US3633604A

US3633604A - Fluid-operated control apparatus

Info

Publication number: US3633604A
Authority: US
Inventors: Gerhard Klee
Original assignee: Samson Apparatebau AG
Current assignee: Samson Apparatebau AG
Priority date: 1966-09-05
Filing date: 1969-10-20
Publication date: 1972-01-11
Anticipated expiration: 1989-01-11
Also published as: SE335025B; DE1523657A1; US3489064A; DE1523657B2; GB1201386A

Abstract

The specification describes a rotary summating apparatus comprising a rotary disc which is arranged to be rotated by a fluid pressure-mechanical transducer in steps. Pressures whose values are to be added together are fed in sequence to the transducer so that the rotary part moves in steps proportional to the values of the pressures.

Description

United States Patent [72] Inventor Gerhard Klee Frankfurt am Main-Ginnheim, Germany [21] Appl. No. 867,780 [22] Filed Oct. 20, 1969 [45 Patented Jan. 11, 1972 [73] Assignee Samson Apparatebau A.G.

Frankfurt am Main, Germany [32] Priority Sept. 5, 1966 [3 3 Germany [3 I] S 105696 Original application Sept. 5, 1967, Ser. No. 665612, now Patent No. 3,489,064, dated Jan. 13, 1970. Divided and this application Oct. 20, 1969, Ser. No. 867,780

[54] FLUID-OPERATED CONTROL APPARATUS 4 Claims, 16 Drawing Figs.

[52] US. Cl 137/83, 91/44, 91/388 [51] Int. Cl F151) 5/00 [50] Field of Search 137/83; 91/41, 44; 235/91 [56] References Cited UNITED STATES PATENTS 3,064,464 11/1962 Black 91/44 X 3,068,650 12/1962 Phillips 91/44 X Primary Examiner-Alan Cohan Attorney-Watson, Cole, Grindle & Watson ABSTRACT: The specification describes a rotary summating apparatus comprising a rotary disc which is arranged to be rotated by a fluid pressure-mechanical transducer in steps. Pressures whose values are to be added together are fed in sequence to the transducer so that the rotary part moves in steps proportional to the values of the pressures.

PATENTED JAN! 1 B72 SHEET 1 BF 8 Fig. 2

INVENTQRI @eI-Lar-d h Zea PATENTEUJANHIQYZ SHEET 50F 3 PATENIED JAPH 1 1972 SHEET 8 OF 8 mnmwzmmvm FIG. 76

I.\'VE.\ I ()RI G rhard K268 FLUID-OPERATED CONTROL APPARATUS This application is a division of application Ser. No. 665,612, filed Sept. 5, 1967, now US Pat. No. 3,489,064, issued Jan. 13,1970.

The present invention relates to control systems and more particularly to pneumatically operated regulators and servomotors in which the input and output quantities are compared with each other to find the deviation or error between the two quantities.

In previously proposed regulating equipment exhibiting proportional integral behavior comparison between input and output quantities is generally carried out simultaneously, that is to say an element is simultaneously responsive to both quantities and its instantaneous position or state represents the instantaneous difference or sum of the two quantities. In many cases such a construction leads to complexity. If, for example, the output quantity is discontinuously measured, as occurs in manufacturing and process technology, known regulators must be connected with additional devices or equipment, for example for storing the output quantity.

The present invention consists in a control system comprising means for acting on a controlled member, means for sensing an output quantity of the controlled member and providing a corresponding output quantity signal, a comparison element arranged to move in response both to the output quantity and an input quantity signal to provide a signal for controlling action of the controlled member, means for repeatedly locking such movement of the comparison element and means for connecting the comparison element alternately with the input and output signals respectively during periods when the comparison element is free to move, the comparison element being arranged to move in accordance with a difference in magnitude of the input and output quantity signals and to remain locked after so moving.

