Patent Publication Number: US-7210911-B2

Title: Controller for variable displacement compressor and control method for the same

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a variable displacement compressor that forms a refrigerating circuit of, for example, a vehicle air conditioner, and more particularly, to a controller for controlling displacement of a variable displacement compressor. 
   The refrigerant circuit of a typical air conditioner includes a gas cooler, an expansion valve, which functions as a depressurizing device, an evaporator, and a compressor. The compressor draws in refrigerant gas from the evaporator, compresses the refrigerant gas, and discharges the compressed gas to a gas cooler. The evaporator functions to perform heat exchange between the refrigerant flowing through the refrigerant circuit and the air in the passenger compartment. The heat transferred from the air that passes by the vicinity of the evaporator to the refrigerant flowing through the evaporator is in accordance with the level of the heating load or cooling load. Accordingly, the pressure of the refrigerant gas at the outlet and downstream side of the evaporator reflects the level of the cooling load in addition to the ambient temperature of the evaporator. 
   Variable displacement swash type compressors are often installed in automobiles. Such a compressor incorporates a displacement control mechanism that either maintains the ambient temperature of the evaporator at a predetermined target value (temperature setting) or maintains the pressure at the outlet of the evaporator (suction pressure) at a predetermined target value (suction pressure setting). To adjust the flow rate of the refrigerant in accordance with the cooling load, the displacement control mechanism feedback controls the displacement of the compressor, or the inclination angle of the swash plate, using the ambient temperature of the evaporator or the suction pressure as a control index. 
   A typical displacement control mechanism is a control valve referred to as an internal control valve. The internal control valve senses the suction pressure with a pressure sensing member, such as a bellows or a diaphragm. The pressure sensing member moves in accordance with the suction pressure. This, in turn, moves a valve body and adjusts the open amount of the valve. Accordingly, the internal control valve adjusts the pressure (crank pressure) of a swash plate chamber (crank chamber) so as to determine the swash plate angle. 
   A simple internal control valve using only one suction pressure setting cannot finely control the air conditioner. Japanese Laid-Open Patent Publication No. 10-318418 describes an example of a variable suction pressure setting control valve that solves this problem. An external device electrically controls this control valve to vary the suction pressure setting. The variable suction pressure setting control valve is formed by combining the above-described internal control valve with an actuator such as an electromagnetic solenoid that electrically adjusts an urging force. Accordingly, the variable suction pressure setting control valve is externally controlled to vary mechanical spring force that is applied to the pressure sensing member to determine the suction pressure setting of the internal control valve. 
   In the variable suction pressure setting control valve, when the actual suction pressure is not included in the range of the variable suction pressure setting (i.e., the range in which the suction pressure setting may be set), the valve body does not move even if the actual suction pressure changes or even if the suction pressure setting changes. For example, cool-down (rapid cooling) may be started in a state in which the actual suction pressure is greater than the variable suction pressure setting range. In such a case, the displacement of the compressor remains maximum until the actual suction pressure falls into the variable suction pressure setting range. The discharge pressure of the compressor increases when the compressor operates in the maximum displacement state. If the actual suction pressure is much greater than the variable suction pressure setting range when cool-down is started due to a high heating load or other reasons, the operation of the compressor in the maximum displacement state is prolonged. This excessively increases the discharge pressure. 
   Instead of using the above-described variable suction pressure setting control valve to control the displacement of the variable displacement compressor, a pressure sensor for detecting the suction pressure or a temperature sensor for detecting the ambient temperature of the evaporator may be used. More specifically, an external device controls the open amount of a control valve, which is an electromagnetic valve (electromagnetic actuator and valve body), so that the pressure detected by the pressure sensor becomes equal to the suction pressure setting or so that the temperature detected by the temperature sensor becomes equal to a predetermined temperature setting. In this case, however, the operation of the compressor in the maximum displacement state is also prolonged when the pressure detected by the pressure sensor is much greater than the suction pressure setting or when the temperature detected by the temperature sensor is much greater than the temperature setting. 
   Therefore, when controlling the displacement of the variable displacement compressor to adjust the cooling load by maintaining the ambient temperature of the evaporator at the temperature setting or by maintaining the suction pressure at the suction pressure setting, the discharge pressure may be excessively increased regardless of whether the control valve is controlled by an internal autonomous device or an external device. 
   Excessive increase of the discharge pressure affects the durability of each device and pipe in the refrigerant circuit. The refrigerant circuit normally includes a pressure relief valve (PRV). The PRV releases refrigerant out of the refrigerant circuit when the discharge pressure excessively increases, such as when a device does not function properly. In this manner, the PRV protects normally functioning devices and pipes. However, the PRV may be activated even though the compressor is functioning properly. In such a case, troublesome work, such as charging refrigerant, would be required for subsequent air-conditioning. 
   The discharge pressure is especially increased when using carbon dioxide as the refrigerant in comparison to when using, for example, FREON as the refrigerant. In this case, since the tolerance margin with respect to durability for the compressor and the pipes are small, the PRV has a tendency of being activated. Further, the critical temperature of the carbon dioxide refrigerant is low. Thus, the carbon dioxide refrigerant may be in a critical state when the ambient temperature is high, such as during the summer. In such a state, the discharge pressure of the carbon dioxide refrigerant tends to increase more suddenly and excessively, compared to a liquid refrigerant, when the compressor is operated in the maximum displacement state. Thus, the PRV would also have a tendency of being activated in this state. 
