WO2011123896A1

WO2011123896A1 - Monitoring device

Info

Publication number: WO2011123896A1
Application number: PCT/AU2011/000403
Authority: WO
Inventors: Tim Hausler; Tony Mathison
Original assignee: MIPAC Pty Ltd
Current assignee: MIPAC Pty Ltd
Priority date: 2010-04-07
Filing date: 2011-04-07
Publication date: 2011-10-13
Anticipated expiration: 2012-10-07
Also published as: AU2011238427B2; AU2011238427A1

Abstract

A monitoring device comprising sensing means adapted to measure one or more operational parameters of at least one electrode in an electrowinning or electrorefining cell, transmission means adapted to transmit information about the one or more operational parameters and retention means adapted to retain the monitoring device on the at least one electrode.

Description

Monitoring Device

Field of the Invention

The present invention relates to a monitoring device. In particular, the present invention relates to a monitoring device that allows for the monitoring of one or more parameters relating to the operation of an electrowinning or electrorefining cell.

Background Art

Conventional electrowinning or electrorefining cells typically contain a multiple, and sometimes dozens, of cathode and anode plates. In these cells, it is common to monitor the electrical current and voltage across the entire cell in order to ensure that the cell is operating efficiently.

In general, fluctuations in the cell voltage or current indicate a fault within the cell (for instance a short circuit or the like). However, while fluctuations in the cell voltage or current may be identified, there is no indication as to the location of these faults within the cell. Thus, in order to determine where within the cell the fault may lie, an operator must test individual anode/cathode pairs within the cell until the location of the fault is found. In cells in which large numbers of anode/cathode pairs are present, this is a time-consuming and laborious process.

Thus, there -would be an advantage if it were possible to provide a means of monitoring one or more operational parameters of an electrowinning or electrorefining cell to enable simple and fast identification of the location, and even the nature, of a fault within the cell.

It will be . clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country'.

Throughout this specification, the term "comprising" and its grammatical equivalents shall be taken to have an inclusive meaning unless the context of use indicates otherwise. Summary of the Invention

It is an object of the present invention to provide a monitoring device which may overcome at least some of the abovementioned disadvantages, or provide a useful or commercial choice.

In one aspect, the invention resides broadly in a monitoring device comprising sensing means adapted to measure one or more operational parameters of at least one electrode in an electrowinning or electrorefining cell, transmission means adapted to transmit information about the one or more operational parameters and retention means adapted to retain the monitoring device on the at least one electrode.

While it will be understood that the monitoring device could be adapted to be retained on either an anode or a cathode, it is envisaged that the monitoring device is most likely to be used in connection with a cathode.

Any suitable sensing means may be used in the monitoring device of the present invention. Preferably, however, the sensing means comprises one or more electronic or electrical sensors. The sensing means may be formed with the monitoring device^'in any suitable manner. However, in a preferred embodiment of the invention, the sensing means may be associated with the retention means.

The sensing means may be adapted to monitor any suitable operational parameter of the electrode. For instance, the sensing means may be adapted to monitor the current in an electrode, and, preferably, a cathode. In a preferred embodiment, a plurality of cathodes within an electrowinning or electrorefining cell may each be provided with a monitoring device. In this way, information regarding individual cathodes within a cell may be rapidly determined. In some embodiments of the present invention, the monitoring device may be provided with a plurality of independent sensing means. Any suitable sensing means may be used to monitor the current. For example, the sensing means may comprise one or more Hall Effect sensors for monitoring the cathode current. In embodiments of the invention in which the current of the cathode is monitored, the sensing means may further comprise one or more sensors adapted to determine contact and plate resistance. In this embodiment, one or more additional electrical contacts may be provided between the monitoring device and the cathode plate.

The monitoring device may include computational means for computing one or more operational parameters from the data measured by the sensing means. Alternatively, the monitoring device may send or transmit data to a computational means for determining the one or more operating parameters from the data measured by the one or more sensors.

