Note: Descriptions are shown in the official language in which they were submitted.
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Electro-Pneumatic Freight Brake Control System
Background Of The Invention
The present invention is related to electro-pneumatic type
brake systems for railroad freight cars and particularly to a car
load compensating arrangement for preventing a wheel slide in the
event of a fail-safe emergency brake application causing the
electronic brake control to experience a power loss.
This application is a division of copending commonly owned
Canadian Patent Application No. 2,153,853 filed July 13, 1995.
Electro-pneumatic brake systems that are suitable for
railroad cars typically employ solenoid operated electro-
pneumatic valves for directly pressurizing the car brake cylinder
device under control of a microprocessor. Such systems have the
potential for eliminating the need for a pneumatic back-up or
emergency brake, since the solenoid valves can be arranged in a
fail-safe configuration in which a source of compressed air is
connected to the brake cylinder in a deenergized state. Such
electro-pneumatic brake systems that employ a microprocessor have
the further ability to perform the load control function
electrically, thereby achieving more accurate, reliable, and
economical brake control operation.
It is important to note, however, that without some means of
retaining the load control function when a power loss occurs, the
maximum pneumatic emergency brake cylinder pressure will be
delivered in accordance with the fail-safe operation of the
system solenoid valves. Accordingly, the potential exists for an
empty or partially loaded car to slide its wheels when such an
emergency occurs, which is undesirable from the standpoint of the
high cost of wheel damage, not to mention the potential for
derailment.
In order to realize the foregoing benefits attributed to an
electro-pneumatic brake system, as above discussed, it is the
objective of the present invention to load limit brake cylinder
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pressure obtained in accordance with the fail-safe operation of
the system solenoid valves in the event of a power loss that
disables the electric load control.
It is an extension of the foregoing objective to proportion
brake cylinder pressure in accordance with the car load condition
to provide the aforementioned limit pressure.
Briefly, these objectives are carried out by means of a
pressure limiting valve that is interposed in the delivery line
leading from the application and release solenoid valves. An
electric load sensor sets the maximum load limited brake cylinder
pressure capable of being delivered by the limiting valve. The
limiting value arrangement is such that the load limited pressure
setting is retained in the event a power loss occurs, causing the
electric load sensor to be disabled.
Brief Description Of The Drawings
These and other objectives and advantages of the present
invention will become more apparent from the following detailed
explanation when taken in conjunction with the accompanying
drawings in which:
FIG. 1 is a diagrammatic view showing an electro-pneumatic
brake system for a railroad car employing application and release
solenoid valves arranged in a fail-safe configuration to provide
emergency break-in-two protection, and an electric load control
arrangement including a pressure limiting valve for load limiting
the fail-safe emergency brake pressure according to the car load
condition; and
FIGS. 2, 3 and 4 are sectional assembly views showing
various alternate pressure limiting valve arrangements suitable
for use in the brake system of FIG. 1.
Description And Operation
The electro-pneumatic brake control system 10 shown in FIG.
1 includes an air supply pipe 12 that extends the length of a
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railroad car and is arranged with hoses and couplings 14 at each
end for connection with the hose couplings on adjacent cars in a
train to provide a continuous train line via which a source of
compressed air is connected from the train locomotive to a
storage reservoir 16. Also extending the length of the railroad
car is a control wire 18 that is connected to the control wire of
adjacent cars in the train by suitable connectors 20 to provide a
continuous train line control wire to which a source of
electrical power is also supplied at the locomotive. Control
wire 18 carries brake command signals and power to operate
application and release control valves 22, 24 under control of a
microprocessor 26 to which the control wire 18 is connected.
Microprocessor 26 interprets the brake command signals and
energizes appropriate application and release control wires 28,
30 for operating application and release control valves 22, 24 to
obtain the desired brake control.
Application and release control valves 22, 24 are two-
position, solenoid-operated, spring-returned valves having an
inlet l, an outlet 2, and an outlet 3. Inlet 1 of application
control valve 22 is connected by a pipe 32 to air supply pipe 12
via a one-way check valve 34; outlet 2 is connected by piping 36
to inlet 1 of release control valve 24 and to the inlet port 38
of a pressure limiting valve 40; outlet 3 of application valve 22
is blanked and outlet 3 of release valve 24 is open to
atmosphere.
tn7hen a brake command signal is transmitted via control wire
18, it is processed by microprocessor 26 in accordance with an
electrical feedback signal corresponding to the car brake
cylinder pressure and to another feedback signal corresponding to
the car load weight. The brake cylinder pressure feedback signal
is provided by a pressure transducer 42 that sends a signal to
microprocessor 26 via a wire 44 indicative of the effective air
pressure at brake cylinder 46. The car load weight feedback
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signal is generated by an electric load sensor 48 that provides a
signal at wire 50 corresponding to the spring deflection of the
car body relative to its truck, as a measure of the car load. An
averaging circuit 52 modifies this load weight signal to negate
the effect of car body rock and roll, in turn connecting the
modified load weight feedback signal to controller 26 via wire 54
and to an electric motor associated with pressure limiting valve
40 via wire 56.
