Patent Publication Number: US-2022235984-A1

Title: Cryocooler, and diagnosis device and diagnosis method of cryocooler

Description:
RELATED APPLICATIONS 
     The contents of Japanese Patent Application No. 2019-188402, and of International Patent Application No. PCT/JP2020/037467, on the basis of each of which priority benefits are claimed in an accompanying application data sheet, are in their entirety incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     Certain embodiments of the present invention relate to a cryocooler, and a diagnosis device and a diagnosis method of a cryocooler. 
     Description of Related Art 
     In the related art, there is known a Gifford-McMahon (GM) cryocooler in which an expansion piston is connected to a drive motor via a crank mechanism and can reciprocate in the expansion cylinder. 
     SUMMARY 
     According to an embodiment of the present invention, there is provided a cryocooler including a motor; a displacer; a cylinder that guides linear reciprocating motion of the displacer and forms an expansion chamber for the working gas between the cylinder and the displacer; a pressure switching valve that determines an intake start timing of the working gas into the expansion chamber and an exhaust start timing of the working gas from the expansion chamber; a motion conversion mechanism that converts rotating motion output by the motor into the linear reciprocating motion of the displacer, and includes a first component and a second component slidably connected to each other; a measuring instrument that is connected to the motor to output time-series data indicating power consumption of the motor or a current flowing through the motor; and a processor configured to detect abrasion of a sliding surface between the first component and the second component of the motion conversion mechanism based on section data including the intake start timing or the exhaust start timing in the time-series data. 
     According to another embodiment of the present invention, there is provided a diagnosis device of a cryocooler. The cryocooler includes a motion conversion mechanism that converts rotating motion output by a motor into linear reciprocating motion of a displacer and includes a first component and a second component slidably connected to each other. The diagnosis device includes a measuring instrument that is connected to the motor to output time-series data indicating power consumption of the motor or a current flowing through the motor; and a processor configured to detect abrasion of a sliding surface between the first component and the second component of the motion conversion mechanism based on section data including an intake start timing of a working gas into an expansion chamber of the cryocooler or an exhaust start timing of the working gas from the expansion chamber in the time-series data. 
     According to still another embodiment of the present invention, there is provided a diagnosis method of a cryocooler. The cryocooler includes a motion conversion mechanism that converts rotating motion output by a motor into linear reciprocating motion of a displacer and includes a first component and a second component slidably connected to each other. The method includes acquiring time-series data indicating power consumption of the motor or a current flowing through the motor; and detecting abrasion of a sliding surface between the first component and the second component of the motion conversion mechanism based on section data including an intake start timing of a working gas into an expansion chamber of the cryocooler or an exhaust start timing of the working gas from the expansion chamber in the time-series data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view schematically showing a cryocooler according to an embodiment. 
         FIG. 2  is a view schematically showing the cryocooler according to the embodiment. 
         FIG. 3  is a view showing an exemplary valve timing used in the cryocooler according to the embodiment. 
         FIG. 4A  is a schematic perspective view showing an exemplary motion conversion mechanism, and  FIG. 4B  is an exploded perspective view schematically showing the motion conversion mechanism in  FIG. 4A . 
         FIGS. 5A and 5B  are schematic views showing a rolling bush. 
         FIGS. 6A and 6B  are schematic views showing an operation of a motion conversion mechanism in a cryocooler. 
         FIG. 7  is a block diagram of a diagnosis device according to the embodiment. 
         FIG. 8  is a flowchart showing a diagnosis method of the cryocooler according to the embodiment. 
         FIGS. 9A to 9F  are diagrams showing waveform data obtained when time-series data indicating power consumption of a motor is input to a processing unit according to the embodiment. 
         FIG. 10  is a diagram showing waveform data obtained when time-series data indicating a current flowing through the motor is input to the processing unit according to the embodiment. 
         FIG. 11  is a diagram showing waveform data obtained when time-series data indicating a current flowing through the motor is input to the processing unit according to an embodiment. 
         FIG. 12  is a block diagram of the diagnosis device according to an embodiment. 
         FIG. 13  is a diagram showing waveform data obtained when time-series data indicating a current flowing through the motor is input to the processing unit according to the embodiment. 
         FIG. 14  is a diagram showing waveform data obtained when time-series data indicating a current flowing through the motor is input to the processing unit according to the embodiment. 
         FIG. 15  is a diagram showing waveform data obtained when time-series data indicating a current flowing through the motor is input to the processing unit according to the embodiment. 
         FIG. 16  is a graph plotting the maximum value of a sliding surface abrasion parameter for each of examples 1 to 4. 
     
    
    
     DETAILED DESCRIPTION 
     The present inventor has studied a cryocooler having a built-in motion conversion mechanism, such as a GM cryocooler, and has come to recognize the following fact. In such a cryocooler, as an operation is continued for a long period of time, abrasion of movable components of the motion conversion mechanism may progress, and thus a gap between the components may gradually expand. Consequently, abnormal noise may be generated from the motion conversion mechanism during operation of the cryocooler. This abnormal noise is collision noise between components generated due to backlash between components. As the abrasion progresses, the gap between the components becomes larger, and abnormal noise may become noticeable. This is not desirable as it is often perceived as unpleasant noise by cryocooler users. When the abrasion further progresses, the components will eventually need to be replaced. 
     A cumulative operation time of a cryocooler may be an indicator of the degree of abrasion. For example, abrasion is considered to have occurred after a certain operation time. However, in reality, the progress of abrasion is greatly affected by individual circumstances such as individual differences between cryocoolers and how individual users use the cryocoolers. Thus, a length of the operation time and the degree of abrasion cannot be immediately associated with each other, and it is difficult to accurately identify the progress of abrasion of the components of the motion conversion mechanism from the cumulative operation time. 
     After all, there has been no effective way to automatically detect the abrasion of the motion conversion mechanism built into the cryocooler. 
     It is desirable to provide a diagnosis technique for detecting abrasion of a motion conversion mechanism of a cryocooler. 
     Any combination of the components described above and a combination obtained by replacing the components and expressions of the present invention between methods, devices, and systems are also effective as an embodiment of the present invention. 
     According to the present invention, it is possible to provide a diagnosis technique for detecting abrasion of a motion conversion mechanism of a cryocooler. 
     Hereinafter, an embodiment 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 processing will be assigned with the same reference symbols, and redundant description thereof will be omitted as appropriate. The scales and shapes of shown components are set for convenience in order to make the description easy to understand, and are not to be understood as limiting unless stated otherwise. The embodiment is merely an example and does not limit the scope of the present invention. All characteristics and combinations to be described in the embodiment are not necessarily essential to the invention. 
       FIGS. 1 and 2  are views schematically showing a cryocooler  10  according to an embodiment.  FIG. 3  is a diagram showing an exemplary valve timing used in the cryocooler  10  according to the embodiment.  FIG. 1  shows an appearance of the cryocooler  10 , and  FIG. 2  shows an internal structure of the cryocooler  10 . The cryocooler  10  is, for example, a two-stage type Gifford-McMahon (GM) cryocooler. 
     The cryocooler  10  includes a compressor  12  and an expander  14 . The compressor  12  includes a measuring instrument  50  and a processing unit or a processor  100 . The expander  14  includes a motor  42  and a motion conversion mechanism  43 . Although the details will be described later, a diagnosis device of the motion conversion mechanism  43  is configured with the motor  42 , the measuring instrument  50 , and the processing unit  100 . 
     The compressor  12  is configured to collect a working gas of the cryocooler  10  from the expander  14 , to pressurize the collected working gas, and to supply the working gas to the expander  14  again. The working gas is also called a refrigerant gas, and other suitable gases may be used although a helium gas is typically used. 
     In general, both of the pressure of a working gas supplied from the compressor  12  to the expander  14  and the pressure of a working gas collected from the expander  14  to the compressor  12  are considerably higher than the atmospheric pressure, and can also be called a first high pressure and a second high pressure, respectively. For convenience of description, the first high pressure and the second high pressure are simply called a high pressure and a 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, and is, for example, about 0.8 MPa. For better understanding, a direction in which the working gas flows is shown with arrows. 
