Isolating interface with a differentiating circuit comprising a capacitive barrier and method for transmitting a signal by means of such isolating interface

A known method for parallel two-way symmetrical signal transmission by means of an isolating interface with a differentiating circuit comprising a capacitive barrier is improved. When restarting communication in the selected direction after a longer break, a pilot signal is conducted via the transmitting plates for the communication in the reverse direction and capacitive compensators to one of the receiving plates for communication in the selected direction. Threshold levels for comparisons of the signals of the first and second time derivative are decreased, the capacitance of capacitive compensators is then set to reduce output the output signal and finally communication is reestablished. Transmitting plates for communication in the reverse direction are now connected to the receiving plates for communication in the selected direction through the capacitive compensators with the capacitance adjusted as described above. This provides satisfactory signal transmission even when a thick layer of an electrically well conductive liquid appears between the plates of the isolating interface.

This is a national stage of PCT/SI08/000039 filed Jun. 20, 2008 and published in English, which has a priority of Slovenia no. P-200700146 filed Jun. 21, 2007, hereby incorporated by reference.

The invention relates to an isolating interface with a differentiating circuit comprising a capacitive barrier and a method for a signal transmission by means of such isolating interface in most demanding conditions, like in the case of penetration of an electrically conductive liquid inbetween the plates of the isolating interface.

An isolating interface with a capacitive barrier is often used for a contactless signal transmission. Known isolating interfaces of this kind operate in a satisfactory manner in an environment, in which parasitic capacitances and especially parasitic conductivities are low.

A known isolating interface ii′1sstwith a capacitive barrier for a one-way symmetrical signal transmission by means of signal replicas being in mutual phase opposition in two parallel paths is represented in FIG. 1 (EP 0 744 750 and WO/2006/045148). The isolating interface ii′1sstcomprises a transmitting circuit, into which an input signal Ui enters and which is comprised of an invertor i, amplifiers a+, a− and transmitting plates tp+, tp−, and a receiving circuit, which is comprised of receiving plates rp+, rp−, resistors r+, r−, comparators c+, c− and a flipflop ff, from which an output signal Uo emerges. Between the transmitting plates tp+, tp− in the first and second path and the receiving plates rp+, rp− in the first and second path, respectively, there exists an interface boundary IB acting as a capacitive barrier. If an electrically conductive liquid penetrated inbetween the interface plates, the situation on the interface boundary IB may be represented by an equivalent circuit inFIG. 1by means of parasitic resistances r1+, r2+, r1−, r2− and r±. The signal level is reduced from its level existing on the transmitting plates tp+, tp− by a voltage divider being represented by the parasitic resistances r1+, r2+, r1−, r2− and r±. Therefore, the distance between two transmitting plates and two receiving plates, at which the functioning of the isolating interface is satisfactory, is drastically decreased. Additionally, the parasitic resistance r± between two paths for the signal replicas being in mutual phase opposition shortens the duration of pulses in the communication signal, as well, because the fall rate of the potential difference between both paths depends mainly on the sum of all capacitances between said plates and on the parasitic resistance r± between two paths. Such pulse form, however, especially disables an effective rejection of disturbances resulting from variations of potential difference between supply voltages of the transmitting and receiving electronic circuits. Till recently, this used to be an even more considerable and usually an unsolvable difficulty in the transmission of the communication signal through a nonideal capacitive barrier.

In their patent SI 200300001 (WO 2004/062221, EP 1 582 039 A1, US 6,819,169), the authors and applicants of the present technical solution suggested a technical solution to a reliable signal transmission in complicated conditions, such as presence of an electrically conductive liquid between the plates of an isolating interface. They proposed the isolating interface ii′1sstwith a capacitive barrier for a one-way symmetrical signal transmission as represented inFIG. 1wherein the capacitive barrier is part of a differentiating circuit in a receiving circuit of the interface in either path of two signal replicas being in mutual phase opposition. Namely, they required that the time constant of either said differentiating circuit should be smaller than a rise time and a fall time of said signal replicas. A level decrease in the signal in point p on either path due to the parasitic conductivities does not cause a proportional signal decrease in the point γ, on the contrary, the signal level at the input of comparators c+ and c− depends on the rate of variations of the input signal Ui.

