US2930360A - Servo mechanisms - Google Patents

Description

March 29, 1960 s. YANDo sERvo MECHANIsMs 2 Sheets-Sheet 1 Filed March 28, 1958 A Il; Il!! Iii. Illlllllultdli rillllllillIlllll IN V EN TOR. STEPHEN YANDO ATTORNEYS.

March 29, 1960 s. YANDo sERvo MEcHANrsMs 2 Sheets-Sheet 2 Filed March 28. 3.958

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a AKI )N1/wrox STEPHEN vANDo ATTORNEYS.

Unite States Patent SERV() MECHANISMS Stephen Yando, Huntington, N.Y.

Application March 28, 1958, Serial No. 724,683

Claims. (Cl. 121-41) This invention relates to servo mechanisms. Servo mechanisms are designed to make large forces and massive equipment sensitively and accurately responsive to light control forces. They have many valuable uses including, for example, power brake and power steering `strongly driven by the fluid actuator in either of two l opposite directions from a zero position, to assume a position uniquely characteristic of the measure and sense of the input control voltage supplied from a control station. The output member is accordingly arranged to control a feedback yvoltage unit in such a wayl that the feedback voltage is always opposite in sense to the input voltage and is cau-sed to increase progressively in absolute value as the displacement of the output member from the zero position increases, until the feedback voltage cancels out the input` voltage, causing the net voltage applied tothe electromagnetic controll member to be zero, and thereby causing the valve to be restored to the neutral condition in which fluid can neither enter nor leave the fluid actuator. kind involves a number of distinct operating units of consid'erable aggregate weight and cost, each subject to its owninii'rmities, and a rather complex and elaborate circuitry.

It is the primary object of the presentinvention to provide a servo mechanism, capable oi?Y performing the services outlined above in an improved way, which mechanisni is reduced to a very simple electrical" or other etiective control means,` together with a single composite unit comprising a control valve, a power` actuatorl and an output member.

Important advantages of the invention reside in the reduction of cost due to simplicity of organization, parts and assembly, reduction'ofY space required, lightness of construction adapting the mechanism to be airborne, ruggedness, dependability, precision of response; andsta.- bilityV under load. f

It is an important objectA ofthe invention to providea feedbacksystem which acts, not to eliminate the appli cation of the yielding control force to the valve when the desired position of the outputl member has been attained, but rather to develop automatically, as a function of the extent of displacementv of the pistonga force which will; balance the control force and return, the valveV to the neutral condition when the extent of output An organization of this vof construction.

cause the feedbackr forces to operate according to any y n 2,930,360 Patented Mar. 29, 1960 has of electrical circuits, which is capable ofautomaticallyV opposing a progressively increasing resistance to the con-y trolwforce, through the valve, as the displacement of the output member progresses, causing the control force to be balanced and the valve to be'returned to, and stably maintained in, the neutral or non-operating relation by said Vbalanced forces.

To these ends, and in accordance with a preferred illustrative embodiment of the invention, itis a featurey that provision is made of a ported cylinder,.a piston in the cylinder, a slide valve associated with the cylinder andV normally occupying a neutral position relative toy the ports so that uid under pressure can neither enter nor leave the cylinder, magnetic means carried Vby the valve and by the piston and arranged to cause the valve normally to occupy a neutral position, and control means operable yieldingly to displace the valve, the construction and arrangement being such that the piston is caused to be operated in a direction opposite tol that in which the valve has been displaced, and. is-ellective magnetically to repel either end of the Valve which it approaches with progressively increasing force until the net eiect of the force exertedby the piston on the valve becomesequal.v to the yieldingly applied control. force, returns the valvel to neutralposition over the resistance of said control force, and thereby establishes a stable condition of the output member, free 'from substantial hunting; effects, with the output. member in: a; deiinite position uniquely characteristic of the magnitude anddirection of theA con;- trol force acting on the valve.

