Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: ALL WHEEL DRIVE FOR MOTOR GRADER
BACKGROUND OF THE INVENTION
The present invention relates to drive control
arrangements for motor graders and in particular, relates
to an all wheel drive arrangement for motor graders and
other vehicles.
All wheel drive arrangements for motor graders have
the advantage that the front drive wheels can compensate
for poor traction conditions experienced by the tandem
drive. The front wheel drive can operate in a passive mode
where the front wheels only form part of the primary drive
when there is slippage of the tandem drive. This passive
mode basically allows the front wheel drive to respond when
slippage has occurred on the tandem drive while in good
traction conditions, the grader acts as if it is only
driven by the tandem drive.
All wheel drive systems allow a variation in the
level of aggression of the front wheel drive and the front
wheel drive can be set to be faster than the tandem drive.
This aggressive mode is not the most cost efficient manner
of operating the grader but it is desirable for certain
applications.
Existing front wheel drive arrangements for motor
graders use a hydraulic motor which is supplied with
hydraulic fluid under pressure for providing the necessary
drive of the front wheels. There is a common hydraulic
pump that supplies hydraulic fluid to each of the hydraulic
motors and a flow control valve distributes the amount of
hydraulic fluid to each motor. Control between the two
motors is based on control of the distributing valve.
Various sensors are used for sensing the speed of the front
wheels, rear wheels, ground speed and other parameters
which sensors are connected to a controller which controls
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the speed of the pump as well as the position of the
distribution valve. With this arrangement, fine adjustment
between the drive of the front wheels is difficult to
achieve.
The present invention in a preferred embodiment
uses a different drive arrangement where each front wheel
has its own hydraulic circuit and is hydraulically
separated from the hydraulic circuit of the other drive.
This drive arrangement provides increased control and
sensitivity in adjustment of the system.
In a further embodiment a bypass valve is used to
control the switching of the hydraulic circuit across the
motor from an open circuit to a closed circuit. This by-
pass valve automatically adjusts to changing conditions and
appropriately switches the conditions of the drive
arrangement.
The present invention allows an all wheel drive
which additional works in a creep mode where only the front
wheels are driven.
The particular drive arrangement for each front
wheel drive allows simplified control logic due to the
automatic response of the by-pass valve to changing
hydraulic conditions as opposed to electrically driving
such a by-pass valve to respond to changing conditions.
The particular hydraulic circuit automatically responds to
the changing conditions and causes the hydraulic circuit to
appropriately respond.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are shown in
the drawings, wherein:
Figure 1 is a perspective view of a motor grader;
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Figure 2 is a top view of a motor grader showing
the tandem and front wheel drives and various sensors;
Figures 3 is a simplified schematic vie of the
hydraulic circuit of one of the front wheel drives;
Figure 4 I a schematic view showing the tandem
drive and two separate drives for the front wheel of the
grader and a control arrangement;
Figure 5 is a sectional view of a variable
displacement. pump;
Figures 6 and 7 are schematic views of a particular
hydraulic motor for the front wheels;
Figure 8 is a control logic chart for the all wheel
drive controller;
Figures 9A, 9B and 9C collectively show the hydraulic
circuit of t:he two front wheel drive systems;
Figures 10 through 14 show various conditions of
the bypass valve;
Figure 15 shows the overall all wheel drive logic
flow chart;
Figures 16A and 16B show the logic associated with the
calibration mode of the system;
Figures 17A and 17B show the control logic associated
with the creep mode;
Figures 18A, 18B and 18C show the control logic
associated with the normal all wheel drive mode; and
Figures 19 through 22 show the bypass valve and
solenoid valve in various conditions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 schematically illustrates a motor grader 1
having a frame 3, an operator's cab 7, and a motor 9 which
drives a tandem drive 20. The grader has a mold board 5
suspended beneath the frame and includes front wheel drive
arrangements to either side of the grader.
