Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
,
Field of the Invention
This invention relates to methods and apparatus for
detecting and obtaining informatian about (particularly locating)
changes in variables, and is related to the invention disclosed
and claimed in Canadian Patent Application Serial No. 457,882,
filed on June 29, 1984, in the name of Raychem corporation refer-
red to herein as the Masia/Reed application.
Introduction to the Invention
A number of methods have been used (or proposed for
use) to detect changes in variables along an elongate path, e.g.
the occurrence of a leak (of water or another liquid or gas),
insufficient or excessive pressure, too high or too low a temper-
ature, the presence or absence of light or another form of
electromagnetic radiation, or a change in the physical position
of a movable member, e.g. a valve in a chemical process plant
or a window in a building fitted with a burglar alarm system.
Changes of this kind are referred to in this specification by the
generic term "event". Such detection methods are for example
highly desirable to detect leaks from steam lines into thermal
insulation surrounding such lines, leaks from tanks and pipes
containing corrosive or noxious chemicals, or leakage or conden-
sation of water under floors or within telecommunication or
electrical power systems. Some of these known methods not only
signal when the event takes place, but also indicate the location
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of the event. However, the methods which were known prior to the
filing of the Masia/Reed applications and which indicate the loca-
tion of the event suffer from serious disadvantages. For example,
they make use of time domain reflectometer techniques (and are,
therefore, expensive), andfor give unreliable results when used
over usefully long elongate paths or under conditions when there
may be a substantial and unknown variation along the length of the
path of a variable which affects the accuracy of the measurement
(especially temperature), and/or make use of electrical conduc-
tors whose primary purpose is to carry a current (e.g. a telecom-
munication signal) under normal operating conditions (and which
therefore have resistance and uniformity characteristics consis-
tent with that purpose), and/or cannot be used when the event
causes electrical connection between two conductors through a
connection which is of high or indeterminate resistance, e.g. an
ionically conductive connecting element. Reference may be made
for example to U.S. Patents Nos. 1,084,910, 2,581,213, 3,248,646,
3,384,493, 3,800,216, and 3,991,413, U.K. Patent No. 1,481,850 and
German Offenlegungschriften Nos. 3,001,150.0 and 3,225,742.
Canadian Application Serial No. 457,882 describes an
improved method and apparatus for monitoring for the occurrence
of an event, and for detecting and obtaining infor-
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mation about the event upon its occurrence (i.e. as soon as it
occurs or at some time after it has occurred). In that method,
upon occurrence of the event, an electrical connection is made
between a source member and a locating member of known impedance
characteristics, the connection(s) being effective at a first
point at which the event takes place (or whose location is defined
by some other characteristic of the event). A current of known
size is then driven through the electrical connection and down the
locating member to a second point whose location is known. The
voltage drop between the first and second points is measured and
the location of the first point can then be determined. The system
preferably makes use of a locating member having an impedance
which is selected to give a desired degree of accuracy in locating
the first point, and which preferably does not vary substantially
within the temperature range in which the system operates. For
many uses, a particularly important advantage is that the infor-
mation obtained can be independent of the impedance of the connec-
tion to the locating member, i.e. the information obtained remains
the same even if a substantial and unknown change is made in the
impedance of the connection.
In the system of Canadian Patent Application Serial No.
457,882, the voltage drop between the first and second points is
preferably determined by means of a voltage-measuring device which
forms part of a reference circuit, the reference circuit comprising:
(a) the voltage-measuring device,
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(b) that part of the locating member which lies
between the first and second points, and
(c) an electrically conductive return member which
(i) is electrically connected to the locating
member at the second point and at another point
on the locating member whose distance from the
second point is at least as great as the distance
from the second point to the first point, both
distances being measured along the locating member,
and (ii) is otherwise insulated from the locating
member,
the voltage-measuring device having an impedance which is ~ery
high by comparison with any unknown part of the impedance of the
other components of the reference circuit.
Canadian Patent Application Serial No. 478,813 filed
on April 11, 1985 in the name of Raychem Corporation describes a
~ystem which is somewhat similar to that referred to in the pre-
vious paragraph, but differs from it in that (a) the source mem-
ber is connected to the locating member, so that current can pass
through the locating member in the absence of an event, and,
(b) upon occurrence of the event, electrical connection is made
between the locating member and the return member. Thus Canadian
application 478,813 provides a novel method for monitoring for the
occurrence of an event, and for detecting and obtaining informa-
tion about the event upon its occurrence, which
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method comprises providing a system in which, upon
occurrence of the event,
(1) electrical connection is made between an
electrically conductive locating member and an
electrically conductive return member;
the connection to the locating member being effective
at a first point whose location is defined by at
least one characteristic of the event;
the making of the connection enabling the formation
of a test circuit which comprises (a) the connection,
(b) that part of the locating member which lies between
the first point and a second point having a known
location on the locating member, and (c) a voltage-
measuring device which has an impedance which is very
high by comparison with any unknown part of the impedance
of the other components of the test circuit;
~2) there is provided a reference circuit which
comprises
(a) a source member which is electrically
connected to the second point on the locating
member and to a point on the locating member
whose distance from the second point is at
least as great as the distance from the
second point to the first point, both distances
being measured along the locating member, and
which is otherwise electrically insulated
from the locating member,
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(b) that part of the locating member which lies
between the first and second points, and
(c) a power source which causes an electrical current
of known size to be transmitted between the first
and second points on the locating member; the cur-
rent and the locating member being such that, by
measuring the voltage drop between the first and
second points, the spatial relationship between
the first and second points can be determined;
(3) the voltage-measuring device is used to measure the
voltage drop between the first and second points; and
(4) information concerning the event is obtained from
the measurement made in step (3).
