US2923307A - Means compensating for the time delay of a transmission line - Google Patents

Description

E. c. GROGAN 2,923,307

2 Sheets-Sheet 1 Feb. 2, 1960 MEANS COMPENSATING FOR THE TIME DELAY OF A TRANSMISSION LINE Filed Dec. 29, 1955 INVENTOR.

EDWARD c. GROGAN BYi ATTORNEY.

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MEANS COMPENSATING FOR THE TIME DELAY OF A TRANSMISSION LINE Filed D80. 29, 1955 FIG. 2

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EDWARD C. GROGAN BY 6 5 7y ATTORNEY.

United States Patent MEANS COMPENSATING FOR THE TIME DELAY OF A TRANSMISSION LINE Edward C. Grogan, Philadelphia, Pa., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Application December 29, 1955, Serial No. 556,319 Claims. (Cl. 137-86) This invention relates to apparatus operated by an elastic fluid, such as air, which apparatus comprises: a measuring element, responsive to variations in a measured variable; a transmitter, which converts the variations sensed by the measuring element into variations in an air pressure; an elongated transmission line having a small cross section and adapted to conduct the variations in air pressure; a receiver connected to the transmission line so as to receive the variations in pressure therefrom; a motor operated by variations in pressure; and a final control element operated by said motor.

It is an object of this invention to provide means for compensating for the time delays in said transmission line due to distributed resistance and distributed capacitance in the transmission line.

More specifically, it is an object of this invention to provide a receiver which will feed power back into the transmission line and to thereby compensate for the time delays caused by the transmission line.

A further object of this invention is to provide an elasticfluid-pressure operated circuit in which there is a receiver which has bi-lateral action. Bi-lateral action may be defined as an action having two eifects. One of these is the elfect obtained by feeding back a portion of the output of the receiver to its input in such a way as to compensate for the time delays due to the transmission line leading to the receiver. The other of these elfects is the effect obtained by feeding the remaining portion of the output of the receiver forward to a motor for a final control valve or the like. This forward feeding is advanced in phase as compared to the phase of the input, and serves to compensate for any time lags due to the transmission and not already compensated for by the feedback to the input to the receiver.

It is yet a further object of this invention to provide a circuit in which a single pneumatic unit acts as a receiver and performs as two unilateral, ideal, proportional plus rate units, acting in tandem. This arrangement therefore serves to minimize considerably the noise problem and to simplify present-day unilateral circuit configurations designed to offer compensation for multi-capacity pneumatic circuits such as tubing connected as a transmission line.

The various features of novelty which characterize this invention are pointed out with particularity in the claimsannexed to and forming a part of this specification. For a better understanding of the invention, however, its ad vantages and specific objects obtained with its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

In the drawings:

Fig. 1 is a diagram showing the circuit connections and the schematic outlines of the apparatus forming the device;

Fig. 2 is a block diagram forming part of a mathematical analysis;

Fig. 3 is a circuit diagram forming part of this mathematical analysis; and

Fig. 4 is a circuit diagram forming part of this mathematical analysis.

In that embodiment of my invention elected from among others for illustration and in the drawings and description in the specification, my device is shown as follows.

The measuring element M is shown as a flow meter although it may be a measuring element of many various kinds adapted to sense a variation in a variable quantity or condition. The measuring element M is adapted to measure the flow of a fluid through a pipe P. Measuring element M comprises an orifice plate 1 having a central opening therethrough coaxial with the pipe P. Perforated discs 2 and 3 are located on opposite sides of the measuring or orifice plate 1 and serve to conduct the pressure on the opposite sides of the orifice in plate 1 through pipes 4 and 5 to the transmitter T.

The transmitter T converts the variations in the measured variable into variations in the pressure of a supply of air.- Transmitter T comprises a measurement-responsive element formed by two rigid, cup-shaped casings 6 and 7 each of which has a wall thereof formed by a flexible diaphragm 8 or 9. The centers of diaphragms 8 and 9 are connected together by a shaft or pin 10 to which a flapper 11 is connected so as to move therewith.

