Yarn tension sensor

A yarn tension sensor is disclosed which is adapted for use with a textile yarn processing machine. The sensor includes a spring arm which is contacted by a running yarn, and such that the tension in the yarn acts to deflect the arm. In order to render the deflection of the arm relatively insensitive to external or machine vibrations, while permitting an accurate response to fluctuations in yarn tension, there is provided a structure for damping the spring arm, which includes a magnetic fluid having a portion of the spring arm immersed therein.

BACKGROUND OF THE INVENTION 
The present invention relates to a yarn tension sensor adapted for use with 
a textile yarn processing machine. 
In the production and processing of multi-filament yarns, such as the false 
twist texturing operation, the yarn tension is an important process 
parameter. Specifically, temporary fluctuations of the yarn tension will 
have a significant influence on the quality of the yarn, and for this 
reason, a satisfactory yarn tension sensor must have an adequately high 
natural frequency so as to be adapted to follow the frequency of the 
fluctuations of the tension. This requirement presents difficulties, 
however, since the vibrations of the machine itself may lead to 
oscillations of the tension sensor. 
It is accordingly an object of the present invention to provide a yarn 
tension sensor which is able to follow the fluctuations of the yarn 
tension, and which is relatively insensitive to machine vibrations and 
other external disturbances. 
SUMMARY OF THE INVENTION 
This and other objects and advantages of the present invention are achieved 
in the embodiments illustrated herein by the provision of a yarn tension 
sensor which comprises an elongate spring arm having one end adapted to be 
fixedly mounted to a textile yarn processing machine and an opposite free 
end which is deflectable in a predetermined deflecting direction with 
respect to the machine, and guide surface means mounted adjacent to the 
free end of the arm for engaging a running yarn, and such that the tension 
in the running yarn acts to deflect the free end of said spring arm in the 
deflecting direction. The sensor also includes means for damping the 
movement of the free end of the spring arm, with the damping means 
including a receptacle supporting a magnetic fluid, and with the spring 
arm having a portion thereof immersed in the magnetic fluid. By this 
arrangement, the movements of the spring arm are dampened, irrespective of 
the deflected position of the arm. 
In one embodiment, the portion of the spring arm which is immersed in the 
magnetic fluid comprises a projection mounted at the free end of the 
spring arm, and which takes the form of a damping piston which extends in 
the direction of deflection. Also, the receptacle of the magnetic fluid 
comprises an open cup-like member which is mounted in alignment with the 
free end of the spring arm, and so that the projection extends into the 
receptacle so as to be immersed in the magnetic fluid. Also, a magnet, 
such as a permanent magnet, is preferably positioned adjacent the bottom 
of the receptacle to hold the magnetic material in place. Further, it is 
preferred that the projection and the receptacle be made of a non-magnetic 
material. 
An advantage resulting from the use of a magnetic fluid resides in the fact 
that the sensor can be mounted in any orientation. For example, in a yarn 
false twisting machine, the yarns normally run in a vertical direction, 
either up or down, and the sensor of the present invention may be mounted 
either horizontally or vertically, and with the deflecting direction in 
either case being horizontal. 
The spring arm may take the form of a pair of parallel, spring plates which 
are interconnected at each of the opposite ends, with the free end of the 
arm mounting the yarn guide surface means as well as the projection or 
damping piston adapted to be immersed in the magnetic fluid contained in a 
receptacle as described above. Here again, a magnet is preferably mounted 
adjacent the bottom of the receptacle, either below the receptacle or in 
its interior, to hold the magnetic fluid in place. In this embodiment, the 
free end of the spring arm is movable only in the deflecting direction, 
and as a result, the direction of the reaction force acting on the yarn 
does not change. 
In another embodiment of the present invention, the receptacle for the 
magnetic fluid has opposite side walls with aligned openings extending 
therethrough, and the spring arm extends through the openings in the side 
walls. The openings in the side walls are sized to permit the deflecting 
movement of the spring arm, and suitable magnets are positioned on the 
other opposing side walls of the fluid receptacle so as to retain the 
magnetic fluid within the receptacle. 
The extent of the deflection of the spring arm may be determined by 
suitable measuring instruments, such as for example strain gauges. 
However, the present invention contemplates a measuring system which is 
essentially independent of temperature changes, and which comprises a 
deflection measuring instrument which directly measures the distance of 
the deflection of the spring arm. In particular, the deflection measuring 
instrument of the present invention takes the form of a cooperating 
arrangement of an optical signal transmitting means or emitter and an 
optical signal receiving means or receiver, and wherein the quantity of 
the received signal varies with the extent of the deflection. 