     The compressor  12  includes a compressor main body  22  and a compressor casing  23  that houses the compressor main body  22 . The compressor  12  will also be referred to as a compressor unit. 
     The compressor main body  22  is configured to internally compress the working gas sucked from a suction port and to discharge the working gas from a discharge port. The compressor main body  22  may be, for example, a scroll type pump, a rotary type pump, or other pumps that pressurize the working gas. In the embodiment, the compressor main body  22  is configured to discharge the working gas at a fixed and constant flow rate. Alternatively, the compressor main body  22  may be configured to change the flow rate of the working gas to be discharged. The compressor main body  22  will be referred to as a compression capsule in some cases. 
     The compressor  12  may include a compressor controller  24  that controls the compressor  12 . The compressor controller  24  may not only control the compressor  12  but also control the cryocooler  10  in an integrated manner, and may also control, for example, the expander  14  (for example, the motor  42 ). The compressor controller  24  may be attached to the compressor  12 , and may be installed on, for example, an outer surface of the compressor casing  23  and housed in the compressor casing  23 . Alternatively, the compressor controller  24  may be disposed away from the compressor  12  and connected to the compressor  12  via, for example, a control signal line. 
     The expander  14  includes a cryocooler cylinder  16  and a displacer assembly  18 . The cryocooler cylinder  16  guides linear reciprocating motion of the displacer assembly  18  and forms expansion chambers ( 32  and  34 ) for the working gas with the displacer assembly  18 . The expander  14  includes a pressure switching valve  40  that determines an intake start timing of the working gas into the expansion chamber and an exhaust start timing of the working gas from the expansion chamber. 
     In the present specification, in order to describe a positional relationship between components of the cryocooler  10 , for convenience of description, a side close to a top dead center of axial reciprocation of a displacer will be referred to as “up” and a side close to a bottom dead center will be referred to as “down”. The top dead center is the position of the displacer at which the volume of an expansion space is maximum, and the bottom dead center is the position of the displacer at which the volume of the expansion space is minimum. Since a temperature gradient in which the temperature drops from an upper side to a lower side in an axial direction is generated during the operation of the cryocooler  10 , the upper side can also be called a high temperature side and the lower side can also be called a low temperature side. 
     The cryocooler cylinder  16  includes a first cylinder  16   a  and a second cylinder  16   b . The first cylinder  16   a  and the second cylinder  16   b  each are, for example, a member that has a cylindrical shape, and the second cylinder  16   b  has a diameter smaller than that of the first cylinder  16   a . The first cylinder  16   a  and the second cylinder  16   b  are coaxially disposed, and a lower end of the first cylinder  16   a  is strongly connected to an upper end of the second cylinder  16   b.    
     The displacer assembly  18  includes a first displacer  18   a  and a second displacer  18   b  that are connected to each other, and the displacers move integrally. The first displacer  18   a  and the second displacer  18   b  each are, for example, a member that has a cylindrical shape, and the second displacer  18   b  has a diameter smaller than that of the first displacer  18   a . The first displacer  18   a  and the second displacer  18   b  are coaxially disposed. 
     The first displacer  18   a  is accommodated in the first cylinder  16   a , and the second displacer  18   b  is accommodated in the second cylinder  16   b . The first displacer  18   a  can reciprocate in the axial direction along the first cylinder  16   a , and the second displacer  18   b  can reciprocate in the axial direction along the second cylinder  16   b.    
     As shown in  FIG. 2 , the first displacer  18   a  accommodates a first regenerator  26 . The first regenerator  26  is formed by filling a tubular main body portion of the first displacer  18   a  with, for example, a wire mesh made of, such as copper, or other appropriate first regenerator material. An upper lid portion and a lower lid portion of the first displacer  18   a  may be provided as members separate from the main body portion of the first displacer  18   a , or the first regenerator material may be accommodated in the first displacer  18   a  by fixing the upper lid portion and the lower lid portion of the first displacer  18   a  to the main body through appropriate means such as fastening and welding. 
     Similarly, the second displacer  18   b  accommodates a second regenerator  28 . The second regenerator  28  is formed by filling a tubular main body portion of the second displacer  18   b  with, for example, a non-magnetic regenerator material such as bismuth, a magnetic regenerator material such as HoCu 2 , or other appropriate second regenerator material. The second regenerator material may be molded into a granular shape. An upper lid portion and a lower lid portion of the second displacer  18   b  may be provided as members separate from the main body portion of the second displacer  18   b , or the second regenerator material may be accommodated in the second displacer  18   b  by fixing the upper lid portion and the lower lid portion of the second displacer  18   b  to the main body through appropriate means such as fastening and welding. 
     The displacer assembly  18  forms, inside the cryocooler cylinder  16 , a room temperature chamber  30 , a first expansion chamber  32 , and a second expansion chamber  34 . In order to exchange heat with a desired object or medium to be cooled by the cryocooler  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 lid portion of the first displacer  18   a  and an upper portion of the first cylinder  16   a . The first expansion chamber  32  is formed between the lower lid portion of the first displacer  18   a  and the first cooling stage  33 . The second expansion chamber  34  is formed between the lower lid portion of the second displacer  18   b  and the second cooling stage  35 . The first cooling stage  33  is fixed to a lower portion of the first cylinder  16   a  to surround the first expansion chamber  32 , and the second cooling stage  35  is fixed to a lower portion of the second cylinder  16   b  to surround the second expansion chamber  34 . 
     The first regenerator  26  is connected to the room temperature chamber  30  through a working gas flow path  36   a  formed in the upper lid portion of the first displacer  18   a , and is connected to the first expansion chamber  32  through a working gas flow path  36   b  formed in the lower lid portion of the first displacer  18   a . The second regenerator  28  is connected to the first regenerator  26  through a working gas flow path  36   c  formed from the lower lid portion of the first displacer  18   a  to the upper lid portion of the second displacer  18   b . In addition, the second regenerator  28  is connected to the second expansion chamber  34  through a working gas flow path  36   d  formed in the lower lid portion of the second displacer  18   b.    
     In order to introduce working gas flow between the first expansion chamber  32 , the second expansion chamber  34 , and the room temperature chamber  30  to the first regenerator  26  and the second regenerator  28  instead of a clearance between the cryocooler cylinder  16  and the displacer assembly  18 , a first seal  38   a  and a second seal  38   b  may be provided. The first seal  38   a  may be mounted on the upper lid portion of the first displacer  18   a  to be disposed between the first displacer  18   a  and the first cylinder  16   a . The second seal  38   b  may be mounted on the upper lid portion of the second displacer  18   b  to be disposed between the second displacer  18   b  and the second cylinder  16   b.    
     As shown in  FIG. 1 , the expander  14  includes a cryocooler housing  20  that accommodates the pressure switching valve  40 . The cryocooler housing  20  is coupled to the cryocooler cylinder  16 , and accordingly a hermetic container that accommodates the pressure switching valve  40  and the displacer assembly  18  is configured. 
     As shown in  FIG. 2 , the pressure switching valve  40  is configured to include a high pressure valve  40   a  and a low pressure valve  40   b  and to generate periodic pressure fluctuations in the cryocooler cylinder  16 . A working gas discharge port of the compressor  12  is connected to the room temperature chamber  30  via the high pressure valve  40   a , and a working gas suction port of the compressor  12  is connected to the room temperature chamber  30  via the low pressure valve  40   b . The high pressure valve  40   a  and the low pressure valve  40   b  are configured to open and close selectively and alternately (that is, such that when one is open, the other is closed). 
