Patent Publication Number: US-11649998-B2

Title: Cryocooler

Description:
RELATED APPLICATIONS 
     The contents of Japanese Patent Application No. 2018-056178, and of International Patent Application No. PCT/JP2019/009601, 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 embodiment of the present invention relates to a cryocooler. 
     Description of Related Art 
     A cryocooler including a compressor and an expander, which is also called a cold head, is known in the related art. The compressor compresses a working gas of the cryocooler to a high pressure and supplies the working gas to the expander. The working gas is expanded by the expander to generate cold. The expansion decreases a pressure of the working gas. The low-pressure working gas is collected in the compressor and is compressed again. 
     SUMMARY 
     According to an aspect of the present invention, there is provided a cryocooler including a cold head, a plurality of compressor main bodies that are connected to the cold head in parallel, a plurality of state detection sensors that are provided to correspond to the plurality of compressor main bodies respectively and each detect a state of a corresponding compressor main body to output a state detection signal, and a compressor control unit that is configured to, in a case where the state detection signal from any one state detection sensor of the plurality of state detection sensors indicates that the corresponding compressor main body is stopped, stop also the other compressor main bodies. 
     Any combination of the components, or a configuration where the components or expressions of the present invention are mutually substituted between methods, devices, systems is also effective as an aspect of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram schematically showing a cryocooler according to one embodiment. 
         FIG.  2    is a flowchart showing an example of a compressor stopping process for the cryocooler according to the embodiment. 
         FIG.  3    is a schematic diagram showing an example of a configuration of a compressor that can be adopted in the cryocooler according to the embodiment. 
         FIG.  4    is a schematic diagram showing another example of the configuration of the compressor that can be adopted in the cryocooler according to the embodiment. 
         FIG.  5    is a schematic diagram showing still another example of the configuration of the compressor that can be adopted in the cryocooler according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present inventor has carried out intensive studies on a cryocooler in which a plurality of compressors are connected to one cold head in parallel, and as a result, has come to recognize the following problems. The design of such a cryocooler is suitable for a cryocooler having a large cold head that provides a large cooling capacity since a working gas can be supplied at a high flow rate to the cold head by simultaneously operating a plurality of compressors. 
     When a situation where one of the plurality of compressors stops abnormally for some reasons is assumed, the working gas can flow back from the operating compressor to the stopped compressor since the other compressor continues to operate normally at this time. Backflow is not desirable since the backflow can have an adverse effect on components of the compressor. By adding a backflow countermeasure component, such as a check valve, to the compressor, the backflow can be prevented or mitigated. However, since such a backflow countermeasure can cause a pressure loss in forward flow of the working gas, a cooling performance of the cryocooler can be decreased. In addition, the addition of a new component causes a rise in manufacturing costs. 
     It is desirable to provide a countermeasure against the backflow of the working gas while suppressing a rise in manufacturing costs for the cryocooler having the plurality of compressors. 
     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 processes will be assigned with the same reference signs, and redundant description will be omitted as appropriate. The scales and shapes of the illustrated parts 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. 
       FIG.  1    is a diagram schematically showing a cryocooler  10  according to the embodiment. 
     The cryocooler  10  includes a compressor  12  and a cold head  14 . The compressor  12  is configured to collect a working gas of the cryocooler  10  from the cold head  14 , to pressurize the collected working gas, and to supply the working gas to the cold head  14  again. The cold head  14  is also called an expander and has a room temperature section  14   a  and a low-temperature section  14   b  which is also called a cooling stage. The compressor  12  and the cold head  14  configure a refrigeration cycle of the cryocooler  10 , and thereby the low-temperature section  14   b  is cooled to a desired cryogenic temperature. The working gas is also called a refrigerant gas, and other suitable gases may be used although a helium gas is typically used. To facilitate understanding, a direction in which the working gas flows is shown with an arrow in  FIG.  1   . 
     Although the cryocooler  10  is, for example, a single-stage or two-stage Gifford-McMahon (GM) cryocooler, the cryocooler may be a pulse tube cryocooler, a Sterling cryocooler, or other types of cryocoolers. Although the cold head  14  has a different configuration depending on the type of the cryocooler  10 , the compressor  12  can use the configuration described below regardless of the type of the cryocooler  10 . 
