NO132067B

NO132067B -

Info

Publication number: NO132067B
Application number: NO4883/70A
Authority: NO
Inventors: G Charbonnier; G Gauthier
Original assignee: Automatisme Cie Gle
Priority date: 1969-12-22
Filing date: 1970-12-21
Publication date: 1975-06-02
Also published as: DE2063290A1; NL7018476A; DE2063290B2; NO132067C; NL168946C; GB1272030A; LU62245A1; NL168946B; FR2071233A5; DE2063290C3; SE369647B; BE760276A

Description

Foreliggende oppfinnelse angår et apparat for overvåking av en akkumulators ladningstilstand, under vanlig drift mens akkumulatoren utlades gjennom en belastning. The present invention relates to an apparatus for monitoring an accumulator's state of charge, during normal operation while the accumulator is being discharged through a load.

En akkumulators ladningstilstand er bestemt ved dens indre motstand r. Ladningstilstanden kan således overvåkes ved å bestemme verdien for akkumulatorens indre motstand. Hvis den elektro-motoriske kraft for akkumulatoren er E og akkumulatoren avgir en strom i, er dens klemmespenning bestemt ved ligningen: An accumulator's state of charge is determined by its internal resistance r. The state of charge can thus be monitored by determining the value of the accumulator's internal resistance. If the electromotive force for the accumulator is E and the accumulator emits a current i, its terminal voltage is determined by the equation:

På denne bakgrunn er det kjent å bestemme akkumulatorens ladningstilstand ved å måle spenningen IL mellom utgangsklemmene ved en kjent utladningsstrom i. Under forutsetning av at akkumulatorens elektromotoriske kraft E er en konstant, kan det lett ved hjelp av ovenfor angitte ligning beregnes en verdi av akkumulatorens indre motstand r, som vil være et uttrykk for akkumulatorens ladetilstand. On this background, it is known to determine the accumulator's state of charge by measuring the voltage IL between the output terminals at a known discharge current i. Under the assumption that the accumulator's electromotive force E is a constant, a value of the accumulator's internal can easily be calculated using the above equation resistance r, which will be an expression of the accumulator's state of charge.

Den ovenfor angitte antagelse at akkumulatorens elektromotoriske kraft er konstant, vil imidlertid i mange tilfeller ikke holde stikk i tilstrekkelig grad til at den kan benyttes i praksis, da den elektromotoriske kraft i virkeligheten vil variere betraktelig med elektrodenes alder og slittasje så vel som med elektrolyttens spesifike tetthet. The assumption stated above that the electromotive force of the accumulator is constant will, however, in many cases not hold true to a sufficient degree that it can be used in practice, as the electromotive force will in reality vary considerably with the age and wear of the electrodes as well as with the specifics of the electrolyte density.

Det er- derfor et formål for oppfinnelsen å fremskaffe et apparat som under pågående utladning kan bestemme en akkumulators indre motstand, og dermed dens ladetilstand, direkte, uten beregning på grunnlag av en antatt verdi av akkumulatorens elektromotoriske kraft E. It is therefore an object of the invention to provide a device which, during ongoing discharge, can determine an accumulator's internal resistance, and thus its state of charge, directly, without calculation on the basis of an assumed value of the accumulator's electromotive force E.

Prinsipielt oppnås dette i henhold til oppfinnelsen ved at en strom I -under en kort tid overlagres s tr ommen i, således at det vil foreligge annen spenning Up mellom klemmene under strømpulsene, og denne spenning er bestemt ved: In principle, this is achieved according to the invention by a current I -for a short time being superimposed on the current i, so that there will be a different voltage Up between the terminals during the current pulses, and this voltage is determined by:

Klemmespenningsvariasjonen A U for akkumulatoren er således: The terminal voltage variation A U for the accumulator is thus:

Hvis A TJ og I er kjent, kan derfor r bestemmes.. If A TJ and I are known, r can therefore be determined.

