U.S. patent application number 14/748832 was filed with the patent office on 2016-12-29 for simulated eh braking system and safety protection.
This patent application is currently assigned to Caterpillar Inc.. The applicant listed for this patent is Caterpillar Inc.. Invention is credited to Jason M. Buckmier, Joshua D. Callaway, Todd R. Farmer, Steven J. Juricak, Korby A. Koch, Brian F. Taggart.
Application Number | 20160375880 14/748832 |
Document ID | / |
Family ID | 57600999 |
Filed Date | 2016-12-29 |
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United States Patent
Application |
20160375880 |
Kind Code |
A1 |
Farmer; Todd R. ; et
al. |
December 29, 2016 |
SIMULATED EH BRAKING SYSTEM AND SAFETY PROTECTION
Abstract
Systems and methods for simulating electro-hydraulic (EH)
braking of a work machine using a combination of hydraulic braking
and engine braking may include determining a commanded brake
pressure requested by an operator, determining an available engine
braking force, and comparing the available engine braking force to
a maximum available bleed pressure that is available to reduce a
hydraulic braking force. An engine braking output and a pressure
reducing valve output pressure may be determined based on the
available engine braking force and the maximum available bleed
pressure. A pressure reducing valve may be actuated so that the
hydraulic braking force is equal to the commanded brake pressure
minus the pressure reducing valve output pressure, and a
transmission may be actuated so that the engine applies an engine
braking force equivalent to the engine braking output to brake the
work machine.
Inventors: |
Farmer; Todd R.; (Apex,
NC) ; Juricak; Steven J.; (Cary, NC) ;
Buckmier; Jason M.; (Cary, NC) ; Callaway; Joshua
D.; (Cary, NC) ; Koch; Korby A.; (Holly
Springs, NC) ; Taggart; Brian F.; (Angier,
NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Caterpillar Inc. |
Peoria |
IL |
US |
|
|
Assignee: |
Caterpillar Inc.
Peoria
IL
|
Family ID: |
57600999 |
Appl. No.: |
14/748832 |
Filed: |
June 24, 2015 |
Current U.S.
Class: |
701/50 |
Current CPC
Class: |
B60T 1/062 20130101;
B60T 13/662 20130101; B60T 17/221 20130101; B60T 13/686 20130101;
B60T 7/042 20130101; B60T 8/1701 20130101; B60T 8/171 20130101;
B60T 7/085 20130101 |
International
Class: |
B60T 8/17 20060101
B60T008/17; B60T 13/68 20060101 B60T013/68; B60T 8/171 20060101
B60T008/171 |
Claims
1. A simulated electro-hydraulic (EH) braking system for a work
machine, comprising: a brake control valve in fluid communication
with a pressurized fluid source and operatively connected to a
brake control to move between a normally closed position and an
open position in response to a displacement of the brake control to
produce a commanded brake pressure at a brake valve outlet that
corresponds to the displacement of the brake control; a brake
control sensor operatively coupled to the brake control to sense
the displacement of the brake control and output a brake control
sensor signal that corresponds to the displacement of the brake
control; a pressure reducing valve having an open position and a
maximum bleed position, a pressure reducing valve inlet in fluid
communication with the brake valve outlet, a first pressure
reducing valve outlet in fluid communication with a brake cylinder,
and a second pressure reducing valve outlet in fluid communication
with a low-pressure reservoir, wherein a pressure reducing valve
output pressure at the second pressure reducing valve outlet
increases from zero at the normally open position to a maximum
bleed pressure at the maximum bleed position; a controller
operatively connected to the brake control sensor and the pressure
reducing valve, wherein: the controller is configured to determine
the commanded brake pressure based on the brake control sensor
signal, the controller is configured to determine an available
power source braking pressure for a power source, the controller is
configured to compare the available power source braking pressure
to a maximum available bleed pressure of the pressure reducing
valve, the controller is configured to determine a power source
braking output pressure and the pressure reducing valve output
pressure based on the available power source braking pressure and
the maximum available bleed pressure, the controller is configured
to cause the pressure reducing valve to move to a position so that
a hydraulic braking output pressure communicated from the first
pressure reducing valve outlet to the brake cylinder is equal to
the commanded brake pressure minus the pressure reducing valve
output pressure, and the controller is configured to actuate a
transmission of the work machine so that the power source applies a
power source braking force equivalent to the power source braking
output pressure to reduce a speed of the work machine.
2. The simulated EH braking system of claim 1, wherein the
controller is programmed to set the power source braking output
pressure equal to the available power source braking pressure and
the pressure reducing valve output pressure equal to the maximum
available bleed pressure minus the available power source braking
pressure in response to determining that the available power source
braking pressure is greater than zero and less than the maximum
available bleed pressure.
3. The simulated EH braking system of claim 1, wherein the
controller is programmed to set the power source braking output
pressure and the pressure reducing valve output pressure equal to
zero in response to determining that the available power source
braking pressure is not greater than zero.
4. The simulated EH braking system of claim 1, wherein the
controller is programmed to set the power source braking output
pressure equal to the maximum available bleed pressure and the
pressure reducing valve output pressure equal to the maximum bleed
pressure in response to determining that the available power source
braking pressure is greater than the maximum available bleed
pressure.
5. The simulated EH braking system of claim 1, wherein the
transmission of the work machine comprises a hydrostatic
transmission having a hydraulic pump and an hydraulic motor, and
wherein the controller is configured to cause the hydraulic pump
and the hydraulic motor to have a hydraulic pump displacement and a
hydraulic motor displacement, respectively, so that the power
source and the transmission apply the power source braking force
equivalent to the power source braking output pressure.
6. The simulated EH braking system of claim 1, comprising a
low-pressure valve operatively connected to the controller and
having a first low-pressure valve inlet in fluid communication with
the brake valve outlet, a second low-pressure valve inlet in fluid
communication with the pressurized fluid source, and a low-pressure
valve outlet in fluid communication with the pressure reducing
valve inlet, wherein the low-pressure valve places the brake valve
outlet in fluid communication with the pressure reducing valve
inlet in an open position, and places the pressurized fluid source
in fluid communication with the pressure reducing valve inlet in an
under pressure fail position.
7. The simulated EH braking system of claim 1, wherein the
controller is configured to determine the available power source
braking pressure by receiving a power source power sensor signal
from a power source power sensor and determining the available
power source braking pressure based on the power source power
sensor signal.
8. A method for simulating electro-hydraulic (EH) braking of a work
machine using a combination of hydraulic brake system braking and
power source braking, comprising: determining a commanded brake
pressure requested by an operator of the work machine; determining
an available power source braking pressure; comparing the available
power source braking pressure to a maximum available bleed pressure
that is available to reduce a hydraulic braking output pressure;
determining a power source braking output pressure and a pressure
reducing valve output pressure based on the available power source
braking pressure and the maximum available bleed pressure;
actuating a pressure reducing valve of the work machine so that the
hydraulic braking output pressure is equal to the commanded brake
pressure minus the pressure reducing valve output pressure; and
actuating a transmission of the work machine so that a power source
of the work machine applies a power source braking force equivalent
to the power source braking output pressure to reduce a speed of
the work machine.
9. The method for simulating EH braking of the work machine of
claim 8, wherein determining the power source braking output
pressure and the pressure reducing valve output pressure comprises
setting the power source braking output pressure equal to the
available power source braking pressure and the pressure reducing
valve output pressure equal to the maximum available bleed pressure
minus the available power source braking pressure in response to
determining that the available power source braking pressure is
greater than zero and less than the maximum available bleed
pressure.
