GB2505190A

GB2505190A - Level sensing in a vehicle fuel tank using electromagnetic fields

Info

Publication number: GB2505190A
Application number: GB1214861.5A
Authority: GB
Inventors: David Lynn; Jonathan Primrose
Original assignee: Schrader Electronics Ltd
Current assignee: Schrader Electronics Ltd
Priority date: 2012-08-21
Filing date: 2012-08-21
Publication date: 2014-02-26
Also published as: GB201214861D0; WO2014029776A1

Abstract

A level sensing apparatus 11 includes an antenna structure 22 having an active component 24 to which an electrical signal is applied during use, and a reference component 26. A controller 20 causes the electrical signal to be applied to the antenna structure 22 to cause the active component 24 to radiate an electromagnetic field which interacts with the liquid in the container 14. The controller also monitors the electrical characteristics of the antennas electrical circuit. The controller causes the apparatus to switch between first and second modes of operation by causing a switch 30 to connect the reference component to, or isolate it from, the electrical circuit by connecting to or disconnecting from electrical ground 28. In the first mode of operation the reference 26 is connected by switch 30 to ground 28 and the electromagnetic field radiated by active component 24 is contained primarily between antenna components 24, 26. In the second mode, reference component 26 is disconnected from ground 28 by switch 30 and the electromagnetic field radiated by active component 24 is not contained between antenna components 24, 26, instead it propagates until it interacts with a conductive reference body 12. In the case where 14 is a vehicle fuel tank 12 is the vehicle body. The level sensor is more accurate for levels up to the height h of the antenna structure when operated in the first mode and more accurate for levels above the height h of the antenna structure when operated in the second mode. The invention allows more accurate sensing of level when the level is above the height of the sensor.

Description

Adaptable System and Method for Level Sensing in a Container

Field of the Invention

The present invention relates to level sensing in a container, especially vehicle tanks, for example fuel tanks or urea tanks. The invention relates particularly to the use of electromagnetic fields to sense liquid or fluid level.

Background to the Invention

It is known to use electromagnetic (EM) fields to sense liquid levels. Typical systems include an EM field-generating sensor located in or beside the liquid to be monitored. Interaction between the liquid and the electromagnetic field causes the liquid to act as an electrical load on a sensing circuit. The load is dependent on the liquid level and so the liquid level can be detected by the circuit. United States patent US 7,926,341 discloses an example of such a system in the context of a vehicle fuel tank.

A problem with conventional systems is that the sensor is not well suited to sensing changes in liquid level when the liquid level is above the height of the sensor. A solution to this problem is to match the size of the sensor to the size of the container. However, this may be impractical. For example economic considerations may dictate that the same system be suitable for different sizes of container.

It would be desirable therefore to provide a liquid or other fluid level sensing system that is better suited for use in containers of varying sizes.

Summary of the Invention

A first aspect of the invention provides a level sensing apparatus comprising: an electrical circuit, typically a resonant circuit, including an antenna structure having an active component to which and electrical signal is applied during use, and a reference component; a controller configured to cause said electrical signal to be applied to said antenna structure and to monitor at least one electrical characteristic of said electrical circuit; and a switch for selectably isolating said reference component from said electrical circuit, wherein said controller is configured to cause said apparatus to switch between first and second modes of operation by causing said switch to connect said reference component to, or isolate said reference component from, said electrical circuit respectively.

Typically, said controller is configured to cause said apparatus to switch between said first and second modes of operation depending on said at least one monitored electrical characteristics. In particular, the controller may be configured to determine a fluid level from said at least one electrical characteristic and to cause said apparatus to switch between said first and second modes of operation depending on said determined fluid level. In a preferred embodiment, the controller is configured to cause said apparatus to adopt said first mode of operation when said determined fluid level is not above said antenna structure during use, and preferably to cause said apparatus to adopt said second mode of operation when said deteimined fluid level is above said antenna structure during use.

The active component and/or the reference component typically comprises an inductor, typically a coil, preferably a spiral coil.

In preferred embodiments, the active and reference components comprise respective elongate structures that are substantially parallel with one another, and which preferably overlap with one another. Advantageously, the active and reference components are offset with respect to one another in a common axial direction.

Typically, the active and reference components each comprises a respective coil, respective turns of each coil being offset and/or interleaved with respect to one another.