The top view of the motor grader as illustrated in
Figure 2 has additional components used to control the all
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wheel drive arrangement of the grader. Each front wheel
drive system 2 includes a variable output pump 4, a
hydraulic motor 8, which is preferrably driven by a
variable output pump 4. The motor drives the front wheels
of the grader. The drive system also includes a condition
sensing by-pass valve 6 associated with the hydraulic motor
8. Brake and clutch sensing mechanism 28 is shown as well
as the all wheel drive control panel 24 and the
transmission control all wheel drive control module 26.
Each of the front wheel drive systems 2 include
their own accumulating tanks 7 for excess hydraulic fluid.
The grader frame is articulated generally about a point 23
in front of the motor 9. The front wheel drive system 2 is
preferrably duplicated to either side of the motor grader.
Each drive system works independently and one such drive
system is schematically shown in Figure 3. The variable
output pump 4 is connected to the hydraulic motor 8 and the
automatic condition sensing bypass valve 6 is in parallel
with the motor. The pump 4 and the hydraulic motor 8 is
shown in greater detail in Figures 5 and 6 respectively.
The controller 34 of the pump receives a current
input signal and based on the current, provides adjustment
of the output of the pump. The pump is by-directional and
therefore allows the motor to operate in either a forward
or reverse direction. The pump can also be in a neutral
position where it does not pump any hydraulic fluid. The
hydraulic motor 8 has an operating position where the
pistons of the motor are connected to an output drive
member and a free wheel position where the pistons are
separated from the drive member. In this free wheel
position, the pistons of the motor are biased to a clear
position and the motor can free wheel.
The automatic sensing bypass valve 6 is either open
or closed. In the closed position, the motor is driven by
the pump as a typical hydrostatic drive, whereas in the
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open position, hydraulic fluid discharged by the motor is
returnable through the bypass valve to the inlet side of
the motor. This arrangement avoids hydrostatic braking and
also avoids cavitation when the motor is overdriven by the
front wheel.
As shown in Figure 4, the all wheel drive
arrangement of the motor grader has two separate front
wheel drive systems 2 which as shown, work independently of
one another hydraulically, yet are controlled by the
controller 40. The controller 40 effectively coordinates
the front wheel drive systems with the tandem drive system
of the grader. The controller receives a number of inputs
including inputs from the sensors generally shown as 42 and
from the operator controller 44. The coordination of the
drive arrangement between the front wheel drive system and
the tandem drive system is simplified due to the automatic
condition sensing by-pass valves 6. In addition if
steering angle and/or articulation angle are sensed, each
pump can be adjusted to provide appropriate aggressive
front wheel drive during cornering.
These bypass valves cause the hydraulic circuit of
each motor 8 to assume a bypass condition when the front
wheels are being driven by the pump at a lesser rate than
necessary to keep up with the drive of the wheels of the
tandem drive arrangement 20. This is often referred to as
a passive mode for the front wheel drive. Basically the
front wheels are being driven by each pump 4, however, the
front wheels are overdriving the motors 8 to keep up with
the movement caused by the tandem drive arrangement.
This can produce a motor cavitation condition that
is corrected by the bypass valve. The automatic condition
sensing by-pass valves 6 produce an open circuit across
each motor and thus protect the motors 8. Basically the
motor can overrun as if connected to an overrunning clutch,
however, this function is accomplished hydraulically. In
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the event that the tandem drive wheels start to slip, due
to a loss of traction, the overdrive condition of the front
wheels will cease and the automatic condition sensing by-
pass valves 6 close due to an increase in pressure caused
by the fact that the motors are no longer functioning as a
pump. This increase in pressure is sensed by the bypass
valves and closes the bypass valves. The front wheels are
then driven by the respective motors and the front wheels
become the dominant traction component of the grader until
such time as the tandem wheels cease to experience the slip
condition. The bypass valves 6 sense these conditions
hydraulically and as the pump is being operated as if it
was driving the front wheel, it immediately has the
necessary flow of hydraulic fluid to drive the respective
motor and the closing of the bypass valve forces the
hydraulic fluid from the pump too power the hydraulic
motor. Thus the system rapidly switches from passive to
aggressive front wheel drive while the drive system is a
full hydrostatic system. The slowing of the grader, due to
slippage of the tandem drive, causes the motor to cease
acting as a pump, allowing the drive pressure to the motor
to increase and close the bypass valve.
