FR2524667A1 - LEVEL LIMIT SWITCH FOR CONDUCTIVE FILLING WASTE - Google Patents

Abstract

A PROBE 4 PLACED AT CRITICAL LEVEL, CONSISTS OF A GROUND ELECTRODE 9 AND A DETECTION ELECTRODE 7. AN INTERMEDIATE ELECTRODE 8 IS MOUNTED BETWEEN THE TWO. WHEN THE LEVEL IS NOT REACHED, THE DEPOSITS 17 FORM BETWEEN ELECTRODES 7 AND 8 on the one hand, 7 and 9 on the other hand, A VOLTAGE DIVIDER. WHEN THE LEVEL IS REACHED, THE DENREE CONSTITUTES SUPPLEMENTARY EQUIVALENT RESISTANCES R3, R4 WHICH DO NOT MODIFY THE BALANCE OF THE DIVIDER. ON THE OTHER HAND, AN EQUIVALENT RESISTANCE R5, ESTABLISHED BY THE SUPPLY BETWEEN DETECTION ELECTRODE 7 AND CONDUCTIVE TANK 2 MODIFIES THE BALANCE. A RESISTOR 16 IS PROVIDED FOR OPERATION IN THE ABSENCE OF DEPOSIT. USE TO DETECT THE LEVEL WITHOUT PARASITIC PHENOMENA DUE TO DEPOSIT.

Description

1 '2524667

  The invention relates to a switch of

  filling level limit for filling

  wise electrically conductive, comprising a probe

  placed at the level of the level to be ascertained, which

  carries a detector electrode which comes into electrical contact with the filling material when it reaches the level to be observed, and an electrode

  mass which is in contact with the foodstuff

  wise at least when it has reached the level of

  tater, a device applying an alternating voltage to the electrodes, and a processing circuit, connected to the detector electrode and to the ground electrode, and

  responds to changes in the electrical signal.

  In level-limit switches of this type, the filling commodity, as soon as it enters

  in contact with the detector electrode, forms a con-

  electrical conductivity between the detector electrode and the ground electrode, so that a current flows

  because of the applied alternating voltage

  bake treatment reacts either to the current or to the voltage drop caused by the current, and it indicates

  hence the level to be ascertained is reached.

  When level limit switches of this type are used for filling stuffs which tend to form deposits, there is a risk of erroneous indications.

  such as fruit juices,

  fitures, mustard and other pasty media or

  After the probe has been covered with

  If taken over by such a commodity, it is formed on

  the probe a deposit that has substantially the same conductivity

  This is why the deposit forms, even when the level has not reached the height of the

  probe, a conductive connection between the electrode

  As a result of the passage of current through this conductive connection, the processing circuit also indicates that the level to be observed is reached, while the probe does not dive into the commodity. The specific conductivity of commodities for which such level limit switches are used varies to a large extent and can

  also, for the same foodstuff, undergo significant variations

  Over time, therefore, the values of the

  alternating current flowing between the detecting electrode

  trice and the ground electrode vary accordingly.

  The processing circuit indicates that the level to be observed is reached when this current exceeds a predetermined threshold value. The threshold to which the treatment circuit responds must therefore be adjusted each time according to the conductivity of the commodity and false indications are incurred. if the conductivity of the food of

filling varies.

  The object of the invention is to perform a switching

  level limiter for electrically conductive media, which is largely insensitive to deposition formations on the probe and which can be exploited in

  a wide range of conductivity of the foodstuff

  pleating without tare being necessary.

  According to the invention, the limit switch

  level is characterized in that an inter-

  medial is arranged between the detector electrode and

  the ground electrode, and that the alternating voltage

  tive is applied between the ground electrode and

the intermediate electrode.

  When, in the case of a

  level limit according to the invention, the probe is

  by the filling material, the electrode detects

  trice forms the setting of a voltage divider whose resistant branches are constituted by'filler filling which shunts on the one hand the gap between the detector electrode and the ground electrode, and on the other hand the interval between the detector electrode and the intermediate electrode A deposit on the probe between the sensor electrode and the ground electrode occurs in the same way as the filling material shunting this gap. an AC voltage which, in accordance with the voltage divider ratio of the voltage divider, is smaller than the AC voltage applied to the intermediate electrode The filling commodity, which shunts the gap between the intermediate electrode and the ground electrode, forms a conductance which is placed in parallel on the voltage divider and thus does not influence the voltage division. It is the same for a deposit which is fo between the intermediate electrode and the ground electrode. f On the other hand, if the filling material does not reach the level determined by the height at which the probe is installed and therefore does not touch the probe and its electrodes, the full voltage

  alternative, which is applied to the intermediate electrode.

  diode, is transmitted to the detector electrode via the conductance of the deposit which shunts

  the interval between the intermediate electrode and the elec-

  detecting trode The part of the deposit shunting the in-

  between the intermediate electrode and the ground electrode has no influence on this transmission of

voltage.

