EP0927982B1 - Transducer power supply - Google Patents

Description

The invention relates to a transmitter supply device for supply a transmitter with electrical energy from a DC voltage source via a two-wire connection, over that in the opposite direction that detected by the transmitter Measured value by a variable between two limit values Direct current is transmitted, whereby for electrical isolation into the connection between the transmitter and the DC voltage source a transformer is inserted, whose primary winding via a chopper to the DC voltage source is connected and a secondary winding Rectifier circuit connected to their Output connections one by rectifying the over the Transmitters of chopped electricity generated direct current with the size determined by the transmitter supplies.

Such a transmitter supply device is known, for example, from US Pat. No. 3,764,880.

A transmitter supply unit of this type is designed to one located in a hazardous area passive transmitter with a two-wire connection to supply electrical energy and at the same time the transmission of the measurement signal supplied by the passive transmitter in the form of a variable between two limit values Enable current signal in the opposite direction. one According to the usual standard, the current signal is between 4 mA and 20 mA variable. A passive transmitter contains not its own electrical voltage source, but it relates the energy required for its operation via the two-wire connection from a remote DC voltage source, and it forms the measurement signal in that it the DC voltage source in addition to the supply current takes a supplementary stream, which is dimensioned so that the total current drawn from the DC voltage source corresponding current signal that corresponds between the both limit values of 4 and 20 mA, for example. This current signal can also communication signals superimposed in the form of impulsive changes which means that digital data is transmitted in both directions can be. Since the total current is only in one Direction, namely from the voltage source to the transmitter is transmitted, there is a galvanic isolation between the Voltage source and the transmitter through a transformer possible by taking from the DC voltage source Total current according to the principle of a DC converter chopped on the primary side of the transmitter and on the Secondary side of the transformer is rectified. A such galvanic isolation is a particularly advantageous one Protective measure for transmitters used in potentially explosive atmospheres Zones are arranged. The galvanic isolation by means of the Transformer of a DC converter does not allow only the transmission of the direct current supply and the Measured value representing DC signal, but also the bidirectional transmission of communication signals in Form of impulsive changes superimposed on the total current provided that the chopper frequency is significantly higher than the frequency of the communication signals.

In the case of a transmitter / power supply unit, the one described above Type, however, there is the problem that it is not possible an active transmitter instead of the passive transmitter to join. An active transmitter differs of a passive transmitter in that it is connected to a own electrical energy supply and the measurement signal in the form of the variable between two limit values DC signal from this own energy supply generated and released at its outputs. It is not possible, the DC signal supplied by the active transmitter in the direction of transmission of the DC / DC converter to transmit in the opposite direction.

The object of the invention is to provide a transmitter power supply of the type specified at the outset, while maintaining caused by the galvanic isolation Protective measure either with a passive transmitter or can be operated with an active transmitter.

According to the invention, this object by the features of claims 1 and 6.

In the transmitter power supply according to the invention, the between the active transmitter and the rectifier circuit inserted matching circuit that the primary side arranged DC voltage source via the rectifier circuit and the transformer in the same way as loaded with a direct current by a passive transmitter which corresponds to the measurement signal to be transmitted. Seen from the primary side, it is therefore not clear whether an active or a passive transmitter on the secondary side connected. The one via the rectifier circuit and the transformer from the primary-side DC voltage source current drawn also contains the power required to operate the matching circuit required supply current. the Total current can be in the same way as when loaded by a passive transmitter communication signals in the form of impulsive changes are superimposed on the bidirectional are transmitted via the transmitter. The through the galvanic isolation protective measure for explosive Zones remain regardless of whether an active one or a passive transmitter is connected receive.

Advantageous refinements and developments of the invention are marked in the subclaims.

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 result from the following description of an exemplary embodiment with reference to the drawings. The drawings show:

Fig. 1

1 shows the circuit diagram of a known transmitter supply device for supplying a passive transmitter with electrical energy and for transmitting the measurement signal via a two-wire connection,

Fig. 2

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

Fig. 3

2 with the circuit diagram of an embodiment of the matching circuit.

In Fig. 1 of the drawing, the right of the interrupted Line A-A circuit components shown a transmitter power supply 10 according to the state of the art for supply a passive transmitter 11 with electrical energy from a DC voltage source 12 via the two conductors 13, 14 of a two-wire connection, via which in the opposite direction the measured value signal generated by the transmitter 11 is transmitted. The two-wire connection 13, 14 is interrupted shown to indicate that they are of arbitrary Length can be. It connects the passive transmitter 11 with two terminals 15, 16 of the transmitter supply unit 10.

