FR2487516A1 - PRESSURE DIFFERENCE TRANSDUCER HAVING A VERY FLEXIBLE AND CONSTANT VOLTAGE DOOR-BEAM BEAM - Google Patents

Abstract

THE PRESENT INVENTION CONCERNS A PRESSURE DIFFERENCE TRANSDUCER OF THE TYPE USING A cantilever beam, having piezoelectric strain gauges which are mounted therein or which are diffused therein, a pressure-sensitive membrane and a pressure-sensitive membrane forming MEMBRANE AND BEAM IN A WAY TO PRODUCE TENSION IN THE BEAM AT THE LOCATION OF THE STRAIN GAUGES IN RESPONSE TO A PRESSURE APPLIED TO THE MEMBRANE. ACCORDING TO THE INVENTION, MEMBRANE 26 HAS A SHAPE FACTOR THAT IS AT LEAST ABOUT 6 AND BEAM 60B HAS A FLEXIBILITY AT LEAST 2.6 TIMES THAT OF THE MEMBRANE. THE INVENTION APPLIES IN PARTICULAR TO MEASURING EQUIPMENT.

Description

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  The low deviation / force ratio of such devices gives a low signal-to-force ratio, and makes such devices far from the best for small pressure difference ranges. that is, in the range of 0.0689 to 0.3447 bar. Furthermore, in such devices according to the prior art, it is necessary to take a high degree of care to thermally match, as precisely as possible, all the components and connections sensitive to pressure and responding to pressure, because the effects of the ambient temperature which produce even a small deflection of the beam, result in a degradation of the signal accuracy. In short, such devices, without a precise thermal correspondence of the components, do not have a relatively high signal-to-noise ratio because of the effects of the anbiante temperature. The condition of such thermal correspondence increases the price of

  such devices according to the prior art.

  Moreover, in order to protect such devices

  overpressure situations, the components must be con-

  truits to very precise tolerances. Thus, for example, if overpressure stops are placed at twice the full-scale deviation, and the full-scale deviation is small, the tolerances of the overpressure stop mechanism must be proportionally accurate to maintain the function. required overpressure. Finally, in the devices according to the prior art, using a combination of stiff beam / flexible membrane, the measurable range of pressure difference is determined by the physical characteristics (ie the stiffness) of the beam. Such devices are typically operative only over a relatively narrow range of pressure. Therefore, different ranges of pressure can be taken into account only by changing the beam for another having a different stiffness. This factor contributes to the costs and disadvantages of using such devices in different pressure ranges, since the beam is usually the most expensive component, as well as the most difficult to replace. Thus, the usual attempt is to have several transducers on hand, each designed for optimal operation in a different range of pressure, where a wide variety of variations can be expected.

pressure ranges.

  The need to have a low-flexibility beam (stiff), in combination with a flexible membrane in the devices according to the prior art, arises from the fact that the beam in such devices is structured so that its bending action, in response to the force on the membrane, being distributed along its entire length, without being concentrated in the zone containing the strain gage elements. This results in a significant loss of efficiency, up to about 50%. Thus, such devices have a relatively low signal-to-force ratio, requiring a design where there is accumulation of membrane forces on the beam, so that

  to obtain an appropriate response from the strain gauge.

  This criterion is achieved by using a membrane that is very flexible in comparison to the beam. The beam, being relatively stiff, has a high degree of stress for a given degree of deflection or deformation thus obtaining the high signal-to-deviation ratio mentioned above. Thus, the devices according to the prior art make it possible to obtain an appropriate response of the strain gauge by a mechanism which rests on a beam of low flexibility, giving a non-optimal performance of the pressure range, as well as other limits in the design of the steep beam, which was mentioned above. Thus, a pressure difference transducer is required to achieve appropriate signal quality over a wide range of pressure differences, including very small pressure differences, and to achieve a certain accuracy and accuracy.

tolerances in case of overpressure.

  In addition, a device with these capabilities with a design suitable for saving production costs is necessary. Finally, such a device is necessary, making it possible to obtain efficiencies

  almost perfect operation.

  In the present invention, a beam of great flexibility and specially designed in combination with a relatively stiff sensing membrane is used to produce a pressure difference transducer meeting the above conditions. At the center of the present invention lies the cantilever beam which has a constant tension configuration, and the bending of the beam is concentrated in the area of

  strain gauge sensors that are linked or diffused.

