Patent Publication Number: US-2022214296-A1

Title: Method of Examining the Electrical Properties of Objects using Electric Fields

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 16/185,447, filed on Nov. 9, 2018, which claims priority from United Kingdom Patent Application number 1718678.4, filed on Nov. 11, 2017. The whole contents of U.S. patent application Ser. No. 16/185,447 and United Kingdom Patent Application number 1718678.4 are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method of examining electrical properties of objects using electric fields. 
     It is known to examine electrical properties of objects using electric fields, as described in U.S. Pat. No. 8,994,383, assigned to the present applicant. Electrodes may be supported by a dielectric membrane. A strobing voltage may be applied to energise a first input electrode as a transmitter and an output voltage may be monitored on an output receiver electrode. An external electric field is generated that may pass through an object, such that an output signal will be influenced by electrical properties of the object, including the permittivity of the object. The output signal is usually sampled at a sample point during each strobing operation to facilitate digital processing. 
     A problem with known systems is that it can be difficult to obtain sufficient information to fully identify properties of an object. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of examining electrical properties of objects, using electric fields, comprising the steps of: arranging an object on an apparatus having a first electrode and a second electrode; energising said first electrode during a first strobing operation of a scanning cycle; monitoring said second electrode during said first strobing operation; energising said second electrode during a second strobing operation of said scanning cycle; and monitoring said first electrode during said second strobing operation. 
     Thus, in this way, the first electrode is not dedicated as a transmitter electrode and the second electrode is not dedicated as a receiver electrode. Each of these electrodes can perform both functions, thereby enhancing the amount of information that can be derived from the apparatus, without making significant modifications to the apparatus itself. 
     In an embodiment, additional electrodes are provided. The method may then further comprise the steps of energising all of the additional electrodes during respective strobing operations of the scanning cycle; and monitoring all of said additional electrodes at appropriate strobing operations of the scanning cycle. 
     Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings. The detailed embodiments show the best mode known to the inventor and provide support for the invention as claimed. However, they are only exemplary and should not be used to interpret or limit the scope of the claims. Their purpose is to provide a teaching to those skilled in the art. 
     Components and processes distinguished by ordinal phrases such as “first” and “second” do not necessarily define an order or ranking of any sort. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows an environment in which an examination apparatus is deployed; 
         FIG. 2  details the examination apparatus identified in  FIG. 1 ; 
         FIG. 3  shows a schematic representation of the functionality of the apparatus shown in  FIG. 2 ; 
         FIG. 4  illustrates the generation of electric fields; 
         FIG. 5  shows an alternative configuration of the apparatus shown in  FIG. 3 ; 
         FIG. 6  illustrates electric fields generated in response to the configuration of  FIG. 5 ; 
         FIG. 7  shows an alternative configuration of the apparatus shown in  FIG. 3  and  FIG. 5 ; 
         FIG. 8  shows resulting electric fields from the configuration of  FIG. 7 ; 
         FIG. 9  illustrates an examination period; 
         FIG. 10  shows a schematic representation of the examination apparatus shown in  FIG. 2 ; 
         FIG. 11  shows a schematic representation of a strobing circuit of the type identified in  FIG. 9 ; 
         FIG. 12  shows an example of a multiplexing environment of the type identified in  FIG. 10 ; 
         FIG. 13  shows an example of a monitoring circuit of the type identified in  FIG. 10 ; 
         FIG. 14  shows an overview of procedures performed by the processor identified in  FIG. 10 ; 
         FIG. 15  details procedures for scanning electrodes identified in  FIG. 14 ; and 
         FIG. 16  shows an alternative examination apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     
       FIG. 1 
     
     An examination apparatus  101  is shown in  FIG. 1  for examining electrical properties of objects, using electric fields. The examination apparatus  101  communicates with a data processing system  102  via a data communication cable  103 , possibly designed in accordance with established USB protocols. 
     