In some embodiments, the monitoring device of the present invention further comprises transmission means adapted to transmit information regarding the one or more operational parameters. Any suitable transmission means may be used, including transmission means that transmit the information to electronic receiving means such as, but not limited to, hardwired transmission means (i.e. communications cables, electrical cables, fibre optics and the like), wireless transmission means or a combination thereof. In embodiments of the invention in which wireless transmission means are used, the wireless transmission means may comprise radio transmission means. A skilled addressee will understand that any suitable radio transmission means may be used, including Bluetooth, cellular transmission means and the like. In these embodiments of the invention, the information transmitted by the transmission means may be received by one or more receiving means. Any suitable receiving means may be used, such as, but not limited to, one or more servers, routers, PDAs, mobile telephones, remote computer stations or the like, or any combination thereof.

In other embodiments, the transmission means may comprise one or more indicators for transmitting a visual and/or aural signal to a user, such as an audible warning signal or a light signal. The visual and/or aural signal may be transmitted at all times, for instance to indicate normal operation of the cell, or a visual and/or aural signal may be transmitted only when a fault is detected. In other embodiments of the invention, a first visual and/or aural signal may be transmitted when normal operation is experienced, while a second visual and/or aural signal may be transmitted when a fault is detected.

Any suitable visual signal may be used. For instance, a flashing light may indicate a fault has been detected. Alternatively, a first visual signal (such as a green light) may be transmitted during normal operation, while a second visual signal (such as a red light) may be transmitted when a fault is detected. In some embodiments, the visual signal to indicate a fault may change depending on the nature of the fault detected. For instance, a light may flash in a particular sequence or at a particular rate to indicate a particular type of fault that has been detected. A visual signal may also be provided if a fault is detected in the monitoring device itself. The visual signal may be provided by a conventional light bulb, halogen light bulb, LED, compact bulb or any other suitable light-emitting device. In this embodiment of the invention, the status of the visual signal may be monitored either manually (for instance, an operator may periodically check the status of the visual signal) or automatically. Automatic monitoring of the visual signals may be achieved using any suitable technique. For instance, a camera may be used to transmit images of the visual signals to a control room or the like. Alternatively, an image analysis system may be used to monitor the visual signals. The image analysis system may be of any suitable type. In some embodiments, the image analysis system may be programmed to recognise when a visual signal is activated (such as when a fault occurs). Alternatively, the image analysis system may be programmed to recognise when a change in visual signal occurs (such as a change in a green light to a red light, indicating a fault). In a preferred embodiment, the image analysis system may be adapted to transmit information to any suitable receiving means, such as, but not limited to, one or more servers, routers, PDAs, control systems (such as a DCS), mobile telephones, remote computer stations or the like, or any combination thereof. Similarly, an aural signal, such as an alarm or siren, may be transmitted when a fault is detected.

Any suitable operational parameters of the electrode may be monitored by the sensing means. For instance, the sensing means may monitor one or more of the following parameters: current, contact resistance, plate resistance, plate current efficiency, plate power efficiency, temperature or the like. Plate current efficiency may be calculated by integrating the current over the time the plate has spent in the electrowinning or electrorefining cell, and comparing this figure to the mass of metal stripped from the cathode. Individual plate power efficiency may be calculated in a similar manner. Temperature sensors may be used to monitor the temperature of the anode and cathode plates. As plate temperature rises, information regarding the temperature may be transmitted to warn a user that the temperature is approaching the temperature of the ignition point of vapour above the cell. In this way, the present invention not only provides for improved cell performance, but also acts as a safety device. Temperature sensors may also be used to correct readings taken from other monitoring devices in same cell, it being appreciated that readings from other sensors may vary with temperature.

The monitoring of the current in cathodes within the cell allows for the fast and simple detection of the location of a short circuit within the cell. An indication that a short circuit has occurred may be relayed to a user either by transmission of data electronically (for instance to a server, the information being presented to a user on a monitor, screen, control panel or the like) or by the presence of a visual and/or aural signal to indicate the exact location of the short circuit. Alternatively, both techniques may be used to alert a user to the existence and/or location of a short circuit.

In some embodiments of the invention, the monitoring device may further be provided with identification means adapted to identify the electrode to which the monitoring device is attached. This will allow the determination of cathode plate performance and enable predictive maintenance to be performed on under-performing cathodes. Suitable identification means for this purpose may include RFID . or contact technology sensors, a barcode, QR code or the like, or a combination thereof. As will be understood, a number of components of the monitoring device may require the input of power in order to function, such as the transmission means, computational means, visual signals, aural signals, and so on. Any suitable power source may be used, including any suitable external power source connected to the monitoring device (one or more batteries, solar cells, generators, or sources of mains powers or the like). Alternatively, the monitoring device may be adapted to draw power directly from the electrowinning or electrorefining cell on which it is used. For instance, in a preferred embodiment of the invention, the monitoring device may be adapted to draw its power from stray inductance (electrical, magnetic etc) in the electrode (and particularly the cathode).

