Patent Publication Number: US-2012043980-A1

Title: Wear sensor

Description:
FIELD OF THE INVENTION 
     The present invention relates to sensors for detecting wear. 
     BACKGROUND OF THE INVENTION 
     Plant and equipment in many industries are subject to wear by the passage or flow of abrasive materials. For example in mining, ore may be passed through chutes onto conveyors for subsequent processing. These chutes are subject to substantial wear by the passage of large, heavy and hard rocks. To extend the service life of such plant and equipment it is known to fix sacrificial wear plates to the surfaces which would otherwise be in contact with the abrasive material. Irrespective of whether or not wear plates are used, in order to optimally manage and maintain the plant and equipment it is common practice to monitor for wear. This may be done by manual inspection or by the use of sensors. In some situations where manual inspection is physically impossible or requires substantial down time, the use of sensors is the only viable option. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention there is provided a wear sensor comprising: 
     an electrical circuit comprising a plurality of discrete elements, each element contributing to a measurable electrical characteristic of the circuit; each element being capable of being electrically decoupled from the circuit by action of wear on the sensor, such that when wear occurs on the sensor one or more of the elements are electrically decoupled thereby changing the measurable electrical characteristic. 
     The wear sensor may comprise a substrate which is capable of being disposed along a path subject to wear and wherein the circuit is supported by the substrate. 
     The elements may be connected at different locations in the circuit such that the elements are sequentially decoupled from the circuit as wear progresses along the path. 
     The circuit may comprise two conductors supported by the substrate wherein the elements are electrically shunted across the conductors to form a parallel connection between the elements. 
     The substrate may comprise a circuit board and the two conductors comprise respective conductive tracks. 
     The two conductors may be embedded in or sandwiched between two nonconductive layers of material. 
     In one embodiment the circuit board is a composite circuit board comprising a first board and a second board which are arranged one on top the other and wherein the conductive tracks extend between the first and second boards. 
     In one embodiment each element has the same nominal electrical characteristic, e.g. the same resistance, or capacitance or inductance. 
     In the same or an alternate embodiment the elements are arranged in a plurality of groups, with each element in each group having the same nominal electrical characteristic. 
     In a further embodiment the elements are located along the wear path so that elements of one value of nominal electrical characteristic are electrically decoupled from the circuit due to action of wear on the sensor before elements of a second different value of a same nominal electrical characteristic are electrically decoupled by the action of wear on the sensor. 
     In an embodiment the plurality of elements comprises at least three elements which are arranged along the wear path with progressively reduced spacing of respective connections to the circuit in a direction of increased wear. 
     One or more of the elements may be a surface mount electronic component or a thick film printed element. 
     In one embodiment electrical elements are resistors. 
     In one possible arrangement the sensor may comprise a first portion and a second portion and the elements are supported on the second portion and are electrically coupled in the circuit by respective conductors that extend along the first portion such that as the sensor wears the respective conductors are worn away thereby electrically decoupling the respective elements from the circuit. 
     In this arrangement the first and second portions are configured to be selectively connectable together wherein the first portion is a sacrificial portion which wears away and the second portion is disposed outside of the wear path. 
     A second aspect of the invention provides a wear sensing system comprising: 
     one or more wear sensors according to the first aspect of the invention, the or each wear sensor capable of being disposed in or along a wear path of an object subject to wear; and, 
     one or more measuring devices for measuring the electrical characteristic of the or each sensor. 
     The wear sensing system may comprise a processor for processing the measured characteristic for the or each sensor and controlling the activation of sensory alarm when the measured characteristic of at least one of the sensors reaches a value indicative of a target amount of wear of the object. 
     The wear sensing system may comprise a visual display controlled by the processor to provide a visual representation of wear of the object. 
     A third aspect of the invention provides a method of sensing wear occurring to an object comprising: 
     measuring an electrical characteristic of a circuit of a wear sensor located in the object at a location subject to wear, wherein the circuit comprises a plurality discrete of elements, wherein the sensor is arranged so that the elements are sequentially electrically decoupled from the circuit due to wear of the wear sensor, each element contributing to a measurable electrical characteristic of the circuit, so that as wear occurs to the object and thus the wear sensor, one or more of the wear elements are electrically decoupled, thereby changing the measurable electrical characteristic. 