På dette grunnlag har .oppfinnelsens apparat som særtrekk at det omfatter en drivkrets koblet mellom akkumulatorens klemmer og innrettet for, overlagret akkumulatorens utladningsstrom, å sende strompulser av forut bestemt varighet gjennom akkumulatoren med forut bestemte, mellomrom; en folerkrets innkoblet mellom akkumulatorklemmene for avfoling av forskjellen, A U mellom foreliggende klemmespenninger, henhv. for og under hver strompuls; On this basis, the device of the invention has as a distinctive feature that it comprises a drive circuit connected between the accumulator's terminals and arranged to, superimposed on the accumulator's discharge current, send current pulses of predetermined duration through the accumulator at predetermined intervals; a sensing circuit connected between the accumulator terminals for detecting the difference, A U between the present terminal voltages, resp. before and during each current pulse;

samt innrettet for å frembringe et differanse signal som angir denne spenningsforskjell^ en komparatorkrets anordnet for å mot- as well as arranged to produce a difference signal indicating this voltage difference^ a comparator circuit arranged to counter-

ta differansesignalet og innrettet for å sammenligne det med et referansesignal Us, som angir et forut bestemt ladningsnivå og frembringes av en referansekrets, samt under hver strompuls å take the difference signal and arrange to compare it with a reference signal Us, which indicates a predetermined charge level and is produced by a reference circuit, and during each current pulse to

avgi et binært signal, hvis ene mulige verdi angir tilstanden /AU/^rUs og annen mulig verdi angir tilstanden AU>/Us/. emit a binary signal, one possible value of which indicates the state /AU/^rUs and another possible value indicates the state AU>/Us/.

Ved' kjent konstant verdi av I vil A U være et uttrykk for .akkumulatorens ladetilstand, og Us kan være avpasset for etter onske å With a known constant value of I, A U will be an expression of the accumulator's state of charge, and Us can be adapted to

angi såvel fullt oppladet som helt utladet tilstand for.akkumulatoren. enter both a fully charged and fully discharged state for the accumulator.

Oppfinnelsen vil nå bli beskrevet mer detaljert ved hjelp av et ut-forelseseksempel og under henvisning til den vedfoyde skjematiske tegning, der: Fig. 1 viser et blokkskjerna for et apparat i henhold til oppfinnelsen, og som er innrettet for overvåking ag styring av en akkumulators ladningstilstand; og Fig. 2 viser kurveformer på forskjellige steder i skjemaet i fig. 1. Det-skal forst henvises til fig. 1', der det er vist at apparatet er oppdelt i tre hoved-seksjoner, nemlig: en overvåker 1 av ladningstilstanden, logiske kretser 2 samt en styrekrets 3 for gjenoppladning av akkumulatoren. The invention will now be described in more detail by means of an embodiment example and with reference to the attached schematic drawing, where: Fig. 1 shows a block core for a device according to the invention, and which is arranged for monitoring and controlling an accumulator state of charge; and Fig. 2 shows curve shapes at different places in the diagram in Fig. 1. Reference must first be made to fig. 1', where it is shown that the device is divided into three main sections, namely: a monitor 1 of the state of charge, logic circuits 2 and a control circuit 3 for recharging the accumulator.

""Akkumulatoren h består av seks celler, idet dens elektriske midt-punkt er forbundet med jord eller et ekvivalent potensial. ""The accumulator h consists of six cells, its electrical center being connected to earth or an equivalent potential.

En belastningskrets 5 e*" forbundet med akkumulatorens klemmer. En .oppladningskrets 6 er likeledes forbundet mellom klemmer for akkumulatoren h. Oppladningen startes av et element 7 og stoppes av et element 8, slik som det vil bli nærmere forklart i det folgende. Elementet 7 styres av de kombinerte utgangssignaler fra seksjonene 1 og 2, mens elementet 8 styres av utgangssignalet A load circuit 5 e*" connected to the terminals of the accumulator. A charging circuit 6 is likewise connected between terminals for the accumulator h. Charging is started by an element 7 and stopped by an element 8, as will be explained in more detail below. The element 7 is controlled by the combined output signals from sections 1 and 2, while element 8 is controlled by the output signal

■ fra seksjon 3»■ from section 3»

Akkumulatoren k avgir'normalt en strom i gjennom belastnings-kret^sen 5. Era apparatet i henhold til oppfinnelsen fores imidlertid en strompuls med amplitudeverdi I og kort varighet gjennom akkumulatoren. Den resulterende forandring av akkumulatorens klemmespenning overvåkes, idet den gir et mål for akkumulatorens ladningstilstand, slik som det allerede er forklart. Apparatet frembringer én rekke sådanne strompulser med forutbestemte jevne mellomrom, og avgir, under hver sådan strompuls, et utgangssignal som angir ladningstilstanden for akkumulatoren. The accumulator k normally emits a current i through the load circuit 5. In the device according to the invention, however, a current pulse with an amplitude value I and a short duration is fed through the accumulator. The resulting change in the accumulator's terminal voltage is monitored, providing a measure of the accumulator's state of charge, as already explained. The device produces a series of such current pulses at predetermined regular intervals, and emits, during each such current pulse, an output signal indicating the state of charge of the accumulator.

