Note: Descriptions are shown in the official language in which they were submitted.
CA 02322125 2000-10-03
EH 380 CA
September 25, 2000
Measuring instrument
The invention relates to a measuring instrument and a
measuring arrangement having at least one measuring
instrument.
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.
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).
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-
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wire measuring instruments. In standard form, such
measuring instruments are fed with 12 V, and the
measuring instrument cont=rols 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 o:E 20 mA, depending on the
instantaneous measured value.
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.
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
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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.
~ 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.
To this end, one aspect of the invention provides
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:
- 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.
In another aspect of the invention, there is
provided 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
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second pair of lines, said measuring instrument detecting at
least one physical variable, 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
of said at least one physical variable, wherein the signal
and supply currents are powered from said higher-order unit,
and wherein the supply current and at least a portion of the
signal current supply said measuring instrument.
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According to a further embodiment, the minimum signal
current is 4 mA and the maximum signal current is
20 mA.
According to a further embodiment, a current/voltage
limner is connected to the input side of each pair of
lines.
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.
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).
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.
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.
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
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measuring instrument and three exemplary embodiments of
a measuring arrangement are illustrated: identical
elements are provided in the figures with the same
reference symbols.
Fig. 1 shows a block diagram of a measuring instrument
according to the invention having two pairs of
lines, which supply two separate circuits:
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;
Fig. 3 shows a measuring arrangement having at least
one measuring instrument according to the
invention
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
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.
Fig. 1 illustrates a block diagram of a measuring
instrument that can be connected to a highex
order unit having at least a first and an identical
second pair of terminals.
To this end, the measuring instrument has a first pair
of lines 1 to be connected to the first pair of
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terminals and a second pair of lines 3 to be connected
to the second pair of terminals.
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 interferen~~e signals.
A current/voltage limiter is connected to the input
side of each pair of lines l, 3. Such a current/voltage
limner 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.
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 2ener 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.
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. l, and, in order to register
the voltage currently present, is connected to the
respective second line 7, 11.
Following the above-described current/voltage limner
on the input side, the first line 5, 9 of each pair of
lines l, 3 in each case has a controllable current
source 21, 23, which sets a current flowing via the
respective pair of lines l, 3 to a specific value as a
function of a contrcl signal.
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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.
In the exemplary embodiment of Fig. l, 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. l,
galvanic isolation 33 is implemented by means of two
optocouplers.
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.
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.
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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.
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.
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.
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The measuring instrument may have, as required, an on-
site display 39, an operating panel 41 and/or a
programming interface 43. The on-site display 39 is
used for example to disp:Lay 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.
Display 39, operating panel 41 and/or programming
interface 43 can be connected to the electronics 25, as
illustrated in Fig. l, and are supplied by the
electronics 25 via the first pair of lines 21.
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.
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
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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.
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.
From the view of the higher-order unit, the two pairs
of lines 1, 3 are identi~~al 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.
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.
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The significant difference resides in the
division of the power taken up by the measuring.
instrument.
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.
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.
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.
By means of the transformer 45, galvanic isolation
between the two circuits is ensured in this exemplary
embodiment as well.
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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.
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
mA.
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.
Figures 3 to 5 show three different measuring
arrangements having measuring instruments
according to the invention.
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.
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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.
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.
In the exemplary embodiments illustrated, conventional
2-wire measuring instruments 59 are connected to the
1St, the 2°d, the 5t''', the 8th, the 9th and the lOt° pair
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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.
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"d
pair of terminals and the second pair of lines 3 being
connected to the 4th 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 6th pair of
terminals and its second pair of lines 3 being
connected to the 7th pair of terminals.
With regard to the electrical connection, the measuring'
instrument 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, i~ 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.
One difference between the conventional 2-wire
measuring instruments 59 and the measuring instruments
61 according to the invention resides in the tact that
the current flowing via the. respective first pairs of
lines 1 is a signal current, which represents a
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September 25, 20000
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.
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.
Each transmitter feed unit 79 can be connected via its
pair of terminals to a measuring instrument, it feeds
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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.
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.
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).
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
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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.
