Note: Descriptions are shown in the official language in which they were submitted.
1 1~i389~t
ELECTRO-HYDR;~UL'I'C' A'CTIJATO'R SYSTEM
This invention relates in general to hydraulic systems, and
in particular, to a hydraulic system having an improved solenoid
actuated pilot spool.
A typical electro-hydraulic servo system will include a main
spool valve for controlling the direction and quantity of fluid from
a source of pressurized fluid, such as a pump, to a fluid actuator,
such as a motor. A pilot valve is used to control the position of the
main spool by selectively directing fluid from another pressurized
source to the ends of the main spool. The pilot valve is itself a
spool valve, whose spool position is determined by a force motor, such
as a solenoid, acting on one end of the pilot spool in order to con-
trol the fluid directed to the ends of the main spool. Linear sole-
noids are designed to generate a near constant force over the entire
distance that the plunger (armature) travels. In other words, linear
~olenoids exert a constant force as the solenoid air gap changes.
The pilot spool is held in a central or neutral position by
relatively light balance springs disposed on opposite ends of the
pilot spool. When a solenoid acts on the pilot spool to displace it
from its neutral position, a solenoid force is applied to one end of
the spool. As the spool moves, it compreses a balance spring until
the spring generates an equal and opposite spring force to counteract
the solenoid force. Examples of such systems may be found in U.S.
Patent Nos. 4,031,813; 3,874,269~ and 3,763,746.
Although a solenoid is an inherently non-linear apparatus,
one can be modified to operate over a linear range; see, e.g. U.S.
Patent No. 3,740,5g4. Linearly operating solenoids have been used to
control pilot spools because such solenoid exert a near constant force
over the full length of their air gap for a given electrical input.
Thus, in theory, a linear solenoid should yield a one-for-one corres-
pondence between the operator input (an electrical signal), the dis-
placement of the pilot spool (which is resisted by a spring with a
linear spring constant), and the resulting displacement of the main
spool and the motor. ~
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Unfortunately, such systems do not achieve their desired
results. I have observed that such systems will inherently suffer
from a reduction in volume of fluid transferred through the pilot
spool and in a slower response time. The latter phenomena are belie-
ved to be caused by a third, non-linear force which acts against the
solenoid force. The source of this third force is believed to be the
fluid itself. This force, sometimes referred to as a Bernoulli force,
arises from the flow of fluid through the chambers of the pilot spool
and acts on the spool to oppose its displacement. The dynamic or
Bernoulli force thus opposes the constant force of the linear sole-
noid, thereby negating the overall desired result of one-for-one cor-
relation between input and output.
It is an object of this invention to provide a non-linear
solenoid actuated pilot spool.
It i8 a further object of this invention to increase the
flow capability of a pilot spool actuated valve and to increase the
respon~e time of such a valve.
Contrary to the traditional use of linear solenoids, I have
dlscovered that a non-linear solenoid whose actuating force increases
ag the air gap decreases will more effectively counteract the dynamic,
Bernoulli forces and thereby yield an overall linear result. Accor-
dingly, a main feature of my invention is the use of a solenoid having
exaggerated, non-linear characteristics for acting upon a pilot spool.
Such non-linear characteristics are usually present in solenoids hav-
ing sloped or slanted pole faces which are known, per se, rather than
the flat pole faces of linear solenoids. With the non-linear Bernou-
lli forces counteracted by the non-linear solenoid forces, the overall
system achieves the desired linear response. Hence, the combination
of a pilot spool valve with a non-linear solenoid overcomes the afore-
mentioned problems associated with earlier pilot spool and linearsolenoid combinations.
The foregoing objects and features of the invention as well
as a more complete understanding of it, will be learned from the
following detailed description and the accompanying drawings wherein:
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Drawings
Figure 1 is a schematic representation of a pilot operated
hydraulic directional control valve system.
Figure 2 is an enlarged view of the solenoid operated pilot
valve of Figure 1.
Figure 3 and 4 are graphical representations of the forces
acting upon a pilot spool
Figure 5 is an electrical schematic represe~tation of a cir-
cuit for energizing the coils of the solenoid actua~or of Figure 2.
