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Claims  |
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What is claimed:
1. Antilocking installation for a road vehicle having a hydraulic
multi-circuit brake system which includes at least one static brake
circuit which is connected to an outlet pressure space of a brake unit
means for providing brake pressure to the wheel brakes;
the outlet pressure space of the brake unit is connected to the wheel
brakes of the static brake circuit by means of an electrically actuated
brake pressure control valve means;
the brake pressure system providing for controlling brake pressure
reduction, brake pressure retention and brake pressure restoration phases
for antilocking control installation, and operating on the pump-back
principle, at least when the antilocking control system demands high
magnitude pressure reductions according to which, in a pressure reduction
phase, brake fluid is drained from the wheel brakes subject to the control
system and is pumped back into the output pressure space of the brake
unit;
a pumping means is provided for this pumping back and includes:
a stepped cylinder with two bore sections of different diameters, offset
relative to one another by means of a radial housing step;
a displaceable step piston means located in these bore sections and having
piston flanges of correspondingly different diameters;
the stepped piston means being guided so that it can be displaced in a
pressure-tight manner;
the larger diameter flange of the stepped piston means forming an axially
movable boundary of a drive pressure space;
an electrically actuatable antilocking control valve means for connecting
the drive pressure space with a high pressure outlet of a hydraulic
auxiliary pressure source and for alternatively connecting the pressure
space to a sump tank of the auxiliary pressure source for displacing the
stepped piston means;
the small diameter flange of the stepped piston means forming a movable
boundary of a functional chamber acting as the pump chamber in pump-back
operation of the brake pressure setting means;
said functional chamber being connected via an outlet non-return valve
means to the output pressure space associated with the static brake
circuit and via an inlet valve means to the wheel brakes of the static
brake circuit;
a return spring means for biasing the stepped piston means into a position
associated with a maximum volume of the functional chamber which can be
connected to the wheel brakes;
an electronic control means providing actuation signals for switching of a
brake pressure control valve means and the electrically actuatable
antilocking control valve means in response to a process of comparing and
differentiating electrical output signals from wheel rotational speed
sensors which are a measure of wheel peripheral speeds in signal level
and/or frequency;
wherein the inlet valve means is designed as a 2/2-way valve controlled by
motion of the stepped piston;
wherein the inlet valve means is held in an open position when the stepped
piston is positioned to cause a minimum volume of the functional chamber
which can be connected to the wheel brakes,
wherein the inlet valve means, starting from this open position, can only
reach its shut off position by a displacement of the stepped piston means
taking place in the sense of increasing the volume of functional chamber;
wherein when the stepped means piston has reached a position in the
immediate vicinity of its end position associated with a maximum volume of
the functional chamber, the inlet valve means returns to an open position
caused by a displacement of the stepped piston means taking place in the
sense of reducing the volume of the functional chamber after the stepped
piston means has executed a minimum stroke, relative to this end position;
a position indicator means for generating electrical output signals which
are characteristic of the positions of the stepped piston means and which
continually vary with changes in the position;
the output signal of the position indicator being applied as an additional
information input to the electronic control unit means which generates
signals in response to the position of the stepped piston means for
switching over the antilocking control valve means to a pump-back
operation of the stepped piston;
and wherein a difference in volume of the functional chamber between its
maximum and minimum values is between 1/5 and 1/3 of that volume of brake
fluid which must be forced into the wheel brakes of the static brake
circuit in order to achieve the maximum brake pressure permitted by the
brake installation.
2. Antilocking system according to claim 1, wherein during a return
movement of the stepped piston means a quality of brake fluid, whose
volume is between 1/4 and 1/2 of the difference between the minimum and
maximum values of the volume of the functional chamber of the stepped
cylinder, can be pumped back into the outlet pressure space of the brake
unit.
3. Antilocking system according to claim 2, wherein the antilocking system
control valve means includes two 2/2-way valve means which can be actuated
electrically;
wherein the basic position of one of these two valve means causes the
output pressure of the auxiliary pressure source to be connected to the
control pressure space of the stepped cylinder and an actuated position
causes the control pressure space to be shut off relative to the auxiliary
pressure source; and
wherein the basic position of the second of these two valve means causes
the control pressure space to be shut off relative to the non-pressurised
sump tank of the auxiliary pressure source and an actuated position
connects the control pressure space to the sump tank of the auxiliary
pressure source.
