Fluid-pressure and analogous brake systems – Speed-controlled – Having a valve system responsive to a wheel lock signal
Reexamination Certificate
1999-12-15
2001-02-20
Oberleitner, Robert J. (Department: 3613)
Fluid-pressure and analogous brake systems
Speed-controlled
Having a valve system responsive to a wheel lock signal
C303S119100
Reexamination Certificate
active
06189982
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to hydraulic braking systems of the brake-by-wire type for vehicles of the kind in which hydraulic fluid from a high pressure source is supplied to a wheel brake under the control of a proportional solenoid-operated valve which, in turn, is actuated in response to signals from a pressure transducer associated with a brake-applying pedal.
In known brake-by-wire systems of the kind set forth the high pressure source comprises an hydraulic accumulator which is adapted to be charged by operation of an hydraulic pump. The pressure in the accumulator is determined by a pressure sensor working in conjunction with an isolating valve which is operative to isolate the pump from the accumulator when the pressure in the accumulator attains a given value. This valve effectively isolates the accumulator from the proportional operated solenoid valves so that the accumulator is not discharged due to leakage through the proportional operated solenoid valves when left to stand for a long period.
In one known system of the kind set forth the pressure sensor is located in the first line between the pump and proportional valve, and the isolating valve is located in a second line between the accumulator and a connection in the first line between the pressure sensor and the proportional valve. The relative positions of the pressure sensor and the isolating valve, under certain circumstances, may lead to concerns about reliability, power consumption, safety and packaging of the system. This is because the isolating valve must be opened in order to measure the accumulator pressure.
With regard to reliability the pressure sensor, isolating valve and proportional valve may all be subjected to large-magnitude pressure transients if pressure in a down stream high-pressure supply gallery decays before the isolating valve re-opens. This would occur if pressure in the first line decays over time due to leakage and then the isolating valve is suddenly opened introducing the accumulator pressure to the first line.
Excessive power consumption, leakage and consequent wear and energy consumption (recharging accumulator) will occur if the isolating valve is held open between successive brake applications in order to reduce accumulator-sensor fatigue due to supply gallery decays before the isolating valve re-opens. In addition excess heat dissipation from the solenoid of the isolating valve may occur. The isolating valve must incorporate a spring capable of opposing maximum accumulator pressure to keep the accumulator charged when the supply line pressure has decayed through leakage. Thus a high solenoid force must be maintained after opening to oppose this high spring force.
Since the accumulator pressure is monitored only when the isolating valve is open, periodic opening takes place for monitoring purposes but such movements may exacerbate the reliability problems discussed above. As a result since accumulator faults may not be detected immediately the driver may be unaware of faults when braking is required.
BRIEF SUMMARY OF THE INVENTION
According to our invention in an hydraulic braking system of the brake-by-wire type of the kind set forth in which the hydraulic pressure source comprises an hydraulic accumulator adapted to be charged by a pump, an accumulator isolating valve is located in a main supply line between the accumulator and a feed line to the solenoid-operated proportional valve, the isolating valve being adapted to act as a one-way valve in a first mode of operation to prevent the flow of fluid from the accumulator to the feed line, and adapted to allow fluid to flow in both directions across the isolating valve in a second mode of operation.
Specifically the valve may comprise a valve member adapted to be biased against a valve seat into a closed position in opposition to the flow from the pump to the accumulator. The valve may be biased into this closed position by a spring. The valve may be adapted to be opened by a force generated by a solenoid acting in opposition to the spring force.
The accumulator isolating valve may be so constructed and arranged as to be biased in such a direction as to produce both a pull-in force and a sustained force required to hold the valve open.
In the first mode of operation, the valve may be closed and held in the closed position by a combination of the pressure drop across the valve between the accumulator and the feed line and a biasing force applied to the valve member.
Preferably an accumulator pressure sensor is disposed between the accumulator and the isolating valve.
Conveniently a main pressure relief valve is connected to a line between the pressure sensor and the isolating valve. Alternatively, this may be between the accumulator and the pressure sensor. This allows excess pressure in the accumulator to be relieved through the relief valve instead of the isolator valve. The relief valve may have a manual override to allow pressure in the accumulator to be relieved during servicing.
This arrangement has the advantage that continuous monitoring of accumulator pressure can be achieved without fatigue concerns or unnecessary leakage.
It will be apparent that this arrangement differs from the prior art in that the biasing force applied to the valve acts in opposition to flow from the pump to the accumulator, as distinct from acting in opposition to flow from the accumulator to the pump.
The isolating valve may comprise a solenoid-operated valve. In the first mode of operation, the solenoid may be de-energised and may be energised in the second mode of operation to apply a force opposed to the biasing force.
Because the spring in the isolating valve applies a biasing force to the valve member in opposition to the flow of fluid from the pump to the accumulator, it can be chosen to have a lower spring pressure than when the valve is arranged the other way round when it must exceed the maximum operating pressure in the accumulator. Thus in the present invention, the first current needed to produce a first solenoid force required to open the isolator valve (which must exceed the sum of the spring force and the pressure difference across the valve from the accumulator to the feed line) can be reduced to a second lower current producing a low operating force once the valve is opened and the pressure across the valve has balanced to be substantially equal to the spring force.
As a result less heat dissipation from the solenoid will occur due to a correspondingly reduced hold-in current. In addition packaging is improved due to reduced hold-in forces. Since the relief valve protects the accumulator directly relief threshold is more predictable.
Thus, the isolating valve may be adapted to be driven by a first current upon initial movement from its first mode of operation (valve normally closed) to its second mode of operation (valve open), and the valve held open in its second mode of operation by a second current which is lower than the first current.
The low operating force produced by the second current may be substantially equal to the biasing spring force.
In addition to requiring reduced current in the solenoid, it helps to reduce problems of overheating due to internal resistance in the solenoid.
The pump may be driven by a motor, and a diagnostic means may be provided whereby the integrity of the isolating valve may be checked during operation.
The diagnostic means may be adapted to calculate the motor load when the isolating valve is in the second mode of operation and measure the change in a motor load when the isolation valve is in the first mode of operation. The expected difference in motor load, for example when the accumulator is being charged from the pump, is equal to the biasing force applied to the valve member by the biasing spring.
The diagnostic means may be adapted to produce an output signal indicative of a fault if the difference in the measured motor loads does not exceed a predetermined safe threshold level.
The diagnostic means may be adapted to measure the motor torque and motor speed e
Fuller Robert Gregory
Harris Alan Leslie
Mortimer Ivan
Uzzell Robert George
Lucas Industries Limited
MacMillan Sobanski & Todd LLC
Oberleitner Robert J.
Sy Mariano
LandOfFree
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