Whether a shorter pulse duration due to the parasitic conductivities will impair the efficiency of the isolating interface inFIG. 1largely depends on the value of the time constant of the controlled shortening of the pulse at the input of the comparators c+ and c−, i.e. on the resistances of the resistors r+ and r− with respect to the value of time constant of the controlled pulse shortening due to the parasitic resistance r±. Simulations of pulse time behaviour in points α, β and γ in the known isolating interface ii′1sstas represented inFIG. 1with resistances r+=r−=10 kΩ are represented inFIG. 3a, and simulations of the pulse time behaviour in the same points in the isolating interface ii′1sstfromFIG. 1with resistances r+=r−=100 kΩ, i.e. according to the patent SI 200300001, are represented inFIG. 3b, each time for the parasitic resistances r±=50 kΩ (dashed) and r±=2 kΩ (full line) at the capacitance of 0.5 pF between the plates tp+ and rp+ as well tp− and rp−. The known isolating interface ii′1sstas represented inFIG. 1with the resistance r+=r−=10 kΩ functions adequately only at low parasitic conductance, whereas the signal at the input of the comparators c+ and c− is too low at high parasitic conductance, this means, when an electrically conductive liquid has penetrated between the interface plates. The known isolating interface ii′1sstfromFIG. 1according to the patent SI 200300001 functions at high parasitic conductance (r±)−1as well. It distinguishes itself in that it can operate also in a very adverse environment.

The isolating interface ii′2sstwith the capacitive barrier for the parallel two-way symmetrical signal transmission by means of the signal replicas being in mutual phase opposition in two parallel paths in either direction1and2(FIG. 2) is a further development of the isolating interface ii′1sstaccording to the patent SI 200300001. Said isolating interface ii′1sstcomprises a first transmitting circuit tc1′ and a first receiving circuit rc1′ for the first communication direction1and a second transmitting circuit tc2′ and a second receiving circuit rc2′ for the second communication direction2. The first transmitting circuit tc1′ and the first receiving circuit rc1′ on the one and another side of the interface boundary IB for the first communication direction1are electrically coupled to each other due to the capacitances C+1; C−1 between pairs of their transmitting and receiving plates tp+1, rp+1; tp−1, rp−1; in the same way the second transmitting circuit tc2′ and the other receiving circuit rc2′ for the second communication direction2are electrically mutually coupled due to the capacitances C+2; C−2 between pairs of their transmitting and receiving plates tp+2, rp+2; tp−2, rp−2. Furthermore, the first receiving circuit rc1′ for the first communication direction1in both paths, situated symmetrically with respect to the ground potential, is provided with a first differentiating circuit r+1, C+1 and a second differentiating circuit r−1, C−1, respectively, which are made of a resistor r+1; r−1 and the capacitance C+1; C−1 existing each time between one transmitting plate and the receiving plate rp+1; rp−1 pertinent thereto at the capacitive barrier and with a first comparator c+1 and a second comparator c−1, which are connected to said first differentiating circuit and the second differentiating circuit, respectively.

