GB1587651A - Method and apparatus for measuring web tension - Google Patents

Description

(54) METHOD AND APPARATUS FOR MEASURING WEB TENSION (71) We, SVENSKA TRAFORSKNINGSTITUTET, of Box 5604, S-11486 Stockholm, Sweden, a Swedish State-owned Research Institute, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to methods of measuring tension in a web of, for example paper or foil and to apparatus for carrying out such methods. The invention relates especially to a method of measuring web tension in paper webs or thin foils by means of induced oscillations substantially perpendicular to the web surface.

In the production or treatment of different kinds of web, e.g., the printing of paper, plastics of other types of foils, long webs of these materials are usually fed between guide rollers, which on the one hand guide the web and on the other hand support it. In this connection it is of the greatest importance that the web tension of the material can be kept under control, and preferably as constant as possible, as an alteration in web tension in one part of the web will give rise to reactions along the whole web. It is also of the utmost importance that the web tension is maintain at the same level across the whole web, as otherwise the web can be warped with the risk of quilling and other unfavourable effects.

Therefore, it is desirable to be able to monitor the web tension continuously at defined locations along a running paper we or the like and also preferably at several different locations across the web at the same time. When continuously monitoring the web tension it is possible to directly apply control measures when a change in the web tension has occurred.

The most commonly used type of web tension meter in modern machines, especially printing machines, where webs of paper or plastics run over guide rollers, is based on some kind of load cell arrangement, on which both ends of the rollers are mounted. When deflection occurs in the web, forces arise at the guide rollers, which are indicated by the load cells and which can be utilized for determination of the web tension. In order that the cells can measure rapid variations of the web tension, the weight of the rollers must be as low as possible. This is contrary to the fact that mechanical vibrations will always arise, when the rollers rotate, which causes disturbances in the signal measured, and that, therefore, the rollers must be made rigid to reduce these vibrations, which in turn results in a high weight of the rollers, especially for machines handling wide materials. Consequently the part of the force on the load cells deriving from the web tension will be small relative to the weight of the rollers. Therefore, the demands on the sensitivity of the load cell will be very great, which causes great problems.

Great efforts have been made to find a solution of the above-mentioned problems.

Especially there are already several devices known, in which the whole paper web between two rollers is caused to vibrate up and down. In such cases the fact is utilized that the resonant frequency of a material has a very special and definite relationship to the tension of the material. This relationship can be expressed as:

where f = the resonant frequency T = the tension of the material n = the overtone for which the web is in resonance = = the length between the points of support of the web and m = mass per surface unit of the web material.

If n, 1 and m are constants the relationship if simplified to f = k In this connection the web can primarily be compared to a string on a musical instrument, for which the resonant frequency of the string, as is well-known, is increased the more the string is stretched. The resonant frequency of the paper web is consequently indicated in accordance with these known devices and the resonant frequency is a measure of the web tension.

There are also prior art devices, where an intermittent oscillation is applied to a part of the web and the travel time required for the oscillation to reach another point on the web is indicated. This travel time has a definite relationship to the web tension.

Both the methods mentioned above suffer from the draw-back that they are sensitive to external disturbances, such as noise. As they are generally to be used in a very noisy environment, the noise must be shielded at the measuring place, which is both troublesome and expensive. Moreover, various overtones may occur, which during the measuring are often indicated instead of the fundamental tone indicating the web tension. This often leads to the fact that the result on a measuring device operating according to the abovementioned principle will be quite misleading. Therefore, these methods have not been used to a large extent in practice.

It is the object of the invention to offer a method of measuring the web tension, especially in paper webs, which method is to operate satisfactorily independently of the environment.

According to the invention there is provided a method for measuring the tension in a running web wherein a defined surface region of the web is subjected to oscillations directed substantially perpendicularly to the surface of the web. said oscillations being produced by an electromagnetic oscillation generating means and having a fixed maximum amplitude and a frequency which is adjustable but is constant during measurement, the resultant oscillations indiced in the web influencing the oscillations generated by the oscillation generating means whereby the impedance of the oscillation generating means is altered and the change in impedance is measured. the measured value of the impedance representing a function of the web tension and the oscillation frequency. the frequency of the generated oscillation being selected to correspond to, or by repeated measurement being adjusted to lie closely adjacent to. the anticipated resonant frequency of the web for a given web tension.

