FR2528168A1 - Pressure transmitter for use with various manometers - has saw-tooth generator and processing circuits for produced differential signal - Google Patents

Abstract

The pressure transmitter has at least two electric windings (B1,B2), fed respectively across two charge resistances (Tr1,Tr2) by the same signal generator (G) producing alternating saw-tooth (Ue) signals. A metallic plate (P) is mounted in the field of one of the windings. Coupled to the plate is a Bourdon monometerwhich is deformable under the effect of the pressure to be measured and produces a relative displacement of the windings and of the plate. A differential amplifier (Ad) forms a differential signal (Ud) corresponding to the difference between the voltages at the terminals of the two windings. The continuous component (Uc) of the differential signal is eliminated by a capacitor (C). The residual alternating signal (Ua) is passed through a rectifying circuit (K) and its mean (Um) or peak value formed. The transmitter produces a signal which varies as linearly as possible as a function of the measured pressure.

Description

 The present invention relates to a pressure transmitter, that is to say a device producing an electrical signal which varies as linearly as possible as a function of the pressure measured.

The article published in the German magazine
FEINWERKTECHNIK + MICRONIC, volume 77, August-September 1973, pages 276 and 277, and entitled "Winkelstellungsmelder", describes a pressure transmitter, which comprises two fixed, electric windings, supplied respectively through two load resistors, by the same generator of periodic rectangular pulses t a metal member can be moved in the field of electrical windings under the effect of the pressure to be measured, as described for example in French patent No 0 1 427 236 of the KAMAN AIRCRAFT CORPORATION, and illustrated in Figures 1, 2, 14 and 15. the pressure transmitter described in the article cited from the German journal FEINWERE-
TECHNIK + NIaRONlO also includes a circuit to form a differential signal, corresponding to the difference between the voltages across the terminals of the two electro windings.

 pressure transmitters of this type offer various drawbacks s the voltage which appears at the terminals of each electrical winding being proportional to the derivative with respect to the time of the current which is sent in this winding by the generator of periodic rectangular pulses, this voltage takes a high value at each rising or falling edge of the rectangular pulses, the slopes of which are very high. On the other hand, the differential signal obtained by applying unipolar rectangular pulses to the two electrical windings comprises a more or less significant continuous component, which, however , is susceptible to random variations, and in particular to a drift over time, linked to various physical phenomena, in particular thermal phenomena, of which the solid electronic components which constitute the differential circuit are located. Consequently, the output signal of a pressure transmitter of this known type is affected by random errors tones and drift over time, which seriously affect its precision and fidelity.

The pressure transmitter according to the present invention is of the known type, previously mentioned, that is to say that it comprises at least two electrical windings, supplied respectively through two load resistors, by the same generator of periodic electrical signals, at least one metal pallet, mounted in the field of at least one of the electrical windings, a deformable member under the effect of the pressure to be measured and arranged so as to produce a relative displacement of the windings and the pallet, and a circuit for forming a differential signal, corresponding to the difference between the voltages across the two windings0 It does not, however, have any of the drawbacks of pressure transmitters of this known type, which have been mentioned previously0
The pressure transmitter according to the present invention is characterized in that the generator produces alternating sawtooth signals, and that circuits are further provided for processing the differential signal formed, first by eliminating its continuous component, then by rectifying the residual AC signal, and forming its mean or peak value
Due to the sawtooth shape of the signals applied to the two electrical windings of the pressure transmitter according to the present invention, the overvoltages applied to these windings are substantially proportional to the slopes of the saw teeth On the other hand, the processing circuits eliminating firstly the continuous component of the differential signal formed, the output signal of the transmitter according to the present invention no longer comprises any component capable of random variations or drift over time, which considerably increases the precision and fidelity of the measurements of pressure made with this transmitter.

The elimination of the DC component of the differential signal makes it possible in particular to use, to constitute the corresponding differential circuit, an amplifier of modest performances, since the possible derivative of its output signal over time has no effect on the final result of measurement.

 It has been found that the output signal from the pressure transmitter according to the present invention is not a perfectly linear function of the pressure to be measured, but a function comprising a term, of small amplitude, proportional to the square of said pressure. This is why, in a preferred embodiment of the pressure transmitter according to the present invention, a linearization circuit is inserted between the output of the processing circuits and the alternating sawtooth signal generator, so as to reduce the slope said sawtooth signals, proportional to the amplitude of the output signal from said processing circuits; for a suitable dimensioning of the linearization circuit, it is possible to practically cancel the term of the output signal which is proportional to the square of the pressure to be measured, so that this output signal is exactly proportional to said pressure.

