US2570624A - Follow-up pneumatic servomotor - Google Patents

Description

Patented Oct. 9, 1951 UNITED STATES v PATENT OFFICE FOLLOW-UP BNEUMATIC SERVOMOTOR Ralph D. Wyckoi'f, Pittsburgh, Pa., a'ssignor to Gulf Research & Development Company, Pittsburgh, Pa., a' corporation of Delaware Application November 19, 1946, Serial No. 710,745

' Claims. (Cl. 121-41) This invention relates to actuating devices and -to controls therefor.

An object of the invention is to provide an actuating device, which, under the influence of a source of power, moves at a predetermined speed.

Another object of the invention is to provide an actuating device, which, under the influence of a motivating force, may move at any selected vide a servomotor including a power cylinder with a pneumatically actuated piston movable therein to drive a power shaft, and having valve means for regulating the flow of compressed gas into the cylinder to drive the piston, with L 'iflce means cooperatively disposed relative to the exhaust of said cylinder to regulate the rate of exhaust therefrom and accordingly to regulate the speed of movement of the piston and its power shaft.

Stillanother object of the invention is to provide a pneumatic actuator system which is capable of typical servo action, and which, by means of a critical orifice device incorporated therein, is capable of attaining substantially constant though adjustable speed of operation over very wide ranges in load and gas supply pressure.

Another object of the invention is to provide a pneumatic actuator device having characteristics closely simulating a constant volume hydraulic system without the use of complex speed control devices, such devices being replaced by a simple orifice which may be adjusted at will.

- Another object of the invention is to provide a pneumatic actuator in which the rest position of the operating piston may be controlled at will.

A further object of the invention is to provide a constant speed pneumatic actuator device which may be employed for the control of elevator and rudder flaps on dirigible high-angle bombs, where the hinge moments are of such magnitude as to preclude the use of electric 'motor drive due to excessive size and other considerations.

Other objects and advantages of the invention will become apparent from the following description of a preferred embodiment thereof as illustrated in the accompanying drawing, and in which,

Figure 1 is a sectional view of a pneumatic actuator according to the invention, showing in the lower portion thereof an actuator cylinder, piston and piston rod, and in the upper portion manifold with its associated disc and regulating orifice,

Figure 3 is an enlarged sectional detail view of a portion of the exhaust manifold with alternative type of orifice which may be employed, and

Figur 4 is a sectional view of the right hand endof the valve'section of Figure 1 when arranged for servo operation.

Pneumatic actuators of the type including a cylinder having a piston driven shaft, are-employed in many and quite diverse applications. One such application is the type of pneumatic actuators particularly intended for the control of elevator and rudder flaps on dirigible highangle bombs, where the hinge moments are of such magnitude, that is, of the order of 1000 pounds maximum thrust per pair, as to preclude the use of electric motor drive due to the excessive size of gear reducers and internal battery power requirements, as well as other considera- 1710118. I Y I Insuch a dirigible high-angle bomb, the only practical source of self-contained motivating power comprises gas, stored under pressure in suitable tanks. ,A further requirement of the actuator for the rudders and elevatorsis that their rate of angular deflection. should at no timeexceed a specified value which is of the order of ,10 degrees per second, nor should-the spee d- ,'of operation fall below this specified rate. vThese requirements must, according to some specifications, be fulfilled regardlessof the fact that the load on the actuator, that is, the hinge moments due to aerodynamic forces on the rudders and elevators, will increase by a factor approximately in the ratio of 25 to 1 at the end of the flight of the bomb as compared with the aerodynamic forces involvedat release of the bomb.

Evidently such operation is characteristic of a hydraulic system in which the actuator cylinder is fed by aconstant volume pump, but is the antithesis of the operation characteristic of normal pneumatic systems. A further and quite importantrequirement of the actuator system is the greatest possible simplicity so as to eliminate excess apparatus and all possible sources of failure.

The above is a summary of the problems to be solved in one particular type of application of the present invention, but it will be understood that such summary is only by way of example, and that the invention may be employed in many types of applications which will readily occur to those skilled in the art.

