Patent GB-796056-A (GB)
Title: Signal responsive systems
Inventor: HASSLER NORMAN A
IPC: G05D1/02
Filed: 1954-10-22
Abstract: 796,056. Electric correspondence control. SPERRY RAND CORPORATION. Oct. 22, 1954, No. 30489/54. Class 40 (1). [Also in Group XL (c)] Relates to a system in which an electrical control signal is produced by combining a first signal with a second signal proportional to the rate of change of the first signal and, if desired, with a third signal proportional to the rate of change of the second signal. If one of the input signals is filtered to eliminate random fluctuations a phase lag is introduced and according to the present invention such phase lag is compensated by using an electro-mechanical servo with velocity feedback as the filter and by combining the input to or output from the filter with a compensating signal whose phase leads that of the signal to which it is added such that the resultant output signal is more nearly in phase with the input signal. The invention is described as applied to an aircraft navigational aid of the type disclosed in Specification 690,985, [Group XL (c)], in which the pointer of a centrezero indicator is deflected by a control signal comprising the algebraic sum of two or three component signals proportional respectively to the displacement of the aircraft from a radiodefined or altitude-controlled flight path, the first time derivative of said displacement and the second time derivative of said displacement such that if the aircraft is manoeuvred to maintain the pointer in its central position, the aircraft will approach the flight path asymptotically. The first derivative signal is produced by differentiating a smoothed displacement signal, Figs. 10, 11, and 16, 17 (not shown), or by differentiating a noisy displacement signal and then smoothing the resultant noisy rate signal, Figs. 12, 13; 14, 15 and 18, 19 (not shown). In the first case the compensating signal may comprise an unfiltered or filtered heading error or pitch error signal since such a signal is substantially proportional to the rate of change of the displacement and therefore leads the displacement signal and in the second case the compensating signal may comprise an unfiltered or filtered signal proportional to the lateral or vertical acceleration of the aircraft, Figs. 12 and 14 (not shown), or a heading error signal, Fig. 18 (not shown). Since the first derivative signal is derived directly from the displacement signal the effect of wind drift is automatically corrected. The second derivative signal is produced by differentiating the smoothed first derivative signal or it may comprise the lateral acceleration signal. Fig. 3 shows an embodiment in which horizontal and vertical guidance is indicated by the horizontal and vertical displacements of crossed pointers 40, 42 respectively. A D.C. signal from a receiver 21 proportional to the horizontal displacement of the aircraft from a flight path defined by a radio approach or omni-range beacon is converted to A.C. in a modulator 23 and applied to a summing amplifier 34, and a similar D.C. signal from the receiver 21 is differentiated in a rate circuit 26, converted to A.C. in a modulator 25 and applied to a servo amplifier 27 together with signals from a vertical gyro 32 and a rigidly mounted accelerometer 33 proportional respectively to the roll angle and the acceleration along the lateral axis of the aircraft; the combination of these two latter signals provides a compensating signal proportional to the horizontal component of the lateral acceleration as explained with reference to Figs. 4-6 (not shown). The output from the servo amplifier 27 controls a motor 28 coupled to a selsyn 29 to generate an A.C. follow-up signal for application to the input of amplifier 27 such that the follow-up signal is equal to the smoothed first derivative of displacement. The smoothing is improved by also applying to the input of amplifier 27 a second derivative displacement signal from a tacho-generator 30 coupled to motor 28. The first and second derivative displacement signals from selsyn 29 and generator 30 are also applied to the summing amplifier 34 and the resultant output signal is applied through a limiter 37, summing amplifier 36 and demodulator 39 to control the vertical pointer 40. In order to limit the maximum roll attitude called for to produce a, zero control signal, a roll signal from the gyro 32 is applied directly to the amplifier 36 and through an attenuator 65 to the amplifier 34 such that in the absence of other inputs to amplifier 34 the two components, of the roll signal reaching the amplifier 36 are equal in amplitude and opposite in phase. Thus when the resultant output from amplifier 34 is less than the level set by the limiter 37 the two component roll signals balance out but when the output is greater than the limiter level the final control signal contains a roll component which reduces the deflection of the pointer 40. For vertical guidance, the vertical displacement of the pointer 42 is controlled by the algebraic sum of (1) a signal from a receiver 44 or altimeter 43 proportional to the vertical displacement of the aircraft from a radio defined glide slope or from a constant height path, and (2) the first derivative of said vertical displacement which is produced by a tacho-generator 53 in a servo system 51-54 which operates in a similar manner to the servo system 27-30 but in this case the phase-compensating signal applied to the amplifier 51 comprises a signal from the vertical gyro 32 which is proportional to the error in pitch attitude and the maximum pitch adjustment called for by the system is limited by applying the pitch signal to summing amplifiers 49 and 59. Alternatively, the vertical displacement of the pointer 42 may be controlled by a signal porportional to the algebraic sum of the vertical displacement and its first and second derivatives by using an arrangement similar to that employed for controlling the vertical pointer 40; in this case compensation for phase lag is effected by a signal proportional to the vertical acceleration of the craft. Fig. 1 (not shown) illustrates a system for horizontal guidance only in which a smoothed displacement rate signal is derived from a noisy displacement signal by means of a servo system, a heading error signal is injected into the servo amplicier as a phase compensating signal and a roll signal is used as the displacement acceleration signal.