 Â By way of examples, several embodiments of the pressure transmitter according to the present invention have been described below and illustrated diagrammatically in the accompanying drawing.

 FIG. 1 is a block diagram of the electronic part of an embodiment of the pressure transmitter according to the present invention.

 FIG. 2 is the detailed electrical diagram of a possible embodiment of the circuits of FIG. 1.

 Figures 3, 5 and 7 are elevational views of the respective mechanical parts of three different embodiments of the pressure transmitter according to the present invention.

 Figures 4 and 6 are sectional views respectively along lines IV-IV and VI-VI of the figures. 3 and 5.

 FIG. 8 is a view in the direction of the arrow VIII in FIG. 7.

 In FIG. 1, G denotes a generator of alternating sawtooth signals, UE, whose amplitudes, A, and slopes, ss, are very constant.

These EU voltage signals are applied in parallel to the inputs of two current voltage transformers, Tri and Tr2, the outputs of which send two sawtooth alternating electric currents, i1 and i2, respectively, in two electric windings and and B2, which have not been shown in detail in Figure 1, but which are arranged opposite one another, on either side of a metal pallet P. As will be described later in more detail, this pallet P is coupled to a deformable member under the effect of the pressure to be measured (this member is also not shown in FIG. 1). The other ends of the windings 31 and B2, opposite to Tri and Tr2, are connected to a source of reference direct potential, UO. The voltages Ul and U2 which appear at the terminals of the two windings B1 and B2 are applied, respectively through resistors R17 and R18, to the respectively positive and negative inputs of a differential amplifier Ad. By designating the respective inductances by LI and L2, and by R1 and R2 the respective resistances of the two windings B1 and B2, one can write the relations:
U1 = L1 di1 / dt + R1i1 (1)
U2 s L2 di2 + R2i2. (2)
dt
The output voltage, UD, of the differential amplifier Ad is given by the relation
UD = g (U1 - U2) = # g. # (L1 - L2), (3), in which g denotes the gain of the amplifier Ad, and 5t denotes the absolute value of the slopes of the sawtooth signals, c is to say
# = ta α = -tgss (4).

Formula (3) above, which assumes that
R1 R2 and i1 - i2, shows that the output voltage, i.e. the differential signal, is in the form of rectangular bipolar pulses, as shown by the waveform designated by UD in Figure 1 Furthermore, formula (3) clearly shows that the amplitude of the alternating rectangular pulses is zero if LI - L2, that is to say if there is equality of the inductances of the two electric windings,
B1 and B2, each coupled with the metal pallet P; this can happen for example if the two windings s B1 and B2 are strictly identical and if the pallet P is arranged in their mediating plane.

When the member deformable under the effect of pressure moves the pallet P away from this mediating plane, the amplitude of the rectangular pulses UD ceases to be zero.

 We see on the other hand, on the waveform designated by UD in FIG. 1, that the amplitudes a1 and a2 of the positive and negative alternations of the rectangular pulses are unequal, so that the train of pulses has a continuous component Uc.

This continuous component may be subject to random variations and to edge phenomena over time, linked to various physical phenomena, in particular thermal phenomena, which occur, in operation, in the solid electronic components which constitute in particular the differential amplifier. Ad.

According to the present invention, the DC component Uc of the differential signal UD is eliminated by inserting a capacitor C, of appropriate value, between the output of said differential amplifier Ad and the input of a rectifier R; the waveform designated by Ua in FIG. 1 shows that the input of the rectifier R thus receives alternating rectangular pulses, the positive and negative alternations of which have exactly the same amplitude a, which can be deduced from the formula
aa? 2 a1 + 2a2 (5).

The voltage Ur which appears at the output of the rectifier R is for example formed exclusively by the negative half-waves of the alternating pulses Ua, and a circuit M forms a DC voltage equal or proportional to the absolute value of their average value, Um This DC voltage Um is transformed into an output current, ig, by a voltage transformer into current,
Tr3.

 As indicated previously, the output current ig of the pressure transmitter according to the present invention, illustrated in FIG. 1 and previously described, is largely free from any drift over time and from any random variation, thanks to the elimination of the direct component Uc, of the differential voltage UD, by the capacitive link C. One of the advantages of eliminating the direct component Uc of the differential voltage UD lies in the possibility of using, to constitute the differential amplifier Ad, a commercial operational amplifier, having modest performances, and, consequently, a low cost; indeed, an operational amplifier of this type is capable of affecting its output signal of a fairly high Uc component, but this is without disadvantage on the precision and the fidelity of the pressure transmitter according to the present invention, since this component continues is eliminated by the capacitive link C.

