757
MAINTENANCE MANUAL
(b)
Two IRS speed data inputs (L and R) provide airplane speed signal to the four wheel cards in the control unit. The left IRS supplies input to cards 2-6 and 3-7, and the right IRS supplies input to cards 1-5 and 4-8. If the left or right IRS input is not present, the captain and the first officer can place the IRS select switch on panel P1 or P3 in the ALTN position. This allows L or R IRS select relay to energize to provide the alternate IRS speed data from the center IRS system.
(c)
The PSEU (Proximity Switch Electronics Unit) provides gear down signal to all wheel cards for antiskid system operation.
(3)
Antiskid control schematic (Fig. 13)
(a)
Antiskid wheel control, and logic
1) Individual wheel control
a) The microprocessor in each wheel card provides control for the fore and aft wheel pair. The card receives and processes wheel speed signals, receives and decodes IRS data, provides valve driver signals and communicates with the test and fault inputs from the BITE card. A digital/analog converter in the card provides the valve command to drive the respective valve for each wheel.
2) Paired wheel control
a) The normal valve driver generates a second signal for the alternate valve driver. The second signal is OR'd with the paired wheel signal from the mating wheel card. The higher signal of the two drives the alternate valve for the laterally paired wheel.
3) Valve driver logic
a) The wheel speed compares the brake applied pressure in three modes. These are proportional, integral and derivative. The proportional mode involves brake applied pressure with proportional wheel speed as the brake pressure changes to maintain a deceleration short of a skid. The integral mode checks the past performance of wheel speed. The derivative mode checks rate of change of the wheel speed. The three modes provide the data input required to produce a driving signal to the normal and alternate valve drivers. An alternate brake selector valve (not shown in Fig. 13) determines which valve driver is used.
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MAINTENANCE MANUAL
4) Antiskid locked wheel protection
a) Locked wheel protection, a secondary antiskid function, prevents lockup of individual wheels during all braking above 25 knots. The tandem pairing of locked wheel shown in Fig. 12 compares the wheel speed of paired wheels. It provides a full brake-release signal to the antiskid valve when the speed of the controlled wheel is less than 30 percent of the paired wheel.
5) Hydroplane/touchdown logic
a) Hydroplane protection provided to the aft wheels to protect against hydroplane-induced wheel lockups also provides touchdown protection (Fig. 12). The control unit compares the IRS speed data with the wheel speed to generate a full brake release signal. The signal goes to the respective antiskid valve when the speed of the controlled wheel is at least 50 knots below the IRS ground speed. The hydroplane/touchdown protection requires valid IRS inputs and that landing gear is down and locked. Loss of IRS signal, however, does not affect other antiskid functions. Hydroplane/touchdown protection for the forward wheel is provided indirectly thru locked wheel protection.
6) Gear retract braking logic
a) The gear up signal (when the left and right landing gears are not down and locked) inhibits the alternate valve drivers for 12.5 |0.5 seconds. This is to allow the alternate system brake pressure (left system pressure) stops the wheels before gear is retracted into wheel wells.
7) Low speed dropout logic
a) A low speed dropout at 7.5 knots inhibits the valve drivers in the wheel control circuit. Below the dropout speed, the antiskid system provides no brake release signal.
(4)
Antiskid hydraulic operation (Fig. 13)
(a)
Brake pressure input of 260 psi or more to the normal antiskid module opens the shutoff valve to allow system pressure supply to the antiskid valves. On alternate system, pilot metered pressure supplies to the valve. The valve varies the output pressure to the brakes by moving the slide valve between apply and release, using an electrical signal from the control unit to the servo valve torque motor. The pressure flow thru the fuses to the shuttle valve module and then on to the brakes.
(b)
Wheel speed signal from transducer provides the required data input to the control unit. The unit compares the wheel speed with the airplane ground speed to provide a control signal to the normal and alternate valves. The valves limit the pressure to the brakes. System shown in Fig. 13 hydraulic schematic is for the left landing gear. System for right landing gear of wheels 3, 4, 7, and 8 is similar.
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本文链接地址：757 AMM 飞机维护手册 起落架 CHAPTER 32 - LANDING GEAR 2(167)
