GB2492047A

GB2492047A - Non-contact stress monitoring by measuring induced current

Info

Publication number: GB2492047A
Application number: GB1109635.1A
Authority: GB
Inventors: Obi Ezenwa; Simon Fawcett
Original assignee: WIRE BELT Co Ltd
Current assignee: WIRE BELT Co Ltd
Priority date: 2011-06-09
Filing date: 2011-06-09
Publication date: 2012-12-26
Also published as: GB201109635D0

Abstract

Measuring the stress applied to a target material by inducing a primary current in either the target material or a sensor material 18 incorporated therein; measuring a secondary current that is subsequently induced in a sensing transformer 14; and calculating the applied stress based on the measured secondary current. The primary current may be induced by applying a voltage to an inducing transformer, e.g. a hinged (24, Fig.2) toroidal transformer 12 through which the target material passes. The sensor material may be an elongate 1mm-diameter shape memory alloy (e.g. Nitinol) wire embedded in a moving target rope, cable, wire or belt (52, Fig.5). The calculation may be based on known linear or non-linear relationships between electrical resistance and stress exhibited by the materials. The calculation may rely on baseline calculation made when the target material is unstressed and further calculation made when it is under stress.

Description

STRESS MONITORING METHOD AND APPARATUS

The present invention relates to a method and apparatus for monitoring stress in an object. In particular, the invention relates to a non-contact monitoring system.

It is often desirable to monitor the stress applied to objects, such as ropes, cables, wires and belting, to ensure that the applied stress does not reach a level that could cause the object to fail. Such monitoring occurs in a wide variety of industries including manufacturing, construction, stone cutting, shipping, and the automotive industry.

Known systems for monitoring the stress applied to an object include strain gauges and load cells. A strain gauge comprises an insulating flexible backing which supports a metallic foil pattern. As the target object is deformed, the foil also becomes deformed, causing its electrical resistance to change. The resistance change is related to the strain and thus measurement of the resistance change (for example by using a Wheatstone Bridge) leads to measurement of the strain applied to the target object. If the Young's Modulus of the target material is known, measurement of strain can be used to calculate the applied stress using the equation E = a/c, where E is Young's Modulus, a is applied stress and c is applied strain.

A load cell is a mechanical arrangement in which the strain applied to a target object is sensed, converted into an electrical signal and fed into an algorithm which calculates the force applied to the load cell. The strain applied to the target object is typically sensed using a strain gauge (or, often, four strain gauges in a Wheatstone Bridge arrangement). In industrial applications, hydraulic (or hydrostatic) sensors are often used, and other types of known sensor include piezoelectric load cells and vibrating wire load cells.

The above described systems require the sensing element to be in contact with the target object, such that the sensing element can deform as the object is stressed. This requirement limits these systems to the finite stiffness of the sensing element used, and can lead to measurement inaccuracies as a result of the sensing element oscillating about its natural frequency. Further, the above described systems are limited to measuring the stress applied to static objects, and would therefore not be suitable for measuring the stress applied to a moving rope or cable, for example.

Non-contact stress measurement systems have been developed. One such system is based on a magnetoetastic effect which occurs in response to applied stress (Yanping Shi and Cheng Qian, Industrial Electronics and Application, 2009).

Sensing is achieved remotely through the measurement of resonance frequency.

However, magnetic induction and detection techniques used for remote sensing in magnetoelastic sensors are susceptible to signal interference.

In view of the above, there is a need for a non-contact stress measurement system which can measure the stress applied to a moving object such as a rope or cable, which is not susceptible to signal interference, and in which shear stress and tensile stress can both be measured using the same sensing element.

According to a first aspect of the present invention, there is provided a method of measuring the stress applied to a target material, the method comprising the steps of: inducing a primary current in either the target material or a sensor material incorporated in the target material; measuring a secondary current that is subsequently induced in a sensing transformer; and calculating the stress applied to the target material based on the measured secondary current.