Apparatets to utgangsklemmer er henhv. forbundet til den negative The device's two output terminals are respectively connected to the negative

(-) og den positive (+) pol for akkumulatoren. (-) and the positive (+) pole for the accumulator.

Apparatets forste seksjon 1 omfatter en fbrste operasjonsforsterker 20 med en forste inngang forbundet direkte til den positive pol for akkumulatoren h. Den annen inngang er forbundet med samme akkumulatorpol gjennom en omkobler 21 som normalt er sluttet, og bare åpnes under hver strompuls. Den annen inngang for forsterkeren 20 er også forbundet med en lagringsinnretning i form av en kondensator 22, som er forbundet mellom den annen inngang for forsterkeren og et jordpunkt eller et ekvivalent potensial. The apparatus's first section 1 comprises a first operational amplifier 20 with a first input connected directly to the positive pole of the accumulator h. The second input is connected to the same accumulator pole through a switch 21 which is normally closed and only opens during each current pulse. The second input for the amplifier 20 is also connected to a storage device in the form of a capacitor 22, which is connected between the second input for the amplifier and a ground point or an equivalent potential.

En spenningsregulator 23 er forbundet med akkumulatorens h negative pol og frembringer ved sin utgang en konstant spenning som tilfores en spenningsd.eler 2h. En del av denne spenning tappes ut av glidearmen 2h og tilfores en forste inngang for en annen operasjonsforsterker 25, som gjor tjeneste som en impedansom-fqrmer. Utgangen for den annen forsterker 25 er forbundet med en forste inngang for. en'tredje operasjonsforsterker 26, i det denne inngang også er forbundet.med utgangen fra den forste operasjonsforsterker 20. Den annen inngang for forsterkeren 26 er forbundet med et jordpunkt eller et tilsvarende potensial. Den tredje forsterker 26 virker som en mettet komparator, og dens utgang er tilsluttet en indikator-innretning 27 samt den forste inngang for en OG-port 17 for underseksjonen 2. A voltage regulator 23 is connected to the negative pole of the accumulator h and produces at its output a constant voltage which is supplied to a voltage divider 2h. Part of this voltage is tapped off by the sliding arm 2h and supplied to a first input for another operational amplifier 25, which serves as an impedance comparator. The output of the second amplifier 25 is connected to a first input for. a third operational amplifier 26, in that this input is also connected to the output of the first operational amplifier 20. The second input for the amplifier 26 is connected to a ground point or a corresponding potential. The third amplifier 26 acts as a saturated comparator, and its output is connected to an indicator device 27 as well as the first input for an AND gate 17 for subsection 2.

Utgangsspenningen for den tredje forsterker 26 kan anta en av The output voltage of the third amplifier 26 can assume one of

to mulige verdier, avhengig av om utgangsspenningen A U fra den forste forsterker er storre eller mindre enn utgangsspenningen Uc for forsterkeren 25. Indikatorinnretningen 27 utloses når ^ U er storre enn Ug. two possible values, depending on whether the output voltage A U from the first amplifier is greater or less than the output voltage Uc of the amplifier 25. The indicator device 27 is triggered when ^ U is greater than Ug.

Den annen under-seksjon 2 omfatter en monostabil krets 12, hvis inngang gjennom omkobleren 11 i den forste urider-seksjon 1 er koblet til akkumulatorens h positive pol. Denne forbindelse er avbrutt i figuren for oversiktens skyld, idet forbindelsen er symbolisert med a-a. Omkoblerne 11 og 21 i under-seksjonen 1 påvirkes synkront av en spole 10 som energiseres synkront' med starten av hver strompuls. The second sub-section 2 comprises a monostable circuit 12, whose input through the switch 11 in the first urider section 1 is connected to the positive pole of the accumulator h. This connection is interrupted in the figure for the sake of clarity, as the connection is symbolized by a-a. The switches 11 and 21 in sub-section 1 are synchronously affected by a coil 10 which is energized synchronously with the start of each current pulse.

