WO1989006346A1

WO1989006346A1 - Tank level gauge with servo-controlled feeler

Info

Publication number: WO1989006346A1
Application number: PCT/FR1988/000644
Authority: WO
Inventors: Danny James KHOÏ
Original assignee: Khoi Danny James
Priority date: 1987-12-29
Filing date: 1988-12-28
Publication date: 1989-07-13
Also published as: FR2625313A1; FR2625313B1

Abstract

The gauge comprises a drum (200) on which is wound a line at the end of which is suspended a feeler which, when taking a measurement, comes to the level to be gauged, its weight being balanced both by the reaction of the medium whose level is detected and by a reaction force imposed to the line via a reaction torque applied to the drum rotor; a reversible servo-motor (160) for driving the drum; means for detecting variations of said reaction force, indicating a variation of the relative position of the feeler with respect to the level to be gauged; servo-motor control means, operating in response to the detection means so as to actuate the servo-motor until the feeler is repositioned to the level to be gauged; means for measuring the drum rotation, delivering a signal which is a function of the unwound line length, representative of the level to be gauged. According to the invention, the servo-motor is a stepping servo-motor (160), the detection means are means which indicate the presence or absence of a reaction force variation and, in the case of variation, the direction of such variation; the control means are means which cause, upon a detected variation, the rotation of the motor by one step in the direction contrary to the direction of the detected variation; and the measuring means are means for counting up/counting down the number of rotation steps of the motor.

Description

RESERVOIR LEVEL GAUGE WITH CONTROLLED PROBE

The present invention relates to a tank level gauge with ⁵ slave probe.

A gauge of this type is for example described in FR-A-1 430 366, of the same inventor ^' ; this device uses a probe suspended from a rope wound on a drum driven by a servomotor controlled by the reaction of the drum to the forces exerted on the rope. When the probe is at the level to be measured, its own weight is balanced by the combined actions of the vertical reaction of the medium whose level is measured (if this medium is a liquid, this vertical reaction is constituted by Archimedes' thrust) and a reaction force, equal to the apparent weight

^~ - ° of the partially submerged probe, imposed on the rope via a corresponding reaction torque applied to the drum rotor. If the level changes, the apparent weight of the probe will vary and the corresponding change in the reaction force will be detected and will cause the reversible actuator to start up until the probe is repositioned at

20 level to gauge.

This type of gauge has been used successfully for almost 25 years; it in fact provides absolute precision of the order of ± 1 mm over 25 meters} precision which can only be achieved or exceeded by much more complex and less robust devices. In addition, the aforementioned device

^ has a structure which makes it conform to very strict safety standards, taking into account the risk of explosion inherent in many liquids whose level must be measured, especially liquid hydrocarbons.

One of the aims of the present invention is to propose such a gauge which, without compromising on the measurement accuracy, is of a more rational design in its structure, in order to reduce the manufacturing cost and to increase it. reliability.

Another object of the present invention is to provide such a gauge which is suitable for modern techniques of digital signal transmission and processing, by directly providing a digitized measurement signal without passing through an analog / digital converter circuit as was the case. case with conventional gauges.

Another object of the present invention is to provide a gauge whose zero adjustment 0 ("time setting") can be achieved by remote control.

Another object of the present invention is to provide a gauge which makes it possible to automatically detect two different levels within the same tank (for example, in addition to the liquid / gas interface corresponding to the surface of the liquid, the level d 'a

- ~ ^ oil / water interface within the liquid) fully automatically and without significantly increasing the overall complexity of the system.

Another object of the present invention is to provide a gauge which can be made entirely autonomous, both as regards its power supply and as regards the transmission of data to a remote control center; this will avoid the use of connecting cables (power and data transmission) which can be ^• very long (typically several kilometers, in the case of refineries for example, or even several tens of kilometers in the case of oil pipelines and pipelines).

The cost of these cable connections is in fact higher the more the whole system must be made, in most cases, entirely intrinsically safe (concept defined by the CENELEC standard), which imposes a certain number specific design choices for the telemetry system. Regarding this last point, reference is made in particular to FR-A-2,545,248, by the same inventor, which offers such an intrinsically safe telemetry system).

In this regard, the apparatus of the present invention has a number of design features which ensure that it consumes much less electrical energy than conventional apparatus, thereby making it possible to envisage the possibility of self- power (for example from photovoltaic cells), while such a possibility was incompatible with the level of consumption of current devices.