10. The method for simulating EH braking of the work machine of
claim 8, wherein determining the power source braking output
pressure and the bleed valve output pressure comprises setting the
power source braking output pressure and the pressure reducing
valve output pressure equal to zero in response to determining that
the available power source braking pressure is not greater than
zero.
11. The method for simulating EH braking of the work machine of
claim 8, wherein determining the power source braking output
pressure and the pressure reducing valve output pressure comprises
setting the power source braking output pressure equal to the
maximum available bleed pressure and the pressure reducing valve
output pressure equal to a maximum bleed pressure in response to
determining that the available power source braking pressure is
greater than the maximum available bleed pressure.
12. The method for simulating EH braking of the work machine of
claim 8, wherein the transmission of the work machine comprises a
hydrostatic transmission having a hydraulic pump and an hydraulic
motor, and wherein actuating the transmission comprises actuating
the hydraulic pump and the hydraulic motor to have a hydraulic pump
displacement and a hydraulic motor displacement, respectively, so
that the power source and the transmission apply the power source
braking force equivalent to the power source braking output
pressure.
13. The method for simulating EH braking of the work machine of
claim 8, wherein determining the commanded brake pressure
comprises: sensing a displacement of a brake control of the work
machine; and determining the commanded brake pressure based on an
amount of the displacement of the brake control.
14. The method for simulating EH braking of the work machine of
claim 8, wherein determining the available power source braking
pressure comprises: sensing a power source speed of the power
source; and determining the available power source braking pressure
based on the power source speed of the power source.
15. A simulated electro-hydraulic (EH) braking kit for a work
machine with a hydraulic brake system having a brake control valve
that moves between a normally closed position and an open position
in response to a displacement of a brake control to produce a
commanded brake pressure at a brake valve outlet that corresponds
to the displacement of the brake control, the simulated EH braking
kit comprising: a pressure reducing valve having an open position
and a maximum bleed position, a pressure reducing valve inlet
configured to be placed in fluid communication with the brake valve
outlet, a first pressure reducing valve outlet configured to be
placed in fluid communication with a brake cylinder of the work
machine, and a second pressure reducing valve outlet configured to
be placed in fluid communication with a low-pressure reservoir of
the work machine, wherein a pressure reducing valve output pressure
at the second pressure reducing valve outlet increases from zero at
the open position to a maximum bleed pressure at the maximum bleed
position; and a simulated EH braking kit controller operatively
connected to the pressure reducing valve and to a machine
controller of the work machine, wherein: the simulated EH braking
kit controller is configured to receive from the machine controller
a commanded brake pressure signal and an available power source
braking pressure signal corresponding to an available power source
braking pressure for a power source of the work machine, the
simulated EH braking kit controller is configured to compare the
available power source braking pressure to a maximum available
bleed pressure of the pressure reducing valve, the simulated EH
braking kit controller is configured to determine a power source
braking output pressure and the pressure reducing valve output
pressure based on the available power source braking pressure and
the maximum available bleed pressure, the simulated EH braking kit
controller is configured to cause the pressure reducing valve to
move to a position so that a hydraulic braking output pressure
communicated from the first pressure reducing valve outlet to the
brake cylinder is equal to the commanded brake pressure minus the
pressure reducing valve output pressure, and the simulated EH
braking kit controller is configured to transmit a power source
braking control signal to the machine controller to cause the
machine controller to actuate a transmission of the work machine so
that the power source applies a power source braking force
equivalent to the power source braking output pressure.
16. The simulated EH braking kit of claim 15, wherein the simulated
EH braking kit controller is programmed to set the power source
braking output pressure equal to the available power source braking
pressure and the pressure reducing valve output pressure equal to
the maximum available bleed pressure minus the available power
source braking pressure in response to determining that the
available power source braking pressure is greater than zero and
less than the maximum available bleed pressure.
17. The simulated EH braking kit of claim 15, wherein the simulated
EH braking kit controller is programmed to set the power source
braking output pressure and the pressure reducing valve output
pressure equal to zero in response to determining that the
available power source braking pressure is not greater than
zero.
18. The simulated EH braking kit of claim 15, wherein the simulated
EH braking kit controller is programmed to set the power source
braking output pressure equal to the maximum available bleed
pressure and the pressure reducing valve output pressure equal to
the maximum bleed pressure in response to determining that the
available power source braking pressure is greater than the maximum
available bleed pressure.
19. The simulated EH braking kit of claim 15, comprising a
low-pressure valve operatively connected to the simulated EH
braking kit controller and having an open position and an under
pressure fail position, a first low-pressure valve inlet configured
to be in fluid communication with the brake valve outlet, a second
low-pressure valve inlet configured to be in fluid communication
with a pressurized fluid source, and a low-pressure valve outlet in
fluid communication with the pressure reducing valve inlet, wherein
the low-pressure valve places the brake valve outlet in fluid
communication with the pressure reducing valve inlet when the
low-pressure valve is in the open position, and the low-pressure
valve places the pressurized fluid source in fluid communication
with the pressure reducing valve inlet when the low-pressure valve
is in the fail high position.
20. The simulated EH braking kit of claim 15, wherein the simulated
EH braking kit controller is implemented in the machine controller
by programming code.
Description
TECHNICAL FIELD
[0001] The present disclosure relates generally to braking systems
in work machines and, more particularly, to a work machine and a
method managing hydraulic braking forces and engine braking forces
to reduce brake wear while providing a natural braking feel to an
operator.
BACKGROUND
[0002] Work machines equipped with hydraulic braking systems may at
times struggle to manage the interaction between the braking force
provided by the braking system in response to an operator input at
a braking input device, such as a brake pedal, and engine braking
force that is applied to the wheels through a transmission that
downshifts when the operator eases off on an input speed control,
such as a gas pedal. The hydraulic braking system responds to the
operator input with braking force that is proportional to the
displacement of the braking input and will result in a natural
feeling braking response for the operator. However, when an engine
braking force is added, the work machine decelerates at a greater
rate than expected, particularly for inexperienced operators, and
thereby producing an unnatural feel for the operator.
[0003] Though also present in work machines having gear-type
transmissions, the unnatural braking phenomenon may be particularly
acute in a work machine equipped with a variable hydrostatic
transmission. An example of a work machine having a hydrostatic
transmission providing a braking force to the traction devices of
the work machine is provided in U.S. Pat. Appl. Publ. No. US
2013/0104532 A1, published for Ries et al. on May 2, 2013, entitled
"Hystat Drive System Having Coasting Functionality." In the
publication, a drive system for a machine may have an engine, a
pump driven by the engine to pressurize fluid, a motor connected to
the pump via an inlet passage and an outlet passage, and a traction
device driven by the motor. The drive system may also have an
operator input device movable from a neutral position through a
range to a maximum displaced position to affect a speed of the
engine, and a controller in communication with the input device and
at least one of the pump and motor. The controller may be
configured to gradually adjust a displacement of the at least one
of the pump and motor to slow the traction device over a period of
time after the operator input device is returned to the neutral
position. The machine may also be equipped with an
electro-hydraulic (EH) braking system having a hydraulic actuated
braking device operatively associated with one of the traction
devices of the work machine and providing a braking force to the
traction device when commanded to do so by a controller in response
to a braking signal received from a braking input. The publication
teaches a method for controlling the drive system to utilize
braking forces of the EH braking system and the hydrostatic
transmission.
