U.S. patent number 4,348,635 [Application Number 06/097,107] was granted by the patent office on 1982-09-07 for detecting and measuring the position of a break in solid formations by measuring the capacitance of an enlongated element embedded therein.
This patent grant is currently assigned to Joy Manufacturing Company. Invention is credited to Ivor Hawkes, David Wright.
United States Patent |
4,348,635 |
Wright , et al. |
September 7, 1982 |
Detecting and measuring the position of a break in solid formations
by measuring the capacitance of an enlongated element embedded
therein
Abstract
This invention relates to an apparatus and method for detecting
and measuring the position of a fracture in solid formations, such
as for example a grout filled borehole. An electrical element is
placed the length of the borehole prior to filling the hole with
grout. When the hole is filled and the grout solidified, any
fracture in the grout also breaks the electrical element. The
element is constructed such that its electrical capacitance is a
known function of its length. The element is constructed of easily
frangible materials so that it is severed in close proximity to the
grout fracture. The capacitance of the element after the fracture
can be measured using suitable instrumentation. The position of the
break is a function of the capacitance as measured after the
break.
Inventors: |
Wright; David (Vershire,
VT), Hawkes; Ivor (Hanover, NH) |
Assignee: |
Joy Manufacturing Company
(Pittsburgh, PA)
|
Family
ID: |
25349702 |
Appl.
No.: |
06/097,107 |
Filed: |
November 23, 1979 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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867393 |
Jan 6, 1978 |
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Current U.S.
Class: |
324/559; 324/519;
324/725; 340/690; 73/784 |
Current CPC
Class: |
G01N
27/24 (20130101); G01B 7/042 (20130101) |
Current International
Class: |
G01B
7/02 (20060101); G01N 27/24 (20060101); G01N
27/22 (20060101); G01B 7/04 (20060101); G01B
007/22 (); G01R 031/08 (); G01L 001/14 () |
Field of
Search: |
;324/61R,61P,52,51,326
;73/762,768,594,775,595,780,781,784,799,803 ;340/690,564,563,562
;361/280,283,324 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Strecker; Gerard R.
Attorney, Agent or Firm: Helms; Ernest E.
Parent Case Text
This is a continuation, of application Ser. No. 867,393, filed Jan.
6, 1978, now abandoned.
Claims
What is claimed is:
1. An apparatus for determining the position of a structural break
in a solid formation comprising:
a continuous, elongated, linearly extending electrical element
adapted to be embedded in solid formation, said element
comprising;
two flat continuous, elongated, linearly extending frangible
conductive tape means for conducting an electric current and
storing an electric charge and adapted to be connected to a
capacitance measuring instrument;
a flat continuous, elongated, linearly extending dielectric paper
tape disposed intermediate said two flat continuous, elongated,
linearly extending frangible conductive tape means with one said
flat continuous, elongated, linearly extending frangible conductive
tape means abutting the first surface of said flat continuous,
elongated, linearly extending flexible dielectric paper tape along
said flat continuous, elongated, linearly extending flexible
dielectric paper tape length and another said flat continuous,
elongated, linearly extending frangible conductive tape means
abutting the second surface of said flat continuous, elongated,
linearly extending flexible dielectric paper tape along said flat
continuous, elongated, linearly extending flexible dielectric paper
tape length whereby the electrical capacitance formed between said
two flat continuous, elongated, linearly extending frangible
conductive tape means is a function of the length of said flat
continuous, elongated, linearly extending frangible conductive tape
means and;
grouting means surrounding said electrical element rigidly
embedding said electrical element in such solid formation, whereby
a fracture in said grouting means due to structural break in the
solid formation causes a severing of said flat continuous,
elongated, linearly extending frangible conductive tape means in
close proximity to said fracture and wherein the location at which
said severing occurs may be determined by the capacitance measured
between said two flat continuous, elongated, linearly extending
frangible conductive tape means by said capacitance measuring
means.
