CA 02417313 2003-O1-27
HYDROSTATIC AUXILIARY DRIVE SYSTEM
Field of the Invention
The present invention is directed to a dual path hydrostatic auxiliary drive
system having hydraulic pressure, motor speed and power boundaries to limit
system output.
Background of the Invention
Motor graders are large work vehicles. Typically they are provided with a
main frame that is pivotally coupled to a forwardly extending frame about a
vertical
pivot axis. The main frame is provided with four driven wheels. Two
longitudinally
separated wheels are located on each side of the main frame. The main frame
houses the internal combustion engine for powering thf~ grader. The forwardly
extending frame is supported on twa front wheels. The forwardly extending
frame is
provided with a drawbar having a grader circle. A grader blade is mounted to
the
grader circle by a blade frame. The position of the grader blade relative to
the
grader frame is controlled by hydraulic cylinders. A saddle is mounted to the
forwardly extending frame above the grader circle. The saddle is provided with
hydraulic cylinders for manipulating the position of the grade circle relative
to the
forwardly extending frame.
Some graders are all wheel drive machines. Hydraulic wheel motors drive
the front wheels. The motors selectively engage the front wheels by hydraulic
clutches. The front wheels may be driven in an overdrive mode, where they are
driven at a faster speed than the rear wheels; at an equal speed mode where
they
are driven at the same speed as the rear wheels; or in an underdrive mode,
where
they are driven slower than the rear wheels.
One manufacturer of all wheel drive motor graders uses mechanical
overrunning clutches in place of hydraulically applied clutches. Another
manufacturer markets an all wheel drive motor grader fnaving a dual path
hydrostatic
auxiliary drive system.
Summary
It is an object of the present invention to provide an improved dual path
hydrostatic auxiliary drive system having an auxiliary controller that
controls the
CA 02417313 2003-O1-27
speed of the auxiliary drive wheels as a function of the speed of the main
drive
wheels in conjunction with various boundary condition;.
A vehicle having a frame that is propelled by main drive wheels that are
driven by an engine and main transmission is provided with an auxiliary drive
system
for left and right auxiliary drive wheels. The left and right auxiliary drive
wheels are
driven by two independent hydraulic circuits that are independently controlled
by an
electronic auxiliary drive controller. Each of the hydrauilic circuits has a
variable
displacement pump, a hydraulic motor and a hydraulically actuated clutch
located
between the motor and the respective auxiliary drive wheel. A reverser valve
is
located in the hydraulic circuit between the variable displacement pump and
the
hydraulic motor.
The auxiliary drive controller is in communications with a number of sensors.
A main speed sensor provides a main speed signal indicating the speed of the
main
drive wheels. Auxiliary wheel speed sensors provide auxiliary wheel speed
signals
indicating the speed of the auxiliary drive wheels. Clutch pressure sensors
provide
clutch pressure signals indicating the hydraulic pressure being applied to the
clutches. An engine speed sensor provides an engine speed signal indicating
the
speed of the engine.
The auxiliary drive controller independently controls the displacement and
thereby the hydraulic output of the variable displacement pumps. The auxiliary
drive
controller drives the auxiliary drive wheels at a speed that is a fixed ratio
to the
speed of the main drive wheels. It does this by monitoring the main speed
signal
and comparing it to the auxiliary wheel speed signals.
A number of boundary conditions are stored in the auxiliary controller.
There are three basic control boundaries stored in the auxiliary controller: a
hydraulic
pressure boundary, a motor speed boundary and a po~nrer limit boundary. If any
of
these boundaries are exceed by the auxiliary wheel speed signal, clutch
pressure
signal or a combination of these two signals, the auxiliary drive controller
reduces the
output of the respective variable displacement hydraulic; pump. An engine
speed
boundary is also stored in the auxiliary controller. If the engine speed
signal falls
below this boundary the auxiliary drive system is shut down by reducing the
flow of
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both variable displacement pumps to the lowest level possible.
A main clutch is drivingly positioned between the main transmission and the
engine for modulating the speed of the main transmission. A clutch activation
switch
signals the main transmission controller and the auxiliary drive controller
when the
clutch is engaged and disengaged. The auxiliary controller is provided with a
clutch
mode selector switch having first and second modes oi' operation. In its
first, OFF,
position the auxiliary drive system shuts down by reducing the output of the
variable
displacement pumps to the lowest level possible whenever the main clutch is
engaged. In its second, ON , position the auxiliary drive controller attempts
to mirror
the operation of the main clutch in controlling the outpuit of the variable
displacement
pumps. The auxiliary drive controller uses the engine :>peed signal and the
main
speed signal to calculate the slip between the engine and main transmission
when
mirroring the operation of the main clutch.
