DE2401773A1 - DIFFERENTIAL PRESSURE MEASURING DEVICE WITH FORCE COMPARISON - Google Patents

Description

GENEHAL SIGNAL CORPORATION

280 Park Avenue New York, NYVSt.A.

Differential pressure measuring device
with force comparison

The invention relates to measuring systems that operate according to the force comparison method, and in particular to a force comparison system that makes levers, pivot bearings and critical bends superfluous.

A force comparison system is used in a differential pressure measuring device used. Such a measuring device provides a milliampere current output signal, which directly is proportional to the differential pressure at the input of the system to a pressure sensor, for example

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a corrugated pipe, a bellows or the like., Is exercised.

It is known to be a very common method to generate an electrical force to balance or compensate for the force exerted by the differential pressure is to excite the coil of a force transducer (feedback motor) with a current whose Polarity causes the formation of a magnetic field in such a way that the coil is pressed against the differential pressure whereby the system is rebalanced or brought into equilibrium. A particular example of such a measuring system is described in US Pat. No. 3,216,250.

A prerequisite for such a measurement system is a method of sensing the relative balance of the compressive force and the electric force. In systems such as the one described in the aforementioned US patent, can be a relatively large force exerted by the diaphragm or the bellows at the entrance is substantially compensated for by the relatively small force generated by a feedback coil when taking advantage of the force multiplication obtained by using a lever system reached, use is made.

Because of the relatively great complexity of levers and the due to this inaccuracy in force adjustment or The invention is particularly aimed at force comparison systems aimed at eliminating such levers.

The elimination of levers and the like. Is achieved according to the invention in that a series construction, i.e., an in-line configuration is provided whereby the feedback coil is directly applied to the pressure sensing Bellows works. The in-line or "in-line"

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Structure is realized in that the system works with a narrow differential pressure range and that a special structure is used for the magnet.

Another feature of the differential pressure measuring device according to the invention is that the size of each Magnet, which is used according to the series construction, lies in narrower areas than in the connection so far magnets used with lever systems. This makes it possible to tailor each series magnet for its particular application to optimize.

An additional feature of the pressure measuring device is the arrangement of the magnetic poles, i.e. the magnetic poles are spatially arranged such that the outer housing for the magnet to the yoke for one of the magnetic poles will. Not only does this arrangement alone bring about an obvious advantage, but it also results in an extra advantage from the fact that external magnetic interference on the magnet is considerably reduced.

According to a further aspect of the invention, a reset or reset function is provided in the system. Zero control effect present. It turns out that in conventional control systems in the steady state there is a small attitude error signal must be present in order to maintain the output drive current that actuates the power transmitter. the provided for the control device according to the invention is that in the circuit on the Output of the position error sensor, instead of a proportional effect, an integral effect is present. The one from The advantage resulting from the integral action is that no control deviation is required in the idle state, The integrator essentially "remembers" the last positional error that existed before a more precise one

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Balance of forces occurred. The integrator output supports the necessary control for the Stromumfomier to maintain an exact force adjustment or comparison.

There are some major advantages to using an integrator. For example, the one different from zero Controller error can manifest itself in the typical force comparison system as an inaccuracy, namely due to changes in the sensing element displacement in the range of the input variables. By providence of an integrator, the control system has a control deviation that is equal to zero in the idle state, and thereby trying to eliminate any force error due to the spring stiffness of the sensing element. In addition, the position sensor, which is in the form of a linearly variable differential transformer, is also one Generate imprecision if within the range of the Input variables there is a change in its position. The linearity characteristic of the sensor has a direct effect on the output signal. The control system with integrator eliminates this possibility because its in the idle state Deviation is zero. The conversion of an input differential pressure into a Output current therefore comes closer to the ideal.

In addition to the better accuracy of the system, there is a minimal displacement volume as the volume a filling fluid on both sides of the sensing element (bellows, corrugated pipe or the like) in the entire area the input variable is kept constant. The reason for this is that the integral action ensures that the sensing element is always returned to the same neutral position. It also forces the integral action a corrective response or correction for df.n slightest error, so that the size of the temporary

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Displacement volume is also reduced to a minimum. As a result of the low displacement volume the transmitter has a better response speed. In addition, there is a saving because between the generator of the main variables and the force comparison transducer small-diameter pipelines can be used.