In addition, as will be seen below, the apparatus of the present invention makes it possible to provide a subsidiary function of concentrator of the signals collected by the various sensors of the same reservoir (levels, pressures, temperatures, etc.), this information being possibly directly processed in situ by the device (for example, to calculate the volume and or the mass of the liquid remaining in the tank), the information, raw or processed, thus collected is then transmitted in block by means of wireless transmission to a remote control station.

Another object of the present invention is to be able to significantly increase the measurement height, while maintaining the same accuracy. Indeed, conventional devices such as those described in the aforementioned FR-A-1 430 366, are limited to a maximum tank height of 25 meters, corresponding for example to the storage spheres of liquefied petroleum gas. It will indeed be seen that the invention, because of its particular measurement system, makes it possible to raise this maximum height without difficulty to approximately 50 meters, always with an accuracy of ± 1 mm, which has the advantage of allowing use of this gauge with liquefied natural gas tanks, the height of which is generally 45 meters.

To this end, the present invention provides a feeler type gauge enslaved, that is to say comprising (as in FR-A-1 430 366):

- a drum on which is wound a rope at the end of which hangs a probe which, during a measurement, must come to the level to be gauged, its weight being balanced there at once by the reaction of the medium which is detected the level and by a reaction force imposed on the rope via a reaction torque applied to the drum rotor,

- a reversible servomotor for driving this drum,

Means for detecting variations in this reaction force, _Q indicating a variation in the relative position of the probe relative to the level to be gauged,

- servomotor control means, operating in response to the detector means so as to actuate the servomotor until the probe is repositioned at the level to be gauged, - means for measuring the rotation of the drum, delivering a signal depending on the length of unrolled line, representative of the level to be gauged,

Characteristically of the present invention, this gauge has the following combination of characteristics:

- the servomotor is a stepper servomotor,

The detector means are means indicating the existence or not of a variation of the reaction force and, in the event of variation, the direction of this variation,

The control means are means for causing, in the event of a detected variation, the rotation of a motor pitch in the opposite direction to the direction of the detected variation, and

- The measurement means are means for counting / counting down the number of engine rotation steps.

According to a certain number of other advantageous characteristics of the invention:

The number of steps per revolution of the engine and the diameter of the drum are chosen so that, for each pitch of rotation of the engine, the drum rotates at its periphery by an arc length equal to a unit of level measurement, so that the counting / down counting of the number of engine rotation steps directly delivers a numerical value representative of the level measured in the tank;

The detection means comprise a first magnetic crown integral in rotation with the drum rotor and provided with a plurality of permanent magnets, a second magnetic crown integral in rotation with the rotor of the booster and provided with a plurality of permanent magnets, so that the reaction torque applied to the drum rotor results from the component tangential of the magnetic forces due to the angular offset deux between the two magnetic rings, and means for detecting variations Δθ of the angular offset θ between the two rings, this variation of the offset corresponding to variations in the reaction force and therefore to variations the relative position of the probe relative to the level to be gauged;

The means for detecting angular offset variations are photoelectric means comprising a photoreceptor photoemitter assembly mounted on a support integral in rotation with one

10 of the rings, cooperating with an angular encoder disc integral in rotation with the other ring;

- means are provided for detecting variations in a second angular offset value, making it possible to detect a second level inside the tank, in particular the level of a

--- * oil / water interface;

- Means are provided for detecting variations of a third angular offset value corresponding to a reaction force greater than the self-weight of the probe, so as to cause the latter to rise and come into abutment against a surface d

20 reference and to deliver a reset pulse to the up / down counting means when the tension force of the rope exceeds the corresponding reaction force value;

- Means are provided for adjusting the angular position of the support of the photoemitter / photoreceptor assembly, so that

25 adjust the angular offset between this support and the reference position of the angular encoder disc on the corresponding position for a medium of given density, at the exact position of the level probe to be gauged, these adjustment means advantageously include, driven by a a radial rod, u

30 eccentric movable in a tangential plane cooperating with a light of the support of the photoemitter / photoreceptor assembly;

- the angular encoder disc is made integral in rotation with the drum roto by means of a magnetic coupling don the homologous elements are located on either side of a paro

*** non-magnetic also separating the two magnetic crowns

- Autonomous means of supply and means of remote transmission of the information delivered by the measuring means are provided.