Preferably the system includes means for acting on a controlled member, means for sensing an output quantity of the controlled member and providing a corresponding output quantity signal, a comparison element arranged to move in response both to the output quantity and an input quantity signal to provide a signal for controlling action of the controlled member, means for repeatedly locking such movement of the comparison element and means for connecting the comparison element alternately with the input and output signals respectively during periods when the comparison element is free to move, the comparison element being arranged to move in accordance with a difference in magnitude of the input and output quantity signals and to remain locked after so moving.

The system can further comprise a mechanical-fluid pressure transducer arranged to be operated by one of the two movable parts. The mechanical-fluid pressure transducer can comprise a vane arranged to act upon a jet of fluid while the fluid pressure-mechanical transducer connected with the two movable parts is in the form of a bellows element.

In accordance with a still further preferred feature of the invention the two movable parts are in the form of aligned rods extending in opposite directions from the fluid pressuremechanical transducer and the clamping means are arranged to bring pressure to bear on the rods in a direction substantially towards the axes to the rods, the clamping means being fluid pressure operated.

One such rod can be arranged to operate a mechanical-fluid pressure transducer and the system can further comprise fluidpressure-operated coarse setting means arranged for moving the other rod. The bellows can be arranged to be acted on firstly on both sides by one of the quantity signals and then by the two quantity signals on both of its sides simultaneously.

In accordance with a further form of the invention the two movable parts are in the form ofa hollow rod and a rod fitting inside it, the two rods extending in the same direction from the fluid pressure-mechanical transducer, the system further comprising two sets of clamping jaws arranged to engage the two rods respectively, screw-threaded means for fixing the jaws in the sets of jaws, and pressure-operated means for operating the screwthreaded means. The system can further comprise a cutting tool connected with one of the rods.

In accordance with a still further embodiment of the invention the movable parts are arranged to rotate and the fluid pressure-mechanical transducer is arranged to produce relative rotational movement between them.

In one form of the invention in which the comparison element comprises movable parts which are arranged to rotate, the parts are in the form of circular discs and the system further comprises radially acting pressure-operated brake means for engaging the peripheries of the discs, and a vane arranged to be rotated by one of the discs for controlling a nozzle.

In accordance with a still further construction in accordance with the invention the comparison element is in the form of a single-rotary part carrying a scale, the system further including brakes for acting on the periphery of the driven part and brake and clutch means for connecting an arm, which is arranged to pivot co axially with the driven part, with the driven part and releasing it from it.

In systems in accordance with the invention the comparison element is moved in steps in one direction or the opposite direction in accordance with each controlled deviation between the input and the output quantities sensed, the direction of the displacement depending uponthe prefix sign of the controlled deviation while the size of the displacement depends upon the amount of the controlled deviation taking into account the P-range of operation of the controller.

A system in accordance with the invention can not only be used for carrying out control operations with discontinuously measured control quantities, but also for control operations in the case of continuously measured control quantities, such as temperature or pressure values, measuring means in the latter case being fed with the aid of any suitable control means alternately with the input and output quantities present at the input or inlet of the regulator. In both cases the invention enables an additional regulator to be dispensed with.

Various embodiments of the invention are now described with reference to the accompanying drawings.

FIGS. 1 and 2 diagrammatically show a regulating circuit with a regulating device showing application to a machine tool.

FIG. 3 is a diagrammatic longitudinal section through a regulating device in accordance with a first embodiment of the invention;

FIG. 4 is a cross section ofa clamp for the device of FIG. 3; FIG. 5 is a table of the regulating steps for the device of FIG.

FIG. 6 shows a diagrammatic longitudinal section through a regulating device in accordance with a second embodiment of the invention;

FIG. 7 is a table ofthe regulating functions of FIG. 6;

FIG. 8 is a partial longitudinal section of a regulating device in accordance with a third embodiment of the invention;

FIG. 9 is an enlarged cross section ofa part of the device of FIG. 8;

FIG. 10 shows a partial longitudinal section of a regulating device for a lathe or similar machine tool in accordance with a fourth embodiment of the invention;

FIG. 11 shows in perspective and partiallyin section a re gulating device in accordance with the invention in which the measuring means moves angularly;

FIG. 12 is a table of the operating steps of the regulating equipment of FIG. 11;

FIGS. 13 and 14 show a further embodiment of a device in accordance having the invention with a rotary servomotor;

FIGS. 15 and 16 are tables representing the operational steps of the apparatus in FIGS. 13 and 14.