       FIG. 2 
     
     The examination apparatus  101  is shown in greater detail in  FIG. 2 . It includes a plurality of parallel electrodes, including a first electrode  201  and a second electrode  202 , along with a plurality of additional electrodes  203 . Electrodes  201  to  203  are supported by a dielectric insulating membrane  204  and the electrodes are then covered by an insulating material to ensure that the surface of the examination apparatus is non-conductive. 
     The examination apparatus  101  is arranged to examine the electrical properties of entities, such as an object  205 . In known systems of the type illustrated in  FIG. 2 , electrodes have dedicated functionality; in that they are either energised, to provide a transmitter electrode, or monitored to provide a receiver electrode. Thus, in known systems, each electrode is identified exclusively as a dedicated input electrode or a dedicated output electrode. 
     The present embodiment provides an apparatus in which a first electrode  201  is configured to be energised during a first strobing operation of a scanning cycle and a second electrode  202  is configured to be monitored during this first strobing operation. Thus, in conventional systems, the first electrode  201  would be dedicated as a transmitter electrode and the second electrode  202  would be dedicated as a receiver electrode. However, in accordance with the present invention, during part of the same scanning cycle, the second electrode  202  is configured to be energised during a second strobing operation and the first electrode  201  is configured to be monitored during this second strobing operation. Thus, within the same overall scanning cycle, data is obtained by using the first electrode  201  as a transmitter and the second electrode  202  as a receiver. Subsequently, these roles are reversed, such that additional data is received by energising the second electrode  202  as the transmitter, with the first electrode  201  being scanned as a receiver. To achieve this, additional electronics are required and operations performed by a microcontroller must ensure that any one electrode is not energised and scanned simultaneously as part of the same strobing operation. 
     In an embodiment, as illustrated in  FIG. 2 , the additional electrodes  203  are also configured to be energised during respective strobing operations, such that the first electrode  201 , the second electrode  202  and the additional electrodes are all configured to be monitored at appropriate strobing operations of a scanning cycle. 
     
       FIG. 3 
     
     In the embodiment of  FIG. 2 , the first electrode  201 , the second electrode  202  and the plurality of additional electrodes  203  define substantially parallel tracks, as shown schematically in  FIG. 3 . Electric fields are generated between adjacent ones of said tracks, as will be described with reference to  FIG. 4 . Alternative arrangements of tracks are possible, an example of which will be described with reference to  FIG. 16 . 
     As used herein, a scanning cycle consists of unique sequential strobing operations performed in a particular order. During an examination, the scanning cycle may be repeated and characteristics of the scanning cycle may be adjusted. However, without making any adjustments of this type, the cycle is repeated periodically at a rate primarily determined by electrical characteristics of the examination apparatus  101 , clock speed and the number of strobing operations performed within each cycle. In this embodiment, a strobing operation consists of energising a selected electrode to generate an electric field. This in turn capacitively couples to other electrodes; such that the scanning operation is completed by selecting an adjacent electrode to be monitored. This provides an output signal that is sampled and then processed within the digital domain. 
     As previously stated, known apparatus dedicate each electrode to being either an input (transmitter) electrode or an output (receiver) electrode. In accordance with an embodiment of the present invention, any electrode  201  to  203  can be selected to receive an energising signal or can be selected to be monitored and thereby produce an output signal. 
     A schematic representation for achieving this functionality is illustrated by a switching device  301 . The switching device  301  receives input energising signals on an input line  302 . Similarly, the switching device  301  provides output signals on an output line  303 . 
     The first electrode  201  is connected to a first switch  304  within the switching device  301 . Similarly, the second electrode  202  is connected to a second switch  305 . In this embodiment, a third electrode  306 , of the additional electrodes  203 , is connected to a third switch  307 . A fourth electrode  308  is connected to a fourth switch  309 . A fifth electrode  310  is connected to a fifth switch  311  and a sixth electrode  312  is connected to a sixth switch  313 . Similarly, a seventh switch  314  connects to a seventh electrode  315 , with an eighth electrode  316  being connected to an eighth switch  317 . 
     Each of switches  304 ,  305 ,  307 ,  309 ,  311 ,  313 ,  314  and  317  includes a first contact  318 , a second contact  319  and a third contact  320 . For each of the switches, the first contact  318  is connected to the input line  302 . Similarly, the third contact  320  is connected to the output line  303 . The second contact  319 , positioned between the first contact  318  and the third contact  320 , does not provide a connection at all and, in this embodiment, presents an open circuit to its respective electrode such that, in the terminology of the art, the electrode is left floating when connected to the second contact  319 . In an alternative embodiment, the second contact  319  may be connected to ground. 
     In the configuration shown in  FIG. 3 , the first switch  304  connects the input line  302  to the first electrode  201  such that, during the next strobing operation, the first electrode  201  will be energised. Furthermore, in the configuration of  FIG. 3 , the third contact  320  of the second switch  305  connects the second electrode  202  to the output line  303 . Thus, during a strobing operation identified above, the second electrode  202  will be monitored while the first electrode  201  is energised. A schematic representation of this strobing operation, when viewed in the direction of arrow  400 , is illustrated in  FIG. 4 . 
     
