Vibration sensor

Abstract

The invention discloses a vibration sensor, wherein a transducer device (2) excites a mechanically oscillating unit (1) to mechanical vibrations and / or receives mechanical vibrations from the mechanically oscillating unit (1). The transducer device (2) has a piezoelectric actuator (20) and four electrodes (5, 6) on one side (200) of the piezoelectric actuator (20).

Description

[0001] The invention relates to a vibration sensor. The vibration sensor is used, for example, to determine and / or monitor a process variable of a medium. The process variable is, for example, the fill level or density. The medium is, for example, a liquid, a gas, or a bulk solid.

[0002] In the prior art, tuning forks are known as an example of a vibration sensor that oscillates with different frequencies and qualities depending on medium properties, such as density or viscoelasticity. The two oscillating elements, also known as fork tines, are located on a membrane and can contact the medium. They form a mechanical resonator or closed oscillating circuit, whereby the oscillation frequency and oscillation amplitude depend only on the medium properties. The energy generated by resonant oscillations in such a vibration sensor is retained in the sensor regardless of the type and design of the process connection. Energy losses are possible due to damping in the resonator and due to dissipation in the medium. For example, in viscous liquids, energy is lost due to molecular friction.

[0003] The low internal energy loss in the resonator of the vibration sensor is an important prerequisite for the proper detection of gases or liquids. Opposing reaction forces of the vibrating elements, i.e., the prongs, should be balanced in the membrane.

[0004] With regard to the application of the sensors, there are differences in the evaluation of the vibration signals: The frequency of the mechanical vibrations is a measurement parameter for applications with liquids. This is due to the significantly higher densities than with gases and the fact that low-viscosity liquids hardly dampen the vibrations. Conversely, the amplitude of the vibrations is a preferred measurement parameter for bulk solids, as such media always exhibit high damping regardless of the bulk density. Therefore, the quality factor or vibration amplitude is a measure for bulk solid detection.

[0005] A uniform deposit of sediment on both tines of the tuning fork does not impair sensor performance within a certain range. However, if there is a risk of solid structures such as chips or lumps becoming trapped between the tines, or of crystallization processes or the hardening of material (e.g., bitumen) occurring on the fork tines, so-called single-rod sensors can be used as mechanically oscillating units. This means that there is only one oscillating element in the form of a rod. In such single-rod sensors, additional elements provide mechanical compensation to prevent vibration energy from being lost via the process connection. Such compensation elements do not come into contact with the medium. If a buildup of the medium forms on the single-rod, the reaction forces are no longer compensated, or no longer sufficiently compensated.Therefore, the oscillations may collapse or the oscillation frequency may be unexpectedly high or low.

[0006] Single rods are mostly used for bulk solids applications and are mechanically compensated for air, i.e., when uncovered. When immersed in the bulk solid, compensation is no longer important because the bulk solids dampen the vibrations so strongly that the sensor does not vibrate at all when covered. This is in contrast to a liquid as the medium. The single rod continues to vibrate when covered, and the medium imparts an additional moment of inertia to the tines, depending on the density of the medium. This causes a change in frequency and also means that the sensor is no longer compensated. Without compensation, however, vibration energy is lost. In conjunction with the influence of components coupled to the sensor, such as the container wall or fittings, the vibrations can break down or the vibrating unit can oscillate at an incorrect frequency.

[0007] Another issue arises from the temperature dependence of the stiffness and damping of the material of the oscillating unit. For example, the stiffness of the material of a tuning fork decreases with increasing temperature, leading to a reduction in the oscillation frequency. The mechanical quality factor of the resonator is also temperature-dependent and can vary by up to a factor of 10 depending on the application conditions. Temperature changes can, for example, lead to switching thresholds being exceeded incorrectly. Therefore, to expand the application range with regard to the process temperature, temperature sensors are used, for example, to account for temperature drift in the switching threshold. However, this makes switching reliability dependent on these temperature sensors.

[0008] Overall, there is a need to develop vibration sensors that function reliably under as many application conditions as possible and preferably independent of temperature and approach.

[0009] For example, it is known to excite the mechanically oscillating unit to oscillate in different directions in order to obtain different information about a medium to be measured or about the mechanically oscillating unit itself (see, for example, DE 100 14 724 A1).