In one illustrated embodiment, the optical emitter and the optical receiver 
of the deflection measuring instrument are respectively mounted to the 
free end of the spring arm, and a support member which is mounted to the 
fixed end of the spring arm. In another embodiment, the emitter and 
receiver are mounted to a common support member, and the support member is 
fixedly mounted to the fixed end of the spring arm. Also, a reflector, 
such as an optical prism, is mounted to the free end of the arm. In this 
latter arrangement, it is preferable that the reflecting angle of the 
prism differs slightly from 90.degree., and also it is preferable that the 
longitudinal distance between the reflector and the support for the 
emitter and receiver be adjustable. This adjustment provides for a 
"zero-balance" calibration in a simple manner. In particular, a 
temperature change will be understood to effect a change in the relative 
distance between the reflector and the support member for the emitter and 
receiver, and the received quantity of the signal will also change since 
the reflected signals do not extend parallel to the emitted signals. 
Temperature compensation may thus be carried out by adjusting the relative 
distance between the reflector and support member for the emitter and 
receiver so that the output signal may be calibrated to the unloaded 
condition. With a prism having a reflecting angle of exactly 90.degree., 
the reflected signals and emitted signals would be parallel, and thus 
there would be no change in the received signals upon a change in 
separation distance. 
A further embodiment of the present invention relates to a specific 
construction of the spring arm which comprises a unitary hollow casing 
having a rectangular or square outline in cross section. The side walls of 
the casing which extend parallel to the direction of the deflection, i.e. 
parallel to the direction in which the yarn tension is operative on the 
yarn sensor, each include elongate openings formed therein to facilitate 
the deflection of the bar. The elongate openings are "bone shaped" in 
outline, which is here understood to mean the openings have elongate 
parallel side edges which extend along a major portion of the length of 
the bar, and a generally circular enlargement at each of the ends thereof. 
Also, the distance measuring instrument as described above may be mounted 
within the casing and extend from the fixed end to a location closely 
adjacent the free end of the arm. The yarn guide surface is mounted 
adjacent the free end of the casing and takes the form of an extension 
which extends along the central longitudinal axis of the casing. Where the 
deflection measuring instrument includes a reflecting prism as described 
above, the reflecting prism may be mounted within the casing adjacent the 
free end, and the support member of the deflection measuring instrument is 
preferably made of the same material as the casing for the purpose of 
temperature compensation. In a further embodiment, the other two sides of 
the casing may also be provided with openings, so as to adapt the 
characteristic curve of the spring bar and thus the measured deflections 
of the casing to the anticipated yarn tensions. 
With respect to the above embodiment, the casing may mount a projection in 
the form of a damping piston adjacent the free end thereof, and which 
extends in the direction of the deflection and which is immersed in a 
magnetic fluid positioned in an open cup-like receptacle in the manner 
described above. Here again, a magnet, such as a permanent magnet, may be 
positioned on the bottom of the receptacle or in its interior, and it is 
also advantageous to make the receptacle which contains the magnetic fluid 
of a non-magnetic material. 
The yarn tension sensor of the present invention is adapted to measure high 
frequency fluctuations of the tension, of for example 30-40 Hz. The 
measuring range may be further expanded by the use of a measuring 
amplifier, having a characteristic curve which rises in the range of the 
natural frequency of the yarn sensor, and which as a result compensates 
either wholly or in part the frequency dependent characteristic curve of 
the yarn sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring more particularly to the drawings, FIG. 1 illustrates a yarn 
tension sensor 1 which embodies the features of the present invention, and 
which comprises an elongate spring arm consisting of a pair of identical, 
parallel flat spring plates 2, 2' which are interconnected at respective 
opposite ends by the intermediate blocks 3 and 4. A mounting spacer 6 is 
positioned at the end of the arm which includes the block 3, and this end 
is fixed to the frame 7 by means of the bolts 5. The frame 7 typically 
comprises a portion of the frame of a textile yarn processing machine. The 
opposite or free end of the arm, and which includes the block 4, mounts a 
yarn guide surface 8 which extends in the direction of the arm. Also, a 
projection or damping piston 9 is mounted to the free end of the arm and 
extends in the direction of the force 11 applied by the tension of the 
running yarn 10, i.e., the direction in which the yarn tension is 
operative on the guide surface 8. As noted above, the spring arm usually 
would be mounted either vertically or horizontally, with the direction of 
deflection 11 being horizontal so as to permit engagement with a 
vertically running yarn. In addition, when the spring arm is mounted so as 
to extend horizontally, the deflection direction extends in a horizontal 
direction which is perpendicular to the horizontal direction of the spring 
arm. A receptacle 12, which has the form of an open cup, is positioned in 
alignment with the free end of the arm, and the receptacle 12 is 
substantially filled with a magnetic fluid 13. In the embodiment of FIG. 
