Abstract:
A system and method for controlling the displacement of a variable displacement compressor by feeding back crankcase pressure as part of a control scheme is disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national stage of International Application No. PCT/US2006/000880, which claims priority to U.S. Provisional Patent Application Ser. No. 60/644,097, filed Jan. 14, 2005. The disclosures of which are both incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with United States Government support under cooperative agreement number 70NANB2H3003 awarded by the National Institute of Standards and Technology (NIST). The United States Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates in general to control of variable displacement compressors. 
         [0005]    2. Description of Related Art 
         [0006]    U.S. Pat. No. 4,428,718 entitled “Variable Displacement Compressor Control Valve Arrangement” to Skinner (Skinner &#39;718), the disclosures of which are hereby incorporated herein by reference, describes a variable displacement compressor, a conventional pneumatic control valve, general function of the variable displacement compressor, and interaction of the control valve with the compressor. 
         [0007]    Referring now to the drawings,  FIG. 1  shows a variable displacement refrigerant compressor  210  as described by Skinner &#39;718. The variable displacement refrigerant compressor  210  is of the variable angle wobble plate type connected in an automotive air conditioning system having a normal condenser  212 , an orifice tube  214 , an evaporator  216  and an accumulator  218  arranged in that order between the compressor&#39;s discharge and suction sides. The compressor  210  comprises a cylinder block  220  having a head  222  and a crankcase  224  sealingly clamped to opposite ends thereof. A drive shaft  226  is supported centrally in the compressor  210  at the cylinder block  220  and the crankcase  224  by bearings. The drive shaft  226  extends through the crankcase  224  for connection to an automotive engine (not shown) by an electromagnetic clutch  236  which is mounted on the crankcase  224  and is driven from the engine by a belt  238  engaging a pulley  240  on the clutch  236 . 
         [0008]    The cylinder block  220  has five axial cylinders  242  through it (only one being shown), which are equally spaced about and away from the axis of the drive shaft  226 . The cylinders  242  extend parallel to the drive shaft  226  and a piston  244  is mounted for reciprocal sliding movement in each of the cylinders  242 . A separate piston rod  248  connects the backside of each piston  244  to a non-rotary, ring-shaped, wobble plate  250 . 
         [0009]    The non-rotary wobble plate  250  is mounted at its inner diameter  264  on a journal  266  of a rotary drive plate  268 . The drive plate  268  is pivotally connected at its journal  266  by a pair of pivot pins (not shown) to a sleeve  276  which is slidably mounted on the drive shaft  226 , to permit angulation of the drive plate  268  and the wobble plate  250  relative to the drive shaft  226 . The drive shaft  226  is drivingly connected to the drive plate  268 . The wobble plate  250  while being angularable with the rotary drive plate  268  is prevented from rotating therewith by a guide pin  270 . 
         [0010]    The angle of the wobble plate  250  is varied with respect to the axis of the drive shaft  226  between the solid line large angle position shown in  FIG. 1 , which is full-stroke, to the zero angle phantom-line position shown, which is zero stroke, to thereby vary the stroke of the pistons  244  and thus the displacement or capacity of the compressor  210  between these extremes. There is provided a split ring return spring  272  which is mounted in a groove on the drive shaft  226  and has one end that is engaged by the sleeve  276  during movement to the zero wobble angle position and is thereby conditioned to initiate return movement. 
         [0011]    The working ends of the cylinders  242  are covered by a valve plate assembly  280 , which is comprised of a suction valve disk and a discharge valve disk, clamped to the cylinder block  220  between the latter and the head  222 . The head  222  is provided with a suction area  282 , which is connected through an external port  284  to receive a gaseous refrigerant from the accumulator  218  downstream of the evaporator  216 . The suction area  282  is open to an intake port  286  in the valve plate assembly  280  at the working end of each of the cylinders  242  where the refrigerant is admitted to the respective cylinders on their suction stroke each through a reed valve formed integral with the suction valve disk at these locations. Then on the compression stroke, a discharge port  288  open to the working end of each cylinder  242  allows the compressed refrigerant to be discharged into a discharge area  290  in the head  222  by a discharge reed valve which is formed integral with the discharge valve disk. The compressor&#39;s discharge area  290  is connected to deliver the compressed gaseous refrigerant to the condenser  212  from whence it is delivered through the orifice tube  214  back to the evaporator  216  to complete the refrigerant circuit as shown in  FIG. 1 . 
         [0012]    The wobble plate angle and thus the compressor displacement can be controlled by controlling the refrigerant gas pressure in the sealed interior  278  of the crankcase  224  behind the pistons  244  relative to the suction pressure. In this type of control, the angle of the wobble plate  250  is determined by a force balance on the pistons  244 . When the crankcase-suction pressure differential exceeds a set amount (the suction pressure control set-point) the net force on the pistons  244  results in a sufficiently large turning moment about the wobble plate pivot pins (not shown) that the wobble plate angle is reduced (i.e., moved toward the angle shown in phantom in  FIG. 1 ) thereby reducing the compressor capacity by reducing the length of stroke of the pistons  244 . 
         [0013]    An important element of the variable displacement compressor  210  is a pneumatic control valve  300  inserted into the head portion  222  of the compressor  210 . The control valve  300  senses an air conditioning load by sensing the pressure state of the refrigerant gas returning to the compressor  210  (the suction pressure). The control valve  300  is operably connected to the crankcase chamber  278 . There are channels in the cylinder block  220  and the head  222  of the compressor  210  for gas flow between the control valve  300  and suction area  282 , discharge area  290  and crankcase chamber  278  of the compressor  210 . The control valve  300  controls the displacement of the piston  244  within the compressor  210  by controlling the pressure of gas in the crankcase chamber  278  that acts on the backside of the pistons  244  and the wobble plate  250 . 
         [0014]    The control valve  300  inserts into a stepped, blind control valve cavity  298  formed in the compressor head  222 . The blind end of the control valve cavity  298  communicates directly with discharge area  290  through port  292 . Control valve cavity ports  294  and  295  communicate with the crankcase chamber  278 . A control valve cavity port  296  communicates with the suction area  282 . The control valve  300  is sealed into the control valve cavity  298  so that particular features of the control valve  300  align with the ports  292 ,  294 ,  295  and  296 . 
         [0015]    U.S. Pat. No. 6,769,667 entitled “Control Valve for a Variable Displacement Compressor” to Kume et al. (Kume &#39;667), the disclosures of which are hereby incorporated herein by reference, describes a control valve that uses electro/pneumatic control with a solenoid and a suction-pressure referenced bellows. 
         [0016]    U.S. Pat. No. 6,390,782 entitled “Control Valve for a Variable Displacement Compressor” to Booth et al. (Booth &#39;782), the disclosures of which are hereby incorporated herein by reference, describes a control valve that uses electro/pneumatic control with a solenoid and a suction-pressure referenced bellows. 
         [0017]    A variable set point control valve (variable control valve)  10  is represented in the diagram of  FIG. 2  according to the prior art disclosed in Booth &#39;782. In  FIG. 2 , the variable control valve  10  is depicted in cross-sectional view and has a shape and feature placements appropriate to fit the control valve cavity  298  of the Skinner &#39;718 variable displacement compressor described previously (see  FIG. 1 ). The variable control valve  10  is coupled to a compressor  100 , which compresses a gas. The variable control valve  10  controls the amount of gas and the degree to which it is pressurized in compressor  100 . In the embodiment shown in  FIG. 2 , the gas compressed in compressor  100  is a refrigerant such as is used in an air conditioning unit, for instance, an air conditioning unit found in an automobile. 