       FIG. 4 
     
     A cross-sectional view of the examination apparatus of  FIG. 2  is illustrated in  FIG. 4 , when viewed in the direction of arrow  400  (of  FIG. 3 ). The object  205  has been placed on the examination apparatus  101 . The first electrode  201  is shown in cross-section, along with the second electrode  202 , the third electrode  306 , the fourth electrode  309  and the fifth electrode  311 . In the embodiment, the apparatus extends to the right to include electrodes  313 ,  314  and  317 . The first electrode  201 , the second electrode  202  and the additional electrodes are supported by the dielectric membrane  204 . This is in turn covered by an insulating coating  401 , thereby insulating the electrodes  201 ,  202  etc. from the object  205 . 
     A conducting ground plane  402  is provided to shield the apparatus from external electrical noise. An intermediate layer  403  is also provided between the dielectric membrane  204  and the ground plane  402  that, in an embodiment, may include response enhancement properties. 
     During a strobing operation, an electric field is produced between the first electrode  201  and the second electrode  202 , as illustrated by electric field lines  404 . These represent capacitive coupling, that occurs given that the first electrode  201  is providing the functionality of a transmitter electrode and the second electrode  202  is providing the functionality of a receiver electrode. During a strobing cycle, the first electrode  201  is energised and the second electrode  202  is monitored. 
     The electric field lines  402  show that the electric field penetrates the object  205 . A useful depth of penetration is indicated at  405 . The distance between the electrodes is indicated at  406 . Experiments conducted by the inventor suggest that the useful depth of penetration  405  is approximately half of the distance  406  between the electrodes. 
     
       FIG. 5 
     
     In this embodiment, for the next strobing operation of the scanning cycle, the first switch  304  is activated to connect the third contact  320  to the first electrode  201 . Similarly, the second switch  305  is activated to connect the second electrode  202  to a second first contact  501 . Thus, in this configuration, electrode  202  now performs a transmitter function, with the first electrode  201  performing a receiver function. 
     
       FIG. 6 
     
     The result of this switching operation, from the configuration of  FIG. 3  to the configuration of  FIG. 5 , results in a reversal of functionality, such that the second electrode  202  becomes a transmitter and the first electrode  201  becomes a receiver. 
     Thus, as illustrated in  FIG. 6 , the direction of the electric field lines  404  has reversed. 
     
       FIG. 7 
     
     In this embodiment, for the next strobing operation, the first switch  304  is activated, the second switch  305  is activated and the third switch  307  is activated. The first switch  304  connects the first electrode  201  to the second contact  319 , such that the first electrode  201  will not contribute to the next strobing operation. Switch  305  remains in position, connected to the second first contact  501 , such that, again, the second electrode  202  will provide the functionality of a transmitter. However, on this strobing operation, the third switch  307  has been activated to connect a third electrode  701  to the output line  303 , thereby causing the third electrode  306  to provide the functionality of a receiver. 
     
       FIG. 8 
     
     Upon initiating a strobing operation for the configuration described with reference to  FIG. 7 , an electric field  801  is generated, as illustrated in  FIG. 8 . Thus, the second electrode  202  provides transmitter functionality and the third electrode  306  provides receiver functionality. 
     Thus, in this embodiment, each electrode sequentially adopts the functionality of a transmitter. When given this functionality, a first strobing operation monitors an electrode immediately to the left, followed by a second strobing operation that monitors the electrode immediately to the right. The sequencing then advances and the roles of the electrodes are changed. 
     
