Note: Descriptions are shown in the official language in which they were submitted.
:1235~
Description
~Ydrostatic Vehicle Control
Technical Field
This invention relates generally to a control
system for a hydrostatic vehicle, and more particularly
to an electronic device for controlling engine speed
and hydraulic pump displacement in response to loads
subjected on a hydrostatic vehicle.
Background Art
In the field of hydrostatic vehicles, for
example, excavators, variable displacement hydraulic
pumps are typically driven by a prime mover, providing
hydraulic power to a plurality of work implements as
well as to the drive system. Excavators, being
extremely versatile machines, are useful in performing
a large number of different and varied tasks (e.g.
pipelaying, mass excavation, trenching, logging, etc.),
each task having its own unique hydraulic flow and
pressure requirements. For example, during mass
excavation, hydraulic power requirements are quite high
with brief periods of reduced need, but in pipelaying,
sustained periods of low flow during waiting are common
with sessions of moderate to high flow.
Prior art has shown that a substantial fuel
savings can be realized by reducing engine speed to low
idle during these sustained periods of waiting. While
this method does address the most obvious area for fuel
savings, it is silent on the possibility of conserving
fuel during active times where less than maximum engine
speed and pump flow would be re~uired. For example,
U.S. Patent 4,395,199, issued to Izumi et al. on July
26, 1983, discloses an electronic control system for a
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hydraulic excavator which controls swash plate
inclinatiun on a variable displacement pump in response
to operator input via a control lever. In this way,
the system provides the hydraulic flow requested by the
operator, reduces load on the engine during periods of
less than ma~imum power requirements and, subsequen-tly,
reduces fuel consumption. Although the system does
save fuel, it will not minimize fuel requirements, due
primarily to the inefficiencies resulting from operation
of the hydraulic pumps at reduced displacement and
continued operation of the engine at a single,
compromising revolutionary speed. While the operator
could manually adjust engine speed to maintain pump
displacement relatively high during actual working, it
is recognized that operation o~ an excavator requires
the operator use both hands and both feet. In view of
the fact that a majority of excavator operators lack a
useful fifth limb, manual adjustment of engine speed is
necessarily given a rather low priority.
The present invention is directed to over-
coming one or more of the problems as set forth above.
Disclosure of the Invention
In accordance with one aspect of the present
invention, an apparatus for controlling an internal
combustion engine having a fuel injection pump
actuator, and at least one variable displacement
hydraulic pump with a load sensing means for detecting
hydraulic load and adjusting inclination of a swash
plate in response to hydraulic flow and load
requirements. The apparatus includes a first means
which detects the displacement of the hydraulic pump
and delivers a first signal responsive to the
displacement of the hydraulic pump. A second means
detects the rotational speed of the engine and delivers
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a second signal responsive to the rotational speed. A
control means recei~es the first signal and delivers a
third signal responsive to the magnitude of the first
signal. An underspeed control means receives the
second and third signals, compares the second and third
signals, and delivers a fourth signal in response to
the third signal being greater than the second signal.
A swash plate actuator means receives the fourth signal
and controls the angle of inclination of the swash
plate in response to the magnitude of the fourth
signal. A fuel control means receives the second and
third signals, compares the second and third ~ignals,
and delivers a fifth signal in response to the third
signal being less than the second signal. A rack
actuation means receives the fifth signal and controls
the supply of fuel to the engine, responsive to the
magnitude of the fifth signal.
Brief Description of the Drawings
Fig. 1 illustrates in block diagrammatic form
a hydrostatic control system, engine, and hydraulic
pump arrangement;
Fig. 2 illustrates a load sensing means for
controlling swash plate inclination;
Fig. 3 illustrates a fuel injection pump
actuator partly in sectional detail and partially in
block diagrammatic form;
Fig. 4 is a block diagram explanation of an
embodiment of the pump control method;
Fig. 5 is a block diagram explanation of an
embodiment of the fuel control method;
Fig. 6 is a detailed block diagram explanation
of the engine speed setting function; and,
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~ ig. 7 is a diagrammatic view showing one
example of the characteristi.c of desired engine speed
with respect to hydraulic pump displacement as
described in Fig. 6.