4. Antilocking system according to claim 2, wherein the inlet valve means
comprises a spool valve with a housing configured as an axial extension of
a section of the stepped cylinder forming a boundary of the functional
chamber;
a valve bore in said spool valve housing for accepting a valve body of the
spool valve and being designed as a blind hole which starts at the smaller
bore step from a fixed boundary of the functional chamber;
wherein the blind hole is offset by a housing step relative to this smaller
bore step; and
wherein the spool valve has a seal connection with said blind hole so that
it can be displaced relative to the functional chamber.
5. Antilocking system according to claim 4, wherein the valve spool is
provided with an end flange which forms a movable boundary of a balance
space which is in continuous communicating connection, via a longitudinal
and transverse duct of the spool valve housing and wherein the functional
chamber of the stepped cylinder is used for accepting the brake pressure
during brake modulation.
6. Antilocking system according to claim 5, wherein the end flange of the
valve spool forming the boundary of the balance space also forms a part of
the valve body of the inlet valve; and
wherein the inlet valve in its shut off position, closes both a valve inlet
duct entering the blind hole and an overflow duct leading from the blind
hole to the functional chamber.
7. Antilocking system according to claim 4, wherein the valve spool of the
inlet valve means is provided with a radial flange on an end section
located within the functional chamber of the stepped cylinder;
said radial flange cooperating with a stop flange, pointing radially
inwards on the stepped piston means; and
wherein said two flanges determine a maximum magnitude h of a possible
relative stroke motion between the stepped piston and the valve spool.
8. Antilocking system according to claim 7, wherein the valve spool is
provided with an end flange which forms a movable boundary of a balance
space which is in continuous communicating connection, via a longitudinal
and transverse duct of the spool valve housing and wherein the functional
chamber of the stepped cylinder is used for accepting the brake pressure
during brake modulation.
9. Antilocking system according to claim 8, wherein the end flange of the
valve spool forming the boundary of the balance space also forms a part of
the valve body of the inlet valve; and
wherein the inlet valve in its shut off position, closes both a valve inlet
duct entering the blind hole and an overflow duct leading from the blind
hole to the functional chamber.
10. Antilocking system according to claim 1, wherein the inlet valve means
comprises a spool valve with a housing configured as an axial extension of
a section of the stepped cylinder forming a boundary of the functional
chamber;
a valve bore in said spool valve housing for accepting a valve body of the
spool valve and being designed as a blind hole which starts at the smaller
bore step from a fixed boundary of the functional chamber;
wherein the blind hole is offset by a housing step relative to this smaller
bore step; and
wherein the spool valve has a seal connection with said blind hole so that
it can be displaced relative to the functional chamber.
11. Antilocking system according to claim 10, wherein the antilocking
system control valve means includes two 2/2-way valve means which can be
actuated electrically;
wherein the basic position of one of these two valve means causes the
output pressure of the auxiliary pressure source to be connected to the
control pressure space of the stepped cylinder and an actuated position
causes the control pressure space to be shut off relative to the auxiliary
pressure source; and
wherein the basic position of the second of these two valve means causes
the control pressure space to be shut off relative to the non-pressurised
sump tank of the auxiliary pressure source and an actuated position
connects the control pressure space to the sump tank of the auxiliary
pressure source.
12. Antilocking system according to claim 10, wherein the valve spool is
provided with an end flange which forms a movable boundary of a balance
space which is in continuous communicating connection, via a longitudinal
and transverse duct of the spool valve housing and wherein the functional
chamber of the stepped cylinder is used for accepting the brake pressure
during brake modulation.
13. Antilocking system according to claim 12, wherein the antilocking
system control valve means includes two 2/2-way valve means which can be
actuated electrically;
wherein the basic position of one of these two valve means causes the
output pressure of the auxiliary pressure source to be connected to the
control pressure space of the stepped cylinder and an actuated position
causes the control pressure space to be shut off relative to the auxiliary
pressure source; and
wherein the basic position of the second of these two valve means causes
the control pressure space to be shut off relative to the non-pressurised
sump tank of the auxiliary pressure source and an actuated position
connects the control pressure space to the sump tank of the auxiliary
pressure source.
