Methods of and apparatus for detecting openings in cable jackets

An opening in a jacket (15) of a cable (16) is detected as the jacketed cable is moved from an extruder (14) through a probe (21) which is positioned in a cooling trough (17) and which comprises first and second aligned, spaced sensors (24, 26) with each of the sensors being connected to an arm of a bridge transformer (37). Electrical circuitry is designed to cause an unbalance voltage in a secondary (43) of the transformer (37) in response to conductance unbalance as the opening in the jacket (15) is advanced through the first sensor (24) and to cause an unbalance voltage of opposite polarity in the secondary (43) in response to conductance unbalance as the opening in thejacket (15) is advanced through the second sensor (26). Logic circuitry (66) tests the unbalance voltages for threshold and peak values to discern jacket holes despite the presence of extraneous signals such as those which are caused by noise.

TECHNICAL FIELD 
This invention relates to methods of and apparatus for detecting openings 
in cable jackets, and, more particularly, to methods of and apparatus for 
detecting openings in a thin-wall cable jacket made of a plastic material 
by a test for conductance unbalance as the cable is advanced along a 
manufacturing line. 
BACKGROUND ART 
One kind of defect that can occur during the manufacture of cables is an 
opening in a plastic jacket which encloses a core that comprises a 
plurality of individually insulated conductors. Unwanted jacket openings 
may be caused in several ways during the manufacture of a cable, such as, 
for example, by line and drive system disturbances. An incomplete breakup 
of plastic pellets, which are supplied to a jacketing extruder, or a loose 
binder wrapped about the core can cause a protuberance such that a plastic 
melt comprising an extrudate tears about the protuberance as it is pulled 
down into engagement with the core. 
Such openings are more likely to occur in thin-wall jackets, i.e. on the 
order of 20 to 40 mils thick, which are customary for inside wiring and 
switchboard cables, than in those cables used in other environments where 
jacket thicknesses are generally on the order of 45 to 100 mils. This 
problem has become more critical in recent years because of plastic 
material shortages. With petroleum-based plastics not infrequently in 
short supply, there has been a trend toward the use of thinner jackets of 
perhaps different plastics which offer the same amount of mechanical and 
electrical protection notwithstanding the thinner wall. 
Until recently, there had been no industry wide effort in 
telecommunications cable manufacture to detect jacket openings along a 
manufacturing line on which a cable core is jacketed. This may be due, at 
least in part, to the heretofore uninterrupted supply of plastic and hence 
the absence of a need to minimize jacket thicknesses. 
Commercially available equipment which generally is designed to detect 
coaxial capacitance unbalance caused by jacket openings does not 
consistently and reliably differentiate between jacket openings and noise. 
Coaxial capacitance as measured from a probe to the cable core varies 
because the jacket is not always concentrically disposed about the core 
with the jacket thickness varying within acceptable limits. Inasmuch as 
capacitance is an area-sensitive electrical characteristic, a further 
drawback to the use of capacitance unbalance to detect jacket openings is 
that an opening of significant size must occur to cause a measurement 
reading to vary beyond its normal range. 
The prior art includes U.S. Pat. No. 3,047,799 which discloses 
interference-free spaced dual probes for detecting deleterious occlusions 
within insulated conductors. The methods used in this patent require (1) 
that the conductor under test be grounded, (2) that a high voltage be 
applied as the conductor is passed through the dual probe to cause corona 
at the insulation occlusion, (3) that distilled water be used as a 
couplant in order to obtain a high resistance between an exciting ring and 
the insulated conductor under test, and (4) that a center conductor be 
excited and that signals radiated from the occlusions be detected. 
Also included in the prior art is a bare wire detecting device which 
includes two conductive zones that are insulated and spaced from each 
other by a predetermined distance. A relatively high D.C. voltage is 
impressed across the two zones to cause bare portions of an insulated 
conductor which are equal to or longer than the distance between the zones 
to complete a circuit to indicate the presence of unacceptable lengths of 
bare wire. See U.S. Pat. No. 3,277,365. This device is inapplicable to a 
test for jacket openings since there is no bare wire; moreover, this 
arrangement, which relies on high voltage for its test, is not desirable 
for use in a factory environment. 