   When using, for example, a suction pressure setting variable control valve to control the displacement of a variable displacement compressor, the maximum value of the variable suction pressure setting range may be increased to solve the above problem. This would readily decrease the actual suction pressure to the variable suction pressure setting range without prolonging the operation of the compressor in the maximum displacement state during cool-down. If the actual suction pressure is in the variable suction pressure setting range, the sensing member functions to decrease the displacement of the compressor. This suppresses excessive increase of the discharge pressure. 
   However, the suction pressure is much higher when using a carbon dioxide refrigerant in comparison to when using a FREON refrigerant. Accordingly, when using a carbon dioxide refrigerant, the sensing member must be much smaller than that used for a FREON refrigerant to obtain the same displacement control characteristics. Nevertheless, it is presently difficult to make the sensing member more compact. For this reason, it is difficult to further widen the range of the variable suction pressure setting when using a carbon dioxide refrigerant. 
   SUMMARY OF THE INVENTION 
   The present invention provides a controller that suppresses excessive increase of the discharge pressure while maintaining the displacement of the variable displacement compressor at a high level. 
   One aspect of the present invention is a controller for a variable displacement compressor. The controller includes a cooling load detecting means for detecting cooling load. A cooling load controlling means controls displacement of the compressor so that the load detected by the cooling load detecting means is converged to a predetermined load setting. A discharge pressure detecting means detects the pressure of a discharge pressure. A discharge pressure controlling means controls the displacement of the compressor so that the pressure detected by the discharge pressure detecting means is converged to a predetermined discharge pressure setting. A switching means switches control of the compressor between the cooling load controlling means and the discharge pressure controlling means in accordance with the pressure detected by the discharge pressure detecting means. The switching means switches the control of the compressor from the cooling load controlling means to the discharge pressure controlling means when the pressure detected by the discharge pressure detecting means is greater than a threshold pressure, which is set greater than or equal to the discharge pressure setting. 
   A further aspect of the present invention is a method for controlling a variable displacement compressor. The method including detecting pressure of a suction pressure region, detecting pressure of a discharge pressure region, and controlling displacement of the compressor so that the pressure of the discharge pressure region is converged to a predetermined discharge pressure setting when the pressure of the discharge pressure region is greater than a threshold pressure, which is set greater than or equal to the discharge pressure setting, and so that the pressure of the suction pressure region is converged to a predetermined suction pressure setting when the pressure of the discharge pressure region is less than the threshold pressure. 
   Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
       FIG. 1  is a cross-sectional diagram of a variable displacement compressor controlled by a controller according to a preferred embodiment of the present invention; 
       FIG. 2A  is a cross-sectional diagram of a control valve in a first mode; 
       FIG. 2B  is a cross-sectional diagram of a control valve in a second mode; 
       FIG. 3  is a flowchart illustrating a main routine; 
       FIG. 4  is a flowchart illustrating a suction pressure control routine; and 
       FIG. 5  is a flowchart illustrating a discharge pressure control routine. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A controller according to a preferred embodiment of the present invention will now be discussed. The controller controls a variable displacement compressor in a refrigerant circuit of an air conditioner for an automobile. 
     FIG. 1  is a cross-sectional view of the variable displacement compressor (hereinafter simply referred to as compressor). The left side as viewed in  FIG. 1  will be described as the front side of the compressor, and the right side as viewed in  FIG. 1  will be described as the rear side of the compressor. The compressor has a housing including a cylinder block  11 , a front housing  12  fixed to the front end of the cylinder block  11 , and a rear housing  14  fixed to the rear end of the cylinder block  11  with a valve plate  13  arranged therebetween. 
   A crank chamber  15  (control chamber) is defined in the compressor housing between the cylinder block  11  and the front housing  12 . A drive shaft  16  extending through the crank chamber  15  is rotatably supported between the cylinder block  11  and the front housing  12 . A clutchless (constant transmission) type power transmission mechanism PT connects the drive shaft.  16  to an engine E, which functions as a drive source of the vehicle. Accordingly, when the engine E is running, the drive shaft  16  is powered by the engine E and constantly rotated. 
   A rotor  17  is fixed to the drive shaft  16  in the crank chamber  15  to rotate integrally with the drive shaft  16 . A generally disk-like swash plate  18 , which functions as a cam plate, is accommodated in the crank chamber  15 . The central portion of the swash plate  18  is fitted to the drive shaft  16  and supported so that the swash plate  18  rotates integrally with the drive shaft  16  in an inclinable manner. A hinge mechanism  19  is arranged between the rotor  17  and the swash plate  18 . 
   The hinge mechanism  19  includes two rotor projections  20   a  (only one shown in  FIG. 1 ), which extend from the rear surface of the rotor  17 , and a swash plate projection  20   b , which extends from the front surface of the swash plate  18  toward the rotor  17 . The swash plate projection  20   b  has a distal end arranged between the two rotor projections  20   a . Accordingly, the rotation force of the rotor  17  is transmitted to the swash plate  18  by the rotor projections  20   a  and the swash plate projection  20   b.    