2. An apparatus of claim 1 wherein said flat continuous, elongated,
linearly extending frangible conductive tape means and said flat
continuous, elongated, linearly extending flexible dielectric paper
tape are generally of uniform cross section throughout their
length, whereby the capacitance formed is a linear function of the
length of said flat continuous, elongated, linearly extending
frangible conductive tape means.
3. The apparatus of claim 1 wherein said two flat continuous,
elongated, linearly extending frangible conductive tape means are
electrically connected at one end of said flat continuous,
elongated, linearly extending flexible dielectric paper tape to
form one conductive path.
4. An apparatus of claim 1 wherein a cross section taken transverse
to the maximum dimension of said flat continuous, elongated,
linearly extending flexible dielectric paper tape has a maximum
cross sectional dimension greater than the maximum cross sectional
dimension of said two flat continuous, elongated, linearly
extending frangible conductive tape means.
5. The apparatus of claim 1 wherein said apparatus has superimposed
thereupon an exterior nonconductive material.
6. An apparatus of claim 1 wherein said flat continuous, elongated,
linearly extending flexible dielectric paper tape comprises a tape
less than two inches in width and less than five thousandths of an
inch thick; and said two flat continuous, elongated linearly
extending frangible conductive tape means comprises an aluminum
tape less than one and one-half inches wide and less than five
thousandths of an inch thick.
7. An apparatus as described in claim 1 which further comprises
connective means for electrical connection to each of said flat
continuous, elongated, linearly extending frangible conductive tape
means at one end of said apparatus and capacitance measuring means
for determining the electrical capacitance of said flat continuous,
elongated, linearly extending conductive tape means by contacting
said connective means.
8. The apparatus of claim 7 further comprising shorting means for
electrical connection between said flat continuous, elongated,
linearly extending frangible conductive tape means on the end of
said apparatus opposite from said connective means.
9. The apparatus of claim 8 further comprising continuity check
means operatively associated with said connective means for
indicating the continuity of the series arrangement of said
shorting means and said two flat continuous, elongated, linearly
extending frangible conductive tape means.
10. The apparatus of claim 9 further comprising resistance
measuring means for determining the electrical resistance between
said two flat continuous, elongated, linearly extending frangible
conductive tape means.
11. An apparatus of claim 9 further comprising drying means for
applying an electric current to said two flat continuous,
elongated, linearly extending frangible conductive tape means
whereby causing a resistance heating element in said flat
continuous, elongated, linearly extending flexible dielectric paper
tape.
Description
BACKGROUND OF THE INVENTION
In many applications involving large solid structures it is
desirable to detect internal disturbances. It is additionally
valuable to be able to measure the position of such disturbances
without further disturbing the integrity of the structure. In
mining and construction it is often required to detect faults or
disturbances and their positions in earth and rock formations. To
detect such faults an electrical element can rigidly be embedded
into the solid structure or earth. When a portion of the solid
breaks or moves relative to the electrical elements it causes the
electrical element to shear or break. Prior methods have used a
ladder like arrangement of parallel resistors as taught by U.S.
Pat. No. 3,477,019. Such systems lack accuracy because of their use
of discrete components. The accuracy of such system is directly
proportional to the number of components and cost. Because these
devices determine position by measuring resistance between parallel
conductors, any shunting resistance such as moisture paths between
the conductors results in an error in position determination. My
invention overcomes these inherent faults by using capacitance as a
measure of position and by using one continuous inexpensive
element.
An elongated electrical element is embedded in a solid formation.
This element is made of easily shearable materials and has a
capacitance that varies as a function of length. The element acts
as an end-feed capacitor having accessible leads attached to the
conductive surfaces at one end of the elongated element. When a
meaningful disturbance occurs in the formation the frangible
element breaks in the area of the disturbance effectively severing
the element into at least two lengths and reducing the effective
capacitance connected to the leads. The position of the break can
be determined by measuring the capacitance at the leads and
relating it to the length by the previously known function.
