Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 A STEERIN~ SYSTEM AND COMPENSATING VALVE
Background of the In~ention
Field of the Invention
This invention relates to a steering system which uses a
unique compensating valve and more particularly, to a
hydrostatic power steering control system with variable volume
feedback compensation.
Description of the Preferred Embodiment
The use of hydraulic steering cylinders to control the
10 position of the steerable wheels on a vehicle, especially off-
road type vehicles, is commonplace. Normally, a mechanical
linkage is employed which connects the two steering arms
together in such a manner as to cause the outside wheel to
generate a greater turning radius than the inside wheel. Since
15 it is also common practice to connect the steering cylinders to
extensions of the steering arms~ the cylinders experience
differences in piston velocities during a steering cycle. These
variations in piston velocities cause fluctuations of the fluid
flow rates that are detrimental to the control of the vehicle if
20 they are not corrected.
Several solutions to correct for the differences in piston
velocities have been proposed. One such solution is the u~e of
an equal area displacement cylinder as taught by L. Blatt et al
in ~.S. Patent 3,949,650 issued April 13, 1976, in the steering
25 systemO Another solution proposed by applicant is the use of
two interconnected equal area displacement cylinders with
constant volume feedback. Both of these systems are
satisfactory but do not allow for a steering control system
wherein hydraulic cylinders having only two internal chambers
30 are used. The use of cylinders having two such chambers reduces
the need for closer piston rod tolerances, lessen manufacturing
time and lower the cost of the cylinders. Now a steering
control system with variable volume feedback compensation has
been invented which uses hydraulic cylinders having only two
35 internal chambers.
Summar~ of the Invention
Briefly, this invention relates to a hydrostatic power
steering control system with variable volume feedback
compensation. The steering control system includes first and
40 second hydraulic cylinders having a housing which contains a
1 --
1 piston and an attached piston rod which extends through one end
of the housing. One end of the cylinders is connected to a pair
of steerable wheels and also to each other by a mechanical link
which causes the piston rods in the cylinders to move
simultaneously. Within each of the housings are two chambers
denoted as a head chamber and a feedback chamber. The head
chamber is the area formed by the head of the piston and the
internal surEace of the cylinder housing and the ~eedback
chamber is the area within the cylinder housing which contains
10 the piston rod. Both chambers contain fluid ports which
communicate with fluid passages. A first pair of fluid passages
are respectively connected to the head chambers oE the cylinders
and a second pair of fluid passages are respectively connected
to the feedback chambers of the first and second cylinders.
15 pressurized fluid source is selectively connectible to the first
pair o~ passages by way of a pressure-operated control valve to
which the first pair of fluid passages are connected. The
control valve is connected between the second pair of fluid
passages and its movement is caused by pressure differences in
20 the passages. A pressure difference is created by turning a
manually operable steering wheel which actuates a metering pump
that interconnects the second pair of fluid passages which
connect with the two feedback chambers. In addition, a
compensating valve and a pair of check valves are fluidly
25 connected in parallel with each other across the second pair of
fluid passages.
As the steering wheel is turned in a first direction, the
metering pump causes a pressure increase in one of the pair of
fluid passages connected to the feedback chambers. This
30 pressure increase acts on one end of the control valve to cause
the control valve to shift to a posi-tion which allows
pressurized fluid to pass from the fluid source to the head
chamber of one of the cylinders. Simultaneously, the control
valve connects the head chamber of the other cylinder to the
35 reservoir. The incoming pressurized fluid into the one head
chamber causes the piston rod to extend and this movement is
relayed by the mechanical link to the piston rod in the other
cylinder which will retract. Such movement of the two piston
rods allows the wheels to turn in a similar directionO As the
40 piston rod of the one cylinder extends, fluid is forced out of
- -- 2 --
1 ~he feedback chamber thereof at a rate or velocity which differs
from the rate or velocity that fluid may enter the feedback
chamber of the cylinder with the re~racting piston rodO
Therefore, the compensating valve is connected across the second
pair of fluid passages, and compensates for fluid fluc-tuations
caused by the differences in piston rod stroke during a steering
cycle. The compensating valve functions to dump excess fluid to
the reservoir in response to excessive pressures in the pair of
passages connected to the feedback chambers.