   The rotor projections  20   a  have a basal portion defining a cam  21 . The rear end surface of the cam  21  defines a cam surface  21   a  facing towards the swash plate  18 . The distal ends of the swash plate projections  20   b  are in contact with the cam surface  21   a  of the cam  21  in a slidable manner. Accordingly, the hinge mechanism  19  guides the inclination of the swash plate  18  so that the distal ends of the swash plate projections  20   b  move along the cam surface  21   a  of the cam  21  toward or away from the drive shaft  16 . 
   A plurality of equally spaced cylinder bores  22  extend through the cylinder block  11  in the longitudinal direction (sideward as viewed in  FIG. 1 ) about the axis L of the drive shaft  16 . A single-headed piston  23  is retained and reciprocated in each cylinder bore  22 . The cylinder bore  22  has a front opening closed by the piston  23  and a rear opening closed by the front side of the valve plate  13 . A compression chamber  24  is defined in the cylinder bore  22 . The reciprocation of the piston  23  in the cylinder bore  22  varies the volume of the compression chamber  24 . Each piston  23  is connected to the swash plate  18  by a pair of shoes  25 . Accordingly, rotation of the drive shaft  16  rotates the swash plate  18  and sways the swash plate  18  in the axial direction of the drive shaft  16 . The swaying of the swash plate  18  reciprocates the pistons  23  back and forth. 
   A suction chamber  26  (suction pressure region) and a discharge chamber  27  (discharge pressure region) are defined in the compressor housing between the valve plate  13  and the rear housing  14 . A suction port  28  and a suction valve  29  are formed in the valve plate  13  between each compression chamber  24  and the suction chamber  26 . A discharge port  30  and a discharge valve  31  are formed in the valve plate  13  between each compression chamber  24  and the discharge chamber  27 . 
   Carbon dioxide is used for the refrigerant of the refrigerant circuit. The refrigerant gas is drawn into the suction chamber  26  of the compressor from an evaporator  36  of an external refrigerant circuit  35 , which forms the refrigerant circuit. Then, as each piston  23  moves from its top dead center position to its bottom dead center position, the refrigerant gas is drawn into the associated compression chamber  24  through the corresponding suction port  28  and suction valve  29 . The refrigerant gas drawn into the compression chamber  24  is compressed to a predetermined pressure as the piston  23  moves from the bottom dead center position to the top dead center position and is then discharged into the discharge chamber  27  through the corresponding discharge port  30  and discharge valve  31 . The refrigerant gas discharged into the discharge chamber  27  is sent to and cooled by a gas cooler  37  of the external refrigerant circuit  35 . Subsequently, the refrigerant gas is depressurized by an expansion valve  38  and sent to an evaporator  36  to be vaporized. 
   A pressure relief valve (PRV)  39  having a known structure is arranged in the rear housing  14  and connected to the discharge chamber  27 . The PRV  39  is activated to release the refrigerant out of the refrigerant circuit if discharge pressure Pd(t) excessively increases (e.g., to 16 MPa or greater) when, for example, a device in the refrigerant circuit fails to function properly. In this manner, the PRV  39  protects the normally functioning devices and pipes. 
   A displacement control mechanism of the compressor will now be described. 
   Referring to  FIG. 1 , a bleed passage  32 , a gas supply passage  33 , and a control valve  34  are provided in the compressor housing. The bleed passage  32  connects the crank chamber  15  to the suction chamber  26 . The gas supply passage  33  connects the discharge chamber  27  to the crank chamber  15 . The control valve  34  is arranged in the gas supply passage  33 . 
   The open amount of the control valve  34  is adjusted to control the balance between the amount of high pressure discharge gas sent into the crank chamber  15  through the gas supply passage  33  and the amount of gas sent out of the crank chamber  15  through the bleed passage  32 . This determines the internal pressure Pc of the crank chamber  15 . As the internal pressure Pc of the crank chamber  15  changes, the difference between the internal pressure Pc of the crank chamber  15  and the internal pressure of the compression chambers  24  changes. This alters the angle of the inclination of the swash plate  18 . As a result, the stroke of the pistons  23 , or the displacement of the compressor  10 , is varied. 
   For example, a decrease in the open amount of the control valve  34  decreases the internal pressure Pc of the crank chamber  15 . This increases the inclination angle of the swash plate  18 , lengthens the stroke of the pistons  23 , and increases the displacement of the compressor. Conversely, an increase in the open amount of the control valve  34  increases the internal pressure Pc of the crank chamber  15 . This decreases the inclination angle of the swash plate  18 , shortens the stroke of the pistons  23 , and decreases the displacement of the compressor. 
   The structure of the control valve  34  will now be described. The control valve  34  is configured to vary the suction pressure setting. 
   Referring to  FIG. 2A , the control valve  34  includes a valve housing  41 . A valve chamber  42 , a communication passage  43 , and a pressure sensing chamber  45  are defined in the valve housing  41 . The valve chamber  42  is connected to the discharge chamber  27  through the upstream portion of the gas supply passage  33 . The communication passage  43  is connected to the crank chamber  15  through the downstream portion of the gas supply passage  33 . The valve chamber  42  and the communication passage  43  form a control valve interior passage of the gas supply passage  33 . The pressure sensing chamber  45  is connected to the suction chamber  26  through a pressure sensing passage  46  extending through the rear housing  14 . Accordingly, the pressure of the pressure sensing chamber  45  is equal to the pressure of the suction chamber  26  (suction pressure Ps). 
