Fluid powered actuator system

A fluid powered actuator system includes two identical fluid powered actuators operable by respective supply pressures which are controlled by respective identical valve arrangements. Operating positions of flow control elements of the valve arrangements are applied through respective couplings to a differential arrangement which operates, if these positions differ by an unacceptable amount, to apply an error indication through respective coupling to prevent the supply pressures from being applied to the respective actuators.

This invention relates to a fluid powered actuator system, and in 
particular to such a system which includes duplicated actuators coupled to 
provide a combined output, and duplicated control valves for the 
respective actuators. 
In such a system it is desirable that the flow control elements should move 
by equal amounts. It is known to provide for comparison between the 
positions of input operating devices for the two valves, but such prior 
art systems do not monitor the positions of the value control elements 
themselves. 
It is an object of the invention to provide a system having duplicated 
fluid powered actuators and control valves, in which operating positions 
of control elements of the valves are sensed directly, and in which an 
unacceptable difference between these operating positions results in 
removal of a fluid pressure supply to the valves. 
It is a further object of the invention to provide that input devices for 
the control valves are operated in accordance with a difference between 
desired and sensed positions of an actuator device to which fluid is 
supplied by the valves. 
According to the invention there is provided a fluid powered actuator 
system comprising two fluid powered actuators which are coupled to provide 
a combined output, two valve devices operable to apply fluid pressures to 
respective ones of said actuators, a first differential arrangement 
responsive to operating positions of both of said valves for providing an 
error output when said positions differ by more than a predetermined 
amount, and valve means for removing a pressure supply from said valve 
devices in response to said error output.