It is often required to know the position of a disturbance in a
solid formation such as earth, rock, or formed conglomerates. This
information is especially valuable in excavations such as are found
in mining and construction for example. In such applications
boreholes are normally drilled to test or reinforce the strata or
solids. In one application of my invention an elongated electrical
element is securely embedded generally axially within these
boreholes. The electrical element does not interfere with other
objects such as for example, structural rods or bolts that may be
included within the same borehole. A hard cementitous material such
as for example concrete can be used to embed the electrical element
within the borehole. If it is desired to detect a very slight earth
movement or no additional strengthening of the earth formation is
required, a weaker, more brittle material can be used as the
embedding grout. The grout used need only be capable of
transmitting the movement or force of the disturbance in sufficient
amount to shear or break the electrical element. The electrical
element is made of frangible materials that are easily severed by
the forces present during a detectable disturbance. Firmly grouting
the electrical element into the solid and constructing the element
from easily shearable material, causes the element to sever at a
point corresponding to the location of a disturbance or movement in
the solid.
The point at which the electrical element is severed can be
calculated by comparing the electrical characteristics before the
disturbance with those after. The electrical element is constructed
so that the electrical characteristics are of a known function of
the physical length of the elongated element. While any known
function is sufficient, it will be desirable to use a continuous
linear function so as to simplify the calculations. In certain
applications it may be desirable to use a non-linear function which
better suits the physical parameters of the disturbance; such as
designing the element so that the electrical characteristics vary
greater per unit length in the area where disturbances are
anticipated so that the highest resolution and accuracy will be
obtained in that area.
Variation in the capacitance for non-linear elements may be made by
varying the distance between electrical conductive surfaces,
increasing or decreasing the area of the electrical conductive
surface, or using electrical insulating material of varying
dielectric constants between the conductive surfaces.
If the elongated element is constructed to have a capacitance which
is a linear function of the elongation, such as a parallel plate
capacitor; then the position of the disturbance is directly
proportional to the capacitance measured at the exposed leads.
Additionally, the ends of the capacitor plates opposite the exposed
leads can be shorted together to allow a continuity check from the
exposed leads. A positive continuity check indicates an unbroken
capacitor and no further capacitance measurement need be taken.
Accordingly, one object of this invention is to provide a means to
accurately and economically determine the position of a movement
within a solid formation.
Another object is to provide for the detection of breaks in the
grout material as a function of the elongated frangible
element.
Another object is to provide an inexpensive electrical element that
can readily be inserted into a borehole and easily fashioned to the
exact depth of the borehole.
Additional objects and features of the present invention will
become apparent to those skilled in the art as the following
description of certain present preferred embodiments thereof
proceeds.
DRAWING DESCRIPTION
In the accompanying drawings are shown present preferred
embodiments and methods of practicing the same in which:
FIG. 1 is a perspective view of an elongated element using foil
conductive surfaces and a strip dielectric, having the foils
shorted on one end;
FIG. 2 is a cross-sectional view taken transverse to the elongated
direction of an element similar to that shown in FIG. 1;
FIG. 3 is a cross-sectional view of an element similar to that of
FIG. 2 except having an outer insulating covering;
FIG. 4 is a cross-section view of an element using a coaxial
arrangement of conducting surfaces and an outer insulating
covering;
FIG. 5 is a cross-sectional view of a borehole, such as in a mine
roof showing an element embedded in grout;
FIG. 6 is similar to FIG. 5 but shows a disturbance and resulting
fracture of the electrical element occuring at a distance B from
the connecting end of the element;
FIG. 7 is a schematic block diagram showing the apparatus and modes
for determining the position of a break;
FIG. 8a is a block diagram of a circuit to measure the capacitance
of the element;
FIG. 8b is a block diagram of a circuit used to measure the
capacitance of the element using two monostable vibrators; and
FIG. 8c is a diagram of a circuit which may be used to detect
continuity and provide variable heating current.