40 Other characteristics and advantages of the present invention will appear on reading the detailed description below of an embodiment, with reference to the appended drawings in which:

FIG. 1 is a sectional view, in elevation, of the gauge of the present invention, FIG. 2 is a cross-section along the line II-II in FIG. 1, showing the two magnetic rings facing each other,

- Figure 3 is a detailed view of the angular encoder disc and the optical readers with which it cooperates, - Figures 4 and 5 are detailed views of the support plate of one of these optical readers, to illustrate the way in which their setpoint adjustment is adjusted, this plate being seen respectively along line IV-IV of FIG. 3 and along line VV of FIG. 4,, - Figures 6 and 7 represent the position of the various photoelectric elements of the reader optical with respect to the sectors of the encoder disc, respectively in the case of detection of the level and in that of resetting the counter to zero,

FIG. 8 is an electrical diagram showing the configuration of the various measurement circuits,

- • Figure 9 is a detail showing the shape of the brushes of the collector, and

- Figure 10 is an overview showing the gauge placed at the top of a tank and equipped with its photovoltaic supply e of its means of wireless data transmission.

In FIG. 1, the reference 100 generally designates the gauge housing, for example made of aluminum, this housing essentially comprising a central element 101 entirely dividing, and sealingly, the gauge in two parts: part I, at left of the middle broken line, brings together the purely mechanical organs and, after closing by a cover 102, will form a sealed enclosure in communication with the sky of the tank; the fact that this part I contains only mechanical parts, it is not necessary to take special precautions with regard to safety, except to ensure sufficient resistance to the pressure prevailing in the tank (typically 25 to 50 bars in the case of liquefied gases).

On the other hand, the second part II, to the right of the middle broken line, contains all of the electromechanical (in particular the servomotor 150) and electronic organs, which will be isolated on the one hand from the interior volume of part I due to the central block 101 which has no opening, and on the other hand from the ambient atmosphere by a closing cover 103 forming an explosion-proof enclosure; this cover 103 may optionally include a window 104 in the rear part allowing an operator to read indications presented by a display device (for example a liquid crystal display) placed inside the explosion-proof housing.

Finally, a conduit 105 is provided for the passage of the power and antenna cables, as will be explained in more detail with reference to FIG. 10; this passage will of course be completely closed by a cable gland so as to maintain the seal and the character explosion-proof enclosure.

The echanic part I is of classical construction, comparable to that of the current gauges. 5 It essentially comprises a drum 200 on which is wound a rope (not visible in the section of FIG. 1) at the end of which is suspended a probe (generally denser than the medium to be gauged, partially submerged on the surface of the medium Typically, the rope is made up of a metallic wire with a very low 0 temperature coefficient of 6/10 ^e of mm in diameter, the probe is for example a metallic aluminum disc of 150 mm in diameter and mass 150 g.

The casing is extended in the lower part by a foot 106 which will be fixed to the top of the tank to be gauged (FIG. 10), this foot being provided with the central part of an orifice 107 for guiding the probe wire.

This rope is wound on the drum 200 whose outer peripheral surface 201 is machined with very high dimensional precision so that the measured angle of rotation of the drum corresponds as closely as possible to the length of rope wound or unwound by this.

* rotation.

The drum 200 is provided with a central hub 202 integral in rotation - but not in translation - with a central drive shaft 21 (note: the elements integral in rotation with the drum will always be designated by a reference of the form 2xx) . The rotary drive 2 of the hub 202, and therefore of the drum 200, is provided by a key 20 sliding axially in a groove homologous to the drive shaft 210.

The drive shaft 210 is supported at one of its ends by a bearing 211 mounted on a chair 110 ensuring the support of the assembly

30 shaft-drum, allowing the coiled rope to hang freely on the latter; at its other end, the shaft 210 is supported by a bearing 21 mounted on the central part 101 of the casing.

The chair 110 is rigidly mounted on the central part 101 of the card by means of fixing screws 111-.

^ 5 Opposite the drive shaft, the hub 202 is closed by an end piece comprising a threaded orifice 204 engaged on a hollow endless vi 120 mounted on the chair 110 (note: the fixed elements c 'is to say integral with the housing 100, will all be designated by a reference of the form lxx). The hollow worm is crossed by the shaft

40 210 to allow the support thereof on the bearing 211 forming an end bearing; a minimal rotational friction torque is thus obtained, typically ensuring a detection sensitivity of variation of the reaction force exerted on the probe of the order of 0.03 N (3 gf).

