# Patent

## Title
Measuring device for measuring a precess variable in industrial measurements and control technology

## Abstract

A measuring device, comprising: an input circuit having a supply line and a return line. Energy supply occurs over a line-pair. Input voltage is applied to the measuring device via the line pair. A series regulator for controlling an electrical current flowing via the line-pair, a shunt regulator installed in a parallel branch, and an apparatus for controlling a voltage drop across the series regulator are provided.

## Background
TECHNICAL FIELD
The invention relates to a measuring device for measuring a process variable, wherein the measuring device includes a line-pair, namely a supply line and a return line, connectable to a superordinated unit. During operation, energy supply to the measuring device and signal transmission, especially of an output signal representing the process variable, between measuring device and superordinated unit occurs over the line-pair. During operation, an input voltage is applied to the measuring device via the line-pair. The measuring device further includes: an input circuit having a series regulator installed in the supply line for controlling an electrical current flowing via the line-pair, a shunt regulator connected after the series regulator and installed in a parallel branch connecting the supply line with the return line; and a measuring sensor connected to the input circuit and supplied with energy via the input circuit for measuring the process variable.
BACKGROUND DISCUSSION
Measuring devices of this type are applied especially in industrial measurements and control technology, as well as in automation and process control technology for measuring process variables.
As a function of the process variables to be measured, today, a great number of different measuring devices are applied for this, such as e.g. pressure-, temperature-, flow- and/or fill level measuring devices.
These measuring devices deliver an output signal, which corresponds to the measured value of the registered process variable. This output signal is transmitted in an industrial plant to a superordinated unit, e.g. a central control unit, such as e.g. a control room or a process control system, connected to the measuring device. As a rule, the entire process control of the manufacturing- and/or treatment process running in the industrial plant occurs via the superordinated unit, where the output signals of various measuring devices are evaluated and, based on the evaluation, control signals are produced for actuators, which control the process flow.
Preferably, measuring devices are applied, which are connectable to the superordinated unit via a single line-pair, via which the energy supply of the measuring device as well as also the signal transmission between the measuring device and the superordinated unit occurs.
The signal transmission of these devices, which are frequently referred to also as 2-wire measuring devices, occurs preferably according to standards usual in the industry.
According to one of these standards, the signal transmission occurs by setting the electrical current flowing via the line-pair from the measuring device to an electrical current value representing the measured process variable, which is then evaluated by the superordinated unit and correspondingly interpreted. For this, today, the electrical current is regularly controlled to values between 4 mA and 20 mA as a function of the measured process variable. Additionally, a communication signal can be superimposed on this electrical current representing the measured process variable. Thus, the electrical current is modulated about the value predetermined by the process variable in accordance with a predetermined communication protocol. Widely used for this today is the communication protocol defined by the HART standard, in the case of which there is superimposed on the electrical current lying between 4 mA and 20 mA a high-frequency oscillation of +/−0.5 mA carrying the information of the communication signal.
Forming another group are measuring devices connectable to a digital data bus. In the case of these measuring devices, the electrical current flowing through the line-pair is set to a predetermined electrical current value independently of the measured process variable, and there is superimposed on this electrical current the communication signal in the form a high-frequency oscillation. Known standards for this are the PROFIBUS, FOUNDATION FIELDBUS and CAN-BUS standards.
Measuring devices of this type usually have an input circuit with a line-pair composed of a supply line and a return line also connectable to the superordinated unit. During operation, energy supply of the measuring device and signal transmission, especially of an output signal representing the process variable, occurs between measuring device and superordinated unit on this line pair.
Connected to the input circuit is a measuring sensor supplied with energy via the input circuit and serving for determining the process variable and for producing a measurement signal representing the process variable.
The input circuit includes, for example, a series regulator installed in the supply line for controlling an electrical current flowing via the line-pair, and a shunt regulator connected after the series regulator and installed in a parallel branch connecting the supply line with the return line. The measuring sensor is connected to the input circuit behind the parallel branch in parallel with the shunt regulator. The shunt regulator is in the simplest case a Zener diode, via which the voltage applied to the input of the measuring sensor is predetermined.
