System for measuring cable footage

A system for accurately marking cables employs optical devices to measure the actual distance between periodic impressions and apparatus to adjust the marking apparatus to maintain a fixed distance between such impressions.

DESCRIPTION 
1. Technical Field 
The field of the invention is the field of measuring and marking the length 
of cables. 
2. Background Art 
In the field of manufacturing electrical cables, such as multiconductor 
cables used in communication systems, it is important that the length be 
accurately measured. Since it is standard industry practice to attach the 
connectors to long lengths of cable in a shop and then to position the 
cable in the field, it is important that the cables not be too short, for 
then a splice would have to be made in an inaccessible region inside a 
conduit. For this reason, it has been standard industry practice to 
deliberately manufacture long cables to an excess length. A side effect of 
this practice has been that it is necessary to provide a way to store the 
excess length underground, with the result that it is necessary to 
excavate large regions in underground cable systems in order to coil up 
the excess length of cable. As an example, a 5,000 foot long cable that is 
manufactured 1% excess in length will have 50 excess feet to be coiled up 
in order to make the connection. This practice of deliberately making 
cables excessively long not only results in an extra manufacturing cost 
for the cable but an extra expense in excavation and installation and an 
improved method of accurately measuring cables during production has long 
been sought in the industry. At present, the practice is to drive a 
marking wheel by friction between a driving wheel and the cable. There is 
some slippage between the cable and the driving wheel which is corrected 
in part by an additional driving motor. This system typically controls the 
cable length to .+-.1% so that an extra amount of 1% of the cable length 
is run off to ensure that the correct amount is actually delivered. 
DISCLOSURE OF INVENTION 
The invention relates to an apparatus for measuring the spacing between 
position marks in a cable dynamically and for adjusting the marking system 
to compensate for variations in that spacing.