It is also old to use light to detect the presence of pinholes during the 
manufacture of a metallic strip such as tinplate which may be moved at 
speeds up to approximately 1524 meters per minute. The use of a light 
source and a conventional detector tube on opposite sides of a jacketed 
cable core would not be adaptable to the detection of jacket openings for 
obvious reasons. 
Nowhere in the prior art does there appear to be a simple, on-line 
apparatus which may be used to detect jacket openings without the 
necessity of special couplants such as distilled water in cooling troughs, 
without the necessity of relying on an operator for externally grounding 
the cable, and without the need to externally excite the cable under test. 
DISCLOSURE OF THE INVENTION 
The foregoing problems are overcome by this invention for testing an 
advancing cable which includes a multi-conductor core enclosed in a 
plastic jacket to detect the presence of openings in the cable jacket. In 
accordance with the invention, a method of detecting openings in a cable 
includes the steps of sensing conductance across a plastic jacket of each 
of a first pair of adjacent sections of a cable which includes a 
multiconductor core that is enclosed in the plastic jacket, sensing 
conductance across the plastic jacket of each of a second pair of adjacent 
sections of the cable in which one of the adjacent sections of the second 
pair is common to the first pair, detecting conductance unbalance, if any, 
that exists between the first and second pairs of cable sections, and 
measuring the magnitude of the conductance unbalance to determine the 
occurrence of openings in the cable jacket. 
An apparatus in accordance with this invention includes facilities for 
advancing a cable which includes a multi-conductor core that is enclosed 
in a plastic jacket along a path through spaced first and second sensors, 
facilities for continuously sensing conductive current across each 
adjacent section of cable within the first and second sensors, facilities 
for continuously detecting conductive current unbalance, if any, between 
the adjacent sections of cable within the first and second sensors such 
that each new section which is advanced into the first sensor is compared 
to the section present in the second sensor, and facilities for measuring 
the conductive current unbalance to determine the occurrence of openings 
in the cable jacket.

DETAILED DESCRIPTION 
An apparatus, designated generally by the numeral 10, (see FIG. 1), in 
accordance with this invention for detecting openings in cable jackets, 
particularly those made of polyvinyl chloride (PVC), operates on the 
principle of conductance unbalance as opposed to capacitance unbalance. 
Since capacitance is an area-sensitive measurement, a significant area of 
a capacitor or effective capacitor must be influenced in order to 
unbalance a capacitance measurement to the point of being measurable. On 
the other hand, conductance is affected greatly by changes in resistance 
so that an opening in a plastic insulating material will significantly 
influence that parameter and readily facilitate the determination of 
unbalance. 
Advantageously, the apparatus 10 is designed to be used on a manufacturing 
line where a cable core 11 which comprises a plurality of individually 
insulated conductors 12--12 is to be jacketed. The core 11 is advanced 
along a path from a payoff reel 13 through an extruder 14 where the core 
is covered with a jacket 15 of a plastic material such as, for example, 
polyvinyl chloride (PVC) to form a cable 16. Then the jacketed cable 16 is 
advanced through a cooling trough 17 and through the apparatus 10 which is 
immersed in the cooling trough. Subsequently, the jacketed core 11 is 
advanced out of the trough 17 at a line speed of about 366 meters/minute 
by a capstan 18 and taken up on a reel 19. 
Referring now to FIGS. 1A and 2, it can be seen that the apparatus 10 
includes a probe 21 which is positioned in the cooling trough near the 
exit end thereof. The probe 21 comprises two cylindrical spaced guard 
sections 22 and 23 with a first cylindrical sensor 24 and an adjacent 
second cylindrical sensor 26 being positioned between the two guard 
sections. In a preferred embodiment, each of the guard sections 22-23 and 
each of the sensors 24 and 26 is about 2.5 cm along and is made of 
commercially available brass. The guard sections 22 and 23 and the sensors 
24 and 26, which are disposed concentrically about the path along which 
the jacketed cable 16 is advanced, are spaced apart about 0.16 cm by flat 
sealing rings 25--25 which are made of a material such as neoprene. The 
sealing rings 25--25 prevent a shorting out between adjacent sections of 
the probe and also perform a hermetic function. 
