Abstract:
A vehicle climate control system develops a fog index based on relative humidity at or near the inner surface of the windshield, and uses the fog index to variably override the normal control settings. The relative humidity is combined with a pair of humidity thresholds to form a dimensionless fog index, and fog index values within a predetermined range are applied to a variable path function for transitioning between existing control settings and control settings that maximize defogging.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/678,532, filed on Oct. 3, 2003, and assigned to the assignee of the present invention. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to climate control in a motor vehicle, and more particularly to a method of operation for automatically preventing window fogging.  
       BACKGROUND OF THE INVENTION  
       [0003]     In general, vehicle climate control systems include a controller that regulates a number of parameters such as blower motor speed, refrigerant compressor activation and/or capacity, air mixing door position, and discharge temperature. In a manual system, the operator directly or indirectly controls the parameters, while in an automatic system, the parameters are automatically controlled in response to a number of inputs, including cabin air temperature, outside air temperature and solar loading, to regulate the cabin air temperature at a set temperature selected by the operator. In either type of system, front and rear window defogging functions are ordinarily manually activated by the operator when the presence of fogging is noticed.  
         [0004]     The desirability of providing automatic activation of front and rear defogging functions has been recognized in the prior art. See, for example, the U.S. Pat. Nos. 4,920,755; 5,653,904; 5,701,752; and 6,311,505, the German Patent No. DE 19942286, and the Japanese Patent No. 60-248423. These are typically on/off systems that override existing control settings when fogging or the potential of fogging is detected. Additionally, the U.S. Pat. No. 6,508,408 to Kelly et al. discloses a technique for calculating a fog factor that quantifies the potential of fogging, and using the fog factor to variably bias the control settings toward values that maximize defogging. However, calculating the fog factor of Kelly et al. requires knowledge of the relative humidity and air temperature near the windshield, as well as the air temperature elsewhere in the passenger compartment, and the cost of sensors for measuring these parameters can make the system too expensive for many production vehicles. Accordingly, what is needed is an accurate and cost-effective way of characterizing windshield fogging potential, and a control method for implementing an automatic defogging control based on the fogging potential.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention is directed to an improved vehicle climate control system that develops a fog index based on relative humidity at or near the inner surface of a window, and uses the fog index to variably override the existing control settings. In a preferred embodiment, the relative humidity is combined with a pair of humidity thresholds to form a dimensionless fog index, and fog index values within a predefined range are applied to a variable path function for transitioning between existing control settings and maximum defog settings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a block diagram of a vehicle climate control system according to this invention, including a microprocessor based control unit;  
         [0007]      FIG. 2  depicts a three-zone automatic defog control according to this invention.  
         [0008]      FIG. 3  depicts a variable path control function used in connection with the three-zone defog control of  FIG. 2 ;  
         [0009]      FIG. 4  depicts a climate control override table look-up based on the variable control path function of  FIG. 3 .  
         [0010]      FIG. 5  is a flow diagram representative of a climate control carried out by the control unit of  FIG. 1  according to this invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0011]     Referring to  FIG. 1 , the method of this invention is described in the context of an automatic climate control system, generally designated by the reference numeral  10 . In the illustrated embodiment, the system  10  includes a variable capacity refrigerant compressor  12  having a stroke control valve  17  that is electrically activated to control the compressor pumping capacity. The compressor input shaft is coupled to a drive pulley  14  via an electrically activated clutch  16 , and the drive pulley  14  is coupled to a rotary shaft of the vehicle engine (not shown) via drive belt  18 , so that the compressor  12  can be turned on or off by respectively engaging or disengaging the clutch  16 . Other compressor arrangements are also possible, such as a fixed displacement compressor that is cycled on and off to control compressor capacity, or a variable displacement compressor that is directly driven by the vehicle engine without a clutch.  
         [0012]     The system  10  further includes a condenser  20 , an orifice tube  22 , an evaporator  24 , and an accumulator/dehydrator  26  arranged in order between the compressor discharge port  28  and suction port  30 . The electric drive motor  34  of cooling fan  32  is controlled to provide supplemental airflow for removing heat from high pressure refrigerant in condenser  20 . The orifice tube  22  allows the cooled high pressure refrigerant in line  38  to expand in isenthalpic fashion before passing through the evaporator  24 . The accumulator/dehydrator  26  separates low pressure gaseous and liquid refrigerant, directs gaseous refrigerant to the compressor suction port  30 , and stores excess refrigerant that is not in circulation. In an alternative system configuration, the orifice tube  22  is replaced with a thermostatic expansion valve (TXV); in this case, the accumulator/dehydrator  26  is omitted, and a receiver/drier (R/D) is inserted in line  38  upstream of the TXV to ensure that sub-cooled liquid refrigerant is available at the TXV inlet.  
