WO2008006406A1

WO2008006406A1 - External electrical energy supply for a field device

Info

Publication number: WO2008006406A1
Application number: PCT/EP2006/064232
Authority: WO
Inventors: Michael Kirst
Original assignee: Endress+Hauser Flowtec Ag
Priority date: 2006-07-13
Filing date: 2006-07-13
Publication date: 2008-01-17

Abstract

The present invention relates to field device electronics, which are fed by an external electrical energy supply, for a field device, wherein the external electrical energy supply provides a supply voltage and produces a variable supply current driven thereby, which field device electronics comprise an internal monitoring and control unit for controlling the field device, and at least one internal supply circuit, which feeds the internal monitoring and control unit, wherein the supply current is a pulsating current, which is formed by superimposition and/or modulation of an AC component on a DC component, and wherein the internal supply circuit uses, at least temporarily and/or proportionately, even the AC component of the pulsating current for covering an instantaneous energy requirement of the monitoring and control unit.

Description

Description External electrical power supply for a field device

The invention relates to a powered by an external electrical power supply field device electronics for a field device, wherein the external electrical power supply provides a supply voltage and supplies a variable supply current driven therefrom.

The advances in microelectronics in recent years have led to miniaturization of devices and integration of functionalities, which caused an effective and cost-effective application of integrated decentralized systems in automation technology. Thus, not only the measured values are determined in the sensors and actuators, but also the measured values are preprocessed, linearized and even a self-diagnosis of the sensor or actuator is implemented. The prerequisite for introducing these decentralized functionalities into a closed automation concept with "intelligent" sensors and actuators is increased information and data exchange of these decentralized units with each other and with a control system, which is why in automation technology there has been plenty in recent years Fieldbus systems that cover either company-specific application areas (eg BITBUS, CAN, MODBUS, RACKBUS) or those that are subject to international standardization (eg HART, PROFIBUS-PA, Foundation FIELDBUS, Ethernet) The large number of fieldbus systems currently available in are used in industrial automation technology and process control technology, are still referred to only with the general term "fieldbus or fieldbus system".

The usual field devices are network-fed four-wire gauges and have to have at least two electrical leads or conductors for the power supply of the field device. Furthermore, two further signal lines are required, which transmit the measured value measuring signal or other communication data signals between the decentralized units and the control center. Generally, this measurement signal or communication data signal will be in accordance with a standard standard for this, e.g. according to the between 4 mA and 20 mA lying current loop standard, a standard frequency standard or a digital standard, generated and transmitted.

In addition, it is also common in automation technology to build the field devices in a so-called two-wire technology and connect to each other, so that the power supply and communication between the devices can be done via a single pair of lines, reducing the wiring and thus the wiring costs of networked decentralized automation systems can be reduced.

Such two-wire field devices can be found, inter alia, the documents EP 0 883 097 B1, EP 0 895 209 B1, EP 1 158 274 A1. 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 outside, as well as for the transmission of the measuring signal.

Another aspect that must be considered in automation technology and in particular 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 known from DE 10049 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.

Based on the above-mentioned prior art, therefore, an object of the invention is to enable the communication of the field device via a two-wire line and also the field device via this two-wire line steadily with the necessary energy, the standards and regulations of the Explosion protection, to supply.

To solve the problem, the invention consists in a field of an external electrical power supply (25) powered field device electronics (5) for a field device (1), wherein the external electrical power supply (25) provides a supply voltage (U) and a supplied variable supply current (I) provides which field device electronics (5) an internal control and control unit for controlling the field device, and at least one internal control and control unit feeding, internal supply circuit, wherein the supply current to a mixed stream acts, which is formed by a superposition and / or a modulation of an alternating current component to a DC component, and wherein the internal supply circuit at least temporarily and / or at least partially also uses the AC component of the mixed current to cover a current energy demand of the control and control unit comprises.

According to an advantageous embodiment of the solution according to the invention is proposed that the DC component of the mixed current serves as an information carrier or communication means for data transmitted via a two-wire connection line.