As shown in FIG. 1 the system comprises a double acting fluid powered 
actuator 10 which effectively comprises two actuator devices 10A, 10B 
coupled for movement in unison and responsive to pressure signals on 
respective pairs of control lines 11, 12 and 13, 14. The actuator 10 
includes a brake device 15 which can be maintained inoperative by 
pressures on lines 16 and 17. The actuator 10 also includes means for 
providing mechanical position feedback signals on two shafts indicated at 
18 and 19, and shown more clearly in FIG. 3. The system also includes two 
identical valve arrangements 20, 21, the arrangement 20 being shown in 
more detail in FIG. 2. The arrangements 20, 21 are connected to separate 
fluid pressure supply lines P1, P2 and separate return lines R1, R2 and 
are operative to control the pressures on lines 11, 12 and lines 13, 14 
respectively. 
A first differential arrangement 34 is shown more clearly in FIG. 4 and is 
responsive to a discrepancy between the operating positions of valves 22 
in the arrangements 20, 21 to isolate these valves 22 from the respective 
pressures P1, P2, by means of respective linkages 35, 36. 
A second differential arrangement 30, shown in more detail in FIG. 3 is 
responsive to an input movement from an actuator position selector 31 and 
to the rotational positions of the shafts 18, 19 to provide mechanical 
outputs on shafts 32, 33 to the respective valve arrangements 20, 21, as 
shown more clearly in FIG. 3. 
As shown in FIG. 2 the valve 22 in the arrangement 20 includes a valve 
spool 37 linearly movable by the shaft 32 to connect the lines 11, 12 
selectively to the supply pressure P1 or return pressure R1. The spool 37 
has a sliding collar 38 and a further collar 39 which abuts a fixed part 
of the spool. A compression spring 40 acts between the collars 38, 39 and 
a further compression spring 41 acts between the collar 38 and a 
relatively fixed part 42. The arrangement is such as to bias the spool 37 
to a central position (shown) in which the lines 11, 12 are isolated from 
the supply and return pressures. The supply pressure P1 is applied to the 
valve spool 37 by way of a shut-off valve 43 having a spool 44 
spring-biased towards a shut position. The spool 44 can be urged to its 
open position (shown) by the pressure in a chamber 45 derived from the 
pressure P1 through a normally shut bypass valve 46. The bypass valve 46 
is urged towards to an open position by a spring 47 but is normally 
restrained against opening movement by a roller 48 on a pivotally mounted 
arm 49 which is shown in more detail in FIG. 4 and which forms part of the 
linkage 35. The operating position of the spool 37 is transmitted through 
a linkage 50, also shown in more detail in FIG. 4 to the differential 
device 34. 
It is to be understood that the valve arrangement 21 corresponds to the 
arrangement 20 described above and is responsive to position signals on 
the shaft 33 and provides valve position signals through a linkage 51 to 
the differential device 34. 
As shown in FIG. 3 the differential device 30 comprises two identical gear 
trains 60, 61, only the train 60 being described in detail. The train 60 
includes a bevel gear 62 drivingly coupled to the position selector 31 and 
an opposed bevel gear 63 drivingly connected through a worm and wheel 64 
to the feedback shaft 19 from the actuator 10. A third bevel gear 67 
meshes with the gears 62, 63 and is mounted for free rotation on a stub 
shaft 66 secured to the actuating shaft 32 for the valve spool 37, the 
shaft 32 passing axially through the bevel gears 62, 63. The arrangement 
is such that rotation of the bevel gear 62 results in rotation of the 
shaft 32 in the same direction, and consequent movement of the spool 37. 
Subsequent movement of the actuator 10 causes rotation of the bevel gear 
63 in the opposite direction to that of the gear 62 and thus returns the 
shaft 32 and spool 37 to its initial position. It will be seen that in 
normal operation movements of the spool 37 and of the corresponding spool 
65 in the valve arrangement 21 will be identical. The differential device 
34 operates in a manner to be described to isolate the spools 37, 65 from 
their respective supply pressures P1, P2 in the event that the spool 
movements differ significantly. Drive to the gear trains 60, 61 from the 
position selector 31 is by way of respective friction clutches 52, 53, so 
malfunction of either of the spools 37, 65 or of the input couplings 
thereto does not adversely affect other parts of the system. 
As shown in FIGS. 3 and 4 the valve arrangements 20, 21 and the 
differential device 34 are mounted in a housing block 70 which is 
indicated in outline only in FIG. 4. As shown in FIG. 4 the linkage 50 
includes a shaft 71 pivotally mounted in the housing 70 and having a 
projection 72 engaging a recess in the valve spool 37. A lever arm 73 on 
the shaft 71 engages a further lever arm 74 on a further shaft 75 also 
pivotally mounted in the housing block 70. A forked lever 76 engages one 
end of an arm 77 which is mounted for movement about a pivot 78 in a 
bracket 79. The bracket 79 is itself mounted for movement about a pivot 80 
supported in the housing block 70. The other end of the arm 77 is engaged 
by a forked lever 81 which corresponds to the lever 76 and forms part of 
the linkage 51 which co-acts with the valve spool 65. In normal operation 
the spools 37, 65 move by equal amounts in opposite directions so that 
movements of the forked levers 76, 81 are equal, and the arm 77 moves 
about the pivot 78, but the bracket 79 does not itself move about the 
pivot 80. However, difference in movement between the spools 37 and 65 
causes pivotal movement of the bracket 79 and this bracket has a cranked 
end 82 which engages a roller 83 forming part of the linkage 35. 
In addition to the lever 49 and roller 48 the linkage 35 includes a shaft 
90 which is pivotally mounted in the housing block 70 and on which the 
lever 49 is supported. The shaft 90 has a crank arm 91 on which the roller 
83 is carried and the roller 83 is biassed against the cranked end of the 
bracket 79 by the spring 47 acting on the bypass valve 46. The arrangement 
is such that pivotal movement of the bracket 79 by more than a 
predetermined amount allows the crank arm 91 and the lever 49 to move 
anti-clockwise and the valve 46 to move under the influence of its spring 
47 to connect the chamber 45 (FIG. 2) to the return line R1, shutting the 
valve 43 and isolating the spool 37 from the pressure supply P1. At the 
same time the pressure in line 16 falls to that of the return pressure R1 
and the brake device 15 (FIG. 1) in the actuator 10 is operated. 
As shown in FIG. 4 the linkage 36 is generally similar to the linkage 35, 
but the crank arm 92, corresponding to the arm 91, does not carry a roller 
but merely engages the end of the arm 91. Pivotal movement of the bracket 
79 permits the arm 91 and a lever 100 to move clockwise, and a further 
valve (not shown), corresponding to the valve 46, to isolate the spool 65 
in the arrangement 21 from the supply pressure P2 in a like manner to that 
described above. 
As shown in FIG. 3 the drive paths between the position selector 31 and the 
differential gear trains 60, 61 each include a ball clutch 95 which is 
loaded by springs 96. This arrangement ensures that jamming of either of 
the trains 60, 61, or of their associated drives 32, 33, or of the spools 
37, 65 results in slipping of the clutch 95 and prevents damage to the 
system. Differential movement of the spools 37, 65, resulting from 
slipping of a clutch 95 causes both spools to be isolated from their fluid 
pressure supplies P1, P2. 
During normal operation of the system the spring loading of the valve 43 
serves to maintain a pressure in the chamber 45, and therefore in the line 
16, against transient pressure fluctuations which might otherwise occur as 
a result of operation of the valve spool 37. 
Differential movement betwwen the spools 37, 65 may result from, for 
example, fracture of the engagement between the spool 37 and its 
connection to the shaft 32, in which case the springs 40, 41 (FIG. 2) will 
maintain the spool 37 in a central position. Alternatively if the spool 37 
breaks between its connections to the shaft 32 and the linkage 50, 
operation of the shaft 32 to move the adjacent part of the spool 37 away 
from the break will cause the linkage 50 to be maintained in its central 
position by the spring 40, 41, resulting in shut-off of pressures P1, P2 
as described above. If the shaft 32 is operated to move the adjacent part 
of the spool 37 against a break therein, the spool may act in a normal, or 
near-normal manner until an attempt is made to move it in the opposite 
direction. 
If a connection of the shaft 32 fails, and one of the springs 40, 41 also 
fails, the spool 37 will be urged in one direction only by the remaining 
spring but the force applied by that spring will by itself be 
insufficient, when applied through the linkage 50, to pivot the lever 76 
and thereby to result in shut-off of the pressures P1, P2. In this 
condition if the spool 65 (FIG. 4) is moved in a direction which 
corresponds to a requirement to move the spool 37 against the remaining 
spring 40 or 41, the spool 37 will not so move and the differential spool 
movement will cause pressures P1, P2 to be shut off. If, however, in this 
last condition of failure the spool 65 is moved in a direction which 
corresponds to a requirement to move the spool 37 in a direction assisted 
by the remaining spring, the spool 37 will be allowed to move in its 
proper direction as the lever 77 (FIG. 4) is permitted to turn about the 
pivot 78 by the forked lever 81. In this last operating condition the 
spool 37 will act normally or near-normally. 
The arrangement of the present invention thus provides either for shut-down 
or for continued near-normal operation under all mechanical malfunctions 
of the valves 22 or the input drives thereto.