[0010] If a flat prong vibrates parallel to the prong surface, a covering liquid will only slightly reduce the oscillation frequency, and the frequencies in air and the medium will be approximately the same. However, if such a prong vibrates perpendicular to the prong surface, the covering with a liquid creates a large frequency difference between the oscillations and the oscillations in air. This is due to the co-moving mass of medium covering the prong, which depends on the density and effective area of ​​the oscillating element, as well as on the viscoelasticity of the medium. The same applies to the oscillation amplitude, which, for resonators with forced oscillations, depends on the mechanical quality of the vibration sensor.

[0011] Thus, a measurement can be performed by evaluating, for example, the frequency or amplitude ratio between two orthogonal oscillations, especially between parallel and perpendicular oscillations. The change in frequency and amplitude depends on the type of medium. For example, parallel and perpendicular oscillations do not differ in frequency and amplitude in gaseous media. In liquids, the frequency of parallel oscillations is higher than that of perpendicular oscillations. The same applies to the amplitude. In highly viscous liquids or bulk materials, no oscillation may be possible at all.

[0012] As already mentioned above, the oscillating unit is no longer optimally compensated due to the medium being covered and transfers energy to the process connection. The extent to which the amplitude or frequency of the oscillations changes depends on the coupling of the oscillator to the process connection. However, since a relative measurement of the two oscillation directions is performed and the energy losses are the same for both directions, the energy loss due to the covering can be neglected.

[0013] To excite or receive vibrations, a transducer device is usually present that converts between electrical signals and mechanical vibrations. Piezoelectric elements are often used for this purpose. These can be arranged in a stack, for example (see, for example, WO 01 / 66269 A1). Alternatively, they are so-called bimorph drives (see, for example, DE 10 2004 010 992 B3 or DE 100 14 724 A1). These are usually discs polarized in opposite directions.

[0014] However, the prior art does not contain any concrete description of how a vibration sensor is to be designed with respect to the transducer device in such a way that the mechanically oscillatable unit can execute oscillations in different directions.

[0015] The invention is based on the object of proposing a vibration sensor whose mechanically oscillating unit can oscillate in different directions.

[0016] The object is achieved by a vibration sensor having at least one mechanically oscillatable unit and a transducer device, wherein the transducer device excites the mechanically oscillatable unit to mechanical oscillations and / or receives mechanical oscillations from the mechanically oscillatable unit, and wherein the transducer device has at least one piezo drive and four electrodes on one side of the piezo drive.

[0017] The vibration sensor according to the invention comprises a mechanically oscillatable unit and a transducer device that excites the mechanically oscillatable unit to mechanical vibrations and / or receives mechanical vibrations from the mechanically oscillatable unit. The mechanically oscillatable unit comprises at least one vibration element.

[0018] The transducer device has a piezoelectric drive—that is, at least one piezoelectric element consisting at least partially of a ceramic polarized in at least one direction—and four electrodes on one side of the piezoelectric drive. In one embodiment, for example, the electrodes are separated from each other by two insulating gaps that intersect at a right angle.

[0019] The process variable of the medium is, for example, the fill level, whereby the medium can be a bulk material or a liquid.

[0020] One embodiment consists in that the mechanically oscillatable unit has two oscillating elements, that the two oscillating elements are designed such that the two oscillating elements each have different dimensions in two orthogonal directions, in that the oscillating elements each have a wide side and a narrow side, that the four electrodes form two pairs of electrodes, and wherein the wide side of the oscillating elements is arranged above a pair of electrodes.

[0021] In this aforementioned embodiment, the mechanically oscillatable unit has - preferably exactly - two oscillating elements, each of which has different dimensions in two orthogonal directions (e.g., length and width). In one embodiment, they can therefore also be referred to as flat paddles, for example. This results in a wide side (in the paddle plane) and a narrow side (perpendicular to the paddle plane). The four electrodes form two electrode pairs. The two electrodes of each pair are preferably located next to one another. This results in one embodiment in which the electrodes of one pair are located next to one another all the way around and are then alternated by the electrodes of the other pair.