         [0018]    The variable control valve  10  comprises a compressor displacement control portion  30  and a variable set point control portion  80 . The compressor displacement control portion  30  controls the flow of the gas from compressor  100  in and out of the variable control valve  10  while the variable set point control portion  80  controls the operation of the compressor displacement control portion  30 . A valve body  12  of the variable control valve  10  is formed with many variable control valve functional elements, which will be described later. In the embodiment illustrated in  FIG. 2 , the valve body  12  is substantially cylindrical in shape as may be inferred from the cross-sectional view shown. O-ring retaining grooves  14  are indicated on the exterior of the valve body  12  in three locations. When the variable control valve  10  is inserted into a control valve cavity of a compressor  100  (see for example,  FIG. 1 ), it is assembled with o-ring seals that allow different pressure sources to be communicated to different portions and ports of the variable control valve  10 . 
         [0019]    The compressor displacement control  30  comprises a suction pressure chamber  32  formed in the lower end  16  of the valve body  12  which is in gas communication with the suction area  120  of the compressor  100  through a variable control valve suction port  34  formed in the valve body  12  and a suction pressure path  112 . A refrigerant circuit line  111  feeds low pressure gas into a compression chamber  114  of the compressor  100  via the suction area  120  and a compressor valve plate  126 . The refrigerant circuit line  111  is a line returning low pressure refrigerant gas from the accumulator  144  of an air conditioning system (not shown). 
         [0020]    The compressor  100  further comprises a piston  116 , a crankcase chamber  118 , and a discharge area  124 . In simple terms, the operation of the compressor  100  is as follows. The refrigerant gas in the compression chamber  114  is compressed by the stroke of the piston  116  as the piston  116  moves towards the compressor valve plate  126 . The compressor valve plate  126  admits high pressure gas to the discharge area  124 . The refrigerant circuit line  111  is connected to the discharge area  124 . The greater the displacement (stroke)  128  along the compression chamber  114  of the piston  116 , the greater the pressure and the flow volume of the refrigerant gas as it passes through the compressor valve plate  126 . The refrigerant gas then passes from refrigerant circuit line  111  to a condenser  140  where it condenses to a liquid in the condenser coils. The liquid then flows to an evaporator  142 , where the liquid expands at an orifice within the evaporator  142 , and evaporates. The air passing over the coils gives off heat energy that provides the energy for the state change from liquid to gas. The cooled air is then blown into the passenger cabin of the automobile, or into whatever chamber the air conditioning system is required to cool. After expanding, the refrigerant gas is in a low pressure state and is returned to the compressor  100  through the refrigerant circuit line  111 . 
         [0021]    The compressor  100  is a variable compressor, meaning that the stroke of piston  116  varies dependent upon the required air conditioning system load. For instance, if a user requires additional cooling of the air passing over the evaporator coils, the flow volume of the refrigerant discharged into the refrigerant circuit line  111  is increased. The stroke  128  of the piston  116  is increased to increase the flow volume. 
         [0022]    A pressure is applied within the crankcase chamber  118  to the back of the piston  116 . The greater the pressure within the crankcase chamber  118 , relative to the suction pressure, the shorter the return stroke  128  of the piston  116  after compression due to the high pressure force exerted against the piston  116  on the return (away from the valve plate  126 ). Conversely, the lower the pressure within the crankcase chamber  118 , relative to the suction pressure, the greater the return stroke of the piston  116  after compression due to the low pressure force exerted against the piston  116 . By varying the pressure within the crankcase chamber  118 , thus varying the displacement  128  of the piston  116  and ultimately the pressure of the discharge through refrigerant circuit line  111 , the temperature of the air from the evaporator is controlled. 
         [0023]    The compressor displacement control portion  30  has a middle chamber  40  formed as a bore centered in the valve body  12  leading from the suction pressure chamber  32 . A first middle port  42  is formed in the valve body  12  and communicates with a middle chamber  40 . The first middle port  42  is in gas communication with the crankcase chamber  118  through a first crankcase pressure path  130 . The variable control valve  10  further comprises a pressure sensitive member, a diaphragm  36 , exposed to the suction pressure chamber  32 . A suction pressure valve, comprising a suction valve closing member, suction valve ball  38 , and a suction valve seat  37  formed in the valve body  12 , is provided to open and close a gas communication path between the suction pressure chamber  32  and the middle chamber  40 . 
         [0024]    The suction valve ball  38  is urged against the suction valve seat  37  by a rigid member  41 , which is in floating contact with the diaphragm  36 . A bias spring  44 , retained in the middle chamber  40 , urges the suction valve ball  38  off the suction valve seat  37 , that is, urges the suction valve portion to open. It is also seen that the bias spring  44  opposes a movement of the diaphragm  36  towards the suction valve seat  37  and so acts as an equivalent pressure, a spring bias pressure, adding to the action of the suction pressure on the pressure receiving area of diaphragm  36 . The variable control valve suction pressure valve opens and closes a gas communication path between the suction area  120  and the crankcase chamber  118  of the compressor  100 . 
         [0025]    A discharge pressure valve portion of the variable control valve  10  is comprised of a discharge valve member, a discharge valve ball  50 , and a discharge valve seat  52  formed in the valve body  12 . The discharge valve ball  50  is positioned in a discharge pressure chamber  60  formed in an upper end  18  of the valve body  12 . A valve insert  64  has a stepped through bore  62  that positions the discharge valve ball  50  in alignment with the discharge valve seat  52 . A ball centering spring  58  may be used to further condition the nominal position of the discharge valve ball  50 . A particle filter cap  74  sealably covers the end of valve body  12 , completing the discharge pressure chamber  60 . When the variable control valve  10  is inserted into the compressor  100 , the upper end  18  of the valve body  12  is sealed in a blind end of a control valve cavity such as the cavity  298  illustrated in  FIG. 1 . A discharge pressure path  110  from the discharge area  124  of the compressor is communicated to the blind end of the control valve cavity  298 . Discharge pressure gas is thereby communicated to the variable control valve discharge pressure chamber  60  through the filter  74 . 
         [0026]    The variable control valve  10  has a central stepped bore  70  formed through the valve body  12 . The central bore  70  has a large diameter bore portion at the upper end adjacent the discharge chamber  60  where at the discharge valve seat  52  is formed. The central bore  70  and the middle chamber  40  are aligned with each other. A second middle port  56  is formed in the valve body  12  and communicates with the large bore portion of the central bore  70 . The second middle port  56  is in gas communication with the crankcase chamber  118  through the second crankcase pressure path  132 . When the discharge valve ball  50  is moved off the discharge valve seat  52 , discharge pressure gas can flow through the bore  70  to the second middle port  56  and then to the crankcase chamber  118  via the second crankcase pressure path  132 . 
         [0027]    A valve rod  54 , inserted in the central bore  70 , partially links the actions of the suction valve portion and the discharge valve portions of the variable control valve  10 . The valve rod  54  has a diameter slightly smaller than the small bore portion of the central bore  70 . The valve rod  54  freely slides in the central bore  70  yet substantially blocks gas communication between the middle chamber  40  and the discharge chamber  60 . The length of the valve rod  54  is chosen so that it simultaneously touches the seated discharge valve ball  50  and the suction valve ball  38  in a fully open (fully unseated) position. This arrangement links the suction and discharge valve portions in a partial open-close relationship. As the suction valve ball  38  moves in a valve-closing direction, the valve rod  54  pushes the discharge ball  50  in a valve-opening direction. As the discharge valve ball  50  moves in a valve closing direction, the valve rod  54  pushes the suction ball  38  in a valve-opening direction. 
