US3750534A

US3750534A - Fluidic regulating apparatus for a reversible hydraulic pump or motor

Info

Publication number: US3750534A
Authority: US
Inventors: V Leutner; R Romes
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1971-04-22
Filing date: 1972-04-20
Publication date: 1973-08-07
Anticipated expiration: 1990-08-07
Also published as: IT1006535B; GB1386472A; DE2119647A1; FR2136563A6; CH535455A

Abstract

A regulating apparatus for a reversible hydraulic machine operating as pump or hydraulic motor in both directions of rotation. The machine can be adjusted to different displacement volumes by fluidic actuating means controlled by an output control signal of an additive control amplifier which compares a first control signal representing the actual displaced volume with a second control signal representing the desired displaced volume. The second control signal is generated in a fluidic control device which includes a fluidic limiting device limiting the second control signal, and being controlled by a first fluidic circuit responsive to the product of the actual and desired volume and fluid pressure of the machine, and by a second fluidic circuit responsive to the highest actual fluid pressure of the machine.

Description

nited States Patent [1 1 Leutner et al. Aug. 7, 1973 [54] FLUIDIC REGULATING APPARATUS FOR A 3,592,083 7/1971 Kawabata 137/805 REVERSIBLE HYDRAULIC M 0 3,499,599 3/1970 Meininger 137/805 MOTOR 3,488,948 1/1970 Comett et a1 137/805 3,478,731 11/1969 Morton et a1......... 137/805 Related 0.8. Application Data Continuation-impart of Ser. No. 232,477, March 7, 1972.

Foreign Application Priority Data Apr. 22, 1971 Germany P 21 19 647.4

US. Cl 91/506, 137/805, 417/217 Int. Cl. F0lb 13/04, FOlb 25/00 Field of Search 91/506; 417/217, 417/222; 137/805 References Cited UNlTED STATES PATENTS 3/1972 Molly 417/217 Primary Examiner-William L. Freeh AttorneyMichael S. Striker [57] ABSTRACT A regulating apparatus for a reversible hydraulic machine operating as pump or hydraulic motor in both directions of rotation. The machine can be adjusted to different displacement volumes by fluidic actuating means controlled by an output control signal of an additive control amplifier which compares a first control signal representing the actual displaced volume with a second control signal representing the desired displaced volume. The second control signal is generated in a fluidic control device which includes a fluidic limiting device limiting the second control signal, and being controlled by a first fluidic circuit responsive to the product of the actual and desired volume and fluid pressure of the machine, and by a second fluidic circuit responsive to the highest actual fluid pressure of the machine.

10 Claims, 3 Drawing Figures FLUIDIC REGULATING APPARATUS FOR A REVERSIBLE HYDRAULIC PUMP OR MOTOR REFERENCE TO A RELATED APPLICATION The present application is a continuation-in-part application of our copending patent application, Ser. No. 232,477 entitled Regulating Apparatus for a Hydro static Pump, filed Mar. 7, 1972.

BACKGROUND OF THE INVENTION The present invention relates to a fluidic regulating apparatus for a reversible hydraulic machine operating as pump or hydraulic motor, and in both directions of rotation. Our above-mentioned U.S. patent application filed Mar. 7, 1972 discloses a regulating apparatus for a hydrostatic pump which is particularly advantageous for a pump operating in open circulation so that regulating and limiting operations can be carried out only when the machine operates as pump in a predetermined direction of rotation, corresponding to a quadrant of a graphical representation of the power output depending on the displaced volume and the pressure of the pump. Since the main regulating circuit is associated with the power output, it would be necessary to reverse the sign of the desired value of the power output for reversing the pump in a closed circulation, in order to obtain a reversal of the sign of the adjusting position value while the actual value of the power output is fed back correctly.

SUMMARY OF THE INVENTION It is the object of the present invention to provide a fluidic regulating apparatus which permits operation of a reversible hydraulic machine as a pump or hydraulic motor, and in either condition, in both directions of rotation so that in a graphical representation, regulation could be carried out in four quadrants respectively associated with operation as pump in forward direction, with operation as hydraulic motor in forward direction, with operation as pump in rearward direction, and with operation as hydraulic motor in rearward direction.