  This configuration thus allows almost perfect efficiencies of operation, because substantially all the force transmitted from the diaphragm to the beam is converted into a stress which, in turn, is converted into an electrical signal by the strain gages. Thus, the beam gives a high signal-to-force ratio which, in turn, allows the use of a very flexible beam

  in conjunction with a soft membrane.

  The advantages of a very flexible beam are numerous.

  First, and perhaps most importantly, because of the high flexibility of the beam, it is sensitive to very small forces transmitted to it by the pressure sensing diaphragm by means of the pulling wire . Thus, the beam is sensitive to. slight differences in pressure, provided that

  a sufficiently sensitive membrane is used (ie

say flexible).

  Secondly, since a very flexible beam may have a much larger full-scale deflection than a rigid beam, or a low-flexibility beam, without exceeding the optimum limits of tension of the material, it

  may have a lower signal-to-deviation ratio.

  Thus, it is less sensitive to errors due to transient causes such as, for example, changes in ambient temperature. From this point of view, it will be noted that deviations due to changes in ambient temperature will represent a relatively small proportion of the full-scale deviation, thus ensuring

  a high signal-to-noise ratio in such circumstances.

  Likewise, the tolerances in the stop mechanism in case of excess pressure or overpressure are less critical with a beam having a large full scale deflection. Thus, in situations of excess pressure, the voltages produced in the beam are well within the elastic limits thereof. Therefore, overpressure stops or stops can easily be established, for example, at twice the full-scale nominal deviation to allow normal manufacturing tolerances while maintaining tension in the system.

  the beam well within the elastic limits of it.

  Therefore, the need for special machining of

  components to precise tolerances to severely limit-

  the maximum deflection of the beam is eliminated,

  which lowers the manufacturing price.

  Finally, the use of a very flexible beam allows a much greater range of operation for it. The range of operation of the transducer is thus limited by the stiffness (flexibility) of the detection membrane. Therefore, if the transducer is to be used in different pressure ranges, only the diaphragm needs to be changed, the higher pressure ranges being accommodated by changing to a softer (stiffer) diaphragm. This is advantageous because the membrane is a relatively inexpensive ingredient,

  and is more ready to replace than the beam.

  The invention will be better understood and other purposes, features, details and advantages thereof

  will appear more clearly during the description

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  transducer. The isolation device 28, in combination with the O-ring 31 and the plate 35 has the function of forming a non-corrosive barrier between the detection cell 24 and the fluid whose pressure must be measured by this pressure transducer. The construction of this barrier differs from the typical construction according to the prior art o the welded isolation membrane

  to the sensor body, is inside the O-ring.

  Such a construction according to the prior art requires an additional support plate or the like, which must be in the same material as the isolation membrane to make the weld non-corrosive. This additional support plate or the like in the prior art represents an additional expense. Moreover, the construction of the present invention (isolation membrane and spacer ring welded to the outside of the joint

  toric) does not require a backing plate or the like.

  This chamber 30 is filled with an incompressible liquid

  such as oil, through a filling port 32 which is closed by means of a ball joint 34. Similarly, on the opposite side of the body 12, a second chamber 40 is formed between a second diaphragm isolation 42 and the inner side of the detection membrane 26. This

  chamber 40 is also filled with the same incomprehensible liquid

  than the first chamber 30 and is closed

a ball joint 50.

  The inlet pressure chambers are made using two pressure port plates 35 in combination with two O-rings 31 and four bolts and nuts (not shown). These plates 35 have appropriate inputs. While the pressure difference is applied, through the inlet pressure ports 37a and 37b, the membrane 26 drives the beam 60b due to

  coupling by incompressible liquid.

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2487516.

  accomplished if the P-type strain gauge elements 68 are scattered along <110> 41TO> directions on a wafer in the <100> plane, and the wafer is cut so that the wafer 64 can be oriented, when is bonded to the parts 61 and 62 of the beam, with the strain gauges arranged longitudinally

and transversely.

  The bridge circuits formed by the strain gauges 68 are provided with metallized pads and contact 70 for connection, by wires 72, to the lower portions of terminals 74 which pass through the collector 18. Wires 76 are fixed to the other end of the terminals 74, these wires passing through the tube 22 to suitable connectors (not shown)

  in the signal processing circuit.