Reference may now be had to the drawings in which like reference characters indicate similar parts throughout the several views. Figure 1 shows aconventional cylinder 2 held betweensuitable heads 4 and s by through-bolts 8. Opcrating within the cylinder chamber In is piston l2 carrying a conventional packing ring l4 constrained within a suitable circumferential groove l6 as is customary. The piston I2 is mounted upon a continuous piston rod l8 in such manner as to preclude leakage between the rod and the piston. Rod I8 operates through thecylinder heads 4 and 6, with suitable packing members and 22 interposed inapertures 24 and 28 respectively in the cylinder heads. The piston rod I8 is terminated at each end by a suitable clevis arrangement at 28 and 30 whereby the load member may be attached, to be moved in one direction or the other 'as the piston I2 is reciprocated within the cylinder chamber l0.

Suitable ports 32 and 34 are provided in each cylinder head, to which are attachedpipes 36 and 38 respectively, for conducting gas into and out of the cylinder, as the piston I2 is thus driven. The actuator cylinder is hence a conventional double-ended piston rod device, which, as gas is admitted throughport 34, causes the piston l2 to move to the left, the leftward movement of the piston forcing any gas remaining in the cylinder chamber between the piston and theleft cylinder head 4 out throughport 32 and into thepipe 36. Similarly, when gas is admitted to the cylinder through theleft hand port 32, the piston 12 will travel to the right, forcing an gas remaining in the cylinder chamber between the piston and theright cylinder head 6 out throughvport 34 and into thepipe 38.

It is thus apparent that thepipes 36 and 38 may serve as both gas inlet and gas exhaust pipes depending upon the desired direction of movement of the piston l2 and the piston rod l8. And it is also apparent that by means of a suitable valve, illustrated in the upper portion of Figure 1, the direction of movement of gas into and out of thecylinder 2 through thepipes 36 and 38 may be controlled at will.

As shown, there is avalve block 40 which com prises a fixed mounting for the entire valve assembly. Theblock 40 has avalve cylinder 42 extending therethrough, thepipes 36 and 38 extending through theblock 48 and terminating in two accurately spacedvalve ports 44 and 48 respectively leading into the interior of thecylinder 42. Twoexhaust ports 48 and 50 lead topipes 49 and 5| which are connected together by apipe 52, terminating in an orifice employed herein. One form of critical orifice is shown in detail in Figure 2, and an alternative form in Figure 3. A supply pipe I0 supplies gas under this critical value.

4 sure equalization characterizing a balanced valve system.

With this valve arrangement, when the valve rod is moved to the right of center, as is shown in the view, high pressure gas from asupply pipe 10 enters through the central section port 12, and flows between thepistons 88 and 82, thence throughport 46 toactuator conduit 38. At the same time,conduit 38, throughvalve port 44, is open toexhaust chamber port 48, to manifold 68, throughcritical orifice 56, to atmosphere.

Obviously, the connections ofconduits 36 and 38 are reversed when thevalve stem 58 is moved to the left of center. In the central portion of thevalve stem 58, bothvalve ports 44 and 46 are closed, and no gas entersconduits 36 and 38 except leakage due to valve clearance. However, as will be seen below, such leakage becomes of no consequence as regards drift of the actuator piston position, and in fact for certain purposes some leakage may be desired.

Attention may now be invited to the critical dimensions of theorifice 58 and the contiguous conduits.

