US3411413A

US3411413A - Fluid-operated step motor

Info

Publication number: US3411413A
Application number: US594942A
Authority: US
Inventors: John H Macneill; Ralph P Means
Original assignee: SOROBAN ENGINEERING Inc
Current assignee: SOROBAN ENGINEERING Inc
Priority date: 1966-11-16
Filing date: 1966-11-16
Publication date: 1968-11-19
Anticipated expiration: 1985-11-19

Description

NOV. 19, 1968 J, H. ac ETAL 3,411,413

FLUID-OPERATED STEP MOTOR Filed Nov. 16, 1966 4 Sheets-Sheet 1 60 50 &

llllllll Ill INVENTORS 5|" JOHN H.MocNE|LL a RALPH P. MEANS tJ BY M1 4 M ATTORNEYS Nov. 19, 1968 J, H. M NElLL. ETAL 3,411,413

FLUID-OPERATED STEP MOTOR 4 Sheets-Sheet 2 Filed Nov. 16, 1966 MES oL LA m m I 6 Wm l l W MOP 2 W Hm N m WM J I ATTORNEY Nov. 19, 1968 Filed Nov; 16, 1966 J. H. M NEILL ETAL 3,411,413

FLUID-OPERATED STEP MOTOR 4 Sheets-Sheet 3 EGA- I B3 B9 4 m 3 52 c3 Bl A2 Al I27 123 ISI n9 n7 M PRESSURE |53 SOURCE INVENTORS JOHN H.MGCNE|LL 8 RALPH P. MEANS BY W, 1 M

ATTORNEYS Nov. 19, 1968 J, MacNElLL ETAL 3,411,413

FLUID-OPERATED STEP MOTOR 4 Sheets-Sheet 4 Filed Nov. 16, 1966 INVENTORS JOHN H. MclcNElLL 8 P. MEANS RALPH ATTORNEYS United States Patent 3,411,413 FLUID-OPERATED STEP MOTOR John H. MacNeill, Indialantic, and Ralph P. Means, Melbourne, Fla., assignors to Soroban Engineering, Inc., Melbourne, Fla., a corporation of Florida Filed Nov. 16, 1966, Ser. No. 594,942 13 Claims. (Cl. 91357) ABSTRACT OF THE DISCLOSURE A fluid-operated step motor is disclosed comprising a movable member disposed in a housing and dividing the housing into two pressure-isolated compartments, the compartments each being supplied with fluid at equal pressures. A plurality of ports are defined through the housing and are selectively exhausted to a low pressure environment, the member being moved into alignment with a selected exhaust port by the pressure differential developed across the member by the exhausting of fluid from one of the compartments via the selected exhaust ports. Selection of an exhaust port is accomplished by selective actuation of one of only three control valves, one valve controlling flow to the low pressure environment from the first of every three sequentially disposed exhaust ports, a second valve controlling flow to the low pressure environment from the second of every three sequentially disposed exhaust ports, and the third valve controlling flow to the low pressure environment from the third of every three sequentially disposed exhaust ports. A masking disc rotatable with the vane permits only the ports disposed on opposite sides of the vane to. communicate with the chamber at any given time.

This invention relates to accurate positioning devices, and particularly to fluid-operated step motors capable of rapid bi-directional rotational motion.

Conventional hydraulic step motors employ the principle that differential fluid pressures, acting on respective sides of a vane which is free to move relative to the motor housing, may be utilized to move the vane to a pre-determined position. In such a motor, a series of exhaust ports, spaced along the housing wall, are selectively operable to effect a pressure reduction on one side of the vane or the other to create the required differential fluid pressures. When the vane moves into alignment with the selected exhaust port, the port is blocked and the fluid pressures on both sides of the vane become equalized, serving to center the vane on said port.

Devices of the type described above have been found to operate quite accurately, but nevertheless do exhibit certain disadvantages, one of which results from the fact that an individual valve means is required for each of the position-defining exhaust ports. The large number of valves tends to make the motor expensive and large.

The present invention, by requiring only three individual valve means to control all the exhaust ports, substantially does away with the above-mentioned disadvantages. This use of only three valves to control a step motor is achieved by manifolding the ports into three groups with the first of every three sequentially spaced ports connected to a first manifold, the second of every group of three ports connected to a second manifold and the third of every group of three ports connected 'with a third manifold. Each manifold is provided with a valve and the valves are operated in sequence to step the vane. To insure that the vane advances only to the next adjacent port a masking disc is secured to the vane so as to rotate therewith, the disc being such that when the vane is centered over a particular exhaust port, only one port immediately adjacent each side of the vane Patented Nov. 19, 1968 can conduct fluid from the chamber. The masking disc thus permits only one exhaust port at a time to create pressure changes on respective sides of the vane even though one-third of all of the exhaust ports are simultaneously opened.