         [0028]    In the embodiment of  FIG. 2 , the valve rod  54  is not attached to either of the valve closing balls  38 ,  50 . The valve rod  54  operates to open either the discharge or the suction valve portions of the variable control valve but not to close either. The forces which act to close the discharge valve portion are the pressure of the discharge gas on an effective pressure receiving area of the discharge valve ball  50  and a small spring force imparted by a ball centering spring  58 . The force that acts to close the suction pressure valve portion derives from a movement of the pressure sensitive diaphragm  36  via the rigid member  41 . Other embodiments of the prior art of U.S. Pat. No. 6,390,782 in which both valve closing members are attached to a coupling means such as the valve rod  54  will be apparent to those skilled in the control valve art. If both valve members are rigidly linked, then a full open-close relationship will exist. 
         [0029]    Reference is made specifically now to the variable set point control portion  80  of the variable control valve  10 . The variable set point control  80  comprises a closed reference chamber  90  bounded by the variable control valve diaphragm  36 , walls  91  formed at the lower end  16  of the valve body  12  when the suction pressure chamber  32  was formed, and a valve end cap  20 . The diaphragm  36  is positioned and sealed against an interior step  93  in the suction pressure chamber  32  by a reference valve carrier  81 . The diaphragm  36  has a first side  43  with a suction pressure receiving area exposed to suction pressure in the suction-pressure chamber  32  and a second side  39  with a reference pressure receiving area exposed to the reference pressure in the reference chamber  90 . The diaphragm  36  is arranged to seal the reference chamber  90  from direct gas communication with the suction pressure chamber  32 , the discharge pressure chamber  60 , the middle chamber  40 , or the central bore  70 . 
         [0030]    Two pressure bleed passageways, a discharge bleed passageway  68  and a suction bleed passageway  72  are provided in the valve body  12  and align with two holes in the diaphragm  36  that is sealed against a valve body interior step  93 . A valve insert  64  has a valve insert bleed hole  69  provided to communicate the discharge chamber  60  with the discharge bleed passageway  68 . The bleed passageways, the valve insert bleed hole, and the corresponding diaphragm holes, provide a source of suction pressure gas and discharge pressure gas to the reference chamber  90 . The feature depicted of supplying the discharge pressure gas to the reference chamber  90  from the variable control valve discharge pressure chamber  60  is important because this design uses a filter  74  to protect the components and passages in reference chamber  90  from foreign material. 
         [0031]    The variable control valve components contained in the reference chamber  90  are illustrated more clearly in  FIG. 3 . The reference chamber valve means are further illustrated at higher detail level in  FIG. 4 . The same elements in  FIGS. 2-4  are labeled with the same numbers. 
         [0032]    Referring now to  FIGS. 2-4 , the reference valve carrier  81  is formed as a thick-walled cylinder with outside walls that sealably fit against the interior of walls  91  formed at the lower end  16  of the valve body  12 . The upper end of reference valve carrier  81  seals against the diaphragm  36 . Two small blind chambers, a suction bleed chamber  96  and a discharge bleed chamber  98  are formed in the reference valve carrier  81  from the upper end that is sealed against the diaphragm  36 . The open end of the suction bleed chamber  96  aligns with the suction bleed passageway  72  and the open end of the discharge bleed chamber  98  aligns with the discharge bleed passageway  68 . Reference chamber valve means are generally indicated as a reference inlet valve  88  and a reference outlet valve  86 . 
         [0033]    Turning to  FIG. 4 , the reference inlet valve  88  is comprised of a reference inlet valve closing member  162 , a reference inlet through hole  160 , and a reference inlet valve seat  164 . The reference inlet through hole  160  is formed from an interior surface of the cylindrical reference valve carrier  81  through to the discharge bleed chamber  98 . The reference inlet valve seat  164  is formed around the inlet through hole  160  where it emerges from the reference valve carrier  81  that is into the reference chamber  90 . The reference inlet valve closing member  162  is attached to an inlet valve push rod  167 , which is part of an inlet solenoid actuator  94 . When an electrical current signal is applied to inlet solenoid leads  85 , an inlet valve push rod  167  is pulled into the center of the solenoid actuator  94 , urging reference the inlet valve closing member  162  against the reference inlet valve seat  164 , closing off the reference inlet through hole  160 . The reference inlet through hole  160  communicates reference chamber  90  with discharge bleed chamber  98 . Thus, opening and closing the reference inlet valve  88  by means of electrical signals applied to the inlet solenoid actuator  94  controls the flow of discharge pressure gas to the reference chamber  90 . 
         [0034]    An inlet solenoid leaf spring  168  is arranged to bias the inlet valve push rod  167  in a retracted position as is illustrated in  FIG. 4 . This inlet solenoid spring bias configuration means that the reference inlet valve  88  will open the reference chamber  90  to the flow of discharge pressure gas in the absence of an electrical signal to energize the coil of the inlet solenoid actuator  94 . The depicted reference inlet valve is said to be normally open. The opposite arrangement of spring biasing the reference inlet valve  88  to a normally closed condition is an alternate configuration of the reference inlet valve means that may also be employed successfully in another embodiment of the prior art of U.S. Pat. No. 6,390,782. 
         [0035]    The reference outlet valve  86  is comprised of a reference outlet valve closing member  172 , a reference outlet through hole  170 , and a reference outlet valve seat  174 . 
         [0036]    The reference outlet through hole  170  is formed from an interior surface of the cylindrical reference valve carrier  81  through to the suction bleed chamber  96 . The reference outlet valve seat  174  is formed around the outlet through hole  170  where it emerges from the reference valve carrier  81  that is into the reference chamber  90 . The reference outlet valve closing member  172  is attached to an outlet valve push rod  177 , which is part of the outlet solenoid actuator  92 . When an electrical current signal is applied to the outlet solenoid leads  87 , the outlet valve push rod  177  is pulled into the center of the solenoid actuator  92 , pulling the reference outlet valve closing member  172  away from the reference outlet valve seat  174 , opening the reference outlet through hole  170 . The reference outlet through hole  170  communicates the reference chamber  90  with the suction bleed chamber  96 . Thus, opening and closing the reference outlet valve  86  by means of electrical signals applied to the outlet solenoid actuator  92  controls the flow of suction pressure gas to the reference chamber  90 . 
         [0037]    An outlet solenoid leaf spring  178  is arranged to bias the outlet valve push rod  177  in an extended position as is illustrated in  FIG. 4 . This outlet solenoid spring bias configuration means that the reference outlet valve  86  will close the reference chamber  90  to the flow of suction pressure gas in the absence of an electrical signal to energize the coil of the outlet solenoid actuator  92 . The depicted reference outlet valve  86  is therefore normally closed. The opposite arrangement of spring biasing the reference outlet valve  86  to a normally open condition is an alternate configuration of the reference outlet valve means that may also be employed successfully in another embodiment of the prior art of U.S. Pat. No. 6,390,782 
         [0038]    It should also be appreciated that, while solenoid actuators are discussed herein and depicted in  FIGS. 2-4 , any electrically-driven physical actuator means could be employed to open and close the reference inlet valve  88  and the reference outlet valve  86 . 