As can be seen in FIG. 2, the sensors 24 and 26 are resistively coupled 
through the water in the trough 17 to the cable jacket 15 from which a 
current is capacitively coupled to the multi-conductor core 11. After the 
current is capacitively coupled to the core 11, the current observes a low 
impedance in each direction along the cable 16 because of the relatively 
long length of the trough 17 compared to the length of each of the sensors 
24 and 26, i.e. 457 cm compared to about 2.5 cm. Outside of the probe 21, 
electrical currents which have been capacitively coupled to the core 11 
are returned along a resistive path to the trough 17 through the water 
which functions to short out the current from the jacket 15 to the trough. 
The probe 21 is designed to test sections of the cable 16 for openings or 
holes 30--30 along a predetermined length of its path through the trough 
17 with minimum interference from the cooling water. 
As can best be seen in FIG. 2, an oscillator, designated generally by the 
numeral 31, is connected along a lead 32 to the cooling trough 17. The 
oscillator 31 is also connected along a lead 33 into a center tap 34 of a 
primary winding 36 of a bridge transformer 37. The bridge transformer 37 
has a low impedance primary winding 36 at a frequency of 10 KHz so that 
the impedance of the water and the capacitive coupling to the core 11 
govern the flow of current in the primary winding. One end of one coil 38 
of the transformer primary 36 is connected along a lead 39 to the upstream 
sensor 24 of the probe 21 while the other oppositely wound coil 41 is 
connected along a lead 42 to the downstream sensor 26. As can be seen by 
the schematic of FIG. 2, an outer surface of the cable jacket 15 is 
effectively connected by ordinary tap water to the adjacent sensors 24 and 
26 of the probe 21 which are in turn connected to the bridge transformer 
37. Under balanced conditions, i.e. no holes in the jacket 15, each of the 
sensor sections 24 and 26 will draw equal current so that the net voltage 
across the primary winding 36 is zero with no voltage being induced in a 
secondary winding 43 of the bridge transformer 37. 
The secondary winding 43 of the bridge transformer 37 is connected along 
two leads 44 and 46 to a commercially available bridge amplifier 51 such 
as one which may be purchased, for example, from Princeton Applied 
Research Company and identified as Model No. 113. The voltage on the input 
and on the output side of the bridge amplifier 51 during those times when 
openings in the jacket 15 are being moved through the sensors 24 and 26 is 
a 10 KHz sine wave. 
An output 52 of the bridge amplifier 51 is connected along a lead 53 to 
demodulator and output buffer circuitry, designated generally by the 
numeral 54. The demodulator and output buffer circuitry 54 also has an 
input 56 which is connected along a lead 57 to digital phase shift 
circuitry, designated generally by the numeral 61, which receives an input 
along a lead 62 from the oscillator 31. The demodulator and buffer 
circuitry 54 inverts alternate half cycles of a sine wave input from the 
bridge amplifier 51 and provides the resulting wave form which is shown in 
FIG. 6(b) and is referred to hereinafter as the "hole signal". 
An output of the demodulator and buffer circuitry 54 is connected along a 
lead 64 to an input of logic circuitry, designated generally by the 
numeral 66. The output signal of the logic circuitry 66 may be fed along a 
lead 67 to a display device 68 which signifies to an operator both the 
presence and location of openings in the jacket 15. Signals from the logic 
circuitry 66 may also be fed along a line 71 to a feedback control 
apparatus, designated generally by the numeral 72, which controls the 
operation of line equipment such as the extruder 14 or the capstan 18 to 
avoid the occurrence of openings in the cable jacket 15. 