[0022] The oscillating elements, which are to be excited to oscillate or whose oscillations are to be detected, are arranged relative to the four electrodes in such a way that the wide side of the oscillating element is arranged above the two electrodes of an electrode pair. Each oscillating element is thus located above a different electrode pair. In one embodiment, the oscillating elements are preferably arranged so that the wide sides are parallel to one another. Due to this orientation, a first oscillating element is located primarily above the two electrodes of a first electrode pair, and the other, thus the second, oscillating element is located above the electrodes of the other, thus the second, electrode pair. Depending on how the electrodes are subjected to excitation signals, the oscillating elements move in different directions, i.e. in particular parallel or perpendicular to the respective wide side.Conversely, vibrations of the vibrating elements lead to different reception signals.

[0023] In one embodiment, the vibrating elements oscillate either parallel to their broad side (p-component), which in one embodiment can also be referred to as the tine surface, or perpendicular to it (s-component). A superposition of both types of vibration is also possible. In one embodiment, a measure of the viscosity of the medium is determined as a process variable from the ratio of the vibration amplitudes.

[0024] One embodiment involves the piezo drive being designed as a bimorph drive. This embodiment involves a piezo drive with at least two segments polarized in opposite directions. One embodiment features four segments, each polarized in opposite directions in pairs.

[0025] In an alternative embodiment, the piezo drive is a piezoelectric element that is uniformly polarized in one direction.

[0026] Alternatively, the piezo actuator has several individual elements that are separately and force-fitted to the membrane. In one embodiment, four individual piezoelectric elements are force-fitted to the membrane, e.g., by adhesive or soldering.

[0027] One embodiment provides that the two oscillating elements are mounted above a membrane and the transducer device is mounted below it, and that the side of the piezo drive on which the four electrodes are arranged faces away from the membrane. In this embodiment, the transducer device and the oscillating elements are indirectly coupled to one another via a membrane. The side of the piezo drive on which the four electrodes are fixed faces away from the membrane. This arrangement particularly simplifies the contacting of the electrodes.

[0028] One embodiment consists in the four electrodes being configured essentially as equally sized circular sectors. In this embodiment, the four electrodes together form a circle. They are each the same size. Therefore, they also excite equally sized sections of the piezo drive to oscillate. The same applies to the reception of the mechanical oscillations and the conversion into electrical signals. In this embodiment, in particular, each pair of electrodes covers essentially half of the side of the piezo drive covered by the electrodes. This embodiment is preferably accompanied by the piezo drive being configured essentially circular.

[0029] One embodiment provides that the two oscillating elements are essentially designed as paddles. Preferably, each is a flat paddle. The paddles are preferably arranged vertically on a membrane, to which they are coupled or connected for the transmission of forces and torques.

[0030] One embodiment consists in the vibration sensor having a control device, which applies excitation signals to the four electrodes, and which are rectangular signals. The excitation signals serve to generate mechanical vibrations. In an extension, the control device can also be used for evaluating or processing the received signals. Thus, the control device becomes a control and evaluation device.

[0031] One embodiment involves connecting the vibrating elements to a housing. The housing preferably adjoins the diaphragm and preferably also houses the connection components for the transducer device.

[0032] An alternative embodiment consists in the transducer device having at least four separate piezoelectric elements, and in the four electrodes being arranged on the end faces of the piezoelectric elements. The four individual piezoelectric elements are, in particular, polarized in only one direction and each have an electrode on one end face. Preferably, the respective end faces are all facing toward or away from the mechanically oscillating unit.

[0033] One embodiment consists in the sum of the surface areas of the electrodes assigned to the same electrode pair being substantially equal. In this embodiment, the sum of the surface areas of the electrodes of one electrode pair is substantially equal to the sum of the surface areas of the other electrode pair. In a further embodiment, the surface areas of the four electrodes are substantially identical.

[0034] The invention is explained in more detail with reference to the following figures. Fig. 1 shows a spatial representation of a vibration sensor, Fig. 2 shows a longitudinal section through the vibration sensor of the Fig. 1, Fig. 3 shows a view inside the housing of the vibration sensor of the Fig. 1 on the side of the piezo drive which is provided with the four electrodes, Fig. 4 shows two examples of the excitation of vibrations in the piezo drive of the Fig. 3, Fig. 5 shows a first example of an electrical circuit of a piezo drive with equivalent circuit diagram, Fig. 6 shows a second example of a wiring with equivalent circuit diagram, Fig. 7 shows a third example of a wiring with equivalent circuit diagram, Fig. 8 shows a fourth example of a wiring with equivalent circuit diagram and Fig. Figure 9 shows a view of an alternative design of the transducer device with four individual piezo elements.

