A Real-Time Computer System for the Control of Refrigerant Flow 970108
This paper presents a real-time computer system for the control of refrigerant flow in an automotive air conditioning system. This is an experimental system used to investigate the potential advantages of electronic flow control over conventional flow control (using an orifice tube or thermal expansion valve). Two features of this system are presented. First, the system organization is described. Second, the control and interface software are presented. The emphasis is on the software.
The system is organized as a closed loop control system. The inputs to the controller are measurements of the refrigerant system. In particular, thermocouples are used to measure the refrigerant temperature before and after the evaporator. The analog thermocouple signals are converted to digital form by an off-the-shelf, portable, data acquisition system (DAQ). Via a parallel port link, these digital measurements are transfered to a laptop computer. The laptop software processes the measurements and determines a desired refrigerant valve opening. Via the parallel port, the desired valve opening is passed back to the DAQ. The DAQ converts the desired setting from a digital value to a low power analog signal. Using this signal as an input, an analog power amplifier drives the valve to the desired opening. The new valve opening increases or decreases flow of refrigerant into the evaporator. This change in flow effects the temperatures measured by the thermocouples. This closes the control loop.
The software includes a real-time digital controller and a user interface. The software was custom designed using Visual Basic and runs under Windows. The DOS/Windows environment is generally problematic for real-time control, but has proven to be suitable for refrigerant flow control. Successful operation under Windows is made possible by resources provided by the DAQ, by careful limitation of Windows activities, and by the relatively slow dynamics of the air conditioner. In addition to the real-time controller, the software simultaneously provides a graphical user interface (GUI). The GUI allows the user to select and change controllers on the fly in real-time. The GUI also provides the user with a “virtual” strip chart. The strip chart aides the user in tuning the controller during operation. This software has been used on the road in a test vehicle and has proven to be very effective in the development of refrigerant flow control methods.