14. Antilocking system according to claim 12, wherein the end flange of the
valve spool forming the boundary of the balance space also forms a part of
the valve body of the inlet valve; and
wherein the inlet valve in its shut off position, closes both a valve inlet
duct entering the blind hole and an overflow duct leading from the blind
hole to the functional chamber.
15. Antilocking system according to claim 14, wherein the antilocking
system control valve means includes two 2/2-way valve means which can be
actuated electrically;
wherein the basic position of one of these two valve means causes the
output pressure of the auxiliary pressure source to be connected to the
control pressure space of the stepped cylinder and an actuated position
causes the control pressure space to be shut off relative to the auxiliary
pressure source; and
wherein the basic position of the second of these two valve means causes
the control pressure space to be shut off relative to the non-pressurised
sump tank of the auxiliary pressure source and an actuated position
connects the control pressure space to the sump tank of the auxiliary
pressure source.
16. Antilocking system according to claim 10, wherein the valve spool of
the inlet valve means is provided with a radial flange on an end section
located within the functional chamber of the stepped cylinder;
said radial flange cooperating with a stop flange, pointing radially
inwards on the stepped piston means; and
wherein said two flanges determine a maximum magnitude h of a possible
relative stroke motion between the stepped piston and the valve spool.
17. Antilocking system according to claim 16, wherein the antilocking
system control valve means includes two 2/2-way valve means which can be
actuated electrically;
wherein the basic position of one of these two valve means causes the
output pressure of the auxiliary pressure source to be connected to the
control pressure space of the stepped cylinder and an actuated position
causes the control pressure space to be shut off relative to the auxiliary
pressure source; and
wherein the basic position of the second of these two valve means causes
the control pressure space to be shut off relative to the non-pressurised
sump tank of the auxiliary pressure source and an actuated position
connects the control pressure space to the sump tank of the auxiliary
pressure source.
18. Antilocking system according to claim 16, wherein the valve spool is
provided with an end flange which forms a movable boundary of a balance
space which is in continuous communicating connection, via a longitudinal
and transverse duct of the spool valve housing and wherein the functional
chamber of the stepped cylinder is used for accepting the brake pressure
during brake modulation.
19. Antilocking system according to claim 18, wherein the end flange of the
valve spool forming the boundary of the balance space also forms a part of
the valve body of the inlet valve; and
wherein the inlet valve in its shut off position, closes both a valve inlet
duct entering the blind hole and an overflow duct leading from the blind
hole to the functional chamber.
20. Antilocking system according to claim 1, wherein the antilocking system
control valve means includes two 2/2-way valve means which can be actuated
electrically;
wherein the basic position of one of these two valve means causes the
output pressure of the auxiliary pressure source to be connected to the
control pressure space of the stepped cylinder and an actuated position
causes the control pressure space to be shut off relative to the auxiliary
pressure source; and
wherein the basic position of the second of these two valve means causes
the control pressure space to be shut off relative to the non-pressurised
sump tank of the auxiliary pressure source and an actuated position
connects the control pressure space to the sump tank of the auxiliary
pressure source. |
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Claims |
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Description |
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BACKGROUND AND SUMMARY
The invention concerns an antilocking system for a road vehicle with a
hydraulic multi-circuit brake installation which includes at least one
static brake circuit connected to an outlet pressure space of a brake unit
provided for brake pressure supply to the wheel brakes of a vehicle. The
outlet pressure of the brake unit can be connected to the wheel brakes of
the static brake circuit by means of an electrically actuated brake
pressure control valve of a brake pressure setting device for controlling
brake pressure reduction, brake pressure retention and brake pressure
restoration phases of the antilocking control system. At least when the
antilocking control system demands high magnitude pressure reductions, the
brake pressure setting device operates on the pump-back principle
according to which, in a brake pressure reduction phase, brake fluid is
drained from the wheel brakes subject to the control system and is pumped
back into the output pressure space of the brake unit. A pumping device is
provided for this purpose and includes a stepped cylinder with two bore
sections of difference diameters, offset relative to one another by means
of a radial housing step. A stepped piston with piston flanges of
correspondingly different diameters is guided in these bore sections so
that it can be displaced in a pressure-tight manner. The larger step of
the stepped piston forms an axially movable boundary of a drive pressure
space, which space can be connected by means of an electrically actuated
antilocking system control valve arrangement to the high pressure outlet
of a hydraulic auxiliary pressure source and can, as an alternative to
this, be relieved to the sump tank of the auxiliary pressure source. A
smaller step of this stepped piston forms the movable boundary of a
functional chamber acting as a pump chamber, in pump-back operations of
the brake pressure setting device. This functional chamber can be
connected, via an outlet non-return valve to the brake unit pressure
outlet associated with the static brake circuit and, via a inlet valve, to
the wheel brakes of the static brake circuit. A return spring is provided
for biasing the stepped piston into its end position associated with
maximum volume of the functional chamber, which can be connected to the
wheel brakes. Actuation signals necessary for the correct switching of the
brake pressure control valve and the electric solenoid valve arrangement
controlling the stepped cylinder drive are created by an electronic
control unit which generates these signals by a process of comparing and
differentiating, with respect to time, electrical output signals from
wheel rotational speed sensors whose output signals are a measure of the
wheel peripheral speeds of the vehicle wheels in level and/or frequency.