The invention also provides an apparatus for measuring the tension in a running web by the method defined in the preceding paragraph. wherein the oscillation generating means comprises an electrodynamic generator including an electro-magnet and means to energise the electromagnet bv an alternating current of predetermined maximum amplitude and frequency, the electromagnet being mounted in the apparatus in close proximity to the surface of the web in the defined surface region which is delimited by point or linear support means in sliding contact with the web such that the surface region of the web can be subjected to said substantially perpendicular oscillations. the tension of the web influencing the amplitude of the resonant oscillation to modifv the impedance of the electro-magnet the oscillation generating means being connected to an impedance measuring device which is adapted to indicate changes in the impedance caused by changes in the tension of the web.

The measuring system can be adapted according to the existing need. A simple instrument with a measuring head can be made for manual handling and be used for control and regulation of a small machine or for surveying the tensions in a large machine. The measuring system is also verv suitable for the supervision of many detectors of the web tension at the same time. which will enable an optimal control of the web tension in large machines. Especiallv in a paper making machine or a printing machine for newspapers having long paper webs. it is possible to arrange several measuring heads in a line or across the paper web. the output signals from said measuring heads being indicated on a common instrument.

The carrying of the invention into effect will now be described by way of example and with reference to the accompanying drawings. in which: Figure 1 shows curves of the amplitude as a function of the frequency of a paper web with two different web tensions.

Figure 2 shows a curve of the amplitude as a function of the web tension of a paper web with applied oscillation of a given frequency.

Figure 3 is a diagram containing a block circuit diagram of an arrangement according to the invention, Figure 4 shows a section of an embodiment of a measuring head according to the invention and Figure 5 is a circuit diagram for a web tension meter according to the invention.

Figure 1 shows two curves of the amplitude of oscillation as a function of the frequency, where a web, e.g. of paper, has been subjected to oscillations acting perpendicularly to the web surface between two well-defined points of support along the web by means of a sinusoidally varying force of a predetermined amplitude. It is apparent from this figure that the frequency, f at which the region of the paper web to be measured will get into resonance, is different for different tensions T of the web. At the web tension T1 the resonant frequency isf1 and at the tension T2 the resonant frequency iso2. It is also evident from Figure 1 that there is a range around the resonant frequency for the same tension in the material, within which the amplitude is variable with increasing value towards the resonant frequency.

As indicated above the relationship between the frequency and the web tension can be written f = ko when the mass per surface unit of the web and the length between the points of support is constant. This shows that the configuration of each of the curves shown in Figure 1 would be of the same type if instead the frequency f was maintained constant and the web tension T was varied. Such a curve has been drawn in Figure 2. This Figure shows that a resonance of the paper web will occur for the web tension To, as the amplitude increases asymptotically at this web tension. It is also evident from Figure 2 that there is one range (T1-T2 and T3-T4) on each side of To with increasing amplitude towards the resonant web tension lying in the range T2-T3. According to a feature of the invention either of these ranges is utilized to determine the web tension of the material. The web is subjected to substantially perpendicular oscillations between two support points along the web by means of a sinusoidal force with predetermined amplitude and frequency, and the amplitude of oscillation of the web material is determined, and is a measure of the web tension. The frequency of the oscillation should then be either somewhat above or somewhat below each expected fundamental resonant frequency of the material for the web tensions in question.

A calibration may be achieved by drawing the part of the curve lying to the left or to the right of To for known web tensions, and when utilizing this measuring method for a paper web having otherwise similar properties only the amplitude of the oscillation in the material need to be read and the web tension will be obtained by means of the calibration curve. If the web tension measured should be far from the highest or the lowest web tension expected, the amplitude of the oscillation of the web then being low, the measuring can be repeated. At this measuring a lower or a high frequency then the previously selected frequency has to be selected. However, this frequency is selected so that it lies closer to the frequency that according to the previous measurement should correspond to the fundamental resonant frequency of the web for the web tension value being indicated. This case will give a curve by which small variationsin web tension will result in large changes of amplitude. By using this possibility of changing the frequency of the applied force, a measuring range can always be selected on each occasion that gives a high resolution of the signal. Switching between different measuring ranges can be performed automatically.

As shown in Figure 3, a measuring head consists of an electrodynamic loudspeaker 1 with a circular supporting ring 2 disposed around the front or outer part of the loudspeaker cone.

The supporting ring of the loudspeaker is placed against the web, the tension of which is to be determined, with the ring in sliding contact with the material. This is preferably achieved by providing the ring 2 with an air suction channel having apertures open towards the web for enabling the ring to be sucked against the web. Other types of adhesion between the ring and the web are of course possible, such as a counter-pressure ring 3 arranged on the opposite side of the web tension meter, by means of which the web is guided between the supporting ring 2 and the counterpressure ring 3.