The regulating circuits schematically shown in FIGS. 1 and 2 represent two embodiments of the invention with discontinuous measurement of the control quantities with reference to a control process in which the diameter of workpieces being turned on a lathe is to be regulated. In this arrangement the output quantity of a finished workpiece l is determined by a pneumatic measuring head 2 and fed to an incremental regulator 3 via a duct 5. This incremental regulator compares the instantaneous output quantity with the input quantity which is indicated either pneumatically via a duct 4 or mechanically by means of an adjustable spring or the like. Within the incremental regulator 3 the control deviation determined is converted by means of an output transducer into a pneumatic output quantity which is fed via a duct 6 to a servomotor 7 as a servocontrol signal. The pneumatically operated servomotor 7, which can be constructed in a known manner, is arranged on the lathe slide 8 of an automatic lathe and acts on the tool carrier 12 whose tool 9 is machining the workpiece 10. When the servo-operating signal or quantity changes owing to a control deviation, the stroke of the servomotor 7 is altered and with it the setting of the turning tool 9. Consequently the workpiece 10 held in the lathe is turned to an accordingly greater or smaller diameter, the control or regulating cycle being repeated as required until the workpiece corresponds with the input quantity.

The regulating device shown in FIG. 2 differs from that of FIG. 1 substantially in that the incremental regulator 11 also serves as a servomotor. The output signal of the regulator 1] acts directly, that is to say without a pneumatic output device, on the tool holder 12 and thus on the turning tool 9. In other respects the manner of operation of this regulating circuit corresponds with that of the regulating circuit according to FIG. 1.

In the case of the regulator shown in FIGS. 3 and 4 a measuring means 15 with a metal bellows is used whose parts 16 are held together by means of strip springs 17 and whose end walls are connected with

rods

18 and 19. Rods l8 and 19 can be held in

clamps

20 and 21 and constitute the comparison element of the system. Each clamp consists of a block 25 fixed on the baseplate 26 of the regulator and has a groove in which the

rods

18 and 19 are held. The rod can be clamped by means of a spring 27 between a jaw 28 and the groove of the block 25. The jaw 28 is connected by means of a piston rod 29 with a piston 30 sliding in a cylinder 32. When the cylinder space 31 below the piston 30 is put under pressure, the clamp is released and the

rods

18 and 19 can be moved axially. The cylinder is connected firmly with the block 25 by means of massive straps. At the free end of the rod 19 for the output signal of the measuring means 15 a vane 36 is attached sliding between the blowing and receiving nozzle 37 and output device 35. This output device constitutes a mechanicalfluid pressure transducer. The blowing and receiving nozzle fed by the supply duct Z are arranged in the cover of a spring mounted metal bellows 38 whose end 39 is mounted on the base plate 26 of the regulator. The output pressure is supplied from the metal bellows 38 in direction of the arrow A, for example, through the duct 6 of the control circuit shown in FIG. 1 to the servomotor 7. The pressure measuring means 15 is supplied with the input quantity W and the output quantity X sequentially, that is to say one after the other, by means ofa switch S. In the input part of the pressure measuring means 15 there is also a pressure-distributing circuit which consists of a constant value choke 42 and an adjustable choke 41 and serves for setting the proportional range of the controller. Instead of this, the proportional range of the output device 35 can be made adjustable. The cycle for comparing the input and output quantities with this device is carried out in a number of sequential steps shown in the table of FIG. 5.