   A rod  47 , which is movable in the axial direction, is arranged in the valve chamber  42  and the communication passage  43  of the valve housing  41 . The rod  47  has an upper portion that disconnects the communication passage  43  from the pressure sensing chamber  45 . Further, the rod  47  has a middle portion located in the valve chamber  42  and defines a valve body  48 . A valve seat  49  is defined at the boundary between the valve chamber  42  and the communication passage  43 . The upper end of the valve body  48  contacts the valve seat  49 . Axial movement of the rod  47  alters the amount of the valve opening between the valve body  48  and the valve seat  49 . This adjusts the open amount of the communication passage  43 . 
   A pressure sensing member  50 , which is formed by a bellows, is arranged in the pressure sensing chamber  45  of the valve housing  41 . A socket  50   a  engaged with the upper end of the rod  47  is provided in the bottom portion of the pressure sensing member  50 . A spring  51  is arranged in the pressure sensing member  50  to apply an urging force that expands the pressure sensing member  50 . A pressure sensing mechanism of the control valve  34  includes the pressure sensing chamber  45 , the pressure sensing member  50 , and the spring  51 . The pressure sensing mechanism functions as a suction pressure detecting means and a suction pressure controlling means. 
   The valve housing  41  has a lower portion, connected to an electromagnetic actuator  52  including a casing  53 . A cylindrical sleeve  54 , which has a closed bottom, is arranged in the center of the casing  53 . A cylindrical fixed steel core  55  is secured to an upper portion of the sleeve  54 . A lower portion of the rod  47  is inserted through the fixed steel core  55  in a movable manner. A movable steel core  56  is arranged in a lower portion of the sleeve  54  in a movable manner so that it contacts the fixed steel core  55  and moves away from the fixed steel core  55 . The movable steel core  56  is integrally fixed to the lower end of the rod  47 . A spring  57  is arranged in the sleeve  54  between the fixed steel core  55  and the movable steel core  56  to urge the movable steel core  56  away from the fixed steel core  55 . 
   A coil  58  is wound around the sleeve  54  and across the fixed steel core  55  and the movable steel core  56 . The coil  58  is connected to an air conditioner ECU  61 , which configures a controller, by a valve drive circuit  62 . The air conditioner ECU  61  supplies the coil  58  with drive current through the valve drive circuit  62 . The air conditioner ECU  61  adjusts the voltage applied to the coil  58  when exciting the coil  58 . In the preferred embodiment, the air conditioner ECU  61  controls the duty ratio of the current supplied to the coil  58  to adjust the voltage applied to the coil  58 . Further, in the preferred embodiment, the air conditioner ECU  61  supplies the coil  58  with current having a high frequency (e.g., about 400 Hz) or current having a low frequency (e.g., about 15 Hz). 
   When the air conditioner ECU  61  supplies the control valve  34  with the high frequency current (drive current), relatively small vibrations are produced in the rod  47  during one cycle of the drive current due to the high frequency. In this case, the valve body  48  changes the valve open amount only slightly. Thus, the displacement of the compressor varies subtly. 
   Conversely, when the air conditioner ECU  61  supplies the control valve  34  with the low frequency current (drive current), the rod  47  moves a relatively large amount during one cycle of the drive current due to the low frequency. In this case, the valve body  48  changes the valve open amount a great amount and varies the displacement of the compressor. More specifically, when the air conditioner ECU  61  supplies the coil  58  with an ON signal (signal for exciting the coil  58 ) during one cycle of the low frequency drive current, electromagnetic attraction force (i.e., the force that moves the movable steel core  56  to the fixed steel core  55  with the magnetic flux penetrating the coil  58 ) becomes maximum. The electromagnetic attraction force remains maximum for a certain period of time and moves the rod  47  upward. As a result, the valve body  48  decreases the valve open amount and increases the displacement of the compressor. Further, when the air conditioner ECU  61  supplies the coil  58  with an OFF signal (signal for de-exciting the coil  58 ) during one cycle of the low frequency drive current, the electromagnetic attraction force is eliminated. The elimination of the electromagnetic attraction force for a certain period moves the rod  47  downward. As a result, the valve body  48  increases the valve open amount and decreases the displacement of the compressor. 
   When the high frequency current is used as the drive current of the control valve  34  and the coil  58  is not excited (duty ratio Dt 1 =0%), the urging force of the spring  57 , which urges the movable steel core  56 , dominantly determines the position of the rod  47 . Thus, the rod  47  moves to the lowermost position, and the top surface  47   a  of the rod  47  moves away from the inner surface in the socket  50   a  of the pressure sensing member  50  (as shown in the state of  FIG. 2B ). In this state, the valve body  48  of the rod  47  is separated from the valve seat  49  by the maximum distance to fully open the communication passage  43  without the movement of the pressure sensing member  50  effecting the position of the rod  47 . As a result, the internal pressure Pc of the crank chamber  15  increases to the maximum value possible under the present circumstance. In this state, the inclination angle of the swash plate  18  is minimum. Thus, the displacement of the compressor is minimum. In such a state in which the coil  58  is not excited, the control valve  34  is in a second mode. 