A nozzle 12 cooperates with flapper 11 and is supplied With an elastic fluid, such as compressed air, under pressure from a filtered air supply F.A.S through a restric- .tion 13 and a pipe 14. A rigid casing 15 is divided into four chambers 16, 17, 18, and 19 by two diaphragms 20, 21, and by a rigid, perforated wall 22. Diaphragms 20 and 21 are connected together by a hollow tube 23 having a closed, upper end and an open, lower end and an opening 24 through the wall thereof. Air is supplied to inlet chamber 19 by a pipe 28. Ball 26 constitutes an inlet valve in cooperation with the rim of a circular opening in wall 22. Spring 27 presses inlet valve ball 26 into engagement with the seat formed by the rim of the opening through wall 22. Exhaust pipe 23 forms an exhaust valve by means of the cooperation of the open, lower end of pipe 23 with ball 26 so that fluid may pass up through the interior of pipe 23, through hole 24 into chamber 17 whence the air escapes to atmosphere through hole 25.

The part just described forms a non-bleed high capacity pilot valve or relay which operates in the following way. When the relay is in its normal, steady state, the pressure in chamber 16 is equal to the pressure in chamber 18 (or at some other, preselected ratio thereto), and the inlet valve 26-22 and the exhaust valve 23-26 are both closed.

Now, if the pressure in chamber 16 increases with respect to the pressure in chamber 18, the pressure on diaphragm 20 causes pipe 23 to move down (as viewed in the drawing) and moves ball 26 away from its cooperating valve seat formed by the rim of the opening through Wall 22. This opens the inlet valve and admits air under pressure from chamber 19 into chamber 18. At the same time, the exhaust valve 2326 remains closed because of the engagement with the lower end of the exhaust pipe 23 with the ball 26.

The inlet valve 26-22 remains open until the pressure in the chamber 18 is equal to (or at some preselectedratio to) the pressure in chamber 16. When this occurs, the pipe 23 again moves upward (as seen in the drawing) and the inlet valve 26-22 closes. I

If, on the contrary, the pressure in chamber 16 decreases with relation to the pressure in chamber 18, the inlet valve 26-22 remains closed. The decrease in pressure in chamber 16, however, allows pipe 23 to move upward (as seen in the drawing). This opens the exhaust valve 23--26 because the lower end of the pipe 23 moves out of contact with the ball 26. Opening the exhaust valve 23--26 connects the chamber 18 to the chamber 17 through the opening 24- so that fluid in chamber" 18 can escape to the atmosphere. When the desired ratio between the pressure in the chamber 16 and the pressure in the chamber 18 is again restored, the pipe 23 moves downward thereby closing the exhaust valve 2326 and again restoring the normal, steady state of the relay.

The output of this relay and of transmitter T is connected by pipe 29 to the input to the transmission line L. Transmission line L comprises a tube of approximately A inch internal diameter and may be as much as 1,000 feet in length. I

The output end of transmission line L is connected to the input of the receiver R. Receiver R comprises an input motor formed of a rigid, cup-shaped casing 31 having one wall thereof formed by a flexible diaphragm 32. A negative feedback motor is formed of a rigid, cupshaped casing 33 having a flexible diaphragm 34 as a wall thereof. Flexible diaphragms 32 and 34 are connected by a shaft 35 which supports the flapper 36.

Flapper 36 cooperates with a nozzle 37 to which air under pressure is supplied from a filtered air supply F.A.S. through a restriction 38 and a pipe 39. A rigid casing 40 is divided into four chambers 41, 42, 43, and 44 by two flexible diaphgrams 45 and 46 and a rigid wall 47. Air is supplied to input chamber 44 by pipe 53. Ball 51 is biased by spring 52 into engagement with the rim of a circular opening in the center of rigid wall'47. An exhaust valve is formed by pipe 48, which is closed at its upper end and open at its lower end and connects diaphragms 45 and 46. Exhaust pipe 48 has a hole 49 in its side wall communicating with exhaust chamber 42 which communicates with the atmosphere through hole 50 in casing 40.

The foregoing elements comprise a second, non-bleed, high capacity pilot valve or relay which operates in the same way as relay 15-27, described above.

The output of this pilot valve or relay is conducted through pipe 54, 55, and 56, which form part of a negative feedback circuit including a series restriction R2 and a pipe 57, which communicates with the negative feedback motor 33, 34. A bypass connection is formed by a bypass restriction R which communicates, at its inlet, with pipe 57 and, at its outlet, with a bypass chamber 60, which has a large capacity as compared with the capacity of input motor 31, 32 or with the capacity or negative feedback motor 33, 34.

