Arrangement for signal transmission in a measuring arrangement.

p0001The invention relates to an arrangement for signal transmission in a measuring arrangement with a transmitter, which is connected to a remote therefrom arranged evaluation unit through a two-wire line, on the one hand, required for the operation of the transmitter DC power on the evaluation unit to the transmitter and on the other hand, the measured variable representing measured value signal from transmitters are transmitted to the evaluation unit, the transmitter is connected to the two-wire line via a transmitter interface, which removes the DC power required by the transmitter from the two-wire line and applies the measured value signal to the two-wire line, and wobie the evaluation with the two-wire line via an evaluation interface is connected, which is designed for applying the DC supply voltage to the two-wire line and for receiving the over the two-wire line transmitted measuring value, and at least one communication unit that can be connected in parallel via a communication interface to the transmitter to the two-wire line, wherein in the transmitter interface , in the evaluation interface and in each communications interface in each case a communication interface circuit is provided that includes a signal transmitter for transmitting a distinguishable from the measured value signal communication signal over the two-wire line and a signal receiver for receiving the next of other communication interface circuits communication signals.

p0002Measuring arrangements are those in which the transmitter and the processing unit separately and connected only by a two-wire line, are very widely used. Through the communication unit, which is also connected to the two-wire line and send and receive communication signals over the two-wire line can, it is isbesondere possible at the location of transducer order from another point of balancing, adjustment, inspection or maintenance work taking advantage of processing unit to perform. The communication unit is for example a calculator-like device with a keyboard and a numeric or alphanumeric display. By operating the keyboard, the operator can retrieve the information needed by the evaluation unit, and the information submitted in response to the query from the evaluation can be made visible on the display of the communication unit. The transferred for this information exchange communication signals are pulse trains that are going through the two-wire line and modulated according to information to be transmitted. There is the problem that the measured value signal may be affected or disturbed by the communication signals. This is particularly true, when the power for the communication signals of the same contained in the evaluation unit power source is removed, which also supplies the DC power for the transmitters and the energy for the measured value signals, as is the case with the commonly used measuring arrangements, in which the measured value signal is formed by a variable 4-20 mA DC current, which also contains the dc supply current for the transmitter. Another limitation of known arrangements of this kind exists in the limited communication options; usually the communication units can only with a remote site, for example, to the evaluation, exchange information.

p0003The object of the invention is to provide an arrangement of the type described, in which as many connected to the two-wire line subscriber stations, including the transmitter and the evaluation include, can exchange communication signals, without prejudice to the simultaneous transmission of the measured value signal over the same two-wire cable is disturbed or harmed.

p0004According to the invention this object is achieved in that the signal transmitter of each communication interface circuit is configured to pulse-like reduction of the supply direct voltage on the two-wire line in accordance with a communication signal representing the pulse modulation, and that the signal receiver each communications interface circuit to the pulse-shaped voltage variations on the two-wire cable responsive.

p0005In the signal transmission device according to the invention, the signal transmitter affects each communication interface circuit in pulses applied by the evaluation of the Zweidrahleitung voltage, and thus appearing on the two-wire line pulse voltage changes can be received by the signal receivers of all communications interface circuits. Thus, there is no limitation to the number and type of devices connected to the two-wire line portion Nahm Create. In particular, a communication unit not only with the transmitter or the evaluation device contact, but also with other devices connected to the same two-wire line communication units. It is particularly advantageous that also the transmitter and the processing unit, to exchange communication signals in both directions regardless of the presence of a communication unit, in addition to the transmission of the measured value signal yet. This allows the transmitter to the evaluation unit to provide additional information that can be used for the evaluation of the measured value signal and for monitoring the operation of the transmitter, and the evaluation can affect the operation of the transmitter by control signals.

p0006Since all communication signals are pulse-like voltage changes, they can be clearly distinguished from the measured value signal when the measured value signal is formed by a current signal, as is the case in particular with the standardized measurement value signal in the form of a variable 4-20 mA DC current. The transmission of the measured value signal can therefore be carried out without interference or disturbance at the same time with the transmission of communication signals.