   Further, when the coil  58  is supplied with high frequency current having a duty ratio that is greater than or equal to the minimum duty ratio DT 1  (min) (&gt;0) of a variable duty ratio range, the upward urging force applied to the movable steel core  56  overcomes the downward urging force of the spring  57 . This starts the upward movement of the rod  47 . Accordingly, as shown in the state of  FIG. 2A , the top surface  47   a  of the rod  47  contacts the inner surface in the socket  50   a  of the pressure sensing member  50 . Further, the spring  51  produces a force that expands the pressure sensing member  50 . Thus, either one of the rod  47  and the pressure sensing member  50  follows the movement of the other one of the rod  47  and the pressure sensing member  50 . That is, the rod  47  and the pressure sensing member  50  move integrally with each other. 
   In this manner, when the rod  47  and the pressure sensing member  50  are connected, an upward magnetic urging force, which is decreased by the lower urging force of the movable steel core urging spring  57 , counters a downward pushing force, which is produced by the suction pressure Ps and increased by the downward urging force of the pressure sensing member urging spring  51 . Accordingly, the control valve  34 , which positions the rod  47  in accordance with changes in the actual suction pressure Ps, functions as an internal autonomous device that continuously maintains a control target for the suction pressure Ps (suction pressure setting) determined by the electromagnetic urging force. The duty ratio Dt 1  is changed to adjust the electromagnetic urging force so that the suction pressure setting is variable between a maximum value corresponding to the minimum duty ratio DT 1  (min) and a minimum value corresponding to the maximum duty ratio (duty ratio Dt 1 =100%). When the coil  58  is excited in a state greater than or equal to the minimum duty ratio Dt 1 (min), the control valve  34  is in a first mode. 
   When the air conditioner ECU  61  supplies the control valve  34  with the low frequency current (drive current), when the drive current is an OFF signal in one cycle of the drive current, the control valve  34  is in a state similar to the state in which the control valve  34  is excited at duty ratio Dt 1 =0% when the high frequency current is used as the drive current. Further, when the high frequency drive current is an ON signal in one cycle, the control valve  34  is in a state similar to the state in which the control valve  34  is excited at a duty ratio of Dt 1 =100% when the high frequency current is used as the drive current. That is, when the low frequency current is used as the drive current of the control valve  34 , the first mode and the second mode of the control valve  34  are alternately repeated in accordance with a duty ratio Dt 2  of the drive current (states excluding duty ratios of Dt 2 =0% and 100%). The control valve  34  substantially functions as an ON/OFF valve. 
   The controller of the compressor will now be described. 
   The air conditioner ECU  61  is a computer-like control unit including a CPU, a ROM, a RAM, and an I/O interface. Referring to  FIG. 2A , the I/O interface has an input terminal connected to an external information detecting means  63 , which provides various types of external information, and an output terminal connected to the valve drive circuit  62 . 
   Based on the various types of external information provided from the external information detecting means  63 , the air conditioner ECU  61  selects either one of the high frequency current and the low frequency current that is more proper as the drive current of the control valve  34 , calculates the duty ratio Dt 1  and Dt 2  of the drive current, and instructs the output of that drive current to the valve drive circuit  62 . The valve drive circuit  62  supplies the coil  58  of the control valve  34  with the selected drive current. 
   The external information detecting means  63  is a function realizing means covering different types of sensors. The external information detecting means  63  includes an A/C switch  64  (ON/OFF switch of the air conditioner that is operated by a vehicle occupant), a temperature sensor  65  for detecting the passenger compartment temperature Te(t), a temperature setting device  66  for setting a preferable temperature setting Te(set) of the passenger compartment, and a discharge pressure sensor  67  (discharge pressure detecting means) for detecting the pressure (discharge pressure Pd(t)) of the discharge chamber  27 . 
   Duty ratio control of the control valve  34 , which is executed by the air conditioner ECU  61 , will now be discussed with reference to the flowchart of  FIGS. 3 to 5 . 
   Referring to  FIG. 3 , the air conditioner ECU  61  starts processing a main routine RF 1 , which functions as the core of an air conditioner control program, when the A/C switch  64  is turned ON. In step S 101 , the air conditioner ECU  61  determines whether the pressure Pd(t) detected by the discharge pressure sensor  67  is greater than or equal to a predetermined threshold pressure Pd(set). The threshold pressure Pd(set) is set lower than the activation pressure (16 MPa) of the PRV  39 . More specifically, the threshold pressure Pd(set), which takes into consideration a certain margin for activation of the PRV  39 , is set at, for example, 13 MPa. 
   When the determination of step S 101  is YES, the air conditioner ECU  61  proceeds to step S 102  and sets a flag F (F=1). The flag F is reset (F=0) when the processing of the main routine RF 1  starts. Then, the air conditioner ECU  61  proceeds to a discharge pressure control routine RF 3 , which is shown in  FIG. 5 . If the determination of step S 101  is NO, the air conditioner ECU  61  proceeds to step S 103  and determines whether or not the flag F is set. When the determination of step S 103  is YES, the air conditioner ECU  61  proceeds to the discharge pressure control routine RF 3  of  FIG. 5 . If the determination of step S 103  is NO, the air conditioner ECU  61  proceeds to a suction pressure control routine RF 2 , which is shown in  FIG. 4 . 
   In the suction pressure control routine RF 2 , for example, after the discharge pressure Pd(t) increases to greater than or equal to the threshold pressure Pd(set), the air conditioner ECU  61  proceeds to the discharge pressure control routine RF 3 . Conversely, in the discharge pressure control routine RF 3 , after the discharge pressure Pd(t) decreases to less than the threshold pressure Pd(set) and the flag F is reset, the air conditioner ECU  61  proceeds to the suction pressure control routine RF 2 . The air conditioner ECU  61 , which functions as a switching means, processes the main routine RF 1 . 
