Abstract:
A method for controlling a vehicle air-conditioning system for cooling an interior of a vehicle is disclosed. The vehicle air conditioning system has a compressor coupled to an electronic control valve. The method includes reading a user manipulatable switch, determining a desired vehicle interior temperature based on the read user manipulatable switch, reading a plurality of sensors indicative of an interior and an exterior climate of the vehicle, determining a heat load on the vehicle air conditioning system, determining a desired evaporator discharge temperature, evaluating a humidity level inside the vehicle by determining a humidity ratio, filtering the updated electronic control valve duty cycle to obtain a new electronic control valve duty cycle based on the desired evaporator discharge temperature, and sending the new electronic control valve duty cycle to a compressor controller, wherein the controller is in communication with the electronic control valve and commands the valve to operate at the new duty cycle.

Description:
TECHNICAL FIELD 
     The present invention relates to systems and methods for controlling the operation of automotive air conditioning compressors, especially variable displacement compressors which may be regulated for optimal operation for a particular engine operating state and environmental condition. 
     BACKGROUND 
     Electronically controlled automotive air conditioning compressors are well known in the prior art. Typically, prior art electronically controlled compressor systems include an electronic control module in communication with various sensors for measuring vehicle interior and exterior environmental conditions, switches for actuating various air conditioning system modes, output ports for relaying output signals to actuate various system components, such as vent doors, blower motor, fans, and valves. 
     These electronically controlled compressors require a control strategy to optimize system operation. Without a control strategy capable of optimizing the performance of the air conditioning system, there is little justification for electronically controlling the compressor as compared to mechanically controlling the compressor. Generally, electronically controlled compressor systems weigh more, are more expensive, and require more sensors than their mechanical counterpart. 
     However, with optimum control of the electronically controlled compressor systems, the inefficiencies of mechanically controlled compressors, that are operated at lower evaporator temperatures than otherwise required (typically around 35 F) may be avoided. Such air conditioning systems having mechanically controlled compressors, thus do more work than is required in the vast majority of operating conditions. 
     Therefore, what is needed is a new and improved method for controlling electronically controlled automotive air conditioning compressors. The new and improved method must not run the compressor unnecessarily. Moreover, it must not create a passenger compartment environment that is prone to fogging or is too humid. 
     SUMMARY 
     A method for controlling a vehicle air-conditioning system for cooling an interior of a vehicle is provided. In an aspect of the present invention the vehicle air conditioning system has a compressor coupled to an electronic control valve. In another aspect of the present invention, the method includes reading a user manipulatable switch, determining a desired vehicle interior temperature based on the read user manipulatable switch, reading a plurality of sensors indicative of an interior and an exterior climate of the vehicle, determining a heat load on the vehicle air conditioning system, determining a desired evaporator discharge temperature, evaluating a humidity level inside the vehicle by determining a humidity ratio, filtering the updated electronic control valve duty cycle to obtain a new electronic control valve duty cycle based on the desired evaporator discharge temperature, and sending the new electronic control valve duty cycle to a compressor controller, wherein the controller is in communication with the electronic control valve and commands the valve to operate at the new duty cycle. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic diagram of an air conditioning system for an automobile having a variable displacement compressor, in accordance with the present invention; 
     FIG. 2 is a schematic diagram of a variable displacement compressor that is selectively driven by the engine, in accordance with the present invention; and 
     FIGS. 3-5 are a flowcharts illustrating a method for controlling the variable displacement compressor, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1 an automotive air conditioning or climate control system  10  is schematically represented, in accordance with the present invention. System  10  includes an air conditioning duct which defines an air passage  14  for directing conditioned air into a passenger compartment. 
     Air conditioning duct  12  includes a plurality of inlets and outlets for drawing in outside air and for directing conditioned air into the passenger compartment. For example, the inlets include an outdoor air inlet  16  for drawing in outside air, and an inside air recirculation inlet  18  for recirculating air contained within the passenger compartment. A mode selector door  20  driven by a small motor  22  is provided to allow a passenger to select between an outside intake mode and an inside air recirculation mode. 