In the flow of gas through a sharp-edged orifice such as.56 in Figs. 2 and 3, the mass rate of flow will increase approximately as the square root of the pressure drop across the orifice until the linear velocity of flow, such as in feet per second, reaches the velocity of sound. The weight of gas flow through the orifice varies linearly with the pressure difference across the orifice when the pressure difference is such as to exceed In this critical flow region the mass of exhausted gas is substantially proportional to the pressure difierence across the orifice. Because of the mass-volume relationship of gases this effect results in a. volume rate of efllux through the orifice which is substantially constant as measured at the high pressure side of the orifice. Neglecting certain modifying details it may be said that in the region beyond the critical point the pressure difference across the orifice may be increased many fold without increase of the volume rate of efllux of gas through the orifice, measured as volume of gas at the high pressure side of the orifice. Moreover, the pressure difference required to reach the critical value is relatively small. It is well known that if P1 is the pressure at the input and P: the pressure at the output the critical velocity maximum efllux rate is reached when the ratio P2/P1 is approximately 0.5. In the case of a sharp-edged orifice discharging gas into the atmosphere at pressure P2=15 pounds per square inch, a substantially constant rate of discharge expressed as volume per second at pressure P1 is reached when P1 is equal to or greater than about 30 pounds per square inch.

My invention makes use of the above phenomena in pneumatic servo mechanisms for the attainment of constant rate or constant velocity of operation. Thus a pneumatically driven piston l2, such as shown in the actuator of Fig. 1, may be made to operate at a substantially constant velocity provided the gas in the end of the cylinder l8 exhausting to the atmosphere is at a pressure P1 equal to or greater than 30 pounds per square inch, and further provided the exhaust is effected through a thin-plate sharpedgedorifice 56 of small enough diameter relative to other conduits so that it alone provides substantially all of the throttling effect. Under these conditions there is constant volume rate of efllux at the pressure P1 exhausting from cylinder I0, and such constant volume rate of e-fiiux results in a constant velocity of motion of piston I2.

The requirement that the exhaust pressure P1 within thecylinder 2 is always greater than the required critical pressure of 30 pounds per square inch, is attained by making certain that the actuating pressure P3, that is, the pressure of the gas entering through pipe I0, is always greater than the equivalent load pressure (P4) plus 30 pounds. Thus, as an example in the case of the actuator illustrated:

Maximum load- 1000 pounds Piston I2 area-: square inches Equivalent load pressure=1000/15=66.6 lbs/sq.

Required P3 66.6+30=96.6 lbs/sq. in. (approx.)

Hence the operating pressure P; may be adjusted say to 100 lbs/sq. in., since the 1000 pounds maximum load pressure on rod I8 was based on a reasonable though small factor of safety, whereupon the piston I2 will operate at constant velocity at all loads up to maximum. Furthermore, the supply pressure Pa may exceed 100 lbs/sq. in. or may fluctuate in value above 100 lbs;/sq. in. without affecting the constant velocity operation of piston I2.

By this expedient is attained a pneumatic actuator having characteristics closely simulating a constant volume hydraulic system without the use of complex speed control devices, such devices being replaced by a, simple orifice which may be adjusted at will. The important requirements to assure properoperation of thecritical orifice 56 are. (1) the circular orifice must be'a sharp-edged hole in a thin plate, and (2), that it be placed in anentrance box 68 whose inside diameter is not less than about 15 times the diameter of theorifice 56. and (3) that there should be no exit nozzle following the orifice.

Figs. 2 and 3 show examples of a thin-plate sharp-edgedorifice 56 which may be used. In Fig. 2 a cover plate.54 covers the opening of theexhaust manifold 68. Theorifice 56 comprises a tapered opening having a sharp edge on the inner side and flared outward at a large angle. The same effect may be attained by means of the orifice shown in Fig. 3 in which thecover plate 54 is reduced in thickness over a central portion 15 in which theorifice 56 is located. It has been found that an orifice in the portion .15 behaves practically as a sharp-edged orifice if the portion I5 is of a thickness'0.032 inch or le s. In both Figs. 2 and 3 the inside face ofplate 54 should be substantially unbroken.

The rate at which the actuator piston operates may be controlled by the size of theopening 56. To this end thecover plate 54 with its orifice may be removably attached to the manifold 66 by means of screws I32 which clamp the cover plate to flange I33 ofmanifold 68. An orifice plate having the desired flow rate value may thus be attached tomanifold 68.

