Antiskid System Component Location  5C9 
32-42-0  Figure 1 (Sheet 1)  Feb 20/85 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

591  Antiskid System Component Location 
Feb 20/85  Figure 1 (Sheet 2)  32-42-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

C.  The antiskid system consists of individual wheel speed sensors or transducers, an antiskid control unit, and four antiskid control valves (Fig. 1). The system also incorporates a test circuit and indicators which enable the system to be checked for proper operation on the ground. Test circuits monitor antiskid operation continuously during airplane operation. Two microswitches, operated by the landing gear lever, disable the antiskid during the landing gear retraction cycle.
2.  Transducer
A.  The transducer is a speed sensing device containing only one moving part, a rotor which rotates inside a fixed stator (Fig. 2). The stator is firmly attached to the axle of each wheel. The rotor is attached to the hubcap and moves with the wheel. The transducer drive is an assembly consisting of a mounting plate, drive coupling and related mounting hardware. The drive assembly, mounted inside the hubcap of each landing gear wheel, is mated to the transducer rotor shaft.
B.  The rotor has poles, or lobes, extending outward from its outer circumference. The stator has an equal number of lobes extending inward. The stator also has a coil around it, which is connected to a controlled source of dc power. When the stator coil is energized, the stator becomes an electromagnet. When the wheel is turning, the lobes on the rotor pass by the lobes on the stator, thus changing the distance between them. The change in distance causes a change in the magnetic field of the stator because of the variation in magnetic coupling (magnetic reluctance) between the stator and rotor. This change in magnetic field generates in the stator coil an ac voltage which is superimposed on the controlled dc energizing voltage. The frequency of the generated voltage is directly proportional to the rotational speed of the wheel. This ac signal is fed into the antiskid control unit.
C.  The transducer produces a sine wave having 150 pulses for one complete rotation of the wheel. The AC signal ranges from an output of 1.5 volts minimum at 150 Hz to an output of 3.0 volts minimum at 5000 Hz.
3.  Antiskid Control Unit
A.  The antiskid control unit is located in the electrical/electronic equipment compartment. It contains all the circuits necessary for antiskid control, and those circuits necessary to respond to autobrake commands for automatic braking control. The control unit contains four identical wheel circuit cards (one for each main whee1), one autobrake deceleration card with microprocessor system, two identical power supplies (inboard and outboard paired wheels), antiskid pilot relays for speed brake deployment logic, and failure monitoring cards. Each main wheel card includes antiskid control, test logic, speed brake spinup control and locked wheel and squat (air safety) logic. The autobrake deceleration card, during automatic braking, controls hydraulic brake pressure upstream of the antiskid valves. Antiskid control is on an individual wheel basis. Locked wheel control is on a paired wheel basis, inboard or outboard.
5C9 
32-42-0 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 20/86 

B. On the face of the control shield are two test switches (component and system) and four indicator lights which assist in isolation of antiskid system troubles on the ground. 
4.  Antiskid Control Valve
A.  The antiskid control valve is an electrically controlled hydraulic valve, which controls hydraulic pressure applied to the brakes in accordance with signals received from the antiskid control shield. Each of the control valves, in turn, consists of an electrically controlled 1st-stage or servo valve, and a hydraulically controlled 2nd-stage valve. (See figure 3.) Each of the control valves operates independently of the other to control the brake on one wheel.
B.  The 1st-stage valve contains a torque motor, which controls the position of a flapper between two restricted hydraulic ports. One port is pressure into the 1st-stage valve chamber; the other is flow to return from the chamber. Movement of the flapper toward one port or the other further restricts pressure flow into the valve chamber or flow to return from the chamber in order to increase or decrease pressure in the chamber. The 2nd-stage valve is spring offset and pressure bias controlled. The end of the 2nd-stage spool opposing spring force senses brake pressure. The end assisting spring force senses 1st-stage valve pressure. When the torque motor receives no control current, the 2nd-stage valve spool is spring offset to port pressure entering the antiskid valve directly to the brakes. The return port is blocked. When a decrease in brake pressure is required, a current is sent to the torque motor to restrict pressure flow into the 1st-stage valve chamber. This decreases pressure in the chamber and on one end of the 2nd-stage valve spool, causing it to open and bleed some pressure to return. The amount of pressure bled to return depends on torque motor current which, in turn, depends on the reduction in brake pressure required. Torque motor current is decreased to reduce pressure bias on the 2nd-stage valve spool, allowing it to move toward the spring offset direction to increase brake pressure.
　
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