     In general, both of a pressure of the working gas to be supplied from the compressor  12  to the cold head  14  and a pressure of the working gas to be collected from the cold head  14  to the compressor  12  are considerably higher than the atmospheric pressure, and can 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 will also be simply referred to as 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, approximately 0.8 MPa. 
     The compressor  12  includes a plurality of compressor main bodies  16  and a common compressor casing  18  that accommodates the compressor main bodies  16 . The plurality of compressor main bodies  16  are disposed inside the compressor casing  18  and are connected to the cold head  14  in parallel. The compressor  12  is also called a compressor unit. 
     The compressor main body  16  is configured to internally compress the working gas, which is sucked from a suction port thereof, and to discharge the working gas from a discharge port thereof. The compressor main body  16  may be, for example, a scroll type pump, a rotary type pump, or other pumps that pressurize the working gas. The compressor main body  16  may be configured to discharge the working gas at a fixed and constant flow rate. Alternatively, the compressor main body  16  may be configured to change the flow rate of the working gas to be discharged. The compressor main body  16  is called a compression capsule in some cases. 
     Although the compressor  12  has the two compressor main bodies  16  in the embodiment shown in  FIG.  1   , the number is not limited thereto. The compressor  12  may have three or more compressor main bodies  16  connected to the cold head  14  in parallel. 
     By operating the plurality of compressor main bodies  16  at the same time, the working gas can be supplied to the cold head  14  at a higher flow rate compared to a case where only one compressor main body  16  is operated. Accordingly, the cryocooler  10  can adopt the large cold head  14  that provides a larger cooling capacity. 
     Although details will be described later, the compressor  12  includes a plurality of state detection sensors  20  provided to correspond to the plurality of compressor main bodies  16 , respectively. Each of the state detection sensors  20  detects a state of the corresponding compressor main body  16  and outputs a state detection signal S 1 . The compressor  12  is configured to, in a case where the state detection signal S 1  from any one state detection sensor  20  of the plurality of state detection sensors  20  indicates that the corresponding compressor main body  16  is stopped, stop the other compressor main body  16  as well. The compressor  12  may be configured to output a stop command signal S 2  to the compressor main bodies  16  based on the state detection signal S 1 . The compressor main bodies  16  are configured to be stopped in response to the stop command signal S 2 . The compressor main bodies  16  are switched from on to off in response to the stop command signal S 2 . 
     In addition, the compressor  12  includes a discharge port  22 , a suction port  24 , a discharge flow path  26 , and a suction flow path  28 . The compressor casing  18  accommodates the discharge flow path  26  and the suction flow path  28  in addition to the compressor main bodies  16 . 
     The discharge port  22  is an outlet of the working gas that is provided in the compressor casing  18  in order to send the working gas, which is pressurized to a high pressure by the compressor main bodies  16 , from the compressor  12 , and the suction port  24  is an inlet of the working gas that is provided in the compressor casing  18  in order for the compressor  12  to receive the low-pressure working gas. 
     The discharge ports of the plurality of compressor main bodies  16  are connected to the discharge port  22  by the discharge flow path  26 , and the suction port  24  is connected to the suction ports of the plurality of compressor main bodies  16  by the suction flow path  28 . Accordingly, the discharge flow path  26  merges from the plurality of compressor main bodies  16  to the discharge port  22 , and the suction flow path  28  is diverted from the suction port  24  to the plurality of compressor main bodies  16 . 
     The discharge flow path  26  is configured to allow backflow. A check valve is not provided in the discharge flow path  26 . Depending on a pressure difference between the discharge port of the compressor main body  16  and the discharge port  22 , the working gas can flow in any one of a forward direction or a reverse direction of the discharge flow path  26 . The arrow shown in  FIG.  1    indicates the forward direction. In a normal operation state of the compressor  12  in which the plurality of compressor main bodies  16  are operating, the working gas flows from the discharge ports of the compressor main bodies  16  to the discharge port  22  in the forward direction of the discharge flow path  26 . In this case, a pressure at the discharge port  22  becomes somewhat lower than a pressure at the discharge port of the compressor main body  16  due to a flow path resistance of the discharge flow path  26 . In addition, since a pressure difference between the discharge port of one compressor main body  16  and the discharge port of the other compressor main body  16  does not occur substantially, the working gas does not mutually flow between the plurality of compressor main bodies  16 . 