The step of inducing a primary current may comprise applying a voltage to an inducing transformer to induce the primary current in either the target material or the sensor material.

The step of calculating the stress applied to the target material advantageously comprises a first calculation when the target material is unstressed and a second calculation when the target material has stress applied thereto.

Preferably, either the target material or the sensor material exhibits a known linear relationship between electrical resistance and stress. Alternatively, either the target material or the sensor material may exhibit a known non-linear relationship between electrical resistance and stress.

Preferably, the target material comprises an elongate element.

A sensor material may be incorporated in the target material, such that the sensor material provides the property of a linear relationship between electrical resistance and stress. The sensor material may comprise an elongate element, and may comprise a shape memory alloy. The sensor material may comprise Nitinol, and may have a diameter of 1 mm.

The inducing and sensing transformers may be respective toroidal transformers. The target material may then pass through the centre of the or each toroidal transformer. The or each toroidal transformer may comprise two separate parts joined by a hinge.

Preferably, the voltage is applied to the inducing transformer by an autotransformer.

Preferably, the secondary current is amplified before being measured.

According to a second aspect of the present invention, there is provided apparatus for measuring the stress applied to a target material, the apparatus comprising: means for inducing a primary current in either the target material or a sensor material incorporated in the target material; means for measuring a secondary current that is subsequently induced in a sensing transformer; and means for calculating the stress applied to the target material based on the measured secondary current.

In one such apparatus, the means for inducing a primary current and the means for measuring a secondary current are separate elements.

In an alternative apparatus, the means for inducing a primary current and the means for measuring a secondary current comprise the same element.

Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a system according to the present invention; Figure 2 is a schematic representation showing the signal inducing toroidal transformer of Figure 1; Figure 3 is a schematic representation showing the signal sensing toroidal transformer of Figure 1 Figure 4 is a schematic representation of an alternative system according to the present invention; Figure 5 is a schematic representation of another alternative system according to the present invention; and Figure 6 is a schematic representation of a further alternative system according to the present invention.

As can be seen in Figure 1, the stress sensing apparatus 10 of a preferred embodiment of the present invention comprises a signal inducer in the form of a toroidal transformer 12 and a signal sensor in the form of a toroidal transformer 14.

This apparatus is used to sense the stress in a cable 16.

The cable 16 has embedded therein a sensor wire 18. The sensor wire 18 is made of Nitinol, a shape memory metal formed from a nickel-titanium (NiTi) alloy that "remembers" its geometry. The sensor wire 18 in this embodiment has a diameter of 1 mm and is heat treated prior to use to obtain a wire exhibiting superelasticity at working temperatures, a known relationship between electrical resistance and stress, and cyclic stability (that is, the ability for the wire to be stressed and return back to its original shape upon removal of the stress force, in numerous cycles without destroying the properties of the wire).

The cable 16 and sensor wire 18 are associated such that a change in stress applied to the cable 16 will equally affect the sensor wire 18.

The sensor wire 18 is arranged to pass through the transformers 12, 14 and is then arranged to provide an elongate portion 19. The sensor wire 18 is in a closed loop. The elongate portion 19 enables the sensor wire 18 to be incorporated into a cable 16 such that the stress applied to the cable 16 can be monitored.

Figure 2 shows the signal inducing toroidal transformer 12 of the preferred embodiment, which comprises a ferromagnetic core 20 around which is wound a primary coil 22. The transformer 12 is constructed from two semi-circular halves, the two halves being joined by a hinge 24. A locking device 26 is provided diametrically opposite the hinge 24 to lock the two halves together. The locking device 26 is of known form.

The primary coil 22 is connected in series with a variable autotransformer 28 such that the signal inducing toroidal transformer 12 is powered by the variable autotransformer 28.

Figure 3 shows the signal sensing toroidal transformer 14 of the preferred embodiment, which comprises a ferromagnetic core 30 around which is wound a secondary coil 32. As with the signal inducing toroidal transformer 12, the signal sensing toroidal transformer 14 is constructed from two semi-circular halves, the two halves being joined by a hinge 34. A locking device 36 is provided diametrically opposite the hinge 34 to lock the two halves together. The locking device 36 is of known form.

