Method for controlling a pulsed expansion valve

Abstract

A method of controlling the duty cycle of a pulse width modulated expansion valve in order to achieve low and stable evaporator superheat is disclosed. Duty cycle is incremented/decremented if superheat is above/below a dead band, and, in a preferred embodiment, incremented/decremented if the time derivative of superheat is above/below a preset value.

1 . A method for stable control of evaporator superheat in a vapor compression refrigerator, evaporator superheat symbolized herein by SH, SH defined as the difference between evaporator outlet and inlet temperatures, the refrigerator including a compressor, a condenser, a pulse width modulated expansion valve, an evaporator, and a thermostatic control for turning the compressor on when the temperature of the interior of the refrigerator rises above a preset value and turning it off when the interior temperature falls below a preset limit, the method consisting generally of controlling the duty cycle of the pulse width modulated expansion valve in response to evaporator superheat, duty cycle defined as the ratio of the time the expansion valve is open to the time interval between successive openings of the expansion valve, duty cycle symbolized herein by DCY, the method specifically comprising the following elements, 
 1) temperature sensors responsive to SH,    2) an electronic control which periodically responds to SH signal from the temperature sensors by adjusting DCY as follows: 
 if SH is within a preset range referred to herein as the “dead band”,  
 the control does not change existing DCY;  
 if SH is above the dead band, the control increments DC; and  
 if superheat is below the dead band, the control decrements DCY.  
   
   
   
       2 . A method according to  claim 1 , in combination with the following additional operations,  
     
       
         
           
             
               
                 
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                         SH 
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                   first 
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                   preset 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   value 
                 
               
               , 
               
                 increment 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 DCY 
               
               , 
               
                 
 
               
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                   if 
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                   ⁢ 
                   
                     
                       ⅆ 
                       
                         ( 
                         SH 
                         ) 
                       
                     
                     
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                       t 
                     
                   
                 
                 ≤ 
                 
                   second 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   preset 
                   ⁢ 
                   
                       
                   
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                   value 
                 
               
               , 
               
                 decrement 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 DCY 
               
               , 
             
             ⁢ 
             
                 
             
           
         
       
     
     where, in the above, t represents time.