lt is not essential that the feedbackmeans utilize permanent magnetsg'since electromagnets orother eld producing means may be utilized. The number and arrangement of the eld producing means may also be varied, asy can the valve arrangement andv various other features The Vmechanism may be contrived to desired law in relation to the extent. of piston, displace ment. With permanent magnets, each magnet` operatesV according to a square law, the repelling forcee increasing. in; inverse relation to thel square of.A the distance between thel piston carried and valve carried polesunder consideration. This is advantageous because of the stability which servodevice' embodying'features of the invention, the'V partsbeing 'shown in a neutral condition;

Fig. 2 is a transverse sectional view, takeu'on 2 2 of Fig.l l, looking in the direction of the arrows;

' Fig. 3- isl a reduced longitudinal section* similar to`V Fig. l wherein aslide valve is shown'in a position to establish liuid. connections for driving the piston upward;

Fig. 4 is a view similar to Fig. 3 wherein theslide'valve' is shown in a position to establish connections foldriv- Ving the piston downward;

` Fig. 7vis a reduced sectionsimil'a'rltoflsigY S wherei'nltheV slide valve is shown. in a position to establishil'uid'c'onfy nections for driving the piston upward;.and

Fi'g: 8`is a view similar to `Fig. `7wherein the slidefvalve the line is shown in a position to establish uid connections for driving the piston downward.

An illustrative electro-hydraulic servo device embodying features of the invention is disclosed in Figs. 1 to 4, inclusive. The device 10 comprises an elongated housing 12, shown'as of cyclindrical configuration. The housing 12 is fluid-tight, being closed `at its upper end by a wall 14 and at its lower end by a wall 16. The lower end wall 16 is provided with an opening in which there ispmounted an inner valve sleeve 18. The valve sleeve 18 forms an integral part of the housing 12, being secured thereto at its upper end through a radial wall 20, and at its lower end through the wall 16.

. The valve sleeve 18 is provided with a plurality of inlet ports 22 and 24, adjacent the walls 16 and 20, respectively. The ports let into annular space 26 that is deiined between the sleeve 18 and the outer wall of the housing 12. Inlet port 22 provides for uid entrance into the lower portion of the longitudinally extending uid space 26, while inlet port 24 serves as a Huid inlet at the upper end of the fluid space 26.

To provide for the discharge of fluid longitudinally spaced fluid exhaust or outlet ports 28 and 30 are provided in the valve sleeve 18, adjacent, respectively, to the walls and 16. The outlet port 28 permits the discharge of fluid from the upper portion of the longitudinal space 26, while the lower outlet port 30 permits the discharge of fluid from the lower portion of the space 26. The annular fluid space 26 is provided into lower and upper working chambers 32 and 34 by the interposition of an output member in the form of a uid actuated piston 36. The piston 36, which is the initial member of a power output mechanism, is suitably secured through slide rods 38 for transmitting motion of the piston to any external output member such as a crossbar 40 and an eye member 42 which is unitary therewith. Although, in the instant embodiment, members 38 are shown in the form of rods threaded at their lower ends 44 to the piston 36 and at their upper ends 46 to the crossbar 40, it will be understood that they may take any other convenient and practical shape.

The piston 36 is selectively moved in the working space 26 of the cylinder by the introduction of uid under pressure to either one of the chambers 32, 34 and the simultaneous discharge of fluid from the other of said chambers. For this reason, a conduit 52 is provided as a means for supplying uid -under pressure from any suitable external source of energy such as a hydraulic pump (not shown) to the interior of the sleeve 18 through an opening 54, the opening 54 being formed in an end portion of the valve sleeve 18 which projects beyond the lower end 16 of the housing 12. The uid under pressure is transmitted selectively either to chamber 32 or to chamber 34 by a slide valve 56. The slide valve 56 has a snug but free sliding t in the valve sleeve 18. A spline 56x is provided which prevents rotation of the slide valve 56 but does not restrain the desired sliding motion. It is provided with inlet and exhaust passages S8 and 60. The inlet passage 58 has an entrance port 62 that is in constant communication with the inlet aperture 54 and the pressure fluid supply means or conduit 52. The pressure fluid is therefore in constant communication with the inlet passage 58. The passage S8 is provided with a uid port 64 for alignment with the inlet port 22 of the valve sleeve 18, and with a tluid port 66 for alignment with the inlet port 24 of the sleeve 18, so that fluid under pressure may be selectively transmitted to the chamber 32 or the chamber 34 under the control of the slide valve 56. Normally, however, the valve occupies a neutral position like that shown in Fig. 1, in which the passage 58 communicates neither with the port 22 nor with the port 24. The discharge passage 60 of the slide valve 56 is provided with side passages or ports 68 and 70 for cooperating, respectively, with the discharge ports 28 and 30. In the `neutral relation of Fig. l, however, the passage 60 is not in communication with either the port 28 or the port 30.