The controller conveniently comprises a processor programmed to monitor said at least one characteristic of said electrical circuit, and may include a signal generator for generating said electrical signal, preferably an RE signal generator.

Said at least one electrical characteristics of said circuit may comprise at least one voltage level in the circuit and/or one or more resonant characteristics of the circuit.

From another aspect the invention provides a level sensing system comprising the level sensing apparatus of the first aspect of the invention, the system further comprising a conductive reference body, typically being electrically connected to electrical ground, said antenna structure being sufficiently close to said conductive reference body that said conductive reference body acts as a reference plane to said active component in said second mode of operation. The conductive reference body may be electrically connected to a current return path of said electrical circuit.

Typically, the system further includes a power source, wherein said conductive reference body is electrically connected to a negative terminal of said power source.

In some embodiments, said level sensing apparatus is installed in a vehicle, and said conductive reference body comprises a part of the body of the vehicle, conveniently a wheel arch. The antenna structure may be installed on or in a tank or container of said vehicle, e.g. a urea or fuel tank.

Another aspect of the invention provides a level sensing method using an antenna structure comprising an active component and an isolatable reference component, the method comprising: causing an electrical signal to be applied to the active component of said antenna structure; monitoring at least one electrical characteristic of an electrical circuit associated with said antenna structure; and switching between first and second modes of operation by causing said reference component to be connected to, or isolated from, said electrical circuit respectively.

Further advantageous aspects of the invention will become clear to those ordinarily skilled in the art upon review of the following description of a preterred embodiment and with reference to the accompanying drawings.

Brief Description of the Drawings

An embodiment of the invention is now described by way of example and with reference to the accompanying drawing in which: Figure 1 is a schematic view of a level sensing system embodying one aspect of the invention, including a level sensing apparatus embodying another aspect of the invention; and Figure 2 is a block diagram of a preferred embodiment of the level sensing apparatus of Figure 1.

Detailed Description of the Drawings

Referring now to Figure 1 of the drawings, there is shown, generally indicated as 10, a level sensing system embodying one aspect of the invention. The system comprises a level sensing apparatus 11 installed in a vehicle, which is represented in Figure 1 by part of the vehicle body 12, for example a wheel arch.

The system 10 is configured to monitor the level of liquid (not shown) in a container 14, which may tor example comprise a fuel tank, urea tank or other tank used by the vehicle for storing liquid.

The system 10 includes an electrical power source 16, which may comprise a battery, conveniently the battery of the vehicle, although any other convenient electrical power source may be provided.

The negative terminal of the power source 16 is connected to a reference voltage, typically electrical earthlground 28. In the illustrated embodiment, the earth/ground 28 is a circuit earth/ground for the electrical components of the system 10 and is connected to an external earth/ground 29 via a filter circuit 31.

The filter circuit 31 provides protection for the system 10 against EMC disturbances, and any suitable filter topology may be used for this purpose. The external ground 29 is assumed to be the vehicle ground in this example.

As is described in more detail hereinafter, the vehicle body 12 serves as a conductive reference body in a second mode of use of the system 10. To this end, the vehicle body 12 is connected to a reference voltage 33, or potential, typically electrical ground/earth. The body 12 is electrically connected to the negative terminal of the power source 16 to provide a return path for current in the second mode of use. Any other convenient electrical connection may alternatively or additionally be provided to provide a current return path from the body 12 to the circuitry of the system 10. Conveniently, reference points 29, 33 may be the same point electrically.

The level sensing apparatus 11 includes a sensor 18 (Fig. 2) and a control system, or controller 20. The sensor 18 comprises an antenna structure 22 which, in preferred embodiments, is located inside the container 14. Typically, the antenna structure 22 is positioned within the container 14 such that it is wholly or partly immersed in the liquid depending on the level of liquid in the container 14. The arrangement is such that, in use, an electromagnetic (EM) field generated by the antenna structure 22 extends into the liquid in which it is wholly or partly immersed.

The antenna structure 22 comprises an active component 24 to which, in use, an electrical signal is applied by the controller 20 to cause the active component 24 to radiate an EM field. Typically, the active component 24 is configured to resonate at at least one resonant frequency, and the electrical signal is applied at an operating frequency, or within an operating frequency band, that causes the active component 24 to resonate thereby radiating the EM field. Conveniently, the operating frequency matches or substantially matches the, or one of, the resonant frequencies.

