GB936086A - Improvements in and relating to control systems for aircraft wheel brakes - Google Patents

Abstract

936,086. Fluid pressure braking systems. G. W. YARBER. April 28, 1960, No. 15006/60. Class 103 (1). [Also in Group XXXV] The landing gear of an aircraft comprises wheel 110 coupled to alternators 40, 50, and having a brake 114 actuated by pump 120 associated with a valve system 130 responsive to signals from the alternators. When the wheel is accelerating or rotating at uniform speed, transistor 190 is cut-off and relay 210 is de-energized, so producing zero voltage drop across resistor 196. As a result, transistor 230 is cut-off and relay 220 is de-energized. The circuits for valve solenoids 142, 152, 162, are therefore open and the brake 114 is responsive to manual control at lever 126. If the wheel decelerates at a moderate rate, the output of rectifier 170 also decreases gradually so that the rate of discharge of capacitor 180 is insufficient to render transistor 190 conductive. However, if the wheel starts to skid, the rate of discharge of capacitor 180 is sufficient to energize relay 210 through transistor 190. Thus, main valve 140 and control valve 160 are both actuated to cut-off pressure supply to the brake and permit the existing brake pressure to decrease slowly through control valve 150. If incipient skidding is thus checked, the relay 210 is de-energized and the valve 140 is returned to its normal position, the valve 160 remaining actuated by discharge of capacitor 244 to limit the rate of increase of braking pressure. If the skidding is too severe to be checked in the manner described, the current passing through transistor 190 reaches a value sufficient to operate relay 220. Control valve 150 is therefore shifted to its fully-open condition for rapid release of the brake. Capacitor 240 and resistor 232 limit the rate at which rapid changes in the voltage across resistor 196 are supplied to the base of the transistor 230. Excessive mechanical oscillation of the landing gear is prevented by associating a time-delay circuit with load resistor 182, such circuit comprising capacitors 173, 174, forming part of a voltage doubler. The voltage delivered by the differentiating circuit comprising capacitor 180, and the brake action controlled thereby, are correspondingly delayed. The transistor 190 is a silicon transistor having a temperature characteristic which compensates for changes in the output voltage of alternator 40 due to variations in temperature of that machine. Means are provided for releasing the brake of any wheel which has substantially ceased to rotate, this control action taking place for each wheel independently of the other wheels when the aircraft is airborne. After touchdown, however, the control action takes place only when one of the other wheels is rotating faster than a predetermined speed. For this purpose, relay 270 is actuated by generator 50 when the speed of wheel 110 exceeds a low value. This causes release of the brake when oil switch 300 is closed, indicating that the aircraft is in the air, and the relay 270 is released. Alternatively, the brake is released, provided that the relay 270 is released and relay 290 is actuated by the high-speed condition of one of the other wheels during the initial portion of a landing run. If mechanical or electrical failure of the system causes the brake to be released for an excessive period, the charge on capacitor 346 becomes sufficient to activate transistor 340 whereby relay 320 is released and the brake is returned to manual control, this condition being indicated by signal lamp 330. This fail-safe arrangement is disabled by switch 300 when the aircraft is airborne. In a modification, diode 192 is replaced by two resistors, one of which is connected in a fullwave rectifying circuit to actuate relay 210 during excessive acceleration and deceleration periods of the wheel. After a skid has been controlled and acceleration proceeds at a moderate rate, re-application of the brake is delayed until normal speed has been reestablished. Brake re-application is initiated earlier if the rate of acceleration is higher, although the rate of such re-application is limited, Fig. 5 (not shown). In another arrangement, the rate of brake re-application is limited when the wheel acceleration exceeds a predetermined low value throughout the period of recovery after skidding, Fig. 6 (not shown). Generator 40 has its rotor 42 fixed to shaft 30, the stator 46 comprising core member 47 with a plurality of poles 48, each carrying windings 49. The rotor is formed with corresponding poles and is permanently magnetized. Generator 50 comprises permanent magnet 52 of annular form magnetized parallel to axis 21, and core member 56 associated with windings 54. The pole formations of rotor 59 resemble teeth, the opposing surface of the stator member 56 being scalloped to produce pole formations equal in number to the teeth of the rotor. The resultant variations in the effective width of the air gap as the rotor revolves produces a voltage in winding 54 whose frequency is proportional to speed.