(2)
The converter output signal (representing wheel speed) is sent to a velocity reference loop consisting of deceleration and velocity reference circuits. The output of the velocity reference loop is a voltage, which represents instantaneous (reference) airplane velocity. The velocity reference output is sent to a velocity comparator circuit, which also receives converter (wheel speed) signals. The comparator circuit compares wheel speed with reference velocity. The output is a velocity error voltage, positive or negative, which is proportional to the magnitude of the difference between wheel speed and reference velocities.
(3)
Velocity comparator output is sent to a pressure bias modulator circuit, which is the main controlling component for brake pressure correction. The pressure bias modulator circuit is a memory circuit in terms of pressure control. The modulator varies brake pressure in an attempt to hold velocity error output from the comparator at a set value, which is the modulator threshold. Reference velocity decreases more rapidly than wheel velocity, causing velocity error voltage output from the comparator to decrease. The pressure bias modulator reacts by increasing brake pressure to slow the wheel. Conversely, the modulator decreases brake pressure for velocity error voltages above threshold.
(4)
Lead and transient control circuits form parallel paths with the pressure bias modulator for control of the antiskid valves. The lead circuit anticipates when the tire is about to skid and responds by decreasing brake pressure before the tire can go into a deep skid. The transient control circuit provides rapid recovery from very sudden, unpredicted deep skids, such as could occur when a tire suddenly comes into contact with an ice patch, paint strip, etc.
(5)
Output signals from the pressure bias modulator and transient and lead circuits are sent to a summing amplifier, which provides a composite signal to the valve driver. The valve driver provides a current to the servo coil in the antiskid valve to modulate brake pressure as required.
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Antiskid System Control Circuit 5C9
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5C9 Antiskid System Control Circuit
Feb 20/85 Figure 5 (Sheet 2) 32-42-0
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B. Touchdown Protection Mode
(1) Touchdown protection circuits prevent touchdown with locked wheels. The inboard wheels are controlled by two air safety relay inputs which prevent brake application in the airborne mode. Touchdown protection for the outboard wheels is controlled by wheel spinup of the inboard wheels. Wheel spinup inputs override the air safety brake release inputs to allow brake application when wheel velocity reaches a preset level, even though the air safety relay may stil1 be in the airborne mode.
C. Locked Wheel Protection Mode
(1) Locked wheel circuits compare wheel speeds in inboard and outboard wheel pairs. Valve overdrive signals are sent to release brake pressure at the slow wheel of an antiskid pair when the slow wheel speed has dropped to 25% of its associated wheel speed. Discriminator circuits ignore wheel speed differences due to turning. Locked wheel protection drops out at speeds between 11 and 25 knots.
D. Automatic Braking Mode
(1) The autobrake system operates independently of the antiskid system. Signals from the autobrake circuit card in the antiskid unit control the autobrake module which bypasses the brake metering valves and meters fluid to the braking system up stream of the antiskid control valves. No autobrake inputs to the antiskid system are required because the antiskid valves are not used to decelerate the airplane but only to control skid and locked wheel conditions.
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E. Antiskid control is energized when the two antiskid switches on the center panel are turned on, but actual antiskid control is not possible until the air safety relay (touchdown protection) is in the ground mode, or wheel speed exceeds a preset value and overrides air safety protection. In the normal antiskid mode of operation, brake pressure applied by the pilot through brake metering valves is transmitted to the brakes unmodified by the antiskid valves. When an impending skid is detected, a control signal is sent from the antiskid control sheild to the appropriate antiskid valve, causing the valve to open by an amount dependent on the strength of the signal. The open antiskid valve bleeds an appropriate amount of brake metering valve pressure to return to reduce pressure and prevent a skid. When the skid has been prevented and wheel speed has returned to normal, control signals to the antiskid valves are reduced or eliminated. All other brake hydraulic system components operate as normal to provide brake pressure. In the automatic breaking mode of operation, however, autobrake pressure control module bypasses the brake metering valves, using system pressure directly as the pressure source. It modulates fluid through the antiskid valves to the brakes based velocity error voltages generated by the autobrake circuit card in the antiskid control unit. Refer to 32-43-01 for Automatic Braking System Operation.
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