  The processing circuit thus measures, between the detector electrode and the ground electrode, the full AC voltage when the probe is not covered by the filling material, and a much lower AC voltage when the probe is covered. voltage difference is a criterion

  sure and unequivocal as to whether the level of

pleating has reached or not the probe.

  Since the two resistant branches of the voltage divider are formed by the foodstuff of

  filling or depositing the said goods, the variations

  conductibility of the latter come from the

  same way on the two branches of resistance

  the voltage divider port of the voltage divider therefore remains in principle constant and independent of conductivity variations. The level limit switch can therefore be used for various types of goods or for products of fluctuating conductivity without any modification of the tare either

necessary.

  Deposition formation on the probe is practically

  without influence on the operation of the switching

  Because of the insensitivity to deposition, probes of dimensions much smaller than those of

  In addition, the probes can take

very varied forms.

  Preferably, the detector electrode and the

  the intermediate electrode are connected to each other by a

resistance.

  When the tank for the foodstuff

  pleating is metallic, the ground electrode can be

  constituted by the wall of the tank.

  According to one embodiment, the probe is rod-shaped, while the detector electrode and the intermediate electrode are arranged remotely.

  each other with respect to the longitudinal direction

nal of the probe rod.

  According to another embodiment, the intermediate electrode is of annular shape and arranged

  concentrically to the detector electrode.

  Other peculiarities and advantages of

  will still result from the description below.

  In the attached drawings, given as examples

  non-limiting plies: Figure 1 is a schematic view of a tank provided with a level limit switch according to the invention and containing an electrically conductive material;

  FIG. 2 is a simplified representation

  clarifying the functions of the fill level limit switch when the probe does not dive into the filling commodity; Figure 3 is the equivalent diagram of the state shown in Figure 2;

  FIG. 4 is a simplified representation

  , explaining the operation of the fill level limit switch when the probe is diving into the filling commodity; Figure 5 is the equivalent diagram of the state shown in Figure 4; Figure 6 is a front view of another embodiment of the probe; and FIG. 7 is a sectional view of the

probe of Figure 6.

  FIG. 1 very schematically shows a tank 1 whose wall 2 is made of metal. The tank 1 contains an electrically conductive filler 3. A rod-shaped probe 4 is fixed at a height h above the bottom of the tank 1. 4 consists of a probe rod 5 which is fixed in such a way to a threaded head 6 screwed into an opening in the wall 2 of the tank, that it is

  protruding horizontally into the interior of the tank 1.

  A detector electrode 7 is installed at the end

  free of the probe rod 5 An intermediate electrode

  8 is installed between the sensor electrode 7 and the threaded head 6, towards the middle of the rod of

  probe 5, so as to be distant from both electricity and

  detecting trode 7 only of the threaded head 6 The head

  6 is electrically connected to the metal wall

  than 2 of the tank with which it forms an elec-

  The electrodes 7 and 8 are insulated from each other and from the ground electrode 9. For this purpose, the probe rod 5 may be made of insulating material; in the case where it is metallic, the electrodes 7 and 8

  are appropriately isolated from the probe rod 5.

  In FIG. 1, the probe 4 is represented for

  clarity on an enlarged scale with respect to

  In addition, electricity connections are

  trodes are figured only very schematically; in reality they are brought to the electrodes through

  the threaded head 6 and the probe rod 5.

  The probe 4 serves, in conjunction with electronic circuits, to establish whether the filling level in the tank 1 has reached or not the height h It

  An alternating voltage generator is provided for this purpose.

  tive 10 which delivers at its output terminals 10a, l Ob a

  alternating voltage of appropriate frequency and amplitude.

  Terminal 10 a, which is grounded, is connected to the

  threaded head 6, therefore at the ground electrode 9 Electricity

  Intermediate trode 8 is connected to the terminal lob.

  Thus, the alternating output voltage of the generator 10 is present between the intermediate electrode 8 and

the ground electrode 9.

  The alternating voltage taken from the detector electrode 7 is led to the input of a processing circuit 11, which contains an amplifier 12, an input terminal of which is connected to the detector electrode 7.

  and whose other input terminal is grounded.

  Thus the alternating voltage existing between

  detecting trode 7 and the ground electrode 9, is applied

  at the input of the amplifier 12.

  In the processing circuit 11, the amplifier is followed by a rectifier circuit 13 which rectifies

  the alternating voltage amplified by the amplifier 12.