The transmitter 11 contains a sensor for the one to be measured physical size and an electronic circuit for Conversion of the sensor signal into the measured value signal to be transmitted. A passive transmitter does not contain its own Energy source, but it relates to the operation of the electronic circuit required energy over the Two-wire connection 13, 14 from the DC voltage source 12 in the remote transmitter supply unit 10. Forms according to a common standard the transmitter 11 the measured value signal in that it off sets the current drawn from the DC voltage source 12 in such a way that the measured value is between 4 mA and 20 mA DC current is expressed. The direct current is through an evaluation circuit arranged at the location of the DC voltage source 12 18 measured and to determine the measured value the physical quantity detected by the transmitter 11 is evaluated. In addition, the transmitter 11 can be designed in this way be that he the current signal digital communication signals superimposed in the form of impulsive changes, so that Measured values and parameters are read and written digitally can be. There is then a requirement for such communication signals bidirectionally between the transmitter 11 and to transmit the evaluation circuit 18.

If the passive transmitter 11 is in a potentially explosive atmosphere Zone is arranged, additional safety precautions must be taken to be hit. A particularly effective protective measure for hazardous areas there is a galvanic one Separation between the transmitter 11 on the one hand and the DC voltage source 12 and the evaluation circuit 18 on the other hand. The transmitter power supply shown in Fig. 1 10 is designed with such a galvanic isolation.

The transmitter power supply is electrically isolated 10 of Fig. 1 by a transformer 20 with a primary winding 21 and a secondary winding 22. Die DC voltage source 12 is between a center tap 23 the primary winding 21 and ground connected. Everyone who two outer connections 24 and 25 of the primary winding 21 via a switch 26 or 27 with one connection 28 a resistor 29 connected to the other terminal Mass lies. The two switches 26 and 27 are through a clock 30 with a relatively high clock frequency controlled by, for example, 200 kHz in push-pull, so that switch 26 is open when switch 27 is closed, and vice versa. Thus the flows from the DC voltage source 12 supplied current in time with the Switch actuation alternately in opposite directions by one or the other half of the primary winding 21, but always in the same direction by the resistor 29. In the primary winding 21 is the DC voltage to a square wave AC voltage chopped, which is transferred to the secondary winding 22. On the secondary winding 22 is a full wave rectifier circuit 31 with four diodes 32 and a filter capacitor 33 connected by rectifying the square wave AC voltage the operating DC voltage for the passive Transmitter 11 generated. It can thus be seen that the Transmitter 20 in connection with that of the switches 26, 27 and the chopper 30 formed chopper and with the Rectifier circuit 31 a DC converter known type forms. The switches 26, 27 that simplified are shown as mechanical switch 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 contains in a hazardous area the rectifier circuit 31 via a fuse 34 connected voltage limiter 35, the Zener diode is shown. Between the output terminals 36, 37 of the Rectifier circuit 31 and for the connection of the passive Transmitter 11 certain terminals 15, 16 of the transmitter power supply protective resistors 38 and 39 are inserted. The protective resistors 38, 39 prevent an increase of the electricity in the hazardous area via a permissible limit, and the voltage limiter 35 limits in connection with fuse 34 the voltage in the potentially explosive Zone to an allowable value.

The passive transducer 11 takes a direct current I _MP from the rectifier circuit 31, the value of which is set in the range from 4 to 20 mA in such a way that it represents the measured value of the physical quantity detected by the sensor. This direct current is supplied via the transformer 20 from the direct voltage source 12, so that with a transmission ratio 1: 1 of the transformer 20 a direct current of the same size flows through the resistor 29. The direct voltage dropping across the resistor 29 is thus proportional to the measuring current I _MP set by the passive measuring transducer 11. It is fed to the evaluation circuit 18 connected to the connection 28.

If the measuring current I _MP is superimposed by the passive transmitter 11 communication signals in the form of pulse-shaped changes, these pulse-shaped changes are also transmitted via the transmitter 20 so that they are expressed 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 lower than the clock frequency of the clock generator 30. The evaluation circuit 18 preferably contains a low-pass filter at the input, the cut-off frequency of which is set so that the clock frequency of the clock generator 30 is suppressed, but the superimposed pulse-shaped communication signals are transmitted.