     In an embodiment repeated measurements are taken over time so that changes in the electrical characteristic are reflected in the measurements. 
     In an embodiment the changes in the electrical characteristic are used to determine the extent of wear to the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to provide a better understanding of the present invention, embodiments of the invention are described, by way of example only, with reference to the accompanied drawings, in which: 
         FIG. 1  is a schematic representation of a bolt securing a wear plate to a structure in which is disposed a wear sensor according to an embodiment of the present invention; 
         FIG. 2  is a perspective view of an embodiment of a wear sensor according to the present invention; 
         FIG. 3  is a perspective view of the wear sensor of  FIG. 2  having been subjected to wear; 
         FIG. 4  is a schematic circuit diagram of the embodiment of the present invention; 
         FIG. 5  is a schematic representation of a second embodiment of the wear sensor; 
         FIG. 6  is a schematic block diagram of a wear system according to an embodiment of the present invention; 
         FIG. 7  is a graph representing measurements taken from a plurality of wear sensors according to an embodiment of the present invention; 
         FIG. 8  is a schematic representation of a circuit diagram for a further embodiment of the wear sensor; and, 
         FIG. 9  is a schematic circuit diagram of a further embodiment of the wear sensor. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In one embodiment a wear sensor comprises a substrate supporting a plurality of electrical elements connected in a parallel configuration. The substrate is intended to be disposed so as to wear away with a surface of an object being monitored for wear. Wear occurs simultaneously to the surface and an end of the substrate adjacent the surface. This results in the progressive removal and thus electrical decoupling (or open circuiting) of the electrical elements and consequently a change in an electrical characteristic to a circuit formed by the parallel elements. The change of the electrical characteristic can be measured and is reflective of the extent of wear to the wear sensor and thus the surface being monitored. Indeed the change in electrical characteristic can be calibrated to the amount of wear of the surface in terms of a distance or thickness (eg a measure in mm) or as a percentage of the total thickness of the surface. 
       FIG. 1  shows an example of a wear sensor system  10 . In this example there is an object in the form of a wear plate  20 , which is subject to wear. The wear plate  20  is secured by a fastener  14  to a structural element  22 . The fastener  14  is in the form of a bolt having a head  16  and shaft  18 . The head  16  locates in a complementary shaped recess in the wear plate  20 . A nut  24  threadingly engages with the shaft  18  so that the wear plate  20  is secured to the structural element  22  by applying a clamping force between the head  16  and the nut  24 . 
     Inside the head  16  and shaft  18  is a passage  30  which extends through the length of the bolt  14 . An embodiment of a wear sensor  12  is disposed within the passage  30 . This coincides with a wear path for the wear plate  20  and bolt  14 . Extending from the wear sensor  12  are wires  32  which exit the shaft  18  and are connected by connector  34  to a communication lead  36 . The communication lead  36  would usually be wired to a junction box, although it could be connected to a wireless communication device, and then to a measuring device  38 . 
     The surface of the wear plate  20  and bolt  16  and the end of the wear sensor  12  adjacent these surfaces are subjected to wear. In time these surfaces and the end of the sensor  12  will wear down to the dashed line  40 . All of the material above the line  40  will have been worn away (i.e. removed) including part of the wear plate  20 , part of the head  16  and part of the wear sensor  12 . Assuming that there is one or more electrical elements or at least connections of those elements to the circuit on the part of the sensor that has been worn away then such elements will have also be worn away or otherwise electrically decoupled or open circuited form the electrical circuit to which they were previously parallel connected. 
       FIG. 2  shows the wear sensor  12  in more detail. The wear sensor  12  comprises a substrate in the form of a printed circuit board (PCB)  50  which has a pair of tracks  52  and  54  extending along its length. In an embodiment the tracks  52  and  54  are parallel to each other. Spaced apart along the length of the PCB  50  is a plurality of elements  56 . The elements  56  are placed so as to be in electrically shunted across the tracks  52  and  54 . In this arrangement the elements  56  are electrically connected in parallel. The elements  56  in one example are surface mount components such as surface mount resistors or thick film resistors which are printed onto the PCB  50 . However, in other embodiments the elements  56  may be other types of electrical components such as capacitors, inductors, semiconductors or combinations of these. Connected to and extending from each of the tracks  52  and  54  are a respective one of the wires  32 . 