When a portion of the jacket 15 which includes a hole 30 is advanced 
through the sensor 24, the current in that sensor increases while that in 
the sensor 26 remains unchanged, thereby causing a net voltage across the 
primary winding 36 of the transformer. As the portion of the cable 20 
which includes the jacket hole 30 is advanced to a point intermediate the 
sensors 24 and 26 of the probe 21, equal currents will flow and there is 
no voltage unbalance across the primary winding 36 of the bridge 
transformer 37. Then when the portion of the jacket 15 having the hole 30 
is advanced into the sensor 26, the voltage across the primary winding 36 
again becomes unbalanced, but shifted 180.degree. from the unbalance when 
the hole was in the sensor 24. 
Thus by using a phase sensitive rectifier, to transform a voltage which is 
produced in the transformer winding 43, and amplified, a signal is 
produced that increases positively as the hole 30 moves to the center of 
the sensor 24, then decreases to zero as the hole passes between the two 
sensors 24 and 26, and decreases to a negative peak when the opening 
reaches the center of the sensor 26. The apparatus 10 is capable of 
reliably detecting holes 30--30 which are smaller than 0.16 cm in diameter 
at line speeds in excess of 366 meters per minute. 
The change in polarity of the unbalance voltage as the hole 30 progresses 
through the sensors 24 and 26, as well as the values of their amplitudes, 
characterizes the output of the demodulator and output buffer circuitry 54 
which is fed to the logic circuitry 66. Provided that the just-described 
pattern exists, the logic circuitry 66 has the capability of determining 
whether or not a hole 30 exists. 
While the guard sections 22 and 23 are not required, they are preferred in 
order to cause any current flow through the jacket 15 to occur between the 
guard sections and radially inwardly of the core 11 instead of being 
dispersed, thereby avoiding "fringing". The guard sections 22 and 23 are 
effective to electrically cut off the cable 16 and isolate portions of the 
cable in the sensors 24 and 26 so that each sensor is effectively testing 
a short length or section of the cable jacket 15. 
The components which are included in the circuitry 31, 51, 54 and 61 
represent an arrangement for achieving a high degree of accuracy in the 
process for monitoring the presence of openings in the cable jacket 15. It 
is to be understood, however, that other circuit arrangements for 
maintaining the integrity of the cable jacket 15 might also be used in 
conjunction with the principle of detecting cable jacket openings 30--30 
by detecting changes in conductance. The following description is directed 
to the circuitry of the blocks 31, 51, 54 and 61; however the logic 
circuitry 66 will not be described in detail since its construction should 
be apparent to one skilled in the art. 
The oscillator circuitry 31 (see FIG. 3) includes a 2.56 MHz crystal 
oscillator 81 which is connected along a lead 82 to a scaler counter 
circuit 81. A read-only-memory (ROM) device 84 is addressed by the counter 
83 to "look-up" binary numbers which are stored in memory addresses 
numbered from 1 to 256 and which each correspond to a voltage along a sine 
wave. For example, at address or location number 128, there is stored a 
binary number which corresponds to the peak value of the positive 
excursion of a sine wave. The binary number output from the 
read-only-memory device 84 is applied as an input to a digital-to-analog 
converter 86 which synthesizes a 10 KHz sine wave having a half period of 
50 .mu.s. The sine wave output from the digital-to-analog converter 86 is 
applied via a lead 87 to a high frequency, smoothing amplifier 88 and then 
along the lead 33 to the transformer 37. The counter 83 also provides an 
input over a lead 89 to an amplifier 91 which through the 
digital-to-analog converter 86 produces a reference square wave which is 
in synchronization with the 10 KHz sine wave. The reference square wave 
also appears as the waveform shown in FIG. 6(a) and is used as a clock 
input to the logic circuitry 66. 
The digital phase shift circuitry 61 synthesizes a 10 KHz square wave which 
is in phase with the 10 KHz sine wave that is received from the bridge 
amplifier 51 in order to operate the demodulator circuitry 54. The 
reference square wave which is fed out of the amplifier 91 of the 
oscillator circuitry 31 is used as a clock pulse in the logic circuitry 66 
whereas the phase-shifted, square wave which is provided by the digital 
phase shift circuitry 61 is in synchronization and in phase with the 
unbalance voltage. 