It is to be noted that unlike the inlet passage means 58, the exhaust passage 60 extends the complete length of a comparator 88 (to be more fully described and explained presently) of which the valve 56 forms a part, opening at its upper end into a lluid receiving pressure equalization chamber 72 defined in the upper end of the housing 12 between the end wall 14 and the impermeable transverse wall 20 thereof. The lower end of exhaust passage 60 opens into a uid pressure equalization chamber 74 defined in the projecting end of the sleeve 18. Stops 72x and 74x are provided at the ends of the chambers 72 and 74, respectively, for keeping the passage 60 in free communication with the chambers and for limiting the stroke of the valve 56. An outlet opening 76 is provided in the projecting end of the sleeve 18 to permit the discharge of fluid in the direction of the arrow 78 from the device 10 to a receiving or recirculating chamber, not shown. Valve means, not shown, may be provided in a discharge line (not shown) for assuring the presence of uid under predetermined limited pressure at all times in the passage 60 and in both the chambers 72 and 74.

The position of the piston 36 and its related output member 40 is precisely determined and maintained by the unique comparator 88 which serves as a novel feedback means. 'Ihe comparator 88 comprises the slide valve 56 and a plurality of eld responsive members, illustratively shown as permanent bar magnets. These include bar magnets 80 and 82 iixedly carried and mechanically joined by the slide valve 56 beyond the opposite ends of the range of piston travel. The magnets 80 and 82 are, themselves, iield producing, but since the comparator with its magnets 80 and 82 is designed to yield to magnetic and electromagnetic forces, the magnets 80 and 82 are referred to as field responsive. A further bar magnet 84, ixedly secured upon the piston 36, constitutes a further field producing means and serves as one input to the comparator, the other input being an arbitrarily produced dis placing-force yieldingly applied to the comparator as will be more fully explained. The magnets 80, 82 and 84 all have their axes disposed parallel to the axis of cylinder 32. The magnets 80 and 82 both have their north poles disposed in one direction and their south poles disposed in the opposite direction, but the arrangement of the magnet 84 is the reverse of that of the magnets 80 and 82.

In the absence of other forces acting on the comparator 88, this magnetic polar relationship causes the slide valve 56 to assume the neutral relation of Fig. l when the piston is in its median or zero position, as shown in Fig. 1.

Selective controlled operation and displacement of the piston 36 away from the zero output position is produced by applying an extraneously produced, yielding displacing force to the comparator 88 through a displacing force applying means. The displacing force may be applied mechanically or in any other suitable manner. As illustrated an electromagnetic coil 86, adapted to be energized selectively by current flow of any selected value and in either direction desired, is fixed on the protruding end of the sleeve 18 in position to have the magnet 80 received `within it to a greater or lesser degree. Displacement of the valve 56 from neutral is etected in one direction by the flow of current in one direction through the coil 86 and in the opposite direction by the ow of current in the opposite direction through the coil 86, the comparator being attracted by the action of the coil on the magnet 80 in one case and repelled by the action of the coil on the magnet 80 in the other.

Since the valve balancing forces exerted upon the comparator magnets 80 and 82 in the zero position of the piston 36 are Vlight forces, even a light yielding force applied to the valve 56, mechanically or through the coil 86, will suice to disturb the balanced condition of the comparator and the neutral relation of the valve tothe cylinder ports.