A positive feedback circuit is formed of pipe 58,'which communicateswith pipes 56 and-55 and with the inlet to a positive feedback restriction R Positive feedback restriction R is connected, at its output side, to pipe 30 intermediate the end of transmission line L and the input to input motor 31, 32.

The output from receiver R is conducted from output chamber 43 by means of pipes 54 and 61 to the motor V.M., which actuates the final control element. Motor V.M. comprises a rigid casing 62 having a fiexiblediaphragm 63 forminga wall thereof. A valve 'stem 64 is connected to the center of diaphragm 63. A spring 65 bears at one end against diaphragm 63 and, at its opposite end, against a portion of the rigid casing 66.

The'final control element V is shown as avalve controlling the flow of liquid through a pipe PP which, however, may be the same as pipe P or a diflerent one. The final control element V comprises a rigid casing 66-dividedintotwo chambers by a perforated partition 68. A valve plug 67, mounted on the end of valve stem 64 cooperates with the hole in partition 68 so as to open, partially open, or close this hole and totlier'eby permit or to shut d the flow of fluid throughthe pipe P.

The operation of the apparatus of this invention is as follows. Assuming the transmitter pilot valve or relay'a'nrl g the receiver pilot valve or relay to be in their-"steady" state 4. or balanced condition, if a variation in the flow through pipe P is sensed by the measuring element M, a difference in pressure exists between the discs 2 and 3. This difference in pressure is conducted, by means of pipes 4 and 5, to the chambers formed by the elements 6, 8 and the elements 7, 9, respectively. Any difference between the pressures in each of these chambers causes shaft 10 to move flapper 11 relative to nozzle 12 in one sense or the other. This movement of flapper 11 varys the flow of fluid through nozzle 12 and causes a corresponding. variation of the pressure in the nozzle chamber 16. Since the pressure in nozzle chamber 16 is balanced against the pressure in outlet chamber 18, a change in pressure in nozzle chamber 16 causes either inlet valve 26 or exhaust valve 23 to open and to admit or exhaust air from the output chamber 18. This changes the air pressure in chamber 18, which change continuesuntil the pressure'in chamber 18 again balances the pressure in nozzle chamber 16. When this balance takes place, the inlet or exhaust valve closes and the pressure ceases to change.

This change in air pressure is transmitted, by means of pipe 29, to the inlet to transmission line L and passes through the transmission line L with more or less of a time delay depending upon the resistance and capacitance of the transmission line L. At the output of the trans mission line L, the pipe 30'connects this change in air pressure to the inlet to positive feedback restriction R1 and to the input motor 31, 32. This change in air pressure flows through the restriction R very slowly because of the resistance of the restriction R This change in air pressure is applied directly to the input motor 31, 32 and causes immediate action of the motor to the full extent of the change in pressure. When the device is in balanced condition, the pressure in input motor 31, 32 is balanced by the pressure in the negative feedback motor 33, 34. If a dilference exists between the pressurein inputmotor 31, 32 and in the negative feedback motor 33, 34, shaft 35 is moved. This moves flapper 36 relative to nozzle 37 in one sense or the other. Flapper 36 and nozzle 37 constitute first valve of the receiver R. This first valve 36, 37 controls the pressure in nozzle chamber 41. The pressure in nozzle chamber 41 is normally balanced by the pressure in output chamber 43. If the pressure in nozzle chamber 41 changes, either inlet valve '51, or exhaust valve 48 is opened and air is admitted to or exhausted 'fromthe output chamber 43. Thisadmission or exhaust of air'from the output chamber 43 continues until the pressure in nozzle chamber- 41 again balances the pressure in output chamber 43. Inlet valve 51 and exhaust valve 48 thus constitute the second valve of the receiver R.

The output air pressure from the pilot valve or relay of receiver 'R is conducted from output chamber 43 through pipes 54 and '61 to the motor 62, 63 of the final control element. A change in the pressure in the motor VM causes valve stem 54 to move valve plug 67 relative to the orifice in partition 68 and to thereby vary the flow of fluid through pipe PP.

Any change in pressure of the output of receiver R is also fed from output chamber 43 through pipes 54, 5S, and 56 to series restriction R through which it passes and thence bymeans of pipe57 to the negative feedback motor 33, 34. Any change in the pressure in negative feedback motor 33, 34 moves shaft 35 until the pressures in the input motor 31, 32 and in negative feedback motor 33, 34 are equal. When this occurs, flapper 36 is held at one position relative to nozzle 37 and the device is in stable or balanced condition.