     The arrangement of parallel elements  56  connected between the tracks  52  and  54  form an electrical circuit  58  having a measurable electrical characteristic determined by the elements  56 . In the case of the elements being resistors, the electrical characteristic will be the total resistance resulting from each of the resistors being in a parallel arrangement. Alternatively another electrical characteristic (for example voltage or current) may be determined that is directly related to the resulting resistance. The electrical characteristic is measurable from the wires  32 . The total resistance R of the circuit  58  is calculated as: 
         R =1÷((1 ÷R   1 )+(1 ÷R   2 ) . . . (1 ÷Rn )),
 
     where R 1 , . . . Rn is the resistance value of each of n resistors connected to the circuit at any one time. 
     The resistance can be measured by an ohmmeter. Alternatively the current or voltage can be measured, where the voltage or current (respectively) is known, and the resistance R calculated by the well known formula V=IR, where V is the voltage across the tracks  52  and  54  and I is the current through one of the wires  32 . 
     In the case of the elements being capacitors the capacitance of the set of elements  56  will be the sum of the capacitance of each element  56 . 
       FIG. 3 , illustrates the PCB  50  of  FIG. 2  but when worn down to the level  40  shown in  FIG. 1 . The worn away portion is shown in phantom line. This indicates that three elements  56   a,    56   b  and  56   c  (resistors in this embodiment) have been removed or decoupled from the circuit  58  which now comprise the remaining seven resistors  56   d - 56   j  connected in parallel with each other. The removal/decoupling of the three resistors  56   a - 56   c  will change the total resistance of the circuit. This change is measured and can be calibrated with actual depth of wear of the wear plate  20  and bolt  14  so that the change in resistance gives a quantifiable measure of depth of wear. 
     A circuit diagram  60  of the circuit  58  is shown in  FIG. 4 . Elements  56   a,    56   b,    56   c,    56   d,    56   e,    56   f,    56   g,    56   h,    56   i  and  56   j  are connected in parallel by being shunted across the tracks  52  and  54 . The electrical characteristic is measured by measuring device  38 . The measuring device may be an ohmmeter, or more preferably a voltmeter where a known current enters either track  52  or  54  from one of the wires  32 , or an ammeter where a known voltage is applied across the tracks  52  and  54 . The measuring device can take other forms depending on the nature of the elements, for example, a capacitance meter, or a frequency meter. 
     Various examples of amounts of wear are shown, for example by lines  40 ,  42 ,  44  and  46 . In the case of line  42 , the wear sensor  12  is worn to the point where resistor  56   a  is removed. The resistance of the circuit would be determined by the remaining resistors i.e.  56   b - 56   j.  In the case of wear being at line  40 , the resistance of the circuit would be as calculated by the value of resistors  56   d - 56   j.  When the wear is at the extent of line  44  the value of the resistance of the circuit would be as calculated by the values of the resistors  56   g - 56   j.  When the extent of wear is at the line indicated by line  46 , the value of the resistance of the circuit would be calculated by resistors  56   i  and  56   j.  Other combinations are omitted for brevity. 
       FIGS. 2 and 3  schematically represent the tracks  50  and  52  as being laid or printed onto of a printed circuit board  50  with the elements  56  electrically coupled across the tracks  52  and  54 . However in an alternate embodiment, in order to minimise the risk of short circuiting of the circuit  58  for example by a conductive material spanning across the tracks  52  and  54 , the tracks  52  and  54  may be embedded in or sandwiched between two nonconductive layers. For example, the printed circuit board  50  may be in the form of a composite circuit board formed from first and second circuit boards which are arranged one on top of the other and where the conductive tracks  52  and  54  extended between the first and second boards. This is illustrated schematically in  FIG. 5  which shows the printed circuit board  50 ′ which is composed of first and second boards  51   a  and  51   b  which are arranged one on top of the other. The tracks  52  and  54  are printed on a face of the circuit board  51   a  which contacts an opposing face of the circuit board  51   b  of the composite PCB  50 ′. Thus the tracks  52  and  54  are in effect sandwiched between the two boards  51   a  and  51   b  together which form a nonconductive layer surrounding or wholly encasing the tracks  52  and  54 . The leads  32  can electrically contact the tracks  52  and  54  by use of conventional through hole coupling. The components  56  may either be surface mounted on say the first board  51   b  to electrically couple between the tracks  52  and  54 ; or alternately, the elements  56  may themselves be sandwiched between the two boards  51   a  and  51   b.  Providing the elements  56  as thick film print elements may be particularly suited to such an embodiment. 
