Design and Test Evaluation of Full Authority Digital Electronic Control System for an Auxiliary Power Unit

A helicopter auxiliary power unit (APU) is initially equipped with a hydro-mechanical control system (HMC). Because HMC's complex structure is difficult to be modified to realize sophisticated control algorithms and the APU is faced with the need for performance improvement, it is urgently necessary to carry out digital control modification of HMC. Based on the analysis of control laws of the original HMC and differences between HMC and digital control system, key techniques involved in the digital control system are studied, such as overall structure, control laws and fuel system based on electric fuel pump, and finally a full authority digital electronic control system (FADEC) is developed for APU. Functions, performances and key techniques of the FADEC system are evaluated on test rig, and the test results show functions of original control system are enhanced and performances of APU are improved more effectively under the control of the designed FADEC compared with the original HMC.