       FIG. 3  shows a valve timing of the pressure switching valve  40 . One rotation of the pressure switching valve  40 , that is, one refrigeration cycle of the cryocooler  10 , includes an intake step A 1  and an exhaust step A 2 . Since one refrigeration cycle is shown in association with 360 degrees, 0 degrees corresponds to the start time of the cycle and 360 degrees corresponds to the end time of the cycle. 90 degrees, 180 degrees, and 270 degrees correspond to ¼ cycle, half cycle, and ¾ cycle, respectively. Here, for convenience, as an example without limitation, the start of the intake step A 1  is set to 0 degrees, and the start of the exhaust step A 2  is set to 180 degrees. 
     The high pressure valve  40   a  sets an intake start timing T 1 . That is, the intake step A 1  is started when the high pressure valve  40   a  is opened. In the intake step A 1 , the low pressure valve  40   b  is closed. A high pressure working gas flows from the compressor  12  into the room temperature chamber  30  through the high pressure valve  40   a , 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 . The pressures in the first expansion chamber  32  and the second expansion chamber  34  rapidly increase at the intake start timing T 1 . When the high pressure valve  40   a  is closed, the intake step A 1  ends. The first expansion chamber  32  and the second expansion chamber  34  are maintained at a high pressure. 
     The low pressure valve  40   b  sets an exhaust start timing T 2 . That is, an exhaust step A 2  is started when the low pressure valve  40   b  is opened. In the exhaust step A 2 , the high pressure valve  40   a  is closed. Since the high pressure first expansion chamber  32  and the high pressure second expansion chamber  34  are opened to the low pressure working gas suction port of the compressor  12  at the exhaust start timing T 2 , the working gas is expanded in the first expansion chamber  32  and the second expansion chamber  34 , and the working gas which has a low pressure as a result 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 . The pressures in the first expansion chamber  32  and the second expansion chamber  34  rapidly decrease at the exhaust start timing T 2 . The working gas is collected from the expander  14  to the compressor  12  through the low pressure valve  40   b . When the low pressure valve  40   b  is closed, the exhaust step A 2  ends. The first expansion chamber  32  and the second expansion chamber  34  are maintained at a low pressure. 
     As shown in  FIG. 3 , there may be a period in which both the high pressure valve  40   a  and the low pressure valve  40   b  are closed from the end of the intake step A 1  to the start of the exhaust step A 2 . There may be a period in which both the high pressure valve  40   a  and the low pressure valve  40   b  are closed from the end of the exhaust step A 2  to the start of the intake step A 1 . 
     The pressure switching valve  40  may take a form of a rotary valve. That is, the pressure switching valve  40  may be configured such that the high pressure valve  40   a  and the low pressure valve  40   b  are alternately opened and closed by rotational sliding of a valve disk with respect to a stationary valve main body. In this case, the motor  42  may be connected to the pressure switching valve  40  to rotate the valve disk of the pressure switching valve  40 . For example, the pressure switching valve  40  is disposed such that a valve rotation axis is coaxial with a rotation axis of the motor  42 . 
     Alternatively, the high pressure valve  40   a  and the low pressure valve  40   b  each may be a valve that can be individually controlled, and in this case, the pressure switching valve  40  may not be connected to the motor  42 . 
       FIGS. 1 and 2  will be referred to again. The motor  42  is attached to the cryocooler housing  20 . The motion conversion mechanism  43  is accommodated in the cryocooler housing  20  like the pressure switching valve  40 . 
     For example, the motor  42  is connected to a displacer drive shaft  44  via a motion conversion mechanism  43  such as a scotch yoke mechanism. The motion conversion mechanism  43  converts rotating motion output by the motor  42  into 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 lid portion of the first displacer  18   a . The rotation of the motor  42  is converted into the axial reciprocation of the displacer drive shaft  44  by the motion conversion mechanism  43 , and the displacer assembly  18  linearly reciprocates in the axial direction in the cryocooler cylinder  16 . 
     Incidentally, the cryocooler  10  is supplied with power from a power source  46  such as a commercial power source (three-phase alternating current power source). The power source  46  is connected to the compressor  12  and the motor  42  via a power supply wiring  48 . Since the motor  42  is connected to the power source  46  via the compressor  12 , the compressor  12  may also be regarded as a power source of the motor  42 . The compressor  12  and the motor  42  may be connected to individual power sources. 
     The motor  42  is, for example, a three-phase motor. The motor  42  operates at a constant rotation speed based on the frequency of the power source  46 . 
     The measuring instrument  50  is connected to the motor  42  such that time-series data D 1  indicating power consumption of the motor  42  or a current flowing through the motor  42  is output. Therefore, the time-series data D 1  indicates a time change of the power consumption of the motor  42  or the current flowing through the motor  42  during the operation of the cryocooler  10 . The measuring instrument  50  is installed at the power supply wiring  48  in order to acquire the time-series data D 1 . 
     As an exemplary configuration, the measuring instrument  50  may employ, for example, a three-phase wattmeter based on the two-power metering method, or may be another type of power sensor that measures the power consumption of the motor  42 . Alternatively, the measuring instrument  50  may be a three-phase ammeter that simultaneously measures three-phase currents flowing through the motor  42  individually, or may be another type of current sensor that measures a current flowing through the motor  42 . 
     The measuring instrument  50  outputs the time-series data D 1  to the processing unit  100 . The measuring instrument  50  is communicatively connected to the processing unit  100  by wire or wirelessly. In the illustrated example, the measuring instrument  50  is built in the compressor  12 , but the present embodiment is not limited to this. The measuring instrument  50  may be provided in the expander  14 , such as mounted on the motor  42 , or may be provided in another location on the power supply wiring  48 . 
     The processing unit  100  is configured to receive the time-series data D 1  from the measuring instrument  50  and diagnose the motion conversion mechanism  43  on the basis of the time-series data D 1 . The processing unit  100  is mounted on the compressor  12  and configures a part of the compressor controller  24 , but the present embodiment is not limited to this. The processing unit  100  may be disposed away from the compressor  12 , and in that case, may be connected to the measuring instrument  50  via a signal wiring. The processing unit  100  may be mounted on the expander  14 . However, the processing unit  100  is disposed in a room temperature environment such as the cryocooler housing  20 . Details of the processing unit  100  will be described later. 
     When the compressor  12  and the motor  42  are operated, the cryocooler  10  causes periodic volume fluctuations in the first expansion chamber  32  and the second expansion chamber  34  and pressure fluctuations of the working gas in synchronization therewith. Typically, the displacer assembly  18  is moved up from the bottom dead center to the top dead center in the intake step A 1  to increase the volumes of the first expansion chamber  32  and the second expansion chamber  34 , and the displacer assembly  18  is moved down in the exhaust step A 2  from the top dead center to the bottom dead center to reduce the volumes of the first expansion chamber  32  and the second expansion chamber  34 . 
     As described above, for example, a refrigeration cycle such as a GM cycle is provided, and the first cooling stage  33  and the second cooling stage  35  are cooled to a desired cryogenic temperature. The first cooling stage  33  may be cooled to a first cooling temperature within a range of, for example, about 20 K to about 40 K. The second cooling stage  35  may be cooled to a second cooling temperature (for example, about 1 K to about 4 K) lower than the first cooling temperature. 
       FIG. 4A  is a schematic perspective view showing an exemplary motion conversion mechanism  43 .  FIG. 4B  is an exploded perspective view schematically showing the motion conversion mechanism  43  in  FIG. 4A . The shown motion conversion mechanism  43  is configured as a scotch yoke mechanism. The motion conversion mechanism  43  includes a crank  60  and a scotch yoke  70 . The crank  60  is fixed to a rotary shaft  42   a  of the motor  42 . The scotch yoke  70  is disposed on a side opposite to the rotary shaft  42   a  of the motor  42  with respect to the crank  60 . The crank  60  has a connecting shaft  62  eccentrically connected to the rotary shaft  42   a . The connecting shaft  62  extends from the crank  60  toward the scotch yoke  70  in parallel to the rotary shaft  42   a . The rotary shaft  42   a  and the connecting shaft  62  extend along an axis X. 