     With reference to  FIG. 2 , a similar structure may be obtained by overlaying the surface of the board  50  containing the tracks  52  and  54  and the elements  56  with a nonconductive layer of material which is adhered thereto. This may also be achieved by encapsulating the printed circuit board  50  in a conventional epoxy resin encapsulant typically used in the electronics industry for the encapsulation of electronic circuits and components. 
       FIG. 6  shows an alternative example of a wear sensor system  10 , which has a plurality of wear sensors  12   a - 12   f  installed at different locations in the object  20  subject to wear. Each sensor  12   a - 12   f  has wires  32  leading to a junction box  134 . In one embodiment the junction box  134  connects the wires  32  to a cable  136  which in turn is connected to a processor  138 . In another embodiment the junction box  134  includes a measuring device which measures the electrical characteristic of each of the wear sensors  12   a - 12   f  as described above. This measurement may then be transferred as an analogue signal via cables  136  or may be converted into a digital signal and sent across cables  136  in the form of a bus to the processor  138 . 
     The processor  138  in one embodiment measures the electrical characteristic provided by the cable  136  or receives a signal representing the measurements from the measurement device in the junction box. The processor  138  in one embodiment is configured to store each of the measurements in a storage device  140 . The processor  138  may also be arranged to compare each of the measurements to a threshold. When the threshold is reached the processor  138  activates an output device  142  which can comprise some type of sensory alert such as an audible siren or a visual alarm (e.g. turning an extinguished light ON or flashing a light). 
     In one embodiment the processor  138  includes measuring circuitry. Typically the processor  138  is on the form of a computer comprising a microprocessor operated under the control of the instructions of a computer program. The computer program is typically stored in a computer readable storage medium such as a memory, flash drive, CD, DVD, hard disc drive etc. The storage device  140  may be for example a memory flash drive, hard disc drive, network storage etc. The output device  142 : in addition, or as an alternate, to providing some type of sensory alert; may be controlled by the processor  138  to provide a visual representation on a display  200  (shown in  FIG. 7 ) of the measurements taken from each of the wear sensors over time or an instantaneous view of the extent of wear that has occurred to each of the wear sensors. 
       FIG. 7  provides an example of an image on the visual display  200  of the extent of wear occurring to wear sensors  12   a,    12   b  and  12   c  from  FIG. 6 . The image on the visual display  200  is in the form of a graph. The graph includes a dashed line  202  which indicates the starting level of each of the wear sensors  12   a - 12   c.  At a given point in time the extent of wear to each of the wear sensors  12   a - 12   c  is determined and graphically represented by the drop in the height of the bars  204 ,  206  and  208  respectively from line  202 . In other words the height of the bars reflects the number of elements remaining on each sensor  12   a - 12   c.  Another dashed line  210  represents a threshold level, which if crossed will trigger an alarm indicating that the extent of wear to the object, such as a wear plate, is at a safe working limit and needs to be replaced. Solid line  212  represents an approximation of the surface profile of the object  20  according to the individual heights of bars  204 ,  206 ,  208  and the extremities of the object  20 . 
     In an alternate embodiment a more sophisticated form of graphical representation can be provided. For example a three dimensional representation could be provided by also providing bars representing the extent of wear to  12   d,    12   e  and  12   f.  Indeed a three dimensional relief map may be constructed from the output of sensors  12  to provide a visual representation of the geometry of the surface of the object  20 . 
     The extent of wear that has occurred is the difference between line  202  and the height of the respective bar. This difference can be shown as an amount above the base of the graph, rather than as a drop from the line  202 . Other forms of data visualisation can be provided, such as slices through selected wear sensors. 
     Rate of wear with the respect to time can be calculated and used to extrapolate when the wear will reach a particular threshold. Furthermore deviations from the expected rate can also be determined and used to extrapolate, for example, the hardness of material or some other property of the material to which the object  20  is exposed. 
     In order to provide greater sensitivity as wear progresses to a critical level the contribution of the electrical elements to the measured electrical characteristic of the circuit can be varied with distance from the surface of the object  20  (prior to wear). For example the contribution by each of the elements may be grouped with each contribution in the group according to one criterion, such as them all being the same resistance or being linear within the group. The contribution of each of the elements within another group can meet another criteria, such as for example their contribution being linear but at a different rate or being exponential, or meeting some other criteria. 
