EP1932791B1 - Method for measuring the tensile strength of a moving web - Google Patents

Abstract

The method involves using at least one sensor with at least one Wheatstone bridge with at least one force transducer (7) influenced by the tension in the web. It involves amplifying the bridge diagonal voltage in an amplifier that produces a tension signal and is periodically loaded with at least one resistance via an intermittently operated switch while the moving web is loaded by the tension. The operability of the sensor(s) is determined from the controlled influence of the tension signal by the load and output as an error signal.

Description

The invention relates to a method for measuring the tension of a moving web according to the preamble of patent claim 1.

From the DE 101 18 887 C1 a device for detecting the tension of a moving web is known, which detects the bearing force of a web deflecting the web. For this purpose, this device has two double bending beams, which are equipped with force transducers in the form of strain gauges. These strain gauges are connected in the form of a Wheatstone bridge in order to achieve the lowest possible temperature dependence and drift of the sensor. This sensor has been well proven in practice and forms the starting point of the present invention. A disadvantage of this known sensor has been found that in case of failure of the strain gauges, for example by breakage or short circuit, the entire sensor provides nonsensical values, which are then interpreted accordingly by subsequent units. If the sensor is included in the control loop of a web tension control, for example, it may happen, depending on the type of failure, that the control completely eliminates the web tension or greatly overstretches the running web. In the simplest case, this can lead to a web break if the web can no longer withstand the introduced tension or gets caught on machine parts due to the lack of tension. In particular, in the regulation of endless belts in paper machines, this can even lead to tearing out of rolls from their warehouses and thus a considerable risk to man and machine.

Another device which is considered to be the closest prior art is disclosed in U.S. Pat EP 0 582 947 A1 discloses the same applies mutatis mutandis to the corresponding method.

The invention has for its object to provide a method for measuring the tension of a moving web of the type mentioned, which also detect the failure of electronic components and can respond accordingly.

This object is achieved with the features of claim 1.

The method according to claim 1 is used for measuring the tension of a moving web with a sensor. It does not matter whether the train is self-contained or continuous. Also, the material of the running track plays no role in the application of this method. The sensor has a Wheatstone bridge containing at least one force transducer. As load cell different sensor principles come in Question, which are able to convert a force or a mechanical deformation into an electrical signal. Strain gauges are preferably used as force sensors, which are mounted on a mechanical component - for example, a double bending beam - which is deformed by the action of the force to be measured. Basically, it is sufficient to form only a resistance of the Wheatstone bridge as a force transducer. To achieve the lowest possible temperature dependence and drift of the sensor, however, all resistors of the Wheatstone bridge are preferably realized as force transducers. The diagonal voltage of the Wheatstone bridge is a measure of the acting force. This diagonal voltage is amplified by an amplifier, which has the main task of keeping resistive loads from the Wheatstone bridge, which could distort the measurement result. In addition, the amplifier can also realize a voltage gain to bring the measurement signal to an easy-to-process voltage range. However, this is not absolutely necessary and depends in particular on the specific choice of load cell. This amplifier outputs at its output from a signal which is proportional to a possibly to be taken into account offset the measured tensile stress and hereinafter referred to as Zugspannungssignal. If one of the load cells fails, this leads to a short circuit or interruption within the Wheatstone bridge, depending on the cause of the defect. In any case, the tensile signal is thereby extremely distorted, so that it can no longer be used for display or control purposes. To one to detect such a defect within the sensor and to be able to react appropriately, an error signal is output in addition to the tension signal. This error signal is inactive during normal operation and is brought to an active state upon the occurrence of a detectable fault within the sensor. In order to be able to detect an error within the sensor, the Wheatstone bridge is periodically loaded by at least one resistor during the stress by the tension of the moving web by means of at least one intermittently controlled switch. This load resistance detunes the Wheatstone bridge in a defined manner, wherein the effect of this load can be determined directly on the basis of a comparison of the tension signal with and without ohmic load. This test is carried out in the operation of the burdened by the web sensor, so that its functioning is checked in a timely manner. In the event that one of the force transducer of the loaded voltage divider should have an internal short circuit, it will be noted that the tensile signal does not change due to the stress of this voltage divider. The same applies to the case where the force transducer connected in series with the load resistor should be interrupted. If the force transducer, which is connected in parallel to the load resistor, should have an interruption, the result is a dependence of the tension signal on the load, but this is twice as high as in the case of the functioning sensor. This can be clearly checked from the dependence of the tension signal on the load, whether the sensor is still functional is. Within certain limits, drifts of the load cells can also be detected. According to the result of this test, the error signal is then activated or deactivated. The additional output of this error signal, subsequent components such as displays or controllers can be informed of the error of the measured signal. The subsequent components that are to evaluate the tension signal can then switch on receipt of an active error signal in a mode in which they no longer evaluate the tension signal, whereby damage to people or machines are avoided.