In yet another aspect, the present invention is concerned with improving the required in the operation of a differential pressure measuring device electronic equipment that is to be operable at a location that is different from a common Power source is removed. In particular, a feature of the present invention resides in a particular arrangement to obtain the bipolar voltages at the location of the sensing instrument (differential pressure measuring device) are required from the signal current that flows in a two-wire line connected to a central Power source is connected. In other words, the same two-wire supply line that has the task of changing current forward to appropriate secondary equipment as an indication of a change in differential pressure, is also designed to provide the required bipolar voltages.

Other objects, features, and advantages of the invention will be apparent from the detailed description that follows in conjunction with the accompanying drawings. Show it:

Fig. 1 is a schematic representation of a her

conventional power / energy balance system,

Fig. 2 is an elevation, partly in section, of a

Pressure measuring device, which is a preferred "embodiment of a force comparison

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systems according to the invention and their electronic circuits are only shown in block form,

3 is a block diagram of a control system;

which shows an integrator built into the feedback loop, and

Fig. 4 is a circuit diagram of the electronic

Circuits of the control system.

Referring first to FIG. 1, the figures show the operation of a conventional force comparison system to recognize. A differential pressure is applied to the inputs labeled HP (high pressure) and LP (low pressure), so that with increasing differential pressure the bellows 10 is pushed upwards, whereby the bellows becomes longer. This change in system balance is achieved by a linearly variable differential transformer 20 sensed, which emits an electrical signal at its output. This electrical signal is generated by a Rectifier 30 rectified and amplified by an amplifier 40. The resulting current from the amplifier 40 is then fed to a force-generating coil 42, which along its length is in a gap with is immersed in a magnetic field which is generated by the permanent magnet 44.

The electromagnetic field thus generated by the current flowing in the coil 42 drives the coil out of the Gap of the magnet 44 out, whereby the core 46 of the linearly variable differential transformer 20 is reset will. This leads to the pressure-sensing bellows 10 being brought closer by means of the lever system 48 is returned to its original position. It should be noted, however, that with this change in pressure a

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new equilibrium is present. In other words, the described follow-up movement generates variable, small error signals according to the additional differential pressure change.

Regardless of whether the system is of the type specifically shown in FIG. 1 or of another type, such as of the kind described in the aforementioned US patent, the result is that a relatively large force exerted by the bellows or other input device, again is balanced by using a lever system such as lever 48 in Fig. 1, use is made of a force multiplication.

Reference is now made to Fig. 2, which shows a preferred embodiment of the invention, wherein the electronic part in simplified form or in block form is shown. The differential pressure transducer according to the invention is housed in a housing 50, which has an upper part or a jfcappe 52, which is essentially is cylindrical and open at one end, and has a lower portion or cap 54, which also is substantially cylindrical and open at one end. A middle portion 56 is between The three housing sections 52, 54 and 56 are coaxial with one another arranged in alignment and fastened to one another by suitable means, such as screws 58, for example.

A partition wall 60, which is arranged substantially perpendicular to the axis of the housing sections, is divided the interior of the housing in two pressure chambers 62 and a passage 66 for the introduction of fluid with eirem relatively high pressure in the lower chamber 64 is caused by the partition 60 and a radially inwardly directed

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limited federal government 68. A corresponding opening or a corresponding passage 70 which is provided at the upper end of the housing, serves to introduce fluid at a low pressure in the upper chamber 62nd

A pressure-sensing bellows 72 is arranged in the lower pressure chamber 64 and seals this pressure chamber in that its upper end is connected to a safety device 76 in such a way that there is an opening 74 encloses in this device. Another opening 78 is formed in this device so that the pressure of the upper chamber 62 also prevails in the interior of the bellows 72.

The displacement of the bellows 72 is effected by a rod 80 directly onto a core 82 of a differential transformer or a difference generating device 84. A spring device 86, the upper end of which in seated in a yoke 88, exerts a prestressing force on the rod 82.

