EP0927982B2 - Transducer power supply - Google Patents

Description

The invention relates to a transmitter power supply for supplying a transmitter with electrical energy from a DC voltage source via a two-wire connection, is transmitted in the opposite direction of the measured value detected by the transmitter by a variable between two limits direct current, wherein the galvanic isolation in the connection between the Transmitter and the DC voltage source, a transformer is inserted, the primary winding is connected via a chopper to the DC voltage source and to the secondary winding of a rectifier circuit is connected, which generated at its output terminals by rectification of the transferred via the transformer, chopped current generated direct current with the by the transmitter certain size supplies.

Such a transducer feeder is for example from the US-A-3,764,880 known.

From the DE-PS-3207785 Another transmitter power supply is known, which is connectable either with a passive transmitter or via an appropriate matching circuit with an active DC generator.

A transmitter power supply of this type is intended to provide a arranged in a hazardous zone passive transducer via a two-wire connection with electrical energy and at the same time to allow the transmission of the measurement signal supplied by the passive transducer in the form of a variable between two limits current signal in the opposite direction. According to a standard, the current signal varies between 4 mA and 20 mA. A passive transmitter does not contain its own electrical power source, but draws the energy required for its operation via the two-wire connection from a remote DC voltage source, and forms the measurement signal by extracting from the DC voltage source, in addition to the supply current, a supplemental current so dimensioned in that the total current drawn from the DC voltage source corresponds to the current signal to be transmitted, which lies between the two limit values of, for example, 4 and 20 mA. This current signal can also be superimposed on communication signals in the form of pulse-shaped changes, whereby digital data can be transmitted in both directions. Since the total current is transmitted in one direction, namely from the voltage source to the transmitter, a galvanic isolation between the voltage source and the transmitter by a transformer is possible by the chopped from the DC voltage source removed total current on the principle of a DC-DC converter on the primary side of the transformer and rectified on the secondary side of the transformer. Such galvanic isolation is a particularly advantageous protective measure for transducers which are arranged in potentially explosive atmospheres. The galvanic isolation by means of the transformer of a DC-DC converter allows not only the transmission of the DC supply current and the measured value representing DC signal, but also the bidirectional transmission of communication signals in the form of the total current superimposed pulse-shaped changes, provided that the chopper frequency is much higher than the frequency the communication signals.

However, in a transmitter power supply of the kind described above, there is the problem that it is not possible to connect an active transmitter instead of the passive transmitter. An active transmitter differs from a passive transmitter in that it is equipped with its own electrical power supply and generates the measurement signal in the form of varying between two limits DC signal from its own power supply and outputs at its outputs. It is not possible to transmit the DC signal supplied by the active transducer in the direction opposite to the transmission direction of the DC-DC converter.

The object of the invention is to provide a transmitter power supply of the type described, which can be operated while maintaining the galvanic separation caused by the protective measure either with a passive transmitter or with an active transmitter.

According to the invention, this object is achieved by the features of Ansprüchs 1.

In the transmitter-feeder device according to the invention, the matching circuit inserted between the active transmitter and the rectifier circuit causes the primary-side DC voltage source to be loaded via the rectifier circuit and the transformer in the same manner as by a passive transmitter with a DC current corresponding to the measurement signal to be transmitted. Therefore, it can not be seen from the primary side whether an active or a passive measuring transducer is connected on the secondary side. The current drawn via the rectifier circuit and the transformer from the primary-side DC voltage source also contains the supply current required for the operation of the matching circuit. The total current can be superimposed in the same way as when loaded by a passive transmitter communication signals in the form of pulse-shaped changes that are transmitted bidirectionally via the transformer. The protective measure for potentially explosive areas caused by the galvanic isolation remains fully intact regardless of whether an active or a passive transmitter is connected.