Utgangsspenningen fra den monostabile krets tilfores inngangen for et omkoblerelement 13 som, når det aktiviseres, forbinder en shunt-motstand 1*f i parallell med belastningskretsen 5 for akkumulatoren h. The output voltage from the monostable circuit is fed to the input of a switching element 13 which, when activated, connects a shunt resistor 1*f in parallel with the load circuit 5 of the accumulator h.

Utgangssignalet fra'den monostabile krets 12 tilfores også inngangen for en krets 15 som forsinker utgangssignalet med en forut bestemt verdi, som er mindre enn strompulsens varighet, samt differensierer dens front-flanke for å frembringe en meget kort pul s. The output signal from the monostable circuit 12 is also fed to the input of a circuit 15 which delays the output signal by a predetermined value, which is less than the duration of the current pulse, and differentiates its leading edge to produce a very short pulse.

Fig. 2 viser utgangssignalet S^g f°r den monostabile krets 12 og den resulterende,tilleggs-strom I som passerer gjennom akkumulatoren <*>f ved innkobling av shunt-motstanden 1^-. Denne strompuls I har transiente svingninger både ved sin for- og bakflanke. Utgangssignalet fra forsinkelse- og differensieringskretsen 15 er vist ved Dette ,signal tilfores inngangen for en annen monostabil krets 16 som er innrettet for å frembringe en utgangspuls hvis varighet er slik at når den legges til forsinkelsen i.kretsen 15} vil det resulterende totale tidsintervall bli mindre enn strompulsens varighet, Utgangssignalet fra den monostabile krets 16 er vist ved S^g i fig._2, og det vil observeres at utgangspulsen fra denne monostabile krets ligger helt innenfor den del av strømpulsen I som ikke er beheftet med transiente svingninger. Fig. 2 shows the output signal S^g for the monostable circuit 12 and the resulting additional current I which passes through the accumulator <*>f when switching on the shunt resistor 1^-. This current pulse I has transient oscillations both at its leading and trailing edges. The output signal from the delay and differentiation circuit 15 is shown at This signal is applied to the input of another monostable circuit 16 which is arranged to produce an output pulse whose duration is such that when added to the delay in the circuit 15 the resulting total time interval will be less than the duration of the current pulse, The output signal from the monostable circuit 16 is shown at S^g in fig._2, and it will be observed that the output pulse from this monostable circuit lies entirely within the part of the current pulse I which is not affected by transient oscillations.

De tidsverdier som er angitt langs tidsaksen i fig. 2, er bare. angitt som eksempel. The time values indicated along the time axis in fig. 2, is just. given as an example.

Utgangssignalet fra den monostabile krets 16 tilfores en annen inngang for den tidligere nevnte OG-port 17. Utgangssignalet fra denne OG-port tilfores styreelementet 7> som således mottar utgangsspenningen fra den tredje operasjonsforsterker 26 under varigheten av utgangspulsen fra den monostabile krets 16. The output signal from the monostable circuit 16 is fed to another input for the previously mentioned AND gate 17. The output signal from this AND gate is fed to the control element 7> which thus receives the output voltage from the third operational amplifier 26 during the duration of the output pulse from the monostable circuit 16.

Den tredje under-seksjon 3 omfatter en spenningsregulator 30 forbundet med den negative pol for akkumulatoren h. Den konstante utgangsspenning fra regulatoren 30 tilfores en spenningsdeler 31 The third sub-section 3 comprises a voltage regulator 30 connected to the negative pole of the accumulator h. The constant output voltage from the regulator 30 is supplied to a voltage divider 31

og en uttappet spenningsdel fra denne tilfores en forste inngang for en fjerde operasjonsforsterker 32. Den annen inngang for denne forsterker er forbundet med et jordpunkt eller et tilsvarende potensial, og en varme-folsom motstand 33 er koblet mellom ut-ganger for forsterkeren 32 og dens forste inngang. Denne motstand 33 er, når apparatet er i bruk, nedsenket i elektrolytten i akkumulatoren ^f, og tjener derved til å kompensere for temperaturvaria-sjonene i elektrolytten. and a tapped voltage portion from this is supplied to a first input for a fourth operational amplifier 32. The second input for this amplifier is connected to a ground point or a corresponding potential, and a heat-sensitive resistor 33 is connected between the outputs of the amplifier 32 and its first entrance. This resistor 33 is, when the device is in use, immersed in the electrolyte in the accumulator ^f, and thereby serves to compensate for the temperature variations in the electrolyte.

