The present invention relates to an apparatus and method for determining tension in an elongate element. In particular, but not exclusively, the present invention provides a non-contact method for determining the tension in a yarn or wire. The yarn or wire which forms a target strand may be stationary or moving.
The measurement of tension of yarns produced by or supplied to textile machines is important for related quality assurance and process control functions. While the term “yarn” is used in the following text by way of example it will be understood that the present invention is applicable to many varieties of target strands such as cords, braidings, cables or lines which are relatively strong or even rovings or slivers which are weaker. The term strand is therefore to be broadly construed since the present invention can provide a mechanism for determining the tension in any elongate element.
Physically, a yarn may consist of a number of continuous filaments or be spun out of relatively short fibres. As such a yarn may have a twist given to it and a degree of unevenness of cross section along its length. Spun yarns also have a certain amount of hairiness. Yarns are often dyed to impart to them a colour which is required by an ultimate product. On a textile machine the yarns move at some speed ranging from a few millimetres per second to tens of meters per second.
Tension measurement of a yarn is carried out over a suitable span usually between two yarn guides. Contact type tension measuring instruments are known which employ the well-known three-point measuring principle and these are most commonly used by the industry. This type of measurement is simple in that it gives a direct reading of the tension in the yarn. However the measurement suffers from a number of drawbacks the greatest of which is the significant measurement error introduced by frictional drag on the yarn caused by measuring tips. This can lead to considerable (5%-15%) measurement errors. Also the tension of a target strand may be affected by the intrusion caused by probe tips. Another disadvantage is that physical contact with the strand may abrade or otherwise damage the target. Another problem with this known technique is the need for mechanical manipulation for threading in of the yarn. Also difficulties may be experienced in measuring tension of moving thread lines.
It is an aim of the present invention to at least partly mitigate the above-mentioned problems.
It is an aim of embodiments of the present invention to provide a non-contact method for determining a tension in a target strand.
It is an aim of embodiments of the present invention to provide apparatus which can very conveniently be used to determine the tension in a running thread or a stationary thread.
According to a first aspect of the present invention there is provided a non-contact method for determining a tension in a target strand, comprising the steps of:
providing a plurality of radiation detecting elements each arranged to provide an output signal for indicating a level of radiation incident at a respective detecting element;
detecting radiation incident at said plurality of detecting elements when said strand vibrates;
repeatedly identifying one or more detecting elements providing an output indicating a predetermined characteristic; and
determining the tension in said strand responsive to which of said detecting elements are identified.
According to a second aspect of the present invention there is provided apparatus for determining a tension in a strand comprising:
a plurality of radiation detection elements each for providing an output signal responsive to a respective level of incident radiation;
means for identifying one or more of said detecting elements providing a respective output indicating a predetermined characteristic; and
means for determining the tension in said strand responsive to which of said detecting elements is identified.
Embodiments of the present invention provide the advantage that no physical contact is required on a strand to measure the tension in that strand. As a result there is no need for mechanical manipulation of the strand for the purpose of tension measurement and consequently physical contact which may abrade or otherwise damage the strand is obviated. The non-contact technique also means that errors in the measurement are much reduced over known techniques.
Embodiments of the present invention provide the advantage that the tension in a strand may be determined regardless of the slender optical profile of the strand and without a requirement for strong illumination. Also variations in the strand, for example in the case of yarn by twist and hairiness, has no effect upon the accuracy of the tension measurement.
Embodiments of the present invention will now be described hereinafter, by way of example only, with reference to the accompanying drawings in which:
In the drawings like reference numerals refer to like parts.
The moving strand 11 moves over two spaced-apart strand supports 12 and 13. These help support and guide the yarn during its movement along a desired path. They also precisely define a distance e. As will be appreciated a textile yarn passing over two such guides is likely to undergo transverse vibrations at a frequency determined by its tension. Also a textile machine produces a certain amount of vibration in its operation and these tend to induce natural vibrations in open runs of the yarn found on it. Furthermore yarn motion aided by guide friction also tends to induce such vibrations. The frequency of such natural vibration has a clear relationship to the tension in the yarn as described later. This relationship has been known for a long time. However problems associated with detecting the tension satisfactorily have, until now, thwarted the realisation of a reliable general purpose non-contact yarn tension measuring instrument based upon that principle.
