U.S. patent application number 10/694349 was filed with the patent office on 2004-05-06 for measuring instrument.
Invention is credited to Capt, Jean-Gyl, Gerst, Peter, Lubcke, Wolfgang.
Application Number | 20040085076 10/694349 |
Document ID | / |
Family ID | 30118778 |
Filed Date | 2004-05-06 |
United States Patent
Application |
20040085076 |
Kind Code |
A1 |
Lubcke, Wolfgang ; et
al. |
May 6, 2004 |
Measuring instrument
Abstract
A description is given of a measuring instrument to be connected
to a higher-order unit having at least a first and an identical
second pair of terminals, which can be connected electrically, very
simply and without errors, to the higher-order unit, the measuring
instrument comprising: a first pair of lines, to be connected to
the first pair of terminals, via which a signal current flows
during operation, the signal current being a measure of an
instantaneous measured value, and a second pair of lines, to be
connected to the second pair of terminals, via which a supply
current flows during operation, whose value is greater than or
equal to a minimum signal current and less than or equal to a
maximum signal current.
Inventors: |
Lubcke, Wolfgang; (Steinen,
DE) ; Gerst, Peter; (Weil am Rhein, DE) ;
Capt, Jean-Gyl; (Habsheim, FR) |
Correspondence
Address: |
Felix J. D'Ambrosio
Jones, Tullar & Cooper, P.C.
Eads Station
P.O. Box 2266
Arlington
VA
22202
US
|
Family ID: |
30118778 |
Appl. No.: |
10/694349 |
Filed: |
October 28, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10694349 |
Oct 28, 2003 |
|
|
|
09677725 |
Oct 2, 2000 |
|
|
|
6684340 |
|
|
|
|
60197545 |
Apr 17, 2000 |
|
|
|
Current U.S.
Class: |
324/609 |
Current CPC
Class: |
G08C 19/02 20130101 |
Class at
Publication: |
324/609 |
International
Class: |
G01R 027/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 7, 1999 |
EP |
99 11 9840.9 |
Claims
What is claimed is:
1. A measuring arrangement comprising: a measuring instrument and a
higher-order unit, said measuring instrument and said higher-order
unit being electrically connected with each other by a first pair
of lines and a second pair of lines, wherein during operation a
signal current flows via said first pair of lines and a supply
current flows via said second pair of lines, said signal current
representing an instantaneous measured value and said supply
current and at least a portion of the signal current supply said
measuring instrument.
2. The measuring arrangement as claimed in claim 1 wherein the
supply current varies in accordance with a current power demand of
said measuring instrument.
3. The measuring arrangement as claimed in claim 1 wherein the
higherorder unit comprises at least two transmitter feed units,
each of said transmitter feed units being operable to supply a
conventional two-wire measuring instrument with electrical
power.
4. The measuring arrangement as claimed in claim 3 wherein each of
said first and said second pairs of lines is connected,
respectively, with one of said at least two transmitter feed
units.
5. The measuring arrangement as claimed in claim 3 wherein each of
said at least two transmitter feed units is connected with one of
said first and said second pairs of lines, respectively.
6. The measuring arrangement as claimed in claim 1 wherein each of
said first and said second pairs of lines is connected to a
current/voltage limiter.
7. The measuring arrangement as claimed in claim 1 wherein said
first and said second pairs of lines are galvanic isolated from
each other.
8. The measuring arrangement as claimed in claim 1 wherein the
measuring instrument comprises a sensor for detecting at least one
physical variable.
9. The measuring arrangement as claimed in claim 8 wherein the
higherorder unit comprises a bus line for transmitting measured
values representing said at least one physical variable.
10. An adapter circuit system for an electrically powered measuring
device, wherein two ports are provided constituting a two-wire
interface for connecting a dual-conductor cable by way of which
electric power is fed to the measuring device and a measuring
signal from the measuring device is transmitted wherein at least
one additional port is provided for connecting a second cable and
wherein the said second cable allows the feeding of additional
electric power to the measuring device.
