Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL FIBRE TENSIONING DEVICE AND METHOD OF
CONTROLLING THE TENSION APPLIED TO AN OPTICAL FIBRE
DESCRIPTION
The present invention relates to an optical fibre
tensioning device and a method of controlling the
tension applied to an optical fibre, particularly
during the winding of the fibre.
Typically, optical fibres are formed in draw
towers from preforms of glass material. A draw tower is
generally provided with a traction device ("capstan"),
for example a motorized pulley, capable of drawing the
optical fibre downwards and feeding it to a storage
device; the latter_- usually comprises a motorized reel
onto which the opti.c:al fibre is wound.
In many operating situations, the speed at which
the optical fibre is drawn by the traction device (the
drawing speed) may not coincide with the speed at which
the optical fibre is fed into the storage device, for
example owing to fluctuations of the speed of the
motors which drive the traction device and the storage
device. The difference between these speeds gives rise
to variations of tension in the fibre with respect to a
target tension; in particular, a fibre tension greater
than the target tension may damage the fibre, while a
tension lower than the target tension may make the
storage process inefficient.
To solve this problem, there is a known practice
of using tensioning devices which are to be interposed
between the tractiorl device and the storage device and
are capable of controlling the tension of the optical
fibre. In particular, there are kriown tensioning
devices comprising at least one fixed pulley and one
movable pulley which can guide the optical fibre. The
movement of the movable pulley eriables the tension of
the optical fibre 1_o be modified. Here and in the
remainder of the description, the term "fixed pulley"
denotes a pulley which is free to rotate about its axis
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and in which the position of the axis is fixed, while
the term "movable pulley" denotes a pulley which is
free to rotate about its axis and in which the position
of the axis is variable.
US Patent 4,712,866 describes a tensioning device
positioned between a traction device of a draw tower
and a storage device comprising a take-up reel. This
tensioning device comprises a pair of fixed pulleys and
a movable pulley located on an oscillating arm, also
called a "dancer arm", which carries a weight. The
dancer arm actuates a conventional speed control so
that the fibre is wound onto the take-up reel at a
tension determined by the aforesaid weight.
US Patent 5,790,292 relating to an optical fibre
transmission line describes, among other things, a draw
tower in which a tensioning device with two pulleys is
interposed between a traction device and a take-up reel
(Figure 7).
US Patent 4,138,069 describes a winding device for
optical glass filarnents in which a traction device
draws the filament: f:rom a source of molten glass and
feeds it through a-:ensioning device to a set of take-
up reels. The tensioning device comprises two fixed
pulleys and one movable pulley mounted on one end of a
dancer arm. The dancer arm extends from an apparatus in
which a spring or other means applies a constant force
to move the movable pulley away from the fixed pulleys.
Tensioning devices can also be used in apparatus
other than draw towers.
US Patent 5,076,104 describes an apparatus for
measuring the load at failure of an optical fibre, in
which tensioning devices comprising a pair of fixed
pulleys and one movable pulley are provided. Also
provided is a tensioning device comprising one fixed
grooved wheel and orie movable grooved wheel; the latter
is positioned below the fixed grooved wheel, is movable
in a vertical direction and is connected to a
predetermined weight. The optical fibre, which in its
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path is wound around both the fixed grooved wheel and
the movable grooved wheel, is thus kept at a constant
tension.
US Patent 4,505,222 describes an extrusion coating
device for optical fibres, in which a tensioning device
is positioned immediately upstream of a take-up reel.
US Patent 4,127,370 describes an extrusion coating
apparatus for an optical fibre, in which a first, a
second and a third pulley are positioned at the entry
to the apparatus, immediately downstream of the fibre
take-off reel. The first and second pulley are spaced
apart from each other and from the take-off reel. The
third pulley has no support and is carried on the
optical fibre between the first and second pulley, in
such a way that the fibre passes, as a result of the
weight of the third pulley, along a U-shaped path
between the first and second pulley. A pair of
photocells is used to detect the vertical positiori of
the third pulley and to control the speed of rotation
of the take-off reel in such a way that the vertical
position of the third pulley, and therefore the tension
of the fibre, remai.ns essentially constant.