If the comparator 88 is yieldingly pulled downward, a condition like that of Fig. 3 will be producedgin which inlet port 2,2 and outlet port 28 are rendered active by the valve 56. The piston will be displaced in an upward direction, causing the magnet 84 to approach the com- .K parator magnet 82 andV repel it with increasing force, and vto separate from the comparator magnet 80 and repel the y magnet 80 with diminishing force.

Since the net effect of magnet 84 on the comparator system will be the vector sum of these two forces, an increasingforce tending to move the comparator, including the valve 56, in an upward direction toward the neutral position is developed. When this developed force becomes suicient to balance the control force yieldingly applied to the comparator 88,' as by the coil 86, the valve is restored to the normal, neutral relation of the valve to the ports of sleeve 18 illustrated in Fig. l. If the pres? sure uid employed is hydraulic uid, and it preferably is for most purposes', the piston 36 will be positivelyv locked in a precise, predetermined position which is characteristic of the control force yieldingly applied to, and maintained upon, the comparator S8 by the coil 86. With this kind of arrangement the position of the valve is stably maintained and this, in turn causes the positionA ofthe piston 36 to be stably maintained.

Should the downwardly acting control force be increased, the same valve ports, 22 and 23, 'will be uncovered and a further action like the one already described will be eiected to carry the piston further upward and lock it in a new position characteristic of the increased control force. f vA diminution of the downward control force applied to the valve 56, or reversal to an upward force applied to the valve willl cause the valve to befmoved upward to a condition like that shown in Fig. 4 This causes the inlet port 24 to be at least partially uncovered for admitting pressure iluid to chamber 34 for driving the piston 36 downward, and the discharge port 30 to be simultaneously uncovered at least part way for permitting escape of lluid from the chamber 32. The valve will again be closed and the piston locked, when the net force exerted on the comparator magnets is exactly equalV inabsolute value, but opposite in direction, to-the controlA force yieldingly applied and maintained by the. coilSG.

In practice it frequently happens that the device 1i?. is subjected to sudden stressesv caused, for example, by, change of speed or direction of the body on which the device is carried. Since these stresses are unavoidably applied to all parts of the servo mechanism, ncludingfthe hydraulic iluid andthe comparator 88, it is important that measures be taken to avoid displacement of the valve through inertia eifects. To this end the average density of the comparator 88 is made equalor substantially equal v of Figs. 1 to 4; It differs chiefly 'in the factura the chambers at opposite Vsides of the pistondiffer substan tially in their cross-sectional dimensions, that the chamber of smaller cross-section is constantly and freely in communication with the pressure fluid supplying means, and

that the valve controls admission and discharge of uid r ing parts with the subscript a added in each instance, and` Ythe description and explanation will be substantiallyy couned to specific dilferences. Y,

Instead of the slide rods 38 and the cross bar 40 of Figs. l to 4, a thick walled output sleevek 38a and a cap member, screwed thereon, are made unitary with the piston which, in this instance, constitutes in its entirety a bar magnet 84a of annular form. A hole 90 is provided incap member 40a to avoid pressure andv suction elects. The sleeve has the eiect of reducing the cross,-

Y sectional area of the upper chamber 34a and the upper tween the chambers 72 and 74, and since the chambers ,I

72 and 74 are filled with hydraulic tluid atall times, the comparator 83 is compelled to move in unison with, and not relative to, the iixed parts of the` servomechanism such as the housing l2 and the sleeve 1S. Thisequality of densitiesV may be achievedby employing a hydraulic iluid of comparatively high density and a valve 56 which consists of magnesium or aluminum or an alloy of one or both of these metals, hollowed out to whatever extent maybe necessary for achieving the stated purpose.

A In Figs. 5 to 8, inclusive, another practical and advantageous illustrative form of seryo-mechanismliia einbodyinggfeatures of the invention is disclosed. The device 10d has many features in common with the' device 10k `ports 54a and 22a.

piston surface 50a very materially as Vcompared with the cross sectional area of the lower chamber 26a and theVV lower piston face 48a, desirably to a ratio of substantially l to 2.