In particular, in cases where both voltage dividing branches of the Wheatstone bridge have at least one force transducer, the stress test of only one voltage divider for determining the functionality of the sensor is insufficient. In this case, it is favorable according to claim 2, when both output lines of the Wheatstone bridge are loaded by at least one switch with at least one resistor. This allows the resistance values of all active elements of the Wheatstone bridge to be checked. Preferably, upon failure of any active element within the Wheatstone bridge, an active error signal is output.

In order to be able to reliably detect as many defects as possible within the sensor, it is advantageous according to claim 3, if the two output lines of the Wheatstone bridge by the at least one resistor alternately be charged. This ensures that even cases in which two load cells are simultaneously defective, reliably detected by the two load tests to be performed.

To achieve the most meaningful error analysis, it is favorable according to claim 4, when the difference between the Zugspannungssignalen without and with load of the Wheatstone bridge is calculated and compared with a lower limit. If the lower limit value is undershot, an error signal is output. In this way, most causes of errors of the sensor can be detected and reacted accordingly. In particular, no shortage of a force transducer within the Wheatstone bridge results in any change in the diagonal voltage with or without load.
This short-circuits of the force transducer can be detected quite reliably in this way. If the force transducer to load resistance in series, it can be reliably detected in this way, an interruption of the force transducer. In this case too, the load on the Wheatstone bridge does not result in any change in the diagonal voltage compared to the unloaded case. On the other hand, if the Wheatstone bridge is fully functional, it will result in a detuning of the bridge symmetry, which leads to a change in the diagonal voltage. This change depends only on the resistance values of the Wheatstone bridge in relation to the resistance value of the load resistance and is therefore a known quantity.

For dimensioning the lower limit of the tension signal, the range of values has been proven in accordance with claim 5. The upper limit of this value range must not be exceeded, otherwise a correctly functioning Wheatstone bridge would be detected as defective. The lower limit is given only for reasons of practicability, in particular to realize a sufficient signal-to-noise ratio of the diagonal voltage of the Wheatstone bridge. Otherwise, there is a risk that a defective Wheatstone bridge will be erroneously considered functional only due to noise.

In order to reliably detect all possible defects of the Wheatstone bridge, it is advantageous according to claim 6, when the difference between the tension signal with and without load of the Wheatstone bridge is also compared with an upper limit. When the upper limit value is exceeded, an active error signal is also output. Thus, further errors can be detected, which are evidenced by an excessively high dependence of the diagonal voltage on the load. For example, in this way an interruption of that force transducer can be detected, which is directly loaded. This interruption doubles the dependence of the diagonal voltage on the load, which can be checked quite easily by comparison with a corresponding limit value. In addition, very unlikely defects can be reliably detected in this way, in which both load cells are defective at the same time. In the event that both load cells should have a short circuit, the diagonal voltage Zero, because the supply voltage of the Wheatstone bridge breaks down in this case. However, if both force transducers have an interruption, in the unloaded case, an input voltage determined only by the amplifier sets in, which is generally approximately at half the operating voltage. Due to the load with the resistor, however, the input voltage is pulled to ground, so that there is a voltage swing of the size of half the operating voltage. This behavior can also be determined by comparing the tension signals with and without loading with an upper limit.

For the upper limit, the dimensions have been proven in accordance with claim 7, in order to detect all conceivable failures within the Wheatstone bridge safely.