BEST MODE FOR CARRYING OUT THE INVENTION 
In FIG. 1, cable 102 travels past driving wheel 104 which is turned by 
friction between it and the cable. Driving wheel 104 drives print wheel 
110 by means of belt 106, connecting with hub 108 of wheel 110. Wheel 110 
includes printer 112 which may make an impression in the cable or may 
provide an ink or paint mark on the cable. Marks 120, 122 and 124 are 
shown in the drawing, 120 and 122 being adjacent marks separated by 
standard distance 125 and mark 124 being spaced further along the cable. 
Light reflected from mark 124 is focused by lens 128 onto optical detector 
130. At the same time, mark 122 passes in the vicinity of lens 126 and 
optical detector 132. Optical detector 132 is composed of a number of 
smaller detectors 133 which provide a series of outputs that represent the 
distribution of light from mark 122 and thus represent the position of 
mark 122. A suitable self-scanned diode array is available from RETICON 
and has 512 elements, so that with proper imaging optics, the position of 
mark 122 can be located with an accuracy of 0.00262 inches. Signals from 
detectors 130 and 132 pass to electronics circuits in controller 140 which 
respond to a trigger signal from detector 130 to compute the deviation of 
distance 123 from its nominal value and to control torque motor 114 in 
order to compensate for slippage between driving wheel 104 and cable 102. 
In the prior art, voltage to torque motor 114 was not adjusted, but rather 
was set at some estimated value so that the distance 125 was 
systematically higher or lower than the correct value. 
In operation, lens 128 forms an image of mark 24 on detector 130, which 
provides a trigger signal to controller 140, which may be a 
general-purpose digital computer or a special-purpose array of logic 
circuits. Controller 140 then interrogates the elements 133 of detector 
132 and computes by means of a weighted average the location of mark 122. 
As illustrated in FIG. 1, the distance 123 between marks 122 and 124 is an 
integral multiple (nL') of the basic distance L'(125). Illustratively, the 
basic distance between adjacent marks is two feet and detectors 130 and 
132 are located in any convenient position along the cable manufacturing 
machine such that distance 123 is an integral multiple of two feet. If n 
is greater than 1, then the measuring error in detector 132 will be a 
smaller percentage of the total length and the accuracy of the measurement 
will be increased. 
Detector 130 may be a photo diode located behind a mask having an aperture 
positioned so that the output signal from detector 130 will pass a 
predetermined threshold when mark 124 passes the correct position. A 
number of other detector arrangements will be evident to those skilled in 
the art. It does not matter if the threshold corresponds to the leading 
edge of a light pulse or the maximum amount so long as the triggering 
point is repeatable. At the triggering time, the amount of light in the 
several optical elements 133 of array 132 is measured and the location of 
mark 122 is calculated. 
Each time optical array 132 is interrogated, a value for the distance 123 
will be computed and the voltage Vo of torque motor 114 will be changed to 
V'=Vo+.DELTA.V. This correction signal .DELTA.V may be proportional to the 
current deviation of length 123 from its nominal value, but such immediate 
correction tends to overcompensate and also to put excess stress on motor 
114 as it attempts to react to changing signals. It is preferable to 
include circuits within controller 140 which sum the differential length 
error .DELTA.L, plus or minus as the case may be, and to adjust the 
correction signal to motor 114 in proportion to the total deviation along 
the length of the cable, so that V'=Vo+C.SIGMA..DELTA.L.sub.i, where C is 
an empirical constant and .DELTA.L.sub.i is the ith deviation of the 
length measurement. This produces a gradual and smooth correction rather 
than a sharp and abrupt one. 
Distance 125, the distance L' that is to be controlled, is not equal to L, 
the inter-mark spacing when the cable is in use, because in the cable 
machine the cable is under tension and is at an elevated temperature and 
is therefore elongated compared to its dimension in use. It will be 
necessary to calibrate the system by actually measuring a suitable length 
of cable after it has been manufactured in order to calculate the correct 
ratio between L', the length under tension, and L, the length at rest. It 
will be necessary to maintain the tension in the cable machine at the same 
value that was used in the test, of course, but it is not necessary that 
the precise value of the cable tension be known so long as it is 
maintained at the same value. It is not necessary that the tension in the 
cable machine be held constant to the same degree of precision as the 
desired length, because an increase in cable tension of a certain 
percentage produces a corresponding percentage increase in cable length 
that is reduced by the constant of proportionality between tension and 
elongation. 
FIG. 2 illustrates an alternate embodiment of the invention, in which lens 
128 and detector 130 function as before, serving as a trigger to 
controller 141, but in which the length measurement is effected by a 
conventional rotary encoder 109 which measures the angle of the shaft of 
wheel 110. When a mark passes detector 130 the encoder 109 is interrogated 
and the deviation of the shaft of print wheel 110 from its correct 
position is translated in controller 141 into a measurement of length 125 
according to the formula L.sub.i =L'+R.DELTA..phi., where L.sub.i is the 
actual measured length, L' is the nominal length, R is the radius of print 
wheel 110 and .DELTA..phi. is the angular deviation. The actual length 125 
is compared to its nominal length and the torque assist motor 114 is 
adjusted as before. Encoder 109 is of course rigidly fixed to wheel 110 so 
that the encoder position translates directly to the length 125 with no 
error. The system is calibrated by means of a measurement of a finished 
cable without tension being applied as in the previous embodiment. 
Compared with the previous embodiment, this present embodiment is simpler 
in that one of the optical detectors is eliminated and the step of 
computing the position of mark 122 from the measured voltages in array 132 
is eliminated. It has the disadvantage that encoders are expensive and 
delicate mechanical devices. It is not necessary in either of the 
preceding embodiments that printing wheel 110 be driven by friction off 
the cable as adjusted by a torque motor. If desired, wheel 110 would be 
driven by a variable speed motor and a similar procedure used to control 
the motor speed to position the marks correctly. 
Another embodiment which has the virtue of no moving parts is illustrated 
in FIG. 3, in which the optical measurement is performed as in FIG. 1 with 
corresponding elements having the same number as in that figure. The 
difference is that the mark is imposed on the cable by light from laser 
214, illustratively a He-Ne laser, which is focused to a line focus by 
cylindrical lens 212. Laser 214 is pulsed to achieve a short pulse of 
sufficiently high intensity to melt or otherwise leave a mark on the cable 
jacket. In operation, the length 123 is computed as before and a signal is 
sent out to delay unit 150. The distance 127 between the center of array 
132 and the point where the mark is to be applied by laser 214 is less 
than the nominal distance L' so that delay unit 150 will trigger laser 214 
at an appropriate time to compensate for a standard delay D.sub.o 
dependent on length 127 and a variable delay .DELTA.D dependent on the 
error in length 123. Controller 151 contains circuits which measure length 
123 from the signals from detectors 130 and 132 and also contains circuits 
which measure the time between successive pulses from detector 130 and 
combine that time with the measured distance 123 to compute the velocity 
of the cable. This value of the velocity of the cable, together with the 
error in length 123 results in the computation of an appropriate delay 
which is provided by delay circuit 150 so that the cable is marked at a 
time which compensates for the error in length 123. 
If laser 214 is pulsed for one microsecond, then the cable may be moving as 
fast as 1,000 inches per second and the cable will still move only 0.001 
inch during the laser pulse. Methods for generating short high-power laser 
pulses, such as by Q-switching, are well known in the art. By virtue of 
the fact that the cable marking system no longer is in mechanical contact 
with the cable, the problem of slippage is eliminated. This system will 
respond to velocity changes of the cable, in contrast to prior art systems 
which respond to velocity and also to changes in the friction between the 
cable and the driving wheel. 
An alternative embodiment employs a solenoid in place of laser 214. This 
solenoid would be pulsed after an appropriate delay, either printing or 
making an impression on the cable. The solenoid will be in contact with 
the cable for a longer period then the laser pulse, of course, but it will 
tend to be more reliable in operation than the laser. 
Yet another embodiment employs an ink-yet printing unit that ejects a small 
quantity of ink or paint at a controlled time. Such a device is 
functionally equivalent to the laser or solenoid in the two preceding 
embodiments in that it is not in mechanical contact with the cable and in 
that it is pulsed after a variable delay. 
An advantageous feature of a system constructed according to the present 
invention is that the system measures not only the length of the cable but 
also the velocity of the cable as it passes through the machine more 
accurately than is possible with mechanical devices. Various features of a 
cable machine such as the pressure applied to extrude a jacket onto the 
cable or the heat or curing time to harden elements of the cable, will be 
dependent upon the velocity. Also, the velocity will depend in part on the 
tension of the cable where the cable is pulled through the machine so that 
a measurement of the velocity may be used to adjust various parameters of 
the machine to improve performance.