Controlling superheat in a refrigeration system

DETAILED DESCRIPTION The present invention provides a unique and novel way of electrically controlling the flow of refrigerant in a vehicle air conditioning system and utilizes a microcomputer receiving a temperature input signal from a single thermistor for generating a width-modulated pulse control signal and having the pulse width or duty cycle thereof varied to provide proper control of refrigerant flow to maintain the passenger compartment at the desired comfort level.
The control signal for the electrically operated refrigerant expansion valve of the present invention is generated by modulating the pulse width or duty cycle about a predetermined value in accordance with an algorithm or table of values.
The predetermined value of the control signal pulse width is developed from calibrating the system at steady state thermal loading for a desired amount of superheat of the vaporized refrigerant returning to the compressor from the evaporator.
The algorithm for modulating the pulse width of the control signal about the predetermined duty cycle is based upon coefficients developed from experimentally operating the system under varying thermal loading conditions.
The present invention employs a self-heated thermistor which senses the presence of liquified refrigerant in vaporized refrigerant returning to the compressor from the evaporator.
The thermistor has a limited current flowing therethrough sufficient to maintain the thermistor at a temperature above the temperature of the refrigerant in the vaporized condition.
The change in resistance of the heated thermistor due to the presence of any liquid in the refrigerant flow is detected.
The change is resistance is converted to a digital representation in the form of an electrical signal pulse time.
A reference pulse time is determined by calibrating the system for a predetermined desired amount of superheat with steady state thermal loading on the system.
In service, under varying thermal loading conditions on the system, the microcomputer compares the reference pulse time with the varying pulse signals; and, in accordance with an algorithm, or predetermined table of values, determines a suitable width-modulated pulse signal for operating the expansion control valve to maintain the system at the desired level of superheat