US2638874A - Pneumatically-operated control instrument with follow-up - Google Patents

Description

y 1953 E. H. WOODHULL PNEUMATICALLY-OPERATED CONTROL INSTRUMENT WITH FOLLOW-UP Filed March 17, 1950 INVENTOR. ELLIOT H. WOODHULL BY A) (Ifl ATTORNEY Patented May 19, 1953 PNEUMATICALLY- OPERATED CONTROL INSTRUMENT WITH FOLLOW-UP Elliot H. Woodhull, Rochester, N. Y., assignor to Taylor Instrument Companies, Rochester, N. Y., a corporation of New York Application March 17, 1950, Serial No. 150,139

10 Claims.

This invention relates to a pneumatically-opderivative, and/or reset responses.

The present invention has for its purpose the provision of a controller of the mentioned type in which the sensitivity thereof can be adjusted over an especially wide range without the use of a system of mechanical levers; in which the follow-up mechanism is essentially free of friction; and in which the derivative response can be adjusted to a time constant as low as three seconds without the need of using an unusually large capacity in the controller output line.

The various features of the invention will appear from the detailed description and claims when taken with the drawings in which:

Fig, 1 diagrammatically illustrates the control instrument of the present invention incorporated in a temperature control system; and

Fig. 2 is a side view of the follow-up unit forming a part of the instrument.

In the control system illustrated in Fig. 1, there is provided a tank T containing material, the temperature of which is to be accurately maintained at a given value. A temperature regulating medium such as steam, is introduced into this tank through a pipe P, under the control of a diaphragm motor valve V. In order to sense variations in the temperature of the material in the tank, so that the valve V can be positioned to accurately govern the flow of the temperature correcting medium to the tankT, there is provided a thermosensitive tube system comprising a bulb communicating through a capillary tube 6 with a hollow spiral Bourdon spring I. The tube system, the bulb 5 of which projects into the material in the tank, is filled with a sensitive fluid.

The inner end of the Bourdon spring I is fixedly mounted at 8 while its outer or free end is provided with a take-off arm 9. This take-ofi arm is connected by a link to one end of a baflle lever II, the other end of which is pivotally mounted at [2 on a fixed support. The intermediate portion of the lever carries a baflle l3, herein illustrated as being cylindrical in form. A nozzle I4 carried on the free end of the nozzle pipe [5 wastes air to the atmosphere under the control of the baffle 23. The nozzle pipe is pivoted at l6 and at this region there are introduced several coils in the pipeto give it flexibility. The nozzle pipe communicates through the conduit I1 and a restriction [8 with a source of compressed air or the like. It will be understood that as the temperature changes at the bulb 5 of the tube system, the Bourdon spring I will move the baflle I3 toward or away from the nozzle [4 depending on the direction of the temperature change. This will vary the back pressure in the nozzle pip l5 which communicates through conduit [9, restriction 20 and capacity 2|, with the capsular chamber 23 of a power relay valve 24. A source of compressed air or the like is-supplied to the valve 24 which under the control of the back pressure in the capsular chamber 23 effects an amplified pressure change at the output of relay valve 24 that is, for example, ten times greater than the back pressure change. The compressed air in the output of relay valve when controlled in the manner just described, is applied through conduit 26, to the diaphragm motor top of the valve V. It will be understood that the restriction 20 and capacity 2| serve to keep pneumatically stable, the follow-up mechanism to be described. These elements also serve to render the relay valve 24 less susceptible to mechanical vibration of the several parts of the control mechanism since the mentioned elements do not permit this valve to give a full amplitude response to balile motions, having a frequency larger than, for example, one cycle per second.

If, for example, the temperature at the bulb 5 increases, the Bourdon spring 1 will raise the baffle a very slight distance away from the nozzle thereby reducing the back pressure in the nozzle pipe l5 and the capsular chamber 23 tending to open the inlet port of the relay valve 24, a slight amount. Compressed air at an increased pressure is supplied through this relay valve to the diaphragm motor of the valve V, tending to shut ofi the steam to the tank T. A drop in temperature at the bulb efiects a similar operation but in the reverse sense.