p0007Finally, the pulse voltage changes on the two-wire line can be generated and received by very simple and reliable circuits. The amplitude of the pulse voltage changes can be very large, creating a very high interference immunity of signal transmission is achieved. In the limiting case the voltage on the two-wire line at each voltage pulse can be brought to zero by full short-circuit.

p0008Advantageous refinements and developments of the invention are characterized in the dependent claims.

p0009Further features and advantages of the invention will become apparent from the following description of an embodiment with reference to the drawing. In the drawings:<ul><li>Fig. 1 shows the principle scheme of a measuring arrangement, in which the invention is applicable,</li><li>Fig. 2. is a circuit diagram of the three interfaces of the measuring arrangement of Fig. 1 in greater detail, and</li><li>Fig. 3 shows diagrams of the time course of signals.</li></ul>

p0010FIG. 1 shows a measuring arrangement with a transducer 10 which is connected through a two-wire line 11 with a distance of arranged evaluator 12th The transmitter 10 includes a sensor 13 for detecting the measured physical variable (eg temperature, pressure, humidity, level) and one connected to the sensor 13 electronic transducer 14 which emits an electrical signal representing a the instantaneous value of the measured variable. The transmitter 10 does not contain its own power source, but includes the necessary for its operation Gleichstomenergie via the two-wire line 11 from a requirement in the evaluation unit 12 voltage source 15 via the same two-wire line is transmitted to the instantaneous value of the measured variable representing measured value signal from the transducer 10 to the evaluation unit 12th The transducer 10 is connected to the two-wire line 11 via a transmitter interface 16, on the one hand ensures the Energeiversorgung of Meßumsetzers 10 from the two-wire line 11 and on the other hand converts the output of the transducer 14 in a for transmission via the two-wire line 11 suitable measured value signal. accordingly One common technique is the measurement value signal via the two-wire line 11 flowing direct current I_MWhich extends from the supply direct current I₀ the transmitter and a correction current_K composed. The correction current I_K is also the voltage source 15 and removed from the transmitter 10 taking into account the size of the supply DC current I₀ adjusted so that the total current I_M between the current values ​​4 and 20 mA to be transmitted represents the measured value. Finally, the transmitter 10 contains a communication electronics 17, which is also connected via the transmitter interface 16 with the two-wire line.

p0011To connect the interface device 12 with the two-wire line 11 an evaluation interface 18 that causes the one hand, the transmission of information required by transducer 10 DC power from the power source 15 to the two-wire line 11, and other part of the over the two-wire line 11 flowing is total current I_M a suitable for the display of the measured value or for further processing signal derives. The evaluation unit 12 also entchält a communication electronics 19, which is connected via the Asuwerte interface 18 with the two-wire line 11th

p0012In Fig. 1, a communication unit 20 is also shown, which is parallel to the transmitter 10 connected to the two-wire line 11 and designed so that it can carry out an exchange of information with the transmitter 10 or the evaluation unit 12, without the normal operation of the measuring arrangement characterized is impaired. The communication unit 20 is a calculator-like device with a keyboard 21 and a digital display 22 and with the requisite electronics for signal processing. The connection with the two-wire line 11 via a communication inter-face 23 and a two-wire connection line 24 which can be clamped by means of terminal Strangulated 25,26 as required to the two-wire line 11th

p0013In the described embodiment, it is assumed that the communication unit 20 is equipped with its own power source (eg battery). However, it would also be possible to take the required power supply to the DC Komunikationseinheit also the voltage source 15 in the evaluation unit 12 via the two-wire line.

p0014Fig. Figure 2 shows the diagrams of the three interfaces 16, 18 and 23 of FIG. 1 in nähreren details.

p0015In the transmitter interface 16, the voltage of the two-wire line 11 via a diode 27 and a constant current generator 28 is applied to the terminals of a capacitor 29, the Zener diode 30 is connected in parallel. The Zener diode 30 bestimmmt the operating voltage U_B for the electronics of the transmitter. The capacitor 29 serves as an energy store, which bridges the power supply at a voltage decrease or a short circuit on the two-wire line eleventh The diode 27 prevents that, in this case, the capacitor 29 is discharged via the two-wire line eleventh

p0016To produce a measured value representing the measured current I_M contains the transmitter interface 16 a shunt branch 31 containing a controllable constant current generator 32nd Via the shunt path 31 flows a continuous direct current, which is also taken from the voltage source 15 and to the two wire line 11 to the supply direct current I₀ superimposed. The constant current generator 32 is controlled by a continuously variable output signal of the transducer 14 so that the current flowing through the shunt branch 31 DC the correction current I_K forms, together with the Versorgungsgelichstrom I₀ the variable 4-20 mA measuring current I_M forms.