Brief Description of the Drawin s
Figure 1 is a perspective view of a motor gradE=r.
Figure 2 is an electricallhydraulic schematic of the subject auxiliary drive
system.
Figure 3 is a graph of the operating boundaries.
Detailed Description
Motor grader 10 is illustrated in Figure 1. Grader 10 comprises a main
frame 12 and a forwardly extending frame 14. A vertical pivot 16 pivotally
couples
the frames 12 and 14 to one another. Hydraulic cylinders 17, only one shown,
articulate the position of the front frame 14 relative to the rear frame 12.
The rear
frame 12 is provided with four main drive wheels 18 arranged in a right pair
and a left
pair, only the front drive wheels of each pair are shown.. The front frame 14
is
provided with left and right auxiliary drive wheels 20 and 21, respectively.
The main
drive wheels 18 are driven by an engine 40 through a main transmission 42. The
engine 40 is provided with an electronic engine controller 44 and the main
transmission 42 is provided with an electronic main transmission controller
46.
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The front frame is also provided with a rearwardly extending drawbar 22
having a grader circle 24. A grader blade 26 is mounted to the grade circle 24
by
blade frame 28. Hydraulic cylinders 30 control the pivotal position of the
grader
blade 26 relative to the grader frame 28. The lateral position of the grader
blade 26
relative to the grader frame 28 is controlled by hydraulic cylinder 32, only
the rod end
being shown. Hydraulic cylinders 34 that are mounted to saddle 36 control the
position of the drawbar 22 relative to the front frame 14~. The grader circle
is
rotatively positioned relative to the drawbar 22 by a grader circle motor 38.
The left and right auxiliary drive wheels 20 and 21 are driven by left and
right
hydraulic motors 50 and 51. Left and right hydraulically actuated clutches 52
and 53
are positioned between the left and right auxiliary drive wheels 20 and 21 and
the left
and right hydraulic motors 50 and 51. The left and right hydraulic motors 50
and 51
are variable displacement motors each having two displacement configurations.
The
left clutch 52 is supplied pressurized hydraulic fluid through left clutch
hydraulic line
54, and right clutch 53 is supplied pressurized hydraulic fluid through right
clutch
hydraulic line 55. A left variable displacement hydraulic pump 56 directs
pressurized
fluid to the left hydraulic motor 50 through a closed left hydraulic circuit
58. Similarly,
a right variable displacement hydraulic pump 57 directs. pressurized hydraulic
fluid to
the right hydraulic motor 51 through a closed right hydraulic circuit 59. A
left
reverser valve 60 is hydraulically positioned between the left hydraulic motor
50 and
the left variable displacement hydraulic pump 56. A right reverser valve 61 is
hydraulically positioned between the right hydraulic motor 51 and the right
variable
displacement hydraulic pump 57. Both reverser valves 60 and 61 are three-
position
valves actuated by dual solenoids 62. The central position of the three-
position
valve defines a neutral or free wheeling position wherein the left and right
auxiliary
drive wheels 20 and 21 can freewheel.
The displacement configurations of the left and right variable displacement
pumps 56 and 57 are independently controlled by an electronic auxiliary drive
controller 64 through left and right solenoids 66 and 67 that position the
swash plates
an the pumps. The positions of the left and right reverser valves 60 and 61
are
controlled by auxiliary drive controller 64 by selectively energizing the
solenoids 62.
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a
The auxiliary drive controller 64 also controls the displacement configuration
of the
left and right variable displacement hydraulic motors 50 and 52 through left
and right
solenoids 90 and 91.
Left and right auxiliary wheel speed sensors 68 and 69 generate left and
right auxiliary wheel speed signals that are transmitted to the electronic
auxiliary
drive controller 64. The auxiliary drive controller 64 also receives a speed
ratio
signal from a speed ratio control 70 located in the operator's cab of the
vehicle 10.