Available to the measuring device via the line-pair is, as a rule, only a very limited amount of energy, as determined by the applied input voltage and the electrical current set by the series regulator.
Accordingly, today, especially in connection with measuring sensors with high energy requirement, such as e.g. measuring transducers of fill-level measuring devices working with microwaves or with ultrasound, methods are applied for efficient use of the available energy.
To this end, German Patent DE 10 2006 046 243 A1 discloses a method, in the case of which the measuring sensor is operated as needed, and, in times when it is not required, it is switched off or placed in a stand-by mode.
In international published application, WO 02/07124 A1, a measuring device for measuring a process variable is described, in the case of which the measurement activities of the measuring transducer are matched to the power available via the line-pair.
The measuring device includes a line-pair composed of a supply line and a return line connectable to a superordinated unit. During operation, energy supply of the measuring device and signal transmission, especially of a measurement signal, between measuring device and superordinated unit occurs via the line-pair, and an input voltage is applied to the measuring device. A measuring device variant is described, which has an input circuit with an electrical current stage installed in the supply line for controlling the electrical current flowing via the line-pair, as well as having a circuit connected after the electrical current stage and lying in a parallel branch connecting the supply line with the return line. This circuit serves to take exactly sufficient electrical current that the voltage drop across the electrical current stage is as small as possible for lessening the power loss. Correspondingly, the voltage drop across the electrical current stage is reduced to a minimum value required for operating the electrical current stage.
A further example of a measuring device is described in international published application WO 00/75904 A1. In this case, the energy supply and the output of the measured process variable occur via a single line-pair. Also in such case, the measuring device includes an input circuit, via which the thereto connected measuring sensor is supplied with energy. The input circuit includes an electrical current controller installed in the incoming supply line, which controls the electrical current flowing via the line-pair to a value representing the process variable. Behind the electrical current controller, a switching power supply is provided in the line-pair, via which the measuring transducer is fed. Also, in the case of this measuring device, the electrical current draw of the measuring sensor is controlled in such a manner for lessening the power loss that the voltage drop across the electrical current controller is as small as possible. In the case of switching power supplies with an input-side capacitance, abrupt changes of the electrical current value to be controlled could, due to the energy stored in the capacitor, lead to the fact that the output of the electrical current controller lies at a higher potential than its input. In order to prevent this, there is placed before the electrical current controller between the two lines of the line pair a further electrical current controller, which is used exclusively when required for preventing this problem.
The procedure described in the state of the art of holding the voltage drop across the input-side series regulator as small as possible offers the advantage that basically more energy is available for the measuring transducer.
It is, however, also, depending on application, connected with considerable disadvantages. One problem is that these measuring devices are more disturbance sensitive due to the small voltage drop across the series regulator. Thus, e.g. voltage fluctuations of the input voltage, such as can arise e.g. from external disturbances, from digital communication signals superimposed on a bus, or from fluctuations of the energy consumption of other bus participants connected to the same bus, can be cancelled less adequately by the series regulator, the lower the voltage drop across such is.
A further problem is that the input impedance of the measuring device is determined decisively by the voltage drop across the input-side series regulator.
A low input impedance leads, however, in the case of a connection of the measuring device to a bus, to an increased loading of the bus. If there are a number of measuring devices connected parallel to one another on a bus, then the maximum number of measuring devices connectable to the same bus is decisively limited by their input impedances. The lower the input impedances of the measuring devices, the smaller is the maximum number of connectable measuring devices parallel to one another on the same bus.
Moreover, low input impedances lead to an increased attenuation of communication signals superimposed on the electrical current flowing via the line-pair to be received by the measuring device.
SUMMARY OF THE INVENTION
To this end, the invention resides in a measuring device for measuring a process variable and comprising