     The scotch yoke  70  includes a yoke plate  72  and a rolling element (hereinafter, also referred to as a rolling bush)  74 , and is movable in an axial direction (indicated by an arrow Z) orthogonal to the axis X. An upper shaft  45  and a displacer drive shaft  44  are fixed to the yoke plate  72 . The upper shaft  45  extends upward from the center of an upper frame of the yoke plate  72 , and the displacer drive shaft  44  extends downward from the center of a lower frame of the yoke plate  72 . The upper shaft  45  and the displacer drive shaft  44  each are supported by the cryocooler housing  20  (refer to  FIG. 1 ) so as to be slidable in the axial direction. 
     The yoke plate  72  has a laterally elongated yoke window  72   a  (indicated by an arrow Y) orthogonal to the axis X and the axial direction Z. The rolling bush  74  is disposed in the yoke window  72   a . The rolling bush  74  has a shaft hole  74   a  at the center, and the connecting shaft  62  penetrates through the shaft hole  74   a . The connecting shaft  62  is in sliding contact with the rolling bush  74  through the shaft hole  74   a , and the connecting shaft  62  and the rolling bush  74  are slidably connected to each other through the shaft hole  74   a . The rolling bush  74  acts as a non-lubricated sliding bearing that supports the connecting shaft  62 . The rolling bush  74  is in rolling contact with the yoke plate  72  at the yoke window  72   a , and the rolling bush  74  is rolled and slidably connected to the yoke plate  72  at the yoke window  72   a.    
     When the rotary shaft  42   a  rotates due to the drive of the motor  42 , the crank  60  rotates together with the rotary shaft  42   a , and the connecting shaft  62  and the rolling bush  74  connected to the connecting shaft  62  rotate in a circle around the rotary shaft  42   a . In this case, the connecting shaft  62  slides while rotating with respect to the rolling bush  74  in the shaft hole  74   a . The rolling bush  74  reciprocates in the lateral direction Y while rolling in the yoke window  72   a , and reciprocates in the axial direction Z together with the yoke plate  72 . The axial reciprocation of the yoke plate  72  causes the displacer drive shaft  44  and the displacer assembly  18  to reciprocate in the axial direction. As described above, the rotating motion output by the motor  42  is converted into the linear reciprocating motion of the displacer. 
     The connecting shaft  62  may further extend through the shaft hole  74   a . In a case where the pressure switching valve  40  is configured as a rotary valve, a tip  62   a  of the connecting shaft  62  is connected to a valve disk  41   a  of the pressure switching valve  40 , and the valve disk  41   a  rotates with respect to a stationary valve main body  41   b  due to rotation of the crank  60 . Therefore, the pressure switching valve  40  can rotate in synchronization with the motion conversion mechanism  43 . 
       FIGS. 5A and 5B  are schematic views showing the rolling bush  74 . As shown in  FIG. 5A , the rolling bush  74  is a disk-shaped member having the circular shaft hole  74   a . As described above, since the shaft hole  74   a  is a sliding surface on which the connecting shaft  62  slides, the rolling bush  74  is made of a resin material having excellent abrasion resistance, such as fluororesin. In this case, an outer peripheral surface  74   b  of the rolling bush  74 , which is a rolling sliding surface with respect to the yoke plate  72 , is also made of an abrasion-resistant material. The abrasion-resistant rolling bush  74  can be provided. 
     As shown in  FIG. 5B , the rolling bush  74  may include a bush inner ring  76  having the circular shaft hole  74   a  and a bush outer ring  78  having the outer peripheral surface  74   b . The bush inner ring  76  and the bush outer ring  78  are coaxially disposition, and the bush inner ring  76  is fixed to the bush outer ring  78 . The bush inner ring  76  is made of a resin material having excellent abrasion resistance, such as fluororesin. The bush outer ring  78  is made of a material different from that of the bush inner ring  76 , such as a general-purpose resin material. Since the abrasion-resistant material is relatively expensive, the rolling bush  74  can be made inexpensive by using the abrasion-resistant material for only a part of the rolling bush  74 . 
       FIGS. 6A and 6B  are schematic views showing an operation of the motion conversion mechanism  43  in the cryocooler  10 . In the newly manufactured cryocooler  10 , components of the motion conversion mechanism  43  are combined with each other with design tolerances, and there is no unnecessary backlash between the components. However, as the cryocooler  10  is operated for a long period of time, abrasion of the movable components of the motion conversion mechanism  43  progresses. The sliding surface between the components is prone to abrasion, and, thus, for example, the shaft hole  74   a  of the rolling bush  74  gradually expands such that a gap  80  is generated between the rolling bush  74  and the connecting shaft  62 . 
       FIG. 6A  shows that the scotch yoke  70  is approaching the bottom dead center at the end of the exhaust step A 2 . Since the connecting shaft  62  is rotating and pushing the rolling bush  74  and the yoke plate  72  downward, the gap  80  is above the connecting shaft  62  in the shaft hole  74   a . In this case, the first expansion chamber  32  and the second expansion chamber  34  of the expander  14  are filled with the low pressure working gas. 
     Assuming that the intake start timing T 1  arrives immediately after this and the intake step A 1  starts, the high pressure working gas flows from the high pressure valve  40   a  into the room temperature chamber  30  as described above. Until the inflowing gas flows into the first expansion chamber  32  and the second expansion chamber  34 , the differential pressure between the room temperature chamber  30  and these expansion chambers acts downward on the displacer assembly  18 . The scotch yoke  70  is fixed to the displacer assembly  18 . 
     Thus, at the intake start timing T 1 , as shown in  FIG. 6B , a downward force  82  transiently acts on the scotch yoke  70 . Consequently, the scotch yoke  70  moves with respect to the connecting shaft  62  by a size of the gap  80 . The connecting shaft  62  may collide with the rolling bush  74  in the shaft hole  74   a , and thus abnormal noise may be generated. 
     The direction of the force is reversed upside down, but the same phenomenon may occur at the exhaust start timing T 2 . When the exhaust step A 2  starts, a transient differential pressure acts on the displacer assembly  18  in the expander  14 , and this force acts upward on the scotch yoke  70 , and the scotch yoke  70  moves with respect to the connecting shaft  62  by the size of the gap  80 . The connecting shaft  62  may collide with the rolling bush  74  in the shaft hole  74   a , and thus abnormal noise may be generated. 
     However, since the cryocooler  10  is usually installed with the low temperature side facing downward, the influence of the upward force acting on the scotch yoke  70  is alleviated by the gravity (that is, the downward force) acting on the displacer assembly  18 . Therefore, the abnormal noise may be louder at the intake start timing T 1  than at the exhaust start timing T 2 . 
     As described above, during the operation of the cryocooler  10 , especially when the intake and exhaust of the working gas are switched, the direction of the gas pressure acting on the motion conversion mechanism  43  is reversed, and thus abnormal noise may be generated from the motion conversion mechanism  43 . Abnormal noise may also be generated when the motion direction of the motion conversion mechanism  43  is reversed. As the abrasion progresses, the gap  80  also becomes larger, and abnormal noise may become noticeable. In the typical operation of the cryocooler  10 , the intake start timing T 1  is as high as once per second. Such frequent occurrence of abnormal noise may be offensive to cryocooler users. Even if the cryocooler  10  is operated in an unmanned environment, such frequent collision between the components may adversely affect the life of the motion conversion mechanism  43 . 
     A method of estimating the progress of abrasion on the basis of the cumulative operation time of the cryocooler  10  is not very practical because the progress of abrasion differs depending on individual cryocoolers, as described at the beginning of the present specification. 
     A typical cryocooler may be provided with an ammeter that measures a motor current in order to detect an abnormal increase in the motor current that may occur when an abnormally large load is applied to the motor. However, since the expansion of the gap  80  due to abrasion does not increase a load on the motor  42 , the abrasion of the motion conversion mechanism  43  cannot be effectively detected even by this method. 