The operator sets this control 70 to the desired aggressiveness of the
auxiliary drive
system. For example, the operator can select an equal speed mode where the
auxiliary drive wheels 20 and 21 are driven at the same speed as the main
drive
wheels 18; an underdrive mode where the auxiliary drive wheels are driven 20
and
21 slightly slower that the main drive wheels 18; or an overdrive mode where
the
auxiliary drive wheels 20 and 21 are driven slightly faster than the main
drive wheels
18. The underdrive and overdrive driving modes have various gradations within
each mode so the operator can better tailor his operations. A, main speed
sensor 72
communicates the speed of the main transmission 42 to the main transmission
controller 46 which in turn directs a main speed signal to the auxiliary drive
controller
64 indicating the speed of the main drive wheels 18. Tlhe auxiliary drive
controller 64
adjusts the displacement configurations of the left and right variable
displacement
hydraulic pumps 56 and 57 to adjust the speed of the auxiliary drive wheels 20
and
21 to the selected speed ratio of the main drive wheels 18 as indicated by
speed
ratio control 70. ft should be noted that the main speed signal can be
transmitted
directly from a sensor on the fnal drives of the main drive wheels 18, or a
sensor
measuring the speed of the main drive wheels 18 themselves, or it can be
derived
from the main transmission controller 46. The left and right auxiliary wheel
speed
sensors 68 and 69 can sense the output speed of the respective hydraulic motor
50
and 51, or the speed of the final drives for the auxiliary drive wheels 20 and
21.
The main transmission controller 46 also signals the electronic auxiliary
drive controller 64 with a gearing signal that indicates what gear or speed
has been
selected and in what direction the vehicle is moving as dictated by the
operator
through shift lever 74. For example, motor grader 10 may have a eight-speed
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transmission with eight forward gears and eight reversE: gears. If the
auxiliary drive
system is turned on by switch 77, the main transmission controller 46 signals
the
electronic auxiliary drive controller 64 with the selected gearing and the
direction of
vehicle movement. Based on this information, the auxiliary drive controller 64
adjusts the swash plates on the left and right variable dlisplacement
hydraulic pumps
56 and 57 for the selected gearing and adjusts the left .and right reverser
valves 60
and S1 to correctly orient the flow of hydraulic fluid in the left and right
hydraulic
circuits 58 and 59. The auxiliary drive controller 64 also adjusts the
displacement of
the left and right hydraulic motors 50 and 51 based upon the selected gearing.
For
gears one through four a first displacement configuration for the left and
right
hydraulic motors 50 and 51 may be used, for gears fiver through seven a second
displacement configuration may be used, and for gear eight the auxiliary drive
system maybe shut down altogether. Various signals between the controllers 44,
46
and 64 are transmitted through a CAN bus 76.
Controlling speed of the auxiliary drive wheels 20 and 21 as a function of the
speed of the main drive wheels 18 is the basic function of the auxiliary drive
controller 64. However a number of operational boundary conditions have been
programmed into the auxiliary drive controller 64 to make it more effective.
An
engine speed sensor 78 transmits an engine speed signal to the controller the
engine controller 44. A minimum engine speed boundary is stored in the
auxiliary
drive controller 64. The minimum engine speed boundary is a level at which the
engine 40 is lugging down and the operator needs to turn off parasitic
systems, such
as the auxiliary drive system. When the auxiliary drive controller 64 detects
an
engine speed signal that is less than the minimum engine speed boundary, the
auxiliary drive controller 64 reduces the flow of pressurized hydraulic fluid
out of the
left and right variable displacement pumps 56 and 57 to the lowest level
possible.
As such, the left and right clutches 52 and 53 are disengaged because of the
reduction of pressure in hydraulic lines 54 and 55.
As shown in Figure 3, three basic boundary co~~nditions influence the normal
operation of the auxiliary drive system. The first boundary is a maximum
pressure
boundary. A maximum hydraulic pressure boundary is stored in the auxiliary
drive
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controller 64. Left and right clutch pressure sensors 80 and 81 generate left
and
right clutch pressure signals that are transmitted to the auxiliary drive
controller 64.
The clutch pressure sensors 80 and 81 are hydraulically coupled to their
respective
clutch hydraulic lines 54 and 55. The auxiliary drive controller 64 compares
the
stored maximum pressure boundary against the left and right hydraulic clutch
pressure signals. If one of the clutch pressure signals exceeds the maximum
pressure boundary, the output of the respective variable displacement pump is
reduced. The hydraulic clutch pressure signals are directly related to the
torque
being transmitted by the clutches to the wheels and by the wheels to the
ground.