When partial capacitances PIC between the plates situated on the same side of the interface boundary IB grow high enough due to ellectrically conductive impurities between the interface plates, the coupling PIC of the second transmitting circuit tc2for the second communication direction2with the first receiving circuit rc1for the first communication direction1sufficiently intensifies for a variation of the potential on the plates tp+2, tp−2 as a consequence of a parallelly transmitted signal Ui2in the second communication direction2to produce a pulse high enough on the receiving plates rp+2, rp−2 so that the comparators c+1 and c−1 switch over due to these pulses. Yet this means an error in the signal received by the first receiving circuit rc1for the communication in the first direction1. Such error results from a crosstalk between the second transmitting plates tp+2, tp−2 and the first receiving plates rp+1, rp−1 if a discussion is limited just to the consideration of the received output signal Uo1in the parallel transmission in the first communication direction1. Said crosstalk can be done away within predictable situations by minimizing parasitic partial capacitances between the plates. In the differential isolating interface it can be achieved by an appropriate mutual arrangement of interface plates. However, an electrically conductive liquid can reach the interface plates in an unpredictable way. The parasitic capacitances then change in time and may also emerge where they were not present before. A disadvantage of the isolating interface ii′2sstwith the differentiating circuit comprising the capacitive barrier for the parallel two-way symmetrical signal transmission exists in that communication herewith is disturbed due to crosstalks in situations of high parasitic conductivities.

Difficulties in communication also arise in an isolating interface ii′1sstaccording to the patent SI 200300001 with the differentiating circuit comprising the capacitive barrier for symmetrical signal transmission by means of two signal replicas being in mutual phase opposition in two parallel paths whenever it must operate in extremely demanding situations when a thick layer of an electrically well conductive liquid appears between the plates of the isolating interface.

The invention is based on the technical problem as how to improve said isolating interface with the differentiating circuit comprising the capacitive barrier for the parallel two-way communication as well as one-way communication, in order to make it adequate to operate in difficult conditions when a thick layer of an electrically well conductive liquid appears between the plates of the isolating interface and to propose a method for signal transmission by means of an improved isolating interface with the differentiating circuit comprising the capacitive barrier.

The set problem is managed by embodiments of the invention of an isolating interface and a method.

An outstanding advantage of the isolating interface and the method of the invention lies in that a satisfactory signal transmission is achieved even when a thick layer of an electrically well conductive liquid appears between the plates of the isolating interface.

An isolating interface ii2sstof the invention with a differentiating circuit comprising a capacitive barrier for a parallel two-way symmetrical signal transmission by means of signal replicas being in mutual phase opposition is represented inFIG. 4and shows only a side thereof, which is situated on the right with respect to an interface boundary IB and is an improvement of the known isolating interface represented inFIG. 2.

It comprises a first transmitting circuit and a first receiving circuit rc1for a first communication direction1and a second transmitting circuit tc2and a second receiving circuit for a second communication direction2. The first receiving circuit rc1is provided in either path lying symmetrically with respect to the ground potential with a first differentiating circuit r+1, C+1 and a second differentiating circuit r−1, C−1, respectively, which are assembled of a resistor r+1; r−1 and a capacitive barrier between each transmitting plate and a receiving plate rp+1; rp−1 pertaining thereto the capacitance therebetween being C+1; C−1 and with a first comparator c+1 and a second comparator c−1, which are connected to an output of the first differentiating circuit r+1, C+1 and the second differentiating circuit r−1, C−1, respectively.

The known isolating interface ii′2sstis further equipped according to the invention as follows.

The first receiving circuit rc1is provided with a compensation control circuit ccc, which inputs are connected to the output of the first comparator c+1 and the output of the second comparator c−1, respectively.

It is further provided with a controlled changeover switch s, which is controlled by the compensation control circuit ccc through a first control signal CPS so that a pilot signal PS is conducted to the second transmitting circuit tc2instead of an input signal Ui2for communication in the second direction2. The frequency of the pilot signal PS is of the same order of magnitude as the frequency of communication signals.

The isolating interface ii′2sstis further provided with a first and a second controlled grounding switch s+1, s−1, which are controlled by the compensation control circuit ccc through a second control signal C+ and a third control signal C−, respectively, to close themselves and to ground the first receiving plate rp+1 and the second receiving plate rp−1, respectively, of the first receiving circuit rc1.