     Further, a blower  24  such as a centrifugal blower is provided within air conditioning duct  12  for producing air flow from the air inlets to the air outlets. Blower  24  further includes a centrifugal fan  26  and a motor  28 . Motor  28  is controlled by a motor driver circuit  30 . 
     Air conditioning duct  12  further includes a plurality of air outlets for directing air conditioned air to various parts of the passenger compartment. More specifically, a defroster outlet  32  is provided for directing conditioned air to a vehicle windshield  34 . A defroster mode is selected by actuating a defroster door  36 . Further, an upper body air outlet  40  is provided for directing conditioned air toward a vehicle occupant&#39;s upper body. An upper body selection mode is selected by actuating an upper body air mode door  42 . Similarly, a foot air outlet  44  is provided for directing conditioned air towards the feet of vehicle occupants. Preferably, a foot air mode door  46  is provided for selecting a foot air mode. 
     With continuing reference to FIG. 1, a heater unit  50  having a heater core is provided for heating cold air passing by an evaporator unit  52 . Typically, the heater core is supplied with heated cooling water from the engine  11 . During the heating cycle of the air conditioning system, the heater unit  50  acts as a heat exchanger using the heater cooling water to heat the cold air passing through the evaporator  52 . An air regulator door  54  is provided for regulating the amount of air heated by the heater unit  50 . 
     Evaporator  52  is in fluid communication with a compressor  60 . Compressor  60  is preferably a variable displacement compressor, or a fixed displacement compressor or a mechanically controlled compressor, that draws in refrigerant, compresses the refrigerant and discharges the refrigerant. Evaporator  52  is also in communication with an expansion valve  62 . Expansion valve  62  expands the liquid refrigerant fed from a receiver  64 . Receiver  64  performs vapor liquid separation of the refrigerant fed from a condenser  66 . Condenser  66  condenses and liquefies the refrigerant fed from compressor  60  through heat exchange with outdoor air. Condenser  66  is cooled by a cooling fan  68  which is driven by a driver motor  70 . 
     Compressor  60  further includes an electromagnetic clutch  72  that is in communication with a compressor drive pulley  76  for engaging and disengaging a drive belt  78  driven by engine  11 . However, in alternative embodiments of the present invention compressor  60  does not include an electromagnetic clutch and thus is in continuous engagement with engine  11 . 
     An air-conditioning system control unit  82  (ACU) is further provided for controlling the operation of the air conditioning system in accordance with the present invention. Air-conditioning control unit  82  includes a microprocessor  84 , read only memory (ROM)  86 , and random access memory (RAM)  88  and other conventional computer components. The ACU is supplied power by the vehicle battery  90  when the ignition switch  92  is switched on. A plurality of switches and sensors are in communication with ACU  82  for sending to the ACU electrical signals indicative of air conditioning environmental factors necessary for determining how to optimally air condition the passenger compartment. The sensors include, for example, an indoor air temperature sensor  94  for determining the temperature of the air inside the passenger compartment, an outdoor air temperature sensor  96  for determining the temperature of the outside air, a solar radiation sensor  98  for determining the intensity of the solar radiation incident on the passenger compartment, a post evaporator temperature sensor  100  detects the actual air cooling by the evaporator, a humidity sensor  102  for detecting a relative humidity of air inside the passenger compartment and a rotational speed sensor  104  for detecting rotational speed of engine  11 . 
     The switches for manual control of the air conditioning system  10  include, for example, a temperature setting switch  106  for setting a desired indoor air temperature to a desired temperature level, an indoor/outdoor air selector switch  108  for selecting outdoor air intake mode or indoor air recirculation mode, an air conditioning on/off switch  110  for turning on and off the air conditioning system, and an automatic mode switch  112  for selecting automatic air conditioning operation. Further, control unit  82  has a plurality of output ports  114  for sending control signals to the various air conditioning system components. For example, control signals are sent to the various vent doors, fan motors, and the variable displacement compressor  60 . 
     Referring now to FIG. 2, a schematic diagram of variable displacement compressor  60  is shown in greater detail, in accordance with the present invention. Compressor  60  includes a driveshaft  140  that is operatively coupled to an external drive source such as vehicle engine  18  by electromagnetic clutch  72  and to electric motor  20 . A swashplate  142  is rotatably secured to shaft  140  and is pivotable about the driveshaft. A pair of guide arms  161  and  162  are attached to swashplate  142  at a first end and to pistons  150  and  151  at a second end. The engagement between guide arms  161 ,  162  and the associated pistons guides the inclination of the swashplate  142  and rotates the swashplate with respect to the driveshaft  140 . Driveshaft  140  and swashplate  142  are positioned within a crankcase chamber  147 . The pressure in crankcase chamber  147  controls the angle of inclination of the swashplate. 
