DE102005001601A1 - Field device has sensor for detecting, an actuator for influencing and chemical or physical process parameter, field device also comprises bus interface configured to external field bus system - Google Patents

Abstract

Field device (1) has a sensor (18) for detecting, an actuator (19) for influencing, and at least a chemical or physical process parameter (P). The field device also comprises a bus interface (3) configured to an external field bus system (2) as double-wire lead (4) as well as a supply circuit and operating circuit (5) to communicate with bus interface. The supply of the field device takes place over the double-wire lead at least, temporarily with a mixed stream, supplied from bus interface, through an overlay or a modulation of alternating component formed on a direct component of mixed stream. The field device sends or receives data over the double wire line to the external field bus system. The alternating component of mixed stream is used for the supply of the field device with electricity. The field device covers its power requirement, at least partly, over the alternating component of mixed stream.

Description

The The invention relates to a field device with at least one sensor for detecting and / or with at least one actuator for influencing at least one chemical and / or physical process parameter, which field device Furthermore, a bus interface to an external fieldbus system, the is designed as a two-wire cable, as well as one with the bus interface communicating supply and operating circuit includes.

The Advances in microelectronics in recent years have too a miniaturization of devices and integration of functionalities led that in automation technology an effective and cost-effective application of integrated distributed systems. So be in the Sensors and actuators not only determines the measured values, but already preprocessed the readings, linearized and even one Self-diagnostics of the sensor or actuator implemented. The condition for the Implementation of these decentralized functionalities in a closed automation concept with "intelligent" sensors and actuators is an elevated one Information and data exchange between these decentralized units and with a control system. In automation technology are off For this reason, a wealth of fieldbus systems have emerged in recent years, either company-specific applications (e.g., BITBUS, CAN, MODBUS, RACKBUS) or those of an international Standardization (e.g., HART, PROFIBUS-PA, Foundation FIELDBUS, Ethernet) subject. The big number of fieldbus systems, currently in industrial automation and process control technology are still only with the general term "fieldbus or field bus system ".

The usual field devices are mains-powered four-wire measuring instruments and must have at least two electrical Supply lines or conductors for power supply of the field device have. Furthermore, two more signal lines are required, which is the Measured value measuring signal or other communication data signals transferred between the decentralized units and the control center. Generally, this measurement signal or communication data signal according to a customary standard, e.g. according to the current loop standard between 4 mA and 20 mA, a usual Frequency standard or a digital standard, generated and transmitted.

Furthermore It is also common in automation technology, the field devices in one to build and connect so-called two-wire technology, so that the energy supply and the communication between the devices over one single pair of leads can be made, reducing the wiring effort and thus the wiring costs of networked decentralized automation systems can be reduced.

Such two-wire field devices are among other things the writings EP 0 883 097 B1 . EP 0 895 209 B1 . EP 1 158 274 A1 refer to. These two-wire field devices generate as measurement signal an output current whose instantaneous values represent as much as possible a transducer signal generated by means of a physical-electrical measuring transducer. The two conductors are used both for the controlled and regulated energy supply, for which purpose a DC voltage source is to be applied to the two conductors from the outside, as well as for the transmission of the measuring signal.

Another aspect which has to be taken into account in automation technology and especially in process automation technology is that the operating capability of the field device in an explosive environment in which the intrinsic safety of the field device and the fieldbus is maintained. Such intrinsically safe, low-reflection fieldbus systems are from the DE 100 49 233 A1 . DE 101 27 561 A1 . DE 102 45 273 A1 , and the cis energy supply concept of the Physikalisch-Technische Bundesanstalt Braunschweig, which deals with the problem of AC or AC signals on field buses. Common to all these documents is that the energy transmission via the fieldbus is effected solely by a high-frequency alternating signal and this fieldbus is protected against intrinsic safety by protective measures and / or is embodied in anechoic reflection by adapting the characteristic impedance.

outgoing The above-mentioned prior art therefore has an object the invention therein, the communication of the field device via a two-wire line to enable and further the field device via them Two-wire line steady with the necessary energy that meets the standards and regulations of explosion protection.