The apparatus for determining the tension in the target strand includes an optical sensing head 14 and associated electronic circuits for the detection of the lateral movement of the yarn due to vibrations and an electronic processing unit for data acquisition, analysis and display of tension readings.
The sensor head 14 employs an optical sensor of the charge coupled device (CCD) linear array type 20. This includes 64 radiation detecting elements arranged in a predetermined array at a predetermined pitch. It will be understood that any type of array of detecting elements could be used. Radiation in the form of visible light (illustrated by lines 21 in
The radiation 21 illustrated in
The CCD array 20 produces an output signal which is fed to a detection circuit 23. The signal 24 output from the detection circuit is detected and an output signal 25 is developed which corresponds the transverse positioning of the target strand. A signal processor 26 is then used to sample this signal repeatedly at a fixed rate and carry out frequency analysis on the acquired data so as to identify the natural frequency of a vibration of the target yarn. As the distance between the two yarn guides and the mean linear density of the yarn may be predetermined and thus known the tension of the yarn can be calculated by using the relationship:
Here f is the natural frequency of vibration, l is the distance between the strand supports, ρ is the linear density of the yarn, T is the tension in the yarn and n is an integer value corresponding to the mode of vibration of the strand. The fundamental natural frequency of the strand is normally encountered so n=1.
This equation is well known. Occasionally depending on the level of tension a higher harmonic vibration may be encountered. In order to avoid an incorrect determination of tension the apparatus can be set up to select the fundamental frequency (i.e. the first harmonic) as will be understood by those skilled in the art.
It will be understood that embodiments of the present invention can be used without the need for specific fighting sources. Ambient light may be sufficient.
a and 4b show a general form of the output from the CCD array. The wave form or frame shown is one complete sequential output available from the linear array. Each frame represents a snapshot in time of the radiation falling on the linear array. Each detecting element output is allocated a respective bin so that the output from one bin 40 corresponds to the output from a respective one of the detecting elements in the CCD array. The waveform is characterised by an initial brief dropping voltage level following the output sequence which corresponds to an internal reset operation of the output sequence. The signal may have a number of small peaks which may result from many external factors such as hairiness of the strand. However there is a major peak 41 which corresponds to the position of the yarn at a point in time when data is collected from the detecting elements. As the yarn vibrates its transverse position with respect to the sensor varies and this is observed in the varying position of the peak in successive frames of the wave form.
As this variation is proportional to the lateral movement of the yarn, the yarn vibration can be detected from the CCD output signal. As the detection is based on the position of the peak, and hence which element in the array and not the actual amplitude of the signal the determination is not affected by fluctuations of illumination level or the variation of reflected light due to reflectivity, hairiness or unevenness variations of the yarn. This is so as long as the level of illumination remains above a minimum level, so that the signal output is sufficiently high to enable the peak to be identified.
The peak 41 will be tall and sharp when the yarn is well focused on the CCD array as shown in
At the normal tension levels encountered and range of yarn counts and span lens encountered textile yarns are found to vibrate at frequencies normally below 500 Hz. This suggests a minimum speed of 1 kHz for sampling the yarn vibration. The output signal amplitude from the CCD array essentially depends on the exposure time or the time per output cycle. A sampling rate of 1 kHz has been found to be acceptable although faster sampling rate may be used and for strands vibrating at frequencies substantially below 50 Hz a lower sampling rate may be used.