Shown in FIG. 1 is an embodiment of a parallel plate element that
uses the characteristics of capacitance and continuity to detect a
fault and determine the position of the disturbance. The element
generally indicated by the reference 12, has been drawn broken to
effectively show both ends of the elongated element. The element 12
can be of any length from a few feet to several hundred feet. The
center portion of this element is composed of a dielectric strip or
tape 13 which generally extends the length of the element. This
dielectric strip may be, for example, a paper or plastic tape.
While a dielectric material of any thickness will function in
forming a capacitance in this parallel plate element, a 3 mil thick
tape will tend to create a reasonable capacitance per linear foot
of the elongated element and allow the element to be flexible
during storage prior to installation.
Firmly attached to each side of the dielectric strip 13 are
parallel electrically conductive means or foils, 14a and 14b for
storing electrical charge. The conductive foil strips or commonly
referred to as tapes 14a and 14b have respective conducting
surfaces 14c and 14d abutting the dielectric strip 13. The
conductive foils and respective conducting surfaces are held in
fixed relationship to the dielectric strip 13 by adhesive layers
17a, 17b.
The conductive foils 14a and 14b can be made of any electrically
conductive material that is frangible when subjected to the forces
present in the particular disturbance desired to be detected. The
conductive foil is made so as to sever generally transverse to the
length of the element in close proximity to differences in forces
along its length. These forces may be caused for example by a
displacement in the grouting material resulting from a shift or
fault movement in the solid. While only one conductive foil need be
frangible it will usually be desirable to have both conductive
foils made of similar materials.
At one end of the element corresponding for example to the maximum
depth in a borehole installation, the conductive foils 14a and 14b
are electrically connected by shorting means, for example, shorting
fold 16. While any means for electrically connecting these two
conductive foils may be used, such as wiring, stapling or
mechanically joining, it has been found that removing a portion of
the dielectric allows one conductive foil strip such as 14a, to be
folded on the other conductive foil strip such as 14b.
At the end of the element 12 opposite the shorting fold 16, the two
conductive foils 14a and 14b have been extended to produce
connecting ends 15a and 15b, respectively. These connecting ends
15a and 15b can be either directly connected to electrical
instrumentation or connected to other conductors or terminals that
provide for connection to electrical devices. In other embodiments
provision can be made for connecting wires or terminals directly to
the ends of foils 14a and 14b without extending the foils. In such
embodiments, the object is to provide connective means for the
respective ends of the conductive surfaces. This allows the
conductive surfaces to be electrically charged or discharged
through the connective means into instrumentation which measures
the capacitance of the element. Such connective means also allows a
test of the continuity through the path of series connected
conductive foils 14a, shorting fold 16 and conductive foil 14b.
FIG. 2 is a cross-section, of a parallel plate element similar to
that shown in FIG. 1 taken transverse to the elongations showing
the respective layers within the parallel plate element 12.
Dielectric strip 13 is intermediate conductive foils 14a and 14b.
Inner conductive surfaces 14c and 14d, of 14a and 14b respectively,
are held in parallel arrangement with the dielectric strip 13 by
the adhesive layers 17a and 17b. The capacitance characteristics of
the parallel plate element is readily visible in the laminated
arrangement of FIG. 2. The distance between conductive surfaces 14c
and 14d correspond to the distance between conducting surfaces in a
parallel plate capacitor. The capacitive dielectric is composed of
the dielectric strip 13 and the adhesive layers 17a and 17b.
It is well known that the capacitance, C, for such a parallel plate
capacitor is calculated by the formula C=KeA/D, where K is the
dielectric constant, e is the permittivity constant, A is the
surface area of one of the conductive surfaces and D is the
distance between the parallel plates. If the electrical element is
constructed as shown in FIG. 2 with uniform cross section
throughout its length, then the equation for capacitance becomes
C=(KeW/D)L wherein W is the width of the conductive surface and L
is the length of the conductive surface. While any value of
capacitance can be used values in the range of 1 to 100 picrofarads
per inch are easily obtainable. Some installations may use such
elements having much larger electrical capacitance per inch.