dans laquelle le coefficient k2 est très inférieur au coefficient k1, mais cependant suffisamment grand pour que cette fonction s'écarte d'une loi linéaire d'environ + 1,5 %0
Pour améliorer la linéarité du courant de sortie is, c'est-à-dire pour annuler pratiquement le coefficient k2 dans la formule (6), il est prévu, dans le montage de la figure 1, de transmettre, à partir du transformateur de tension en courant, Tr3, un signal électrique proportionnel au courant de sortie is, à un circuit de linéarisation L, qui est associé au générateur de signaux en dents de scie, G, de manière à réduire la pente des signaux en dents de scie UE, proportionnellement à l'amplitude du courant de sortie ig. Il est ainsi possible de parvenir à ce que la relation (6) ci-dessus ne s'écarte pas d'une loi linéaire de plus de + O,9ffi. Experience has shown that the output current is of the transmitter in FIG. 1 varies as a function of the pressure p to be measured substantially depending on the function:

in which the coefficient k2 is much lower than the coefficient k1, but nevertheless large enough for this function to deviate from a linear law of approximately + 1.5% 0
To improve the linearity of the output current is, that is to say to practically cancel the coefficient k2 in formula (6), provision is made, in the assembly of FIG. 1, to transmit, from the transformer of current voltage, Tr3, an electrical signal proportional to the output current is, to a linearization circuit L, which is associated with the sawtooth signal generator, G, so as to reduce the slope of the sawtooth signals UE , proportional to the amplitude of the output current ig. It is thus possible to achieve that the relation (6) above does not deviate from a linear law of more than + O, 9ffi.

 It should be noted that the pressure transmitters of the known type, mentioned above, in which rectangular pulses are applied to the two electrical windings, do not make it possible to obtain as good linearity of their output signal; this is due to the fact that the differential voltage which can be formed has a logarithmic shape, the linearization of which would require a circuit for converting a logarithmic signal into a linear signal; moreover, in the case of these known transmitters, the amplitude of the differential signals is practically equal to the supply voltage, which makes it very difficult to measure their peak voltage. This drawback is certainly avoided in the case of the transmitter according to the present invention. the circuit of which is shown in FIG. 1, since it comprises a circuit M for determining the average voltage, Um, of the differential pulses, however, as a variant, it would also be possible to determine the peak value (+ a) of the rectified pulses Ur (FIG. 1) since, as already indicated, this value of droppings a is perfectly defined, and different from the supply voltage of the circuit.

In the embodiment illustrated in FIG. 2, A1 to A8 denote operational amplifiers which may be of a current type, relatively inexpensive, insofar as drift phenomena, possibly affecting their output signals, have little effect. disadvantage on the performance of the pressure transmitter according to the present invention, as explained above. In FIG. 2, D has been designated conventional diodes, by R ohmic resistors, by C capacitors, by Z Zener diodes, by g transistors, by P potentiometers or variable resistors. OV terminals. and 24V. a DC voltage source, components D1, R1,
Z1, CI, C2, C3, R2, R3, Al and C4, as well as a regulator r allow to form a reference potential of 8V and a supply voltage of 15V. The potential of 8V. corresponds to the reference potential UO of the diagram in FIG. 1. The alternating sawtooth signal generator (G in FIG. 1) consists essentially of the components
R4, R5, R6, A2, C5, C6, R9, R10, R11, R12, TI, T2,
Z2, Z3, R13, R14, A3, C7 and C8. The sawtooth alternating voltage (UE in FIG. 1) is applied to the mobile socket of a potentiometer PI, the other two terminals of which are connected respectively to the two electrical windings, B1 and B2, through the resistors R15 and R16 . This potentiometer PI makes it possible to equalize the resistances of the two branches in parallel where the two windings B1 and B2 are inserted respectively. The voltages at their respective terminals (Uî and U2 in Figure 1) are applied respectively. through resistors R17 and R18 at the input terminals, negative and positive, of a differential amplifier A4-R19-R20. The differential signal (UD in Figure 1) which appears at the output of amplifier A4 is transmitted through the capacitor C9 to an input of an amplifier A5, so as to eliminate the DC component of the differential signal. The rectification circuit formed by the components R23, R24, R25, A6, D2 and D3 makes it possible to eliminate the positive half-waves of the alternating rectangular pulses transmitted by the output of the amplifier.
AT 5. The average value (Um in FIG. 1) of the rectified negative rectangular pulses is determined by the circuit formed by the components R26,
R27, C11, C12. The direct voltage is then converted into direct current by the components A7, 013, C14, C15, R28, R29, R30, R31, R32, T3 and D5.