   In the suction pressure control routine RF 2 ,  FIG. 4  illustrates the procedures related with the air conditioner capability for controlling the suction pressure Ps. When the processing proceeds to the suction pressure control routine RF 2 , the air conditioner ECU  61  selects the high frequency current as the drive current of the control valve  34 . In step S 201 , the air conditioner ECU  61  determines whether or not the detected temperature Te(t) is greater than the temperature setting Te(set), which is set by the temperature setting device  66 . If the determination is NO in step S 201 , the air conditioner ECU  61  proceeds to step S 202  and determines whether or not the detected temperature Te(t) is less than the temperature setting Te(set). If the determination of step S 202  is NO, the detected temperature Te(t) is substantially equal to the temperature setting Te(set). Thus, the air conditioner ECU  61  does not change the duty ratio Dt 1 , which adjusts the cooling capability. 
   When the determination of step S 201  is YES, it is assumed that the passenger compartment is hot and the heating load is high. Thus, the air conditioner ECU  61  proceeds to step S 203  to increase the duty ratio Dt 1  by a unit amount ΔD 1  and instruct the valve drive circuit  62  to change the duty ratio Dt 1  to the corrected value Dt 1 +ΔD 1 . This slightly reduces the valve open amount of the control valve  34  and increases the displacement of the compressor. As a result, the heat elimination capacity of the evaporator  36  in the external refrigerant circuit  35  is increased, and the temperature Te(t) is decreased. 
   When the determination of step S 202  is YES, it is assumed that the passenger compartment is cool and the heating load is low. Thus, the air conditioner ECU  61  proceeds to step S 204  and decreases the duty ratio Dt 1  by a unit amount ΔD 1  and instructs the valve drive circuit  62  to change the duty ratio Dt 1  to the corrected value Dt 1 −ΔD 1 . This slightly increases the valve open amount of the control valve  34  and decreases the displacement of the compressor. As a result, the heat elimination capacity of the evaporator  36  in the external refrigerant circuit  35  is decreased, and the temperature Te(t) is increased. In step S 204 , the air conditioner ECU  61  decreases the duty ratio Dt 1  within a range in which the minimum duty ratio Dt 1 (min) is the lower limit. In other words, the control valve  34  is maintained in the first mode. 
   In this manner, the air conditioner ECU  61  corrects the duty ratio Dt 1  in step S 203  and/or step S 204  to gradually optimize the duty ratio Dt 1  even if the detected temperature Te(t) is deviated from the temperature setting Te(set). Further, the internal autonomous adjustment of the valve open amount in the control vale  34  converges the temperature Te(t) to a value close to the temperature setting Te(set). 
   In the discharge pressure control routine RF 3 , the procedures related with the air conditioning capability for controlling the discharge pressure Pd(t) is illustrated in  FIG. 5 . In the discharge pressure control routine RF 3 , the air conditioner ECU  61  selects the low frequency current as the drive current of the control valve  34 . In step S 301 , the air conditioner ECU  61  determines whether or not the detected discharge pressure Pd(t) is greater than the threshold pressure Pd(set), which is a discharge pressure setting. When the determination of step S 301  is NO, in step S 302 , the air conditioner ECU  61  determines whether or not the detected discharge pressure Pd(t) is less than the threshold pressure Pd(set). When the determination of step S 302  is NO, the detected pressure Pd(t) is substantially equal to the threshold pressure Pd(set). Thus, the air conditioner ECU  61  does not change the duty ratio Dt 2 , which would lead to a change in the discharge pressure Pd(t). 
   When the determination of step S 301  is YES, in step S 303 , the air conditioner ECU  61  decreases the duty ratio Dt 2  by a unit amount ΔD 2  and instructs the valve drive circuit  62  to change the duty ratio Dt 2  to the corrected value Dt 1 −ΔD 2 . Accordingly, the ratio of the control valve  34  in the first mode for one cycle of the drive current slightly decreases, while the ratio of the second mode slightly increases. As a result, the average displacement of the compressor for one cycle decreases and lowers the discharge pressure Pd(t). 
   When the determination of step S 302  is YES, in step S 304 , the air conditioner ECU  61  increases the duty ratio Dt 2  by a unit amount ΔD 2  and instructs the valve drive circuit  62  to change the duty ratio Dt 2  to the corrected value Dt 1 +ΔD 2 . Accordingly, the ratio of the control valve  34  in the first mode for one cycle of the drive current slightly increases, while the ratio of the second mode slightly decreases. As a result, the average displacement of the compressor for one cycle increases and raises the discharge pressure Pd(t). In step S 305 , the air conditioner ECU  61  determines whether the duty ratio Dt 2  is maximum, or 100%. In other words, the air conditioner ECU  61  determines whether it can be assumed that the displacement of the compressor is maximum. 