The differential transformer 84 is provided with a coil or winding arrangement 85, the two connected in series Having secondary windings and a primary winding excited by a suitable power source. Since this species of differential transformer is known, details of its construction are for a description of the present Invention insignificant. However, there is an electrical connection from the secondary windings of the transformer 84 shown by means of ladders 90 to a pressure-tight clamp 92. This terminal is next to a rectifier 94, as shown schematically, and the output of the comparator 94 is connected to an amplifier 96 tied together. The output of the amplifier 96 is fed back to a pressure terminal 97 which is connected by means of conductors 98 to the Coil or winding 100 is connected. The coil 1OO is seated 409831/07 5 6

- Q _

2ΑΠ1773

on a sleeve 102 attached to the lower end of the bellows 72 is attached. The coil 100 is part of a moving coil or electrodynamic system, i.e. the Coil 100 on the sleeve 102 is movable in an air gap of the magnet assembly 104, which the pole pieces and 108. A closed path for the magnetic circuit is through the lower part 54 of the housing formed, which is made of magnetic material.

The pressure cell or measuring device according to the invention uniquely measures an input differential pressure because between movement of spool 100 and a direct action is provided for the pressure-sensing bellows 72. This series or "in-line" relationship which can be seen from Fig. 2, between the sleeve 102 carrying the spool and the bellows 72 required more force from the coil 100 than previously with conventional ones Constructions has been required. However, the requirement of the additional force is met by the Construction of the force transmitter 105, which has the magnet assembly 104 and the coil 100, namely in such a way that it has a sufficiently strong magnetic field to deliver the necessary force through the coil movement, when current is passed through coil 100. When applying the present invention, it has been shown that that a magnetic induction of approximately 5000 Gauss in the air gap is sufficient in typical cases, in which encountered a range of input differential pressures from 0 to 186.2 mm QS (3.6 pounds / square inch) will.

As can be seen, the differential pressure to which exerted pressure-sensing bellows 72 when the individual pressures on the chambers 62 and 64 are different. The low pressure in the upper chamber 62 is achieved by means of the openings 78 and 74 in the safety device

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transferred into the interior of the bellows 72. The length of the bellows 72 changes according to the pressure difference. As a result, if, for example, the bellows becomes shorter, which is the case when the differential pressure increases, the core 82 of the transformer 84 moves against the biasing force of the spring 86 upward. That list an output signal which is applied to the input of the rectifier 94. The rectified output signal is then amplified by amplifier 96 and transmitted to coil 100 by conductor 98. The sink 100 is moved in the air gap a distance substantially equal to the displacement of core 82, but in the opposite direction, i.e. downwards in this case, around the bellows essentially in its starting position bring back. The difference between the two displacement movements represents the small displacement which is required / to generate an error signal, which is sufficient to keep the system in a stabilized or steady state. However, it is to recognize that the generation of an error signal is only required in a type "0" control system with proportional action and that when a Type "1" restoring or integral action is present, as is preferably the case in that If the measuring system to be described is the case of the electronic circuit part, no error signal is generated will.

It can be seen that the direct effect of that exerted by the coil 100 on the pressure sensing bellows Restoring force leads to an extremely high level of accuracy, because rotary bearings and their frictional properties as well as rotary bending with their alignment difficulties are eliminated.

The electronic features of the invention are shown in FIG

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has been shown in simplified form. For the sake of simplicity, only the feedback loop to the force-generating coil 100 is shown in FIG. In addition, as mentioned above, the integrating function has not been shown in detail in FIG. 1. However, referring to Figure 4, the details of the zero control or integration feature of the invention will become apparent. In the following, however, reference is first made to the control system shown in the form of a block diagram in FIG. In FIG. 3 it is shown that an integrator _{is inserted into the system following the position error sensing device, ie the block labeled K 0} , and immediately before the block labeled K. As mentioned above, the integrator ensures that the sensing element is always brought back to the same neutral point. In other words, the integrator output provides the necessary control for the current converter in order to supply the force-generating coil with sufficient current so that an exact force balance is maintained.

The implementation of the integrator shown in block form in FIG. 3 can be seen from FIG. The schematic diagram of FIG. 4 shows an integrating operational amplifier 110 which has a feedback capacitor 112. Such an operational amplifier is only part of an integrated semiconductor chip, which also has the operational amplifiers 114 and 116. Such a chip is supplied, for example, by the company Siliconex Corporation under the designation L143 or L144.

It can be seen that the operational amplifier 114 is connected to the primary winding circuit of the linearly variable differential transformer 84 previously shown in FIG.