During the step I the clamp 20 is released, that is to say the rod 18 of the measuring means 15 can slide freely, and the clamp 21 is tightened so that the rod 19 of the measuring device 15 is held firmly. In the table of steps a minus sign is used to indicate released and a plus sign to indicate tightened. If switch S is in the position W, so that the measuring means 15 receives the input quantity, the measuring means consequently makes a movement to the left corresponding to the input quantity. During the two following steps II and III both

clamps

20 and 21 are tightened so that the measuring means 15 cannot move. However, on passing from step II to step II the switch S is switched over from the position W into the position X. In this position, shown in FIG. 3, the measuring means 15 receives the output quantity X. In step IV the clamp 21 is released, that is to say the rod 19 is freed. If the input and output quantities are now equal, the measuring device is not moved and the position of the rod 19 is also not changed. ll, however, the output quantity is greater than the input quantity, corresponding to a positive control deviation, the measuring means 15 moves the rod 19 to the right. If on the other hand the output quantity is smaller than the input quantity, corresponding to a negative control deviation, the measuring means 15 draws the rod 19 back towards the left. The position reached by the rod 19, forming the output signal to the measuring device 15, must now be fixed. For this purpose in step v both clamps 20 and 21 are tightened. The switch S, which in step V is at first connected with the output quantity X, is now switched over in step VI to be connected with the input quantity W. With this the comparison cycle is completed. The terminal position of the measuring means, the position of the rod 19, remains fixed without alteration until the next comparison between the input-output quantities begins again in step 1. During the change in position of the rod 19 the position of the vane 36 in the nozzle system 37 of the output unit 35 varies so that the displacement is transformed into a change in pressure proportional to the movement of the rod 19. In this manner a corresponding pneumatic output signal of the regulator is obtained. The control vane 36 is arranged in relation to the output unit 35 in the input-output quantity comparison cycle in such a manner that a positive control deviation corresponds with a reduction in the output pressure at A, that is to say, the direction of operation of the regulator in this case is rising/falling. A reversal in the direction of operation so as to be rising/rising, in which a positive control deviation corresponds with an increase in the output pressure, is brought about in a simple manner by changing the association between the actuation of the

clamps

20 and 21 and the position of the switch S. In the two last vertical columns of the table of FIG. 5 the switch positions for these two directions of operation are indicated. The control of the regulator shown in FIGS. 3 and 4 in accordance with this table can be brought about with a known following control which is not shown in the drawing.

The embodiment of the regulating device illustrated in FIG. 6, instead of the pressure measuring means 15 of- FIG. 3, is provided with a differential pressure measuring means 45 in which the control deviations are determined. A membrane 48 is mounted in a control housing 47 between the springs 49. As was the case with the device of FIG. 3, a rod 18 is connected with the housing 47 and is mounted in a clamp 20 while the rod 19 passing through the housing 47 and connected with the membrane 48 is mounted in the clamp 21. At the free end of the rod 19 the control vane 36 of an output unit 35 is mounted as is shown in FIG. 3. The rod 18 is extended to the left beyond the clamp 20 and connected with an additional driving system 46 which enables a rapid movement of the measuring means as far as the terminal position, or a similar travel of the measuring means to be carried out. This drive system consists of a rolling membrane or cuff 52 mounted between a housing 50 and a housing cover 51, a membrane carrier 53, and a spring 54. The housing 50 is connected with the baseplate 26 of the regulator. The free end of the rod 18 inserted into the housing 50 is connected with the membrane carrier 53. When the membrane or cuff 52 is acted upon by pressure entering through inlet 55 after both the

clamps

20 and 21 have been released, the rod 18 serves to displace the measuring means 45 and thus also rod 19. For example the drive system 46 can be so constructed that the measuring means can be displaced with a variation in pressure of 0.2-1.0 (atmospheric gauge pressure) from one end or terminal position to the other terminal position. The manner of operation of this regulating device can readily be understood with reference to the table of steps and switching operations given in FIG. 7 and the explanation given in connection with the embodiment of FIG. 3. The positions of the switch S which in one position transmits the input quantity W to both chambers or spaces and in the other position transmits the output quantity X into the one measuring means chamber while the other measuring means chamber remains connected with the input quantity, are given in the table again for both directions of the regulator.

