U.S. patent number 3,817,150 [Application Number 05/319,165] was granted by the patent office on 1974-06-18 for hydraulic actuator with mechanical feedback.
This patent grant is currently assigned to SLI Industries. Invention is credited to Robert M. Cox.
United States Patent |
3,817,150 |
Cox |
June 18, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
HYDRAULIC ACTUATOR WITH MECHANICAL FEEDBACK
Abstract
A hydraulic actuator having a selectively positioned, rotary
output shaft. The actuator includes a control assembly in which a
spool valve controls the admission and return of working fluid
through a valve port. The spool valve position is controlled by a
pilot assembly which communicates with valve chambers exerting
pressure on opposite ends of the spool valve. The pilot assembly
includes opposed nozzles connected with the valve chambers and a
flapper between the nozzles which may be moved back and forth
between them to variably restrict their output and hence the
pressure in the valve chambers. The flow of fluid from the control
assembly is directed to an output assembly which has first and
second movable pistons mounted in piston chambers. The first piston
chamber is continuously connected to a source of working fluid at
supply pressure, while admission of fluid to the second piston
chamber is controlled by the spool valve via the valve port. The
pistons are connected by a link secured to an output shaft and
exert opposing torques on the link. When fluid is directed into the
second piston chamber, the second piston exerts the greater torque
rotating the shaft in one direction. When fluid is vented from the
second piston chamber, the first piston rotates the output shaft in
the opposite direction. A finger feeds back a portion of the motion
of the link to the flapper via a spring to center the flapper when
the output shaft is in a selected position.
Inventors: |
Cox; Robert M. (Northridge,
CA) |
Assignee: |
SLI Industries (Van Nuys,
CA)
|
Family
ID: |
23241120 |
Appl.
No.: |
05/319,165 |
Filed: |
December 29, 1972 |
Current U.S.
Class: |
91/186; 91/365;
91/387; 137/625.62 |
Current CPC
Class: |
F15B
9/12 (20130101); Y10T 137/86598 (20150401) |
Current International
Class: |
F15B
9/12 (20060101); F15B 9/00 (20060101); F01b
001/00 () |
Field of
Search: |
;91/365,461,186,387
;92/73 ;137/625.62 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Geoghegan; Edgar W.
Assistant Examiner: Hershkovitz; Abraham
Attorney, Agent or Firm: Fulwider Patton Rieber Lee &
Utecht
Claims
I claim:
1. A hydraulic actuator comprising, a control assembly having, a
first housing, an enclosed bore within said first housing, a valve
port communicating with said bore, inlet means adapted for
connection to a source of fluid under pressure for admitting fluid
under pressure to said bore, outlet means for outflow of fluid from
said bore, two valve chambers at opposite ends of said bore in
communication with said inlet means, valve means subjected to the
pressures within said valve chambers movably mounted within said
bore for controlling communication between said valve port and said
inlet and outlet means, said valve means occupying a neutral
position within said bore in which said valve port is closed when
the pressures in said valve chambers are equal, unequal pressures
in said valve chambers moving said valve means in opposite
directions from said neutral position to positions in which said
valve port communicates with one at a time of said inlet means and
said outlet means; and two flow passages, each communicating with
one of said chambers,
a pilot assembly having a second housing, a cavity in said second
housing, opposed nozzles opening into said cavity and communicating
with said flow passages to direct fluid therefrom into said cavity
as opposed jets, a flapper disposed between said nozzles in spaced
relation therewith and movable back and forth between them to
variably restrict said nozzles and thereby vary the pressures in
said valve chambers, and actuating means for urging said flapper
towards either said nozzle with a selected variable force,
an output assembly having a third housing, first and second piston
chambers in said third housing, said first piston chamber
communicating with said inlet means, said second piston chamber
communicating with said valve port, first and second pistons
slidably mounted in said piston chambers, each said piston having a
piston rod extending outwardly from the associated said piston
chamber, an output shaft rotatably mounted in said third housing, a
link fixedly secured to said output shaft, the ends of said link
acted upon by said piston rods to transmit opposing torques to said
output shaft, the torque exerted by said second piston exceeding
the torque exerted by said first piston when both piston chambers
communicate with said inlet means thereby causing rotation of said
output shaft in one direction, the torque exerted by said first
piston exceeding the torque exerted by said second piston when said
second chamber communicates with said outlet means thereby causing
rotation of said shaft in an opposite direction; and
feedback means connected with said flapper and with said link
responsive to movement thereof for urging the flapper to a centered
position between said nozzles when said output shaft is in a
selected position.