Broadly such an antilocking system can be considered as known from DE-OS
No. 33 47 618, although the control system on the static brake circuit,
the front axle brake circuit, is designed as a single wheel control system
with the brake pressure control valves individually associated with each
of the vehicle wheels and a common brake pressure control system on both
wheels of a brake circuit is only explicitly revealed for the rear axle
brake circuit, designed as an open brake circuit.
For the static brake circuit, the known antilocking system provides a low
pressure reservoir which initially accepts the brake fluid drained from a
wheel brake, subject to the control system, in a pressure reduction phase.
This brake fluid is then pumped back by means of a hydraulically driven
return pump and into an output pressure space, associated with the static
brake circuit, of the brake unit. This return pump is designed in such a
way that between 0.2 and 0.4 cm.sup.3 of brake fluid can be pumped back
per piston stroke into the output pressure space of the brake unit. This
corresponds to between 1/20 and 1/10 of that quantity of brake fluid which
is forced into the brake circuit in a vehicle of the higher power class
(vehicle weight approximately 1.5 tons and maximum speed approximately 200
km/h to 220 km/h) when braking occurs with the maximum pressure permitted
by the brake installation design.
The valves provided as the brake pressure control valves are 3/3-way
solenoid valves whose basic position is the brake pressure build-up
position. These valves can be driven by a control signal of defined
control current strength into a shut off position (the brake pressure
retention position) and can be driven into a brake pressure reduction
position by a control signal of defined higher strength (e.g. double,
control current strength), in which position the wheel brakes of the
static brake circuit, subjected to the control system are connected to a
low pressure reservoir from which the brake fluid is pumped back into the
outlet pressure space of the brake unit by means of the hydraulically
driven feed pump.
In such a design of the brake antilocking system, pumping back to meet the
requirements requires a drive for the hydraulic return pump with a stroke
repetition frequency of at least 10 Hz which, because of the associated
pressure shocks, leads to unpleasant noise in control operation. Such
noise is not only disadvantageous because of the associated adverse effect
on vehicle driving comfort but, above all, because particularly careful
drivers who, due to a "defensive" driving style only cause the antilocking
system to respond extremely rarely and are therefore not "accustomed" to
such a noise, can be startled by it. This may cause them to react even
worse in a moment of danger with which a braking situation requiring an
antilocking system response is always associated. Also disadvantageous in
this known antilocking system, is the necessary technical complication
involved in achieving it. This is because of the low pressure reservoir
and the complicated construction of the brake pressure control valves
provided in addition to the hydraulically driven return pump.
The disadvantage of the unpleasant noise is avoided in an antilocking
system known from DE-OS No. 29 08 482, in which the wheel brakes,
subjected to an antilocking control, are each associated with a pressure
modulator, which provides a brake pressure control action on the principle
of producing brake pressure change by changing the volume of a modulator
outlet pressure space to which the wheel brake cylinder of the particular
wheel brake is directly connected. This modulator outlet pressure space
has a movable axial boundary formed by a first piston flange of a control
piston. The control piston also has a second piston flange solidly
connected to the first flange by means of an elongated piston rod. This
second piston flange is guided so that it can be displaced in a
pressure-tight manner in a larger step of a stepped bore of the modulator
housing where it forms the axially movable boundary of a reaction space.