The embodiment of the invention shown in FIGS. 8 and 9 is in the form of a regulating device in which a deviation from a desired control function is formed in a measuring means 60 and converted into a displacement and transformed by means of an output unit 61 into a pressure proportional to the control deviation. The two clamps 62 and 63 mounted on the regulator base plate 64 can be constructed as was the case with the previous embodiments of the invention or in accordance with FIG. 9. A receiving body 65, welded on the baseplate 64, has a receiving groove 66 for the

rod

67 or 68 to be clamped. On the receiving body 65 a holding arm 70 is held by means of a spring band 69. Through a hole in this holding arm 70 there extends the upper end of a bolt which is fixed in the receiving body 65 and has on its thread a nut which can be screwed to act upon Belville washers 73. The nut 72 can be used to tighten the

rod

67 or 68. This can be carried out pneumatically as shown in FIG. 8. The nuts 72 of the two

clamps

62 and 63 are for this purpose connected by levers 75 and coupling rods 76 with a double-membrane system 77. When the

membranes

78 and 79 are acted upon by the pressure introduced into the

chambers

80 and 81, the nuts 72 are pulled tight and the

clamps

62 and 63 tightened. Conversely, the clamps are released if the membranes are acted upon by pressure introduced into the

chambers

82 and 83. Since all chambers can also be fed with pressure independently of one another it is also possible to tighten or release only one of the

clamps

62 and 63. The rod 67 is connected with the membrane plate 86 of the measuring means 60 and is mounted in the clamp 63 so as to be movable. It also carries a control vane 87 for the output unit 61. The rod 68 has a hole in the longitudinal direction and is firmly connected with the membrane housing 88. The rod is mounted in the clamp 62, the rod 67 passing through the hole in the rod 68. The method of operation of this control device basically corresponds with that of the regulator in accordance with FIG. 3 with the only difference that, instead of the metal bellows type measuring means 15, use is made of a measuring means 60 with a spring loaded membrane 85 acted upon by the pressure coming through the inlet 89 in correspondence with the input or output quantity.

In the construction in accordance with FIG. the regulator also forms the servomotor. The membrane system or measuring means 95 in this case preferably includes steel membranes 97 which owing to machined away parts in the zone 98 are so dimensioned that they also form membranes. These membranes are very stiff so as to ensure that the counter forces due to the working tool 99 on the tool carrier 96 can only affect the measuring means 95 to a sight degree. The membranes 97 therefore only move through small distances. Preferably several such membranes 97 are arranged one behind the other in order to increase the thrust produced. The output signal from the measuring means 95 in this case determines mechanically in a direct manner the setting of the workpiece carrier 96 attached to the rod 67 and thus the position of the working tool 99 in relation to the workpiece 100.

Embodiments of the invention in which the measuring means performs rotational or angular movements instead of translatory ones offer the advantage that the moving parts of the device can be balanced as to weight and can be directly supported so that bearing errors and deviations, for example due to vibrations, can be avoided. A further advantage of the use of a system with angular movements is that the movement of the servomotor is not limited. One can thus arrange the system so that the angular movement is greater than 360.

A device with rotary or angular movement of the measuring means can for example be constructed in accordance with the invention in such a manner that the measuring means con nects two rotary terminal parts, such as discs or similar bodies, in such a manner that variations in pressure in the measuring means bring about alterations in the angle of one end part in relation to the other. The corresponding rotary end parts, such as discs or the like, are held by clamps or the like or released by them, in accordance with a further important feature of the invention. The angular movement of one rotary end part, for example one of the discs, is converted by means of an output unit into a proportional pressure in a manner similar to a device working with translatory movement.

In accordance with a further advantageous embodiment of the invention with rotary or angular movements, the device is provided with a rotary terminal part, for example a discshaped or similar body, which can be held or released by a clamp, the pressure measuring means being capable of being connected by means of a clamp with the end or terminal part, for example the disc, while the clamp iscapable of being released by means of a pressure-responsive part.