The applicant has observed that tensioning devices
of known types, requiring the use of gravity and/or
predetermined elastic action for controlling the
position of the movable pulley, apply ari uncontrollable
predetermined force to the movable pulley. Therefore it
is not possible to modulate in any way the force
applied to the movable pulley in order to modify the
response of the tensioning device. Consequently, the
tension control ob1.=a.i.ned by means of the known devices,
being of the non-modulatable type, might be inadequate
in some operating c:or:d:itions.
The applicant has found t=hat the aforesaid problem
can be at least partially overcome by using an optical
fibre tensioning device in which there is a movable
pulley on which the optical fibre runs, a device for
measuring the tension applied to --he optical fibre and
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a device for movirig the movable pulley, capable of
controlling the position of the movable pulley in
accordance with the signal generated by the tension
measuring device.
The tensioning device according to the present
invention is capable of receiving at its input. an
optical fibre fronl a fibre guide or return element or
from a device of another type, for example a traction
device or a take-off device, and is capable of
supplying the optical fibre from its output, after
having automatically controlled its tension, to a
further guide or return element, or to a further
device, for example a storage device (particularly a
reeling device) for the optical fibre.
In greater detail, the tensioning device according
to the present invention comprises a fixed pulley
carried by a supporting structure and a movable pulley
carried by a slide which is slidable along a
rectilinear guide c-arried by the support.ing structure.
The optical fibre entering the tensioning device
is wound around ttie fixed pulley for a certain length,
and passes from here to the movable pulley around which
it is wound f:or a f:urther length. The distance between
the axes of the fixed pulley and the movable pulley is
variable to modify t:he length of the portion of fibre
which extends between the two pulleys and consequently
the tension applied to the fibre. To vary the distance
between the axes of the fixed pulley and the movable
pulley there are used, as mentioned above, a fibre
tension measuring device, for example a load cell
associated with the fixed pulley, and a device for
moving the slide which can control. the distance between
the axes of the pulleys in accordance with the signal
supplied by the tension measuring device, to
automatically keep the tension in the fibre practically
constant. The movement device can comprise an electric
motor and a system. for transmitting the motion from the
motor to the slide, for example a screw and nut
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coupling in which the screw extends parallel to the
rectilinear guide and is rotationally movable by the
force of the motor and in which the nut is coupled to
the slide, or a belt transmission, for example a
5 toothed belt transmission.
Since it automatically controls the tension of the
fibre without using the force of gravity or other
predetermined forces, the aforesaid optical fibre
tensioning device cari be used to modulate in the
desired way the terision applied to the optical fibre.
In this way, the movement of the movable pulley is
made entirely independent of the force of gravity or
predetermined elastIc forces, and is achieved by the
application of a controllable force.
Thus a tensioning device which automatically
adapts itself to any variatiori of the operating
conditions is provi_ded.
In a first aspect, the present invention relates
to an optical fibre tensioning device, for use iri an
optical fibre advance path, comprising at least one
movable pulley arourid which the optical fibre is wound
for a certain length. at least; the position of the said
movable pulley being variable to modify the tension
applied to the opti_cal fibre,
and additionally comprising a tension measuring
device capable of rneasuring the said applied tension
and of generating a signal indicating the tension, and
a motorized device for moving the said movable pulley,
capable of receiviriq the said signal indicating the
tension and of moving the movable pulley in accordance
with the said signal indicating the tension, to
automatically keep the tension applied to the optical
fibre essentially constant.
Preferably, the tensioning device also comprises a
fixed pulley around which the optical fibre is wound
for a certain length at least, the distance between the
axes of the said fixed pulley and the said movable
pulley being variable to modify the terlsion applied to
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the optical fibre, and the said motorized movement
device being capable of controlling the said distance
to automatically keep the tension applied to the
optical fibre essentially constant.
The said motorized movement device may comprise a
guide along which a slide supporting the said movable
pulley can be moved and an actuator device for moving
the said slide aloriq the said guide.
The said actuator device may comprise a screw
movable rotationally by the force of an electric motor
and a nut connected. to the said slide, or an electric
motor and a motion transmission system of the belt type
(for example, one using a toothed belt) to connect the
motor to the slide.
The tensioning device may comprise an electronic
control device which receives reference signals and
measurement signals at its input, and is capable of
generating, in response to the said input signals,
operating signals for the closed-loop control of the
said motorized movement device.
