Method for controlling a pulsed expansion valve

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.