In addition to the single-step operation made possible by the invention as outlined above, the step motor of this invention is also capable of rapidly moving through a large portion of its cycle by rapid operation of the three control valves in sequence for a pre-determined number of operations, said number being controlled by a counting means.

It is therefore an object of the present invention to provide an improved fluid operated step motor having far fewer control valves than the conventional step motor.

It is another object of this invention to provide a fluid operated step motor which requires only three control valves for normal operation.

It is still a further object of this invention to provide an improved fluid operated step motor employing a fluid mask secured to a rotating vane element and requiring only three control valves to effect a stepping operation.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawing, wherein:

FIGURE 1 is a view in perspective of one embodiment of this invention;

FIGURES 2 and 3 are partial cross-sectional views taken through

lines

22 and 33 of FIGURE 1 and a partial schematic representation of the various controls employed for that embodiment;

FIGURE 4 is a schematic representation of a second embodiment of this invention; and

FIGURE 5 is a schematic representation of still another embodiment of this invention.

Referring specifically to FIGURES 1, 2 and 3, there is illustrated one embodiment of the step motor of the present invention wherein the motor housing 10 comprises cylindrical wall 11 extending between two

circular end walls

13 and 15 so as to form chamber 16 therebetween. Rotatably mounted in the

end walls

13 and 15 and coaxial with the end walls and the cylindrical wall 11 is shaft 17 which is connected to a load (not shown). A sector 19 extends radially between wall 19 and the shaft 17 and axially between the

walls

13 and 15. The sector 19 is rigidly secured and sealed to end wall 15 and cylinder wall 11. A resilient seal 22 may be attached to the end of the sector 19 adjacent rotatable shaft 17. Fluid

pressure inlet ports

33 and 35, open into chamber 16, through cylindrical wall 11 on opposite sides of the sector 19.

Ports

33 and 35 communicate with

fluid output passages

47 and 49 respectively of a valve 51 for connection to an external fluid source, both of which are to be described subsequently.

Rigidly secured to rotatable shaft 17 so as to rotate therewith is vane 23 extending radially from said shaft to the wall 11 and axially throughout the length of chamber 16. A resilient seal 25 may be provided along two 01 the three outer edges of vane 23, said seal engaging end wall 15 and cylindrical wall 11 as shaft 17 rotates. The third outer edge of vane 23 is secured to a rotatable masking disc 27 described in more detail below. Vane 23 thus divides chamber 16 into two fluid isolated compartment: which are each respectively defined by a different side 01 the vane and a different side of sector 19. Each side 0: sector 19 also defines the extreme position for the movement of vane 23. This is achieved in conjunction witl input fluid received by chambers 16 via inlet ports 33 ant 35 which are positioned adjacent respective opposite side:

of sector 19. This fluid is trapped between the respective sector side and the vane 23 as the latter approaches the sector, thereby providing a resilient cushion and avoiding high speed impact between the vane and the sector.

The radial cross-section of the vane 23 may be any appropriate shape such as rectangular, a sector (as best illustrated in FIGURE 2), etc. As will be seen from the subsequent description, however, the width of the vane has certain limitations.

Circularly arranged about the center of end wall 13 and extending axially therethrough are a plurality of exhaust ports (A1, A2, A3, B1, B2, B3, C1, C2, C3, etc.) which are suitable for conducting pressurized fluid out of chamber 16. These exhaust ports may take any convenient shape, however, they must be slightly larger than the arcuate length of the vane 23 in the region of the ports so that a rather small amount of fluid may pass from both sides of the vane through the port over which the vane is to be centered. This permits accurate positioning of the vane 23 since a slight deviation of the vane to one side of a central position over an open exhaust port will cause a pressure decrease on the other side of the vane, thereby destroying pressure equilibrium across the vane and tending to move the vane towards the central position. Further the exhaust ports must be not appreciably larger than the arcuate length of vane 23 in the region of the ports so that the vane can be positioned to substantially block passage of fluid through any individual exhaust port. In addition, the exhaust ports must be spaced such that vane 23 can substantially block only one such port at any time.