The general object of this invention is to provide a
hydrostatic power steering control system having variable volume
feedback compensation. A more specific object of this invention
is to provide a hydrostatic power steering control system using
a compensating valve which will allow fluid to be discharged
15 from the system once a predetermined pressure range value has
been exceeded.
Another object of this invention is to provide a hydrostatic
power steering control system with variable volume feedback
compensation to correct variable volume feedback caused by
20 differences in piston stroke in a pair of hydraulic cylinders.
Still another object of this invention is to provide a
hydrostatic power steering control system using a compensating
valve which is suitable for use with hydraulic cylinders having
only two internal chambers.
A further object of this invention is to provide a
compensating valve having a uni~ue shuttle pin and check ball
arrangement wherein fluid flow can be both regulated and
restricted.
Other objects and advantages of the present invention will
30 become more apparent to those skilled in the art in view of the
following description and the accompanying drawings.
Brief Description of the Drawings
Fig. 1 is a schematic view of a steering control system with
variable volume feedback compensation.
Fig. 2 is a cross-sectional view of a compensating valve
used to regulate pressure differences across two fluid
passages.
Fig. 3 is a sectional view of Fig. 2 along line 3--3.
Fig. 4 is a perspective view of the shuttle pin shown in
40 Fig- 2-
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2~
1 Detalled scription of the Preferred Embodiment
Referring now to Fig. 1, a hydrostatic power steeringcontrol systern 10 with variable volume feedback compensation is
shown. The steering control system 10 includes first and second
hydraulic cylinders, 12 and 13 respectively. The cylinders 12
and 13 respectively, include pistons 14 and 15 to which are
respectively attached piston rods 16 and 17 which are in turn
reciprocably mounted in housings 18 and 19, respectively.
Outward ends 20 and 21, respectively, of the piston rods 16 and
10 17 are respectively connected to respective first ends of
support arms 22 and 23 which respectively pivot about points 24
and 25. The support arms 22 and 23 are attached together by a
link 26, preferably a mechanical linkl which has its opposite
ends respectively connected to the support arms 22 and 23 at
15 respective locations at the opposite sides of the pivot points
24 and 25 from the connection points of the piston rod ends 20
and 21 with the support arms 22 and 23. The support arms 22 and
23, together with the link 26, form what is conventionally known
as an Ackerman linkage.
The Ackerman linkage supports a pair of steerable wheels 30
and 31 which are attached to the support arms 22 and 23,
respectively, in a commonly known manner.
Each of the hydraulic cylinders 12 and 13, respectively,
contain two internal chambers denoted as head and feedback
25 chambers. Each head chamber is located between a first end of a
respective one of the cylinders and the associated piston while
each feedback chamber is located between a second end of the
cylinders and the associated piston. As is common terminology
in the hydraulic industryr the head chamber is the area which
30 does not contain a piston rod while the feedback chamber is the
area which does. The volume of both the head and feedback
chambers will vary as the respective piston and piston rod
reciprocate within the cylinder housings 18 and 19.
Fluid to actuate the hydraulic cylinders 12 and 13 is
35 contained in a reservoir 32. A supply pump 34, which is
connected to the reservoir 32, supplies pressurized fluid
through a passage 36 to a control valve 38. The control valve
38, which is preferably a four-way, three-position directional
control valve, has a movable member 40 which is shiftable from a
40 normally neutral position to a first position or a second
4 -
- 1 position. As the movable member 40 is shifted within the
control valve 38, passage of pressurized fluid Erom the supply
pump 34 is selectively controlled to one of the head chambers of
the cylinders 12 or 13 through respective flu~d passages 42 and
43 and respective Eluid ports 44 and 45. Additional fluid ports
46 and 47 communicate with the respective feedback chambers of
the cylinders 12 and 13 and with fluid passages 48 and 49. The
fluid passages 48 and ~9 are connected together by a metering
pump 50, which is actuatable by a manually operable steering
10 element 52 such as a steering wheel. Preferably, the metering
pump 50 is a bi-directional pump.