This configuration allows, when the drum is rotated

45 unwinding or winding a certain length of rope, to subject it simultaneously an axial translation (arrow 205) making it possible to keep the rope still in line with the orifice 107, despite the winding or unwinding of a certain length thereof at the periphery of the drum. On the side of the central part 101 of the casing, the shaft 210 carries a crown 220 (also visible in FIG. 2) comprising a plurality of peripheral magnets 221 in even number (eight in the example shown), including the north axis south is oriented radially and whose magnetization is reversed by a magnet to the neighbor, all the poles facing inwards of these magnets being joined by an annular yoke 222 made of mild steel.

As will be described later, this configuration makes it possible, through the continuous wall 108 (of non-magnetic material, for example aluminum) of the central part 101 of the casing, to slide a magnetic coupling with a ring of homologous magnets 320 comparable, integral with the servomotor rotor.

With regard to the production of this crown (as well as that of the crowns 320, 230 and 240, which will be described later), the magnets can be screwed or glued to the cylinder head (the magnets must indeed resist forces of attraction exerted by the homologous magnets located opposite) and embedded or inserted in a protective covering (resin or nylon for example).

The shaft 210 also carries a second ring 230 of magnets 231, for example four in number (FIG. 2), of cylindrical shape with their main magnetic axis oriented axially, these magnets being provided at their free end with a pole piece 231 allowing them to conform to the inclination of the wall 108 of the central part 105 of the casing which is opposite, and are joined at their other end by a culase 232 made of mild steel in the form of a flat crown. This second series of magnets 230 cooperates, as will be indicated below, with a homologous ring 240 of magnets 241 placed opposite, on the other side of the wall 109, in order to cause rotation of an angular encoder disc 246.

Having described the purely mechanical part I (on the left in FIG. 1), we will now describe the electromechanical and electronic part II (on the right in FIG. 1); as already mentioned above, these two parts are physically separated by the central part 101 of the casing 100 which provides total sealing between its two faces, the coupling being only ensured by magnetic effect through the watertight and continuous walls 108 , 109.

The essential part located in this part II is a movable assembly 300 driven by the shaft 311 of the booster 160, the stator of which is mounted on a fixed plate 130 secured to the casing 100 (note: in the same manner as above, all the members secured in rotation of this moving part will bear a reference of the form 3xx). First of all, the mobile assembly carries a crown 320 of magnets 321 (also visible in FIG. 2), homologous to the crown 220 of magnets 221 secured to the drive shaft of the drum. These magnets are in number equal to that of the magnets of the crown 220, with a reverse magnetization direction so as to achieve an attraction effect between the magnets facing each other; all the poles facing outwards of the magnets 321 are joined by a cylinder head 322 made of mild steel.

As can be seen in Figure 2, there is between the magnet rings 321 and 221 an angular offset θ due to the reaction force 0 compensating for the apparent weight F of the partially submerged probe. This sliding angle θ is thus characteristic of the probe immersion level, for a given probe weight and for a medium of given density. Any variation Δθ in more or less of this sliding angle θ will indicate that the apparent weight of the probe has varied, either in one direction or in the other, therefore that the probe is no longer exactly at the gauging level and that it It will then be necessary to actuate the reversible servomotor 160 in the opposite direction until the initial immersion level is restored.

The detection of this condition, that is to say the detection of the variations Δθ of the sliding angle θ around a predetermined value, is carried out

20 by photoelectric means essentially comprising a photoemitter / photoreceptor assembly 330 (for example, a light-emitting diod (LED) placed opposite a photodiode), which will be called hereafter "optical reader". This optical reader is of conventional type, and one can for example use a model such as l

- ** KLH 212 marketed by MCB, or any comparable reader.

This optical reader is integral with the crew 300 (donating identically to the crown 320), and cooperates with an angular coding disc (that is to say a disc comprising concentric tracks alternately having alternately opaque sectors e

30 transparencies interposed between the LED and the photodiode of the optical reader), this disc rotating identically at the crown 220, that is to say identically at the drum.