This reaction space is permanently connected to the high outlet pressure
of an auxiliary pressure source and a drive pressure space, whose axially
movable boundary is formed by the second larger flange of the control
piston and whose axially fixed boundary is formed by an end wall of the
modulator housing.It is possible to alternatively connect this drive
pressure space (by means of electrically triggered inlet and outlet
valves) to the high pressure outlet of the auxiliary pressure source and
to its non-pressurized reservoir. A return spring is supported on the end
wall of the modulator housing forming the boundary of the drive pressure
space and fixed relative to the housing of the drive pressure space. This
return spring acts on the larger piston flange of the control piston and
biases into its basic position associated with the minimum volume of the
outlet pressure space of the modulator. This minimum volume is associated
with normal brake operation, i.e. brake operation not subjected to the
control system. In this basic position of the control piston, a central
valve, located in its smaller piston flange, is held in its open position
against the action a valve spring, because its valve body is supported by
a stop pin fixed relative to the housing. In this open position of the
central valve, the modulator outlet pressure space (connected to the wheel
brake cylinder) is connected to an inlet space which is connected to the
outlet pressure space of a brake unit in which an actuating force
proportional to the brake pressure can be built up by actuation of a brake
pedal. The second axial boundary of this inlet space is formed by the
smaller diameter step of an annular piston designed as a stepped piston,
which is sealed so that it can be displaced, both against the smaller bore
step of the housing bore and against the piston rod connecting together
the two piston flanges of the control piston. Between the two bore steps,
closed by the end walls of the modulator housing, is interposed a third
bore step, which has the largest diameter. The piston is sealed against
the third bore step so that it can be displaced. A radial annular piston
flange, which follows on from the pistons smallest step, forms the
boundary of the inlet space. At an axial distance from this flange, the
annular piston has a second radial flange which is sealed, so that it can
be displaced against that bore step in which the larger flange of the
control piston is also guided. Thus, it too can be displaced in a
pressure-tight manner. The annular piston is pan-shaped on an end facing
towards the reaction space in such a way that its outer shell surrounds,
at a radial distance, the piston-rod shaped central section of the control
piston and, together with the latter, forms the boundary of an inner
annular space. This space communicates through a transverse duct of the
annular piston with an outer annular space, whose axial boundaries are
formed by the two radial flanges of the annular piston. The outer annular
space is permanently connected to the non-pressurized reservoir of the
auxiliary pressure source. The largest diameter flange of the annular
piston and a housing step form the axial boundaries of an annular space
used as the drive pressure space. A valve which, due to an axial
displacement of the control piston acting to increase the volume of the
modulator outlet pressure space, opens into the annular space in a
functional position so that it communicates with the reaction space and is
therefore also subject to the high outlet pressure of the auxiliary
pressure source. The valve body of this valve is designed as an
essentially cylindrical sleeve which is sealed on the reaction space side
against the piston rod of the control piston so that it can be displaced.
Otherwise however, the sleeve outer shell surrounds the central piston rod
of the control piston with a small radial distance between them so that
there is an inner annular gap within the valve body communicating with the
non-pressurized annular space. The valve body is sealed on the outside
against the outer shell of the pan-shaped annular piston part so that is
can be displaced. The sleeve-shaped valve body has, on its section sealed
against the piston rod of the control piston, an external conically shaped
sealing surface which forms a boundary of an external groove of the valve
body. The external groove extends between this conical sealing surface and
the section of the valve body sealed against the annular piston and
together with the valve body, forms an annular space which is connected to
communicate via transverse and longitudinal ducts with the non-pressurized
annular space of the annular piston and with the annular space which can
be used as the drive pressure space. The valve body is forced by a valve
spring into sealing contact of its conical surface with the edge of the
reaction space end opening of the annular piston.