Masking disc 27, secured to vane 23 so as to rotate therewith, is circular in shape and is coaxial with the shaft 17. The disc 27 has a radius which is just slightly less than the radius of cylindrical wall 11, and may employ means such as seal 37 to prevent leakage from chamber 16 to exhaust ports A1, B1, C1, etc. Disc 27 passes between sector 19 and Wall 13, a relatively tight seal being required to prevent excessive leakage across the sector 19. To this end there may be provided a radially extending seal 21 secured to the sector 19.

Disc 27 has two cut-

outs

39 and 41 which are situated on either side of vane 23 at a distance from the center of the disc which is substantially equal to the radial distance of exhaust ports A1, B1, C1, etc. from the center of wall 13. Cut-

outs

39 and 41 are slightly larger than one exhaust port opening but not large enough to encompass two such port openings when the vane 23 is positioned to block any single port. As a result of the above structure fluid in chamber 16 normally has access to the exhaust ports only through cut-

outs

39 and 41 in disc 27.

An additional exhaust port 43, provided for reset purposes, extends through wall 11 adjacent to the sector 19. The port 43 is axially aligned with the counterclockwisemost port X1 of the aforesaid exhaust ports.

A fluid pressure source 53 for the motor feeds fluid at some constant pressure to fluid conducting means 54 and thence to self-centering shuttle valve 51. The body of the valve 51 is connected via

variable orifices

52 and 56 to

output passages

47 and 49, respectively, of valve 51 which are connected to supply fluid to input

ports

33 and 35 respectively of the motor 10. Valve 51 comprises a shuttle 65, containing

springs

57 and 59 and respective adjustment screws 61 and 63 for the springs. The compressive forces on the

springs

57 and 59 are normally adjusted such that for equal back pressures in

output passages

47 and 49, the shuttle 65 is located in the center of the valve chamber, thereby providing equal blockage of

orifices

52 and 56. Thus equal impedances to flow are provided between the sources 53 and the

passages

47 and 49. A reduction in back pressure on either end of the shuttle causes it to move towards that end and block the

adjacent orifice

52 or 56, thus permitting fluid from the source to be conducted only to the unblocked output passage (47 or 49) which is at the higher pressure.

A series of fluid conducting means A4, A5, A6, B4, B5, B6, C4, C5, C6, etc. are connected respectively to the exhaust ports A1, A2, A3, B1, B2, B3, C1, C2, C3, etc. through appropriate fittings (not illustrated) at the outer portion of wall 13. As best illustrated in FIGURE 2, the first of every three sequentially spaced exhaust ports has its respective fluid conducting means connected to a first manifold, thus fluid conducting means A4, B4, C4, etc. are connected to manifold path 40. Similarly, the second of every three sequentially spaced exhaust ports has its respective fluid conducting means connected to a second manifold. Thus fluid conducting means A5, B5, C5, etc. are all connected to manifold 50. In like manner, the third of every three sequentially spaced exhaust ports has its respective fluid conducting means connected to a third manifold. Thus fluid conducting means A6, B6, C6, etc. are connected to manifold 60. Each manifold 40, 50, and is provided with a

difierent control valve

44, 55, 66 each of which controllably vents its associated manifold to reference pressure which is lower than the pressure provided by source 53. If the system operates on pressurized air, the atmosphere may be used as the lower pressure.

Valves

44, 55, 66 are individually actuable by controller 67 which may be any appropriate valve mechanism, either manual or automatic. The valves may be of any appropriate type, and may be either mechanically or electrically actuable.

A further fluid conduction means 69 is connected to reset exhaust port 43 by appropriate fittings (not illustrated) in end wall 13. Reset control valve 71 controllably vents fluid in conduction means 69 to some pressure lower than that produced by source 53. Reset control valve 71 is illustrated as controlled by controller 67 which also provides commands for

valves

44, 55, and 66. It is clear, however, that an independent controller may be employed for reset purposes if desired.

In explaining the operation of the above-described embodiment, it will first be assumed that the

control valves

44, 55, 66 and 71 are all closed, thereby precluding existence of any venting path for fluid in chamber 16 on either side of vane 23. Thus the pressurized fluid from source 53 has access to the portions of chamber 16 on both sides of the vane, and the equal pressure in both chamber portions is reflected back through

inlet ports

33, 35 and

outputs

47 and 49 of valve 51 to keep shuttle centrally positioned with respect to the valve cylinder. With the equal fluid pressures existing on both sides of the vane 23, the vane remains in whatever position in which it was previously placed. If it is assumed that this initial position is such that the vane is located at exhaust port C2 as in FIGURE 2, the cut-

outs

39 and 41 of masking disk 27 must assume positions whereby they permit fluid in chamber 16 to have access to exhaust ports C1 and C3, respectively. Since the control valves are all assumed to be closed, the fluid in chamber 16 cannot be vented via ports C1 and C3.

