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Aviation History
1960
1960 - 2233.PDF
FLIGHT, 1 October 1960 573 AUTOPILOT DEMAND SYSTEM Xdt COMPARISONDEMAND SYSTEM MONITOR (DISCONNECT Fig 7. A simplified pitch autopilot using basic comparison monitoring principles. The initials are: ADS, air data senser; VG, vertical gyro; PRG, pitch rate gyro; Comp, comparison AUTOPILOT DEMAND SYSTEM Ai COMPARISON DEMAND SYSTEM Az B2 C? SERVO MOTOR ERROR MONITOR & DISCONNECT HI- JL Fig 8A. A pitch autopilot with dupli-cate demand system AUTOPILOT DEMAND SYSTEM Fig 8B. A pitch autopilot with a simpli- fied comparison channel TO DISCONNECT COMBINED AUTOPILOT & COMPARISON DEMAND SERVO I MOTOR Fig 9. A simplified, fail-safe pitchautopilot ERRORMONITOR & DISCONNECT In practice, failure of the system to disconnect following anautopilot failure will occur only if both the autopilot and the disconnection device fail. The likelihood of this is remote as itinvolves a product of small probabilities in the landing phase. The acceptance of an electrically actuated disconnect devicepermits further simplifications of the duplicate channel, with an increase in system reliability and a saving in weight. In thisconcept, the second servo motor or veto-servo can be omitted. The operation can be checked in a different way by comparing thedemand of the second autopilot with the effective demand of the first which is obtained by suitably processing the actual controloutput with the approximate inverse transfer function of the servo motor control loop (see Fig 6). The steady state discrepancies between the two signal chainscan now be reduced by the inclusion of simple washout (d.c. blocking) filters so that the threshold setting on the disconnectunit may be tightened to improve protection against the hard-over type of failure. Provided the washout time-constant is madereasonably small compared with that of the integrators, the need for a duplicate integrator and cross-synchronization are eliminated.The inclusion of washouts also permits the removal of the duplicate pitch and roll autostabilizer rate gyros and filters if it can bedemonstrated that the signals from these are small under normal conditions. Protection against slow runaways, i.e., slow-overs, isto some extent eliminated by the washout filters, but it can be re-introduced by measuring the error in the controlled quantity(e.g., height error when on height lock) and disconnecting the autopilot when a set level is exceeded. Error thresholds can thenbe chosen separately to satisfy simultaneously any requirements for any controlled quantities. For example, with height lockengaged, different limits can be set for pitch angle error and height error signals. Because the duplicate sensers are used forcomparison and not for actual control, they can be considerably simplified and therefore made more reliable and lighter thanthose used in the autopilot; and the inherent differences make them less liable to fail from a common environmental cause. Even further simplification can be achieved. The general prin- ciples can be understood by reference to Figs 8A and 8B whichshow how the extent of the redundancy in the comparison channel can be further reduced by eliminating equivalent elements whicheither have a very high degree of reliability—simple mechanical devices—or can be made to give an absolute indication of failure.For example, item Ai is of such a nature that its failure can be positively indicated without comparison with another element.Item Bi is a mechanical assembly so designed that the probability of its failing is acceptably remote, while the failure of item Ci canbe positively detected only by comparison with a similar unit. Items AL> and Ba have thus been eliminated. Item Ai indicatesdirectly to the disconnect unit whether or not it is operating satis- factorily, while item Bi feeds electrically independent outputs toboth Ci and G>, the latter being the only item remaining in the comparison demand system, though in a simplified form. For thosesensers which either cannot be absolutely monitored or for which absolute monitoring techniques would prove uneconomical (e.g.,an air data senser or vertical gyro) it is possible to re-arrange the comparison sensers so that they do not feed directly into the com-parison channel, but only monitor their equivalent sensers in the main autopilot. By this means all the autopilot sensers are directlychecked for correct functioning and can therefore be integrated into the system in the same way as an absolutely monitored devicelike Ai in Figs 8A and 8B. In this scheme, each autopilot senser has three electricallyisolated output devices on a common mechanical output shaft: one output device is used in the main autopilot channel, anotherin the comparison channel and the third is fed to the disconnect unit where its magnitude is compared with the output of thecorresponding comparison senser. By this means, effective signal tolerance errors and hence the slow-over protection is consider-ably improved over that of the previous system. Because the out- puts into the two signal channels are now identical, it remains onlyto set the error threshold for the individual sensers and this can be separately set independently of the remaining control system. The principle is illustrated in Fig 9 which represents asimplified autopilot pitch channel of this kind. The air data senser
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