Apparatus in accordance with the present specification can also, in principle, be used for carrying out calculation operations such as for example addition and subtraction. With a device in accordance with the invention it is thus possible to carry out summational operations of all types or determine averages. This means that the device can for instance be applied to the measurement of quantities of liquids in a number of containers by adding the levels. A particularly advantageous field of application for their respective invention is in the counting or measurement of quantities of flowing media. In a construction designed for this purpose in accordance with the invention the operational steps must be precisely timed and the input pressure must be proportional to the indication ofa flow meter or the like. For this application a device whose measuring means forms rotary movements is particularly advantageous because a counter can be driven directly. Also the total counting of a number of flow stations can be carried out comparatively simply in accordance with the invention. In this case the step cycles of the individual flow meters, whose values are to be compiled, are conveniently arranged so as to occur in sequence and for determining the numerical value the total tim of all individual cycles is sued.

Some examples of devices in accordance with the invention in which the measuring means forms angular or rotary displacements and the displacement steps are angular rotations instead of translatory movements, are shown in FIGS. 11, 13, and 14 with the respective associated table of steps shown in FIGS. 12, 15 and 16.

FIG. 11 shows a device corresponding in principle to the device according to FIG. 3 but arranged to perform angular movements instead of translatory ones. The rotary discs or

wheels

118 and 119 are connected together by the pressure measuring means in such a manner that a change in pressure in the measuring means 115 causes a change of the angle of the disc or wheel 119 in relation to the disc 118. The latter disc can be held by a clamp 120 while the disc 119 can be held by a clamp 121 because in this case a spring 127 presses the brake shoe 128 against the periphery of the corresponding wheel or disc. However, if the pressure is transmitted to the cylinder chamber 131, the spring 127 is compressed so that the brake shoe 128 is raised and the corresponding disc is released for angular movement. The angular movements of the disc 119 are converted by means ofan output unit into a proportional pressure at A. For this purpose, for example, a spiral-shaped disc 136, which is connected so as to rotate with the disc 119, is mounted in the slot ofa blowing and receiving nozzle system 137 arranged on the head of a resilient bellows 138. The bottom 139 of the resilient bellows remains stationary. I

The manner of operation of this device can easily be understood from the table of FIG. 12 and the description of FIG. 3. FIGS. 11 and 12 show the arrangement for an increasing pressure at A when the pressure of the output quantity is greater than the input quantity. A reversal can be achieved either by reversing the switch positions W and X of the switch S (input and output 136.

A further device with a rotating terminal servo part is diagrammatically shown in FIGS. 13 and 14. FIG. 13 shows the quantities) or by reversing the spiral disc device from the side, the drive of the clamps 170 being omitted for the sake of clarity. FIG. 14 shows a view in which the clamps 160 are omitted for the same reason. FIG. 15 shows the corresponding table of operational steps.

A disc or wheel 151 is connected with a shaft 152 and the two can rotate when they are released by the clamp 160. The clamp consists of two levers 153 each with a brake shoe 154 pressed by a spring 155 against the periphery of the ,disc 151 so as to prevent the disc from rotating. The levers 153 are also connected by a cylinder 156 and piston 157. If pressure is communicated to the cylinder space or chamber 158, the braking levers 153 are pushed away from each other against the force of the spring 155 and the disc or wheel 151 is released. The pressure measuring means 180, which consists of a spring loaded membrane, is connected to lever 181, pivoting on a shaft, by means of a link 182. The lever 181 carries a clamp 170 by means of which it can be clamped by the action of a spring 173 on the disc 151. The clamp 170 is, however, also connected with a piston 174 and a cylinder 175. When the cylinder space 176 is put under pressure, the lever 177 is raised against the action of the spring 173 away from the clamp and the disc 151 is released from its connection with the lever 181. The operation step table of F I6. 15 explains the manner of operation of this device when the latter is used as a regulator.