         [0039]    The variable set point control portion  80  further comprises an electronic control unit  82 , a pressure sensor  84 , an electrical circuit carrier  83 , and variable control valve electrical leads  89 . The pressure sensor  84  is an optional feature of the embodiment of the prior art of U.S. Pat. No. 6,390,782. It is a transducer device that produces an electrical signal that is related to a gas pressure impinging on its sensitive portion. The pressure sensor  84  is mounted on the electrical circuit carrier  83  so as to respond to the gas pressure within the closed reference chamber  90 . It is not necessary for the practice of the prior art of U.S. Pat. No. 6,390,782 that the pressure sensor  84  be mounted directly in the interior of the reference chamber  90 . An alternative embodiment could mount the pressure sensor at some other position as long as the pressure sensitive portion of the sensor  84  is brought into gas communication with the reference chamber  90 . 
         [0040]    The electronic control unit  82  is an optional feature of the embodiment of the prior art of U.S. Pat. No. 6,390,782. The control unit  82  may contain electronic circuitry to control the reference chamber valve means or to receive and process the electrical signals produced by the pressure sensor  84 . In an embodiment of this optional feature of the prior art of U.S. Pat. No. 6,390,782, the electrical components of the control unit  82  are co-located with the pressure sensor  84  by means of the electrical circuit carrier  83 . Other functions of the optional control unit  82  will be described later. 
         [0041]    The variable control valve electrical leads  89  are routed from the electrical circuit carrier  83  through a sealed opening in the valve end cap  20 . The number of electrical leads needed by the variable control valve  10  will depend on the functions performed by the optional electronic control unit  82  and the device characteristics of the optional pressure sensor  84 . When neither the electrical control unit  82  nor the reference chamber pressure sensor  84  are employed, then variable control valve electrical leads  89  need comprise only those needed to carry electrical signals to activate the reference chamber valve means. 
         [0042]    The variable set point control portion  80  controls the operation of the compressor displacement control portion  30 . By controlling a pressure within the reference chamber  90 , the variable set point control  80  is able to regulate the open/close conditions of the suction pressure valve portion and the discharge pressure valve portion of the variable control valve  10 . For instance, if the pressure in the reference chamber  90  exerts a force against the diaphragm  36  which is less than the force exerted by the pressure in the suction pressure chamber  32  and the bias spring  44 , the diaphragm  36  will distort into the reference chamber  90 , that is in the direction of the reference inlet let actuator  94 . This motion moves the suction valve ball  38  from the suction valve seat  37 , thus opening the flow of gas from the first crankcase pressure path  130  to the suction pressure chamber  32 . At the same time, the discharge pressure valve portion is closed by the pressure of discharge gas forcing the discharge valve ball  50  onto the discharge valve seat  52 . By opening the flow through the suction valve portion of the variable control valve  10 , gas from the crankcase chamber  118  will flow into the suction pressure chamber  32  and out to the suction area  120  of the compressor  100  via the suction pressure path  112 . With the bleeding of gas out of the crankcase chamber  118 , less force is exerted on piston  116  giving piston  116  greater displacement. The flow of refrigerant gas flowing into the evaporator of the system is thus increased. 
         [0043]    If the pressure in the reference chamber  90  exerts a force against diaphragm  36  which is greater than the force exerted by the pressure in the suction pressure chamber  32  and the bias spring  44 , the diaphragm  36  will distort into the suction pressure chamber  32 , that is, in the direction of the suction valve seat  37 . This action closes the variable control valve suction valve portion and, at the same time, opens the variable control valve discharge valve portion by pushing the discharge valve ball  50  away from the discharge valve seat  52  by means of the valve rod  54 . As the discharge valve portion is opened, high pressure gas from the discharge pressure path  110  flows through the discharge pressure chamber  60 , the stepped central bore  70 , the second middle port  56  and the second crankcase pressure path  132  to the crankcase chamber  118 . Pressure will build up in the crankcase chamber  118 , thus applying a force against the piston  116 . The displacement  128  of the piston  116  is thus restricted and the amount of the refrigerant gas passing into the evaporator of the system is reduced. 
         [0044]    The force that the bias spring  44  exerts on the diaphragm  36  is an important design variable for the overall performance of the variable control valve  10 . It has been found through experimentation that it is most beneficial if the spring force is adjusted to be equivalent to from 2 to 20 psi of suction pressure, and most preferably, from 4 to 10 psi. This range of spring bias force allows for sufficient operational range of the variable control valve  10  in the condition of very low compressor capacity usage, that is, when the compressor is near full de-stroke operation. 
         [0045]    The pressure within the reference chamber  90  is controlled by the opening and closing of the reference outlet valve  86  and the reference inlet valve  88 . Each of these are optionally controlled by the pressure sensor  84  and the electronic control unit  82 . Specifically, the pressure within the reference chamber  90  is in gas communication with the pressure sensor  84 . The pressure sensor  84 , interfaced to the electronic control unit  82 , measures the pressure of the gas in the reference chamber  90  and communicates that pressure to the electronic control unit  82 . The electronic control unit  82  receives control signals and information from a compressor control unit  146 . Passenger comfort level settings and other information about environmental conditions and vehicle operation conditions are received by the compressor control unit  146 . The compressor control unit  146  uses stored compressor performance algorithms to calculate a necessary amount of gas to be compressed within the compression chamber  114  by the piston  116  to cause a desired condition to occur, namely that the passenger comfort level settings are optimally achieved within the constraints imposed by environmental and vehicle operational factors. 
         [0046]    The calculated compressor displacement requirements, the pressure information from pressure sensor  84 , and known physical response characteristics of the variable control valve  10  elements are utilized by variable control valve performance algorithms to calculate a necessary pressure within the reference chamber  90  to meet the compressor displacement requirements. This calculated reference pressure, necessary to meet the requirements determined by the compressor control unit, is called a predetermined reference pressure. The variable displacement compressor  100  is thereby controlled by the determining of the predetermined reference pressure and the maintenance of the gas pressure in the reference chamber  90  to this predetermined pressure level. 
         [0047]    Alternatively, if the pressure sensor  84  is not employed, the predetermined reference pressure may be selected from a stored set of reference pressure levels that has been pre-calculated based on the known nominal characteristics of the variable control valve  10  or, in addition, customized for the variable control valve by means of a calibration set-up procedure. In the case of this alternate embodiment of the prior art of U.S. Pat. No. 6,390,782, the calculated compressor displacement requirements are used to determine, in look-up table fashion, the predetermined reference pressure that is optimal for achieving the desired compressor displacement control. 
         [0048]    Control of the reference outlet valve  86  and the reference inlet valve  88  comes from the electronic control unit  82  through the actuators  92  and  94 , respectively. Dependent upon the outputs of the algorithms within the electronic control unit  82 , the electronic control unit  82  will open and close the reference outlet valve  86  by actuating the outlet actuator  92  and open and close the reference inlet valve  88  by the inlet actuator  94 . For instance, when the pressure within the reference chamber  90  is to be increased, the inlet actuator  94  will retract the reference inlet valve member  162  allowing high pressure gas to flow from the discharge pressure chamber  60  through the valve insert bleed hole  69 , the discharge pressure bleed passageway  68  and the discharge bleed chamber  98  into the reference chamber  90 . At the same time, the outlet actuator  92  closes the reference outlet valve  86 , thus allowing the pressure in the reference chamber  90  to increase. Inversely, to decrease the pressure in the reference chamber  90 , the electronic control unit  82  will actuate the outlet actuator  92  to retract the reference outlet valve member  172  to open flow from the reference chamber  90  through the suction bleed chamber  96  to the suction pressure bleed passage  76  to the suction pressure chamber  32 , thereby bleeding off pressure. At the same time, the actuator  94  is signaled by the electronic control unit  82  to extend the reference inlet valve member  162  to close off discharge pressure flow into the reference chamber  90 . 