11. The adapter circuit system as In claim 10, wherein two ports
are provided, constituting a second two-wire interface for
connecting a second dual-conductor cable.
12. The adapter circuit system as in claim 11, wherein the current
emanating from the first two-wire interface and/or the current
emanating from the second two-wire interface is limited.
13. The adapter circuit system as in claim 11, wherein the first
two-wire interface and the second two-wire interface connect to a
voltage regulator provided at the input of the measuring device
14. The adapter circuit system as in claim 11, wherein the current
emanating from the first two-wire interface is regulated by means
of a first linear regulating transistor controlled by the voltage
regulator or by the measuring device.
15. The adapter circuit system as in claim 11, wherein the current
emanating from the second two-wire interface is regulated by means
of a second linear regulating transistor controlled by a
circuit.
16. The adapter circuit system as in claim 11, wherein the current
emanating from the second two-wire interface is regulated by means
of a second linear regulating transistor controlled by the voltage
regulator or by the measuring device.
17. The adapter circuit system as in claim 11, wherein a rectifier
is connected in series respectively with the first two-wire
interface and/or the second two-wire interface.
18. A measuring device wherein two ports are provided constituting
a two-wire interface for connecting a dual-conductor cable by way
of which electric power is fed to the measuring device and a
measuring signal is transmitted from the measuring device to an
evaluation circuit, wherein at least one additional port is
provided for connecting a second cable, the said second cable
allowing the feeding of additional electric power to the measuring
device, providing the possibility to either use the measuring
device in an intrinsically safe mode by only using the said two
ports of the two-wire interface or to use the measuring device in a
non-intrinsically safe mode by using the said two ports of the
two-wire interface and the said additional port to which the said
second cable is connected.
Description
[0001] The invention relates to a measuring instrument and a
measuring arrangement having at least one measuring instrument.
[0002] In the applications which are common in measurement and
control engineering, for example in the monitoring, control and/or
automation of complex processes, it is usual for a number of
measuring instruments, for example pressure, temperature, flow
and/or level measuring instruments, to be in use at the same
time.
[0003] A measuring instrument generally comprises a sensor, which
registers a physical measured variable and converts it into an
electrical variable, and electronics which convert the electrical
variable into a measurement signal. The measuring instruments have
to be connected individually, that is to say they have to be
supplied with power and the measurement signal has to be fed to a
higher-order unit. The core of the higher-order unit is usually a
control and/or regulating unit, which registers the measurement
signals, evaluates them and supplies display, control and/or
regulating signals for the monitoring, control and/or automation of
a process as a function of the instantaneous measured values.
Examples of this are programmable logic controllers (PLC),
distributed control systems (DCS) or personal computers (PC).
[0004] In order to keep the work which is entailed during the
installation of the measuring instrument to a low level, in
measurement and control engineering use is preferably made of
measuring instruments having only one pair of lines, via which both
the supply to the measuring instrument and its signal transmission
take place. These instruments are often referred to as
two-measuring instruments are fed with 12 V, and the measuring
instrument controls a current flowing via the pair of lines as a
function of an instantaneous measured value. The measurement signal
is a signal current in the case of these measuring instruments.
According to a standard which is common in measurement and control
engineering, the signal current is set to values between a minimum
signal current of 4 mA and a maximum signal current of 20 mA,
depending on the instantaneous measured value.
[0005] Since both the supply and the signal transmission take place
via the pair of lines, given a feed voltage of 12 V and a signal
current of 4 mA, there is only a power of 48 mW available to the
measuring instrument. This is completely adequate for a very large
number of measuring instruments. In large plants, therefore,
terminal blocks are usually provided which have a large number of
identical pairs of terminals for these pairs of lines to be
connected to the higher-order unit. As a result of this
standardization of the method of connection, a large number
measuring instruments can be connected up very simply and quickly
and therefore cost-effectively. Since all the pairs of lines and
all the pairs of terminals are identical, wiring errors are
virtually ruled out.