To achieve the object, the invention consists in a field device with at least one sensor for detecting and / or with at least one actuator for influencing at least one chemical and / or physical process parameter, which field device further comprises: a bus interface to an external fieldbus system, which is a two-wire Line is designed, as well as a communicating with the bus interface supply and Be drive circuit, wherein the supply of the field device via the two-wire line at least temporarily takes place with a supplied from the bus interface mixed stream, which is formed by a superposition and / or a modulation of an AC component to a DC component of the mixed stream, wherein the field device via the two-wire line Sends data to the external field bus system and / or receives data from the external field bus system, wherein at least the alternating component of the mixed current for supplying the field device with electrical energy, and wherein the field device at least partially covers its energy demand on the alternating component of the mixed stream.

According to one advantageous embodiment of the solution according to the invention proposed that the DC component of the mixed stream as an information carrier or Means of communication for transmitted via two-wire line Data serves.

A expedient embodiment consists of the field device according to the invention in that the DC component of the mixed stream values within a current range from 4 mA to 20 mA. For two-wire field devices accordingly the mentioned, current loop standard between 4 mA and 20 mA a certain current value within this current range is exactly one electrical Sensor signal or actuating signal. Therefore, only the current range can of 4 mA for the ensured Power supply to the electronics of the field device with a two-wire cable planned and used. This requires that the power supply only with a power of the order of magnitude of 50 mW available hereinafter referred to as low power. Two-wire field devices with the mentioned, Current loop standards lying between 4 mA and 20 mA are due to these facts especially suitable for use in potentially explosive atmospheres Environments.

A advantageous embodiment of the solution according to the invention suggests that the mixed stream at least one RMS value that is less than or equal to one highest permissible limit value is.

According to one Great Embodiment of the field device according to the invention is suggested that the highest allowed Limit value of the effective value of the mixed current is 20 mA accepts. In order not to violate the regulations of the explosion protection the maximum effective value of the mixed flow should not be greater than be the predetermined limit.

A Advantageous embodiment of the solution according to the invention is that the effective value of the modulated alternating component of the mixed flow one if necessary existing difference value of the DC component of the mixed flow to highest permissible Limit value of the mixed stream. So that the field device always with the maximum allowable current can be supplied, the difference of the DC component of the mixed stream by the effective value of, for example, the rectified alternating component the mixed flow is supplemented, so that their sum is the maximum permissible limit taking. This makes the field device steady with the maximum Energy or power supplied.

According to one advantageous embodiment of the field device according to the invention it is proposed that a negative rms value of the modulated alternating component of the Mixed current excess energy of the field device derives and thus regulates the energy demand or energy balance of the field device.

Becomes a negative RMS value of the AC component at the basis of the DC component generated in the AC / DC converter, in which, for example, by a certain Conversion of the alternating component of the mixed flow, a negative rectified alternating component the mixed flow is generated, can excess power from the field device be derived.

A very advantageous variant of the solution according to the invention can be seen in that the alternating component of the mixed flow has a sinusoidal waveform. Preferably, the alternating component of the mixed current is sinusoidal signals used because their signal spectrum has no harmonics. whereby prescriptions regarding the "electromagnetic compatibility (EMC) "easier to let. Also a superposition with other DC or DC signals is easier, because the time average of sinusoidal waveforms is zero and the coding thus means-free.

A Particularly advantageous development of the solution according to the invention proposes that the alternating component of the mixed flow is a rectangular and / or has pulsed waveform.

A advantageous embodiment of the solution according to the invention suggests that a controlled AC source generates the alternating component of the mixed stream.

A very advantageous variant of the solution according to the invention can be seen in that a switching unit generates the alternating component of the mixed stream.

A preferred embodiment of the inventions According to the invention field device provides that a control and control unit is integrated in the supply and operating circuit, which is used to control the switching unit based on the current and future energy needs or energy balance of the field device and / or further based on the current and future electrical sensor signal or control signal the field device for controlling and regulating the current controller, which takes over the adjustment of the DC component of the mixed stream as an information carrier, is used.

According to one advantageous embodiment of the solution according to the invention suggested that the control and control unit the future energy needs of the field device considering and estimate the future running processes and applications are calculated and calculated.

One expedient embodiment consists of the field device according to the invention in that at least one of a chemical and / or physical Process parameter sensor-sensing transducer is provided which provides an electrical sensor signal.

A very advantageous variant of the solution according to the invention can be seen in that at least one controlled by an electrical control signal Signal converter is provided, the at least one chemical and / or physically adjusts physical process parameters.

One expedient alternative embodiment consists of the field device according to the invention in that as an integral part of the supply and operating circuit at least one AC / DC converter and at least one DC / DC converter is provided is.