The said electronic control device is preferably a
device for the closed-loop control of the tension
applied to the optical fibre, comprising:
- a subtractor to which is supplied a tension
reference signal, proportional to a target tension
of the optical. fibre, and a measurement signal,
consisting of the said signal indicating the
tension of the optical fibre; and
- a controller, oreferably a proportional-integral
derivative controller, receiving at its input a
tension error signal supplied from the output of
the said subtractor, and generating at its output
a drive signal which can be used to operate the
said motorized movement device and to control the
said distance.
Thus a closed-loop control of the tension present
in the fibre is provided, by means of which this
tension is constantly monitored, causing an immediate
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intervention of ttie movement device whenever the
measured tension deviates from the reference tension.
The tension in the fibre is thus kept constantly equal
to the reference tension, providing a particularly
precise control.
Advantageously, the said tension measuring device
may comprise a load cell connected to the said fixed
pulley and capable of generating at its output the said
signal indicating the tension.
Preferably, the said motorized movement device
comprises a guide along which a slide supporting the
said movable pulley can be moved, and an actuator
device for moving the said slide along the said guide;
the said electrorlic control device implementing a
system for the closed-loop control of the positiori of
the said slide alorig the said guide.
Preferably, the tensioning device is connected for
operation to a device for storing the optical fibre,
capable of receiving the optical fibre from the
tensioning device, arld the said svstem for the closed-
loop control of the position of the said slide along
the said guide comprises:
- a further subt.ractor to which is supplied a
position reference signal, correlated with a
reference position of the said slide along the
said guide, and. a measurement signal, proportional
to the actual position of the said slide along the
said guide; and
- a further controller, preferably a proportional-
integral deriva-.ive controller, receiving at its
input a position error signal supplied from the
output of the said further subtractor, and
generatirlg at its output a further drive signal
which can be used to operate the said storage
device.
The said storage device preferably comprises at
least one motorized r-eel on which the optical fibre
from the said tensioning device is wound, the said
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motorized reel beinq rotationally movable about an axis
of rotation under the action of an electric motor, and
the said further drive signal being usable for the
control of the said electric motor.
The said electronic control device preferably
comprises an integrator receiving at its input a signal
correlated with the speed of movement of the said slide
along the said guide and generating at its output the
said measurement signal proportional to the actual
position of the saicl slide along the said guide.
In this way, it is ensured that the movable pulley
is always kept in ttle proximity of a reference position
from which it is always possible to move the movable
pulley in a useful way to compensate for a variation of
tension; thus the slide is not permitted to be
positioned at a terminal position at which it is no
longer possible to rnove the movable pulley in one of
the directions of movement.
In a further aspect, the present invention relates
to a method of controlling the tension applied to an
optical fibre aloncl a path of advance of an optical
fibre, the said method comprising the stage of forming
a portion of the said path having a length which can be
varied to modify the tension applied to the said fibre,
and the stages of measuring the tension applied to the
optical fibre and controlling the said length iri an
automatic way, by means of a motor, in accordance with
the said measured tension, to keep the tension of the
optical fibre practic:ally constant.
Preferably, the method comprises the stage of
closed-loop control of the tension applied to the said
optical fibre in accordance with at least one reference
signal, proportiona-L to a target tension of the fibre,
and a measurement signal which is generated in this
stage of ineasurinq the tension and is proportional to
the tension to which the optical fibre is actually
subjected.
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The said stage of closed-loop control of the
tension preferably comprises the stages of:
- comparing the said tension reference signal with
the said measurement signal, generating a tension error
signal; and
- processing the said error signal, generating at
the output a drive signal which can be used to modify
the said length by means of a motor and to keep the
tension in the fibre essentially constant.
The method preferably comprises the stage of
closed-loop control of the position of a movable member
in accordance with at least one position reference
signal which is proportional to a target positiorr of
the said movable member, and a measurement signal which
is proportional to the instantaneous position of the
said movable member, the variation of the positiori of
the said movable member causing the variation of the
said length.
The method preferably also comprises the stage of
storing the optical fibre at the end of the said
advance path; the said stage of closed-loop contro:L of
the position of the said movable member comprises the
stages of:
- comparing the said position reference signal with
the said position measurement signal, generating a
position error signal; and
- processirlg the said position error signal,
generating at the output a drive signal which carl be
used to modify the speed at which the optical fibre is
stored.
