WO2002088013A1

WO2002088013A1 - Thread tension measuring device based on a photoelastic measuring principle

Info

Publication number: WO2002088013A1
Application number: PCT/EP2002/003691
Authority: WO
Inventors: Stefano Dr.-Ing. Lamprillo
Original assignee: Iropa Ag
Priority date: 2001-04-10
Filing date: 2002-04-03
Publication date: 2002-11-07
Also published as: DE10117878A1

Abstract

The invention relates to a device (M) for measuring the tension of a thread deflected on a stationary deflector (D). An evaluating circuit (6) produces output signals (i) corresponding to the load (K) associated to the thread tension (t), and at least one transducer system (E) is placed in the load transmission path. According to the invention, said transducer system (E) comprises a transparent photoelastic element (S) which is housed in such a way as to be elastically deformable, as well as an optoelectronic scanning system (A) connected to the evaluating circuit (6) for evaluating at least one optical characteristic of the photoelastic element (S) which varies as a function of the load.

Description

THREAD TENSION GAUGE BASED ON A PHOTOLASTIC MEASUREMENT PRINCIPLE

The invention relates to a thread tension meter according to the preamble of claim 1.

Thread tension meters have to provide precise information on the thread tension when the thread is stationary or running from the very small load of the thread deflected at the deflector. From the deformation resulting from the small load of the thread, the thread tension in a thread processing system, e.g. a weaving machine with weft thread delivery devices, despite work-related vibrations and with the thread running, varying frictional influences can be determined accurately. In practice, thread tension meters have been developed for this purpose, which work with strain measuring elements or piezoelectric elements, which are analyzed by the evaluation circuit. In order to ensure a clear measurement of the thread tension within a large measuring range and in spite of the influences originating from the thread and the external influences, a high level of electronic and technical equipment is required. The known thread tension meters are expensive and sensitive devices, which are usually installed in large numbers in thread processing systems. In the case of strain gauges or piezoelectric elements, the phenomenon is used that their materials change their electrical properties under a mechanical load and, via the changes, allow conclusions to be drawn about the load causing the mechanical stress states. For example, the internal electrical resistance or a deformation-induced electrical voltage varies. To derive reasonably usable useful signals, however, high electronic expenditure is necessary.

The invention has for its object to provide a structurally simple and reliable, inexpensive to manufacture thread tension meter of another type, which has a large measuring range and can be derived with moderate electronic effort clear output signals.

The object is achieved with the features of claim 1. The phenomenon of birefringence of a photo-elastic, deformable element is used to measure the thread tension in a departure from the conventional principle of directly measuring and evaluating the elastic deformation in the transducer device electronically. From the deformation of the photoelastic element, scanning is carried out optoelectronically with polarized light via the change in the refractive behavior on the basis of the intensity of the emerging light, which changes with a change in the internal stress state in the photoelastic element. The photo-elastic element consists of a dielectric material that is transparent and usually at least largely isotropic. This material becomes anisotropic as soon as it is subjected to a mechanical stress condition. The anisotropy induced thereby results in different refractive indices for different lightwave polarization directions with reference to the main stresses inside the element. The voltage condition can be detected via the intensity of the emerging light. An approximately perpendicularly incident light wavefront of polarized light is divided into two waves which propagate at two different speeds in the interior of the element, since there are different refractive indices along the main axes in the element. This results in a phase difference in the emerging light; the element acts like a uniaxial crystal, in which the intensity of the emerging light follows an analytical equation that can be easily derived. Since in practice the main stresses in the element are hardly oriented parallel to the polarization axes, the stress conditions can easily be sensed by means of the emerging light by means of its isochromate. Practically over the entire extent of the element, the special arrangement leads to an almost constant phase offset or phase difference, as a result of which the two continuous light waves influence one another. By evaluating the light that is finally collected in relation to the light acting on it, the instantaneous tension conditions in the photo-elastic element can be assessed with little electronic effort within a wide measuring range. The thread tension can be derived precisely from this. The output characteristic is not linear, but square in the first approximation. However, the thread tension can be measured with high precision using suitable, known linearization methods. Since the thread tension indirectly via the internal tension condition of the elastic element determined by optoelectronic means determined, using the effect of photo elasticity or birefringence and supported by the geometric conditions, strong and meaningful useful signals for measuring the thread tension can be achieved. This is done with moderate electronic effort. The mechanical construction of the thread tension meter is simple. The thread tension meter can be produced inexpensively, is insensitive, compact and therefore optimal to install in thread processing systems. According to the invention, the phenomenon of photoelastic material is used, which changes its optical properties under internal tension states caused by the mechanical load of the thread tension and thus enables conclusions to be drawn about the extent of the load and the thread tension. This means that photoelastic material changes its refractive behavior or the ratio between applied and transmitted light with the load when illuminated or fluorinated, for example with polarized light, whereby the geometry of the photoelastic element is also used to derive strong and meaningful output signals within a wide measuring range , Since the output signal varies widely and meaningfully within a wide range and there is a clear relationship between the change in the mechanical stress state and the change in the optical properties, the electronic effort required to evaluate the output signal remains low. In addition, the thread tension meter can be designed inexpensively, simply and compactly, the use of the medium light being extremely advantageous for the precise measurement of forces as low as result from the thread tension of a thin thread. In addition, the photo-elastic element as the main active component of the thread tension meter is only scanned contact-free by the light, which simplifies the construction, since no direct electrical tap is required on the photo-elastic element.