   In a state in which the discharge pressure Pd(t) is less than the threshold pressure Pd(set) when the displacement is maximum, the discharge pressure Pd(t) does not become greater than or equal to the threshold pressure Pd(set) even if the suction pressure control routine RF 2  is executed. Accordingly, when the determination of step S 305  is YES, the air conditioner ECU  61  resets the flag F in step S 306 . When the flag F is reset, the determination given by the air conditioner ECU  61  is NO in step S 103  of the main routine RF 1  illustrated in  FIG. 3 . Thus, the air conditioner ECU  61  switches the processing from the discharge pressure control routine RF 3  to the suction pressure control routine RF 2 . When the determination of step S 305  is NO, the flag F is not reset. Since the flag F remains set, the determination of the air conditioner ECU  61  for step S 103  in the main routine RF 1  of  FIG. 3  is YES. Accordingly, the air conditioner ECU  61  continues the discharge pressure control routine RF 3  even if the discharge pressure Pd(t) is less than the threshold pressure Pd(set). 
   As described above, the air conditioner ECU  61  gradually optimizes the duty ratio Dt 2  by correcting the duty ratio Dt 2  in step S 303  and/or step S 304  even if the detected pressure Pd(t) is deviated from the threshold voltage Pd(set). Accordingly, the detected pressure Pd(t) is converged to a value close to the threshold pressure Pd(set). In this manner, the air conditioner ECU  61  functions as a discharge pressure controlling means to process the discharge pressure control routine RF 3 . 
   The controller of the first embodiment has the advantages described below. 
   (1) When the displacement of the compressor remains high due to the cool-down demand when, for example, the compressor is controlled in the suction pressure control routine RF 2 , the discharge pressure Pd(t) may exceed the threshold pressure Pd(set). In such a state, the air conditioner ECU  61  included in the controller of the first embodiment switches the control of the compressor from the suction pressure control routine RF 2  to the discharge pressure control routine RF 3 . In this manner, the air conditioner ECU  61  maintains the high displacement of the compressor, or the high cooling capacity of the refrigerant circuit, while suppressing excessive increase of the discharge pressure Pd(t). Accordingly, the controller of the preferred embodiment optimally performs cool-down and prevents the PRV  39  from being activated when the compressor is functioning normally. 
   (2) When the discharge pressure Pd(t) is less than the threshold pressure Pd(set) and the displacement of the compressor is maximum, the discharge pressure Pd(t) does not become greater than or equal to the threshold pressure Pd(set) even if the compressor is controlled in the suction pressure control routine RF 2 . In this case, the air conditioner ECU  61  switches the control of the compressor from the discharge pressure control routine RF 3  to the suction pressure control routine RF 2 . Accordingly, a state in which the discharge pressure Pd(t) becomes greater than or equal to the threshold pressure Pd(set) immediately after switching the control is avoided. That is, a state is avoided in which hunting occurs and switches the control of the compressor back to the discharge pressure control routine RF 3 . 
   (3) The control valve  34 , which mechanically detects the suction pressure Ps, moves the rod  47  (valve body  48 ) so as to offset changes in the detected pressure Ps and adjusts the valve open amount in an internally autonomous manner. In the prior art, the size of the pressure sensing member  50  must be reduced to increase the upper limit of the range of the variable suction pressure setting and prevent excessive increase of the discharge pressure Pd(t). However, as described in the BACKGROUND OF THE INVENTION, the reduction of the size of the pressure sensing member  50  when employing a carbon dioxide refrigerant is presently difficult. In the preferred embodiment, excessive increase of the discharge pressure Pd(t) is prevented using the control valve  34 , which has the same structure as that of the prior art, without reducing the size of the pressure sensing member  50 . That is, the controller of the preferred embodiment enables employment of the carbon dioxide refrigerant while providing these advantages. 
   (4) In the discharge pressure control routine RF 3 , when the pressure Pd(t) detected by the discharge pressure sensor  67  is greater than the threshold pressure Pd(set), the air conditioner ECU  61  gradually decreases the ratio the control valve  34  is in the first mode in one cycle of the drive current (step S 303 ). In this manner, the air conditioner ECU  61  fixes the control valve  34  in the second mode when the pressure Pd(t) detected by the discharge pressure sensor  67  is greater than the threshold voltage Pd(set). When the detected pressure Pd(t) is less than the threshold pressure Pd(set), the air conditioner ECU  61  gradually increases the ratio the control valve  34  is in the first mode in one cycle of the drive current (step S 304 ). In this manner, when the pressure Pd(t) detected by the discharge pressure sensor  67  is less than the threshold voltage Pd(set), the air conditioner ECU  61  further suppresses sudden and excessive change in the displacement of the compressor in comparison to when the control valve  34  is fixed in the first mode. The controller of the preferred embodiment sets the first and second modes in this manner. Thus, the discharge pressure Pd(t) is easily converged to a value close to the threshold pressure Pd(set). Accordingly, the controller of the preferred embodiment keeps the compressor displacement high while suppressing excessive increase of the discharge pressure Pd(t). 
   (5) Carbon dioxide is used as the refrigerant of the refrigerant circuit. In comparison to when using a FREON refrigerant, the discharge pressure Pd(t) has a tendency of increasing suddenly and excessively when using a carbon dioxide refrigerant. Accordingly, since the controller of the preferred embodiment is applied to a compressor that compresses carbon dioxide, advantages (1) to (4) are further prominent. 
   It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms. 
   In the preferred embodiment, the suction pressure detecting means and the suction pressure controlling means include the pressure sensing mechanism (pressure sensing member  50 , etc.) incorporated in the control valve  34 . Instead, a suction pressure sensor that detects the suction pressure Ps may function as the suction pressure detecting means, and the air conditioner ECU  61  may function as the suction pressure controlling means. More specifically, the air conditioner ECU  61  may control the valve open amount of a control valve, which is formed by an electromagnetic valve (electromagnetic actuator and valve body) so that the detected pressure of the suction pressure sensor becomes equal to a predetermined suction pressure setting. In this case, the suction pressure setting may be constant or may be varied in accordance with the cooling load like in the preferred embodiment. 
