Fluid-pressure and analogous brake systems – Multiple control – Fluid and electric
Reexamination Certificate
1999-01-08
2001-02-27
Butler, Douglas C. (Department: 3613)
Fluid-pressure and analogous brake systems
Multiple control
Fluid and electric
C303S126000, C303S006010, C303S009610, C303S014000
Reexamination Certificate
active
06193326
ABSTRACT:
The present invention relates to controlled braking of a set of aircraft wheels, and more particularly it relates to apparatus for braking a set of wheels in which the wheels are generally distributed in two groups organized symmetrically on either side of a longitudinal midplane of the aircraft, of the type in which each wheel is fitted with a brake actuated by means of brake pedals.
BACKGROUND OF THE INVENTION
There already exist numerous brake apparatuses fitted with a first hydraulic circuit powered by an associated hydraulic source and used for braking under normal conditions, and with an emergency hydraulic circuit powered by a different source of hydraulic pressure and used only in the event of a breakdown.
Traditionally, each of the two hydraulic circuits (normal and emergency) is connected to each of the various brakes via a brake valve which is a servo-valve that is electrically controlled by a cockpit unit which measures information corresponding to brake pedal depression, and in general also to the speed of the wheel in question. For the state of the art, reference can be made to the following documents: EP-A-0 443 213, U.S. Pat. Nos. 4,834,465, 5,050,940, 5,024,491, 3,926,479, FR-A-2 038 001, and DE-B-1 118 020. Another example of a particularly elaborate electro-hydraulic brake apparatus for a set of aircraft wheels is described in the Applicant's document U.S. Pat. No. 5,397,173.
The various brake apparatuses described in the above-mentioned documents thus comprise electrically controlled servo-valves of structure that always has two hydraulic stages interconnected by a hydraulic link and respectively associated with control and with distribution, the control stage acting on the slider of the distribution stage. The hydraulic control stage is electrically controlled, and the feed pressure input to the servo-valve is modulated in the distribution stage by through sections (nozzles) operating on the principle of hydraulic leakage that is generally at a rate of about one liter per minute. Such hydraulic leakage is always required in that kind of design, and the magnitude thereof makes it difficult to use such servo-valves when the hydraulic source is constituted by a hydraulic accumulator, as may be the case for an emergency circuit. The volume of fluid consumed by the permanent leakage of such servo-valves is no longer available for powering the brakes, thereby considerably diminishing the duration and number of times the brakes can be applied before the accumulator has been emptied.
Although the structure of presently-used braking servo-valves is generally well understood, specialists are fully aware of the drawbacks and operating constraints on brake valves of those types.
The operating pressure recovered at the outlet from a servo-valve, which pressure is a function of the magnitude of the reference current, is, in fact, obtained with relatively poor accuracy. Firstly, the mechanical adjustment of the servo-valve nozzles cannot achieve dispersion on utilization pressure of less than a few bars, or indeed of about ten bars at some points, given the non-linearity and hysteresis phenomena specific to any type of hydraulic equipment. Furthermore, the servo-valve is sensitive to fluctuations in the feed pressure and in the temperature of the hydraulic fluid, which can give rise to operating pressure variations of several bars, even in normal operation. Finally, mechanical wear of the nozzles inevitably gives rise to drift over time.
OBJECTS AND SUMMARY OF THE INVENTION
The invention seeks specifically to solve this problem and to design brake apparatus that provides better performance, while being less sensitive to the phenomena of temperature and wear, and to fluctuations in feed, and while nevertheless remaining very accurate concerning operating pressure, without it being necessary to perform difficult periodic adjustments.
An object of the invention is thus to provide apparatus for breaking a set of aircraft wheels, in which the structure of the apparatus makes it possible to obtain braking that is precise and constant over time, while avoiding the above-mentioned drawbacks inherent to using brake servo-valves having two hydraulic stages.
According to the invention, this problem is solved by braking apparatus for braking a set of aircraft wheels, in which each wheel is fitted with a brake actuated from brake pedals, said braking apparatus comprising two pressure sources each feeding a respective hydraulic circuit, comprising a normal circuit and an emergency circuit for use in the event of a breakdown, the two hydraulic circuits leading to each of the various brakes via associated brake valves, the brake valves of the normal circuit and/or of the emergency circuit being constituted by direct drive valves that are electrically controlled by associated electronic control units.
Surprisingly for the person skilled in the art, direct drive valves (DDVs) are highly effective when integrated in aircraft braking systems. Until now, such direct drive valves have been used only in machine tools for controlling position or displacement (e.g. in rolling mills), or else in motor vehicle suspensions. In known uses of such direct drive valves, provision is generally made for servo-controlling flow rate (not pressure), in which the position of the valve slide is monitored by a position sensor.
In a first embodiment of the brake apparatus of the invention, the brake valves of the emergency circuit are the only valves constituted by direct drive valves, with the brake valves of the normal circuit being servo-valves of traditional type. This provides a considerable advantage resulting from the difficulty in using conventional servo-valves with two hydraulic stages in the emergency circuit, when the hydraulic source of the circuit is constituted by a hydraulic accumulator. Provision can then be made for each of the direct drive brake valves to be associated with the brakes of a pair of wheels.
In a variant embodiment, the brake valves of the normal circuit and of the emergency circuit are all constituted by direct drive valves. This provides numerous advantages which are described in greater detail below, inherent to using direct drive valves for the entire brake circuit, i.e. both for normal use and for use in the event of a breakdown.
According to advantageous characteristic, at least some of the direct drive valves are servo-controlled in pressure.
Such pressure servo-control is extremely advantageous when direct drive brake valves are used in a brake circuit, unlike position servo-control as is sometimes used for such valves in technological environments that are completely different.
Under such circumstances, each pressure servo-controlled direct drive valve preferably includes a hydraulic distributor valve and an actuating electric motor controlled by associated control electronics, and on the delivery outlet of the hydraulic distributor valve, a pressure sensor which provides said control electronics with an electrical signal representative of the measured pressure. Where desirable, the direct drive valve under pressure servo-control could also be subjected to position control by means of a position sensor associated with the actuating electric motor which sends a corresponding electrical signal to the associated control electronics.
Since the direct drive valve does not have the first hydraulic stage of traditional servo-valves (e.g. a quadruple hydraulic potentiometer), the power for controlling the slide of the valve is now electrical (it is constituted solely by controlling the motor for driving the slide) and no longer hydraulic (by leakage from the first hydraulic stage of the servo-valve). In addition, contrary to that which is encountered with traditional servo-valves whether or not they are servo-controlled in pressure, leakage from a direct drive valve (feed to return) is now restricted to the slide of the valve, and is thus by construction less than the leakage of a traditional servo-valve. This opens the way to genuine electrical braking, in particular in
Butler Douglas C.
Friedman Stuart J.
Messier-Bugatti
Nixon & Peabody LLP
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