PatentDe  


Dokumentenidentifikation EP0419475 29.04.1993
EP-Veröffentlichungsnummer 0419475
Titel ENERGIESPARENDE WAAGE.
Anmelder Premark FEG Corp., Wilmington, Del., US
Erfinder KASINOFF, A., Harvey, Troy, OH 45373, US
Vertreter derzeit kein Vertreter bestellt
DE-Aktenzeichen 3879717
Vertragsstaaten DE, FR, GB
Sprache des Dokument En
EP-Anmeldetag 03.11.1988
EP-Aktenzeichen 889102976
WO-Anmeldetag 03.11.1988
PCT-Aktenzeichen US8803913
WO-Veröffentlichungsnummer 8904466
WO-Veröffentlichungsdatum 18.05.1989
EP-Offenlegungsdatum 03.04.1991
EP date of grant 24.03.1993
Veröffentlichungstag im Patentblatt 29.04.1993
IPC-Hauptklasse G01G 3/142

Beschreibung[en]

This invention relates to load cell scales and, more particularly, to load cell scales which are battery powered. Such scales are portable and are able to operate in remote areas where alternating current is not readily available. A principal problem with such scales is the fact that the batteries run down rather rapidly and have to be recharged or replaced.

A typical 350 ohm load cell scale requires at least about 25 milliamperes of current for operation at the accuracy typically required by governmental weights and measures officials for use in sales applications. At such a current drain prior art battery operated scales have typically required battery recharging or replacement after only about 10 hours of use. This life has been extended, however, by supplemental use of photo-voltaic cells. Longer battery life has also been obtained by adding resistance (either resistors or higher impedance load cells) to reduce the current drain. This latter technique causes a sacrifice in weighing accuracy.

It is an object of the present invention to provide a load cell scale which weighs an object at high accuracy while conserving power during periods when objects are not being weighed. It is another object to provide a battery operated load cell scale which has a long battery life and yet meets governmental specifications for sales related use.

Thus, from one aspect, the invention consists in a load cell scale comprising: a load cell for sensing the weight of an object supported thereby and generating a weight indicating signal corresponding thereto, current supply means for supplying electrical current to said load cell, said current supply means being switchable between a first state in which it supplies a normal operating current to said load cell and a second state in which it supplies to said load cell an energy saving current, lower than said operating current but high enough for enabling said load cell to detect the presence of said object and generate a load detection signal, control means connected to said current supply means to control the switching thereof between said first and second states, said control means being operative in the absence of said object to cause said current supply means to switch to said second state and responsive to said load detection signal to cause said current supply means to switch to said first state, and indicating means for creating an indication of the weight represented by said weight indicating signal.

This invention provides high accuracy operation of a load cell scale at a low enough average current drain for long-term operation from a modestly sized battery power pack. Such energy efficiency is obtained by powering the load cell during waiting periods from a current supply which is too low for normal load cell operation but which is high enough for detection of the presence of an object on the scale. When an object is detected the load cell generates a load detection signal which is relayed to the control means. The latter then causes a switching operation which applies a normal operating current flow to the load cell. Thus the load cell in effect senses the presence of a load and controls its own current supply.

From another aspect, the invention consists in a method of operating a load cell scale comprising the steps of: supplying electrical current to a load cell in an energy saving amount which is insufficient for normal weighing operation but sufficient for detecting the presence of an object on said scale, sensing the presence of said object on said scale, applying a normal operating current to said load cell in response to sensing the presence of said object, and causing the generation of a weight indicating signal therefrom, and using said weight indicating signal to create an indication of the weight represented thereby.

In a preferred embodiment of the present invention, the scale has a pair of transistor switches at one side of the load cell and another pair of transistor switches at the other side of the load cell. The current supply means is split between the two sides, so that the transistor switches provide the load cell with current at three different non zero levels. The first level is a normal operating level at which the load cell operates at full weight reading accuracy. At the second level the load cell operates at reduced accuracy and is used only for detecting the presence of an object to be weighed. At the third level the load cell receives only enough current to keep it electrically centered and ready to operate. A programmed microprocessor causes the transistor switches to switch periodically from the third level to the second level. If a weight is placed on the scale during a period of time when the load cell is being powered at the third level, there is a slight delay in the weighing thereof. As soon as the second current level is applied to the load cell, the object is detected, and the circuit switches to the first level for generation of a weight indicating signal.