  As can be seen in Figure 1, the metallic wire

  The tie rod 66 passes through a pin 78 into the sensing cell 24, and its end is fixed to the center of the sensing diaphragm 26. On the wire 6b, butt ends of the first and second abutments 80a and 80b of overpressure, which bear against O-rings 58a and 58b, respectively, to prevent excess voltage of the beam in overpressure situations respectively concerning overpressures of the left

  and from the right looking at Figure 1.

  Referring back to the specific beam

  shown in FIG. 2, the beam 60a can advantageously

  It should be chosen to have a nominal operating stress of 0.30 y / mm and a nominal full-scale deviation of 0.165 mm. This gives a signal / deviation ratio of the order of 2.0 pm / m, which is quite low, indicating a relatively high flexibility compared to the beams according to the prior art. This high flexibility is the result, at least partially, of the reduced thickness of the silicon wafer 64 in comparison with the beam. In the example given (which is only to illustrate the invention), the pellet 64 is about 0.3175 mm thick, while the elements 61 and 62 of

  the beam have a thickness of about 1.65 mm.

  This reduced thickness of the pellet compared to the beam is important from another point of view. Since the length of the pellet is relatively small compared to the entire length of the moment arm (typically less than 25%), and almost all the bending of the beam occurs in the pellet area (due to its greatly reduced thickness), the voltage across the strain gage area does not vary greatly and in fact can be considered to be approximately constant throughout this area, which thus develops

  substantially all the tension developed by the beam.

  Since the voltage is concentrated in the area of the detection elements, the beam has a relatively high signal-to-force ratio, for example of the order of 1.65. Thus, for example, a full-scale deviation, which gives a voltage ( or signal) 3401 / mm is obtained with an applied force of the order of 0.206 J, thereby to give the above-mentioned ratio of 1.65. This high efficiency of operation (which can be considered almost perfect, in the sense that almost all the applied force is converted into a strain on the strain gauges), makes it possible to obtain appropriate levels of signals despite the low signal ratio. / deviation (high flexibility) As the beam is very flexible, a good choice of the detection membrane can give a transducer giving acceptable results even at very small pressure differences, for example of the order of 0.0689 bar or less. The main criterion for choosing the right membrane is the ratio of the stiffness of the latter to the stiffness of the beam (the stiffness being the opposite or the opposite

  flexibility in this case).

  Each membrane has a "form factor" o which is defined by the geometry of the particular undulations of the membrane. More particularly, ob can be defined as follows: c equals the square root of J times the square root of K, where J is the ratio of the distance, along the surface of the membrane, between corresponding points - of two successive undulations (ie, peak-to-peak), the wavelength of the ripple (the distance in a straight line between the corresponding points of two successive ripples), and K is the ratio of the moment of inertia d a strip taken along a radius of the corrugated membrane, at the moment of inertia of a flat strip having the same thickness and the same length as the corrugated strip. It will be noted, on reading the above definition of form factor o6, that J and K will be equal to 1 for a flat (non-corrugated) membrane. Thus, for such a membrane, there will be 4 = 1, whereas a corrugated membrane will have a value cb i4 greater than 1, the particular value being defined by the geometry of the corrugations. For typical membranes used in pressure difference transducers of the type disclosed herein, cd values are

  usually from about 5 to about 30.

  For such membranes, the total force produced by applied pressure (the collected force) is given by the formula: (1) Fe = R2P; o Fc is the collected force, R is the radius of the membrane (assuming a circular membrane) and P is the pressure applied to the membrane. In order to obtain an appropriate signal at the output of the transducer, the force collected must be

  converted into a force applied to the beam (applied force).

  The applied force Fa can be expressed by a fraction of the force collected thus (2) Fa = F / C C; o G) is a number equal to or greater than 1. By solving according to GJ, we have:

(3) C F F IF

  The present inventor has empirically determined that for membranes with DG values of the order of 6 and above, the value of O does not exceed 2.6. This means that for such membranes, 2.6 units of force collected on the membrane produce at least one unit of force applied to the beam (the force on the membrane being transmitted to the beam by the wire 66, which is connected to the center of the detection membrane 26, as previously described) It can be seen that for small deviations of the membrane, the amount of deflection is roughly proportional to the force collected and roughly

  inversely proportional to the stiffness of the membrane.