The negative fcdback circuit formed of pipes 54, 55, and 56, series restriction R and pipe 57, also includes a bypass restriction R which controls the supply of fluid to a bypass chamber 60, Whose capacity is great as compared with'the' capacityof input'motor 31, 32 and the capacity of negativ. feedback motor 33, 34.

Receiver R also includes'a positive feedback circuit formed of pipe 58 which connects, at its input, with pipes 55 and 56 and, at its output, with the positive feedback restriction R The output of positive feedback restriction R communicates by means of pipe 59 with pipe 30, intermediate the end of transmission line L and the input to input motor 31, 32.

The operation of the pneumatic circuit'described above depends on using single derivative action for effective compensation on distributed lags through pneumatic transmitting elements which behave as multi-capacity systems (tubing, etc.). This is based upon bi-lateral action across a compensated rate unit (the receiver), which avoids the need for second derivative response that would ordinarily be required in unilateral arrangements.

The simple pneumatic circuit arrangement shown permits adjustments on rate amplitude and rate time. 'Bilateral action is obtained by resistance coupling between the input and the output of the receiver, which permits the receiver to feed air back into the line in which it is terminated. Since the pressuredeveloped at the output of the receiver leads the transmitted input signal, a regenerative feedback effect will occur on the pressure signal which is propagated in a line feeding into the receiver. This action will minimize the effects of line capacitance and hence speed up signal transmission to the receiver.

The pressure developed at the output 54 of the receiver R can be said to lead in phase the input signal transmitted through the pipe 30 to the input chamber of the relay R for the following reasons. Anychange of the pressure in the input chamber of the relay R causes a corresponding change in the output pressure of the relay R in the chamber 43 and consequently in the pipe 54. This change in pressure is immediately transmitted through the pipe 61 to the diaphragm 63 of the valve motor VM.

This change in pressure is also transmitted through the positive feedback restriction R to the input chamber of the relay R. The change in pressure is felt in the input chamber of the relay R after a time delay which depends upon the resistance of the restriction R The change in pressure is also fed through pipe 57 to the negative feedback chamber 33 of the relay R after a time delay which depends upon the resistance of the restrictions R and R and upon the capacitance 60. The resistance of restrictions R R and R and the capacitance of chamber 60 can be so adjusted that the positive feedback restriction R causes a variation in flow to be conducted to the transmission line 30 and to the input motor 3132. This variation in flow is large enough to compensate for the time delay in the transmission of the variations in the pressure passing through the supply line L and caused by the resistance and capacitance of the transmission line L.

The interaction described in the above suggests that the receiver serves as an impedance matching device on a line in which it is terminated. Later herein, it will be shown that the input impedance of the receiver appears as a negative resistance in series with a negative capacitor to ground, which act as current sources. In feeding back into a multi-capacity system, the receiver will .of course be limited in its ability to nullify, completely, the effects of distributed lags on the propagation of a signal transmitted to its input chamber. However, in acting for ward, the receiver transmits a pressure signal (to a valve tion may be analyzed mathematically as follows. Re ferring to Fig. 2, A=Etfective area of diaphragms (assumed equal) on which the input (P and feedback (P pressures react. k=Gradient (low) of spring loading on the stack. \,,=Slope of flapper-nozzle curve (psi/inch, etc.), G,,=Pressure gain through the pilot valve relay.

The equations of response across the unit may be developed as follows:

Referring to Figure 1: 1) P.= A.G. P.- P.)

where A, k, X and G, are defined above.

For the case where (If G co v 16 Referring again to Figure 2:

P,,-P,, P,,P, (3) I r- R3 assuming:

(a) Ohms law for fluid flow through a pneumatic restriction.

(b) Negligible volume at the junction between R and R3.

From the law for a perfect gas:

where C,=the volumetric capacitance of the volume chamber in which R, is terminated. Substituting Equation 4 into Equation 3:

actuator, etc.), which leads the signal .transmitted to its input motor chamber, and hence will offer additional compensation for distributed resistance-capacitance eifects in a transmission line.

In theory, the bi-lateral action of the device described in the above will thereby permit a single compensated proportional plus rate receiver to perform as two unilateral uncompensated proportional plus rate receivers acting in tandem.