In order that the present invention may be more readily understood, one embodiment thereof will now be described with reference to the accompanying drawings, in which:-

Fig. 1 is a schematic diagram of switching control apparatus for an energy efficient load cell scale.

Fig. 2 is a timing diagram illustrating the timing of power levels applied to a load cell.

Figs. 3A and 3B are a flow chart illustrating the logical sequence followed by the apparatus of Fig. 1.

As illustrated in Fig. 1, a scale constructed in accordance with the present invention may comprise a load cell 10 characterized by strain gauges connected together in a bridge arrangement. Load cell 10 preferably may be a device commercially known as a 350 ohm load cell, but other resistance values are suitable. The load cell is powered by current flowing through control lines 47, 49 and produces output signals on lines 51, 53 representing the weight of an object placed upon a platter supported by load cell 10. Output signals on lines 51, 53 are applied to an instrumentation amplifier 44 for comparison. The output signal from instrumentation amplifier 44 represents the weight of the object in analogue form. This signal is applied to A/D converter 12 via line 54. A/D converter 12 converts the weight signal to digital form for application to microprocessor 14 via data lines 13. Microprocessor 14 formats the weight signal for application to display 16 via line 15. Display 16 may be a visual display, a label printer, or any other convenient device for presenting an indication of object weight.

A/D converter 12 operates in the ratiometric mode, so that variations in supply voltage are not interpreted as weight variations. Thus resistors 46, 48, 50, 52 function as a voltage divider to provide a reference voltage across lines 56,58 for A/D converter 12. The resistor network 46, 48, 50, 52 is driven by the load cell supply appearing on lines 47, 49.

The drive current for load cell 10 is controlled by four FET switches 18, 20, 22, 24. Switches 18, 20 are P-channel devices, whereas switches 22, 24 are N-channel devices. FET switches 18, 22 are arranged for parallel operation under control of the signals supplied by microprocessor 14 to output line 60 and inverted by amplifier 40. Switching control signals for FET 18 are amplified by voltage follower 33, whereas switching control signals for FET 24 are amplified by amplifier 38. FET switches 20, 22 operate in parallel under switching control of signals supplied by microprocessor 14 to line 62 and inverted by amplifier 42. FET 20 is serviced by voltage follower 34, whereas FET 22 is serviced by amplifier 36.

FET switches 18, 20, 22, 24 selectively cause current to flow through load cell 10 at any of three different levels. When switches 20, 22 are closed, load cell 10 is connected directly across the positive and negative terminals of the power supply, which comprise eight D-cell alkaline batteries. At this current level load cell 10 is fully operational and provides weight signals at an accuracy of 1 part in 30,000, which equates to +/-4.5 gm on a 13.6 kg scale (+/- 0.01 lb. on a 30 lb. scale). At this point the system is in an operational condition hereinafter referred to as Mode I. In this mode all components of the scale are fully energized, and about 38 milliamperes of current are drawn from the power supply.

During periods of time when there is no load on the scale the system switches into Mode II, a low power mode in which load cell 10 receives about 40% of full power. Display 16 and other peripherals (not illustrated) are deenergized at this time. The current drain during Mode II is about 20 milliamperes. During this mode FET switches 20, 22 are switched OFF and FET switches 18, 24 are switched ON. In this mode load cell 10 is connected to the power supply through resisters 26, 30 and does not receive sufficient current for full reading accuracy. However, the current supply is sufficient to enable generation of output weight indications which are accurate within about 4.5 gm (.01 lbs). These weight indications serve as load detection signals. When such load detection signals are sensed during Mode II operation, the microprocessor switches the system to Mode I operation. This involves closing FET switches 20, 22 and opening FET switches 18, 24.

The system also has a low power mode, termed Mode III, in which all of FET switches 18, 20, 22, 24 are opened. During this mode load cell 10 is connected to the power supply via resisters 26, 28, 30, 32, and the system as a whole draws only about 7 milliamperes of current. The current flow through load cell 10 is insufficient for generation even of load detection signals, but the load cell is kept electrically centered. Microprocessor 14 cycles FET switches 18, 24 ON and OFF cyclically to produce the power utilization plot of Fig. 2. It will be appreciated that a load detection signal may not be generated and an object may not be weighed when the system is in Mode III. If an object is placed on the platter when the system is in Mode III, the system takes no immediate responsive action. However, as soon as the system makes its next periodic switch to Mode II, the object is detected, and an automatic switch to Mode I is made.