The pipe l9 also communicates through the adjustable needle valve 21 and a capacity 28 as Well as through a pipe 29 with the capsular diaphragm 30 of a force balance type of amplifying relay valve 3| which is herein indicated within the broken line rectangle. It should be mentioned that the needle valve El and capacity 28 tend to introducean adjustable time delay in the follow-up action to be described, thereby providing what is known in the art as derivative action. For those applications where derivative action is not required pipe l9 may communicate directly through a pipe with capsular diaphragm 30. The diaphragm 30 is provided with an upwardly extending stud 32 in'a position to engage the under side of the movable end of a bafiie lever 33." This baflie lever is pivotally supported at its left end;

on a leaf hinge 3d. The right end of this lever carries a bafile in operative relation to a nozzle 35. A spring 37 having one end engaging a fixed part of the frame and its other end engaging the top of the baffle lever tends to hold this lever 33 in contact with the stud 32. The nozzle 36 communicates through a pipe 38 and the restriction 39, with a source of compressed air or the like. Pipe 38 also communicates through a restriction 40 with a capsular diaphragm M. This diaphragm is provided with a downwardly directed stud 42 positioned to engage the upperside of the baffle lever 33. By this construction the two capsular chambers since they tend to move the baille lever in opposite directions, will be effective to position the baflle 35 with respect to the nozzle until the change in pressure on capsular diaphragm 4| tends to produce a moment in bafile lever 33 which opposes any moment produced by changes in pressure on diaphragm 30. The several parts may be so designed that the amplification between input pressure and output pressure is, 'for example, ten. It should also be pointed out that the restriction 49 and the capacity of the capsular diaphragm chamber 41 serve to keep pneumatically stable the followup mechanism to be described. As built, these elements 60 not permit diaphragm M 'to give a full amplitude response to pressures in 38 having a frequency larger than about 1 cycle per second.

The pipe $8 communicates with a bellows 44 or a followup and reset bellows unit, the construction of which is illustrated in Fig. 2. This bellows uni-t comprises a fixed support 45 to the lower surface of which there is sealed the open ens-0r the bellows M. A. second bellows 46 has its open'end sealed to the upper end of the support "so that the free end of this bellows projects upward. A rectangular yoke 49 engages the free ends of the bellows moved thereby. Thus when the pressure in bell'ows '45 is less than that in bellows M, the .yoke 49 will move downward and since this yoke is connected by a leaf hinge 50 to the nozzle pipe 15, this pipe will-also move downward 'to-separ'ate the nozzle Hi from the baflle l3 a slight amount. Ina-"si-inila'rmanner when the pressure i'n bell'ows 46 is greater than that in bellows "M, the several 5 'rts will operate in the reverse sense to make t nozzle '44 approach the ba'file l3. It should ber'nentioned that the spring 5i attached to yoke 49 can have 'its tension adjusted by the knurled "adjusting knob '52 so that one or the other of the bellows 44 and All can be stretched and the other con'ipressed until their eiiective areas are equal. "It was found experimentallythatastandard bellows changes its effective area with position such that as the bellows is extended from i-t's free length the eire'ctive area decreases and as it compressed its efiective area increases. This effective area is not changed by the magnitude of pressure in the bellows -and dep'ends upon'pos'ition alone. Bym eans of biasing spring, such as5 I, it is-possibl'e to take two bellows which are of the same nominal area and extend one and simultaneously com-press the other until the areas "are exactly matched.

'A branch-pipe connection 54, which'communicates with pipe 38, also communicates through the sensitivity adjusting needle valve and "through pipes 56 and ill to the interior of bellows 46. This last mentioned connection together with a connectionincluding restriction -66 and pipe 57, to be described, supply -.pressur-e =fiuid to bellows '46 which tends to oppose the 44 and the bellows "46 to be the'c-ap'sular chamber 3i).

followup action of bellows 44 by an adjustable amount to provide adjustable sensitivity where adjustable sensitivity is defined as the amount of opening or closing of valve V in response to a given movement of baffle 13. The preferred locations of restrictions 55 and 6 are as shown in Fig. 1, although, they may be interchanged. The branch pipe 54 also communicates through pipe El, reset time adjusting needle valve 58, capacity 59 and pipeiiil, to the diaphragm chamber 6| of the one to one relay valve '53. By the use of such a relay valve, the output pressure thereof to restriction 66 is equal to the input pressure in pipe '60 leading thereto. Thus this relay functionsto apply the pressure of line 60 to restriction '6i5 without permitting direct flow between Bil and 66. Any one-to-one relay will be suitable for this purpose.