In embodiments of the invention in which the monitoring device is adapted to draw- power from the cell, the monitoring device may draw the correct current directly from the cathode. Alternatively, it may be necessary to convert cathode current to a suitable current for use by the monitoring device. Any suitable conversion means may be used to convert the current, such as a transformer of the like.

In an embodiment of the invention, one or more AC power supplies may be used to power the monitoring devices. The frequency of the AC power supply is preferably at least 1 kHz. Preferably, the frequency of the AC power supply is between 5kHz and l OOkHz. In a preferred embodiment, the frequency is between 10kHz and 30kHz.

In some embodiments of the invention in which DC current is used to power the cells, AC current from the additional AC power supplies may be superimposed onto the DC current to power the monitoring device(s). Preferably, there is an equal amount of current flowing through each set of anode and cathode plates. Preferably, the current flowing through each set of anode and cathode plates is flowing in the same direction.

In some embodiments, the AC power supply induces current in the monitoring devices to hereby power the monitoring devices.

In some embodiments of the invention, the monitoring device(s) may include resonant AC current transformers to induce power from the superimposed AC current. Preferably, the frequency of the AC power supply is sufficient to generate the desired waveform for powering the monitoring device(s).

Preferably, capacitors are connected on either side of the AC power supplies to prevent interference from the DC current. Switches and/or potentiometers may be connected in parallel with any number of cells in order to control the current flowing through them (e.g. to disable one or more cells).

In some embodiments of the invention, the monitoring device may be adapted to automatically detennine the location of an electrode within an electrowinning or electrorefining cell (or indeed an electrowinning or electrorefining plant). In this way, a user may remove the monitoring device from the cathode (for instance, for maintenance) then replace the monitoring device in any position in the plant without resulting in the monitoring device transmitting incorrect or corrupted data.

The monitoring device may be adapted to determine its location using any suitable technique. Preferably, however, the monitoring device may be adapted to detennine its location on the cell using infrared communication. In this embodiment of the invention, the monitoring device may be provided with infrared communication means such that adjacent monitoring devices may communicate with one another to determine the location of adjacent monitoring devices. From this, any given monitoring device may determine its own correct location.

Preferably, the monitoring device is provided with infrared communication means allowing the monitoring device to communicate with external communications such as a server or router. In this embodiment of the invention, a first monitoring device may be able to communicate with a server or router either directly or via an adjacent second monitoring device using infrared communication should communications between the first monitoring device and the server or router be blocked. In this embodiment, the first monitoring device communicates with the adjacent second monitoring device which in turn communicates with the server or router. Conversely, a server or router may communicate with a monitoring device either directly or through an adjacent monitoring device, for instance when reprogramming of the monitoring device is required. In this way, a single monitoring device may be reprogrammed via a server or router and the remaining monitoring devices may be reprogrammed by communication between adjacent monitoring devices, thereby reducing the cost and time taken to reprogram the monitoring devices.

5

Reprogramming of the monitoring devices may be required when changes are being made to the layout of the electrowinning or electrorefining circuit, when updated software is being installed or when a software fault has been detected in order to rectify the software fault.

10

The retention means are adapted to retain the monitoring device on the electrode. Thus, any suitable retention means may be used that fulfill this criterion. For instance, the retention means may comprise one or more hooks, loops, clamps, clips, or the like. Alternatively, the retention means may comprise one or more hook and loop patches 15 (e.g. Velcro) adapted to engage with similar patches on the cathode.

In some embodiments, the retention means may comprise one or more apertures into which fixing means may be located to fix the monitoring device to the electrode. Any suitable fixing means may be used, such as, but not limited to, one or more nails, 20 bolts, rivets, ties or the like, or a combination thereof.

In further embodiments of the invention, the retention means may comprise one or more straps. In this embodiment of the invention, the one or more straps may be adapted to encircle at least a portion of the cathode or the cathode hanger bar. In this 25 way, the monitoring device may be fixed to the cathode. The one or more straps may be adjustable so that the straps may be tightened or loosened as required.