Fig. 1

das Schaltbild eines Meßumformer-Speisegeräts bekannter Art zur Versorgung eines passiven Meßumformers mit elektrischer Energie und zur Übertragung des Meßsignals über eine Zweidrahtverbindung,

Fig. 2

die Abänderung des Meßumformer-Speisegeräts von Fig. 1 zum wahlweisen Anschluß eines aktiven Meßumformers anstelle eines passiven Meßumformers und

Fig. 3

das Meßumformer-Speisegerät von Fig. 2 mit dem Schaltbild einer Ausführungsform der Anpassungsschaltung.

Further features and advantages of the invention will become apparent from the following description of a Embodiment with reference to the drawings. In the drawings show:

Fig. 1

the diagram of a transducer power supply of known type for supplying a passive transducer with electrical energy and for transmitting the measuring signal via a two-wire connection,

Fig. 2

the modification of the transmitter power supply of Fig. 1 for selectively connecting an active transmitter instead of a passive transmitter and

Fig. 3

the transmitter supply unit of Fig. 2 with the diagram of an embodiment of the matching circuit.

In Fig. 1 In the drawing, the circuit components shown on the right of the broken line AA form a prior art transducer feeder 10 for supplying a passive transducer 11 with electrical energy from a DC power source 12 via the two conductors 13, 14 of a two-wire connection across the opposite direction transmitted by the transducer 11 measured value signal is transmitted. The two-wire connection 13, 14 is shown interrupted to indicate that it may be of any length. It connects the passive transducer 11 with two terminals 15, 16 of the transducer power supply 10th

The transmitter 11 includes a sensor for the physical quantity to be measured and an electronic circuit for converting the sensor signal into the measured value signal to be transmitted. A passive transmitter does not contain its own power source, but it draws the energy required for the operation of the electronic circuit via the two-wire connection 13, 14 from the DC voltage source 12 in the remote located transducer supply unit 10. According to a conventional standard, the transmitter 11 forms the Measurement signal in that it adjusts the current drawn from the DC voltage source 12 so that the measured value is expressed by a lying between 4 mA and 20 mA direct current. The DC current is measured by an evaluation circuit 18 arranged at the location of the DC voltage source 12 and evaluated to determine the measured value of the physical variable detected by the measuring transducer 11. Additionally, the transmitter 11 may be configured to superimpose digital communication signals in the form of pulsed changes on the current signal so that measurements and parameters may be digitally read and written. There is then the requirement to transmit such communication signals bidirectionally between the measuring transducer 11 and the evaluation circuit 18.

If the passive transmitter 11 is located in a hazardous area, additional safety precautions must be taken. A particularly effective protective measure for hazardous areas is a galvanic isolation between the transmitter 11 on the one hand and the DC voltage source 12 and the evaluation circuit 18 on the other. This in Fig. 1 shown transducer supply unit 10 is formed with such a galvanic isolation.

The galvanic isolation is carried out at the transmitter supply unit 10 of Fig. 1 by a transformer 20 having a primary winding 21 and a secondary winding 22. The DC voltage source 12 is connected between a center tap 23 of the primary winding 21 and ground. Each of the two outer terminals 24 and 25 of the primary winding 21 is connected via a switch 26 and 27 to one terminal 28 of a resistor 29 whose other terminal is grounded. The two switches 26 and 27 are controlled by a clock 30 with a relatively high clock frequency of, for example, 200 kHz in push-pull, so that the switch 26 is opened when the switch 27 is closed, and vice versa. Thus, the current supplied by the DC voltage source 12 alternately flows in opposite directions through the one or the other half of the primary winding 21, but always in the same direction through the resistor 29. In the primary winding 21, the DC voltage is chopped into a rectangular AC voltage, the is transferred to the secondary winding 22. To the secondary winding 22, a full-wave rectifier circuit 31 with four diodes 32 and a filter capacitor 33 is connected, which generates by rectifying the rectangular AC voltage, the DC operating voltage for the passive transducer 11. It will thus be appreciated that the transmitter 20 in combination with the chopper formed by the switches 26, 27 and the clock 30 and with the rectifier circuit 31 forms a DC-DC converter of known type. The switches 26, 27, which are shown in simplified form as mechanical switching contacts, are in reality of course fast electronic switches, for example field effect transistors.