The isolating interface ii′2sstis further provided with a first and second capacitance compensator cc+, cc−, which are controlled by the compensation control circuit ccc through a fourth control signal CC+ and a fifth control signal CC−, respectively, in such a way that a first transmitting plate tp+2 and a second transmitting plate tp−2 of the second transmitting circuit tc2are capacitively connected to a first receiving plate rp+1 of the first receiving circuit rc1through a connection Utp+2 and Utp−2, respectively, through the first capacitance compensator cc+ and a connection compens+ after the first capacitance compensator cc+ have begun to receive the fourth control signal CC+, and/or the first transmitting plate tp+2 and the second transmitting plate tp−2 of the second transmitting circuit tc2are capacitively connected to the second receiving plate rp−1 of the first receiving circuit rc1through a connection Utp+2 and Utp−2, respectively, through the second capacitance compensator cc− after the second capacitance compensator cc− has started to receive the fifth control signal CC−.

The compensation control circuit ccc is designed in a way to stop communication in the first direction1whenever said communication restarted, to transmit the first control signal CPS, to close a controlled changeover switch s and to conduct the pilot signal PS to the second transmitting circuit tc2.

The compensation control circuit ccc then transmits the third control signal C− and/or the second control signal C+ to set the first capacitance compensator cc+ and/or the second capacitance compensator cc−, respectively, so that the amplitude value of that part of a signal U+1 at the output of the first comparator c+1 and/or of that part of a signal U−1 at the output of the second comparator c−1, which originates or originate from the pilot signal PS due to the coupling of the first receiving circuit rc1with the second transmitting circuit tc2, lies below a predetermined value.

The compensation control circuit ccc then freezes such setting of the first capacitance compensator cc+ and/or the second capacitance compensator cc− and stops transmitting the pilot signal PS, the first control signal CPS and the third control signal C− and/or the second control signal C+ to return the pertaining switches into the original position and to further allow the communication in the first direction1.

Whenever communication in the first direction1restarted and was stopped thereafter the compensation control circuit ccc, by means of a sixth and/or a seventh control signal TC+, TC−, advantageously decreases the threshold of the first comparator c+1 and/or that of the second comparator c−1, respectively, and increases said threshold to the original level after the first capacitance compensator cc+ and/or the second capacitance compensator cc− has been set.

The capacitance compensator cc+ can be layed out as schematically shown inFIG. 5. It is made of a capacitive block cb, into which the connections Utp+2 and Utp−2 of the first transmitting plate tp+2 and the second transmitting plate tp−2, respectively, of the second transmitting circuit tc2are conducted and from which the connection compens+ to the receiving plate rp+1 of the receiving circuit rc1is conducted. A control circuit cbcc provided to set the capacitive block cb controls switches connecting individual capacitors within the block into the connection of the transmitting plates p+2, tp−2 to the receiving plate rp+1 as well determines the compensation control circuit ccc by means of the signal CC+.

A known method for the parallel two-way symmetrical signal transmission by means of the isolating interface with the differentiating circuit comprising a capacitive barrier as represented inFIG. 2comprises:a generation of two replicas being in mutual phase opposition of the input signal for the communication in either directiona differentiation of the signal replicas transmitted through the capacitive barrier in the first and second differentiating circuit whereat said differentiating circuits comprise the capacitive barriers between two pairs of the interface plates for the transmission in the selected direction as in one of both possible communication directions and the time constants of said differentiating circuits are smaller than the rise time and the fall time of the signal replicas,a generation of the first output signal replica by comparing the signal of the first time derivative to that of the second time derivative anda generation of the second output signal replica by comparing the signal of the second time derivative to that of the first time derivative anda generation of a signal transmitted in said direction as an output signal of a flip-flop, to which inputs said output signal replicas are conducted.

The described known method for the parallel two-way symmetrical signal transmission by means of the isolating interface with the differentiating circuit comprising a capacitive barrier is further developed according to the invention as follows.

Whenever communication in the selected direction as in one of both possible communication directions has restarted after a longer time interval communication is stopped and a pilot signal instead of the input signal for communication in the selected direction is conducted to the input of the isolating interface for the reverse transmission direction.