     However, when the pressure at the discharge port  22  is higher than the pressure at the discharge port of the compressor main body  16 , the working gas can flow from the discharge port  22  to the discharge port of the compressor main body  16  in the reverse direction of the discharge flow path  26 . In addition, when a pressure difference between the discharge port of the one compressor main body  16  and the discharge port of the other compressor main body  16  occurs, the working gas can flow in any direction depending on the pressure difference. When the one compressor main body  16  stops operating and the other compressor main body  16  continues operating for some reasons, the working gas can flow back to the one compressor main body  16  since the discharge port of the one compressor main body  16  has a pressure lower than the discharge port of the other compressor main body  16 . 
     Similarly, the suction flow path  28  is configured to allow backflow. A check valve is not provided in the suction flow path  28 . Depending on a pressure difference between the suction port of the compressor main body  16  and the suction port  24 , the working gas can flow in any one of a forward direction or a reverse direction of the suction flow path  28 . In the normal operation state of the compressor  12 , the working gas flows from the suction port  24  to the suction ports of the compressor main bodies  16  in the forward direction of the suction flow path  28 . In addition, when a pressure difference between the suction port of the one compressor main body  16  and the suction port of the other compressor main body  16  occurs, the working gas can flow in any direction depending on the pressure difference. 
     Each of the plurality of compressor main bodies  16  includes a compressor motor  30  and a motor current sensor which is an example of the state detection sensor  20 . The motor current sensor is configured to be connected to the compressor motor  30  to detect a motor current flowing in the compressor motor  30 , and to output a motor current signal which is an example of the state detection signal S 1 . The motor current sensor may be a non-contact type current sensor, for example, a current transformer (CT) type current sensor. 
     The state detection signal S 1  indicates whether the corresponding compressor main body  16  is in an on state or an off state. In a case where the state detection sensor  20  is a motor current sensor, the state detection signal S 1  indicates whether or not a current is flowing in the corresponding compressor motor  30 , that is, whether the compressor motor  30  is turned on or off. In a case where the compressor motor  30  is turned on, the corresponding compressor main body  16  is operating (that is, in the on state). In a case where the compressor motor  30  is turned off, the corresponding compressor main body  16  is stopped (that is, in the off state). 
     The state detection sensor  20  is not limited to the motor current sensor. The state detection sensor  20  may be any type of sensor provided in the compressor motor  30  to output a voltage, a current, or other suitable electric signals indicating the on or off state of the compressor motor  30  as the state detection signal S 1 . 
     The compressor motor  30  may be, for example, an electric motor, or any other suitable type of motor. The compressor motor  30  may include a motor protection circuit  31 , for example, a thermal relay. The motor protection circuit  31  may be configured to, for example, forcibly cut off power supply to the compressor motor  30  when a temperature of the compressor motor  30  has excessively increased during operation and to stop the compressor motor  30 . 
     In addition, the cryocooler  10  includes a working gas line  32  that allows the working gas to circulate between the compressor  12  and the cold head  14 . The working gas line  32  includes a high pressure line  33  through which the working gas is supplied from the compressor  12  to the cold head  14  and a low pressure line  34  through which the working gas is collected from the cold head  14  to the compressor  12 . The room temperature section  14   a  of the cold head  14  includes a high pressure port  35  and a low pressure port  36 . The high pressure port  35  is connected to the discharge port  22  by a high-pressure pipe  37 , and the low pressure port  36  is connected to the suction port  24  by a low-pressure pipe  38 . The high pressure line  33  includes the high-pressure pipe  37  and the discharge flow path  26 , and the low pressure line  34  includes the low-pressure pipe  38  and the suction flow path  28 . 
     Therefore, the working gas to be collected from the cold head  14  to the compressor  12  enters the suction port  24  of the compressor  12  from the low pressure port  36  of the cold head  14  through the low-pressure pipe  38 , and further returns to the plurality of compressor main bodies  16  via the suction flow path  28  so as to be compressed and pressurized by each of the compressor main bodies  16 . The working gas to be supplied from the compressor  12  to the cold head  14  exits from the discharge port  22  of the compressor  12  through the discharge flow path  26  from the plurality of compressor main bodies  16 , and is further supplied into the cold head  14  via the high-pressure pipe  37  and the high pressure port  35  of the cold head  14 . 