An expedient embodiment of the field device according to the invention is that the DC component of the mixed current assumes values within a current range of 4 mA to 20 mA. In the case of two-wire field devices corresponding to the aforementioned current loop standard lying between 4 mA and 20 mA, a specific current value within this current range corresponds exactly to an electrical sensor signal or actuating signal. Therefore, only the current range of 4 mA can be planned for the guaranteed power supply of the electronics of the field device with a two-wire line and used. This implies that the power supply is only available with a power in the order of 50 mW, which is referred to below as low power. Two-wire field devices with the above-mentioned current loop standard lying between 4 mA and 20 mA are particularly well suited for use in potentially explosive environments due to these circumstances.

According to an advantageous embodiment of the solution according to the invention, it is proposed that the alternating current component of the mixed current assumes a frequency of greater than 10 Hz (Hertz), in particular greater than 50 Hz (Hertz).

An advantageous embodiment of the solution according to the invention suggests that the mixed stream assumes at least an effective value which is less than or equal to a maximum permissible limit.

According to a favorable embodiment of the field device according to the invention it is suggested that the maximum permissible limit value of the effective value of the mixed current is 20 mA. In order not to violate the explosion protection regulations, the maximum effective value of the mixed flow should not be greater than the specified limit value.

An advantageous embodiment of the solution according to the invention is that the rms value of the modulated alternating current component of the mixed current assumes an optionally existing difference value of the direct current component of the mixed current to the highest permissible limit value of the mixed current. So that the field device can always be supplied with the maximum permissible current, the difference of the DC component of the mixed current is supplemented by the effective value of, for example, rectified AC component of the mixed current, so that their sum occupy the maximum permissible limit. As a result, the field device is constantly supplied with the maximum energy or power.

According to an advantageous embodiment of the field device according to the invention, it is proposed that a modulated one to the DC component negative AC component of the mixed current dissipates existing excess energy of the field device and thus regulates the energy requirements or energy balance of the field device in this way, if too much energy of the field device electronics is available. If a negative effective value of the alternating current component in the AC / DC converter is generated at the base of the DC component, in which, for example, a DC component of the mixed current which is more negative than the DC component results in a DC component of the mixed current, excess current can be dissipated from the field device.

A very advantageous variant of the solution according to the invention is to be seen in that the AC component of the mixed stream has a sinusoidal waveform. Preferably sinusoidal signals are used as the alternating current component of the mixed current, since their signal spectrum has no harmonics. Which makes it easier to comply with specifications regarding "electromagnetic compatibility (EMC)". It is also easier to superimpose with other DC components or DC signals, since the time average of sinusoidal signal forms is zero and the coding is thus free of mean values.

A particularly advantageous development of the solution according to the invention suggests that the alternating current component of the mixed current has a rectangular and / or pulsed signal form.

An advantageous embodiment of the solution according to the invention suggests that a controlled AC power source in the external power supply generates the AC component of the mixed stream.

A very advantageous variant of the solution according to the invention is the fact that a switching unit in the field device electronics of the field device generates the AC component of the mixed current by means of load modulation. Due to the changing current load of the external energy supply by the variable with a corresponding frequency current requirements in the field device, the AC component of the mixed stream is generated.

A preferred embodiment of the field device according to the invention provides that a control and control unit is integrated in the field device electronics, 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 actuating signal of 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 an advantageous embodiment of the solution according to the invention it is proposed that the control and control unit, the future energy needs of the field device taking into account and estimate the future expiring Processes and applications determined and calculated.

An expedient embodiment of the field device according to the invention is that at least one sensor which senses a chemical and / or physical process parameter is provided, which supplies an electrical sensor signal.

A very advantageous variant of the solution according to the invention is to be seen in that at least one actuated by an electrical actuating signal actuator is provided which adjusts the at least one chemical and / or physical process parameters actuarially.

A useful alternative embodiment of the invention

Field device is that at least one AC / DC converter and / or at least one DC / DC converter is provided as an internal power supply circuit.

An advantageous embodiment of the solution according to the invention is that the data transmitted by means of the field device electronics derived from the sensor signal and / or by means of the external power supply adapted measured values.