The control valve 38 is fluidly connected to the fluid
passages 48 and 49 by fluid lines 51 and 53 and has pressure
actuatable means which shiEt the movable member 40 within the
15 control valve 38. As a predetermined pressure range difference
in the fluid passages 43 and 49 is exceeded, the movable member
40 will be repositioned so as to control the flow of fluid
through the control valve 38.
Connected between the fluid passages 48 and 49, in parallel
20 with the control valve 38, are a compensating valve 54 and a
pair of one-way check valves 56 and 57. The compensating valve
54, better seen in Figs. 2-4, includes a housing 60 defining a
pair of axially aligned bores 62 and 63. The bores 62 and 63
respectively have inlet ports 64 and 65 and are connected
25 together by a through passge 66 which communicates with an
outlet port 68. ~ovably positioned in the through passage 66 is
a shuttle pin 70 having first and second ends 72 and 73 which
respectively extend into the bores 62 and 63 when the shuttle
pin 70 is in a normal neutral position, as shown in Fig. 2. The
30 shuttle pin 70 is deslgned to allow fluid to flow from the bores
62 and 63 into the through passage 66. Therefore, the shuttle
pin 70 should either have a smaller diameter than the interior
diameter of the through passage 66 or be constructed with a
different cross-sectional shape. Preferably, the shuttle pin 70
35 can be constructed to satisfy both criteria, as is shown in
Figs. 2-4. Although the configuration of the shuttle pin 70 can
vary, preferably the first and second ends 72 and 73 are flat to
allow free seatiny of contactable check elements.
Positioned in the bore 62 are first and second check
40 elements 74 and 76, respectively, and positioned in the bore 63
1 are first and second check elements 75 and 77, respectively.
The first check elements 74 and 75 are movably positioned
adjacent inner ends of the inlet ports 64 and 65 while the
second check elements 76 and 77 respectively, are movably
positioned adjacent valve seats 78 and 79 formed at the opposite
ends of the passage 66 which respectively open into the bores 62
and 63.
Positioned between the check elements 74 and 76, which are
preferably check balls, is a biasing means 80, such as a
10 compression spring. Likewise, positioned between the check
elements 75 and 77, which are preferably check balls, is a
biasing means 81, such as a compression spring. The compression
springs 80 and 81 should have approximately the same compressive
strength so that the outer two check balls 74 and 75 open at an
15 equal pressure value. The inner two check balls 76 and 77
continually contact the first and second ends 72 and 73,
respectively, of the shuttle pin 70 and are normally spaced from
the respective valve seats 78 and 79. In the normal neutral
position, see Fig. 2, the outer check balls 74 and 75 close off
20 and block fluid 10w through the inlet ports 64 and 65 while the
inner check balls 76 and 77 are spaced from the valve seats 78
and 79.
Referring again to Fig. 1, the outlet port 68 of the
compensating valve 54 is connected to a fluid passage 89. The
25 fluid passage 89 is in turn connected to a return line 90 which
is positioned between the control valve 38 and the reservoir
32. Located in the return line 90 is a pressure relief valve 92
which maintains a positive pressure in the return line 90. This
pressure relief valve 92 is set to open at a higher pressure
30 than that needed to open the check valves 56 and 57 to insure
that adequate fluid is always present in the fluid passages 48
and 49. The check valves 56 and 57, which are conventional
check ball type valves, allow fluid flow only in one directiona
When the pressure in either one of the passages 48 or 49 goes
35 below a minimum predetermined value, the respective check valves
56 or 57 will open and permit fluid in the fluid passage 89 and
the return line 90 to be routed into the respective ~luid
passage 48 or 49. This feature prevents air from being drawn
into the fluid passages 48 and 49, which could adversely affect
40 the steering operation.