The antenna structure 22 also comprises a reference component 26 that is selectably connectable to a reference point 35 having a reference potential, typically electrical ground. In this example the reference point 35 is conveniently the circuit ground 28. In any event, the reference point 35 is electrically connected to the apparatus 11 to provide a return path for current. The apparatus 11 comprises a switch 30 operable to electrically connect or isolate the reference component 26 to or from the reference point 35. The switch 30 is operated by the controller 20 as is described in more detail hereinafter.

The level sensing apparatus 11 is operable in first and second modes. In the first mode, the reference component 26 is connected to the reference point 35.

Hence, the reference component 26 is included in an electrical circuit that includes the controller 20 and sensor 18, and in which the reference component 26 serves as a reference plane, or ground plane, for the active component 24 of the antenna structure 22. Consequently, the EM field radiated by the active component 24 is contained primarily between the antenna components 24, 26.

However, the configuration is such that a portion of the EM field, which may be referred to as fringe fields, extends beyond the components 24, 26 and so may interact with the liquid in the container 14. By way of example, the fringe fields typically extend no more than approximately 5 mm beyond the antenna structure 22.

In the second mode of operation, the reference component 26 is disconnected from the reference point 35. Hence, the reference component 26 is disconnected from the electrical circuit that includes the controller 20 and sensor 18, and does not serves as a reference plane, or ground plane, for the active component 24.

Consequently, the EM field radiated by the active component 24 is not contained between the antenna components 24, 26 and propagates beyond the antenna structure 22, i.e. it propagates substantially freely from the antenna structure 22 until it interacts with a conductive reference body.

In the second mode, the part 12 of the vehicle body serves as a conductive reference body, and in particular as a reference, or ground, plane with which the EM field radiating from the active component 24 interacts. In this mode, the body 12 provides a current return path in contrast with the first mode where the current return path is provided via the reference component 26.

The active component 24 may comprise any physical structure that is suitable for radiating an EM field, for example a conductive plate, wire or track, any of which may be formed as a discrete component or a trace on a PCB board as convenient. It is preferred that the active component 24 comprises a coil, or inductor. In preferred embodiments, the active component 24 comprises a coil inductor formed by one or more conductive tracks in or on a PCB or other suitable substrate. The coil may be spiral-like comprising coplanar or substantially coplanar coil turns, e.g. concentric or substantially concentric coil turns, each for example being provided as a conductive track in a FCB or other suitable substrate. Alternatively, the coil may comprise a solenoid.

The reference component 26 may comprise any physical structure that is suitable for serving as a reference or ground plane, for example a conductive plate, wire or track, any of which may be formed as a discrete component or a trace on a POB board as convenient. It is preferred that the reference component 26 comprises a coil, or inductor. In preferred embodiments, the reference component 26 comprises a coil inductor formed by one or more conductive tracks in or on a PCB or other suitable substrate. The coil may be spiral-like comprising coplanar or substantially coplanar coil turns, e.g. concentric or substantially concentric coil turns, each for example being provided as a conductive track in a PCB or other suitable substrate. Alternatively, the coil may comprise a solenoid.

The active and reference components 24, 26 typically comprise respective elongate structures that are substantially parallel with, and/or overlapping with, one another. The spacing between the structures, or between respective

B

adjacent parts, e.g. coil turns, of the structures, may for example be approximately 0.5mm to 10mm. It is preferred that the respective structures are not perfectly aligned with one another, e.g. that they are offset with respect to one another in their longitudinal directions. This mis-alignment facilitates the creation of the fringe EM field of the sensor 18. In the preferred embodiment where both structures comprise coils, it is preferred that the respective loops of each coil are offset with respect to one another, and, preferably still, interleaved with one another. Typically, the structures 24, 26 are substantially vertically disposed during use, or substantially perpendicular to the ground surface supporting the vehicle. As would be apparent to a skilled person, the length of the structures 24, 26 can be chosen to suit the application and may depend on one or more of the following factors: depth of container 14, desired inductance or desired 0 value.

In use, when the liquid level in the container 14 is below a threshold level, the apparatus 11 operates in the first mode. The liquid in which the sensor 18 is wholly or partly immersed interacts with the EM field generated by the sensor 18.