More specifically, the disc 246 is mounted coaxially on an equipment rack receiving a ring 240 of magnets 241 joined by a pole piece 242,

35 equal in number to the magnets 231 located opposite the shaft 210, on the other side of the non-magnetic wall 109. Like the magnets 231, these magnets 241 have their axially oriented magnetic axis and are provided with a pole piece 243 conforming to the wall 109. This equipment is supported by two bearings 244 (mounted on the part

4 central 101 of the casing) and 245 (mounted on the moving element 300), and it is mounted insane so that, functionally, it can be considered comm integral in rotation with the shaft 210 (it is for this reason that all these elements are referenced in the form 2xx), disregarding a system with magnetic coupling constituted by the magnets 231, 24

4 which shows no slippage (unlike the coupling constituted by magnets 221, 321).

Finally, the electrical connections between the moving part 300 (which carries the optical reader 330) and the rest of the circuits mounted fixed on the casing are provided by a rotary joint formed by a series of annular tracks 340 d sposées on a plate 312 (these tracks are shown schematically in FIG. 8) and cooperating with a series of brushes 140 fixed on the plate 130 already supporting the servomotor 160.

Preferably, as illustrated in more detail in FIG. 9, the brushes 140, which consist of conventional bent elastic blades, are arranged alternately from one track to another and are in even number, so that the forces of friction of these brushes 140 on the tracks 340 does not produce a different friction torque from one direction of rotation to the other, so as not to distort the measurements.

The plate 312 also supports a series of electronic cards represented schematically at 400, arranged in a square around the servomotor 160.

Furthermore, and characteristically of the invention, a drum circumference 200 is chosen which, measured in millimeters, is a multiple of the number of steps per revolution of rotation of the booster; this makes it possible to directly have an indication of the length of rope unwound or wound up by counting or counting down the number of motor rotation steps (ie the number of control pulses sent to it): the cumulative value of this count gives directly the absolute value of the length of the unwound rope and therefore the level in the tank.

This avoids going through an angular encoder (in fact, the angular encoder 140, 246 is only used to detect the presence or absence of a variation Δθ with respect to the set value θ and does not carry out any measurement of this variation Δθ nor of the angle θ) or an analog sensor (as in FR-A-1 430 366) followed by an analog / digital converter, since the servomotor itself performs the double drive function d on the one hand and measurement on the other hand, this measurement being by nature very digital since it is a stepping motor.

For example, there is provided a drum whose circumference is 480 mm, with a servomotor 160 whose output shaft rotates by one revolution for 480 steps (for example a 48-step motor is used with an output train gears ratio 10: 1): thus, for each step of rotation of the motor, we will have unwound or wound exactly 1 mm of rope, and it will suffice to count or count the number of steps, without any conversion, to obtain directly the length of the rope in metric units (for other units of measurement, it suffices to choose a drum with a circumference of different value). FIG. 3 describes in more detail the structure of the device for detecting the relative difference between the crew 300 and the drum 200, comprising the encoder disc 246 integral in rotation with the drum on which the fili is wound (we ignore the 'magnetic coupling 231, 241, which plays no functional role), and the optical reader 330 integral in rotation with the moving element 300 driven by the servomotor.

This assembly is intended, as indicated above, to detect the appearance of a variation Δθ of the angular offset θ, also indicating the direction of this variation (increase or decrease in θ). To this end, the optical reader 330 includes two LED / photodiod pairs

335A and 335B slightly offset on either side of a median radius. The encoder disc 246 carries the sectors illustrated in FIG. 6 such that, if there is no variation, the photodiodes 335A and 335B are all obscured. If, for example, the level in the tank drops, the apparent weight of the probe will increase due to its lower immersion e thus causing an imbalance increasing the angle θ: due to this variation, one of the photodiodes, for example the photodiode 335A will be obscured by the corresponding transparent track of the disc 246, thus becoming illuminated by the LED located opposite. On the other hand, the photodiode 335B will not change state, since it will always be in front of an opaque window of the disc 246.

This information will be transmitted to servo means (described in more detail below with reference to FIG. 8) which will cause the sending of a pulse to the servomotor step by step in the direction tending to restore the level d 'initial immersion, i.e. in the direction allowing an additional length of rope to be unwound in order to bring the probe down to the new level to be gauged.

Conversely, in the event of a rise in the liquid, the photodiode 335B will be deactivated, causing the servomotor to rotate in the opposite direction in order to raise the probe until it returns to its initial level of immersion, corresponding to the new level to be gauged.

Advantageously, a second optical reader 330 ′ is provided, identified with the optical reader 330, and cooperating with tracks on the same disc 24 arranged in an identical manner. The only difference from the optical reader 330 is that this optical reader 330 'is set to a sliding angle θ' different from the angle θ corresponding to the level of the liquid / gas interface.