A particularly advantageous development of the solution according to the invention suggests that the received data are to be adjusted by means of the external power supply and / or to be converted into the control signal by means of the field device electronics.

According to a further advantageous embodiment of the solution according to the invention it is provided that a further AC component of the mixed stream, for example the hard signal, which is distinguishable from the AC component and / or separated in the frequency band, modulated as an information carrier. Field devices using an additional Hart signal exchange their data over the two-wire lines of the 4 to 20 mA current loop standard. With the aid of this communication, the field devices can be very flexibly parameterized and put into operation, or measured values or data determined and stored can be read out. The Hart protocol enables bidirectional communication even in potentially explosive environments. Hart enables a point-to-point transmission of at least two subscribers. The discretized and digitized data will be transmitted and received to a special modem which must be present in each subscriber in a hard protocol with the FSK method. The FSK (Frequency Shift Keying) method uses different frequencies of a sinusoidal signal to differentiate the binary states (eg Bell 202 standard: "0" 2200 Hz, "1" 1200 Hz). Since this method operates largely level-independent, it also offers high interference immunity for signal attenuation and changing loads. Of course, it must be ensured that the transmission medium is able to transmit the frequencies used for coding. Is used to power the field device, a sinusoidal waveform used in the AC component, the frequency of the hard signal must be different from that of the AC component of the mixed stream, so that the two AC components can be re-separated by, for example, a filter and processed separately.

According to an advantageous embodiment of the solution according to the invention it is proposed that the energy requirement of the field device is partially covered by the DC component of the mixed stream.

The invention and selected embodiments will be explained in more detail with reference to the following drawings. For simplicity, identical parts have been given the same reference numerals in the drawings, but they have been repeated only if deemed appropriate. It shows:

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

2 shows 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 , iε) of the first exemplary embodiment of the circuit of a field device according to the invention from FIG. 2,

FIG. 4 shows a second exemplary embodiment of the solution according to the invention of a field device which is connected to the field bus via a bus interface, FIG.

5 shows a first diagram of the currents (I _M , iε) of the second exemplary embodiment of the circuit of a field device according to the invention from FIG. 4, [0034] FIG.

6 shows a second diagram of the currents (I _M , iε) of the second exemplary embodiment of the circuit of a field device according to the invention from FIG. 4, [0035] FIG.

The block diagram shown in FIG. 1 represents the most important subcomponents of a decentralized automation system for process parameter acquisition, evaluation and transmission. Furthermore, field devices 1 are frequently designed in such a way that they are connected via a conventional external fieldbus system 2 to a remote control center 24 or communicate with another field device 28 and provide itself with the required energy. On the one hand, this can be done by direct connection to the fieldbus system 2, eg according to the FTELDBUS protocol. On the other hand, the cooperation can take place indirectly with the interposition of a bus coupler 3, for example in accordance with the so-called HART protocol. The components of the decentralized automation system 1 shown on the right in FIG. 1 are interconnected in such a way that a field device 1, which operates, for example, according to the PROFIBUS PA protocol, is indirectly connected to the external fieldbus system 2 via a bus interface or a bus coupler 3. A direct connection of the field device 1 to the external fieldbus system 2, for example, according to the FTELDBUS protocol is not explicitly shown in Fig. 1, but it is also possible that Invention at a direct connection of the field device 1 to a fieldbus system 2 implement.

The field devices 1 generally consists of a field device electronics 5 and a sensor 18 or an actuator 19. An integral part or an affiliated part of the field device electronics 5 represents the transmitter 27, which is the electrical actuating signal setting of the actuator 19th or the electrical sensor signal Sens of the sensor 18 is detected and converted into corresponding data D or measurement data. For example, at least the control and control unit 17 can be integrated in the transmitter 27.

[0038] Examples of such sensors 18 are in particular flow-through

Measuring sensor, which determine the flow of an at least temporarily flowing medium according to the Coriolis principle or magnetic-inductive measuring principle, or fill level and / or Grenzwertaufnehmer for determining the level in a container according to the transit time principle of high-frequency or acoustic measurement signals to call. In process automation, for example, an actuator for a valve is an actuator 19.