The conduit 52a is a'branched conduit, o'ne branch leading to port 54a and the other being` connected to communicate freely with the chamber 34a at all times.`

The end wall .14a is thickened and circumferentially rabbeted, the reduced end extendingbelow the adjacent inlet of conduit 52a to prevent cutting o of the port byl the piston, while maintaining a clear passage between conduit 52a and the upper cylinder chamber 34u.

The sleeve 18a is extended at its upper end beyondthe housing 12a and within the output sleeve 38a, being closed at its upper end to provide the pressure equalizing` chamber 72a. The Vsleeve 18a includes ports 22a and 30a for admitting pressure fluid to the chamber 26a and dis#` charging it therefrom, respectively, but no'ports corre-f vided. Y

The comparator valve 56a includes an intakey passage 58a having ports 62a and 64a for cooperating with the A discharge passage 60a extends through the comparator 88a from equalizing chamber 72a to equalizing chamber 74a, and' has a port 70afor cooperating with discharge port 30a;

The action of the comparator magnets a andk 82a; piston magnet 84a, and the coil 86a andjtheir effects upon the valve 56a are as previously described. The hydraulic action, however, requires a word of explanation. When the .valve is pulled downward from the position of Fig. 5y to a position like that of Fig. 7, pressure uid is admitted spending to the ports24- and 2S ofl Figs. 1 to 4,`are proto the chamber 26a. Since the-force of the uid pressurev against the lower face of piston S4Zz'is` greater than that` against the upper face of the piston, because of the v`difference o'f areas, the piston moves upward, forcing ilu'id" in the upper chamber 34a, which is under the same pres- Sure, back into the pressure supply conduit 52a. This actionY continues until the valve 56a has beenrestored to the piston downward, and driving the pressure il'uid out 'i of chamber 26a until the comparator forces balance the control force, closing the valve and leaving the pisto'n in a new position characteristic of the magnitude and direction of the control force maintained upon the valve.

While certain preferred embodiments of the invention have been illustrated and described in detail, it is to be understood that changes may be made therein and the invention embodied in other structures. It is not, therefore, the intention to limit the patent to the specific construction illustrated, but to cover the invention broadly in whatever form its principles may be utilized.

I claim:

l. A servo mechanism comprising, in combination, means supplying fluid under pressure, a ported, chambered housing, a fluid actuated piston in the housing, a slide valve normally maintained in a neutral relation to the housing ports but movable relative to the housing ports to control admission and discharge of fluid to and .from at least o'ne chamber of the housing, field producing means, causing fields to be established at opposite ends of the slide valve, field producing means carried by the piston and acting to apply a progressively increasing repelling force to either end of the slide valve toward which the piston is moved, and control means associated with the slide valve for yieldingly urging the slide valve in either o'f two opposite directions from the neutral relation, the construction and arrangement being such that the piston will be fluid operated in the direction opposite to that in which the valve is displaced to a definite position uniquely characteristic of the direction and magnitude of the control force applied to the valve, and in which the valve is restored to the neutral relation through a counterbalancing of the control force.

2. A servo mechanism comprising, in combination, means supplying uid under pressure, a ported cylinder, a fluid actuated piston in the cylinder, a slide valve normally maintained in a neutral relation to the cylinder ports but movable relative to the cylinder to control admission of fluid from said supplying means to the cylinder and discharge of iiuid from the cylinder, field responsive means carried by the slide valve at opposite ends thereof, field producing means carried by the piston and acting to repel either end of the slide valve toward which the piston is moved, and control means associated with the slide valve and operable yieldingly to urge the slide valve in either of two opposite directions from the neutral relation, the construction and arrangement being such that the piston will be fluid operated in the direction opposite to that in which the valve is displaced to a definite position uniquely characteristic of the direction and magnitude of the control force applied to the slide valve.