Due to the load of the Wheatstone bridge this is deliberately detuned, so that the measurement results are falsified accordingly. In order to avoid that measurement results of the detuned Wheatstone bridge are passed on to subsequent components, it is advantageous according to claim 8, when the sensor outputs Zugspannungsmeßwerte only for those measuring cycles in which the switch is open. When using multiple switches is to ensure that all switches are open. This ensures that measurement results are transmitted to the following components only when the Wheatstone bridge is actually unloaded. The measurement results with loaded Wheatstone bridge are therefore exclusive internally processed to determine the error signal.

To avoid erroneous measurements, it is advantageous according to claim 9, when the position of the switch is synchronized with the measuring cycles of the sensor. It is ensured that the switch position is not changed during the entire measurement cycle, so that each measurement cycle is based on a defined switch position.

For operation of the sensor, the use of a revision cycle has been proven according to claim 10. This revision cycle includes several measurement cycles of the sensor and is repeated periodically. In each revision cycle, at least one measuring cycle with a closed switch and at least one measuring cycle with an open switch is provided. This periodically outputs measured values and the entire sensor is also checked periodically.

When testing both voltage divider of the Wheatstone bridge, it is advantageous according to claim 11, if in each inspection cycle at least one measuring cycle with closed switch of the first output line and at least one measuring cycle with closed switch of the second output line of the Wheatstone bridge is provided. In this way, it is ensured that the Wheatstone bridge is completely tested within each revision cycle and at least one measured value for the tensile stress is generated with unloaded Wheatstone bridge.

Especially with control applications of the sensor, a short reaction time of the sensor is important. Often, an output of a measured value is no longer sufficient for every third measuring cycle in order to guarantee a clean regulation. In this case, it is favorable according to claim 12, if in each inspection cycle more measuring cycles are provided with an open switch than with the switch closed. Therefore, the sensor generates useful measurement results substantially at a time interval of its cycle time, wherein at certain predetermined intervals, an internal test of the sensor is made, so that then an isolated measurement cycle for the generation of the tensile signal fails. Of course, the last generated measured value can be stored and made available to the following components in order to bridge this failure.

In order to achieve a web tension control, it is advantageous according to claim 13, when the tension signal output by the sensor is used as the actual value in the control. When the error signal is active, however, the control is blocked in order to prevent undefined or even destructive reactions of the control.

The load of the Wheatstone bridge results in an additional voltage swing in the diagonal voltage, which must be coped with by a subsequent amplifier and possibly analog-to-digital converter. This basically results in the analog-to-digital converter using part of its bit width for the load test. In case of a slight load of Wheatstone Bridge, this usually does not play a significant role. However, this results in a relatively large susceptibility to failure of the functional test of the Wheatstone bridge. If you want to use the entire dynamic range of the amplifier and the analog-to-digital converter with high significance of the functional test, it is favorable according to claim 14, when the load of the Wheatstone bridge and their supply voltage is changed. The supply voltage change is usually chosen so that it counteracts the effect of the load. Preferably, the supply voltage is selected in the loaded and unloaded case so that in case of a functional Wheatstone bridge in about the same diagonal voltage occurs. Thus, the entire dynamic range of the amplifier and analog-to-digital converter can be used for the measurement task. In the case of a defect in the Wheatstone bridge, a change in the diagonal voltage which can be detected by the analog-to-digital converter results in this case. The latter may possibly result in an overflow condition that is very easily detectable. An exact measurement of the voltage swing is not required in this case, since for this purpose, only the func tion is required efficiency as a yes-no decision.

To achieve a particularly secure system, it is advantageous according to claim 15, if at least two Wheatstone bridges are provided. These Wheatstone bridges each deliver diagonal voltages, which are evaluated via amplifiers and analog-to-digital converters. Both Wheatstone bridges are in the monitored as above. When an error signal occurs for one of the Wheatstone bridges, the generation of the tension signal is taken over by the other Wheatstone bridge. The same principle can be realized with more than two Wheatstone bridges. In this case, the individual Wheatstone bridges are preferably prioritized or their tension signal is averaged for better accuracy. Wheatstone bridges that show an active error signal are excluded from the calculation.