The field device 1 of the illustrated on the right in Fig. 1 decentralized automation system is connected for example via a two-wire connection line 4 directly to the control center 24. In this embodiment, the communication and the supply of the field device 1 via the two-wire connection line 4 by the external power supply 25 and / or the controlled AC power source 15 in the control center 24, for example, according to the HART standard and / or 20mA current loop standard or teletypes standard. According to the HART standard and / or TTY standard, further field devices 28 can also be connected to this two-wire connection line 4, although this was not explicitly carried out in FIG. 1.

In the field device 1, a field device electronics 5 is integrated, via the

Two-wire connection line 4 to the module of the controlled AC power source 15 and / or the external power supply 25 in the respective units, e.g. the control center 24, the bus coupler 3 or other field devices 28, is connected. The chemical or physical process parameter P acts on the sensor or transducer 18 or is changed or provided by the actuator or signal converter 19. The detected or adjusted change of the chemical or physical process parameter P by the sensor 18 or the actuator 19 is further processed by the field device electronics 5 and transmitted via the two-wire connection line 4, for example to the bus interface 3 or directly to the control center 24.

By protective measures in the field device 1 and any measures in the

Bus coupler 3 is the two-wire connection line 4 and the field device 1 intrinsically safe is executed 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 protection area Ex, in which, for example, an explosive gas atmosphere prevails.

In Fig. 2, these sub-components of Fig. 1 are shown in more detail, wherein in the bus interface 3, an external power supply 25 is symbolically integrated with a DC voltage source 26, 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 measurement voltage U _M is further processed, for example, in the bus interface 3, digitized and output to the external fieldbus system 2. Furthermore, it is also possible to transmit this measuring voltage U _M directly via the connecting line 4 to the control station 24 or another field device 28. The capacitor C is connected in parallel to the resistor R and forms together with this in the external power supply 25, a low-pass filter TP, the disturbing AC signals having a higher frequency than the cutoff frequency of the low-pass filter TP, in the measurement voltage signal U _{M of} the measured value MW or Control value SW attenuates. Instead of the elements described above - R, C - may be integrated in the bus interface 3, another evaluation and supply unit in the current loop, which couples data and signals from the external fieldbus system 2 in the current loop of the two-wire connecting line 4 or converts the measuring current I _M. and sends out to the same according to the standard of the external fieldbus system 2 or the connection line 4.

As a further component of the bus interface 3 of the field bus system 2 or the control center 24 is a controlled AC power source 15 connected in parallel, which feeds the one feed current i _E in the two-wire connection line 4. This controlled AC power source 15 may also be integrated in the external power supply 25 of the bus interface 3, the transmitter 27 or the control center 24. An advantage of a modular construction of the controlled AC power source 15 and the external power supply 25 is that existing external power supplies 25 having only a DC power source 26 can be easily retrofitted by an AC power source module. The AC power source 15 and the DC voltage source 26 communicate via a separate control and data line 21 and an intermediate current control unit 23 for controlling the current ratio of the AC component 7 and the DC component 8 of the mixed stream 6. At the terminals of the connecting line 4 is thus a supply voltage U, the a supply current I from the feed current i _E and the measuring current I _M drive. This flow control unit 23 is there to, as the case may be A large amount of measuring current I _M requested by the field device 1 or fed by the external fieldbus system through the internal electronics of the Buskopplers 3, in a certain ratio, a correspondingly adapted, fed AC i _{E in} addition to the measuring current I _{M is} mixed, so that the effective value of sum current (i _M + iεW below a certain set threshold is established. In the simplest case, I _M of the external power supply 25 is measured by the flow control unit 23 of the measuring current and the controlled alternating current power source 15 corresponding to the deficit to the maximum effective value of the sum current (V ^ _ES driven.

This embodiment of the external power supply 25, the controlled AC power source 15 and the current control unit 23 in Fig. 2 can also be implemented in a control center 24 or as a stand-alone unit and are not explicitly carried out here due to the equivalence.