[0004] The combination of the EH braking system and the hydrostatic
transmission is desirable so that the work machine can command a
desired speed reduction by actively scaling the braking force
versus the transmission retarding force to yield a natural feel
during braking. However, EH braking systems are very expensive to
implement due to redundancies that must be designed into the system
to ensure that an acceptable level of braking can be achieved if an
electronic component of the braking system fails during the braking
cycle. In view of this, opportunities exist for providing a more
economical system for combining braking forces and transmission
retarding forces with a natural feel while also ensuring required
minimum levels of braking in the event of an electrical fault in
the braking system.
SUMMARY OF THE DISCLOSURE
[0005] In one aspect of the present disclosure, a simulated EH
braking system for a work machine is disclosed. The simulated EH
braking system my include a brake control valve in fluid
communication with a pressurized fluid source and operatively
connected to a brake control to move between a normally closed
position and an open position in response to a displacement of the
brake control to produce a commanded brake pressure at a brake
valve outlet that corresponds to the displacement of the brake
control, a brake control sensor operatively coupled to the brake
control to sense the displacement of the brake control and output a
brake control sensor signal that corresponds to the displacement of
the brake control, a pressure reducing valve having an open
position and a maximum bleed position, a pressure reducing valve
inlet in fluid communication with the brake valve outlet, a first
pressure reducing valve outlet in fluid communication with a brake
cylinder, and a second pressure reducing valve outlet in fluid
communication with a low-pressure reservoir, wherein a pressure
reducing valve output pressure at the second pressure reducing
valve outlet increases from zero at the normally open position to a
maximum bleed pressure at the maximum bleed position, and a
controller operatively connected to the brake control sensor and
the pressure reducing valve. The controller may be configured to
determine the commanded brake pressure based on the brake control
sensor signal, to determine an available power source braking
pressure for a power source, and to compare the available power
source braking pressure to a maximum available bleed pressure of
the pressure reducing valve. The controller may further be
configured to determine a power source braking output pressure and
the pressure reducing valve output pressure based on the available
power source braking pressure and the maximum available bleed
pressure, to cause the pressure reducing valve to move to a
position so that a hydraulic braking output pressure communicated
from the first pressure reducing valve outlet to the brake cylinder
is equal to the commanded brake pressure minus the pressure
reducing valve output pressure, and to actuate a transmission of
the work machine so that the power source applies a power source
braking force equivalent to the power source braking output
pressure to reduce a speed of the work machine.
[0006] In another aspect of the present disclosure, a method for
simulating EH braking of a work machine using a combination of
hydraulic brake system braking and power source braking is
disclosed. The method may include determining a commanded brake
pressure requested by an operator of the work machine, determining
an available power source braking pressure, and comparing the
available power source braking pressure to a maximum available
bleed pressure that is available to reduce a hydraulic braking
output pressure. The method may further include determining a power
source braking output pressure and a pressure reducing valve output
pressure based on the available power source braking pressure and
the maximum available bleed pressure, actuating a pressure reducing
valve of the work machine so that the hydraulic braking output
pressure is equal to the commanded brake pressure minus the
pressure reducing valve output pressure, and actuating a
transmission of the work machine so that a power source of the work
machine applies a power source braking force equivalent to the
power source braking output pressure to reduce a speed of the work
machine.
[0007] In a further aspect of the present disclosure, a simulated
EH braking kit for a work machine with a hydraulic brake system is
disclosed. The hydraulic brake system may include a brake control
valve that moves between a normally closed position and an open
position in response to a displacement of a brake control to
produce a commanded brake pressure at a brake valve outlet that
corresponds to the displacement of the brake control. The simulated
EH braking kit may include a pressure reducing valve having an open
position and a maximum bleed position, a pressure reducing valve
inlet configured to be placed in fluid communication with the brake
valve outlet, a first pressure reducing valve outlet configured to
be placed in fluid communication with a brake cylinder of the work
machine, and a second pressure reducing valve outlet configured to
be placed in fluid communication with a low-pressure reservoir of
the work machine, wherein a pressure reducing valve output pressure
at the second pressure reducing valve outlet increases from zero at
the open position to a maximum bleed pressure at the maximum bleed
position. The simulated EH braking kit may further include a
simulated EH braking kit controller operatively connected to the
pressure reducing valve and to a machine controller of the work
machine, with the simulated EH braking kit controller being
configured to receive from the machine controller a commanded brake
pressure signal and an available power source braking pressure
signal corresponding to an available power source braking pressure
for a power source of the work machine, to compare the available
power source braking pressure to a maximum available bleed pressure
of the pressure reducing valve, and to determine a power source
braking output pressure and the pressure reducing valve output
pressure based on the available power source braking pressure and
the maximum available bleed pressure. The simulated EH braking kit
controller may further be configured to cause the pressure reducing
valve to move to a position so that a hydraulic braking output
pressure communicated from the first pressure reducing valve outlet
to the brake cylinder is equal to the commanded brake pressure
minus the pressure reducing valve output pressure, and to transmit
a power source braking control signal to the machine controller to
cause the machine controller to actuate a transmission of the work
machine so that the power source applies a power source braking
force equivalent to the power source braking output pressure.
[0008] Additional aspects are defined by the claims of this
patent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a side elevation view of a work machine in the
form of a wheel loader in which a simulated EH braking system
strategy in accordance with the present disclosure may be
implemented;
[0010] FIG. 2 is a schematic view of a hydraulic braking system in
accordance with the present disclosure that may be implemented in
the work machine of FIG. 1;
[0011] FIG. 3 is a graph of brake pedal rotation angle versus brake
outlet pressure for the hydraulic braking system of FIG. 2;
[0012] FIG. 4 is a schematic illustration of an exemplary
electronic control unit and control components that may be
implemented in the work machine of FIG. 1; and
[0013] FIG. 5 is a flow diagram of an exemplary simulated EH
braking routine in accordance with the present disclosure that may
be implemented in the work machine of FIG. 1.
DETAILED DESCRIPTION
[0014] The operation of a simulated EH braking system strategy may
be discussed with reference to an exemplary work machine in which
the strategy may be implemented. Those skilled in the art will
understand that the strategy in accordance with the present
disclosure may be implemented in other types of work machines. FIG.
1 illustrates an embodiment of an exemplary work machine in the
form of a wheel loader 10. The wheel loader 10 includes a body
portion 12 and a non-engine end frame 14 connected by an
articulating joint 16. The body portion 12 houses a power source,
such as an engine 18, and a transmission 20 that drive rear wheels
22, and includes an elevated cab 24 for the operator. The end frame
14 has front wheels 26 that are turned by the steering mechanism,
with the articulating joint 16 allowing the end frame 14 to move
from side-to-side to turn the wheel loader 10. In the illustrated
embodiment, an implement in the form of a bucket 28 is mounted at
the front of the end frame 14 on a coupler 30. The bucket 28 and
coupler 30 may be configured for secure attachment of the bucket 28
during use of the wheel loader 10, and for release of the bucket 28
and substitution of another implement.