V~hen the machine is moving and the front wheels are
being overdriven (i.e. low aggression setting) the motors
are rotating faster (or require more oil) than the oil flow
being supplied by the pump. Therefore, pump flow is not
being "restricted" drive pressure will not result. The
motor is acting as pump, and as a result, there will be
some "hydrostatic braking pressure". The amount of
backpressure, or the difference between the inlet and
outlet, pressure of the motor will be equal the spring
setting of the by-pass valve (or logic element).
Overrunning in forward. Drive pressure will drop
to charge pressure (approximately 400 PSI)
- Area A2 (motor in) will see charge pressure (400
PSI)
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- Area A3 (motor in) will see charge pressure (400
PSI)
- Area A1 (motor out) will build up 400 + spring =
480 PSI
- The valve will shift to the open position
V~hen the rear wheels slip, the whole machine slows
down and the front wheels slow down. The pump continues to
supply the same amount of oil. then the machine (and
wheels) slow to a rate that is less than the flow rate
supplied by the pump, pump flow will be restricted and
drive pressure increase. The bypass valve will close and
the motors will be positively driven.
- Area A2 (motor in) will see drive pressure
(assume 1000 PSI)
- Area A3 (motor in) will see drive pressure
(assume 1000 PSI)
- Area A1 (motor out) will see charge pressure (400
PSI)
- The valve will close.
The all wheel drive system is also capable of being
driven in an aggressive mode where the front wheels are
over driven relative to the tandem drive. In this
condition, which is a setting inputted by the operator
using controller 44, the bypass valves are closed and the
front drive wheels are active. Basically each pump is
driven to produce a pressure that closes the bypass valve
as the motor now drives the front wheel as opposed to be
driven by the front wheel. The bypass valves still function
to help protect the hydraulic motors 8 even in the
aggressive mode. For example, when the grader is being
turned and the motor speeds are such that one motor is
overdriven. This condition is sensed by the by-pass valve
and again, it operates to protect the motor from
cavitation.
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Figure 5 shows a cross section of the variable
output pump of which is an axial piston pump of the type
manufactured by Sauer-Sundstrand. The pump includes a
barrel and piston rotation arrangement where the pistons
slide on a swashplate. The swashplate is adjustable by
varying the angle of the swashplate to vary the
displacement of the pump. An electric displacement control
unit 34 varies the position of the swashplate. This allows
the output of the pump to be tailored to the needs of that
particular front wheel.
Figure 6 and 7 illustrate a radial piston pump of
the type manufactured by Valmet Hydraulics. Figure 7 shows
the hydraulic motor schematically in closed hydraulic
circuit condition. This is a low speed high torque radial
piston cam/lobe type motor. Oil ported to the pistons
force the pistons against the cam, forcing the cam to
rotate. The motor housing and wheel rotate with the cam.
Figure 7 shows the motor in a free wheeling mode
where the pistons are not in contact with the cam. The cam
housing and wheel rotate freely. This is the condition
when the front wheel drive is off. The hydraulic oil which
is normally in contact with the pistons that drive the
pistons has been drained and is essentially not acting on
the pistons. Spring arrangements and hydraulic pressure
can be used to drive the pistons to the non contacting
position of Figure 7.
The bypass valve 6 automatically senses a
cavitation condition of the motor (overdrive condition) and
opens the bypass valve to feed additional hydraulic fluid
to the inlet of the motor. During actual braking or
clutching the bypass valve is opened (by movement of the
solenoids) such that hydrostatic braking is avoided.
The front wheel drive systems define a full
hydrostatic drive that operates in a passive or aggressive
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mode and is actually driven in these modes to be available
if slippage of tandem drive occurs and the front systems
are in passive mode. In such a condition the motors of the
front systems prior to slippage are driven by the
respective motors and additional fluid is provided by the
bypass valves to avoid cavitation. Slippage of the tandem
drive results in a change in hydraulic pressures and
closing of the bypass valves resulting in front wheel
drive. The pumps are always pumping and the pistons of the
motor are in contact with theses drive components.