2. A hydraulic actuator as defined in claim 1 wherein said feedback
means includes,
a finger slidably mounted in the adjacent portions of said housing,
one end of said finger contacting said link and moved thereby in a
linear direction toward and away from said flapper by pivoting
motion of said link, a first spring disposed between one side of
said flapper and the opposite end of said finger, said first spring
holding said finger against said link, and a second spring acting
against the other side of said flapper and opposing said first
spring to urge said flapper to a centered position between said
nozzles when said output shaft is in said selected position.
3. A hydraulic actuator as defined in claim 1 wherein said first
and second pistons are of equal cross-sectional area and wherein
the axis of rotation of said output shaft is closer to said first
piston rod than to said second piston rod to provide sufficient
leverage to said second piston to enable it to exert a greater
torque on said output shaft than said first piston when both said
piston chambers are connected to said inlet means.
4. A hydraulic actuator as defined in claim 1 wherein said bore is
of generally cylindrical configuration and said valve means is a
spool valve mounted for reciprocation within said bore, said spool
valve including a generally cylindrical body concentric with and
spaced from said bore, axially spaced, peripherally extending first
and second lands fixedly connected with said spool body in sliding
sealing contact with said bore; said lands, when said spool is in
the neutral position, closely overlapping the opposite edges of
said valve port on opposite sides thereof and preventing flow of
fluid through said valve port; displacement of said spool from the
neutral position in either direction opening said valve port for
flow of fluid between the valve port and the interior of said
bore.
5. A hydraulic actuator as defined in claim 4 further
including,
third and fourth lands spaced on opposite sides of said first and
second lands, respectively, said third and fourth lands secured to
said spool body extending into sliding sealing contact with said
bore, said third and fourth lands being acted upon by the fluid
pressures in said chambers to cause axial motion of said spool
valve when the pressures are unequal.
6. A hydraulic actuator as defined in claim 1 wherein said valve
means further includes,
means for preventing fluid communication between said outlet means
and said first and second valve chambers.
7. A hydraulic actuator as defined in claim 2 further including, a
fluid filled sump in said third housing enclosing said link, an
annular opening extending between said finger and the adjacent
portions of said housings through which fluid may pass from said
sump, and a passage connecting an outlet end of said annular
opening with said outlet means.
8. A hydraulic actuator as defined in claim 3 wherein said feedback
means includes.
a finger slidably mounted in the adjacent portions of said
housings, one end of said finger contacting said link and moved
thereby in a linear direction toward and away from said flapper by
pivoting motion of said link, a first spring disposed between one
side of said flapper and the opposite end of said finger and
holding said finger against said link, and a second spring acting
against the other side of said flapper and opposing said first
spring to urge said flapper to a centered position between said
nozzles when said output shaft is in a selected position, the point
of contact of said finger with said link being intermediate said
output shaft and said second piston rod whereby the linear stroke
of said finger is less than the linear displacement of said second
piston.
9. A hydraulic actuator as defined in claim 5 wherein said inlet
means communicates with said bore in a region thereof between said
second and forth lands on said spool valve and wherein said outlet
means communicates with said bore in a region thereof between said
first and third lands on said spool valve, said spool body further
including passage means through said spool body and internally
thereof for conducting fluid from said inlet means to said first
and second valve chambers.
10. In a hydraulic actuator having a pilot valve assembly including
a flapper, two orifices for directing jets of hydraulic fluid
toward opposite sides of said flapper, actuating means for
selectively urging the flapper toward each of the orifices to
variably restrict the jet flow therefrom, and feedback means for
acting on said flapper in opposition to said actuating means in
accordance with the response of a controlled device, and including
a movable feedback element; and a control assembly having an inlet
for actuating fluid, on outlet for said actuating fluid, and a
valve port, valve means movable back and forth along a
predetermined path within the control assembly for selectively
connecting and disconnecting said valve port with one at a time, or
neither, of said inlet and said outlet in different positions along
said path, and two pressure chambers for applying actuating
pressure to said valve means to variably position the latter along
said path, the improvement comprising:
an output assembly having a rotary output member, first and second
crank arms on said output member for turning the latter, first and
second hydraulic cylinders having actuating elements engaging said
first and second crank arms, respectively, to apply opposed
actuating torques to said output member and also having first and
second actuating chambers for said actuating elements, said second
chamber communicating with said valve port, means for delivering
fluid under a preselected pressure to said first chamber to apply
torque to said output member through said crank arm that is less
than the torque applied through said second crank arm when said
inlet is connected to said valve port;
and a feedback connector acting between said feedback element and
one of said crank arms at a point spaced from said output member to
transmit rotary motion of said output member to said feedback
element.