[0006] However, there are also measuring instruments, such as
highly accurate level measuring instruments operating with
microwaves, level measuring instruments operating with ultrasound
or flowmeters, for which this low power is not adequate. In order
that these measuring instruments can nevertheless be used in
conjunction with the previously described standard, these measuring
instruments usually have two pairs of lines. The measuring
instrument is supplied via one of the pairs of lines, and a signal
current corresponding to the previously described standard flows
via the other pair of lines. For the supply, it is usually
necessary to connect a transformer and a rectifier to the normal
power line, which carries 230 V alternating voltage, for example,
in order for example to provide a supply voltage of usually 24 V DC
for the measuring instrument. This is very complicated, and there
is the risk that the two pairs of lines can be transposed during
the connection of the instrument.
[0007] It is an object of the invention to specify a measuring
instrument which can be electrically connected, very simply and
without errors, to a higher-order unit.
[0008] To this end, the invention consists in a measuring
instrument to be connected to a higher-order unit having at least a
first and an identical second pair of terminals, which
comprises:
[0009] a first first [sic] pair of lines, to be connected to the
first pair of terminals,
[0010] via which a signal current flows during operation,
[0011] the signal current being a measure of an instantaneous
measured value, and
[0012] a second second [sic] pair of lines, to be connected to the
second pair of terminals,
[0013] via which a supply current flows during operation,
[0014] whose value is greater than or equal to a minimum signal
current and less than or equal to a maximum signal current.
[0015] According to one embodiment of the invention, the supply
current and at least a proportion of the signal current are
available to supply the measuring instrument.
[0016] According to a further embodiment, the minimum signal
current is 4 mA and the maximum signal current is 20 mA.
[0017] According to a further embodiment, a current/voltage limiter
is connected to the input side of each pair of lines.
[0018] According to a further embodiment, the first pair of lines
is connected to a first circuit and the second pair of lines is
connected to a second circuit, and the first and the second
circuits are galvanically isolated from each other.
[0019] In addition, the invention consists in a measuring
arrangement having at least one measuring instrument according to
the invention, in which the higher-order unit comprises a control
and/or regulating unit, in particular a programmable logic
controller (PLC), a distributed control system (DCS) or a personal
computer (PC).
[0020] According to one embodiment of the measuring arrangement,
the higher-order unit has one or more batteries of transmitter feed
units, at least one battery having at least two transmitter feed
units and each transmitter feed unit having a pair of
terminals.
[0021] According to a further embodiment, each battery of
transmitter feed units is connected to the control and/or
regulating unit via a bus access circuit and a bus line in order to
transmit the measured values from the measuring instruments
connected thereto.
[0022] The invention and further advantages will now be explained
in more detail using the figures of the drawing, in which two
exemplary embodiments of a measuring instrument and three exemplary
embodiments of a measuring arrangement are illustrated; identical
elements are provided in the figures with the same reference
symbols.
[0023] FIG. 1 shows a block diagram of a measuring instrument
according to the invention having two pairs of lines, which supply
two separate circuits;
[0024] FIG. 2 shows a block diagram of a measuring instrument in
which power supplied via the second pair of lines is distributed to
a number of end users via a transformer having a number of
outputs;
[0025] FIG. 3 shows a measuring arrangement having at least one
measuring instrument according to the invention;
[0026] FIG. 4 shows a measuring arrangement in which the
higher-order unit has a control and/or regulating unit and a
battery of transmitter feed devices arranged remotely therefrom;
and
[0027] FIG. 5 shows a measuring arrangement which has a number of
batteries of transmitter feed units which are each connected to a
control and/or regulating unit via a bus access circuit and a bus
line.
[0028] FIG. 1 illustrates a block diagram of a measuring instrument
that [sic] can be connected to a higher-order unit having at least
a first and an identical second pair of terminals.
[0029] To this end, the measuring instrument has a first pair of
lines 1 to be connected to the first pair of terminals and a second
pair of lines 3 to be connected to the second pair of
terminals.