Therefore, the transition is accomplished smoothly and
quickly.
Hydrostatic braking is avoided by opening the
bypass valve whereby normal braking using the tandem
braking continues. This opening of the bypass occurs
automatically as the front wheels are driving the motor and
this hydraulic condition opens the bypass valve. This
occurs due to the motor being overdriven. The operator
depressing the brake or clutch changes a solenoid valve
which changes the hydraulic conditions acting on the bypass
valve and moves it to a bypass position if not already
there. A further advantage of the front wheel drive system
is the protection of the hydraulic motors. The bypass
valve automatically senses cavitation conditions and opens
to supply the necessary additional fluid while continuing
to be driven
A full hydrostatic front wheel drive system is used
which recognizes that pump adjustment to compensate for
many changing conditions is impractical. The bypass valve
provides an automatic clutch like function, provides a
differential function for cornering, provides cavitation
protection, provides passive and aggressive modes in a full
hydrostatic drive, allows the systems to avoid hydrostatic
braking, and maintains the pump and motor in a full
function condition for fast return to an aggressive drive
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condition. The bypass valve also allows greater
flexibility in changing drive settings on the fly.
In creep mode the bypass valve is preferably closed
by adjustment of the solenoids such that a full hydrostatic
drive with hydrostatic braking is provided. The operator
uses the throttle position to accelerate or decelerate.
The tandem braking system remains active and can open the
bypass valve when actuated. This allows normal braking of
the grader to occur.
Figure 8 shows details of the control logic used
for the all wheel drive arrangement. Each of the pumps are
initially driven to a nominal start value to provide
hydraulic fluid to the motors. Input is received regarding
the desired front wheel target speed and a comparison is
made between the actual front wheel speed of each drive
wheel system and its own target. If the wheel speed is
within a very tight tolerance, the process is repeated. If
the wheel speed is outside this close tolerance and is
within a somewhat less demanding tolerance, then a decision
is made to produce a small size signal correction which is
fed to the controller of the pump.
If the wheel speed is well is outside this close
tolerance, a further decision is made whether the pump
signal is within upper and lower limits, and if it is
within the upper and lower limits which define normal
operating ranges then a large signal correction is made.
In contrast, if the pump is either the upper or lower limit
then a decision is made to hold the signal at the limit.
In this way, upper and lower limits are set for each pump
and different incremental adjustments are made to the pump
signal in accordance with the condition sensed. With this
system, the hydraulic motors are always ready to provide
drive. The lower limit of the pump keeps the output at a
value where power can be provided quickly when necessary.
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The detailed hydraulic circuit of Figures 9A, 9B and 9C
illustrate vaz:ious controls of the front wheel drive
systems. On the right hand side of the drawing, a right
pump 4A and a left pump 4B are shown. Right pump 4A is
connected to a right side free wheeling valve 31A and a
left free wheeling valve 31B is connected to left pump 4B.
These valves cause each of the respective motors 8A and 8B
to operate in the free wheeling position as shown in Figure
7. Each circuit also has the bypass valve 6A and 6B
respectively. The circuits include a common charged pump
which provides a minimum pressure to the systems.
The automatic condition sensing by-pass valves 6
can be better understood from a review of Figures 10
through 14. Each by-pass assembly consists of two solenoid
operated two position three-way valves, one pilot operated
logic element and one shuttle valve. The logic element
(bypass) shifts to either the open or close position
depending upon pressure acting on three differential areas.
The solenoids are controlled to determine whether the
device operates in a forward or reverse direction. In
addition, DCV3 and DVC4 will be de-energized when a clutch
or brake is actuated and/or when the all-wheel drive on/off
switch is in the on position and the grader is in neutral
or in 8th gear. The 8th gear is the highest gear and the
all wheel drive is not in use in this position. Similarly,
when, the grader is in neutral this system is off. The
bypass valve is open in 8th gear but the pump and motor are
operating should a shift to 7th gear be made. The
operating parameters of 8th gear cause the valve to open.