As a further protective measure for the use of the passive transmitter 11 in a hazardous area, the rectifier circuit 31 includes a connected via a fuse 34 voltage limiter 35, which is shown as a Zener diode. Protective resistors 38 and 39, respectively, are inserted between the output terminals 36, 37 of the rectifier circuit 31 and the terminals 15, 16 of the transmitter power supply unit intended for connection of the passive transmitter 11. The protective resistors 38, 39 prevent the current in the hazardous area from rising above an allowable limit, and the voltage limiter 35, in conjunction with the fuse 34, limits the voltage in the circuit hazardous area to a permissible value.

The passive transmitter 11 extracts from the rectifier circuit 31 a DC current I _MP whose value is set in the range of 4 to 20 mA so as to represent the measured value of the physical quantity detected by the sensor. This direct current is supplied via the transformer 20 from the DC voltage source 12, so that at a transmission ratio of 1: 1 of the transformer 20, a DC current of the same magnitude flows through the resistor 29. The drop across the resistor 29 DC voltage is thus proportional to the set by the passive transducer 11 measuring current I _MP . It is fed to the evaluation circuit 18 connected to the connection 28.

If the measurement current I _MP 11 communication signals are superimposed by the passive transducer in the form of pulse-shaped changes, these pulse-shaped changes are also transmitted via the transformer 20 so that they express themselves in pulse-shaped voltage changes in the voltage drop across the resistor 29. These voltage changes are also detected and evaluated by the evaluation circuit 18. The repetition frequency of the pulse-shaped changes is substantially less than the clock frequency of the clock 30. The evaluation circuit 18 preferably contains at the input a low-pass filter whose cut-off frequency is set so that the clock frequency of the clock 30 is suppressed, but the superimposed pulse-shaped communication signals are transmitted.

Fig. 2 shows the schematic diagram of a transmitter power supply unit 40, which makes it possible, instead of the passive transducer 11 optionally connect an active transmitter 41. In contrast to a passive transmitter, an active transmitter contains its own electrical power supply, and at the output it outputs a direct current supplied by this power supply whose size again corresponds in the range of 4 to 20 mA to the measured value of the physical quantity detected by the sensor. It is immediately apparent that it would not be possible to simply replace the active transducer 41 in place of the passive transducer 11 with the terminals 15, 16 of the circuitry of FIG Fig. 1 because the DC current supplied by the active transducer 41 could not be transmitted to the primary side of the transformer 20 via the rectifier circuit 31 and the transformer 20. The transmitter supply unit 40 therefore has two further terminals 42 and 43, to which the active transmitter 41 is connected via the two conductors 44 and 45 of a two-wire connection.

For simplicity, in Fig. 2 only the circuit components of the transmitter-power supply 40 located on the secondary side of the transformer 20 are shown; the circuit components lying on the primary side are identical to those of Fig. 1 identical. As far as the circuit components in Fig. 2 with those of Fig. 1 match, they are given the same reference numerals as in Fig. 1 and they have the same function as previously associated with Fig. 1 has been described. It can be seen immediately that for the passive transducer 11, the same circuitry as in Fig. 1 is present, with the only difference that between the terminal 36 of the rectifier circuit 31 and the protective resistor 38, a changeover switch 50 is inserted. When the change-over switch 50 is set in the position in which it connects the rectifier circuit 31 to the terminal 15 via the protective resistor 38, the circuit arrangement is identical to that of FIG Fig. 1 identical.