The relay herein disclosed comprises the input or diaphragm chamber 6! defined by the diaphragm 62 and the lower part of the shallow cylindrical casing 64. The upper part of the casing together with the diaphragm 62 defined the output chamber 65 of the relay, which communicates through the restriction 6% with the pipe 51. Asource'of pressure fluid such as compressed air is supplied through pipe 6'} and restriction 68 to the output chamber. The top of the casing is alsoprovided with an opening which functions as nozzle 68, while a ball valve l0 actuated by the diaphragm B2, acts as a baiile to govern the waste of compressed air from the output chamber to the atmosphere until the pressures in the output and input chambers are substantially equal. The effective area of the top surface of the diaphragm is reduced by the area of the nozzle 69. This tends to make the gain of the relay greater than unity, since a greater change in pressure in the upper chamber than in the lower chamber is required to bring about an equilibrium condition in these chambers. However, by properly selecting the spring rate of spring 1! the gain can be made almost exactly unity. 'Theinitial-compression in spring H must be greater than s'upplypressure times port area '69 so that the ball 70 will never fail to make contact with plate ill.

I The arrangement of the power relay 24 in the pneumatic circuits of the present controller, makes it possible to operate from the outputpressure of this power relay, a valve V with a diaphragm motor of any desired volume without in any way changing the proportional, reset and derivative responses which are all eiiected directly from the back pressure in the nozzle pipe.

Ithas been pointed out above howan increase in temperature at the bulb '5 operates the Bourdon spring 7 to raise the baffle 63 a very slight distance away from the nozzle, thereby reducing the backpr'es'sure in the nozzl pipe l5, andalso that this 'reduction in back pressure actuates the relay valve 24 which supplies compressed air at increased pressure to the diaphragm -motor of the'valve V, tending to shut off the steam to the tank T. -At -the saine time the reduction in back pressure in-nozzle pipe E5 is supplied through the needle valve '21, theca'pacity 28, and pipe 29 to This results in a slight withdrawal of the baffle 35 from the nozzle 36 thereby modifying the pressure of the airsupplied through the restriction 39, pipe-38, to the capsular diaphragm ll. The pressure of the air thus supiplied is reduced until the pressure in the capsular diaphragm 4! is equal to that in the capsular diaphragm 36.

The reduction'in pressure in pipe 38 is communicated to the bellows 44 which tends to contract, thereby moving the nozzle pipe and its nozzle toward the bailie 30 at the same time, but at a delayed rate the reduced pressure in the pipe 34 is communicated through pipe 60 and the one-to-one relay valve 63 and pipe 51, leading to the bellows 46. Bellows 46 tends to cancel out the follow-up action of the bellows 44, thereby tending torestore the initial bafiie nozzle separation. A drop in pressure at the bulb '5 assures a similar operation of the controller but in the reverse sense.

It should be noted that the sensitivity adjustment in the present controller, is effected by simply turning the knob of the needle valve 55', the operation of which is such that its reference scale can have uniformly spaced graduations throughout. This construction obviates the need of levers and pivots necessary in conventional sensitivity reducing arrangements.

As compared with prior controllers of this type, the present controller has a follow-up unit in which the number of pivots has been so reduced that friction has been greatly minimized with the resultant increase in accuracy of operation.