p0017The constant current generator 28 supplies the constant supply DC I₀ for the electronics of the transmitter. The determined by constant current generator 28 current is controlled by the Konstantsromgenerator 32 or is fixed at the current value I₀ set. The constant current generators 28 and 32 can be combined into one unit. Instead of the constant current generator 28 may also include a voltage regulator.

p0018In the evaluation interface 18 is in the head of a two-wire line 11, a resistor 33 is inserted, through which the measuring current I_M= I₀+ I_K flows. can at resistor 33 thus a voltage are tapped that the measuring current I_M proportional and contains the measured value information. This voltage can be used to display the measured value or processed in any manner for evaluating the measured value information. In the simplest case it would also be possible to insert a suitably calibrated flow meter instead of the resistor 33 in the two-wire cable, which would then directly display the measured value.

p0019The parts of the transmitter interface 16 and the evaluation interface 18 described so far correspond to a normal formation of measuring units with two-wire connection, in which the measured value information changeable by a zwsichen 4 and 20 mA DC current is transmitted.

p0020The communication interface 23 of the communication unit 20 consists essentially of a connected to the electronics 34 of the communication unit communication interface circuit 40. A communication interface circuit 50 completely the same kind is arranged in addition to the above-described conventional ingredients in the transmitter interface 16, and the evaluation interface 18 includes a further communication interface circuit 60 of the same type. Since the three KommunicationsSchnittstellenschaltungen 40, 50 and 60 have the same structure and the same function, the following description of the communication interface circuit 40 also applies to the other two communication interface circuits 50 and 60th

p0021The communication interface circuit 40 includes a signal generator 41 and a signal receiver 42, which are connected via the connection line 24 with the two-wire line eleventh Similarly, the communication interface circuit 10 includes 50 of the transmitter a signal generator 51 and a signal receiver 52, and the Kommunications interface circuit 60 of the processing unit 12 includes a signal generator 61 and a signal receiver 62nd

p0022The signal generator 41 consists essentially of a controlled shunt branch 43, which bridges the connection line 24 and thus also the two-wire line eleventh In the shunt path 43, a switch, a switching transistor is in the illustrated example is 44. To the base of the switching transistor 44, a control signal is applied from the electronics 34, which is pulse-modulated according to the information to be transmitted.

p0023When the switching transistor 44 of the signal generator 41 is energized, the two-wire line 11 is virtually shorted, and the shunt branch 43, a current flows, which is taken from the voltage source 15th For this short-circuit current is limited to a permissible value, a current limiter 35 is inserted in series with the voltage source 15 in the two-wire line 11 in the evaluation interface 18th The current limiter 35 may be formed in the simplest case by a sufficiently high resistance, but is a controlled constant current generator used in a later-explained reason, in the illustrated embodiment as a current limiter.

p0024As a result of the short breaks the voltage U on the two-wire line 11 together so that they rest from the value U₀ goes to 0. Each current pulse generated by the signal generator 41 thus corresponds to a negative-going voltage pulse on the two-wire line 11 shown 3 in the diagram A of Fig., And the result of this voltage pulse is designed by the switching transistor 44 supplied control signal.

p0025Instead of the two-wire line 11 is completely short-circuited by the switching transistor 44, it is also possible to arrange a resistor in series with the switching transistor 44 is 45, as shown in phantom in Fig. 2. The voltage on the two-wire line 11 then drops at each current pulse from the quiescent value U₀ to a pre-defined voltage value U₁ from, which is different from 0 as the plot B of Fig. 3. Fig.