The operation of the pneumatic circuit of this inven- Substituting (6) into the equality (given in (5)),

n( r( 3+ 2) o( 1 3 From Equation 2:-

RAMP;- Thus, substituting Equation 2 into Equation 7:

- t( r( 3+ 2) o( 1- 8 +1) which is the transfer function across a compensated proportional' plus rate unit; where a=rate amplitude ratio, and T =rate time. (Note that adjustments maybe made on both a and T Referring again to Figures 1 and 2 and noting that air flow (19 into the unit is as surned positive: t

Starting with the equation:

J) P.- R P,

and substituting Equation 1 from column 7, into the above;

For Z,-(P)=- %:(P)=input impedance of the unit:

Note that since oc 1, the input impedance of the unit has a negative sign associated with it. -A'circuit with an analogous input impedance as illustrated in Fig. 3. Referring to Figure 3,

Comparing Equations'Z and 3:

(Rs appears to be dead ended).

for a and (RH-R C, for T,

and rearranging terms in Equations 4 and 5:-the settings on R R and R required for desired values of R 0,, and'T, may be determined from the following equations:

a de) 12.0. T, 0. Rio.

Note that required values of R; and C when substituted in the above, will be negative in sign.

Note, also, that adjustments in R R and R will satisfy -a complete range in values for R1, l, and T in cases where optimum performance requires the rate time (T,) of the receiver toexceed the time constant (R C of its input matching impedance. Otherwise compromises will have to be made in the bi-lateral compensating nature of the receiver v From a theoretical analysis made on the characteristics of tubing when loaded, the following have been determined as optimum settings for R R and R in terms of the length of standard in. ID. tubing terminated by the receiver, when compensating the efiects of distributed transmission lag:

R a.04n p.s.i./in. 3/sec.

tubing length (as feet The above is based on an operating pressure level of 9 p.s.i.g. and a S-in. volume, terminating thepressure divider circuit in the negative feedback circuit of the receiver.(i.e., C,=.2l in. ./p.s.i.).

While, in accordance with the provisions of the statutes, :I have illustrated and described the best forms of the invention now known to me, it will be apparent to those skilledtinthe art that .changesmay be made in the form of the apparatus disclosed without departing from 9 the spirit of the invention as set forth in the appended claims, and that in some cases certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

Having now described my invention what I claim as new and desire to secure by Letters Patent is as follows:

1. A circuit operated by the pressure of an elastic fluid, including, a transmitter responsive to a measuring element which senses variations in a variable, said transmitter controlling the pressure of a first supply of elastic fluid, an elongated transmission line having a small cross section and connected to said first supply and causing time delays in the transmission of the variations in the pressure of said first supply because of the resistance and capacitance of said transmission line, and a receiver connected to said transmission line and controlling the operation of a final control element operated in response to the variations in the variable, said receiver comprising, an input motor connected to said transmission line and having a small capacitance compared to the capacitance of said transmission line and responsive to the variations in the pressure of said first supply, a first valve actuated by said input motor in one sense and controlling the pressure of a second supply of elastic fluid, a second valve operated by the pressure of said second supply of fluid and controlling the pressure of a third supply'of fluid adapted for connection to the final control element, a negative feedback motor connected to said third supply and actuating said first valve in a sense opposite to that in which said first valve is actuated by said input motor, a manually variable series restriction connected between said third supply and said negative feedback motor, a manually variable bypass restriction connected at its input to the output of said series restriction and to said negative feedback motor, a bypass chamber having a large capacitance compared to the capacitance of said negative feedback motor and connected to the output of said bypass restriction, and a manually variable positive feedback restriction connected at its input to said third supply and to the input of said series restriction and connected at its output to the input to said input motor, the resistance of each of said restrictions and the capacitance of said bypass chamber being so adjusted that said positive feedback restriction causes a variation in flow to be conducted to said transmission line and to said input motor, said variation in flow being large enough to compensate for the time delay in the transmission of the variations in the pressure of said first supply because of the resistance and capacitance of said transmission line.

2. In a connecting circuit conducting an elastic fluid under pressure and connecting a measuring instrument to a final control element spaced from said measuring instrument, means for compensating for the time delay due to the resistance and capacitance of said circuit and including, an input motor actuated by the pressure of an elastic fluid, a valve actuated in one sense by said motor and controlling the pressure of a supply of fluid from a source of fluid under pressure, a negative feed-back motor actuated by the pressure of said supply of fluid and actuating said valve in the opposite sense to that in which it is actuated by said input motor, a negative feed-back conduit conducting an elastic fluid under pressure and connecting said supply of fluid to said negative feed-back motor and including resistance and capacitance, and a positive feed-back circuit conducting elastic fluid under pressure and connecting said supply of fluid to sa1d mp ut motor and having resistance and capacitance, sa1d circuits cooperating to compensate for the time delay due to the resistance and capacitance of said connecting circuits.