What I claim is:

1. In a controller, mechanism for sensing changes in a variable condition, a source of elastic fluid underpressure, a pneumatic couple comprising a nozzle element and a bafile element relatively movable with respect to each other, a restriction through which said fluid is supplied to said nozzle, means including said mechanism for relatively moving said baflle and nozzle whereby the back pressure in the nozzle is changed in amounts corresponding to changes in the variable, a pneumatically operated control device, means responsive in correspondence with said back pressure for supplying throttled pressure fluid to such device, a follow-up unit tending to reduce any given relative movement of said elements without moving said mechanism, said follow-up unit including two pressure chambers and means responsive to the difierence in pressure in said chambers for moving one of said elements in a direction tending to reduce any relative movement therebetween, means for applying to one of said chambers'a pressure corresponding to said back pressure, and governing means for supplying fluid under modifying 0 pressure to said other chamber, said governing means comprising two restrictions each of which communicates with said other chamber, one of which restrictions is supplied with fluid under a given pressure, the other of the restrictions being, supplied with fluid under a pressure proportional to said back pressure.

2. In a controller, mechanism for sensing changes in a variable condition, a source of elastic fluid under pressure, a pneumatic couple comprising a nozzle element and a bafile element relatively movable with respect to each other, a restriction through which said fluid is supplied to said nozzle, means including said mechanism for relatively moving said baffle and nozzle in response to changes in said variable whereby the back pressure in the nozzle is changed in amounts corresponding to said changes, a pneumatically operated control device, means responsive in correspondence with said back pressure for supplying throttled pressure fluid to such device, a followup unit tending to reduce any given relative movement of said elements, said followup unit including two pressure chambers, means responsive to the difference in pressure in said chambers for moving one of said elements in a direction tending to reduce any relative movement therebetween without substantially interfering with the free deflection of said mechanism, means for applying to one of said chambers a pressure corresponding to said back pressure, a pressure divider circuit consisting of a fixed restriction. and a variable restriction in series, means for supplying a fixed pressure to one end of said divider circuit, means for supplying a pressure corresponding to said back pressure to the other end of said pressure divider circuit, and means for supplying the resultant pressure occurring between the fixed and variable restrictions of said pressure divider to said other pressure chamber.

3. In a controller, mechanism for sensing changes in a variable condition, a source of elastic fluid under pressure, a pneumatic couple comprising a nozzle element and a bafile element relatively movable with respect to each other, a restriction through which said fluid is supplied to said nozzle which nozzle wastes fluid to the atmosphere, means including said mechanism for relatively moving said battle and nozzle in response to changes in said variable whereby the back pressure in the nozzle is changed in amounts corresponding to said changes, a pneumatically operated control device, means responsive in correspondence with said back pressure for supplying throttled pressure fluid to such device, a followup unit tending to reduce any given relative movement of said elements, said followup, unit including two pressure chambers, means responsive to the difference in pressure in said chambers for moving one of said elements in a direction tending to reduce anyrelative movement therebetween, means for applying to one of said chambers a pressure corresponding to said back pressure, a pressure divider circuit consisting of a fixed restriction and a variable restriction in series, means for supplying a fixed pressure to one end of said divider circuit, means for supplying a pressure corresponding to said back pressure to the other end of said pressure divider circuit, and means for supplying the resultant pressure occurring between the fixed and variable restrictions of said pressure divider to said other pressure chamber.

4. In a controller, mechanism for sensing changes in a variable condition, a source of elastic fluid under pressure, a pneumatic couple comprising a nozzle element and a bafile element relatively movable with respect to each other, a restriction through which said fluid is supplied to said nozzle, means including said mechanism for moving one of said elements relative to the other in response to changes in said variable whereby the back pressure in the nozzle is changed in amounts corresponding to said changes, a pneumatically operated control device, means responsive in correspondence with said back pressure for supplying throttled pressure fluid to such device, a followup unit tending to reduce any given relative movement of said elements, said followup unit including two pressure chambers, means responsive to the difference in pressure in said chambers for moving one of said elements in a direction tending to reduce any relative movement therebetween while applying substantially no force tosaid mechanism, means for applying to one of said chambers an actuating pressure corresponding to said back pressure, a pressure divider circuit consisting of a fixed restriction and a variable restriction in series, means for nearer-4 supplying said actuatmg pressure to one. end of the pressure divider :circuit, means which include a restrirtionand acapacity for supplying a mo difled actuating-pressure to the other end of the pressurerdivider circuit, and means for supplying the resultant pressure occurring between the fixed and variable restrictions or" said pressure divider te-said other pressure chamber.