Best Mode For Carrying Out The Invention
Referring now to the drawings, wherein a
preferred embodiment of the present apparatus 10 is
sho~n, Fig. 1 illustrates an electronic control system
1~ 12 for a prime mover 14, preferably being an internal
combustion engine 16 controlled by a rack 18 of a fuel
injection pump 20. The rack 18 is positioned by a
known electrohydraulic rack actuator means 22 under
direction from the control system 12. Variable
lS displacement hydraulic pumps 24,26 are driven by the
engine 16 while a hydromechanical load sensing
apparatus 28 (shown in greater detail in Fig. 2 and
discussed later in this specification), controls
inclination of the swash plates 30,32 in response to
~O detected hydraulic load. The control system 12 can be
divided into three major components: fuel control means
34, underspeed control means 36, and control means 38.
The control means 38 receives first signals
from a first means 39 over lines 40,42 responsive to
the displacements of each of the hydraulic pumps 24,26,
computes a desired engine speed responsive to the first
signal of greatest magnitude, and delivers a third
signal via a line 44 representative of the desired
engine speed to both the fuel control means 34 and the
underspeed control means 36. A second means 46 detects
the actual rotational speed of the engine 16 and
delivers a second signal indicative of the actual
engine speed to both the fuel and underspeed control
means 34,36. The fuel control rneans 34 receives the
second and third signals representing actual and
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desired engine speed, respectively, compares the two
signals, and delivers a fifth signal in response to the
desired ensine speed being less than actual engine
speed. A rack actuator means 22 receives the fifth
signal and controls the supply of fuel to the engine 16
responsive to the magnitude of the fifth signal.
Similarly, the underspeed control means 36 receives the
second and third signals, compares the two signals, and
delivers a fourth signal in response to the desired
engine speed being greater than the actual engine
speed. A swash plate actuator means 50 receives the
fourth signal and controls the angle of inclination of
the swash plate responsive to the magnitude of the
fourth signal, More simply stated, when actual engine
speed "lugs" below desired engine the underspeed
control means 36 acts to reduce pump displace~ent and
allow the engine speed to increase under lower load
constraints. Should the actual engine speed rise above
desired speed, the fuel control means will reduce the
supply of fuel to the engine allowing the engine to
slow to a more efficient operating point.
Fig. 2 illustrates one embodiment of a hydro-
mechanical load sensing apparatus 28. The apparatus 28
includes the hydraulic pump 24 with positionable swash
plate 30~ a plurality of operator actuated valves 52,54
for respectively controlling hydraulic fluid flow to a
plurality of work implements 56,58, a flow priority
control valve 60, and a ball resolver valve 62 for
delivering a load pressure signal of greatest magnitude
to the swash plate actuator 64. The flow priority
control valve 60 operates to give the implement 56
priority of hydraulic fluid flow over the implement
58. Fully actuating the valve 52 causes the control
valve 60 to be biased in a direction where all
hydraulic flow is directed to the implement 56.
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Alternatively, not actuating the valve 52 results in a
pressure signal biasing the flow control valve 60 in
the opposite direction and thereb~ directs flow to the
valve 54. Yarying degrees of operation of the valve 52
result in an appropriate quantity of flow being
delivered to the implement 56 with the remaining flow
available to the implement 58. The ball resolver valve
62 receives load pressure signals from each of the
valves 52,54 corresponding to the load applied to the
implement cylinders. The signal of greatest magnitude
is passed to the swash plate actuator 64 where the
position oE the swash plate 30 is set corresponding to
the magnitude of the signal. A pump discharge pressure
signal is also delivered via line 65 to the swash plate
actuator 64 for maintaining pump output pressure at,
for example, a 300 psi differential above that requested
by the load pressure signal.