     The cryocooler  10  includes a compressor control unit  40  that controls the compressor  12 . The compressor control unit  40  may be physically mounted on the compressor  12 , or for example, may be attached to an outer surface of the compressor casing  18  or be accommodated in the compressor casing  18 . Alternatively, the compressor control unit  40  may be physically separated from the compressor  12 , and be connected by signal wiring for transmitting and receiving control signals (for example, the state detection signal S 1  and the stop command signal S 2 ) to and from the compressor  12 . 
     The compressor control unit  40  is configured to, in a case where the state detection signal S 1  from any one state detection sensor  20  of the plurality of state detection sensors  20  indicates that the corresponding compressor main body  16  is stopped, stop the other compressor main body  16  as well. The compressor control unit  40  is configured to, in a case where the state detection signal S 1  from one state detection sensor  20  indicates that the corresponding compressor main body  16  is stopped, output the stop command signal S 2  to all of the compressor main bodies  16  (or all of the other compressor main bodies  16 ). 
     The compressor control unit  40  is configured to, in a case where the state detection sensors  20  are motor current sensors and a case where a motor current signal from any one of the motor current sensors indicates that the corresponding compressor motor  30  is stopped, stop the other compressor motor  30  as well. The compressor control unit  40  is configured to, in a case where the state detection signal S 1  from one motor current sensor indicates that the corresponding compressor motor  30  is stopped, output the stop command signal S 2  to all of the compressor motors  30  (or all of the other compressor motors  30 ). 
     The compressor control unit  40  is electrically connected to each of the state detection sensors  20  to acquire the state detection signal S 1  from each of the plurality of state detection sensors  20 . In addition, the compressor control unit  40  is electrically connected to each of the compressor main bodies  16  (for example, the compressor motors  30 ) to supply the stop command signal S 2  to each of the plurality of compressor main bodies  16 . 
     The compressor control unit  40  may include a state determination unit  42  and a motor control unit  44 . 
     The state determination unit  42  is configured to determine whether or not there is a disagreement between states (that is, the on state and the off state) of the plurality of compressor main bodies  16 . The state determination unit  42  is configured to determine whether or not only one compressor main body  16  of the plurality of compressor main bodies  16  is turned off. The state determination unit  42  is configured to regularly receive the state detection signal S 1  from each of the plurality of state detection sensors  20 , and to determine whether or not the state detection signal S 1  from at least one of the state detection sensors  20  indicates that the compressor motor  30  is stopped. The state determination unit  42  is configured to provide the determination result to the motor control unit  44 . 
     The motor control unit  44  is configured to control the on or off state of each of the plurality of compressor motors  30  in accordance with the determination result from the state determination unit  42 . The motor control unit  44  is configured to transmit the stop command signal S 2  to each of the compressor motors  30  such that all of the compressor motors  30  are stopped in a case where the state determination unit  42  determines that at least one of the compressor motors  30  is stopped. The motor control unit  44  may be a motor driver or any other motor control circuit for controlling the compressor motors  30 . 
     The compressor control unit  40  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 the compressor control unit is shown in  FIG.  1    as a functional block realized in cooperation therewith. It is clear for those skilled in the art that the functional blocks can be realized in various manners in combination with hardware and software. 
       FIG.  2    is a flowchart showing an example of a compressor stopping process for the cryocooler  10  according to the embodiment. The compressor stopping process described below is repeatedly executed by the compressor control unit  40  at a predetermined cycle during the operation of the cryocooler  10 . The compressor stopping process is applicable to the cryocooler  10  having the plurality of compressor main bodies  16  as in the cryocooler  10  shown in  FIG.  1   . 
     As shown in  FIG.  2   , the state determination unit  42  of the compressor control unit  40  determines whether or not any one compressor main body  16  of the plurality of compressor main bodies  16  is turned off (S 10 ). Specifically, the state determination unit  42  determines whether or not the state detection signal S 1  from any one state detection sensor  20  of the plurality of state detection sensors  20  indicates that the corresponding compressor motor  30  is turned off. 
     In a case where none of the compressor main bodies  16  are turned off, that is, in a case where the state detection signals S 1  from all of the state detection sensors  20  indicate that the compressor motors  30  are turned on (N in S 10 ), the state determination unit  42  allows the compressor  12  to continue to operate (S 12 ). In this case, the motor control unit  44  does not output the stop command signal S 2  to any one of the compressor motors  30 . Accordingly, all of the compressor motors  30  are kept on, and all of the compressor main bodies  16  continue working gas compressing operation. In this manner, the compressor control unit  40  finishes the compressor stopping process. The compressor stopping process is executed again at a predetermined cycle as described above. 