The object of the invention is obtained by passing the desired value signal obtained by the regulating circuit for the volume adjusting means of the machine, through a bipolar fluidic limiting device to which a limiting signal in the form of a hydraulic value is transmitted, which represents information for the desired value and actual value of the power output and of the pump pressure, and which regulates and limits these values. The regulating apparatus of the invention includes elements of the regulating apparatus disclosed in our prior patent application and uses in the same manner, hydraulic differential pressures, while the introduction of the desired values may be carried-out by means of electro-fluidic transducers.

An embodiment of the invention comprises adjusting means for varying the volume of fluid displaced by the machine, actual position measuring means responsive to the position of the adjusting means to generate a first fluidic control signal representing the actual volume displaced by the machine, a fluidic control device for generating a second fluidic control signal representing the desired position of the adjusting means and thereby the volume desired to be displaced by the machine, a fluidic control amplifier receiving the first and second control signals and generating a differential output control signal representing the difference between the actual displaced volume and the volume desired to be displaced by the machine, and actuating means responsive to the output signal to vary the position of the adjusting means until the adjusting means is in the desired position and the machine displaces the desired volume.

In accordance with the invention, the control device includes a transducer producing the second control signal, a fluidic limiting device receiving the second control signal and transmitting the limited second control signal to the control amplifier, and fluidic control means responsive to the actual value and to the desired value of the power output and of the fluid pressure of the machine to transmit to the fluidic limiting device, a fluidic limiting signal representing the difference between the actual values and the desired values of the power output and fluid pressure of the machine.

Preferably, the control means include a fluidic additive limit amplifier transmitting the limiting signal to the limiting device, a first fluidic circuit responsive to the actual and desired volume and fluid pressure of the machine to transmit a first input signal to the additive limit amplifier, and a second fluidic circuit responsive to the actual and desired fluid pressure of the machine to transmit a second input signal to the additive limit amplifier so that the first and second input signals control the limiting signal.

It is an advantage of the regulating apparatus of the invention that, in addition. to the protection of the prime mover of the machine against stalling and free rotation, due to the limiting of the power output, a complete control of the hydrostatic pressures is obtained for inertia loads in all four operational conditions described above.

The control of the pressures when the machine operates as a hydraulic motor in forward or rearward direction, is obtained in accordance with the invention by hydraulic logic means which determine whether the machine operates undersuch conditions. In this event, the adjusting speed of the machine is regulated so tha no excessively high pressure develops.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF TI'IEDRAWING 4 FIG. I is 'a diagrammatic view illustrating a regulating apparatus according to the invention for regulation and limitation of the power output, the pressure, and the displaced volume of an adjustable hydraulic machine, which may operate as a pump or hydraulic motor in either direction of rotation;

FIG. 2 is a diagrammatic view illustrating details of a fluidic circuit shown schematically in FIG. 1', and

FIG. 3 is a diagram graphically representing the operation of the hydraulic machine in four above-explained operational conditions, represented by hyperbolas in the four quadrants of the diagram shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, a hydraulic machine 11, which can operate as a pump or hydraulic motor, has

adjusting means, and may be constructed as an axial piston pump with an adjustable swash plate, as described, for example, in the U. S. Pat. No. 3,489,094. The position of the adjusting means of the hydraulic machine 11 is measured by a position measuring means 12 which transmits a control signal representing the adjusted position and the volume displaced in the adjusted position of the hydraulic machine 11. The control amplifier 13 receives another control signal A from an electro-fluidic transducer, "the signal A passing through a bipolar fluidic limiting device, which may be of the type described in the U. S. Pat. No. 3,516,428. The output control signal of the control amplifier 13 is transmitted to a bipolar fluidic limiting device 14 of the same type and then transmitted to a servo valve and force amplifier 15 for the adjusting means of the hydraulic machine 11. The above-described elements are shown surrounded by a rectangular chain line, and form a unit L for regulating the position of the adjusting means of the hydraulic machine 11 and thereby the pumped volume.