[0035] The vibration sensor of the Fig. 1 has two oscillating elements 10 for the mechanically oscillating unit 1, which are designed in the form of flat paddles. It is important that the oscillating elements 10 extend significantly further in one direction—in the paddle plane—than in a direction perpendicular to it. The length and width therefore differ significantly from each other, so that the oscillating elements 10 have a wide side 11 and a narrow side 12. Here, the oscillating elements 10 are flat. Alternatively, at least one element has a structure.

[0036] The oscillating elements 10 are mounted on an elastic membrane 3, the inside of which is located in a housing 4 that is connected to the membrane 3.

[0037] Furthermore, a control device 8 is provided, which, together with the converter device discussed below, excites the oscillating elements 10 to oscillate. These oscillations are, in particular, in two orthogonal directions, as indicated here by the arrows. Thus, in one excitation mode, the oscillating elements 10 move along their wide sides 11 (these are the so-called parallel oscillations) and, in another excitation mode, perpendicular to the wide side 11 and thus along their narrow side 12 (this is referred to here as perpendicular oscillation).

[0038] If the frequency and / or amplitude values ​​of the two (i.e., parallel or perpendicular) oscillations are compared to each other, a process variable of interest in a medium, e.g., fill level or density, can be determined very reliably. This processing of the received signals preferably takes place in a processing unit (not shown here), which may, if necessary, be combined with the control device 8 to form a common electronic unit.

[0039] In the Fig. Figure 2 shows that the transducer device 2 is located below the membrane 3 and thus below the oscillating elements 10. The transducer device 2 converts back and forth between electrical signals and mechanical vibrations. Thus, when the transducer device 2 is subjected to an excitation signal, it generates the mechanical vibrations of the oscillating elements 10. Conversely, the transducer device 2 converts mechanical vibrations of the oscillating elements 10 into electrical signals, which are evaluated with respect to the measured variable of interest.

[0040] The transducer device 2 is protected within the housing 4 and has a bimorph drive 20 designed as a flat disc, an example of a drive consisting of at least one piezoelectric element. One part of the piezoelectric element is polarized toward the membrane 3, and another part is polarized in the opposite direction, away from the membrane 3. One end face 201 of the bimorph drive 20 is frictionally connected to the inside of the membrane 3 and carries a common electrode 202. The other face 200 faces away from the membrane 3 and—as explained below—carries the electrode structure with the four electrodes 5, 6. The four electrodes 5, 6 are located opposite the common electrode 202 here.

[0041] Consider the case where an excitation signal from electrodes 5, 6 causes both sections of the bimorph drive 20 below the electrodes 5, 6 – or more precisely: between the two electrodes 5, 6 and the common electrode 202 – to contract or expand parallel to the plane of the membrane 3. This causes the membrane 3 as a whole to bend outwards or inwards. As a result, the oscillating elements 10 move in antiphase to one another. However, if one section of the bimorph drive 20 expands while another section contracts, one area of ​​the membrane 3 bends inwards and another outwards. This causes the oscillating elements 10 to tilt in phase. Depending on the control of the electrodes 5, 6, different oscillations can thus be generated. Conversely, electrical signals can also be generated from the mechanical oscillations.

[0042] The Fig. Figure 3 shows the four electrodes 5, 6 on the side of the bimorph drive which, when installed, faces away from the membrane 3. The membrane 3, which is circular here, surrounds the bimorph drive, which is also circular. There are four electrodes 5, 6, which are implemented as equally sized circular segments and are divided into two electrode pairs. The two electrodes 5, 6 of each pair are arranged adjacent to one another. The electrodes of one pair thus follow one another around the circumference and are then alternated by the electrodes of the other pair. The circular segments of the two pairs each have an axis of symmetry. Between the electrodes 5, 6 there are two insulating gaps 7 which are perpendicular to one another at a center point of the arrangement of the electrodes 5, 6.

[0043] The position of the oscillating elements 10 relative to the four electrodes 5, 6 is shown. It can be seen that the paddle shape of the oscillating elements 10 results in a rectangle when projected onto the side of the bimorph drive 2. The oscillating elements 10 are arranged parallel to each other, and each oscillating element 10 is located above a different pair of electrodes 5.

