For improving reliability and security of unmanned helicopter system under harsh environment, this paper proposes a dual redundant rotary brushless motor based on FPGA, and separately introduce the operating principle of rotary transformer, demodulation interface circuit of excitation, signal acquisition of FPGA, and fault detection, diagnosis and switching of rotary transformer. Analyze the rotary fault detection method, and put forward the calculation that the dual redundant rotary transformation process is stable. Simulation results show that the system work is stable and reliable and it improves normal working performance when the rudder control system was under harsh environments.