[0015] The coupler 30 is connected to the end frame 14 by a pair of
lift arms 32. One end of each lift arm 32 is pivotally connected to
the end frame 14 and the other end is pivotally connected to the
coupler 30 proximate the bottom. The lift arms 32 rotate about the
point of connection to the end frame 14, with the rotation of the
lift arms 32 being controlled by corresponding lift cylinders 34
pivotally coupled to the end frame 14 and the lift arms 32 that
extend to raise the lift arms 32 and retract to lower the lift arms
32. Rotation of the bucket 28 and the coupler 30 may be controlled
by a Z-bar linkage that may include a tilt lever 36 pivotally
connected to a tilt lever support 38 mounted on the lift arms 32.
At one end of the tilt lever 36, a tilt link 40 has one end
pivotally connected to the end of the tilt lever 36, and the
opposite end pivotally connected to the coupler 30. A tilt cylinder
42 couples the opposite end of the tilt lever 36 to the end frame
14 with pivotal connections at either end. For a given position of
the lift arms 32, the bucket 28 and the coupler 30 are rotated
toward the racked position by extending the tilt cylinder 42, and
rotated in the opposite direction toward the dump position by
retracting the tilt cylinder 42.
[0016] The transmission 20 of the wheel loader 10 may be any
appropriate type of transmission for transferring torque from the
engine 18 to the rear wheels 22. In the illustrated embodiment, the
transmission 20 may be a hydrostatic (hystat) transmission 20
having a variable displacement bi-directional axial piston pump 44
operatively coupled to and driven by the engine 18 to pump
hydraulic fluid to a fixed or variable displacement bi-directional
axial piston hydraulic motor 46 that is operatively connected to a
rear axle 48 and the rear wheels 22. A controller 50 of the wheel
loader 10 may be operatively connected to the actuators (not shown)
for swash plates (not shown) of the hystat pump 44 and the hystat
motor 46. The controller 50 may respond to operator commands at an
input speed control (not shown) in the cab 24 by causing angles of
the swash plates in the pump 44 and the motor 46 to increase and
decrease and thereby vary the hydraulic flow between the pump 44
and the motor 46 and, correspondingly, the speed of the rear wheels
22 to propel the wheel loader 10 in the manner known in the art.
The hystat transmission 20 is exemplary only, and any other
appropriate transmission may be implemented, such as gear-type
automatic transmissions or electric drive transmissions.
[0017] Braking force for the wheel loader 10 may be provided by a
hydraulic braking system that is responsive to operator input at an
input braking control, such as a brake pedal 52, located in the cab
24. The brake pedal 52 may be operatively connected to a brake
control valve 54 that responds to movement of the brake pedal 52 to
selectively fluidly connect a pressurized fluid source, such as a
pump or an accumulator 56, to a rear brake cylinder 58 and a front
brake cylinder 60. The brake cylinders 58, 60 may be operatively
connected to corresponding wheel brakes (not shown), such as disk
brakes or drum brakes, that increase braking forces on the wheels
22, 26, respectively, when pressurized fluid from the fluid source
is delivered to the brake cylinders 58, 60. The controller 50 may
be operatively connected to a brake control sensor (not shown) and
an actuator (not shown) of the brake control valve 54 to control
the operation of the brake control valve 54 and the hystat
transmission 20 in response to displacement of the brake pedal 52
as will be discussed further below.
[0018] The elements of an embodiment of a hydraulic braking system
100 and their interconnections are illustrated in the schematic
diagram of FIG. 2. The hydraulic braking system 100 may include a
plurality of the accumulators 56 that may provide pressurized
braking fluid to the brake control valve 54. The accumulators 56
may be placed in fluid communication to receive pressurized brake
fluid from a high-pressure fluid source such as a brake fluid pump
102. The pump 102 may be configured to draw brake fluid from a
low-pressure reservoir or tank 104, pressurize the brake fluid to a
desired level, and discharge the brake fluid to the accumulators 56
via high-pressure supply lines 106, 108.
[0019] The brake control valve 54 may be a manually-operated,
variable position, three-way valve that is mechanically coupled to
the brake pedal 52 so that displacement of the brake pedal 52 is
converted to corresponding linear displacement of brake valve
elements 110, 112 between a normally-closed position and an open
position for a variable rate of flow through the brake control
valve 54 that is proportional to the displacement of the brake
pedal 52. Each of the brake valve elements 110, 112 corresponds to
one of the brake cylinders 58, 60, respectively, to control the
flow of brake fluid thereto. The brake valve elements 110, 112 may
be separate valve elements that are coupled to move in unison, or
may be a single valve element configured to produce separate outlet
flows to the brake cylinders 58, 60. The brake control valve 54 may
have brake valve inlets placed in fluid communication with the
high-pressure supply lines 106, 108 and the accumulators 56 by
brake valve supply lines 114, 116, and brake valve outlets
connected to brake valve output lines 118, 120 such that the brake
valve supply lines 114, 116 are cut off from the brake valve output
lines 118, 120 when the brake valve elements 110, 112 are in the
normally-closed position, and in fluid communication with the brake
valve supply lines 114, 116 when the brake valve elements 110, 112
move toward the open position. The brake control valve 54 may also
have brake valve return outlets that are connected to the tank 104
by a brake valve return line 122.
[0020] The hydraulic braking system 100 may further include a
low-pressure valve 124 located downstream from the brake control
valve 54. The low-pressure valve 124 may be a solenoid-operated,
two-position, three-way valve that is movable in response to a
command from the controller 50 to selectively convey brake fluid
from the brake control valve 54 and the pump 102 and accumulators
56 to the brake cylinders 58, 60. The low-pressure valve 124 may
include a pair of low-pressure valve element 126, 128 that
correspond to the brake valve elements 110, 112, respectively, and
are movable between a normally-open position and a under pressure
fault position. The low-pressure valve elements 126, 128 may be
spring biased toward the open position, and have a low-pressure
valve actuator 130 operatively connected to the controller 50 to
receive control signals causing the low-pressure valve actuator 130
to move the low-pressure valve elements 126, 128 toward the under
pressure fault position. The low-pressure valve 124 may have first
low-pressure valve inlets placed in fluid communication with the
brake valve outlets by the brake valve output lines 118, 120, and
second low-pressure valve inlets placed in fluid communication with
the high-pressure supply line 108 by a low-pressure valve supply
line 132. The low-pressure valve 124 may also include low-pressure
valve outlets connected to low-pressure valve output lines 134,
136. When the low-pressure valve 124 is in the normally-open
position, unrestricted flow is provided from the brake valve output
lines 118, 120 to the low-pressure valve output lines 134, 136.
When the low-pressure valve 124 is in the under pressure fault
position, orifices of the low-pressure valve elements 126, 128
allow restricted flow of brake fluid from the low-pressure valve
supply line 132 to the low-pressure valve output lines 134, 136.
Those skilled in the art will understand that configurations of the
solenoid-operated valves are exemplary only. In each case, the
solenoids and springs may be implemented to provide a desired
response. For example, in the low-pressure valve 124, the spring
may be installed to bias the low-pressure valve elements 126, 128
toward the under pressure fault position, and the low-pressure
solenoid actuator 130 may be energized to move the low-pressure
valve elements 126, 128 to the open position. Similar alternate
configurations of the other solenoid-operated valves will be
apparent.
[0021] The hydraulic braking system 100 may also include a pressure
reducing valve 138 located between the low-pressure valve 124 and
the brake cylinders 58, 60. The pressure reducing valve 138 may be
a solenoid-operated, variable position, three-way valve that is
movable in response to a command from the controller 50 to
selectively bleed off pressure from the brake fluid output by the
brake control valve 54 in a manner discussed more thoroughly below.