Referring now to FIG. 4, it can be seen that inputs to the digital phase 
shift circuitry 61 are fed along lines 101 and 66 from the 2.56 MHz 
oscillator 81 and the counter 83, respectively, of the oscillator 
circuitry 31. The counter 83 is capable of counting up to 256 and is 
designed to generate a binary number every cycle, i.e. every 400 
nanoseconds which is fed from the binary counter 83 into a comparator 102. 
The digital phase shift circuitry 61 also includes an adjustable binary 
switch or binary pattern bit generator 103 which includes eight binary 
switches that can be set to any one of 256 patterns. The switch 103 is 
designed to shift the square wave relative to the 10 KHz sine wave which 
is provided by the bridge amplifier 51 in order to operate the demodulator 
circuitry 54. 
The binary pattern bit generator 103 is preset so that seven of the eight 
switches enter a seven bit binary number for shifting the phase of the 
square wave through almost 180 degrees. Since seven switch bits correspond 
to 128 discrete steps of phase shift, a resolution of 1.4 degrees per step 
is achieved. Experiments have shown that this phase shift technique is 
well suited for precisely phase matching the square wave and the 10 KHz 
sine wave which are fed into the demodulator circuitry 54; however, for 
other applications, a different resolution may be required and may be 
determined empirically. 
The binary number which is generated by the preset switch 103 is sufficient 
to produce a square wave, which is shifted with respect to the reference 
square wave, such that each of its crossovers is aligned with the 
crossovers of the sine wave signal which is received from the transformer 
37. The binary number which is generated by the binary switch 103 is 
applied as an input to the comparator 102. When that number compares equal 
to that received from the counter 83, a transition point on the phase 
shifted square wave is determined. This transition point is shifted in 
time from the zero crossing of the 10 KHz sine wave of the oscillator 
output along the lead 33 by the number of 2.56 MHz clock cycles which are 
set through the binary switch 103. 
When the outputs from the counter 83 and from the switch 103 are compared 
equal by the comparator 102, it generates a signal at that particular 
clock pulse which is applied over a lead 104 to an input 106 of a 
flip-flop 107. Also, the oscillator 81 applies a 2.56 MHz clock signal 
over a line 108 to an input 109 of the flip-flop 107. When signals are 
present on both of the inputs 106 and 109 of the flip-flop 107, it is set 
and in turn activates a gate 111 which also receives an input from the 
switch 103 over a lead 112. The input to the gate 111 over the lead 112 is 
provided by the most significant of the switches in the switching device 
103. In a preferred embodiment, the gate 111 is an exclusive "OR" gate 
which produces an output signal when a high or "1" signal is present on 
either of its inputs. This causes the output of the gate 111, which is a 
phase-shifted square wave with its transition points corresponding to the 
transition points of the sine wave from the bridge amplifier 51 because of 
the adjustment in the binary switch 103, to be inverted for operational 
convenience. 
The phase-shifted square wave is applied over the lead 57 to a switching 
device 121 (see FIG. 5) of the demodulator and output buffer circuitry 54 
which operates between input positive and input negative positions in 
synchronization with the signal which is received along the lead 53 from 
the bridge amplifier 51. The demodulator and output buffer circuitry 54 
functions as a switching type detector that operates on the amplified 
bridge unbalance voltage to produce a D.C. output envelope which is 
proportional to the conductance unbalance in the probe 21. The demodulator 
and buffer circuitry 54 provides for both positive and negative phase 
sensitive signal detection, which are identical signals except that they 
are 180 degrees out of phase. Differential amplifiers 122--122, which are 
well known in the art and available commercially, produce outputs that are 
then fed along leads 123 and 124 through a buffer amplifier 126 which 
averages the positive and negative fully rectified outputs from the 
amplifiers to provide differential outputs of conductance deviation along 
leads 127 and 128 and to the logic circuitry 66. 