This second set value can in particular correspond to a liquid liquid interface such as an oil / water interface. If the optical reader 330 ′ is used in place of the optical reader 330, the position of the probe is no longer controlled on the actual level of liquid in the reservoir, but on a lower apparent weight corresponding to an interface liquid / liquid (e.g. 1.4 N (140 gf instead of 1.85 N (185 gf), for a 250 g mass probe and a liquid with density δ = 1.6): the servo then operates as an "interface follower" instead of operating as a "follower level".

5 _Q is also provided, always cooperating with the same encoder disc

246, a third optical reader 330 ", of a structure comparable to optical readers 330 and 330 ', but comprising (FIG. 7) a third LED / photodiode couple 335C aligned on the median radius and cooperating with a third track of the encoder disc comprising only "a narrow window, appreciably the width of the photodiode 335C. The photodiodes 335A and 335B control the servomotor in exactly the same way as in the case of the optical readers 330 and 330", but with an adjustment of the set position on a sliding angle θ "corresponding to an apparent weight greater than the self-weight of the probe in air (for example a setting at 4N (400 gf) for a probe having a mass of 250 g).

It is understood that, in this way, the servo-control will cause the probe to rise over the entire height of the tank without finding an equilibrium position, since the weight of the probe will always be less than the value

²⁰ setpoint for stopping the servo-control. This rise will occur until the probe comes into abutment against the underside 150 (Figure 1) of the gauge support. At this instant, the probe being immobilized, the prolonged rotation of the servomotor will energize the portion of filiii between the drum and the probe, and this

^ tension force will very quickly reach the setpoint force of the servo (4 N (400gf) in the example given), stabilizing the servomotor at this value. At the precise moment when the set value will be reached, the window of the internal track of the disc 246 will obscure for a short time the photodiode 335C, producing in

3 output from it a brief electrical pulse.

This brief electrical pulse will be used as a reset pulse for the servomotor step counter so that, after reconnecting to one of the optical readers 330 or 330 'to make a level measurement, the cumulative value of the number of steps

35 of the actuator control will give the absolute value of the length of rope unwound from the stop position, ie the absolute value of the level counted from the reference surface 150.

A very simple and fully automatic reset of the system is thus obtained ("time setting"), since it is obtained simply by adding a third optical reader 330 "to the encoder disc 246, and since the resetting the time is done by simple switching of the optical readers This switching can therefore be remote-controlled, for example, or triggered automatically at periodic intervals, without the intervention of an operator and a fortiori without the latter having ^ need to physically access the gauge organs; this last point is particularly interesting when you remember that these gauges are often placed at the top of tanks several tens of meters high (in oil refineries, for example), are eg accessible, and can also be very distant (from several kilometers or tens of kilometers) from the control room.

We will now describe the mechanical assembly of the optical readers 330, 330 'and 330 ". (Identical for each of these three readers), montag whose design makes it possible to simplify the adjustment of setpoint positions as much as possible (i.e. angles θ, θ 'and θ ") corresponding to each of the three optical readers.

To this end, each optical reader, for example the optical reader 330 is fixed on a plate 331 (seen in isolation from above in FIG. 4 and in fac in FIG. 5 in isolation) comprising two oblong slots 332 coming to engage in counterparts 351 secured to the crew body

300 (FIG. 1): the optical reader 330 is thus guaranteed a degree of freedom and rotation around the central axis of the gauge (ie the axis of the angular coder 246).

The fine adjustment of the desired position is achieved by means 35 comprising an eccentric 353 movable inside a homologous lumen 333 of the plate 331. This eccentric, which moves in rest in a tangential plane, is driven by a radial rod 352 emerging in a radial orifice 355 easily accessible from outside the body of the crew 300. A spring 354 allows the rod 352 to be immobilized in the desired position.

In addition to their particularly simple structure, these adjustment means 350 are extremely easy to handle by the operator, to which it suffices to remove the cover 103 (FIG. 1) to directly access the three orifices 355, 355 'and 355 "allowing the respective adjustment. setpoint positions of each of the three optical readers, this adjustment being for example carried out by means of a simple screwdriver.

We will now describe schematically the electrical and electronic circuits of the gauge of the invention, with reference to FIGS. 8 e 10.