Assuming microprocessor 26 determines that brake cylinder
pressure is inadequate to meet the load weighed brake command
signal at wire 18, application and release wires 28 and 30 are
deenergized, causing solenoid valves 22 and 24 to assume the
positions shown, in which compressed air is connected from
storage reservoir 16 to inlet port 38 of pressure limiting valve
40. Depending on the car load condition, as reflected by the
averaged load weight signal effective at wire 54 and branch wire
56, pressure limiting valve 40 establishes a maximum pressure
setting below which value the pressure at inlet port 38 is
connected to outlet port 58 and thence to pilot port 60 of a
relay valve 62 via pipe 64. Compressed air stored in reservoir
16 is connected via pipe 66 to supply port 68 of relay valve 62
and via delivery port 70 and pipe 72 to brake cylinder 46 at a
pressure corresponding to the pilot port pressure, the air flow
capacity of relay valve 62 being such as to quickly pressurize
brake cylinder 46.
At the appropriate brake cylinder pressure as determined by
microprocessor 26 in accordance with the instantaneous brake
cylinder feedback signal from transducer 42 and the car load
weight feedback signal provided by averaging circuit 52,
application wire 28 is energized and application solenoid valve
22 is accordingly reset to its closed positioned. In this
position, inlet 1 is connected to closed outlet port 3 to cut-off
further supply of air to the pressure limiting valve inlet port
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38, thereby establishing a lap condition in which a load
compensated pneumatic brake pressure is obtained under electric
control.
When it is desired to release the brakes, microprocessor 26
energizes release wire 30, as well as application wire 28,
causing release valve 24 to be operated to its open position in
which inlet 1 is connected to outlet 3. Pilot pressure effective
at port 60 of relay valve 62 is thus vented to atmosphere via
pressure limiting valve 40 and release solenoid valve 24. Relay
valve 62 accordingly operates in response to the reduction of
pressure at pilot port 60 to exhaust brake cylinder pressure at
its vent port 74.
In the event a train break-in-two occurs, electric power
supplied to microprocessor 26 via control wire 18 will be
interrupted, thereby disabling the above-explained electric
control of the car brakes. Application wire 28 and release wire
30 are thus deenergized, resulting in application solenoid valve
22 being forced to the shown open position by its return spring,
while concurrently, release solenoid valve 24 is forced to the
shown closed position by its return spring. This fail-safe
configuration of the application and release solenoid valves is
such that compressed air is supplied from reservoir 16 to brake
cylinder 46 via the application and release control valves, the
same as during the above-discussed electrically controlled brake
application, irrespective of the fact that the supply of electric
power has been interrupted. Since the above-mentioned train
break-in-two results in a loss of air via the open supply pipe 12
at the location where the break-in-two occurs concurrent with the
interruption of power to control wire 18, check valve 34 is
provided to prevent the compressed air stored in reservoir 16
from escaping to atmosphere. This assures the availability of
compressed air to provide emergency braking during a brake-in-
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two, as above-discussed, without the need to provide a separate
pneumatic back-up brake control system on each car.
In that the interruption of electric power during a train
break-in-two may also result in the averaged load weight signal
at wire 54 being lost, it is important to note that the character
of pressure limiting valve 40 is such that its previously set
upper pressure limit is latched in, so that the loss of power to
pressure limiting valve 40 does not affect the limiting valve
pressure setting. This is the basis of the present invention,
since without electric power, application and release control
valves 22 and 24 remain in their fail-safe position, without the
ability to control the degree of brake application.
Pressure limiting valve 40, however, having a locked-in
maximum pressure setting according to the car load condition,
operates to cut-off the supply of air from reservoir 16 to
control port 60 of relay valve 62, when a pressure appropriate
for the car load weight is realized. The brake cylinder pressure
delivered to brake cylinder 46 via relay valve 62 is thus load
compensated to prevent the undesirable condition of wheel slide
under the emergency brake condition discussed.