    an input circuit having
    a line-pair, namely a supply line and a return line, connectable to a superordinated unit, wherein, during operation, energy supply to the measuring device and signal transmission, especially of a output signal representing the process variable, between measuring device and superordinated unit occurs over the line-pair, and wherein, during operation, an input voltage is applied to the measuring device via the line-pair,
    a series regulator installed in the supply line for controlling an electrical current flowing via the line-pair,
    a shunt regulator connected after the series regulator and installed in a parallel branch connecting the supply line with the return line, and
    an apparatus for controlling a voltage drop across the series regulator, wherein the apparatus sets the voltage drop across the series regulator as a function of the applied input voltage for input voltages, which are greater than a predetermined minimum input voltage, to a value predetermined by a characteristic curve for the particular input voltage and lying above a minimum value required for operation of the series regulator, and
    a measuring transducer connected to the input circuit and supplied with energy via the input circuit for measuring the process variable.


In a further development, the characteristic curve is a step shaped curve, which has a hysteresis in the region of at least one of the steps of the step shaped curve.
In an embodiment, the value of the voltage drop predetermined by the characteristic curve for input voltages, which lie above the predetermined minimum input voltage, rises with rising input voltage to a predetermined maximum value.
In an embodiment, the apparatus sets the voltage drop across the series regulator for input voltages, which lie below a predetermined minimum input voltage, to a minimum value required for operation of the series regulator.
In a first form of embodiment, the series regulator controls the electrical current as a function of a measurement signal of the measuring unit representing the measured process variable to a value representing the process variable.
In an embodiment applicable alternatively or supplementally to the first form of embodiment, the series regulator modulates the electrical current as a function of a signal to be transmitted from the measuring device to the superordinated unit.
In a further development, the apparatus for setting the voltage drop includes an intelligent electronics unit, especially a microprocessor or a microcontroller, which, by a corresponding operation of the shunt regulator, sets the voltage drop across the series regulator, based on the input voltage applied to the measuring device and the voltage drop across the series regulator, to the value predetermined by the characteristic curve.
In a first further development of the further development, the measuring device includes a receiving system, via which the characteristic curve can be specified to the intelligent electronics unit.
In a second further development of the further development, the measuring device includes a receiving system, via which the maximum value and/or an input voltage threshold, in the case of which the maximum value is reached, can be specified to the intelligent electronics unit.
In an additional further development,

    a selection of different characteristic curves is stored in the measuring device, and
    the measuring device includes a bidirectional communication system, via which the characteristic curve is selectable from the predetermined selection.


Additionally, the invention includes a measuring arrangement having at least one measuring device according to the first embodiment, in the case of which

    the measuring device is connected via its line-pair to a bus in parallel with at least one additional measuring device, and
    the maximum value is a value predetermined as a function of a number of the measuring devices connected to the bus parallel to one another.


A essential advantage of the invention is that the measuring devices have, due to the lifting of the voltage drop across the series regulator as a function of the input voltage to a value lying above the minimum voltage drop required for its operation, a clearly smaller disturbance susceptibility relative to fluctuations of the input voltage. The measuring device detects, based on the input voltage and the characteristic curve, automatically whether sufficient energy is available for this lifting. As soon as this is the case, a greatest possible insensitivity to disturbances from fluctuations of the input voltage is reached via the characteristic curve automatically for the conditions at the location of use, without that the conditions at the location of use must be known earlier at the factory.
A further advantage is that, via this targeted lifting of the voltage drop matched to the conditions at the location of use, the input impedance of the measuring device is increased. A high input impedance offers the advantage of a smaller attenuation of communication signals transmitted to the measuring device.
Moreover, via the targeted increasing of the voltage drop and therewith the increasing of the input impedance of the measuring device, the opportunity is provided to increase the number of measuring devices connectable parallel to one another to a single bus.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention and other advantages will now be explained in greater detail based on the figures of the drawing presenting an example of an embodiment. Equal elements are provided in the figures with the equal reference characters. The figures of the drawing show as follows:
FIG. 1 is a block diagram of a measuring device of the invention;
FIG. 2 is a characteristic curve for controlling voltage drop across the series regulator of FIG. 1 as a function of the input voltage;
FIG. 3 is a step-shaped characteristic curve;
FIG. 4 is a step-shaped characteristic curve having hysteresis in the regions of steps; and
FIG. 5 shows a number of measuring devices of the inventions connected parallel to one another to a bus.



DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS
FIG. 1 shows a block diagram of a measuring device of the invention for measuring a process variable, such as e.g. a pressure, a flow or a fill level.
The measuring device includes an input circuit 1 and a thereto connected, measuring sensor 3 supplied with energy via the input circuit 1. The measuring sensor 3 is, depending on the process variable to be measured, for example, a pressure measuring sensor, a flow sensor or a fill level measuring unit. The measuring sensor 3 serves for determining the process variable and for producing a measurement signal M representing the process variable. For this, it includes, for example, a measuring electronics 5 and, connected thereto, a measuring transducer 7, e.g. a probe.
The measuring device is connectable via a single line-pair, namely a supply line 9 and a return line 11, to a superordinated unit (not shown). During operation, energy supply of the measuring device and signal transmission, especially of an output signal representing the process variable, occurs between the measuring device and the superordinated unit via the line-pair. In operation, the superordinated unit provides an input voltage Uin across the two terminals A1 and A2 of the input circuit. The input circuit 1 includes, installed in the supply line 9, a series regulator 13 (illustrated here only schematically), which serves to set an electrical current 1, which flows in the line-pair.
This happens preferably according to one of the above described standards.
Thus, the electrical current I flowing in the line-pair can be set by the series regulator 13, based on the measurement signal M supplied to the series regulator 13 from the measuring sensor 3, to an electrical current representing the measured process variable value, preferably an electrical current lying between 4 mA and 20 mA, which then is evaluated and correspondingly interpreted by the superordinated unit. Additionally, there can be superimposed on this electrical current representing the measured process variable value a communication signal Ks supplied to the series regulator 13, for example, likewise by the measuring electronics 5 of the measuring sensor 3. This is done by modulating, in accordance with a predetermined communication protocol, the electrical current value predetermined by the process variable. Preferably used for this is the communication protocol defined by the HART standard, in the case of which there is superimposed on the electrical current lying between 4 mA and 20 mA a high-frequency oscillation of +/−0.5 mA reflecting the communication signal KS.
Alternatively, the electrical current flowing in the line-pair can be set to a predetermined electrical current value, independently of the measured process variable, for connecting the measuring device to a digital data bus, on which a communication signal, in the form a high-frequency oscillation is superimposed, for example, a communication signal KS produced by the measuring electronics 5 of the measuring sensor 3, especially a signal reflecting the measured process variable. Known standards for this are the PROFIBUS, FOUNDATION FIELDBUS and CAN-BUS standards.
In series after the series regulator 13, a shunt regulator 15 is connected in a parallel branch 17 connecting the supply line 9 with the return line. The input voltage Uin corresponds therewith to a sum of a voltage drop ΔU present across the series regulator 13 and a voltage UM falling across the parallel branch 17. Voltage UM corresponds at the same time also to the supply voltage of the measuring sensor 3 arranged parallel to the parallel branch 17, behind the parallel branch 17.
According to the invention, an apparatus 19 for controlling the voltage drop ΔU across the series regulator 13 is provided, which sets the voltage drop ΔU as a function of the applied input voltage Uin to a value predetermined by a characteristic curve ΔUdesired(Uin). The characteristic curve ΔUdesired(Uin) assigns to the possible input voltage values of the input voltage Uin desired values ΔUdesired for the voltage drop ΔU, which is set by the apparatus 19.
For this, the apparatus 19 determines via a first measurement line 21 connected to the supply line 9 before the series regulator 13 the input voltage Uin and via the input voltage Uin and a second measurement line 23 connected to the supply line 9 behind the series regulator 13 the voltage drop ΔU. The control of the voltage drop ΔU across the series regulator 13 occurs preferably via a corresponding operating of the shunt regulator 15, with which the apparatus 19 is connected for this via a control line 25.
The apparatus 19 can be a purely analog control circuit designed corresponding to the characteristic curve ΔUdesired(Uin) and influencing, via a corresponding control of the shunt regulator 15, the voltage divider ratio of series regulator 13 and shunt regulator 15 relative to one another corresponding to the characteristic curve ΔUdesired(Uin) in such a manner that there arises across the series regulator 13 the voltage drop ΔUdesired predetermined for the particular input voltage Uin.