In a further aspect, the present invention relates
to an optical fibre processing system, for example a
draw tower, an ex,--rusion line or an apparatus for
measuring the breakirig strength of an optical fibre, in
which there is a ri optical fibre tensioning device
capable of receivinq at its input an optical fibre from
a source of optical fibre, for example an optical fibre
traction device, and capable of supplying the optical
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fibre from its output to a device for storing the
optical fibre, for example a motorized reel, the said
tensioning device comprising at least one movable
pulley around which the said optical fibre is wound for
5 a certain length at least, the position of the said
movable pulley beirig variable to modify the tension
applied to the said fibre, the tensioning device
additionally comprising a tension measuring device
capable of ineasuri.ng the said applied tension and of
10 generating a sigrial indicating the tension, and a
device for the motorized movement of the said movable
pulley which is capable of receiving the said signal
indicating the tension and of moving the movable pulley
in accordance with the said signal indicating the
tension to automatically keep the tension in the fibre
practically constant.
Preferably, t:he system comprises an electronic
control device capable of providing a system of closed-
loop control of the tension applied to the optical
fibre, and compris:ing:
- a subtractor to which is supplied a tension
reference signal, proportional to a target tension
of the optical fibre, and a measurement signal,
consisting of the said signal indicating the
tension; and
- a controller, preferably a proportional-integral
derivative controller, receiving at its input a
tension error signal supplied frorn the output of
the said subtractor, and generating at its output
a drive signal which can be used to operate the
said motorized movement device.
Preferably, the said motorized movement device
comprises a guide, along which a slide supporting the
said movable pulley can be moved, and an actuator
device for moving t:~e said slide along the said guide;
the said electroni.c control device providing a system
of closed-loop control of the position of the said
slide along the sa:icl guide.
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Preferably, the tensioning device is connected for
operation to the optical fibre storage device, and the
said system for the closed-loop control of the position
of the said slide along the said guide comprises:
- a further subtractor to which is supplied a
position reference signal, correlated with a
reference position of the said slide along the
said guide, anci a measurement signal, proportional
to the actual position of the said slide along the
said guide; arld
- a further controller, preferably a proportional-
integral derivative controller, receiving at its
input a posit:.ion error signal supplied from the
output of the said further subtractor, and
generating at its output a further drive signal
which can be used to operate the said storage
device.
The invention will now be illustrated with
particular referencis to the attached drawings which
represent, without restriction, a preferred embodiment
of the invention in which:
= Figure 1 shows, =i_n plan view, a tensioning device for
optical fibres, made according to the present
invention; and
= Figure 2 shows a block diagram of an electronic unit
for controlling the device of Figure 1, made
according to the present invention.
In Figure 1, the number 1 indicates, as a whole,
an optical fibre tensioning device.
The tensioning device 1 cornprises a supporting
panel 3 (made from metal, for example) carried by a
frame (not illustrated) which allows it to be
positioned in a fixed way, for example in a horizontal
or a vertical posit:i.cn.
The tensioning device 1 also comprises at least
one rectilinear guide 5, which in this particular case
is formed by two cylindrical rods 12a, 12b carried by
the supporting parel 3, and a slide 7 movable with a
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reversible motion along the rectilinear guide 5 and
supporting a first pulley 9, referred to hereafter as
the movable pulley. The direction of movement of the
slide 7 is indicated. by D in Figure 1.
The cylindrical rods 12a, 12b are positioned
parallel to each other, and have their respective end
portions carried by pairs of supports 15a, 15bfixed to
the supporting panel 3. The slide 7 can have a
rectangular shape in plan view, and can be provided
with sliding elements 7a, 7b engaged with the rods 12a,
12b respectively; for example, the slidable engagement
between the slide 7 and the rods 12a, 12b can be formed
by means of sliding bearings 18, or ball-bearing
collars.
The movable pulley 9 is preferably carried by a
pin 20 which extends from, and is perpendicular to, the
slide 7, in such a way that the axis of rotation of the
movable pulley 9 is perpendicular to the direction D of
movement of the slide 7.
The slide 7 can be moved by the force of an
electrical motor 24, preferably a motor having an
output shaft connected to a screw 26 located between
the rods 12a, 12b and extending parallel to these. The
motor 24 can be niounted in a projecting way on a
bracket 28 fixed t:o the panel 3, and the ends of the
screw 26 are preferably supported by corresponding
supports 30a, 30b which are also fixed to the panel 3.