In embodiments of the invention in which one or more straps are provided, it is envisaged that the sensing means may be associated with the one or more straps. For 30 instance, the sensing means may be formed integrally with at least one of the one or more straps such that the sensing means also encircles at least a portion of the cathode or the cathode hanger bar. Alternatively, the sensing means may be formed separately from the one or more straps and then placed in association with the one or more straps when the monitoring device is in use.

For instance, the sensing means may comprise a current measurement loop that, when placed so as to encircle a portion of the cathode hanger bar, may generate information relating to the current in the cathode. In addition, the sensing means located in this way may also function to provide power to the monitoring device, such as by acting as a transformer for the power supply.

Preferably, the monitoring device is powered only when the cathode to which it is fixed is drawing power. In other words, once the cathode is removed from the cell, the monitoring device will not be operational.

In some embodiments, two or more different types of retention means may be provided.

The sensing means may be of any suitable form, as previously described. However, in a preferred embodiment of the invention, all components of the sensing means are provided on one or more printed circuit boards (PCBs). In some embodiments, the sensing means and/or the computational means and/or the transmission means may be hermetically sealed within the monitoring device, for instance within a body or housing. In a preferred embodiment of the invention, the body or housing may be adapted to be placed in abutment with a cathode or cathode hanger bar. For instance, the body or housing may be adapted to be placed against an upper surface of a cathode or cathode hanger bar.

The monitoring device may be fabricated from any suitable material. For instance, the body or housing of the monitoring device may be fabricated from metal, plastic, fibreglass or the like, or a combination thereof. Preferably, however, the monitoring device is fabricated from a material that is capable of tolerating the conditions encountered in and around electrowinning or electrorefining cells. Thus, in a preferred embodiment of the invention, the monitoring device may be fabricated from chemical-resistant plastic and, in particular, acid-resistant plastic. The monitoring device may be fabricated using any suitable technique. Preferably, however, the monitoring device is fabricated using a moulding process.

It is envisaged that the monitoring device will be associated with a single cathode for substantially the entire working life of the monitoring device.

Brief Description of the Drawings.

An embodiment of the invention will be described with reference to the following drawings in which:

Figure 1 illustrates a monitoring device according to an embodiment of the present invention; and

Figure 2 illustrates monitoring devices according to an embodiment of the present invention installed in an electrorefining cell.

Figure 3 is an electrical diagram illustrating one way in which the electrorefining cells and the monitoring devices can be powered according to an embodiment of the present invention.

Figure 4 illustrates the direction of current flow through each electrorefining cell according to an embodiment of the present invention.

Figure 5 illustrates a circuit model for the electrorefining cells according to one embodiment of the present invention.

Detailed Description of the Drawings.

It wili be appreciated that the drawings have been provided for the purposes of illustrating preferred embodiments of the present invention and that the invention should not be considered to be limited solely to the features as shown in the drawings.

In Figure 1 there is illustrated a monitoring device 10 according to an embodiment of the present invention. The monitoring device 10 comprises a body 1 1 adapted to abut a cathode hanger bar (not shown). The computational means and transmission means (obscured) are sealed within the body 1 1 in order to protect the computational means and transmission means from physical or chemical damage.

The monitoring device 10 is provided with retention means in the form of a pair of apertures 12 that allow the monitoring device 10 to be bolted to a cathode hanger bar (not shown). In addition, the monitoring device 10 is provided with a pair of retention loops 13, 14 that are adapted to be looped around the cathode hanger bar (not shown), thereby retaining the monitoring device 10 in place.

In the embodiment of the invention shown in Figure 1 , loop 14 is provided with sensing means in the form of a current measurement loop 15. The current measurement loop 15 may be embedded within the loop 14 or may be located on a surface of the loop 14. The current measurement loop 15 may be used to measure the current in the cathode hanger bar (and therefore the cathode) and this information may then be relayed to the computational means (obscured) which compute whether the operational current of the cathode is in a normal operating range.

In Figure 1, the transmission means (obscured) transmit information calculated by the computational means regarding the current to a pair of LEDs including a red LED 16 and a green LED 17. During periods in which the cathode current is within a normal operating range, the green LED 17 will be illuminated. However, if the computational means detects a fault (such as a variation in the current as measured by the current measurement loop 15 outside a normal operating range), the transmission means will transmit a signal to illuminate the red LED 16 instead of the green LED 17. In this way, a visual signal may be sent to clearly and quickly indicate that a fault has occurred.