Activating the brake or clutch stops the direct
drive of the front wheels. When oil flows in the motor
through the B port, this defines the forward operation
whereas when oil is provided through the A port, this
defines the reverse operation. Basically, the forward
bypass solenoid and the reverse bypass solenoid are set
by the operator and similarly the input regarding the brake
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and clutch merely de-activate the system. The bypass valve
generally forms the hydraulic equivalent of a sensitive
over running clutch in function.
In a passive mode, the bypass valve receives input
from the motor causing a hydraulic pressure which is higher
than the pressure being provided by the pump. The bypass
valve operates to bypass fluid either side of the motor and
the wheels can merely react to the drive of the tandem
drive without damaging the motor. In contrast, when the
tandem drive starts to slip the,front wheels are no longer
over driven causing a reduction in pressure, causing the
pump pressure to be higher and the bypass valve
automatically closes and the pump drives the front wheels.
This is accomplished automatically and quickly.
Figure 11 shows the bypass valve where the all-
wheel drive is switched on and a grader is in neutral. The
bypass valve is shifted to a bypass position due to the
fact the grader is in neutral and the pump pressure is low.
Figure 12 shows the all-wheel drive on with the
grader moving forward. The bypass valve is closed and oil
flow is directed to the motor.
Figure 13 shows the position of a valve when the
operator actuates the clutch or brake. This results in the
bypass valve being shifted to the open circuit condition.
Figure 14 illustrates the position of the bypass
valve when the operator makes a right turn. In such a
case, the left wheel is driven faster by the grader and the
full rate of oil supplied by the pump does not increase as
the wheel speed increases. Drive pressure drops off
eventually to the charged pressure. Oil flow returning
from the motor increases as motor speed increases and this
increase in braking pressure causes the bypass valve to
open once the braking pressure is higher than the bypass
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valve spring pressure. This allows the left wheel to
assume the correct speed as the output of the motor is connected
to the motor input and then provides the
necessary additional fluid and avoids cavitation. The pistons
remain in contact with the cam and are ready when the condition
is removed.
Figure 15 shows the overall all-wheel drive logic
flow chart. In this case when the controller is powered
up, it looks to see whether there are abnormal conditions
and also checks whether calibration has been performed or
should be performed. It then checks to see whether the
all-wheel drive is on or off and further checks whether it
has been placed in creep mode. In creep mode, the tandem
drive is disconnected and the grader is driven merely by
the front wheels. The maximum creep mode speed is less
than 3 m.p.h. and preferably 2 m.p.h. or less.
Figures 16A and 16B show detail of the logic associated with
the calibration mode. Figures 17A and 17B show control logic
associated with the creep mode of operation and Figures 18A, 18B and 18C
show detail associated with normal all-wheel drive mode.
In the creep mode, the current provided to the
hydraulic pump of each drive system 2 is a function of the
engine RPM. Creep mode is used for very fine finishing
grading. With such grading it is desirable to disconnect
the tandem drive to avoid any scuffing or damage caused by
turning. A large engine RPM range is used to allow the operator
fine control with respect to the particular creep speed.
Figures 19 through 22 are similar to Figures 10
through 14 but provide additional details regarding the
automatic response of the bypass valve.
Figure 19 is a schematic showing the position of
the bypass valve and the forward solenoid valve and the
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reverse solenoid valve when the drive arrangement is in all
wheel drive forward mode. The drive pressure from the pump
is provided to the motor along line 202 and the charge
pressure on the low side of the motor is returned on line
204. There is a large pressure differential the high side
and the low side across the motor. As shown in the Figure,
the forward solenoid valve 206 is activated and therefore
the high pressure is provided through the solenoid valve
206 to the shuttle valve 208 and to the bypass valve at
210. This high pressure acts on a large area of the logic
valve which is twice the size of the individual areas on
the opposite side of the logic valve.