In contrast, when the switch 50 in the in Fig. 2 shown position, it connects the terminal 36 of the rectifier circuit 31 via a connecting conductor 51, a separating capacitor 52, a protective resistor 53 and a diode 54 to the terminal 42. The terminal 37 of the rectifier circuit 31 is permanently connected via a connecting conductor 55 and a protective resistor 56 connected to terminal 43. As previously explained, the active transducer 41 includes its own electrical power supply and outputs at the output a direct current I _MA whose magnitude in the range of 4 to 20 mA corresponds to the measured value of the physical quantity sensed by the sensor. Between the active transmitter 41 and the rectifier circuit 31, a matching circuit 60 is inserted, which takes the rectifier circuit 31, a DC I _MS equal to or proportional to the supplied by the active transmitter 41 direct current I _MA . The matching circuit 60 includes a resistor 61 connected to the terminals 42 and 43 through the diode 54, a control circuit 62 whose input terminals are connected to the terminals of the resistor 61, and a controllable current source 63 connected between the connecting conductors 51 and 55, the control input thereof the output of the control circuit 62 is connected. Thus, the controllable current source 63 bridges the two output terminals 36 and 37 of the rectifier circuit 31, when the switch 50, the in Fig. 2 shown position corresponding to the connection of the active transmitter 41. The control circuit 62 receives at the input a DC voltage which corresponds to the voltage drop across the resistor 61 caused by the current I _MA , and it is designed so that its output signal, the controllable current source 63 so adjusted that taken from the rectifier circuit 31 current I _{MS of} the active transducer I _MA is proportional to a predetermined constant factor. Preferably, this factor has the value 1, so that the current I _{MS is} equal to the current I _MA . Thus, the current I _MS taken from the rectifier circuit 31 gives the same effect as the current determined in the other position of the changeover switch 50 by the passive transmitter 11 I _MP : It is mirrored on the primary side of the transformer 20 and causes a proportional voltage drop across the resistor 29. This voltage drop is thus proportional to the measuring current I _MA delivered by the active measuring transducer 41.

Fig. 3 shows the circuit diagram of an embodiment of the controllable matching circuit 60 of Fig. 2 , The circuit components corresponding to those of Fig. 2 are identical with the same reference numerals as in Fig. 2 designated. The controllable current source 63 is formed by a field effect transistor 70 which is connected in series with a resistor 71 between the connecting conductors 51 and 55. The control circuit 62 includes an operational amplifier 72 whose power supply terminals are connected to the connecting conductors 51 and 55, so that the operational amplifier 72 is powered by the rectifier circuit 31 when the switch 50 is in the position corresponding to the terminal of the active transmitter 41st equivalent. The inverting input of the operational amplifier 72 is connected to the connection conductor 55 via a resistor 73. In the connecting conductor 55, a resistor 74 is inserted between the terminals of the controllable current source 63, the operational amplifier 72 and the resistor 73 on the one hand and the output terminal 37 of the rectifier circuit 31 on the other hand, via which thus both the current determined by the controllable current source 63 and the supply current of the operational amplifier 72 flows. The non-inverting input of the operational amplifier 72 is connected to the tap of a voltage divider of two resistors 75 and 76 which are connected in series between the connected via the diode 54 to the terminal 42 of the terminal 61 and the terminal 37 of the rectifier circuit 31. The output of the operational amplifier 72 is connected to the gate terminal of the field effect transistor 70.

Designating the resistance values of the resistors 61, 74, 75 and 76 with R ₆₁ , R ₇₄ , R ₇₅ and R ₇₆ , respectively, the following relationship exists between the current I _MA flowing through the resistor 61 and the resistor 74 to the input terminal 37 of the rectifier circuit 31 flowing current I _MS : $$\mathrm{IMS}\mathrm{=}{\mathrm{I}}_{\mathrm{MA}}\mathrm{\cdot }\frac{{\mathrm{<figure-callout\; id="61"\; label="R"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathrm{61}}\mathrm{\cdot }{\mathrm{R}}_{\mathrm{76}}}{{\mathrm{<figure-callout\; id="74"\; label="R"\; filenames="imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathrm{74}}\mathrm{\cdot }\mathrm{R}\mathrm{75}}$$