3. A servo mechanism comprising, in combination, means supplying tiuid under pressure, a ported cylinder, a uid actuated piston in the cylinder, a slide valve no'rmally maintained in a neutral relation to the cylinder ports but movable relative to the cylinder to control admission and discharge of duid to and from the cylinder, magnetic means carried by the slide valve at opposite ends thereof, magnetic means carried by the piston and acting to repel either end of the slide valve to'ward which the piston is moved, and electromagnetic means associated with the slide valve and energizable yieldingly to urge the slide valve in either of two opposite directions from the neutral relation, the construction and arrangement being such that the piston will be liuid operated in a direction opposite o'f the valve displacement, to a delinite position uniquely characteristic of the direction and magnitude of the control force applied to the slide valve.

4. A servo mechanism comprising, in combination, means supplying hydraulic iiuid under pressure, a ported cylinder, a liuid actuated piston in the cylinder, a slide valve normally maintained in a neutral relation to the cylinder ports but movable relative to the cylinder to control admission and discharge of hydraulic fluid to and from one chamber, at least, of the cylinder, field producing means providing opposite polarities in confronting relation to the piston at opposite ends of the slide valve, iield producing means carried by the piston and acting to exert a repelling force upon either end of the slide valve toward which the piston is moved, and control means associated with the slide valve for applying yielding force to urge the slide valve in either of two opposite direction from the neutral relation, the construction and arrangement being such that the piston will be hydraulically operated in a direction opposite to that in which the valve is displaced until the valve is compelled, against the resistance of said yielding force and while said force is still maintained, to reassume the normal relation to the cylinder ports, so that the piston will be caused to assume a definite position uniquely characteristic of the direction and magnitude of whatever control force may be applied to the slide valve.

5. A servo mechanism comprising, in combination, means supplying uid under pressure, a cylinder, a fixed ported sleeve extending into the cylinder and throughout the major portion of the length thereof, a fluid actuated piston in the cylinder, a slide valve having a snug sliding fit within the sleeve and normally maintained in a neutral relation to the ports, but movable relative to the sleeve and cylinder to control admission and discharge of hydraulic fluid to and from the cylinder, iield responsive means providing opposite poles in confronting relation to the piston at opposite ends of the slide valve, field producing means carried by the piston and acting to exert a progressively increasing repelling force upon either end of the slide valve toward which the piston is moved, and control means associated with the slide valve for yieldingly urging the slide valve in either of two opposite directions from the neutral relation, the construction and arrangement being such that the piston will be fluid operated in a direction opposite to that in which thc valve is displaced by the control force until the valve is compelled, against the resistance of said yielding control force and while said force is still maintained, to reassume the normal relation to the cylinder ports, so that the piston is caused to assume a definite position uniquely characteristic of the direction and magnitude of whatever control force, within the limits of the mechanism, may be applied to the slide valve.

6. A servo mechanism as set forth in claim 5 in which the slide valve has intake and discharge passages therein, and segregated chambers are provided in which opposite ends of the slide valve play, the discharge passage of the valve being extended completely through the valve from end to end thereof in free communication with both chambers, whereby suction effects tending to interfere with the free and intended operation of the valve are avoided.

7. A servo mechanism as set forth in claim 6 in which the uid is hydraulic and the hydraulic tiuid and the slide valve are of equal density, whereby the slide valve is rendered immune to acceleration eiects acting on the mechanism as a whole.

8. A servo mechanism as set forth in claim 5 wherein the ield responsive and field producing means referred to consists of permanent magnets carried, respectively, by the opposite ends of the slide valve and by the piston, the valve carried magnets having like poles disposed in the same direction lengthwise of the valve, and the piston carried magnet having its poles reversely disposed.

9. A servo mechanism as set forth in claim 5 in which the sleeve and the slide valve extend beyond the cylinder at one end, and in which the control means comprises a coil surrounding a protruding portion of the sleeve and adapted to be variably energized by current flow of varied strength in either direction.

10. A servo mechanism as set forth in claim 5 in which the fluid is hydraulic and sufficient clearance ix provided between the sleeve and the slide valve to cause a thin lm of hydraulic fluid to bemaintained continuonsly between the sleeve and the valve for lubricating purposes.