From an energy savings point of view it is desirable to keep the duration of Mode III settings as long as possible. However, this slows down the weighing operation. Thus as illustrated in Fig. 2 the preferred embodiment compromises by fixing the Mode III periods at two seconds. This means that the system (when energized) will always respond to a load in two seconds or less.

Referring now to Fig. 2, it will be seen that the system goes immediately to full power (mode I) upon energization and maintains that condition for one minute. Following a one minute warm-up the system is zeroed, and then a transition is made to medium power (mode II). Medium power is maintained for five seconds and then the system begins periodically switching between low power (Mode III) and medium power. Following each two second period in low power, the system switches to medium power for one second and thereafter transitions again to low power. Cyclical switching between low power and medium power proceeds for a five minute period, after which the system switches to full power for five seconds in order to check high power scale zero. The five minute cycle then repeats. Each time the system switches to full power the scale is again zeroed. As noted above, cyclical switching from medium power back to low power is interrupted whenever an object is detected on the platter. This operating technique provides about 1,000 hours of life from a battery pack consisting of eight D-cell alkaline batteries.

Fig. 3 illustrates the logic which is programmed into microprocessor 14. As shown therein the microprocessor begins by setting Mode I, and then going through a one minute warm-up. Mode II is then entered for five seconds, after which the system sets counters which will produce a five minute time out and a two second time out. Thereafter the system enters Mode III and begins a logical process causing continuance in Mode III for two seconds followed by one second in Mode II. While the system is in Mode II it checks to see if the weight reading is greater than 3 gm (.007 lbs). If so, then a transition is made to Mode I. Otherwise the system stays in Mode II and continues weight testing for a one second period. After each weight check the five minute timer is checked. If a five minute period has elapsed, the timer is reset and the system enters Mode I.

Immediately upon entry into Mode I the system sets a five second timer and then begins checking to determine whether there is no weight on the platter (absolute value of a weight less than 0.9 gm (.002 lbs).). If the load cell senses a weight greater than 0.9 gm (.002 lbs). then a weighing routine is entered. Such weighing routines are well-known in the art and need not be further described here. If there is no weight on the scale then the system continues checking for a maximum of five seconds, zeros the scale, sets the one second timer, sets Mode II and branches back to the Mode III checkpoint.

It will be appreciated that it is not essential for FET switches 18, 24 to be opened when FET switches 20, 22 are closed. Furthermore it is possible to obtain substantial although reduced energy efficiency by operating only in the medium power and high power modes.


Anspruch[de]
  1. Wägezellenwaage mit:

       einer Wägezelle (10) zum Erfühlen des Gewichtes eines Gegenstandes, der von der Zelle getragen wird, und zum Erzeugen eines das Gewicht anzeigenden Signales, welches dem Gewicht entspricht,

    einer Stromzufuhreinrichtung (18, 20, 22, 24, 26, 30) für die Zufuhr eines elektrischen Stromes zu des Wägezelle, wobei die Stromzufuhreinrichtung zwischen einem ersten Zustand, in welchem sie der Wägezelle einen normalen Betriebsstrom zuführt, und einem zweiten Zustand, in welchem sie der Wägezelle einen Energiesparstrom zuführt, hin- und herschaltbar ist, wobei letzterer kleiner ist als der Betriebsstrom, jedoch groß genug, um die Wägezelle in die Lage zu versetzen, daß sie die Gegenwart des Gegenstandes erfaßt und ein Lasterfassungssignal erzeugt,

    einer Steuereinrichtung (14), die mit der Stromzufuhreinrichtung verbunden ist, um das Umschalten derselben zwischen dem erwähnten ersten und zweiten Zustand zu steuern, wobei die Steuereinrichtung so betreibbar ist, daß sie bei Abwesenheit des Gegenstandes bewirkt, daß die Stromzufuhreinrichtung auf den zweiten Zustand umgeschaltet wird, und daß sie auf das Laterfassungssignal anspricht, um zu bewirken, daß die Stromzufuhreinrichtung auf den ersten Zustand umschaltet, und

    einer Anzeigeeinrichtung (16), um eine Anzeige des Gewichtes zu erzeugen, welches durch das Gewichtsanzeigesignal repräsentiert wird.
  2. Wägezellenwaage nach Anspruch 1, wobei