       FIG. 9 
     
     During a working period, many objects may be examined. The duration of an examination is illustrated in  FIG. 9 . Similar procedures are performed for each object and a particular examination of an object starts by arranging the object on the apparatus, as described with reference to  FIG. 2 . 
     During an examination process  901 , electrodes are energised sequentially and the procedure may be referred to informally as “scanning”. As used herein, a complete scan cycle is performed when all unique combinations of transmitters and receivers have been exercised. Thus, during the examination  901 , many scan cycles may be performed. For the purposes of this illustration, during examination procedure  901 , a first scan cycle  902  is performed, followed by a similar second scan cycle  903  and a similar third scan cycle  904 . 
     During each scan cycle, such as scan cycle  902 , many strobing operations are performed, including a first strobing operation  905 , a second strobing operation  906 , and a third strobing operation  907  etc. Each strobing operation is unique, in terms of the particular electrode selected as the transmitter in combination with the particular electrode selected as the receiver. Each strobing operation consists of energising the selected transmitter electrode and monitoring the selected receiver electrode. 
     Due to capacitive coupling, each monitoring process monitors a voltage at the receiver electrode. To determine electrical properties of objects, a measurement is required. In a preferred embodiment, this measurement is achieved by performing a process of analog to digital conversion, thereby allowing the result of this conversion to be processed within the digital domain. 
     In  FIG. 9 , strobing operation  905  takes place within a monitored duration  908 . Within the monitored duration  908 , a sampling instant  909  occurs, representing an instant within the monitored duration at which an output voltage is sampled. 
     In order to optimise results received from the examination process, the sampling instant does not occur immediately following the generation of an input strobing signal. Although, in an embodiment, a sharp, rapidly-rising strobing input signal is supplied to the transmitters, the shape of resulting output signals will not rise so steeply; as a result of the electrical properties of the device and the electrical properties of the objects. Thus, to optimise the value of the information derived from the procedure, the sampling instant  909  is delayed by a predetermined delay period  910 . 
     
       FIG. 10 
     
     A schematic representation of the examination apparatus  101  is illustrated in  FIG. 10 . This provides an apparatus for examining electrical properties of objects, using electric fields. A number of substantially parallel electrodes are supported on a dielectric membrane, as described with reference to  FIG. 2 . In the representation of  FIG. 10 , the dielectric membrane, with parallel electrodes, is included within a multiplexing environment  1001 . In addition to the dielectric membrane, the multiplexing environment  1001  includes a de-multiplexer for selectively de-multiplexing multiplexed energising input voltage pulses for application to each of the electrodes, along with a multiplexer for selectively multiplexing output signals monitored from each of the electrodes, as described with reference to  FIG. 12 . 
     A processor  1002 , implemented as a microcontroller, controls the de-multiplexer and the multiplexer to ensure that the same electrode cannot both be energised and monitored during a strobing operation. 
     An energising circuit  1003  is energised by a power supply  1004  that in turn may receive power from an external source via a power input connector  1005 . A voltage control line  1006  from (a digital to analog convertor within) the processor  1002  to the energising circuit  1003  allows the processor  1002  to control the voltage (and hence energy) of energising signals supplied to the multiplexing environment  1001 , via a strobing line  1007 . The timing of each strobing signal is controlled by the microcontroller  1002  via a trigger signal line  1008 . 
     An output from the multiplexing environment  1001  is supplied to an analog processing circuit  1009  over a first analog line  1010 . A conditioning operation is performed by the analog processing circuit  1009 , allowing analog output signals to be supplied to the microcontroller  1002  via a second monitoring line  1011 . The processor  1002  also communicates with a two way data communication circuit  1012 , thereby allowing a data interface  1013  to connect with the data communication cable  103 . 
     In operation, the processor  1002  supplies addresses over address busses  1014  to the multiplexing environment  1001  in order to achieve the functionality described with reference to  FIGS. 3 to 8 . Thus, having supplied addresses to the multiplexing environment  1001 , a strobing voltage is supplied via strobing line  1007 , resulting in an output signal being supplied to the processor  1002 . At the processor  1002 , an analog input signal is sampled to produce a digital representation and, in an embodiment, this digital data is uploaded to the data processing system  102  via the data interface  1013 . 
     
       FIG. 11 
     
     An example of the energising circuit  1003  is shown in  FIG. 11 . The energising circuit  1003  consists of a voltage control circuit  1101  connected to a strobing circuit  1102  via a current limiting resistor  1103 . 
     A voltage input line  1104  receives energising power from the power supply  1004  to energise an operational amplifier  1105 . The operational amplifier  1105  is configured as a comparator and receives a reference voltage via feedback resistor  1106 . This is compared against a voltage control signal, received on the voltage control line  1006 , to produce an input voltage for the strobing circuit  1102 . 
     In the embodiment of  FIG. 11 , the strobing circuit  1102  includes two bipolar transistors configured as a Darlington pair, in combination with a MOSFET. This creates strobing pulses with sharp rising edges and sharp falling edges that are conveyed to the strobing line  1008 . 
     