Threshold levels for both comparations of the signals of the first and the second time derivative are decreased and the capacitances of the capacitive compensators, through which the transmitting plates for communication in the reverse direction are connected to the one or other receiving plate for communication in the selected direction, are set in a way that the amplitude of the output signal of the flip-flop, to which inputs said output signal replicas are conducted, lies below a predetermined value.

Communication in the selected direction is reestablished. Signal transmission now takes place through the isolating interface, in which the transmitting plates for communication in the reverse direction are connected to the receiving plates for communication in the selected direction through the capacitive compensators having the capacitance adjusted as described above.

An isolating interface ii1astwith a differentiating circuit comprising a capacitive barrier (SI 200300001) for a one-way asymmetrical signal transmission is represented inFIG. 6a. An asymmetrical forward path for of the signal through the isolating interface ii1astand a reverse path ret of the signal through capacitive connections are represented, which are parts of a loop provided for the one-way signal transmission from a first electronic circuit eel to a second electronic circuit ec2through the isolating interface ii1ast. Such communication loop can operate in extremely difficult situations when a thick layer of an electrically well conductive liquid appears between the plates of the isolating interface.

The isolating interface ii1astfor a one-way asymmetrical signal transmission comprises an amplifier a and a transmitting plate tp connected to the output of the amplifier a in a transmitting circuit and, in a receiving circuit, a differentiating circuit, which consists of a resistor r and a capacitance C of the capacitive barrier between the transmitting plate tp and a receiving plate rp, and a comparator c, which is connected with its first input to the differentiating circuit and of which a second input is connected to the ground. An output signal of the comparator c is an output signal of the isolating interface ii1ast.

The communication loop provided for the one-way asymmetrical signal transmission from the first electronic circuit ec1to the second electronic circuit ec2through the isolating interface ii1astis made of a forward directed path, in which the first electronic circuit ec1is connected to the second electronic circuit ec2through the isolating interface ii1astof the invention for the one-way signal transmission, and of a reversely directed path ret, in which the second electronic circuit ec2is connected to the first electronic circuit ec1in a manner that the ground g2of the second electronic circuit ec2is capacitivelly connected to the ground g1of the first electronic circuit ec1. Said capacitive connection can be carried out by means of connecting capacitors Cg1cgand Cg2cgto the common ground cg (FIG. 6a). Two variants of the capacitive connection in the reversely directed path ret are shown inFIGS. 6aand6b.

A method for the one-way asymmetrical signal transmission in the represented loop through the isolating interface is comprised of the following steps:a signal transmitted in the forward direction through the capacitive barrier of the isolating interface is differentiated in the differentiating circuit, which comprises the capacitive barrier between the plates of the isolating interface and of which the time constant is smaller than the rise time and the fall time of the signal,an output signal of the isolating interface is generated in that the signal of said time derivative is compared to the ground potential andthe signal is transmitted in the backward direction from the second electronic circuit to the first electronic circuit through the ground of the second electronic circuit and the ground of the first electronic circuit, said grounds being capacitivelly interconnected.

An isolating interface ii2astof the invention with a differentiating circuit comprising a capacitive barrier for a parallel two-way asymmetrical signal transmission is represented inFIG. 7with only its right side with respect to the interface boundary IB. The isolating interface ii2astcomprises a first transmitting circuit and a first receiving circuit arc1for a first communication direction1as well a second transmitting circuit atc2and a second receiving circuit for a second communication direction2.

The first receiving circuit arc1is provided with a differentiating circuit r1, C1, which consists of a resistor r1and the capacitive barrier between the transmitting plate and the receiving plate rp1pertinent thereto, said plates having a mutual capacitance C1, and with a comparator c1, which is connected to the differentiating circuit r1, C1and of which a second input is connected to the ground.