       FIG. 7  is a block diagram of a diagnosis device according to the embodiment. The diagnosis device of the motion conversion mechanism  43  includes the motor  42 , the measuring instrument  50 , and the processing unit  100 . The processing unit  100  includes a memory  102 , a parameter calculation unit  104 , and a comparison unit  110 . The diagnosis device may include notification means  120  for providing a visual notification of information indicating a diagnosis result, and the notification means  120  may include, for example, a display  122 . The notification means  120  may provide a notification of a diagnosis result by voice such as using a speaker. The notification means  120  may transmit a diagnosis result to a remote device via a network such as the Internet. 
     The processing unit  100  detects abrasion of a sliding surface between a first component and a second component of the motion conversion mechanism  43  on the basis of section data D 2  including the intake start timing T 1  or the exhaust start timing T 2  in the time-series data D 1 . In the present embodiment, the processing unit  100  detects abrasion of the sliding surface of the motion conversion mechanism  43  on the basis of the section data D 2  over at least one cycle of the linear reciprocating motion of the displacer in the time-series data D 1 . The first component and the second component are, for example, the connecting shaft  62  and the rolling bush  74 . The processing unit  100  calculates a sliding surface abrasion parameter D 4  on the basis of the section data D 2 , and detects the abrasion of the sliding surface on the basis of comparison between the sliding surface abrasion parameter D 4  and a parameter threshold value. 
     The measuring instrument  50  outputs the time-series data D 1  indicating power consumption of the motor  42  or a current flowing through the motor  42  to the memory  102 . The memory  102  stores the time-series data D 1 . In addition to the time-series data D 1 , the memory  102  may store or preserve in advance various pieces of output data intermediately or finally generated or output by the processing unit  100 , or data related to the cryocooler  10 . 
     The parameter calculation unit  104  reads the section data D 2  from the memory  102  and calculates the sliding surface abrasion parameter D 4  on the basis of the section data D 2 . As described above, the section data D 2  corresponds to data measured for a time corresponding to one cycle (typically, for example, about 1 second) of the linear reciprocating motion (that is, the refrigeration cycle) of the displacer in the time-series data D 1 . In a case where the intake start timing T 1  (or the exhaust start timing T 2 ) can be specified in the time-series data D 1 , the data measured for a predetermined time including the intake start timing T 1  (or the exhaust start timing T 2 ) in the time-series data D 1  may be used as the section data D 2 . 
     In a case where the time-series data D 1  indicates the power consumption of the motor  42 , the parameter calculation unit  104  may calculate the sliding surface abrasion parameter D 4  by performing a smoothing process and time differentiation on the section data D 2 . Therefore, the parameter calculation unit  104  may include a smoothing unit  106  and a differentiation calculation unit  108 . The smoothing unit  106  performs a smoothing process on the section data D 2  to generate smoothed section data D 3 . The differentiation calculation unit  108  performs time differentiation (for example, primary differentiation) on the smoothed section data D 3  to calculate the sliding surface abrasion parameter D 4 . 
     The smoothing process may include a process of taking a moving average of the section data D 2  in a time frame based on a cycle of a power supply frequency (for example, 50 Hz or 60 Hz) of the motor  42 . Therefore, the smoothing unit  106  takes a moving average of the section data D 2  for a time length of, for example, one cycle (or an integer multiple thereof) of the power supply frequency of the motor  42 , and generates the smoothed section data D 3 . Consequently, a ripple corresponding to the power supply frequency of the motor  42  included in the section data D 2  can be effectively removed. The smoothing unit  106  may include other suitable smoothing filter that remove noise. 
     The time differentiation means a process of differentiating waveform data input to the differentiation calculation unit  108  with respect to time or a variable corresponding to time. The variable corresponding to time may be, for example, an operation angle of the cryocooler  10 . The operation angle is perfectly associated with time. For example, as described with reference to  FIG. 3 , one refrigeration cycle of the cryocooler  10  is associated with an operation angle of 360 degrees. 
     The time-series data D 1 , that is, the section data D 2  is often discrete data. In that case, the differentiation calculation unit  108  performs a differential process on the smoothed section data D 3  and calculates the sliding surface abrasion parameter D 4 . For example, a time differentiation ΔP ave /Δt of the moving average P ave  of the power consumption of the motor  42  is calculated according to ΔP ave /Δt=(P ave (t)−P ave (t′))/(t−t′) when a measured value of the power consumption at the measurement time t is set to P ave (t) and a measured value of the power consumption at the next measurement time t is set to P ave (t′). A value of the time differentiation ΔP ave /Δt obtained as described above is used as the sliding surface abrasion parameter D 4 . An absolute value |ΔP ave /Δt| of the time differentiation may be used as the sliding surface abrasion parameter D 4 . 
     In a case where the time-series data D 1  indicates the current flowing through the motor  42 , the parameter calculation unit  104  may calculate the sliding surface abrasion parameter D 4  by performing a smoothing process on the section data D 2 . The smoothing unit  106  performs a smoothing process on the section data D 2 , and outputs the smoothed section data D 3  as the sliding surface abrasion parameter D 4 . The processing unit  100  does not have to include the differentiation calculation unit  108 . 
     In this case, only one phase of the measured three-phase currents may be used as the section data D 2 . Alternatively, two-phase or three-phase currents may be used as the section data D 2 . The smoothing unit  106  performs a smoothing process on each of the two-phase or three-phase currents, and may output one of the smoothed two-phase or three-phase currents, or a maximum value or an average value thereof as the sliding surface abrasion parameter D 4 . 
     The comparison unit  110  generates abrasion diagnosis data D 5  on the basis of comparison between the sliding surface abrasion parameter D 4  and the parameter threshold value. The abrasion diagnosis data D 5  indicates whether or not abrasion is detected on the sliding surface between the first component and the second component of the motion conversion mechanism  43 . The parameter threshold value is preset and stored in the memory  102 . The parameter threshold value may be set as appropriate on the basis of empirical knowledge of a designer, experiments or simulations by the designer, or the like. 
     The abrasion diagnosis data D 5  is sent to the notification means  120 , and a user is notified of a diagnosis result by displaying the diagnosis result, for example, on the display  122 . In a case where abrasion is detected, the notification means  120  may notify a user with an alarm sound. Instead of (or with) providing an immediate notification as described above, the abrasion diagnosis data D 5  may be stored in the memory  102  such that the data can be presented to the user as necessary. 
     An internal configuration of the processing unit  100  is realized by an element or a circuit including a CPU and a memory of a computer as a hardware configuration and is realized by a computer program as a software configuration, but is shown in  FIG. 1  as a functional block realized in cooperation therebetween. It is clear for those skilled in the art that such a functional block can be realized in various manners through combination between hardware and software. 
     For example, the processing unit  100  can be implemented by combining a processor (hardware) such as a central processing unit (CPU) or a microcomputer and a software program executed by the processor (hardware). Such a hardware processor may be configured by a programmable logic device such as a field programmable gate array (FPGA), or may be a control circuit such as a programmable logic controller (PLC). The software program may be a computer program causing the processing unit  100  to perform diagnosis on the cryocooler  10 . 
       FIG. 8  is a flowchart showing a diagnosis method of the cryocooler  10  according to the embodiment. First, as shown in  FIG. 8 , during the operation of the cryocooler  10 , the time-series data D 1  indicating power consumption of the motor  42  or a current flowing through the motor is acquired (S 10 ). Then, abrasion of the sliding surface between the first component and the second component of the motion conversion mechanism  43  is detected on the basis of the section data D 2  (S 20 ). 
     In S 20 , the sliding surface abrasion parameter D 4  is calculated on the basis of the section data D 2  (S 21 ). The calculated sliding surface abrasion parameter D 4  is compared with the parameter threshold value M (S 22 ). In a case where the sliding surface abrasion parameter D 4  exceeds the parameter threshold value M (Y in S 22 ), the comparison unit  110  determines that abrasion has occurred on the sliding surface (S 23 ), and outputs the abrasion diagnosis data D 5  indicating that fact. In a case where the sliding surface abrasion parameter D 4  is equal to or less than the parameter threshold value M (N in S 22 ), the comparison unit  110  determines that no abrasion has occurred on the sliding surface (S 24 ), and outputs the abrasion diagnosis data D 5  indicating that fact. In this way, the diagnosis process ends. 