3. In a connecting circuit, means for compensating for the time delay due to the resistance and capacitance of said circuit and located adjacent a final control element and including, an input motor actuated by the pressure of an elastic fluid, a first valve actuated in one sense by said input motor'andcontrolling the pressure of a first supply of fluid from a source of fluid under pressure, a second valve'actuated by the pressure of said first supply of fluid and controlling the pressure of a second supply of fluid from a source of fluid under pressure, a negative feedback motor actuated by the pressure of an elastic fluid and connected to said first valve to actuate it in the opposite sense to that in which it is actuated by said input motor, anegative feed-back conduit conducting an-elastic fluid under pressure and connecting sa1d second supplyof fluid and said negative feed-back motorand having a capacitance and resistance, and a posiuve feed-back conduit conducting an elastic fluid under pressure and connecting said second supply of fluid and said input motor and having a resistance and capacitance, said circuits cooperating to compensate for the time delay due to the capacitance and resistance of said connecting circuits 4. In a connecting circuit, means for compensating for the time delay due to the resistance and capacitance of said circuit and located adjacent to a final control element and including, an input motor actuated by the pressure of an elastic fluid, a valve actuated by said input motor in one sense and controlling the pressure of a supply of fluid from a source of fluid under pressure, a negative feed-back motor actuated by the pressure of an elastic fluid and actuating said valve in the sense opposite to that in which said valve is actuated by said input motor, a negative feed-back circuit conducting elastic fluid under pressure and connecting said supply of fluid and said negative feed-back motor and having resistance and capacitance, a restriction to the flow of elastic fluid in said negative teed-back circuit connected in series between said supply of fluid and said negative feed-back motor, and a positivetfeed-back circuit conducting an elastic fluid under pressure and connecting said supply of fluid and said input motor and having resistance and capac ltance, said circuits cooperating to compensate for the time delay due to the resistance and capacitance of said connecting circuits.

5. In a connecting circuit connectinga measuring element and a final control element spaced from said measurlng element, means for compensating for the time delay caused by the resistance and capacitance of said circu1t and including, an input motor actuated by the pressure of an elastic fluid, a valve actuated by said input motor in one sense to control the pressure of a supply of fluid from a source of fluid under pressure, a negative feed-back motor actuated by an elastic fluid under pressure and connected to said valve to actuate it in a sense opposite to that in which said valve is actuated by said input motor, a negative feed-back circuit conducting an elastic fluid under pressure and connecting said supply of fiuld to said negative feed-back motor and having a res1stance and capacitance, a positive feed-back circuit conducting an elastic fluid under pressure and connecting said supply of fluid and said input motor, and a restriction to the flow of fluid connected in said positive feed back motor between said supply of fluid and said input motor, said circuits cooperating to compensate for time delay due to the resistance and capacitance of said connecting circuit.

6. In a connecting circuit conducting an elastic fluid under pressure, means for compensating for the time delays due to the resistance and capacitance of said circuit and located adjacent a final control element and includmg, an input motor actuated by an elastic fluid under pressure, a valve actuated in one sense by said input motor to control the pressure of a supply of fluid from a source of fluid under pressure, a negative feed-back motor actuated by an elastic fluid under pressure and connected to said valve so as to actuate it in a sense opposite to that in which said valve is actuated by said input motor, a negative feed-back circuit conducting an elastic fluid un- 11 de p e su and c nnecting said' vpp y o fl id and sa negative feed-back motor, a restriction to the flow of fluid connected in series-between the said supply of fluid and said negative feed-back motor, a restriction to the flow of fluid connected in said negative feed-back circuit in parallel with said negative feed-back motor, a closed chamber having a greater capacity for fluid than said negative feed-back motor and connected to the output side of said bypass restriction, and a positive feed-back conduit conducting an elastic fluid under pressure and connecting said supply of fluid to said input motor and having a resistance and capacitance, said conduits coperating to compensate for time delay due to the resistance and capacitance of said connecting circuit.