     Furthermore the location of each of the elements along a wear path of each wear sensor may be equally spaced or may be spaced differently, according to the position of the element within the wear path. For example a higher level of granularity may be required as the amount of wear reaches a critical level. Consequently the spacing made of elements may be closer together at a certain position within the wear path than the spacing of the elements at another position. 
     When the elements are resistors, to accommodate the change in resistance as elements in one example the resistors can be arranged so that resistors with a lower resistance are worn away before higher values of resistance are worn away. For some implementations, this produces a more effective change in the value of the resistance of a circuit as the elements are worn away. 
     The length of the wear sensor  12  can be as anticipated required according to the thickness of the object being measured that is, the length of the wear path. Furthermore the spacing of the elements or their connection in the circuit  58  need not be uniform. For example as the amount of wear increases, higher granularity may be required, thus the elements may be closer to each other in a region within which the amount of wear becomes critical. An example of this is illustrated schematically in  FIG. 8  which shows an embodiment of an electric circuit  58   a  where the electrical components  56   a - 56   j  are progressively closer together in a direction of wear W of the objection which is subject to wear and being monitored by a wear sensor comprising the circuit  58   a.  Thus in this example, the electrical element  56   a  is the first of the elements that will electrically decoupled from a circuit  58   a  as wear progresses. The element  56   j  will be the last element removed or electrically decoupled. Further, the spacing between any three consecutive elements progressively decreases. For example, looking at elements  56   b,    56   c  and  56   d,  the spacing between elements  56   c  and  56   d  is smaller than the spacing between elements  56   b  and  56   c.  The smallest spacing is between the lines  56   i  and  56   j.  Thus as wear increases, there is a increased rate of change in the measured electrical characteristic (in this case total resistance) of the circuit  58   a.    
     The value of the electrical elements may be grouped so that the value of the change of resistance changes according to a desired function of the extent of wear. The value of the resistors may be selected so that the change in the resistance is substantially linear. The wear sensor  12  is inserted into the head  16  and any air gaps may be filled by a non-conductive filler. 
     The wear sensor may be used to measure the extent of wear to an object along a path which need not extend into the object. Instead the path may be for example across a surface of the object. 
       FIG. 9  illustrates an alternate circuit  58   b  which may be incorporated in another embodiment of the wear sensor. In this embodiment, the circuit  58   b  comprises electrical elements  56   a - 56   j  and the conductive tracks  52  and  54 . However, the geometry or configuration of the connection of the elements  56  between the tracks  52  and  54  is different. In this embodiment, one end of each element  56  has a first short lead or conductor that is connected to the track  52 , while an opposite end of the elements  56   a - 56   j  have long leads or conductors  57   a - 57   j  respectively (hereinafter referred to in general as “conductor  57 ”) which follow a rectangular like path to connect at an opposite end to the track  54 . In this embodiment, the conductor  57  of each of the elements  56  may for example be printed conductive tracks on a portion of a print circuit board  50 . Furthermore, the circuit board  50  can comprise a first portion  50   a  which carries the substantive length of the tracks  57 , and a second portion  50   b  that carries the components  56 . The sensor can be arranged so that, with reference to  FIG. 1  the portion  50   a  is disposed in a portion of the passage  30  which extends through the head  16  of the fastener  14  while the second portion  50   b  in a part of the passage  30  within the shank  18  of the fastener  14 . In this embodiment, as wear progresses, the conductors  57  are progressively worn away thereby electrically decoupling the elements  56  from the circuit  58   b  although the elements  56  themselves are never subject to wear. This provides the possibility of forming the printed circuit board and the circuit  58  as two components, a first sacrificial component comprising the conductors  57  on portion  50   a,  and a second reusable component or portion which comprises the resistive elements  56  on portion  50   b.  In this embodiment the first and second portions  50   a  and  50   b  can be electrically and physically selectively connectable together by means of pins and sockets. Indeed the connection may be for example by a ribbon cable so that the second portion is located distant or remote from the object  20 . 
     Modifications and variations as would be apparent to a person skilled in the art are deemed to be within the scope of the present invention the nature of which is to be determined by the above description and the appended claims.