       FIG. 12 
     
     An example of a multiplexing environment  1001  is detailed in  FIG. 12 . The switching functionality, described with reference to  FIG. 3 , is achieved by the provision of a first multiplexing device  1201  and a second multiplexing device  1202 . In this alternative embodiment, a dielectric membrane  1203  supports sixteen parallel electrodes  1204 . 
     The address busses  1014  include an input address bus  1205  and an output address bus  1206 , for addressing the first multiplexing device  1201  and the second multiplexing device  1202  respectively. The addressing space for the input address bus  1205  and the output address bus  1206  may be similar, which may assist in terms of ensuring that the same address cannot be supplied simultaneously to both the input address bus  1205  and the output address bus  1206 . 
     The first multiplexing device  1201  also includes a first enabling line  1207 . Similarly, the second multiplexing device  1202  includes a second enabling line  1208 . In operation, addresses are supplied to the input address bus  1205  and to the output address bus  1206  but line selection does not actually occur until the multiplexing devices receive a respective enabling signal. 
     The first multiplexing device  1201  receives an input pulse from the energising circuit  1003  via the strobing line  1008 . Multiple strobing operations are performed, such that an input energising voltage is supplied sequentially to electrodes performing a transmitter function. Strobing signals are distributed to multiple inputs; therefore, the first multiplexing device  1201  should be seen as performing a de-multiplexing operation. 
     The second multiplexing device  1202  performs a multiplexing operation, in that multiple output signals are selected sequentially and then combined onto the first monitoring line  1010  for reception by the monitoring circuit  1009 . Thus, in this embodiment, the multiplexing environment is established by a single first multiplexing device for input signals and a single second multiplexing device for output signals, both of which are connected to all sixteen of the available electrodes. Furthermore, if a greater number of electrodes are present upon a dielectric membrane, it is possible for additional multiplexing devices to be provided such that, for example, a pair of multiplexing devices may provide the input de-multiplexing function and a further pair of multiplexing devices may provide the multiplexing output function; provided that an appropriate addressing space has been established. 
     During a strobing operation, an input address is supplied on the input address bus  1205  and an output address is supplied on the output address bus  1206 . The addresses are enabled such that, at a particular point in time, the output multiplexer  1202  is enabled and as such is then configured to monitor output signals on the addressed output electrodes. The selected input electrode is then energised by the application of a strobing pulse, which may be considered to occur at the start of arrow  910  shown in  FIG. 9 . 
     A short, predetermined delay, for the duration of arrow  910 , occurs before the sampling instant  909  occurs; taking a sample of the voltage monitored on the output electrode. In this embodiment, the first monitoring line  1010  applies an output analog voltage to the analog processing circuit  1009  for the duration of the strobing operation, such as strobing operation  905 . The analog voltage is conditioned by the analog processing circuit  1009 , which in turn supplies a conditioned voltage to the processor  1002  via the second monitoring line  1011 . Digital-to-analog conversion then takes place within the processor  1002 , such that the point at which the sampling instant  909  occurs is determined by the processor. 
     
       FIG. 13 
     
     An example of an analog processing circuit  909  is illustrated in  FIG. 13 . Signals received on the first monitoring line  1010  are supplied to a buffering amplifier  1301  via a decoupling capacitor  1302 . During an initial set-up procedure, a variable feedback resistor  1303  is trimmed to optimise the level of monitored signals supplied to the processor  1002  via the second monitoring line  1011 . A Zener diode  1304  prevents excessive voltages being supplied to the processor  1002 . 
     
       FIG. 14 
     
     An overview of procedures performed by the processor  1002  is illustrated in  FIG. 14 . After an initial switch-on, possibly initiated by the data processing system  102 , the sensor array is calibrated at step  1401 . This enables a reference level to be established, prior to the application of an object, such as object  205 . 
     After the application of an object, the electrodes are scanned at step  1402 . As previously described, each scan consists of a plurality of strobing operations with each strobing operation consisting of a unique combination of transmitter electrode and receiver electrode. 
     At step  1403 , data is processed and the degree of local data processing will depend upon the processing capabilities provided by the processor  1002 . 
     In an embodiment, the level of received monitored signals may be compared against a reference and, where appropriate, a control voltage on the voltage control line  906  may be adjusted. 
     More sophisticated processing may be achieved by the data processing system  102 , therefore the data is supplied as an output to the data processing system  102  at step  1404 . Thereafter, further scanning is performed at step  1402  and the procedures are repeated until a de-energisation command is received. 
     