The opposite side of the logic valve has a first
area a1 which is exposed to the charge pressure and a
second area A2 which, in this case, is exposed to the high
drive pressure. The high pressure is also operating on the
opposite end of the logic valve on an area A3 where A3 is
twice the size of A1 or A2, and a1 equals A2. A spring 212
exerts a force equivalent to 80 psi. In the forward
position as shown, the high pressure acting in area A3 in
combination with the spring 212 produces a force which
overwhelms the force exerted on A2 and A1. As can be
appreciated A1 is only acted upon by the lower charge
pressure, and as such, the force created on area 3 moves
the bypass valve to the closed condition. Each of the
check valves are closed.
In this forward condition, the motor is
functioning to provide power and the operator has not
actuated the clutch or the brake. If the operator actuates
the clutch or the brake, a problem would occur as he would
anticipate braking, however, the motor would continue to
drive the front wheels as there has been no change to the
output of the pump. To address this situation, a signal
corresponding to a brake signal or a clutch action of the
operator causes the forward solenoid valve 206 to be de-
energized. In this case, the solenoid valve moves rapidly
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upwardly and connects line 215 associated with the tank,
with line 217 associated with the shuttle valve and thus
brings area A3 under the pressure of the tank which is
essentially at zero pressure. Under these conditions, the
force exerted on areas A2 and A1 vastly overpower the
spring pressure and the pressure on A3 and the bypass valve
moves rapidly to the bypass condition. In this way, the
hydraulic fluid from the pump can pass the motor and the
front wheels are then allowed to free wheel such that
normal braking or clutching occurs. A further benefit is
that the pump for powering of the motor remains stroked and
as soon as the clutching or braking condition is removed,
the solenoid valve is activated returning the circuit to
the condition of Figure 19 and power is returned to the
motor.
From the above, it can be seen that the
activation of the solenoid or the deactivation of the
solenoid valve rapidly causes the bypass valve to move from
a closed position of Figure 19 to a bypass position and
thus controls the motor hydraulically acting much in the
manner of a mechanical clutch to the wheel during braking
or clutching or an overdrive condition.
Figure 11 shows an arrangement where the all
wheel drive switch is on and then grader is in neutral. In
this case, the forward solenoid valve is de-energized much
as it would be during clutching or braking, and as such,
the bypass valve has assumed the bypass condition.
Therefore, the forward solenoid valve is controlled in
accordance with a braking signal, clutching signal or a
neutral signal to force the bypass valve to the bypass
condition hydraulically.
During normal operation of the all wheel drive,
there are several conditions which can result in a
overrunning of the front wheel drive motors. Such an
overrunning condition can occur during cornering of the
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motor grader as the outside wheel will be overdriven and
can occur during passive operation of the front wheel
drive. In the passive mode, the hydraulic motor is driving
the front wheel, however, it is not driving at sufficient
speed to match the speed caused by the tandem drive. In
such a condition, the motor is being overdriven and the
discharge from the hydraulic motor is actually higher than
the inlet pressure of the motor. Such an overdrive
condition is shown in Figure 14 and in Figure 20.
As can be appreciated, due to the position of
the forward solenoid valve, area A3 is still exposed to the
inlet pressure to the motor. Similarly, area A2 is exposed
to the inlet pressure to the motor. Area A1 is now exposed
to the higher outlet pressure from the motor, thus the
pressure acting on A1 has changed from a low value to a
high value, due to the motor being effectively overdriven,
and the motor functioning as a pump. This change in
hydraulic conditions causes the bypass valve to move
rapidly to the bypass condition as clearly shown in Figure
20. Such a overrunning condition occurs whenever the motor
is effectively overdriven. This can occur during cornering
and is the general condition when the all wheel drive is
on, and it is working in passive mode. The passive mode
implies that the motor is being under driven by the pump
associated therewith and it will only effectively kick in
if there is slippage of the tandem drive.