The control signals of the electronic circuits (figure 8) are those delivered by the three optical readers 330 330 'and 330 "which we have just described. The three photodiodes 335A (figures 6 and 7) are coupled in parallel on the one of the circuits of the rotary joint 140 ₁ 340, the corresponding sign being brought to a control unit 410 (for example, microcomputer), on an S + input thereof. Similarly, the three photodiodes 335B are mounted in parallel and connected to the S- d input of this same control unit 410. As for the photodiode 335C of the optical reader 335 ", its signal is applied to a reset RZ input while the common point of all the photodiodes is connected at the common mass. Switching to one of the three optical readers 330, 330 'or 330 "s is done by means of a switch 420 which selectively supplies only one of the three inputs E, E' and E" for supplying the LEDs of the optical readers. 5 In position A, the optical reader 330 will be switched to slave the system to a set point corresponding to the level of the liquid to be gauged; in position A ', the optical reader 330' will be switched, slaving the system on an oil / water interface, while for position A ", the probe will be raised until it comes into abutment with the plane reference, this coming into abutment producing, as indicated above, a brief impulse which will be found on the reset input RZ of the control unit 410.

The control of the switch 420 can be carried out manually, automatically by the control unit 410 (which triggers, for example at periodic intervals, a "reset to the time" of the system), or even by remote control from a remote control station.

When the control unit 410 detects the appearance of a variation with respect to the set point, it sends to a unit 430 for controlling the servomotor 160 a pulse causing the latter to rotate in

20 the direction tending to create an antagonistic variation, and simultaneously increments or decrements (depending on the direction of rotation) by one unit an internal counter.

The cumulative value of this counter is indicated on a display 440, for example visible through the window 104 (FIG. 1), this display ^& giving directly, as indicated above, the length of the unwound rope, and therefore the level of the tank , without any other form of conversion.

Very advantageously, the information produced by the control unit 410 is transmitted remotely on a bus 411, for example using the RS-232 standard, this bus can also transmit, in addition

30 level information prepared by the gauge of the invention, pressure information, temperature, etc. delivered by 460,470 sensors.

Optionally, not only the raw level, pressure and temperature information is transmitted, but also information

35 derivatives, calculated in situ by the control unit 410, for example the density of the liquid, the volume of the latter in the reservoir, its mass, etc.

In this regard, reference may be made to the aforementioned FR-A-2,545,248, which indicates the manner of preparing and multiplexing this information on a common transmission line.

40 Very advantageously, to make the system autonomous, provision is made for the transmission of this data by radio means by means of a high frequency transmitter circuit 450 delivering signals to an antenna 451 which can be placed (FIG. 10) above the gauge and supported by it; as the gauge is generally located at the top of a large tank, it is advantageous to be able to place an antenna in this high point.

The high frequency circuit 450 can also include receiver circuits receiving orders sent from a remote control room and transmitting these to the control unit via the bus 411, which is then bidirectional. For example, you can remotely switch from level measurement to that of the oil / water interface, resetting the system time, etc.

To make the system completely autonomous, it is advantageously added to it a power supply 480 comprising a panel of photovoltaic cells 481 supplying a regulator 482 making it possible to charge a series of batteries 483 (for example Cd-Ni batteries ensuring the supply of the various electrical circuits and electronic.

It is important to note that, due to their design, the electromechanical members of the gauge of the present invention are all members with low consumption, a necessary condition to be able to supply them with solar energy (the efficiency of the photovoltaic cells is indeed generally very weak and only allows a low voltage). Indeed, the use of a stepping servomotor ensures that the consumption thereof is practically zero when the system is in the equilibrium position, then no impulse is sent to it as long as this equilibrium remains. In addition, the use of purely magnetic couplings (with permanent magnets) ensures that these couplings do not consume any energy to maintain the equilibrium position. It is the same for the detection of variations with respect to the setpoint positio, previously carried out by a differential transformer comprising two series of windings movable with respect to each other. Furthermore, all of the circuits are compatible with the low voltage available at the output of a panel of photovoltaic cells, typically a voltage of 12 V, which allows direct use of the voltage across the terminals of the 483 battery without the need for a DC / DC converter (to possibly raise the voltage up to 24 o 48 V), or especially a DC / AC converter, which will be essential to supply the electromagnetic organs used in current gauges using differential transformers e transmissions to selsyns.