Utgangen for forsterkeren 32 er forbundet til en forste inngang The output of the amplifier 32 is connected to a first input

for en femte operasjonsforsterker 3<*>+, idet denne inngang også er forbundet med den-positive pol for akkumulatoren h. Den annen inngang for forsterkeren 3^ er forbundet med et jordpunkt eller et for a fifth operational amplifier 3<*>+, as this input is also connected to the positive pole of the accumulator h. The other input for the amplifier 3^ is connected to a ground point or a

tilsvarende potensial. Utgangssignalet fra forsterkeren 3<*>+ tilfores styree'nheten 8, hvis ut gangs signal gjennom en f or sinkelses-krets 35 tilfores en tilsvarende inngang for oppladningskretsen 6. corresponding potential. The output signal from the amplifier 3<*>+ is fed to the control unit 8, whose output signal is fed through a delay circuit 35 to a corresponding input for the charging circuit 6.

Apparatet virker på folgende måte: Spolen 10, omkobleren 11, den monostabile enhet 12, omkoblerelementet, 13 og shunt-motstanden 1<1>)-danner, en drivkre.ts som er koblet mellom den negative og positive .pol for akkumulatoren k, idet denne,krets er innrettet for å til-fore strompulser til akkumulatoren. Forsterkeren 20, omkobleren 21 og lagringsinnretningen 22, danner en foler-krets for avf.oling av spenningen mellom akkumulatorens klemmer og frembringer et forste signal som angir differansen mellom klemmespenningen for og under hver strompuls. Dette forste signal er utgangsspenningen U for forsterkeren 20. The apparatus works as follows: The coil 10, the switch 11, the monostable unit 12, the switch element, 13 and the shunt resistor 1<1>) form a drive circuit which is connected between the negative and positive poles of the accumulator k, as this circuit is designed to supply current pulses to the accumulator. The amplifier 20, the switch 21 and the storage device 22 form a sensor circuit for sensing the voltage between the accumulator's terminals and produces a first signal indicating the difference between the terminal voltage before and during each current pulse. This first signal is the output voltage U of the amplifier 20.

Spenningsregulatoren 23, spenningsdeleren 2h og forsterkeren 25 danner en referansekrets som frembrin ger et referansesignal i form av utgangsspenningen Ug for forsterkeren 25. En komparatorkrets som utgjores av forsterkeren 26 mottar det forste signal The voltage regulator 23, the voltage divider 2h and the amplifier 25 form a reference circuit which produces a reference signal in the form of the output voltage Ug for the amplifier 25. A comparator circuit formed by the amplifier 26 receives the first signal

AU og sammenligner det med referansesignalet Ug. AU and compares it with the reference signal Ug.

Hvis utgangssignalet fra forsterkeren 26 angir at batteriets ladningsnivå har nådd et forut bestemt minimums-nivå påvirkes styreelementet 7 og aktiverer oppladningsinnretningen 6, for opplad-ning av akkumulatoren. If the output signal from the amplifier 26 indicates that the battery's charge level has reached a predetermined minimum level, the control element 7 is affected and activates the charging device 6, for charging the accumulator.

Spenningsregulatoren 30, spenningsdeleren 31, forsterkeren 32 med sin tilordnede motstand 33 samt forsterkeren 3<*>+ dannet en krets for overvåking av ladningsnivået og stopp av oppladningsinnretningen 6, når akkumulatoren atter er fullt oppladet. Den fullt oppladede tilstand er temmelig noyaktig definert ved en bestemt verdi for akkumulatorens klemmespenning. The voltage regulator 30, the voltage divider 31, the amplifier 32 with its assigned resistance 33 and the amplifier 3<*>+ formed a circuit for monitoring the charge level and stopping the charging device 6, when the accumulator is fully charged again. The fully charged state is quite precisely defined by a specific value for the accumulator's terminal voltage.