11. A servo mechanism comprising, in combination, means supplying hydraulic fluid under pressure, a cylinder, a fixed ported sleeve extending into the cylinder and throughout the major portion of the llength thereof, a fluid actuated piston dividing the cylinder into two opposed working chambers, a slide valve having intake and discharge passages therein, and havinga snug slidingl fit within lthe sleeve and normally maintained in a neutral relation to the ports, but movable relative to the sleeve and cylinder selectively to effect admission of hydraulic uid to either chamber and simultaneous discharge thereof from the other, field responsive means providing opposite poles in confronting relation to. the piston at opposite ends of the slide valve, field producing means carried by the piston and acting to exert a progressively increasing repelling force upon either end of ther slide valve toward which the piston is moved, and control means associated with the slide valve for yieldingly urging the slide valve in either of two opposite directions in said radial space in response to fluid pressure acting` thereon, a slide valve movable longitudinally in said sleeve, said slide valve having passages adapted respectively to admit andrexhaust fluid under pressure, uid ports in `said sleeve communicable with the respective valve passages to admit and exhaust Huid under pressure to and from said radial space, field responsive means on said valve and field producing means on the piston col operating normally to ycausev said valve to close the fluid from the neutral relation, the constructionand arrangement being such that the piston willvbe hydraulically operated in a direction opposite to that-in which the valve is displaced by the control force until the valve is compelled, againstj'the resistance of said yielding con- -trol force and while said force is still maintained, to reassume the normal relation to the cylinderY ports, so that the piston will be caused to assume a definite position uniquely characteristic of the direction and magnitude of whatever control force within the limits of the mechanism, may be applied to the slide valve.`

l2. A servo Amechanism comprising, in combination, l

means supplying fluid under pressure, a cylinder, a fixed ported sleeve extending into the cylinder and throughout the major portion of the length thereof, a uid actuated piston dividing the cylinder into opposed working chambers, output means of substantial cross-section connected to the piston and extending through an end of the cylinder, said output means having the effect of cutting down the working area of the piston in said chamber bya substantial amount as compared with the working area of the piston in the other chamber, said supplying means being directly and freely connected at all times with said one chamber, a slide valve having a snug sliding fit within the sleeve and normally maintained in a neutral relation to the ports, but movable relative to the sleeve and cylinder to control admission rand discharge of liuid to and from said other chamber, field responsive means providing opposite poles in confronting relation to the piston at opposite ends of the slide valve, field producing means carried by the piston and acting to exert a progressively increasing repelling force upon either end of the slide valve toward which the pistonA is moved, and control means associated with the slide valve for yieldingly urging the slide valve in either of two opposite directions from the neutral relation, the construction and arrangement being such that the piston will be fluid operated in a direction opposite to that in which the valve is displaced by the control force until the valve is compelled, against the resistance of said yielding control force and while said force is still maintained, to reassume the normal relation to the cylinder ports, so that the piston will be caused to assume a definite position uniquely characteristic of the direction and magnitude of whatever communication between said valve and sleeve, and yieldable control means associated with said valve for yieldingly moving the valve in either of two opposite directions from the normal, communication closing position, the construction and arrangement being such that the piston will be hydraulically operated in the direction opposite to that in which the valve is displaced, to a definite stable position uniquely characteristic ofthe direction and magnit/nde of the control force applied to the slide valve.

l5. A hydraulic servo device comprising a source of fluid under pressure, inner and outer longitudinally extending radially spaced members, a piston operable longitudinally in said radial space in response to the huid pressure acting thereon, valve means movable in said'inner member to communicate fluid pressure to said piston, said piston including field producing means, a comparator including the valve as a unitary part thereof and providing l longitudinally spaced fields at opposite sides of the piston and in constant cooperation with said piston field producing means, and yieldable control means operable to yieldingly displace saidV comparator in either of two opposite directions from the normal communication closing position, the construction and arrangement being such that the piston will be hydraulically operated in the direction opposite to Athat in which the valve is displaced, to a definite stable positionV uniquely characteristic of the direction and magnitude of the control force applied to v the comparator.