The pressure reducing valve 138 may include a pair of pressure
reducing valve elements 140, 142 that correspond to the brake valve
elements 110, 112 and the low-pressure valve elements 126, 128,
respectively, and are movable between a normally-open position and
a maximum bleed position. The pressure reducing valve elements 140,
142 may be spring biased toward the open position, and have a
pressure reducing valve actuator 144 operatively connected to the
controller 50 to receive control signals causing the pressure
reducing valve actuator 144 to move the pressure reducing valve
elements 140, 142 toward the maximum bleed position. The pressure
reducing valve 138 may have pressure reducing valve inlets placed
in fluid communication with the low-pressure valve outlets by the
low-pressure valve output lines 134, 136, and pressure reducing
valve outlets placed in fluid communication with the brake
cylinders 58, 60, respectively, by pressure reducing valve output
lines 146, 148. The pressure reducing valve 138 may also include
pressure reducing return outlets connected to the tank 104 by a
pressure reducing valve return line 150. When the pressure reducing
valve 138 is in the normally-open position, unrestricted flow is
provided from the low-pressure valve output lines 134, 136 to the
pressure reducing valve output lines 146, 148 and the brake
cylinders 58, 60. When the pressure reducing valve 138 moves toward
the maximum bleed position, orifices of the pressure reducing valve
elements 140, 142 bleed off a portion of the brake fluid from the
low-pressure valve output lines 134, 136 to the tank 104, with the
remaining brake fluid being communicated to the brake cylinders 58,
60 at a fluid pressure reduced by the pressure of the fluid bled to
the tank 104.
[0022] A parking brake valve 152 may control the flow of
pressurized brake fluid to a parking brake cylinder 154 for
application of a parking brake 156. The parking brake valve 152 may
be a solenoid-operated, two-position, three-way valve that is
movable in response to a command from the controller 50 to
selectively convey brake fluid from the high-pressure supply line
106 to the parking brake cylinder 154. The parking brake valve 152
may be movable between a normally-closed position and an open
position. The parking brake valve 152 may be spring biased toward
the normally-closed position, and have a parking brake valve
actuator 158 operatively connected to the controller 50 to receive
control signals causing the parking brake valve actuator 158 to
move the parking brake valve 152 toward the open position. The
parking brake valve 152 may have parking brake valve inlet placed
in fluid communication with the high-pressure supply line 106 by a
parking brake valve supply line 160, and the parking brake valve
outlet placed in fluid communication with the parking brake
cylinder 154 by a parking brake valve output line 162. The parking
brake valve 152 may further include a parking brake valve return
placed in fluid communication with the tank 104 by a parking brake
valve return line 164. When the parking brake valve 152 is in the
normally-open position, the parking brake valve inlet is cut off
from the parking brake valve outlet, and the parking brake valve
outlet is in fluid communication with the parking brake valve
return so that the parking brake cylinder 154 can drain to the tank
104. When the controller 50 detects actuation of a parking brake
control (not shown), the controller 50 may cause the parking brake
valve actuator 158 to move the parking brake valve 152 to the open
position for unrestricted flow of brake fluid to the parking brake
cylinder 154 to apply the parking brake 156.
[0023] In this arrangement of the hydraulic braking system 100, the
pressure reducing valve 138 allows pressure from the brake control
valve 54 to the brake cylinders 58, 60 to be selectively reduced so
that engine braking force may be applied without giving an
unnatural feel to the operator. FIG. 3 provides a graph 170 of the
brake pedal performance characteristics of the hydraulic braking
system 100. The characteristics are expressed as the brake outlet
pressure in the pressure reducing valve output lines 146
communicated to the brake cylinders 58, 60 versus the rotation
angle of the brake pedal 52 in degrees. As a reference, a curve 172
represents one example of the response of a current mechanical
braking system that attempts to integrate engine braking. The
current brake provides no response for approximately the first
14.degree. of rotation of the brake pedal 52. During this time,
only engine braking forces are used to slow the work machine 10. At
approximately 14.degree., the hydraulic brake pressure increases to
approximately 1000 kPa, and then increases by approximately 511
kPa/degree until reaching approximately 5600 kPa at approximately
23.degree.. As the mechanical braking system responds according to
the curve 172, it does so without regard to the amount of engine
braking force available. Consequently, the combination of the
mechanical braking forces and the engine braking forces may be
greater than or less than expected, thereby providing an unnatural
feel to the operator.
[0024] A brake valve response curve 174 represents the pressure
response of the brake control valve 54 in response to displacement
of the brake pedal 52 by the operator. The data in the curve 172
presumes that no solenoid current is applied to the valve actuators
130, 144, and the low-pressure valve 124 and the pressure reducing
valve 138 are in their normally open positions so that the pressure
at the brake valve outlets and the brake valve output lines 118,
120 is communicated to the brake cylinders 58, 60 with only minimal
losses due to flow through the intervening fluid elements. The
curve 172 begins with an industry-standard deadband built into the
connection between the brake pedal 52 and the brake control valve
54 so that the brake valve elements 110, 112 do not respond to open
the brake control valve 54 for an initial portion of the
displacement of the brake pedal 52. In the illustrated embodiment,
the brake outlet pressure is 0 kPa for approximately the first
2.degree. of rotation of the brake pedal 52. After the deadband,
the displacement of the brake pedal 52 may cause the brake valve
elements 110, 112 to move toward the open position with an
approximately linear response. As shown, the brake valve outlet
pressure increases at approximately 270 kPa/degree, and reaches a
maximum brake valve outlet pressure of approximately 5,600 kPa at
approximately 23.degree. of rotation. The curve 174 represents the
normal response of the hydraulic braking system 100 without engine
braking to provide a "natural" feel to the operator. The curve 174
also represents an over pressure response of the hydraulic braking
system 100 where a fault condition results in the valve elements
126, 128, 140, 142 remaining in the open positions shown in FIG.
2.
[0025] A maximum bleed curve 176 illustrates a response of the
hydraulic braking system 100 when full solenoid current is provided
to the pressure reducing valve actuator 144 to move the pressure
reducing valve 138 to the maximum bleed position, and no solenoid
current is provided to the low-pressure valve actuator 130 and the
low-pressure valve 124 remains in the normally open position. Past
the deadband area, as the brake pedal 52 begins to open the brake
control valve 54, the orifices of the pressure reducing valve
elements 140, 142 are large enough so that substantially all of the
brake fluid bleeds to the low-pressure reservoir 104, and the brake
pressure at the brake cylinders 58, 60 remains essentially at 0
kPa. At approximately 10.degree. rotation of the brake pedal 52,
and approximately 2,100 kPa at the brake valve outlets, a maximum
bleed pressure is reached and the pressure reducing valve orifices
cannot divert more fluid flow to the low-pressure reservoir 104.
From this point forward in the displacement of the brake pedal 52,
the brake pressure will increase at approximately the same rate as
the brake valve response curve 174, but will be offset by
approximately 2,100 kPa. As will be apparent, brake pressures
within an area 178 between the curves 174, 176 may be achieved by
varying the current to the pressure reducing valve actuator 144 and
correspondingly by varying the position of the pressure reducing
valve elements 140, 142 to produce a pressure reducing valve outlet
pressure between 0 kPa and the maximum pressure reducing valve
outlet pressure. This flexibility in the pressure output to the
brake cylinders 58, 60 will allow seamless integration of the
available engine braking force with a natural feel to the operator
as will be discussed further below.