At least one field device 1 is connected to the two-wire connecting line 4, the at least one AC / DC converter 9, which serves for the evaluation and conversion of the AC component 7 of the mixed stream 6 or Einseisungsstroms i _E , at least one DC / DC converter 10, which is used for evaluation and conversion of the DC component 8 of the mixed stream 6 or the measuring current I _M , at least one control and control unit 17, which evaluates the electrical sensor signal Sens of the sensor 18 and adjusts the electrical control signal setting of the actuator 19 and the controller and Regulation of the communication and power supply takes over, and at least one power controller 20, which adjusts the communication of the current of the current loop or measuring current I _M includes. If only one field device 1 is connected to the external supply unit via a connecting line 4, the supply current I is equal to the total current I _M + I _E , otherwise the supply current I is a multiple of the total current I _M + i _E , depending on the number of connected field devices 1. The chemical or physical process parameter P is detected by the transducer 18 and transmitted as an electrical sensor signal Sens via an electrical connection to the control and control unit 17, which evaluates this sensor signal Sens and controls the current controller 20 accordingly, so that a proportional measurement current I _M in the current loop flows and that a corresponding measured value MW of the chemical or physical process parameter P is transmitted to the bus coupler 3 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 1 separate into two separate current branches. For this purpose, in the node K a crossover or a filter - which is not explicitly shown in FIG. 2 and FIG. 4 - be integrated into the circuit. The DC / DC converter 10 and the AC / DC converter 9 are connected via a control and data line 21, over which For example, a transmission of the current values and / or a control of the current conversion takes place, connected to the control and control unit 17. This control and control unit 17 is supplied via the supply line 22 by the AC / DC and DC / DC converters (9,10) with the necessary energy. The control and control unit 17 is 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, such 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 only the control of the current controller 20 to understand, but may include a targeted control of the DC / DC converter 10 and the AC / DC converter 9 and optionally in the field device 1 energy buffer memory - not explicitly shown in Fig. 2 and Fig. 4 - the power supply to the field device 1 are regulated. The supply of the field device 1 can be controlled via the control of the AC / DC converter 10 and the associated conversion of the symmetrical, mean-free alternating current component 7 of the mixed current 6 into an appropriate effective value 11, so that, for example, the maximum energy required by the field device 1 always belongs to this field device 1 is available. From the external power supply 25 and the module with the controlled AC power source 15, the maximum effective value of the total current (WiE) EfT is always transmitted to the field device 1. The difference in energy required by the field device 1 and the energy provided as the maximum effective value of the sum current (I _M + I _E W from the external power supply 25 and the controlled AC power source 15) is via the AC / DC converter 9 can be regulated in that only a certain amount of the alternating current component 7 is converted into a direct current component 8. In the case of an energy surplus in the field device 1, a negative effective value 11 of the symmetrical, mean-free alternating current component 7 of the mixed current 6 can be converted to the base of the direct current component 8 of the mixed current 6 or the measuring current I _M, excess energy or current can be derived from the field device 1 and does not have to be converted into heat or any other form of energy in the field device, as has been the case in the past The measuring current I _M or the DC component 8 of the mixing current ms 6 caused by the current controller 20 or a switching unit 16 in the current loop is indicated by a trapezoidal function with portions of constant values of the current value. Since changes in the chemical and physical process parameters P usually take a certain amount of time and do not change immediately to the other value, there is no difference between the constant values in which the current value is not changed, a certain period of time in which the current value, for example, linearly passes from a constant value in the other constant value. The DC component 8 or measuring current I _M assumes standard values between 4 mA and 20 mA after current grinding. This DC component 8 of the mixed stream 6 is an amplitude modulated AC component 7 with an example sinusoidal waveform 13 of the mixed stream 6 - shown in the Fig. 3 shown as a dashed line - modulated. The amplitude of the alternating current component 7 changes in such a way that the effective value of the sum current (i _M + iεWaus the AC component 7 and the DC component 8 of the mixed flow or from the measuring current I _M and the feed stream i _E , in the AC / This constant value is, for example, the current value of the field device 1 currently required for the power supply or a maximum constant limit value 12. The disadvantage of amplitude-modulated signals is that the signal amplitude within a network can change, which also changes the rms value 11. The rms value 11 of an alternating current component 7 of an electric current or alternating current is the value which generates the same amount of heat in an effective resistance as an equal DC component 8 or DC 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 at which 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 known from DE 100 49 233 A1, DE 101 27 561 A1, DE 10245 273 A1.