Electronic control of the load sensing
apparatus 28 is obtained by the use of a pilot supply
66, proportional pressure valve 68, and a solenoid 70.
The proportional valve 68 controls the pressure of the
pilot supply 66 delivered to the swash plate actuator
64. Operation oE the solenoid 70 under direction from
the underspeed control means 36 regulates the
proportional pressure valve 68 controls the pressure
delivered to the swash plate actuator 64, and
consequently effects the swash plate's position.
For example, during operation of the excavator,
assume desired engine speed equals actual engine speed;
therefore, the underspeed control means 36 will take no
action to alter swash plate position. Should actual
engine speed drop below desired engine speed, the load
sensing apparatus will continue to increase pump
displacement to provide the requested flow; however,
the underspeed control means 36 will act to reduce pump
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displacement by actuating the solenoid 70 and providing
an underspeed pressure signal to the swash plate
actuator 64. The magnitude of the underspeed pressure
signal is varied by the underspeed control means 36 as
S a function of the difference between desired and actual
engine speed (discussed in greater detail later in this
text).
Fig. 3 shows an electrohydraulic rack actuator
means 22 for controllably positioning a rack 18 of a
fuel injection pump 20. As is conventional, the fuel
injection pump 20 includes a fuel injection pump
housing 70 and a reciprocating ~uel rack 18 axially
movable in opposite fuel-increasing and fuel-decreasing
directions (shown in Fig. 3 as being to the left and to
the right, respectively).
The actuator means 22 further includes a rack
control member 72 which is also movable in opposite
fuel-increasing the fuel-decreasing directions. In the
particular system illustrated herein, the rack control
member 72 is in the form of an annular sleeve or
collar~ A hydraulic servo system 74 is provided to
function as a means for moving the fuel rack 18 in its
fuel-increasing and fuel-decreasing directions in
response to corresponding movements of the rack control
member 72 and with a force greater than that required
to move the rack control member 72. The hydraulic
servo system 74 particularly illustrated herein
includes a cylinder 76, a piston 78, a sleeve 8~, and a
pilot valve spool 82.
The cylinder 76 is secured to the fuel
injection pump housing 70 and has a passage 84
communicating with the interior of the pump housing 70
through which pressurized engine lubricating oil may
flow. The piston 78, which is ported and stepped and
connected to the fuel rack 18 for axial movement
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therewith, is disposed for axial movement in the
cylinder 76. The diameter of the left end 86 of the
piston 78 is less than the diameter of the right end 88
of the piston 78 which slides in the sleeve 80 fixed
within the cylinder 76, and both such diameters are
less than that of the intermediate piston head 90. The
left end 86 of the piston 78, the piston head 90, and
the cylinder 76 define an annular chamber 92. The
piston head 90 has an annular surface 94 on the right
side thereof.
The pilot valve spool 82 is mounted ~ithin the
piston 78 for limited axial movement relative thereto,
and has a reduced diameter recess 96 in conti~uous
communication with piston ports 98. The axial length
of the recess 96 is sized relative to the piston ports
100 and 102 such that the recess 96 does not
communicate with either of the piston ports 100 and 102
when the pilot valve spool 82 is in the balanced
position of Fig. 3, but will communicate with the
piston ports 100 or 102 when moved to the right or the
left, respectively, relative to the piston 78.
The rack control member 72 is mounted for
limited axial sliding movement on the left end stem 104
of the pilot valve spool 82. The rack control member
72 is biased towards the right by a spring 106 which
shoulders against a spring retainer 108, with rightward
movement of the rack control member 72 being limited by
a retainer clip 110 fixed to the pilot valve spool stem
104. The rack control member 72 has a pair of radially
extending flanges 112 on one side thereof to provide a
pair of oppositely facing shoulders 114 and 116.