Depending upon the length of the solid being monitored and the
magnitude of the break or fracture desired to be detected, larger
elements and corresponding larger capacitance could be used. In
mining installations a convenient size element can be made using
conductive foil less than 5 mil thick and less than one and one
half inches wide, with a dielectric of similar thickness and less
than two inches wide.
In referring to FIG. 2 is should be noted that in cross-section the
dielectric strip 13 has a larger width than the respective
conductive foil 41a and 14b. The wider dielectric strip in this
embodiment acts as a protective barrier between the respective
conducting surfaces 14a and 14d. In a one inch wide dielectric
strip is used with one half inch wide conductive foils centered on
the dielectric, a one quarter inch barrier exists to prevent the
foils from shorting together along each edge of the element.
It has been found that 3 mil hard aluminum foils as the conductive
foils, and 3 mil paper tape as the dielectric strip with a 1 mil
adhesive layer, produces a parallel plate element having good
flexibility during insertion into the borehole and such strip is
easily sheared by disturbances in the solid, for example earth
disturbances in mining installations.
FIG. 3 shows a cross-section of another embodiment in which a
parallel plate arrangement is encased in an outer insulation. The
dielectric slab 51 is intermediate the two conductive plates 52a
and 52b. An outer insulation covering 53 has been added. Such an
insulation can be used to add additional rigidity to the element
and/or simultaneously to protect the element from intrusion of
water, acid, gas or other foreign materials. The element shown in
FIG. 3 has a capacitance that can be calculated by the same
equation as given for the element in FIG. 2.
FIG. 3 is one example of an element that does not use an adhesive
to maintain the proper spacing between the conductive means such as
plates or foils for example. The adhesive layers may be omitted if
the conductive means is bonded directly on the dielectric such as
when a conductive metal coating is used as the conductive means. In
some embodiments the dielectric itself is the adhesive as when the
conductive foil is attached to a plastic dielectric, for
example.
Referring now to FIG. 4 there is shown a cross-section of an
element having a coaxial arrangement. The coaxial element is
composed of a center conductor 56 having a circular outer
conducting surface 56a. Coaxially surrounding the center conductor
56 is a dielectric tube 57 having uniform wall thickness.
Concentric with the dielectric tube is outer conductor sleeve 58
having an inner conducting surface 58a. The element is then encased
in an outer protective covering 59. The protective covering could
be omitted if the outer conductor 58 is made sufficiently durable
for the specific application. While the embodiment shown in FIG. 4
does not have an adhesive layer shown, such a layer could be
used.
If the coaxial element has a uniform cross section throughout its
length, then its capacitance will be a linear function of the
length of the element. Such a coaxial element has a capacitance
given by the equation C=2.pi.KeL/ln (a/b). Where a is the radius of
the inner conductor; and L is the total length of one of the
conductive surfaces; and b is the distance from the center of the
inner conductor to the inner surface of the outer conductor; and K
is the dielectric constant; and e is the permittivity constant.
Similar to the element shown in FIG. 1 a coaxial element can have
the conductive means or one end shorted so as to allow for a
continuity check prior to capacitance measuring. This can be done
by electrically connecting one end of the center conductor 56 to an
adjacent surface on the outer conductor 58.
While two specific types of elongated elements, parallel plate and
coaxial, have been described it is to be understood that elements
composed of variations of these or other known types of capacitor
design are included within the scope of this invention. Such other
embodiments would include elements wherein at least one of the
conducting surfaces is a rigid support member such as for example
an anchoring or roof bolt. In such systems only one of the
conducting means in the element need be frangible or easily
shearable. Depending upon the desired accuracy and dimensions some
applications may use the actual grout material itself as a
dielectric between two conducting surfaces.