The power supply of the field device 1 according to the invention by a mixed flow 6 can also be used if, for example, another alternating current component 7a, e.g. according to the HART standard, to which DC component 8 of the 20 mA current loop measurement signal for modulating data D was modulated. In order to separate the alternating current component 7 required for the energy supply of the field device 1 and the further alternating current component 7a of the hard signal, the frequencies of the two alternating signals 7, 7a should be in different frequency bands. The alternating signals 7, 7a lying in different frequency bands can be metrologically separated again by appropriate filters and further processed separately.

In FIG. 4, in contrast to FIG. 2, the alternating current component 7 of the current is not controlled by an external energy supply 25 connected upstream, controlled alternating Current source 15 is generated on the current loop or the two-wire connection line 4, but a switching unit 16 is driven so that the necessary power or the necessary energy by load modulation in the appropriate frequency of the field device 1 of the external power supply 25 in the control center 24, is taken in the bus coupler 3 or another field device 28.

This embodiment of the external power supply 25 with a DC voltage source 26 of FIG. 4 can also be implemented in a control center 24 or in a stand-alone unit. Since this structure is equivalent to the description of the bus coupler 3 in Figure 2, this is no longer explicitly discussed.

The chemical or physical process parameter P is detected by the sensor 18 or adjusted by the actuator 19, in which the sensor 18 sends an electrical sensor signal Sens to the control and control unit 17, which is further processed there or the actuator 19 of the Control and control unit 17 an electrical control 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, the corresponding DC component 8 and measuring current I _M in the current loop of the two-wire connecting line 4 and the DC branch generated in the field device 1. This current flows through the DC / DC converter, which supplies the control and control unit 17 with the microcontroller .mu.C and other components in the field device electronics 5 of the field device 1 with the required energy. The difference between the required energy in the field device 1 and the energy generated by the DC component 8 or measuring current I _M is inventively by an AC component

7 of a stream balanced. This alternating current component 7 is generated by 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, whereby the field device 1 with the two-wire connection line 4 is designed intrinsically safe. The DC / DC converter 10 and the AC / DC converter 9 on the one hand converts the AC component 7 and the DC component

8 of the mixed stream 6 and thus supplies the control and control unit 17 via the supply line 22 with the necessary energy on the other hand, the AC component 7 and the DC component 8 of the mixed stream 6 is measured and via the control and data line 21 to the control and control unit 17th transmitted. With these measured data and the current and future energy requirement of the field device 1, the control and control unit 17 calculates the ratio of the alternating current component 7 to the direct current component 8 of the mixed current 6 and accordingly controls speaking the switching unit 16 at.

The control and control unit 17, which also includes the energy management of

Field device 1 controls, can regulate the DC / DC converter 10 and the AC / DC converter 9 via the control and data line 21, that only as much energy is converted, as currently or in the future also required in the field device 1. By generating an effective negative value 11 of the alternating current component 7 which is negative for the base of the direct current component 8, it is even possible to derive the excess energy or current which the field device 1 does not need for the supply.