Potentiometer P2 is used for zero adjustment of the pressure transmitter. The variable resistor P3 is used to adjust its measurement scale. The linearization circuit (L in Figure 1) consists of the components A8, R33, R34, R35, R36, D4 and C16.

It produces a linearity correction signal, which appears on C16, and which is transmitted through a resistor R37, to the negative terminal of the operational amplifier A3, which also receives the sawtooth signals, so as to decrease the slope of their negative half-wave, so as to reduce the coefficient k2 (formula (6) and thus improve the linearity. The output current i2 appears on the other electrode of transistor T3, through a diode D5 in series with a resistance R28.

 In the embodiment illustrated in FIG. 2, the current voltage transformers (Irl and Tr2 in FIG. 1) consist essentially of the fixed resistors, R15 and R16, in series with the corresponding fractions of the zero adjustment potentiometer , PI. This is allowed by the fact that, in formulas (1) and (2) above, the voltage drops across these two resistors, which are each equal for example 2200 ohms, have very high values not only compared in the ohmic terms of the voltages across the electrical windings B1 and B2, but also in relation to the inductive terms, of the form L.di / dt.

Figures 3 and 4 illustrate the adaptation of a pressure transmitter according to the present invention to a Bourdon pressure gauge of the type comprising a deformable tube in the form of C, T, one end of which is embedded laterally in a connector Ra, through which arrives the fluid whose pressure is to be measured, while its other end can be free, or else coupled in a known manner, by means of a mechanical device A, with a mechanism for processing a device d pressure display, for example a movable needle in front of a scale (not shown). S denotes a plate, for example of triangular shape, which serves to support the assembly described above and to secure it to the bottom of the button (not shown) of the pressure gauge. According to the present invention, a metal pallet P, for example in a non-ferromagnetic metallic material, such as brass or aluminum, has a bent end, PI, which is fixed, for example by welding or brazing, in the concavity of the deformable tube T; the other end of the pallet P extends in the interval between the two electrical windings B1 and B2, which are supported opposite one another by a U-shaped part, E, made of non-magnetic material, for example stainless steel, which is itself fixed, for example welded, against a plate Q, mounted cantilever on the connection Ra, by means of pins, t1 and t2, and a tightening screw V. Preferably, the assembly is made in such a way that the pallet
P is located near the mediating plane of the electrical windings B1 and B2 when the pressure prevailing inside the deformable tube T has a predetermined value, Po, for example equal to atmospheric pressure. When the pressure ç increases above this value Po, the free end of the deformable tube t undergoes a displacement in the direction of the arrow F, which has the effect of bringing the pallet P closer to the electric winding B1, whose self-inductance L1 therefore decreases, while that, L2, of the electrical winding B2 increases. The above formula (3) shows that this results in an increase in the differential voltage UD, that is to say the amplitude of the alternating rectangular pulses, amplitude which moreover had a very low value, or even possibly zero for the pO value of the pressure to be measured.

In FIGS. 5 and 6, the same references have been used as in FIGS. 3 and 4 to designate homologous components. The embodiment of FIGS. 5 and 6 differs from that of FIGS. 3 and 4 only by the constitution of the deformable tube T, of the Bourdon pressure gauge: it comprises a first extreme part in an arc, T1, embedded laterally in the connection Ra, a middle part formed by several helical turns T2, constituting a sort of helix with an axis perpendicular to the plane of FIG. 5, and a second end part, T3, substantially rectilinear; the end of the second end part T3 is free or else coupled to a pressure display mechanism via a member A. The pallet P, which also consists of a non-ferromagnetic material, is shaped of L, its part P1 being fixed, for example welded to the second end part, 3, of the deformable tube T, while the active part proper of the pallet
P is arranged in the interval between the two windings B1 and B2; these are still supported opposite one another by a U-shaped piece, E, welded to one end of an elongated plate Q, the other end of which is itself fixed to the connector Ra.

The variations in pressure p result in displacements of the second extreme part, T3, of the deformable tube T, in the direction of the double arrow f, which causes displacements of the pallet P in the interval between the electric windings B1 and
B2, these displacements being such that the pallet P approaches one of the two windings B1 and B2, away from the other.

 Figures 7 and 8 show a pressure transmitter according to the present invention, equipped with a pressure capsule, Ca, of a known type. The latter is supported by a circular plate S, by means of a connector, Ra, which passes through it at its center, and to which the fluid whose pressure 2 must be measured reaches. At the center of the face of the capsule Ca, opposite the support X, is fixed a first end of a rod t, made of a non-ferromagnetic material, the other end of which supports a pallet P, also made of non-ferromagnetic material.