An electrically energizable brushless direct
current torque motor 118 is mounted in fixed relation
to the cylinder 76 of the servo system 74, the motor
118 has a rotatable rotor 120 movable in opposite fuel-
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increasing and fuel-decreasing directions. It is a
functional characteristic of such a torque motor 118
that its rotor 120 will turn freely in its bearings
when no electrical current is supplied to the motor
118. When electrical current is applied, the rotor 120
will exert a preselected torque in one direction, the
degree of torque being proportional to the amount of
the current applied. In the preferred embodiment,
applied current is controlled by regulating the duration
of the applied signal as detailed later in this text.
A coupling means 122 is provided for connecting
the rotor 120 of the torque motor 118 to the rack
control member 72 to move the rack control member 72 in
one of its fuel-increasing or fuel-decreasing
directions in response to movement of the rotor 120 in
its corresponding fuel-increasing or fuel-decreasiny
direction. In the particular system shown herein, the
coupling means 122 comprises a control lever 124 fixed
to the rotor 120 and having a free end 126 confined
between the shoulders 114,116 of the rack control
member 72.
In the system illustrated in Fig. 3, the
torque motor 118 is arranged so that current applied
thereto will cause a torque to be exerted on the
control lever 124, urging it to move in a clockwise,
fuel-increasing direction, in turn urging the rack
control member 72 in its leftward, fuel-increasing
direction~ A bias means 128 is provided for biasing
the rack control member 72 in a direction opposite to
the direction that the coupling means 122 will move the
rack control member 72 when the torque motor 118 is
energized. In the particular rack actuator means 22
shown in Fig. 3, the bias means 128 comprises a low
rate compression spring 130 confined between a fixed
spring seat 132 and an extension 134 of the control
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lever 124. With this arrangement, the spring 130
biases control lever 124 in its fuel-decreasing
direction, with the free end 126 of control lever 124
acting on the shoulder 114 of the rack control member
72 to bias such rack control member 72 for movement in
its fuel-decreasing direction.
Operation of the underspeed control means is
illustrated in block diagram form by Fig. 4. One
implementation of the underspeed control means 36 is
shown as a first proportional plus derivative feedback
means 136 for controlling the magnitude of the fourth
signal and thereby control hydraulic pump displacement.
The actual engine speed signal is received from the
second means 46 and delivered to a low pass filter 138,
eliminating transients associated with individual
cylinder ignition. The filtered engine speed signal is
then delivered to a first summing means 140 where it is
added to a negative representation of the desired
engine speed signal. The resulting signal is
indicative of an error signal or the difference between
the actual and desired engine speed. The error signal
is then multiplied by a first preselected coefficient
Kp2 and delivered to a second summing means 142 as
the proportional term of the control equation.
Simultaneously, the actual engine speed signal is
delivered to a second low pass filter 144 and passed to
a negative input of a third summing means 146. A
positive input of the third summing means 146 receives
the unfiltered actual engine speed signal; and
resultingly, the third summing means delivers a signal
based on the difference between the filtered and
unfiltered signals, and more particularly a signal
indicative of rate of change of engine speed or the
derivative of actual engine speed. The derivative
signal is multiplied by a second coefficient KD and
123~ 7
delivered to the second surnming means 142. A first
actuator setpoint means 1~8 delivers a seventh signal
of constant magnitude representative oP a rnaxiJnum angle
of inclination of the swash plate to the second sumtning
means 142. The second summing means 142 adds the
proportional, derivative, and constant signals and
delivers this sum as an eighth signal for controlling
the magnitude of the fourth signal. A processing means
150 receives the eighth signal and accesses a
preselected memory location indicative of the magnitude
of the fourth signal. A software table look-up routine
determines the magnitude of the eighth signal and
retrieves a hinary number from a memory location
determined by the magnitude of the eighth signal. The
binary number determines the duration of the fourth
signal and; consequently, controls the hydraulic pump
displacement. For example, retrieving the number
00000000 would result in delivering a fourth signal of
minimum pulse width and retrieving the number 11111111
causes the processing means to deliver a pulse width of
maximum duration. Binary numbers of varying magnitude
between the two extremes result in pulse widths of
corresponding variable duration. Those skilled in the
art of electronic design will recognize that the
implementation of the proportional plus derivative
feedback control equation, as shown in Fig. 4, could be
implemented as a hardware arrangement, a software
program, or a combination of the two. For example, low
pass filters are commonly available hardware circuits
and software configurations of low pass filters are
also known in the art. Similarly, summing means can be
provided by either software or hardware.