As can be seen in FIG. 5, the switching device 121 reverses the polarity of 
the bridge unbalance voltage which is fed into the amplifiers 122--122 at 
each zero crossing of the sine wave under control of the phase-shifted 
square wave. In order to provide for both positive and negative phase 
sensitive detection, the switching device 121 of the demodulator circuitry 
54 includes a pair of solid state switches which are represented as 
mechanical switches 132 and 133 in FIG. 5 for simplicity. 
The phase-shifted square wave is effective to cause the switches 132 and 
133 to operate at each zero crossing of the signal which is received from 
the bridge transformer 37. A positive input to a lead 136 is applied over 
the switch 132 along a lead 137 and applied as a positive input 138 at the 
amplifier 122--122. A negative input on a lead 141 is fed through the 
other switch 133 shown in the unbroken position in FIG. 5 and along a lead 
142 and applied as an input to the negative terminal 143 of the amplifiers 
122--122. Since the transition points of the square wave which is fed into 
the switching device 121 correspond to the transition points of the sine 
wave which is received from the bridge transformer 37, the next excursion 
of the shifted square wave causes the switching device 121 to be 
controlled to move the switches 132 and 133 to input negative positions as 
shown by the broken line positions in FIG. 5 and designated 132' and 133'. 
At that time, a positive input along the lead 141 is routed along a lead 
144 through switch 132' and the lead 137 to the positive input 138 of the 
amplifier 122--122 whereas a negative input along the line 136 is applied 
along a lead 146 through the switch 133' and the lead 142 to the terminal 
143. As a result, phase-sensitive detection occurs with the resultant wave 
forms being shown on the output side of the amplifiers 122--122 in FIG. 5. 
The outputs from the amplifiers 122--122 are fed along leads 123 and 124 to 
the buffer amplifier 126 which increases the amplitude of the signals 
while lowering their impedance. The signals which are fed from the 
demodulator circuitry 54 along a lead 64 to the logic circuitry 66 must be 
driven with a low impedance because of the high capacitance present in 
lengthy leads that extend from the demodulator circuitry, which is 
generally at the probe 21, back to a central instrument panel. The buffer 
amplifier 126 is effective to avoid a filtering out of the signal which 
could result in a loss of signal at the monitoring station. In a preferred 
embodiment, the logic circuitry 66 is located at the probe 21 thereby 
obviating the need for the buffer amplifier 126. This follows preferred 
design practice of monitoring as close as possible to point of signal 
pick-up to avoid the effects of extraneous signals. 
The logic circuit 66 is constructed with hole counting logic which is 
effective to test signals which are received from the demodulator 
circuitry 54 to verify that certain characteristics do or do not occur. 
Operation of the hole counting logic is best understood by referring to 
FIG. 6. Referring now to FIG. 6(a), there is shown a clock pulse with up 
counts and down counts and in FIG. 6(b) a hole signal, which is 
representative of a hole in the cable jacket 15. As can be seen in that 
figure, a jacket hole 30 causes an almost symmetrical signal to be 
generated because the cable 16 is advanced through each of the sensors 24 
and 26 at the same speed. On the other hand, a noise signal is usually of 
single polarity and assymetric. 
There is no unbalance in the bridge transformer of the electrical circuit 
when sections of the cable 16 having no openings in the jacket 15 are 
advanced through the probe 21. Since the current is the same in each of 
the oppositely wound coils 38 and 41 of the primary 36 of the bridge 
transformer 37, there is no unbalance in the secondary winding 43. Also, 
there is no unbalance, if a hole is present simultaneously in each section 
of the cable jacket 15 within the sensors 24 and 26; however, it has been 
found that the probability of such an occurrence is essentially zero. 