Advantageously, the screw 26 is coupled to a
circulating ball nut (not illustrated) fixed to the
slide 7 on the sidE: opposite the movable pulley 9.
Alternatively, the transmission of motion from the
motor 24 to the movable pulley 9 could be provideci by
means of a belt transmission, with a toothed belt for
example (not illust:r.ated).
The tensioning device 1 also comprises a second
pulley 32, referred to hereafter as the fixed pulley,
carried by a pin 33 which preferably extends
perpendicularly to the panel 3, for example in a
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portion of the panel 3 opposite that occupied by the
motor 24.
The fixed pulley 32 preferably has the same
diameter as the movable pulley 9, and the axes of
rotation of the pulleys 9 and 32 are preferably
parallel to each other.
The angular rotation of the electric motor 24
produces the linear movement of the slide 7 along the
guide 5, thus modi:Eying the distance Dp between the
axes of rotation of the movable pulley 9 and the fixed
pulley 32.
In particular, the distance Dp is continuously
variable between a minimum value (DpMin) for which the
slide 7 is located at a first terminal position (not
illustrated) in which it bears on the brackets 15a, 15b
which are farther from the motor 24, and a maximum
value (DpMax) for which the slide 7 is located at a
second terminal position (not shown) at which it bears
on the brackets 15a, 15b which are closer to the motor
24.
The tensioning device 1 can also comprise a
transparent protective enclosure (carter - not shown)
which can be connected to the supporting panel 3 and
can house all the parts carried by the panel 3.
The tensioning device 1 is capable of being fitted
in an advance path of an optical fibre 35, for example
in a draw tower (not illustrated) for the optical
fibre, and is capabl.e of receiving the optical fibre
from a source of optical fibre 37 (shown
schematically). Ir:. the non-restrictive example
considered here, the source of optical fibre 37 is a
traction device ("capstan") of arl optical fibre draw
tower (not illustrated). Alternatively, the source of
optical fibre 37 could be any member or device for
returning or guiding t:he optical fibre, or an optical
fibre take-off reel.
The optical i=ibre 35 extends from the traction
device 37 to the fixed pulley 32, forming a first
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rectilinear portion 35a, is wound (through 180 for
example) around the fixed pulley 32, extends between
the fixed pulley 32 and the movable pulley 9, forming a
second rectilinear portion 35b, is wound (through 180
for example) around the movable pulley 9, and finally
extends between the movable pulley 9 and a further
device 40, thus forming a third rectilinear portion
35c. In the non-restrictive example considered here,
the device 40 (showrl schematically) is a storage device
comprising a motorized reel 42 on to which the optical
fibre 35 can be wound. The motorized reel 42 is movable
rotationally about. an axis of rotation 43 under the
action of an electric motor 45. Alternatively, the
device 40 can be any member or device for returning or
guiding the optical fibre.
Preferably, but not necessarily, the rectilinear
portions 35a, 35b, _35c are parallel to each other.
Between the device 37 and the tensioning device 1,
and between the tensioning device 1 and the device 40,
there may advantageously be return pulleys which, for
simplicity, are not shown in Figure 1.
An electronic control unit 50 is capable of
controlling, through corresponding power driver
circuits ("drivers") 24p, 45p, the electric motor 24
and the electric motor 45.
Advantageously, the tensioning device 1 comprises
a tension measuring device 56, for example a load cell,
preferably connected to the pin 3:3 of the fixed pulley
32. The tension measuring device 56 is capable of
generating a signal 'I'mis proportional to the tension to
which the optical fibre 35 is subjected, and is
connected to the electronic control unit 50 to supply
the signal Tmis to the electronic contrc>l unit 50.
With particular reference to Figure 2, the
electronic unit 50 is illustrated in detail. The
electronic unit 50 comprises a subtractor 60 to which
is supplied a signal To, proportional to a target
tension of the fibre, and the sigrlal Tmis, proportional
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to the tension to which the optical fibre 35 is
actually subjected. 'rhe subtractor 60 generates at its
output an error signal s= AT = To - Tmis (tension
error) and supplies this error signal to a controller
5 62, particularly a PID (proportional-integral
derivative) controller, which generates at its output a
signal (o (a voltage signal, compatible with the drive
circuit 24p which is operated by means of voltage).