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is shown. Accordingly, the core 82 and the winding arrangement 85 correspond to the respective individual parts in FIG. 2. Because of the convenient connection of the capacitors 120 and 122 to the common busbar 130 and the pins 9 and 6 (minus input) of the operational amplifier 114 and further because of Connecting the primary winding of differential transformer 84 to the same pins, amplifier 114 operates as a Colpitts oscillator which resonates at a frequency of approximately 2 kHz. It can also be seen that pin 5 (plus input) of operational amplifier 114 is connected to common busbar 130 through a resistor 132.

The secondary circuit of the differential transformer 84 has two secondary windings in a symmetrical series circuit arrangement which has pairs of rectifying diodes 140, 141 and symmetrical or balanced resistors 142. Connections to the ends of the secondary windings are made by leads 150 and 152. In addition, connections are made by line 154 from the midpoint of the secondary windings to the midpoint between resistors 142 having the same values. A capacitor 160 is connected between the ends of the secondary windings. A capacitor is connected between the outputs of the rectifier pairs 140, 141 and is also connected to pin 3 (minus input) of the integrating operational amplifier 110 through the resistor 164. For stabilization purposes, ie to prevent unwanted oscillations, a capacitor 166 is connected to the same input and to the common busbar 130. The other input (plus input) on pin 4 is connected directly to busbar 130.

As a result of the symmetrical arrangement in the secondary circuit k 0 9 8 3 1/0 7 S 6

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is the operational amplifier 110, which is known to be fundamental is a differential amplifier, for potential differences respond at its two inputs to generate an output signal. The output of the amplifier 110 (on pin 11) is connected to the Connected to pin 12 (plus input) of op amp 116, and this input is also through the Stabilization capacitor 176 is connected to the common bus bar 130. The output of amplifier 116 is in addition to the input of a power output stage 180 through a stabilizing capacitor 181 as well connected to the common busbar. A feedback path is from the emitter follower output of the stage via resistor 182 to pin 13 (negative input) of amplifier 116. The feedback connection is on the upper end of a load resistor 184, the other end to the common busbar connected is. The voltage to be fed back is of course derived from that through the resistor 184 flowing output current. The value of this resistor is expediently 50 ohms, if the output current range is from 4 to 20 mA; on the other hand, this value is 20 ohms when an output current range from 10 to 50 mA can be handled.

The particular arrangement for delivering bipolar voltages can be seen from the schematic diagram of FIG recognize. It can be seen that at the top of the diagram there are two clamps 200 and 202 which be in communication with a suitable transmission line connected to a remote battery which provides a voltage in the range of 50 to 75 volts. Thus, the same transmission line serves as the connected to these terminals to transmit the change in the current signal when the differential pressure increases changes, in addition to this, from the removed battery the

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supply voltages required for the various amplifiers shown in FIG. These amplifiers are the operational amplifiers 110, 114 and 116. Bipolar voltages are required for these amplifiers. It is highly desirable that the required Voltages are supplied from a remote central station instead of from where the differential pressure is sensed.

Achieving the goal of obtaining the desired bipolar voltages requires extremely precise regulation these voltages are required. Thus, as can be seen from Figure 1, to the power output stage 180, i. A power supply arrangement 2Ο6 is connected to the output of the transistor pair 204 arranged in a Darlington circuit, which has a current regulating diode 210 and a Zener diode 212 in series therewith. The cathode the Zener diode is connected to the base of a transistor 214, while its anode is connected to the bus bar 130 connected is. The collector of transistor 214 is connected to the anode of current regulating diode 210. The current regulating diode The purpose of 210 is to ensure that the zener diode 212 is within prescribed current limits works to provide the required, well regulated voltage across its terminals. As a result, the tension is also well regulated from base to emitter of transistor 214, with the result that a positive Supply voltage of approximately 7.0 V on pin 14 of the various operational amplifiers, i.e. the amplifiers 110, 114 and 116 is available. By using the series resistor 220, pin 1 has the aforementioned Amplifier provides a slightly reduced positive supply voltage. It can be seen that appropriate Capacitors 222 and 224 are also provided in this half of the voltage regulator circuit.