     On the other hand, in a case where any one of the compressor main bodies  16  is turned off, that is, in a case where the state detection signal S 1  from any one state detection sensor  20  of the plurality of state detection sensors  20  indicates that the corresponding compressor motor  30  is turned off (Y in S 10 ), the state determination unit  42  prohibits the operation of the compressor  12  (S 14 ). In this case, the motor control unit  44  outputs the stop command signal S 2  to all of the compressor motors  30 . Accordingly, all of the compressor motors  30  are switched to off, and all of the compressor main bodies  16  finish the working gas compressing operation. In this manner, the compressor control unit  40  finishes the compressor stopping process. 
     In a case where the motor protection circuit  31  is built in the compressor motor  30  as described above, the motor protection circuit  31  operates and only a specific compressor main body  16  can be stopped. In a typical configuration, the motor protection circuit  31  can operate independently of the compressor main body  16  by the compressor control unit  40  (that is, even when the compressor control unit  40  has commanded the compressor main body  16  to be turned on, the motor protection circuit  31  can ignore the command and switch the compressor main body  16  to off). In addition, in most cases, according to specifications thereof, the motor protection circuit  31  is configured such that the presence or absence of the operation is not output to the outside such as the compressor control unit  40 . In this case, the operation stop of the compressor motor  30  or the compressor main body  16  caused by the operation of the motor protection circuit  31  is not directly detected by the compressor control unit  40 . 
     Alternatively, the plurality of individual compressor main bodies  16  can stop abnormally, for example, due to various factors such as severe fluctuations that exceed assumptions on environments where the compressor is provided, including a temperature, humidity, and atmospheric pressure, and defects of cooling facilities of the compressor, including an abnormal cooling quality decrease of a refrigerant such as cooling water. 
     Even when only a specific compressor main body  16  is stopped for some reasons, the working gas can flow back from the discharge port of the operating compressor main body  16  to the discharge port of the stopped the compressor main body  16  since the other compressor main body  16  is operating at this time. Alternatively, the working gas can flow back from the suction port of the stopped compressor main body  16  to the suction port of the operating compressor main body  16 . When such backflow of the working gas continuously occurs, unexpected inconvenience can occur, for example, an oil for cooling or lubricating the compressor main bodies  16  can excessively flow out from the discharge port or the suction port of the stopped compressor main body  16  together with the working gas. Accordingly, the backflow of the working gas is not desired. 
     By adding a backflow countermeasure component, such as a check valve, to the compressor  12 , the backflow of the working gas can be prevented or mitigated. For example, the check valve can be disposed on each of a discharge side and a suction side for each of the compressor main bodies  16 . However, since the check valve also functions as a flow path resistance, the check valve can cause a pressure loss in forward flow of the working gas and decrease a cooling performance of the cryocooler  10 . In addition, the addition of a new component causes a rise in manufacturing costs. 
     The compressor  12  is configured to, in the cryocooler  10  according to the embodiment, in a case where the state detection signal S 1  from any one state detection sensor  20  of the plurality of state detection sensors  20  indicates that the corresponding compressor main body  16  is stopped, stop the other compressor main body  16  as well. In this manner, when one of the compressor main bodies  16  abnormally stops, the other compressor main body  16  can be stopped synchronously by using the plurality of state detection sensors  20  provided to correspond to the plurality of compressor main bodies  16  respectively. 
     Therefore, even when one compressor main body  16  stops abnormally, the other compressor main body  16  can also be stopped promptly. The backflow of the working gas that can occur in the compressor  12  due to a disagreement between the on and off states of the plurality of compressor main bodies  16 , such as some of the compressor main bodies  16  stop and the rest of the compressor main bodies  16  operate, can be mitigated or prevented. Even when the backflow occurs, the backflow occurs only temporarily or momentarily, and an effect of the backflow is slight. For this reason, since it is not necessary to add a backflow countermeasure component such as a check valve to the compressor  12 , a pressure loss of the working gas that is assumed in a case where the backflow countermeasure component is added and the accompanying decrease in the cooling performance do not occur. In addition, since the backflow countermeasure component is not added, a rise in manufacturing costs can be suppressed. 