The subject invention is exemplified with reference to the drawing, without limiting the scope.

Figur 1

eine Schnittdarstellung durch eine Kraftmeßwalze einer laufenden Warenbahn,

Figur 2

eine schematische Darstellung eines Sensors und

Figur 3

ein Ablaufdiagramm zum Betrieb des Sensors gemäß Figur 2.

It shows:

FIG. 1

a sectional view through a load roller of a moving web,

FIG. 2

a schematic representation of a sensor and

FIG. 3

a flowchart for the operation of the sensor according to FIG. 2 ,

The FIG. 1 shows a sectional view through a Kraftmeßwalze 1, on which a web 2 is deflected. The web 2 exerts a force 3 on the Kraftmeßwalze 1, which depends only on the tension of the web 2 and the wrap around the Kraftmeßwalze 1. To measure the tension of the web 2, it is therefore sufficient to measure the bearing force of the force measuring roller 1 at a known wrap angle

The force measuring roller 1 has a stationary body 4, which is connected via double bending beam 5 with a machine-fixed axis 6. Depending on the load of the force measuring roller 1 by the force 3, the double bending beams 5 are deformed more or less strongly S-shaped. On the double bending beam 5 force transducer 7 are applied, which are preferably formed by strain gauges. These force transducers are essentially ohmic resistors which change their resistance when bent. The force transducer 7 are mounted in the end regions of the double bending beam 5, where the curvature of the double bending beam 5 is strongest. The stationary body 4 is connected via a roller bearing 8 with a shell 9, which forms the outer contour of the force measuring roller 1. This shell 9 is detected directly by the web 2.

The FIG. 2 shows a schematic diagram of a sensor 10, which detects the bearing force of the force measuring roller 1 and thus indirectly the tension of the web 2. The sensor 10 has a Wheatstone bridge 11, which is formed by two voltage dividers 12, 13. The voltage dividers 12, 13 are formed by the force sensors 7, which are applied to the double bending beam 5. Through the use of four force transducers 7, which are connected to the Wheatstone bridge 11, results in an advantageous temperature compensation of the force transducer 7. In addition, this drift of the force transducer 7 is substantially eliminated.

The Wheatstone bridge 11 is supplied via a changeover switch 14 'optionally with a supply voltage 14 which is formed stable and low noise. From the Wheatstone bridge 11 go two output lines 15, 16 away, between which a diagonal voltage 17 drops. This diagonal voltage 17 is the actual measurement signal, which is obtained from the force transducers 7. The output lines 15, 16 are supplied to an amplifier 18, which is designed as a differential amplifier. The amplifier 18 has high-impedance inputs in order not to burden the Wheatstone bridge 11 as possible. In addition, the amplifier 18 can amplify the diagonal voltage 17 by a gain factor that allows easy evaluation of the diagonal voltage 17.

The amplifier 18 is on the output side with an analog-to-digital converter 19 in operative connection, which generates from the output signal of the amplifier 18, a proportional thereto digital word. This digital word is supplied via a bus 20 to a processor 21 in which it is processed. The processor 21 can initiate a measurement cycle in the analog-to-digital converter 19 via a control line 22. In response, the processor 21 receives the information via a signal line 23 that the measurement cycle of the analog-to-digital converter 19 has been completed and that a new data word is present on the bus 20.

In order to be able to determine whether the force transducers 7 are still functional and thus the Wheatstone bridge 11 outputs meaningful values, the two output lines 16, 17 can be loaded with a load resistor 26 via switches 24, 25. This load resistor 26 ensures a one-sided detuning of the Wheatstone bridge 11, so that a defined change in the diagonal voltage 17 is to be expected. This change in the diagonal voltage 17 is supplied via the amplifier 18 and the analog-to-digital converter 19 via the bus 20 to the processor 21, which applies corresponding mathematical operations on this data word. In this case, in addition to a tension signal 27, which essentially corresponds to the value on the bus 20 with unloaded Wheatstone bridge 11, an error signal 28 is output. This error signal 28 indicates in the activated state that the Wheatstone bridge 11 is defective and therefore the output tension signal 27 is not usable. In addition, the processor 21 gives the following components a handshake signal 29 to synchronize with the data output of the processor 21.