16., A fluid operated servo mechanism comprising, in combination, means providing a supply of pressure fluid, a housing chamber, an output piston movable in said chamber in response to uid pressure acting thereon, a valveV movable to. control communication between said housing chamber and said fluid supply means, force field producing means on said piston, comparator force field responsive means on said valve, said piston and valve force field producing and responsive means coacting Vvwith progressively increasing force as the piston approaches either end of the valve to move said valve to closesaid communication, and control means yieldingly operable' to move the valve away from closing position over the opposition of the piston carried force field producing means and to maintain the valve open until, through movement of the piston, the valve closing effect of the piston carried force field producing means upon the comparator means is sufficiently increased to offset the yieldeither of two opposite directions in response to fluid pres- Y sure acting thereon, a housing in which the piston operates, supply means for liuid under pressure, valve means between said housing and Vsaid pressure fluid supplyfmean's operable to vary the communication therebetween and to control the direction of application of the fluid pressure on said piston, said piston having magnetic means, a comparator including the valveimeans and magnetic means predeterminately spaced on said valve means at opposite sides of the piston and cooperable with :said piston magnetic means normally to move said valve to close said communication, and electromagnetic control means operable on said valve, and variably energizable to "cause selective movement of said valve, the construction and arrangement being such that the piston is caused, t0

11' assume a positionV uniquely characteristic of the energization of the control means.

18. A hydraulic servo device comprising, in combination, a housing having a longitudinally extending working space therein, a piston movable in said space and dividing said space into two chambers, a pressure uid supply means, valve means movable in one direction to admit pressure tluid from said supply means to one of said chambers, and in the opposite direction to discharge uid from the same chamber, a comparator including the valve means and magnetic means on said valve means, said piston including magnetic means coacting with said comparator to bias said valve means toward closed position, yielding control means operable to shift the valve in one direction to admit pressure uid to said one chamber and to shift the valve in the opposite direction to discharge fluid from said one chamber, the arrangement being such that the comparator means acts progressively to balance the force of the piston magnetic means against the control force and close the valve in response to piston movement, and means acting on the piston continuously to 4exert yielding force on the piston inthe direction opposite that in which the piston is moved by the admission of pressure fluid to said one chamber.

19. A servo mechanism comprising, in combination, an output member possessing field producing means, a control member also possessing field producing means, the field producing means of the output member and of the control member being arranged to interact with one another, means for applying small displacing forces yieldingly in either of two opposite directions to the control member, and an external source of energy for applying operating power to the output member in response to displacement of the control member from a neutral position by said displacing force applying means, the construction and arrangement being such that with no displacing force applied to the control member the output member will be maintained in a normal or neutral position with the control member balanced by the field producing means in a neutral position, but that when the control member is displaced in either direction from neutral by the displacing force applying means, the output member will be operated powerfully in the direction opposite to that in which the control member is displaced until the output member has been moved to a position characteristic of the magnitude and direction of the cornparatively small displacing force applied to the control member, in which position the field producing means will have supplied a net feed-back force to the control member through the field producing means of the output member and the control member suicient to counterbalance the applied displacing force, thereby to return the control member to its neutral position and arrest the output member.

20. A servo mechanism comprising, in combination, an output member possessing tield producing means, a control member also possessing field producing means, the field producing means of the output member and of the control member being arranged to interact with one another, means for applying small displacing forces yieldingly to the control member, and an external source of energy for applying power to the output member in response to displacement of the control member by said displacing force applying means, the construction and arrangement being such that with no displacing force applied to the control member by the displacing force applying means the output member will be maintained in a normal position and the control member held in a normal condition, but that when the control member is displaced from its normal condition to an operating condition by the displacing force applying means, the output member will be operated powerfully in the opposite direction until the output member has been moved to a position characteristic of the magnitude of the comparatively small displacing force applied to the control member, in which position the tield producing means will have supplied a net feed-back force to the control member suflicient to drive the control member against the resistance of the displacing force to a neutral position.

References Cited in the tile of this patent UNITED STATES PATENTS 2,372,813 Darling Apr. 3, 1945 2,638,125 Parsons May 12, 1953 2,688,953V Burns Sept. 14, 1954

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