         [0049]    By controlling the pressure within the reference chamber  90  to the predetermined reference pressure, the electronic control unit  82 , through the actuators  170  and  172 , controls the deflection of the diaphragm  36 , thus controlling the varying of the displacement  128  of the piston  116 . For the embodiment depicted in  FIGS. 2-4 , the reference chamber pressure can be continuously or periodically monitored by means of the pressure sensor  84 . This pressure information can be used as a feedback signal by the control unit  82  in a pressure servo control algorithm to maintain the reference chamber  90  at the predetermined reference pressure within chosen error boundaries. 
         [0050]    It is anticipated that an important benefit of the variable control valve design disclosed herein is the ability to maintain valve control performance by tightly maintaining the predetermined reference pressure. The disclosed design also enables the system to electronically change the predetermined reference pressure to a different value, thereby changing the suction pressure set-point about which the variable displacement compressor operates. This allows the vehicle to adjust the compressor control in the face of changing environmental factors to achieve a desired balance of passenger comfort and vehicle performance. The benefits of the balance of passenger comfort and vehicle performance are more fully realized the more responsive the pressure in the reference chamber is to the control. 
         [0051]    The responsiveness of the reference pressure control system depends in part on the characteristics of the flow of discharge pressure gas through inlet valve  88  and the flow out of the outlet valve  86  to suction pressure.  FIGS. 5 and 6  illustrate some important geometrical feature details of the reference inlet valve  88  and the reference outlet valve  86 . 
         [0052]    Referring first to  FIG. 5 , the inlet valve closing member  162  is illustrated in a fully closed position holding off the force of discharge pressure gas impinging an effective pressure receiving area, AI, on the inlet valve member  162 . Also indicated in  FIG. 5  is the diameter, DI, of the reference inlet port  160  leading from the discharge bleed chamber  98 . A large value of DI will promote quick response to commands to increase reference chamber pressure by admitting a large flow of discharge pressure. The size of DI needed to achieve a given reference chamber pressure rise time will depend on the reference chamber gas volume. A larger reference inlet port  160  will be required for a larger reference chamber gas volume to achieve the same increase in reference chamber pressure rise time as for a smaller reference chamber gas volume. 
         [0053]    However, a large value of DI necessitates a correspondingly large value of AI, the effective inlet valve member pressure receiving area. This, in turn, would mean that the closing force that would be needed from the inlet valve actuator  94  would also be large. A large closing force might require a physically large actuator or require excessive power to maintain the inlet valve in a closed state. Consequently, the choice of the reference inlet port  160  diameter, DI, and the pressure receiving area, AI, involves a balance of competing requirements. 
         [0054]    The effective inlet valve member pressure receiving area, AI, is the net, unbalanced, area of the inlet valve closing member that is exposed to the discharge pressure when the inlet valve is fully closed. That is, the area that effectively receives the force of the discharge pressure, AI, may be calculated by measuring the force exerted on the inlet valve closing member by the discharge pressure, and dividing by the discharge pressure. It has been found through experimentation effective inlet valve pressure receiving area, AI, may be beneficially chosen to be less than 30,000 square microns and preferably, less than 7500 square microns when the reference chamber gas volume is approximately 2 milliliters. Under typical automotive air conditioner compressor operating conditions, a reference inlet valve closing force of less than 1 pound will suffice if the effective inlet valve member pressure receiving area, AI, is less than approximately 7500 square microns. 
         [0055]    Referring to  FIG. 6 , the outlet valve closing member  172  is illustrated in a fully open position with gas flowing out of the reference chamber  90  through an effective gas flow area. Many geometrical designs of the reference outlet port  170  may be chosen to have the same result in terms of the gas volume flow for a given pressure differential between the reference chamber  90  and the suction bleed chamber  96 . The effective flow area is chosen to balance competing performance characteristics. In order to insure quick response to a command to lower the reference chamber pressure, it is desirable to have a large outlet valve  86  effective flow area On the other hand, to help restrain rapid pressure increases in the reference chamber when opening the inlet valve  88  to discharge pressure, and to bring down reference pressure overshoots that may occur, it is helpful to have a small outlet valve  86  effective flow area 
         [0056]    The effective gas flow area of the reference outlet valve  86  may be beneficially chosen as a ratio to the effective flow area of the inlet valve  88 . Alternatively, the diameter, DO, of the reference outlet port  170 , may be chosen as a ratio of the reference inlet port  160  diameter, DI. It has been determined by experimentation and analysis that the beneficial range of the ratio DO to DI is from 0.5 to 5.0, and, most preferably, from 0.7 to 2.0. The corresponding beneficial ratio of inlet-port to outlet-port cross-sectional areas, the inlet-to-outlet port areal ratio, is 0.25 to 25.0, and, most preferably, 0.5 to 4.0. When the geometries of inlet and outlet gas flow areas are more complex than the circular passageways illustrated in  FIGS. 5 and 6 , the gas flow cross-sectional areas may be analyzed or experimentally determined and the inlet-to-outlet port areal ratio design guideline followed. 
         [0057]    It has been found through experimentation, for example, that when the reference chamber  90  gas volume is approximately 2 milliliters, a reference outlet port  170  diameter DO of 100 microns is an effective choice when the reference inlet port  160  diameter DI is 100 microns, a reference outlet port diameter to reference inlet port diameter ratio of 1.0. With these parameter values, and under typical automotive air conditioner compressor operating conditions, the reference chamber pressure can be controllably changed, or tracked to a predetermined reference pressure, at the rate of 10 psi/second. 
         [0058]    For alternative embodiments of the variable control valve  10  without a pressure sensor, the compressor control unit  146  may periodically recalculate the compressor displacement conditions required to maintain performance of the cooling system. Based on the magnitude and time behavior of changes in these calculations, the compressor control unit  146  may send instruction signals to the variable control valve electronic control unit  82  to increase or decrease the reference chamber pressure to re-establish the pre-determined reference pressure level. It will be appreciated by those skilled in the art that this method of affecting servo control of the pressure in the reference chamber to the predetermined level will be less timely than can be implemented using a direct measurement of reference chamber pressure. Nonetheless, this loose-servo method can be effective and appropriate for a low cost embodiment of the prior art of U.S. Pat. No. 6,390,782. 
         [0059]    The functions attributed to the variable control valve electronic control unit  82  and the compressor control unit  146  could be performed by other computational resources within the overall system employing the variable control valve  10 , the compressor  100  and the cooling equipment. For example, if the overall system is an automobile with a central processor, then all of the control information and calculations needed to select and maintain the predetermined reference pressure could be gathered and performed by the automobile central processor. Signals to and from the pressure sensor  84  could be routed to an input/output port of the central processor and the reference inlet and outlet valve actuation signals could be sent to the variable control valve  10  from another input/output port of the central processor. Alternately, a compressor control unit  146  could perform all the control functions needed to manage the variable control valve  10 . And finally, the variable control valve control unit  82  could be provided with circuitry, memory and processor resources necessary to perform the compressor displacement requirement calculation as well as selecting and maintaining the predetermined reference pressure 
       BRIEF SUMMARY OF THE INVENTION 
       [0060]    This invention relates in general to variable displacement compressors and more specifically to systems and methods for controlling the displacement of a variable displacement compressor. In one embodiment of the present invention, displacement of a variable displacement compressor is controlled by utilizing crankcase pressure. 