5. In a controller, mechanism for sensing changes in a variable condition, a source of elas .tic fluid under pressure, azpneuniatic .coup'lescomprising a nozzle element and a baffle element relatively movable with respect to each other, a restriction through which .said fluid is supplied to said nozzle which nozzle wastes fluid to the atmosphere, means including said mechanism for relatively moving said baffle and nozzle in re- ;sponse to changes in said variable whereby the back pressure in the nozzle ischanged inamounts corresponding to said changes, a pneumatically operated control device, means responsive in correspondence with said :back pressure for supplyi'n'g throttled pressure fluid to such device, a iollowup unit tending to reduce any given relative movement of said elements, said followup unit including two pressure chambers, means responsive :to the difference in pressure in said chambers for moving one of said elements in a direction tending to reduce any relative movement therebetween, means for applying to one of said :chambers can actuating pressure corresponding toisaid back pressure, a pressure divider circuit consisting of a fixed restriction and a variable restriction in series, 'means for supplying said actuating pressure to one end of the pressure divider circuit, means which include another variable restriction and acapacity for supplying :a :modified actuating pressure to the other end of the pressure divider circuit, and means for supplying the resultant pressure occurring between the fixed and variable restrictions of said pressure divider to said other pressure chamber.

6. In .a controller, mechanism for sensing cihanges'ina variable condition, a source of elastic fluid under pressure, a pneumatic couple comprising a nozzle element and a bafiie element relatively movable with respect to each other, a restriction through which said fluid is supplied to said nozzle, means including said mechanism for moving one of saidlelements relative to the other in'response'ito'changes in said variable'whereby the back pressure in the nozzle is changed umountsrorresponding to said changes, apneuinatically operated control :device, means respon- -sive :in icorrespondence with said ham pressure for supplying throttle-d pressure fluid to -such device, a-iollowup-unit tending to :reduce any-given :rla'tive movement-of said "elements, said followup =unit including two pressure chambers, means responsive to the difference in pressure in said 'dhanibers for moving one of said elements in a 'direction tending to reduce any relative movement therebetween without substantially interferi'ng vvlth the free deflection of said mechanism, :means ior :applying to one of .asai'd chambers an actuating pressure corresponding to said back. pressure, a pressure divider circuit consisting of a fixed restriction and a variable restriction in series, means for supplying saidactuating pres- .1sure to one end :of the pressure divider circuit, means which include, another variable restriction, inapacity and relay valve forsupplying a modified actuating pressure to the:other endof the pres- :sure 'diVldI';.ClI8Uit, and means for supplying .the :resu-ltant pressure occurring between the. fixed '8 and variable restrictions of said pressure divider to said other pressure chamber.

7. In a controller, mechanism for sensin changes ma variable condition, a source of elastic fluid. under pressure, a pneumatic couple comprising a nozzle element and a baffle element relatively movable with respect to each other, a restriction through which said fluid is supplied to said. nozzle which nozzle wastes fluid to the atmosphere, means including said mechanism for relatively moving said battle and nozzle in response to changes in said variable whereby the back pressure in the nozzle is changed in amounts corresponding to said changes, .a pneumatically operated control device, means responsive incorrespondence with said back pressure .for supplying throttle-d pressure fluid to said device, a 01- :lowup unit tending to reduce any given relative movement of said elements, said followup .unit including two pressure chambers, means responsive to the difierence in pressure in said cham- Lbers for moving one of said elements in a direc- .tion tending to reduce any relative movement therebetween, means for applying to one of said chambersianactuating pressure corresponding tosaid back pressure, a pressure divider circuit .consisting of a fixed restriction and ,a variable restriction in series, means for supplying said actuating pressure :to one end of the pressure divider circuit,smeans which nclude another variable restriction, capacity, and relay valve for supplying modified actuating pressure to the other end .of :i-he pressure divider circuit, and means for supplying :the resultant pressure occurring bethe fixed and variable restrictions of said pressure divider to said other pressure chamber.