A advantageous embodiment the solution according to the invention consists in that the data sent by means of the supply and operating circuit derived from the sensor signal and adapted by means of the bus interface Measured values MW are.

A Particularly advantageous development of the solution according to the invention proposes that the received data to be adjusted by means of the bus interface and by means of the supply and operating circuit in the control signal are to be implemented positions.

According to one further advantageous embodiment of the solution according to the invention provided that a further alternating component of the mixed stream, e.g. the hard signal, which is distinguishable from the other alternating component, as an information carrier or modulated as a means of communication to the signal of the mixed stream is. field devices the one additional Using Hart signal, exchange their data over the two-wire lines of the 4 to 20 mA current loop standard. With the help of this communication let the field devices be very flexible parameterization and commissioning or determination and read out stored measured values or data. The Hart protocol allows one bidirectional communication even in potentially explosive atmospheres Surroundings. Hard allows a point-to-point transmission of at least two participants. The discretized and digitized Data will be available to a special modem in each participant must transmit and receive in a hard protocol using the FSK method. The FSK (Frequency Shift Keying) uses the distinction of binary conditions different frequencies of a sinusoidal signal (e.g. Bell 202 standard: "0" 2200 Hz, "1" 1200 Hz). Because of this Method largely level independent It also offers signal attenuation and changing Loads a high immunity to interference. Of course it must be ensured that the transmission medium is capable is to transmit the frequencies used for coding. Is used to power the field device, a sinusoidal waveform of the AC component uses the frequency of the hard signal different from the alternating part of the mixed flow, so the two alternating components by, for example, a filter again can be processed separately and separately.

According to one advantageous embodiment of the solution according to the invention suggested that the energy demand of the field device partially by the DC component of the mixed flow is covered.

The Invention and selected embodiments will be explained in more detail with reference to the following drawings. For simplicity, in the drawings identical parts with the same reference numerals which were only repeated when it was was considered useful. It shows:

1 : a schematic representation of a field device which is connected to the field bus via a bus interface,

2 a first exemplary embodiment of the circuit of a field device according to the invention, which is connected to the field bus via a bus interface,

3 : Diagram of the currents (I _M , i _E ) of the first embodiment of the circuit of a field device according to the invention 2 .

4 a second embodiment of the inventive solution of a field device, the connected to the fieldbus via a bus interface,

5 a first diagram of the currents (I _M , i _E ) of the second embodiment of the circuit of a field device according to the invention from 4 .

6 : A second diagram of the currents (I _M , i _E ) of the second embodiment of the circuit of a field device according to the invention from 4 .

This in 1 The block diagram shown represents the most important subcomponents of a decentralized automation system for process parameter acquisition, evaluation and transmission. Furthermore, field devices 1 with a two-wire cable 4 Often designed to work with one of the usual external fieldbus systems 2 can work together. On the one hand, this can be achieved by direct connection to the fieldbus system 2 , eg according to the FIELDBUS protocol. On the other hand, the cooperation can be done with the interposition of a Buskopplers 3 , eg according to the so-called HART protocol, indirectly. The components in 1 are connected to each other so that a field device 1 This works according to the PROFIBUS PA protocol via a bus interface or bus coupler 3 indirectly to the external fieldbus system 2 connected.

In the field device 1 is a supply and operating circuit 5 integrated, over the two-wire line 4 with the module of controlled AC power source 15 and the following bus interface or the bus coupler 3 connected is. The chemical or physical process parameter P acts on the sensor or transducer 18 on or off by the actuator or signal converter 19 changed or provided. The detected or adjusted change of the chemical or physical process parameter P by the sensor 18 or the actor 19 is through the supply and operating circuit 5 processed further and over the two-wire line 4 to the bus interface 3 transmitted.

By protective measures in the field device 1 and possible measures in the bus coupler 3 is the two-wire cable 4 and the field device 1 intrinsically safe and separates the explosion protection area Ex from the non-explosion protection area Ex , The field device 1 can be used by this intrinsically safe separation in an explosion-proof area Ex, in which, for example, an explosive gas atmosphere prevails.