A peak detector and hold stage circuit 53 receives the sequential output bin by bin of a frame. The value of each bin is compared with a peak voltage value. For the first bin the value of the detecting element corresponding to that bin will be the peak value for that frame. The value of the next bin corresponding to the detecting element adjacent to the first detecting element is then compared with that pre-stored value. If the new bin has a higher value than the earlier bin or bins then a new high value is stored. In this way a voltage value corresponding to the peak 41 is detected and stored and output to a comparator 54. The comparator 54 compares the identified, so far, peak value of a frame with the bin by bin serial value output from the amplifier stage 52. In this way when a bin value is equal to the peak value so far for that frame the comparator output issues an enable signal on line 55. For a generally increasing slope as shown on the left hand side of
The latch 56 is continually supplied with an eight-bit digital count signal from counter 57 through bit lines B0 to B7. When a new frame is examined, indicating a snap shot of the location of a strand at any instant, the counter 57 is reset. In this way the counter stage is zeroed as each new frame of output signal is started. It thus counts the clock pulses continuously until the completion of the frame. Since it is an eight-bit counter the count can be in the range of 0 to 256 which is more than sufficient to count the 64 detecting element outputs. As such, arrays up to 256 elements can be accommodated by an 8 bit counter. As suggested by
In embodiments of the present invention the digital to analogue converter 58 stage is not used and the digital count is supplied directly to a data processor. In such a case it is necessary for the oscillator 51 to run at a precise frequency, for example 65 kHz for a CCD of 64 elements to provide 1000 Hz sampling rate and also possibly provide a synchronising pulse. The 65 kHz rate is useful when a CCD array texas instruments TSL 214 is utilised. It will be understood that other forms of detecting element array are applicable according to other embodiments of the present invention.
The tension of a yarn normally has a continuous variation and therefore the data captured over the measuring interval will reflect this variation. It is possible to take into account the signal amplitudes other than the highest to derive the profile of tension variation over a continuous measuring interval.
Embodiments of the present invention provide an instrument which is particularly suitable for measuring the average level of tension in textile yarns for process control purposes. The readings provided by the method compare well with those obtained by conventional methods. In fact since the non-contact methods are not affected by contact friction readings achieved more closely resemble true values. Embodiments of the present invention can be used as a handheld device or are suitable for machine mounted yarn tension monitoring involving single or multiple thread line situations.
Embodiments of the present invention provide a method and apparatus for detecting tension in a strand which may be very thin and perhaps too delicate for mounting any sensor directly onto. Also yarns in a moving state, sometimes many metres per second, can have their tension monitored.
Embodiments of the present invention can also provide a method and apparatus for determining the tension in a target strand which is non-metallic in nature which otherwise rules out the use of capacitive or magnetic sensing.
Although embodiments of the present invention have been described by way of example in a reflective mode it will be understood that a transmitive mode can be used whereby light obscured by a suitably located target strand is measured. In such circumstances a predetermined characteristic which is identified is the detecting element having the lowest level of incident radiation. This corresponds to the minimum rather than peak value. This also relates to a position when the strand is most directly between a light source and a detecting element array.
It will be understood that embodiments of the present invention need not identify just the maximum or minimum value from a detecting element array. It would be possible to detect any other predetermined characteristic. By using a linear optical detector detection in a horizontal axis can be achieved. This obviates the problems of prior known techniques in which variation in a vertical (amplitude) axis due for example to changes in detected values because of hairiness causes problems. Equally embodiments of the present invention provide a method and apparatus for determining the tension in a target strand which does not require strong illumination, which is not susceptible to a variation of the amount of reflected light caused by variations of yarn twist, reflectivity, cross-section and hairiness. Also 100 Hz flicker caused by fluorescent lighting is not unduly problematical.
Although embodiments of the present invention have been described with respect to a moving strand it will be understood that embodiments of the present invention are applicable to a stationary strand. The strand should have a vibration introduced into it for example by plucking or blowing on the strand at some location. In this way a natural resting position of the strand is interrupted and the detecting steps can be used whilst the strand returns to the resting position.
Embodiments of the present invention have been described above by way of example only. It will be understood that modifications may be made to the specifically described embodiments without departing from the scope of the present invention.
Number | Date | Country | Kind |
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0323906.8 | Oct 2003 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB04/04308 | 10/11/2004 | WO | 2/9/2007 |