Canadian Patent Application Serial No. 468,974 filed
on November 29, 1984 in the name of Raychem Corporation describes
preferred ways of assembling the locating, return and source
members, at least one of the members having a wrapped configura-
tion.
Canadian Patent Application Serial No. 478,817 filed on
April 11, 1985 in the name of Raychem Corporation describes a
modification of the systems of Canadian applications 457,882 and
478,813 in which a reference impedance is placed in series with
the locating member, and the location of the first point on the
locating member is determined from the ratio of the voltage drops
down the locating member and the reference impedance.
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Canadian Patent Application Serial No. 478,819 filed on
April 11, 1985 in the name of Raychem Corporation discloses a
modification of the systems of Canadian applications 457,882 and
478,813 in which the locating member comprises
(a) a plurality of spaced-apart discrete impedant
components, each of which has substantial
impedance, and
(b) a plurality of elongate intermediate components,
each of which physically separates and elec-
trically connects a pair of impedant components.
When such a locating member is used, the system isarranged so that, upon occurrence of an event, connection is made
to the locating member at one or more of a plurality of available
connection points, each of which lies between two impedant com-
ponents.
Canadian Patent Application Serial No. 478,818 filed
on April 11, 1985 in the name of Raychem Corporation discloses a
modification of the system of Canadian application 457,882 in
which the system is so arranged that the only variable impedance
in the test circuit is the impedance of the connection between
the source and locating members.
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In the systems described in the aforementioned applica-
tions, a key requirement is that a current of known size is driven
down the locating member. In order to supply such a current, a
controlled current source is preferably employed.
SUMMARY OF THE I~VE~TIO~
In making use of systems as described in Canadian
application Serial No. 457,882, using a controlled current source,
delivering a "fixed" current, in order to detect the presence of
electrolytes, we found that false information was sometimes
obtained. We further discovered that this false information was
obtained when a very small amount of electrolyte was present, so
that the locating and source members were connected to each other
through a connection whose resistance was so high that the
"controlled current" source was no longer able to supply the
expected current because its compliance voltage (i.e. the maximum
voltage which the current source can provide) was insufficient to
drive the "fixed" current in the test circuit. The present
invention solves that problem by including in the system one or
more components which
(a) monitor the current in the test circuit, and
(b) prevent the delivery of information if the current
falls below the "fixed" value.
It can also be desirable, in a Masia/Reed System including a
controlled current source, to prevent the
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delivery of information when the impedance of the con-
nection is within some predetermined range ~e.g. above
some predetermined level), even when part or all of
that predetermined range is not such as to cause the
current in the test circuit to fall below the "fixed"
level. Under these circumstances, the information will
be correct, but unwanted, and in accordance with the
present invention, the system is modified to include
one or more components which
(a) monitor the output voltage of the current
source (the voltage required to deliver the
"fixed" current to a particular test circuit) and
(b) prevent delivery of information if the output
voltage is within a predetermined range.
Similarly, when a constant voltage source is used
in the Masia/~eed system it may be desirable to prevent
the delivery of information when the impedance of the
connection is within some predetermined range (e.g.
above a certain level or below a certain level). Under
these circumstances, too, the information will be
correct, but unwanted, and in accordance with the pre-
sent invention, the system is modified to include one
or more components which
(a) monitor the current in the test circuit; and
(b) prevent delivery of information if tbe
current is within a predetermined range.
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In one aspect, the invention provides a method for
monitoring for the occurrence of an event which is expected to
occur at some unknown time in the future, and for detecting and
obtaining information about the event upon its occurrence, which
method comprises
(1) providing a system
(a) which comprises an elongate electrically
conductive locating member and an elongate
electricall~ conductive source member, and
(b) in whlch, upon occurrence of the event,
electrical connection is made between the
locating member and the source member,
the connection to the locating member being
effective at a first point whose location is
defined by at least one characteristic of the
event;
the making of the connection enabling the formation
of a test circuit which comprises (a) the
connection, (b~ that part of the locating member
between the flrst point and a second point having a
known location on the locating member, and (c) a
power source which has an output voltage of V volts
and which causes an electrical current I amps of
known size to be transmitted between the first and
second points on the locating member; and the
current and the locaiing member being such that, by
measuring the voltage drop between the first and
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27065-186
second points, the spatial relationship between the
first and second points can be determined;
(2) monitoring the system to determine when a said
connection has been made;
(3) when it is determined that a said connection has
been made, determining whether the value of the
ratio V/I is within a predetermined range, and if
it is, measuring the voltage drop between the first
and second points; and
(4) obtaining information concerning the event from the
voltage drop measured in step (3).