In 2 these sub-components are shown in more detail, wherein in the bus interface 3 symbolically a DC voltage source U _{= is} integrated, which supplies a measuring current I _M. This measuring current I _M flowing through a resistor R and generates a dependent of the measuring current I _M U _M voltage, which represents the measured value MW. This voltage U _M is in the bus interface or the bus coupler 3 further processed, digitized and to the external fieldbus system 2 output. The capacitor C is connected in parallel to the resistor R and together with it forms a low-pass filter TP which attenuates disturbing AC voltage signals which have a higher frequency than the cut-off frequency of the low-pass filter TP in the voltage signal of the measured value MW. Instead of the elements described above - R, C - may be in the bus interface 3 an evaluation and feed unit to be integrated into the current loop, which data and signals from the external fieldbus system 2 in the current loop of the two-wire line 4 or converts the measuring current I _M and according to the standard of the external fieldbus system 2 sent to the same.

Another component is the bus interface or the bus coupler 3 a controlled AC power source I _~ connected in parallel, the one feed current i _E in the two-wire line 4 feeds. The AC power source I _~ and the DC voltage source U ₌ communicate via a separate control and data line 21 and an intermediate power control unit 23 for controlling the current ratio of the alternating component 7 and the DC share 8th of the mixed stream 6 , This flow control unit 23 This is because, depending on how much measuring current I _M from the field device 1 requested or from the external fieldbus system by the internal electronics of the bus coupler 3 is fed, in a certain ratio a correspondingly adapted, fed AC i _{E in} addition to the measuring current I _{M is} mixed, so that sets the effective value of the sum current (I _M + i _E ) _Eff below a certain set limit.

To the two-wire line 4 is at least a field device 1 connected, that at least one AC / DC converter 9 for the evaluation and conversion of the alternating share 7 of the mixed stream 6 or Einseisungsstroms i _E , at least one DC / DC converter 10 , which evaluates and converts the DC component 8th of the mixed stream 6 or the measuring current I _M , at least one control and control unit 17 , which is the electrical sensor signal Sens of the sensor 18 evaluates and the electrical control signal Actuator 19 adjusts and takes over the control and regulation of communication and power supply, and at least one power controller 20 , which adjusts the current of the current loop or measuring current I _M for communication comprises. The chemical or physical process parameter P is determined by the transducer 18 detected and as an electrical sensor signal Sens via a electrical connection to the control and control unit 17 transmit, which evaluates this sensor signal Sens and the current controller 20 correspondingly drives, so that a proportional measuring current I _{M flows} in the current loop and that a corresponding measured value MW of the chemical or physical process parameter P to the bus coupler 3 is transmitted and can be further processed there. At the node K, the two currents, the measuring current I _M and the feed current i _E in the field device separate 1 into two separate branches. For this purpose, in the node K a crossover or a filter - not explicitly in the 2 and the 4 shown - be integrated into the circuit. The DC / DC converter 10 and the AC / DC converter 9 are via a control and data line 21 , via which, for example, a transmission of the current values and / or a control of the current conversion takes place, with the control and control unit 17 connected. This control and control unit 17 is via the supply line 22 from the AC / DC and DC / DC converters ( 9 . 10 ) supplied with the necessary energy. The control and control unit 17 represents the heart of the field device 1 with a microcontroller .mu.C as at least an integral part. About this control and control unit 17 the entire process parameter acquisition, evaluation and transmission is controlled, as well as the energy management of the field device 1 and optionally further bus components regulated. Under the energy management of the field device 1 is not just the controller of the power controller 20 to understand, rather, can via a targeted control of the DC / DC converter 10 and the AC / DC converter 9 and optionally one in the field device 1 include energy buffer - which is not explicitly in 2 and 4 shown is - the power supply to the field device 1 be managed. About the control of the AC / DC converter 10 and the associated conversion of the symmetric, mean-free alternating component 7 of the mixed stream 6 in a corresponding rms value 11 is the supply of the field device 1 controllable, so that, for example, always from the field device 1 needed maximum energy this field device 1 is available. From the energy sources of the bus interface or the bus coupler 3 and the module with the controlled current source 15 always the maximum effective value of the total current (I _M + i _E ) _Eff to the field device 1 transfer. The difference between the energy required by the field device and the energy required as the maximum effective value of the total current (I _M + i _E ) _Eff from the bus interface 3 and the controlled current source is provided via the AC / DC converter 9 thereby adjustable, that only a certain measure of the Wechselanteil 7 into a DC component 8th is converted. With an energy surplus in the field device 1 can by generating a negative RMS value 11 of the symmetric, mean-free alternating component 7 of the mixed stream 6 to the basis of the DC component 8th of the mixed stream 6 or the measuring current I _M excess energy or current from the field device 1 be derived out and does not need to be converted in the field device eg in heat or another form of energy.