     In addition, a motor current sensor is used as the state detection sensor  20 . With this, since the presence or absence of a motor current directly indicates the on or off state of the compressor motor  30 , that is, the compressor main body  16 , the on or off state of the compressor main body  16  can be reliably detected. In addition, the compressor main body  16  typically has the compressor motor  30  and the motor current sensor. Configuring a control system for simultaneously stopping the plurality of compressor main bodies  16  by using such existing components is advantageous in suppressing a rise in manufacturing costs, and mounting is also easy. 
       FIG.  3    is a schematic diagram showing an example of a configuration of the compressor  12  that can be adopted in the cryocooler  10  according to the embodiment. Similar to the compressor  12  shown in  FIG.  1   , the compressor  12  shown in  FIG.  3    includes the plurality of compressor main bodies  16  and the common compressor casing  18  that accommodates the compressor main bodies  16 . Each of the compressor main bodies  16  includes the compressor motor  30 . The compressor motor  30  may include or may not include a motor current sensor  20   a , which is an example of the state detection sensor, and the motor protection circuit  31 . In addition, the compressor  12  includes the discharge port  22 , the suction port  24 , the discharge flow path  26 , and the suction flow path  28 . The components already described with reference to  FIG.  1    will be assigned with the same reference signs in  FIG.  3   , and redundant description will be omitted as appropriate. 
     In  FIG.  3   , for easy understanding, the flow paths of the working gas are shown with thick lines, and a flow path of an oil and a flow path of a refrigerant are shown with thin lines respectively. 
     In the embodiment shown in  FIG.  3   , the compressor  12  includes a storage tank  46 , a working gas cooling unit  48 , an oil separator  50 , a bypass flow path  52 , and an adsorber  54  for each of the plurality of compressor main bodies  16 . The working gas cooling unit  48 , the oil separator  50 , and the adsorber  54  are disposed in the discharge flow path  26 , and the storage tank  46  is disposed in the suction flow path  28 . 
     The storage tank  46  is provided as a volume for removing pulsation included in the low-pressure working gas returning from the cold head  14  to the compressor  12 . The working gas cooling unit  48  is provided in order to cool the high-pressure working gas heated by compression heat generated through the compression of the working gas in the compress or main body  16 . The oil separator  50  is provided in order to separate an oil mixed in the working gas out from the working gas by causing the working gas to pass through the compressor main body  16 . The adsorber  54  is provided in order to remove, for example, a vaporized oil and other contaminants remaining in the working gas from the working gas through adsorption. 
     The working gas flowing into the compressor  12  from the suction port  24  is collected into the suction port of the compressor main body  16  via the storage tank  46  on the suction flow path  28 . Since the storage tank  46  is provided for each of the compressor main bodies  16  as described above, the suction flow path  28  is branched between the suction port  24  and the storage tank  46 . 
     The working gas sent from the discharge port of the compressor main body  16  exits the compressor  12  from the discharge port  22  via the working gas cooling unit  48 , the oil separator  50 , and the adsorber  54  on the discharge flow path  26 . The discharge flow path  26  merges between the adsorber  54  and the discharge port  22 . 
     The bypass flow path  52  connects the discharge flow path  26  to the suction flow path  28  to bypass the corresponding compressor main body  16 . For example, the bypass flow path  52  connects the oil separator  50  between the storage tank  46  and the compressor main body  16 . At least one bypass valve  56  is disposed in the by pass flow path  52 . The by pass valve  56  is provided in order to control the flow rate of the working gas in the bypass flow path  52  and/or in order to equalize pressures of the discharge flow path  26  and the suction flow path  28  when the compressor  12  is stopped. 
     The compressor  12  includes an oil line  58  that allows an oil to be circulated for each of the plurality of compressor main bodies  16 . The oil flowing in the oil line  58  is used for cooling and/or lubricating the compressor main body  16 . The oil lines  58  of the respective compressor main bodies  16  are separated from each other. That is, the oil does not flow between the oil lines  58 . 
     Providing the oil line  58  individually for each of the compressor main bodies  16  helps maintain an appropriate amount of oil in each of the oil lines  58 . When the oilcan flow between the plurality of oil lines  58 , the oil flows from one of the oil lines  58  to the other oil line  58  during the operation of the compressor  12 , and an imbalance of the amounts of oil can occur between the plurality of oil lines  58 . However, in a case where such an imbalance of the amounts of oil falls within an allowable range, the plurality of oil lines  58  may be connected to each other. 