7. In a connecting circuit, means for compensating for the time delay due to the resistance and capacitance of said circuit and located adjacent a final-control element and including, an input motor actuated bythe pressure of an elastic fluid, avalve actuated in one sense by said input motor to control the pressure of a supply of fluid from a source of fluid under pressure, a negative feedback motor actuated by' the pressure of an elastic fluid and connected to said valve so as to actuate it in a sense opposite to that in which said valve is actuated by said input motor, a negative feed-back circuit conducting an elastic fluid under pressure and connecting said supply of fluid and said negative feed-back motor, a restriction to the flow of fluid connected in series between said supply of fluid and said negative feed-back motor, a restriction to the flow of fluid connected in said negative feedback circuit in parallel to said negative feed-back motor, a closed chamber having a greater capacity for fluid than said negative feed-back motor and connected to the output side of said by-pass restriction, a positive feed-back circuit conducting an elastic fluid under pressure and connecting said supply of fluid and said input motor and having a resistance and capacitance, and a restriction to the flow of fluid connected in said positive feed-back circuit between the input to said input motor and the input to said negative feed-back circuit, said conduits cooperating to compensate for the time delay due to the resistance and capacitance of said connecting circuit.

8. In a connecting circuit connecting a measuring element to a final control element, an elongated transmission line conducting elastic fluid under pressure, means for compensating for the time delay is due to the resistance and capacitance of said transmission line and including, an input motor actuated by the pressure of an elastic fluid and connected to that end, of said transmission line adjacent to final control element, a valve connected to said input motor so as to be actuated thereby in one sense to control the pressure of a supply of fluid from a source of fluid under pressure, a negative feed-backtmotor actuated by the pressure of an elastic fluid and connected to said valve to actuate said valve in a sense opposite to that in which said valve is actuated by said input motor, a negative feed-backcircuit conducting an elastic fluid under pressure and connected to said supply of fluid and to said negative feed-back motor and having a resistance and capacitance, and a positive feed-back conduit conducting elastic fluid under pressure and connecting said supplygof fluid and said input motor and having a resistance and capacitance, said circuits cooperating to compensate for the time delay due to the resistance and capacitance of said connecting circuit.

9. A connecting circuit conducting an elastic fluid under pressure and connecting a measuring element to a final control element and including, a measurement-responsive element, a transmitter valve actuated by said measurement-responsive element and varying the pressure of a first supply of fluid from a source of fluid under pressure, an elongated transmission line connected to said first supply of fluid, an input motor actuated by the pressure of an elastic fluid and connected to the other end of said transmission line and located adjacent to said final control element, a receiver valve actuated in one sens-e by said input motor and controlling the pressure of a second supply of fluid from a source of fluid under pressure, a negative feed-back motor actuated by the pressure of an elastic fluid and connected to said receiver valve and actuating it in the sense opposite to the sense in which said receiver valve is actuated by said input motor, a negative feedback circuit conducting elastic fluid under pressure and connecting said second supply of fluid to said negative feed-back motor and having a resistance and capacitance, and a positive feed-back circuit conducting elastic fluid under pressure and connecting said second supply of fluid to said input motor and having a resistance and capacitance, said circuits cooperating to compensate for the time delay due to the resistance and capacitance of said connecting circuit.

10. In a connecting circuit conducting an elastic fluid under pressure and connecting a measuring element to a final control element, means for compensating for the time delay due to the resistance and capacitance of said circuit and including, an elongated transmission line con ducting an elastic fluid at a pressure which is variable in response to a deviation in a measured variable, an input motor connected to one end of said transmission line and adjacent to said final control element and responsive to variations in said measured variable pressure, a valve mechanically connected to said input motor and operable thereby to change the pressure of an elastic fluid, a nega tive feed-back motor responsive to the variations in the pressure of said fluid and mechanically connected to said valve to operate it in the opposite sense to that in which it is moved in response to said deviation, a series restriction connected between said first fluid under pressure and said negative feed-back motor, a bypass restriction connected at one end to the output of said series restriction and to said negative feed-back motor, a bypass chamber connected to the opposite side of said bypass restriction and containing an elastic fluid under pressure, and a third restriction connected at its input to said elastic fluid under pressure and to the output of said series restriction and connected at output to said input motor and manually adjustable and connected so as to conduct a variation in the pressure of the elastic fluid to said input motor, which variation is of a suitable size to compensate for the time delay in the transmission of said measured variable to said input motor.

References Cited in the file of this patent UNITED STATES PATENTS Farrington July 24, 1956

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