1, the receptacle 12 is placed on a magnet 14, preferably a permanent 
magnet, and the receptacle and magnet are fixedly mounted to the frame 7. 
The yarn 10, which is shown contacting the guide surface 8, may also be 
guided through a guide opening (not shown) which is positioned adjacent 
the guide surface 8. 
Magnetic fluids which are adapted for use with the present invention are 
well known in the art, and are commonly called "ferrofluids" in the 
literature. Such fluids are a dispersion of extremely small particles of 
magnetic materials, for example, Fe.sub.3 O.sub.4, which are suspended in 
a carrier fluid such as kerosene or silicon oils. Magnetic fluids may be 
purchased from Ferrofluidics Corporation of Nashua, N.H. Further 
information can be found, for example, in U.S. Pat. No. 4,107,063 and in 
"Magnetic-fluid seals", published in Laser Focus Magazine, April 1979, pp. 
56-63, Advanced Technology Publications, 1001 Watertown St., Newton, MA 
02165. 
Another embodiment of the present invention is illustrated in FIGS. 2, 2A 
and 3. This embodiment incorporates a deflection measuring instrument as 
further described below, and by reason of the relatively high sensitivity 
of this instrument, the elasticity constant of the sensor can be 
relatively high, and as a result thereof the natural frequency is also 
relatively high. As a result, high frequency fluctuations of the yarn 
tension may be detected. In particular, the illustrated damping means, 
which comprises the piston 9, receptacle 12, and magnetic fluid 13, 
permits the detection of fluctuations in the yarn tension at frequencies 
in the range of about 50 Hz, where an essentially inertia-less deflection 
measuring instrument of the type described below is employed. The 
compensation of the frequency response characteristic of the sensor by 
means of an amplifier with a frequency compensation, permits the sensor to 
detect tension frequencies of several hundred Hertz. 
Referring again to the embodiment of FIGS. 2, 2A and 3, the illustrated 
spring arm comprises a unitary hollow casing 16 having a rectangular 
outline in cross section, with the casing having two opposite side walls 
22, 22' (FIG. 3) which lie in parallel planes which are parallel to the 
deflection direction. The end of the casing 16 which includes the block 3 
is fixedly mounted to the spacer 6 and frame 7 by means of bolts 5, 
whereas the free end of the casing mounts a head 15 having a yarn guide 
surface 8 and a projection or damping piston 9 mounted thereto. The two 
opposite side walls 22, 22' of the casing 16 each include elongate 
openings 17 formed therein to facilitate the deflection of the casing. The 
elongate openings 17 are symmetrically configured, and each opening 
comprises parallel side edges which extend along a major portion of the 
length of the casing, and a generally circular enlargement at each of the 
ends thereof. The diameter of the enlargements correspond to the inside 
cross sectional dimension of the casing 16 in the illustrated embodiment. 
As a result, the openings may be said to be " bone-shaped", and the cross 
section of the casing exhibits a reduced bending resistance adjacent the 
centers of the circular enlargements. As best seen in FIG. 3, the top and 
bottom walls of the casing also contain similar openings 17 for the 
purpose of favorably influencing the rigidity of the casing. 
The damping piston 9 is immersed in the magnetic fluid 13 in the receptacle 
12, so that the oscillations generated by the machine are damped. The 
magnetic fluid is stabilized and retained in the receptacle 12 with the 
aid of the magnet 14, which is preferably a permanent magnet, and which 
renders the yarn tension sensor of the present invention independent of 
deflected position. 
A deflection measuring instrument is rigidly mounted inside the casing, and 
the instrument includes a support member 23 which extends longitudinally 
from the fixed end of the arm to a location closely adjacent the head 15 
at the free end of the arm. A mounting head 19 is mounted in alignment 
with the support member 23, and the head 19 is secured inside the casing 
adjacent the free end, and such that the head 19 follows the movements 
caused by the yarn tension. A mounting head 20 is mounted at the free end 
of the support member 23, and the mounting head 20 in turn mounts a source 
of radiation 29. Also, a receiver 30 is mounted on the head 19. The 
radiation source 29 and receiver 30 are further described below with 
respect to FIG. 6, and they provide an essentially inertialess scanning of 
the deflecting movements, and a reliable measuring is possible, even at 
high frequencies of yarn tension fluctuations. 