Fig. 2 shows the schematic diagram of a transmitter supply device 40, which makes it possible to replace the passive transmitter 11 optionally connect an active transmitter 41. In contrast to a passive transmitter an active transmitter contains its own electrical one Power supply, and it gives one of these at the output Voltage supply supplied direct current, its size again in the range of 4 to 20 mA the measured value from the sensor recorded physical size corresponds. It is immediate to recognize that it would not be possible to use the active transmitter 41 simply instead of the passive transmitter 11 to connect the terminals 15, 16 of the circuit arrangement of FIG. 1, because that supplied by the active transmitter 41 DC current could not go through the rectifier circuit 31 and the transmitter 20 to the primary side of the transmitter 20 be transmitted. The transmitter power supply 40 therefore has two further terminals 42 and 43 to which the active transmitter 41 via the two conductors 44 and 45 of a two-wire connection connected.

For simplification, only those on the secondary side are shown in FIG. 2 of the transformer 20 lying circuit components the transmitter power supply 40 shown; the on the Primary circuit components are with those of Fig. 1 identical. So much for the circuit components in FIG. 2 correspond to those of FIG. 1, they are designated by the same reference numerals as in FIG. 1, and they have the same function as before has been described in connection with FIG. 1. It is immediately recognize that for the passive transmitter 11th the same circuit arrangement as in FIG. 1 is present, with the only difference that between the connection 36 the rectifier circuit 31 and the protective resistor 38 a switch 50 is inserted. If the switch 50 in the position is in which he is the rectifier circuit 31 via the protective resistor 38 with the terminal 15 connects, the circuit arrangement is that of Fig. 1 identical.

On the other hand, when the changeover switch 50 is brought into the position shown in FIG. 2, it connects the connection 36 of the rectifier circuit 31 via a connecting conductor 51, an isolating capacitor 52, a protective resistor 53 and a diode 54 to the terminal 42. The connection 37 of the rectifier circuit 31 is permanently connected to terminal 43 via a connecting conductor 55 and a protective resistor 56. As previously explained, the active transducer 41 has its own electrical voltage supply, and it outputs a direct current I _MA at the output, the size of which in the range from 4 to 20 mA corresponds to the measured value of the physical quantity detected by the sensor. Between the active transducer 41 and the rectifier circuit 31, an adaptation circuit 60 is inserted, which draws a direct current I _MS from the rectifier circuit 31, which is equal or proportional to the direct current I _MA supplied by the active transducer 41. The matching circuit 60 contains a resistor 61 connected via the diode 54 to the terminals 42 and 43, a control circuit 62, the input connections of which are connected to the connections of the resistor 61, and a controllable current source 63 connected between the connecting conductors 51 and 55, the control input of which is connected to the output of the control circuit 62. The controllable current source 63 thus bridges the two output connections 36 and 37 of the rectifier circuit 31 when the changeover switch 50 assumes the position shown in FIG. 2, which corresponds 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 such that its output signal adjusts the controllable current source 63 so that the current I _MS taken from the rectifier circuit 31 corresponds to that of the active transmitter 41 supplied current I _MA is proportional to a predetermined constant factor. This factor preferably 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 has the same effect as the current I _MP determined in the other position of the switch 50 by the passive measuring transducer 11: it is mirrored on the primary side of the transmitter 20 and causes a proportional voltage drop across the resistor 29 , This voltage drop is thus proportional to the measuring current I _MA supplied by the active transmitter 41.

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

If one designates the resistance values of the resistors 61, 74, 75 and 76 with R ₆₁ , R ₇₄ , R ₇₅ and R ₇₆ , the following relationship exists between the current I _MA flowing through the resistor 61 and that through the resistor 74 to the input terminal 37 of the rectifier circuit 31 flowing current I _MS : IMS = I MA · R 61 · R 76 R 74 R75

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 the current I _{MS is then} equal to the current I _MA . For example, this applies to the following resistance values:

R ₆₁ = 250 Ω

R ₇₄ = 50 Ω

R ₇₅ = 100 kΩ

R ₇₆ = 20 kΩ

From Figures 2 and 3 it can also be seen that at each position of the switch 50 with respect to the explosive zone, protective measures taken, namely the galvanic isolation by the transformer 20, the voltage limitation by the voltage limiter 35 and the fuse 34 and the current limitation through the protective resistors 38, 39 or through the protective resistors 53, 56 remain fully effective. The isolation capacitor 52 causes a direct current separation of the active transmitter 41 from the rectifier circuit 31 but the transmission of the superimposed communication signals.