     Generally, compressor  60  further includes a cylinder housing  148  having cylindrical bores  144  and  145  extending therethrough. Each bore  144  and  145  accommodates one piston  150 ,  151 . Each piston and bore define compression chambers  153 ,  155 . Alternatively, each piston may be coupled to the swashplate by a pair of shoes (not shown). Rotation of the swashplate is converted into reciprocation of pistons  150 ,  151  in bores  144 ,  145  by means of the shoes, as well known in the art. 
     Further, compressor  60  includes a rear housing  170  having a suction chamber  172  and  173  and a discharge chamber  174 . Suction ports  176  and  177  and discharge ports  178  and  179  are also provided at each chamber. A suction valve (not shown) is provided at each suction port for opening and closing the suction port. A discharge valve (not shown) is provided at each discharge port for opening and closing the discharge port. Further, a bypass port or orifice  175  is provided between crankcase chamber  147  and suction chamber  172 . 
     As each piston  150 ,  151  moves from a fully extended position to a fully retracted position refrigerant is drawn into the corresponding suction port from the suction chamber to enter the associated compression chamber. Conversely, when each piston moves from a fully retracted position to a fully extended position, the refrigerant is compressed in compression chambers  153 ,  155  and the discharge valve opens allowing refrigerant to flow into discharge chamber  174  through associated discharge ports  178 ,  179 . The inclination of swashplate  148  varies in accordance with the difference between the pressure in crankcase chamber  147  and the pressure in compression chambers  153 ,  155 . More specifically, the difference between the pressure in crankcase chamber  147  (PC) and the pressure in the suction chambers  172 ,  173  (PS) or the pressure difference “PC−PS” determines the inclination of the swashplate. PC is maintained at a pressure value that is higher than the suction pressure PS (PC&gt;PS). An increase in the pressure difference PC−PS decreases the inclination of the swashplate. This shortens the stroke of each piston  150 ,  151  and decreases the displacement of compressor  60 . On the other hand, a decrease in pressure difference PC−PS increases the inclination of swashplate  142 . This lengthens the stroke of each piston  150 ,  151  and increases the displacement of compressor  60 . 
     In FIG. 2 swashplate  142  is indicated by solid-lines (a) in a first position (position a). When the swashplate is in position (a) the pistons  150 ,  151  do not reciprocate within chambers  153 ,  155 . Compressor  60  is at its minimum displacement. As indicated by dashed-lines (b) the swashplate may be disposed in a second position (position b). Position (b) illustrates the maximum angle of inclination the swashplate can achieve. This is also the position in which compressor  60  achieves its maximum displacement. Depending on the pressures in crankcase chamber  147 , suction chamber  172  and discharge chamber  174  the swashplate may be inclined at any angle between position (a) and (b) achieving variable displacement. 
     An electronic control valve  200  is in communication with the discharge chamber  174 , through a refrigerant/oil separator  202 , and with the crankcase chamber. Electronic control valve  200  regulates the pressure in crankcase chamber  147 , suction chamber  172  and discharge chamber  174 , by selectively opening and closing communication ports connecting the crankcase chamber to the discharge chamber. A control strategy for actuating valve  200  will be described hereinafter. 
     The electromagnetic control valve  200  serves to regulate the discharge capacity of compressor  60  by changing a set level of suction pressure of the compressor according to a control current supplied by the air conditioning electronic control unit  82 . 
     In a preferred embodiment of the present invention a control strategy for controlling the operation of electromagnetic control valve  200  is implemented in software, or in hardware or in both software and hardware. For example, control logic for controlling the operation of control valve  200  in one embodiment is stored in the ACU&#39;s read only memory  86 . 
     Referring now to FIG. 3, a variable compressor and valve control strategy  201  is illustrated in flow chart form, in accordance with the present invention. The initial step of the control strategy is to determine the load acting on the AC system. The thermal load is determined by analyzing four elements (1) the fresh air and body leakage air intake load, (2) the convection and conduction losses through the body of the car, (3) the solar gain load through the car, and (4) the thermal inertia which must be overcome to bring the interior temperature of the car down to a desired level. The fresh air and body leakage load is calculated as a function of blower speed, the blend door position, the recirculation door position, and the interior and exterior temperatures. The blower speed and flow rate determines how much of the fresh air is being injected into the vehicle. This control strategy is based on the assumption that if the blower is in recirculation mode, then 20% of the flow is outside air and 80% of the flow is inside air. If the mode doors are set for floor/defrost or defrost, then this strategy assumes that the AC system is set in fresh air mode. The fresh air and body leakage load may be described by the following equation: 
     