Alternatively, the apparatus 19 for controlling the voltage drop ΔU can comprise—such as shown in FIG. 1—an intelligent electronics unit 27, especially a microprocessor or a microcontroller, which sets the voltage drop ΔU across the series regulator 13 to the value predetermined by the characteristic curve ΔUdesired(Uin) based on the input voltage Uin applied to the measuring device and the voltage drop ΔU across the series regulator 13 by a corresponding operating of the shunt regulator 15. In that case, the characteristic curve ΔUdesired(Uin) is stored in a memory 29 associated with the intelligent electronics unit 27.
The characteristic curve ΔUdesired(Uin) is, for example, the characteristic curve illustrated in FIG. 2, in the case of which the desired value ΔUdesired for the voltage drop ΔU across the series regulator 13 in the case of low input voltages Uin corresponds to a minimum value ΔUmin required for operation of the series regulator 13, and, as soon as the input voltage Uin exceeds a predetermined minimum input voltage Uinmin, rises with rising input voltage Uin to a predetermined maximum value ΔUmax. The slope of the rise of the characteristic curve in the region between the minimum input voltage Uinmin and a predetermined input voltage threshold UinT, in the case of which the maximum value ΔUmax is reached, is preferably matched to the energy requirement of the measuring sensor 3 taking into consideration the form selected for the transmission of the output signal representing the process variable, such as an electrical current value representing the process variable or such as a communication signal superimposed on a fixed electrical current value.
In this way, on the one hand, it is effected that the measuring sensor 3 in the case of low input voltages Uin is fed as much energy as possible, in that here the losses across the series regulator 13 are held as small as possible. As soon as the measuring device, however, has more energy available via an input voltage Uin exceeding the minimum input voltage Uinmin, the voltage drop ΔU across the series regulator 13 is increased above the minimum value ΔUmin, and therewith the initially described disturbance sensitivity of the measuring device relative to fluctuations of the input voltage Uin is clearly reduced.
In this way, the measuring sensor 3 is only fed as much energy as possible—to the detriment of insensitivity to disturbances—, when the measuring sensor 3 also actually absolutely requires such. As soon as this, due to a higher input voltage Uin, is no longer absolutely required, the voltage drop ΔU across the series regulator 13 is automatically increased via the apparatus 19, and therewith the insensitivity to disturbances improves.
FIGS. 3 and 4 show two other examples of embodiments for the characteristic curve ΔUdesired(Uin), via which the voltage drop ΔU across the series regulator 13 is set.
In the example of an embodiment illustrated in FIG. 3, the value ΔUdesired predetermined for the voltage drop ΔU rises above the predetermined minimum input voltage Uinmin as a function of the input voltage Uin in the form of steps to the maximum value ΔUmax.
In the example of an embodiment illustrated in FIG. 4, the characteristic curve ΔUdesired(Uin) above the predetermined minimum input voltage Uinmin has likewise a step shaped rise to the maximum value ΔUmax. In contrast to the variant illustrated in FIG. 3, the characteristic curve ΔUdesired(Uin) here has, however, at least in the region of one of the steps S1, S2, preferably—such as shown here—in the region of all steps S1, S2, a hysteresis of predetermined width ΔUH(S1), ΔUH(S2). The shown characteristic curve ΔUdesired(Uin) includes, rising directly above the minimum input voltage Uinmin, a first step S1, in the case of which the desired value ΔUdesired of the voltage drop ΔU rises from the minimum value ΔUmin to a step of value ΔUS1, and, rising directly below the input voltage threshold UinT, a second step S2, in the case of which the desired value ΔUdesired of the voltage drop ΔU rises from the step value ΔUS1 to the maximum value ΔUmax. The hystereses mean that the increasing of the voltage drop ΔU in the case of increasing input voltages Uin occurs at the respective steps S1, S2 at higher input voltages Uin, than its lowering in the case of lessening input voltages Uin. In the illustrated example, the desired value ΔUdesired of the voltage drop ΔU in the case of rising input voltage Uin accordingly rises at the input voltage Uinmin+ΔUH(S1) to the step value ΔUS1, while the desired value ΔUdesired in the case of sinking input voltage Uin falls at the minimum input voltage Uinmin from the step value ΔUS1 to the minimum value ΔUmin. Correspondingly, the desired value ΔUdesired of the voltage drop ΔU in the region the second step S2 in the case of rising input voltage Uin rises only at the input voltage threshold UinT to the maximum value ΔUmax, while the desired value ΔUdesired in the case of sinking input voltage Uin, falls from the maximum value Umax to the step value ΔUS at the input voltage UinT-ΔUH(S2) lying below the input voltage threshold UinT by the hysteresis width ΔUH(S2).