Referring now to FIG. 2, there is shown a limiting valve 40A
having a piston assembly 74 and a valve assembly 75, the latter
controlling communication of compressed air between an inlet port
38 and an outlet port 58 according to the pressure setting of the
piston assembly. This pressure setting of the piston assembly is
determined by the position of a moveable fulcrum member 76 that
bears against a balance beam 77, the ends of which are pivotally
connected to a control diaphragm piston 78 and a feedback
diaphragm piston 79 of the piston assembly.
Valve assembly 75 comprises a disc valve element 80 housed
in a cartridge 81 that is mounted on control piston 78 adjacent a
fixed annular valve seat 82 in the limiting valve body. A
release spring 83 between disc valve element 80 and cartridge 81
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urges the valve element toward engagement with seat 82. A bias
spring 84 between the limiting valve body and control piston 78
deflects piston 78 downwardly to a position in which cartridge 81
captures valve element 80 and maintains valve element 80 spaced
apart from seat 82.
When a brake application is made, in accordance with the
foregoing explanation relative to FIG. l, compressed air is
connected to inlet port 38 where it acts on piston 78 and passes
via open valve element 80 to outlet port 58 and to feedback
piston 79. When the force exerted by the air pressure acting on
piston 79 and amplified by the effective ratio of balance beam 77
is sufficient to counter-act the combined force of air acting on
piston 78 and the force of bias spring 84, piston 78 is forced
upwardly to seat valve 80. Depending upon the position of
fulcrum member 76, the relative sizes of pistons 78, 79 and the
strength of bias spring 84, the delivery pressure at outlet 58
will be a predetermined proportion of the air pressure effective
at inlet 38 when valve 80 closes. In that, the delivery pressure
provided by limiting valve 40A of FIG. 2 is load modulated, the
averaged load weight signal feedback to microprocessor 26 via
wire 54 in FIG. 1 may be eliminated when employing the limiting
valve 40A of FIG. 2.
The position of fulcrum member 76 along balance beam 77 is
controlled by a motor 85 having an internally threaded output
shaft 86 to which a threaded adjusting screw 87 of fulcrum member
76 is connected. Preferably, motor 85 is a stepping motor that
incrementally rotates its output shaft in proportion to the load
weight signal derived from averaging circuit 52 (FIG. 1) to in
turn adjust the linear displacement of adjusting screw 87 and
thereby locate the position of fulcrum member 76 in accordance
with the car load condition. In the mid-position of fulcrum
member 76 along balance beam 77, the car is assumed to be in a
full load condition, so that if the relative sizes of pistons 78,
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79 are equal, bias spring 84 will maintain valve element open by
virtue of the balance beam being rotated in a counterclockwise
direction. Accordingly, the pressure at inlet port 38 is
effective at outlet port 58 on a l:l basis consistent with the
assumed full load condition.
As the car load condition changes toward empty, the stepping
motor output shaft is rotated in a direction to extend adjusting
screw 27 and thus move fulcrum member 76 leftward from its mid-
position to thereby change the effective ratio of balance beam 77
and accordingly change the proportion of air pressure effective
at outlet port 58.
When a brake release is required, pressure at inlet port 38
is vented, allowing the greater pressure effective at outlet port
58 to unseat valve element 80 against its return spring 83. This
in turn allows the outlet port pressure to follow the reducing
pressure at inlet port 38 until the force exerted by pressure
acting on feedback piston 79 is insufficient to overcome the
opposing force of bias spring 84. When this occurs, piston 78 is
forced downward to its normal position in which valve element 80
is captured by cartridge 81 and pulled off of its valve seat 82
to allow complete release of the pressure at outlet port 58.
It will now be appreciated that in the force balanced
condition of piston assembly 74, as shown, valve element 80 is
seated to interrupt pressure communication between inlet port 38
and outlet port 58, thereby isolating the brake cylinder delivery
pressure from the brake cylinder supply pressure. In this
manner, the proportioned pressure effective at outlet port 58
corresponds to the pressure setting dictated by the position of
fulcrum member 78. It will be noted that in this position, as
shown, balance beam 77 is substantially parallel with threaded
rod 87 so that no linear counter force is transmitted to the
screw 87 and the motor output shaft. Consequently, the position
of fulcrum member 76 will remain unchanged in the event power is
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lost at motor 85, so that the proportioned pressure effective at
outlet port 58 will be maintained constant irrespective of the
emergency pressure developed at inlet port 38 in accordance with
the aforementioned fail-safe operation of solenoid valves 22, 24
resulting from such power loss.