The first receiving circuit arc1is also provided with a compensation control circuit accc, which is connected to an output of the comparator c1, a controlled changeover switch s, which is controlled by the compensation control circuit accc through a first control signal CPS, so that a pilot signal PS instead of an input signal Ui2for the communication in the second direction2is conducted to the second transmitting circuit atc2, and with a capacitive compensator acc. The frequency of the pilot signal PS is of the same order of magnitude as the frequency of the communication signals.

The compensation control circuit accc controls a capacitive compensator acc through a second control signal CC so that the transmitting plate tp2of the second transmitting circuit atc2is capacitively connected to a receiving plate rp1of the first receiving circuit arc1through an inverter i2, a connection Utp2and the capacitive compensator acc after the capacitive compensator acc has started to receive the second control signal CC.

The compensation control circuit accc transmits the first control signal CPS whenever communication in the first direction1has restarted and transmits the pilot signal PS and sets the capacitive compensator acc so that a value of the amplitude of that part of a signal Uo1originating from the pilot signal PS at the output of the comparator c1lies below the predetermined value. The compensation control circuit accc then freezes such setting of the capacitive compensator acc and stops transmitting the first control signal CPS as well the pilot signal PS and reestablishes communication in the first direction1, which communication was interrupted for a short time.

Whenever communication in the first direction1was reestablished and stopped thereafter the compensation control circuit accc in the isolating interface ii2astof the invention advantageously decreases the threshold of the comparator c1by means of a third control signal TC and increases said threshold to the original level after the capacitive compensator acc has been set.

A communication loop of the invention provided for the parallel two-way asymmetrical signal transmission between electronic circuits ec1and ec2through the isolating interface ii2astconsists of two parts. On the one hand, a signal input and a signal output of the first electronic circuit eel and a signal input and a signal output of the second electronic circuit ec2are connected through the isolating interface ii2astfor the parallel two-way signal transmission, and, on the other hand, a ground g1of the first electronic circuit ec1is capacitively connected to a ground g2of the second electronic circuit ec2in a way as represented for the one-way asymmetrical signal transmission inFIGS. 6a,6band6c. The capacitance of the interconnection of said grounds g1, g2of the communicating electronic circuits ec1, ec2must be at least three times larger than the capacitance between the transmitting plate and the receiving plate of the isolating interface ii2astfor either communication direction. If the capacitance of the interconnection of said grounds g1, g2were lower a variation of the potential of the transmitting plate would cause a considerable potential difference between the electronic circuits ec1and ec2, which would be reflected as a parasitic input signal on the receiving plate.

A method of the invention for the parallel two-way asymmetrical signal transmission in the presented loop through the isolating interface ii2astconsists of the following steps:whenever communication in the selected direction being one of both possible communication directions restarted, the communication is stopped,the pilot signal instead of the input signal for communication in this direction is conducted to the input of the isolating interface for the reverse transmission direction,in the circuit for the selected communication direction, the threshold level for the comparation of the signal from the output of the differentiating circuit to the ground potential is decreased,the capacitance of the capacitive compensator is set, through which and through an invertor there is connected the transmitting plate for the communication in the reverse direction to the receiving plate for the selected communication direction in a waythat the amplitude of the output signal for the communication in the selected direction lies below the predetermined value,the signal transmission in the selected direction then takes place through the isolating interface, in which the transmitting plate for communication in the reverse direction is connected to the receiving plate for communication in the selected direction through the capacitive compensator, of which the capacitance was adjusted in said way,the signal for the communication in the selected direction is transmitted through the capacitive barrier of the isolating interface,the signal is differentiated in the first differentiating circuit, which comprises said capacitive barrier between the transmitting plate and receiving plate for communication in the selected direction within the isolating interface and of which the time constant is smaller than the rise time and the fall time of the signal,the output signal of the isolating interface for the communication in the selected direction is generated in that the signal from the output of the differentiating circuit is compared to the ground potential andthe signal for communication in the selected direction is transmitted through the reverse path from the second electronic circuit to the first electronic circuit through the capacitively interconnected ground of the second electronic circuit and ground of the first electronic circuit.