Patent GB-1049563-A (GB)
Title: Surveillance system for speeds of rotation
IPC: C04B24/42
Filed: 1963-07-09
Abstract: 1,049,563. Automatic speed control. BELL AEROSPACE CORPORATION. July 9, 1963 [July 16, 1962], No. 27100/63. Heading G3R. [Also in Divisions G1 and H3] In a system for monitoring and controlling a speed of rotation e.g. of the motor or rotor of a helicopter, a signal whose frequency depends upon the speed is applied to an AND gate through a delay circuit and in parallel through a frequency dependent delay circuit to produce an output signal in dependance upon coincidence of the input signals at the gate. The signal may be fed back to correct the speed. A tachometer generator 40 operated by the rotor of a helicopter produces an A.C. signal having a frequency dependent upon the rotor speed. This signal is passed to an AND gate 54 through a network 44 producing pulses having a fixed phase delay and also through a network 46, 48 producing pulses phase delayed by an amount dependent upon the signal frequency. If the rotor speed is correct the pulses coincide and the resultant output is passed through a further AND gate 58 connected to a peak detector and relay circuit to disable audio and visual alarms. If the frequency is not correct the pulses do not coincide, the relay is released and the alarms are operated. A similar circuit is associated with an engine driven tachometer 42 so that the alarm sounds if either rotor or engine speed is incorrect. A further AND gate (X<SP>1</SP> Fig. 4, not shown) may be supplied with the frequency dependent pulses and through a further fixed delay circuit IV<SP>1</SP>) with fixed delay pulses so that as the frequency moves towards a second value, coincidence occurs in the second gate. The resulting output signal may be passed through a further detector XI<SP>1</SP> to operate the alarm and may be combined with the output signal from the first gate to produce a frequency and thus speed dependent error signal which may be used to control the engine. A circuit corresponding in function to units 40, 44, 46, 48, 54 and 70 Fig. 3 (and I to XII Fig. 4) is shown in Fig. 2, and the waveforms wherein in Figs. 2(a)-2(e) (not shown). The A.C. signal 2(a) at A is clipped and squared by diodes 80, 82 and by T<SP>1</SP> to produce waveforms 2(b) and 2(c) at B and C. A long time constant circuit 96, 98 changes the waveform to 2(d) at D and the peak portion of the negative half cycle is squared by T2 to produce waveform 2(e) at E the leading edge of which is delayed with respect to the leading edge of 2(d) by half a cycle. This is differentiated by 110 and 112 to produce 2(f) at F and the negative peak is squared by T3 to produce a narrowed pulse 2(g) at G 1  for application to one terminal of a gate T7. The A.C. input signal 2(a) at A is also applied to a circuit 118, 120 producing a 90 degrees phase shift at the centre frequency and clipped and squared by 124, 126, T4 and T5 to produce pulses 2(h) at H which are delayed by the phase shift circuit by an amount which depends upon the signal frequency By differentiating these pulses and squaring the negative peaks, narrow pulses 2(j) with a frequency dependent delay appear at J and these are applied to the other input of the AND gate T7. The delays are chosen such that when the frequency is correct the pulses overlap. The resulting output pulses are rectified and smoothed by 150, 152 to produce a voltage dependent on the overlap and if the overlap is sufficient a relay 156 is operated and the alarm circuit is broken at 158.