  Therefore, as the force applied to the beam is approximately equal to 0.38 times the force collected, so that the beam undergoes a full scale deviation for a given collected force, it must have a stiffness not

  not more than about 0.38 times the stiffness of the membrane.

  Conversely, the beam may have a flexibility of

  at least about 2.6 times that of the membrane.

  Using the above described relationship between the flexibility of the beam and the flexibility of the diaphragm, a full-scale deflection of the beam can be achieved with pressure differences only reaching

  0.0689 bar with the beam constructed as previously

  described. Thus, the highly flexible beam will have a flexibility at least about 2.6 times greater than the relatively flexible sensing membrane used to measure such small pressure differences. If higher pressure ranges are to be measured, stiffer membranes are put in place, and the flexibility factor is necessarily increased above the minimum of 2.6. In this way, we can take into account, by a good replacement of the membranes, a range of differences of

pressure from 0.0689 to 4.137 bar.

  It will of course be noted that, especially when small pressure differences are measured, the insulation membranes 28 and 42 must have negligible stiffness

  in comparison to the detection membrane 26.

  FIG. 3 illustrates in detail another embodiment of the beam, that is to say the beam 60b illustrated with the transducer of FIG. 1. This beam 60b is a one-piece metal element, preferably made of steel 17-4 or 17-7 (17% chromium, 4% -7% nickel, which can be quenched by precipitation), and is cantilevered at one end with respect to the manifold 18. The free end of the beam b carries a wire 66 forming pulling, as in

  the embodiment previously described.

  The portion of the beam 60b close to the collector 18 is relatively thin and comprises a slight depression

  deep 88 and a flat area of reduced thickness 90.

  Area 90, which is not more than one-third of the total length of the beam, shall not be more than one-third the thickness of the remainder of the beam. As a specific example, if the body of the beam, of semicircular cross section, has a maximum thickness of the order of 1.65 mm, the zone of reduced thickness should preferably not have more than about 0.533 mm

thick.

  On the beam, at its reduced thickness area 90, two pairs of stress piezoelectric gauges 82 are glued, one pair on each side (only one pair of strain gauges 82 is shown for the sake of clarity). The strain gauge pairs on each side of the beam are wired in the half-bridge configuration. The strain gauges 82 are provided with metallized contact pads 84 for connecting the wires 72 which lead to the terminals 74. A terminal pad 86 is located in the depression 88 on one side of the beam, and another terminal pad (not shown ) is placed on

  the opposite side of the beam relative to the stud 86.

  With the configuration described above, a beam can be constructed which will have, for example, a full-scale deviation of 0.165 mm with a stress of 0.350 F / mm, to give a signal-to-noise ratio of the order of 2, 1 F / mm / mm. The full scale deviation can be obtained with a force of 0.687 J, to give a report

signal / strength of 0.509.

  As with the beam 60a of FIG. 2, substantially all of the tension due to the bending of the beam 60b is concentrated in the area of reduced thickness 90. Thus, the reduced thickness area 90 of the beam 60b, like the pellet of reduced thickness 64 of the beam 60a, gives the beam a high flexibility, while also making it possible to obtain appropriate levels of signals even with a relatively low signal / deviation ratio due to the concentration of the deflection force in the area of the strain gauge sensors, to thereby give a relatively high signal-to-force ratio. Furthermore, since the area of reduced thickness is short in length relative to the total length of the beam, and the strain gauges typically occupy only about one-third to one-half the length of the area of the beam. reduced thickness, the measured voltage does not vary greatly over the length of the strain gauges, and thus can be considered approximately constant over the entire length of the strain gages,

  which improves the efficiency and accuracy of the transducer.

  The metal beam 60b may be incorporated into a transducer, as described above, where the detection membrane 26 has a shape factor of at least about 6, and a stiffness of at least 2.6 times that of the beam, to obtain the advantages described above. Thus the specific examples described above can be made with a beam stiffness of only about 41.55 N / cm.