11. A hydraulic actuator comprising, an output assembly having a
first housing, first and second piston chambers in said first
housing, said first piston chamber adapted for continuous
connection to a source of fluid at a supply pressure, first and
second pistons slidably mounted in said piston chambers, an output
shaft rotatably mounted in said first housing, crank arms connected
with said output shaft and said pistons to transmit opposing
torques to said output shaft, the torque exerted by said second
piston exceeding the torque exerted by said first piston when both
piston chambers are at the same pressure thereby causing rotation
of said output shaft in one direction, the torque exerted by said
first piston exceeding the torque exerted by said second piston
when said second chamber is at a return pressure lower than the
supply pressure thereby causing rotation of said shaft in an
opposite direction;
a control assembly having a second housing provided with an inlet
for incoming flow fluid at the supply pressure, an outlet for exit
of fluid at the return pressure and a port communicating with said
second piston chamber; and valve means movable relative to said
second housing for closing said port and for selectively placing
said port in communication with either of said inlet and said
outlet,
a pilot assembly having a third housing and a movable member
resiliently biased in two opposite directions toward a neutral
position, signal controlled means for exerting a force of
predetermined magnitude on said movable member to displace it in
either of said two directions on application of a signal to said
signal controlled means, means responsive to the position of said
movable member connected with said valve means for causing movement
thereof, said responsive means causing said valve means to close
said port when said movable member is in the neutral position and
causing said valve means to place said second piston chamber in
communication with either of said inlet and outlet when said
movable member is displaced in the opposite directions; and
feedback means connected with at least one of said crank arms and
with said movable member for urging the latter to said neutral
position when said output shaft is in a selected position.
Description
BACKGROUND OF THE INVENTION
This invention relates to a hydraulic actuator having a rotary
output shaft in which the position of the output shaft is detected
and fed back to a positioning mechanism within the actuator. More
particularly, the actuator includes an output assembly in which a
shaft is turned by the action of two pistons acting through a
linkage; a control assembly by which working fluid is selectively
directed to the output assembly through the use of a spool valve;
and a pilot assembly to direct the positioning of the spool
valve.
Hydraulic actuators of the rotary shaft output type, are widely
used in diverse industrial applications e.g., one application would
be for controlling the position of a butterfly valve in a flow
line. The hydraulic actuator itself may be controlled from a remote
location by an operator who can apply an electrical or other signal
remotely to the actuator to direct the necessary degree of shaft
rotation to accomplish the desired result in the controlled device.
Such units often include an output assembly comprising the working
parts to which hydraulic fluid is applied and a control assembly
which directs the delivery of the working fluid in the appropriate
direction and rate to operate the output assembly.
One prior control assembly, disclosed in applicant's prior U.S.
Pat. No. 3,698,437 issued Oct. 17, 1972, is intended to control the
operation of a linear hydraulic motor comprising a double acting
piston mounted in a cylinder. Flow of actuating fluid to and from
the hydraulic cylinder is controlled by a spool valve movable
within a spool valve housing in opposite directions from a neutral
position, in response to a difference in pressures in chambers
positioned adjacent the ends of the spool. A pilot valve assembly,
mounted on the end of the spool valve housing, includes an
electromagnetically actuated flapper for variably restricting flows
from the end chambers. Two balanced springs engage opposite sides
of the flapper to urge it toward a centered position. A
longitudinally slidable feedback pin between one spring and the end
of the spool valve adjusts the force of the spring in accordance
with changes in the position of the spool.
Although satisfactory for its intended use, certain difficulties
may be encountered with a valve of the prior type described. For
example, during manufacture, it is necessary to insure that the
various fluid directing surfaces on the exterior of the spool valve
are machined with a very high degree of precision to insure that as
one side of the hydraulic cylinder is connected to the fluid
supply, the other side is simultaneously connected to exhaust and
vice versa. Manufacturing difficulties inherent in maintaining
precise dimensioning can lead to high scrappage rates and increased
costs, and it would therefore be desirable to find a construction
less demanding in its manufacture.
In addition the prior device, as described, derived the feedback
motion to recenter the flapper from the motion of the spool
controlling the delivery of fluid to the hydraulic motor rather
than directly from the position of the controlled member i.e., the
piston rod. However as it is the position of the controlled member
that is of ultimate interest it would be preferable to sense the
position of that directly for the feedback response, as there is
always a possibility of slight variance between the relative
motions of the spool and the controlled member due to overshooting,
the development of slight play in the spool movement and like
causes.