In this case the interacting EM field comprises the fringe field(s) around the antenna structure 22. As is described in more detail hereinafter, the interaction between the liquid and EM field affects one or more electrical characteristics of the sensor 18, the extent of the, or each, effect depending on the liquid level.

The control system 22 is configured to monitor one or more of the affected electrical characteristics and so to determine the liquid level in the container 14.

When the liquid level in the container 14 exceeds the threshold level, the apparatus 11 operates in the second mode. The liquid in which the sensor 18 is immersed interacts with the EM field established between the sensor 18 (and in particular the antenna structure 22) and the vehicle body 12. As is described in more detail hereinafter, the interaction between the liquid and EM field affects one or more electrical characteristics of the sensor 18, the extent of the, or each, effect depending on the liquid level. The control system 22 is configured to monitor one or more of the affected electrical characteristics and so to determine the liquid level in the container 14.

It is found that liquid level measurements taken in the first mode of operation are accurate when the liquid level does not exceed the height /i of the sensor 18, i.e. when the liquid level is not above the sensor 18. However, when the liquid level is above the sensor 18, the sensor 18 is less responsive to changes in liquid level. In the second mode of operation, the sensor 18 is sufficiently responsive to changes in liquid level when the level is above the sensor 18 to provide accurate liquid level measurements. Accordingly, the threshold level is advantageously selected to be substantially at the in-use upper end of the sensor 18 (which in the preferred embodiment corresponds with the in-use upper end of the antenna structure 22). Alternatively, the threshold level may be set at a level above the in-use upper end of the sensor 18, e.g. at a level where the first mode is deemed to yield insufficiently accurate measurements. The threshold level may alternatively be set below the in-use upper end of the sensor 18. Moreover, since the second mode is effective irrespective of liquid level, it may be used instead of the first mode for all liquid levels. However, the first mode of operation is found to provide more accurate measurements than the second mode when the liquid level is below the upper end of the sensor 18 and so it is advantageous to switch between modes depending on the liquid level.

The controller 20 is configured to cause the apparatus 11 to switch between the first and second modes depending on the detected liquid level, i.e. when the level crosses the threshold. In the illustrated embodiment, the controller 20 achieves this by operating the switch 30 to connect or disconnect the reference component 26 to or from the reference point 35.

Referring now to Figure 2, an embodiment of the sensor 18 and of the controller are shown by way of example. The sensor 18 typically comprises a resonant circuit 40 comprising resistive, inductive and/or capacitive components. One or more resistors, inductors and/or capacitors may be provided as suits the application, and may take any suitable form, for example discrete components (e.g. mounted on a circuit board) or trace components (e.g. in or on a POB).

Alternatively or in addition, any one or more of the resistive, inductive or capacitive components may be present parasitically. The values of one or more of the resistive, inductive or capacitive components, and their topological arrangement in the circuit 40 are selected to provide the resonant circuit 40 with at least one resonant frequency that is compatible with the operational frequency of the apparatus 11. Typically, the operating frequency is in the range 6-15MHz.

In the illustrated embodiment, the operating frequency is approximately 8MHz.

In the preferred embodiment, the antenna structure 22 forms part of the resonant circuit 40 and may contribute to, or provide, one or more of the resistive, inductive and capacitive components of the circuit 40. In the preferred embodiment where the active component 24 comprises a coil/inductor, the active component typically provides or at least contributes to the inductive and resistive components of the circuit 40. In the first mode of operation, the active component 24 and reference component 26 together provide or at least contribute to the capacitive component of the circuit 40. In the second mode of operation, the active component 24 and vehicle body 12 together provide or at least contribute to the capacitive component of the circuit 40. In a simple embodiment, the antenna structure 22 may provide all of any one or more of the resistive, inductive and capacitive components of the circuit 40, although typically one or more resistors, inductors and/or capacitors are provided in order to achieve the desired resonant frequency. In Figure 2, the switch 30 is shown as part of the resonant circuit 30 but this need not be the case -the switch 30 may be provided in any convenient manner.