    die Stromzufuhreinrichtung ein erstes Stromzufuhrmittel aufweist, um der Wägezelle (10) elektrischen Strom in einer Menge zuzuführen, welche den Betrieb der Wägezelle mit ihrer vollen normalen Auflösung ermöglicht,

    ein zweites Stromzufuhrmittel (26, 30) aufweist, um der Wägezelle elektrischen Strom in einer Menge zuzuführen, die ausreichend gering ist, um elektrische Energie einzusparen, jedoch groß genug, um zu ermöglichen, daß die Wägezelle die Anwesenheit des Gegenstandes erfaßt und das Lasterfassungssignal erzeugt, und

    eine Schalteinrichtung (18, 20, 22, 24) aufweist, um wahlweise das erste Stromzufuhrmittel und das zweite Stromzufuhrmittel mit den Wägezellen zu verbinden, wobei die Steuereinrichtung (14) bei Abwesenheit des Gegenstandes so betreibbar ist, daß sie bewirkt, daß die Schalteinrichtung (18, 20, 22, 24) das erste Stromzufuhrmittel von der Wägezelle trennt und das zweite Stromzufuhrmittel (26, 30) mit dieser verbindet, und die Steuereinrichtung auf das Lasterfassungssignal anspricht, indem sie bewirkt, daß die Umschalteinrichtung die erste Stromzufuhreinrichtung wieder mit der Wägezelle verbindet.
  3. Wägenzellenwaage nach Anspruch 2, wobei die Steuereinrichtung (14) eine abwechseinde Verbindung der ersten und zweiten Stromzufuhrmittel mit der Wägezelle (10) bewirkt.
  4. Wägenzellenwaage nach Anspruch 2, wobei die Steuereinrichtung (14) bei Abwesenheit des erwähnten Gegenstandes so betreibbar ist, daß sie ein periodisches Unterbrechen und Wiederherstellen der Verbindung des zweiten Stromzufuhrmittels (26, 30) mit der Zelle (10) bewirkt.
  5. Wägezellenwaage nach Anspruch 2, 3 oder 4, einschließlich eines dritten Stromzufuhrmittels (26, 28, 30, 32), um der Wägezell (10) elektrischen Strom in einer Menge zuzuführen, die geringer ist als die durch die zweite Stromzufuhreinrichtung (26, 30) bereitgestellte Mengen, jedoch ausreichend groß ist, um die Wägezelle in einem elektrisch ausgeglichenen (zentrierten) Zustand zu erhalten, wobei die Steuereinrichtung (14) in Abwesenheit eines Gewichtes auf der Wägezelle so betreibbar ist, daß sie ein periodisches Umschalten der Wägezelle zwischen der zweiten Stromzufuhreinrichtung und der dritten Stromzufuhreinrichtung bewirkt.
  6. Wägezellenwaage nach Anspruch 2, wobei die erste und zweite Stromzufuhreinrichtung auf die Wägezelle (10) verteilt sind, wobei die Umschalteinrichtung ein paar von Transistorschaltern (18, 20, 22, 24) auf jeder Seite der Wägezelle aufweist.
  7. Verfahren zum Betreiben einer Wägezellenwaage mit den folgenden Schritten:

       Zuführen eines elektrischen Stromes zu einer Wägezelle (10) in einer Energiesparmenge, die für einen normalen Wägevorgang unzureichend ist, jedoch ausreich, um die Gegenwart eines Gegenstandes auf der Waage zu erfassen,

    Erfühlen der Gegenwart des besagten Gegenstandes auf der Waage,

    Aufbringen eines normalen Betreibsstromes auf die Wägezelle unter Ansprechen auf das Erfassen der Gegenwart des Gegenstandes und Bewirken, daß ein Gewichtsanzeigesignal daraus erzeugt wird, und

