Automatic control of Stirling cycle liquid nitrogen plant

The authors describe the design of an automatic controller, based on the Intel 8748 single-chip microprocessor, for a Stirling cycle nitrogen liquefying plant. The microcontroller has a real-time clock and a battery back-up system to monitor mains power failures and takes corrective action after the restoration of power. It controls the plant automatically without requiring the presence of an operator round the clock. The plant was automated by using solenoid valves at specific points to control electrically the existing manual valves. The controller continuously monitors the various subsystems such as cooling water flow rate, the pressure of the lubricant oil, etc., and switches off the plant when an error condition is detected. It displays in such cases a single digit error code corresponding to the error detected. It waits in a check-loop until the error is rectified by an operator and then starts the plant automatically.<>