The monitoring device 10 of Figure 1 is provided with identification means in the form of a QR code 18. The QR code 18 provides a unique identifier for the monitoring device 10, meaning that the performance of a particular cathode may be tracked throughout its operational life, regardless of the location of the cathode within the electrorefining or electrowinning plant. In Figure 2 there is shown a plurality of monitoring devices 10 located on adjoining cathode hanger bars 39 within an electrorefining cell. The monitoring devices 10 are located on an upper surface of the hanger bars 19 so that the LEDs 1 6, 17 may be easily seen by an operator, conventional camera or image analysis camera. In a typical electrorefining plant, cathodes 20 are periodically removed from the electrorefining cell to allow metal (such as copper) to be stripped from the cathode 20. The stripped cathode may then be returned to a cell, although it is likely that the cathode 20 will be placed in a different location within the plant. Thus, by associating a monitoring device 10 with a single cathode 20, the performance of the cathode 20 may be tracked regardless of where in the plant the cathode 20 is positioned. If the cathode 20 is underperforming consistently, it may be removed for maintenance or in order to be discarded. This is aided by the fact that the monitoring device 1 0 is provided with a unique QR code 18 identifier which allows the cathode to be quickly and easily identified.

Further, by attaching the monitoring devices 10 to the cathodes 20 only, the construction of the monitoring device 10 is simplified in that it is only necessary to provide an electrical or magnetic contact with the cathode, and no electrical connections to the intermediate busbar are required.

In Figure 3, there is provided an electrical circuit diagram illustrating electrowinning or electrorefining cells 31 ; and how the monitoring devices (not shown in Figure 3) can be powered. Each electrowinning or electrorefining cell 31 in Figure 3 comprises a series of cathode and anode plates 32 (each pair of cathode and anode plate is herein referred to as a electrode pair) and two bus bars 33, 34 located at two sides of the cell 31. The cells 3 1 are connected in series across a rectifier 35 outputting DC current. The rectifier 35 may convert AC current from a main supply into DC current for powering the cells 31. Any suitable number of cells 31 may be connected in series across the rectifier 35, and the cells may be grouped into sections 37 for ease of operation and maintenance. Each section 37 may comprise any number of cells 31 , for example, four cells are grouped together for each section 37 as illustrated in Figure 3. A switch 36 is connected in parallel with each section 37 of cells 31. When the switch 36 is closed, the cells 31 and their respective monitoring devices in the corresponding section 37 are disabled (or 'turned off) as the current is bypassed through the switch 36. ·

In some embodiments, if the monitoring devices are powered by induced power from an AC signal, the DC current from the rectifier 35 may not be able to power them. In these cases, one or more additional AC power supplies may be introduced into the circuitry, for example in Figure 3, two AC power supplies 38, 39 are utilised, with a first AC power supply 38 coupled between the positive terminal of the rectifier 35 and point 42 in the circuit between two cells 31 , and a second AC power supply 39 coupled between the negative terminal of the rectifier 35 and point 42 in the circuit between two cells 31. In this embodiment, the frequency of the AC current is between 10kHz to 30kHz so that a desired waveform may be generated to power the monitoring devices. To prevent interference from the DC current, capacitors 40 are utilised on either side of the AC power supplies.

In the electrical circuit diagram of Figure 3, the high frequency AC voltage is effectively superimposed onto the DC voltage, with the DC- voltage typically being significantly higher than the AC voltage to ensure the polarity of the cells is not reversed. In the illustrated embodiment,- the DC component is utilised to power the ceils and the AC component is utilised to power the monitoring devices.

Figure 4 is an illustration showing the direction of current flow through each electrowinning or electrorefining cell 31 according to an embodiment of the present invention. Current through the cell 31 flows from one bus bar 33, through the anode plates, the electrolyte, the cathode plates and the monitoring devices 41 (only one shown in Figure 4), to the second bus bar 34.