A PID controller is a controller which, having
10 received an input signal Si, generates an output signal
So, typically given by:
r
Sõ (t) = KP S; (t) + ~ JS, (t) dt + Td = d ~(t)
~o
A PID controller can therefore be identified by
the three constants Kpõ Ki=KP/Ti and Kd=KP = Td.
15 The signal t) is used, after having been converted
and transformed into a power signal by the drive
circuit 24p (of a known type), to operate the motor 24.
The speed of rotation of the motor 24 is therefore
correlated with tt-ie signal o). The signal (o is also
supplied to a conversion unit 64, which generates at
its output a signal V correlated with the linear speed
of movement of the slide 7 alorig the guide 5.
In practice, if vmaX is the maximum speed of the
motor 24 (expressed in r.p.s.), c)maX is the (electric
voltage) value of the signal w with which the maximum
speed vmaX is associ.ated, p is the oitch of the screw 26
and c.o(t) is the generic value of the signal co at the
instant t (supplied from the output of the controller
62), the conversion unit 64 generates at its output a
signal V whose value V(t) at the instant t is given by:
VM = v", - Wt) P
(~)max
In practice, the conversion unit 64 implements the
transfer function which relates the angular rotation of
the screw 26 to the lirlear speed of the slide 7.
The signal V i_s supplied to a further conversion
unit 66, which, by integrating the signal V, generates
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at its output a signal Pmis (electric voltage) relating
to the instantaneous position of the slide 7 along the
guide 5.
The position signal Pmis is supplied tc> a
subtractor 70 to which a signal Po, proportional to a
target position of the slide 7 along the guide 5, is
also supplied. Preferably, the target position of the
slide is the position intermediate between the first
and second terminal positions.
The subtractor 70 generates at its output an error
signal E= AP = Po-Pmisr and supplies this error signal
to a controller 72 (in particular, a PID (proportional-
integral derivative) controller), which generates at
its output a signal cob which is used, after having been
converted and transformed into a power signal by the
drive circuit 45p (of a known type), to operate the
motor 45. The signal (ob is correlated with the speed of
rotation of the reel 42.
In operation, 'when the storage device imparts to
the incoming optica:L fibre 35 a higher speed than the
speed at which it is supplied from the output of the
traction device 37, the tension Tmis in the fibre
increases and deviates from the target value To.
Consequently, the tension error AT (which is
negative in this case) increases in absolute value, and
the controller 62, through the drive circuit 24p,
causes a rotation of the motor 24 such that the slide 7
is made to move towards the first terminal position,
and consequently the distance Dp and the length of the
portion 35b of fibre located between the two pulleys 9
and 32 are decreased; the decrease of the length of the
portion 35b is thus opposed to the increase of tension
in the fibre, in such a way that the tension in the
fibre is kept essentially constant.
Conversely, when the storage device 40 imparts to
the incoming optical fibre 35 a lower speed than the
speed at which it is supplied from the output of the
traction devi_ce 37, the tension Tmis in the fibre
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decreases and devi_ates from the target value To.
Consequently, the tension error AT (which is positive
this time) increases, and the controller 62, through
the drive circuit 24p, causes a rotation of the motor
24 such that the slide 7 is made to move towards the
second terminal position, and consequently the distance
Dp and the length of the portion 35b are increased; the
increase of the lerlgth of the portion 35b is opposed to
the decrease of tension in the fibre in such a way that
the tension in the fibre is kept essentially constant.
Thus a motorized closed-loop control of tension is
provided, which returns the tension in the fibre to the
target value To.
Additionally, the unit 66, by integrating the
signal V (correlated with the linear speed of the slide
7), determines the instantaneous position Pmis of the
slide 7 along the guide 5. This instantaneous position
is compared with the objective position Po, and the
position error AP, processed by the conversion unit 72,
produces a variation of the speed of rotation of the
motor 45. In particular, when the slide 7 is closer
than the reference position to the first terminal
position (with the slide 7 moved to the left in Figure
1), the control system produces a decrease of the speed
of rotation of the motor 45 in such a way that the
tension in the fibre 35 decreases and the tension
control loop, in order to compensate for this variation
of tension, moves the slide 7 (by rneans of the motor_ 24
and the screw 26) towards the second terminal position
(in other words t(D the right in Figure 1), thus
compensating for the variation of position.