Detailed Descri~ion
In Figure 1 there i5 ~hown an electro-hydraulic actuated
system 10. That system includes a fluid motor, such as the cylinder
11 with a differential piston 12 which can be translated in either
direction by selective application of fluid pressure and flow via
service lines 13 and 14. The direction and quantity of fluid applied
to service lines 13 and 14 is controlled by main spool valve 15.
Hydraulic fluid is drawn from a reservoir 19 and i8 pressurized by a
pump 16 that is in turn connected to spool valve inlet line 17. An
outlet line 18 carries fluid away from the spool valve 15. A pressure
relief valve 9 is connected between lines 17 and 18 in a manner well
known in the art.
Main spool valve 15 operates in a conventional manner. To
this end, when the spool valve is displaced to the right, service
line 13 i8 connected with inlet 17 thereby displacing the piston 12
to the right. Likewise, fluid is exhausted from cylinder 11 via the
service line 14 which is connected to drain or outlet 18. In a
similar manner, the piston 12 can be driven in the opposite direction
when spool valve 15 is shifted to the left.
The position of main spool valve 15 is hydraulically con-
trolled by a pilot spool actuator system 20. Pilot service lines 21,
22 connect the ends of the main spool valve 15 with the pilot actua-
tor system 20. Accordingly, main spool valve 15 is shifted to the
left or the right depending upon the fluid pressure and flow applied
via service lines 21, 22 which act against the centering springs 5
and 6 of the main spool. - ~
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The pilot actuator system 20 includes a pilot spool valve 24
which controls the direction and ~uantity of pilot fluid supplied via
pilot lines 21 and 22. The position of pilot spool 24 is itself
determined by a centering spring 23 and a solenoid actuated force
motor 25. Pilot fluid is drawn from the same reservoir 19 and is
pressurized by a pilot pump 26 t~at is connected to pilot inlet line
27. A pilot outlet line 29 is provided for exhausting return fluid
through the pilot valves 24 to the reservoir 19. A pilot relief
valve 28 is connected between the inlet and outlet lines 27, 29 for
relieving excessive pilot pressure in a manner well known in the art.
Turning now to Figure 2, there is shown an enlarged cross-
sectional view of the pilot actuator system 20. As can be seen, the
pilot spool valve 24 includes a housing 30 having a cylindrical bore
35 with a cylindrical spool 31 is slideably mounted therein. A
plurality of raised lands 32, 33, 34 on spool 31 are provided for
controlling the direction of fluid from the inlet line 27 to the
pilot service line ports 21a and 22a in a.
The solenoid force motor 25 includes a ferromagnetic housing
40. The housing includes an armature cavity 37 having a pair of op-
positely dispo~ed conical pole faces 38, 39. An armature 42 is reci-
procally and slideably mounted within the cavity 39. Armature 42 has
a central cylindrical body portion and two oppositely disposed, coni-
cally shaped end faces 48, 4g that face pole faces 38, 39, respec-
tively. A pair of electromagnetic coils 44, 45 are enclosed in the
housing 40, closely adjacent to the armature 42. The coils 44, 45
are separated near the middle of cavity 37 by an insulating disc 46.
The armature 42 will move towards the left when coil 44 is energized;
armature 42 will be urged to the left when coil 44 is energized;
armature 42 will be urged to the right when coil 45 is energized.
Electrical energy is supplied to the respective coils 44, 45 by a
pair of leads 54, 55 (see Figure 1). An actuator rod 41 axially
mounted on the armature 42 extends from the force motor housing 40
through the pilot spool housing 34 to engage one end of pilot spool
31. Accordingly, the position of the armature 42 and thus the pilot
S_
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spool 31 will be determined upon the net electromeahanical force
exerted on the armature 42 via the coils 44, 45.
The energizing circuit for coils 44 and 45 is schematically
shown in Figure 5. There, a supply voltage V is applied across a
rheostat 51 and a parallel combination of ~hree series resistors, Rl,
R2, and R3. The output of rheostat 51 is applied to an oscillator 56
whose output is in turn a saw tooth shaped voltage signal whose
amplitude is representative of the voltage drop between the wiper arm
51a of the rheostat and the ground. The oscillator signal is applied
to a comparator 57. The comparator 57 will compare the input oscil-
lator voltage with two fixed voltages, i.e., the voltages between
resistors Rl-R2 and R2-R3. As a result, the output of the comparator
57 will be a modulated triangular pulse signal whose amplitude is
representative of the relative displacement of the rheostat arm 51a
from its mid or null point. The comparator output is applied to a
pair of pulse width modulating amplifiers 58, 59 whose outputs yield
pul~e width modulated current signals which are respectively pro-
portional to the relative position of the wiper arm of rheostat 51.