The hysteresis offers the advantage that a time stable voltage drop ΔU can be set, even when the input voltage Uin fluctuates slightly around the threshold value of the respective step S1, S2, here the minimum input voltage Uinmin in and the input voltage threshold UinT. This is especially advantageous when superimposed on the electrical current flowing in the line-pair are communication signals KS or KE generated by the measuring device or sent to the measuring device, since this can, in given circumstances, bring about small fluctuations of the input voltage Uin.
While in measuring devices with purely analog apparatus for setting the voltage drop ΔU across the series regulator 13 only a fixedly predetermined characteristic curve ΔUdesired(Uin) can be implemented, the apparatus 19 shown here offers, due to the intelligent electronics unit 27, the opportunity to provide different characteristic curves ΔUdesired(Uin) matched to particular fields of use for the measuring device. For this, the particular application-specific characteristic curve ΔUdesired(Uin) can be stored in the memory 29 at the factory. Alternatively, interfaces of the measuring device present, as a rule, in any event can be utilized for implementing the respective application-specific characteristic curves ΔUdesired(Uin). Thus, for example, a number of different characteristic curves ΔUdesired(Uin), for example, distinguished by the rise of the desired value ΔUdesired for the voltage drop ΔU, the input voltage threshold UinT and/or the maximum value ΔUmax, can be stored in the memory 29, from which the user selects, in the context of start-up of the device, via an interface enabling bidirectional communication, such as e.g. an user interface, the characteristic curve ΔUdesired(Uin) best suitable for its application. Alternatively, also only the maximum value ΔUmax and/or the input voltage threshold UinT can be selected application-specifically from a predetermined selection or specified by the user.
In the case of measuring devices, which—such as presented here—have a receiving system 31 for the receipt of communication signals KE superimposed on the electrical current I flowing in the line-pair, the specification of an application-specific characteristic curve ΔUdesired(Uin), an application-specific maximum value ΔUmax and/or an application-specific input voltage threshold UinT can also occur via the superordinated unit connected to the line-pair. The receiving system 31 is, for example, a modem, to which the communication signal KE is fed via a capacitor C connected to the supply line 9 on the input side before the series regulator 13. For this, the relevant communication signals KE can be fed in the measuring device either directly to the apparatus 19 for controlling the voltage drop ΔU, or first processed in an additional circuit, for example, a circuit integrated in the measuring electronics 5, from which then the required information is fed to the intelligent unit 27.
In connection with measuring devices having a receiving system 31, the increasing the voltage drop ΔU across the series regulator 13 purposely performed as a function of the available input voltage Uin offers the advantage that the entering communication signals KE are attenuated less, the higher the voltage drop ΔU across the series regulator 13 is.
In the case of measuring devices—such as the measuring device shown here—which include a bidirectional communication system for communication over the line-pair, the user can naturally likewise select via the superordinated unit from a predetermined selection of characteristic curves ΔUdesired(Uin) stored in the measuring device, input voltage threshold values UinT and/or maximum values ΔUmax.
FIG. 5 shows a measuring arrangement in which a measuring device of the invention is connected via its line-pair to a bus 33 in parallel with additional measuring devices, in the example of an embodiment, measuring devices likewise of the invention. In these applications, the invention offers the advantage that the maximum value ΔUmax for the voltage drop ΔU can be predetermined as a function of the number of measuring devices connected, or to be connected, parallel to one another on the bus 33. In such case, the increasing of the voltage drop ΔU effects an increasing of the input impedance of the respective measuring device and leads as a consequence to a smaller loading of the bus 33 by the particular measuring device. Correspondingly, more measuring devices can be connected to the bus 33, as a function of how much higher their input impedances are.
Due to the setting of the voltage drop ΔU across the series regulator 13 controlled via the characteristic curve ΔUdesired(Uin), the measuring device can now be applied extremely flexibly and offers in each application the maximum achievable insensitivity to disturbances for the input voltage ratios present there.

