 may determine the current operation mode of the cryogenic refrigerator 10, and when the cryogenic refrigerator 10 is in cool-down operation, control the inverter 70 so that the operating frequency of the expander motor 42 is higher than when the cryogenic refrigerator 10 is in steady operation. The operating frequency of the expander motor 42 in cool-down operation may be higher than the input frequency (e.g., 50 Hz or 60 Hz) from the external power supply 80 to the inverter 70, and the operating frequency of the expander motor 42 in steady operation may be equal to or lower than this input frequency.

[0068] For example, the controller 110 may control the operating frequency of the expander motor 42 within a first range in the cool-down operation and control the operating frequency of the expander motor 42 within a second range in the steady operation. The second range may be an operating frequency lower than the first range. Or, the controller 110 may control the operating frequency of the expander motor 42 from a first initial value in the cool-down operation and control the operating frequency of the expander motor 42 from a second initial value in the steady operation. The second initial value may be an operating frequency lower than the first initial value. The first range (or the first initial value) may be higher than the input frequency to the inverter 70, and the second range (or the second initial value) may be equal to or lower than the input frequency to the inverter 70.

[0069] In the above-described embodiment, the first pressure sensor 54 and the second pressure sensor 55 are used as pressure measurement units for measuring the differential pressure between the high pressure line 63 and the low pressure line 64. However, in some embodiments, a differential pressure sensor provided in, for example, the bypass line 56 or the relief valve 60 may be used as the pressure measurement unit.

[0070] Pressure measuring units such as the first pressure sensor 54 and the second pressure sensor 55 do not necessarily need to be provided in the compressor 12, and may be provided in any location where pressure can be measured, such as the gas line 62 or the expander 14. For example, the first pressure sensor 54 may be provided in any location in the high pressure line 63, and the second pressure sensor 55 may be provided in any location in the low pressure line 64. Similarly, the bypass line 56 and the relief valve 60 do not necessarily need to be provided in the compressor 12, and may be disposed outside the compressor 12 to connect the high pressure line 63 and the low pressure line 64.

[0071] In the above-described embodiment, the compressor 12 is configured to discharge a fixed constant working gas flow rate. However, in an embodiment, the compressor 12 may be configured to vary the working gas discharge flow rate. In this case, the controller 110 may perform the above-described accelerated cooling when the compressor 12 is controlled to discharge a constant working gas flow rate. Alternatively, the controller 110 may control the compressor 12 to maintain (or increase) the operating frequency of the expander motor 42 and simultaneously increase the working gas discharge flow rate of the compressor 12 when the measured differential pressure ΔPM is below the target pressure PT.

[0072] In the above embodiment, the cryogenic refrigerator 10 is a two-stage GM refrigerator, but the present invention is not limited to this. The cryogenic refrigerator 10 may be a single-stage or multi-stage GM refrigerator, or may be another type of cryogenic refrigerator, such as a GM pulse tube refrigerator, that includes an expander motor for driving an expander.

[0073] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-mentioned embodiments, that various design changes are possible, that various modifications are possible, and that such modifications are also within the scope of the present invention. [Explanation of symbols]

[0074] 10 cryogenic refrigerator, 12 compressor, 14 expander, 56 bypass line, 60 relief valve, 63 high pressure line, 64 low pressure line, 70 inverter, 110 controller.

Claims

1. A cryogenic refrigerator capable of performing a cool-down operation in which the refrigerator is rapidly cooled from room temperature to a cryogenic temperature upon startup, and a steady operation in which the refrigerator is maintained in a state cooled to a cryogenic temperature by the cool-down operation, A compressor; an expander having a motor and driven by the motor; an inverter for controlling an operating frequency of the motor; a high pressure line connecting the compressor to the expander so as to supply high pressure working gas from the compressor to the expander; a low pressure line connecting the compressor to the expander so as to return low pressure working gas from the expander to the compressor; a pressure measuring unit configured to measure a pressure in the high pressure line and a pressure in the low pressure line, or to measure a differential pressure between the high pressure line and the low pressure line; a controller that performs accelerated cooling by comparing a differential pressure between the high pressure line and the low pressure line with a target pressure based on an output of the pressure measurement unit during at least the cool-down operation, and controlling the inverter to increase an operating frequency of the motor when the differential pressure exceeds the target pressure; a bypass line connecting the high pressure line to the low pressure line; a relief valve provided in the bypass line and opening when a differential pressure equal to or greater than a set pressure acts between an inlet and an outlet; The cryogenic refrigerator is characterized in that the target pressure is set to a pressure value smaller than the set pressure, and a difference between the target pressure and the set pressure is within 0.1 MPa.

2. The cryogenic refrigerator according to claim 1 , wherein the controller controls the inverter so as to reduce an operating frequency of the motor when the differential pressure falls below the target pressure.

3. 3. The cryogenic refrigerator according to claim 1, wherein the difference between the target pressure and the set pressure is in a range of 0.03 MPa to 0.07 MPa.

4. A cryogenic refrigerator as described in any one of claims 1 to 3, characterized in that the controller performs the accelerated cooling during the cool-down operation and does not perform the accelerated cooling during the steady-state operation.

5. A cryogenic refrigerator as described in any one of claims 1 to 4, characterized in that the controller controls the inverter so as to increase the operating frequency of the motor during the cool-down operation compared to the steady-state operation.

6. A control method for a cryogenic refrigerator, the cryogenic refrigerator being capable of performing a cool-down operation for rapidly cooling from room temperature to a cryogenic temperature at start-up, and a steady operation for maintaining a state cooled to a cryogenic temperature by the cool-down operation, the cryogenic refrigerator comprising: a compressor; an expander having a motor and driven by the motor; an inverter for controlling an operating frequency of the motor; a high-pressure line connecting the compressor to the expander so as to supply a high-pressure working gas from the compressor to the expander; a low-pressure line connecting the compressor to the expander so as to recover a low-pressure working gas from the expander to the compressor; a bypass line connecting the high-pressure line to the low-pressure line; and a relief valve provided in the bypass line, the method comprising the steps of: Measuring a differential pressure between the high pressure line and the low pressure line; comparing the measured differential pressure between the high pressure line and the low pressure line to a target pressure; controlling the inverter to increase an operating frequency of the motor when the differential pressure exceeds the target pressure; The relief valve opens when a pressure difference between the inlet and outlet is equal to or greater than a set pressure, The method according to claim 1, wherein the target pressure is set to a pressure value smaller than the set pressure, and the difference between the target pressure and the set pressure is within 0.1 MPa.

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