In 3 a diagram of the currents in a decentralized automation system is shown. The measuring current I _M or the DC component 8th of the mixed stream 6 the by the power controller 20 or a switching unit 16 in the current loop is shown by a trapezoidal function with sections of constant values of the current value. Since changes in the chemical and physical process parameters P usually take some time and do not change immediately to the other value, between the constant values in which the current value does not change, a certain period of time in which the current value is, for example, linear from one constant value to the other Constant value passes. The DC component 8th or measuring current I _M assumes standard values between 4 mA and 20 mA after current grinding. This DC component 8th of the mixed stream 6 is an amplitude modulated alternating component 7 with a sinusoidal waveform, for example 13 of the mixed stream 6 - in the 3 shown as a dashed line - modulated. The amplitude of the alternating component 7 changes in such a way that the effective value of the sum current (I _M + i _E ) _Eff from the alternating component 7 and the DC component 8th the mixed current or from the measuring current I _M and the feed current i _E , in the AC / DC converter 9 Two way rectified, assumes a certain constant value. This particular constant value is, for example, the current value of the field device currently required for the power supply 1 or a maximum constant limit 12 , The disadvantage of amplitude modulated signals is that, especially at long distances, the signal amplitude within a network can change, which also causes the RMS value 11 is changed. As effective value 11 a change share 7 an electric current or alternating current, the value is given, which generates the same amount of heat in an effective resistance, as an equal DC component 8th or direct current causes.

The maximum positive amplitude A and the maximum negative amplitude A are in a low current range at a certain frequency F of the AC component 7 whose energy is below an upper limit, where an explosive atmosphere is ignited. The dependence of the ignition behavior of alternating quantities of currents or alternating currents and the associated "intrinsic safety" of field devices 1 is from the DE 100 49 233 A1 , of the DE 101 27 561 A1 , of the DE 102 45 273 A1 known.

In 4 is unlike 2 of the alternating component 7 the current is not from one of the bus interface 15 upstream controlled AC power source to the current loop or two-wire line 4 switched, but a switching unit 16 is controlled so that the necessary power or the necessary energy from the field device 1 the external fieldbus system 2 is removed. The bus coupler or the bus interface 3 has the same structure as in 2 and will not be discussed in detail here.

The chemical or physical process parameter P is from the sensor 18 captured or by the actor 19 set in which the sensor 18 an electrical sensor signal Sens to the control and control unit 17 sends, which is further processed there or the actuator 19 from the control and control unit 17 an electrical actuating signal Stell receives. The control and control unit 17 is responsible for the processing of the electrical actuating signal Stell or sensor signal Sens and controls based on the processed data, the switching unit 16 that have a corresponding DC component 8th or measuring current I _M in the current loop of the two-wire line 4 and the DC branch in the field device 1 generated. This current flows through the DC / DC converter, which is the control and control unit 17 with the microcontroller μC and other components in the supply and operating circuit 5 of the field device 1 supplied with the required energy. The difference between the required energy in the field device 1 and that of the DC component 8th or measuring current I _M generated energy is inventively by an AC component 7 a current balanced. This alternating share 7 is through the switching element 16 which generates, for example, a pulse width modulated (PWM) or frequency modulated (FWM) step signal. By limiting the sum current I _M + i _E in the switching unit 16 , the maximum positive amplitude A of the sum current I _M + i _{E has} an upper limit, which is below the ignition point of an explosive atmosphere, causing the field device 1 with the two-wire cable 4 is designed intrinsically safe. The DC / DC converter 10 and the AC / DC converter 9 on the one hand converts the alternating component 7 and the DC component 8th of the mixed stream 6 and thus supplies the control and control unit 17 over the supply line 22 with the necessary energy on the other hand, the alternating component 7 and the DC component 8th of the mixed stream 6 measured and via the control and data line 21 to the control and control unit 17 transmitted. With these measurement data and the current and future energy requirements of the field device 1 calculates the control and control unit 17 the ratio of the alternating component 7 to the DC component 8th of the mixed stream 6 and accordingly controls the switching unit 16 at.