The phenomenon of photoelasticity using measuring devices for large forces, pressures or other measured values are known from US 4466295 A and US 3 971 934 A.

The thread deflector is expediently supported in the bearing directly by means of the photoelastic element, so that any force resulting from the thread tension is introduced into the photo-elastic element and induces a mechanical stress state in it, which varies strictly depending on the load and can be scanned with light insensitive to interference.

A photo-elastic element in plate form is favorable, from which the thread deflector protrudes on one side and thereby has a thread contact area which is spaced from the nearest plate surface by a torsion lever arm. In this way, the photoelastic element is primarily subjected to torsion with shear loads, possibly in connection with a bend, since the photoelastic element is particularly sensitive to torsion. The optical axis of the optoelectronic scanning device passes through the plate transversely to the course of the plate, preferably approximately perpendicular to the plate surfaces, so that critical light collimation within an excessively long light path in the photo-elastic element is avoided. The stored mass of the photo-elastic element is only a minimum, so that, especially for use in thread tension measurement, a favorable natural resonance frequency, for example in the order of magnitude of about 1 kHz, with good direct response is achieved.

A strip-shaped plate is very practical, which is fixed in the storage at opposite longitudinal ends, the plate width being a multiple of the plate thickness. The optical axis of the optoelectronic scanning device passes through the strip-shaped plate perpendicular to the main plane of the plate and at a distance from the thread deflector and a clamped longitudinal end, i.e. in a range in which the mechanical torsional stress in the photo-elastic element is almost homogeneous, which simplifies the positioning of the optical axis of the optoelectronic scanning device.

Alternatively, the photo-elastic element could be a plate clamped on one side, which is contacted at a distance from that in the area of attachment directly from the thread and thus forms the thread deflector itself, or carries a thread guide for contact with the thread. In this embodiment, the plate is essentially subjected to bending under the load from the thread tension. Isotropic or quasi-isotropic plastic or isotropic optical glass is particularly suitable as the material for the photo-elastic element. Plastic or optical glass with a reasonably defined, preferably production-related, optical light transmission axis is also suitable for use in the thread tension meter. Such materials are usually isotropic and become anisotropic in a mechanical stress state. A photo-elastic plastic such as polycarbonate, which is commercially available with the desired material specifications, is particularly useful. The induced anisotropy results in different refractive indices for different light polarization directions with respect to the main stresses inside the photoelastic element. The voltage condition can be detected by measuring the intensity of the emerging light, the photoelastic element being placed between two linear polarization elements with crossing polarization axes.

Since it is important for the use of the photoelastic element in the thread tension meter to have as little manufacturing-related tension as possible in the largely stress-free state, at least the free-running cut edges should be ground on a plate cut out of a plastic film in order to eliminate "frozen" tension , The plate is cut to size and then, at least in the exposed cut edges, brought to the necessary plate width by grinding in order to minimize undesirable influences on the measurement accuracy.