         [0061]    Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0062]      FIG. 1  is a cross-sectional view of a variable displacement compressor for use in an automobile from the prior art of U.S. Pat. No. 4,428,718. 
           [0063]      FIG. 2  is a cross-sectional view of a variable set point control valve according to a preferred embodiment of the prior art of U.S. Pat. No. 6,390,782. 
           [0064]      FIG. 3  shows a cross-section of the variable set point control portion of the variable control valve of  FIG. 2 . 
           [0065]      FIG. 4  shows a cross-section of the reference chamber valve means of the variable control valve of  FIGS. 2 and 3 . 
           [0066]      FIGS. 5 and 6  show cross-sections of the valve members and valve seats of the reference chamber valve means of the variable control valve of  FIGS. 2-4 . 
           [0067]      FIG. 7  is a cross-sectional view of a variable set point control valve according to a first embodiment of the present invention. 
           [0068]      FIG. 8  is a cross-sectional view of a variable set point control valve according to a second embodiment of the present invention. 
           [0069]      FIG. 9  is a flow chart illustrating a method for controlling the displacement of a variable displacement compressor by utilizing crankcase pressure according to the present invention. 
           [0070]      FIG. 10  is a schematic illustration of an arrangement for pneumatic control of a variable displacement compressor according to another embodiment of the invention. 
           [0071]      FIG. 11  is a schematic diagram of a compressor system according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0072]    Referring again to the drawings, there is illustrated in  FIG. 7  a system  410  according to a first embodiment of the present invention.  FIG. 7  is a view generally similar to  FIG. 2 , except as will be discussed below, and similar components are labeled with the same numbers. 
         [0073]    The system  410  includes a dividing wall  412 . The dividing wall  412  separates the reference chamber  90  into two isolated chambers, an upper chamber  490   a  and a lower chamber  490   b . The upper chamber  490   a  is defined on opposite ends by the diaphragm  36  and the dividing wall  412 . The lower chamber  490   b  is defined on opposite ends by the dividing wall  412  and the valve end cap  20 . 
         [0074]    A lower crankcase port  414  is in fluid communication with the crankcase chamber  118  through a third crankcase pressure path  416 . Thus, the pressure sensor  84  is operable to measure the actual pressure of the crankcase chamber  118 . As used in this application the term “pressure sensor” means any sensor which measures pressure or any other parameter from which pressure can be inferred. The system  410  controls crankcase pressure to control the displacement of the compressor  100 , for example by a method that will be discussed below. 
         [0075]      FIG. 8  is a view similar to  FIG. 7 , except showing a system  420  for controlling the displacement of the compressor  100  by utilizing pressure of the crankcase chamber  118 , according to a second embodiment of the present invention, and similar components are labeled with the same numbers. 
         [0076]    The system  420  includes a sensor  422 . The sensor  422  is an electronic pressure sensor, although such is not required. The sensor  422  is located within the crankcase chamber  118  and is operable to measure the actual crankcase pressure. It must be understood, however, that the sensor  422  need not be located entirely inside the main cavity of the crankcase chamber  118 . For example, the sensor  422  may be located in a passage, subchamber, or any other suitable location where the sensor  422  may sense the pressure in the crankcase chamber  118 . The sensor  422  is located such that the sensor  422  does not interfere with movement of the piston  116  or other moving parts within the compressor  100 . 
         [0077]    The sensor  422  is electrically connected to the compressor control unit  146  by a sensor lead  424  for electrical communication. In the present embodiment, the sensor  422  measures pressure in the crankcase  118  and transmits a pressure sensor signal to the control unit  146 . The control unit  146  then alters a control signal to the variable control valve  10  to effect a change in the position of the variable control valve  10  based upon the pressure sensor signal received. The system  410  is thus responsive to the measured actual crankcase pressure to control the displacement of the compressor  100 . 
         [0078]    The present embodiments have been described as having electrical control of the displacement of the compressor  100 . This electrical control may be achieved, for example, by use of a computer chip, a pressure transducer measuring actual crankcase pressure, and an electrically actuated device. In such a case, electrical control may be achieved by the computer chip setting a target crankcase pressure, and comparing the target crankcase pressure and the actual crankcase pressure, and determining an amount to move the variable control valve  10  based on the difference of the actual crankcase pressure from the target crankcase pressure. Then electrically actuated device or devices, such as a microvalve or a solenoid operated macro-sized valve, such as the reference inlet valve  88  and/or the reference outlet valve  86  are actuated in a manner to cause the desired control valve response (i.e., as described above in the Background Of The Invention) to change the actual crankcase pressure. For electronic control, where a computer chip is used, a program is preferably written that can cause the variable control valve  10  to respond in a fine-tuned method, as will be further described below. 
         [0079]    Although the present embodiments have been described as having electrical control, the system may have, and the method may use, any suitable control, such as pneumatic control, electro-pneumatic control, hydraulic control, or any other suitable control. 
         [0080]    An arrangement for pneumatic control of an air conditioning system  600  (which is only partially illustrated) is schematically illustrated in  FIG. 10 . As shown, a diaphragm or bellows  610  controls the position of a three way valve  620  suitable to selectively increase the pressure in the crankcase  118 , maintain the pressure in the crankcase  118 , or decrease the pressure in the crankcase  118 . In the illustrated example, the interior of the bellows is controlled at a reference pressure to generate a target crankcase pressure. The reference pressure may be generated in any suitable manner, including a variable pneumatic pressure regulator. As illustrated, however, the target crankcase pressure is generated by a thermostatic system  630  with a sensor bulb  632  disposed in an air stream of the air conditioning system generated by a fan  634 . The bulb  632  is located in the outlet air stream downstream of the evaporator  216 , so that when temperature of the air stream rises, pressure in the bulb  632  (and thus inside the bellows  610 ) will rise, as a fluid medium inside the bulb  632  heats up and expands. Conversely, when temperature of the air stream falls, pressure inside the bulb  632  and thus target crankcase pressure inside the bellows  610  will also fall. Thus, in this instance, the reference pressure is an inverse of a target crankcase pressure, since when an increase in cooling is desired, as would be the case when temperature of the air stream around the bulb  632 , the crankcase pressure of the compressor  100  must go down to increase stroke of the pistons and increase cooling. It should also be noted that, like many conventional thermostatic systems, the thermostatic system  630  may be provided with a mechanism (not illustrated) for adjusting the response of the air-conditioning s system  600  to a given temperature of the air stream within which the bulb  632  is disposed, to enable, for example, a user to select a cooler or warmer airflow from the air-conditioning system  600 . 
         [0081]    The bellows  610  is disposed within a chamber  640  that is in fluid communication with the crankcase  118 , and thus contains actual crankcase pressure. The bellows  610  thus expands or contracts in response to the difference in pressure between the reference pressure within the bellows  610  and the actual crankcase pressure outside the bellows  610 , within the chamber  640 . The moving end of the bellows  610  is mechanically connected to a moving element  650  of the valve  620 . 