In order to achieve a pressure reduction phase of the antilocking control
system and a drive pressure reduction phase of the antilocking control
system, the drive pressure space of the modulator is shut off against its
reaction space and is instead connected via the outlet valve to the
non-pressurized sump reservoir of the auxiliary pressure source. By this
means, the control piston experiences an introductory displacement, such
as to increase the modulator outlet pressure space, which is connected to
the wheel brake cylinder of the wheel brake which can be subjected to the
control system. In consequence, the central valve of the piston flange
forming the boundary of the modulator outlet pressure space closes; the
communicating connection between the non-pressurized annular space of the
annular piston and the annular space and annular gap, bounded by the valve
body and the annular piston is also interrupted by contact between a
central stop and sealing flange of the control piston and an inner edge of
the sleeve-shaped valve body. Due to the displacement of the control
piston, and therefore also of the sleeve-shaped valve body, the conical
sealing surface of the latter rises from the seat formed by the annular
piston and the pressure present in the reaction space is also connected
into the annular drive pressure space. This results in the annular piston
always following the motion of the control piston, which is itself
displaced so as to increase the volume of the outlet pressure space in the
reaction space (if the pressure in the drive pressure space bounded by the
larger flange of the control piston is relieved). On the basis of the
construction of the pressure modulator and its function explained up to
this point, it follows that the stroke of the control piston must be
dimensioned so as to be sufficiently large for complete pressure reduction
at its maximum stroke and that pump-back operation by means of the
pressure modulator is impossible. Although the antilocking system known
from DE-OS No. 29 08 482 has the advantage that practically no noise
occurs when it responds, it does have the severe disadvantage that in no
case where the antilocking system responds, is the driver given an obvious
pedal reaction to indicate this fact,
A compromise, advantageous relative to the above, between undesired noise
and a pedal reaction which is desirable in the case of an "extreme"
response of the antilocking system (which will be correspondingly rare) is
provided by Applicants' own German patent application P No. 36 37 781.3.
This describes an antilocking system with pressure modulators which have a
piston which can be displaced in a housing, the piston forming the
boundary of an outlet pressure space which can be alternatively connected
to the high pressure outlet and the non-pressurized reservoir of the
modulator. This piston displacement occurs by action of the pressure
present at auxiliary pressure source to provide control of the brake
pressure reduction and brake pressure build-up phases of an antilocking
control cycle. These pressure modulators have at least one return spring
which urges the piston into a position associated with maximum volume of
the primary chamber for controlling the pressure build-up and pressure
reduction phases of an antilocking system control valve arrangement. The
piston of the particular pressure modulator is designed as a stepped
piston, whose smaller piston step forms a boundary of the outlet pressure
space and whose larger piston step forms a boundary of the drive pressure
space. In normal brake operation, the drive pressure space is subject to
the high outlet pressure of the auxiliary pressure source so that the
piston is forced against the action of a powerful return spring into its
basic position associated with minimum volume of the modulator outlet
pressure space. In this basic position, an inlet valve is mechanically
driven into its open position in which the outlet pressure space,
associated with the controllable brake circuit of the brake unit of the
brake installation, is connected to communicate with the outlet pressure
space of the pressure modulator.
This inlet valve is designed as a non-return valve which has already taken
up its shut off position in the introductory phase of a pressure reduction
stroke of the modulator piston, which is controlled by relieving the
pressure in its drive pressure space. After the inlet valve has taken up
this shut off position, an increasing pressure reduction occurs in the
modulator outlet pressure space with further displacement of the modulator
piston. The increase in the modulator outlet pressure space, attainable by
means of a piston displacement, is smaller than the volume of the liquid
quantity which can be forced, at maximum braking pressure, into the wheel
brake(s) of the brake circuit connected to the pressure modulator. The
wheel brake(s) controllable by such a pressure modulator can be shut off
from the outlet pressure space of the pressure modulator by means of a
solenoid valve whose basic position is the open position. If the brake
pressure reduction attainable by a single pressure reduction stroke of the
modulator is not sufficient for the antilocking control, the particular
wheel brake is shut off from the modulator outlet pressure space and then
the drive pressure space of the modulator is again subject to drive
pressure so that it now operates as a return pump, which pumps brake fluid
previously drained from a wheel brake back into the brake unit, in order
to reduce brake pressure at the wheel brakes.