In order to minimize the influence of extraneous light or the like, the photo-elastic element should have an opaque cover outside the passage area of the optical axis of the optoelectronic scanning device, e.g. a paint job.

In the optoelectronic scanning device, the light source should emit at least quasi-monochromatic light. A first polarizing element between the light source and the photo-elastic element only linearly polarizes the light that strikes the photo-elastic element. A second polarizing element is placed on the opposite side. is offset by a predetermined angle, for example 90%, above the polarization axis of the first polarizing element. The receiver is positioned behind it. By coordinating the rotational positions of the first and second polarizing elements relative to one another and relative to the optical light transmission axis of the photo-elastic element, a positioning is determined in which minimal light transmission occurs without loading the photo-elastic element or with a minimal loading, while the light transmission within a wide scanning range applied load and increasing thread tension increases legally up to a maximum. It can be expedient to arrange the polarizing elements so that they can be rotated in order to be able to set the best orientation for them in relation to the photo-elastic element. It is thus always possible to find two orthogonal positions in which the light extinction between the crossing polarizing elements is almost perfect, for example better than 0.1%. It is expedient to subject the photo-elastic element only to a largely pure torsion so that the thread tension meter shows a low sensitivity to vibration.

A red light LED is expedient as the light source, while the receiver can be a photo element such as a photo transistor. The output characteristic is not linear, but in a first approximation it is square, i.e. the intensity of the light emerging from the second polarizing element follows an analytical expression that can be easily derived. For example, the intensity of the emerging light is proportional to the squared value of the torque applied by the load, so that the thread tension is determined directly via the constant torsion arm of the load.

The thread deflector is expediently a ceramic rod or ceramic tube, preferably with a round cross section, which is fixed to the plate with a holder and extends approximately perpendicular to the plate surface. Ceramic material has good wear resistance against the abrasion of the thread, is light, insensitive to temperature and damping.

In a specific embodiment, the plate is fixed upright between two bearing blocks so that it covers the space between the bearing blocks. bridged. A bearing block contains the first and second plate-shaped polarizing elements facing the two plate surfaces, the rotational position of which is expediently adjustable. The light source and the receiver, which are also housed in this pedestal, define the optical axis of the optoelectronic scanning device, which penetrates the plate perpendicular to the plate surface, at a distance from the point at which the thread deflector out of the load a torsion in of the plate, and also at a distance from the area of definition of the plate. This construction concept leads to a low sensitivity to vibration of the thread tension meter and to a good and immediate response of the thread tension meter to changes in thread tension.

In a structurally simple manner, a bridge-shaped holder with thread guides is also arranged on the base body, which fixes the thread path through the thread tension meter with the thread deflector. If necessary, an additional thread guide is placed in the immediate vicinity of the thread deflector so that the thread acts on the thread deflector at a constant distance from the plate, even if the distances to the thread guides in the thread running direction are chosen to be relatively large. A small deflection angle of the thread is sufficient, which minimizes its mechanical load (friction, diffraction).

Embodiments of the subject matter of the invention are explained with reference to the drawings. Show it:

1 is a schematic perspective view of a thread tension meter,

Fig. 2 is a schematic representation of the thread tension meter of Fig. 1, and

Fig. 3 schematically shows a modified embodiment of a thread tension meter.

A thread tension meter M in FIG. 1 is provided for use on thread processing systems to measure the tension in a running or resting thread F. to measure or to provide an output signal representing the thread tension, possibly for controlling at least one assigned component such as a controllable thread brake (not shown). A typical place of use for the thread tension meter M is the thread path between a thread delivery device and a weaving machine.

The thread tension meter M has a base body 1 and a bridge-like holder 2 for two aligned thread guides 3, which define a predetermined, straight thread path through the thread tension meter M. The thread F is deflected at a deflector D, in the embodiment shown on a rod or tube 12, for example made of ceramic material and with a round cross section. The thread F rests on the thread deflector D in a predetermined contact area 14, in which the thread F can be held by a further thread guide, not shown.

On the base body 1 17 bearing blocks 4, 5 are arranged on both sides of an intermediate space. In the bearing block 5, an optoelectronic scanning device A is arranged, consisting of a light source LS, for example a red light LED 12, a first polarizing element Pj in the form of a square or round plate which can be rotated in the bearing block 5; a second polarizing element Po, located at a distance from one another, likewise in the form of a square or round plate, which can be arranged so as to be rotatably adjustable, and a receiver R, for example a photo element or phototransistor 13 , which provides a signal i.

