Patent GB-869265-A (GB)
Title: Grounded aircraft trainers
Inventor: COHEN CHARLES L
IPC: G09B9/28
Filed: 1958-12-22
Abstract: 869,265. Grounded aircraft trainers. ACF INDUSTRIES Inc. Dec. 22, 1958, No. 41350/58. Class 4. A grounded aircraft trainer having means for loading a simulated aircraft control operable by the trainee, comprises means for deriving a signal representing displacement of the control from a reference position, and a circuit connected between the signal deriving means and the loading means to amplify small signals representing the initial displacement of the control at a high rate to simulate a preloaded condition of the control, and to amplify larger signals representing subsequent displacement of the control at a lower rate. In Fig. 1, a simulated control column 10 is movable fore- and-aft on a shaft 18 mounted in a yoke 16, and is movable laterally about the shaft 20 of yoke 16. The column is connected to drive the wipers 32, 42 of potentiometers 30, 40 to derive A.C. signals representing the deviations of the column from a neutral position in the fore-and-aft and lateral directions respectively. These signals are fed to amplifiers 90, 92, described later, which energize torque motors 28, 38 connected to apply torque to the column in the sense to tend to restore the column to its neutral position. The characteristics of the amplifiers ensure that for very small deflections of the column, considerable torque is applied, to simulate the action of preloaded centering springs. For larger deflections, the torque applied increases linearly with the deflection. Fig. 5 shows a suitable amplifier circuit. A conventional phase inverting amplifier 50 has an input terminal 52 and output terminals 54. A feed back lead 56 from one output terminal is connected to the junction 58 of a diode pair 60, 62 connected to terminal 52 through equal resistors 64, 66 respectively. The input signal is fed from terminal 68 over resistor 70 to terminal 52. A transformer 72 has its primary winding 71 energized by a reference source of the same frequency and phase as the signal on terminal 68 (derived from wiper 32 or 42 of Fig. 1) and its secondary winding is grounded at a centre tap 75 and connected across terminals 74, 76, as one diagonal of a bridge comprising diodes 78, 80, 82, and 84. The other diagonal of the bridge is connected across diodes 60 and 62. Assuming that there is no signal on terminal 68, the diode bridge 78-84 acts as a full wave rectifier and backbiases diodes 60 and 62 with a reference voltage, and the diodes cannot thus conduct. The symmetrical position of terminal 52 ensures that it remains at ground potential. Thus amplifier 50 has no input signal and no output signal. If now a signal is added at terminal 68, an amplified signal appears on lead 56. If this amplified signal is less than the reference voltage maintained across diodes 60, 62, the latter remain non- conducting, and no feed back from lead 56 reaches terminal 52. The amplifier thus operates at its maximum gain, and energizes torque motor 28 or 38 of Fig. 1 to apply considerable torque for a small input signal. When the feed back signal on lead 56 exceeds the back-bias on diodes 60, 62, each diode conducts on the appropriate half cycle. For example, during one half cycle diode 62 conducts, and back-biases diodes 82, 84, which now do not conduct. Thus the feed back voltage is fed to terminal 52 over resistor 66. Diodes 78, 80 continue to conduct, and the reference voltage is fed to terminal 52 over resistor 64. Since the feed back voltage is greater than the reference voltage, there will be a resultant potential at terminal 52. On the other half cycle, diodes 78, 80 are backbiased, and diodes 82, 84 conduct, and a resultant potential again appears at terminal 52, which is thus fed with a resultant A.C. feedback signal. The gain of amplifier 50 thus drops to a value depending on the impedance of the feed back network and the input impedance of amplifier 50. A graph of relationship between the signal on terminals 54 and that on terminal 68 is shown in Fig. 6, and comprises a very steep portion passing through the origin, and two linear portions of lesser slope, this graph simulating closely the relationship between centering force and control column displacement when preloaded centering springs are used. In a modification, a feed back resistor is connected between lead 56 and terminal 52, to lessen the slope of the steep portion of the graph in Fig. 6. Specification 869,266 and U.S.A. Specification 2,678,177 are referred to.