  The rectified voltage is fed to the input of a threshold circuit, whose output voltage takes one or the other of two values depending on whether the rectified voltage applied to the input is above or below below a set threshold voltage Threshold circuit 14

  can be formed for example of a Schmitt rocker.

  The signal leaving the threshold circuit 14 may be

  to an indicating device 15 in the usual manner, which indicates whether or not the level in the tank 1 has reached the height h; naturally, we can

  desired, also trigger switching effects.

  with the same output signal.

  The operating mode of the fill level limit switch shown in Figure

  1 will be explained using Figures 2 to 5.

  FIG. 2 again shows in simplified representation the device of FIG. 1 in the state of

  operation where the level in the

  see 1 did not reach the height of probe 4 On

  also supposes in Figure 2 that the foodstuff

  wise formed a deposit 17 on the rod 5 of the probe.

  The deposit 17 has substantially the same conductivity

  than the filling material 3, and it is

  Thus, an electrical connection with a certain resistance between the electrode 7 and the electrode

  intermediate 8 and the ground electrode 9

  electrical energy equivalent to deposit 17 are

  The resistance Ri represents the resistance of the portion of the deposit 17 connecting the intermediate electrode 8 to the ground electrode 9, and the resistance R 2 represents the resistance of the part of the deposit 17 connecting the detecting electrode 7 with the ground electrode 8 The resistor R 2 is in parallel with the resistor 16, the value of which

  resistance is designated R 16.

  Figure 3 shows the equiva-

  are the value of the device in the state of Figure 2 The alternating voltage UG provided by the AC voltage generator 10 is applied between the intermediate electrode 8 and the ground electrode 9 The intermediate electrode 8 is, on the one hand connected to the ground electrode 9 through the resistor Ri and, on the other hand, connected to the detector electrode 7 through the mounting

  in parallel with the resistors R 2 and R 16.

  Thus, the voltage Us taken between the electricity

  detector trode 7 and the ground electrode 9 is equal

  to the alternating voltage UG supplied by the generator 10.

  The resistance Ri is a burden for the general

  10, but it has no influence on the US voltage. Figure 4 shows the state of operation where the filling commodity 3 has reached the height of the probe 4 The commodity now produces, in addition

  deposit 17, a conductive connection between the electro-

  7, 8 and 9 The conductive connection created by the commodity between the intermediate electrode 8 and the ground electrode 9 is represented symbolically in FIG. 4 by the resistor R 3, while the resistor R 4 represents the conducting connection created. by the commodity

  between the detector electrode 7 and the intermediate electrode

8.

  In addition, the filling material produces a conductive connection between the sensing electrode 7 and all the surfaces of the wall 2 of the reservoir which are

  in contact with the filling material This link

  her driver is represented symbolically by the

resistance R 5.

  The corresponding equivalent circuit diagram is shown in FIG. 5. It is distinguished first from the diagram of FIG. 3 in that the resistor R 3 is put in parallel on the resistor Ri, and the resistor R 4 is paralleled on the resistance R 2 By this parallel connection, however, there are only the effective values of the set of resistances which are reduced without for this the operation

  the device is fundamentally modified.

  On the other hand, the resistance R 5 is now

  in series with the parallel mounting of

  R 2, R 4 and R 16, between the electrodes 8 and 9 of

  so that these resistors form a divisor of

  The voltage Us taken from the detector electrode 7 is therefore, in accordance with the voltage division ratio of this voltage divider, smaller than the voltage.

UG of the voltage generator.

  The threshold value of the threshold circuit 14 in the processing circuit 11 (FIG. 1) is set

  so that the DC voltage, obtained by rectifying

  the output voltage Us, is in the case of Figure 3 above this threshold value, and in the case of Figure 5, below the threshold value The processing circuit can therefore do

  clearly the distinction between the two

  shown in Figure 2 and Figure 4, and provide

  output an outgoing signal which, in the operating state,

  Figure 2 has one of the signal values and the other value in the operating state

of Figure 4.

  It should be noted that the resistance R 5 has, in comparison with the resistors R 2 and R 4, a relatively small resistance value, since it represents the cross-sectional area of the current of the entire filling commodity in the reservoir. the voltage divider US / UG of the voltage divider, formed on the one hand by the resistors R 2, R 4 and on the other hand by the resistor R 5, is therefore much lower than 1/1,

  so that the values of the voltage Us are clearly

  different from each other in the two states

  of operation of Figure 2 and Figure 4.

  The processing circuit 11 can therefore distinguish one from the other these voltage values with a good margin

of security.

  Resistance 16 is only intended to

  In the operating state of FIG. 2, the voltage of the intermediate electrode 8 is transmitted to the detector electrode 7, even if no deposit has yet formed on the probe. It is therefore possible to choose a large value. for the resistor R 16 with respect to the values of the resistors R 2 and R 4, so that the voltage division ratio is not substantially

affected by resistance R 16.