Although the illustrated embodiment of FIG. 1 makes no mention of a similar 
yarn tension measuring instrument, it will be understood that a similar 
instrument could be employed with the embodiment of FIG. 1. In such case, 
the support member 23 would be rigidly mounted between the two leaf 
springs 2, 2' at the fixed end adjacent the block 3, whereas the measuring 
head 19 would be secured to the block 4. It will be understood however 
that other suitable deflection measuring instruments, such as a pneumatic 
system could be employed. 
Preferably, the damping piston or projection 9 and the receptacle 12 are 
made of a non-magnetic material. To avoid different coefficients of 
thermal expansion, it is advantageous to make the intermediate blocks 3 
and 4 as shown in FIG. 1, and similarly the casing 16 and holder 24 and 
support member 23 as seen in FIG. 2, of the same material. 
The damping means of the present invention can be used to successfully 
suppress measuring errors caused by the natural oscillation of the yarn 
sensor, in all yarn tension measuring devices in which the relative 
movement is detected between a yarn sensor moved by the yarn tension 
relative to its mounting support. 
In the embodiment of FIGS. 4 and 5, the spring arm of the sensor again 
comprises two parallel leaf springs or plates 2, 2', which extend 
horizontally through a damping chamber receptacle 25. The two spring 
plates are fixedly held to the frame 7 at the right end of the spring arm 
as seen in FIG. 4 by the block 3, spacer 6, and bolt 5. The yarn 10 is 
guided into contact with the guide surface 8, and the guide surface 8 is 
attached to the free end adjacent the block 4. Deflection measuring 
instruments, such as strain gauges may be disposed on the spring arm to 
measure the deflection in the direction of the force 11. Also, the damping 
receptacle 25 includes opposite side walls with aligned pairs of openings 
26, 27 extending therethrough, and the spring plates 2, 2' extend through 
respective ones of the openings in each side wall. These openings closely 
enclose the cross section of the spring plates 2, 2', but only to the 
extent that the spring plates are permitted to move unhindered in the 
direction 11. The receptacle 25 is completely filled with a magnetic 
fluid, and permanent magnets 14 are mounted on opposite sides of the 
receptacle for retaining the fluid therein, note FIG. 5. 
Another embodiment of the deflection measuring instrument of the present 
invention is illustrated in greater detail in FIG. 6. Specifically, the 
instrument includes the support member 23, which mounts the head 20 at the 
forward end thereof. The opposite or right hand end of the support member 
23 is slideably mounted in the support guide 32, and the support guide 32 
is fixedly mounted to the fixed end of the support arm. 
The mounting head 19 is fixed to the head 15 at the free end of the arm, 
and mounts a prism 28. The prism includes a reflecting angle alpha which 
is slightly less than 90.degree.. The mounting head 20 mounts a source of 
radiation 29 and a receiver 30. The source of radiation may, for example 
be a source of light, and the receiver may be a photo diode 31. Since the 
reflecting angle alpha is not equal to 90.degree., the emitted light beams 
are not parallel to the reflected beams, and as a result, the reflected 
beams strike the center of the photo diode 31 to produce the optimum 
measured deflection, represented by the voltage U, only at a predetermined 
distance A between the heads 19 and 20. This measured deflection is 
entered as a calibrated signal of the unloaded condition of the yarn 
sensor, and if the distance A changes because of temperature fluctuations 
and the corresponding expansion of the casing 16 and support member 23, 
there thus results a different calibrated signal U at zero load. The 
effect of the temperature may be equalized by displacing the support 
member 23 in the guide 32, until the calibrated signal U is restored. To 
this end, the support member 23 is slideably mounted in the guide 32, and 
to displace the support member, an adjusting screw 33 is provided. In 
particular, the adjusting screw 33 is rotatably supported in the guide and 
threadedly engages the support 23, and the support member 23 is biased on 
its rear side by a compression spring 34. 
In the drawings and specification, there has been set forth a preferred 
embodiment of the invention, and although specific terms are employed, 
they are used in a generic and descriptive sense only and not for purposes 
of limitation.