C. Anti-skid
(1) The inputs and outputs of the anti skid system are shown in the schematic (Ref. Fig. 045)
NOTE : The Braking/Steering Control Unit is designed for use with two
____ types of brakes. The selection is made through shunts located at the rear of the unit, on the wiring side of the aircraft.
(2)
Principle (Ref. Fig. 046)
The speed of each main gear wheel is compared to the reference speed. With braking ordered, when the speed of a braked wheel decreases to below the input control speed (Vc), the anti skid system sends a brake release order. This order keeps the wheel speed value at the input control speed. A slip law function of the reference speed is introduced. The anti skid system has the corrective networks necessary to stabilize the feedback loop. The servovalve is the pressure-servoed electro-hydraulic component (Ref. Para. 2. F.). The static gain of the brake is between 300 and 450 mN/bar depending on the brake type. The tachometer transforms the angular velocity of the braked wheel into a frequency (Ref. Para. 2. I.). A converter transforms the input frequency into a digital signal that the microprocessor can use directly.
(3)
Description
(a) Calculation of the aircraft speed
1_ In flight:
The reference speed is given by a fixed value Vo = 50 m/s
VREF = Vo.
2_ On impact of the main landing gears, during acceleration of
the main gear wheels, a logic monitoring the acceleration of
the wheels and hydroplaning determines, from the smallest of
the highest wheel speeds per gear (VR) and from its
derivative, the end of the wheel acceleration phase. The
brakes are released as long as this phase is not over. During
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Anti Skid - Schematic
Figure 045
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Anti Skid - Principle
Figure 046
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this phase, the reference speed is computed from a
deceleration forced at 0.35 g and from VR as follows, so as to
obtain the best reference speed evaluation for the following
phases :
(Ref. Fig. 047, 048)
Vref (t) = MAX (Vref (t-dt) - 3.5dt ; VR (t)) (in m/s)
3_ During the roll, without braking control, the reference speed
is obtained in the same manner as during the wheel
acceleration phase, as a function of a deceleration forced at
0.25 g and of VR :
Vref (t) = MAX (Vref (t-dt) - 3.5dt ; VR (t)) (in m/s)
4_ During braking, the reference speed is obtained from the speed
VR and the aircraft deceleration (gamma) determined from the
airframe longitudinal acceleration and pitch angle given by
the ADIRUs 1, 2 and 3.
(Ref. Fig. 049)
The reference speed is calculated as follows :
Vref (t) = MAX (Vref (t-dt) + gamma (t)dt ; VR(t)) (in m/s)
5_ The logic which modifies the reference speed calculation as a
function of the FLIGHT/GROUND information is obtained from
LGCIU discretes (main gear shock absorbers compressed), the
tachometer speed VR and the ground speed V GROUND. V GROUND is
calculated from the ground speeds delivered by the ADIRUs 1, 2
and 3.
(Ref. Fig. 050, 051)
(b) Regulation The control speed of the speed servoing of each wheel (4 independent wheels) is calculated by decreasing the reference speed by the value of the slip ratio. (Ref. Fig. 052)
Vc (t) = 0.935 VRef (t)- 0.9 (in m/s) The regulator compares the control speed to the braked wheel speed to establish, if necessary, a brake release order so as to decrease wheel slip. The corrective networks and the static gain of the regulator of one wheel are adjusted to maintain a high level of stability and accuracy.
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Generation of the Reference Speed - Principle
Figure 047
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Generation of the Reference Speed - Flow Chart
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