From the above description, one can see that
during operation of the electronic control system 12
when actual engine speed is above desired engine speed,
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the underspeed control means 36 ~Jill deliver an eighth
signal calling for a pump displacement greater than the
maximum pump displacement. An eighth signal requesting
a pump displacement greater than maximum will have no
additional affect on pump displacement as the pump can
supply no more than the maximum. Consequently, the
underspeed control means will act to alter the pump
displacement only when actual speed "lugs n below
desired speed.
lQ Operation of the fuel control means 34 is
illustrated in block diagram form by Fig. 5. One
implementation of the fuel control means 34 is shown to
be similar to the underspeed control means 36, in that
a second proportional plus derivative feedback means
152 controls the magnitude of the fifth signal and
thereby controls the supply of fuel to the engine.
~ he actual engine speed signal is received
from the second means 46 and delivered to the negative
input of a third low pass filter 154. The filtered
engine speed signal is then delivered to a fourth
summing means 156 where it is added to the desired
engine speed signal. The resulting signal is, once
again, indicative of an error signal or the difference
~etween the actual and desired engine speed, but
opposite in sign to the corresponding error signal in
the underspeed control means. The error signal is then
multiplied by a third preselected coefficient Kpl and
delivered to a fifth summing means 158 as the
proportional term of the control equation.
Simultaneously, the actual engine speed signal is
delivered to a fourth low pass filter 160 and passed to
a negative input of a sixth summing means 162. A
positive input of the sixth summing means 162 receives
the unfiltered actual engine speed signal; and
resultingly, the sixth summing means 162 delivers a
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signal based on the derivatlve of actual engine speed.
The derivative signal is multiplied by a fourth
coefficient KDl and delivered to the fifth sumlming
means 158. A second actuator setpoint means 164
delivers a tenth signal of constant magnitude
representative of a maximum allowable rack position to
the fifth summing means 158. The fifth summing means
158 adds the proportional, derivative, and constant
signals and delivers this sum as an eleventh signal for
controlling the magnitude of the ~ifth signal. A
processing means 166 receives the eleventh signal and
accesses a preselected memory location indicative of
the magnitude of the fifth signal. A software look-up
routine determines the magnitude of the eleventh signal
and retrieves a binary number Erom a memory location
detarmined by the magnitude of the eleventh signal, as
described in the operation of the underspeed control
means 36. The binary nuTnber determines the duration of
the fifth signal and; consequently, controls the rack
position and fuel supply~
Operation of the fuel control means 34 is
similar to that of the underspeed control means 36
except for the difference in sign of the proportional
term. With the constant term being set to call for
maximum rack, only negative proportional terms will
have an influence on reducing rack position, or more
precisely, the fuel control means will act to reduce
the supply of fuel when actual engine speed exceeds
desired engine speed. Unlike the underspeed control
means 36, calling for greater than maximum allowable
rack 18 will have the result of increasing rack 18
beyond its rated position. To prevent this phenomenon
from occurring, the additional step of setting the
proportional term to zero in response to the actual
engine speed signal being less than the desired engine
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speed signal has been added to the fuel control means.
Checking to see if the error signal is greater than
zero and setting the error to zero if the condition
exists is shown as occurring prior to multiplyiny
the error signal by the constant Kpl. Should the
error be less than zero, then the signal is passed
unaltered.
