Fault-Tolerant Sensor Using Model Based Simulated Value for Space Environment Simulation Applications

Integrated spacecrafts and associated appendages have to be tested for performance in simulated space environments before launch. These tests involve simulation of thermal and vacuum environments for the space which are performed in thermo-vacuum chambers as well as thermal cycling tests for appendages. Temperature variations in these tests are performed by radiatively coupling the test object with an active surface heated and cooled by the media flowing within. Usually thermocouples placed on the test object act as the process value to which the control system responds by manipulating the temperature of the media in the active surface. Unlike other process applications where a fault in the control channel value can be tolerated or rectified, sensors in thermo-vacuum chambers are inaccessible. The failure of this thermocouple leads to loss of control or wrong control action. This paper introduces the application of model identification techniques to simulate the test object thermocouple data. The same simulated value is used as the process value for the control system. The developed application identifies linear models viz. ARX and ARMAX and non-linear models viz. NLARX and Neural. The simulated process value for the control system is automatically selected in case of constraint violation by the actual sensor value. Process dynamics can be observed and either PID or Fuzzy control can be applied. Both model based process value simulation and control gave excellent results.