The loudspeaker coil is fed with an alternating current having a fixed maximum amplitude and frequency that is held constant during a measurement. This current is generated by means of an oscillator 4, feeding one side of a bridge resistance circuit 5, in which side the loudspeaker coil in included. Another branch of the bridge contains a separate loudspeaker coil Rcon,p with the same characteristics as the coil for the loudspeaker 1, the coil R comp being located closely adjacent to the coil for the loudspeaker 1.

The coil R is so arranged that it will reach the same temperature as the coil for loudspeaker 1. The two other branches of the bridge comprises impedance elements having a constant impedance, and which are preferably fixed resistors.

The web of material acts as an external membrane attached to the loudspeaker cone, which membrane tends to damp the oscillation of the cone of loudspeaker 1 and will give rise to a change in the impedance of the loudspeaker coil in dependence on the damping which is itself affected by web tension. This impedance variation is determined by feeding the resultant signal across the other diagonal of the bridge circuit to an amplifier 6 and then rectifying it in a rectifier 7. the output signal from the rectifier 7 is fed to an averaging device 8, the output of which represents the web tension. The relationship between the web tension and the signal level is determined by calibration against known web tensions.

Figure 4 shows a cross section of a preferred embodiment of the measuring head in a web tension meter according to the invention. The frame of the measuring head is shown in a diagonal cross section, whereas for illustrative purposes the loudspeaker in the centre part is shown in side view. As is apparent from the Figure the frame is divided into two portions, that at the front of the loudspeaker cone, i.e. the support ring 2, being provided with an annular air channel 9, from which radial channels 10 extend. Perpendicularly to these radial channels 10 suction channels 11 lead to the surface facing the web. These can either be made as separate channels or as a partly annular groove. Air is blown into the air channels 9 and flows out through the channels 10. By means of the consequent ejector effect a suction will result in the channels 11 drawing the web against the face of the ring 2.

The loudspeaker 1 is mounted to the centre part of the frame and consists of an electrodynamic loudspeaker of known design, but because the measuring head in this embodiment is to be used for measuring the web tension of a paper web in a paper making machine, said web having a high humidity content and a temperature that may amount to about 100"C, the measuring transducer is heated for compensating for the influence of condensation and temperature variations and is provided with a control system (not shown), keeping its temperature constant. Heating is performed electrically by means of a heating coil 12, would around the loudspeaker magnet 13. Around the magnet there is arranged a cap 14 for heat insulation. A thermostat (not shown) is incorporated in the cap and is connected in the circuit of the heating coil 12. Additionally, a second loudspeaker coil 15 is arranged on a bobbin.

In Figure 5 there is shown a wiring diagram of an embodiment of a web tension meter according to the invention. In this Figure the circuit blocks in Figure 3 are indicated in such a way that the circuit in Figure 5 has been divided into sections corresponding to these blocks with a corresponding reference numeral under each section. The oscillator 4 is based on an Integrated Circuit (IC-circuit) of the type LM555, which is a so-called timer and for which the frequency and the pulse duration are determined by the resistors R4 and R5 and the capacitor C1. A triangular output signal is derived from the capacitor C1 and is fed via a resistor R6 and a capacitor C4 connected in series with this resistor to an amplifier F1. At the junction between theresistor R6 and the capacitor C4 a capacitor C2 is connected to earth. A substantially sinusoidal output signal from the amplifier F1 is applied across the one diagonal of two bridge circuits connected in parallel and having two resistors R12 and R13 as common branches and having a loudspeaker in a further resistance branch in each one of said circuits. In this embodiment one loudspeaker H1 is used for the web tension measuring and the other loudspeaker H2 is a loudspeaker corresponding to the first loudspeaker, i.e. matched thereto. According to this embodiment this other loudspeaker is allowed to perform free oscillations and is placed close to the first loudspeaker H1, thus operating at the same temperature as the first loudspeaker. The loudspeaker H2 is thus used for temperature compensation. The common resistors R12 and R13 of the bridge circuits are connected across one bridge diagonal. The signal in each one of the other bridge diagonals of the two bridges is further supplied to one operational amplifier each. Thus, the signal through the resistor R12 and the loudspeaker H1 is supplied to the operational amplifier F2, and the signal through the resistor R12 and the loudspeaker H2 to the operational amplifier F3. The output signals from said two amplifiers F2 and F3 are supplied to a respective rectifier by means of capacitors Cg and C10 respectively. In these a diode D2 and D4 respectively with an earthed anode is included between the input capacitors Cg and C10 respectively and earth. Moreover, diodes Dl and D3 respectively, each with its anode connected to the connection point between the input capacitor and the first diode, are included. In addition there is a parallel connection of a resistor R22 and R23 respectively and a capacitor C11 and C12 connected between the cathode of the second diodes D1, D3 and earth. The output signals from the two rectifiers are supplied to another operational amplifier F4, the output signal from which is detected either with an ammeter or a recording instrument or in another suitable way and is an unambiguous function of the impedance change in the coil of the loudspeaker H1 due to the influence of the web, because the basic impedance of the coil of the loudspeaker, i.e. the impedance, that should appear without the influence of the web, is completely compensated by the coupling to the additional matched loudspeaker H2.