[0030] The first and the second pair of lines 1, 3 each have a
first and a second line 5, 7, 9, 11, each of which is earthed via a
capacitor 13. The capacitors 13 are used to filter out interference
signals. A current/voltage limiter is connected to the input side
of each pair of lines 1, 3. Such a current/voltage limiter protects
the measuring instrument against excessively high currents and/or
voltages. If the current and voltage are limited to values at which
the formation of sparks in the measuring instrument can be ruled
out with certainty, the use of the measuring instrument in
hazardous areas is possible.
[0031] In the exemplary embodiment illustrated in FIG. 1, the
current is limited by means of a fuse 15 inserted in each case into
the first line 5, 9 of a pair of lines 1, 3. The voltage is limited
by a Zener diode 17 connected between the respective first and
second line 5, 9, 7, 11 of the first and of the second pair of
lines 1, 3.
[0032] In addition to the Zener diodes 17, a voltage stabilizing
means 19 can be provided in each case. This is inserted into the
respective first line 5, 9, for example as shown in FIG. 1, and, in
order to register the voltage currently present, is connected to
the respective second line 7, 11.
[0033] Following the above-described current/voltage limiter on the
input side, the first line 5, 9 of each pair of lines 1, 3 in each
case has a controllable current source 21, 23, which sets a current
flowing via the respective pair of lines 1, 3 to a specific value
as a function of a control signal.
[0034] Connected to the first pair of lines 1 are electronics 25.
The second pair of lines 3 supplies sensor electronics 27 and a
sensor 29 connected thereto. The sensor 29 registers a physical
measured variable, for example a pressure, a level in a container
or a flow rate through a pipe, and converts this into an electrical
variable, for example a voltage, a current, a resistance change, a
capacitance change or a signal. The electrical variable is
registered by means of the sensor electronics 27 and made
accessible for further evaluation and/or processing.
[0035] In the exemplary embodiment of FIG. 1, the sensor
electronics 27 are connected to the electronics 25 by signal lines
31, via which the information can be exchanged. In the exemplary
embodiment shown, this connection is bidirectional and preferably
has galvanic isolation 33. In the exemplary embodiment of FIG. 1,
galvanic isolation 33 is implemented by means of two
optocouplers.
[0036] The final measured value is determined, for example, by the
sensor electronics 27 and transmitted to the electronics 25.
Equally well, however, a raw signal can also be transmitted from
the sensor electronics 27 to the electronics 25, which then
determine the measured value from the raw signal.
[0037] During operation, the electronics 25 generate a control
signal, which depends on the instantaneous measured value and is
applied to the current source 23 via a signal line 35. The control
signal has the effect that, during operation, the current source 23
causes a signal current to flow via the first pair of lines 1 which
is a measure of an instantaneous measured value.
[0038] According to a standard which is common in measurement and
control engineering, the signal current varies as a function of the
measured value between a minimum signal current of 4 mA and a
maximum signal current of 20 mA. The necessary power is provided by
the higher-order unit. A signal current of more than 20 mA or less
than 4 mA is normally recognized by the higher-order unit as a
malfunction and effects the triggering of an alarm and/or the
initiation of process-specific handling directed toward safety.
[0039] During operation, the sensor electronics 27 likewise
generate a control signal which is applied to the current source 21
via a signal line 37. This control signal is independent of the
instantaneous measured value. During operation, the control signal
has the effect that the current source 21 causes a supply current
to flow via the second pair of lines 3.
[0040] According to the invention, the control signal is designed
in such a way that the supply current in normal operation always
has a value which is greater than or equal to the minimum signal
current and less than or equal to the maximum signal current. The
standard of 4 mA to 20 mA which is common in measurement and
control engineering is likewise used here. In the case of a
measuring instrument which always needs a great deal of power, the
supply current will preferably always be equal to the maximum
signal current. In the case of a measuring instrument which needs a
high power on the basis of the measurement operation, for example
only at specific time intervals, but otherwise manages with
considerably less power, it is advisable to vary the supply current
via the control signal in accordance with the current power
demand.
