Hydraulic braking system

Fluid-pressure and analogous brake systems – Multiple systems

Reexamination Certificate

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Details

C303S003000, C303S015000

Reexamination Certificate

active

06254197

ABSTRACT:

This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/DK97/00532 which has an International filing date of Nov. 20, 1997 which designated the United States of America.
The present invention relates to a hydraulic braking system, to a method for controlling the hydraulic braking system and to a control system for controlling the hydraulic braking system. The hydraulic braking system may be applied to any kind of moving elements or systems, e.g., turbines, in particular wind turbines, conveyer systems etc.
The invention is suited for hydraulic brakes and is able to secure a soft, graduated braking sequence through a simple and cheap electro-hydraulic regulation system. A hydraulic braking system according to the invention provides a system that prevents the high braking torque that is the result of a normal braking with full torque applied.
The very high braking torque results in lifetime loss in the attached transmission system. Especially the overshoot and oscillations occurring after the rotor has been stopped are damaging.
The usual approach of solving this problem is to control the braking torque by regulating the braking pressure using one or several proportional valves. Though proportional valves allow an effective and accurate control of the braking pressure, their use is an expensive way of obtaining a controlled braking torque.
Thus, the aim of the invention is to provide a hydraulic braking system, a method for operating it, and a control system therefore, where the braking pressure is controlled by at least one on/off valve, in particular by at least one three-way valve.
From the prior art, hydraulic braking systems comprising three-way valves and alternative kinds of valves are known.
U.S. Pat. No. 3,646,959 discloses an alternative valve for controlling the motion of a crane. The control valve comprises two three-way valves for controlling the hydraulic pressure at a motor. The difference between the two pressures provided from the two valves determines the direction and the velocity of the motor. However, the valve is manually controlled, and the working principle of the valve is based on the presence of at least two sources of hydraulic fluid under pressure.
An alternative valve is disclosed in U.S. Pat. No. 3,990,352 concerning a control valve for controlling the hydraulic pressure at a hydraulic clutch. The valve is designed with a spool and a sliding lever arranged in place in the main body of the valve in such a manner as to be free to slide therein, the valve thus constituting an alternative form of a proportional valve. This valve is manually controlled.
A major problem of these known systems is that they do not allow an automatic control of the braking pressure and the speed of the machine to be braked. Further, their mechanical design is rather complex and they comprise a large number of parts.
The present invention provides a novel, simple and effective alternative to known hydraulic braking systems. The present invention also provides a braking system that, besides from being cheaper than prior art braking systems, also allows a more effective and accurate control of the braking sequence. The invention also provides a method of controlling the braking system according to the invention and a control system as such.
In one aspect, the invention relates to a hydraulic braking system comprising
a hydraulic brake with a hydraulic fluid chamber,
a source of hydraulic fluid for supplying hydraulic fluid at a pressure greater than ambient pressure, and
a first hydraulic valve means having
a first port connected to the source of hydraulic fluid,
a second port connected to the hydraulic fluid chamber,
a third port connected to the ambience,
a first switch which in a first position connects the first port to the second port and closes the third port and which in a second position closes the first port and connects the second port to the third port,
whereby the hydraulic pressure in the hydraulic fluid chamber can be controlled by selectively positioning the switch in its first and second positions so as to control the braking pressure exerted by the hydraulic brake.
Thus, the invention provides a hydraulic braking system in which the braking pressure may be controlled by a valve means having a switch having an on- and an off-position. By selectively positioning the switch of the valve in its on- or off-position, the hydraulic pressure in the hydraulic fluid chamber of the brake mechanism may be increased or decreased, depending upon whether the braking mechanism comprises a positive or negative braking mechanism. When the hydraulic pressure is to be increased, the switch is positioned in a position connecting the hydraulic chamber of the brake to the source of hydraulic fluid under pressure. When the braking pressure is to be decreased, the switch is put in a position connecting the hydraulic chamber of the brake to, e.g., a fluid container at a pressure lower than or equal to the pressure of the hydraulic fluid chamber.
The hydraulic system according to the invention may further comprise a first hydraulic accumulator for the hydraulic fluid under pressure for preservation of the hydraulic pressure during a time period after pressure loss in the hydraulic fluid supplied by the source. The first accumulator preferably secures a sufficient amount of hydraulic fluid so as to feed the brake during a braking sequence in case of a power cut, when the source of hydraulic fluid under pressure is dependent on a power supply. Further, the first accumulator tends to smoothen the pressure variations that may exist in the hydraulic system.
Preferably, the hydraulic fluid is oil. However other fluids with suitable properties, e.g., high viscosities, may be used.
The hydraulic system according to the invention may further comprise a second valve means, preferably a three-way valve, having
a first port connected to the source of hydraulic fluid, when said first switch is in its first position, and is connected to the ambience, when said first switch is in its second position,
a second port connected to the hydraulic fluid chamber,
a third port connected to the ambience,
a second switch which in a first position connects the first port to the second port and closes the third port and which in a second position closes the first port and connects the second port to the third port,
whereby the hydraulic pressure in the hydraulic fluid chamber is increased, when the second switch is in its first position, and when the first switch is in its first position, and whereby the hydraulic pressure in the hydraulic fluid chamber is decreased, when the second switch is in its second position.
By introducing the second valve means in series with the first valve means, the second valve means may constitute an “emergency” valve in case the braking mechanism is a negative braking mechanism. When positioning the second switch in a position that connects the hydraulic fluid chamber to a low pressure, e.g., ambient pressure, the brake will brake at its maximum braking torque irrespective the position of the first switch. For a positive braking mechanism, the system would work in an analogous way.
The hydraulic system according to the invention may further comprise a throttling valve connected in series with the first and the second valve means for throttling the flow so that volume flow rate of the hydraulic fluid flowing into or out of the hydraulic chamber is substantially constant. The physical characteristics of the throttling valve further influence the response time of the pressure in the hydraulic fluid chamber to control signals controlling the operation of the switches. The throttling valve may thus constitute a substantial part of the control system.
The hydraulic system according to the invention may further comprise a second hydraulic accumulator of hydraulic fluid under pressure connected to the hydraulic system between the first valve means and the hydraulic fluid chamber for increasing the displaced hydraulic fluid volume to or from the f

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