The variable pump associated with the motor is
adjusted in accordance with the speed of the grader and in
a passive mode, under drives the motor. However, when
slippage occurs, the forward drive becomes active as the
front wheels are no longer being overdriven by the tandem
drive which is slipping. As such, the passive mode causes
the hydraulic motor to act as a motor. The motor is no
longer overdriven due to the tandem drive slipping and the
change in hydraulic conditions causes the bypass valve to
close and the front wheel drive provides positive drive.
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Once the slippage condition of the tandem drive is removed,
the system returns to its passive mode (bypass valve open).
As it can be appreciated, the sensed hydraulic conditions
cause the rapid movement of the bypass valve from a bypass
condition, to a non bypass position. The bypass valve
automatically responds to the changes in hydraulic pressure
and rapidly moves to appropriately assume the bypass or the
closed mode.
It is also possible to run the hydraulic motor
in an aggressive mode merely by increasing the output of
the pump which supplies the hydraulic fluid to the motor.
In the aggressive mode, the hydraulic motor is acting as a
motor, and as such, the motor bypass valve is closed. It
should be noted that if an overrunning condition occurs due
to cornering, for example, the bypass valve senses the
changes in hydraulic conditions and automatically assumes a
bypass mode.
Figure 21 shows the position of the valves when
the all wheel drive is in a reverse drive mode. In this
case, the high pressure hydraulic fluid is provided to the
motor along line 204 and line 202 is at the charge
pressure. The forward solenoid valve 206 has been
deactivated, and thus the pressure exerted on the shuttle
valve 208 is low, i.e., the tank pressure. In contrast,
the reverse solenoid 230 has been activated and now
connects the high pressure line 204 through the solenoid
valve to line 232 to the shuttle valve 208 and thus
provides high pressure to area A3. Area A1 is also exposed
to this high pressure. In contrast, area A2 is now exposed
to the discharge pressure of line 202. As can be
appreciated, the forward and reverse solenoid valves have
basically reversed and areas A1 and A2 have been reversed.
A3 is still exposed to the high pressure from the pump.
The bypass valve will operate in the identical manner as
previously described to sense an overrunning condition and
the reverse solenoid valve 230 can be deactivated in the
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case of a clutch or braking action, or if the grader is in
neutral.
The pump is controlled as a function of the gear
selection. When the AWD is on and the grader is in
neutral, the pump will be at zero displacement. The pump
is connected to the motor via the freewheeling valve.
When the machine is shifted into gear, a current
signal is immediately sent to the pump that causes the pump
to go to a nominal displacement appropriate for the gear
selected. The nominal displacement is approximately equal
to the displacement required to drive the front wheels at
the same speed as the rear wheels.
The control system then monitors the speed
signals from the front wheels, the output speed of the
transmission (rear wheel speed) and the aggression switch.
The current supplied to the pump is adjusted accordingly to
drive the front wheels at the appropriate speed.
When the clutch and/or brake are actuated and
the bypass valve opens to allows oil to bypass the motor,
the pump remains stroked (supplies oil to the motors) and
speed sensing is still operating.
When the clutch and/or brake are released and
the bypass valve closes, the pump is at the correct
displacement for good positive responsive front wheel
drive.
When a new gear is selected, a new nominal pump
displacement is selected appropriate for the gear.
The system as described uses a hydraulic circuit
to sense overrunning conditions during passive mode and
during cornering and opens the bypass valve to protect the
motor and improve handling. A solenoid valve is used to
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alter the hydraulic pressures to the bypass valve and
causes the bypass valve to assume the bypass condition
during braking or clutching, as well as accommodating a
neutral condition. The all wheel drive is controlled by a
variable displacement pump and the output of the pump can
continue its normal operation during braking or clutching.
Similarly, the pump does not have to and does not respond
to conditions which would result in overrunning of the
motor. This greatly simplifies the control of the pump and
allows extreme flexibility in setting of the pump as these
conditions or the hydraulic conditions of the motor are
protected by the bypass valve/solenoid combination.