: - 6 ~
1 The return line 90 can be constructed to pass through the
metering pump 50 so that the heat generated by the flowing fluid
in the return line 90 can be transferred to the metering pump
50. This is beneficial when the steering system 10 is mounted
on a vehicle which is operated in cold weather for a warm
metering pump 50 will facilitate the movement of fluid
therethrough as well as heating up this bypassing Eluid.
_peration
The operation of the steering system 10 will now be
10 e~plained. Starting from a position wherein the wheels 30 and
31 are aligned straight ahead and with the engine of the vehicle
running, pressuri~ed fluid from the supply pump 34 will be
present in the fluid passage 36. This pressurized fluid is
blocked from passing to one of the head chambers of the
15 cylinders 12 or 13 by the control valve 38 which is in a neutral
position. It should be noted that during a turn, the inside
wheel will have to turn through a greater arc than the outside
wheel. Therefore, the stroke of the piston rods 16 and 17 in
the respective cylinders 12 and 13 will be different. This
20 difference in piston rod stroke will cause fluid fluctuations
within the system 10 which will have to be compensated for in
order to prevent failure of the system componentsl such as
breakage of the hydraulic hoses.
As the steering wheel 52 is turned, the metering pump 50
25 will be actuated to facilitate fluid movement between the fluid
passages 48 and 49. This fluid movement creates a pressure
difference between the fluid ~assages 48 and 49 which in turn
causes the movable member 40 of the control valve 38 to shift
from its neutral position to either its first position which is
30 completely to the right or its second position which is
completely to the left. For example, as the operator turns the
steering wheel 52 counterclockwise for a left-hand turn, as
viewed in Fig. 1, the metering pump 50 will transfer fluid from
the passage 48 into the passage 49. This action will increase
35 the fluid pressure in the passage 49 over that in the passage
480 This pressure difference will be felt in the passages 51
and 53 and because the higher pressure is present in the passage
53, the movable member 40 of the control valve 38 will be
shifted to the right.
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2~
1 With the control valve 38 in its first position, the
pressurized fluid from the suppl~ pump 34 ls able to pass
through the control valve 38 and the passage 42 to the port ~4
which communicates with the head chamber of the Eirst cylinder
12. Simultaneously, the head chamber of the second cylinder 13
is fluidly connected to the reservoir 32 by the port 45, the
passage 43, the control valve 38 and the return line 90.
The pressurized fluid entering the head chamber of the first
cylinder 12 will cause the piston rod 16 to extend. This
10 extension of the piston rod 16 causes the support arm 22 to
pivot about the point 24 thereby turning the wheel 30 to the
left. At the same time as the inside wheel 30 is turning, the
mechanical link 26 is forced to the left b~ the support arm 22
and causes the support arm 23 to pivot about the point 25. As
15 the support arm 23 pivotst the piston rod 17 is retracted and
the outer wheel 31 is turned leftward.
As was mentioned above, the inner wheel during a turn will
turn through a greater arc than the outer wheel. In our left-
hand turn example, the inner wheel is wheel 30. This means that
2G the piston rod 16 will have to move a greater distance than the
piston rod 17 and, therefore the volume of the feedback chamber
of the first cylinder 12 will be decreasing at a faster rate
than the volume of the feedback chamber of the second cylinder
13 is increasing. With the momentary increase in fluid leaving
25 the feedback chamber of the first cylinder 12, which is more
than can be directed into the feedback chamber of the second
cylinder 13, fluid fluctuations are caused in the system 10.
These fluctuations are regulated by the compensating valve 54.
In our example, once the pressure difference across the
30 compensating valve 54 exceeds a maximum predetermined pressure
range, the increased pressure in the passage 49 will cause the
check ball 75 to open. With the check ball 75 open, fluid can
flow through the inlet port 65 and the passage 66 to the outlet
port 68. From the outlet port 68 the fluid is directed through
35 the passage 89 and the return line 90 to the reservoir 32. When
the operator is satisfied with the leftward turning of the
vehicle, he stops turning the steering wheel 52 which in turn
stops the actuation of the metering pump 50. With the metering
pump 50 stopped, the fluid from the collapsing feedback chamber
40 of the first cylinder 12 will increase the fluid pressure in the
-- 8
~ ~A~D3L
1 passage 48 and this pressure will shift the movable member 40 of
the control valve 38 back to its neutral position.
As the operator starts to turn the steering wheel 52
clockwise or to the right, the metering pump 50 is again
actuated and a pressure difference is again created between the
passages 48 and 49. This time, the higher pressure is in the
passage 48 and this higher pressure causes the movable member 40
of the control valve 38 to shiEt leftward to its second
position. In its second position, the control valve 38 allows
10 pressurized fluid from the supply pump 34 to flow through the
control valve 38, the passage 43 and the port 45 to the head
chamber of the second cylinder 13. Simultaneously, the fluid in
the head chamber of the first cylinder 12 is connected by the
port 44, the passage 42, the control valve 38 and the return
15 line 90 to the reservoir 32. With continuous movement of the
steering wheel 52, the piston rod 16 will retract and the piston
rod 17 will extend. The retraction of the piston rod 16 will be
Easter than -the extension of the piston rod 17 until both piston
rods 16 and 17 are approximately at the center position in their
20 respective cylinders. Accordinglyr an additional volume of
fluid is required to maintain the passages 48 and 49 full of
fluid. This additional fluid is drawn in through the check
valve 57 by the opertion of the metering pump 50 in transferring
fluid from the passage 49 to the passage 48 while maintaining
25 sufficient pressure in the passage 48 to hold the control valve
38 in its second position.
In rare situations, such as when the steering wheel 52 is
being used to break loose a stuck control valve 38, ~he pressure
in the bore 63 of the compensating valve 54 may increase enough
30 during a left turn to close the inner check ball 77 against the
valve seat 73. This stops flow through the bore 63 and causes
the outer check ball 75 to again block the inlet port 65. When
this occurs, the increase in fluid pressure in the passage 49 is
limited only by the strength of the operator.
It should be pointed out that both of the outer check balls
74 and 75 could conceivably open at the same time if for some
reason an equal pressure value was present at both oE the ports
64 and 65, and this equal pressure value exceeded the maximum
redetermined pressure range value. The maximum predetermined
_ g _
%~
1 pressure range value is established by the compressive strength
in the compression springs 80 and 81.
It should also be noted that the flat ends 72 and 73 on the
shuttle pin 70 aid in allowing the inner check balls 76 and 77,
respectively, to close against the valve seats 78 and 79,
respectively, when the fluid pressure within the bores 62 and 63
respectively, becomes too great. This feature is important for
it permits a sufficient pressure buildup within the system 10 to
release the control valve 38 should it become stuck to one
10 side.
Turning the steering wheel 52 further clockwise, past the
straight ahead alignment position of the wheels 30 and 31, for a
right-hand turn will result in steering the wheels 30 and 31 to
the right.
The right-hand turn results in having the movable member 40
of the control valve 38 shifted leftward to the second
position. The inside wheel will now be the wheel 31 which will
turn through a greater arc than the outside wheel 30. This
means that the piston rod 16 will retract a lesser distance than
20 the piston rod 17 will extend. Again, fluid fluctuations and
fluid excesses will result which will be corrected by the
compensating valve 54 as explained above.
While the invention has been described in conjunction with a
specific embodiment, it is to be understood that many
alternatives, modifications, and variations will be apparent to
those skilled in the art in light of the aforegoing
description. Accordingly, this invention is intended to embrace
all such alternatives, modifications, and variations which fall
within the spirit and scope of the appended claims.
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