Figure 5, is an illustration of a circuit model^' for the electrowinning or electrorefining ^' cells 31 according to one embodiment of the present invention. Each cell 31 may be modelled by resistors 51 , wherein the resistance of each operating electrode pair is modelled by the resistance of various elements within the ceils 31 connected in series with each other. For instance, resistor 51 a represents the resistance of the cathode plate, resistor 51b represents the resistance of the electrolyte, and resistor 51 c represents the resistance of the corresponding anode plate. Resistors 51 a, 51 b and 51 c are connected in series. The total resistance 55 is then connected in parallel with the total resistance of other electrode pairs 55 in the cell 31. This model may be used to determine the required output current and voltage for the additional AC power supplies 38, 39.

For example, in a plant of 500 cells per rectifier, 45 electrode pairs per cell and the cathode plate, electrolyte, and corresponding anode plate for each operating electrode pair having the nominal resistance values as specified below:

Using the circuit model in Figure 5, the total resistance of each cell R_r , may be calculated as follows:

If each monitoring device requires 3 - 5A to operate, and one monitoring device is allocated to each electrode pair, the current (I) required to power the monitoring devices in each cell would be approximately:

This would also be the output current requirement for the AC power supplies as shown in Figure 3.

The voltage (V) output required from each AC power supply, if arranged substantially as shown in the circuit diagram of Figure 3 (although not all 500 cells are shown in Figure 3), may be calculated using:

Therefore, two AC power supplies with an output current of 135A 1» 225A and an output voltage of 0.34V and 0.56V would be required to power the monitoring devices installed in a plant of 500 cells per rectifier, according to the embodiment shown in Figure 3.

Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It will be understood that the present invention encompasses all such variations and modifications that fall within its spirit and scope.

Claims

The claims defining the invention are as follows:

1 . A monitoring device comprising sensing means adapted to measure one or more operational parameters of at least one electrode in an electrowinning or electrorefining cell, transmission means adapted to transmit information about the one or more operational parameters and retention means adapted to retain the monitoring device on the at least one electrode.

2. A monitoring device according to claim 1 , wherein the sensing means is associated with the retention means.

3. A monitoring device according to any one of the preceding claims, wherein the sensing means is adapted to monitor the current in the at least one electrode.

4. A monitoring device according to claim 3, wherein the sensing means comprises one or more Hall Effect sensors.

5. A monitoring device according to any one of the preceding claims, wherein the at least one electrode is a cathode.

6. A monitoring device according to claim 5, wherein the sensing means further comprises one or more sensors adapted to measure contact and plate resistance of the cathode.

7. A monitoring device of claim 5 or 6, wherein one or more electrical contacts are provided between the monitoring device and the cathode.

8. A monitoring device according to any one of the preceding claims, wherein the monitoring device further comprises computational means for computing the one or more operational parameters measured by the sensing means.

9. A monitoring device according to any one of the preceding claims, wherein the transmission means comprises wifeless transmission means.

10. A monitoring device according to any one of the preceding claims, wherein the information transmitted by the transmission means is received by one or more receiving means.

1 1 . A monitoring device of claim 10, wherein the receiving means comprises one or more servers, routers, PDAs, mobile phones, remote computer stations or the like, or any combination thereof.

12. A monitoring device according to any one of the preceding claims, wherein the monitoring device further comprises an indicator for transmitting a signal to a user to indicate when the electrowinning or electrorefining cell is operating normally and/or when a fault has been detected.

13. A monitoring device of claim 12, wherein the signal transmitted by the indicator is a visual and/or an aural signal.

14. A monitoring device according to any one of the preceding claims, wherein the monitoring device further comprises an identification means adapted to identify the electrode to which the monitoring device is attached.

15. A monitoring device of claim 14, wherein the identification means comprises one or more RFID or contact technology sensors, barcodes, QR codes or the like, or any combination thereof.

16. A monitoring device according to any one of the preceding claims, wherein the monitoring device is adapted to automatically determine the location of an electrode within an electrowinning or electrorefining cell.

17. A monitoring device according to any one of the preceding claims, wherein the monitoring device has infrared communication means for communication with a server or router.

18. A monitoring system comprising at least one monitoring device as claimed in any one of the preceding claims, wherein at least one AC power supply is used to power the at least one monitoring device.

19. A monitoring system of claim 18, wherein the AC power supply is substantially between 5kHz and 100 kHz.

20. A monitoring system as claimed in claim 18 or 19, wherein the AC power supply induces current in the at least one monitoring device to power the at least one monitoring device.