ForsinkeJLseskretsen 35 gjor det mulig å forsinke avslutningen av oppladningen med et forut be.stemt tidsintervall, således at det oppstår en lett overladning av akkumulatoren. Dette kan være fordelaktig for omrbring av elektrolytten når denne er en væske. Det nettopp beskrevne apparat er særskilt fordelaktig når automatisk styring av en akkumulators ladningstilstand er påkrevet, og over-våkingen av en operator er vanskelig eller umulig. Sådanne anvendelser foreligger, f.eks. i boyer og fyrtårn, som tilfores elektrisk energi fra en gruppe akkumulatorer, men periodisk kon-troll av en operator er meget vanskelig, hvis ikke umulig. The delay circuit 35 makes it possible to delay the end of charging by a predetermined time interval, so that a slight overcharging of the accumulator occurs. This can be advantageous for transferring the electrolyte when it is a liquid. The apparatus just described is particularly advantageous when automatic control of an accumulator's state of charge is required, and monitoring by an operator is difficult or impossible. Such applications exist, e.g. in buoys and lighthouses, which are supplied with electrical energy from a group of accumulators, but periodic control by an operator is very difficult, if not impossible.

Det vil forstås at oppladningskretsen 6 og dens styrekrets 3 kan utelates i anvendelser der det bare er onskelig å få en anvisning på akkumulatorens ladningstilstand. Der det er tilstrekkelig' bare å frakoble akkumulatoren når den er fullt oppladet, kan utgangen fra OG-porten 17 tilfores en passende krets for utforelse av denne frakobling. It will be understood that the charging circuit 6 and its control circuit 3 can be omitted in applications where it is only desirable to obtain an indication of the accumulator's state of charge. Where it is sufficient only to disconnect the accumulator when it is fully charged, the output of the AND gate 17 can be fed to a suitable circuit for carrying out this disconnection.

Claims

1. Apparatus for monitoring an accumulator's state of charge during normal operation while the accumulator (h) is being discharged through a load (5), characterized in that the device comprises a drive circuit (10, 11, 12, 13, ik) connected between the accumulator's terminals and arranged to, superimposed on the accumulator's discharge current, send current pulses of predetermined duration through the accumulator (h) at predetermined intervals; a sensing circuit (20, 21, 22) connected between the accumulator terminals for sensing the difference, A U between the available terminal voltages or before and during each current pulse; and arranged to produce a difference signal indicating this voltage difference; a comparator circuit (26) arranged to receive the difference signal and arranged to compare it with a reference signal Us, which indicates a predetermined charge level and is produced by a reference circuit (23, 2h, 25), and to emit a binary signal during each current pulse, if one possible value indicates the state / ^ U/>Us and other possible value indicates the state AU^/ Us/.

2. Apparatus as stated in claim 1, characterized in that it further comprises an instruction circuit (27) connected to the comparator circuit (26) to receive the "binary signal from this, as well as arranged to produce an indication of the accumulator's state of charge.

3. Apparatus as stated in claim 1, characterized in that the reference circuit comprises a voltage regulator (23) which is connected to the accumulator ( h) and emits a constant output voltage, which via a voltage divider (2<*>+) is supplied to an input for an operational amplifier (25). h.

Apparatus as stated in claims 1 and 3? characterized in that the felt circuit (20, 21, 22) comprises another operational amplifier (20) with a first input (U2) connected directly to one of the accumulator's terminals (+) and a second input (U1) connected directly to a storage device (22) , as well as to said accumulator terminal (+) above a first switching element (21), which is open during each current pulse through the accumulator.

5. Apparatus as stated in claim 3, characterized in that the storage device is a capacitor (22) connected between said second input (U1) and a grounded point or a corresponding potential.

6. 'Apparatus as specified in claims 1-5, characterized in that the comparator circuit comprises a third operational amplifier (26), which emits said binary signal on its output side.

7. Apparatus as specified in claims 1-6, characterized in that the drive circuit (10, 11, 12, 13, 1^-) comprises another switch element (11) which is connected in said space of previously determined duration between current pulses, for energizing a first monostable circuit (12), if output pulse is mainly of the same duration as the strb pulse and is fed to a third switching element (13) arranged to connect a shunt resistance (1<*>+) between the accumulator terminals for the entire duration of the output pulse.

8. Apparatus as specified in claims 1-7> characterized by. that it comprises the following logic circuits: a delay circuit 05) which is controlled by current pulse signals from said drive circuit (10, 11, 12, 13, 1<*>+) and is arranged to emit release signals delayed in relation to the corresponding current pulses with a time interval smaller than the duration of the current pulses; a further monostable circuit (16) which is triggered by each trigger signal to produce an output pulse whose duration is at most equal to the difference between the duration of the current pulse and the delay delay; and an AND circuit (17) connected to receive the output signals from both the comparator circuit (26) and the further monostable circuit (16).