Fig. 6 illustrates the control means 38 in
block diagram form and can best be explained in
conjunction with the graphical representation of engine
speed versus pump displacement shown in Fig. 7. As
discussed earlier, the control means 38 functions to
determine a desired engine speed based on the hydraulic
pump displacement. While the block diagrams of Fig. 6
can be most easily explained as steps in a software
routine, those skilled in the art of electronic control
design recognize that either portions, or all of the
software may be replaced by hardware circuitry without
departing from the spirit of the present invention.
Fig. 7 shows the desired engine speed signal being
controllably settable to one of a plurality of
preselected levels in response to the hydraulic pump
displacement signal being within one of a plurality of
corresponding ranges. More particularly, the desired
engine speed signal is a first preselected level in
response to the pump displacement signal being less
than a first preselected magnitude for a preselected
duration of time. For example, the desired engine
speed is set to a standby speed of about 1140 rpm in
response to the pump displacement being less than 5%
for approximately 2 seconds. Further, the desired
engine speed signal is a second preselected level in
response to said pump displacement signal being in a
range between the first preselected magnitude and a
second preselected magnitude. More precisely, the
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desired engine speed is set to some operator selected
working speed when pump displacement is between 5% and
40%. A third range exists where the desired engine
speed signal is directly proportional to the first
signal when the first signal is greater than the second
preselected magnitude. The ramp portion of the graph
between the working speed and a maximum speed
illustrates one possible proportional curve. However,
the working speed is adjustable to a plurality of
discrete levels, one example being the dashed line of
Fig. 7, necessitating the slope of the ramp portion be
adjusted to fit between the maximum speed and the new
working speed. Maximum desired engine speed is
advantageously set to correspond with maximum pump
displacement.
Fig. 6 illustrates one implementation of the
graph of Fig. 7. Pump displacement signals are
received by the block 200 for each of the pumps 24,26
over the lines 40,42. The signal of greatest magnitude
is selected and delivered to the block 202 where the
signal is filtered to remove transient displacements
which can occur at very low pump displacement. Block
204 receives the filtered signal and sets a variable
DESNE5 to one of two values. The first value
corresponds to standby engine speed and will be
assigned to the variable DESNE5 if the pump
displacement signal falls below 5% for a period greater
than two seconds. The second value corresponds to
maximum desired engine speed and will be assigned to
the variable DESNE5 at all times when the pump
displacement signal rises above 5%.
Block 206 also receives the pump displacement
signal of greatest magnitude and uses a software table
look-up routine to assign a proportional desired engine
speed to the variable DESNE3~ The table look-up
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routine accesses a memory location based on the pump
displacement and retrieves a desired enyine speed
stored there. For example, the graph of Fig. 7 shows a
pump displacement of approximately 50~ corresponding to
a desired engine speed of about 1700 rpm. In this
example, the table loolc-up routine would access the
memory location corresponding to 50~ pump displacement,
retrieve the desired engine speed of 1700 rpm, and set
the variable DESNE3 to 1700 rpm.
Block 208 is similar in operation to block 206
as it also employs the use of a table look-up routine
to set a desired engine speed variable. The variable
DESNEl is set to a working speed in response to an
operator positionable thumbwheel switch 210. The block
208 receives a signal from the thumbwheel switch 210
indicative of the operator selected level, accesses an
appropriate memory location, and assigns the value
stored in that memory location to the variable DESNEl.