Additionally, when the slide 7 is closer than the
reference position to the second terminal position
(with the slide 7 moved to the right in Figure 1) , the
control systern produces an increase of the speed of
rotation of the motor 45 in such a way that the tension
in the fibre 35 increases and the tension control loop,
in order to compensate for this variation of tension,
CA 02329822 2000-12-28
PCB47
18
moves the slide 7 (by means of the motor 24 and the
screw 26) towards the first terminal position (in other
words to the left in Figure 1), thus compensating for
the variation of position.
Thus a closed-loop position control is provided,
which returns the slide to the target position Po.
If preferred, the position controller could be
dispensed with; in this case, the control device would
not include the elements indicated by the numbers 64,
66, 70, 72 and 45p.
The above descr:iption clearly shows the advantages
of the present invention, in that the movement of the
movable pulley 9 depends solely on the action of the
electric motor 24 and is entirely independent of the
force of gravity or other predetermined forces (of the
elastic type, for example) applied to the movable
pulley 9. This, a force whose value can be
automatically controlled in accordance with the
operating signal applied to the motor 24 is applied to
the optical fibre 3:3. The tensioning device 1 adapts
itself automatical:Ly to any change in operating
requirements (such as a change in the drawing speed, or
a change in the position of the slide 7) and returns
the value of the tension applied to the fibre to a
target value.
Additionally, it is ensured that the movable
pulley 9 is always kept close to a reference position
(in particular, the central position of the guide 5)
from which it can always be moved in a useful way (to
the right or to the left) to effectively oppose a
change iri tension. In other words, the slide 7 is
prevented from being positioned close to the first or
second terminal pos:i_tion, where the movement of the
slide in one of the two directions is no longer
possible.
The tensioning device 1 can be used in apparatus
of different types, for example in a draw tower, in an
extrusion line or in a test apparatus (also known as a
CA 02329822 2000-12-28
PC EI 4 7
19
"screening apparatus") for testing the optical fibre in
respect of the breaking stresses.
A draw tower typically comprises a furnace for
melting a preform of glass material, a traction member
(or "capstan", for example one of the single pulley or
double pulley type) for drawing downwards the optical
fibre generated frorn the melted preform, a coating
device positioned between the furnace and the traction
member to apply a protective surface coating (typically
made from acrylate) to the optical fibre, and a fibre
take-up reel receiving the fibre from the traction
member. The tensioning device 1 can advantageously be
positioned, as merltioned above, between the traction
member and the take-up reel.
An extrusion l.i_ne is a processing line along which
a plurality of optical fibres are made to advance with
a wire-shaped supporting element, and along which a
polymer mater_ial i.s extruded over the supporting
element in such a way as to incorporate the optical
fibres and hold them in a fi.xed position around the
supporting element, thus formirig the optical core of: an
optical fable. An extrusion line t:ypically comprises a
take-off reel for the supporting element, a plura-i~ity
of fibre take-off reels, an extruder capable of
receiving both the fibres and the supporting element at
its inlet, a coolirig vessel receiving from the extruder
the optical core formed in the extruder, and a final
take-up reel. In an extrusion line, the tensioriing
device 1 can, for example, be posi_tioned downstream of
each of the fibre take-off reels.
A screening apparatus is typically used to check
that the fibre is capable of withstanding predetermined
tensions, and is therefore free of significant
structural defects. A screen apparatus typically
comprises a take-(-:)ff reel for the optical fibre, a
take-up reel for the optical fibre and a plurality of
guide and/or r_eturn elements capable of forming a path
for the optical fibre between the take-off and take-up
CA 02329822 2000-12-28
PC1347
reels and capable of imparting a predetermined tension
to the optical fibre during its passage. In a screening
apparatus, the device 1 can be positioned at any point
of the aforesaid pat.h.
5 Finally, it is clear that modifications and
variations can be made to the tensioning device
described without: departing from the scope of
protection of the present invention.
In particular, the system for moving the slide 7
10 (screw 26 and nut. coupled to the slide 7) could be
different from that described, and could comprise, for
example, a toothed belt (not illustrated) connected to
the slide 7 and movable between two end pulleys.
Finally, the position Pmis of the slide 7 along
15 the guide 5 could bi= determined not in an indirect way
(in other words, following the measurement of the
speed) as illustrated in Figure 2, but in a direct way,
using a row of position sensors (not illustrated), for
example a row of photocells, or an optical strip (in
20 other words a linear photosensitive component, for
example a linear CCD - not illustrated), arranged along
the guide 5.