   In the preferred embodiment, the suction pressure detecting means and the suction pressure controlling means include the pressure sensing mechanism (pressure sensing member  50  etc.) incorporated in the control valve  34 . Instead, the temperature sensor  65  detecting the temperature Te(t) may function as the cooling load detecting means, and the air conditioner ECU  61  may function as the cooling load controlling means. More specifically, the air conditioner ECU  61  may control the valve open amount of a control valve, which is formed by an electromagnetic valve (electromagnetic actuator and valve body) so that the temperature detected by the temperature sensor  65  becomes equal to the temperature setting Te(Set). 
   In the preferred embodiment, the control target (discharge pressure setting) in the discharge pressure control routine RF 3  is set to the threshold pressure Pd(set) used in the determination of step S 101  in the main routine RF 1 . Instead, the control target (discharge pressure setting) may be set to a pressure that is lower than the threshold pressure Pd(set) by 5% to 20%. 
   In the control valve of the preferred embodiment, the control valve  34  is a so-called suction side control valve, which adjusts the open amount of the gas supply passage  33 . Instead, the control valve may be a so-called discharge side control valve, which adjusts the open amount of the bleed passage  32 . 
   In the preferred embodiment, if the discharge pressure Pd(t) is less than the threshold pressure Pd(set) and the displacement of the compressor is maximum (or presumed to be maximum), the air conditioner ECU  61  switches the control of the compressor from the discharge pressure control routine RF 3  to the suction pressure control routine RF 2 . 
   Alternatively, for example, if the compressor is controlled in the discharge pressure control routine RF 3  continuously for a predetermined time, the air conditioner ECU  61  may switch the control of the compressor from the discharge pressure control routine RF 3  to the suction pressure control routine RF 2 . Continuous control of the compressor in the discharge pressure control routine RF 3  for a predetermined time significantly decreases the suction pressure Ps. In this state, it may be determined that the discharge pressure Pd(t) does not become greater than or equal to the threshold pressure Pd(set) when the compressor is controlled in the suction pressure control routine RF 2 . 
   In the preferred embodiment, the air conditioner ECU  61  switches the control of the compressor from the discharge pressure control routine RF 3  to the suction pressure control routine RF 2  when the discharge pressure Pd(t) is less than the threshold voltage Pd(set) and the displacement of the compressor is maximum (or presumed to be maximum). Instead, the air conditioner ECU  61  may switch the control of the compressor from the discharge pressure control routine RF 3  to the suction pressure control routine RF 2  when the discharge voltage Pd(t) becomes less than a pressure setting that is set to a value lower than the threshold pressure Pd(set). 
   In the preferred embodiment, the air conditioner ECU  61  switches the control of the compressor from the discharge pressure control routine RF 3  to the suction pressure control routine RF 2  when the discharge pressure Pd(t) is less than the threshold pressure (pd(set) and the displacement of the compressor is maximum. In addition, the air conditioner ECU  61  may switch the control of the compressor from the discharge pressure control routine RF 3  to the suction pressure control routine RF 2  regardless of the discharge pressure Pd(t) and the displacement when the discharge pressure Pd(t) is decreased by change in a parameter, such as decrease in the speed of the engine E (i.e., the rotation speed of the compressor) or decrease in the rotation speed of a blower motor, which controls the air flow amount. 
   In the discharge control routine RF 3  of the preferred embodiment, the air conditioner ECU  61  gradually decreases the ratio that the control valve is in the first mode in one cycle of the drive current (step S 303 ) when the detected pressure Pd(t) of the discharge pressure sensor  67  is greater than the threshold pressure Pd(set) (step S 304 ). The air conditioner ECU  61  gradually increases the ratio at which the control valve  34  is set in the first mode in one cycle of the drive current when the detected pressure Pd(t) is less than the threshold pressure Pd(set). Instead, the air conditioner ECU  61  may fix the control valve  34  in the second mode when the detected pressure Pd(t) of the discharge pressure sensor  67  is greater than the threshold pressure Pd(set). Conversely, the air conditioner ECU  61  may fix the control valve  34  in the first mode when the detected pressure Pd(t) is less than the threshold pressure Pd(set). In this case, the air conditioner ECU  61  switches control from the discharge pressure control routine RF 3  to the suction pressure control routine RF 2  after the discharge pressure control routine RF 3  is continued over a predetermined time. 
   The controller of the preferred embodiment adjusts the internal pressure Pc of the crank chamber  15 , which connects the suction chamber  26  to the discharge chamber  27 , to control the displacement of the compressor. Instead, an actuator, such as a fluidal pressure cylinder connected to the swash plate  18 , may be used to control the displacement of the compressor. More specifically, the actuator may be externally controlled so that the controller adjusts the inclination angle of the swash plate  18 , that is, the displacement of the compressor. 
   The present invention may be applied to a controller used for a wobble type variable displacement compressor. 
   The present invention may be applied to a variable displacement compressor that does not use pistons. 
   The present invention may be applied to a variable displacement compressor that is not used in a refrigerant circuit. 
   The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.