[0044] In the Fig. 4 are two different ways of stimulating the Fig. 3. The view is in the direction of the membrane 3, with the positions of the oscillating elements also indicated.

[0045] In the Fig. 4 a) The electrodes of the two pairs are each subjected to a voltage signal with a different sign. This is indicated by the "+" and "-" symbols, with the neighboring electrodes of the other electrode pair also receiving a different voltage signal sign. Thus, the sign of the excitation signal alternates for all four electrodes. As a result, the two oscillating elements perform movements along their wide sides, i.e., in their "prong plane." This is indicated by the arrows.

[0046] In the Fig. Figure 4 b) shows that all four electrodes are subjected to the same excitation signal. This is indicated here by the "-" sign. Membrane 3 performs a movement as a unit, which causes the two oscillating elements to tilt outward (indicated by the arrows). If the excitation signal has the opposite sign (here: "+"), the oscillating elements move toward each other. The oscillation occurs perpendicular to the wide side of the oscillating elements.

[0047] If the transducer device with a piezo drive is intended to excite and receive mechanical vibrations, for example, an electrode on one side of the piezoelectric element is connected to ground via a resistor, and an electrode on the opposite side is supplied with a square-wave signal. A signal tapped between one electrode and ground carries the sinusoidal signals of the vibrations of the mechanically oscillating unit. This signal corresponds to a differentiation of the excitation signal via the RC element generated by the transducer device.

[0048] The illustrations Fig. 5 and Fig. 6 each refer to a piezo drive 20 which has a uniform polarization in only one direction.

[0049] The Fig. Figure 5 shows how a piezo drive 20—an alternative term is piezo element—with four electrodes 5, 6 and a single polarization excites the oscillating elements 10 to oscillate parallel to the wide side (i.e., the p-mode). The control device 8 applies a rectangular signal to two electrodes 5, 6 from different electrode pairs. The differentiated signal, which carries information about the mechanical oscillations, is tapped from the other two electrodes 5, 6.

[0050] The equivalent circuit diagram shows four piezoelectric sub-elements, each of which receives an excitation signal via the conductor shown at the top, and from which the differentiated signal is tapped via the conductor shown at the bottom. The upper and lower electrodes 5, 6 are connected in parallel, with the two tapping electrodes connected to ground via a resistor. The common electrode located opposite the four electrodes 5, 6 on the full surface of the other side of the piezo drive (see Fig. Fig. 2) is not contacted with the excitation electronics and serves as a virtual ground.

[0051] If the piezo drive 20 of the example is to be used Fig. 5 an excitation of the s-mode (vibrations perpendicular to the wide side of the oscillating elements) takes place, the Fig. 6, how the four electrodes 5, 6 can be switched so that all four segments of the piezo drive 20 bend with the same phase.

[0052] For this purpose, all four electrodes 5, 6 on the side shown are connected in parallel. This also connects the four segments of the piezo drive 20 in parallel. The control device 8 applies a rectangular signal to the electrodes 5, 6. The common electrode 202, located on the rear side (not visible here), is connected to ground via a resistor. The received signal is tapped between the common electrode 202 and the resistor, along with the measurement parameters dependent on the mechanical vibration, such as frequency, amplitude, and / or phase.

[0053] The equivalent circuit diagram shows the wiring with four individual piezo elements connected in parallel, with the common electrode connected to the resistor.

[0054] The illustrations Fig. 7 and Fig. 8 shows the case where the piezo drive 20 is a so-called bimorph drive. The piezoelectric element thus has at least two segments or sections polarized in different directions. The four electrodes 5, 6, in particular, comprise four segments, each polarized in pairs in opposite directions.

[0055] The Fig. Figure 7 shows the circuit for the excitation of the p-mode, i.e. the oscillations parallel to the wide side of the oscillating elements 10.

[0056] The control device 8 outputs a rectangular signal to the two electrodes 5, 6 of the two adjacent electrode pairs. One segment of the piezo drive 20 is positively polarized, and the other segment is negatively polarized. The other two electrodes 5, 6 are each connected to ground via a resistor, and the received signals are also tapped from them. The two resistors serve to balance the oscillating elements 10 for the p-mode. The equivalent circuit diagram illustrates the excitation and shows that the common electrode (cf. Fig. 2) is not used.