[0041] The measuring instrument may have, as required, an onsite
display 39, an operating panel 41 and/or a programming interface
43. The on-site display 39 is used for example to display the
current measured value or else, in conjunction with the operating
panel 41, to display the data entered via the operating panel 41.
Via the operating panel 41 it is possible, for example, for a
configuration, a calibration and/or a setting of a measuring range
of the measuring instrument to be carried out at the point of use.
A handheld terminal, for example, can be connected via the
programming interface 43.
[0042] Display 39, operating panel 41 and/or programming interface
43 can be connected to the electronics 25, as illustrated in FIG.
1, and are supplied by the electronics 25 via the first pair of
lines 21.
[0043] Both the sensor electronics 27 and the electronics 25 can
contain voltage regulators, which transform [sic] a voltage applied
by the higher-order unit to the first and to the second pair of
lines 1, 3 to values which are matched to the requirements of the
electronics 25, the sensor electronics 27, the sensor 29, the
display 39, the operating panel 41 and the programming interface
43.
[0044] During the design of the measuring instrument, the procedure
is preferably such that the functional blocks of the measuring
instrument are divided up into analog and digital functional
blocks. The analog functional blocks are preferably integrated into
the sensor electronics 27, and the digital functional blocks are
preferably integrated into the electronics 25. This offers the
advantage that it is possible to manage with very few voltage
regulators. As a rule, the digital functional blocks have a
considerably lower power requirement than the analog functional
blocks and the sensor 29. Consequently, during the above-described
division, the functional blocks with the lower power requirement
are supplied with the signal current via the first pair of lines 1.
The functional blocks with the higher power requirement are
supplied with the supply current via the second pair of lines 3.
Thus, the supply current and at least a proportion of the signal
current are available to supply the measuring instrument.
[0045] The first pair of lines 1 is connected to a first circuit,
which contains the electronics 25. The second pair of lines 3 is
connected to a second circuit, which contains the sensor
electronics 27 and the sensor 29. The two circuits are isolated
from each other and connected only via the signal lines 31. Since
the signal lines 31 have galvanic isolation 33, the two circuits
are also galvanically isolated from each other.
[0046] From the view of the higher-order unit, the two pairs of
lines 1, 3 are identical with regard to their power supply. For the
higher-order unit, the measuring instrument behaves electrically in
exactly the same way as if two 2-wire measuring instruments were
connected. Both 2-wire measuring instruments meet the above
mentioned standard, common in measurement and control engineering,
in which the signal current assumes values from 4 mA to 20 mA.
[0047] FIG. 2 shows a further exemplary embodiment of a measuring
instrument according to the invention. Because of the relatively
far-reaching agreement, only the differences from the exemplary
embodiment illustrated in FIG. 1 will be described specifically
below.
[0048] The significantly [sic] difference resides in the division
of the power taken up by the measuring instrument.
[0049] In the second circuit, a transformer 45 is provided, which
on the primary side is fed via the second pair of lines 3 and on
the secondary side has two outputs 47, 49. The sensor electronics
27 and the sensor 29 are supplied via the first output 47. The
second output 49 is connected to the electronics 25. The
electronics 25 are therefore on the one hand supplied via the
signal current flowing in the first pair of lines 1, exactly as in
the exemplary embodiment illustrated in FIG. 1, but in addition
they also draw power via the second output 49 of the transformer
45, said power being fed to the measuring instrument via the second
pair of lines 3.
[0050] In the exemplary embodiment shown in FIG. 2, it is also the
case that the sensor electronics 27 supply a control signal which
is used to set the supply current flowing in the primary circuit
via the second pair of lines 3. The control signal is applied via
the signal line 37 to a regulating unit 53 which is arranged in the
primary circuit and which sets the supply current
appropriately.
[0051] According to the invention, the control signal is also
designed here in such a way that, in normal operation, the supply
current always has a value which is greater than or equal to the
minimum signal current and less than or equal to the maximum signal
current.
[0052] By means of the transformer 45, galvanic isolation between
the two circuits is ensured in this exemplary embodiment as
well.