The means for actuating the valve mechanism and the actuator piston may now be further described. Both ends of thevalve rod 58 are provided with extensions to which are pinned soft iron plungers I4 and I6 which operate slidably within solenoid coils I8 and 80 respectively. Theouter iron shells 82 and 84 serve as the outer magnetic circuit for the solenoid: as well as protective housings; and provide mountingsleeves 66 and 88 whereby the solenoids are accurately aligned concentric with the axis of the valve cylinder.

The inner shell of each solenoid comprises a thin brass tube 90 which prevents iron to iron contact and consequent sticking in the annular air gap between theplungers 14 and I6 and the concentric pole face comprising the inner ends of the solenoid shell. At the outer end of each solenoid shell the magnetic circuit is completed by a pole piece comprising aniron core 92 and 94, having a smoothly sliding fit within the inner brass cylinders and fixedly attached to brackets which in turnare rigidly attached to guide 96, which slides smoothly inguide hole 98 bored through thevalve block 40 with its axis parallel to the axis of thevalve cylinder 42.

As is customary in solenoid design to improve the traction curve, theplungers 14 and I6 andpole pieces 92 and 94 are provided with conical faces and non-magnetic spacers I00 to prevent sticking.Pole pieces 92 and 94 are also provided with axial holes I02 thro gh which non-magnetic valve rod extensions I04 and I06 of therod 58 pass freely; The extensions I04 and I06 are adjusted to such length after assembly that the terminus of each is exactly flush with the outer ends of thepole pieces 92 and 94 and hence cen-v tered between the guide-rod end posts. Centering springs I08 and H0 are attached by screws H2 and bear on the extensions I04 and I06 with suflicient contact pressure to hold thevalve stem 58 firmly centered between the guide-rod posts when the solenoid coils are not energized. With the valve rod so centered and restrained by springs I08 and H0, thevalve rod 58 will partake of any motion of guide-rod 96 if the spring tensions are sufficiently great with respect to friction between the valve and the valve cylinder. With the valve guide-rod 96in centerposition pole pieces 92 and 94 will be symmetrically located in their respective solenoids. If solenoid coil be then energized through wires II4, it is evident that the plunger I6 will overcome the thrust of spring IIO on extension I06, with the result that thevalve port 46 leading toconduit 38 will be supplied with gas pressure while at the same time.conduit 36 will be opened to exhaust throughports 44, 48,conduits 49, 52 andorifice 56. This will urge piston I2 to' the extreme left position and the rate at which piston I2 travels will be dependent only on the size of theorifice 56, provided the supply pressure in. I0 is higher than the required critical pressure P3 determined as previously stated.Orifice 56 may of course be made of such size as to give the desired rate of operation.

Similarly, with valve guide-rod 96 centered. the energization ofsolenoid coil 18 through wires II6 will reverse the valve action. And, clearly, when both solenod coils are de-energized, the springs I08 and H0 will return thevalve stem 58 and itsdiscs 60, 62, 64 and 66 to center.

Control of the valve action other than by solenoid means may be rendered possible by moving oif center the valve guide-rod 06, which carries the entire valve rod assembly. Thus rod 96may be provided, for example, with a threaded extension I I8 on which rotates a threaded adjusting nut or pulley I20 confined longitudinally ator other than merely urging the piston I2 to extreme right or left rest positions as described above.

Referring now to Figure 1, in order to obtain servo-type action of the actuator, it is necessary that the motion of the piston rod I8 be transmitted back in a proportional manner to the guide-rod 96 of the valve assembly. An expedient manner of doing this in the present instance is by means of a suitable flexible cable I26 from attachment at a point on the left end of piston rod I8, over suitable guide pulleys I21, around pulley wheel I20 of the valve assembly, and back via return section of cable I28, pulley I30, to the right end of the piston rod I8. The cable is adjusted to be free of slack motion and may have customary tension adjustment or slack removing means not shown.