         [0013]     The evaporator  24  is formed as an array of finned refrigerant conducting tubes, and an air intake duct  40  disposed on one side of evaporator  24  houses a ventilation blower  42  driven by blower motor  43  for forcing air past the evaporator tubes. The duct  40  is bifurcated upstream of the blower  42 , and an inlet air control door  44  is adjustable as shown to control inlet air mixing; depending on the door position, outside air may enter blower  42  through duct leg  44   a  as indicated by arrow  48 , and passenger compartment air may enter blower  42  through duct leg  44   b  as indicated by arrow  50 .  
         [0014]     An air outlet duct  52  disposed on the downstream side of blower  42  and evaporator  24  houses a heater core  54  formed as an array of finned tubes through which flows engine coolant. The heater core  54  effectively bifurcates the outlet duct  52 , and a re-heat door  56  next to heater core  54  is adjustable as shown to divide the airflow through and around the heater core  54 . The heated and un-heated air portions are mixed in a plenum portion  62  downstream of re-heat door  56 , and a pair of mode control doors  64 ,  66  direct the mixed air through one or more outlets, including a defrost outlet  68 , a panel outlet  70 , and a heater outlet  72 . The mode control door  64  is adjustable as shown to switch the outlet air between the defrost and panel outlets  68 ,  70 , as indicated by arrows  76 ,  78 , respectively. The mode control door  66  is adjustable as shown to control airflow through the heater outlet  72 , as indicted by arrow  82 .  
         [0015]     The above-described components of system  10  are controlled by the microprocessor-based control unit  90 , which is responsive to the normal automatic climate control inputs such as outside air temperature (OAT), solar loading (SOLAR), passenger compartment air temperature (PCAT), a set temperature (SET) and discharge air temperature (DAT), and a relative humidity input (RELHUM) for determining fogging potential. Other inputs not shown in  FIG. 1  include the usual operator demand inputs, such as the override controls for mode, blower motor  43  and rear window defogger grid  120 . A relative humidity sensor  94  for generating the RELHUM input is located on an inside surface  96  of windshield  98  as shown.  
         [0016]     In response to the inputs mentioned above, the control unit  90  develops output signals for controlling the compressor clutch  16 , the capacity control valve  17 , the fan motor  34 , blower motor  43 , and the air control doors  44 ,  56 ,  64  and  66 . In  FIG. 1 , the output signals CL, STR, FC and BL for clutch  16 , stroke control valve  17 , condenser fan motor  34 , and blower motor  43  appear on lines  104 ,  105 ,  108  and  107 , respectively. The air control doors  44 ,  56 ,  64 ,  66  are controlled by corresponding actuators  110 ,  112 ,  114 ,  116  via lines  106 ,  113 ,  115  and  117 , respectively. Additionally, the control unit  90  generates an output signal RDEF on line  119  for controlling activation of rear window defogger grid  120 .  
         [0017]     The present invention is directed to a control carried out by the control unit  90  that automatically overrides the default climate control settings (manual or automatic) for the purpose of preventing the formation of fog on the windshield  98  or eliminating fogging as quickly as possible. Since windshield fogging can occur at different times and for a number of different reasons, reliable prevention of fogging requires an accurate and reliable judgment of the fogging potential. Fundamentally, this invention recognizes that the potential for fogging can be reliably indicated simply by quantizing the nearness of RELHUM to 100%, since fogging is deemed to be present when RELHUM equals 100%. Thus, the fogging potential can be defined as the difference (RELHUM−RH_LO), where RH_LO is a relative humidity threshold such as 90%. However, since the distribution of relative humidity near the windshield  98  tends to be non-uniform, the fogging potential is preferably defined in reference to a second relative humidity threshold RH_HI representing an upper control band limit. In this case, the threshold values RH_LO and RH_HI effectively divide the range of fogging potential into three zones: (1) a first zone (RELHUM≦RH_LO) where the air is sufficiently dry that no defogging action is required; (2) a second zone (RELHUM≧RH_HI) where heavy fogging is expected and maximum defogging action is required; and (3) a third zone (RH_LO&lt;RELHUM&lt;RH_HI) of possible fogging between the first and second regions. The first, second and third zones are identified in  FIG. 2  as ZONE I, ZONE II and ZONE III, respectively. The thresholds RH_LO and RH_HI thus essentially define a transition band in which the control transitions between default climate control settings and maximum defog settings.  
         [0018]     The trace(s) depicted in  FIG. 2  represent the amount or degree of defogging control action to be taken by control unit  90 . In Zone I where RELHUM&lt;RH_LO, the defog control is inactive, and the control unit outputs are the default control settings. In Zone II where RELHUM&gt;RH_HI, the maximum control setting override is put into effect, and in Zone III, a partial control setting override is put into effect. The control path function in Zone III may be proportional as indicated by the linear trace  130 , or nonlinear as indicated by the traces  132  and  134 . The nonlinear control path designated by the trace  132  provides a relatively aggressive defogging response; it can be used to initiate preemptive defogging response in anticipation of fog formation, or can be used to compensate for the slow response of certain relative humidity sensors. On the other hand, the control path designated by the trace  134  provides a subdued response at the early stage of fogging and a steep ramp-up near the upper limit; this response characteristic can be used to optimize passenger comfort at the early stages of fogging while still ensuring sufficient defogging when the fogging risk is more significant.  