[0026] An additional curve 180 illustrates a under pressure fault
condition response of the hydraulic braking system 100 when the
valve elements 126, 128, 140, 142 move to the under pressure fault
and the maximum bleed position, respectively. In this fault
condition, the low-pressure valve elements 126, 128 place the
low-pressure valve supply line 132 in fluid communication with the
low-pressure valve output lines 134, 136. The orifices of the
low-pressure valve elements 126, 128 reduce the pressure from the
low-pressure valve supply line 132 before providing the brake fluid
to the pressure reducing valve 138. The under pressure fault
condition places the pressure reducing valve elements 140, 142 in
the maximum bleed position such that the fluid pressure in the
low-pressure valve output lines 134, 136 is further reduced by the
maximum bleed pressure before being output to the brake cylinders
58, 60 through the pressure reducing valve output lines 146, 148.
With the low-pressure valve 124 in the under pressure fault
position, the brake control valve 54 is cut off from the brake
cylinders 58, 60, and the hydraulic braking system 100 is not
responsive to displacements of the brake pedal 52. Instead, a
constant load is placed on the brake cylinders 58, 60 sufficient to
meet the standards for an under pressure fault condition. In the
present example, shown by the curve 180, the fluid pressure output
by the low-pressure valve 124 to the low-pressure valve output
lines 134, 136 may be approximately 3,900 kPa so that the maximum
bleed pressure of approximately 2,100 kPa results in a constant
under pressure fault pressure of approximately 1,800 kPa being
output on the pressure reducing valve output lines 146, 148 to the
brake cylinders 58, 60.
[0027] The electrical and control components that may be required
to execute a simulated EH braking control strategy in the hydraulic
braking system 100 are illustrated in FIG. 4. The controller 50 may
include a microprocessor 200 for executing specified programs that
control and monitor various functions associated with the wheel
loader 10, including functions that are outside the scope of the
present disclosure. The microprocessor 200 includes a memory 202,
such as a read only memory (ROM) 204, for storing a program or
programs, and a random access memory (RAM) 206 which serves as a
working memory area for use in executing the program(s) stored in
the memory 202. Although the microprocessor 200 is shown, it is
also possible and contemplated to use other electronic components
such as a microcontroller, an ASIC (application specific integrated
circuit) chip, or any other integrated circuit device.
[0028] The controller 50 electrically connects to the control
elements of the work machine 10, as well as various input devices
for commanding the operation of the work machine 10 and monitoring
their performance. As a result, the controller 50 may be
electrically connected to input devices detecting operator input
and providing control signals to the controller 50 that may include
an input speed control 210, such as a gas pedal or accelerator,
that is manipulated by the operator to regulate the speed of the
work machine 10. The input speed control 210 may transmit speed
control signals that are interpreted by the controller 50 to
determine a commanded speed. A brake control sensor 212 may be
operatively connected to and detect displacement of the brake pedal
52, and transmit brake control sensor signals to the controller 50
that are interpreted to determine a commanded brake output
pressure. A parking brake sensor 214 may be operatively connected
to a parking brake control device to detect actuation of the device
and transmit parking brake control signals to the controller 50 for
actuation of the parking brake 156. The controller 50 may also be
connected to sensing devices providing control signals with values
indicating real-time operating conditions of the work machine 10,
such as an engine power sensor 216 that may be operatively
connected to the power source 18. The engine power sensor 216 may
be configured to detect a power source output shaft speed, a
transmission speed, or any other parameter of the work machine 10
that may be indicative of an amount of power available for engine
braking. The engine power sensor 216 may transmit engine power
sensor signals that are received and interpreted by the controller
50 to determine an available power source braking pressure.
[0029] The controller 50 may also be electrically connected to
output devices to which control signals are transmitted and from
which control signals may be received by the controller 50, such
as, for example, the low-pressure valve actuator 130, the pressure
reducing valve actuator 144 and the parking brake valve actuator
158 discussed above. The valve actuators 130, 144, 158 may be
solenoids or other type of actuators to which the controller 50
outputs control signals or solenoid current to move the
corresponding valve elements to desired positions. The controller
50 may also be electrically connected to a hystat pump actuator 220
and a hystat motor actuator 222 that may be operatively connected
to the swash plates of the hystat pump 44 and the hystat motor 46.
The actuators 220, 222 may respond to control signals transmitted
from the controller 50 to adjust the angles of the corresponding
swash plates and vary the displacement of the pump 44 and the motor
46 to control the speed and power transferred from the power source
18 to the rear wheels 22. An engine throttle 218 may be provided to
control the speed of the power source 18. When the input speed
control 210 transmits speed control signals, the controller 50 may
respond by transmitting appropriate control signals to the engine
throttle 218 to change the engine speed and, correspondingly, the
speed of the work machine 10, as commanded by the operator. Those
skilled in the art will understand that the input devices, output
devices and operations of the controller 50 described herein are
exemplary only, and that additional and alternative devices may be
implemented in the work machine 10 in accordance with the present
disclosure to monitor the operations of the work machine 10 and
inputs provided by operators of the work machine 10, and to control
the power source 18, the transmission 20, the hydraulic braking
system 100, and other systems of the work machine 10 to operate in
a desired manner.
[0030] FIG. 5 illustrates an exemplary simulated EH braking routine
250 that may be programmed into the controller 50 to integrate the
hydraulic braking system 100 and available engine braking capacity
to provide a natural braking feel to an operator. The routine 250
may start at a block 252 where the controller 50 may determine
whether the brake pedal 52 has been displaced by the operator based
on the brake force sensor signals transmitted by the brake control
sensor 212. If the brake pedal 52 has not been displaced, the
controller 50 may continue to evaluate the brake force sensor
signals from the brake control sensor 212 until displacement of the
brake pedal 52 is detected.
[0031] If the controller 50 determines that the brake pedal 52 has
been displaced, control may pass to a block 254 to determine the
brake pressure commanded by the displacement of the brake pedal 52.
For a given displacement, the commanded brake pressure will
correspond to the point along the brake valve response curve 174.
The controller 50 may be programmed with a formula for determining
the brake pressure based on the value in the brake force sensor
signal from the brake control sensor 212. Alternatively, data
representing the curve 174 may be stored in tabular form in the
memory 202, with the commanded brake pressure being retrieved by
the controller 50. Depending on the granularity of the data in the
table, interpolation may be required between the two nearest data
points to the actual displacement of the brake pedal 52 to
determine the commanded brake pressure.
[0032] After the commanded brake pressure is determined at the
block 254, or prior to or concurrently there with, control may pass
to a block 256 to determine an available engine braking force, and
a corresponding engine braking pressure, that may be provided by
the power source 18 and the transmission 20. The controller 50 may
use the data in the engine power sensor signals to determine the
power available from the power source 18 to apply an engine braking
force to the rear wheels 22 based on the characteristics of the
power source 18 and the components of the transmission 20. In the
illustrated hystat transmission 20, the engine braking force may be
determined based on an engine speed and the ability to manipulate
the hystat pump 44 and the hystat motor 46 to apply force to the
rear axle 48. In a typical automatic transmission 20, the engine
braking force may be dependent on the capacity to downshift to a
lower gear to reduce the axle speed. In an electric drive
transmission 20, the braking force may be dependent on the
retarding force created when the electric drive is engaged to
charge a battery or batteries using the momentum of the work
machine 10. Once determined, the available braking force may be
converted into an equivalent engine braking pressure that can be
applied to the brake cylinders 58, 60 to brake the work machine 10
in the same manner. The equivalent braking pressure may be used
with the command brake pressure to determine how to adjust the
brake pressure of the brake cylinders 58, 60 to use both braking
components and produce a natural braking feel.