     The processing unit  100  periodically and repeatedly executes such a diagnosis process. Since abrasion of the sliding surface of the motion conversion mechanism  43  is a long-term phenomenon that gradually progresses over a long span, the diagnosis process is practically sufficient when the diagnosis method is performed occasionally during the operation of the cryocooler  10 . Alternatively, the diagnosis process may be performed at all times during the operation of the cryocooler  10 . 
     In order to avoid misdiagnosis due to noise, the comparison unit  110  may determine that abrasion has occurred on the sliding surface in a case where the sliding surface sliding surface abrasion parameter D 4  exceeds the parameter threshold value M continuously for a certain period of time, determine that no abrasion has occurred on the sliding surface is not abrasion in other cases. The comparison unit  110  may calculate the maximum value of the sliding surface abrasion parameter D 4  for a plurality of pieces of (for example, 10 or more or 100 or more) section data D 2 , and in a case where all of these values exceed the threshold value, determine that abrasion has occurred on the sliding surface. The plurality of pieces of section data D 2  may be acquired at different timings, and may be acquired, for example, during a plurality of consecutive reciprocating motions of the displacer. Each piece of section data D 2  includes the intake start timing T 1  (or the exhaust start timing T 2 ). 
       FIGS. 9A to 9F  are diagrams showing waveform data obtained when the time-series data D 1  indicating the power consumption of the motor  42  is input to the processing unit  100  according to the embodiment. The signal waveform shown in each figure is based on the power consumption of the motor  42  for one cycle (that is, 360 degrees) measured by the measuring instrument  50 . The intake start timing T 1  is set to about 300 degrees, and the exhaust start timing T 2  is set to about 120 degrees. 
       FIGS. 9A, 9B, and 9C  show the section data D 2 , the smoothed section data D 3 , and the sliding surface abrasion parameter D 4 , respectively. These signal waveforms are obtained by performing a diagnosis process on the cryocooler  10  that operates normally (that is, there is no abrasion in the motion conversion mechanism  43  and there is no unnecessary backlash between the connecting shaft  62  and the rolling bush  74 ). 
     From the time-series data D 1 , the section data D 2  for one refrigeration cycle of the cryocooler  10  is acquired. As shown in  FIG. 9A , the section data D 2  vibrates finely because a ripple corresponding to the power supply frequency occurs. The ripple is removed through a smoothing process, and as shown in  FIG. 9B , the smoothed section data D 3  is obtained. The section data D 3  is smoothed by taking a moving average of the section data D 2  with a time length of one cycle of the power supply frequency of the motor  42 . The smoothed section data D 3  indicates fluctuations in power consumption according to an operating state such as a load of the motor  42 . By performing time differentiation on the smoothed section data D 3 , the sliding surface abrasion parameter D 4  shown in  FIG. 9C  is obtained. 
     It can be seen that the sliding surface abrasion parameter D 4  has a substantially constant value near zero in the normal (sufficiently small degree of abrasion) cryocooler  10 . In this case, the sliding surface abrasion parameter D 4  does not exceed the parameter threshold value M. 
       FIGS. 9D, 9E, and 9F  show the section data D 2 , the smoothed section data D 3 , and the sliding surface abrasion parameter D 4 , respectively. However, these are obtained by performing a diagnosis process on the cryocooler  10  in which abrasion of the sliding surface of the motion conversion mechanism  43  has already progressed. In the cryocooler  10 , a certain amount of abnormal noise is generated due to backlash between the connecting shaft  62  and the rolling bush  74  during operation. 
     In the same manner as in the normal cryocooler  10 , the section data D 2  shown in  FIG. 9D  is oscillating, and is subjected to a smoothing process such that the smoothed section data D 3  is obtained as shown in  FIG. 9E . By performing time differentiation on the smoothed section data D 3 , the sliding surface abrasion parameter D 4  shown in  FIG. 9F  is obtained. 
     As shown in  FIG. 9F , in the period other than the intake start timing T 1 , the sliding surface abrasion parameter D 4  has a substantially constant value near zero, as in the normal case. However, the sliding surface abrasion parameter D 4  remarkably fluctuates at the intake start timing T 1  and exceeds the parameter threshold value M. It is considered that this large fluctuation is caused by switching between intake and exhaust of the working gas in the cryocooler  10  and the backlash between the components of the motion conversion mechanism  43 . Therefore, it is possible to detect abrasion of the sliding surface of the motion conversion mechanism  43  on the basis of the sliding surface abrasion parameter D 4  at the intake start timing T 1 . 
       FIGS. 10 and 11  are diagrams showing waveform data obtained when time-series data D 1  indicating a current flowing through the motor  42  is input to the processing unit according to the embodiment.  FIG. 10  shows the sliding surface abrasion parameter D 4  for the normal cryocooler  10 , and  FIG. 11  shows the sliding surface abrasion parameter D 4  for the cryocooler  10  in which abrasion has progressed. 
     From the time-series data D 1  of three-phase currents (a U-phase, a V-phase, and a W-phase) of the motor  42  measured by the measuring instrument  50 , the section data D 2  for one refrigeration cycle of the cryocooler  10  is acquired. The section data D 2  is smoothed, for example, by taking a moving average for the time length of one cycle of the power supply frequency of the motor  42 . The smoothed section data D 3  is used as the sliding surface abrasion parameter D 4 . 
     As shown in  FIG. 10 , the sliding surface abrasion parameter D 4  is near zero in the normal cryocooler  10 . The sliding surface abrasion parameter D 4  does not exceed the parameter threshold value M. 
     On the other hand, as shown in  FIG. 11 , in a case where abrasion has occurred on the sliding surface of the motion conversion mechanism  43 , the sliding surface abrasion parameter D 4  remarkably fluctuates at the intake start timing T 1  and exceeds the parameter threshold value M. In the period other than the intake start timing T 1 , the sliding surface abrasion parameter D 4  remains near zero in the same manner as in the normal case. Therefore, it is possible to detect abrasion of the sliding surface of the motion conversion mechanism  43  on the basis of the sliding surface abrasion parameter D 4  at the intake start timing T 1 . 
     As described above, according to the embodiment, the cryocooler  10  can measure power consumption of the motor  42  or a current flowing through the motor  42  at the intake start timing T 1 , and detect abrasion of the motion conversion mechanism  43  on the basis of the measurement result. 
     As described above, even at the exhaust start timing T 2 , the pressure of the working gas may act on the backlash between the components existing in the motion conversion mechanism  43 . Therefore, depending on the specifications and operation conditions of the cryocooler  10 , it is possible to detect abrasion of the motion conversion mechanism  43  on the basis of a measurement result at the exhaust start timing T 2 . 
     In a case where the progress of abrasion of the sliding components is left unattended, the cryocooler  10  may eventually fail. In a case where the cryocooler  10  fails, an operation of a cryogenic system (for example, a superconductivity equipment or an MRI system) that uses the cryocooler  10  is required to be stopped until maintenance such as repair of the cryocooler or replacement with a new one is completed. In the case of a sudden failure, the time required for recovery tends to be relatively long. 
     However, according to the embodiment, it is possible to diagnose the sliding components of the cryocooler  10  and notify a user of the cryocooler  10  or a service person who performs maintenance of the cryocooler  10  of a diagnosis result. It is possible to take measures to minimize the impact on an operation of the cryogenic system on the basis of the diagnosis result. 
     The sliding surface abrasion parameter D 4  shown in  FIGS. 9F and 11  indicates the experimental results for the cryocooler  10  in which abnormal noise has been actually generated. However, it is assumed that the sliding surface abrasion parameter D 4  will fluctuate in the same manner as the abrasion progresses, even before abnormal noise is generated. Therefore, according to the embodiment, it is expected that abrasion can be detected before abnormal noise is generated. By performing maintenance of the cryocooler  10  at that time, abnormal noise can be prevented. 