In Fig. 5, a current waveform is shown, which was generated by a circuit construction as shown in Fig. 4. The measurement current I _M , which corresponds to a proportionality to the measured or set chemical and physical process parameter P of the field device 1, is represented by a trapezoidal function in a thicker lines. This measurement current I _M is a rectangular and / or pulsed waveform 14 aufmoduliert by the switching unit 16 is controlled in the manner of the control and control unit 17 clocked that briefly momentary currents as AC components 7 of the mixed stream 6 with a maximum positive amplitude A. be modulated on the DC component 8 of the mixed stream 6. The duration or the number of pulses of a pulse sequence varies, depending on how much current is required in addition to the measuring current I _M or DC component 8 in the field device 1. 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 + iE) Effinit the currently required current value the power requirement for power supply of the field device 1 matches. In FIG. 5, the maximum power requirement of the field device has been assumed, so that the effective value of the sum current (I _M + i _E ) _Eff assumes the limit value 12 of 20 mA and the field device 1 is thus supplied with maximum and constant maximum current. The rectangular and / or pulsed waveforms of alternating current components 7 have the disadvantage that these signal forms have a large number of harmonics and thus cover a large frequency range, whereby filters - which are not explicitly shown in FIG. 4 - are inserted into the circuit of FIG. 4 must be so that the measuring current I _M and the DC components 8 of the mixed stream 6 is not or only slightly changed by these pulse trains. It can also be supplied to the field device 1 only as much energy as far as the field device 1 currently requires by negative pulse trains whose amplitudes have a lower current value than the current measuring current I _M. This avoids that the field device 1 draws only as much energy from the field bus as it currently requires and the DC component 8 of the mixed stream 6 or the measuring current I _M almost unaffected in dependence the change of the process parameter P can be set.

The diagram in FIG. 6 differs from the diagram in FIG. 5 in the sense that individual pulse train packets with the same frequency f are not modulated in a time window, but in this diagram rectangular functions are shown, the time duration of which Suspend the measuring current I _M. This type of modulation is called a pulse width modulated square wave signal (PWM). This type of modulation may be according to the invention in the manner transpose by at small differences of the measuring current I _M and the DC component 8 of the mixed stream 6 from the desired effective value of the sum current (I _M + I _E W _the> a correspondingly smaller value between If, on the other hand, the measuring current I _M in the current loop is currently very low, a large value in the pulse-pause ratio, eg due to a short pause duration and longer pulse durations, is achieved.

Other forms of modulation of current signals and voltage signals may also be used - e.g. Frequency-modulated (FMW) alternating signals 7 - which are no longer explicitly shown as an embodiment due to the similarity to the previous embodiments.

[0056] List of Reference Numerals

1st field device

2. external fieldbus system

3. Bus interface, Bus Coupler

4. Two-wire connection cable

5. Field device electronics

6. Mixed stream

7. AC component

7a. additional alternating current component, HART signal

1. DC component

2. AC / DC converter

3. DC / DC converter

4. RMS value

5th limit

6. sinusoidal waveform

7. rectangular and / or pulsed waveform

8. controlled AC power source

9. Switching unit

10. Control and control unit

11. Instrument transformer, sensor

12. Signal converter, actuator 13. Current regulator

14. Control and data line

15. Supply line

16. Flow control unit

17. Control center

18. external energy supply

19. DC voltage source

20. Transmitter

21. another field device

22. internal supply circuit

[0058] SW control value

[0059] MW measured value

D data

P (chemical or physical) process parameter

Sens electrical sensor signal

Set electrical control signal

[0064] μC microcontroller

Explosion proof area, intrinsically safe area

Ex no explosion protection area, not intrinsically safe area

R resistance

C capacitor

TP low pass

[0070] I supply current

[0071] I _M measuring current, current loop current

[0072] i _E feed stream

[0073] I _M + I _E total current

(I _M + i _E ) _Eff Effective value of the total current

[0075] U _M measuring voltage

[0076] U supply voltage

K node

A maximum positive amplitude

[0079] -A maximum negative amplitude

[0080] f frequency

Claims

claims

1. Field devices powered by an external electrical power supply (25)

Electronics (5) for a field device (1), wherein the external electrical power supply (25) provides a supply voltage (U) and supplies a variable supply current (I) driven thereby, which field device electronics (5) comprises: - at least one internal control - and control unit (17) for controlling the field device (1), and - at least one internal control and control unit (17) feeding, internal supply circuit (29), - wherein it is the supply current (I) to a mixed stream (6 ), which is formed by a superposition and / or a modulation of a, in particular higher-frequency, AC component (7) on a, in particular variable, DC component (8), and - wherein the internal supply circuit (29) at least temporarily and / or at least proportionately also uses the AC component (7) of the mixed stream (6) to cover a current energy demand of the control and control unit (17).