The control signal A can be supplied to the regulating unit L mechanically, hydraulically or by means of an electro-fluidic transducer 16 of the type described in the U. S. Pat. No. 3,521,654. The limiting and regulating of the power output and pressure of the hydraulic machine 11 is effected by limiting the control signal A in the bipolar fluidic limiting device 17 which receives a fluidic limiting signal B from the output of an additive limit amplifier 23. The limiting signal B includes information about the desired value and actual value of the power output and pressure of the machine 11.

A first fluidic circuit 18 to 22 includes a fluidic multiplier 18, which may be of the type disclosed in the U. S. Pat. No. 3,516,605, which forms the actual value of the power output as a product of the pressure P and the displaced volume 0. The signal representing the product passes through a fluidic rectifier 19, which may be of the type disclosed in the U. S. Pat. No. 3,552,414, which forms the absolute value of the actual power output. This absolute value is supplied as an actual power output signal to an additive power signal amplifier 20, which also receives a desired power output representing signal C from the electrofluidic transducer 31. The additive power signal amplifier forms and amplifies the difference between the absolute value of the actual power output value and the desired power output value.

If the difference at the additive power signal amplifier 20 is negative, which means that the desired power output value is greater than the actual power output value, amplifier 20 is shunted by the selective fluidic transmitter 22, which may be of the type described in the U. S. Pat. No. 3,504,689, and preferably includes check valve means, or means for limiting the signal of the power signal amplifier 20.

The regulating apparatus of the invention is particularly suitable for hydraulic machines which operate in a closed circulation, in all four quadrants ofa graphical representation of the machine operations, as shown in FIG. 3. The abscissa represents the pressure P of the hydraulic apparatus 11, and the ordinate represents the displaced fluid volume Q. In all four quadrants, the 6 graphs are hyperbolas representing a constant power output N P.Q. Horizontal lines represent the value Q is constant, and in quadrants I and III, limiting lines representing the pressure P to be substantially constant, are shown.

In quadrant I and III, the hydraulic machine operates as a pump in forward direction in quadrant I and in rearward direction of rotation in quadrant III. In quadrants II and IV the machine operates as a hydraulic motor in forward and rearward directions of rotation, respectively.

The pressure limitation in quadrants I and III, and the limiting of the speed of pump adjustment in quadrants II and IV, is obtained by a second fluidic circuit 24, shown as a box in FIG. 1, and illustrated in detail in FIG. 2. The hydraulic circuit 24 determines the quadrant and operational condition in which the machine operates. Hydraulic circuit 24 receives, for example from a fluidic transducer 25, the desired value of the pressure P, the electro-fluidic transducer 25 being shown both in FIGS. 1 and 2. The output signals E and E of the fluidic selector circuit 24 corresponds to the difference between the actual pressure P and the desired pressure D. As long as the output signal is negative, which means that the desired pressure is greater than the actual pressure, the input signal for the additive limit amplifier 23 is suppressed, but if signal E is positive, during operation of the machine in quadrant I or III, the input signal E is transmitted to the limit amplifier 23. During operations in quadrant ll, IV, the signal E is supplied to the bi-polar fluidic amplifier 14 of the actuating means l4, 15 of the adjusting means of the hydraulic machine 11. Signal E determines the speed of adjustment of the hydraulic machine 11.

The bipolar fluidic limiting device 17 operates in such a manner that its output control signal to the additive control amplifier 13, cannot be greater than the difference between q, b, wherein q is the value of the entire range of the signal from zero to saturation, and b is the value of the limiting signal B. As long as signal B is zero, the amount of displaced fluid volume is not limited.

Only if from the fluidic transmitter 22 a signal representing the actual value of the power output to be greater than the desired value of the power output is transmitted, or if the selective fluidic circuit 24- transmits a signal indicating the exceeding of the permissible discharge pressure P, the value of the desired position of the adjusting means and of the desired discharged volume of the machine 11 is limited. This can be mathematically formulated as follows:

qt q, in a stable condition wherein q,, q, fluid volume N,, N, desired and actual value of power output P, discharge pressure desired and actual value of discharged a amplifier factor of additive amplifier 20. It follows:

(qs qsmar) a s/ sma1)] a auru where N q P If a is sufficiently great, e.g. a

qs a 1 follows substantially which corresponds to a true power output regulation.