     In the embodiment, it is not intended to diagnose a failure of the motor  42  itself. According to the embodiment, it is possible to diagnose the components of the motion conversion mechanism  43  instead of the motor  42  by using the motor  42  and the measuring instrument  50  that monitors an operation of the motor  42 . 
     The motor  42  of the cryocooler  10  is often provided with a sensor that measures power consumption of the motor  42  or a current flowing through the motor  42 , like the measuring instrument  50 . Therefore, the embodiment is also advantageous in that the motion conversion mechanism  43  can be diagnosed without adding a new sensor to the cryocooler  10 . 
     According to the embodiment, the diagnosis process is performed on the basis of the section data D 2  over at least one cycle of the linear reciprocating motion of the displacer in the time-series data D 1 . In the above-described way, it is not necessary to specify the intake start timing T 1  (or the exhaust start timing T 2 ) when the measuring instrument  50  performs measurement (or when the section data D 2  is generated). In order to detect these intake/exhaust switching timings (T 1  and T 2 ), a timing detection sensor such as a working gas pressure sensor in the cryocooler cylinder  16  may be required, but the embodiment is advantageous in that such a timing detection sensor does not have to be newly provided in the cryocooler  10 . The cryocooler  10  may be provided with a timing detection sensor. 
     In the above-described embodiment, a case where a rotation speed of the motor  42  is kept constant has been described, but a rotation speed of the motor  42  may be variable. Since power consumption or a current of the motor  42  may change when a motor rotation speed changes, the sliding surface abrasion parameter D 4  may also change due to the influence thereof. This may cause an error in detecting abrasion of the motion conversion mechanism  43 . Therefore, in order to reduce such an error, the processing unit  100  may monitor a rotation speed of the motor  42 . For example, the processing unit  100  may start the above diagnosis process when a rotation speed of the motor  42  is kept constant. Alternatively, in a case where a rotation speed of the motor  42  is kept constant (for example, when a fluctuation of the rotation speed is less than a threshold value) during execution of the diagnosis process, the processing unit  100  may continue the diagnosis process and in a case where the rotation speed of the motor  42  fluctuates (for example, in a case where the rotation speed fluctuation is more than the threshold value), stop the diagnosis process. 
       FIG. 12  is a block diagram of the diagnosis device according to the embodiment. In the present embodiment, a cryocooler  10  is different from the cryocooler  10  of the above embodiment with reference to  FIGS. 1 to 11  in that an inverter  90  that controls a rotation speed of the motor  42  of the expander  14  is provided. The inverter  90  is installed on the power supply wiring  48  that connects the compressor  12  as a power source of the motor  42  to the motor  42 . The motor  42  can operate at a rotation speed corresponding to an output frequency of the inverter  90  (also called an operation frequency of the cryocooler  10 ). 
     A diagnosis device  200  shown in  FIG. 12  is configured as a diagnosis device of the motion conversion mechanism  43  as in the above-described embodiment, and includes the motor  42  and a diagnosis unit  202 . The diagnosis unit  202  includes a measuring instrument  50  and a processing unit or a processor  100  together with the inverter  90 . An internal configuration of the processing unit  100  may have the same configuration as, for example, that of the processing unit  100  shown in  FIG. 7 . The diagnosis unit  202  may include notification means  120  for providing a notification (for example, visual notification) of information indicating a diagnosis result. 
     The measuring instrument  50  is installed on the power supply wiring  48  between the inverter  90  and the motor  42 , and is configured to output time-series data D 1  indicating a current flowing through the motor  42  to the processing unit  100 . For example, the measuring instrument  50  may be configured to individually and simultaneously measure three-phase currents output from the inverter  90  to the motor  42 , and to output, for example, a voltage signal indicating a magnitude of each of the measured three-phase currents as the time-series data D 1  to the processing unit  100 . 
     The inverter  90  is configured to output, to the processing unit  100 , output frequency information D 6  indicating an output frequency of the inverter  90 . For example, the output frequency of the inverter  90  may change in a range of 30 Hz to 100 Hz. 
     Alternatively, instead of the processing unit  100  receiving the output frequency information D 6  from the inverter  90 , the processing unit  100  may calculate the output frequency information D 6  from the time-series data D 1  input from the measuring instrument  50 . For example, the processing unit  100  may calculate the output frequency of the inverter  90  by counting the number of current peaks per unit time from a waveform of a current flowing through the motor  42 . 
     In order to reduce or prevent adverse effects on the motor  42  due to radio frequency noise that may be generated by the inverter  90 , a noise suppression component such as a ferrite core may be provided on the power supply wiring  48  (for example, between the inverter  90  and the measuring instrument  50 ). In order to reduce or prevent adverse effects on the measuring instrument  50  due to radio frequency noise that may be generated by the inverter  90 , a conductive shielding plate that surrounds at least a part of the inverter  90  may be provided in the diagnosis unit  202 . 
     An operation of the diagnosis device  200  shown in  FIG. 12  will be described with reference to  FIGS. 13 and 14 .  FIGS. 13 and 14  are diagrams showing waveform data obtained when the time-series data D 1  indicating a current flowing through the motor  42  is input to the processing unit  100  according to the embodiment.  FIGS. 13 and 14  show the section data D 2  and the smoothed section data D 3 , respectively. 
     However, these data are obtained by performing a diagnosis process on the cryocooler  10  in which abrasion of the sliding surface of the motion conversion mechanism  43  has already progressed. In this cryocooler  10 , a certain amount of abnormal noise is generated due to backlash between a first component and a second component (for example, the connecting shaft  62  and the rolling bush  74  shown in  FIGS. 4 and 6 ) of the motion conversion mechanism  43 . 
     From the time-series data D 1  of three-phase currents (a U-phase, a V-phase, and a W-phase) of the motor  42  measured by the measuring instrument  50 , the section data D 2  for one refrigeration cycle of the cryocooler  10  is acquired. As shown in  FIG. 13 , the section data D 2  is oscillating in the same manner as in the normal cryocooler  10 . As an example,  FIG. 13  shows three-phase real currents for one second when the output frequency of the inverter  90  is 60 Hz. 
     Here, the processing unit  100  may determine a length of the section data D 2  on the basis of the output frequency information D 6 . As is known, the output frequency of the inverter  90  can be converted into a rotation speed of the motor  42 , and one rotation of the motor  42  corresponds to one refrigeration cycle of the cryocooler  10 . Therefore, the processing unit  100  may determine time for one refrigeration cycle from the output frequency information D 6 , and cut out the section data D 2  measured for this time from the time-series data D 1 . In the above-described way, even in a case where the rotation speed of the motor  42  fluctuates, it is guaranteed that the section data D 2  includes the intake start timing T 1  or the exhaust start timing T 2 . 
     Alternatively, as an alternative, since the longest time required for one refrigeration cycle can be obtained in advance from the lowest output frequency of the inverter  90  (that is, the lowest possible rotation speed of the motor  42 ), the processing unit  100  may cut out the section data D 2  measured for the longest time or longer from the time-series data D 1  and may use this section data D 2  to calculate the sliding surface abrasion parameter D 4 . In this case, a length of the section data D 2  is fixed regardless of the output frequency of the inverter  90 . 
     Next, the processing unit  100  takes a moving average of the section data D 2  for a time length of, for example, one cycle (or an integer multiple thereof) of the output frequency of the inverter  90 , and generates the smoothed section data D 3 . The smoothed section data D 3  is used as the sliding surface abrasion parameter D 4 . An absolute value of the smoothed section data D 3  may be used as the sliding surface abrasion parameter D 4 . The processing unit  100  may be provided with another suitable smoothing filter (for example, a low pass filter) for removing noise. 