Step (4) is preferably carried automatically and the
information obtained is fed to a display.
In another aspect the invention provides apparatus which
is suitable for use in the method defined above and which
comprises
(1) a first terminal;
(2) a second terminal;
(3) a third terminal;
(4) means for connecting the first and second terminals
to a power source;
(5) a voltage-measuring device for measuring the
voltage drop between the second and third terminals;
(6) a display unit for displaying information derived
from a voltage drop measured by the voltage-measuring
device; and
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27065-186
(7) a device which, when the first and second terminals
are connected to a power source and are connected to
each other through a locating member and
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MP0924-CA
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a source member, thus forming a test circuit, prevents
the display unit from displaying information if the
value of the ratio V/I is outside a predetermined
range, where I is the current in the test circuit
in amps and v is the output voltage of the power
source.
~ his apparatus will often comprise a support member
to which components (1) to (7) are secured, and/or a
housing which provides physical and/or electrical
protection for the components.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated in the accompanying
drawings in which Figures 1, 2 and 3 are schematic cir-
cuit diagrams of the method of the invention.
DETAILED DESCRIPTION OF THE INVENTION
As previously noted, a key feature of the earlier
Masia/Reed applications is that, upon occurrence of an
event, a current of known size is driven down the
locating member. When using a fixed current source, as
is preferred, it is assumed, in calculating the loca-
tion of the first point, that the current driven down
the locating member is the "fixed" current. However,
this assumption will not be correct if the impedance of
the test circuit is so high that the compliance voltage
of the current source is insufficient to drive the
n fixed" current, and as a result, the location of the
first point will not be correct when it is calculated
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from the voltage drop between the first and second points.
The impedance of the test circuit, as defined by
Masia/Reed, includes as variables the resistance (RL) of that
part of the locating member which lies between the first and
second points on the locating member, and the resistance (RE) of
the event- sensitive connection means. RL is a variable because
the first point can occur anywhere along the locating member, but
the upper limit of RL is fixed. RE is a variable because
different events typically produce connections having different
resistance, and the variation can be very large.
If RL and RE are the only variables in the test
circuit, then it is not possible to fix precisely the range of
values of RE for which information is obtained. In other words,
the sensitivity of the system depends on the location of the first
point. This sensitivity decreases as the ratio of RE to the
total resistance of the locating member increases. Accordingly,
the critical impedance of the connection (or the lowest value if
it i8 a range) is generally at least 2 times, preferably at least
5 times, particularly at least 10 times, the impedance of the full
length of the locating member.
By making use of the invention described in Canadian
Patent application Serial No. 478,818, the values of RE can be
fixed precisely. Thus Reeder's systems include in the test cir-
cuit a component which (1) is connected in series with that part
of the locating member which
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lies between the first and second points and (2) has an
impedance substantially equal to the difference between
the total impedance of the locating member and the
impedance of that part of the locating member. Such a
component is preferably provided by making use of a
source member which has the same impedance charac-
teristics as the locating member.
The range of values for the ratio V/I within which
information will be reported can easily be changed (for
example so that leaks of different minimum sizes are
signalled). When using a fixed current source, sen-
~itivity can be changed by changing the compliance
voltage of the source, and/or by including a known
impedance in the test circuit, and/or by changing the
preselected range of output voltages. When using a
fixed voltage source, the size of the voltage can be
changed, and/or a known impedance can be included in
the test circuit, and/or the preselected current value
can be changed.
By way of example, in a system in which (1) the
source and locating members have a diameter of 30
mils, are separated by a distance of 0.25 inch, and are
assembled in a braided configuration; and (2) the
"fixed~ current is 250 microamps, then, if RE is set at
16.5 kilohms, a water leak less than 2 inches in length
is ignored by the system and a water leak in excess of
2 inches in length is detected by the system.
Attention is now directed to Figures 1, 2 and 3
which show schematic circuit diagrams of the method of
the present invention.
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Figure 1 includes a system 10 comprising source,
S, locating, L, and return, R, terminals. The source
terminal receives input along a line 12 from a power
supply 14, shown as a controlled current supply. The
locating and return terminals are connected along lines
16 and 18 respectively to a display 20. An overrange
blanking unit 22 is connected along lines 24 and 26 to
the power supply line 12 and display 20.
The overrange blanking unit 22 continuously moni-
tors the output voltage of the controlled current
source ~see Figure 2). If the variable sum (RL+RE)
exceeds a predetermined value, so as to increase the
output voltage to the compliance voltage, then the
overrange blanking unit 22 blanks out the display 20 so
that it will not display incorrect information as to
event location.
Figure 3 is similiar to Figure 2 except that the
power supply 14 is a constant voltage source. The
variable current in the test circuit, ILoop~ is moni-
tored by an ammeter A. If the ammeter indicates that
ILoop has fallen below a predetermined size, then the
overrange blanking unit 22 blanks out the display 20.