The limiting valve 40B shown in FIG. 3 is constructed and
operates similar to that of FIG. 2, with corresponding reference
numerals beings assigned to like ports. It will be seen that
valve element 80 in FIG. 3 is associated with feedback piston 79
instead of control piston 78, as is bias spring 84. Also in FIG.
3, a separate release check valve 88 is provided between the
supply and delivery sides of valve element 80, whereas in FIG. 2,
the single valve element 80 serves as the release check valve, as
well as the supply valve. Finally, the output shaft 86 of
stepping motor 85 in FIG. 3 is formed with external screw threads
that engage with internal threads in fulcrum member 76 to provide
adjustment of the linear position of fulcrum member 76. An
indicator rod 89 is attached to fulcrum member 76 and projects
externally of the limiting valve body to provide a visual
indication of the adjusted position of the fulcrum member and
thus the railroad car load weight.
Limiting valve 40C shown in FIG. 4 differs from the previous
arrangements in that a single differential piston assembly 89 is
provided to proportion the pressure effective at outlet port 58
instead of separate control and feedback pistons; and a load
adjustable proportioning spring 90 is provided to load modulate
the output pressure instead of an adjustable fulcrum member
acting through a balance beam. In this FIG. 4 arrangement, the
same reference numerals as used in the arrangements of FIGS. 2
and 3 are used to identify like parts.
The output shaft 86 of stepping motor 85 is provided with
shallow lead threads that engage with corresponding threads in an
axially adjustable spring seat 91 that is keyed against rotation.
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Differential piston assembly 89 includes a larger area piston
member 92 having an annular valve element 80 on one side that is
engageable with a fixed valve seat 82, and a smaller area piston
member 93 connected to the larger area piston 92 adjacent its one
side. Disposed between smaller area piston 93 and spring seat 91
is proportioning spring 90, which urges piston assembly 89 in a
direction to unseat valve element 80.
On a railroad car that is fully loaded, the car load weight
signal causes stepping motor 85 to rotate its output shaft in a
direction to cause spring seat 91 to move rightwardly a distance
sufficient to obtain maximum compression of spring 90. In the
aforementioned unseated condition of supply valve 80, compressed
air effective at inlet port 38 flows past the open supply valve
to outlet port 58 and concurrently acts on the side of piston
member 93 opposite spring 90. The maximum force is selected such
that full pressure at inlet port 38 is insufficient to compress
spring 90 enough to seat valve element 80. Consequently, full
air pressure is supplied to outlet port 58 consistent with the
assumed full load weight signal at stepping motor 85.
Progressively lighter car load weights cause stepping motor
85 to retract spring seat 91 and thereby relax the force exerted
by spring 90. Compressed air at inlet port 35 initially flows
past unseated valve element 80 to outlet port 58 until the
pressure acting on the side of piston member 93 exerts a force
sufficient to overcome the force of spring 90. When this occurs,
piston assembly 89 moves leftward until valve element 80 engages
seat 82 to cut-off further flow of air to outlet port 58.
At this point, any further build-up of pressure at inlet
port 35 causes piston valve assembly 89 to proportion the
pressure effective at outlet port 58. The degree of
proportioning is dependent on the relative pressure areas of
piston members 92, 93, as well as the force exerted by
proportioning spring 90.
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In the event of a power loss such that the load weight
signal controlling stepping motor 85 is absent, the existing
compression of proportioning spring 90 will remain unchanged,
since the shallow lead of the threads on the motor output shaft
86 preclude the spring force from retracting spring seat 86 from
its set position, even though torque on the motor output shaft is
absent due to the loss of motor power. Therefore, the pressure
effective at outlet port 58 is assured of being load modulated to
prevent the possibility of sliding wheels, as could otherwise
occur due to the aforementioned fail-safe operation of solenoid
valves 22, 24. As in the case of limiting valves of FIGS. 2 and
3, the fact that the delivery pressure is load modulated by the
limiting valve itself makes it possible to eliminate the averaged
load weight signal feedback to microprocessor 26 via wire 54 in
FIG. 1.
When a brake release is required, the reduced pressure at
inlet port 38 effective on the spring side of a release check
valve 94 allows the greater pressure at outlet port 58 to open
the check valve and follow the exhausting pressure from inlet 38.
This causes the differential force on piston assembly 89 to be
reversed, allowing valve element 80 to be unseated by virtue of
rightward movement of piston assembly 89. When this occurs,
output port pressure is exhausted via the unseated valve element
80 and inlet port 38.
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