Next let it be assumed that control valve 55 receives a command from controller 67 to open. With valve 55 opened exhaust ports A2, B2, C2, etc. are vented; however masking disk 27 prevents all of these particular ports except C2 from venting fluid from chamber 16. C2 is substantially blocked by vane 23 which is centered thereover; thus no appreciable path exists for the fluid in chamber 16 on either side of vane 23 and the pressure on both sides remains equal. This equal pressure at both

output passages

47 and 49 of valve 15 keeps shuttle 65 centered and

orifices

52 and 56 partially and equally blocked. Vane 23 therefore remains stationary in its illustrated position.

Next assume that valve 66 is open,

valves

44, 55 and 71 being closed. Under such conditions exhaust ports A3, B3, C3, etc. are vented, but all but C3 are blocked by masking disc 27. Since C3 is positioned under cut-out 41 in the disc, it is able to vent the pressurized fluid from that portion of chamber 16 to the right of vane 23. Since the pressure on the left side of the vane remains unchanged, a pressure diiferential is created across the vane tending to move shuttle 65 to the right to further block orifice 56 and further open orifice 52. In addition, the higher pressure at the left side of vane 23 moves the vane clockwise towards exhaust port C3. As the vane begins to block port C3 the pressure to the right of the vane begins to build up, thereby increasing the back pressure at valve output passage 49. This causes shuttle 65 to gradually return to its central position so as to gradually increase fluid flow to the right side of the vane. This proportioning feature of the shuttle valve tends to prevent overshoot of vane 23 past the open exhaust port. Once the vane substantially blocks the exhaust port C3, the pressure across the vane is equalized since there is no further appreciable exhaust path from chamber 16. Vane 23 thus remains stationary, and shuttle 65 remains centrally positioned over exhaust port C3.

By similar operation, the motor may be stepped in the counterclockwise direction. Assume, for example, that the vane is initially positioned over port C2 and that all

valves

44, 55, 66, and 71 are closed. If now valve 44 is opened so as to provide a vent to ports A1, B1, C1, etc. it is apparent that of these only C1 will not be blocked by disc 27, cut-out 39 being positioned thereover. The pressure on the left side of vane 23 decreases due to this vent path at port C1 while the pressure on the right side of the vane remains unchanged. The resulting pressure differential moves shuttle 65 to block orifice 52 and moves the vane counterclockwise until it is positioned over port C1. As discussed above, shuttle 65 gradually moves back towards its central position as the vane begins to block port C1, thereby providing a gradual flow increase to the left side of chamber 16. The gradual flow increase prevents vane 23 from shooting past port C1 and thus provides a stabilizing effect. Once the vane sub stantially blocks C1, masking disk 27 blocks all other exhaust ports except B3, C1, C2 and 43 which are respectively prevented from venting chamber 16 by closed valve 66, vane 23, closed valve 55, and closed valve 71. The vane remains stationary due to the equal pressures thereacross, and shuttle 65 returns to its central position.

For any position of vane 23, if valve 71 is opened and

valves

44, 55, and 66 are closed, vane 23 will be moved to reset position adjacent abutment member 19 and in blocking relation to exhaust port X1. This occursbecause any time valve 71 is open, chamber 16- is vented via reset port 43, fluid path 69 and valve 71. Since no other valves except 71 are open, the pressure on the right side of vane 23 exceeds that on the left side and the vane is forced into counterclockwise movement until it is centered over vent passage through port 43. It should be apparent that the reset position need not be located at an end of the path travelled by vane 23, but may be located anywhere along said path by appropriately positioning reset port 43.

It is also possible for vane 23 to be moved rapidly between any two non-adjacent exhaust ports by sequentially activating

valves

44, 55, and 66 until the vane moves the required number of steps to the desired location. This may be done automatically by conventional electronic counting circuitry, or by conventional controls, either or both of which may be designed into controller 67. In the electronic type controller, it is evident that conventional computer circuitry may be employed to program any desired sequence of positional commands to control valves.

The motor of the present invention is particularly useful in high speed electric typewriters or printers for stepping a carriage at rates in the order of 100 steps per second. Such a typewriter is described in copending patent application Ser. No. 501,060 filed on Oct. 22, 1965 in the name of Ralph P. Means and assigned to the same assignee as the present invention.