FIG. 2 illustrates in greater detail a selective fluidic circuit 24 connected to the closed circulation of a hydrostatic transmission including the hydraulic machine 11 and a hydraulic motor 27. The hydraulic motor 27 drives an apparatus, for example a winch 27a which drives the hydraulic motor 27 when the load drops due to gravity so that hydraulic motor 27 operates as a pump whereby the hydraulic machine 11 is operated as a hydraulic motor. A pressure selecting valve 28 responds to the highest pressure in the

conduits

29, 30, 31, 32 to supply an actual pressure signal through conduit 34 to the additive pressure signal amplifier 33 where the actual pressure signal is compared with the desired pressure signal generated by the electro-fluidic transducer 25. The difference between the pressure signals is amplified, and if negative, blocked by a selective fluidic transmitter 35 of the type described in U. S. Pat. No. 3,504,689. The output signal F of the fluidic transmitter 35 is supplied to fluidic logic means 36 which includes

slide valves

37 and 38. The logic valve means 36 shifts the fluidic signal F dependent on the operational condition at which the hydraulic machine operates in one of quadrants I to IV, to one of the two outputs 39, 40 whereby the slide valve 37 cancels the selection of the pressure selecting valve which was required for the processing of the signals at the pressure signal amplifier 33. During operation in the quadrants I and III, the signal F is supplied to conduit 39, and during operation in quadrants II and IV to conduit 40.

The slide valve 38 is controlled by a signal representing the actual value 0 representing the position of the adjusting means of hydraulic machine 1 l, and the associated fluid volume, derived from the regulating unit L,

If the signal 0 is positive, the signal F, if supplied to conduit 39, is shifted to conduit 41, and supplied to the additive limit amplifier 23, as shown in FIG. 1. If the signal F is supplied to conduit 40, it is shifted to output 42 and from there to the bipolar fluidic amplifier 14 which controls the actuating means 15 of the adjusting means of the hydraulic machine 11. During operation in the first and third quadrants as shown in FIG. 3, signal F is supplied to output 41. During operation in the second and fourth quadrants, signal F is supplied to output 42.

The hyperbolas represent in all four quadrants, the maximum constant value of the power output. In quadrants II and IV, broken lines 0 indicate a substantially constant value for the pressure P.

The fluidic devices used in the apparatus of the invention are well-known, and not an object of the invention. The

devices

16, 25, 21 are electro-fiuidic transducers as described in the U. S. Pat. No. 3,521,654; the

devices

13, 20, 23, 33 are additive amplifiers of the type disclosed in the U. S. Pat. No. 3,554,206; the devices 14 and 17 are bipolar fluidic limiting devices as described in the U. S. Pat. No. 3,516,428; the measuring device 12 is connected with the swash plate, for example, of an axial piston pump as described in the U. S. Pat. No. 3,489,094; device 19 is a fluidic rectifier as described in the U. S. Pat. No. 3,552,414; the

devices

22 and 25 are fluidic transmitters as described in the U. S. Pat. No. 3,504,689; and device 18 is a fluidic multiplier as described in the U. S. Pat. No. 3,516,605.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of regulating apparatus for a hydraulic machine differing from the types described above.

While the invention has been illustrated and described as embodied in a regulating apparatus-for reversible hydraulic machine operating as pump or hydraulic motor in either direction of rotation and being regulated to maintain a constant power output, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new anddesired to be protected by Letters Patent is set forth in the appended claims.

We claim: I

l. Regulating apparatus for a reversible hydraulic machine, comprising adjusting means having a plurality of positions for varying the volume of fluid displaced by said machine; actual position measuring means responsive to the position of said adjusting means to generate a first fluidic control signal representing the actual volume displaced by said machine; a fluidic control device for generating a second fluidic control signal representing the desired position of said adjusting means and thereby the volume desired to be displaced by said machine; a fluidic control amplifier receiving said first and second control signals and generating a differential output-control signal representing the difference between the actual displaced volume and the volume desired to be displaced by said machine; actuating means responsive to said output signal to vary the position of said adjusting means until said adjusting means is in said desired position and said machine displacesthe desired volume; said control device including a transducer producing said second control signal, a fluidic limiting device receiving said second control signal and transmitting the limited second control signal to said control amplifier, and fluidic control means responsive to the actual value and to the desired value of the power output and of the fluid pressure of said machine to transmit to said fluidic limiting device, a fluidic limiting signal representing the difference between the actual values and the desired values of the power output and fluid pressure of the machine.