         [0019]     The thresholds RH_LO and RH_HI are calibrated in a humidity-controlled environmental chamber. If testing shows that the location of relative humidity sensor  94  tends to lag in fog formation, RH_HI should be assigned a value less than 100% (95%, for example) to gain a more aggressive response for the fogging part of windshield  98 . On the other hand, RH_HI should be assigned a value greater than 100% (105%, for example) if the location of sensor  94  tends to lead in fog formation. Also, RH_LO and RH_HI can be calibrated to vary as a function of the outside air temperature OAT to provide accurate operation in any season.  
         [0020]     In a preferred implementation, the control unit  90  computes a normalized FOG_INDEX as follows:  
                                       FOG_INDEX = (RELHUM − RH_LO)/(RH_HI − RH_LO)   (1)                  
 
         [0021]     In Zone I, FOG_INDEX≦0, in Zone III, 0&lt;FOG_INDEX&lt;1, and in Zone II, FOG_INDEX≧1. The term FOG_INDEX varies linearly with the severity of fogging risk and gives a sense of the fogging severity in understandable terms: 0% (i.e., 0) is no fogging and 100% (i.e., 1) is heavy fogging. The control path calibration for Zone III can be advantageously implemented by defining a fog factor a that is a power function of FOG_INDEX as follows:  
                                                       α = FOG_INDEX n     (2)                      
 
 The exponent “n” in the power function allows different control paths to be selected as illustrated in  FIG. 3 . When the exponent n is in the range of zero to one, the corresponding control paths provide relatively aggressive application of defogging control response, as mentioned above in respect to trace  132  of  FIG. 2 . When the exponent n equals one, a linear or proportional response is realized. And when the exponent n is greater than one, the corresponding control paths provide delayed response characteristics, as mentioned above in respect to trace  134  of  FIG. 2 . 
 
         [0022]      FIG. 4  illustrates possible defogging control responses as a function of the fog factor a for 0&lt;α&lt;1. The trace  140  represents an offset for the commanded discharge air temperature, and illustrates that as the fog factor a increases above zero, the discharge air temperature is immediately increased, while the other control parameters are kept unchanged. The trace  142  represents an offset for the commanded inlet air door position, and shows that when the fog factor a reaches a predefined threshold, the air inlet door  44  is switched to the full outside air position if it is not there already. The trace  144  represents a minimum blower speed, and illustrates how the blower speed is increased with increasing values of fog factor a to ensure sufficient airflow. The trace  146  represents an air delivery mode setpoint, and shows that the air delivery mode of the HVAC system is gradually shifted toward Defrost mode from a pure Vent mode. Finally, the trace  148  represents an offset for the commanded stroke or capacity of compressor  12 , and shows that the compressor stroke increases with increasing values of fog factor α. When the fog factor a reaches one, the discharge air temperature will be at full hot, the air inlet door  44  will be set to full outside air, the blower motor  43  will be operating at maximum speed, and the compressor  12  will be at full stroke. Each of these measures is designed to increase the defogging capability of the air stream impinging on the windshield  98 . In the outside air mode, fresh air from outside of the vehicle is introduced into the cabin to flush out the humidity accumulated in the vehicle cabin. Turning on the compressor  12  (when ambient temperature permitting) allows the air stream to be dehumidified in the evaporator  24 . Heating the outlet air to a higher temperature before discharge reduces the relative humidity level. Higher airflow rate across the windshield  98  during defogging operation increases the heat and mass transfer coefficient, which allows the speedy removal of accumulated moisture.  
         [0023]      FIG. 5  designates a flowchart representative of a software routine executed by the control unit  90  for carrying out the above-described control. The blocks  150  and  152  obtain the input RELHUM and set the thresholds RH_LO and RH_HI. The block  154  calculates FOG_INDEX using equation (1) and block  156  calculates the fog factor a using equation (2). The block  158  then selects defog control responses as a function of fog factor α by table look-up, completing the routine.  
         [0024]     In summary, the control of this invention provides a reliable and cost-effective method of variably overriding normal climate control settings to automatically prevent window fogging. The control response region and the aggressiveness of the response can be calibrated to suit the vehicle manufacturer. While this invention has been described in reference to the illustrated embodiment, it is expected that various modifications in addition to those mentioned above will occur to those skilled in the art. For example, the control may be applied to any vehicle window, and in vehicles having very large windshields, it may be desirable to use more than one humidity sensor on the windshield. Also, the control is applicable to so-called manual control systems in which the vehicle operator manually controls compressor activation, fan speed, mode and air inlet functions. Thus, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.