Any motion of the piston rod I8 will be transmitted to the guide-rod 96 of the valve assembly by means of the cable I26-I28 running over V threaded pulley I20, and thence to thevalve rod 58 in the same sense, that is, a leftward motion of the piston rod I8 will move thevalve rod 58 to the left by virtue of the particular arrangement illustrated; And the relative magnitude of motion of the piston rod I8 and thevalve rod 58 will be determined by the diameter of the pulley I20 and'the pitch of the threads on the .end II9 of the guide rod extension. The determination of the proper pulley size for a particular case will become evident.

There may now be considered the case, by way of example, in which it is desired that the neutral position of the actuator piston I2 should be the midpoint of its stroke. Cable I28I28 may then be adjusted so that with solenoids deenergized thevalve rod 58 is centered, that is, itsdiscs 60 and 62cover ports 44 and 46, when the actuator piston I2 is in the desired center position. This is ascertained best with the apparatus in operating position. If now, for ex ample, theright solenoid coil 80 is energized from wires H4,port 46 will be opened and high pressure gas from input pipe I0 will flow viaconduit 38 to move piston I2 to the left.

However, at the same time, because of the mechanical coupling between the actuator and the valve, thevalve rod 58 will also be carried to the left, gradually closingports 44 and 46 until whenvalve pistons 60 and 62 symmetrically cover theirrespective ports 44 and 46 the actuator becomes locked in the corresponding position, which, by proper adjustment of the coupling ratio, may be at the end of its maximum possible stroke as limited by the length of the cylinder. Now, ifsolenoid 80 is de-energized, spring IIO forces thevalve rod 58 to center with respect to the valve guide posts onrod 96. Whereupon, since the valve guide assembly is now displaced to the left of the valve-port center,port 44 will be uncovered to high pressure gas from inlet pipe I0, andport 46 to exhaust, thus returning the actuator piston I2 to the right and carrying the valve guide also to the right. When the actuator has reached center, the valve guide will be found centered with respect to the valve ports and the actuator piston will lock in neutral position. Any tendency to drift from proper neutral, as by variation in loading or leakage at the valve-will be corrected immediately by'valve action. By making the valve-overlap on the valve port small, the accuracy of position lock-in will be enhanced.

Having thus described the functional arrangeend of the valve assembly of Figure .1.

ment of the valve and actuator, it will be evident that variations in design and functioning are possible. Thus, instead of a solenoid arrangement which is obviously a full-right, full-left control, one might provide for any desired type of stroke. Thus a screw adjustment may be used as in Figure 4 which shows the right hand Like parts of Figure 4 carry the same numerals as in Figure 1. The solenoid (of Figure 1) is replaced by a sleeve I90 pressed into the end 01' thevalve cylinder 40. The valve stem I08 may be threaded and provided with a keyway I96 into which projects,a pin I92 to prevent its tuming.Nut 200, retained betweenguide rod post 94 and its arm I94 will permit displacement of the valve rod irom central position between posts 94 and 92 (Figure 1). The left hand end of the valve system may be similar to Figure 4 except that the rod I06 need not be threaded or keyed and no nut would be provided at this end.

Such displacement would be proportionately repeated in the power stroke of the actuator piston I2 and its position held under load. Thus the system is capable of typical servo action under control ofnut 200 or any other control means by which this nut may be actuated. Moreover, by virtue of the critical orifice device, illus trated in detail in Figure 2, substantially constant though adjustable speed of operation may be obtained over very wide ranges in load and 7 gas supply pressure. Evidently bell-crank and variable linkage couplings may be used to replace the flexible cable and pulley arrangement illustrated.

Although I have described a preferred embodiment of the invention in specific terms, it is to be understood that various changes may be made in size, shape, materials and arrangement without departing from the spirit and scope of the invention, as claimed herein. Furthermore, my invention may be employed for a variety of uses other than that of actuating the control surfaces of a dirigible bomb as mentioned, examples of such uses being automatic door closers, brake mechanisms, ships rudder controls, throttle and valve controls, and many others in which constant speed of operation is desirable.