In a system of this type an electronic counter may I stepped each time a symbol is received and the valve 7 may be activated each time a carriage return signal received.

FIGURE 4 illustrates another embodiment of the ir vention is which the exhaust ports are located in cylir drical wall 111 rather than in circular end wall 113 a above. In this embodiment the ports A1, A2, A3, B1, B1 B3, C1, C2, C3, etc. are of any convenient shape, thei size being limited by the fact that the outer edge of van 123 must be slightly smaller than any one port opening These ports are circularly arranged along cylindrical waI 111 and are connected according to position in alternat groups of three as above to respective manifolds 1411 150, and 160. Pressures in these manifolds are controller by

control valves

144, 155, and 166, respectively, which it turn are responsive to commands from controller 167 ii a manner identical to that described for the embodimen illustrated in FIGURE 2. Pressure source 153 and valvl 151 also operate as described above in providing pres surized fluid to chamber 116 via fluid inlet ports 133 ant 135 located in abutment member 119. Similarly, rese control valve 171 and its associated fluid path 169 operah to reset the vane 123 by venting fluid from chamber 11( via reset exhaust port 143. The reset exhaust port 143 i: illustrated as being located in abutment member rather than in end wall v113 as above, but it is to be understood that any appropriate location may be utilized.

An annular mask 127 of this embodiment is cylindrical rather than circular as in the previous embodiment, sc as to conform to the shape of wall 111 in which the exhaust ports are located. The annulus 127 is secured to the vane 123 at that edge of the vane which communicates with Wall 111. It should be noted that for this embodiment, abutment member 119 is mounted to the two end walls (not illustrated) of the housing, not to cylindrical wall 111 as in the previous embodiment in order to permit the mask to revolve between abutment member 119' and wall 111. Cut-

outs

139 and 141 in the mask 127 permit only the two exhaust ports immediately adjacent vane 123 to be accessible to fluid in chamber 116. Operation of this embodiment is similar to that of the FIGURE 2 embodiment. Opening any of the

control valves

144, 155, or 166 creates a pressure imbalance across vane 123 until the vane is moved by said pressure to cover the appropriate venting exhaust port. Annular mask 127 prevents more than one of the plurality of exhaust ports associated with each control valve from venting chamber 116 at any one time.

FIGURE 5 illustrates still another embodiment of this invention wherein the exhaust ports are arranged in a straight line as opposed to a circle. Housing 210 may be either cylindrical or box shaped. Piston 223- is shaped to conform to the cross-sectional shape of the housing and thus divides chamber 216 into two fluid-isolated compartments. Exhaust ports A1, A2, A3, B1, B2, B3, C1, C2, C3, etc. are arranged in a straight line in wall 211 and may be of any size or shape as long as they are slightly larger than the edge of piston 223. Masking plate 227 is contoured to the shape of wall 211 and is secured to the portion of the edge of piston 223 which engages the exhaust ports. Mask 227 is of such size and shape that it covers all exhaust ports A1, A2, A3, B1, B2, B3, C1, C2, C3, etc. except the two immediately adjacent the piston position, these being coincident with cut-

outs

239 and 241. Abutment member 219 is located to prevent piston 223 from moving beyond a pre-determined reset position when reset control port 243 is actuated by valve 271. The operation of this embodiment is similar to the operation of the two embodiments described above, with the control valves causing stepped lateral movement of piston 223 and shaft 217 along the longitudinal axis of the shaft rather than the rotational motion of the vane and shaft created in the previous cases.

It is clear from the above description that this invention provides a novel means for reducing the size, cost and mplexity of fluid-drive step positioning devices. Spelically, this result is accomplished by reducing the num- :r of control valves from one per port to a total of three, 1d by grouping the exhaust ports such that one of ery three ports is controlled by a common valve. The asking means is employed to ensure that only the ports ljacent to the wane or piston are eflecti've to produce raft motion even though one third of all the ports are :tuated simultaneously.

It is to be understood that appropriate fluid sealing teams are to be employed at the various joints throughut the above embodiments so as to prevent undesired uid leakage paths. In addition, various other details nd modifications are deemed to be within the scope of 1is invention. For example, the invention need not be mited to the particular shuttle valve illustrated in FIG- ]RE 2, but may take the form of any two-output fluid .evice which blocks one or the other output path as a unction of pressure differential in said paths. Further, he fluid medium to be employed may be air, oil, or any )ther fluid, and the materials used for the various elements may be suitable metals, plastics, and the like.