When the web tension is to be meausred at several places across a web, several web tension meters of the above-mentioned type can preferably be located besides each other and be fed by an oscillator common to all the meters. The output signals from the different meters can preferably be indicated on instruments arranged in line beside each other and having a distinct vertical level indication. In this way a difference between the output signals will be easily detectable also by a glance at the instruments.

The output signal obtained by means of the instrument is not directly proportional to the web tension, and therefore a linearisation network may be connected before the output.

Such a network can e.g. consist of series connections of a resistor and a Zener diode connected in parallel and mutually matched so that a curve representing the output signal as a function of the web tension will be linear and equal for all the meters when several web tension meters are operated in parallel.

An example of component values for Figures 5 is listed below.

R1 = 6, 8kohm R11 = 4, 7kohm R2 = 3, 3kohm R12 = 25 ohm R3 = 1, 2kohm R13 = 100 ohm R4 = 1, 2kohm R14 = R15= 25 ohm R5 = 1, Skohm R16 = R17 = 5, 1kohm R6 = 100 ohm R19 = R20 = 4, 7kohm R7 = 180 kohm R18 = R21 = 100 kohm Rg = 33 kohm R22 = R23 = 15 kohm R10 = 68 ohm R24 = R25 = 1 kohm R26 = 82 kohm C8 = 1000uF R28 = 1, 2kohm C11 = C12 = 15 uF R29 = 1, 2kohm C13 = 0, 1uF R30 = 10 kohm C14 = 100 nF R31 = 50 kohm C15 = 47 uF C1 = C2 = C3 = C9 = C10 = 1 uF F1 = TBA 915 C4 = C5 = 10 uF F2 = F3 = F4 = LM 308 = = C7 = 100 uF D1 = D4 = AA 119 Tests have shown that a web tension meter in accordance with the invention has a series of advantages over the tension meters previously known. It has great accuracy, no restrictions of the measuring range, the transmitter cannot be overloaded, it has a high stability and an insignificant zero drift and no hysteresis. The calibration constants are determined only by the material on which measuring is to be effected. It is easy to apply the transmitter to the material. It can be used at an arbitrary location in a machine without influence on its calibration. It is insensitive to mechanical vibrations and electrical disturbances, nor has it any inertia and is able to respond to sudden variationsin web tension. It has small dimensions and low weight and is suitable to be moved along the web and thereby to measure profiles of web tension. The measuring system is also well adapted for the supervision of several transmitters at the same time.

WHAT WE CLAIM IS: 1. A method of measuring the tension in a running web wherein a defined surface region of the web is subjected to oscillations directed substantially perpendicularly to the surface of the web, said oscillations being produced by an electromagnetic oscillation generating means and having a fixed maximum amplitude and a frequency which is adjustable but is constant during measurement, the resultant oscillations induced in the web influencing the oscillations generated by the oscillation generating means whereby the impedance of the oscillation generating means is altered and the change in impedance is measured, the measured value of the impedance representing a function of the web tension and the oscillation frequency, the frequency of the generated oscillation being selected to correspond to, or by repeated measurement being adjusted to lie closely adjacent to, the anticipated resonant frequency of the web for a given web tension.

2. An apparatus for measuring the tension of a running web by the method according to Claim 1, wherein the oscillation generating means comprises an electrodynamic generator including electro-magnet and means to energise the electromagnet by an alternating current of predetermined maximum amplitude and frequency, the electro-magnet being mounted on the apparatus in close proximity to the surface of the web in the defined surface region which is delimited by point or linear support means in sliding contact with the web such that the surface region of the web can be subjected to said substantially perpendicular oscillations, the tension of the web influencing the amplitude of the resultant oscillation to modify the impedance of the electromagnet, the oscillation generating means being connected to an impedance measuring device which is adapted to indicate changes in the impedancecaused by changes in the tension of the web.