The prior device described is, in addition, disclosed for use with
a linear hydraulic motor. There are many applications for which
there is a need for an electro-hydraulic actuator which has a
rotary shaft output. Desirable features of such an actuator are
that it should have a high flow rate and a high pressure capability
to provide high torque output and rapid operation.
SUMMARY OF THE INVENTION
The present invention provides a hydraulic actuator having a rotary
shaft output, which is characterized by rapid operation and high
torque output. Although the invention utilizes internal components
of the spool valve type, the construction is such that the need to
maintain high precision tolerances on the spool valve dimensions
during manufacture is substantially reduced so that a lower
production cost can be achieved. In addition, a hydraulic actuator
according to the invention provides a feedback directly responsive
to the position of the controlled member.
More particularly, a hydraulic actuator constructed in accordance
with the invention includes an output assembly which rotates an
output shaft in either direction when actuating fluid is directed
to it, a control assembly provided with a movable spool valve for
directing the flow of actuating fluid to the output assembly, and a
pilot assembly for positioning the spool valve of the control
assembly. The control assembly includes a first housing having an
internal bore and inlet and outlet passages which admit and exhaust
fluid to and from the bore, respectively. A spool valve movably
mounted within the bore controls flow of fluid through a valve
port. Valve chambers are positioned at opposite ends of the bore
and communicate with the inlet passage. The spool valve is acted
upon by the difference in pressures within the valve chambers for
motion in opposite directions from a neutral position, in which the
valve port is closed when the pressures in the valve chambers are
equal, to opposite positions in which the valve port communicates
with either the inlet passage or the outlet passage dependent upon
the direction of the pressure differential between the valve
chambers. Flow passages communicate with the valve chambers for
outflow of fluid therefrom. The pilot assembly includes a second
housing, connected with the first, having an internal cavity into
which two opposed nozzles open. The nozzles communicate with the
flow passages and direct fluid from them into the cavity as opposed
jets. A flapper disposed between the nozzles in spaced relation to
them is movable back and forth. An electromagnetic actuator, on
receipt of an input signal, urges the flapper towards either of the
nozzles with a selected variable force. The flapper variably
restricts the nozzles and thus varies the pressure in the valve
chambers (and hence the position of the spool valve). The output
assembly includes a third housing having first and second pistons
slidably mounted in corresponding piston chambers. The first piston
chamber is in continuous communication with the inlet passage while
the second piston chamber communicates with the valve port. Each
piston has a piston rod extending outwardly from its associated
chamber. The output shaft is rotatably mounted in the third housing
and has a link fixed to it which is acted upon by the piston rods
to transmit opposing torques to the shaft. When the spool valve
connects the valve port to the inlet passage so that both piston
chambers are both at the same supply pressure, the torque exerted
by the second piston is greater than the torque exerted by the
first piston and the link rotates the shaft in one direction. When
the spool valve is in the opposite position to vent the second
chamber, the torque exerted by the first piston is substantially
unopposed and the shaft is turned in the opposite direction. A
feedback unit connected with the flapper and the link is responsive
to the link movement to urge the flapper to a centered position
between the nozzles when the output shaft is in the selected
position.
It will be appreciated that this arrangement, in which one of the
pistons is continuously connected to the supply source, provides a
construction in which it is only necessary to control a single
valve port to obtain rotary motion in opposite directions. This
offers significant manufacturing advantages because it is
relatively much less expensive to drill or ream a single valve port
to match the dimension between the controlling portions of the
spool valve than it is to machine the flow controlling portions on
the exterior of the spool valve with a high degree of
precision.
In the preferred embodiment, the desired torque relationship is
arranged by utilizing pistons of equal cross sectional area but
pivoting the link at a location closer to the first piston rod than
to the second piston rod so that the latter exerts the greater
torque when both piston chambers are at the same pressure.
The feedback unit includes a finger extending from the link towards
the flapper and moved linearly in that direction by pivoting motion
of the link. The finger is slidably mounted in the adjacent
portions of the housings and has one of its ends contacting the
link while its other end contacts a first spring which abuts the
flapper. A second spring acting on the other side of the flapper
opposes the first spring to urge it to a centered position between
the nozzles of the pilot assembly when the spool is in the selected
position.
By coupling the motion of the finger to the link, two advantages
are attained. Firstly, the feedback response is directly responsive
to the position of the controlled member which is the matter of
ultimate concern rather than to the position of an intermediate
component, the spool valve. The second advantage is that by
positioning the point of contact of the finger close to the pivot
point, it is possible to obtain a reduction in the linear travel of
the finger as compared with the piston travel. Such reduction is
necessary because the range of available movement of the flapper
between the nozzles is relatively much smaller.