What I claim as my invention is:

1. A pneumatic actuator comprising a cylinder having gas inlet and outlet ports, a piston movably disposed in said cylinder for movement under the influence of gas admitted to said cylinder through said inlet port, a gas outlet duct connected to said outlet port, a thin disc closing said outlet duct and having a sharp-edged circular orifice formed in said disc for regulat-.

ing the speed of movement of said piston by regulating the speed of exhaust of gases to atmosphere therethrough, the inside diameter of said outlet duct being at least fifteen times the diameter of the orifice, and means for supply ing gas to the inlet of the actuator at a pressure higher than that necessary to produce criti-.

connectable at its outer end to a source of gas under pressure which is at least 30 pounds per square inch in excess of that required to move said piston under maximum load, a gas exhaust manifold for conducting exhaust gas from said cylinder to atmosphere, critical orifice means disposed in the path of gas exhausting from said manifold to regulate the rate of exhaust thereof, said critical orifice means including a thin disc closing the outlet end of said exhaust manifold and having a sharp-edged circular orifice formed in said disc, the inside diameter of the outlet end of said manifold and of said thin disc being about fifteen times the diameter of the orifice, and valve means for connecting said gas inlet duct to said first conduit andfor connecting said gas exhaust manifold at its inner end to said second conduit whereby gas is admitted into said cylinder for driving said piston, said valve means also being so constructed and arranged as to connect said gas inlet duct to said second conduit and for connecting said gas exhaust manifold at its inner end to said first conduit for driving said piston in the reverse direction.

3. The construction according toclaim 2, characterized further in that said valve means comprises a housing having a valve cylinder formed therein, a valve rod movable in said cylinder, spaced valve discs carried by said valve rod and slidable in contact with the cylinder walls, said cylinder having first and second ports communicating respectively with said first and second conduits, said cylinder having third and fourth ports communicating with said exhaust manifold, said cylinder having a fifth port communicating with said gas inlet duct, said valve discs being so arranged as to be slidable to effect communication between said fifth and second ports for admitting gas into said drive cylinder for moving said piston in one direction and at the same time to effect communication between said first and third ports to allow gas under the infiuence of said drive piston to exhaust from said drive cylinder into said manifold and through said orifice under conditions of critical flow, said valve discs being so arranged as to be movable also to effect reverse motion of said drive piston.

4. The construction according toclaim 2, chartherein, a valve rod movable in said cylinder,

spaced valve disc'carried by said valve rod and slidable in contact with said cylinder walls, said cylinder having first, second, third, fourth and fifth ports communicating respectively with said first and second conduits, said exhaust manifold, and said gas inlet duct, whereby upon appropriate movement of said valve rod the drive piston may be moved in either direction as desired, solenoid means for moving said valve rod in either direction as desired, and automatic means for returning said valve rod to a predetermined rest position upon de-energization of said solenoid means.

5. A pneumatic actuator comprisin a drive cylinder having first and second ports, a drive piston movable in said cylinder under the infiuence of gas admitted to the cylinder, a piston rod movable by said piston for driving a load, a

source of compressed gas connectable to said cylinder for admitting gas therein through one of said ports, a thin disc-like member closing the outlet from the other of said ports, said disclike member having a sharp-edged orifice formed therein for regulating the speed of exhaust gas moving therethrough to atmosphere under the influence of said moving drive piston, the inside diameter of said outlet port and said disc-like mem-. her being at least about fifteen times the diameter of the orifice, the pressure of said gas admitted'to said cylinder being sufi'iciently greater than the equivalent load pressure required to drive said load as to cause the pressure of exhaust gas in said outlet port on the high pressure side of .said

acterized further in that the valve means comprises a housing having a valve cylinder formed orifice to be at least double the pressure on the low pressure outer side of said orifice, whereby the speed of movement of said drive piston may be regulated to remain at a substantially constant predetermined level.

RALPH D. WYCKOFF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS.

Number Name Date 526,930 Maxson Oct. 2, 1894 913,853 Rowntree- Mar. 2, 1909 2,288,297 Naiman June 30, 1942 2,410,967 Eaton Nov. 12, 1946

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