         [0082]    The valve  620  has an inlet chamber  622  in fluid communication with the discharge  652  of the compressor  100 , and an outlet chamber  654  in fluid communication with the suction  120  of the compressor  100 , and a load chamber  656  in fluid communication with the crankcase  118 . 
         [0083]    As the bellows  610  expands, when to the reference pressure is greater than the actual crankcase pressure, the three way valve  620  is adjusted rightward, as viewed in  FIG. 10 , toward an decrease of pressure position, in which the crankcase  118  is connected via the chambers  656  and  654  of the valve  620  to the suction flow path of the compressor  100 , and more specifically to the suction  120 . Since the pressure at the suction  120  is the lowest pressure in the air conditioning system, the pressure in the crankcase  118  falls when the crankcase  118  is vented thusly to the suction  120 . 
         [0084]    As the bellows  610  contracts, when the reference pressure is less than the actual crankcase pressure, the three way valve  620  is adjusted leftward, as viewed in FIG.  10 , toward an increase of pressure position, in which the crankcase  118  is connected via the chambers  656  and  652  of the valve  620  to the discharge flow path of the compressor  100 , and more specifically to the discharge  124 . Since the pressure at the discharge  124  is the highest pressure in the air conditioning system, the pressure in the crankcase  118  rises when the crankcase  118  is connected to the discharge  124 . 
         [0085]    It will be apparent that the reference pressure could be connected to the chamber  640 , outside the bellows  610 , and the crankcase  118  connected instead to the inside of the bellows  610 , in the reverse of what is shown in  FIG. 10 . In such a case, all that need be done is to connect the discharge  124  to the chamber  654  and the suction  120  to the chamber  652  for this alternative embodiment to work generally as described above. 
         [0086]    Additionally, instead of the illustrated thermostatic system  632 , it is contemplated that any suitable arrangement may be provided to generate the reference pressure. 
         [0087]    An alternative arrangement which might be suitably substituted for the bellows  610  and the valve  620 , would be a three-way pressure actuated microvalve. One valve which might be suitably adapted is the pilot-operated valve  10  described in U.S. Pat. No. 6,694,998 (the &#39;998 patent), the disclosures of which are incorporated herein by reference. (Note: In the following discussion, the reference numbers refer to components previously discussed in this disclosure, unless specifically noted as being reference numbers ‘of the &#39;998 patent’). A reference pressure (either from a thermostatic system  632  or from any other suitable arrangement, including an electrically controlled pilot microvalve, such as the microvalve 9 of the &#39;998 patent) is introduced into the control chamber 125 of the &#39;998 patent, and acts against the axial face of the second end 276 (of the &#39;998 patent) of the slider element 240 of the &#39;998 patent. The port 220 of the &#39;998 patent is connected to the discharge  124  of the compressor  100 , the port  230  is connected to the suction  120  of the compressor  100 , and the port 226 of the &#39;998 patent is connected to the crankcase  118 . As described in the &#39;998 patent, the slider element 240 of the &#39;998 patent will operate to maintain the port 226 of the &#39;998 patent at the pressure in the control chamber 125 of the &#39;998 patent, in this case, the reference pressure (the desired target crankcase pressure, in this case). 
         [0088]    Furthermore, while the valve  620  is shown as a three-way valve directly actuated by the bellows  610  and directly controlling the connection between the crankcase  118  and the suction  120  and the discharge  124 , a pilot valve and pilot operated valve arrangement is also contemplated. The pilot valve (not shown) could be operated by the difference in pressure between the reference pressure and the actual crankcase pressure to direct fluid pressures to the pilot operated valve. The pilot operated valve (not shown) could be selectively positioned by the fluid pressures of the pilot valve to a pressure increase position connecting the discharge  124  to the crankcase  118 , a pressure decrease position connecting the suction  120  to the crankcase  118 , and a pressure hold position, in which the crankcase  118  is isolated from the suction and discharge flow paths. Such a pilot and pilot-operated valve arrangement could otherwise operate generally similarly to the system  600  shown in  FIG. 10 . It is also contemplated that the pilot valve and possibly the pilot operated valve could be microvalves. Of course, the suitability of using microvalves to substitute for the valve  620  illustrated in  FIG. 10 , either as discussed in this paragraph, or in the previous paragraph depend upon the flow requirements of the particular system in which they are to be installed, and the flow capacity of the valves themselves. 
         [0089]    In the case of electro-pneumatic control, the reference pressure in a reference chamber (not shown) may set to obtain to a desired crankcase pressure, similarly as described above for pneumatic control. A diaphragm or bellows may effectively measure the difference between the reference pressure and the actual crankcase pressure by the amount of expansion or contraction of the diaphragm or bellows. A sensor (not shown) is provided which measures the movement of the diaphragm or bellows, generating a signal representative of the difference between the reference pressure and the actual crankcase pressure. An electrically actuated device, such as an electrically actuated micro- or macro-sized valve or valves, may then operate to affect a control valve response, based at least in part upon the sensor&#39;s signal, to alter actual crankcase pressure. In another embodiment, the micro- or macro-sized valve or valves directly port or vent pressure from the crankcase  118  to change actual crankcase pressure based upon the sensor&#39;s signal. 
         [0090]      FIG. 9  is a flow chart illustrating a method  510  for controlling displacement of a variable displacement compressor by utilizing crankcase pressure according to the present invention. For example, the method  510  may be implemented in the system  410 , as shown in  FIG. 7 , in the system  420 , as shown in  FIG. 8 , or in the system  600 , as shown in  FIG. 10 . 
         [0091]    In a first step  512  according to the method  510  of the present invention, a reference pressure is set. The reference pressure is a pressure related to the target (desired) crankcase pressure that is expected to result in the compressor  100  operating at a capacity resulting in the desired heat transfer. As discussed above, the reference pressure may be the target crankcase pressure (as in the system  410  and  420 ) or may be a pressure related to the target crankcase pressure in some fashion (i.e., the reference pressure is a function of the target crankcase pressure), such as the reference pressure of the system  600 . 
         [0092]    In a second step  514 , actual pressure in the crankcase  118  is measured. This actual (measured) crankcase pressure may be acquired in any suitable manner, such as by the sensor  84  in the system  410 , by the sensor  422  in the system  420 , or by direct connection to a diaphragm or bellows such as the bellows  610  in the system  600 . 
         [0093]    In a third step  516 , the reference crankcase pressure and the actual crankcase pressure are compared. For example, the comparison may be made mechanically, such as by action of a differential pressure across a diaphragm or bellows, such as the bellows  610 . Alternatively, a calculation to compare the target crankcase pressure and the crankcase pressure measured may be made by the control unit  146 . 