While an equation for the capacitance, such as the two stated
previously, can be determined for any given geometric structure by
an analysis of electric fields using Gauss's law. Such equations
are not necessary to determine the capacitance as a function of
length of the elongation as empirical methods can be used. After
any element has been formed having uniform cross-sectional
dimensions and materials, such an element will have a capacitance
which is a linear function of the length of the elongation. Using
instrumentation the capacitance of the complete element can be
measured; such measurement may be taken after the element is
inserted into the solid and prior to any disturbance. This measured
total capacitance divided by the total length is a constant u. For
linear varying capacitance this relationship can be written as
C=F(x)=ux, where x is the length of an element.
Since the constant u is the same before and after the break, this
equation can be used to solve for the value of x after the break by
dividing the capacitance measured after the break by the constant
u. The new value for x will indicate the length of the element
after the break.
While any dielectric material can be used it may be desirable in
some applications to use a highly breakable dielectric such as
glass. Generally a somewhat flexible dielectric such as paper or
plastic will result in easy storage prior to insertion of the
element into a borehole. The use of materials such as aluminum
foil, and plastic tape allow for easy transportation and cutting of
laminated materials to the exact length at the insertion site
during grouting operations.
FIG. 5 shows a cross-section of an installation of a parallel plate
element firmly embedded in a grout filled borehole. The borehole 21
is drilled into the face 26 of an earth formation 20. A parallel
plate element 12 is inserted into the borehole 21 so that the
element 21 extends generally axially within the borehole 21. When
the element is in proper position the hole is next filled with the
grout material 22, such as for example, concrete. The embodiment
shown in FIG. 5 has provision for the borehole to be filled with
grout material 22 by means of hollow grout pipe 24. A seal plug 27
is fitted into the mouth of the borehole and has provision for the
grout pipe 24 and the connector 25 to extend through the seal plug
27.
While many means can be used to provide electrical connection from
the element 12 to a position outside the borehole at the face 26,
the embodiment in FIG. 5 uses electrical connections 23a and 23b
which are in electrical contact with the respective conductive
foils of 14a and 14b and the grout pipe 24 and connector rod 25.
When so connected an electrical current path exists through the
series arrangement of grout pipe 24, electrical connection 23a,
conductive foil 14a, shorting fold 16, conductive fold 14b,
electrical connection 23b and connector rod 25.
If a continuity detector is connected to the grout pipe 24 and the
connector rod 25 as shown in FIG. 5, such continuity detector will
show a very low resistance in the current path. This low resistance
indicates that the element 12 is intact and no disturbance of the
earth has occurred. In normal testing it would not be necessary to
take further capacitive measurements; embodiment not using shorting
fold 16 would indicate the total capacitance C, corresponding to an
element of length L.
The total capacitance, C of the element 12 is known either from
calculation or from actual measurement prior to any break of the
element. If an element having its capacitance varying as a linear
function of length such as in FIG. 5 is used, then the capacitance
per unit length, u, is C/L.
FIG. 5 shows a typical installation with an element length L where
the element is set back or recessed a distance S from the face 26.
While the scope of this invention encompasses any distance L,
experimentation has been done in which L varied from a few feet to
several hundred feet. In installations where L is several hundred
feet or where accuracy is not critical the dimension S may be
neglected in calculating the position of a break. In other
installations the end of the element 12 may be extended to the face
so that the set back dimension S is zero.
The installation shown in FIG. 5 is in a vertical borehole having
an open bottom. Such an installation would be typical of a mine
roof bolt hole. Other installation sites would include for example
horizontal boreholes, vertical top opening holes, and cast concrete
structures.