[0033] With the commanded brake pressure and the available engine
braking pressure determined, control may pass to a block of 258 to
determine whether any engine braking pressure is available. If the
engine braking pressure is not greater than 0 kPa and no engine
braking pressure is available at the block 258, control may pass to
a block 260 to set an engine braking output pressure and a pressure
reducing valve output pressure equal to 0 kPa to indicate that the
brake valve response curve 174 should be followed to achieve a
natural braking response. With the output pressures set to 0 kPa,
control may pass to a block 262 where the controller 50 may
transmit control signals to actuate the actuators 144, 220, 222 to
create the corresponding braking forces. With the engine braking
output pressure set to 0 kPa, the controller 50 may allow the swash
plates of the hystat pump 44 and the hystat motor 46 to remain in
position as no engine braking force is available. Alternatively, to
ensure that no residual braking force is applied, the controller 50
may transmit control signals to cause the actuators 220, 222 to
shallow the swash plates so there is no displacement by the hystat
pump 44 and the hystat motor 46. With an automatic or electric
drive transmission 20, the controller 50 may transmit control
signals causing the transmission 20 to shift to neutral. At the
same time, the controller 50 may cut off current to the pressure
reducing valve actuator 144 so the pressure reducing valve 138
remains in its normally open position and no hydraulic braking
pressure from the brake control valve 54 is bled off. At the same
time, the brake pedal 52 has mechanically opened the brake control
valve 54 to transmit the commanded brake pressure to the brake
cylinders 58, 60 with the natural response and feel corresponding
to the displacement of the brake pedal 52. Once the braking forces
are set, control may pass back to the block 252 to monitor
subsequent changes to the displacement of the brake pedal 52.
[0034] If the available engine braking pressure is greater than 0
kPa at the block 258, and engine braking forces available for
integration with the hydraulic braking system 100, control may pass
to a block 264 to determine whether the available engine braking
pressure is greater than a maximum available bleed pressure at the
pressure reducing valve 138. The maximum available bleed pressure
is dependent on the displacement of the brake pedal 52 and the
corresponding braking pressure commanded by the operator. For
example, in the graph 170 of FIG. 3, the pressure reducing valve
138 is capable of bleeding the entire fluid pressure from the brake
control valve 54 until the brake pedal 52 is displaced by
approximately 10.degree. of rotation, and can bleed the maximum
bleed pressure of approximately 2,100 kPa after the brake pedal 52
is displaced by greater than 10.degree. of rotation. Consequently,
the maximum available bleed pressure is less than the maximum bleed
pressure, and is equal to the brake valve output pressure according
to the brake valve response curve 174, for displacement of the
brake pedal 52 by less than 10.degree. of rotation, and is equal to
the maximum bleed pressure of approximately 2,100 kPa for greater
displacements of the brake pedal 52. This adjustment to the maximum
available bleed pressure is important to avoid applying a greater
engine braking pressure than is available to be bled off from the
brake valve output pressure.
[0035] If the available engine braking pressure is greater than the
maximum available bleed pressure at the block 264, control may pass
to a block 266 to set the engine braking output pressure equal to
the maximum available bleed pressure, and to set the pressure
reducing valve output pressure equal to maximum bleed pressure to
indicate that the pressure bled off of the brake valve output
pressure will be offset by only that portion of the available
engine braking pressure needed to replace the maximum available
bleed pressure. With the output pressures set according to the
maximum available bleed pressure, control may pass to the block 262
where the controller 50 may transmit control signals to actuate the
actuators 144, 220, 222 to create the corresponding braking forces.
With the engine braking output pressure set to the maximum
available bleed pressure, the controller 50 may transmit control
signals to cause the actuators 220, 222 to adjust the swash plates
so that the displacement by the hystat pump 44 and the hystat motor
46 yields a braking force on the rear axle 48 corresponding to the
maximum available bleed pressure. With an automatic transmission
20, the controller 50 may transmit control signals causing the
transmission 20 to downshift to a gear that will apply a braking
force that is no greater than the maximum available bleed pressure.
With an electric drive transmission 20, the controller 50 may
transmit control signals causing the electric drive to engage in a
manner that applies a retarding force that is no greater than the
maximum available bleed pressure in charging the battery or
batteries. At the same time, the controller 50 may transmit control
signals or solenoid current to the pressure reducing valve actuator
144 to move the pressure reducing valve elements 140, 142 to the
maximum bleed position so that the maximum available bleed pressure
is bled off from the brake valve output pressure created by the
displacement of the brake pedal 52 and corresponding displacement
of the brake valve elements 110, 112.
[0036] If the available engine braking pressure is less than the
maximum available bleed pressure at the block 264, the bleed
pressure must be reduced for the total braking force to match the
brake valve response curve 174 and provide the operator with a
natural breaking feel. Under these conditions, control may pass to
a block 268 to set the engine braking output pressure equal to the
available engine braking pressure, and to set the pressure reducing
valve output pressure equal to the maximum available bleed pressure
minus the available engine braking pressure so the pressure bled
off of the brake valve output pressure is reduced by an appropriate
amount. With the output pressures set to reflect the available
engine braking pressure being less than the maximum available bleed
pressure, control may pass to the block 262 where the controller 50
may transmit control signals to cause the actuators 220, 222 to
adjust the swash plates so that the available engine braking force
is applied to the rear axle 48, or to cause an automatic
transmission 20 to maintain a gear, or an electric drive
transmission 20 to engage in a manner, that will apply the
available braking force. At the same time, the controller 50 may
transmit control signals or solenoid current to the pressure
reducing valve actuator 144 to move the pressure reducing valve
elements 140, 142 to an intermediate bleed position so that only
the bleed pressure necessary to offset the available engine braking
pressure is bled off from the brake valve output pressure from the
brake valve elements 110, 112.
INDUSTRIAL APPLICABILITY
[0037] The simulated EH braking system 100 illustrated and
described herein provides functionality found in EH braking systems
without the expense required to implement the necessary redundancy
functionality required for failure of electronic components during
the braking cycle. The available engine braking capacity is
integrated into the hydraulic braking forces provided by the
braking system 100 to provide the operator of a work machine such
as the wheel loader 10 with a natural response to pressing the
brake pedal 52 regardless of the current operating conditions and
the available engine braking forces. When no engine braking force
is available, the braking system 100 will not bleed any of the
breaking valve output pressure, and instead will use the normal
response of the brake control valve 54 to displacement of the brake
pedal 52. When some engine braking forces are available, but not
more than can be offset by the pressure reducing valve 138, the
braking system 100 will balance the amount of pressure bled from
the brake valve output pressure to reduce the response of the brake
control valve 54 by the amount of the available engine braking
force. Finally, when the available engine braking pressure exceeds
the amount of pressure that can be bled from the brake valve output
pressure, the braking system 100 will only use as much of the
available engine braking pressure as is necessary to offset the
maximum available bleed pressure. In this way, regardless of the
operating conditions, the response of the braking system 100 will
track the brake valve response curve 174 to provide a consistent
response when the operator presses the brake pedal 52.