The system is not prone to damage and conditions
which could produce cavitation of the hydraulic motor are
corrected by the bypass valve automatically moving to the
bypass position. Preferrably, the pump and motor act
together as part of a drive for one front wheel alone. It
can be appreciated that the bypass valve solenoid
combination essentially protects the hydraulic motor used
to drive the front wheel and any suitable combination for
providing hydraulic fluid for driving of the motor will be
acceptable. A separate pump for each drive wheel
simplifies control of the pump output.
When the system is used in a motor grader, each
of the front wheels acts independently of the other. A
speed sensor can be associated with each front wheel and a
controller can use the particular gear and engine speed to
set the output of the variable displacement pump associated
with the particular motor. This controller can adjust the
variable displacement pump either up and down, to bring the
sensed wheel speed into agreement with a predetermined
speed based on the gear and engine speed and level of
aggression set by the operator. In this way, each system
acts independently and adjust itself to the correct speed.
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The system is also capable of changing on the
fly from a passive mode to an aggressive mode and a host of
settings in between. Basically, the operator sets a
particular level of passive or aggressive mode, and each of
the variable displacement pumps adjust to produce the
correct speed condition. The system also relies on the
bypass valve such that the pump output will not drop below
a certain minimum level and cannot go beyond a certain
maximum level. In this way, power is quickly transferred
to the front wheels as the pumps are in an operating
condition and the bypass valve is normally bypassing until
it is required. This will be the case where the system is
in the passive mode and the tandem drive starts to slip.
The front wheel drive becomes active due to the ground
speed decreasing and the bypass valve closing.
Figure 22 shows the various valves when the all
wheel drive is in creep mode. In this position, the bypass
valve does not work and front wheel drive is provided as
well as front wheel hydraulic braking until the brake is
activated. As shown, the forward solenoid valve 206 is
activated and as such, drive pressure is provided as area
A3. Area A2 is also subject to the drive pressure. Area
A1 is subject to the charge pressure, and as such, the
bypass valve is in the closed position.
In creep mode, both the forward and reverse
solenoid operated valves are energized (or shifted).
Engine speed and current supplied to pump are controlled
with throttle pedal.
The bypass valve is forced to stay in the closed
position (except when the clutch and/or brake are actuated)
at all times. The motors cannot be in an "overrunning"
condition as overrunning has been discussed in this
application.
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Moving forward on a level grade or uphill.
- Area A2 (motor in) will see drive pressure
(assume 1000 PSI)
- Area A3 (motor in) will see drive pressure
(assume 1000 PSI)
- Area A1 (motor out) will see charge pressure
(400 PSI)
Moving forward downhill.
- Area A2 (motor in) will see charge pressure.
- Area A3 (now motor out) will see brake
pressure (assume 1000 PSI)
- Area A1 (motor out) will see brake pressure
(assume 1000 PSI)
Both solenoid-operated valves are shifted and
either motor inlet or outlet pressure can be directed to
Area A3. The high pressure will win.
While turning, front wheel speed is not being
controlled and matched to rear wheel speed and pump flow is
such that the motors will not run out of oil. In the worst
case, during a very sharp turn, the operating pressure may
decrease at the outside wheel and increase at the inside
wheel.
In creep mode, a true closed loop, hydrostatic
drive with hydrostatic braking, is provided unless the
brake or clutch is engaged.
The present all-wheel drive arrangement has
recognized that there are significant control logic
advantages to having separate drive systems for each front
wheel of the grader. The actual hydraulic flow to each
hydraulic motor can be more precisely controlled by varying
the output of the particular pump. In addition the
condition sensing bypass valve allows rapid opening and
closing or switching of the hydraulic circuit between an
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opened and a closed configuration. This allows the drive
wheels to assume an active mode when they are in a passive
condition and slippage of the tandem drive occurs. The
automatic bypass valve simplifies the control logic.
Electric control arrangements which operate valves are not
as fast and the sophistication of the control logic is
significantly greater particular in the light of many
different modes and speeds in which the system operates.
Although various preferred embodiments of the
present invention have been described herein in detail, it
will be appreciated by those skilled in the art, that
variations may be made thereto without departing from the
spirit of the invention or the scope of the appended
claims.
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