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The other half of the required bipolar voltage is in the lower left part of the diagram (Fig. 4) to be recognized. The required negative voltage is therefore obtained by connecting a Zener diode 230 supplied with a negative terminal 200. It has been found that the use of the Zener diode 230 produces satisfactory results without additional control elements. It can be seen that the capacitor 232 is connected in parallel to the Zener diode 230. It turns out that as a result of the Zener diode circuit a pin 10 the required negative supply voltage of approximately 7.9V for the operational amplifiers Available.

The operation of the electronic part of the control system will now be described. As mentioned above, becomes the primary of differential transformer 84 with vibrations from the Colpitts oscillator already explained provided. These vibrations are present regardless of the position of the core 82. This leads to the result that the same alternating voltages are induced in the two secondary windings when the core is in a neutral position or zero position. The half-wave components of the two voltages appearing at the output of the demodulators 140 and 141 are therefore same. Thus, the voltage mean is between the point labeled 250 (at the top of the pair of resistors 142) and the common busbar 130 zero. Accordingly, the DC voltage across the Capacitor 162 zero.

It is assumed that the core 82 then moves up from the zero position; the voltage induced in the upper secondary winding then increases in its fourth, so that the mean value measured between point 250 and the common busbar is a positive equal

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voltage value is. The integrating operational amplifier 110 therefore "sees" this positive DC voltage value. As mentioned above, the amplifier 110 connects by means of the feedback capacitor 112 a suitable time constant with resistor 164, whereby the integrating or restoring effect is made possible. In the event that the selectivity of the Regular operation is required, i.e. if, for example, a proportional action is required in conjunction with an integral action, a resistor 113 becomes in the feedback path of the amplifier 110 and a suitable value is selected for this resistor. But if only a proportional action if desired, capacitor 112 is removed from the circuit.

The operational amplifier 116 effects a DC voltage amplification and accordingly amplifies that at the output of the Operational amplifier 110 appearing voltage signal. This amplifier 116 has its own emitter follower output Mistake. However, the required current gain is provided by the Darlington pair Reached a pair of transistors contained in the output stage. The output signal from stage 180 controls the current signal (output current) in the loop formed by a conductor 150 in the the force generating coil 100 is located. The return line for the loop is given by one in the diagram on the left visible conductor 252 is formed. The current signal that is looped by the assumed core displacement is required and which has changed the state of the integrating operational amplifier, leads to a movement the force-generating coil 100 in a direction opposite to the initial movement of the core 82, so that the core is returned to its zero position.

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Accordingly, if it is assumed that the
Core 82 shifts downwards / instead of upwards, as was initially assumed, so the voltage induced in the lower secondary winding is increased in value so that the mean value at the input of amplifier 110 is a negative DC voltage. This ultimately has the result that a current signal is present in the loop which causes the coil 100 to move in the opposite direction to the initial downward movement of the core 82, which in turn brings the core back to its zero position.

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Claims (8)

24D1773 Patent claims: Differential pressure measuring device with force comparison, characterized by a housing which has two pressure chambers by a pressure sensing device for measuring the pressure difference between the two Chambers, through a linearly variable differential transformer with a core displaceable as a function of the movement of the pressure sensing device, and by a feedback device having a feedback coil that receives power from the transformer and returns the pressure sensing device substantially to its original position. 2. Device according to claim 1, characterized by devices for direct action of the coil on the pressure sensing device. 3. Device according to claim 1 or 2, characterized in that the feedback device is one on the Pressure sensing device mounted sleeve on which the spool is mounted and having a magnet assembly, which is axially aligned with the pressure sensing device and provided with an air gap in which the coil is immersed. 4. Device according to one of claims 1 to 3, characterized in that the pressure sensing device is an in one of the two chambers, which is under high pressure, is axially arranged bellows, the interior of which is with the other chamber, which is under low pressure, communicates. 5. Device according to one of claims 1 to 4, marked 409831/0756 24-1773 characterized by a transmission loop with a common DC source and through facilities for transmitting a signal current depending on the location of the core to the feedback device and about this loop. 6- Device according to one of claims 1 to 5 , characterized by an integrating device as part of the feedback device for creating a zero point deviation. 7. Device according to claim 5 or 6, characterized by amplifying devices in the feedback device and by devices which the amplifying devices supply with bipolar voltages and which a device for removing the bipolar Have voltages from the common power source. 8. Device according to claim 1, characterized in that the device for removing the bipolar voltages is connected to the transmission loop. 409831/0756