The device can, however, also be used for other purposes such as calculating, for instance addition and subtraction. In this case the measuring means 180 is then fed with a pressure corresponding to the zero value instead of the input quantity and, instead of the output quantity, a pressure corresponding to the value to be measured. The pressures which are greater than the pressure corresponding to the zero value are then added and those which are smaller are subtracted. The setting of the disc or wheel 151 gives the result. The disc can be provided with a scale or can drive a counting device directly. Instead of the pressure measuring means 180 it is also possible to use a differential pressure measuring means. On one side of the differential pressure measuring means those pressures are fed which are to be added while on the other side pressures are fed which are to be subtracted.

A further practical field of application of the invention is, for example, the counting of quantities on the basis of flow measurements. The flow is measured for example in kg. per minute. If the flow is measured for each minute and the values are added together the quantity in kilograms flowing through the measuring device is obtained. The device shown in FlGS. 13 and 14 is suitable for determining quantities if the stepping cycle is precisely timed. The input quantity is then zero pressure and the output quantity is the pressure which corresponds to the instantaneous flow value, both these pressure values being fed to the pressure measuring means. Preferably a counting device is provided driven by the shaft 152.

A counting of several partial flows, which are measured by several flow meters, can be carried out in accordance with a further embodiment of the invention. The table of operational steps of FIG. 16 shows the manner of operation. In the table, 0 denotes zero pressure while the numbers indicate the flow measured at the various measuring positions. The duration of the whole cycle 18 steps) is used as a basis for determining the quantities counted. If the device is used for determining an average or mean or for the determination of the contents of several containers by determining the differences in height, the device must be set at zero after each cycle, this being simple to carry out with known means.

Whatl claim is: l. A system for generating a pressure output in response to first and second pressure inputs, comprising;

means for alternately sensing said first and second pressure inputs and generating respective corresponding output responses, means selectively rotatable in accordance with said output responses for comparing said first and second pressure inputs with res ect to one another, means for eye ically locking said means for comparing to prevent rotation thereof, means for connecting said means for alternately sensing with said first and second pressure inputs only during periods when said means for comparing is free to rotate, driver means rotated in accordance with the selectable rotation of said means for comparison, and means for providing said pressure output and operated by said driver means whereby said pressure output is proportional to said first and second pressure inputs.

2. A system as in claim 1 wherein said means for comparing includes two discs each independently rotatable through a predetermined angle in accordance with a respective one of said output responses and interconnected with each other by said means for sensing, whereby the comparison of said first and second pressure inputs is represented by the rotation of one of said discs relative to the other of said discs.

3. A system as in claim 1 wherein said means for cyclically locking said means for comparing includes a pair of independently operable brake means, each one of said pair of brake means engaging an associated one of said discs subsequent to a rotative movement thereofl 4. A system as in claim 3 wherein said driver means includes a cam mounted to rotate with said one disc the rotation of which represents said comparison and said means for providing said pressure output is a resilient bellows device having a bellows member engaging said cam to be moved thereby.

Claims

1. A system for generating a pressure output in response to first and second pressure inputs, comprising; means for alternately sensing said first and second pressure inputs and generating respective corresponding output responses, means selectively rotatable in accordance with said output responses for comparing said first and second pressure inputs with respect to one another, means for cyclically locking said means for comparing to prevent rotation thereof, means for connecting said means for alternately sensing with said first and second pressure inputs only during periods when said means for comparing is free to rotate, driver means rotated in accordance with the selectable rotation of said means for comparison, and means for providing said pressure output and operated by said driver means whereby said pressure output is proportional to said first and second pressure inputs.

3. A system as in claim 1 wherein said means for cyclically locking said means for comparing inCludes a pair of independently operable brake means, each one of said pair of brake means engaging an associated one of said discs subsequent to a rotative movement thereof.

4. A system as in claim 3 wherein said driver means includes a cam mounted to rotate with said one disc the rotation of which represents said comparison and said means for providing said pressure output is a resilient bellows device having a bellows member engaging said cam to be moved thereby.