3. Apparatus according to Claim 2, wherein the electrodynamic oscillation generator is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. meters can preferably be indicated on instruments arranged in line beside each other and having a distinct vertical level indication. In this way a difference between the output signals will be easily detectable also by a glance at the instruments. The output signal obtained by means of the instrument is not directly proportional to the web tension, and therefore a linearisation network may be connected before the output. Such a network can e.g. consist of series connections of a resistor and a Zener diode connected in parallel and mutually matched so that a curve representing the output signal as a function of the web tension will be linear and equal for all the meters when several web tension meters are operated in parallel. An example of component values for Figures 5 is listed below. R1 = 6, 8kohm R11 = 4, 7kohm R2 = 3, 3kohm R12 = 25 ohm R3 = 1, 2kohm R13 = 100 ohm R4 = 1, 2kohm R14 = R15= 25 ohm R5 = 1, Skohm R16 = R17 = 5, 1kohm R6 = 100 ohm R19 = R20 = 4, 7kohm R7 = 180 kohm R18 = R21 = 100 kohm Rg = 33 kohm R22 = R23 = 15 kohm R10 = 68 ohm R24 = R25 = 1 kohm R26 = 82 kohm C8 = 1000uF R28 = 1, 2kohm C11 = C12 = 15 uF R29 = 1, 2kohm C13 = 0, 1uF R30 = 10 kohm C14 = 100 nF R31 = 50 kohm C15 = 47 uF C1 = C2 = C3 = C9 = C10 = 1 uF F1 = TBA 915 C4 = C5 = 10 uF F2 = F3 = F4 = LM 308 = = C7 = 100 uF D1 = D4 = AA 119 Tests have shown that a web tension meter in accordance with the invention has a series of advantages over the tension meters previously known. It has great accuracy, no restrictions of the measuring range, the transmitter cannot be overloaded, it has a high stability and an insignificant zero drift and no hysteresis. The calibration constants are determined only by the material on which measuring is to be effected. It is easy to apply the transmitter to the material. It can be used at an arbitrary location in a machine without influence on its calibration. It is insensitive to mechanical vibrations and electrical disturbances, nor has it any inertia and is able to respond to sudden variationsin web tension. It has small dimensions and low weight and is suitable to be moved along the web and thereby to measure profiles of web tension. The measuring system is also well adapted for the supervision of several transmitters at the same time. WHAT WE CLAIM IS:

1. A method of measuring the tension in a running web wherein a defined surface region of the web is subjected to oscillations directed substantially perpendicularly to the surface of the web, said oscillations being produced by an electromagnetic oscillation generating means and having a fixed maximum amplitude and a frequency which is adjustable but is constant during measurement, the resultant oscillations induced in the web influencing the oscillations generated by the oscillation generating means whereby the impedance of the oscillation generating means is altered and the change in impedance is measured, the measured value of the impedance representing a function of the web tension and the oscillation frequency, the frequency of the generated oscillation being selected to correspond to, or by repeated measurement being adjusted to lie closely adjacent to, the anticipated resonant frequency of the web for a given web tension.

2. An apparatus for measuring the tension of a running web by the method according to Claim 1, wherein the oscillation generating means comprises an electrodynamic generator including electro-magnet and means to energise the electromagnet by an alternating current of predetermined maximum amplitude and frequency, the electro-magnet being mounted on the apparatus in close proximity to the surface of the web in the defined surface region which is delimited by point or linear support means in sliding contact with the web such that the surface region of the web can be subjected to said substantially perpendicular oscillations, the tension of the web influencing the amplitude of the resultant oscillation to modify the impedance of the electromagnet, the oscillation generating means being connected to an impedance measuring device which is adapted to indicate changes in the impedancecaused by changes in the tension of the web.

3. Apparatus according to Claim 2, wherein the electrodynamic oscillation generator is

an electrodynamic loudspeaker placed in proximity to the web.

4. Apparatus according to claim 3, wherein the outer edge of the loudspeaker cone is provided with a support ring adapted to be placed in close contact with the web to form said points of support, and provided with means for maintaining it in contact with the web.

5. Apparatus according to claim 4, wherein said means for maintaning the support ring in contact with the web consists of air channels arranged in the support ring and having suction apertures directed towards the web.

6. Apparatus according to claim 3, wherein an additional coil is arranged around the magnet of the electrodynamic loudspeaker for providing a means for compensating temperature variations of said loudspeaker.

7. Apparatus according to claim 6, wherein the additional coil has substantially the same characteristics as the electrodynamic coil of the loudspeaker and is arranged to attain substantially the same temperature as said coil of the loudspeaker, said additional coil each being connected in a respective branch of an impedance bridge.

8. A method for measuring the tension in a web substantially as hereinbefore described and with reference to the accompanying drawings.

9. Apparatus for measuring the tension in a web substantially as hereinbefore described and with reference to the accompanying drawings.