A photoelastic element E, for example in the form of an elongated, thin plate S, is clamped with a longitudinal end in a fixing area 7 in the bearing block 7 and, for example, as here also with the other longitudinal end in a fixing area 8 in the bearing block 5. The plate S extends upright here between the fixing areas 7 and 8. The plate S has mutually parallel plate surfaces 9 and also mutually parallel plate edge surfaces 10. On the plate S, the thread deflector D is fixed with a holder 11 in a motion-transmitting manner. Since the thread F the thread deflector D in the contact area 14 with a Load K is applied, which is derived as a reaction force from the thread tension, the thread deflector projects freely on one side, and the contact region 14 has a torsion lever arm to the plate S, pure torsion is generated by the load K in the plate S, which inside the plate S causes a torsional stress state, which depends on the load K to the extent. For this purpose, it is expedient to fix the plate S at two opposite end regions and to twist between them.

The plate S consists for example of dielectric, transparent, normally at least quasi-isotropic material such as optical glass or plastic (amorphous organic glass, or plastic with a cubic crystal lattice) and becomes anisotropic in the case of an internal torsional stress condition. The anisotropy results in different refractive indices for different lightwave polarization directions with respect to the main stresses in the interior of the photoelastic element.

1, the photoelastic element E is arranged between two linear polarizing elements Pj, P _σ , such that the optical axis of the optoelectronic scanning device A is at a distance from the fixing area 8 and from the holder 11 and approximately is directed perpendicular to the plate surfaces 9 through the plate S, then the voltage condition can be detected by measuring the intensity of the light emerging from the illuminated plate. The polarizing elements are arranged so that their polarization axes cross each other. By coordinating the relative rotational positions of the polarizing elements with the light transmission axis of the photo-elastic element, extensive light extinction can be set with the condition of the photo-elastic element largely free of tension. With increasing torsional tension inside the photo-elastic element, the intensity of the emerging light increases. This intensity variation is registered by the receiver R and converted in the evaluation circuit 6 into an output signal i, from which the thread tension can be determined.

The light intensity is at least approximately proportional to the squared value of the applied torque, which causes the torsional stress, resulting from the load K and the torsion arm of the thread contact area 14 to the plate S. A polycarbonate, for example known with the trade name optical LE-XAN, can be used as the plate with a thickness of, for example, 1 to 2 mm. The polarizing elements Pi, Po are so-called polaroids. The light source LS generates, for example, a quasi-monochromatic light. Sheet polycarbonate is usually not isotropic. Due to the manufacturing process, however, it can have an optical light transmission factor, even if it is not under mechanical tension. The two polarizing elements can be used to find two orthogonal positions with which the light extinction between the polarizing elements is almost perfect, for example better than 0.1%. Since the photoelastic element can only be twisted, it works with low sensitivity to vibrations.

The behavior under pure torsional stress is explained with reference to FIG. 2. The plate S, for example made of polycarbonate, has a width h, a length I and a thickness b. It is defined at the left end in definition area 7. A torque T is applied to the other end. As a result, a uniform stress status appears in the plate S as a stress tensor. This modifies the dielectric tensor so that the plate S has an anisotropic behavior. The main voltages lie on the XY plane, so that their direction is never parallel to the axes of the two polarizing elements Pj, P ₀ , the axes of which are mainly oriented in the X direction. Each light wave front perpendicular to the plate S is divided into two waves which propagate in the plate S at two different speeds, as a result of the different refractive indices along two main axes. This results in a phase difference, with the chosen arrangement resulting in a constant phase delay over almost the entire plate extension, at least as far as the isochromates of light are concerned. The light emerging from the second polarizing element P ₀ follows an analytical equation that can be easily derived.

Since the two wave or vibration components propagate through the plate with a total phase difference, and the amplitudes of the light waves or light vibrations emerging from the second polarizing element can be found out easily, an interference occurs between the two waves, which ultimately is responsible for the variation of the emerging and registered light by the receiver.