2. Regulating apparatus as claimed in claim 1 wherein said control means include a fluidic additive limit amplifier transmitting said limiting signal to said limiting device, a first fluidic circuit rep'onsive to the actual and desired volume and fluid pressure of the machine to transmit a first input signal to said additive limit amplifier, and a second fluidic circuit responsive to the actual and desired fluid pressure of the machine to transmit a second input signal to said additive limit amplifier so that said first and second input signals control said limiting signal.

3. Regulating apparatus as claimed in claim 2 wherein said first fluidic circuit includes a fluidic multiplier responsive to the actual displaced volume and to the fluid pressure in the machine to generate an actual power output signal, electro-fluidic transducer means for generating a desired power output signal, and an additive power signal amplifier receiving actual power and desired power signals to generate said first input signal.

4. Regulating apparatus as claimed in claim 3 wherein said first fluidic circuit includes check valve means for shunting said additive power signal amplifier when said first input signal is negative and the desired power output is greater than the actual power output.

5. Regulating apparatus as claimed in claim 1 wherein said second fluidic circuit includes selector valve means responsive to the actual fluid pressure in said machine to transmit an actual pressure signal, electro-fluidic transducer means for transmitting a desired pressure signal, an additive pressure signal amplifier receiving said pressure signals generating a differential pressure signal and a fluidic pressure transmitter for suppressing a negative differential pressure signal and transmitting a positive differential pressure signal to said additive limit amplifier.

6. Regulating apparatus as claimed in claim 5 wherein said selector valve means include check valve means for shunting said pressure signal amplifier when said second input signal is negative and said desired fluid pressure is greater than said actual fluid pressure.

7. Regulating apparatus as claimed in claim 5 wherein said second fluidic circuit includes hydraulic logic means connected with conduits of said machine and with said fluidic differential pressure transmitter for determining whether said machine operates as a pump or hydraulic motor and for determining the direction of rotation of said machine whereby the quad rant of a graphical representation of the power output depending on pressure and displaced volume is determined, the first and the third quadrant representing operation of said machine as pump in opposite directions of rotation, respectively, and the second and fourth quadrant representing operation of said machine as hydraulic motor in opposite directions of rotation, respectively, said hydraulic logic means including a first output connected with said limit amplifier for supplying said second input signal when the actual power output is greater than the desired power output, and a second output connected with said actuating means for supplying an actuating signal to said actuating means when the desired power output is greater than the actual power output.

8. Regulating apparatus as claimed in claim 7 wherein said hydraulic logic means supplies during operation of the machine as pump in both directions of rotation said second input signal through said limit amplifier to said fluidic limiting device, and supplies during operation as a hydraulic motor in both directions of rotation said actuating signal to said actuating means, said actuating means including a fluidic limiting device receiving said output control signal from said control amplifier, and said actuating signal from said logic means for controlling the adjusting speed of said adjusting means of said machine. 7

9. Regulating apparatus as claimed in claim 8 wherein said actuating means includes a servo valve; and wherein said fluidic actuating limiting device is an amplifier transmitting a signal to said servo valve.

10. Regulating apparatus as claimed in claim 1 wherein said control means include a fluidic additive limit amplifier transmitting said limiting signal to said limiting device, a first fluidic circuit responsive to the actual and desired volume and fluid pressure of the machine to transmit a first input signal to said additive limit amplifier, and a second fluidic circuit responsive to the actual and desired fluid pressure of the machine to transmit a second input signal to said additive limit amplifier so that said first and second input signals control said limiting signal; wherein said first fluidic circuit includes a fluidic multiplier responsive to the actual displaced volume and to the fluid pressure in the machine to generate an actual power output signal, electro-fluidic transducer means for generating a desired power output signal, and an additive power signal amplifier receiving said actual power and desired power signals to generate said first input signals; wherein said second fluidic circuit includes selector valve means responsive to the actual fluid pressure in said machine to transmit an actual pressure signal, electrofluidic transducer means for transmitting a desired pressure signal, an additive pressure signal amplifier receiving said pressure signals generating a differential pressure signal and a fluidic pressure transmitter for suppressing a negative differential pressure signal and transmitting a positive differential pressure signal to said additive limit amplifier. i