BRIEF DESCRIPTION OF THE DRAWINGS
A hydraulic actuator constructed in accordance with the preferred
embodiment of the invention is illustrated in the accompanying
drawings in which:
FIG. 1 is a perspective view of a hydraulic actuator constructed in
accordance with the preferred embodiment of the invention, shown
mounted on an exemplary control device and connected thereto by a
parallel arm linkage;
FIG. 2 is a cross-sectional side view of the hydraulic actuator
shown in FIG. 1, taken along the lines 2--2 therein;
FIG. 3 is a cross-sectional end view of the hydraulic actuator
shown in FIG. 2, taken along the lines 3--3 therein;
FIG. 4 is a cross-sectional end view of the hydraulic actuator
shown in FIG. 2, taken along the lines 4--4 therein, with a spool
valve, forming a part of the invention, shown in a neutral
position;
FIG. 5 is a cross-sectional end view of a portion of the hydraulic
actuator shown in FIG. 2, taken along the lines 5--5 therein;
FIG. 6 is a cross-sectional bottom view of a portion of the
hydraulic actuator shown in FIG. 2, taken along the lines 6--6
therein, but with the spool valve shifted from the neutral
position; and
FIG. 7 is a cross-sectional side view of the hydraulic actuator
shown in FIG. 6, taken along the lines 7--7 therein.
DETAILED DESCRIPTION
Referring to FIG. 1 of the drawings, a hydraulic actuator 2 is
there shown, secured to an underlying controlled device 4. The
shaft output of the actuator 2 is connected to the input of the
controlled device 4 by a connecting parallel arm linkage 6. The
controlled device is shown purely for the purposes of
exemplification and may be, for example, a valve positioner, a pump
controller or any of a very diverse range of devices customarily
controlled by a hydraulic actuator having a shaft output.
The hydraulic actuator of the invention, includes three principal
assemblies; an output assembly 8 for applying power to turn an
output shaft 10 progressively in either direction to any selected
position between two extremes of angular movement, a control
assembly 12 for controlling the admission and direction of
actuating fluid to the output assembly, and a pilot 14 for
receiving an electrical positioning signal and actuating the
control assembly to deliver fluid to the operating assembly to
achieve the desired motion of the output shaft. Actuating fluid
under a pressure P is supplied to the control assembly 12 through
an inlet connection 16 from a source such as a pump (not
shown).
The output assembly 8 (FIGS. 2 and 4) includes a housing 20
provided with spaced, parallel, first and second piston chambers 22
and 24, respectively, of identical configuration. First and second
pistons 26 and 28 are sealingly and slidingly received within the
first and second chambers and have piston rods of equal length
extending outwardly from the chambers in one direction. Actuating
fluid, at the supply pressure P, is continuously directed to the
first chamber 22 via a vertical inlet passage 30 in a housing 32
(forming a part of the control assembly) and a transverse channel
34 extending between the inlet passage and the first chamber.
Admission of fluid to and from the second chamber 24 is controlled
by a spool valve 36 in the control assembly 12, which is movable
relative to a valve port 38 (FIG. 6) communicating with the second
chamber. In a neutral position of the spool valve 36, it closes the
valve port so that the piston remains locked in a stationary
position. However, the spool valve may be moved selectively, under
the control of the pilot assembly 14, in one direction in which it
places the valve port 38 in communication with the inlet passage 30
so that fluid at the supply pressure P is admitted to the second
chamber 24, or in an opposite direction in which the valve port 38
communicates with an outlet vent from the hydraulic actuator. When
it is in communication with the outlet vent the pressure in the
second chamber 24 is reduced to a relatively negligible pressure
compared with the supply pressure P.
The first and second pistons 26 and 28 apply torque in opposite
directions to the output shaft 10 through a pivoting link 40 (FIG.
2) fixedly secured to the output shaft by a pin 41. The link 40 is
provided with bearing units 42 at its opposite ends abutting the
free extremities of the piston rods. The shaft 10 is mounted in
bearings 44 (FIG. 3) supported by a cover portion 46 of the output
assembly 8, at a location closer to the first piston 26 than to the
second piston 28. Because the point of pivotal connection of the
link (which is the same as the axis of rotation of the output shaft
10) is closer to the first piston, the second piston has greater
leverage about the pivot point than the first piston. When the
spool valve is moved to put the valve port 38 in communication with
the inlet passage thereby causing the supply pressure P to be
equally applied to both piston chambers, the second piston 28
exerts a greater torque on the output shaft 10 than the opposite
torque exerted by the first piston 26, because of the off-center
pivotal connection of the link. The resultant torque causes the
output shaft to be rotated in an anti-clockwise direction as shown
in FIG. 2. However, when the spool valve is shifted in the opposite
direction to place the valve port 38 in communication with the
outlet vent so that the pressure in the second chamber drops to a
relatively negligible value, then the torque exerted by the first
piston 26 is substantially unopposed and the link is rotated in a
clockwise direction.