## Claims

1. A measuring device for measuring a process variable, comprising:
an input circuit, having:
a line-pair, namely a supply line and a return line, connectable to a superordinated unit, wherein, during operation, energy supply to the measuring device and signal transmission, of an output signal representing the process variable, between the measuring device and the superordinated unit occurs over said line-pair, and wherein, during operation, an input voltage is applied to the measuring device via said line-pair;
a series regulator installed in said supply line for controlling an electrical current flowing via said line-pair;
a shunt regulator connected after said series regulator and installed in a parallel branch connecting said supply line with said return line; and
an apparatus for controlling a voltage drop across said series regulator, wherein:
said apparatus sets the voltage drop across said series regulator as a function of the applied input voltage for input voltages, which are greater than a predetermined minimum input voltage, to a value predetermined by a characteristic curve for a particular input voltage and lying above a minimum value required for operation of said series regulator;

a measuring sensor connected to said input circuit and supplied with energy via said input circuit for measuring the process variable; and
the predetermined value of the voltage drop for input voltages, which lie above the predetermined minimum input voltage, rises with rising input voltage to a predetermined maximum value.

2. The measuring device as claimed in claim 1, wherein:
the characteristic curve has a step shaped curve, which has a hysteresis in the region of at least one of the steps of the step shaped curve.

3. The measuring device as claimed in claim 1, wherein:
said apparatus sets the voltage drop across said series regulator for input voltages, which lie below said predetermined minimum input voltage, to a minimum value required for operation of said series regulator.

4. The measuring device as claimed in claim 1, wherein:
said series regulator controls the electrical current as a function of a measurement signal, said measuring sensor representing the measured process variable to a value representing the process variable.

5. The measuring device as claimed in claim 1, wherein:
said series regulator modulates the electrical current as a function of a signal to be transmitted from the measuring device to the superordinated unit.

6. The measuring device as claimed in claim 1, wherein:
said apparatus for setting the voltage drop includes a microprocessor or a microcontroller intelligent electronics unit, which, by a corresponding operation of said shunt regulator, sets the voltage drop across said series regulator, based on the input voltage applied to the measuring device and the voltage drop across the series regulator, to the value predetermined by the characteristic curve.

7. The measuring device as claimed in claim 6, further comprising:
a receiving system, via which the characteristic curve is specified to said intelligent electronics unit.

8. The measuring device as claimed in claim 6, further comprising:
a receiving system, via which the maximum value and/or an input voltage threshold, at which the maximum value is reached, is specified to said intelligent electronics unit.

9. The measuring device as claimed in claim 6, wherein:
a selection of different characteristic curves is stored in the measuring device; and
the measuring device includes a bidirectional communication system, via which the characteristic curve is selectable from the predetermined selection.

10. A measuring arrangement having at least one measuring device with an input circuit having:
a line-pair, namely a supply line and a return line, connectable to a superordinated unit, wherein, during operation, energy supply to the measuring device and signal transmission, of an output signal representing the process variable, between the measuring device and the superordinated unit occurs over said line-pair, and wherein, during operation, an input voltage is applied to the measuring device via said line-pair;
a series regulator installed in said supply line for controlling an electrical current flowing via said line-pair;
a shunt regulator connected after said series regulator and installed in a parallel branch connecting said supply line with said return line; and
an apparatus for controlling a voltage drop across said series regulator, wherein said apparatus sets the voltage drop across said series regulator as a function of the applied input voltage for input voltages, which are greater than a predetermined minimum input voltage, to a value predetermined by a characteristic curve for the particular input voltage and lying above a minimum value required for operation of said series regulator; and
a measuring sensor connected to said input circuit and supplied with energy via said input circuit for measuring the process variable, wherein:
said measuring device is connected via its line-pair to a bus in parallel with at least one additional measuring device; and
a maximum value voltage is a value predetermined as a function of a number of the measuring devices connected to the bus parallel to one another.

11. A measuring device for measuring a process variable, comprising:
an input circuit, having:
a line-pair, namely a supply line and a return line, connectable to a superordinated unit, wherein, during operation, energy supply to the measuring device and signal transmission, of an output signal representing the process variable, between the measuring device and the superordinated unit occurs over said line-pair, and wherein, during operation, an input voltage is applied to the measuring device via said line-pair;
a series regulator installed in said supply line for controlling an electrical current flowing via said line-pair;
a shunt regulator connected after said series regulator and installed in a parallel branch connecting said supply line with said return line; and
an apparatus for controlling a voltage drop across said series regulator, wherein:
said apparatus sets the voltage drop across said series regulator as a function of the applied input voltage for input voltages, which are greater than a predetermined minimum input voltage, to a value predetermined by a characteristic curve for a particular input voltage and lying above a minimum value required for operation of said series regulator;
a measuring sensor connected to said input circuit and supplied with energy via said input circuit for measuring the process variable; and
the characteristic curve has a step shaped curve.

12. The measuring device as claimed in claim 11, wherein:
said step shaped curve has a hysteresis in the region of at least one of the steps of said step shaped curve.