  Therefore, to achieve proper operability over a wide range of pressure differences (up to about 0.0689 bar), the membrane should have a stiffness of at least 108 N / cm. Again, as steeper membranes are used to accommodate higher ranges of pressure, the flexibility ratio of beam to membrane will necessarily increase beyond the

minimum value of 2.6.

  Thus, a pressure difference transducer has been described where the cantilever beam is constructed to obtain appropriate signal levels while exhibiting relatively high flexibility. This high flexibility, in turn, allows this transducer to accommodate a wide variety of pressure ranges, up to 0.0689 bar or less, by simply changing the sensing membrane (provided of course, this membrane has a shape factor of at least about 6 and a stiffness of at least about 2.6 times that of the beam, which is an easy criterion to obtain.

  given that the beam is very flexible).

  On the other hand, with the very flexible beam, with its large full-scale deviation, the transducer is less sensitive to inaccuracies due to ambient temperature variations, as described above, than are beam transducers. steep. Likewise, as previously indicated, the tolerances required in machining the components of the overpressure shutdown mechanism become less critical, as previously indicated, leading to manufacturing cost savings. % u-lFor a qqgo anbeqo ap suld ap puaidmoo ITnb ao ua 9STaD.OJeo '5 uoTDoTpuGAaJ el uolas inalonpsu-ea * 9 * uoTsseaJd I alqTsuas aueiqmae e.Tpul ape [at q uOsTeaEdmoo ua elqeaSTISu inapTei aunu2% uuamaeosTp aoue-qmam aeTp-e 'saujaexe xnaTpTm O sap uoTssaad aI q aeqTsueas aeBqumw eTpel laeosT% uuAnOd% a uoTssaed el elqTsuas auuaqmam BI q aJnaTj, 9xa (8Z) uamalosTp aueaqmam aun: snId ap puaeadmoo IT1nb ao ue 9sTloueo' L uoTeoTpuAeaa sl uoles anaeonpsuelJ * g * ael-npga inass Tdagp auoz el ap allaeo sToj sTo.x sutom ne q? B ak2% its anod BI ap a7eIeo.nan2uoT el anb ao ua egsTJl9oereo 'uoTeoTpuaea. InaeonSpectual sluiceS aeod aPul ap aleioz nan2uoI BeI B% ueuzaTauuopqJodoad al..noo e.uama.eaG a.znanmuo1 aun-e a.TJnppa iessTedggp ouoz oz eaTpul iaelnod and ap (06) eT-npga inessTuadgp ouoz aun You need JavaScript enabled to view it SAU SHEEEUID% uos saeqTogad aqeuTv- uoo ep sea2n sal enb ao ue 9sTgl9.OJBO 4. UOeoTPUG.aeI el uo0Is ataonpguil * osaa ^ Togiad a% .uT-afuoo 3p se2nu sap auoz BI suep aeTnpoJd% sa aienod 91 a% .peI suep eTnpoid uoTsua% eI eno. ueameIaqTsuas anb aIIa uoT.ltelSuoo aun e a9% To9d aainod el anb ao ua cs anouaeo '1 uoTeoTpueAa Ml uoles 1no J onpsu = Il' z aueJqmame aPPe ap aelao sToj 9'z Bc suTom ne a [ega asseldnos aun B (qo09 09 ) aelnod e9Tpul ea 9 uoTAue suToum 0 nsep aemoj ap mnaeosj a e (9Z) aueeqmm aupîe anb ao ua 9sTa9aOeauO aueleqmam aelpeI B eanbTIdde uoTssaid aun asuodaa uea auTuuoo ap senee.Tpsap% .Toipuaei q aelnod apeI sUmp uoTsua. aunTnpoJad uoô; ap aalxnod a.Tpel has auirqmum alTp% 'ueTTa [%: uImJo $ luaemz [r a% a uoTssaed There are alqTsuas at: tqmam at a 4aosnIJTp 1uos &: nb no seluom% uos X pTb atTexuoo ap sanbTaoaeIoz9Td sagner sap XUB xnej-s -alaod ua alznod aun'lueXoîdma ad4X. p u u 0 a a ap ap anal S S S S S S S S S S S S S S S S anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal anal and a pressure plate (35) which, in combination with the aforesaid isolation membrane and said O-ring, forms an inlet pressure chamber on one side of said transducer, said isolation membrane being welded to said side of said transducer at one end thereof; position external to the position of said O-ring.