It will be appreciated that it is the leverage provided by the
pivotal link 40 that enables the system to be operated by control
of the fluid supply to only one of the two working pistons so that,
while the other exerts a constant torque, the controlled piston can
exert either a greater opposite torque using the same supply
pressure to rotate the shaft in one direction or a zero torque
permitting the shaft to be rotated in the opposite direction. The
portions of the link extending on opposite sides from the pivot
point to the two pistons constitute two connected crank arms. This
ability to operate by control of fluid admission to only one of the
working pistons considerably simplifies the spool value structure
(to be described hereinafter) leading to considerable manufacturing
savings.
This torque relation could also be achieved in whole or part by
utilizing pistons of different cross-sectional area in the first
and second chambers. For example it would be possible to utilize a
link pivoted at its mid point and a second piston twice the area of
the first piston to achieve a comparable result.
Because the pistons are of relatively large cross-sectional area, a
hydraulic actuator in which the resultant output torque is high, is
provided. Also, the large flows of actuating fluid that occur in
and out of the large piston chambers 22 and 24 provide for
particularly rapid response of the device in achieving the desired
positional output of the drive shaft 10.
Fluid flow to and from the second chamber 24 is, as previously
mentioned, regulated by the spool valve 36 (FIGS. 6 and 7). The
spool valve 36 is movably mounted within an enclosed horizontal
bore 50 extending transversely within the lower part of the housing
32 of the control assembly. The valve port 38 intersects the bore
50 at approximately its mid point and extends horizontally through
the housing 32 to communicate with the second chamber. The spool
valve 36 includes a hollow cylindrical valve body 52 spaced
concentrically within the bore 50 and of shorter length than the
bore. It is provided with axially spaced, first and second annular
lands 54 and 56, respectively, which extend into sliding sealing
contact with the walls of the bore 50. When the spool valve 36 is
in a neutral position, the outer edges of the lands 54 and 56
closely overlap the outer edges of the valve port 38 to close the
valve port from fluid communication with the interior of the bore
50 outside the lands (FIG. 4). However, it takes only a slight
axial shift of the spool valve to open the valve port 38 for flow
of fluid between the second chamber 24 and the bore 50 (FIGS. 6 and
7). This arrangement has a particularly significant advantage from
the viewpoint of manufacturing construction. Because only one valve
port for the passage of actuating fluid is controlled, the only
manufacturing alignment that is needed is to ream the valve port 38
until its diameter is approximately equal to, but slightly less
than, the distance between the outer surfaces of the first and
second lands 54 and 56. Precise machining of the axial dimensions
of the lands themselves is thus rendered unnecessary, thereby
substantially reducing manufacturing costs and losses due to
scrappage of defectively machined spool valves.
The spool valve 36 also includes third and fourth lands 58 and 60
adjacent and spaced from the first and second lands 54 and 56. The
third and fourth lands 58 and 60 are spaced from the adjacent ends
of the spool body 52 and extend annularly about the spool body into
sliding sealing contact with the bore 50. Incoming actuating fluid
passing down the previously mentioned inlet passage 30, which
communicates at its lower end with the bore 50, enters the bore 50
between the second and fourth lands 56 and 60 of the spool valve
and fills the annular space between the lands and the spool body.
The spool valve 36 may be shifted axially in one direction to
partially expose the valve part 38 so that the incoming fluid
passes from the inlet passage 30 via the space between the lands 56
and 60 through the valve port 38 into the second chamber 24. The
axial spacing between the second and fourth lands 56 and 60 is
sufficient to insure that the inlet passage 30 remains in fluid
communication with the space between the lands, even in the extreme
positions of displacement of the spool valve from the neutral
position.
To enable outlfow of the actuating fluid from the bore 50, an
outlet or vent passage 62 is also provided in the housing 32 of the
control assembly. The vent passage 62 extends vertically in spaced
parallel relation to the inlet passage 30, but on an opposite side
of the valve port 38 from the inlet passage. Adjacent its upper
end, which is blocked, the vent passage 62 communicates with an
outlet connection 64 to return the actuating fluid to a suitable
reservoir (not shown). The pressure of the fluid in the vent
passage is substantially lower than the pressure P at which the
actuating fluid is supplied and may, for the purposes of
description, be regarded as at negligible pressure. The lower end
of the vent passage 62 communicates with the bore 50 in the region
between the first and third lands 54 and 58 on the spool valve, so
that when the spool valve is shifted in an opposite axial direction
from its neutral position, the valve port 38 is placed in
communication with the vent passage 62 and fluid within the second
chamber 24 can escape therefrom as illustrated in FIG. 6. The axial
spacing between the lands 54 and 58 is sufficient to insure that
the vent passage 62 communicates with the space between the lands,
even in the extreme positions of displacement of the spool valve
36.