In order that braking can still take place in the case of a failure of the
auxiliary pressure source, the modulator piston takes up its end position
associated with the maximum volume of the modulator outlet pressure space
in which the inlet valve is shut off. The modulator is provided with a
mechanical controlled bypass valve which is open in the end position of
the modulator piston with the faulty function mentioned. This permits
connection of brake pressure from the brake unit via the modulator outlet
pressure space into the wheel brake(s). However, as soon as the modulator
position is displaced, even if only slightly, from its end position so as
to produce brake pressure restoration or to produce pump-back operation,
the bypass valve closes again. The valve body of the bypass valve is
forced against its valve seat with a closing force which becomes greater
as the modulator piston becomes "closer" to its position corresponding to
the minimum volume of the modulator outlet pressure space. A disadvantage
of this arrangement is that situation in which the auxiliary pressure
source has only "partially" failed, in such a way that its outlet pressure
is just insufficient to hold the modulator piston in the position
corresponding to minimum volume of the outlet pressure space against the
effect of the return spring and the outlet pressure of the brake unit
connected into its outlet pressure space. This holds the modulation piston
in a position slightly displaced from the ideal, and although the inlet
valve is closed, the bypass valve cannot open and the brake installation
is practically inoperative. An antilocking system with pressure modulators
of the type described in the German patent application P No. 36 37 781.2
is therefore questionable for safety reasons.
The object of the invention is therefore to improve an antilocking system
of the general type mentioned at the beginning in such a way that, without
adverse effect on the control functions, the technical complexity
necessary to achieve the system is reduced, the noise due to the control
system is substantially avoided and, in addition, high functional
reliability of the brake installation overall is ensured.
The invention achieves the object by having the inlet valve designed as a
2/2-way valve, controlled by stroke motions of the stepped piston 27, and
held in its open position when the position of the stepped piston is
associated with minimum volume of the functional chamber which can be
connected to the wheel brakes. This 2/2-way valve, starting from this open
position, can only reach its shut off position by a displacement of the
stepped piston taking place in the sense of increasing the functional
chamber. When the stepped piston has reached a position in the immediate
vicinity of its end position associated with maximum volume of the
functional chamber and starting from this end position, the 2/2-way valve
48 only returns to the open position by means of a displacement of the
stepped piston taking place in the sense of reducing the volume of the
functional chamber, after the stepped piston has executed a minimum stroke
h, relative to this end position. A position indicator is provided which
generates electrical output signals which are characteristic of the
positions of the piston and continually vary with changes to them. The
output signals of the position indicator are fed as additional information
inputs to an electronic control unit which, by processing them, generates
signals for switching over the antilocking control valve arrangement by
which the pump-back operation of the stepped cylinder 24 can be
controlled. The difference in the volumes of the functional chamber,
between its maximum and minimum values, is between 1/5 and 1/3 of that
volume of brake fluid which must be forced into the wheel brakes of the
static brake circuit in order to achieve the maximum brake pressure
permitted by the design of the brake installation.
The antilocking system of the invention is suitable for control both on the
so-called select-low principle, according to which brake pressure changes
caused by the control system in each wheel brake of the static brake
circuit take place in the same sense and with the control system coming
into action as soon as a locking tendency occurs on only one of the wheel
brakes of the static brake circuit and being maintained as long as wheel
brake of this static brake circuit exhibits a locking tendency; and
control on the so-called select-high principle, according to which control
is only initiated when all the wheel brakes of the static brake circuit
show a locking tendency but in which control is maintained as long as
there is a locking tendency even if on only one of the wheel brakes of the
static brake circuit. A control system employing the first-name principle
is advantageous where the static brake circuit is the rear axle brake
circuit of the vehicle whereas control according to the second principle
is suitable for a static brake circuit including both front wheels. For
both types of control, the construction per brake circuit of the
antilocking system uses: only one brake 2/2-way solenoid pressure control
valve; a stepped cylinder with a mechanically controlled function valve,
which provides automatic switching as required from pressure modulation
operation to pump-back operation of the stepped cylinder;, and only one
antilocking system control valve arrangement, by means of which the drive
control of the stepped cylinder takes place. This construction is very
simple, because it is substantially easier to control a two-position
solenoid valve than a three-position solenoid valve in contrast to the
known antilocking system. This alone provides a substantial control
technique simplification relative to the known antilocking system. Since,
from the statistical point of view, an overwhelming majority (more than
95%) of braking situations involve actuating the brake installation with a
force which corresponds to at most one fourth, or a smaller fraction, of
that actuation force that must be employed in order to generate the
maximum brake pressure permitted by the brake installation design, the
increase in functional chamber volume achievable by a single "suction"
stroke of the piston of the stepped cylinder also suffices even if it is
necessary in a corresponding high, proportion of brake situations
requiring control to achieve the largest possible brake pressure reduction
at the wheel brake(s) subject to the control system so that switching the
stepped cylinder to pump-back operation only occurs on rare occasions.