The control and control unit 17 which also includes the energy management of the field device 1 controls, can via the control and data line 21 the DC / DC converter 10 and the AC / DC converter 9 regulate that only as much energy is converted as currently or in the future also in the field device 1 is needed. By generating one to the base of the DC component 8th negative RMS value 11 of the alternating component 7 can even use the excess energy or electricity that the field device 1 not needed to supply, are derived.

In 5 a current curve is shown by a circuit structure like 4 was generated. The measuring current I _M , the proportionality to the measured or set chemical and physical process parameters P of the field device 1 is represented by a trapezoidal function in a thicker line. This measurement current I _M is a rectangular and / or pulsed waveform 14 modulated by the switching unit 16 in the way of the control and control unit 17 is controlled clocked that briefly momentary currents as alternating shares 7 of the mixed stream 6 with a maximum positive amplitude A on the DC component 8th of the mixed stream 6 be modulated. The time duration or the number of pulses of a pulse sequence varies, depending on how much current in addition to the measuring current I _M or DC component 8th in the field device 1 is needed. That is, depending on how large the difference of the power demand is to the power supply, current pulses of a certain frequency f are added to the measuring current IM for a corresponding time window, so that the effective value of the sum current (I _M + i _E ) _Eff with the current required current value of the power requirement for the power supply of the field device 1 matches. In the 5 the maximum power requirement of the field device was assumed, so that the effective value of the total current (I _M + i _E ) _{Eff is} the limit value 12 of 20 mA and the field device 1 thus constantly and constantly supplied with maximum current. The rectangular and / or pulsed signal forms of alternating components 7 have the disadvantage that these waveforms have a variety of harmonics and thus cover a large frequency range, which in the circuit of 4 Filters - which are not in 4 are explicitly shown - must be inserted so that the measuring current I _M and the DC components 8th of the mixed stream 6 is not or only slightly changed by these pulse sequences. It may also be due to negative pulse trains whose amplitudes have a lower current value than the current measuring current I _M , the field device 1 only as much energy is supplied as much the field device 1 currently needed. This will avoid the field device 1 only draws as much energy from the fieldbus as it currently needs and the DC component 8th of the mixed stream 6 or the measuring current I _M can be adjusted almost uninfluenced in dependence of the change of the process parameter P.

The diagram in 6 is different from the diagram in 5 from the aspect that individual pulse train packets with the same frequency f is not modulated on in a time window, but in this diagram rectangular functions are shown whose duration depends on the measuring current I _M. This type of modulation is called a pulse width modulated square wave signal (PWM).

Other forms of modulation are also contemplated - such as frequency modulated (FMW) alternating signals 7 - which are not explicitly shown.

1

field device

2

external fieldbus system

3

bus interface, Bus Coupler

4

Two-wire line

5

supply and operating circuit

6

mixed stream

7

alternating component

7a

Another Alternating component, HART signal

8th

DC component

9

AC / DC converter

10

DC / DC converter

11

RMS

12

limit

13

sinusoidal waveform

14

rectangular and / or pulsed waveform

15

controlled AC power source

16

switching unit

17

inspection and control unit

18

Transducers sensor

19

Signal converter, actuator

20

power plate

21

Tax- and data line

22

supply line

23

Flow control unit

MW

reading

D

dates

P

(chemical or physical) process parameters

Sens

electrical sensor signal

parking

electrical actuating signal

.mu.C

microcontroller

Ex

Explosion protection area intrinsically safe area

Ex

no Explosion protection area, non-intrinsically safe area

R

resistance

C

capacitor

TP

lowpass

U ₌

DC voltage source

I _~

controlled AC power source

I _M

Measuring current, Current loop current

i _e

feed stream

I _M + i _E

total current

(I _M + i _E ) _Eff

RMS of the total current

K

junction

A

maximum positive amplitude

-A

maximum negative amplitude

f

frequency

Claims (20)