The secondary coil 32 of the signal sensing toroidal transformer 14 is connected to a burden resistor 38, and further connected to amplification and monitoring means so that the signal can be amplified and monitored.

The hinged arrangement of the signal inducing toroidal transformer 12 and the signal sensing toroidal transformer 14 allows these transformers to be placed around the cable 16 to allow sensing and monitoring of the stress applied thereto.

That is to say, the cable 16 passes though the centres of both toroidal transformers 12, 14. The toroidal transformers 12, 14 thus surround the cable 16 without coming into contact with it. The cable 16 can be stationary, or can be moving in a longitudinal direction, in which case the cable 16 will travel through the toroidal transformers 12, 14 without coming into contact with them.

In order to monitor the stress applied to the cable 16 and sensor wire 18, a baseline measurement is firstly taken in that the resistance of the sensor wire 18 is measured when the sensor wire 18 is unstressed. A voltage is then applied to the primary coil 22 of the toroidal transformer 12 by means of the variable autotransformer 28. This creates a magnetic field which is contained within the centre of the signal inducing toroidal transformer 12. The magnetic field is thus contained within the area through which the cable 16 and sensor wire 18 pass.

The magnetic field in turn causes a current to flow in the Nitinol sensor wire 18 by means of electromagnetic induction. A current signal (primary current) is therefore induced in the sensor wire 18. The applied voltage is maintained at a constant level, and as such the current induced in the sensor wire 18 is also maintained at a constant level.

The current signal induced in the sensor wire 18 in turn creates a magnetic field around the closed loop, which produces a secondary current in the secondary coil 32 of the signal sensing toroidal transformer 14 by means of electromagnetic induction. This secondary current is proportional to the primary current.

The secondary coil 32 of the signal sensing toroidal transformer 14 is connected to a burden resistor 38, and a low-level voltage is thus obtained. This voltage is amplified for monitoring and control. The amplification and monitoring means are of known type and are omitted from the Figures for clarity.

The secondary current induced in the secondary coil 32 of the signal sensing toroidal transformer 14 is proportional to the level of the voltage applied to the primary coil 22 and inversely proportional to the resistance of the sensor wire 18, in keeping with Ohm's Law (i.e. I = VIP, where I is current, Vis voltage and P is resistance). As the voltage applied to the primary coil 22 is maintained at a constant level, the secondary current induced in the secondary coil 32 changes in response to a change in the resistance of the sensor wire 18.

In keeping with Poisson's Effect, the application of a uniaxial stress to the cable 16 and sensor wire 18 causes a reduction in the cross-sectional area of both cable 16 and sensor wire 18, in a direction perpendicular to the longitudinal direction of the cable 16. It is essential that the relationship between the resistance of the sensor wire 18 and the stress applied thereto is known and repeatable as the wire undergoes numerous stress cycles. In this connection, sensing electronics are calibrated to allow for a non-linear relationship between resistance and applied stress in a given wire. The relationship is known and repeatable for a particular wire being used in a system such that a stress applied to the wire will result in a predictable change in resistance.

The secondary current induced in the secondary coil 32 can thus be used as a measure of the resistance in the sensor wire 18. As such, a change in the resistance of the sensor wire 18 resulting from the application of a stress force to the cable and sensor wire leads to a proportional change in the secondary current, which can be measured by means of a suitable meter connected to the secondary coil32.

An alternative embodiment of the present invention is shown in Figure 4. A separate transceiver unit 40 houses both signal inducing and signal sensing elements. In this connection, a primary coil 42 acts as both the signal inducer and the signal sensor. The primary coil is connected to an electronic device 44.

In this alternative embodiment, a sensor wire 46 is embedded in a cable 48, with the ends of the sensor wire 46 being shaped into a coil 50. The coil 50 forms the secondary coil of the system. The sensor wire 46 forms a closed loop.