    Verwenden des Gewichtsanzeigesignales, um die Anzeige eines Gewichtes zu erzeugen, daß dadurch wiedergegeben wird.
  8. Verfahren nach Anspruch 7 mit dem Schritt, daß periodisch und zyklisch der elektrische Energiesparstrom auf einen noch niedrigeren Wert reduziert wird, der ausreichend ist, um die Wägezelle elektrisch ausgeglichen zu erhalten jedoch unzureichend für das Erfassen der Gegenwart eines Päckchens auf der Waage ist.
  9. Verfahren nach Anspruch 8 mit den Schritten, daß der normale Betriebsstrom periodisch und zyklisch auf die Wägezelle aufgebracht wird, und daß die Waage während des Aufbringens des Stromes auf Null gestellt wird.
Anspruch[en]
  1. A load cell scale comprising:

       a load cell (10) for sensing the weight of an object supported thereby and generating a weight indicating signal corresponding thereto,

       current supply means (18,20,22,24,26,30) for supplying electrical current to said load cell, said current supply means being switchable between a first state in which it supplies a normal operating current to said load cell and a second state in which it supplies to said load cell an energy saving current, lower than said operating current but high enough for enabling said load cell to detect the presence of said object and generate a load detection signal,

       control means (14) connected to said current supply means to control the switching thereof between said first and second states, said control means being operative in the absence of said object to cause said current supply means to switch to said second state and responsive to said load detection signal to cause said current supply means to switch to said first state, and

       indicating means (16) for creating an indication of the weight represented by said weight indicating signal.
  2. A load cell scale according to claim 1 wherein

       the current supply means comprises first current supply means for supplying electrical current to the load cell (10) in an amount enabling operation of said load cell at its full normal resolution,

       second current supply means (26,30) for supplying electrical current to said load cell in an amount sufficiently low for conserving electrical energy but high enough for enabling said load cell to detect the presence of said object and generate the load detection signal, and

       switch means (18,20,22,24) for selectively connecting said first current supply means and said second current supply means to said load cells, said control means (14) being operative in the absence of said object for causing said switch means (18,20,22,24) to disconnect said first current supply means from said load cell and connect said second current supply means (26,30) thereto, and said control means being responsive to said load detection signal by causing said switch means to reconnect said first current supply means to said load cell.
  3. A load cell scale according to claim 2 wherein said control means (14) causes alternate connection of said first and second current supply means to said load cell (10).
  4. A load cell scale according to claim 2 wherein said control means (14) is operative in the absence of said object for causing periodic disconnection and reconnection of said second current supply means (26,30) from and to said load cell (10).
  5. A load cell scale according to claim 2,3 or 4 including third current supply means (26,28,30,32) for supplying electrical current to said load cell (10) in an amount less than provided by said second current supply means (26,30) but sufficiently high to maintain said load cell in an electrically centered condition; said control means (14) being operative in the absence of a weight on said load cell for causing periodic switching of said load cell between said second current supply means and said third current supply means.
  6. A load cell scale according to claim 2 wherein said first and second current supply means are split across said load cell (10); said switch means comprising a pair of transistor switches (18,20,22,24) at each side of said load cell.
  7. Method of operating a load cell scale comprising the steps of:

       supplying electrical current to a load cell (10) in an energy saving amount which is insufficient for normal weighing operation but sufficient for detecting the presence of an object on said scale,

       sensing the presence of said object on said scale,

       applying a normal operating current to said load cell in response to sensing the presence of said object, and causing the generation of a weight indicating signal therefrom, and

       using said weight indicating signal to create an indication of the weight represented thereby.
  8. Method according to claim 7 comprising the step of periodically and cyclically reducing said energy saving electrical current to a still lower level sufficient for keeping said load cell electrically centered but insufficient for detecting the presence of a package on said scale.
  9. Method according to claim 8 comprising the steps of periodically and cyclically applying said normal operating current to said load cell, and zeroing said scale during said current application.
Anspruch[fr]
  1. Balance à cellule de charge comportant :

       une cellule (10) de charge destinée à capter le poids d'un objet qu'elle supporte et à générer un signal d'indication de poids qui lui correspond,

       des moyens (18, 20, 22, 24, 26, 30) d'alimentation en courant destinés à alimenter en courant électrique ladite cellule de charge, lesdits moyens d'alimentation en courant étant commutables entre un premier état dans lequel ils fournissent un courant de travail normal à ladite cellule de charge et un second état dans lequel ils fournissent à ladite cellule de charge un courant d'économie d'énergie, inférieur audit courant de travail, mais assez intense pour permettre à ladite cellule de charge de détecter la présence dudit objet et de générer un signal de détection de charge,

       des moyens (14) de commande connectés auxdits moyens d'alimentation en courant afin d'en commander la commutation entre lesdits premier et second états, lesdits moyens de commande ayant pour effet, en l'absence dudit objet, d'amener lesdits moyens d'alimentation en courant à passer par commutation dans ledit second état et, en réponse audit signal de détection de charge, d'amener lesdits moyens d'alimentation en courant à passer par commutation dans ledit premier état, et