The diode 54 is polarized so that it allows the current I _MA supplied by the active transmitter 41 to flow in the forward direction via the resistor 61, but prevents a current flow from the transmitter supply device 40 to the active transmitter 41. Due to the current and voltage limitation already contained in the circuit of FIG. 1, when a passive transmitter is connected, there is sufficient safety for the transmitter / power supply unit because the maximum energy available in the event of a fault is too low to ignite a spark. When connecting an active transmitter, however, the case could arise that a current flowing from the transmitter supply device, which would be too weak in itself to ignite a spark, is superimposed on a current originating from the active transmitter outside the transmitter supply device, so that the The sum of the two currents could be sufficient to ignite a spark. This danger is eliminated by the diode 54 since it prevents a current from flowing from the transmitter supply unit to the active transmitter.

Claims (7)

1. A device for determining a measurement value representing a physical magnitude, which device comprises a measuring-transducer feed appliance (40), through which a direct current (I_MS ) passes and which is suitable for supplying a measuring transducer - in particular arranged in a region at risk from explosion - with electrical energy by way of a two-wire connexion, and a measuring transducer (41), coupled to the measuring-transducer feed appliance (40), characterized in that the measuring transducer (41) is an active measuring transducer which is provided with a separate energy supply and which delivers an output current (I_MA ) representing the measurement value, and in order to connect the active measuring transducer (41) to the measuring-transducer feed appliance (40) an adapter circuit (60) is provided which is connected to the measuring transducer (41) and the measuring-transducer feed appliance (40), in such a way that both the output current (I_MA ) and the direct current (I_MS ) flow through it, and which sets the direct current (I_MS ) flowing in the measuring-transducer feed appliance (40) in such a way whilst using the output current (I_MA ) that the said direct current (I_MS ) is proportional to the output current (I_MA ) of the active measuring transducer (41).
2. A device according to Claim 1, characterized in that a direct-current voltage driving the direct current (I_MS ) is delivered by a source (12) of direct-current voltage, and a d.c. voltage transformer (20, 26, 27, 30, 31), through which the direct current (I_MS ) flows on the secondary side, is inserted into the connexion between the measuring transducer (41) and the source (12) of direct-current voltage for galvanic separation.
3. A device according to Claim 2, characterized in that the d.c. voltage transformer comprises a transmitter (20) from which a primary winding (21) is coupled to the source (12) of direct-current voltage by way of a chopper (26, 27, 30) and from which a secondary winding (22) is connected to a rectifier circuit (31) which at its output terminals delivers the direct current (I_MS ) - produced by rectification of the chopped current transmitted by way of the transmitter (20) - at the magnitude determined by the output current (I_MA).
4. A device according to one of Claims 1 to 3, characterized in that the adapter circuit (60) is arranged in the measuring-transducer feed appliance (40).
5. A device according to Claim 4, characterized in that the measuring-transducer feed appliance (40) comprises a change-over switch (50) which lets the direct current (I_MS ) flow optionally either through a two-wire connexion connected to the measurin-transducer feed appliance (40) or through the adapter circuit (60).
6. A method of measuring a variable physical magnitude, which method comprises the following steps:

   connecting a measuring-transducer feed appliance (40) to an active measuring transducer (41) with the interposition of an adapter circuit (60), wherein the active measuring transducer (41) is provided with a separate energy supply,

   detecting the physical magnitude by means of the active measuring transducer (41),

   producing an output current (I_MA )- flowing through the adapter circuit (60) and proportional to the physical magnitude - by means of the measuring transducer (41) connected to the adapter circuit (60),

   producing a variable direct current (I_MS ) - flowing through the adapter circuit (60) - by means of the measuring-transducer feed appliance (40) connected to the adapter circuit (60),

   regulating the direct current (I_MS ) flowing in the adapter circuit (60) whilst using the output current (I_MA ) flowing through the adapter circuit (60), so that the direct current (I_MS ) is equal or proportional to the output current (I_MA ), and

   measuring the direct current (I_MS ) and determining a measurement value for the physical magnitude detected by the measuring transducer (41).
7. A method according to Claim 6, which comprises the following further steps in order to effect a galvanic separation between the measuring transducer (41) and a source (12) of direct-current voltage driving the direct current (I_MS ):

   chopping the direct current (I_MS ),

   transmitting the chopped direct current (I_MS ) by means of a transmitter (20), and

   rectifying the direct current (I_MS ) transmitted via the transmitter (20).

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