Each of the variables DESNEl~ DESNE3, DESNE5
are received by block 212 where a variable DESNE is
first assigned the value contained within variable
DESNEl. The variables DESNE, DESNE3 are compared and
if the variable DESNE3 is greater than the variable
DESNE then the variable DESNE is redefined to be equal
to the variable DESNES. More simply stated, the
working speed is compared to the proportional speed, if
the proportional speed is greater than the working
speed then desired engine speed is set to the
proportional speed. The variable DESNE is then
compared to the variable DESNE5 and if the variable
DESNE is greater than the variable DESNE5 then the
variable DESNE is redefined to be equal to the variable
DESNE5. This step compares the desired engine speed,
which has been set to either the working or
porportional speed, to either the maximum speed or the
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standby speed, depending upon which value block 204 has
assigned to the variable DESNE5. If the desired engine
speed is greater than maximum speed, then an overspeed
condition exists and the desired engine speed is
S redefined to be equal to the maximum speed.
Alternately, if the pump displacement has been less
than 5% for more than two seconds, then the variable
DESNE5 has been set to standby speed. I~, at this
time, the desired engine speed is greater than the
standby speed, then desired engine speed is redefined
to be the standby speed.
The variable DESNE is delivered to a filter
214 to prevent sudden changes in engine speed. For
example, with the engine running at maximum speed,
should the operator discontinue an operation which
requires high output then desired enyine speed will
change somewhat drastically in a very short period,
giving the appearance of ~erky operation. The filter
214 causes the desired engine speed to change more
slowly and operation appears much smoother. The
desired engine speed is delivered to both the fuel
control and underspeed control means 34,36 as discussed
earlier.
Industrial Applicabilit~
In the overall operation of the excavator,
assume that the operator is trenching, and at this
particular portion oE the work cycle he is positioning
the bucket to make a cut. The load experienced by the
3~ hydraulic implements 56,58 is low to moderate and the
hydraulic load sensing apparatus has positioned the
swash plate 30 to provide, for example, approximately
25% pump displacement. The pump displacement is
detected and the control means 38 sets desired engine
speed to the working speed requested by the operator.
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As the bucket begins the cut, the hydraulic
load on the implements 56,58 increases. The load
sensing apparatus 28 responds by increasing pump
displacement to approximately 90~ to provide the
additional flow required. A corresponding increase in
desired engine speed to 1900 rpm occurs in response to
the increased pump displacement, but the increased
hydraulic load reduces the engine responsiveness, and
the actual engine speed lugs below the desired engine
speed. The underspeed control means 36 responds by
stroking back the pump 30 according to the proportional
plus derivative means 136 shown in Fig. 4. This
reduced swash plate position is detected by the control
means 38 which summarily reduces the desired engine
speed signal to correspond to the new swash plate 30
position. The fuel control means 3~ maintains full
rack as long as desired engine speed is greater than
actual engine speed. Thus, the engine is accelerating
under the reduced load and the underspeed control means
36 increases swash plate position as the difference
between desired and actual speed diminishes~ However,
as the swash plate position increases, so too does the
desired engine speed signal. Judicious selection of
the gains Kp2, KD2 allow the control means 38 and
the underspeed control means 36 to interact when
operating on the proportional portion o~ the curve
shown in Fig. 7, and provide the desired relationship
between engine speed and pump displacement.
At the end of the cut, hydraulic load
decreases, the load sensing apparatus reduces
displacement, desired engine speed is reduced, and the
fuel control means responds to actual engine speed
being greater than desired engine speed by reducing
rack displacement until desired equals actual engine
speed. The first proportional plus derivative feedback
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means forces the fuel control means to reduce the rack
position less as the difference between actual and
desired engine speed becomes less at an increasiny rate.
At any point during the work cycle, should the
operator pause and allow the load sensing apparatus to
stroke the pump displacement to less than 5~ for longer
than two seconds, then the control means 38 will set
desired speed to the standby speed of about 1140 rpm.
The fuel control means will reduce rack position and
force the engine to slow actual speed to the targeted
standby speed.
While the present invention has been described
primarily in association with hydraulic excavators, it
is recognized that the invention could be implemented
on most any prime mover and hydraulic pump arrangement.
Other aspects, objects, and advantages of this
invention can be obtained from a study of the drawings,
the disclosure, and the appended claims.
