Computer Control of an External Active Thermal Control System for the International Space Station

An overview of the computer control of an ammonia-based External Active Thermal Control System (EATCS) for the International Space Station Alpha (ISSA) is presented. The control system relies on the use of Honeywell-provided Multiplexer-Demultiplexers (MDMs) and code produced using the Matrix_x rapid-prototyping tool. The functions of the EATCS (collecting, distributing, and disposing of thermal energy) are managed by an MDM designated as the external (EXT). The EXT MDM validates commands and initiates control sequences in response to operator commands by sending separate commands to lower-level MDMs. Two of the lower-level MDMs are the S1-1 and P1-2 (otherwise known as the S1/P1 MDMs), named after the ISSA truss segments on which they arrive (Starboard segment 1 and Port segment 1). These computers, in conjunction with the system's Pump and Control Valve Package (PCVP) firmware controllers (FWCs), are responsible for controlling the pumps, ammonia tanks, regulating/control valves, and isolation/ bypass valves for the distribution of the ammonia. Another pair of lower-level MDMs are the S0-1 and S0-2 MDMs, again named after the truss segment on which they arrive on-orbit. These MDMs control valves at the inlet and outlet of the heat exchanger Orbit Replaceable Units (ORUs) that acquire thermal energy from the pressurized elements. The last set of lower-level computers are named for the thermal radiator wings upon which they reside (Starboard Thermal Radiator and Port Thermal Radiator). These MDMs are responsible for controlling the disposal of thermal energy, including the radiator valving and radiator deployment/retraction mechanisms. An overview of the EATCS hardware is provided for background purposes. The software requirements for the EATCS computers (detection of failures and functional recovery, sensor monitoring, system operations, command execution, and status reporting) are described.