In use, a current (primary current) is caused to flow through the primary coil 48 by the device 44. This induces a current (secondary current) in the secondary coil 50, thus causing a current to flow in the sensor wire 46. The primary current is maintained at a constant level, and as such the secondary current induced in the sensor wire 46 is also maintained at a constant level.

Again, it is essential that the sensor wire 46 exhibits a known relationship between applied stress and resistance, such that the secondary current induced in the sensor wire changes in a predictable way in response to a change in the resistance of the sensor wire. This change in the secondary current is sensed by the primary coil 42 acting as a signal sensor. A burden resistor (not shown) is provided on the primary coil 42, such that the electronic device 44 can measure the change in secondary current, and this the change in resistance of the sensor wire 46 resulting from the application of a stress force to the cable 48 and sensor wire 46.

In view of the above, systems according to the present invention provide means for real time monitoring of the stress applied to a cable 16, through continuous measurement of the secondary current induced in the secondary coil.

This monitoring is effectively enabled through a combination of Ohm's Law and Poisson's Effect.

An advantage of the system according to the described embodiment is the provision a non-contact method for monitoring the stress applied to the cable. The sensing element thus does not become deformed in any way as the cable is stressed.

A further advantage of the system according to the described embodiment is that, due to the non-contact nature of the system, it is possible to monitor the stress applied to a cable whether the cable is stationary or moving.

In the preferred embodiment the apparatus is used to sense and monitor the stress applied to a cable. However the apparatus could be used equally as effectively to sense and monitor the stress applied to any other elongate object, such as a rope, wire or belt. An arrangement in which the apparatus is used to monitor the stress applied to a belt 52 is shown in Figure 5.

In addition, the number of turns of sensor wire around the inducing transformer and/or the sensing transformer may be varied when long loops of sensing wire are used, such as when the stress applied to a rope is being monitored. An arrangement in which the sensor wire 18 comprises a number of turns around the inducing transformer 12 and the sensing transformer 14 is shown in Figure 6.

It is essential that the relationship between electrical resistance and stress of the sensor wire is known. Ideally, the relationship between resistance and stress is a linear relationship. The sensor wire material can be made to approach a linear relationship through heat treatment and subsequent training of the material.

In some applications, the relationship would be non-linear and would be in the form of a curve that is repeatable after training. In this case, the sensing electronics would be calibrated to allow for the non-linear response.

In addition, the known linear or non-linear response of the sensor wire may not commence with zero loading of the wire. That is to say, the working region of a stress-resistance graph for a given sensor wire (the region in which the relationship between resistance and stress is known) may not commence with the point where stress is zero. In this case, the system would be calibrated to the particular characteristics of the sensor wire being used.

Further, variations in the temperature of the sensor wire may also affect the resistance of the wire. A temperature monitoring and compensation system integrated with the sensing electronics may therefore be required in systems exposed to a large variation in temperature, or in systems which require a high degree of accuracy.

The voltage, current and frequency of the induced signal can be specifically chosen for a given application, so as to optimise the monitoring characteristics for that application.

In the preferred embodiment, the signal inducing and signal sensing elements are both toroidal transformers, with the cable passing through the centres thereof. However, another suitable form of non-contact signal inducer and/or sensor could be used. Although in the preferred embodiment the toroidal transformers are constructed from two semi-circular halves, other hinged arrangements could be used to enable the toroidal transformers to surround the cable. The transformers could be formed of a unitary piece of material, with the cable being passed through the centres of the transformers prior to having its ends joined to form a ioop.

In other arrangements, the cable does not have to pass through the transformers at all, and the arrangement could be such that the cable passes close enough to the transformers to induce a current flow therein. The sensor wire at least must, however, be in a closed loop.