     The oil line  58  includes an oil circulation line  60  and an oil return line  62 . The oil circulation line  60  has an oil cooling unit  64 . The oil circulation line  60  is configured such that an oil flowing out from the compressor main body  16  is cooled by the oil cooling unit  64  and flows into the compressor main body  16  again. The oil return line  62  connects the oil separator  50  to the compressor main body  16  in order to return the oil collected by the oil separator  50  to the compressor main body  16 . 
     The compressor  12  includes a cooling system  66  that cools the compressor main bodies  16 , for example, using a refrigerant such as cooling water. The cooling system  66  includes the working gas cooling units  48  and the oil cooling units  64 . The working gas cooling unit  48  cools the working gas through heat exchange between the working gas compressed by the compressor main body  16  and the refrigerant. In addition, the oil cooling unit  64  cools the oil through heat exchange between the oil flowing out from the compressor main body  16  and the refrigerant. 
     The cooling system  66  has a refrigerant inlet port  68  and a refrigerant outlet port  70  which are provided in the compressor casing  18 , and a refrigerant supplied from the refrigerant inlet port  68  is discharged from the refrigerant outlet port  70  via the working gas cooling units  48  and the oil cooling units  64 . The refrigerant exiting from the refrigerant outlet port  70  may be cooled by, for example, a chiller (not shown) and be supplied again to the refrigerant inlet port  68 . In this manner, compression heat generated by the compressor main bodies  16  is removed to the outside of the compressor  12  together with the refrigerant. 
     In addition, the compressor  12  includes some sensors that can be used as the plurality of state detection sensors provided to correspond to the plurality of compressor main bodies  16  respectively. The compressor  12  includes a first pressure sensor  20   b , a second pressure sensor  20   c , a first temperature sensor  20   d , a second temperature sensor  20   e , and a third temperature sensor  20   f  for each of the plurality of compressor main bodies  16 . 
     The first pressure sensor  20   b  is configured to detect a pressure of the working gas discharged from the corresponding compressor main body  16  and to output a first pressure detection signal P 1  as a state detection signal. The first pressure sensor  20   b  is disposed in the discharge flow path  26  between the adsorber  54  and the discharge port  22  to measure the pressure of the working gas. The second pressure sensor  20   c  is configured to detect the pressure of the working gas to be sucked into the corresponding compressor main body  16  and to output a second pressure detection signal P 2  as a state detection signal. The second pressure sensor  20   c  is disposed in the suction flow path  28  between the storage tank  46  and the compressor main body  16  to measure the pressure of the working gas. 
     The first temperature sensor  20   d  and the second temperature sensor  20   e  are configured to detect a temperature of the working gas discharged from the corresponding compressor main body  16  and to output temperature detection signals (T 1  and T 2 ) as state detection signals. The first temperature sensor  20   d  is disposed in the discharge flow path  26  between the compressor main body  16  and the working gas cooling unit  48  to measure the temperature of the working gas, and the second temperature sensor  20   e  is disposed in the discharge flow path  26  between the working gas cooling unit  48  and the oil separator  50  to measure the temperature of the working gas. 
     The third temperature sensor  20   f  is configured to detect a temperature of a refrigerant that cools the working gas discharged from the corresponding compressor main body  16  and to output a temperature detection signal T 3  as a state detection signal. For example, the third temperature sensor  20   f  is disposed in the cooling system  66  between the oil cooling unit  64  and the refrigerant outlet port  70  to measure the temperature of the refrigerant. 
     The first pressure sensor  20   b , the second pressure sensor  20   c , the first temperature sensor  20   d , the second temperature sensor  20   e , and the third temperature sensor  20   f  are connected to output the state detection signals (P 1 , P 2 , and T 1  to T 3 ) to the compressor control unit  40 . 
     The first pressure detection signal P 1  from the first pressure sensor  20   b  indicates the pressure of the working gas discharged from the corresponding compressor main body  16 . Accordingly, when the compressor main body  16  is stopped, the first pressure detection signal P 1  indicates a pressure lower than a pressure during the operation of the compressor main body  16 . The second pressure detection signal P 2  from the second pressure sensor  20   c  indicates the pressure of the working gas discharged from the corresponding compressor main body  16 . Accordingly, when the compressor main body  16  is stopped, the second pressure detection signal P 2  indicates a pressure higher than the pressure during the operation of the compressor main body  16 . Similarly, the temperature detection signals (T 1 , T 2 , and T 3 ) from the first temperature sensor  20   d , the second temperature sensor  20   e , and the third temperature sensor  20   f  also indicate temperatures different from a temperature during the operation of the compressor main body  16  when the corresponding compressor main body  16  is stopped. 