The polycarbonate plate has, for example, a length of 50 mm, a width of 20 mm and a thickness of 2 mm and can be reliably scanned with the optoelectronic scanning device A to derive a meaningful output signal, in particular by applying a torsional tension to measure the thread tension to be able to.

Another arrangement of the photo-elastic element E is shown schematically in FIG. The photoelastic element E is again a plate S with mutually parallel plate surfaces 9 and plate edge surfaces 10. As in the embodiment of the plate S in FIGS. 1 and 2, the plate is cut out of a polycarbonate film parallel to its main direction, this main direction should coincide with de main dimension of the film. The plate is cut out with a larger width than required. The correct dimension is produced by grinding at least the longitudinal cut edges in order to eliminate frozen or internal stress states from the production and / or the cutting of the plate. The surfaces of the plate that are not used are expediently provided with opaque shielding, e.g. with a paint application C in Fig. 1st

In the variant in FIG. 3, the plate S is clamped on one side only in the bearing block 4 in the fixing area 7. It cantilevers with the other end. The sheet S can serve directly as the thread-deflector D by the thread F is diverted directly from the _^ plate and in the contact region 14 to the load K dispense. The optoelectronic scanning device A is arranged between the fixing region 7 and the contact region 14 in such a way that its optical axis passes through the plate approximately perpendicular to the plate surfaces 9 and at a distance from the longitudinal edges.

In the dashed alternative in FIG. 3, a thread guide 15 is fixed at the free end of the plate S directly or with a holder 16, which forms the contact area 14 for the thread F. In this design, the plate is mainly subjected to bending, so that with the optoelectronic scanning device A, the inner bending Tensions are detected and the thread tension is deduced from the bending tension conditions by varying the intensity of the emerging light. The scanning principle can also be described with the birefringence phenomenon. The evaluation circuit is relatively simple. It also serves to activate the light source LS and to polarize the photo element, for example a phototransistor 13. The output signal is read off, for example, via an approximately 10 K-ohm load resistor, in connection with a 10 nF parallel filter capacitor, and can be connected directly to an oscilloscope managed, without further reinforcement or conditioning. The visual representation is carried out using 16 average patterns, for example. The connecting lines to the evaluation circuit, and then further, should expediently be well shielded in order to superimpose as little noise as possible on the thread tension signal.

An arrangement similar to that shown in Fig. 1, i.e. a plate-shaped photo-elastic element which is loaded with the load K from the thread tension essentially purely on torsion and is clamped at both ends. By scanning only the inner torsional tensions, with the short passage of light through the small thickness of the plate and perpendicular to the plate surfaces, with the coordinated alignment of the crossing polarization axes of the linear polarizing elements, and with, for example, red light, the thread tension can be expanded within a wide range , d. H. Measure the spread area precisely, with a slight change in the thread tension already leading to a significant change in the intensity of the emerging light. Due to the pure torsional tension in the photo-elastic element, the thread tension meter is relatively insensitive to vibrations which are oriented differently, as is usually the case in thread processing systems, e.g. on a weaving machine are inevitable. The thread tension meter M is compact, structurally simple and consists of a few and inexpensive parts. It is reliable and can be used universally.

Claims

claims

1.Thread tension meter (M) for a thread (F) deflected within a defined thread path on a stationary deflector (D), with an electronic evaluation circuit (6) which consists of the mechanical and with the thread tension (D) taken up by the deflector (D) t) Corresponding load (K) derives output signals (i), at least one transducer device (E) deformable in dependence on the load and monitored by the evaluation circuit being provided in the load transmission path from the deflector (D) to the bearing (4, 5), characterized in that that the transducer device (E) has a translucent photoelastic element (S) arranged elastically deformable under the thread tension (t) in the thread tension meter (M) and an optoelectronic scanning device (A) connected to the evaluation circuit (6) for at least one load-dependent Varying optical property of the photo-elastic element (S) exposed to light (L).

2. Thread tension meter according to claim 1, characterized in that the thread deflector (D) is supported by means of the photo-elastic element (S) in the bearing (4, 5).