       des moyens d'indication (16) destinés à produire une indication du poids représenté par ledit signal d'indication de poids.
  2. Balance à cellule de charge selon la revendication 1, dans laquelle

       les moyens d'alimentation en courant comprennent un premier moyen d'alimentation en courant destiné à alimenter en courant électrique la cellule de charge (10) à une valeur permettant le fonctionnement de ladite cellule de charge à sa résolution normale complète,

       un second moyen (26, 30) d'alimentation en courant destiné à alimenter en courant électrique ladite cellule de charge à une valeur suffisamment basse pour économiser de l'énergie électrique, mais assez élevée pour permettre à ladite cellule de charge de détecter la présence dudit objet et de générer le signal de détection de charge, et

       des moyens de commutation (18, 20, 22, 24) destinés à connecter sélectivement ledit premier moyen d'alimentation en courant et ledit second moyen d'alimentation en courant auxdites cellules de charge, lesdits moyens de commande (14) ayant pour effet, en l'absence dudit objet, d'amener lesdits moyens de commutation (18, 20, 22, 24) à déconnecter ledit premier moyen d'alimentation en courant de ladite cellule de charge et à lui connecter ledit second moyen (26, 30) d'alimentation en courant, et lesdits moyens de commande réagissant audit signal de détection de charge en amenant lesdits moyens de commutation à reconnecter ledit premier moyen d'alimentation en courant à ladite cellule de charge.
  3. Balance à cellule de charge selon la revendication 2, dans laquelle ledit moyen (14) de commande provoque une connexion alternée desdits premier et second moyens d'alimentation en courant à ladite cellule de charge (10).
  4. Balance à cellule de charge selon la revendication 2, dans laquelle lesdits moyens (14) de commande ont pour effet, en l'absence dudit objet, de provoquer une déconnexion et une reconnexion périodiques dudit second moyen (26, 30) d'alimentation en courant à ladite cellule (10) de charge et de ladite cellule (10) de charge.
  5. Balance à cellule de charge selon la revendication 2, 3 ou 4, comprenant un troisième moyen (26, 28, 30, 32) d'alimentation en courant destiné à alimenter en courant électrique ladite cellule (10) de charge à une valeur inférieure à celle fournie par ledit deuxième moyen (26, 30) d'alimentation en courant, mais suffisamment élevée pour maintenir ladite cellule de charge dans un état électriquement centré ; lesdits moyens (14) de commande ayant pour effet, en l'absence d'un poids sur ladite cellule de charge, de provoquer une commutation périodique de ladite cellule de charge entre ledit deuxième moyen d'alimentation en courant et ledit troisième moyen d'alimentation en courant.
  6. Balance à cellule de charge selon la revendication 2, dans laquelle lesdits premier et deuxième moyens d'alimentation en courant sont divisés de part et d'autre de ladite cellule de charge (10) ; lesdits moyens de commutation comprenant deux commutateurs à transistors (18, 20, 22, 24) de chaque côté de ladite cellule de charge.
  7. Procédé pour faire fonctionner une balance à cellule de charge, comprenant les étapes qui consistent :

       à alimenter en courant électrique une cellule de charge (10) en une quantité économisant de l'énergie qui est insuffisante pour une opération normale de pesée, mais suffisante pour détecter la présence d'un objet sur ladite balance,

       à capter la présence dudit objet sur ladite balance,

       à appliquer un courant de travail normal à ladite cellule de charge en réponse à la détection de la présence dudit objet, et à provoquer la génération d'un signal qui en indique le poids, et

       à utiliser ledit signal d'indication de poids pour produire une indication du poids qu'il représente.
  8. Procédé selon la revendication 7, comprenant l'étape qui consiste à réduire périodiquement et cycliquement ledit courant du signal d'économie d'énergie à un niveau encore plus bas suffisant pour maintenir ladite cellule de charge électriquement centrée, mais insuffisant pour détecter la présence d'un colis sur ladite balance.
  9. Procédé selon la revendication 8, comprenant les étapes qui consistent à appliquer périodiquement et cycliquement ledit courant de travail normal à ladite cellule de charge, et à mettre à zéro ladite balance durant ladite application de courant.






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