     The compressor control unit  40  is configured to, in a case where the state detection signal (P 1 , P 2 , or T 1  to T 3 ) from any one state detection sensor ( 20   a  to  20   f ) of the plurality of state detection sensors ( 20   a  to  20   f ) indicates that the corresponding compressor main body  16  is stopped, stop the other compressor main body  16  as well. The compressor control unit  40  is configured to, in a case where the state detection signal (P 1 , P 2 , or T 1  to T 3 ) from one state detection sensor ( 20   a  to  20   f ) indicates that the corresponding compressor main body  16  is stopped, output the stop command signal S 2  to all of the compressor main bodies  16  (or all of the other compressor main bodies  16 ). 
     The compressor control unit  40  may be configured to determine a state of the corresponding compressor main body  16  from the state detection signal from one type of sensor of the motor current sensor  20   a , the first pressure sensor  20   b , the second pressure sensor  20   c , the first temperature sensor  20   d , the second temperature sensor  20   e , and the third temperature sensor  20   f . Alternatively, the compressor control unit  40  may be configured to determine a state of the corresponding compressor main body  16  from the state detection signal from a plurality of types of sensors of the motor current sensor  20   a , the first pressure sensor  20   b , the second pressure sensor  20   c , the first temperature sensor  20   d , the second temperature sensor  20   e , and the third temperature sensor  20   f.    
     In this manner, when one of the compressor main bodies  16  abnormally stops, the other compressor main body  16  can be stopped synchronously by using various sensors mounted on the compressor  12 . The backflow of the working gas that can occur in the compressor  12  due to a disagreement between the on and off states of the plurality of compressor main bodies  16  can be mitigated or prevented. Similar to the embodiment shown in  FIG.  1   , the embodiment shown in  FIG.  3    also provides a countermeasure against the backflow of the working gas while suppressing a rise in manufacturing costs. 
     In addition, some components of the compressor  12  may be shared by the plurality of compressor main bodies  16 . By doing so, the number of components can be reduced and manufacturing costs can be suppressed. 
       FIG.  4    is a schematic diagram showing another example of the configuration of the compressor  12  that can be adopted in the cryocooler  10  according to the embodiment. In the embodiment shown in  FIG.  4   , some components provided in the suction flow path  28  are shared by the plurality of compressor main bodies  16 . The rest of the configuration is the same as the embodiment described above, and the same reference signs will be assigned in  FIG.  4    as well, and redundant description will be omitted as appropriate. 
     The compressor  12  may include the common storage tank  46  provided in the suction flow path  28  between the suction port  24  and a diverting portion  72  to the plurality of compressor main bodies  16 . In addition, also the first pressure sensor  20   b , the second pressure sensor  20   c , and the bypass valve  56  may be shared by the plurality of compressor main bodies  16 . 
       FIG.  5    is a schematic diagram showing still another example of the configuration of the compressor  12  that can be adopted in the cryocooler  10  according to the embodiment. In the embodiment shown in  FIG.  5   , some components provided in the discharge flow path  26  are shared by the plurality of compressor main bodies  16 . The rest of the configuration is the same as the embodiment described above, and the same reference signs will be assigned in  FIG.  5    as well, and redundant description will be omitted as appropriate. 
     The compressor  12  may include the common adsorber  54  provided in the discharge flow path  26  between a merging portion  74  from the plurality of compressor main bodies  16  and the discharge port  22 . 
     The present invention has been described hereinbefore based on the embodiment. 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. 
     Various characteristics described related to one embodiment are also applicable to other embodiments. A new embodiment generated through combination also has the effects of each of the combined embodiments. 
     Although the plurality of compressor main bodies  16  are accommodated in the single compressor casing  18  in the embodiment, the invention is not limited thereto. Each of the compressor main bodies  16  may be accommodated in a separate compressor casing. Accordingly, the compressor  12  may include the plurality of compressor main bodies  16  connected to the cold head  14  in parallel and a plurality of compressor casings that each accommodate one compressor main body  16 . 
     It is possible to use the present invention in the field 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.