To control the two switches 24, 25, the processor 21 has two control outputs 30, 31, which ensure that the switches 24, 25 are closed only during a test cycle, the switches 24, 25 are not closed simultaneously, but only alternately , During a normal measuring operation, in which a new tension signal 27 is to be determined, the two switches 24, 25 are open.

In addition, for the duration of the test cycle, the supply voltage 14 of the Wheatstone bridge 11 can be switched by the processor 21. This switching causes a proportional change in the diagonal voltage 17, so that the voltage swing caused by the load becomes smaller. It is also thought to change the supply voltage of the Wheatstone bridge 11 so that it counteracts the load exactly. In this case, there is no load-dependent change in the diagonal voltage 17 when the Wheatstone bridge 11 is functional. In the case of a defective Wheatstone bridge 11, however, a characteristic voltage swing of the diagonal voltage 17 results in this case.

The FIG. 3 shows a flowchart for the operation of the processor 21. In an initialization step 32, the two switches 24, 25 are opened and the error signal 28 is activated. This prevents a random value at the output 28 from being interpreted as a measured value.

After the initialization step 32, a loop follows that defines a revision cycle 33. Therefore, this revision cycle 33 is repeated periodically as often as desired after initialization 32.

In the revision cycle 33, the switch 25 is first opened and a measuring cycle 34 is started. The measurement takes place in this case with unloaded Wheatstone bridge 11. The data obtained from the measurement cycle data stored in a variable Z ₀ . alternative to FIG. 3 It would also be possible to start several measuring cycles 34 in succession and to output the measuring results if the error signal 28 is deactivated.

Subsequently, the switch 24 is closed, whereby the output line 15 of the Wheatstone bridge 11 is loaded by the load resistor 26. Subsequently, a new measuring cycle 35 is started and the measured value of the analog-to-digital converter 19 determined in this case is stored in a variable Z ₁ . Subsequently, the absolute value of the difference between the values Z ₀ and Z _{1 is} calculated and stored in a variable F ₁ . alternative to FIG. 3 could now follow several measuring cycles 34 with open switches 24, 25, the measurement results are output only when the error signal is disabled.

In the following step, the positions of both switches 24, 25 are reversed, so that now the output line 16 of the Wheatstone bridge 11 is loaded by the load resistor 26. Subsequently, a further measuring cycle 36 is started. The value determined by the analog-to-digital converter 19 is stored again in the variable Z ₁ . Now again, the absolute value of the difference between the variables Z ₀ and Z _{1 is} determined and stored in a variable F ₂ . The variables F ₁ and F ₂ therefore contain measures for influencing the Wheatstone bridge 11 by the two types of load applied.

In a following comparison step 37, the variables F ₁ and F _{2 are} compared with predefined lower threshold values U and upper threshold values O. Only in the event that both variables F ₁ and F _{2 are} within the band defined by the thresholds U and O, the sensor 10 is interpreted as functional and the value Z _{0 is} output. The value of Z ₀ contains the measured value with no load Wheatstone bridge 11. In addition, the error signal 28 is reset in this case, to indicate that subsequent components of the output measurement value is reliable.

LIST OF REFERENCE NUMBERS

1 force measuring 30 control output 2 web 31 control output 3 force 32 initialization 4 stationary body 33 revision cycle 5 Double beam 34 unloaded measuring cycle 6 axis 35 loaded measuring cycle 7 Load cell 36 loaded measuring cycle 8th roller bearing 37 comparison step 9 Bowl 10 sensor 11 Wheatstone bridge 12 voltage divider 13 voltage divider 14 supply voltage 14 ' switch 15 output line 16 output line 17 diagonal voltage 18 amplifier 19 Analog-to-digital converter 20 bus 21 processor 22 control line 23 control line 24 switch 25 switch 26 load resistance 27 tensile stress 28 error signal 29 hand-shake signal

Claims (15)