[0038] The simulated EH braking system 100 may provide benefits in
addition to the improved sensory experience for the operator. By
utilizing available engine braking forces, the amount of usage and
the corresponding wear on the brakes of the work machine 10 are
reduced, thereby prolonging the useful life of the brake parts.
Moreover, as discussed above, the braking system 100 may provide an
EH braking response without the attendant expense of redundancies
for the electronic components of an EH braking system. The
low-pressure valve 124 and the pressure reducing valve 138 may
function in combination to satisfy the specifications for handling
over pressure and under pressure fault conditions. During an over
pressure situation, where faults at the valve actuators 130, 144
move the valve elements 126, 128, 140, 142 to the open positions,
the braking system 100 will respond to the operator braking
commands via the brake control valve 54 without bleeding any
pressure at the pressure reducing valve 138. It is possible that
engine braking forces during the over pressure fault condition may
make the brakes feel to the operator like excess pressure is being
applied and the brakes are grabbing, but the breaking force will be
applied despite the lack of response by the valve actuators 130,
144. During an under pressure fault situation, when the solenoid
current or absence thereof moves the low-pressure valve 124 to the
under pressure fault position, and the pressure reducing valve 138
to the maximum bleed position, the feed of pressurized fluid
directly from the pump 102 and the accumulators 56 through the
low-pressure valve 124 ensures that at least a minimum required
amount of brake pressure will be applied to the brake cylinders 58,
60 to stop the work machine 10.
[0039] The simulated EH braking system 100 may provide additional
benefits in being adjustable to handle varying operating conditions
to which the work machine 10 may be subjected. For example, it may
be desirable to provide greater braking force when the work machine
10 is traveling downhill or at a higher rate of speed than when
traveling at low speeds or on a level surface. In such cases, the
controller 50 may be configured to adjust the control signals to
the actuators 144, 220, 222 to decrease the bleed pressure and
increase the engine braking force applied to the rear axle 48 so
that the total braking force applied by the hydraulic braking
system 100 may be greater than that dictated by the brake valve
response curve 174. Conversely, when the work machine 10 is
traveling uphill or at low speeds, the controller 50 may be
configured to adjust the control signals to the actuators 144, 220,
222 to increase the bleed pressure and decrease the engine braking
force so that the total braking force applied by the hydraulic
braking system 100 may be less than that dictated by the brake
valve response curve 174 and slow the work machine 10 at a lower
rate.
[0040] The low-pressure valve 124 may also be utilized in operating
conditions other than in the over pressure and under pressure fault
conditions discussed above. For example, in conditions such as when
the work machine 10 has a fully loaded bucket 28 and is travelling
down grade, the speed of the work machine 10 can exceed the engine
speed commanded by the operator and cause the power source 18 to
over-rev if the operator does not apply the brakes, either
unintentionally or intentionally. In this type of over-speed
conditions, the controller 50 may detect the condition based on
sensor signals from a machine speed sensor indicating that the work
machine 10 is travelling faster than a speed commanded by the
operator at the input speed control 210. In previous work machines
10, this condition may cause an alarm to sound in the cab 24 to
warn the operator. In the work machine 10 in accordance with the
present disclosure, the controller 50 may transmit control signals
to the actuator 130 to cause the valve elements 126, 128 of the
low-pressure valve 124 to move toward the under pressure fault
position to place the accumulators 56 in fluid communication with
the brake cylinders 58, 60 and apply the brakes and slow the work
machine 10 even if the operator fails or chooses not to displace
the brake pedal 52. The controller 50 may continue to actuate the
low-pressure valve 124 until the work machine 10 is travelling at
an acceptable speed relative to the speed commanded by the operator
that will not risk damage to the power source 18 or other rotating
components such as the hydraulic pump 102.
[0041] The low-pressure valve 124 may also be utilized in
conditions where braking force from the service brakes will be a
helpful compliment to the available engine braking force. For
example, the controller 50 may actuate the low-pressure valve 124
during high speed directional shifts to increase the deceleration
of the work machine 10 beyond what is capable through engine
braking alone. When an operator commands a directional shift at low
speeds, the controller 50 may manipulate the transmission 20 to
apply a maximum engine braking force to slow the work machine 10
before driving the work machine 10 in the opposite direction. The
controller 50 may cause the actuators 220, 222 to reverse the fluid
flow between the hystat pump 44 and the hystat motor 46 to apply
the engine braking force as the power source 18 continues producing
power. However, at high speeds, the inertia of the work machine 10
can cause the power source 18 to overrun its maximum output speed
and risk damaging the power source 18. Typically during directional
shifts, the operator does not act to apply the service brakes, and
instead allows the work machine 10 to execute the shift and
corresponding direction change. In prior work machines 10 under
this condition, the controller 50 causes the engine throttle 218 to
essentially shut off the power source 18 and allow just the torque
required to drive the output shaft of the shut off power source 18,
the parasitic loads within the work machine 10 and its own weight
to slow the work machine 10 until the controller 50 could safely
cause the engine throttle 218 to start up the power source 18 and
reverse the travel direction of the work machine 10. In the present
work machine 10 during high speed directional shifts, the
controller 50, in a similar manner as the over-speed condition
discussed above, can transmit control signals to the actuator 130
to move the low-pressure valve 124 toward the under pressure fault
position. The low-pressure valve 124 creates brake pressure in the
brake cylinders 58, 60 that will supplement the engine braking
force and cause the work machine 10 to slow at a faster rate, with
the benefit of executing the high speed directional shift in less
time.
[0042] The simulated EH braking functionality discussed herein may
be added to existing work machines having hydraulic braking systems
that did not compensate for the available engine braking forces.
The pressure reducing valve 138, the low-pressure valve 124 and a
simulated EH braking controller may be provided in an aftermarket
kit. The valves 124, 138 may be installed between the brake control
valve 54 and the brake cylinders 58, 60 as shown in FIG. 2. The
simulated EH braking controller may be operatively connected to the
controller 50 and the other control elements of the work machine 10
as necessary to execute the logic of the simulated EH braking
routine 250. Appropriate changes to the programming of the
controller 50 may also be implemented when the kit is installed.
The simulated EH braking controller may be implemented as a
separate control unit, or may be implemented in the controller 50
with programming code that will modify the logic executed by the
controller 50 to simulate the EH braking response.
[0043] While the preceding text sets forth a detailed description
of numerous different embodiments, it should be understood that the
legal scope of protection is defined by the words of the claims set
forth at the end of this patent. The detailed description is to be
construed as exemplary only and does not describe every possible
embodiment since describing every possible embodiment would be
impractical, if not impossible. Numerous alternative embodiments
could be implemented, using either current technology or technology
developed after the filing date of this patent, which would still
fall within the scope of the claims defining the scope of
protection.
[0044] It should also be understood that, unless a term was
expressly defined herein, there is no intent to limit the meaning
of that term, either expressly or by implication, beyond its plain
or ordinary meaning, and such term should not be interpreted to be
limited in scope based on any statement made in any section of this
patent (other than the language of the claims). To the extent that
any term recited in the claims at the end of this patent is
referred to herein in a manner consistent with a single meaning,
that is done for sake of clarity only so as to not confuse the
reader, and it is not intended that such claim term be limited, by
implication or otherwise, to that single meaning.
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