Hence, when rheostat 51 is set in its center or null position, the
signals imposed upon coil lead 47, 48 will have the same relative
width and hence the armature 42 will be held in its central or
neutral position. However, as the wiper arm is moved away from its
null position, the relative width of the current pulse signals appear-
ing on the respective lines 47, 48 will change and the armature will
be moved in the general direction of the larger pulse width signal.
The interaction between the spring, solenoid, and dynamic
forces is best explained with reference to Figures 3 and 4. When the
force motor 25 is actuated either to the left or to the right, it
applies a solenoid force designated Fs on to the pilot spool 31.
That force Fs is opposed by two forces which act upon a pilot spool
31 in a direction opposite to the solenoid force. These two opposing
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forces include the spring force Fk exerted on the spool by centering
spring 23 and a fluid dynamic force, Fd, otherwise known as a
Bernoulli force. The latter is so designated because Bernoulli was
the first to explain how the passage of a fluid through a variable
restricted orifice generated opposing forces upon the means used to
constrict the orifice. Such Bernoulli forces are non-linear and are
generally represented by the graphical presentation shown in Figure
4.
There it will be seen that the Bernoulli forces Fd will
generally reach peak Fd max after which they will fall off. In the
case of a pilot spool, the Bernoulli forces can be related to the
spool displacement which is the cause of the restriction of the fluid.
Hence, it can be said that the Bernoulli forces will reach a maximum
(Fd max.) at a given spool displacement, in this case, d max.
I have discovered that such Bernoulli forces can be counter-
balanced by a non-linear solenoid which increases its applied force
at a rate similar to the Bernoulli forces as the air gap of the sole-
noid decreases. I have chosen to use a solenoid with conical pole
forces 38, 39 and a conical-ended armature 42 in order to emphasize
the non-linearity of the force applied by the solenoid as the arma-
ture 42 approaches either of the opposite pole faces 38, 39. The
conical pole forces 38, 39 serve to concentrate the magnetic field at
the end of air gap, thereby exerting an increasing force on armature
42 as it moves toward one of the pole faces 38, 39.
The length of air gap d on the solenoid is chosen to match
the spool displacement which will encounter the maximum Bernoulli
forces. As the fluid dynamic forces Fd increase with the spool dis-
placement, the solenoid forces increase in a proportionate amount
thereby canceling one another out and leaving a net force at Fn which
approximates a desired lineal response. Accordingly, the displace-
ment of the pilot spool, and ultimately, of the main spool itself,
is directly proportional to the position of the wiper arm of rheostat
51. As the wiper arm is moved 10 percent from its null position,
the pilot spool will move _ ~ -
1 163897!10 ?ercent from its null position and thereby displace the main
5pco1 10 percent from its null position.
Such a true, overall linear correlation between an
inp-ut signal and the output displacement of the main spool has not
been possible with the usual linear solenoid foxce motor. Such
force motors generally apply a constant force across the whole
air ga~. So, as the Bernoulli forces increase with the spool
dis?lacement, the linear solenoid maintains a constant force and
the net force applied to the pilot spool decreases. Of course, to
lo counteract the opposing Bernoulli forces, an operator could
increase the power supplied to the coils 44, 45 by adjusting the
position of the rheostat 51. As a result, an operator might have
to move the rheostat 30 percent of its displacement in order to
achieve a 10 percent displacement of the main spool. Hence, in
operation, an operator was required to overshoot the rheostat
adjustment in order to achieve the desired displacement for the
main spool.
However, with my invention, an operator can now achieve
the tesired one-for-one correlation between the rheostat position
and the displacement of the main spool 15. Thus, the invention
will provide for greater precision in both the manual and the
A auto~atic operation of electrohydraulic ~e~e systems.
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