Field device ( 1 ) with at least one sensor ( 18 ) for detecting and / or with at least one actuator ( 19 ) for influencing at least one chemical and / or physical process parameter (P), which field device further comprises: a bus interface ( 3 ) to an external fieldbus system ( 2 ), which can be used as a two-wire cable ( 4 ), and one with the bus interface ( 3 ) communicating supply and operating circuit ( 5 ), whereby the supply of the field device ( 1 ) via the two-wire line ( 4 ) at least temporarily with one of the bus interface ( 3 ) supplied mixed stream ( 6 ), which is determined by an overlaying and / or a modulation of an alternating component ( 7 ) to a DC component ( 8th ) of the mixed stream ( 6 ), wherein the field device ( 1 ) via the two-wire line ( 4 ) Data (D) to the external fieldbus system ( 2 ) and / or data (D) from the external fieldbus system ( 2 ), wherein at least the alternating component ( 7 ) of the mixed stream ( 6 ) for the supply of the field device ( 1 ) is used with electrical energy, and wherein the field device ( 1 ) at least in part its energy demand via the alternating share ( 7 ) of the mixed stream ( 6 ) covers. Field device according to Claim 1, in which the DC component ( 8th ) of the mixed stream ( 6 ) as an information carrier or communication means for via two-wire line ( 4 ) transmitted data (D) is used. Field device according to Claims 1 and 2, in which the DC component ( 8th ) of the mixed stream ( 6 ) Assumes values within a current range of 4 mA to 20 mA. Field device according to Claim 1, in which the alternating component ( 7 ) of the mixed stream ( 6 ) at least one effective value ( 11 ) which is less than or equal to a maximum permissible limit ( 12 ). Field device according to Claim 4, in which the maximum permissible limit value ( 12 ) of the RMS value ( 11 ) of the mixed stream ( 6 ) assumes a value of 20 mA. Field device according to Claims 4 and 5, in which the effective value of the modulated alternating component ( 7 ) of the mixed stream ( 6 ) an optionally existing difference value of the DC component ( 8th ) of the mix stream ( 6 ) to the highest permissible limit ( 12 ) of the mixed stream ( 6 ). Field device according to Claim 1, in which a negative effective value ( 11 ) of the modulated alternating component ( 7 ) of the mixed stream ( 6 ) excess energy of the field device ( 1 ) and thus the energy requirement of the field device ( 1 ) regulates. Field device according to Claim 1, 6 or 7, in which the alternating component ( 7 ) of the mixed stream ( 6 ) a sinusoidal waveform ( 13 ) having. Field device according to Claim 1, 6 or 7, in which the alternating component ( 7 ) of the mixed stream ( 6 ) a rectangular and / or pulsed waveform ( 14 ) having. Field device according to Claim 8 or 9, in which a controlled alternating current source ( 15 ) the alternating component ( 7 ) of the mixed stream ( 6 ) generated. Field device according to Claim 9, in which a switching unit ( 16 ) the alternating component ( 7 ) of the mixed stream ( 6 ) generated. Field device according to Claim 1, 10 or 11, in which a control and control unit ( 17 ) in the supply and operating circuit ( 5 ), which is used to control the switching unit ( 16 ) based on the current and future energy requirements of the field device ( 1 ) and / or further on the basis of the current and future electrical sensor signal (Sens) or actuating signal (Stell) of the field device ( 1 ) for controlling and regulating the current controller ( 20 ), which determines the setting of the DC component ( 8th ) of the mixed stream ( 6 ) as the information carrier, serves. Field device according to Claim 12, in which the control and control unit ( 17 ) the future energy demand of the field device ( 1 ), taking into account and estimating future processes and applications. Field device according to Claim 1, in which at least one chemical and / or physical process parameter (P) comprises sensor-detecting measuring transducers ( 18 ) is provided which provides an electrical sensor signal (Sens). Field device according to Claim 1, in which at least one signal converter controlled by an electrical actuating signal (actuator) ( 19 ) is provided, which adjusts the at least one chemical and / or physical process parameter (P) actorisch. Field device according to Claim 1, in which, as an integral part of the supply and operating circuit ( 5 ) at least one AC / DC converter ( 9 ) and at least one DC / DC converter ( 10 ) is provided. Field device according to Claim 1 or 2, in which the data (D) sent are transmitted by means of the supply and operating circuit ( 5 ) derived from the sensor signal (Sens) and by means of the bus interface ( 3 ) are measured measured values MW. Field device according to Claim 1 or 2, in which the received data (D) is transmitted by means of the bus interface ( 3 ) and by means of the supply and operating circuit ( 5 ) are in the control signal (Stell) to be converted positions. Field device according to Claim 1, in which a further alternating component ( 7a ) of the mixed stream ( 6 ), eg the Hart signal, which is distinguishable from the other alternating component ( 7 ), as information carrier to the signal of the mixed stream ( 6 ) is modulated. Field device according to Claim 1, in which the energy requirement of the field device ( 1 ) partially by the DC component ( 8th ) of the mixed stream ( 6 ) is covered.