         [0094]    For example, one method of comparison is to minimize the differences between the target crankcase pressure and the actual crankcase pressure by using an optimization algorithm. Any suitable optimization algorithm may be used. Many optimization algorithms are available, but they generally fall into three categories: derivative, simulated annealing, and genetic. In one embodiment, a simulated annealing algorithm is utilized where key optimization parameters are developed through prior testing. These parameters are dependent upon the particular system configuration. Another method of comparison would be to use a variable step function that depends on the difference between the target crankcase pressure and the actual crankcase pressure. For example, when the difference is relatively high, a relatively large step toward the target would be taken; that is, a relatively large change in position of the control valve  10  would be commanded (in embodiments utilizing the control valve  10 ). As the actual crankcase pressure approaches the target pressure, so the difference is relatively smaller, a relatively smaller step would be commanded. As will be discussed below, the steps of the method illustrated in  FIG. 9  are repeated in an iterative process, so that each time the third step  516  is repeated, a smaller step will be commanded, although such repetition is not required. As the actual crankcase pressure approaches the target crankcase pressure, smaller steps are taken to minimize the tendency to overshoot and oscillate. The amount of the reduction in step size may be based on the magnitude of difference between target crankcase pressure and actual crankcase pressure, or may be time based, that is reduced stepwise at certain intervals. It is contemplated that the magnitude of a step could be anything from zero (no position modification) to a maximum position modification signal; for example, when utilizing Pulse Width Modulated signals to the associated valve(s) to control the porting of pressure to the crankcase  118  and venting of pressure from the crankcase  118 , a zero signal would be zero voltage applied for the full application interval (no pulse), and a maximum signal would be a full power pulse applied for the full application interval. Further, the present invention contemplates that when the actual crankcase pressure is relatively close to the target crankcase pressure, there would be no change in the signal. 
         [0095]    The illustrated method  510  then concludes with a step  518  where the position of the control valve  10  is modified based upon the comparison of the reference and actual crankcase pressures, thereby changing the position of the control valve  10  to the desired position. For example, in the systems  410  and  420 , the control unit  146  will send an appropriate signal to the reference outlet valve  86  and/or the reference inlet valve  88  to change state based, at least in part, upon the comparison of the target pressure and the actual crankcase pressure to reposition the control valve  10 , thereby adjusting the actual crankcase pressure toward the target crankcase pressure. As a further example, in the system  600 , the valve  620  is repositioned by the differential pressure acting across the bellows  610  to cause crankcase pressure to change toward the target crankcase pressure. 
         [0096]    In an alternative embodiment of the present invention, the method illustrated in  FIG. 9  is a continuously iterative process, such that the method  510  continues to loop continuously through the steps  512  to  518 , while the variable displacement compressor  100  is in operation, as indicated by the dashed line  520 . 
         [0097]    In testing, a control program, implementing the method illustrated in  FIG. 9 , was developed using LabVIEW computer development software (available from National Instrument Corporation, of Austin, Tex.), and was loaded on a computer control system. A compact variable compressor manufactured by Delphi Corporation (Troy, Mich.) was connected to traditional automotive air conditioning system components. An electrical pressure sensor was disposed located in a crankcase chamber of the compact variable compressor. The sensor was suitable to monitor crankcase pressure conditions in the crankcase chamber. A Microstaq™ microvalve, manufactured by Microstaq, Inc. (Bellingham, Wash.) was connected to the compact variable compressor for controlling the pressure in the crankcase chamber. Using crankcase feedback, i.e., a pressure measurement from the sensor, the control program instructed the Microstaq™ microvalve to regulate the pressure in the crankcase chamber. By monitoring crankcase pressure conditions, rather than suction pressure conditions, and using this crankcase pressure as a feedback signal to control compressor displacement, among other inputs, compressor control was achieved that was superior to that achieved by the prior art (which used suction pressure as a signal to control compressor displacement). 
         [0098]    Shown in  FIG. 11  is a schematic diagram of a compressor system  710  according to the present invention. The compressor system  710  includes a compressor  712 . The compressor  712  is a variable displacement compressor in which the capacity of the compressor is controlled by the crankcase pressure, such as the variable displacement refrigerant compressor  210  of  FIG. 1 , the compressor  100  of  FIG. 2 , or any suitable compressor. The compressor  712  includes a crankcase port, as indicated at  714 , in fluid communication with a crankcase, not shown, within the compressor  712 . The compressor  712  includes a discharge port, as indicated at  716 , and a suction port, as indicated at  718 . The compressor system  710  includes an A/C system  720  having input  722  in fluid communication with the discharge port  716  and having an output  724  in fluid communication with the suction port  718 . The A/C system  710  may be, for example, the automotive air conditioning system of  FIG. 1  having a normal condenser  212 , an orifice tube  214 , an evaporator  216  and an accumulator  218  arranged in that order between the compressor discharge port  716  and the compressor suction port  718 , the air conditioning unit of  FIG. 2  having a condenser  140 , an evaporator  142 , and accumulator  144 , or any suitable air conditioning system. 
         [0099]    The compressor system  710  includes a control mechanism  726  having a crankcase interface, as indicated at  728 , in fluid communication with the crankcase port  714 . The control mechanism  726  has a discharge interface  730  in fluid communication with the discharge port  716 . The control mechanism  726  has a suction interface  732  in fluid communication with the suction port  718 . The control mechanism includes a valve arrangement for providing selective communication between the crankcase interface  728 , the discharge interface  730 , and the suction interface  732  in order to control crankcase pressure in a manner previously described. The control mechanism  726  may, for example, include an arrangement such as: 
         [0100]    a pneumatic control valve similar to the control valve  300  of  FIG. 1 ; 
         [0101]    an electronically controlled valve and control unit similar to the variable control valve  10  and the control unit  146  of  FIG. 2 ; 
         [0102]    one or more microvalves and/or one or more macro-sized valves, which valves may be actuated by a differential pressure or may be electrically, pneumatically, or electro-pneumatically operated under the control of an electronic control unit (not shown), and which may be direct acting valves or which may be arranged as pilot valves and pilot-operated valves; 
         [0103]    a bellows and valve arrangement, or a diaphragm and valve arrangement, such as the arrangement illustrated in  FIG. 10 ; or 
         [0104]    any other suitable control arrangement for controlling the pressure in the crankcase of the compressor  712  which operates the valve portion of the control mechanism to control crankcase pressure depending upon the difference between the actual crankcase pressure and a reference pressure related to a target crankcase pressure. 
         [0105]    The present invention provides for significant enhancement of compressor output control, as compared to the prior art. This is achieved by reducing the time between system input (change in the crankcase pressure) and feedback (pressure sensor signal) for a given change. In the prior art compressor control method, suction pressure has been used as a feedback reference. The control valve changes the pressure in the crankcase to change the output of the compressor. Due to such factors as the need for a compressor piston to stoke to change suction pressure, refrigerant compressibility, and the volume of the air conditioning system, I have discovered that there is a relatively long time delay between a change in the position of control valve and the resulting suction pressure change. There is also an inherent instability in the prior art compressor system that tends to drive the compressor to a particular state, for example, minimum output as the variable control is opened and the crankcase pressure increases to maximum. These factors cause the prior art compressor to tend to go to extremes, i.e., to maximum output or to minimum output, with small changes in the control valve setting. 
         [0106]    In the present invention, where crankcase pressure is monitored as the feedback reference, the effect of a control valve change upon the pressure in the crankcase, and thereby the compressor output, is recognized much sooner, as compared to the prior art, as the feedback reference is not communicated through the compressor mechanism and the air conditioning refrigerant volume. Consequently, the tendency of the compressor to overshoot is reduced as changes in the crankcase pressure quickly recognized and the variable control valve can be adjusted to minimize or eliminate compressor overshoot. 
         [0107]    Although the preferred embodiments have been described in relation to a compressor suitable for use in an automotive air conditioning system, it must be understood that the invention may be practiced with any suitable compressor or compressor system where crankcase pressure controls the capacity of the compressor. 
         [0108]    The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.