[0053] In a completely analogous way, it is of course also possible
for part of the power available via the signal current to be fed,
galvanically isolated, to the sensor electronics 27 and/or the
sensor 29, by a transformer being placed in the first circuit and
being connected via one output to the electronics 25 and via a
further output to the sensor electronics 27 and/or the sensor
29.
[0054] From the view of the higher-order unit, the two pairs of
lines 1, 3 are identical with regard to their power supply in this
exemplary embodiment as well. For the higher-order unit, the
measuring instrument behaves electrically in exactly the same way
as if two 2-wire measuring instruments were connected. Both 2-wire
measuring instruments meet the above mentioned standard, common in
measuring and control engineering, in which the signal current
assumes values from 4 mA to 20 mA.
[0055] A particular advantage is that the measuring instruments
according to the invention do not have to be supplied by mains
voltage. As a result, in measuring instruments according to the
invention, only the low signal and supply currents occur. This
increases safety, in particular in plants or points of use where,
for example, there is a considerable risk of explosion.
[0056] FIGS. 3 to 5 show three different measuring arrangement
[sic] having measuring instruments according to the invention.
[0057] FIG. 3 illustrates a measuring arrangement having a
higher-order unit 57, to which six conventional 2-wire measuring
instruments 59 and two measuring instruments 61 according to the
invention are connected.
[0058] The higher-order unit 57 is, for example, a programmable
logic controller or a distributed control system. In the exemplary
embodiment shown, for reasons of clarity it has only 10 identical
pairs of terminals, numbered consecutively from 1. to 10. Each pair
of terminals is designed as standard for the connection, the supply
and the transmission of a measured value in the form of a signal
current of a 2-wire measuring instrument.
[0059] The higher-order unit 57 has a power supply unit 65 which is
connected to a voltage source 63 and via which the individual pairs
of terminals 1. to 10. are supplied. Each pair of terminals 1. to
10. is assigned a pick-up unit, which registers a current flowing
via a pair of terminals 1. to 10. and generates a signal
corresponding to the current and feeds it to an intelligent core 67
of the higher-order unit 57, for example a microprocessor. In the
intelligent core 67, all the incoming measured values are monitored
and, in accordance with a flow chart stored in the intelligent core
67, display, control, regulating or switching operations are
triggered as a function of the instantaneous measured values. This
is illustrated symbolically in FIG. 3 by a first output, via which
the higher-order unit 57 controls a valve 69, a second output, via
which the higher-order unit 57 controls a switch 71, and a third
output, via which the higher-order unit 57 controls a display 73.
The display used can of course also be a personal computer, which
not only displays a measured value but, for example, can also
visualize a process sequence of an entire plant.
[0060] In the exemplary embodiments illustrated, conventional
2-wire measuring instruments 59 are connected to the 1.sup.st, the
2.sup.nd, the 5.sup.th, the 8.sup.th, the 9.sup.th and the
10.sup.th pair of terminals. The current flowing in each case via
one of these pairs of terminals 1., 2., 5., 8., 9., 10. corresponds
to a measured value from the respective conventional 2-wire
measuring instrument 59.
[0061] A measuring instrument 61 according to the invention is
connected to the two pairs of terminals 3. and 4., by the first
pair of lines 1 being connected to the 3.sup.rd pair of terminals
and the second pair of lines 3 being connected to the 4.sup.th pair
of terminals. A further measuring instrument 61 according to the
invention is connected to the pairs of terminals 6. and 7., by its
first pair of lines 1 being connected to the 6.sup.th pair of
terminals and its second pair of lines 3 being connected to the
7.sup.th pair of terminals.
[0062] With regard to the electrical connection, the measuring
instrument [sic] 61 according to the invention in no way differ
from the conventional 2-wire measuring instruments 59. In each
case, one pair of lines is connected to a pair of terminals in the
case of all the instruments. In the flow chart in the intelligent
core 67 of the higher-order unit 57, it is defined which pair of
terminals 1. to 10. is assigned what significance. For example, the
fact is stored there that the measured value obtained via the first
pair of terminals 1. is a level in a specific container. In the
flow chart, it is also possible, for example, to define that when a
specific level is reached, an outlet valve which responds to an
output from the higher-order unit 57 and belongs to this container
is to be opened.