In the preferred embodiment the sensor wire has a diameter of 1 mm, however a sensor wire of any diameter suitable for its purpose may be used. For example, a larger diameter sensor wire may support the monitoring of a larger diameter cable, and vice versa. Further, in the preferred embodiment the sensor wire embedded in the cable is made of Nitinol, however the wire could be made of any other suitable superelastic shape memory alloy.

The sensor wire may be incorporated into the cable (or other object being sensed) by any method which is suitable for the particular object in question. For example, if the object is a nylon rope, the sensor wire may be woven as one of the threads of the rope. If the object is a cutting wire, the sensor wire may replace the usual steel core wire. If the object is a steel wire rope, the sensor wire may be insulated and then woven in two halves.

Alternatively, the target material could be such that the primary current can be induced directly into the target material, and the secondary current can be subsequently induced by the target material, without the need for a separate sensor wire. For this to be possible, however, the target material must exhibit a linear relationship between applied stress and resistance and the target material must be in a closed loop.

In the preferred embodiment, the signal inducing toroidal transformer is powered by a variable autotransformer. However, in alternative embodiments the signal inducing toroidal transformer cold be powered by other means, such as by being connected to a mains supply.

In the preferred embodiment, the secondary current is passed through a burden resistor and subsequently amplified, with the amplified signal being fed into a monitoring means. Modified systems could see the secondary current being fed directly to a suitable meter or relay.

Claims

CLAIMS1. A method of measuring the stress applied to a target material, the method comprising the steps of: inducing a primary current in either the target material or a sensor material incorporated in the target material; measuring a secondary current that is subsequently induced in a sensing transformer; and calculating the stress applied to the target material based on the measured secondary current.
2. A method according to claim 1, wherein the step of inducing a primary current comprises applying a voltage to an inducing transformer to induce the primary current in either the target material or the sensor material.
3. A method according to claim I or 2, wherein the step of calculating the stress applied to the target material comprises a first calculation when the target material is unstressed and a second calculation when the target material has stress applied thereto.
4. A method according to any preceding claim, wherein either the target material or the sensor material exhibits a known linear relationship between electrical resistance and stress.
5. A method according to any one of claims 1 to 3, wherein either the target material or the sensor material exhibits a known non-linear relationship between electrical resistance and stress.
6. A method according to any preceding claim, wherein the target material comprises an elongate element.
7. A method according to any preceding claim, wherein a sensor material is incorporated in the target material, the sensor material providing the property of a linear relationship between electrical resistance and stress.
8. A method according to claim 7, wherein the sensor material comprises an elongate element.
9. A method according to claim 7 or 8, wherein the sensor material comprises a shape memory alloy.
10. A method according to claim 9, wherein the sensor material comprises Nitinol.
11. A method according to any one of claims 8 to 10, wherein sensor material hasadiameterofl mm.
12. A method according to any one of claims 2 to 11, wherein the inducing transformer is a toroidal transformer.
13. A method according to any preceding claim, wherein the sensing transformer is a toroidal transformer.
14. A method according to claim 12 or 13, wherein the target material passes through the centre of the or each toroidal transformer.
15. A method according to any one of claims 12 to 14, wherein the or each toroidal transformer comprises two separate parts joined by a hinge.
16. A method according to any preceding claim, wherein the voltage is applied to the inducing transformer by an autotransformer.
17. A method according to any preceding claim, wherein the secondary current is amplified before being measured.
18. Apparatus for measuring the stress applied to a target material, the apparatus comprising: means for inducing a primary current in either the target material or a sensor material incorporated in the target material; means for measuring a secondary current that is subsequently induced in a sensing transformer; and means for calculating the stress applied to the target material based on the measured secondary current.
19. Apparatus according to claim 18, wherein the means for inducing a primary current and the means for measuring a secondary current are separate elements.
20. Apparatus according to claim 18, wherein the means for inducing a primary current and the means for measuring a secondary current comprise the same element.
21. A method of measuring the stress applied to a target material substantially as described herein with reference to the accompanying drawings.
22. Apparatus for measuring the stress applied to a target material substantially as described herein with reference to the accompanying drawings.