2. field device electronics according to claim 1, wherein the DC component (8) of the

Mixed stream (6) as an information carrier or communication means for via a two-wire connection line (4) transmitted data (D) is used.

3. Field device electronics according to one of the preceding claims, wherein the

DC component (8) of the mixed current (6) assumes values within a current range of 4 mA to 20 mA.

4. field device electronics according to one of the preceding claims, wherein the

AC component (7) of the mixed stream (6) assumes a frequency of greater than 10 Hz, in particular greater than 50 Hz.

5. Field device electronics according to one of the preceding claims, wherein the

AC component (7) of the mixed flow (6) assumes at least one effective value (11) which is less than or equal to a maximum permissible limit value (12).

6. field device electronics according to claim 5, wherein the most permissible

Limit value (12) of the effective value (II) of the mixed current (6) is 20 mA.

7. field device electronics according to claim 4 or 5, wherein the rms value of the modulated AC component (7) of the mixed stream (6) an optionally existing difference value of the DC component (8) of the mixed stream (6) to the maximum permissible limit (12) of the mixed stream (6).

8. Field device electronics according to one of the preceding claims, in which a for

DC component (8) of negative modulated AC component (7) of the mixed stream (6) dissipates existing excess energy of the field device (1). 9. field device electronics according to one of the preceding claims, wherein the

AC component (7) of the mixed stream (6) has a sinusoidal waveform (13).

10. Field device electronics according to one of the preceding claims, wherein the

AC component (7) of the mixed stream (6) has a rectangular and / or pulsed waveform (14).

11. Field device electronics according to one of the preceding claims, wherein a controlled AC power source (15) in the external power supply (25) generates the AC component (7) of the mixed stream (6).

12. Field device electronics according to one of the preceding claims, wherein a

Switching unit (16) in the field device electronics (5) of the field device (1) generates the AC component (7) of the mixed stream (6) by means of load modulation.

13. Field device electronics according to one of the preceding claims, wherein the

Control and control unit (17) for controlling a switching unit (16) based on the current and future energy requirements of the field device (1) is used and / or further based on the current and future electrical sensor signal (Sens) or control signal (Stell) of the field device (1) for controlling and regulating a current controller (20) which takes over the adjustment of the direct current component (8) of the mixed current (6) as information carrier.

14. Field device electronics according to claim 13, wherein the control and

Control unit (17) determines and calculates the future energy requirements of the field device (1) taking into account and estimating the future running processes and applications.

15. Field device electronics according to one of the preceding claims, in which at least one chemical and / or physical process parameter (P) sensor-sensing sensor (18) is provided which provides an electrical sensor signal (Sens).

16. Field device electronics according to one of the preceding claims, wherein at least one of an electrical control signal (actuator) controlled actuator (19) is provided which adjusts the at least one chemical and / or physical process parameters (P) actuarially.

17. Field device electronics according to one of the preceding claims, wherein at least one AC / DC converter (9) and / or at least one DC / DC converter (10) is provided as the internal supply circuit (29).

18. Field device electronics according to one of the preceding claims, wherein the transmitted data (D) by means of the field device electronics (5) derived from the sensor signal (Sens) and / or by means of the external power supply (25) adapted measured values (MW ) are. 19. Field device electronics according to one of the preceding claims, wherein the received data (D) by means of the external power supply (25) to be adapted and / or by means of the field device electronics (5) in the control signal (Stell) to be converted positions ( SW) are.

20. Field device electronics according to one of the preceding claims, wherein a further AC component (7a) of the mixed stream (6) distinguishable from the AC component (7) and / or modulated separately in the frequency band is modulated as an information carrier.

21. Field device electronics according to one of the preceding claims, wherein the

Energy requirement of the field device (1) is partially covered by the DC component (8) of the mixed stream (6).