1. Method for measuring the tensile stress of a running web (2) using at least one sensor (10) which has at least one Wheatstone bridge (11) containing at least one force sensor (7) which is influenced by the tensile stress of the running web (2), a diagonal voltage (17) of the at least one Wheatstone bridge (11) being amplified by an amplifier (18) which outputs a tensile stress signal (Z₀), characterized in that the at least one Wheatstone bridge (11) is periodically loaded by at least one resistor (26) using at least one intermittently driven switch (24, 25) during loading by the tensile stress of the running web (2), the functionality of the at least one sensor (10) being determined from the extent to which the tensile stress signal (Z₁) is influenced by the loading and being output in the form of an error signal (28).
2. Method according to Claim 1, characterized in that both output lines (15, 16) of the Wheatstone bridge (11) are loaded with at least one resistor (26) by means of at least one switch (24, 25).
3. Method according to Claim 2, characterized in that the output lines (15, 16) of the Wheatstone bridge (11) are alternately loaded by the at least one resistor (26).
4. Method according to at least one of Claims 1 to 3, characterized in that the difference (F₁, F₂) between the tensile stress signals (Z₀, Z₁) with and without loading of the Wheatstone bridge (11) is calculated and is compared with a lower limit value (U), an active error signal (28) being output when said lower limit value is undershot.
5. Method according to Claim 4, characterized in that the lower limit value (U) is between 0.05 and 0.5 times the value $$\frac{U_{\mathit{W}}{\mathit{VR}}_{\mathit{K}}}{{\mathit{R}}_{\mathit{K}}\mathit{+}{\mathit{R}}_{\mathit{S}}}$$

   

   
   where U_w is the supply voltage of the Wheatstone bridge (11), V is the gain factor, R_S is the loading resistance and R_K is the resistance of the force sensor (7).
6. Method according to Claim 4 or 5, characterized in that the difference (F₁, F₂) is compared with an upper limit value (O), an active error signal (28) being output when said upper limit value is exceeded.
7. Method according to Claim 6, characterized in that the upper limit value (0) is less than 0.5 U_W and less than $$\frac{U_{\mathit{W}}{\mathit{VR}}_{\mathit{K}}}{{\mathit{R}}_{\mathit{K}}\mathit{+}{\mathit{R}}_{\mathit{S}}}$$

   

   
   where U_W is the supply voltage of the Wheatstone bridge (11), V is the gain factor, R_S is the loading resistance (26) and R_K is the resistance of the force sensor (7).
8. Method according to at least one of Claims 1 to 7, characterized in that the sensor (10) outputs tensile stress measured values (Z₀) only for those measuring cycles (34) in which the at least one switch (24, 25) is open.
9. Method according to at least one of Claims 1 to 8, characterized in that the position of the at least one switch (24, 25) is synchronized with the measuring cycles (34, 35, 36) of the sensor (10).
10. Method according to Claim 9, characterized in that provision is made of a revision cycle (33) which comprises a plurality of measuring cycles (34, 35, 36) of the sensor (10), at least one measuring cycle (35, 36) with the switch (24, 25) closed and at least one measuring cycle (34) with the switch (24, 25) open being provided in each revision cycle (33).
11. Method according to Claim 10, characterized in that at least one measuring cycle (35) with the switch (24) of the first output line (15) of the Wheatstone bridge closed and at least one measuring cycle (36) with the switch (25) of the second output line (16) of the Wheatstone bridge (11) closed are provided in each revision cycle (33).
12. Method according to Claim 10 or 11, characterized in that more measuring cycles (34) with the switch (24, 25) open than with the switch (24, 25) closed are provided in each revision cycle (33).
13. Method according to at least one of Claims 1 to 12, characterized in that the web tension is regulated, the tensile stress signal (27) output by the sensor (10) being used as an actual value, the regulating operation being blocked in the case of an active error signal (28).
14. Method according to at least one of Claims 1 to 13, characterized in that the supply voltage (14) of the Wheatstone bridge (11) is also changed when the latter is loaded.
15. Method according to at least one of Claims 1 to 14, characterized in that at least two of the Wheatstone bridges (11) are provided, in which case, in the event of an error signal (28) for one of the Wheatstone bridges (11), at least one of the other Wheatstone bridges (11) generates the tensile stress signal (Z₀).

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