a a a a

Claims

1. Regulating apparatus for a reversible hydraulic machine, comprising adjusting means having a plurality of positions for varying the volume of fluid displaced by said machine; actual position measuring means responsive to the position of said adjusting means to generate a first fluidic control signal representing the actual volume displaced by said machine; a fluidic control device for generating a second fluidic control signal representing the desired position of said adjusting means and thereby the volume desired to be displaced by said machine; a fluidic control amplifier receiving said first and second control signals and generating a differential output control signal representing the difference between the actual displaced volume and the volume desired to be displaced by said machine; actuating means responsive to said output signal to vary the position of said adjusting means until said adjusting means is in said desired position and said machine displaces the desired volume; said control device including a transducer producing said second control signal, a fluidic limiting device receiving said second control signal and transmitting the limited second control signal to said control amplifier, and fluidic control means responsive to the actual value and to the desired value of the power output and of the fluid pressure of said machine to transmit to said fluidic limiting device, a fluidic limiting signal representing the difference between the actual values and the desired values of the power output and fluid pressure of the machine.

2. Regulating apparatus as claimed in claim 1 wherein said control means include a fluidic additive limit amplifier transmitting said limiting signal to said limiting device, a first fluidic circuit reponsive to the actual and desired volume and fluid pressure of the machine to transmit a first input signal to said additive limit amplifier, and a second fluidic circuit responsive to the actual and desired fluid pressure of the machine to transmit a second input signal to said additive limit amplifier so that said first and second input signals control said limiting signal.

7. Regulating apparatus as claimed in claim 5 wherein said second fluidic circuit includes hydraulic logic means connected with conduits of said machiNe and with said fluidic differential pressure transmitter for determining whether said machine operates as a pump or hydraulic motor and for determining the direction of rotation of said machine whereby the quadrant of a graphical representation of the power output depending on pressure and displaced volume is determined, the first and the third quadrant representing operation of said machine as pump in opposite directions of rotation, respectively, and the second and fourth quadrant representing operation of said machine as hydraulic motor in opposite directions of rotation, respectively, said hydraulic logic means including a first output connected with said limit amplifier for supplying said second input signal when the actual power output is greater than the desired power output, and a second output connected with said actuating means for supplying an actuating signal to said actuating means when the desired power output is greater than the actual power output.

8. Regulating apparatus as claimed in claim 7 wherein said hydraulic logic means supplies during operation of the machine as pump in both directions of rotation said second input signal through said limit amplifier to said fluidic limiting device, and supplies during operation as a hydraulic motor in both directions of rotation said actuating signal to said actuating means, said actuating means including a fluidic limiting device receiving said output control signal from said control amplifier, and said actuating signal from said logic means for controlling the adjusting speed of said adjusting means of said machine.

10. Regulating apparatus as claimed in claim 1 wherein said control means include a fluidic additive limit amplifier transmitting said limiting signal to said limiting device, a first fluidic circuit responsive to the actual and desired volume and fluid pressure of the machine to transmit a first input signal to said additive limit amplifier, and a second fluidic circuit responsive to the actual and desired fluid pressure of the machine to transmit a second input signal to said additive limit amplifier so that said first and second input signals control said limiting signal; wherein said first fluidic circuit includes a fluidic multiplier responsive to the actual displaced volume and to the fluid pressure in the machine to generate an actual power output signal, electro-fluidic transducer means for generating a desired power output signal, and an additive power signal amplifier receiving said actual power and desired power signals to generate said first input signals; wherein said second fluidic circuit includes selector valve means responsive to the actual fluid pressure in said machine to transmit an actual pressure signal, electrofluidic transducer means for transmitting a desired pressure signal, an additive pressure signal amplifier receiving said pressure signals generating a differential pressure signal and a fluidic pressure transmitter for suppressing a negative differential pressure signal and transmitting a positive differential pressure signal to said additive limit amplifier.