  A change in the specific conductivity

  that the filling material 3 in the tank has virtually no influence on the voltage division ratio, because it modifies in the same ratio the values of the resistors R 2, R 4 and R 5 The operation of the limit switch of level remains substantially unaltered during a change in the specific conductivity of the food, without the need to renew the

  setting of the threshold value in the processing circuit

  11 The same applies to the case where the same

  level limiter is used for various types of

  different conductivity products.

  It will be further understood that even the formation of the deposit 17 on the probe 4 does not have a troublesome effect on the operation of the level limit switch. When the deposit 17 does not exist, this means that the resistors Ri and R 2 are deleted

  the disappearance of the resistance Ri has the sole consequence

  In the case of FIGS. 2 and 3, the resistor R 16 assumes the role of the resistor R 2, and in the case of FIG. 5, the voltage divider.

  is formed by the resistors R 4 and R 5.

  It should be noted that the load supplied by this AC voltage generator 10 varies greatly in

  function of the conductivity of the foodstuff

  However, the correct operation of the level limit switch assumes that the output voltage UG remains substantially constant regardless of the load. The generator 10 must therefore be designed with a very low internal resistance that can supply power. a charge

  In practice, it proved to be sufficient

  alternative voltage, provided by the general

  voltage transformer 10, has an effective value

  about 0.4 V With a global value of resistance

  of the food in the tank of about 1 Q, as is the case in practice, the generator must

  then provide a current of about 0.4 A.

  The frequency of the AC voltage supplied by the generator 10 can be chosen from a wide range It is perfectly possible to operate

  with the frequency of 50 Hz of the sector; alternating voltage

  native can then be elaborated from the tension

  sector alternative But for food stuffs

  Since it has a low resistance, it has proved advantageous to work with higher frequencies; frequencies up to 20 kHz could

be used successfully.

  The shape and position of the probe electrodes can be very different for each use case The level limit switch is not limited to the use of shaped probes

  of rods, as in the example previously described.

  By way of example, FIGS. 6 and 7 show an annular probe, where the annular intermediate electrode 8 'concentrically surrounds the detector electrode 7' remotely and is in turn surrounded remotely by the electrode 9 'mass

  As shown in Figure 7, electricity

  9 'mass trode may consist of a plate-shaped metal plate 18, which has a flat recess in which is installed a disk 19 of insulating material The electrodes 7' and 8 'are formed by metal deposits on the disk 19 Connections, not shown, for the electrodes 7 'and 8' can be fed through the metal plate

18 and the disc 19.

  To obtain the described operation for each selected probe construction, it is only necessary that the intermediate electrode, to which the AC voltage is applied, be arranged between the detector electrode and the ground electrode so that the intervals are shunted. by one

possible deposit formation.

  The use of the level limit switch described above is also not limited to the case where the tank for the filling commodity

  is metallic In the application to an iso-

  lant, it is only necessary to

  mass trunk of sufficient size so that then

  it is at least in contact with the foodstuff

  wise, apart from a possible deposition formation, when the commodity covers the probe This can be obtained for example with a metal plate placed on the bottom of the tank, or by a metal surface of sufficient size surrounding the threaded head of the probe .

Claims (1)

1 fill level limit switch for electrically conductive fillers, comprising a probe (4) located at the level of the level to be observed, which comprises a detector electrode (7) which comes into electrical contact with the filling material (3) when it reaches the level to be observed, and a ground electrode (9) which is in electrical contact with the filling material at least when it has reached the level to be observed. , a device (10) applying an alternating voltage to the electrodes, and a processing circuit (11), connected to the detector electrode (7) and to the ground electrode (9), and which responds to the modifications of the electrical signal between the detector electrode (7) and the ground electrode (9), characterized in that an intermediate electrode (8) is arranged between the detector electrode (7) and the ground electrode ( 9), and in that the alter voltage native is applied between the ground electrode (9) and the intermediate electrode (8). 2 switch according to claim 1, characterized in that the sensor electrode (7) and the intermediate electrode (9) are interconnected by a resistor (16). 3 switch according to one of claims 1 or 2, characterized in that the tank (1) for the filling food (3) is metallic, and in that the ground electrode (9) is constituted by the wall ( 2) of the tank (1). 4 switch according to one of claims 1 to 3, characterized in that the probe (4) is rod-shaped (5) and in that the sensor electrode (7) and the intermediate electrode (8) are arranged at a distance from each other relative to the longitudinal direction of the probe rod (5). Switch according to one of the claims   1 to 3, characterized in that the intermediate electrode (8) is annular and concentrically disposed at the sensor electrode (7).