[0057] For the excitation of the s-mode, the segments are arranged as in the Fig. 8. In the piezoelectric element, the two electrodes 5, 6 of each pair of electrodes are connected in parallel. The segments are polarized positively ("+") and negatively ("-"), respectively. The equivalent circuit illustrates the commutation of the segments. The common electrode (cf. Fig. 2) is also not used.

[0058] The Fig. Figure 9 shows an embodiment in which the transducer device 2 is formed by four separate piezoelectric elements 30, which are arranged at the same distance from a common center point of the circular membrane 3. The piezoelectric elements 30 are each polarized in the same direction, here as an example out of the plane of the drawing. The electrodes 5, 6 are shown, which are located on the end faces 31 of the circular-cylindrical piezoelectric elements 30. It can be seen that - as in the embodiment of the Fig.3 - the oscillating elements 10 are arranged so that they are each located above a pair of electrodes.

[0059] In general, the vibration sensor offers the following advantages: • The sensor is particularly suitable for digital excitation of the mechanically oscillating unit and for the evaluation of the oscillations or the signals generated from the oscillations. • A uniform switching logic can be used for different media such as liquids and bulk materials. • There is an alternative to temperature-compensated vibration sensors that uses temperature sensors. This is based on the fact that temperature affects both frequencies Fp and Fs equally, and that the temperature dependency is automatically taken into account using a ratio of the frequencies (e.g., Fp / Fs or Fs / Fp). • Sensors with two orthogonal vibration modes can operate with significantly more approach than other sensors because the approach affects the frequencies of the two vibrations essentially the same. • Sensor calibration is not required. • The shape of the oscillating unit, in particular that of the oscillating element, allows it to be used with media that tend to crystallize (ice formation, melting, etc.) or that contain components that can jam (e.g. fruit pulp in yogurt, fibers). List of reference symbols 1 mechanically oscillating unit 2 converter device 3 membranes 4 housings 5 electrode of an electrode pair 6 Electrode of another electrode pair 7 Insulation gap 8 Control device 9 Connection unit 10 Oscillating element 11 wide side of the oscillating element 12 narrow side of the oscillating element 20 bimorph drive 30 piezo element 31 front side 200 one side of the piezo drive 201 opposite side of the piezo drive 202 common electrode QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 100 14 724 A1 [0009, 0013] WO 01 / 66269 A1

[0013] DE 10 2004 010 992 B3

[0013]

Claims

[1] Vibration sensor, with at least one mechanically oscillatable unit (1) and a transducer device (2), wherein the transducer device (2) excites the mechanically oscillatable unit (1) to mechanical oscillations and / or receives mechanical oscillations from the mechanically oscillatable unit (1), and wherein the transducer device (2) has at least one piezo drive (20) and four electrodes (5, 6) on one side (200) of the piezo drive (20). [2] Vibration sensor according to claim 1, wherein the mechanically oscillatable unit (1) has two oscillating elements (10), wherein the two oscillating elements (10) are designed such that the two oscillating elements (10) each have different dimensions in two orthogonal directions, in that the oscillating elements (10) each have a wide side (11) and a narrow side (12), wherein the four electrodes (5, 6) form two pairs of electrodes, and wherein the wide side (11) of the oscillating elements (10) is arranged above a pair of electrodes. [3] Vibration sensor according to claim 1 or 2, wherein the piezo drive (20) is designed as a bimorph drive. [4] Vibration sensor according to one of claims 1 to 3, wherein the two oscillating elements (10) are mounted above and the transducer device (2) below a membrane (3), and wherein the side (200) of the piezo drive (20) on which the four electrodes (5, 6) are arranged faces away from the membrane (3). [5] Vibration sensor according to one of claims 1 to 4, wherein the four electrodes (5, 6) are designed substantially as equally sized circular sectors. [6] Vibration sensor according to one of claims 1 to 5, wherein the two oscillating elements (10) are designed essentially as paddles. [7] Vibration sensor according to one of claims 1 to 6, wherein the vibration sensor has a control device (8), and wherein the control device (8) applies excitation signals to the four electrodes (5, 6), and where the excitation signals are square wave signals.

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