3. Thread tension meter according to claim 1, characterized in that the photo-elastic element (S) is plate-shaped and is fixed to at least one plate edge area in the bearing (4, 5) that the thread deflector (D) at a distance from the fixing area ( 7, 8) is cantilevered on one side with a direction transverse to the plate course starting from the fixing area (7, 8) on the photoelastic element (S) and has a predetermined thread contact area (14) which is from the plate surface (9) or plate closest to the thread contact area. tenrandfläche (10) is spaced with a torsion arm, and that for measuring the thread tension (t) based on a torsion (T) generated by the load (K) with shear loads in the photo-elastic element (S), the optoelectronic scanning device (A) has an optical axis ( X), which the photo-elastic element (S) transversely to the plate course and in the direction of the plate course at a distance from the face passes through the deflector (D) and the fixing area (7, 8), preferably approximately perpendicular to a plate surface (9)

4. Thread tension meter according to claim 3, characterized in that the photo-elastic element (S) is a strip-shaped plate and fixed at opposite longitudinal ends in the storage (4, 5) that the thread deflector (D) approximately transverse to the longitudinal direction of the Plate is oriented and held on the plate between the longitudinal ends, and that the optoelectronic scanning device (A) between the holder (11) of the thread deflector (D) and a longitudinal end of the plate with its optical axis (X) substantially perpendicular to the plate surface (10) is arranged.

5. Thread tension meter according to claim 1, characterized in that the photo-elastic element (S) as the thread deflector (D) comprises a in the storage (4) at least on one side fixed plate, either directly through the contacting thread (F) or indirectly Via a thread guide (15) arranged on the plate, the load (K) is applied in a bending sense approximately perpendicular to the plate surface (10).

6. Thread tension meter according to claim 1, characterized in that the photo-elastic element (S) consists of isotropic plastic or optical glass.

7. Thread tension meter according to claim 1, characterized in that the photo-elastic element (S) consists of a plastic or optical glass with a defined, preferably production-related, optical light transmission axis.

8. Thread tension meter according to claim 1, characterized in that the photo-elastic element (S) made of polycarbonate, for example trade name optical LEXAN.

9. Thread tension meter according to claim 1, characterized in that the photo-elastic element (S) is a cut out of a film, flat and translucent plate, the cutting area surfaces (10) extending from the fixing area (7, 8) or extending between the end fixing areas are ground ,

10. Thread tension meter according to claim 1, characterized in that the photo-elastic element (S) outside the passage area of the optical axis (X) of the optoelectronic scanning device (A) has an opaque cover (C), preferably an application of paint.

11. Thread tension meter according to claim 1, characterized in that the optoelectronic scanning device (A) is a light source (LS) for at least quasi-monochromatic and / or polarized light (L), a first between the light source (LS) and the photo-elastic element (S) Polarizing element (Pj), in the direction of the optical axis (X) and on the opposite surface of the photo-elastic element (S) a second polarizing element (P ₀ ) with a polarization axis rotated about the optical axis (X) at a predetermined angle with respect to the polarization axis of the first polarizing element , which preferably cause birefringence together and have a receiver (R) behind them.

12. Thread tension meter according to claim 11, characterized in that the light source (LS) a red light LED (12) and the receiver (R) are a photo element (13).

13. Thread tension meter according to claim 3, characterized in that the thread deflector (D) is a ceramic rod or tube (12), preferably with a round cross section, which is fixed with its holder (11) on the plate and approximately extends perpendicular to the plate surface (10).

14. Thread tension meter according to at least one of the preceding claims, characterized in that on a base body (1) of the thread tension knife (M) two bearing blocks (4, 5) spaced apart with an intermediate space (17) are provided so that the plate is fixed at both ends in the bearing blocks (4, 5), preferably lying on edge, and extending through the intermediate space (17) extends that the deflector (D) is attached to the plate in the intermediate space and protrudes with an end having the thread contact area (14) opposite the bearing blocks (4, 5), the first facing the two plate surfaces (10) in a bearing block (5) and second plate-shaped polarizing elements (Pj, B) are contained, and that the light source (LS) and the receiver (R) in the bearing block (5) are placed on the first and second polarizing elements (Pj, P _σ ).

15. Thread tension meter according to claim 14, characterized in that a bridge-shaped holder (2) with two aligned thread guides (3) is arranged on the base body (1) below the intermediate space (17) and the end of the thread deflector (D) use the thread deflector (D) to define the thread path through the thread tension meter (M).