Shifting of the spool valve axially in the bore is accomplished by
exerting differential pressure on its end portions. The end regions
of the bore 50, between its right end and the fourth land 60 of the
spool valve and between the left end of the bore and the third land
58, constitute first and second valve chambers 66 and 68,
respectively. Centering springs 70 between the ends of the bore 50
and the adjacent lands 58 and 60 center the spool in the neutral
position in which the valve port 38 is closed when the pressures in
the first and second chambers 66 and 68 are equal. Suitable
adjustment members (not shown) are provided for adjusting the
spring pressures to center the spool during calibration. The
actuating fluid at inlet pressure P is admitted to the first and
second chambers 66 and 68 via the hollow interior of the spool body
36, through two openings 72 in the spool body in the region between
the second and fourth lands 56 and 60. A fixed barrier 74,
positioned within the interior of the spool body 52, prevents
direct fluid communication between the first and second valve
chambers 66 and 68. To permit controlled outflow of fluid from the
first and second valve chambers 66 and 68, they are provided with
first and second flow passages 76 and 78, respectively, (FIG. 6).
If flow out of one of the valve chambers is restricted to a greater
extent than flow out of the other, the pressure in the one valve
chamber will become relatively higher than in the other. As a
result the spool valve will move in the direction of the reduced
pressure chamber. Thus, positional control of the spool valve, and
hence control of the operation of the output assembly, is effected
by varying the rate at which fluid is permitted to escape from the
first and second valve chambers 66 and 68, respectively.
The differential control of the outflow of fluid from the valve
chambers of the control valve assembly is effected by the pilot
assembly 14, which is of substantially the same construction as the
corresponding pilot assembly shown in applicant's aforementioned
U.S. Pat. No. 3,698,437. The pilot assembly includes a third
housing 80 (FIGS. 2 and 6) having a vertical cavity 82 therein and
two fixed horizontal nozzles 84 and 86 extending in spaced,
opposed, aligned relation into the cavity 82. Between the second
and third housings 32 and 80, which are fixedly connected together,
there is interposed a plate 88 (FIG. 5) having cut-out openings in
which are mounted O-rings to define channels for the passage of
fluid. The plate 88 provides a first channel 90 (FIG. 6) which
connects the flow passage 78 from the second valve chamber with a
chamber 91 communicating with the nozzle 84. The flow passage 76
from the first chamber communicates with a vertical channel 92 in
the plate 88, which, at its upper end, communicates with an
inclined passage 94 (FIG. 2) extending downwardly into a chamber 95
communicating with the other nozzles 86. Thus, flows of actuating
fluid from the two valve chambers 66 and 68 are directed into the
cavity 82 as opposed fluid jets. A drain passage 96 (FIG. 2) from
the lower end of the cavity 82 returns fluid to a lower end portion
of the previously mentioned vent passage 62 extending below the
bore 50 (FIG. 7).
Disposed between the two nozzles 84 and 86 is a so-called flapper
98 (FIGS. 2 and 6) which has oppositely facing flat sides adjacent,
but spaced from, both nozzles when the flapper is centered. The
flapper is movably mounted for swinging of each flat side toward
the adjacent nozzle, thereby progressively increasing the
restriction of the flow through that nozzle, while simultaneously
reducing the restriction of the flow through the other nozzle. As a
result, the back pressure in the one of the valve chambers 66 and
68 connected to the restricted nozzle is increased, while the
pressure of the other chamber is reduced, thereby unbalancing the
forces of the spool valve 36 to cause it to move away from the
increased-pressure chamber and toward the reduced-pressure
chamber.
The flapper 98 is the flattened lower end of an elongated rod 99
supported adjacent its upper end by a fitting 100 which provides
for pivoting movement of the rod. The upper end of the flapper rod
extends into a electromagnetic torque motor 104 which may be
electrically controlled to apply a predetermined force to the
flapper rod, which is made of a metal subject to magnetic
attraction, so that the rod is pivoted on the fitting 100 to cause
the flapper rod to move adjacent one or other of the nozzles 84 and
86 with a selected variable force dependent on the magnitude and
direction of the electrical input signal. Inasmuch as a complete
description of the fitting and the torque motor are provided in
applicant's prior U.S. Pat. No. 3,698,437, such description is
incorporated herein by reference.
