Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Indication or control of braking – acceleration – or deceleration
Reexamination Certificate
2001-12-17
2004-01-27
Louis-Jacques, Jacques H. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Indication or control of braking, acceleration, or deceleration
C701S070000, C701S080000, C303S148000, C303S149000, C303S150000, C244S075100, C244S111000, C188S18100R
Reexamination Certificate
active
06684147
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to aircraft landing gear braking systems, and more particularly concerns an improved system for controlling aircraft brake pressure.
A conventional skid detection system used in aircraft braking systems typically includes a wheel speed transducer for each wheel brake of the wheels of the aircraft, for measuring wheel speed and generating wheel speed signals that are a function of the rotational speed of the brake wheel. The wheel speed signal is typically converted to a signal representing the velocity of the aircraft, and compared with a desired reference velocity, to generate wheel velocity error signals indicative of the difference between the wheel velocity signals from each braked wheel and the reference velocity signal. The output of the velocity comparator is referred to as velocity error. The velocity error signals typically are adjusted by a pressure bias modulator (PBM) integrator, a proportional control unit, and a compensation network, the outputs of which are summed to provide an anti-skid control signal received by the command processor. The PBM integrator in the antiskid loop dictates the maximum allowable control pressure level during braking. When no skid is detected, this integrator allows full system pressure to the brakes.
The conventional PID controller for aircraft brake control systems deals with various conditions such as aerodynamics, landing gear dynamics, &mgr;-slip profile, different landing conditions, and the like. One major problem is that tuning of controller parameters to guarantee high efficiency in different landing conditions and conditions affecting the tire-runway coefficient of friction (&mgr;) of the aircraft braking system is often a difficult task.
Such algorithms usually take only one input, i.e., wheel velocity (Vw), and determine a reference velocity (Vref) with an apparatus. Then the Vref and Vw signals pass through the PID control logic, which generates a command signal. The command signal is supplied to a hydraulic servo valve and the output of servo valve, fluid pressure generates a brake torque through a brake. The algorithms show good antiskid performance—robustness and adaptability.
In spite of success of the PID type controller, related industry engineers and researchers have been continuously investigating other control schemes, partially because of difficulty in antiskid braking control parameter tuning. A need therefore still exists for an antiskid braking controller that can facilitate and shorten the process of antiskid braking control parameter tuning. The present invention meets these and other needs.
SUMMARY OF THE INVENTION
Briefly, and in general terms, the present invention provides for a sliding integral proportional (SIP) controller for aircraft antiskid braking control that improves and shortens the time required for antiskid braking control parameter tuning, and that also provides higher braking efficiency, robustness, and adaptability, since the antiskid braking control parameters to be tuned are adjusted based on an accurate adaptive threshold and an velocity error ratio or modified slip ratio (S
mod
) signal with an estimated net wheel torque, a few integral gains, and a proportional gain. The proposed SIP controller requires only one input, and shows excellent braking efficiencies, robustness, and adaptability with only a fraction of tuning effort and time.
The present invention accordingly provides for a sliding, integral, and proportional (SIP) controller for providing anti-skid braking control for an aircraft. The SIP controller includes a reference velocity subsystem generating a reference velocity signal based upon an input wheel velocity signal; a velocity error ratio subsystem generating a modified slip ratio signal (S
mod
) based upon a ratio of the difference between the reference velocity and the wheel velocity to the reference velocity; and a main controller subsystem receiving the reference velocity signal and the modified slip ratio signal, and generating a control command output signal indicative of a command braking pressure.
In one embodiment, the reference velocity subsystem receives a plurality of sampled wheel velocity signals, determines a minimum value of the sampled wheel velocity signals, and compares the minimum value with an individual wheel velocity signal. If the minimum value of the sampled wheel velocity signals is greater than the wheel velocity signal, a predetermined desired reduction amount is subtracted from the minimum value of the sampled wheel velocity signals and the result is output as the reference velocity of the reference velocity subsystem. Otherwise the wheel velocity signal is output as the reference velocity of the reference velocity subsystem. In one aspect, the sampled wheel velocity signals have a predetermined fixed sampling time. In a present embodiment, the modified slip ratio signal (S
mod
) is determined based upon the equation:
S
mod
=
Velerror
Vref
where S
mod
is the velocity error ratio or modified slip ratio, Vref is the reference velocity in radians per second, and Velerror is the velocity error in radians per second, determined from the equation Vref−Vw, where Vw is the wheel velocity in radians per second.
In a present embodiment, the main controller subsystem includes a one dimensional sliding mode controller subsystem to determine an estimated net wheel torque signal; an adaptive threshold subsystem for generating an adaptive threshold based upon the modified slip ratio signal (S
mod
) and a clock signal; a first integral gain subsystem for comparing the estimated net wheel torque signal with the adaptive threshold to determine dominance between the tire drag torque and braking torque, and outputting a corresponding gain value; a second integral gain subsystem exponentially generating a deep skid signal (deep_skid) when the S
mod
signal is greater than a predetermined limit and a change in wheel velocity indicates a deep skid situation; a third integral gain subsystem to avoid S
mod
signals that are too small or negative and to modify the initial braking command signal; a proportional controller subsystem generating an output signal to prevent sudden deep skids; and a pressure limiter for limiting the command braking pressure. In one aspect of the invention, the output of the main controller subsystem is a command signal indicative of a torque, which is converted to a command brake pressure signal by multiplication of a predetermined gain.
The estimated net wheel torque may be determined based upon the velocity estimation error. One-dimensional sliding surface condition takes a form as:
1
2
ⅆ
ⅆ
t
s
2
=
s
∂
Vref
∂
t
-
Gain2
Im
w
&LeftBracketingBar;
s
&RightBracketingBar;
(
1
)
where s=Vref−{circumflex over (V)}, Vref is the reference velocity in radians per second, {circumflex over (V)} is the observed or estimated wheel velocity in radians per second, Gain
2
is determined as the largest possible net wheel torque in ft-lbs, and Imw is the wheel/tire/brake mass moment of inertia in slug-ft
2
. The equation (1) is always less than zero, and thus, the sliding condition is satisfied. The net wheel torque signal may be determined according to the equation:
∂
V
^
∂
t
=
Gain2
Im
w
sgn
(
s
)
(
2
)
where sg n(s) is the sign of s. The net wheel torque signal optionally may be determined according to the equation:
NWTe=DF×sgn
(
s
)×Gain
2
(3)
where NWTe is the estimated net wheel torque in ft-lbs, and DF is a discrete filter of time constant, 0.1 sec. The low pass filter DF may be defined according to the equation:
DF
=
0.04877
z
-
0.9512
where z is a complex variable.
In one embodiment of the invention, a plurality of skid levels are established to effectively maintain a tire drag friction coefficient (&mgr;) approaching the peak value of &mgr; without undesirable deep skid. In one present aspect, three skid levels are established. T
Cook Robert D.
Park Duk-Hyun
Fulwider Patton Lee & Utecht LLP
Hydro-Aire, Inc.
Louis-Jacques Jacques H.
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