As in the previous embodiments, the pallet P is thus supported in the interval between the electrical windings B1 and B2. These are mounted, one opposite the other, on the two branches of a U-shaped part, E, which is itself fixed to a plate Q1, movably mounted below a fixed plate Q, secured to the support S by balusters Co. The movements of the plate Q1 relative to the fixed plate Q are guided by guide rods, g1 and g2, fixed to the plate QI, and freely passing through corresponding openings of plate Q; the spacing between the plates Q and QI is maintained by a spring r, the compression of which is adjusted by means of a screw V, the head of this bearing on the face of the plate Q opposite to the plate QI1 which comprises a tapped hole into which the rod of screw V is screwed. This device makes it possible to place the two electric windings B1 and B2 at equal distance from the pallet P when the pressure prevailing in the manometric capsule has a predetermined value, Po. the capsule Ca when the pressure p increases or decreases from this value pO produces, by means of the rod t, displacements of the pallet P in the direction of one of the two electric windings B1 and B2.

 The present invention is not limited to the embodiments previously described. It encompasses all their variants. The pressure transmitter according to the present invention can be adapted to most known pressure gauges, the principle of which is based on the displacement of a moving part as a function of variations in the pressure to be measured; it can be applied in particular to all types of bellows pressure gauges. Instead of a single metal pallet P, arranged in the interval between the two electrical windings, B1 and B2, mounted one opposite the other, it is possible to associate the two windings B1 and B2 respectively, identical to each other, but arranged in any way, one, a fixed metal pallet, and the other, a movable metal pallet; with this arrangement, the self-inductances of the two windings coupled respectively to the two pallets are substantially equal when the respective spacings between, on the one hand, the electrical windings and, on the other hand the pallets associated with them, are equal to the one to

Claims (7)

R16), by the same generator of periodic electrical signals (G), at least one metallic pallet (P), mounted in the field of at least one of the electrical windings (B1, B2), a member deformable under the effect of the pressure to be measured and arranged so as to produce a relative displacement of the windings and of the pallet, and a circuit (Ad) to form a differential signal (UD), corresponding to the difference between the voltages at the terminals of the two windings (B ?, B2), transmitter characterized in that the generator (G) produces alternating sawtooth signals (UE), and that circuits (C, R, M;) are furthermore provided for processing the differential signal formed ( UD), first by eliminating its continuous component (UC), then by rectifying the residual alternating signal and by forming its mean or peak value.  CLAIMS 1. Pressure transmitter, comprising at least two electrical windings (B1, B2), supplied respectively through two load resistors (R15, 2. Pressure transmitter according to claim 1, characterized in that the values of the two load resistors are adjusted to obtain a differential signal (UD) formed of rectangular alternating pulses, said load resistors being for example formed by two fixed resistors (R15, R16) of equal values, between which a potentiometer (p1) is inserted, the movable plug of which is connected to the output of the generator (G) of alternating sawtooth signals (UE). 3. Transmitter according to any one of claims 1 and 2, characterized in that a linearization circuit (L) is inserted between the output of the processing circuits (C, R, M, Tr3) and the generator (G) of alternating sawtooth signals (UE), so as to reduce the slope (5T) of said sawtooth signals (use), proportional to the amplitude of the output signal of said processing circuits. 4. Pressure transmitter according to any conch of claims 1 to 3, characterized in that the metal pallet (P) has one end (P1) fixed in the concavity of the deformable tube (T), in the form of an arc of circle, of a Bourdon manometer, and that the other end of said pallet extends in the interval between the two electrical windings (B1, B2), which are supported one opposite the other, tzar example by a magpie ce (Q) mounted cantilever on the connection (Ra) of the deformable tube (T). 5. Pressure transmitter according to any one of claims 1 to 3, characterized in that the metal pallet (P) has one end (P1) fixed near the free end (T3) of the deformable tube (T) with turns helical (T2) of a Bourdon pressure gauge, and that the other end of said pallet extends in the interval between the two electric windings (B1, B2), which w are supported one opposite the other , for example by a part (Q) cantilevered on the connection (Ra) of the deformable tube (T). 6. Pressure transmitter according to any one of claims 1 to 3, characterized in that the metal pallet (P) is coupled to a menonetric capsule (Ca), for example by a rod (t) supporting said pallet (P) in the interval between the two electrical windings (B1, B2), which are themselves supported one opposite the other, for example by a part (Q) integral with the support (S) of the manometric capsule ( It). 7. Pressure transmitter according to claim 6, characterized in that means, comprising for example a spring (r) and an adjustment screw (V), are provided to allow the position of two electrical windings (B1, B2) relative to the metal pallet (P).