Referring to FIG. 6, this shows the installation of FIG. 5 after a
disturbance has occurred in the earth formation. The displaced
earth 20b has caused the grout material to separate into three
sections, a severed grout 22a, a displaced grout 22b and a
remaining grout 22c. The displaced grout 22b has caused the element
12 to break into three portions. A portion of the element 12b has
become displaced and severed; and a portion 12a has become severed.
The remaining portion 12c is intact and electrically connected to
the connector rod 25 and grout pipe 24. Because the severed element
12a containing the shorting fold 16, is no longer in electrical
connection with the conductive foils of element portion 12c, a
complete current path does not exist between the grout pipe 24 and
connector rod 25. When an ohmeter or other means for indicating
continuity means is attached to the grout pipe 24 and connector rod
25, an open circuit is indicated by the high resistance measured.
The continuity detector could be any known circuit such as for
example, a lamp or voltage source in series, or an ammeter and
battery in series. In normal installations a simple continuity test
can be made to detect disturbances as a prerequisite to the more
exacting capacitance measurement. A series of such elements may be
electrically interconnecting so that an automatic monitor of
continuity could indicate a disturbance in the system.
To determine the position of the break in element 12 of FIG. 6 a
capacitance measuring instrument is connected to the connector rod
25 and the grout pipe 24. The capacitance measured after an earth
disturbance or fault, herein referred to as the break capacitance,
is related to the length of the element portion 12c by the same
function as previously calculated or measured for the unbroken
element 12. The distance B can be found by substituting into that
equation the break capacitance and solving for the length of the
element which corresponds to B in FIG. 6. The sum of S and B will
correspond to the position of the break.
While the earth movement in FIG. 6 has resulted in a single
stratified movement of earth 20b, actual earth disturbances may
cause additional movements or be of such magnitude so as to
displace both section 12a and 12b of the element. The procedure
previously described allows position determination of the
disturbance in closest proximity to the face 26. Should the
installation allow additional electrical connectors to the end of
the element opposite the face such as on portion 12a in lieu of the
shorting fold 16, then the position of the break area could be
ascertained relative to both ends of the element by using the
measured break capacitances of both portions 12a and 12c. Such
additional electrical connector would be brought out of the solid
in a direction opposite the face 26, so as not to be affected by
the disturbance. Such additional connection would allow the portion
12a to be measured in the same way as 12c. These additional
electrical connections can be brought to the face 26 or another
measuring position in any manner such that they do not electrically
disconnect during a disturbance.
In the preferred embodiments specifically described the position of
the disturbance is indicated by a reduction in the measured
capacitance of the element. This reduction occurs when at least one
of the conductive surfaces of the element is severed. In the
drawings, both conductive foils and surfaces, and the dielectric
have been shown as severed, but it is to be understood that only
one surface need be broken to indicate the position of a
disturbance. For this reason it may be desirable, where fine
sensitivity is required to have one conductive surface made of a
thin conducting metal coating which is easily broken.
FIG. 7 shows a block diagram for a disturbance tester connected to
an element similar to FIG. 6 after a break has occurred. The tester
uses a switch or mode selector 43 to electrically connect one of
three circuits to the elements by means of connections 23a and 23b.
The element having original length L is shown severed having a
remaining length B. The element is composed of a dielectric slab 13
intermediate two conductive foils 14a and 14b. At the end of the
element opposite the electrical connections 23a and 23b is a
shorting fold 16.
Prior to the fault the capacitance and length of the element have
been determined and recorded. In the normal sequence of operations
the tester is periodically connected to the element preferably
keeping all leads relatively short to avoid stray capacitance. The
mode selector is placed in position "a" so as to connect the
continuity detector 40 to the element. The detector 40 may be any
known means for continuity indication such as for example an
ohmeter. If the element is unbroken, a low resistance current path
exists between conducting foils 14a and 14b through the shorting
fold 16. If as shown in FIG. 7 the element is broken the detector
40 will show a high resistance path or open circuit indicating an
earth disturbance.
