Phase locked loop control of turboprop aircraft engines for cancellation of interior or exterior noise

Abstract

Interior and exterior noise generated by multi-engine, turboprop aircraft remains a serious problem. In this paper, two phase locked loop schemes are used for engine synchro-phasing to effect noise cancellation in a twin engine OV-ZOA aircraft. For interior noise, a sampled microphone technique serves as the phase detector. For exterior noise, a digital timer/wunter board monitoring engine pulse tachometers is used Both schemes use high bandwidth, inner frequency lock loops to improve capture range. Principle of Noise CancellationThe noise generated by turboprop aircraft engmes poses problems to crew and passengers in the cabin during flight, and to surrounding communities in the far field during take-off and landing. Multi-engine turboprop aircraft noise can be minimized if the proper speed and phase relationship are maintained between propellers or engine shafts [ 1-51. This approach is illustrated in Figure 1 for a twin-engine aircraft. Unlike conventional Power lever (Cabin) ave engine l/rev si pnal Figure 1. Nobe cancella~on co~fig~ratioR for OV-10A. active noise control metho$s which require adding canceling sources, the phase lock oach &criM in this paper is simple and relatively heTraditional engine synchro phase angle between the propeller shafts thus avoiding the acoustic beats arising from varying speed and phase differences between them. A conventional phase locked loop consists of a phase detector, a loop compensator and a voltage controlled oscillator (the engine and propeller, in our case)[ 6 , q . The reference frequency is the master engine speed while the output frequency is the slave engine speed. In the approach presented in this paper, an inner frequency locked loop is used to extend capture range, while an the phase difference trated in Figure 2. The frequency frequency while the phase locked loop uses feedback of phase. The frequency difference detector and phase detector can be realized through analog or digital means [6,7].yields a fixed angle that may not m m t for varying ~~~ In this paper, the approach for in that " h s the cabin noise at a desired ~~a t i Q n by cons monitoring noise at that location using a ~~~h o~.Once per revolution pulses from m a s t e r and slave engine shafts are used for the phase locked li troller is used with erence angle is de interior and exterior c~e~~t i Q n 1104