$8. In a controller, mechanism for sensing changes ina variablecondition, asource oi elas- :ticifiuid underpressure, .a pneumatic couple-comprising :a nozzle element and a baffle element relatively movable with respect to each other, a restriction through which said fluid is supplied-to said nozzle, means incl ng said mechanism for relatively moving said bafile and nozzle in re sponse to changes in said variable whereby the back pressure in the nozzle is changed in amounts corresponding to .said changes, a pneumatically operated-control device, means responsive in cor-- .respondence with said back pressure for supplying throttled pressure fluid to such device, a -.f ollo,wup unit tending to reduce any given rela- .tive ,movement of said elements, followup unit including two pressure cha means responsive to the difference in pres lie in said ..cliambers vfor moving one of elements in a direction tending to reduce any relative move ment there'between without moving said mechanism, means for applying to one of said chainhers a pressure corresponding to said throttled pressure, a pressure divider circuit consisting of a fi-Xed restriction and a variable restriction in series, ,means for supplying a fixed pressure to one end of said divider circuit, means for supplying pressure corresponding to said 'throttled pressure to the other end or" said pressure divider circuit, and means "for supplying the resultant pressure occurringbetween the fixed and variable restrictions of said pressure divider to said other pressure chamber.

9. In a controller, mechanism for changes in a variable condition, a source of elastic fluid under pressure, a pneumatic couple comprising a nozzle element and a bafile element relatively movable with respect to each other, a restriction through which ,said :fluid is supplied to said nozzle, means including said mechanism for moving one of said elements relative to the other in response to changes in said variable whereby the back pressure in the nozzle is changed in amounts corresponding to said changes, a pneumatically operated control device, means responsive in correspondence with said back pressure for supplying throttled pressure fluid to such device, means for producing an actuating pressure which means includes a resistance, capacity and relay valve to which said back pressure is applied, a followup unit tending to reduce any given relative movement of said elements, said followup unit including two pressure chambers, means responsive to the difference in pressure in said chambers for moving one of said elements in a direction tending to reduce any relative movement therebetween, means for applying to one of said chambers said actuating pressure, a pressure divider circuit consisting of a fixed restriction and a variable restriction in series, means for supplying said actuating pressure to one end of the pressure divider circuit, means which include another variable restriction, capacity, and relay valve for supplying a modified actuating pressure to the other end of the pres-- sure divider circuit, and means for supplying the resultant pressure occurring between the fixed and variable restrictions of said pressure divider to said other pressure chamber.

10. In a controller, mechanism for sensing changes in a variable condition, a source of elastic fluid under pressure, a pneumatic couple comprising a nozzle element and a baiile element relatively movable with respect to each other, a restriction through which said fluid is supplied to said nozzle, means including said mechanism for moving one of said elements relative to the other in response to changes in said variable whereby the back pressure in the nozzle is changed in amounts corresponding to said changes, a pneumatically operated control device, means responsive in correspondence with said back pressure for supplying throttled pressure fluid to such device, means for producing an actuating pressure which means includes a restriction, capacity and relay valve to which said back pressure is applied, a followup unit tending to reduce any given relative movement or" said elements, said followup unit including two pressure chambers, means responsive to the difference in pressure in said chambers for moving the one of said elements in a direction tending to reduce any relative movement therebetween, means for applying to one of said chambers said actuating pressure, a pressure divider circuit consisting of a fixed restriction and a variable restriction in series, means for supplying said actuating pressure to one end of the pressure divider circuit, means for supplying a given pressure to the other end of the pressure divider circuit, and means for supplying the resultant pressure occurring between the fixed and variable restrictions of said pressure divider to said other pressure chamber.

ELLIOT H. WOODHULL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,117,800 Harrison et al May 17, 1938 2,292,761 Krogh Aug. 11, 1942 2,299,884 Edwards Oct. 27, 1942 2,400,048 Jones May 7, 1946 2,440,183 Nisbet Apr. 20, 1948 2,443,891 Buerschaper June 22, 1948 2,475,894 Hermanny July 12, 1949 2,484,557 Eckman Oct. 11, 1949

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