[0063] One difference between the conventional 2-wire measuring
instruments 59 and the measuring instruments 61 according to the
invention resides in the fact that the current flowing via the
respective first pairs of lines 1 is a signal current, which
represents a measured value which is registered and used by the
higher-order unit 57. The supply current flowing via the respective
second pair of lines 3 is either ignored completely by the
higher-order unit 57, for example by its not appearing at all in
the flow chart, or else it can be allocated an alarm function or
the like. An alarm function could be configured, for example, in
such a way that the higher-order unit 57 triggers an alarm or
reports a malfunction if the supply current is greater than the
maximum signal current or less than the minimum signal current. In
addition, a sequence of actions directed toward safety can be
provided in-the flow chart for the eventuality of a malfunction of
the measuring instrument.
[0064] FIG. 4 shows a further exemplary embodiment of a measuring
arrangement having at least one measuring instrument 61 according
to the invention. The significant difference from the measuring
arrangement illustrated in FIG. 3 consists in that the higher-order
unit 75 of FIG. 4 comprises a control and/or regulating unit 77,
for example a programmable logic controller (PLC) or a distributed
control system (DCS), and a battery, arranged physically separately
from the latter, of series-connected transmitter feed units 79. The
battery is supplied via a power supply unit 83 connected to a
voltage source 81. Each transmitter feed unit 79 has a pair of
terminals for a 2-wire measuring instrument. In order that a
measuring instrument according to the invention can be connected,
the battery must have at least two transmitter feed units 79.
However, it is usual for such batteries to have considerably more
than two, for example 10 or 64, transmitter feed units.
[0065] Each transmitter feed unit 79 can be connected via its pair
of terminals to a measuring instrument, it feeds the measuring
instrument, registers a current flowing via the pair of lines
connected to its pair of terminals and, via a signal line 85,
outputs a signal to the control and/or regulating unit 77
corresponding to the current. In this exemplary embodiment, too, a
number of identical pairs of terminals is therefore provided and,
for the connection of conventional 2-wire measuring instruments 59
and measuring instruments 61 according to the invention, that which
was said previously in conjunction with the exemplary embodiment
illustrated in FIG. 3 applies.
[0066] FIG. 5 shows a further exemplary embodiment of a measuring
arrangement. The measuring arrangement has a number of batteries of
transmitter feed units 79, which are in each case fed via a power
supply unit 83 connected to a voltage source 81. Exactly as in the
case of the exemplary embodiment illustrated in FIG. 4, each
transmitter feed unit 79 here also has a pair of terminals, and
both conventional 2-wire measuring instruments 59 and measuring
instruments 61 according to the invention are connected to the
transmitter feed units 79.
[0067] In order to transmit the measured values, from the measuring
instruments 59, 61 connected to it, each battery of transmitter
feed units is connected via a bus access circuit 87 and a bus line
89 to a control and/or regulating unit 91, for example a
programmable logic controller (PLC), a distributed control system
(DCS) or a personal computer (PC).
[0068] All three measuring arrangements produce the advantages of
the measuring instruments 61 according to the invention to a
considerable extent. Thus, although these instruments need more
power than the 2-wire measuring instruments 59, in which, as
previously described, the supply and the transmission of the
measured value is certainly carried out via one and the same pair
of lines, and therefore only a limited power is available, they can
readily be used in a measuring arrangement which is intrinsically
designed only for 2-wire measuring instruments. Additional supply
terminals, such as conventional measuring instruments with a higher
power demand have, are no longer necessary, because of the design
according to the invention of the measuring instruments 61. The
measuring instruments 61 according to the invention are connected
to the higher-order unit together with the 2-wire measuring
instruments and in an identical way. An additional operation is not
required, and errors on account of transpositions of the terminals
of these instruments are ruled out.
* * * * *