Finally, FIG. 10 shows the general layout of the various elements of the power supply 480, which can in particular be placed in a parallelepiped trunk of small thickness overhanging the gauge, but of length and width sufficiently large to be able to receive a substantial number of cells photovoltaic 481 and, in addition, provides a sun visor and / or weather protection function for the gauge located below. It can also be seen that this configuration does not prevent easy access to the various organs of the gauge, access which will be obtained simply by removing the protective casings 102 and 103, the supply cabinet remaining mounted on the central part 101 of the casing of the gauge, itself mounted on the foot 106 fixed to the tank.

Claims

1. A tank level gauge of the servo-sensor type, comprising: 5

- A drum (200) on which is wound a rope at the end of which hangs a feeler which, during a measurement, must come to the level to be gauged, its weight being balanced there at once by the reaction of the medium whose the level is detected and by a reaction force 0 imposed on the rope via a reaction torque applied to the rotor of the drum,

- a reversible servomotor (160) for driving this drum,

Means for detecting variations in this reaction force, indicating a variation in the relative position of the probe relative to the level to be gauged,

- actuator control means, operating in response to the detector means so as to actuate the actuator until the probe is repositioned at the level to be gauged,

Means for measuring the rotation of the drum, delivering a signal which is a function of the length of the unwound rope, representative of the level to be gauged,

characterized in that:

2 - the servomotor is a stepping servomotor (160),

The detector means are means indicating the existence or no of a variation in the reaction force and, in the event of a variation, the sen of this variation,

The control means are means for causing, in the event of

30 variation detected, the rotation of a step of the motor in the opposite direction of the direction of the detected variation, and

The measuring means are means for counting / counting down the number of motor rotation steps., Or

⁰⁰ 2. The gauge of claim 1, wherein the number of steps pa revolution of the motor and the drum diameter are selected so that for each step of motor rotation, the drum is rotated at its periphery with a length arc equal to a level measurement unit, so that the counting / counting down of the number of rotation steps of the

40 engine directly delivers a numerical value representative of the level measured in the tank.

3. The gauge of one of claims 1 or 2, in which the detection means comprise:

45 A first magnetic ring (220) integral in rotation with the drum rotor and provided with a plurality of permanent magnets (221),

A second magnetic ring (320) integral in rotation with the rotor of the booster and provided with a plurality of permanent magnets (321),

so that the reaction torque applied to the drum rotor results from the tangential component of the magnetic forces due to the angular offset θ between the two magnetic rings, and

- Detector means (246, 330) of the variations Δθ of the angular offset θ between the two rings, these variations of the offset corresponding to variations in the reaction force and therefore to variations in the relative position of the probe relative to the level at gauge.

4. The gauge of claim 3, wherein the means for detecting angular offset variations are photoelectric means comprising a photoemitter / photoreceptor assembly (330) mounted on a support (300) integral in rotation with one of the crowns (320 ), cooperating with an angular encoder disc (246) integral in rotation with the other ring (220).

5. The gauge of one of claims 3 or 4, wherein there is further provided means for detecting (246, 330 ') variations of a second angular offset value, making it possible to detect a second level at the inside the tank, in particular the level of an oil / water interface.

6. The gauge of one of claims 3 to 5, wherein there is further provided means for detecting (246, 330 ") variations of a third angular offset value corresponding to a reaction force greater than the dead weight of the probe, so as to cause it to rise and come into abutment against a reference surface (150) and to deliver a reset pulse to the counting / counting means when the tension force of the rope crosses the corresponding reaction force value.

7. The gauge of one of claims 4 to 6, wherein there is provided means (350) for adjusting the angular position of the support (331) of the photoemitter / photoreceptor assembly, so as to adjust the offset angle between this support and the reference position of the angular encoder disc on the position corresponding, for a medium of given density, to the exact position of the probe at the level to be gauged.

8. The gauge of claim 7, wherein the adjustment means (350) comprise, driven by a radial rod (352), an eccentric u (353) movable in a tangential plane cooperating with a light (332) of the support ( 331) of the whole

₅ photoemitter / photoreceptor.

9. The gauge of one of claims 4 to 8, wherein the angular encoder disc (246) is made to rotate with the drum rotor by means of a magnetic coupling, the elements of which

10 counterparts (231, 241) are located on either side of a non-magnetic wall (109) also separating the two magnetic rings (220, 320).

10. The gauge of one of claims 1 to 9, further comprising

-p. autonomous means (481, 482) for supplying and means (450, 451) for remote transmission of the information delivered by the measuring means.

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