Thus, an electrical signal to the torque motor moving the flapper
towards one of the nozzles will cause a shift in the position of
the spool valve from its centered position and the admission or
exhaust of fluid from the second piston chamber, so that the link
40 will be rotated turning the output shaft 10. To cause the
rotation to be terminated once the desired angular position has
been reached, a feedback mechanism is incorporated into the
actuator. For this purpose a feedback finger 110 (FIG. 2),
responsive to the movement of the link 40, is provided. The
feedback finger includes a pin 112 extending through the housings
of the control assembly and the output assembly, and a second pin
114 slidably mounted in the housing of the pilot assembly. The pin
112 has one end in contact with a notch 116 in the mid point of the
link 40 and its other end in contact with the pin 114. Both pins
are moved linearly towards the flapper rod as the link 40 pivots in
a clockwise direction. A spring 118 is compressed between the
second pin 114 and the flapper in response to movement of the pin
by the link. A similar spring 120 acts against the opposite side of
the flapper to counterbalance the force of the first spring 118
thereby holding the flapper centered between the nozzles 84 and 86
when the spool is in the neutral position. The detailed mounting
structure of the second pin 114, the springs 120 and 118, and of
structure for adjusting the centered position of the flapper rod
between the springs is described fully in applicant's previously
mentioned U.S. Pat. No. 3,698,437, and such description is
incorporated herein by reference.
In operation, when an electrical signal is applied to the torque
motor to apply a selected force to the flapper rod to move the
flapper towards the nozzle 84, flow out of the second chamber 68 is
restricted and the pressure in the chamber 68 is relatively
increased, so that the spool shifts connecting the second chamber
24 to the vent passage as shown in FIG. 6. The first piston 26 then
moves outwardly, rotating the link 40, and the output shaft 10, in
clockwise direction as shown in FIG. 2. The motion of the link 40
causes the finger 110 to move toward the flapper rod compressing
the spring 118 and exerting a centering force on the flapper rod in
opposition to the signal force, to urge the flapper to a centered
position between the nozzles when the output shaft reaches the
selected position. As the flapper reaches the centerd position, it
causes the spool to move to its neutral position, so that flow to
and from the second chamber 24 is shut off and the shaft remains
locked in the selected position.
When the electrical input signal moves the flapper towards the
other nozzle 86, the spool is moved in the opposite direction to
admit actuating fluid into the pressure chamber 24 to cause the
piston 28 to rotate the link 40 in an anticlockwise direction. The
finger 110 then moves away from the flapper rod reducing the degree
of compression of the spring 118 so that the counterforce exerted
by the other spring 120 urges the flapper rod back to the centered
position.
A further advantage of the link structure described is that by
mounting the feedback finger so that it contacts the link adjacent
its mid point, the effective length of stroke of the feedback
finger is reduced relative to the extent of travel of the pistons.
Such reduction in stroke simplifies construction because the range
of movement available to the flapper rod between the nozzle is much
less than the range of travel through which the piston is
moved.
The space within the interior of the cover 46 of the output
assembly housing constitutes a fluid filled sump into which leakage
of fluid past the pistons drains. The leakage fluid can escape from
the sump to the vent passage 62 along an annular channel 122 (FIG.
2) surrounding the finger 112, an opening 124 in the plate 88 and a
connecting duct 126 intersecting the vent passage. Suitable grooves
are provided in an enlarged head of the pin 112 to permit fluid to
bypass the enlargement.
Summarizing, a hydraulic actuator according to the present
invention possesses high output torque capability by virtue of the
large working surfaces of the pistons, and is capable of rapid
action due to the large flow of working fluid controlled by the
motion of the spool valve which is moved between its flow
controlling positions by relatively insignificant control flows of
the fluid. The arrangement by which one piston remains permanently
subjected to the full working pressure, while the other piston is
provided with a linkage permitting it to exert a greater or less
torque than the first piston utilizing the same source of pressure
fluid, allows a valving structure wherein only one valve port to
the working pistons needs to be controlled. As a result, the valve
and port structure is considerably simplified and the need for
precision machining of the external projecting surfaces of the
spool valve is avoided.
A further advantage of the linkage described is the manner in which
it enables the stroke of the feedback finger to be reduced to
conform with the limited range of flapper movement available,
without requiring complicated reduction mechanisms.
It will be apparent to those skilled in the art that although a
particular form of the invention has been illustrated and
described, various modifications can be made without departing from
the spirit and scope of the invention.
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