• R₆₁ = 250 Ω
• R₇₄ = 50 Ω
• R₇₅ = 100 kΩ
• R₇₆ = 20 kΩ

Thus, the current I _{MS is} proportional to the current I _MA with a constant factor determined by the resistors. This constant factor can be made equal to 1 by suitable dimensioning of the resistors, so that then the current I _{MS is} equal to the current I _MA . This applies, for example, to the following resistance values:

• R ₆₁ = 250 Ω
• R ₇₄ = 50 Ω
• R ₇₅ = 100 kΩ
• R ₇₆ = 20 kΩ

From the Figures 2 and 3 It can also be seen that at each position of the switch 50, the protective measures taken with regard to the hazardous area, namely the galvanic isolation by the transformer 20, the voltage limitation by the voltage limiter 35 and the fuse 34 and the current limiting by the protective resistors 38, 39 or by the protective resistors 53, 56 remain fully effective. The isolating capacitor 52 effects a DC isolation of the active transmitter 41 from the rectifier circuit 31, but allows the transmission of the superimposed communication signals.

The diode 54 is poled so that it flows in the forward direction through the resistor 61 supplied by the active transducer 41 current I _MA , but prevents a flow of current from the transducer supply unit 40 to the active transmitter 41. By the already in the circuit of Fig. 1 contained current and voltage limit is given when connecting a passive transducer sufficient security for the transmitter power supply, because the maximum energy available in a fault is too low to ignite a spark. When connecting an active transmitter but could occur the case that a current flowing from the transmitter power supply, which would be too weak for the ignition of a spark too weak, is superimposed outside of the transmitter power supply originating from the active transmitter current, so that the Sum of the two currents could be sufficient to ignite a spark. This hazard is precluded by the diode 54 because it prevents current from flowing from the transmitter supply to the active transmitter.

Claims (1)

1. A unit for determining a measured value that represents a physical variable, said unit comprising a transmitter power supply unit (40), through which a direct current (I_MS) flows, which is suitable for providing electrical energy to a transmitter - particularly one arranged in a hazardous zone - via a two-wire connection, and further comprising a transmitter (41) coupled to the transmitter power supply unit (40), whereby:

   - the transmitter (41) is an active transmitter which is equipped with its own power supply system and delivers an output direct current (I_MA) that represents the measured value

   - an adaptive circuit (60) is provided to connect the active transmitter (41) to the transmitter power supply unit (40), said circuit being connected to the transmitter (41) and the transmitter power supply unit (40) in such a way that both the output direct current (I_MA) and the direct current (I_MS) flow through the circuit; and said circuit using the output current (I_MA) to set the direct current (I_MS) flowing through the transmitter power supply unit (40) such that this direct current (I_MS) is proportional to the output direct current (I_MA) of the active transmitter (41), characterized in that:

   - a DC voltage generating the direct current (I_MS) is supplied by a DC voltage source (12), and a DC voltage converter (20, 26, 27, 30, 31) is integrated in the connection between the transmitter (41) and the DC voltage source (12) for the purpose of galvanic isolation. Said converter exhibits a transformer, a chopper, which is connected to a primary winding of the transformer and chops the DC voltage, and a rectifier circuit connected to a secondary winding of the transformer, characterized in that the adaptive circuit (60) is integrated between the active transmitter (41) and the rectifier circuit (31) and in that direct current (I_MS) flows through the DC voltage converter at a secondary side. The adaptive circuit (60) is arranged in the transmitter power supply unit (40) and the transmitter power supply unit (40) comprises a changeover switch (50) which allows the direct current (I_MS) to either flow through a two-wire connection connected to the transmitter power supply unit (40) or through the adaptive circuit (60).

   - a DC voltage, which drops over a resistor and is proportional to the direct current (I_MS), is supplied to an evaluation circuit (18) to determine the measured value.

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