     As shown in  FIG. 14 , in a case where abrasion has occurred on the sliding surface of the motion conversion mechanism  43 , the sliding surface abrasion parameter D 4  remarkably fluctuates at the intake start timing T 1  and exceeds the parameter threshold value M. The sliding surface abrasion parameter D 4  does not exceed the parameter threshold value M during a period other than the intake start timing T 1 . Considering that the numerical value on the vertical axis in  FIG. 14  is 1/10 of that in  FIG. 13 , the sliding surface abrasion parameter D 4  is considered to be substantially constant during the period other than the intake start timing T 1 . This is the same as a behavior of the sliding surface abrasion parameter D 4  in the normal cryocooler  10 . The parameter threshold value M may be set as appropriate on the basis of empirical knowledge of a designer, experiments or simulations by the designer, or the like in the same manner as in the above-described embodiment. Therefore, it is possible to detect abrasion of the sliding surface of the motion conversion mechanism  43  on the basis of the sliding surface abrasion parameter D 4  at the intake start timing T 1 . 
     As described above, in the same manner as in the above-described embodiment, the processing unit  100  calculates the sliding surface abrasion parameter D 4  on the basis of the section data D 2  including the intake start timing T 1  or the exhaust start timing T 2  in the time-series data D 1 . In this case, the processing unit  100  calculates the sliding surface abrasion parameter D 4  by performing a smoothing process on the section data D 2 . The smoothing process includes a process of taking a moving average of the section data D 2  in a time frame based on a cycle of the output frequency of the inverter  90 . The processing unit  100  detects abrasion of the sliding surface on the basis of comparison between the sliding surface abrasion parameter D 4  and the parameter threshold value M. As described above, it is possible to detect abrasion of the sliding surface between the first component and the second component (for example, the connecting shaft  62  and the rolling bush  74  shown in  FIGS. 4 and 6 ) of the motion conversion mechanism  43 . 
     It can be seen that the sliding surface abrasion parameter D 4  shown in  FIG. 14  can have a steady deviation X (for example, U phase). Since a magnitude of the steady deviation X is not always known in advance, this can contribute to making it difficult to set the appropriate parameter threshold value M. Therefore, in order to reduce or eliminate this steady deviation X of the sliding surface abrasion parameter D 4 , the sliding surface abrasion parameter D 4  may be acquired by subtracting a simple average of the section data D 2  from the above moving average of the section data D 2 . Here, the simple average of the section data D 2  refers to an average value of the section data D 2  for a time (for example, a time corresponding to one refrigeration cycle) sufficiently longer than, for example, a time length of one cycle of the output frequency of the inverter  90 . An absolute value of a difference between the moving average of the section data D 2  and the simple average of the section data D 2  may be used as the sliding surface abrasion parameter D 4 . 
       FIG. 15  exemplifies the sliding surface abrasion parameter D 4  obtained by a difference between the moving average of the section data D 2  and the simple average of the section data D 2 . The sliding surface abrasion parameter D 4  becomes a substantially constant value near zero in the period other than the intake start timing T 1  as in the normal case, and does not exceed the parameter threshold value M. On the other hand, the sliding surface abrasion parameter D 4  remarkably fluctuates at the intake start timing T 1  and exceeds the parameter threshold value M. As shown in  FIG. 15 , since the steady deviation of the sliding surface abrasion parameter D 4  is removed, the parameter threshold value M can be set to a smaller value, and thus abrasion can be detected with higher accuracy. 
       FIG. 16  is a graph in which the maximum value of the sliding surface abrasion parameter D 4  is plotted for each of examples 1 to 4. The graph of the example 1 is obtained by performing a diagnosis process on a normal cryocooler (that is, the motion conversion mechanism  43  is abrasion-free or has sufficiently small abrasion and there is no unnecessary backlash between the connecting shaft  62  and the rolling bush  74 ). The examples 2 to 4 are obtained by performing a diagnosis process on a cryocooler in which abrasion of the sliding surface of the motion conversion mechanism  43  has already progressed. In the cryocoolers in the examples 2 to 4, a certain amount of abnormal noise is generated due to backlash between the connecting shaft  62  and the rolling bush  74  during operation. Abrasion progresses in the order of the example 2, the example 3, and the example 4, and when a size of the backlash (for example, the gap  80  shown in  FIG. 6 ) in the cryocooler of the example 3 is 1, backlash sizes in the example 2 and the example 4 are 0.75 and 1.2, respectively. 
     In these examples, the sliding surface abrasion parameter D 4  is acquired by taking the moving average of the current flowing through the motor  42  in a time frame based on the cycle of the output frequency of the inverter  90 , as described with reference to  FIGS. 12 to 15 . In  FIG. 16 , peak values of the absolute value of the moving average of the current obtained as described above are plotted for a plurality of different output frequencies. 
     In the example 1 related to a normal cryocooler without abrasion, the maximum value of the sliding surface abrasion parameter D 4  is almost constant regardless of the output frequency of the inverter  90 , and is closest to zero. In the examples 2 to 4 in which abrasion is progressing, the maximum value of the sliding surface abrasion parameter D 4  increases as the output frequency of the inverter  90  increases. 
     In  FIG. 16 , the circled plot represents an operation mode in which abnormal noise can be clearly heard. For example, in the example 2, the maximum value of the sliding surface abrasion parameter D 4  exceeded about 50 mA at 70 Hz, and abnormal noise was heard at this time. In the example 3 in which abrasion was more progressing than in the example 2, abnormal noise was heard at both 60 Hz and 70 Hz. In the example 4 in which abrasion was even more progressing, abnormal noise was heard at 50 Hz, 60 Hz, and 70 Hz. As described above, as the abrasion progresses, abnormal noise is heard at a lower frequency, and the maximum value of the sliding surface abrasion parameter D 4  also increases. In the example shown in  FIG. 16 , when the maximum value of the sliding surface abrasion parameter D 4  exceeds about 25 mA, it can be seen that abnormal noise is heard. 
     According to the results shown in  FIG. 16 , when the maximum value of the sliding surface abrasion parameter D 4  is in the range of, for example, about 10 to 25 mA, no clear abnormal noise is heard during the operation of the cryocooler, but it is considered that somewhat abrasion occurs in the motion conversion mechanism  43  compared with in the normal cryocooler of the example 1. Therefore, by setting the parameter threshold value M within this range, it is possible to detect abrasion before abnormal noise actually occurs. In this case, abnormal noise can be prevented by performing maintenance on the cryocooler  10 . 
     The present invention has been described on the basis of the embodiments. It is clear for those skilled in the art that the present invention is not limited to the embodiments, various design changes are possible, various modification examples are possible, and such modification examples are also within the scope of the present invention. The various features described in relation to one embodiment are also applicable to other embodiments. New embodiments resulting from the combination have the effects of each of the combined embodiments. 
     In a certain embodiment, the cryocooler  10  may be a single-stage GM cryocooler, or another type of cryocooler with a motion conversion mechanism such as a scotch yoke mechanism. 
     In the above-described embodiments, the connecting shaft  62  and the rolling bush  74  are slidably connected to each other, but the connecting shaft  62  may be fixed to the rolling bush  74 . In that case, since the motion conversion mechanism  43  has a sliding surface between the rolling bush  74  and the yoke plate  72 , the processing unit  100  may detect abrasion of the sliding surface between the rolling bush  74  and the yoke plate  72  by using the same diagnosis process. 
     In one embodiment, the processing unit  100  may be a part of a cryogenic system (for example, a superconductivity equipment or an MRI system) provided with the cryocooler  10  instead of forming a part of the cryocooler  10 . 
     The present invention has been described by using specific terms and phrases on the basis of the embodiments, but the embodiments show only one aspect of the principles and applications of the present invention, and various modifications and disposition changes are permitted in the embodiments within the scope without departing from the idea of the present invention defined in the claims. 
     The present invention can be used in the fields of cryocoolers, and diagnosis devices and diagnosis methods of cryocoolers. 
     It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention.