An electrohydraulic proportional valve wherein an electromagnetic force motor sends a mechanical signal to a pilot actuator which moves a main spool. The pilot actuator includes a pilot spool slidably received in a pilot sleeve. A feedback spool which is movable with the main spool has a camming surface against which the pilot sleeve rides. A load check valve is provided that isolates the load pressure from system pressure when desired.

TECHNICAL FIELD 
This invention relates to electrohydraulic proportional valves suitable for 
use in selective control valve applications. In particular, it relates to 
a three-way, electrohydraulic flow control valve with continuously 
variable output flow proportional to an electrical signal from an 
operator. 
BACKGROUND OF THE PRIOR ART 
The prior art is aware of electrohydraulic control devices. One example of 
the prior art is disclosed in assignee's U.S. Pat. No. 4,290,447, issued 
Sept. 2, 1981. In that patent, a hydraulic bridge is established having 
two fixed orifices and two variable orifices. Here, additional advantages 
are obtained by establishing a bridge with four variable orifices and a 
load holding check. 
OBJECTS OF THE INVENTION 
It is a general object of the invention to provide an electrohydraulic 
proportional valve unit with improved fluid control functions. 
It is a particular object of the invention to provide such a valve having a 
"float" position wherein a load member, for example, which is traversing 
ground contours, can float or move therewith. 
It is a further object of the invention to provide a means of opening a 
load-holding check valve depending on the main spool position. 
It is a still further object of the invention to provide such a valve in 
which battery rundown or line losses do not limit the ability of the pilot 
actuator to move the main spool to its center position. 
Other objects and advantages of the present invention will become apparent 
from the appended detailed description of a preferred embodiment taken in 
conjunction with the accompanying in which:

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION 
Referring now to the drawings wherein like elements are indicated by like 
numerals, the numeral 10 refers generally to the valve of this invention. 
The valve is comprised generally of a force motor 12 and a 
pilot-controlled valve assembly and housing 14. The force motor 12 is 
received by the housing 14 and can be secured and detached by mounting 
screws such as 13. 
The electromagnetic force motor 12 is a bi-directional device producing a 
linear output displacement proportional to the magnitude and polarity of 
an electric signal. The magnetic circuit of the force motor contains 
permanent magnets which create a polarizing magnetic flux in working air 
gaps. The coil flux interacts with a permanent magnet flux to move the 
armature in one direction or the other depending on the polarity of the 
electric signal. The armature of the force motor is spring-centered so 
that it returns to a neutral position upon the loss of the electrical 
signal. The armature is suspended from the rest of the force motor 
assembly. Thus, there are no rubbing contacts between the armature and the 
components. Hysteresis is reduced due to the elimination of frictional 
forces acting on the armature. Additionally, the force motor cavity is 
flooded with oil in order to eliminate the use of small dynamic seals 
which would be subjected to a large number of cycles and which would place 
undesired frictional forces on the armature assembly. The force motor 12 
has an output member 16 which moves in accordance with an electrical 
signal transmitted to the force motor by way of the electrical conduits 15 
which lead to an operator's position. 
The housing 14 has a pilot control receiving bore 18 in which a pilot 
control sleeve 20 is slidably received. The pilot control sleeve 20 has a 
central bore 22 slidably receiving a generally hollow pilot control spool 
24. The upper end of the pilot spool is in engagement with the output 
member 16. There is an opening 29 formed at the upper end of spool 24. The 
lower end of sleeve 20 is closed by a follower plug 26. Intermediate its 
length, the sleeve 20 is formed with openings 28, 30, 32 and 34. 
As best seen in FIG. 2, the pilot control spool 24 is formed with a land 36 
having a width commensurate with opening 34, a reduced portion 38 and a 
second land 40 having a width commensurate with opening 30. The opening 32 
is in communication with a passageway 45 leading to pilot control pressure 
source. The opening 34 is in communication with passageway 43. The opening 
30 is in communication with the passageway 42. In interest of clarity 
passageway 42 is shown diagramatically. Passageway 42 leads to chamber 84 
described below. 
A spring 46 has its bottom resting on plug 26 and is disposed to urge spool 
24 upwards into engagement with the force motor output member 16. A spring 
48 urges the pilot sleeve downwardly against the feedback spool 71. 
The housing 14 has a main bore 50 extending throughout its length. The bore 
50 is enclosed at one end by a plug 52 and at its other end by plug 54. 
Between the plugs, the bore 50 slidably receives a main operating spool 
56. From the right, the spool is provided with a land 58, a reduced 
portion 60, a land 62, a reduced portion 64, a land 66, a reduced portion 
68, a land 70, a reduced portion 72 (having the truncated conical section 
71), a land 74 and a spool extension 76. 
Between plug 52 and land 74, a control chamber 78 is formed in which the 
extension 76 is received and to which the passageway 43 is communicated. A 
centering spring assembly 80 is secured about extension 76 so as to 
preload the spool 56 when there is no electrical signal (null) from the 
force motor 12. As seen in FIG. 1, the components are at this null 
position. Centering spring 80 is preloaded in the assembly. The spring is 
captured on the spool stem 76 between two cup-like spring guides. The left 
spring guide is prevented from moving to the left relative to the spool by 
retaining ring 75 in a groove 79 in the stem. The right-hand spring guide 
is prevented from moving to the right relative to the spool and stem 
because the spring guide rests against the spool land 74. The spring 
guides and preloaded spring are captured in the valve body by end plug 52 
on the left and by a step 77 in valve body 14. 
It should be noted that the space allotted for the spring guides and the 
preloaded spring by the valve body equals the dimension from the left end 
of the left-hand spring guide to the right end of the right-hand spring 
guide if the assembly were not in the valve body. The preload of the 
captured centering spring must be overcome whether the spool 56 is moved 
either to the right or to the left. The preload assembly 80 holds the main 
spool in its "null" position any time there is zero or equal hydraulic 
pressure acting on the ends of the main spool 56. 
The reduced portion 72 of spool 56 forms a part of a chamber 82, the 
reduced portion 68 forms a part of chamber 85, the reduced portion 64 
forms a part of chamber 86 and the reduced portion 60 forms a part of 
chamber 88. The chamber 88 is communicated with tank via passageway 89, 
the chamber 86 is communicated to a load-holding check valve assembly. 
The housing 14 is also formed with a bore 100 which receives the load 
holding check valve assembly. The check valve assembly is held in place by 
a plug 101 which is threadably received at the outer end of bore 100. 
Within the bore 100 is a sleeve 102 that provides a seat for a poppet 92 
intermediate its length. Poppet 92 has an inner bore 105 that receives the 
spring 104. Spring 104 urges check ball 106 against its valve seat 108. 
The interior chamber of the load-checking assembly is communicated to 
"load" pressures through the passageway 103 and the cylinder port 
diagrammatically shown at 107. Thus, when the poppet is seated because of 
load pressures and the bias of spring 104, fluid cannot drain from the 
pressurized side of the working cylinder. 
A reduced bore extension 112, inwardly and axially of bore 100, receives a 
plunger 114. Plunger 114 is slidably received in bore 112 and has an arm 
116 extending in the direction of the check ball 106. The plunger can 
reciprocate between the position shown in FIG. 1 to a position against 
annular flange 118 wherein check ba11 106 is displaced from its seat. 
Passageway 120 communicates the other side of plunger 114 to drain chamber 
82 or to the pump pressure chamber 85 depending on the position of land 
70. Openings 117 are provided about sleeve 102 to communicate the interior 
thereof to passageway 90. 
When an electrical signal is applied to force motor 12, it moves pilot 
spool 24 an amount proportional to the electrical signal. For instance, 
when spool 24 is moved downwardly against the bias of spring 46; or 
upwardly by spring 46 the location of pilot spool 24 will determine 
whether pilot pressure is communicated to chamber 78 (to the left of the 
main spool) via passageway 43 or to chamber 84 (to the right of the main 
spool) via passageway 42. When the spool 24 moves downwardly, pilot 
pressure is transmitted to chamber 84 via passageway 42 and the main spool 
is moved to the left. When additional pressure is transmitted to chamber 
78, the main spool moves to the right. The pilot control sleeve 20 
operates similarly to that explained in U.S. Pat. No. 4,290,447, i.e, the 
pilot sleeve will move in the same direction as spool 24 to close the 
variable orifice which opens when the pilot spool is moved. In other 
words, the positional feedback employed in Pat. No. '447 is also used 
here. However, in Pat. No. '447, the ends of the piston that actuates the 
main spool are at tank pressure via fixed orifices A-3. 
In the instant application, the pilot spool communicates pressures to the 
ends of the main spool and to tank through the variable orifices. For 
instance, when the spool moves downwardly, there is developed a variable 
orifice between upper surface of land 36 and opening 34 (A-3) and between 
upper surface of land 40 and opening 30 (A-2). When the spool moves 
upwardly a variable orifice is developed between the lower surface of land 
36 and opening 34 (A-1); and the lower surface of land 40 and opening 30 
(A-4). The schematic of this system can be seen best in FIG. 4b. In the 
proportional valve of Pat. No. '447, A-1 and A-2 are variable orifices and 
A-3 fixed. In the instant case, the orifices A-3 and A-4 are also 
variable. This precisely controls the position of the main spool. The 
structure shown herein provides a true four-way pilot control that 
positions the main spool in proportion to the electrical signal received 
from the force motor. 
While the invention has an infinite number of positions and variable flows, 
the three basic positions of its components can be characterized as: 
"Hold"--The position shown in FIG. 1, wherein the main spool 56 is in its 
spring-centered position. In hold, the left side of the load-holding check 
valve plunger 114 is connected to drain pressure and the right side is 
connected to tank via notch 63; therefore, the plunger tends to move away 
from the load holding check valve if there is a slight pressure at the 
tank port. The load-holding check valve is against its seat and blocks 
flow from the cylinder port. The load is held in position. 
"Up"--The main spool is moved to the right. As land 66 moves right, flow 
passes through the load-holding check valve to the cylinder port 107. This 
flow moves the load against gravity or other loads. 
"Down"--The main spool is moved to the left. In this position, system 
pressure in passageway 120 acts on the left side of the load-holding check 
valve plunger 114 causing it to move to the right wherein the plunger arm 
116 upsets the ball 106 from its seat 108 against the bias of the check 
ball holding spring 104 opening the interior and back of poppet 92 to tank 
pressure through opening 117, passageway 90 and chamber 88. Passageway 103 
restricts the flow of fluid from cylinder port 107 thus reducing pressure 
in the interior and back of poppet 92. Load pressure is present in 
cylinder port 107 and acts on relieved portion 109 of poppet 92 thus 
opening the load-holding check valve poppet 92. Poppet 92 moves to the 
right and the cylinder port 107 is opened to tank through the load-holding 
poppet and across land 62 of the main spool. Before the main spool 56 
first begins to move to the left, pressure in passage 90 is metered to 
tank through the notches 63 on land 62. As the spool moves to the left, 
pressure from chamber 85 is metered to passageway 120 and plunger 114 by 
means of spool land 70. The load-holding check valve is caused to open and 
fluid in load port 107 is metered across land 62 through chamber 88 
passageway 89 to tank. When the main spool has been moved carefully to the 
left, free flow is permitted between the load port 107 and tank. 
Therefore, gravity, acting on the load, permits the load to be lowered. 
When the load is resting on the ground, the load member is free to "float" 
up and down, as it traverses the ground contours. 