       
           {dot over (Q)}   fresh   ={dot over (m)}   blower   ·K   door   ·C   air ·( T   amb   −T   set ) 
       
     
     where: 
     K door =0.8 fresh    
     {dot over (m)}=mass flow rate of blower 
     T amb =ambient air temperature 
     T set =set temperature 
     The body conductivity losses should be based on actual or simulated test data recorded at 110 F. Body leakage is a function of the inside and outside air temperature difference and the thermal insulation characteristics of the vehicle. 
     The convection losses through the body of the vehicle are determined first by conducting thermal testing of the vehicle in question to determine the heat absorption rate at a given temperature. Using this data a convection constant (K con ) is determined, and the following equation describes the convection load: 
     
       
           {dot over (Q)}   con   =K   con ·( T   amb   −T   set ) 
       
     
     where:          K   con     =     0.012   +       S   veh     ·         1.0     110   -   70       -     0.75     110   -   70           96   -   48                                  
     S veh =Speed of Vehicle (km/hr) 
     The sun load is a function of the measurements from a sun load sensor and also particular characteristics of a given vehicle. Again, vehicle testing would be required to determine the amount of energy a vehicle absorbs under full sun load. The sun load may be described by the following equation: 
     
       
         
           {dot over (Q)} 
           sun 
           =K 
           sun 
           ·T 
           sun 
         
       
     
     where: 
     K sun =0.67 m 2    
     1 kWm −2 ≧T sun ≧0 kWm −2    
     The remaining load determines the thermal inertia load. This load is a function of the interior temperature and the vehicle occupant&#39;s desired interior temperature. Desired interior temperature is determined by reading control switches and buttons, as represented by block  202 . In an embodiment of the present invention, the compressor is operated at a maximum capacity until the desired temperature is reached. Preferably, the load is based on the difference between the current interior temperature and the desired temperature. This allows the two temperatures to converge asymptotically and thus avoid overshoot. An acceleration timer can be used to increase the speed of convergence. The thermal inertia load may be described by the following equation: 
     
       
           {dot over (Q)}   inertia   =K   acc   ·{dot over (m)}   blower   ·C   air ·( T   int   −T   set ) 
       
     
     Thus, the total load is calculated by summing the above loads as described by the following equation: 
     
       
         
           {dot over (Q)} 
           tol 
           ={dot over (Q)} 
           fresh 
           +{dot over (Q)} 
           con 
           +{dot over (Q)} 
           sun 
           +{dot over (Q)} 
           inertia 
         
       
     