If the detector 40 indicates an open circuit the operator changes
the mode selector 43 to position "b" thereby connecting a
capacitance measuring device 41 to the element. The capacitance
measuring device can be of any known type and for ease of operation
can be calibrated so that it reads directly in units of length. If
the capacitance measuring device reads in units of capacitance, the
length B can be calculated by the equation previously given.
In some environments, especially where moisture is present, the
dielectric strip 13 may develop leakage current paths, represented
in FIG. 7 as RL. This leakage path shown as a leakage resistance
RL, is often present when a moisture absorbant material such as
paper is used for the dielectric. Leakage resistance should
generally be larger than 20,000 ohms to facilitate accurate
capacitance readings from measuring device 41. If the leakage
resistance is low, indicating sizable current paths between
conductive foils 14a and 14b, an electrical heating power source 42
may be connected to the element by position "c" on the mode
selector 43. This source 42 provides current to produce I.sup.2 R,
resistance heating within the element.
FIG. 8a is a block diagram of a circuit for a capacitance measuring
device which can be used as the device 41 in FIG. 7. The square
wave generator or SWG 70 in the form shown is of the type with an
output frequency which is a function of the connected capacitance
C. The output pulses of the SWG 70 are used to trigger a gating
module GM, reference numeral, 73. The period of the output of SWG
70 is a known function, usually linear, of the capacitance C. This
period is used as the "on" interval for the GM 73. The clock, 71
feeds a series of pulses to the scaling function generator or SFG
72 which scales the frequency so that the readout will be in proper
engineering units of length. The output of the SFG 72 is permitted
to pass through the gating module 73 during a period of the SWG.
This string of pulses is proportional to C and are counted on the
digital counter or DC 74. This count can then be shown on the digit
display or DD 75. The actual circuits used in each of the blocks 71
through 75 are well known in the art and a variety of known
circuits can be used for any of the circuits represented by the
blocks.
The circuit diagram shown in 8b is an example of an embodiment of a
capacitance measuring device which compares the output of two
matched monostable multivibrators. The output of one monostable
multivibrator MSMV-1, 81, is a fixed pulse having width T. The
output of the other monostable multivibrator MSMV-2, 82, is a pulse
having a width T+t where t is proportional to the capacitance C
added in the external circuit. The values for C1 and R2 are fixed
and may be chosen so as to enhance the relation of T to t so that
desired accuracy can be achieved. While R1 will normally be equal
to R2, R1 may be variable so as to provide a calibration.
Both MSMV-1, 81 and MSMV-2, 82 are initiated simultaneously by the
triggering unit or Tu, 80. The output pulses, as shown on FIG. 8b
are fed to a pulse width comparator or PWC, 83. The PWC subtracts
the output from MSMV-1 from the output of MSMV-2 and feeds the
remaining signal to the amplifier or AMP, 84.
The AMP, 84 amplifies the signal t which is proportional to C. The
signal can also be scaled by the amplifier so that when it is fed
into the indicator or I, 85, the units will read directly in units
of length. If for example, I is a meter it can be calibrated to
read in feet, meters or other units of length. The individual
blocks of FIG. 8b are well known to those skilled in the art. While
any capacitance measuring device can be used; it is desirable that
the device be designed to operate accurately even when a leakage
resistance is present.
Referring now to FIG. 8c, which shows a circuit that can be used
both as the continuity detector 40 and the heating power source 42.
When the terminals OC and COM are used the battery V1 and the
ammeter A are in series to function as an ohmeter and a means for
indicating continuity. If the terminals OH and COM are used the
circuit can function as a heating current source with the battery
V1 and the variable resistance RV in series.
While the specification has shown and described certain present
preferred embodiments it is to be distinctly understood that the
invention is not limited thereto but may be embodied in other
alternatives, modifications and variations apparent to those
skilled in the art. Accordingly, it is intended to embrace all such
alternatives, modifications, and variations as fall within the
spirit and scope of the appended claims.
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