p0026The signal receiver 42 of the communication interface circuit 40 is designed such that it responds to negative-going pulse-like voltage changes the dargestaellten in the diagrams A and B of FIG. 3 type. It comprises, in the illustrated embodiment has a Schmitt trigger 46, the voltage of the two-wire line 11 via a zugefürht consisting of a capacitor 47 and a resistor 48 RC element. The Schmitt trigger 46 thus responds only to pulse-shaped voltage variations on the two-wire line and converts it to a digital signal which is fed to the electronics 34th

p0027Similarly, the signal transmitter 51 generates in the communication interface circuit 50 of the transducer 10 under control by the communication electronics 17 to the two wire line 11, negative-going pulse-like voltage changes on responsive to the signal receiver 42 in the communication unit 20 and the signal receiver 62 in the evaluation unit 12 , and the signal generator 61 in the communication interface circuit 60 of the processing unit 12 generates, under control of the communication electronics 19 to the two wire line 11, negative-going pulse-like voltage changes, to which address the signal receiver 42, 52 of the two other communication interface circuits 40, 50th

p0028To follow all can connect with each other and exchange information on the two-wire cable connected to the two-wire line 11 units at will. All of communication interface circuits are formed in an identical manner, and each interface may receive the communication signals transmitted from any other interface. After a technique known per se is achieved by appropriate coded address signals that each subscriber station evaluates only the information intended for them. All interface to generate the communication signals, the same power source, namely voltage source 15 in the evaluation unit, and generate the communication signals in the same manner, namely, by pulse-like reduction in the voltage on the two-wire line. The use of voltage pulses as communication signals allows a clear and unambiguous differentiation of the measured value information, which is represented by a current value. Represented by the communication signals to be transmitted information (data), for example, may be included in the repetition frequency of the voltage pulses (pulse frequency modulation), in the encoded pulse trains (pulse code modulation), or in a combination of these two types of modulation.

p0029The number of subscriber stations that can contact each other in this way is not limited. It is readily possible to clamp a plurality of communication units by way of communication unit 20 at the same time to the two-wire line 11th All communication units can then use the transmitter 10, the evaluation unit 12 and exchange information with each other. The communication units make it possible in particular to carry out of any point of balancing, adjustment or inspection work without the normal operation of the measuring arrangement being affected. In addition, it is possible to affect the operation of the transmitter by the control signals coming from the evaluation in the form of communications signals, and the transmitter can be used to provide evaluation data signals that allow the evaluation to monitor the operation of the transmitter. These control and data signals can be received by each communication unit, which is clamped to the two-wire line 11th

p0030The existing in the transmitter interface 16 capacitor 29, which acts as an energy store, prevented in conjunction with the diode 27, the short pulse voltage changes on the two-wire line 11 interfere with the operation of the transducer 10th

p0031In some cases it may be regarded as a disadvantage that the two-wire line via the transmitted measuring current I_M is interrupted during each negative voltage pulse of a communication signal. To avoid this phenomenon is in the evaluation interface 18 of FIG. 2, a current value storage ( "sample &amp; hold") 36 is provided, which continuously monitor the instantaneous value of the measuring current I_M stores, flowing via the two-wire line 11, when no communication signals are present. To determine the value of the measuring current I_M is inserted, a resistor 37 in the line and the tapped off at the resistor 37 voltage is supplied to the current value storage 36th The output of the instantaneous value memory 36 is connected via a controlled by the output of the signal receiver 62 of the communication interface circuit 60 switch 38 to an actuator 39, which influences the setting of the current limiter 35 forming the constant current generator. The switch 38 is closed at each reception of a pulse voltage, and the actuator 39 then sets the determined by the current limiter 35 current value to the current value stored in the Momentantwertspeicher 36. Thus, during each voltage pulse across the resistor 33 the same current flows as before the arrival of the voltage pulse. Since the measuring current I_M changes only slowly, as a rule, the display is not distorted significantly by the temporary bridges.

p0032The signal transmission described with reference to FIG. 2 requires a minimum of components for the signal generation and signal reception. Furthermore, it is to be expected because of the high signal level with relatively low susceptibility to faults. Finally, the signal transmission is particularly advantageous for use in hazardous environments by reducing the normal voltage where signal transmission by pulse voltage increases can cause problems.