       FIG. 15 
     
     Procedures  1402  for scanning the electrodes are detailed in  FIG. 15 . At step  1501 , an input electrode is selected; which would be the first electrode  201  on the first iteration. At step  1502  a question is asked as to whether there is an N minus one (N−1) electrode which, if present, is selected at step  1503 . Thereafter, the input electrode N selected at step  1501  is energised and the electrode before it, N minus one, is monitored at step  1504 . 
     On a first iteration, the first electrode will have been selected; therefore, an N minus one electrode does not exist. Consequently, the question asked at step  1502  will be answered in the negative. 
     At step  1505  a question is asked as to whether there is an N plus one electrode (N+1) which, when answered in the affirmative, results in a selection of this electrodes as a monitoring electrode at step  1506 . Thus, at step  1507  electrode N is energised and electrode N plus one (N+1) is monitored. On the first iteration, an N plus one electrode is present, therefore the energisation at step  1507  is as illustrated in  FIG. 4 , with the first electrode  201  being a transmitter and the second electrodes  202  being a receiver. Thus, output data is generated. 
     Thereafter, a question is asked at step  1508  as to whether another input electrode is present which, when answered in the affirmative, results in the next input being selected at step  1501 . In this example, this will result in the second electrode  202  being selected at step  1501  and the question then asked at step  1502  will be answered in the affirmative, given that the first electrode  201  will now be selected as the N minus one (N−1) electrode. Consequently, the second electrode  202  is energised and the first electrode  201  is monitored, as illustrated in  FIG. 6 . 
     Thereafter, at step  1505  a question is asked as to whether there is an N plus one (N+1) electrode, which would be answered in the affirmative; resulting in the third electrode  306  being selected at step  1506 . Consequently, at step  1507 , the second electrode  202  will be energised and the third electrode  306  will be monitored, as illustrated in  FIG. 8 . Thereafter, the question asked at step  1508  will be answered in the affirmative and the process will be repeated, this time with the third electrode  306  being the energised transmitter electrode N. 
     Thus, the question asked at step  1508  will continue to be answered in the affirmative until all of the electrodes have been selected. This will result in the establishment of a complete cycle such that, at step  1509 , a question is asked as to whether the cycle is to repeat. When answered in the affirmative, the first electrode is selected again at step  1501 . 
     The procedure provides a method of examining electrical properties of objects, using electric fields. An object is arranged on an apparatus having a first electrode and a second electrode, as described with reference to  FIG. 2 . The first electrode is energised during a first strobing operation of a scanning cycle and a second electrode is monitored during this first strobing operation. Usually, this would establish electrodes as being specifically dedicated for a transmitting operation or a reception operation. However, by providing a sophisticated multiplexing environment, as described with reference to  FIG. 12 , it is possible to then energise the second electrode during a second strobing operation, while monitoring the first electrode during this second strobing operation; as part of the same scanning cycle. 
     
       FIG. 16 
     
     An alternative examination apparatus  1601  is shown in  FIG. 16 . A first electrode  1602  and a first plurality of additional electrodes provide a first set of substantially parallel tracks; substantially similar to the arrangement described with respect to  FIG. 12 . However, the second electrode  1603  and a second plurality of electrodes provide a second set of substantially parallel tracks; where both sets of tracks are mounted on opposite sides of a dual-sided membrane  1604 . 
     The second set of electrodes, including second electrode  1603 , is substantially orthogonal to the first set of electrodes, including the first electrode  1602 , and electrically insulated therefrom. The first electrode and the first plurality of electrodes are energised (sequentially) while the second electrode and the second plurality of electrodes are sequentially monitored. Thereafter, the second electrode and the second plurality of electrodes are energised while the first electrode and the first plurality of electrodes are monitored. 
     A first alternative multiplexing device  1605  supplies energising signals to the first set of electrodes. A second alterative multiplexing device  1606  receives output signals from the first set of electrodes. Similarly, a third alterative multiplexing device  1607  supplies energising signals to the second set of electrodes and a fourth alternative multiplexing device  1608  receives output signals from the second set of electrodes. 
     During a scanning cycle, the first alterative multiplexing device  1605  is operative, in combination with the fourth alternative multiplexing device  1608 , such that, for part of the cycle, the first set of electrodes are energised and the second set of electrodes are scanned. Thereafter, as part of the same cycle, the third alternative multiplexing device  1607  is energised; thereby energising the second set of electrodes and the second alternative multiplexing device  1606  is addressed.