     At block  204 , the various system sensors described above are read. Three conditions are checked at blocks  206 ,  208  and  210 . All of these conditions must be met to continue strategy  201 . The first condition, represented by block  206  is to determine whether the ambient outside air temperature is greater than a predefined minimum temperature, and whether the vehicle ignition is “on”. If the ambient air temperature is greater than the predefined temperature and the ignition is “on”, the next condition is checked, at block  208 . However, if the ambient temperature is not greater than the predefined minimum temperature and/or the ignition is “off”, then control valve  200  is not activated, as represented by block  212 . The next condition checked is whether climate control system  10  has been activated, as represented by block  208 . If the system is “on”, then the third condition is checked, as represented by block  210 . If system  10  is not “on”, then control valve  200  is not activated, as represented by block  212 . At block  210 , the strategy determines whether the electromagnetic clutch  72  is engaged. If the clutch is not engaged, then valve  200  is not activated, as represented by block  212 . However, if the clutch is engaged then the desired evaporator discharge air temperature is determined, as represented by block  214  and further in FIG.  4 . 
     In FIG. 4, a method  280  for determining the desired evaporator discharge air temperature (T et ) is illustrated, in accordance with the present invention. If climate control system  10  has been requested, the system sets the T et  to the lower of the driver (T des1 ) or passenger (T des2 ) desired temperatures in a dual zone system, at block  290 . At block  300 , the system determines whether defrost or floor/defrost modes are selected. If defrost or floor/defrost modes are activated, then T et  is set for maximum dehumidification. However, if defrost or floor/defrost modes are not activated, then the system determines if the temperature is set to maximum cooling mode, as represented by block  304 . 
     If the temperature is set to maximum cooling, then T et  is set for maximum cooling, as represented by block  306 . However, if temperature is not set to maximum cooling, the system determines whether the temperature is set for maximum heating, as represented by block  308 . If the system determines that the temperature is set to maximum heating, then valve  200  is not activated and T et  is set equal to T desired , where T desired  is equal to the maximum system temperature (T max ), as represented by block  310 . However, if the temperature is not set to maximum heating, then the system sets T et  to the greater of T et  and the minimum temperature (T min ), as represented by block  316 . 
     The next step, as indicated by block  216 , is to evaluate the humidity level in the vehicle and determine what steps are necessary to prevent fogging. With reference to FIG. 5, a method  318  for evaluating the humidity level in the passenger compartment to prevent fogging is illustrated. This is accomplished by setting a target temperature for air passing through the evaporator and modulating the compressor accordingly to achieve the target temperature. Having calculated the load and knowing the air mass flow rate ({dot over (m)}) from previous calculations shown above, T desired  and T et  may be described by the following equation:          T   desired     =       T   ei     -         Q   .     tot           m   .     blower     ·     C   air                                  
     T et =T desired    
     T et =evaporator inlet temperature 
     As illustrated in this equation, the evaporator capacity is modulated based on the thermal loading on the system. 
     At block  320  the humidity level within the passenger compartment is measured by a humidity sensor. If the humidity level is too high, irrespective of the interior or exterior conditions, the air within the passenger compartment must be cooled to remove the humidity from the air. A target evaporator discharge temperature of approximately 55° F. is selected, which falls within the normal comfort level, as defined by ASHRAE. When the air is reheated, the air will fall into a comfortable region. In order to determine if the humidity is too high, a humidity ratio must be evaluated. The humidity ratio is evaluated by referring to table 1 below and by measuring the humidity, using the humidity sensor, and the temperature using the temperature sensor, as represented by blocks  320  and  322 . The humidity ratio is then evaluated, as represented by block  324 . Preferably, table 1 is stored in system memory. The humidity ratio is compared to a target humidity ratio such as approximately 0.009, as reprsented by block  326 . If at a given temperature the relative humidity is greater than the relative humidity shown in the table 1, then the humidity ration is determined to be greater than the target humidity ratio. The air must then be dehumidified, as represented by block  328 . Table 1 below shows the temperature versus humidity values for a humidity ratio of 0.009 kg water/kg air. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Temperature Vs. Relative Humidity at 0.009 Humidity Ratio 
               
             
          
           
               
                   
                 Temperature 
                 Relative Humidity 
               
               
                   
                   
               
             
          
           
               
                   
                 54 
                 100 
               
               
                   
                 57 
                 90 
               
               
                   
                 60 
                 80 
               
               
                   
                 64 
                 70 
               
               
                   
                 69 
                 60 
               
               
                   
                 74 
                 50 
               
               
                   
                 81 
                 40 
               
               
                   
                 90 
                 30 
               
               
                   
                 103 
                 20 
               
               
                   
                   
               
             
          
         
       
     
     If the measured interior air has a humidity ratio above 0.009 kg water/kg air, then the air must be cooled to 55° F. This is due to the fact that the humidity ratio at 55° F. and 100% relative humidity is 0.009 kg water/kg air. The following control logic statement may be used in the control strategy to accomplish this objective: 
     IF((T et )55 F) and (HR)0.009))T et =55° F. 
     Finally, the control strategy determines if fogging is probable, as represented by  332 . If fogging is likely, the compressor will be operated to produce the lowest evaporator discharge temperature possible to remove or dilute the moisture in the air, as represented by blocks  332  and  334 . The following control logic statement may be used to accomplish this objective: 
     IF(Fogging Probability=High)T et =35° F. 
     Fogging occurs when the humidity in the vehicle is high enough that water condenses on the interior of the car. The strategy returns to the main program at block  336 . 
     Having decided upon the target evaporator outlet temperature, the strategy returns to FIG.  3 . The next step is to determine the output current for the compressor, as represented by block  218 . The control current/depends on current control setting for the compressor and the difference between the actual evaporator outlet temperature (T et ) and the real evaporator temperature (T evapout ). The following closed loop control logic may be used: 
     ΔT=T et −T evapout           I     l   +   1       =       I   t     +       I   max     ·   K   ·       Δ                 T     10                                
      where. 
     I l+1 ≦I max    
     ΔT≦10 
     As any person skilled in the art of electronic control automotive air conditioning compressors will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention defined in the following claims.