In operation, signals are transmitted through lines 15 which will move 
output member 16. When output member 16 moves downwardly, it moves spool 
24 downwardly causing land 36 to uncover opening 34 and land 40 to uncover 
opening 30. Pilot pressure in conduit 45 is isolated from conduit 43 and 
communicated through the metered orifice to conduit 42 leading to chamber 
84 at the right of main spool 56. This pressure causes the main spool to 
move to the left. at the same time, chamber 78 is communicated to tank via 
the metered opening between the upper surface of land 36, opening 34, bore 
29 and opening 28. Conversely, if output member 16 is moved upwardly, 
spool 24 moves upwardly and pilot pressure from conduit 45 is communicated 
to chamber 78 through the metered orifice between the lower surface of 
land 36 and opening 34 and chamber 84 is communicated to tank through the 
metered orifice between the lower surface of land 40, opening 30, and 
opening 28. 
When the main spool is moved to the right, system or pump pressure is 
connected to the cylinder port 107 through the passage system that opens 
between land 66 and passageway 90 and through the load-holding check 
valve. Resulting flow depends on the amount of opening caused by the main 
spool motion and the pressure difference between the system pressure and 
the load pressure at the cylinder port 107. If the load pressure is 
constant, flow will be proportional to the electrical signal given by the 
force motor. 
When the main spool 56 is moved to the left, system pressure is connected 
to the load-holding check valve plunger 114 via the opening created by 
land 70 uncovering passageway 120. The plunger opens the load-holding 
check ball 106 and load holding check poppet 92 permitting fluid to return 
to tank via the check valve opening, conduit 90 and the opening provided 
by the movement to the left of land 62. 
Before the main spool 56 is moved to the left, the passage between the main 
spool and the load-holding check valve passage 90 is connected to tank 
through a small metering notch 63 of land 62 (See FIG. 3) allowing a bleed 
down of pressure to tank. As the main spool moves to the left, the 
load-holding check valve opens, which applies load pressure to the bleed 
down orifice. Then the main spool moves fully to the left allowing 
unrestricted flow to tank. If the load is constant, the flow to tank will 
be proportional to the electrical signal. 
In many hydraulic systems, pressure to the valve is held constant by the 
pump. Referring to FIG. 1, for systems of this type, the pressure in 
chamber 85 will be constant. If load pressure at port 107 is constant, for 
a given input electrical signal to force motor 12, flow through the valve 
will be constant. As load pressure increases or decreases, flow through 
the valve also increases or decreases. Since it is desirable to have flow 
through the valve constant, many prior art devices provided an additional 
spool valve to maintain a constant pressure difference across the valve 
spool even though load pressure was varying. This is called pressure 
compensation. While pressure compensation devices accomplish the 
objective, it adds to the cost and size of an additional spool valve. In 
this invention a similar effect is accomplished by means of a combination 
of the contour of land 66 and taper 66a, the shape of chambers 85 and 86 
and the means of supplying pressure to chambers 78 and 84. 
When spool 56 moves to the right, fluid from pump P moves across land 66 
and taper 66a. Because of the high fluid velocity and the contour of land 
66 and taper 66a, and the shape of chambers 85 and 86, a force due to flow 
is generated on main operating spool 56. The flow forces are used and 
enhanced to provide the desirable effect of pressure compensation. These 
flow forces tend to move main operating spool toward a position of reduced 
opening for flow. Therefore as flow increases, due to reduced load 
pressure at 107, the flow forces urge spool 56 toward a closed position 
and the spool closes slightly. As flow decreases due to increased load 
pressure at 107, the flow forces urging spool 56 toward a closed position, 
decrease and the spool opens slightly. It should be understood that the 
characteristic or stiffness of the control system supplying control 
pressure to chambers 78 and 84 will affect the amount of spool opening or 
closing due to the flow forces. Summarizing, as flow tends to increase due 
to decreased load pressure, the main spool tends to close. As flow tends 
to decrease due to increased load pressure, the main spool tends to open. 
This effect tends to maintain a constant flow through the valve and in 
fact provides the pressure compensation prior art accomplished by means of 
an additional spool valve. 
In describing the invention, reference has been made to a preferred 
embodiment and illustrative advantages of the invention. Those skilled in 
the art, however, and familiar with the instant disclosure of the subject 
invention, may recognize additions, deletions, modifications, 
substitutions and/or other changes which fall within the purview of the 
subject invention and claims.