While we have described and illustrated one specific :mbodirnent of our invention, it will be clear that variaion of the details of construction which are specifically llustrated and described may be resorted to without deaarting from the spirit and scope of the invention as deined in the appended claims.

We claim:

1. A fluid-operated step-positioning device, comprislng:

housing means forming a chamber and having a plurality of exhaust ports disposed along a determinable path in at least one of its walls;

movable means within said chamber and dividing said chamber into two substantially fluid-isolated compartments;

means producing equal fluid pressures in said compartments and across said movable means for holding the latter stationary;

means producing a fluid pressure diflFerential across said movable means for moving the latter along said determinable path in the direction of least pressure to block a selected one of said exhaust ports, said means producing a fluid pressure differential comprising said plurality of exhaust ports, three control valve means for selectively venting fluid from said chamber via said selected one of said plurality of exhaust ports, and fluid path means connecting the first of every three sequentially positioned exhaust ports to a first of said three control valves, the second of every three sequentially positioned exhaust ports to a second of said three control valves, and the third of every three sequentially positioned exhaust ports to a third of said three control valves;

means for selectively controlling said three control valves;

masking means rigidly secured to said movable means for permitting substantial amounts of fluid to be vented from said chamber only via the two exhaust ports which :are at any time positioned on opposite sides of and immediately adjacent to said movable means.

2. The device of claim 1 wherein said equal pressure producing means comprises:

a source of fluid pressure;

a fluid inlet means for each of said compartments;

self-centering shuttle valve means having an input and two outputs and for providing equal pressure at its two outputs when said outputs are equally loaded but zero pressure at one of its outputs when its two outputs are loaded substantially unequally;

means connecting said source to said valve input and each of said valve outputs to respective ones of said fluid inlet means.

3. The device of claim 1 further comprising reset means for positioning said movable means to some pre-determined initial location along said determinable path.

4. The device of claim 1 wherein said housing is of cylindrical shape, wherein said movable means comprises a vane secured to a shaft which is movably mounted along the longitudinal axis of the cylinder, and wherein said masking means is secured to the edge of the vane which traverses said determinable path.

5. The device of claim 4 wherein said housing means includes an abutment member for determining the extreme positions of said vane.

6. The device of claim 5 wherein said exhaust ports are disposed in a circular path along an end wall of said cylindrical housing and wherein said masking means is a circular disk having two cut-outs therein, each of said cut-outs being large enough to expose only one exhaust port and each being situated immediately adjacent to and on opposite sides of said vane.

7. A device of claim 6 further comprising reset means for moving said vane to some pre-deterrnined initial position, said reset means comprising:

an additional exhaust port in a cylindrical wall of said housing;

reset control means for selectively venting said chamber via said additional exhaust port.

8. The device of claim 6 further comprising reset means for moving said vane to some pre-determined initial po sition, said reset means comprising:

a reset exhaust port located in said abutment member;

reset control means for selectively venting said chamber via said reset exhaust port.

9. The device of claim 5 wherein said exhaust ports are disposed in a circular path around the cylindrical wall of said housing and wherein said masking means is cylindrical in shape with two cut-outs therein, each of said cut-outs being large enough to expose only one exhaust port and each being located immediately adjacent to and on opposite sides of said vane.

10. The device of claim 9 further comprising reset means for selectively venting said chamber so as to return said vane to a pre-determined initial position.

11. The device of claim 1 wherein said movable means comprises a piston and said housing comprises a cylinder for said piston, wherein said exhaust ports are disposed on the cylinder wall parallel to the longitudinal axis of the cylinder, and wherein said masking means comprises a plate contoured to the shape of said cylinder wall and having two cut-outs therein, each of said cut-outs being shaped to expose only one exhaust port and each being situated immediately adjacent to and on opposite sides of said piston.

12. The device of claim 11 further comprising reset means for selectively venting said chamber so as to return said piston to a pre-determined initial position.

13. The device of claim 4 wherein each of said exhaust ports is slightly larger than said edge of said vane which traverses said determinable path.

References Cited UNITED STATES PATENTS 1,004,541 9/1911 Martin 91-357 2,171,005 8/1939 McNeil et al 91-357 2,398,997 4/1946 Berry et al. 91-357 2,954,755 10/1960 Pecchenina 91-357 3,058,450 10/1962 Lissau 9l357 PAUL E. MASLOUSKY, Primary Examiner.