During use interaction between the EM field generated by the antenna structure 22 and the liquid in the container 14 affects one or more electrical characteristics of the resonant circuit 40 in a manner that is dependent on the liquid level. In particular, the respective impedance value of one or more of the resistive, inductive or capacitive components of the circuit 40 may change depending on liquid level. This may occur as a result of changes in electrical loading of the circuit 40 by the liquid at different levels. Impedance changes affect the resonant characteristics of the circuit, for example it may change the resonant frequency of the circuit. Alternatively or in addition, the EM field strength may vary depending on liquid level. For example, in the first mode of operation, liquid level is detected by interaction of liquid with the fringe field generated by the sensor 18 as it encroaches along the sensor 18. In this mode, changes in the conductivity of the liquid around the sensor 18 has a significant role in the operation of the sensor 18. In the second mode of operation, the active component 24 and the body 12 are typically separated by the liquid in the container 14, the container wall and an air gap between the container wall and the body 12. As such liquid conductivity has a less significant affect on sensor operation. In the second mode, the controller 20 may be configured to detect changes in capacitance, especially parasitic capacitance, in order to determine the liquid level.

The controller 20 is configured to detect changes in at least one of the electrical characteristics affected by the changes in liquid level, and to interpret the detected changes as corresponding changes in liquid level. For example, this the controller 20 may achieve this by monitoring one or more voltage or current levels in the circuit 40, and/or to determine changes in the resonant frequency or other resonant characteristics of the circuit 40. The detection and interpretation performed by the controller 20 may be performed in any convenient conventional manner. US 7926341 provides examples of how this may be done.

The controller 20 includes a processor 42 which may comprise a suitably programmed microprocessor, microcontroller or other processor. The processor 42 receives signals from the sensor 18 (e.g. representative of one or more monitored voltage levels trom circuit 40) from which it determines changes in liquid level, and so acts as a level detector. The processor 42 may be further configured to report such determinations to a remote computer, for example an ECU or other control unit of the vehicle. An analogue-to-digital converter (ADC) 44 may be provided for converting signals received from the sensor 18. The ADC 44 may also be used by the processor 42 to monitor changes in resonant frequency of the circuit 40 in conventional manner.

The controller 20 typically also includes means for energising the antenna structure 22 by applying an (AC) electrical signal at the operating frequency (or within an operating frequency band). The energising means typically includes a signal generator 46, usually an RF generator, which may conveniently be controlled by the processor 42. The energising means may also include an antenna driver 48 for applying the generated signal to the antenna structure 22.

The sensor 18 may optionally be provided in a liquid-tight housing and is typically provided on a PCB or other circuit board. The sensor 18, in particular the antenna structure 22, may be provided separately from all or some of the other system components, e.g. on a separate circuit board, so that it may more readily be positioned with respect to the container 14.

In preferred embodiments, the sensor 18 is located inside the container 14.

Alternatively, it may be provided outside of the container 14, e.g. mounted on an outside surface of a wall of the container 14. In any event, the arrangement is such that the EM fields generated by the sensor 18 are able to interact with the liquid in the desired manner.

The apparatus 11 is connected to, or may include, any suitable electrical power source, typically a battery, e.g. vehicle battery 16.

To calibrate the system 10 and apparatus 11 in respect of the first mode of operation, multiple readings may be taken from the level sensor 18 when the container 14 is both empty and full. Characteristics such as fluid level, frequency, amplitude (e.g. frequency and/or amplitude of one or more signals measured from or derived from measurements taken from circuit 40), temperature (which may be measured by a separate temperature sensor) and liquid effect may be recorded for both full and empty. Using a suitable mathematical model, e.g. comprising a polynomial equation, a best fit trend line can be established for the behaviour of the sensor 18 at various liquid levels between 0-100% (empty to full). For the read above sensor mode (i.e. the second mode), a similar calibration method may be used. The liquid should still behave in accordance to the trend found in respect of the first mode. However some characteristic(s) of the sensor's response (e.g. the rate of change of the sensor's response to changing fluid levels) may change depending on the characteristics of the container and vehicle.

Further measurement of the behaviour of the sensor 18 when mounted in a particular vehicle may be made as necessary to assess the need adjust the trend lines.

It will be understood that the invention is not limited to use with containers, e.g. tanks, of vehicles and may be used in any other application where level sensing is required and where the container comprises, is incorporated into or located adjacent a structure that includes a conductive body that can serve as the refeience body in the second mode of operation (instead of the vehicle body 12).

Foi example, systems embodying the invention may be used in a stand alone tank, e.g. a domestic or commercial fuel tank, or a silo.