Therefore, in the overwhelming majority of braking actions subject to an
antilocking control system, the stepped cylinder also fulfills the
function of a pressure reservoir provided in the known antilocking system,
which can therefore be omitted in the antilocking system of the invention.
This permits a further substantial reduction in the technical complication
necessary for the development of the antilocking system of the invention.
The antilocking system according to the invention therefore operates in
the overwhelming majority of braking situations requiring control,
corresponding to the statistical frequency mentioned, by so-called
pressure modulation operation, i.e. in that mode of operation in which
brake fluid which is accepted in the functional chamber of the stepped
cylinder (considered in this case as the modulation chamber) in order to
reduce the pressure in the wheel brakes, is forced back into the wheel
brakes in brake pressure restoration phases of the antilocking control
system so that a single pressure reduction stroke and restoration stroke
of the stepped piston is required to achieve the brake pressure changes
necessary for control. This is of benefit both with respect to the
sensitivity of the control system and with respect to the "control
comfort" because it substantially avoids otherwise disturbing noise.
The design of the stepped cylinder, wherein a pump-back stroke of the
stepped piston can pump back a quantity of brake fluid into the outlet
pressure space of the brake unit whose volume is between 1/4 and 1/2 of
the difference between the minimum and maximum values of the volume of the
functional chamber of the stepped cylinder, allows the cylinder to act
mainly in pressure modulation operation, i.e. as pressure modulator. This
achieves the result that, even with suitably small spatial dimensions of
the piston cylinder, a repetition of the pump-back stroke cycle of the
piston of the stepped cylinder only becomes necessary on very rare
occasions, which again is of benefit to control comfort.
It is advantageous if the inlet valve is designed as a spool valve whose
housing, is an axial extension of the stepped cylinder, from its section
forming the boundary of the functional chamber. A valve bore containing
the valve body of the spool valve is designed as a bind hole starting at
the bore step forming the fixed boundary, of the functional chamber The
blind hole is offset by a housing step relative to the cylinder bore step,
in the immediate vicinity of which the valve spool is sealed so that it
can be displaced relative to the functional chamber. The valve spool of
the inlet valve is provided with a radial flange at an end section located
within the functional chamber of the stepped cylinder. The radial flange
is, in turn, encompassed by a stop flange, pointing radially inwards of
the stepped piston. These two flanges determine the maximum magnitude h of
a possible relative stroke motion between the stepped piston and the valve
spool. The valve spool is also provided with an end flange which forms the
movable boundary of a balance space which is in continuous communicating
connection with the functional chamber of the stepped cylinder used for
brake pressure modulation via a longitudinal and a transverse duct of the
valve spool. This end flange of the valve spool forms the boundary of the
balance space at one end and also forms the valve body of the inlet valve,
which in its shut off position, closes both the valve inlet duct entering
the blind hole and an overflow duct leading from the blind hole to the
functional chamber. This provides for a simple design means of the
function switch-over valve of the stepped cylinder of the antilocking
system.
The configuration of the antilocking system control valve arrangement
includes two valves which can be actuated electrically and are each
designed as 2/2-way valves. The basic position of one valve causes the
output pressure of the auxiliary pressure source to be connected to the
control pressure space of the stepped cylinder and its actuated position
causes the control pressure space to be shut off relative to the auxiliary
pressure source. The basic position of the second valve causes the control
pressure space to be shut off relative to the non-pressurized sump tank of
the auxiliary pressure source and its actuated position connects the
control pressure space to the sump tank of the auxiliary pressure source.
This has the advantage that when the brake pressure control valve is shut
off, brake pressure retention phases of the antilocking control system can
be controlled in a simple manner by means of the antilocking system
control valve arrangement alone.
Other objects, advantages and novel features of the present invention will
become apparent from the following detailed description of the invention
when considered in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an electro-hydraulic block circuit diagram of an antilocking
system according to the invention with a pressure modulator provided for
controlling brake pressure change phases and also providing the function
of a return pump; and
FIGS. 2a, 2b, 2c, 2d, 2e, 2f, and 2g show the pressure modulator in various
functional states, each intended to explain the antilocking system
according to FIG. 1 and with each shown in section along a plane of its
central longitudinal axis.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the components essential to the function of an antilocking
system wherein the system is intended for a road vehicle with static brake
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