An up/down count is used to determine the measuring intervals in which both 
positive and negative peaks must exceed the preset thresholds in order to 
be a valid signal which represents a hole in the jacket 16 FIGS. 6(c) and 
(d) illustrate the up count interval and the total counting intervals, 
respectively. It is important to recognize that the frequency of the 
hole-generated signals at line speeds up to about 460 meters/minute is 
relatively low compared to the frequency of the oscillator 81. At a line 
speed of about 15 meters/minute for example, the hole signal may have a 
half period of 100 milliseconds whereas if the line speed is increased to 
152 meters/minute, the half period decreases to 10 milliseconds. This 
causes the logic circuitry 66 to be able to perform its tests on each half 
of the sine wave which represents the conductor current unbalance caused 
by a hole 30 notwithstanding changes in line speed. The measuring interval 
self-adjusts for different manufacturing line speeds in that the up-clock 
pulses are counted during the positive excursion of the hole signal. Then 
the count is decremented until zero is reached which in time occurs near 
the end of the hole signal and which occurs independent of line speed in a 
range of 38 to 457 meters/minute. Once the threshold is exceeded in the 
first half of the hole signal, the logic circuitry 66 is designed to look 
for a peak value within the same time period on the negative half wave. 
The 10 KHz output of the oscillator gives 40 cycles of the sine wave 
during the four milliseconds that a portion of the cable is within each 
2.5 cm sensor section which permits ample opportunity for the demodulator 
circuitry 54 to develope an envelope. 
A hole in the jacket 15 is detected by establishing the hole logic with 
respect to characteristics of its signal in a particular type of cable 
jacket 15. First, a positive excursion must exceed an initial threshold 
value of 0.5 volt. As soon as this occurs, the clock or the 2.56 KHz 
oscillator counts up as indicated with the small upwardly directed arrows 
in FIG. 6(a). The measurement of the threshold value is accomplished with 
a comparator which is a well known device in the art. Another comparator 
is used to determine if the signal exceeds a +3 volt value. Then as the 
signal drops below the +0.5 volt value, the clock begins to count down 
while still another comparator tests the signal to determine if it exceeds 
a negative excursion level of -3.0 volts before a counter (not shown) in 
the logic circuitry 66 reaches zero. 
The determination that openings 30--30 are occurring in the cable jacket 15 
is used to control the manufacturing line to eliminate subsequent 
occurrences of openings. If these characteristics of a hole signal are 
satisfied by the signal output of the demodulator circuitry 54, and the 
wave form shown in FIG. 6(b) is determined to represent the occurrence of 
an opening 30 in the jacket 15, then a hole indicating pulse (see FIG. 
6(e)) is generated and fed over the line 71 (see FIG. 2) to the feedback 
control apparatus 72. The feedback control apparatus 72, through 
appropriate circuitry which is well known in the art and in response to 
hole count pulses generated by the logic circuitry 66, controls the 
extruder 14 to increase the thickness of the jacket 15. The feedback 
control apparatus 72 in response to hole count pulses being fed thereto 
may also generate signals to control the capstan 18 to reduce the line 
speed at which the cable 16 is advanced to effectively increase the 
thickness of the jacket 15. Obviously, the feedback control apparatus 72 
may generate signals to control either one, or both, of the 
above-mentioned variables, as the jacket 15 is continuously monitored by 
the probe 21. Further, the logic circuitry 72 may generate a pulse as 
shown in FIG. 6(f) for operating a relay (not shown) which in turn drives 
a solenoid to cause a footage counter (not shown) to mark the location of 
the opening or a cable marking apparatus (not shown) to mark the cable 16 
directly. 
As an alternative to or as an addition to the feedback control apparatus 
71, an analog display of the wave form shown in FIG. 6(b) may be provided 
such as, for example, on a strip chart. During the operation of the 
apparatus 10, the logic circuitry 66 may generate signals to cause the 
results of the continuous monitoring of the cable jacket 15 to be recorded 
on a recording device (not shown). The recording may be a continuous line 
interrupted with spikes at those points along the length of the cable 16 
where the openings occur or there may be a digital print out which will 
signify to an operator that a jacket hole 30 has occurred. This is 
expensive since the line may run for hours without the occurrence of a 
hole; however, one advantage of a strip chart, i.e. the provision of a 
permanent record, is provided by using the logic circuitry 66 and the 
feedback device 71 in combination with a footage card device to record the 
footages at which holes occur. 
It is to be understood that the above-described arrangements are simply 
illustrative of the invention. Other arrangements may be devised by those 
skilled in the art which will embody the principles of the invention and 
fall within the spirit and scope thereof.