It will be understood that the invention is not limited to use with sensing liquid level. Embodiments of the invention may be used to sense the level of other material in a container, for example fluids, including gases or flowable (e.g. particulate) solids.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims

CLAIMS: 1. A level sensing apparatus comprising: an electrical circuit including an antenna structure having an active component to which and electrical signal is applied during use, and a reference component; a controller configured to cause said electrical signal to be applied to said antenna structure and to monitor at least one electrical characteristic of said electrical circuit; and a switch for selectably isolating said reference component from said electrical circuit, wherein said controller is configured to cause said apparatus to switch between first and second modes of operation by causing said switch to connect said reference component to, or isolate said reference component from, said electrical circuit respectively.
2. An apparatus as claimed in claim 1, wherein said controller is configured to cause said apparatus to switch between said first and second modes of operation depending on said at least one monitored electrical characteristics.
3. An apparatus as claimed in claim 2, wherein said controller is configured to determine a fluid level from said at least one electrical characteristic and to cause said apparatus to switch between said first and second modes of operation depending on said determined fluid level.
4. An apparatus as claimed in claim 3, wherein said controller is configured to cause said apparatus to adopt said first mode of operation when said determined fluid level is not above said antenna structure during use.
5. An apparatus as claimed in claim 3 or 4, wherein said controller is configured to cause said apparatus to adopt said second mode of operation when said determined fluid level is above said antenna structure during use.
6. An apparatus as claimed in any preceding claim, wherein said active component comprises an inductor.
7. An apparatus as claimed in any preceding claim, wherein said active component comprises a coil, preferably a spiral coil.
8. An apparatus as claimed in any preceding claim, wherein said reference component comprises an inductor.
9. An apparatus as claimed in any preceding claim, wherein said reference component comprises a coil, preferably a spiral coil.
10. An apparatus as claimed in any preceding claim wherein the active and reference components comprise respective elongate structures that are substantially parallel with one another.
11. An apparatus as claimed in any preceding claim wherein the active and reference components comprise respective elongate structures that overlap with one another.
12. An apparatus as claimed in any preceding claim wherein said active and reference components are offset with respect to one another in a common axial direction.
13. An apparatus as claimed in any preceding claim, wherein the active and reference components each comprises a respective coil, respective turns of each coil being offset with respect to one another.
14. An apparatus as claimed in any preceding claim, wherein the active and reference components each comprises a respective coil, respective turns of each coil being interleaved with one another.
15. An apparatus as claimed in any preceding claim, wherein said electrical circuit comprises a resonant circuit.
16. An apparatus as claimed in any preceding claim, wherein said controller comprises a processor programmed to monitor said at least one characteristic of said electrical circuit.
17. An apparatus as claimed in any preceding claim, wherein said controller comprises a signal generator for generating said electrical signal, preferably an RE signal generator.
18. An apparatus as claimed in any preceding claim, wherein said at least one electrical characteristics of said circuit comprise at least one voltage level in the circuit and/or one or more resonant characteristics of the circuit.
19. A level sensing system comprising a level sensing apparatus as claimed in any preceding claim, the system further comprising a conductive reference body, said antenna structure being sufficiently close to said conductive reference body that said conductive reference body acts as a reference plane to said active component in said second mode of operation.
20. A system as claimed in claim 19, wherein said conductive reference body is electrically connected to electrical ground.
21. A system as claimed in claim 19 or 20, wherein said conductive reference body is electrically connected to a current return path of said electrical circuit.
22. A system as claimed in any one of claims 19 to 21, further including a power source, wherein said conductive reference body is electrically connected to a negative terminal of said power source.
23. A system as claimed in any one of claims 19 to 22, wherein said level sensing apparatus is installed in a vehicle, and said conductive reference body comprises a part of the body of the vehicle, conveniently a wheel arch.
24. A system as claimed in claim 23, wherein said antenna structure is installed on or in a tank or container of said vehicle, e.g. a urea or fuel tank.
25. A level sensing method using an antenna structure comprising an active component and an isolatable reference component, the method comprising: causing an electrical signal to be applied to the active component of said antenna structure; monitoring at least one electrical characteristic of an electrical circuit associated with said antenna structure; and switching between first and second modes of operation by causing said reference component to be connected to, or isolated from, said electrical circuit respectively.