The second boundary is the maximum motor speed boundary. A maximum
motor speed boundary is stored in the auxiliary drive controller 64. The
stored
maximum motor speed boundary is compared against 'the left and right auxiliary
wheel speed signals from wheel speed sensors 68 and 69. If one of the
auxiliary
wheel speed signals exceeds the maximum motor speed boundary, the output of
the
respective variable displacement pump is reduced.
If the engine 40 had infinite power or a rather Large power reserve the
maximum pressure boundary and the maximum motor speed boundary would be
adequate. However, good engineering dictates sizing i:he engine for the
appropriate
work. As such, a power limit boundary is stored in the auxiliary drive
controller 64.
The power limit boundary is proportional to the maximum power output of the
engine
40. For example it can be one-third of the maximum power output of the engine.
In
this way one-third of engine torque would be consumed by auxiliary drive
system
and two-thirds would be consumed by the main transmission 42. The proportion
of
the engine power used by the auxiliary drive system can be controlled by
adjusting
the power limit boundary stored in the auxiliary drive controller 64. If one
of the
clutch pressure signals when combined with the respecaive auxiliary wheel
speed
signals exceeds the power limit boundary, the output oi~ the respective
variable
displacement pump is reduced.
The auxiliary drive controller 64 drives the left and right auxiliary drive
wheels 20 and 21 independently of one another. As such, it is possible that
the
speed of one of the auxiliary drive wheels would be corstrolled by the speed
ratio
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control 70 and the other auxiliary drive wheel would be controlled by the
pressure or
power boundary conditions. This optimizes tractive effort and efficiency in
conditions
where one front wheel has good traction and the other does not.
The operator's cab of the motor grader 10 is provided with a clutch pedal 88
for actuating the main clutch 86. In motor graders this clutch pedal 88 is
typically
known as an inching pedal. By fully depressing the clutch pedal 88 the clutch
86
disengages the main transmission 42 from the engine a40. By lifting the clutch
pedal
88 the main clutch 86 passes through a modulation zone where the main clutch
86
modulates the speed of the transmission. The operation of the main clutch 86
like
the operation of the main transmission 42 is controlled by a main transmission
controller 46. The main transmission controller 46 is alerted to the movement
of the
clutch pedal 88 by a clutch activation switch 84 that is <actuated by the
initial
movement of the clutch pedal 88. Clutch activation swiitch 84 is also directly
connected to the auxiliary drive controller 64. So the auxiliary drive
controller 64 is
alerted to a main clutching event directly by clutch activation switch 84 and
also by
the main transmission controller 4C through CAN bus 76.
Clutch mode selector switch 82 is connected to the auxiliary drive controller
64. The clutch mode selector switch 82 has two modes: a first OFF mode where
the
auxiliary drive system is turned off when the main clutch 86 is applied, and a
second
ON mode where the speed of the auxiliary drive wheels 20 and 21 is modulated
in
tandem with the main drive wheels 18. The clutch modle selector 82 is located
in the
operator's cab.
If the clutch mode selector switch is in its first OFF mode, when the clutch
pedal 88 is depressed the auxiliary drive controller 64 i;a alerted and the
controller 64
reduces the output of the left and right variable displacement hydraulic pumps
56
and 57 to the lowest level possible. As such, the left arid right clutches 52
and 53
are disengaged because of the reduction of pressure ins hydraulic lines 54 and
55.
As the clutch pedal 88 is raised the clutch activation svuitch 84 signals the
auxiliary
drive controller 64 to reapply driving force to the auxiliary drive wheels 20
and 21.
If the clutch mode selector switch 82 is in its second ON position, when the
clutch pedal 88 is depressed the auxiliary drive controller 64 is alerted and
the
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controller 64 attempts to mirror the modulation of the main clutch 86. The
auxiliary
drive controller 64 receives a main speed signal indicating the speed of the
main
transmission 42 from transmission controller 46 and an engine speed signal
indicating the speed of the engine from engine controller 44. From these
signals the
auxiliary controller 64 can calculate slip between the engine 40 and the main
transmission 42 and adjust the output of the left and rigiht variable
displacement
hydraulic pumps 56 and 57 accordingly.
Having described the preferred embodiment, it: will become apparent that
various modifications can be made without departing from the scope of th.e
invention
as defined in the accompanying claims.
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